Polymer-Assisted 3D Printing of Inductor Cores

Poly(glycerol monomethacrylate) (PGMA) prepared by reversible addition–fragmentation chain transfer polymerization was investigated as an additive for high-loading iron oxide nanoparticle (IOP) 3D printable inks. The effect of adjusting the molar mass and loading of PGMA on the rheology of IOP suspensions was investigated, and an optimized ink formulation containing 70% w/w IOPs and 0.25% w/w PGMA98 at pH 10 was developed. This ink was successfully 3D printed onto various substrates and into several structures, including rectangles, high aspect ratio cylinders, letters, spiral- and comb-shaped structures, and thin- and thick-walled toroids. The effect of sintering on the mechanical properties of printed artifacts was investigated via four-point flexural and compressive testing, with higher sintering temperatures resulting in improved mechanical properties due to changes in the particle size and microstructure. The printed toroids were fabricated into inductors, and their electrical performance was assessed via impedance spectroscopy at a working frequency range of 0.001–13 MHz. There was a clear trade-off between electrical properties and sintering temperature due to a phase change between γ-Fe2O3 and α-Fe2O3 upon heating. Nevertheless, the optimized devices had a Q factor of ∼40 at 10 MHz, representing a superior performance compared to that of other inductors with iron oxide cores previously reported. Thus, this report represents a significant step toward the development of low-cost, fully aqueous, high loading, and 3D printable ceramic inks for high-performance inductors and functional devices.

Thermogravimetric analysis (TGA).TGA was carried out using an SDT 650 simultaneous thermal analyzer (TA Instruments).To prepare samples, 0.2 g IOPs were added into 20 g deionized water containing different concentrations of PGMA, followed by 5 min mechanical stirring and 30 min ultrasonication.Then, mixtures were centrifuged at 6000 rpm for 30 min.The sediments were collected and redispersed in 30 mL deionized water followed centrifugation at 6000 rpm for 30 min.Afterwards, the sediment was collected and dried at 60 °C for 48 h.The obtained dried powders were used for TGA.TGA was conducted in an air atmosphere by heating from 25 °C to 800 °C at 10 °C min -1 .The mass of adsorbed additives was calculated by comparing the weight loss between polymer coated IOPs and pristine IOPs.

X-ray diffraction (XRD).
XRD measurements were conducted using a Bruker D8 DISCOVER instrument with a Cu target (λ = 1.5418Å) at a generating voltage of 40 kV, and current of 40 mA.The scattering angle was varied from 20 to 80° in the θ−2θ scan mode with a 0.01° step size and 2 s dwell time.During testing, samples were rotated at a speed of 10 rpm.Measurements were conducted at an IOP concentration of 0.1% w/w in the presence of 1 mM KCl as a background electrolyte.

Calculation of the adsorption ratio and amount of absorbed PGMA98
x is defined as the amount of added PGMA98 based on the mass of IOPs present.
Mreference is defined as the residual mass fraction after performing TGA on pristine IOPs, and was set as the reference.
Mx is defined as the residual mass fraction after TGA of IOPs with varying amounts of PGMA98 added.The adsorbed amount of PGMA98 was calculated using: and Adsorption ratio = Adorbed PGMA 98 ÷  (2) Figure S1.Synthesis of PGMAm via RAFT solution polymerization in ethanol at 70 °C.

Figure S3 .
Figure S3.GPC chromatogram for PGMA98.DMF was used as the eluent.Mn and Mw/Mn values were determined using poly(methyl methacrylate) calibration standards.

Figure S4 .
Figure S4.(a) Zeta potential and (b) intensity-average diameter as a function of pH obtained for iron oxide particles with 1% w/w PGMA98, based on IOP concentration (red dots), and IOPs without additional polymer (blue dots).Measurements were conducted at an IOP concentration of 0.1% w/w in the presence of 1 mM KCl as a background electrolyte.

Figure S5 .
Figure S5.TGA data for IOPs with different amounts of PGMA98 added IOPs.(a) Red, green, blue and black curves represent 0.75%, 0.9%, 1.0% and 0.0% w/w PGMA98 added, respectively.(b) Red, green, blue, brown and black curves represent 1.7%, 2.5%, 4.6%, 5.8% and 0.0% w/w PGMA98 added, respectively.(c) Amount of absorbed PGMA98 (blue) and adsorption ratio (red) calculated from TGA curves.The adsorbed amount of PGMA98 is the amount of the polymer adsorbed to IOPs based on the mass of IOPs.The amount of added PGMA98 is the amount of PGMA98 added based on the mass of IOPs present.The adsorption ratio is the mass of added PGMA98 divided by the mass of absorbed PGMA98, as detailed below.

Figure S6 .
Figure S6.Particle size distribution histograms determined from SEM image analysis of (a) pristine IOP powder, and the outer surfaces of 3D printed thin-walled toroidal cores after being sintered at different temperatures: (b) no sintering, (c) sintering at 400 °C for 2 hours, (d) sintering at 600 °C for 2 hours and (e) sintering at 800 °C for 2 hours.

Figure S7 .
Figure S7.XRD patterns at room temperature and XRD reference cards.(a) Reference card of γ-Fe2O3, (b) pristine IOPs, (c) the printed sample without sintering treatment, (e) reference card of α-Fe2O3, and the printed samples sintered at (d) 400 °C, (f) 600 °C and (g) 800 °C for 2 hours.The ink used for printing contained 70% w/w IOPs and 0.25% w/w PGMA98, based on IOP concentration.

Figure S8 .
Figure S8.(a) Printing set-up of the I&J7300-LF 3D printer.The printer had a movable metal base, a movable head and pressure unit.3D printed letters on (b) cardboard, (c) a piece of blue tissue paper, and (d) a piece of nitrile rubber glove to test the ink printability on different substrates at room temperature.(e) A 3D printed spiral-shaped structure (height × outer diameter × wall thickness = 6.4×20×1 mm) and (f) a 3D printed comb-like thin-walled structure (length × width × wall thickness = 20×20×1 mm).The printing speed was 8 mm s -1 .

Figure S9 .
Figure S9.Mechanical test instrumentation.(a) Four-point flexural testing of a rectangular block (the sample was sintered at 400 °C for 2 hours); (b) compressive testing of a thin-walled toroidal core (the sample was sintered at 400 °C for 2 hours).All mechanical testing was conducted at room temperature.

Table S2 .
Mechanical properties of 3D-printed IOP samples a

Sintering Temperature b Flexural Mechanical Properties c Compressive Mechanical Properties g
Shapes for testing were printed using 70% w/w IOP at pH 10 with 0.25% w/w PGMA98, based on IOP concentration.b.Sintering profiles are shown in Figure 4a.c.Measured using 3D-printed rectangular blocks.d.Stress at failure point in four-point flexural testing.e. Strain at failure point in four-point flexural testing.f.Obtained by fitting the slope between flexural stress and strain.h.Stress at failure point in compressive testing.i. Strain at failure point in compressive testing.j.Obtained by fitting the slope between compressive stress and strain.

Table S3 .
Electrical properties and geometries of different iron oxide-based inductors.This work.b.Work of Hodaei et al. 1 c.Work of Yun et al. 2