Knudsen EML - Technology demonstrator for measuring evaporation rates of metals and alloys

- High-precision determination of material constants under zero-g conditions -

- Preparatory work for measurements on the International Space Station (ISS) -

Material constants are required to develop new materials for the aerospace, automotive and energy industries. With these highly accurate experimentally determined data, complex metallurgical melting processes can be simulated later, for example. The microstructure of the new alloy can be specifically adjusted and alloy properties such as strength, hardness, temperature and corrosion resistance can be predicted. The necessary number of metallurgical melting experiments can thus be minimized. These can only be carried out on an industrial scale at high cost and expense.

Material constants and fundamental thermodynamic data such as enthalpy, entropy and Gibbs energy of the evaporation reaction as well as the partial and molar mixing parameters can be determined via the temperature dependence of the vapor pressure of a sample.

The vapor pressure can be determined using the Knudsen-effusion method in a high vacuum. For this purpose, a sample is heated in a so-called Knudsen cell. The crucible of the Knudsen cell is closed with a cap with a small opening. At a constant temperature, the sample is in equilibrium with its gas phase. Due to the large free path length of the gas species, particles only escape randomly from the opening of the Knudsen cell. This is called effusion. Thermodynamic data are calculated from the measured temperature-dependent mass loss.

In a joint project with the TU Clausthal, the German Aerospace Center (DLR-MP) and Airbus Space and Defense, a test setup is to be developed that will make it possible to conduct a Knudsen effusion experiment on the International Space Station (ISS). Vapor pressures are to be measured in a Knudsen cell with a high-resolution nanoscale.

The sample is positioned in the middle of the Knudsen cell by a magnetic field. Thus the free evaporation and the resulting thermodynamic data can be investigated without the influence of gravity.

The Institute for Applied Thermo- and Fluid Dynamics (ATF) at the Mannheim University of Applied Sciences is simulating the Knudsen Effusion in the new experimental set-up. Two different methods are used.

• An analytical method based on the Hertz-Knudsen-Langmuir equation
• Direct simulation Monte Carlo method (DSMC method)

With the analytical method it is possible to calculate a mass flow depending on the temperature, the opening geometry and the partial pressure in the Knudsen cell. With the DSMC method it is possible to calculate a molecular flow in which the temperature, velocity and position of the simulated gas species is known. Influences such as temperature gradients, magnetic field and geometry of the Knudsen cell can be simulated with this method.