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. 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 798 ZERODUR blanks including spares to form its primary mirror out of 798 hexagonal tiles and will create its monolithic mirrors with ZERODUR as well. SCHOTT is currently investing heavily 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.
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.
"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.
ZERODUR's success story owes itself to the material's exceptional properties, such as the near-zero thermal expansion that gave it its name. This isotropic, homogeneous material also has excellent polishing qualities.
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.
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.
