Sol-gel synthesis of nanoparticulate mullite precursor with the highest solid content

Abstract

Mullite is an alumina silicate with the general formula Al₂(Al_(2+2x)Si_(2-2x))O_(10-x). Specifically, when x=0.25, mullite is represented as 3:2 (3Al₂O₃·2SiO₂). In recent years, it has gained recognition as a critical raw material in the production of refractory materials and advanced engineering ceramics. To synthesize high-purity mullite at low temperatures with a uniform and homogeneous nanostructure, the sol-gel technique is a highly effective method. Achieving a mullite sol with a high solid content is also essential. This research aims to develop an innovative approach to produce a mullite precursor sol with the highest possible solid content using aluminum nitrate nonahydrate (Al(NO₃)₃·9H₂O) and colloidal silica. The primary focus of the study is to investigate the effects of pH and the stoichiometric ratio of Al₂O₃ to SiO₂ on the solid content and the types of phases forming in the sol. At an acidic pH (3-4) with a 3:2 ratio, only mullite was formed after calcination at 1250°C. In contrast, at higher pH values, intermediate phases of alumina and cristobalite were also observed. By varying the stoichiometric ratio of Al2O3 to SiO2 and increasing the amount of silica (3:3, 3:4), the solid content of the mullite sol increased, with the highest solid content of 25.50% achieved in the 3:4 sample. X-ray diffraction (XRD) results of the powders synthesized at 1250°C show that the mullite phase is present in all three samples, although traces of intermediate phases of alumina are noted in the 3:3 and 3:4 samples. As the calcination temperature is raised to 1400°C, weak mullite peaks appear in all samples, indicating the penetration of alumina and silica due to the high heat treatment temperature. Thus, the mullite formation process is completed in all samples.