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

多孔陶瓷介质有效导热系数的确定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，其中沿着任何一种方式都对多孔介质的固体，气体和“辐射”热导率起着巨大的作用。此外，随着孔径的减小，另一个因素影响气体热导率：热适应系数α。该系数描述了在气体分子和固体材料（孔）之间的碰撞期间发生的能量传输的效率。因此，它对克努森数升高时孔径中的能量传输有巨大的影响。在现有的导热系数计算模型中，α通常被设置为1，这还没有得到实验的证实。为此，开始了一项研究项目，重点是陶瓷表面热适应系数的实验测定（测量条件：100-500 mbar，0°C bis 100°C/303 K bis 373 K）。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。依赖性并不总是明显的，并且经常被（脱）吸附过程所掩盖。在本项目的框架内，可以获得第一个测量结果（αCalciumsilikat = 0，2），但必须对其进行进一步检查（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，这个结果非常有趣，因为它不支持多孔介质α = 1的常见假设。因此，热适应系数不能随意确定，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。

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.