Percolation Transition Under Thermal Reduction of Graphene Oxide

The conductivity of thermally reduced graphene oxide (GO) measured at various reduction stages shows that the samples demonstrate transition to the conducting state within a narrow range of annealing temperatures (150-170 °С). The conductivity in this temperature region increases by about five orders of magnitude to indicate that the percolation mechanism is responsible for this transition. According to this mechanism, the above temperature range is associated with formation of a certain number of conductive channels composed of GO fragments reduced to the conducting state. Thus, it is a new type of percolation transition, since increased conductivity is a result of increased conductivity of the particles due to thermal treatment rather than a result of increased concentration of conducting particles. A further temperature growth is associated with increased number of conducting GO fragments and the number of conducting channels, and some smoother conductivity increase. The XPS spectra of partially reduced GO samples indicate correlation between increased conductivity of GO fragments and plasmon oscillations revealed as XPS peaks. The current-voltage characteristics measured for the samples of partially reduced GO indicate a non-ohmic conductivity which manifests as increased conductivity versus voltage dependence. This is due to the fact that electrical resistance of the percolation channels is the sum of resistances of conductive GO fragments and contact resistances which depend on the voltage drop at the contact and, therefore, on the voltage applied.     

Multifunctional nanocomposites based on tetraethylenepentamine-modified graphene oxide/epoxy

Composites based on epoxy/graphene were investigated for thermal-mechanical performance. Initially, few-layer graphene oxide (GO) was modified with tetraethylenepentamine (GO-TEPA) in a reaction assisted by microwave radiation. GO and GO-TEPA samples were characterized for their structure and morphology. Composites containing 0.1, 0.3 and 0.5 wt.% of GO and GO-TEPA were prepared, and the effect of fillers on the morphology of cryofractured regions of epoxy matrix was observed through electron microscopy images. Dynamic mechanical thermal analysis (DMA) tests revealed increases of approximately 20 °C in glass transition. Moreover, when compared to neat polymer, composites containing 0.5 wt.% of GO-TEPA gained up to 103% in thermal conductivity (obtained by flash laser). Finally, nanoindentation analyses showed increases of 72% in Young's modulus and 143% in hardness for the same sample. The system is characterized as multifunctional nanocomposites because of the simultaneous gains in thermal and mechanical properties. The best results of the multifunctional composites were strongly associated with the chemical modification of the GO by TEPA.     

Enhancement of tensile,electrical and thermal properties of epoxy nanocomposites through chemical hybridization of polypyrrole and graphene oxide

This paper presents the properties of epoxy nanocomposites, prepared using a synthesized hybrid Polypyrrole-Graphene Oxide (PPy-GO) filler, via in-situ chemical polymerization, at various filler loadings (i.e., 0.5–2 w. t %). The microstructures and properties of the PPy-GO hybrids and epoxy nanocomposites were studied via Fourier transform infrared (FTIR), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), mechanical (Tensile Properties), electrical, Dynamic mechanical thermal analysis (DMTA) and thermogravimetric analyses (TGA). Morphological study demonstrated that varying the nanofiller nature (PPy-GOs, PPy or GO) lead to different states of dispersion. Mechanical, electrical and thermal analysis demonstrated that the hybrid concentration and its architecture (PPy:GO ratio) are interesting factors significantly affected the properties of the epoxy based nanocomposites. On the other hand, the mechanical performance of the cured nanocomposites outperformed the PPy-GO, with enhancements of 78% and 51% of Young's modulus and strength, respectively. Here it has been established that the embedding of PPy-GO hybrids into pristine epoxy endows optimum dispersion of PPy and GO as well as better interfacial adhesion between the fillers and matrix, which results in a significant improvement in load transfer effectiveness. Electrical conductivity measurements showed that conductivity of epoxy filled nanocomposites increased up 10⁻⁴ S/cm for Epoxy/PPy-GO nanocomposites. DMTA test indicated that incorporation of PPy-GO resulted in a significantly increase in Tg of the resultant nanocomposites, which is attributed to the highly exfoliation structure and the stronger interfacial interaction. The PPy-GO particles enhanced electrical, thermal and mechanical properties of nanocomposites, confirming the synergistic effect of PPy-GO as multifunctional filler.     

Synergistic effect of hybrid graphene and boron nitride on the cure kinetics and thermal conductivity of epoxy adhesives

In the present study, the synergistic effect of hybrid boron nitride (BN) with graphene on the thermal conductivity of epoxy adhesives has been reported. Graphene was prepared by chemical reduction of graphite oxide (GO) in a mixture of concentrated H₂SO₄/H₃PO₄ acid. The particle size distribution of GO was found to be ~10 μm and a low contact angle of 54° with water indicated a hydrophilic surface. The structure of prepared graphene was characterized by Fourier transform infrared (FTIR), X‐ray diffraction (XRD), Raman spectroscopy and atomic force microscopy (AFM). The thermal conductivity of adhesives was measured using guarded hot plate technique. Test results indicated an improvement in the thermal conductivity up to 1.65 W/mK, which was about ninefold increase over pristine epoxy. Mechanical properties of different epoxy formulations were also measured employing lap shear test. The surface characterization of different epoxy adhesive systems was characterized through XRD, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies. Fourier transform infrared also served to determine the nature of interactions between filler particles and epoxy resin. Non‐isothermal differential scanning calorimetric (DSC) technique was used to investigate the effects of graphene and BN particles on the cure kinetics and cross‐linking reaction of epoxy cured with amine curing agent. The Kissinger equation, the model‐free isoconversional Flynn–Wall–Ozawa method and the Ozawa model were used to analyze the kinetic parameter. Copyright © 2017 John Wiley & Sons, Ltd.     

纯水中氧化石墨烯的超声裁剪及机理研究

采用超声在纯水中对氧化石墨烯(GO)进行裁剪, 研究了超声时间对GO片层尺寸、 官能团种类和含量的影响, 探讨了超声裁剪GO的机理. 用原子力显微镜(AFM)和透射电子显微镜(TEM)对产物进行观测, 结果表明, 随着超声时间延长, GO片层平均直径从2000 nm减小至200 nm. 红外光谱(FTIR)、 X射线光电子能谱(XPS)及热分析结果表明, 随着超声时间延长, GO的含氧官能团种类没有变化, 但是GO含氧官能团总量逐渐升高, 其中环氧基团含量逐渐下降, 羧酸基团含量变化不大, 酮基基团含量先下降后升高, 羟基基团含量先升高后下降, 醚氧基团含量逐渐升高. 基于上述结果, 提出了超声裁剪GO的机理: 超声空化现象形成的剪切力使GO的碳碳键、 环氧、 酮基基团断裂, GO片层尺寸变小; 在超声诱导下水分子形成的羟基自由基和氢自由基与片层上断裂形成的碳自由基结合生成羟基或醚氧基团, 未发生水解的碳自由基形成新的碳碳键, 长时间的超声作用还会使羟基基团发生断裂并转化成酮基基团. 利用超声在纯水中裁剪GO的条件温和, 不使用其它化学试剂, 可以有效控制GO片层尺寸, 是一种具有应用前景的二维材料裁剪技术.     

Proton Conductivities of Graphene Oxide Nanosheets: Single,Multilayer, and Modified Nanosheets

Proton conductivities of layered solid electrolytes can be improved by minimizing strain along the conduction path. It is shown that the conductivities (σ) of multilayer graphene oxide (GO) films (assembled by the drop‐cast method) are larger than those of single‐layer GO (prepared by either the drop‐cast or the Langmuir‐Blodgett (LB) method). At 60 % relative humidity (RH), the σ value increases from 1×10^?6 S cm^?1 in single‐layer GO to 1×10^?4 and 4×10^?4 S cm^?1 for 60 and 200 nm thick multilayer films, respectively. A sudden decrease in conductivity was observed for with ethylenediamine (EDA) modified GO (enGO), which is due to the blocking of epoxy groups. This experiment confirmed that the epoxide groups are the major contributor to the efficient proton transport. Because of a gradual improvement of the conduction path and an increase in the water content, σ values increase with the thickness of the multilayer films. The reported methods might be applicable to the optimization of the proton conductivity in other layered solid electrolytes.     

Proton Conductivities of Graphene Oxide Nanosheets: Single,Multilayer, and Modified Nanosheets

Proton conductivities of layered solid electrolytes can be improved by minimizing strain along the conduction path. It is shown that the conductivities (σ) of multilayer graphene oxide (GO) films (assembled by the drop‐cast method) are larger than those of single‐layer GO (prepared by either the drop‐cast or the Langmuir‐Blodgett (LB) method). At 60 % relative humidity (RH), the σ value increases from 1×10⁻⁶ S cm⁻¹ in single‐layer GO to 1×10⁻⁴ and 4×10⁻⁴ S cm⁻¹ for 60 and 200 nm thick multilayer films, respectively. A sudden decrease in conductivity was observed for with ethylenediamine (EDA) modified GO (enGO), which is due to the blocking of epoxy groups. This experiment confirmed that the epoxide groups are the major contributor to the efficient proton transport. Because of a gradual improvement of the conduction path and an increase in the water content, σ values increase with the thickness of the multilayer films. The reported methods might be applicable to the optimization of the proton conductivity in other layered solid electrolytes.     

高分子阴离子电导的研究

制备了含Ｉ＾－榻解质薄膜,用阻抗谱法测量了样品的Ｉ＾－电导率,分析了样品的频响特性曲线,用Ｗａｇｎｅｒ直流极化法测量了样品的电子电导。实验发现,此阴离子导体几乎和相应的含Ｎａ＾＋有相同的离子电导率,且随着碘含量的增加, 率增大。离子电导实验结果表明,在此阴离子导体中,电子电导占总电导率的９％,而空穴的导导率比电子电导率小１￣２个数量级。     

High thermal conductivity EP adhesive based on the GO/EP interface optimized by TDI

Graphene oxide (GO) was used as the filler to modify the epoxy resin (EP) adhesive, and the GO/EP interface was optimized by toluene diisocyanate (TDI) in order to improve the thermal conductivity and T peel strength performance of the adhesive. Through the characterization of the GO product, which was modified by TDI, TDI was grafted onto the surface of GO, and there were NCO groups remaining; thus the chemical bonds were built onto the interface which was non‐wetting between GO and EP. The results of the properties characterization of the adhesive indicated that the bonding properties were significantly enhanced, especially the T peel strength, which was up to 9.62 N/mm, which was contributed by the optimized GO/EP interface. The thermal conductivity of the adhesive increased to 0.624 W m^?1 K^?1, as the interface thermal resistance was reduced after the interface between GO/EP was optimized by TDI. The insulation performance of the adhesive was also improved, since the well‐dispersed GO formed a micro‐capacitance model in EP, and the surface of GO was covered by the EP so that the electronic paths were blocked by the formed chemical bonds.     

Validation of experimental results for graphene oxide-epoxy polymer nanocomposite through computational analysis

The change in interfacial interaction behavior of epoxy resin nanocomposites with the incorporation of graphene oxide (GO) was explored experimentally and computationally. GO with different weight (wt) loading was incorporated in epoxy resin by a three-way dispersion method. GO formed mechanical interlocking with epoxy resin, thereby resulting in a remarkable enhancement in mechanical and thermo-mechanical properties of GO-epoxy nanocomposite. In 0.3 wt% GO-epoxy nanocomposites, improvement of 26.7% in flexural strength and 39.2% in flexural modulus was reported. Using dynamic mechanical analysis (DMA), thermomechanical analysis (TMA) and differential scanning calorimetry (DSC), glass transition temperature (T_g) of 182.7°C and maximum thermal stability was reported for 0.3% GO-epoxy nanocomposite. The effect of GO on cross-linking in GO-epoxy nanocomposite was analyzed by DSC and Raman spectroscopy. The X-ray photoelectron spectroscopy (XPS) study was utilized to determine the interfacial interaction, and further was verified by density functional theory (DFT). By experimental and computational study, H-bonding was observed to improve interfacial interaction in GO-epoxy nanocomposite.     

In‐Situ “molecular welding” preparation of graphene/polyimide hybrid film with superior thermal conductivity and flexibility

With the rapid development of electronic industry, thermal management has become a critical issue that severely restricts the application of portable devices. In this work, we fabricate a flexible and free‐standing graphitized‐graphene/polyimide (I‐g‐GO/PI) film via an in‐situ “molecular welding” strategy. With the help of in‐situ polymerization, PI can be well‐dispersed with GO and serves as a solder to enlarge the grain size of GO, resulting in an enhanced thermal conductivity of the film. The 7 wt % addition of PI into GO (I‐g‐GO/PI‐7%) leads to an in‐plane thermal conductivity as high as 1269.700 ± 1.498 W/m/K, which is 81.8% higher than that of the pristine graphene and also superior to that fabricated via solution blending method by 58.3%. Simultaneously, the hybrid film exhibits an excellent flexibility and survives from a 2000 cycles bending test. The large‐area hybrid film prepared by such an in‐situ “molecular welding” method provides a promising way to fabricate graphene‐based film for highly efficient thermal management. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1215–1223     

Characteristics of Raman spectra for graphene oxide from ab initio simulations

The Raman spectra of several locally stable structures of the graphene oxide (GO) have been simulated by ab initio calculations. Compared to graphite, the G band of GO is broadened and blueshifted due to the emergence of a series of new Raman peaks. The Raman intensities and positions of the D and G bands depend sensitively on the local atomic configurations. In addition to the normal epoxy and hydroxyl groups, other oxidation groups such as epoxy pairs are also studied. Epoxy pairs induce large blueshift of G band with respect to that of the graphite.     

Preparation and characterization of functionalized graphene oxide/carbon fiber/epoxy nanocomposites

Graphene oxide (GO) was functionalized using three different diamines, namely ethylenediamine (EDA), 4,4′-diaminodiphenyl sulfone (DDS) and p-phenylenediamine (PPD) to reinforce an epoxy adhesive, with the aim of improving the bonding strength of carbon fiber/epoxy composite. The chemical structure of the functionalized GO (FGO) nanosheets was characterized by elemental analysis, FT-IR and XRD. Hand lay-up, as a simple method, was applied for 3-ply composite fabrication. In the sample preparation, the fiber-to-resin ratio of 40:60 (w:w) and fiber orientations of 0°, 90°, and 0° were used. The GO and FGO nanoparticles were first dispersed in the epoxy resin, and then the GO and FGO reinforced epoxy (GO- or FGO-epoxy) were directly introduced into the carbon fiber layers to improve the mechanical properties. The GO and FGO contents varied in the range of 0.1–0.5 wt%. Results showed that the mechanical properties, in terms of tensile and flexural properties, were mainly dependent on the type of GO functionalization followed by the percentage of modified GO. As a result, both the tensile and flexural strengths are effectively enhanced by the FGOs addition. The tensile and flexural moduli are also increased by the FGO filling in the epoxy resin due to the excellent elastic modulus of FGO. The optimal FGO content for effectively improving the overall composite mechanical performance was found to be 0.3 wt%. Scanning electron microscopy (SEM) revealed that the failure mechanism of carbon fibers pulled out from the epoxy matrix contributed to the enhancement of the mechanical performance of the epoxy. These results show that diamine FGOs can strengthen the interfacial bonding between the carbon fibers and the epoxy adhesive.     

Liquid crystalline epoxies bearing biphenyl ether and aromatic ester mesogenic units: Synthesis and thermal properties

Two liquid crystalline epoxies containing biphenyl ether and aromatic ester mesogenic units, oxybis(4,1-phenylene)bis(4-(oxiran-2-ylmethoxy)benzoate)(LCE1) and oxybis(4,1-phenylene) bis(4-(4-(oxiran-2-yl)butoxy)benzoate)(LCE2), were synthesized and characterized. Subsequently, the epoxy monomers were cured with diaminodiphenylsulfone (DDS). From DSC, XRD and POM results, monomers did not show liquid crystalline phase while the cured samples exhibited nematic phase. The cured samples showed good mechanical properties with strength of 99.1MPa and excellent thermal stabilities with high glass transition temperature up to 168.0?°C, 5% weight loss temperature at 343?°C and high char yield of 24.5% at 800?°C. The relationship between thermal conductivity and network structure was discussed in this work. Due to the introduction of mesogenic units into epoxy networks, the cured resins showed high thermal conductivity as high as 0.292?W/(m*K), more than 1.5times higher than conventional epoxy resins. By introducing alumina (Al₂O₃) into LCE1/DDS cured system, composites of LCE1/DDS/Al₂O₃ with the highest thermal conductivity of 1.61?W/(m*K) was obtained with the content of 80?wt% while that of diglycidyl ether of bisphenol A (DGEBA, E51) epoxy resin/DDS/Al₂O₃ was 1.10?W/(m*K). The as-prepared epoxy resins showed high glass transition temperature and excellent thermal stabilities, indicating the potential of application in microelectronics.     

Effect of aspect ratio of vertically aligned copper nanowires in the presence of cellulose nanofibers on the thermal conductivity of epoxy composites

The composites comprising vertically aligned network of copper nanowires (CuNWs) in the presence of cellulose nanofibers were fabricated by using the freeze‐templating method and the effect of aspect ratio (A/R) of CuNWs on the thermal conductivity of epoxy composites was investigated. The thermal conductivity of epoxy composites increased to 0.79 W m^?1 K^?1 at 1.12 vol% of high A/R CuNWs loading, corresponding to the thermal conductivity enhancement of 365% as compared to the pure epoxy. The thermal conductivity of vertically aligned higher A/R CuNWs/epoxy, which is 38.5% and 51.9% higher than those of the lower A/R CuNWs and the randomly aligned CuNWs, respectively. The application of the epoxy composites in heat dissipation was demonstrated by the temperature changes of composites on a hot plate with the increase of heating time. These results indicate that the thermally conductive composites in this study could be applied for thermal dissipating materials in electronic devices.     

Reduced graphene oxide conjugated Cu2O nanowire mesocrystals for high-performance NO2 gas sensor

Reduced graphene oxide (rGO)-conjugated Cu(2)O nanowire mesocrystals were formed by nonclassical crystallization in the presence of GO and o-anisidine under hydrothermal conditions. The resultant mesocrystals are comprised of highly anisotropic nanowires as building blocks and possess a distinct octahedral morphology with eight {111} equivalent crystal faces. The mechanisms underlying the sequential formation of the mesocrystals are as follows: first, GO-promoted agglomeration of amorphous spherical Cu(2)O nanoparticles at the initial stage, leading to the transition of growth mechanism from conventional ion-by-ion growth to particle-mediated crystallization; second, the evolution of the amorphous microspheres into hierarchical structure, and finally to nanowire mesocrystals through mesoscale transformation, where Ostwald ripening is responsible for the growth of the nanowire building blocks; third, large-scale self-organization of the mesocrystals and the reduction of GO (at high GO concentration) occur simultaneously, resulting in an integrated hybrid architecture where porous three-dimensional (3D) framework structures interspersed among two-dimensional (2D) rGO sheets. Interestingly, "super-mesocrystals" formed by 3D oriented attachment of mesocrystals are also formed judging from the voided Sierpinski polyhedrons observed. Furthermore, the interior nanowire architecture of these mesocrystals can be kinetically controlled by careful variation of growth conditions. Owing to high specific surface area and improved conductivity, the rGO-Cu(2)O mesocrystals achieved a higher sensitivity toward NO(2) at room temperature, surpassing the performance of standalone systems of Cu(2)O nanowires networks and rGO sheets. The unique characteristics of rGO-Cu(2)O mesocrystal point to its promising applications in ultrasensitive environmental sensors.     

Graphene oxide/epoxy composites with enhanced fracture toughness for liquid hydrogen storage

Graphene oxide (GO)/epoxy composites cured by aliphatic dibasic acids have been prepared. The influences of structure of aliphatic dibasic acid and loading of GO on curing process and mechanical properties of epoxy composites were studied. The results show that the reaction activities, gel time of corresponding epoxy-acid system and tensile strength of the formed epoxy resins decrease with the increase of the chain length of aliphatic dibasic acids. Both fracture toughness (>1.96 MPa⋅m^1/2) and elongations at break (>6%) increase with the increase of the chain length of aliphatic dibasic acids. The introduction of GO is helpful to increase the mechanical properties and the gas transmission coefficient of GO/epoxy composites. A maximum of tensile strength and elongations at break were obtained when the loading of GO is 0.6 wt%. The gas transmission coefficient of GO/epoxy composite increases with the increase of GO loading. The excellent mechanical properties and gas leakage resistance coefficient of the formed epoxy composites provides potential application in many fields where conventional brittle epoxy resins are inapplicable.     

Ozonated Graphene Oxide Film as a Proton‐Exchange Membrane

Graphene oxide (GO) contains several chemical functional groups that are attached to the graphite basal plane and can be manipulated to tailor GO for specific applications. It is now revealed that the reaction of GO with ozone results in a high level of oxidation, which leads to significantly improved ionic (protonic) conductivity of the GO. Freestanding ozonated GO films were synthesized and used as efficient polymer electrolyte fuel cell membranes. The increase in protonic conductivity of the ozonated GO originates from enhanced proton hopping, which is due to the higher content of oxygenated functional groups in the basal planes and edges of ozonated GO as well as the morphology changes in GO that are caused by ozonation. The results of this study demonstrate that the modification of dispersed GO presents a powerful opportunity for optimizing a nanoscale material for proton‐exchange membranes.     

水溶液体系中氧化石墨烯的γ射线辐射还原

石墨烯是一种碳原子以二维蜂窝状晶格结构构成的单片层材料,由于其具有优异的电传导性、力学性能和热传导性近年来受到广泛关注.本文采用γ射线辐射技术分别处理水溶液和对苯二胺(PPD)水溶液中的氧化石墨烯(GO),得到辐照还原氧化石墨烯(RGO)和胺基化修饰的还原氧化石墨烯(RGON).通过傅里叶变换红外(FTIR)光谱、X射线光电子能谱(XPS)、拉曼(Raman)光谱、X射线衍射(XRD)和热失重分析(TGA)等表征分析产物的化学结构和元素组成;通过四探针测试仪和接触角测量仪研究产物的导电性能和亲水性.实验结果表明,在水溶液及PPD水溶液中γ射线辐射均可高效还原GO,还原后得到的RGO和RGON电导率均显著增大.PPD的胺基在辐射还原过程中还可以修饰到石墨烯的表面,因此RGON的亲水性比RGO好,但胺基的存在会干扰石墨烯表面π电子的传导,导致其电导率下降.     

Synergistic effect of functional carbon nanotubes and graphene oxide on the anti‐corrosion performance of epoxy coating

In this work, we reported the synergistic effect of functional carbon nanotubes (CNTs) and graphene oxide (GO) on the anticorrosion performance of epoxy coating. For this purpose, the GO and CNTs were firstly modified by the 3‐aminophenoxyphthalonitrile to realize the nitrile functionalized graphene oxides (GO‐CN) and carbon nanotubes (CNTs‐CN). As modified GO‐CN and CNTs‐CN were characterized and confirmed by Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, and gravimetric analyzer. It was found that about 19 and 24 wt% of 3‐aminophenoxyphthalonitrile were grafted onto the surface of the GO and CNTs, respectively. The electrochemical impedance spectroscopy results showed that the GO‐CN&CNTs‐CN hybrid materials exhibit a remarkable superiority in enhancing the anticorrosion performance of epoxy coatings. Significant synergistic effect of the lamellar structural GO‐CN and CNTs‐CN on the anticorrosion performance of epoxy composite coatings was designed. Besides, the epoxy coating with 1 wt% of the GO‐CN&CNTs‐CN hybrid exhibited the best anticorrosion performance, in which the impedance showed the largest one (immersion in 3.5 wt% of NaCl solution for 168 hr). Copyright © 2016 John Wiley & Sons, Ltd.