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Now showing items 1 - 16 of 85

  • Room-Temperature Solid Solution Softening in Fe-V Binary System

    Park, Jihye   Shim, Jae-Hyeok   Lee, Young-Kook  

    Molecular dynamics simulations of the glide of an edge dislocation in the bcc matrix of Fe-V alloys were performed to investigate the room-temperature solid solution softening by V atoms. For this purpose, the glide velocity of an edge dislocation was calculated as a function of V concentration under the shear stresses of 100-300 MPa using the Fe-V cross-potential constructed newly in the present study. Whereas the solid solution hardening occurred as the V concentration is less than 0.13 at% or more than 0.5 at%, the room-temperature solid solution softening was observed in Fe-(0.13-0.5) at% V alloys. The solid solution softening occurring in Fe-(0.13-0.5) at% V alloys was caused by the accelerated growth velocity of kinks by solute V atoms. The increase in kink velocity happened when the interatomic distance between solute V atoms was similar to the length of dislocation kinks.
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  • Atomistic simulation of hydrogen diffusion at tilt grain boundaries in vanadium

    Shim, Jae-Hyeok   Ko, Won-Seok   Suh, Jin-Yoo   Lee, Young-Su   Lee, Byeong-Joo  

    Molecular dynamics simulations of hydrogen diffusion at I 3 pound and I 5 pound tilt grain boundaries in bcc vanadium (V) have been performed based on modified embedded-atom method interatomic potentials. The calculated diffusivity at the grain boundaries is lower than the calculated bulk diffusivity in a temperature range between 473 and 1473 K, although the difference between the grain boundary and bulk diffusivities decreases with increasing temperature. Compared with that of the other directions, the mean square displacement of an interstitial hydrogen atom at the I 3 pound boundary is relatively small in the direction normal to the boundary, leading to two dimensional motion. Molecular statics simulations show that there is strong attraction between the hydrogen atom and these grain boundaries in V, which implies that the role of grain boundaries is to act as trap sites rather than to provide fast diffusion paths of hydrogen atoms in V.
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  • Pressure-enhanced dehydrogenation reaction of the LiBH(4)-YH(3) composite

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

    The increase in hydrogen back pressure unexpectedly enhances the overall dehydrogenation reaction rate of the 4LiBH(4) + YH(3) composite significantly. Also, argon back pressure has a similar influence on the composite. Gas back pressure seems to enhance the dehydrogenation reaction by kinetically suppressing the formation of the diborane by-product.
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  • A modified embedded-atom method interatomic potential for the V-H system

    Shim, Jae-Hyeok   Lee, Young-Su   Fleury, Eric   Cho, Young Whan   Ko, Won-Seok   Lee, Byeong-Joo  

    An interatomic potential for the vanadium-hydrogen binary system has been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) potential formalism, in combination with the previously developed potentials for V and H. Also, first-principles calculation has been carried out to provide data on the physical properties of this system, which are necessary for the optimization of the potential parameters. The developed potential reasonably reproduces the fundamental physical properties (thermodynamic, diffusion, elastic and volumetric properties) of V-rich bcc solid solution and some of the vanadium hydride phases. The applicability of this potential to the development of V-based alloys for hydrogen applications is discussed. (C) 2011 Elsevier Ltd. All rights reserved.
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  • Hydrogenation reaction of CaH(2)-CaB(6)-Mg mixture RID C-3834-2012

    Kim, Yoonyoung   Reed, Daniel   Lee, Young-Su   Shim, Jae-Hyeok   Han, Heung Nam   Book, David   Cho, Young Whan  

    Hydrogen desorption/absorption cycling properties of Ca(BH(4))(2)+MgH(2) mixture having 9.1 wt% of theoretical hydrogen capacity have been investigated. The hydrogenation reaction sequence starting from the dehydrogenated product CaH(2)+CaB(6)+Mg was thoroughly followed using X-ray diffraction, and that can be summarized as (i) Mg+H(2) -> MgH(2), (ii) 4CaH(2)+3MgH(2 ->)Ca(4)Mg(3)H(14), (iii) Ca(4)Mg(3)H(14)+2CaB(6)+20H(2)-> 6Ca(BH(4))(2)+3MgH(2). The steps (i) and (ii) proceeded rather fast, and the total amount of reabsorbed hydrogen was governed by the step (iii). The kinetics of the step (iii) was critically affected by the temperature of dehydrogenation/hydrogenation reaction, which we mainly attributed to the microstructure of the dehydrogenated product such as particle size. We indirectly proved our hypothesis by showing that inserting high-energy ball-milling process during hydrogenation is more effective than simply increasing the reaction time. Optimal reversibility was achieved from the sample which was dehydrogenated at 673 K and hydrogenated at 623 K. (c) 2009 Elsevier B.V. All rights reserved.
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  • Rehydrogenation and cycle studies of LiBH(4)-CaH(2) composite

    Lim, Jae-Hag   Shim, Jae-Hyeok   Lee, Young-Su   Suh, Jin-Yoo   Cho, Young Whan   Lee, Joonho  

    Rehydrogenation behavior of 6LiBH(4) + CaH(2) composite with NbF(5) has been studied between 350 and 500 degrees C after dehydrogenation at 450 degrees C. The composite exhibits the best rehydrogenation feature at 450 degrees C in terms of the overall rehydrogenation rate and the amount of absorbed hydrogen. It is found that about 9 wt% hydrogen is absorbed at 450 degrees C for 12 h. Up to 10 dehydrogenation-hydrogenation cycles have been carried out for the composite. It is demonstrated that 6LiBH(4) + CaH(2) with 15 wt% NbF(5) maintains a reversible hydrogen storage capacity of about 6 wt% at 450 degrees C after a slight degradation between the 1st and 5th cycles. The addition of NbF(5) seems to improve the cycle properties by retarding microstructural coarsening during cycles. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
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  • Polymorphism and Thermodynamics of Y(BH(4))(3) from First Principles

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

    Structure and thermodynamics of the recently discovered two polymorphs of Y(BH(4))(3) are investigated by first-principles calculation. Simulated X-ray and neutron diffraction patterns combined with structure analysis from first principles enable us to assign a space group Fm (3) over barc to the high-temperature polymorph among the several proposed space groups. An orientational disorder of [BH(4)](-) groups is considered and compared with NaBH(4), which has a disordered [BH(4)](-) arrangement at room temperature. In the case of NaBH(4), the structure stays in a local energy minimum irrespective of the [BH(4)](-) orientation, but in Y(BH(4))(3), [BH(4)](-) reorientation is suppressed by a strong repulsive force created by close H-H contacts and the structure becomes unstable, thus favoring an ordered [BH(4)](-) arrangement. The calculated high energy barrier for the [BH(4)](-) reorientation partly accounts for the slow phase transition observed in Y(BH(4))(3), again making a good contrast with the facile [BH(4)](-) flipping in NaBH(4). The thermodynamics of Y(BH(4))(3) appears quite attractive, exhibiting a lower dissociation temperature than Mg(BH(4))(2) under 1 bar of H(2).
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  • Multiscale modeling of point defect interactions in Fe-Cr alloys

    Wong, Kwan L.   Lee, Hyon-Jee   Shim, Jae-Hyeok   Sadigh, Babak   Wirth, Brian D.  

    Predictive performance models of ferritic/martensitic alloys in fusion neutron irradiation environments require knowledge of point defect interactions with Cr, which can be investigated by a multiscale modeling approach. Molecular dynamics simulations, using Finnis-Sinclair-type potentials, have been used to investigate the interstitial diffusion and reveal that the extremes of attractive and repulsive binding between Cr and interstitials change the characteristics of interstitial migration and the Cr-to-Fe diffusivity ratio. Ab-initio calculations have been performed to determine the vacancy-Cr interactions, and these calculations reveal complex electronic and magnetic interactions between Cr and Fe. The ab-initio values have been used to calculate the Cr-to-Fe diffusivity ratio by a vacancy mechanism using the LeClaire multi-frequency model and a kinetic lattice Monte Carlo model, both of which indicate that Cr diffuses faster than Fe. The modeling results are discussed in the context of the radiation-induced segregation of Cr at grain boundaries in BCC Fe-Cr alloys. (C) 2009 Published by Elsevier B.V.
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  • Cyclic oxidation of yttria dispersed austenitic stainless steels RID D-1605-2011

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

    Cyclic oxidation of austenitic steels (Fe-20Ni-14Cr-2.5Mo-2Mn-2.5Al-wt.%) dispersed with 0, 0.5 and 5 wt.% yttria was carried out at 800 degrees C in air. The scale surface and cross-section were characterized using X-ray diffraction, scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and X-ray photoelectron spectroscopy (XPS). Yttria additions improve the resistance to spallation. The increase in the resistance to spallation appears to be related to the presence of mixed oxides between yttria and base metal oxides. (C) 2010 Elsevier Ltd. All rights reserved.
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  • Evaluation of W-Si-C thick coating as a plasma facing material

    Seok, Hyun Kwang   Jung, Kyung Ho   Kim, Yu Chan   Shim, Jae-Hyeok   Kim, Dong-Ik   Han, Seung-Hee   Baik, Kyeong Ho   Cha, Pil-Ryung  

    We present tungsten alloy coating of 150-200 pm thickness with improved plasma erosion resistance fabricated by plasma spraying of granular W-SiC composite powders. During increasing the SiC concentration to 8 wt%, we observed the increase in the hardness of the coating from 250 to 440 Hv. The plasma erosion depth of the coating decreased by 10 times compared with pure tungsten in the same erosion environment. (C) 2009 Elsevier B.V. All rights reserved.
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  • Effect of Time-Dependent Pinning Pressure on Abnormal Grain Growth:Phase Field Simulation

    Kim, Jeong Min   Min, Guensik   Shim, Jae-Hyeok   Lee, Kyung Jong  

    The effect of the time-dependent pinning pressure of precipitates on abnormal grain growth has been investigated by multiphase field simulation with a simple precipitation model. The application of constant pinning pressure is problematic because it always induces abnormal grain growth or no grain growth, which is not reasonable considering the real situation. To produce time-dependent pinning pressure, both precipitation kinetics and precipitate coarsening kinetics have been considered with two rates: slow and fast. The results show that abnormal grain growth is suppressed at the slow precipitation rate. At the slow precipitation rate, the overall grain growth caused by the low pinning pressure in the early stage indeed plays a role in preventing abnormal grain growth by reducing the mobility advantage of abnormal grains. In addition, the fast precipitate coarsening rate tends to more quickly transform abnormal grain growth into normal grain growth by inducing the active growth of grains adjacent to the abnormal grains in the early stage. Therefore, the present study demonstrates that the time dependence of the pinning pressure of precipitates is a critical factor that determines the grain growth mode.
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  • New Equation for Prediction of Martensite Start Temperature in High Carbon Ferrous Alloys

    Park, Jihye   Shim, Jae-Hyeok   Lee, Seok-Jae  

    Since previous equations fail to predict M (S) temperature of high carbon ferrous alloys, we first propose an equation for prediction of M (S) temperature of ferrous alloys containing > 2 wt pct C. The presence of carbides (Fe3C and Cr-rich M C-7(3)) is thermodynamically considered to estimate the C concentration in austenite. Especially, equations individually specialized for lean and high Cr alloys very accurately reproduce experimental results. The chemical driving force for martensitic transformation is quantitatively analyzed based on the calculation of T (0) temperature.
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  • Thermal decomposition behavior of calcium borohydride Ca(BH(4))(2)

    Kim, Jae-Hun   Jin, Seon-Ah   Shim, Jae-Hyeok   Cho, Young Whan  

    The thermal decomposition behavior of adduct-free Ca(BH(4))(2), prepared by heating Ca(BH4)(2)center dot 2THF powder under vacuum, was investigated by X-ray diffraction and thermal analyses. It has been found that Ca(BH(4))(2) undergoes a polymorphic transformation at 440 K and eventually decomposes in two steps between 620 and 770 K. CaH(2) and an unknown intermediate compound form after the first step, but CaH(2) is the only crystalline phase observed after the second step with a total weight loss of about 9.0 wt.%. (c) 2007 Elsevier B.V. All rights reserved.
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  • Dehydrogenation behavior of LiBH4/CaH2 composite with NbF5

    Lim, Jae-Hag   Shim, Jae-Hyeok   Lee, Young-Su   Cho, Young Whan   Lee, Joonho  

    The dehydrogenation behavior of 6LiBH(4) + CaH2 + 0.2NbF(5) composite has been investigated. It is demonstrated that the addition of NbF5 is effective in lowering the dehydrogenation temperature of this composite. The composite starts dehydrogenation before the melting of LiBH4 and releases 9.1 wt.% hydrogen up to 400 degrees C. The equilibrium dehydrogenation temperature under I bar of hydrogen is estimated to be 309 degrees C with a reaction enthalpy change of 56.5kJ mol(-1) H-2, which is consistent with thermodynamic calculation. Crown Copyright (c) 2008 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.
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  • Reversible hydrogen storage in LiBH(4)-Al-LiH composite powder

    Jin, Seon-Ah   Shim, Jae-Hyeok   Cho, Young Whan   Yi, Kyung-Woo   Zabara, Oleg   Fichtner, Maximilian  

    A composite powder has been prepared by ball milling a mixture of LiBH(4) and LiAlH(4) together with 3 mol.% TiF(3). The powder starts to decompose between 450 and 520 K and the weight loss reaches up to 7.2 wt.%. The formation of AlB(2) is clearly observed after desorption at 673 K for 4 h. It is also confirmed that dehydrogenated product can be hydrogenated to about 5.1 wt.% H(2) at 623 K for 6 h under 70 bar of hydrogen. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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  • Decomposition Reactions and Reversibility of the LiBH(4)-Ca(BH(4))(2) Composite

    Lee, Ji Youn   Ravnsbaek, Dorthe   Lee, Young-Su   Kim, Yoonyoung   Cerenius, Yngve   Shim, Jae-Hyeok   Jensen, Torben R.   Hur, Nam Hwi   Cho, Young Whan  

    LiBH(4) is one of the promising candidates for hydrogen storage materials because of its high gravimetric and volumetric hydrogen capacity. However, its high dehydrogenation temperature and limited reversibility has been a hurdle for its use in real applications. In an effort to overcome this barrier and to adjust the thermal stability, we make a composite system LiBH(4)-Ca(BH(4))(2). In order to fully characterize this composite system we study xLiBH(4) + (1 - x)Ca(BH(4))(2) for several x values between 0 and 1, using differential scanning calorimetry, in situ synchrotron X-ray diffraction, thermogravimetric analysis, and mass spectrometry. Interestingly, this composite undergoes a eutectic melting at ca. 200 degrees C in a wide composition range, and the eutectic composition lies between x = 0.6 and 0.8. The decomposition characteristics and the hydrogen capacity of this composite vary with x, and the decomposition temperature is lower than both the pure LiBH(4) and Ca(BH(4))(2) at intermediate conpositions, for example, for x approximate to 0.4, decomposition is finished below 400 degrees C releasing about 10 wt % of hydrogen. Partial reversibility of this system was also confirmed for the first time for the case of if mixed borohydride composite.
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