Effects of Precursor pH on Synthesizing Behavior and Morphology of Mullite in Stoichiometric Composition

Abstract

Stoichiometric mullite (<TEX>$3Al_2$</TEX><TEX>$O_3$</TEX>. <TEX>$2SiO_2$</TEX>) precursor sol has been prepared by sol-gel method. The effects of the precursor pH and sintering temperature on the synthesizing behavior and morphology of mullite have been studied. Mullite precursor sol was prepared by dissolution of aluminum nitrate enneahydrate (Al(<TEX>$NO_3$</TEX>)<TEX>$_3$</TEX>.9H<TEX>$_2</TEX>O) into the mixture of silica sol. Precursor pH of the sols was controlled to acidic condition (<TEX>$PH\leq$</TEX> 1~1.5) and to basic condition (<TEX>$pH\geq$</TEX>8.5~9). The synthesized aluminosilicate sols were formed under 20 MPa pressure after drying at <TEX>$150^{\circ}C$</TEX> for 24 hours, and then sintered for 3hours in the temperature range of <TEX>$1100~1600^{\circ}C$</TEX>. From TGA/DTA analysis, total weight loss in the aluminosilicate gel of the acidic sample was (equation omitted) 56% and that of the basic sample was (equation omitted) 85%, indicating that the synthesizing temperature of mullite phase for acidic and basic samples was above <TEX>$1200^{\circ}C$</TEX> and <TEX>$1300^{\circ}C$</TEX>, respectively. The morphologies of the synthesized mullite were fine and needle-like (or rod-like) for acidic sample, and granular for basic sample that has been sintered above <TEX>$1300^{\circ}C$</TEX>. It was found that the morphology of mullite particle was predominantly governed by precursor pH and sintering temperature.