极化电压对聚丙烯压电驻极体膜压电性能的影响*

摘要

压电驻极体是具有压电效应的微孔结构空间电荷驻极体材料,其压电性能与材料的微结构和空间电荷密切相关.本文首先利用压缩气体膨化工艺对聚丙烯(PP)的微结构进行改性,然后利用接触极化方法,研究了极化电压与PP膜空间电荷密度之间的关系,及其对压电性能的影响.结果表明对于极化前厚度为100µm的PP膜,其内部建立有序空间电荷分布的阈值极化电压为2 kV；一旦有序空间电荷建立起来, PP膜即具有压电效应.随着极化电压的提高, PP膜的空间电荷密度逐步增大,压电效应显著增强.当峰值电压为8 kV时, PP膜电极上的电荷密度、准静态压电系数和品质因数FOMv(d33· g33)分别为0.56 mC/m2,379 pC/N和8.6(GPa)-1. PP压电驻极体膜的FOMv比聚偏氟乙烯(PVDF)铁电聚合物膜高2个量级以上,且声阻抗非常低(∼0.025 MRayl),因此该压电膜在超声波发射-接收系统或脉冲-回波系统中具有明显的优势.%Piezoelectrets are a kind of space-charged electret material with a void-structure. The piezoelectric effect in such a material is related with its microstructure and space charge. In this paper the micro-structure of polypropylene (PP) films is first modified by using a pressed gas expansion process to enhance the charging capability of the films, and then a direct contact charging is carried out to polarize the expanded films. The relationship between the applied voltage and the space charge density, and the influence of the applied voltage on the piezoelectric performance of PP films are investigated. Results show that for 100 µm thick modified PP films, the critical voltage necessary for the build-up of the “macro-dipoles” in the inner voids is approximately 2 kV; once the “macro-dipoles” are built up, the PP films will exhibit piezoelectric effect. With increasing polarization voltage, the space charge density gradually increases, resulting in significant enhancement of piezoelectric effect. For the PP films polarized at a peak voltage of 8 kV, the space charge density, d33 coefficient, and the figure of merit FOMv(d33 ·g33) are 0.56 mC/m2, 379 pC/N and 8.6 (GPa)-1, respectively. Since not only the FOMv of the PP films is almost two orders of magnitude larger than that of PVDF, but also the acoustic impedance in such a material is very low (∼0.025 MRayl), the PP films have an obvious advantage as applied in airborne ultrasonic transmit-receive or pulse-echo systems.