ELT secondary mirror blank successfully cast

Zerodur (notation of the manufacturer: ZERODUR®),
registered trademark
of

Schott AG Schott AG is a German multinational glass company specializing in the manufacture of glass and glass-ceramics. Headquartered in Mainz, Germany, it is owned by the Carl Zeiss Foundation. The company's founder and namesake, Otto Schott, is cre ...

, is a lithium-

aluminosilicate Aluminosilicate minerals ( IMA symbol: Als) are minerals composed of aluminium, silicon, and oxygen, plus countercations. They are a major component of kaolin and other clay minerals. Andalusite, kyanite, and sillimanite are naturall ...

glass-ceramic 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 ...

produced by

since 1968. It has been used for a number of very large telescope mirrors including GTC, Keck I, Keck II, and

SOFIA Sofia ( ; bg, София, Sofiya, ) is the capital and largest city of Bulgaria. It is situated in the Sofia Valley at the foot of the Vitosha mountain in the western parts of the country. The city is built west of the Iskar river, and h ...

, as well as some smaller telescopes (such as the

GREGOR Solar Telescope GREGOR is a solar telescope, equipped with a 1.5 m primary mirror, located at 2,390 m altitude at the Teide Observatory on Tenerife in the Canary Islands. It replaces the older Gregory Coudé Telescope and was inaugurated on May 21, 2012. First ...

). With its very low

coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...

(CTE) it can be used to produce mirrors that retain acceptable

figures Figure may refer to: General *A shape, drawing, depiction, or geometric configuration * Figure (wood), wood appearance *Figure (music), distinguished from musical motif * Noise figure, in telecommunication * Dance figure, an elementary dance patt ...

in extremely

cold Cold is the presence of low temperature, especially in the atmosphere. In common usage, cold is often a subjective perception. A lower bound to temperature is absolute zero, defined as 0.00K on the Kelvin scale, an absolute thermodynamic ...

environments such as deep space. Although it has advantages for applications requiring a

less than that of

borosilicate glass Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), m ...

, it remains very expensive as compared to borosilicate. The tight tolerance on CTE, ±0.007 K⁻¹, allows for its use in high-precision applications.

Applications

# Optics #

Microlithography Microlithography is a general name for any manufacturing process that can create a minutely patterned thin film of protective materials over a substrate, such as a silicon wafer, in order to protect selected areas of it during subsequent etchin ...

# Measurement technology

Properties

Zerodur has both an amorphous (vitreous) component and a

crystalline A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

component. Its most important properties are: * Particularly low

thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...

: in the range 0 to 50 °C it has a mean of 0 ± 0.007 K⁻¹, which is two orders of magnitude better than that of fused quartz.
/ref> * High 3D Homogeneity (physics), homogeneity with few inclusions, bubbles and internal stria (as contrasted to Cer-Vit). * Hardness similar to that of

, so that it can be ground and polished more easily than

fused quartz Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses in which other ingredients are added which change ...

. * High affinity for

coatings A coating is a covering that is applied to the surface of an object, usually referred to as the substrate. The purpose of applying the coating may be decorative, functional, or both. Coatings may be applied as liquids, gases or solids e.g. Powd ...

. * Low helium permeability. * Non-porous (as contrasted to sintered ceramics). * Good chemical stability similar to that of

. * Fracture toughness approximately 0.9 MPa·m^1/2.

Physical properties

Dispersion Dispersion may refer to: Economics and finance * Dispersion (finance), a measure for the statistical distribution of portfolio returns * Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variat ...

: (''n''_F − ''n''_C) = 0.00967 *

Density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

: 2.53 g/cm³ at 25 °C *

Young's modulus Young's modulus E, the Young modulus, or the modulus of elasticity in tension or compression (i.e., negative tension), is a mechanical property that measures the tensile or compressive stiffness of a solid material when the force is applied le ...

: 9.1 Pa *

Poisson ratio In materials science and solid mechanics, Poisson's ratio \nu ( nu) is a measure of the Poisson effect, the deformation (expansion or contraction) of a material in directions perpendicular to the specific direction of loading. The value of Poi ...

: 0.24 *

Specific heat capacity In thermodynamics, the specific heat capacity (symbol ) of a substance is the heat capacity of a sample of the substance divided by the mass of the sample, also sometimes referred to as massic heat capacity. Informally, it is the amount of heat t ...

at 25 °C: 0.196 cal/(g·K) = 0.82 J/(g·K) *

Coefficient of thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kineti ...

(20 °C to 300 °C) : 0.05 ± 0.10/K *

Thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...

: at 20 °C: 1.46 W/(m·K) * Maximum application temperature: 600 °C * Impact resistance behavior is substantially similar to other

glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling ( quenching ...

References

External links

{{Glass makers and brands Glass types Low-expansion glass Low thermal expansion materials Glass-ceramics Glass trademarks and brands Transparent materials German brands

Applications

Properties

Physical properties

See also

References

External links