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

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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.