Effect of chemical composition on sintering and properties of Al2O3-SiO2 system derived from sillimanite beach sand

Reaction sintering of beach sand sillimanite and alumina is an innovative as well as inexpensive method of mullite formation. Beach sand sillimanite, a by-product generated during the separation of rare earth compounds, and calcined alumina were used as starting materials and were mixed in appropriate proportion and sintered in compacted form at 1500-1575 degrees C. The study reveals that the use of submicron size powder enhances the sintering process. In this investigation, effect of chemical composition on sintering and mechanical/thermomechanical properties has been studied. It was found that the densification process is dependent on the Al2O3/SiO2 ratio. Silica rich composition achieves highest density at 1525 degrees C whereas the alumina rich composition requires 1550 degrees C. Flexural strength measured at room temperature and at 1200 degrees C initially de-creases with alumina content, with a minima at 71-74% Al2O3 and then increases again. Microstructure of the sintered is also dependent on the Al2O3/SiO2 ratio of the batch. Mullite made from silica rich composition is needle shape in nature whereas in alumina rich composition they are non-acicular in nature. (C) 1998 Elsevier Science Limited and Techna S.r.l. All rights reserved