Thermal Decomposition of Niobium and Tantalum Monocarbides

Abstract

The thermal decomposition of polycrystalline NbC was investigated from 2000° to 3200°C, and TaC from 1890° to 3320°C in the presence of one atmosphere of helium. It was found that both compounds lose carbon preferentially, and that the final carbon/metal molar ratio obtained may be represented by an equation of the form C/M=A—B exp(λt), where t is the maximum temperature of heating for a constant time (30 min in both cases). Similarly the resultant lattice constant may be expressed as a0=A′—B′ exp(λ′t), where a0 is in A at 25°C. For a maximum temperature of 3000°C and heating times of 30 min to 12 hr, a0=4.459084+0.0093071 exp(—0.18916 τ) for NbC, where τ is the time in hours. It was found that a0=4.414712+0.056862 (C/Nb) for the C/Nb range 0.885 to 0.981 and that a0=4.385779+0.070204 (C/Ta) for the C/Ta range 0.906 to 0.996. By extrapolation, the lattice constants of stoichiometric NbC and TaC should be 4.47157±0.00012 A at 25°C and 4.45598±0.00038 A at 25°C, respectively.