Modellering en eksperimentele ondersoek van Sb-oppervlak segregasie in Cu-enkelkristalle
Terblans, Jacobus Johannes
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In this study, the segregation of Sb to a Cu(111) and a Cu(110) surface was studied by (i) modelling the segregation process theoretically and (ii) measuring it experimentally. The aim of this study was to determine the influence of surface orientation on the segregation kinetics. Theoretically, the segregation of Sb from the bulk to the surface was modelled by an improved Darken model, since Darken's current segregation model does not include vacancy diffusion. Darken's model was improved by adding vacancy diffusion and rewriting the activation energy (E) in terms of the energy of migration (Em) and the vacancy formation energy (Ev). From the literature, it is clear that the energy (ES) of a surface depends on the surface orientation and in this study, the surface energy was linked to the vacancy formation energy and an improved Darken model was developed. This improved Darken model was used and the segregation kinetics to different surface orientations were modelled. The results of these calculations showed that the rate of segregation to the (110) surface is higher than segregation to the (111)-surface and that the activation energy for diffusion in the bulk just under the (111)-surface is approximately 15% higher than under the (110)-surface. Experimentally, the segregation of Sb from the bulk to the surface of two Cu single crystals were measured. The surface orientations of the two Cu single crystals were (110) and (111) and the bulk concentrations of Sb were ¼ 0.09 at%. The Sb surface concentration was monitored with Auger Electron Spectroscopy (AES) while the temperature of the crystal was increased linearly. Both the AES data and the temperature of the crystal were controlled and recorded by a computer. The conclusion is that the bulk diffusion coefficient in a crystal is position dependent and it is determined by the surface orientation of the crystal.