Effective Ionic Charge Research Articles

Determination of effective charge of small ions, polyelectrolytes and nanoparticles by capillary electrophoresis

In this paper, a systematic and comparative study related to the effective charge determination of three kinds of solutes (small ions, polyelectrolytes and nanoparticles) was performed. Four approaches were compared regarding their conditions of validity and their advantages/disadvantages. Three of them allow the effective charge determination from the electrophoretic mobility and the hydrodynamic radius of the solutes using electrophoretic mobility modelings based on Nernst–Einstein (NE), O’Brien–White–Ohshima (OWO) and Yoon and Kim (YK) equations. Electrophoretic mobility and hydrodynamic radius were determined by capillary electrophoresis and Taylor dispersion analysis, respectively, using the same instrumentation in similar conditions, on a large set of samples. A fourth experimental approach based on the sensitivity of detection in indirect UV detection mode (IUV) was compared to the previously mentioned methods. OWO and YK modelings are well adapted for the effective charge determination of small ions and nanoparticles, while IUV is the only method adapted for polyelectrolytes.

Rotational dynamics of benzene and water in an ionic liquid explored via molecular dynamics simulations and NMR T1 measurements

The rotational dynamics of benzene and water in the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride are studied using molecular dynamics (MD) simulation and NMR T(1) measurements. MD trajectories based on an effective potential are used to calculate the (2)H NMR relaxation time, T(1) via Fourier transform of the relevant rotational time correlation function, C(2R)(t). To compensate for the lack of polarization in the standard fixed-charge modeling of the IL, an effective ionic charge, which is smaller than the elementary charge is employed. The simulation results are in closest agreement with NMR experiments with respect to the temperature and Larmor frequency dependencies of T(1) when an effective charge of ±0.5e is used for the anion and the cation, respectively. The computed C(2R)(t) of both solutes shows a bi-modal nature, comprised of an initial non-diffusive ps relaxation plus a long-time ns tail extending to the diffusive regime. Due to the latter component, the solute dynamics is not under the motional narrowing condition with respect to the prevalent Larmor frequency. It is shown that the diffusive tail of the C(2R)(t) is most important to understand frequency and temperature dependencies of T(1) in ILs. On the other hand, the effect of the initial ps relaxation is an increase of T(1) by a constant factor. This is equivalent to an "effective" reduction of the quadrupolar coupling constant (QCC). Thus, in the NMR T(1) analysis, the rotational time correlation function can be modeled analytically in the form of aexp (-t/τ) (Lipari-Szabo model), where the constant a, the Lipari-Szabo factor, contains the integrated contribution of the short-time relaxation and τ represents the relaxation time of the exponential (diffusive) tail. The Debye model is a special case of the Lipari-Szabo model with a = 1, and turns out to be inappropriate to represent benzene and water dynamics in ILs since a is as small as 0.1. The use of the Debye model would result in an underestimation of the QCC by a factor of 2-3 as a compensation for the neglect of the Lipari-Szabo factor.

Scaling of linear microtearing stability for a high collisionality National Spherical Torus Experiment discharge

Linear gyrokinetic simulations are performed based on a high collisionality NSTX discharge that is part of dimensionless confinement scaling studies. In this discharge, the microtearing mode is predicted to be unstable over a significant region of the plasma (r/a = 0.5–0.8), motivating comprehensive tests to verify the nature of the mode and how it scales with physical parameters. The mode is found to be destabilized with sufficient electron temperature gradient, collisionality, and beta, consistent with previous findings and simple theoretical expectations. Consistent with early slab theories, growth rates peak at a finite ratio of electron-ion collision frequency over mode frequency, νe/i/ω ∼ 1–6. Below this peak, the mode growth rate decreases with reduced collisionality, qualitatively consistent with global confinement observations. Also, in this region, increased effective ionic charge (Zeff) is found to be destabilizing. Experimental electron beta and temperature gradients are two to three times larger than the inferred linear thresholds. Increasing magnetic shear (s) and decreasing safety factor (q) are both destabilizing for ratios around the experimental values s/q = 0.6–1.3. Both the Zeff and s/q scaling are opposite to those expected for the ETG instability offering an opportunity to experimentally distinguish the two modes. Finally, we note that the kinetic ballooning mode is found to compete with the microtearing mode at outer locations r/a ≥ 0.8.

High resolution charge-exchange spectroscopic measurements of aluminum impurity ions in a high temperature plasma

Charge-exchange recombination spectroscopy, which is generally used to measure low-Z impurities in fusion devices, has been used for measuring Al+11 and Al+13 impurities in the Madison Symmetric Torus reversed field pinch. To obtain the impurity ion temperature, the experimental emission spectrum is fitted with a model which includes fine structure in the atomic transition. Densities of these two ionization states, calculated from charge-exchange emission brightness, are used in combination with a collisional radiative model to estimate the abundance of all other charge states of aluminum in the plasma and the contribution of aluminum to the effective ionic charge of the plasma.

Electronic structure of Al-doped ZnO transparent conductive thin films studied by x-ray absorption and emission spectroscopies

This study used O K-, Zn L3-, Zn K-, and Al K-edges x-ray absorption near-edge structure (XANES) and O K-edge x-ray emission spectroscopy (XES) measurements to investigate the electronic structure of transparent Al-doped ZnO (AZO) thin film conductors. The samples were prepared on glass substrates at a low temperature near 77 K by using a standard RF sputtering method. High-purity Ne (5N) was used as the sputtering gas. The crystallography of AZO thin films gradually transformed from the ZnO wurtize structure to an amorphous structure during sample deposition, which suggests the suitability to grow on flexible substrates, eliminating the severe degradation due to fragmentation by repeated bending. The O K- and Zn L3-edges XANES spectra of AZO thin films revealed a decrease in the number of both O 2p and Zn 3d unoccupied states when the pressure of Ne was increased from 5 to 100 mTorr. In contrast, Al K-edges XANES spectra showed that the number of unoccupied states of Al 3p increased in conjunction with the pressure of Ne, indicating an electron transfer from Al to O atoms, and suggesting that Al doping increases the negative effective charge of oxygen ions. XES and XANES spectra of O 2p states at the O K-edge also revealed that Al doping not only raised the conduction-band-minimum, but also increased the valence-band-maximum and the band-gap. The results indicate that the reduction in conductivity of AZO thin films is due to the generation of ionic characters, the increase in band-gap, and the decrease in density of unoccupied states of oxygen.

Theoretical investigations of the electronic and magnetic structures for the [ CoCl 4] 2− cluster in Cs 3CoCl 5 crystal

On the basis of the complete energy matrices for a d 7 configuration ion in D 2 d symmetry ligand-field, the local structure parameter θ = 52.60° for [ CoCl 4 ] 2− clusters in Cs 3CoCl 5 crystal is determined. It is shown that the ligand-field calculation results of θ are associated with the equivalent positions of the effective charges of chlorine ions, which are slightly different from the nuclear positions of chlorine ions θ 0 = 53.0° ± 0.2° found by X-ray diffraction experiment. This difference means that there exists an obvious polarization effect for the electron clouds of chlorine ions in Cs 3CoCl 5 crystal. Furthermore, the theoretical g-factors and paramagnetic susceptibility are in agreement with the experimental findings.

NSTX plasma response to lithium coated divertor

NSTX experiments have explored lithium evaporated on a graphite divertor and other plasma-facing components in both L- and H- mode confinement regimes heated by high-power neutral beams. Improvements in plasma performance have followed these lithium depositions, including a reduction and eventual elimination of the HeGDC time between discharges, reduced edge neutral density, reduced plasma density, particularly in the edge and the SOL, increased pedestal electron and ion temperature, improved energy confinement and the suppression of ELMs in the H-mode. However, with improvements in confinement and suppression of ELMs, there was a significant secular increase in the effective ion charge Zeff and the radiated power in H-mode plasmas as a result of increases in the carbon and medium-Z metallic impurities. Lithium itself remained at a very low level in the plasma core, <0.1%. Initial results are reported from operation with a Liquid Lithium Divertor (LLD) recently installed.

Potential of a Bayesian Integrated Determination of the Ion Effective Charge via Bremsstrahlung and Charge Exchange Spectroscopy in Tokamak Plasmas

Reliable and accurate estimates of the ion effective charge <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z_{\rm eff}$ </tex></formula> in tokamak plasmas are of key importance with respect to the impurity transport studies and the establishment of thermonuclear burn criteria. These issues are of fundamental interest to ITER and reactor operational scenarios in general. However, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z_{\rm eff}$</tex></formula> estimates derived from bremsstrahlung spectroscopy on the one hand and from the weighted summation of individual impurity concentrations obtained via charge exchange spectroscopy (CXS) on the other hand often are not compatible. This is a long-standing problem in fusion plasma diagnosis. A rigorous analysis of uncertainty sources and their propagation in the experimental determination of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z_{\rm eff}$</tex></formula> can contribute significantly to the derivation of a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z_{\rm eff}$</tex></formula> value with reduced uncertainty that is consistent with both the bremsstrahlung and CXS data sets. In this paper, Bayesian probability theory is used in an integrated approach as a powerful tool for an advanced error analysis in the derivation of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z_{\rm eff}$</tex></formula> , even in the presence of systematic errors on the data. A simple probabilistic model is proposed for the estimation of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z_{\rm eff}$</tex></formula> , first assuming only statistical uncertainty and then taking into account also the systematic deviations. The obtained <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z_{\rm eff}$</tex></formula> estimates have smaller error bars than the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$Z_{\rm eff}$ </tex></formula> values derived from the individual bremsstrahlung and CXS measurements, approaching ITER requirements. The estimates are shown to be consistent with all available information. In addition, the systematic errors on the data are quantized through the requirement of data consistency between different time slices in the acquired measurements.

Integrated analysis and consistency measurement of bremsstrahlung and charge exchange spectroscopy data for the determination of the ion effective charge

In the context of Bayesian probability theory, we discuss a model for estimating the plasma ion effective charge Z(eff), integrating data from both bremsstrahlung spectroscopy and individual impurity concentrations obtained via charge exchange spectroscopy (CXS). The validity of the model, taking into account statistical as well as systematic uncertainties, is shown via the deviance information criterion. The consistency of the continuum and CXS data regarding Z(eff) is improved, as measured by the symmetrized Kullback-Leibler divergence and the geodesic distance between the respective Z(eff) marginal posterior densities.

Ferroelectric dead layer driven by a polar interface

Based on first-principles and model calculations we investigate the effect of polar interfaces on the ferroelectric stability of thin-film ferroelectrics. As a representative model, we consider a TiO2-terminated BaTiO3 film with LaO monolayers at the two interfaces that serve as doping layers. We find that the polar interfaces create an intrinsic electric field that is screened by the electron charge leaking into the BaTiO3 layer. The amount of the leaking charge is controlled by the boundary conditions which are different for three heterostructures considered, namely Vacuum/LaO/BaTiO3/LaO, LaO/BaTiO3, and SrRuO3/LaO/BaTiO3/LaO. The intrinsic electric field forces ionic displacements in BaTiO3 to produce the electric polarization directed into the interior of the BaTiO3 layer. This creates a ferroelectric dead layer near the interfaces that is non-switchable and thus detrimental to ferroelectricity. Our first-principles and model calculations demonstrate that the effect is stronger for a larger effective ionic charge at the interface and longer screening length due to a stronger intrinsic electric field that penetrates deeper into the ferroelectric. The predicted mechanism for a ferroelectric dead layer at the interface controls the critical thickness for ferroelectricity in systems with polar interfaces.

Estimation of profiles of the effective ion charge at ASDEX Upgrade with Integrated Data Analysis

The knowledge of the effective ion charge Zeff which indicates the degree of plasma pollution is of high interest, since the tolerable impurity concentration to achieve successful ignition of a fusion plasma is limited. The quantification of Zeff at ASDEX Upgrade has been carried out by means of Integrated Data Analysis in the framework of Bayesian probability theory which allows the combined analysis of different bremsstrahlung and charge exchange diagnostics to obtain one joint Zeff profile. With this tool it is possible to assess the quality of the data acquired by the individual diagnostics and to find a reasonable selection of data for the routine Zeff analysis. The Zeff results are validated by the examination of different discharge scenarios and successfully applied to simulations of the neutron rate and the loop voltage. The investigation of Zeff for nitrogen seeded discharges reveals a correlation between the confinement improvement and the increase in Zeff.

Structure and Dynamics of 1-Ethyl-3-methylimidazolium Acetate via Molecular Dynamics and Neutron Diffraction

The liquid state structure of the ionic liquid, 1-ethyl-3-methylimidazolium acetate ([C(2)mim][OAc]), an excellent nonderivitizing solvent for cellulosic biomass, has been investigated at 323 K by molecular dynamics (MD) simulation and by neutron diffraction using the SANDALS diffractometer at ISIS to provide experimental differential neutron scattering cross sections from H/D isotopically substituted materials. Ion-ion radial distribution functions both calculated from MD and derived from the empirical potential structure refinement (EPSR) model to the experimental data show the alternating shell structure of anions around the cation, as anticipated. Spatial probability distributions reveal the main anion-to-cation features as in-plane interactions of anions with the three imidazolium ring hydrogens and cation-cation planar stacking above/below the imidazolium rings. Interestingly, the presence of the polarized hydrogen-bond acceptor (HBA) anion (acetate) leads to an increase in anion-anion tail-tail structuring within each anion shell, an indicator of the onset of hydrophobic regions within the anion regions of the liquid. MD simulations show the importance of scaling of the effective ionic charges in the basic simulation approach to accurately reproduce both the observed experimental neutron scattering cross sections and ion self-diffusion coefficients.

Electromigration Force on a Proton with a Bound State

The driving force on an ion in a metal due to an applied electric field, called the electromigration force, is built up out of two contributions, a wind force and a direct force. The wind force is due to the scattering of the current carrying electrons off the ion. The direct force works on the effective charge of the ion. In the present work we concentrate on the direct force on a migrating proton embedded in an electron gas. For this force a sign change is obtained as soon as a bound state is formed. In recent calculations hardly a sign change was seen, although a bound state was found in a self-consistent-potential for lower electron densities. Here we show that a supplementary term shows up, as soon as one accounts for the bound state explicitly. By this the problem has been solved regarding a possible lack of completeness of the published formalism. The results presented are based on square-well model potentials. By using different depths it is possible to show results for potentials without a bound state and accommodating one bound state.

The influence of alkaline earth ions on the structural organization of acetone probed by the noncoincidence effect of the ν(CO) band: experimental and quantum chemical results

We have investigated the Raman noncoincidence effect (NCE = nu(aniso)-nu(iso), where nu(aniso) and nu(iso) are the anisotropic and the isotropic Raman frequencies) of the nu(C=O) band of acetone arising from the interactions of this solvent with the metal ions in acetone electrolytic solutions of alkaline earth metal (Mg, Ca, Sr, Ba) perchlorates. Assisted by the results of ab initio molecular orbital (MO) calculations carried out at the Hartree-Fock (HF) level with the 6-31+G(2df,p) and LanL2DZ basis sets, we have been able to attribute the anisotropic and isotropic components of this band to the formation of acetone-metal ion clusters, (acetone)(n)M(2+), and to interpret its high and negative NCE, opposed to the positive NCE of the bulk liquid, as the consequence of the large separation between the higher frequency of the in-phase mode (active in the Raman isotropic spectrum) and the lower (average) frequency of the n- 1 out-of-phase modes (predominantly active in the Raman anisotropic spectrum). The negative sign of the NCE is compatible with the transition dipole coupling (TDC) mechanism. The comparison between the observed NCE for each electrolytic solution at the concentrations used in this study and those calculated for the different solvation numbers n of each (acetone)(n)M(2+)cluster gives a clear indication of the highest stability of the hexa-coordinated cluster for the Mg(2+) ion, but leaving uncertain (n = 6 or 8) this conclusion for the acetone clusters of the remaining M(2+) ions. We have interpreted the observed and calculated decrease of the magnitude of NCE with the ion size through the ion polarizing power in the light of the ion effective charge and its distance (M(2+)...O=C) from the C=O oscillators.

Plasma response to lithium-coated plasma-facing components in the National Spherical Torus Experiment

Experiments in the National Spherical Torus Experiment (NSTX) have shown beneficial effects on the performance of divertor plasmas as a result of applying lithium coatings on the graphite and carbon-fiber-composite plasma-facing components. These coatings have mostly been applied by a pair of lithium evaporators mounted at the top of the vacuum vessel which inject collimated streams of lithium vapor toward the lower divertor. In neutral beam injection (NBI)-heated deuterium H-mode plasmas run immediately after the application of lithium, performance modifications included decreases in the plasma density, particularly in the edge, and inductive flux consumption, and increases in the electron and ion temperatures and the energy confinement time. Reductions in the number and amplitude of edge-localized modes (ELMs) were observed, including complete ELM suppression for periods of up to 1.2 s, apparently as a result of altering the stability of the edge. However, in the plasmas where ELMs were suppressed, there was a significant secular increase in the effective ion charge Zeff and the radiated power as a result of increases in the carbon and medium-Z metallic impurities, although not of lithium itself which remained at a very low level in the plasma core, <0.1%. The impurity buildup could be inhibited by repetitively triggering ELMs with the application of brief pulses of an n = 3 radial field perturbation. The reduction in the edge density by lithium also inhibited parasitic losses through the scrape-off-layer of ICRF power coupled to the plasma, enabling the waves to heat electrons in the core of H-mode plasmas produced by NBI. Lithium has also been introduced by injecting a stream of chemically stabilized, fine lithium powder directly into the scrape-off-layer of NBI-heated plasmas. The lithium was ionized in the SOL and appeared to flow along the magnetic field to the divertor plates. This method of coating produced similar effects to the evaporated lithium but at lower amounts.

Nano- and micrometer-scale thin-film-interconnection failure theory and simulation and metallization lifetime prediction, Part 1: A general theory of vacancy transport, mechanical-stress generation, and void nucleation under electromigration in relation to multilevel-metallization degeneration and failure

The physical principles are considered of on-chip interconnection degeneration and failure in sub-1-µm technologies from the viewpoint of multilevel-metallization reliability. A general theory and simulation results are presented that deal with electromigration-induced failure of thin-film conducting tracks on the micro- and nanometer scales. They provide a detailed treatment of micro-, meso-, and nanoscopic mechanisms underlying the deformation and breaking of actual interconnection configurations. Parts 1 and 2 contain (i) the derivation and analysis of basic kinetic equations; (ii) the formulation and solution of three-dimensional boundary-value problems representing the transport of vacancies (both in the bulk and along grain boundaries) and the development of deformation and stress; (iii) models of void nucleation and development; (iv) an analysis of a multilevel-metallization lifetime; and (v) the identification of potential break locations in a multilevel metallization in relation to current density, metallization structure and layout, temperature, and the grain structure of the conducting material. Part 3 deals with electromigration in doped on-chip polycrystalline interconnections in terms of lengthening their lifetime. To predict electromigration resistance, models are constructed that represent the influence of texture and other grain-boundary properties on the effective charges of native and dopant ions.

Effect of Co, Ni, and Cu substitution on the electronic structure of hexagonal YMnO3 studied by x-ray absorption spectroscopy

X-ray absorption spectroscopy measurements have been performed to elucidate local electronic and atomic structures of orthorhombic 3d-transition metal-doped yttrium manganites (YMnO3) with chemical formulae YMn2/3Me1/3O3 (Me=Co, Ni, and Cu). The Mn L3- and K-edges x-ray absorption near-edge structure (XANES) demonstrate the direct substitution of Me2+ for Mn3+, so that the positive effective charge of Mn ions are increased. Me-doping is also found to induce substantial broadening of the Mn L3-edge feature, which suggests enhancement of the delocalization of Mn 3d eg subbands and conductivity. Local spin density approximation (LSDA)+U (Hubbard U parameter) calculations were used to understand their electronic structures.

A method for calculating the effective charge of ions decelerated in a hot dense plasma

A method for calculating the effective charge of fast ions decelerated in a hot dense plasma is proposed. The method is based on the known experimental dependence of the effective charge of an ion decelerated in cold matter on its velocity. The ion velocity in this dependence is replaced with the velocity of an ion relative to plasma electrons, averaged over the Fermi-Dirac distribution. Using results of numerical calculations performed in a wide range of plasma parameters (from a Maxwellian plasma to a fully degenerate one), a scale-invariant representation of the effective charge of a decelerating ion as a function of its initial velocity and the plasma temperature and density is obtained. An analytical formula fitting the calculated results to within 5% is derived. The obtained dependences of the effective charge are incorporated in the model describing deceleration of fast ions in plasma. Using this model, the stopping powers of krypton and lead ions in a relatively cold rarefied gas-discharge plasma and hot ICF plasma are calculated. The results of calculations are shown to agree satisfactorily with available experimental data.

Geometric ferroelectricity in rare-earth compoundsRGaO3andRInO3

We have studied the stability and ferroelectric properties of hexagonal $R{\text{GaO}}_{3}$ and $R{\text{InO}}_{3}$ ($R$: rare-earth elements) by first-principles calculations. Computed spontaneous polarization in the series shows a systematic increase with the rare-earth elements, with values being larger in $R{\text{InO}}_{3}$ than in the corresponding $R{\text{GaO}}_{3}$. The largest polarization found is about $10\text{ }\ensuremath{\mu}\text{C}/{\text{cm}}^{2}$ for ${\text{ErInO}}_{3}$, which is about twice as large as those observed in hexagonal $R{\text{MnO}}_{3}$. The polarization can be further increased by applying in-plane compressive stress. The Born effective charges of constituent ions in the compounds are found to be similar to their formal values, implying that the ferroelectric displacements are merely driven by the ionic size effect. A transition to the high-symmetry phase at around 1500 K was confirmed in ${\text{GdInO}}_{3}$ and ${\text{DyInO}}_{3}$ by in situ high-temperature powder x-ray diffractometry. The present systems should belong to the family of geometric ferroelectrics.

Low-temperature synthesis of supported hydrodesulfurization catalysts based on chevrel phases

A new method has been developed for the synthesis of finely dispersed, highly active, supported hydrodesulfurization catalysts based on Chevrel phases. It is hypothesized that the modification of MoS2 with cobalt or nickel, which enhances the catalytic activity, and the same modification of Chevrel-type systems are of the same nature. The modifiers act through electron density donation into the conduction band of the active component. The increase in catalytic activity is due to the decrease of the effective charge of the molybdenum ion. The catalysts undergo partial restructuring during the reaction.