To study the influence of construction interfaces on dynamic characteristics of roller compacted concrete dams(RCCDs),mechanical properties of construction interfaces are firstly analyzed. Then, the viscous-spring art...To study the influence of construction interfaces on dynamic characteristics of roller compacted concrete dams(RCCDs),mechanical properties of construction interfaces are firstly analyzed. Then, the viscous-spring artificial boundary(VSAB) is adopted to simulate the radiation damping of their infinite foundations, and based on the Marc software, a simplified seismic motion input method is presented by the equivalent nodal loads. Finally, based on the practical engineering of a RCC gravity dam, effects of radiation damping and construction interfaces on the dynamic characteristics of dams are investigated in detail. Analysis results show that dynamic response of the RCC gravity dam significantly reduces about 25% when the radiation damping of infinite foundation is considered. Hot interfaces and the normal cold interfaces have little influence on the dynamic response of the RCC gravity dam.However, nonlinear fracture along the cold interfaces at the dam heel will occur under the designed earthquake if the cold interfaces are combined poorly. Therefore, to avoid the fractures along the construction interfaces under the potential super earthquakes,combination quality of the RCC layers should be significantly ensured.展开更多
The influence of high temperature repressing treatment on the segregation of phosphorus in the 93W alloy was carefully investigated by means of Auger electron spectroscope,EPMA, TEM and SEM.The segregation of phosphor...The influence of high temperature repressing treatment on the segregation of phosphorus in the 93W alloy was carefully investigated by means of Auger electron spectroscope,EPMA, TEM and SEM.The segregation of phosphorus has been observed at the tungsten-tungsten grain boundaries,particularly at the tungsten-matrix interphases when the specimens were kept at the temperature in the range of 1200—1500℃ ,followed by furnace cooling.However, no segregation of phosphorus was observed at the interfaces after the specimens were re- pressed at the range of temperature,followed by furnace cooling.After investigation,the dis- locations in the matrix phase of as-repressed specimens directly influenced the phosphorus segregation to the interfaces.After annealing,the mechanical properties were reduced because of the phosphorus segregation at the interface boundaries.展开更多
Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated...Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated cellular recrystallization accompanied with formation of twin structure and TCP phase of P during annealing at sub-solvus temperature of 1,100 °C. The precipitation of P phase is considered to be attributed to the coarsening of c0 phase in the recrystallized aggregates which lower the activation energy for atomic migration.展开更多
The presence of Dirac delta function in differential equation can lead to a discontinuity,which may degrade the accuracy of related numerical methods.To improve the accuracy,a secondorder numerical method for elliptic...The presence of Dirac delta function in differential equation can lead to a discontinuity,which may degrade the accuracy of related numerical methods.To improve the accuracy,a secondorder numerical method for elliptic equations with singular sources is introduced by employing a local kernel flter.In this method,the discontinuous equation is convoluted with the kernel function to obtain a more regular one.Then the original equation is replaced by this fltered equation around the singular points,to obtain discrete numerical form.The unchanged equations at the other points are discretized by using a central difference scheme.1D and 2D examples are carried out to validate the correctness and accuracy of the present method.The results show that a second-order of accuracy can be obtained in the fltering framework with an appropriate integration rule.Furthermore,the present method does not need any jump condition,and also has extremely simple form that can be easily extended to high dimensional cases and complex geometry.展开更多
Selective reduction of CO_(2) into liquid products such as ethanol through electrochemical catalysis is promising in storing renewable energy in more deliverable chemicals and balancing the carbon footprint in the env...Selective reduction of CO_(2) into liquid products such as ethanol through electrochemical catalysis is promising in storing renewable energy in more deliverable chemicals and balancing the carbon footprint in the environment.However,the lack of efficient catalysts for electrochemical CO_(2) reduction reaction(eCO_(2)RR)makes the promise challenging because the formation of C2+alcohols requires coupling reactions between the shallow reduction intermediates and deep reduction intermediates that are usually difficult to form on uniform catalyst surfaces simultaneously with appropriate transient kinetics.Herein,we report a new strategy for synthesizing bimetallic nanostructures with high densities of interfaced Ag/Cu boundaries,which facilitate the coupling reaction of the high‐oxidation‐number intermediates(CO)formed on the Ag surface and the low‐oxidation‐number intermediates(CHx)formed on the Cu surface.The synthesis relies on the electrochemical reduction of bilayered nanoplates made of silver thiolate and copper thiolate,resulting in Ag/Cu nanostructures exposing Ag surface,Cu surface,and the Ag/Cu interfaced boundaries.Balancing the accessible surface areas of the Ag surface,Cu surface,and Ag/Cu boundaries is beneficial for maximizing the activity and selectivity of eCO_(2)RR towards ethanol production.Faradaic efficiency of forming ethanol has been observed as high as about 50%using the Ag/Cu nanostructure catalyst with molar ratio nAg:nCu of 1:1.Moreover,the promoted coupling reaction at the Ag/Cu boundaries and surface modification with thiolate anions significantly suppress the undesirable hydrogen evolution reaction,particularly at high cathodic potentials,maintaining high energy efficiency for eCO_(2)RR.展开更多
The near-wall domain decomposition method(NDD)has proved to be very efficient for modeling near-wall fully turbulent flows.In this paper the NDD is extended to non-equilibrium regimeswith laminar-turbulent transition(...The near-wall domain decomposition method(NDD)has proved to be very efficient for modeling near-wall fully turbulent flows.In this paper the NDD is extended to non-equilibrium regimeswith laminar-turbulent transition(LTT)for the first time.The LTT is identified with the use of the e^(N)-method which is applied to both incompressible and compressible flows.TheNDD ismodified to take into account LTT in an efficientway.In addition,implementation of the intermittency expands the capabilities of NDD to model non-equilibrium turbulent flows with transition.Performance of the modified NDD approach is demonstrated on various test problems of subsonic and supersonic flows past a flat plate,a supersonic flow over a compression corner and a planar shock wave impinging on a turbulent boundary layer.The results of modeling with and without decomposition are compared in terms of wall friction and show good agreement with each other while NDD significantly reducing computational resources needed.It turns out that the NDD can reduce the computational time as much as three times while retaining practically the same accuracy of prediction.展开更多
In this paper we analyze a long standing problem of the appearance of spurious,non-physical solutions arising in the application of the effective mass theory to low dimensional nanostructures.The theory results in a s...In this paper we analyze a long standing problem of the appearance of spurious,non-physical solutions arising in the application of the effective mass theory to low dimensional nanostructures.The theory results in a system of coupled eigenvalue PDEs that is usually supplemented by interface boundary conditions that can be derived from a variational formulation of the problem.We analyze such a system for the envelope functions and show that a failure to restrict their Fourier expansion coeffi-cients to small k components would lead to the appearance of non-physical solutions.We survey the existing methodologies to eliminate this difficulty and propose a simple and effective solution.This solution is demonstrated on an example of a two-band model for both bulk materials and low-dimensional nanostructures.Finally,based on the above requirement of small k,we derive a model for nanostructures with cylindrical symmetry and apply the developed model to the analysis of quantum dots using an eight-band model.展开更多
An efficient and accurate numerical scheme is proposed for solving the transverse electric(TE)mode electromagnetic(EM)propagation problemin two-dimensional earth.The scheme is based on the alternating direction finite...An efficient and accurate numerical scheme is proposed for solving the transverse electric(TE)mode electromagnetic(EM)propagation problemin two-dimensional earth.The scheme is based on the alternating direction finite-difference time-domain(ADI-FDTD)method.Unlike the conventional upward continuation approach for the earth-air interface,an integral formulation for the interface boundary is developed and it is effectively incorporated to the ADI solver.Stability and convergence analysis together with an error estimate are presented.Numerical simulations are carried out to validate the proposed method,and the advantage of the present method over the popular Du-Fort-Frankel scheme is clearly demonstrated.Examples of the electromagnetic field propagation in the ground with anomaly further verify the effectiveness of the proposed scheme.展开更多
We report recent advances in the experimental and theoretical study of grain size(GS)effects on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy(SMA).It is shown that whe...We report recent advances in the experimental and theoretical study of grain size(GS)effects on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy(SMA).It is shown that when GS<60 nm,the superelastic stress-strain hysteresis loop area(H)of the polycrystal decreases rapidly with GS and tends to vanish as GS approaches 10 nanometers.At the same time,the temperature dependence of the transition stress also decreases with GS and eventually approaches zero,leading to a wide superelastic temperature window and breakdown of the Clausius-Claperyon relationship.Rate dependence of the stress-strain responses is significantly reduced and the cyclic stability of the material is improved by the nanocrystallization.It is proposed that the emergence of such significant changes in the behavior of the material with GS reduction originate from the large increase in the area-to-volume ratios of the nanometer-thick interfaces(grain boundary and Austenite-Martensite(A-M)interface)in the polycrystal.In particular,with GS reduction,interfacial energy terms will gradually become dominant over the bulk energy of the crystallite,eventually bring fundamental changes in the phase transition responses of the material.Modelling strategy leading to the establishment of quantitative relationships among GS,grain boundary,A-M interfaces and the macroscopic responses of the material are outlined.展开更多
基金Projects(20120094110005,20120094130003)supported by the Research Fund for the Doctoral Program of Higher Education of ChinaProjects(51379068,51139001,51279052,51209077,51179066)supported by the National Natural Science Foundation of China+1 种基金Project(NCET-11-0628)supported by the Program for New Century Excellent Talents in University,ChinaProjects(201201038,201101013)supported by the Public Welfare Industry Research Special Fund Project of Ministry of Water Resources of China
文摘To study the influence of construction interfaces on dynamic characteristics of roller compacted concrete dams(RCCDs),mechanical properties of construction interfaces are firstly analyzed. Then, the viscous-spring artificial boundary(VSAB) is adopted to simulate the radiation damping of their infinite foundations, and based on the Marc software, a simplified seismic motion input method is presented by the equivalent nodal loads. Finally, based on the practical engineering of a RCC gravity dam, effects of radiation damping and construction interfaces on the dynamic characteristics of dams are investigated in detail. Analysis results show that dynamic response of the RCC gravity dam significantly reduces about 25% when the radiation damping of infinite foundation is considered. Hot interfaces and the normal cold interfaces have little influence on the dynamic response of the RCC gravity dam.However, nonlinear fracture along the cold interfaces at the dam heel will occur under the designed earthquake if the cold interfaces are combined poorly. Therefore, to avoid the fractures along the construction interfaces under the potential super earthquakes,combination quality of the RCC layers should be significantly ensured.
文摘The influence of high temperature repressing treatment on the segregation of phosphorus in the 93W alloy was carefully investigated by means of Auger electron spectroscope,EPMA, TEM and SEM.The segregation of phosphorus has been observed at the tungsten-tungsten grain boundaries,particularly at the tungsten-matrix interphases when the specimens were kept at the temperature in the range of 1200—1500℃ ,followed by furnace cooling.However, no segregation of phosphorus was observed at the interfaces after the specimens were re- pressed at the range of temperature,followed by furnace cooling.After investigation,the dis- locations in the matrix phase of as-repressed specimens directly influenced the phosphorus segregation to the interfaces.After annealing,the mechanical properties were reduced because of the phosphorus segregation at the interface boundaries.
基金financially supported by National 973 Project of China(No.2015CB654902)National Nature Science Foundation of China(Nos.11374174 and 51390471)
文摘Sub-solvus recrystallization behavior of a second-generation single-crystal superalloy has been studied by transmission electron microcopy and scanning transmission electron microcopy. Surface local stress facilitated cellular recrystallization accompanied with formation of twin structure and TCP phase of P during annealing at sub-solvus temperature of 1,100 °C. The precipitation of P phase is considered to be attributed to the coarsening of c0 phase in the recrystallized aggregates which lower the activation energy for atomic migration.
基金supported by the National Natural Science Foundation in China(Grant Nos.51076006,11202013)BUAA SJP ‘‘111’’ Program(Grant No.B08009)+1 种基金the National Basic Research Program of China(2012CB720200)the Open Research Fund of MOE Key Lab-oratory of High-speed Railway Engineering,Southwest Jiao-tong University and the European Community’s Seventh Framework Program(FP7/2007-2013)under Grant agreement 225967‘‘NextMuSE’’
文摘The presence of Dirac delta function in differential equation can lead to a discontinuity,which may degrade the accuracy of related numerical methods.To improve the accuracy,a secondorder numerical method for elliptic equations with singular sources is introduced by employing a local kernel flter.In this method,the discontinuous equation is convoluted with the kernel function to obtain a more regular one.Then the original equation is replaced by this fltered equation around the singular points,to obtain discrete numerical form.The unchanged equations at the other points are discretized by using a central difference scheme.1D and 2D examples are carried out to validate the correctness and accuracy of the present method.The results show that a second-order of accuracy can be obtained in the fltering framework with an appropriate integration rule.Furthermore,the present method does not need any jump condition,and also has extremely simple form that can be easily extended to high dimensional cases and complex geometry.
文摘Selective reduction of CO_(2) into liquid products such as ethanol through electrochemical catalysis is promising in storing renewable energy in more deliverable chemicals and balancing the carbon footprint in the environment.However,the lack of efficient catalysts for electrochemical CO_(2) reduction reaction(eCO_(2)RR)makes the promise challenging because the formation of C2+alcohols requires coupling reactions between the shallow reduction intermediates and deep reduction intermediates that are usually difficult to form on uniform catalyst surfaces simultaneously with appropriate transient kinetics.Herein,we report a new strategy for synthesizing bimetallic nanostructures with high densities of interfaced Ag/Cu boundaries,which facilitate the coupling reaction of the high‐oxidation‐number intermediates(CO)formed on the Ag surface and the low‐oxidation‐number intermediates(CHx)formed on the Cu surface.The synthesis relies on the electrochemical reduction of bilayered nanoplates made of silver thiolate and copper thiolate,resulting in Ag/Cu nanostructures exposing Ag surface,Cu surface,and the Ag/Cu interfaced boundaries.Balancing the accessible surface areas of the Ag surface,Cu surface,and Ag/Cu boundaries is beneficial for maximizing the activity and selectivity of eCO_(2)RR towards ethanol production.Faradaic efficiency of forming ethanol has been observed as high as about 50%using the Ag/Cu nanostructure catalyst with molar ratio nAg:nCu of 1:1.Moreover,the promoted coupling reaction at the Ag/Cu boundaries and surface modification with thiolate anions significantly suppress the undesirable hydrogen evolution reaction,particularly at high cathodic potentials,maintaining high energy efficiency for eCO_(2)RR.
文摘The near-wall domain decomposition method(NDD)has proved to be very efficient for modeling near-wall fully turbulent flows.In this paper the NDD is extended to non-equilibrium regimeswith laminar-turbulent transition(LTT)for the first time.The LTT is identified with the use of the e^(N)-method which is applied to both incompressible and compressible flows.TheNDD ismodified to take into account LTT in an efficientway.In addition,implementation of the intermittency expands the capabilities of NDD to model non-equilibrium turbulent flows with transition.Performance of the modified NDD approach is demonstrated on various test problems of subsonic and supersonic flows past a flat plate,a supersonic flow over a compression corner and a planar shock wave impinging on a turbulent boundary layer.The results of modeling with and without decomposition are compared in terms of wall friction and show good agreement with each other while NDD significantly reducing computational resources needed.It turns out that the NDD can reduce the computational time as much as three times while retaining practically the same accuracy of prediction.
文摘In this paper we analyze a long standing problem of the appearance of spurious,non-physical solutions arising in the application of the effective mass theory to low dimensional nanostructures.The theory results in a system of coupled eigenvalue PDEs that is usually supplemented by interface boundary conditions that can be derived from a variational formulation of the problem.We analyze such a system for the envelope functions and show that a failure to restrict their Fourier expansion coeffi-cients to small k components would lead to the appearance of non-physical solutions.We survey the existing methodologies to eliminate this difficulty and propose a simple and effective solution.This solution is demonstrated on an example of a two-band model for both bulk materials and low-dimensional nanostructures.Finally,based on the above requirement of small k,we derive a model for nanostructures with cylindrical symmetry and apply the developed model to the analysis of quantum dots using an eight-band model.
基金research was supported in part by the Natural Sciences and Engineering Research Council of CanadaWanshan Liwas supported by the China Scholarship Coun-cil for her visit to the University of Alberta,and Dong Liang would like to acknowledge the partial support by the Natural Science Foundation of China under grant 11271232.
文摘An efficient and accurate numerical scheme is proposed for solving the transverse electric(TE)mode electromagnetic(EM)propagation problemin two-dimensional earth.The scheme is based on the alternating direction finite-difference time-domain(ADI-FDTD)method.Unlike the conventional upward continuation approach for the earth-air interface,an integral formulation for the interface boundary is developed and it is effectively incorporated to the ADI solver.Stability and convergence analysis together with an error estimate are presented.Numerical simulations are carried out to validate the proposed method,and the advantage of the present method over the popular Du-Fort-Frankel scheme is clearly demonstrated.Examples of the electromagnetic field propagation in the ground with anomaly further verify the effectiveness of the proposed scheme.
基金supported by the Hong Kong Research Grant Council(RGC)(Grant No.619113)the National Natural Science Foundation of China(Grant No.11128204)the State Key Lab of Water Resources and Hydropower Engineering of WHU,China
文摘We report recent advances in the experimental and theoretical study of grain size(GS)effects on the thermal and mechanical properties of nanostructured NiTi polycrystalline shape memory alloy(SMA).It is shown that when GS<60 nm,the superelastic stress-strain hysteresis loop area(H)of the polycrystal decreases rapidly with GS and tends to vanish as GS approaches 10 nanometers.At the same time,the temperature dependence of the transition stress also decreases with GS and eventually approaches zero,leading to a wide superelastic temperature window and breakdown of the Clausius-Claperyon relationship.Rate dependence of the stress-strain responses is significantly reduced and the cyclic stability of the material is improved by the nanocrystallization.It is proposed that the emergence of such significant changes in the behavior of the material with GS reduction originate from the large increase in the area-to-volume ratios of the nanometer-thick interfaces(grain boundary and Austenite-Martensite(A-M)interface)in the polycrystal.In particular,with GS reduction,interfacial energy terms will gradually become dominant over the bulk energy of the crystallite,eventually bring fundamental changes in the phase transition responses of the material.Modelling strategy leading to the establishment of quantitative relationships among GS,grain boundary,A-M interfaces and the macroscopic responses of the material are outlined.
