Highly Thermally Conductive and Superior Electrical Insulation Polymer Composites via In Situ Thermal Expansion of Expanded Graphite and In Situ Oxidation of Aluminum Nanoflakes

Role of four-phonon processes in thermal conductivity of two-dimensional materials and thermal-transport enhancement arising from interconnected nanofiller networks in polymer/nanofiller composites

Recent research has shed light on the importance of four-phonon scattering processes in the thermal conductivity (k) of 2D materials. The inclusion of 4 phonon scattering processes from first-principles has been shown to lead to a thermal conductivity of ∼1290 W m-1 K-1 in graphene at 300 K, significantly lower than the values predicted to be in excess of 4000 W m-1 K-1 based only on 3 phonon scattering processes. Four phonon processes are shown to be most significant for flexural ZA phonon modes, where the reflection symmetry selection rule (RSSR) is less restrictive for 4-phonon than 3-phonon scattering processes. This combined with the low frequencies of ZA phonon modes, leading to high populations, leads to higher 4-phonon than 3-phonon scattering of low frequency ZA phonon modes in graphene at 300 K. In this review, the role of parameters such as atomic structure, phonon dispersion and temperature on 4-phonon scattering processes in a wide range of 2D materials is reviewed. Materials such as graphene nanoplatelets (GnPs) have been extensively investigated for enhancement of the thermal conductivity of polymer composites. However, such enhancement is limited by the poor interfacial thermal conductance between the polymer and filler material. Interconnected filler networks overcome this issue through highly efficient continuous percolative heat transfer paths throughout the composite. Such 3D networks have been shown to enable ultra-high polymer thermal conductivities, approaching ∼100 W m-1 K-1, and even exceeding those of several metals. In this review, different techniques used to achieve such interconnected 3D filler networks, namely, aerogels, foams, ice-templating, expanded graphite, hot pressing of filler coated polymer particles, the synergistic effect between multiple fillers, and the stitching of filler sheets, are discussed and their impact on thermal conductivity enhancement are presented.

Enhancement of Mechanical Properties of High-Thermal-Conductivity Composites Comprising Boron Nitride and Poly(methyl methacrylate) Resin through Material Design Utilizing Hansen Solubility Parameters

Materials for heat sinks in automotive heat dissipation systems must demonstrate both high thermal conductivity and stress resistance during assembly. This research proposes a composite material, comprised of thermally conductive ceramic fillers and matrix resins, as a suitable option for such application. The strategy for designing this material interface is directed with Hansen solubility parameters (HSP). A composite material featuring a honeycomb-like structure made of poly(methyl methacrylate) (PMMA) and boron nitride (BN) particles was successfully fabricated through press molding. This yielded a continuous BN network exhibiting high thermal conductivity and moderate mechanical strength. The HSP evaluation led to the suggestion of introducing highly polar functional groups into the matrix resin to enhance the affinity between PMMA resin and BN fillers. In line with this recommendation, a nitrile (CN) group─a highly polar group─was introduced to PMMA (CN-PMMA), significantly enhancing the composite's maximum bending stress without noticeably degrading other properties. Surface HSP evaluation through contact angle measurements revealed an "interface enrichment effect", with the CN groups concentrating at the resin-filler interface and effectively interacting with the surface functional groups on the BN particles, which resulted in an increase in the maximum bending stress. These findings emphasize the advantage of employing HSP methodologies in designing high-performance composite materials.

Constructing Hierarchical Polymer Nanocomposites with Strongly Enhanced Thermal Conductivity

The rapid advancement of communication technology has substantially increased the demand for advanced electronic packaging materials with high thermal conductivity and outstanding electrical insulation properties. In this study, we design polyvinyl alcohol/polydopamine-modified boron nitride nanosheet (PVA/BNNS@PDA) nanocomposites with hierarchical structures by combining electrospinning, vacuum filtration deposition, and hot pressing. The modified BNNS@PDA improves the interaction between the filler and the polymer matrix while reducing the interfacial thermal resistance, resulting in superior thermal conductivity, excellent insulation, and perfect flexibility. The PVA/BNNS@PDA nanocomposites possess an ultrahigh in-plane thermal conductivity of 16.6 W/(m·K) at 35.54 wt % BNNS@PDA content. Even after 2000 folds, the nanocomposites do not undergo any crack, showing their ultrahigh thermal conductivity behavior. Furthermore, the nanocomposites exhibit a volume resistivity above 1014 Ω·cm, which is well above the standard for insulating materials. Based on these results, this work provides a novel method to produce nanocomposites with high thermal conductivity, offering a new perspective to design advanced thermal management materials.

Fabricating Well-Dispersed Poly(Vinylidene Fluoride)/Expanded Graphite Composites with High Thermal Conductivity by Melt Mixing with Maleic Anhydride Directly

Maleic anhydride (MA) is introduced to fabricate poly(vinylidene fluoride)/expanded graphite (PVDF/EG) composites via one-step melt mixing. SEM micrographs and WAXD results have demonstrated that the addition of MA helps to exfoliate and disperse the EG well in the PVDF matrix by promoting the mobility of PVDF molecular chains and enhancing the interfacial adhesion between the EG layers and the PVDF. Thus, much higher thermal conductivities are obtained for the PVDF/MA/EG composites compared to the PVDF/EG composites that are lacking MA. For instance, The PVDF/MA/EG composite prepared with a mass ratio of 93:14:7 exhibits a high thermal conductivity of up to 0.73 W/mK. It is 32.7% higher than the thermal conductivity of the PVDF/EG composite that is prepared with a mass ratio of 93:7. Moreover, the introduction of MA leads to an increased melting peak temperature and crystallinity due to an increased nucleation site provided by the uniformly dispersed EG in the PVDF matrix. This study provides an efficient preparation method for PVDF/EG composites with a high thermal conductivity.

Significantly enhanced thermally conductive epoxy composite composed of caterpillar-like structured expanded graphite/ boron nitride nanotubes

The trend toward miniaturization, integration and multifunctionality of modern electronics has led to a rapid increase in power density, which makes heat dissipation a critical issue. Despite the great potential of graphite-related nanocomposites in dissipating excess heat to ensure high efficiency and long lifetime of electronic devices, the practical application of these composites is limited by the ultra-low vertical thermal conductivity due to the interfacial thermal resistance between graphite layers. Here, a caterpillar-like hybrid filler was fabricated by the in situ intercalation of boron nitride nanotubes (BNNTs) between expanded graphite (EG) layers based on chemical vapor deposition technology. Owing to the optimized interfacial thermal resistance by forming covalent C-N bonding at the interface of EG and BNNT, the through-plane thermal conductivity of epoxy-based nanocomposites can be up to 5.18 Wm−1 K−1. In addition, the composite possessed electromagnetic interference shielding performance of 33.34 dB while maintaining electrical insulation due to the hierarchical structure. This work provided a new strategy for fabricating polymer-based composites with excellent through-plane thermal conductivity in thermal management applications.

Pressure-Induced Self-Interlocked Structures for Expanded Graphite Composite Papers Achieving Prominent EMI Shielding Effectiveness and Outstanding Thermal Conductivities

High-performance films via layer-by-layer assembly of two-dimensional (2D) materials would provide all possibilities for the development of modern integrated electronics. However, the stacked structure between nanosheets and large-scale fabrication still remain a great challenge. Herein, Fe₃O₄/expanded graphite (EG) papers are fabricated via in situ oxidation of ferrocene onto EG nanosheets, followed by a continuous roll-in process. Upon mechanical compaction, the self-interlocked structures driven by close overlapping and hooking of nanosheets in Fe₃O₄/EG (FG) composites remarkably facilitate the construction of phonon and electron transmission channels and improve mechanical strength. FG papers exhibit prominent shielding effectiveness (67.1 dB at ∼100 μm) with enhanced absorptivity (∼0.1, surpassing lots of conductive film materials), stemming from the synergistic effect of electrical and magnetic properties. Also, the electromagnetic interference (EMI) shielding performance shows prominent reliability after bending (2000 cycles) and ultrasonic treatment (30 min). The corresponding tensile strength reaches 35.8 MPa; meanwhile, the corresponding in-plane thermal conductivity coefficient is as high as 191.7 W/(m·K), which can rapidly and efficiently accelerate heat dissipation. In particular, FG papers also reveal rapid response, controllable, and highly stable Joule heating performance and present promising prospects in the fields of radiation-proof clothing, flexible heaters, portable wearable devices, and aerospace.

Millefeuille-Inspired Thermal Interface Materials based on Double Self-Assembly Technique for Efficient Microelectronic Cooling and Electromagnetic Interference Shielding

Owing to the increasing power density of miniaturized and high-frequency electronic devices, flexible thermal interface materials (TIMs) with the electromagnetic interference (EMI) shielding property are in urgent demand to maintain the system performance and reliability. Recently, carbon-based TIMs receive considerable attention due to the ultrahigh intrinsic thermal conductivity (TC). However, the large-scale production of such TIMs is restricted by some technical difficulties, such as production-induced defects of graphite sheets, poor microstructure architecture within the matrix, and nonnegligible interfacial thermal resistance result from the strong phono scattering. In this work, inspired by the structure and production process of millefeuille cakes, a unique double self-assembly strategy for fabricating ultrahigh thermal conductive TIMs with superior EMI shielding performance is demonstrated. The percolating and oriented multilayered microstructure enables the TIM to exhibit an ultrahigh in-plane TC of 233.67 W m^-1 K^-1 together with an outstanding EMI shielding effectiveness of 79.0 dB (at 12.4 GHz). In the TIM evaluation system, a nearly 45 °C decrease is obtained by this TIM when compared to the commercial material. The obtained TIM achieves the desired balance between thermal conduction and EMI shielding performance, indicating broad prospects in the fields of military applications and next-generation thermal management systems.

Highly Enhancing Electrical, Thermal, and Mechanical Properties of Polypropylene/Graphite Intercalation Compound Composites by In Situ Expansion during Melt Mixing

Polypropylene/graphite intercalation compound (PP/GIC) composites are prepared via melt mixing at three different temperatures (180, 200, and 220 °C). The dispersion of GICs in the composites is clearly improved due to the increased interlamellar spacing caused by in situ expansion of GICs at higher temperatures, which facilitates the intercalation of PP molecular chains into the GIC galleries. As a result, the PP/GIC composite with 10 wt% GICs prepared at 220 °C (PG220) presents a dielectric constant of about 1.3 × 10⁸ at 10³ Hz, which is about six orders higher than that of the composite prepared at 180 °C (PG180). Moreover, the thermal conductivity of the PG220 sample (0.63 Wm^-1K^-1) is 61.5% higher than that of the PG180 sample. The well-dispersed GICs accelerates the crystallization of PP by increasing the nucleation point and enhances the thermal stability of the composites. The PG220 sample shows a Young's modulus that is about 21.2% higher than that of the PG180 samples. The results provide an efficient approach for fabricating polymer/GIC composites without complex exfoliation and dispersion processes.