Effects of water content during cold sintering process of NaCl ceramics

Abstract NaCl ceramics were prepared by cold sintering at room temperature with the water content varying from 0 to 10 wt%. The relative density of the NaCl ceramics first decreases rapidly from 99.3% to 95.0% with increasing the water content from 0 to 3 wt%, and it decreases slowly to 93.4% with further increasing the water content up to 10 wt%. The decrease in relative density is mainly attributed to the volume increase caused by the evaporation of the residual water in NaCl ceramic body. The dielectric constant exhibits the same tendency as the relative density, while the Qf value increases from 27,600 GHz to 49,600 GHz with increasing the water content from 0 to 4 wt%, and it does not change much for higher water content. The improved Qf value is attributed to the improved microstructure homogeneity that benefits from the fast mass transport of the water-activated dissolution-precipitation mechanism. The results clearly show that the water content should be optimized but not determined arbitrarily or empirically to achieve the good combination of high density, microstructure homogeneity and good properties. To avoid the negative effect of the residual water, NaCl ceramics were also cold-sintered at 120 °C, in which both high relative density (98.8–99.6%) and high Qf value (48,100–56,000 GHz) were obtained.