Chemical diffusion in titanium carbide crystals

Single crystals of TiC_x (x=0.84) were exposed to a carbon source at temperatures near 2000°C. The penetration of carbon into the structure, reflected in a gradient in the value of x, was evaluated with the electron microprobe analyzer. The chemical diffusivity D̃ was then determined using the Boltzmann-Matano method of analysis. The experimental value of D̃ was found to increase with decreasing carbon vacancy concentration, 1-x, and to have the average value of 220 exp (-97 700/RT) cm²/sec. This value of D̃ is in good agreement with one generated from Sarian's ¹⁴C tracer measurements on TiC crystals and DePoorter's semitheoretical model for activity. Because of the abundance of vacancies in the carbon fcc sublattice of TiC_x, the experimental activation energy represents an energy of motion only. Lower experimental values for the diffusion activation energy have been obtained by other workers with the phase-boundary advance method. The present work shows that the difference between the activation energy measured by the tracer method and that measured by the phase-boundary advance technique cannot be ascribed to the influence of a chemical gradient. Hence it is probably due to grain-boundary short circuiting.