SCHOTT’s ZERODUR® glass-ceramic to be used in European Southern Observatory telescope

ZERODUR® has been the company’s material answer for high-tech and astronomy applications for 50 years.

Half a century ago, materials specialists from Schott used an ingenious process technology to develop ZERODUR®, a special glass-ceramic with a coefficient of expansion of nearly zero. This property makes the material ideal for applications requiring the highest precision in fields such as astronomy, IC lithography, the semiconductor industry, metrology and flat panel display production.

While most materials expand with heat and contract with cold, this isn’t the case with ZERODUR® glass-ceramic.  It remains highly dimensionally stable in response to temperature fluctuations precisely because the positive thermal expansion of the glassy portion is almost completely offset by the negative thermal expansion of the crystalline content.

ZERODUR® glass-ceramic consists of a crystalline and a residual glass phase, which together enable 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 ZERODUR® 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.

“Its secret lies in the balanced mixture of 30 to 50-nanometer crystallites embedded in a glass matrix of lithium, aluminium, and silicon oxides,” explained Dr. Thomas Westerhoff, Director for Strategic Marketing ZERODUR® at Schott Advanced Optics.

In the material’s most recent milestone, the European Southern Observatory (ESO) selected ZERODUR® for the Extremely Large Telescope (ELT) project. Schott will supply the ELT with ZERODUR® blanks including spares to form its primary mirror out of 798 hexagonal tiles and will create its monolithic mirrors with ZERODUR® as well.

Telescope mirrors in astronomy have used glass-ceramics as a substrate material since the early 1970s. In 1968, Schott produced its first mirror substrate on behalf of the Max Planck Institute for Astronomy. Today, the main components of many of the major optical telescopes worldwide are made of ZERODUR® glass-ceramic. That includes the over 8-meter diameter primary mirrors of the ESO’s Very Large Telescope, the segmented primary mirrors of the Gran Telescopio Canarias on La Palma, the two 10-meter Keck telescopes on Hawaii, the Big Bear Solar telescope in California, the GREGOR solar telescope on Tenerife, the 4-meter DKIST solar telescope on the Heleakala volcano on Hawaii, and the flying observatory SOFIA on board a jumbo jet.

ZERODUR® is particularly in demand in the aerospace industry because it is very easy to grind. Schott produces very light and extremely stable mirrors by shaping the material on the backs of mirror substrates into a honeycomb or isogrid structure.

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 Himalayas of India, the world’s telescopes rely on ZERODUR. The material’s presence will grow exponentially in the coming years, and by 2024, Schott will supply more than 900 blanks for the M1 segments of the ELT.

ZERODUR® glass-ceramic is ideal for more than just seeing into space. It also enables commercial high-tech applications with high-precision requirements in the fields of metrology and aviation and in semiconductor and FPD technology. For instance, the material is used as a substrate for measuring standards in instruments; in ring laser gyroscopes, ZERODUR® is used as a carrier structure for lasers in aircraft and submarine navigation; in IC lithography, it enables precise positioning of wafers. ZERODUR® is also used in FPD production as an optical mirror material for precise light guidance.

Schott is currently investing heavily in its glass-ceramic competence centre in Mainz Germany. With new melting capacities and post-processing options for a wide range of technical applications, the company is ideally positioned for the future.7