Vaporization Behavior of Niobium Carbide

Abstract

The vaporization behavior of niobium carbide was studied over the temperature range from 2260° to 2940°K utilizing the Langmuir method. NbC was observed to lose carbon preferentially down to a composition of NbC_0.75, at which composition the vaporization proceeds congruently at a temperature of 2940°K. The vapor pressure of carbon above nearly stoichiometric NbC could be represented by the equation log P (atm) = 5.296–3.276×10⁴/T with a corresponding second law heat of vaporization of 150 kcal/mole of C vaporized. However, third law calculations and heat of formation data indicate a ΔH⁰₂₉₈ of from 180 to 189 kcal/mole. Several possible explanations of this disagreement are discussed. The total rate of evaporation from NbC_x was found to increase by a factor of two as the value of x decreased from 0.97 to 0.80. Finally, a semi‐quantitative evaluation of the diffusion constant of C in NbC was made, resulting in a value of D≅8×10⁻⁷ cm²/sec at 2940°K.