Developments in Ceramic Materials Research

212
R. Ramesh, H. Kara, Ron Stevens and C. R. Bowen
piezocomposites made up of porous piezoceramics are very useful for wide-band
hydrophone applications. A detailed picture of these studies and the salient features of
these materials will be discussed.
Keywords: Piezoelectric, piezoceramic, porous, piezocomposite, hydrophone, Finite
Element Modelling.
1. INTRODUCTION
Piezoelectric ceramic materials, such as Lead Zirconate Titanate (PZT), are widely used
as active components in transducers used for sonar, under-water communication, under-water
imaging, medical imaging and non-destructive evaluation, because of their high
electromechanical coupling coefficient and low electrical and mechanical losses [1].
However, their higher specific acoustic impedance, higher weight factor and poor mechanical
properties are undesirable in certain applications. On the other hand, the piezoelectric
polymers like polyvinylidene difluoride (PVDF) have very low specific acoustic impedance
offering excellent impedance matching with water or human tissues [2] The serious
drawbacks of these materials are very low values of electromechanical coupling coefficient
and dielectric constant. Recently, piezocomposite materials have been developed by
combining active piezoelectric materials with passive polymers, thereby taking advantages of
the desirable features of both the constituent materials [3]. They exhibit superior
electromechanical properties compared to the piezoceramic materials.
Some of the advantages of the piezocomposites are, higher electromechanical coupling
coefficients, low specific acoustic impedance, a wide range of dielectric constant, low
dielectric and mechanical losses, variable sound velocity, low mode- coupling, ease of
subdividing into acoustically isolated array elements and formability into complex curved
shapes, etc [4] However, all the benefits are not been achieved simultaneously and
composites with the best characteristics are generally achieved through optimization of
various parameters [5] Piezocomposite transducers with predetermined properties can be
tailor-made for a specific application by optimizing the type and proportion of ceramic and
polymer components and their relative arrangements in the composite structure [6].
In piezocomposites, the piezoceramic and polymer phases are physically connected in
three dimensions in a systematic arrangement called connectivity [7]. Although
piezocomposite of various connectivities do exist theoretically, only certain categories, such
as, 1-3, 2-2 and 3-3 connectivities are technically feasible to fabricate and are found useful for
practical applications [8, 9, 10].
1-3 piezocomposites have been studied extensively and various modelling and
experimental studies have been reported in literature [11, 12]. Although, 1-3 composites are
highly useful for transducer applications, their production is tedious and expensive [8]. 3-3
piezocomposites prove to be an alternative, with comparable material properties and
relatively simpler method of synthesis [8]. 3-3 piezocomposites in the form of porous PZT
materials show considerably improved transducer characteristics. Experimental studies on
porous piezoelectric structures indicate that they have high hydrostatic figure-of-merit [13]
and high receiving sensitivity [14, 15]. However, their depth-handling capability and the