Glass-ceramics are polycrystalline materials produced through controlled crystallization of base glass, producing a fine uniform dispersion of crystals throughout the bulk material. Crystallization is accomplished by subjecting suitable glasses to a carefully regulated heat treatment schedule, resulting in the nucleation and growth of crystal phases. In many cases, the crystallization process can proceed to near completion, but in a small proportion of processes, the residual glass phase often remains. Glass-ceramic materials share many properties with both glasses and ceramics. Glass-ceramics have an amorphous phase and one or more crystalline phases and are produced by a so-called "controlled crystallization" in contrast to a spontaneous crystallization, which is usually not wanted in glass manufacturing. Glass-ceramics have the fabrication advantage of glass, as well as special properties of ceramics. When used for sealing, some glass-ceramics do not require brazing but can withstand brazing temperatures up to 700 °C. Glass-ceramics usually have between 30% [m/m] and 90% [m/m] crystallinity and yield an array of materials with interesting properties like zero porosity, high strength, toughness, translucency or opacity, pigmentation, opalescence, low or even negative thermal expansion, high temperature stability, fluorescence, machinability, ferromagnetism, resorbability or high chemical durability, biocompatibility, bioactivity, ion conductivity, superconductivity, isolation capabilities, low dielectric constant and loss, corrosion resistance, high resistivity and break-down voltage. These properties can be tailored by controlling the base-glass composition and by controlled heat treatment/crystallization of base glass. In manufacturing, glass-ceramics are valued for having the strength of ceramic but the hermetic sealing properties of glass. Glass-ceramics are mostly produced in two steps: First, a glass is formed by a glass-manufacturing process, after which the glass is cooled down. Second, the glass is put through a controlled heat treatment schedule. In this heat treatment the glass partly crystallizes. In most cases nucleation agents are added to the base composition of the glass-ceramic. These nucleation agents aid and control the crystallization process. Because there is usually no pressing and sintering, glass-ceramics have no pores, unlike sintered ceramics. A wide variety of glass-ceramic systems exist, e.g., the Li2O × Al2O3 × nSiO2 system (LAS system), the MgO × Al2O3 × nSiO2 system (MAS system), and the ZnO × Al2O3 × nSiO2 system (ZAS system).
History Réaumur, a French chemist, made early attempts to produce polycrystalline materials from glass, demonstrating that if glass bottles were packed into a mixture of sand and gypsum, and subjected to red heat for several days, the glass bottles turned opaque and porcelain-like. Although Réaumur was successful in the conversion of glass to a polycrystalline material, he was unsuccessful in achieving the control of the crystallization process, which is a key step in producing true practical glass ceramics with the improved properties mentioned above. The discovery of glass-ceramics is credited to a man named Donald Stookey, a renowned glass scientist who worked at Corning Inc. for 47 years. The first iteration stemmed from a glass material, Fotoform, which was also discovered by Stookey while he was searching for a photo-etch-able material to be used in television screens. Soon after the beginning of Fotoform, the first ceramic material was discovered when Stookey overheated a Fotoform plate in a furnace at 900 degrees Celsius and found an opaque, milky-white plate inside the furnace rather than the molten mess that was expected. While examining the new material, which Stookey aptly named Fotoceram, he took note that it was much stronger than the Fotoform that it was created from as it survived a short fall onto concrete. In the late 1950s two more glass-ceramic materials would be developed by Stookey, one found use as the radome in the nose cone of missiles, while the other led to the line of consumer kitchenware known as Corningware. Corning executives announced Stookey's discovery of the latter "new basic material" called Pyroceram which was touted as light, durable, capable of being an electrical insulator and yet thermally shock resistant. At the time, there were only few materials which offered the specific combination of characteristics that Pyroceram did and the material was rolled out as the Corningware kitchen line August 7, 1958. Some of the success that Pyroceram brought inspired Corning to put an effort towards strengthening glass which became an effort by the technical director's of Corning titled Project Muscle. A lesser known "ultrastrong" glass-ceramic material developed in 1962 called Chemcor (now known as Gorilla Glass) was produced by Corning's glass team due to the Project Muscle effort. Chemcor would even be used to innovate the Pyroceram line of products as in 1961 Corning launched Centura Ware, a new line of Pyroceram that was lined with a glass laminate (invented by John MacDowell) and treated with the Chemcor process. Stookey continued to forge ahead in the discovery of the properties of glass-ceramics as he discovered how to make the material transparent in 1966. Though Corning would not release a product with his new innovation, for fear of cannibalizing Pyrex sales, until the late 1970s under the name Visions.
Nucleation and crystal growth The key to engineering a glass-ceramic material is controlling the nucleation and growth of crystals in the base glass. The amount of crystallinity will vary depending on the amount of nuclei present and the time and temperature at which the material is heated. It is important to understand the types of nucleation occurring in the material, whether it is homogeneous or heterogeneous. Homogeneous nucleation is a process resulting from the inherent thermodynamic instability of a glassy material. When enough thermal energy is applied to the system, the metastable glassy phase begins to return to the lower-energy, crystalline state. The term "homogeneous" is used here because the formation of nuclei comes from the base glass without any second phases or surfaces promoting their formation. The rate of homogenous nucleation in a condensed system can be described with the following equation, proposed by Becker in 1938.
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