Microstructure control and enhancement of piezoelectric properties via solid-state conversion of high quality single crystals in the $(Na_{1/2}Bi_{1/2})TiO_3-BaTiO_3-(K_{1/2}Na_{1/2})NbO_3$ System

Abstract

Piezoelectric materials, which allow interconversion between electrical and mechanical energy, are widely used for various electronic devices. Lead-containing piezoelectric materials still dominate the commercial market owing to their excellent piezoelectric performance. However, the toxicity of lead with respect to the environment and human health has led to a demand for alternative eco-friendly materials. Therefore, several lead-free systems have been studied in depth as potential lead-free piezoelectric candidate materials, in particular, the $(Na_{1/2}Bi_{1/2})TiO_3-BaTiO_3-(K_{1/2}Na_{1/2})NbO_3$ (NBT-BT-KNN) system has excellent inverse piezoelectric properties. Piezoelectric properties are sensitively affected by their crystallographic orientation and therefore these parameters can be maximized by producing of single crystals with a particular orientation. Conventionally, single crystals have been fabricated from a liquid or a vapor. However, these melting methods would be inappropriate for fabricating lead-free single crystals because it is difficult to guarantee chemical homogeneity of single crystals produced in this manner. In addition, most of the lead-free piezoelectric materials contain volatile elements such as K, Bi, Na, etc. and have a complex chemical composition, and thus the fabrication of single crystals is more difficult. These problems can be overcome by the solid-state single crystal growth (SSCG) method developed by our group. Because single crystals grow via solid-state conversion of polycrystals into single crystals at low temperature, the chemical homogeneity and volatile elements at the A-site of the perovskite can be maintained effectively. The solid-state conversion of single crystals from polycrystals is based on a microstructural evolution principle, and therefore it is necessary to understand the principle first. In this thesis, we produced lead-free NBT-BT-KNN single crystals based on the principle of microstructural evolution. The produced single crystals exhibited excellent piezoelectric properties.