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Optical Tunability and Characterization of Mg–Al, Mg–Ti, and Mg–Ni Alloy Hydrides for Dynamic Color Switching Devices
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Energy, Environmental, and Catalysis Applications

Optical Tunability and Characterization of Mg–Al, Mg–Ti, and Mg–Ni Alloy Hydrides for Dynamic Color Switching Devices
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ACS Applied Materials & Interfaces

Cite this: ACS Appl. Mater. Interfaces 2023, 15, 1, 1010–1020
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https://doi.org/10.1021/acsami.2c17264
Published December 25, 2022

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Abstract

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Mg shows great potential as a metal hydride for switchable optical response and hydrogen detection due to its ability to stably incorporate significant amounts of hydrogen into its lattice. However, this thermodynamic stability makes hydrogen removal difficult. By alloying Mg with secondary elements, the hydrogenation kinetics can be increased. Here, we report the dynamic optical, loading, and stress properties of three Mg alloy systems (Mg–Al, Mg–Ti, and Mg–Ni) and present several novel phenomena and three distinct device designs that can be achieved with them. We find that these materials all have large deviations in refractive index when exposed to H2 gas, with a wide range of potential properties in the hydride state. The magnitude and sign of the optical property change for each of the alloys are similar, but the differences have dramatic effects on device design. We show that Mg–Ti alloys perform well as both switchable windows and broadband switchable light absorbers, where Mg0.87Ti0.13 and Mg0.85Ti0.15 can achieve a 40% transmission change as a switchable window and a 55% absorption change as a switchable solar absorber. We also show how different alloys can be used for dynamically tunable color filters, where both the reflected and transmitted colors depend on the hydrogenation state. We demonstrate how small changes in the alloy composition (e.g., with Mg–Ni) can lead to dramatically different color responses upon hydrogenation (red-shifting vs blue-shifting of the resonance). Our results establish the potential for these Mg alloys in a variety of applications relating to hydrogen storage, detection, and optical devices, which are necessary for a future hydrogen economy.

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1. Introduction

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The chemical process of hydrogenation has been implemented as a mechanism to dynamically modify the optical properties of metallic thin films and nanostructures, ranging from pure elements to metallic compounds. (1−8) Mg absorbs a large amount of hydrogen upon H2 exposure and goes through dramatic changes in its optical properties. One challenge of using pure Mg in dynamic metal hydride devices is that the hydride state is too thermodynamically stable and must be unloaded at a high temperature. (9) The kinetics of both the absorption and the desorption are also slow, especially in bulk Mg, due to MgH2 being a poor proton conductor, which limits the diffusion of H through the material. (10) In order to improve upon the kinetic and thermodynamic properties of Mg, alloying has been extensively used to destabilize the hydride phase and increase hydrogen diffusion. Alloying has been attempted with many metals including Co, (11−14) Fe, (11,14−16) and Mn, (14,17) among others, but the most common secondary alloying elements used in the literature are Al, (18−22) Ti, (7,23−27) and Ni. (28−38) These alloys have increased kinetics when compared to pure Mg (21,39−41) and were each found to have interesting optical responses upon hydrogenation.
The Mg–Ni system in particular has been of interest in the optical community due to Mg2Ni forming an intermediate optical “black state” during its loading process. (33−35) This black state is characterized by an intermediate loading state where the sample absorbs >75% of incident visible light when illuminated through a transparent substrate. The process has been investigated with 15N nuclear reaction analysis hydrogen depth profiling (35) to determine the H atom vertical distribution in the sample. It was found that the black state formation is caused by preferential loading from the Mg2Ni/substrate interface, as opposed to the hydride being seeded near the dissociation sites at the Mg2Ni/Pd interface. A multi-layer interference effect occurs, where the bottom layer is fully hydrogenated Mg2NiHx and the top layer remains metallic Mg2Ni. This layered orientation creates the observed black state. In the fully hydrogenated state, some Mg–Ni alloys have also been found to have good switchable window properties, switching from a reflecting state to a transparent state. (32)
The Mg–Ti system has been investigated for switchable solar absorbers, (25) switchable mirrors, (42) and hydrogen sensor applications. (43) These materials have the advantage of having a highly absorbing state for wavelengths across the visible region for thick films (>200 nm) in their fully hydrogenated state, as opposed to Mg2Ni which obtains this highly absorbing state only during intermediate loading. (26) This high-absorbing state occurs in MgyTi1–y for y = 0.7–0.8. This alloying system is also of interest due to a decrease in degradation over many hydrogenation cycles, along with faster kinetics during these cycles, when compared to pure Mg. (26) For very thin Mg–Ti films (<40 nm), the hydride state is more color-neutral than Mg–Ni alloys (which have a yellow tint), allowing for more aesthetically pleasing switchable windows, albeit with fairly low transmission amounts ∼20%. (42)
Mg–Al alloys are of interest for their high hydrogen weight percent (weight of hydrogen in a material divided by the total weight of the material) for hydrogen storage along with much faster hydrogenation kinetics at room temperature when compared to Mg. (22) These materials also have been suggested as switchable window devices as Mg–Al alloys with ∼70% Mg have been demonstrated to have color-neutral transmission. (21)
To date, all optical measurements reported in the literature of these alloys consist of using normal incidence reflection and transmission measurements to determine their properties, such as the absorption coefficient and/or the optical band gap (e.g., in Mg–Ti (26) and Mg–Ni alloys (31,44)) and the dielectric function by of the reflection and transmission data with a Drude–Lorentz model (e.g., in Mg–Ni alloys (35,45)). These methods of obtaining thin-film optical properties are much less sensitive than variable-angle spectroscopic ellipsometry, especially for very thin films <50 nm. (46) There have been no reports in the literature of the optical properties of Mg–Al alloys, or of any Mg–Ti, Mg–Ni, or Mg–Al alloys in intermediate loading states. Our system allows for in situ ellipsometry to dynamically investigate the optical properties of thin films (<50 nm) of these materials with high sensitivity as they are hydrogenated. (1,2,47) By using similar deposition parameters to fabricate the different alloy systems, these measurements also allow us to quantitatively compare the responses of these different systems.
In this work, we measure in situ the dynamic optical properties of different atomic ratios of thin-film Mg–Al, Mg–Ti, and Mg–Ni alloys as they are exposed to H2 gas. We also quantify the loading and stress values of these alloys during their exposure to H2. We find large optical changes for all the alloys investigated, with Mg0.85Ti0.15 exhibiting the largest optical changes for any alloy. We confirm the optical black state in the Mg–Ni samples through transfer-matrix method (TMM) simulations using our measured optical properties and observe the highest absorbing state for Mg0.73Ni0.27, as expected with that atomic composition being the closest to Mg2Ni. We demonstrate three potential dynamic devices: switchable windows, switchable light absorbers, and tunable color filters. The Mg–Ti alloys exhibit the best properties for each application, with Mg–Ni alloys showing the widest range of different property changes depending on the alloy composition. Overall, our in situ experiments, quantitative analysis, and computational designs lay the foundation for scalable and robust devices. Our material selection and their intrinsic optical behavior are well-suited for photonic devices where reversible color changes are required.

2. Experimental Section

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Each thin-film sample is fabricated by room-temperature physical vapor deposition cosputtering. Two separate AT-cut 5 MHz quartz crystal microbalances (QCMs), a glass slide, and a Si chip are included in the deposition chamber for each deposition. Prior to deposition, samples are cleaned with acetone, methanol, isopropanol, and water. The alloys are deposited through a 12.5 mm diameter shadow mask centered on the QCM substrate. The direct current powers of the sputtering tool ranged from 50 to 450 W to attain the different alloy compositions. These minimum and maximum values are determined by the minimum voltage necessary to maintain a plasma and the voltage limit of the tool, respectively. Each sample was capped with a 3 nm Pd layer without breaking vacuum to catalyze the hydrogenation reaction and prevent surface oxidation of the sample. The composition of each alloy is determined with EDX taken on the Si chip included in the deposition chamber, taking an average of five measurements at different points on the sample to ensure uniform alloying.
The optical properties of the materials are determined with in situ spectroscopic ellipsometry using the system described in Palm et al. (1) To determine the thickness of the Mg alloy films, the glass slide sample that is included in the deposition chamber is measured with ellipsometry and transmission measurements. The raw Ψ and Δ data are then fit with an optical model with the properties of the Mg alloy and the thickness of that alloy as fit parameters. The optical properties of the glass substrate are taken before the metal deposition and are defined in the model. The properties of the sputtered Pd are measured separately and are defined in the model. The thickness of the Pd layer is set to be 3 nm in the model for all alloy samples. With the transmission measurements, this fitting procedure allows for unambiguous determination of the thickness of the Mg alloy films. The thicknesses of the films we investigated ranged from 19 to 42 nm. As a consistency check for our thickness measurements, we use the same model properties and thicknesses on the data taken on the Au QCM electrode substrate and find good agreement without refitting the model parameters.
The volume expansion of these Mg-based alloys is an important factor in the thin-film optical model fittings. The Mg alloy thin films investigated here have appreciable transmission, and the optical effect of the substrate must be taken into account. For these alloys, the volume expansion for each is found to be ∼15% over the atomic ratio region that we are investigating, (35,48) and we use this value for all of our samples. To include this expansion in the model, we use the dynamic loading data to determine how much H is in the film at each optical time step and then scale the total thickness expansion by the same ratio as the current loading to the final loading value (i.e., if a film’s final calculated loading value is 1 and at time step t, we calculate that the loading is H/M = 0.33, then we define the volume expansion at this time step to be 0.33 × 15% = 5%). H/M is defined to be the number of hydrogen atoms per metal atom in the metal alloy lattice, with the total number of metal atoms equaling the number of Mg atoms added to the number of atoms from the secondary alloying element. We also define the loading amount of the Pd capping layer by this same ratio. At each time step, with the thicknesses and Pd cap properties defined, we then use a B-spline model with 0.3 eV node spacing to fit the optical properties of the Mg-alloy.
The hydrogen loading for each alloy is measured on the second QCM sample with the method outlined in a study by Murray et al. (47) To convert from the QCM frequency to the H/M loading ratio, the density of each sample must be input into the calculation. For Mg–Ti, both Mg and Ti have hexagonal close-packed lattice structures, and no known intermetallic states form. Because of this behavior, we believe that a linear weighting of the densities of Mg and Ti in the same ratio as their atomic percent in the alloy is a reasonable approximation of the density. As a check of this assumption, we compare densities calculated from linear weighting with densities calculated from lattice constant measurements using X-ray diffraction measurements (23) and find good agreement (<6% difference). For the Mg–Al system, we also choose to use this linear weighting scheme because there are no other alloy phases found for this material when the atomic Mg percent >60%. (21) For the Mg–Ni system, we use a slightly different method. Mg2Ni is a known intermetallic with a density of 3.48 g/cm3 and because this forms a separate phase within the Mg–Ni alloys, a linear combination of pure metal densities is not applicable. From ref (44), we know for MgyNi1–y if 0.67 < y < 0.89, the alloy forms a varying mixture of crystalline Mg2Ni and amorphous Mg0.89Ni0.11, but the lattice constant remains constant in this region. If 0.89 < y < 0.95, the lattice constant begins to expand, and the alloy is mainly nanocrystalline Mg2Ni and crystalline Mg. Using this knowledge of the phases, we use the following equation to calculate the densities of the Mg–Ni alloys for 0.67 < y < 0.89
ρMgyNi1−y=ρMg2NiAMgy+ANi(1−y)0.67AMg+0.33ANi
(1)
where ρMg2Ni is the density of Mg2Ni, AMg is the atomic mass of Mg, and ANi is the atomic mass of Ni. Here, we calculate the density by multiplying the mass ratio of the alloy to Mg2Ni with the known density of Mg2Ni. This result is using the fact that the volume of the lattice is not changing from the Mg2Ni size, thus allowing us to only account for mass. For higher Mg percent with 0.89 < y < 0.95, we have
ρMgyNi1−y=11+y−0.891−y(ρMg2NiAMgy+ANi(1−y)0.67AMg+0.33ANi)+y−0.891−y1+y−0.891−yρMg
(2)
where ρMg is the density of pure Mg. Here, we linearly weight the density of Mg0.89Ni0.11 (calculated with eq 1) with the density of Mg. This rationale uses the fact that for samples with up to 89% Mg, there is no expansion in the lattice, and with additional Mg added above the 89% point, we are adding crystalline Mg with the density ρMg. Using these calculated densities and the thicknesses found from the optical fittings, we can then calculate the loadings and stresses with the method outlined in a study by Murray et al. (47)

3. Results and Discussion

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3.1. Optical Properties of Mg Alloy Hydrides

In the following sections, we report and discuss the dynamic optical properties of our fabricated Mg–Al, Mg–Ti, and Mg–Ni alloys as they are exposed to 1 atm H2 gas.

3.1.1. Mg–Al Hydrides

Figure 1 shows the measured optical properties ñ = n + ik of four different compositions of Mg–Al alloys and how these properties change under complete hydrogenation under 1 bar H2 pressure. For these materials, we observe a small spread in the initial and final properties of the materials, which is as expected due to all the alloys being close together in the atomic composition. The reason for this proximity in composition is a relative lack of sensitivity of the different materials to different deposition voltages (i.e., 200 W Mg and 200 W Al powers compared to 450 W Mg and 50 W Al powers only give a 7% difference in atomic Mg percent). We find that both the real and imaginary parts of the index of refraction increase with longer wavelengths for the alloy in the metallic state as we expect from most lossy metals. Interestingly, we find that metallic Mg0.77Al0.23 exhibits the largest n and k across the spectrum, as well as the largest optical change in k. We find that for all the Mg–Al alloys, the change in the optical properties upon hydrogenation follow the same trend. For Δn, there is little variation from sample to sample. Each sample exhibits a positive Δn for shorter wavelengths and a negative change for longer wavelengths, with the crossover point from positive to negative Δn occurring between 1100 and 1225 nm. All samples exhibit a decrease in k across the measured spectrum, with the largest decreases occurring in the NIR and the smallest decreases in the visible spectrum. Despite having the largest Δk, fully loaded Mg0.77Al0.23Hx still has the largest k across the visible spectrum, although only by a small margin, and the second highest in the NIR. In the final hydride state, as shown in Figure 1c, we find that each of the materials still has significant attenuation in the long-wavelength visible and into the NIR spectrum, with k > 3 after ∼1000 nm for each alloy. This behavior is despite the fact that the NIR optical properties saw the largest decreases in k. For the real part of the index of refraction, we find a minimum in the mid-visible spectrum that increases to large values (>3) in the NIR. The hydride samples are still somewhat optically metallic, not exhibiting a complete transition to a dielectric material.

Figure 1

Figure 1. Optical properties ñ = n + ik of four different Mg–Al alloy hydrides. (a) Optical properties in the pristine metallic state before hydrogenation. (b) Change in optical properties upon hydrogenation, defined here as the pure metal optical properties subtracted from the hydride optical properties. (c) Optical properties in the fully hydrogenated hydride state. Each colored line on the plot represents a different atomic ratio of metal hydride, with darker shades representing higher atomic Mg percent ratios.

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As shown in Figure 2, we observe the intermediate states during the hydrogen loading. These plots show a smooth transition of the optical properties from the metallic (lighter colored curves) to the hydride states (darker colored curves). Note that the chosen optical curves on this plot are not linearly spaced with time but are instead chosen to show the range of properties of the intermediate states. Loading generally begins slowly as small amounts of H2 are introduced to the chamber, then increases quickly during the beginning of the α to β phase transition, and finally slows down to a long tail for the final ∼10% of the load when most of the material is in the β phase. Most of our materials show similar time dynamics, with the total time of loading ranging from 10–15 min for most samples.

Figure 2

Figure 2. Optical properties of different Mg–Al alloys as they are exposed to H2 gas. The lightest colored line depicts the alloy in the pristine metallic state. As H2 is introduced to the system, the material begins to hydrogenate, denoted by the lines getting darker in the plot, with the darkest line indicating the full hydride state. Each line is not linearly spaced in time and is instead chosen to aesthetically show the range of possible intermediate states. The alloys shown here are (a) Mg0.77Al0.23, (b) Mg0.81Al0.19, (c) Mg0.83Al0.17, and (d) Mg0.84Al0.16.

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3.1.2. Mg–Ti Hydrides

Figure 3a shows the metallic optical properties of five different fabricated Mg–Ti alloys with the atomic Mg percent ranging from 82–91%. We find that the n values in the metallic state are fairly close together, with the exception of Mg0.89Ti0.11, which exhibits a lower n for the state. The higher Mg percent alloys exhibit higher k values, again except for Mg0.89Ti0.11 (we will discuss the discrepancies of the Mg0.89Ti0.11 sample further down). Higher attenuation for samples with more Mg is expected as Mg has much higher k values than Ti in their unalloyed form. The metallic properties of these Mg–Ti samples also align fairly closely with the metallic Mg–Al alloys investigated in the previous section. As we hydrogenate these samples, we find similar types of changes in the optical properties when compared to the Mg–Al samples, with n increasing in the visible and decreasing the in the NIR spectrum, and with k decreasing for all wavelengths, with larger decreases for longer wavelengths. There is a much broader range in the changes in properties for these alloys, and two alloys, Mg0.85Ti0.15 and Mg0.87Ti0.13, exhibit larger changes than the Mg–Al samples. Figure 3c shows the properties of the hydride states. The three lowest Mg percent samples show somewhat constant n in the visible spectrum, with peaks in the ultraviolet, and increasing values for longer wavelengths into the NIR spectrum. The two highest Mg percent samples have smaller n values in the visible, with sharper minima that monotonically increase with longer wavelengths, similar to the Mg–Al hydrides. These two samples also have the largest k by a significant margin. The lower Mg percent samples have relatively smaller k values, with Mg0.85Ti0.15 exhibiting almost no attenuation across the visible and NIR spectrum.

Figure 3

Figure 3. Optical properties ñ = n + ik of five different Mg–Ti alloy hydrides. (a) Optical properties in the pristine metallic state before hydrogenation. (b) Change in optical properties upon hydrogenation, defined here as the pure metal optical properties subtracted from the hydride optical properties. (c) Optical properties in the fully hydrogenated hydride state. Each colored line on the plot represents a different atomic ratio of metal hydride, with darker shades representing higher atomic Mg percent ratios.

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As shown in Figure 4, we observe the intermediate states during the hydrogen loading. These plots mostly show smooth optical transitions, except for Mg0.85Ti0.15 and Mg0.87Ti0.13, which show a resonance-like dip in n between 500 and 600 nm for high hydrogen content states. The Mg0.82Ti0.18 sample also slightly shows this effect. For the Mg0.85Ti0.15 sample, we also observe a decrease in n in the NIR spectrum until the almost fully hydrogenated state and then a sharp increase in n when the hydrogenation is complete. Note that again the chosen curves on this plot are not linearly spaced with time but are instead shown to depict the range of properties of the intermediate states.

Figure 4

Figure 4. Optical properties of different Mg–Ti alloys as they are exposed to H2 gas. The lightest colored line depicts the alloy in the pristine metallic state. As H2 is introduced to the system, the material begins to hydrogenate, denoted by the lines getting darker in the plot, with the darkest line indicating the full hydride state. Each line is not linearly spaced in time and is instead chosen to aesthetically show the range of possible intermediate states. The alloys shown here are (a) Mg0.82Ti0.18, (b) Mg0.85Ti0.15, (c) Mg0.87Ti0.13, (d) Mg0.89Ti0.11, and (e) Mg0.91Ti0.09.

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3.1.3. Mg–Ni Hydrides

Lastly, we investigate the properties of Mg–Ni alloys. Figure 5a shows the properties in the metallic state, where we see a much larger spread in initial n values for these materials, but n and k still follow the same trend as was found with the other Mg alloys (n and k generally increase with increasing wavelength). This larger spread in initial properties is expected as we are able to obtain a larger range of atomic ratios for this alloy system compared to the other two, ranging from 59–92% Mg. In Figure 5b, we see the same trends of optical property change as Mg–Al and Mg–Ti with large decreases in k across the spectrum upon hydrogenation and increases in n in the visible spectrum with decreases in the NIR spectrum. In the hydride state in Figure 5c, we see a large range of potential final optical properties depending upon the alloy ratio. Generally, the lower Mg percent hydrides have a higher n across the spectrum, with the Mg0.90Ni0.10 sample demonstrating the lowest n across most of the spectrum and Mg0.59Ni0.41 the highest. Most of the hydrides have low attenuation in the visible spectrum, with k < 2. The higher Mg percent hydrides then have larger attenuation into the NIR spectrum, while the lower Mg percent samples exhibit a more constant k across the measured spectrum.

Figure 5

Figure 5. Optical properties ñ = n + ik of six different Mg–Ni alloy hydrides. (a) Optical properties in the pristine metallic state before hydrogenation. (b) Change in optical properties upon hydrogenation, defined here as the pure metal optical properties subtracted from the hydride optical properties. (c) Optical properties in the fully hydrogenated hydride state. Each colored line on the plot represents a different atomic ratio of metal hydride, with darker shades representing higher atomic Mg percent ratios.

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Figure 6 shows the dynamic transition-state data for the Mg–Ni samples. For each of the samples, the transitions are mostly monotonic without any exotic features. For the intermediate loading states, here we are modeling the Mg–Ni film as homogeneous throughout the thickness of the film. As was discussed in the Introduction section, it has been found that Mg–Ni films do not load homogeneously but instead with preferential phase formation from the substrate of the film that propagates to the Pd cap. Our modeling process is not contradictory to this process and still allows for the bulk characterization of the properties of the film in this orientation with illumination through the Pd cap. The low mean squared error (MSE) obtained in our optical fits indicates that our model accurately captures the optical properties of our samples. For our dynamic fits, the MSE for any individual fit was never greater than 10, indicating a good fit. However, for a different illumination orientation of the device (i.e., backside illumination through a transparent substrate), our intermediate model fits would not account for the layering effects of the loading. To model those bulk responses with our setup, the optical properties would have to be measured in that same orientation. Note that this layering effect would only affect the properties in the intermediate states and have no effect on the modeling of the metallic or fully hydrogenated states, as shown in Figure 5.

Figure 6

Figure 6. Optical properties of different Mg–Ni alloys as they are exposed to H2 gas. The lightest colored line depicts the alloy in the pristine metallic state. As H2 is introduced to the system, the material begins to hydrogenate, denoted by the lines getting darker in the plot, with the darkest line indicating the full hydride state. Each line is not linearly spaced in time and is instead chosen to aesthetically show the range of possible intermediate states. The alloys shown here are (a) Mg0.59Ni0.41, (b) Mg0.73Ni0.27, (c) Mg0.84Ni0.16, (d) Mg0.85Ni0.15, (e) Mg0.90Ni0.10, and (f) Mg0.92Ni0.08.

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To determine whether any of our materials would exhibit this black state with backside illumination, we modeled the multi-layer loading process with TMM simulations. Figure 7a shows the simulation architecture. The samples consist of illumination through a SiO2 substrate. The layers proceed from the substrate with the fully hydrogenated Mg–Ni hydride, the fully metallic Mg–Ni alloy, and lastly a 3 nm PdHx capping layer. The hydrogenation of the material is simulated by beginning with the Mg–Ni–H layer equal to 0 nm and then increasing this layer size while decreasing the Mg–Ni layer by the same amount, until the sample is completely hydrogenated. We defined the alloy thickness to be 25 nm for these simulations. Using this method, we find a high-absorbing intermediate state for multiple samples, with the largest absorption occurring for the Mg0.73Ni0.27 sample, which is expected as this material is the closest composition to Mg2Ni for which the black state was initially discovered. In Figure 7b, we show the absorption curves for different loading thicknesses for this alloy, where the peak absorption occurs at a hydride thickness of 12 nm, which is equal to half of the thin film being loaded.

Figure 7

Figure 7. Modeled absorption from backside illumination of Mg0.73Ni0.27. (a) Simulation architecture for the sample loading. Sample consists of a SiO2 substrate, followed by a fully hydrogenated Mg–Ni–H layer, then a fully metallic Mg–Ni layer, and finally a 3 nm Pd capping layer. The total Mg alloy thickness is defined to be 25 nm. Simulated hydrogenation is defined as an increase in the thickness of the hydride layer and a decrease of the same magnitude of the metal layer. (b) Calculated absorption for different loading thicknesses using Mg0.73Ni0.27 optical properties.

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3.2. Stress and Loading Properties

With the second QCM crystal included in the deposition chamber, we dynamically measure the loading and stress properties of the films. The loading values for all the samples are shown in Figure 8. Figure 8a shows the loading values of the Mg–Al samples. We can see that the samples have loadings near H/M = 1, with the highest measured loading at 1.28 for Mg0.77Al0.23 and the lowest loading at 0.75 for Mg0.81Al0.19. These values are at the low end of the range of hydrogen loading measurements reported in the literature for alloys in this composition range, which find loading values between H/M = 0.85–1.5. (22,44) The Mg0.77Al0.23 also exhibited the largest optical property change compared to the other Mg–Al samples. Future work looking at these samples should investigate fabrication of higher Al atomic percentages to determine if there is a correlation of higher loading for higher Al percent for any range of compositions and if the optical property change is greater in this region.

Figure 8

Figure 8. Measured maximum loading values under 1 bar H2 for different thin films for (a) Mg–Al, (b) Mg–Ti, and (c) Mg–Ni alloy samples. The horizontal and vertical error bars represent one standard deviation uncertainty in the atomic Mg % and the loading, respectively.

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The loading values of the Mg–Ti films are shown in Figure 8b. Three of these values are in general agreement in the literature, which finds that the average amount of loading of Mg–Ti alloys in this atomic composition range is H/M = 1.55. (26,41) We find two samples that fall measurably below this average, with Mg0.82Ti0.18 at 0.96 and Mg0.89Ti0.11 at 0.79. We also find that the Mg0.89Ti0.11 sample had the lowest optical change of any of the investigated alloys. We suspect that there was an issue with the fabrication of this sample that prevented a complete loading. This could have been caused by an incomplete Pd capping layer that did not fully encapsulate the sample, allowing for oxidation of the surface of the alloy, or potential alloying between the Pd capping layer and Mg near the surface of the Mg–Ti alloy.
For Mg–Ni alloys in Figure 8c, we see generally lower calculated loadings than that for the other two alloys. We also find a slightly negative correlation between the loading amount and the atomic Mg percent. This does not agree with previously found data in the literature, which found that there should be a slightly positive correlation between these values and that they should fall between 1.2 and 1.4 H/M. (31) This loading difference could be attributed to differences in sample preparation. Other thin-film Mg–Ni alloys have been fabricated with multi-layer metal deposition followed by a high-temperature anneal, as opposed to our cosputtering method. These different fabrication conditions could potentially be forming different alloying phases within the metal, which would affect the total loading amount. Further studies on the crystal structures of Mg–Ni alloys fabricated with these two techniques should be carried out to determine if there is any difference.
In Figure 9, we show the total stress change of each of the measured Mg alloys upon full hydrogenation. These stress changes are compressive and are defined to be positive. We find the stresses for the samples to be consistent within a material system. Taking the averages of the stresses in each system, the Mg–Al samples have the highest stresses with an average of 0.56 GPa, next is Mg–Ti with an average of 0.48 GPa, and finally the Mg–Ni alloys have the lowest measured stress with an average of 0.36 GPa.

Figure 9

Figure 9. Measured total stress change values for different thin-film samples for (a) Mg–Al, (b) Mg–Ti, and (c) Mg–Ni alloy samples. The change in stress reported here is the stress change from the initial pristine metal mounted in the environmental chamber to the fully hydrogenated state under 1 bar H2 (does not include intrinsic stress of the initial pristine alloy). The horizontal and vertical error bars represent one standard deviation uncertainty in the atomic Mg % and the stress change, respectively.

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3.3. Applications

In this section, we use our measured optical properties of the alloys to demonstrate three potential applications. We use the TMM to simulate thin-film responses of these materials for different thicknesses and on distinct substrates. The first application of these materials is for switchable window technologies. As an example, we simulate the transmission through the alloys in their metallic and hydride states and compare the transmission amounts. We use a metallic alloy thickness of 25 nm with a 3 nm Pd capping layer. We find that these thicknesses are in the ideal range for switchable window purposes because it is just thick enough in the metallic state to create high reflection, while remaining thin enough to allow appreciable transmission in the hydride state. These simulations also consider the 15% volume expansion of the alloys upon hydrogenation. Another important factor in window technologies is to have color-neutral transmission. Windows with non-neutral color transmission tint the light as it transmits through the window, which is not ideal when attempting to make a clear window. To model this color distortion, we use the CIE 1931 XYZ color space and plot the perceived colors of the transmitted spectrum. Note that only x and y need to be plotted to fully characterize the color because x + y + z = 1. On these plots, color-neutral is the x = y = 0.333 data point, which creates the ideal window. We show these switchable window properties for all the investigated alloys in Figure 10.

Figure 10

Figure 10. Switchable windows with Mg alloys. Simulated transmission values of thin films (a) Mg–Al, (b) Mg–Ti, and (c) Mg–Ni alloys on SiO2. The stack is defined as an SiO2 substrate, a 25 nm Mg alloy film, and a 3 nm Pd capping layer. Transmission in the hydride (metal) state is represented by solid (dashed) lines. (d) CIE 1931 chromaticity diagram with the transmission color points for the different alloys. Colors of the points match with the hues in part a–c. (e) Zoom-in of chromaticity diagram. The intersection of the black dashed lines represents color neutral at x = y = 0.333.

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For the Mg–Al alloys, we see poor transmission in the hydride state, with the Mg0.84Al0.16 alloy having the highest transmission through the visible spectrum with values <40% transmission for most of the spectrum. These materials generally only exhibit ∼20% absolute change in transmission upon hydrogenation, agreeing with results previously reported in the literature for MgxAl1–x with x > 0.69. (21) This behavior is due to the materials still exhibiting a high attenuation in the visible, even in the hydride state. These windows are close to color neutral, even with their low transmission, adding a small blue–green tint to the transmitted light.
Some of the Mg–Ti alloys perform much better as switchable window technologies with transmissions values >60% across most of the visible spectrum for Mg0.85Ti0.15 and Mg0.87Ti0.13. For these samples, we observe a transmission change of ∼40%. Higher transmission can be achieved in the hydride state by a thinning of the sample; however, this thinning causes the transmission in the metallic state to also become significantly higher. We find the transmission colors of these windows to be similar to those of the Mg–Al samples, being mostly color neutral with a slight blue–green tint. Mg0.85Ti0.15 is the most color neutral, in addition to having the highest transmission across most of the visible spectrum.
The Mg–Ni alloys measured in this manuscript have poor switchable window characteristics, with low transmissions in the hydride state in the shorter wavelength visible region, with similar results for a Mg-rich Mg–Ni films having been found previously where a peak transmission of 25% was found for an 80 nm thin film. (32) For the longer wavelength visible, we see increased transmission but still only achieve values of ∼50%. However, we do see ∼40% transmission changes in this region. Some Mg–Ni samples exhibit very good color neutrality, with Mg0.92Ni0.08 having a transmission color value of x = 0.325 and y = 0.338. The other alloys have a slight yellow tint, as opposed to the blue–green tint of the Mg–Al and Mg–Ti alloys.
As mentioned in the Introduction, Mg–Ti alloys have also been investigated as broadband switchable light absorbers, with a highly reflecting state in the metallic form and a highly absorbing state when hydrogenated. The high absorption states for these materials have been found with thicker samples >200 nm, much thicker than measured here, and it has been demonstrated that the total absorption of the film can be significantly tuned by varying the thickness of the film. (26,49) To see if our measured properties show any potential for switchable absorption, we modeled a 300 nm Mg–Ti film on an SiO2 substrate with a 10 nm Pd capping layer. The results of these simulations are shown in Figure 11. We see that for three of our measured Mg–Ti alloys, we achieve large amounts of switchable absorption throughout the visible spectrum, with absorption tailing off into the NIR spectrum. In the visible wavelength region, Mg0.87Ti0.13, Mg0.85Ti0.15, and Mg0.82Ti0.18 all achieve >80% absorption in the hydride state with <25% absorption in the metallic state (corresponding to high reflection in this state). This is a very large absorption change upon hydrogenation for these alloy compositions and shows their potential for broadband switchable light absorbers. These results agree with the previously reported literature showing that MgxTi1–x alloys with x ≈ 0.2 is an excellent chemical composition for solar absorption. (25)

Figure 11

Figure 11. Broadband switchable light absorbers using Mg–Ti alloys. (a) Schematic of switchable light absorber consisting of a 300 nm Mg–Ti alloy with a 10 nm Pd cap on a SiO2 substrate. (b) Absorption plots for this structure for different alloy compositions. Solid (dashed) lines are absorption in the hydride (metallic) state. Colors represents different alloy compositions.

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Finally, we investigate the potential of utilizing these metal alloys as tunable color filters. Our proposed structure uses a cavity structure, consisting of a 20 nm Au backplane on SiO2, a variable length SiO2 cavity, capped with 25 nm of Mg alloy with a 3 nm Pd cap. We find that the Mg–Ti alloys have the most vibrant color change, with Mg87Ti17 exhibiting a change from a pale orange to a vibrant blue with an SiO2 cavity length of 260 nm. The Mg–Ni alloys showed an interesting effect upon hydrogenation, with some alloys exhibiting a blue-shift in color change upon hydrogenation, while other exhibit a red-shift (Figure 12g–l). Combining these different alloys into a single structure could allow for an even greater relative color change in a device. The Mg–Al shows the smallest color change of the alloys investigated in either the reflection or transmission spectrum.

Figure 12

Figure 12. Dual-functionality reflective and transmissive color filters with Mg alloy hydrides. (a) Simulated transmission plots of the Mg81Al19 color filter in both the metallic and hydride states with 150, 200, and 250 nm SiO2 cavity thickness. The inset shows the schematic of the proposed tunable color filter structure and defines the cavity thickness d. (b) Reflected and (c) transmitted colors of the device at different hydrogenation levels and cavity thicknesses. Similar plots are shown for (d–f) Mg87Ti13, (g–i) Mg73Ni27, and (j–l) Mg59Ni41.

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4. Conclusions

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In conclusion, we have for the first time experimentally measured the complex optical properties of different compositions of Mg–Al, Mg–Ti, and Mg–Ni alloys during the hydrogenation process using variable-angle spectroscopic ellipsometry. By quantifying changes in the refractive index of these alloys, we accurately designed three classes of devices: switchable windows, solar absorbers, and color filters. Our results show the chemical composition dependence on the optical properties of each of these devices, agreeing with the literature on overlapping compositional data points for the switchable windows and solar absorbers, with no literature suggesting tunable color filters with these materials to the authors’ knowledge. Surprisingly, we found a wide range in optical properties for the alloys in the final hydrogenated state, albeit most of the samples showing similar properties in the metallic state. We dynamically measured the loadings and stresses of all samples and found that the loadings of our materials are less than those previously reported in the literature, suggesting that these material properties are likely sensitive to fabrication differences. Future work will focus on elucidating the effects of different fabrication techniques (evaporation vs sputtering, annealing affects, substrate preparation, etc.) on the different material properties of these alloys. We also explored the implementation of these materials into optical devices, showing that Mg0.87Ti0.13 and Mg0.85Ti0.15 can achieve 40% transmission changes as switchable windows and 55% absorption changes as switchable solar absorbers. The alloys could also be used as active layers in tunable color filters. The switching process of the dual-reflective-transmissive filter introduced here is scalable and reveals a previously overlooked class of materials, Mg-based-alloys, as a model system for color pixels to be applied in situations where reconfigurability is desired, for example, encryption, tunable windows, or holography.

Author Information

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  • Corresponding Author
  • Authors
    • Kevin J. Palm - Department of Physics, University of Maryland, College Park, Maryland 20742, United States;  Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, Maryland 20742, United States;  Orcidhttps://orcid.org/0000-0003-4066-4584
    • Micah E. Karahadian - Department of Electrical and Computer Engineering, University of California, Davis, California 95616, United States
    • Marina S. Leite - Department of Materials Science and Engineering, University of California, Davis, California 95616, United States;  Orcidhttps://orcid.org/0000-0003-4888-8195
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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M.S.L. thanks the financial support from the National Science Foundation under grant no. 2016617 (DMR). KJP and JNM thank Google LLC for additional financial support. The authors also acknowledge support from the FabLab at the University of Maryland Nanocenter and additional funds from the University of California, Davis. K.J.P. is supported by a National Defense Science and Engineering Graduate Fellowship.

References

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This article references 49 other publications.

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    Ternary hydrides Mg2CoH5 and Mg6Co2H11 were obtained by a combined technique that included ball milling of a 2Mg-Co mixt. under Ar followed by sintering in hydrogen atm. (5900 kPa) at 425 °C. Formation and decompn. Van't Hoff diagrams for these hydrides were deduced from pressure-compn. isotherms and related changes of enthalpy and entropy were afterwards estd. While decompn. enthalpy values for both hydrides are lower than previously published ones, formation enthalpy for Mg2CoH5 is closed to the only reported data. Calcd. formation enthalpy for Mg6Co2H11 is a novel contribution for the thermodn. characterization of the system. Based on Van't Hoff diagrams for hydride formation obtained in this work, several synthesis routes were designed as consecutive sintering stages of const. temp. and hydrogen pressure. They allow the study of the influence of both parameters and treatment duration on reaction kinetics and resulting phases. Synthesis procedures for prodn. of MgH2, Mg6Co2H11 or a mixt. of ternary hydrides were identified and involved mechanisms during hydride formation were proposed. These expts. evidenced the key role that the dependence of Mg and Co mobilities with temp. has in the prodn. of these ternary hydrides.
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  13. 13
    Shao, H.; Liu, T.; Wang, Y.; Xu, H.; Li, X. Preparation of Mg-Based Hydrogen Storage Materials from Metal Nanoparticles. J. Alloys Compd. 2008, 465, 527– 533,  DOI: 10.1016/j.jallcom.2007.11.003
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    Preparation of Mg-based hydrogen storage materials from metal nanoparticles
    Shao, Huaiyu; Liu, Tong; Wang, Yuntao; Xu, Hairuo; Li, Xingguo
    Journal of Alloys and Compounds (2008), 465 (1-2), 527-533CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    By hydrogen plasma metal reaction method, we obtained Mg, Ni, Co, Cu and Fe nanoparticles. Mg nanoparticles show larger av. particle size than Ni, Co, Cu and Fe ones. From these metal nanoparticles, Mg-based hydrogen storage alloys (Mg2Ni, Mg2Co and Mg2Cu) and hydrides (Mg2NiH4, Mg2CoH5 and Mg2FeH6) were prepd. by solid-solid and gas-solid reactions. Powder X-ray diffraction (XRD) was used to define the structure and compn. information. The prepn. results in different atm. were compared and discussed. Hydrogen and nanostructure play important roles in the prepn. of Mg-based hydrogen storage alloys/hydrides in convenient conditions from metal nanoparticles.
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  14. 14
    Richardson, T. J.; Slack, J. L.; Farangis, B.; Rubin, M. D. Mixed Metal Films with Switchable Optical Properties. Appl. Phys. Lett. 2002, 80, 1349– 1351,  DOI: 10.1063/1.1454218
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    Mixed metal films with switchable optical properties
    Richardson, T. J.; Slack, J. L.; Farangis, B.; Rubin, M. D.
    Applied Physics Letters (2002), 80 (8), 1349-1351CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Thin, Pd-capped metallic films contg. Mg and 1st-row transition metals (Mn, Fe, Co) switch reversibly from their initial reflecting state to visually transparent states when exposed to gaseous H or following cathodic polarization in an alk. electrolyte. Reversion to the reflecting state is achieved by exposure to air or by anodic polarization. The films were prepd. by cosputtering from one Mg target and one Mn, Fe, or Co target. Both the dynamic optical switching range and the speed of the transition depend on the Mg-transition metal ratio. IR spectra of films in the transparent, hydrided (deuterided) states support the presence of the intermetallic hydride phases Mg3MnH7, Mg2FeH6, and Mg2CoH5.
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  15. 15
    Lima, G. F.; Jorge, A. M.; Leiva, D. R.; Kiminami, C. S.; Bolfarini, C.; Botta, W. J. Severe Plastic Deformation of Mg-Fe Powders to Produce Bulk Hydrides. J. Phys. Conf. 2009, 144, 012015,  DOI: 10.1088/1742-6596/144/1/012015
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    Berlouis, L. E. A.; Cabrera, E.; Hall-Barientos, E.; Hall, P. J.; Dodd, S. B.; Morris, S.; Imam, M. A. Thermal Analysis Investigation of Hydriding Properties of Nanocrystalline Mg-Ni- and Mg-Fe-Based Alloys Prepared by High-Energy Ball Milling. J. Mater. Res. 2001, 16, 45– 57,  DOI: 10.1557/JMR.2001.0012
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    16
    Thermal analysis investigation of hydriding properties of nanocrystalline Mg-Ni- and Mg-Fe-based alloys prepared by high-energy ball milling
    Berlouis, L. E. A.; Cabrera, E.; Hall-Barientos, E.; Hall, P. J.; Dodd, S. B.; Morris, S.; Imam, M. A.
    Journal of Materials Research (2001), 16 (1), 45-57CODEN: JMREEE; ISSN:0884-2914. (Materials Research Society)
    The hydrogen loading characteristics of nanocryst. Mg, Mg-Ni (Ni from 0.1 to 10 at.%), and Mg-Fe (Fe from 1 to 10 at.%) alloys in 3 MPa H2 were examd. using high pressure differential scanning calorimetry and thermogravimetric anal. All samples showed rapid uptake of hydrogen. A decrease in the onset temp. for hydrogen absorption was obsd. with increasing Ni and Fe alloy content, but the thermal signatures obtained suggested that only Mg was involved in the hydriding reaction; i.e., no clear evidence was found for the intermetallic hydrides Mg2NiH4 and Mg2FeH6. Hydrogen loading capacity decreased with temp. cycling, and this was attributed to a sintering process in the alloy, leading to a redn. in the sp. surface available for hydrogen absorption.
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  17. 17
    Lu, Y.; Kim, H.; Sakaki, K.; Hayashi, S.; Jimura, K.; Asano, K. Destabilizing the Dehydrogenation Thermodynamics of Magnesium Hydride by Utilizing the Immiscibility of Mn with Mg. Inorg. Chem. 2019, 58, 14600– 14607,  DOI: 10.1021/acs.inorgchem.9b02253
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    17
    Destabilizing the Dehydrogenation Thermodynamics of Magnesium Hydride by Utilizing the Immiscibility of Mn with Mg
    Lu, Yanshan; Kim, Hyunjeong; Sakaki, Kouji; Hayashi, Shigenobu; Jimura, Keiko; Asano, Kohta
    Inorganic Chemistry (2019), 58 (21), 14600-14607CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Hydrogen storage is a key technol. for the advancement of hydrogen and fuel cell power technologies in stationary and portable applications. MgH2, one of high-capacity hydrogen storage materials, has two major material challenges for practical applications: slow hydrogen desorption kinetics and high hydrogen desorption temp. Numerous studies have reported enhancement in kinetics but only few in thermodn. Here, we present a simple but effective way to improve in both aspects by utilizing the immiscibility of Mn, a non-hydrogen absorbing metal, with Mg. Mg0.25Mn0.75, prepd. through ball milling MgH2 and Mn powders, is nanocomposite where nanometer-sized MgH2 domains are randomly embedded in a Mn matrix. This sample readily absorbs and desorbs deuterium reversibly even at a temp. of 200°C without adding any catalysts. This is nearly 180°C lower than the typical operating temp. of conventional bulk Mg. Furthermore, at a given temp., its deuterium desorption pressure is clearly elevated compared to that of pure Mg indicating the destabilization of MgD2. The av. crystallite size of MgD2 in deuterated Mg0.25Mn0.75 detd. from X-ray diffraction data is around 9 nm. NMR spectroscopy study shows that MgD2 domains are heavily strained and some of D atoms are coordinated by a few Mn atoms suggesting that a large no. of lattice defects including the partial substitution of Mg with Mn are introduced during ball milling. Furthermore, the Mn matrix firmly locks nanosized MgD2 preventing the agglomeration of MgD2 below 250°C. Our study suggests that a synergistic effect created by nanosizing, large lattice distortion, and robust interface between MgD2 and the Mn matrix can effectively improve the kinetics and thermodn. of MgD2 in Mg0.25Mn0.75 concurrently. Our work demonstrates the possibility of utilizing the immiscibility of metals with Mg to synthesize robust nanostructure that can alter the kinetics and stability of MgH2. The thermodn. of MgD2 is destabilized in the Mg-Mn immiscible system, which is attributed to a synergistic effect created by nanosizing, large lattice distortion, and robust interface between MgD2 and the Mn matrix.
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  18. 18
    Gremaud, R.; Borgschulte, A.; Lohstroh, W.; Schreuders, H.; Züttel, A.; Dam, B.; Griessen, R. Ti-Catalyzed Mg(AlH4)2: A Reversible Hydrogen Storage Material. J. Alloys Compd. 2005, 404–406, 775– 778,  DOI: 10.1016/j.jallcom.2005.01.140
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    18
    Ti-catalyzed Mg(AlH4)2: A reversible hydrogen storage material
    Gremaud, R.; Borgschulte, A.; Lohstroh, W.; Schreuders, H.; Zuettel, A.; Dam, B.; Griessen, R.
    Journal of Alloys and Compounds (2005), 404-406 (), 775-778CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    Mg-Al thin films with a compositional gradient were co-sputtered from off-centered magnetron sources and capped with a thin Pd layer. Hydride formation by these films were studied by monitoring their optical transmission during hydrogenation under defined pressure and temp. conditions. Mg(AlH4)2 was already formed from the elements at p(H2)=1 bar and T=100°. A thin layer of Ti acts as a catalyst, but doping of Mg-Al with Ti has a neg. influence on H absorption.
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    Zaluska, A.; Zaluski, L.; Ström-Olsen, J. O. Structure, Catalysis and Atomic Reactions on the Nano-Scale: A Systematic Approach to Metal Hydrides for Hydrogen Storage. Appl. Phys. A 2001, 72, 157– 165,  DOI: 10.1007/s003390100783
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    19
    Structure, catalysis and atomic reactions on the nano-scale: a systematic approach to metal hydrides for hydrogen storage
    Zaluska, A.; Zaluski, L.; Strom-Olsen, J. O.
    Applied Physics A: Materials Science & Processing (2001), 72 (2), 157-165CODEN: APAMFC; ISSN:0947-8396. (Springer-Verlag)
    A review with 48 refs. We show how reducing structure, catalysis and at. reactions to the nano-scale may be used in a systematic way to substantially enhance the hydrogenation properties of metal hydrides. We examine, with examples from a wide range of hydrides, the direct impact of nano-scale structure, subsequent improvements in kinetics through nanoscale solid state catalysis, the special properties of nanocomposites, and the role played by nano-scale reactions.
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  20. 20
    Bouaricha, S.; Dodelet, J. P.; Guay, D.; Huot, J.; Boily, S.; Schulz, R. Hydriding Behavior of Mg-Al and Leached Mg-Al Compounds Prepared by High-Energy Ball-Milling. J. Alloys Compd. 2000, 297, 282– 293,  DOI: 10.1016/S0925-8388(99)00612-X
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    20
    Hydriding behavior of Mg-Al and leached Mg-Al compounds prepared by high-energy ball-milling
    Bouaricha, S.; Dodelet, J. P.; Guay, D.; Huot, J.; Boily, S.; Schulz, R.
    Journal of Alloys and Compounds (2000), 297 (1-2), 282-293CODEN: JALCEU; ISSN:0925-8388. (Elsevier Science S.A.)
    The structure and hydrogen absorption properties of Mg:Al alloys prepd. by high-energy ball milling were studied over the whole compositional range. These materials were prepd. in their as-milled and Al-leached forms. The latter are obtained from the former materials by leaching out Al in a 1N NaOH soln. The structure of the various alloys was detd. by X-ray diffraction. The structure of the material in the hydrided state was also detd. in some cases. In the as-milled state, hcp Mg(Al) with a small proportion of Mg17Al12 and fcc Al(Mg) are formed at Mg:Al (90:10) and (20:80) compns., resp. At intermediate (58:42) and (37:63) compns., the intermetallic Mg17Al12 and Mg3Al2 phases are formed, resp. Following leaching, the Al content of Mg:Al (90:10) and (20:80) varies from 10.4 and 77.0 to 3.0 and 51.0 at.%, resp. In both cases, noticeable change in the XRD pattern confirms that bulk dissoln. of Al has been achieved. There is a two-fold increase in the sp. surface area of Mg:Al (90:10) following leaching of Al. In the case of Mg:Al with intermediate compns., dissoln. of Al, if any, does not lead to discernable modification in the structure of the material. The measured hydrogen capacity of the as milled material decreases with Al content, from H/M = 1.74 for pure un-milled Mg, to 1.38 for Mg:Al (90:10), and then to 1.05 for Mg:Al (75:25). In each case, there is a further 10-15% decline of the hydrogen absorption capacity after leaching. In the case of Mg:Al (58:42), which basically only contains a nanocryst. Mg17Al12 intermetallic phase, hydriding leads to the formation of MgH2 and Al. This reaction is totally reversible and Mg17Al12 is recovered upon dehydriding. In each case, there is an increase in the kinetics of hydrogen absorption and desorption following Al leaching.
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  21. 21
    Gremaud, R.; Borgschulte, A.; Chacon, C.; van Mechelen, J. L. M.; Schreuders, H.; Züttel, A.; Hjörvarsson, B.; Dam, B.; Griessen, R. Structural and Optical Properties of MgxAl1–xHy Gradient Thin Films: A Combinatorial Approach. Appl. Phys. A 2006, 84, 77– 85,  DOI: 10.1007/s00339-006-3579-z
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    21
    Structural and optical properties of MgxAl1-xHy gradient thin films: a combinatorial approach
    Gremaud, R.; Borgschulte, A.; Chacon, C.; van Mechelen, J. L. M.; Schreuders, H.; Zuettel, A.; Hjoervarsson, B.; Dam, B.; Griessen, R.
    Applied Physics A: Materials Science & Processing (2006), 84 (1-2), 77-85CODEN: APAMFC; ISSN:0947-8396. (Springer)
    The structural, optical and d.c. elec. properties of MgxAl1-x (0.2≤x≤0.9) gradient thin films covered with Pd/Mg are studied before and after exposure to H2. Hydrogenog., a novel high-throughput optical technique, was used to map simultaneously all the hydride forming compns. and the kinetics thereof in the gradient film. Metallic Mg in the MgxAl1-x layer undergoes a metal-to-semiconductor transition and MgH2 is formed for all Mg fractions x studied. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH2 and Al occurs. This uncommon large-scale segregation is evidenced by metal and H profiling using Rutherford backscattering spectrometry and resonant nuclear anal. based on the reaction 1H(15N,αγ)12C. Besides MgH2, an addnl. semiconducting phase is found by elec. cond. measurements around an at. [Al]/[Mg] ratio of 2 (x = 0.33). Probably the film is partially transformed into Mg(AlH4)2 at around this compn.
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    Fritzsche, H.; Saoudi, M.; Haagsma, J.; Ophus, C.; Luber, E.; Harrower, C. T.; Mitlin, D. Neutron Reflectometry Study of Hydrogen Desorption in Destabilized MgAl Alloy Thin Films. Appl. Phys. Lett. 2008, 92, 121917,  DOI: 10.1063/1.2899936
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    22
    Neutron reflectometry study of hydrogen desorption in destabilized MgAl alloy thin films
    Fritzsche, H.; Saoudi, M.; Haagsma, J.; Ophus, C.; Luber, E.; Harrower, C. T.; Mitlin, D.
    Applied Physics Letters (2008), 92 (12), 121917/1-121917/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    We studied the H absorption and desorption properties of thin Pd-covered Mg1-xAlx alloy films as a function of temp. and alloy compn. Using neutron reflectometry, we were able to det. the H content and the H distribution within these MgAl films in situ. For all films, H was uniformly dispersed within the MgAl film and no H was observable in the Pd cover layer. The Mg0.7Al0.3 film shows an appreciable 4.1 wt % stored H and improved desorption characteristics with complete desorption at a temp. of 448 K. (c) 2008 American Institute of Physics.
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  23. 23
    Vermeulen, P.; Graat, P. C. J.; Wondergem, H. J.; Notten, P. H. L. Crystal structures of MgyTi100–y thin film alloys in the as-deposited and hydrogenated state. Int. J. Hydrogen Energy 2008, 33, 5646– 5650,  DOI: 10.1016/j.ijhydene.2008.07.014
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    23
    Crystal structures of MgyTi100-y thin film alloys in the as-deposited and hydrogenated state
    Vermeulen, P.; Graat, P. C. J.; Wondergem, H. J.; Notten, P. H. L.
    International Journal of Hydrogen Energy (2008), 33 (20), 5646-5650CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    In situ X-ray diffraction was used to identify the crystal structures of as-deposited and hydrogenated MgyTi100-y thin film alloys contg. 70, 80, and 90 at.% Mg. The preferred crystallog. orientation of the films in both the as-prepd. and hydrogenated state made it difficult to unambiguously identify the crystal structure up to now. In this work, identification of the unit cells was achieved by in situ recording diffraction patterns at various tilt angles. The results reveal a hexagonal closed packed structure for all alloys in the as-deposited state. Hydrogenating the layers under 105 Pa H2 transforms the unit cell into face-centered cubic for the Mg70Ti30 and Mg80Ti20 compds., whereas the unit cell of hydrogenated Mg90Ti10 has a body-centered tetragonal symmetry. The (de)hydrogenation kinetics changes along with the crystal structure of the metal hydrides from rapid for fcc.-structured hydrides to sluggish for hydrides with a bct. symmetry and emphasizes the influence of the crystal structure on the hydrogen transport kinetics.
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  24. 24
    Kim, H.; Schreuders, H.; Sakaki, K.; Asano, K.; Nakamura, Y.; Maejima, N.; Machida, A.; Watanuki, T.; Dam, B. Unveiling Nanoscale Compositional and Structural Heterogeneities of Highly Textured Mg0.7Ti0.3Hy Thin Films. Inorg. Chem. 2020, 59, 6800– 6807,  DOI: 10.1021/acs.inorgchem.0c00059
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    24
    Unveiling Nanoscale Compositional and Structural Heterogeneities of Highly Textured Mg0.7Ti0.3Hy Thin Films
    Kim, Hyunjeong; Schreuders, Herman; Sakaki, Kouji; Asano, Kohta; Nakamura, Yumiko; Maejima, Naoyuki; Machida, Akihiko; Watanuki, Tetsu; Dam, Bernard
    Inorganic Chemistry (2020), 59 (10), 6800-6807CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Thin films often exhibit fascinating properties, but the understanding of the underlying mechanism behind such properties is not simple. This is partially because of the limited structural information available. The hurdle in obtaining such information is esp. high for textured thin films such as Mg-rich MgxTi1-x, a promising switchable smart coating material. Although these metastable thin films are seen as solid soln. alloys by conventional crystallog. methods, their H-induced optical transition is hardly understood by a solid soln. model. The authors collect at. pair distribution function (PDF) data for a Mg0.7Ti0.3Hy thin film in situ on hydrogenation and resolve TiH2 clusters of an av. size of 30 Å embedded in the Mg matrix. This supports the chem. segregated model previously proposed for this system. The authors also observe the emergence of a previously unknown intermediate face-centered tetragonal phase during hydrogenation of the Mg matrix. This phase appears between Mg and MgH2 to reduce lattice mismatch, thereby preventing pulverization and facilitating rapid H uptake. This work may shed new light on the H-induced properties of Mg-rich MgxTi1-x thin films. Nanoscale compositional and structural heterogeneities of highly textured Mg0.7Ti0.3Hy thin films were studied during the hydrogenation process using the at. pair distribution function technique.
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  25. 25
    Borsa, D. M.; Baldi, A.; Pasturel, M.; Schreuders, H.; Dam, B.; Griessen, R.; Vermeulen, P.; Notten, P. H. L. Mg-Ti-H thin films for smart solar collectors. Appl. Phys. Lett. 2006, 88, 241910,  DOI: 10.1063/1.2212287
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    Mg-Ti-H thin films for smart solar collectors
    Borsa, D. M.; Baldi, A.; Pasturel, M.; Schreuders, H.; Dam, B.; Griessen, R.; Vermeulen, P.; Notten, P. H. L.
    Applied Physics Letters (2006), 88 (24), 241910/1-241910/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Mg-Ti-H thin films have attractive optical properties: they absorb 87% of the solar radiation in the hydrogenated state and only 32% in the metallic state. In the absorbing state Mg-Ti-H has a low emissivity - at 400 K only 10% of blackbody radiation is emitted. The transition between the optical states is fast, robust, and reversible. These properties highlight the applicability of such materials as switchable smart coatings in solar collectors.
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  26. 26
    Borsa, D. M.; Gremaud, R.; Baldi, A.; Schreuders, H.; Rector, J. H.; Kooi, B.; Vermeulen, P.; Notten, P. H. L.; Dam, B.; Griessen, R. Structural, optical, and electrical properties of MgyTi1–yHx thin films. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 205408,  DOI: 10.1103/PhysRevB.75.205408
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    26
    Structural, optical, and electrical properties of MgyTi1-yHx thin films
    Borsa, D. M.; Gremaud, R.; Baldi, A.; Schreuders, H.; Rector, J. H.; Kooi, B.; Vermeulen, P.; Notten, P. H. L.; Dam, B.; Griessen, R.
    Physical Review B: Condensed Matter and Materials Physics (2007), 75 (20), 205408/1-205408/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The structural, optical, and elec. transformations induced by hydrogen absorption and/or desorption in Mg-Ti thin films prepd. by co-sputtering of Mg and Ti are investigated. Highly reflective in the metallic state, the films become highly absorbing upon H absorption. The reflector-to-absorber transition is fast, robust, and reversible over many cycles. Such a highly absorbing state hints at the coexistence of a metallic and a semiconducting phase. It is, however, not simply a composite material consisting of independent MgH2 and TiH2 grains. By continuously monitoring the structure during H uptake, we obtain data that are compatible with a coherent structure. The av. structure resembles rutile MgH2 at high Mg content and is fluorite otherwise. Of crucial importance in preserving the reversibility and the coherence of the system upon hydrogen cycling is the accidental equality of the molar volume of Mg and TiH2. The present results point toward a rich and unexpected chem. of Mg-Ti-H compds.
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    Niessen, R. a. H.; Notten, P. H. L. Electrochemical Hydrogen Storage Characteristics of Thin Film MgX (X = Sc, Ti, V, Cr) Compounds. Electrochem. Solid-State Lett. 2005, 8, A534,  DOI: 10.1149/1.2012238
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    27
    Electrochemical Hydrogen Storage Characteristics of Thin Film MgX (X = Sc, Ti, V, Cr) Compounds
    Niessen, R. A. H.; Notten, P. H. L.
    Electrochemical and Solid-State Letters (2005), 8 (10), A534-A538CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
    The hydrogen storage characteristics of thin film MgX (X = Sc, Ti, V, Cr) compds. were investigated electrochem. The successful prepn. of these metastable, cryst. single-phase, MgX compds. was achieved by means of electron-beam deposition at room temp. The reversible hydrogen storage capacity of these compds. is excellent and up to six times higher than com. AB5-type materials. The gravimetric storage capacities of these new materials were detd. to be 1790 mA-h/g for Mg80Sc20, 1750 mA-h/g for Mg80Ti20, 1700 mA-h/g for Mg80V20, and 1325 mA-h/g for Mg80Cr20, corresponding to 6.7, 6.5, 6.4, and 4.9 wt.% hydrogen, resp. The hydrogen absorption and desorption kinetics are profoundly influenced by the element X incorporated in the MgX compd. Galvanostatic measurements show that the rate capability of the Sc- and Ti-contg. compds. is significantly better than that of the V- and Cr-contg. compds. Isotherms of these systems are obtained using galvanostatic intermittent titrn. technique, revealing that the equil. potential of the main charge/discharge plateau, apart from the Mg80Sc20 compd., only slightly depends on X in MgX. The electrochem. measurements show that low-cost Ti is an excellent substitute for the expensive Sc in MgSc, without introducing detrimental effects.
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  28. 28
    Westerwaal, R. J.; Borgschulte, A.; Lohstroh, W.; Dam, B.; Kooi, B.; ten Brink, G.; Hopstaken, M. J. P.; Notten, P. H. L. The Growth-Induced Microstructural Origin of the Optical Black State of Mg2NiHx Thin Films. J. Alloys Compd. 2006, 416, 2– 10,  DOI: 10.1016/j.jallcom.2005.07.068
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    28
    The growth-induced microstructural origin of the optical black state of Mg2NiHx thin films
    Westerwaal, R. J.; Borgschulte, A.; Lohstroh, W.; Dam, B.; Kooi, B.; ten Brink, G.; Hopstaken, M. J. P.; Notten, P. H. L.
    Journal of Alloys and Compounds (2006), 416 (1-2), 2-10CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
    Hydrogen absorption by a thin Mg2Ni film capped with Pd results in the nucleation of the Mg2NiH4 phase at the film/substrate interface. On further hydrogenation, a self-organized two-layer system consisting of a Mg2NiH0.3/Mg2NiH4 bottom layer and a Mg2NiH0.3 top layer is formed. This leads to an intermediate optical black state in Mg2Ni thin films, which transforms from metallic/reflective to semiconducting/transparent upon hydrogenation. This hydrogen absorption behavior is completely unexpected, since the hydrogen enters the film through the Pd-capped film surface. To explain the preferential nucleation of Mg2NiH4 at the substrate/film interface, we det. the chem. homogeneity of these thin films by RBS and SIMS. Furthermore by STM, TEM and SEM, we analyze the microstructure. We find that up to a film thickness of 50 nm, the film consists of small grains and clusters of small grains. On further growth, a columnar structure develops. We propose that the nucleation barrier for the formation of the Mg2NiH4 phase is smaller for the small loosely packed grains at the interface, while the columnar grain boundaries promote the hydrogen diffusion to the substrate.
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    Orimo, S.; Fujii, H. Materials Science of Mg-Ni-Based New Hydrides. Appl. Phys. A 2001, 72, 167– 186,  DOI: 10.1007/s003390100771
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    Materials science of Mg-Ni-based new hydrides
    Orimo, S.; Fujii, H.
    Applied Physics A: Materials Science & Processing (2001), 72 (2), 167-186CODEN: APAMFC; ISSN:0947-8396. (Springer-Verlag)
    A review, with 144 refs., of published data and authors's systematic studies on the relation between nanometer- or atomistic-scale structures and the specific hydriding properties of the Mg-Ni binary system that exhibit the reversible hydriding reaction. The following intermetallics are considered:(1) nanostructured (n)-Mg2Ni, (2) a mixt. of n-Mg2Ni and amorphous (a)-MgNi, (3) pure a-MgNi, and (4) n-MgNi2. The studies on (5) an a-MgNi-based system for clarifying the effect of the short-range ordering on the structural and hydriding properties and (6) a MgNi2-based system for synthesizing the new Laves phase structure are also presented. The studies of MgNi-based hydrides will provide indispensable knowledge for practically developing the Mg alloys as hydrogen-storage materials.
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    Ludwig, A.; Cao, J.; Dam, B.; Gremaud, R. Opto-mechanical characterization of hydrogen storage properties of Mg-Ni thin film composition spreads. Appl. Surf. Sci. 2007, 254, 682– 686,  DOI: 10.1016/j.apsusc.2007.05.093
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    Opto-mechanical characterization of hydrogen storage properties of Mg-Ni thin film composition spreads
    Ludwig, A.; Cao, J.; Dam, B.; Gremaud, R.
    Applied Surface Science (2007), 254 (3), 682-686CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Thin film compn. spreads of Mg-Ni were deposited by co-sputtering on micromachined Si-cantilevers. The studied compns. range from about Mg60Ni40 to about Mg80Ni20. Structural properties as well as mech. stress before and after hydrogenation were measured with XRD and laser profilometry, resp. The compn. spreads were hydrogenated in a special pressure vessel, which allows measuring optically the H-induced deflection (stress-change) of 16 cantilevers as a function of H pressure and/or temp. The H-induced stress is correlated with the compn. and microstructure of the films. Highest H-induced stress changes were found for compns. close to the cryst. Mg2Ni phase.
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    Johansson, E.; Chacon, C.; Zlotea, C.; Andersson, Y.; Hjörvarsson, B. Hydrogen Uptake and Optical Properties of Sputtered Mg-Ni Thin Films. J. Phys. Condens. Matter 2004, 16, 7649– 7662,  DOI: 10.1088/0953-8984/16/43/008
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    Hydrogen uptake and optical properties of sputtered Mg-Ni thin films
    Johansson, Emil; Chacon, Cyril; Zlotea, Claudia; Andersson, Yvonne; Hjoervarsson, Bjoergvin
    Journal of Physics: Condensed Matter (2004), 16 (43), 7649-7662CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
    The H uptake and distribution in wedged Mg-Ni films, with compn. ranging from Mg0.85Ni0.15 to Mg0.55Ni0.45, were studied. Upon H loading at 298 K these films undergo a transition from a mirror-like metallic to a semiconducting transparent state. After exposure to a H pressure of 1 bar, the samples exhibit large variation in optical appearance, ranging from a pale yellowish (Mg rich side) to a brownish shade (Ni rich side). The change in the effective optical band gap Eeffg as a function of sample compn. and H concn. was detd.; it showed changes from 3.6 eV in the Ni poor domain to 2.4 eV in the Ni rich domain. Compn. anal. using the 15N nuclear resonance method showed close to homogeneous H distribution throughout the film and close to linear increase in the H uptake with increasing Mg content. The thermal stability of the films is limited; annealing >393 K results in significant redistribution of the constituents. Mg is enriched at the surface, reacting with Pd and thereby degrading the capping layer through the formation of Mg6Pd and MgO, as detd. by x-ray diffraction, XPS and Rutherford backscattering studies. This redistribution results in a severe decrease of the H uptake rate, as monitored by in situ resistivity measurements.
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    Richardson, T. J.; Slack, J. L.; Armitage, R. D.; Kostecki, R.; Farangis, B.; Rubin, M. D. Switchable mirrors based on nickel-magnesium films. Appl. Phys. Lett. 2001, 78, 3047– 3049,  DOI: 10.1063/1.1371959
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    Switchable mirrors based on nickel-magnesium films
    Richardson, T. J.; Slack, J. L.; Armitage, R. D.; Kostecki, R.; Farangis, B.; Rubin, M. D.
    Applied Physics Letters (2001), 78 (20), 3047-3049CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    An electrochromic mirror electrode based on reversible uptake of hydrogen in nickel magnesium alloy films is reported. Thin, magnesium-rich Ni-Mg films prepd. on glass substrates by co-sputtering from Ni and Mg targets are mirror-like in appearance and have low visible transmittance. Upon exposure to hydrogen gas or on cathodic polarization in alk. electrolyte, the films take up hydrogen and become transparent. When hydrogen is removed, the mirror properties are recovered. The transition is believed to result from reversible formation of Mg2NiH4 and MgH2. A thin overlayer of palladium was found to enhance the kinetics of hydrogen insertion and extn., and to protect the metal surface against oxidn.
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    Isidorsson, J.; Giebels, I. a. M. E.; Griessen, R.; Di Vece, M. Tunable Reflectance Mg-Ni-H Films. Appl. Phys. Lett. 2002, 80, 2305– 2307,  DOI: 10.1063/1.1463205
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    Tunable reflectance Mg-Ni-H films
    Isidorsson, J.; Giebels, I. A. M. E.; Griessen, R.; Di Vece, M.
    Applied Physics Letters (2002), 80 (13), 2305-2307CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    Mg2NiHx switchable mirrors, which switch from shiny metallic Mg2Ni to transparent Mg2NiH4, exhibit dramatic reversible changes in reflectance at remarkably low H concns. Already for x ≈ 0.3 a 232 nm thick Mg2NiHx film has an absorptance of ∼90% over the whole visible optical spectrum. The transition from highly reflective to black occurs in the concn. interval 0.1<x<0.3. The vol. changes involved are 1 order of magnitude smaller than in the transition from reflecting to transparent. This is expected to enhance the lifetime of future devices based on these materials.
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    Lohstroh, W.; Westerwaal, R. J.; Noheda, B.; Enache, S.; Giebels, I. A. M. E.; Dam, B.; Griessen, R. Self-Organized Layered Hydrogenation in Black Mg2NiHx Switchable Mirrors. Phys. Rev. Lett. 2004, 93, 197404,  DOI: 10.1103/PhysRevLett.93.197404
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    Self-Organized Layered Hydrogenation in Black Mg2NiHx Switchable Mirrors
    Lohstroh, W.; Westerwaal, R. J.; Noheda, B.; Enache, S.; Giebels, I. A. M. E.; Dam, B.; Griessen, R.
    Physical Review Letters (2004), 93 (19), 197404/1-197404/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    In addn. to a mirrorlike (Mg2Ni) and a transparent (Mg2NiH4) state, thin films of Mg2NiHx exhibit a remarkable black state with low reflection over the entire visible spectrum, essentially zero transmission and a low elec. resistivity. Such a black state is not explicable for a homogeneous layer since a large absorption coeff. always yields substantial reflection. We show that it results from a self-organized and reversible double layering of metallic Mg2NiH0.3 and semiconducting Mg2NiH4.
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    Lohstroh, W.; Westerwaal, R. J.; van Mechelen, J. L. M.; Chacon, C.; Johansson, E.; Dam, B.; Griessen, R. Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 165411,  DOI: 10.1103/PhysRevB.70.165411
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    Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading
    Lohstroh, W.; Westerwaal, R. J.; Van Mechelen, J. L. M.; Chacon, C.; Johansson, E.; Dam, B.; Griessen, R.
    Physical Review B: Condensed Matter and Materials Physics (2004), 70 (16), 165411/1-165411/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
    The structural, thermodn. and optical properties of Mg2Ni thin films covered with Pd are investigated upon exposure to hydrogen. Similar to bulk, thin films of metallic Mg2Ni take up 4 hydrogen per formula unit and semiconducting transparent Mg2NiH4-δ is formed. The dielec. function ~ε of Mg2Ni and fully loaded Mg2NiH4-δ is detd. from reflection and transmission measurements using a Drude-Lorentz parametrization. Besides the two "normal" optical states of a switchable mirror-metallic reflecting and semiconducting transparent-Mg2NiHx exhibit a third "black" state at intermediate hydrogen concns. with low reflection and essentially zero transmission. This state originates from a subtle interplay of the optical properties of the constituent materials and a self-organized double layering of the film during loading. Mg2NiH4-δ preferentially nucleates at the film/substrate interface and not-as intuitively expected-close to the catalytic Pd capping layer. Using ~εMg2Ni and ~εMg2NiH4 and this loading sequence, the optical response at all hydrogen concns. can be described quant. The uncommon hydrogen loading sequence is confirmed by x-ray diffraction and hydrogen profiling using the resonant nuclear reaction 1H(15N,αγ)12C. Pressure-compn. isotherms suggest that the formation of Mg2NiH4-δ at the film/substrate interface is mainly due to locally enhanced kinetics.
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    Gremaud, R.; Broedersz, C. P.; Borgschulte, A.; van Setten, M. J.; Schreuders, H.; Slaman, M.; Dam, B.; Griessen, R. Hydrogenography of MgyNi1–yHx gradient thin films: Interplay between the thermodynamics and kinetics of hydrogenation. Acta Mater. 2010, 58, 658– 668,  DOI: 10.1016/j.actamat.2009.09.044
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    Pasturel, M.; Slaman, M.; Borsa, D. M.; Schreuders, H.; Dam, B.; Griessen, R.; Lohstroh, W.; Borgschulte, A. Stabilized Switchable Black State in Mg2NiH4/Ti/Pd Thin Films for Optical Hydrogen Sensing. Appl. Phys. Lett. 2006, 89, 021913,  DOI: 10.1063/1.2221412
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    Stabilized switchable "black state" in Mg2NiH4/Ti/Pd thin films for optical hydrogen sensing
    Pasturel, M.; Slaman, M.; Borsa, D. M.; Schreuders, H.; Dam, B.; Griessen, R.; Lohstroh, W.; Borgschulte, A.
    Applied Physics Letters (2006), 89 (2), 021913/1-021913/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    A triple layer thin film (30 nm Mg2Ni/100 nm Ti/10 nm Pd sputtered on glass) switches reversibly from a shiny metallic to a black state upon exposure to moderate H pressure (≈5 × 103 Pa). This black state resembles that obtained in thick Mg2NiHx layers and has the great advantage of being stable and easily controlled. Both the reversible high optical contrast (Rreflective/Rblack≈10 in the red wavelength range) and the fast kinetics of H absorption and desorption make this material interesting for applications as optical H sensors.
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    Pivak, Y.; Palmisano, V.; Schreuders, H.; Dam, B. The Clamping Effect in the Complex Hydride Mg2NiH4 Thin Films. J. Mater. Chem. A 2013, 1, 10972– 10978,  DOI: 10.1039/C3TA11937H
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    The clamping effect in the complex hydride Mg2NiH4 thin films
    Pivak, Yevheniy; Palmisano, Valerio; Schreuders, Herman; Dam, Bernard
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (36), 10972-10978CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    The thermodn. properties and hysteresis behavior of Mg2NiH4 thin films were studied using hydrogenog. in the 303-513 K temp. range. For the enthalpy of hydride formation and decompn., as estd. from a Van't Hoff anal., we find ΔHabs = -51.2 kJ mol-1 H2 and ΔHdes = -76.3 kJ mol-1 H2. These values are affected by the clamping of the film to the substrate. However, the av. enthalpy of the (de-)hydrogenation reaction, ΔH = (ΔHabs + ΔHdes)/2 = -63.5 kJ mol-1 H2, is in remarkable agreement with the av. bulk data, -64 kJ mol-1 H2. The (de-)hydrogenation pressure hysteresis in Mg2NiH4 thin films is >3 orders of magnitude at 303 K, which is substantially larger than in MgH2 films. This behavior is believed to originate from the fact that the phase transition in Mg2NiH4 films occurs between two preferentially oriented phases. We assume that the energy involved in the necessary re-orientation is related to the monoclinic to hexagonal structure transformation. By adding this term, the stress-strain anal. is able to explain the enormous difference between absorption and desorption equil. pressures in Mg2NiH4.
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  39. 39
    Akiba, E.; Nomura, K.; Ono, S.; Suda, S. Kinetics of the Reaction between Mg-Ni Alloys and H2. Int. J. Hydrogen Energy 1982, 7, 787– 791,  DOI: 10.1016/0360-3199(82)90069-6
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    Kinetics of the reaction between magnesium-nickel alloys and molecular hydrogen
    Akiba, E.; Nomura, K.; Ono, S.; Suda, S.
    International Journal of Hydrogen Energy (1982), 7 (10), 787-91CODEN: IJHEDX; ISSN:0360-3199.
    The kinetics of the hydriding and dehydriding reaction of Mg2Ni and Mg-10 wt.% Ni was investigated. The reaction rate at various hydride compns. was measured by the pressure change in a const. vol.. For Mg2Ni, at the beginning of the reaction the amt. of absorbed/desorbed H can be expressed by n = k (po-Peq) (n-no), where N is the ratio of H to Mg in the solid, po and Peq are the initial H pressure of the system and the equil. pressure, and no and n3 are the values of n at the start of the reaction and a const. around 2, resp.. After a transient period of approx. 1 min., a diffusion-controlled stage took place as dn/dt = k'(p-peq)/t, where p is the pressure at time t. The rate for Mg-10% Ni is also given.
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    Zhou, C.; Fang, Z. Z.; Lu, J.; Luo, X.; Ren, C.; Fan, P.; Ren, Y.; Zhang, X. Thermodynamic Destabilization of Magnesium Hydride Using Mg-Based Solid Solution Alloys. J. Phys. Chem. C 2014, 118, 11526– 11535,  DOI: 10.1021/jp501306w
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    Thermodynamic Destabilization of Magnesium Hydride Using Mg-Based Solid Solution Alloys
    Zhou, Chengshang; Fang, Zhigang Zak; Lu, Jun; Luo, Xiangyi; Ren, Chai; Fan, Peng; Ren, Yang; Zhang, Xiaoyi
    Journal of Physical Chemistry C (2014), 118 (22), 11526-11535CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Thermodn. destabilization of magnesium hydride is a difficult task that has challenged researchers of metal hydrides for decades. In this work, solid soln. alloys of magnesium were exploited as a way to destabilize magnesium hydride thermodynamically. Various elements were alloyed with magnesium to form solid solns., including: In, Al, Ga, and Zn. Thermodn. properties of the reactions between the magnesium solid soln. alloys and hydrogen were investigated. Equil. pressures were detd. by pressure-compn.-isothermal (PCI) measurements, showing that all the solid soln. alloys that were investigated in this work have higher equil. hydrogen pressures than that of pure magnesium. Compared to magnesium hydride, the enthalpy (ΔH) of decompn. to form hydrogen and the magnesium alloy can be reduced from 78.60 kJ/(mol H2) to 69.04 kJ/(mol H2), and the temp. of 1 bar hydrogen pressure can be reduced to 262.33°C, from 282.78°C, for the decompn. of pure magnesium hydride. Further, in situ XRD anal. confirmed that magnesium solid solns. were indeed formed after the dehydrogenation of high-energy ball-milled MgH2 with the addn. of the solute element(s). XRD results also indicated that intermetallic phases of Mg with the solute elements were present along with MgH2 in the rehydrogenated magnesium solid soln. alloys, providing a reversible hydrogen absorption/desorption reaction pathway. However, the alloys were shown to have lower hydrogen storage capacity than that of pure MgH2.
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    Vermeulen, P.; Niessen, R. A. H.; Notten, P. H. L. Hydrogen storage in metastable MgyTi(1–y) thin films. Electrochem. Commun. 2006, 8, 27– 32,  DOI: 10.1016/j.elecom.2005.10.013
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    Hydrogen storage in metastable MgyTi(1-y) thin films
    Vermeulen, P.; Niessen, R. A. H.; Notten, P. H. L.
    Electrochemistry Communications (2006), 8 (1), 27-32CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
    The electrochem. properties of MgyTi(1-y) thin films with 0.50≤y≤0.95 during (de)hydrogenation were studied. These metastable alloys were successfully prepd. by electron-beam deposition at room temp. XRD confirmed that cryst. single-phase materials were obtained. Galvanostatic dehydrogenation measurements show that substituting Mg with Ti affects the H storage properties. MgyTi(1-y) alloys with <20 at.% Ti exhibit a low rate-capability, whereas increasing the Ti-content enhances rate-capability. A superior reversible H storage capacity, along with good rate-capability, was found for the Mg0.80Ti0.20 alloy. To study the effect of Ti on the electrochem. H storage properties, pure Mg thin films were also included. The electrochem. deep-discharging behavior of pure Mg shows evolution of the overpotential which may be due to nucleation and a growth process. Galvanostatic hydrogenation of MgyTi(1-y) alloys revealed the formation of products that do not correspond to the intrinsic thermodn. properties of the individual Mg and Ti hydrides, suggesting that no segregation occurs. Also, the analogy of the electrochem. behavior of MgyTi(1-y) and MgySc(1-y) alloys points to a fcc.-structured hydride.
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    Bao, S.; Tajima, K.; Yamada, Y.; Okada, M.; Yoshimura, K. Magnesium-Titanium Alloy Thin-Film Switchable Mirrors. Sol. Energy Mater. Sol. Cells 2008, 92, 224– 227,  DOI: 10.1016/j.solmat.2007.02.024
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    Magnesium-titanium alloy thin-film switchable mirrors
    Bao, S.; Tajima, K.; Yamada, Y.; Okada, M.; Yoshimura, K.
    Solar Energy Materials & Solar Cells (2008), 92 (2), 224-227CODEN: SEMCEQ; ISSN:0927-0248. (Elsevier B.V.)
    The authors show gasochromic and electrochromic switching properties of Pd top capped Mg-Ti (Mg-Ti) thin films prepd. by d.c. magnetron sputtering. These films show excellent switchable mirror properties. By exposing to 4% H2 in Ar, Pd (4 nm)/Mg0.82Ti0.18 (40 nm) film changed from the metallic state to the transparent state drastically within 5 s. By exposing to air, it goes back to the metallic state within 60 s. The transmittance spectrum in the hydride state is quite flat in the wavelength range from 400 to 2500 nm. It looks complete color neutral and its chromaticity coordinates are x = 0.326 and y = 0.340. Simple electrochromic device of Mg-Ti thin film using a liq. electrolyte works very well. It can be switched between the mirror state and the color-neutral transparent state.
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    Slaman, M.; Dam, B.; Schreuders, H.; Griessen, R. Optimization of Mg-Based Fiber Optic Hydrogen Detectors by Alloying the Catalyst. Int. J. Hydrogen Energy 2008, 33, 1084– 1089,  DOI: 10.1016/j.ijhydene.2007.09.036
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    Optimization of Mg-based fiber optic hydrogen detectors by alloying the catalyst
    Slaman, M.; Dam, B.; Schreuders, H.; Griessen, R.
    International Journal of Hydrogen Energy (2008), 33 (3), 1084-1089CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    Pd-capped gasochromic metal hydrides can be used as sensing layer in fiber optic hydrogen detectors. Using a sensing layer consisting of a 50. nm thick Mg70Ti30 film capped with a 30. nm Pd catalytic layer the authors demonstrate a drop in reflectance by a factor of 10, at hydrogen levels down to 10% of the lower flammability limit. The switching takes place within a few seconds, depending on the actual amt. of hydrogen in the vicinity of the detector. The authors report on the sensitivity of the fiber optic detector for hydrogen, both in argon and oxygen rich environments. Further the sensing behavior in oxygen rich gas mixts. and in normal air can be improved by adding silver to the catalytic Pd cap layer.
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  1. Lars J. Bannenberg, Isa M. Veeneman, Folkert I. B. Straus, Hsin-Yu Chen, Christy J. Kinane, Stephen Hall, Michel A. Thijs, Herman Schreuders. Thin Film TaFe, TaCo, and TaNi as Potential Optical Hydrogen Sensing Materials. ACS Omega 2024, Article ASAP.
  2. Rui Li, Yuzhang Liang, Haonan Wei, Hui Zhang, Svetlana Kurilkina, Wei Peng. Dynamic Spectral Modulation Enabled by Conductive Polymer-Integrated Plasmonic Nanodisk-Hole Arrays. ACS Applied Materials & Interfaces 2023, 15 (49) , 57486-57495. https://doi.org/10.1021/acsami.3c10853
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  • Abstract

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    Figure 1

    Figure 1. Optical properties ñ = n + ik of four different Mg–Al alloy hydrides. (a) Optical properties in the pristine metallic state before hydrogenation. (b) Change in optical properties upon hydrogenation, defined here as the pure metal optical properties subtracted from the hydride optical properties. (c) Optical properties in the fully hydrogenated hydride state. Each colored line on the plot represents a different atomic ratio of metal hydride, with darker shades representing higher atomic Mg percent ratios.

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    Figure 2

    Figure 2. Optical properties of different Mg–Al alloys as they are exposed to H2 gas. The lightest colored line depicts the alloy in the pristine metallic state. As H2 is introduced to the system, the material begins to hydrogenate, denoted by the lines getting darker in the plot, with the darkest line indicating the full hydride state. Each line is not linearly spaced in time and is instead chosen to aesthetically show the range of possible intermediate states. The alloys shown here are (a) Mg0.77Al0.23, (b) Mg0.81Al0.19, (c) Mg0.83Al0.17, and (d) Mg0.84Al0.16.

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    Figure 3

    Figure 3. Optical properties ñ = n + ik of five different Mg–Ti alloy hydrides. (a) Optical properties in the pristine metallic state before hydrogenation. (b) Change in optical properties upon hydrogenation, defined here as the pure metal optical properties subtracted from the hydride optical properties. (c) Optical properties in the fully hydrogenated hydride state. Each colored line on the plot represents a different atomic ratio of metal hydride, with darker shades representing higher atomic Mg percent ratios.

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    Figure 4

    Figure 4. Optical properties of different Mg–Ti alloys as they are exposed to H2 gas. The lightest colored line depicts the alloy in the pristine metallic state. As H2 is introduced to the system, the material begins to hydrogenate, denoted by the lines getting darker in the plot, with the darkest line indicating the full hydride state. Each line is not linearly spaced in time and is instead chosen to aesthetically show the range of possible intermediate states. The alloys shown here are (a) Mg0.82Ti0.18, (b) Mg0.85Ti0.15, (c) Mg0.87Ti0.13, (d) Mg0.89Ti0.11, and (e) Mg0.91Ti0.09.

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    Figure 5

    Figure 5. Optical properties ñ = n + ik of six different Mg–Ni alloy hydrides. (a) Optical properties in the pristine metallic state before hydrogenation. (b) Change in optical properties upon hydrogenation, defined here as the pure metal optical properties subtracted from the hydride optical properties. (c) Optical properties in the fully hydrogenated hydride state. Each colored line on the plot represents a different atomic ratio of metal hydride, with darker shades representing higher atomic Mg percent ratios.

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    Figure 6

    Figure 6. Optical properties of different Mg–Ni alloys as they are exposed to H2 gas. The lightest colored line depicts the alloy in the pristine metallic state. As H2 is introduced to the system, the material begins to hydrogenate, denoted by the lines getting darker in the plot, with the darkest line indicating the full hydride state. Each line is not linearly spaced in time and is instead chosen to aesthetically show the range of possible intermediate states. The alloys shown here are (a) Mg0.59Ni0.41, (b) Mg0.73Ni0.27, (c) Mg0.84Ni0.16, (d) Mg0.85Ni0.15, (e) Mg0.90Ni0.10, and (f) Mg0.92Ni0.08.

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    Figure 7

    Figure 7. Modeled absorption from backside illumination of Mg0.73Ni0.27. (a) Simulation architecture for the sample loading. Sample consists of a SiO2 substrate, followed by a fully hydrogenated Mg–Ni–H layer, then a fully metallic Mg–Ni layer, and finally a 3 nm Pd capping layer. The total Mg alloy thickness is defined to be 25 nm. Simulated hydrogenation is defined as an increase in the thickness of the hydride layer and a decrease of the same magnitude of the metal layer. (b) Calculated absorption for different loading thicknesses using Mg0.73Ni0.27 optical properties.

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    Figure 8

    Figure 8. Measured maximum loading values under 1 bar H2 for different thin films for (a) Mg–Al, (b) Mg–Ti, and (c) Mg–Ni alloy samples. The horizontal and vertical error bars represent one standard deviation uncertainty in the atomic Mg % and the loading, respectively.

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    Figure 9

    Figure 9. Measured total stress change values for different thin-film samples for (a) Mg–Al, (b) Mg–Ti, and (c) Mg–Ni alloy samples. The change in stress reported here is the stress change from the initial pristine metal mounted in the environmental chamber to the fully hydrogenated state under 1 bar H2 (does not include intrinsic stress of the initial pristine alloy). The horizontal and vertical error bars represent one standard deviation uncertainty in the atomic Mg % and the stress change, respectively.

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    Figure 10

    Figure 10. Switchable windows with Mg alloys. Simulated transmission values of thin films (a) Mg–Al, (b) Mg–Ti, and (c) Mg–Ni alloys on SiO2. The stack is defined as an SiO2 substrate, a 25 nm Mg alloy film, and a 3 nm Pd capping layer. Transmission in the hydride (metal) state is represented by solid (dashed) lines. (d) CIE 1931 chromaticity diagram with the transmission color points for the different alloys. Colors of the points match with the hues in part a–c. (e) Zoom-in of chromaticity diagram. The intersection of the black dashed lines represents color neutral at x = y = 0.333.

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    Figure 11

    Figure 11. Broadband switchable light absorbers using Mg–Ti alloys. (a) Schematic of switchable light absorber consisting of a 300 nm Mg–Ti alloy with a 10 nm Pd cap on a SiO2 substrate. (b) Absorption plots for this structure for different alloy compositions. Solid (dashed) lines are absorption in the hydride (metallic) state. Colors represents different alloy compositions.

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    Figure 12

    Figure 12. Dual-functionality reflective and transmissive color filters with Mg alloy hydrides. (a) Simulated transmission plots of the Mg81Al19 color filter in both the metallic and hydride states with 150, 200, and 250 nm SiO2 cavity thickness. The inset shows the schematic of the proposed tunable color filter structure and defines the cavity thickness d. (b) Reflected and (c) transmitted colors of the device at different hydrogenation levels and cavity thicknesses. Similar plots are shown for (d–f) Mg87Ti13, (g–i) Mg73Ni27, and (j–l) Mg59Ni41.

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  • References

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      Wadell, Carl; Syrenova, Svetlana; Langhammer, Christoph
      ACS Nano (2014), 8 (12), 11925-11940CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      A review. In this review, the evolution of localized surface plasmon resonance and surface plasmon resonance hydrogen sensors based on nanostructured metal hydrides, which has accelerated significantly during the past 5 years are discussed. The authors put particular focus on how, conceptually, plasmonic resonances can be used to study metal-hydrogen interactions at the nanoscale, both at the ensemble and at the single-nanoparticle level. Such efforts are motivated by a fundamental interest in understanding the role of nanosizing on metal hydride formation processes in the quest to develop efficient solid-state hydrogen storage materials with fast response times, reasonable thermodn., and acceptable long-term stability. Therefore, a brief introduction to the thermodn. of metal hydride formation is also given. However, plasmonic hydrogen sensors not only are of academic interest as research tool in materials science but also are predicted to find more practical use as all-optical gas detectors in industrial and medical applications, as well as in a future hydrogen economy, where hydrogen is used as a carbon free energy carrier. Therefore, the wide range of different plasmonic hydrogen sensor designs already available is reviewed together with theor. efforts to understand their fundamentals and optimize their performance in terms of sensitivity. In this context, the authors also highlight important challenges to be addressed in the future to take plasmonic hydrogen sensors from the lab. to real applications in devices, including poisoning/deactivation of the active materials, sensor lifetime, and cross-sensitivity toward other gas species.
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    7. 7
      Baldi, A.; Borsa, D. M.; Schreuders, H.; Rector, J. H.; Atmakidis, T.; Bakker, M.; Zondag, H. A.; van Helden, W. G. J.; Dam, B.; Griessen, R. Mg-Ti-H Thin Films as Switchable Solar Absorbers. Int. J. Hydrogen Energy 2008, 33, 3188– 3192,  DOI: 10.1016/j.ijhydene.2008.01.026
      7
      Mg-Ti-H thin films as switchable solar absorbers
      Baldi, A.; Borsa, D. M.; Schreuders, H.; Rector, J. H.; Atmakidis, T.; Bakker, M.; Zondag, H. A.; van Helden, W. G. J.; Dam, B.; Griessen, R.
      International Journal of Hydrogen Energy (2008), 33 (12), 3188-3192CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      The reflection and transmission spectra of Pd capped Mg y Ti 1 - y thin films ( y = 0.7 , 0.8 and 0.9) are measured in the 0.5-5.5 eV energy range, both in the as-prepd. and hydrogenated states. Upon hydrogenation these films switch reversibly from a shiny metallic state into a "black" absorbing one. The compn. and thicknesses can be tailored to achieve high solar absorptance and low thermal emittance in the hydrogenated state. The combination of these two characteristics is interesting for the application of this material as switchable absorber in solar collectors. The use of a Mg y Ti 1 - y switchable absorber in solar collectors allows to lower the stagnation temp. from 180 to 80 °C. The collector efficiency is affected only minimally.
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    8. 8
      Darmadi, I.; Khairunnisa, S. Z.; Tomeček, D.; Langhammer, C. Optimization of the Composition of PdAuCu Ternary Alloy Nanoparticles for Plasmonic Hydrogen Sensing. ACS Appl. Nano Mater. 2021, 4, 8716– 8722,  DOI: 10.1021/acsanm.1c01242
      8
      Optimization of the Composition of PdAuCu Ternary Alloy Nanoparticles for Plasmonic Hydrogen Sensing
      Darmadi, Iwan; Khairunnisa, Sarah Zulfa; Tomecek, David; Langhammer, Christoph
      ACS Applied Nano Materials (2021), 4 (9), 8716-8722CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)
      Alloying is a long-standing central strategy in materials science for the tailoring and optimization of bulk material properties, which more recently has started to find application also in engineered nanomaterials and nanostructures used in, among other, nanoplasmonic hydrogen sensors. Specifically, alloying Pd nanoparticles to form binaries and ternaries with the coinage metals Au and Cu has proven efficient to mitigate hysteresis in the sensor response, improve response and recovery times, boost sensitivity in the low hydrogen concn. sensing range, and reduce the detrimental impact of carbon monoxide poisoning. However, when surveying the corresponding studies, it is clear that there is a trade-off between the sensitivity enhancement and the CO-poisoning resistance effects provided by Au and Cu alloyants, resp. Therefore, in this work, we systematically screen the impact of the Au and Cu concn. in PdAuCu ternary alloy nanoparticles used for plasmonic hydrogen sensing, to obtain a champion system with maximized sensitivity and CO-poisoning resistance based on an evaluation using the stringent ISO 26142 test protocol. As the main results, we find that the best hysteresis-free and sensitive response combined with deactivation resistance to 500 ppm CO in synthetic air is obtained for the Pd65Au25Cu10 ternary alloy system, which also exhibits good long-term stability during operation under severe CO poisoning conditions.
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    9. 9
      Stampfer, J. F.; Holley, C. E.; Suttle, J. F. The Magnesium-Hydrogen System1-3. J. Am. Chem. Soc. 1960, 82, 3504– 3508,  DOI: 10.1021/ja01499a006
      9
      Magnesium-hydrogen system
      Stampfer, J. F., Jr.; Holley, C. E., Jr.; Suttle, J. F.
      Journal of the American Chemical Society (1960), 82 (), 3504-8CODEN: JACSAT; ISSN:0002-7863.
      It is shown that MgH2 is a stoichiometric compd. from the compn. of the solid phase of MgH2 and its decompn. pressure from 440 to 560° and up to 200 atm. In the given temp. range the H soly. in Mg varied from 2 to 10 atom %. An isotherm for MgD2 also was obtained at 510°. The equation of state is given in terms of fugacity for H and D, and thermodn. functions of MgH2 and MgD2 are estd.
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    10. 10
      Stander, C. M. Kinetics of Decomposition of Magnesium Hydride. J. Inorg. Nucl. Chem. 1977, 39, 221– 223,  DOI: 10.1016/0022-1902(77)80003-1
      10
      Kinetics of decomposition of magnesium hydride
      Stander, C. M.
      Journal of Inorganic and Nuclear Chemistry (1977), 39 (2), 221-3CODEN: JINCAO; ISSN:0022-1902.
      The kinetics of MgH2 decompn. in an atm. of H were studied gravimetrically. At H pressures appreciably different from the equil. dissocn. pressure, the form of the rate expression indicates that movement of the phase boundary is the rate-detg. step. At pressures near the equil. dissocn. pressure, a different rate expansion holds; this indicates that nucleation could be random instead of fast at these pressures, but the exptl. data support the conclusion that nucleation is kinetically unimportant and that the rate-detg. mechanism is phase boundary movement.
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    11. 11
      Reiser, A.; Bogdanović, B.; Schlichte, K. The Application of Mg-Based Metal-Hydrides as Heat Energy Storage Systems. Int. J. Hydrogen Energy 2000, 25, 425– 430,  DOI: 10.1016/S0360-3199(99)00057-9
      11
      The application of Mg-based metal-hydrides as heat energy storage systems
      Reiser, A.; Bogdanovic, B.; Schlichte, K.
      International Journal of Hydrogen Energy (2000), 25 (5), 425-430CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Science Ltd.)
      Mg-based metal hydride systems are potential high temp. heat storage media. In this paper the features and possibilities of the systems Mg/MgH2, Mg-Ni/Mg2NiH4, Mg-Fe/Mg2FeH6 and Mg-Co-H are discussed. All the materials show cyclic stability in certain temp. ranges. The thermal energy which is released by these systems covers a temp. range from 250° to 550° in which high thermal energy densities of up to 2257 kJ/kg are reached.
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    12. 12
      González Fernández, I.; Gennari, F. C.; Meyer, G. O. Influence of Sintering Parameters on Formation of Mg-Co Hydrides Based on Their Thermodynamic Characterization. J. Alloys Compd. 2008, 462, 119– 124,  DOI: 10.1016/j.jallcom.2007.08.038
      12
      Influence of sintering parameters on formation of Mg-Co hydrides based on their thermodynamic characterization
      Gonzalez Fernandez, I.; Gennari, F. C.; Meyer, G. O.
      Journal of Alloys and Compounds (2008), 462 (1-2), 119-124CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      Ternary hydrides Mg2CoH5 and Mg6Co2H11 were obtained by a combined technique that included ball milling of a 2Mg-Co mixt. under Ar followed by sintering in hydrogen atm. (5900 kPa) at 425 °C. Formation and decompn. Van't Hoff diagrams for these hydrides were deduced from pressure-compn. isotherms and related changes of enthalpy and entropy were afterwards estd. While decompn. enthalpy values for both hydrides are lower than previously published ones, formation enthalpy for Mg2CoH5 is closed to the only reported data. Calcd. formation enthalpy for Mg6Co2H11 is a novel contribution for the thermodn. characterization of the system. Based on Van't Hoff diagrams for hydride formation obtained in this work, several synthesis routes were designed as consecutive sintering stages of const. temp. and hydrogen pressure. They allow the study of the influence of both parameters and treatment duration on reaction kinetics and resulting phases. Synthesis procedures for prodn. of MgH2, Mg6Co2H11 or a mixt. of ternary hydrides were identified and involved mechanisms during hydride formation were proposed. These expts. evidenced the key role that the dependence of Mg and Co mobilities with temp. has in the prodn. of these ternary hydrides.
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    13. 13
      Shao, H.; Liu, T.; Wang, Y.; Xu, H.; Li, X. Preparation of Mg-Based Hydrogen Storage Materials from Metal Nanoparticles. J. Alloys Compd. 2008, 465, 527– 533,  DOI: 10.1016/j.jallcom.2007.11.003
      13
      Preparation of Mg-based hydrogen storage materials from metal nanoparticles
      Shao, Huaiyu; Liu, Tong; Wang, Yuntao; Xu, Hairuo; Li, Xingguo
      Journal of Alloys and Compounds (2008), 465 (1-2), 527-533CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      By hydrogen plasma metal reaction method, we obtained Mg, Ni, Co, Cu and Fe nanoparticles. Mg nanoparticles show larger av. particle size than Ni, Co, Cu and Fe ones. From these metal nanoparticles, Mg-based hydrogen storage alloys (Mg2Ni, Mg2Co and Mg2Cu) and hydrides (Mg2NiH4, Mg2CoH5 and Mg2FeH6) were prepd. by solid-solid and gas-solid reactions. Powder X-ray diffraction (XRD) was used to define the structure and compn. information. The prepn. results in different atm. were compared and discussed. Hydrogen and nanostructure play important roles in the prepn. of Mg-based hydrogen storage alloys/hydrides in convenient conditions from metal nanoparticles.
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    14. 14
      Richardson, T. J.; Slack, J. L.; Farangis, B.; Rubin, M. D. Mixed Metal Films with Switchable Optical Properties. Appl. Phys. Lett. 2002, 80, 1349– 1351,  DOI: 10.1063/1.1454218
      14
      Mixed metal films with switchable optical properties
      Richardson, T. J.; Slack, J. L.; Farangis, B.; Rubin, M. D.
      Applied Physics Letters (2002), 80 (8), 1349-1351CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Thin, Pd-capped metallic films contg. Mg and 1st-row transition metals (Mn, Fe, Co) switch reversibly from their initial reflecting state to visually transparent states when exposed to gaseous H or following cathodic polarization in an alk. electrolyte. Reversion to the reflecting state is achieved by exposure to air or by anodic polarization. The films were prepd. by cosputtering from one Mg target and one Mn, Fe, or Co target. Both the dynamic optical switching range and the speed of the transition depend on the Mg-transition metal ratio. IR spectra of films in the transparent, hydrided (deuterided) states support the presence of the intermetallic hydride phases Mg3MnH7, Mg2FeH6, and Mg2CoH5.
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    15. 15
      Lima, G. F.; Jorge, A. M.; Leiva, D. R.; Kiminami, C. S.; Bolfarini, C.; Botta, W. J. Severe Plastic Deformation of Mg-Fe Powders to Produce Bulk Hydrides. J. Phys. Conf. 2009, 144, 012015,  DOI: 10.1088/1742-6596/144/1/012015
      There is no corresponding record for this reference.
    16. 16
      Berlouis, L. E. A.; Cabrera, E.; Hall-Barientos, E.; Hall, P. J.; Dodd, S. B.; Morris, S.; Imam, M. A. Thermal Analysis Investigation of Hydriding Properties of Nanocrystalline Mg-Ni- and Mg-Fe-Based Alloys Prepared by High-Energy Ball Milling. J. Mater. Res. 2001, 16, 45– 57,  DOI: 10.1557/JMR.2001.0012
      16
      Thermal analysis investigation of hydriding properties of nanocrystalline Mg-Ni- and Mg-Fe-based alloys prepared by high-energy ball milling
      Berlouis, L. E. A.; Cabrera, E.; Hall-Barientos, E.; Hall, P. J.; Dodd, S. B.; Morris, S.; Imam, M. A.
      Journal of Materials Research (2001), 16 (1), 45-57CODEN: JMREEE; ISSN:0884-2914. (Materials Research Society)
      The hydrogen loading characteristics of nanocryst. Mg, Mg-Ni (Ni from 0.1 to 10 at.%), and Mg-Fe (Fe from 1 to 10 at.%) alloys in 3 MPa H2 were examd. using high pressure differential scanning calorimetry and thermogravimetric anal. All samples showed rapid uptake of hydrogen. A decrease in the onset temp. for hydrogen absorption was obsd. with increasing Ni and Fe alloy content, but the thermal signatures obtained suggested that only Mg was involved in the hydriding reaction; i.e., no clear evidence was found for the intermetallic hydrides Mg2NiH4 and Mg2FeH6. Hydrogen loading capacity decreased with temp. cycling, and this was attributed to a sintering process in the alloy, leading to a redn. in the sp. surface available for hydrogen absorption.
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    17. 17
      Lu, Y.; Kim, H.; Sakaki, K.; Hayashi, S.; Jimura, K.; Asano, K. Destabilizing the Dehydrogenation Thermodynamics of Magnesium Hydride by Utilizing the Immiscibility of Mn with Mg. Inorg. Chem. 2019, 58, 14600– 14607,  DOI: 10.1021/acs.inorgchem.9b02253
      17
      Destabilizing the Dehydrogenation Thermodynamics of Magnesium Hydride by Utilizing the Immiscibility of Mn with Mg
      Lu, Yanshan; Kim, Hyunjeong; Sakaki, Kouji; Hayashi, Shigenobu; Jimura, Keiko; Asano, Kohta
      Inorganic Chemistry (2019), 58 (21), 14600-14607CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Hydrogen storage is a key technol. for the advancement of hydrogen and fuel cell power technologies in stationary and portable applications. MgH2, one of high-capacity hydrogen storage materials, has two major material challenges for practical applications: slow hydrogen desorption kinetics and high hydrogen desorption temp. Numerous studies have reported enhancement in kinetics but only few in thermodn. Here, we present a simple but effective way to improve in both aspects by utilizing the immiscibility of Mn, a non-hydrogen absorbing metal, with Mg. Mg0.25Mn0.75, prepd. through ball milling MgH2 and Mn powders, is nanocomposite where nanometer-sized MgH2 domains are randomly embedded in a Mn matrix. This sample readily absorbs and desorbs deuterium reversibly even at a temp. of 200°C without adding any catalysts. This is nearly 180°C lower than the typical operating temp. of conventional bulk Mg. Furthermore, at a given temp., its deuterium desorption pressure is clearly elevated compared to that of pure Mg indicating the destabilization of MgD2. The av. crystallite size of MgD2 in deuterated Mg0.25Mn0.75 detd. from X-ray diffraction data is around 9 nm. NMR spectroscopy study shows that MgD2 domains are heavily strained and some of D atoms are coordinated by a few Mn atoms suggesting that a large no. of lattice defects including the partial substitution of Mg with Mn are introduced during ball milling. Furthermore, the Mn matrix firmly locks nanosized MgD2 preventing the agglomeration of MgD2 below 250°C. Our study suggests that a synergistic effect created by nanosizing, large lattice distortion, and robust interface between MgD2 and the Mn matrix can effectively improve the kinetics and thermodn. of MgD2 in Mg0.25Mn0.75 concurrently. Our work demonstrates the possibility of utilizing the immiscibility of metals with Mg to synthesize robust nanostructure that can alter the kinetics and stability of MgH2. The thermodn. of MgD2 is destabilized in the Mg-Mn immiscible system, which is attributed to a synergistic effect created by nanosizing, large lattice distortion, and robust interface between MgD2 and the Mn matrix.
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    18. 18
      Gremaud, R.; Borgschulte, A.; Lohstroh, W.; Schreuders, H.; Züttel, A.; Dam, B.; Griessen, R. Ti-Catalyzed Mg(AlH4)2: A Reversible Hydrogen Storage Material. J. Alloys Compd. 2005, 404–406, 775– 778,  DOI: 10.1016/j.jallcom.2005.01.140
      18
      Ti-catalyzed Mg(AlH4)2: A reversible hydrogen storage material
      Gremaud, R.; Borgschulte, A.; Lohstroh, W.; Schreuders, H.; Zuettel, A.; Dam, B.; Griessen, R.
      Journal of Alloys and Compounds (2005), 404-406 (), 775-778CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      Mg-Al thin films with a compositional gradient were co-sputtered from off-centered magnetron sources and capped with a thin Pd layer. Hydride formation by these films were studied by monitoring their optical transmission during hydrogenation under defined pressure and temp. conditions. Mg(AlH4)2 was already formed from the elements at p(H2)=1 bar and T=100°. A thin layer of Ti acts as a catalyst, but doping of Mg-Al with Ti has a neg. influence on H absorption.
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    19. 19
      Zaluska, A.; Zaluski, L.; Ström-Olsen, J. O. Structure, Catalysis and Atomic Reactions on the Nano-Scale: A Systematic Approach to Metal Hydrides for Hydrogen Storage. Appl. Phys. A 2001, 72, 157– 165,  DOI: 10.1007/s003390100783
      19
      Structure, catalysis and atomic reactions on the nano-scale: a systematic approach to metal hydrides for hydrogen storage
      Zaluska, A.; Zaluski, L.; Strom-Olsen, J. O.
      Applied Physics A: Materials Science & Processing (2001), 72 (2), 157-165CODEN: APAMFC; ISSN:0947-8396. (Springer-Verlag)
      A review with 48 refs. We show how reducing structure, catalysis and at. reactions to the nano-scale may be used in a systematic way to substantially enhance the hydrogenation properties of metal hydrides. We examine, with examples from a wide range of hydrides, the direct impact of nano-scale structure, subsequent improvements in kinetics through nanoscale solid state catalysis, the special properties of nanocomposites, and the role played by nano-scale reactions.
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    20. 20
      Bouaricha, S.; Dodelet, J. P.; Guay, D.; Huot, J.; Boily, S.; Schulz, R. Hydriding Behavior of Mg-Al and Leached Mg-Al Compounds Prepared by High-Energy Ball-Milling. J. Alloys Compd. 2000, 297, 282– 293,  DOI: 10.1016/S0925-8388(99)00612-X
      20
      Hydriding behavior of Mg-Al and leached Mg-Al compounds prepared by high-energy ball-milling
      Bouaricha, S.; Dodelet, J. P.; Guay, D.; Huot, J.; Boily, S.; Schulz, R.
      Journal of Alloys and Compounds (2000), 297 (1-2), 282-293CODEN: JALCEU; ISSN:0925-8388. (Elsevier Science S.A.)
      The structure and hydrogen absorption properties of Mg:Al alloys prepd. by high-energy ball milling were studied over the whole compositional range. These materials were prepd. in their as-milled and Al-leached forms. The latter are obtained from the former materials by leaching out Al in a 1N NaOH soln. The structure of the various alloys was detd. by X-ray diffraction. The structure of the material in the hydrided state was also detd. in some cases. In the as-milled state, hcp Mg(Al) with a small proportion of Mg17Al12 and fcc Al(Mg) are formed at Mg:Al (90:10) and (20:80) compns., resp. At intermediate (58:42) and (37:63) compns., the intermetallic Mg17Al12 and Mg3Al2 phases are formed, resp. Following leaching, the Al content of Mg:Al (90:10) and (20:80) varies from 10.4 and 77.0 to 3.0 and 51.0 at.%, resp. In both cases, noticeable change in the XRD pattern confirms that bulk dissoln. of Al has been achieved. There is a two-fold increase in the sp. surface area of Mg:Al (90:10) following leaching of Al. In the case of Mg:Al with intermediate compns., dissoln. of Al, if any, does not lead to discernable modification in the structure of the material. The measured hydrogen capacity of the as milled material decreases with Al content, from H/M = 1.74 for pure un-milled Mg, to 1.38 for Mg:Al (90:10), and then to 1.05 for Mg:Al (75:25). In each case, there is a further 10-15% decline of the hydrogen absorption capacity after leaching. In the case of Mg:Al (58:42), which basically only contains a nanocryst. Mg17Al12 intermetallic phase, hydriding leads to the formation of MgH2 and Al. This reaction is totally reversible and Mg17Al12 is recovered upon dehydriding. In each case, there is an increase in the kinetics of hydrogen absorption and desorption following Al leaching.
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    21. 21
      Gremaud, R.; Borgschulte, A.; Chacon, C.; van Mechelen, J. L. M.; Schreuders, H.; Züttel, A.; Hjörvarsson, B.; Dam, B.; Griessen, R. Structural and Optical Properties of MgxAl1–xHy Gradient Thin Films: A Combinatorial Approach. Appl. Phys. A 2006, 84, 77– 85,  DOI: 10.1007/s00339-006-3579-z
      21
      Structural and optical properties of MgxAl1-xHy gradient thin films: a combinatorial approach
      Gremaud, R.; Borgschulte, A.; Chacon, C.; van Mechelen, J. L. M.; Schreuders, H.; Zuettel, A.; Hjoervarsson, B.; Dam, B.; Griessen, R.
      Applied Physics A: Materials Science & Processing (2006), 84 (1-2), 77-85CODEN: APAMFC; ISSN:0947-8396. (Springer)
      The structural, optical and d.c. elec. properties of MgxAl1-x (0.2≤x≤0.9) gradient thin films covered with Pd/Mg are studied before and after exposure to H2. Hydrogenog., a novel high-throughput optical technique, was used to map simultaneously all the hydride forming compns. and the kinetics thereof in the gradient film. Metallic Mg in the MgxAl1-x layer undergoes a metal-to-semiconductor transition and MgH2 is formed for all Mg fractions x studied. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH2 and Al occurs. This uncommon large-scale segregation is evidenced by metal and H profiling using Rutherford backscattering spectrometry and resonant nuclear anal. based on the reaction 1H(15N,αγ)12C. Besides MgH2, an addnl. semiconducting phase is found by elec. cond. measurements around an at. [Al]/[Mg] ratio of 2 (x = 0.33). Probably the film is partially transformed into Mg(AlH4)2 at around this compn.
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    22. 22
      Fritzsche, H.; Saoudi, M.; Haagsma, J.; Ophus, C.; Luber, E.; Harrower, C. T.; Mitlin, D. Neutron Reflectometry Study of Hydrogen Desorption in Destabilized MgAl Alloy Thin Films. Appl. Phys. Lett. 2008, 92, 121917,  DOI: 10.1063/1.2899936
      22
      Neutron reflectometry study of hydrogen desorption in destabilized MgAl alloy thin films
      Fritzsche, H.; Saoudi, M.; Haagsma, J.; Ophus, C.; Luber, E.; Harrower, C. T.; Mitlin, D.
      Applied Physics Letters (2008), 92 (12), 121917/1-121917/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We studied the H absorption and desorption properties of thin Pd-covered Mg1-xAlx alloy films as a function of temp. and alloy compn. Using neutron reflectometry, we were able to det. the H content and the H distribution within these MgAl films in situ. For all films, H was uniformly dispersed within the MgAl film and no H was observable in the Pd cover layer. The Mg0.7Al0.3 film shows an appreciable 4.1 wt % stored H and improved desorption characteristics with complete desorption at a temp. of 448 K. (c) 2008 American Institute of Physics.
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    23. 23
      Vermeulen, P.; Graat, P. C. J.; Wondergem, H. J.; Notten, P. H. L. Crystal structures of MgyTi100–y thin film alloys in the as-deposited and hydrogenated state. Int. J. Hydrogen Energy 2008, 33, 5646– 5650,  DOI: 10.1016/j.ijhydene.2008.07.014
      23
      Crystal structures of MgyTi100-y thin film alloys in the as-deposited and hydrogenated state
      Vermeulen, P.; Graat, P. C. J.; Wondergem, H. J.; Notten, P. H. L.
      International Journal of Hydrogen Energy (2008), 33 (20), 5646-5650CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      In situ X-ray diffraction was used to identify the crystal structures of as-deposited and hydrogenated MgyTi100-y thin film alloys contg. 70, 80, and 90 at.% Mg. The preferred crystallog. orientation of the films in both the as-prepd. and hydrogenated state made it difficult to unambiguously identify the crystal structure up to now. In this work, identification of the unit cells was achieved by in situ recording diffraction patterns at various tilt angles. The results reveal a hexagonal closed packed structure for all alloys in the as-deposited state. Hydrogenating the layers under 105 Pa H2 transforms the unit cell into face-centered cubic for the Mg70Ti30 and Mg80Ti20 compds., whereas the unit cell of hydrogenated Mg90Ti10 has a body-centered tetragonal symmetry. The (de)hydrogenation kinetics changes along with the crystal structure of the metal hydrides from rapid for fcc.-structured hydrides to sluggish for hydrides with a bct. symmetry and emphasizes the influence of the crystal structure on the hydrogen transport kinetics.
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    24. 24
      Kim, H.; Schreuders, H.; Sakaki, K.; Asano, K.; Nakamura, Y.; Maejima, N.; Machida, A.; Watanuki, T.; Dam, B. Unveiling Nanoscale Compositional and Structural Heterogeneities of Highly Textured Mg0.7Ti0.3Hy Thin Films. Inorg. Chem. 2020, 59, 6800– 6807,  DOI: 10.1021/acs.inorgchem.0c00059
      24
      Unveiling Nanoscale Compositional and Structural Heterogeneities of Highly Textured Mg0.7Ti0.3Hy Thin Films
      Kim, Hyunjeong; Schreuders, Herman; Sakaki, Kouji; Asano, Kohta; Nakamura, Yumiko; Maejima, Naoyuki; Machida, Akihiko; Watanuki, Tetsu; Dam, Bernard
      Inorganic Chemistry (2020), 59 (10), 6800-6807CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Thin films often exhibit fascinating properties, but the understanding of the underlying mechanism behind such properties is not simple. This is partially because of the limited structural information available. The hurdle in obtaining such information is esp. high for textured thin films such as Mg-rich MgxTi1-x, a promising switchable smart coating material. Although these metastable thin films are seen as solid soln. alloys by conventional crystallog. methods, their H-induced optical transition is hardly understood by a solid soln. model. The authors collect at. pair distribution function (PDF) data for a Mg0.7Ti0.3Hy thin film in situ on hydrogenation and resolve TiH2 clusters of an av. size of 30 Å embedded in the Mg matrix. This supports the chem. segregated model previously proposed for this system. The authors also observe the emergence of a previously unknown intermediate face-centered tetragonal phase during hydrogenation of the Mg matrix. This phase appears between Mg and MgH2 to reduce lattice mismatch, thereby preventing pulverization and facilitating rapid H uptake. This work may shed new light on the H-induced properties of Mg-rich MgxTi1-x thin films. Nanoscale compositional and structural heterogeneities of highly textured Mg0.7Ti0.3Hy thin films were studied during the hydrogenation process using the at. pair distribution function technique.
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    25. 25
      Borsa, D. M.; Baldi, A.; Pasturel, M.; Schreuders, H.; Dam, B.; Griessen, R.; Vermeulen, P.; Notten, P. H. L. Mg-Ti-H thin films for smart solar collectors. Appl. Phys. Lett. 2006, 88, 241910,  DOI: 10.1063/1.2212287
      25
      Mg-Ti-H thin films for smart solar collectors
      Borsa, D. M.; Baldi, A.; Pasturel, M.; Schreuders, H.; Dam, B.; Griessen, R.; Vermeulen, P.; Notten, P. H. L.
      Applied Physics Letters (2006), 88 (24), 241910/1-241910/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Mg-Ti-H thin films have attractive optical properties: they absorb 87% of the solar radiation in the hydrogenated state and only 32% in the metallic state. In the absorbing state Mg-Ti-H has a low emissivity - at 400 K only 10% of blackbody radiation is emitted. The transition between the optical states is fast, robust, and reversible. These properties highlight the applicability of such materials as switchable smart coatings in solar collectors.
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    26. 26
      Borsa, D. M.; Gremaud, R.; Baldi, A.; Schreuders, H.; Rector, J. H.; Kooi, B.; Vermeulen, P.; Notten, P. H. L.; Dam, B.; Griessen, R. Structural, optical, and electrical properties of MgyTi1–yHx thin films. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 205408,  DOI: 10.1103/PhysRevB.75.205408
      26
      Structural, optical, and electrical properties of MgyTi1-yHx thin films
      Borsa, D. M.; Gremaud, R.; Baldi, A.; Schreuders, H.; Rector, J. H.; Kooi, B.; Vermeulen, P.; Notten, P. H. L.; Dam, B.; Griessen, R.
      Physical Review B: Condensed Matter and Materials Physics (2007), 75 (20), 205408/1-205408/9CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The structural, optical, and elec. transformations induced by hydrogen absorption and/or desorption in Mg-Ti thin films prepd. by co-sputtering of Mg and Ti are investigated. Highly reflective in the metallic state, the films become highly absorbing upon H absorption. The reflector-to-absorber transition is fast, robust, and reversible over many cycles. Such a highly absorbing state hints at the coexistence of a metallic and a semiconducting phase. It is, however, not simply a composite material consisting of independent MgH2 and TiH2 grains. By continuously monitoring the structure during H uptake, we obtain data that are compatible with a coherent structure. The av. structure resembles rutile MgH2 at high Mg content and is fluorite otherwise. Of crucial importance in preserving the reversibility and the coherence of the system upon hydrogen cycling is the accidental equality of the molar volume of Mg and TiH2. The present results point toward a rich and unexpected chem. of Mg-Ti-H compds.
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    27. 27
      Niessen, R. a. H.; Notten, P. H. L. Electrochemical Hydrogen Storage Characteristics of Thin Film MgX (X = Sc, Ti, V, Cr) Compounds. Electrochem. Solid-State Lett. 2005, 8, A534,  DOI: 10.1149/1.2012238
      27
      Electrochemical Hydrogen Storage Characteristics of Thin Film MgX (X = Sc, Ti, V, Cr) Compounds
      Niessen, R. A. H.; Notten, P. H. L.
      Electrochemical and Solid-State Letters (2005), 8 (10), A534-A538CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
      The hydrogen storage characteristics of thin film MgX (X = Sc, Ti, V, Cr) compds. were investigated electrochem. The successful prepn. of these metastable, cryst. single-phase, MgX compds. was achieved by means of electron-beam deposition at room temp. The reversible hydrogen storage capacity of these compds. is excellent and up to six times higher than com. AB5-type materials. The gravimetric storage capacities of these new materials were detd. to be 1790 mA-h/g for Mg80Sc20, 1750 mA-h/g for Mg80Ti20, 1700 mA-h/g for Mg80V20, and 1325 mA-h/g for Mg80Cr20, corresponding to 6.7, 6.5, 6.4, and 4.9 wt.% hydrogen, resp. The hydrogen absorption and desorption kinetics are profoundly influenced by the element X incorporated in the MgX compd. Galvanostatic measurements show that the rate capability of the Sc- and Ti-contg. compds. is significantly better than that of the V- and Cr-contg. compds. Isotherms of these systems are obtained using galvanostatic intermittent titrn. technique, revealing that the equil. potential of the main charge/discharge plateau, apart from the Mg80Sc20 compd., only slightly depends on X in MgX. The electrochem. measurements show that low-cost Ti is an excellent substitute for the expensive Sc in MgSc, without introducing detrimental effects.
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    28. 28
      Westerwaal, R. J.; Borgschulte, A.; Lohstroh, W.; Dam, B.; Kooi, B.; ten Brink, G.; Hopstaken, M. J. P.; Notten, P. H. L. The Growth-Induced Microstructural Origin of the Optical Black State of Mg2NiHx Thin Films. J. Alloys Compd. 2006, 416, 2– 10,  DOI: 10.1016/j.jallcom.2005.07.068
      28
      The growth-induced microstructural origin of the optical black state of Mg2NiHx thin films
      Westerwaal, R. J.; Borgschulte, A.; Lohstroh, W.; Dam, B.; Kooi, B.; ten Brink, G.; Hopstaken, M. J. P.; Notten, P. H. L.
      Journal of Alloys and Compounds (2006), 416 (1-2), 2-10CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      Hydrogen absorption by a thin Mg2Ni film capped with Pd results in the nucleation of the Mg2NiH4 phase at the film/substrate interface. On further hydrogenation, a self-organized two-layer system consisting of a Mg2NiH0.3/Mg2NiH4 bottom layer and a Mg2NiH0.3 top layer is formed. This leads to an intermediate optical black state in Mg2Ni thin films, which transforms from metallic/reflective to semiconducting/transparent upon hydrogenation. This hydrogen absorption behavior is completely unexpected, since the hydrogen enters the film through the Pd-capped film surface. To explain the preferential nucleation of Mg2NiH4 at the substrate/film interface, we det. the chem. homogeneity of these thin films by RBS and SIMS. Furthermore by STM, TEM and SEM, we analyze the microstructure. We find that up to a film thickness of 50 nm, the film consists of small grains and clusters of small grains. On further growth, a columnar structure develops. We propose that the nucleation barrier for the formation of the Mg2NiH4 phase is smaller for the small loosely packed grains at the interface, while the columnar grain boundaries promote the hydrogen diffusion to the substrate.
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    29. 29
      Orimo, S.; Fujii, H. Materials Science of Mg-Ni-Based New Hydrides. Appl. Phys. A 2001, 72, 167– 186,  DOI: 10.1007/s003390100771
      29
      Materials science of Mg-Ni-based new hydrides
      Orimo, S.; Fujii, H.
      Applied Physics A: Materials Science & Processing (2001), 72 (2), 167-186CODEN: APAMFC; ISSN:0947-8396. (Springer-Verlag)
      A review, with 144 refs., of published data and authors's systematic studies on the relation between nanometer- or atomistic-scale structures and the specific hydriding properties of the Mg-Ni binary system that exhibit the reversible hydriding reaction. The following intermetallics are considered:(1) nanostructured (n)-Mg2Ni, (2) a mixt. of n-Mg2Ni and amorphous (a)-MgNi, (3) pure a-MgNi, and (4) n-MgNi2. The studies on (5) an a-MgNi-based system for clarifying the effect of the short-range ordering on the structural and hydriding properties and (6) a MgNi2-based system for synthesizing the new Laves phase structure are also presented. The studies of MgNi-based hydrides will provide indispensable knowledge for practically developing the Mg alloys as hydrogen-storage materials.
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    30. 30
      Ludwig, A.; Cao, J.; Dam, B.; Gremaud, R. Opto-mechanical characterization of hydrogen storage properties of Mg-Ni thin film composition spreads. Appl. Surf. Sci. 2007, 254, 682– 686,  DOI: 10.1016/j.apsusc.2007.05.093
      30
      Opto-mechanical characterization of hydrogen storage properties of Mg-Ni thin film composition spreads
      Ludwig, A.; Cao, J.; Dam, B.; Gremaud, R.
      Applied Surface Science (2007), 254 (3), 682-686CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Thin film compn. spreads of Mg-Ni were deposited by co-sputtering on micromachined Si-cantilevers. The studied compns. range from about Mg60Ni40 to about Mg80Ni20. Structural properties as well as mech. stress before and after hydrogenation were measured with XRD and laser profilometry, resp. The compn. spreads were hydrogenated in a special pressure vessel, which allows measuring optically the H-induced deflection (stress-change) of 16 cantilevers as a function of H pressure and/or temp. The H-induced stress is correlated with the compn. and microstructure of the films. Highest H-induced stress changes were found for compns. close to the cryst. Mg2Ni phase.
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    31. 31
      Johansson, E.; Chacon, C.; Zlotea, C.; Andersson, Y.; Hjörvarsson, B. Hydrogen Uptake and Optical Properties of Sputtered Mg-Ni Thin Films. J. Phys. Condens. Matter 2004, 16, 7649– 7662,  DOI: 10.1088/0953-8984/16/43/008
      31
      Hydrogen uptake and optical properties of sputtered Mg-Ni thin films
      Johansson, Emil; Chacon, Cyril; Zlotea, Claudia; Andersson, Yvonne; Hjoervarsson, Bjoergvin
      Journal of Physics: Condensed Matter (2004), 16 (43), 7649-7662CODEN: JCOMEL; ISSN:0953-8984. (Institute of Physics Publishing)
      The H uptake and distribution in wedged Mg-Ni films, with compn. ranging from Mg0.85Ni0.15 to Mg0.55Ni0.45, were studied. Upon H loading at 298 K these films undergo a transition from a mirror-like metallic to a semiconducting transparent state. After exposure to a H pressure of 1 bar, the samples exhibit large variation in optical appearance, ranging from a pale yellowish (Mg rich side) to a brownish shade (Ni rich side). The change in the effective optical band gap Eeffg as a function of sample compn. and H concn. was detd.; it showed changes from 3.6 eV in the Ni poor domain to 2.4 eV in the Ni rich domain. Compn. anal. using the 15N nuclear resonance method showed close to homogeneous H distribution throughout the film and close to linear increase in the H uptake with increasing Mg content. The thermal stability of the films is limited; annealing >393 K results in significant redistribution of the constituents. Mg is enriched at the surface, reacting with Pd and thereby degrading the capping layer through the formation of Mg6Pd and MgO, as detd. by x-ray diffraction, XPS and Rutherford backscattering studies. This redistribution results in a severe decrease of the H uptake rate, as monitored by in situ resistivity measurements.
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    32. 32
      Richardson, T. J.; Slack, J. L.; Armitage, R. D.; Kostecki, R.; Farangis, B.; Rubin, M. D. Switchable mirrors based on nickel-magnesium films. Appl. Phys. Lett. 2001, 78, 3047– 3049,  DOI: 10.1063/1.1371959
      32
      Switchable mirrors based on nickel-magnesium films
      Richardson, T. J.; Slack, J. L.; Armitage, R. D.; Kostecki, R.; Farangis, B.; Rubin, M. D.
      Applied Physics Letters (2001), 78 (20), 3047-3049CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      An electrochromic mirror electrode based on reversible uptake of hydrogen in nickel magnesium alloy films is reported. Thin, magnesium-rich Ni-Mg films prepd. on glass substrates by co-sputtering from Ni and Mg targets are mirror-like in appearance and have low visible transmittance. Upon exposure to hydrogen gas or on cathodic polarization in alk. electrolyte, the films take up hydrogen and become transparent. When hydrogen is removed, the mirror properties are recovered. The transition is believed to result from reversible formation of Mg2NiH4 and MgH2. A thin overlayer of palladium was found to enhance the kinetics of hydrogen insertion and extn., and to protect the metal surface against oxidn.
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    33. 33
      Isidorsson, J.; Giebels, I. a. M. E.; Griessen, R.; Di Vece, M. Tunable Reflectance Mg-Ni-H Films. Appl. Phys. Lett. 2002, 80, 2305– 2307,  DOI: 10.1063/1.1463205
      33
      Tunable reflectance Mg-Ni-H films
      Isidorsson, J.; Giebels, I. A. M. E.; Griessen, R.; Di Vece, M.
      Applied Physics Letters (2002), 80 (13), 2305-2307CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      Mg2NiHx switchable mirrors, which switch from shiny metallic Mg2Ni to transparent Mg2NiH4, exhibit dramatic reversible changes in reflectance at remarkably low H concns. Already for x ≈ 0.3 a 232 nm thick Mg2NiHx film has an absorptance of ∼90% over the whole visible optical spectrum. The transition from highly reflective to black occurs in the concn. interval 0.1<x<0.3. The vol. changes involved are 1 order of magnitude smaller than in the transition from reflecting to transparent. This is expected to enhance the lifetime of future devices based on these materials.
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    34. 34
      Lohstroh, W.; Westerwaal, R. J.; Noheda, B.; Enache, S.; Giebels, I. A. M. E.; Dam, B.; Griessen, R. Self-Organized Layered Hydrogenation in Black Mg2NiHx Switchable Mirrors. Phys. Rev. Lett. 2004, 93, 197404,  DOI: 10.1103/PhysRevLett.93.197404
      34
      Self-Organized Layered Hydrogenation in Black Mg2NiHx Switchable Mirrors
      Lohstroh, W.; Westerwaal, R. J.; Noheda, B.; Enache, S.; Giebels, I. A. M. E.; Dam, B.; Griessen, R.
      Physical Review Letters (2004), 93 (19), 197404/1-197404/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      In addn. to a mirrorlike (Mg2Ni) and a transparent (Mg2NiH4) state, thin films of Mg2NiHx exhibit a remarkable black state with low reflection over the entire visible spectrum, essentially zero transmission and a low elec. resistivity. Such a black state is not explicable for a homogeneous layer since a large absorption coeff. always yields substantial reflection. We show that it results from a self-organized and reversible double layering of metallic Mg2NiH0.3 and semiconducting Mg2NiH4.
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    35. 35
      Lohstroh, W.; Westerwaal, R. J.; van Mechelen, J. L. M.; Chacon, C.; Johansson, E.; Dam, B.; Griessen, R. Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 165411,  DOI: 10.1103/PhysRevB.70.165411
      35
      Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading
      Lohstroh, W.; Westerwaal, R. J.; Van Mechelen, J. L. M.; Chacon, C.; Johansson, E.; Dam, B.; Griessen, R.
      Physical Review B: Condensed Matter and Materials Physics (2004), 70 (16), 165411/1-165411/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)
      The structural, thermodn. and optical properties of Mg2Ni thin films covered with Pd are investigated upon exposure to hydrogen. Similar to bulk, thin films of metallic Mg2Ni take up 4 hydrogen per formula unit and semiconducting transparent Mg2NiH4-δ is formed. The dielec. function ~ε of Mg2Ni and fully loaded Mg2NiH4-δ is detd. from reflection and transmission measurements using a Drude-Lorentz parametrization. Besides the two "normal" optical states of a switchable mirror-metallic reflecting and semiconducting transparent-Mg2NiHx exhibit a third "black" state at intermediate hydrogen concns. with low reflection and essentially zero transmission. This state originates from a subtle interplay of the optical properties of the constituent materials and a self-organized double layering of the film during loading. Mg2NiH4-δ preferentially nucleates at the film/substrate interface and not-as intuitively expected-close to the catalytic Pd capping layer. Using ~εMg2Ni and ~εMg2NiH4 and this loading sequence, the optical response at all hydrogen concns. can be described quant. The uncommon hydrogen loading sequence is confirmed by x-ray diffraction and hydrogen profiling using the resonant nuclear reaction 1H(15N,αγ)12C. Pressure-compn. isotherms suggest that the formation of Mg2NiH4-δ at the film/substrate interface is mainly due to locally enhanced kinetics.
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    36. 36
      Gremaud, R.; Broedersz, C. P.; Borgschulte, A.; van Setten, M. J.; Schreuders, H.; Slaman, M.; Dam, B.; Griessen, R. Hydrogenography of MgyNi1–yHx gradient thin films: Interplay between the thermodynamics and kinetics of hydrogenation. Acta Mater. 2010, 58, 658– 668,  DOI: 10.1016/j.actamat.2009.09.044
      There is no corresponding record for this reference.
    37. 37
      Pasturel, M.; Slaman, M.; Borsa, D. M.; Schreuders, H.; Dam, B.; Griessen, R.; Lohstroh, W.; Borgschulte, A. Stabilized Switchable Black State in Mg2NiH4/Ti/Pd Thin Films for Optical Hydrogen Sensing. Appl. Phys. Lett. 2006, 89, 021913,  DOI: 10.1063/1.2221412
      37
      Stabilized switchable "black state" in Mg2NiH4/Ti/Pd thin films for optical hydrogen sensing
      Pasturel, M.; Slaman, M.; Borsa, D. M.; Schreuders, H.; Dam, B.; Griessen, R.; Lohstroh, W.; Borgschulte, A.
      Applied Physics Letters (2006), 89 (2), 021913/1-021913/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      A triple layer thin film (30 nm Mg2Ni/100 nm Ti/10 nm Pd sputtered on glass) switches reversibly from a shiny metallic to a black state upon exposure to moderate H pressure (≈5 × 103 Pa). This black state resembles that obtained in thick Mg2NiHx layers and has the great advantage of being stable and easily controlled. Both the reversible high optical contrast (Rreflective/Rblack≈10 in the red wavelength range) and the fast kinetics of H absorption and desorption make this material interesting for applications as optical H sensors.
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    38. 38
      Pivak, Y.; Palmisano, V.; Schreuders, H.; Dam, B. The Clamping Effect in the Complex Hydride Mg2NiH4 Thin Films. J. Mater. Chem. A 2013, 1, 10972– 10978,  DOI: 10.1039/C3TA11937H
      38
      The clamping effect in the complex hydride Mg2NiH4 thin films
      Pivak, Yevheniy; Palmisano, Valerio; Schreuders, Herman; Dam, Bernard
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2013), 1 (36), 10972-10978CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      The thermodn. properties and hysteresis behavior of Mg2NiH4 thin films were studied using hydrogenog. in the 303-513 K temp. range. For the enthalpy of hydride formation and decompn., as estd. from a Van't Hoff anal., we find ΔHabs = -51.2 kJ mol-1 H2 and ΔHdes = -76.3 kJ mol-1 H2. These values are affected by the clamping of the film to the substrate. However, the av. enthalpy of the (de-)hydrogenation reaction, ΔH = (ΔHabs + ΔHdes)/2 = -63.5 kJ mol-1 H2, is in remarkable agreement with the av. bulk data, -64 kJ mol-1 H2. The (de-)hydrogenation pressure hysteresis in Mg2NiH4 thin films is >3 orders of magnitude at 303 K, which is substantially larger than in MgH2 films. This behavior is believed to originate from the fact that the phase transition in Mg2NiH4 films occurs between two preferentially oriented phases. We assume that the energy involved in the necessary re-orientation is related to the monoclinic to hexagonal structure transformation. By adding this term, the stress-strain anal. is able to explain the enormous difference between absorption and desorption equil. pressures in Mg2NiH4.
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    39. 39
      Akiba, E.; Nomura, K.; Ono, S.; Suda, S. Kinetics of the Reaction between Mg-Ni Alloys and H2. Int. J. Hydrogen Energy 1982, 7, 787– 791,  DOI: 10.1016/0360-3199(82)90069-6
      39
      Kinetics of the reaction between magnesium-nickel alloys and molecular hydrogen
      Akiba, E.; Nomura, K.; Ono, S.; Suda, S.
      International Journal of Hydrogen Energy (1982), 7 (10), 787-91CODEN: IJHEDX; ISSN:0360-3199.
      The kinetics of the hydriding and dehydriding reaction of Mg2Ni and Mg-10 wt.% Ni was investigated. The reaction rate at various hydride compns. was measured by the pressure change in a const. vol.. For Mg2Ni, at the beginning of the reaction the amt. of absorbed/desorbed H can be expressed by n = k (po-Peq) (n-no), where N is the ratio of H to Mg in the solid, po and Peq are the initial H pressure of the system and the equil. pressure, and no and n3 are the values of n at the start of the reaction and a const. around 2, resp.. After a transient period of approx. 1 min., a diffusion-controlled stage took place as dn/dt = k'(p-peq)/t, where p is the pressure at time t. The rate for Mg-10% Ni is also given.
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    40. 40
      Zhou, C.; Fang, Z. Z.; Lu, J.; Luo, X.; Ren, C.; Fan, P.; Ren, Y.; Zhang, X. Thermodynamic Destabilization of Magnesium Hydride Using Mg-Based Solid Solution Alloys. J. Phys. Chem. C 2014, 118, 11526– 11535,  DOI: 10.1021/jp501306w
      40
      Thermodynamic Destabilization of Magnesium Hydride Using Mg-Based Solid Solution Alloys
      Zhou, Chengshang; Fang, Zhigang Zak; Lu, Jun; Luo, Xiangyi; Ren, Chai; Fan, Peng; Ren, Yang; Zhang, Xiaoyi
      Journal of Physical Chemistry C (2014), 118 (22), 11526-11535CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Thermodn. destabilization of magnesium hydride is a difficult task that has challenged researchers of metal hydrides for decades. In this work, solid soln. alloys of magnesium were exploited as a way to destabilize magnesium hydride thermodynamically. Various elements were alloyed with magnesium to form solid solns., including: In, Al, Ga, and Zn. Thermodn. properties of the reactions between the magnesium solid soln. alloys and hydrogen were investigated. Equil. pressures were detd. by pressure-compn.-isothermal (PCI) measurements, showing that all the solid soln. alloys that were investigated in this work have higher equil. hydrogen pressures than that of pure magnesium. Compared to magnesium hydride, the enthalpy (ΔH) of decompn. to form hydrogen and the magnesium alloy can be reduced from 78.60 kJ/(mol H2) to 69.04 kJ/(mol H2), and the temp. of 1 bar hydrogen pressure can be reduced to 262.33°C, from 282.78°C, for the decompn. of pure magnesium hydride. Further, in situ XRD anal. confirmed that magnesium solid solns. were indeed formed after the dehydrogenation of high-energy ball-milled MgH2 with the addn. of the solute element(s). XRD results also indicated that intermetallic phases of Mg with the solute elements were present along with MgH2 in the rehydrogenated magnesium solid soln. alloys, providing a reversible hydrogen absorption/desorption reaction pathway. However, the alloys were shown to have lower hydrogen storage capacity than that of pure MgH2.
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    41. 41
      Vermeulen, P.; Niessen, R. A. H.; Notten, P. H. L. Hydrogen storage in metastable MgyTi(1–y) thin films. Electrochem. Commun. 2006, 8, 27– 32,  DOI: 10.1016/j.elecom.2005.10.013
      41
      Hydrogen storage in metastable MgyTi(1-y) thin films
      Vermeulen, P.; Niessen, R. A. H.; Notten, P. H. L.
      Electrochemistry Communications (2006), 8 (1), 27-32CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
      The electrochem. properties of MgyTi(1-y) thin films with 0.50≤y≤0.95 during (de)hydrogenation were studied. These metastable alloys were successfully prepd. by electron-beam deposition at room temp. XRD confirmed that cryst. single-phase materials were obtained. Galvanostatic dehydrogenation measurements show that substituting Mg with Ti affects the H storage properties. MgyTi(1-y) alloys with <20 at.% Ti exhibit a low rate-capability, whereas increasing the Ti-content enhances rate-capability. A superior reversible H storage capacity, along with good rate-capability, was found for the Mg0.80Ti0.20 alloy. To study the effect of Ti on the electrochem. H storage properties, pure Mg thin films were also included. The electrochem. deep-discharging behavior of pure Mg shows evolution of the overpotential which may be due to nucleation and a growth process. Galvanostatic hydrogenation of MgyTi(1-y) alloys revealed the formation of products that do not correspond to the intrinsic thermodn. properties of the individual Mg and Ti hydrides, suggesting that no segregation occurs. Also, the analogy of the electrochem. behavior of MgyTi(1-y) and MgySc(1-y) alloys points to a fcc.-structured hydride.
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    42. 42
      Bao, S.; Tajima, K.; Yamada, Y.; Okada, M.; Yoshimura, K. Magnesium-Titanium Alloy Thin-Film Switchable Mirrors. Sol. Energy Mater. Sol. Cells 2008, 92, 224– 227,  DOI: 10.1016/j.solmat.2007.02.024
      42
      Magnesium-titanium alloy thin-film switchable mirrors
      Bao, S.; Tajima, K.; Yamada, Y.; Okada, M.; Yoshimura, K.
      Solar Energy Materials & Solar Cells (2008), 92 (2), 224-227CODEN: SEMCEQ; ISSN:0927-0248. (Elsevier B.V.)
      The authors show gasochromic and electrochromic switching properties of Pd top capped Mg-Ti (Mg-Ti) thin films prepd. by d.c. magnetron sputtering. These films show excellent switchable mirror properties. By exposing to 4% H2 in Ar, Pd (4 nm)/Mg0.82Ti0.18 (40 nm) film changed from the metallic state to the transparent state drastically within 5 s. By exposing to air, it goes back to the metallic state within 60 s. The transmittance spectrum in the hydride state is quite flat in the wavelength range from 400 to 2500 nm. It looks complete color neutral and its chromaticity coordinates are x = 0.326 and y = 0.340. Simple electrochromic device of Mg-Ti thin film using a liq. electrolyte works very well. It can be switched between the mirror state and the color-neutral transparent state.
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    43. 43
      Slaman, M.; Dam, B.; Schreuders, H.; Griessen, R. Optimization of Mg-Based Fiber Optic Hydrogen Detectors by Alloying the Catalyst. Int. J. Hydrogen Energy 2008, 33, 1084– 1089,  DOI: 10.1016/j.ijhydene.2007.09.036
      43
      Optimization of Mg-based fiber optic hydrogen detectors by alloying the catalyst
      Slaman, M.; Dam, B.; Schreuders, H.; Griessen, R.
      International Journal of Hydrogen Energy (2008), 33 (3), 1084-1089CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      Pd-capped gasochromic metal hydrides can be used as sensing layer in fiber optic hydrogen detectors. Using a sensing layer consisting of a 50. nm thick Mg70Ti30 film capped with a 30. nm Pd catalytic layer the authors demonstrate a drop in reflectance by a factor of 10, at hydrogen levels down to 10% of the lower flammability limit. The switching takes place within a few seconds, depending on the actual amt. of hydrogen in the vicinity of the detector. The authors report on the sensitivity of the fiber optic detector for hydrogen, both in argon and oxygen rich environments. Further the sensing behavior in oxygen rich gas mixts. and in normal air can be improved by adding silver to the catalytic Pd cap layer.
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    44. 44
      Gremaud, R.; van Mechelen, J. L. M.; Schreuders, H.; Slaman, M.; Dam, B.; Griessen, R. Structural and Optical Properties of MgyNi1-yHx Gradient Thin Films in Relation to the as-Deposited Metallic State. Int. J. Hydrogen Energy 2009, 34, 8951– 8957,  DOI: 10.1016/j.ijhydene.2009.08.051
      44
      Structural and optical properties of MgyNi1-yHx gradient thin films in relation to the as-deposited metallic state
      Gremaud, R.; van Mechelen, J. L. M.; Schreuders, H.; Slaman, M.; Dam, B.; Griessen, R.
      International Journal of Hydrogen Energy (2009), 34 (21), 8951-8957CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
      Thin MgyNi1-yHx films with a gradient in chem. compn. are investigated by optical spectrophotometry, dc resistivity and X-ray diffraction measurements before and after exposure to hydrogen. In the metallic state cryst. Mg2Ni is present for 0.6 ≤ y ≤ 0.8 and coexists with amorphous Mg and/or Ni. The hydride state is mainly characterized by the presence of Mg2NiH4 around the stoichiometric [Mg]/[Ni] = 2 compn. and some MgH2 on the Mg-rich side. The abrupt microstructural changes found in the as-deposited metallic state around the Mg-Mg2Ni eutectic point correlate well with the compositional dependence of the optical properties in the hydride state. We conclude that the formation of the hydride depends directly on the detailed nature of the metallic parent phase. Furthermore, we demonstrate that high-throughput compositional screening via fiber-optic spectrophotometry is useful for hydride identification. When no structural long-range order is present, this provides a new tool for the search for hydrogen storage materials.
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    45. 45
      Lohstroh, W.; Westerwaal, R. J.; van Mechelen, J. L. M.; Schreuders, H.; Dam, B.; Griessen, R. The Dielectric Function of MgyNiHx Thin Films (2<y<10). J. Alloys Compd. 2007, 430, 13– 18,  DOI: 10.1016/j.jallcom.2006.04.075
      45
      The dielectric function of MgyNiHx thin films ( 2 ≤ y ≤ 10 )
      Lohstroh, W.; Westerwaal, R. J.; van Mechelen, J. L. M.; Schreuders, H.; Dam, B.; Griessen, R.
      Journal of Alloys and Compounds (2007), 430 (1-2), 13-18CODEN: JALCEU; ISSN:0925-8388. (Elsevier B.V.)
      MgyNi (2 ≤ y ≤ 10) thin films covered with a Pd cap layer are hydrogenated in 105 Pa H2 between room temp. and 80° and their dielec. function .vepsiln. ~ is detd. from reflection and transmission measurements. The hydrogenated MgyNiHx thin films show a continuous shift of the optical absorption towards higher photon energies with increasing y. Comparison of the obtained dielec. functions with predictions from an effective medium theory show that a considerable doping of the Mg2NiH4 host takes place at least for y ≤ 3.5 while no signature of MgH2 is obsd. in that compn. range in the optical spectra. This is in contrast to the predictions from the bulk phase diagram where a mixt. of semiconducting Mg2NiH4 (energy gap Eg = 1.6 eV) and MgH2 (Eg = 5.6 eV) is expected.
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    46. 46
      King, R. J.; Talim, S. P. A Comparison of Thin Film Measurement by Guided Waves, Ellipsometry and Reflectometry. Opt. Acta Int. J. Opt. 1981, 28, 1107– 1123,  DOI: 10.1080/713820674
      There is no corresponding record for this reference.
    47. 47
      Murray, J. B.; Palm, K. J.; Narayan, T. C.; Fork, D. K.; Sadat, S.; Munday, J. N. Apparatus for Combined Nanoscale Gravimetric, Stress, and Thermal Measurements. Rev. Sci. Instrum. 2018, 89, 085106,  DOI: 10.1063/1.5040503
      47
      Apparatus for combined nanoscale gravimetric, stress, and thermal measurements
      Murray, Joseph B.; Palm, Kevin J.; Narayan, Tarun C.; Fork, David K.; Sadat, Seid; Munday, Jeremy N.
      Review of Scientific Instruments (2018), 89 (8), 085106/1-085106/10CODEN: RSINAK; ISSN:0034-6748. (American Institute of Physics)
      We present an app. that allows for the simultaneous measurement of mass change, heat evolution, and stress of thin film samples deposited on quartz crystal microbalances (QCMs). We show device operation at 24.85 ± 0.05 °C under 9.31 ± 0.02 bars of H2 as a reactive gas. Using a 335 nm palladium film, we demonstrate that our app. quantifies curvature changes of 0.001 m-1. Using the QCM curvature to account for stress induced frequency changes, we demonstrate the measurement of mass changes of 13 ng/cm2 in material systems exhibiting large stress fluctuations. We use a one-state nonlinear lumped element model to describe our system with thermal potentials measured at discrete positions by three resistance temp. devices lithog. printed on the QCM. By inputting known heat amts. through lithog. defined Cr/Al wires, we demonstrate a 150 μW calorimetric accuracy and 20 μW min. detectable power. The capabilities of this instrument will allow for a more complete characterization of reactions occurring in nanoscale systems, such as the effects of hydrogenation in various metal films and nanostructures, as well as allow for direct stress compensation in QCM measurements. (c) 2018 American Institute of Physics.
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    48. 48
      Slaman, M.; Dam, B.; Pasturel, M.; Borsa, D. M.; Schreuders, H.; Rector, J. H.; Griessen, R. Fiber Optic Hydrogen Detectors Containing Mg-Based Metal Hydrides. Sens. Actuators, B 2007, 123, 538– 545,  DOI: 10.1016/j.snb.2006.09.058
      48
      Fiber optic hydrogen detectors containing Mg-based metal hydrides
      Slaman, M.; Dam, B.; Pasturel, M.; Borsa, D. M.; Schreuders, H.; Rector, J. H.; Griessen, R.
      Sensors and Actuators, B: Chemical (2007), 123 (1), 538-545CODEN: SABCEB; ISSN:0925-4005. (Elsevier B.V.)
      The authors report on the implementation of Pd-capped chemo-chromic metal hydrides as a sensing layer in fiber optic H detectors. Due to the change in optical properties of Mg-based alloys on H absorption, a drop in reflectance by a factor of 10 is demonstrated at H levels down to 15% of the lower explosion limit. The switching takes place in only a few seconds. Comparing Mg-Ni and Mg-Ti based alloys, the latter has superior optical and switching properties. Using a 50 nm thick Mg70Ti30 film capped with a 30 nm Pd catalytic layer as the sensing layer, the authors report on the sensitivity of the fiber optic detector for H both in Ar and O, the temp. behavior and the reversibility.
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    49. 49
      Farangis, B.; Nachimuthu, P.; Richardson, T. J.; Slack, J. L.; Meyer, B. K.; Perera, R. C. C.; Rubin, M. D. Structural and Electronic Properties of Magnesium-3D Transition Metal Switchable Mirrors. Solid State Ionics 2003, 165, 309– 314,  DOI: 10.1016/j.ssi.2003.08.041
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      Structural and electronic properties of magnesium-3D transition metal switchable mirrors
      Farangis, B.; Nachimuthu, P.; Richardson, T. J.; Slack, J. L.; Meyer, B. K.; Perera, R. C. C.; Rubin, M. D.
      Solid State Ionics (2003), 165 (1-4), 309-314CODEN: SSIOD3; ISSN:0167-2738. (Elsevier Science B.V.)
      Reversible mirror-to-transparent state switching was obsd. in a variety of mixed metal thin films contg. Mg and 1st-row transition elements including Ni, Fe, Co, Mn, and Ti. The very large changes in both reflectance and transmittance on loading these films with H are accompanied by significant structural and electronic transformations. The valence states and coordination of metal atoms during H loading were followed using dynamic in situ transmission-mode x-ray absorption spectroscopy. Time-resolved Mg K-edge and Ni, Co, Mn, and Ti L-edge spectra reflect both reversible and irreversible changes in the metal environments. These spectra are compared to those of ref. materials and to predictions from calcns.
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