Effect of CuO on the sintering and piezoelectric properties of 0.95(Na _0.5K_0.5)NbO₃-0.05SrTiO₃ lead-free piezoelectric ceramics

CuO was used to enhance the sinterability of 0.95(Na_0.5K _0.5)NbO₃-0.05SrTiO₃ (0.95NKN-0.05ST) ceramics. The poor sinterability of 0.95NKN-0.05ST ceramics was improved by the addition of 1.5 mol% CuO, so that they were well sintered even at 960°C with grain growth. The densification and grain growth were attributed to liquid-phase sintering, probably due to the presence of a liquid phase containing SrO and CuO. The grain size decreased when the sintering temperature exceeded 960°C, due to the presence of a large amount of liquid phase. The microstructure of the 0.95NKN-0.05ST ceramics was considerably affected by even small variations of sintering temperature. The d₃₃, k_p, and Q_m values increased with CuO addition due to the increased density and grain size. The high piezoelectric properties of k_p=0.35, d₃₃=200, and ε₃₃^T/ε_o=1235 were obtained from the 0.95NKN-0.05ST ceramics containing 1.5 mol% CuO sintered at 960°C for 10 h.