Development of Thermo-Stable Piezoelectric Polymers and Their Applications to Ultrasonic Transducers

本研究项目是为了开发机电耦合系数kt大于0.25且在高温环境下耐用的新型压电聚合物材料。聚偏氟乙烯在六方相中的结晶只在高压和温度下出现，生成了一层压实了β晶体结构的扩展链片状晶体的薄膜。得到的薄膜的kt值为0.27，并保持了其压电活性，直到略低于熔点，205ﾟC的温度。与常规的聚偏氟乙烯相比，高压结晶聚偏氟乙烯薄膜的压电性能得到了很大的改善，后者的kt值为0.2，在120ﾟC时失去了活性。其中包括：(1)测定了PVDF和P(VDF-TrFE)(VDF-TrFE)的居里温度和熔点与压力的关系.(2)用透射电子显微镜首次观察到P(VDF-TrFE)单晶片中的铁电畴结构.(3)用AT切石英谐振器检测到P(VDF-TrFE)单晶片中链分子的各向异性运动；顺电相中的分子像液体一样沿平行于链轴的方向移动，而在横跨链轴的剪切力作用下，顺电相中的分子则呈锯齿状。(4)采用特殊的处理工艺制备了高度双取向的P(VDF-TrFE)薄膜(类单晶膜)。

This research project was performed for development of new piezoelectricpolymer materials having the electromechanical coupling factor kt more than 0.25 and durable for use under high-temperature ambient condition. Crystallization of poly ( vinylidene-fluoride ) in the hexagonal phase, which appears only at highpressure and temperatures, yielded a film comprizing extended chain lamellar crystals of the beta-form crystal structure. The resulting film has a kt value of 0.27, and retains its piezoelectric activityup to the temperatures just below the melting point, 205ﾟC.High-pressure crystallized PVDF film thus much improved piezoelectric properties as compared to convential PVDF,which has a kt value of 0.2, and loses its activity at 120ﾟC.The usefulness of the newly developed PVDF film as an ultrasonic transducer material working at the megahertz frequency range was demonstrated by constructing ultrasonic transducers which operated effectively at 180-200ﾟC.In addition to the above mentioned results, we have many additional results important both technologically and scientifically. Some of them are as follows :( 1 ) Pressure dependence of Curie temperature and melting point for PVDFand P ( VDF-TrFE ) ( VDF Trifluoroethylenecopolymers ) was determined.( 2 ) Ferroelectic domain structure in a single crystal lamellae of P ( VDF-TrFE ) was first observed by TEM.( 3 ) Anisotropic motion of chainmolecules in a single crystalline lamella of P ( VDF-TrFE ) was detected by usingAT-cut quartz resonators ; the moleculesin the paraelectric phase are very mobilealong the direction parallel to the chain axis like liquid, while they are ridgid for the shear stress across the chainaxis.( 4 ) Highly-doublly-oriented films ( single crystal like films ) of P (VDF-TrFE ) were found to be fabricated by a special treatment techmique.