Speaker
Description
In materials proposed for Librti, it can be difficult to directly interpret the measured diffusivity of hydrogen isotopes because a majority of gas atoms may be trapped at defects such as impurities, dislocations, and grain boundaries. Unless taken at high temperature, so that the entropy dominates, or at high hydrogen concentrations where traps are filled, the measured diffusivity can be orders of magnitude lower than the ideal pure lattice material.
We use molecular dynamics simulations to directly compute the retention and effective diffusivity of hydrogen gas atoms in homogeneous distributions of monovacancies and voids. We compare to an analytic model for the effective diffusivity we have recently derived for multi-occupancy traps [1]. This new model allows us to transition seamlessly from modelling the diffusion of hydrogen in vacancies with a few occupancy states to voids with many hundreds of occupancy states without using coupled cluster dynamics. Instead, we can fully characterize the effective diffusivity of mobile interstitial hydrogen atoms as a function of temperature, and trap and gas density, using molecular statics calculations for the binding energies as a function of void occupancy [2]. No parameters are fitted to experiment, or to Arrhenius plots.
We find our theory gives a quantitatively better agreement to full atomistic simulation, validating the analytic model for diffusivity for materials containing nanoscale defects characteristic of radiation damage, and improving on the standard McNabb-Foster (single occupancy trap) model without significantly impacting its computational efficiency. We conclude the single-occupancy trap approximation breaks down during plasma loading. Finally, we demonstrate the value of the method by comparing multiple occupancy hydrogen retention Finite Element simulations using MOOSE to experiment.
[1] Kaur et al, (2025) Phys Rev Mat 9, 125404 https://doi.org/10.1103/nbwm-bs8m
[2] Tirumala et al, (2026) J. Phys.: Condens. Matt. https://doi.org/10.1088/1361-648X/ae37bc
| Speaker affiliation | Karlsruhe Institute of Technology - KIT |
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