Microstructured BN Composites with Internally Designed High Thermal Conductivity Paths for 3D Electronic Packaging

Simulation-Directed Construction of Bamboo-Forest-Like Heat Conduction Networks to Enhance Silicon Rubber Composites' Heat Conduction Properties

Highly vertically thermally conductive silicon rubber (SiR) composites are widely used as thermal interface materials (TIMs) for chip cooling. Herein, inspired by water transport and transpiration of Moso bamboo-forests extensively existing in south China, and guided by filler self-assembly simulation, bamboo-forest-like heat conduction networks, with bamboo-stems-like vertically aligned polydopamine-coated carbon fibers (VA-PCFs), and bamboo-leaves-like horizontally layered Al2O3(HL-Al2O3), are rationally designed and constructed. VA-PCF/HL-Al2O3/SiR composites demonstrated enhanced heat conduction properties, and their through-plane thermal conductivity and thermal diffusivity reached 6.47 W (mK)-1 and 3.98 mm2 s-1 at 12 vol% PCF and 4 vol% Al2O3 loadings, which are 32% and 38% higher than those of VA-PCF (12 vol%) /SiR composites, respectively. The heat conduction enhancement mechanisms of VA-PCF/HL-Al2O3 networks on their SiR composites are revealed by multiscale simulation: HL-Al2O3 bridges the separate VA-PCF heat flow channels, and transfers more heat to the matrix, thereby increasing the vertical heat flux in composites. Along with high volume resistivity, low compression modulus, and coefficient of thermal expansion, VA-PCF/HL-Al2O3/SiR composites demonstrate great application potential as TIMs, which is proven using multiphysics simulation. This work not only makes a meaningful attempt at simulation-driven biomimetic material structure design but also provides inspiration for the preparation of TIMs.

Thermally Conductive Polydimethylsiloxane-Based Composite with a Three-Dimensional Vertically Aligned Thermal Network Incorporating Hexagonal Boron Nitride Nanosheets and Nanodiamonds

Thermal interface materials play a pivotal role in efficiently transferring heat from heating devices to thermal management components, thereby reducing the risk of component degradation due to overheating. In this study, we propose a strategy for enhancing the out-of-plane thermal conductivity (TC) of composite materials by fabricating a three-dimensional (3D) thermal network within a polydimethylsiloxane (PDMS) matrix. Specifically, the composite material was designed to incorporate a dense thermal network comprising hexagonal boron nitride nanosheets (BNNSs) and nanodiamonds (NDs). The fabrication process commenced with the preparation of BNNSs through liquid-phase exfoliation, followed by the creation of a 3D BNNSs-NDs/polyimide aerogel thermal framework using a unidirectional solidification ice templating method and subsequent heat treatment. Vacuum impregnation and curing were then employed to finalize the production of the 3D BNNSs-NDs/PDMS composite material. Characterization analyses indicated that the addition of NDs filled the voids between BNNSs, leading to the densification of the thermal framework pore walls and the establishment of additional thermal pathways. Impressively, with concentrations of BNNSs and NDs of 17.99 and 7.71 wt %, respectively, the out-of-plane TC of the 3D BNNSs-NDs/PDMS composite material reached 1.623 W m-1 K-1, marking notable enhancements of 754.21% and 256.70% compared to those of pure PDMS and composites prepared via direct blending with randomly distributed BNNSs and NDs, respectively. Furthermore, the 3D BNNSs-NDs thermal framework improved the compressive strength and the dimensional stability of the composite material. Finite element simulations additionally confirmed the synergistic improvement of the TC achieved through the combination of BNNSs and NDs, demonstrating that the 3D BNNSs-NDs/PDMS composite material displayed superior heat conduction and a greater density of thermal pathways compared to those of its counterparts, including 3D BNNSs/PDMS and 3D NDs/PDMS composite materials. In summary, this work presents a strategy for enhancing the out-of-plane TC of polymer-based composite materials by incorporating vertically aligned thermal networks.

Highly oriented BN-based TIMs with high through-plane thermal conductivity and low compression modulus

In the pursuit of effective thermal management for electronic devices, it is crucial to develop insulation thermal interface materials (TIMs) that exhibit exceptional through-plane thermal conductivity, low thermal resistance, and minimal compression modulus. Boron nitride (BN), given its outstanding thermal conduction and insulation properties, has garnered significant attention as a potential material for this purpose. However, previously reported BN-based composites have consistently demonstrated through-plane thermal conductivity below 10 W m-1 K-1 and high compression modulus, whilst also presenting challenges in terms of mass production. In this study, low molecular weight polydimethylsiloxane (PDMS) and large-size BN were utilized as the foundational materials. Utilizing a rolling-curing integrated apparatus, we successfully accomplished the continuous preparation of large-sized, high-adhesion BN films. Subsequent implementation of stacking, cold pressing, and vertical cutting techniques enabled the attainment of a remarkable BN-based TIM, characterized by an unprecedented through-plane thermal conductivity of up to 12.11 W m-1 K-1, remarkably low compression modulus (55 kPa), and total effective thermal resistance (0.16 °C in2 W-1, 50 Psi). During the TIMs performance evaluation, our TIMs demonstrated superior heat dissipation capabilities compared with commercial TIMs. At a heating power density of 40 W cm-2, the steady-state temperature of the ceramic heating element was found to be 7 °C lower than that of the commercial TIMs. This pioneering feat not only contributes valuable technical insights for the development of high-performance insulating TIMs but also establishes a solid foundation for widespread implementation in thermal management applications across a range of electronic devices.

Significantly Promoting the Thermal Conductivity and Machinability of Negative Thermal Expansion Alloy via In Situ Precipitation of Copper Networks

Rapid advancements in electronic devices yield an urgent demand for high-performance electronic packaging materials with high thermal conductivity, low thermal expansion, and great mechanical properties. However, it is a great challenge for current design philosophies to fulfill all the requirements simultaneously. Here, an effective strategy is proposed for significantly promoting the thermal conductivity and machinability of negative thermal expansion alloy (Zr,Nb)Fe2 through eutectic precipitation of copper networks. The eutectic dual-phase alloy exhibits an isotropic chips-matched thermal expansion coefficient and a thermal conductivity enhancement exceeding 200% compared with (Zr,Nb)Fe2, along with an ultimate compressive strength of 550 MPa. The addition of copper reorganizes the composition of (Zr,Nb)Fe2, which smooths the magnetic transition and shifts it toward higher temperature, resulting in linear low thermal expansion in a wide temperature range. The highly fine eutectic copper lamellae construct high thermal conductivity networks within (Zr,Nb)Fe2, serving as highways for heat transfer electrons and phonons. The in situ forming of eutectic copper lamellae also facilitates the mechanical properties by enhancing interfacial bonding and bearing additional stress after yielding of (Zr,Nb)Fe2. This work provides a novel strategy for promoting thermal conductivity and mechanical properties of negative thermal expansion alloys via eutectic precipitation of copper networks.

Highly Thermally Conductive and Flexible Thermal Interface Materials with Aligned Graphene Lamella Frameworks

Highly thermally conductive and flexible thermal interface materials (TIMs) are desirable for heat dissipation in modern electronic devices. Here, we fabricated a high-crystalline aligned graphene lamella framework (AGLF) with precisely controlled lamella thickness, pore structure, and excellent intergraphene contact by manipulating the thermal expansion behavior of scanning centrifugal casted graphene oxide films. The rational design of the AGLF balances the trade-off between the thermal conductivity and flexibility of TIMs. The AGLF-based TIM (AGLF-TIM) shows a record thermal conductivity of 196.3 W m-1 K-1 with a graphene loading of only 9.4 vol %, which is about 4 times higher than those of reported TIMs at a similar graphene loading. Meanwhile, good flexibility remains comparable to that of commercial TIMs. As a result, an LED device achieves an additional temperature decrease of ∼8 °C with the use of AGLF-TIM compared to high-performance commercial TIMs. This work offers a strategy for the controlled fabrication of graphene macrostructures, showing the potential use of graphene as filler frameworks in thermal management.

Thermal shock protection with scalable heat-absorbing aerogels

Improving thermal insulation is vital for addressing thermal protection and energy efficiency challenges. Though silica aerogel has a record-low thermal conductivity at ambient pressure, its high production cost, due to its nanoscale porous structure, has hindered its widespread use. In this study, we introduce a cost-effective and mild method that enhances insulation by incorporating phase change materials (PCMs) into a micron-porous framework. With a thermal conductivity at 0.041 W m-1K-1 on par with conventional insulation materials, this PCMs aerogel presents additional advantages for thermal protection from transient high-temperature loads by effectively delaying heat propagation through heat absorption. Moreover, the PCMs aerogel remains stable under cyclic deformation and heating up to 300 °C and is self-extinguishing in the presence of fire. Our approach offers a promising alternative for affordable insulation materials with potential wide applications in thermal protection and energy conservation areas.

Soft and Damping Thermal Interface Materials with Honeycomb-Board-Mimetic Filler Network for Electronic Heat Dissipation

High-power-density electronic devices under vibrations call for soft and damping thermal interface materials (TIMs) for efficient heat dissipation. However, integrating low hardness, high damping, and superior heat transfer capability into one TIM is highly challenging. Herein, soft, damping, and thermally conductive TIMs are designed and prepared by constructing a honeycomb-board-mimetic boron nitride nanosheet (BNNS) network in a dynamic polyimine via one-step horizontal centrifugal casting. The unique filler network makes the TIMs perform a high through-plane thermal conductivity (> 7.69 W m-1 K-1) and a uniform heat transfer process. Meanwhile, the hierarchical dynamic bonding of the polyimine endows the TIMs with low compressive strength (2.16 MPa at 20% strain) and excellent damping performance (tan δ > ≈0.3 at 10-2-102 Hz). The resulting TIMs also exhibit electrical insulation and remarkable recycling ability. Compared with the commercial ones, the TIMs provide better heat dissipation (4.1 °C) for a high-power 5G base station and less temperature fluctuation (1.8 °C) for an automotive insulated gate bipolar transistor (IGBT) under vibrations. This rational design offers a viable approach to prepare soft and damping TIMs for effective heat dissipation of high-power-density electronic devices under vibrations.

Polymeric Nanocomposites of Boron Nitride Nanosheets for Enhanced Directional or Isotropic Thermal Transport Performance

Polymeric composites with boron nitride nanosheets (BNNs), which are thermally conductive yet electrically insulating, have been pursued for a variety of technological applications, especially those for thermal management in electronic devices and systems. Highlighted in this review are recent advances in the effort to improve in-plane thermal transport performance in polymer/BNNs composites and also the growing research activities aimed at composites of enhanced cross-plane or isotropic thermal conductivity, for which various filler alignment strategies during composite fabrication have been explored. Also highlighted and discussed are some significant challenges and major opportunities for further advances in the development of thermally conductive composite materials and their mechanistic understandings.

Natural Deep Eutectic Solvent-Assisted Construction of Silk Nanofibrils/Boron Nitride Nanosheets Membranes with Enhanced Heat-Dissipating Efficiency

Natural polymer-derived nanofibrils have gained significant interest in diverse fields. However, production of bio-nanofibrils with the hierarchical structures such as fibrillar structures and crystalline features remains a great challenge. Herein, an all-natural strategy for simple, green, and scalable top-down exfoliation silk nanofibrils (SNFs) in novel renewable deep eutectic solvent (DES) composed by amino acids and D-sorbitol is innovatively developed. The DES-exfoliated SNFs with a controllable fibrillar structures and intact crystalline features, novelty preserving the hierarchical structure of natural silk fibers. Owing to the amphiphilic nature, the DES-exfoliated SNFs show excellent capacity of assisting the exfoliation of several 2D-layered materials, i.e., h-BN, MoS2, and WS2. More importantly, the SNFs-assisted dispersion of BNNSs with a concentration of 59.3% can be employed to construct SNFs/BNNSs nanocomposite membranes with excellent mechanical properties (tensile strength of 416.7 MPa, tensile modulus of 3.86 GPa and toughness of 1295.4 KJ·m-3) and thermal conductivity (in-plane thermal conductivity coefficient of 3.84 W·m-1·K-1), enabling it to possess superior cooling efficiency compared with the commercial silicone pad.

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.

Thermal-Rectified Gradient Porous Polymeric Film for Solar-Thermal Regulatory Cooling

Solar-thermal regulation concerning thermal insulation and solar modulation is pivotal for cooling textiles and smart buildings. Nevertheless, a contradiction arises in balancing the demand to prevent external heat infiltration with the efficient dissipation of excess heat from enclosed spaces. Here, a concentration-gradient polymerization strategy is presented for fabricating a gradient porous polymeric film comprising interconnected polymeric microspheres. This method involves establishing an electric field-driven gradient distribution of charged crosslinkers in the precursor solution, followed by subsequent polymerization and freeze-drying processes. The resulting porous film exhibits a significant porosity gradient along its thickness, leading to exceptional unidirectional thermal insulation capabilities with a thermal rectification factor of 21%. The gradient porous film, with its thermal rectification properties, effectively reconciles the conflicting demands of diverse thermal conductivity for cooling unheated and spontaneously heated enclosed spaces. Consequently, the gradient porous film demonstrates remarkable enhancements in solar-thermal management, achieving temperature reductions of 3.0 and 4.1 °C for unheated and spontaneously heated enclosed spaces, respectively, compared to uniform porous films. The developed gradient-structured porous film thus holds promise for the development of thermal-rectified materials tailored to regulate solar-thermal conditions within enclosed environments.

Low thermal contact resistance boron nitride nanosheets composites enabled by interfacial arc-like phonon bridge

Two-dimensional materials with ultrahigh in-plane thermal conductivity are ideal for heat spreader applications but cause significant thermal contact resistance in complex interfaces, limiting their use as thermal interface materials. In this study, we present an interfacial phonon bridge strategy to reduce the thermal contact resistance of boron nitride nanosheets-based composites. By using a low-molecular-weight polymer, we are able to manipulate the alignment of boron nitride nanosheets through sequential stacking and cutting, ultimately achieving flexible thin films with a layer of arc-like structure superimposed on perpendicularly aligned ones. Our results suggest that arc-like structure can act as a phonon bridge to lower the contact resistance by 70% through reducing phonon back-reflection and enhancing phonon coupling efficiency at the boundary. The resulting composites exhibit ultralow thermal contact resistance of 0.059 in2 KW-1, demonstrating effective cooling of fast-charging batteries at a thickness 2-5 times thinner than commercial products.

A Novel Thermal Interface Material Composed of Vertically Aligned Boron Nitride and Graphite Films for Ultrahigh Through-Plane Thermal Conductivity

The relentless drive toward miniaturization in microelectronic devices has sparked an urgent need for materials that offer both high thermal conductivity (TC) and excellent electrical insulation. Thermal interface materials (TIMs) possessing these dual attributes are highly sought after for modern electronics, but achieving such a combination has proven to be a formidable challenge. In this study, a cutting-edge solution is presented by developing boron nitride (BN) and graphite films layered silicone rubber composites with exceptional TC and electrical insulation properties. Through a carefully devised stacking-cutting method, the high orientation degree of both BN and graphite films is successfully preserved, resulting in an unprecedented through-plane TC of 23.7 Wm-1 K-1 and a remarkably low compressive modulus of 4.85 MPa. Furthermore, the exceptional properties of composites, including low thermal resistance and high resilience rate, make them a reliable and durable option for various applications. Practical tests demonstrate their outstanding heat dissipation performance, significantly reducing CPU temperatures in a computer cooling system. This research work unveils the possible upper limit of TC in BN-based TIMs and paves the way for their large-scale practical implementation, particularly in the thermal management of next-generation electronic devices.

Application-Driven High-Thermal-Conductivity Polymer Nanocomposites

Polymer nanocomposites combine the merits of polymer matrices and the unusual effects of nanoscale reinforcements and have been recognized as important members of the material family. Being a fundamental material property, thermal conductivity directly affects the molding and processing of materials as well as the design and performance of devices and systems. Polymer nanocomposites have been used in numerous industrial fields; thus, high demands are placed on the thermal conductivity feature of polymer nanocomposites. In this Perspective, we first provide roadmaps for the development of polymer nanocomposites with isotropic, in-plane, and through-plane high thermal conductivities, demonstrating the great effect of nanoscale reinforcements on thermal conductivity enhancement of polymer nanocomposites. Then the significance of the thermal conductivity of polymer nanocomposites in different application fields, including wearable electronics, thermal interface materials, battery thermal management, dielectric capacitors, electrical equipment, solar thermal energy storage, biomedical applications, carbon dioxide capture, and radiative cooling, are highlighted. In future research, we should continue to focus on methods that can further improve the thermal conductivity of polymer nanocomposites. On the other hand, we should pay more attention to the synergistic improvement of the thermal conductivity and other properties of polymer nanocomposites. Emerging polymer nanocomposites with high thermal conductivity should be based on application-oriented research.

Thermal Rectification in Modularly Designed Bulk Metamaterials

Thermal rectification is a phenomenon of great practical importance where heat transfer is preferential in one direction. Programmable control of heat transfer in 3D space is key to enable thermal rectification at the macroscale but is rarely realized in natural materials or in current existing devices that are constructed at the nano and microscales with high system complexity. Here, modularly designed bulk metamaterials that can break the symmetry of heat transfer from one direction to the other are created, leading to thermal rectification in convergent or divergent states by tuning the metamaterial microstructural design. These thermal metamaterials are microstructured composites made using one material composition, however, they offer sufficient microstructural design freedom to allow tunable local thermal properties for unusual macroscopic heat transfer. The strategy and performance achieved are promising for next-generation thermal management.

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.

Effective Surface Modification of 2D MXene toward Thermal Energy Conversion and Management

Thermal energy management is a crucial aspect of many research developments, such as hybrid and soft electronics, aerospace, and electric vehicles. The selection of materials is of critical importance in these applications to manage thermal energy effectively. From this perspective, MXene, a new type of 2D material, has attracted considerable attention in thermal energy management, including thermal conduction and conversion, owing to its unique electrical and thermal properties. However, tailored surface modification of 2D MXenes is required to meet the application requirements or overcome specific limitations. Herein, a comprehensive review of surface modification of 2D MXenes for thermal energy management is discussed. First, this work discusses the current progress in the surface modification of 2D MXenes, including termination with functional groups, small-molecule organic compound functionalization, and polymer modification and composites. Subsequently, an in situ analysis of surface-modified 2D MXenes is presented. This is followed by an overview of the recent progress in the thermal energy management of 2D MXenes and their composites, such as Joule heating, heat dissipation, thermoelectric energy conversion, and photothermal conversion. Finally, some challenges facing the application of 2D MXenes are discussed, and an outlook on surface-modified 2D MXenes is provided.

Tetris-Style Stacking Process to Tailor the Orientation of Carbon Fiber Scaffolds for Efficient Heat Dissipation

As the miniaturization of electronic devices and complication of electronic packaging, there are growing demands for thermal interfacial materials with enhanced thermal conductivity and the capability to direct the heat toward heat sink for highly efficient heat dissipation. Pitch-based carbon fiber (CF) with ultrahigh axial thermal conductivity and aspect ratios exhibits great potential for developing thermally conductive composites as TIMs. However, it is still hard to fabricate composites with aligned carbon fiber in a general approach to fully utilize its excellent axial thermal conductivity in specific direction. Here, three types of CF scaffolds with different oriented structure were developed via magnetic field-assisted Tetris-style stacking and carbonization process. By regulating the magnetic field direction and initial stacking density, the self-supporting CF scaffolds with horizontally aligned (HCS), diagonally aligned and vertically aligned (VCS) fibers were constructed. After embedding the polydimethylsiloxane (PDMS), the three composites exhibited unique heat transfer properties, and the HCS/PDMS and VCS/PDMS composites presented a high thermal conductivity of 42.18 and 45.01 W m-1 K-1 in fiber alignment direction, respectively, which were about 209 and 224 times higher than that of PDMS. The excellent thermal conductivity is mainly ascribed that the oriented CF scaffolds construct effective phonon transport pathway in the matrix. In addition, fishbone-shaped CF scaffold was also produced by multiple stacking and carbonization process, and the prepared composites exhibited a controlled heat transfer path, which can allow more versatility in the design of thermal management system.

Engineering of the Core-Shell Boron Nitride@Nitrogen-Doped Carbon Heterogeneous Interface for Efficient Heat Dissipation and Electromagnetic Wave Absorption

The effective integration of multiple functions into electromagnetic wave-absorbing (EWA) materials is the future development direction but remains a huge challenge. A rational selection of components and the design of structures can make the material have excellent EWA performance and heat dissipation. Herein, the core-shell structured boron nitride@nitrogen-doped carbon (BN@NC) is prepared by using waterborne polyurethane (WPU) as the carbon source via a facile pyrolysis treatment process, where NC is used as the conductive loss shell, and BN serves as an impedance matching core and dominant heat transfer media. As a result, the BN@NC-900 filled with paraffin wax yields a minimum reflection loss of -42.2 dB at 2.2 mm and an effective absorbing bandwidth of 4.48 GHz at 1.8 mm, and its thermal conductivity reaches up to 0.92 W/m·K in epoxy resin. Most importantly, flexible BN@NC/WPU films are prepared and simultaneously achieve the dual-functional capability of efficiently dissipating heat and electromagnetic waves (-50.0 dB). Besides, an attractive multiband absorption feature (>99%) from C to Ku bands is realized and a strong absorbing over -27.0 dB at the S band (2.88 GHz) is even achieved. This study may pave a new route for the rational design of multifunctional EWA materials.

Enhanced Thermal Conductivity of Epoxy Composites by Introducing 1D AlN Whiskers and Constructing Directionally Aligned 3D AlN Filler Skeletons

With the miniaturization of current electronic products, ceramic/polymer composites with excellent thermal conductivity have attracted increasing attention. For regular ceramic particles as fillers, it is necessary to achieve the highest filling fraction to obtain high thermal conductivity, yet leading to higher production cost and reduced mechanical properties. In this paper, AlN whiskers with a high aspect ratio were successfully prepared using a modified direct nitriding method, which was further paired with AlN particles as fillers to prepare the AlN/epoxy composites. It is indicated that AlN whiskers could form bridging links between AlN particles, which favored the establishment of thermal pathways inside the polymer matrix. On this basis, we constructed the 3D AlN skeletons as a thermal conductivity pathway by the freeze-casting method, which could further enhance the thermal conductivity of the composites. The synergistic enhancement effect of 1D AlN whiskers and directional filler skeletons on the composite thermal conductivity was further demonstrated by the actual heat transfer process and finite element simulations. More significantly, the experimental results showed that the addition of one-dimensional fillers could also effectively improve the thermal stability and mechanical properties of the composites, which was beneficial for preparing high-performance TIMs.