Highly Thermally Conductive Yet Electrically Insulating Polymer/Boron Nitride Nanosheets Nanocomposite Films for Improved Thermal Management Capability

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

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

A stretchable laminated GNRs/BNNSs nanocomposite with high electrical and thermal conductivity

The rapid development of modern electronics has accelerated the demand for stretchable components with high thermal management capability because increasing the power density and miniaturization of electronic devices generate greater heat. However, stretchable electronics with enhanced heat dissipation have been rarely reported. In this study, a stretchable laminated nanocomposite-based conductor with both robust electric conductivity and enhanced thermal management capability was fabricated. With the optimized GNRs and BNNS contents, this conductor exhibited a thermal conductivity enhancement of 266%, leading to a decrease in the working temperature from 57.4 °C to 29.2 °C. Even under 100% strain, the fluctuation of the equilibrium operational temperature was within 10%. Moreover, the conductor showed outstanding electric performance under 200% strain with an R/R₀ value of 1.46. Whether stretched and tested in a Moebius-belt shape or under hard-environmental conditions such as in seawater, crude oil, and even integrated in a wireless charging circuit, the significant reliability of this conductor was recorded. Thus, our results are promising to provide a practical approach for the fabrication of stretchable electronic devices working in high temperature environments associated with extreme thermal stresses and under extreme circumstances such as sea rescue operations and marine oil pollution remediation.

Highly Thermally Conductive Polymer Composite Originated from Assembly of Boron Nitride at an Oil-Water Interface

Thermally conductive polymer packaging material is of great significance for the thermal management of electronics. Inorganic thermally conductive fillers have been demonstrated as a convenient approach to achieve this goal by sacrificing the lightweight and processability of the polymer. To address this problem, an effective 3D boron nitride (BN) network was constructed as a heat conduction pathway in a polystyrene (PS) matrix based on an oil-water interface assembly in this work. Styrene oil droplets were stabilized by BN sheets in the water phase to form Pickering emulsions, and then in situ polymerization was trigged to synthesize PS microspheres with ultrathin BN layer-covered surfaces (PS@BN microspheres). Composite substrates were fabricated through hot-compressing the PS@BN microspheres to form BN networks based on the original microsphere template. Benefited from the network structure, the maximum thermal conductivity of the composite substrate reached 0.94 W/mK at 33.3 wt % BN, which is 626% folds of that of pure PS. It was also demonstrated that the storage modulus and thermal stability of the composite substrate were dramatically improved by the BN network. The reported composite substrate and its fabrication strategy are promising in the development of thermal management of electronics.

Insight into the Directional Thermal Transport of Hexagonal Boron Nitride Composites

Ideal dielectric materials for microelectronic devices should have high directionally tailored thermoconductivity with low dielectric constant and loss. Hexagonal boron nitride (hBN) with excellent thermal and dielectric properties shows a promise for the fabrication of thermoconductive dielectric polymer composites. Herein, a simple method for the fabrication of lightweight polymer/hBN composites with high directionally tailored thermoconductivity and excellent dielectric properties is presented. The solid polymer/hBN composites are manufactured by melt-compounding and injection molding. The porous composites are successfully manufactured in an injection molding process through supercritical fluid (SCF) foaming. X-ray tomography provides direct visualization of the internal microstructure and hBN orientation, leading to an in-depth understanding of the directionally dependent thermoconductivity of the polymer/hBN composite. Shear-induced orientation of hBN platelets in the solid HDPE/hBN composites leads to a significant anisotropic thermal conductivity. The solid HDPE/23.2 vol % hBN composites show an in-plane thermoconductivity as high as 10.1 W m^-1 K^-1, whereas the through-plane thermoconductivity is limited to 0.28 W m^-1 K^-1. However, the generation of a porous structure via SCF foaming imparts in situ exfoliation, random orientation, and interconnectivity of hBN platelets within the polymer matrix. This results in highly isotropic thermoconductivity with higher bulk thermal conductivity in the lightweight porous composites as compared to their solid counterparts. Furthermore, the electrically insulating composites developed in this study exhibit low dielectric constant and ultralow dielectric loss. Thus, this study presents a simple fabrication method to develop lightweight dielectric materials with tailored thermal conductivity for modern electronics.

Millefeuille-Inspired Thermally Conductive Polymer Nanocomposites with Overlapping BN Nanosheets for Thermal Management Applications

Increasing power density makes modern electronic devices and power equipment generate excess heat, which greatly restricts the applications of polymeric materials because of their poor thermal conductivity. In the present work, inspired by the structure and production process of millefeuille cakes, we show that electrostatic spraying of boron nitride nanosheets (BNNSs) onto electrospun poly(vinyl alcohol) (PVA) nanofibers can produce highly thermally conductive, electrically insulating, flexible, and lightweight nanocomposites via a scalable method of building a multilayer PVA/BNNS nanonetwork structure. The PVA/BNNS nanocomposites exhibit an ultrahigh in-plane thermal conductivity of 21.4 W/(m·K) at 22.2 vol % BNNS addition, realized by an orientated BNNS network structure with overlapping interconnections. The BNNS networks exhibit low thermal resistance and interfacial heat scattering between BNNSs. Moreover, for heat dissipation applications, the nanocomposites with an overlapping BNNS network show higher efficiency in dissipating hot spots than randomly dispersed BNNS or directly hot-pressed BNNS composites. These PVA/BNNS nanocomposites can be used as high-performance lateral heat spreaders in next-generation thermal management systems.

Flexible Films for Smart Thermal Management: Influence of Structure Construction of a Two-Dimensional Graphene Network on Active Heat Dissipation Response Behavior

In recent years, the development and wide application of micro-electronic technology brings forward high demands for active thermal management systems. However, such systems are not only costly, but also usually tethered, and need constant power to operate. To avoid such a limitation, smart thermal management systems have been developed to achieve active thermal management. Here, inspired by the temperature control principle of a butterfly, a shape memory polymer was used to endow the thermally conductive graphene-polymer hybrid film with intelligence. As the device temperature reaches 60 °C, the bud-shaped hybrid film started to bloom, which is a visually active heat dissipation process. As a result, this active process promoted the thermal management capacity of the hybrid film and increased the temperature-raising time of the light-emitting diode. Through the construction of a bilayer structure, the transmission channel for phonon transfer was optimized, which lead the hybrid film to attain a remarkable thermal conductivity of 21.83 W·m^-1·K^-1 with 30 wt % graphene. This graphene-polymer hybrid film shows potential application in the smart thermal management field.

3D Vertically Aligned BNNS Network with Long-Range Continuous Channels for Achieving a Highly Thermally Conductive Composite

Construction of a three-dimensional (3D) vertically aligned filler network in a polymer matrix has been believed to be an effective method to attain a large through-plane thermal conductivity enhancement at relatively low filler loading. However, it is still a challenge to construct a vertically aligned filler network composed of many long-range continuous pore channels in a polymer matrix for the high-flux heat-conduction. To address this problem, herein, nanofibrillated cellulose (NFCs) assisted unidirectional freeze-drying of a boron nitride nanosheets (BNNSs) slurry was used to prepare a novel epoxy composite containing a 3D vertically aligned BNNS network with long-range continuous pore channels. The vertically aligned and nacre-mimetic channels make the composite possess a high through-plane thermal conductivity of 1.56 W m^-1 K^-1 at an extremely low BNNSs loading of 4.4 vol %, and a significant thermal conductivity enhancement efficiency of 167.3 per 1 vol % filler. Therefore, we think this work is expected to give a significant insight into the preparation of polymer composite with high heat-conduction efficiency to address the heat dissipation of modern electronics.

Optimization of Boron Nitride Sphere Loading in Epoxy: Enhanced Thermal Conductivity and Excellent Electrical Insulation

Thermally conductive but electrically insulating materials are highly desirable for thermal management applications in electrical encapsulation and future energy fields, for instance, superconducting magnet insulation in nuclear fusion systems. However, the traditional approaches usually suffer from inefficient and anisotropic enhancement of thermal conductivity or deterioration of electrical insulating property. In this study, using boron nitride sphere (BNS) agglomerated by boron nitride (BN) sheets as fillers, we fabricate a series of epoxy/BNS composites by a new approach, namely gravity-mix, and realize the controllable BNS loading fractions in the wide range of 5-40 wt%. The composites exhibited thermal conductivity of about 765% and enhancement at BNS loading of 40 wt%. The thermal conductivity up to 0.84 W·m^-1·K^-1 at 77 K and 1.66 W·m^-1·K^-1 at 298 K was observed in preservation of a higher dielectric constant and a lower dielectric loss, as expected, because boron nitride is a naturally dielectric material. It is worth noting that the thermal property was almost isotropous on account of the spherical structure of BNS in epoxy. Meanwhile, the reduction of the coefficient of thermal expansion (CTE) was largely reduced, by up to 42.5% at a temperature range of 77-298 K.

Aerogel Perfusion-Prepared h-BN/CNF Composite Film with Multiple Thermally Conductive Pathways and High Thermal Conductivity

Hexagonal boron nitride (h-BN)-based heat-spreading materials have drawn considerable attention in electronic diaphragm and packaging fields because of their high thermal conductivity and desired electrical insulation properties. However, the traditional approach to fabricate thermally conductive composites usually suffers from low thermal conductivity, and cannot meet the requirement of thermal management. In this work, novel h-BN/cellulose-nano fiber (CNF) composite films with excellent thermal conductivity in through plane and electrical insulation properties are fabricated via an innovative process, i.e., the perfusion of h-BN into porous three dimensional (3D) CNF aerogel skeleton to form the h-BN thermally conductive pathways by filling the CNF aerogel voids. When at an h-BN loading of 9.51 vol %, the thermal conductivity of h-BN/CNF aerogel perfusion composite film is 1.488 W·m⁻¹·K⁻¹ at through plane, an increase by 260.3%. The volume resistivity is 3.83 × 10¹⁴ Ω·cm, superior to that of synthetic polymer materials (about 10⁹~10¹³ Ω·cm). Therefore, the resulting h-BN/CNF film is very promising to replace the traditional synthetic polymer materials for a broad spectrum of applications, including the field of electronics.

High-Performance and Rapid-Response Electrical Heaters Based on Ultraflexible, Heat-Resistant, and Mechanically Strong Aramid Nanofiber/Ag Nanowire Nanocomposite Papers

High-performance and rapid response electrical heaters with ultraflexibility, superior heat resistance, and mechanical properties are highly desirable for the development of wearable devices, artificial intelligence, and high-performance heating systems in areas such as aerospace and the military. Herein, a facile and efficient two-step vacuum-assisted filtration followed by hot-pressing approach is presented to fabricate versatile electrical heaters based on the high-performance aramid nanofibers (ANFs) and highly conductive Ag nanowires (AgNWs). The resultant ANF/AgNW nanocomposite papers present ultraflexibility, extremely low sheet resistance (minimum R_s of 0.12 Ω/sq), and outstanding heat resistance (thermal degradation temperature above 500 °C) and mechanical properties (tensile strength of 285.7 MPa, tensile modulus of 6.51 GPa with a AgNW area fraction of 0.4 g/m²), benefiting from the partial embedding of AgNWs into the ANF substrate and the extensive hydrogen-bonding interactions. Moreover, the ANF/AgNW nanocomposite paper-based electrical heaters exhibit satisfyingly high heating temperatures (up to ∼200 °C) with rapid response time (10-30 s) at low AgNW area fractions and supplied voltages (0.5-5 V) and possess sufficient heating reliability, stability, and repeatability during the long-term and repeated heating and cooling cycles. Fully functional applications of the ANF/AgNW nanocomposite paper-based electrical heaters are demonstrated, indicating their excellent potential for emerging electronic applications such as wearable devices, artificial intelligence, and high-performance heating systems.

Interfacial Characteristics of Boron Nitride Nanosheet/Epoxy Resin Nanocomposites: A Molecular Dynamics Simulation

The interface between nanofillers and matrix plays a key role in determining the properties of nanocomposites, but the interfacial characteristics of nanocomposites such as molecular structure and interaction strength are not fully understood yet. In this work, the interfacial features of a typical nanocomposite, namely epoxy resin (EP) filled with boron nitride nanosheet (BNNS) are investigated by utilizing molecular dynamics simulation, and the effect of surface functionalization is analyzed. The radial distribution density (RDD) and interfacial binding energy (IBE) are used to explore the structure and bonding strength of nanocomposites interface. Besides, the interface compatibility and molecular chain mobility (MCM) of BNNS/EP nanocomposites are analyzed by cohesive energy density (CED), free volume fraction (FFV), and radial mean square displacement (RMSD). The results indicate that the interface region of BNNS/EP is composed of three regions including compact region, buffer region, and normal region. The structure at the interfacial region of nanocomposite is more compact, and the chain mobility is significantly lower than that of the EP away from the interface. Moreover, the interfacial interaction strength and compatibility increase with the functional density of BNNS functionalized by CH3–(CH2)4–O– radicals. These results adequately illustrate interfacial characteristics of nanocomposites from atomic level.

Highly Thermally Conductive Fluorinated Graphene Films with Superior Electrical Insulation and Mechanical Flexibility

Graphene-based heat-spreading films have captured high attention in academic study and commercial applications because of their extremely high thermal conductivity and desired flexibility. However, the electrical conductivity limits their utilizations in many electronic fields. Herein, to address this problem, fluorinated graphene (F-graphene) that is exfoliated from commercial fluorinated graphite was first used to prepare the flexible free-standing composite film via vacuum filtration of uniform poly(vinyl alcohol)-assisted F-graphene suspension. The well-organized alignment of F-graphene lamellas makes the composite film show an ultrahigh in-plane thermal conductivity of 61.3 W m^-1 K^-1 at 93 wt % F-graphene. Despite at such high filler loading, the fabricated F-graphene film still possesses a superior electrical insulation property. Therefore, these results suggest that F-graphene, as the novel thermally conductive filler, demonstrates fascinating characters in the preparation of a thermally conductive yet electrically insulating nanocomposite.

Melamine Foam-Supported Form-Stable Phase Change Materials with Simultaneous Thermal Energy Storage and Shape Memory Properties for Thermal Management of Electronic Devices

Paraffin wax (PW) is widely used as a phase change material (PCM) in the thermal energy storage field, whereas the leakage and strong rigidity of PW have hindered its practical applications. In this work, binary melamine foam (MF)/PW blends with simultaneous thermal energy storage and shape memory properties were prepared through vacuum impregnation. Herein, PW performs as a latent heat storage material and as a switching phase for shape fixation and MF serves as a supporting material to prevent the leakage and as a permanent phase for shape recovery. Due to the light weight and super-elasticity of MF, the MF/PW PCMs possess not only good encapsulation ability and a high latent heat, but also excellent shape-fixing and recovery properties (shape-fixing and recovery ratios are about 100%). Besides, the MF/PW PCMs can be fabricated into arbitrary shapes using MF as a template, and they exhibit excellent shape memory cyclic performance and thermal reliability. A temperature-sensitive and temperature-controlled deployable panel is further established, which can be installed in the electronic device and used for temperature protection. With high thermal energy storage capability, excellent shape memory properties, shape designability, and stable cycling reliability, this multifunctional MF/PW PCM shows a promising application in thermal energy management systems.

The combustion synthesis of highly crystalline boron nitride nanosheets and their application in thermoconductive polymeric composites

We report scalable fabrication of single crystalline BNNS by a magnesiothermic reduction combustion synthesis method and their applications in thermoconductive polymeric composites.