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

The (Na1-xKx)NbO3-based (NKN-based) lead-free piezoelectric ceramics have been extensively investigated from the beginning of this century to replace the Pb(Zr1-xTix)O-3-based piezoelectric ceramics. The sintering of the NKN-based ceramics is difficult because they are soluble in water, and Na2O 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.