Aluminium and oxygen tracer diffusion in doped alpha-Al2O3

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Alpha-Al2O3 is an important high temperature material with multiple technological applications: e. g. as an in-situ growing self-healing protective scale, as a single-crystalline or polycrystalline bulk material for very different tasks or as a reinforcement fibre in composite materials. Modelling the ongoing high temperature processes (creep, sintering, scale growth) requires data on the bulk diffusion of oxygen and aluminium. Doping with lower-valent or higher-valent foreign elements enables a targeted control of the oxygen and aluminium transport and is, therefore, a prime concept to influence high temperature process kinetics. Because of severe methodological difficulties there is only very little information available on aluminium self-diffusion, which means that the influence of dopants on the aluminium diffusion is unknown as yet. Comparing the oxygen diffusion data of different experimental groups published to date for doped alpha-Al2O3 single crystals (i. e. in order to quantitatively asses the influence of dopant concentrations on oxygen diffusion) is extremely hindered because of different background impurities (of the crystals studied) and different experimental methods applied.Contrary to the experimental programmes of the published studies the present project will be the first to make consequently use of the unique possibility to measure both oxygen tracer diffusion and aluminium tracer diffusion on the same material as a function of the concentration of a lower-valent dopant (Mg) and a higher-valent dopant (Ti), respectively, as well as a function of temperature. The correlations originating from this joint data set will, for the first time, supply robust evidence on the point defects, which are operational in the studied crystals. Further, this data will enable to asses the defect formation energies and the migration energies, which have been published by different theoretical groups.