Crystals in glass-ceramics

Fig. 5.24. Variation of hardness Hy with volume fraction Vf of gahnite crystals in glass-ceramic.

Microstructure is of equal importance to composition. This feature is the key to most mechanical and optical properties, and it can promote or diminish the characteristics of key crystals in glass-ceramics. It is clear that microstructure, is not an independent variable. It obviously depends on the bulk composition and crystalline phase assemblage, and it also can be modified, often dramatically, by varying the thermal treatment. 

The neutral silver is formed by this reaction. During the subsequent heat treatment of the glass at approximately 600 C, colloids of metallic silver are formed (Beall 1992). This colloidal silver forms heterogeneous nuclei of approximately 80 A for the subsequent crystallization of a lithium metasilicate primary crystal phase, Li2Si03. Lithium metasilicate possesses a chain silicate structure. The crystallization of this compound proceeds dendritically. The dentritic growth of crystals in glass-ceramics is discussed in Section 3.2.4. 

The formation of P-quartz solid solution and gahnite crystals in glass-ceramics from the Si02-Al203—ZnO allowed specific properties, such as translucency and even transparency, to be combined with variable coefficients of thermal expansion of... 

FIuorosihca.tes, Compared to the simple sUicates, these crystals have more complex chain and sheet stmctures. Examples from nature iaclude hydrous micas and amphiboles, including hornblende and nephrite jade. In glass-ceramics, fluorine replaces the hydroxyl ion fluorine is much easier to iacorporate ia glass and also makes the crystals more refractory. Eour commercial fluorosUicate glass-ceramic compositions and thek properties are Usted ia Table 2. 

Silicates comprise more than 95% by weight of the earth s crust and mantle, and are widely used in glasses, ceramics, sieves, catalysts, and electronic devices. Crystals of silicates are often hard, and may show considerable extinction in their diffraction pattern, which means not only that small samples must be used, but also that ambient temperatures may be adequate for charge density studies.2... 

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). 

By comparing the emission spectra of Cr in glass-ceramics with that of Cr " in single crystals one finds that the tendency to form pairs is greater in the glass-ceramics. In mixed crystals of spinel the appearance of Cr " pairs is again significant. 

Spherulitic crystals also are often found in glass ceramics. Spherulitic growth, or branching growth from a central point, is common in viscous systems where ionic mobility is the rate-limiting step. Models developed for polymer systems apply to glass ceramics and qualitatively explain the tendency to form spherulites. ... 

Silica, the main component of silicates, is widely used as mentioned earlier. In its crystalline and noncrystalline polymorphs, silica is used industrially as a raw material for glasses, ceramics, foundry molds, in the production of silicon, and more recently in technical applications such as quartz oscillators and optical waveguides for longdistance telecommunications. Of the crystalline forms, only a-quartz is commonly used as sand or as natural and synthetic single crystals. Cristobalite is often utilized as the synthetic phase in glass-ceramics. 

A. Marotta, A. Buri, F. Branda and S. Saiello, Nucleation and crystallisation of Li20.2Si02 Glass—A DTA Study, in Advances in Ceramics, Vol. 4, Nucleation and Crystallization in Glasses, American Ceramic Society, Columbus, OH (1982), pp. 146-152. 

M, Mortier, A, Bensalah, G, Dantelle, G, Patriarche, D, Vivien, Rare-earth doped oxyfluoride glass-ceramics and fluoride ceramics Sintesis and optical properties. Opt. Mat., 29, 1263-1270 (2005), Patterning of non-linear optical crystals in glass by laser-induced crystallization, J. Am. Ceram. Soc. 90, 699-705 (2007),... 

W. Pannhorst, Nucleation and Crystallization in Glass and Liquids, ed. M. Weinberg, American Ceramics Society, WesterviUe.OH261 (1993). 

D.R.Stewart, Advances in Nucleation and Crystallization in Glasses, The American Ceramic. Society, 83-90 (1971). 

The two most important factors in glass ceramic production are the composition of the melt and the microstmcture of the final product. These are interrelated, of course. The composition controls the ability of the substance to form a glass with the correct viscosity and workability, as the starting solid is completely glassy in nature. Composition also controls what nuclei can form in the glass and the types of crystal that can grow. Most crystals have a definite crystal habit, and this factor greatly influences the microstructure of the final solid. 

Since the early DM work on metals with unautomated torsion pendulum systems [21], few papers have been published, perhaps because of the limited temperature range available. In 1989 Duncan presented some DM traces on common metals to 800 C [124]. Glass has had a couple of papers recently. Of the major glass companies in Europe, only Schottglas use this technique to my knowledge, and of the few published papers in the open literature Hill [35] and Boeder et al. [36] are examples. Hill showed that liquid-liquid phase separation prior to crystal nucleation and real time studies of the kinetics of crystal-ization could be studied in glass ceramics, and activation energies could be matched to... 

Many other methods of strengthening are based on formation of composites by inclusion of fibers or whiskers or by crystallization to form glass-ceramics. Phase separation may also affect strength by altering crack propagation mechanisms. Transformation toughening has also been attained by formation of a small concentration of zirconia crystals in glasses. 

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