Studies on Properties of Ferroelectric Polymers at Low Temperatures and Their Applications

本研究的主要目的是阐明铁电聚合物在低温下的机电、热释电及相关性质，并将铁电聚合物应用于低温超声换能器，以研究功能物质的低温声学性能。为此，我们介绍了一台低温恒温器(RD-204)，它可以工作到8K，并可选择配备高频测量设施，并构建了一套在低温下精确测量声速和吸收的系统。主要研究的铁电聚合物是偏氟乙烯和三氟乙烯的共聚物[P(VDF-TrFE)]。主要研究结果如下：(1)尽管介电常数和刚性常数对温度有很强的依赖性，但许多铁电聚合物的机电耦合常数k_t几乎与温度无关(8K-300K)[PVDF，P(VdF-TrFE)，P(VdF-…)更多Tefe)]。这是因为这些聚合物的电致伸缩系数与温度无关。(2)P(VDF-TrFE)的kt值即使在很低的温度下也很大(0.3)，因此它是工作在低温(LTS)下的超声换能器的良好换能器。(3)P(VDF-TrFE)在斩波光照射下产生的动态热释电电压随着温度的降低而增加，这表明P(VDF-TrFE)有可能成为灵敏的LT热释电探测器。测量的热容和热释电系数与温度的关系可以用三维和一维振动模式的晶格振动理论很好地描述。(4)首次发现对苯二酚和甲醇的包合物是居里温度为68K的第一OEDER铁电晶体。在该温度下，我们观察到了声速的不连续和介电常数的峰值。除了这些结果，我们还得到了许多关于P(VDF-TrFE)、液晶聚合物、碳纤维复合塑料和氧化铜超导体在低温下的声学性质的重要信息。我们发现了P(VDF-TrFE)中铁电畴的形成，并阐明了它的作用机理。较少

The main purpose of this study is to clarify the electromechanical, pyroelectric and related properties of ferroelectric polymers at low temperatures, to apply ferroelectric polymers to ultrasonic transducers operating at low temperatures for studying low temperature acoustic properties of functionlty substances. For this purpose we introduced an cryostat (RD-204), which can works down to 8K and is optionally equipped with facilities for high frequency measurements, and we constructed a system for precise measurement of sound velocity and absorption at low temperatures. The ferroelectric polymers mainely studied are copolymers of vinlyidene fluoride and trifluoroethylene [P(VDF-TrFE)]. The main research results we obtained are as follows:(1) Despite of strong temperature dependence of the dielectric constant and stiffness constant.,the electromechanical coupling constant k_t is almost independent of temperature (8K-300K) for many ferroelectric polymers studied[PVDF, P(VDF-TrFE), P(VDF- … More TeFE)]. This stems from the fact that the electrostriction coefficients of these polymers are temperature-independent.(2) The value of k_t of P(VDF-Trfe) is large (0.3) even at very low temperatures, and therefore it is a good transducer for ultrasonic transducers working at low temperatures (LTs).(3) Dynamical pyroelectric voltage in P(VDF-TrFE) generated on irradiation of chopped light increases with decreasing temperature owing to more rapid decreasing of heat capacity, showing that P(VDF-TrFE) has a potential for sensitive LT pyroelectric detectores. The measured temperature dependence of heat capacity and pyroelectric coefficient are found to be well described by the lattice vibration theory for 3- and 1- dimensional and optical vibrational modes.(4) We find for the first time that the clathrate compound of hydroquinone and methanol is a ferroelectric crystal of the first oeder whose Curie temperature is at 68K. We observed a discontinuity in sound velocity and a peak in dielectric permittivity at that temperature. We consider that the alignment of dipoles of OH-groups is responsible for the ferroelectricity.(5) In addition to these results, we get a lot of important information on LT acoustic properties of P(VDF-TrFE), liquid crystalline polymers, carbon-fiber composite plastics, and a copper oxide superconductor, using P(VDF-TrFE) films as ultrasonic at low temperature. We found the formation of ferroelectric domains in P(VDF-TrFE) and clarified ots mechanism. Less