Jong-Chul Lee

Lithography-free centimeter-long nanochannel fabrication method using an electrospun nanofiber array

Novel cost-effective methods for polymeric and metallic nanochannel fabrication have been demonstrated using an electrospun nanofiber array. Like other electrospun nanofiber-based nanofabrication methods, our system also showed high throughput as well as cost-effective performances. Unlike other systems, however, our fabrication scheme provides a pseudo-parallel nanofiber array a few centimeters long at a speed of several tens of fibers per second based on our unique inclined-gap fiber collecting system. Pseudo-parallel nanofiber arrays were used either directly for the PDMS molding process or for the metal lift-off process followed by the SiO2xa0deposition process to produce the nanochannel array. While the PDMS molding process was a simple fabrication based on one-step casting, the metal lift-off process followed by SiO2xa0deposition allowed finetuning on height and width of nanogrooves down to subhundred nanometers from a few micrometers. Nanogrooves were covered either with cover glass or with PDMS slab and nanochannel connectivity was investigated with a fluorescent dye. Also, nanochannel arrays were used to investigate mobility and conformations of λ-DNA.

Power Generation Using Magnetic Nanofluids in Millimeter-Sized Channel With In-Phase Mode of Magnetization

Magnetic nanofluids (MNFs) are an interesting energy harvesting source. In this paper, the flow energy harvesting was experimentally and numerically investigated in a millimeter-sized channel using an externally applied permanent magnet to control the magnetizing direction of the magnetic nanoparticles (MNPs). Oil- or water-based MNF includes a certain percentage of magnetized nanoparticles and has unique features that vary with the strength of the external electromagnetic field. When the MNF flows through a cross-sectional area of the coil loop, the electromotive force can be obtained by following Faradays law, because the MNPs act as permanent magnets. When the MNFs are used for flow energy harvesting, the main issue is the in-phase mode alignment of the MNPs magnetization with the coil loop. Without the in-phase mode, the electric power cannot be generated, because the net magnetization of the MNF is zero. Most of the previous research works, however, have not considered it. Thus, to implement this mode, we proposed an externally applied magnetic field generated by a cylindrically shaped permanent magnet. Short and closed Teflon tubing with a 1.5 mm inner diameter, containing the MNF, was located inside long silicon tubing and moved along the positive and negative directions by a pump. Then, the generated voltages were measured, and exhibited similar results to those obtained analytically. In the same way, we calculated and experimentally tested a chain type of Teflon tubing.

Numerical Simulation on the Induced Voltage Across the Coil Terminal by the Segmented Flow of Ferrofluid and Air-Layer

Nanoparticles and nanofluids have been implemented in energy harvesting devices, and energy harvesting based on magnetic nanofluid flow was recently achieved by using a layer-built magnet and microbubble injection to induce a voltage on the order of 10-1 mV. However, this is not yet suitable for some commercial purpose. The air bubbles must be segmented in the base fluid, and the magnetic flux of the ferrofluids should change over time to increase the amount of electric voltage and current from energy harvesting. In this study, we proposed a novel technique to achieve segmented flow of the ferrofluids and the air layers. This segmented ferrofluid flow linear generator can increase the magnitude of the induced voltage from the energy harvesting system. In our experiments, a ferrofluid-filled capsule produced time-dependent changes in the magnetic flux through a multi-turn coil, and the induced voltage was generated on the order of about 101 mV at a low frequency of 2 Hz. A finite element analysis was used to describe the time-dependent change of the magnetic flux through the coil according to the motion of the segmented flow of the ferrofluid and the air-layer, and the induced voltage was generated to the order of 102 mV at a high frequency of 12.5 Hz.

Feasibility Study on a Segmented Ferrofluid Flow Linear Generator for Increasing the Time-Varying Magnetic Flux

Nanoparticles and nanofluids have been implemented in energy harvesting devices, and energy harvesting based on magnetic nanofluid flow was recently achieved by using a layer-built magnet and micro-bubble injection to induce a voltage on the order of 10-1 mV. However, this is not yet suitable for some commercial purpose. In order to further increase the amount of electric voltage and current from this energy harvesting the air bubbles must be segmented in the base fluid, and the magnetic flux of the segmented flow should be materially altered over time. The focus of this research is on the development of a segmented ferrofluid flow linear generator that would scavenge electrical power from waste heat. Experiments were conducted to obtain the induced voltage, which was generated by moving a ferrofluid-filled capsule inside a multi-turn coil. Computations were then performed to explain the fundamental physical basis of the motion of the segmented flow of the ferrofluids and the air-layers.

Interaction Model Between an Electrical Arc and a Material for Arc Discharge Synthesis of Metal Nanoparticles

Metal nanoparticles are used in applications ranging from bio-diagnostics to catalysis due to the expectation to improve attributes or the performance of specific products or processes. The electric arc can be used to produce metal nanoparticles by evaporating the anode and forming the anode vapor. In order to synthesize the nanoparticles of the desired properties, the influence of various input parameters on the growth kinetics has to be fully understood. In this study, we presented two and three dimensional results of numerical simulation of the transferred electric arc taking into account the interaction model between an electric arc and two electrodes. It was found that the predicted temperature of the arc column with two electrodes was in good agreement with the measured data, and the main advantage of this model over our previous one was to predict the temperature distribution of the arc column with two electrodes by two- and three-dimensional computations.

Numerical Study on Alternating Current Breakdown Mechanism Between Sphere–Sphere Electrodes in Transformer Oil-Based Magnetic Nanofluids

A numerical simulation was developed for magnetic nanoparticles in a liquid dielectric to investigate the AC breakdown voltage of the magnetic nanofluids according to the volume concentration of the magnetic nanoparticles. In prior research, we found that the dielectric breakdown voltage of the transformer oil-based magnetic nanofluids was positively or negatively affected according to the amount of magnetic nanoparticles under a testing condition of dielectric fluids, and the trajectory of the magnetic nanoparticles in a fabricated chip was visualized to verify the related phenomena via measurements and computations. In this study, a numerical simulation of magnetic nanoparticles in an insulating fluid was developed to model particle tracing for AC breakdown mechanisms happened to a sphere-sphere electrode configuration and to propose a possible mechanism regarding the change in the breakdown strength due to the behavior of the magnetic nanoparticles with different applied voltages.

Two-Dimensional Computations on the Thermal Behaviors Between Arc Plasmas and Their Electrodes for the Switching Chamber Design

A SF6 self-blast switching chamber belongs to a new generation of high-voltage switching devices, which take advantage of the auto-expansion principle and arc rotation to improve the switching performance on thermal and dielectric interruptions. The thermal behaviors between the arc plasma and the electrodes in the device are very complex to understand only through experimental studies. Since the late nineteen-eighties, significant progress has been made in computational methods describing the physical processes occurring in thermal plasmas. The final goal of a computer simulation on thermal plasmas is to predict the switching performance on thermal and dielectric interruptions from an engineering point of view. In this paper, we have conducted computations to predict the thermal and dielectric breakdown capabilities of a SF6 self-blast switching chamber from the results of the thermal behaviors during the entire switching process, such as a high-current period, pre-current zero period, and current-zero period. Through the complete work, the temperature of the residual thermal plasmas as well as the breakdown index after the current zero should be good criteria to predict the thermal and dielectric capabilities of the switching chambers.

Study on temperature-rise of a 72.5kV vacuum circuit breaker for the higher rated current

In the development of ultra high-voltage (UHV) vacuum circuit breakers (VCB), higher current-rating and improved thermal performance become more and more important to provide safe function and reliability in spite of scale-down and increasing performance demanded of modern devices. Due to the high complexity of heat generation and loss processes, it is not easy to predict temperature-rise of the breaker under various load conditions. In order to prevent the abnormal temperature-rise inside of the breaker the designer should pay attention to heat transfer mechanism of the breaker. Related international standards including IEC and ANSI limit that the temperature-rise of the various parts of the switchgear should not exceed its specified values. Therefore, the thermal analysis on vacuum breakers should be carried out. In this study, systematic approaches have been undertaken using combination of experiments, numerical analysis, and thermal network finite difference analysis technique to understand the heat transfer mechanism of a 72.5kV vacuum circuit breaker with various design alternatives. It was found that the temperature-rise of the vacuum circuit breaker through numerical analysis and the error was less than 10% with both two methods, CFD and TNA.

Magnetic field and thermal characteristics simulation of axial magnetic field contacts in a vacuum interrupter

In this study, we considered two different types of axial magnetic field (AMF) electrodes, the coil type and the cup type, which are used widely in industries, to investigate the effective arc area and the magnetohydrodynamic characteristics by high-current vacuum arcs in a vacuum interrupter (VI) through three-dimensional FEM analysis. In order to validate the present calculation, the calculated results were compared with the results obtained using a commercial package, MAXWELL 3D, which is a reliable analysis tool for electro-magnetic fields, to validate the present calculation. From the results of two different types of AMF electrodes, the maximum axial magnetic flux (Bz) was locally observed with the respective number of slots on the contact surfaces. Results also showed that the cup type electrode led to a more uniform AMF distribution and Joule heat concentration at the contact than the coil type electrode.