Composite Piezoelectric Sensors and Actuators
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ABSTRACT Composite materials have found a number of structural applications, but their use in the electronics industry has been relatively limited. As the function of electroceramic composites are better understood, we can expect this picture to change. In this paper some of the piezoelectric composite sensor and actuator studies carried out in our laboratory during the past decade will be reviewed. The ideas that provide a basic understanding of functional composites have previously been discussed [1]. This paper describes recent advances in the processing and properties of composites possessing 0-3, 1-3, and 2-2 connectivity made of polymers, metals and ferroelectric ceramics. The introduction of open spaces in the previous connectivity patterns allows the development of new designs of several piezoelectric composites with connectivity patterns of 0(0)3, 1(0)-3, 2(0)-2-2, and 2-0-2. As in most electronic systems that are developed with improved properties a push toward lower cost and smaller sizes of the piezoelectric ceramics, together with improved reliability and performance occurs. The piezocomposites with open spaces in their structures clearly demonstrate the growth of the functional ceramics into the field of the smart ceramics as the materials for the beginning of the next century. INTRODUCTION PZT is widely used as a transducer material because of its high piezoelectric coefficients. However, for hydrophones applications, PZT is a poor material for several reasons. The hydrostatic piezoelectric coefficient, dh (=d 33 + 2d 31 ), is very low. The piezoelectric voltage coefficients, g33 and gh, are low because of the high dielectric constant of PZT (1800). The acoustic matching of PZT with water is poor because of its high density (7.9 g/cm 3). Moreover, it is a brittle, non-flexible ceramic. In the last decade, several investigators have tried to fabricate composites of PZT and polymers to overcome the above problems of PZT. The fabrication of piezoelectric composites requires careful replacement of a portion of the ceramic with a polymer. This replacement allows the reduction of the acoustic impedance of the piezoelectric ceramics for application in a system containing water or a living body. It has been shown that it is possible to improve upon the piezoelectric properties of homogeneous PZT by the composite approach. The concept that the connectivity of the individual phases controls the resulting properties has been demonstrated in a number of composites with different geometry and different connectivity of the individual phases. The hydrostatic piezoelectric properties of these composites are superior to single-phase PZT. However, some of the earlier composites suffer from disadvantages due to difficulty in preparation or reduction in hydrostatic sensitivity with increasing pressure. Thus, there still exists a need to further improve the piezoelectric properties of these composites. A primary goal is to reproducibly fabricate composite transducers with high figures of merit and minimal pressure sensit
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