Zerodur

Zerodur (notation of the manufacturer: ZERODUR®), a registered trademark of Schott AG,^[1] is a lithium aluminosilicate glass-ceramic^[2] produced by Schott AG since 1968.^[3] It has been used for a number of very large telescope mirrors including Keck I, Keck II,^[4] and SOFIA. With its very low coefficient of thermal expansion it can be used to produce mirrors which retain acceptable figures in extremely cold environments such as deep space.^[5] 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 allows for highly accurate applications which require high-precision.

Zerodur has both an amorphous (vitreous) component and a crystalline component. Its most important properties^[6] are:

• Particularly low thermal expansion: in the range 0 to 50°C it has a mean of 0 ± 0.007 × 10⁻⁶/K which is two orders of magnitude better than that of fused quartz^[7]
• 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 [2] [8]}

Applications

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

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

Properties^[6]

• 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 application temperature: 600 °C
• Impact resistance behavior is substantially similar to other glass^[9]

See also

• CorningWare
• Macor
• Ring laser gyroscope
• Sitall

References

1. ↑ http://www.unitedlens.com/page175.html
2. ↑ 2.0 2.1 Viens, Michael J (April 1990). "Fracture Toughness and Crack Growth of Zerodur" (PDF). NASA Technical Memorandum 4185. NASA. Retrieved 28 August 2011. 
3. ↑ 3.0 3.1 Schott AG Zerodur description
4. ↑ Döhring, Thorsten; Peter Hartmann, Ralf Jedamzik, Armin Thomas, Frank-Thomas Lentes. "Properties of Zerodur Mirror Blanks for Extremely Large Telescopes" (PDF). Proc. of SPIE Vol. 6148 61480G-8. SPIE. Retrieved 26 August 2011.  Cite uses deprecated parameter |coauthors= (help)
5. ↑ Baer, JW; WP Lotz. "Figure testing of 300 mm Zerodur mirrors at cryogenic temperatures" (PDF). Retrieved 26 August 2011. 
6. ↑ 6.0 6.1 Schott AG Zerodur properties
7. ↑ SCHOTT CTE Grades
8. ↑ Hartmann, P. (18 December 2012). "ZERODUR - Deterministic Approach for Strength Design" (PDF). NASA. Retrieved 11 September 2013. 
9. ↑ Senf, H; E Strassburger; H Rothenhausler (1997). "A study of Damage during Impact in Zerodur". J Phys Iv France 7 (Colloque C3, Suppltment au Journal de Physique I11 d'aotit 1997). Retrieved 31 August 2011. 

External links