Recent Developments in (K, Na)NbO₃-Based Lead-Free Piezoceramics

(K_0.5Na_0.5)NbO₃ (KNN)-based ceramics have been extensively investigated as replacements for Pb(Zr, Ti)O₃-based ceramics. KNN-based ceramics exhibit an orthorhombic structure at room temperature and a rhombohedral–orthorhombic (R–O) phase transition temperature (T_R–O), orthorhombic–tetragonal (O–T) phase transition temperature (T_O–T), and Curie temperature of −110, 190, and 420 °C, respectively. Forming KNN-based ceramics with a multistructure that can assist in domain rotation is one technique for enhancing their piezoelectric properties. This review investigates and introduces KNN-based ceramics with various multistructures. A reactive-templated grain growth method that aligns the grains of piezoceramics in a specific orientation is another approach for improving the piezoelectric properties of KNN-modified ceramics. The piezoelectric properties of the [001]-textured KNN-based ceramics are improved because their microstructures are similar to those of the [001]-oriented single crystals. The improvement in the piezoelectric properties after [001] texturing is largely influenced by the crystal structure of the textured ceramics. In this review, [001]-textured KNN-based ceramics with different crystal structures are investigated and systematically summarized.