Heat Conduction Path Research Articles

Enhanced thermal conductivity of epoxy composites by constructing thermal conduction networks via adding hybrid alumina filler

AbstractHigh thermal conductivity of polymer composites could be achieved through constructing 3D continuous thermally conductive networks into polymer composites. Herein, we demonstrate a simple, effective method to optimize the segregated alumina networks by using ternary mixture of alumina particles with different diameter ratios (5.8 μm + 2.6 μm + 0.5 μm) at the fixed total content of 60 wt% via conventional melt blending method. The thermal properties of epoxy composites were investigated as a function of the weight fraction and particle size of alumina particles. The thermal conductivity (TC) values of composites with coarse fillers (13.2 μm) are in agreement with Agari model at high filler loading (57 ~ 67 wt%). And the value of C2 was ~0.949 (close to 1), indicating that the formation of heat conduction paths in the composites. The addition of 60 wt% smaller fillers (2.6 μm) facilitates a higher interaction degree between the polymer/fillers, consequently, leads to a higher Tg of 147.8°C. A high TC of 1.109 W m−1 K−1 is achieved as the ratio of alumina filler with different particle sizes is 7/2/1, corresponding to a TC enhancement of 23%–32% compared to the composites with a single particle size filler. A suitable proportion of hybrid fillers is crucial for constructing three‐dimension continuous network, in which nano‐particles filled between the micro‐particles could increase the particle‐to‐particle connectivity.

Molecular dynamics simulation on thermal enhancement for carbon nano tubes (CNTs) based phase change materials (PCMs)

In the study, CNTs based nanocomposite phase change materials (NPCMs) is considered for breaking through the constraint of the low thermal conductivity of conventional PCMs for thermal energy storage. A molecular dynamics simulation method is newly established to predict the structural, diffusive and thermal properties of composite PCMs based on CNTs. It is found that the PCMs is preferentially distributed as either one or two ring-shaped layers that discretely separated from the CNTs wall with a distance of 3.8 Å and 8.0 Å, respectively. The mean square displacement (MSD) and the self diffusion coefficient are characterized for featuring the molecule mobility and predicting the melting temperature of the composite PCMs. Interestingly, the dense phase existing between the CNTs wall and the PCMs is proven to be the dominant factor that negates the phase change enthalpy of the composite system. In realistic experiments using the CNTs with a radius of 10∼15 Å, the thermal conductivity is highly potential to be improved to 5∼15 W/m·K, indicating an augmentation of the thermal conductivity by approximate 32∼100 times compared to pure PCMs. The formation of efficient heat conduction paths explains for the intrinsic mechanism of the tremendous improvement in the CNTs based NPCMs, which could be achieved in the premise of strong binding effect of materials and reduced contact thermal resistance among CNTs in macroscopic PCM modules.

Flexible Fiber Membrane Based on Carbon Nanotube and Polyurethane with High Thermal Conductivity.

The development of high thermally conductive polymer composites with low filler content remains challenging in the field of thermal interface materials (TIMs). Herein, we fabricated a series of flexible fiber membranes (TMMFM) with high thermally conductive based on thermoplastic polyurethane (TPU) and acidified multiwalled carbon nanotubes (a-MWCNTs) via electrospinning and ultrasonic anchoring method. The SEM and TEM results demonstrated that the a-MWCNTs aligned along the fiber orientation in the membrane and anchored on the membrane surface strongly, which can establish the heat conduction path both in the horizontal and vertical directions. With the incorporation of 10 wt% a-MWCNTs, the horizontal direction (λ∥) and vertical direction (λ⊥) thermal conductivity value of TMMFM-5 was 3.60 W/mK and 1.79 W/mK, respectively, being 18 times and 10 times higher compared to pure TPU fiber membranes. Furthermore, the TMMFM maintained favorable flexibility of the TPU matrix because the small amount of a-MWCNTs only slightly hinders the mobility of the TPU molecular chain. The performance of the obtained TMMFM unveils their potential as a promising choice of flexible TIMs.

High Thermal Conductivity and Mechanical Strength Phase Change Composite with Double Supporting Skeletons for Industrial Waste Heat Recovery.

The "solid-liquid" leakage and low thermal conductivity of organic phase change materials limit their wide range of applications. In this paper, a novel carbon fiber/boron nitride (CF/BN)-based nested structure was constructed, and then, a series of poly(ethylene glycol) (PEG)-based phase change composites (PCCs) with high thermal conductivity and mechanical strength were prepared via the simple vacuum adsorption technology by employing the CF/BN nested structure as the heat conduction path and supporting material and the in situ obtained cross-linking epoxy resin as another supporting material. The thermal conductivity of the obtained PCC is as high as 0.81 W/m K, which is 7.4 times higher than that sample without the CF/BN nested structure. The support of the double skeletons confers the obtained PCCs with excellent mechanical strength. Surprisingly, there is not any deformation for PCCs under the pressure of 128.5 times its own weight during the phase change process. In addition, the phase change enthalpy of the obtained PCC is as high as 107.9 J/g. All the results indicate that the obtained PEG-based PCCs possess huge application potential in the field of industrial waste heat recovery.

Dual-pace transient heat conduction in vertically aligned carbon nanotube arrays induced by structure separation

Vertically aligned arrays of carbon nanotubes (CNTs) are attractive for a wide range of macroscopic applications which can exploit the remarkable properties of individual nanotubes. In this work, an abnormal behavior of CNT bundles in heat conduction is discovered under the transient electro-thermal characterization. The measured voltage change over the sample shows a dual-pace thermal response (DTR), which could not be fitted using a single thermal diffusivity heat transfer model. Instead, two thermal diffusivities are determined from the DTR phenomenon. Three rounds of cryogenic experiments are conducted to investigate the physics behind DTR phenomenon. It starts to emerge when the temperature is reduced to a certain level. After two rounds of cryogenic experiments, the DTR phenomenon becomes permanent from 295 K to 10 K. The nano-scale structure separation induced by the increased thermal strain leads to macroscale structure separation, which results in two parallel heat conduction paths responsible for the DTR phenomenon. By building a new parallel heat transfer model taking both the transient and steady-state electrical and thermal response into consideration, the area ratio of separated CNTs is determined. This result uncovers the existence of coiled morphology and nano-structure evolution of CNTs under cryogenic state and its effect on thermal and electrical transport, especially on their transient behaviors. Knowledge of the thermal transport in these CNTs arrays is important considering the fact that transient thermal response strongly affects their mechanical, optical, and electrical behaviors.

Why the thermal conductivity of graphene nanofluids is extremely high? A new model based on anisotropy and particle-free renovation

Many studies have shown that low concentration of graphene can appreciably increase the thermal conductivity of nanofluids that is difficult to explain by conventional thermal conductivity model. In this paper, a new model considering the effects of graphene’s length and thickness, interface layer and anisotropic characteristics is established. The model is further modified by considering the possibility of absence of graphene in the actual thermal conduction path, that is only bulk liquid along the heat conduction path in some cases. It is found that the thermal conductivity predicted by the present model is positively correlated with the temperature, concentration and length of nanoparticles, while is slightly negatively correlated with thickness. This model is applied to predict thermal conductivities of graphene nanofluids in different cases compared to other models. The results indicate that the present model has a strong ability to predict graphene nanofluids especially at relatively low concentrations (<0.1 vol%).

An experimental investigation of a PCM-based heat sink enhanced with a topology-optimized tree-like structure

A heat sink filled with phase change material (PCM) is an efficient thermal management device which utilizes the high latent heat of fusion of PCM in the cooling process. To improve the thermal performance of the PCM-based heat sink, a topology optimization (TO) strategy is devised to develop a new class of enhanced structures. This is achieved by carrying out a comprehensive numerical study to identify the effects of various thermal transport mechanisms on the TO design by considering two different heat transfer problems, i.e., steady-state heat conduction and transient heat conduction with phase change. To enable easy fabrication and performance evaluation of the new heat sink design predicted by the TO process, the resulting heat sink with tree-like structure was fabricated by selective laser melting (SLM), a metal additive manufacturing (AM) technique. Experimental characterization of the TO heat sink was carried out using three different types of PCMs, i.e., RT35, RT35HC and RT44HC and heat fluxes ranging from 4.00 kW/m2 to 7.24 kW/m2. Our experimental results show that the TO tree-like structure heat sink has better performance, exhibiting up to 4 °C lower wall temperatures, than the conventional fin-structure heat sink. At low heat fluxes, the best thermal performance can be obtained with RT35HC whereas at high heat fluxes, lower wall temperatures were achieved with RT44HC. In addition, the tree-like structure increases operational time by up to 13% as compared to the fin-structure heat sink. The better thermal performance of the tree-like structure heat sink is due to its optimized heat conduction paths that allow heat from the concentrated heat source to be efficiently dissipated to the PCM. This work not only demonstrates the potential of enhancing electronics cooling with TO PCM-based heat sinks, but it also outlines key guidelines for the design and implementation of TO structures for other cooling applications.

Enhanced Electron Heat Conduction in TaS3 1D Metal Wire.

The 1D wire TaS3 exhibits metallic behavior at room temperature but changes into a semiconductor below the Peierls transition temperature (Tp), near 210 K. Using the 3ω method, we measured the thermal conductivity of TaS3 as a function of temperature. Electrons dominate the heat conduction of a metal. The Wiedemann–Franz law states that the thermal conductivity of a metal is proportional to the electrical conductivity σ with a proportional coefficient of L0, known as the Lorenz number—that is, . Our characterization of the thermal conductivity of metallic TaS3 reveals that, at a given temperature T, the thermal conductivity κ is much higher than the value estimated in the Wiedemann–Franz (W-F) law. The thermal conductivity of metallic TaS3 was approximately 12 times larger than predicted by W-F law, implying . This result implies the possibility of an existing heat conduction path that the Sommerfeld theory cannot account for.

Analysis of the heat conduction mechanism for Al2O3/Silicone rubber composite material with FEM based on experiment observations

Based on the microcosmic configuration of samples on cross-section, a three-dimensional particle random distribution model of spherical Al2O3/SIR composites was built. Through sample measurement and model calculation, the effects of volume fraction, particle size and binary filling schemes of the Al2O3 fillers on thermal conductivity of the composites were studied. The particle contact probability which aiming at single sized particles filling case and different sized particles filling case were proposed to quantitatively characterize the formation probability of heat conduction path in the materials. Increasing the volume fraction of fillers can increase the contact probability of filler particles. When the volume fraction of Al2O3 fillers exceeds a certain threshold, the thermal conductivity of composites increases slightly first and then decreases with the increment of particle size. For Al2O3/SIR composites filled with different particle size fillers, as the volume fraction of the reinforced fillers with small particle size increases, the thermal conductivity of composites rises. As the particle size of the reinforced fillers increases, the thermal conductivity rises first and then reduces. The optimal volume ratio between the main fillers and reinforced fillers will be affected by the total filling content of the Al2O3 fillers.

Mathematical modeling of heat transfer in GO-doped reinforce polymer for anti-/deicing of wind turbines

Icing of wind turbine blades seriously affects the efficiency of wind power generation and threatens the safe operation of wind turbines. In order to improve the heat transfer efficiency of wind turbine blade anti-deicing components, a mathematical heat transfer model of graphene oxide (GO) doped reinforced polymer was established based on the thermal resistance law and the equivalent thermal conductivity. The heat transfer characteristic of glass fiber/epoxy resin (GF/EP) reinforced polymer composites modified with GO as heat transfer filler was presented by combining numerical calculation with finite element method. The correctness of the heat transfer mathematical model of GO/GF/EP composites was verified by comparing the numerical and experimental results with the finite element simulation results, which shows the maximum error is 7.11%. With the increasing of the content of GO in GO/GF/EP composites, the heat conduction paths composed of GO are formed in the composites. Thus, from the macroscopic point of view, the overall thermal conductivity of the material is greatly improved by 6.41%. The number of thermal paths in the composites is directly proportional to the content of GO. The increase of thermal conductivity channels promotes the heat transfer capacity of the composites, which making up for the low thermal conductivity of glass fiber and epoxy resin. The results show that the modification of GO can significantly improve the heat transfer performance of GF/EP composites, which can be employed in the anti−/deicing field of the wind turbine blades in the future.

Incorporating MXene into Boron Nitride/Poly(Vinyl Alcohol) Composite Films to Enhance Thermal and Mechanical Properties.

Aggregated boron nitride (ABN) is advantageous for increasing the packing and thermal conductivity of the matrix in composite materials, but can deteriorate the mechanical properties by breaking during processing. In addition, there are few studies on the use of Ti3C2 MXene as thermally conductive fillers. Herein, the development of a novel composite film is described. It incorporates MXene and ABN into poly(vinyl alcohol) (PVA) to achieve a high thermal conductivity. Polysilazane (PSZ)-coated ABN formed a heat conduction path in the composite film, and MXene supported it to further improve the thermal conductivity. The prepared polymer composite film is shown to provide through-plane and in-plane thermal conductivities of 1.51 and 4.28 W/mK at total filler contents of 44 wt.%. The composite film is also shown to exhibit a tensile strength of 11.96 MPa, which is much greater than that without MXene. Thus, it demonstrates that incorporating MXene as a thermally conductive filler can enhance the thermal and mechanical properties of composite films.

Highly thermal conductive epoxy nanocomposites filled with 3D BN/C spatial network prepared by salt template assisted method

The construction of heat conduction paths in the polymer matrix is essential to improve the thermal management performance of polymer composites. A three-dimensional (3D) thermally conductive network with regular filler structures is very attractive for building fast conductive paths in polymer composites. Herein, a unique 3D interconnected tannic acid modified boron nitride (BN) and C network (M-BN/C) was successfully fabricated by the carbonization of M-BN/thermoplastic polyurethane (TPU) skeletons, which were obtained via simple salt template assisted strategy to enhance the thermal transfer properties of composites. The highly thermally conductive epoxy composites (M-BN/C/EP) were then prepared by impregnating epoxy resin (EP) into the 3D M-BN/C network. The thermal conductivity of the composites with a M − BN loading of 23 wt% is as high as 1.524 W/(m·K), which exhibits a significant enhancement of 702% compared with pure EP. In addition, our composite exhibited outstanding thermal behaviors during heating and cooling processes. Furthermore, the finite element heat conduction simulation further analyzes the heat conduction mechanism of epoxy composites from the theoretical level. This work provides a new idea to significantly enhance the thermal conductivity of thermal management materials.

Path Planning for Reconfigurable hTetro Robot Combining Heat Conduction-Based and Discrete Optimization

Self-reconfigurable robots present advanced solutions for various automation applications in domains, e.g., planetary exploration, rescue missions, cleaning, and maintenance. These robots have the ability to change their morphology according to given requirements or adapt to new circumstances, which, for example, can overcome constraints while navigating within a working environment. However, the autonomous navigation of self-reconfigurable robots is more complex than that of robots with fixed shape because of the intrinsic complexity of robot motions, especially in complicated obstacle environments. To address this challenge, we present a novel path planning method for reconfigurable robots in this study. The technique is inspired by the similarity between a robot motion path and a heat conduction path at the steady-state. In the heat transfer analysis domain, feasible moving locations are modeled as materials with high conductivity, while obstacles are considered thermal insulators, and the initial and destination positions are assigned as heat sink and heat source, respectively. The temperature profile and gradient calculated by finite element analysis are used to indicate the possible moving directions from the heat sink to the heat source. Based on the temperature gradient ascent, a step-wise conductivity reaching algorithm is developed to optimize robot paths using customized multi-objective functions that take the costs of morphology changes, path smoothness, and safety into account. The proposed path planning method is successfully applied to the hinged-tetro self-reconfigurable robot and demonstrated on several virtual environments and a real-world testbed environment.

Microscopic regulation of plant morphological pores on mechanical properties of porous mullite materials

AbstractGenerally, a multilayer structure is present inside a walnut shell, and the residual structure of the walnut shell is retained after impregnation and firing. When the walnut shell is used as a pore‐forming agent, this structure helps in improving the mechanical and thermal insulation properties of the lightweight porous materials. In this study, porous mullite materials (PMMs) with plant morphological structure pores were prepared using a‐Al2O3 and silica powder as the raw materials with addition of sol‐impregnated walnut shell powder (WSP). The influence of sol type and firing temperature on the pore structure of the PMMs was analyzed, which affected the compressive strength and thermal conductivity. The plant morphological porous structure was observed in the samples after sol impregnation. After firing at different temperatures, the porous structure gradually contracted and supported the pores, improving the mechanical properties, while the complex porous structure increased the heat conduction path, thereby improving the insulation performance. Using WSP impregnated with silica‐sol and zirconia‐sol as pore‐forming agents, PMMs with higher compressive strength and relatively low thermal conductivity (TC) were prepared.

Thermal and dielectric properties of epoxy-based composites filled with flake and whisker type hexagonal boron nitride materials

In this work, the epoxy-based composites filled with 3-Aminopropyltriethoxysilane (KH-550) modified binary filler of hexagonal boron nitride (h-BN) flakes and h-BN whiskers were fabricated, and the thermal and dielectric properties of composites were systematically investigated. Adding h-BN whiskers to the filler could effectively form heat conduction paths and improve the phonon transmission efficiency, thereby improving the thermal conductivity of the composites. According to our results, the thermal conductivity of composite with 27 wt% h-BN flakes and 3 wt% h-BN whiskers reached up to 0.819 W·m−1·K−1, which was 3.9 times that of pure epoxy resin. At the same time, the dielectric loss value and dielectric constant of this composite at 1 MHz were 0.02678 and 4.55, respectively, while pure epoxy resin were 0.03602 and 4.33. In addition, introducing h-BN whiskers into the matrix could effectively improve the thermal stability and heat dissipation performance of composites.

Improved Cauer thermal network considering thermal coupling effects of multi‐chip modules

The Cauer thermal network has made a realistic physical representation of thermal behaviour of a multi-chip module (MCM) theoretically possible. However, the traditional method assumes the heat conduction is critical, with an approximate one-dimensional heat conduction path for calculation and fixed heat spreading angle to make up for the compensation brought by the approximation. It will limit the accuracy and cause a significant error in estimating the junction temperature of MCM. In this study, the heat spreading angle in the area of two adjacent chips is studied by using a cross-section to change the problem to a two-dimensional heat transfer question. Based on the heat flow got from the results of calculation by using Fourier's law, an expression for heat spreading angle as a function of boundary conditions is given, which is capable of estimating the junction temperature by adjusting effective heat conduction area of an MCM whose thermal behaviour is seriously coupled among neighbour chips. The accuracy of the proposed method and expression is validated by both the finite-element method and experimental results and they both show a good agreement with the calculation.

Enhanced thermal conductivity and isotropy of polymer composites by fabricating 3D network structure from carbon‐based materials

AbstractGraphene‐based composite is promising as thermal interface material (TIM) due to its outstanding thermal properties. However, there are some bottlenecks to excellent performance, such as agglomeration of particles and undesirable voids between nanoplatelets. In this work, a composite with three‐dimensional (3D) thermally conductive network has been assembled, which combines three kinds of nanofillers varying geometric dimensions. Thermal conductivity (TC) of composite with graphene nanoplatelets (GNPs) and carbon‐nanotubes (CNTs) at a weight ratio of 3:1 is around 9% higher than that of GNP‐based composite. By the introduction of carbon spheres (CSs), the TC is further increased by 28%. The enhanced thermal property further demonstrated by FLIR infrared camera is attributed to the formation of 3D heat conduction paths by GNPs, CNTs, and CSs, where the GNPs play the role of thermally conductive backbones. The other two components are introduced to attenuate the aggregation and strong thermal anisotropy. Moreover, the TC is confirmed nearly isotropic, which is different from most graphene‐based TIMs because of the in‐plane alignment. Our results indicate that the apparent synergy endows this 3D nanofiller great potential for heat dissipation applications requiring heat removal in two directions.

Investigations of optimized fin structures in a compact thermal energy storage panel

In this paper, the thermal behaviours of phase change material (PCM) within a compact thermal energy storage panel installed with conductive metal fins were numerically investigated. To improve the thermal performance of the metal fins, a topological structure optimization approach was employed to generate innovative fin structures with the so-called best heat conduction paths. The conventional plate fin structures with the same metal volume fractions as the optimized structures were considered for comparative studies. Four metal volume fractions including 0.05, 0.10, 0.15 and 0.20 were investigated. Results showed that at the same metal volume fraction, the optimized fin design can significantly enhance the melting of PCM as compared to the plate fin design. When f=0.9, the average PCM melting speeds obtained by the optimized fin structures were 1.10, 1.19, 1.31, and 1.69 times as that by the plate fin designs at the melt volume fractions of 0.05, 0.10, 0.15 and 0.20, respectively. Moreover, the optimized fin structure with a metal volume fraction of 0.10 can achieve the comparable melting speed as the plate fin design with a metal volume fraction of 0.15, and the melting speed achieved by the optimized fin structure with a metal volume fraction of 0.15 is even faster than that by the plate fin design with a metal volume fraction of 0.20.

Numerical evaluation of inclined heat pipes on suppressing spontaneous coal combustion

Suppressing spontaneous coal combustion with heat pipe (HP) is an effect fire fighting method in practice. To improve the heat transfer capacity of HPs in coal piles as much as possible, this work explores the optimal inclination angle of HPs to remove heat in the coal stockpiles. Based on the theory of fluid mechanics and heat transfer, a physical-mathematical model of coal-HP-air was established, and the spontaneous combustion process of coal piles and the effect of HPs at four inclination angles (θ) of 30°, 45°, 60°, and 90° on the spontaneous combustion of coal piles were studied. The results show that the presence of HPs can change the heat conduction path in a coal pile and help the coal pile to dissipate heat in time, and this process makes the temperature contours on both the windward side and the leeward side deform and exhibits a ‘saddle’-like cooling mode at the periphery of the HPs. As θ of the HPs increases, the area (ΔS) by which the spontaneous combustion danger zone (SCDZ) is decreased first increases and then decreases, and the maximum temperature (Tmax) drop values (ΔT) first increases and then decreases. Moreover, the thermal performance of the HPs at inclination angle of 60° is found to be higher than other tested inclination angles. When θ = 60°, the heat is largely removed, and the integrity of the SCDZ is destroyed, so that ΔS is 11.106 m2, ΔT is 13.1 °C, and the natural ignition period is extended by approximately 25 days. Consequently, this study provides a reference for the design and optimization of coal fire fighting engineering with HP application in the future.

Highly thermally conductive, ductile biomimetic boron nitride/aramid nanofiber composite film

Combining a large volume of boron nitride platelets with a little polymer to fabricate ductile, thermally conductive composite materials for the thermal management of flexible electronic devices is a challenge. Herein, we imitate the hierarchical layered architecture of mollusk nacre to design high-loading boron nitride composite film. Boron nitride/aramid nanofiber composite films are prepared by a sol-gel-film transformation approach and show nacre-like hierarchical layered composite structure. High-strength aramid nanofibers form a three-dimensional framework as matrix, which hosts 40–70 wt% oriented BN platelets. Thanks to the high interconnectivity of aramid nanofiber in the three-dimensional framework, the composite films have large ductility, up to 18.3–49.4%. Meanwhile, the composite films have high thermal conductivity (47.4–122.5 W m−1 K−1) because high-loading boron nitride platelets form a percolative, oriented heat conduction path. Moreover, the outstanding ductility and thermal conductivity are integrated with high volume resistivity (1014–1016 Ω cm) and thermal decomposition temperature (515 °C), enabling the structured boron nitride/aramid nanofiber composite films to be promising as flexible substrate for cooling electronic components.