Diffusion mechanism of H and O at the interstitial sites of a Ti-Zr alloy sorbent: A first-principles investigation

Abstract

In the ultra-high vacuum environment of fusion devices, sorbent materials were required not only to remove H₂, but also to capture impurity gases (i.e., H₂O, CO₂, O₂, etc.). The adsorption of gases by sorbent materials was affected by a combination of surface adsorption and bulk diffusion. In this work, the first-principles method was employed to analyze the stability of interstitial sites for H and O atoms in Ti-Zr binary alloy sorbent. Diffusion pathways and barriers for H and O atoms among interstitial sites were calculated using the Climbing Image Nudged Elastic Band method (CI-NEB). The stability of interstitial sites occupied by solute atoms was found to affect the ease of diffusion of solute atoms between adjacent interstitial sites. The higher the stability of these interstitial sites, the more difficult it was for the atom to diffuse to other sites. However, solute atoms were more likely to diffuse from less stable interstitial sites to those with greater stability. The Tb tetrahedral interstitial site in Ti_0.5Zr_0.5 was most readily occupied by the H atom. The energy barrier for diffusion from the OC octahedral interstitial site to the Tb tetrahedral site was found to be the lowest, which was equal to 0.135 eV. The O atom was most likely to occupy the OC octahedral interstitial sites. When diffusing along the 2Ta3-3Ta3 path parallel to the C-axis, the lowest diffusion barrier for O atom was 5.425 eV. The next most favorable diffusion pathway was from the Ta tetrahedral interstitial site to the OC octahedral site, with a diffusion barrier of 5.435 eV. The main findings of the diffusion mechanism of H and O in the bulk structure of binary titanium-zirconium alloys revealed in this work are expected to provide a theoretical reference and potential feasibility analysis for the application of new titanium-zirconium-based sorbent materials.