Stability Of Helium Research Articles

Disordering of helium gas bubble superlattices in molybdenum under ion irradiation and thermal annealing

Self-organization of gas bubbles causes the formation of an ordered array of nanoscale gas bubbles (a gas bubble superlattice), a highly efficient mechanism for gas storage under irradiation. The stability of helium (He) gas bubble superlattices in molybdenum (Mo) under krypton (Kr) ion irradiation and thermal annealing has been investigated. The He gas bubble superlattices gradually become disordered under Kr ion irradiation at 300°C, and the order-disorder transformation process completes at 2.5 dpa. Both transmission electron microscopy (TEM) and synchrotron-based small-angle X-ray scattering (SAXS) reveal that the order-disorder transformation of He gas bubble superlattices is associated with a slight increase in the average bubble size. Phase-field modeling indicates that the inhomogeneous growth and coarsening of bubbles/voids cause the disordering of imperfect superlattices under irradiation and implies that highly ordered superlattices could potentially exhibit much stronger resistance to irradiation damage. Under thermal annealing, the He gas bubble superlattices in Mo become unstable and disordered at 1000°C with the bubble size increasing from ∼1.1 to ∼1.6 nm. The finding in this research provides insights into the disordering mechanisms of defect superlattices as well as guidance for designing stable defect superlattices in harsh environments.

Interactions between helium, hydrogen and intrinsic point defects in TaC crystal

Tantalum carbide is not only an important precipitate in reduced-activation steels, but also an ultrahigh-temperature ceramic useful for protective coating. Under 14 MeV neutron irradiation, He and H atoms are produced from the (n, α) and (n, p) transmutation reactions, respectively, along with certain amount point defects created by cascades in the materials. In order to understand the role of TaC precipitates on the clustering of He/H in RAFM steels under irradiation, the energetics and stability of helium (He) atoms, hydrogen (H) atoms and intrinsic point defects inside TaC crystal have been systemically investigate by first-principles calculations. From energetic point of view, the C vacancy and interstitial defects are more stable than those of Ta. H impurity has two stable interstitial sites in TaC, while other atoms (He, C, or Ta) has only one stable site. With a pre-existing vacancy, H prefers to occupy an interstitial site in the space of the monovacancy rather than the vacancy center, while He sits the vacancy center. Both He and H exhibit high diffusion barriers (0.73 eV for He and 0.31–0.44 eV for H) between two interstitial sites. He-He pair has stronger attraction than H-H, while the binding of C/Ta/Fe-He pair is stronger than C/Ta/Fe-H pair. These results suggest that He atoms are easier to form cluster with C/Ta/Fe interstitial atom. We further discuss the stability of small HexHy, Hex-vacancy and Hy-vacancy clusters. The formation energy of mixed HexHy cluster increases linearly with the total number of He and H atoms, while the pure Hy clusters is more stable than the Hex clusters.

Interaction between helium and intrinsic point defects in 3C-SiC single crystal

Silicon carbide (SiC) is a candidate structural material for fission and fusion reactors as well as an important wide band-gap semiconductor for electronic devices. Using first-principles calculations, we systemically investigate the energetics and stability of helium (He) atoms and intrinsic point defects inside single-crystalline 3C-SiC. We find that the formation energy of interstitial He is lower than those of point defects. Inside 3C-SiC, the He-C interaction is stronger than He-Si. Hence, the interstitial He atom in the Si tetrahedral site has a stronger interaction with the six C atoms in the second nearest neighbor than the four nearest neighboring Si atoms. For interstitial He atoms, the equilibrium He-He distance is about 1.81 Å with a weak attraction of 0.09 eV. According to the binding energies of Hen (n = 2–4) clusters, He interstitials can form He bubbles without involving other types of structural defects. Moreover, a Si (C) monovacancy can accommodate up to 11 (9) He atoms. The Hen clusters trapped in the Si or C monovacancy induce large internal pressure in the order of magnitude of GPa and thus facilitate the creation of a new vacancy at the nearby lattice site.

Stability of helium and hydrogen films

We use the variational theorem to evaluate the domain of parameter space such thatsome film exists on a given surface. This is accomplished by determining the binding energy variationally for a monolayer film at T=0. The He results are for the most part consistent with those of Cheng, et al., for the domain of wetting films. New results for H2 are compared with calculated parameters of noble gas adsorption potentials.

Flow and stability of helium II between concentric cylinders

We discuss our present knowledge of the flow and stability of helium II between concentric cylinders. The flow problem for helium II leads us to consider the formation of quantized vortices in the uniform rotation of helium II in an open bucket as well as quantized circulation states and vortices in a rotating annulus. We then consider how to treat the first appearance of vortices in the presence of shear, which allows us to characterize the basic flow which must be examined for stability. The results suggest an explanation for heretofore unexplained experiments. Future directions for research on the stability of helium II are suggested.

A Study of Flow Stability in Helium Cooling Systems

Methods have been developed for the computation of the frequency response and stability of helium cooling systems in the frequency range of the density wave instability. While more generally applicable, these methods were developed for a study of superconducting power transmission. Special features are the use of helium thermodynamic properties derived at every point from the exact equation of state, and the full accounting of the effect of compressibility. Classical linear control theory is employed throughout. By using a finite difference approach to the integration of the conservation equations over the space coordinate, the accuracy obtainable is limited only by the computation time available. Examples are given for representative transmission line parameters with the helium at a variety of supercritical pressures. It is concluded that, while density wave instability is a real potential problem in superconducting power transmission lines, it is not difficult to select operating parameters in a safe stable regime.

Phase Coherence and Stability of Helium II in Narrow Channels

Phase Coherence and Stability of Helium II in Narrow Channels

Experiments on the Hydrodynamic Stability of Helium II Between Rotating Cylinders

Experiments on the Hydrodynamic Stability of Helium II Between Rotating Cylinders