Electromigration and chemical diffusion in titanium carbide

Abstract

The ionic charge and lattice diffusion of carbon in titanium carbide have been investigated through studies of diffusion under the influences of kan applied dc-electric field and of a chemical potential. The electron microprobe was used to measure the diffusion concentration profiles. Electromigration studies of carbon in titanium carbide yielded a positive effective charge which increases from +0.4 at l8250 C to possibly as high as +2.8 at 2480oC. The observed sign and temperature dependence of/the effective charge are interpreted as indicating that during the diffusion jump the average carbon-ion (charge seen by the electric field is positive and that the whole window is increasing with respect to the lielectron winduu with increasing temperature. It is suggested that this positive charge supports the prediction of Lye's energy-band calculation that charge is transferred from titanium to carbon electronic states but still is consistent with qualitative results of recent ESCA and electron mapping experiments that show that the carbon atom is negative at its equilibrium site. The chemical diffusivity in single crystals of titanium carbide was determined for 19200 C ~ T ~ 2328°C using the Boltzmann-Matano method of analysis. The experimental chemical diffusivity increases with decreasing carbon vacancy concentration and has an average value of 220 exp(-97.7!RT) cm !sec. A concentration-dependent diffusivity with an average value of 300 exp(-99~3/RT) cm2/sec is calculated from Sarian's tracer diffusivity data and DePoorter's semi-theoretical model for activity. The present work shows that the chemical diffusivity in titanium carbide depends on concentration through the tracer activation energy but not through the pre-exponential factor. A comparison between these single crystal results and diffusivities reported for chemical diffusion in layer growth samples demonstrates that the latter represent short-circuit enhanced diffusion. Also~ the present work substantiates the results of Sarian for tracer diffusion in single crystals of titanium carbide, and it connects the polycrystalline layer-growth diffusivities with the single crystal tracer diffusivity results reported in the literature.U of I OnlyThesi