Chemical Diffusion in Titanium Carbide Crystals

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Abstract
Single crystals of TiCx (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 TiCx, 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.

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