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.

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.

Fabrications and evaluations of hydrogen permeation on Al₂O₃/CeO₂/graphene (ACG) composites membrane by Hot Press Sintering (HPS)

Lee, N.R.,Lee, S.S.,Kim, K.I.,Kim, W.G.,Ju, H.,Kim, D.M.,Hong, T.W. Pergamon Press ; Elsevier Science Ltd 2013 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.38 No.18

Ceramic membrane has high permeation rate of hydrogen and chemical stability. Al<SUB>2</SUB>O<SUB>3</SUB> indicates stable at high temperature and a relatively large surface area. In addition, Al<SUB>2</SUB>O<SUB>3</SUB> of porous is used as hydrogen separation membranes support, because of the high hydrogen permeability based on Knudsen diffusion mechanism. In this work, metal alkoxides as starting materials was used Aluminum isopropoxide powder. Then CeO<SUB>2</SUB> as catalyst at the partial oxidation and Graphene Oxide as electrical conductivity are added, Al<SUB>2</SUB>O<SUB>3</SUB>-CeO<SUB>2</SUB>-Graphene oxide (ACG) composites are synthesized by sol-gel process. ACG membrane was fabricated type of disk by Hot Press Sintering. A synthesized ACG composite material was characterized by EDS, SEM, TG/DTA, XRD, BET. The hydrogen permeation property of ACG membrane was measured by Sievert's type hydrogen permeation membrane equipment. The hydrogen permeability of ACG membranes was examined at RT-673 K under increasing pressure. Results, hydrogen permeability was obtained to 2.62 x 10<SUP>-7</SUP> mol/ms Pa<SUP>½</SUP> at 673 K under 0.3 MPa.

Effects of syngas addition on flame propagation and stability in outwardly propagating spherical dimethyl ether-air premixed flames

Song, W.S.,Jung, S.W.,Park, J.,Kwon, O.B.,Kim, Y.J.,Kim, T.H.,Yun, J.H.,Keel, S.I. Pergamon Press ; Elsevier Science Ltd 2013 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.38 No.32

Experiments in outwardly propagating spherical flame were carried out to investigate unstretched laminar burning velocity and flame instability by adding 25%, 50%, and 75% syngas to DME-air mixtures at room temperature and elevated pressures up to 0.3 MPa. The measured unstretched laminar burning velocities were compared to numerical predictions using PREMIX code with Zhao reaction mechanism and good agreement was found between them. Flame instability was also investigated through evaluating Markstein length and cellular instability. Behavior of the Markstein lengths was described well by the deficient reactant Lewis number and highly affected by the amount of syngas addition to the DME-air mixtures. Effects of syngas addition and increased initial pressure on cell formation on the flame surface were also examined through evaluating the Lewis number, flame thickness, and thermal expansion ratio. Regardless of syngas addition, the cellular instability was enhanced mainly by the hydrodynamic instability due to decreased flame thickness while diffusional-thermal instability was minor.

Impact of biogenic volatile organic compounds on ozone production at the Taehwa Research Forest near Seoul, South Korea

Kim, S.Y.,Jiang, X.,Lee, M.,Turnipseed, A.,Guenther, A.,Kim, J.C.,Lee, S.J.,Kim, S. Pergamon Press ; Elsevier [distribution] 2013 Atmospheric environment Vol.70 No.-

The importance of biogenic volatile organic compounds (BVOCs) in understanding of air-quality and climate on regional to global scales has been highlighted in a number of modeling and observational studies. At the same time, another important emerging research topic in atmospheric chemistry is the regional and global impacts of fast growing East Asian megacities. These two research topics must be integrated in order to adequately understand and address air quality challenges emerging from Eastern Asian megacities surrounded by planted or natural forest areas. We present initial measurement results for May, June and September 2011 from the Taehwa Research Forest (TRF) which has been developed to serve as a long term observatory for investigating biosphere-atmosphere interactions at the edge of the Seoul Metropolitan Area (population of ~23.5 million). The comprehensive measurement datasets of ozone and its precursors such as CO, NO<SUB>x</SUB>, SO<SUB>2</SUB> and VOCs shows that high ozone episodes in the suburban site could not be explained by just anthropogenic pollutants alone. In addition, isoprene (C<SUB>5</SUB>H<SUB>8</SUB>) and monoterpenes (C<SUB>10</SUB>H<SUB>16</SUB>) were observed as two of the most important OH chemical sinks inside of the forest canopy. In order to understand the impacts of these BVOCs on ozone and related photochemistry, we conducted model sensitivity simulations using a coupled meteorology-chemistry model (WRF-Chem) for conditions including with and without BVOC emissions. The modeling results suggest that BVOC emissions could enhance regional daytime ozone production from 5 to 20 ppbv. The observed temporal variations in ozone correspond well with the variations in BVOCs, which likely reflects the influence of BVOCs on ozone formation. These findings strongly suggest that interactions between anthropogenic pollutants and BVOCs must be understood and quantified in order to assess photochemical ozone formation in the regions surrounding East Asian megacities.

Minimization of hot spot in a microchannel reactor for steam reforming of methane with the stripe combustion catalyst layer

Jeon, S.W.,Yoon, W.J.,Baek, C.,Kim, Y. Pergamon Press ; Elsevier Science Ltd 2013 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.38 No.32

Hot spot formation is inevitable in a heat exchanger microchannel reactor used for steam reforming of methane because of the local imbalance between the generated and absorbed heat. A stripe configuration of the combustion catalyst layer was suggested to make the catalytic combustion rate uniform in order to minimize the hot spot near the inlet. The stripe configuration was optimized by response surface methodology with computational fluid dynamics. With the optimal catalyst layer, the hot spot was not observed near the inlet and the maximum temperature decreased by 130 K from that of the uniform catalyst layer without any conversion loss. The maximum relative particle diameters of the uniform and the optimal stripe catalyst layer were about 3.68 and 2.51, respectively, and the surface-averaged particle diameter of the optimal stripe catalyst layer was 7.64% less than that of the uniform stripe catalyst layer.

Systems biology and biotechnology of Streptomyces species for the production of secondary metabolites

Hwang, K.S.,Kim, H.U.,Charusanti, P.,Palsson, B.O.,Lee, S.Y. Pergamon Press ; Elsevier Science Ltd 2014 Biotechnology advances Vol.32 No.2

Streptomyces species continue to attract attention as a source of novel medicinal compounds. Despite a long history of studies on these microorganisms, they still have many biochemical mysteries to be elucidated. Investigations of novel secondary metabolites and their biosynthetic gene clusters have been more systematized with high-throughput techniques through inspections of correlations among components of the primary and secondary metabolisms at the genome scale. Moreover, up-to-date information on the genome of Streptomyces species with emphasis on their secondary metabolism has been collected in the form of databases and knowledgebases, providing predictive information and enabling one to explore experimentally unrecognized biological spaces of secondary metabolism. Herein, we review recent trends in the systems biology and biotechnology of Streptomyces species.

Two selective growth modes for graphene on a Cu substrate using thermal chemical vapor deposition

Song, W.,Jeon, C.,Youn Kim, S.,Kim, Y.,Hwan Kim, S.,Lee, S.I.,Jung, D.S.,Jung, M.W.,An, K.S.,Park, C.Y. Pergamon Press ; Elsevier Science Ltd 2014 Carbon Vol.68 No.-

Here we provide evidence of two selective growth modes, namely the 'surface adsorption (SA) mode' and the 'diffusion and precipitation (DP) mode' for the synthesis of graphene on Cu foil by thermal chemical vapor deposition. Using acetylene feedstock, the number of graphene layers was controlled simply by adjusting the injection time, and the DP growth mode was clearly verified by the existence of a carbon-diffused Cu layer with expansion of the Cu lattice. With methane feedstock, either SA or DP growth modes could be selected for the growth of graphene at low or high partial pressure of carbon feedstocks, respectively. The critical pressure for switching the growth modes depends on reactivity of carbon feedstock to Cu substrate.

Isothermal exfoliation of graphene oxide by a new carbon dioxide pressure swing method

Lee, S.Y.,Park, S.J. Pergamon Press ; Elsevier Science Ltd 2014 Carbon Vol.68 No.-

Graphene oxide (GO) was prepared by a modified Hummers' method. GO was modified using a simple CO<SUB>2</SUB> pressure swing technique to obtain exfoliated GO. The microcrystalline structures and morphologies were characterized using the X-ray diffraction and scanning electron microscope/transmission electron microscope measurements. The textural properties were investigated by N<SUB>2</SUB> (77K) adsorption/desorption isotherms. CO<SUB>2</SUB> (298K, 30bar) and H<SUB>2</SUB> (298K, 100bar) adsorption experiments were conducted to investigate their adsorption behaviors. The results indicated that the best sample had a specific surface area of 547m<SUP>2</SUP>/g and total pore volume of 2.468cm<SUP>3</SUP>/g. According to the results, CO<SUB>2</SUB> pressure swing method can be used to increase the efficiency of exfoliation and expansion of the graphitic interlayers in GO.

Synthesis and properties of an atomically thin carbon nanosheet similar to graphene and its promising use as an organic thin film transistor

Joh, H.I.,Lee, S.,Kim, T.W.,Hwang, S.Y.,Hahn, J.R. Pergamon Press ; Elsevier Science Ltd 2013 Carbon Vol.55 No.-

We synthesize an atomically thin carbon nanosheet (CNS) analogous to graphene with properties suitable for an organic thin film transistor (OTFT). The synthesis of graphene by chemical vapor deposition has serious drawbacks such as wrinkles, grain boundaries, and defects due to catalyst removal and transfer process. Here the CNS is directly synthesized on a silicon wafer by heat-treatment of spin-coated polyacrylonitrile and shows a higher electrical conductivity (>1600Scm<SUP>-1</SUP>) than that of chemically converted graphene. The CNS on glass, transferred from a silicon wafer, exhibits approximately 92% optical transmittance. We have used our CNS as the electrodes of OTFTs, and recorded a mobility (0.25-0.35cm<SUP>2</SUP>V<SUP>-1</SUP>s<SUP>-1</SUP>) that exceeds that of gold electrodes (0.2-0.25cm<SUP>2</SUP>V<SUP>-1</SUP>s<SUP>-1</SUP>).