Oxygen-enriched sintering for improved piezoelectric performance of (K0.5Na0.5)(Ta0.3Nb0.7)O3 lead-free ceramics: The impact of defects

In this work, (K0.5Na0.5)(Ta0.3Nb0.7)O3 (KNNT) powders synthesized by the microwave-hydrothermal method were sintered under different oxygen partial pressure (PO2 = 0, 0.498, 0.995 atm) atmospheres. Effects of PO2 on the phase composition, microstructure, piezoelectric properties, and defects in the KNNT ceramics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS). Results indicated that PO2 had almost no effect on the phase structure and grain morphology of KNNT ceramics. Abnormal grain growth was observed with increasing PO2; simultaneously, the density of the ceramics decreased. However, the d33 value reached its maximum of 205 pC/N when sintered at the highest PO2, and kp showed a similar trend. TEM observations indicated that the microscopic domains had the most regular domain orientation at a PO2 of 0.998 atm. These phenomena were due to the different oxygen vacancy concentrations caused by varying PO2. UV-Vis DRS revealed that the defect concentration decreased with increasing PO2; moreover, the oxygen vacancy concentrations were determined by XPS to be 23.5%, 21.1%, and 19.1% corresponding to PO2 values of 0, 0.498, and 0.995 atm, respectively, indicating that oxygen-enriched sintering can reduce defects and enhance piezoelectric properties.