Enhancement of electrical and thermal conductivity of polypropylene by graphene nanoplatelets

One of disadvantages of polymer composites is poor electrical and thermal conductivity. As a first step in this direction, graphene‐modified polypropylene polymer is being developed to improve its electrical and thermal conductivity. Two techniques were investigated: surface coating and extrusion. In the case of coating technique, the percolation threshold was found to be 0.5 wt % of graphene and electrical conductivity of polypropylene increased around 13 log cycles. Coating technique breaks the agglomerations due to magnetic stirring followed by sonication and gives homogeneous graphene‐coated polypropylene pellets. When polymer melts under compression molding, the graphene platelets network formed on the surface of polypropylene pellets as well as through‐the‐thickness of the molded disk, which provide continuous network of graphene. However, in extrusion technique, graphene segregated and did not disperse properly in polypropylene. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45833.     

Enhancement of weak localization for nitrogen-doped graphene by short range potentials

We have prepared pristine graphene and nitrogen-doped graphene on copper foils by chemical vapor deposition. Compared with the pristine graphene, an increased disorder in nitrogen-doped graphene was confirmed by Raman spectra studies. Temperature dependent Hall resistances and magnetoresistances were measured for both samples. The carrier densities can be extracted from the experimental datum. Abrupt decreases of magnetoresistances near zero magnetic field strongly suggest weak localization effects for both samples. Furthermore, more obvious decreases of magnetoresistances near zero magnetic field and valleys at higher magnetic fields were observed due to an enhancement of weak localization for nitrogen-doped graphene. The whole field dependence of magnetoresistance at different temperatures can be well fitted by a revised McCann model. By defining characteristic magnetic fields, visual phase diagrams were obtained. In addition, larger weak localization area was found for nitrogen-doped graphene than the one for pristine graphene. Our results manifest that an increased elastic intervalley scattering which comes from the increased disorder with short range potentials should account for the expected enhancement of the weak localization for nitrogen-doped graphene.     

Enhancement in the fluorescence of graphene quantum dots by hydrazine hydrate reduction

A simple and effective chemical method was reported to enhance the fluorescence of graphene quantum dots (GQDs). Specifically, water-soluble GQDs, prepared by solvothermal synthesis from graphene oxide, are chemically reduced by hydrazine hydrate to produce reduced GQDs (rGQDs). The results show that the hydrazine hydrate reduction not only decreases the O/C atomic ratio of GQDs, also changes the bonding type of N atoms. Such surface/edge chemical bond change of GQDs results in that as-made rGQDs exhibit more than two times fluorescence intensity as strong as that of the pristine GQDs.     

Enhancement of physical properties of electroactive polyimide nanocomposites by addition of graphene nanosheets

Electroactive polyimide (EPI) nanocomposites with amino‐capped aniline trimer and 4′‐(4,4′‐isopropylidene‐diphenoxy)bis(phthalic anhydride) as monomers, and functionalized with carboxyl‐graphene nanosheets, were prepared by thermal imidization. The as‐prepared electroactive polyimide/graphene nanocomposite (EPGN) materials were then characterized by Fourier transform infrared spectroscopy and transmission electron microscopy. In situ monitoring of the redox behavior of the as‐prepared EPGN materials was performed by cyclic voltammetry studies. The effects of material composition on the mechanical, thermal, thermal transport, dielectric and molecular barrier properties of EPGN membranes were investigated by dynamic mechanical analysis, TGA, DSC, the transient plane source technique, LCR meter and gas permeability analyzer, respectively. It should be noted that all the properties of the EPGN membranes were found to improve substantially over those of non‐electroactive polyimide and EPI. For example, upon loading of 1 wt% graphene, EPGN membranes were found to have an increase of over 20%, 5%, 65% and 20% in mechanical strength, thermal stability, thermal conductivity and dielectric constant, respectively, and a reduction of over 20% in gas permeability. © 2013 Society of Chemical Industry     

Exceptional thermal interface properties of a three-dimensional graphene foam

We have uncovered some unusual thermal interface properties of a three-dimensional, flexible and interconnected graphene foam (GF). The thermal interfacial resistance of GF at Si–Al interface is as low as 0.04 cm²K W⁻¹, which is one order of magnitude lower than conventional thermal grease and thermal paste-based thermal interfacial material (TIM). The thermal contact resistance was found to dominate the overall interfacial resistance of GF-based TIM, in as much as the bulk thermal conductivity of GF is rather high. The contact pressure-dependent thermal interfacial resistance of GF exhibits an asymptotic behavior, which converges into a plateau value at an ultralow contact pressure (∼0.1 MPa). Significantly, the GF-based TIM has shown a superior performance to vertically aligned carbon nanotubes currently held as the gold standard (at least ∼75% improvement in thermal interfacial resistance at Si–Al interface), thus providing a strong candidate for the next generation of high-performance carbon-based TIM.     

Enhancement of barrier properties of cement mortar with graphene nanoplatelet

The transport properties of cement mortar with graphene nanoplatelet (GNP) are investigated experimentally in this study. GNP, a low cost carbon-based nano-sheet, was added to mortar at contents of 0, 2.5, 5.0 and 7.5%, by weight of cement. The water penetration depth, chloride diffusion coefficient and chloride migration were determined for cement mortar with GNP and compared with plain cement mortar specimens. Test results showed that the addition of 2.5% GNP can cause significant decrease of 64%, 70% and 31% for water penetration depth, chloride diffusion coefficient and chloride migration coefficients respectively. The reduced water and ions ingress can be partially attributed to a reduction in the critical pore diameter of about 30%. This refinement of the microstructure by the GNP is validated by the mercury intrusion porosimetry (MIP) results. The impermeable GNP also contributes to the reduced permeability due to the increased tortuosity against water and aggressive ions ingress.     

Tuning the interface of graphene platelets/epoxy composites by the covalent grafting of polybenzimidazole

This paper reported a facile one-pot strategy for covalent functionalization of graphene platelets (GnPs) by polybenzimidazole, and the fabrication of their composites with epoxy resin. The functionalized GnPs (fGnPs) was prepared by subsequently acylation reaction between dicarboxylic acid and GnPs, and in-situ polymerization of polybenzimidazole. Spectroscopic studies and elemental analysis confirmed the successful grafting of polymer chains and the highly integrated structure of fGnPs, while TEM images demonstrated the well exfoliated state of fGnPs in organic solvent. As a consequence of the good dispersion state of fGnPs in matrix, and the covalent interactions between fGnPs and epoxy, the fGnPs/Epoxy composites showed significantly improved Young's modulus, tensile strength and fracture toughness as compared to neat epoxy or unmodified GnPs reinforced epoxy. The improved dynamic mechanical properties and thermal stabilities of composites filled with fGnPs were also demonstrated.     

Hydrogen bonding at the water/quasi-freestanding graphene interface

Vibrational spectroscopy has been used to study the interaction of water with quasi-freestanding graphene grown on Pt(1 1 1). A sharp O–H (O–D) vibrational band centered at 457 (337) meV is a direct evidence of the existence of non-H- (non-D-) bonded water molecules at the water/graphene interface. This finding is expected to play a significant role in understanding the behavior of water at hydrophobic surfaces. Such finding is in agreement with results reported for water adsorbed on other hydrophobic surfaces and with the behavior of water confined in carbon nanotubes and between graphene sheets.     

基于气泡动力学分段调控浸润性强化核态沸腾

基于多孔或微结构表面润湿性改性的核态沸腾强化传热,已得到广泛研究。利用CFD-VOF数值模拟方法,针对单晶硅微柱表面单气泡的生长及脱离过程,进行表面浸润性分段调控,实现气泡沸腾换热的全程强化。分别调控初始接触角为48°、60°、90°和110°后,同一时刻 （t = 0.152 ms） 变接触角为20°,对比研究分段调控浸润性对气泡动力学过程与表面换热性能的影响。结果表明：疏水性可提高气泡生长速率,增强微柱表面对气泡的黏附力,促进气泡在微结构缝隙内的横向铺展;t = 0.150 ms时接触角为110° 表面上气泡与底面接触面积增加1.3倍,微层蒸发功率增加1.2倍。需要指出的是,毛细效应随颗粒粒径变化趋势受到多孔介质复杂孔隙结构特征的影响。在当前粒径范围内,认为其具有正相关关系,但在更大范围内的对应关系,还需要在未来进一步深入揭示。     

Intrinsic energy dissipation in CVD-grown graphene nanoresonators

We utilize classical molecular dynamics to study the quality (Q)-factors of monolayer CVD-grown graphene nanoresonators. In particular, we focus on the effects of intrinsic grain boundaries of different orientations, which result from the CVD growth process, on the Q-factors. For a range of misorientation angles that are consistent with those seen experimentally in CVD-grown graphene, i.e. 0° to ～20°, we find that the Q-factors for graphene with intrinsic grain boundaries are 1-2 orders of magnitude smaller than that of pristine monolayer graphene. We find that the Q-factor degradation is strongly influenced by both the symmetry and structure of the 5-7 defect pairs that occur at the grain boundary. Because of this, we also demonstrate that the Q-factors of CVD-grown graphene can be significantly elevated, and approach that of pristine graphene, through application of modest (1%) tensile strain.     

Ultrafast modulation of optical transitions in monolayer and multilayer graphene

We study ultrafast modulations of absorption spectra for both monolayer and multilayer graphene, by performing time-resolved transmission measurements with tuning probe photon energy. While reduced absorptions by photo-excited carriers are observed in monolayer graphene irrespective of the probe energy, multilayer graphene shows increased absorption at around 0.6 eV, which is explained by the optical transitions between subband states. Intraband carrier relaxation and electron–hole recombination times are found to be as fast as 0.5 and 10 ps, respectively. Modifications of ultrafast carrier dynamics are also studied with changing temperature and excitation density.     

Enhancement of solid-state polymerization with microwave energy

This work reports on using microwave energy to increase the rate of solid-state polymerization for PET and nylon 66. Theoretical analysis and experimental evidence show that the increase in polymerization rate is not due to an increase in the bulk temperature. Instead, the effect is consistent with directed heating of the condensate leading to enhanced diffusion rates. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1203–1212, 1998     

Operating characteristics and energy distribution in a nitrogen transferred arc plasma

The electrical characteristics of a nitrogen transferred arc plasma were measured in a closed chamber, which simulated a plasma furnace. The total arc voltage depended strongly on the arc length and much less on current and plasma gas volumetric flow rate, with a voltage gradient of about 12 V/cm. The various mechanisms accounting for the transfer of the energy in the arc to the system were identified and measured. The two major components of the energy distribution were by electron transfer to the anode (whether water-cooled solid copper or molten metal) and by radiation from the plasma column. The significance of these measurements for the design and optimization of plasma furnaces is discussed.     

Epoxy/graphene platelets nanocomposites with two levels of interface strength

Graphene platelets (GP) are a novel class of nanofillers due to its good compatibility with most polymers, high aspect ratio, high absolute strength and cost-effectiveness. We in this study synthesised two types of epoxy/GP nanocomposites with different interface strength using the combination of sonication and chemical modification. Although the surface-modified graphene platelets (m-GP) formed clusters, a higher degree of dispersion and exfoliation of graphene was observed in each cluster owning to the improved interface by modification. The scrolling of graphene was found predominantly in the interface-modified nanocomposite. At 4 wt%, the modified nanocomposite shows fracture energy release rate G_1c 613.4 J m⁻², while the unmodified nanocomposite indicates 417.3 J m⁻², in comparison with neat epoxy G_1c 204.2 J m⁻². The interface modification enhanced the glass transition temperature of neat epoxy from 94.7 to 108.6 °C, 14.7% increment. Toughening mechanisms are attributed to the voiding, microcracking and breakage of GP, while matrix may not consume as much fracture energy as m-GP do.     

Scanning probe microscopy study of chemical vapor deposition grown graphene transferred to Au(111)

Graphene is grown by chemical vapor deposition (CVD) on copper films and transferred ex situ to atomically flat Au(111) films, after which the sample is annealed in ultra-high vacuum (UHV) prior to scanning tunneling microscopy (STM) investigation. STM imaging at 78 K reveals large, clean and defect-free atomically flat areas that are separated by graphene wrinkles and grain boundaries. In addition to the graphene atomic structure, the flat surface regions exhibit patterns with larger periodicity that can be interpreted as Moiré patterns formed by the atomic lattices of the graphene and the gold. Our findings show that the CVD growth and ex situ transfer of graphene (G) to atomically flat Au(111) surfaces allows obtaining clean and high-quality G/Au surfaces that are suitable for in situ deposition of, e.g., molecules and atoms, for UHV investigation purposes. This approach may offer a higher degree of freedom in preparing bare and doped graphene on atomically flat surfaces compared to a full in situ approach.     

石墨烯/聚苯胺纳米复合材料界面相互作用的分子动力学模拟

石墨烯具有独特的二维结构和优异的力学、电学性能,将其与聚苯胺复合得到的石墨烯/聚苯胺(Gr/PANI)纳米复合材料在微波吸收、超级电容、电子器件等领域具有广泛的应用前景。为研究Gr/PANI纳米复合材料界面相互作用的微观机理,利用分子动力学方法考察了Gr/PANI体系的相互作用能、相互作用构型以及石墨烯与PANI之间的对关联函数。温度、能量演化曲线和相互作用能分析表明,Gr/PANI体系在较短的时间内达到平衡,Gr/PANI体系为热力学稳定体系。相互作用构型显示PANI分子与石墨烯之间存在较强的相互吸引作用。对关联函数分析表明,Gr/PANI纳米复合材料界面存在近程强非键相互作用,较强的界面相互作用主要源于石墨烯与PANI都具有sp²杂化的π共轭结构。     

Benzyne-functionalized graphene and graphite characterized by Raman spectroscopy and energy dispersive X-ray analysis

The benzyne functionalization of chemical vapor deposition grown large area graphene and graphite was performed using a mixture of o-trimethylsilylphenyl triflate and cesium fluoride that react with the carbon surface. The reaction requires at least 2 days of treatment before the appearance of Raman and energy-dispersive X-ray spectral signatures that verify modification. Raman spectra of modified graphene and graphite show a rich structure of lines corresponding to CCC, CH, and low frequency modes of surface-attached benzyne rings.     

Heat conduction across metal and nonmetal interface containing imbedded graphene layers

Thermal conductance at the interface between metal and non-metal materials in the presence or absence of an inserted graphene layer is measured using a time domain transient thermoreflectance technique. The insertion of a single layer graphene between thermal evaporation Al film and Si substrate enhances the interfacial thermal conductance, because the graphene works as a mask to prevent the metal atoms diffusing into the substrate and causes the reduction of the intermixing layer thickness. Conversely, for the Al/Si interface with the Al film prepared by magnetron sputtering, the insertion of a single layer graphene increases the number of interface and leads to the decrease of the interfacial thermal conductance.