Effective moduli and electro-acoustic performance of epoxy-ceramic 1-3 piezocomposites

The inclusion of piezoceramic rods in a passive polymer can greatly enhance the piezoelectric effect - a fact which has been exploited in the manufacturing of sensitive pulse-echo transducers. We analyze this phenomenon from the point of view of the theory of composite materials, focusing on the evaluation of d_h, the hydrostatic piezo-electric coefficient, d_hg_h, the hydrophone figure of merit, and k_h, the hydrostatic coupling factor measuring the efficiency of energy conversion. We show how these quantities can be expressed as algebraic functions of a single microstructural parameter, p. In the limit of large elastic contrast (soft matrix, hard ceramic), this theory gives a first-principles explanation of the decoupling effect of the composite on the hydrostatic piezo-electric coefficient, as well as the role played by the porosity and Poisson's ratio of the matrix phase. Using a Differential Effective Medium (DEM) type scheme, we compute the aforementioned properties for two commercial polymer-piezoceramic composites. It is thus shown that this effective medium approach provides a simple yet self-consistent framework for the design of effective 1-3 piezocomposites.