The advent of the 5G era has stimulated the rapid development of high power electronics with dense integration.Three-dimensional(3D)thermally conductive networks,possessing high thermal and electrical conductivities a...The advent of the 5G era has stimulated the rapid development of high power electronics with dense integration.Three-dimensional(3D)thermally conductive networks,possessing high thermal and electrical conductivities and many different structures,are regarded as key materials to improve the performance of electronic devices.We provide a critical overview of carbonbased 3D thermally conductive networks,emphasizing their preparation-structure-property relationships and their applications in different scenarios.A detailed discussion of the microscopic principles of thermal conductivity is provided,which is crucial for increasing it.This is followed by an in-depth account of the construction of 3D networks using different carbon materials,such as graphene,carbon foam,and carbon nanotubes.Techniques for the assembly of two-dimensional graphene into 3D networks and their effects on thermal conductivity are emphasized.Finally,the existing challenges and future prospects for 3D carbon-based thermally conductive networks are discussed.展开更多
The microstructure and surface state of three kinds of polyacrylonitrile-based carbon fibers (T700, T300 and M40) before and after high temperature treatment were investigated. Also, the pyrocarbon and thermal condu...The microstructure and surface state of three kinds of polyacrylonitrile-based carbon fibers (T700, T300 and M40) before and after high temperature treatment were investigated. Also, the pyrocarbon and thermal conductivity of carbon/carbon composites with different carbon fibers as preform were studied. The results show that M40 carbon fiber has the largest crystallite size and the least d002, T300 follows, and TT00 the third. With the increase of heat treatment temperature, the surface state and crystal size of carbon fibers change correspondingly. M40 carbon fiber exhibits the best graphitization property, followed by T300 and then T700. The different microstructure and surface state of different carbon fibers lead to the different microstructures of pyrocarbon and then result in the different thermal conductivities of carbon/carbon composites. The carbon/carbon composite with M40 as preform has the best microstructure in pyrocarbon and the highest thermal conductivity.展开更多
A conduction heat transfer process is enhanced by filling prescribed quantity and optimized-shaped high thermal conductivity materials to the substrate. Numerical simulations and analyses are performed on a volume to ...A conduction heat transfer process is enhanced by filling prescribed quantity and optimized-shaped high thermal conductivity materials to the substrate. Numerical simulations and analyses are performed on a volume to point conduction problem based on the principle of minimum entropy generation. In the optimization, the arrangement of high thermal conductivity materials is variable, the quantity of high thermal-conductivity material is constrained, and the objective is to obtain the maximum heat conduction rate as the entropy is the minimum.A novel algorithm of thermal conductivity discretization is proposed based on large quantity of calculations.Compared with other algorithms in literature, the average temperature in the substrate by the new algorithm is lower, while the highest temperature in the substrate is in a reasonable range. Thus the new algorithm is feasible. The optimization of volume to point heat conduction is carried out in a rectangular model with radiation boundary condition and constant surface temperature boundary condition. The results demonstrate that the algorithm of thermal conductivity discretization is applicable for volume to point heat conduction problems.展开更多
In CH HCSB TBM(China Helium-Cooled Solider Breeder Test Blanket Module), the structural material (Eurofer), the neutron multiplier (Be), the breeder material(Li4SiO4), and the coolant(He) are used in the des...In CH HCSB TBM(China Helium-Cooled Solider Breeder Test Blanket Module), the structural material (Eurofer), the neutron multiplier (Be), the breeder material(Li4SiO4), and the coolant(He) are used in the design. The parameter selections of materials are very important to the safety design. This paper will provide some parameters of materials property(such as specific heat, thermal conductivity) through calculation.展开更多
The behavior of building materials in constructions of civil structures is influenced by the surrounding moisture and it is a crucial for intensively examined field of the construction physics. Most standard building ...The behavior of building materials in constructions of civil structures is influenced by the surrounding moisture and it is a crucial for intensively examined field of the construction physics. Most standard building materials are characterized by a porous structure, which results in the ability to receive water in a liquid as well as gaseous form in the inner pores. The water fills the storage space of pores under certain conditions; it is transported and transferred back to the surroundings. Many technical studies show that the moisture monitoring is prevailingly based on experiments. Previous calculating methods introduced, e.g., by Glaser, which became the basis for the standard calculations in many European countries in the 1960s, are not always sufficient with respect to the demands of the civil structures. The moisture influences thermo-insulating properties of the material. By a change of the thermo insulation properties of the construction also the thermal and diffusion scheme of the construction is changed and its thermal resistance is decreasing. Faults in the thermo-technical projects occur when thermal conductivity coefficient L values for material in a dry state are substituted.The aim of the research is to determine the capillary conductivity coefficient as a characteristic material moisture parameter of the building materials by the means of a non-destructive method using the experimentally assembled apparatus developed at the Department of Civil Engineering, Brno University of Technology. Keywords: Capillary conductivity coefficient, moisture transfer, EMWR (electromagnetic microwave radiation), diffusion展开更多
The one-dimensional calculation of the gas/particle flows of a supersonic two-stage high-velocity oxy-fuel(HVOF) thermal spray process was performed. The internal gas flow was solved by numerically integrating theequa...The one-dimensional calculation of the gas/particle flows of a supersonic two-stage high-velocity oxy-fuel(HVOF) thermal spray process was performed. The internal gas flow was solved by numerically integrating theequations of the quasi-one-dimensional flow including the effects of pipe friction and heat transfer. As for the supersonicjet flow, semi-empirical equations were used to obtain the gas velocity and temperature along the centerline. The velocity and temperature of the particle were obtained by an one-way coupling method. The material ofthe spray particle selected in this study is ultra high molecular weight polyethylene (UHMWPE). The temperaturedistributions in the spherical UHMWPE particles of 50 and 150 m accelerated and heated by the supersonic gasflow was clarified.展开更多
We present a rigorous homogenization approach for elcient computation of a class of physical problems in a one-dimensional periodic heterogeneous material. This material is represented by a spatially periodic array of...We present a rigorous homogenization approach for elcient computation of a class of physical problems in a one-dimensional periodic heterogeneous material. This material is represented by a spatially periodic array of unit cells with a length of More specifically, the method is applied to the diffusion, heat conduction, and wave propagation problems. Heterogeneous materials can have arbitrary position-dependent continuous or discontinuous materials properties (for example heat conductivity) within the unit cell. The final effective model includes both effective properties at the leading order and high-order contributions due to the microscopic heterogeneity. A dimensionless heterogeneity parameter ~ is defined to represent high-order contributions, shown to be in the range of [-1/12, 0], and has a universal expression for all three problems. Both effective properties and heterogeneity parameter 13 are independent oft, the microscopic scale of heterogeneity. The homogenized solution describing macroscopic variations can be obtained from the effective model. Solution with sub-unit-cell accuracy can be constructed based on the homogenized solution and its spatial derivatives. The paper represents a general approach to obtain the effective model for arbitrary periodic heterogeneous materials with position-dependent properties.展开更多
This paper presents experimental results of thermal radiative properties of xonotlite-type calcium silicate insulation material. Transmittance spectra were first taken using Fourier transform infrared spectrometer (F...This paper presents experimental results of thermal radiative properties of xonotlite-type calcium silicate insulation material. Transmittance spectra were first taken using Fourier transform infrared spectrometer (FTIR) for the samples with p = 234 kg/m^3. Specific extinction coefficient spectra were then obtained by applying Beer's law. Finally, by using the diffusion approximation, the specific Rossland mean extinction coefficients and radiative thermal conductivities were obtained for various temperatures. The results show that the specific spectral extinction coefficient of xonotlite is larger than 7 m^2/kg in the whole measured spectra, and diffusion approximation equation is a reasonable description of radiative heat transfer in xonotlite insulation material. The specific Rossland mean extinction coefficient of xonotlite has a maximum value at 400 K and the radiative thermal conductivity is almost proportional to the cube of temperature.展开更多
An inverse method is presented for estimating the unknown boundary incident radiation heat flux onone side of one-dimensional semitransparent planar slab with semitransparent boundaries from theknowledge of the radiat...An inverse method is presented for estimating the unknown boundary incident radiation heat flux onone side of one-dimensional semitransparent planar slab with semitransparent boundaries from theknowledge of the radiation intensities exiting from the other side. The inverse problem is solved usingconjugate gradient method of minimization based on discrete ordinates method (DOM) of radiativetransfer equation. The equations of sensitivity coefficients are derived and easily solved by DOM, withthe result that the complicated numerical differentiation commonly used in solving sensitivity coefficients is avoided. The effects of anisotropic scattering, absorption coefficient, scattering coefficient,boundary reflectivity, fluid temperature outside the boundaries, convection heat transfer coefficients,conduction coefficient of semitransparent media and slab thickness on the accuracy of the inverse analysis are investigated. The results show that the boundary incident radiation heat flux can be estimatedaccurately, even with noisy data.展开更多
Manipulating thermal conductivities are fundamentally important for controlling the conduction of heat at will. Thermal cloaks and concentrators, which have been extensively studied recently, are actually graded mater...Manipulating thermal conductivities are fundamentally important for controlling the conduction of heat at will. Thermal cloaks and concentrators, which have been extensively studied recently, are actually graded materials designed according to coordinate transformation approaches, and their effective thermal conductivity is equal to that of the host medium outside the cloak or concentrator. Here we attempt to investigate a more general problem: what is the effective thermal conductivity of graded materials? In particular, we perform a first-principles approach to the analytic exact results of effective thermal conductivities of materials possessing either power-law or linear gradation profiles. On the other hand, by solving Laplace's equation, we derive a differential equation for calculating the effective thermal conductivity of a material whose thermal conductivity varies along the radius with arbitrary gradation profiles.The two methods agree with each other for both external and internal heat sources, as confirmed by simulation and experiment. This work provides different methods for designing new thermal metamaterials(including thermal cloaks and concentrators), in order to control or manipulate the transfer of heat.展开更多
Interfacial solar steam generation is an efficient water evaporation technology which has promising applications in desalination,sterilization,water purification and treatment.A common component of evaporator design i...Interfacial solar steam generation is an efficient water evaporation technology which has promising applications in desalination,sterilization,water purification and treatment.A common component of evaporator design is a thermal-insulation support placed between the photothermal evaporation surface and bulk water.This configuration,common in 2-dimensional(2 D)evaporation systems,minimizes heat loss from evaporation surface to bulk water,thus localizing the heat on the evaporation surface for efficient evaporation.This design is subsequently directly adopted for 3-dimensional(3 D)evaporators without any consideration if it is appropriate.However,unlike 2 D solar evaporators,the 3 D evaporators can also harvest additional energy(other than solar light)from the air and bulk water to enhance evaporation rate.In this scenario,the use of thermal insulator support is not proper since it will hinder energy extraction from water.Here,the traditional 3 D evaporator configuration was completely redesigned by using a highly thermally conductive material,instead of a thermal insulator,to connect evaporation surfaces and the bulk water.Much higher evaporation rates were achieved by this strategy,owing to the rapid heat transfer from the bulk water to the evaporation surfaces.Indoor and outdoor tests both confirmed that evaporation performance could be significantly improved by substituting a thermal insulator with thermally conductive support.These findings will redirect the future design of 3 D photothermal evaporators.展开更多
With the large latent heat and low cost, the paraffin has been widely used in battery thermal management(BTM) system to improve the efficiency and cycling life of power battery. The numerical model of paraffin melting...With the large latent heat and low cost, the paraffin has been widely used in battery thermal management(BTM) system to improve the efficiency and cycling life of power battery. The numerical model of paraffin melting in a cavity has been established, and the effects on the solid–liquid phase change process have been investigated for the purpose of enhancing the heat transfer performance of paraffin-based BTM system. The results showed that the location of the heating wall had great effects on the melting process. The paraffin in the cavity melted most quickly when the heating wall located at the bottom. Furthermore, the effects of thermal conductivity and the velocity of the slip wall have been considered. The gradient of liquid fraction increased with the increase in thermal conductivity, and the melting process could be accelerated or delayed by the slip wall with different velocity.展开更多
We present a general homogenization method a periodic heterogeneous material with piecewise constants for diffusion, heat conduction, and wave propagation in The method is relevant to the frequently encountered upsca...We present a general homogenization method a periodic heterogeneous material with piecewise constants for diffusion, heat conduction, and wave propagation in The method is relevant to the frequently encountered upscaling issues for heterogeneous materials. The dispersion relation for each problem is first expressed in the general form where the frequency co (or wavenumber k) is expanded in terms of the wavenumber k (or frequency ω). A general homogenization model can be directly obtained with any given dispersion relation. Next step we study the unit cell of the heterogeneous material and derive the exact dispersion relation. The final homogenized equations include both leading order terms (effective properties) and high order contributions that represent the effect of the microscopic heterogeneity on the macroscopic behavior. That effect can be lumped into a single dimensionless heterogeneity parameter β, which is bounded between -1/12≤β≤ 0 and has a universal expression for all three problems. Numerical examples validate the proposed method and demonstrate a significant computational saving.展开更多
文摘The advent of the 5G era has stimulated the rapid development of high power electronics with dense integration.Three-dimensional(3D)thermally conductive networks,possessing high thermal and electrical conductivities and many different structures,are regarded as key materials to improve the performance of electronic devices.We provide a critical overview of carbonbased 3D thermally conductive networks,emphasizing their preparation-structure-property relationships and their applications in different scenarios.A detailed discussion of the microscopic principles of thermal conductivity is provided,which is crucial for increasing it.This is followed by an in-depth account of the construction of 3D networks using different carbon materials,such as graphene,carbon foam,and carbon nanotubes.Techniques for the assembly of two-dimensional graphene into 3D networks and their effects on thermal conductivity are emphasized.Finally,the existing challenges and future prospects for 3D carbon-based thermally conductive networks are discussed.
基金Project(201012200233)supported by the Freedom Explore Program of Central South University,China
文摘The microstructure and surface state of three kinds of polyacrylonitrile-based carbon fibers (T700, T300 and M40) before and after high temperature treatment were investigated. Also, the pyrocarbon and thermal conductivity of carbon/carbon composites with different carbon fibers as preform were studied. The results show that M40 carbon fiber has the largest crystallite size and the least d002, T300 follows, and TT00 the third. With the increase of heat treatment temperature, the surface state and crystal size of carbon fibers change correspondingly. M40 carbon fiber exhibits the best graphitization property, followed by T300 and then T700. The different microstructure and surface state of different carbon fibers lead to the different microstructures of pyrocarbon and then result in the different thermal conductivities of carbon/carbon composites. The carbon/carbon composite with M40 as preform has the best microstructure in pyrocarbon and the highest thermal conductivity.
基金Supported by the National Key Basic Research Program of China(2013CB228305)
文摘A conduction heat transfer process is enhanced by filling prescribed quantity and optimized-shaped high thermal conductivity materials to the substrate. Numerical simulations and analyses are performed on a volume to point conduction problem based on the principle of minimum entropy generation. In the optimization, the arrangement of high thermal conductivity materials is variable, the quantity of high thermal-conductivity material is constrained, and the objective is to obtain the maximum heat conduction rate as the entropy is the minimum.A novel algorithm of thermal conductivity discretization is proposed based on large quantity of calculations.Compared with other algorithms in literature, the average temperature in the substrate by the new algorithm is lower, while the highest temperature in the substrate is in a reasonable range. Thus the new algorithm is feasible. The optimization of volume to point heat conduction is carried out in a rectangular model with radiation boundary condition and constant surface temperature boundary condition. The results demonstrate that the algorithm of thermal conductivity discretization is applicable for volume to point heat conduction problems.
文摘In CH HCSB TBM(China Helium-Cooled Solider Breeder Test Blanket Module), the structural material (Eurofer), the neutron multiplier (Be), the breeder material(Li4SiO4), and the coolant(He) are used in the design. The parameter selections of materials are very important to the safety design. This paper will provide some parameters of materials property(such as specific heat, thermal conductivity) through calculation.
文摘The behavior of building materials in constructions of civil structures is influenced by the surrounding moisture and it is a crucial for intensively examined field of the construction physics. Most standard building materials are characterized by a porous structure, which results in the ability to receive water in a liquid as well as gaseous form in the inner pores. The water fills the storage space of pores under certain conditions; it is transported and transferred back to the surroundings. Many technical studies show that the moisture monitoring is prevailingly based on experiments. Previous calculating methods introduced, e.g., by Glaser, which became the basis for the standard calculations in many European countries in the 1960s, are not always sufficient with respect to the demands of the civil structures. The moisture influences thermo-insulating properties of the material. By a change of the thermo insulation properties of the construction also the thermal and diffusion scheme of the construction is changed and its thermal resistance is decreasing. Faults in the thermo-technical projects occur when thermal conductivity coefficient L values for material in a dry state are substituted.The aim of the research is to determine the capillary conductivity coefficient as a characteristic material moisture parameter of the building materials by the means of a non-destructive method using the experimentally assembled apparatus developed at the Department of Civil Engineering, Brno University of Technology. Keywords: Capillary conductivity coefficient, moisture transfer, EMWR (electromagnetic microwave radiation), diffusion
文摘The one-dimensional calculation of the gas/particle flows of a supersonic two-stage high-velocity oxy-fuel(HVOF) thermal spray process was performed. The internal gas flow was solved by numerically integrating theequations of the quasi-one-dimensional flow including the effects of pipe friction and heat transfer. As for the supersonicjet flow, semi-empirical equations were used to obtain the gas velocity and temperature along the centerline. The velocity and temperature of the particle were obtained by an one-way coupling method. The material ofthe spray particle selected in this study is ultra high molecular weight polyethylene (UHMWPE). The temperaturedistributions in the spherical UHMWPE particles of 50 and 150 m accelerated and heated by the supersonic gasflow was clarified.
文摘We present a rigorous homogenization approach for elcient computation of a class of physical problems in a one-dimensional periodic heterogeneous material. This material is represented by a spatially periodic array of unit cells with a length of More specifically, the method is applied to the diffusion, heat conduction, and wave propagation problems. Heterogeneous materials can have arbitrary position-dependent continuous or discontinuous materials properties (for example heat conductivity) within the unit cell. The final effective model includes both effective properties at the leading order and high-order contributions due to the microscopic heterogeneity. A dimensionless heterogeneity parameter ~ is defined to represent high-order contributions, shown to be in the range of [-1/12, 0], and has a universal expression for all three problems. Both effective properties and heterogeneity parameter 13 are independent oft, the microscopic scale of heterogeneity. The homogenized solution describing macroscopic variations can be obtained from the effective model. Solution with sub-unit-cell accuracy can be constructed based on the homogenized solution and its spatial derivatives. The paper represents a general approach to obtain the effective model for arbitrary periodic heterogeneous materials with position-dependent properties.
文摘This paper presents experimental results of thermal radiative properties of xonotlite-type calcium silicate insulation material. Transmittance spectra were first taken using Fourier transform infrared spectrometer (FTIR) for the samples with p = 234 kg/m^3. Specific extinction coefficient spectra were then obtained by applying Beer's law. Finally, by using the diffusion approximation, the specific Rossland mean extinction coefficients and radiative thermal conductivities were obtained for various temperatures. The results show that the specific spectral extinction coefficient of xonotlite is larger than 7 m^2/kg in the whole measured spectra, and diffusion approximation equation is a reasonable description of radiative heat transfer in xonotlite insulation material. The specific Rossland mean extinction coefficient of xonotlite has a maximum value at 400 K and the radiative thermal conductivity is almost proportional to the cube of temperature.
文摘An inverse method is presented for estimating the unknown boundary incident radiation heat flux onone side of one-dimensional semitransparent planar slab with semitransparent boundaries from theknowledge of the radiation intensities exiting from the other side. The inverse problem is solved usingconjugate gradient method of minimization based on discrete ordinates method (DOM) of radiativetransfer equation. The equations of sensitivity coefficients are derived and easily solved by DOM, withthe result that the complicated numerical differentiation commonly used in solving sensitivity coefficients is avoided. The effects of anisotropic scattering, absorption coefficient, scattering coefficient,boundary reflectivity, fluid temperature outside the boundaries, convection heat transfer coefficients,conduction coefficient of semitransparent media and slab thickness on the accuracy of the inverse analysis are investigated. The results show that the boundary incident radiation heat flux can be estimatedaccurately, even with noisy data.
基金Support by the National Natural Science Foundation of China under Grant No.11725521the Science and Technology Commission of Shanghai Municipality under Grant No.16ZR1445100
文摘Manipulating thermal conductivities are fundamentally important for controlling the conduction of heat at will. Thermal cloaks and concentrators, which have been extensively studied recently, are actually graded materials designed according to coordinate transformation approaches, and their effective thermal conductivity is equal to that of the host medium outside the cloak or concentrator. Here we attempt to investigate a more general problem: what is the effective thermal conductivity of graded materials? In particular, we perform a first-principles approach to the analytic exact results of effective thermal conductivities of materials possessing either power-law or linear gradation profiles. On the other hand, by solving Laplace's equation, we derive a differential equation for calculating the effective thermal conductivity of a material whose thermal conductivity varies along the radius with arbitrary gradation profiles.The two methods agree with each other for both external and internal heat sources, as confirmed by simulation and experiment. This work provides different methods for designing new thermal metamaterials(including thermal cloaks and concentrators), in order to control or manipulate the transfer of heat.
基金financial support from the Australian Research Council(ARC Future Fellowship FT190100485)financial support from the China Scholarship Council for his PhD Scholarshipthe Future Industries Institute for a top up scholarship。
文摘Interfacial solar steam generation is an efficient water evaporation technology which has promising applications in desalination,sterilization,water purification and treatment.A common component of evaporator design is a thermal-insulation support placed between the photothermal evaporation surface and bulk water.This configuration,common in 2-dimensional(2 D)evaporation systems,minimizes heat loss from evaporation surface to bulk water,thus localizing the heat on the evaporation surface for efficient evaporation.This design is subsequently directly adopted for 3-dimensional(3 D)evaporators without any consideration if it is appropriate.However,unlike 2 D solar evaporators,the 3 D evaporators can also harvest additional energy(other than solar light)from the air and bulk water to enhance evaporation rate.In this scenario,the use of thermal insulator support is not proper since it will hinder energy extraction from water.Here,the traditional 3 D evaporator configuration was completely redesigned by using a highly thermally conductive material,instead of a thermal insulator,to connect evaporation surfaces and the bulk water.Much higher evaporation rates were achieved by this strategy,owing to the rapid heat transfer from the bulk water to the evaporation surfaces.Indoor and outdoor tests both confirmed that evaporation performance could be significantly improved by substituting a thermal insulator with thermally conductive support.These findings will redirect the future design of 3 D photothermal evaporators.
基金supported by the National Natural Science Foundation of China(51406223)
文摘With the large latent heat and low cost, the paraffin has been widely used in battery thermal management(BTM) system to improve the efficiency and cycling life of power battery. The numerical model of paraffin melting in a cavity has been established, and the effects on the solid–liquid phase change process have been investigated for the purpose of enhancing the heat transfer performance of paraffin-based BTM system. The results showed that the location of the heating wall had great effects on the melting process. The paraffin in the cavity melted most quickly when the heating wall located at the bottom. Furthermore, the effects of thermal conductivity and the velocity of the slip wall have been considered. The gradient of liquid fraction increased with the increase in thermal conductivity, and the melting process could be accelerated or delayed by the slip wall with different velocity.
文摘We present a general homogenization method a periodic heterogeneous material with piecewise constants for diffusion, heat conduction, and wave propagation in The method is relevant to the frequently encountered upscaling issues for heterogeneous materials. The dispersion relation for each problem is first expressed in the general form where the frequency co (or wavenumber k) is expanded in terms of the wavenumber k (or frequency ω). A general homogenization model can be directly obtained with any given dispersion relation. Next step we study the unit cell of the heterogeneous material and derive the exact dispersion relation. The final homogenized equations include both leading order terms (effective properties) and high order contributions that represent the effect of the microscopic heterogeneity on the macroscopic behavior. That effect can be lumped into a single dimensionless heterogeneity parameter β, which is bounded between -1/12≤β≤ 0 and has a universal expression for all three problems. Numerical examples validate the proposed method and demonstrate a significant computational saving.