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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.

The spectroscopic properties of Ni(2+)- doped nanocrystalline glass-ceramic fibers are reported. The cerammed fibers show strong fluorescence with peak wavelength at 1250 nm, 3-dB bandwidth at ~250nm, measured lifetimes at >1ms, and low-fluorescence saturation powers (~35mW) for 980-nm diode pumping. Current diode-pumped output powers are ~100microW .

Thin-film polysilicon solar cells on foreign substrates are often considered as a promising low cost alternative to bulk silicon solar cells. Until now however, the obtained efficiencies and open-circuit voltages are far below those of other technologies. In this paper, we show how the open-circuit voltage can be enhanced significantly using an amorphous silicon ? crystalline silicon heterojunction emitter instead of a diffused homojunction emitter. Open-circuit voltages up to 536 mV were obtained for polysilicon layers with a heterojunction emitter. This is the highest open-circuit voltage obtained for polysilicon solar cells with a p-n structure on foreign substrates.

Efficient thin-film polycrystalline-silicon (pc-Si) solar cells on foreign substrates could lower the price of photovoltaic electricity. Aluminum-induced crystallization (AIC) of amorphous silicon followed by epitaxial thickening at high temperatures seems a good way to obtain efficient pc-Si solar cells. Due to its transparency and low cost, glass is well suited as substrate for pc-Si cells. However, most glass substrates do not withstand temperatures around 1000°C that are needed for high-temperature epitaxial growth. In this paper we investigate the use of experimental transparent glass-ceramics with high strain-point temperatures as substrates for pc-Si solar cells. AIC seed layers made on these substrates showed in-plane grain sizes up to 16 μm. Columnar growth was observed on these seed layers during high-temperature epitaxy. First pc-Si solar cells made on glass-ceramic substrates in substrate configuration showed efficiencies up to 4.5%, fill factors up to 75% and open-circuit voltage (Voc) values up to 536 mV. This is the highest Voc reported for pc-Si solar cells on glass and the best cell efficiency reported for cells made by AIC on glass.

Summary from only given. The dramatic improvement in the spectroscopic properties obtained by using a nanocrystalline environment for the active ion is clearly worth the trouble of using a glass-ceramic fiber. This, coupled with the improved formability of a glass-ceramic over a single crystal host, is the driving force behind the work presented here.

Transition metals are well known as optically active dopants in crystalline hosts because they fluoresce broadly in the near-infrared; examples include Ti3:sapphire (A1203) and Cr4:forsterite (Mg2SiO4). Because transition element ions yieldstronger optical activity in a crystalline, rather than amorphous, environment, glass-ceramics can potentially combine theadvantages of glass (such as fiberizability) with those of doped crystals (such as broadband optical gain or absorption.) Theglass-ceramic process can, in many cases, also allow the growth in glass of crystals which are difficult or impossible toobtain in single crystal form. Material properties and spectroscopic data are presented for transparent glass-ceramics basedon transition element-doped forsterite, willemite, mixed Li-Zn-Mg-orthosilicates, and spine! crysta!s. The transitionelement ions include Cr4 in tetrahedral coordination and Ni2 in octahedral coordination. The optical propertymeasurements of the transparent glass-ceramics, including absorbance, fluorescence, and fluorescence lifetimes, duplicatethose reported in the literature for single crystals or powders. Cr4- and Ni2tdoped glass-ceramics exhibit broad emissionacross the telecommunications wavelength range of 1 1 00- 1 700 nm.Keywords: Glass-ceramic, transition metal, fluorescence

Summary form only given. Glass-ceramics are two-phase systems consisting of a base glass within which crystals are grown by heat treatment, a process known as ceramming. Recent progress on fiberization of these materials led to the demonstration of a rare earth doped fiber laser and amplifier with an efficiency uncompromised by the presence of the crystals embedded within the core of the single mode fiber. Transitions metal ions are notoriously inefficient when incorporated into a glass host, due to the difficulty of controlling the local environment, to which they are very sensitive. However to date, the properties of Cr/sup 4+/-doped glass-ceramic systems have received little attention, despite the apparent advantages of crystal site control. This paper investigates the fluorescence properties of Cr/sup 4+/-doped forsterite glass-ceramics and shows that many of the important characteristics of the single crystal are reproduced in the glass-ceramic system. Furthermore, many of the spectroscopic parameters are also observed in glass-ceramic fiber, highlighting the potential for a new class of fiber laser and amplifier made from transition metal-doped glass-ceramics.