the d33 value decrease at a more rapid rate than that of d31, so
the dh value decreased appropriately.
With the increase of porosity, the lowered εr and high value of
dh led to promoted value of hydrostatic voltage coefficient (gh),
and hence hydrostatic figure of merit (HFOM) with respect to
dense PZT ceramics. As can be seen from Fig. 7, no matter how
the compositions of suspension changed, the HFOM increased
obviously with the increase of porosity with the highest value of
19,520 10 −15 Pa−1, much higher than that of previous research
[23]. However, the low value of εr corresponding to high porosity
is always harmful for the stability and reliability of specimen. The
capacitance, C, which characterizes the stability of specimen,
could be calculated from the inversion formula