Composition of Low-Expansion Glass Ceramics

High-quartz solid solutions are stuffed derivates of the quartz modification of Si02 [3.31]. Because the Si02 lattice allows for many substitutions, high-quartz solid solutions in glasses can have rather complicated compositions, which may vary with the ceramization conditions. According to Petzoldt the composition can be described in general by [3.32]  

By variation of the composition of the h-quartz s.s. the CTE can be adjusted to a low absolute value for a particular temperature range, for example, less than a2o,7oo 0.15 x 10 K between 20 and 700°C for use as cooktop panels. 

commercial low-expansion glass ceramics contain at least about eight oxides, each one having a particular function. Table 3.5 shows the typical composition range and the major role of the constituents of low-expansion glass ceramics. 

Alumina has to be kept above about 18 wt% in order to achieve a low CTE and a transparent glass ceramic. In order to avoid the detrimental formation of mullite during glass forming, the alumina content should not exceed about 25%. 

The Si02 level is more or less constrained by that of the other components. A high silica level leads to an undesired highly viscous and seedy glass, whereas a lower content, coupled with a high alumina level, enhances the probability of mullite formation. 

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