Critical behavior of ultrasonic wave velocities in porous piezoelectric ceramics

We present results on lead zirconate-titanate (PZT) porous ceramics with pore volume fraction up to 60%, prepared by tape casting which, unlike uniaxial powder pressing, allows to obtain isotropic samples. Tape casted samples have been subjected to thermal treatments at different sintering temperatures and their pore size distribution and microstructure have been analyzed through intrusion mercury porosimetry and scanning electron microscopy (SEM). After poling the ultrasonic wave velocities as well as the elastic, dielectric and piezoelectric properties of the samples have been measured. It has been found that they vary continuously with increasing porosity and vanishes (or diverges) at a certain critical value of the porosity, following power laws like in other critical phenomena e.g. thermal phase transitions.