Thermal generation of vacancies in high-quality aluminium crystals

Abstract

The dislocation density in high-purity (99.9999%) aluminium has been decreased to 10⁵–10⁷ m⁻² by cyclic annealing. In such a crystal, there are only a few sources for vacancy generation, so that it is expected to take a long time to attain the thermal equilibrium concentration of vacancies. In order to study the generation rate of vacancies, the electrical resistance has been measured accurately at high temperatures. It is found that it takes at least several hours to attain the thermal equilibrium concentration. For comparison, generation profiles from pre-existing dislocations and interstitial-type dislocation loops are estimated with a diffusion-limited model. It is concluded that, even in a high-quality aluminium crystal, most vacancies are generated from pre-existing dislocations, and only a small fraction is generated by the growth of interstitial-type dislocation loops. The surface oxidizes at a rate of about an atom layer per an hour and is not effective for the generation of vacancies.