Improvement of crystallization and particle size distribution of boric acid in the processing of a boron carbide precursor

Abstract

In the processing of crystalline boron carbide, boron oxide and carbon precursors are used as feedstock into a carbothermal reduction process reactor. The uniformity of the precursor blends has a dramatic effect on the homogeneity of the resultant boron carbide. In boron carbide precursor preparation via a wet process, boric acid is usually dissolved into water at 80 °C and then mixed with carbon and dried at 120 °C, followed by calcination then comminution to obtain the desired feed particle size. However, controlling the crystallization and particle size of boric acid is challenging and there are no prior published studies dealing with this issue. In this study, a simple process demonstrated that adding hexane to the boric acid water solution during the drying can achieve a highly crystallized powder with a narrow particle size distribution. Hexane generates supersaturation during the recrystallization of boric acid by increasing boric acid nucleation. The hexane can inhibit boric acid crystal growth by isolating the formed nuclei thus providing a barrier to molecular diffusion. The water to hexane volumetric ratio, suitable processing parameters, and optimized dispersant amount were investigated. It was found that the process of mixing the optimum solution at room temperature and then drying it at 120 °C produced the relatively uniform 0.3 μm particles. By adding 1.5 wt% dispersant (Triton X-100) to the mixture, 20 nm boric acid particle size can be obtained. The boron carbide powder fabricated by carbothermal process from the modified precursor demonstrated a finer and more uniform particle size with lower free carbon content. The precursor calcination temperature could be lowered from 600 °C to 450 °C.