Thermal conductivity measurements on supporting structures of the mercury probe Bepi Colombo

S. Vidi1, S. Rausch1, H. Ebert1 and D. Petry2

1Bavarian Center for Applied Energy Research (ZAE Bayern), Germany
2Astrium GmbH, Germany

Keywords: guarded hot plate
property: thermal conductivity
material: carbon fiber-reinforced polymer structures

Space applications have always pushed the material and system properties to extremes. This is also valid for highly efficient solar panels as well as for their supporting structure. The latter need the combination off many seemingly opposing properties such as high mechanical stability, light weight and, in the case of the structure carrying a solar panel, a high thermal conductivity through the structure, in order to guarantee the efficient removal of waste heat from the damageable panels. Typically, such a structure consists of a very thin Carbon fiber-Reinforced Polymer (CRP) core with a honeycomb like structure glued to thicker CRP plates. The challenge lies in the raising the heat flow without sacrificing stability or gaining weight. This can be done by optimizing the core structure as well as the bond between the core and the covering plates. This design leads to a structure with a remarkably anisotropic effective thermal conductivity with high lateral heat transport and comparably small heat transport perpendicular to the specimen, making the determination of thermal properties a challenging task. One of the few measuring methods suitable for the examination of these CRP structures is the Guarded Hot Plate (GHP) Method. The strong effective lateral thermal conductivity of the samples and the high thermal contact resistances between the specimen and the apparatus, due to the high rigidity of the surfaces, pose difficulties to be solved. Good results were achieved using specialized measurement setups: Firstly, using an elastic material between a highly conducting reference specimen and the apparatus plates in addition to an adjustable external pressure on the stack, a well defined thermal contact resistance could be achieved. By using the same setup on the CRP specimens it is possible to extract the specimen's effective thermal conductivity from the total measured thermal resistance. Secondly, the specimens were cut to the exact diameter of only the hot plate and measured with an additional insulation surrounding the specimen in the area of the guard rings. This leads to a drastically reduced lateral heat flow during the measurement. The remaining lateral heat flow is considered mathematically using a correction method including measurements with different temperature gradients. An analysis of the measurement uncertainty was performed.

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