Effect of Metal Oxides on Surface Grain Growth of Alumina

Abstract

The rate and the activation energy of the surface grain growth of alumina containing 2 or 5wt% of MgO, Co3O4, TiO2 and MnO2 were measured in the temperature range from 1400° to 1650°C.In general, the relation between grain size and heating time can be described by the following equation:D=Atm (1)In the present investigation D was expressed by the mean value of the diameter of 20 largest grains. The results obtained were as follows:1) The equation (1) held good for all of the specimens.2) The m value increased with temperature rise and its increasing rate was larger according as the grain growth was remarkable.3) The relation between grain growth and sintering was proportional.4) The addition of the above metal oxides promoted grain growth, but the extent was different by materials and their quantities. The specimens with 5% of MnO2 showed remarkable grain growth particularly.The grain growth was generally related to time by the following equation.D-D0=Ktn (2)Exactly the value of n was dependent on temperature but approximately constant at 0.33.The addition of metal oxide decreased the activation energy for grain growth. Particularly the value for the specimen with MnO2 (75kcal/mole) was about half of that for the specimen without additives (135kcal/mole).