Structure evolution and piezoelectric properties of K0.40Na0.60Nb0.95Sb0.05O3–Bi0.5K0.5HfO3–SrZrO3 ternary lead-free ceramics with R–O–T phase boundary

A conventional ceramic preparation method was used to fabricate the newly designed ternary (K,Na)NbO3-based lead-free ceramics, which have the general formula of (0.97-x)K0.40Na0.60Nb0.95Sb0.05O3–0.03Bi0.5K0.5HfO3–xSrZrO3. An X-ray diffraction analysis, together with dielectric-temperature measurements, was performed for determining their phase structures. The crystal structure was found to transition from a rhombohedral-orthogonal phase coexistence to a three-phase coexistence of rhombohedral-orthogonal-tetragonal phases with the increase in SrZrO3 concentration, and then to a rhombohedral-tetragonal phase coexistence, and finally to a single rhombohedral phase. Near the three-phase coexistence zone, the ceramics exhibit an obviously enhanced electrical activity, with an electromechanical coupling coefficient kp = 0.364 and a piezoelectric constant d33 = 280 pC/N. Through a microstructure analysis, a slightly excessive amount of SrZrO3 was found to result in an inhibition for the grain growth of the studied ceramics. These results show that incorporating SrZrO3 into ternary (K,Na)NbO3-based ceramics is still an effective means of constructing phase boundaries and improving the electromechanical properties.