Heat Conduction Mechanism Research Articles

Ballistic heat conduction characteristics of graphene nanoribbons

One of the fundamental physical properties of graphene nanoribbons is its ability to conduct heat. Unfortunately, the precise microscopic mechanisms of heat conduction have also not been fully elucidated at room temperature when heat transport occurs in the two-dimensional material ballistically. The transport mechanism of ballistic heat conduction in graphene nanoribbons was investigated experimentally and theoretically. The dimensions of the two-dimensional material are substantially equal to or shorter than the average length that the phonons can travel freely. The thermal conductivity was determined experimentally and predicted theoretically under different dimension conditions, and the ability of the graphene nanoribbons to conduct heat ballistically was characterized. The boundary scattering effect arising from the crystallographic edge structure was evaluated, and the ballistic and diffusive transport characteristics were investigated. The results indicated that the heat conduction can be ballistic or diffusive, depending on the dimensions. With the decrease of the dimensions, heat conduction may begin to transition from diffusive to ballistic. This will inevitably lead to a considerable decrease in thermal conductivity. Transport can be ballistic even at room temperature, and ballistic effects may be noticeable. Graphene nanoribbons with larger dimensions may disadvantageously be used for applications in thermal management.

Thermal conductivity of scrap tire rubber-sand composite as insulating material: Experimental investigation and predictive modeling

Scrap tire rubber of good thermal insulating properties can be reused with soils as a new sustainable construction material. While the heat conduction mechanism of this tire rubber-soil composite has not been fully understood. The present study investigated the thermal behavior of scrap tire rubber-sand composite for using it as an insulating material in thermo-active applications. Several series of thermal conductivity tests were conducted on scrap tire rubber-sand composite prepared with different sand/shredded tire particle size ratios, shredded tire mixing ratios, and degrees of saturation. A predictive model was developed within the framework of normalized thermal conductivity concept that could capture the experimental data. The results showed that addition of shredded tires leads to a considerable reduction in thermal conductivity of sand, indicating great potential of using recycled scrap rubber tires in the construction of insulation layers. With an increase in degree of saturation, the order of thermal conductivity increase was found to be: sand > scrap tire rubber-sand composite > shredded tires. The spatial distribution of solid particles changed from ‘rubber-connected insulation layer’ to ‘sand-encompassed conduction path’ with increasing sand/shreds tire particle size ratio. The normalized thermal conductivity exhibited a linearly increasing trend with shredded tire mixing ratio, and the increase rate mainly depended on degree of saturation and particle size ratio. The predicted thermal conductivity showed satisfactory accuracy for engineering design, as reflected by the absolute percentage difference being controlled under 25%. The validity of the prediction model was also demonstrated based on the experimental results reported in the published literature. Additional research is recommended to assess the effects of particle morphology on thermal conduction of the binary-solid particle system.

Thermally conductive silicone rubber composites with vertically oriented carbon fibers: A new perspective on the heat conduction mechanism

The continual increase in power density and consumption of modern electronic devices calls for high-performance thermal interface materials (TIMs). In this work, silicone rubber composites with vertically oriented magnetic carbon fibers (o-MCF/SR) were successfully prepared via the cast molding and filler orientation upon uniform magnetic field in a vacuum environment. The through-plane thermal conductivity of o-MCF/SR composites showed an unusual power-law growth with increasing MCF loading. At a 9 vol% MCF loading, the o-MCF/SR composite exhibited a maximum through-plane thermal conductivity of 4.72 W/(m·K) with 2045.5 % TCE, about 3.55 W/(m·K) higher than that of SR composites with randomly dispersed MCF (λ = 1.17 W/(m·K), TCE = 431.8 %). To reveal the underlying through-plane heat conduction mechanism of o-MCF/SR composites, finite element simulation combined with classical effective medium theory model and machine learning method was used. A homogeneous unit fiber-filled composite (HUFC) was modelled, and a new parameter influencing the through-plane thermal conductivity, i.e., shape factor (b/a), was introduced here for the first time. Theoretically, this shape factor was dependent on the filler content and indirectly reflected the assembly of matrix and 1D filler in HUFC fit to a parallel-series hybrid model. Furthermore, a decrease of b/a was practically found with increasing MCF loading, and the dominant series-type assembly gradually transformed to parallel-type assembly of the o-MCF/SR composites. This work not only developed a method to fabricate desirable TIMs that hold great promise for advanced thermal management, but also introduced a new parameter to reveal the underlying mechanism for heat conduction capacity of polymer composites.

A novel lumped thermal characteristic modeling strategy for the online adaptive temperature and parameter co-estimation of vehicle lithium-ion batteries

Accurate modeling of thermal characteristics is critical to the safe use and reliable management of lithium-ion batteries. However, limitations in sensors and testing methods make online real-time acquisition of internal temperatures extremely difficult. This paper uses the similarity of dynamic system modeling to construct a lumped thermal characteristic model of the battery. By analyzing the heat conduction mechanism inside the battery, the optimized heat path model is combined with the classical Bernardi equation to realize the state description of the battery thermal characteristic system. In addition, the forgetting factor recursive least squares algorithm is used to realize the online identification of the parameters of the lumped thermal characteristics model. Meanwhile, the identification of the external thermal resistance is coupled with the estimation of the internal temperature, and a novel online adaptive co-estimation strategy based on the forgetting factor recursive least squares — joint Kalman filter is proposed, which solves the problem that the external thermal resistance cannot be accurately identified adaptively in a complex environment. The experimental results show that the maximum root-mean-square error of the model under different experiments is 0.53 °C, which verifies the high-accuracy of the lumped thermal characteristics modeling strategy.

Study on dominant heat transfer mechanism in vertical smooth/finned-tube thermal energy storage during charging process

• Heat transfer mechanism of heat conduction is dominant in smooth-tube TES. • The heat transfer in finned-tube TES is dominated by secondary flow. • Heat conduction enhancement rate was defined to characterize the heat conduction intensity. • Heat conduction enhancement rate increases with the increase of the number of fin. • Aluminum finned TES obtains the best thermal storage efficiency. Thermal energy storage (TES) is an indispensable part of solar energy utilization system. This paper intends to explore the role of heat conduction, natural convection and secondary flow on the charging performance of phase change material in vertical smooth/finned-tube TES respectively. It was found that heat conduction is dominant in smooth-tube TES. The role of natural convection is to cause non-uniformity of melting rate of the lower and upper part. Meanwhile, secondary flow is dominant in finned-tube TES due to the generation of vortices between adjacent fins. Fin material has little effect on the melting characteristic of phase change material when fin pitch is larger than 40 mm in this study. Therefore, nonmetallic fin and cheap metallic fin could be used in finned-tube TES to increase thermal storage density and reduce cost of investment. Compared to conduction model, the total melting time was reduced by 9.7% and 34.2% in smooth-tube TES and aluminum finned-tube TES when convection model is considered in the computation. Besides, the heat conduction intensity and secondary flow intensity were explored in aluminum finned-tube TES. The heat conduction enhancement rate φ was defined to characterize the heat conduction intensity. With the fin pitch decreasing from 40 mm to 8 mm, the heat conduction enhancement rate increases from 15.2% to 39.6%, indicating that heat conduction plays a crucial role in the finned-tube TES. The aluminum finned TES exhibit the best thermal storage characteristic as a result of the best heat transfer enhancement and smaller density.

Thermal conductivity of polydisperse hexagonal BN/polyimide composites: Iterative EMT model and machine learning based on first principles investigation

Demand for thermal management materials (TMMs) with efficient in-plane heat dissipation has grown with the advancement of intelligent wireless communication equipment. Herein, polydisperse hexagonal boron nitride (ae-BN) in the range of micrometers to nanometers via aqueous-assisted exfoliation. First principles investigation revealed that ae-BN possess high intrinsic thermal conductivity. A series of ae-BN/PI composites were then fabricated through facile methods: vacuum-filtration and hot-pressing. The ae-BN/PI composites with 30 vol% ae-BN loading exhibited superior in-plane thermal conductivity (6.57 W/(m·K) compared to pristine h-BN/PI composite (3.92 W/(m·K)). SEM images and structural modeling of ae-BN/PI composites revealed that thermal conduction pathways constructed in the composites continuously increased with ae-BN content, attributing to an increased contact probability in composites with higher content of ae-BN. Reduction in thermal boundary resistance of ae-BN/PI composites was proved by our iterative EMT model. In-plane thermal conductivity of ae-BN/PI composites with different fillers’ contents at variable temperatures were predicted by machine learning technique, viz. artificial neural network (ANN) model. In brief, ae-BN/PI composites with high thermal conductivity, electrical insulation, thermal stability, and mechanical strength were successfully fabricated. The heat conduction mechanism of ae-BN/PI composites was investigated, serving as an important piece of puzzle for the advancement in TMMs of the advanced electronic devices.

Functionalized graphene origami metamaterials with tunable thermal conductivity

Graphene with tunable thermo-mechanical property is of great importance for next-generation thermal management devices. Distinct from previously reported porous graphene materials that tune the thermal conductivity at the cost of degrading their mechanical properties, non-porous hydrogenated graphene origami metamaterial exhibits a unique combination of tunable thermal conductivity, high strength, and enhanced stretchability. Through molecular dynamics simulation, a vast range of thermal conductivity can be found by tuning the geometrical parameters of the Miura-ori graphene origami, altering the adatom types and density, designing new origami patterns, and applying mechanical strains. By analyzing and comparing the numerical results from atomistic and continuum-based simulations, the effect of length scale on the thermal property of graphene origami metamaterials is explored. The temperature distribution, phonon density of states, phonon group velocity, and the atomic heat flux of the graphene origami are examined to illustrate the heat conduction mechanism. Finally, 3D graphene origami metamaterials are constructed by assembling the graphene origami strips, followed by evaluating their thermo-mechanical performance. Negative coefficients of thermal expansion are also observed in the functionalized graphene origami nanotubes. The introduced strategy for controlling the thermo-mechanical properties of graphene metamaterials can open up new avenues for developing thermoelectric devices, heat management systems, and flexible nanoelectronics.

Mechanisms for thermal conduction in molten salt-based nanofluid

The addition of nanoparticles to molten salt can enhance its utility as a thermal energy storage material for concentrating solar power plants. The heat transfer property of solar salt loaded with Al2O3 nanoparticle is investigated by molecular dynamics simulations. The results show that the introduction of Al2O3 nanoparticles improves the thermal conductivity of solar salt, but its enhancement effect is not as significant as SiO2 nanoparticles. The increased thermal conductivity cannot be explained by the previously proposed hypothesis, such as the Brownian motion of nanoparticles, microconvection of base liquid, ordered layer of base liquid and coulombic potential energy. New insights into heat conduction performance of molten salt based nanofluid are given from the perspectives of material components and heat flux fluctuation modes. It is found that the nanofluid thermal conductivity is mainly contributed by the atom motions and interactions between atoms in Al2O3 nanoparticle, rather than in base fluid. The heat flux is concentrated in nanoparticle, which promotes the heat transfer. Furthermore, the collision involving the work done by the interatomic forces or the virial interaction dominates the heat conduction in nanofluid with lower mass fraction (< 6%), while the potential energy and collision both contribute to the nanofluid with higher mass fraction (> 6%). These new findings make a step forward in understanding the heat conduction mechanisms of molten salt based nanofluid.

Effect of Microstructural Damage on the Thermomechanical Properties of Electrodes in Proton Exchange Membrane Fuel Cells.

Advanced functional materials composed of multiple nanoscale phases, including pores and interfaces, have been extensively applied in the fields of new energy, architecture, and aerospace. However, insufficient knowledge of the thermomechanical properties resulting from material failures, such as interfacial delamination and porosity deformations, which limit the durability and lifetime of these materials, has hindered their further application, demanding a deeper understanding of microstructural changes. Based on the fuel cell electrode, we explore a multiscale prediction model that correlates the atomic interactions between interfaces with a microscopic thermomechanical model to illuminate the effects of interface binding characteristics on the materials' mechanical response and heat conduction mechanisms. Compared with experimental measurements and theoretical calculations at the macroscopic scale, our model excels in predicting the initiation and propagation of interfacial debonding and the thermal conductivity of the electrode, with the resistance factors for the interface, pores, and cracks taken into consideration. This work provides guidance for designing robust electrodes resistant to thermomechanical failure and serves as a reference method for predicting damage in heterogeneous porous materials.

Research progress of thermal transport in graphene-based thermal interfacial composite materials

With the rapid increase of the thermal power density of microelectronic devices and circuits, controlling its temperature has become an urgent need for the development and application of the electronic industry. By virtue of the ultrahigh thermal conductivity of graphene, developing high-performance graphene-based composite thermal interface materials has attracted much research attention and become one of hot research topics. The understanding of phonon transport mechanism in graphene-based composites at atomic scale can be helpful to enhance the heat conductive capability of composites systems. In this review, focused on graphene-based thermal interfaces materials, the heat conduction mechanism and the regulating strategy are introduced on both the internal thermal resistance and interfacial thermal resistance. Finally, the reseach progress and opportunities for future studies are also summarized.

Modelling and analyzing the glass-like heat transfer behavior of rare-earth doped alkaline earth fluoride crystals

Based on the “three-stage” heat transfer behavior of rare-earth doped CaF2 crystals, numerical models at different stages for the thermal conductivity of rare earth doped fluorides are proposed to study the mechanism of glass-like heat conduction.

Discussions on the Heat Conduction Mechanisms of Metal-Water Nanofluid Based on the Phonon Density of State

Discussions on the Heat Conduction Mechanisms of Metal-Water Nanofluid Based on the Phonon Density of State

FEA-Based Structural Heat Transfer Characteristic of 3-D Orthogonal Woven Composite Subjected to the Non-Uniform Heat Load

Abstract The thermodynamic behavior of 3-D orthogonal woven composite is studied to explore its structural heat transfer mechanism in a non-uniform heat load field based on finite element analysis (FEA). The temperature distribution characteristics of the resin matrix and the fabric reinforcement are observed to compare the heat absorption. Furthermore, the dynamic expansion and distribution characteristics of temperature in the 3-D orthogonal woven composite structure have also been quantitatively studied, together with simultaneously obtaining the path characteristics of the heat transfer in each system (i.e., warp yarns, weft yarns, and Z-yarns). In addition, the spatial temperature distribution characteristics of each yarn system in the fabric reinforcement are also explored. Thus, the structural mechanism of heat conduction for 3-D orthogonal woven composite is obtained.

Analysis and Design of Oil Cooling Structure in Motor Shaft of New Energy Vehicle

Due to the small volume and high-power density of new energy vehicle motor, a large number of losses in the working process are converted into heat accumulation, resulting in temperature rise, which affects its efficient operation. Based on the heat conduction mechanism, four kinds of shaft oil cooling models with different structures are designed, which are comprehensively analysed by using the thermal-fluid-structure coupling analysis method, and the most effective cooling shaft oil cooling model is solved. The simulation is based on Ansoft Maxwell, and the loss results of each component of the motor are obtained, and the loss data is imported into the Fluent software for fluid-structure coupling analysis. By keeping the other variables consistent, the oil flow rate, pressure drop, and temperature rise of four kinds of in shaft oil cooling structures are analysed and compared. The experimental results show that the rectangle around type is the optimal oil cooling structure. In addition, based on the rectangle around oil duct model , the thermal-fluid-structure coupling analysis of the whole motor is carried out, and compared with the motor without cooling system. The temperature rise cloud diagram of the two motors shows that the former has more obvious heat dissipation effect than the latter, and effectively reduces the temperature rise of the motor, especially the rotor and permanent magnet parts, which verifies the rationality of the shaft cooling structure design.

Thermal Conductivity of Sodium Silicate Glasses and Melts: Contribution of Diffusive and Propagative Vibration Modes

The thermal conductivity of silicate melts and glasses is an important physical property for understanding the temperature distribution in high-temperature metallurgical processes; however, the mechanism of heat conduction in these non-crystalline materials remains unclear. Two types of vibration modes must be considered to understand the mechanism of heat conduction, namely, propagative and diffusive vibration modes. In the present study, we carefully derived the thermal conductivity of pure silica and sodium disilicate glasses and melts, and estimated the contribution of the diffusive vibration mode using a recently developed model. The results indicated that the diffusive vibration mode was not dominant in the silicate non-crystalline materials, whereas the propagative vibration mode (i.e., phonons) was dominant in the heat conduction of silicate glasses and melts, which is in contrast with borate glasses.

Functionalized Graphene Origami Metamaterials with Tunable Thermal Conductivity

Graphene with tunable thermo-mechanical property is of great importance for next-generation thermal management devices. Distinct from previously reported porous graphene materials that tune the thermal conductivity at the cost of degrading their mechanical properties, non-porous hydrogenated graphene origami metamaterial exhibits a unique combination of tunable thermal conductivity, high strength, and enhanced stretchability. Through molecular dynamics simulation, an extremely broad range of thermal conductivity can be obtained by tuning the geometrical parameters of the Miura-ori nanoarchitecture of graphene origami, altering the adatom types and density, designing new origami patterns, and applying mechanical strains. By analyzing and comparing the results from atomistic and continuum-based simulations, the effect of length scale on the thermal property of graphene origami metamaterials is explored. The temperature distribution and the phonon density of states of the proposed graphene origami are examined to illustrate the heat conduction mechanism. Finally, 3D graphene origami metamaterials are constructed based on the coupling and assembling of graphene origami strips, and their thermo-mechanical performance is elucidated. Negative coefficients of thermal expansion are obtained in graphene origami nanotubes. The introduced strategy for controlling the thermo-mechanical properties of graphene metamaterials can open up new avenues for developing thermoelectric devices, heat management systems, and flexible nanoelectronics.

Excellent Thermally Conducting Ni Plating Graphite Nanoplatelets/Poly(phenylene sulfone) Composites for High-Performance Electromagnetic Interference Shielding Effectiveness.

First, nickel particles were deposited on the surface of graphite nanoplatelets to fabricate highly conductive GnPs@Ni core-shell structure hybrid fillers via electroplating. The modified GnPs were blended with polyphenylene sulfone via the solution blending method, followed by the hot-pressing method to achieve high thermally conducting GnPs@Ni/PPSU composites for high performance electromagnetic interference effectiveness. The results showed that in-plane and through-plane thermal conductivity of the composite at the 40 wt% filler loading could reach 2.6 Wm−1K−1 and 3.7 Wm−1K−1, respectively, which were 9.4 and 20 times higher than that of pure PPSU resin. The orientation degree of fillers was discussed by XRD and SEM. Then, heat conduction data were fitted and analyzed by the Agari model, and the heat conduction mechanism was further explored. The testing results also demonstrated that the material exhibited good conductivity, electromagnetic shielding effectiveness and superior thermal stability. Overall, the GnPs@Ni/PPSU composites had high thermal conductivity and were effective electromagnetic shielding materials at high temperatures.

Effects of interface conditions on heat and mass transfer during modeling of laser dissimilar welding

An improved 3 D heat and mass transfer model was developed to study the effects of interface conditions during modelling of laser dissimilar welding. In detail, the interface conditions consist of the physical processes at gas/liquid surface (e.g. free surface deformation and optical absorptance), substrate interface (e.g. mixture properties in liquid phase and thermal contact condition) and solid/liquid interface (e.g. fusion line). Their effects on heat and mass transfer are numerically and experimentally analyzed, which are all non-negligible in the welding modelling. In conclusion, free surface deformation influences convection flow and should be considered in the situation of micro-welding and high energy-input welding. Besides, the energy transfer between laser and substrate is more reasonably described by the optical absorptance expressed in polynomial function. The mass transfer induced variation of mixture properties is well described by the method based on time-dependent mixture fraction. Thermal resistance between clamp and substrate should be considered in the modelling of temperature field on macroscale. The joint conductance at substrate interface could be neglected when modelling heat and mass transfer inside the melt pool, while it should be calculated in the simulation of temperature distribution based on the mechanism of heat conduction. The obtained results in this paper provide a vital insight into the interface conditions in laser dissimilar welding process.

Finite element simulation of disk‐shaped HfB2 ceramics during spark plasma sintering process

AbstractSpark plasma sintering is an interesting manufacturing method, which benefits electric current heating. In the present study, the sintering process of hafnium diboride (HfB2) ceramics was simulated using the finite element method. The governing equations of electric potential, current conservation, and energy conservation were solved using COMSOL Multiphysics software. The electric potential of the whole system versus time and current distribution at different time steps were obtained. The heat generation was calculated using the Joule heating effect, and the needed temperature for sintering was obtained. The required temperature for process progression was 2100°C that achieved after 1380 s. Moreover, the maximum electric current was 5942.863 A, which remained constant until the end of the operation. Since HfB2 is an electrical conductor, the current easily passed through the sample with an approximately uniform distribution. To provide a better understanding of the sintering mechanisms, an electrically resistive material (Al2O3) was also simulated. The results showed a more uniform current distribution in HfB2 compared to Al2O3. It was observed that the needed temperature for sintering of alumina was obtained only by heat conduction from graphite die toward the sample, whereas HfB2 benefited both mechanisms of heat conduction and Joule heating inside the sample.

Ultralow thermal conductivity in tetrahydrofuran clathrate hydrate

The detailed knowledge of the low thermal conductivity of host–guest compounds is essential to improve our fundamental understanding of heat conduction in complex solids and develop high-performance thermoelectric materials. In this Letter, the intrinsic ultralow thermal conductivity (0.44 ± 0.06 W m−1 K−1 in 140–190 K) of the tetrahydrofuran (THF) clathrate hydrate is characterized by the time-domain thermoreflectance technique. The underlying heat conduction mechanism is further investigated by non-equilibrium molecular dynamics simulations. We find that trapped THF molecules do harmonic motions and behave as parts of a crystalline structure, thus playing negligible roles in thermal conductivity reduction. The large unit cell and complex cage-like host structure dominate the low thermal conductivity of the THF hydrate.