Zerodur

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

The main applications for Zerodur include telescope optics in astronomy^[2] and space applications,^[3] lithography machines for microchips and displays,^[4] and inertial measurements systems for navigation.^[5][6]

In astronomy, it is used for mirror substrates in large telescopes such as the Hobby-Eberly Telescope,^[7] the Keck I and Keck II telescopes,^[8] the Gran Telescopio Canarias,^[9] the Devasthal Optical Telescope,^[10] the European Southern Observatory's 8.2 m Very Large Telescope,^[11] and the 39 m Extremely Large Telescope.^[12]

In space, it has been used for the primary mirror of SOFIA’s telescope,^[13] for the imager in Meteosat Earth observation satellites,^[14] and for the optical bench in the LISA Pathfinder mission.^[15]

In microlithography, Zerodur is used in wafer steppers and scanner machines for precise and reproducible wafer positioning.^[16][17] It is also used as a component in refractive optics for photolithography.^[18]

In inertial measurement units, Zerodur is used in ring laser gyroscopes.^[19]

Zerodur has both an amorphous (vitreous) component and a crystalline component. Its most important properties^[20] are:

• The material exhibits a particularly low thermal expansion, with a mean value of 0 ± 0.007×10⁻⁶ K⁻¹ within the temperature range of 0 to 50 °C.^[21]
• High 3D homogeneity^[21] 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·m^1/2.^[22][23]

• Dispersion: (n_F − n_C) = 0.00967
• Density: 2.53 g/cm³ at 25 °C
• Young's modulus: 9.1×10¹⁰ 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 ± 0.10×10⁻⁶/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^[24]

Schott, under Jürgen Petzoldt, began developing glass-ceramics in the 1960s, in response to demand for low expansion glass ceramics for telescopes.^[25]

In 1966, Hans Elsässer, the founding director of the Max Planck Institute for Astronomy, asked the company if it could produce large castings of almost 4 meters using low-expansion glass-ceramic for telescope mirror substrates. The order for a 3.6 m (12 ft) mirror blank, along with ten smaller mirror substrates was accepted in November 1968. Development ended by December 1969, and the mirrors were delivered by late 1975.^[25]

In 1984, the mirror substrate went into operation in a telescope at the Calar Alto Observatory in Spain. Further orders for mirror blanks subsequently followed.^[26]

• CorningWare
• Macor
• Ring laser gyroscope
• Sitall