Intergranular Stress Induced Phase Transition in CaZrO3 Modified KNN-Based Lead-Free Piezoelectrics

The phase transition temperatures of CaZrO3-doped (Na0.49K0.49Li0.02)(Nb0.8Ta0.2)O-3 lead-free piezoelectrics (LKNNT-CZ5) have been studied with X-ray diffraction and Raman scattering. It is found that LKNNT-CZ5 ceramics and powders with the same crystallite size (similar to 2-5m) exhibit distinctly different phase transition behaviors versus temperature. The ceramic bulk changes its orthorhombic phase to tetragonal structure abruptly after being pulverized to powders at room temperature(RT), and follows a phase transition sequence with increasing temperature: orthorhombic (RT-65 degrees C), orthorhombic and tetragonal (65 degrees C-105 degrees C), and tetragonal phase (105 degrees C to similar to 220 degrees C). Contrastively, the powders hold a tetragonal structure without any phase transition within the whole temperature region (RT-220 degrees C). These differences might be induced by the existing internal stress or residual strain, which could be released during the grinding process.