Thermally activated reaction–diffusion-controlled chemical bulk reactions of gases and solids

Abstract

The chemical kinetics of the reaction of thin films with reactive gases is investigated. The removal of thin films using thermally activated solid-gas to gas reactions is a method to in-situ control deposition inventory in vacuum and plasma vessels. Significant scatter of experimental deposit removal rates at apparently similar conditions was observed in the past, highlighting the need for understanding the underlying processes. A model based on the presence of reactive gas in the films bulk and chemical kinetics is presented. The model describes the diffusion of reactive gas into the film and its chemical interaction with film constituents in the bulk using a stationary reaction-diffusion equation. This yields the reactive gas concentration and reaction rates. Diffusion and reaction rate limitations are depicted in parameter studies. Comparison with literature data on tokamak co-deposit removal results in good agreement of removal rates as a function of pressure, film thickness and temperature.

Diffusion model of the impact of helium and argon impurities on deuterium retention in tungsten

“…Open porosity is known to significantly increase the specific surface area and to make the material quasi transparent for gas phases (percolation). One example for this effect is the thermo-chemical removal of deposited layers [12], with removal rates increasing due to the high specific surface area and open pathways for gas particles. At a volume fraction of more than 16% [13], the individual bubbles form a porous structure with open pathways to the surface of the sample.…”