Normal sintering of Li-doped and Li/Ta-codoped potassium sodium niobate (KNbO 3 -NaNbO 3 , KNN)-based ceramics was investigated to clarify the optimal sintering condition for densification, microstructure, and electrical properties. It was found that density increased greatly within a narrow temperature range but tended to decrease when the sintering temperature slightly exceeded the optimal one, accompanied by the appearance of abnormal grain growth, which was considered to be due to the intensified volatilization of alkali metal oxides. Piezoelectric and dielectric properties also showed a similar relationship between the density and sintering temperature, but the highest piezoelectric strain coefficients were obtained at the temperatures lower than that for the highest density, because both densification and composition affect the electrical properties. The highest d 33 value of 206 pC/N was obtained for the Liand Ta-codoped KNN ceramics prepared at 10901C.

A unique core-shell structure was observed in coarse grains in (K,Na)NbO 3 (KNN)-based lead-free piezoelectric ceramics. It is morphologically different from the chemical inhomogeneityinduced core-shell grain structure reported previously in BaTiO 3 -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 101-201C. 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.

Microstructure characteristics, phase transition, and electrical properties of (Na0.535K0.485)0.926Li0.074(Nb0.942Ta0.058)O3 (NKN‐LT) lead‐free piezoelectric ceramics prepared by normal sintering are investigated with an emphasis on the influence of sintering temperature. Some abnormal coarse grains of 20–30 μm in diameter are formed in a matrix consisting of about 2 μm fine grains when the sintering temperature was relatively low (980°C). However, only normally grown grains were observed when the sintering temperature was increased to 1020°C. On the other hand, orthorhombic and tetragonal phases coexisted in the ceramics sintered at 980°–1000°C, whereas the tetragonal phase becomes dominant when sintered above 1020°C. For the ceramics sintered at 1000°C, the piezoelectric constant d33 is enhanced to 276 pC/N, which is a high value for the Li‐ and Ta‐modified (Na,K)NbO3 ceramics system. The other piezoelectric and ferroelectric properties are as follows: planar electromechanical coupling factor kp=46.2%, thickness electromechanical coupling factor kt=36%, mechanical quality factor Qm=18, remnant polarization Pr=21.1 μC/cm2, and coercive field Ec=1.85 kV/mm.

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