Creat membership Creat membership
Sign in

Forgot password?

Confirm
  • Forgot password?
    Sign Up
  • Confirm
    Sign In
home > search

Now showing items 33 - 48 of 61

  • Interatomic Potential of Li-Mn-O and Molecular Dynamics Simulations on Li Diffusion in Spinel Li(1-x)Mn(2)O4

    Lee, Eunkoo   Lee, Kwang-Ryeol   Lee, Byeong-Joo  

    An interatomic potential of the Li-Mn-O ternary system has been developed on the basis of the second-nearest-neighbor modified embedded-atom method(2NN MEAM) formalism combined with a charge equilibration (Qeq) concept. The potential reproduces fundamental physical properties (structural, elastic, thermodynamic and migration properties) of various compounds well, including lithium oxides, manganese oxides, and lithium manganese ternary oxides. Through molecular dynamics (MD) simulations using the developed potential, lithium diffusion properties (activation energy for lithium migration and diffusion coefficient) in spinel Li1-xMn2O4 are also reproduced in good agreement with experiments. We have found that the effect of the lithium vacancy concentration is marginal on the activation energy for lithium diffusion in the Li1-xMn2O4 cathode, but it is significant in the lithium diffusion coefficient. The potential can be further utilized for atomistic simulations of various materials phenomena (phase transitions, defect formation, lithiation/delithiation, etc.) in LIB cathode materials.
    Download Collect
  • Humidity effect on friction behaviors of nano-undulated diamond-like carbon films RID A-4224-2010 RID E-1242-2011

    Yi, Jin Woo   Park, Se Jun   Moon, Myoung-Woon   Lee, Kwang-Ryeol   Kim, Seock-Sam  

    Humidity dependency of friction behavior of nano-undulated diamond-like carbon (DLC) films was investigated by a home-made ball-on-disk type tribometer under controlled relative humidity of 0, 50, and 90%. Nano-undulated DLC films with surface roughness ranging from 0.2 to 13.4 nm were prepared by deposition of DLC film on the Si substrate with Ni nanodots. Friction coefficient of the flat DLC surface increased with the relative humidity, while that of the nano-undulated surfaces revealed smaller dependence on the relative humidity. When the surface roughness increased to 13.4 nm, friction behavior was observed to be independent of the relative humidity. The analysis of chemical composition and atomic bond structure of the debris and the transfer layer revealed that the humidity dependence on the nano-undulated surface was minimized by suppressing the graphitization of the transfer layer even with high concentration of Fe in the debris. (C) 2008 Elsevier B.V. All rights reserved.
    Download Collect
  • Precursor gas effect on the structure and properties of diamond-like carbon films

    Lee, Kwang-Ryeol   Baik, Young-Joon   Eun, Kwang Yong   Han, Seunghee  

    Download Collect
  • Long-Lasting Hydrophilicity on Nanostructured Si-Incorporated Diamond-Like Carbon Films RID A-4224-2010 RID E-1242-2011

    Yi, Jin Woo   Moon, Myoung-Woon   Ahmed, Sk Faruque   Kim, Haeri   Cha, Tae-Gon   Kim, Ho-Young   Kim, Seock-Sam   Lee, Kwang-Ryeol  

    We investigated the long-lasting hydrophilic behavior of a Si-incorporated diamond-like carbon (Si-DLC) film by varying the Si fraction in DLC matrix through oxygen and nitrogen plasma surface treatments. The wetting behavior of the water droplets on the pure DLC and Si-DLC with the nitrogen or oxygen plasma treatment revealed that the Si element in the oxygen-plasma-treated Si-DLC films played a major role in maintaining a hydrophilic wetting angle of < 10 degrees for 20 days in ambient air. The nanostructured patterns with a roughness of similar to 10 run evolved because of the selective etching of the carbon matrix by the oxygen plasma in the Si-DLC film, where the chemical component of the Si-Ox bond was enriched on the top of the nanopatterns and remained for over 20 days.
    Download Collect
  • Improvement of adhesion of DLC coating on nitinol substrate by hybrid ion beam deposition technique RID A-4224-2010

    Roy, Ritwik Kumar   Ahmed, Sk. Faruque   Yi, Jin Woo   Moon, Myoung-Woon   Lee, Kwang-Ryeol   Jun, Youngha  

    Diamond-like carbon (DLC) films were prepared for a protective coating on nitinol substrate by hybrid ion beam deposition technique with an acetelene as a source of hydrocarbon ions. An amorphous silicon (a-Si) interlayer was deposited on the substrates to ensure better adhesion of the DLC films followed by Ar ion beam treatment. The film thickness increased with increase in ion gun anode voltage. The residual stresses in the DLC films decreased with increase in ion gun anode voltage and film thickness, while the stress values were independent of the radio frequency (RF) bias voltage. The adhesion of the DLC film was improved by surface treatment with argon ion beam for longer time and by increasing the thickness of a-Si interlayer. (c) 2009 Elsevier Ltd. All rights reserved.
    Download Collect
  • High Performance Gas Diffusion Layer with Hydrophobic Nano layer under a Supersaturated Operation Condition for Fuel Cells

    Ko, Tae-Jun   Kim, Sae Hoon   Hong, Bo Ki   Lee, Kwang-Ryeol   Oh, Kyu Hwan   Moon, Myoung-Woon  

    Reliable operation of a proton exchange membrane fuel cell requires proper water management to prevent water flooding in porous carbon materials such as the gas diffusion layer (GDL). In contrast to the conventional GDL that uses the "wet" dip-coating process with solvent and expensive polytetrafluoroethylene, we have proposed a novel GDL with a controlled hydrophobic silicone (i.e., hexamethyldisiloxane) nanolayer by a highly efficient and cost-effective "dry" deposition process. The GDL with the nanolayer exhibited an increased contact angle, decreased contact angle hysteresis, and suppressed water condensation. Even though the GDL with the nanolayer had a higher electrical resistance than the pristine GDL, the cell performance of the GDL with an optimum nanolayer thickness of 8.6 nm was practically the same as that of the pristine GDL under normal operating conditions. Under a supersaturated condition, the GDL with optimum nanolayer thickness exhibited much higher cell performance than the pristine GDL over all current densities due to enhanced hydrophobicity. Long-term operational stability and dynamic response of the GDL with the nanolayer were much improved over those of the pristine GDL.
    Download Collect
  • Anisotropic rearrangement of the substrate atoms during Ar bombardment on Pd(0 0 1) surface RID G-9233-2011 RID E-1242-2011

    Kim, Sang-Pil   Kim, Byung-Hyun   Kim, Haeri   Lee, Kwang-Ryeol   Chung, Yong-Chae   Seo, Jikeun   Kim, Jae-Sung  

    Using a three-dimensional molecular dynamics (MD) simulation, we investigated the atomic scale rearrangement that occurs on a Pd(0 0 1) surface after energetic bombardment by Ar at room temperature. High energy Ar bombardment provoked the significant rearrangement of Pd atoms in a ballistic manner with a fourfold symmetric lateral distribution aligned along the < 1 1 0 > direction. The MD simulation of uniform Ar bombardment at normal incidence on a Pd surface reproduced the experimentally observed fourfold symmetric nano-scale surface structure. The present result supports that the ballistic rearrangement of the substrate atoms plays an important role in the ion induced surface structure evolution. (C) 2011 Elsevier B.V. All rights reserved.
    Download Collect
  • Measurements of to Temperatures of Supersaturated Si-As Alloys

    Lee, Kwang-Ryeol   West, Jeffrey A.   Smith, Patrick M.   Aziz, M. J.   Knapp, J. A.  

    Download Collect
  • Measurement of Friction Coefficients Between Diamond-Like Carbon Coated VCR Head Drum And VCR Tapes

    Lee, Kwang-Ryeol   Eun, Kwang Yong  

    Download Collect
  • A first-principles study on the bond characteristics in carbon containing Mo, Ag, or Al impurity atoms RID E-1242-2011

    Choi, Jung-Hae   Lee, Seung-Cheol   Lee, Kwang-Ryeol  

    The atomic bond characteristics between carbon and metal atoms are investigated by the first-principles calculation using a tetrahedron bond model. Both the charge density distribution of the highest occupied molecular orbital and the partial density of states were examined to reveal the bond characteristics. The chemical bond characteristics between Mo and carbon are nonbonding, which would reduce the bond directionality On the other hand, noble metals such as Ag show antibonding characteristic with carbon. The bond between Al and carbon has ionic characteristics and was found to prefer a two-dimensional configuration. The effect of metallic impurities in carbon materials could be explained from the difference in the atomic bond characteristics. (c) 2007 Elsevier Ltd. All rights reserved.
    Download Collect
  • Atomistic Observation of the Lithiation and Delithiation Behaviors of Silicon Nanowires Using Reactive Molecular Dynamics Simulations

    Jung, Hyun   Lee, Minho   Yeo, Byung Chul   Lee, Kwang-Ryeol   Han, Sang Soo  

    For the practical use of silicon nanowires (Si NWs) as anodes for Li-ion batteries, understanding their lithiation and delithiation mechanisms at the atomic level is of critical importance. Here, we report the mechanisms for the lithiation and delithiation of Si NWs determined using a large-scale molecular dynamics (MD) simulation with a reactive force field (ReaxFF). The ReaxFF is developed in this work using first-principles calculations. Our ReaxFF-MD simulation shows that an anisotropic volume expansion behavior of Si NWs during lithiation is dependent on the surface structures of the Si NWs; however, the volumes of the fully lithiated Si NWs are almost identical irrespective of the surface structures. During the lithiation process, Li atoms penetrate into the lattices of the crystalline Si (c-Si) NWs preferentially along the < 110 > or < 112 > direction, and then the c-Si changes into amorphous LixSi (a-LixSi) phases due to the simultaneous breaking of SiSi bonds as a result of the tensile stresses between Si atoms. Before the complete amorphization of the Si NWs, we observe the formation of silicene-like structures in the NWs that are eventually broken into low-coordinated components, such as dumbbells and isolated atoms. However, during delithiation of the LixSi NWs, we observe the formation of a small amount of c-Si nuclei in the a-LixSi matrix below a composition of Li1.4Si approximate to Li1.5Si, in which the volume fraction of formed c-Si phases relies on the delithiation rate. We also demonstrate that the two-phase structure can be thermodynamically more favorable than the single-phase a-LixSi. We expect that our comprehensive understanding of the lithiation and delithiation mechanisms along with the developed ReaxFF for Li-Si systems will provide helpful guidelines in designing Si anodes to obtain better performing Li-ion batteries.
    Download Collect
  • Optical properties of surface modified polypropylene by plasma immersion ion implantation technique RID A-4224-2010 RID E-1242-2011

    Ahmed, Sk Faruque   Moon, Myoung-Woon   Kim, Chansoo   Jang, Yong-Jun   Han, Seonghee   Choi, Jin-Young   Park, Won-Woong   Lee, Kwang-Ryeol  

    The optical band gap and activation energy of polypropylene (PP) induced by an Ar plasma immersion ion implantation technique were studied in detail. It was revealed that the structural alternation with an increase in polymer chain cross-linking in the ion beam affected layer enhanced the optical properties of PP. The optical band gap, calculated from the transmittance spectra, decreased from 3.44 to 2.85 eV with the Ar plasma ion energy from 10 to 50 keV. The activation energy, determined from the band tail of the transmittance spectra, decreased while the electrical conductivity increased with the Ar plasma ion energy. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3481417]
    Download Collect
  • First principles investigation of interaction between impurity atom (Si, Ge, Sn) and carbon atom in diamond-like carbon system

    Li, Xiaowei   Wang, Aiying   Lee, Kwang-Ryeol  

    The interaction between impurity atom (Si, Ge, and Sn) and carbon atom in diamond-like carbon (DLC) system was investigated by the first principles simulation method based on the density functional theory. The tetrahedral configuration was selected as the calculation model for simplicity. When the bond angle varied in a range of 90 degrees-130 degrees from the equivalent state of 109.471 degrees, the distortion energy and the electronic structures including charge density of the highest occupied molecular orbital (HOMO) and partial density of state (PDOS) in the different systems were calculated. The results showed that the addition of Si, Ge and Sn atom into amorphous carbon matrix significantly decreased the distortion energy of the system as the bond angles deviated from the equilibrium one. Further studies of the HOMO and PDOS indicated that the weak covalent bond between Si(Ge, Sn) and C atoms was formed with the decreased strength and directionality, which were influenced by the electronegative difference. These results implied that the electron transfer behavior at the junction of carbon nano-devices could be tailored by the impurity element, and the compressive stress in DLC films could be reduced by the incorporation of Si, Ge and Sn because of the formation of weaker covalent bonds. (C) 2012 Elsevier B.V. All rights reserved.
    Download Collect
  • Networked interpenetrating connections of icosahedra Effects on shear transformations in metallic glass RID E-1242-2011 RID A-3232-2010

    Lee, Mirim   Lee, Chang-Myeon   Lee, Kwang-Ryeol   Ma, Evan   Lee, Jae-Chul  

    The local structures of metallic glasses have been analyzed previously in term of various types of short range order (SRO) However, the SRO alone, neglecting the interconnection of neighboring icosahedra to medium range and beyond, is insufficient to account for the structure property relationship in metallic glasses In this study, we use molecular dynamics (MD) simulations of Cu-Zr binary metallic glasses to examine the effects of the next level of structural hierarchy the interpenetrating connection of icosahedra (ICOI) and the linkage of the medium-range ICOI patches to form networks of icosahedra over extended range The mechanical properties of these metallic glasses, especially the shear transformations that mediate plasticity, are found to be dependent on the degree of ICOI and development of the ICOI network The evolution of the ICOI network during shear deformation, as well as the composition dependence, has been monitored and discussed (C) 2010 Acta Materialia Inc Published by Elsevier Ltd All rights reserved
    Download Collect
  • Roles of an oxygen Frenkel pair in the photoluminescence of Bi3+-doped Y2O3: computational predictions and experimental verifications

    Choi, Heechae   Cho, So Hye   Khan, Sovann   Lee, Kwang-Ryeol   Kim, Seungchul  

    Bi3+ as a dopant in wide-band-gap yttria (Y2O3) has been used as a green light emission center or a sensitizer of co-doped rare earth elements. Because the photoluminescence (PL) properties of Y2O3:Bi3+ vary remarkably according to heat treatment, the roles of point defects have been an open question. By using first-principles calculations and thermodynamic modeling, we have thoroughly investigated the formation of point defects in Y2O3:Bi3+ at varying oxygen partial pressures and temperatures, as well as their rotes in PL. The photoabsorption energies of the Bi3+ dopant were predicted to be 3.1 eV and 3.4 eV for doping at the S-6 and the C-2 sites, respectively, values that are in good agreement with the experimental values. It was predicted that an oxygen interstitial (01) and an oxygen vacancy (V-O) are the dominant defects of Y2O3:Bi3+ at ambient pressure and an annealing temperature of 1300 K (3.19 x 10(16) cm(-3) for 1% Bi doping), and the concentrations of these defects in doped Y2O3 are approximately two orders of magnitude higher than those in undoped Y2O3. The defect V-O(2+) in Y2O3:Bi3+ was predicted to reduce the intensity of PL from Bi3+ at both S-6 and C-2 sites. We verify our computational predictions from our experiments that the stronger PL of both 410 and 500 nm wavelengths was measured for the samples annealed at higher oxygen partial pressure.
    Download Collect
  • Enhancement of electron field emission property with silver incorporation into diamondlike carbon matrix RID A-4224-2010 RID E-1242-2011

    Ahmed, Sk. Faruque   Moon, Myoung-Woon   Lee, Kwang-Ryeol  

    Effects of silver doping on the electron field emission properties of diamondlike carbon films deposited on silicon substrates by the rf reactive sputtering technique were studied in detail. It was found that the threshold field and effective emission barrier were reduced by Ag doping and the emission current strongly depends on the Ag doping percentage. The threshold field was found to decrease from 6.8 to 2.6 V/mu m with a variation of Ag at. % from 0 to 12.5. The field enhancement factor was calculated and we have explained the emission mechanism. (C) 2008 American Institute of Physics.
    Download Collect
1 2 3 4

Contact

If you have any feedback, Please follow the official account to submit feedback.

Turn on your phone and scan

Submit Feedback