Abnormal grain growth and new core-shell structure in (K,Na)NbO3-based lead-free piezoelectric ceramics

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 degrees-20 degrees 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.