In the next ­decade, eso plans to realize the Extremely Large Telescope, a new astro project with a 42-meter mirror. Photo: ESO
Gerhard Samulat

Outsmarting Mother Nature

For four decades, Zerodur® glass ceramic has been offering the highest possible precision.

Forty years ago, a new material was born: a glass ceramic with the brand name Zerodur®. Its name also happens to be its motto. A team of developers at SCHOTT headed by Dr. Jürgen Petzoldt, an expert on materials and physicist, succeeded in producing a material with a coefficient of expansion close to zero using rather clever process technology. In doing so, the experts who work for the company based in Mainz basically outsmarted Mother Nature, because all known materials usually expand when they are subjected to heat and shrink at low temperatures. This is not the case with the Zerodur® glass ­ceramic, however. Shifts in temperature hardly have any effect on it and it always remains in its current shape.

The secret lies in the well-balanced mixture of crystallites 30 to 50 nanometer in size embedded inside a glass matrix of lithium, aluminum and silicon oxides. Long before the buzz word ”nanotechnology” became popular, SCHOTT had already been using this technique.
Zerodur® glass ceramic can be processed in a highly precise manner (right). It is used in a wide variety of applications, such as projects like the ESO Very Large Telescope for which SCHOTT manufactured an 8.2 meter mirror substrate (left). Photo (left): W. Feldmann, Photo (right): SCHOTT
Due to its excellent material characteristics – it is isotropic, homogeneous and can be polished rather easily – this material is in great demand with many applications. It mainly leverages its strengths in areas where the highest possible precision is important, optics, for example, where light and shadow lie only a wavelength apart. Oftentimes, this is only a matter of a few thousandths of micrometers.

But, what lead to the development of this unique material? Well, it is hardly surprising to learn that it was science, astronomy, to be more specific. In the nineteen sixties, Germany was faced with the prospect of falling behind the international competition in this field. For this reason, the observatory at the University of Heidelberg contacted SCHOTT and asked the company to develop a mirror substrate for a large telescope. ­Following extensive tests, Petzoldt and his group gave the go-ahead. Then, in November of 1968, SCHOTT received the order from the Max Planck Institute for Astronomy in Heidelberg that had been founded as a result of these discussions.
Zerodur® Glaskeramik as a round disk for lithography devices, where the highest accuracy of shape really counts. Photo: SCHOTT/T. Bauer
Nevertheless, they still had a long way to go before completing the final instrument. Nearly 150 specially-trained employees worked for nearly nine years before the heart piece for Germany’s first large telescope was finished: what looked to be a record-breaking mirror substrate with a diameter of over 3.6 meters that was nearly 60 centimeters thick. ”We were all very proud of our team’s achievement, definitely something very special back in the 1960s,” Petzoldt notes. Later, he became a member of the Board at SCHOTT and was responsible for ­research and development, among other areas.

Nevertheless, the efforts certainly paid off. This instrument is still delivering first class results even today. With its help, the astronomers of the Max Planck Society were able to discover the largest black hole ever found. This galactic monster that is capable of swallowing everything that not even light can escape is located 3.5 billion light years away from the earth in the constellation of  Cancer. According to the astronomer’s calculations, it is around 18 billion times heavier than our sun.
An electronic measurement device feels for the geometry of the Zerodur® glass ceramic surface. Photo: SCHOTT/S. Oelsner
No wonder that many other observatories knocked on the door at SCHOTT following this success story. Today, the main components of nearly all of the world’s important optical telescopes are made of the Zerodur® glass ceramic. This also applies for the heart piece of the world’s largest reflector telescope, the Gran Telescopio Grantecan on the Canary Island of La Palma, as well as the two 10 m KECK Telescopes on Hawaii and the Big Bear Solar Telescope in the U.S. state of California or the flying observatory SOFIA on board a jumbo jet from the German Aerospace Center and NASA.

In the area of aerospace, the fact that this material can be easily processed using grinding techniques truly pays off. By milling out material on the backside of the mirror substrates, SCHOTT engineers are able to produce ultra-light mirror substrates that still remain extremely stable. If necessary, Zerodur® glass ceramic can even be etched into millimeter-thin structures to form delicate satellite optics.

Fit for a variety of applications

Even for the large telescopes of the future, there will hardly be any alternatives. For example, the United States and Canada are planning to work together to build a telescope with a 30-meter diameter, and the European Southern Observatory eso is even planning one that spans 42 meters. Several hundred segmented mirrors will be needed and SCHOTT is capable of mass producing them.

Nevertheless, Zerodur® is not only designed to provide a clear view of outer space. It is also suited for a number of down-to-earth applications. Here, the optics are always the common denominator. For example, the Federal Agency for Cartography and Geodesy and the Research Group Satellite Geodesy use a ring-shaped laser that stands on top of a huge glass ceramic support structure to precisely measure the earth’s rotation. This is extremely important for the navigation of ships, airplanes, cars and satellites.
A glass ceramic disk 4.25 meters in diameter serves as the heart of the world‘s most precise ring laser gyroscope at the Fundamental Station Wettzell, Germany. Four additional ­Zerodur® bars extend the surface that is closed in by laser beams that face in opposite directions to 4x4 square meters. Photo: Fundamentalstation Wettzell
Precision is also a top priority, when it comes to manufacturing semiconductor chips. The sizes of the structures on the silicon wafers continue to shrink to only a few nanometers or millionths of millimeters. To expose the wafer, light that has the right type of short wavelength is needed. There are no more transmitting materials that can do this, however, and lenses are therefore out of the question. For this reason, the semiconductor industry has been increasingly working with mirror systems, just as astronomers have been doing for years. Here, too, the product that SCHOTT offers happens to be one material of choice, due to its dimensional stability. After all, even the smallest defect on a mask multiplies itself by millions of times during mass production of computer components.

LCD (Liquid Crystal Display) flat screen monitors for televisions, laptops and cell phones are manufactured in a similar fashion as computer chips. After all, they also consist of thousands or even millions of circuits that produce color schemes. For this reason, many leading vendors are switching over to producing their liquid crystal display screens with mirror optics made of  Zerodur®.

The sky is the limit, when it comes to the possibilities. For instance, SCHOTT project manager, Dr. Thorsten Döhring, is already focusing on other industries. ”The growing pressure to increase the precision of manufacturing accelerates the demand for Zerodur® components in fine mechanics and measurement technology,” he is convinced. It seems that the next chapters of the success story on this glass ceramic are already pre-programmed.
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