A thermogravimetric study of the reactions of molybdenum and tungsten disilicides with anhydrous hydrogen fluoride and fluorine

Abstract

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