ELECTRON-RADIATION DAMAGE OF ALPHA-ALUMINA

Electron irradiation studies of alpha-alumina were conducted in both high vacuum and ultra-high vacuum (UHV) environments. In the high vacuum (10(-5) Pa) of a conventional high resolution electron microscope (C-HREM), the crystal surfaces damaged to form characteristic "dark-line" facets under low-flux (10 A/cm2) conditions. However, in the UHV (10(-8) Pa) environment and similar electron flux, the surfaces did not facet and only suffered from oxygen loss. Under continued irradiation crystallites of aluminum formed on the surfaces. The damage process was accelerated at 100 keV, a regime where relative rate of ionization is favored and ballistic knock-on processes are minimized. These results signify that the damage process in alpha-alumina is associated with an ionization mechanism where the reaction products are greatly controlled by the ambient vacuum in the microscope.