Adsorption of methane on a tungsten field emitter

Abstract

The adsorption of methane on tungsten at temperatures between 78 K and 2400 K was investigated by field emission techniques. The results indicate that CH 4 adsorbs rapidly at 78 K forming weak and strongly bonded layers. The former are desorbed at T > 240 K while the latter undergo decomposition which is essentially complete at ∼ 700 K. Above this temperature surface diffusion of carbon is observed and at ∼ 1000 K initial stages of tungsten-carbon structures are seen. These coalesce into prominent ridges at higher temperatures. While adsorption and/or decomposition are also observed at higher adsorption temperatures, the extent is much reduced due to the low initial sticking probabilities. Rough estimates of the latter gave values of ∼ 5 × 10 −3 and 5 × 10 −4 at 200 and 300 K. The sticking probability rises when the adsorption temperature is > 800 K where adsorption, decomposition, surface diffusion and tungsten-carbon structure formation are simultaneous processes. At 1200 K the value is ∼ 1 × 10 −2. At 78 K, the average work function undergoes a large decrease initially but with time, increases until it exceeds the clean value. For adsorption temperatures above 200 K the work function increases very slightly.