The vapor pressure over a liquid or solid material is a crucial physical property. It is possible to calculate directly basic thermodynamic data, if the composition of the vapor and its temperature behavior is known. Such high precision thermodynamic data are a basis for designing new materials and predict the corrosion resistant and phase stability for new alloys for automotive or aerospace applications especially for high temperatures.
The vapor pressure can be measured by the method of Knudsen Effusion in a high vacuum atmosphere. Therefore, a sample is heated up in a so called Knudsen Cell. The crucible of the Knudsen Cell is closed with a lid which has a small orifice between 0.1-1 mm. At constant temperature the gas phase is in equilibrium with the sample. Due to the mean free path in vacuum, the molecules of the gas atmospheres escape from the Knudsen Cell only stochastically. The molecules effuse out of the Knudsen Cell and will be ionized and then characterized quantitatively and qualitatively by a mass spectrometer. Thereby the equilibrium between gas phase and sample will not be disturbed.
The method to measure and to characterize the vapor pressure is known as:
Knudsen Effusion Mass Spectrometry (KEMS). Vapor pressure between 1.0E-7 - 1 mbar and 290 K – 3000 K can be determined with this method.
A completely new designed experimental device is set up in our laboratory. The aim of the project is to build up a new KEMS apparatus which exceeds the current state of the art of measuring vapor pressures in sensitivity and usability.
The feasibility to perform Knudsen Effusion experiments at the International Space Station (ISS) will be demonstrated in a joint project developing a new experimental set up to determine partial pressures of species effusing from a Knudsen Cell under Zero Gravity using a high resolution nano balance. The experimental conditions allow measuring thermodynamic data without influence of gravity driven effects such as convection, sedimentation and natural draft. This is very important for the development of physical models to describe solidification processes quantitatively.
In our work the Knudsen Effusion process in the new experimental set up is simulated by the Direct Simulation Monte Carlo method (DSMC-Method). With the DSMC-Method it is possible to simulate a molecular flow out of a Knudsen Cell. The trajectory, temperature, velocity, position of each gas species is known at each simulation timestep.