LOW-FREQUENCY GRAIN-BOUNDARY RELAXATION IN ALUMINA

The internal friction spectrum of polycrystalline, sintered alumina was determined at 15 to 25 cps and 25° to 1550°C. Two relaxation mechanisms appeared to be operative. A high‐temperature relaxation interpreted as stemming from diffusion‐controlled grain‐boundary motion was dominant above 1150°C. Assuming a mean diffusion path of 10−4 cm gave a diffusivity of D = 5.2 × 10−2 exp (−52,000/RT) which was independent of impurity content. A lower‐temperature relaxation was observed in specimens of higher impurity content at 900° C. This relaxation reached a maximum but did not behave as a Debye‐type relaxation. Single‐crystal alumina exhibited neither of these relaxations. Copyright © 1968, Wiley Blackwell. All rights reserved