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Just as the microstructures in glass-ceramics encompass the range from nanocrystalline transparent materials to microcrystalline tough materials, so the paths of microstructural evolution in glass-ceramics vary widely. Evolution can proceed in numerous ways, their genesis being a perturbation of some type, including the surface nucleation used in glass frit processing, crystallization of the primary phase or phases upon distinct crystalline nuclei, and nucleation promoted by nano- or micro-scale amorphous phase separation in the parent glass. Examples of the crystallization history of several glass-ceramic materials are described, with emphasis on how their microstructural evolution influences their ultimate physical and optical properties.

Future applications for glass-ceramics are likely to capitalize on designed-in, highly specialized properties for the transmission, display, and storage of information. Glass-ceramics with microstructures comprised of uniformly dispersed crystals

Transparent, refractory glass-ceramics based on 10 nm crystals of spinel solid solution in a highly siliceous residual glass can be produced from compositions in the SiO 2 –Al 2 O 3 –ZnO–MgO–TiO 2 –ZrO 2 system. The glass-ceramics possess excellent chemical durability as well as thermal expansion coefficients of 35–40 × 10 −7 /°C and strain point temperatures of over 900°C. Titania serves as both an extremely effective nucleating agent in these glasses and as an integral component of the spinel crystals. The materials are designed such that fluxes in the glass partition into the spinel crystals during the crystallization process, leaving a continuous residual glass very high in silica. The strain points of the resultant glass-ceramics approach those of vitreous silica and quartz materials which require significantly more expensive manufacturing processes. These glass-ceramics are excellent substrate candidates for high temperature, high quality polysilicon thin films. Potential products include solar cells and active matrix liquid crystal displays.

Strong and tough glass-ceramic materials based on the complex chain silicate miserite (KCa5open square(Si2O7)(Si6O15)F) have been prepared using standard frit sintering and internal nucleation processes. The miserite may be accompanied by crystals of fluorite, cristobalite, xonotlite, a canasite-like phase, and fluorapatite. The highly crystalline glass-ceramics have a microstructure composed primarily of interlocked, lath- or log-shaped miserite crystals with pronounced cleavage planes. This microstructure provides abraded flexural strength values as high as 235 MPa (34 000 psi) and fracture toughness values >3.0 MPa·m1/2. These strength and toughness values are quite high for a glass-derived material. Miserite glass-ceramics may be useful for many applications in which such strength and toughness are desired.

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Nanocrystalline glass-ceramics with high elastic modulus and moderate strength and toughness can be produced from a wide area in the SiO 2 -Al 2 O 3 -MgO-ZnO-TiO 2 system. Phase assemblages include those based primarily on spinel crystals as well as those based on enstatite and spinel; common accessory phases include Mg-petalite, Mg,Al-titanate solid solutions, quartz solid solutions and rutile. These glass-ceramics can have ultra-fine grain sizes, with spinel crystals less than 25 nm and enstatite crystals less than 100 nm in size. Such nanocrystalline microstructures are unique in the case of an enstatite glass-ceramic and provide extremely smooth polished surfaces, with average roughness of about 5-10 A (0.5-1.0 nm). These materials provide strength and toughness values higher than might otherwise be expected from such a fine-grained microstructure, due in part to the toughening effect of the lamellar twinning in the enstatite phase. The morphology of the initial phase separation provides a template for the ultimate crystalline microstructure, essentially locking in its nanocrystalline nature such that regardless of heat treatment crystals grow no larger than 200 nm. © 1997 Elsevier Science B.V.