Origin of the Hydrogen Signal in Atom Probe Tomography
Identifying mechanisms for hydrogen contamination of solid materials will help the quantification of H concentration in materials by atom probe tomography (APT) and elucidate open questions regarding H-involving mechanisms, like H embrittlement.
An accurate determination of the impurity content in a sample is vital for understanding its properties. Hydrogen is an omnipresent element, which is well known to affect materials properties, leading for example to H embrittlement. One technique considered to be extremely promising to determine H content in materials is Atom probe tomography (APT), due to its high spatial resolution and chemical sensitivity. There seems to be an understanding that H-related species (e.g., H+, H2+, and H3+) are always produced at the specimen’s surface during APT measurements, due to the used high fields, but the origin of H impurities is still subject of (a long-standing) debate. This indeterminacy limits our ability to precisely quantify H concentration in materials by APT. Hence, advancing our understanding of the H behavior during APT experimental conditions is essential and would ultimately help elucidate numerous open questions in physics, chemistry, and materials science related to hydrogen trapping or grain boundary segregation of H, if we consider the example of hydrogen embrittlement.
To solve this debate we perform APT analysis and DFT calculations on two very different metals - sodium (Na) and platinum (Pt) . The APT mass spectrum of the first-ever reported analysis of pure Na metal, interestingly, shows no H-related peaks, whereas that of Pt shows high H peaks also observed in the APT analysis of other metals. Our DFT thermodynamic analysis of Na and Pt surfaces allows us to understand the stability of the surfaces in a H2 gas environment and the given experimental conditions. The combined analysis we carried out reveals that Na surfaces have higher resistances to surface contamination with H compared to Pt surfaces, which explains the absence of H-related peaks in the APT mass spectrum of Na. Based on our findings we propose that the origin of the H signal detected in APT measurements is primarily due to H contamination of the specimen’s surface occurring during specimen preparation and transport, or the adsorption of residual background H2 gas, but not ionization of residual hydrogen in the chamber.
The project is a collaborative activity with the group of Atom Probe Tomography at MPIE.