Papers by Author: Chong Oh Kim

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Authors: Jong Gwan Ahn, Dong Jin Kim, Y.N. Jang, Chong Oh Kim, Hun Saeng Chung, Hoang Tri Hai
Abstract: In order to produce monodispersed particles, two requirements are considered; they are a complete separation of nucleation and growth steps for the formation of particles and an avoidance of their aggregation during the nucleation and growth. In this study, a new method was presented for preparing narrowly size distributed copper powders from CuO slurry by wet chemical reduction with hydrazine used as a reductant. The rapid nucleation was first induced by a very fast reduction of Pd2+ resulted from batch process. Subsequently copper species which contribute to the growth on the formed Pdo nuclei was slowly generated by semibatch process. A complete separation between nucleation and growth therefore was obtained, resulting in a narrow size distribution of final Cu powder product. The effect parameters on the growth such as Pd2+ concentration and temperature were also investigated.
Authors: L.V. Tho, K.E. Lee, Cheol Gi Kim, Chong Oh Kim, W.S. Cho
Abstract: Alloys of CoFe-rich magnetic films are well known as typical soft magnetic alloys. They are used for many kinds of electric and electronic parts such as magnetic recording heads, transformers and inductors. In order to get superior soft magnetic properties of the CoFe-based nanocrystalline thin films, the effect of O2 partial pressure on magnetic properties of Co-Fe-Hf-O nanocrystalline thin films have been investigated. It is found that the soft magnetic properties and electrical property of these films show a dependence on the partial pressure of reactive gases, which presumably changes the microstructure of the films and related magnetic anisotropy. With optimal conditions, thin film exhibit excellent soft magnetic properties: saturation magnetization (4πMs) of 21 kG, magnetic coercivity (Hc) of 0.18 Oe, anisotropy field (Hk) of 49 Oe, and an electrical property is also shown to be as high as 300 μcm. The combination of high 4πMs and relatively high Hk in these films are believed to be partly responsible for the excellent ultra-high-frequency behavior
Authors: L.T. Tu, N.V. Vuong, N.T. Hieu, Cheol Gi Kim, Chong Oh Kim
Abstract: The squareness factor γ used to evaluates for criterion allowing quick estimation of the quality of NdFeB melt-spun powders on Stoner–Wohlfarth model. For the powder compaction of 6.4 g/cm3 mass density, the measured value γ=0.48 serves the evidence of the Stoner – Wohlfarth behaviour of the powder grains and the preparation conditions are optimal for producing Stoner – Wohlfarth particles. For the powder compaction of given mass densities the calibration curve of the squareness factor γ is presented. We can observe one more thing that for the ideal, full dense compaction of NdFeB Stoner – Wohlfarth particle the squareness factor γ can not be large than 0.53.
Authors: Anh Tual Le, Chong Oh Kim, Hee Bok Lee
Abstract: A novel excellent response of giant magnetoimpedance effect was found out in a magnetic LC-resonator consisting of a glass-coated amorphous Co83.2B3.3Si5.9Mn7.6 microwire and two capacitive cylindrical electrodes at the end of the microwire. The shapes of the impedance curves plotted vs. applied external dc-field varied dramatically with increasing frequency. The phase angle was also strongly found to be dependent on this field. The impedance curves were changing abruptly at near the resonance frequency. Because the permeability of ultra soft magnetic microwire is changing rapidly as a function of external magnetic field, the resonance frequency as well as impedance of the LC-resonator also changes drastically with respect to the external magnetic field. The maximum magnetoimpedance ratio value was reached as much as 1,600%. The sudden changes of phase angle as much as 180 degree evidenced the occurrence of resonance phenomenon. These results are promising for developing ultra-high sensitive magnetic sensor applications.
Authors: Pratap Kollu, Seok Soo Yoon, Chong Oh Kim, Cheol Gi Kim
Abstract: A highly sensitive magnetic sensor using the GMI (Giant Magneto Impedance) effect has been developed. The sensor performance is studied and estimated. The sensor circuitry consists of a Square wave generator (driving source), a sensing element in a form of composite wire of a 20μm copper core electrodeposited with a thin layer of soft magnetic material (Ni80Fe20), two amplifier stages for improving the gain from the sensor element, switching mechanism, scaler circuit, an ac power source driving the permeability of the magnetic coating layer of the sensing element into a dynamic state, and a signal pickup LC circuit formed by a pickup coil and an capacitor .The results showed that for high sensitivity and resolution (10-7 T), the frequency and magnitude of the ac driving current through the sensing element each has an optimum value, the resonance frequency of the signal pickup LC circuit should be equal to or twice as the driving frequency on the sensing element, and the anisotropy of the magnetic coating layer of the sensing wire element should be longitudinal.
Authors: Li Jin, Y.W. Rheem, Cheol Gi Kim, De Sheng Sun, Chong Oh Kim, Seong Cho Yu
Abstract: The magnetic properties were investigated on Co66Fe4B15Si15 amorphous ribbons of annealed at temperature range from 473 K to 773 K in vacuum (A-batch) and open air (B-batch). Static permeability from domain wall motion decreases with annealing temperature both in A- and B-batch sample. The permeability from magnetization rotation increases up to 700 K annealing temperature, but then decreases. The GMI profiles show a symmetric curve in A-batch samples irrespective of annealing temperature. In B-batch samples, symmetric profiles are shown up to 650 K annealing temperature, while they becomes asymmetric for further annealing temperature, due to the exchange bias-field induced by surface crystalline layer.
Authors: Hoang Tri Hai, Dong Jin Kim, Y.D. Kim, Chong Oh Kim, Hun Saeng Chung, Jong Gwan Ahn
Abstract: Ultrafine dispersed copper powders were prepared from copper (I) oxide (Cu2O) slurry by a wet chemical reduction with hydrazine as a reductant. Palladium chloride (PdCl2) was used as a nucleator along with its polymer stabilizer, polyvinylpyrrolidone (PVP). Sodium pyrophosphate (Na4O7P2) was added as a surfactant. It was found that the use of Cu2O and Na4O7P2 exhibit very strong impact on dispersity of the prepared Cu powders. In this study, we indicate the critical conditions at which the particles aggregation still occurs even though the powders produced at high zeta potential resulted from an appropriate addition of Na4O7P2.
Authors: Seong Min Hong, Cheol Gi Kim, Chong Oh Kim
Abstract: The magnetic and structural properties of FINEMET [Fe73.5Si13.5B9Nb3Cu1 wt %] amorphous powder were investigated after nitrification and mechanical milling. Fe-based amorphous powder were nitrified and crystallized simultaneously at 550°Cusing by ammonia(NH3) gas. Nitrified powder exhibited iron nitride phase such as γ′-Fe4N, Fe3N and α″-Fe16N2. Nitrified particles were more brittle than raw particles. As a result, nanometer sized nitride powder were fabricated by high energetic ball milling method. The saturation magnetization(Ms) and coercivity(Hc) of nitrified powder were increased due to nitride phase.
Authors: Hee Bok Lee, Wan Shik Cho, Taek Soo Kim, Kyeong Jae Lee, Chong Oh Kim
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