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Ultrastable materials investigated in depth

Thermal expansion measured at low temperatures for future space missions

PTB-News 2.2016
Especially interesting for

space research

semiconductor physics

materials research

Space telescopes such as the infrared observatory Herschel of the European Space Agency (ESA) observe radiation in the far infrared. In this context, it is of vital importance that the instruments are cooled so that they do not emit disturbing infrared radiation. The mirrors of these telescopes, which are used at temperatures below –190 °C, are made of special, ultrastable ceramics. In a cooperation project with ESA, PTB measured the thermal expansion of the materials used and of single-crystal silicon very precisely. The investigations will be useful for future space missions, but have also shown that the values used to date for single-crystal silicon as a reference material must be corrected.

Space telescope Herschel (2009-2013) allowed fascinating insights into the birth of stars. (Photo: ESA)

From outer space, space telescopes can investigate spectral ranges that are not accessible from the Earth. How critical it is to know the exact thermal expansion of the materials used when setting up such telescopes was clearly demonstrated during the Herschel mission, 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-depth investigations of the materials used were required. Within the scope of the ESA project, the scientists from PTB investigated the thermal expansion of the special, ultrastable ceramics (such as silicon carbide) in a temperature range from −266 °C to +20 °C with nanometer accuracy. In vast parts of this temperature range, the accuracy attained corresponds to a relative change in length of approx. one billionth per degree Celsius.

PTB's Ultra Precision Interferometer, which was used for this purpose, is deemed the most accurate worldwide. 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, single-crystal silicon, has also been investigated within the scope of the project. Across a vast temperature range, the values obtained turned out to deviate significantly from the reference values used to date for single-crystal silicon.

The results are of importance for further space missions that have already been planned. A clever procedure can prevent any radiation heat from occurring, so that the James Webb space telescope will be used at temperatures below −220 °C. In the case of the Space Infrared Telescope for Cosmology and Astrophysics (SPICA), the temperatures may be even lower.


René Schödel
Department 5.4 Interferometry on Material Measures
+49 (0)531 592-5400

Scientific publication

T. Middelmann, A. Walkov, G. Bartl, R. Schödel: Thermal expansion coefficient of single-crystal silicon from 7 K to 293 K. Phys. Rev. B 92, 174113 (2015)