The influence of residual gases and oxygen on the activation energy for surface self-diffusion of tungsten in the presence of high electric fields

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Two build-up stages could be distinguished during the heating of tungsten field emitters at various residual gas pressures, p, while an electric field, F, was applied: (a) build-up around the 〈111〉 poles and (b) build-up around {100} planes. A plot of log p versus Q F, the activation energy for surface self-diffusion with field, indicated a linear relation with a positive slope for {100} build-up and a linear relation with a negative slope for 〈111〉 build-up. The value of Q F also, is a linear function of F, indicating a permanent dipole migration mechanism. In the presence of oxygen, 〈111〉 build-up occurred predominantly. The Q F values match the pressure dependence for residual gas 〈111〉 build-up. It could be determined: Q F (W-atom) = 1.9 eV/particle for F = 25 MV/cm , Q F = 0 (dipole) = 4.8 eV/particle at P residual = 4 × 10 −9 Torr, and an effective dipole moment of μ ⋍ 6 D . The increase of Q F {100} with increasing residual gas pressure is proposed to be due to pinning by carbon. The final build-up form ({110}, {100}, and {112}) is discussed in comparison with other single-crystal growth forms of tungsten.