Kinetics of the iodide titanium process by the thermal decomposition of titanium tetraiodide

An extensive study of the kinetics of the iodide titanium process, performed in a flow system by the thermal decomposition of titanium tetraiodide over a tungsten filament substrate, is presented in this paper. The influence of the substrate temperature, T-F, and reactant pressure, P-Til4(o), on the process rate has been analyzed. It has been found that the overall process is rate limited by the transfer of gases in the neighborhood of the substrate. In addition, it has been determined that the rate of titanium transfer is modulated at the filament surface by a certain deposition efficiency that depends on reactant pressure and reaction temperature. The available thermodynamic data for the gaseous titanium iodides at high temperatures fail to explain such an efficiency factor, although the calculations described herein indicate that such data are probably incorrect. Eventually, the titanium growth rate results from the noninterfering contributions of titanium deposition and evaporation rates. The overall reaction rate for the process can be expressed as a function of the operational parameters by the following equation: r(G)(T-F, P-Til4(o)) = 0.9 epsilon(D)(T-F, P-Til4(o))(PTil4TF)-T-o - r(E)(T-F) mg cm(-2) min(-1).epsilon(D) represents the determined deposition efficiency and r(E) the titanium evaporation rate in vacuum. Owing to the similarities between the iodide process for titanium acid zirconium, the general characteristics for the titanium growth kinetics here described are expected to be also fulfilled for the zirconium case. (C) 2000 The Electrochemical Society.