Thermal properties of multilayer graphene and hBN reinforced copper matrix composites

The unique properties of graphene make it a very attractive application, although there are still no commercial products in which graphene would play a key role. Good thermal conductivity is undoubtedly one of the attributes which can be easily used both in materials involving large monoatomic layers, that are very difficult to obtain, as well as multilayer graphene flakes, which have been commercially available on the market for several years. The article presents the results of tests on the characteristic thermal properties of composites with the addition of 2–15% of multilayer graphene (MLG) crystals. The motivation of the study was literature reports showing the possibility of increasing the thermal conductivity of composites with MLG participation in the copper matrix. Since the production of composites with increased properties is associated with obtaining a strong orientation of the flakes in the structure, composites with hBN flakes exhibiting significantly worse but also directional thermal properties were produced for comparison. The paper showed a strong influence of flake morphology on the possibility of creating a directional structure. The obtained Cu/MLG composites with the addition of only 2% MLG were characterized by an increase in the thermal conductivity coefficient of about 30% in relation to sinters without the participation of MLG.

     

Controlled synthesis of ZnS quantum dots and ZnS quantum flakes with graphene as a template

Novel ZnS quantum dots (QDs) and ZnS quantum flakes (QFs) were successfully prepared with graphene nanosheets (GNs) as a special template, and two unique heterostructures of ZnS/GNs were also obtained. Due to the structure-directing template effect of GNs, the as-synthesized ZnS with different morphologies, dots or flakes, were uniformly distributed on the surface of GNs by controlling nucleation and growth. The two different heterostructures of ZnS/GNs exhibited obvious photovoltaic response, and ZnS/GN QFs-on-sheet heterostructures show higher photovoltage than that of ZnS/GN QDs-on-sheet.     

Role of method of synthesis on the size of flakes,dispersion stability and thermophysical properties of aqua based reduced graphene oxide nanofluids

Journal of Thermal Analysis and Calorimetry - The role of method of synthesis on the size of flakes, dispersion stability and thermophysical properties of aqua based reduced graphene oxide (rGO)...     

Solutions of negatively charged graphene sheets and ribbons

Negatively charged graphene layers from a graphite intercalation compound spontaneously dissolve in N-methylpyrrolidone, without the need for any sonication, yielding stable, air-sensitive, solutions of laterally extended atom-thick graphene sheets and ribbons with dimensions over tens of micrometers. These can be deposited on a variety of substrates. Height measurements showing single-atom thickness were performed by STM, AFM, multiple beam interferometry, and optical imaging on Sarfus wafers, demonstrating deposits of graphene flakes and ribbons. AFM height measurements on mica give the actual height of graphene (ca. 0.4 nm).     

High-yield synthesis of few-layer graphene flakes through electrochemical expansion of graphite in propylene carbonate electrolyte

High-yield production of few-layer graphene flakes from graphite is important for the scalable synthesis and industrial application of graphene. However, high-yield exfoliation of graphite to form graphene sheets without using any oxidation process or super-strong acid is challenging. Here we demonstrate a solution route inspired by the lithium rechargeable battery for the high-yield (>70%) exfoliation of graphite into highly conductive few-layer graphene flakes (average thickness <5 layers). A negative graphite electrode can be electrochemically charged and expanded in an electrolyte of Li salts and organic solvents under high current density and exfoliated efficiently into few-layer graphene sheets with the aid of sonication. The dispersible graphene can be ink-brushed to form highly conformal coatings of conductive films (15 ohm/square at a graphene loading of <1 mg/cm(2)) on commercial paper.     

Effect of the C/O ratio in graphene oxide materials on the reinforcement of epoxy-based nanocomposites

The effect of the C/O ratio of graphene oxide materials on the reinforcement and rheological percolation of epoxy-based nanocomposites has been studied. As-prepared graphene oxide (GO) and thermally-reduced graphene oxide (TRGO) with higher C/O ratios were incorporated into an epoxy resin matrix at loadings from 0.5 to 5 wt %. Tensile testing revealed good reinforcement of the polymer up to optimal loadings of 1 wt %, whereas agglomeration of the flakes at higher loadings caused the mechanical properties of the composites to deteriorate. The level of reduction (C/O) of the graphene oxide filler was found to influence the mechanical and rheological properties of the epoxy composites. Higher oxygen contents were found to lead to stronger interfaces between graphene and epoxy, giving rise to higher effective Young's moduli of the filler and thus to superior mechanical properties of the composite. The effective modulus of the GO in the nanocomposites was found to be up to 170 GPa. Furthermore, rheological analysis showed that highly oxidized graphene flakes did not raise the viscosity of the epoxy resin significantly, facilitating the processing considerably, of great importance for the development of these functional polymeric materials. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 281–291     

Structures and electronic states of halogen-terminated graphene nano-flakes

Halogen-functionalized graphenes are utilized as electronic devices and energy materials. In the present paper, the effects of halogen-termination of graphene edge on the structures and electronic states of graphene flakes have been investigated by means of density functional theory (DFT) method. It was found that the ionization potential (Ip) and electron affinity of graphene (EA) are blue-shifted by the halogen termination, while the excitation energy is red-shifted. The drastic change showed a possibility as electronic devices such as field-effect transistors. The change of electronic states caused by the halogen termination of graphene edge was discussed on the basis of the theoretical results.     

Electrochemical reduction and electric conductivity of graphene oxide films

The types of cells and methods of electrochemical reduction of graphene oxide films were described. The possibility of creating ultrathin membrane–electrode assemblies of supercapacitor cells was demonstrated. The peculiarities of the electrochemical behavior of films of different thicknesses that contact with carbon and metal current collectors were shown. The limiting charge (1500–2000 C/g) for complete electrochemical reduction of graphene oxide was determined. Possible mechanisms of proton conductivity along the basal faces of graphene oxide flakes were proposed. The nature of the current collector was shown to affect the electron–hole conductivity of graphene oxide films and the observed contact potential difference.     

Morphology,structure, and photoactivity of two types of graphene oxide-TiO<Subscript>2</Subscript> composites

Two types of graphene oxide-TiO₂ composites were prepared: one by including graphene oxide flakes in the TiO₂ sol, followed by thermal treatment (GI composite) at 300°C, and the second by including graphene oxide flakes in the calcined (at 500°C) TiO₂ xerogel (GII composite). The composites were characterized by SEM, TEM-EDS, TEM-SADP, STEM-HAADF, HRTEM coupled with FT, XRD, and XPS. Photocatalysis results were fitted to different kinetic models (pseudo-first and pseudo-second kinetics, intraparticle Weber-Morris diffusion, film diffusion, and external mass transfer). The results showed that by introducing graphene oxide flakes in the TiO₂ sol, followed by thermal treatment at 300°C (GI composite), an efficient graphene oxide-TiO₂ catalyst with high specific surface area, heterogeneity, and many graphitized areas can be obtained. Complete crystallization of the composite is not the key issue for the best photoactivity achievement. The rate limiting step in the photocatalytic process is the photooxidation of SA molecules on the TiO₂ surface.     

Highly concentrated aqueous dispersions of graphene exfoliated by sodium taurodeoxycholate: dispersion behavior and potential application as a catalyst support for the oxygen-reduction reaction

A high-yielding exfoliation of graphene at high concentrations in aqueous solutions is critical for both fundamental study and future applications. Herein, we demonstrate the formation of stable aqueous dispersions of pristine graphene by using the surfactant sodium taurodeoxycholate under tip sonication at concentrations of up to 7.1 mg mL(-1). TEM showed that about 8% of the graphene flakes consisted of monolayers and 82% of the flakes consisted of less than five layers. The dispersions were stable regardless of freezing (-20 °C) or heat treatment (80 °C) for 24 h. The concentration could be significantly improved to about 12 mg mL(-1) by vacuum-evaporation of the dispersions at ambient temperature. The as-prepared graphene dispersions were readily cast into conductive films and were also processed to prepare Pt/graphene nanocomposites that were used as highly active electrocatalysts for the oxygen-reduction reaction.     

Solvent-exfoliated graphene at extremely high concentration

We describe three related methods to disperse graphene in solvents with concentrations from 2 to 63 mg/mL. Simply sonicating graphite in N-methyl-2-pyrrolidinone, followed by centrifugation, gives dispersed graphene at concentrations of up to 2 mg/mL. Filtration of a sonicated but uncentrifuged dispersion gives a partially exfoliated powder that can be redispersed at concentrations of up to 20 mg/mL. However, this process can be significantly improved by removing any unexfolaited graphite from the starting dispersion by centrifugation. The centrifuged dispersion can be filtered to give a powder of exfoliated few-layer graphene. This powder can be redispersed at concentrations of at least 63 mg/mL. The dispersed flakes are ~1 μm long and ~3 to 4 layers thick on average. Although some sedimentation occurs, ~26-28 mg/mL of the dispersed graphene appears to be indefinitely stable.     

Direct synthesis of noble metal/graphene nanocomposites from graphite in water: photo-synthesis

We report on the direct and facile method for noble metal/graphene nano-composites from graphite without reducing agents. In this system, the irradiant white-light instead of the chemical reducing agent exerts the influence on the synthesis of noble metal nanoparticles on graphene. Noble metal salts adsorbed on graphene flakes which were functionalized with ionic surfactants were reduced by irradiation with white light. In particular, noble metal nanoparticles were more evenly distributed on the surface of graphene which was functionalized with SDS than with CTAB.     

Theoretical study of nitrogen‐doped graphene nanoflakes: Stability and spectroscopy depending on dopant types and flake sizes

As nitrogen‐doped graphene has been widely applied in optoelectronic devices and catalytic reactions, in this work we have investigated where the nitrogen atoms tend to reside in the material and how they affect the electron density and spectroscopic properties from a theoretical point of view. DFT calculations on N‐doped hexagonal and rectangular graphene nanoflakes (GNFs) showed that nitrogen atoms locating on zigzag edges are obviously more stable than those on armchair edges or inside flakes, and interestingly, the N‐hydrogenated pyridine moiety could be preferable to pure pyridine moiety in large models. The UV–vis absorption spectra of these nitrogen‐doped GNFs display strong dependence on flake sizes, where the larger flakes have their major peaks in lower energy ranges. Moreover, the spectra exhibit different connections to various dopant types and positions: the graphitic‐type dopant species present large variety in absorption profiles, while the pyridinic‐type ones show extraordinary uniform stability and spectra independent of dopant positions/numbers and hence are hardly distinguishable from each other. © 2018 Wiley Periodicals, Inc.     

Steam etched porous graphene oxide network for chemical sensing

Oxidative etching of graphene flakes was observed to initiate from edges and the occasional defect sites in the basal plane, leading to reduced lateral size and a small number of etch pits. In contrast, etching of highly defective graphene oxide and its reduced form resulted in rapid homogeneous fracturing of the sheets into smaller pieces. On the basis of these observations, a slow and more controllable etching route was designed to produce nanoporous reduced graphene oxide sheets by hydrothermal steaming at 200 °C. The degree of etching and the concomitant porosity can be conveniently tuned by etching time. In contrast to nonporous reduced graphene oxide annealed at the same temperature, the steamed nanoporous graphene oxide exhibited nearly 2 orders of magnitude increase in the sensitivity and improved recovery time when used as chemiresistor sensor platform for NO(2) detection. The results underscore the efficacy of the highly distributed nanoporous network in the low temperature steam etched GO.     

The Kernel energy method: Application to graphene and extended aromatics

The quantum chemistry of finite aperiodic graphene flakes is a matter of considerable interest because of the anticipated technological importance of such objects. Since real aperiodic graphene flakes will in general be composed of many thousands of carbon atoms, theoretical methods appropriate to such large molecules would need to be used for the ab initio quantum calculation of their properties. The Kernel energy method is discussed here, and it is shown to be accurately applicable to graphenes and analogous extended aromatic molecules. It is necessary to define the kernels of a graphene molecule in a new way because of the extensive aromaticity, which defines its electronic structure. The kernels used in the reconstruction of the full graphene sheet preserve the total number of π‐electrons, Clar sextets, and the approximate overall aromaticity. Sivaramakrishnan et al. [J Phys Chem A, 2005, 109, 1621] define similar “ring conserved isodesmic reactions (RCIR).” The principal innovation of this article is the suggestion that kernels may be mathematically extracted from an extended aromatic molecule such as graphene by a fissioning of aromatic bonds. Hartree Fock (HF) and Møller‐Plesset (MP2) chemical models using a Gaussian basis of 3‐21G orbitals are used to calculate the total energy of a graphene flake. This demonstration calculation is performed on a graphene flake in which dangling bonds are saturated with hydrogens (C₇₈H₂₆) composed of a total of 104 atoms arranged in 27 benzenoid rings. The KEM with both types of chemical model are shown to be accurate to nearly 1 kcal/mol, of a total energy, which is nearly 3000 atomic units, that is, with an absolute error within “chemical accuracy” and a relative error of the order of 5 × 10⁵% of the total energy. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Electrochemical synthesis and the functionalization of few layer graphene in ionic liquid and redox ionic liquid

Electrochemical reductive exfoliation of graphite to few layered graphene(FLG) in presence of 1-ethyl-2,3-dimethyl imidazolium bis(trifluoromethylsulfonyl) imide ionic liquid and redox ionic liquid based ferrocene has been investigated. Thus, by applying a mild negative potential(-2.7 V vs. Fc/Fc~+) to carbon electrode in ionic liquid graphene flakes could be generated. The generated materials have been characterized by X-ray photoelectron spectroscopy, Raman spectroscopy, high resolution transmission electron microscopy and atomic force microscopy. XPS and Raman analysis show that the electrochemical reductive exfoliation provides the formation of FLG. The thickness of the resulting FLG was found to be ranged between 4 and1 nm. HR-TEM images reveal the formation of few graphene layers and in some cases single graphene layer was observed.Moreover, this electrochemical route conduces to the formation of ionic liquid functionalized FLG. Finally, the reductive exfoliation was further investigated in the presence of redox ionic liquid. XPS and electrochemical measurements confirm the presence of ferrocene.     

Thermal properties enhancement of epoxy resins by incorporating polybenzimidazole nanofibers filled with graphene and carbon nanotubes as reinforcing material

Enhancement of thermal properties of epoxy resins was achieved by incorporation of polybenzimidazole (PBI) fibermats filled with carbon nanomaterials, prepared by the solution electrospinning technique. Different type of carbon nanostructures (carbon nanotubes, graphite flakes, graphene nanoplatelets and carbon black) were compared as fillers in polybenzimidazole fibers. The carbon-PBI-fibermats showed remarkable thermal transport properties and therefore, they were studied as thermal reinforcement material for epoxy composites. Mechanical and thermal properties of produced composites were evaluated and the effectiveness of different types of carbon fillers examined. Results showed that the produced carbon filled fibermats can be used effectively as a thermal reinforcing material in epoxy resins, offering several advantages.     

Enhanced thermal and mechanical properties of poly (vinylidene fluoride) nanocomposites reinforced by liquid-exfoliated graphene

We report the preparation and enhanced thermal and mechanical properties of poly (vinylidine diflouride) (PVDF) nanocomposites reinforced by few-layer graphene flakes which are produced by the direct liquid-phase exfoliation of pristine graphite. Graphene flakes are found to homogeneously disperse in PVDF, reduce the bubble defects and thus the porosity of PVDF, and change PVDF’s crystallinity. Thermogravimetric analysis indicates that graphene can accelerate the fracture of hydrogen bond connecting PVDF and N-Methyl pyrrolidone molecules. 1.5?wt% graphene loading leads to around 20?°C enhancement in the melting temperature of PVDF. The mechanical properties like Young’s modulus (EIT), yield stress (σy), and hardness (H) of the nanocomposites are investigated by nanoindentation technique. A 1.0?wt% loading of graphene is found to increase EIT, σy, and H of PVDF by ～337%, ～102%, and ～228%, respectively.

• Highlights

• Few-layer graphene was produced by liquid-phase exfoliation.

• Graphene were added to PVDF to enhance thermal and mechanical properties of polymer.

• Mechanical properties of PVDF/graphene composite films were investigated by nanoindentation.

     

Thiourea aldehyde resin-based carbon/graphene composites for high-performance supercapacitors

Thiourea aldehyde resin-based heteroatom doping carbon and graphene composites (RHDC/GN) were prepared by an in situ polymerization and carbonization. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that thiourea aldehyde resin deposited on lamellar GO flakes during the polymerization and RHDC/GN composites had a hierarchical structure. The specific capacitance of the RHDC/GN composites was high up to 355 F g^?1, much higher than that of the pure thiourea aldehyde resin-based heteroatom doping carbon (RHDC) with specific capacitance of 135 F g^?1 at a current density of 1.0 A g^?1 in 6-M KOH electrolyte. And the hetroatoms in RHDC/GN composites increase the specific capacitance, and GN enhances the conductivity of the electrodes which is beneficial to improving electrochemical cycling stability of the electrode significantly. The specific capacitance retains 90.97% after 5000 charge-discharge processes at 10 A g^?1, which provides potential as supercapacitors.     

Cobalt oxyhydroxide/graphene oxide nanocomposite for amelioration of electrochemical performance of lithium/sulfur batteries

Cobalt oxyhydroxide combination with graphene oxide (CoOOH@GO) as a novel conductive matrix is developed for high performance lithium/sulfur batteries. Enhancement retention of polysulfide species into matrix of cobalt oxyhydroxide anchored on graphene oxide flakes by strong chemical binding of carbon-sulfur is demonstrated. Sulfur incorporated in the sheet-like morphology of CoOOH@GO delivers high initial discharge specific capacity of 1190.85 mAh/g, which raises 260 mAh/g with respect to graphene oxide/sulfur (GO/S) as a cathode material. Furthermore, CoOOH@GO/S maintains the average coulombic efficiency of 96 % after 300 cycles at 1 C rate with capacity retention of about 61 %. Good current rate capability of CoOOH@GO/S cathode reveals that the resulting composite is open platform for electrolyte diffusion and fast ion transportation leading to the improved electrochemical performance of lithium/sulfur batteries.