Glass ceramic

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Typical glass ceramic hob

Glass ceramic , also known as vitro ceramic , is a group of materials that consists of a polycrystalline and a glassy phase .

Glass ceramic has a structure similar to ceramic , but is manufactured differently. In a glass , crystallites grow in a controlled manner with nucleating agents (partial crystallization ). The workpieces are cast as with glasses , followed by a heat treatment.

Manufacturing and history

A glass ceramic is created from a glass melt in which crystal growth is specifically promoted. For this purpose, various oxides or carbonates are melted at high temperature and mixed homogeneously. When it cools down, a glass is created in which the formation of crystalline areas has been classified as disturbing and procedural errors. For further processing into glass ceramics, the smallest crystallites, so-called nuclei, are produced by tempering in a first step. They arise at relatively low temperatures and are only a few nanometers (millionths of a millimeter) in size. This controlled crystallization was first achieved by Stanley Donald Stookey in the 1950s at the Corning glass works in the USA by promoting nucleation by adding titanium oxide to the raw materials. In a second step, the crystallites are allowed to grow at a slightly higher temperature, so that they ultimately make up 30 to 95 percent of the material mass (the rest remains amorphous). This proportion as well as the size and also the shape of the crystallites can be adjusted via the temperature profile. The chemical composition of the melt and, to a certain extent, the tempering treatment can even determine the crystal structure. Thus, unlike sintered ceramics, the entire microscopic structure of the material can be controlled regardless of shape and compression. In 1958, the first glass-ceramic cooking pots were made from the material, and the first glass-ceramic hobs were brought onto the market by Schott in 1972. The glass ceramic material Zerodur, which is characterized by its very low thermal expansion, was developed at the end of the 1960s as a basic mirror material for large telescopes. In 1973, the first 3.5 m mirror was cast from the material.

Properties and uses

There are many different glass-ceramic systems. Some of the most important are the MgO x Al 2 O 3 x nSiO 2 system (MAS system), the ZnO x Al 2 O 3 x nSiO 2 system (ZAS system), glass ceramics made from lithium disilicate and glass ceramics with phlogopite as Basic system.

By far the most important system, however, uses lithium oxide , aluminum oxide and silicon dioxide as the main components . This system, which is most important for the glass ceramic industry, is also known as the LAS system and exists in many variations. Zirconium (IV) oxide in combination with titanium (IV) oxide is usually added as an epitaxial nucleating agent. Hummel and Smoke have carried out fundamental work on the main crystal phases found in this system, a high-quartz mixed crystal (HQMK) and a keatite mixed crystal (KMK).

Glass ceramics of the LAS system with HQMK as the main crystal phase have a very good thermal shock resistance due to their low coefficient of thermal expansion of around 0.1 · 10 −6 1 / K (in the range from 20 to 700 ° C). If the chemical composition of the pure Li 2 O x Al 2 O 3 x nSiO 2 system is n> 3.5, the HQMK is converted into keatite mixed crystal from around 950 ° C. The phase change is irreversible and reconstructive, i.e. coupled with the breaking of bonds. Nevertheless, as Li was able to show, the structures of the two crystal phases are very similar. After the conversion, the coefficient of thermal expansion of the glass ceramic increases to around 1 · 10 −6 1 / K (in the range of 20 to 700 ° C) due to the higher coefficient of thermal expansion of the KMK .

Characteristic of this glass ceramic, which is to be understood as a composite material made of glass and crystals, is therefore a very low or even negative coefficient of thermal expansion in different temperature ranges, which prevents breakage due to thermal shock. Glass ceramics with excellent thermal shock properties and also very good mechanical strength can therefore be produced with these phases. The thermal expansion coefficient of a glass ceramic can be adapted to a wide variety of requirements through the quantitative ratio of the glass phase to the crystal phase.

Applications arise in many ways as a material for laser gyroscopes or as protective glasses with high thermal shock resistance, as well as in the household sector as a hob and cookware. Mirror carriers for large telescopes are now made from glass ceramics, as are high-performance reflectors for digital projectors. In laboratories, glass ceramic plates have replaced asbestos wire nets as a base when heated.

The largest manufacturers of glass ceramics - Schott , Nippon Electric Glass and Corning (Eurokera) - have mainly focused on these applications. Well-known are product names such as Ceran and KeraBlack in the field of hobs and Zerodur for telescopic mirror supports and those of the transparent glass ceramic Robax and Pyroceram for fireplace panels, as well as Firelite and Neoceram.

See also

  • Fortadur , fiber reinforcement in glass ceramic
  • MACOR , a machinable glass ceramic

Individual evidence

  1. ^ Armin Petzold: Inorganisch-nichtmetallische Werkstoffe , Springer-Verlag Vienna 1981, page 130f
  2. ↑ Spectrum of Science: New Glass Ceramic Material Class - Spectrum of Science , accessed on August 23, 2018
  3. ↑ Wolfram Höland: glass ceramic . vdf Hochschulverlag AG, 2006, ISBN 978-3-8252-2813-2 ( limited preview in the Google book search).
  4. ↑ Fraunhofer IKTS: Glass ceramics: When scientists swing the wooden spoon on the ceramic hob - Fraunhofer IKTS , accessed on August 23, 2018
  5. ^ Helmut A. Schaeffer, Roland Langfeld: Material glass - old material with a great future . Springer-Verlag, 2013, ISBN 978-3-642-37231-5 , p. 74 ( limited preview in Google Book search).
  6. ↑ Hummel FA: Thermal expansion properties of some synthetic lithia minerals , Journal of the American Ceramic Society , 1951, Volume 34 (8), pp. 235-239.
  7. ↑ Smoke EJ: Ceramic compositions having negative linear thermal expansion , Journal or the American Ceramic Society, 1951, Volume 34, pp. 87-90.
  8. ↑ Li CT: Transformation mechanism between high-quartz and keatite phases of LiAlSi 2 O 6 composition , Acta Crystallographica , 1971, B27, pp. 1132-1140; doi : 10.1107 / S0567740871003649 .

literature

  • PW McMillan: The glass phase in glass-ceramics , Glass Technology, 1974, Vol. 15 (1), pp. 5-15.
  • H. Bach (ed.): Low thermal expansion glass ceramics , Springer Verlag 1995.

Web links