Properties of graphitized carbon fiber-halogen residue compounds

Br₂ or ICl residue compound formation in TP 4104B and GY-70 fibers increases their electrical conductivity 3–5 times. It also decreases the tensile strength of the GY-70 fibers by increasing their cross-sectional area. In contrast, the tensile strength of the TP 4104B fiber remains unchanged on Br₂ or ICl residue compound formation. The TP 4104B/Br₂ residue compound loses only 1–2% of its weight and increases its resistance by only 5–10% when heated to 400°C in air; the ICl residue compounds of both fibers are unstable above 150°C. The temperature coefficient of resistivity of the unintercalated fibers (in the range 2–673 K) is attributed to a temperature dependent variation in the number of current carriers in the intercrystalline regions of the fibers and not to a finite resistance percolation path through the graphite crystallites. The magnetoresistance of the fibers from 2–200 K is attributed to a large resistance increase within the graphite crystallites. Both the temperature coefficient of resistivity and the magnetoresistance of the residue compounds are consistent with a metallic-like electron structure. The preparation of bromine residue compounds of individual TP 4104B fibers that had different bromine concentrations was not successful; fibers either intercalated Br₂ and formed the “full” residue compound or remained unintercalated.     

The intercalation of bromine in graphitized carbon fibers and its removal

When the bromine residue in graphite is heated and cooled between 20° and 900°C there is an emission of Br₂ over certain relatively small temperature ranges on both heating and cooling. It is shown that two of these emissions are associated with observed decreases in X-ray diffraction spacing between carbon layers—one on heating around 175°C and one on cooling around 100°C. The intercalation isotherms of Br₂ and of ICl on graphitized carbon fibers show that the threshold pressure on the residue is lower than on the original fiber. This is believed to be caused in fibers by the production of microscopic cracks in the interlocking amorphous carbon networks between the graphitized domains during the initial intercalation. Hence, when again exposed to Br₂ the compresssion-tension builds to a lower value so that the threshold pressure is lower and the rate of intercalation is higher. The anomalous bromine emission behavior in fiber is attributed to differential dimensional changes of the isotropic carbon network and the anisotropic crystallites. In natural graphites the anomalous emission originates in a differential dimensional change between the polycrystalline structure of the total sample of graphite and the anisotropic single crystal components of the graphite.     

The microstructure of intercalated graphite fibers

A study of the microstructure of two types of graphite fibers GY70, a polyacrylotrinitrile (PAN) based fiber and UC4104B a pitch based fiber, is reported. These high modulus fibers were modified by intercalation with the donors K, Rb, Cs and the acceptors FeCl₃, AlCl₃. Several experimental techniques were used, to elucidate the effects of intercalation on the microstructure of the graphite fibers. These include scanning and transmission electron microscopy and small and wide angle X-ray diffraction.     

Exfoliation of nitric acid intercalated carbon fibers: effects of heat-treatment temperature of pristine carbon fibers and electrolyte concentration on the exfoliation behavior

Effect of heat treatment temperature of mesophase pitch-based carbon fibers on the exfoliation behavior of derived intercalation compounds with nitric acid was studied. Carbon fibers heat-treated above 2500 °C gave intercalation compounds with mass increase of more than 80 mass% and resulted in a marked exfoliation by a rapid heating to 1000 °C, where no memory of original single fiber was observed. On those below 2000 °C, on the other hand, their residue compounds showed mass increase less than 80 mass% and the appearance after exfoliation at 1000 °C was similar to the original single fiber. On 1150 °C-treated carbon fibers, mass increase was only 13 mass% and no evidence of intercalation was detected even after electrolysis and, as a consequence, the formation of only small fissures along their fiber axis was observed, with no apparent exfoliation. The dependence on electrolyte concentration was also examined on 3000 °C-treated carbon fibers.     

Preparation and electrical conductivity of fluorine-graphite fiber intercalation compound

Fluorine-graphite fiber intercalation compounds were prepared in the presence of copper fluoride, and their electrical conductivities and stability were evaluated. Major products were a mixture of the 2nd and 3rd stages or the 3rd stage with intercalate 12–16 wt% corresponding to C_8–12F. A mixture of the 3rd and 4th stages contained 8–12 wt% intercalate corresponding to C_12–18F. No 1st and pure 2nd stage compounds were prepared under the present condition (F₂:1 atm, 10 days). The highest electrical conductivities at room temperature were 1.1 × 10⁵ 2.0 × 10⁴ and 7.0 × 10³ Scm⁻¹ for VGCF, pitch fiber and PAN fiber-based GIC's, respectively. They were 8.5, 8 and 6 times those of the pristine fibers. The decrease in the conductivity in air was less than 18% after 100 days for PAN fiber-based GIC, and 30% after 130 days for pitch fiber-based GIC. However, VGCF-based GIC decomposed to graphite after 1 month. DTA and TG analyses showed that the decomposition of GIC to a high stage compound started at 150°C, and the high stage GIC started to decompose to graphite at 360–390°C for PAN and pitch fiber-based GIC's. VGCF-based GIC decomposed at lower temperatures than those of other two fibers.     

Influence of bonding carbon on low carbon Al2O3-C refractory composites

The carbonized behavior of binders (carbores pitch, mesophase pitch, high temperature pitch and phenolic resins) was investigated. Meanwhile, the influence of binders, Ni-catalyst and heat treatment on structure and properties of low carbon Al₂O₃-C materials was researched. Mesophase pitch and high molecular weight resin provided higher carbon yield. The synergistic interaction of pitch and phenolic resin provided the higher carbon yield than that of single component. Ni-catalyst changed structure and morphology of bonded carbon promoting the transition of amorphous carbon to crystalline carbon increasing carbon yield and degree of graphitization. High molecular weight phenolic resins, mesophase or carbores pitch and Ni-catalyst worked together providing low carbon Al₂O₃-C samples of excellent strength performance in a wide temperature range and high resistance of thermal shock. Embedding coke in N₂ could promote the generating of carbon fibers which obviously enhanced thermal shock resistance of low carbon (6% flake graphite) Al₂O₃-C materials after heat treatment.     

大直径中间相沥青基炭纤维的制备研究

以萘系中间相沥青为原料,通过熔融纺丝和随后的预氧化、炭化以及石墨化处理制备了中间相沥青基圆形炭纤维.研究了热处理温度对纤维导电性能和力学性能的影响,并采用红外光谱仪、元素分析仪、扫描电子显微镜和X射线衍射仪对纤维的组成、形貌和微观结构进行了表征.研究结果表明：纤维在预氧化时形成的羟基、酰基等含氧官能团在随后的炭化、石墨化处理过程中消失;随热处理温度的升高,石墨微晶逐渐发育、长大,并沿纤维轴向高度取向,纤维的电阻率不断降低,力学性能不断增强;3 000℃石墨化纤维电阻率为1.3μΩ·m,对应的强度和模量值为1.6GPa和380 GPa.     

Fluorine-intercalated carbon fibers-I. structural and transport properties

The intercalation of fluorine in various types of carbon fibers (PAN-based or pitch-based, asreceived or high-temperature treated) has been investigated at room temperature in the presence of gaseous HF. Stage-1 compounds with C_2.5F to C₄F compositions are obtained for 10 bar F₂ pressures, whereas lower pressures (1 bar F₂) lead to stage-2 compounds. Although in higher stages (≥2) the electrical conductivity is generally larger than in the pristine fiber, in stage-1 compounds a drastic increase of resistivity is observed, ρ being more than one order of magnitude larger than that of the starting material. Finally, fluorine-intercalated GICs have been found appropriate to investigate the effects of disorder and reduced dimensionality.     

The electronic and structural characteristics of carbon fibers from mesophase pitch

The electronic properties (resistivity, magnetoresistance, and electron spin resonance) of mesophase pitch-based carbon fibers have been studied in relation to the fiber structure and processing conditions. Reproducible correlations between the electronic properties and the structure demonstrate that the electronic properties are a sensitive indicator of the degree of graphite order in the fibers. Fibers having radial transverse structure are more easily graphitized than fibers with random transverse structure. The effect of differences in the thermosetting procedure is relatively unimportant. The ultimate degree of graphitization attainable by these fibers is comparable to that of similarly heat-treated pyrolytic carbons, whereas PAN-based carbon fibers are ultimately capable only of properties comparable to pitch-based fibers treated in the 1700–2300°C range.     

Role of particle size on the magnetoresistance of nano-crystalline graphite

We report on microstructure and magnetotransport behavior of ball milled graphite to investigate the effect of particle size on magnetoresistance (MR) in graphite. The observed results show, large values of resistivity and a negative temperature coefficient of resistivity indicating non-metallic, semiconductor nature of the compacts. As the milling time increases, the resistivity value increase which is attributed to the particle size differences of the samples. Graphite exhibits the positive MR at room temperature as well as at low temperatures (4 K). 600 °C annealed samples exhibit less resistivity compared to as-milled samples due to the grain growth during annealing but show large MR values compared to as-milled sample.     

The structure and properties of ribbon-shaped carbon fibers with high orientation

Using a naphthalene-derived mesophase pitch as a starting material, highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. The preferred orientation, morphology, and microstructure, as well as physical properties, of the ribbon-shaped carbon fibers were characterized. The results show that, the ribbon-shaped fibers possessed uniform shrinkage upon heat treatment, thereby avoiding shrinkage cracking commonly observed in round-shaped fibers. As heat treatment progressed, the ribbon-shaped graphite fibers displayed larger crystallite sizes and higher orientation of graphene layers along the main surface of the ribbon-shaped fiber in comparison with corresponding round-shaped fibers. The stability of the ribbon-shaped graphite fibers towards thermal oxidation was significantly higher than that of K-1100 graphite fibers. The longitudinal thermal conductivity of the ribbon fibers increased, and electrical resistivity decreased, with increasing the heat treatment temperatures. The longitudinal electrical resistivity and the calculated thermal conductivity of the ribbon-shaped fibers graphitized at 3000 °C are about 1.1 μΩ m and above 1100 W/m K at room temperature, respectively. The tensile strength and Young’s modulus of these fibers approach 2.53 and 842 GPa, respectively.     

A novel route towards high quality fullerene-pillared graphene

A new approach for the synthesis of graphite intercalation compounds (GICs), by the help of co-intercalant molecules, has been observed. In the present work, we demonstrate the successful incorporation of fullerene (C₆₀) molecules between the graphene sheets aided by the preceding intercalation of nitric acid. The presence of intercalated C₆₀ between the graphene sheets is deduced from X-ray diffraction patterns, while Raman and X-ray photoelectron spectroscopy (XPS), verify that the quality of the graphene layers is not compromised by the intercalation. A quantification of the interaction yield was derived from thermogravimetric analysis and XPS studies, giving that fullerene molecules intercalated in the pillared structure amount to about 25 wt%. The present method opens new perspectives for the intercalation of various guest molecules in graphite also because graphite nitrate allows for functionalization processes following the very well established carbon chemistry.     

Graphite intercalation compound with iodine as the major intercalate

Halogenated graphite CBr_xI_y (1 < y/x < 10) was made by exposing graphite materials to either pure Br₂ or an I₂/Br₂/HBr mixture to initiate the reaction, and then to iodine vapor containing a small amount of Br₂/HBr/IBr to complete the intercalation reaction. Wetting of the graphite materials by the I₂/Br₂/HBr mixture is needed to start the reaction, and a small amount of Br₂/HBr/IBr is needed to complete the charge transfer between iodine and carbon. The interplanar spacings for the graphite materials need to be in the 3.35 to 3.41 Å range. The X-ray diffraction data obtained from the halogenated HOPG indicate that the distance between the two carbon layers containing intercalate is 7.25 Å. Electrical resistivity of the fiber product is from 3 to 6.5 times the pristine value. The presence of a small amount of isoprene rubber in the reaction significantly increased the iodine-to-bromine ratio in the product. In this reaction, rubber is known to generate HBr and to slowly remove bromine from the vapor. The halogenation generally caused a 22% to 25% weight increase. The halogens were found uniformly distributed in the product interior. However, although the surface contains very little iodine, it has high concentrations of bromine and oxygen. It is believed that the high concentrations of bromine and oxygen in this surface cause the halogenated fiber to be more resistant to structural damage during subsequent fluorination to fabricate graphite fluoride fibers.     

碳纤维增强PEEK复合材料的摩擦学性能研究

用磨损试验机对碳纤维增强聚醚醚酮（PEEK）复合材料进行室温干滑动磨损试验。考察了碳纤维的含量，石墨润滑剂，对靡时间及载荷对材料靡损量及摩擦系数的影响，并用电子显微镜对其磨损表面进行了观察与分析，同时对材料的磨损机理进行了探讨，研究结果表明，随着载荷的升高和对磨时间的延长，材料的摩擦系数逐渐降低并趋于稳定，磨损量呈上升趋势，加入碳纤维可以明显地降低材料的摩擦系数和磨损量，当碳纤维含量为5％－10％时复合材料的摩擦系数和磨损量最低；加入适量固体石墨可进一步降低复合材料的摩擦系数和磨损量。     

氧化石墨烯的制备及其掺杂的炭/炭复合材料导热性能研究

设定两种不同配比强酸氧化剂,以鳞片石墨为原料,采用Hummers法,制备了氧化石墨烯,再经过高温炭化得到热处理氧化石墨烯。并分别以中间相沥青为基体炭前驱体,炭纤维为增强相,氧化石墨烯及其热处理物为热疏导功能体,制备出掺杂氧化石墨烯的炭/炭复合材料。TEM、SEM等表征表明,选用强酸氧化剂组合配比用量较少的制备出的氧化石墨烯,其形貌整体上要优于用量较多的,具有独特的褶皱结构;相比于氧化石墨烯,掺杂其热处理物的复合材料界面覆盖均匀平滑且结合更优良,且其导热系数可达到60 W.m-1.K-1,是无掺杂的纯复合材料两倍多,导热系数得到了较大幅度提高。     

The electric resistivity of compacts of graphite intercalation compounds with antimony pentafluoride and with antimony pentachloride

The electric resistivities were measured for textured compacts of the intercalation compounds of graphite with SbF₅ and with SbCl₅. Starting materials were coarse flakes of natural graphite and graphite foils (0.35 and 2 mm thick). First stage compounds and mixtures of first and second stage compounds were studied. The resistivities of the compacts from flakes were strongly pressure dependent, maximum conductivity was not yet reached at 600 bar. The resistivities of preparations from graphite foils were much less pressure dependent. The resistivity of the foils decreased to 36 × 10^?8 Ωm, that is 3–4% of the resistivity of the parent foils. In general, the intercalation compounds with SbF₅ showed somewhat higher resistivities than the SbCl₅ compounds. This seems to be caused by formation of an insulating layer on the surface of the particles due to partial fluorination of carbon atoms with SbF₅. CF bonds were detected by IR spectroscopy and by XPS. The first stage SbF₅ compound is formed already at room temperature, contrary to literature reports, provided sufficient time is given.     

Graphite intercalation compounds as PEMFC electrocatalyst supports

Intercalation of metal chlorides into fine natural graphite flakes of 2 × 10⁻⁶ m in diameter was attempted. CuCl₂ was satisfactorily intercalated from the vapour phase. NiCl₂ and PdCl₂ were also intercalated from the liquid phase using chloroform solvent. The reduction products of the obtained graphite intercalation compounds were used as carbon supports of the platinum electrocatalyst for a proton exchange membrane fuel cell. Oxygen reduction activity was evaluated and it can be seen that natural graphite flakes treated using the intercalation technique provide different levels of activity from that of pristine natural graphite flakes. In particular, the catalytic activity was enormously improved when CuCl₂ was used as the intercalate.     

Chemistry of graphite intercalation by nitric acid

Gas-phase intercalation of graphite by nitric acid is accompanied by evolution of a brown gas which has been identified as nitrogen dioxide by chemiluminescence measurements. The reaction is inhibited by the presence of oxygen or water vapor, but not by nitrogen. These results, and the existence of induction times of 5–20 min before intercalation begins, is interpreted as evidence that intercalation is effected by oxidation of graphite by adsorbed nitronium ions with the release of NO₂. The graphite lattice then intercalates NO₃^? ions along with neutral HNO₃ molecules.     

鳞片石墨对无粘结剂炭材料性能的影响

：采用煤沥青中加入一定量鳞片石墨制备的类中间相原料,再经过成型、烧结等工艺制备无粘结剂炭材料。实验表明：在类中间相原料制备过程中加入一定量鳞片石墨,所制得的无粘结剂炭材料的电阻率相对传统无粘结剂炭材料有所下降,其抗折强度是传统炭材料的3倍左右。当鳞片石墨质量分数为沥青的6％时,所制备的无粘结剂炭材料的体积密度为1．46g／cm^3,电阻率为46．0μ·Ω,抗折强度为29．8MPa。     

Resistivity, Hall coefficient, magnetoresistance, and microtexture of cellulose carbon films

Dense carbon films of about 20 μm in thickness were prepared from a commercially available cellulose film of 40 μm thick by heat treatment up to 900°C. Carbon films thus obtained were heat treated at temperatures between 1800 and 3000°C for 30–60 min. The heat-treated carbon films were investigated by the measurements of resistivity, Hall coefficient and magnetoresistance at liquid nitrogen temperature. Reflected and transmitted X-ray diffraction experiments and observations by a high-resolution scanning electron microscope (SEM) were also carried out. The results of the resistivity and Hall coefficient for high-temperature-treated specimens indicated that the carbonized film was more graphitizable than bulk glass-like carbons. The magnetoresistance and X-ray diffraction profiles for the high-temperature-treated specimens suggested that each of these specimens substantially consisted of the matrix of granular microtexture and the graphite layer skin. The skin was clearly observed by SEM for the specimens heat-treated at 2800 and 3000°C and their thickness was 100–300 nm.