Field emission study of the formation and desorption of oxide layers on tungsten surfaces

Abstract

The interaction of oxygen with tungsten has been re-examined in the field emission microscope (FEM) as a sequel to a detailed temperature-programmed flash-desorption study of the kinetics of adsorption and desorption in the interaction of oxygen with polycrystalline tungsten. FEM patterns and Fowler-Nordheim average work functions which accompany the four identifiable stages in the formation and high-temperature evaporation of oxide layers on tungsten have been observed and distinguished from changes due to redistribution or rearrangement processes in the adlayer. Using distinctive FEM patterns as end-points for each of the stages of desorption of the adsorbate, activation energies E_d, and first-order rate-constant pre-exponentials ν₁ were determined for the loss of each layer; the overall work function change accompanying the loss of each layer was also characterized: layer I (desorption product: O atoms, 1900–2500 K), E_d= 543 kJ/mol; log ν₁= 13.5; Δϕ∼–0.2 eV. layer II (desorption product: W oxides, 1600–1800 K), E_d= 506 kJ/mol; log ν₁= 16.5; Δϕ=–1.0 eV. layer III (desorption product: W oxides, 1300–1500 K), E_d= 458 kJ/mol; log ν₁= 16.0; Δϕ=–0.9 eV.