Appearance Process of Transparency by Vacuum Sintering of Alumina Obtained by Thermal Decomposition of Aluminum Sulfate

Abstract

Thermally decomposed alumina from aluminum sulfate has a superior sinterability. Transparent polycrystalline alumina body may be obtained from this alumina atuarelatively low temperature and short sintering time in vacuum.The main subject of this study was to investigate the appearance process of the transparency of the above bodies. Two series of specimens were prepared, i.e. with and without addition of 0.05wt% MgO. Specimens were sintered in vacuum (5×10-4mmHg) at the temperature range from 1450°C to 1750°C.The relationship among porosity, transmission, and grain size for the sintered bodies at varied temperatures were examined and discussed.The results obtained were as follows:1) The relationship between the porosity (P) and the transmission (T) can be described by the following equation:T=Ae-mPThe values of A and m obtained from this experiment were 56% and 0.35 respectively.2) From MgO added specimens transparent bodies were obtained by firing at above 1600°C. This might be ascribed to the superior sinterability of the decomposed alumina material.3) The porosity for the series of specimen fired at above 1550°C without additives showed no decrease and the value was maintained at about 7%. The specimens of this showed poor transparency. The closed pore which might affect the transparency increased in the temperature range from 1500°C to 1550°C.The above temperature range was in accordance with that of exaggerated grain growth. The existence of some relationship between the occurrence of closed pore and exaggerated grain growth may be suggested.