Migration mechanisms of helium in copper and tungsten

Point defect stability in a semicoherent metallic interface

“…A NEB simulation of the migration between two of these latter vacancy sites has been performed and a value of 0.72 eV has been obtained. This value is higher than the migration energy in the bulk, 0.59 eV, as obtained using the same methodology as for W by means of a 4x4x4 cubic unit cell 27 , and in good agreement with experimental data 29 . Adding the calculated migration energy and the difference between both plateaus, the energy cost for the movement of the Nb vacancies out of the MDIs results to be 1.56 eV.…”

“…Although for a complete understanding of the migration mechanisms similar NEB calculations should be performed for each pair of points, including some other more complex directions, this value can be considered as a lower bound of the energy barrier for one He atom to move out of an MDI. This energy seems to be much higher than the migration barrier in the perfect metallic bulks, namely, 0.125 eV 27 and 0.31 eV, respectively, suggesting that isolated He atoms will only move at high temperatures in the interface. Consequently, we conclude that isolated He atoms can be trapped at MDIs present at the interface, which is the first step in He cluster nucleation.…”

“…Additionally, the energy of the system increases by 0.62 eV when the vacancy is created along the red line. A NEB calculation has been performed between the two highest energy sites giving an energy barrier of 1.06 eV, again much higher than the 0.73 eV of the perfect bulk 27 . The total migration energy can be estimated as the difference between the most stable site and the point calculated between the two highest energy sites using the NEB methodology.…”

Object kinetic Monte Carlo simulations of irradiated tungsten for nuclear fusion reactors

“…where and Iglesias [108]). Thus, this maximum binding energy for a He atom to a He n V m cluster was set in the parameterization in this thesis.…”

“…With regard to migration parameters, He n V m and He n I m have been considered as immobile [66], with the only exception of HeV, HeV 2 and HeV 3 [108]. Both single He atoms and He n clusters can migrate (see Table 3.4).…”