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

确定高孔陶瓷介质的有效导热系数仍然是一个广泛的科学课题。许多影响因素，如原材料的组成、制造过程，包括孔隙率和孔径分布，都对多孔介质的固体、气体和辐射导热系数起着巨大的作用。此外，随着孔径的减小，另一个影响气体导热系数的因素是：热适应系数α。这个系数描述了气体分子与固体材料(气孔)碰撞过程中能量传输的效率。因此，在较高的克努森数下，它对孔洞大小的能量传输有很大的影响。在现有的计算导热系数的模型中，α往往被设定为1，这还没有得到实验的批准。为此，开始了一个研究项目，重点是实验测定陶瓷表面的热适应系数(测量条件：100-500mbar，0°C/100°C/303K/373K)。热适应系数取决于气体种类、表面材料种类、温度、表面结构和表面污染。这种依赖关系并不总是明显的，而且常常被(去)吸附过程所笼罩。在这个项目的框架内，可以获得第一个测量结果(αCalciumsilikat=0，2)，但还需要进一步检查(例如，对表面结构影响的影响)，并且必须使用不同的参考材料重复测量。然而，这一结果非常有趣，因为它不支持多孔介质的α=1的常见假设。因此，热适应系数不能随机确定，而必须通过实验测量来验证现有的气体导热系数计算模型的有效性。

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.