Piezoelectric properties of Li- and Ta-modified (K0.5Na0.5)NbO3 ceramics

Lead-free, potassium sodium niobate piezoelectric ceramics substituted with lithium (K_0.5-x/2,Na_0.5-x/2,Li_x)Nb_O3 or lithium and tantalum (K_0.5-x/2,Na_0.5-x/2,Li_x)(Nb_1-y,Ta_y)O₃ have been synthesized by traditional solid state sintering. The compositions chosen are among those recently reported to show high piezoelectric properties [Y. Saito, H. Takao, T. Tani, T. Nonoyama, K. Takatori, T. Homma, T. Nagaya, and M. Nakamura, Nature (London) 42, 84 (2004); Y. Guo, K. Kakimoto, and H. Ohsato, Appl. Phys. Lett. 85, 4121 (2004); Mater. Lett. 59, 241 (2005)]. We show that high densities and piezoelectric properties can be obtained for all compositions by pressureless sintering in air, without cold isostatic pressing, and without any sintering aid or special powder treatment. Resonance and converse piezoelectric (strain-field) measurements show a thickness coupling coefficient k_t of 53% and converse piezoelectric coefficient d₃₃ around 200pm/V for the Li-substituted ceramics, and a k_t of 52% and d₃₃ over 300pm/V for the Li- and Ta-modified samples. The unipolar strain-field hysteresis is small and comparable to that measured under similar conditions in hard Pb(Zr ,Ti)O₃. A peak of piezoelectric properties can be noted close to the morphotropic phase boundary. These ceramics look very promising as possible, practicable, lead-free replacements for lead zirconate titanate.