Chemistry:Zerodur

Short description: Extremely durable glass-ceramic

Opening of the ELT secondary mirror ZERODUR® blank mould containing the glass at first annealing at the Schott AG 4-meter blank annealing facility in Mainz, Germany.

Zerodur (notation of the manufacturer: ZERODUR®), a registered trademark of Schott AG, 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 GTC, Keck I, Keck II, and SOFIA, as well as some smaller telescopes (such as the GREGOR Solar Telescope). With its very low coefficient of thermal expansion (CTE) it can be used to produce mirrors that retain acceptable figures in extremely cold environments such as deep space. 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

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

1. Optics
2. Microlithography
3. Measurement technology

Properties

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

• Particularly low thermal expansion: in the range 0 to 50 °C it has a mean of 0 ± 0.007×10⁻⁶ K⁻¹, which is two orders 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 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.

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

See also

• CorningWare
• Macor
• Ring laser gyroscope
• Sitall