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Now showing items 65 - 80 of 85

  • Mechanochernical synthesis of ultrafine TiAl(3) powder and its catalytic effect on dehydrogenation of Li(3)AlH(6)

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

    Ultrafine TiAl(3) powder has been mechanochemically synthesized using a reaction between TiCl(3), AlCl(3) and Mg. MgCl(2) byproduct was removed from the product mixture by dissolving it selectively in distilled water. This ultrafine TiAl(3) whose primary particle size is about 100 nm shows a quite good catalytic effect on reducing the dehydrogenation temperature of Li(3)AlH(6). Although its catalytic ability does not exceed well-known titanium chlorides, the inevitable loss in hydrogen storage capacity by introducing catalyst is less with TiAl(3) than with TiCl(3). (c) 2005 Elsevier B.V. All rights reserved.
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  • Synthesis of ultrafine titanium aluminide powders and their catalytic enhancement in dehydrogenation kinetics of NaAlH4

    Lee, Gil-Jae   Kim, Ji Woo   Shim, Jae-Hyeok   Cho, Young Whan   Lee, Kyung Sub  

    Titanium aluminide powders with various sizes have been synthesized using four different mechanochemical reactions. While the reaction between TiCl3, AlCl3 and Mg produces TiAl3 nanopowder with a primary particle size of about 50 nm, the use of LiH instead of Mg results in the formation of Ti2Al5. These titanium aluminide powders reveal a considerable catalytic effect on dehydrogenation of NaAlH4. It is also found that the catalytic effect is particle-size dependent. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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  • Effect of alloying elements (Al, Co, Fe, Ni) on the solubility of hydrogen in vanadium: A thermodynamic calculation

    Kim, Ki-Hyun   Shim, Jae-Hyeok   Lee, Byeong-Joo  

    CALPHAD type thermodynamic assessments for the V-M-H (M = Al, Co, Fe, Ni) hydrogen membrane systems have been carried out on the basis of a newly assessed Co H binary description, a partly modified Fe-H description and existing thermodynamic descriptions for the other M-H (V-H, Al-H, Ni-H) and V-M (V-Al, V-Co, V-Fe, V-Ni) binary systems. A special attention was paid to estimate the thermodynamic descriptions for the non-existing bcc Al-H, Co-H and Ni-H alloys. Thermodynamic parameters for those binary alloys were estimated by using a theoretical approach (atomistic computation) and fitting limited amount of experimental data for the hydrogen solubility in V-rich bcc ternary alloys. The proposed thermodynamic descriptions predict phase equilibria, especially the effect of alloying elements on the hydrogen solubility in the V-rich bcc alloys, in good agreement with available experimental data. The present thermodynamic descriptions can be easily extended to higher order alloy systems and can provide useful information for alloy design of metallic hydrogen membranes with well-balanced hydrogen permeability and mechanical properties. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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  • Low temperature formation of Mg2FeH6 by hydrogenation of ball-milled nano-crystalline powder mixture of Mg and Fe

    Fadonougbo, Julien O.   Jung, Jee-Yun   Suh, Jin-Yoo   Lee, Young-Su   Shim, Jae-Hyeok   Cho, Young Whan  

    Low temperature formation of Mg2FeH6 is demonstrated by hydrogenation of Mg-Fe elemental powder mixture at a temperature as low as 350 degrees C which is lower than the conventional process temperature, 500 degrees C. To enable the low temperature synthesis, the powder mixture of Mg and Fe has been prepared by high energy ball milling using different process control agents (PCAs). A systematic study on the ball milling and hydrogenation conditions has been carried out to maximize the yield of the ternary line compound. The hydrogenation conditions together with the particle size of the starting materials turn out to play a significant role in the hydrogenation kinetics of the system. An optimized condition has demonstrated a significant hydrogenation as well as a robust cycling ability at low temperature which suggests the strong potential of the process for practical applications. (C) 2017 Elsevier Ltd. All rights reserved.
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  • Enhanced high temperature hydrogen permeation characteristics of V-Ni alloy membranes containing a trace amount of yttrium

    Oh, Ju-Young   Ko, Won-Seok   Suh, Jin-Yoo   Lee, Youg-Su   Lee, Byeong-Joo   Yoon, Woo Young   Shim, Jae-Hyeok  

    The hydrogen permeation behaviors of V85Ni15 and V84.8Ni15Y0.2 membranes with palladium coating layers were compared at 480 degrees C, which is considered high temperature for conventional vanadium alloy-based membranes. Although the hydrogen permeability of the V85Ni15 membrane drastically decreased over time, that of the V84.8Ni15Y0.2 membrane was sustained for 12 days. The role of yttrium in the membrane was to effectively hinder interdiffusion between the Pd coating layers and the V alloy substrate during hydrogen permeation. The results of the present study provide important clues for the development of hydrogen permeable metallic membranes for high temperature applications. (C) 2016 Elsevier Ltd. All rights reserved.
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  • Determining the effect of added zirconium on the bond character in TiFe alloys using scanning Kelvin probe force microscopy

    Han, Huijun   Kim, Han-Jin   Kim, Hayoung   Sohn, So-Dam   Shim, Jae-Hyeok   Suh, Jin-Yoo   Shin, Hyung-Joon  

    Alloying elements are known to enhance the first hydrogenation of TiFe alloys, although the mechanism has not yet been fully understood. The effects of Zr on the bond character of a TiFe alloy was investigated using scanning Kelvin probe force microscopy. Adding Zr led to increased formation of the TiFe2 and Ti2Fe phases. As for TiFe2 phase, where the added Zr substituted Fe, the substitution caused the bond character of the phase to become more covalent and less ionic due to the lower electronegativity of Zr compared to Fe. Subsequently, the work functions increased as a function of Zr content.
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  • Effect of post-weld heat treatment on the microstructure and hardness of P92 steel in IN740H/P92 dissimilar weld joints

    Seo, Wi-Geol   Suh, Jin-Yoo   Shim, Jae-Hyeok   Lee, Hansang   Yoo, Keunbong   Choi, Shi-Hoon  

    The microstructural factors that contribute to hardening mechanisms were investigated to explain the effect of post-weld heat treatment (PWHT) on the hardness of P92 steel in IN740H/P92 dissimilar weld joints. This study presents experimental analysis of the distribution of microstructural factors such as precipitate size, precipitate fraction, grain size and dislocation density in the heat-affected zone (HAZ) and base metal (BM) of the martensitic heat-resistant steel. Although precipitates mainly decorated the prior-austenite grain boundaries (PAGBs), packet boundaries (PBs), and block boundaries (BBs), their spatial distribution strongly depended on the distance from fusion line and the PWHT conditions. The grain size and the densities of geometrically necessary dislocations (GNDs) also exhibited a non-uniform distribution. The individual contributions of the microstructural factors to hardness were explained by introducing a simple hardening equation that considers the independent effect of different hardening mechanisms.
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  • A finite outlet volume correction to the time lag method:The case of hydrogen permeation through V-alloy and Pd membranes

    Lee, Young-Su   Shim, Jae-Hyeok   Suh, Jin-Yoo  

    The time-lag method is a technique to derive the permeability of a membrane from the time-variant outflux of a permeate. The original formulation assumes an invariant outlet pressure; however, for practical reasons, the permeate is often contained in a finite outlet volume, and the outflux is evaluated using the increase in the outlet pressure. The outlet pressure change inevitably introduces an error in the diffusivity and in the solubility, i.e., Sieverts' constant. We demonstrate that when a time lag is obtained from the tangent line at the maximum flux, the error in the diffusivity stays within ca. 2% under usual experimental conditions. On the other hand, Sieverts' constant is underestimated to a larger degree if the outlet pressure change is not considered. To solve this problem, we propose a finite outlet volume correction to the time-lag method for the case of hydrogen permeation. The proposed scheme is an approximate solution that is valid when a linear hydrogen concentration profile is developed across the membrane. When the correction method is applied to characterize V-alloy and Pd membranes, the simulated outlet pressure from the corrected Sieverts' constant closely approximates the experimental data. The proposed method does not require any additional measurements and greatly improves the accuracy of the permeability measurement.
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  • CALPHAD-based alloy design for advanced automotive steels - Part II:Compositional and microstructural modification for advanced carburizing steels

    Cha, Sung Chul   Hong, Seung-Hyun   Kim, Myung-Yeon   Park, Jihye   Shim, Jae-Hyeok   Jung, Woo-Sang   Rath, Markus   Kozeschnik, Ernst  

    The CALPHAD-based computational techniques established in the first part of this series [1] have been applied for the development of advanced carburizing steels used for gears in vehicle transmissions. To improve the strength and hardenability of a conventional carburizing steel, the alloying composition has been modified based on the calculation of phase equilibria and the prediction of mechanical properties such as the yield and tensile strengths, hardness, and volume fractions of martensite and bainite. The size and density of carbides precipitated with V, Nb or Ti microalloying elements as well as their austenite grain size have been predicted by thermo-kinetic simulation to optimize the microstructure. The reliability of the computational results has been experimentally confirmed by comparing the austenite grain size and the hardness of the newly developed carburizing steels with those of the conventional carburizing steels. (C) 2016 Elsevier Ltd. All rights reserved.
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  • Modeling and experiment on microstructure evolutions and mechanical properties in grade 600 MPa reinforcing steel rebar subjected to TempCore process

    Bandyopadhyay, Kaushik   Lee, Joonho   Shim, Jae-Hyeok   Hwang, Byoungchul   Lee, Myoung-Gyu  

    In this study, finite element (FE) modeling of the microstructure evolutions and resultant mechanical properties in the grade 600 MPa steel bar subjected to the TempCore process, a thermo-mechanically controlled process involving quenching and self-tempering, and related experiments are presented. The phase transformation kinetics based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and Koistinen-Marburger equations were implemented in the user-defined subroutine of FE software to consider diffusional and martensitic transformations, respectively. Moreover, a robust simulation approach for solving complex thermo-mechanical problems induced by the quenching (i.e., external water cooling), internally generated heat due to phase transformations, and heat transfers between core and surface were addressed. The developed model can also simulate deformations associated with temperature change, phase transformations, and mechanical plasticity. The developed model was validated by estimating evolutions of various phase fractions and hardness in steel bars produced by both TempCore and normal air-cooling, which were compared with corresponding experimental results. Finally, prediction of flow stress curves and their experimental validation were also performed.
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  • Modeling and experiment on microstructure evolutions and mechanical properties in grade 600 MPa reinforcing steel rebar subjected to TempCore process

    Bandyopadhyay, Kaushik   Lee, Joonho   Shim, Jae-Hyeok   Hwang, Byoungchul   Lee, Myoung-Gyu  

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  • The role of Fe particle size and oxide distribution on the hydrogenation properties of ball-milled nano-crystalline powder mixtures of Fe and Mg

    Fadonougbo, Julien O.   Jung, Jee Yun   Suh, Jin-Yoo   Lee, Young-Su   Shim, Jae-Hyeok   Fleury, Eric   Cho, Young Whan  

    In the aim of evidencing the relationship between Mg2FeH6 synthesis and the size of Fe particles, several specimens have been prepared by applying various milling energies (milling time) on a 2.1 Mg and 1Fe powder mixture doped with a small fraction of Unsaturated Fatty Amine (UFA). The resulting nanocrystalline composite structures display a broad Fe particle size distribution as a function of milling time. The hydrogenation of those complex powders has been conducted at temperatures lower than 400 degrees C under 60 bar of hydrogen pressure. As expected, the Fe particle size significantly influenced the hydrogenation kinetics. Also, the inevitable distribution of a minor fraction of oxides occurring during the milling process affected greatly the hydrogen storage capacity. Under the low pressure and temperature conditions selected in the frame of this study, lower than 100 bar and 500 degrees C conventionally used for synthesis of high purity Mg2FeH6, the hydrogenation reaction was demonstrated to be almost completed within 6 h, confirming the fast hydrogen absorption capability of the prepared materials. Plus, nearly 84 wt% of Mg2FeH6 was achieved under the afore mentioned moderate conditions and a minor fraction of unreacted Fe still remained due to diffusion constraints existing at low temperatures. (C) 2019 Elsevier B.V. All rights reserved.
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  • Modeling precipitation thermodynamics and kinetics in type 316 austenitic stainless steels with varying composition as an initial step toward predicting phase stability during irradiation

    Shim, Jae-Hyeok   Povoden-Karadeniz, Erwin   Kozeschnik, Ernst   Wirth, Brian D.  

    The long-term evolution of precipitates in type 316 austenitic stainless steels at 400 degrees C has been simulated using a numerical model based on classical nucleation theory and the thermodynamic extremum principle. Particular attention has been paid to the precipitation of radiation-induced phases such as gamma' and G phases. In addition to the original compositions, the compositions for radiation-induced segregation at a dose level of 5, 10 or 20 dpa have been used in the simulation. In a 316 austenitic stainless steel, gamma' appears as the main precipitate with a small amount of G phase forming at 10 and 20 dpa. On the other hand, G phase becomes relatively dominant over gamma' at the same dose levels in a Ti-stabilized 316 austenitic stainless steel, which tends to suppress the formation of gamma'. Among the segregated alloying elements, the concentration of Si seems to be the most critical for the formation of radiation-induced phases. An increase in dislocation density as well as increased diffusivity of Mn and Si significantly enhances the precipitation kinetics of the radiation-induced phases within this model. (C) 2015 Elsevier B.V. All rights reserved.
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  • Modeling precipitation thermodynamics and kinetics in type 316 austenitic stainless steels with varying composition as an initial step toward predicting phase stability during irradiation

    Shim, Jae-Hyeok   Povoden-Karadeniz, Erwin   Kozeschnik, Ernst   Wirth, Brian D.  

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  • Effects of crystallographic structure and Cr on the rate of void nucleation in BCC Fe: An atomistic simulation study

    Lee, Byeong-Joo   Shim, Jae-Hyeok   Kwon, Junhyun  

    Atomistic Monte Carlo simulations based on modified embedded-atom method (MEAM) interatomic potentials have been carried out to clarify the differences in swelling rates between bcc and fcc Fe and between pure bcc Fe and bcc Fe-Cr alloys. Assuming that the transient regimes prior to the onset of steady-state swelling correspond to the void nucleation stage, the effect of crystallographic structure (bcc vs. fcc) or Cr alloying on the void nucleation rate under a given amount of supersaturated vacancies was examined. It was found that the void nucleation rate is much higher in fcc Fe than in bcc Fe. Randomly distributed Cr atoms slightly increase the void nucleation rate in bcc Fe, but microstructural evolutions such as the precipitation of Cr-rich phase have more decisive effects, serving as a vacancy sink. The reasons for the individual results are rationalized in terms of the binding energy of vacancy clusters and the size difference between Fe and Cr atoms.
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  • Molecular dynamics simulation of primary irradiation defect formation in Fe-10%Cr alloy

    Shim, Jae-Hyeok   Lee, Hyon-Jee   Wirth, Brian D.  

    Molecular dynamics simulations of displacement cascades in Fe and Fe-10%Cr have been performed for primary knock-on energies from I to 20 keV using two different Finnis-Sinclair style interatomic potentials. The different potentials were fit to describe the extremes of positive (attractive) and negative (repulsive) binding between substitutional Cr atoms and Fe self-interstitial atoms. As expected, the effect of Cr on the collisional stage of cascade evolution and on the number of point defects and point defect clusters produced is quite minimal. However, the quantity of mixed Fe-Cr dumbbells produced is sensitive to the choice of potential. (c) 2006 Elsevier B.V. All rights reserved.
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