Showing papers on "Charge density published in 1991"

First-principles study of the electronic properties of graphite

Abstract: The electronic properties of hexagonal graphite have been studied in the framework of the density-functional technique, using nonlocal ionic pseudopotentials and a large number of plane waves. The valence charge density and the density of states are presented, as well as the band structure and the charge-density contributions of some typical wave functions. The electronic energies, at the Fermi level, are parametrized by the Slonczewski-Weiss-McClure model, and compared with the parametrization of experimental data. The numerical accuracy of the calculation has been controlled, in order to provide a reliable comparison between theory and experiment. In particular, the agreement obtained in the framework of the density-functional theory for electronic energies at the Fermi level is surprisingly good.

Electrostatic effects in proteins: comparison of dielectric and charge models.

Abstract: Two approaches for calculating electrostatic effects in proteins are compared and ana analysis is presented of the dependence of calculated properties on the model used to define the charge distribution. Changes in electrostatic free energy have been calculated using a screened Coulomb potential (SCP) with a distance-dependent effective dielectric permittivity to model bulk solvent effects and a finite difference approach to solve the Poisson-Boltzmann (FDPB) equation. The properties calculated include shifts in dissociation constants of ionizable groups, the effect of annihilating surface charges on the binding of metals, and shifts in redox potentials due to changes in the charge of ionizable groups. In the proteins considered the charged sites are separated by 3.5-12 A. It is shown that for the systems studied in this distance range the SCP yields calculated values which are at least as accurate as those obtained from solution of the FDPB equation. In addition, in the distance range 3-5 A the SCP gives substantially better results than the FDPB equation. Possible sources of this difference between the two methods are discussed. Shifts in binding constants and redox potentials were calculated with several standard charge sets, and the resulting values show a variation of 20-40% between the 'best' and 'worst' cases. From this study it is concluded that in most applications, changes in electrostatic free energies can be calculated economically and reliably using an SCP approach with a single functional form of the screening function.

Mechanism of negative‐bias‐temperature instability

Abstract: Although negative‐bias‐temperature instability in metal‐oxide‐semiconductor integrated circuits has been minimized empirically, the exact mechanism is unknown. We argue in this paper that the mechanism of negative‐bias‐temperature instability can be modeled by a first‐order electrochemical reaction between hydrogenated trivalent silicon, a neutral water‐related species located in the oxide near the Si‐SiO2 interface, and holes at the silicon surface to form neutral trivalent silicon and a positively charged water‐related species. To show that such a reaction describes the phenomenon, we show that (1) water must be present in the oxide near the Si‐SiO2 interface, (2) induced interface and oxide‐fixed charge densities are equal, (3) the saturation interface‐trap and oxide‐fixed charge densities depend on the initial hole concentration at the silicon surface or aging field, (4) the buildup of these charge densities follows first‐order reaction kinetics, and (5) time constants for this charge buildup are inde...

Electric field soundings through thunderstorms

Abstract: Twelve balloon soundings of the electric field in thunderstorms are reported. The maximum magnitude of E in the storms averaged 96 +/-28 kV/m, with the largest being 146 kV/m. The maximum was usually observed between vertically adjacent regions of opposite charge. Using a 1D approximation to Gauss' law, four to ten charge regions in the storms are inferred. The magnitude of the density in the charge regions varied between 0.2 and 13 nC/cu m. The vertical extent of the charge regions ranged from 130 to 2100 m. None of the present 12 storms had charge distributions that fit the long-accepted model of Simpson et al. (1937, 1941) of a lower positive charge, a main negative charge, and an upper positive charge. In addition to regions similar to the Simpson model, the present storms had screening layers at the upper and lower cloud boundaries and extra charge regions, usually in the lower part of the cloud.

Structure of adsorption layers of ionic surfactants at the solid/liquid interface

Abstract: A large number of experimental results of different surfactant adsorption systems (mainly on the silicas) obtained from both equilibrium and kinetic studies under different conditions are interpreted by a model of small individual surface aggregates. The adsorption model is contrasted with the influences of various factors, including electrostatic interaction, hydrophobic interaction, concentrations, types of coions, types of counterions, surfactant structure, alkyl chain length, types of head groups, neutral electrolytes, pH, adsorbent structure, porosity, surface charge density, and surface polarity.

Time‐dependent electron emission from ferroelectrics by external pulsed electric fields

Abstract: Results of experiments investigating the influence of the amplitude, the growth rate, and the repetition rate of the exciting electric field pulses on the electron current and charge density emitted from ferroelectrics are reported. The behavior of two types of lead‐lanthanum‐zirconium‐titanate (PLZT) ceramics, 2/95/5 and 8/65/35, was studied. The temporal shift between the applied HV pulses and the emitted electron current pulses was different for the two materials. Regular electron emission was observed at repetition rates of up to 2 MHz in the PLZT‐2/95/5 material, showing that the recovery of the emitting sample back to the original state may happen in less than a microsecond.

Charge accumulation on insulating spacers for HVDC GIS

Abstract: A capacitive probe measurement that can be used to quantify the charge density on solid insulators is introduced. Studies performed in the development of a +or-125 kV high-voltage DC (HVDC) gas-insulated converter station and +or- 500 kV HVDC switchgear (HVDC-GIS) are reviewed. The properties and mechanisms of surface charging, the optimum design of the spacer and its breakdown characteristics are summarized. Problems associated with capacitive probe measurement of surface charge and a practical solution to obtain the charge distribution on the spacer are presented. >

Organo-Bentonites with Quaternary Alkylammonium Ions

Abstract: A B S T R A C T: Three bentonites, from Bavaria, Wyoming and Brazil, were separated into various fractions. The layer charge was determined by alkylammonium ion exchange and increases with particle size from 0.25 Eq/(Si,A1)4Ol0 (<0.06 gm) to 0-28 Eq/(Si,Al)4010 (l-10 #m). The charge density corresponds to an interlayer cation density of 0-75-0.80 mEq/g silicate. Total amounts of 0.90-1-0 mEq of different surfactant cations (dimethyl dioctadecylammonium, trimethyl tetradecylammonium, alkylammonium ions) are bound per gram silicate. The difference between the total and the interlayer amount of surfactant ions decreases with increasing particle size. The amounts exceeding the interlayer CEC are bound at the edges. Tetramethylammonium (TM) ions restrict the interlayer adsorption of long-chain quaternary alkylammonium ions such as trimethyl tetradecylammonium (TMTD) ions, and only monolayers of flat-lying surfactants are formed. A ratio of TMTD and TM is attained which leads to densely packed monolayers of organic ions. The collapsing effect is smaller for tetraethylammonium ions so that considerable amounts of TMTD ions are adsorbed in bilayers. When bentonites are reacted with quaternary alkylammonium ions of technical quality some selectivity is observed according to particle size and layer charge. Smaller particles with lower charge density preferentially bind the longer chain compounds, whereas large particles with higher charge density select smaller sized surfactants.

Ion condensation on solid particles: Theory and simulations

Abstract: For polyelectrolytes the Manning theory predicts an effective charge density, which increases with the bare charge density up to the point where the thermal energy of an ion balances the reversible work necessary to remove the ion from the polymer. From that point on, the effective charge remains constant, as no more ions will desorb. For spherical colloidal particles, a similar maximum effective charge can be expected, which indeed is predicted by the Poisson–Boltzmann theory. To check these predictions quantitatively, Monte Carlo simulations have been performed on the ionic distribution of a salt‐free colloid at a volume fraction of 36%, both in the spherical cell model and in a periodic system of cubic symmetry. In the latter system the interactions were evaluated using the Ewald summation method. These simulations show that the effective charge of a colloid at this volume fraction does not reach a plateau value for large bare charges, but instead the effective charge passes through a maximum, and decr...

Ion solvation dynamics in an interaction-site model solvent

Abstract: The molecular theory of the frequency-dependent and wavevector-dependent longitudinal dielectric function ϵL(k, ω) is derived for fluids comprising interaction-site model molecules in which point charges are located on the interaction sites. We find that ϵL(k, ω) is a simple functional of a particular charge susceptibility χμϕ0 (k, ω), which in turn is related to a collective charge-charge equilibrium time correlation function. The electrostatic part FBorn (t) of the time-dependent free energy of solvation of a solute that instantaneously changes its charge state is, on the other hand, determined by a charge susceptibility χμϕ0(1) (k, kt, ω) of the solvent in the presence of the solute molecule in its initial charge state. Using an approximate relation between χμϕ0(1) and χμϕ0 we express F Born(t) in terms of χμϕ0. The resulting theory is applied to calculate the solvation time correlation function of the solute immersed in a dipolar hard sphere (DS) and in dipolar dumbbell (DD) model solvent; the mean spherical approximation and an extended mean spherical approximation are used to compute the structure of the DS and DD solvent models, respectively. With parameters chosen so that the two models have the same molecular volume and the same electric dipole moment, it is found that they have very nearly the same ϵL(k, ω) except at large wavevector, but significantly different solvation time correlation functions.

Capacitance of a circular symmetric model of a via hole including finite ground plane thickness

Abstract: The capacitance of a simplified model of a via hole is calculated based on an integral equation approach for the surface charge density. The formulation of the problem is based on an integral equation for the surface charges combined with an analytical solution at the ground plane opening. The finite ground plane thickness is explicitly taken into account. Numerical data are obtained for a large range of realistic geometrical data. The relative importance of the contribution to the total capacitance coming from the ground plane opening is explicitly evaluated. It is found that the via capacitance is proportional to the square root of its height, at least for the range of geometrical data considered. >

Simulation of corona in wire-duct electrostatic precipitator by means of the boundary element method

Abstract: A new approach for modeling the space density and the electric field distribution in electrostatic precipitators is presented in this paper. Two numerical techniques are used iteratively until a self-consistent solution is obtained: the boundary element method, for evaluation of the electric potential; and the characteristic method, determining the charge distribution. This hybrid approach is applied to simulate the electrical conditions in a wire-duct electrostatic precipitator and obtained results are compared with the experimental data. >

Modified potential-induced-breathing model of potentials between close-shell ions.

Abstract: We have developed a simple ab initio model for the calculation of the thermoelastic properties of ionic compounds. The model is based on the Gordon-Kim-type electron-gas theory with spherically symmetric relaxation of ionic charge densities. The relaxation is controlled by a spherically averaged potential due to the total crystal charge density. The potential is self-consistent with the charge distribution, and contains Coulomb, exchange, and correlation contributions. We find that this potential yields anions that are slightly smaller than those stabilized by a point-ion Coulomb potential only. In the case of MgO, this results in a zero-pressure density that differs from experiment by less than 1%, a significant improvement over models that include only point-ion stabilization potentials. Further, the calculated equations of state and B1-B2 phase-transition pressures of NaCl, KCl, MgO, CaO, and SrO are in equally good agreement with data. The calculated equation of state and structure of the more covalent and less symmetric ${\mathrm{SiO}}_{2}$ stishovite is only slightly less accurate.

Rotational dielectric friction on a generalized charge distribution

Abstract: The dielectric friction on a solute molecule reorienting in a liquid solution is computed by modeling the solute as a rigid collection of point charges rotating within a spherical cavity in a dielectric continuum. Such a calculation on an extended charge distribution is a logical progression from existing theories, which treat only single point charges or point dipoles. It is shown how a more realistic charge distribution can change the calculated friction coefficient by several orders of magnitude, and the generalized theory is applied to rotational diffusion data for three phenoxazine dyes in dimethylsulfoxide.

Valence‐electron contributions to the electric‐field gradient and the crystal field at rare‐earth sites in intermetallic compounds

Abstract: From first‐principles band‐structure calculations, the electric‐field gradient Vzz at Gd nuclei in Gd2Fe14B, Gd2Co14B, Gd2Fe17, Gd2Co17, GdCo5, GdNi5, GdFe3, and elemental Gd has been calculated. The crystal‐field parameter A02 has been calculated for Gd2Fe14B. A02 and Vzz are mainly determined by the asphericity of the charge density of the 5d and 6p valence shells of the Gd atoms. We discuss the validity of a frequently used proportionality relation between A02 and Vzz.

Periodic charge-density modulations on graphite near platinum particles.

Abstract: We have imaged ten different superstructures on highly oriented pyrolytic graphite near adsorbed platinum particles using a scanning tunneling microscope. These superstructures are localized in small areas near the platinum particles and decay within a distance of 2--5 nm into the graphite lattice. When Fourier transformed, the observed structures contain up to three pairs of first-order Fourier components in addition to those of graphite. These Fourier components have different intensities and phases in different directions, but they have the same period of 1.5a (a is the lattice constant of graphite) and are rotated 30\ifmmode^\circ\else\textdegree\fi{} relative to the graphite lattice. We show that the observed features are mainly due to a periodic charge-density modulation superimposed onto the graphite lattice, i.e., the platinum particles perturb the surface charge density of graphite giving rise to the superstructures in the nearby regions.

Fluid electrification measurements of transformer pressboard/oil insulation in a Couette charger

Abstract: A Couette charger (CC) facility has been built to simulate flow electrification processes in transformers where transformer oil fills the annulus between coaxial cylindrical electrodes. Transient measurements with a step change in temperature have shown the charge density to change from an initial value to a new steady-state value, including cases of polarity reversal, both values dependent on the equilibrium moisture levels in oil and pressboard. Through a model that accounts for diffusion of charge from the paper-oil interface into the bulk of the oil, these measurements are used to deduce the equilibrium wall charge density. To help understand the effects of energization and to scale laboratory measurements at low voltages and low frequencies to operating transformers, a charge injection model was refined to examine the migration of double layer and injected charge in the imposed sinusoidally time-varying electric field. Good fits between this model and measurements are achieved. >

Structure and properties of partially neutralized poly(acrylic acid) gels

Abstract: We investigated, using small-angle neutron scattering, the effect of the charge density on the polymer chains of polyacrylic acid gels. The structure factor was found to have a peak at a finite wavevector q*. The position and the amplitude of this peak depend on the polymer concentration, the charge density on the chains and the ionic content of the solvent. The results obtained are fairly well understood with the advent of recent theoretical studies [1, 2], in which it is shown that such materials are liable to form mesophases at a microscopic scale.

Multilevel measurement of the electric field underneath a thundercloud: 2. Dynamical evolution of a ground space charge layer

Abstract: In August 1989 the electric field was measured below a thunderstorm at Kennedy Space Center for more than 1 hour at several levels up to 800 m. No substantial precipitation reached the ground at the experimental site until after this data collection period, which included the thunderstorm approach and the triggering of four lightning flashes. The sensors at altitude were suspended from a tethered balloon and that at the surface was a standard field mill. The maximum value detected was 65 kV/m at 603 m while at the same time the steady surface field did not exceed 5 kV/m. Applying Gauss's law to the electric field data, we have calculated average charge densities within the layers formed by the levels where the sensors were located. Corona ions produced at the surface were detected up to the upper layer between 436 m and 603 m, and the densities calculated reached values close to 1 nC m−3 within each layer. The current densities were also evaluated at the sensor levels; the one calculated at 436 m ranges about 1.4 nA m−2 which means that a large proportion of corona ions were carried up to several hundreds of meters above ground.

Transient electric and magnetic fields associated with establishing a finite electrostatic dipole, revisited

Abstract: The authors reexamine the analysis of M.J. Master and M.A. Uman (1983), who derived and plotted the electromagnetic field waveforms generated by a square wave of current propagating with constant speed along a finite linear path, resulting in the formation of a finite electrostatic dipole. It was concluded that such a square pulse of current had charge only at its front and rear. The authors argue that errors were made in that analysis, correct these errors, and extend the overall analysis: (1) to show that the decomposition of the electromagnetic fields into electrostatic, induction, and radiation components on the basis of distance dependence is not general; (2) to demonstrate the use of the continuity equation in finding the charge distribution along a current path when the current distribution is known; and (3) to show how a seemingly simple analytical technique, the treatment of an abrupt step of current as a linearly rising current in the limit that the risetime goes to zero, can lead to erroneous results if there is not a good physical and mathematical understanding of all the variables involved. >

The bonding charge density of β′NiAl

Abstract: The (100), (110), (111) and (200) low-angle structure factors of β′NiAl have been accurately measured by high energy electron diffraction (HEED) and it was found that, except for the (200) reflection, these agreed closely with the values deduced by interpolation between the best pure element crystal atomic scattering (form) factors; the (200) structure factor was found to be about 2% lower than the best equivalent pure element value. The trend of these results is somewhat similar to those obtained from powder X-ray diffraction measurements made on β′NiAl over 25 years ago and indicates that there is no ionic component in the bonding scheme for this alloy. This was confirmed by the generation of deformation electron density distributions (DEDDS) from the structure factor data which showed that bonding is this alloy is achieved by the depletion of electrons from both the nickel and aluminum atom sites with extensive charge build-up at the mid-points between nearest-neighbor (n.n.) Ni Al atoms (〈111〉 directions). This shows that bonding in this intermetallic compound is essentially covalent with some metallic character and no ionic component. The DEDDs also showed a large depletion of electrons between second n.n. Al Al atoms (〈100〉 directions) which also occurred, but to a lesser extent second n.n. Ni Ni atoms. This suggests that as well as the atomic size difference, there are electronic reasons why n.n. and second n.n. Al Al bonds are not favored in this B2 alloy type. This explains why β′FeAl, β′CoAl and β′NiAl are usually fully ordered and do not easily tolerate Al antistructure atoms on their transition metal sublattices. The build-up of electrons between n.n. atoms along 〈111〉 and depletion between second n.n. Ni Ni and Al Al atoms along 〈100〉 also gives indications of the electronic origins of the ω phase (collapse of atoms along 〈111〉 directions towards vacancy or antistructure atom defects) and the elastic softening associated with shear along [011] on (011), which occurs in β′NiAl containing more than about 53 at.% Ni.

Does the Harris energy functional possess a local maximum at the ground-state density?

Abstract: We prove that for any system, and within the framework of any reasonable local approximation to the Hohenberg-Kohn-Sham exchange-correlation energy functional, the Harris energy functional posseses either a saddle point or a local minimum at a self-consistent ground-state charge density. This invalidates the common suggestion that this functional has a local maximum at this charge density. Some numerical results in support of our arguments are presented.

Monte Carlo electrostatic persistence lengths compared with experiment and theory

Abstract: An off‐lattice rotational isomeric state model Monte Carlo algorithm for a polyelectrolyte with Debye–Hueckel screening and no hard core repulsion is used to generate short (up to 150 unit) chains at (1) different ionic strengths, (2) varying uniform charge densities, and (3) pH–pK0 governed ionization. The mean square radii of gyration are related to the apparent total persistence lengths of the polyelectrolytes via the wormlike chain model. Near the random coil limit the apparent electrostatic persistence length varies approximately as the inverse square root of the ionic strength and linearly with charge density. The persistence length behavior is very similar in cases (2) and (3). These approximate power laws agree well with those found experimentally for hyaluronate and variably ionized polyacrylic acid. The original electrostatic persistence length theory, which does not contain excluded volume effects, predicts power law exponents which are twice these. Corrections due to polyelectrolyte excluded v...

Electronic properties of α-quartz under pressure

Abstract: The electronic structure of \ensuremath{\alpha}-quartz is investigated within the local-density formalism using recently developed soft pseudopotentials. We present results for the band structure, density of states, and charge density of \ensuremath{\alpha}-quartz at ambient pressure and near the crystalline-to-amorphous transition. The results at ambient pressure compare well with experiment, and are generally consistent with earlier theoretical analysis. Pressure is found to induce a strong interaction between the Si-O bonding and O 2p lone-pair valence states. The gap between the corresponding bands vanishes in the region of the transition to the glass phase. The fundamental band gap increases, instead, with increasing pressure. Trends in the pressure dependence of the electronic structure are discussed in connection with the distortion of the oxygen sublattice towards an ideal close-packed configuration.