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Search Results: 1 - 10 of 426104 matches for " S Zhang "
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A physical model for one-dimension and time-dependent ionosphere. Part II. Results and discussion
S. Zhang,S. M. Radicella
Annals of Geophysics , 1993, DOI: 10.4401/ag-4251
Improved Cyclability of Liquid Electrolyte Lithium/Sulfur Batteries by Optimizing Electrolyte/Sulfur Ratio
Sheng S. Zhang
Energies , 2012, DOI: 10.3390/en5125190
Abstract: A liquid electrolyte lithium/sulfur (Li/S) cell is a liquid electrochemical system. In discharge, sulfur is first reduced to highly soluble Li 2S 8, which dissolves into the organic electrolyte and serves as the liquid cathode. In solution, lithium polysulfide (PS) undergoes a series of complicated disproportionations, whose chemical equilibriums vary with the PS concentration and affect the cell’s performance. Since the PS concentration relates to a certain electrolyte/sulfur (E/S) ratio, there is an optimized E/S ratio for the cyclability of each Li/S cell system. In this work, we study the optimized E/S ratio by measuring the cycling performance of Li/S cells, and propose an empirical method for determination of the optimized E/S ratio. By employing an electrolyte of 0.25 m LiSO 3CF 3-0.25 m LiNO 3 dissolved in a 1:1 (wt:wt) mixture of dimethyl ether (DME) and 1,3-dioxolane (DOL) in an optimized E/S ratio, we show that the Li/S cell with a cathode containing 72% sulfur and 2 mg cm ? 2 sulfur loading is able to retain a specific capacity of 780 mAh g ?1 after 100 cycles at 0.5 mA cm ?2 between 1.7 V and 2.8 V.
Fully-compliant statically-balanced mechanisms without prestressing assembly: concepts and case studies
G. Chen ,S. Zhang
Mechanical Sciences (MS) , 2011, DOI: 10.5194/ms-2-169-2011
Abstract: The purpose of this paper is to present new concepts for designing fully-compliant statically-balanced mechanisms without prestressing assembly. A statically-balanced compliant mechanism can ideally provide zero stiffness and energy free motion like a traditional rigid-body mechanism. These characteristics are important in design of compliant mechanisms where low actuation force, accurate force transmission or high-fidelity force feedback are primary concerns. Typically, static balancing of compliant mechanisms has been achieved by means of prestressing assembly. However, this can often lead to creep and stress relaxation arising in the flexible members. In this paper two concepts are presented which eliminate the need for prestressing assembly of compliant mechanisms: (1) a weight compensator which employs a constant-force compliant mechanism, (2) a near-zero-stiffness mechanism which combines two multistable mechanisms. In addition to the advantages provided by statically-balanced compliant mechanisms, two other notable features of these statically-balanced mechanisms are their ability to be monolithically fabricated and to return to their as-fabricated position without any disassembly when not in use.
Semilinear parabolic problems on manifolds and applications to the non-compact Yamabe problem
Qi S. Zhang
Electronic Journal of Differential Equations , 2000,
Abstract: We show that the well-known non-compact Yamabe equation (of prescribing constant positive scalar curvature) on a manifold with non-negative Ricci curvature and positive scalar curvature behaving like $c/d(x)^2$ near infinity can not be solved if the volume of geodesic balls do not increase "fast enough". Even though both existence and nonexistence results have appeared in the case when the scalar curvature is negative somewhere([J], [AM]), or when the scalar curvature is positive ([Ki], [Zhan5]), the current paper seems to give the first nonexistence result in the case that the scalar curvature is positive and $Ricci ge 0$, which seems to be the fundamental part of the noncompact Yamabe problem. We also find some complete non-compact manifolds with positive scalar curvature which are conformal to complete manifolds with constant and with zero scalar curvature. This is a new phenomenon which does not happen in the compact case.
Sulfurized Carbon: A Class of Cathode Materials for High Performance Lithium/Sulfur Batteries
Sheng S. Zhang
Frontiers in Energy Research , 2013, DOI: 10.3389/fenrg.2013.00010
Abstract: Liquid electrolyte lithium/sulfur (Li/S) batteries cannot come into practical applications because of many problems such as low energy efficiency, short cycle life, and fast self-discharge. All these problems are related to the dissolution of lithium polysulfide, a series of sulfur reduction intermediates, in the liquid electrolyte, and resulting parasitic reactions with the Li anode. Covalently binding sulfur onto carbon surface is a solution to completely eliminate the dissolution of lithium polysulfide and make the Li/S battery viable for practical applications. This can be achieved by replacing elemental sulfur with sulfurized carbon (SC) as the cathode material. This article reviews the current efforts on this subject and discusses the syntheses, electrochemical properties, and prospects of the SC as a cathode material in the rechargeable Li/S batteries.
Status, Opportunities, and Challenges of Electrochemical Energy Storage
Sheng S. Zhang
Frontiers in Energy Research , 2013, DOI: 10.3389/fenrg.2013.00008
High Energy Continuum Spectra from X-Ray Binaries
S. N. Zhang
Physics , 1996,
Abstract: A variety of high energy (>1 keV) spectra have been observed in recent years from Black Hole (BH) and Neutron Star (NS) X-ray Binaries (XB). Some common physical components exist between BHXBs and NSXBs, resulting in some high energy spectral features. A common component between a BHXB and a weakly magnetized NSXB is the inner accretion disk region extending very close to the surface (for a NS) or the horizon (for a BH). The inner disk radiation can be described by a multi-color blackbody (MCB) spectral model. The surface radiation of the NS can be approximated by a Single Color Blackbody (SCB) spectrum. For a strongly magnetized NSXB, the high energy emission is from its magnetosphere, characterised by a thermal bremsstrahlung (TB) spectrum. In both BHXBs and weakly magnetized NSXBs, a hot electron cloud may exist, producing the hard X-ray power law (photon index -1.5 to -2.0) with thermal cutoff (50-200 keV). It has been recently proposed that a converging flow may be formed near the horizon of a BH, producing a softer power law (photon index about -2.5) without cutoff up to several hundred keV. Based on these concepts we also discuss possible ways to distinguish between BH and NS XBs. Finally we discuss briefly spectral state transitions in both BH and NS XBs.
Thermally assisted magnetization reversal in the presence of a spin-transfer torque
Z. Li,S. Zhang
Physics , 2003, DOI: 10.1103/PhysRevB.69.134416
Abstract: We propose a generalized stochastic Landau-Lifshitz equation and its corresponding Fokker-Planck equation for the magnetization dynamics in the presence of spin transfer torques. Since the spin transfer torque can pump a magnetic energy into the magnetic system, the equilibrium temperature of the magnetic system is ill-defined. We introduce an effective temperature based on a stationary solution of the Fokker-Planck equation. In the limit of high energy barriers, the law of thermal agitation is derived. We find that the N\'{e}el-Brown relaxation formula remains valid as long as we replace the temperature by an effective one that is linearly dependent of the spin torque. We carry out the numerical integration of the stochastic Landau-Lifshitz equation to support our theory. Our results agree with existing experimental data.
Magnetization dynamics with a spin-transfer torque
Z. Li,S. Zhang
Physics , 2003, DOI: 10.1103/PhysRevB.68.024404
Abstract: The magnetization reversal and dynamics of a spin valve pillar, whose lateral size is 64$\times$64 nm$^2$, are studied by using micromagnetic simulation in the presence of spin transfer torque. Spin torques display both characteristics of magnetic damping (or anti-damping) and of an effective magnetic field. For a steady-state current, both M-I and M-H hysteresis loops show unique features, including multiple jumps, unusual plateaus and precessional states. These states originate from the competition between the energy dissipation due to Gilbert damping and the energy accumulation due to the spin torque supplied by the spin current. The magnetic energy oscillates as a function of time even for a steady-state current. For a pulsed current, the minimum width and amplitude of the spin torque for achieving current-driven magnetization reversal are quantitatively determined. The spin torque also shows very interesting thermal activation that is fundamentally different from an ordinary damping effect.
Summary Of Working Group On Single Beam High Luminosity Issues
S. Guiducci,C. Zhang
Physics , 2004,
Abstract: The aim of the Working Group on Single Beam Behaviour was to concentrate on the items limiting the achievement of high luminosity. Some are related to high current and short bunch distance, as electron cloud instability (ECI), the others to the lattice design, as Dynamic Aperture (DA), wigglers, Interaction Region (IR) design, lifetime and background. These arguments have been discussed to explore the feasibility of a very high luminosity Phi-factory.
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