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851.
Jianpeng Li Dingshu Xiao Peichu Wang Hongyan Chen Minghua Deng Degang Zhu Jie Yu 《Particle & Particle Systems Characterization》2023,40(1):2200141
Transition metal compounds anchored on N-doped carbon (NC) show intrinsic activity and stability for oxygen reduction reaction (ORR). However, the interaction between the transition metal compounds and NC still needs to be strengthened for electron transfer at the compounds/carbon interface. Herein, Fe/Fe3C hybrid nanoparticles encapsulated into N-doped carbon (Fe@NC) are used as high-performance ORR catalysts. Benefiting from the strong interaction at Fe/Fe3C nanoparticles/NC interface, the electrons can transfer from Fe/Fe3C hybrid nanoparticles to NC, redistributing the electron density of active sites and promoting the ORR process. The as-synthesized Fe@NC exhibits outstanding ORR catalytic activity with an onset potential of 1.01 V and a half-wave potential of 0.92 V in alkaline media. It also shows prominent cycling stability and tolerance to methanol crossover, superior to Pt/C catalyst. The theoretical analysis reveals that the Fe nanoparticles have regulated the electron distributions at the heterojunction interface. The Gibbs free energy diagrams for ORR illustrate that the rate-determining step is the conversion of OH* to OH−. In situ Raman spectra give evidence of O-containing intermediates to prove the ORR process. 相似文献
852.
This paper presents an indirect adaptive stabilization scheme forfirst-order continuous-time systems under saturated input which isdescribed by a sigmoidal function. The singularities are avoided througha modification scheme for the estimated plant parameter vector so thatits associated Sylvester matrix is guaranteed to be nonsingular andthen the estimated plant model is controllable. The modificationmechanism involves the use of a hysteresis switching function. Analternative hybrid scheme, whose estimated parameters are updated atsampling instants is also given to solve a similar adaptivestabilization problem. Such a scheme also uses hysteresis switching formodification of the parameter estimates so as to ensure thecontrollability of the estimated plant model. 相似文献
853.
B. A. Schrefler 《Meccanica》1991,26(2-3):93-99
Geomaterials are modelled as deforming multiphase porous media with a solid, liquid and/or gaseous phase. The models differ according to the mass transfer mechanism taking place at high, medium or low water content. The Finite Element Method is used for the discretization in space of the governing equations.
Sommario Geomateriali, cioè terre, rocce, calcestruzzi, sono modellati come mezzi porosi multifase, deformabili, composti quindi da una fase solida, da une fase liquida e/o una gassosa. I modelli relativi differiscono secondo il meccanismo di trasporto di massa che prevale ad alto, medio o basso contenuto d'acqua. Il metodo degli elementi finiti è utilizzato per la discretizzazione spaziale e temporale delle equazioni differenziali che governano il problema.相似文献
854.
We consider the problem of laminar mixed convection flow between parallel, vertical and uniformly heated plates where the governing dimensionless parameters are the Prandtl, Rayleigh and Reynolds numbers. Using the method based on the centre manifold theorem which was derived from the general theory of dynamical systems, we reduce a three-dimensional simplified model of ordinary differential amplitude equations emanating from the original Navier-Stokes system of the problem in the vicinity of a trivial stationary solution. We have found that when the forcing parameter, the Rayleigh number, increases beyond the critical value Ras, the stationary solution is a pitchfork bifurcation point of the system. 相似文献
855.
J.C. Umavathi 《International Journal of Non》2005,40(1):91-101
The problem of combined free and forced convective magnetohydrodynamic flow in a vertical channel is analysed by taking into account the effect of viscous and ohmic dissipations. The channel walls are maintained at equal or at different constant temperatures. The velocity field and the temperature field are obtained analytically by perturbation series method and numerically by finite difference technique. The results are presented for various values of the Brinkman number and the ratio of Grashof number to the Reynolds number for both equal and different wall temperatures. Nusselt number at the walls is determined. It is found that the viscous dissipation enhances the flow reversal in the case of downward flow while it counters the flow in the case of upward flow. It is also found that the analytical and numerical solutions agree very well for small values of ε. 相似文献
856.
Variation of the stress intensity factor along the front of a 3-D rectangular crack subjected to mixed-mode load 总被引:3,自引:0,他引:3
Summary The singular integral equation method is applied to the calculation of the stress intensity factor at the front of a rectangular
crack subjected to mixed-mode load. The stress field induced by a body force doublet is used as a fundamental solution. The
problem is formulated as a system of integral equations with r
−3-singularities. In solving the integral equations, unknown functions of body-force densities are approximated by the product
of polynomial and fundamental densities. The fundamental densities are chosen to express two-dimensional cracks in an infinite
body for the limiting cases of the aspect ratio of the rectangle. The present method yields rapidly converging numerical results
and satisfies boundary conditions all over the crack boundary. A smooth distribution of the stress intensity factor along
the crack front is presented for various crack shapes and different Poisson's ratio.
Received 5 March 2002; accepted for publication 2 July 2002 相似文献
857.
An analysis is carried out to study the effects of localized heating (cooling), suction (injection), buoyancy forces and magnetic field for the mixed convection flow on a heated vertical plate. The localized heating or cooling introduces a finite discontinuity in the mathematical formulation of the problem and increases its complexity. In order to overcome this difficulty, a non-uniform distribution of wall temperature is taken at finite sections of the plate. The nonlinear coupled parabolic partial differential equations governing the flow have been solved by using an implicit finite-difference scheme. The effect of the localized heating or cooling is found to be very significant on the heat transfer, but its effect on the skin friction is comparatively small. The buoyancy, magnetic and suction parameters increase the skin friction and heat transfer. The positive buoyancy force (beyond a certain value) causes an overshoot in the velocity profiles.A
mass transfer constant
- B
magnetic field
- Cfx
skin friction coefficient in the x-direction
- Cp
specific heat at constant pressure, kJ.kg–1.K
- Cv
specific heat at constant volume, kJ.kg–1.K–1
- E
electric field
- g
acceleration due to gravity, 9.81 m.s–2
- Gr
Grashof number
- h
heat transfer coefficient, W.m2.K–1
- Ha
Hartmann number
- k
thermal conductivity, W.m–1.K
- L
characteristic length, m
- M
magnetic parameter
- Nux
local Nusselt number
- p
pressure, Pa, N.m–2
- Pr
Prandtl number
- q
heat flux, W.m–2
- Re
Reynolds number
- Rem
magnetic Reynolds number
- T
temperature, K
- To
constant plate temperature, K
- u,v
velocity components, m.s–1
- V
characteristic velocity, m.s–1
- x,y
Cartesian coordinates
-
thermal diffusivity, m2.s–1
-
coefficient of thermal expansion, K–1
- ,
transformed similarity variables
-
dynamic viscosity, kg.m–1.s–1
- 0
magnetic permeability
-
kinematic viscosity, m2.s–1
-
density, kg.m–3
-
buoyancy parameter
-
electrical conductivity
-
stream function, m2.s–1
-
dimensionless constant
-
dimensionless temperature, K
- w,
conditions at the wall and at infinity 相似文献
858.
The steady laminar flow and thermal characteristics of a continuously moving vertical sheet of extruded material are studied close to and far downstream from the extrusion slot. The velocity and temperature variations, obtained by a finite volume method, are used to map out the entire forced, mixed and natural convection regimes. The effects of the Prandtl number (Pr) and the buoyancy force parameter (B) on the friction and heat transfer coefficients are investigated. Comparisons with experimental measurements and solutions by others in the pure forced and pure natural convection regions are made. In the mixed convection region, the results are compared with available finite-difference solutions of the boundary layer equations showing excellent agreement. The region close to the extrusion slot is characterized as a non-similar forced-convection dominated region in which NuxRex−1/2 drops sharply with increasing Richardson number (Rix). This is followed by a self-similar forced-convection dominated region in which NuxRex−1/2 levels off with increasing Rix until the buoyancy effect sets in. The existence and extent of the latter region depend upon the value of B. A non-similar mixed convection region where increasing buoyancy effect enhances the heat transfer rate follows. Finally, this region is followed downstream by a self-similar natural-convection dominated region in which NuxRex−1/2 approaches the pure natural convection asymptote at large Rix. Critical values of Rix to distinguish the various convection regimes are determined for different Pr and B. 相似文献
859.
Wei Fang Dr. Fangyi Liang Dr. Zhengwen Cao Frank Steinbach Prof. Dr. Armin Feldhoff 《Angewandte Chemie (International ed. in English)》2015,54(16):4847-4850
To combine good chemical stability and high oxygen permeability, a mixed ionic‐electronic conducting (MIEC) 75 wt % Ce0.85Gd0.1Cu0.05O2?δ‐25 wt % La0.6Ca0.4FeO3?δ (CGCO‐LCF) dual‐phase membrane based on a MIEC–MIEC composite has been developed. Copper doping into Ce0.9Gd0.1O2?δ (CGO) oxide enhances both ionic and electronic conductivity, which then leads to a change from ionic conduction to mixed conduction at elevated temperatures. For the first time we demonstrate that an intergranular film with 2–10 nm thickness containing Ce, Ca, Gd, La, and Fe has been formed between the CGCO grains in the CGCO‐LCF one‐pot dual‐phase membrane. A high oxygen permeation flux of 0.70 mL min?1 cm?2 is obtained by the CGCO‐LCF one‐pot dual‐phase membrane with 0.5 mm thickness at 950 °C using pure CO2 as the sweep gas, and the membrane shows excellent stability in the presence of CO2 even at lower temperatures (800 °C) during long‐term operation. 相似文献
860.
Direct Detection of the Superoxide Anion as a Stable Intermediate in the Electroreduction of Oxygen in a Non‐Aqueous Electrolyte Containing Phenol as a Proton Source
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Prof. Zhangquan Peng Dr. Yuhui Chen Prof. Peter G. Bruce Prof. Ye Xu 《Angewandte Chemie (International ed. in English)》2015,54(28):8165-8168
The non‐aqueous Li–air (O2) battery has attracted intensive interest because it can potentially store far more energy than today′s batteries. Presently Li–O2 batteries suffer from parasitic reactions owing to impurities, found in almost all non‐aqueous electrolytes. Impurities include residual protons and protic compounds that can react with oxygen species, such as the superoxide (O2?), a reactive, one‐electron reduction product of oxygen. To avoid the parasitic reactions, it is crucial to have a fundamental understanding of the conditions under which reactive oxygen species are generated in non‐aqueous electrolytes. Herein we report an in situ spectroscopic study of oxygen reduction on gold in a dimethyl sulfoxide electrolyte containing phenol as a proton source. It is shown directly that O2?, not HO2, is the first stable intermediate during the oxygen reduction process to hydrogen peroxide. The unusual stability of O2? is explained using density functional theory (DFT) calculations. 相似文献