A study of the reactions between molybdenum and tungsten disilicide (MoSi2 and WSi2) with anhydrous hydrogen fluoride and fluorine was carried out to investigate the chemical behaviour of the materials. These two compounds were used as alternatives that resemble the chemical behaviour of uranium silicide (U3Si2). An extensive literature survey of U3Si2 processing techniques is included, which guided the process selection for this work. The thermogravimetric results of a study into the dry fluorination of molybdenum and tungsten disicilides using both anhydrous hydrogen fluoride and dilute fluorine gas as fluorinating agents are reported. For both solids the observed reactions with fluorine follow the thermodynamically predicted routes, in which the formation of the volatile metal hexafluorides, along with gaseous silicon tetrafluoride was observed. The disilicides get fully converted at roughly 300 to 400 °C respectively. The expected products for the reactions of both solids with hydrogen fluoride are solid tungsten metal, solid molybdenum metal, hydrogen gas, and gaseous silicon tetrafluoride. The metal fluorides (WF4 and MoF3) were not obtained because they form at low temperatures only. Therefore the metals of molybdenum and tungsten were obtained as final products respectively from both reactions; and were verified with the aid of XRF and XRD analyses. Mass-transfer phenomena are shown to play a role in the reactions between hydrogen fluoride and both disilicides, preventing unrestrained complete fluorination of the two solids. Kinetic parameters are reported and the rate limiting mechanisms identified. Copyright / Dissertation (MSc)--University of Pretoria, 2012. / Chemical Engineering / unrestricted

This work presents a theoretical study of stability of phases in selected metallic systems. We propose a model of structural transformations in transition metal disilicides MoSi2, CrSi2, VSi2 and TiSi2 and in Pd thin films grown on cubic substrates W(001) and Nb(001). The obtained results yield the total energy proles for the structural transformations studied, the activation energies needed for each individual transformation and an estimate of the temperature at which the structure can transform. The total energies are calculated by full-potential linearized augmented plane waves (FLAPW) method incorporated in the WIEN2k code. Both generalized gradient approximation (GGA) and local density approximation (LDA) are employed for the exchange-correlation term. It turns out that temperatures corresponding to the activation energies of structural transformations in transition metal disilicides exceed their melting temperatures. Comparing the resulting total energy proles to those obtained by the semiempirical Bond Order interatomic potentials (BOP) substantially helps to adjust the fitting parameters of the BOPs. The estimated temperature of 168 K needed to transform the hcp structure of an innite Pd crystal into the dhcp structure explains the behavior of the Pd thin lm on W(001) and Nb(001) substrates. Pd lms deposited on W(001) substrate and thicker than about 100 monolayers undergo this transformation already at room temperature. Thinner lms need to be annealed at 400 K rst, due to their stronger interaction with the substrate. The difference between the computed result and a real temperature at which the hcp Pd lm transforms its structure to the dhcp can be explained by both the interaction between the lm and the substrate and by the inuence of the domain topology of the lm. Analyzing different models of transformation of the initial hcp Pd structure to the ground state fcc structure, we identied the optimum model that respects the domain topology of the Pd lm.