Acoustoelectric charge injection in semiconductors

A considerable change of trapped and free electric charge is observed in piezoelectric semiconductors in the presence of a traveling acoustic wave. The electric field, induced by the ultrasound, alters the electric equilibrium of the semiconductor sample, resulting in an accumulation of majority carriers at the surface with a consequent decrease in surface resistance. In specific cases, charge injection occurs at the semiconductor-metal contact area due to the large electric field induced by the acoustic wave. This effect, here referred to as the Acoustoelectric Charge Injection, was also investigated for the case in which a Surface Acoustic Wave (SAW) is propagating along the metalized surface of a semiconductor. The injected charge is experimentally measured having a exponential time decay typical of a deep trap level, thus suggesting that Acoustic Charge Injection can modify the transient behavior of high-speed analog signal processing devices based on SAW, acoustooptic, and acoustic charge transport (ACT) phenomena. Experimental results and theoretical calculations are presented for CdS samples and for a metalized GaAs-epilayer grown on semi-insulating GaAs substrate     

Formation of stable stacking zones in a flow stream for sample immobilization in microfluidic systems

Electrocapture is a multifunctional microfluidic tool that can be used for concentration, sample cleanup, multistep reactions, and separation of biomolecules. Herein, we investigate the mechanisms underlying the electrocapture principle. A microfluidic electrocapture device was found to be capable of generating regions of different electric field, which are maintained in the flow by electric and hydrodynamic forces, with the zones of lower electric field strength upstream of those with higher strength. In addition to detection of the local electric fields by direct measurements, the existence of the zones was observed by the capture of a solution containing Coomassie and myoglobin. The two molecules were captured at different spots in a steady-state manner and were released (separated) at different electric fields. Considering these observations and the experimental values for the electric field strengths, flow velocities, and electrophoretic mobilities of DNA, proteins, and peptides, it is concluded that the macromolecules are captured between the field zones by a stacking mechanism.     

Optical fiber sensor for electric field and electric charge using low-coherence, Fabry-Perot interferometry

An optical fiber sensor for electric field and electric charge, based on the deflection of a small cantilever, has been developed. When the sensor head is placed in an electric field, induced charging produces deflection of the cantilever, which is measured using low-coherence, Fabry-Perot interferometry. The sensor has been used to measure the electric field in the vicinity of a Van de Graaff generator, in the range 135-650 V/cm. The measured deflections are in good agreement with the predictions of a simple model equating the electrostatic and mechanical forces acting on the cantilever.     

Effect of rotor cage's outer and inner radii on the inner flow field of the turbo air classifier

The effects of rotor cage's outer and inner radii on flow field of the turbo air classifier are comparatively analyzed by numerical simulation using ANSYS-FLUENT. The results of quantitative analysis show when the rotor cage's outer and inner radii are increased, the tangential velocity, radial velocity and upward axial velocity decrease in the annular region and near the entrance of the rotor cage. However, when the rotor cage's outer and inner radii are too large or too small, the tangential velocity and radial velocity will be fluctuated greatly. Moreover, the rotor cage's outer and inner radii directly influence the radial velocity distribution in the rotor cage channel. The rotor cage's outer and inner radii should not be too large or too small. Therefore, in the seven contrast rotor cage models, model 100–70 and 90–60 are selected to carry out the calcium carbonate classification experiments due to their small tangential velocity and radial velocity fluctuations and well-distribution in the rotor cage channel. The experimental results reflect the characteristics of the numerically simulated flow field in the classifier.     

Field weakening for radial force reduction in brushless permanent-magnet DC motors

In brushless permanent-magnet dc (BLDC) machines, the attraction between the rotor permanent magnets and the stator iron causes radial stator forces that excite the stator structural response and radiate unwanted acoustic noise. In this paper, we develop an analytical model that predicts rotor torque and radial force ripple as functions of the stator currents. The model shows that field weakening of sinusoidally commutated BLDC machines can reduce radial forces but requires higher currents to maintain the desired torque. We confirmed the analytical results numerically on a BLDC motor using ANSYS finite-element analysis and found a 30% reduction in force ripple at no load.     

径向极化压电陶瓷管的径向振动研究

结合平面应变和三维压电弹性力学理论,对径向极化厚壁压电陶瓷细长管的径向振动进行了研究,得出位移函数、电势函数的精确解。利用振子静电电荷方程导出电位移及电场强度函数,解决了电压与电场强度非线性关系问题,进而辅助maple软件首次对厚壁管形振子等效电导纳进行研究,推出了精确的谐振反谐振频率方程。利用数值法得出了不同尺寸管形振子的谐振反谐振频率,并通过有限元分析,结果表明了本文理论的准确性与精确性,为压电陶瓷厚壁振子的理论研究及设计提供了依据。     

Analysis of the collective behaviour of evaporated gold nuclei: modifications induced by electric fields

The experimental results on the collective behaviour of gold nuclei on (100) KCl substrates are summarized in order to demonstrate that the repulsive forces between nuclei can be modified by the application of an electric field in the plane of the film. An analysis of the radial distribution of nuclei gives direct evidence of the role of an electric field in the kinetics of coalescence.     

Comparative study of carbon nanotubes- and fullerenes-doped liquid crystal for different electrophoretic parameters

This research focuses on the study of different electrokinetic parameters of carbon nanotubes (CNTs) and fullerene aggregates in liquid crystal (LC) host medium, which are investigated in the homogeneously aligned nematic LC cells driven by in-plane field. The colloidal CNTs in LC medium are observed to move towards the negative electrodes whereas the fullerenes in LC medium are observed to move towards positive electrode at low frequencies. We propose a model to estimate the charge and zeta potential of colloidal particles by incorporating both the dielectrophoresis and electrophoresis forces in order to probe the reason of moving the colloidal particles in opposite direction. Interestingly, charge and zeta potential values on CNTs and fullerenes estimated from given model were positive and negative, respectively. The CNTs and fullerenes at high frequency and field are found to stretch along the direction of electric field. The CNTs dispersed whereas fullerenes start to move in perpendicular direction to the applied electric field with increasing electric field at high frequency.     

Electrically charged filter materials

Filters made from ordinary textile fibres are too coarse to remove micrometre-sized dust particles, which are responsible for much respiratory disease. If, however, an electric charge is applied to the fibres, a filter can provide a very good respiratory protection. Fibres can be charged by triboelectric exchange, by corona or by induction; and charge levels can give rise to electric fields in the interstices of the filter, approaching the breakdown field of the air. Some of the effect of the charge is lost as the filters become loaded with dust, but charge stability during storage is high, with shelf lives of years being attainable. The author discusses the capture of particles by electric forces and electrically charged filter materials and their production methods. The measurement of electric charge on filters, charge stability during storage, filter performance when loaded with dust and the advantages and disadvantages of such filters, are also discussed     

Characterization of noise and vibration sources in interior permanent-magnet brushless DC motors

This work characterizes electromagnetic excitation forces in interior permanent-magnet (IPM) brushless direct current (BLDC) motors and investigates their effects on noise and vibration. First, the electromagnetic excitations are classified into three sources: 1) so-called cogging torque, for which we propose an efficient technique of computation that takes into account saturation effects as a function of rotor position; 2) ripples of mutual and reluctance torque, for which we develop an equation to characterize the combination of space harmonics of inductances and flux linkages related to permanent magnets and time harmonics of current; and 3) fluctuation of attractive forces in the radial direction between the stator and rotor, for which we analyze contributions of electric currents as well as permanent magnets by the finite-element method. Then, the paper reports on an experimental investigation of influences of structural dynamic characteristics such as natural frequencies and mode shapes, as well as electromagnetic excitation forces, on noise and vibration in an IPM motor used in washing machines.     

Numerical Studies of Axial and Radial Magnetic Forces Between High Temperature Superconductors and a Magnetic Rotor

The magnetic bearing is considered as one of the most prospective applications of high temperature superconductors (HTSs) which can realize stable levitation in a magnetic field generated by permanent magnet devices or coils. The exploration of this kind of HTS bearing through numerical investigation is usually made by assuming the induced current circulates only within the ab-plane and thus simplifying the actual three-dimensional problem to a two-dimensional one. In this paper, on the basis of the three-dimensional model of the HTS bulk established before, we further introduce the developed finite-element software to calculate the magnetic field generated by a magnetic rotor which is composed of permanent magnet (PM) rings and ferromagnet (FM) shims, and in this way, we can investigate the magnetic forces (radial force and axial force) of a simplified HTS bearing model, i.e., two symmetric HTS bulks and a magnetic rotor, at a three-dimensional level. The investigations performed in this paper lead to the observations: the favorable configuration to construct the HTS bearing is that the axial height of each HTS element should be equivalent to the axial heights of PM ring plus FM shim; the increase of the radial thickness of PM ring will improve both the radial force and the axial force considerably, but its margin decreases; the enhancement of critical current density of HTSs due to the decrease of operating temperature can result in a higher increase of both the radial and axial force with a lower nominal gap between the HTSs and the magnetic rotor.     

Estimation of the electron lifetime in tetramethylsilane

The variation of the collected charge with electric field in a diode-type ionization chamber is used to estimate the electron lifetime in tetramethylsilane (TMS). The primary ionizing particles are the 976 keV conversion electrons emitted from a ²⁰⁷Bi source. In the best purity sample of TMS we obtained an electron lifetime of 11 ± 2 μs which corresponds to an attenuation length at 10 kV/cm, equal to 11 cm. The free ion yield at zero electric field, G_f(0), is determined to be 0.51 ± 0.05.     

Two-fluid MHD model of migma: Equilibrium and stability

MHD equations describing the stable stationary equilibrium of migma plasma are shown to result from the natural evolution of a magnetofluid in which magnetic helicity, cross helicity, and modified kinetic helicity are conserved. These equations are solved in two dimensions to yield distributions of azimuthal velocity, mass density, magnetic field, and current density. The radial expansion of migma as a function of plasma beta (diamagnetism) and the maximum obtainable beta as a function of the uniformity of the vacuum magnetic field are also found. If one assumes that the kinetic pressure is entirely due to Maxwellian electrons, then self-consistent expressions for the electron temperature and radial electric field can be found. The radial ion motions make a non-negligible contribution to the total internal energy, however. Modification to the basic equations arising from finite ion pressure is derived, and some preliminary conclusions are drawn.     

Effects of electric field and charge distribution on nanoelectronic processes involving conducting polymers

The injection of charge carriers in conducting polymer layers gives rise to local electric fields which should have serious implications on the charge transport through the polymer layer. The charge distribution and the related electric field inside the ensemble of polymer molecules, with different molecular arrangements at nanoscale, determine whether or not intra-molecular charge transport takes place and the preferential direction for charge hopping between neighbouring molecules. Consequently, these factors play a significant role in the competition between current flow, charge trapping and recombination in polymer-based electronic devices. By suitable Monte Carlo calculations, we simulated the continuous injection of electrons and holes into polymer layers with different microstructures and followed their transport through those polymer networks. Results of these simulations provided a detailed picture of charge and electric field distribution in the polymer layer and allowed us to assess the consequences for current transport and recombination efficiency as well as the distribution of recombination events within the polymer film. In the steady state we found an accumulation of electrons and holes near the collecting electrodes giving rise to an internal electric field which is greater than the external applied field close to the electrodes and lower than the one in the central region of the polymer layer. We also found that a strong variation of electric field inside the polymer layer leads to an increase of recombination events in regions inside the polymer layer where the values of the internal electric field are lower.     

Tunable aqueous virtual micropore

A charged microparticle can be trapped in an aqueous environment by forming a narrow virtual pore--a cylindrical space region in which the particle motion in the radial direction is limited by forces emerging from dynamical interactions of the particle charge and dipole moment with an external radiofrequency quadrupole electric field. If the particle satisfies the trap stability criteria, its mean motion is reduced exponentially with time due to the viscosity of the aqueous environment; thereafter the long-time motion of particle is subject only to random, Brownian fluctuations, whose magnitude, influenced by the electrophoretic and dielectrophoretic effects and added to the particle size, determines the radius of the virtual pore, which is demonstrated by comparison of computer simulations and experiment. The measured size of the virtual nanopore could be utilized to estimate the charge of a trapped micro-object.     

Anisotropic spinodal decomposition of a polar dielectric in a strong electric field: Molecular dynamics simulation

The evolution of a one-component, initially single-phase, dielectric medium that represents a two-dimensional system of dipole molecules has been studied by the molecular dynamics method after the application of a uniform electric field. It is shown that the initially homogeneous medium under the action of the applied field can separate into two phases, liquid and gaseous. In the two-dimensional model system, these phases appear as bands extended along the field direction.     

Modelling the effect of structure and base sequence on DNA molecular electronics

DNA is a material that has the potential to be used in nanoelectronic devices as an active component. However, the electronic properties of DNA responsible for its conducting behaviour remain controversial. Here we use a self-consistent quantum molecular dynamics method to study the effect of DNA structure and base sequence on the energy involved when electrons are added or removed from isolated molecules and the transfer of the injected charge along the molecular axis when an electric field is applied. Our results show that the addition or removal of an electron from DNA molecules is most exothermic for poly(dC)-poly(dG) in its B-form and poly(dA)-poly(dT) in its A-form, and least exothermic in its Z-form. Additionally, when an electric field is applied to a charged DNA molecule along its axis, there is electron transfer through the molecule, regardless of the number and sign of the injected charge, the molecular structure and the base sequence. Results from these simulations provide useful information that is hard to obtain from experiments and needs to be considered for further modelling aiming to improve charge transport efficiency in nanoelectronic devices based on DNA.     

Electroosmotic flows in microchannels with finite inertial and pressure forces

Emerging microfluidic systems have spurred an interest in the study of electrokinetic flow phenomena in complex geometries and a variety of flow conditions. This paper presents an analysis of the effects of fluid inertia and pressure on the velocity and vorticity field of electroosmotic flows. In typical on-chip electrokinetics applications, the flow field can be separated into an inner flow region dominated by viscous and electrostatic forces and an outer flow region dominated by inertial and pressure forces. These two regions are separated by a slip velocity condition determined by the Helmholtz-Smoulochowski equation. The validity of this assumption is investigated by analyzing the velocity field in a pressure-driven, two-dimensional flow channel with an impulsively started electric field. The regime for which the inner/outer flow model is valid is described in terms of nondimensional parameters derived from this example problem. Next, the inertial forces, surface conditions, and pressure-gradient conditions for a full-field similarity between the electric and velocity fields in electroosmotic flows are discussed. A sufficient set of conditions for this similarity to hold in arbitrarily shaped, insulating wall microchannels is the following: uniform surface charge, low Reynolds number, low Reynolds and Strouhal number product, uniform fluid properties, and zero pressure differences between inlets and outlets. Last, simple relations describing the generation of vorticity in electroosmotic flow are derived using a wall-local, streamline coordinate system.     

Electrophoretic analysis of N-glycans on microfluidic devices

We report rapid and efficient electrophoretic separations of N-glycans on microfluidic devices. Using a separation length of 22 cm and an electric field strength of 750 V/cm, analysis times were less than 3 min, and separation efficiencies were between 400,000 and 655,000 plates for the N-glycans and up to 960,000 plates for other sample components. These high efficiencies were necessary to separate N-glycan positional isomers derived from ribonuclease B and linkage isomers from asialofetuin. Structural isomers of N-glycans derived from a blood serum sample of a cancer patient were also analyzed to demonstrate that clinically relevant, complex samples could be separated on-chip with efficiencies similar to those derived from model glycoproteins. In addition, we compared microchip and capillary electrophoresis under similar separation conditions, and the microchips performed as well as the capillaries. These results confirmed that the noncircular cross section of the microchannel did not hamper separation performance. For all experiments, the glycan samples were derivatized with 8-aminopyrene-1,3,6-trisulfonic acid to impart needed charge for electrophoresis and a fluorescent label for detection.     

Steering rigidified nonplanar conjugation based perylene diimide supramolecular for enhancing photocatalytic activity

π-Conjugated supramolecular with higher delocalization of electrons has attracted considerable attention in enhancing the charge transfer in photocatalysis. However, those conjugated macromolecules often possess varied rotational geometries, which will significantly deteriorate charge mobility but still inexplicitly. Herein, we reported diversified PDI polymers with intramolecular angles of 94.7°, 149.7° and 176.3° to explore the role of π-conjugated non-planar molecules. Density functional theory (DFT) calculations and experimental results show that vertical structural PDIMH has antibonding in anisotropic polarizable monomer to generate a macro-dipole, which greatly expands the built-in electric field and facilitates charge transfer and exciton dissociation. On the other hand, the vertical angle is favorable for the face-to-face overlap of the homogeneous molecules, which will create a carrier migration channel and promote carrier separation. Notably, PDIMH exhibited highly effective photocatalytic sterilization (near 100% in 2 h) and benzylamine oxidation (conversion rate up to 300 mmol g⁻¹h⁻¹), which is superior to other ever reported catalysts. This work provides a new interpretation for regulating molecular geometry in developing highly efficient photocatalyst to solve future sustainable issues.