This
results from their ability to accomplish coupling between
mechanical and electric signals as well as their possibility of
obtaining very high values for some piezoelectric coefficients
compared with dense materials. Intense attention has been
attached to the study of porous PZT ceramics mainly for the
following several reasons. First, since piezoelectric coefficients
d33 and d31 are opposite in sign, hydrostatic piezoelectric strain
coefficient, dh (=d33 + 2d31), has a small value. Previous
research [4,5] has demonstrated that increasing porosity can
result in moderate decline in d33 but rapid reduction in d31,
contributing to the value of dh. Second, inclusion of air phase
reduces dielectric constant ( e) of the bodies considerably,
leading to increase in hydrostatic piezoelectric voltage
coefficient gh, which can be calculated by dh/e.