Characterization of High-Temperature Polymer Thin Films for Power Conditioning Capacitors

Army Research Laboratory, Adelphi, Maryland

Wide bandgap semiconductors (e.g., silicon carbide) will enable operation of military systems at temperatures above 150 °C, which eases thermal management. However, such systems cannot be designed efficiently unless capacitors are available that can operate at similarly high temperatures.

Metallized polymer film capacitors have the advantage of self-healing, which allows graceful failure, i.e., a gradual loss in capacitance, rather than catastrophic failure as in ceramic capacitors. As a result, the capacitor dielectric can be operated at an electric field near the dielectric breakdown strength, thus achieving higher energy density. The state of the art in capacitor films is biaxially oriented polypropylene (BOPP), which has a low loss (tan δ~1x10^-4) that is independent of frequency, and a high dielectric strength (~700 MV/m). The disadvantage of BOPP is that at temperatures above 85 °C, the operating voltage must be derated, and the maximum operating temperature is limited to about 105 °C.

Figure 1. The configuration for Breakdown Strength Measurement showing (a) the four individual layers separated and (b) the configuration during measurement.

Other commercially available polymer film capacitors that can be operated over a wider temperature range include poly(ethylene terephthalate) (PET), poly(ethylene napthalate) (PEN), polycarbonate (PC), poly(phenylene sulfide) (PPS), and Teflon. However, only PPS and Teflon can be operated at 150 °C and up to 200 °C, respectively. PPS has a poor self-healing capability, and Teflon has a low breakdown strength. As a result, neither of the presently available high-temperature capacitor dielectrics is likely to satisfy military requirements for reliability and operating temperature on power electronics.

Poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI) are two commercially available thin films (<12 μm) that are candidates for high-temperature applications, as their glass transition temperature is above 150 °C. This work characterizes these two polymer films over a wide range of temperatures and compares them to BOPP and PPS.

The materials studied were 12 μm (nominal) PEEK from Victrex, 6 μm (nominal) PEI from General Electric, 9 μm (nominal) PPS from Toray, and 7 μm (nominal) BOPP from Kopafilm. PPS and BOPP were used as benchmarks.

Figure 2. Dielectric Constant of the Polymers under study at different temperatures. The dielectricconstant of BOPP is shown at 95 °C, and that of the other three polymers is shown at 200 °C, along with the data at 30 °C for comparison.

The breakdown strength measurements and dielectric properties at elevated temperatures for PEEK and PEI were compared to those for PPS and BOPP, which represent the present state of the art. The breakdown strength at room temperature for PEEK was the lowest at about 320 MV/m, while strength for both PPS and PEI was 500 MV/m and for BOPP was 720 MV/m. At 150 °C, the decrease in breakdown strength relative to room temperature for PEI was about 16% and about 13% for PEEK, while PPS remained unchanged. For BOPP, the maximum test temperature was 100 °C, at which the breakdown strength decreased by about 11%.

Based on the results of breakdown strength measurements, PEEK and PEI appear to offer no improvement over PPS, from which capacitors are already available. However, results from dielectric loss measurements seem to indicate that PPS has a greater electrical conductivity at 200 °C than PEEK or PEI. Moreover, PEI has a lower loss than PEEK at temperatures above 150 °C and frequencies higher than 1 kHz. Therefore, PEI appears to be the better candidate for power conditioning capacitors.

This work was done by Janet Ho and Richard Jow of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. ARL-0079