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

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 less than or equal to 10 mu m and compacted to bodies. These bodies were sintered at 1300 degrees C and subsequently high-temperature treated at 1600, 1800, and 2000 degrees C. The treated bodies were investigated to determine boron content, bulk density, crystallite parameters, and mechanical properties. Boron catalyzes graphitization already above 1300 degrees C; an influence of boron concentration was detected only above 1800 degrees C. Carbons produced with a low boron content exhibit a flexural strength of 135 MPa, a Young's modulus of 14 Cpa 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%. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.