Diffusion Cross Sections Research Articles

Cold collisions between alkali metals and alkaline-earth metals in the heteronuclear atom-ion system Li+Ba+

In a recent experiment [P. Weckesser et al., Nature (London) 600, 429 (2021)], the quantum $s$-wave regime was attained for an alkali-metal and alkaline-earth atom-ion combination $({\mathrm{LiBa}}^{+})$. We investigate possible outcomes from the interaction of this ion-atom pair at quantum regimes from a theoretical point of view. For this purpose, Born-Oppenheimer potential energy surfaces are constructed for the three lowest dissociation channels of the ${(\mathrm{Ba}\text{\ensuremath{-}}\mathrm{Li})}^{+}$ molecular system using a multireference configuration interaction electronic-structure calculation. We present elastic, spin-exchange (SE), and diffusion cross sections in different energy regimes. The collisional properties of this system are calculated in terms of the scattering phase shifts and scattering cross sections, and the semiclassical behavior at a relatively high energy limit is also examined. For SE collisions, phase locking is obtained towards lower partial waves.

Gas Diffusion Characteristics and Size Effect in Coal Particles with Different Degrees of Metamorphism

Coalbed methane (CBM) is a green unconventional energy source where the gas diffusion efficiency in coal directly limits the production capacity. Therefore, different gas adsorption experiments were used to determine the diffusion characteristics of coal samples with varying degrees of metamorphism. The influence law based on the gas type, pore structure, and coal sample size on the gas diffusion coefficient was analyzed, and the synergistic control effect of various factors on diffusion was discussed. The results show that when the particle size is the same, the diffusion coefficient of different gases shows the change rule of CO2 > N2 > CH4, and the diffusion coefficient of different samples is positively correlated with the degree of pore development. When the particle size is different, the diffusion coefficient is negatively correlated with the pore development degree and positively correlated with the sample particle size. The effective diffusion coefficient is consistent with the diffusion coefficient when the particle size of the sample is the same, and the effective diffusion coefficient is opposite to the diffusion coefficient when the particle size decreases. The analysis reveals that the diffusion coefficients of different gas are limited by the diameter of gas molecular dynamics, and the diffusion coefficient is reduced with the decrease in the diameter of the gas molecular dynamics. The change of diffusion coefficient caused by particle size change is positively correlated with the change of diffusion cross section caused by size effect. The difference between the effective diffusion coefficient and the diffusion coefficient mainly comes from whether the effective diffusion path is calculated. Therefore, when the particle size is constant, the effective diffusion coefficient can replace the diffusion coefficient to characterize the diffusion behavior. When the particle size is different, it is more appropriate to use the diffusion coefficient to characterize the gas diffusion behavior. The significance of the research findings in revealing gas transport in the coal matrix and coalbed methane formation is substantial.

The investigation of mechanisms on pipe diffuser leading edge vortex generation and development in centrifugal compressor

Pipe diffuser can adapt the high Mach number distorted flow in high pressure ratio centrifugal compressors because of its special scallop leading edge. However, the special leading edge could generate pair vortices and it is not clear if the pair vortices have positive or negative effect on pipe diffuser performance. Besides, the generation and development of pair vortices are also need to be investigated. This work is aimed at uncovering the influence of vortices on aerodynamic loss in pipe diffuser. Pipe diffusers with several different geometries are designed for a high pressure ratio centrifugal compressor used in a small gas turbine. The generation and evolution of pair vortices in pipe diffuser passages are studied by validated state-of-the-art CFD method. Detailed understandings of the influence of different geometries on pair vortices and pipe diffuser performance are provided. Through these works, it is found that the pair vortices could enhance the flow mixing of high and low energy flow in pipe diffuser passage near throat, thus the boundary layer becomes thin, and the flow separation is suppressed near pipe diffuser throat under large negative incidence. However, pair vortices also move low momentum flow to the pressure side and induced separation in pipe diffuser cone after throat, eventually. The diffuser inlet-to-impeller exit radius ratio and diffuser cross section shape could affect the compressor performance by changing the intensity of pair vortices. The diffuser with inlet-to-impeller exit radius ratio 1.02 and the pipe diffuser with the cross section with big fillets such as 3.37 mm are recommended for their good performance. The diffuser throat area and passage count can also have impact on pair vortices, but the compressor performance is not only influenced by pair vortices when these two parameters are changed.

Homonuclear ion-atom collisions: Application toLi+−Li

We present a theoretical framework for homonuclear, ground-state ion-atom collisions wherein the indistinguishability of direct elastic and resonant charge exchange (RCE) scattering channels is discussed. The discussion in terms of the total cross section(${\ensuremath{\sigma}}_{\text{tot}}$) is motivated and derived. The standard cross-section expressions for direct elastic(${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\sigma}}}_{\text{el}}$) and RCE (${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\sigma}}}_{\text{ce}}$) channels are obtained as high-energy approximations to ${\ensuremath{\sigma}}_{\text{tot}}$. We show that the total cross section resolves the inconsistency, at low energies, between the diffusion cross section (${\ensuremath{\sigma}}_{D}$) and the high-energy approximation to the total cross section (${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\sigma}}}_{\text{tot}}$). The validity of the equivalence relation between the ${\ensuremath{\sigma}}_{D}$ and $2\ifmmode\times\else\texttimes\fi{}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\sigma}}}_{\text{ce}}$ is also discussed. The differences which result in cross sections which are frequently used in the description of ion-atom collision is illustrated with specific calculations for an ${\mathrm{Li}}^{+}\ensuremath{-}\mathrm{Li}$ system. The use of ${\ensuremath{\sigma}}_{\text{tot}}$ is advocated as a correct and consistent way to represent cross sections.

DONJON5/CLASS coupled simulations of MOX/UO2 heterogeneous PWR core

Most fuel cycle simulation tools are based either on fixed recipes or assembly calculations for reactor modeling. Due to the high number of calculations and extensive computational power requirements, full-core computations are often seen as not viable for this purpose. However, this leads to additional hypotheses and modeling biases, thus limiting the realism of the resulting fuel cycle. For several applications, the current modeling method is sufficient, but precise calculations of discharged fuel composition may require further refinements. CLASS (Core Library for Advanced Simulation Scenarios) is a dynamic fuel cycle simulation code developed since 2012 with reactor models based on neural networks to produce nuclear data and physical quantities. Past work has shown a first coupling between CLASS and DONJON5 to quantify neural networks approach biases. This work assesses the applicability of 3D full-core diffusion calculations using the DONJON5 code coupled with nuclear scenario simulations involving a realistic PWR core at equilibrium cycle conditions. DONJON5 interpolates burnup dependent diffusion coefficients and cross sections generated beforehand by DRAGON5, a deterministic lattice calculation tool. Whereas previous studies considered only homogeneous reactors (i.e. homogeneous assembly in terms of composition and enrichment as well as homogeneous core), the present contribution focuses on the integration of full-core calculations in CLASS for fuel cycles involving a MOX/UO2 PWR core (i.e. 1/3 MOx–2/3 UOx). The DONJON5 model considered in this work describes a core with critical boron concentration at each time step partially loaded with MOx heterogeneous assemblies composed of three enrichments. In fuel cycle calculations, the main issue is to adapt, in the fabrication stage, the fresh fuel composition for the reactor with regards to the isotopic composition of the available stocks. This work presents a fuel loading model based on power peaking factors minimization that respects irradiation cycle length, 235U enrichment as well as Pu concentration and fissile quality, hence, ensuring a more uniform power distribution in the core.

A Review on Chemical Permeation Enhancers used in the Formulation of Transdermal Drug Delivery System

Skin penetration enhancement technology is a rapidly evolving area that will greatly increase the quantity of transdermal drug delivery medications. Penetration enhancers are used to facilitate the movement of drugs through the skin barrier. Numerous methods exist for extending partition enhancement. The enhancers' contact with the polar head of the lipid groups is the potential means for increasing the penetration. Penetration enhancers improve the amount of free water molecules between the bilayer, leading to an improvement of the polar drug diffusion cross section. This article focuses on the different compounds assessed for improving penetration activity like sulphoxides, azones, pyrrolidones, alcohols and alkanols, glycols, surfactants and terpenes.

Analytical approximation of cross sections of collisions of electrons with atoms of inert gases

The paper presents an analysis of data on the cross sections of elastic and inelastic collisions of electrons with noble gas atoms. The transport (diffusion) cross sec-tion, the excitation and ionization cross sections are considered. For the selected sets of experimental and theoretical data, optimal analytical formulas are found and approximation coefficients are selected for them. The obtained semi-empirical formulas allow us to reproduce the cross section values in a wide range of colli-sion energies from 0.001 to 10000 eV with an accuracy of several percent.

Cross sections of electron collisions with noble gases atoms

Consideration is given to the analysis of data on the cross sections of elastic and inelastic col-lisions of electrons with noble gas atoms. The transport (diffusion) cross section, the excita-tion and ionization cross sections are studied. For the selected sets of experimental and theo-retical data, optimal analytical formulas are found and approximation coefficients are select-ed for them. The obtained semi-empirical formulas allow us to reproduce the cross section values in a wide range of collision energies from 0.001 to 10000 eV with an accuracy of sev-eral percent.

Analytical approximation of cross sections of collisions of electrons with atoms of inert gases

The paper presents an analysis of data on the cross sections of elastic and inelastic collisions of electronswith noble gas atoms. The transport (diffusion) cross section, the excitation and ionization cross sectionsare considered. The bibliography on the cross sections of electron-atomic collisions includes manythousands of works. But the critical analysis of the results of experimental data in the review work is verydifficult due to the fact that the necessary initial data can only be available to the authors of the work. Theerrors of the order of 1-3% given in the original works are contrasted with each other, sometimesdiffering by 50%. Comparisons of the electron cross sections sets in noble gases was made. For theselected sets of experimental and theoretical data, optimal analytical formulas are found andapproximation coefficients are selected for them. The obtained semi-empirical formulas allow us toreproduce the cross-section values for them in a wide range of collision energies from 0.001 to 10000 eVwith an accuracy of several percent. Key words: electron atomic collisions, transport cross section, excitation cross section, ionization crosssection, approximation of cross sections, noble gases.

Measuring Dynamic Structural Changes of Nanoparticles at the Atomic Scale Using Scanning Transmission Electron Microscopy.

We propose a new method to measure atomic scale dynamics of nanoparticles from experimental high-resolution annular dark field scanning transmission electron microscopy images. By using the so-called hidden Markov model, which explicitly models the possibility of structural changes, the number of atoms in each atomic column can be quantified over time. This newly proposed method outperforms the current atom-counting procedure and enables the determination of the probabilities and cross sections for surface diffusion. This method is therefore of great importance for revealing and quantifying the atomic structure when it evolves over time via adatom dynamics, surface diffusion, beam effects, or during insitu experiments.

Experimental Investigation of FCCI Using Diffusion Couple Test Between UZr Fuel with Sb Additive and Cladding

Alloying additions are introduced into U-Zr fuel in order to bind lanthanides (e.g., cerium) and prevent their migration to the fuel-cladding interface. Antimony (Sb) is being investigated as a candidate additive. The present study focuses on the diffusion couple behavior of U-10Zr (wt%) alloy with Sb against cladding (iron or HT9) at 640°C. The diffusion cross sections were analyzed using a scanning electron microscope and X-ray diffraction. Zr-rind was found at the interface of the fuel alloy, Sb was found to be bound in Sb-Zr precipitates or Sb-Ce precipitates, and no reaction was found between Sb precipitates and the cladding materials.

Ab initio interaction potentials of the Ba, Ba+ complexes with Ar, Kr, and Xe in the lowest excited states.

The complexes of the Ba atom and Ba+ cation with the rare gas atoms Ar, Kr, and Xe in the states associated with the 6s → 5d, 6p excitations are investigated by means of the multireference configuration interaction techniques. Scalar relativistic potentials are obtained by the complete basis limit extrapolation through the sequence of aug-cc-pwCVnZ basis sets with the cardinal numbers n = Q, T, 5, combined with the suitable effective core potentials and benchmarked against the coupled cluster with singles, doubles, and non-iterative triples calculations and the literature data available for selected electronic states. Spin-orbit coupling is taken into account by means of the state-interacting multireference configuration interaction calculations performed for the Breit-Pauli spin-orbit Hamiltonian. The results show weak spin-orbit coupling between the states belonging to distinct atomic multiplets. General trends in the interaction strength and long-range anisotropy along the rare gas series are discussed. Vibronic spectra of the Ba and Ba+ complexes in the vicinity of the 1S → 1P° and 2S → 2P° atomic transitions and diffusion cross sections of the Ba(1S0, 3DJ) atom in high-temperature rare gases are calculated. Comparison with available experimental data shows that multireference calculations tend to underestimate the interaction strength for excited complexes.

Validation of the Serpent 2-DYNSUB code sequence using the Special Power Excursion Reactor Test III (SPERT III)

The Special Power Excursion Reactor Test III (SPERT III) is studied using the Serpent 2-DYNSUB code sequence in order to validate it for modeling reactivity insertion accidents (RIA) in PWRs. The SPERT III E-core was a thermal research reactor constructed to analyze reactor dynamics. Its configuration resembles a commercial PWR on terms of fuel type, choice of moderator, coolant flow and system pressure. The initial conditions of the rod ejection accident experiments (REA) performed cover cold startup, hot startup, hot standby and operating power scenarios. Eight of these experiments were analyzed in detail. Firstly, multi-dimensional nodal diffusion cross section tables were created for the three-dimensional reactor simulator DYNSUB employing the Monte Carlo neutron transport code Serpent 2. In a second step, DYNSUB stationary simulations were compared to Monte Carlo reference three-dimensional full scale solutions obtained with Serpent 2 (cold startup conditions) and Serpent 2/SUBCHANFLOW (operating power conditions) with a good agreement being observed. The latter tool is an internal coupling of Serpent 2 and the sub-channel thermal-hydraulics code SUBCHANFLOW. Finally, DYNSUB was utilized to study the eight selected transient experiments. Results were found to match measurements well. As the selected experiments cover much of the possible transient (delayed super-critical, prompt super-critical and super-prompt critical excursion) and initial conditions (cold and hot as well as zero, little and full power reactor states) one expects in commercial PWRs, the obtained results give confidence that the Serpent 2-DYNSUB tool chain is suitable to model REAs and other RIAs in PWRs.

Study on the adhering failure mechanism of cemented carbide inserts and element diffusion model during the heavy-duty cutting of water chamber head

In view of the serious adhering failure of cemented carbide inserts during the heavy-duty cutting of water chamber head, the adhering failure mechanism of inserts and the element diffusion model of the adhering process were investigated. First, based on cutting simulations, the cutting force was found to be as high as 45.2 kN while the maximum cutting temperature was 930 °C. Under the combined action of mechanical and thermal loads, element diffusion occurred between insert and workpiece and formed an adhering layer, resulting in the adhering failure of inserts. After that, the element diffusion dynamics equation of the adhering process was proposed according to Fick’s law. Based on the semi-infinite long-bar diffusion model and its initial conditions and boundary conditions, the dynamics equation was solved, with the element diffusion concentration model obtained. In the end, the experimental scheme of element diffusion was put forward, and the element diffusion between the cemented carbide insert and workpiece at elevated temperatures was experimentally investigated. The diffusion distance of Co in 508III steel was measured as 8 μm. The element diffusion concentration within the diffusion distance was compared with the value predicted by the model, with the effectiveness of the element diffusion model validated. The concentration distribution of diffusing elements in the diffusion cross section was characterized through experiments and compared with the theoretical concentration distribution. The research results can provide a basis for the in-depth study of the formation mechanism of insert-chip adhering layers and their influence on the cutting performance and service life of inserts.

Collisional relaxation of vibrational states of SrOH with He at 2 K

Vibrational relaxation of strontium monohydroxide (SrOH) molecules in collisions with helium (He) at 2 K is studied. We find the diffusion cross section of SrOH at 2.2 K to be and the vibrational quenching cross section for the (100) Sr–O stretching mode to be . The resulting ratio is more than an order of magnitude smaller than for previously studied few-atom radicals (Au et al 2014 Phys. Rev. A 90 032703 ). We also determine the Franck–Condon factor for SrOH () to be .

Scattering of near-zero-energy electrons and positrons byH2

The parameters for $S$-wave elastic scattering of near-zero-energy electrons and positrons by ${H}_{2}$ molecules are calculated using the stabilization method with explicitly correlated Gaussians. The confined variational method is applied to optimize the Gaussians to describe the short-range interaction of incident ${e}^{\ifmmode\pm\else\textpm\fi{}}$ with ${H}_{2}$ in the fixed-nuclei approximation. For ${e}^{+}$-${H}_{2}$ scattering the scattering length of previous work [Phys. Rev. Lett. 103, 223202 (2009)] is substantially improved. More importantly, for ${e}^{\ensuremath{-}}$-${H}_{2}$ scattering, from first principles, the scattering length is computed as a function of the internuclear distance. In the case that the two nuclei are at the equilibrium distance the results are in a good agreement with values derived from fitting experimental total and diffusion cross sections to the modified effective range theory.

Quantum effects in the case of6Li+and7Li+ions evolving in a neutral6Li gas at a wide range of temperatures

This work deals with the quantum-mechanical calculation of the temperature-dependent mobility of ionic lithium atoms diffusing in their parent gas. The computation of the quantal phase shifts in connection with the gerade and ungerade potential-energy curves, through which Li(+) approaches Li(2s), leads to the computation of the charge-transfer and diffusion cross sections. The behavior of the coefficients of diffusion and mobility with temperature is also examined. Throughout this work, the isotopic effects in the (6)Li(+)-(6)Li and (7)Li(+)-(6)Li collisions are emphasized.

Scattering properties of ground-state spin-polarized atomic hydrogen

In this paper, the scattering properties of ground-state spin-polarized atomic hydrogen (H↓) are studied at 0K using the Lippmann–Schwinger formalism. The total, diffusion and viscosity cross sections, as well as the S-wave scattering length, are calculated. The S-wave scattering cross section is found to be the most significant partial wave contributing to the total cross section at low energy. The contribution of the higher angular momentum waves, especially the D-wave (ℓ=2), to the scattering increases with increasing relative momentum k. Our calculations are performed for three triplet-state potentials: Morse-type, Silvera and Born–Oppenheimer potentials. It is also noted that as k→0, the results of the Morse potential are larger than those of the Silvera and Born–Oppenheimer potentials. This is because of the exponential tail of the Morse potential which falls off more rapidly than the r−6 behavior of the Silvera and Born–Oppenheimer potentials. Also, the Morse potential is relatively shallower than the other two potentials. For high k, the Morse cross sections approach the corresponding Silvera cross sections. This is because these triplet-state potentials have almost the same short-range part. The total cross sections reflect the quantum oscillations arising from the diffraction caused by the repulsive short-range part of the potential. Our results are consistent with those obtained by other methods.

Atomic transport collisions with Lorentzian electron distribution: nonthermal shielding

The nonthermal plasma screening effects on the electron–atom transport collision are investigated in partially ionized Lorentzian plasmas. The effective polarization potential and partial-wave theory are applied to obtain the scattering phase shift and transport cross section as functions of the spectral index, collision energy, Debye length and polarizability. The result shows that the nonthermal plasma screening effect suppresses the scattering phase shift as well as the transport cross section in the low-energy domain. It is found that the nonthermal plasma screening effect on the diffusion cross section decreases with an increase of the collision energy. In addition, the nonthermal plasma shielding effects on the excited states are found to be more significant than those on the ground state of the target atom in Lorentzian plasmas. It is also found that the nonthermal screening effect on the 2p state is more significant than that on the 2s state.

Binary scattering model for Lennard-Jones potential: Transport coefficients and collision integrals for non-equilibrium gas flow simulations

A Lennard-Jones (LJ) binary interaction model for dilute gases is obtained by representing the exact scattering angle as a polynomial expansion in non-dimensional collision variables. Rigorous theoretical verification of the model is performed by comparison with exact values of diffusion and viscosity cross sections and related collision integrals. The collision quantities given by the polynomial approximation model agree within 3.5% with those of the exact LJ scattering. The proposed model is compared in detail with the generalized soft sphere (GSS) model which is the closest in terms of fidelity among existing direct simulation Monte Carlo collision models. The GSS model's performance for the collision integral used in the first approximate of viscosity coefficient is comparable to the proposed model for most reduced temperatures. However, other collision integrals deviate significantly, even at moderate reduced temperatures. The high fidelity of the proposed model at low reduced temperatures enables non-equilibrium simulations of gases with deep LJ potential well such as metallic vapors. The model is based on the scattering angle as opposed to viscosity or diffusion coefficients and provides a direct link to molecular dynamics simulations.