Mullite formation from mixtures of alumina and silica sols: mechanism and pH effect

Carla C Osawa,Celso A Bertran

doi:10.1590/s0103-50532005000200019

Abstract

This work reports the effect of pH on the process of mullite formation from mixtures of alumina and silica sols. The pH of the mixtures determines the charges of particle surfaces and affects their interactions and distributions. Mullite formation from amorphous precursors with an Al:Si molar ratio of 1:3, prepared from the sols mixture, was not affected by pH. In this case, the higher concentration of silica determined its distribution around alumina, which led to tetragonal mullite formation, according to the Sundaresan and Aksay mechanism. However, for mullite formation from precursors with Al:Si = 3:1, the pH played an important role on the interactions between alumina and silica particles, as well as on the predominant aluminum species. At pH 1, octahedrically coordinated Al3+ ions predominated in the alumina sol while tetrahedrically coordinated Al3+ ions predominated in the sol at pH ~6. The interactions between silica and alumina particles and their distributions in these precursors determined the minimum temperature required for orthorhombic mullite formation.

Highlights

Mullite (3Al O .2SiO ) is a very important ceramic material, with high temperature applications, due to its excellent mechanical properties, such as high strength, low thermal expansion coefficient, low thermal conductivity, high thermal shock resistance and creep resistance.[1,2,3,4,5,6]Mullite is an interesting model for the development of new procedures of ceramic material synthesis, due to the simplicity of its composition.[7]
Studies of mullite formation from alumina and silica sols can be useful to evaluate the interactions of alumina and silica particles in an aqueous medium, as a function of the pH, and, the effect of these interactions on mullite synthesis
Mullite formation was employed as a model to investigate particle interactions in mixtures composed of two different sols where the charges on the particle surfaces are determined by the pH value

Summary

Introduction

Mullite (3Al O .2SiO ) is a very important ceramic material, with high temperature applications, due to its excellent mechanical properties, such as high strength, low thermal expansion coefficient, low thermal conductivity, high thermal shock resistance and creep resistance.[1,2,3,4,5,6]Mullite is an interesting model for the development of new procedures of ceramic material synthesis, due to the simplicity of its composition.[7]. The effect of pH on mullite formation was analyzed by the minimum temperature required to transform amorphous mullite precursors into crystalline mullite and is related to the interactions among alumina and silica particles in the mixtures of alumina and silica sols.

Results

Conclusion