Evaporation of Tungsten Under Various Pressures of Argon

Theory of the mechanism of the evaporation of a tungsten filament in the presence of an inert gas.---A theory of the evaporation of a heated tungsten filament immersed in an inert gas is developed in which it is assumed that the evaporating atoms diffuse through a film of stationary gas surrounding the filament. The diameter of the film is calculated from Langmuir's equation for the heat loss from a filament. The theory leads to the relation maPlogb/a=constant, where m is the rate of evaporation, P is the pressure, and a and b are the diameters of the filaments and gas film respectively.Test of the theory. Rate of evaporation of a tungsten filament at 2870^∘{}K in a gas mixture, 86% argon, 14% nitrogen.---By measuring the loss in weight of the filament the rate of evaporation for various gas pressures was calculated. The rates varied from 230×{}10^-9 gr/cm² sec. in a vacuum to 2×{}10^-9 gr/cm² sec. at a pressure of 165 cm. It is shown that when b is calculated from Langmuir's equation for heat loss the expression maPlogb/a is actually constant for gas pressures in excess of 10 cm. The fact that the round filaments developed a hexagonal cross-section in accord with the normal crystalline shapes of tungsten grains is further evidence of the backward diffusion of evaporated metal with consequent condensation on the filament.