Design optimization of ink in electrohydrodynamic jet printing: Effect of viscoelasticity on the formation of Taylor cone jet

Yu, M.,Ahn, K.H.,Lee, S.J. Elsevier Ltd 2016 Materials & Design Vol.89 No.-

<P>The formation of Taylor cone jet is a key process in electrohydrodynamic printing that is used to produce high resolution patterns. Even though the inks are complex fluids composed of particles, binder, and solvent, most of previous researches have assumed the ink as a Newtonian fluid. In this study, we investigate the effect of viscoelasticity of the ink on Taylor cone jet using two model systems designed to control the elasticity and viscosity of the ink independently. The elasticity and viscosity improve the stability by expanding the operating windows for Taylor cone jet. The results can be summarized in terms of two parameters: elasticity parameter, xi, and viscosity parameter, chi. The increase in elasticity widens the range of voltage for Taylor cone jet zone, while the range of flow rate remains independent of elasticity. The effect of elasticity is dominant for xi>1 while it is nearly negligible for xi<1. When the viscosity is increased, the Taylor cone jet zone is widened mainly by the flow rate when chi<1, while the voltage stabilizes the Taylor cone jet for chi>1. This study will contribute to the optimal design of EHD printing ink by providing the operating window maps in terms of material properties. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Mechanical behavior of ultrafine-grained high-Mn steels containing nanoscale oxides produced by powder technology

Jeon, J.,Nam, S.,Kang, S.,Shin, J.,Choi, H. Elsevier Ltd 2016 Materials & Design Vol.92 No.-

<P>A new class of ultrafine-grained high-Mn steels containing nanoscale oxides has been developed by spark plasma sintering of ball-milled powders. During spark-plasma sintering, nanoscale manganese oxides were generated in Fe-15Mn steel, while nanoscale aluminum oxides were produced in Fe-15Mn-3A1-3Si steel because of the high affinity of aluminum for oxygen. Ultrafine-grained high-Mn steels that contain nanoscale oxides exhibited superior strength without significant loss of toughness, owing to the combined effects of grain refinement, twinning-induced plasticity (TWIP), and dispersion of nanoscale oxides. These new materials have potential for application in powder metallurgy components used in the automotive industry, such as gear sets, connecting rods, and bearing caps, which require high surface hardness as well as good core toughness. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Study of pulsed-DC sputtering induced Ge₂Sb₂Te₅ thin films using facile thermoelectric measurement

Kumar, M.,Vora-ud, A.,Seetawan, T.,Han, J.G. Elsevier Ltd 2016 Materials & Design Vol.98 No.-

<P>Thermoelectric measurement is an exhaustive exercise for the case of thin films, requiring meticulous attention to the thermal contact interfaces and the instrumentation. Usually, different set-ups are combined for the temperature dependent measurement of different thermoelectric key quantities. Here, a facile 6-probe measurement set-up is presented, which can measure Seebeck coefficients and electrical properties of thin films in the temperature range of 300 K-600 K. Using this set-up, the thermoelectric properties of Ge2Sb5Te5 thin films, prepared with pulsed DC magnetron sputtering method are studied. The effects of working pressure, post-deposition thermal treatment and variation of film thickness on the microstructure, surface, electrical and thermoelectric properties are investigated systematically. Plasma diagnostics, performed using optical emission spectroscopy provided the information about various radicals' excitations and the electron temperature. Microstnictural studies show the phase transformation from amorphous to metastable cubic phase. FESEM study exhibits highly dense films with uniform grains compactness. It has been found that lowering average crystallite size by optimum electron temperature and pressure conditions governs the enhancement in Seebeck coefficient. The throughput of process >400 nm/min, and obtained Seebeck coefficients values 271.50 mu V/K are highly promising for industrial utilization. (C) 2016 Elsevier Ltd. All lights reserved.</P>

Surrogate-based Pareto optimization of annealing parameters for severely deformed steel

Ghiabakloo, H.,Lee, K.,Kazeminezhad, M.,Kang, B.S. Elsevier Ltd 2016 Materials & Design Vol.92 No.-

<P>Severe plastic deformation (SPD) is a metalworking technique that is used for the enhancement of the strength and hardness of metallic materials. As SPD causes ductility deterioration, materials typically necessitate annealing for ductility increase; however, annealing may conversely affect strength and hardness. Thus, to optimally balance strength, hardness, and ductility, this study determined annealing conditions with a severely deformed low carbon steel sheet by adjusting annealing time and temperature. For the facilitation of the annealing process optimization, measurements of strength, hardness, and ductility under various annealing conditions were represented by regression Kriging. Then, because of the conflicting nature of the desired metal properties, a set of optimal annealing conditions was identified by Pareto multi-objective optimization. Finally, the best combination on the Pareto front was selected with TOPSIS. The results of Pareto optimization with regression Kriging showed that the best candidates for annealing conditions can be determined at a significantly reduced experimental cost. (C) 2015 Published by Elsevier Ltd.</P>

Resistance of coal bottom ash mortar against the coupled deterioration of carbonation and chloride penetration

Jang, J.G.,Kim, H.J.,Kim, H.K.,Lee, H.K. Elsevier Ltd 2016 Materials & Design Vol.93 No.-

<P>This paper investigated the resistance of bottom ash mortar against the coupled deterioration of carbonation and chloride penetration. To evaluate the resistance of the mortar, a comparative study was conducted with variables of ordinary mortar, lightweight mortar using expanded shale, fly ash cement mortar, and slag cement mortar. Test results showed that a combination of carbonation and chloride penetration accelerated the rate of chloride penetration into the mortar regardless of the mortar type. The replacement of sand with bottom ash moderated the penetration depth of chloride under coupled-deterioration environments, while the overall penetrated-chloride contents showed no significant differences. It can be concluded in a coupled-deterioration environment that the chloride resistance of the bottom ash mortar was greater than that of both ordinary mortar and lightweight mortar. The type of cement paste on the surface of a carbonated mortar had a governing impact on the chloride penetration. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Robust fault-tolerant control for power systems against mixed actuator failures

Kaviarasan, B.,Sakthivel, R.,Kwon, O.M. Elsevier Ltd 2016 NONLINEAR ANALYSIS HYBRID SYSTEMS Vol.22 No.-

<P>This paper employs linear matrix inequality (LMI) based optimization algorithm to develop a method for designing fault-tolerant state feedback controller with mixed actuator failures for power systems subject to random changes. Meanwhile, the random abrupt changes are determined by a finite set Markov chain so the considered system is equivalently represented as a discrete-time Markov jump linear system (MJLS). Further, due to the variations of loading conditions in power system, an uncertainty term is incorporated to MJLS. For the proposed system, we construct a novel actuator fault model containing both linear and nonlinear terms which is more general than the conventional actuator fault models. The main purpose of this paper is to design the robust fault-tolerant controller such that for all possible actuator failures, time-varying delays and admissible parameter uncertainties, the closed-loop uncertain discrete-time MJLS is robustly stochastically stable. Based on free-weighting matrix approach and linear matrix inequality theory, anew set of sufficient conditions that guaranteeing the robust stochastic stability is presented by choosing an appropriate Lyapunov-Krasovskii functional candidate. In addition, a single-machine infinite-bus (SMIB) power system is considered as an application example and its simulation results demonstrate the effectiveness of the proposed design techniques. (C) 2016 Elsevier Ltd. All rights reserved.</P>

Development of Mo@?Ni@?Si@?B metallic glass with high thermal stability and H versus E ratios

Kim, J.,Kyeong, J.S.,Ham, M.H.,Minor, A.M.,Kim, D.H.,Park, E.S. Elsevier Ltd 2016 Materials & Design Vol.98 No.-

<P>We report a novel Mo-Ni-Si-B metallic glass which can be solidified into fully amorphous state by melt-spinning process, with high crystallization onset temperature of over 1100 K, extremely high Vickers hardness of 27.5 +/- 2.2 GPa and relatively low Young's modulus of 364.3 +/- 6.6 GPa. The dense cluster-packing model suggests that the addition of boron up to 10 at.% can occupy vacant cluster-interstices of (Mo, Ni)-Si cluster arrays, which results in a more efficiently dense-packed cluster structure, destabilizes the formation of nanocrystalline phases, and systematically increases the glass-forming ability (GFA) in Mo-Ni-Si-B alloys, The GFA parameters that do not directly rely on T-g, such as Delta T and epsilon parameter, show greater reliability to evaluate GFA for Mo-Ni-Si-B metallic glass exhibiting no clear T-g. The H/E and H-2/(2E) ratios of the newly developed Mo-Ni-Si-B metallic glass, which reflect wear resistance and resilience, exhibit the highest values among various hard ceramic materials as well as metallic glass-forming alloys developed up to now. These advantages of Mo-Ni-Si-B metallic glass can be used more widely to forma high temperature wear-resistant coating layer on various substrates. Furthermore, the same idea might be used to form a metallic glass-nitride nanocomposite coating layer by reactive deposition in N-2 ambient, with highly lubricative properly and high wear-resistance, especially at high temperature. (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>

Growth and residual stresses in the bonded compliant seal of planar solid oxide fuel cell: Thickness design of window frame

Jiang, W.,Zhang, Y.C.,Zhang, W.Y.,Luo, Y.,Woo, W.,Tu, S.T. Elsevier Ltd 2016 Materials & Design Vol.93 No.-

<P>Bonded compliant seal (BCS) is a new sealing method for planar solid oxide fuel cell. The BCS design uses a thin foil to bond the cell and window frame, which generates a multilayer structure. However, the high temperature bonding generates large residual stresses that greatly affect the fracture. This paper presents a numerical method and neutron diffraction measurement to study the residual stress, and effect of window frame thickness has been discussed. A grain boundary diffusion model incorporated with a power-law creep constitutive model is developed to calculate the growth stress in the oxide film. Then, the thermal elasto-plastic finite element method is applied to calculate the thermal stress. A neutron diffraction experiment is performed to measure the through thickness stresses. A good agreement is found between the calculation results and the neutron diffraction measurements. Compressive stress is generated in the oxide scale because of the substrate constraint. Furthermore, a competition exists between the generation of growth stress and the creep relaxation in the oxide layer. The residual stresses in the oxide layer decrease with the decrease in the substrate thickness. The thicknesses of the window frame and foil are designed to be 500 and 50 mu m, respectively. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Formation mechanism of typical onion ring structures and void defects in friction stir lap welded dissimilar aluminum alloys

Yoon, T.J.,Yun, J.G.,Kang, C.Y. Elsevier Ltd 2016 Materials & Design Vol.90 No.-

<P>The formation mechanism for typical onion ring structure and void defect with heat input during FSLW was continuously visualized by an exit-hole continuous observation technique. Based on this result, the compatibility between microstructure, microtexture, element maps and strain maps using electron backscattered diffraction (EBSD) with the chemical indexing assisted by EDS analysis was simultaneously investigated. The results revealed that the threaded probe was significantly correlated to typical onion ring structure and the onion structure formed as soon as it touched the probe. This result is different from the results so far. On the other hand, the remnant of original interface between top and bottom plates after FSLW and asymmetrical flow around rotating tool were significantly correlated to the formation of void defect in low heat input condition. (C) 2015 Elsevier Ltd. All rights reserved.</P>