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Open access publications by faculty, postdocs, and graduate students in the Department of Mechanical Engineering.
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Item A parallel fictitious domain method for the interface-resolved simulation of particle-laden flows and its application to the turbulent channel flow(Hong Kong Polytechnic University, Department of Civil and Structural Engineering, 1/20/16) Yu,Zhaosheng; Lin,Zhaowu; Shao,Xueming; Wang,Lian-Ping; Zhaosheng Yu, Zhaowu Lin, Xueming Shao , Lian-Ping Wang; Wang, Lian-PingA parallel direct-forcing (DF) fictitious domain (FD) method for the simulation of particulate flows is reported in this paper. The parallel computing strategies for the solution of flow fields and particularly the distributed Lagrange multiplier are presented, and the high efficiency of the parallel code is demonstrated. The new code is then applied to study the effects of particle density (or particle inertia) on the turbulent channel flow. The results show that the large-scale vortices are weakened more severely, and the flow friction drag increases first and then reduces, as particle inertia is increased.Item Composite Membrane Based on Graphene Oxide Sheets and Nafion for Polymer Electrolyte Membrane Fuel Cells(Electrochemical Society, 2014-10-29) Wang, Liang; Kang, Junmo; Nam, Jae-Do; Suhr, Jonghwan; Prasad, Ajay K.; Advani, Suresh G.; Liang Wang, Junmo Kang, Jae-Do Nam, Jonghwan Suhr, Ajay K. Prasad and Suresh G. Advani; Wang, Liang; Prasad, Ajay K.; Advani, Suresh G.A composite membrane for fuel cell applications was prepared by incorporating custom-made graphene oxide (GO) in Nafion resin. The GO was used to provide mechanical reinforcement to Nafion. Transmission electron microscopy confirmed the formation of highly crystalline and individually-dispersed graphene oxide sheets. Tensile strength, water uptake, swelling, proton conductivity and electrical conductivity of the composite membranes were measured and compared with pure Nafion. The polarization curves indicated that the fuel cell performance of the 3wt% GO/Nafion composite membrane was similar to that of the pure Nafion membrane, but the composite membrane was superior to Nafion in terms of mechanical properties.Item A Precise, Reduced-Parameter Model of Thin Film Electrolyte Impedance(The Electrochemical Society, 2015-03-07) McNealy, Benjamin E.; Jiang, Jun; Hertz, Joshua L.; Benjamin E. McNealy, Jun Jiang, and Joshua L. Hertz; McNealy, Benjamin E.; Jiang, Jun; Hertz, Joshua L.he extreme shape factors inherent in characterizing thin film electrolytes can present a challenge to quantitative interpretation f impedance spectra. Here, the impedance of a thin film ceramic electrolyte with surface microelectrodes is modeled via direct umerical solution of current conservation. Faradaic and non-faradaic currents at the electrode-electrolyte interface are modeled phe- omenologically using a formulation based on the Butler-Volmer equation. The model is able to reproduce complex, experimentally btained impedance spectra for Pt/YSZ and Pt/GDC cells using only four adjustable, physically intuitive parameters: electrolyte onductivity, permittivity, exchange current density, and double layer capacitance. Equivalent circuit models typically used to fit hese spectra instead require six or more adjustable parameters with ambiguous physical meaning. Notably, the model described here s able to capture a heretofore unexplained intermediate frequency arc seen in the experimental results. A parametric study enables he mechanism of the intermediate frequency feature to be identified as a spreading resistance in the electrolyte that vanishes at high requencies due to low-impedance dielectric transport of current across the electrode-electrolyte interface. The fitting results are validated by comparison of the parameter values with literature reports.Item TiO2 enhanced ultraviolet detection based on a graphene/Si Schottky diode(Royal Society of Chemistry., 2015-03-11) Zhu, Miao; Zhang, Li; Li, Xinming; He, Yijia; Li, Xiao; Guo, Fengmei; Zang, Xiaobei; Wang, Kunlin; Xie, Dan; Li, Xuanhua; Wei, Bingqing; Zhu, Hongwei; Miao Zhu, Li Zhang, Xinming Li, Yijia He, Xiao Li, Fengmei Guo, Xiaobei Zang, Kunlin Wang, Dan Xie, Xuanhua Li, Bingqing Wei and Hongwei Zhu; Wei, BingqingGraphene/Si has been proved to form a quality Schottky junction with high photoelectric conversion efficiency at AM 1.5. However, for the ultraviolet portion of the incident light, the photoelectric performance will degrade significantly due to severe absorption and recombination at the front surface. Herein, to realize enhanced ultraviolet detection with a graphene/Si diode, TiO2 nanoparticles (NPs, 3–5 nm) are synthesized and spin-coated on the graphene surface to improve the photoresponse in the ultraviolet region. According to our results, the conversion efficiency of the graphene/Si diode at 420 nm and 350 nm increases by 72.7% and 100% respectively with TiO2 coating. Then C−2–V measurements of both TiO2 and graphene/Si diode are performed to analyze the electronic band structure of the TiO2/graphene/Si system, based on which we finally present the enhancement mechanism of photodetection using TiO2 NPs.Item An all-copper plasmonic sandwich system obtained through directly depositing copper NPs on a CVD grown graphene/copper film and its application in SERS(Royal Society of Chemistry, 2015-05-11) Li, Xuanhua; Ren, Xingang; Zhang, Yongxing; Choy, Wallace C. H.; Wei, Bingqing; Xuanhua Li, Xingang Ren, Yongxing Zhang, Wallace C. H. Choy and Bingqing Wei; Wei, BingqingA simple, low-cost, all-copper sandwich system has been obtained through directly depositing Cu nanoparticles (NPs) onto a graphene sheet, which has already been grown on a Cu foil (Cu-NGF). The new design inherits two key advantages: (1) the materials of the NGF coupling system are composed of only cheaper Cu instead of Au and Ag, (2) direct fabrication of the system without transferring graphene will greatly lower the fabrication cost. More importantly, the Cu-NFG system shows a high sensitivity in surface-enhanced Raman scattering (SERS) with the highest enhancement factor (EF, over 1.89 × 107) reported to date in Cu plasmonic systems. Experimental and theoretical results reveal that the strong EF is mainly because of the strong near-field coupling between Cu NPs and Cu films at the optimal angle of incidence, opening up a new route for Cu materials in SERS applications.Item Synthesis of ultralong MnO/C coaxial nanowires as freestanding anodes for high-performance lithium ion batteries(Royal Society of Chemistry, 2015-05-20) Wang, Jian-Gan; Zhang, Cunbao; Jin, Dandan; Xie, Keyu; Wei, Bingqing; Jian-Gan Wang, Cunbao Zhang, Dandan Jin, Keyu Xie and Bingqing Wei; Wei, BingqingA facile synthesis strategy is reported for the preparation of a freestanding membrane of ultralong MnO/C coaxial nanowires using a novel in situ interfacial polymerization technique. The MnO/C membrane possesses interconnected porous structures with a nanowire diameter of ca. 100 nm and a length of up to hundreds of micrometers. When used as a freestanding anode for lithium ion batteries, the coaxial MnO/C nanocomposites exhibit a high reversible capacity of 832 mA h g−1 at a current density of 100 mA g−1 after 100 cycles, good rate capability and outstanding cycling stability with a specific capacity of 480 mA h g−1 being retained after 600 cycles at a high current density of 1000 mA g−1. The uniform carbon coating formed along the ultralong one-dimensional nanostructure surface is the key-enabling factor that not only improves the electrode reaction kinetics, but also renders excellent cycling performance by accommodating the large volume variation of MnO during charge/discharge processes. The superior electrochemical properties suggest that the facile synthesis strategy can be extended to the fabrication of other freestanding films for potential application in energy storage systems.Item Inhibition of T-Type Voltage Sensitive Calcium Channel Reduces Load-Induced OA in Mice and Suppresses the Catabolic Effect of Bone Mechanical Stress on Chondrocytes(PLOS (Public Library of Science), 2015-05-26) Srinivasan, Padma P.; Parajuli, Ashutosh; Price, Christopher; Wang, Liyun; Duncan, Randall L.; Kirn-Safran, Catherine B.; Padma P. Srinivasan, Ashutosh Parajuli, Christopher Price, Liyun Wang, Randall L. Duncan, Catherine B. Kirn-Safran; Srinivasan, Padma P.; Parajuli, Ashutosh; Price, Christopher; Wang, Liyun; Duncan, Randall L.; Kirn-Safran, Catherine B.Voltage-sensitive calcium channels (VSCC) regulate cellular calcium influx, one of the earliest responses to mechanical stimulation in osteoblasts. Here, we postulate that T-type VSCCs play an essential role in bone mechanical response to load and participate in events leading to the pathology of load-induced OA. Repetitive mechanical insult was used to induce OA in Cav3.2 T-VSCC null and wild-type control mouse knees. Osteoblasts (MC3T3- E1) and chondrocytes were treated with a selective T-VSCC inhibitor and subjected to fluid shear stress to determine how blocking of T-VSCCs alters the expression profile of each cell type upon mechanical stimulation. Conditioned-media (CM) obtained from static and sheared MC3T3-E1 was used to assess the effect of osteoblast-derived factors on the chondrocyte phenotype. T-VSCC null knees exhibited significantly lower focal articular cartilage damage than age-matched controls. In vitro inhibition of T-VSCC significantly reduced the expression of both early and late mechanoresponsive genes in osteoblasts but had no effect on gene expression in chondrocytes. Furthermore, treatment of chondrocytes with CM obtained from sheared osteoblasts induced expression of markers of hypertrophy in chondrocytes and this was nearly abolished when osteoblasts were pre-treated with the T-VSCC-specific inhibitor. These results indicate that T-VSCC plays a role in signaling events associated with induction of OA and is essential to the release of osteoblast-derived factors that promote an early OA phenotype in chondrocytes. Further, these findings suggest that local inhibition of T-VSCC may serve as a therapy for blocking load-induced bone formation that results in cartilage degenerationItem Fast and stable redox reactions of MnO2/CNT hybrid electrodes for dynamically stretchable pseudocapacitors(Royal Society of Chemistry, 2015-06-04) Gu, Taoli; Wei, Bingqing; Taoli Gu and Bingqing Wei; Gu, Taoli; Wei, BingqingPseudocapacitors, which are energy storage devices that take advantage of redox reactions to store electricity, have a different charge storage mechanism compared to lithium-ion batteries (LIBs) and electric double-layer capacitors (EDLCs), and they could realize further gains if they were used as stretchable power sources. The realization of dynamically stretchable pseudocapacitors and understanding of the underlying fundamentals of their mechanical–electrochemical relationship have become indispensable. We report herein the electrochemical performance of dynamically stretchable pseudocapacitors using buckled MnO2/CNT hybrid electrodes. The extremely small relaxation time constant of less than 0.15 s indicates a fast redox reaction at the MnO2/CNT hybrid electrodes, securing a stable electrochemical performance for the dynamically stretchable pseudocapacitors. This finding and the fundamental understanding gained from the pseudo-capacitive behavior coupled with mechanical deformation under a dynamic stretching mode would provide guidance to further improve their overall performance including a higher power density than LIBs, a higher energy density than EDLCs, and a long-life cycling stability. Most importantly, these results will potentially accelerate the applications of stretchable pseudocapacitors for flexible and biomedical electronics.Item Design, Operation, Control, and Economics of a Photovoltaic/Fuel Cell/Battery Hybrid Renewable Energy System for Automotive Applications(MDPI AG, 2015-06-09) Whiteman, Zachary S.; Bubna, Piyush; Prasad, Ajay K.; Ogunnaike, Babatunde A.; Zachary S. Whiteman, Piyush Bubna, Ajay K. Prasad and Babatunde A. Ogunnaike; Whiteman, Zachary S.; Bubna, Piyush; Prasad, Ajay K.; Ogunnaike, Babatunde A.Meeting rapidly growing global energy demand—without producing greenhouse gases or further diminishing the availability of non-renewable resources—requires the development of affordable low-emission renewable energy systems. Here, we develop a hybrid renewable energy system (HRES) for automotive applications—specifically, a roof-installed photovoltaic (PV) array combined with a PEM fuel cell/NiCd battery bus currently operating shuttle routes on the University of Delaware campus. The system’s overall operating objectives—meeting the total power demand of the bus and maintaining the desired state of charge (SOC) of the NiCd battery—are achieved with appropriately designed controllers: a logic-based “algebraic controller” and a standard PI controller. The design, implementation, and performance of the hybrid system are demonstrated via simulation of real shuttle runs under various operating conditions. The results show that both control strategies perform equally well in enabling the HRES to meet its objectives under typical operating conditions, and under sudden cloud cover conditions; however, at consistently high bus speeds, battery SOC maintenance is better, and the system consumes less hydrogen, with PI control. An economic analysis of the PV investment necessary to realize the HRES design objectives indicates a return on investment of approximately 30% (a slight, but nonetheless positive, ~$550 profit over the bus lifetime) in Newark, DE, establishing the economic viability of the proposed addition of a PV array to the existing University of Delaware fuel cell/battery bus.Item Understanding the nanoscale local buckling behavior of vertically aligned MWCNT arrays with van der Waals interactions(Royal Society of Chemistry, 2015-07-20) Li, Yupeng; Kim, Hyung-ick; Wei, Bingqing; Kang, Junmo; Choi, Jae-boong; Nam, Jae-Do; Suhr, Jonghwan; Yupeng Li, Hyung-ick Kim, Bingqing Wei, Junmo Kang, Jae-boong Choi, Jae-Do Nam and Jonghwan Suhr; Li, Yupeng; Wei, BingqingThe local buckling behavior of vertically aligned carbon nanotubes (VACNTs) has been investigated and interpreted in the view of a collective nanotube response by taking van der Waals interactions into account. To the best of our knowledge, this is the first report on the case of collective VACNT behavior regarding van der Waals force among nanotubes as a lateral support effect during the buckling process. The local buckling propagation and development of VACNTs were experimentally observed and theoretically analyzed by employing finite element modeling with lateral support from van der Waals interactions among nanotubes. Both experimental and theoretical analyses show that VACNTs buckled in the bottom region with many short waves and almost identical wavelengths, indicating a high mode buckling. Furthermore, the propagation and development mechanism of buckling waves follow the wave damping effect.Item Processing and Characterization of a Novel Distributed Strain Sensor Using Carbon Nanotube-Based Nonwoven Composites(MDPI AG, 2015-07-21) Dai, Hongbo; Thostenson, Erik T.; Schumacher, Thomas; Hongbo Dai, Erik T. Thostenson, and Thomas Schumacher; Dai, Hongbo; Thostenson, Erik T.; Schumacher, ThomasThis paper describes the development of an innovative carbon nanotube-based non-woven composite sensor that can be tailored for strain sensing properties and potentially offers a reliable and cost-effective sensing option for structural health monitoring (SHM). This novel strain sensor is fabricated using a readily scalable process of coating Carbon nanotubes (CNT) onto a nonwoven carrier fabric to form an electrically-isotropic conductive network. Epoxy is then infused into the CNT-modified fabric to form a free-standing nanocomposite strain sensor. By measuring the changes in the electrical properties of the sensing composite the deformation can be measured in real-time. The sensors are repeatable and linear up to 0.4% strain. Highest elastic strain gage factors of 1.9 and 4.0 have been achieved in the longitudinal and transverse direction, respectively. Although the longitudinal gage factor of the newly formed nanocomposite sensor is close to some metallic foil strain gages, the proposed sensing methodology offers spatial coverage, manufacturing customizability, distributed sensing capability as well as transverse sensitivity.Item Experimental characterization of tensile properties of epoxy resin by using micro-fiber specimens(SAGE Publications, 2016) Misumi, Jun; Ganesh, Raja; Sockalingam, Subramani; Gillespie, John W. Jr.; Jun Misumi, Raja Ganesh, Subramani Sockalingam and John W Gillespie Jr.; Misumi, Jun; Ganesh, Raja; Sockalingam, Subramani; Gillespie, John W. Jr.In unidirectional carbon fiber reinforced plastic (CFRP) laminates, the distance between fibers can vary from submicron to micron length scales. The mechanical properties of the matrix at this length scale are not well understood. In this study, processing methods have been developed to produce high quality epoxy micro-fibers with diameters ranging from 100 to 150 um that are used for tensile testing. Five types of epoxy resin systems ranging from standard DGEBA to high-crosslink TGDDM and TGMAP epoxy systems have been characterized. Epoxy macroscopic specimens with film thickness of 3300 um exhibited brittle behavior (1.7 to 4.9% average failure strain) with DGEBA resin having the highest failure strain level. The epoxy micro-fiber specimens exhibited significant ductile behavior (20 to 42% average failure strain) with a distinct yield point being observed in all five resin systems. In addition, the ultimate stress of the highly cross-linked TGDDM epoxy fiber exceeded the bulk film properties by a factor of two and the energy absorption was over 50 times greater on average. The mechanism explaining the dramatic difference in properties are discussed and is based on size effects (the film volume is about 2000 times greater than the fiber volume within the gage sections) and surface defects. Based on the findings 3 presented in this paper, the microscale fiber test specimens are recommended and provide more realistic stress-strain response for describing the role of the matrix in composites at smaller length scales.Item A parallel fictitious domain method for the interface-resolved simulation of particle-laden flows and its application to the turbulent channel flow(Taylor & Francis, 2016-01-20) Yu, Zhaosheng; Lin, Zhaowu; Shao, Xueming; Wang, Lian-Ping; Zhaosheng Yu, Zhaowu Lin, Xueming Shao & Lian-Ping Wang; Wang, Lian-PingA parallel direct-forcing (DF) fictitious domain (FD) method for the simulation of particulate flows is reported in this paper. The parallel computing strategies for the solution of flow fields and particularly the distributed Lagrange multiplier are presented, and the high efficiency of the parallel code is demonstrated. The new code is then applied to study the effects of particle density (or particle inertia) on the turbulent channel flow. The results show that the large-scale vortices are weakened more severely, and the flow friction drag increases first and then reduces, as particle inertia is increased.Item Recent Advances in Modeling and Experiments of Kevlar Ballistic Fibrils, Fibers, Yarns and Flexible Woven Textile Fabrics – A Review(Sage Publications, 2016-05-02) Sockalingam, Subramani; Chowdhury, Sanjib C.; Gillespie, John W. Jr.; Keefe, Michael; Subramani Sockalingam, Sanjib C. Chowdhury, John W. Gillespie Jr and Michael Keefe; Sockalingam, Subramani; Chowdhury, Sanjib C.; Gillespie, John W. Jr.; Keefe, MichaelBallistic impact onto flexible woven textile fabrics is a complicated multi-scale problem given the structural hierarchy of the materials, anisotropic material behavior, projectile geometry-fabric interactions, impact velocity and boundary conditions. Although this subject has been an active area of research for decades, the fundamental mechanisms such as material failure, dynamic response, multi-axial loading occurring at the lower length scales during impact are not well understood. This paper reviews the recent advances in modeling and experiments of Kevlar ballistic fibrils, fibers, yarns and flexible woven textile fabrics pertinent to the deformation modes occurring during impact and serves to identify topics worthy of further investigation that will advance the basic understanding of the phenomena governing transverse impact. This review also explores on aspects such as homogeneous versus heterogeneous behavior of yarns consisting of individual fibers and the inelastic transverse behavior of the fiber which is not considered in the previous review papers on this topic.Item Biomechanical Analysis of Gait Termination in 11-17 Year Old Youth at Preferred and Fast Walking Speeds(Elsevier, 2016-07-15) Ridge, Sarah Trager; Henley, John; Manal, Kurt; Miller, Freeman; Richards, James G.; Sarah Trager Ridge, John Henley, Kurt Manal, Freeman Miller, and James G. Richards; Ridge, Sarah Trager; Manal, Kurt; Richards, James G.In populations where walking and/or stopping can be difficult, such as in children with cerebral palsy, the ability to quickly stop walking may be beyond the child’s capabilities. Gait termination may be improved with physical therapy. However, without a greater understanding of the mechanical requirements of this skill, treatment planning is difficult. The purpose of this study was to understand how healthy children successfully terminate gait in one step when walking quickly, which can be challenging even for healthy children. Lower extremity kinematic and kinetic data were collected from 15 youth as they performed walking, planned, and unplanned stopping tasks. Each stopping task was performed as the subject walked at his/her preferred speed and a fast speed. The most significant changes in mechanics between speed conditions (preferred and fast) of the same stopping task were greater knee flexion angles (unplanned: +16.49±.54°, p=.00; planned: +15.75±1.1°, p=.00) and knee extension moments (unplanned: +.67±.02 N/kgm, p=.00; planned: +.57±.23 N/kgm, p=.00) at faster speeds. The extra range of motion in the joints and extra muscle strength required to maintain the stopping position suggests that stretching and strengthening the muscles surrounding the joints of the lower extremity, particularly the knee, may be a useful intervention.Item Dynamic effects of single fiber break in unidirectional glass fiber-reinforced composites(Sage Publications, 2016-09-15) Ganesh, Raja; Sockalingam, Subramani; Haque, Bazle Z. (Gama); Gillespie, John W. Jr.; Raja Ganesh, Subramani Sockalingam, Bazle Z. (Gama) Haque and John W. Gillespie, Jr.; Ganesh, Raja; Sockalingam, Subramani; Haque, Bazle Z. (Gama); Gillespie, John W. Jr.In a unidirectional composite under static tensile loading, breaking of a fiber is shown to be a locally dynamic process which leads to stress concentrations in the interface, matrix and neighboring fibers that can propagate at high speed over long distances. To gain better understanding of this event, a fiber-level finite element model of a 2-dimensional array of S2-glass fibers embedded in an elastic epoxy matrix with interfacial cohesive traction law is developed. The brittle fiber fracture results in release of stored strain energy as a compressive stress wave that propagates along the length of the broken fiber at speeds approaching the axial wave-speed in the fiber (6 km/s). This wave induces an axial tensile wave with a dynamic tensile stress concentration in adjacent fibers that diminishes with distance. Moreover, dynamic interfacial failure is predicted where debonding initiates, propagates and arrests at longer distances than predicted by models that assume quasi-static fiber breakage. In the case of higher strength fibers breaks, unstable debond growth is predicted. A stability criterion to define the threshold fiber break strength is derived based on an energy balance between the release of fiber elastic energy and energy absorption associated with interfacial debonding. A contour map of peak dynamic stress concentrations is generated at various break stresses to quantify the zone-of-influence of dynamic failure. The dynamic results are shown to envelop a much larger volume of the microstructure than the quasi-static results. The implications of dynamic fiber fracture on damage evolution in the composite are discussed.Item Experimental characterization of tensile properties of epoxy resin by using micro-fiber specimens(Sage Publications, 2016-09-21) Misumi, Jun; Ganesh, Raja; Sockalingam, Subramani; Gillespie, John W. Jr.; Jun Misumi, Raja Ganesh, Subramani Sockalingam, John W Gillespie; Misumi, Jun; Ganesh, Raja; Sockalingam, Subramani; Gillespie, John W. JrIn unidirectional carbon fiber-reinforced plastic laminates, the distance between fibers can varies from submicron to micron length scales. The mechanical properties of the matrix at this length scale are not well understood. In this study, processing methods have been developed to produce high quality epoxy micro-fibers with diameters ranging from 100 to 150 µm that are used for tensile testing. Five types of epoxy resin systems ranging from standard DGEBA to high-crosslink TGDDM and TGMAP epoxy systems have been characterized. Epoxy macroscopic specimens with film thickness of 3300 µm exhibited brittle behavior (1.7 to 4.9% average failure strain) with DGEBA resin having the highest failure strain level. The epoxy micro-fiber specimens exhibited significant ductile behavior (20 to 42% average failure strain) with a distinct yield point being observed in all five resin systems. In addition, the ultimate stress of the highly cross-linked TGDDM epoxy fiber exceeded the bulk film properties by a factor of two and the energy absorption was over 50 times greater on average. The mechanism explaining the dramatic difference in properties is discussed and is based on size effects (the film volume is about 2000 times greater than the fiber volume within the gage sections) and surface defects. Based on the findings presented in this paper, the microscale fiber test specimens are recommended and provide more realistic stress–strain response for describing the role of the matrix in composites at smaller length scales.Item Facile Synthesis of V2O5 Hollow Spheres as Advanced Cathodes for High-Performance Lithium-Ion Batteries(MDPI, 2017-01-18) Zhang, Xingyuan; Wang, Jian-Gan; Liu, Huanyan; Liu, Hongzhen; Wei, Bingqing; Xingyuan Zhang, Jian-GanWang, Huanyan Liu, Hongzhen Liu and Bingqing Wei; Wei, BingqingThree-dimensional V2O5 hollow structures have been prepared through a simple synthesis strategy combining solvothermal treatment and a subsequent thermal annealing. The V2O5 materials are composed of microspheres 2–3 m in diameter and with a distinct hollow interior. The as-synthesized V2O5 hollow microspheres, when evaluated as a cathode material for lithium-ion batteries, can deliver a specific capacity as high as 273 mAh g���1 at 0.2 C. Benefiting from the hollow structures that afford fast electrolyte transport and volume accommodation, the V2O5 cathode also exhibits a superior rate capability and excellent cycling stability. The good Li-ion storage performance demonstrates the great potential of this unique V2O5 hollow material as a high-performance cathode for lithium-ion batteries.Item Coaxial MoS2@Carbon Hybrid Fibers: A Low-Cost Anode Material for High-Performance Li-Ion Batteries(MDPI AG, 2017-02-13) Zhou, Rui; Wang, Jian-Gan; Liu, Hongzhen; Liu, Huanyan; Jin, Dandan; Liu, Xingrui; Shen, Chao; Xie, Keyu; Wei, Bingqing; Rui Zhou, Jian-GanWang, Hongzhen Liu, Huanyan Liu, Dandan Jin, Xingrui Liu, Chao Shen, Keyu Xie and Bingqing Wei; Wei, BingqingA low-cost bio-mass-derived carbon substrate has been employed to synthesize MoS2@carbon composites through a hydrothermal method. Carbon fibers derived from natural cotton provide a three-dimensional and open framework for the uniform growth of MoS2 nanosheets, thus hierarchically constructing coaxial architecture. The unique structure could synergistically benefit fast Li-ion and electron transport from the conductive carbon scaffold and porous MoS2 nanostructures. As a result, the MoS2@carbon composites—when serving as anodes for Li-ion batteries—exhibit a high reversible specific capacity of 820 mAh·g−1, high-rate capability (457 mAh·g−1 at 2 A·g−1), and excellent cycling stability. The use of bio-mass-derived carbon makes the MoS2@carbon composites low-cost and promising anode materials for high-performance Li-ion batteries.Item Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces(American Physical Society, 2017-02-22) Ye, Ning; Feser, Joseph P.; Sadasivam, Sridhar; Fisher, Timothy S.; Wang, Tianshi; Ni, Chaoying; Janotti, Anderson; Ning Ye, Joseph P. Feser, Sridhar Sadasivam, Timothy S. Fisher, Tianshi Wang, Chaoying Ni, and Anderson Janotti; Ye, Ning; Feser, Joseph P.; Wang, Tianshi; Ni, Chaoying; Janotti, AndersonSilicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of TiSi2, CoSi2, NiSi, and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no dependence of interface conductance on the detailed structure of the interface. In particular, there is no difference in the thermal interface conductance between epitaxial and nonepitaxial silicide/silicon interfaces, nor between epitaxial interfaces with different interface orientations.While these silicide-based interfaces yield the highest reported interface conductances of any known interface with silicon, none of the interfaces studied are found to operate close to the phonon radiation limit, indicating that phonon transmission coefficients are nonunity in all cases and yet remain insensitive to interfacial structure. In the case of CoSi2, a comparison ismade with detailed computational models using (1) full-dispersion diffuse mismatch modeling (DMM) including the effect of near-interfacial strain, and (2) an atomistic Green’ function (AGF) approach that integrates near-interface changes in the interatomic force constants obtained through density functional perturbation theory. Above 100 K, the AGF approach significantly underpredicts interface conductance suggesting that energy transport does not occur purely by coherent transmission of phonons, even for epitaxial interfaces. The full-dispersion DMM closely predicts the experimentally observed interface conductances for CoSi2, NiSi, and TiSi2 interfaces, while it remains an open question whether inelastic scattering, cross-interfacial electron-phonon coupling, or other mechanisms could also account for the high-temperature behavior. The effect of degenerate semiconductor dopant concentration onmetal-semiconductor thermal interface conductance was also investigated with the result that we have found no dependencies of the thermal interface conductances up to (n or p type) ≈1 × 1019 cm−3, indicating that there is no significant direct electronic transport and no transport effects that depend on long-range metal-semiconductor band alignment.