Investigation of the thermal accommodation coefficient on ceramic surfaces up to a gas pressure of 10-4 mbar

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The determination of the effective thermal conductivity of highly porous ceramic media is still a broad topic of science. Many influencing factors such as the composition of raw materials, the process of manufactoring including the porosity and pore size distribution which come along either way are playing a huge role concerning the solid, gas and „radiative“ thermal conductivity of the porous media. Furthermore, with lowering the pore size another factor is influencing the gas thermal conductivity: the thermal accommodation coefficient α. This coefficient describes the efficiency of the energy transport occuring during a collision between a gas molecule and the solid material (pore). Therefore it has a huge impact on the energy transport in pore sizes at elevated Knudsen numbers. In existing models for calculating the thermal conductivity α is often set to one, which hasn‘t been approved yet experimentally. For this reason a research project was started with focus on the experimental determination of the thermal accommodation coefficient on ceramic surfaces (measurement conditions: 100-500 mbar, 0°C bis 100°C/303K bis 373K). The thermal accommodation coefficient is dependent on the kind of gas, kind of surface material, the temperature, the surface structure and the surface contamination. The dependencies are not always distinct and are often clouded by (de-)adsorption processes. In the frame of this project a first measurement result could be achieved (αCalciumsilikat = 0,2), but it has to be further examined (e.g. impact on influence of surface structure) and measurements have to be repeated with different reference materials. However, this result is highly interesting as it does not support the common assumption of α = 1 for porous media. Therefore the thermal accommodation coefficient can not be determined randomely, but it has to be measured experimentally to (not) validate the existing models for calculating the gas thermal conductivity.