Zerodur

Zerodur ( spelling of the manufacturer: Zerodur ®) is a glass ceramic material of Schott AG, which is prepared by the controlled volume crystallization.

Properties

Zerodur contains a crystalline phase and a residual vitreous phase, through which the exceptional characteristics of the material are determined. These special features include an extremely low expansion coefficient, good material homogeneity, chemical stability, long term stability and hardly varying mechanical properties.

Production

Initially prepared glass pieces are heated again for the production of Zerodur. At about 800 degrees centigrade, the molten glass form extraneous material nuclei, where with increasing temperature tiny crystals ( average size about 50 nm) grow. These have the property to contract when heated. Thus, they counteract the thermal expansion of pure glass. The art of this process, which is referred to as ceramization, is the ratio of crystal phase to glass phase adjusted so that the resulting thermal expansion of minimally, in certain temperature ranges, even equal to zero. This is the case, when about 70 weight percent of the melt present in crystalline form. The process of ceramicization can last up to several months, depending on the size of the glass blank.

Use

The material Zerodur is typically used as substrate material of optical elements in astronomy, for example, in comet probes, as a mirror substrate for modern large astronomical telescopes such as the Very Large Telescope in Chile ( four mirrors each 8.2 m in diameter), the Keck Observatory in Hawaii (two mirror each 10.0 m in diameter ) or the Gran Telescopio Canarias in La Palma ( a mirror 10.4 meters in diameter ) and used in precision optics and precision metrology. Zerodur is used because of its excellent thermal properties as a material for the frame and carrier of lithography and nano- measuring machines. An example of this is the nanopositioning and nano- measuring machine (NMM -1 ) of the Technical University of Ilmenau. For use in space Zerodur is interesting because of the possible reduction in the take-off weight over other mirror materials.

History and material variants

Zerodur is put on the market in 1968. 1971 followed ceramic, in which Zerodur glass ceramic for " Ceran" hobs used. Since the material to numerous material options such as " Zerodur K20 " has been expanded. It is produced by thermal transformation from the semitransparent starting material Zerodur, is highly thermally stable and does not change during multiple temperature cycles. "Zerodur K20 " glass ceramic has a high long -term temperature stability up to 850 ° C. The material can be used, for example for mechanical and optical components in high-power lasers or as a molding material for use in the hot-forming (glass or plastic) can be used.