KINETICS OF CARBOTHERMAL REDUCTION SYNTHESIS OF BORON-CARBIDE

Carbothermal reduction kinetics to synthesize boron carbide are studied under conditions of high carbon/boron oxide precursor heating rates (> 10(3) K/s) and intermediate temperatures (1803 K < T < 2123 K). For these conditions, fractional carbon conversion, X, can be described by a nucleation-growth rate expression: In (1 - X) = - (kt)3, where k(o) = 3.86 x 10(6) s-1 and E(a) = 301 +/- 55 kJ/mol for the temperature range (1803 < T < 1976 K) and k(o) = 2 x 10(20) s-1 and E(a) = 820 +/- 89 kj/mol for the temperature range (1976 < T < 2123 K), respectively. Boron carbide formation is highly dependent upon the phase change of reactant boron oxide from solid to liquid to gaseous boron suboxides and the effect of reaction environment (i.e., heating rate and ultimate temperature) on the rate at which the phase changes occur.