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

Army Research Laboratory, Adelphi, Maryland

Tuesday, December 01 2009

High-temperature capacitor dielectrics in power electronics increase with the electrification of military propulsion and weapons systems.

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.

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.