Zerodur^[2] is a lithium-aluminosilicate glass-ceramic^[3] produced by Schott AG since 1968.^[4] It has been used for a number of very large telescope mirrors including GTC, Keck I, Keck II,^[5] and SOFIA, as well as some smaller telescopes (such as the GREGOR Solar Telescope). With its low coefficient of thermal expansion (CTE), it is suitable for producing mirrors that maintain acceptable figures in extremely cold environments, such as deep space.^[6] Although it has advantages for applications requiring a coefficient of thermal expansion less than that of borosilicate glass, it remains very expensive as compared to borosilicate. The tight tolerance on CTE, ±0.007×10⁻⁶ K⁻¹, allows for its use in high-precision applications.

Applications

1. Optics
2. Microlithography
3. Measurement technology^[4]

Properties

Zerodur has both an amorphous (vitreous) component and a crystalline component. Its most important properties^[7] 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. This thermal expansion performance is comparatively superior to that of fused quartz by two orders of magnitude.^[8][9]
• High 3D homogeneity^[9] with few inclusions, bubbles and internal stria (as contrasted to Cer-Vit).
• Hardness similar to that of borosilicate glass, so that it can be ground and polished more easily than fused quartz.
• High affinity for coatings.
• Low helium permeability.
• Non-porous (as contrasted to sintered ceramics).
• Good chemical stability similar to that of fused quartz.
• Fracture toughness approximately 0.9 MPa·m^1/2.^[3][10]

Physical properties

• 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^[11]