Review of Sintering Technologies, Structural Characteristics, and Piezoelectric Properties of NKN-Based Lead-Free Ceramics

The (Na 1−x K x )NbO 3 -based (NKN-based) lead-free piezoelectric ceramics have been extensively investigated from the beginning of this century to replace the Pb(Zr 1−x Ti x )O 3 -based piezoelectric ceramics. The sintering of the NKN-based ceramics is difficult because they are soluble in water, and Na 2 O evaporation, which degrades the piezoelectric properties of the specimen, occurs during sintering. CuO was used as a sintering aid to improve the sinterability and to reduce the sintering temperature of the NKN ceramics. The NKN-based ceramics with various polymorphic phase boundary (PPB) structures such as orthorhombic–tetragonal PPB, rhombohedral–orthorhombic PPB, and rhombohedral–tetragonal PPB structures were developed to enhance the piezoelectric properties. The specimen with the rhombohedral–tetragonal PPB structure has been reported to show excellent piezoelectric properties. Recently, NKN-based ceramics with various pseudocubic-based PPB structures were reported, and they exhibited enhanced piezoelectric properties with good thermal stability. In this manuscript, the sintering of NKN-based ceramics and the various PPB structures developed for improving their piezoelectric properties are discussed in detail.