Kinetics of the Iodide Titanium Process by the Thermal Decomposition of Titanium Tetraiodide

Abstract

The iodide process was developed by van Arkel and de Boer in the 1920s of the present century for the production of high-purity metals. 1 This preparation method follows a cyclic behavior in a closed vessel where elemental iodine acts as a gaseous transport agent. It starts with the reaction of iodine and impure titanium metal in excess at a temperature high enough to release volatile metal iodides. The evolved iodide vapors then contact an incandescent tungsten filament where they decompose. The filament serves as growth substrate for the refined metal, whereas the liberated iodine diffuses back to the impure titanium in order to complete the cyclic process. The main interest of this method lies in the fact that inter-stitial gaseous impurities from the crude metal are not transferred to the deposited material due to the very low reactivity of iodine against most metal oxides, carbides, and nitrides. A systematic survey of the many metals that can be purified by the iodide process has been made by Rolsten. 2 This process has been shown to be particularly suited for the refinement of titanium and zirconium, as a negligible content of gaseous impurities in both metals is required for their production in a ductile condition. Up to the 1960s, the iodide process was extensively studied due to the potential use of zirconium and titanium for nuclear and aircraft applications, respectively. However, owing to the inherent characteristics of this deposition technique, it was difficult to obtain these materials on an industrial scale, and active investigation of this technique almost stopped. At that time, some fundamental physico-chemical mechanisms of the process remained unclear and, as an important example, a deep controversy prevailed about the rate-limiting step in the kinetics of metal deposition. Many investigations, such as those of Holden and Kopelman, 3 Runnalls and Pidgeon, 4 and Shelton, 5-7 pointed out that the transport of iodide vapors should determine the process kinetics. On the other hand, the formation of lower iodides 8-10 (namely, TiI 3 and TiI 2 in the case of titanium depo-sition) and the reaction between elemental iodine and the crude metal 11,12 have also been stated to influence the kinetics of the process. Besides, although vapor transport is frequently accepted to control the overall rate of metal deposition, it has not been established whether the slow step is that of the transport of gases between the crude metal and the filament region or just their transfer in the close neighborhood of the filament. This lack of knowledge is connected with contradictory observations. Some authors have found that the metal deposition rate is proportional to the filament surface area, 4,13 whereas others have obtained a null dependence. 12,14 Nowadays, several aspects make the research on this process again valuable. On one hand, the continuous necessity of preparing high-purity metals to carry out accurate fundamental studies, such as, for example, the assessment of the Ti-H phase diagram. 15,16 On the other hand, it is now possible to acquire a better understanding of the iodide process kinetics with the help of the powerful analytical chemical vapor deposition (CVD) tools that have been developed in the last thirty years. Additionally, further research on the properties of titanium tetraiodide is, at present, highly desirable, since this compound is a promising precursor for the preparation of TiN diffusion barriers to be used in integrated circuits. 17 The scope of the present paper is to establish the main characteristics of the iodide process kinetics. To this end, an extensive research program on the preparation of iodide titanium was performed in our laboratory using titanium tetraiodide as starting material. 18,19 This modification of the original process was adopted in the past by Ingra-ham and Pidgeon 20 and Kesler, 21 but definite kinetic results were not obtained at that time. In this way, the process acquires a flow behavior and the problems related to the reaction between the crude titanium and iodine (i.e., the formation of lower titanium iodides and surface passivation of the crude titanium) are avoided. Furthermore, the preparation method becomes, in principle, the easiest example of a CVD process: the thermal decomposition of a volatile reactant to produce a solid deposit. 22 In order to determine the rate-limiting step of this process, we have studied the dependence of the titanium growth rate on the substrate temperature, reactant pressure, and substrate surface area. In a previous publication, 23 a detailed description of the experimental system was given together with a phenomenological model which allows monitoring of the titanium growth rate from the variation of the substrate electrical resistance. In that publication, the dependence of the titanium growth rate on the substrate surface area was analyzed. It was found that these parameters fulfill a linear dependence. That result suggested that the titanium growth rate is controlled by the transfer of gases in the vicinity of the substrate, however, a more detailed work was said to be prudent to confirm this statement. Such a work is presented in this paper. Experimental The growth of iodide titanium films has been accomplished in a self-designed CVD reactor made entirely of glass, as dictates the high reactivity of iodide vapors against most metals. It comprises a central reaction chamber, where a tungsten substrate is located, and two side arms to place, respectively, a weighed quantity of TiI 4 powder (1 g) and a gas by-product cooling trap. The tungsten substrate 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 o TiI4 , 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 o TiI4) 0.9 D (T F , P o TiI4)P o TiI4 T F r E (T F) mg cm 2 min 1. 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 and zirconium, the general characteristics for the titanium growth kinetics here described are expected to be also fulfilled for the zirconium case.