Positron lifetime study of vacancy-type defects in amorphous and polycrystalline nanometer-sized alumina

Amorphous and polycrystalline nanometer-sized alumina were prepared by direct oxidation and hydrolysis, respectively, of pure aluminum. The positron-lifetime parameters and their variations with grain growth during annealing were measured. Three lifetime components were observed in both amorphous Al2O3 and nanocrystalline Al2O3 specimens. The short-lifetime (170 +/- 120 ps), intermediate-lifetime (410 +/- 20 ps) and long-lifetime components correspond to three different kinds of defects: monovacancy-like free volumes, microvoids, and larger voids. The appearance of lifetimes in the range 1-5 ns indicates the formation of positronium. The influence of thermal annealing from 873 K to 1373 K on positron lifetime parameters was also analyzed. The components with lifetimes tau(1) = 170 ps and tau(2) = 410 ps persisted even after the grains had grown to 100 nm in size, while the long-lifetime component declined significantly when grain sizes exceeded 10 nm. The interface characteristics of polycrystalline nano-Al2O3 prepared by the two methods were compared by analyzing the variations of the positron-lifetime parameters with grain growth.