Ultrastable materials investigated in depth

The latest issue of the scientific journal Physical Review B contains a report dedicated to the latter of these two subjects.Space telescopes such as Herschel explore spectral ranges that are not accessible from Earth; they can therefore only be used in space. How critical it is to know the exact thermal expansion of the materials used when setting up such telescopes was clearly demonstrated during one of the latest ESA missions, as it was revealed that the simulations performed previously were not in agreement with the manufactured mirrors. The discrepancies were fortunately not discovered in space, but still led to unnecessary delays. To prevent such unpleasant surprises from recurring in the future, in-depth investigations of the materials used were required. For their investigations within the scope of the ESA project, René Schödel's research group used PTB's ultra-precise interferometer to measure the length of the samples across the whole temperature range with nanometer accuracy.

This interferometer is the only one of its kind in the world. To allow measurements to be taken with similar accuracy but with less effort, even at other institutes, reference materials whose exact thermal expansion is known are usually used for comparison. One such reference material is single-crystal silicon, which is characterized by a continuous lattice structure with very few defects; it was also investigated by the researchers. Similar to some of the ultrastable ceramic materials, silicon exhibits a peculiar behavior: at low temperatures, it starts re-expanding below a certain temperature. This dynamic characteristic -- which is rather unexpected in everyday life -- was also exactly measured by the scientists from PTB. Their measurements yielded an important result: across a vast temperature range, they discovered significant deviations from the reference values used to date for single-crystal silicon. This suggests that the reference values must be corrected.

The results of the project are of importance for further space missions that have already been planned, such as the James Webb Space Telescope (JWST), for which temperatures of use below ‑220 °C are planned, or the Space Infrared Telescope for Cosmology and Astrophysics (SPICA), for which even lower temperatures of use are envisaged.