Compositional Design of Dielectric, Ferroelectric and Piezoelectric Properties of (K, Na)NbO₃ and (Ba, Na)(Ti, Nb)O₃ Based Ceramics Prepared by Different Sintering Routes

Lead free piezoelectric materials are being intensively investigated in order to substitute lead based ones, commonly used in many different applications. Among the most promising lead-free materials are those with modified NaNbO₃, such as (K, Na)NbO₃ (KNN) and (Ba, Na)(Ti, Nb)O₃ (BTNN) families. From a ceramic processing point of view, high density single phase KNN and BTNN ceramics are very difficult to sinter due to the volatility of the alkaline elements, the narrow sintering temperature range and the anomalous grain growth. In this work, Spark Plasma Sintering (SPS) and high-energy ball milling (HEBM), following heat treatments (calcining and sintering), in oxidative (O₂) atmosphere have been used to prepare single phase highly densified KNN ("pure" and Cu²⁺ or Li¹⁺ doped), with theoretical densities ρ_th > 97% and BTNN ceramics (ρ_th - 90%), respectively. Using BTTN ceramics with a P4mm perovskite-like structure, we showed that by increasing the NaNbO₃ content, the ferroelectric properties change from having a relaxor effect to an almost "normal" ferroelectric character, while the tetragonality and grain size increase and the shear piezoelectric coefficients (k₁₅, g₁₅ and d₁₅) improve. For KNN ceramics, the results reveal that the values for remanent polarization as well as for most of the coercive field are quite similar among all compositions. These facts evidenced that Cu²⁺ may be incorporated into the A and/or B sites of the perovskite structure, having both hardening and softening effects.