New Approach to the Synthesis of Nanocrystalline Boron Carbide

The use of nanoparticles in ceramic matrix composites provides lower sintering temperatures and higher densities at a given temperature than common coarse-grained materials. Nanocrystalline B₄C was synthesized by an inexpensive carbothermal reduction method using carbon black and B₂O₃ as precursor. Full conversion was achieved at 1623 K for annealing times of 480 minutes or with a large excess of B₂O₃ and oxidation of the remaining carbon after 30 minutes of annealing. The average particle size of the synthesized B₄C powder was 260 nm, which was reduced to 70 nm after separation of the small particle fraction from the larger particles by sedimentation. A mixture of the as-prepared powder and commercial coarse-grained B₄C yielded an increase of the density of low temperature hot pressed samples by 25% in comparison to pure commercial B₄C. Possible chemical reactions and mechanisms in the synthesis of B₄C were examined with the Gibbs free energies of reactions. The most likely reaction was the reduction of B₂O₃ vapor at the surfaces of the carbon particles after its vapor transport from the liquid B₂O₃. An observed reduction of B₄C yield above 1623 K was probably caused by loss of B₂O₃ vapor from the reaction mixture.