An apparently irrefutable fact of physics is that materials expand when it is hot and contract when it is cold. However, ZERODUR^® glass-ceramic has been ignoring this principle for more than 50 years. Because it can. A material which seems to come from another world. A material which always seems to be ahead of its time. A material which hardly ever expands?

To describe the heroic strength of ZERODUR^® glass-ceramic in numbers, you would need several zeroes and a graphic comparison. Try and picture this: ten kilometers of railway track expand by two meters when temperatures increase by 20° Celsius. Under the same conditions, if ZERODUR^® glass-ceramic was utilized, the change amounts to a single millimeter. On a mirror segment of the Extremely Large Telescope (ELT) with a converted diameter of 1.45 meters, this would translate to a deviation of one millionth in the millimeter range – or 0.000001 millimeter – due to the low thermal expansion coefficient of glass-ceramic. An unbelievably tiny value. And that is how ZERODUR^® glass-ceramic rightly earned its name: zero thermal expansion.

But where do these superpowers – including a fully isotropic structure, extremely high homogeneity and a non-porous surface – come from? Chemically speaking, ZERODUR^® glass-ceramic is a lithium-aluminosilicate, or a hybrid of glass and crystals. The secret is the carefully balanced mixture of 30 to 50 nanometers of tiny crystallites, which are embedded in a glass matrix. The basic formula has remained nearly unchanged since its patent registration in 1967. Utilizing clever process technologies, a team of material specialists working with the former SCHOTT Executive Director of Research & Development, Dr. Jürgen Petzoldt, developed this special glass-ceramic.

Despite a nearly identical formula to the original one, the material has continually developed over the last five decades simply because SCHOTT’s knowledge has also deepened over time. Today, customers can count on customized ZERODUR^® glass-ceramic products, including an individualized distribution of the linear expansion, also known as the Alpha value. “Our capabilities for this material run very deep, and I am certain no other competitor has the expertise to match,” assures Thomas Werner. The 54-year-old is head of the SCHOTT internal ELT project and was the measurement engineer during the manufacture of the mirror substrates for the Very Large Telescope (VLT). “Working together with the customer we continously optimized ZERODUR^®,” explained Werner. Currently, SCHOTT has made a mid-doubledigit million euro investment in its glass-ceramic competence center in Mainz, Germany.

Today, the primary components of the world’s most important reflector telescopes are made of ZERODUR^®. The list includes the VLT’s four primary mirrors, each with a diameter of 8.20 meters, produced utilizing the largest monolithic glass components ever. “ZERODUR^® not only ‘fl exes its muscles’ for the space industry, but also for commercial high-tech applications which demand the highest precision including measurement technology, the aerospace and semiconductor industries and FPD technology,” says Dr. Thomas Westerhoff, Head of Strategic Marketing for ZERODUR^®. A success that would not have been possible had it not been for its use in spectacular astronomy projects. Thus, the European Southern Observatory (ESO) has paid tribute to the technological milestone in the development of materials in an unusual manner. Even if the ELT does not receive its first light until 2024 in Chile, you can still stand in awe today and touch a hexagonal segment of ZERODUR^® glass-ceramic in the new ESO Supernova Planetarium and Visitor Center near Munich.

July 13th, 2018