Glass grains

A number of other phosphate-based glass-ceramic systems have been investigated. Fine-grained glass-ceramics based on crystals isostructural with NZP (NaZt O ) can be prepared from certain transition-metal orthophosphate and silicophosphate glasses (27). The extensive solid solution possible in the NZP phase provides for a wide range in thermal expansion, with coefficients ranging from —20 to 60 x 10-7 /°C. 

As opposed to composites reinforced with SiC(w), the residual stresses formed at the interface should be lower as the CTE of the matrix and the reinforcement phase are similar. Several modeling studies have shown that the composition of the grain/glass interface has a negligible effect on the magnitude of the residual stresses and therefore has no significant influence on the fracture behaviour.19... 

The phase composition of fly ash can be very different even in particular grains. Glass is a main constituent its content in the sihceous fly ash is generally higher than 80%. The crystalline phases are quartz, mrrlhte, hematite and magnetite. In Table 7.6 some examples of different fly ash composition are shown. 

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. 

B. Andrianasolo, B. Champagnon, C. Esnouf Ultrafine grained glass-ceramics obtained with Cr203 additions , J. Non. Cryst. Sol. 126, 103-110 (1990)... 

The development of the principles of nucleation and growth eady in the twentieth century (2) ultimately led to the discovery that certain nucleating agents can induce a glass to crystallize with a fine-grained, highly uniform microstmcture that offers unique physical properties (3). The first commercial glass-ceramic products were missile nose cones and cookware. 

Fig. 2. RepHca electron micrograph of the fracture surface of enstatite—P-spodumene—zirconia glass-ceramic, showing twinning in the enstatite grains...

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