Self‐Diffusion of Beryllium in Polycrystalline Beryllium Oxide

Abstract

The self‐diffusion of beryllium in polycrystalline beryllium oxide was measured from 1330° to 2135°C with the 7 Be tracer. Results are compared with the findings of other investigators, and it is concluded that the differences reported are due to variations in impurity and porosity contents. Diffusion has been shown to be via a vacancy mechanism, with such impurities as aluminum and silicon being the principal contributors of vacancies. A roll‐over in the plot of log D versus 1/ T , where D is the diffusion coefficient and T is absolute temperature, was found in this study as in some others. Above about 1500°C the activation energy is no higher than 62 kcal, whereas it is 92 kcal at lower temperatures. It is suggested that this roll‐over is due to aluminum impurity. There appears to be a temperature‐independent anisotropy of diffusion, with the rate in the 〈 c 〉 direction being 30% higher than that in the 〈 a 〉 direction. It is concluded that for this and other investigations the diffusion coefficient of beryllium in beryllium oxide above the rollover temperature is governed by impurities and in this region it is given by the formula image where F p is the fraction of porosity and N v is the molar fraction of vacancies introduced by impurities. There is sufficient scatter in individual 〈 a 〉 and 〈 c 〉 diffusion values so that there is no value in discriminating between directions in this formula.