Lei Li,Wen Bin Hong,Shuang Yang,Han Yan,Xiang Ming Chen
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.
The NaCl:xEu 3+ (x = 0.002, 0.004, 0.006, 0.008, 0.01, 0.02) ceramics were synthesized using a cold sintering method under varying temperature and pressure conditions. The impact of Eu 3+ ions on the structural and optical properties of NaCl ceramics was examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), analytical scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), UV–visible spectroscopy, reflectance spectroscopy, photoluminescence (PL) spectroscopy, and fluorescence lifetime measurements. XRD analyses confirmed the incorporation of Eu ions into the matrix lattice. SEM results indicated that the ceramics achieved high density when the uniaxial pressure exceeded 300 MPa. EDS analysis demonstrated a well-distributed presence of rare earth ions, devoid of impurities. PL spectroscopy revealed that the intensity of the emission spectra increased the increase of Eu 3+ concentration and observed the optimal luminescence performance at x = 0.01 mol% and a sintering temperature of 125 °C. XPS results indicated that the Eu ions were predominantly in a trivalent state within the NaCl ceramics. Collectively, the above results provide valuable insights for the preparation of rare-earth doped alkali halide ceramics using cold sintering technology.
NaCl ceramics were prepared by room-temperature cold sintering using moistened NaCl powder with 4 wt% water and dry pressing using dehydrated powder. When the applied uniaxial pressure is low, the relative density of dry-pressed NaCl ceramic is significantly lower than that of cold-sintered ceramic, while the former is 98.5%-99.3% and much higher than the latter (94.3%-94.6%) for high applied pressure of 200-300 MPa. The uniaxial pressure-induced plastic deformation dominates the densification of dry-pressed NaCl ceramic, and also plays a role during cold sintering as well as the dissolution-precipitation process. The lower density of cold-sintered NaCl ceramic under high applied pressure is attributed to the trapped water in ceramic body during cold sintering. Besides, the presence of water always promotes the microstructural homogeneity, which is responsible for the much higher Qf value of cold-sintered NaCl ceramic. The optimal microwave dielectric properties with ε r = 5.55, Qf = 49 600 GHz, and τ f = −173 ppm/°C are obtained in cold-sintered NaCl ceramic under the applied pressure of 300 MPa, indicating that it is a promising candidate as a microwave dielectric material.
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