Two-shell Model Research Articles

Analysis of Chemical Composition Formula of Sealing Alloys Using Nearest-neighbor Two-shell Model

Analysis of Chemical Composition Formula of Sealing Alloys Using Nearest-neighbor Two-shell Model

Local structure of Fe2+ and Fe3+ in organic solvents

We systematically investigated the local structure of Fe2+ and Fe3+ in 11 solvents by extended X-ray absorption fine structure (EXAFS) analysis and X-ray absorption near-edge structure (XANES). The EXAFS analyses with a one-shell model indicate that Fe2+ and Fe3+ are coordinated by six oxygen of the ligand molecules (L) and form a FeL 6-type complex in all solvents. The XANES suggests that the [FeL 6]3+ complex shows noncentrosymmetric deformation in several solvents. In protic solvents, the EXAFS analyses with a two-shell model revealed that the coordination number (n C) of the second nearest carbon around Fe3+ significantly depends on L. We interpreted the observation in terms of the distribution of the Fe-C distance.

A two-nearest-neighbor structure model and analysis of low-expansion Fe–Ni alloys

The main goal of the study is to introduce a two-nearest-neighbor structure model for analyzing low-expansion Fe–Ni alloys. A two-shell atomic structure model is proposed to accurately locate the compositions of the alloy. In the presented model, the Cowley parameters of the alloy short-range ordered structure are considered. The selection of the first-nearest-neighbor cluster is determined, and the number and composition ratio of the second-nearest-neighbor atoms are evaluated by means of the spherical periodic oscillation model. The results show that the developed formula can provide a practical procedure for the composition design of low-expansion Fe–Ni alloys.

Local structure and medium-range order in a glassy Ti-Ta-based surface alloy after low-temperature annealing studied by electron nano-beam diffraction

The rejuvenated metallic glasses subjected to heat treatments show the improved thermal stability and properties. The atomic structure in the glassy alloys inherited from the liquid state possesses the medium-range order that could be significantly modified upon heating. In this work, we utilized the nano-beam diffraction technique to resolve the medium-range order in a glassy Ti-Ta-based surface alloy upon isothermal annealing. After annealing at T = 773 K the as-received Ti-Ni-Ta surface alloy, fabricated by a liquid-phase mixing of the system [Ti-Ta film/TiNi substrate], devitrified through the formation of a nanocrystalline structure: η′-(Ti,Ta)2Ni + β(Ti,Ta) + α″(Ti,Ta). The transmission electron microscopy methods were employed to determine the parameters of the short-range order in the amorphous phase both in the reciprocal and real spaces. For interpretation of the results of the electron scattering in the reciprocal space, the coherent and diffuse scattering intensities were calculated in the framework of the multi-shell cluster model. We suggested a scheme describing the transformations of icosahedral nanoclusters during the temperature-induced rejuvenation. A methodology on the selection of the validated two-shell cluster model reproducing the medium-range order was proposed.

Local configurations and atomic intermixing in as-quenched and annealed Fe1−xCrx and Fe1−xMox ribbons

Local atomic configuration, phase composition and atomic intermixing in Fe-rich Fe1−xCrx and Fe1−xMox ribbons (x = 0.05, 0.10, 0.15), of potential interest for high-temperature applications and nuclear devices, are investigated in this study in relation to specific processing and annealing routes. The Fe-based thin ribbons have been prepared by induction melting, followed by melt spinning and further annealed in He at temperatures up to 1250 °C. The complex structural, compositional and atomic configuration characterisation has been performed by means of X-ray diffraction (XRD), transmission Mössbauer spectroscopy and differential scanning calorimetry (TG-DSC). The XRD analysis indicates the formation of the desired solid solutions with body-centred cubic (bcc) structure in the as-quenched state. The Mössbauer spectroscopy results have been analysed in terms of the two-shell model. The distribution of Cr/Mo atoms in the first two coordination spheres is not homogeneous, especially after annealing, as supported by the short-range order parameters. In addition, high-temperature annealing treatments give rise to oxidation of Fe (to haematite, maghemite and magnetite) at the surface of the ribbons. Fe1−xCrx alloys are structurally more stable than the Mo counterpart under annealing at 700 °C. Annealing at 1250 °C in He enhances drastically the Cr clustering around Fe nuclei.

Hydrogen accumulation around dislocation loops and edge dislocations: from atomistic to mesoscopic scales in BCC tungsten

In a fusion tokamak, the plasma interacts with the metallic wall and the divertor. Hydrogen isotopes penetrate and diffuse into the material and interact with defects where they are trapped. Neutrons produced by the fusion reactions in the plasma are stopped in the material, creating defects, including vacancy and interstitial clusters, and dislocation loops. The trapping of hydrogen in vacancies has been extensively investigated. In our recent paper (De Backer et al 2017 Nucl. Fusion), we proposed a multi-scale model for H trapping and accumulation around interstitial defects, dislocation loops and dislocation lines. These defects create a long-range elastic field that attracts and may retain H atoms. A two-shell model with a short-range core region and a long-range elastic shell has been parameterized using a database of density functional theory (DFT) calculations. This model gives the number of H atoms forming the Cottrell atmosphere of a defect at finite temperature. In this paper, we present new DFT calculations of large dislocation loops decorated with up to 80 H, and explore our two-shell model in fusion relevant conditions. We conclude that large dislocation loops and edge dislocations can trap a significant number of hydrogen atoms in the core at temperatures up to 800 K, and also in the elastic field if the background hydrogen concentration is high.

Multiscale modelling of the interaction of hydrogen with interstitial defects and dislocations in BCC tungsten

In a fusion tokamak, the plasma of hydrogen isotopes is in contact with tungsten at the surface of a divertor. In the bulk of the material, the hydrogen concentration profile tends towards dynamic equilibrium between the flux of incident ions and their trapping and release from defects, either native or produced by ion and neutron irradiation. The dynamics of hydrogen exchange between the plasma and the material is controlled by pressure, temperature, and also by the energy barriers characterizing hydrogen diffusion in the material, trapping and de-trapping from defects. In this work, we extend the treatment of interaction of hydrogen with vacancy-type defects, and investigate how hydrogen is trapped by self-interstitial atom defects and dislocations. The accumulation of hydrogen on dislocation loops and dislocations is assessed using a combination of density functional theory (DFT), molecular dynamics with empirical potentials, and linear elasticity theory. The equilibrium configurations adopted by hydrogen atoms in the core of dislocations as well as in the elastic fields of defects, are modelled by DFT. The structure of the resulting configurations can be rationalised assuming that hydrogen atoms interact elastically with lattice distortions and that they interact between themselves through short-range repulsion. We formulate a two-shell model for hydrogen interaction with an interstitial defect of any size, which predicts how hydrogen accumulates at defects, dislocation loops and line dislocations at a finite temperature. We derive analytical formulae for the number of hydrogen atoms forming the Cottrell atmosphere of a mesoscopic dislocation loop or an edge dislocation. The solubility of hydrogen as a function of temperature, pressure and the density of dislocations exhibits three physically distinct regimes, dominated by the solubility of hydrogen in a perfect lattice, its retention at dislocation cores, and trapping by long-range elastic fields of dislocations.

Distribution of Cr atoms in a strained and strain-relaxed Fe89.15Cr10.75 alloy: a Mössbauer effect study

An Fe89.15Cr10.75 alloy in a heavily strained (by cold rolling) state and in strain-relaxed states was studied by means of conversion electrons Mössbauer spectroscopy. Analysis of the spectra in terms of a two-shell model revealed significant differences between the studied samples, particularly in values of the hyperfine field and the distribution of Cr atoms within the first two neighbour shells. The latter is quantified in terms of short-range order parameters.

Complexation of the Zn, Co, Cd, and Pb ions by metallothioneins: A QM/MM simulation

Complexation of divalent metal ions (Zn, Co, Cd and Pb) with metallothioneins (MT), typical low-molecular proteins of diverse higher organisms, has been theoretically studied within the all-electron approximation B3LYP/DZP. The active site of MT (metal ions and cysteine residues) adopts the structure, in which cadmium, cobalt and zinc ions reveal a tetrahedral location of the sulfur atoms. A pyramidal structure is typical for the coordination environment of the lead complexes in the MT active site. The Gibbs energies of the active site formation were calculated accounting the influence of water medium in terms of the IEFPCM approximation. For all studied metallothioneins, the metal–active site complexes demonstrate close Gibbs formation energies per one metal ion. The influence of the amino acid environment of the MT active sites has been studied with the two-shell ONIOM model and B3LYP/DZP:UFF approximation. A simple scheme for calculation of relative metal–MT binding energies has been developed. Even under the pronounced destabilization of the lead complexes by a polypeptide shell, the total energetic effect is sufficient to make accumulation of lead ions by metallothioneins more efficient than Cd and, especially, Zn and Co. The following general trend of stability of the MT complexes has been found: Co≈Zn≪Cd≪Pb. These results compared with the experimental regularities of bioaccumulation of heavy metal salts by bacteria Bacillus are briefly discussed.

PROBING CURVATURE EFFECTS IN THEFERMIGRB 110920

Curvature effects in Gamma-ray bursts (GRBs) have long been a source of considerable interest. In a collimated relativistic GRB jet, photons that are off-axis relative to the observer arrive at later times than on-axis photons and are also expected to be spectrally softer. In this work, we invoke a relatively simple kinematic two-shell collision model for a uniform jet profile and compare its predictions to GRB prompt-emission data for observations that have been attributed to curvature effects such as the peak-flux--peak-frequency relation, i.e., the relation between the $\nu$F$_\nu$ flux and the spectral peak, E$_{pk}$ in the decay phase of a GRB pulse, and spectral lags. In addition, we explore the behavior of pulse widths with energy. We present the case of the single-pulse Fermi GRB 110920, as a test for the predictions of the model against observations.

Phase-decomposition-related short-range ordering in an Fe–Cr alloy

A redistribution of Cr atoms related to a phase decomposition (PD) caused by an isothermal annealing at 415°C in a 15.15at.% Cr Fe–Cr alloy was studied in an ex situ way by the conversion electrons Mössbauer spectroscopy. Analysis of the spectra in terms of a two-shell model enabled the determination of probabilities of 17 different atomic configurations and average numbers of Cr atoms within the first (1NN) and the second (2NN) neighbor shells vs. annealing time, separately. The annealing-time evolution of these numbers, expressed in terms of the Cowley–Warren short-range order (SRO) parameters, was shown to follow the Johnson–Mehl–Avrami–Kolgomorov equation. The SRO parameter averaged over the 1NN–2NN shells was revealed to be linearly correlated with the average hyperfine field. Signatures in favor of the nucleation and growth mechanisms responsible for PD are also discussed.

Magnetic-Field Dependence of Tunnel Couplings in Carbon Nanotube Quantum Dots

By means of sequential and cotunneling spectroscopy, we study the tunnel couplings between metallic leads and individual levels in a carbon nanotube quantum dot. The levels are ordered in shells consisting of two doublets with strong- and weak-tunnel couplings, leading to gate-dependent level renormalization. By comparison to a one- and two-shell model, this is shown to be a consequence of disorder-induced valley mixing in the nanotube. Moreover, a parallel magnetic field is shown to reduce this mixing and thus suppress the effects of tunnel renormalization.

Transmembrane potential induced on the internal organelle by a time-varying magnetic field: a model study

BackgroundWhen a cell is exposed to a time-varying magnetic field, this leads to an induced voltage on the cytoplasmic membrane, as well as on the membranes of the internal organelles, such as mitochondria. These potential changes in the organelles could have a significant impact on their functionality. However, a quantitative analysis on the magnetically-induced membrane potential on the internal organelles has not been performed.MethodsUsing a two-shell model, we provided the first analytical solution for the transmembrane potential in the organelle membrane induced by a time-varying magnetic field. We then analyzed factors that impact on the polarization of the organelle, including the frequency of the magnetic field, the presence of the outer cytoplasmic membrane, and electrical and geometrical parameters of the cytoplasmic membrane and the organelle membrane.ResultsThe amount of polarization in the organelle was less than its counterpart in the cytoplasmic membrane. This was largely due to the presence of the cell membrane, which "shielded" the internal organelle from excessive polarization by the field. Organelle polarization was largely dependent on the frequency of the magnetic field, and its polarization was not significant under the low frequency band used for transcranial magnetic stimulation (TMS). Both the properties of the cytoplasmic and the organelle membranes affect the polarization of the internal organelle in a frequency-dependent manner.ConclusionsThe work provided a theoretical framework and insights into factors affecting mitochondrial function under time-varying magnetic stimulation, and provided evidence that TMS does not affect normal mitochondrial functionality by altering its membrane potential.

Two-Shell Ionospheric Model for Indian Region: A Novel Approach

In the U.S. Wide Area Augmentation System, European Geostationary Navigation Overlay Service, and Indian Global Positioning System Aided Geo Augmented Navigation, a near real-time grid-based single-shell model is proposed to correct the ionospheric delay at the user aircrafts. The single-shell model is based on the assumption that the whole ionosphere is compressed at a fixed altitude at 350 km. This assumption may not be appropriate for the Indian region, which falls in the Equatorial Ionospheric Anomaly belt. In this paper, a two-shell model which incorporates two different shells, at 300- and 500-km altitudes, having different weights at different time domains has been implemented. A statistical comparison between single- and two-shell models has been done for all quiet days of year 2005. Based on the results, it is observed that there is at least 60% improvement in the performance of the two-shell model in comparison to the single-shell model for the Indian region.

Wave propagation in double-walled carbon nanotubes conveying fluid

Carbon nanotubes may hold scientific promise in nanotechnology as nanopipes conveying fluid. In this paper, the wave propagation in double-walled carbon nanotubes (DWCNTs) conveying fluid is studied based on the Euler–Bernoulli beam theory. The influences of internal moving fluids, such as flow velocity and mass density of fluids, on the sound wave propagation of DWCNTs or the DWCNTs embedded in an elastic matrix are investigated in detail. The DWCNTs are considered as a two-shell model coupled together through the van der Waals interaction between two adjacent nanotubes. According to the proposed theoretical approach, the results indicate that fluid flow through carbon nanotubes affects the wave speed and the critical frequency in the carbon nanotubes. The amplitude ratios of the inner to outer nanotubes are largely affected by the fluid velocity and density when the vibrational frequency in nanotubes is larger than 1.5 Hz. The theoretical investigation may give a useful reference for potential application and design of nanoelectronics and nanodevices.

Effects of Oscillatory Electric Fields on Internal Membranes: An Analytical Model

We derive an analytical model of the potential differences induced across plasma and internal organelle membranes in suspended cells exposed to oscillatory electric fields. Multiple shells are modeled using iterative applications of the single-shell calculation with mobile charges. This work is motivated, in part, by recent results suggesting the ability to use alternating current (ac) fields to noninvasively monitor enzyme activity within internal membranes, particularly the mitochondrial electron transport chain. Previous work, on induced transmembrane voltages in cells subjected to ac fields, has mainly been limited to oscillatory potentials across the plasma membrane. Here we first develop a three-membrane model, consisting of a plasma membrane surrounding inner and outer membranes representing an internal organelle, such as a mitochondrion. Frequency-dependent transmembrane potentials are modeled for spherical, weakly conducting membrane shells enclosing a conductive cytoplasm surrounding an idealized internal organelle. We then use a two-shell model to simulate induced ac membrane potentials of a suspended isolated mitochondrion in which the outer membrane is usually much more permeable than the inner membrane.

Scattered emission from a relativistic outflow and its application to gamma-ray bursts

We investigate a scenario of photons scattering by electrons within a relativistic outflow. The outflow is composed of discrete shells with different speeds. One shell emits radiation for a short duration. Some of this radiation is scattered by the shell(s) behind. We calculate in a simple two-shell model the observed scattered flux density as a function of the observed primary flux density, the normalized arrival time delay between the two emission components, the Lorentz factor ratio of the two shells and the scattering shell's optical depth. Thomson scattering in a cold shell and inverse Compton scattering in a hot shell are both considered. The results of our calculations are applied to the Gamma-Ray Bursts and the afterglows. We find that the scattered flux from a cold slower shell is small and likely to be detected only for those bursts with very weak afterglows. A hot scattering shell could give rise to a scattered emission as bright as the X-ray shallow decay component detected in many bursts, on a condition that the isotropically equivalent total energy carried by the hot electrons is large, $\sim 10^{52-56}$ erg. The scattered emission from a faster shell could appear as a late short $\gamma$-ray/MeV flash or become part of the prompt emission depending on the delay of the ejection of the shell.

What can quantum chemistry tell us about Pa(v) hydration and hydrolysis?

Published liquid–liquid extraction studies of Pa(V) were interpreted with aqueous mono-, di- and trications. B3LYP DFT is applied here to such cations surrounded by two explicit hydration layers: Linear or tetrahedral geometries are found for the Pa(V) aquo ions. PaO2+ is similar to the other AnO2+ cations, but has strong apical bonds, resulting from the highly negative Oyl charge, which decreases along the An(V) series. This explains the instability of PaO2+ in water, and the differences with the heavier An(V). PaO2+ diprotonates to give Pa(OH)23+ and can further dihydrolyse to give Td-Pa(OH)4+, which might very well be the most stable Pa(V) monocation. PaOOH2+ is confirmed to be the Pa(V) aqueous dication invoked in the literature for pH ≤ 1.4 ± 0.7. PaO3+ is confirmed in sulfate solution, with a bond length close to 180 pm. Pa(OH)23+ cannot be excluded in other conditions. The strong influence of the solvent was not fully taken into account in most previous theoretical studies that focused only on bare or partially solvated PaO2+. Toraishi et al. have studied hydrated Pa(V) and our work confirms this study and its qualitative interpretation. The new tetrahedral Pa(OH)4+ geometry that is shown here to be important opens the field to further quantum chemical studies of Pa(V) and other f-elements. As a test for the two-shell model approach for Pa(V), fluoride coordination to Pa(V) is studied and compared with published EXAFS data: an excellent fit is obtained with the well-established species PaF72−, but most other stoichiometries tested are precluded.

Vibrational analysis of fluid-filled carbon nanotubes using the wave propagation approach

This paper presents a method for vibrational analysis of fluid-filled double-walled carbon nanotubes using the wave propagation approach. Simplified Flügge shell equations are proposed as the governing equations of vibration for the carbon nanotubes. The double-walled nanotubes are considered as a two-shell model coupled together through the van der Waals interaction between two adjacent nanotubes. Based on the proposed theoretical approach, we investigate the influences of parameters, such as fluid properties and vibrational modes, on the vibrational characteristics of carbon nanotubes. In this study, we consider the double-walled nanotubes with an inner diameter of 2.2 nm and an outer diameter of 3.0 nm. The theoretical investigation may give a useful reference for potential application and design of nanoelectronics and nanodevices.

Protein influence on the plasma membrane dielectric properties: In vivo study utilizing dielectric spectroscopy and fluorescence microscopy

We have investigated the origin of the dielectric response of the plasma membrane of living yeast cells ( Saccharomyces cerevisiae) by using radiofrequency dielectric spectroscopy. The cells were genetically engineered to overexpress in the membrane of yeast cells a G protein-coupled receptor – the Sterile2-α factor receptor protein (Ste2p) – fused to the green fluorescent protein (GFP). Presence of the Ste2-GFP proteins in the plasma membrane was confirmed by exciting the cells at 476 nm and observing with a confocal microscope the emission characteristic of the GFP from individual cells. The dielectric behavior of cells suspended in KCl solution was analyzed over the frequency range 40 Hz–110 MHz and compared to the behavior of control cells that lacked the ability to express Ste2p. A two-shell electrical cell model was used to fit the data starting from known structural parameters and adjustable electrical phase parameters. The best-fit value for the relative permittivity of the plasma membrane showed no significant difference between cells expressing Ste2p (1.63 ± 0.11) and the control cells (1.75 ± 0.16). This result confirmed earlier predictions that the dielectric properties of the plasma membrane in the radiofrequency range mostly reflect the properties of the hydrophobic layer of the membrane, which is populated by the hydrocarbon tails of the phospholipids and hydrophobic segments of integral membrane proteins. We discuss ways by which dielectric spectroscopy can be improved to be used for tag-free detection of proteins on the membrane.