Graphene Network Structure Research Articles

Multifunctional nanocomposites reinforced by aligned graphene network via a low-cost lyophilization-free method

Compared with a disordered network, the ordered framework is more beneficial to improving the multifunctional properties (including thermal, electrical, and mechanical properties) of epoxy-based composites. However, current methods for building aligned structures are both energy and time-consuming on account of the tedious and lengthy lyophilization process. Herein, a new strategy is proposed to obtain low-density graphene aerogels (GAs) with long-range oriented structures through a lyophilization-free processes. Due to the minimum thermal boundary resistance, the prepared aligned graphene/epoxy composite exhibits an ultrahigh thermal conductivity of ∼11.6 W/(m·K) at a graphene content of 1.84 vol%, i.e., an enhancement efficiency of 3640% per 1 vol%. In addition, the compressive strength of the composite is increased by 2.6 times compared to epoxy (from 0.29 MPa to 0.76 MPa). Because the composite shows a modulus as low as 0.5–1.0 MPa, it has excellent deformability and compression performance. Moreover, the obtained 3-mm-thick composite achieves an electromagnetic interference shielding effectiveness (EMI SE) of 40 dB due to the interconnected graphene network structure, which meets the requirements of commercial EMI shielding applications. Hence, this strategy can be used to synthesize aligned graphene/epoxy composites with excellent multifunctional performances, which can be simultaneously used as elastic thermal interface materials (TIMs) and EMI shielding materials in the fields of advanced electronic packaging.

Finite Element Simulation of Graphene Thermal Conductivity Framework

With the rapid development of electronic devices towards miniaturization and high integration, the problem of overheating and heat dissipation forces people to find thermal conductive materials with good comprehensive properties to meet the needs of development. Inspired by the three-dimensional boron nitride network, this paper creatively proposed the construction of a composite thermal conductive material with 3D graphene network structure and PDMS as filling matrix, and carried out finite element analysis and simulation with COMSOL. The results show that the thermal conductivity of the composite thermal conductive material with graphene 3D network structure is significantly better than that of the single thermal conductive material without graphene and the thermal conductive material with random distribution of graphene, and most of the heat flow forms a connected thermal conductive path along the heat transfer direction in the graphene of 3D network composite. It can be seen that the prepared composites with 3D thermal conduction network structure have excellent thermal conductivity, and will have broad market prospects in the fields of heat dissipation materials, computers, smart phones, laser weapons and so on.

Lightweight Low‐Frequency Sound‐Absorbing Composites of Graphene Network Reinforced by Honeycomb Structure

AbstractEfficient, facile, lightweight, and low‐frequency sound‐absorbing materials are in great demand for noise elimination and insulation. However, the applications of some candidate materials are limited by their large space, mass, or unsatisfactory sound‐absorption performance especially in low‐frequency sound‐wave region. Graphene network structure can be promising sound‐absorbing materials due to ultrahigh porosity, ultralight weight, and excellent intrinsic properties. Conventionally, allowing for poor mechanical strength, such graphene materials are needed to be reinforced by polymer, metal, or ceramic. Here, a novel strategy of graphene network reinforced by honeycomb structure is proposed, in which graphene is used as a sound absorber, and meanwhile flexible aramid honeycomb is introduced to enhance the graphene composite. In this work, the compressive strength of graphene network structure is improved 1000 times by the flexible aramid honeycomb, and meanwhile the density is only increased 2.5 times. Moreover, the sound‐absorption coefficient of graphene network is about 0.9 at 700 Hz for the thickness of 30 mm. Further, the sound‐absorption coefficient is above 0.9 in the frequency range of >1000 Hz. Therefore, the graphene composite is expected to be saving space and weight for the engineering design of sound absorbers in low‐frequency regions.

Constructing 3D Graphene Network in Rubber Nanocomposite via Liquid-Phase Redispersion and Self-Assembly

A three-dimensional graphene (GE) segregated network structure is of significance for improving the conductivity of composites. However, constructing such a GE network structure in composites still remains a challenge. Here, we demonstrate a facile process, that is, liquid-phase redispersion and self-assembly (LRS) to prepare polymer nanocomposites with graphene segregated networks. High shear liquid-phase mixing accompanied by the diffusion of dissolved polymer chains into the interstices and voids of the loose graphene powders can lead to redispersion of GE in polymer solution. Once the stirring is stopped, the self-assembly and segregation of redispersed GE occurs in a poor solvent driven by π-π interaction. After solvent evaporation, the GE assembly structures are retained as networks in the GE/polymer composite prepared by hot pressing. The graphene/(isobutylene-isoprene rubber) nanocomposite (GE/IIR) was investigated as a demonstration for the advantages of the LSR method. The morphologies of GE assemblies in the liquid phase and GE networks in the solid composite were observed. Due to the existence of the homogeneously distributed graphene segregated networks, the tensile strength and elongation at break for GE/IIR nanocomposites increase by ∼410 and ∼126%, respectively, and the electrical conductivity reaches ∼100 S m-1 at a GE content of 3.76 vol %. The LRS method was also successfully tried for systems with different polymer matrixes and different solvents, suggesting the robustness of the proposed method. The prepared flexible GE/IIR nanocomposites with GE networks are sensitive to tiny strain and can be applied in wearable sensors for the detection of human physiological signals.

A graphene rheostat for highly durable and stretchable strain sensor

AbstractStrain sensors for human health monitoring are of paramount importance in wearable medical diagnostics and personal health monitoring. Despite extensive studies, strain sensors with both high durability and stretchability are still desired, particularly with the stability for different environmental conditions. Here, we report a series of strain sensors possessing the graphene network with a high density of intermittent physical interconnections, which produces the relative resistance change by varying the overlap area between the neighboring graphene sheets under stretching and releasing, analogous to the slide rheostat working in electronics. Our in‐situ transmission electron microscope observation reveals the full recoverability of the structure from large deformation upon unloading for ensuring the exceptional cycle stability of our material on monitoring full‐range body movements. The stable response is also demonstrated over wide temperature range and frequency range, because the peculiar dynamic structure can be maintained through the self‐adjustment to the thermal expansion of the bulk material. Based on the working mechanism of graphene “slide rheostat,” the sensing properties of the strain sensor are tailored by tuning the graphene network structure with different mass densities using different concentrations of graphene oxide dispersion, while the stretchability and sensitivity can be separately optimized for different application requirements.

Preparation of electroactive shape memory polyurethane/graphene nanocomposites and investigation of relationship between rheology, morphology and electrical properties

In this study, electroactive shape-memory polyurethane/graphene nanosheets nanocomposites were prepared. Shape-memory polyurethane (SMPU) was synthesized from polycaprolactone-diols (PCL-diols) with a molecular weight of 4000 g/mol, hexamethylene diisocyanate (HDI), and 1,4-butanediol (BDO) with a molar ratio of 5 (HDI)/4 (BDO)/1 (PCL) via solution polymerization. Nanocomposites were prepared from neat SMPU and mono-layered fluffy graphene nanosheets through solution mixing method. Electrical resistivity test was employed to investigate the electrical conductivity threshold of the nanocomposites. The results showed that the nanocomposites with graphene content higher than 1.5 wt% had a reasonable conductivity to respond to the electrical current. Fourier transform infrared (FTIR) spectroscopy confirmed the interaction between graphene nanosheets and SMPU chains by variation of hydrogen bonding content. Differential scanning calorimetry (DSC) thermograms and X-ray diffraction (XRD) patterns showed that the crystalline structures of SMPU have been changed by incorporating of graphene nanosheets. Rheological examination showed that the percolation threshold to create a graphene network structure was 1.5 wt%. Field emission scanning electron microscopy (FE-SEM) images revealed percolation of graphene nanosheets in the electrically conductive nanocomposites. Mechanical and shape-memory tests revealed that the mechanical properties and shape-memory parameters were improved by the addition of graphene nanosheets. The changes in surface temperature under the examination with voltage of 75 V indicated that the temperature of the electrically conductive samples was significantly increased by flowing of electrical current. This increase in temperature led responsivity to the electrical current in SMPU nanocomposites containing 2 and 3 wt% graphene nanosheets.

Synthesis of novel nanocomposites reinforced with 3D graphene/highly-dispersible nanodiamonds nano-hybrids

The phenolic resin-based ternary nanocomposites were prepared successfully by filling a three-dimensional (3D) graphene network structure embedded with highly-dispersible nanodiamonds into the resin matrix. Firstly, 3D graphene/nanodiamonds hydrogels were fabricated via self-assembly approach in a self-made dispersing medium, in which the volume ratio of ethanol absolute to deionized water was equal to 1:1. Then water molecules within the as-prepared hydrogels were replaced with resol by means of soaking and appropriate heat treatment. In-situ polymerization technique was employed subsequently to afford the ternary nanocomposites of phenolic resin/3D graphene/nanodiamonds, while the dispersion of nanodiamonds remained decent. The obtained nanocomposites exhibited high hardness and compressive strength, where the measured values were 31.1% and 24.7% higher than that of the composites procured from direct dispersion of nanodiamonds in phenolic resin matrix, respectively.

Improved strength and toughness of semi-aromatic polyamide 6T-co-6(PA6T/6)/GO composites via in situ polymerization

In this paper, poly(hexamethylene terephthalamide)-co-polycaprolactam (PA6T/6)/graphene oxide (GO) composites were prepared via in-situ polycondensation and/or melt blending. To improve the comprehensive properties, PA6T/6 chains were grafted onto GO, taking advantage of condensation reaction between amino group of the polymer chain terminal and carboxyl group on the GO surface. The results demonstrated that the polymer chain grafted GO reached a more homogeneous dispersion in PA6T/6 matrix compared with pristine GO, which was ascribed to the intermolecular force such as hydrogen bond, generating among amides (CONH) emerged separately on grafted and free PA6T/6 chains. Furthermore, the length of grafted chain was varied by controlling the grafting process and its effect on the thermal and mechanical properties of (PAT/6)/GO composites was studied in detail. Hence, long PA6T/6-chain-grafted graphene sheet gradually improved the tensile strength to 73.6 MPa (by 45.7%) due to strong interaction, excellent dispersion and physical entanglement with PA6T/6 matrix even with a small amount of GO (2 wt‰), and a balance of strength and toughness was obtained owing to the formation of polymer-bridged graphene network structure. Notably, short PA6T/6 (prePA6T/6) chain grafted graphene sheet together with moderate amount of prePA6T/6, which remarkably promoted the crystallization of PA6T/6, might play a synergistic reinforcement role and increased the tensile strength to 71.9 MPa with just 1 wt‰ of GO content. That synergistic reinforcement effect might open a new avenue to further enhance those graphitic carbon-based polymer nanocomposites with extremely small amount of fillers, especially for those crystallizable polymers.

A green method for preparing conductive elastomer composites with interconnected graphene network via Pickering emulsion templating

The exploitation of generic strategy for preparation of conductive elastomer composites with interconnected filler network is of significant scientific and technical interests. Herein, we report a novel, green and effective strategy to prepare conductive natural rubber (NR) composites with interconnected graphene network structure. It is found that the Pickering emulsion template is effective in constructing 3D interconnected graphene oxide (GO) networks while the thermal treatment during hot pressing step is able to convert GO to conductive thermal reduced graphene (TRG). This strategy does not rely on the toxic and corrosive reducing agents and the adopted solvents in Pickering emulsions can be recycled and reused, which makes the process environmentally friendly. As a result, interconnected graphene network is well constructed within NR matrix, which significantly enhances the electrical conductivity, mechanical properties and lowers the percolation threshold of the resulted composites. Specifically, with the incorporation of 1.90 vol% TRG, the electrical conductivity of NR composites with interconnected graphene network structure is more than 7 orders of magnitude higher than that of NR composites with uniform graphene dispersion. More intriguingly, this GO stabilized Pickering emulsion template strategy can also be used to realized the selective distribution of carbon nanotubes and assemble them into interconnected network structure, further facilitating the conductivity improvement of the resulted composites. Overall, we envision that the present work offers new insights into the design of conductive elastomer composites.

Topological Nodal-Net Semimetal in a Graphene Network Structure.

Topological semimetals are characterized by the nodal points in their electronic structure near the Fermi level, either discrete or forming a continuous line or ring, which are responsible for exotic properties related to the topology of bulk bands. Here we identify by abinitio calculations a distinct topological semimetal that exhibits nodal nets comprising multiple interconnected nodal lines in bulk and have two coupled drumheadlike flat bands around the Fermi level on its surface. This nodal net semimetal state is proposed to be realized in a graphene network structure that can be constructed by inserting a benzene ring into each C─C bond in the bct-C_{4} lattice or by a crystalline modification of the (5,5) carbon nanotube. These results expand the realm of nodal manifolds in topological semimetals, offering a new platform for exploring novel physics in these fascinating materials.

Nitrogen-Superdoped 3D Graphene Networks for High-Performance Supercapacitors.

An N-superdoped 3D graphene network structure with an N-doping level up to 15.8 at% for high-performance supercapacitor is designed and synthesized, in which the graphene foam with high conductivity acts as skeleton and nested with N-superdoped reduced graphene oxide arogels. This material shows a highly conductive interconnected 3D porous structure (3.33 S cm-1 ), large surface area (583 m2 g-1 ), low internal resistance (0.4 Ω), good wettability, and a great number of active sites. Because of the multiple synergistic effects of these features, the supercapacitors based on this material show a remarkably excellent electrochemical behavior with a high specific capacitance (of up to 380, 332, and 245 F g-1 in alkaline, acidic, and neutral electrolytes measured in three-electrode configuration, respectively, 297 F g-1 in alkaline electrolytes measured in two-electrode configuration), good rate capability, excellent cycling stability (93.5% retention after 4600 cycles), and low internal resistance (0.4 Ω), resulting in high power density with proper high energy density.

Three-dimensional Nitrogen-Doped Graphene Supported Molybdenum Disulfide Nanoparticles as an Advanced Catalyst for Hydrogen Evolution Reaction.

An efficient three-dimensional (3D) hybrid material of nitrogen-doped graphene sheets (N-RGO) supporting molybdenum disulfide (MoS2) nanoparticles with high-performance electrocatalytic activity for hydrogen evolution reaction (HER) is fabricated by using a facile hydrothermal route. Comprehensive microscopic and spectroscopic characterizations confirm the resulting hybrid material possesses a 3D crumpled few-layered graphene network structure decorated with MoS2 nanoparticles. Electrochemical characterization analysis reveals that the resulting hybrid material exhibits efficient electrocatalytic activity toward HER under acidic conditions with a low onset potential of 112 mV and a small Tafel slope of 44 mV per decade. The enhanced mechanism of electrocatalytic activity has been investigated in detail by controlling the elemental composition, electrical conductance and surface morphology of the 3D hybrid as well as Density Functional Theory (DFT) calculations. This demonstrates that the abundance of exposed active sulfur edge sites in the MoS2 and nitrogen active functional moieties in N-RGO are synergistically responsible for the catalytic activity, whilst the distinguished and coherent interface in MoS2/N-RGO facilitates the electron transfer during electrocatalysis. Our study gives insights into the physical/chemical mechanism of enhanced HER performance in MoS2/N-RGO hybrids and illustrates how to design and construct a 3D hybrid to maximize the catalytic efficiency.

Graphene Nanoplatelets as Rheology Modifiers for Polylactic Acid: Graphene Aspect-Ratio-Dependent Nonlinear Rheological Behavior

Two types of graphene were used to make polylactic acid (PLA)–graphene nanocomposites with various concentrations of graphene through a solution casting method. Because of the differences in surface area, thickness, and aspect ratio between the two types of graphene, the graphene–graphene and graphene–polymer interactions were different in these two nanocomposites. As a result, the two types of graphene nanoplatelets assembled into different interconnected structures in the matrix. Steady-state shear, stepwise small-amplitude oscillatory shear (SAOS) and large-amplitude oscillatory shear (LAOS), and frequency sweep tests immediately after the stepwise shear were used to determine and study the two different graphene aggregate structures. The disruption (under LAOS) and recovery (under SAOS) of the structures were monitored and the great disparities between the two types of graphene were ascribed to their structural roots. The formation of a percolated graphene network structure in the matrix significantly...

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.

The structural evolution of multi-layer graphene stacks in carbon fibers under load at high temperature – A synchrotron radiation study

Single carbon fibers were directly heated and tensile tested in vacuum in a specially designed in-situ X-ray testing device. We followed the structural evolution by diffraction of synchrotron radiation X-rays and determined the effect of time, temperature and applied stress on the growth of nanocrystallites as well as their orientational change with respect to the fiber axis. As the nanocrystallites are stacks of graphene sheets, this is a direct measurement of the development of multi-layer graphene with temperature. In the same experiment, size and orientation of pores within the material were obtained from simultaneous in-situ small-angle X-ray scattering. It turned out that the final temperature is decisive for structural evolution, significantly more than time or load. The multi-layer graphene network structure in fibers after heat treatment without stress is stable, whereas this is not the case in fibers without heat treatment, no matter if the carbon fibers were produced from polyacrylonitrile or from mesophase pitch precursor.

Electrophoretic Deposition of a Reduced Graphene–Au Nanoparticle Composite Film as Counter Electrode for CdS Quantum Dot‐Sensitized Solar Cells

A reduced graphene (RG)-Au nanoparticle composite film is successfully fabricated by electrophoretic deposition and used as counter electrode for quantum dot-sensitized solar cells. The RG-Au composite is prepared by one-step microwave-assisted reduction of chloroaurate in alkaline solution with graphite oxide dispersion. Under one sun illumination (AM 1.5 G, 100 mW cm(-2)), the cell with a RG-Au counter electrode shows an energy conversion efficiency of 1.36 %, which is higher than those of cells employing conventional Pt or Au counter electrodes, due to the superior combination of highly catalytic Au nanoparticles and the conductive graphene network structure.

Structure–process–property relationship of the polar graphene oxide-mediated cellular response and stimulated growth of osteoblasts on hybrid chitosan network structure nanocomposite scaffolds

We here describe the structure–process–property relationship of graphene oxide-mediated proliferation and growth of osteoblasts in conjunction with the physico-chemical, mechanical, and structural properties. Chitosan–graphene network structure scaffolds were synthesized by covalent linkage of the carboxyl groups of graphene oxide with the amine groups of chitosan. The negatively charged graphene oxide in chitosan scaffolds was an important physico-chemical factor influencing cell–scaffold interactions. Furthermore, it was advantageous in enhancing the biocompatibility of the scaffolds and the degradation products of the scaffolds. The high water retention ability, hydrophilic nature, and high degree of interconnectivity of the porous structure of chitosan–graphene oxide scaffolds facilitated cell attachment and proliferation and improved the stability against enzymatic degradation. The cells infiltrated and colonized the pores of the scaffolds and established cell–cell interactions. The interconnectivity of the porous structure of the scaffolds helps the flow of medium throughout the scaffold for even cell adhesion. Moreover, the seeded cells were able to infiltrate inside the pores of chitosan–graphene oxide scaffolds, suggesting that the incorporation of polar graphene oxide in scaffolds is promising for bone tissue engineering.