Search

Powder processing by ball milling is an effective approach for materials engineering. Although various methods for material processing are available, only high-energy shaker/vibratory or planetary mills have been intensively utilized to develop mechanical milling or alloying routes for structural control of MgB2 superconducting materials. Herein, we have attempted structural modification by using a low-rotation shaker, which is categorized as a low-energy and economical mill in terms of industrial applications. The operation speed was kept constant at 40 rpm, which is much lower than typical conditions employed for planetary mills. Instead of adjusting the low rotational speed, the other processing parameters were controlled to enhance the energy transfer from the balls to powders. The applied milling conditions were ultimately found to cause severe plastic deformation of the raw powders. The shape and size changed drastically, depending on the processing time. The morphological variation of the processed powders as precursors for the MgB2 materials influenced the void structure and the composition including amorphous phases. By considering these results, we also elucidated the mechanism underlying the structural changes upon ball milling and their effects on the transport critical current performance. The present approach for powder processing offers potential as an effective milling route for structural modification of superconducting materials.

Structural disorder within grains formed by carbon doping is widely known to enhance the in-field transport critical current performance of MgB2 wire. The structural formation usually requires high-temperature sintering above 600 °C due to low reactivity of the dopant. In this study, we selected a carbohydrate (malic acid) that has a low-decomposition temperature as the carbon dopant and performed low-temperature sintering (below 600 °C) to introduce greater disorder within the MgB2 grains. We revealed that the doping and low-temperature sintering process cause severe disorder, especially along the out-of-plane (c-axis) direction. The disordered formation, in addition to the in-plane disorder combined with well-connected grains, eventually raised the in-field transport performance.

We report an ultra-low resistance superconducting joint using unreacted multifilament MgB2 wires produced by tailoring the powder compaction pressure within the joint with heat treatment conditions. The joint demonstrated an ultra-low resistance of 5.48 × 10−15 Ω and critical current (Ic) of 91.3 A at 20 K in self-field. The microstructural and composition studies of the joint revealed cracks and a high amount of MgO, respectively. These two features reduced the Ic of the joint to some extent; nevertheless, the joint resistance was not affected by it. Our tailored joining process will play a pivotal role in the superconducting joints development.

Nanocomposites have been actively studied for understanding the superior electrical characteristics and for utilizing in the electric devices of HVDC systems. Nanocomposites in electrical devices are not used unilaterally, but rather as the hetero-dielectric composites, which form an interface with other insulators. In this study, the hetero-dielectric composites comprised of nanocomposites and transformer oil were analyzed using the migration–migration model incorporating the finite element method. Until now, hetero-dielectric composites have been usually studied by using the Migration–Ohmic model, where the specific space charge behavior cannot be analyzed. Although interface conditions are significant for the transport and local accumulation of charge, the BCT models, including conditions for interface with other insulators, have not been reported. To investigate the space charge behavior of the hetero-dielectric composites, charge carriers of positive, negative ions and electrons are considered in the liquid region and electrons and holes in the nanocomposites. These hetero-dielectric composites are applied to the needle-bar electrode system to analyze the surface discharge phenomena at the interface. The BCT model was applied to the nanocomposites, and the drift dominated charge continuity model was applied to the transformer oil. These numerical results were compared with those from the previous research works validated with the experimental results. We analyzed the propagation of the streamer with an equivalent parallel current circuit and adopted the effective mobility of the electrons according to the field strength to improve the applicability of the numerical analysis model.

A superconducting joint architecture to join unreacted carbon-doped multifilament magnesium diboride (MgB

As an insulator, dielectric composites should be guaranteed the specific level of insulation performance to maintain the electric device. It is essential to understand the physical phenomena at the interface of the dielectric composites, where the electric field and charge distribution are concentrated. We, therefore, focused on the surface discharge propagated on the interface forming with dielectric composites, composed of nanocomposites as a solid region and oil as a liquid region. Nanocomposites have a broad range of conductivities depending on the content of nanoparticles. Thus, the conductivities of nanocomposites were analyzed from 0 to 10−3, 10−6, 10−7, 10−8, and 10−9 S/m with the experimentally verified numerical model. Our numerical model was expanded incorporating with the migration-ohmic model to link the physical phenomena between liquid and solid. With extending the simple model that handles only one charge carrier, three charge carriers were employed to analyze the temporal discharge dynamics in the liquid. Using the fully coupled finite element method, the patterns of the streamer were investigated with different conductivities. The higher the conductivity, the slower the propagation speed of the streamer. Conductivity changes the amount of the surface charge accumulated on the surface. This surface charge affects the speed of the streamer. Furthermore, the equivalent circuit model was implemented to explain the relations between the propagation speed and conductivity of solid.

A cryogen recondensing cooling system was designed for a 7 T superconducting solenoid magnet in an electron beam ion source (EBIS) system. In an EBIS system, it is difficult to access the apparatus due to strong X-ray emission when in full operation. The proposed cooling system was adapted for liquid-helium recondensation. An EBIS is a typical ion source that is feasible for highly charged ion production. Thus, the EBIS system can meet various requirements of end applications, and the development of such a system is strongly required. Estimations of the thermal and structural stability levels are very important during the design of a cryostat; therefore, these values were calculated by means of a numerical analysis. In the thermal analysis, the heat loads were calculated according to the temperature gradient after calculating the temperature distribution. The cooling margins were estimated at the first and second stages of the cryocooler, respectively, with respect to the calculated heat load. It was also estimated that the thermal stress due to the temperature gradient. Finally, it was confirmed that the proposed cryostat design was thermally and structurally stable and suitable for application to a superconducting magnet for an EBIS system.

The propagation of surface discharge due to interfacial polarization was numerically analyzed at the oil-nanocomposite interface using fully coupled finite-element analysis incorporating the relative permittivity from experiments. To improve the insulation ability, a new nanodielectric insulating material has been proposed in which a pressboard is coated with epoxy resin mixed with silica nanoparticles; this nanocomposite material can enhance the breakdown voltage in power systems with a certain level of silica nanoparticles. To specify the electric breakdown performance of this nanocomposite material, we measured the bulk relative permittivity of epoxy resin containing different percentages of silica nanoparticles on the pressboard. Surface discharge, or creepage discharge, tends to propagate along the solid–liquid interface and then leads to flashover. The mechanism of surface discharge, therefore, is a critical issue for understanding the dielectric breakdown strength in solid–liquid interface problems. To quantitatively analyze and explain the characteristics of surface discharge, here, the fully coupled finite-element analysis technique has been applied and tested with various relative permittivity values of nanocomposite materials. This phenomenon has been simulated using the fully coupled governing equations using Poisson’s equation for electric field and charge continuity equations, including surface charge accumulation for charge transport. After verification of our numerical setup in a conventional oil-pressboard system, a needle-bar electrode system was proposed and applied to the analysis of surface discharge propagation for the new nanocomposite materials with bulk dielectric permittivity. The propagation speed at the oil-nanocomposite interface was compared with different percentages of nanosilica. Finally, the physical mechanism of surface discharge due to the interfacial polarization was analyzed with the space, bounded, and surface charge densities at the oil-nanocomposite interface based on the numerical results.

Persistent-mode operation is a key feature of magnetic resonance image systems to improve the required field stability. The superconducting joint is known to be beneficial for reducing all the resistant components in an electrically closed-circuit. The joint technique of magnesium diboride (MgB2) multifilamentary wire, however, is the main obstacle to the use of magnet in practical applications. In response, herein, we designed and developed a unique configuration of superconducting joint to further enhance the interconnection of exposed cores between two 18-multifilamentary wires. It was confirmed that developed joint samples achieved high critical current similar to a non-jointed wire. The proposed joint technique was directly applied to the MgB2 single-turn coil and MgB2 magnet for estimating a joint property through persistent-mode operation. This work provides fundamental insights into the design of persistent-mode MgB2 magnets to boost magnetic resonance image systems.

A method has been developed for demonstrating a flow nanogenerator by using a mixture of magnetic nanofluid (MNF) and bubbles in a fluid circulating system, and notable phenomena related to the power generation properties of the nanogenerator have been explored. MNF is widely used in various areas because of its interesting magnetic properties under an external magnetic field. The objective of the proposed technique is to obtain the induced electromotive force (EMF) based on Faraday’s law due to the flow of MNF in a closed-circulating system. To maximize the induced EMF, magnetic nanoparticles (MNPs) should pass through the induction coil with a perpendicular magnetization direction in accordance with Faraday’s law. To control the magnetization direction of the MNPs, a permanent magnet was employed to produce an external magnetic field that considers the Brownian and Neel motions. To obtain a continuously induced voltage, a circulation system was implemented ensuring the flow of the MNF in the closed cycle. Further, power generation properties were investigated considering electric, magnetic, and fluidic effects. To analyze this complicated physics, a multiphysics analysis was used to calculate the flow pattern of the MNF according to its magnetic properties, and the acquired results were compared with those obtained from the experiment. From these experiments, we investigated the generation properties of the nanogenerator considering the flowrate of the MNF as well as the presence or absence of bubbles within the MNF. Our experimental tests demonstrated that the continuous power generation mode was successfully achieved with a mixture of MNF and bubbles.

Superconducting joints are essential for persistent-mode operation in a superconducting magnet system to produce an ultra-stable magnetic field. Herein, we report rationally designed niobium-titanium (Nb-Ti) superconducting joints and their evaluation results in detail. For practical applications, superconducting joints were fabricated by using a solder matrix replacement method with two types of lead-bismuth (Pb-Bi) solder, including Pb42Bi58 as a new composition. All the joints attained a critical current of >200 A below 1.43 T at 4.2 K. Our optimized superconducting joining method was tested in a closed-loop coil, obtaining a total circuit resistance of 3.25 × 10−14 Ω at 4.2 K in self-field. Finally, persistent-mode operation was demonstrated in an Nb-Ti solenoid coil with a persistent-current switch. This work will pave the way to developing high-performance Nb-Ti superconducting joints for practical applications.

Simple and efficient characterization of local structural states in materials is desirable for evaluating and tailoring the material performance. Herein, we have focused on micro X-ray diffraction and conducted a local structure characterization for superconducting wires having MgB2 polycrystalline cores. The critical current properties of the superconducting materials were controlled and improved by a low-temperature sintering approach with malic acid doping as a carbon source. The transport performance depending on the sintering and doping effects was found to be closely associated with the characterized properties of the local structures. The evaluation method will therefore be useful to develop composite conductors.

Evaluation and control of amorphous phases in materials are very important for optimizing their properties. Herein, we focus on polycrystalline MgB2 materials prepared with hydrocarbon doping and study the effects of residual amorphous impurities on the superconducting performance. Carbon is known to be an effective element for enhancing the transport critical current under an external magnetic field. The doped samples were prepared under two different nominal conditions, MgB2(C16H10)x/16 and MgB2−x(C16H10)x/16, which respectively correspond to additional and substitutional type doping of the MgB2 composition. Regardless of the doping type, both fabrication methods retarded the formation of the MgB2 phase due to the dopant, leading to an increase in amorphous impurities. However, the apparent phenomena that arise from the additional and substitutional types are still elusive. Ultimately, the structural differences due to the impurity effects caused significant changes in the transport critical current performance. The present quantitative analysis of the amorphous impurities thus paves the way to further optimize the doping methodology for MgB2 superconducting materials.

Use of the systematic process of design optimization was proposed using the sequential approximation technique for the 10 MeV cyclotron magnet, which is a positron emission tomography cyclotron for producing radioactive isotopes. To design cyclotrons with the conventional routine approach, too much time and an iterative process are required, because even a very small change in the magnet shape is directly linked to success or failure, as verified using the beam orbit and energy level. To resolve this sensitive and tricky problem, designers need a systematic numerical approach for developing a new design. Until now, however, related research articles have not been seen in the literature. To build a systematic optimization process for the cyclotron magnet, we adopted the Latin hypercube sampling method, one of the design of experiments, and generated sampling data. On the basis of these data, an approximation model was modeled using the Kriging technique. To find the optimal shape, an iterative searching technique was implemented, which incorporates the approximation model, restricted evolution strategy, and boundary shifting technique. The initial magnet model was obtained from the conventional routine procedure for a cyclotron. To verify our proposed model, the final magnet model was tested with a beam simulation for checking the trajectory of $^{18}\text{F}$ at a level of 10 MeV at the extraction point.

Positive and negative discharge mechanisms in atmospheric air have been numerically analyzed using a fully coupled finite element analysis. Until now, many research works have examined discharge analyses, but the discharge mechanism has not yet been explained in detail numerically. In a positive discharge, the electric field intensity at the streamer head can be enhanced owing to the superposition of a Laplacian field and a field from the positive space charge density. With this increased electric field intensity and attraction force between the negative electrode and positive space charge density, the positive streamer can move straight from the positive tip electrode to the negative plane electrode. In a negative discharge, the polarity of the electrode is reversed, and the positive space charge covers the negative electrode. Excess negative ions move toward the positive electrode, and a Trichel pulse is formed in the discharge current. To analyze this space charge propagation, we employed charge continuity equations for carriers such as electrons and positive and negative ions. These equations include ionization, attachment, and recombination effects, and Poisson's equation for electric fields. A secondary emission effect is included as a boundary condition on the cathode surface. Tip-plane electrodes were implemented in 2D axial symmetric model with a gap radius of 1 mm. To verify our numerical setup, the numerical results were compared with experimental data found in the previous literature.

Shape and dynamic behavior of nonmagnetic materials immersed in magnetic nanofluid was analyzed by using the level-set method for two-phase flow driven by the magnetic force density. To verify suitable magnetic force density, the total forces due to the surface and body force density methods were compared with experimental results obtained from the rising bubble setup with a sensing coil and cylindrical permanent magnet. After that, the shape of nonmagnetic material was investigated due to the different force densities.

Measurement of concentration fields in a micro-channel is the crucial technology in the area of Lab-on-a-chip to be used for various bio-chemical applications. It is well-known that the only possible way to measure the concentration field in the micro-channel is using micro-LIF(Laser Induced Fluorescence) method. However, an accurate concentration field at a given cross plane in a micro-channel has not been made so far due to the limit of light illumination. The present study demonstrates a novel method to provide an ultra thin laser sheet beam having 5 microns thickness by a micro focus laser line generator. Nile Blue A was used as fluorescent dye for LIF measurement. The laser sheet beam illuminates an exact plane of concentration measurement in the micro-channel to increase the signal to noise ratio and reduce the depth uncertainty considerably.Copyright © 2004 by ASME

The development of a 9 MeV compact cyclotron for the production of radioisotopes for medical applications has been recently completed. The machine accelerates negative hydrogen ions generated from an internal PIG (Penning Ion Gauge) ion source following spiral orbits. Some of the structures designed for early beam acceleration, including a pair of center poles providing ions a circular direction, the head of the ion source, and the electrodes, are located in the center of the cyclotron. In this paper we discuss and evaluate the design of the central region that pulls the ions from the chimney of the ion source and directs them into the equilibrium orbit. The magnetic field produced by the center poles was analyzed using the magnetic solver in OPERA-3D TOSCA, and the phase error and ion equilibrium orbit, which is dependent on the kinetic energy within the designed field, were calculated using CYCLONE v8.4. The electric field produced in the acceleration gap was designed using an electrostatic solver. Then, the single beam trajectory was calculated by our own Cyclotron Beam Dynamics (CBD) code. The early orbits, vertical oscillation, acceptable RF phase and the energy gain during the early turns was evaluated. Final goal was to design the central region by the iterative optimization process and verify it with 1 MeV beam experiment.