Stable resonance characteristics in CuO-modified lead-free 0.94(K0.5Na0.5)NbO3-0.06LiNbO3 ceramics sintered at optimal temperature

Lead-free piezoelectric ceramics with the nominal composition of [(K_0.5Na_0.5)_0.94Li_0.06]NbO₃+xmol%CuO [KNLN+ xmol%CuO] had been synthesized by conventional solid-state sintering. Effects of the addition of CuO on the phase structure, micro-morphology evolution and electric properties of the ceramics were investigated. The ceramics synthesized at 1000-1060 degC showed a phase transition from orthorhombic to tetragonal symmetry, which is analogous to the morphtropic phase boundary (MPB). Because of this polymorphic phase transition, high piezoelectric coefficient d₃₃~206pC/N and electromechanical coupling factor k_p~31% were obtained in the pure KNLN ceramic sintered at 1060degC, however, the mechanical quality factor Q_m, a key parameter for frequency devices, was relatively low. In order to improve the resonance characteristics, CuO had been doped in KNLN ceramics. Our results showed that the doping of CuO was effective in promoting the densification of the ceramics and hardening of piezoelectric properties.