Novel Ultrahigh-Performance ZnO-Based Varistor Ceramics

The nonlinear response of a material to an external stimulus is vital in fundamental science and technical applications. The power-law current–voltage relationship of a varistor is one such example. An excellent example of such behavior is the power-law current–voltage relationship exhibited by Bi2O3-doped ZnO varistor ceramics, which are the cornerstone of commercial varistor materials for overvoltage protection. Here, we report on a sustainable, ZnO-based varistor ceramic, without the volatile Bi2O3, that is based on Cr2O3 as the varistor former and oxides of Ca, Co, and Sb as the performance enhancers. The material has an ultrahigh α of up to 219, a low IL of less than 0.2 μA/cm2, and a high Eb of up to 925 V/mm, making it superior to state-of-the-art varistor ceramics. The results provide insights into the design of materials with specific characteristics by tailoring states at the grain boundaries. The discovery of this ZnO-Cr2O3-type varistor ceramic represents a major breakthrough in the field of varistors for overvoltage protection and could drastically affect the world market for overvoltage protection.


■ INTRODUCTION
Varistors, better described as variable resistors, are used in billions of low-power electronic devices and heavy-duty electrical-energy-distribution systems to protect circuits from transient voltage surges by means of their nonlinear current− voltage (I−V) characteristics. 1−3 Specifically, a varistor is highly resistive at a low applied voltage but becomes conductive very quickly when the applied voltage exceeds a material-specific threshold known as the breakdown voltage E b . Above E b , the nonlinear I−V characteristics are empirically described by a power-law function I = bV α , where b is a constant and α is a nonlinear coefficient. The value of α is thus a gauge of how responsive the varistor is to the transient voltage surge. As illustrated in Figure 1a, the value of α is physically governed by the height of the double-Schottky barrier (DSB), φ B , at the grain boundary (GB); 4,5 φ B = |E FB − E FG |, where E FB and E FG are the Fermi levels in the GB and the grain, respectively. Besides the high α value, a high-performance varistor entails a small leakage current (I L ), which is the current at 0.75 × E b . Low values of I L ensure a low chance of thermal runaway, good stability and aging behavior, and also a low power consumption. 6 Among the metal−oxide varistors (MOVs) that are the state-of-the-art varistors, ZnO-based varistors have the highest DSB heights φ B and superior nonlinear properties (high α values and low I L values). 6 Note that pristine ZnO displays a very weak, nonlinear I−V characteristic, reflecting E FB and E FG that are nearly equal. It takes varistor former(s), also known as varistor activator(s), to create defect complexes at the GB, which shift the E FB to a deeper position in the band gap, and the resulting φ B gives rise to the DSB and in turn elicits nonlinear I−V characteristics in the ZnO. 5 In parallel with the varistor former(s) implemented at the GB, performance enhancer(s) can be doped into the grains to actively raise the E FG and thus increase φ B to higher α and smaller I L values. 6−8 Other than α and I L , a high E b value is desired for surge protection and a reduction of device size in high-voltage dc power transmission and heavy-duty power-supply systems, thereby broadening the scope of applications. However, the increase of E b tends to worsen α and I L simultaneously. 3,9 As a result, the state-of-the-art ZnO-based varistors, with varistor formers such as Bi 2 O 3 , Pr 6 O 11 , and V 2 O 5 along with their corresponding performance enhancers, such as Sb, Co, Mn, Ni, and Cr, typically achieve an α value of less than 60, an I L values higher than 1 μA/cm 2 , and an E b value in the range 200−500 V/mm. 10−12 After exploring a large phase space (Supporting Information), here we report on a novel ZnO-based varistor ceramic that uses Cr as the varistor former and Ca, Co, and Sb as the performance enhancers. These ceramics have record-high nonlinear characteristics, manifested in an ultrahigh α value up to 219, a very low I L value of less than 0.2 μA/cm 2 , and a high E b value of 925 V/mm, making them superior to today's varistor ceramics. For conciseness, we only list the performance of ZnO-based varistors for comparison in Figure 1b. Moreover, the materials sustainability (volatility and toxicity) of these ultrahigh-performance varistors is significantly improved. Specifically, the compositions avoid the use of highly volatile bismuth oxide, 13  The starting powders were wet mixed by ball milling for 8 h, followed by drying at about 120°C for 5 h, calcining at 450°C for 2 h, and then cold pressed as pellets with a diameter of 12 mm and thickness of 1 mm. Finally, the pellets were sintered at 1200°C for 3 h. Silver pastes were covered on the sample's opposite surfaces and then dried at 560°C for 15 min as electrodes.
Characterization. X-ray diffraction using Cu Kα radiation (D/ max 2550 V, Rigaku, Tokyo, Japan) was used to analyze the phase  Microstructural observations were carried out on the polished surfaces using a field-emission scanning electron microscope (Magellan 400, FEI Co., USA). Energy-dispersive X-ray spectroscopy (Oxford Instrument, UK) was performed to detect elements and distributions. Spherical aberration-corrected transmission electron microscopy (HF 5000, Hitachi, Japan) with energy-dispersive X-ray spectroscopy (Oxford X-Max 100TLE, UK) and transmission electron microscopy (Tecnai-F20, FEI Co., USA) were used to observe the microstructures and compositions of the grain boundaries (GBs). The micro-Raman spectra were recorded with a Renishaw InVia Confocal micro-Raman system using the 532 nm line as an excitation source. The I−V curves were recorded with a high-voltage digital sourcemeter (Keithley 2410, Keithley Instruments Inc., USA). By convention, the breakdown voltage (E b ) was measured at a current of 1 mA/cm 2 , and the leakage current density (I L ) was measured at an electric field of 0.75 × E b . The nonlinear coefficient (α) was fitted by the equation I = bV α when the voltage is larger than E b . The dielectric spectra were measured using a broad-band dielectric spectrometer (NovoControl, Hundsangen, Germany) in the frequency range from 0.1 Hz to 1 MHz with an amplitude voltage of 1 V. Ag paste (fired at 560°C for 15 min) electrodes were used. The dc conductivity (f < 1 Hz) was obtained with dielectric spectroscopy at f = 0.1. The data from the impedance measurements were analyzed using the commercial software (Z-VIEW, version 3.1). Kelvin probe force microscopy (KPFM) measurements were performed on the atomic force microscopy (AFM) platform to observe the surface potential images. An ac modulation voltage of 2 V at 55 kHz in the lift mode with a distance of approximately 10 nm between the tip and the sample and a conductive Pt/Ir-coated tip (PR-EX-KPFM-5) were used. Additional information is available from the Wiley Online Library or from the author.

■ RESULTS AND DISCUSSION
Unlike previous reports, Cr is, in this work, found to be a varistor former rather than a performance enhancer for ZnObased varistors. Cr was thought to be a performance enhancer and, in this role, together with other performance enhancers (i.e., Sb, Co, Mn, Ni, etc.) was added to ZnO along with the well-established varistor formers such as Bi and Pr. 17,19 To verify whether Cr is a varistor former at the GB or a performance enhancer in the grains, a series of samples with nominal compositions Zn 1−x Cr 2x O 1+2x was synthesized. The Xray powder-diffraction measurements ( Figure S1) reveal that a pure hexagonal wurtzite structure is obtained when x ≤ 0.1%, above which a secondary phase, ZnCr 2 O 4 , forms. These arguments are corroborated by the backscattered electron (BSE) micrographs and the energy-dispersive X-ray spectroscopy (EDS) measurements ( Figure S2). As shown in Figure  2a, no intergranular phases or amorphous regions are observed at the GB by high-resolution transmission electron microscopy (HRTEM). While no Cr is detected in the grains, within the EDS detection limit, for all of the samples ( Figure S2) Cr is observed at the GB (inset of Figure 2a). Micro-Raman spectra also detect a strong vibration mode centered at ∼830 cm −1 , which was previously reported in Cr-added ZnO nanocrystals, 26 at the GB, but absent in the grains (Figure 2b). All of the data suggest a preference for Cr to stay at the GB instead of in the grain. Meanwhile, the oxygen content is higher while the Zn content is lower at the GB compared to within the grain, indicating that the segregation of the Cr at the GBs leads to an O-rich GB compared to the grain ( Figure S3). In light of the case study of ZnO-Bi 2 O 3 -based and ZnO-Pr 6 O 11 -based varistors, 27,28 the segregation of Cr at the GB leads to the formation of a Cr Zn + V Zn + O i defect complex, which acts as a varistor former to shift the E FB to a deeper position in the band gap to yield the DSBs and thus the nonlinear characteristics (Figure 2c).
The varistor former Cr elicits nonlinear I−V characteristics in ZnO, but the α value is only about 2 in the ZnO-0.1% Cr 2 O 3 sample (Figure 2c). Introducing performance enhancers is thus a must to enlarge the difference between E FB and E FG , i.e., φ B . To this end, the results of our study show that codoping with Ca, Co, and Sb acts as an effective performance enhancer in the presence of the varistor former Cr. Figure S4a, which shows elemental mapping, indicates that Ca, Co, and Sb are homogeneously distributed in the grains in the absence of Cr within the EDS detection limit. Hereafter, the chemical formula (Ca, Co, Sb)-doped ZnO with a varistor former derived from x% Cr 2 O 3 will be written in the form (Ca, Co, Sb)-doped ZnO-x% Cr 2 O 3 . Figure 2c shows that the nonlinear current density vs. electric field (J−E) characteristics are observed in all of the Cradded ZnO ceramics, but the α values vary by several orders of magnitude with the specific compositions of the performance enhancers ( Table 1). The detailed electrical properties (α, I L and E b , shown in Tables S1−S3) of the ZnO ceramics with various Ca-, Co-, and Cr-doping ratios are presented in the Supporting Information. In the following, we will focus on the samples with 0.1 mol % Cr 2 O 3 , 2 mol % CaCO 3 , 0.5 mol % Co 3 O 4 , and 0.15 mol % Sb 2 O 3 ratios as an illustration and for conciseness. As shown in Figure 2d and Table 1, the α value increases from about 2 for ZnO-0.1% Cr 2 O 3 , to 27 for Cadoped ZnO-0.1% Cr 2 O 3 , to 73 for (Ca, Co)-codoped ZnO-0.1% Cr 2 O 3 , and finally to an exceptional value of 219 in (Ca, Co, Sb)-codoped ZnO-0.1% Cr 2 O 3 . It is not rare to observe increased α values with an increasing number of enhancers (dopants); 29 however, the magnitude of the improvement attained here is unprecedented (Figure 2d). Importantly, the (Ca, Co, Sb)-codoped ZnO-0.1% Cr 2 O 3 ceramic has a very low I L of less than 0.2 μA/cm 2 , also superior to those of previous state-of-the-art varistors (Figure 1b). 12,16,18,19 Furthermore, a high E b of 925 V/mm is obtained in the (Ca, Co, Sb)-codoped ZnO-0.1% Cr 2 O 3 ceramic due to the greatly decreased grain size ( Figure S5).
The very low I L and greatly decreased grain size suggest a high GB resistance. This argument is corroborated by the results of impedance spectroscopy. As shown in Figure S6, the resistances of the GBs are much larger than those of the grains in all of the Cr-containing ZnO ceramics. In particular, the (Ca, Co, Sb)-codoped ZnO-0.1% Cr 2 O 3 ceramic shows an extremely high resistance at the GBs, an indication of the asformed DSB. 2,30 The record-high varistor performance in 0.2% Cr-added (Co, Ca, Sb)-codoped ZnO ceramics with a high α value, a very low I L value, and a high E b value needs to be related to the band structure and the added performance enhancers ( Figure  3a). As discussed above, E FB is close to E FG in the pristine and 0.2% Cr-added ZnO ceramics, yielding small α values. When adding the performance enhancers in ZnO, shallow donor levels form near the bottom of the conduction band of the grains, thus raising E FG , φ B , and α.
To probe the shallow donor levels in the grains, we performed temperature-dependent dc conductivity measurements. The activation energy (E a ) is estimated by fitting the experimental data for the dc conductivity to the Arrhenius relation T e ( ) , where σ 0 is a constant, k B is the Boltzmann constant, and T is the absolute temperature. As shown in Figure 3b and Figure S7, the high-temperature σ data (373−473 K) and the low-temperature σ data (below 253 K) point toward two distinct E a values, hereafter termed E aL and E aH , respectively. With the composition varying from ZnO-0.1% Cr 2 O 3 , to Ca-doped ZnO-0.1% Cr 2 O 3 , to (Ca, Co)doped ZnO-0.1% Cr 2 O 3 , and finally to (Ca, Co, Sb)-doped ZnO-0.1% Cr 2 O 3 , E aL and E aH evolve with opposite trends: E aH systematically increases (cf. Figure 3b), whereas E aL systematically decreases ( Figure S7b). The decreasing E aL indicates that the donor level becomes shallower in the band gap. Notably, the dc conductivities are nearly temperature independent at T < 253 K for (Co, Ca)-ZnO-0.1% Cr 2 O 3 and (Co, Ca, Sb)-ZnO-0.1% Cr 2 O 3 , suggesting that the Fermi levels of the grain E FG in these two compositions shift into the conduction band with practically zero activation energy. The E FG value is enhanced with an increasing number of dopants (enhancers). The increasing E FG value helps increase the φ B and α values (Figure 2d) and substantiates the band-structure diagram plotted in Figure 3a. More supporting evidence will be provided in the following.
The high-temperature dc conductivities are known to be sensitive to φ B . 34 The results, including the fitted hightemperature activation energy E aH , are presented in Figure 3b. The E aH value is proportional to φ B , i.e., |E FB − E FG |. 34 The E aH value of the (Co,Ca)-codoped ZnO ceramic is found to be 0.14 eV, similar to that of pristine ZnO, as shown in Figure S8. This implies that the E FG and E FB values are close, giving rise to a very weakly nonlinear I−V behavior ( Figure S9). These results also indicate that Ca and Co are not effective varistor formers. In contrast, the E aH value for the ZnO-1% Cr 2 O 3 ceramic is 0.43 eV, significantly higher than that of ZnO and (Co, Ca)-doped ZnO. Thus, Cr is an effective varistor former. The E aH value is further increased to 0.82 eV in Co-ZnO-0.1% Cr 2 O 3 , to 0.93 eV in (Co, Ca)-ZnO-0.1% Cr 2 O 3 , and to 0.95 eV in (Co, Ca, Sb)-ZnO-0.1% Cr 2 O 3 . The E aH value can also be cross-checked by decomposing the high-temperature complex impedance Z* into the grain resistance R g and the GB resistance R b and fitting the temperature dependence of R b into the Arrhenius model ( Figure S10). As shown in Figure 3c, the E aH values derived from the high-temperature dc conductivities and high-temperature impedance spectra agree well with each other.
All of the results presented and discussed in the last two paragraphs thus support the schematic diagram shown in Figure 3a, in which φ B is significantly enhanced with an increasing number of dopants (enhancers). The systematically enhanced Schottky barrier height is further confirmed by the results from scanning Kelvin probe microscopy (Figure 3d and Figure S11). The surface potential across the GBs in ZnO-0.1% Cr 2 O 3 is low and under the detection limit, but it quickly increases to about 5 mV in Ca-doped ZnO-0.1% Cr 2 O 3 , to 14 mV in (Co, Ca)-doped ZnO-0.1% Cr 2 O 3 , and finally to 35 mV in (Co, Ca, Sb)-ZnO-0.1% Cr 2 O 3 , consistent with the trend in the variation of E aH .

■ CONCLUSIONS
A novel type of ZnO-based varistor ceramic with Cr as the varistor former and Ca, Co, and Sb as performance enhancers was discovered with simultaneously an ultrahigh nonlinear coefficient, a high breakdown voltage, and a low leakage current, superior to all of the state-of-the-art varistor ceramics, including the existing ZnO-based ones. The breakthrough is not only in the varistor's performance but also in terms of the materials sustainability in terms of chemical stability, being environmental friendly, and having low cost. Specifically, the element Cr at the GBs as a varistor former and the proper performance enhancers (Ca, Co, Sb) doped in the grain work in tandem to yield a record-high varistor performance for the novel ZnO-Cr 2 O 3 type of varistor ceramics.
The discovery of a novel ZnO-Cr 2 O 3 -type varistor ceramic represents the first such major breakthrough in the field of overvoltage protection after several decades that were dominated by the ZnO-Bi 2 O 3 -based ceramics. The results also provide insights into the design of the material and the development of nonlinear varistor ceramics with enhanced performance.