Among the techniques that allow obtaining a sintered bulk ceramic material with perovskite structure, the cold sintering process (CSP) attracts attention because it allows obtaining a dense (more than 90%) sample at a sintering temperature of about 300 ^oC. This method requires uniaxial pressure (it can be up to 700 MPa), an additional substance that serves as a transient liquid phase (TLP) during the sintering process, and a temperature up to 300 ^oC when this temperature value is required [1]. For example, some materials have also been cold sintered at room temperature with a dense microstructure, and usually such materials are water soluble and water serves as TLP. The main purpose of TLP is to promote the dissolution-precipitation process between grains during the cold sintering process. The additional influence of uniaxial pressure, which promotes not only mass diffusion between grains but also rearrangement of particles at the beginning of the cold sintering process, enables the production of perovskite material with dense microstructure. The main problem in cold sintering of perovskite material is to find a suitable substance that can serve as TLP.

In our research, we cold sintered sodium potassium niobate ((K_0.5Na_0.5)NbO₃) with different particle size distribution. The starting powder was prepared by solid state synthesis with 0.5% Sr (KNNSr) and no dopants (KNN). For cold sintering, a mixture of KOH and NaOH in a 1:1 molar ratio was dissolved in water and 0.075 ml of this solution was added per 0.6 g of KNN powder. A uniaxial pressure of 650 MPa and a temperature of 300 °C were applied for 2-6 hours. As a result, bulk samples with density up to 97% was obtained. We investigated the influence of the initial powder particle size, the duration of CSP and the uniaxial pressure on the final functional properties such as the specific electrical conductivity, dielectric constant and dielectric losses, polarization and displacement at different electric fields. We have also shown why cold-sintered samples should be annealed in oxygen at 450 °C after CSP and why this is necessary to avoid any interaction with moisture. Our results show that cold-sintered KNN has completely different functional properties and the microstructure does not contain large grains (the size of the grains of cold-sintered KNN has the same range as the size of the initial powder), which is typical for sintered KNN materials.

References
1. H. Guo et al., “Cold Sintering Process: A Novel Technique for Low-Temperature Ceramic Processing of Ferroelectrics,” J. Am. Ceram. Soc., 99, 11, 3489–3507, 2016.