Gap Density Research Articles

Spectral analysis of permanent magnet electric machines

PurposeThis paper aims to provide the mathematical background for representation of permanent magnet AC machines in terms of rotating magnetic field waves of any shape instead of being restricted to the sinusoidal. The general idea is to replace the inductances in the mathematical model of the machine by means of adequate time functions of the flux linkage.Design/methodology/approachOn the basis of several 2D or 3D solutions obtained by the finite element (FE) approach, the set of basis functions is generated for further post‐processing. These functions enable fast and accurate computations of back EMF time shape at any load conditions, which in turn gives the instantaneous values of terminal quantities like torque or voltage, depending on the regime of interest.FindingsThe permanent magnet machine (PMM) has been represented by means of the traveling non‐dispersive waves of the flux density in the air gap rotating with specified group velocity. The conversion between distributions of the flux density in space and flux linkage in time is obtained through filtering in the spectral domain using 2D or 3D discrete Fourier transform. The change of magnetic saturation due to arbitrary value of the machine load is incorporated by the interpolation between known magnitudes of the basis functions at given a priori RMS values of phase currents. It has been proved that a sinusoidal field machine is particular to the presented theory.Research limitations/implicationsThe paper deals with the steady state of PMM; however, the extension towards the transient analysis is possible.Practical implicationsThe paper presents a fast and accurate model of PMM for the analysis of its basic electromagnetic quantities.Originality/valueThe analysis of terminal quantities being different in time from sinusoidal or constant distributions, both electrical and mechanical, is usually performed by means of a time stepping approach. The required computing effort is still too high for real time applications. The presented method starts from single FE solutions and converts their accuracy on the set of mutually orthogonal functions having the clear representation in the spectral, mode‐frequency domain. The magnitudes of these basic functions enable one to express the electromagnetic power in a form equivalent to classic dq representation, but not constrained by sinusoidal input quantities.

Flux Pump Excited Double-Helix Rotor for Use in Synchronous Machines

High temperature superconducting (HTS) machines have been demonstrated to benefit applications requiring high power density and torque. However, power density of existing superconducting machinery is limited by achievable flux density in the air gap and could be significantly increased with larger rotor currents. We propose a rotor based on MgB2 conductor running in persistent current mode. A novel MgB2 HTS flux pump transformer enables currents of several thousand amps in the rotor in a persistent mode. Large cryogenic heat loads associated with current leads are diminished by using a flux pump transformer for rotor excitation. All coils in the rotor and flux pump transformer are based on the double-helix winding configuration, which offers unmatched rotor robustness and reliability. This winding configuration also facilitates large bending radii, leading to an outstanding stress management and thus improved current carrying capacity of stress sensitive HTS conductors. The research presented in this paper analyses the performance of such a system through the design of a synchronous rotor for motors, generators and synchronous condensers.

Magnetic-field-enhanced synthesis of single-wall carbon nanotubes in arc discharge

The ability to control the properties of single-wall nanotubes (SWNTs) produced in the arc discharge is important for many practical applications. Our experiments suggest that the length of SWNTs significantly increases (up to 4000 nm), along with the purity of the carbon deposit, when the magnetic field is applied to arc discharge. Scanning electron microscopy and transmission electron microscopy analyses have demonstrated that the carbon deposit produced in the magnetic-field-enhanced arc mainly consists of the isolated and bunched SWNTs. A model of a carbon nanotube interaction and growth in the thermal plasma was developed, which considers several important effects such as anode ablation that supplies the carbon plasma in an anodic arc discharge technique, and the momentum, charge, and energy transfer processes between nanotube and plasma. It is shown that the nanotube charge with respect to the plasma as well as nanotube length depend on plasma density and electric field in the interelectrode gap. For instance, nanotube charge changes from negative to positive value with an electron density decrease. The numerical simulations based on the Monte Carlo technique were performed, which explain an increase in the nanotubes produced in the magnetic-field-enhanced arc discharge.

A theoretical study of the impact of magnetic field of Hall current on the parameters of a high-current vacuum-arc discharge

It is demonstrated that the magnetic field of Hall current in a short high-current vacuum-arc discharge significantly distorts the external axial magnetic field and affects the distribution of current density in the discharge gap. This effect decreases with increasing external magnetic field, with decreasing arc current, and with decreasing ratio of the length of discharge gap to its transverse dimension. A 2D magnetohydrodynamic mathematical model is used to calculate the discharge parameters for different values of induction of external magnetic field. The calculation results are compared with similar results obtained using a 1.5D model in which the impact made by magnetic field of Hall current is ignored. It is inferred that the simpler 1.5D model may be employed in calculations of parameters of vacuum arc in a wide range of variation of arc current and of induction of external magnetic field.

Calculation of free-energy differences and potentials of mean force by a multi-energy gap method

A method is proposed to significantly accelerate the convergence of free-energy calculations. It introduces a bias factor in Monte Carlo simulations or, equivalently, a bias force in molecular dynamics simulations. The bias factor targets the energy gap, i.e., the difference in energy function between two states, and is therefore specifically designed for calculating free-energy differences. The goal is to make the probability density of the energy gap as uniform as possible, thus allowing for its accurate determination. An iterative procedure, based on simulations at higher temperatures, is devised to obtain the bias factor. The same method naturally extends to the calculation of potentials of mean force. The generalized coordinate, for which the potential of mean force is to be calculated, now plays the role of the energy gap. Applications to model systems confirm the expected increase in accuracy of calculated free-energy differences and potentials of mean force.

Electrocoagulation study for the removal of arsenic and chromium from aqueous solution

This study was undertaken to investigate the removal of arsenic (As) and chromium (Cr) from aqueous solution using electrocoagulation (EC) technique. Batch EC studies were performed using iron electrodes to evaluate the influence of various experimental parameters on the removal of metal ions. The parameters were initial pH (pH 0), electrolysis time (t), initial concentration (C 0), electrode gap (g), stirring rate (r) and current density (j). Effect of pH 0 was studied in the range 2.0 to 8.0 while C 0 was varied from 10 to 100 mg/L. As and Cr removal by EC was governed by the chemical dissolution of iron, and the formation of metal-hydrous ferric oxide complexes, which in turn was strongly, influenced by pH 0 and j. Optimum value of pH 0 and j for As and Cr removal were found to be 4.0 and 2.0; and 75 and 50 A/m2, respectively. Removal efficiency increased with decrease in the value of C 0 and g. The r value of 100 rpm produced sufficient agitation for the proper agglomeration of flocs and optimum removal of ions.

Individual gold dimers investigated by far‐ and near‐field imaging

Plasmon resonances in 3D nanoparticle arrangements can produce strong localized optical fields, which are of importance for any application involving interaction of light with subwavelength volumes of matter down to the molecular level. In particular, remarkable field enhancement and confinement occur in a dimer geometry formed by two identical closely spaced particles. Although, recent advances in nanofabrication have rendered the fabrication of complex plasmon architectures more accessible, addressing their local fields in a nonperturbative fashion remains not straightforward, because metallic nanostructures are rather sensitive to their local environment. Here we study gold dimers fabricated by e-beam lithography. Individual dimers are imaged both by far- and near-field methods. First, the near-field electromagnetic interaction in an ensemble of dimers is investigated by scattering spectroscopy, using dark field microscopy. Next, to probe their local field, we explore the luminescence of individual gold dimers utilizing a confocal microscope with single molecule detection sensitivity. We provide a statistical analysis of the dimer luminescence for different incident polarizations, with direct comparison to single particles (monomers). Finally, the near-field transmission of the resonant dimers is mapped with a subwavelength resolution using polarized controlled near-field scanning optical microscopy. Surprisingly, no clear evidence of the high mode density in the dimer gap is observed. This result may be attributed to the limited coupling of the field emitted by the aperture probe to the dimer mode.

Landscape context and microenvironment influences on liana communities within treefall gaps

AbstractQuestions:How do gap aspect, openness, age and gap density in the surroundings affect diversity and composition of lianas within treefall gaps? Are the variation patterns in liana communities within treefall gaps associated with their climbing mechanisms?Location:Subtropical montane forests in northwestern Argentina.Methods:We used ordination analyses (NMDS) and multiple regressions to describe liana communities (species and climbing groups, stems ≥ 1 cm) in 35 gaps located in a 6‐ha plot and to assess relationships with aspect, canopy and subcanopy openness, age and density of recent (three to eight years old) and old (> 8 years old) gaps.Results:Treefall gaps segregated in the ordination diagrams based on their liana species and climbing mechanisms composition: gaps surrounded by a higher density of recent gaps showed higher densities of lianas species using tendrils, spines/hooks and voluble stems. Liana density and richness (independent of stem density) were also positively associated with the density of recent gaps in the surroundings, and negatively associated to gap age. The number of liana species corrected per area was negatively associated to gap aspect and subcanopy openness, and absolute number of liana species was positively associated to gap canopy openness.Conclusions:This study integrates the analysis of different factors influencing liana communities within treefall gaps, and shows that spatial context of gap density (a variable largely neglected in previous studies) is particularly important on lianas composition and diversity, probably by increasing propagule input and the availability of small trellises for support.

Repeated disturbances and canopy disturbance regime in a tropical semi-deciduous forest

Abstract:The canopy disturbance regime and the influence of gap methods on the interpretation of forest structure and dynamics were evaluated in a tropical semi-deciduous forest in south-eastern Brazil. We encountered a gap density of 11.2 gaps ha−1and an average size which varied from 121 to 333 m2depending on the gap delimitation method considered (minimum gap size was 10 m2). Although average size was slightly higher, the median value obtained (78 m2) was comparable to other tropical forest sites and the gap size-class distribution found supported the pattern described for such forest sites. Among 297 gap makers, snapping and uprooting were the most common modes of disturbance. The number and basal area of gap makers were good predictors of gap size. Almost 25% of all gaps suffered from repeated disturbance events that brought about larger gap sizes. Such processes, along with delimitation methods, strongly influenced the estimation of turnover rate and therefore the interpretation of forest dynamics. These results demonstrated the importance of further studies on repeated disturbances, which is often neglected in forest studies.

Structure determination, magnetic and optical properties of a new chromium(II) thioantimonate, [Cr((NH 2CH 2CH 2) 3N)]Sb 4S 7

The chromium(II) antimony(III) sulphide, [Cr((NH 2CH 2CH 2) 3N)]Sb 4S 7, was synthesised under solvothermal conditions from the reaction of Sb 2S 3, Cr and S dissolved in tris(2-aminoethyl)amine (tren) at 438 K. The products were characterised by single-crystal X-ray diffraction, elemental analysis, SQUID magnetometry and diffuse reflectance spectroscopy. The compound crystallises in the monoclinic space group P2 1 /n with a=7.9756(7), b=10.5191(9), c=25.880(2) Å and β=90.864(5)°. Alternating SbS 3 3− trigonal pyramids and Sb 3S 6 3− semi-cubes generate Sb 4S 7 2− chains which are directly bonded to Cr(tren) 2+ pendant units. The effective magnetic moment of 4.94(6) μ B shows a negligible orbital contribution, in agreement with expectations for Cr(II): d 4 in a 5A ground state. The measured band gap of 2.14(3) eV is consistent with a correlation between optical band gap and framework density that is established from analysis of a wide range of antimony sulphides.

Surface Microscopic Characteristics Analysis on Electrochemical Mechanical Finishing of Stainless Steel in NaNO3 Electrolyte

The mechanism of electrochemical mechanical finishing (ECMF) process was investigated. The ECMF experimental system and control unit were developed and some vital procedure parameters such as operating voltage, electrolyte component and concentration, machining temperature, electrode gap and current density were also evaluated and optimized, then the optimal procedure parameter match was obtained. Furthermore, the important surface roughness characteristics before and after ECMF process such as height and spacing characteristics of surface roughness, surface waviness characteristics, surface microscopic appearance and light reflection characteristics were compared. The experiment and measurement results indicate that ECMF process can distinctly improve surface quality, eliminate the surface scratch marks and defects and reduce surface roughness.

Do finer gap mosaics provide a wider niche for Quercus gilva in young Japanese cedar plantations than coarser mosaics? Simulation of spatial heterogeneity of light availability and photosynthetic potentialThis article is one of a selection of papers published in the Special Forum IUFRO 1.05 Uneven-Aged Silvicultural Research Group Conference on Natural Disturbance-Based Silviculture: Managing for Complexity.

Both the size and the density of gaps affect seedling growth, but these two parameters have a reciprocal relationship at a given gap ratio. The objective of the present study was to clarify the appropriate gap mosaic coarseness required to facilitate the growth of Quercus gilva Blume seedlings in Cryptomeria japonica D. Don plantations. The spatial heterogeneity of the photosynthetic photon flux density on the forest floor was predicted using mimicked hemispherical diagrams under five levels of gap mosaic coarseness ranging from the finest gap mosaic with a gap size of 25 m2 and a gap density of 100·ha–1 to the coarsest mosaic of 400 m2 and 6.25·ha–1. Photosynthetic potentials (PP) were calculated by combining the predicted photosynthetic photon flux density and photosynthetic relationships of Q. gilva and two nontree major competitors ( Mallotus japonicus (Thunb.) Muell. Arg. and Miscanthus sinensis Anderss.). The coarser gap mosaic formed a more heterogeneous and bimodal PP frequency and resulted in a wider site in which the three species had high growth potential without considering competition among species. However, an intermediate mosaic with a gap width to mean canopy height ratio of 0.7 formed the widest realized niche in which Q. gilva would show good growth with a relative PP > 70%, whereas the competitor species would be suppressed with a relative PP < 70%.

An experimental study of the reactive ion etching (RIE) of GaP using BCl 3 plasma processing

A detailed study of the reactive ion etching (RIE) of GaP, through BCl 3 based plasma processing is reported. We discuss the effects on the etch rate through the studies of RF power, reactant and carrier gas (Ar) flow and chamber pressure. Atomic force microscopy (AFM) characterization, along with photoluminescence (PL) spectroscopy, is used to investigate the surface quality and correlate the material damage. Compared to previous dry etching studies, we find that the etching rate is enhanced to 850 nm/min with slight increase in surface roughness. PL spectroscopy indicates a progressive degradation of the surface quality with increased RF power, which is not due to increased surface roughness but presumably due to either a change of the surface state density or depletion layer thickness in GaP.

Excited state quantum phase transitions in many-body systems

Phenomena analogous to ground state quantum phase transitions have recently been noted to occur among states throughout the excitation spectra of certain many-body models. These excited state phase transitions are manifested as simultaneous singularities in the eigenvalue spectrum (including the gap or level density), order parameters, and wave function properties. In this article, the characteristics of excited state quantum phase transitions are investigated. The finite-size scaling behavior is determined at the mean-field level. It is found that excited state quantum phase transitions are universal to two-level bosonic and fermionic models with pairing interactions.

Modelling of corona discharge in cylinder-wire-plate electrode configuration

Numerical computation of the electric field strength and ionic space charge density in electrode systems consisting of ionizing wire and non-ionizing cylinder, connected to the same DC high-voltage supply and facing a grounded plate, is a difficult problem, which is of interest to several electrostatic processes applications. In a previous study a simple numerical method has been proposed to calculate the spatial distributions of electric field and ionic space charge in a case of a continuum and uniform corona discharge originating at the surface of the wire. The aim of the present paper is to improve the physical model of the corona discharge in this particular electrode configuration, by assuming a more realistic law of charge injection on the wire circumference. The computations were carried out for an ionizing wire of radius r=0.1 mm, located at different distances h from a metallic tubular support of radius R=13.4 mm.The initial conditions of the corona discharge took into account the non-uniformity of the charge injection around the ionizing wire electrode. The computational results were compared with those obtained under the assumption of uniform corona discharge. The comparison pointed out that neither the non-uniformity of the electric field nor that of the charge injection can be neglected. They depend on the geometry of the electrode system and affect the distribution of the electric field and of the space charge density in the inter-electrode gap.

Filling dependence of the zigzag Hubbard ladder for the quasi-one-dimensional superconductorPr2Ba4Cu7O15−δ

We investigate filling dependence of the zigzag Hubbard ladder, using density matrix renormalization group method. We illustrate the chemical-potential vs. electron-density and spin gap vs. electron density curves, which reflect characteristic properties of the electron state.On the basis of the obtained phase diagram, we discuss the connection to a novel quasi-one-dimensional superconductor Pr$_2$Ba$_4$Cu$_7$O$_{15-\delta}$.

Sapling responses to variations in gap densities and spatial configurations modeled using SORTIE

The objective of this study is to measure the sapling responses to variation in gap densities and spatial configurations in simulated stands using the SORTIE model. The hypothesis is that the cumulative effects and the interaction of several small increases in direct and diffuse light can have important effects on the growth of saplings. Four gap densities were introduced by removing trees until 10%, 20%, 35% and 50% of total basal area was removed. The configuration of the gaps surrounding the saplings was characterized by recording the gaps present in six radial distance intervals, and the number of gaps directly positioned above the saplings. The analysis of the sapling growth before and after the introduction of gaps shows an increase in basal area and in the number of growth release episodes that varies with gap density, with a stronger trend observed in shade intolerant species. Spatial analyses reveal that the combination of increases in light originating from several gaps has an important effect on saplings growth and can cause growth releases similar to those attributed to a direct gap. These results confirm the value of combining modeling and spatial analysis to study gap dynamics at fine spatial and temporal scales.

Experimental and theoretical investigation of the adsorption behaviour of new triazole derivatives as inhibitors for mild steel corrosion in acid media

Three triazole derivatives (4-chloro-acetophenone-O-1′-(1′,3′,4′-triazolyl)-metheneoxime (CATM), 4-methoxyl-acetophenone-O-1′-(1′,3′,4′-triazolyl)-metheneoxime (MATM) and 4-fluoro-acetophenone-O-1′-(1′,3′,4′-triazolyl)-metheneoxime (FATM)) have been synthesized as new inhibitors for the corrosion of mild steel in acid media. The inhibition efficiencies of these inhibitors were evaluated by means of weight loss and electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and polarization curves. Then the surface morphology was studied by scanning electron microscopy (SEM). The adsorption of triazole derivatives is found to obey Langmuir adsorption isotherm, and the thermodynamic parameters were determined and discussed. The relationship between molecular structure of these compounds and their inhibition efficiency has been investigated by ab initio quantum chemical calculations. The electronic properties such as the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) energy levels, energy gap (LUMO–HOMO), dipole moment and molecular orbital densities were computed.

Factors affecting synthesis of single wall carbon nanotubes in arc discharge

In this paper a model of single wall carbon nanotube (SWNT) interactions with thermal plasma is developed. Several effects such as momentum, charge and energy transfer between the SWNT and plasma are considered. It is shown that the SWNT charge and potential with respect to the plasma, as well as SWNT aspect ratio, depend on plasma density and electric field in the interelectrode gap. For instance, the SWNT charge changes from negative to positive value with electron density decrease. This model suggests several ways to improve the controllability of SWNT synthesis in arc discharge, such as an increase in plasma density and the application of an electric field. It is predicted that higher plasma density in the SWNT formation region and electric field lead to the formation of SWNT with a very high aspect ratio.

Rotor Pole Design in Spoke-Type Brushless DC Motor by Response Surface Method

This paper proposes a novel rotor pole shape which consists of a uniform surface and an eccentric surface to make a sinusoidal magnetic flux density in air gap in order to reduce cogging torque, torque ripple, and harmonics of back-electromotive force waveform in spoke-type brushless DC motor. In the process of design and analysis, response surface methodology and the 2-D finite-element method are employed to obtain the rotor geometry and verify the results of the novel rotor pole shape