Studies on piezoelectricity and pyroelectricity of Aramid film by mean of poling at high temperature and high voltage

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This research was proposed to explore the possibility of finding the polymeric piezoelectric membrane with higher heat-resistance than the fluorinic membranes evaluated until now. The origin of the research idea was in finding the fact that the molecules of poly (p-phenyleneterephthalamide) (PPTA) rotate in the crystal of PPTA around 230ﾟC by the angle of 90ﾟ in spite of its high heat-resistance of 510ﾟC. By utilizing this motion, if the poling at high temperature and high electric field is carried out, the membrane with high heat-resistance will be realized.In 1990, the membrane of PPTA was taken up and the poling was carried out at the temperatures around 230ﾟC which corresponds to the crystalline relaxation temperature. The piezoelectric constant was found however to be as low as 0.05pC/N. The reason for it was ascribed to the unusually stable crystal structure of PPTA and high degree of crystallinity, which brought very few. amide groups available for piezoelectricity. Thus, in 1991, poly (m-phenyleneisophthalamide) (PRIA) was used for the piezoelectric film, which has lower crystallinity than PPTA. After many trials, it was found unexpectedly that the poling of PNIA film at 100ﾟC lower than the glass transition temperature of the amorohous region gave the piezoelectric constant was 0.4pC/N which was held as high as 300ﾟC. Structural inspection revealed that the 60% of piezoelectricity is assigned to the amorphous region and 40% to the crystalline region. Further improvement is expected by using uniaxially uniplanar oriented film of PNIA.