Separation glass phases

Aluminosilicate ash (Class F) Quart/ and unburm carbon Melt, mullite. Ie4),. Fed), Glass-in-glass phase separation secondary mullite... 

Other Aluminosilicates, Transparent mullite glass-ceramics can be produced from modified binary Al C —Si02 glasses (21). In these materials, the bulk glass phase separates into tiny alumina-rich droplets in a siliceous matrix. Further heat treatment causes these droplets to crystallize to mullite spherulites less than 0.1 Jim in size. When doped with ions such as Cr3+, transparent mullite glass-ceramics can be made to absorb broadly in the visible while fluorescing in the near-ii (22,23), thereby making them potentially useful for luminescent solar collectors. 

Figure 12.07. Electron micrographs of glasses phase separated by spinodal (left) and nucleation and growth (right) mechanisms. (After Yamane and Asahara, 2000).

During heat treatment Zerodur base glass phase separates first, Fig. 2.24, and later small nuclei of about 5 nm in size precipitate. Fig. 2.25. If the temperature is not increased too rapidly, a large number of nuclei are formed before any crystal nucleation and growth of h-quartz s.s. is initiated. If this condition is observed during ceramization, then a high density of small h-quartz s.s. crystals can be obtained. Crystal densities for the h-quartz s.s. crystals are estimated to be about 10 cm, whereas the density of the nuclei is not known but may be even higher. 

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