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SMV-2022-08: Methods of thermal radiation measurements
Frolec, Jiří
The research activity was focused on experimental methods for testing the relative rate of materials to emit or absorb thermal radiation, i. e. emissivity and absorptivity. The main output is a review of available methods, with an emphasis on those operated at low temperatures. The ordering party's goal is to build dedicated workplace capable of performing the mentioned type of measurements. For this purpose, it was necessary to create an overview of the methods and assess the optimal one for the given purpose. The required research was carried out by members of the Group of Cryogenics and Superconductivity from ISI Brno, based on the available literature as well as on theoretical and practical experience gained during two decades of experiments with their own cryogenic apparatus for measuring thermal radiation interacting with various materials.
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SMV-2022-07: Cryogenic tests of aluminium foils with low emission and absorption of thermal radiation
Frolec, Jiří
The research was focused on the heat transfer by thermal radiation, specifically the interaction of thermal radiation with several different types of aluminium foils, used mainly in the so-called multilayer insulation (MLI). From the supplied raw material, samples suitable for testing were fabricated at the ISI Brno and then examined according to the requirements of the company. We focused on the temperature dependence of the relative ability of the samples to emit or absorb thermal radiation at all wavelengths and in all directions. The experimental verification then took place in a dedicated apparatus cooled by a liquid helium bath for a temperature range from about -253 °C to room temperature, which required choosing an optimal configuration of the apparatus and verifying its calibration. After the individual measurements, the received data were processed, the temperature stability of the foils at low temperatures was evaluated and their thermal-radiation properties were compared with our own low-temperature database of cryogenic measurements. The project contributed to selection of the optimal foil for the intended application, i. e. for use in MLI.
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Factors Influencing Thermal Radiative Properties of Metals
Králík, Tomáš ; Hanzelka, Pavel ; Musilová, Věra ; Srnka, Aleš
Data on thermal absorptivity and emissivity are important for calculations of heat flows in cryogenic devices. Although thermal radiative properties of materials used in cryogenics are measured, published and used for more than sixty years we have noticed that sometimes users of that data don’t well understand the significance of individual factors influencing thermal radiative properties. The sensitivity of the thermal radiative properties of metals to the condition of the surface and the difficulty of measurement, especially at low temperatures is probably the reason of dispersion of the published values. The effect of the material and its purity, finishing (machining, mechanical or chemical polishing) and of the protective coatings or accidental layers (water, oxidation) on the thermal radiative properties is not always obvious. Influence of the material, surface treatment, roughness and layers on the surface is discussed on the basis of our 20 year experience with measurement of the emissivity and absorptivity in the range from 20 K to 300 K.
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