Novel Lead-Free 2–2-Type Composites with High Piezoelectric Sensitivity and Strong Hydrostatic Response: Examples of 2–1–2 Connectivity

A modification of the well-known laminar composite structure with 2–2 connectivity leads to changes of the effective physical properties and their dependences on the composite content and microgeometric characteristics. The present chapter reports new results on novel three-component 2–1–2 composites with a high piezoelectric sensitivity and large hydrostatic parameters. In the 2–1–2 composite, there are parallel-connected layers of two types. The first type represents a ferroelectric domain-engineered [0 0 1]-poled lead-free single crystal based on solid solutions of alkali niobates-tantalates with the perovskite-type structure. In the second type of the layers, ferroelectric ceramic rods in the form of an elliptic cylinder are regularly aligned in a large polymer matrix, and the ceramic component is chosen among the perovskite-type compositions being either leaf-free or lead-containing. The hydrostatic piezoelectric coefficients d_h^* and g_h^*, related figure of merit d_h^*g_h^*, and their non-monotonic behaviour are studied by taking an orientation effect into account. This orientation effect is caused by rotations of the ceramic rod bases in the polymer medium. The aforementioned hydrostatic parameters are compared to those of the conventional two-component ceramic-polymer composites, and some advantages of the studied lead-free 2–1–2 composites are discussed. These novel composites are suitable for hydroacoustic applications.