In-plane Thermal Conductivity Research Articles

Structural effects on thermal conductivity of micro-thick Li4Ti5O12-based anode

This study investigates the structural effects on the cross-plane thermal conductivity of Li4Ti5O12-based anode active material. Three structures are investigated: a basic structure consisting of LiBr/LiCl/Li4Ti5O12, polyvinylidene difluoride, and Super P (sample #1); a structure without Li4Ti5O12 (sample #2); and a structure without LiBr/LiCl (sample #3). Despite its high porosity level (77%), sample #1 exhibits higher thermal conductivity than sample #3 (64% porosity) in both air and vacuum conditions, potentially due to the extra structural bonding provided by LiBr/LiCl. The observed difference in cross-plane thermal conductivity between air and vacuum conditions provides insights into the configuration of the anode's active material in the heat transfer direction. The lower limit corresponds to the parallel thermal circuit configuration of active material and air, which is the product of the sample's porosity and thermal conductivity of air. Our analysis suggests that in sample #2, the anode's active material and air inside the pores demonstrate a more serial configuration, while in sample #3, they exhibit a more parallel configuration in the heat transfer direction. However, the thermal conductivity difference observed for sample #1 falls below the theoretical lower bound indicating significant thermal radiation within the pores. Furthermore, the in-plane thermal conductivity is predominantly controlled by the copper foil. Sample #2 exhibits the lowest in-plane thermal conductivity. This is attributed to the severe oxidization of the copper foil by LiBr/LiCl, which is confirmed by structure characterization.

Interweaved filler network in epoxy resin with reduced interface thermal resistance via in-situ high-temperature “welding” for significantly improved thermal conductivity

For heterogeneous integration in harsh environments, polymeric materials are required to integrate excellent insulating property with significant thermal conductivity. Traditional discrete fillers used in polymeric matrix to create thermal pathways introduce numerous contact interfaces, leading to limited thermal conductivity enhancement efficiency (TCE) and unstable dielectric properties. Here, a distinctive strategy, involving the preparation of an interweaved thermally conductive filler with inherent heat channels through self-templated electrospinning and in-situ high-temperature “interface welding”, is proposed to reduce the interface thermal resistance (ITR). The growth of grains at high temperature facilitates the process of “interface welding”, while self-templated electrospinning establishes intrinsic thermal conduction pathways. Consequently, the epoxy resin fortified with this interweaved filler exhibits an exceptionally TCE of 162.15 % and enhanced in-plane thermal conductivity of 3.75 W m−1 K−1 at ultra-low filler ratio of 7.10 vol%. The combination of high electrical insulation of filler and low filler content results in excellent electrical insulating property (>1017 Ω cm) and stable dielectric properties over a frequency range of 103-106 Hz, enabling the successful implementation of heterogeneous integration in challenging environments. This work has the potential to offer a groundbreaking approach for achieving interconnected functional networks within polymer-based composites.

3D hierarchical graphene-based composite for ultra-high heat-conducting film

In order to pursue lightness and portability, modern electronic devices usually use a compact layout to place various components, but this highly integrated design increases the heat density of the device and causes heat accumulation. Excessive heat accumulation will reduce the reliability of the device and even cause thermal failure. The advantages of light weight, easy processing and low cost of polymer-based thermally conductive composites have become main concerns of modern thermal management materials, but the low thermal conductivity (TC) has seriously limited their application. Here, 3D hierarchical anisotropic oriented graphene-based composites with high TC are fabricated by layer-by-layer self-assembling of one-dimensional carboxylated cellulose nanofibers (CNF–C) and poly dopamine modified nickel particle-coated two-dimensional graphene nanosheets (PDA-Ni@GNS). Owing to the well-defined flaky packing layers and the hydrogen bonding, the composite exhibits not only sufficiently high in-plane TC of 28.8 W m−1 K−1 and tensile strength of 124 MPa, but also exhibits hydrophobic property. A generalized four-parameter model (FPM) is found to reasonably predict the entire CPU temperature evolution versus time. This work presents a facile strategy for constructing 3D hierarchical interfacial structure to obtain excellent versatility of anisotropic thermal management composites.

Anomalous size effects of ultra-small graphene sheets on the thermal properties of macroscopic films

Graphene films assembled from graphene nanosheets have achieved a wide range of applications in thermal management due to their ultra-high thermal conductivity. Although it is well established in the micron range that increasing the lateral size of graphene sheets enhances the thermal conductivity of graphene films. However, the preparation of large-size graphene sheets and their assembly into ordered film structures remain challenging, which impedes the development of high-performance thermally conductive graphene films. Herein, by unifying the properties of graphene sheets and preparation processes, we correlate the thermal conductivity of graphene films and the lateral size of starting sheets ranging from 0.32 to 20.32 μm and probe the underlying mechanism from the perspectives of macroscopic and microscopic structural defects. Surprisingly, decreasing lateral size to 0.32 μm achieved as high in-plane thermal conductivity (K//, 1550.06 ± 12.99 W/mK) and significantly higher through-plane thermal conductivity (K⊥, 8.11 ± 0.08 W/mK) of graphene films as the case of 20.32 μm. Besides, the commonly overlooked size effect of K⊥ shows an unexpected negative correlation, and the size effect of K// is also not monotonically increasing. These phenomena are correlated with micro-structure defects and lattice defects. Mechanism analysis reveals that thermally induced gas generation and the complex gas-sheet interactions during micro-structure evolution play a critical role in the size effect and may explain the unexpected negative size effect in the sub-micro range. These findings in size effects provide theoretical guidance for the selection of raw materials in fabricating highly thermally conductive graphene films.

The copper matrix composite with "sandwich" structure and three-dimensional networks, showcasing significantly improved thermal conducting performance

Copper matrix composites can be used to address the issue of heat dissipation in electronic devices. Nevertheless, the thermal conductivity (TC) of carbon fiber (CF)/copper (Cu) composites falls significantly below expectations. This is primarily attributed to the challenge of achieving a well-ordered arrangement of CF within the matrix, coupled with the issue of low interfacial bonding strength. A highly effective and uncomplicated approach to fabricate copper matrix composites featuring a "sandwich structure" of Cu-CF-Cu is introduced in this paper. This configuration, achieved through structural design facilitated by hot-press sintering and vacuum suction filtration, exhibited a three-dimensional graphene-like networks (3D-GLNs) and meticulously ordered CF layers over long distances. More importantly, in comparison to pure copper (350 W m−1 K−1), the laminated composite with 40 vol% CF of the CF layer had an in-plane TC of 510 W m−1 K−1, which was approximately 1.5 times greater than that of pure copper. Additionally, the coefficient of thermal expansion (CTE) of the laminated composite with 45 vol% was 9.98 ppm/K. The in-plane TC of the composite was comparable to that of the graphene/Cu composites with the same content. This underscores the advantages of the structural design, even in cases where the TC of the reinforced phase differs by a factor of two. The aforementioned findings demonstrate that this methodology for developing a unique "sandwich" structure can effectively enhance in-plane TC in CF/metal composites, presenting potential for utilization in electronic packaging requiring efficient directional heat transfer.

Unveiling the magic of twist angle: Thermal transport in Two-dimensional Graphene/MoS2/Graphene Heterostructures

Thermal transport properties of two-dimensional vdW heterostructures have attracted considerable research interests in electron-related devices due to their powerful capability of multi-functional integration. Herein, we investigated the in-plane and out-of-plane thermal conductivity of graphene/MoS2 heterostructures under various twist angles utilizing molecular dynamics simulations. It was found that the in-plane thermal conductivity of MoS2 layer in heterostructures decreases with twist angle. Notably, the lowest thermal conductivity occurs at a twist angle of 10.893°, just 47.3 % of that in untwisted MoS2 layers in heterostructure. In addition, the interlayer interfacial thermal resistance between graphene and MoS2 layer increases with the twist angle but decreases with the layer numbers of graphene. The atomic position deviation, potential energy surface, phonon state of density are calculated to reveal the regulation mechanism of thermal conductivity by twist angle. Interestingly, it is found that the twist angles and the layer numbers of graphene can control the atomic vibration arising from potential energy surfaces and phonon transport through the overlap of phonon density of states between graphene and MoS2 layers. This study can provide valuable insights into the thermal conductivity modulation and phonon transport mechanism of heterostructures.

Flexible yet impermeable composites with wrinkle structured BNNSs assembling for high-performance thermal management

Efficient thermal management has become one of the most critical issues of electronics because of the high heat flux generated from highly integrated, miniaturized, and increased power. Here we report highly flexible composites with aligned and overlapping interconnected boron nitride nanosheets (BNNSs) assembled in wrinkle structures. Besides high in-plane thermal conductivity of more than 26.58 W m−1 K−1, such structure rendered enhanced through-plane conduction along with increasing pre-stain. As thermal interface materials (TIMs) of both rigid and flexible devices, the composites revealed an outstanding thermal cooling capability outperforming some commercial TIMs. During a record-long bending process of more than 3000 cycles, the maximum temperature fluctuation of the flexible device with 100%-prestrained composite was only within 0.9 °C, less than one-third of that with commercial thermal pad. Moreover, the composite revealed a superior impermeability for flexible seals. Our results illustrate that the composites could be an ideal candidate for the thermal management of emerging flexible electronics.

Sizing agent induces the oriented attachment of 2D nanosheets for preparing high-performance carbon fiber composites

Inducing highly oriented 2D nanosheets to create a mussel-inspired interface phase on the carbon fiber (CF) surface to enhance its composites’ strength and toughness has become a great challenge. In this study, a novel coating was synthesized to induce the graphene oxide (GO) oriented distributed on the CF surface while constructing a biomimetic "Brick-and-Mortar" structural interface phase, which simultaneously promoted the strength and toughness of CF composites. Acting as a sizing agent, di-pyrene terminated PEG (py-PEG-py) facilitates abundant π-π* stacking interactions, and enables flexible chain segment motion during interface fracture of composites, as well as effectively improving the toughness of composites. Eventually, the formation of a superior mechanism on the CF surface results in remarkable properties for composites, including an interfacial shear strength (IFSS) of 99.3 MPa, interlaminar shear strength of 93.2 MPa, and flexural strength of 1430 MPa. The tensile strength and toughness increased to 1777 MPa and 45.0 MJm−3, respectively. Simultaneously, the continuous thermal conduction pathway produced by the oriented attachment of GO on the CF surface increased in-plane thermal conductivity. The reinforcing mechanisms of the unique mussel-inspired coating were systematically investigated. This work opens up an effective and scalability avenue for obtaining high-performance CF composites.

In-plane thermal conductivity of hexagonal boron nitride from 2D to 3D

The in-plane thermal conductivity of hexagonal boron nitride (h-BN) with varying thicknesses is a key property that affects the performance of various applications from electronics to optoelectronics. However, the transition of the thermal conductivity from two-dimensional (2D) to three-dimensional (3D) h-BN remains elusive. To answer this question, we have developed a machine learning interatomic potential within the neuroevolution potential (NEP) framework for h-BN, achieving a high accuracy akin to ab initio calculations in predicting its thermal conductivity and phonon transport from monolayer to multilayers and bulk. Utilizing molecular dynamics simulations based on the NEP, we predict the thermal conductivity of h-BN with a thickness up to ∼100 nm, demonstrating that its thermal conductivity quickly decreases from the monolayer and saturates to the bulk value above four layers. The saturation of its thermal conductivity is attributed to the little change in phonon group velocity and lifetime as the thickness increases beyond four layers. In particular, the weak thickness dependence of phonon lifetime in h-BN with a nanoscale thickness results from its extremely high phonon focusing along the in-plane direction. This research bridges the knowledge gap of phonon transport between 2D and 3D h-BN and will benefit the thermal design and performance optimization of relevant applications.

Preparation of high performance thermal conductive composites from aluminum plastic packaging waste via Solid-state Drawing

Aluminum-plastic multilayer films have been widely used as the top-grade packaging material due to their exceptional barrier properties. However, the recycling of aluminum plastic packaging waste (APPW) poses a great challenge. To address this issue, we proposed a facile method for recycling APPW using solid-state drawing technology to produce high performance thermal conductivity composites for the first time. The ultrafine APPW with good processability was obtained by solid-state shear milling (S3M), with a low electrical conductivity of less than 10−12 S/cm. The subsequent solid-state drawing of both the aluminum sheet and polymer chain resulted in the formation of an oriented crystalline structure, with the Al sheet acting as an effective nucleating agent. This resulted in a significant increase in tensile strength for HDPE/APPW (30 %) composites, from 21.4 MPa to 116.1 MPa at the drawing ratio of 5. In addition, the in-plane thermal conductivity of HDPE/APPW (50 %) composites reached an impressive 2.5 W/mK, with the tensile strength of 78.8 MPa. Our findings demonstrate the potential of utilizing APPW for the production of value-added composites without requiring separation, with possible applications in heat management for emerging electrical systems and electronic devices.

Directional thermal transport feature in binary filler-based SiR composites with horizontally oriented h-BN

Efficiently thermal management in electronic devices calls for creating polymer composites that have superior thermal transport abilities. Compared with single-filler composites, hybrid-filler composites have gained significant attention as they not only utilize the advantages of each filler individually but also allow for the potential cooperation in constructing filler networks between them. Differing from the previous research on hybrid-filler size, geometry, composition, and content and their impact on thermal conductivity (TC) in composites, this study aims to examine the spatial distribution effects of hybrid-fillers. A binary-filler strategy comprising of anisotropic 2D h-BN and 0D isotropic S–Al2O3 has been proposed, with the primary h-BN filler aligned to filly utilize its anisotropy and intrinsic TC properties. Furthermore, positioning a small quantity of S–Al2O3 between adjacent parallel h-BN considerably improves TC in the non-oriented direction of silicon rubber (SiR) composites. A special focus is given to the influence of the interstitial S–Al2O3 filler on the individual through-plane TC (λ⊥) and in-plane TC (λ⫽) variation of the binary-filler hybrid SiR composites, which are perpendicular and parallel to the oriented h-BN. When incorporating 40 vol% h-BN and 5 vol% S–Al2O3, the binary SiR composites exhibit the highest concurrent λ⫽ and λ⊥ values of 14.581 and 2.132 W/m·K, respectively. The corresponding effects on the ductility, hardness, and dielectric properties of the SiR composites were explained. Moreover, the use of commercially available TC fillers, along with the effortless preparation method, enables the potential to produce h-BN based TIMs materials on a large scale for thermal management usage.

Enhanced thermal conductivity of CS/BN/Ti3O5 coatings to promote electron transport for ultra-sensitive fire sensing

Intelligent fire warning system is a crucial protective equipment reduce fire hazards. A recent increasing concern is the development of sensitive fire warning materials that can be integrated with combustible materials to provide real-time alarms in the fire. However, current research on improving the sensitivity of fire warning sensors based on temperature-resistance response focuses on constructing continuous conductive networks. The effect of the heat transfer process is ignored, which limits the further improvement of alarm sensitivity. Here, we developed a strategy to improve thermal conductivity. Based on the chitosan (CS) conductive network in Ti3O5-based fire sensing system, aminated 2D h-BN nanosheets with high temperature resistance and ultrahigh in-plane thermal conductivity were further introduced. The h-BN improves the thermal conductivity of the fire warning coating from 1.43 W/m·K to 2.29 W/m·K. Intelligent fire warning sensors with ultra-sensitivity (1.16 s), low response temperature (170 °C), duration time (12 min) and durability (100 cycles) were successfully prepared by improving thermal conductivity to rapidly transfer heat to Ti3O5, which leads to a rapid decrease in resistance as the core mechanism. This work proposed an ingenious strategy for the construction of high fire safety and thermal conduction fireproof coatings, which revealed enticing prospects in the field of intelligent coatings.

Increased thermal conductivity and decreased electron–phonon coupling factor of the aluminum scandium intermetallic phase (Al3Sc) compared to solid solutions

Aluminum scandium alloys and their intermetallic phases have arisen as potential candidates for the next generation of electrical interconnects. In this work, we measure the in-plane thermal conductivity and electron–phonon coupling factor of aluminum scandium alloy thin films deposited at different temperatures, where the temperature is used to control the grain size and volume fraction of the Al3Sc intermetallic phase. As the Al3Sc intermetallic formation increases with higher deposition temperature, we measure increasing in-plane thermal conductivity and a decrease in the electron–phonon coupling factor, which corresponds to an increase in grain size. Our findings demonstrate the role that chemical ordering from the formation of the intermetallic phase has on thermal transport.

Anisotropic Fourier Heat Conduction and phonon Boltzmann transport equation based simulation of time domain thermo-reflectance experiments

The anisotropic Fourier Heat Conduction Equation (FHCE) and the multidimensional phonon Boltzmann Transport Equation (BTE) were solved numerically in cylindrical coordinates and in time domain to simulate a Time Domain Thermo-Reflectance (TDTR) experimental silicon/aluminum substrate/transducer setup. The out-of-phase response of the probe laser was predicted at various beam offset distances for a pump laser pulse frequency of 80 MHz and modulation frequency of 10 MHz and compared against experimental measurements for a silicon substrate. The isotropic FHCE was also solved for comparison. Results show that the isotropic FHCE with bulk thermal conductivity of 145 W/m/K significantly underpredicts the out-of-phase temperature difference, particularly at smaller beam offsets. With an isotropic thermal conductivity of 105 W/m/K, the computed results match experimental data at smaller beam offsets well, but overpredicts the experimental data at larger beam offsets. An almost-perfect match is obtained by using an anisotropic thermal conductivity wherein the radial (in-plane) thermal conductivity is set to 85 W/m/K and the axial (through-plane) conductivity is set to 130 W/m/K. The multidimensional frequency and polarization dependent phonon BTE is next solved. The BTE results for the out-of-phase temperature difference match experimental observations well at small and intermediate beam offsets, but overpredicts the experimental data at larger beam offsets. FHCE results are fitted to the BTE predictions, and the extracted (best fit) thermal conductivity is found to be 110 W/m/K.

Flash soldering of boron nitride nanosheets for all-ceramic films

The assembly of boron nitride nanosheets (BNNSs) into strong, flexible thin films capable of withstanding extreme conditions, such as high/low working temperatures and corrosive environments, holds promise for numerous applications. However, Assmebling and sintering of BNNS into dense thin film can be challenging. We report the rapid fabrication of dense BNNS all-ceramic films (BACFs) in 30 s by combining the fabrication strategies for 2D and ceramic materials. In this process, densely arranged and highly oriented BNNSs are flash soldered into a monolith achieved by precisely controlled ultrafast sintering. This fusion of BNNS leads to an ultrahigh in-plane thermal conductivity of 134.5 W·m−1·K−1. The as-prepared BACF exhibits the combined advantages of sintered ceramics (durability, stability, acid-alkali resistance) and 2D material films such as flexibility and shock resistance, which is important for practical uses. It also has excellent cooling performance, dielectric properties, and good thermal stability, which is promising for demanding thermal management applications in emerging technologies.

Boron Nitride/Carbon Fiber High-Oriented Thermal Conductivity Material with Leaves-Branches Structure.

In the realm of thermal interface materials (TIMs), high thermal conductivity and low density are key for effective thermal management and are particularly vital due to the growing compactness and lightweight nature of electronic devices. Efficient directional arrangement is a key control strategy to significantly improve thermal conductivity and comprehensive properties of thermal interface materials. In the present work, drawing inspiration from natural leaf and branch structures, a simple-to-implement approach for fabricating oriented thermal conductivity composites is introduced. Utilizing carbon fibers (CFs), known for their ultra-high thermal conductivity, as branches, this design ensures robust thermal conduction channels. Concurrently, boron nitride (BN) platelets, characterized by their substantial in-plane thermal conductivity, act as leaves. These components not only support the branches but also serve as junctions in the thermal conduction network. Remarkably, the composite achieves a thermal conductivity of 11.08 W/(m·K) with just an 11.1 wt% CF content and a 1.86 g/cm3 density. This study expands the methodologies for achieving highly oriented configurations of fibrous and flake materials, which provides a new design idea for preparing high-thermal conductivity and low-density thermal interface materials.

Manipulating heat transfer at graphene/silicon interface with nitrogen doping

We perform a numerical evaluation on manipulating the thermal transfer at graphene/silicon by adjusting nitrogen doping modes. The results show that nitrogen doping can reduce the in-plane thermal conductivity of graphene/silicon heterostructure, but when doping is regular and distributed throughout graphene, the thermal conductivity will increase abruptly. This phenomenon can be attributed to the coupling between nitrogen phonons, which provides a new heat transfer channel for in-plane heat transfer. We also detected that the in-plane thermal conductivity of regular doping is higher than that of random doping in all concentration ranges. In the other, we found that nitrogen doping can significantly reduce the interfacial thermal resistance of graphene/silicon interface. The above-mentioned findings provide new ideas and theoretical support for controlling the heat transfer between graphene based heterogeneous interfaces.

Thermally conductive, electrically insulating, and radiative cooling PVDF/BNNS/Al2O3/cordierite nanocomposites

For now, the introduction of fillers with highly intrinsic thermal conductivity (TC) into polymers is the most effective way to prepare composites with excellent heat dissipation capability. However, reducing the filler loading without lowering the TC is the most challenging. In this article, patterned electrospinning and hot pressing were employed to prepare thermally conductive and electrically insulating polyvinylidene fluoride (PVDF)-based composites containing 20 wt% boron nitride nanosheets (BNNSs) as the dominant filler and 5 wt% Al2O3 nanopowders as the auxiliary filler. We have proven “1 + 1>2″ in terms of enhancing the TC of composites at nanoscale by the synergistic effect of nanosheet and nanopowder. The results showed that the in-plane TC (8.55 W/(m·K)), volume resistivity (1.56 × 1015 Ω cm), and breakdown strength (406.2 kV/mm) of the composites were improved by 4759.1 %, 649.0 %, and 34.1 % compared with the neat polymer, respectively. Furthermore, the potential application of such composites in contact heat dissipation and radiative cooling was demonstrated by the experiment of integrating cordierite particles with high infrared radiation onto the films.

Construction of three-dimensional network using boron nitride microspheres and the thermal conductivity/stability of epoxy resin composites

Efficient heat dissipation has become the focus of electronic packaging materials, constructing three-dimensional thermally conductive filler networks to prepare composites can realize efficient heat transfer. However, there are few studies comparing the effects of different ways of constructing three-dimensional networks on the thermal conductivity enhancement of polymers. In this study, BNMS self-assembled from BNNSs was used as a thermally conductive filler, and the three-dimensional skeleton was constructed by both the powder mixing pressing method and the ice template method, and scanned to analyze the morphological and structural characteristics. At the same time, the epoxy composites were prepared, and the advantages and disadvantages of the thermal conductivity of the composites prepared in different ways were analyzed by combining the SEM structures. In the powder mixing pressing method, the in-plane and through-plane thermal conductivity of the epoxy composites with the BNMS additive amount of 53.92 wt% reached 10.961 W·m−1·K−1 and 9.955 W·m−1·K−1, respectively, which was increased by 58 and 52 times. The thermal stability has been improved significantly that the T60 was in creased by 60.89°C compared to pure epoxy. Using the ice template method with 7.67 wt% of BNMS addition, the through-plane thermal conductivity of the epoxy composites reaches 0.977 W·m−1·K−1, which is an improvement of 428.1 % compared with pure EP. In addition, both composites maintain low dielectric constants and dielectric losses. It is instructive for the preparation method of subsequent construction of three-dimensional thermally conductive filler networks.

Impact of crystallinity on thermal conductivity of RF magnetron sputtered MoS2 thin films

This study investigates the effects of sulfur atomic defects and crystallinity on the thermal conductivity of MoS2 thin films. Utilizing scanning transmission electron microscopy (STEM), X-ray diffraction (XRD), and Raman spectroscopy, we examined MoS2 films, several nanometers thick, deposited on Si/SiO2 substrates. These films were prepared via a combination of RF magnetron sputtering and sulfur vapor annealing (SVA) treatment. Structural analyses, including cross-sectional STEM and in-plane and out-of-plane XRD measurements, revealed an increase in the S/Mo ratio and grain size of the MoS2 films following SVA treatment. Notably, the in-plane thermal conductivity of MoS2 films treated with SVA was found to be at least an order of magnitude higher than that of films without SVA treatment. This research suggests that the in-plane thermal conductivity of MoS2 thin films can be significantly enhanced through crystallinity improvement via SVA treatment.