David J. Duval, Subhash H. Risbud, and James F. Shackelford

Abstract Mullite is the only stable intermediate phase in the alumina-silica system at atmospheric pressure. Although this solid solution phase is commonly found in human-made ceramics, only rarely does it occur as a natural mineral. Yet mullite is a major component of aluminosilicate ceramics and has been found in refractories and pottery dating back millennia. As the understanding of mullite matures, new uses are being found for this ancient material in the areas of electronics and optics, as well as in high temperature structural products. Many of its high temperature properties are superior to those of most other metal oxide compounds, including alumina. The chemical formula for mullite is deceptively simple: 3Al2O32SiO2. However, the phase stability, crystallography, and stoichiometry of this material remain controversial. For this reason, research and development of mullite is presented in an historical perspec­tive that may prove useful to engineers and scientists who encounter this material under nonequilibrium conditions in their work. Emphasis is placed on reviewing studies where the primary goal was to create single-phase mullite monoliths with near theoretical density.

1 Introduction

Mullite is a solid solution phase of alumina and silica commonly found in ceramics. Only rarely does mullite occur as a natural mineral. According to introductory remarks made by Schneider and MacKenzie at the conference “Mullite 2000”[1], the geologists Anderson, Wilson, and Tait of the Scottish Branch of His Majesty’s Geological Survey discovered the mineral mullite less than a century ago. The trio was collecting mineral specimens from ancient lava flows on the island of Mull off the west coast of Scotland when they chanced upon the first known natural deposit of this ceramic material. The specimens were initially identified as sillimanite, but later classified as mullite.

Being the only stable intermediate phase in the Al2O3-SiO2 system at atmospheric pressure, mullite is one of the most important ceramic materials. Mullite has been fabricated into transparent, translucent, and opaque bulk forms. These materials may have optical and electronic device applications. Mullite’s temperature stability and

J. F. Shackelford and R. H. Doremus (eds.), Ceramic and Glass Materials: 27

Structure, Properties and Processing.

© Springer 2008

refractory nature are superior to corundum’s in certain high-temperature structural applications. Another characteristic of this aluminosilicate is its temperature-stable defect structure, which may indicate a potential use in fuel cell electrolytes.

In this chapter, developments in the understanding of mullite over the last few decades are reviewed. A discussion of crystal structures and phase stability is pre­sented to provide the reader with an overview of certain characteristics of this mate­rial. The next part of this chapter examines the effect of process chemistry on the synthesis and microstructure of mullite. The role of various synthetic methods that are used to modify mullite formation will be discussed, followed by a compilation of selected materials properties.