The effect of microstructural inhomogeneity on the growth paths of surface-cracks in copper processed by equal channel angular pressing

Goto, M.,Ando, Y.,Han, S.Z.,Kim, S.S.,Kawagoishi, N.,Euh, K. Pergamon Press ; Elsevier Science Ltd 2010 Engineering fracture mechanics Vol.77 No.11

The growth behavior of cracks is monitored on specimens of ultrafine grained copper produced by equal channel angular pressing. Temporary retardation of crack growth under low stress amplitudes occurs when the crack length reaches about 0.1mm, but there is no similar retardation at high stress amplitudes. Dependent on stress amplitude, different crack growth path morphologies develop. Analysis of the fracture surfaces is conducted by scanning electron microscopy, showing planer, granular and striated surfaces. The physical background of growth path and fracture surface formation is discussed by considering crack growth mechanism and microstructural inhomogeneity.

Orbital hybridization mechanism for the enhanced photoluminescence in edge-functionalized sp² carbon clusters

Kang, B.,Choi, Y.,Kim, B.S.,Youn, I.S.,Lee, G. Pergamon Press ; Elsevier Science Ltd 2016 Carbon Vol.109 No.-

<P>Using the first principles methods, we performed systematic study on the effect of edge-functional groups on the electronic energy levels and the optical properties of sp(2) carbon clusters. It is found that the intrinsic pi and pi* orbitals are weakly altered by oxygen-bearing functional groups, but it is significantly disrupted by pyrrolic groups. Thereby the oscillator strength of the lowest-energy transition is found to be much stronger for the pyrrolic group functionalized cluster than for the carboxyl group. From our results being consistent with the experimental reports, we suggest that the photoluminescence enhancement is caused by a perturbation of the intrinsic, frontier molecular orbitals by edge groups. (C) 2016 Elsevier. Ltd. All rights reserved.</P>

Effects of the degree of polymerization on the structure of sodium silicate and aluminosilicate glasses and melts: An ¹⁷O NMR study

Lee, S.K.,Stebbins, J.F. Pergamon Press ; Elsevier Science Ltd 2009 Geochimica et cosmochimica acta Vol.73 No.4

Revealing the atomic structure and disorder in oxide glasses, including sodium silicates and aluminosilicates, with varying degrees of polymerization, is a challenging problem in high-temperature geochemistry as well as glass science. Here, we report <SUP>17</SUP>O MAS and 3QMAS NMR spectra for binary sodium silicate and ternary sodium aluminosilicate glasses with varying degrees of polymerization (Na<SUB>2</SUB>O/SiO<SUB>2</SUB> ratio and Na<SUB>2</SUB>O/Al<SUB>2</SUB>O<SUB>3</SUB> ratio), revealing in detail the extent of disorder (network connectivity and topological disorder) and variations of NMR parameters with the glass composition. In binary sodium silicate glasses [Na<SUB>2</SUB>O-k(SiO<SUB>2</SUB>)], the fraction of non-bridging oxygens (NBOs, Na-O-Si) increases with the Na<SUB>2</SUB>O/SiO<SUB>2</SUB> ratio (k), as predicted from the composition. The <SUP>17</SUP>O isotropic chemical shifts (<SUP>17</SUP>O δ<SUB>iso</SUB>) for both bridging oxygen (BO) and NBO increase by about 10-15ppm with the SiO<SUB>2</SUB> content (for k=1-3). The quadrupolar coupling products of BOs and NBOs also increase with the SiO<SUB>2</SUB> content. These trends suggest that both NBOs and BOs strongly interact with Na; therefore, the Na distributions around BOs and NBOs are likely to be relatively homogenous for the glass compositions studied here, placing some qualitative limits on the extent of segregation of alkali channels from silica-enriched regions as suggested by modified random-network models. The peak width (in the isotropic dimension) and thus bond angle and length distributions of Si-O-Si and Na-O-Si increase with the SiO<SUB>2</SUB> content, indicating an increase in the topological disorder with the degree of polymerization. In the ternary aluminosilicate glasses [Na<SUB>2</SUB>O]<SUB>x</SUB>[Al<SUB>2</SUB>O<SUB>3</SUB>]<SUB>1-x</SUB>SiO<SUB>2</SUB>, the NBO fraction decreases while the Al-O-Si and Al-O-Al fractions apparently increase with increasing Al<SUB>2</SUB>O<SUB>3</SUB> content. The variation of oxygen cluster populations suggests that deviation from ''Al avoidance'' is more apparent near the charge-balanced join (Na/Al=1). The Si-O-Si fraction, which is closely related to the activity coefficient of silica, would decrease with increasing Al<SUB>2</SUB>O<SUB>3</SUB> content at a constant mole fraction of SiO<SUB>2</SUB>. Therefore, the activity of silica may decrease from depolymerized binary silicates to fully polymerized sodium aluminosilicate glasses at a constant mole fraction of SiO<SUB>2</SUB>.

An efficient decal transfer method using a roll-press to fabricate membrane electrode assemblies for direct methanol fuel cells

Mehmood, A.,Ha, H.Y. Pergamon Press ; Elsevier Science Ltd 2012 International journal of hydrogen energy Vol.37 No.23

This study has focused on the development of a roll-press based decal transfer method to fabricate membrane electrode assemblies (MEAs) for direct methanol fuel cells (DMFCs). This method exhibits an outstanding transfer rate of catalyst layers from substrates to the membrane, despite hot-pressing at a considerably lower pressure and for a much shorter duration than the flat-press based conventional decal method. The MEA produced by a roll-press (R-MEA) delivers an excellent single-cell performance with power densities more than 30% higher than that fabricated using a flat-press (F-MEA). The new method considerably improves catalyst active sites in both electrodes and renders a high cathode porosity. The superior pore structure of the cathode makes the R-MEA more efficient in terms of performance and operation stability under lower air stoichiometries. Moreover, MEAs can be prepared in a continuous mode using this new method due to the unique design of the roll-press. All these advantages demonstrate the superiority of this method over the conventional flat-press decal method and make it suitable for use in the commercial manufacturing of MEAs for direct methanol fuel cells.

Real-time measurement of submicron aerosol particles having a log-normal size distribution by simultaneously using unipolar diffusion charger and unipolar field charger

Park, D.,Kim, S.,An, M.,Hwang, J. Pergamon Press ; Elsevier Science Ltd 2007 Journal of aerosol science Vol.38 No.12

Recently, Park et al. [(2007). Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution. Journal of Aerosol Science, 38, 420-430] have introduced a methodology for performing simple and fast measurements of submicron aerosol particles having a log-normal size distribution, using a unipolar diffusion charger, an electrometer, and a condensation particle counter (CPC). The methodology can be applied to particles of 30-700nm and requires an assumption of their geometric standard deviation in size. In this paper we propose a much cheaper but faster method which involves substituting a unipolar field charger and another electrometer for the CPC. With the data obtained using this dual-charger system, we developed a data inversion algorithm and estimated the particle size distribution by minimizing the differences between the measured aerosol currents and the calculated values. To compare the size distribution with the data measured using a scanning mobility particle sizer (SMPS), sodium chloride (NaCl) particles smaller than 0.1μm in diameter, and dioctyl sebacate (DOS) particles with a diameter of 0.1-0.7μm, were used. The estimated results for the NaCl and DOS particles were within 10% of the data measured with the SMPS, while a 33% deviation from the SMPS results was obtained in Park et al. [(2007). Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution. Journal of Aerosol Science, 38, 420-430]. Furthermore, the detection time obtained with the use of our dual-charger system was faster (<3s) than the 5s obtained by Park et al. [(2007). Development and performance test of a unipolar diffusion charger for real-time measurements of submicron aerosol particles having a log-normal size distribution. Journal of Aerosol Science, 38, 420-430].

The effect of heating rate on porosity production during the low temperature reduction of graphite oxide

Yang, S.J.,Kim, T.,Jung, H.,Park, C.R. Pergamon Press ; Elsevier Science Ltd 2013 Carbon Vol.53 No.-

We studied the relationship between the thermal conditions and the evolution of the porosity in thermally reduced graphite oxide (TH-rGO) and found that the heating rate, rather than reduction temperature and atmospheric condition, played a crucial role in the evolution of porosity during the thermal reduction of GO at low temperatures. Higher heating rates increased the porosity of the TH-rGO. A slow heating rate facilitated the evolution of H<SUB>2</SUB>O and CO<SUB>2</SUB> whereas a higher heating rate released CO<SUB>2</SUB> and CO gases with the concurrent development of a folded and crumpled morphology. We further demonstrated that the higher heating rate resulted in a highly porous texture with lower reduction temperature (below 140<SUP>o</SUP>C) and shorter reduction time (less than 5min), indicating that the reduction time and temperature are found to be dependent on the heating rate.

Contribution of small phytoplankton to total primary production in the Chukchi Sea

Lee, S.H.,Sun Yun, M.,Kyung Kim, B.,Joo, H.,Kang, S.H.,Keun Kang, C.,Whitledge, T.E. Pergamon Press ; Elsevier Science Ltd 2013 Continental shelf research Vol.68 No.-

Given a projection of thriving small phytoplankton in the Arctic Ocean under climate-induced environmental changes, it is important to estimate the contribution of small phytoplankton (0.7-5μm) to the total primary production in the Chukchi Sea, which is an important conduit of organic matter from the North Pacific to the Arctic Ocean. Based on a <SUP>13</SUP>C-<SUP>15</SUP>N dual isotope tracer technique, small phytoplankton productivity measurements were taken during two consecutive cruises in the Chukchi Sea in 2004. The total phytoplankton carbon uptake rates ranged from 0 to 25.38mgCm<SUP>-3</SUP>h<SUP>-1</SUP>, whereas the uptake rates of small phytoplankton ranged from 0 to 2.87mgCm<SUP>-3</SUP>h<SUP>-1</SUP>. In comparison with the carbon uptake rates, total phytoplankton nitrate uptake rates ranged from 0 to 4.40mgNm<SUP>-3</SUP>h<SUP>-1</SUP> while small phytoplankton nitrate uptake rates ranged from 0 to 0.39mgNm<SUP>-3</SUP>h<SUP>-1</SUP>. Ammonium uptake rates ranged from 0 to 8.34mgNm<SUP>-3</SUP>h<SUP>-1</SUP> and from 0.01 to 2.18mgNm<SUP>-3</SUP>h<SUP>-1</SUP>, for total and small phytoplankton, respectively. Small phytoplankton contributed 24.80% (S.D.=+/-23.0%) to the total chlorophyll-a concentration, and 59.41% (S.D.=+/-52.12%) to the total carbon biomass due to its higher particulate organic carbon per chlorophyll-a unit during the two cruises in 2004. In the Chukchi Sea, the average contributions of small phytoplankton to carbon and total nitrogen (nitrate+ammonium) uptake rates were 31.72% (S.D.=+/-23.59%) and 37.31% (S.D.=+/-26.06%), respectively.

Quantitative analysis of hysteretic reactions at the interface of graphene and SiO₂ using the short pulse I-V method

Lee, Y.G.,Kang, C.G.,Cho, C.,Kim, Y.,Hwang, H.J.,Lee, B.H. Pergamon Press ; Elsevier Science Ltd 2013 Carbon Vol.60 No.-

Unstable characteristics of graphene field effect transistors (FETs) have generated concerns about the feasibility of graphene electronic devices. Two dominant mechanisms of instability, charge trapping and interfacial redox reaction, and their quantitative contributions were investigated for chemical vapor deposited graphene by analyzing the transient responses of the hysteretic characteristics in microseconds to milliseconds range. In contrast to previous reports emphasizing the role of the interfacial redox reaction, we have found that charge trapping at the interface is responsible for 78-87% of the hysteresis and that the interfacial redox reaction at the graphene/SiO<SUB>2</SUB> interface contributes only 13-22%. Systematic analysis on the temperature and ambient dependence of instability suggest that graphene FETs can operate more reliably with a proper passivation to create an oxygen deficient environment.

Automatic pose-independent 3D garment fitting

Lee, Y.,Ma, J.,Choi, S. Pergamon Press ; Elsevier Science Ltd 2013 Computers & graphics Vol.37 No.7

Given a virtual garment model on a reference human model, we propose an automated 3D garment fitting system that fits the garment model to a target human model. The proposed method can transfer garment models between human models without any user guidance even when the reference and target human models have different poses. Our goal is not to resize or deform the original garment model according to the target human model but to yield realistic fitting results of the given garment on the target human models. Using pose-independent segmentation and cloth simulation, we achieve realistic and automatic fitting results in reasonable running time. Our method can replace the time-consuming manual fitting process that is necessary for many applications that use virtual garments, such as games, animations, CAD tools and online clothing stores.

Effects of CrN/Cr coating layer on durability of metal bipolar plates under a fuel recirculation system of direct methanol fuel cells

Park, Y.C.,Lee, S.H.,Kim, S.K.,Lim, S.,Jung, D.H.,Choi, S.Y.,Kim, J.H.,Peck, D.H. Pergamon Press ; Elsevier Science Ltd 2013 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.38 No.25

Effects of a CrN/Cr coating layer on the durability of metal bipolar plates (stainless steel (STS) 430) are investigated in direct methanol fuel cells (DMFCs) with a fuel recirculation system, since under fuel recirculation the metal bipolar plates can be faced with a tougher corrosion environment. Before the fuel recirculation, the performance losses of the cells consisting of metal bipolar plates are ascribed to cathode degradation, due to a high corrosion level of the cathode side. However, after fuel recirculation, corrosion of the anode metal bipolar plate by pH decrease and overpotential increase brings about severe anode degradation. It is found that damage by corrosion of the cathode metal bipolar plate is limited to degradation of the cathode catalyst, whereas corrosion of the anode metal bipolar plate deteriorates not only the catalysts but also the electrolyte membrane. These durability tests show that the CrN/Cr coatings deposited on the STS 430 improve the corrosion resistance of the metal substrate and lead to low performance degradation.