Poly(aryl-ether-ether-ketone) as a Possible Metalized Film Capacitor Dielectric: Accurate Description of the Band Gap Through Ab Initio Calculation

Report No. ARL-TR-7160
Authors: Janet Ho, Marco Olguin, and Carlos Diaz
Date/Pages: December 2014; 22 pages
Abstract: Poly(aryl-ether-ether-ketone) (PEEK) is a primary candidate for a high temperature capacitor dielectric due to its outstanding mechanical strength and thermal stability. However, its breakdown strength is unsatisfactory (~400 MV/m) and its volumetric resistivity drops significantly at electric fields above 100 MV/m. Our main objective is to study the effect of chemical impurities, such as water molecules and chain ends, on impurity states in the band gap of PEEK using density functional theory (DFT) to gain insight on the factors impacting electrical resistivity under a high field, which may correlate to low breakdown strength. Our focus is to determine the correct geometry of the unit cell and develop a methodology for computing the band gap of PEEK accurately. We used dispersion-corrected DFT to optimize various starting geometries and determined the correct one based on a comparison with experimental crystallographic data. The band gap was determined from the projected density of states, which is calculated using the HSE06 hybrid exchange-correlation functional. The original HSE06 formulation overestimates the band gap of PEEK by 33% compared to the experimental value. So, to obtain a more accurate functional form, we conducted a systematic study on the parameter space of the functional by varying the exchange-correlation mixing and screening coefficients.
Distribution: Approved for public release
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Last Update / Reviewed: December 1, 2014