Base glass microstructures

This formula demonstrates that Li", Mg ", Zn, and Al ions are incorporated into the structure of P-quartz. As a result, the P-quartz is frozen in the structure of the glass-ceramic in a metastable state at room temperature. In addition, AlPO structural units are incorporated. These units are iso-morphous with Si02 and replace the [SiO J tetrahedra in the P-quartz solid solution structure. The microstructure of this glass-ceramic was studied by Maier and Muller (1987). It is evident that the P-quartz solid solution crystals measure approximately 100 nm. This microstructure was developed by heat treating the base glass at 700 C for 4 hours. Subsequently, the base glass was heat treated at 900°C for 10 minutes. Surprisingly, this relatively short heat treatment produced the fine crystals. 

Glass-ceramics with a leucite main crystal phase (Section 2.2.9) are also produced according to the mechanism of controlled surface crystallization of the opal base glass since volume nucleation cannot be controlled. In this case, it is important for the crystallites to achieve a high nuclei density and to uniformly precipitate into the glassy matrix. The coast-and-island microstructure has been specifically developed as the transitional stage (Holand et al., 1996b). 

Figure 3-11 TEM replica of the microstructure of the base glass, showing a phase-separated microstructure and an almost-flat Mo03 crystal test area.

Figure 3-13 House-of-cards microstructure following heat treatment of the base glass at 980°C/1 h.

Two materials are considered here to illustrate the effects of microstructure on crack extension. The first is a cordierite (2Mg0.2Al203.5Si02) based glass-ceramic contain-... 

The idea to produce ceramic-like materials with a fine microstructure by controlled devitrificaton of base glasses was soon extended to procedures other than the controlled volume nucleation and crystallization of base glasses. Relatively fine-grained glass ceramics can also be obtained by sintering and crystallization of glass powders to dense bodies. 

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