Thermal Conductive Pathways Research Articles

Size effect on the thermal expansion of ZrO2/ZrW2O8 composites fabricated by the conventional sintering process

The size effect on the thermal expansion of ZrO2/ZrW2O8 fabricated by the conventional thermal sintering process is investigated systematically. The small-sized sample with dense microstructure and few ZrW2O8 decomposition exhibits near-zero thermal expansion. While the microstructure and thermal expansion property of the large-sized sample deteriorates due to the serious decomposition of ZrW2O8. The finite element analysis is used to investigate the thermal transfer during the quenching, demonstrating that the smaller specific surface area (SSA) of the large-sized sample is the direct reason of the size effect, and the core causes are the instability of ZrW2O8 and the low thermal conductivity of the ZrO2/ZrW2O8. A thermal conductive pathway is further designed for the large-sized sample to verify and attenuate the size effect by increasing the SSA. The technique of ultra-low sintering temperature is urgently recommended to effectively solve the obstruction caused by the size effect.

Synergetic enhancement of thermal conductivity by constructing BN and AlN hybrid network in epoxy matrix

Thermal management has developed into a severe issue with the evolution of electronics. The construction of effectively thermal conduction pathways in the matrix is crucial for highly thermal conductive composites. In this work, a unique thermal conductive channel in the epoxy matrix had been established with hybridizing hexagonal boron nitride (BN) sheets and aluminum nitride (AlN) particles through a solution mixture and hot-pressing method. Synergetic enhancement of thermal conductivity was observed between BN and AlN fillers owing to the better dispersion of hybrid fillers, which created more pathways for phonon transport. With 40 vol% hybrid BN-AlN filler contents, the thermal conductivity of EP composite reached 2.4 Wm−1 K−1, eightfold increasing over the pristine epoxy matrix and two times to that of single BN or AlN filling composite. Moreover, the thermal conductivity of hybrid composite was further improved by more than 10% through the surface treatment of fillers with silane couple agents owing to the enhancement of interaction in composites. This study is of critical importance for composites used in electronics and electric equipment.

Flexible and fire-resistant all-inorganic composite film with high in-plane thermal conductivity

With the enlarging application of thermal conductive composites in aviation field and so on, not only high in-plane thermal conductivity (TC), mechanical flexibility and electrical insulation, but also thermal stability and fire resistance are critically desired. Although with outstanding advantages such as light weight, good flexibility and processability, the intrinsic flammability and high temperature intolerance are Achilles’ heel for most polymer-based thermal conductive composites, which severely hinders their application in harsh environments. To solve this issue, we here proposed a facile strategy to prepare flexible all-inorganic composite films inspired by the recent progress of using inorganic nanowires to prepare fire-resistant paper. In this design, one-dimensional hydroxyapatite nanowires (HAPNWs) synthesized via solvothermal method and two-dimensional boron nitride nanosheets (BNNSs) prepared by liquid phase exfoliation were integrated together by vacuum-assisted filtration to form layer-structured composite films, in which HAPNWs provided flexibility and BNNSs offered orientational thermal conductive pathways. When BNNSs loading content was 30 wt%, the in-plane TC value of the resultant composite film reached 6.4 W m−1 K−1 at 25 °C and was still maintained at 5.5 W m−1 K−1 under 100 °C. As expected, the all-inorganic films exhibited excellent flexibility, fire retardant ability and low coefficient of thermal expansion, which show great potential as thermal conductive materials used in harsh environment.

Sandwich structured RGO/CNF/RGO composite films for superior mechanical and thermally conductive properties

It is urgent and significant to develop a thermal interface material (TIM) with both high thermal conductivity and excellent mechanical strength for high-performance electronic devices. For the first time, we designed and fabricated a sandwich structured TIM, constructed by a cellulose nanofiber (CNF) core layer sandwiched by reduced graphene oxide (rGO) shell, with copper ions as a crosslinker, via a facile and scalable vacuum filtration process. The continuous rGO layers on the surface of the composite provided a good thermal conductive pathway for the composite films. The thermal conductivity of the sandwiched films with 8.0 wt% rGO reached 29.5 W/mK, which is over eight times than the CNF films, and realized an ultrafast thermal diffusion time at 73 ms. The sandwich structure combined with the cross-linker of copper ions also plays a synergistic role in construct mechanical strength. Compared to the CNF, the tensile strength of the sandwiched films with 8.0 wt% rGO unprecedentedly reached 314 MPa (nearly three times of bare CNF films), and the elongation increased 63%. In addition, the films also shows high water stability and excellent flexibility, which makes it a very promising for advanced flexible or wearable electronics. This work provides a new insight in rational structure design and novel scalable fabrication strategy to develop TIM with outstanding mechanical strength and thermal conductivity.

Copper nanowires embedded in boron nitride nanosheet-polymer composites with enhanced thermal conductivities for thermal management

With the rapid development of modern microelectronic devices, high-performance thermal interface polymer materials are of ever-rising importance. However, polymers commonly have very low intrinsic thermal conductivities, thence, introducing the thermal conductive nanofillers was considered to be one of the efficiently ways to enhance the thermal conductivity. The construction of effective thermal conductive pathway is essential for the thermal conductivity of polymer-based composites. In this context, toward the common functionalized boron nitride nanosheet (f-BNNS)/polymer composites, we found that the introduction of a very low amount of one-dimensional (1D) copper nanowires (CuNWs) can bring significantly further improvement in thermal conductivity. The in-plane thermal conductivity of f-BNNS-CuNWs/poly (vinyl alcohol) (PVA) composite film with 0.1 wt% of CuNWs can achieved at ~ 6.5 W/(m•K) which is ~1.76 times as high as that of f-BNNS/PVA composite film at the same filling content of f-BNNS. The use of high-aspect-ratio CuNWs is beneficial to the formation of random bridges or networks with conductive f-BNNS and facilitates the phonon transfer leading to high thermal conductivity. SEM images demonstrated that the adjacent 2D f-BNNS plates were in tandem with 1D CuNWs. The obtained results offer a facile and feasible way to design and fabricate the highly thermal-conductive polymer composites as advanced thermal interface polymer composite materials.

A reduced percolation threshold of hybrid fillers of ball-milled exfoliated graphite nanoplatelets and AgNWs for enhanced thermal interface materials in high power electronics

Thermal interface materials (TIMs) are essential components to deplete the accumulated heat with efficient thermal management in the majority of power electronic systems such that the interfacial thermal resistance (ITR) should be significantly reduced. A reduced percolation threshold of hybrid fillers of ball-milled exfoliated graphite nanoplatelets and AgNWs are used for enhanced thermal interface materials via the assembly of high-quality ball-milled exfoliated graphite nanoplatelets (BMEGN) and AgNWs on a flexible Polydimethylsiloxane (PDMS) substrate. BMEGN modified surface with increased specific area and reduced percolation interface, together with the generation of effective 3D thermal conductive pathways between the 2D graphite and 1D AgNWs, which was experimentally measured to possess the significant improvement in thermal conductivity. The proposed method of thermally conductive film of AgNWs at the loading of 2.0 mg/mL can dramatically increase the in-plane thermal conductivity (K//) to 29.2 W/mK, while the through-plane thermal conductivity (K┴) shows the value of 4.94 W/mK by using the reduced percolation threshold. Anisotropy reaches up to K///K┴ = 5.9 with the proposed percolation, which is much larger than filler only BMEGN, indicating that the effective thermal conductive path (TCP) can be networked on a PDMS polymeric to thermally dissipate high power electronics with enhanced heat management capability. Both CPU and IGBT tests of the proposed TIMs with fillers of AgNWs loading 2.0 mg/mL can effectively minimize temperature rise to ~16 °C and ~17 °C as favorably compared with thermal grease alone. Furthermore, transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman, X-ray diffraction (XRD) and thermogravimetric analysis (TGA) were used for surface morphology, microstructure evolution, disorder evaluation, phase analysis and thermal stability of the composite material.

Synergetic enhancement of thermal conductivity in the silica-coated boron nitride (SiO2@BN)/polymethyl methacrylate (PMMA) composites

High thermal conductivity polymer composites have increased application in modern electronics. To achieve high thermal conductivity at relatively low filler content, two critical approach are used: surface modification of the filler and construction of thermal conductive network in polymer composites. This article provided a new simple and feasible method to modify h-BN consisting of physical absorption of polyvinylpyrrolidone (PVP) and hydrolysis of tetraethyl orthosilicate (TEOS). Then, SiO2@BN/PMMA composites were fabricated via solution-mixing and hot compression. The surface modification of BN enhanced the interface interaction between adjacent BN, which led to reduced thermal resistance and phonon scattering. The solution-mixing and hot compression process ensured the formation of thermal conductive pathway at the same time. The highest thermal conductivity of 40 vol% SiO2@BN/PMMA composite reached 5.583W /m K, which exhibited 3292% and 200% enhancement compared with the pure PMMA and melt-mixing BN/PMMA composites, respectively.

Simultaneously enhanced thermal conductivity and mechanical properties of PP/BN composites via constructing reinforced segregated structure with a trace amount of BN wrapped PP fiber

Segregated structure with continuous filler network but isolated matrix is a classic prototype to increase the thermal conductivity (TC) of the composites. However, such prototype only finds limited applications due to both the poor mechanical properties originated from the discontinuity of the polymer matrix and the mediately improved TC of the continuous filler network arising from the random contact of the fillers. In this work, a reinforced segregated structure in which the continuous phase is made with boron nitride (BN) wrapped a trace amount of PP fiber (PF), is first proposed in order to enhance the TC and mechanical properties simultaneously. In such reinforced segregated structure, BN aligns on the surfaces of PF thus forms a long-range ordered, high thermal conductive pathway. As expected, an enhancement of 244%, 133% and 103% in the TC, elongation at break and tensile strength of the composite with reinforced segregated structure are successfully achieved compared with typical segregated structure. Therefore, this work offers some new insights into the design of a segregated structure to balance the TC and mechanical properties.

Thermally Reduced Graphene Oxide/Carbon Nanotube Composite Films for Thermal Packaging Applications.

Thermally reduced graphene oxide/carbon nanotube (rGO/CNT) composite films were successfully prepared by a high-temperature annealing process. Their microstructure, thermal conductivity and mechanical properties were systematically studied at different annealing temperatures. As the annealing temperature increased, more oxygen-containing functional groups were removed from the composite film, and the percentage of graphene continuously increased. When the annealing temperature increased from 1100 to 1400 °C, the thermal conductivity of the composite film also continuously increased from 673.9 to 1052.1 W m−1 K−1. Additionally, the Young’s modulus was reduced by 63.6%, and the tensile strength was increased by 81.7%. In addition, the introduction of carbon nanotubes provided through-plane thermal conduction pathways for the composite films, which was beneficial for the improvement of their through-plane thermal conductivity. Furthermore, CNTs apparently improved the mechanical properties of rGO/CNT composite films. Compared with the rGO film, 1 wt% CNTs reduced the Young’s modulus by 93.3% and increased the tensile strength of the rGO/CNT composite film by 60.3%, which could greatly improve its flexibility. Therefore, the rGO/CNT composite films show great potential for application as thermal interface materials (TIMs) due to their high in-plane thermal conductivity and good mechanical properties.

Lightweight mesoporous carbon fibers with interconnected graphitic walls for supports of form-stable phase change materials with enhanced thermal conductivity

Thermal storage performance of form-stable phase change materials (PCMs) is often compromised by the heavy use of the supporting materials. Here, lightweight mesoporous carbon fibers (MCFs) with continuous graphitic walls are prepared by carbonization of polyvinylidene fluoride on magnesium oxide fibers for form-stable PCMs. The structure of the MCFs are comprehensively investigated to show their merits for supports of the PCMs. The MCFs are rich in relatively ordered mesopores, which can provide effective loading of paraffin and give high thermal storage capacities per unit weight. The paraffin loadings of the as-obtained MCF-700, MCF-800 and MCF-900 reach 93, 90, and 92 wt% with maintained shape stability, and deliver the thermal storage capacities of 176.54, 183.35, and 177.75 kJ kg−1, respectively. Due to the abundant pore networks and the improved interfacial property from the in-situ fluorine doping, the MCFs-supported PCMs exhibit high running stability and thermal storage efficiency in cycling. More importantly, the penetrating graphitic walls of the MCFs provide effective thermal-conductive pathways from outside to the interior for rapid thermal transfer. The results exhibit that the MCFs with interconnected graphitic walls can be an effective support of form-stable phase change materials for solar harvesting, etc.

Enhancing thermal conductivity of epoxy with a binary filler system of h-BN platelets and Al2O3 nanoparticles

Epoxy resin is a common adhesive bonding material used to join dissimilar materials, especially in the electronics and aerospace industries. However, its low thermal conductivity and high coefficient of thermal expansion limit the direct use of epoxy in practical applications. In order to improve thermo-mechanical properties, we have prepared a series of epoxy composites using a binary system of hexagonal-boron nitride (h-BN) and aluminum oxide (Al2O3) fillers and analyzed the effect of the ratio of these fillers on the thermal conductivity of composites. While h-BN platelets form the main thermal conductive network, Al2O3 nanoparticles bridge the separated h-BN platelets to build more thermal conductive pathways. We proposed the improving of thermal conductivity as well as the mechanical properties of the epoxy matrix by incorporating h-BN and Al2O3 fillers at an optimum ratio.

Self-constructing thermal conductive filler network via reaction-induced phase separation in BNNSs/epoxy/polyetherimide composites

An effective and facile method was developed to construct the thermal conductive path way in BNNSs-NH2/epoxy/polyetherimide (PEI) ternary blends via reaction-induced phase separation (RIPS). The well dispersed BNNSs-NH2 in the blends was prepared by the urea-functionalization process, which built a bridge to each other at the interface between epoxy and PEI phases. The through-plane thermal conductivity (TC) of composites is enhanced by 83% at low content of 1 wt% BNNSs-NH2. Filler thermal conductive efficiency (η) of BNNSs-NH2 reaches 189 and 83 based on volume fraction and weight fraction, respectively. In addition, the BNNSs-NH2 shows a critical impact on the kinetic of RIPS, which promotes the curing reaction (coarsening process) of epoxy phase. The glass transition temperatures (Tg) of epoxy and PEI were both enhanced due to the amino group of BNNSs and its selective distribution.

High thermal conductive shape-stabilized phase change materials of polyethylene glycol/boron nitride@chitosan composites for thermal energy storage

Phase change materials (PCMs) applied in the energy storage and temperature control system are crucial for energy conservation and environmental protection. In this work, boron nitride (BN)@chitosan (CS) scaffolds with three-dimensional (3D) porous structures were fabricated. And effective thermal conductive pathways could be created in the resultant scaffolds. By introducing polyethylene glycol (PEG) into the BN@CS scaffolds, composite PCMs with large latent heat of fusion and excellent shape-stability were obtained. In particular, a high thermal conductivity up to 2.77 W m−1 K−1 could be reached at a relatively low content of BN (27 wt%). Moreover, they also exhibited a satisfactory energy storage density of 136 J g−1. This work demonstrated a facile and environmentally friendly strategy to simultaneously achieve enhancement of thermal conductivity, high energy storage density, shape stability and outstanding thermal repeatability for composite PCMs, which held promising potential in waste heat recovery, cooling system and temperature control system.

Simultaneously enhancing the thermal conductivity and dielectric constant of BN/CF hybrid filled polypropylene/polystyrene composites via in situ reactive processing

AbstractThermal conductive and electrical insulating polymer composites are exerted a tremendous fascination on the field of thermal management. The effective utilization of fillers is indispensable to construct the thermal conductive pathway to fabricate high thermal conductive polymer composites. Herein, a facile approach involves selective localization of boron nitride (BN) and carbon fiber (CF) through in situ polymerization of styrene through reactive processing as well as double‐percolation structure design. These composites can be easily fabricated by blending the BN, CF with polystyrene during the polymerization of styrene monomer and secondary compounding with polypropylene (PP). The selective localization of BN and CF was observed by scanning electron microscope (SEM) characterization. The cocontinuous structure of PP/PS was also confirmed by SEM observation. The thermal conductivity values was promoted from 0.22 W/mk for PP/PS blends to 0.62 W/mk for the in situ reactive blending composites with 14.5 wt% BN and 18 wt% CF. Moreover, the dielectric properties were also higher than that of the simple melt blending composites. Therefore, the method we proposed provides a potentially feasible plan to design and prepare the thermal conductive and electrical insulating composites for thermal management applications.

Effects of selective distribution of alumina micro-particles on rheological, mechanical and thermal conductive properties of asphalt/SBS/alumina composites

Constructing thermal conductive pathways is an effective route to increase the thermal conductivity of thermal interface materials (TIMs). Asphalt/styrene-butadiene-styrene tri-block copolymer (SBS)/alumina composites with alumina micro-particles selectively distributed in SBS (denoted as asphalt/SBS/sd-Al2O3) were fabricated through pre-dispersion of alumina micro-particles in SBS, followed by compounding with asphalt. The effects of alumina distribution on the rheological, mechanical and thermal conductive properties of the asphalt-based composites were systematically studied. Compared with the composites with randomly distributed alumina, i.e., asphalt/SBS/rd-Al2O3, those with selectively and homogeneously distributed alumina micro-particles in SBS formed a continuous thermal conductive SBS/alumina network. Therefore, the asphalt/SBS/sd-Al2O3 composites displayed maximum thermal conductivity enhancement of ~35% at 30 vol% alumina. At 50 vol% alumina, the thermal conductivity of the asphalt/SBS/sd-Al2O3 composites reached ~0.99 W/mK, which is 400% higher than that of the asphalt/SBS blend (0.20 W/mK). Also, the asphalt/SBS/sd-Al2O3 composites possessed higher values of storage modulus, tensile strength and softening point. This work has provided a promising approach to fabricate high-performance and low-cost TIMs.

Nacre-inspired copper nanowires/graphene oxide films with excellent thermal conductivity, flame retardancy and electrical performance

Inspired by the hierarchical microstructure of nacre, via the surface modification of polydopamine (PDA), nacre-mimicking composite films containing graphene oxide (GO) and different mass ratio (5, 15, 25, 35 wt%) of copper nanowires (CuNWs) were prepared with a solvent-induced assembly strategy. The introduction of PDA not only reduced GO, but also protected the CuNWs from oxidation. 1D CuNWs, as a thermal bridge, was introduced into 2D GO nanosheets to construct an interconnected thermal conduction network. This structure can provide an effective thermal conductive pathway. The CuNWs/GO-PDA film containing 25 wt% CuNWs (25CuNWs/GO-PDA) exhibits excellent in-plane thermal conductivity of 6.841 W/mK and cross-plane thermal conductivity of 0.202 W/mK. A high anisotropy index of about 34 for 25CuNWs/GO-PDA film was obtained. Due to the char forming ability of PDA, and unique nanosheets structure of GO, films exhibit excellent flame retardancy. Micro combustion calorimeter tests confirm dramatically reduced 80.9% of peak heat release rate compared with pristine GO. More importantly, electrical conductivity of composites improved with the increase of CuNWs loading. With the addition of 35 wt% CuNWs, the electrical conductivity reaches 6.43 × 106 S/m. This type of bioinspired film with multifunctional properties is expected to be used as a promising candidate for electronic materials.

Boron nitride/agarose hydrogel composites with high thermal conductivities

Hydrogels are cross-linked polymers suitable for various applications, but the thermal conductivities of hydrogel-based composites have not been thoroughly investigated. In this study, agarose hydrogel-based composites with various boron nitride (BN) fillers were synthesized and their thermal conductivities were systematically investigated. With the increase in the agarose content from 1.5 wt% to 3.0 wt%, the thermal conductivity of the composite decreased. The composites with BN micropowder had larger thermal conductivities than those of the composites with BN nanopowder at the same filler loading, as the BN micropowder provided better thermal conduction pathways in the hydrogel matrix than those provided by the nanopowder. The maximum thermal conductivity of 2.69 W·m−1·K−1 was achieved when 15 wt% microscale BN fillers were added into 1.5 wt% agarose hydrogel, which was 3.5 times larger than that of the pure agarose hydrogel. Additionally, a theoretical model was used to calculate the thermal conductivities of the BN/agarose hydrogel composites; a good agreement was achieved between the experimental and fitting ones. This study demonstrated that the thermal conductivities of hydrogel-based materials can be efficiently and significantly enhanced using BN fillers.

Proposing an optimal tree-like design of highly conductive material configuration with unequal branches for maximum cooling a heat generating piece

Due to the concerns regarding the space and cost of high thermal conductivity materials, researching for a better design of high thermal conductivity pathways embedded into a heat generating piece is important. Constructal design theory is applied and a numerical optimization procedure is carried out to find a new geometric pattern of tree-like highly conductive material pathways with unequal branches. The optimization purpose is the minimization of the peak temperature in the heat generating piece, by varying the lengths of the branches and the initial distance from the middle plane. The constraint is the fixed volume fraction occupied by the high thermal conductivity insert. Heat conduction equations are solved by a finite element method (FEM). The numerical code develops in MATLAB software and use unstructured triangular elements for meshing. Because of the multiple optimization variables existed in this work, the “Pattern Search” algorithm is used. The results indicate that the performance of the tree-like configuration with unequal branches is better than the one with equal branches for the same number of branches, the same volume fraction of the highly conductive material and the same conductive ratio. It is also shown that the new proposed configurations of highly thermal conductive pathways, significantly surpass the latest configurations presented in the literature. Depending on the volume fraction of highly conductive material and thermal conductivity ratio, the performance of the proposed configurations is observed to be 2–61% better than the performance of the best result existed in previous works.

Thermal Metamaterials with Site‐Specific Thermal Properties Fabricated by 3D Magnetic Printing

AbstractRecently, composites with global anisotropic thermal properties, including thermal conductivity and thermal expansion, have drawn increasing attention as promising thermal‐management materials for highly integrated electrical devices. However, fabricating thermal metamaterials with site‐specific thermal properties remains a great challenge. A 3D magnetic printing process to fabricate fiber‐reinforced composites with site‐specific thermal properties by controlling the orientation of the fibers is presented. In proof‐of‐concept experiments, nickel‐coated carbon fibers (NiCFs) dispersed in resin are site‐specifically aligned according to predesigned patterns through design of the magnet trajectory plan. Composites with aligned fibers are fabricated resulting in site‐specific thermal conductivity that can be locally modulated by a factor of up to 1.39, which is mainly attributable to the aligned NiCFs that can form thermal conductive pathways for phonon transport along the direction of the aligned fibers. These results demonstrate that the thermal properties of composites can be controlled by adjusting the orientation of the fibers. Finally, the present method is applied to fabricate a site‐specific thermal metamaterial as a proof‐of‐concept thermal control. The proposed strategy takes full advantage of 3D‐printing technology, opening a new avenue for manufacturing composites with programmable thermal conductivity, which is thus far inaccessible by traditional manufacturing technologies.

Soy protein as a sustainable surfactant to functionalize boron nitride nanosheets and its application for preparing thermally conductive biobased composites

To enhance interfacial performance of thermal conductive polymer composites, low-cost and abundant plant-derived soy protein isolate (SPI) was used in this study to produce surface functionalized boron nitride nanosheets (BNNS), where SPI acts as a green surfactant to disperse and exfoliate BNNS during the process. Then, biobased tannic acid-coated cellulose nanofiber (TA@CNF) motifs were synthesized to serve as a template to anchor SPI-functionalized BNNS (SPI-BNNS), wherein the TA@CNF provided versatile active surfaces to induce the formation of the interconnected network structure. The TA@CNF assembly with SPI-BNNS could easily form high thermal conductive pathways in the composite films, and the resultant films containing only 1.0 wt% of SPI-BNNS showed significantly enhanced in-plane thermal conductivity (TC) of 6.88 W m−1 K−1, which was three times higher than that of pure TA@CNF film. Moreover, the composite films exhibited remarkable improvement in tensile strength and toughness. We attributed the simultaneously increased TC and mechanical properties to construction of the interconnected network and covalent/hydrogen bonds regulated between SPI-BNNS and TA@CNF. This research is of practical importance to value-added and high-efficiency utilization of plant resources in the future.