Enhancement of piezoelectric properties in KNN-based lead-free ceramics through controlled NaNbO₃ seed addition and phase structure engineering

This study investigates the effects of introducing 0–5 mol% NaNbO₃ (NN) seeds on the structural, microstructural, dielectric, ferroelectric, and piezoelectric properties of KNN-based lead-free piezoelectric ceramics. All samples with the final composition 0.96[0.95(K_0.52Na_0.48NbO₃) − 0.05LiSbO₃] − 0.04SrZrO₃–CuO (KNNLS–SZ–C) were sintered at 1060 °C for 6 h. X-ray diffraction analysis revealed a perovskite single phase for 0–5 mol% NN seed contents, with a multiphase coexistence of tetragonal, orthorhombic, and rhombohedral structures. As seed content increased from 0 to 3 mol%, the rhombohedral fraction increased while tetragonal and orthorhombic fractions decreased. SEM micrographs showed abnormal grain growth at 1–2 mol% seeds, transitioning to normal grain growth beyond 3 mol%. Optimal piezoelectric and electromechanical properties including d₃₃ = 323 pC/N, k_p = 0.39 were obtained at 3 mol% NN seed, attributed to the favorable multiphase structure fraction and moderate grain size. This work elucidates the interplay between NN seed addition, phase fraction distribution, and microstructural development in tuning the piezoelectric performance of these lead-free ceramics.