Superconducting Behavior of Interfaces in Graphite: Transport Measurements of Micro-constrictions

We have studied the magnetoresistance (MR) of thin highly oriented pyrolytic graphite mesoscopic samples without and with micro-constrictions of different widths between the voltage electrodes. The MR for fields parallel to the c-axis shows an anomalous hysteresis loop compatible with the behavior expected for granular superconductors. The smaller the constriction width, the larger is the anomalous hysteresis and the higher the temperature for its observation. Our results support the existence of granular superconductivity probably embedded at interfaces between crystalline graphite regions.     

Basal plane pyrolytic graphite modified electrodes: comparison of carbon nanotubes and graphite powder as electrocatalysts

The oxidations of NADH, epinephrine, and norepinephrine are studied using carbon nanotube and graphite powder-modified basal plane pyrolytic graphite electrodes. Immobilization is achieved in two ways: first, via abrasive attachment of multiwall carbon nanotubes or graphite powder by gently rubbing the electrode surface on a fine quality paper supporting the desired material; second, via "film" modification from dispersing either graphite powder or nanotubes in acetonitrile and pipeting a small volume onto the electrode surface and allowing the solvent to volatilize. While electrocatalytic behavior of both types of nanotube-modified electrodes is shown, with enhanced currents and reduced peak-to-peak separations in the voltammetry in comparison with naked basal plane pyrolytic graphite, similar catalytic behavior is also seen at the graphite powder-modified electrodes. Caution is, therefore, suggested in assigning unique catalytic properties to carbon nanotubes.     

Properties of carbon-carbon composites based on exfoliated graphite

We report the first study of the mechanical and electrical properties of carbon-carbon composites based on flexible graphite foils modified with pyrolytic carbon. Our results demonstrate that slight densification with pyrolytic carbon enhances the chemical and thermal stability of the graphite foils, increases their Young’s modulus by a factor of 2.5, and reduces their resistivity by 25%. Decomposition of the polymeric carbon precursors poly(hydridocarbyne) and poly(naphthylhydridocarbyne) in an exfoliated graphite matrix increases the tensile strength of the composite by a factor of 2 compared to the as-prepared graphite foil.     

一种进口石墨箔低温热导率的测量

介绍了一套真空环境的热导率测试实验装置,测量了一种进口热解石墨箔在液氮温区到室温范围内的热导率。通过功率修正的方法得到了石墨薄膜热导率与温度的关系,平均误差在5%以内,为工程应用提供了数据支持。     

氧化镍掺杂酚醛树脂热解炭的结构及抗氧化性

在普通酚醛树脂中直接掺杂氧化镍粉末,研究氧化镍掺量和炭化处理温度对树脂热解炭的结构与氧化过程的影响,用X射线衍射仪、拉曼光谱分析仪、扫描电镜和综合差热分析仪等对掺杂改性树脂热解炭的石墨化度、显微结构及氧化过程分析表征,结果表明,在埋炭条件下掺杂改性树脂在450-750℃的炭化处理中三氧化二镍被逐级还原为一氧化镍和单质镍后,碳原子在镍颗粒上沉积生长,形成晶须、片状或块状结构的热解炭,热解炭石墨化程度取决于炭化温度和氧化镍掺杂量,在高于1050℃炭化处理的热解炭中出现明显的石墨化炭峰,随着掺杂量增加,热解炭石墨化程度大大增加,氧化温度比普通树脂明显提高,且以3~5%的掺杂量为佳。     

退火对高定向热解石墨的输运特性的影响

研究了退火对磁场诱导下高定向热解石墨(HOPG)输运特性的影响.结果表明,退火后HOPG的R-T曲线明显变得光滑,且退火后电阻率增大了近50%,相应温度下的磁电阻效应均减小约40%.分析认为,这是由于HOPG内部存在层错与所含杂质缺陷引起的.磁场下电阻率随温度的变化表明,外加磁场对低温下的金属导电行为存在抑制作用,随着外加磁场的增加,低温下的金属导电行为受到抑制,在H=5.57×104A/m时,低温下的绝缘体-金属转变(I-M)被完全抑制,呈半导体性质.随着磁场的继续增加,在磁场高于H=9.24×105A/m时再次出现绝缘体-金属转变,即再入型金属导电行为,且退火对这一转变过程没有影响,表明这种绝缘体-金属转变是由外加磁场诱导产生的.     

Self-assembly of vertically aligned nanorod supercrystals using highly oriented pyrolytic graphite

Supercrystallization of CdS nanorods (10 nm x 25 nm) into perpendicular superlattices was obtained by controlled evaporation of a nanorod solution trapped between a smooth substrate and a block of highly oriented pyrolytic graphite (HOPG). Hexagonal oriented domains 2 microm2 in size were routinely obtained on a variety of substrates without external electric fields.     

Galvanomagnetic properties of AsF5-intercalated graphite

The galvanomagnetic properties of AsF₅-intercalated, highly-oriented, pyrolytic graphite (HOPG) were determined for stages 1 to 4 in the temperature range from 4.2 to 300 K using contactless eddy current methods forthe measurement of electrical conductivity and magneto-resistance and a new inductive technique forthe Hall effect. From the results p-type conduction, mean carrier mobility, total carrier concentration and charge transfer are deduced. The mobility is diminished upon intercalation due to an effective mass increase when the Fermi level is shifted by charge transferfrom AsF₅ to graphite layers. Charge transfer reaches a maximum of 1/3 for stage-2 compounds. The temperature dependence of galvanomagnetic effects yields carrier scattering at crystallite boundaries for low temperatures and scattering at phonons of both intercalated layers and graphite sheets for higher temperatures. Magnetoresistance of low-sage compounds exhibits anomalies at low temperatures resulting in an overestimation of mobility and saturation at magnetic Wd strengths where the low field approximation still should be vali? For magnetic fields of several kiloGGuss a reduction of the magnetoresistance was observed. It is suggested that these effects are due to trigonal warping of the graphite energy bands and to diffuse scattering at crystallite boundaries.     

The effect of molecular orientation on the potential of porphyrin-metal contacts

The effect of molecular orientation at metal contacts on interface properties was determined by examining the local work function of porphyrin on atomically smooth graphite. The orientation was varied by self-assembly from the vapor phase, and the local potential was quantified by Kelvin force microscopy (scanning surface potential microscopy). When the porphyrin ring is oriented parallel to the substrate, the surface work function is 50 mV less than that of the highly ordered pyrolytic graphite; in contrast, when the porphyrin molecular plane is oriented perpendicular to the substrate, the surface work function is unchanged. The orientation dependence of the surface work function is determined by the geometric relationships between the delocalized charge densities in the molecule and substrate and possible interface bonding. The differences in interface properties with molecular configuration have important design implications to molecular electronic and organic electronic devices.     

Detailed analysis of the electron-transfer properties of azurin adsorbed on graphite electrodes using DC and large-amplitude Fourier transformed AC voltammetry

The analysis of dc cyclic voltammograms of surface-confined metalloproteins is complicated by large background currents, significant ohmic iRu drop, and frequency dispersion related to protein and electrode surface inhomogeneity. The use of large-amplitude Fourier transform ac voltammetry for the quantification of the electron-transfer properties of a thin film of redox-active protein azurin adsorbed onto edge-plane, basal-plane, and highly oriented pyrolytic graphite electrode surfaces has been evaluated and compared to results obtained by dc cyclic voltammetry. In principle, it has been established that fourth and higher harmonic sine-wave data are ideally suited for analysis of electron-transfer processes as they are almost completely devoid of background capacitance current contributions. However, uncompensated resistance has a higher impact on these components, as is the case with fast scan rate dc techniques, so strategies to include this term in the simulations have been investigated. Application of recommended strategies for the evaluation of the electron-transfer properties of azurin adsorbed onto three forms of graphite, each having different background or uncompensated resistance values, is described and compared to results obtained by traditionally used forms of cyclic voltammetry. The electron-transfer rate constant, k0', of azurin at a highly oriented pyrolytic graphite electrode surface was approximately 250 s(-1), compared with > or =1000 s(-1) at edge-plane and basal-plane graphite electrodes. The significantly lower k0' value found at the highly oriented pyrolytic graphite electrode was related to the relatively low level of edge-plane defect sites present at the surface of this electrode. However, analysis of high ac harmonics suggests that frequency dispersion is substantial at all electrode surfaces. Such effects in these diffusionless situations are significantly enhanced relative to solution-phase voltammetry, where overlay of diffusion layers minimizes the impact of heterogeneity.     

铜基石墨复合材料的研究进展

铜基石墨复合材料是一种广泛应用于摩擦材料和电接触材料等领域的金属基复合材料.综述了铜基石墨复合材料在改善铜基体与石墨增强体结合方面的研究进展,主要包括基体合金化、石墨的表面处理、添加粘结剂等,重点介绍了铜基石墨复合材料的制备工艺方法,并概述了其导电导热性能、摩擦磨损性能、工艺性能及应用,最后展望了铜基石墨复合材料的研究重点和发展方向,认为取代现有的易切削铅黄铜合金将成为铜基石墨复合材料今后研究和应用的一个亮点.     

Peculiarities of high-temperature properties of HAPG graphite within the melting region

By means of pulse current heating (3–5 μs), specimens of a new highly oriented pyrolytic graphite (the highly annealing pyrolytic graphite, HAPG) within the temperature range 2500–5000 K were investigated. The temperature, electrical resistance (compared to the initial sizes), and Joule energy of heating were measured. The beginning of graphite melting was registered at 4800–5000 K. It was observed that HAPG is characterized by a lower melting heat as compared to other types of graphite.     

抑制多级降压收集极二次电子发射的研究进展

简述了热解石墨、高纯各向同性石墨和无氧铜材料的二次电子发射特性,以及离子束表面改性对二次电子发射特性的影响。分析表明热解石墨和高纯各向同性石墨的二次电子发射系数均明显低于无氧铜的二次电子发射系数,并且通过离子束表面改性后二次电子发射系数能够得到进一步的降低。同样的,离子束表面改性后的无氧铜的二次电子发射系数也得到一定的降低,并且还有很大的发展空间。     

Modeling of effective material properties of pyrolytic carbon with different texture degrees by homogenization method

The elastic properties of pyrolytic carbon material as a function of texture degree were calculated by means of a homogenization method. The material microstructure is modeled as a system of graphite crystals (inclusions) embedded in an infinite homogeneous matrix with unknown effective (overall) parameters. The texture degrees of carbon planes extracted from the experimental selected-area electron diffraction patterns as well as size of coherent domains extracted from high resolution transmission electron microscopy images have been used as reference points for modeling of material properties. The experimental diffraction curves exhibiting a good fitting with the Gauss density function have been used to simulate the spatial orientation of inclusions. After that the overall elasticity tensor is calculated and the influence of the texture degree of pyrolytic carbon material on the engineering elastic parameters is studied.     

Material Based Structure Design: Numerical Analysis Thermodynamic Response of Thermal Pyrolytic Graphite /Al Sandwich Composites

Amine-grafted multiwalled carbon nanotubes (MWCNTs) based thermally conductive adhesive (TCA) was studied in the previous paper and applied here in thermal pyrolytic graphite (TPG)/Al radiator due to its high thermal conductivity, toughness and cohesiveness. In this paper, in an attempt to confirm the application of TCA to TPG/Al sandwich radiator, the thermodynamic response in TPG/Al sandwich composites associated with key material properties and structural design was investigated using finite element simulation with commercial available ANSYS software. The induced thermal stress in TCA layer is substantial due to the thermal expansion mismatch between Al plate and TPG. The maximum thermal stress is located near the edge of TCA layer with the von Mises stress value of 4.02 MPa and the shear stress value of 1.66 MPa. The reasonable adjustment of physical-mechanical properties including thermal conductivity, thermal expansion, Young^,s modulus and the thickness of TCA layer, Al plate and TPG are beneficial for reducing the temperature of the top surface of the upper skin and their effects on the reduction of thermal structural response in some ways. These findings will highlight the structural optimization of TPG/Al radiator for future application.     

A graphite nanoeraser

We present here a method for cleaning intermediate-size (up to 50 nm) contamination from highly oriented pyrolytic graphite and graphene. Electron-beam-induced deposition of carbonaceous material on graphene and graphite surfaces inside a scanning electron microscope, which is difficult to remove by conventional techniques, can be removed by direct mechanical wiping using a graphite nanoeraser, thus drastically reducing the amount of contamination. We discuss potential applications of this cleaning procedure.     

纳米石墨的制备方法及机理与应用研究进展

纳米石墨的制备与合成方法可分为两大类:一类是将天然鳞片石墨破碎的物理制备法,主要包括机械球磨法、爆轰裂解法、超声破碎法以及电化学插层法;另一类是化学合成法,主要有脉冲激光沉积法、爆轰合成法、化学气相沉淀法、化学合成法等.本文阐述了纳米石墨制备方法的研究动态和目前纳米石墨的主要应用领域,并对其发展前景进行展望.     

The influence of boron content on the fine microstructure of pyrolytic graphite

A study of the fine microstructure of boronated pyrolytic graphite was made utilising optical and X-ray diffraction techniques. The influence of boron content on the laminar structure, the lattice parameter, the lattice strain, the particle size, and the flexural strength of pyrolytic graphite was studied. Significant changes in the behaviour of all investigated parameters were observed in the 0.50 to 0.75 wt % boron range. A mechanism relating these changes to the appearance of a boron carbide deposit is hypothesised.     

Topochemical speciation of intercalated palladium in graphite by valence band X-ray spectrometry in the electron microprobe

The binding state of palladium was studied within the frame of an investigation on the mechanism of analyte fixation during the pyrolysis step in graphite furnace atomic spectrometry. One approach was the determination of the palladium intercalated in the pyrolytic coating of the graphite tube. Due to the low concentrations of intercalated palladium in the pyrolytic coating, precise determination of the shift of certain X-ray lines was chosen. From several investigated valence state sensitive X-ray transitions, the Pd Lbeta2/15 (L3-N4,5) line shift was the one best determinable. The measured line shifts are in the range of -0.14 to 0.71 eV at line widths of 13 eV (fwhm) and a line energy of 3.1729 keV. These very small line shifts were determined by electron probe microanalysis. The detection of the small line shifts was performed with a new method-by evaluation of the change of the intensity in the flanks of the X-ray line. The measurements yielded the following results: inside the pyrolytic graphite, the palladium is distributed inhomogeneously in the form of clusters or islands and in the form of particles on the surface of the pyrolytic graphite. The differentiation between particles and clusters is a very practical one: as long as a particle can be seen in the SEM we talk of particles. Often, however, Pd is detected in an area on the tube or platform surface without detection of a particle. Hence, it can be assumed that the Pd is present in the form of clusters which might even be intercalated in the uppermost graphite layers. The valence state inside these clusters does not appear to be uniform. It can be interpreted as a mixture of Pd with PdO in the center of the clusters or particles (positive peak shift) and of Pd bound to the graphite (strong negative peak shift). On the basis of these observations, a way is proposed to determine how activated Pd atoms in intercalated Pd domains are forming strong covalent bonds to analytes. These bonds are responsible for the analyte fixation of even very volatile analytes.     

热解碳基碳/碳复合材料的内耗特征与机制

以高纯石墨为对照材料,初步研究了热解碳基碳/碳复合材料的内耗行为,并根据实验结果提出了碳/碳复合材料的内耗机制:热滞弹性机制与静滞后型内耗机制。纤维/基体的界面内耗效应对碳/碳复合材料的内耗特性影响很大,它的存在使碳/碳复合材料产生了一些较为反常的内耗现象。