Atomistics of helium diffusion in copper and tungsten

Using previously determined interatomic potentials, the activation energy for migration of a single substitutional helium atom in copper and tungsten has been determined. The mechanism of migration involves the jump of the helium atom out of the vacancy concurrent with the jump of a host atom into that vacancy. The helium then occupies the vacant site created by the jumping host atom, resulting in a substitutional helium at a distance of √2 r ₀ (√3r ₀) from its original site in copper (tungsten). The rate-limiting step in the process is found to be the jump of the helium out of the vacancy, the activation energy for which is 2.15 eV in copper and 4.69 eV in tungsten.     

Equation of state of fluid helium at high temperatures and densities

Hugoniot curves and shock temperatures of gas helium with initial temperature 293 K and three initial pressures 0.6, 1.2, and 5.0 MPa were measured up to 15000 K using a two-stage light-gas gun and transient radiation pyrometer. It was found that the calculated Hugoniot EOS of gas helium at the same initial pressure using Saha equation with Debye-Hückel correction was in good agreement with the experimental data. The curve of the calculated shock wave velocity with the particle velocity of gas helium which is shocked from the initial pressure 5 MPa and temperature 293 K, i.e., theD ≈u relation,D=C ₀+λu (u<10 km/s, λ=1.32) in a low pressure region, is approximately parallel with the fittedD ≈u (λ=1.36) of liquid helium from the experimental data of Nellis et al. Our calculations show that the Hugoniot parameter λ is independent of the initial density p{in0}. TheD≈u curves of gas helium will transfer to another one and approach a limiting value of compression when their temperature elevates to about 18000 K and the ionization degree of the shocked gas helium reaches 10⁻³.     

Equation of state of fluid helium at high temperatures and densities

Helium, hydrogen, and their isotopes are the simplest monoatomic and diatomic molecules. It is relatively easy to describe their properties using the basic principles of quantum mechanics. In condensed matter physics, hydrogen and helium serve as the models for the condensed matter properties at extreme conditions so that both experi- mental and theoretical physicists pay much attention to the study of their properties[1], especially the insulator-metal transition of hydrogen[2]. The aim to st…     

氦泡对铝的弹性性质的影响

主要利用分子动力学方法模拟计算了含氦泡的铝的弹性性质,首先,应用第一性原理的方法计算了Al-He的相互作用势.其次,从两个不同的方面研究了氦泡对铝弹性常数的影响,一是不同的氦泡尺寸(直径分别是16, 20, 25, 30和3.5nm),二是不同的氦泡压力(即固定氦泡大小,氦泡内氦原子的个数与空位的比分别大约是5％,15％,45％ 和85％).结果表明弹性常数随着氦泡半径的增大而减小,对于固定大小的氦泡,随着氦泡内的压力变化弹性常数基本保持恒定.最后通过建立弹性复合体模型,得到的解析解定性上解释了氦泡的大小以及内压对铝的弹性常数的影响,与分子动力学模拟结果吻合. 关键词： 辐照损伤 氦泡 分子动力学 弹性常数     

Modifications in high energy higher order Born approximation

A systematic study is made to find out the differential scattering cross-section in the case of electron-atom collisions. The first and the second Born terms ofO(1/k _i ) are calculated in the framework of Yates high energy higher order Born approximation. The second Born term ofO(1/k _i ² ) is calculated using the second order Wallace term, the third term is calculated using the Glauber-eikonal series of Yates. The method is applied to the elastic scattering of electrons by atomic hydrogen in the energy range 100–400 eV and by helium for energies 200 eV and 400 eV. Comparison is made with other theoretical results and the experimental data.     

电弧特性及其对熔池形貌影响的数值模拟

通过电弧模型与熔池模型耦合数值模拟,研究了氩弧和氦弧特性及其对SUS304不锈钢钨极惰性气体保护（TIG）焊熔池形貌的影响.通过比较氩弧和氦弧的温度轮廓线以及阳极表面电流密度和热流密度分布发现,氦弧的径向距离比氩弧收缩明显,导致更多热量传递给阳极.模拟了氩弧和氦弧下浮力、电磁力、表面张力和气体剪切力分别对熔池形貌的影响.结果表明：不论是在氩弧还是在氦弧下熔池中表面张力是影响熔池形貌的最主要驱动力.在氩弧下,影响熔池形貌的另一个重要的驱动力是气体剪切力,而氦弧下则是电磁力.由于电磁力引起的内对流运动增加了熔深,从而导致相同氧含量时氦弧下的熔深和焊缝深宽比要高于氩弧下的熔深和焊缝深宽比.随着氧含量的增加,氩弧和氦弧下的焊缝深宽比均先增加而后保持不变.焊缝深宽比的模拟结果与实验结果符合较好. 关键词： 氩弧 氦弧 电弧特性 熔池形貌     

Interaction potentials,spectroscopy and transport properties of C+(2PJ) and C+(4PJ) with helium

We calculate accurate interatomic potentials for the interaction of a singly charged carbon cation with a helium atom. We employ the RCCSD(T) method, and basis sets of quadruple-ζ and quintuple-ζ quality; each point is counterpoise corrected and extrapolated to the basis set limit. We consider the two lowest C⁺(²P) and C⁺(⁴P) electronic states of the carbon cation, and calculate the interatomic potentials for the terms that arise from these: ²Π and ²Σ⁺, and ⁴Π and ⁴Σ^?, respectively. We additionally calculate the interatomic potentials for the respective spin–orbit levels, and examine the effect on the spectroscopic parameters. Finally, we employ each set of potentials to calculate transport coefficients, and compare these to the available data. Critical comments are made in the cases where there are discrepancies between the calculated values and measured data.     

Analysis of experimental data on the key thermonuclear reactions going within the low energy range

A comparative analysis of experimental data on hydrogen and helium isotope interaction with deuterons and tritons at low energy is performed. If the energy of an incident particle falls to several keV, the astrophysical factor S(E) rises sharply, indicating that there is an electron screening effect for such interaction. The values of electron screening potentials and enhancement factors are given for key thermonuclear reactions. It is important to allow for the electron screening effect in calculations performed during astrophysical research and in designing different thermonuclear facilities.     

On the Theory of Corona Discharge

Fundamentals of microscopical theory of the corona discharge are considered. Main mechanisms of generation and disappearance of charged and excited particles are discussed for the example of the helium discharge. An important role of pair ionizing collisions of metastable particles is indicated. Field configurations and distributions of charged and excited particles in the discharge gap are obtained, the corona voltage-current characteristics are calculated. It is shown that the presence of nonlinear ionization processes may result in hysteresis phenomena. In particular, on set and offset potentials of the corona discharge may differ considerably. Probable features of the corona discharges in some other gases are discussed qualitatively.     

First principles calculations in iron: structure and mobility of defect clusters and defect complexes for kinetic modelling

Predictive simulations of the defect population evolution in materials under or after irradiation can be performed in a multi-scale approach, where the atomistic properties of defects are determined by electronic structure calculations based on the Density Functional Theory and used as input for kinetic simulations covering macroscopic time and length scales. Recent advances obtained in iron are presented. The determination of the 3D migration of self-interstitial atoms instead of a fast one-dimensional glide induced an overall revision of the widely accepted picture of radiation damage predicted by previously existing empirical potentials. A coupled ab initio and mesoscopic kinetic Monte Carlo simulation provided strong evidence to clarify controversial interpretations of electrical resistivity recovery experiments concerning the mobility of vacancies, self-interstitial atoms, and their clusters. The results on the dissolution and migration properties of helium in α-Fe were used to parameterize Rate Theory models and new inter-atomic potentials, which improved the understanding of fusion reactor materials behavior. Finally, the effects of carbon, present in all steels as the principal hardening element, are also shown. To cite this article: C.C. Fu, F. Willaime, C. R. Physique 9 (2008).     

Theoretical study of Si+(2PJ)–RG complexes and transport of Si+(2PJ) in RG (RG = He–Ar)

We calculate accurate interatomic potentials for the interaction of a singly charged silicon cation with a rare gas atom of helium, neon or argon. We employ the RCCSD(T) method, and basis sets of quadruple-ζ and quintuple-ζ quality; each point is counterpoise-corrected and extrapolated to the basis set limit. We consider the lowest electronic state of the silicon atomic cation, Si⁺(²P), and calculate the interatomic potentials for the terms that arise from this: ²Π and ²Σ⁺. We additionally calculate the interatomic potentials for the respective spin-orbit levels, and examine the effect on the spectroscopic parameters; we also derive effective ionic radii for C⁺ and Si⁺. Finally, we employ each set of potentials to calculate transport coefficients, and compare these to available data for Si⁺ in He.     

Theoretical calculation for the high pressure properties of alkali fluorides

Abstract

The overlap repulsive potentials of ion pairs (Li ⁺?F^?, Na⁺?F^? and K⁺?F^?) are calculated by means of generalized Heitler-London method and the numerical results are fitted in the Born-Mayer form, i.e. Dexp (—αR), Then the parameter D for each pair potential is modified to satisfy the crystal equilibrium condition at the experimentally known lattice constant. The redetermined repulsive potentials are applied to calculate the cohesive energies, bulk moduli, equations of state and NaC1–to-CsC1 structural phase transition pressures of LiF, NaF and KF crystals. The results obtained are compared with some other theoretical values and the available experimental data, and good agreement is reached.     

High power high repetition rate pulsed collisional laser using a He + Eu+ mixture

Studies have been made of the properties of an ionized europium collision laser with=1002 nm in the 1.5 to 6 kW range of pumping powers. For this purpose, a gas-discharge tube made of BeO ceramic, 50 cm long and 2.7 cm diameter was used. The pulse repetition rate ranged from 2 to 10 kHz at a pumping duration of 400 ns.For helium pressures in the range of 0.3 to 1 atm, the efficiency and laser power increases faster than the concentration of helium atoms. At the atmospheric pressure of helium, the optimal lasing conditions are as follows: discharge current amplitude 150 A, the europium vapour pressure corresponds to 640 to 660° C and is dependent on the discharge current. The laser efficiency is independent of the pulse repetition rate in the 3 to 9 kHz range.The maximum laser power achieved was 12.7 W atF=9.5 kHz, and the efficiency amounted to 0.21%, whereas at 11.8 W andF=6.5 kHz the maximum efficiency was 0.24%. The dynamic efficiency of the laser reached 0.4%.     

Dynamical response of helium bubble motion to irradiation with high-energy self-ions in aluminum at high temperature

Brownian-type motion of helium bubbles in aluminum and its dynamical response to irradiation with 100-keV Al⁺ ions at high temperatures has been studied using in situ irradiation and transmission electron microscopy. It is found that, for most bubbles, the Brownian-type motion is retarded under irradiation, while the mobility returns when the irradiation is stopped. In contrast, under irradiation, a small number of bubbles display exceptionally rapid motion associated with the change in bubble size. These effects are discussed in terms of the dynamical interaction of helium bubbles with cascade damage formed by the high-energy self-ion irradiation.     

Positron annihilation study of helium clustering in alpha irradiated copper

Abstract

Helium clustering in alpha irradiated copper has been investigated by positron annihilation spectroscopy. Pure copper samples have been homogeneously helium implanted using a cyclotron, yielding helium concentrations of 100 appm and 400 appm. Post-implantation positron lifetime and Doppler broadened annihilation lineshape measurements have been carried out on these Cu samples as a function of isochronal annealing temperature. An annealing stage observed in the isochronal annealing curve viz., a marked reduction in the resolved lifetime τ₂ and an increase of its intensity I ₂, is explained as due to the formation of helium bubble embryos. At higher annealing temperatures, τ₂ corresponding to helium bubbles increases and saturates while its intensity I ₂ decreases, indicating an increase in the size of the bubble with a concomitant decrease in the bubble concentration. This stage is interpreted to be the bubble growth stage. From an analysis of positron lifetime parameters in the growth stage, helium stom density, bubble size and bubble concentration have been deduced at various annealing temperatures. The bubble characteristics are found to be affected by the helium dose. The present results on direct helium implanted Cu are compared with those of our earlier study on n-irradiated Cu-B, where helium was introduced using ¹⁰B(n, α)⁷ Li reaction.     

Radiation defects induced by helium implantation in gold-based alloys investigated by positron annihilation spectroscopy

The formation of gas bubbles in metallic materials may result in drastic degradation of in-service properties. In order to investigate this effect in high density and medium-low melting temperature (T _M) alloys, positron annihilation spectroscopy measurements were performed on helium-implanted gold–silver solid solutions after isochronal annealing treatments. Three recovery stages are observed, attributed to the migration and elimination of defects not stabilized by helium atoms, helium bubble nucleation and bubble growth. Similarities with other metals are found for the recovery stages involving bubble nucleation and growth processes. Lifetime measurements indicate that He implantation leads to the formation of small and over-pressurized bubbles that generate internal stresses in the material. A comprehensive picture is drawn for possible mechanisms of helium bubble evolution. Two values of activation energy (0.26 and 0.53 eV) are determined below and above 0.7T _M, respectively, from the variation of the helium bubble radius during the bubble growth stage. The migration and coalescence mechanism, which accounts for these very low activation energies, controls the helium bubble growth.     

Coexistence of superfluid and solid helium in aerogel

The results of recent neutron scattering studies of solid helium in silica aerogel are discussed. Previously I.V. Kalinin et al., Pis’ma Zh. éksp. Teor. Fiz. 87 (1), 743 (2008) [JETP Lett. 87 (1), 645 (2008)], we detected the existence of a superfluid phase in solid helium at a temperature below 0.6 K and a pressure of 51 bar, although, according to the phase diagram, helium should be in the solid state under these conditions. This work is a continuation of the above studies whose main goal was to examine the detected phenomenon and to establish basic parameters of the existence of a superfluid phase. We have determined the temperature of the superfluid transition from solid to superfluid helium, T _C = 1.3 K, by analyzing experimental data. The superfluid phase excitation parameters (lifetime, intensity, and energy) have a temperature dependence similar to that of bulk helium. The superfluid phase coexists with the solid phase in the entire measured temperature range from T = 0.05 K to T _C and is a nonequilibrium one and disappears at T _C.     

l-changing electron impact cross sections for high Rydberg atoms with application to helium experiments

The impact parameter method is applied to high Rydberg atoms for estimating Δn=0 dipole transition cross sections. Values for high-l states are determined in large part by polarization and relativistic fine-structure contributions to energy levels. Computed cross sections for low-l helium atoms show good agreement with angular momentum transfer cross sections from electron impact experiments over the range 20<n<80.