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Initial arc formation in high-current drawn vacuum arc was investigated experimentally and analyzed quantitatively in this paper. Experiments were conducted in a demountable vacuum chamber. Characteristics of molten metal bridges evolution and arc voltage indicated that initial arc formation could be divided into three stages. The correlation between the rupture of molten metal bridges and the fluctuation of arc voltage in stage 2 was analyzed. In addition, characteristics in stage 2 under different arc currents and opening speeds were analyzed quantitatively. [ABSTRACT FROM AUTHOR]

Bottom-up nanofabrication, organizing colloidal nanoparticles into ordered clusters with structural and functional complexity, provides an alternative to yield nanometric smart structures unachievable in traditional top-down approaches. Those remarkable structures assembled of colloidal nanoparticles possessing optical properties feature exotic optical responses under the illumination of incident light, showing highly dependent on the particle separation, composition, and arrangement. Here, the nanoscale electromagnetic coupling in magnetic–plasmonic nanoparticle clusters is analyzed by using a 3D full-wave computational model, and the mechanism of the resonances in a split-ring resonator out of magnetic–plasmonic nanoparticle building blocks is also illustrated. In particular, once assembling those artificial molecules (split-ring resonator) into secondary metamaterial superstructures, such as split-ring resonator dimer and Y-shaped structures, a series of exotic optical responses are presented due to the greatly enhanced near-field coupling of nanoparticles and the generation of antiparallel ring currents in the asymmetrical superstructure, exhibiting flexible sensitivity to the changes of the surrounding environment. Besides, the H-shaped arrangement exhibits high sensitivity with Q = 50.5 at λ = 2.02 μ m. Our finding provides a platform for yielding complex structures with flexible tunability of the optical response through arranging those assembled artificial molecules into complex secondary structures, allowing for the development of smart sensing devices. [ABSTRACT FROM AUTHOR]

Due to vacuum interrupter's excellent interruption performance, an increasing number of on-load tap-changers (OLTCs) have chosen it as a load changeover switch in recent years. The stability of the arcing process is crucial for the interruption since the vacuum interrupter in OLTC must break current frequently with the requirement of long electrical life. In this paper, experiments on butt contacts and axial magnetic field (AMF) contacts applied in OLTC were carried out in a demountable vacuum chamber, and the arcing process was investigated by a high-speed camera. Investigations were conducted on the effects of opening speed, opening phase, current amplitude, and AMF on arc characteristics, e.g., arc expansion and arc voltage. The experiment findings demonstrated that expansion and instability of arc increased with higher opening speed, number of cathode spots moving to the inclined surface decreased with shorter arcing time, and that cathode spots' propensity to form group cathode spots increased with higher current. AMF lessened the impact of high opening speed and high current on the arcing process. The investigation carried out in this paper helps to better understand the characteristics of arc evolution and the influence of some key factors, which may serve as a guide for improving the design and vacuum arc control of vacuum OLTCs. [ABSTRACT FROM AUTHOR]

Numerical investigation was carried out on the evolution of inhomogeneous energy deposition of polarity effects in negative polarity electrical explosion of aluminum wire in vacuum. First, radial electric field distribution of the aluminum wire was simulated. The results showed that the initial electric field near electrodes was much smaller than that in the middle of the wire. A model of electrical wire explosion based on ZEUS-EW with artificial limitation on breakdown time of the aluminum wire was used to simulate the behavior of inhomogeneous energy deposition of the polarity effect. The complete evolution process of electrical explosion of the aluminum wire with inhomogeneous energy deposition was further explored and qualitatively divided into three stages according to the simulation results. Finally, the influence of some factors on the inhomogeneous energy deposition simulation was investigated. This study provides some help for a better understanding of the polarity effect in the process of wire explosion. [ABSTRACT FROM AUTHOR]

In this paper, the evolution of vacuum arc under different conditions in the quench protection switch (QPS) based on forced current zero was investigated experimentally and analyzed quantitatively. Experiments were conducted with cup-shaped axial magnetic field (AMF) contacts in a demountable vacuum chamber. Images of the arc column were photographed through the observation window with a high-speed digital camera with exposure time of 2 μs. Arc appearance was analyzed quantitatively through digital image processing. Quantified arc appearance and arc voltage characteristics indicated that the high-current vacuum arc evolution in the QPS could be divided into four stages: arc formation stage, arc column merging stage, diffusing stage, and fast extinguishing stage. The influence of AMF on the evolution of high-current vacuum arc in the diffusing stage was also studied. Experiment results indicated that the vacuum arc at relatively low current had a simpler evolution process and the arc column merging had less correlation with voltage stability. It was found that the vacuum arc with a long gap distance developed faster and the time required to enter the stable burning state was shorter. In addition, the vacuum arc with long arcing time tended to constrict again, which is not beneficial to current interruption. [ABSTRACT FROM AUTHOR]

Patterning a wide range of colloidal plasmonic nanoparticles into prescribed spatial arrangements, analogous to the formation of natural materials, enables the fabrication of functionalized structures with novel optical properties, such as Fano resonance, magneto-optical Kerr effect, negative refraction, etc. Currently, capillarity-assisted particle assembly is always utilized to place colloidal particles at predetermined positions by exploiting the capillary forces resulting from the motion of an evaporating droplet. However, this technique requires specialized equipment and the assembly process is always carried out in an open system, thereby introducing the risk of contamination and limiting its applications. Here, we present a microfluidic and magnetic field-assisted self-assembly of colloidal magnetic-plasmonic nanoparticles by utilizing magnetic dipole–dipole interactions resulting from the localized magnetic gradient field produced by an array of soft-magnetic elements and external magnetic bias field. The magnetized magnetic-plasmonic nanoparticles are controlled to deposit at the predesigned traps microfabricated onto the soft-magnetic elements. After deposition, the inlet velocity of the microchannel is improved to clear away the particles out of the traps, forming arrays of patterns with consistent structures. Furthermore, a Lagrangian–Eulerian model is introduced for the first time to predict the processing of the microfluidic and magnetic field-assisted self-assembly of colloidal magnetic-plasmonic nanoparticles by taking the magnetic and hydrodynamic forces and particle–fluid interaction into account. Our analysis demonstrates that the particle–fluid interaction not only plays a significant role in determining the final self-assembled nanostructures, but provides an opportunity to improve the consistency of the assembled nanostructures. The microfluidic and magnetic field-assisted self-assembly protocol proposed here enables the patterning of colloidal magnetic-plasmonic nanoparticles to be carried out in a controlled environment and also opens up a new direction for assembling complex structures. [ABSTRACT FROM AUTHOR]

• The combination of Ω- and straight-shaped obstacles enhance the mixing performance. • Well mixing is achieved as the obstacles are in contact with the walls of Tesla chambers. • Increasing the radius of Ω-shaped obstacles can improve the performance of micromixer. • The hexagonal chambers are better than triangle and Tesla chambers. An efficient fluid mixing performance is achieved by a Tesla/hexagon-shaped passive micromixer in a wide range of Reynolds numbers with short mixing length (4.2 mm/1.15 mm from the origin). The passive micromixers are composed of Tesla/hexagonal chambers and Ω-shaped obstacles or combined with straight-shaped obstacles that induce the generation of vortex, contraction and expansion process, the changes of the flow direction, and even the split and recombination of streams. The mixing process of two fluids (water and ferrofluid) is numerically investigated using computational fluid dynamics. For the Tesla-shaped micromixer embedded with Ω-shaped obstacles, a sufficient mixing is obtained as the obstacles are in contact with the internal walls of Tesla chambers. In the hexagon-shaped micromixer, the combination of straight- and omega-shaped obstacles greatly enhance the performance of micromixer in particular at moderate flow rate (1< Re <10). Moreover, reasonable increasing the curvature radius of the omega-shaped obstacles can further improve its mixing performance. The designed passive micromixers were also fabricated, tested and compared with the numerical results through the soft-lithography processes, and the good agreement between the experimental and numerical results identifies our predictions. The good performance of the designed micromixers provides a new direction in designing micromixer with high-throughput and short mixing length. [Display omitted] [ABSTRACT FROM AUTHOR]

• A hybrid magnetic field produced by microwires and Halbach magnets is proposed to enhance the mixing between ferrofluid and water. • We numerically and experimentally analyze the effects of the arrangement and the number of microwires on the mixing efficiency. • By adjusting the direction and the magnetite of uniform magnetic field, we receive optimal mixing efficiency 99.06%. With the development in biomedical and biotechnological areas, novel and efficient microfluidic devices integrated with sufficient and rapid mixing function have been attracting enormous scientific attention as versatile and robust platform for low cost and non-contact biological analysis and diagnose. Here, we design and fabricate a magnetic micromixer integrated with special designed microwires, a Y-shaped microchannel and uniform magnetic field for realizing rapid mixing between ferrofluid and deionized water. A comprehensive analysis on the dynamic behavior of ferrofluid is performed by using 3D Eulerian-Eulerian model. Good consistency between experiment and numerical results demonstrates that the enhanced mixing in the presence of longitudinally arranged microwires is due to the introduction of eddies, whose impacts are strengthened with the increase of the number of microwires (n w). However, increasing n w requires a long microchannel and long time to complete mixing. In order to further improve the mixing performance of our micromixer, a horizontally arranged microwire combined with a particular uniform magnetic field is proposed to provide enough magnetic force for enhancing the mixing of ferrofluid and deionized water, receiving optimal mixing efficiency 99.06%. The flexible tunability of external uniform magnetic field enables desired performance of our designed system. [ABSTRACT FROM AUTHOR]

To void breakdown between busbars, the dissociation unit is applied in the arc chamber of an air circuit breaker (ACB) to cool down the hot gas exhausted from the vent of the arc chamber in the interrupting process. Cooling efficiency and flow resistance of the dissociation unit have important roles, which can greatly affect the safety of the low voltage switcher and interrupting performance of the arc chamber. The most common structure style of the dissociation unit is a multilayer metal wire mesh, and it has a complex microstructure. Then, it is unreasonable to directly simulate the dissociation unit due to the huge cost of calculation. In this study, the porous medium model is introduced and a combination of two different porous materials is applied to simplify the simulation model of the dissociation unit. Based on the porous model and simplified arc chamber model, the ACB with the dissociation unit is analyzed. The cooling effect and flow resistance of the dissociation unit are evaluated in the breaking process. By comparing experimental and simulated pressures in the arc chamber, this simulation approach is verified. [ABSTRACT FROM AUTHOR]

Development towards HVDC grids is drawing intense development of HVDC circuit‐breakers (CBs). The increasing rated voltage and capacity requires HVDC CBs with excellent performance to be installed and comprehensive tests on HVDC CBs are indispensable. This paper contributes to clarifying clearer test requirements on the current interruption test, which is the most challenging one and proposing a novel applicable test method. It is the toughest test because when an HVDC CB interrupts fault current, a high transient interruption voltage (TIV) is produced by the operation of metal oxide surge arrester (MOSA), and a large current continues flowing through MOSA simultaneously, which means considerable energy is demanded for the test. To verify an HVDC CB's current interruption ability, voltage withstand ability and energy dissipation ability in single test can be costly and even impractical in extra high voltage classes. Alternatively, a multi‐part test method is reasonable and valuable, in which the HVDC CB can be tested with large current and high voltage one after another, while the energy dissipation ability will be verified separately. At last, a proposed synthetic test circuit is validated by a simulation and a low voltage experiment, creating high equivalence between the simulation and the validation test. [ABSTRACT FROM AUTHOR]

This work investigates the dynamic characteristics of vacuum arc in the initial expansion process of a direct current (dc) circuit breaker. Based on the commercial cup-type axial magnetic field (AMF) contact, a 3-D transient magnetohydrodynamic (MHD) model is established. In the model, transient equations are used to study the transient characteristics of the vacuum arc. The effect of magnetic field hysteresis is also considered in the method. The dynamic mesh technique is adopted to simulate the electrode separation, which can make the simulation results more accurate. The simulation results show that, during the initial expansion process, with the increase in the current, the arc gradually expands in the electrode gap. The ion pressure, ion number density, axial current density, and energy flux density to the anode gradually decrease. Ion rotation can be observed under the influence of the magnetic field, and the rotation speed will increase with the increase in the current. Two different arc ignition positions are also compared. The result shows that igniting the arc at the contact center is more helpful to the success of the interruption of the dc circuit breaker. The appearance of the arc predicted by the model is consistent with the experimental results. [ABSTRACT FROM AUTHOR]

The preparation and storage of aluminum nanoparticles pose significant challenges due to their high chemical reactivity. This paper proposes a one-step method for the production of carbon-coated aluminum nanoparticles using an argon protective arc in a liquid nitrogen environment. The emission spectra of the arc validate that the method proposed in this paper can effectively mitigate the adverse impact of impurity gases such as oxygen and water on the product without the need for expensive vacuum equipment. The obtained nanoparticles underwent characterization through high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman analysis, thermogravimetric analysis, and dynamic light scattering. The characterization results demonstrate the successful synthesis of carbon-coated aluminum nanoparticles with nitrogen-doped fullerene shells. With an increase in arc current, the nanoparticles exhibit a corresponding enlargement in diameter, accompanied by a broader distribution. The characterization of the electrode erosion suggests that the aluminum droplet ejection plays a more important role in the nanoparticle generation process as the arc current increases. Optimized strategies are proposed to increase the percentage of successfully-formed carbon-coated aluminum nanoparticles. • One-step preparation of carbon-coated aluminum nanoparticles. • Arc discharge in liquid environment without vacuum and sealing equipment. • Core-shell structure: Al core wrapped in N-doped fullerenes. • The arc current impacts electrode erosion pattern and product characteristics. [ABSTRACT FROM AUTHOR]

A mid-infrared perfect metamaterial absorber (PMA) consisting of well-engineered unit cells, composed of metallic backed layer (Cu), dielectric layer (Al 2 O 3), matts-shaped meta cell (Cu), and an anti-oxidation layer from bottom to top, is proposed in this paper. The electromagnetic response of the designed PMA is analyzed using 3D full-wave computational model. Our analysis demonstrates that the maximum absorption is enhanced to 99.9% at the resonance wavelength of 7.35 µm, which is caused by magnetic polaritons (MPs) resonance between the incident beam and magnetic polaritons formed by three fluxing currents. Specially, a three-circulation equivalent circuit model is proposed to predict the resonant wavelength of the designed PMA, which comprehensively considers the influence of coupling capacitance not only between neighboring unit cells, but also between the metal lines inside the top metal pattern for the first time. The influence of geometrical parameters on resonant wavelength is predicted by the equivalent circuit model proposed in this paper, and the prediction error is less than 5% as compared with that of computational analysis. With well-engineered plasmonic meta-atoms, the designed PMA presents the characteristics of angle and polarization- independent with a smaller relative size (0.245λ) and a thinner relative thickness (0.053λ). Besides, an obvious red-shift of the resonance frequency of PMA is observed as the permittivity of dielectric layers is increased. The approach of establishing equivalent circuit model will provide methods for the analysis of PMA based on MPs with complex electromagnetic behavior. [ABSTRACT FROM AUTHOR]