Reactions of Oxygen with Pure Tungsten and Tungsten Containing Carbon

Abstract

Oxygen was made to flow at constant rates through a system consisting of a tungsten ribbon, an ion gauge, and a conventional mass spectrometer. The ion gauge recorded total pressures while the spectrometer recorded partial pressures of O2, CO, CO2, H2O, and other gases. From the way these pressures changed with filament temperature and time, we computed absolute rates at which O2 was converted to WO3, CO, CO2, and O or evaporated as O2. The data show that the sticking probabilities, the adsorption and desorption energies, and the rates of reaction change drastically when all the surface tungsten atoms have made first valence bonds with the adsorbed species; they change again after second valence bonds have been completed. Other conclusions are: (1) On a clean W surface consisting of 311 and 411 planes and for oxygen concentrations less than 4×1014 atoms/cm2, the sticking probability is 0.14, the desorption energy is 4.6 ev, and the evaporation rate is proportional to θ12; for concentrations between 5–9×1014 atoms/cm2, the sticking probability is 0.04, the desorption energy is 2.3 ev, and the evaporation rate is proportional to θ22. (2) In the temperature range 1200–1600°K the rate of formation and evaporation of WO3 increases with p(O2) and with T; the activation energy is 1.1 ev. (3) From 2000–2400°K the rate of WO3 formation increases as p(O2)32 and it decreases with T. (4) Less than about 4% of O2 molecules which strike the hot surface are converted to WO3. Most of the molecules evaporate as O2. Only above 2200°K is any oxygen evaporated as O. (5) No CO or CO2 is generated if the tungsten is carbon free. (6) If the tungsten contains carbon, CO and CO2 are formed if the surface is covered with a layer of oxygen and the temperature is high enough. Every carbon atom that diffuses to the surface is oxidized. The diffusion rate of carbon is proportional to the concentration gradient and increases exponentially with (1/T); the activation energy is 2.2 ev. (7) Tungsten can be made carbon free by glowing it at 2200°K in oxygen pressures near 10−6 mm for 10–60 hr.