New insight on two-step sintering process of lead-free (K, Na)NbO3-based piezoelectric ceramics

The 0.96 K0.5Na0.5NbO3-0.04(Ba0.5Na0.5)ZrO3 and 0.98 K0.5Na0.5NbO3-0.02(Ba0.5Na0.5)TiO3 ceramics were fabricated via two-step sintering. Their phase structures were identified by an X-ray diffraction method. Through the Rietveld refinement on the X-ray diffraction data, it was found that the phase structures of both the ceramics did not experience a significant evolution with the change of sintering conditions. Due to the coexistence of fast low temperature type and slow high temperature type grain boundaries, both the ceramics exhibited some microstructural changes that violated the characteristics of thermal activation. A liquid phase was noted to appear in the (Ba0.5Na0.5)TiO3-modified ceramic during sintering, which made it show some different sintering behaviors from the (Ba0.5Na0.5)ZrO3-modified ceramic. Additionally, an investigation was performed on the influence of preparation conditions on the electrical properties, and it was found that only the electrical properties of the (Ba0.5Na0.5)TiO3-modified ceramic could be improved by two-step sintering, while those of the (Ba0.5Na0.5)ZrO3-modified ceramic could not. Furthermore, the d33 of KNN-BNT ceramic would be reduced by less than 10 % compared to its highest value over a wide sintering temperature range of 1050-1110 degrees C, showing better sintering temperature stability than KNN-BNZ ceramic. This work can provide a good reference for the application of two-step sintering in (K, Na)NbO3-based ceramics.