Shkoropado M. High temperature heat and mass transfer and oxidation kinetics of refractory metals in different gaseous media

The successive stages of high temperature heat and mass transfer and oxidation of tungsten and molybdenum filaments are defined. The basic mechanisms of heat transfer, phase transitions and chemical transformations are described. The temperature profiles along the tungsten and molybdenum filaments are measured, which allow to estimate the filaments surface condition at different values of heating current. The critical values of heating current are defined for a number of filament sizes which correspond to non-stationary high temperature oxidation in various media. It is shown, that natural and forced convection affects the critical value of electric current as well as the oxide scale formation. High temperature oxidation of refractory metals is studied in various gaseous media: air at normal pressure, rarified air, argon technical and carbon dioxide. A method is elaborated to evaluate a spectral emissivity coefficient of metal surface at high temperatures. The successive stages of non-stationary oxidation are defined for tungsten and molybdenum filaments electrically heated. For the first time a maximum is discovered experimentally on the oxide scale thickness time dependence. The oxide scale formation mechanism on the tungsten and molybdenum surface is determined. The rates of oxide crystals growth above the primary oxide films are defined. The growth mechanism is studied. The high temperature heat and mass transfer of metal filaments heated electrically and their oxidation is modeled with account of internal heat source. The temperature profile along the filament is expressed analytically supposing the specific resistance to be a temperature linear function. A new expression is obtained to describe a conductive heat flux to cold contacts with account of molecular-convective heat transfer to ambient medium. The computation results are in good agreement with the experimental data.