User:Elisabeth at SCHOTT/Zerodur



Zerodur is a lithium-aluminosilicate glass-ceramic manufactured by Schott AG. Zerodur has a near zero coefficient of thermal expansion (CTE), which is used for high-precision applications in telescope optics, microlithography machines and inertial navigation systems.

History
Schott began developing glass-ceramics in the 1960s. In 1966, Hans Elsässer, the founding director of the Max Planck Institute for Astronomy, asked the company if it could cast an almost 4m telescope mirror substrate from a new low-expansion glass-ceramic it had been developing. In May 1969, the Max Planck Institute for Astronomy ordered 11 mirror substrates, including a 3.6m mirror blank.

Since the late 1960s, Zerodur has also been applied in integrated circuit and flat panel display lithography used to produce computer chips and displays, precision measurement equipment, and ring laser gyroscopes in navigation systems. Additionally, it has been used in Earth observation satellites and space telescopes.

Applications


The main applications for Zerodur include telescope optics in astronomy and space  applications, lithography machines for microchips and displays, and inertial measurements systems for navigation.

In astronomy, it is used for mirror substrates in large telescopes such as the Hobby-Eberly Telescope, the Keck I and Keck II telescopes, the Gran Telescopio Canarias, the Devasthal Optical Telescope, the European Southern Observatory's 8.2 m Very Large Telescope, and the 39 m Extremely Large Telescope.

In space, it has been used for the primary mirror of SOFIA’s telescope, for the imager in Meteosat Earth observation satellites, and for the optical bench in the LISA Pathfinder mission.

In microlithography, Zerodur is used in wafer steppers and scanner machines for precise and reproducible wafer positioning. It is also used as a mirror substrate material in refractive optics for EUV lithography.

In inertial measurement units, Zerodur is used in ring laser gyroscopes.

Properties
Zerodur has both an amorphous (vitreous) component and a crystalline component. Its most important properties are:
 * Near zero coefficient of thermal expansion, with a mean value of 0 ± 0.007×10−6 K−1 within the temperature range of 0 to 50 °C.
 * High 3D homogeneity with few inclusions, bubbles and internal stria.
 * Hardness similar to that of borosilicate glass.
 * High affinity for coatings.
 * Low helium permeability.
 * Non-porous.
 * Good chemical stability.
 * Fracture toughness approximately 0.9 MPa·m1/2.

Physical properties

 * Dispersion: (nF − nC) = 0.00967
 * Density: 2.53 g/cm3 at 25 °C
 * Young's modulus: 9.1 Pa
 * Poisson ratio: 0.24
 * Specific heat capacity at 25 °C: 0.196 cal/(g·K) = 0.82 J/(g·K)
 * Coefficient of thermal expansion (20 °C to 300 °C) : 0.05 ± 0.10/K
 * Thermal conductivity: at 20 °C: 1.46 W/(m·K)
 * Maximum application temperature: 600 °C
 * Impact resistance behavior is substantially similar to other glass