Developments in Ceramic Materials Research

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R. Ramesh, H. Kara, Ron Stevens and C. R. Bowen
improvement over dense PZT ceramic for most of the PZT-polymer composites. The
reduction in dh by polymer infiltration is due to reduction in stress born by the ceramic phase,
which is not the case for PZT-air composites where stress is born by the ceramic phase only.
As explained earlier extensive cracking in BURPS (ATR) samples hinder the stress or charge
transfer between the electroded faces, resulting in a dramatic reduction in dH compared to
BURPS (PEO) samples.
The hydrostatic charge coefficient is shown to be a function of the Young’s modulus of
the polymer [33] increasing with decreasing stiffness due to an increase in stress transfer into
the PZT ceramic phase. The epoxy resin used in this work had the Young’s modulus of 2.8
GPa determined by tensile testing. It has been reported that the softer epoxy yields lower
acoustic impedance and higher electromechanical coupling [34]. Therefore the type of epoxy
utilised will also be influential on the electromechanical properties. Weak bonding of the
polymer to the ceramic might also contribute to a reduction in dH due to reduction in stress
transfer efficiency.
2.3.4. Hydrostatic Voltage Coefficient
As shown in Figure 4b, the hydrostatic voltage coefficient, gH, increases with decreasing
piezoceramic volume fraction. The increase in gH is more pronounced for PZT-air composites
due to the higher dh values. As low permittivity air or polymer replaces the high value
dielectric PZT phase, an increase in gH is expected since gH is inversely related to the
permittivity (ε). In a 3-3 composite, the improved dh and the reduction in permittivity will
increase gH. Furthermore, a decrease in the PZT ceramic volume fraction will lead to the PZT
ceramic component bearing more stress, resulting an increase in electric field per unit
hydrostatic stress. The highest hydrostatic voltage coefficient measured for the PZT-air and
PZT-polymer composites are 49.4x10 -3 and 161x10 -3 Vm -1 Pa -1 , respectively. These values are
considerably higher than that of dense piezoceramic (3.7x10 -3 Vm -1 Pa -1 ). The Young’s
modulus of the polymer is also thought to influence gH [33]. As the compliance of the
polymer increases, stress transfer from the polymer to the PZT ceramic phase will also
increase, thus increasing gH.
As the permittivity decreases with decrease in ceramic content, the capacitance of the
piezocomposite will reduce to an unacceptable level for hydrophone applications. In addition,
there will be a breakdown limit where the piezoceramic phase cannot bear the hydrostatic
stress (mechanical breakdown) or such a high level of stress in the active ceramic phase will
result in de-poling.
2.3.5. Hydrostatic Figure of Merit
The hydrostatic Figure-of-Merit (FoM) defines the hydrophones’ sensing and actuating
capability. The figure of merit is a product of dh and gH, and expresses the optimum value of
an active element for hydrophone applications and is also a figure of merit for the noise level
[10]. Optimum values are shown around 20% piezoceramic volume fraction, as shown in
Figure 4c. Although both PZT-air and PZT-polymer composites follow a similar trend, the
PZT-Polymer composites have smaller values than that of PZT-Air composites due to their
low dh values, the reasons having been explained in the earlier section. Nevertheless, there is
a significant improvement from a dense PZT (368x10 -15 Pa -1 ) compared to a 20% PZT-