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.

Zerodur is produced in a two-step process involving melting and ceramization. Depending on the size of the blanks, each step can take several months.²

First, raw materials including main components of lithium oxide (Li₂O), alumina (Al₂O₃), and silica (SiO₂) are melted at high temperatures of around 1600 °C, poured into molds, and annealed in a controlled cooling process that relieves internal stresses that develop during forming.³⁴ Then the glass undergoes a ceramization process involving controlled volume crystallization, which creates high-quartz nano-crystallites of 30 nm to 50 nm.² The negative CTE of the crystals compensates for the positive CTE of the residual glass matrix, which gives Zerodur its near zero thermal expansion.⁴

The main applications for Zerodur include telescope optics in astronomy⁵ and space applications,⁶ lithography machines for microchips and displays,⁷ and inertial measurements systems for navigation.⁸⁹

In astronomy, it is used for mirror substrates in large telescopes such as the Hobby-Eberly Telescope,¹⁰ the Keck I and Keck II telescopes,¹¹ the Gran Telescopio Canarias,¹² the Devasthal Optical Telescope,¹³ the European Southern Observatory's 8.2 m Very Large Telescope,¹⁴ and the 39 m Extremely Large Telescope.¹⁵ It also has been used for the primary mirror of SOFIA's airborne telescope.¹⁶

ASA (AstroSysteme Austria) also produces some telescopes with Zerodur.¹⁷

In space, it has been used for the imager in Meteosat Earth observation satellites,¹⁸ and for the optical bench in the LISA Pathfinder mission.¹⁹

In microlithography, Zerodur is used in wafer steppers and scanner machines for precise and reproducible wafer positioning.²⁰²¹ It is also used as a component in refractive optics for photolithography.²²

In inertial measurement units, Zerodur is used in ring laser gyroscopes.²³

Zerodur has both an amorphous (vitreous) component and a crystalline component. Its most important properties²⁴ 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.²⁵
• High 3D homogeneity²⁵ 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^1/2.²⁶²⁷

• 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 glass²⁸

Schott began developing glass-ceramics in the 1960s led by Jürgen Petzoldt, in response to demand for low expansion glass ceramics for telescopes.²⁹

In 1966, Hans Elsässer, the founding director of the Max Planck Institute for Astronomy (MPIA), asked the company if it could produce large castings of almost 4 meters using low-expansion glass-ceramic for telescope mirror substrates. In 1969, the MPIA ordered a 3.6 m (12 ft) mirror blank, along with ten smaller mirror substrates. The mirrors were delivered by late 1975,²⁹ and went into operation in 1984 in a telescope at the Calar Alto Observatory in Spain. Further orders for mirror blanks followed.³⁰