Besides its importance for conventional ceramics, mullite has become a choice of material for advanced structural and functional ceramics due to its favourable properties. Some outstanding properties of mullite are low thermal expansion, low thermal conductivity, excellent creep resistance, high-temperature strength, and good chemical stability. The mechanism of mullite formation depends upon the method of combining the alumina- and silica-containing reactants. It is also related to the temperature at which the reaction leads to the formation of mullite (mullitisation temperature). Mullitisation temperatures have been reported to differ by up to several hundred degrees Celsius depending on the synthesis method used. The three synthesis methods used to prepare mullite are discussed in this paper together with an overview of mullite, its properties and the current application of mullite.
Mullite is an alumino-silicate compound that is used extensively in traditional refractory applica- tions. Most traditional ceramic products have mullite as part of their final phase composition since they usually contain some clay and silicon as starting materials. The microstructures of the materials also contain a relatively low melting glass.
Natural mullite is rare in nature; it is named after one of the few known deposits on the Isle of Mull, United Kingdom. (Bowen and Greig, 1924) The occurrence of this compound on the Isle of Mull is thought to be a result of post-Caledonian volcanic activities in which clay mineral deposits, heated through contact with magma, produced a high temperature mullite phase. (Bowen, Greig et al., 1924) It was not until the 1970s that the signi- ficance of the inherently favourable properties of mullite was recognised, when the mechanical behaviour in compression of mullite specimens, free or nearly free of a glassy phase, was first studied.
Mullite is becoming increasingly important in electronic, optical, and high temperature structural applications, because of its low dielectric constant, good transparency for mid-infrared light and excellent creep resistance. Classical uses of mullite include refractories in the metallurgical industries for electric-furnace roofs, hot metal mixers and low- frequency induction furnaces. In the glass Indus- tries, these refractories are employed in the upper structure of the tank in which the glass is melted and for constructing the drawing chambers. Mullite is frequently used as kiln setting slabs and posts for firing ceramic ware as well as for the linings of high-temperature reactors. However, during the last decade, the spectrum of actual or potential employ- ment of mullite now includes its use as a matrix material for high-temperature composite develop- ment, a substrate in multilayer packaging, protective coatings, turbine engine components, and an infrared-transparent window especially for high- temperature application. (Tummala, 1991; Prochaz- ka and Klug, 1983) One approach to improve the environmental durability of silicon-based ceramics is to apply refractory oxide coatings containing no silica or those with a lower silica activity.
Mullite is most promising because of its environmental durability, chemical compatibility, and coefficient of thermal expansion (CTE) similar to that of SiC. Coatings of mullite applied by atmospheric plasma spraying (APS) to SiC ceramic matrix composites provides some degree of corrosion resistance to corrosive industrial environments. (Van Roode, Price et al., 1988)
These new interests have sparked extensive studies on the synthesis and processing of mullite and mullite-based composites by molecular and/or colloidal methods and also on techniques for producing mullite protective layers.
Mullite rarely occurs in nature, so it is usually synthesised, rather than mined. Various prepara- tion methods and starting materials are used for the synthesis of mullite ceramics. The preparation methods can be classified into three different preparation routes as follows: (1) sinter-mullite, (2) fused-mullite (3) chemical-mullite (high-purity mullite). The synthesis of mullite ceramics together with important processing routes to fabricate monolithic mullite ceramics are described in this paper.
References
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