Thermal desorption spectroscopy a combined modelling and experimental approach for the complete determination of the kinetic constants associated with hydrogen trapping and detrapping
Résumé
Thermal desorption mass spectroscopy (TDS) is a widely used/employed experimental method for the investigation and characterization of hydrogen trap site interactions. This method essentially gives access to the hydrogen detrapping activation energy and trap site binding energy. On top of experimental considerations, one key issue associated with TDS is data treatment, i.e. spectral analysis. A model based upon the numerical resolution of the McNabb equations has been developed. This model takes into consideration each step in the specimen history and preparation regarding hydrogen exposure and desorption before simulating a classical TDS spectrum. Key factors which can drastically influence experimental results were identified by comparing experimental data with simulated spectra. These factors (i.e. specimen thickness) were then thereby included in the code as input parameters. A best fit approach was used to determine the interstitial diffusion coefficient of hydrogen in two fcc alloys from specifically designed model materials in which the majority of residual trap sites had been eliminated. Using a second type of model material, in which a single trap site was isolated, and a specially designed pre-TDS aging period, a full set of trapping and detrapping kinetic constants (i.e. pre-exponential and activation energy) associated with the presence of chromium carbides or dislocations was determined. Additionally, thanks to a quantitative analysis of the TDS spectra, the respective trap site densities were assigned. Finally, the possible relevance of Oriani's hypotheses as well as a critical view of selected literature abundant spectral analysis methods will be discussed/presented.