Effect of Li2O and CuO additives on the structural and piezoelectric properties of (Na0.5K0.5)(Nb0.96Sb0.04)O3 lead-free piezoelectric ceramics

[(Na0.5K0.5)1−xLix](Nb0.96Sb0.04)O3 [NKLNS100x] ceramics with 0.0 ≤ x ≤ 0.1 were sintered at 1040 °C for 2 h and the specimens with x ≥ 0.05 had a dense microstructure due to liquid phase sintering. The specimen with x = 0.05 had both orthorhombic and tetragonal phases, and showed good piezoelectric properties of kp = 0.43, d33 = 243 pC/N and ɛ33T/ɛ0 = 990. The [(Na0.5K0.5)0.95Li0.05](Nb0.96Sb0.04)O3 [NKLNS5] ceramics sintered at 960 °C also showed a high density by the addition of CuO, due to the production of the CuO and Na2O-related liquid phase. The Cu2+ ions were considered to replace Nb5+ ions and act as a hardener, which increased the Qm value of the NKLNS5 ceramics. High piezoelectric properties of kp = 0.37, Qm = 560, ɛ33T/ɛ0 = 600 and d33 = 170 pC/N were obtained from the specimen containing 1.5 mol% of CuO sintered at 960 °C for 2 h.