For 50 years, the name has said it all: ZERODUR®. Half a century ago, materials specialists from SCHOTT using ingenious process technology developed this special glass-ceramic with a coefficient of expansion of nearly zero. This makes the material ideal for applications requiring the highest precision – such as in astronomy, IC lithography, the semiconductor industry, metrology and flat panel display production. One recent highlight is the order from the European Southern Observatory (ESO) for the Extremely Large Telescope (ELT) project, for which SCHOTT will supply ZERODUR® segments for the primary mirror consisting of 798 hexagons in addition to several monolithic mirrors. SCHOTT is currently investing a mid-double digit million-euro amount in its glass-ceramic competence center in Mainz. With new melting capacities and post-processing options for a wide range of technical applications, the company is ideally positioned for the future.
Essentially every material expands with heat and contracts with cold. That isn’t the case with ZERODUR® glass-ceramic, however: temperature fluctuations hardly affect it at all as it remains highly dimensionally stable. “Its secret lies in the balanced mixture of 30 to 50-nanometer crystallites embedded in a glass matrix of lithium, aluminum and silicon oxides”, explains Dr. Thomas Westerhoff, Director Strategic Marketing ZERODUR® at SCHOTT Advanced Optics. The isotropic, homogeneous material also has excellent polishing qualities. ZERODUR®’s success story owes itself to the material’s exceptional properties, such as the near-zero thermal expansion which gave it its name.
SCHOTT manufactures special lightweight mirror substrates for space telescopes or satellites where weight is an issue. Photo: SCHOTT
Clear focus on astronomy projects
Glass-ceramics have been used as a substrate material for telescope mirrors in their main field of application, astronomy, since the early 1970s. On behalf of the Max Planck Institute for Astronomy (MPIA), SCHOTT was the first producer of a mirror substrate with a diameter of nearly 4 meters in 1968. Today, the main components of almost all major reflector telescopes worldwide are made of ZERODUR® glass-ceramic. That includes the primary mirrors of the Very Large Telescope (VLT) of the European Southern Observatory (with a diameter of over eight meters), the segmented primary mirrors of the Gran Telescopio Canarias (GTC) on La Palma and those of the two 10-meter Keck telescopes on Hawaii, as well as the Big Bear Solar telescope in California, the GREGOR solar telescope on Tenerife, the 4-meter DKIST solar telescope on the Heleakal volcano on Hawaii and the flying observatory SOFIA, which is on board a jumbo jet. The material is particularly in demand in the aerospace industry because it is very easy to grind. By shaping the material on the backs of the mirror substrates into a honeycomb structure, SCHOTT succeeds in producing super-light yet extremely stable mirrors. Etching can even be used to produce millimeter-thin structures that enable filigree satellite optics.
All in all, SCHOTT has produced several hundred mirror substrates in the form of monoliths or hexagons for many astronomy applications over the past 50 years. Whether in Chile’s Atacama Desert, atop the Mauna Kea volcano on Hawaii or in the high Himalayan mountains of India, the world’s telescopes rely on ZERODUR®. The material’s presence will grow exponentially in the coming years. SCHOTT will supply more than 900 circular discs for the M1 segments of the ELT by 2024.
Material for high-tech applications
ZERODUR® glass-ceramic is ideal for more than just seeing into space, but also commercial high-tech applications with high-precision requirements such as in the fields of metrology and aviation or in semiconductor and FPD technology. In measuring instruments, for instance, the material is used as a substrate for measuring standards; in ring laser gyroscopes, it is used as a carrier structure for lasers in aircraft and submarine navigation; in IC lithography, it enables precise positioning of wafers and it is also used in FPD production as an optical mirror material for precise light guidance.
ZERODUR® glass-ceramic consists of a crystalline and residual glass phase, together enabling extremely low coefficients of thermal expansion nearing zero. The thermal expansion is also highly homogeneous. Even with large material components, fluctuations in mechanical and thermal properties are hardly detectable, making the material ideal for optic applications with special demands on precision and temperature stability. The material’s optical transparency also enables optimal inspection of internal quality. Bubbles, streaks and inclusions can be eliminated in all but the most exceptional cases. ZERODUR® also has high chemical resistance and can be polished to an extremely smooth surface. These properties are stable for both small and large components.
ZERODUR® is a registered trademark of SCHOTT AG.
SCHOTT is a leading international technology group in the areas of specialty glass and glass-ceramics. The company has more than 130 years of outstanding development, materials and technology expertise and offers a broad portfolio of high-quality products and intelligent solutions. SCHOTT is an innovative enabler for many industries, including the home appliance, pharma, electronics, optics, life sciences, automotive and aviation industries. SCHOTT strives to play an important part of everyone’s life and is committed to innovation and sustainable success. The group maintains a global presence with production sites and sales offices in 33 countries. With its workforce of approximately 15,000 employees, sales of 2.05 billion euros were generated in fiscal year 2016/2017. The parent company, SCHOTT AG, has its headquarters in Mainz (Germany) and is solely owned by the Carl Zeiss Foundation. As a foundation company, SCHOTT assumes special responsibility for its employees, society and the environment.