Transparent and Machinable Glass-Ceramics
“Glass-ceramics” are glasses with controlled crystallization having certain extraordinary properties and therefore unique applications. Since their accidental discovery in the early 1950s of the last century, extensive research and development have been carried out leading to commercialization of several products for both consumer and strategic sectors. Glass structure being thermodynamically metastable is prone to get converted to a stable crystallized structure through a diffusion-controlled nucleation a growth mechanism. Crystallization is normally facilitated by adding a nucleating agent, the refractory oxides which do not normally dissolve in the glassy matrix. Microstructurally, glass-ceramic materials invariably contain some residual glassy phase together with one or more crystalline phases.
Glass-ceramics possess a wide range of unusual properties; they are much tougher than conventional glasses with a wide range of thermal expansion coefficient and unlike crystalline ceramics do not contain any porosity. It is easier to seal them with metallic counter parts and therefore used extensively in different kind of seals.
Glass-ceramic materials become transparent to visible light if the dispersed crystallites are much smaller than the wavelength of visible light or the difference between the refractive index of the crystallites and that of glassy matrix is very small. There are several aluminosilicate-based glass-ceramic systems in which these conditions are satisfied and therefore can be referred as “transparent glass-ceramics.” The crystal phases are solid solutions of β-quartz, β-spodumene, spinel, mullite, cordierite, etc. One of the most widely used transparent glass-ceramic products is known as Zerodur® made by Schott AG, Germany. It possesses extremely low coefficient of thermal expansion, which is very close to zero or slightly negative in certain temperature ranges. Its transparency is quite good in the range 400–2,300 nm. “Zerodur” is extensively used for lightweight mirror blanks used in large astronomical telescopes and satellites. Their size ranges from a few centimeters to more than 8 m. The most recent application of transparent glass-ceramics is, however, in the area of lighting systems based on white LEDs for which the glass-ceramics are used as the dispersion medium for the phosphors replacing commonly used organic silicone.
Certain varieties of glass-ceramics particularly containing mica crystals are fairly soft, giving rise to their machinable property. Different manufacturers market them with their trade names. The most common is the MACOR® developed and marketed by Corning Glass Works and is used extensively in different technologies; DICOR® on the other hand is primarily used to make dental crown. MACOR® contains around 55% fluorophlogopite mica (KMg3AlSi3O10F2) and 45% borosilicate glass. Complex shapes can be machined with precision dimensional tolerance, thermally stable up to a temperature of 1000 °C. They possess very good electrical as well as thermally insulating property. Combined with zero porosity, they are excellent materials for fabrication of vacuum feedthroughs. In addition, there are several other applications in electronics, aerospace, defense, and nuclear technologies. They also find extensive use in microwave tube industry.
KeywordsGlass-ceramics Glass transition Crystallization Nucleation and growth Light scattering Transparent glass-ceramics Zero thermal expansion Machinable glass-ceramics White LED
List of Abbreviations
Coefficient of thermal expansion
White light-emitting diodes
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