Piezoelectric Sensitivity and Anisotropy in 1–3-Type Composites Based on Lead-Free Ferroelectrics

Abstract

The piezoelectric effect in novel 1–3-type composites with lead-free components is studied in terms of the effective piezoelectric coefficients d_3j^* and g_3j^*, electromechanical coupling factors k_3j^* and anisotropy factors d₃₃^* / d₃₁^* and k₃₃^* / k₃₁^*. The piezoelectric component is either a domain-engineered [001]-poled single crystal or a poled ceramic, and both the single crystal and ceramic are based on ferroelectric alkali-niobate—alkali-tantalate solid solutions. In the studied composites this component represents a system of long rods that are oriented parallel to the poling axis OX₃, and the rods are surrounded by a large piezo-passive polymer matrix that can be either monolithic (in 1–3 composites) or porous (in 1–3–0 composites). The high level of piezoelectric sensitivity of the lead-free 1–3 and 1–3–0 composites is noted due to the longitudinal piezoelectric coefficient g₃₃^* ~ 10²–10³ mV^.m / N. Conditions for a large anisotropy $d_{33}^{*}$ / | $d_{31}^{*}$ | = g₃₃^* / | g₃₁^* | ≥ 5 and k₃₃^* / | k₃₁^* | ≥ 5 are valid in specific volume-fraction ranges of the ferroelectric component and porosity in the polymer component. The important role of the pore shape in achieving the large anisotropy of d_3j^* and k_3j^* of the 1–3–0 composite is discussed. The studied 1–3-type composites are of interest for the selection of advanced piezoelectric materials for transducer, sensor and related applications, with additional respect to the environment due to the lead-free nature of the composite components.