Electrode Radius Research Articles

Dependence of spark conditioning on breakdown charge and electrode material under a non-uniform electric field in vacuum

Spark conditioning is an effective method for improving dielectric strength in vacuum. In this paper, we discuss the dependence of spark conditioning on breakdown (BD) charge and electrode material in vacuum under various non-uniform electric field and circuit parameters. We applied a negative impulse voltage to a rod-plane electrode configuration with a cathode made of stainless steel (SS), Cu-Cr, or OFHC Cu (Cu), and an anode made of SS or Cu. We changed the gap distance, the tip radius of the rod electrode and the circuit parameters. As a result, the conditioning effect, i.e. improving BD electric field strength by the conditioning, depended on the BD charge (QBD) and reached its maximum value against QBD. The optimal QBD required to achieve the maximum conditioning effect decreased with the cathode melting point. Moreover, when BD caused by cathode heating (CH-BD) occurred readily, with the SS anode, the BD electric field strength at the cathode surface after conditioning depended on QBD, regardless of the gap distance and the tip radius. Thus, for CH-BD, we estimated the withstand voltage that could be achieved by spark conditioning from the electrode configuration and QBD. However, when BD caused by anode heating (AH-BD) occurred easily with the Cu anode, the optimal QBD was larger than that for CH-BD because the cathode temperature was lower for AH-BD than for CH-BD.

The voltammetry of decamethylferrocene and coboltacene in supercritical difluoromethane (R32)

The voltammetry of decamethylferrocene, cobaltocene and decamethylcobaltocene at micro and macrodisc electrodes in supercritical difluoromethane at 360K and 17.6MPa has been studied. In all cases the voltammetry is distorted to some degree by the effects of random convection but these can be suppressed by adding a baffle around the electrode. The voltammetry of decamethylferrocene is well behaved with fast electrode kinetics at Pt microdisc electrodes. The limiting currents, corrected for random convection, obey the normal microdisc equation and are linear in electrode radius for decamethylferrocene up to the highest concentration (11mM) used in this study. Based on the microelectrode studies, we find that the diffusion coefficient of decamethylferrocene in supercritical difluoromethane containing 20mM [NBun4][BF4] at 360K and 17.6MPa is 8.3×10−5cm2s−1. Finally we have briefly investigated the voltammetry of cobaltocene and decamethylcobaltocene in supercritical difluoromethane under the same conditions. We find that reduction of the cobaltocenium cation leads to fouling of the Pt microdisc electrode which limits its use as a model redox system and that reduction of the decamethylcobaltocenium cation was not observed before electrolyte reduction at around −1.6V vs. Pt.

A new soil ionization model for grounding electrodes

This paper proposes a new model of soil ionization, in the form of an equivalent circuit containing capacitive and resistive elements representing a network of soil particles and air voids. In previously published models, the effect of soil ionization is only indirectly considered by changing the soil resistivity and the electrode radius. This has resulted in inferior performance of those models in terms of voltage response accuracy. In this work, the energy balance concept was applied to compute the arc resistance. The soil structure is modeled by using two-layer capacitor elements. The soil ionization process is incorporated inside the two-layer capacitor elements using specially derived differential equations. Unlike previous models, a discrete form of soil ionization is used. The accuracy of the proposed equivalent circuit model was confirmed by the small differences between the computed and experimental results.

Intersection of Paschen's curves for argon

The intersection of Paschen's curves for argon with a same gap length but a different electrode radius was observed. While the breakdown voltage increases with the increase of the nonuniformity in the electric field of the gap at lower pressures, it decreases at higher pressures. The reason for the intersection of Paschen's curves was given based on the mean free path length of the electrons inversely proportional to the gas pressure and the electron impact ionization coefficient exponentially increasing with the electric field. The intersection of the Paschen's curves was qualitatively reproduced by a numerical simulation.

Validation of a Central Approximation in Theories of Regular and Random Electrochemical Electrode Arrays. Extraction of Statistical Information from Amperometric Response of Random Electrode Arrays

Regular electrode arrays generally possess a symmetry which (under certain conditions) allows to reduce their mathematical description to consideration of a single electrode of this array within the laterally bounded domain (known as ‘unit cell’ or ‘diffusion domain’) [1, 2]. In this case by multiplying the simulated current of the single cell by the number of electrodes in the array one would obtain an electrochemical response of the whole array. The electrodes in arrays are generally ordered in a hexagonal or square patterns, which result in hexagonal or square cross-sections of the unit cells, respectively. A straightforward approach to the mass-transport problem in such cells would require complex and time-consuming 3D simulations. Therefore almost always an approximation is made in simulations by replacing the original unit cell by the one with circular cross-section of the same surface area. At one hand, this greatly facilitates the simulations due to the possibility to formulate the mass-transport problem in two dimensions (2D). On the other hand, although this approach was proven to be effective in practice, the error introduced by this approximation, to the best of our knowledge, was never been studied or even estimated. The error associated with the approximation is based on the fact that it is impossible to pave the plane with non-intersecting circles contrary to the perfect plane paving with hexagons or squares as well as due to the different possible diffusion pathways within the single cells with different cross-sections. In order to assess quantitatively this approximation we performed 3D Brownian motion simulations for the single cells of square, hexagon and circular cross-sections equipped with disk electrode. Moreover the situations with inlaid, recessed and protruding electrodes were also considered for each cross-section shape. Notwithstanding the completely different diffusion patterns observed in these systems, the obtained results showed that the approximation gives rise to an experimentally indistinguishable error (less than 5%) for all considered systems [2]. The only exception (i.e. when the error is larger than 5%) was the case when the electrode radius was extremely close to the characteristic size of the cross-section, i.e. the situation when the neighbouring electrodes in the array almost touch each other. Such systems are of low experimental interest and, in addition, exhibit the same behavior as a planar electrode with the surface area of the whole array (except of the very initial times of the experiment). With the same idea in mind, to check the validity of the circular diffusion domain approximation, the random arrays were considered. The arrays with randomly distributed electrodes of the same size were partitioned by means of Voronoi tessellation, which creates a unit cell with polygonal cross-section around each electrode. These polygonal unit cells were replaced then by the unit cells with circular cross-section having the same surface area. Taking into account distribution of the unit cell sizes one can compute the response of the whole array. By comparison the outcome of this procedure with the full scale 3D Brownian motion simulations of the whole array we showed that employment of the circular unit cells in simulations of the responses of the random electrode arrays is validated [3]. These results allow us to develop an easy to evaluate analytical approximation for the electrochemical current of the random array requiring only a distribution of sizes of the unit cells as an input. The analytical approximation in turn makes possible retrieval of the unit cell size distribution from the amperometric response of the random array.

Linear methods for reducing EMG contamination in peripheral nerve motor decodes.

Signals recorded from the peripheral nervous system (PNS) with high channel count penetrating microelectrode arrays, such as the Utah Slanted Electrode Array (USEA), often have electromyographic (EMG) signals contaminating the neural signal. This common-mode signal source may prevent single neural units from successfully being detected, thus hindering motor decode algorithms. Reducing this EMG contamination may lead to more accurate motor decode performance. A virtual reference (VR), created by a weighted linear combination of signals from a subset of all available channels, can be used to reduce this EMG contamination. Four methods of determining individual channel weights and six different methods of selecting subsets of channels were investigated (24 different VR types in total). The methods of determining individual channel weights were equal weighting, regression-based weighting, and two different proximity-based weightings. The subsets of channels were selected by a radius-based criteria, such that a channel was included if it was within a particular radius of inclusion from the target channel. These six radii of inclusion were 1.5, 2.9, 3.2, 5, 8.4, and 12.8 electrode-distances; the 12.8 electrode radius includes all USEA electrodes. We found that application of a VR improves the detectability of neural events via increasing the SNR, but we found no statistically meaningful difference amongst the VR types we examined. The computational complexity of implementation varies with respect to the method of determining channel weights and the number of channels in a subset, but does not correlate with VR performance. Hence, we examined the computational costs of calculating and applying the VR and based on these criteria, we recommend an equal weighting method of assigning weights with a 3.2 electrode-distance radius of inclusion. Further, we found empirically that application of the recommended VR will require less than 1 ms for 33.3 ms of data from one USEA.

Discharge Characteristics of Microwave Plasma in Ethanol Solution

AbstractAn apparatus was designed for generating plasma in ethanol solution. The plasma was generated on the top of the electrode by applying microwave radiation of 2.45 GHz. The results showed that ignition power decreased with increasing temperature of ethanol solution. However, ignition power increased with increasing pressure and point electrode radius of curvature. Plasma and bubbles were generated periodically in the same manner. The electron temperature of the plasma increased with increasing power, while it decreased with increasing pressure. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Numerical study on the discharge characteristics and nonlinear behaviors of atmospheric pressure coaxial electrode dielectric barrier discharges**Project supported by the National Natural Science Foundation of China (Grant Nos. 11447244 and 11405208), the Science Foundation of Hengyang Normal University, China (Grant No. 14B41), and the Hunan Provincial Applied Basic Research Base of Optoelectronic Information Technology, China (Grant No. GDXX010).

The discharge characteristics and temporal nonlinear behaviors of the atmospheric pressure coaxial electrode dielectric barrier discharges are studied by using a one-dimensional fluid model. It is shown that the discharge is always asymmetrical between the positive pulses and negative pulses. The gas gap severely affects this asymmetry. But it is hard to acquire a symmetrical discharge by changing the gas gap. This asymmetry is proportional to the asymmetric extent of electrode structure, namely the ratio of the outer electrode radius to the inner electrode radius. When this ratio is close to unity, a symmetrical discharge can be obtained. With the increase of frequency, the discharge can exhibit a series of nonlinear behaviors such as period-doubling bifurcation, secondary bifurcation and chaotic phenomena. In the period-doubling bifurcation sequence the period-n discharge becomes more and more unstable with the increase of n. The period-doubling bifurcation can also be obtained by altering the discharge gas gap. The mechanisms of two bifurcations are further studied. It is found that the residual quasineutral plasma from the previous discharges and corresponding electric field distribution can weaken the subsequent discharge, and leads to the occurrence of bifurcation.

Inverse parameter determination in the development of an optimized lithium iron phosphate – Graphite battery discharge model

Battery models are riddled with incongruous values of parameters considered for validation. In this work, thermally coupled electrochemical model of the pouch is developed and discharge tests on a LiFePO4 pouch cell at different discharge rates are used to optimize the LiFePO4 battery model by determining parameters for which there is no consensus in literature. A discussion on parameter determination, selection and comparison with literature values has been made. The electrochemical model is a P2D model, while the thermal model considers heat transfer in 3D. It is seen that even with no phase change considered for LiFePO4 electrode, the model is able to simulate the discharge curves over a wide range of discharge rates with a single set of parameters provided a dependency of the radius of the LiFePO4 electrode on discharge rate. The approach of using a current dependent radius is shown to be equivalent to using a current dependent diffusion coefficient. Both these modelling approaches are a representation of the particle size distribution in the electrode. Additionally, the model has been thermally validated, which increases the confidence level in the selection of values of parameters.

A theoretical consideration of ion size effects on the electric double layer and voltammetry of nanometer-sized disk electrodes.

Considering that an electric-double-layer (EDL) structure may significantly impact on the mass transport and charge transfer kinetics at the interfaces of nanometer-sized electrodes, while EDL structures could be altered by the finite sizes of electrolyte and redox ions, the possible effects of ion sizes on EDL structures and voltammetric responses of nanometer-sized disk (nanodisk) electrodes are investigated. Modified Boltzmann and Nernst-Planck (NP) equations, which include the influence of the finite ion volumes, are combined with the Poisson equation and modified Butler-Volmer equation to gain knowledge on how the finite sizes of ions and the nanometer sizes of electrodes may couple with each other to affect the structures and reactivities of a nanoscale electrochemical interface. Two typical ion radii, 0.38 nm and 0.68 nm, which could represent the sizes of the commonly used aqueous electrolyte ions (e.g., the solvated K+) and the organic electrolyte ions (e.g., the solvated TEA+) respectively, are considered. The finite size of ions can result in decreased screening of electrode charges, therefore magnifying EDL effects on the ion transport and the electron transfer at electrochemical interfaces. This finite size effect of ions becomes more pronounced for larger ions and at smaller electrodes as the electrode radii is larger than 10 nm. For electrodes with radii smaller than 10 nm, however, the ion size effect may be less pronounced with decreasing the electrode size. This can be explained in terms of the increased edge effect of disk electrodes at nanometer scales, which could relax the ion crowding at/near the outer Helmholtz plane. The conditions and situations under which the ion sizes may have a significant effect on the voltammetry of electrodes are discussed.

Nanopipette-Based Electroplated Nanoelectrodes.

Here, we report a nanopipette-based electrochemical approach to prepare metal nanoelectrodes with excellent control over electrode size, shape, and thickness of the insulation wall. Nanoelectrodes are prepared by electrochemical plating in a laser-pulled quartz nanopipette tip immersed in a liquid gallium/indium alloy electrode, which not only protects the ultrasmall quartz tip but also starts electrodeposition from the tip orifice. This versatile approach enables reproducible fabrication of electrodes of several different metals, including gold, platinum, silver, and copper. Moreover, nanoelectrodes with varying sizes can be easily prepared by focused ion-beam milling. A unique aspect of this method is the control over the thickness of quartz insulation walls relative to the size of the electroactive surface enabling control of the RG (defined as the radius of the insulating sheath over the radius of the active metal electrode). As such, these nanoelectrodes may be especially attractive as useful nanoprobes in high-resolution imaging applications, such as scanning electrochemical microscopy.

Electrical tree growth characteristics in XLPE cable insulation under DC voltage conditions

The growth characteristics of electrical trees in XLPE cable insulation under DC voltage conditions were studied by a combination of treeing test and microscopic observation system. In the treeing test samples, the tip radius of the needle electrode was 5 μm, and the pin-plane distance was 2 mm. When a 70 kV DC voltage was applied on the samples for totally 10 hours, either continuously or intermittently, no trees appeared. So the trees for growth study were all initially triggered by a 50 kV standard lightning impulse. The DC voltages were applied periodically, so as to make it possible for the required observation and recording. Under the DC voltages, the electrical trees developed gradually from a single slender branch to a sparse bushlike structure, and tree channels were found to be non-conductive. The growth rates were rather slow compared with AC trees under the same condition. Grouping experiments showed that the tree length increased with the increasing number of cycles and amplitude of DC voltage, and it was also related to the voltage duration in each cycle at 60 kV. This tree growth pattern has been well explained by a nonconductive tree model and the equivalent circuit, and the discharges occurred in the tree channels are thought to be the driving force for tree growth. Under the condition of an unexpected grounding of the needle electrode, a new tree would initiate instantly from the needle tip, while an existed tree would grow very rapidly. This is correlated to the material damage caused by the sudden release of the injected charges. The work is useful for the recommendations on test arrangement and operation strategy of HVDC cables.

Electrochemical Nucleation of Stable N2 Nanobubbles at Pt Nanoelectrodes.

Exploring the nucleation of gas bubbles at interfaces is of fundamental interest. Herein, we report the nucleation of individual N2 nanobubbles at Pt nanodisk electrodes (6–90 nm) via the irreversible electrooxidation of hydrazine (N2H4 → N2 + 4H(+) + 4e(–)). The nucleation and growth of a stable N2 nanobubble at the Pt electrode is indicated by a sudden drop in voltammetric current, a consequence of restricted mass transport of N2H4 to the electrode surface following the liquid-to-gas phase transition. The critical surface concentration of dissolved N2 required for nanobubble nucleation, CN2,critical(s), obtained from the faradaic current at the moment just prior to bubble formation, is measured to be ∼0.11 M and is independent of the electrode radius and the bulk N2H4 concentration. Our results suggest that the size of stable gas bubble nuclei depends only on the local concentration of N2 near the electrode surface, consistent with previously reported studies of the electrogeneration of H2 nanobubbles. CN2,critical(s) is ∼160 times larger than the N2 saturation concentration at room temperature and atmospheric pressure. The residual current for N2H4 oxidation after formation of a stable N2 nanobubble at the electrode surface is proportional to the N2H4 concentration as well as the nanoelectrode radius, indicating that the dynamic equilibrium required for the existence of a stable N2 nanobubble is determined by N2H4 electrooxidation at the three phase contact line.

Nonlinear standing wave excitation by series resonance-enhanced harmonics in low pressure capacitive discharges

It is well-known that standing waves having radially center-high rf voltage profiles exist in high frequency capacitive discharges. It is also known that in radially uniform discharges, the capacitive sheath nonlinearities excite strong nonlinear series resonance harmonics that enhance the electron power deposition. In this work, we consider the coupling of the series resonance-enhanced harmonics to the standing waves. A one-dimensional, asymmetric radial transmission line model is developed incorporating the wave and nonlinear sheath physics and a self-consistent dc potential, for both conducting and insulating electrode surfaces. The resulting coupled pde equation set is solved numerically to determine the discharge voltages and currents. A 10 mTorr argon plasma is chosen with density m−3, gap width 2 cm and conducting electrode radius 15 cm, driven by a 500 V rf source with resistance 0.5 . We examine a set of frequencies from near 30 MHz up to frequencies more than three times as high. For most frequencies, no harmonics correspond exactly with the series or spatial resonances, which is the generic situation. Nevertheless, nearby resonances lead to a significantly enhanced ratio of the electron power per unit area on axis, compared to the average. Nearly similar results are found for insulating electrodes. Strong effects are seen for varying source resistance: high (50 ) resistance damps out most of the harmonic activity, while zero source resistance leads to a non-steady discharge with bias voltage relaxation oscillations. Stronger harmonic effects are seen for an increased radius of 30 cm, as lower harmonics become spatially resonant at lower frequencies. The radial dependence of electron power with frequency showed significant variations, with the central enhancement and sharpness of the spatial resonances depending in a complicated way on the amplitudes of the nearby series resonance current harmonics and the phase relations among the voltage harmonics driving these current harmonics. Significant center/average electron power per unit area enhancement is found even at the lowest frequencies for both high and low densities: 4.5 at 30 MHz and m−3, and 2.2 at m−3.

Polarity effects on breakdown of short gaps in a point-plane topology in air

Electrical breakdown in air in a point-plane topology involves complex processes that are still not fully understood. Unlike uniform-field topologies, the highly-divergent fields produced by point-plane topologies create pre-breakdown corona with volumetric space charge. It is known that space charges developed by corona discharge have significant impacts on the breakdown voltage in non-uniform electrode topologies. With large inter-electrode gaps (>cm) the breakdown voltage for a HV point cathode in air at atmospheric pressure is noticeably larger than a HV point anode. However, this paper shows that in shorter point-plane gaps in air (less than ~10 mm), in the air pressure range 0.1-0.35 MPa, an HV point anode has a similar breakdown voltage which eventually is surpassed by the HV point cathode as the inter-electrode gap is increased. The inter-electrode gap at which the HV cathode has a higher hold-off voltage is found to be dependent on the gas pressure and radius of the point electrode.

Characterization of capacitive micromachined ultrasonic transducers

This paper describes numerical and experimental characterization of capacitive micromachined ultrasonic transducers (CMUTs) for ultrasound transmission. Simulations based on a finite elements method to model the electromechanical behaviour of CMUTs and to determine the dimensions of elementary cells are presented. In particular, we analyse how the collapse voltage and the capacitance are affected by different parameters of a circular cell and by different bias voltages. The fill factor is defined as the ratio of the top electrode radius to the membrane radius and we study the influence of the fill factor in the performances of CMUTs. The fabrication process of a CMUT uses anodic bonding of a SOI wafer on a borosilicate glass substrate and we compare experimental results with numerical results in terms of eigenfrequencies, bandwidth, quality factor and capacitance for non-metallized and metallized membranes.

Korteweg-de Vries solitons on electrified liquid jets.

The propagation of axisymmetric waves on the surface of a liquid jet under the action of a radial electric field is considered. The jet is assumed to be inviscid and perfectly conducting, and a field is set up by placing the jet concentrically inside a perfectly cylindrical tube whose wall is maintained at a constant potential. A nontrivial interaction arises between the hydrodynamics and the electric field in the annulus, resulting in the formation of electrocapillary waves. The main objective of the present study is to describe nonlinear aspects of such axisymmetric waves in the weakly nonlinear regime, which is valid for long waves relative to the undisturbed jet radius. This is found to be possible if two conditions hold: the outer electrode radius is not too small, and the applied electric field is sufficiently strong. Under these conditions long waves are shown to be dispersive and a weakly nonlinear theory can be developed to describe the evolution of the disturbances. The canonical system that arises is the Kortweg-de Vries equation with coefficients that vary as the electric field and the electrode radius are varied. Interestingly, the coefficient of the highest-order third derivative term does not change sign and remains strictly positive, whereas the coefficient α of the nonlinear term can change sign for certain values of the parameters. This finding implies that solitary electrocapillary waves are possible; there are waves of elevation for α>0 and of depression for α<0. Regions in parameter space are identified where such waves are found.

Analyse the Electric field of symmetrical and asymmetrical concentric electrodes

The different between two potential voltages can cause the electric field. The electric field is normally distributed along the radius of electrode, and hence it depends on the shape of electrodes. This paper analyses the distribution factor of electric field of symmetrical and asymmetrical concentric electrodes by using Finite Element technique. This allows an analysis the optimum safety clearance distance between two concentric electrodes. The symmetrical concentric electrode refers to Spherical-Spherical concentric electrodes and Cylindrical-Cylindrical concentric electrodes. It must be noted that the symmetrical electrodes are mostly applied for Gas Insulated Substation (GIS) equipments. The asymmetrical electrodes mention to Spherical (inner)-Cylindrical (outer) concentric electrodes and Cylindrical-Cube concentric electrodes, which present as the connection point of High Voltage (HV) cable. The simulations is also complies with the existing standards and regulations in order to ensure the accurate results.

One-Dimensional Semianalytical Model for Optimizing the Standing-Wave Direct Energy Converter

A method to optimize the spacing of decelerator electrodes in the standing-wave direct energy converter has been developed. The standing-wave direct energy converter is introduced as a simplified version of, and possible milestone toward, the traveling-wave direct energy converter to facilitate a general understanding of the physics of the traveling-wave direct energy converter, as well as to simplify the modeling and results. The standing-wave direct energy converter may also stand alone as an alternative to the traveling-wave direct energy converter. The method developed takes advantage of analytical simplifications in capacitance calculations to avoid the use of a computational mesh, thereby modeling the standing-wave direct energy converter system with low simulation runtimes while using explicit numerical methods to advance groups of ions, referred to as ion bunches. The optimization scheme returns the optimal electrode spacing and circuit resistance value given the inputs of the initial ion energy, ion bunch density, operating frequency, decelerator electrode radius, and number of decelerator electrodes. The method approximates the ion bunches as point charges on the axis of the device, and the limitations of this approximation are considered. The expansion of the ion bunch is treated analytically and imposes a limiting tradeoff between the density of the ion bunches and the duration of ion deceleration. Performance trends and other tradeoffs are also presented.

Simulation and sensitivity analysis of controlling parameters in resistance spot welding

This study was performed to investigate the fundamental parameters controlling the nugget growth. The parameters were categorized into four groups, i.e. material parameters, electrical parameters, thermal parameters and geometrical parameters. In order to quantify the sensitivity of nugget growth to changes in these parameters, a numerical model which incorporates the electrical, mechanical and thermal contact was developed. As a result, a sensitivity index table was constructed and analyzed to ascertain the relative importance of these characteristic parameters. It was found that the most important factor in determining the variability of nugget growth behavior is the ratio of contact radius to electrode radius and the ratio of electrode radius to the square of specimen thickness. In general for a variation of 10%, the geometrical parameters are most important, followed by the material parameters. The electrical parameters and the thermal parameters are the least important. The importance of contact at the faying interface is greater for the contact area than for the contact resistance.