Zerodur

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.^[3][4] 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 astronomy^[5] and space applications,^[6] lithography machines for microchips and displays,^[7] and inertial measurements systems for navigation.^[8][9]

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 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.^[20][21] 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 properties^[24] 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 homogeneity^[25] 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.^[26][27]

Schott began developing glass-ceramics in the 1960s lead 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]

• CorningWare
• Macor
• Ring laser gyroscope
• Sitall