Partially sintered lead-free ceramics from piezoelectric powders prepared via conventional firing and spark plasma sintering (SPS) – – characterization of microstructure and dielectric properties

Abstract

This work deals with dielectric properties of lead-free ceramics from piezoelectric powders and focuses on the preparation and characterization of potassium-sodium niobate (K0.5Na0.5NbO3, KNN) and barium titanate (BaTiO3, BT) ceramics. Ceramic samples with different porosity were prepared from commercial KNN and BT powders by conventional firing in air or spark plasma sintering (SPS) at temperatures 600 - 1000 °C for KNN and 900 - 1300 °C for BT, resulting in partially or fully sintered microstructures. Bulk density, apparent density and open porosity were determined using the Archimedes method and closed and total porosities were calculated on the basis of theoretical densities. For both types of ceramics, the porosity decreases with increasing sintering temperature, and for identical temperatures the porosity of SPS samples is lower than for conventional firing, because the pressure applied during SPS promotes densification. For KNN the influence of SPS on the porosity is much larger than for BT. With increasing SPS temperature KNN exhibits a moderate decrease of the alkali content. The results of dielectric property measurements and their frequency dependence via impedance spectroscopy in the range from 10 or 100 Hz to 1 MHz show that the relative permittivity decreases in all cases with frequency and is usually higher for ceramics prepared via SPS than for conventional firing. This can be explained by the lower porosity and smaller grain size. The absolute values of the relative permittivity at 1 kHz are 134 - 532 (conventional firing) and 148 - 3780 (SPS) for KNN, and 753 - 1801 (conventional firing) and 923 - 10 380 (SPS) for BT ceramics.