Approche expérimentale nouvelle pour l'étude de cinétiques réactionnelles de surface à hautes températures (2 OOO-3 OOO K) / New experimental approach for the study of surface reaction kinetics at high temperatures (2000-3000 K)

A fraction of the material sputtered from a sample by an ion beam is collected in a tantalum cell brought at a high temperature where it is thermalized and partly ionized. Ions are extracted from the cell and analyzed in a mass spectrometer. A method based on this principle was proposed a few years ago for the analysis of thin layers. Here we are using the same device for studying the surface reaction kinetics of atoms injected in the cell with the tantalum walls. The interaction of uranium with tantalum is described in the present study. From 3000 K to 2700 K we have observed an adsorption process characterized by an activation energy Ea=2.4 eV and a pre-exponential factor of the resident time 1/ν0⇒3×10-6 s. At a lower temperature a chemical diffusion of uranium in the tantalum wall is observed with the formation of a solid solution, the concentration limit of which imposes a permanent diffusion flux JDT(t→∞)¬=;0. The thickness of this phase is growing in proportion to the diffusion time with a velocity which increases when the temperature is decreasing (from 1.8×10-2 Å/s at 2600 K to 1.4×10-1 Å/s at 1850 K).