Oxidation of carbon nanofibers and materials obtained on their basis

The results of the oxidation of carbon nanofibers and materials obtained on their basis are presented; these results demonstrate that the nanofibers were formed by carbon with different degrees of crystal structure ordering. The experimental data supported previous hypotheses that amorphous carbon results from the decomposition of metal carbides. The subsequent formation of spatial structures and the appearance of crystalline carbon species resulted from catalytic graphitization. It was demonstrated that sorbents can be prepared based on carbon nanofibers after pyrolytic consolidation followed by activation, and these sorbents are more effective than well-known sorbents.     

Bamboo-shaped carbon nanofibers obtained in pyrolytic carbon by thermal gradient chemical vapor deposition

Bamboo-shaped carbon nanofibers were obtained in pyrolytic carbon fabricated by thermal gradient chemical vapor deposition and their micro-and nanostructure were examined by transmission and scanning electron microscopy. The results showed that, bamboo-shaped nanofibers with diameters from a few tens to about 250 nm were distributed homogeneously in the pyrolytic carbon. The nanofibers could be pulled out of the pyrolytic carbon when they were fractured.     

电纺聚丙烯腈基碳纳米纤维在超级电容器中的应用

静电纺丝法制备聚丙烯腈（PAN）基纳米纤维具有较大的比表面积、较高的机械强度、优异的纳米结构及良好的化学稳定性。以PAN纳米纤维为基础,进行多方位设计与合成的电极材料在超级电容器中表现出优异的电化学性能,具有广阔的应用前景。本文根据电极材料分类,主要综述了近年来PAN基多孔结构电极材料、杂原子掺杂电极材料以及与碳系材料、导电聚合物、金属氧化物复合等电极材料的研究进展;讨论了电极材料的结构特征、制备方法及其提高电化学性能的原理;最后指出了上述研究中存在的问题,并对未来PAN基电极材料在超级电容器的发展前景进行了展望。     

Liquid crystal engineering of carbon nanofibers and nanotubes

Four high-aspect-ratio carbon nanomaterials were fabricated by template-directed liquid crystal assembly and covalent capture. By selecting from two different liquid crystal precursors (thermotropic AR mesophase, and lyotropic indanthrone disulfonate) and two different nanochannel template wall materials (alumina and pyrolytic carbon) both the shape of the nanocarbon and the graphene layer arrangement can be systematically engineered. The combination of AR mesophase and alumina channel walls gives platelet-symmetry nanofibers, whose basic crystal symmetry is maintained and perfected upon heat treatment at 2500 °C. In contrast, AR infiltration into carbon-lined nanochannels produces unique C/C-composite nanofibers whose graphene planes lie parallel to the fiber axis. The transverse section of these composite nanofibers shows a planar polar structure with line defects, whose existence had been previously predicted from liquid crystal theory. Use of solvated AR fractions or indanthrone disulfonate produces platelet-symmetry tubes, which are either cellular or fully hollow depending on solution concentration. The use of barium salt solutions to force precipitation of indanthrone disulfonate within the nanochannels yields continuous nanoribbons rather than tubes. Overall the results demonstrate that liquid crystal synthesis routes provide molecular control over graphene layer alignment in nanocarbons with a power and flexibility that rivals the much better known catalytic routes.     

Kinetics of pyrolytic carbon infiltration for the preparation of ceramic/carbon and carbon/carbon composites

Carbon/carbon and zeolite/carbon composites have been prepared by pyrolytic carbon infiltration of organic and inorganic substrates with different porous structures. The chemical vapour infiltration kinetics of these substrates has been studied in a thermogravimetric system at atmospheric pressure, using benzene as pyrolytic carbon precursor. The rate of pyrolytic carbon infiltration seems to depend on the porosity of the substrate available to the pyrolytic carbon precursor, irrespective of the nature of the substrate studied. Activation energy values of about 180 kJ/mol were found for the different substrates used in the temperature range of 700-800 °C, where the cracking reaction of benzene takes place, predominantly, in a heterogeneous form. At higher temperatures homogeneous reactions compete with heterogeneous ones and higher values of activation energies (280-380 kJ/mol) were obtained. The oxidation of the pyrolytic carbon deposited on the different substrates studied takes place in the same range of temperature, which suggests the presence of a similar pyrolytic carbon structure on substrates of different nature or a similar accessibility to the deposited layer.     

Structural analysis of conical carbon nanofibers

Conical carbon nanofibers are a relatively new type of carbon nanomaterial that has received considerable scientific and commercial interest due to its physical properties. However, its structure and growth mechanism have still not been determined conclusively. In this study the structure of these materials was investigated employing molecular models and structural analyses and compared with reported experimental observations, principally of cone apex angles. The results showed that stacked cone models could not explain the wide variety of apex angles observed for these nanofibers and related structures. Cone-helix models, originally proposed for other carbon conical structures, allow a variety of apex angle structures and were found to be applicable for nanofibers as well. An equation was developed that allows for prediction of cone-helix structures with good graphitic alignment. Such structures were also shown to be more compatible with the physical properties and growth mechanism of nanofibers than stacked cone structures. From these results a cone-helix structure, and a new cone-helical growth mechanism for the nanofibers based on heterogeneous nucleation on conical catalyst particles, are proposed.     

Enhanced thermoelectric properties of carbon fiber reinforced cement composites

Thermoelectric properties of carbon fiber reinforced cement composites (CFRCs) have attracted relevant interest in recent years, due to their fascinating ability for harvesting ambient energy in urban areas and roads, and to the widespread use of cement-based materials in modern society. The enhanced effect of the thin pyrolytic carbon layer (formed at the carbon fiber/cement interface) on transport and thermoelectric properties of CFRCs has been studied. It has been demonstrated that it can enhance the electrical conduction and Seebeck coefficient of CFRCs greatly, resulting in higher power factor 2.08 µW m⁻¹ K⁻² and higher thermoelectric figure of merit 3.11×10⁻³, compared to those reported in the literature and comparable to oxide thermoelectric materials. All CFRCs with pyrolytic carbon layer, exhibit typical semiconductor behavior with activation energy of electrical conduction of 0.228-0.407 eV together with a high Seebeck coefficient. The calculation through Mott’s formula indicates the charge carrier density of CFRCs (10¹⁴–10¹⁶ cm⁻³) to be much smaller than that of typical thermoelectric materials and to increase with the carbon layer thickness. CFRCs thermal conductivity is dominated by phonon thermal conductivity, which is kept at a low level by high density of micro/nano-sized defects in the cement matrix that scatter phonons and shorten their mean free path. The appropriate carrier density and mobility induced by the amorphous structure of pyrolytic carbon is primarily responsible for the high thermoelectric figure of merit.     

Flammability of thermoplastic carbon nanofiber nanocomposites

Five commodity thermoplastics (polyethylene, polypropylene, thermoplastic polyurethane, poly(butylene terephthalate), and poly(amide 6)) were melt compounded with vapor grown carbon nanofibers via twin screw extrusion. These materials were then analyzed for flammability behavior by cone calorimeter to determine how the nanofibers would reduce flammability of the polymers. It was found by cone calorimeter that the nanofibers greatly reduced peak heat release rate and improved many other flammability parameters of the samples. However, smoke release was increased in all samples, which may be one drawback of using these materials. Interestingly, the amount of flammability reduction was not uniform across all samples, with nanofiber reducing flammability the most in the thermoplastic polyurethane sample. The mechanism of flammability reduction in the polymers tested in this paper is shown again to be a mass loss rate reduction induced by the formation of thick tangled networks of carbon nanofibers during polymer decomposition. This mechanism was confirmed by studying the mass loss rate curves and electron microscopy analysis of the final chars collected from the cone calorimeter experiments. Copyright © 2010 John Wiley & Sons, Ltd.     

Carbon Nanofibers: Catalytic Synthesis and Applications

Carbon nanofibers (diameter range, 3-100 nm; length range, 0.1-1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (~200 m²/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.     

Index to Volume 31

Carbon nanofibers (diameter range, 3–100 nm; length range, 0.1–1000 µm) have been known for a long time as a nuisance that often emerges during catalytic conversion of carbon-containing gases. The recent outburst of interest in these graphitic materials originates from their potential for unique applications as well as their chemical similarity to fullerenes and carbon nanotubes. In this review, we focus on the growth of nanofibers using metallic particles as a catalyst to precipitate the graphitic carbon. First, we summarize some of the earlier literature that has contributed greatly to understand the nucleation and growth of carbon nanofibers and nanotubes. Thereafter, we describe in detail recent progress to control the fiber surface structure, texture, and growth into mechanically strong agglomerates. It is argued that carbon nanofibers are unique high-surface-area materials (?200 m²/g) that can expose exclusively either basal graphite planes or edge planes. Subsequently, we will present the recently explored applications of carbon nanofibers: polymer additives, gas storage materials, and catalyst supports. The latter application is described in detail. It is shown that the graphite surface structure and the lyophilicity play a crucial role during metal emplacement and catalytic use in liquid-phase catalysis. A case in point is fiber-supported Pd catalysts for nitrobenzene hydrogenation. Finally, we summarize issues with respect to the large-scale production of carbon nanofibers, including production cost estimates and research items to be dealt with in future work.     

Improvement on the electrochemical characteristics of graphite anodes by coating of the pyrolytic carbon using tumbling chemical vapor deposition

The electrochemical characteristics of graphite coated with pyrolytic carbon materials using tumbling chemical vapor deposition (CVD) process have been studied for the active material of anodes in lithium ion secondary batteries. Coating of pyrolytic carbons on the surface of graphite particles, which tumble in a rotating reactor tube, was performed through the pyrolysis of liquid propane gas (LPG). The surface morphology of these graphite particles coated with pyrolytic carbon has been observed with scanning electron microscopy (SEM). The surface of graphite particles can well be covered with pyrolytic carbon by tumbling CVD. High-resolution transmission electron microscopy (HRTEM) image of these carbon particles shows that the core part is highly ordered carbon, while the shell part is disordered carbon. We have found that the new-type carbon obtained from tumbling CVD has a uniform core (graphite)-shell (pyrolytic carbon) structure. The electrochemical property of the new-type carbons has been examined using a charge-discharge cycler. The coating of pyrolytic carbon on the surface of graphite can effectively reduce the initial irreversible capacity by 47.5%. Cyclability and rate-capability of theses carbons with the core-shell structure are much better than those of bare graphite. From electrochemical impedance spectroscopy (EIS) spectra, it is found that the coating of pyrolytic carbon on the surface of graphite causes the decrease of the contact resistance in the carbon electrodes, which means the formation of solid electrolyte interface (SEI) layer is suppressed. We suggest that coating of pyrolytic carbon by the tumbling CVD is an effective method in improving the electrochemical properties of graphite electrodes for lithium ion secondary batteries.     

静电纺丝制备聚丙烯腈纳米碳纤维

利用静电纺丝制备连续的聚丙烯腈纳米碳纤维;介绍了静电纺丝的原理、影响静电纺丝的主要因素以及制备纳米碳纤维、纳米活性炭纤维、纳米碳纤维复合材料的方法和原理;分析了静电纺丝产率低,难以得到单向平铺的纤维等问题,影响静电纺丝的参数主要有溶液特性、纺丝工艺参数、纺丝环境参数。由静电纺丝得到纳米聚丙烯腈纤维,然后再经预氧化和碳化制备纳米碳纤维,或把纳米纤维预氧化,经活化、碳化制备纳米活性炭纤维。并指出纳米碳纤维具有巨大的潜在应用空间。     

Friction behaviors of carbon/carbon composites with different pyrolytic carbon textures

Friction and wear properties of carbon/carbon (C/C) composites with a smooth laminar (SL), a medium textured rough laminar (RL) and a high textured RL pyrolytic carbon texture were investigated with a home-made laboratory scale dynamometer to simulate airplane normal landing (NL), over landing (OL) and rejected take-off (RTO) conditions. The morphology of worn surfaces at different braking levels was observed with scanning electron microscopy. The results show that C/C composites with RL have nearly constant friction coefficients, stable friction curves and proper wear loss at different braking levels, while friction coefficients of C/C composites with SL pyrolytic carbon decrease intensely and their oxidation losses increase greatly under OL and RTO conditions. Therefore, C/C composites with a high and medium textured RL pyrolytic carbon may satisfy the requirements of aircraft brakes. The good friction and wear properties of C/C composites with RL are due to the properties of RL, which leads to a uniform friction film forming on the friction surface.     

氮掺杂石墨烯/碳纳米管/无定形炭复合材料制备及其电化学性能

碳基复合材料被认为是超级电容器广泛应用最有前景的电极材料之一。本文使用氧化石墨烯（GO）、硝酸钴［Co(NO₃)₂］、三聚氰胺为原料,利用钴对高温下热解碳源的催化作用,制备得到了氮掺杂石墨烯/碳纳米管/无定形炭（NC）复合材料,并测试了其电化学性能。探究了金属和三聚氰胺添加量对碳基复合材料结构和性能的影响,研究发现,在添加量分别为0.02mmol和0.3g时,制得的样品具有大比表面积（380.5m²/g）和高掺氮质量分数（6.29%）,并在三电极系统中体现出优异的电化学性能,电流密度为0.5A/g时样品的比电容为137.1F/g,5A/g时比电容为113.5F/g,保持率为88.5%,具有优异的倍率性能,在循环5000圈后样品的容量保持率为104%,具有良好的循环稳定性,这归因于三维结构可以加快充放电过程中的离子转移和氮掺杂可提高材料润湿性和贡献部分赝电容,为超级电容器电极材料的制备提供了理论借鉴。     

Growth of carbon nanofibers on activated carbon fiber fabrics

Activated carbon fiber fabrics, an excellent adsorbent, were used as catalyst supports to grow carbon nanofibers. Because of the microporous structure of the activated carbon fibers, the catalysts could be distributed uniformly on the carbon surface. Based on this concept, the carbon nanofibers can be grown directly on the activated carbon fiber fabrics. We demonstrate that carbon nanofibers with a diameter between 20 and 50 nm for most of the fibers can be synthesized uniformly and densely on activated carbon fiber fabrics, impregnated by nickel nitrate catalyst precursor, using catalytic chemical vapor deposition. Although the carbon nanofibers are not straight with a crooked morphology, they form a three-dimensional network structure. Structure characterizations by TEM and XRD indicate that the carbon nanofibers have a turbostratic graphite structure and the graphite layers are stacked with a herringbone structure.     

Corrosion behaviour of carbon materials exposed to molten lithium chloride–potassium chloride salt

The corrosion behaviour of four carbon materials namely low density graphite, high density graphite, glassy carbon and pyrolytic graphite were investigated in molten LiCl–KCl electrolyte medium at 600 °C for 2000 h under high pure argon atmosphere. Structural and microstructural changes in the carbon materials after exposure to molten chloride salt were investigated from the weight change and using scanning electron microscopy, atomic force microscopy, X-ray diffraction and laser Raman spectroscopic techniques. Microstructural analysis of the samples revealed the poor corrosion resistance of high density and low density graphite and severe attack was observed at several places on the surface. On the other hand, glassy carbon and pyrolytic graphite were relatively inert, while pyrolytic graphite showed the best corrosion resistance to molten salt attack. In the order of increasing corrosion resistance to molten salt, the carbon materials were found to follow the sequence: low density graphite < high density graphite < glassy carbon < pyrolytic graphite.     

Iron impregnation on the amorphous shell of vapor grown carbon fibers and the catalytic growth of secondary nanofibers

Vapor grown carbon fibers (VGCFs) with diameters of several microns were synthesized and investigated by high resolution transmission electron microscopy. It was found that the shell of the VGCFs consisted of densely-packed domains embedded in loosely-packed matrix, and both were highly amorphous. Regular edge planes as observed on the surface of fishbone nanofibers do not exist on VGCFs. Hence, surface treatment is more important for the deposition of catalysts. Ammonium ferric citrate (AFC) was employed for the impregnation of iron, where the high viscosity of the aqueous solution of AFC is beneficial. Calcination was found to be a key step to improve the dispersion of the iron particles, which can be attributed to enhanced interactions between iron and carbon due to the gasification of carbon occurring at the iron–carbon interface. Quantitative analysis by X-ray photoelectron spectroscopy showed that the calcination of the supported AFC led to a higher atomic concentration of iron on the surface, indicating smaller particle size and a higher dispersion. Secondary carbon nanofibers were grown subsequently on the VGCFs from cyclohexane. The specific surface area was enhanced considerably, from less than 1 m² g^? 1 to 106 m² g^? 1 after the growth of the secondary nanofibers. The obtained composites are promising materials as structured support in heterogeneous catalysis.     

Effect of Inorganic and Organosilicon Additives on Pyrolysis of Hydrated Cellulose Fibre Materials

It was found that the use of mixtures of pyrolytic additives has a better effect on pyrolysis of hydrated cellulose fibre materials (HCFM), resulting in an increase in the yield of carbon fibre materials (CFM) and an increase in the strength in comparison to the individual pyrolytic additives. The elevated cross-linking action of organosilicon additive OSC 6 is demonstrated; it increases the strength of CFM when used both individually and combined with other additives. The important possibility of obtaining CFM with a yield controllable in a wide range and strength as a function of the pyrolytic additives is shown.     

Comparative assessment of the effect of carbon-based material surfaces on blood clotting activation and haemolysis

This work presents results of haemolytic reaction and activation of blood clotting in contact with various carbon materials. Synthetic graphite (SG), glass-like carbon (GLC), pyrolytic carbon (LTI), pyrolytic graphite (PG) and diamond-like carbon (DLC) were investigated. Haemolytic reaction was determined by the assessment of haemolytic index (HI), haemolysis percentage and by the morphological evaluation of erythrocytes. The results indicated that, independently of the methods used and the materials studied, values of haemolysis and morphological appearances of erythrocytes were in the range of standards. It was found that LTI carbon surface prolongs the most effectively clotting activation among the carbon materials studied. The most distinct changes in haemolysis were noted for synthetic graphite, while the smallest ones for LTI carbon. Interfacial bonding energy between GLC surface and human fibrinogen was slightly lower than that for LTI carbon, whereas its total surface energy reached the highest value among the carbon materials studied. The LTI and GLC samples were shown to be the most effective in preventing thrombus formation and in prolonging the clotting time as compared with the other carbon surfaces.     

Pyrolytically coated carbon cloth electrodes

The performance of carbon cloth electrodes with a pyrolytic graphite coating deposited on their surface is reported. Cyclic voltammograms recorded with treated and untreated cloth, in the –0·9 to +0·9 V (versus Ag/AgCl) potential range, are compared. The extremely high residual currents observed on untreated cloth electrodes are absent after pretreatment; the latter maintain flat base lines even after repeated potential cycling. Optimum conditions for the pyrolytic process are recommended.