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A survey of published literature reveals a difference in the density of amorphous and crystalline solids (organic and inorganic) on the order of 10%–15%, whereas for metallic alloys, it is found to be typically less than 5%. Standard geometric models of atomic packing can account for the polymeric and inorganic glasses without requiring changes in interatomic separations (bond lengths). By contrast, the relatively small difference in density between crystalline and glassy metals (and metallic alloys) implies variations in interatomic separations due to merging orbitals giving rise to reduced atomic volumes. To test this hypothesis, quantum density functional theory computations were carried out on ordered and irregular clusters of aluminum. The results point to decreasing interatomic distances with decreasing coordination, from which one can deduce that the geometrical method of random hard sphere packing significantly underestimates the densities of amorphous metallic alloys. [ABSTRACT FROM AUTHOR]

Propagation of an electromagnetic (EM) pulse in an underdense plasma can either generate a wakefield or excite soliton wave, which depends on the competition between the linear dispersion and nonlinear self-modulation of the wave. Here, we study the interaction of the EM pulse and relativistic hot plasma analytically and numerically and reveal the physical mechanism of the transition from wakefield generation to soliton excitation in terms of soliton stability and modulation instability (MI) of a plane wave. Starting from the relativistic hot fluid-Maxwell model, a nonlinear Schrödinger equation (NLSE) governing the amplitude of scalar potential is obtained by using a multi-scale perturbation technique. The bright and dark soliton solutions of the NLSE are obtained analytically. The stability phase diagram of solitons is given numerically. Furthermore, the MI of the plane wave is studied, and the stability phase diagram of MI is obtained. The results indicate that, when the plasma density increases, the propagation of the EM pulse in the plasma experiences wakefield–soliton transition, which depends on the thermal effect. Our results provide theoretical evidence for deep understanding of high-power laser plasma interaction. [ABSTRACT FROM AUTHOR]

A study on the effect of fine particulate matter on the surface morphologies of conductors subjected to high positive dc voltages has been undertaken. A global distribution pattern of particulate matter was observed in the shape of parallel chains consisting of particle agglomerations. Based on a coupling model of electric and force fields, the dynamic progress of particles near conductor surfaces was investigated and the effects of forces exerted on the particles were analyzed. The experimental observations agreed well with calculated results. It was found that the interactions between particles played a critical role in the formation of agglomerations and parallel chains. The surface roughness and the corona discharge intensity of the conductors increased significantly as a result of the variation of surface morphologies. [ABSTRACT FROM AUTHOR]

Polyetheretherketone is used in the packaging of high voltage power electronics as a frame material, and the surface discharge behaviors of polyetheretherketone are the main concerns for the packaging design. Pulse width modulation voltage, which shows a variable duty cycle, is applied to polyetheretherketone at working conditions. The surface discharge behaviors of polyetheretherketone under a positive repetitive pulse voltage of 50 Hz with different duty cycles were investigated in this paper. It is generally considered that the surface discharge inception is mainly related to electric field distribution; however, the pulse width of voltage could significantly affect the surface discharge inception characteristics. In this study, the surface discharge inception voltage, time-integrated streamer images, and cumulative surface discharge patterns under a positive repetitive pulse voltage of 50 Hz were obtained. It was found that the surface discharge inception voltage decreased with the increase in the duty cycle. This relationship was explained by the role of time-lag. Time-lag distribution analysis was performed, and the time-lag distribution was mainly determined by stochastic time-lag. The mean stochastic time-lag was obtained by the Laue plot, and the relationship between the mean stochastic time-lag and applied voltage could be well fitted by the inverse power law. Furthermore, the relationship between the surface discharge inception voltage and duty cycle could be exactly fitted by the inverse power law, which was derived from the relationship between the stochastic mean time-lag and applied voltage. The closely related relationships demonstrate that time-lag plays a critical role in the surface discharge inception at different duty cycles. [ABSTRACT FROM AUTHOR]

Electric fields are regarded as a promising means of graphene oxide reduction, but previous studies have only focused on pristine graphene. Here, based on first principles density functional theory calculations, we report on electric field mediated reduction of neutral and charged O and hydroxyl groups from both pristine and defective graphene sheets. The critical electric field strengths for different species are determined in facilitating a progressive and selective graphene oxide reduction. Our results demonstrate that the presence of vacancy defects significantly inhibits the effectiveness of electric fields as a means of reduction of O and OH functionals, due to the complexities that arise between the functional group and vacancy edge atoms in the presence of an applied electric field. [ABSTRACT FROM AUTHOR]

The surface discharge behaviors of polyetheretherketone (PEEK) in nitrogen under both positive and negative semi-square voltage were investigated using the current pulse detection method by non-inductive resistor and optical signal detection method by photomultiplier and digital camera. A 50Hz positive or negative semi-square voltage was applied to a rod-plane electrode system, and two types of surface discharge in a voltage cycle were found: one forward discharge and one back discharge. The characteristics and initiation mechanism of the two types of surface discharge were explored. The experimental results showed that surface discharges between cycles were independent. Based on the independence, the time-lag characteristics of surface discharge were obtained. An improvement on Laue-Zuber time-lag theory was made to fit the time-lag characteristics of forward discharge and back discharge. The mean time-lag decreased with the increase of voltage for both forward discharge and back discharge, and the relation between mean time-lag and applied voltage could be modeled by inverse power law. Comparative analysis on the characteristics of back discharge and forward discharge under opposite voltage was performed, and the characteristics of back discharge and forward discharge under opposite voltage were homogenous. In addition, the initiation mechanism of the two types of discharge was discussed. The initiation of forward discharge is believed to be mainly determined by the time-lag and the electric field caused by the applied voltage, while the initiation of back discharge is believed to be mainly determined by the time-lag and the electric field induced by the residual surface charge left by the forward discharge in the same cycle. Verification tests under positive semi-square voltage with different fall time were conducted to validate the initiation mechanism. [ABSTRACT FROM AUTHOR]

Corona-generated audible noise is induced by the collisions between space charges and air molecules. It has been proven that there is a close correlation between audible noise and corona current from DC corona discharge. Analysis on the correlation between audible noise and corona current can promote the cognition of the generation mechanism of corona discharge. In this paper, time-domain waveforms of AC corona-generated audible noise and corona current are measured simultaneously. The one-to-one relationship between sound pressure pulses and corona current pulses can be found and is used to remove the interferences from background noise. After the interferences are removed, the linear correlated relationships between sound pressure pulse amplitude and corona current pulse amplitude are obtained through statistical analysis. Besides, frequency components at the harmonics of power frequency (50 Hz) can be found both in the frequency spectrums of audible noise and corona current through frequency analysis. Furthermore, the self-correlation relationships between harmonic components below 400 Hz with the 50 Hz component are analyzed for audible noise and corona current and corresponding empirical formulas are proposed to calculate the harmonic components based on the 50 Hz component. Finally, based on the AC corona discharge process and generation mechanism of audible noise and corona current, the correlation between audible noise and corona current in time domain and frequency domain are interpreted qualitatively. Besides, with the aid of analytical expressions of periodic square waves, sound pressure pulses, and corona current pulses, the modulation effects from the AC voltage on the pulse trains are used to interpret the generation of the harmonic components of audible noise and corona current. [ABSTRACT FROM AUTHOR]

The corona-generated electromagnetic interference commonly known as radio interference (RI) has become a limiting factor for the design of high voltage direct current transmission lines. In this paper, a time-domain measurement system is developed to measure the time-domain characteristics of corona-generated RI from a single corona source under a positive corona source. In the experiments, the corona current pulses are synchronously measured through coupling capacitors. The one-to-one relationship between the corona current pulse and measured RI voltage pulse is observed. The statistical characteristics of pulse parameters are analyzed, and the correlations between the corona current pulse and RI voltage pulse in the time-domain and frequency-domain are analyzed. Depending on the measured corona current pulses, the time-domain waveform of corona-generated RI is calculated on the basis of the propagation model of corona current on the conductor, the dipolar model for electric field calculation, and the antenna model for inducing voltage calculation. The well matched results between measured and simulated waveforms of RI voltage can show the validity of the measurement and calculation method presented in this paper, which also further show the close correlation between corona current and corona-generated RI. [ABSTRACT FROM AUTHOR]

Cable-driven exoskeletons have used active cables to actuate the system and are worn on subjects to provide motion assistance. However, this kind of wearable devices usually contains uncertain kinematic parameters. In this paper, a model-based identification method has been proposed for a cable-driven arm exoskeleton to estimate its uncertainties. The identification method is based on the linearized error model derived from the kinematics of the exoskeleton. Experiment has been conducted to demonstrate the feasibility of the proposed model-based method in practical application.

Rehabilitation technologies have great potentials in assisted motion training for stroke patients. Considering that wrist motion plays an important role in arm dexterous manipulation of activities of daily living, this paper focuses on developing a cable-driven wrist robotic rehabilitator (CDWRR) for motion training or assistance to subjects with motor disabilities. The CDWRR utilizes the wrist skeletal joints and arm segments as the supporting structure and takes advantage of cable-driven parallel design to build the system, which brings the properties of flexibility, low-cost, and low-weight. The controller of the CDWRR is designed typically based on a virtual torque-field, which is to plan "assist-as-needed" torques for the spherical motion of wrist responding to the orientation deviation in wrist motion training. The torque-field controller can be customized to different levels of rehabilitation training requirements by tuning the field parameters. Additionally, a rapidly convergent parameter self-identification algorithm is developed to obtain the uncertain parameters automatically for the floating wearable structure of the CDWRR. Finally, experiments on a healthy subject are carried out to demonstrate the performance of the controller and the feasibility of the CDWRR on wrist motion training or assistance.