Cold Sintering of Na₂WO₄ Ceramics using a Na₂WO₄-2H₂O Chemistry

Crystal water is a kind of water molecules that are incorporated into the crystal structure of hydrated salts, which are generally decomposed at very low temperatures (usually 100∼200 °C). Na₂WO₄ is a compound that could react with H₂O forming the Na₂WO₄·2H₂O hydrated crystal salt under certain conditions. In this work, the cold sintering of Na₂WO₄ ceramics is studied using a Na₂WO₄-2H₂O chemistry at a low sintering temperature of 120 °C. Their sintering mechanism has been investigated using SEM, TEM, DSC, FTIR, together with the shrinkage rates of the samples. During the sintering process, the intentionally added liquid water reacts with Na₂WO₄ powders to produce a Na₂WO₄·2H₂O and Na₂WO₄ composite, and the crystal water can be decomposed from the Na₂WO₄·2H₂O phase under a certain sintering condition. The water in both liquid and solid forms contributes to the densification of Na₂WO₄ ceramics. Finally, dense Na₂WO₄ ceramics with excellent properties (permittivity: 5.7; Q × f: 70 000 GHz; TCF: −70 ppm/°C) have been obtained by removing the residual water after a heat treatment. Such sintering process can be generalized to the densification of other inorganic materials that easily react with liquid water to form hydrated salts, which provides a strategy for the cold sintering with crystal water.