Revisited reaction-diffusion model of thermal desorption spectroscopy experiments on hydrogen retention in material

Abstract

Desorption phase of thermal desorption spectroscopy (TDS) experiments performed on tungsten samples exposed to flux of hydrogen isotopes in fusion relevant conditions is analyzed using a reaction-diffusion model describing hydrogen retention in material bulk. Two regimes of hydrogen desorption are identified depending on whether hydrogen trapping rate is faster than hydrogen diffusion rate in material during TDS experiments. In both regimes, a majority of hydrogen released from material defects is immediately outgassed instead of diffusing deeply in material bulk when the evolution of hydrogen concentration in material is quasi-static, which is the case during TDS experiments performed with tungsten samples exposed to flux of hydrogen isotopes in fusion related conditions. In this context, analytical expressions of the hydrogen outgassing flux as a function of the material temperature are obtained with sufficient accuracy to describe main features of thermal desorption spectra (TDSP). These expressions are then used to highlight how characteristic temperatures of TDSP depend on hydrogen retention parameters, such as trap concentration or activation energy of detrapping processes. The use of Arrhenius plots to characterize retention processes is then revisited when hydrogen trapping takes place during TDS experiments. Retention processes are also characterized using the shape of desorption peaks in TDSP, and it is shown that diffusion of hydrogen in material during TDS experiment can induce long desorption tails visible aside desorption peaks at high temperature in TDSP. These desorption tails can be used to estimate activation energy of diffusion of hydrogen in material.

Experimental determination of the deuterium binding energy with vacancies in tungsten

“…As the surface effects can retard hydrogen release and thus complicate TDS analyses, we will assume and consider only the case of a very fast H-H recombination at the surface, so that the surface effects do not influence both the TDS peak position and its shape. In this case two limiting H release regimes were identified: detrapping-limited regime and retrapping-limited 4 regime [20]. In the former case, the concentration of traps N t (in atomic fractions) is so low…”

“…Although all the considerations described above were made for a metal containing only one type of trapping sites (characterised by the values of E b and E tr ) and each trap can accommodate only one H atom, the same dependences are valid for every trap type in the metal as long as they do not evolve during the TDS measurements. The same dependencies should be also valid in the case of trapping of several H atoms by each trap since it can be approximated by several distinct trapping sites [20]. From the experimental point of view, in the case of a material with several types of trapping sites, TDS peaks must be well-resolved to determine their positions accurately.…”

“…As a result, one experimental spectrum can be fitted by using many combinations of fitting parameters [4]. However, under the condition of a high H-H recombination rate at the surface, the H binding energy with a defect can be directly determined from the shift of the desorption maximum in a series of TDS measurements performed with identical samples but with different heating rates [20,21].…”

New rate equation model to describe the stabilization of displacement damage by hydrogen atoms during ion irradiation in tungsten

“…In summary the relatively high D concentration deep in the 450 K sample is due to the fitting process and is most probably not a real feature one should strive to reproduce. We would like to stress that the correct D depth distribution is very important for correctly determining the de-trapping energies and the trap concentrations as the D depth distribution of a trap has a non-negligible effect on the shape of the D desorption spectra [40].…”

Influence of charging conditions on simulated temperature-programmed desorption for hydrogen in metals

“…However, as shown in equation 2, also appears in the intersection with the origin with the constant . This constant has been interpreted by Guterl et al [22] and related to vibration frequencies; it also defines a frontier value between detrapping-limited and retrapping-limited regimes.…”