Piezoelectric semiconductors(PSs)possess both semiconducting properties and piezoelectric coupling effects,making them optimal building blocks for semiconductor devices.PS fiber-like structures have wide applications ...Piezoelectric semiconductors(PSs)possess both semiconducting properties and piezoelectric coupling effects,making them optimal building blocks for semiconductor devices.PS fiber-like structures have wide applications in multi-functional semiconductor devices.In this paper,a one-dimensional(1D)theoretical model is established to describe the piezotronic responses of a PS fiber under gradient temperature changes.The theoretical model aims to explain the mechanism behind the resistance change caused by such gradient temperature changes.Numerical results demonstrate that a gradient temperature change significantly affects the physical fields within the PS fiber,and can induce changes in its surface resistance.It provides important theoretical guidance on the development of piezotronic devices that are sensitive to temperature effects.展开更多
The effects of impurities on ion temperature gradient(ITG)driven turbulence transport in tokamak core plasmas are investigated numerically via global simulations of microturbulence with carbon impurities and adiabatic...The effects of impurities on ion temperature gradient(ITG)driven turbulence transport in tokamak core plasmas are investigated numerically via global simulations of microturbulence with carbon impurities and adiabatic electrons.The simulations use an extended fluid code(ExFC)based on a four-field gyro-Landau-fluid(GLF)model.The multispecies form of the normalized GLF equations is presented,which guarantees the self-consistent evolution of both bulk ions and impurities.With parametric profiles of the cyclone base case,well-benchmarked ExFC is employed to perform simulations focusing on different impurity density profiles.For a fixed temperature profile,it is found that the turbulent heat diffusivity of bulk ions in a quasi-steady state is usually lower than that without impurities,which is contrary to the linear and quasilinear predictions.The evolutions of the temperature gradient and heat diffusivity exhibit a fast relaxation process,indicating that the destabilization of the outwardly peaked impurity profile is a transient state response.Furthermore,the impurity effects from different profiles can obviously influence the nonlinear critical temperature gradient,which is likely to be dominated by linear effects.These results suggest that the improvement in plasma confinement could be attributed to the impurities,most likely through adjusting both heat diffusivity and the critical temperature gradient.展开更多
Thermal management in solid oxide fuel cells(SOFC)is a critical issue due to non-uniform electrochemical reactions and convective fl ows within the cells.Therefore,a 2D mathematical model is established herein to inve...Thermal management in solid oxide fuel cells(SOFC)is a critical issue due to non-uniform electrochemical reactions and convective fl ows within the cells.Therefore,a 2D mathematical model is established herein to investigate the thermal responses of a tubular methanol-fueled SOFC.Results show that unlike the low-temperature condition of 873 K,where the peak temperature gradient occurs at the cell center,it appears near the fuel inlet at 1073 K because of the rapid temperature rise induced by the elevated current density.Despite the large heat convection capacity,excessive air could not eff ectively eliminate the harmful temperature gradient caused by the large current density.Thus,optimal control of the current density by properly selecting the operating potential could generate a local thermal neutral state.Interestingly,the maximum axial temperature gradient could be reduced by about 18%at 973 K and 20%at 1073 K when the air with a 5 K higher temperature is supplied.Additionally,despite the higher electrochemical performance observed,the cell with a counter-fl ow arrange-ment featured by a larger hot area and higher maximum temperature gradients is not preferable for a ceramic SOFC system considering thermal durability.Overall,this study could provide insightful thermal information for the operating condition selection,structure design,and stability assessment of realistic SOFCs combined with their internal reforming process.展开更多
Frost heave in seasonally frozen regions is a one-dimensional process that could severely damage infrastructure subgrades.Stress state,temperature and water migration are important factors for frost heave.This work in...Frost heave in seasonally frozen regions is a one-dimensional process that could severely damage infrastructure subgrades.Stress state,temperature and water migration are important factors for frost heave.This work investigated the effects of soil temperature and volumetric water content on the transient frost heave ratio during the freezing of saturated silty clay in an open system and analyzed the relationships between the transient frost heave ratio and freezing rate and between temperature gradient and frost heave rate.The results show that the frost heave ratio,frost heave rate,and freezing rate are positively correlated with the temperature gradient since the temperature gradient drives the water migration during freezing,indicating the transient temperature gradient could be used to evaluate the frost heave of saturated silty clay.The transient freezing rate and transient frost heave ratio are logarithmically related to the transient frost heave ratio and transient temperature gradient,respectively.The effects of transient temperature gradient on frost heave are the principal mechanism responsible for different frost heave characteristics and uneven frost heave along a subgrade of the same soil type.展开更多
In order to enhance the lead and zinc recovery from the refractory Pb-Zn oxide ore, a new technology was developed based on sulfidation roasting with sulphur by temperature gradient method. The solid-liquid reaction s...In order to enhance the lead and zinc recovery from the refractory Pb-Zn oxide ore, a new technology was developed based on sulfidation roasting with sulphur by temperature gradient method. The solid-liquid reaction system was established and the sulfidation thermodynamics of lead and zinc carbonate was calculated with the software HSC 5.0. The effects of roasting temperature,molar ratio of sulphur to lead and zinc carbonate and reaction time in the first step roasting, and holding temperature and time in the second roasting on the sulfidation extent were studied at a laboratory-scale. The experimental results show that the sulfidation extents of lead and zinc are 96.50% and 97.29% under the optimal conditions, respectively, and the artificial galena, sphalerite and wurtzite were formed. By the novel sulfidizing process, it is expected that the sulphides can be recovered by conventional flotation technology.展开更多
Temperature is one of the important loads for designing slab track. The characteristic of slab track tem- perature varies greatly with different regional climates. In this work, a bi-block slab track model was built u...Temperature is one of the important loads for designing slab track. The characteristic of slab track tem- perature varies greatly with different regional climates. In this work, a bi-block slab track model was built under outdoor conditions in Chengdu area; the statistical characteristic of temperature gradient in track slab and the relationship between temperature gradient and surface air temperature were tested and analyzed. The results show that the track slab temperature gradient will vary periodically according to the surface air temperature, and show a clear nonlinearity along the height direction. The temperature gradient distribution is extremely uneven: the temperature gradient in the top part of the track slab is larger than that in the bottom part; the most frequently occurring temperature gradient of the track slab is around -3.5 ℃/m and more than 75 % locates in the level -10 to 10 ℃/m; concrete with a relatively good heat exchange condition with the surrounding air has a narrower band distribution. In addition, the frequency distribution histogram should exclude the time zone from 00:00 to 06:00 because there is almost no traffic in this period. The amplitude of track slab temperature variation is obviously lower than that of the air temperature variation, and the former is approximately linear with the latter.展开更多
Understanding the behaviours of ice nucleation in non-isothermal conditions is of great importance for the preparation and retention of supercooled water. Here ice nucleation in supercooled water under temperature gra...Understanding the behaviours of ice nucleation in non-isothermal conditions is of great importance for the preparation and retention of supercooled water. Here ice nucleation in supercooled water under temperature gradients is analyzed thermodynamically based on classical nucleation theory(CNT). Given that the free energy barrier for nucleation is dependent on temperature, different from a uniform temperature usually used in CNT, an assumption of linear temperature distribution in the ice nucleus was made and taken into consideration in analysis. The critical radius of the ice nucleus for nucleation and the corresponding nucleation model in the presence of a temperature gradient were obtained. It is observed that the critical radius is determined not only by the degree of supercooling, the only dependence in CNT, but also by the temperature gradient and even the Young's contact angle. Effects of temperature gradient on the change in free energy, critical radius,nucleation barrier and nucleation rate with different contact angles and degrees of supercooling are illustrated successively.The results show that a temperature gradient will increase the nucleation barrier and decrease the nucleation rate, particularly in the cases of large contact angle and low degree of supercooling. In addition, there is a critical temperature gradient for a given degree of supercooling and contact angle, at the higher of which the nucleation can be suppressed completely.展开更多
Laser forraing is a new flexible and dieless forming technique. To achieve the high accuracy forming, the temperature gradient mechanism (TGM) is studied. In the analysis of TGM, the plate bends about x-axis and abo...Laser forraing is a new flexible and dieless forming technique. To achieve the high accuracy forming, the temperature gradient mechanism (TGM) is studied. In the analysis of TGM, the plate bends about x-axis and about y-axis as well. To understand the deformation trend, the numerical simulation of deformation of plate is conducted by choosing different laser powers, laser spot diameters, scanning speeds, lengths, widths and thicknesses. From the results of simulation, it can be seen that the laser spot diameter, the scanning speed, laser power and thickness of plate play dominant roles in the laser forming process. However, the bending angles αx and αy show different trends with the variation of parameters. In addition, in comparison with above four parameters, the effect of length and width of plate on the beading angle may be neglected, but their effects are significant for the bending radius R.展开更多
Unidirectional freezing experiments under overburden pressure were carried out, in order to study the driving force of mois- ture migration of remodeled clay during freezing, through improving the indoor moisture migr...Unidirectional freezing experiments under overburden pressure were carried out, in order to study the driving force of mois- ture migration of remodeled clay during freezing, through improving the indoor moisture migration test device. Overburden pressure and cooling temperature with the same circumstance were changed to determine the influence on water migration of a single factor. Results show that water content increases above the location of the final ice lenses and decreases below the loca- tion. When the overburden pressure increases, water intake gradually decreases and the time starting to absorb water is delayed. The location of the final ice lens is not sensitive to overburden pressure but influenced by the temperature boundary. The im- pact of overburden pressure and maximum temperature is not obvious. Freezing rate is not sensitive to overburden pressure but influenced by temperature, and it increases when the cold temperature decreases. Frost heave and water intake flow in- creases with increasing time and rises up to a peak value, and then decreases. During the freezing process, water intake flow increases when freezing rate decreases. Water intake flow decreases when the overburden pressure increases when the cold temperature decreases. Finally, we expanded the segregation theory, and proposed a model to describe the relationship between water intake flow and freezing rate.展开更多
The temporal interface microstructures and diffusions in the diffusion couples with the mutual interactions of the temperature gradient, concentration difference and initial aging time of the alloys are studied by pha...The temporal interface microstructures and diffusions in the diffusion couples with the mutual interactions of the temperature gradient, concentration difference and initial aging time of the alloys are studied by phase-field simulation, and the diffusion couples are produced by the initial aged spinodal alloys with different compositions. Temporal composition evolution and volume fraction of the separated phase indicate the element diffusion direction through the interface under the temperature gradient. The increased temperature gradient induces a wide single-phase region on two sides of the interface.The uphill diffusion proceeds through the interface, no matter whether the diffusion direction is up or down with respect to the temperature gradient. For an alloy with short initial aging time, phase transformation accompanying the interdiffusion results in the straight interface with the single-phase regions on both sides. Compared with the temperature gradient,composition difference of diffusion couple and initial aging time of the alloy show greater effects on diffusion and interface microstructure.展开更多
The effects of temperature-gradient-induced damage of zirconia metering nozzles were investigated through analysis of the phase composition and microstructure of nozzle samples. The analysis was carried out using X-ra...The effects of temperature-gradient-induced damage of zirconia metering nozzles were investigated through analysis of the phase composition and microstructure of nozzle samples. The analysis was carried out using X-ray diffraction and scanning electron microscopy after the samples were subjected to a heat treatment based on the temperatures of the affected, transition, and original layers of zirconia metering nozzles during the continuous casting of steel. The results showed that, after heat treatment at 1540, 1410, or 1300A degrees C for a dwell time of 5 h, the monoclinic zirconia phase was gradually stabilized with increasing heat-treatment temperature. Moreover, a transformation to the cubic zirconia phase occurred, accompanied by grain growth, which illustrates that the temperature gradient in zirconia metering nozzles affects the mineral composition and microstructure of the nozzles and accelerates damage, thereby deteriorating the quality and service life of the nozzles.展开更多
As a result of the nonlinear effect, acoustic streaming has been widely used for increasing the transport coefficient or driving a rotor, for example, in resonant cavities and non-contact ultrasonic motors. It has bee...As a result of the nonlinear effect, acoustic streaming has been widely used for increasing the transport coefficient or driving a rotor, for example, in resonant cavities and non-contact ultrasonic motors. It has been demonstrated by experiments that a temperature gradient transverse to the wave propagating direction can significantly increase the velocity of the streaming flows in resonant cavities. To check whether the transverse temperature gradient can also increase the working velocity of acoustic streaming-driven motors, we investigate this issue by numerically solving the hydrodynamic equations. It is found that the velocity of the rotor only weakly depends on the transverse temperature gradient, e.g., even with a temperature difference of 40℃ between the rotor and the stator, the velocity increases only -8.8%.展开更多
We propose to generate and reverse the spin accumulation in a quantum dot (QD) by using the temperature difference between the two ferromagnetic leads connected to the dot. The electrons are driven purely by the tem...We propose to generate and reverse the spin accumulation in a quantum dot (QD) by using the temperature difference between the two ferromagnetic leads connected to the dot. The electrons are driven purely by the temperature gradient in the absence of an electric bias and a magnetic field. In the Coulomb blockade regime, we find two ways to reverse the spin accumulation. One is by adjusting the QD energy level with a fixed temperature gradient, and the other is by reversing the temperature gradient direction for a fixed value of the dot level. The spin accumulation in the QD can be enhanced by the magnitudes of both the leads' spin polarization and the asymmetry of the dot-lead coupling strengths. The present device is quite simple, and the obtained results may have practical usage in spintronics or quantum information processing.展开更多
The solidification microstructures and solute segregation of a newly developed hot corrosion resistant single-crystal Ni-base superalloy were investigated with a zone-melting and ultra-high thermal gradient unidirecti...The solidification microstructures and solute segregation of a newly developed hot corrosion resistant single-crystal Ni-base superalloy were investigated with a zone-melting and ultra-high thermal gradient unidirectional solidification apparatus.Compared with the microstructures solidified at conventional low thermal gradient conditions,the dendrite arm spacings,the interdendritic microporosity and γ/γ' eutectic,and the severity of solute segregation of the single-crystal superalloy solidified at ultra-high thermal gradient conditions were considerably reduced.It was shown that the microstructure solidified under ultra-high thermal gradient condition is ideal for the full exploitation of the excellent property potentials of single-crystal superalloys.展开更多
The detailed laser surface remelting experiments of Cu-31.4 wt pct Mn and Cu-26.6 wt pct Mn alloys on a 5 kW CO2 laser were carried out to study the effects of processing parameters (scanning velocity, output power of...The detailed laser surface remelting experiments of Cu-31.4 wt pct Mn and Cu-26.6 wt pct Mn alloys on a 5 kW CO2 laser were carried out to study the effects of processing parameters (scanning velocity, output power of laser) on the growth direction of microstructure in the molten pool and cellular spacing selection under the condition of ultra-high temperature gradient and rapid directional solidification. The experimental results show that the growth direction of microstructure is strongly affected by laser processing parameters. The ultra-high temperature gradient directional solidification can be realized on the surface of samples during laser surface remelting by controlling laser processing parameters, the temperature gradient and growth velocity can reach 106 K/m and 24.1 mm/s, respectively, and the solidification microstructure in the center of the molten pool grows along the laser beam scanning direction. There exists a distribution range of cellular spacings under the laser rapid solidification conditions, and the average spacing decreases with increasing of growth rate. The maximum, λmax, minimum, λmin, and average primary spacing, A, as functions of growth rate, Vb, can be given by,λmax=12.54Vb-0.61, λmin=4.47 Vb-0.52, λ=9.09Vb-0.62, respectively. The experimental results are compared with the current Hunt-Lu model for rapid cellular/dendritic growth, and a good agreement is found.展开更多
The relationship between the partition ratio at a solid-liquid interface and the temperature gradient or the external force field has been theoretically analysed.It is shown that under the influence of a temperature g...The relationship between the partition ratio at a solid-liquid interface and the temperature gradient or the external force field has been theoretically analysed.It is shown that under the influence of a temperature gradient or an external force field,the partition ratio at a solid-liquid interface will deviate from the equilibrium value.展开更多
Based on the car front-wheel-hub forging forming process of numerical simulation, the temperature gradient expression of forging model cavity near the surface layer was got ten, which illustrates that the forging temp...Based on the car front-wheel-hub forging forming process of numerical simulation, the temperature gradient expression of forging model cavity near the surface layer was got ten, which illustrates that the forging temperature gradient is related to forging die materials thermal conductivity, specific heat and impact speed, and the correlation coefficient is 0.97. Under the different thermal conductivity, heat capacity and forging speed, the temperature gradient was compared with each other. The paper obtained the relevant laws, which illustrates the temperature gradient relates to these three parameters in a sequence of thermal conductivity 〉 impact speed〉 specific heat capacity. To reduce thermal stress in the near-surface layer of hot forging cavity, the material with greater thermal conductivity coefficient and specific heat capacity should be used.展开更多
During heat treatment or mechanical processing,most polycrystalline materials experience grain growth,which significantly affects their mechanical properties.Microstructure simulation on a mesoscopic scale is an impor...During heat treatment or mechanical processing,most polycrystalline materials experience grain growth,which significantly affects their mechanical properties.Microstructure simulation on a mesoscopic scale is an important way of studying grain growth.A key research focus of this type of method has long been how to efficiently and accurately simulate the grain growth caused by a non-uniform temperature field with temperature gradients.In this work,we propose an improved 3D Monte Carlo Potts(MCP)method to quantitatively study the relationship between non-uniform temperature fields and final grain morphologies.Properties of the aluminum alloy AA6061-T6 are used to establish a trial calculation model and to verify the algorithms with existing experimental results in literature.The detailed grain growth process of the 6061-T6 aluminum alloy under different temperature fields is then obtained,and grain morphologies at various positions are analyzed.Results indicate that while absolute temperature and duration time are the primary factors determining the final grain size,the temperature gradient also has strong influence on the grain morphologies.The relationships between temperatures,temperature gradients and grain growth process have been established.The proposed MCP algorithm can be applied to different types of materials when the proper parameters are used.展开更多
Electron thermal transport is one of the most complex processes in fusionplasmas. It is generally described by a simple thermal diffusivity in transport analyses ofdischarges, but there is evidence of critical gradien...Electron thermal transport is one of the most complex processes in fusionplasmas. It is generally described by a simple thermal diffusivity in transport analyses ofdischarges, but there is evidence of critical gradient effects with moderate stiffness. By analyzingperiodic perturbations to an equilibrium, one canmeasure the variations in electron energy flux andelectron temperature gradient over the perturbation cycle, obtaining the flux as a function ofgradient over the range of parameters generated by the perturbation. Although time-dependenttransport analysis is very sensitive to noise in the input data, averaging over many cycles of aperiodic perturbation can provide data of sufficient quality. The analyses presented here are basedon the ECE temperature data with high spatial and temporal resolution and full profile coverage onDIII-D for sawteeth and modulated ECH heating.展开更多
基金Project supported by the National Natural Science Foundation of China (Nos.12172326 and 11972319)the National Key Research and Development Program of China (No.2020YFA0711700)the Natural Science Foundation of Zhejiang Province of China (No.LR21A020002)。
文摘Piezoelectric semiconductors(PSs)possess both semiconducting properties and piezoelectric coupling effects,making them optimal building blocks for semiconductor devices.PS fiber-like structures have wide applications in multi-functional semiconductor devices.In this paper,a one-dimensional(1D)theoretical model is established to describe the piezotronic responses of a PS fiber under gradient temperature changes.The theoretical model aims to explain the mechanism behind the resistance change caused by such gradient temperature changes.Numerical results demonstrate that a gradient temperature change significantly affects the physical fields within the PS fiber,and can induce changes in its surface resistance.It provides important theoretical guidance on the development of piezotronic devices that are sensitive to temperature effects.
基金supported by National Natural Science Foundation of China(Nos.U1967206 and 12275071)National Key R&D Program of China(No.2017YFE0301201)。
文摘The effects of impurities on ion temperature gradient(ITG)driven turbulence transport in tokamak core plasmas are investigated numerically via global simulations of microturbulence with carbon impurities and adiabatic electrons.The simulations use an extended fluid code(ExFC)based on a four-field gyro-Landau-fluid(GLF)model.The multispecies form of the normalized GLF equations is presented,which guarantees the self-consistent evolution of both bulk ions and impurities.With parametric profiles of the cyclone base case,well-benchmarked ExFC is employed to perform simulations focusing on different impurity density profiles.For a fixed temperature profile,it is found that the turbulent heat diffusivity of bulk ions in a quasi-steady state is usually lower than that without impurities,which is contrary to the linear and quasilinear predictions.The evolutions of the temperature gradient and heat diffusivity exhibit a fast relaxation process,indicating that the destabilization of the outwardly peaked impurity profile is a transient state response.Furthermore,the impurity effects from different profiles can obviously influence the nonlinear critical temperature gradient,which is likely to be dominated by linear effects.These results suggest that the improvement in plasma confinement could be attributed to the impurities,most likely through adjusting both heat diffusivity and the critical temperature gradient.
基金by the Project of Strategic Importance Funding Scheme from The Hong Kong China Polytechnic University(No.P0035168)the National Natural Science Foundation of China(No.51806241).
文摘Thermal management in solid oxide fuel cells(SOFC)is a critical issue due to non-uniform electrochemical reactions and convective fl ows within the cells.Therefore,a 2D mathematical model is established herein to investigate the thermal responses of a tubular methanol-fueled SOFC.Results show that unlike the low-temperature condition of 873 K,where the peak temperature gradient occurs at the cell center,it appears near the fuel inlet at 1073 K because of the rapid temperature rise induced by the elevated current density.Despite the large heat convection capacity,excessive air could not eff ectively eliminate the harmful temperature gradient caused by the large current density.Thus,optimal control of the current density by properly selecting the operating potential could generate a local thermal neutral state.Interestingly,the maximum axial temperature gradient could be reduced by about 18%at 973 K and 20%at 1073 K when the air with a 5 K higher temperature is supplied.Additionally,despite the higher electrochemical performance observed,the cell with a counter-fl ow arrange-ment featured by a larger hot area and higher maximum temperature gradients is not preferable for a ceramic SOFC system considering thermal durability.Overall,this study could provide insightful thermal information for the operating condition selection,structure design,and stability assessment of realistic SOFCs combined with their internal reforming process.
基金supported by the National Natural Science Foundation of China(No.51808128)the Natural Science Foundation of Fujian Province(No.2022J01091)。
文摘Frost heave in seasonally frozen regions is a one-dimensional process that could severely damage infrastructure subgrades.Stress state,temperature and water migration are important factors for frost heave.This work investigated the effects of soil temperature and volumetric water content on the transient frost heave ratio during the freezing of saturated silty clay in an open system and analyzed the relationships between the transient frost heave ratio and freezing rate and between temperature gradient and frost heave rate.The results show that the frost heave ratio,frost heave rate,and freezing rate are positively correlated with the temperature gradient since the temperature gradient drives the water migration during freezing,indicating the transient temperature gradient could be used to evaluate the frost heave of saturated silty clay.The transient freezing rate and transient frost heave ratio are logarithmically related to the transient frost heave ratio and transient temperature gradient,respectively.The effects of transient temperature gradient on frost heave are the principal mechanism responsible for different frost heave characteristics and uneven frost heave along a subgrade of the same soil type.
基金Project(51204210)supported by the National Natural Science Foundation of ChinaProject(2011AA061001)supported by the High-Tech Research and Development Program of ChinaProject(2012BAC12B04)supported by the National Science&Technology During the12th Five-Year Plan Period,China
文摘In order to enhance the lead and zinc recovery from the refractory Pb-Zn oxide ore, a new technology was developed based on sulfidation roasting with sulphur by temperature gradient method. The solid-liquid reaction system was established and the sulfidation thermodynamics of lead and zinc carbonate was calculated with the software HSC 5.0. The effects of roasting temperature,molar ratio of sulphur to lead and zinc carbonate and reaction time in the first step roasting, and holding temperature and time in the second roasting on the sulfidation extent were studied at a laboratory-scale. The experimental results show that the sulfidation extents of lead and zinc are 96.50% and 97.29% under the optimal conditions, respectively, and the artificial galena, sphalerite and wurtzite were formed. By the novel sulfidizing process, it is expected that the sulphides can be recovered by conventional flotation technology.
基金financially supported by National Natural Science Foundation of China(No.50827102)the Scientific Research Foundation for Ph.D.,Northwest A&F University(No.Z109021103)+1 种基金the Special Fund for Basic Scientific Research of Central Colleges,Northwest A&F University(No.Z109021114)the Fund of the State Key Laboratory of Solidification Processing in NWPU(No.SKLSP201220)
基金supported by the National Key Basic Research Program of China (973 Program) (2013CB036202)the National Natural Science Foundation of China (51008258)Fundamental Research Funds for the Central Universities (SWJTU12CX065)
文摘Temperature is one of the important loads for designing slab track. The characteristic of slab track tem- perature varies greatly with different regional climates. In this work, a bi-block slab track model was built under outdoor conditions in Chengdu area; the statistical characteristic of temperature gradient in track slab and the relationship between temperature gradient and surface air temperature were tested and analyzed. The results show that the track slab temperature gradient will vary periodically according to the surface air temperature, and show a clear nonlinearity along the height direction. The temperature gradient distribution is extremely uneven: the temperature gradient in the top part of the track slab is larger than that in the bottom part; the most frequently occurring temperature gradient of the track slab is around -3.5 ℃/m and more than 75 % locates in the level -10 to 10 ℃/m; concrete with a relatively good heat exchange condition with the surrounding air has a narrower band distribution. In addition, the frequency distribution histogram should exclude the time zone from 00:00 to 06:00 because there is almost no traffic in this period. The amplitude of track slab temperature variation is obviously lower than that of the air temperature variation, and the former is approximately linear with the latter.
文摘Understanding the behaviours of ice nucleation in non-isothermal conditions is of great importance for the preparation and retention of supercooled water. Here ice nucleation in supercooled water under temperature gradients is analyzed thermodynamically based on classical nucleation theory(CNT). Given that the free energy barrier for nucleation is dependent on temperature, different from a uniform temperature usually used in CNT, an assumption of linear temperature distribution in the ice nucleus was made and taken into consideration in analysis. The critical radius of the ice nucleus for nucleation and the corresponding nucleation model in the presence of a temperature gradient were obtained. It is observed that the critical radius is determined not only by the degree of supercooling, the only dependence in CNT, but also by the temperature gradient and even the Young's contact angle. Effects of temperature gradient on the change in free energy, critical radius,nucleation barrier and nucleation rate with different contact angles and degrees of supercooling are illustrated successively.The results show that a temperature gradient will increase the nucleation barrier and decrease the nucleation rate, particularly in the cases of large contact angle and low degree of supercooling. In addition, there is a critical temperature gradient for a given degree of supercooling and contact angle, at the higher of which the nucleation can be suppressed completely.
文摘Laser forraing is a new flexible and dieless forming technique. To achieve the high accuracy forming, the temperature gradient mechanism (TGM) is studied. In the analysis of TGM, the plate bends about x-axis and about y-axis as well. To understand the deformation trend, the numerical simulation of deformation of plate is conducted by choosing different laser powers, laser spot diameters, scanning speeds, lengths, widths and thicknesses. From the results of simulation, it can be seen that the laser spot diameter, the scanning speed, laser power and thickness of plate play dominant roles in the laser forming process. However, the bending angles αx and αy show different trends with the variation of parameters. In addition, in comparison with above four parameters, the effect of length and width of plate on the beading angle may be neglected, but their effects are significant for the bending radius R.
基金supported by the National Basic Research Program("973") of China (2012CB026102)the National Natural Science Foundation of China (No.41271080 and No.41230630)the open fund of Qinghai Research and Observation Base, Key Laboratory of Highway Construction and Maintenance Technology in Permafrost Region Ministry of Transport, PRC(2012-12-4)
文摘Unidirectional freezing experiments under overburden pressure were carried out, in order to study the driving force of mois- ture migration of remodeled clay during freezing, through improving the indoor moisture migration test device. Overburden pressure and cooling temperature with the same circumstance were changed to determine the influence on water migration of a single factor. Results show that water content increases above the location of the final ice lenses and decreases below the loca- tion. When the overburden pressure increases, water intake gradually decreases and the time starting to absorb water is delayed. The location of the final ice lens is not sensitive to overburden pressure but influenced by the temperature boundary. The im- pact of overburden pressure and maximum temperature is not obvious. Freezing rate is not sensitive to overburden pressure but influenced by temperature, and it increases when the cold temperature decreases. Frost heave and water intake flow in- creases with increasing time and rises up to a peak value, and then decreases. During the freezing process, water intake flow increases when freezing rate decreases. Water intake flow decreases when the overburden pressure increases when the cold temperature decreases. Finally, we expanded the segregation theory, and proposed a model to describe the relationship between water intake flow and freezing rate.
基金Project supported by the National Natural Science Foundation of China(Grant No.51571122)the Fundamental Research Funds for the Central UniversitiesChina(Grant No.30920130121012)
文摘The temporal interface microstructures and diffusions in the diffusion couples with the mutual interactions of the temperature gradient, concentration difference and initial aging time of the alloys are studied by phase-field simulation, and the diffusion couples are produced by the initial aged spinodal alloys with different compositions. Temporal composition evolution and volume fraction of the separated phase indicate the element diffusion direction through the interface under the temperature gradient. The increased temperature gradient induces a wide single-phase region on two sides of the interface.The uphill diffusion proceeds through the interface, no matter whether the diffusion direction is up or down with respect to the temperature gradient. For an alloy with short initial aging time, phase transformation accompanying the interdiffusion results in the straight interface with the single-phase regions on both sides. Compared with the temperature gradient,composition difference of diffusion couple and initial aging time of the alloy show greater effects on diffusion and interface microstructure.
基金financially supported by the National Natural Science Foundation of China (No. 51372193)
文摘The effects of temperature-gradient-induced damage of zirconia metering nozzles were investigated through analysis of the phase composition and microstructure of nozzle samples. The analysis was carried out using X-ray diffraction and scanning electron microscopy after the samples were subjected to a heat treatment based on the temperatures of the affected, transition, and original layers of zirconia metering nozzles during the continuous casting of steel. The results showed that, after heat treatment at 1540, 1410, or 1300A degrees C for a dwell time of 5 h, the monoclinic zirconia phase was gradually stabilized with increasing heat-treatment temperature. Moreover, a transformation to the cubic zirconia phase occurred, accompanied by grain growth, which illustrates that the temperature gradient in zirconia metering nozzles affects the mineral composition and microstructure of the nozzles and accelerates damage, thereby deteriorating the quality and service life of the nozzles.
基金Supported by the National Natural Science Foundation of China under Grant No 10874083
文摘As a result of the nonlinear effect, acoustic streaming has been widely used for increasing the transport coefficient or driving a rotor, for example, in resonant cavities and non-contact ultrasonic motors. It has been demonstrated by experiments that a temperature gradient transverse to the wave propagating direction can significantly increase the velocity of the streaming flows in resonant cavities. To check whether the transverse temperature gradient can also increase the working velocity of acoustic streaming-driven motors, we investigate this issue by numerically solving the hydrodynamic equations. It is found that the velocity of the rotor only weakly depends on the transverse temperature gradient, e.g., even with a temperature difference of 40℃ between the rotor and the stator, the velocity increases only -8.8%.
基金Project supported by the National Natural Science Foundation of China (Grant No. 10704011)the SKLSM, China (Grant No.CHJG200901)the LNET, China (Grant No. 2009R01)
文摘We propose to generate and reverse the spin accumulation in a quantum dot (QD) by using the temperature difference between the two ferromagnetic leads connected to the dot. The electrons are driven purely by the temperature gradient in the absence of an electric bias and a magnetic field. In the Coulomb blockade regime, we find two ways to reverse the spin accumulation. One is by adjusting the QD energy level with a fixed temperature gradient, and the other is by reversing the temperature gradient direction for a fixed value of the dot level. The spin accumulation in the QD can be enhanced by the magnitudes of both the leads' spin polarization and the asymmetry of the dot-lead coupling strengths. The present device is quite simple, and the obtained results may have practical usage in spintronics or quantum information processing.
文摘The solidification microstructures and solute segregation of a newly developed hot corrosion resistant single-crystal Ni-base superalloy were investigated with a zone-melting and ultra-high thermal gradient unidirectional solidification apparatus.Compared with the microstructures solidified at conventional low thermal gradient conditions,the dendrite arm spacings,the interdendritic microporosity and γ/γ' eutectic,and the severity of solute segregation of the single-crystal superalloy solidified at ultra-high thermal gradient conditions were considerably reduced.It was shown that the microstructure solidified under ultra-high thermal gradient condition is ideal for the full exploitation of the excellent property potentials of single-crystal superalloys.
基金The financial aid of the National Natural Science Foundation of China under grant No.59771054Postdoctoral Science Foundation of China+1 种基金Postdoctoral Science Foundation of Tsinghua-Zhongda985 Science Foundation of Tsinghua University are gratefully acknowledged.
文摘The detailed laser surface remelting experiments of Cu-31.4 wt pct Mn and Cu-26.6 wt pct Mn alloys on a 5 kW CO2 laser were carried out to study the effects of processing parameters (scanning velocity, output power of laser) on the growth direction of microstructure in the molten pool and cellular spacing selection under the condition of ultra-high temperature gradient and rapid directional solidification. The experimental results show that the growth direction of microstructure is strongly affected by laser processing parameters. The ultra-high temperature gradient directional solidification can be realized on the surface of samples during laser surface remelting by controlling laser processing parameters, the temperature gradient and growth velocity can reach 106 K/m and 24.1 mm/s, respectively, and the solidification microstructure in the center of the molten pool grows along the laser beam scanning direction. There exists a distribution range of cellular spacings under the laser rapid solidification conditions, and the average spacing decreases with increasing of growth rate. The maximum, λmax, minimum, λmin, and average primary spacing, A, as functions of growth rate, Vb, can be given by,λmax=12.54Vb-0.61, λmin=4.47 Vb-0.52, λ=9.09Vb-0.62, respectively. The experimental results are compared with the current Hunt-Lu model for rapid cellular/dendritic growth, and a good agreement is found.
文摘The relationship between the partition ratio at a solid-liquid interface and the temperature gradient or the external force field has been theoretically analysed.It is shown that under the influence of a temperature gradient or an external force field,the partition ratio at a solid-liquid interface will deviate from the equilibrium value.
基金Funded by the Fundamental Research Funds for the Central University (No.2010-II-025)the National Natural Science Foundation of China(No.50675165)
文摘Based on the car front-wheel-hub forging forming process of numerical simulation, the temperature gradient expression of forging model cavity near the surface layer was got ten, which illustrates that the forging temperature gradient is related to forging die materials thermal conductivity, specific heat and impact speed, and the correlation coefficient is 0.97. Under the different thermal conductivity, heat capacity and forging speed, the temperature gradient was compared with each other. The paper obtained the relevant laws, which illustrates the temperature gradient relates to these three parameters in a sequence of thermal conductivity 〉 impact speed〉 specific heat capacity. To reduce thermal stress in the near-surface layer of hot forging cavity, the material with greater thermal conductivity coefficient and specific heat capacity should be used.
基金The authors would like to acknowledge the financial support from China Postdoctoral Science Foundation Project(2018M641128)the National Key Research and Development Program of China(2018YFB0703500).
文摘During heat treatment or mechanical processing,most polycrystalline materials experience grain growth,which significantly affects their mechanical properties.Microstructure simulation on a mesoscopic scale is an important way of studying grain growth.A key research focus of this type of method has long been how to efficiently and accurately simulate the grain growth caused by a non-uniform temperature field with temperature gradients.In this work,we propose an improved 3D Monte Carlo Potts(MCP)method to quantitatively study the relationship between non-uniform temperature fields and final grain morphologies.Properties of the aluminum alloy AA6061-T6 are used to establish a trial calculation model and to verify the algorithms with existing experimental results in literature.The detailed grain growth process of the 6061-T6 aluminum alloy under different temperature fields is then obtained,and grain morphologies at various positions are analyzed.Results indicate that while absolute temperature and duration time are the primary factors determining the final grain size,the temperature gradient also has strong influence on the grain morphologies.The relationships between temperatures,temperature gradients and grain growth process have been established.The proposed MCP algorithm can be applied to different types of materials when the proper parameters are used.
基金the US Department of Energy under grant DE-FG03-97-ER54415
文摘Electron thermal transport is one of the most complex processes in fusionplasmas. It is generally described by a simple thermal diffusivity in transport analyses ofdischarges, but there is evidence of critical gradient effects with moderate stiffness. By analyzingperiodic perturbations to an equilibrium, one canmeasure the variations in electron energy flux andelectron temperature gradient over the perturbation cycle, obtaining the flux as a function ofgradient over the range of parameters generated by the perturbation. Although time-dependenttransport analysis is very sensitive to noise in the input data, averaging over many cycles of aperiodic perturbation can provide data of sufficient quality. The analyses presented here are basedon the ECE temperature data with high spatial and temporal resolution and full profile coverage onDIII-D for sawteeth and modulated ECH heating.