Effects of particle size and O-contamination of LiF on the phase purity and microwave dielectric properties of BaLiF3 ceramic

Abstract

Recently, fluoride ceramics have gained much interest because of their low permittivities and high Q×f values at microwave frequencies, coupled with relatively low sintering temperatures. In this paper, we have explored the effects of the particle-size and O contamination of LiF raw material, sintering/annealing atmosphere on the mechanosynthesis, structural stability, and microwave dielectric properties of BaLiF3 ceramic. Using a fine LiF raw material would facilitate the mechanical synthesis of BaLiF3, being densified at lower temperatures than the coarse one. A high concentration of oxygen contamination in LiF raw material and partial pressure of O2 in the sintering atmosphere would lead to the formation of BaF2 due to the instability of the fluoride perovskite with an increase in the concentration of F-vacancies caused by the partial oxidization during the sintering at high temperature (850 °C/2h). The oxidization of fluoride perovskite preferably occurs at the surface and grain boundaries via O diffusion, which reduces the Q×f value from 83175 to 70228 GHz with little change in εr (∼11) and τf values (∼-71ppm/oC).