Spark plasma sintering of Li/Ta-modified (K,Na)NbO3 lead-free piezoelectric ceramics: Post-annealing temperature effect on phase structure, electrical properties and grain growth behavior

Highly dense Li/Ta co-doped (K,Na)NbO3 lead-free ceramics were prepared by spark plasma sintering (SPS) in a relatively wide temperature window. The influences of post-annealing temperature on phase structure, electrical properties and grain growth were investigated. Through tailoring post-annealing temperature, relative dielectric constant ɛr, and piezoelectric coefficient d33 of SPSed KNN-based samples can be enhanced to 960 and 225 pC/N for the same composition compared with normally sintered values (771 and 206 pC/N). The enhanced electrical properties were discussed from the point view of microstructure (grain size and its distribution, porosity) and phase transition, which occurred when post-annealing temperature exceeded 1050 °C. Also, with increasing post-annealing temperature, three grain growth behavior stages were investigated on the balance between the volatilization of alkali metal oxides and secondary grain growth.