• Journal of the Korean Magnetics Society
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• v.27 no.1
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• pp.35-40
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• 2017

Kerr microscopy was assembled to observe magnetic domain image of ultra thin 3 %Si-Fe by using parts of an optical microscope. Digital images were obtained from CCD camera attached to the microscopy. A method was suggested to decide a boundary between magnetic domain regions in this study. The method was using some operations such as subtraction, integration and least mean square approximation for pixel values in the digital image. The method has a strong point that high priced image processor is not needed in the Kerr microscopy system. From the results that three different domain walls were observed and magnetic flux density of 0.085 [T], this method could be applied in the magnetic domain regions having a straight $180^{\circ}$ domain wall.

• Journal of the Korean Society for Precision Engineering
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• v.30 no.7
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• pp.733-740
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• 2013

The high-velocity electromagnetic forming (EMF) process is based on the Lorentz force and the energy of the magnetic field. The advantages of EMF include improved formability, wrinkle reduction, and non-contact forming. In this study, numerical simulations were conducted to determine the practical parameters for the EMF process. A 2-D axis-symmetric electromagnetic model was used, based on a spiral-type forming coil. In the numerical simulation, an RLC circuit was coupled to the spiral coil to measure various design parameters, such as the system input current and the electromagnetic force. The simulation results show that even though the input peak current levels were at the same level in each case, the forming condition varied due to differences in the frequency of the input current. Thus, the electromagnetic forming force was affected by the input current frequency, which in turn, determined the magnitude of the current density and the magnetic flux density.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.06a
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• pp.375-375
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• 2009

For many large-scale applications of high-temperature superconducting materials, large critical current density($J_c$) in high applied magnetic fields are required. A number of methods have been reported to introduce artificial pinning centers(APCs) in $YBa_2Cu_3O_{7-\delta}$(YBCO) films for enhancement of their $J_c$. We report measurements of critical current in $YBa_2Cu_3O_{7-\delta}$ films deposited by PLD on $SrTiO_3$ substrates decorated with Au nanoparticles. Au nanoparticles were synthesized on STO substrates with self assembled monolayer. Microstructural analysis of the obtained YBCO films was performed by using cross-section transmission electron microscopy(TEM). Phase and textural analysis was done using X-ray diffraction. The surface morphology and surface roughness(Ra) of the layers was measured by atomic force microscopy(AFM).

• Bulletin of the Korean Space Science Society
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• 2008.10a
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• pp.23.4-23.4
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• 2008

In March and April 2001, the apogee (~9 Re) of the Polar spacecraft was located near the subsolar magnetopause with its orbital plane nearly parallel to a magnetic meridian plane. Polar electric and magnetic field data acquired during the two-month interval of solar maximum have been used to study fundamental standing Alfven waves near the subsolar meridian plane (magnetic local time = 1000-1400 hours) at magnetic latitudes from the equator to $\pm45$ degrees and at L values between 7 and 12. In the frequency band from 1.5 to 10 mHz, fundamental mode oscillations were identified based on high coherence (more than 0.7) and an approximately 90-degree phase shift between the azimuthal magnetic and radial electric field components. The L dependence of the fundamental frequencies is studied, and the frequencies are compared with those observed near the solar minimum interval (Takahashi et al. 2001). We found that the average frequencies in solar maximum are lower than those in solar minimum by a factor of ~2. This implies that the mass density in solar maximum is higher than that in solar minimum by a factor of ~4. Since there is a positive correlation between solar irradiance and solar activity, we suggest that the ionosphere in solar maximum produces more ions and load magnetic flux tubes with more ions.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.11
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• pp.2732-2741
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• 2000

Combination of magnetic recording technology and optical recording technology such as Near Field Optical Recording is watched recently. In order to accomplish this technology, the development of an electromagnetic driven mm-sized mirror shifting laser beam in track direction have to needed. In Near Field Optical Recording System, shifting laser beam in track direction mean as fine tracking and means as coarse tracking. Therefore in Near Field Optical Recording, 2-stage actuator is composed of servo controller in reading or recording information on disc layer. In our research, through design and simulation process of driven mm-sized mirror, we arrange systematically design process of driven mm-sized mirror having good frequency transfer characteristics. Design and simulation processes included modal analysis of spring, calculation of magnetic moment according to the number of turns and geometric configuration of coil and magnetic circuit analysis meaning that calculation of magnetic flux density in air gap of magnetic circuit. After that we design and make parts of driven mm-sized mirror, assemble and evaluate our electriomagnetic driven mm-sized mirror. we compared design values with actual characteristic values and present solution scheme. Through these processes we performed manufacturing of an electromagnetic driven mm-sized mirror having good frequency-domain characteristics and high sensitivity characteristics.

• Design & Manufacturing
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• v.11 no.2
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• pp.20-24
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• 2017

Recent industrial developments are constantly advancing, and rapid technological development is demanding high technology level in related fields. The need for polishing is increasing even more to improve quality. In order to improve the surface quality, the final finishing process or polishing process is a very important part. Research on super precise polishing method using MR fluid is actively being carried out in domestic and foreign countries. Fine magnetic abrasive grains are aligned in the direction of a magnetic force line formed by a magnetic field and serve as a brush to polish a metal surface. This method has the advantage that the shape of the tool is not fixed and is not affected by the shape of the workpiece or the machining area. We will design the electromagnets for the MR polish polishing system and apply the magnetic field analysis using the magnetic field analysis program (ANSYS). The data obtained through this process suggests an efficient method to increase the magnetic flux density important for polishing. We will investigate the influence of the Al6061-T6 specimen on the surface of the MR polishing machine based on the optimized design.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2003.07b
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• pp.1168-1171
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• 2003

In contrast to a conventional transformer, the flat transformer is made using a number of small ferrite cores. Two cores for transformer and one core for inductor, which composed one module. Many modules can be connected together to form a flat matrix transformer. This structural arrangement eliminates the single hot spot problem in conventional transformers and permits high current density pertains at high frequency. In this study, the ferrite magnetic cores of Mn-Zn system for the Flat transformer were manufactured and the electrical and magnetic characteristics of its tested. The power loss of sample FO2(Mn-Zn ferrite) sintered at $1350^{\circ}C$ was $350kW/m^3$ in test conditions of 250kHz, 200mT and $100^{\circ}C$, which showed the good power loss property in high frequency. The power loss of FO2 samples has been studied as a function of magnetic flux density and frequency. Steinmetz exponent was 2.82 at 250kHz and 2.73 at 500kHz. These results illustrated the switching of power loss mechanism in ferrite core from hysteresis losses to eddy current losses or others.

• Journal of the Korean Magnetics Society
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• v.12 no.4
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• pp.154-159
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• 2002

Amorphous FeBSiNb alloy ribbons having bulk glass forming ability and high saturation magnetic flux density were produced by single-roller melt spinning apparatus in the thickness range of 22∼102㎛. With the increase of thickness, the coercive force and squareness ratio decreased, while maximum permeability and AC permeability increased. However electrical resistivity was almost constant. Furthermore refined and complex magnetic domain structure was observed in thicker ribbons owing to the change in internal magnetic anisotropy. For the alloy with the thickness of 81㎛, small coercive force of 24 mOe and high effective permeability of 12,000 at 1㎑ were obtained, those are considered to be better comparing to the conventional soft magnetic amorphous alloys (∼20 ㎛). The good soft magnetic properties of the thick FeBSiNb amorphous alloys were attributed to the decrease in surface pinning effect during wall motion, appearance of perpendicular anisotropy and resulted domain refinement.

• Korean Journal of Materials Research
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• v.22 no.11
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• pp.569-574
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• 2012

Large single grain $Gd_{1.5}Ba_2Cu_3O_{7-y}$ (Gd1.5) bulk superconductors were fabricated by a top-seeded melt growth (TSMG) process using an $NdBa_2Cu_3O_{7-y}$ seed. The seeded Gd1.5 powder compacts with a diameter of 50 mm were subjected to the heating cycles of a TSMG process. After the TSMG process, the diameter of the single grain Gd1.5 compact was reduced to 43 mm owing to the volume contraction during the heat treatment. The superconducting transition temperature ($T_c$) of the top surface of the single grain Gd1.5 sample was as high as 93.5 K. The critical current densities ($J_cs$) at 77 K and 1T and 1.5 T were in ranges of 25,200-43,900 $A/cm^2$ and 10,000-23,000 $A/cm^2$, respectively. The maximum attractive force at 77 K of the sample field-cooled using an Nd-B-Fe permanent magnet (surface magnetic field of 0. 527 T) was 108.3 N; the maximum repulsive force of the zero field-cooled sample was 262 N. The magnetic flux density of the sample field-cooled at 77 K was 0.311T, which is approximately 85% of the applied magnetic field of 0.375 T. Microstructure investigation showed that many $Gd_2BaCuO_5$ (Gd211) particles of a few ${\mu}m$ in size, which are flux pinning sites of Gd123, were trapped within the $GdBa_2Cu_3O_{7-y}$ (Gd123) grain; unreacted $Ba_3Cu_5O_8$ liquid and Gd211 particles were present near the edge regions of the single grain Gd1.5 bulk compact.

• Progress in Superconductivity and Cryogenics
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• v.25 no.4
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• pp.19-23
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• 2023

The Second-generation high-temperature superconducting (HTS) Rare-Earth Barium Copper Oxide (REBCO) wire is a composite laminate having a multi-layer structure (8 or more layers). HTS wires will undergo multiple loads including the bending-tension loads during winding, high current density, and high magnetic fields. In particular, the wires are subjected to bending stress and magnetic field stress because HTS wires are wound around a circular bobbin when making a high-field magnetic. Each of the different laminated wires inevitably exhibits damage and fracture behavior of wire due to stress deformation, mismatches in thermal, physical, electrical, and magnetic properties. Therefore, when manufacturing high-field magnets and other applications, it is necessary to calculate the stress-strain experienced by high-temperature superconducting wire to present stable operating conditions in the product's use environment. In this study, the finite element model (FEM) was used to simulate the strain-stress characteristics of the HTS wire under high current density and magnetic field, and bending loads. In addition, the result of obtaining the neutral axis of the wire and the simulation result was compared with the theoretical calculation value and reviewed. As a result of the simulation using COMSOL Multiphysics, when a current of 100 A was applied to the wire, the current value showed the difference of 10^-9. The stress received by the wire was 501.9 MPa, which showed a theoretically calculated value of 500 MPa and difference of 0.38% between simulation and theoretical method. In addition, the displacement resulted is 30.0012 ㎛, which is very similar to the theoretically calculated value of 30 ㎛. Later, the amount of bending stress by the circular mandrel was received for each layer and the difference with the theoretically obtained the neutral axis result was compared and reviewed. This result will be used as basic data for manufacturing high-field magnets because it can be expanded and analyzed even in the case of wire with magnetic flux pinning.