The presence of the Sb5+ ion increased the piezoelectric properties of the (Na0.5K0.5)NbO3 (NKN) ceramics by enhancing the poling state as identified by the increased phase angle (θ). The increased θ value was explained by the increased ferroelectricity due to the large electronegativity of the Sb5+ ion and the increased off centering of the cation due to the small ionic size of the Sb5+ ion. The (Na0.5K0.5)(Nb0.97Sb0.03)O3[NK(N0.97S0.03)] ceramics containing CuO were well sintered even at 960 °C. A small amount of the Cu2+ ions was incorporated into the matrix of the NK(N0.97S0.03) ceramic and behaved as a hardener. The kp, Qm, d33, and ε3T/ε0 values of the CuO-added NK(N0.97S0.03) ceramics were much higher than those of the CuO-added NKN ceramics due to the Sb5+ ion’s enhancement of the piezoelectric properties by increasing the poling state of the specimens. The high piezoelectric properties of kp=0.41, Qm=1333, d33=111 pC/N, and ε3T/ε0=324 were obtained from the 2.0 mol % CuO-added NK(N0.97S0.03) ceramics sintered at 960 °C for 8 h.

When a small amount of CuO was added to (Na 0.5 K 0.5 )NbO 3 (NKN) ceramics sintered at 9601C for 2 h, a dense microstructure with increased grains was developed, probably due to liquidphase sintering. The Curie temperature slightly increased when CuO exceeded 1.5 mol%. The Cu 21 ion was considered to have replaced the Nb 51 ion and acted as a hardener, which increased the E c and Q m values of the NKN ceramics. High piezoelectric properties of k p 5 0.37, Q m 5 844, and e 3 T /e 0 5 229 were obtained from the specimen containing 1.5 mol% of CuO sintered at 9601C for 2 h.

CuO was used to enhance the sinterability of 0.95(Na 0.5 K 0.5 ) NbO 3 -0.05SrTiO 3 (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 9601C 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 9601C, 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 d 33 , 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 5 0.35, d 33 5 200, and e 33 T /e o 5 1235 were obtained from the 0.95NKN-0.05ST ceramics containing 1.5 mol% CuO sintered at 9601C for 10 h.

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