Papers by Author: Chan Gyu Lee

Abstract: We demonstrate that a magnetic tunnel structure irradiated by microwaves can generate a significant voltage signal due to the rectification effect. The measurements were carried out using current-in-plane geometry with a current flowing parallel to the interfaces in the structure. A value of the microwave-induced voltage strongly depends on a bias current and can be driven by a magnetic field. The rectification effect is discussed both in classical terms of nonlinearity of the current-voltage characteristic and using a mechanism that involves the interplay between the spin-polarized current and magnetization dynamics in the magnetic tunnel structure.

Abstract: Ce1-xFexO2 (x=0, 0.01, 0.03 and 0.0 5) thin films were grown by pulsed laser deposition technique on Si and LaAlO3 (LAO) substrates. These films were deposited in vacuum and 200 mTorr oxygen partial pressure for both the substrates. These films were characterized by x-ray diffraction XRD and Raman spectroscopy measurements. XRD results reveal that these films are single phase. Raman results show F2g mode at ~466 cm-1 and defect peak at 489 cm-1 for film that deposited on LAO substrates, full width at half maximum (FWHM) is increasing with Fe doping for films deposited on both the substrates.

Abstract: 2021 series aluminum alloy is used as the matrix material for its wide application in engineering to make AlON coating layers by the electrolytic plasma processing (EPP) method. The experiments were carried out on 2021 Al alloys in alkaline electrolytes which are eco-friendly and low-cost. The experimental electrolyte composition includes: 2g/L NaOH as the electrolytic conductive agent, 6~14g/L Na3PO4 as alumina formative agent, 0.5g/L NaNO3 as a nitrogen inducing agent. The effects of phosphate content variation are evaluated by a combined composition and structure analysis of the coating layer using with Philips-X’Pert X-ray diffractometer, JSM 5610 scanning electron microscopy for the specimens EPP-treated at room temperature in 10 min under a hybrid voltage (260V DC + 200V AC-50Hz). In addition, microhardness of the ceramic coatings was measured to correlate the evolution of microstructure and resulting mechanical properties. The wear tests show that a composite of AlON-Al2O3 high anti-abrasive coating formed as a result of a reactive process between Al in the alloy itself and O-N supplied by the electrolyte.

Abstract: The composites of (1-x)La0.7Ca0.3MnO3/xCoO and (1-x)La0.7Sr0.3MnO3/xCoO (x =0 ~ 0.09) were fabricated by conventional solid state reaction method, and their electrical transport and magnetoresistance (MR%) properties were investigated by physical property measurement system (PPMS). The result of x-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicated that no new phase appeared in the composites except manganites matrix and CoO phases. CoO is mainly distributed at the grain boundaries and surfaces of the matrix. The resistivity LCMO and LSMO based composites were measured in the range 90-320K and 90-400K, respectively. The applied magnetic field is 0T and 0.5T. The observed variation of MR with varying Ca and Sr concentration has been qualitatively investigated.

Abstract: Composites with compositions La0.7Ca0.3MnO3(LCMO) and La0.7Sr0.3MnO3(LSMO)/SnO2 were prepared by a standard ceramic technique. The structure and morphology of the composites have been studied by the X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The XRD and SEM results indicate that no reaction occurs between La based maganite and SnO2 grains, and that SnO2 segregates mostly at the grain boundaries of La based maganite. The variation in resistivity with temperature has been studied and shows a semiconducting behavior, furthermore the composites exhibit metallic percolation. It is interesting to note that an enhanced magnetoresisitance (MR) effect for the composites is found over a wide temperature range from low temperature to room temperature in an applied magnetic field of 0.5 Tesla. The spin-polarized tunneling and the spin-polarized tunneling may be attributed to the enhanced low-field magnetoresistance (LFMR) effect.

Abstract: In Zr-Cu-Ni-Al bulk metallic glasses where there are no dislocations, localized plastic deformation in shear bands occurs largely by the formation and migration of defects such as voids, micropores, shear bands and local variations in composition. Thus, the investigation on defects is critical for the understanding and improvement of plastic deformation in metallic glasses. In this study, microstructures and nano defects in the Zr-Cu-Ni-Al BMGs are characterized by variety of techniques, such as X-ray diffractometry, high resolution transmission electron microscopy, scanning transmission electron microscopy and electron holography.

Abstract: Zr62Cu17Ni13Al8 in the supercooled liquid state is expected to be micro-formable at a relatively low stress. We used X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and quantitative high-resolution TEM (HRTEM) to investigate the microstructures of Zr62Cu17Ni13Al8 amorphous alloy after compression test. The alloy exhibited the homogeneous amorphous microstructure with some crystalline phases dispersed in the matrix. According to the XRD results, under the certain strain rate in the supercooled liquid state, the alloy showed higher crystallization at the higher heat treatment temperature. However, at the same heat treatment temperature, the alloy deformed under low strain rate showed higher crystallization. The β crystalline phase particles with spherical shape were detected by SEM and TEM. The sample with higher strain rate and temperature showed longer shear bands. Nano-voids formed by the coalescence of excess free volume in shear bands were investigated by quantitative HRTEM. Compared with the undeformed area, in the shear band, nanovoids were identified in the deformed area through quantitative HRTEM simulation.

Abstract: The influence of nitriding type and condition on the thermal crack propagation behavior of hot work die steel was investigated. Thermal fatigue tests were carried out using a special apparatus, which is consisted of induction heating and water spray cooling unit. The sum of crack length per unit specimen length, Lm is proposed as an index representing the susceptibility to crack propagation. The Lm values of the gas and ion nitrided specimens were lower than that of as-heat treated specimen. But in the case of maximum and average crack length of nitrided specimen was higher than those of as-heat treated specimen. The nitrogen diffusion layer still remained although the oxide scale was formed and fell off during thermal fatigue test. After 1000 cycles of the thermal fatigue test, the interior region of nitrided specimen showed lower hardness by softening. In particular, ion nitrided specimens of relatively less softening condition showed reduction both the depth and number of cracks compared with the other surface conditions.

Abstract: We indicated synthesization of LCMO by hydrothermal reaction. The results of transmission electron microscopy revealed that the LCMO particles had wide range in size and various in shapes. The LCMO particles had a perovskite-type crystal structure with some other phases. Magnetic property was measured by physical property measurement system. Their crystallinity and magnetization tended to increased with increasing reaction time.

Abstract: It is well known that the main failure mechanisms in die-casting mould are heat checking due to thermal fatigue and melt-out caused by chemical reaction between die and molten alloys. Thermal fatigue tests were carried out using the thermal cycle simulator to establish the proper method to estimate the thermal fatigue resistance of hot die steel. In this study, the thermal shock tester consisted of induction heating and water spray cooling unit was constructed to evaluate thermal crack propagation resistance and the sum of crack length per unit specimen length, Lm is proposed as the index representing the susceptibility to crack initiation and propagation. Also, new concept of measurement for the melt-out behavior was suggested. AISI H13 hot work tool steel was solution treated and tempered at various temperatures, to control the hardness and toughness that have effect on the behavior of thermal crack propagation. The result of thermal fatigue test showed that there is optimum value of hardness and impact energy to maximize the thermal crack propagation resistance. The influence of nitriding on melt-out resistance was also investigated. The dissolution rate due to melt-out phenomenon tended to be smaller for thicker compound layer. Furthermore, the resistance to melt-out was affected by the compound layer thickness rather than that of diffusion layer. The results of the both tests properly reflect the effect of materials properties on failure modes of die-casting mould and it means those test methods are suitable to evaluate the durability of hot work tool steel for die-casting.