Piezoelectric characterisation of porous piezoceramics

The characterization of piezoelectric materials is a well-known matter supported by several international standards -IEEE, -IEC. The assumptions normally accepted about the material properties in the standards are low dielectric and mechanical losses and material homogeneity. Some commercial piezoceramics have a porosity about 10% or even higher. The pore space is normally interconnected and filled with oils or polymers. Therefore the material is no more homogeneous and even the losses are not negligible. These materials, widely used to fabricate broad-band transducers, must be accurately characterized if good modelling work is to be undertaken. Some singular results when characterizing these materials following the standards are reported. In particular, the strong influence of the minor phase on the global properties of a porous ceramic is analyzed. The change of the main elastic, dielectric and piezoelectric parameters of some porous ceramics when different fillers are used is reviewed. Moreover, the dynamic behavior of the ceramic is also strongly affected by the properties of the minor phase. All these differences are justified in terms of a model which takes into account the different ways in which the two phases of the porous ceramic interact. The appearance of forbidden mechanical resonances are reported and analyzed in this paper. A characterization procedure based on the measurements of the complex coefficients is used together with the standard methods.