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      • Erosion wear on Francis turbine components due to sediment flow

        Aslam Noon, A.,Kim, M.H. Elsevier Sequoia [etc.] 2017 Wear: An international journal on the science and Vol.378 No.-

        <P>Sediment particles flowing through the turbine components erode the surface in interaction. Erosive wear of hydro turbine components generally depends on different parameters such as concentration, size and shape of the sediments particle, velocity of flow, properties of the base material of the turbine components and operating hours of the turbine. Tarbela Dam Hydel Project (TDHP) located in the Himalayan range in Pakistan is facing the same problem. The sediments particle have caused damage to the plant equipment, mainly to the turbine components; stay vanes, guide vanes, runner and draft tube. As a result, these components are disassembled and refurbished almost every year. Analysis have been performed on one of the Francis turbine units to predict the effect of sediment particles concentration, size and shape on erosion rate. Gradual removal of the base material has changed the profiles of various components of the turbine and also has weaken its structure. One of the major concerns of these effects is the continuous loss of turbine hydraulic efficiency. The governing equations of fluid flow are solved numerically on an unstructured grid using FEM based software ANSYS CFX. Finnie erosion model is used to compute average erosion rates. Simulation results are compared with the actual site data. The CFD analysis showed good agreement with the results of experimental work done previously using similar kind of geometries and operating conditions. (C) 2017 Elsevier B.V. All rights reserved.</P>

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        In-situ photoisomerization for aggregation-induced emission of dibiphenyl fumaronitrile

        Jeong, H.,Kim, K.M.,Cho, S.,Lee, J.K. Elsevier Sequoia 2015 Journal of photochemistry and photobiology Chemist Vol.311 No.-

        The synthesis and photochemical characteristics of a fumaronitrile (FN)-based π-conjugated molecule, dibiphenyl fumaronitrile (BP-FN), comprising two biphenyl aggregation-induced emission (AIE) activators linked by an FN core were investigated. Upon illumination with a UV light, the emission from the Z-isomer, (Z)BP-FN, was more intense than that of the E-isomer, (E)BP-FN, due to conformational restriction of the former. (E)BP-FN exhibited J-aggregation behavior in mixed THF-H<SUB>2</SUB>O solvents when the H<SUB>2</SUB>O content exceeded 70vol%, illustrative of typical AIE characteristics. (Z)BP-FN presented significant aggregation causing quenching in emission when the H<SUB>2</SUB>O content exceeded 80vol%. In-situ photoisomerization of (E)BP-FN to (Z)BP-FN proceeded successfully even in the H<SUB>2</SUB>O-induced aggregated state in THF solution; however, the emission of the in-situ photoisomerized (Z)BP-FN aggregates was significantly quenched in solutions with high H<SUB>2</SUB>O contents of above 80 vol%.

      • Improving microstructure and ductility in the Mg-Zn alloy system by combinational Ce-Ca microalloying

        Langelier, B.,Nasiri, A.M.,Lee, S.Y.,Gharghouri, M.A.,Esmaeili, S. Elsevier Sequoia 2015 Materials science & engineering. properties, micro Vol.620 No.-

        A strategy is proposed to enhance the microstructure and mechanical properties of Mg-Zn alloys by combining microalloying additions of the rare earth element Ce and the non-rare earth element Ca. The double additions of Ce-Ca are found to significantly increase tensile elongation compared to binary Mg-Zn, or single additions of either Ce or Ca. Microstructure analysis reveals that the Ce-Ca additions increase ductility by modifying texture and refining grain size. Texture modification is attributed to solute effects from the microalloying elements, particularly Ca, while grain refinement is additionally influenced by a fine dispersion of Mg<SUB>6</SUB>Ca<SUB>2</SUB>Zn<SUB>3</SUB> precipitates that form during rolling and pin grain boundaries. The microalloying element additions also lead to large secondary phase particles in the alloys, which can limit ductility enhancement by promoting early fracture. By scaling Zn content in the Mg-Zn-Ce-Ca alloys, the Mg<SUB>6</SUB>Ca<SUB>2</SUB>Zn<SUB>3</SUB> phase fraction and Zn solute content can be controlled for optimum ductility or strengthening potential.

      • Integrated study of first principles calculations and experimental measurements for Li-ionic conductivity in Al-doped solid-state LiGe<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> electrolyte

        Kang, J.,Chung, H.,Doh, C.,Kang, B.,Han, B. Elsevier Sequoia 2015 Journal of Power Sources Vol.293 No.-

        Understanding of the fundamental mechanisms causing significant enhancement of Li-ionic conductivity by Al<SUP>3+</SUP> doping to a solid LiGe<SUB>2</SUB>(PO<SUB>4</SUB>)<SUB>3</SUB> (LGP) electrolyte is pursued using first principles density functional theory (DFT) calculations combined with experimental measurements. Our results indicate that partial substitution Al<SUP>3+</SUP> for Ge<SUP>4+</SUP> in LiGe<SUB>2</SUB>(PO<SUB>4</SUB>)<SUB>3</SUB> (LGP) with aliovalent (Li<SUB>1+x</SUB>Al<SUB>x</SUB>Ge<SUB>2-x</SUB>(PO<SUB>4</SUB>)<SUB>3</SUB>, LAGP) improves the Li-ionic conductivity about four-orders of the magnitude. To unveil the atomic origin we calculate plausible diffusion paths of Li in LGP and LAGP materials using DFT calculations and a nudged elastic band method, and discover that LAGP had additional transport paths for Li with activation barriers as low as only 34% of the LGP. Notably, these new atomic channels manifest subtle electrostatic environments facilitating cooperative motions of at least two Li atoms. Ab-initio molecular dynamics predict Li-ionic conductivity for the LAGP system, which is amazingly agreed experimental measurement on in-house made samples. Consequently, we suggest that the excess amounts of Li caused by the aliovalent Al<SUP>3+</SUP> doping to LGP lead to not only enhancing Li concentration but also opening new conducting paths with substantially decreases activation energies and thus high ionic conductivity of LAGP solid-state electrolyte.

      • Deep traps in GaN-based structures as affecting the performance of GaN devices

        Polyakov, A.Y.,Lee, I.H. Elsevier Sequoia 2015 Materials science & engineering. a review journal. Vol.94 No.-

        New developments in theoretical studies of defects and impurities in III-Nitrides as pertinent to compensation and recombination in these materials are discussed. New results on experimental studies on defect states of Si, O, Mg, C, Fe in GaN, InGaN, and AlGaN are surveyed. Deep electron and hole traps data reported for GaN and AlGaN are critically assessed. The role of deep defects in trapping in AlGaN/GaN, InAlN/GaN structures and transistors and in degradation of transistor parameters during electrical stress tests and after irradiation is discussed. The recent data on deep traps influence on luminescent efficiency and degradation of characteristics of III-Nitride light emitting devices and laser diodes are reviewed.

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        Organic light-Emitting diode based on bis(10-phenylanthracen-9-yl)benzene derivatives with a stable white emission

        Kim, Y.S.,Yoon, J.y.,Lee, H.W.,Kim, J.,Lee, S.E.,Lee, H.W.,Kim, Y.K.,Yoon, S.S. Elsevier Sequoia 2015 Journal of photochemistry and photobiology Chemist Vol.310 No.-

        In this work, we used the Suzuki cross-coupling reaction to synthesize the following two emitting materials: 1,3-bis(10-phenylanthracen-9-yl)benzene (1) and 2,6-bis(10-phenylanthracen-9-yl)pyridine (2). To investigate the electroluminescent (EL) properties of these materials, multilayered organic light-emitting diodes (OLEDs) devices were fabricated in the following sequence: indium-tin-oxide (ITO) (180nm)/4,4'-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) (50nm)/emitting materials (1 and 2) (40nm)/tris(8-hydroxyquinolinato)aluminium (Alq<SUB>3</SUB>) (15nm)/lithium quinolate (Liq) (2nm)/Al (100nm). Of special significance, a device using 1 as an emitting material showed a white emission with maximum luminance, luminous, power, and external quantum efficiency values of 1727cd/m<SUP>2</SUP>, 1.74cd/A, 0.78lm/W, and 0.67% at 20mA/cm<SUP>2</SUP>, respectively, as well as CIE coordinates of (0.31, 0.44) at 7V. Another device using 2 exhibited a sky-blue emission with maximum luminance, luminous, power, and external quantum efficiency values of 2279cd/m<SUP>2</SUP>, 1.95cd/A, 0.97lm/W, and 0.93% at 20mA/cm<SUP>2</SUP>, respectively as well as CIE coordinates of (0.24, 0.29) at 7V.

      • Rapid synthesis of an extra hard metal matrix nanocomposite at ambient temperature

        Ahn, B.,Zhilyaev, A.P.,Lee, H.J.,Kawasaki, M.,Langdon, T.G. Elsevier Sequoia 2015 Materials science & engineering. properties, micro Vol.635 No.-

        The strengthening of metals is essentially controlled by the microstructures of the metal solids and it is well understood that smaller grain sizes lead to higher hardness and increased strength. Nevertheless, true bulk nanostructured materials are difficult to produce using established engineering techniques, especially when considering the practical and societal needs of materials selection. Lightweight Al and Mg are conventional metals having excellent physico-chemical and mechanical properties and with good strength/weight ratios in the finished products. However, the fabrication of high-strength metals consisting of these elements, using mechanical alloying and milling and cladding-type metal working, generally involves long-term processing conducted under extreme conditions using special facilities. The present study demonstrates the very rapid synthesis of a metal matrix nanocomposite (MMNC) of the Al-Mg system which was achieved by stacking metal disks of the two pure metals and processing by high-pressure torsion at ambient temperature for 10 turns. An exceptionally high hardness was achieved, similar to many steels, through rapid stress-induced diffusion of Mg and the simultaneous formation of intermetallic nano-layers and a nanostructured intermetallic compound with a supersaturated solid solution. This unexpected result suggests a potential for simply and expeditiously fabricating a wide range of MMNCs.

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        Facile synthesis of InVO<sub>4</sub>/TiO<sub>2</sub> heterojunction photocatalysts with enhanced photocatalytic properties under UV-vis irradiation

        Perales-Martinez, I.A.,Rodriguez-Gonzalez, V.,Lee, S.W.,Obregon, S. Elsevier Sequoia 2015 Journal of photochemistry and photobiology Chemist Vol.299 No.-

        The InVO<SUB>4</SUB>/TiO<SUB>2</SUB> heterojunction system has been prepared by means of a practical impregnation method in alcoholic media. The obtained photocatalysts were characterized by several techniques, such as X-ray powder diffraction (XRD), UV-vis diffuse reflectance spectroscopy (DRS) and electron microscopy (SEM/TEM). Also, nitrogen adsorption-desorption isotherms were employed in order to determine the surface area (BET) and pore-size distribution (BJH) of the samples. We have observed that the addition of InVO<SUB>4</SUB> did not provide changes in the structural and textural properties of TiO<SUB>2</SUB> but substantially improved its photocatalytic properties. The best photocatalytic performance for the degradation of phenol was achieved for TiO<SUB>2</SUB> with 0.5wt.% loading of InVO<SUB>4</SUB>. From these results it can be inferred that the effective separation of the charge carriers produced an improvement in the photocatalytic performances of the InVO<SUB>4</SUB>/TiO<SUB>2</SUB> heterojunction photocatalysts. In the same way, a possible mechanism is discussed in order to explain the enhanced photoactivity under UV-vis irradiation.

      • Surface grain boundary engineering of Alloy 600 for improved resistance to stress corrosion cracking

        Telang, A.,Gill, A.S.,Tammana, D.,Wen, X.,Kumar, M.,Teysseyre, S.,Mannava, S.R.,Qian, D.,Vasudevan, V.K. Elsevier Sequoia 2015 Materials science & engineering. properties, micro Vol.648 No.-

        In this paper, we demonstrate a novel method for grain boundary engineering in Alloy 600 using iterative cycles of ultrasonic nanocrystal surface modification (UNSM) and strain annealing to modify the near surface microstructure (~250@?m) for improved stress corrosion cracking (SCC) resistance. These iterative cycles resulted in increased fraction of special grain boundaries whilst decreasing the connectivity of random grain boundaries in the altered near surface region. A disrupted random grain boundary network and a large fraction of low CSL boundaries (Σ3-Σ27) reduced the propensity to sensitization. Slow strain rate tests in tetrathionate solutions at room temperature show that surface GBE lowered susceptibility to intergranular SCC. Detailed analysis of cracks using Electron Back-scattered Diffraction showed cracks arrested at J1(1-CSL) and J2 (2-CSL) type of triple junctions. The probability for crack arrest, calculated using percolative models, was increased after surface GBE and explains the increase in resistance to SCC.

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        Enhancing the photoresponse of electrodeposited WO<sub>3</sub> film: Structure and thickness effect

        Kim, Y.O.,Yu, S.H.,Ahn, K.S.,Lee, S.K.,Kang, S.H. Elsevier Sequoia 2015 Journal of Electroanalytical Chemistry Vol.752 No.-

        Electrodeposited WO<SUB>3</SUB> thin films were prepared on the W, Ti, and Nb metal substrates in strongly acidic solution containing a tungsten precursor of (0.025M sodium tungstate dihydrate powder (Na<SUB>2</SUB>WO<SUB>4</SUB>.2H<SUB>2</SUB>O)) and by varying the applied potential. The applied potential determined the thickness and crystallite size of the deposited WO<SUB>3</SUB> thin films, irrespective of the metal substrate. The thickness and crystallite size of the films, as well as the total consumed electric charge (Q), increased as the applied potential was increased from -0.27 to -0.47V. Conversely, the photoelectrochemical (PEC) activity declined as the deposition potential increased; the optimal performance was achieved at a deposition potential of -0.27V for all metal substrates. This potential generated a porous WO<SUB>3</SUB> film or a very thin WO<SUB>3</SUB> layer composed of small nanoparticles, both of which were favorable for electrolyte penetration leading to enhanced charge transport/transfer behavior and providing a large contact area for the electrolyte. Furthermore, the PEC performance of WO<SUB>3</SUB> on the W substrate was higher than those on the Ti and Nb substrates because of the homogenous composition of the W substrate that resulted in the least lattice disturbance. Thus, the maximum photocurrent density of 1.68mA/cm<SUP>2</SUP> at 1.5V (vs. saturated Ag/AgCl) with an IPCE of 31% at 330nm was obtained with the electrodeposited WO<SUB>3</SUB> film grown at a deposition potential of -0.27V on the W substrate. The charge-transport and charge-transfer behavior of the electrodeposited WO<SUB>3</SUB> film were respectively discussed based on linear sweep voltammograms and electrochemical impedance spectroscopy.

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