Thermal metallurgical analysis of GMA welded AH36 steel using CFD-FEM framework

Cheon, J.,Kiran, D.V.,Na, S.J. Elsevier Ltd 2016 Materials & Design Vol.91 No.-

<P>A temporal combination of CFD mass and heat transfer, and FEM conductive heat transfer analysis was conducted using a proper temperature history implantation scheme. The phase distribution in an AH36 steel weldment was predicted and compared with experimental results. The numerical phase fraction estimation was performed using the critical austenite temperature model in the heating process as a function of heating rate. The CCT information based transformation starting and finishing temperature, and the maximum phase fraction models were utilized with instant cooling rate in the cooling process. The thermal analysis result agreed well with the FZ shape and measured temperature history. The calculated hardness slightly overestimated the measured hardness. The steep reduction of hardness in the HAZ and the tempered zone was much more affected by the change in austenite critical temperature than the cooling rate. Based on the potential results of this work, predicting weldment deformation by considering phase transformation will be extended. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Evaluation of compressibility and small strain stiffness characteristics of sand reinforced with discrete synthetic fibers

Choo, H.,Yoon, B.,Lee, W.,Lee, C. Elsevier Applied Science Publishers, Ltd ; Elsevie 2017 Geotextiles and geomembranes Vol.45 No.4

<P>This experimental investigation evaluates the compressibility and small strain stiffness of sand reinforced with discrete synthetic fibers. Varying fiber contents (FC), fiber aspect ratios (AR), and void ratios were selected as testing variables in this study, and the modified oedometer tests were conducted to measure the compression index (C-c) and maximum shear modulus (Gmax) of fiber-reinforced sand. The results of this study demonstrate that the Cc of the tested fiber-reinforced sand increases with an increase in FC because the packing of sand grains in the fiber-reinforced sand is very loose due to a disruption of direct contact between the sand grains due to the presence of long discrete fibers. Additionally, this disruption of direct contact between sand grains due to the fibers results in a reduction of interparticle contact and coordination number between sand grains. Therefore, the Gmax of tested fiber-reinforced sand decreases with an increase in FC. Most notably, the Gmax of the tested fiber-reinforced sand with varying FC and AR can be expressed as a single function of the void ratio at a given applied stress, which implies that the inclusion of fibers just alters the packing state of sand grains, and the interparticle contact stiffness is mainly determined by the contacts between sand grains. (C) 2017 Elsevier Ltd. All rights reserved.</P>

Joining of metal-ceramic using reactive air brazing for oxygen transport membrane applications

Raju, K.,Muksin,Kim, S.,Song, K.s.,Yu, J.H.,Yoon, D.H. Elsevier Ltd 2016 Materials & Design Vol.109 No.-

<P>This study examined the joining of dense Ce0.9Gd0.1O2 (-) (delta)-La0.6Sr0.4Co0.2Fe0.8O3 (-) (delta) (GDC-LSCF) ceramics to high temperature metal alloys for the fabrication of multilayered oxygen transport membrane (OTM) stacks. Reactive air brazing using a silver-based paste was performed at 1050 degrees C for 30 min in air to join GDC-LSCF/high temperature alloys, such as AISI 310S, Inconel 600 and Crofer 22 APU. The effects of the various filler materials, including CuO, GDC, LSCF, and GDC-LSCF mixture, in the Ag paste were also examined. The Ag-10 wt% CuO braze filler ensured in a reliable and compact joining without the formation of cracks and voids at the joining interface, while the addition of other ceramic fillers resulted in incomplete joining. Although none of the GDC-LSCF/metal alloy joints showed gas leakage at room temperature, the GDC-LSCF/Crofer joint only maintained the gas-tightness up to 800 degrees C under pressurized air up to 7 bars, which was explained by the microstructural rigidness of the oxide layer formed on the filler/alloy interface at high temperatures. This was supported by the minimal decrease in shear strength of the GDC-LSCF/Crofer joint, which was 91.1 and 88.3 MPa for the as-brazed and isothermally aged joint at 800 degrees C for 24 h in air, respectively. (C) 2016 Published by Elsevier Ltd.</P>

CoSn(OH)₆ hybridized with anionic and cationic graphenes as a new high-capacity anode for lithium ion batteries

Richard Prabakar, S.J.,Han, S.C.,Jeong, J.,Sohn, K.S.,Pyo, M. Elsevier Ltd 2017 Materials & Design Vol.118 No.-

<P>Herein is the first use of a CoSn(OH)(6)/graphene composites as a high-performance anode in lithium ion batteries (LIBs). CoSn(OH)(6) alone is found to be electrochemically active and to possess electrochemical properties that are superior to those displayed in its dehydrated form (CoSnO3). In contrast to the formation of 3Li(2)O in CoSnO3 during the first discharge (CoSnO3 + 6Lit(+) 6e(-) -> Co + Sn + 3Li(2)O), the production of 6 mol of LiOH in CoSn(OH)(6) (CoSn(OH)(6) + 6Li(+) + 6e(-) -> Co + Sn + 6LiOH) seems to provide nanometric Co/Sn particles with more efficient reversibility for the subsequent Li+-insertion/de-insertion via conversion/alloy reactions. The electrochemical performance of CoSn(OH)(6) is further improved when composited with graphenes. This is accomplished by electrostatically combining negatively charged Co2+/Sn2+-anchored graphene oxide (GO) with positively charged amine-functionalized graphene (GN) in a solution. The subsequent hydrothermal reaction produces CoSn(OH)(6) nanocubes that are tightly held by graphene sheets (GO/CS/GN). The GO/CS/GN delivers unprecedentedly high capacity and excellent cyclability (discharge capacities of 1475 mAh.g(-1) for the 100th charge/discharge (C/D) cycles at a 0.1 A.g(-1)). In contrast to the comparison materials (CoSnO3/graphene composites) the rate performance is also remarkable, delivering a capacity of 650 mAh.g(-1) at 2.0 at A.g(-1). (C) 2017 Published by Elsevier Ltd.</P>

Design principle of super resolution near-field structure using thermally responsive optical phase change materials for nanolithography applications

Park, G.,Lee, J.,Kang, S.,Kim, M.,Kang, S.,Choi, W. Elsevier Ltd 2016 Materials & Design Vol.102 No.-

<P>The super resolution near-field structure (Super-RENS) which is composed of a thin layer of a thermally responsive optical phase change material (PCM) between two dielectric layers, can be a means of resolving the limited resolution of the laser beam for direct laser lithography. In Super-RENS, incident laser irradiation induces the direct, reversible opening and closing of a nanoaperture in the PCM layer, and a nanoscale pattern is realized in the lithography system. Here, we first introduce the complete modeling procedures and optimization methodology for Super-RENS in nanolithography based on a rigorous analysis of near-field structure, thermal analysis in the finite-element method, and analysis of the corresponding feature size on the photoresist (PR) layer. Multiple combinations of the PCM layer and the two dielectric layers with varying dimensions are considered as design parameters to achieve the required resolution in the nanolithography system. The feasible line profiles are investigated at the general operating conditions of the pulsed laser beam, based on varying dimensions of the PCM layer (5-30 nm) and the two dielectric layers (10-200 nm). This work will provide a detailed methodology for the design and optimization of the Super-RENS for applications in the nanolithography system. (C) 2016 Elsevier Ltd. All rights reserved.</P>

Microstructural and kinetic investigation on the suppression of grain growth in nanocrystalline copper by the dispersion of silicon carbide nanoparticles

Akbarpour, M.R.,Farvizi, M.,Kim, H.S. Elsevier Ltd 2017 Materials & Design Vol.119 No.-

<P>In this paper, the thermal stability and grain growth kinetics of nanocrystalline Cu, reinforced with SiC nanoparticles and obtained using a mechanical milling process, were investigated during isothermal annealing. The presence of the nanoparticles in the nanocrystalline copper matrix resulted in a significant decrease in grain growth, the formation of partially textured microstructure and twin boundaries at higher temperatures, and an increase in the volume fraction of recrystallized grains, as estimated by grain orientation spread, in comparison to unreinforced Cu during annealing. The lower volume fraction of recrystallized grains at higher temperatures was attributed to dynamic recovery. Normal grain growth was observed in the annealing range of 400-600 degrees C, and significant abnormal grain growth was observed at higher temperatures. An analysis of the grain growth kinetics in the temperature range of 400-600 degrees C revealed a time exponent of n approximate to 3.6 and activation energy of approximate to 34 kJ mol(-1), based on the parabolic equation. The calculated activation energy for grain growth in the SiC dispersion strengthened Cu was found to be less than that of nanocrystalline Cu. The low activation energy and high thermal stability were attributed to high lattice strain and the retarding effect of nanoparticles by the Zener mechanism. (C) 2017 Elsevier Ltd. All rights reserved.</P>

Molecular simulation for thermoelectric properties of c-axis oriented hexagonal GeSbTe model clusters

Vora-ud, A.,Rittiruam, M.,Kumar, M.,Han, J.G.,Seetawan, T. Elsevier Ltd 2016 Materials & Design Vol.89 No.-

<P>Using a combination of molecular orbital and molecular dynamics simulations, electronic and thermoelectric properties of GeSbTe model clusters are presented. The unit cells of Ge13Sb20Te52, Ge7Sb12Te40 and Ge14Sb6Te26 model clusters are designed corresponding to GeSb2Te4, GeSb4Te7 and Ge2Sb2Te5 compositions in hexagonal phase, oriented in the c-axis direction. The electronic structures of clusters have been simulated by discrete-variational molecular orbital calculation using Hartree-Fock-Slater approximation to determine the electrical conductivity and Seebeck coefficients in Mott expression. For thermal properties, molecular dynamics simulations have been employed on clusters in amorphous, cubic and hexagonal phases using Verlet's algorithm and subsequently using Green-Kubo relation for lattice thermal conductivity. We assumed inter-atomic interaction, defined by the Morse-type potential function added to Busing-Ida potential function, which considers partial electronic charges on the ions, bond length of the cation-anion pair, and depth and shape of the potential. Based on our simulations, detailed variation of electrical conductivity, carrier thermal conductivity, lattice thermal conductivity, Seebeck coefficients, power factor and figure of merit, are presented as a function of temperature in 300-700 K range. Thermoelectric parameters obtained in present study were compared and explained with those of experimentally results of Ge2Sb2Te5 composition in hexagonal phase. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Fluorination of free lithium residues on the surface of lithium nickel cobalt aluminum oxide cathode materials for lithium ion batteries

Kim, H.,Lee, K.,Kim, S.,Kim, Y. Elsevier Ltd 2016 Materials & Design Vol.100 No.-

<P>Lithium nickel cobalt aluminum oxide (LNCA) cathode materials for lithium ion batteries (LIBs) were treated with NH4FHF to convert free Li2CO3 and LiOH, which are natively present on the LNCA surface, into a stable LiF layer. Gas evolution caused by a side-reaction between the reactive free lithium compounds and electrolyte was greatly reduced using this method. The pH of the water dispersion of LNCA was also lower; this indicates that LiOH, which causes rapid polymerization of the polyvinylidene fluoride binder in the slurry for electrode casting, was effectively eliminated. No significant decrease of the electrochemical performance of LNCA was observed with the treatment. Designed as a one-step simple wet process, the developed method is expected to be practically applicable for mass production with suppressed swelling of LIBs and stabilization of the electrode fabrication process. (C) 2016 Elsevier Ltd. All rights reserved.</P>

Mechanical and high temperature wear properties of extruded Al composite reinforced with Al₁₃Fe₄ CMA nanoparticles

Nemati, N.,Emamy, M.,Penkov, O.V.,Kim, J.,Kim, D.E. Elsevier Ltd 2016 Materials & Design Vol.90 No.-

<P>The mechanical and tribological properties of extruded aluminum matrix composites reinforced with various weight percentages (1, 3, 5, 7, 10 wt.%) of Al13Fe4 complex metallic alloys (CMAs) were investigated. The nano-composites were produced using conventional powder metallurgy and a hot extrusion process. The tribological behavior of the composites was investigated under normal loads in the range of 20-80 N using a reciprocating high-temperature tribo-tester over a temperature range of 25-350 degrees C. At an optimized reinforcing agent concentration of 5 wt.%, the composite showed a significant enhancement in Young's modulus (similar to 108 MPa) and hardness (similar to 1.85 GPa). The lowest coefficient of friction of 0.1 was attained at a temperature of 250 degrees C with a reinforcing agent concentration of 5 wt.%. Also, the wear rate was reduced by a factor of similar to 25 compared to the unreinforced aluminum specimen. The significant improvement in the tribological properties of the nanocomposite was attributed to the enhanced mechanical properties due to severe plastic deformation incurred during the extrusion process and incorporation of well distributed CMA nanoparticles in the matrix which provided oobstacles for dislocation motion. Detailed microstructural analyses revealed that incorporation of the second phase to the Al matrix led to microstructure refinement and increased the hardness up to similar to 2 GPa. Furthermore, the nanoparticles aided in the formation of hard and temperature-resistant tribo-layers which reduced the wear rate of the composite (Al-5 wt.% Al13Fe4) down to 1.5 x 10(-4) at 250 degrees C. (C) 2015 Elsevier Ltd. All rights reserved.</P>

New results on passivity-based H_~ control for networked cascade control systems with application to power plant boiler-turbine system

Mathiyalagan, K.,Park, J.H.,Sakthivel, R. Elsevier Ltd 2015 NONLINEAR ANALYSIS HYBRID SYSTEMS Vol.17 No.-

<P>This paper is concerned with the problem of passivity-based H-infinity controller design for a class of networked cascade control systems (NCCSs) with random packet dropouts. The NCCS under consideration is modeled by using state feedback controllers and the network-induced imperfections like packet dropouts and time-varying delays. The model is defined with a stochastic packet-dropout case by using the Bernoulli distributed white sequence with time-varying probability measures. The probability-dependent conditions for stabilization of NCCSs are established to guarantee the resulting closed-loop system to be stochastically stable and achieve a prescribed mixed H-infinity and passivity performance. The Lyapunov stability theory and linear matrix inequality (LMI) approach are used to derive criteria for the existence of the state feedback controllers. The proposed probability-dependent gain scheduled controller can be designed by solving the convex optimization problem by means of a set of LMIs, which can be easily solved by using some standard numerical packages. Finally, a practical application is presented to illustrate the effectiveness and potential of the proposed results. (C) 2015 Elsevier Ltd. All rights reserved.</P>