Effects of addition of seed grains on morphology and yield of boron carbide powder synthesized by carbothermal reduction

B4C powders were synthesized by carbothermal reduction using mixtures of B2O3, carbon black, and B4C seed grains, and the effects of addition of B4C seed grains on the morphologies and yields of the synthesized B4C powders were investigated. The B4C particles added to the starting mixture of B2O3 and C acted as seeds for crystal growth, resulting in an improvement in the B4C yield. The B4C yields of the products obtained by heat-treatment at 1450 °C increased with increasing amounts of B4C seed grains. In the case of heat-treatment at 1550 and 1750 °C, the B4C yields of the products increased with the amount of seed grains up to a certain B4C-seed/B2O3 ratio, and then decreased slightly above that ratio. The B4C yields improved significantly compared with those obtained under the same conditions without seeds in the case of heat-treatment at 1450 °C (B4C-seed/B2O3 ratio >> 0.05) and at 1550 °C. Truncated octahedral B4C particles could be synthesized selectively by addition of small B4C seeds and heat-treatment at 1450–1750 °C. The particle size of the synthesized B4C decreased with increasing amounts of seed grains. In the case of addition of smaller amounts of seeds, the B4C particles synthesized at 1450 and 1550 °C were polyhedral, and the morphologies of the B4C particles synthesized at 1750 °C were dendrite-like and polyhedral. In the case of addition of larger amounts of seeds, the morphologies of the B4C particles synthesized at 1450–1750 °C were mainly truncated octahedral.