Transverse Contacts Research Articles

Frictional Contacts Between Rough Grains With Fractal Morphology

AbstractSurface morphology plays a crucial role in friction between two contacting geomaterial surfaces, yet many questions remain unanswered regarding how detailed frictional responses deviate from analytical solutions for smooth surfaces due to the presence of roughness. In this study, we revisit the Cattaneo‐Mindlin problem for contacts between two fractally rough elastic or elasto‐plastic spheres generated based on ultra‐high degree (e.g., up to 2,000) spherical harmonics with the corresponding wavelength less than a thousandth of the mean grain diameter. Transverse contacts are simulated by finite element method, validated by the extended Cattaneo‐Mindlin solution to full slide regime for smooth sphere contacts. Extensive simulations are conducted to study contacts between two rough spheres with various surface geometries, micro friction coefficients, normal contact distances, relative roughness, fractal dimensions, and wavelength ranges. Out results indicate that: (a) the new analytical solution can approximately predict the macro contact response except for extremely high relative roughness and narrow wavelength range; (b) deviations induced by roughness from smooth sphere contacts can be neutralized by plasticity, high normal contact interference, and high micro friction coefficient; and (c) fractal dimension impacts frictional contacts less than relative roughness. The main cause of these phenomena can be credited to the underlying microscale contact information. Contact area and stress distributions and their evolutions provide concrete evidence of these observed behavior. This work provides a pathway for applying computational contact mechanics to many geophysical fields, such as the asperity model in earthquake science and the mechanics of granular materials.

Thin film deposition of organic-inorganic quinoline-tin dioxide p-n junction for optoelectronic devices

Tin dioxide (SnO2) is an oxide semiconductor with n-type characteristics, with high transparency in the UV–Vis, where the donors are usually associated with oxygen vacancies and interstitial tin ions. Quinoline derivatives (QD) are usually p-type semiconductors with emission in the blue range. We report photo-induced properties of the QD 4-(6-(diethylamino)-4-phenylquinolin-2-yl)benzoic acid and the combination with the inorganic semiconductor oxide SnO2, both layers in the form of thin film, which forms a heterostructure. Thin film is a very convenient format for integration in optoelectronics. Emission of the QD takes place in blue range (470–485 nm) and depends on the solvent when in solution, being used acetone and tetrahydrofuran (THF). However, when in the form of thin film, it does not depend on the solvent. Concerning the heterostructure, it is explored under distinct device architecture: 1) combination in a transport profile perpendicular to the films (transverse contacts) leading to a rectifying behavior similar to a p-n junction, which is evidence of the p-type-like electrical behavior of the QD; 2) in parallel conduction profile, where there seems to exist some sort of interfacial phenomenon similar to a two-dimensional electron gas (2-DEG), a property that can be explored in transparent high-mobility transistors.

Study on the vacuum arc characteristics between novel straight slot transverse magnetic contacts with auxiliary slots on the back

The motion stability of the vacuum arc between TMF contacts significantly affects the breaking capacity of vacuum switches. The vacuum arc movement can be effectively controlled by slots on the TMF contacts to provide the necessary driving force. However, the necessary cross-slot movement of the arc affects its stability. In this paper, a novel straight-slot transverse magnetic field (SSTMF) contact structure with additional auxiliary slots on the back is proposed to improve current distribution and avoid stagnation at the contact center. The simulation analysis was conducted on the force acting on the arc at different positions and the magnetic field distribution between the contacts. Additionally, the arcing experiments were performed in a demountable vacuum chamber for both novel and traditional SSTMF contacts. The SEM analyses were carried out for the contacts after the experiments. Combined with simulation and comparative experimental results, it is verified that the auxiliary slots can improve the control and driving effect on the arc. This can provide ideas and references for future contact design and improve the breaking performance of vacuum circuit breakers.

Experimental study of the vacuum arc motion-diffuse transition in cup-shaped transverse magnetic field contacts

The arc in transverse magnetic field (TMF) contacts transitions to diffusion mode before current zero, affecting arc morphology and motion. TMF-generated force competes with local overheating's stagnation effect, impacting contact surface ablation. Cup-shaped TMF contacts exhibit slow, fast, and unstable transitions. Arc voltage declines gently, rapidly, or in repeated steps. Peak current, contact diameter, contact piece structure, and iron core influence transition characteristics. Transition energy increases linearly with peak current. Larger contact diameter and iron cores make transition constant earlier and transition current higher. Compared to circular contacts, annular contacts delay transition constant and widen transition current range. Scanning electron microscope (SEM) images showed that the ablation cathode surfaces exhibited five morphologies: melting pit, periphery, lamellar, particle, and pore. Energy dispersive spectrum (EDS) analyses show that the periphery has a higher Cu content than the interior, suggesting less Cu loss during fracture and less droplet splashing. The peripheral Cu content is comparable between the two contacts, 62.94 % vs. 62.37 %. However, the interior Cu content of contact NO.4 is higher than NO.6 at 60.54 % vs. 56.26 %. This indicates that the difference in the effect of fast and slow transitions on the cathode surface activity is mainly in the interior region.

Photoelectric properties and persistent photoconductivity of GaN-based ultraviolet photodetectors

Persistent photoconductivity deeply affects the performance of the photodetector, and has been studied in a variety of semiconductor optoelectronic devices. In this work, AlGaN/GaN metal-heterojunction-metal (MHM) ultraviolet (UV) detectors and GaN metal-semiconductor-metal (MSM) ultraviolet detectors were prepared using transverse Schottky contacts, and characterized at different temperatures, light intensity and bias voltage. The two-dimensional electron gas (2-DEG) at the Schottky heterojunction of AlGaN and GaN is used to generate high-speed electron mobility, which significantly improves the response speed and responsivity of photogenerated carriers. Under the illumination of 365 nm center ultraviolet light with an intensity of 4.4 mW/cm2, the photocurrent can reach 434 μA-MHM and 38 μA-MSM (25 °C), and the attenuation rate increase by 2.8 times, 5 times and 3.7 times with temperature (25 °C-250 °C), bias voltage (5/10/15 V), and light intensity (0.33/1.1/4.4 mW/cm2), respectively, and the attenuation rate reaches 50∼80 times.

Experimental investigation on “glow flow” phenomenon during current-zero period between transverse magnetic contacts

The inter-electrode state of the transverse magnetic field contact during the current-zero period is determined by the evolution of the arcing process. It has an important effect on whether the vacuum switch can be successfully broken. In this paper, a series of contacts were subjected to arcing and fixed gap distance experiments. The results show that a special inter-electrode phenomenon (glow flow) appears during the current-zero period when the current exceeds the threshold for the transition from diffused to constricted state of the vacuum arc. The mechanism behind the formation of this phenomenon was elucidated through an analysis of multiple sets of experimental results. It is found that the inter-electrode phenomenon arises from the disruption of the equilibrium process between the pressure from the arc column plasma and the anode vapor flux on the near-anode side at the current-zero. The copper vapor falls and is ionized by collisions with heat-emitting electrons. The experimental results and theoretical analysis also show that this phenomenon has a time interval with the reignition phenomenon. When this phenomenon is intense to a certain degree, the reignition will occur with high probability. The observation and theoretical analysis results of this paper are relatively consistent with the research results of other scholars, which can help to improve the vacuum arc characteristics and interrupting mechanism. This can provide a basis for improving the arc extinction performance of the vacuum switch.

Investigation on the arc root characteristics of vacuum arc between the iron-core cup-shaped transverse magnetic field contacts

The severity of the contact surface ablation determines the breaking capacity of the vacuum circuit breaker, and the interaction between the arc root and the electrode is the main cause of the contact surface ablation. The characteristics of the arc root determine the magnitude of the electromagnetic force received by the arc during the movement stage as well as the shape and speed of the arc movement. It will eventually affect the temperature of the contact surface and the ablation situation. This paper studied the arc root characteristics of high-current constricted vacuum arcs between the iron-core cup-shaped transverse magnetic field contacts and put forward the idea of using the ratio of cathode and anode arc root areas to quantitatively analyze the concentration of vacuum arc. The research on the arc root characteristics of the vacuum arc can help to improve the arc characteristics and arcing mechanism, and the proposition of the ratio can provide a new method for the study of constricted arc. This can provide a basis for improving the arc extinction performance of the switch, reducing the ablation of the contact material, and improving the contact life.

Capacitive making of double‐break vacuum circuit breaker: Transverse magnetic field contacts are superior to axial magnetic field contacts

Previous studies mainly focused on the behaviours of single-break vacuum circuit breakers (VCBs) in switching capacitor banks. The prestrike characteristics of double-break VCBs subjected to axial magnetic fields (AMFs) and transverse magnetic fields (TMFs) in capacitive making operations, to find a proper structure of double-break VCBs for capacitive switching are experimentally investigated here. Special attention is given to the prestrike gap, inrush current interruption, prestrike voltage, prestrike electric field strength and erosion on contact surfaces of double-break VCBs. The results show that the 50% prestrike gap d50 and the scatter in prestrike gaps are significantly reduced by nearly 50% by the TMF double-break VCB compared with the AMF double-break VCB. The 50% prestrike arcing time ta50 of the TMF double-break VCB can be reduced by 37.0% compared with the AMF double-break VCB. The voltage distribution ratio between two vacuum interrupters (VIs) is more balanced in the TMF double-break VCB (6:4) than in the AMF double-break VCB (7:3) when prestrike occurs. The TMF double-break VCB can reduce the erosion caused by inrush currents on contact surfaces compared with the AMF double-break VCB. In switching capacitor banks, the TMF double-break VCB is superior to the AMF double-break VCB.

Numerical simulation of vacuum arc movement between transverse magnetic field contacts

The contact system of the vacuum switch is a crucial factor affecting the breaking performance of the vacuum switch. The study of the arc characteristics has significant impacts in improving the breaking ability of the vacuum switch. It is difficult to fully grasp the characteristics of the vacuum arc by experimental methods. The vacuum arc simulation model can obtain the internal parameters spatial distribution and motion characteristics more effectively. A high-current vacuum arc magnetohydrodynamic model was established, which considered the influence of ion and electron viscosity and the combined effect of the transverse magnetic field (TMF) and the axial magnetic field (AMF). The simulation obtained the plasma parameters of the arc column region as well as the temperature distribution characteristics of the cathode and anode surfaces. In addition, the changes in plasma parameters during arc movement under the action of different TMF and AMF components between TMF contacts were discussed. The simulation results are relatively consistent with the experimental results and the existing research results, which provide theoretical support for the research on the mechanism of vacuum arc motion characteristics.

Study of Vacuum Arc Rotation Characteristics of Different Transverse Magnetic Field Contacts

As a special phenomenon under transverse magnetic field (TMF) contacts, vacuum arc rotation deserves more attention. In this article, rotation characteristics of three typical TMF contacts, i.e., swastika-type, cup-shaped, and double-TMF contacts, are systematically studied. According to arc images, recorded by a high-speed camera, physical terms related to arc rotation such as rotation duration and rotation speed are clearly defined. The magnetic field and Lorentz force are calculated and the force balance of the moving arc is analyzed. On this basis, the effects of arc ignition mode, peak current, and contact type on arc rotation are investigated. Arc voltage noise as well as arc splashing outside caused by rotation is also discussed. The results show that the duration from ignition to rotation in two-point arc ignition (TPAI) mode is usually the shortest because of the rapidest constructed column formation. As the peak current increases, arc rotation becomes easier. Due to stronger TMF, the arc under double-TMF contacts has the longest rotation duration and the highest rotation speed.

Simulation of vacuum arc cathode spot movement between transverse magnetic field contacts

Vacuum switches have been extensively used in the field of medium voltage. Carrying out relevant research on vacuum arc is particularly important for improving the performance and reliability of vacuum switches. The vacuum arc cathode spots provide metal vapor and electrons, which determine the breaking capacity of the vacuum switches to some extent. Therefore, based on the simulation model of the cathode spots between the transverse magnetic field (TMF) contacts, the motion phenomenon and distribution of the cathode spot were simulated and the characteristics of the initial diffusion stage of cathode spots between TMF contacts were studied. The simulation results showed that the probability of generating a cathode spot in the direction of retrograde motion increased with the increase in the TMF component and the direction of motion varied with the angle between the magnetic field and the contact. The diffusion process of cathode spots under wan-shaped contacts, cup-shaped contacts, and spiral-shaped contacts was simulated. The ring-shaped distribution of the cathode spots between cup-shaped contacts with a large current in the initial diffusion stage was formed by the self-generated magnetic field of cathode spots, while the axial magnetic field component and the clustered cathode spots destroyed the ring-shaped distribution. Taking the distribution of cathode spots and current from experiments as initial conditions, the simulation results of cathode spot distribution in the initial diffusion stage and the arc root radius were consistent with the experimental results.

Controlled Localized Domain Wall Writing in Antiferromagnetic Nano Stripes With In-Plane Transverse Currents

In order for textures in intrinsic antiferromagnets to be applied in proposed spintronics devices, localized and controllable writing is required. Here, the feasibility of a simple scheme for spin-Hall torque driven localized writing of chiral antiferromagnetic (AFM) domain wall (DW) pairs at a chosen location in AFM nano stripes is investigated. Short voltage pulses causing in-plane current-injection across transverse contacts in the heavy metal trigger a localized horizontal boundary-nucleated reverse AFM domain that propagates throughout the width to the opposite boundary, defining a locally written DW pair without affecting information in other parts of the device. The nucleation mechanism is examined, indicating exchange torque governing. A diagram of the number of DW-pairs written as a function of applied writing pulse amplitude and pulse duration is shown for a generic antiferromagnet, illustrating the degree of control required of the excitation protocol. Simulations of write and drive pulses are also presented, showing that the slight current bending around the writing contacts does not impose any significant pinning nor nucleation of unwanted textures for a typical driving amplitude. The advantages of this simple scheme are that it does not require any component for vertical spin-polarized current injection nor lasers for writing as in previous proposals.

Investigation of unstable motion characteristics of vacuum arc cathode spots between transverse magnetic field contacts

AbstractWith the improvement of the current level of power grids, the requirements of the opening level of the vacuum switches are also increasing. Vacuum arc cathode spots provide steam and electrons and, to a certain extent, determine the opening capacity of the vacuum switch. In this paper, a vacuum arc cathode spot research platform based on the de‐mountable vacuum chamber is constructed. The characteristics of the vacuum arc cathode spots under the transverse magnetic field (TMF) contacts are assessed by a high‐speed charge coupled device. The experimental results show that the cathode spot diffusion process can be divided into three processes through cathode spot distribution, arc voltage and current: initial diffusion stage of cathode spots, unstable motion stage of cathode spots, and extinguishing stage. The motion mode of cathode spots during unstable motion stage can be divided into cathode spots group stagnation (CSGS) to multi‐cathode jet (MCJ) switch mode, cathode spots group motion (CSGM) to MCJ switch mode, CSGM mode, and MCJ mode. The effects of peak current and contact diameter on unstable motion mode were analysed.

Post-Arc Characteristics of High-Current Arcs in Vacuum Circuit-Breakers

The post-arc (PA) characteristics of vacuum arcs in transverse magnetic field contacts are studied for short-circuit currents of up to 123 kA peak and transient recovery voltages below 875 V. The measured PA currents are interpreted in terms of an Electric Resistance Model and the models of Andrews-Varey, Langmuir-Child, and Slepian-Schmelzle. Whereas in the late PA period, the calculations do not agree well with the measurements, the PA behavior is well described in the early period after current-zero. It is concluded that the PA discharge is amplified by ionization of metal vapor particles in the boundary sheath due to electron impact.

Experimental Investigation on the Postarc Current in Vacuum Circuit Breakers and the Influence of Arcing Memory Effect

The dielectric recovery process is a crucial stage of current interruption in a vacuum circuit breaker (VCB). The postarc current is an important macroscopic electric characteristic of the dielectric recovery process, which has caused many concerns in the past few decades. In this paper, the postarc currents of VCBs with different types of contacts, such as butt contacts, axial magnetic field contacts, and transverse magnetic field contacts with different materials are measured with a high-resolution current-zero diagnostic system. Besides, the appearance of vacuum arcs is recorded using a high-speed camera in the experiments. The relationship between the arc memory effect and the postarc current is analyzed. What is more, the influence of the instability of the vacuum arc shape on the dispersion of postarc current is also studied.

Experimental and Simulation Research on Influence of Axial Magnetic Field Components on Vacuum Arc Between Transverse Magnetic Field Contacts

The vacuum transverse magnetic field (TMF) contacts have the advantages of simple structure, short current path, and low cost. Arcing process and arc characteristics between the TMF contacts directly determine the temperature distribution and ablation of the contact surface, which are closely related to the breaking ability of the contacts. The axial magnetic field (AMF) component generated by the TMF contact structure also affects the arcing process. In this paper, five kinds of contacts with different starting positions of spiral slots were experimentally investigated in a demountable vacuum chamber. The arc appearance and the arc characteristics during different arcing stages and the surface utilization of the contacts were analyzed. The magnetic characteristics of arc in different stages were simulated. It was found that the arc with small AMF component has a large diameter and small range of motion. The arc with large AMF component might jet toward the outside of the interelectrode.

Study on arc mode and shape transformation in the arc motion stage between transverse magnetic field contacts

Transverse magnetic field (TMF) contacts drive the vacuum arc moving along the contact surface by producing a magnetic field perpendicular to the arc current flow, which makes the arc energy distribute uniformly and prevents the contact from serious ablation due to local overheating. The characteristics of arc motion between the TMF contacts directly determine the surface temperature distribution and contact erosion. Also, it is closely related to the breaking ability of the contacts. In this paper, the arc motion between spiral‐type contacts was recorded by a high‐speed camera. By analysing the arc voltage and arc images, the characteristics of arc shape transformation in the motion stage under different currents were studied. At the same time, the influence of arc duration and arc characteristics before the motion stage on the arc motion is discussed. It is found that the arc behaviour before the motion stage did influence the shape transformation and range of arc rotation. After entering the motion stage, there was always a rapid transformation process of the arc shape and the arc would return to where it first became constricted after a short movement. After a continuous movement, it might rotate in a small region where the arc stagnated before the motion stage. In addition, the arc behaviour was also related to its duration. When the opening time was varied from 2 to 5 ms, the arc velocity decreased. Meanwhile, it was more likely that multiple transformations of the arc aggregation degree occurred.

Arc characteristics during the instability stage on transverse magnetic field contacts

The arcing process greatly affects the breaking ability after current zero. The instability stage is the transition stage from the ignition to the movement stage, which affects the arc movement characteristics. In this paper, the arc characteristics during the instability stage on spiral-type contacts were investigated using a high-speed video camera. A multi-column parallel instability mode and a single-column instability mode were found during the instability stage. The arc appearance and constriction degree changed rapidly. The arc voltage usually increased accompanied by fluctuations. In addition, it was found that the current significantly influenced the arc mode and duration in the instability stage. With increased peak current, the probability of a single-column instability mode increased, and the fluctuation range and average time decreased.

Modeling and numerical simulation of anode activity and arc motion in a transverse magnetic field

Vacuum interrupters based on transverse magnetic field contacts now are widely used in medium voltage circuit breakers. The arc motion under the transverse magnetic field (TMF) plays a decisive role in the interruption. In this paper, we focus on the movement of vacuum arc driven by TMF and anode thermal process during the arcing. Then, based on the principle of conservation, a transient two-dimensional anode activity model (subjected to TMF constricted arc) is established. The state change of material, evaporation from contacts and motion of molten pool are also considered in our model. By considering the arc motion in the anode activity model, the transient anode thermal process and the movement of constrict arc under TMF are studied, respectively. The simulation predicted that average speed of arc is not higher than 200m/s. In the simulation, the anode surface is completely melted near the current peak with 200 μm molten layer. In addition, the influence of movement of molten pool on the arc speed is analyzed, finding that the movement of the bath accelerates arc speed, but weakens the surface melting at the same time.

Finite element analysis of a 3D moving vacuum arc for transverse magnetic field contacts based on Gundlach’s formula

A systematic numerical method for analyzing a 3D moving vacuum arc was proposed and tested in this research by using a transverse magnetic field (TMF) contact. The analysis was carried out by employing the finite element method and the experimental energy equation defined by Gundlach’s formula. In the literature, the vacuum interrupter has been widely applied to medium-voltage switching circuits. TMF-type contacts use the Lorentz force density to move a high-temperature arc so as to prevent the contacts from being melted and damaged. The material erosion caused by the arc on the electrode’s surface is an important process that results in the interruptive capabilities of these vacuum interrupters. In a classical arc model, to move the vacuum arc, it is required that the magneto-hydrodynamics be analyzed in the arc region at each step. However, with this approach convergence is difficult, resulting in a very time-consuming. Therefore, we propose a new technique to predict the behaviors of vacuum arc between two electrodes. This new approach adopts the experimental arc voltage equation between two electrodes defined by Gundlach’s formula. We verify our proposed model by comparing its results with the arcing behaviors obtained from earlier experiments.