Investigation of the Surface Reaction of Oxygen with Carbon on Tungsten with the Field Emission Microscope

Abstract

The adsorption of oxygen and carbon on tungsten and the surface reaction between carbon and oxygen on tungsten have been studied by means of the field emission microscope. Emission patterns for oxygen on tungsten, carbon on tungsten, and oxygen and carbon together on tungsten under a variety of temperature pretreatment conditions have been obtained. Average work functions over all exposed crystallographic planes, calculated from measured voltages and emission currents on the basis of the Fowler-Nordheim equation, give a measure of surface coverage in the case of oxygen on tungsten. The similarity of the emission-pattern changes in the evaporation of oxygen from tungsten with those found in the reaction between carbon and oxygen on tungsten suggests that the rate-controlling step in the reaction is desorption of oxygen from the tungsten surface. Diffusion of carbon into the tungsten lattice plays no role in the pattern changes since, in the absence of oxygen, the carbon-on-tungsten surface is found to be very stable.