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Now showing items 33 - 48 of 85

  • Thermodynamic assessment of the NaH <-> Na(3)AlH(6)<-> NaAlH(4) hydride system

    Lee, Byeong-Moon   Jang, Je-Wook   Shim, Jae-Hyeok   Cho, Young Whan   Lee, Byeong-Joo  

    Thermodynamic properties of hydrides, NaH, Na(3)AlH(6) and NaAlH(4) which can be a promising candidate for a hydrogen storage material system, were critically assessed by analyzing available piecewise literature information on thermodynamic properties using the CALPHAD method. Through the assessment, a critical evaluation on the reliability of individual literature data on thermodynamic properties of the NaH-Na(3)AlH(6)-NaAlH(4) hydride system could be made, and a self-consistent thermodynamic description could be obtained. The finally assessed thermodynamic description can reproduce various thermodynamic properties of individual hydrides and phase equilibria among hydrides in reasonable agreement with relevant experimental and high-level calculation data, and provide a reference data that can be used for comparison with high-level calculations and for further thermodynamic assessments of higher order systems. (c) 2006 Elsevier B.V. All rights reserved.
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  • Prediction of elastic properties of precipitation-hardened aluminum cast alloys RID C-3834-2012

    Suh, Jin-Yoo   Lee, Young-Su   Shim, Jae-Hyeok   Park, Hoon Mo  

    A methodology is proposed for predicting the elastic properties of precipitation-hardened alloys by combining different modeling techniques: the CALPHAD method, first-principles calculation, and elasticity models. The proposed procedure was applied to conventional aluminum cast alloys to predict their elastic moduli. The predicted Young's moduli are in reasonable agreement with values reported in the literature, which verifies the potential applicability of the methodology to the development of high-stiffness aluminum cast alloys. (C) 2011 Elsevier B.V. All rights reserved.
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  • Synthesis and liquid phase sintering of TiN/TiB2/Fe-Cr-Ni nanocomposite powder

    Kim, Ji Woo   Chung, Hee-Suk   Shim, Jae-Hyeok   Ahn, Jae-Pyoung   Cho, Young Whan   Oh, Kyu Hwan  

    Nanostructured TiN/TiB2/Fe-Cr-Ni composite powder has been prepared via high energy ball milling. Ti and BN powders are reacted to form a uniform mixture of TiN and TiB2 within 2 h of milling by a mechanically activated self-sustaining reaction (MSR). The crystallite sizes of TiN and TiB2 are about 7 and 16 nm, respectively, after 32 h of milling. Through liquid phase sintering and optimizing the milling time, a compact body with almost full density could be obtained which has a narrow grain size distribution and exceptionally high hardness. (c) 2005 Elsevier B.V. All rights reserved.
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  • Reaction products between TiCl(3) catalyst and Li(3)AlH(6) during mechanical mixing

    Shim, Jae-Hyeok   Lee, Gil-Jae   Cho, Young Whan  

    The reaction between TiCl(3) and LbAlH(6) was investigated to understand reaction products in Ti-catalyzed Li(3)AlH(6) using mechanical milling and thermodynamic calculation. As reaction products, LiCl, TiAl(3) and TiH(2) are observed by X-ray powder diffraction, which is in agreement with the results of thermodynamic calculation. TiAl(3) forms in an L1(2) structure rather than an equilibrium D0(22) structure during milling. It is predicted that the mole fraction of TiAl(3) increases and that of TiH2 decreases as temperature increases during milling. Differential scanning calorimetry confirms that TiAl(3) acts as a dehydrogenation catalyst in Li(3)AlH(6). (c) 2005 Elsevier B.V. All rights reserved.
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  • Microstructural evolution of NbF(5)-doped MgH(2) exhibiting fast hydrogen sorption kinetics

    Kim, Ji Woo   Ahn, Jae-Pyoung   Jin, Seon-Ah   Lee, Sang Hoon   Chung, Hee-Suk   Shim, Jae-Hyeok   Cho, Young Whan   Oh, Kyu Hwan  

    The microstructure of MgH(2) with 1 mol% NbF(5), Prepared by high-energy ball milling (HEBM), was studied using high resolution transmission electron microscopy (HR-TEM) with an X-ray energy dispersive spectrometer (EDS) before and after hydrogen sorption cycles. The TEM samples were prepared without any air exposure by a novel, focused ion beam (FIB) system specially designed for highly air sensitive materials. During HEBM, the doping agent, NbF5, was distributed as an extremely thin, film-like, amorphous phase along the grain boundaries of the nanocrystalline MgH2. After 10 sorption cycles, amorphous Nb-F phase was transformed into crystalline Nb hydrides. It is believed that the Nb hydride played a decisive role in improving the sorption kinetics of MgH(2). (C) 2007 Elsevier B.V. All rights reserved.
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  • Screw dislocation assisted martensitic transformation of a bcc Cu precipitate in bcc Fe

    Shim, Jae-Hyeok   Cho, Young Whan   Kwon, Sang Chul   Kim, Whung Whoe   Wirth, Brian D.  

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  • Atomistic modeling of nanosized Cr precipitate contribution to hardening in an Fe-Cr alloy

    Shim, Jae-Hyeok   Kim, Dong-Ik   Jung, Woo-Sang   Cho, Young Whan   Wirth, Brian D.  

    Molecular dynamics simulations of the interaction between an edge dislocation and nanosized Cr precipitates in bcc Fe have been performed to investigate the hardening effect of alpha' phases in high Cr ferritic/martensitic steels. The critical resolved shear stress needed for an edge dislocation to overcome Cr precipitates of diameter between 3 and 6 nm is larger than for dislocation glide in the bcc Fe lattice containing 10% Cr solute atoms. This indicates that the precipitation of W phases leads to hardening in high Cr ferritic/martensitic steels. The MD simulations reveal that the interspacing of Cr precipitates plays a more crucial role in the hardening of Fe-Cr alloys than the precipitate size. An attractive interaction exists between an edge dislocation and nanosized Cr precipitates, which is evident as a decrease in total energy when an edge dislocation is placed within in a Cr precipitate. (c) 2008 Elsevier B.V. All rights reserved.
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  • Modified embedded-atom method calculation for the Ni–W system

    Shim, Jae-Hyeok   Park, Sung Il   Cho, Young Whan   Lee, Byeong-Joo  

    A semi-empirical interatomic potential of the Ni-W system was developed using a modified embedded-atom method (MEAM) formalism including second-nearest-neighbor interactions. The cross potential was determined by fitting physical properties of tetragonal Ni 4W available in the literature. The MEAM potential was used to predict phase stabilities, lattice constants, and bulk moduli of nonequilibrium and equilibrium phases in the Ni-W system. The results were in good agreement with experimental information or first-principles calculation
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  • Mitigation of degradation in the dehydrogenation behavior of air-exposed MgH2 catalyzed with NbF5

    Park, Mirae   Shim, Jae-Hyeok   Lee, Young-Su   Im, Yeon Ho   Cho, Young Whan  

    This paper addresses the dehydrogenation behavior of MgH2 exposed to air for up to 24 h at room temperature, focusing on the effect of the addition of a small amount of NbF5 and Nb2O5 as a catalytic additive. Although the dehydrogenation reaction of MgH2 with no catalytic additive and Nb2O5 is significantly retarded after air exposure compared with unexposed samples, the degradation in the dehydrogenation behavior of MgH2 with NbF5 due to air exposure is found to be effectively mitigated. Although differential scanning calorimetry of the air-exposed samples indicates that the activation energy for the dehydrogenation reaction significantly increases after air exposure, the activation energy for MgH2 with NbF5 remains the lowest. Scanning electron microscopy with energy dispersive X-ray spectroscopy shows that the amount of oxygen on the surfaces of air-exposed MgH2 particles catalyzed with NbF5 is much lower than that of air-exposed MgH2 with no catalytic additive and Nb2O5. This implies that the addition of NbF5 forms protective layers on the surfaces of MgH2 particles against oxygen and moisture in the air, which seems to be responsible for the positive effect of NbF5 on the degradation of the dehydrogenation behavior of air-exposed MgH2. (C) 2013 Elsevier B.V. All rights reserved.
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  • Effect of Thermal Charging of Hydrogen on the Microstructure of Metastable Austenitic Stainless Steel

    Kim, Han-Jin   Phaniraj, M. P.   Kim, Ju-Heon   Lee, Young-Su   Kim, Dong-Ik   Suh, Jin-Yoo   Lee, Joonho   Shim, Jae-Hyeok   Park, Seong-Jun  

    The tensile behavior of hydrogen-charged 304-type austenitic stainless steel, with and without prestrain, is investigated. The specimens are thermally charged with hydrogen in 15MPa hydrogen gas at 300 degrees C for 72h. Tensile behavior of the specimen is compared with the specimen aged in vacuum at 300 degrees C. The effect of the charging condition on the stability of microstructure is determined by characterizing prestrained specimens before and after charging. The hydrogen content in the specimens is determined using thermal desorption spectroscopy (TDS). Analysis of X-ray diffraction (XRD) data and electron backscattered diffraction (EBSD) shows that the fraction of martensite increases after charging in hydrogen by 5-10%. The fracture surfaces of the uncharged and charged specimens are examined for characteristic features. Flow stress and ductility of the charged and prestrained and charged specimens are discussed in terms of the martensite fraction and hydrogen content.
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  • Molecular dynamics simulation of the crystallization of a liquid gold nanoparticle

    Shim, Jae-Hyeok   Lee, Seung-Cheol   Lee, Byeong-Joo   Suh, Jin-Yoo   Whan Cho, Young  

    Molecular dynamics simulation of the crystallization behavior of a liquid gold (Au) nanoparticle, about 4nm in diameter, on cooling has been carried out based on the modified embedded-atom-method potential. With decreasing cooling rate, the final structure of the particle changes from amorphous to crystalline via icosahedron-like structure. While the outer shell of the icosahedron-like particle shows crystalline feature with {111}-like facets, the inner core remains amorphous. It is found that the structure of the fully crystallized particle is polycrystalline face-centered cubic (fcc). The fcc structure of the gold nanoparticle is proved energetically the most stable form.
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  • Dehydrogenation Reaction Kinetics of the LiBH4-YH3 Composite Promoted by Various Inert Gas Atmospheres

    Kim, Kee-Bum   Shim, Jae-Hyeok   Choi, In-Suk   Cho, Young Whan   Oh, Kyu Hwan  

    The application of back pressure using inert gases, such as helium, neon and argon, significantly influences the dehydrogenation reaction kinetics of the LiBH4-YH3 composite, even though the inert gases do not participate in the dehydrogenation reaction. While an increase in back pressure generally promotes the reaction kinetics, the ability to promote dehydrogenation reaction kinetics differs for different the gas species. The atomic weight of the inert gas appears to play an important role in determining the ability to promote dehydrogenation reaction kinetics. With increasing atomic weight of the inert gas, the reaction tends to occur at lower pressures with a shorter incubation period.
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  • Liquid miscibility gap in the Al–Pb–Sn system

    Shim, Jae-Hyeok   Lee, Hyung-Nae   Ha, Heon Phil   Cho, Young Whan   Yoon, Eui-Pak  

    The liquid miscibility gap boundary of two immiscible liquids, Al- and Pb-rich, has been measured in a temperature range between 1073 and 1293 K and a ternary liquid thermodynamic parameter has been evaluated based on experimental data measured in the present work together with phase equilibrium data available in the literature. From these, the phase diagram of the Al–Pb–Sn system including the liquid miscibility gap has been calculated using the CALPHAD method. The calculated results are in good agreement with the experimental data.
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  • Microstructure development in mechanically alloyed yttria dispersed austenitic steels RID D-1605-2011

    Phaniraj, M. P.   Kim, Dong-Ik   Shim, Jae-Hyeok   Cho, Young Whan  

    Austenitic oxide dispersion strengthened (ODS) alloys containing 0.5 and 5 wt.% yttria were prepared from elemental powders (Fe-20% Ni-14%, Cr-2.5%) Mo-2.5% Al-2% Mn) by mechanical alloying. The powders were analyzed using X-ray diffraction (XRD), X-ray photoelectron spectroscopy and transmission electron microscopy (TEM), paying particular attention to the behavior of yttria. XRD and high-resolution TEM analyses show that yttria does not form a solid solution with austenite; unlike in ferritic ODS alloys, where it dissolves. Milling induces the formation of the thermodynamically favorable yttrium aluminum perovskite (YAP). Alumina from the aluminum powder in the starting blend. formed in the initial stages, of milling using oxyen available from the other elemental powders, combines with yttria to form YAP. The yttria content does riot affect alloy formation but reduces the crystallite size and strain significantly in the 5% yttria composition. TEM analysis of hot-pressed compacts reveals nanocrystalline particles of yttria, yttrium aluminium garnet and YAP. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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  • Abnormal lattice behavior of MgH2 in isotope labeled LiBD4-MgH2 composites at the melting of LiBD4

    Cho, In Hwa   Gang, Seunggi   Choi, Yong Nam   Lee, Heeju   Park, Mirae   Shim, Jae-Hyeok   Noh, Do Young  

    We found abnormal lattice behavior of MgH2 in LiBD4-MgH2 composites at the melting of LiBD4 using high resolution x-ray diffraction measurements. At the LiBD4 melting point, the tetragonal MgH2 unit cell contracted isotropically which is attributed to D atoms diffused from neighboring LiBD4; these D atoms can occupy interstitals or lattice points of MgH2. Endothermic behavior of LiBD4 during melting might also have contriubuted to the contraction of MgH2. In addition, the intensity of MgH(D)(2) Bragg peaks measured in in situ neutron diffraction experiment confirms that H/D exchange between the two hydrides starts below the LiBD4 melting temperature. (C) 2019 Elsevier B.V. All rights reserved.
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  • Monte Carlo Simulation of Phase Separation Behavior in a Cu-Co Alloy Nanoparticle

    Shim, Jae-Hyeok   Lee, Byeong-Joo   Ahn, Jae-Pyoung   Cho, Young Whan   Park, Jong-Ku  

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