Vertically Aligned High-Quality Graphene Foams for Anisotropically Conductive Polymer Composites with Ultrahigh Through-Plane Thermal Conductivities

Popcorn-Inspired Expanded Graphite Microspheres with Controlled Morphology and Considerable Conductivity

The structured processing of graphite is complex and challenging, in which expanded graphite plays a crucial role. Given its superior physical and chemical properties, expanded graphite finds extensive application in diverse domains such as electrochemistry and thermal management. However, the traditional preparation process is inconvenient in effectively meeting the design requirements on the macro and micro scales, which presents a challenge for the structured processing of expanded graphite materials. Here, an innovative method is first proposed for the controllable preparation of expanded graphite microspheres. Inspired by the explosion process of popcorn, the controlled gas release inside the natural flake graphite during chemical expansion is regulated by fuming sulfuric acid, realizing the controllable preparation of expanded graphite microspheres. Subsequently, sulfur trioxide can also intensify the degree of oxidation on the surface of the microspheres. The controllable microsphere morphology endows the composite with good isotropic network bonding, with considerable thermal conductivity of 1.703 W m-1 K-1 at low loading of 10 wt.% and reliable cyclic stability. This work opens up a new way for the morphology control of expanded graphite and provides a novel design thought for the physical and chemical structure control of carbon materials in the future.

2D Materials-Based Thermal Interface Materials: Structure, Properties, and Applications

The challenges associated with heat dissipation in high-power electronic devices used in communication, new energy, and aerospace equipment have spurred an urgent need for high-performance thermal interface materials (TIMs) to establish efficient heat transfer pathways from the heater (chip) to heat sinks. Recently, emerging 2D materials, such as graphene and boron nitride, renowned for their ultrahigh basal-plane thermal conductivity and the capacity to facilitate cross-scale, multi-morphic structural design, have found widespread use as thermal fillers in the production of high-performance TIMs. To deepen the understanding of 2D material-based TIMs, this review focuses primarily on graphene and boron nitride-based TIMs, exploring their structures, properties, and applications. Building on this foundation, the developmental history of these TIMs is emphasized and a detailed analysis of critical challenges and potential solutions is provided. Additionally, the preparation and application of some other novel 2D materials-based TIMs are briefly introduced, aiming to offer constructive guidance for the future development of high-performance TIMs.

Well-designed lamellar reduced graphene oxide-based foam for high-performance solar-driven water purification

Although solar steam generation is promising for seawater desalination, it is less effective in purifying wastewater with both salt/heavy metal ions and organic contaminants. It is thus imperative to develop multifunctional integrated solar-driven water purification systems with high solar-thermal evaporation and photocatalytic degradation efficiencies. Herein, a lamellar reduced graphene oxide (L-RGO) foam with the vertical lamellar structure is fabricated by bidirectional-freezing, lyophilization, and slight chemical reduction for water purification. The unique vertical lamellar structure not only accelerates upward transport of water for facilitating water evaporation but also endows the L-RGO foam with superb high elasticity for tuning the interlayer distance and varying interactions between the oxygen-containing groups and water molecules to adjust water energy state. As a result, the L-RGO foam achieves a superb water evaporation rate of 2.40 kg m-2 h-1 along with an energy efficiency of 95.3 % under the compressive strain of 44.7 % under 1-sun irradiation. Equally importantly, the decoration of L-RGO foam with polypyrrole is capable of efficiently degrading organic pollutants while retaining high solar steam generation performances, exhibiting great potential in the comprehensive treatment of various water sources for relieving freshwater crisis.

Recent Progress of Vertical Graphene: Preparation, Structure Engineering, and Emerging Energy Applications

Vertical graphene (VG) nanosheets have garnered significant attention in the field of electrochemical energy applications, such as supercapacitors, electro-catalysis, and metal-ion batteries. The distinctive structures of VG, including vertically oriented morphology, exposed, and extended edges, and separated few-layer graphene nanosheets, have endowed VG with superior electrode reaction kinetics and mass/electron transportation compared to other graphene-based nanostructures. Therefore, gaining insight into the structure-activity relationship of VG and VG-based materials is crucial for enhancing device performance and expanding their applications in the energy field. In this review, the authors first summarize the fabrication methods of VG structures, including solution-based, and vacuum-based techniques. The study then focuses on structural modulations through plasma-enhanced chemical vapor deposition (PECVD) to tailor defects and morphology, aiming to obtain desirable architectures. Additionally, a comprehensive overview of the applications of VG and VG-based hybrids d in the energy field is provided, considering the arrangement and optimization of their structures. Finally, the challenges and future prospects of VG-based energy-related applications are discussed.

Highly Aligned Graphene Aerogels for Multifunctional Composites

Stemming from the unique in-plane honeycomb lattice structure and the sp2 hybridized carbon atoms bonded by exceptionally strong carbon-carbon bonds, graphene exhibits remarkable anisotropic electrical, mechanical, and thermal properties. To maximize the utilization of graphene's in-plane properties, pre-constructed and aligned structures, such as oriented aerogels, films, and fibers, have been designed. The unique combination of aligned structure, high surface area, excellent electrical conductivity, mechanical stability, thermal conductivity, and porous nature of highly aligned graphene aerogels allows for tailored and enhanced performance in specific directions, enabling advancements in diverse fields. This review provides a comprehensive overview of recent advances in highly aligned graphene aerogels and their composites. It highlights the fabrication methods of aligned graphene aerogels and the optimization of alignment which can be estimated both qualitatively and quantitatively. The oriented scaffolds endow graphene aerogels and their composites with anisotropic properties, showing enhanced electrical, mechanical, and thermal properties along the alignment at the sacrifice of the perpendicular direction. This review showcases remarkable properties and applications of aligned graphene aerogels and their composites, such as their suitability for electronics, environmental applications, thermal management, and energy storage. Challenges and potential opportunities are proposed to offer new insights into prospects of this material.

Scalable microfluidic fabrication of vertically aligned two-dimensional nanosheets for superior thermal management

Two-dimensional (2D) nanosheets have been assembled into various macroscopic structures for wide engineering applications. To fully explore their exceptional thermal, mechanical, and electrical properties, 2D nanosheets must be aligned into highly ordered structures due to their strong structural anisotropy. Structures stacked layer by layer such as films and fibers have been readily assembled from 2D nanosheets due to their planar geometry. However, scalable manufacturing of macroscopic structures with vertically aligned 2D nanosheets remains challenging, given their large lateral size with a thickness of only a few nanometers. Herein, we report a scalable and efficient microfluidics-enabled sheet-aligning process to assemble 2D nanosheets into a large-area film with a highly ordered vertical alignment. By applying microchannels with a high aspect ratio, 2D nanosheets were well aligned vertically under strong channel size confinement and high flow shear stress. A vertically aligned graphene sheet film was obtained and applied to effectively improve the heat transfer of thermal interfacial materials (TIMs). Superior through-plane thermal conductivity of 82.7 W m-1 K-1 at a low graphene content of 11.8 vol% was measured for vertically aligned TIMs. Thus, they demonstrate exceptional thermal management performance for switching power supplies with high reliability.

Vertical Array of Graphite Oxide Liquid Crystal by Microwire Shearing for Highly Thermally Conductive Composites

Excellent through-plane thermally conductive composites are highly demanded for efficient heat dissipation. Giant sheets have large crystalline domain and significantly reduce interface phonon scattering, making them promising to build highly thermally conductive composites. However, realizing vertical orientation of giant sheets remains challenging due to their enormous mass and huge hydrodynamic drag force. Here, we achieve highly vertically ordered liquid crystals of giant graphite oxide (more than 100 µm in lateral dimension) by microwire shearing, which endows the composite with a recorded through-plane thermal conductivity of 94 W m^-1 K^-1 . Microscale shearing fields induced by vertical motion of microwires conquer huge hydrodynamic energy barrier and vertically reorient giant sheets. The resulting liquid crystals exhibit extremely retarded relaxation and impart large-scale vertical array with bidirectional ordering degree as high as 0.82. The graphite array-based composites demonstrate an ultrahigh thermal enhancement efficiency of over 35 times per unit volume. Furthermore, the composites improve cooling efficiency by 93% for thermal management tests compared to commercial thermal interface materials. This work offers a novel methodology to precisely manipulate the orientation of giant particles and promote large-scale fabrication of vertical array with advanced functionalities.

Development and Optimization of 3D-Printed Flexible Electronic Coatings: A New Generation of Smart Heating Fabrics for Automobile Applications

Textile-based Joule heaters in combination with multifunctional materials, fabrication tactics, and optimized designs have changed the paradigm of futuristic intelligent clothing systems, particularly in the automobile field. In the design of heating systems integrated into a car seat, conductive coatings via 3D printing are expected to have further benefits over conventional rigid electrical elements such as a tailored shape and increased comfort, feasibility, stretchability, and compactness. In this regard, we report on a novel heating technique for car seat fabrics based on the use of smart conductive coatings. For easier processes and integration, an extrusion 3D printer is employed to achieve multilayered thin films coated on the surface of the fabric substrate. The developed heater device consists of two principal copper electrodes (so-called power buses) and three identical heating resistors made of carbon composites. Connections between the copper power bus and the carbon resistors are made by means of sub-divide the electrodes, which is critical for electrical-thermal coupling. Finite element models (FEM) are developed to predict the heating behavior of the tested substrates under different designs. It is pointed out that the most optimized design solves important drawbacks of the initial design in terms of temperature regularity and overheating. Full characterizations of the electrical and thermal properties, together with morphological analyses via SEM images, are conducted on different coated samples, making it possible to identify the relevant physical parameters of the materials as well as confirm the printing quality. It is discovered through a combination of FEM and experimental evaluations that the printed coating patterns have a crucial impact on the energy conversion and heating performance. Our first prototype, thanks to many design optimizations, entirely meets the specifications required by the automobile industry. Accordingly, multifunctional materials together with printing technology could offer an efficient heating method for the smart textile industry with significantly improved comfort for both the designer and user.

Constructing Sandwich-Structured Poly(vinyl alcohol) Composite Films with Thermal Conductive and Electrical Performance

With the miniaturization and high integration development in microelectronic devices, the problem of heat dissipation has attracted widespread attention. Highly thermal conductive and electrical insulation polymer composites show great advantages to solve the problems of heat dissipation. Nevertheless, the fabrication of polymer composites with both excellent thermal conductivity and electrical performance is still a great challenge. Herein, to coordinate the thermal and electrical properties of the composite film, the sandwich-structured poly(vinyl alcohol) (PVA)/boron phosphide (BP)-boron nitride nanosheet (BNNS) composite films were prepared, with the PVA/BP composite film as the top and bottom layers and the BNNS layer as the middle layer. When the filler loading was 31.92 wt %, the sandwich-structured composite films showed excellent in-plane thermal conductivity (9.45 W·m^-1·K^-1), low dielectric constant (1.25 at 10² Hz), and excellent breakdown strength. In the composite film, the interconnected BP particles and BNNS layer formed several heat dissipation pathways to increase the thermal conductivity, while the insulated BNNS layer hampered the electron transformation to enhance the electrical resistivity of films. Therefore, the PVA/BP-BNNS composite films showed a potential application in heat dissipation of high power electronic devices.

Fluorobenzene and Water-Promoted Rapid Growth of Vertical Graphene Arrays by Electric-Field-Assisted PECVD

Vertical graphene (VG) arrays show exposed sharp edges, ultra-low electrical resistance, large surface-to-volume ratio, and low light reflectivity, thus having great potential in emerging applications, including field emission, sensing, energy storage devices, and stray light shields. Although plasma enhanced chemical vapor deposition (PECVD) is regarded as an effective approach for the synthesis of VG, it is still challenging to increase the growth rate and height of VG arrays simultaneously. Herein, a fluorobenzene and water-assisted method to rapidly grow VG arrays in an electric field-assisted PECVD system is developed. Fluorobenzene-based carbon sources are used to produce highly electronegative fluorine radicals to accelerate the decomposition of methanol and promote the growth of VG. Water is applied to produce hydroxyl radicals in order to etch amorphous carbon and accelerate the VG growth. The fastest growth rate can be up to 15.9 µm h^-1 . Finally, VG arrays with a height of 144 µm are successfully synthesized at an average rate of 14.4 µm h^-1 . As a kind of super black material, these VG arrays exhibit an ultra-low reflectance of 0.25%, showing great prospect in stray light shielding.

Ultralow Interfacial Thermal Resistance of Graphene Thermal Interface Materials with Surface Metal Liquefaction

Developing advanced thermal interface materials (TIMs) to bridge heat-generating chip and heat sink for constructing an efficient heat transfer interface is the key technology to solve the thermal management issue of high-power semiconductor devices. Based on the ultra-high basal-plane thermal conductivity, graphene is an ideal candidate for preparing high-performance TIMs, preferably to form a vertically aligned structure so that the basal-plane of graphene is consistent with the heat transfer direction of TIM. However, the actual interfacial heat transfer efficiency of currently reported vertically aligned graphene TIMs is far from satisfactory. In addition to the fact that the thermal conductivity of the vertically aligned TIMs can be further improved, another critical factor is the limited actual contact area leading to relatively high contact thermal resistance (20-30 K mm2 W-1) of the "solid-solid" mating interface formed by the vertical graphene and the rough chip/heat sink. To solve this common problem faced by vertically aligned graphene, in this work, we combined mechanical orientation and surface modification strategy to construct a three-tiered TIM composed of mainly vertically aligned graphene in the middle and micrometer-thick liquid metal as a cap layer on upper and lower surfaces. Based on rational graphene orientation regulation in the middle tier, the resultant graphene-based TIM exhibited an ultra-high thermal conductivity of 176 W m-1 K-1. Additionally, we demonstrated that the liquid metal cap layer in contact with the chip/heat sink forms a "liquid-solid" mating interface, significantly increasing the effective heat transfer area and giving a low contact thermal conductivity of 4-6 K mm2 W-1 under packaging conditions. This finding provides valuable guidance for the design of high-performance TIMs based on two-dimensional materials and improves the possibility of their practical application in electronic thermal management.

Recent Studies on Thermally Conductive 3D Aerogels/Foams with the Segregated Nanofiller Framework

As technology advances toward ongoing circuit miniaturization and device size reduction followed by improved power density, heat dissipation is becoming a key challenge for electronic equipment. Heat accumulation can be prevented if the heat from electrical equipment is efficiently exported, ensuring a device's lifespan and dependability and preventing otherwise possible mishaps or even explosions. Hence, thermal management applications, which include altering the role of aerogels from thermally insulative to thermally conductive, have recently been a hot topic for 3D-aerogel-based thermal interface materials. To completely comprehend three-dimensional (3D) networks, we categorized and comparatively analyzed aerogels based on carbon nanomaterials, namely fibers, nanotubes, graphene, and graphene oxide, which have capabilities that may be fused with boron nitride and impregnated for better thermal performance and mechanical stability by polymers, including epoxy, cellulose, and polydimethylsiloxane (PDMS). An alternative route is presented in the comparative analysis by carbonized cellulose. As a result, the development of structurally robust and stiff thermally conductive aerogels for electronic packaging has been predicted to increase polymer thermal management capabilities. The latest trends include the self-organization of an anisotropic structure on several hierarchical levels within a 3D framework. In this study, we highlight and analyze the recent advances in 3D-structured thermally conductive aerogels, their potential impact on the next generation of electronic components based on advanced nanocomposites, and their future prospects.

Multi-scale hybrid spherical graphite composites: a light weight thermal interface material with high thermal conductivity and simple processing technology

In consideration of low density and high intrinsic thermal conductivity, spherical graphite powders can act as promising fillers for light weight thermal interface materials. Herein, spherical artificial graphite derived composites exhibit a similar thermal conductivity and significantly reduced bulk density compared with traditional Al₂O₃-derived composites. Further, based on the particle packing theory, an innovatively optimized calculation method has been proposed by introducing the quadratic programming method into the traditional calculation method to acquire the optimum formulation of multi-scale spherical graphite particles. The thermal conductivity of the optimum formulation-derived composites attains 1.994 W m⁻¹ K⁻¹, which is 1.72 times higher than that of the single particle size-derived composites (1.156 W m⁻¹ K⁻¹), accompanied by a low density of 1.812 g cm⁻³vs. the 2.31 g cm⁻³ of the traditional Al₂O₃-derived composites. Besides, the relationships between the tap density of the graphite powders, thermal conductivity and maximum filling content of the composites are creatively established, which are available for predicting the thermal conductivities of composites by simply testing the tap density of the fillers. This present work provides an instructional strategy to optimize spherical filler particles for thermal management of electronic devices.

A new carbon-based thermal conductive filler, a new calculation method of multi-scale spherical particle mixing and a model for predicting thermal conductivity by tap density.

Hyperbolic Graphene Framework with Optimum Efficiency for Conductive Composites

Constructing conductive filler networks with high efficiency is essential to fabricating functional polymer composites. Although two-dimensional (2D) sheets have prevailed in nanocomposites, their efficiency in enhancing conductive functions seems to reach a limit, as if merely addressing the dispersion homogeneity. Here, we exploit the unrecognized geometric curvature of 2D sheets to break the efficiency limit of filler systems. We introduce the hyperbolic curvature concept to mediate the incompatibility between 2D planar topology and 3D filler space and hold the efficient conductive path through face-to-face contact. The hyperbolic graphene framework exhibits a record efficiency in enhancing electrically and thermally conductive functions of nanocomposites. At a volume loading of only 1.6%, the thermal and electrical conductivities reach 31.6 W/(mK) and 13 911 S/m, respectively. We demonstrate that the conductive nanocomposites with a hyperbolic graphene aerogel framework are useful for thermal management, sensing, and electromagnetic shielding. Our work provides a solution to reconcile the incompatibility between the 2D planar structure of sheets and the highly expected 3D conductive path, presenting a geometrically optimal filler system to break the efficiency limit of multifunctional nanocomposites and broaden the structural design space of 2D sheets by curvature modulation to meet more applications.

A highly orientational architecture formed by covalently bonded graphene to achieve high through-plane thermal conductivity of polymer composites

Combining the advantages of high thermal conductivities and low graphene contents to fabricate polymer composites for applications in thermal management is still a great challenge due to the high defect degree of exfoliated graphene, the poor orientation of graphene in polymer matrices, and the horrible phonon scattering between graphene/graphene and graphene/polymer interfaces. Herein, mesoplasma chemical vapor deposition (CVD) technology was successfully employed to synthesize vertically aligned graphene nanowalls (GNWs), which are covalently bonded by high-quality CVD graphene nanosheets. The unique architecture leads to an excellent thermal enhancement capacity of the GNWs, and a corresponding composite film with a matrix of polyvinylidene fluoride (PVDF) presented a high through-plane thermal conductivity of 12.8 ± 0.77 W m^-1 K^-1 at a low filler content of 4.0 wt%, resulting in a thermal conductivity enhancement per 1 wt% graphene loading of 1659, which is far superior to that using conventional graphene structures as thermally conductive pathways. In addition, this composite exhibited an excellent capability in cooling a high-power light-emitting diode (LED) device under real application conditions. Our finding provides a new route to prepare high-performance thermal management materials with low filler loadings via the rational design of the microstructures/interfaces of graphene skeletons.

Effect of reduction time of functionalized graphene oxide on the morphology and properties of epoxy composite foams

In this study, N-aminoethylpiperazine (AEP)-functionalized and reduced graphene oxide (RGO) with different structures and properties was prepared by simply tuning the reduction time and then its effect on the rheological, curing and foaming behavior of epoxy resin was carefully investigated using the environment-friendly carbamate as a chemical foaming agent. As the reduction time of RGO increased, the reduction degree of RGO first increased and then levelled off but the grafted AEP was little affected by the reduction time. The addition of RGO undergoing long reduction time improved the viscoelasticity of epoxy/reduced graphene oxide (EP/RGO) composites but weakened the interfacial compatibility of RGO and EP. With increasing the reduction time of RGO, the cell size of EP/RGO composite foams decreased and the cell density increased. However, as compared with pure EP foam, the composite foams containing RGO with lower reduction degree had a larger cell size and a lower density. These results were attributed to the complicated effect of RGO, which not only acted as the heterogeneous nucleating and foaming agent but also affected the viscoelasticity of composites. In addition, as the reduction time of RGO increased, the initial thermal decomposition temperature, storage modulus at room temperature, electrical conductivity, thermal conductivity, and compressive properties of EP/RGO composite foams increased while the glass transition temperature remained unchanged. These results were related not only to the intrinsic properties and dispersion of RGO, but also to the density and cell morphology of the composite foams.

Efficient Preconstruction of Three-Dimensional Graphene Networks for Thermally Conductive Polymer Composites

Electronic devices generate heat during operation and require efficient thermal management to extend the lifetime and prevent performance degradation. Featured by its exceptional thermal conductivity, graphene is an ideal functional filler for fabricating thermally conductive polymer composites to provide efficient thermal management. Extensive studies have been focusing on constructing graphene networks in polymer composites to achieve high thermal conductivities. Compared with conventional composite fabrications by directly mixing graphene with polymers, preconstruction of three-dimensional graphene networks followed by backfilling polymers represents a promising way to produce composites with higher performances, enabling high manufacturing flexibility and controllability. In this review, we first summarize the factors that affect thermal conductivity of graphene composites and strategies for fabricating highly thermally conductive graphene/polymer composites. Subsequently, we give the reasoning behind using preconstructed three-dimensional graphene networks for fabricating thermally conductive polymer composites and highlight their potential applications. Finally, our insight into the existing bottlenecks and opportunities is provided for developing preconstructed porous architectures of graphene and their thermally conductive composites.

Large Enhancement in Thermal Conductivity of Solvent-Cast Expanded Graphite/Polyetherimide Composites

We demonstrate in this work that expanded graphite (EG) can lead to a very large enhancement in thermal conductivity of polyetherimide−graphene and epoxy−graphene nanocomposites prepared via solvent casting technique. A k value of 6.6 W⋅m⁻¹⋅K⁻¹ is achieved for 10 wt% composition sample, representing an enhancement of ~2770% over pristine polyetherimide (k~0.23 W⋅m⁻¹⋅K⁻¹). This extraordinary enhancement in thermal conductivity is shown to be due to a network of continuous graphene sheets over long−length scales, resulting in low thermal contact resistance at bends/turns due to the graphene sheets being covalently bonded at such junctions. Solvent casting offers the advantage of preserving the porous structure of expanded graphite in the composite, resulting in the above highly thermally conductive interpenetrating network of graphene and polymer. Solvent casting also does not break down the expanded graphite particles due to minimal forces involved, allowing for efficient heat transfer over long−length scales, further enhancing overall composite thermal conductivity. Comparisons with a recently introduced effective medium model show a very high value of predicted particle–particle interfacial conductance, providing evidence for efficient interfacial thermal transport in expanded graphite composites. Field emission environmental scanning electron microscopy (FE−ESEM) is used to provide a detailed understanding of the interpenetrating graphene−polymer structure in the expanded graphite composite. These results open up novel avenues for achieving high thermal conductivity polymer composites.

Chloroform-Assisted Rapid Growth of Vertical Graphene Array and Its Application in Thermal Interface Materials

With the continuous progress in electronic devices, thermal interface materials (TIMs) are urgently needed for the fabrication of integrated circuits with high reliability and performance. Graphene as a wonderful additive is often added into polymer to build composite TIMs. However, owing to the lack of a specific design of the graphene skeleton, thermal conductivity of graphene-based composite TIMs is not significantly improved. Here a chloroform-assisted method for rapid growth of vertical graphene (VG) arrays in electric field-assisted plasma enhanced chemical vapor deposition (PECVD) system is reported. Under the optimum intensity and direction of electric field and by introducing the highly electronegative chlorine into the reactor, the record growth rate of 11.5 µm h^-1 is achieved and VG with a height of 100 µm is successfully synthesized. The theoretical study for the first time reveals that the introduction of chlorine accelerates the decomposition of methanol and thus promotes the VG growth in PECVD. Finally, as an excellent filler framework in polymer matrix, VG arrays are used to construct a free-standing composite TIM, which yields a high vertical thermal conductivity of 34.2 W m^-1  K^-1 at the graphene loading of 8.6 wt% and shows excellent cooling effect in interfacial thermal dissipation of light emitting diode.

Wood-Derived, Vertically Aligned, and Densely Interconnected 3D SiC Frameworks for Anisotropically Highly Thermoconductive Polymer Composites

Construction of a vertically aligned and densely interconnected ordered 3D filler framework in a polymer matrix is a challenge to attain significant thermal conductivity (TC) enhancement efficiency. Fortunately, many biomaterials with unique microstructures can be found in nature. With inspiration from wood, artificial composites can be rationally designed to achieve desired properties. Herein, the authors report a facile and effective approach to fabricate anisotropic polymer composites by biotemplate ceramization technology and subsequent vacuum impregnation of epoxy resin. The hierarchical microstructure of wood is perfectly replicated in the cellular biomass derived SiC (bioSiC) framework by carbothermal reduction. Owing to the anisotropic architecture of bioSiC, the epoxy composite with vertically aligned dense SiC microchannels shows interesting properties, including a high TC (10.27 W m^-1 K^-1 ), a significant enhancement efficiency (259 per 1 vol% loading), an outstanding anisotropic TC ratio (5.77), an extremely low coefficient of linear thermal expansion (12.23 ppm K^-1 ), a high flexural strength (222 MPa), and an excellent flame resistance. These results demonstrate that this approach is expected to open a new avenue for design and preparation of high performance thermal management materials to address the heat dissipation of modern electronics.

Controlled Vertically Aligned Structures in Polymer Composites: Natural Inspiration, Structural Processing, and Functional Application

Vertically aligned structures, which are a series of characteristic conformations with thickness-direction alignment, interconnection, or assembly of filler in polymeric composite materials that can provide remarkable structural performance and advanced anisotropic functions, have attracted considerable attention in recent years. The past two decades have witnessed extensive development with regard to universal fabrication methods, subtle control of morphological features, improvement of functional properties, and superior applications of vertically aligned structures in various fields. However, a systematic review remains to be attempted. The various configurations of vertical structures inspired from biological samples in nature, such as vertically aligned structures with honeycomb, reed, annual ring, radial, and lamellar configurations are summarized here. Additionally, relevant processing methods, which include the transformation of oriented direction, external-field inducement, template method, and 3D printing method, are discussed in detail. The diverse applications in mechanical, thermal, electric, dielectric, electromagnetic, water treatment, and energy fields are also highlighted by providing representative examples. Finally, future opportunities and prospects are listed to identify current issues and potential research directions. It is expected that perspectives on the vertically aligned structures presented here will contribute to the research on advanced multifunctional composites.

Constructing a three-dimensional nano-crystalline diamond network within polymer composites for enhanced thermal conductivity

In order to meet the requirement of thermal performance with the rapid development of high-performance electronic devices, constructing a three-dimensional thermal transport skeleton is an effective method for enhancing the thermal conductivity of polymer composites. In this work, a three-dimensional porous diamond framework was prepared by depositing nano-crystalline diamond on alumina foam which was impregnated with epoxy to obtain a nano-crystalline diamond@alumina foam/epoxy composite. The epoxy composite with nano-crystalline diamond@alumina foam demonstrated a thermal conductivity of 2.21 W m^-1 K^-1, which was increased by 1063% in comparison with pure epoxy. The thermal conductivity of the epoxy composite measured under various conditions and heat transport applications demonstrates that it possesses excellent thermal transportation and stability properties. This work provides a new idea to significantly enhance the thermal transportation properties of epoxy composites in the application of advanced packaging materials.

Advances in Field-Assisted 3D Printing of Bio-Inspired Composites: From Bioprototyping to Manufacturing

Biocomposite systems evolve to superior structural strategies in adapting to their living environments, using limited materials to form functionality superior to their inherent properties. The synergy of physical-field and Three-dimensional （3D） printing technologies creates unprecedented opportunities that overcome the limitations of traditional manufacturing methods and enable the precise replication of bio-enhanced structures. Here, an overview of typical structural designs in biocomposite systems, their functions and properties, are provided and the recent advances in bio-inspired composites using mechanical, electrical, magnetic, and ultrasound-field-assisted 3D printing techniques are highlighted. Finally, in order to realize the preparation of bionic functional devices and equipment with more superior functions, here an outlook on the development of field-assisted 3D printing technology from three aspects are provided: Materials, technology, and post-processing.

Optimization of Effective Thermal Conductivity of Thermal Interface Materials Based on the Genetic Algorithm-Driven Random Thermal Network Model

Polymer-based thermal interface materials (TIMs) are indispensable for reducing the thermal contact resistance of high-power electronic devices. Owing to the low thermal conductivity of polymers, adding multiscale dispersed particles with high thermal conductivity is a common approach to enhance the effective thermal conductivity. However, optimizing multiscale particle matching, including particle size distribution and volume fraction, for improving the effective thermal conductivity has not been achieved. In this study, three kinds of filler-loaded samples were prepared, and the effective thermal conductivity and average particle size of the samples were tested. The finite element model (FEM) and the random thermal network model (RTNM) were applied to predict the effective thermal conductivity of TIMs. Compared with the FEM, the RTNM achieves higher accuracy with an error less than 5% and higher computational efficiency in predicting the effective thermal conductivity of TIMs. Combining the abovementioned advantages, we designed a set of procedures for an RTNM driven by the genetic algorithm (GA). The procedure can find multiscale particle-matching ways to achieve the maximum effective thermal conductivity under a given filler load. The results show that the samples with 40 vol %, 50 vol %, and 60 vol % filler loading have similar particle size distribution and volume fractions when the effective thermal conductivity reaches the highest. It should be emphasized that the optimized effective thermal conductivity can be improved obviously with the increase in the volume fraction of the filler loading. The high efficiency and accuracy of the procedure show great potential for the future design of high-efficiency TIMs.

Tailoring Highly Ordered Graphene Framework in Epoxy for High-Performance Polymer-Based Heat Dissipation Plates

As the power density and integration level of electronic devices increase, there are growing demands to improve the thermal conductivity of polymers for addressing the thermal management issues. On the basis of the ultrahigh intrinsic thermal conductivity, graphene has exhibited great potential as reinforcing fillers to develop polymer composites, but the resultant thermal conductivity of reported graphene-based composites is still limited. Here, an interconnected and highly ordered graphene framework (HOGF) composed of high-quality and horizontally aligned graphene sheets was developed by a porous film-templated assembly strategy, followed by a stress-induced orientation process and graphitization post-treatment. After embedding into the epoxy (EP), the HOGF/EP composite (24.7 vol %) exhibits a record-high in-plane thermal conductivity of 117 W m^-1 K^-1, equivalent to ≈616 times higher than that of neat epoxy. This thermal conductivity enhancement is mainly because the HOGF as a filler concurrently has high intrinsic thermal conductivity, relatively high density, and a highly ordered structure, constructing superefficient phonon transport paths in the epoxy matrix. Additionally, the use of our HOGF/EP as a heat dissipation plate was demonstrated, and it achieved 75% enhancement in practical thermal management performance compared to that of conventional alumina for cooling the high-power LED.

Graphitized-rGO/Polyimide Aerogel as the Compressible Thermal Interface Material with Both High In-Plane and Through-Plane Thermal Conductivities

Reduced graphene oxide (rGO) aerogels with a three-dimensional (3D) interconnected network provides continuous heat transport paths in multi-directions. However, the high porosity of rGO aerogels commonly leads to very low thermal conductivity (TC), and defects and grain boundaries of rGO sheets result in a high extent of phonon scattering, which is far from satisfying the requirement of thermal interface materials (TIMs). Here, a compressible graphitized-rGO/polyimide (g-rGO/PI) aerogel was prepared by the ice-template method and "molecular welding" strategy. The regular cellular structure and closely packed cell walls bring the g-rGO/PI aerogel high compressibility, which made the aerogel can maintain the continuous thermal transport paths well even in highly compacted status. The rGO sheets in the cell wall surface are welded up by g-PI during imidization and graphitization treatment, providing efficient channels for phonon transportation in the 3D network. The g-rGO/PI aerogel in a compressive strain of 95% has a high TC in the plane of 172.5 W m-1k-1 and a high TC through the plane of 58.1 W m-1k-1, which is superior to other carbon-based TIMs previously reported.

Highly Thermally Conductive 3D Printed Graphene Filled Polymer Composites for Scalable Thermal Management Applications

Efficient thermal transportation in a preferred direction is highly favorable for thermal management issues. The combination of 3D printing and two-dimensional (2D) materials such as graphene, BN, and so on enables infinite possibilities for hierarchically aligned structure programming. In this work, we report the formation of the asymmetrically aligned structure of graphene filled thermoplastic polyurethane (TPU) composites during 3D printing process. The as-printed vertically aligned structure demonstrates a through-plane thermal conductivity (TC) up to 12 W m^-1 K^-1 at 45 wt % graphene content, which is ∼8 times of that of a horizontally printed structure and surpasses many of the traditional particle reinforced polymer composites. The superior TC is mainly attributed to the anisotropic structure design that benefited from the preferable degree of orientation of graphene and the multiscale dense structure realized by finely controlling the printing parameters. Finite element method (FEM) confirms the essential impact of anisotropic TC design for highly thermal conductive composites. This study provides an effective way to develop 3D printed graphene-based polymer composites for scalable thermal-related applications such as battery thermal management, electric packaging, and so on.

A Review of Polymer Composites Based on Carbon Fillers for Thermal Management Applications: Design, Preparation, and Properties

With the development of microelectronic devices having miniaturized and integrated electronic components, an efficient thermal management system with lightweight materials, which have outstanding thermal conductivity and processability, is becoming increasingly important. Recently, the use of polymer-based thermal management systems has attracted much interest due to the intrinsic excellent properties of the polymer, such as the high flexibility, low cost, electrical insulation, and excellent processability. However, most polymers possess low thermal conductivity, which limits the thermal management applications of them. To address the low thermal conduction of the polymer materials, many kinds of thermally conductive fillers have been studied, and the carbon-based polymer composite is regarded as one of the most promising materials for the thermal management of the electric and electronic devices. In addition, the next generation electronic devices require composite materials with various additional functions such as flexibility, low density, electrical insulation, and oriented heat conduction, as well as ultrahigh thermal conductivity. In this review, we introduce the latest papers on thermally conductive polymer composites based on carbon fillers with sophisticated structures to meet the above requirements. The topic of this review paper consists of the following four contents. First, we introduce the design of a continuous three-dimensional network structure of carbon fillers to reduce the thermal resistance between the filler–matrix interface and individual filler particles. Second, we discuss various methods of suppressing the electrical conductivity of carbon fillers in order to manufacture the polymer composites that meet both the electrical insulation and thermal conductivity. Third, we describe a strategy for the vertical alignment of carbon fillers to improve the through-plane thermal conductivity of the polymer composite. Finally, we briefly mention the durability of the thermal conductivity performance of the carbon-based composites. This review presents key technologies for a thermal management system of next-generation electronic devices.

Thermal interface materials with graphene fillers: review of the state of the art and outlook for future applications

We review the current state-of-the-art graphene-enhanced thermal interface materials for the management of heat in the next generation of electronics. Increased integration densities, speed and power of electronic and optoelectronic devices require thermal interface materials with substantially higher thermal conductivity, improved reliability, and lower cost. Graphene has emerged as a promising filler material that can meet the demands of future high-speed and high-powered electronics. This review describes the use of graphene as a filler in curing and non-curing polymer matrices. Special attention is given to strategies for achieving the thermal percolation threshold with its corresponding characteristic increase in the overall thermal conductivity. Many applications require high thermal conductivity of composites, while simultaneously preserving electrical insulation. A hybrid filler approach, using graphene and boron nitride, is presented as a possible technology providing for the independent control of electrical and thermal conduction. The reliability and lifespan performance of thermal interface materials is an important consideration towards the determination of appropriate practical applications. The present review addresses these issues in detail, demonstrating the promise of graphene-enhanced thermal interface materials compared to alternative technologies.

Multiscale Structural Modulation of Anisotropic Graphene Framework for Polymer Composites Achieving Highly Efficient Thermal Energy Management

Graphene is usually embedded into polymer matrices for the development of thermally conductive composites, preferably forming an interconnected and anisotropic framework. Currently, the directional self-assembly of exfoliated graphene sheets is demonstrated to be the most effective way to synthesize anisotropic graphene frameworks. However, achieving a thermal conductivity enhancement (TCE) over 1500% with per 1 vol% graphene content in polymer matrices remains challenging, due to the high junction thermal resistance between the adjacent graphene sheets within the self-assembled graphene framework. Here, a multiscale structural modulation strategy for obtaining highly ordered structure of graphene framework and simultaneously reducing the junction thermal resistance is demonstrated. The resultant anisotropic framework contributes to the polymer composites with a record-high thermal conductivity of 56.8-62.4 W m-1 K-1 at the graphene loading of ≈13.3 vol%, giving an ultrahigh TCE per 1 vol% graphene over 2400%. Furthermore, thermal energy management applications of the composites as phase change materials for solar-thermal energy conversion and as thermal interface materials for electronic device cooling are demonstrated. The finding provides valuable guidance for designing high-performance thermally conductive composites and raises their possibility for practical use in thermal energy storage and thermal management of electronics.

Recent Progress in Graphene/Polymer Nanocomposites

Nanocomposites, multiphase solid materials with at least one nanoscaled component, have been attracting ever-increasing attention because of their unique properties. Graphene is an ideal filler for high-performance multifunctional nanocomposites in light of its superior mechanical, electrical, thermal, and optical properties. However, the 2D nature of graphene usually gives rise to highly anisotropic features, which brings new opportunities to tailor nanocomposites by making full use of its excellent in-plane properties. Here, recent progress on graphene/polymer nanocomposites is summarized with emphasis on strengthening/toughening, electrical conduction, thermal transportation, and photothermal energy conversion. The influence of the graphene configuration, including layer number, defects, and lateral size, on its intrinsic properties and the properties of graphene/polymer nanocomposites is systematically analyzed. Meanwhile, the role of the interfacial interaction between graphene and polymer in affecting the properties of nanocomposites is also explored. The correlation between the graphene distribution in the matrix and the properties of the nanocomposite is discussed in detail. The key challenges and possible solutions are also addressed. This review may provide a constructive guidance for preparing high-performance graphene/polymer nanocomposite in the future.

Recent progress in the development of thermal interface materials: a review

Modern electronic devices are characterized by high-power and high-frequency with excessive heat accumulation. Thermal interface materials (TIMs) are of crucial importance for efficient heat dissipation to maintain proper functions and lifetime for these devices. The most promising TIMs are those polymer-based nanocomposites consisting of polymers and low-dimensional materials with high thermal conductivity (TC). This perspective summarizes the recent progress on the thermal transport properties of newly discovered one-dimensional (1D) nanomaterials and two-dimensional (2D) nanomaterials as well as three-dimensional (3D) nanostructures consisting of these 1D and 2D nanomaterials. Moreover, the applications of various nanomaterials in polymer nanocomposites for advanced TIMs are critically reviewed and the mechanism of TC enhancement is analysed. It is hoped that the present review could provide better understanding of the thermal transport properties of recently developed 2D nanomaterials and various 3D nanostructures as well as relevant polymer-based TIMs, shedding more light on the thermal management research.

Graphene/Graphitized Polydopamine/Carbon Nanotube All-Carbon Ternary Composite Films with Improved Mechanical Properties and Through-Plane Thermal Conductivity

Graphene films (GFs) are promising ultrathin thermally conductive materials for portable electronic devices because of their excellent thermally conductive property, light weight, high flexibility, and low cost. However, the application of GFs is limited due to their poor mechanical properties and through-plane thermal conductivity. Here, a graphene-(graphitized polydopamine)-(carbon nanotube) (G-gPDA-CNT) all-carbon ternary composite film was fabricated by chemical reduction, carbonization, graphitization, and mechanical compaction of the evaporation-assembled (graphene oxide)-PDA@CNT film. The G-gPDA-CNT film exhibited a uniform all-carbon composite structure in which the components of the graphene, gPDA layers, and CNTs were cross-linked by strong covalent bonds. This unique structure promoted the load transfer and energy dissipation between the components by which the mechanical properties of the G-gPDA-CNT film were substantially improved. Furthermore, electron and phonon transfers were also promoted, greatly improving the electrical and thermal conductivities, especially the through-plane thermal conductivity of the G-gPDA-CNT film. The G-gPDA-CNT film showed a tensile strength of 67.5 MPa, 15.1% ultimate tensile strain, toughness of 6.07 MJ/m³, electrical conductivity of 6.7 × 10⁵ S·m^-1, in-plane thermal conductivity of 1597 W·m^-1·K^-1, and through-plane thermal conductivity of 2.65 W·m^-1·K^-1, which were 2.24, 1.44, 3.16, 1.46, 1.15, and 3.90 times that of the pure GFs, respectively.

Recent Advances in Preparation, Mechanisms, and Applications of Thermally Conductive Polymer Composites: A Review

At present, the rapid accumulation of heat and the heat dissipation of electronic equipment and related components are important reasons that restrict the miniaturization, high integration, and high power of electronic equipment. It seriously affects the performance and life of electronic devices. Hence, improving the thermal conductivity of polymer composites (TCPCs) is the key to solving this problem. Compared with manufacturing intrinsic thermally conductive polymer composites, the method of filling the polymer matrix with thermally conductive fillers can better-enhance the thermal conductivity (λ) of the composites. This review starts from the thermal conduction mechanism and describes the factors affecting the λ of polymer composites, including filler type, filler morphology and distribution, and the functional surface treatment of fillers. Next, we introduce the preparation methods of filled thermally conductive polymer composites with different filler types. In addition, some commonly used thermal-conductivity theoretical models have been introduced to better-analyze the thermophysical properties of polymer composites. We discuss the simulation of λ and the thermal conduction process of polymer composites based on molecular dynamics and finite element analysis methods. Meanwhile, we briefly introduce the application of polymer composites in thermal management. Finally, we outline the challenges and prospects of TCPCs.

3D Lamellar-Structured Graphene Aerogels for Thermal Interface Composites with High Through-Plane Thermal Conductivity and Fracture Toughness

HIGHLIGHTS

Lamellar-structured graphene aerogels with vertically aligned and closely stacked high-quality graphene lamellae are fabricated. The superior thermally conductive capacity of the aerogel endows epoxy with a high through-plane thermal conductivity of 20.0 W m⁻¹ K⁻¹ at 2.30 vol% of graphene content. The nacre-like structure endows the epoxy composite with enhanced fracture toughness.

ABSTRACT

Although thermally conductive graphene sheets are efficient in enhancing in-plane thermal conductivities of polymers, the resulting nanocomposites usually exhibit low through-plane thermal conductivities, limiting their application as thermal interface materials. Herein, lamellar-structured polyamic acid salt/graphene oxide (PAAS/GO) hybrid aerogels are constructed by bidirectional freezing of PAAS/GO suspension followed by lyophilization. Subsequently, PAAS monomers are polymerized to polyimide (PI), while GO is converted to thermally reduced graphene oxide (RGO) during thermal annealing at 300 °C. Final graphitization at 2800 °C converts PI to graphitized carbon with the inductive effect of RGO, and simultaneously, RGO is thermally reduced and healed to high-quality graphene. Consequently, lamellar-structured graphene aerogels with superior through-plane thermal conduction capacity are fabricated for the first time, and its superior through-plane thermal conduction capacity results from its vertically aligned and closely stacked high-quality graphene lamellae. After vacuum-assisted impregnation with epoxy, the resultant epoxy composite with 2.30 vol% of graphene exhibits an outstanding through-plane thermal conductivity of as high as 20.0 W m⁻¹ K⁻¹, 100 times of that of epoxy, with a record-high specific thermal conductivity enhancement of 4310%. Furthermore, the lamellar-structured graphene aerogel endows epoxy with a high fracture toughness, ~ 1.71 times of that of epoxy. [Image: see text]

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (10.1007/s40820-020-00548-5) contains supplementary material, which is available to authorized users.

Multifunctional Electromagnetic Interference Shielding Ternary Alloy (Ni-W-P) Decorated Fabric with Wide-Operating-Range Joule Heating Performances

Flexible electromagnetic interference (EMI) shielding textiles with wide-operating-range Joule heating performances are urgently indispensable in the application of artificial intelligence, communication industry, and wearable electronics. Herein, a simple and cost-effective approach is proposed to construct multifunctional textiles by electroless depositing a nickel-tungsten-phosphorus (Ni-W-P) ternary alloy on a polyamide (PA) fabric. The resultant fabric with a thickness of ∼117 μm exhibits a favorable EMI shielding effectiveness (SE) of 43.6 dB within 2-12.5 GHz. Particularly, finite difference time domain (FDTD) simulation was introduced to investigate the effects of the PA fabric mesh number and Ni-W ratio on the EMI SE value, which was validated by experimental results. In addition, the conductive fabric demonstrates excellent heating efficiency (up to 140 °C under 2 V within 60 s), a wide operating range (from 40 to 140 °C), and simultaneously, satisfactory reproducibility by undergoing dozens of heating and cooling cycles. Notably, EMI SE of the multifunctional fabric remains unchanged even after a series of durability measurements including 180 °C heating, ultrasonication treatment, and repetitive peeling tests, respectively. Therefore, the prepared Ni-W-P coated PA fabric with prominent chemical stability and mechanical robustness endows enormous potential in multi-scene applications.

Highly thermally conductive boron nitride@UHMWPE composites with segregated structure

Highly thermally conductive boron nitride (BN)@ultra-high molecular weight polyethylene (UHMWPE) composites with the segregated structure were fabricated by powder mixing and hot pressing. Scanning electron microscopy and polarizing optical microscopy were used to analyze the dispersion of BN particles in the UHMWPE matrix. The morphology observation shows that BN particles are selectively located at the interfaces of UHMWPE particles and form continuous thermally conductive networks after the compression molding process. As a result, the thermal conductivity of the BN@UHMWPE composite increases to 3.37 W m−1 K−1 with 38.3 vol% BN, which is seven times larger than that of the pure UHMWPE. Furthermore, the incorporation of BN also influences the crystallinity and thermal properties of UHMWPE.

Highly Oriented Graphite Aerogel Fabricated by Confined Liquid-Phase Expansion for Anisotropically Thermally Conductive Epoxy Composites

Graphene-based thermally conductive polymer composites are of great importance for the removal of the excess heat generated by electronic devices. However, due to the orientation of graphene sheets in the polymer matrix, the through-plane thermal conductivity of polymer/graphene composites remains far from satisfactory. We here demonstrate a confined liquid-phase expansion strategy to fabricate highly oriented confined expanded graphite (CEG) aerogels. After being incorporated into epoxy resin (EP), the resulting EP/CEG composites exhibit a high through-plane thermal conductivity (4.14 ± 0.21 W m^-1 K^-1) at a quite low filler loading of 1.75 wt % (0.91 vol %), nearly 10 times higher than that of neat EP resin and 7.5 times higher than the in-plane thermal conductivity of the composite, indicating that the CEG aerogel has a high through-plane thermal conductivity enhancement efficiency that outperforms those of many graphite/graphene-based fillers. The facile preparation method holds great industrial application potential in fabricating anisotropic thermally conductive polymer composites.

High Thermal Conductivity Enhancement of Polymer Composites with Vertically Aligned Silicon Carbide Sheet Scaffolds

Owing to the growth of demand for highly integrated electronic devices, high heat dissipation of thermal management materials is essential. Epoxy composites have been prepared with vertically aligned (VA) three-dimensional (3D)-structured SiC sheet scaffolds. The required VA-SiC sheet scaffolds were prepared by a novel approach starting with a graphene oxide (GO) scaffold. The VA-GO scaffolds were reduced to VA-graphene scaffolds in an argon environment, and the latter were subsequently transformed into VA-SiC sheet scaffolds by a template-assisted chemical vapor deposition method. Epoxy resin was filled in the empty spaces of the 3D scaffold of SiC sheets to prepare the composite mass. The material so prepared shows anisotropic thermal property with ultrahigh through-plane conductivity of 14.32 W·m^-1·K^-1 at a SiC sheet content of 3.71 vol %. A thermal percolation is observed at 1.78 vol % SiC filler. The SiC sheet scaffold of covalently interconnected SiC nanoparticles plays a vital role in the formation of the thermal conductive network to significantly enhance the thermal conductivity of epoxy composites. The application of the VA-SiC/epoxy composite as an efficient thermal dissipating material has also been presented. The VA-SiC/epoxy composites have a strong potential for preparing heat-dissipating components in integrated microelectronics.

Tree-inspired radially aligned, bimodal graphene frameworks for highly efficient and isotropic thermal transport

Highly-oriented, interconnected graphene frameworks have been considered as promising candidates to realize high-performance thermal management in microelectronics. However, the obvious thermal boundary resistance and anisotropic heat conduction still remain major bottlenecks for efficient heat dissipation. Herein, a biomimetic design enabled by radially aligned, bimodal graphene frameworks (RG-Fin) is proposed to achieve highly efficient and isotropic thermal transport. An interconnected RG skeleton is prepared via a radial ice-template method, serving as the primary expressway for isotropic heat conduction. Tree-leaf-like graphene nanofins are vertically grown on the RG surface to provide additional thermal pathways for bimodal phonon transportation, which can reduce the thermal boundary resistance without degrading the thermal properties of the skeleton. An RG-Fin composite exhibits a superior thermal conductivity of 4.01 W m^-1 K^-1 (almost 20 times that of a polymer) at an ultralow loading of 1.53 vol%, demonstrating an exceptionally large thermal conductivity enhancement efficiency of 1247%, which far exceeds those of graphene-based polymer composites. Further theoretical analysis and finite element simulations reveal the critical role of the nanofins in significantly decreasing the thermal boundary resistance (by almost 27-fold). Finally, the practical thermal management of running a CPU module is demonstrated, in which the heating-up rate of the RG-Fin composite is ∼2.0 times that of a pure polymer. This strategy provides an innovative avenue for designing radially aligned networks to realize isotropic and efficient thermoconductive composites for thermal management.

General, Vertical, Three-Dimensional Printing of Two-Dimensional Materials with Multiscale Alignment

Two-dimensional (2D) materials (e.g., boron nitride (BN), graphene, and MoS₂) have great potential in emerging energy, environmental, and electronics applications. Assembly of 2D materials into vertically aligned structures is highly desirable (e.g., low tortuosity for rapid ion transport in fast charging-discharging batteries, guiding thermal transport for efficient thermal management), yet extremely challenging due to the energetically unfavorable in processing. Herein, we reported a general three-dimensional (3D) printing method to fabricate vertically aligned 2D materials in multiscale, using BN nanosheet as the proof-of-concept. The 3D-printed macroscale rods are composed of vertically aligned BN nanosheets at the nanoscale. The formation of the hierarchical aligned structure is enabled by the optimized ink that holds a significant shear-thinning behavior and an ultrahigh storage modulus, as identified at a narrow region in the printability diagram. The resulting vertically aligned multiscale structure with 2D nanosheets demonstrated an outstanding through-plane thermal conductivity, up to 5.65 W m^-1 K^-1, significantly higher than the value of conventional BN based structures where the sheets are horizontally aligned. The vertical 3D printing of 2D BN nanosheets can be expanded to other 2D materials in constructing hierarchically aligned structures for a range of emerging technologies such as batteries, membranes, and structural materials.