Understanding the morphological variation in the formation of B4C via carbothermal reduction reaction

Despite the relatively successful effort for the synthesis of fine boron carbide (B4C) powders, very little is known about the underlying interrelationships between processing, compositions and the resulting product morphology for B4C formation via the carbothermal reduction reaction (CTR) process. In this paper, B4C, including uniform submicron powders, were synthesized using CTR of finely mixed boron trioxide and carbon obtained from low-cost water-soluble precursors. To understand the aforementioned interrelationships and to better control product morphology, the effects of factors such as CTR thermal profile, CTR atmosphere and precursors on product phase purity and morphology were systematically studied. Among all the factors, it was found that CTR temperature, along with total reaction time, plays a dominant role in determining the morphology of B4C products. Such observation was understood by analysis of CTR reaction kinetics and comparing it with microstructural evolution: significant non-uniformity in B4C product morphology arises from the competition between nucleation and growth at low to intermediate CTR temperatures (e.g., ~1150–1450 °C) while highly uniform submicron or nano B4C products result from very fast nucleation at high CTR temperatures (e.g., ≥~1750 °C) within a few minutes or even seconds.