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Now showing items 1 - 16 of 65

  • Critical assessment of Pt surface energy – An atomistic study

    Jin-Soo Kim   Donghyuk Seol   Byeong-Joo Lee  

    Highlights • Pt surface energy is calculated by interatomic potential that reproduces the surface segregation in eight Pt-binary alloys. • The finally optimized Pt surface energy is 2036, 2106 and 1502 mJ/m 2 for the (100), (110) and (111) surfaces, respectively. • The optimized surface energy is one of the most accurate ones among those reported during the last 40 years. • We propose our computational procedure as an evaluation tool for surface energy of elements. Abstract Despite the fact that surface energy is a fundamental quantity in understanding surface structure of nanoparticle, the results of experimental measurements and theoretical calculations for the surface energy of pure Pt show a wide range of scattering. It is necessary to further ensure the surface energy of Pt to find the equilibrium shape and atomic configuration in Pt bimetallic nanoparticles accurately. In this article, we critically assess and optimize the Pt surface energy using a semi-empirical atomistic approach based on the second nearest-neighbor modified embedded-atom method interatomic potential. That is, the interatomic potential of pure Pt was adjusted in a way that the surface segregation tendency in a wide range of Pt binary alloys is reproduced in accordance with experimental information. The final optimized Pt surface energy (mJ/m 2 ) is 2036 for (100) surface, 2106 for (110) surface, and 1502 for (111) surface. The potential can be utilized to find the equilibrium shape and atomic configuration of Pt bimetallic nanoparticles more accurately. Graphical abstract Image, graphical abstract
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  • An interatomic potential for the Li-Co-O ternary system

    Eunkoo Lee   Kwang-Ryeol Lee   Byeong-Joo Lee  

    Graphical abstract Abstract Although large-scale atomistic simulations provide useful insights into various material phenomena, such studies on LiCoO 2 , which is the most widely used cathode material for lithium ion batteries (LIBs), have rarely been undertaken due to difficulties in developing adequate interatomic potentials. In this study, an interatomic potential (2NNMEAM + Qeq) for the Li-Co-O ternary system is developed to carry out molecular dynamics (MD) simulation studies on lithium cobalt oxides. Potential parameters are optimized so that the potential can successfully reproduce fundamental materials properties (structural, elastic, thermodynamic and migration properties) of various compounds of sub-binary and lithium cobalt ternary oxide systems. Through MD simulations, we investigate lithium diffusion properties (activation energy for lithium migration and diffusion coefficient) in layered Li 1− x CoO 2 (0 ≤  x  ≤ 0.5) of various lithium vacancy concentrations. We find that the lithium vacancy concentration has a significant influence on the activation energy for lithium diffusion and the lithium diffusion coefficient in the Li 1− x CoO 2 cathode. The developed potential can be further utilized for atomistic simulation studies on other materials phenomena (phase transitions, defect formation, lithiation/delithiation, etc.) in LIB cathode materials.
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  • Thermodynamic properties of phase-field models for grain boundary segregation

    Seong Gyoon Kim   Jae Sang Lee   Byeong-Joo Lee  

    Abstract Impurity atoms segregated in grain boundary (GB) regions can dramatically change the physical and chemical properties of the GBs. Such changes often appear to be attributed to the GB energy reduction and/or solute drag effect. Phase-field models have been utilized to clarify both the thermodynamic and kinetic effects of the GB segregation. In this study, we developed phase-field models for GB segregation that are diffuse interface versions of the classical two-phase model of GB segregation. The thermodynamic state at any point in the system is represented as a mixture of a GB phase and a matrix phase. There are two choices for the thermodynamic relation between the GB phase and the matrix phase that constitute the point: the equal composition condition in model I and the equal diffusion potential condition in model II. Most of the previous PFMs for GB segregation appear to be specific cases of model I. We examined the thermodynamic properties of models I and II, and compared them with each other and the classical two-phase model. Although all the models resulted in the same GB composition, the GB energy and its dependency on the composition at the equilibrium state are quite different from each other. In model I, there is a lower bound to the GB energy, which originates from the equal composition condition. The GB energy from model II shows no such lower bound, and it is represented as the vertical distance between the parallel tangent lines on the free energy diagram, as in the classical two-phase model. Nevertheless, the compositional dependence in the model II is quite different from that in the classical two-phase model. This originates from the different choices for the composition-independent parameter in the models: a constant gradient energy coefficient in model II and a constant GB width in the classical two-phase model. Model I is not suitable for simulations of alloys that show a reduction of the GB energy due to GB segregation below a certain limit (in dilute alloys, about half of the GB energy of pure solvents). Model II is a correct choice for such alloys. Graphical abstract GB energy changes with the matrix composition in various phase-field models and the classical two-phase model for grain boundary segregation. The black, blue and red solid curves are the GB energies for α = 2, 3 and 4, respectively. Solid lines, dashed lines and dotted lines represent the GB energy changes from model II, model I and classical two-phase model, respectively.
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  • Modified embedded-atom interatomic potential for Co–W and Al–W systems

    Wei-ping DONG   Zheng CHEN   Byeong-Joo LEE  

    Abstract A semi-empirical interatomic potential formalism, the second-nearest-neighbor modified embedded-atom method (2NN MEAM), has been applied to obtaining interatomic potentials for the Co–W and Al–W binary system using previously developed MEAM potentials of Co, Al and W. The potential parameters were determined by fitting the experimental data on the enthalpy of formation, lattice parameter, melting point and elastic constants. The present potentials generally reproduce the fundamental physical properties of the Co–W and Al–W systems accurately. The lattice parameters, the enthalpy of formation, the thermal stability and the elastic constants match well with experiment and the first-principles results. The enthalpy of mixing and the enthalpy of formation and mixing of liquid are in good agreement with CALPHAD calculations. The potentials can be easily combined with already-developed MEAM potentials for binary cobalt systems and can be used to describe Co–Al–W-based multicomponent alloys, especially for interfacial properties.
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  • A numerical model to predict mechanical properties of Ni-base disk superalloys

    Young-Kwang Kim   Donghoon Kim   Hong-Kyu Kim   Eun-Yoo Yoon   Youngseon Lee   Chang-Seok Oh   Byeong-Joo Lee  

    Abstract A numerical model to predict mechanical properties of Ni-base superalloys for turbine disk applications is developed by modifying and combining previous models that consider various microstructural factors. Modifications are made so that as many necessary model parameters as possible, whose values have been obtained from mechanical measurements and microstructural observation, can be determined using a thermodynamic calculation and a precipitation kinetics simulation. The model's predictions of tensile properties, creep resistance and fatigue damage tolerance have good correlation with experimental data. The applicability and limits of the present model as an alloy design tool for new Ni-base disk superalloys are discussed in this paper. Highlights • We propose a numerical model to predict tensile properties, creep resistance and fatigue damage tolerance. • The model has been achieved by combining earlier empirical models with modifications. • The model considers various microstructural factors based on deformation mechanisms. • The advantage of our model is to enable to calculate all model parameters from computational approaches. • The model enables to predict mechanical properties without any further experimental measurement.
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  • A summary of the CALPHAD XXXIX conference

    Byeong-Joo Lee   Chang-Seok Ohb   Joonho Leec   Jae-Hyeok Shimd   Hyuck Mo Leee  

    CALPHAD XXXIX (May 23–28, 2010) was held in Jeju, Korea, organized by Byeong-Joo Lee, Chang-Seok Oh, Joonho Lee, Jae-Hyeok Shim and Hyuck Mo Lee. 125 registered participants presented 73 oral contributions and 52 posters, including 5 invited talks by experimental researchers from various materials research fields in a specially inserted session named “experiments that need thermodynamic analysis”. Poster sessions were held on the evening of Monday and Tuesday. A “Young Calphadian Meeting” was also organized on Tuesday evening after the poster session with A. Pelton, B. Sundman and J. Ågren as invited speakers. There was a conference excursion to the Jeju forkvillage during Wednesday afternoon. The Best Paper Award for papers published in 2009 in the CALPHAD journal and a Best Poster Award for one of the conference’s posters were conferred during the conference dinner on Wednesday evening. The abstracts and titles of oral presentations and titles of poster presentations are summarized in this report.
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  • A summary of the CALPHAD XXXIX conference

    Byeong-Joo Lee   Chang-Seok Oh   Joonho Lee   Jae-Hyeok Shim   Hyuck Mo Lee  

    CALPHAD XXXIX (May 23–28, 2010) was held in Jeju, Korea, organized by Byeong-Joo Lee, Chang-Seok Oh, Joonho Lee, Jae-Hyeok Shim and Hyuck Mo Lee. 125 registered participants presented 73 oral contributions and 52 posters, including 5 invited talks by experimental researchers from various materials research fields in a specially inserted session named “experiments that need thermodynamic analysis”. Poster sessions were held on the evening of Monday and Tuesday. A “Young Calphadian Meeting” was also organized on Tuesday evening after the poster session with A. Pelton, B. Sundman and J. Ågren as invited speakers. There was a conference excursion to the Jeju forkvillage during Wednesday afternoon. The Best Paper Award for papers published in 2009 in the CALPHAD journal and a Best Poster Award for one of the conference’s posters were conferred during the conference dinner on Wednesday evening. The abstracts and titles of oral presentations and titles of poster presentations are summarized in this report.
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  • Fabrication of defrost films using graphenes grown by chemical vapor deposition

    Byeong-Joo Lee   Goo-Hwan Jeong  

    We demonstrate the fabrication of defrost films using graphene-based transparent conductive films (TCF). The graphene was catalytically synthesized on Ni thin film using chemical vapor deposition method with methane source and transferred onto glass or flexible polymeric substrates. The fabricated graphene-based TCF showed moderate transmittance over 85% in visible range and low sheet resistance of 750?Ω/sq. with high reliability. We measured heating response of the films as a result of constant or cyclic voltage application to use as defrost films. The maximum temperature of the films increased up to 80?°C under the application of 40?V and the films showed high heating stability for 24?h. Finally, we confirmed that the frost formed on the surface of the graphene-based TCF can be clearly removed within 30?s under the application of 30?V. The results show high feasibility of the graphene-based TCF as defrost films in many areas.
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  • A Semi-Empirical Atomistic Approach in Materials Research

    Byeong-Joo Lee  

    With developments of semi-empirical interatomic potentials for realistic materials systems, atomistic approaches to a wide range of bulk and nanostructured materials become more and more feasible. This article outlines a recently developed semi-empirical interatomic potential model, the second nearest-neighbor modified embedded-atom method that shows a strong applicability to multicomponent systems. It is shown that the interatomic potentials can well reproduce fundamental physical properties of representative materials systems. Examples are used to illustrate the applications of the atomistic approach to calculation of fundamental physical properties of both nano and bulk structural materials such as thermodynamic, elastic, interface, and defect properties necessary to understand the materials behavior and to serve as input to larger-scale simulations.
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  • Method for fabricating 3D structure having hydrophobic surface by dipping method

    A method for fabricating a 3D (three-dimensional) structure is disclosed to provide hydrophobicity to a surface of a 3D structure by using a dipping method in which a predetermined-shaped structure is immersed in a molten metal solution. The method includes: immersing a predetermined-shaped structure in a molten metal solution to coat a molten metal material on the surface of the predetermined-shaped structure; anodizing a metal base coated with the molten metal material; coating a polymer material on an outer surface of the metal-coated base to form a negative replica structure; covering an outer surface of the negative replica structure with an outer formation material; and removing the metal-coated base from the negative replica structure and the outer formation material.
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  • Modified embedded-atom method interatomic potentials for Mg-Nd and Mg-Pb binary systems

    Ki-Hyun Kim   Byeong-Joo Lee  

    Abstract Interatomic potentials for the Mg-Nd and Mg-Pb binary systems have been developed within the framework of the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. The potentials describe a wide range of fundamental materials properties (thermodynamic, structural and elastic properties of compound and solution phases) of relevant systems in reasonable agreement with experimental data or first-principles and CALPHAD calculations. The applicability of the developed potentials to atomistic simulations on deformation behavior in Mg and its alloys is demonstrated by showing that the potentials reproduce related material properties reasonably and are transferable sufficiently. Highlights • 2NN MEAM potentials for Mg-Nd and Mg-Pb binary systems are developed. • The potentials describe well fundamental materials properties of the relevant Mg alloys. • The potentials can be utilized to investigate the deformation behavior of Mg alloys.
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  • Modified embedded-atom method interatomic potentials for pure Zn and Mg-Zn binary system

    Hyo-Sun Jang   Kyeong-Min Kim   Byeong-Joo Lee  

    Abstract Interatomic potentials for pure Zn and Mg–Zn binary system have been developed on the basis of the second nearest-neighbor modified embedded-atom method formalism. The potentials describe fundamental material properties of pure Zn (bulk, defect, and thermal properties) reasonably and reproduce the alloy behavior (thermodynamic, structural, and elastic properties of compounds and solution phases) of Mg-Zn alloys well in good agreement with experiments, first-principles and CALPHAD. The applicability of the developed potentials to atom-scale investigations on the slip behavior of Mg-Zn alloys is also demonstrated by showing that the calculated effects of Zn on the general stacking fault energy on the basal, prismatic and pyramidal planes are consistent with first-principles calculations. Highlights • 2NN MEAM potentials for pure Zn and Mg-Zn binary systems are developed. • The potentials describe fundamental material properties of the pure Zn and Mg-Zn alloys reasonably well. • The potentials can be utilized to atom-scale investigations of slip and other alloy behavior of Mg-Zn binary alloys.
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  • Structure modifications of vertically grown carbon nanotubes by plasma ion bombardment

    Byeong-Joo Lee   Eui-Chul Shin   Goo-Hwan Jeong  

    We report the structural modifications of vertically grown carbon nanotubes (VCNT) by plasma ion bombardments. The VCNT were grown by thermal chemical vapor deposition (CVD) using acetylene feedstock with iron catalyst and alumina supporting layers on silicon substrate. The plasma ion bombardments were performed using DC plasma enhanced CVD with parallel electrodes configuration. As a result, the height of the as-grown VCNT decreased with increasing applied bias voltages, plasma powers and working pressures. In addition, we observed the aggregated morphologies of the VCNT top surface after the plasma treatments which would be useful for field emission display and energy storage applications.
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  • Structure modifications of vertically grown carbon nanotubes by plasma ion bombardment

    Byeong-Joo Lee   Eui-Chul Shin   Goo-Hwan Jeong  

    We report the structural modifications of vertically grown carbon nanotubes (VCNT) by plasma ion bombardments. The VCNT were grown by thermal chemical vapor deposition (CVD) using acetylene feedstock with iron catalyst and alumina supporting layers on silicon substrate. The plasma ion bombardments were performed using DC plasma enhanced CVD with parallel electrodes configuration. As a result, the height of the as-grown VCNT decreased with increasing applied bias voltages, plasma powers and working pressures. In addition, we observed the aggregated morphologies of the VCNT top surface after the plasma treatments which would be useful for field emission display and energy storage applications.
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  • A modified embedded-atom method interatomic potential for the Fe–H system

    Byeong-Joo Lee   Je-Wook Jang  

    A modified embedded-atom method (MEAM) interatomic potential for the Fe-H binary system has been developed using previously developed MEAM potentials of Fe and H. The potential parameters were determined by fitting to experimental data on the dilute heat of solution of hydrogen in body-centered cubic (bcc) and face-centered cubic (fcc) Fe, the vacancy-hydrogen binding energy in bcc Fe, and to a first-principles calculation for the lattice parameter and bulk modulus of a hypothetical NaCl-type FeH. The potential accurately reproduces the known physical properties of hydrogen as an interstitial solute element in bcc and fcc Fe. The applicability of the potential to atomistic approaches for investigating interactions between hydrogen atoms and other defects such as vacancies, dislocations and grain boundaries, and also for investigating the effects of hydrogen on various deformation and mechanical behaviors of iron is demonstrated.[All rights reserved Elsevier].
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  • A modified embedded atom method interatomic potential for silicon

    Byeong-Joo Lee  

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