Controlled Crystallization of Glass

Crystalline Silica. Quartz sand is of course the principal raw material for the production of glass (qv). Cristobalite and 3-quartz are used in glass ceramics (qv), ie, ceramics produced by the controlled crystallization of glass. Silica is a main constituent of ceramics (qv). For example, refractory silica brick containing small amounts of A O is used as roof brick for open-hearth furnaces at temperatures >1600° C (see Refractories). Silica sand or flour (ground quartz) is the raw material for soluble silicates, such as sodium silicate, which is consistently ranked as one of the top 50 U.S. industrial chemicals (98) (see Silicon compounds, synthetic inorganic silicates). 

Glass-ceramics are an important class of materials that have been commercially quite successful. They are polycrystalline materials produced by the controlled crystallization of glass and are composed of randomly oriented crystals with some residual glass, typically between 2 and 5 percent, with no voids or porosity. 

G. Lusvardi, G. Malavasi, L. Menabue, M.C. Menziani, A. Pedone, U. Segre, A computational tool for the prediction of crystalline phases obtained from controlled crystallization of glasses. J. Phys. Chem. B 109(46), 21586-92 (2005)... 

Controlled crystallization of glass samples has been tried with typical annealing temperatures of 15-80°C above Tg and characterized by scanning electron microscopy (SEM) (Fig. 1.2), XRD (Fig. 1.3), and visible and IR transmission spectra (Fig. 1.4). 

Figure 3.17 Schematic time-temperature cycle for the controlled crystallization of a glass-ceramic body.

Transport through Lacking control devitrification of glass Important for glass crystallization e.g., after... 

Glass-ceramics are formed by controlled crystallization of a glass. They consist of a high density (maybe >95 vol%) of small crystals in a glass matrix. The important feature of the processing of glass-ceramics is that the crystallization must be controlled. As usual, crystallization occurs in two stages ... 

Nano-structures can be synthesized by controlled crystallization of metallic glasses also known as de-vitrification method. 

The basic process development for the production of P-quartz solid solution glass-ceramics was conducted by Corning Glass Works. The key to the controlled crystallization of P-quartz solid solution was soon shown to be controlled nucleation, in other words, the correct selection of the nucleating agents and their concentration. 

The controlled crystallization of mica in glass to produce glass-ceramics, however, presents a particular challenge for materials engineers. Mica alone, demonstrating the basic structural formula o(Xq j F2 3 (0H,F)2, is a... 

As stated at the beginning of this book and supported by a number of examples, controlled crystallization of several main crystal phases has been shown to permit the combination of different properties in the development of glass-ceramics, according to special requirements. 

Controlled crystallization of the base glass took place at temperatures between 700 C and 950°C. The formation of magnetite crystals (FcgO ) was determined as the primary phase. The first primary crystals measured 6-30 nm. At heat treatment above 1000 C, the magnetite was converted to hematite (a-Fe202). 

Metaphosphate glasses are of particular interest in the development of glass-ceramics and glass-ceramic fibers (Griffith 1995) by the controlled crystallization of phosphate base glasses. These glasses are composed of (PO ) tetrahedra chains. In chemical terms, these structural units must be called condensed phosphates. 

A NaP03 type metaphosphate glass was used to investigate controlled crystallization and controlled surface crystallization in particular (Gutzow 1987). The stress-strain relations that influence surface crystallization of glasses were also examined in this glass (see Section 1.4). 

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