First principles calculations of alloying element diffusion coefficients in Ni using the five-frequency model

The diffusion coefficients of several alloying elements(Al,Mo,Co,Ta,Ru,W,Cr,Re) in Ni are directly calculated using the five-frequency model and the first principles density functional theory.The correlation factors provided by the five-frequency model are explicitly calculated.The calculated diffusion coefficients show their excellent agreement with the available experimental data.Both the diffusion pre-factor(D 0) and the activation energy(Q) of impurity diffusion are obtained.The diffusion coefficients above 700 K are sorted in the following order:DAl>DCr>DCo>DTa>DMo>DRu>DW>D Re.It is found that there is a positive correlation between the atomic radius of the solute and the jump energy of Ni that results in the rotation of the solute-vacancy pair(E 1).The value of E 2-E 1(E 2 is the solute diffusion energy) and the correlation factor each also show a positive correlation.The larger atoms in the same series have lower diffusion activation energies and faster diffusion coefficients.     

Fuel-dilution effect on differential molecular diffusion in laminar hydrogen diffusion flames

Laminar flame calculations have been made for a Tsuji counterflow geometry to investigate salient features caused by the differential diffusion effect in nitrogen-diluted hydrogen diffusion flames. A strong dependence of the differential diffusion parameter z_H on fuel dilution is found, where z_H is the difference of the mixture fractions based on H and O elements. The strain rate, however, appears to have a relatively minor impact on z_H. A simplified transport equation for the z_H parameter has been derived to explain qualitatively the behaviours exhibited in the numerical solutions. Two source terms of z_H are identified in the transport equation; one is due to mixing among species of different diffusion coefficients and the other one is associated with chemical reactions of H₂. More importantly, the second source term is found to be dominant in reacting flows, and it increases with inert gas dilution. This feature causes the differential diffusion parameter to increase with the amount of fuel dilution. The z_H values at the stoichiometric position are shown to correlate well with the ratio, Y_H2O|_max/(Z_H,1?Z_H,2), which may be useful for quantifying the influence of chemical reactions on the differential diffusion effect. For flames at low strain rates, the scalar dissipation rate exhibits a local minimum near the stoichiometric position. This peculiar feature is found to be caused by the differential diffusion effect modulated by chemical reactions. The local minimum in the scalar dissipation rate disappears at high strain rates when the convective transport overwhelms the molecular diffusion.     

Influence of the interatomic potential on thermotransport in binary liquid alloys: case study on NiAl

Equilibrium molecular dynamics simulation in conjunction with the Green-Kubo formalism is employed to study the transport properties of a model Ni₅₀Al₅₀ melt with the embedded-atom method potential developed in [G.P. Purja Pun, Y. Mishin, Phil. Mag., 2009, 89, 3245]. The principal objective of the work is to quantitatively characterise and analyse thermotransport in the system, i.e. diffusion driven by a temperature gradient. In addition, direct phenomenological coefficients for mass and thermal transport are also evaluated and analysed in the process. Furthermore, the results obtained are compared with previously published data for a different model of Ni₅₀Al₅₀ melt with an alternative embedded-atom method potential for the alloy as well as with experiment where possible. It is found that both potentials are able to consistently predict both direct transport coefficients over a wide temperature range. However, these two potentials are found to be inconsistent in characterising the cross-coupled heat and mass transport, predicting even different directions (sign) of the heat of thermotransport. The origin of this difference is discussed in the paper in detail.     

Cross-Field Diffusion by Charge Changing Processes

The particle transport induced by ionization, recombination and charge exchange processes in magnetically confined plasmas is analyzed. For ions with charge numbers Z ≧ 2 a random walk is caused by the fluttering of the gyro-radius as a result of successive recombination and ionization events. The corresponding diffusion coefficient, however, is very small and may be neglected even in the case of high-energy alpha particles being produced in fusion plasmas. Single and neutral particles are subjected to joint transport across the magnetic field due to their unimpeded motion during the short atomic phases. It is found that this mechanism is more important than classical transport over a wide temperature and density range. The corresponding diffusion coefficient D₁ = f(Te)Ti/mine is independent of the magnetic field strength and shows an electron temperature dependence which can be expressed in terms of the rate coefficients for ionization, recombination and charge exchange. The latter effect leads to a strong reduction of the diffusion process in hydrogen plasmas. Diffusion coefficients comparable with anomalous values of about 1m²/s are therefore reached only for cool (divertor) plasmas with T < 5eV.     

Kinetic analysis of the ionization ratio effects on a three‐species plasma

In partially ionized plasmas, the energy transferred to electrically charged species by the electromagnetic field can be partly channelized to the population of neutrals, due to interspecies collisional processes. Depending on the relative density of neutrals, these effects may govern the collective plasma dynamics by drastically modifying particle dynamics and energy‐transport processes with respect to the fully ionized plasma‐approximation models. In this work, the influence of the ionization ratio r_i on a partially ionized plasma is analysed by means of a three‐species one‐dimensional kinetic model to compute transient and steady state velocity‐dependent distribution functions. The conservative collision operators accounting for charge–charge and charge–neutral interactions allow studying several plasma scenarios with the same entire number of particles per unit of volume but for an increasing r_i parameter, in the presence of a modulated signal‐like electric field. For a sequence of plasma scenarios of fixed r_i, ranging from typical weakly ionized to highly ionized plasma values r_i ～ 10^?7–10^?4, the mass species flows are examined. These flows behave linearly with respect to r_i up to a value r_i ? 10^?5 from which the quasi‐linear dependence is critically altered. The convection–diffusion equations are solved with the semianalytical Propagator Integral Method, which behaves well to deal with conservative operators, density, and field discontinuities, allowing for the use of collision terms of disparate time and spatial characteristic scales. The results can be relevant to a wide class of plasma systems and to analyse the ionization ratio effects on transport coefficients.     

Determination of kinetic transport coefficients for ions in air as functions of electric field and temperature

A method for determining the kinetic coefficients of ion transfer in gases, viz., mobility K and longitudinal (D _L) and transverse (D _T) diffusion coefficients, as functions of the electric field E and gas temperature T is described. The method is based on the measurement of the increments to the ion mobility coefficients as functions of the electric field at a parametrically specified temperature. The kinetic transport coefficients K(E, T) and D _{L, T}(E, T) are determined for positive ions of aniline, pyridine, benzene, orthotoluidine, dimethyl methyl phosphonate, N-methyl aniline, N,N-dimethyl aniline, N,N-diethyl aniline, and diphenyl amine (DPA) formed as a result of β-ionization in air.     

Analysis of interdiffusion via isolated vacancies in strongly ionic crystals

In this paper, we analyse chemical interdiffusion in strongly ionic crystals for diffusion couples AY _m –BY _m , where A and B have the same charge numbers. We employ the exact sum rule given by Moleko and Allnatt relating the phenomenological coefficients for diffusion in the multicomponent random alloy via the agency of monovacancies. It is shown that the ratio of the intrinsic diffusivities can be expressed very simply in terms of the atom–vacancy exchange frequencies without correlation terms. For the case of an immobile anion sublattice and making use of a highly accurate diffusion kinetics theory due to Moleko et al., it is shown that the interdiffusivity is principally proportional only to the off-diagonal phenomenological coefficient relating the two cations.     

Hydrodynamic theory of electron transport in a strong magnetic field

The mode coupling contribution to the transverse transport coefficients of a three-dimensional one-component plasma in a strong external magnetic field is calculated. For very strong fields it is found that the tagged particle diffusion rate, the thermal diffusion rate, and the coefficient of viscosity in the plane orthogonal to the field have a Bohm-like B ^–1 behavior. The mode coupling mechanism responsible for such an effect is always one that involves the finite-frequency upper hybrid modes.     

T-matrix approach to heavy quark diffusion in the QGP

We assess transport properties of heavy quarks in the quark–gluon plasma (QGP) using static heavy-quark (HQ) potentials from lattice-QCD calculations in a Brueckner many-body T-matrix approach to evaluate elastic heavy-quark–light-quark scattering amplitudes. In the attractive meson and diquark channels, resonance states are formed for temperatures up to ∼1.5T _c, increasing pertinent drag and diffusion coefficients for heavy-quark rescattering in the QGP beyond the expectations from perturbative-QCD calculations. We use these transport coefficients, complemented with perturbative elastic HQ gluon scattering, in a relativistic Langevin simulation to obtain HQ p _t distributions and elliptic flow (v ₂) under conditions relevant for the hot and dense medium created in ultrarelativistic heavy-ion collisions. The heavy quarks are hadronized to open-charm and -bottom mesons within a combined quark-coalescence fragmentation scheme. The resulting single-electron spectra from their semileptonic decays are confronted with recent data on “non-photonic electrons” in 200 A GeV Au–Au collisions at the Relativistic Heavy-Ion Collider (RHIC).     

双温度氦等离子体输运性质计算

获得覆盖较宽温度和压力范围内的等离子体输运性质是进行等离子体传热和流动过程数值模拟的必要条件.本文采用Saha方程计算等离子体组分, 采用基于将Chapman-Enskog方法扩展到高阶近似的方法, 计算获得了电子温度(T_e)不等于重粒子温度(T_h)的情形下, 在300 K到40000 K的温度范围内氦等离子体的黏性、热导率和电导率. 研究结果表明压力和热力学非平衡参数(θ =T_e/T_h)对氦等离子体的输运性质有较大的影响. 在局域热力学平衡条件下,计算获得的氦等离子体输运性质和文献报道的数据符合良好.     

Diffusion and transport phenomena in a collisional magnetoplasma having both streaming and temperature anisotropy: Collisional kinetics

Boltzmann-transport equation is analytically solved for two-component magnetoplasma using Chapman-Enskog analysis to include collisional diffusion transport having anisotropies in both streaming velocity and temperature components. The modified collisional integrals are analytically solved with flux integrals and perturbed kinetic equation to arrive at drift diffusion velocity and resulting transport coefficients which are markedly affected by both streaming and temperature anisotropy. The early isotropic results are recovered in the limit V ₀ = 0 and T _‖ = T _⊥ which reduce to eqs (11.30) and (11.31) of [1] and eqs (2.7) and (2.13) of [2]. The electrical resistivity (η_⊥) diminishes sharply in fusion temperature limit kT _⊥ = 1 keV. The shape of the curves for both electrical resistivity and thermal conductivity is rectangular hyperbolic. However, for low thermal ratio (T _‖/T _⊥ < 1), the curves are raised up and for high thermal ratio (T _‖/T _⊥ > 1), they are lowered down the isotropic case (T _‖/T _⊥ > 1), showing comparatively diminished magnitudes of the quantities.      

Muon diffusion in β-V2H

Hydrogen diffusion in the ordered hydride β-V₂H is mainly brought about by a minority fraction of interstitial atoms on antistructural sites. Recently, this mechanism was elucidated in a single crystal QNS study at temperatures close to the critical point (390 K≤T≤440 K) where already an appreciable amount of antistructural sites is occupied. Here we use the positive muon as a radioactive hydrogen tracer in order to show that the same diffusion mechanism is also valid at low temperatures (80 K≤T≤320 K) where the different jump processes are very slow and where the fraction of antistructural atoms is tiny but nevertheless dominates the long range diffusion.     

On the shape of wedding cakes

The large-scale morphology of a growing surface is characterized for a simple model of crystal growth in which interlayer transport is completely suppressed due to the Ehrlich-Schwoebel effect. In the limit where the ratio of the surface diffusion coefficient to the deposition rateD/F the surface consists of wedding-cake-like structures whose shape is given by the inverse of an error function. The shape can be viewed as a separable solution of the singular diffusion equationu ₁=[u ^–2 u _x ] _x . As an application, expressions for the number of exposed layers as a function of coverage and diffusion length are derived.     

本征GaAs量子阱中电子自旋扩散输运的时-空分辨吸收光谱研究

发展了一种时-空分辨圆偏振光抽运-探测光谱及其理论,并用于本征GaAs量子阱中电子自旋扩散输运的实验研究.获得室温下本征GaAs量子阱中的“自旋双极扩散系数”为D_as=37.5±15 cm²/s.此结果比用自旋光栅法测量到的掺杂GaAs量子阱中电子自旋扩散系数小.解释为是由于“空穴库仑拖曳”效应减慢了电子自旋波包的扩散输运. 关键词： 时-空分辨抽运-探测光谱 电子自旋扩散 GaAs量子阱     

Concentration dependence of nickel diffusion in nickel-cobalt alloys

In the present paper the results of measuring the self-diffusion coefficients for Ni in Ni-Co alloy in the composition interval 0–100 at.% Co, and in the temperature range 1065–1290 °C are given. The measurements have been carried out by the Gruzin method using the radioisotope Ni-63. The curves indicating the concentration dependence of the diffusion coefficients calculated for differentaT _m vaules (T _m is the melting point at a given concentration,a is a number from the interval (0, 1)) are straight lines; this feature has been found in other solid solutions which slightly deviate from regularity as well. It results in simple equations for the concentration dependence of self-diffusion characteristicsD ₀ andH.     

Multiphonon contribution to the structure factor of metals

The structure factor of Na is calculated including two-phonon terms and the Debye-Waller factor. The result is compared with the one-phonon approximation usually employed to evaluate the electronic transport coefficients. This multiphonon contribution can amount to 13 per cent at the melting point and 10 per cent at room temperature in the transport sensitive region of wave vector 1.5 k_F < q < 2 k_F, where k_F is the Fermi wave vector. We conclude that calculations of electronic transport coefficients of metals intended to attain a precision better than 10 per cent above the Debye temperature must take into account the contributions of the Debye-Waller factor and the two phonon terms.     

Transport properties of H–N2 mixtures

Transport coefficients (shear viscosity, volume viscosity, thermal conductivity, and mass and thermal diffusion coefficients) of H–N₂ mixtures in the dilute-gas limit have been calculated from the intermolecular potential in the temperature range 300–2000K using the classical trajectory method. The intermediate results pertaining to H–N₂ binary interactions are reported, mainly in terms of cross-section ratios. Cross-sections evaluated with the Mason–Monchick approximation yield very good results for this system, the largest deviations, about 2.5%, being observed for the thermal diffusion coefficient. The accuracy here of this approximation can primarily be attributed to a light H atom and a weakly non-spherical potential resulting in a high rotational collision number. Furthermore, we investigate to which H–N₂ cross-sections and their ratios the values of the mixture transport coefficients are most sensitive. Our results indicate that, for some cross-section ratios, reliance on universal correlations at high temperatures, often used in flame codes, can induce sizeable errors in the thermal conductivity coefficient and especially in the thermal diffusion coefficients. We also observed that the volume viscosity is particularly sensitive to the value of the cross-section for internal energy relaxation in H–N₂ collisions.