What is it about?
In order to enable very sharp high-resolution images a telescope mirror must have a very precise and stable reflecting surface. In order to avoid mechanical warping a mirror needs a minimum thickness. This makes it more sensitive to changing ambient temperature leading to temperatures differences in the mirrors volume. Different parts of the volume will expand non-uniformly thus warping the mirror. An effective remedy is to use a material, which is insensitive to temperature changes. ZERODUR® is such a material with extremely low thermal expansion. Recently its expansion with temperature change could be reduced further to 7 nanometer length change per meter length for one degree temperature change. The material’s expansion can even be tailored i.e. minimized for given temperature courses. Improved methods allows precise measurement of even such low expansion. Also at cryo temperature, ZERODUR® has a low and tailorable expansion. Recent and ongoing research proves the reliability of ZERODUR® in space borne missions because the radioactive environment prevailing here hardly affects its properties at all. Outstanding examples of ZERODUR® long lasting applications in earth-bound and space borne astronomy and in high technology industry, where additionally utmost length stability is a strict requirement, underpin its key enabling role.
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Why is it important?
The article gives an overview over the achievements with the ultra low thermal expansion material ZERODUR from SCHOTT AG Germany. The already very low thermal expansion could be reduced even further. The material can be adapted to specific temperature courses not only in room temperature but also in the cryo temperature range. New measurement equipment allows proving the very low expansion. It is far less sensitive against space radioactive irradiation than thought before, There is a lot of long term applications proving the reliability of the material. The overview is meant to encourage users with extreme requirements to consider applying this outstanding material.
Perspectives
For me this material ZERODUR is a bundle of wonders. The first wonder is its mere existence. Found by a serendipity event it combines glass and crystal in way thought to be impossible before. The second one is the low expansion based on the negative expansion of the nano-crystals. The third one is the possibility to produce items of more than 8 m diameter from this material. The fourth wonder is that it can be controlled so precisely in its properties, that the extremely low expansion can be reproduced to a very high degree. The fifth is the extreme homogeneity with respect to its expansion even across 8 m diameter. This holds also for other bulk material properties, one can read a newspaper through 1 m of Zerodur (ok in the near IR). The sixth is that it allows minimum breakage strength for long time mechanically loaded applications. The seventh wonder is that a brittle material such as Zerodur can be ground to achieve extremely light-weighted structures withstanding high accelerations. The eighth wonder is that is can be polished to (sub?)nanometer figures and roughness. The ninth wonder is that it can be recoated on and on without degradation of the surface. The tenth wonder is that there is still no obstacle in sight, which could prevent further developments.
Dr Peter Hartmann
SCHOTT AG retired
Read the Original
This page is a summary of: Glass ceramic ZERODUR®: Even closer to zero thermal expansion: a review, part 1, Journal of Astronomical Telescopes Instruments and Systems, June 2021, SPIE,
DOI: 10.1117/1.jatis.7.2.020901.
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