3.3. Piezoelectric property
The correlation of longitudinal piezoelectric strain coeffi-
cient (d33) with porosity is shown in Fig. 5. The d33 values of
porous PZT ceramics decreased almost linearly in the porosity
range of 33.0–68.7%. According to the space-charge theory
[22], the existence of pores brings about the increase of spacecharge sites such as lattice vacancies or impurity atoms