A new Ultra Precision Interferometer (UPI) was built at Physikalisch-Technische Bundesanstalt. As its precursor, the precision interferometer, it was designed for highly precise absolute length measurements of prismatic bodies, e.g. gauge blocks, under well-defined temperature conditions and pressure, making use of phase stepping imaging interferometry. The UPI enables a number of enhanced features, e.g. it is designed for a much better lateral resolution and better temperature stability. In addition to the original concept, the UPI is equipped with an external measurement pathway (EMP) in which a prismatic body can be placed alternatively. The temperature of the EMP can be controlled in a much wider range compared to the temperature of the interferometer's main chamber. An appropriate cryostat system, a precision temperature measurement system and improved imaging interferometry were established to permit absolute length measurements down to cryogenic temperature, demonstrated for the first time ever. Results of such measurements are important for studying thermal expansion of materials from room temperature towards less than 10 K.

The purpose of this study was to evaluate the thermal endurance of two types of thermal control foils. This is an important parameter for selecting materials for inner solar system missions. Assessing the thermal endurance by isothermal testing in a thermogravimetric analyser (TGA) is a time-consuming method. A quicker method is to perform kinetic modelling on several decomposition curves. Four temperature scans at different heating rates were recorded with the TGA for two thermal control foils, Kapton HN (DuPont, USA) and Upilex S (UBE Industries, Japan). Two methods of kinetic modelling were used to calculate the activation energy, namely the ASTM E 1641 and the approach of model free kinetics (MFK). The first assumes a constant value for the activation energy whereas the latter calculates it as a function of the conversion. With the activation energy it is possible to make a lifetime prediction that indicates the mass loss that occurs at a certain temperature for a certain duration. The lifetime predictions are used to compare the two materials and also the two modelling methods. The modelling is verified by comparing the results to isothermal TGA tests and thermal ageing of samples in high vacuum chambers at 350'C for durations up to half a year and in combination with UV irradiation screening tests. All the experiments performed indicate that Upilex S has a better thermal endurance than Kapton HN. The results of the kinetic modelling were in good agreement with the isothermal experiments in the TGA and in the vacuum facilities. The MFK approach for kinetic modelling was found to be more adequate than the ASTM method.

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Thermal analysis is the name of a collection of techniques that investigates material properties as a function of temperature. Four main techniques which all look at specific material properties are differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomechnical analysis (TMA) and dynamic mechanical analysis (DMA). The commercially available polyimide foils Kapton HN and Upilex S are often used as external thermal control material on spacecraft. For missions to the inner part of the solar system, these materials need to withstand an elevated service temperature in combination with high intensity solar radiation, such as UV and protons. The thermal stability of Kapton and Upilex has been investigated at around 250 1 C for a mission to Venus and at 350 1 C for a mission to Mercury. Samples were aged at representative temperature levels, also in combination with high intensity UV and proton radiation. It was found that TMA and DMA revealed, in good detail, the environmental effects of the behavior of the materials. Both foils are known to be thermally stable materials. Assessment by TGA and kinetic modeling indicates a better stability of Upilex at 350 1 C. The DMA experiments of foils aged at 350 1 C for up to 3580 h prove that the glass transition of Kapton is largely affected and almost completely disappears. Upilex is also affected but remains more stable. The TMA assessment revealed annealing of residual stresses at elevated temperature that cause (partial) shrinking of the foils. Ageing at around 250 1 C causes such annealing besides other more subtle changes. The added irradiation to high intensity UV and protons does not cause significant changes compared to thermal ageing alone. The DMA results are in good agreement with the TMA. The behavior of Upilex is deemed to be somewhat less affected by the environmental testing than Kapton.

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