The Activation Energy for Grain Growth in Alumina

Abstract

High-purity alumina has been isostatically cold-compacted and sintered. Nickel was used as a grain-growth inhibitor. Grain size distribution in the sintered materials has been measured by electron microscopy. The activation energy for grain growth in the temperature range 1600–1800°C was determined as 138 ± 7 kcal/mole; other workers have found a value of 150 kcal/mole. Electron probe microanalysis has revealed a nickel-rich phase present not only at the original sites of the nickel particles, but also as a fine dispersion whose mean spacing corresponds to the particle diameter of the alumina before compaction. The estimated nickel content of the spinel phase (15 wt.%) accords with that of a nonstoichiometric spinel found by other workers. A grain-growth inhibitor might be expected to increase the activation energy for grain growth, but it is shown that the necessary increase in activation energy is of the same order as the limits of error of the activation energy. However, this is further considered with reference to the Arrhenius rate equation, and it is suggested that the presence of nickel could alter the “constant” term, thus affecting grain growth, while leaving the activation energy for the process essentially unchanged.