Effects of Li+ and/or F− modification methods on the sintering behaviors, microstructures of Mg2SiO4 ceramics

In this paper, we prepared four different Mg₂SiO₄ samples modified by LiF-addition, LiF-doping, Li₂CO₃-addition, and purely F⁻doping, respectively to comparatively investigate the effects of the Li⁺ and/or F⁻ modification methods on the sintering behaviors, microstructures, and microwave dielectric properties of the Mg₂SiO₄ ceramics. It was found that the sintering behaviors, reactions involved in the process, and corresponding microstructures of the modified Mg₂SiO₄ ceramics are sensitive to the Li⁺ and/or F⁻ modification methods. The 2 wt% LiF-doped, F-doped, Li₂CO₃-added, and LiF-added samples could be densified at 1575°C/2 h, 1525°C/2 h, 1400°C/2 h, and 900°C/2 h, respectively. The liquid phase sintering justified the enhancement of sinterability by the LiF or Li₂CO₃-addition. The lower sintering temperature of the LiF-added sample than that of the Li₂CO₃-added one could be understood by considering the lower melting point (∼800°C) and viscosity of the LiF–SiO₂–SiF₄ ternary eutectic liquid phase than that of Li₂SiO₃–SiO₂ binary liquid phase (∼1350°C). The easy evaporation of SiF₄ for the LiF-added Mg₂SiO₄ results in a high porosity of ∼5% after sintering at 900°C. All modified samples demonstrate similar optimized relative dielectric permittivities (ε_r) of ∼7 and Q × f values of ∼83000 GHz despite slight differences in the porosities and phase assemblages.