Radiation-induced electrical degradation in crystalline Al2O3.

Abstract

Recent studies, simulating the behavior of insulating materials in a fusion device environment, show that under concurrent applications of radiation, applied electric field, and elevated temperature, they suffer degradation of their electrical properties. The goal of the present study is to address the mechanism of this radiation-induced electrical degradation and the defects involved. Our results show that when an ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ crystal under a moderate electric field is irradiated with 1.8 MeV electrons at 773 K, the dc conductivity during and after irradiation increases rapidly above a critical dose and saturates after the conductivity increases by a factor of ${10}^{3}$. There are two main conclusions. First, the electrical degradation is due to the charge of the electrons and holes created during radiation, rather than due to displacements of indigenous ions by elastic collisions with the energetic electrons. Second, the defects attending the observed electrical degradation are dislocations. Transmission electron microscopy studies revealed regions of large dislocation density distributed nonuniformly throughout the degraded area, with an overall average density of \ensuremath{\sim}${10}^{9}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$, as opposed to \ensuremath{\sim}${10}^{4}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ in regions which were not electron irradiated nor subjected to an electric field. The concentration of point defects, as characterized by optical absorption and electron paramagnetic resonance, was below detectable limit. In addition, no second phase was observed.