Abnormal Grain Growth and New Core–Shell Structure in (K,Na)NbO3‐Based Lead‐Free Piezoelectric Ceramics

Abstract

A unique core–shell structure was observed in coarse grains in (K,Na)NbO3 (KNN)‐based lead‐free piezoelectric ceramics. It is morphologically different from the chemical inhomogeneity‐induced core–shell grain structure reported previously in BaTiO3‐based ceramics. The core region is composed of highly parallel nanosized subgrains, whereas the shell region consists of larger‐sized but similar self‐assembled subgrains. The electron‐backscattered diffraction analysis and selected area electron diffraction pattern confirmed that coarse grains with a core–shell structure were single‐crystalline‐like grains. The formation process of such coarse grains was then discussed based on mesocrystal growth along with the classical theory of grain growth. The two studied KNN‐based systems showed a similar grain growth transformation: from self‐assembled aggregation clusters with nanosized subgrains to a typical core–shell grain structure when the sintering temperature was increased only by a range of 10°–20°C. The volatilized alkali metal oxides and liquid phase were supposed to accelerate such grain growth transformation. When abnormal grown grains with a core–shell structure occurred, both systems showed the highest densities and dielectric constants along with the lowest dielectric losses, while their piezoelectric properties tended to decline.