Observation of compensation effects during the thermal dissolution of aluminum oxide layers on tungsten and molybdenum 〈111〉 and on tungsten {110} in the presence of electric fields

R. Vanselow, L. R. Pederson

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    Aluminum oxide layer dissolution was studied between 700 and 1200 K in the substrate areas of W〈111〉, Mo〈111〉, and on W{110} by means of FEM. Varying the electric field strength, F, between +45 and +105 MV cm, two types of dissolution could be observed: dissolution by surface diffusion (low F's) and dissolution by ion desorption (high F's). It is assumed that aluminum suboxides - preferentially AlO - are involved in the dissolution processes. The preexponential factors, AF, of an Arrhenius-Frenkel type equation were measured as a function of F. The field dependence of AF is determined by the dissolution mechanism: (a) dissolution by diffusion: log A0F = log A00 - ΔμF 2.3k*T (μ molecular dipole moment, *T ≡ isokinetic for W〈111〉, log A00 = - 6.0 and *T = 940 K; for Mo〈111〉, log A00 = - 3.1 and *T = 860K; and (b) dissolution by ion desorption: log A+F = log A+0 + n 3 2e 3 2F 1 2 2.3k*T; for A+0 = - 22 and *T = 1200 K; for W〈111〉, log A+0 = - 21 and *T = 1200 K. Using earlier proposed safeguards, isokinetic relationships (compensation effects) could be established for each of the two dissolution processes. The coordinates of the isokinetic points have the following average values: log *A00 = 2.5 and *T = 920K for diffusion; log*A+0 = - 1 and *T = 1240K for ion desorption. The entropy changes (at T = *T, zero field strength, and unit pressure) for the phase changes: solid layer → diffusion layer and solid layer → ion gas, are of the order of 30 cal K · mol and 90cal K · mol, respectively. The two dissolution mechanisms can be described by the following Arrhenius-Frenkel type equations: τ0F = *A00 exp[ - (E00 + ΔμF) k*T] exp[( E00 + ΔμF) kT] for diffusion and τ+F = *A+0 exp[ - (E+0 - n 3 2e 3 2F 1 2) k*T] exp[( E+0 - n 3 2e 3 2F 1 2) kT] for ion desorption.

    Original languageEnglish
    Pages (from-to)123-136
    Number of pages14
    JournalSurface Science
    Issue number1
    Publication statusPublished - 1 May 1984


    ASJC Scopus subject areas

    • Condensed Matter Physics
    • Surfaces and Interfaces
    • Surfaces, Coatings and Films
    • Materials Chemistry

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