Electromechanical properties of 1-3 piezoelectric ceramic/piezoelectric polymer composites

Abstract

A study of the fabrication and electromechanical properties of piezoelectric 1-3 ceramic/polymer composites is reported in which the polymer phase, as well as the ceramic phase, is piezoelectric. Composites were prepared by both layering and dice-and-fill techniques. In the dice-and-fill technique, the major problem was preventing damage to the ceramic rods during the filling process. Both types of composite were found to present problems in the poling process. The electromechanical properties of the composites were measured and compared with modeling results. In general, it was found that the composite properties agreed with model predictions, with the exception of the piezoelectric coefficients, which were significantly lower than predicted. lNTRODUCTlONRecently, l-3 composites (shown schematically in Fig. 1) have proven to be useful materials for medical ultrasonic transducer'." and underwater hydrophone3 applications because they have lower acoustic impedance and higher electromechanical coupling than conventional piezoceramics. l-3 composites are usually composed of piezoelectric ceramics and nonpiezoelectric polymers. The present work involved the investigation of l-3 ceramic/polymer composites in which both ceramic and polymer phases are piezoelectric. This material, if properly designed, is expected to provide useful new properties for ultrasonic transducer applications. For example, the composite with two piezoelectric phases can be used to fabricate novel structure transducers such as two intertwined arrays using the ceramic phase to transmit and using the piezopolymer matrix to receive. In addition the interaction of the piezoelectric effect in the two phases presents interesting possibilities. The piezoelectric coefficients of the ceramic and polymer are of opposite signs and thus tend to cancel in the d,, and d,, of the composite. Since the coercive fields of the two phases are significantly different, it may be possible to pole the two phases in opposite directions, giving a composite in which the contributions of d33 and dsr add. However, the hydrostatic coefficient comes about by an addition of d,,, ds2, and d33. In producing the hydrostatic coefficient of the composite, the contributions from the ceramic J,, and the polymer d,, add, while the contributions of the ceramic d,, and polymer d,, subtract4 The relative magnitude of the effects will depend crucially on the mechanical anisotropy of the polymer phase. This means that in the design of the composite it would be possible to choose to enhance d33 or dh .The polymer used in the present work was a 75125 mol % copolymer of vinylidene fluoride (VDF or VFz) and trifluoroethylene (TrFE), obtained from Solvay et Cie of Bel-"Present address: FDMIPA, Ikip Manado di Tonsaru, Sulawesi Utara, Manado, Indonesia 95115.gium. The ceramic used was TLZ-5 (equivalent to PZT-5H from Vernitron) from GEC-Marconi, Australia. The homopolymer polyvinylidene fluoride (PVDF) normally crystallizes in a nonpolar cr crystal form and needs mechanical stretching to conver...

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