Electrically and thermally conductive underwater acoustically absorptive graphene/rubber nanocomposites for multifunctional applications

Research progress and future perspectives on electromagnetic wave absorption of fibrous materials

Electromagnetic waves have caused great harm to military safety, high-frequency electronic components, and precision instruments, and so forth, which urgently requires the development of lightweight, high-efficiency, broadband electromagnetic waves (EMW) absorbing materials for protection. As the basic fibrous materials, carbon fibers (CFs) and SiC fibers (SiCf) have been widely applied in EMW absorption due to their intrinsic characteristics of low density, high mechanical properties, high conductivity, and dielectric loss mechanism. Nevertheless, it has remained a great challenge to develop lightweight EMW-absorbing fibrous materials with strong absorption capability and broad frequency range. In this review, the fundamental electromagnetic attenuation mechanisms are firstly introduced. Furthermore, the preparation, structure, morphology, and absorbing performance of CFs and SiCf-based EMW absorbing composites are summarized. In addition, prospective research opportunities are highlighted toward the development of fibrous absorbing materials with the excellent absorption performance.

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

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

Structural Design and Fabrication of Multifunctional Nanocarbon Materials for Extreme Environmental Applications

Extreme environments represent numerous harsh environmental conditions, such as temperature, pressure, corrosion, and radiation. The tolerance of applications in extreme environments exemplifies significant challenges to both materials and their structures. Given the superior mechanical strength, electrical conductivity, thermal stability, and chemical stability of nanocarbon materials, such as carbon nanotubes (CNTs) and graphene, they are widely investigated as base materials for extreme environmental applications and have shown numerous breakthroughs in the fields of wide-temperature structural-material construction, low-temperature energy storage, underwater sensing, and electronics operated at high temperatures. Here, the critical aspects of structural design and fabrication of nanocarbon materials for extreme environments are reviewed, including a description of the underlying mechanism supporting the performance of nanocarbon materials against extreme environments, the principles of structural design of nanocarbon materials for the optimization of extreme environmental performances, and the fabrication processes developed for the realization of specific extreme environmental applications. Finally, perspectives on how CNTs and graphene can further contribute to the development of extreme environmental applications are presented.

Optimized Properties in Multifunctional Polyphenylene Sulfide Composites via Graphene Nanosheets/Boron Nitride Nanosheets Dual Segregated Structure under High Pressure

The practical application of polymer composites in the electronic and communications industries often requires multi-properties, such as high thermal conductivity (TC), efficient electromagnetic interference (EMI) shielding ability with low electrical conductivity, superior tribological performance, reliable thermal stability and excellent mechanical properties. However, the integration of these mutually exclusive properties is still a challenge, ascribed to their different requirement on the incorporated nanofillers, composite microstructure as well as processing process. Herein, a well-designed boron nitride nanosheet (BN)/graphene nanosheet (GNP)/polyphenylene sulfide (PPS) composite with a dual-segregated structure is fabricated via high-pressure molding. Rather than homogenous mixing of the hybrid fillers, GNP is first coated on PPS particles and followed by encapsulating the conductive GNP layers with insulating BN, forming a BN shell-GNP layer-PPS core composite particles. After hot-pressing, a dual segregated structure is constructed, in which GNP and BN are distinctly separated and arranged in the interfaces of PPS, which on the one hand gives rise to high thermal conductivity, and on the other hand, the aggregated BN layer can act as an "isolation belt" to effectively reduce the electronic transmission. Impressively, high-pressure is loaded and it has a more profound effect on the EMI shielding and thermal conductive properties of PPS composites with a segregated structure than that with homogenous mixed-structure composites. Intriguingly, the synergetic enhancement effect of BN and GNP on both thermal conductive performance and EMI shielding is stimulated by high pressure. Consequently, PPS composites with 30 wt% GNP and 10 wt% BN hot-pressed under 600 MPa present the most superior comprehensive properties with a high TC of 6.4 W/m/K, outstanding EMI SE as high as 70 dB, marvelous tribological performance, reliable thermal stability and satisfactory mechanical properties, which make it promising for application in miniaturized electronic devices in complex environments.

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

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

Bio-Based Eucommia ulmoides Gum Composites with High Electromagnetic Interference Shielding Performance

Herein, high-performance electromagnetic interference (EMI) shielding bio-based composites were prepared by using EUG (Eucommia ulmoides gum) with a crystalline structure as the matrix and carbon nanotube (CNT)/graphene nanoplatelet (GNP) hybrids as the conductive fillers. The morphology of the CNT/GNP hybrids in the CNT/GNP/EUG composites showed the uniform distribution of CNTs and GNPs in EUG, forming a denser filler network, which afforded improved conductivity and EMI shielding effect compared with pure EUG. Accordingly, EMI shielding effectiveness values of the CNT/GNP/EUG composites reached 42 dB in the X-band frequency range, meeting the EMI shielding requirements for commercial products. Electromagnetic waves were mainly absorbed via conduction losses, multiple reflections from interfaces and interfacial dipole relaxation losses. Moreover, the CNT/GNP/EUG composites exhibited attractive mechanical properties and high thermal stability. The combination of excellent EMI shielding performance and attractive mechanical properties render the as-prepared CNT/GNP/EUG composites attractive candidates for various applications.

Fabrication of graphene-based porous materials: traditional and emerging approaches

The anisotropic nature of ‘graphenic’ nanosheets enables them to form stable three-dimensional porous materials. The use of these porous structures has been explored in several applications including electronics and batteries, environmental remediation, energy storage, sensors, catalysis, tissue engineering, and many more. As method of fabrication greatly influences the final pore architecture, and chemical and mechanical characteristics and performance of these porous materials, it is essential to identify and address the correlation between property and function. In this review, we report detailed analyses of the different methods of fabricating porous graphene-based structures – with a focus on graphene oxide as the base material – and relate these with the resultant morphologies, mechanical responses, and common applications of use. We discuss the feasibility of the synthesis approaches and relate the GO concentrations used in each methodology against their corresponding pore sizes to identify the areas not explored to date.

Due to their anisotropic nature, graphene nanosheets can be used to form 3-dimensional porous materials using template-free and template-directed methodologies. These fabrication strategies are found to influence the properties of the final structure.

Enhanced Sound Absorption Properties of Ceramics with Graphene Oxide Composites

Noise pollution is acknowledged as the main environmental problem and is as harmful to human physical and mental health as water and air pollution. However, the acoustic properties of traditional sound absorption materials in low frequency ranges still need to be improved. Herein, the low-frequency sound absorption coefficient of porous ceramics was further improved by coating a graphene oxide (GO) and styrene-butadiene rubber (SBR) composite film inside the porous ceramics. The improved sound absorption coefficient of the porous composite reached 30.4% in the range of 200-800 Hz, which is attributed to the enhancement of the thermal viscous effect and the extension of the dissipation mechanism. Predictably, designing the morphology of three-dimensional interconnected porous structures on the microscale is comparatively useful for developing a porous sound absorbing material effective in middle- and low-frequency noise.

EMI Shielding Nanocomposite Laminates with High Temperature Resistance, Hydrophobicity and Anticorrosion Properties

High-performance multifunctional EMI shielding composite fabricated by low-cost method is increasingly required. Herein, novel EMI shielding nanocomposite laminates, consisting of composite prepreg of carbon fiber/epoxy resin/carbon nanotube film, were manufactured by facile electric heating of carbon nanotube film. The results indicated that composite with excellent specific shielding effectiveness of 0.07 dB/μm, 47 dB cm³/g and metamaterial properties can be designed by composite prepreg, and the primary shielding mechanism of it was reflection loss, along with interface polarization loss and conductive loss, which was superior to lots of shielding materials including carbon nanotube-based, carbon black-based, carbon nanofiber-based and graphene-based materials reported previously. Meanwhile, highly required excellent properties, including the thermostability with initial decomposition temperature up to 300 °C, hydrophobicity over contact angle of 115°, corrosion resistance of the composite with metal-free modification, and function as structural laminate compared with previous studies were demonstrated, which suggested tremendous potentials of the multifunctional EMI shielding composites in harsh environment.

The recovery of nano-sized carbon black filler structure and its contribution to stress recovery in rubber nanocomposites

The hierarchical structural evolution of natural rubber (NR) filled with different contents of nanoscale carbon black (CB) (10 phr-CB10 and 50 phr-CB50) after first loading and recovering for different times was investigated by X-ray nano-CT, wide-angle X-ray scattering (WAXS) and solid state NMR techniques. The CB filler structures as captured by X-ray nano-CT recover gradually with increasing recovering time, but the filler network with different CB contents shows dramatically different structure evolution. For CB10, limited by the filling content, CB particles mainly induces a hydrodynamic effect in spite of deformation or recovering. For CB50, the CB filler forms a 3D connected network, partially destructed during deformation, and the destructed part can be partially recovered during recovery. This suggests that the connected CB filler structure mainly acts as a network reinforcement, whereas the destructed part can induce a hydrodynamic effect. The different effects induced by different CB filling contents are also reflected by the NR matrix, which is reflected by the onset strains εc of strain-induced crystallization (SIC) of NR as captured by WAXS. For CB10, εc remains almost constant, i.e. εc = ca. 1.49, while that of NR with CB50 slightly decreases from initial ca. 1.12 to 0.96 with increasing recovering time up to 50 h. Also, the bound rubber fraction and entangled rubber network remain unchanged after deformation and under different recovery time as detected by the magic sandwich echo (MSE) FID and proton multiple quantum (MQ) NMR. These results demonstrate the key role of the CB filler network in determining the stress-softening behavior of reinforced rubber.

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

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

Active control of graphene-based membrane-type acoustic metamaterials using a low voltage

Membrane-type acoustic metamaterials for acoustic insulation applications have been attracting ever increasing attention. However, the first anti-resonant frequency of these acoustic metamaterials is fixed once the membrane type is chosen. Here, we propose a novel yet convenient strategy to actively adjust the anti-resonant frequency of the membrane. The poly(vinyl alcohol)/graphene (PVA/GR) nanocomposite membrane is introduced into the acoustic metamaterial, the effective modulus of which is tunable by applying an external electric field. As a result, the first anti-resonant frequency of membrane-type acoustic metamaterials can be actively tuned between 369.2 to 420 Hz, leading to excellent sound attenuation properties. The noise reduction frequency can be actively modulated by DC voltage. Moreover, the change in frequency is consistent with the modulus variation of the PVA/GR nanocomposite membrane when the graphene concentration is varied. In addition, the conductive PVA/GR nanocomposite membrane also exhibits good electromagnetic interference shielding performance in the frequency range of 8-12 GHz. Being actively tunable by an external electric field, this PVA/GR nanocomposite membrane-based acoustic metamaterial is very promising for use in frequency-tunable acoustic insulation applications.

A facile route to fabricate thermally conductive and electrically insulating polymer composites with 3D interconnected graphene at an ultralow filler loading

Thermally conductive polymer composites show attractive prospects as thermal management materials in many applications such as microelectronic devices. However, traditional approaches in the preparation of thermally conductive polymer composites usually have the disadvantages of complex processes. In this study, a facile method for highly thermally conductive silicone rubber composites is reported, based on 3D interconnected graphene sponges by using an inorganic salt as a sacrificial template. The composites exhibit a high thermal conductivity of 1.50 W m-1 k-1 and a thermal conductivity enhancement of 752% at a very low graphene loading of 1.46 wt%. More significantly, highly thermally conductive epoxy and styrene-butadiene rubber composites are also fabricated by the same method. The composites also show excellent electrical insulating properties (>1013 Ω cm). Thus, this effective method is proved to be widely used for the facile fabrication of polymer composites of both plastic and rubber matrices which are thermally conductive and have excellent electrical insulating properties.

Fabrication of thermally conductive and electrically insulating polymer composites with isotropic thermal conductivity by constructing a three-dimensional interconnected network

Efficient heat removal via thermal management materials has become one of the most critical challenges in the development of modern microelectronic devices. However, the conventional polymer-based thermally conductive composites with randomly distributed filler particles usually yield an undesired value because of the lack of efficient heat transfer pathways. Therefore, constructing a three-dimensional interconnected filler structure is greatly desirable for realizing high thermal conductivity enhancement in composites. Herein, graphene oxide (GO) was used as a thermally conductive filler due to its excellent thermal conductivity and coated with polydopamine (PDA) to enhance its electric insulation performance. A unique "particle-constructing" method was adopted for fabricating highly ordered three-dimensional GO-based polymer composites, throughout which the GO-PDA formed an intact, uniform and well-defined network structure. The composite, even with a very low GO-PDA loading of 0.96 vol%, exhibited both high in-plane (4.13 W m^-1 K^-1) and through-plane (4.56 W m^-1 K^-1) thermal conductivities and also presented excellent electrically insulating properties (>10¹⁴Ω cm). These composites have promising applications in heat dissipation of next-generation portable and collapsible electronic devices.

Synergistic Effect of Aligned Graphene Nanosheets in Graphene Foam for High-Performance Thermally Conductive Composites

Graphene shows a great potential for high-performance thermally conductive composite applications because of its extremely high thermal conductivity. However, the graphene-based polymer composites reported so far only have a limited thermal conductivity, with the highest thermal conductivity enhancement (TCE) per 1 vol% graphene less than 900%. Here, a continuous network of graphene foam (GF), filled with aligned graphene nanosheets (GNs), is shown to be an ideal filler structure for thermally conductive composite materials. Compared to previous reports, a clear thermal percolation is observed at a low graphene loading fraction. The GNs/GF/natural rubber composite shows the highest TCE of 8100% (6.2 vol% graphene loading) ever reported at room temperature, which gives a record-high TCE per 1 vol% graphene of 1300%. Further analyses reveal a significant synergistic effect between the aligned GNs and 3D interconnected GF, which plays a key role in the formation of a thermal percolation network to remarkably improve the thermal conductivity of the composites. Additionally, the use of this composite for efficient heat dissipation of light-emitting diode (LED) lamps is demonstrated. These findings provide valuable guidance to design high-performance graphene-based thermally conductive materials, and open up the possibility for the use of graphene in high-power electronic devices.

Electromagnetic and acoustic double-shielding graphene-based metastructures

Nowadays, acoustic and electromagnetic pollution problems are becoming increasingly serious. Thus, electromagnetic interference (EMI) shielding materials and high acoustic insulation materials are urgently needed. EMI shielding materials and high acoustic insulation materials or structures have been rapidly developed. However, electromagnetic yet acoustic shielding (double-shielding) structures have not been reported so far. Herein, we propose a novel metastructure based on a graphene paper and artificial honeycomb, which exhibits not only excellent electromagnetic interference shielding property but also robust acoustic shielding efficiency. The EMI shielding effectiveness of the metastructure is up to 74 dB in a broad range from 2.6 to 26.5 GHz. The acoustic shielding effectiveness reaches 25 dB from 200 to 1500 Hz. It is worth noting that the effectiveness of acoustic shielding reaches up to 40 dB at very low frequency of 200 Hz. A typical double-shielding application demo was assembled to prove the attenuation performance against electromagnetic waves and sound. Our unique metastructure is very promising for protecting from electromagnetic and acoustic pollution.

Highly Thermally Conductive Composite Films Based on Nanofibrillated Cellulose in Situ Coated with a Small Amount of Silver Nanoparticles

In this paper, a freestanding flexible nanofibrillated cellulose (NFC)/silver (Ag) composite film with high thermal conductivity (TC) was prepared using the NFC that was in situ coated with a small amount of Ag nanoparticles through mussel-inspired chemistry of dopamine. The results demonstrated that Ag nanoparticles were homogeneously coated on the surface of NFC nanofibers and their incorporation had little influence on the film-forming ability of NFC. The NFC decorated with Ag nanoparticles could easily form thermally conductive pathways in the composite films, and the resultant films containing only 2.0 vol % of Ag showed a high in-plane TC value of 6.0 W/(m·K), which was 4 times that of pure NFC film. Moreover, the composite films exhibited relatively high strength and flexibility. The highly thermally conductive NFC/Ag composite films possess potential applications as lateral heat spreaders in flexible electronic equipment.