Observations on Normal Grain Growth of Magnesia and Calcia

Abstract

It is generally agreed that the rate of grain growth in metallic systems may be expressed by an equation that describes the square of the average diameter as being proportional to the time of annealing at constant temperature. The proportionality constant is exponentially dependent on temperature as normally expected in a rate expression according to the theory of reaction kinetics. The purpose of this study was to check the applicability of these equations to metallic oxide systems. Reagent‐grade MgO and CaO prepared from reagent‐grade CaCl 2 obeyed the grain‐growth expressions reasonably well. Strong temperature dependence of the growth rate of both materials was attributed to impurities. Activation energies of 110 and 60 kcal per mole were calculated for CaO and MgO, respectively. Both values are higher than would be anticipated for grain‐boundary diffusion in high‐purity materials. Although porosity varied considerably for various isotherms, no effect of porosity on grain growth was detected. The data, however, were considered to be insufficient to warrant strong conclusions concerning this variable.