Piezoelectric composites with 0-3 connectivity were prepared by thermosetting lead zirconate-titanate ceramic(PZT) - phenolic resin polymer and barium titanate - phenolic resin polymer under various curing pressure. The ceramic and phenolic resin powder could be homogeneously blended by the dry mixing method. Thus the variation in piezoelectric constant from point to point over the surface of the composite was low(＜10%). These composite were investigated for an influence of microstructure on the dielectric and piezoelectric properties.
There are three kinds pore in 0-3 type ceramic - polymer composite. That is matrix pores, particle pores, and ceramic - polymer interface pores. Among these pores, the effect of particle and interface porosity on the dielectric and piezoeletric constants were investigated. The particle porosity variation of ceramic in composite was changed by sintering temperature and the interface porosity of composite was controlled the curing pressure.
The dependence of the dielectric and piezoelectric constants on interface porosity were discussed especially in terms of porosity factors.
In pure PZT ceramics, the dielectric and piezoelectric strain constant $d_{33}$ were decreased exponentionally as a function of porosity and their porosity factors were nearly equal in 4.64 and 4.89 respectively. Therefore the piezoelectric voltage constant $g_{33}$ was nearly constant, though the porosity of ceramic was changed. However, in PZT - polymer composite, the interface porosity factor of the piezoeletric constant was greater than that of the dielectric constant. It is believed that interface pores play the role of a stress absorber in composites. Therefore, the reduced local stress was transferred to the active prticles. Thus the piezoelectric constant, relative to the dielectric constant, was remarkably diminished by interface pore. On the other hand, when the particle porosity was high, the interface porosity factors of the dielectric and piezoelecric constants were reduced. This means that particle porosity is the dominate factor in determining dielectric and piezoelectric properties.
In pure barium titanate ceramics, the porosity factor of dielectric and piezoelectric constants were nearly equal in 5.69 and 4.97 respectively. However, in $BaTiO_3$ - polymer composite, the interface porosity factor of the piezoelectric constant was greater than that of the dielectric constant, interface porosity factor b in $d_{33}$ was 9.81 and in $\epsilon_r$ 4.57. It means that interface pores in composite play the role of a stress buffer. On the other, the piezoelectric voltage constant $g_{33}$ was independent of the porosity of barium titanate ceramics. But in $BaTiO_3$ - polymer system, the piezoelectric voltage constant $g_{33}$ was decreased with their interface porosity.
Piezoelectric $BaTiO_3$ -polymer composite were investigated for dielectric and piezoeletric properties with the barium titanate active particle size. Under the condition of the same density and ceramic volume ratio of composite, the dielectric and piezoelectric constant of composite are increasing as the ceramic particle size in composite are increasing. The surface layer model was quoted to explain these phenomena in our system and experimentally confirmed.
The connectivity parameter of modified cube model of composite was calculated from the dielectric constant variation as their particle size. The connectivity parameter X and Y were 77.8% and 98.9% respectively. It means that the barium titanate particle distribution in composite nearly approach the parallel mode. It was experimentally confirmed that the surface layer has low dielectric and nonferrolectric properties. Dielectric constant and thickness of surface layer were calculated from the equivalent circuit of composite. The ground particle of the sintered barium titanate has the surface layer of the thickness of 1.59 μm and the relative dielectric constant of 105. So, the dielectric and piezoelectric constant were abruptly decrease in the particle size under the range about 60μm. However, the surface layer thickness may be different as how their particle were prepared. In our system, the dielectric and piezoelectric properties were nearly independent on the composite with ceramic powder over the range about 120 $\mum$.
