Density functional theory study of the effect of helium clusters on tritium-containing palladium lattices

Abstract

Density functional theory (DFT) calculations have been employed to calculate the energetics, structures and migration behaviour of helium in palladium tritides. Increasing the tritium concentration in palladium leads to a decrease in the formation energies of helium clusters, indicating that He clusters can form in the lattices. The calculated results show less lattice expansion in Pd defect-containing lattices compared to the perfect lattice owing to smaller lattice distortions. The lowest energy migration path for helium diffusion is along octahedral-tetrahedral-octahedral sites but the energy barrier increases with increasing tritium concentration. Repulsive interactions occur between Pd d and He s orbitals, suggesting that displacement of the metal atoms in the lattice leads to growth of pressure inside the lattices. This process may change the microstructural properties leading to the degradation of the material.

Three-Dimensional Maps of Helium Nanobubbles To Probe the Mechanisms of Bubble Nucleation and Growth

“…Our 3D electron tomography experiments and subsequent analysis present a more detailed description of the behavior of helium in metals. Transport of helium as atoms or clusters through interstitial sites in a perfect crystal is believed to be quite rapid. ,− However, the helium can be trapped at interstitial or substitutional impurities, self-interstitial atoms, vacancies that may be abundant due to stabilization by hydrogen, , and dislocations that may include those produced by growing bubbles. , Dilute substitutional copper in palladium has been predicted to trap helium atoms with a heat of formation of −0.2 eV; similar results could be expected for the substitutional nickel in the palladium sample presented here . This trapped helium may still be mobile at room temperature, but much less so, especially if several helium atoms are trapped at a defect. − Bubbles that are smaller than those that are resolvable could serve a similar function as traps in the lattice.…”

Microstructure Evolution of Ti Tritides During Aging

“…Das et al have calculated the effect of helium clusters on the perfect and Pd vacancy-containing PdT 0.47 and PdT 0.72 lattices. The formation energy per atom is decreasing with increasing cluster size [1]. This means the migration of helium atoms prefer the new generated helium bubbles to the grow-up helium bubbles, and only the single isolated helium bubbles mostly lie in young sample can grow up into a big one.…”