Piezoelectric properties and hardening behavior of K_5.4Cu_1.3Ta₁₀O₂₉-doped K_0.5Na_0.5Nb O₃ceramics

Lead-free piezoelectric ceramics K0.5 Na0.5 Nb O3 +x mol % K5.4 Cu1.3 Ta10 O29 have been prepared by a conventional ceramic fabrication technique. All the ceramics possess a perovskite structure with orthorhombic symmetry. Our results reveal that the addition of K5.4 Cu1.3 Ta10 O29 is effective in improving the densification of the ceramics. Besides, after the addition of K5.4 Cu1.3 Ta10 O29, the Curie temperature and the tetragonal-orthorhombic phase transition temperature decrease and the P-E loop becomes constricted, in particular, for the ceramic with x=0.75. Based on the symmetry-conforming principle of point defects, it is suggested that defect dipoles are formed by the acceptor dopant ions Cu2+ and O2- vacancies along the polarization direction. As a result of the low migration rate of defects, the defect dipoles remain in the original orientation during the P-E loop measurement and thus provide restoring forces to reverse the switched polarizations. Similarly, the defect dipoles do not response along with the polarization in the normal piezoelectric activities and thus provide "pinning" to the deformed polarization, making the ceramics become "hardened." For the ceramic with x=0.75, the mechanical quality factor Qm becomes maximum at a value of 1530, while the other piezoelectric properties remain reasonably high: piezoelectric coefficient d33 =90 pCN, planar and thickness mode electromechanical coupling factors kP =41 and kt =46%.