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

Abstract 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, A F , of an Arrhenius-Frenkel type equation were measured as a function of F . The field dependence of A F is determined by the dissolution mechanism: (a) dissolution by diffusion: log A 0 F = log A 0 0 − ΔμF 2.3k ∗ T (μ  molecular dipole moment, ∗ T ≡ isokinetic for W〈111〉, log A 0 0 = − 6.0 and ∗ T = 940 K ; for Mo〈111〉, log A 0 0 = − 3.1 and ∗ T = 860 K ; and (b) dissolution by ion desorption: log A + F = log A + 0 + n 3 2 e 3 2 F 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 ∗ A 0 0 = 2.5 and ∗ T = 920 K for diffusion; log ∗ A + 0 = − 1 and ∗ T = 1240 K 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 90 cal K · mol, respectively. The two dissolution mechanisms can be described by the following Arrhenius-Frenkel type equations: τ 0 F = ∗ A 0 0 exp [ − (E 0 0 + ΔμF) k ∗ T ] exp [( E 0 0 + ΔμF) kT ] for diffusion and τ + F = ∗ A + 0 exp [ − (E + 0 − n 3 2 e 3 2 F 1 2 ) k ∗ T ] exp [( E + 0 − n 3 2 e 3 2 F 1 2 ) kT ] for ion desorption.