Small Helium-vacancy Clusters Research Articles

<i>Ab Initio</i> Study of Helium in Tantalum: Interaction, Migration, and Clustering with Helium and Vacancies

Ab initio calculations based on the Density Function Theory (DFT) have been performed to study the interaction between helium and helium, helium and vacancy, migration of helium, and the stability of small helium-vacancy clusters in tantalum. The following results are found: (I) The tetrahedral interstitial helium atoms have weak interactions in tantalum, suggesting that no stable covalent bond is formed between this two helium atoms; (II) The stability of small helium-vacancy clusters is investigated. The interstitial helium atom and vacancy to the clusters are found to be positive in almost all case, i.e., all interactions are attractive; (III) The activation energies for a substitutional helium atom migration by the dissociation or vacancy mechanisms are estimated under the irradiation condition.

Thermal behaviour of helium-implanted spinel single crystals

The study of the microstructural modifications induced in spinel implanted with 4He + at 4.7 at.% and subsequently annealed at 1075 K is addressed in this paper. The combination of three analysis techniques Rutherford backscattering spectrometry in channeling geometry (RBS/C), X-ray diffraction and transmission electron microscopy was used in order to gain information about the damage depth distribution, the nature of radiation defects, and the occurrence of microstructural modifications. In as-implanted crystals the disorder level is weak, and the damage principally consists of small helium-vacancy clusters. These defects induce a tensile strain in the direction normal to the implanted crystal surface. After annealing, a surprising increase of the disorder level is measured by RBS/C. This increased backscattering yield is due to the formation of a particular type of He-vacancy clusters, namely He platelets, which also induce a relaxation of the strain.

Energy calculation of point defects in plutonium by embedded atom method

Plutonium is vulnerable to aging due to α radioactive decay. The properties and behaviors of point defects in plutonium are the basis for understanding plutonium aging. We have employed a molecular dynamics technique to calculate the formation energy and binding energy of point defects and small helium-vacancy clusters in plutonium, using embedded atom method, Morse pair potential and the Lennard-Jones pair potential for describing the interactions of Pu-Pu, Pu-He and He-He, respectively. A single self-interstitial atom’s steady configuration is 〈100〉 dumb-bell. An interstitial helium atom at octahedral site is more stable than that at tetrahedral site. As a result of high binding energy of an interstitial helium atom to a vacancy, helium atoms can combine with vacancies to form helium-vacancy cluster during the process of self-radiation. The formation energy of helium-vacancy cluster increases with the increasing number of helium atoms. When the number of helium atoms equals to the number of vacancy, the helium-vacancy cluster is rather stable. Both substitutional and interstitial helium atoms are trapped at the grain boundary (GB). The binding energy of the self-interstitial atom at GB core is higher than that of helium atom and vacancy.

First-principles approach to the properties of point defects and small helium-vacancy clusters in palladium

First-principles calculations based on density functional theory (DFT) have been performed to study the properties of interstitial helium atoms, the vacancy, substitutional, and small helium-vacancy clusters HemVn (m, n=0–4) in palladium. The result indicates that the vacancy has the strongest ability of capturing helium atoms and the octahedral interstitial configuration is more stable than the tetrahedral one, while the energy difference between them is very small. In the palladium crystal, helium atom will migrate from one octahedral interstitial site to another one through the O–T–O path. The formation energies and binding energies of an interstitial helium atom and an isolated vacancy to the helium-vacancy clusters are also determined in palladium. It is found that the formation energies increase with the increasing of helium atoms and the binding energies mainly depend on the helium to vacancy ratio of the clusters rather than the cluster size.

Stability of helium bubbles in alpha-iron: A molecular dynamics study

Molecular dynamics simulations were performed to estimate the dissociation energies of helium interstitials, vacancies and self-interstitial atoms from small helium–vacancy clusters. Several sets of empirical potentials have been tested and compared with available ab initio calculations in order to provide the best combination of potentials to study the stability of small helium bubbles. The behavior of the cluster seems to be better described using Ackland potential for the Fe–Fe interactions and Juslin potential for the Fe–He interactions. From the calculations, it appears that the dissociation energies mainly depend on the helium-to-vacancy ratio rather than the cluster size. The helium/vacancy crossover slightly varies with increasing number of vacancies, but the crossover defining the loop-punching regime decreases strongly with increasing cluster sizes.

Ab initiostudy of helium inα−Fe: Dissolution, migration, and clustering with vacancies

Density functional theory calculations have been performed to study the dissolution and migration of helium in $\ensuremath{\alpha}$-iron, and the stability of small helium-vacancy clusters ${\mathrm{He}}_{n}{V}_{m}$ ($n$,$m=0$ to 4). Substitutional and interstitial configurations of helium are found to have similar stabilities. The tetrahedral configuration is more stable than the octahedral by 0.2 eV. Interstitial helium atoms are predicted to have attractive interactions and a very low migration energy (0.06 eV), suggesting that He bubbles can form at low temperatures in initially vacancy-free lattices. The migration of substitutional helium by the vacancy mechanism is governed by the migration of the ${\mathrm{HeV}}_{2}$ complex, with an energy barrier of 1.1 eV. The activation energies for helium diffusion by the dissociation and vacancy mechanisms are estimated for the limiting cases of thermal-vacancy regime and of high supersaturation of vacancies. The trends of the binding energies of vacancy and helium to helium-vacancy clusters are discussed in terms of providing additional knowledge on the behavior of He in irradiated iron, necessary for the interpretation of complex experimental data such as thermal He desorption spectra.

The influence of dissociation of small helium-vacancy clusters on the growth of helium bubbles, the accumulation of helium at the grain boundaries and the kinetics of helium desorption from irradiated materials

This paper deals with the effect of thermal dissociation of small helium-vacancy clusters on the currents of helium atoms to gas filled bubbles and to grain boundaries. The theory proposed demonstrates the possibility of accelerated accumulation of helium in the bubbles and at the grain boundaries. Also the desorption of helium from metal foils being irradiated with alpha-particles is considered and the effective coefficient of helium diffusion in the material is determined.