Developments in Ceramic Materials Research

Progress in Porous Piezoceramics 213
stability to hydrostatic pressure have yet to be proved. The mechanical strength of the
transducer can be improved by filling with a polymer as second phase.
In certain cases, depending on the method of synthesis, the 3-3 piezocomposites are
found to coexist with 0-3 composites for intermediate ceramic volume fractions. The material
properties of these composites with mixed connectivities can be evaluated using theoretical
models [16, 17]. Some theoretical models have been proposed to study the material properties
of ideal 3-3 piezocomposites [18, 19]. Bowen et al. [20] proposed a model, in which the ‘unit
cell’ is split into four volumes with series/ parallel combinations of piezoceramic and polymer
components. This model assumes that only certain volumes contribute effectively to the
piezoelectric charge coefficients and dielectric constant, depending on the alignment of
piezoceramic component with respect to the stresses applied in the longitudinal and lateral
directions. Effective material parameters of the composites can be determined from the
material properties of the constituent material phases, in an average sense [21]. These models
are useful to determine the material properties of the porous piezoceramics.
However, the transducer characteristics of these materials have not been studied
extensively, for which Finite Element Modelling (FEM) would be a simple and effective tool.
FEM studies on 1-3 piezocomposite transducers have been reported [22, 23]. In these studies,
only one unit cell of the composite block has been modelled, assuming that it represents the
entire piezocomposite structure. Although these models can give material parameters like
piezoelectric coefficients, to considerable accuracy, they are inadequate to predict the lateralmode
resonance of a transducer of finite dimensions [24]. Hence, real-size 3-dimensional
FEM studies are necessary to evaluate the device characteristics of piezocomposite
hydrophones.
The transducer characteristics of the porous piezoceramics and polymer filled
piezocomposites are evaluated to demonstrate their suitability to underwater applications [14,
15, 25]. The receiving voltage sensitivity of the 3-3 Piezocomposites hydrophones is found to
be higher than that of the hydrophones constructed from dense PZT materials.
This chapter presents a detailed picture of synthesis and characterisation of porous
piezoceramics, variation of material parameters with the ceramic volume fraction, a method
to estimate the effective material parameters of 3-3 piezocomposites, finite element modelling
of the transducer devices fabricated using porous piezoceramics to estimate the acoustic
performance, construction of piezocomposite hydrophones and evaluation of various
underwater transducer characteristics.
2. SYNTHESIS AND CHARACTERISATION OF
POROUS PIEZOCERAMICS
There are a number of techniques to synthesise porous structures, for example, newly
developed rapid prototyping [26] and free form fabrication [27] processes where a desired
porous structure is built up layer by layer, mixing ceramic powder with a fugitive binder [28]
(BURPS technique, an acronym for ‘BURned-out Plastic Spheres’), replicating a porous
structure by coating it with a ceramic slurry and burning it out during sintering (lost wax
replication of a coral skeleton [29] and polymeric sponge [30] techniques). Of these, the
BURPS and polymeric sponge techniques are relatively easier to implement for large-scale