Ultrahigh Thermal Conductive yet Superflexible Graphene Films

Electrical devices generate heat at work. The heat should be transferred away immediately by a thermal manager to keep proper functions, especially for high‐frequency apparatuses. Besides high thermal conductivity (K ), the thermal manager material requires good foldability for the next generation flexible electronics. Unfortunately, metals have satisfactory ductility but inferior K (≤429 W m^?1 K^?1), and highly thermal‐conductive nonmetallic materials are generally brittle. Therefore, fabricating a foldable macroscopic material with a prominent K is still under challenge. This study solves the problem by folding atomic thin graphene into microfolds. The debris‐free giant graphene sheets endow graphene film (GF) with a high K of 1940 ± 113 W m^?1 K^?1. Simultaneously, the microfolds render GF superflexible with a high fracture elongation up to 16%, enabling it more than 6000 cycles of ultimate folding. The large‐area multifunctional GFs can be easily integrated into high‐power flexible devices for highly efficient thermal management.     

Graphene foam/carbon nanotube/poly(dimethyl siloxane) composites for exceptional microwave shielding

Highly porous poly(dimethyl siloxane) (PDMS) composites containing cellular-structured microscale graphene foams (GFs) and conductive nanoscale carbon nanotubes (CNTs) are fabricated. The unique three-dimensional, multi-scale hybrid composites with inherent percolation and a high porosity of 90.8% present a remarkable electromagnetic interference shielding effectiveness (EMI SE) of ∼75 dB, a 200% enhancement against 25 dB of the composites made from GFs alone with the same graphene content and porosity. The corresponding specific EMI SE measured against the composite density is 833 dB cm³/g. These values are among the highest for all carbon filler/polymer composites reported thus far. Significant synergy arises from the hybrid reinforcement structure of the composites: the GFs drive the incident microwaves to be attenuated by dissipation of the currents induced by electromagnetic fields, while the CNTs greatly enhance the dissipation of surface currents by expanding the conductive networks and introducing numerous interfaces with the matrix.     

Assembly of Graphene Sheets into 3D Macroscopic Structures

Integration of graphene sheets, 2D nanoscale building blocks, into 3D macroscopic assemblies and ultimately into a functional system is essential to explore the advanced properties of individual graphene sheets for macroscopic applications. This Concept paper summarizes different ways, such as flow-directed assembly, layer-by-layer deposition, template-directed method, and leavening strategy to assemble graphene sheets into the layered and porous 3D macroscopic structures. The obtained structures show unique properties, such as flexible network, high specific surface area, and outstanding electrical and mechanical properties. Furthermore, the functional systems based on such graphene 3D macroscopic structures have shown enhanced performance in the applications of energy storage, catalysis, environmental remediation, and sensing.     

Morphology, mechanical and thermal properties of graphene-reinforced poly(butylene succinate) nanocomposites

Graphene nanosheets (GNSs) reinforced poly(butylene succinate) (PBS) nanocomposites are facilely obtained by a solution-based processing method. Graphene nanosheets, which are derived from chemically reduced graphite oxide (GO), are characterized by AFM, TEM, XRD and Raman spectra. The state of dispersion of the GNSs in the PBS matrix is examined by SEM observations that reveals homogeneous distribution of GNSs in PBS matrix. A 21% increase in tensile strength and a 24% improvement of storage modulus are achieved by addition of 2.0 wt% of GNS. The electrical conductivity and thermal stability of the graphene-based nanocomposite are also improved. DSC measurement indicates that the presence of graphene sheets does not have a remarkable impact on the crystallinity of the nanocomposites. Therefore, the high performances of the nanocomposites are mainly attributed to the uniform dispersion of GNSs in the polymer matrix and strong interfacial interactions between both components.     

Graphene-based conducting inks for direct inkjet printing of flexible conductive patterns and their applications in electric circuits and chemical sensors

A series of inkjet printing processes have been studied using graphene-based inks. Under optimized conditions, using water-soluble single-layered graphene oxide (GO) and few-layered graphene oxide (FGO), various high image quality patterns could be printed on diverse flexible substrates, including paper, poly(ethylene terephthalate) (PET) and polyimide (PI), with a simple and low-cost inkjet printing technique. The graphene-based patterns printed on plastic substrates demonstrated a high electrical conductivity after thermal reduction, and more importantly, they retained the same conductivity over severe bending cycles. Accordingly, flexible electric circuits and a hydrogen peroxide chemical sensor were fabricated and showed excellent performances, demonstrating the applications of this simple and practical inkjet printing technique using graphene inks. The results show that graphene materials-which can be easily produced on a large scale and possess outstanding electronic properties-have great potential for the convenient fabrication of flexible and low-cost graphene-based electronic devices, by using a simple inkjet printing technique.      

Thermal and electrical conductivity of Al(OH)<Subscript>3</Subscript> covered graphene oxide nanosheet/epoxy composites

Al(OH)₃ functionalized graphene composites (Al–GO) were prepared using a simple sol–gel method. In this protocol, graphene oxide (GO) was prepared according to the Hummers method and functionalized to enhance its reactivity with aluminum isopropoxide by a LiAlH₄ treatment. The functionalized graphene sheets were characterized by X-ray photoelectron spectroscopy, field emission scanning electron microscopy, and transmission electron microscopy. These analyses confirmed that GO had been fabricated and the Al(OH)₃ layer could have a homogeneous distribution with large and dense coverage onto GO sheets. In addition, the thermal and electrical conductivity of the epoxy composites with GO and Al–GO fillers were measured. The thermal conductivities of the composites with graphene-based fillers were enhanced by the addition of fillers. In particular, the thermal conductivity of GO/epoxy composite containing 3 wt% was approximately two times higher than that of pure epoxy resin. In addition, the electrical conductivity of Al–GO embedded composites degenerated compared to GO composites.     

Design and preparation of graphene/poly(ether ether ketone) composites with excellent electrical conductivity

Graphene/poly(ether ether ketone) (m-TRG/PEEK) composites with excellent electrical conductivity were fabricated by hot pressing technique with thermally reduced graphene nanosheets (m-TRG) which were modified by poly(ether sulfone). Moreover, the conductive, thermal, and mechanical properties of PEEK/m-TRG composites were investigated by the precision impedance analyzer, thermal gravimetric analyzer, differential scanning calorimetry, and universal tester, respectively. The electrical conductivity of m-TRG/PEEK composites was greatly improved by incorporating graphene, resulting in a sharp transition from electrical insulator to semiconductor with a low percolation threshold of 0.76 vol.%. A high electrical conductivity of 0.18 S m^?1 was achieved with 3.84 vol.% of m-TRG. The data were compared with those of composites reduced chemically, and the results showed that thermal reduction was an effective method to acquire higher electrical conductive composites. The excellent electrical property should be attributed to the large specific surface area of m-TRG, well dispersion of m-TRG in PEEK matrix, and good compatibility of m-TRG with PEEK matrix, as proven by scanning electron microscope. Besides, m-TRG/PEEK composites also exhibited relatively good thermal and mechanical properties.     

Silica coating onto graphene for improving thermal conductivity and electrical insulation of graphene/polydimethylsiloxane nanocomposites

Graphene possess extremely high thermal conductivity, and they have been regarded as prominent candidates to be used in thermal management of electronic devices. However, addition of graphene inevitably causes dramatic decrease in electrical insulation, which is generally unacceptable for thermal interface materials(TIMs) in real electronic industry. Developing graphene-based nanocomposites with high thermal conductivity and satisfactory electrical insulation is still a challenging issue. In this study,we developed a novel hybrid nanocomposite by incorporating silica-coated graphene nanoplatelets(Silica@GNPs) with polydimethylsiloxane(PDMS) matrix. The obtained Silica@GNP/PDMS composites showed satisfactory electrical insulation(electrical resistivity of over 10~(13)Ωcm) and high thermal conductivity of 0.497 W m-1K-1, increasing by 155% compared with that of neat PDMS, even higher than that of GNP/PDMS composites. Such high thermal conductivity and satisfactory electrical insulation is mainly attributed to the insulating silica-coating, good compatibility between components, strong interfacial bonding, uniform dispersion, and high-efficiency heat transport pathways. There is great potential for the Silica@GNP/PDMS composites to be used as high-performance TIMs in electronic industry.     

Mechanical properties of graphene films enhanced by homo-telechelic functionalized polymer fillers via π–π stacking interactions

Most researches on graphene/polymer composites are focusing on improving the mechanical and electrical properties of polymers at low graphene content instead of paying attention to constructing graphene’s macroscopic structures. In current study the homo-telechelic functionalized polyethylene glycols (FPEGs) were tailored with π-orbital-rich groups (namely phenyl, pyrene and di-pyrene) via esterification reactions, which enhanced the interaction between polyethylene glycol (PEG) molecules and chemical reduced graphene oxide (RGO) sheets. The π–π stacking interactions between graphene sheets and π-orbital-rich groups endowed the composite films with enhanced tensile strength and tunable electrical conductivity. The formation of graphene network structure mediated by the FPEGs fillers via π–π stacking non-covalent interactions should account for the experimental results. The experimental investigations were also complemented with theoretical calculation using a density functional theory. Atomic force microscope (AFM), scanning electron microscope (SEM), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal gravimetric analysis (TGA), UV–vis and fluorescence spectroscopy were used to monitor the step-wise preparation of graphene composite films.     

Graphene-based materials and structures for energy harvesting with fluids – A review

Graphene and graphene-based systems have recently been recognized as promising platforms for energy harvesting, microelectronic components and energy storage owing to their excellent electrical and thermal conductivity, outstanding mechanical properties, good chemical stability, area adaptability, among other significant properties. Integration of energy harvesting systems relying on the graphene/graphene-based materials in contact with fluids has been emphasized in recent years, as well as their potential impact on electric energy generation for a wide range of applications (e.g. innovative medical devices, advanced electronic systems and highly-efficient transduction systems for renewable energy). This review summarizes, for the first time, major breakthroughs carried out in the scope of energy harvesting exploiting graphene-based material systems (comprising graphene films, graphene grids, graphene membranes, 3D graphene composites and tribological structures) in contact with ionic and non-ionic fluids. Several transduction mechanisms for energy harvesting have been thoroughly analyzed. Energy outputs, materials and structures, substrates, types of fluid, manufacture methodologies, and experimental test methodologies are systematically highlighted in this review. Finally, future research directions and innovative applications of these harvesters are proposed.     

Graphene/poly(vinylidene fluoride) composites with high dielectric constant and low percolation threshold

In aiming to obtain highly flexible polymer composites with high dielectric performance, graphene/poly(vinylidene fluoride) (PVDF) composites with a multi-layered structure were proposed and prepared. Graphene sheets were prepared by reducing graphene oxide using phenylhydrazine, which could effectively alleviate aggregation of the graphene sheets. A two-step method, including solution casting and compression molding, was employed to fabricate the graphene/PVDF composites. The composites showed an alternative multi-layered structure of graphene sheets and PVDF. Due to their unique structure, the composites had an extremely low percolation threshold (0.0018 volume fraction of graphene), which was the lowest percolation threshold ever reported among PVDF-based polymer composites. A high dielectric constant of more than 340 at 100?Hz could be obtained within the vicinity of the percolation threshold when the graphene volume fraction was 0.00177. Above the percolation threshold, the dielectric constant continued to increase and a maximum value of as high as 7940 at 100?Hz was observed when the graphene volume fraction was 0.0177.     

Graphene-based bimorph microactuators

A novel graphene-on-organic film fabrication method that is compatible with a batch microfabrication process was developed and used for electromechanically driven microactuators. A very thin layer of graphene sheets was monolithically integrated and the unique material characteristics of graphene including negative thermal expansion and high electrical conductivity were exploited to produce a bimorph actuation. A large displacement with rapid response was observed while maintaining the low power consumption. This enabled the successful demonstration of transparent graphene-based organic microactuators.     

Graphene-based ultracapacitors

The surface area of a single graphene sheet is 2630 m(2)/g, substantially higher than values derived from BET surface area measurements of activated carbons used in current electrochemical double layer capacitors. Our group has pioneered a new carbon material that we call chemically modified graphene (CMG). CMG materials are made from 1-atom thick sheets of carbon, functionalized as needed, and here we demonstrate in an ultracapacitor cell their performance. Specific capacitances of 135 and 99 F/g in aqueous and organic electrolytes, respectively, have been measured. In addition, high electrical conductivity gives these materials consistently good performance over a wide range of voltage scan rates. These encouraging results illustrate the exciting potential for high performance, electrical energy storage devices based on this new class of carbon material.     

Fabrication of Flexible Metal‐Nanoparticle Films Using Graphene Oxide Sheets as Substrates

Graphene‐based sheets that possess a unique nanostructure and a variety of fascinating properties are appealing as promising nanoscale building blocks of new composites. Herein, graphene oxide sheets are used as the nanoscale substrates for the formation of silver‐nanoparticle films. These silver‐nanoparticle films assembled on graphene oxide sheets are flexible and can form stable suspensions in aqueous solutions. They can also be easily processed, forming macroscopic films with high reflectivity. Raman signals of graphene oxide in such hybrid films are increased by the attached silver nanoparticles, displaying surface‐enhanced Raman scattering activity. The degree of enhancement can be adjusted by varying the quantity of silver nanoparticles on the graphene oxide sheets.     

Supraparamagnetic, conductive, and processable multifunctional graphene nanosheets coated with high-density Fe3O4 nanoparticles

The amazing properties of graphene are triggering extensive interests of both scientists and engineers, whereas how to fully utilize the unique attributes of graphene to construct novel graphene-based composites with tailor-made, integrated functions remains to be a challenge. Here, we report a facile approach to multifunctional iron oxide nanoparticle-attached graphene nanosheets (graphene@Fe(3)O(4)) which show the integrated properties of strong supraparamagnetism, electrical conductivity, highly chemical reactivity, good solubility, and excellent processability. The synthesis method is efficient, scalable, green, and controllable and has the feature of reduction of graphene oxide and formation of Fe(3)O(4) nanoparticles in one step. When the feed ratios are adjusted, the average diameter of Fe(3)O(4) nanoparticles (1.2-6.3 nm), the coverage density of Fe(3)O(4) nanoparticles on graphene nanosheets (5.3-57.9%), and the saturated magnetization of graphene@Fe(3)O(4) (0.5-44.1 emu/g) can be controlled readily. Because of the good solubility of the as-prepared graphene@Fe(3)O(4), highly flexible and multifunctional films composed of polyurethane and a high content of graphene@Fe(3)O(4) (up to 60 wt %) were fabricated by the solution-processing technique. The graphene@Fe(3)O(4) hybrid sheets showed electrical conductivity of 0.7 S/m and can be aligned into a layered-stacking pattern in an external magnetic field. The versatile graphene@Fe(3)O(4) nanosheets hold great promise in a wide range of fields, including magnetic resonance imaging, electromagnetic interference shielding, microwave absorbing, and so forth.     

Enhanced electrical conductivity in polystyrene nanocomposites at ultra-low graphene content

We compared the electrical conductivity of multiwalled-carbon-nanotube/polystyrene and graphene/polystyrene composites. The conductivity of polystyrene increases from ～6.7 × 10(-14) to ～3.49 S/m, with an increase in graphene content from ～0.11 to ～1.1 vol %. This is ～2-4 orders of magnitude higher than for multiwalled-carbon-nanotube/polystyrene composites. Furthermore, we show that the conductivity of the graphene/polystyrene system can be significantly enhanced by incorporation of polylactic acid. The volume-exclusion principle forces graphene into the polystyrene-rich regions (selective localization) and generates ～4.5-fold decrease in its percolation threshold from ～0.33 to ～0.075 vol %.     

High-performance graphene-based transparent flexible heaters

We demonstrate high-performance, flexible, transparent heaters based on large-scale graphene films synthesized by chemical vapor deposition on Cu foils. After multiple transfers and chemical doping processes, the graphene films show sheet resistance as low as ～43 Ohm/sq with ～89% optical transmittance, which are ideal as low-voltage transparent heaters. Time-dependent temperature profiles and heat distribution analyses show that the performance of graphene-based heaters is superior to that of conventional transparent heaters based on indium tin oxide. In addition, we confirmed that mechanical strain as high as ～4% did not substantially affect heater performance. Therefore, graphene-based, flexible, transparent heaters are expected to find uses in a broad range of applications, including automobile defogging/deicing systems and heatable smart windows.     

Flyweight,Superelastic, Electrically Conductive,and Flame‐Retardant 3D Multi‐Nanolayer Graphene/Ceramic Metamaterial

A ceramic/graphene metamaterial (GCM) with microstructure‐derived superelasticity and structural robustness is achieved by designing hierarchical honeycomb microstructures, which are composited with two brittle constituents (graphene and ceramic) assembled in multi‐nanolayer cellular walls. Attributed to the designed microstructure, well‐interconnected scaffolds, chemically bonded interface, and coupled strengthening effect between the graphene framework and the nanolayers of the Al₂O₃ ceramic (NAC), the GCM demonstrates a sequence of multifunctional properties simultaneously that have not been reported for ceramics and ceramics–matrix–composite structures, such as flyweight density, 80% reversible compressibility, high fatigue resistance, high electrical conductivity, and excellent thermal‐insulation/flame‐retardant performance simultaneously. The 3D well‐ordered graphene aerogel templates are strongly coupled with the NAC by the chemically bonded interface, exhibiting mutual strengthening, compatible deformability, and a linearly dependent relationship between the density and Young's modulus. Considerable size effects of the ceramic nanolayers on the mechanical properties are revealed in these ceramic‐based metamaterials. The designed hierarchical honeycomb graphene with a fourth dimensional control of the ceramic nanolayers on new ways to scalable fabrication of advanced multifunctional ceramic composites with controllable design suggest a great potential in applications of flexible conductors, shock/vibration absorbers, thermal shock barriers, thermal insulation/flame‐retardant skins, and porous microwave‐absorbing coatings.     

Graphene based materials: Past, present and future

Graphene, a two dimensional monoatomic thick building block of a carbon allotrope, has emerged as an exotic material of the 21st century, and received world-wide attention due to its exceptional charge transport, thermal, optical, and mechanical properties. Graphene and its derivatives are being studied in nearly every field of science and engineering. Recent progress has shown that the graphene-based materials can have a profound impact on electronic and optoelectronic devices, chemical sensors, nanocomposites and energy storage. The aim of this review article is to provide a comprehensive scientific progress of graphene to date and evaluate its future perspective. Various synthesis processes of single layer graphene, graphene nanoribbons, chemically derived graphene, and graphene-based polymer and nano particle composites are reviewed. Their structural, thermal, optical, and electrical properties were also discussed along with their potential applications. The article concludes with a brief discussion on the impact of graphene and related materials on the environment, its toxicological effects and its future prospects in this rapidly emerging field.     

高密度石墨泡沫及其石蜡复合材料的热物理性能(英文)

以中间相沥青和添加中间相炭微球的沥青为原料,调整发泡压力和发泡温度制备沥青泡沫,经1273K炭化和2973K石墨化,制备了高密度石墨泡沫。为了进一步提高石墨泡沫的密度,采用573 K的沥青反复浸渍炭化未添加中间相炭微球的沥青在1273K下所制的泡沫炭,再经2973K石墨化获得增密度后的石墨泡沫。而后制备了相应石墨泡沫/石蜡复合材料。研究了石墨泡沫热物理性能的影响因素和石墨泡沫/石蜡复合材料的热行为。研究表明:沥青组分、发泡温度和发泡压力决定了石墨泡沫的结构和热物理性能,而石墨泡沫的热导率决定了复合材料的热行为。与石蜡相比,石墨泡沫/石蜡复合材料的热扩散系数提高了768至1588倍。石墨泡沫/石蜡复合材料的潜热与石蜡的质量分数成正比。该复合材料是快速响应电子散热材料的良好选择。