Positron lifetime measurements in sintered alumina

Abstract

Sintered alumina samples of grain diameter 1.7 and 4.5 μm, in which silicon impurity dominates, i.e. 90 ppm, were investigated by positron annihilation lifetime spectroscopy at room temperature using a new procedure of computer analysis. A bulk lifetime of 122 ± 4 ps was derived. A trapped positron lifetime of 137 ± 2 ps and a trapping coefficient μ ≥ 6 × 1014 s–1 were assigned to defects located within the grain. These traps are likely cationic vacancies VAl″′ and clusters (nSi•Al : VAl″′) induced by SiO2 dissolution into Al2O3. An annihilation process of small intensity and long lifetime, which can reflect annihilation in vacancy clusters located at grain boundaries, was found. The derived bulk lifetime is confirmed by measurements in Al2O3 single crystal of high purity. The reliability of the computer analyses is discussed.