Glass-ceramic base

Certain glass-ceramic materials also exhibit potentially useful electro-optic effects. These include glasses with microcrystaUites of Cd-sulfoselenides, which show a strong nonlinear response to an electric field (9), as well as glass-ceramics based on ferroelectric perovskite crystals such as niobates, titanates, or zkconates (10—12). Such crystals permit electric control of scattering and other optical properties. 

Glass-Ceramics Based on Silicate Crystals. The principal commercial glass-ceramics fall into this category. These can be grouped by composition, simple siUcates, fluorosiUcates, and aluminosihcates, and by the crystal stmctures of these phases. 

Oxides. Although not widespread commercially, glass-ceramics consisting of various oxide crystals in a matrix of siUceous residual glass offer properties not available with mote common siUcate crystals. In particular, glass-ceramics based on spinels and perovskites can be quite refractory and can yield useful optical and electrical properties. 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

Borates. Litde work has been carried out in this area, but some frit-derived glass-ceramics based on alkaline-earth aluminoborate (RAI2B2O2)... 

Amperometric cells, sensors using, 22 271 Amperometric measurements, 14 612 Amphetamine, 3 89-90 Amphibole asbestos, 1 803 3 288 crystal structure, 3 297-298 exposure limits, 3 316 fiber morphology, 3 294-295 silicate backbone, 3 296 Amphibole potassium fluorrichterite, glass- ceramics based on, 12 637 Amphiphile-oil-water-electrolyte phase diagram, 16 427-428 Amphiphile-oil-water phase diagrams,... 

Non-SI (unacceptable and obsolete) units, l xxiv-xxv 2-26 xxii-xxiii Nonsilicate crystals, glass-ceramics based on, 12 641-642... 

Glass-ceramics based on natural or synthetic basalts, various zeolites, and apatite/britholite were also developed (Saidl Ralkova 1966 Hayward 1988 Wronkiewicz et al. 1996 Sinkler... 

Chemical Properties. The chemical durability is a function of the durability of the crystals and the residual glass. Generally, highly siliceous glass-ceramics with low alkali residual glasses, such as glass-ceramics based on quartz and (3-spodumene, have excellent chemical durability and corrosion resistance similar to that obtained in borosilicate glasses. 

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. 

Glass-ceramics based on the LijO-AljC -SiC can be tailored, principally through varying the alumina content, to have linear thermal expansivities in the range from close to zero to approximately 18 MK-1. The low expansion materials have excellent resistance to thermal shock whilst those with the higher expansivities can be successfully joined to a range of metals. 

Many aluminosilicate glass ceramics are based on framework structures of AIO4 tetrahedra, which, when crystallized, posseses low thermal expansions. This gives the glass ceramics based on them near zero expansions and thus excellent dimensional stability, thermal shock resistance, and mechanical strength. Aluminosilicate glass ceramics are used commercially as telescope mirrors, thermally stable structures for satellites and space probes, gyroscope components, heat-resistant windows, stove tops, and cookware. 

Glass ceramics based on the Pb0-Zn0-B203-Si02 system are used as solder glasses that is, they fuse at low temperatures ( 450°C) and can bond other glasses and... 

Ambekar, P., Randhawa. J.. Bhoga. S.S. and Singh, K. (2004) Galvanic CO2 sensor with Li2O B2O3 glass ceramics based composite. Ionics, 10 (1-2). 45-9. 

Glass-ceramics are also used for dental prosthetics. These materials are primarily based on either lithium disilicate or calcium phosphate glasses. Machinable glass-ceramics based on fluoromica phases have also been developed for machined inlays. All of these materials must be stained to match the surrounding teeth to be acceptable to the patient. 

---