High Piezoelectric Performance in Undoped (K,Na)NbO₃ Piezoceramics over a Wide Range of Compositions

In a previous study, an optimized low-temperature (LT) sintering process for the preparation of high-performance undoped (K,Na)NbO₃ (KNN) ceramics with high density, high reproducibility, and high chemical stability was established for the K = 50% composition. In the current work, this optimized process is applied to other stoichiometries ranging from K = 20% to 90%, aiming at gaining more insights into the stoichiometry dependence of piezoelectric properties. Alike the case of K = 50%, fast melt-quenching and preannealing of calcined raw materials lead to high-crystallinity single-phase powders without parasitic phases regardless of composition. Grain growth upon recrystallization after pulverization can be seen to depend on the composition and recrystallization annealing temperature, which is also reflected in the microstructure of ceramics showing smaller grain sizes and piezodomains in high K-rich stoichiometries. After LT spark-plasma sintering of powders, high-density ceramics with high and stable properties [d₃₃ ∼ (140 to 150) pC/N; k_p and k_t ∼ (40 to 45)%] are obtained over a wide range of middle stoichiometries. Such piezoelectric results contrast with the general assumption of higher piezoelectric performance around K = 50%, where two or more phases (orthorhombic and monoclinic) are supposed to coexist, like in the case of standard Pb(Zr,Ti)O₃ ceramics. Here, it is demonstrated that the best properties are found within the orthorhombic KNN phase for K ≥ 40%. Therefore, this work demonstrates that at present, the main factor for the achievement of high-performance undoped KNN ceramics is not the stoichiometry, but rather the preparation process.