Highly Thermoconductive, Thermostable, and Super-Flexible Film by Engineering 1D Rigid Rod-Like Aramid Nanofiber/2D Boron Nitride Nanosheets

Lightweight and Mechanically Robust Ambient-Electrospun Nanofibrous Sponges Combined with Solar-Driven Active Heating and Low-Temperature Superinsulation

Traditional fibrous warmth retention materials suffer from limited performance improvement due to their micrometer-scale diameter and fail to meet the requirements of lightweight yet high-efficiency cold protection in extreme environments. Herein, we present a novel, facile, and ecofriendly strategy to fabricate a lightweight, mechanically robust nanofibrous sponge with integrated solar-driven active heating and low-temperature superinsulation. The high-porosity structure is achieved through urea-induced phase separation during ambient electrospinning, which overcomes the energy-intensive and unsafe high-humidity processing challenges. Simultaneous in situ incorporation of silicon carbide nanoparticles with photothermal properties enables solar-activated heat generation. This nanofibrous sponge realizes dual functionalities: ultralow thermal conductivity (27.31 mW m-1 K-1) for low-temperature superinsulation and rapid solar heating (50.1 °C temperature rise within 10 min under simulated sunlight), combined with exceptional attributes including lightweight property (volume density of 3.8 mg cm-3), hydrophobicity (water contact angle = 128°), antifouling behavior, and stable mechanical performance. Its superior performance in extreme environments (e.g., high-altitude and polar regions) and medical applications establishes a new paradigm for advanced warmth retention materials with integrated passive/active thermal management functionalities.

Polymer-Guided Exfoliation, Microstructure, Thermal Conduction, and Mechanical Behaviors of Boron Nitride Nanosheets

The size and surface properties of individual nanomaterials significantly influence the microstructures and the performance of their assemblies. Herein, we demonstrated that when using polymeric solutions as ball-milling media of hexagonal boron nitride (h-BN), the molecular structures of the employed polymers would have significant influences on the lateral sizes, surface charges, and attached polymer kinds and contents of the exfoliated boron nitride nanosheets (BNNSs), which in turn determine the microstructures and thermal conductivities (TCs) of the BNNS-based films. The polymers that have high binding energies and dipole-dipole interactions with h-BN are conducive to peeling off large-area BNNSs, but those having strong hydrogen bonding interactions with h-BN can attach more molecular chains on the exfoliated BNNSs and form dense and highly horizontally oriented structures in assembled BNNS films, of which the TCs are balanced by the lateral sizes, compactness, horizontal orientation, and interfacial interactions of the nanosheets. Density functional theory simulations confirmed that the exfoliation ability of the polymers is mainly determined by their binding energies with h-BN.

Flexible solid-liquid nanocomposite with high surface resistivity for effective electromagnetic interference shielding and heat dissipation

The miniaturization of electronics and increased power density pose significant challenges, including short circuits, electromagnetic interference (EMI) and heat accumulation. Developing electrically insulative materials that integrate EMI shielding and heat dissipation capabilities offers an effective solution. However, developing such materials is challenging due to the inherent conflict between creating electrically and thermally continuous pathways for EMI shielding and heat dissipation while maintaining electrical insulation. Herein, we sequentially integrated boron nitride nanosheet-bridging-liquid metal (BLM) and MXene-bridging-liquid metal (MLM) solid-liquid bi-continuous networks into poly-p-phenylene benzobisoxazole (PBO) nanofiber matrices. This yielded a sandwich-structured nanocomposite (S-PBLM/MLM) that demonstrates high electrical insulation (volume resistivity of 1.9 × 1013 Ω cm, breakdown voltage of 139 kV mm-1), promising EMI shielding performance (68.2 dB at a thickness of 25 μm), and excellent in-plane thermal conductivity (50.3 W m-1 K-1). Meanwhile the S-PBLM/MLM nanocomposite demonstrates stable EMI shielding performance even after enduring harsh conditions, including mechanical wear, high humidity storage, ultrasonication treatment, extreme temperatures, thermal shock and direct burning. Furthermore, the nanocomposite displays high mechanical strength (tensile strength: 252.6 MPa, toughness: 8.8 MJ m-3). This nanocomposite has significant potential in the fields of modern electronics, aerospace, and defense.

Biomimetic Network and Microstructure for Multifunctional Recyclable Ramie Fiber-Reinforced Composites: High-Hydrophobic, Wave-Transparent, and Heat-Conducting

Ramie fiber yarn (PRF), characterized by its superior microwave transmittance (MWT), is a novel material that can effectively substitute traditional glass fiber (D-GF) in the fabrication of composites for communication equipment shells. However, the super hydrophilicity and inadequate thermal conductivity (λ) of PRF hinder its overall performance and safety in use. To date, no effective strategy has been developed to prepare multifunctional PRF-reinforced composites, including high hydrophobicity, MWT, and λ. Herein, by integrating the template method with vacuum-assisted spraying technology, a mosquito-eye-like honeycomb network was constructed on the surface of PRF fabric. This network, composed of nacre-like brick-and-mortar microstructures as its fundamental units, achieves both mesoscopic and microscopic order. The biomimetic network and microstructure enable the PRF fabric to transition from superhydrophilic to high-hydrophobic, significantly reducing water absorption in PRF-reinforced composites to levels similar to D-GF-reinforced composites. Furthermore, the biomimetic structures are incorporated into the composite interface via the fabric surface, resulting in MWT and λ values of up to 98% and 1.0582 W/mK, respectively. Importantly, PRF-reinforced composite waste can be fully transformed to multifunctional particles, thereby enabling closed-loop recycling and reuse. This biomimetic network and microstructure offer an efficient and versatile surface modification strategy for multifunctional composites.

Scalable assembly of micron boron nitride into high-temperature-resistant insulating papers with superior thermal conductivity

With the rapid development of modern electrical equipment towards miniaturization, integration, and high power, high-temperature-resistant insulating papers with superior thermal conductivity are highly desirable for ensuring the reliability of high-end electrical equipment. However, it remains a challenge for current insulating papers to achieve this goal. Herein, we demonstrate the design of high-temperature-resistant micron boron nitride (m-BN) based insulating papers with superior thermal conductivity by a universal and scalable one-step assembly strategy. Inspired by the floating shape of jellyfish in the ocean, aramid nanofibers (ANF) resembling the tentacles of jellyfish were employed to support the bell-shaped m-BN, which effectively addresses the kinetically stable dispersion and film-forming ability of m-BN. The resultant m-BN@ANF papers exhibit excellent high-temperature-resistant insulating performance with an ultra-high breakdown strength of 359.0 kV mm-1 even at a high temperature of 200 °C, far exceeding those of these previously reported systems. In addition, the optimal m-BN@ANF paper demonstrates a superior thermal conductivity of 26.4 W m-1 K-1 and an excellent thermostability with an initial decomposition temperature of 486 °C. This outstanding comprehensive performance demonstrates the promise of applying these m-BN@ANF papers in advanced electrical systems operating under high-temperature circumstances.

Ultra-high thermally conductive graphite microplatelet/aramid nanofiber composites with reduced interfacial thermal resistances by engineered interface π-π interactions

Polymer-based thermally conductive composites with ultrahigh in-plane thermal conductivity are ideal candidates for heat dissipation applications in electronics. However, the complex interfaces between the functional filler and polymer matrix limit the significant increase in thermal conductivity of the polymer composites. In this study, we developed a one-pot strategy to prepare highly thermally conductive composite films of freeze-expansion large-size graphite microplatelets (F-GMPs) and aramid nanofibers (ANFs) with π-π interactions. The obtained F-GMP/ANF nanocomposite films present salient in-plane thermal conductivity, considerable flexibility, and outstanding long-term stability. The π-π interactions between the F-GMPs and ANFs promote the freeze-expansion exfoliation of graphite, yielding stable F-GMP/ANF precursor pastes with high-quality graphite platelets. Moreover, the π-π interactions improve the filler-matrix interfacial compatibility and reduce the interfacial thermal resistance, while the large-size F-GMP particles are directly lapped to construct a thermal transfer pathway with a reduction in the filler-filler interfacial thermal resistance. Consequently, the F-GMP/ANF composite films with 30 wt% F-GMPs exhibit unprecedentedly high in-plane thermal conductivity (56.89 W m-1 K-1) and corresponding thermal conductivity enhancement efficiency, presenting great application potential for the effective thermal management of highly integrated electronics.

Light Weight Organic Composites with High Thermal Management Capability

With the advancement of science and technology, effectively addressing the issue of heat dissipation in electronic equipment has become a key research topic. Polymers have attracted attention due to their low price, excellent flexibility, and lightweight characteristics, but thermal conductivity has a limitation. In this work, aiming for all-polymer composites with lightweight and high thermal conductivity, poly(p-phenylene benzobisoxazole) (PBO) fibers were used to construct a long-range ordered heat transfer path in the organosilicon matrix, and an all-organic composite material with a low density of 1.24 g cm-3 and thermal conductivity of 18.44 W/ (m K) was produced. At the same time, the composite material was applied to the cooling performance test of LED lamps, which was 4.8 °C lower than advanced commercial thermal conductive materials, demonstrating its potential in the field of thermal management materials.

Innovations Progress in Emerging Multifunctional Aramid Nanofiber Aerogels

Aerogels have attracted great attention in the academic and industrial fields due to their intrinsic low density, high porosity, and high specific surface area. As an ideal nanobuilding block for advanced aerogels, the research community has shown a strong interest in aramid nanofiber (ANF) aerogel materials and their applications in frontier fields. However, there is a lack of a comprehensive review of the basic development and practical applications of pure ANF aerogels regarding these latest achievements in the past decade. This review aims to fill this gap by comprehensively exploring the preparation, properties, application progress, challenges, and prospects of ANF aerogels. We begin with a summary of the rheological principles of ANF dispersions, different preparation strategies, and drying methods of ANF aerogels. Then, the unique properties of ANF aerogels of various morphologies and outstanding advantages based on these properties are critically reviewed. Besides, the progress of multifunctional applications in the fields of flexible thermal insulation, environmental protection, energy storage, and other fields is summarized. Finally, we explore the main challenges and prospects of ANF aerogels to give insights for further development promotion from lab-scale to industry-scale.

Unlocking the Trade-off Between Intrinsic and Interfacial Thermal Transport of Boron Nitride Nanosheets by Surface Functionalization for Advanced Thermal Interface Materials

The increasing computing power of AI presents a major challenge for high-power chip solution and heat dissipation. Boron nitride nanosheet-based thermal interface materials (BNNS-based TIMs) exhibit excellent electrical insulation property, ensuring the secure and stable operation of chips. However, the efficiency of micro/nano interfacial thermal transport is limited, impeding further enhancements in the thermal conductivity (TC) of BNNS-based TIMs. Here, a strategy of surface functionalization is reported to unlock the trade-off between the intrinsic and interfacial thermal transport of BNNS within TIMs. These results suggest that the surface functionalization maintains the intrinsic high TC of BNNS while significantly increasing binding energy between micro/nano interfaces in BNNS-based TIMs, effectively reducing interfacial thermal resistance of BNNS joint interfaces and interfaces between BNNSs and the matrix by 50% and 26.1%, respectively. The BNNS-based TIMs exhibit excellent TC (≈21-25 W/(m·K)) and ultralow Young's modulus, which can promote the development of flexible high-performance chip cooling technology in the AI industry.

Functional Carbon Springs Enabled Dynamic Tunable Microwave Absorption and Thermal Insulation

Electromagnetic (EM) wave pollution and thermal damage pose serious hazards to delicate instruments. Functional aerogels offer a promising solution by mitigating EM interference and isolating heat. However, most of these materials struggle to balance thermal protection with microwave absorption (MA) efficiency due to a previously unidentified conflict between the optimizing strategies of the two properties. Herein, this study reports a solution involving the design of a carbon-based aerogel called functional carbon spring (FCS). Its unique long-range lamellar multi-arch microstructure enables tunable MA performance and excellent thermal insulation capability. Adjusting compression strain from 0% to 50%, the adjustable effective absorption bandwidth (EAB) spans up to 13.4 GHz, covering 84% of the measured frequency spectrum. Notably, at 75% strain, the EAB drops to 0 GHz, demonstrating a novel "on-off" switchability for MA performance. Its ultralow vertical thermal conductivity (12.7 mW m-1 K-1) and unique anisotropic heat transfer mechanism endow FCS with superior thermal protection effectiveness. Numerical simulations demonstrate that FCS outperforms common honeycomb structures and isotropic porous aerogels in thermal management. Furthermore, an "electromagnetic-thermal" dual-protection material database is established, which intuitively demonstrates the superiority of the solution. This work contributes to the advancement of multifunctional MA materials with significant potential for practical applications.

Anisotropic Heat Transfer in a Fibrous Membrane with Hierarchically Assembled 2D Materials

Effective heat redistribution in specific directions is vital for advanced thermal management, significantly enhancing device performance by optimizing spatial heat configurations. We have designed and fabricated a hierarchical fibrous membrane that enables precise heat directing. By integrating hierarchical structure design with the anisotropic thermal conductivity of two-dimensional (2D) materials, we developed a fibrous membrane for anisotropic heat transfer. Such a structure is fabricated by aligning a 1D structured fiber in the 2D plane to achieve anisotropy at each scale level. The fiber units, where 2D nanosheets circumferentially and axially aligned, achieved a high axial thermal conductivity of 16.8 W·m-1·K-1 and advanced heat directing ability, confirmed by characterizations and simulations. The assembled membrane demonstrated an exceptional tensile strength (365 MPa) and high thermal conductivity (10.5 W·m-1·K-1) along the fiber axis. Our membranes are seen as a refined model for thermal management materials, combining the benefits of heat spreaders and thermal interface materials, thus being proficient in directing heat along programmed pathways. A practical wireless charging cooling demonstration illustrated this. Our methodology also proved versatile with different 2D fillers and various geometries. This research presents a method to achieve precise heat directing at the material's level, facilitating the systematic design of thermal management in electronics.

Robust and Density Tunable Kevlar/Hexagonal Boron Nitride Microribbon Aerogels with Excellent Thermal, Mechanical, and Oil-Absorption Properties

With rapid advancements in aerospace and supersonic aircraft technology, there is a growing demand for multifunctional thermal protective materials. Aerogels, known for their low density and high porosity, have garnered significant attention in this regard. However, developing a lightweight multifunctional aerogel that combines exceptional thermal and mechanical properties through a straightforward and time-efficient method remains a significant challenge. Herein, a facile and universal approach is developed for the preparation of Kevlar/hexagonal boron nitride (h-BN) aerogels, in which a spin-assisted method is applied to create robust microribbons and further accelerate solvent displacement. The resulting microribbon scaffold, with its entangled nanofiber-nanosheet morphologies, exhibits sufficient strength to prevent volume shrinkage during drying, thereby allowing precise control over aerogel density. The porous hybrid aerogels, featuring controllable geometric characteristics and tailored densities ranging from 6.9 to 100 mg cm-3, can be successfully fabricated. These aerogels exhibit excellent thermal insulation properties, and the thermal conductivities of the as-prepared KBX aerogels have a wide distribution in the range of 0.0269-0.0450 W m-1 k-1. The thermal stability of the hybrid aerogels is enhanced to 566 °C. Moreover, the resulting hybrid aerogels exhibit an ultrahigh bearing ratio, supporting more than 2000 times their own weight while maintaining stable structural integrity. These aerogels also demonstrate high compressive strength, hydrophobicity, and excellent sorption performance for various oils and solvents. Additionally, the oil-saturated aerogels can be easily recovered through heat treatment or combustion in air. The features endow hybrid Kevlar/h-BN aerogels with significant potential for applications in thermal management, environmental protection, and neutron protection.

Molecular Simulation of the Water Diffusion Behavior and Electronic Properties of Boron-Nitride-Composited Mineral Oil

Despite the fact that doping nanoparticles into insulating transformer oil has proven to be an effective method of enhancing its dielectric and electrical properties, it remains unclear how different types and surface conditions of nanoparticles may affect their dielectric and electrical properties. Therefore, the effect of doping various types of BN nanoparticles (nanosphere, nanotube, and nanosheet) in insulating mineral oil (MO) on the diffusion properties of water molecules and electrical properties across the BN/MO interface was investigated using molecular dynamics (MD) and Density Functional Theory (DFT) simulations. Our results show that different surface morphology and grafted functional groups in different types of BN nanoparticles have a significant impact both on the water diffusion behavior and the interfacial potential barrier across the interface between BN and MO. In the MO system directly doped by BN nanospheres, water diffusion behavior is not significantly restricted. However, grafting -NH2 polar groups onto the BN nanoparticle surface may significantly limit the diffusion behavior of water due to the strong attraction between the -NH2 polar groups and water molecules; the most significant effect is with nanospheres, followed by nanotubes and nanosheets. In terms of electrical properties across the interface between BN and MO, the h-BN surface (derived from BN nanosheets and nanotubes) acts as a trap for electrons in MO (-0.59 eV), while the c-BN surface (derived from BN nanospheres) acts as a potential barrier for electrons in MO (1.45 eV), and it is noteworthy that the presence of water molecules near the interface between BN and MO has little impact on the potential barriers. Advancing a fundamental understanding of the electrical and water diffusion properties of MO in correlation with the surface morphology of different types of nanoparticles is key to improving the insulation properties of oil-impregnated power transformers.

Consistent Thermal Conductivities of Spring-Like Structured Polydimethylsiloxane Composites under Large Deformation

Flexible and highly thermally conductive materials with consistent thermal conductivity (λ) during large deformation are urgently required to address the heat accumulation in flexible electronics. In this study, spring-like thermal conduction pathways of silver nanowire (S-AgNW) fabricated by 3D printing are compounded with polydimethylsiloxane (PDMS) to prepare S-AgNW/PDMS composites with excellent and consistent λ during deformation. The S-AgNW/PDMS composites exhibit a λ of 7.63 W m-1 K-1 at an AgNW amount of 20 vol%, which is ≈42 times that of PDMS (0.18 W m-1 K-1) and higher than that of AgNW/PDMS composites with the same amount and random dispersion of AgNW (R-AgNW/PDMS) (5.37 W m-1 K-1). Variations in the λ of 20 vol% S-AgNW/PDMS composites are less than 2% under a deformation of 200% elongation, 50% compression, or 180° bending, which benefits from the large deformation characteristics of S-AgNW. The heat-transfer coefficient (0.29 W cm-2 K-1) of 20 vol% S-AgNW/PDMS composites is ≈1.3 times that of the 20 vol% R-AgNW/PDMS composites, which reduces the temperature of a full-stressed central processing unit by 6.8 °C compared to that using the 20 vol% R-AgNW/PDMS composites as a thermally conductive material in the central processing unit.

"Surface-Like Growth" Strategy for the Direct Synthesis of Horizontally Aligned Boron Nitride Nanotubes

The inherent properties of boron nitride nanotubes (BNNTs) can be further enhanced through the control of their anisotropy. In particular, horizontally aligned BNNTs (HABNNTs) exhibit considerable potential for various applications. However, directly synthesizing HABNNTs is difficult owing to the random floating of BNNTs and the absence of directional forces. Here, we employed a simple, efficient, and universal "surface-like growth" strategy to synthesize high-density and high-quality HABNNTs in the W2B5/Zn precursor system. First, the floating range of BNNTs was restricted to the vicinity of the precursor, and then, directional forces were applied to induce BNNT directional growth along the substrate surface. Experiments and simulations confirmed that the HABNNT orientation could be controlled through manipulation of the directional forces. Furthermore, the strategy was employed for HABNNTs synthesis using the MoB2/Zn, further demonstrating the universality of the approach. Overall, this work offers a fresh perspective on the synthesis of HABNNTs, further expanding their potential applications.

High-Throughput and Scalable Exfoliation of Large-Sized Ultrathin 2D Materials by Ball-Milling in Supercritical Carbon Dioxide

The 2D materials exhibit numerous technological applications, but their scalable production is a core challenge. Herein, ball milling exfoliation in supercritical carbon dioxide (scCO2) and polystyrene (PS) is demonstrated to completely exfoliate hexagonal boron nitride nanosheets (BNNSs), graphene, molybdenum disulfide (MoS2), and tungsten disulfide (WS2). The exfoliation yield of 91%, 93%, 92%, and 92% and average aspect ratios of 743, 565, 564, and 502 for BNNSs, graphene, MoS2, and WS2, respectively, are achieved. Integrating exfoliated BNNSS in the polystyrene matrix, 3768 % thermal conductivity in the axial direction and 316% in the cross-plane direction at 12 wt.% loading is increased. Also, the in-plane and cross-plane electrical conductivity of 6.3 × 10-4 S m-1 and 6.6 × 10-3 S m-1, respectively, and the electromagnetic interference (EMI) of 63.3 dB is achieved by exfoliated graphene nanosheets based composite. High thermal and electrical conductivities and EMI shielding are attributed to the high aspect ratio and ultrathin morphology of the exfoliated nanosheets, which exert high charge mobility and form better the percolation network in the composite films due to their high surface area. The process demonstrate herein can produce substantial quantities of diverse 2D nanosheets for widespread commercial utilization.

Highly Thermally Conductive Super-Aligned Boron Nitride Nanotube Films for Flexible Electronics Thermal Management

Flexible electronics toward high integration, miniaturization, and multifunctionality, leading to a dramatic increase in power density. However, the low thermal conductivity of flexible substrates impedes efficient heat dissipation and device performance improvement. In this work, we propose a template-assisted chemical conversion strategy for obtaining boron nitride nanotube (BNNT) films with high thermal conductivity and great flexibility. Aligned carbon nanotube (CNT) films have been adopted as templates; a low-temperature chemical conversion followed by a high-temperature annealing has been carried out to produce a highly ordered BNNT film. Benefiting from the high orientation order, the BNNT film exhibits an exceptional thermal conductivity of 45.5 W m-1 K-1 and presents excellent heat dissipation capability, much superior to the commonly used polyimide film. Furthermore, the BNNT film demonstrated excellent flexibility and high insulation resistance. The test of integration with film resistors demonstrated its potential as a thermally conductive substrate for electronics cooling. This work provides a solution for the effective thermal management of flexible electronics.

Multifunctional Liquid Metal-Bridged Graphite Nanoplatelets/Aramid Nanofiber Film for Thermal Management

Miniaturization of modern micro-electronic devices urges the development of multi-functional thermal management materials. Traditional polymer composite-based thermal management materials are promising candidates, but they suffer from single functionality, high cost, and low fire-resistance. Herein, a multifunctional liquid metal (LM)-bridged graphite nanoplatelets (GNPs)/ aramid nanofibers (ANFs) film is fabricated via a facile vacuum-assisted self-assembly approach followed by compression. ANFs serve as interfacial binders to link LM and GNPs together via hydrogen bondings and π-π interactions, while LM bridges the adjacent layer of GNPs to endow a fast thermal transport by phonons and electrons. The resultant composite films exhibit a high bidirectional thermal conductivity (In-plane: 29.5 W m-1K-1 and through-plane: 5.3 W m-1K-1), offering a reliable and effective cooling. Moreover, the as-fabricated composite films exhibit superior flame-retardance (peak of heat release rate of 4000J g-1), outstanding Joule heating performance (200 °C at supplied voltage of 3.5 V), and excellent electromagnetic interference shielding effectiveness (EMI SE of 62 dB). This work provides an efficient avenue to fabricate multifuntional thermal management materials for micro-electronic devices, battery thermal management, and artificial intelligence.

Correlating Young's Modulus with High Thermal Conductivity in Organic Conjugated Small Molecules

Attaining elevated thermal conductivity in organic materials stands as a coveted objective, particularly within electronic packaging, thermal interface materials, and organic matrix heat exchangers. These applications have reignited interest in researching thermally conductive organic materials. The understanding of thermal transport mechanisms in these organic materials is currently constrained. This study concentrates on N, N'-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8), an organic conjugated crystal. A correlation between elevated thermal conductivity and augmented Young's modulus is substantiated through meticulous experimentation. Achievement via employing the physical vapor transport method, capitalizing on the robust C═C covalent linkages running through the organic matrix chain, bolstered by π-π stacking and noncovalent affiliations that intertwine the chains. The coexistence of these dynamic interactions, alongside the perpendicular alignment of PTCDI-C8 molecules, is confirmed through structural analysis. PTCDI-C8 thin film exhibits an out-of-plane thermal conductivity of 3.1 ± 0.1 W m-1 K-1, as determined by time-domain thermoreflectance. This outpaces conventional organic materials by an order of magnitude. Nanoindentation tests and molecular dynamics simulations elucidate how molecular orientation and intermolecular forces within PTCDI-C8 molecules drive the film's high Young's modulus, contributing to its elevated thermal conductivity. This study's progress offers theoretical guidance for designing high thermal conductivity organic materials, expanding their applications and performance potential.

Highly Thermally Conductive Aramid Nanofiber Composite Films with Synchronous Visible/Infrared Camouflages and Information Encryption

The development of highly thermally conductive composites that combine visible light/infrared camouflage and information encryption has been endowed with great significance in facilitating the application of 5G communication technology in military fields. This work uses aramid nanofibers (ANF) as the matrix, hetero-structured silver nanowires@boron nitride nanosheets (AgNWs@BNNS) prepared by in situ growth as fillers, which are combined to fabricate sandwich structured thermally conductive and electrically insulating (BNNS/ANF)-(AgNWs@BNNS)-(BNNS/ANF) (denoted as BAB) composite films by "filtration self-assembly, air spraying, and hot-pressing" method. When the mass ratio of AgNWs@BNNS to BNNS is 1 : 1 and the total mass fraction is 50 wt %, BAB composite film has the maximum in-plane thermal conductivity coefficient (λ∥ of 10.36 W/(m ⋅ K)), excellent electrical insulation (breakdown strength and volume resistivity of 41.5 kV/mm and 1.21×1015 Ω ⋅ cm, respectively) and mechanical properties (tensile strength of 170.9 MPa). 50 wt % BAB composite film could efficiently reduce the equilibrium temperature of the central processing unit (CPU) working at full power, resulting in 7.0 °C lower than that of the CPU solely integrated with ANF directly. In addition, BAB composite film boasts adaptive visible light/infrared dual camouflage properties on cement roads and jungle environments, as well as the function of fast encryption of QR code information within 24 seconds.

Enhancing Thermal Conductivity in Polymer Composites through Molding-Assisted Orientation of Boron Nitride

Incrementing thermal conductivity in polymer composites through the incorporation of inorganic thermally conductive fillers is typically constrained by the requirement of high filler content. This necessity often complicates processing and adversely affects mechanical properties. This study presents the fabrication of a polystyrene (PS)/boron nitride (BN) composite exhibiting elevated thermal conductivity with a modest 10 wt% BN content, achieved through optimized compression molding. Adjustments to molding parameters, including molding-cycle numbers, temperature, and pressure, were explored. The molding process, conducted above the glass transition temperature of PS, facilitated orientational alignment of BN within the PS matrix predominantly in the in-plane direction. This orientation, achieved at low filler loading, resulted in a threefold enhancement of thermal conductivity following a single molding time. Furthermore, the in-plane alignment of BN within the PS matrix was found to intensify with increased molding time and pressure, markedly boosting the in-plane thermal conductivity of the PS/BN molded composites. Within the range of molding parameters examined, the highest thermal conductivity (1.6 W/m·K) was observed in PS/BN composites subjected to five molding cycles at 140 °C and 10 MPa, without compromising mechanical properties. This study suggests that compression molding, which allows low filler content and straightforward operation, offers a viable approach for the mass production of polymer composites with superior thermal conductivity.

Highly Thermally Conductive Flexible Biomimetic APTES-BNNS/BC Nanocomposite Paper by Sol-Gel-Film Technology

Owing to the evolution of 5G technology, new energy vehicles, flexible electronics, miniaturization and integration of microelectronic devices, high-frequency and high-power devices, and thermal management of materials must consider additional limitations such as electrical insulation, excellent transverse heat transfer, flexibility, and weight. Boron nitride nanosheets (BNNSs) are ideal insulating materials with high thermal conductivity. However, the problem of the 3D thermal conductivity pathway and toughness strength of nanocomposite paper loaded with inorganic thermal conductivity fillers remains a huge challenge. In this study, we propose a new method for preparing ultrathin, large, and uniformly thick BNNS for quantitative production. Bulk hexagonal boron nitride (hBN) layers were exfoliated using a simple and low-cost hydrothermal reaction, and large-scale fewer-layered BNNSs were efficiently prepared by ball milling with a high yield (up to 80%). Based on the aforementioned step, a flexible insulating composite film with high thermal conductivity and a natural "brick-mud" shell structure was constructed via the sol-gel-film conversion method. After prestretching and hot-pressing treatment, the hydrogels became denser, and the modified BNNS formed a three-dimensional (3D) network structure with an ordered orientation and interconnections in the bacterial cellulose (BC) matrix. After 100 folding cycles, the tensile strength of the nanofiber composite film reached 53 MPa, and the strength retention rate exceeded 42%. By optimizing the modified BNNS content, the thermal conductivity reached 24 W/(m·K). This simple approach has wide application potential in the next-generation electronic devices, providing options for designing thermal interface materials with excellent electrical insulation, high thermal stability, and flexibility.

Aqueous Dispersion of Aramid Nanofibers Achieved by Using Tannic Acid for Ultrahigh Strength Films

Polymer nanofibers have established a robust foundation and possess immense potential in various emerging fields such as sensors and biotechnology. In this study, aqueous dispersions of aramid nanofibers (ANFs) were successfully prepared by using tannic acid (TA). Morphological analysis revealed that TA effectively prevented self-aggregation of ANFs, and preserved the nanofiber structure during TA-assisted solvent exchange. Subsequently, the ANF and TA/ANF films were fabricated using casting and vacuum-assisted filtration techniques. Notably, the tensile strength of the casting TA/ANF film reached 393.8 MPa, exhibiting a remarkable improvement of 41.3% compared to that of the pure ANF film. These exceptional mechanical properties can be attributed to the well-dispersed nanostructures, hydrogen-bonding interactions, zigzag structures, and fiber-bridging effects. Furthermore, the TA/ANF film demonstrated superior ultraviolet (UV) shielding capabilities, visible transparency properties, and excellent resistance to chemical reagents. The above-mentioned interesting findings demonstrate its potential as a nanofiber-reinforced material for poly(vinyl alcohol) (PVA) composites.

Interfacial Assembly of 2D Graphene-Derived Ion Channels for Water-Based Green Energy Conversion

The utilization of sustained and green energy is believed to alleviate increasing menace of global environmental concerns and energy dilemma. Interfacial assembly of 2D graphene-derived ion channels (2D-GDICs) with tunable ion/fluid transport behavior enables efficient harvesting of renewable green energy from ubiquitous water, especially for osmotic energy harvesting. In this review, various interfacial assembly strategies for fabricating diverse 2D-GDICs are summarized and their ion transport properties are discussed. This review analyzes how particular structure and charge density/distribution of 2D-GDIC can be modulated to minimize internal resistance of ion/fluid transport and enhance energy conversion efficiency, and highlights stimuli-responsive functions and stability of 2D-GDIC and further examines the possibility of integrating 2D-GDIC with other energy conversion systems. Notably, the presented preparation and applications of 2D-GDIC also inspire and guide other 2D materials to fabricate sophisticated ion channels for targeted applications. Finally, potential challenges in this field is analyzed and a prospect to future developments toward high-performance or large-scale real-word applications is offered.

Unidirectionally Structured Magnetic Phase-Change Composite Based on Carbonized Polyimide/Kevlar Nanofiber Complex Aerogel for Boosting Solar-Thermo-Electric Energy Conversion

To realize highly efficient solar-thermo-electric energy conversion for clean electricity power generation, we have developed a new type of unidirectionally structured magnetic phase-change composite comprising a carbonized polyimide (C-PI)/Kevlar nanofiber (KNF) complex aerogel as a 3D carbon skeleton porous supporting material, CoFe2O4 nanoparticles as a magnetic additive, polyethylene glycol (PEG) as a phase-change material, and polypyrrole as a photothermal absorption coating layer. The as-fabricated C-PI/KNF complex aerogel exhibits a unidirectional microstructure, high porosity, robust skeleton frame, ultralight weight, and high thermal conductance. Featured with such unique structure and characteristics, the complex aerogel can offer an effective heat and electron transfer method to ensure highly efficient solar-thermal conversion and photothermal energy storage of the developed composite. The developed composite exhibits a high latent heat capacity of over 150 J g-1, outstanding shape stability along with a low leakage of 0.2 wt %, good thermal cycling stability, and high photothermal conversion efficiency of 84.8%. Based on the Seebeck effect, a solar thermoelectric generation system (STEGS) was constructed with the hot side coupled with the developed composite and the cold side immersed in air and ice water. Under 2.0 kW m-2 solar irradiation, the developed STEGS in ice water obtained maximum output voltage and current of 259.7 mV and 27.1 mA, respectively, which are significantly higher than those in air. The output power of the developed STEGS in an ice water environment is 50.6% higher than that in air under 4.0 kW m-2 solar irradiation. More importantly, the developed STEGS in ice water continuously generated output voltage and current for about 810 s without solar irradiation thanks to the latent heat release by the PEG component within the developed composite. In addition, the introduction of magnetic CoFe2O4 can accelerate solar-thermal conversion through periodic electron motion by the Néel relaxation or Brownian relaxation. This resulted in an increase in the maximum output voltage and current by 13.7 and 11.5%, respectively, under an alternating magnetic field as a result of the magnetism-accelerated solar-thermo-electric conversion. This study offers an innovative approach for developing PCM-based advanced functional materials for solar energy utilization in clean and sustainable electricity generation.

Sandwich-Like Flexible Breathable Strain Sensor with Tunable Thermal Regulation Capability for Human Motion Monitoring

High-performance flexible strain sensors with synergistic and outstanding thermal regulation function are poised to make a significant impact on next-generation multifunctional sensors. However, it has long been intractable to optimize the sensing performance and high thermal conductivity simultaneously. Herein, a novel flexible sandwich-like strain sensor with advanced thermal regulation capability was prepared by assembling electrospun thermoplastic polyurethane (TPU) fibrous membrane, MXene layer, and TPU/boron nitride nanosheet (BNNS) composite films. The as-prepared sensor demonstrates a wide strain working range (∼100% strain), an ultrahigh gauge factor (2080.9), and a satisfactory reliability. Meanwhile, benefiting from the uniform dispersion and promising orientation of BNNSs in TPU composites, the sensor possesses a high thermal conductivity of 1.5 W·m-1·K-1, guaranteeing wearer comfort. Additionally, the unique structure endows the sensor with high stretchability, breathability, biocompatibility, and tunable electromagnetic interference shielding performances. Furthermore, an integrated wireless motion monitoring device based on this sensor is rationally designed. It exhibits a fast response time, a wide recognition range, and the ability to maintain skin temperature during prolonged physical activity. These encouraging findings provide a new and feasible approach to designing high-performance and versatile flexible strain sensors with broad applications in advanced wearable technology.

Fillers and methods to improve the effective (out-plane) thermal conductivity of polymeric thermal interface materials - A review

The internet of things and growing demand for smaller and more advanced devices has created the problem of high heat production in electronic equipment, which greatly reduces the work performance and life of the electronic instruments. Thermal interface material (TIM) is placed in between heat generating micro-chip and the heat dissipater to conduct all the produced heat to the heat sink. The development of suitable TIM with excellent thermal conductivity (TC) in both in-plane and through-plane directions is a very important need at present. For efficient thermal management, polymer composites are potential candidates. But in general, their thermal conductivity is low compared to that of metals. The filler integration into the polymer matrix is one of the two approaches used to increase the thermal conductivity of polymer composites and is also easy to scale up for industrial production. Another way to achieve this is to change the structure of polymer chains, which fall out of the scope of this work. In this review, considering the first approach, the authors have summarized recent developments in many types of fillers with different scenarios by providing multiple cases with successful strategies to improve through-plane thermal conductivity (TPTC) (k⊥). For a better understanding of TC, a comprehensive background is presented. Several methods to improve the effective (out-plane) thermal conductivity of polymer composites and different theoretical models for the calculation of TC are also discussed. In the end, it is given a detailed conclusion that provides drawbacks of some fillers, multiple significant routes recommended by other researchers to build thermally conductive polymer composites, future aspects along with direction so that the researchers can get a guideline to design an effective polymer-based thermal interface material.

Dual-Effect Coupling for Superior Dielectric and Thermal Conductivity of Polyimide Composite Films Featuring "Crystal-Like Phase" Structure

To match the increasing miniaturization and integration of electronic devices, higher requirements are put on the dielectric and thermal properties of the dielectrics to overcome the problems of delayed signal transmission and heat accumulation. Here, a 3D  porous thermal conductivity network is successfully constructed inside the polyimide (PI) matrix by the combination of ionic liquids (IL) and calcium fluoride (CaF2 ) nanofillers, motivated by the bubble-hole forming orientation force. Benefiting from the 3D thermal network formed by IL as a porogenic template and "crystal-like phase" structures induced by CaF2 - polyamide acid charge transfer, IL-10 vol% CaF2 /PI porous film exhibits a low permittivity of 2.14 and a thermal conductivity of 7.22 W m-1 K-1 . This design strategy breaks the bottleneck that low permittivity and high thermal conductivity in microelectronic systems are difficult to be jointly controlled, and provides a feasible solution for the development of intelligent microelectronics.

Robust and Multifunctional Ti3 C2 Tx /Modified Sawdust Composite Paper for Electromagnetic Interference Shielding and Wearable Thermal Management

Robust, ultrathin, and environmental-friendliness papers that synergize high-efficiency electromagnetic interference (EMI) shielding, personal thermal management, and wearable heaters are essential for next-generation smart wearable devices. Herein, MXene nanocomposite paper with a nacre-like structure for EMI shielding and electrothermal/photothermal conversion is fabricated by vacuum filtration of Ti3 C2 Tx MXene and modified sawdust. The hydrogen bonding and highly oriented structure enhance the mechanical properties of the modified sawdust/MXene composite paper (SM paper). The SM paper with 50 wt% MXene content shows a strength of 23 MPa and a toughness of 13 MJ·M-3 . The conductivity of the SM paper is 10 195 S·m-1 , resulting in an EMI shielding effectiveness (SE) of 67.9 dB and a specific SE value (SSE/t) of 8486 dB·cm2 ·g-1 . In addition, the SM paper exhibits excellent thermal management performance including high light/electro-to-thermal conversion, rapid Joule heating and photothermal response, and sufficient heating stability. Notably, the SM paper exhibits low infrared emissivity and distinguished infrared stealth performance, camouflaging a high-temperature heater surface of 147-81 °C. The SM-based e-skin achieves visualization of Joule heating and realizes human motions monitoring. This work presents a new strategy for designing MXene-based wearable devices with great EMI shielding, artificial intelligence, and thermal management applications.

Flexible and Robust Functionalized Boron Nitride/Poly(p-Phenylene Benzobisoxazole) Nanocomposite Paper with High Thermal Conductivity and Outstanding Electrical Insulation

With the rapid development of 5G information technology, thermal conductivity/dissipation problems of highly integrated electronic devices and electrical equipment are becoming prominent. In this work, "high-temperature solid-phase & diazonium salt decomposition" method is carried out to prepare benzidine-functionalized boron nitride (m-BN). Subsequently, m-BN/poly(p-phenylene benzobisoxazole) nanofiber (PNF) nanocomposite paper with nacre-mimetic layered structures is prepared via sol-gel film transformation approach. The obtained m-BN/PNF nanocomposite paper with 50 wt% m-BN presents excellent thermal conductivity, incredible electrical insulation, outstanding mechanical properties and thermal stability, due to the construction of extensive hydrogen bonds and π-π interactions between m-BN and PNF, and stable nacre-mimetic layered structures. Its λ∥ and λ⊥ are 9.68 and 0.84 W m-1 K-1, and the volume resistivity and breakdown strength are as high as 2.3 × 1015 Ω cm and 324.2 kV mm-1, respectively. Besides, it also presents extremely high tensile strength of 193.6 MPa and thermal decomposition temperature of 640 °C, showing a broad application prospect in high-end thermal management fields such as electronic devices and electrical equipment.

Organic Conjugated Small Molecules with High Thermal Conductivity as an Effective Coupling Layer for Heat Transfer

As the features of electronics are miniaturized, the need for interfacial thermal coupling layers to enhance their thermal transfer efficiency and improve device performance becomes critical. Organic conjugated small molecules possess a unique combination of periodic crystal structures and conjugated units with π electrons, resulting in notable thermal conductivities and molecular structure orientation that facilitates directed heat transfer. Nevertheless, there is a noticeable gap in literatures regarding the thermal properties of organic conjugated small molecules and their potential applications in nanoscale thermal management. Herein, we report the fabrication of high-quality thin films of organic conjugated small molecules. The result reveals that the 2D organic conjugated small molecule thin films exhibit a high cross-plane thermal conductivity of 3.2 W/m K. The increased thermal conductivity is attributed to the well-organized lattice structure and existence of π-electrons induced by conjugated systems. The studied conjugated small molecules engage in π-π stacking interactions with carbon materials and efficiently exchange energy with electrons in metals, promoting rapid interfacial heat transfer. These molecules act as coupling layers, significantly enhancing thermal transfer efficiency between graphite-based thermal pads and copper heat sinks. This pioneering research represents the inaugural investigation of the thermal performance of conjugated organic small molecules. These findings highlight the potential of conjugated small molecules as thermal coupling layers, offering tunable combinations of desirable properties.

High-Performance Graphene Biocomposite Enabled by Fe3+ Coordination for Thermal Management

Emerging biocomposites with excellent heat dissipation capabilities and inherent sustainability are urgently needed to address the cooling issues of modern electronics and growing environmental concerns. However, the moisture stability, mechanical performance, thermal conductivity, and even flame retardancy of biomass-based materials are generally insufficient for practical thermal management applications. Herein, we present a high-performance graphene biocomposite consisting of carboxylated cellulose nanofibers and graphene nanosheets through an evaporation-induced self-assembly and subsequent Fe3+ cross-linking strategy. The Fe3+ coordination plays a critical role in stabilizing the material structure, thereby improving the mechanical strength and water stability of the biocomposite films, and its effect is revealed by density functional theory calculations. The hierarchical structure of the biocomposite films also leads to a high in-plane thermal conductivity of 42.5 W m-1 K-1, enabling a superior heat transfer performance. Furthermore, the resultant biocomposite films exhibit outstanding Joule heating performance with a fast thermal response and long-term stability, improved thermal stability, and flame retardancy. Therefore, such a general strategy and the desired overall properties of the biocomposite films offer wide application prospects for functional and safe thermal management.

Self-standing boron nitride bulks enabled by liquid metals for thermal management

Thermally conductive materials (TCMs) are highly desirable for thermal management applications to tackle the "overheating" concerns in the electronics industry. Despite recent progress, the development of high performance TCMs integrated with an in-plane thermal conductivity (TC) higher than 50.0 W (m K)-1 and a through-plane TC greater than 10.0 W (m K)-1 is still challenging. Herein, self-standing liquid metal@boron nitride (LM@BN) bulks with ultrahigh in-plane TC and through-plane TC were reported for the first time. In the LM@BN bulks, LM could serve as a bonding and thermal linker among the oriented BN platelets, thus remarkably accelerating heat transfer across the whole system. Benefiting from the formation of a unique structure, the LM@BN bulk achieved an ultrahigh in-plane TC of 82.2 W (m K)-1 and a through-plane TC of 20.6 W (m K)-1, which were among the highest values ever reported for TCMs. Furthermore, the LM@BN bulks exhibited superior compressive and leakage-free performances, with a high compressive strength (5.2 MPa) and without any LM leakage even after being crushed. It was also demonstrated that the excellent TCs of the LM@BN bulks made them effectively cool high-power light emitting diode modules. This work opens up one promising pathway for the development of high-performance TCMs for thermal management in the electronics industry.

Coaxial Wet Spinning of Boron Nitride Nanosheet-Based Composite Fibers with Enhanced Thermal Conductivity and Mechanical Strength

Hexagonal boron nitride nanosheets (BNNSs) exhibit remarkable thermal and dielectric properties. However, their self-assembly and alignment in macroscopic forms remain challenging due to the chemical inertness of boron nitride, thereby limiting their performance in applications such as thermal management. In this study, we present a coaxial wet spinning approach for the fabrication of BNNSs/polymer composite fibers with high nanosheet orientation. The composite fibers were prepared using a superacid-based solvent system and showed a layered structure comprising an aramid core and an aramid/BNNSs sheath. Notably, the coaxial fibers exhibited significantly higher BNNSs alignment compared to uniaxial aramid/BNNSs fibers, primarily due to the additional compressive forces exerted at the core-sheath interface during the hot drawing process. With a BNNSs loading of 60 wt%, the resulting coaxial fibers showed exceptional properties, including an ultrahigh Herman orientation parameter of 0.81, thermal conductivity of 17.2 W m-1 K-1, and tensile strength of 192.5 MPa. These results surpassed those of uniaxial fibers and previously reported BNNSs composite fibers, making them highly suitable for applications such as wearable thermal management textiles. Our findings present a promising strategy for fabricating high-performance composite fibers based on BNNSs.

Preparation and Space Charge Properties of Functionalized Zeolite/Crosslinked Polyethylene Composites with High Thermal Conductivity

Nanocomposite doping is an effective method to improve the dielectric properties of polyethylene. Meanwhile, the introduction of thermal conductivity groups in crosslinked polyethylene (XLPE) is also an effective way to improve the thermal conductivity. Nano-zeolite is an inorganic material with a porous structure that can be doped into polyethylene to improve the insulation performance. In this paper, hyperbranched polyarylamide (HBP) with a high thermal conductivity and an auxiliary crosslinking agent (TAIC) was grafted on the surface of ZSM-5 nano-zeolite successively to obtain functionalized nano-zeolite (TAICS-ZSM-5-HBP) (the "S" in TAICS means plural). The prepared functionalized nano-zeolite was doped in polyethylene and grafted under a thermal crosslinking reaction to prepare nanocomposites (XLPE/TAICS-ZSM-5-HBP). The structural characterization showed that the nanocomposite was successfully prepared and that the nanoparticles were uniformly dispersed in the polyethylene matrix. The space charge of the TAICS-ZSM-5-HBP 5wt% nanocomposite under a high electric field was obviously inhibited. The space charge short-circuit test showed that the porous structure of the nano-zeolite introduced more deep traps, which made the trapped charge difficult to break off, hindering the charge injection. The introduction of TAICS-ZSM-5-HBP particles can greatly improve the thermal conductivity of nanocomposites. The thermal conductivity of the XLPE/5wt% and XLPE/7wt% TAICS-ZSM-5-HBP nanocomposites increased by 42.21% and 69.59% compared to that of XLPE at 20 °C, and by 34.27% and 62.83% at 80 °C.

Thermally Insulating and Moisture-Resilient Foams Based on Upcycled Aramid Nanofibers and Nanocellulose

Low-density foams and aerogels based on upcycled and bio-based nanofibers and additives are promising alternatives to fossil-based thermal insulation materials. Super-insulating foams are prepared from upcycled acid-treated aramid nanofibers (upANFA ) obtained from Kevlar yarn and tempo-oxidized cellulose nanofibers (CNF) from wood. The ice-templated hybrid upANFA /CNF-based foams with an upANFA content of up to 40 wt% display high thermal stability and a very low thermal conductivity of 18-23 mW m-1 K-1 perpendicular to the aligned nanofibrils over a wide relative humidity (RH) range of 20% to 80%. The thermal conductivity of the hybrid upANFA /CNF foams is found to decrease with increasing upANFA content (5-20 wt%). The super-insulating properties of the CNF-upANFA hybrid foams are related to the low density of the foams and the strong interfacial phonon scattering between the very thin and partially branched upANFA and CNF in the hybrid foam walls. Defibrillated nanofibers from textiles are not limited to Kevlar, and this study can hopefully inspire efforts to upcycle textile waste into high-performance products.

Inhibited Grain Growth Through Phase Transition Modulation Enables Excellent Mechanical Properties in Oxide Ceramic Nanofibers up to 1700 °C

Oxide ceramics are widely used as thermal protection materials due to their excellent structural properties and earth abundance. However, in extremely high-temperature environments (above 1500 °C), the explosive growth of grain size causes irreversible damage to the microstructure of oxide ceramics, thus exhibiting poor thermomechanical stability. This problem, which may lead to catastrophic accidents, remains a great challenge for oxide ceramic materials. Here, a novel strategy of phase transition modulation is proposed to control the grain growth at high temperatures in oxide ceramic nanofibers, realizing effective regulation of the crystalline forms as well as the size uniformity of primary grains, and thus suppressing the malignant growth of the grains. The resulting oxide ceramic nanofibers have excellent mechanical strength and flexibility, delivering an average tensile strength as high as 1.02 GPa after being exposed to 1700 °C for 30 min, and can withstand thousands of flexural cycles without obvious damage. This work may provide new insight into the development of advanced oxide ceramic materials that can serve in extremely high-temperature environments with long-term durability.

Effect of Nanoscale in Situ Interface Welding on the Macroscale Thermal Conductivity of Insulating Epoxy Composites: A Multiscale Simulation Investigation

Insulating thermally conductive polymer composites are in great demand in integrated-circuit packages, for efficient heat dissipation and to alleviative short-circuit risk. Herein, the continuous oriented hexagonal boron nitride (h-BN) frameworks (o-BN@SiC) were prepared via self-assembly and in situ chemical vapor infiltration (CVI) interface welding. The insulating o-BN@SiC/epoxy (o-BN@SiC/EP) composites exhibited enhanced thermal conductivity benefited from the CVI-SiC-welded BN-BN interface. Further, multiscale simulation, combining first-principles calculation, Monte Carlo simulation, and finite-element simulation, was performed to quantitatively reveal the effect of the welded BN-BN interface on the heat transfer of o-BN@SiC/EP composites. Phonon transmission in solders and phonon-phonon coupling of filler-solder interfaces enhanced the interfacial heat transfer between adjacent h-BN microplatelets, and the interfacial thermal resistance of the dominant BN-BN interface was decreased to only 3.83 nK·m2/W from 400 nK·m2/W, plunging by over 99%. This highly weakened interfacial thermal resistance greatly improved the heat transfer along thermal pathways and resulted in a 26% thermal conductivity enhancement of o-BN@SiC/EP composites, compared with physically contacted oriented h-BN/EP composites, at 15 vol % h-BN. This systematic multiscale simulation broke through the barrier of revealing the heat transfer mechanism of polymer composites from the nanoscale to the macroscale, which provided rational cognition about the effect of the interfacial thermal resistance between fillers on the thermal conductivity of polymer composites.

Electrically insulating PBO/MXene film with superior thermal conductivity, mechanical properties, thermal stability, and flame retardancy

Constructing flexible and robust thermally conductive but electrically insulating composite films for efficient and safe thermal management has always been a sought-after research topic. Herein, a nacre-inspired high-performance poly(p-phenylene-2,6-benzobisoxazole) (PBO)/MXene nanocomposite film was prepared by a sol-gel-film conversion method with a homogeneous gelation process. Because of the as-formed optimized brick and mortar structure, and the strong bridging and caging effects of the fine PBO nanofibre network on the MXene nanosheets, the resulting nanocomposite film is electrically insulating (2.5 × 109 Ω cm), and exhibits excellent mechanical properties (tensile strength of 416.7 MPa, Young's modulus of 9.1 GPa and toughness of 97.3 MJ m-3). More importantly, the synergistic orientation of PBO nanofibres and MXene nanosheets endows the film with an in-plane thermal conductivity of 42.2 W m-1 K-1. The film also exhibits excellent thermal stability and flame retardancy. This work broadens the ideas for preparing high-performance thermally conductive but electrically insulating composites.

Improved thermal conductivity and excellent electrical insulation properties of polysiloxane nanocomposite-incorporated functional boron nitride sheets via in situ polymerization

Benefiting from its high thermal conductivity (κ) and superior insulation, the boron nitride nanosheet (BNNS) is widely investigated as a promising filler for thermal nanocomposites. However, poor dispersibility and weak interaction with polymer matrix hinder the further improvement of BNNS-based thermal composites. Here, inspired by side-chain liquid crystal polysiloxane (SCLCP) with good mesomorphic structures, highly thermoconductive nanocomposites prepared via in situ polymerization using SCLCP with 2D BNNS are reported. The surface of BNNS is silanized with γ-(methacryloxy)propyltrimethoxysilane (KH-570) to introduce double bonds (defined as f-BNNS), and it is directly linked with SCLCP chains during polymerization. Therefore, the alternating stacking of f-BNNS and microscopic ordered structure of SCLCP yielded a high κ of 2.463 W m-1 K-1 at only 30 wt% f-BNNS content, improving dramatically the κ of pure SCLCP by ∼9 times. Further, the volume electrical resistivity reached 2.11 × 1014 Ω cm, which is five orders of magnitude higher than the critical resistance for electrical insulation (109 Ω cm). Also, the f-BNNS/SCLCP composites as thermal management materials decreased the temperature of the LED chip by 17.5 °C, exhibiting superior thermal management performance. Along with high κ and excellent electrical resistance, this type of nanocomposites displays great advantages in thermal properties for electronic packaging and thermal management of electronics.

Nickel nanoparticle decorated silicon carbide as a thermal filler in thermal conductive aramid nanofiber-based composite films for heat dissipation applications

Aramid nanofibers (ANFs) have shown potential applications in the fields of nanocomposite reinforcement, battery separators, thermal insulation and flexible electronics. However, the inherent low thermal conductivity limits the application of ANFs, currently, to ensure long lifetime in electronics. In this work, new nickel (Ni) nanoparticles were employed to decorate the silicon carbide (SiC) filler by a rapid and non-polluting method, in which nickel acetate tetrahydrate (Ni(CH₃COO)₂·4H₂O) and SiC were mixed and heated under an inert atmosphere. The composites as thermal fillers were applied to prepare an aramid nanofiber (ANF)-based composite film. Our results showed that the decoration of SiC by an appropriate amount of Ni nanoparticles played an important role in improving the thermal conductivity, hydrophobicity, thermal stability, and puncture resistance of the ANF composite film. After adjusting the balling time at 10 h, the optimized content of 10 mol% Ni nanoparticles improved the thermal conductivity to 0.502 W m^-1 K^-1, 298.4% higher than that of the original ANF film. Moreover, increasing the content of thermal fillers to 30 wt% realized a high thermal conductivity of 0.937 W m^-1 K^-1, which is 643.7% higher than that of the pristine ANF film. Moreover, the compatibility between thermal fillers and ANFs and thermal stability were improved for the ANF-composite films. The effective heat transfer function of our composite films was further confirmed using a LED lamp and thermoelectric device. In addition, the obtained composite films show certain mechanical properties and better hydrophobicity; these results exhibit their great potential applications in electronic devices.

A Thermochromic, Viscoelastic Nacre-like Nanocomposite for the Smart Thermal Management of Planar Electronics

Cutting-edge heat spreaders for soft and planar electronics require not only high thermal conductivity and a certain degree of flexibility but also remarkable self-adhesion without thermal interface materials, elasticity, arbitrary elongation along with soft devices, and smart properties involving thermal self-healing, thermochromism and so on. Nacre-like composites with excellent in-plane heat dissipation are ideal as heat spreaders for thin and planar electronics. However, the intrinsically poor viscoelasticity, i.e., adhesion and elasticity, prevents them from simultaneous self-adhesion and arbitrary elongation along with current flexible devices as well as incurring high interfacial thermal impedance. In this paper, we propose a soft thermochromic composite (STC) membrane with a layered structure, considerable stretchability, high in-plane thermal conductivity (~ 30 W m-1 K-1), low thermal contact resistance (~ 12 mm2 K W-1, 4-5 times lower than that of silver paste), strong yet sustainable adhesion forces (~ 4607 J m-2, 2220 J m-2 greater than that of epoxy paste) and self-healing efficiency. As a self-adhesive heat spreader, it implements efficient cooling of various soft electronics with a temperature drop of 20 °C than the polyimide case. In addition to its self-healing function, the chameleon-like behavior of STC facilitates temperature monitoring by the naked eye, hence enabling smart thermal management.

Propagation of terahertz elastic longitudinal waves in piezoelectric semiconductor rods

Terahertz elastic waves travelling in piezoelectric semiconductors (PSs) with the deformation-polarization-carrier coupling have a huge potential application in elastic wave-based devices. To reveal wave propagation characteristics of terahertz elastic waves in rod-like PS structures, we present three typical rod models based on the Hamilton principle and the linearization of the nonlinear current, which are extensions of the classical, Love, and Mindlin-Herrmann rod models for elastic media to those for PS materials. Using the derived equations, the analytical dispersion relations of the elastic longitudinal waves propagating in an n-type PS rod are obtained, which can be reduced to those for piezoelectric and elastic rods by sequentially dropping the corresponding electron- and piezoelectricity-related terms. The Mindlin-Herrmann rod model is more accurate for analysis of terahertz elastic longitudinal wave in rod-like PS structures. The effects of the interaction between the piezoelectricity and semiconducting properties on the dispersion behaviors of terahertz elastic longitudinal waves are investigated in detail. Numerical results show that both phase and group velocities have a 50%-60% reduction in the terahertz range in comparison with those in the low frequency range, and the effective tuning range of the initial electron concentration is different for longitudinal waves with different frequencies. It lays the theoretical foundations for the design of terahertz elastic wave-based devices.

Molecular design of a highly matched and bonded interface achieves enhanced thermal boundary conductance

Interfacial binding and phonon mismatch are two crucial parameters in determining thermal boundary conductance. However, it is difficult for polymer/metal interfaces to possess both significant interfacial binding and weak phonon mismatch to achieve enhanced thermal boundary conductance. Herein, we circumvent this inherent trade-off by synthesizing a polyurethane and thioctic acid (PU-TA) copolymer with multiple hydrogen bonds and dynamic disulfide bonds. Using PU-TA/aluminum (Al) as a model interface, we demonstrate that the thermal boundary conductance of the PU-TA/Al interfaces measured by transient thermoreflectance is 2-5 times higher than that of traditional polymer/Al interfaces, which is attributed to the highly matched and bonded interface. Furthermore, a correlation analysis is developed, which demonstrates that interfacial binding has a greater impact than phonon mismatch on thermal boundary conductance at a highly matched interface. This work provides a systematic understanding of the relative contributions of the two dominant mechanisms to thermal boundary conductance by tailoring the polymer structure, which has applications in thermal management materials.

Self-Stacked 3D Anisotropic BNNS Network Guided by Para-Aramid Nanofibers for Highly Thermal Conductive Dielectric Nanocomposites

The enhancement of the heat-dissipation property of polymer-based composites is of great practical interest in modern electronics. Recently, the construction of a three-dimensional (3D) thermal pathway network structure for composites has become an attractive way. However, for most reported high thermal conductive composites, excellent properties are achieved at a high filler loading and the building of a 3D network structure usually requires complex steps, which greatly restrict the large-scale preparation and application of high thermal conductive polymer-based materials. Herein, utilizing the framework-forming characteristic of polymerization-induced para-aramid nanofibers (PANF) and the high thermal conductivity of hexagonal boron nitride nanosheets (BNNS), a 3D-laminated PANF-supported BNNS aerogel was successfully prepared via a simple vacuum-assisted self-stacking method, which could be used as a thermal conductive skeleton for epoxy resin (EP). The obtained PANF-BNNS/EP nanocomposite exhibits a high thermal conductivity of 3.66 W m^-1 K^-1 at only 13.2 vol % BNNS loading. The effectiveness of the heat conduction path was proved by finite element analysis. The PANF-BNNS/EP nanocomposite shows outstanding practical thermal management capability, excellent thermal stability, low dielectric constant, and dielectric loss, making it a reliable material for electronic packaging applications. This work also offers a potential and promotable strategy for the easy manufacture of 3D anisotropic high-efficiency thermal conductive network structures.

Hydroxy silicone oil modified boron nitride for high thermal conductivity and low dielectric loss silicone rubber composites: experimental and molecular simulation studies

Polymer-based composites are widely used in microelectronics and wireless communications, which require high thermal conductivity and low dielectric loss for effective heat dispersion and signal transmission. Different lengths of hydroxyl silicone oil chains modified boron nitride/silicone rubber composites were explored and prepared in this work. Experiments demonstrate that the long-chain modified BN improves the thermal conductivity and decreases the dielectric loss of composites. A molecular dynamics simulation was employed to study the mechanism and affecting variables. The calculated results indicated that the improvement of the thermal and dielectric properties is mainly related to the interfacial behavior, including interfacial compatibility, interfacial bond strength, and phonon matching. Based on the simulated interfacial behavior and thermal conductivity, the thermal and dielectric properties of different chain-length modified boron nitride/silicone rubber composites have been anticipated. The results show that the longer-chain modified boron nitride/silicone rubber composites have better thermal and dielectric properties. This research may give a theoretical foundation for the development of materials with designable performance for electronic devices.

Flexible and Ultrathin Graphene/Aramid Nanofiber Carbonizing Films with Nacre-like Structures for Heat-Conducting Electromagnetic Wave Shielding/Absorption

Electromagnetic interference (EMI) shielding and electromagnetic wave absorption (EWA) materials with good thermal management and flexibility properties are urgently needed to meet the more complex modern service environment, especially in the field of smart wearable electronics. How to balance the relation of electromagnetic performance, thermal management, flexibility, and thickness in material design is a crucial challenge. Herein, graphene nanosheets/aramid nanofiber (C-GNS/ANF) carbonizing films with nacre-like structures were fabricated via the blade-coating/carbonization procedure. The ingenious configuration from highly ordered alignment GNS interactively connected by a carbonized ANF network can effectively improve the thermal/electrical conductivity of a C-GNS/ANF film. Specifically, the ultrathin C-GNS/ANF film with a thickness of 17 μm shows excellent in-plane thermal conductivity (TC) of 79.26 W m^-1 K^-1 and superior EMI shielding up to 56.30 dB. Moreover, the obtained C-GNS/ANF film can be used as a lightweight microwave absorber, achieving excellent microwave absorption performance with a minimum reflection loss of -56.07 dB at a thickness of 1.5 mm and a maximum effective absorption bandwidth of 5.28 GHz at an addition of only 5 wt %. Furthermore, the C-GNS/ANF films demonstrate good flexibility, outstanding thermal stability, and flame retardant properties. Overall, this work indicates a prospective direction for the development of the next generation of electromagnetic wave absorption/shielding materials with high-performance heat conduction.

Room-Temperature Self-Healing Soft Composite Network with Unprecedented Crack Propagation Resistance Enabled by a Supramolecular Assembled Lamellar Structure

Soft self-healing materials are compelling candidates for stretchable devices because of their excellent compliance, extensibility, and self-restorability. However, most existing soft self-healing polymers suffer from crack propagation and irreversible fatigue failure due to easy breakage of their dynamic amorphous, low-energy polymer networks. Herein, inspired by distinct structure-property relationship of biological tissues, a supramolecular interfacial assembly strategy of preparing soft self-healing composites with unprecedented crack propagation resistance is proposed by structurally engineering preferentially aligned lamellar structures within a dynamic and superstretchable poly(urea-ureathane) matrix (which is elongated to 24 750× its original length). Such a design affords a world-record fracture energy (501.6 kJ m^-2 ), ultrahigh fatigue threshold (4064.1 J m^-2 ), and outstanding elastic restorability (dimensional recovery from 13 times elongation), and preserving low modulus (1.2 MPa), high stretchability (3200%), and high room-temperature self-healing efficiency (97%). Thereby, the resultant composite represents the best of its kind and even surpasses most biological tissues. The lamellar 2D transition-metal carbide/carbonitride (MXene) structure also leads to a relatively high in-plane thermal conductivity, enabling composites as stretchable thermoconductive skins applied in joints of robotics to thermal dissipation. The present work illustrates a viable approach how autonomous self-healing, crack tolerance, and fatigue resistance can be merged in future material design.

Enhancing Bioinspired Aramid Nanofiber Networks by Interfacial Hydrogen Bonds for Multiprotection under an Extreme Environment

In nature, many insects have evolved sclerotic cuticles to shelter their soft bodies, which are considered as "body armor". For beetles, the epidermis is composed of cross-linked intertwined fiber structures; such a fiber network structure could provide an anti-impact function for composites. Aramid nanofibers (ANFs) are of great interest in various applications due to their 1D nanoscale, high aspect ratio, excellent strength and modulus, and impressive chemical and thermal stability. In this paper, a kind of ANF network is prepared by a layer-by-layer assembly method. The enhancing ANF networks are developed by introducing carboxylated chitosan acting as a hydrogen-bondin donors as well as a soft interlocking agent (C-ANFs). As a result of the formation of a nanostructure and the hydrogen-bond interactions, the assembled C-ANF networks presented a high tensile strength (551.4 MPa) and toughness (4.0 MJ/m²), which is 2.41 times and 32.69 times those of neat ANF networks, respectively. The excellent mechanical properties endow C-ANF networks with distinguished anti-impact performance. The specific dissipated energy after mass normalization reaches 7.34 MJ/kg, which is significantly superior to traditional protective materials such as steel and Kevlar composites. A nonlinear spring model is also used to explain the mechanical behavior of C-ANF networks. In addition to anti-impact protection, C-ANF networks can realize more than 99% of UV irradiation absorption and have excellent thermal stability. The chemical stability of C-ANF networks make them survive in acid and alkali environments. The above characteristics show that C-ANF networks have great application value in multiscale protection scenarios under an extreme environment.