• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2000.11a
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• pp.93-96
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• 2000

In this study, we investigated the electromagnetic properties of Mn-Zn ferrite doped with rare earth oxide (Dy$_2$O$_3$, Er$_2$O$_3$). The main composition is 52mo1% ${\alpha}$-Fe$_2$O$_3$, 25mol% Mn$_3$O$_4$ 23mo1% ZnO and doped with them(0.05wt% ∼ 0.25wt%, step:0.05wt%). An experimental process has advanced by conventional ferrimagnetism manufacturing that was prepared by standard ceramic techniques. The XRD pattern of all doped sample were observed spinel and secondary phase. The density of sample were measured nearly constant value. As increased the additive, resistivity, initial permeability and real component of the series complex permeability increased with setting limits each other. In case of Mn-Zn ferrite excess doped with them, resistivity, initial permeability and real component of the series complex permeability decreased and magnetic loss increased in proportion to increasing the additive.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.472-473
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• 2006

Maghemite and hematite nanospheres were synthesized by using the Sol-gel technique. The structural properties of these nanosphere powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and pore size distribution. Hematite phase shows crystalline structures. The mean particle size that resulted from BET and XRD analyses were 4.9 nm and 2 nm. It can be seen from transmission electron microscopy that the size of the particles are very small which is in good agreement with the FESEM and the X-ray diffraction. The BET and pore size method were employed for specific surface area determination.

• Journal of Korea Foundry Society
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• v.17 no.5
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• pp.502-509
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• 1997

The mechanical behavior and microstructural evolution in the ignition-proof Mg-Zn-Ca-Zr alloy produced by the semisolid squeeze casting are clarified and the mechanical properties are also compared with those of squeeze cast Mg-Zn-Ca-Zr alloy. The tensile strength and elongation increase slightly as the solid fraction depending on temperature decreases, while the 0.2% proof stress decreases. The size of primary crystal increases with increasing holding time. The tensile strength and 0.2% proof stress of the semi-solid squeeze cast Mg-Zn-Ca-Zr alloy decrease as the size of primary crystal increases, indicating the dependence of strength on the size of primary crystal. The elongation of the semi-solid squeeze cast Mg-Zn-Ca-Zr alloy is two times as large as the squeeze cast Mg-Zn-Ca-Zr alloy and the tensile strength is unchanged despite the growth of primary crystal, resulting from the refining of the melted ${\alpha}Mg$ phase and the brittle eutectic compound as well as the reduction of solidification shrinkage and porosities.

• Nuclear Engineering and Technology
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• v.30 no.6
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• pp.609-616
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• 1998

Cast duplex stainless steels are extensively used for primary pressure boundary components. These components are, however, embrittled due to the precipitation of $\alpha$' phase by spinodal decomposition and other processes when exposed to reactor operating temperature for a design lifetime or life extension conditions. This report presents a procedure for estimating the current condition and the residual life of safety-related stainless steel components by using ANL database and correlations. The database of Charpy impact energy suggests that CF-8M grade is the most susceptible to thermal aging and CF-3 grade is the least. Thus, the integrity of CF-8M alleys may be degraded seriously and the degree of deterioration may exceed acceptance limit after several years of service in the nuclear reactors.

• Journal of the Korean Vacuum Society
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• v.8 no.3A
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• pp.187-193
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• 1999

In this study, PVDF thin films which show the excellent piezoelectricity and pyroelectricity, are prepared by PVD (physical vapor deposition) method, and thir electrical conduction phenomena for analyses of the electrical conduction mechanism and TSC (Thermally Stimulated Current) for identification of the behavior of conductive carriers are investigated. As a result of FT-IR(Fourier Transform Infrared Spectroscopy) spectra, the crystalline phase transforms $\alpha$ type into $\beta$ type with increasing electric field. From XRD (X-Ray diffraction) analyses patterns, the degree of crystallinity increases from 49.8% to 67%, as the substrate temperature increases from $30^{\circ}C$ to $80^{\circ}C$. As a result of electrical conduction phenomena, the electrical conduction mechanism of PVDF thin films is identified as ionic conduction mechanism. From TSC analyses, there are three peaks as P1, P2, P3 with increasing temperature, and with increasing substrate temperature, the peak temperature of TSC increases and the peak intensity of TSC decreases.

• Bulletin of the Korean Chemical Society
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• v.6 no.1
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• pp.40-45
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• 1985

The thermal and optical properties of two new series of thermotropic, liquid crystalline compounds were studied by differential scanning calorimetry and on the hot-stage of a polarizing microscope. The first series contained two identical mesogenic units, 4-(p-phenylbenzoyloxy) benzoate moieties, at both ends of the polymethylene spacer of varying lengths and the second series, mesogenic units with different substituents and the hexamethylene group as the central spacer. A thermodynamic analysis of the phase transitions of the compounds was made and the results were discussed in relation to their chemical structures.

• Bulletin of the Korean Chemical Society
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• v.4 no.3
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• pp.143-148
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• 1983

Two series of thermotropic compounds were prepared and their thermal and liquid crystal properties were examined by differential scanning calorimetry and on the hot-stage of a cross-polarizing microscope. The first series of the compounds has two terminal mesogenic units based on unsubstituted and substituted p-(phenoxycarbonyl) phenyl ethers bracketing a central decamethylene spacer, and the second has 4-(p-phenylphenoxycarbonyl) phenyl ether moiety as the two terminal mesogenic units and central polymethylene spacers of varying lengths. A thermodynamic analysis of the phase transitions was made and explained in relation to structures and thermotropic behavior of the compounds.

• Bulletin of the Korean Chemical Society
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• v.12 no.5
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• pp.537-540
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• 1991

Electrophilic methylation reactions of five-membered heteroaromatic compounds, furan, pyrrole and thiophene, with the dimethylfluoronium ion, ${CH_3}{FCH_3}(+), have been investigated theoretically by the MNDO method. The site selectivity of ${\alpha}, {\beta}$ and heteroatom (X) is related to charge density of the site, indicating that the site selectivity is dictated by electrosatic interaction between two reaction centers. The reactivity order between the three heteroaromatics can not be determined decisively since the order differs depending on which site is compared, with relatively low activation enthalpies, ${\Delta}{H^\neq}$= 20-30 kcal/mol, in all cases. These site and substrate selectivity behaviors are consistent with the gas-phase experimental results.

• Structural Engineering and Mechanics
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• v.71 no.3
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• pp.245-255
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• 2019

In this paper, an inverted-pendulum model consisting of a point supported by spring limbs with roller feet is adopted to simulate human walking load. To establish the kinematic motion of first and second single and double support phases, the Lagrangian variation method was used. Given a set of model parameters, desired walking speed and initial states, the Newmark-${\beta}$ method was used to solve the above kinematic motion for studying the effects of roller radius, stiffness, impact angle, walking speed, and step length on the ground reaction force, energy transfer, and height of center of mass transfer. The numerical simulation results show that the inverted-pendulum model for walking is conservative as there is no change in total energy and the duration time of double support phase is 50-70% of total time. Based on the numerical analysis, a dynamic load factor ${\alpha}_{wi}$ is proposed for the traditional walking load model.

• Journal of the Korean Society for Heat Treatment
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• v.32 no.5
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• pp.195-200
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• 2019

For expanding the applications and workability of TiAl alloy, elongation is very important property. Fine microstructure is needed for elongation and physical properties of TiAl alloys. In this study, The effects of cyclic heat treatment process for fine microstructure of Ti-46Al-Nb-W-Cr-Si-C alloy, which was made by VAR (vacuum arc remelting) and VIM(vacuum induction melting) centrifugal casting process, was investigated. Cycle heat treatment process was very effective for recrystallization of this TiAl system, which has microstructure size of $50{\sim}100{\mu}m$ through pre-heat treatment, cyclic heat treatment in ${\alpha}+{\gamma}$ phase region and solution heat treatment respectively. Refined grain size was finally confirmed by photos of optical microscope and scanning electron microscope.