Resistivity, mullite composites

MuUite is another important crystalline ceramic that is an oxide It is a solid solution of alumina and silica in the compositional range 71-75 wt% alumina. Mullite is represented by the formula, 3 Al203.2Si02. It has excellent strength and creep resistance as well as low thermal expansion and conductivity. For more detailed information on the structure and properties of mullite the reader is referred to Schneider et al. (1994). Table 6.2 provides a summary of the properties of mullite. 

This section centers on fiber coatings for non-oxide eomposites in which either the fiber or the matrix is a non-oxide ceramic. Although oxide fiber-reinforced composites have been studied, most of the research available in the literature has focused on SiC fiber-reinforced composites. For example, mullite (3Al203-2Si02) fiber-reinforced SiC matrix composites have been fabricated by CVI (chemical vapor infiltration). However, SiC fiber-reinforced SiC matrix (SiC/SiC) composites are superior for the following reasons (1) mullite fiber-reinforced composites do not improve resistance to oxidation, one of the major factors limiting the use of non-oxide composites and (2) SiC fibers have mechanical properties superior to those of mullite fibers. This section will be concerned primarily with SiC fiber-reinforced ceramic composites, which offer the best oxidation resistance of any non-oxide fiber at high temperatures (particularly above 1,100°C [2012°F]). 

The thermal shock resistance of mullite-SiC whisker composites and mullite-Zr02-SiC whisker composites was evaluated by Kelly, et al. using samples prepared as described by Ruh andMazdiyasni. Thermal expansion, flexural strength and Young s modulus were... 

Direct current resistivity measurements revealed that the data are sensitive to the volume fraction of SiC whiskers, but no clear distinction can be made between measurements perpendicular versus parallel to the hot-pressing directioa The percolation threshold for SiC whiskers in mullite occurs before the 10% SiC whisker composition. 

A positive note is that there are clear approaches to the development of higher temperature, fine diameter oxide fibers. For example, YAG is the most creep resistant oxide reported [54] and therefore a good candidate for a fiber, and mullite and zircon both have good creep resistance and low thermal expansivities. There are also microstructural approaches to improving creep resistance. It is hoped that improving prospects for all of the other constituents for higher temperature composites will motivate vigorous research on improved fibers. 

Mullite. A mixture of alumina and sUica, mullite is valuable because of its low thermal expansion and resistance to spalling and deformation under load. The specific composition of a given specimen depends on the method of synthesis. The theoretical composition is 71.8 percent alumina and 28.2 percent silica, but alumina content can range as high as 77 percent or as low as 62.6 percent. 

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