Sintering of powders of polyaromatic mesophase to high-strength isotropic carbon-IV powders based on boron-substituted mesophase

Abstract

Boron-substituted polyaromatic mesophases were synthesized by co-pyrolysis of a coal tar pitch with pyridine–borane using boron contents of 0.5 and 1%. After removal of residual isotropic material by extraction with tetrahydrofuran, the mesophases were milled to a particle size of d≤10 μm and compacted to bodies. These bodies were sintered at 1300°C and subsequently high-temperature treated at 1600, 1800, and 2000°C. The treated bodies were investigated to determine boron content, bulk density, crystallite parameters, and mechanical properties. Boron catalyzes graphitization already above 1300°C; an influence of boron concentration was detected only above 1800°C. Carbons produced with a low boron content exhibit a flexural strength of 135 MPa, a Young's modulus of 14 GPa and a strain-to-failure of 1% without optimization of the process steps. These properties exist over the total temperature range investigated, i.e. they are not lowered by progressive graphitization. Carbons with a high boron content show decreasing strength and modulus, but a constant strain-to-failure of about 0.95%.