Knudsen Effusion Mass Spectrometry (KEMS)
The vapor pressure over a liquid or solid sample is an essential physical property. This can be determined exactly using Knudsen Effusion Mass Spectrometry (KEMS). Vapor pressures between 1x10-10 - 1 mbar can be measured in a temperature range between 20 - 3000 °C. The mass spectrometric analysis of the vapor phase also makes it possible to analyse the composition of the gas species. If the temperature dependence of the vapor pressure is known, fundamental thermodynamic data can be determined. These include enthalpies, entropies and Gibbs energy, which are of utmost importance for material development according to the "Integrated Computational Materials Engineering" (ICME) method.
During a KEMS measurement, a material sample is heated in a Knudsen cell with an inner diameter of 7 - 10 mm. The crucible is closed with a lid. There is a small opening of approx. 0.5 to 1 mm in the lid. A gas phase is now formed in the Knudsen cell, which is in thermodynamic equilibrium with the sample. Due to the equilibrium, the vapor pressure in the gas phase is constant. Through the very small Knudsen opening, only a small number of atoms and molecules escape from the Knudsen cell by random. The thermodynamic balance is practically undisturbed. This process is called effusion. The escaped molecules are then ionized by electron bombardment and accelerated in a potential field. Using a mass spectrometer, the molecules can now be separated according to their mass-to-charge ratio and characterized by the analyzer. From the analysis, the species effused from the Knudsen cell can be identified and quantified.
