| Research Abstract |
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-
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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
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