Zerodur

Zerodur®, a registered trademark of Schott Glass Technologies, is a lithium aluminosilicate glass-ceramic produced by Schott AG since 1968. It has been used for a number of very large telescope mirrors including Keck I and Keck II. With its very low coefficient of expansion it can be used to produce mirrors which retain acceptable figures in extremely cold environments such as deep space. Although it has advantages for applications requiring COE less than that of borosilicate glass it remains very expensive as compared to borosilicate.

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

• Low thermal expansion (≈0.2 × 10⁻⁷/K at 0 °C–50 °C) which is an order of magnitude better than that of fused quartz
• High 3D homogeneity with few inclusions, bubbles and internal stria - (as contrasted to Cer-Vit)
• Hardness similar to 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 x m^1/2

Applications

The Keck II Telescope showing the segmented primary mirror made of Zerodur

1. Optics
2. Microlithography
3. Measurement technology

Properties

• Dispersion: (nf − nc) = 0.00967
• Density: 2.53 g/cm³ at 25 °C
• Young's Modulus: 9.1 x 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 temperature: 600 °C
• Impact resistance behavior is substantially similar to other glass

See also

• CorningWare
• Macor
• Ring laser gyroscope
• Sitall