Simulation of Silver Evaporation for A Knudsen Effusion Experiment Under Zero Gravity
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.
The aim of research is to develop a new technology demonstrator for measuring evaporation rates of metals and alloys to determine pressures of species effusing from a Knudsen cell using a high-resolution nanobalance system. Therefore, a new experimental set up will be developed and should be compatible with the electromagnetic levitator system (EML) on board of the International Space Station (ISS).
The feasibility to perform Knudsen effusion experiments under zero-gravity conditions is recently demonstrated during DLR Parabolic Flight Campaigns (Novespace, Bordeaux-Merignac). Such measurements give new insights into free vaporization without influence between sample and Knudsen cell and gravity driven effects (convection, sedimentation and natural drift).
In our work the Knudsen effusion process of the new experimental set up is simulated using the COMSOL Multiphysics® software environment with the Molecular Flow module. The silver evaporation rate of a special designed Knudsen cell is modelled for a levitated silver sample as investigated experimentally during parabolic flights. Simulations were performed for different sample temperatures and results were compared with experimental data obtained during parabolic flights.
