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

The thermogravimetric results of a study into the dry fluorination of tungsten disilicide, WSi2, using hydrogen fluoride and dilute fluorine gas as fluorinating agents, are reported. The reaction between tungsten disilicide and fluorine follows the thermodynamically preferred route, viz. the formation of the volatile tungsten hexafluoride along with gaseous silicon tetrafluoride, with the reaction starting just below 200 °C. The reaction with hydrogen fluoride yields solid tungsten metal and gaseous silicon tetrafluoride, similarly thermodynamically predicted, above 250 °C. No reaction is observed at low temperatures where solid tungsten tetrafluoride is expected to form. The results of a kinetic analysis of the data for the reaction with hydrogen fluoride are reported. At temperatures below 450 °C, the kinetics cannot be reliably determined. In the higher temperature region, mass transport of the reactive hydrogen fluoride through the stagnant gas film surrounding the particle, and penetrating the pitted areas, controls the mass loss rate.

Parallel cracks are observed on the surface of the WSi2 particles after reacting with HF(g), evidently after the removal of the silicon from the crystalline matrix, the more dense tungsten metal forms, leaving unoccupied strips in the particles.Figure optionsDownload as PowerPoint slideHighlights
► Tungsten disilicide (WSi2) and fluorine yield volatile tungsten hexafluoride along with gaseous silicon tetrafluoride.
► Tungsten disilicide with hydrogen fluoride yields solid tungsten metal and gaseous silicon tetrafluoride.
► In the higher temperature region, mass transport of the reactive hydrogen fluoride through the stagnant gas film surrounding the particle, and penetrating the pitted areas, controls the mass loss rate.