Preparation of highly crystalline nanofibers on Fe and Fe-Ni catalysts with a variety of graphene plane alignments

The present study confirmed that highly crystalline nanofibers with controlled structure may be prepared over Fe and Fe-Ni alloy catalysts. The degree of graphitization of various carbon nanofibers (CNFs) was analyzed by using C(0 0 2) peaks from the XRD profiles. The C(0 0 2) peaks of CNFs over Fe catalyst shifted to higher angle and became narrower as the preparation temperature increased from 560 to 620 °C. Tubular CNFs prepared at temperature higher than 630 °C showed lower 2θ angles compared to those of platelet fibers. CNFs prepared over Fe-Ni catalysts tended to resemble those prepared over Fe catalysts. The degree of graphitization of platelet CNFs resembled natural graphite, while d_0 0 2 of the tubular CNFs showed values below the 3.39 Å reported as a theoretical minimum for a cylindrical alignment. Lc_0 0 2 of platelet and tubular CNFs increased by heat treatment at 2000 and 2800 °C though d_0 0 2 changed little. A transverse section of platelet and tubular CNFs had a hexagonal shape, not a round shape. The hexagonal column allows AB stacking of hexagonal planes that can give perfect hexagonal alignment.     

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.     

Effect of carbon nanofibers on the anisotropy of an aromatic thermotropic liquid crystalline polymer

The microstructure of a thermotropic liquid crystalline polymer (TLCP, Vectran V400 P) was investigated in the presence of vapor-grown carbon nanofibers. Percolation threshold was observed at ∼5 wt% carbon nanofibers. During processing, strong flows resulted in severe anisotropy of the semirigid rod-like TLCP molecules. For a given type of flow, however, the nanofibers were found to reduce the overall anisotropy of the TLCP nematic phase in the nanocomposite. We believe that nanofibers provide surface anchoring for the nematic phase that helps disrupt the high degree of molecular order in the TLCP matrix and reduces its anisotropy in the nanocomposite.     

Synthesis and characterization of a nanofiltration carbon membrane derived from phenol–formaldehyde resin

We have synthesized carbon membranes by carbonizing a phenol–formaldehyde (PF) resin in the absence of air, and evaluated their physical properties and separation performance. Carbonization of the PF resin at 500 °C for 30 min resulted in around 20% mass reduction and produced a membrane material which exhibited a moderate BET surface area of 29.6 m²/g, with the majority of its porous structure confined to pores <15 nm. As observed by visible Raman spectroscopy, the carbon membrane seems to consist of a crystalline structure imbedded in an amorphous carbon matrix. The average grain size was found to be around 9.18 nm, which matches closely the results of Raman studies. The crystal structure is based on a hexagonal structure with a=7.8072 Å and c=7.066 Å with the latter length matching that of graphite. The difference in the former parameter was attributed to a motif consisting of a seven-membered plane benzene ring (one ring surrounded by six others) due to internal strain which was determined to be 3.22% by an XRD technique. The molecular weight cut-off (MWCO) of this membrane was found to be about 7500 Daltons. The performance of the membrane was studied by separating hexavalent chromium ions from an aqueous chromic acid solution. The apparent and intrinsic rejection of Cr⁶⁺ ions was of the order of 70 and 90%, respectively, and was found to increase with an increase in applied pressure.     

A novel hybrid of carbon nanotubes/iron nanoparticles: iron-filled nodule-containing carbon nanotubes

A straightforward, one-step method for the preparation of novel carbon nanotube/iron nanoparticle hybrids with some degree of shape control is reported herein. These carbon nanostructures differ from those reported previously: the nanoparticles were not attached to or coated onto the surface of carbon nanotubes but embedded inside the carbon wall. They were synthesized in good yield by thermolysis of ferrocene and thiophene mixtures in a closed steel vessel. The shapes and compositions of these nanostructures can be simply controlled by adjusting the reaction temperature and relative amounts of the precursors. Iron-filled T-junction carbon nanotubes were also obtained easily by this procedure. These iron-filled nodule-containing carbon nanotubes (INCNTs) are either empty or filled with iron or iron carbide (Fe(C)) nanowires. The outer diameters of these nanotubes range from 70 to 150 nm and the lengths reach up to several micrometers. The average size of the Fe(C) nanoparticles (or empty cores) inside the nodules is about 50 nm in diameter. The carbon in the INCNTs is amorphous. Sulfur was found being responsible for the disordered structure and playing a unique role in promoting the growth of INCNTs as well as the formation of T- or Y-junctions.     

Large diameter carbon fibres from mesophase pitch

Large diameter carbon fibres of 15-60 μm were produced from naphthalene-derived mesophase pitch. In the as-spun state, preferred orientation increases with fibre diameter, confirming a previous study. However, the sheath-core structure that develops during carbonisation of large diameter fibres results in a decrease in preferred orientation. As a consequence, the strength, modulus and electrical resistivity also decrease. The coarse optical texture of the partially relaxed core allows a higher degree of graphitisation. With extended stabilisation, the core can be eliminated and the mechanical properties improve whilst the electrical resistivity increases.     

A study of film thickness dependence of the graphitizability of PMDA–ODA polyimide-derived carbon film

PMDA–ODA (pyromellitic dianhydride–4,4′-oxydianiline) polyimide (PI) films with thickness of 2–26 μm were synthesized and heat-treated at 2200–2500°C. The effect of film thickness on both the graphitizability of the PI-derived carbon film and the crystalline organization of the initial PI film was investigated by X-ray diffraction. The degree of graphitization of the resultant carbon film was observed to increase with decreasing initial PI film thickness, corresponding to the film thickness dependence of the crystalline ordering in the initial PI films. Structural factors related to PI film thickness are discussed for understanding the graphitizability of the resultant carbon.     

Preparation of vapor-grown carbon fibers from deoiled asphalt

Vapor-grown carbon fibers (VGCFs) with high-purity have been successfully prepared from the thermal cracking of deoiled asphalt (DOA) with ferrocene as catalyst by chemical vapor deposition (CVD) in argon atmosphere and characterized systematically by field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) techniques. Results showed that VGCFs with a diameter of 150-200 nm and a maximum length of 10-40 μm can be obtained.     

Specification for a standard procedure of X-ray diffraction measurements on carbon materials

In 1963, the 117 Committee of the Japan Society for the Promotion of Sciences (JSPS) specified standard procedures for the determination of lattice constants and crystallite sizes of carbon materials, especially graphitized materials, the so-called “Gakushin” method. In 2002 the 117 Committee started to discuss the revision of the specification in order to accept some modern computing processes, such as automatic step-scanning measurements of diffraction intensities, profile fitting for diffraction lines, etc. In this paper, the English version of the revised specification is presented to the international community of scientists working on carbon materials, and to solicit comments.     

Local chemical vapor deposition of carbon nanofibers from photoresist

The addition of nanofeatures to carbon microelectromechanical system (C-MEMS) structures would greatly increase surface area and enhance their performance in miniature batteries, super-capacitors, electrochemical and biological sensors. Negative photoresist posts were patterned on a Au/Ti contact layer by photolithography. After pyrolyzing the photoresist patterns to carbon patterns, graphitic nanofibers were observed near the contact layer. The incorporation of carbon nanofibers in C-MEMS structures via a simple pyrolysis of modified photoresist was investigated. Both experimental results considered to consist of a local chemical vapor deposition mechanism. The method represents a novel, elegant and inexpensive way to equip carbon microfeatures with nanostructures, in a process that could possibly be scaled up to the mass production of many electronic and biological devices.     

Modeling of fishbone-type carbon nanofibers: A theoretical study

Five models for fishbone-type carbon nanofibers (f-CNFs) have been proposed and their geometries have been optimized through molecular dynamics and molecular mechanics simulations. The angle between graphene layers and fiber axis can take the values 9.71°, 19.60°, 30.03°, 41.99°, and 56.43°, which agree satisfactorily with the experimentally determined values. With reduction of the angle, the interlayer spacing of graphene sheets increases from 0.3376 nm to 0.3392 nm accordingly, due to the strain of the graphitic cones. In addition, X-ray diffraction (XRD) simulations are also implemented. With the high-resolution transmission electron microscopy (HRTEM) results taken into account, the XRD simulations verify that f-CNFs can adopt not only a regular arrangement but also a turbostratic way, and our models are good approximations for describing the bulk microstructures of f-CNFs.     

Preparation and characterization of antibacterial silver-dispersed activated carbon aerogels

Silver-dispersed carbon aerogels (CAs) were obtained by direct immersion of organic aerogels prepared by ambient pressure drying technique in AgNO₃ aqueous solution and then carbonization. The effect of preparation conditions such as the resorcinol/catalyst ratio, the feed AgNO₃ concentration, the ratio of aerogel mass/solution volume, immersion time and carbonization temperature on the bulk density and silver content as well as the BET surface area of the dispersed CAs was studied. The dispersion and structure of silver nanoparticles in obtained materials were investigated by means of scanning electron microscopy, transmission electron microscopy and X-ray diffraction. The Ag-dispersed CAs prepared exhibit strong and long-term antibacterial activity.     

纳米碳纤维化学镀镍-铁-钴-磷合金镀层

在铁片试样上研究了化学镀Ni-Fe-Co-P合金镀层的工艺,利用此工艺在经过敏化、活化处理后的纳米碳纤维表面沉积出Ni-Fe-Co-P合金镀层。采用能量色散X射线谱(EDS)分析得出镀层成分,利用扫描电子显微镜(SEM)观察镀层形貌。结果表明,镀层的沉积速率随镀液中氯化镍质量浓度的增加而增加,随硫酸钴质量浓度的增加而降低;通过控制镀液中c(Co2 )/c(Ni2 )的比值,可控制沉积速率及镀层中镍、钴元素的相对含量;镀后的纳米碳纤维分散性好,获得了连续、均匀的Ni-Fe-Co-P合金镀层。     

Preparation of carbon nanofibers/carbon foam monolithic composite from coal liquefaction residue

Carbon nanofibers/carbon foam composites that are made by growing carbon nanofibers (CNFs) on the surface of a carbon foam (CF) have been prepared from coal liquefaction residues (CLR) by a procedure involving supercritical foaming, oxidization, carbonization, and catalytic chemical vapour deposition (CCVD) treatment. These new carbon/carbon composites were examined using SEM, TEM and XRD. The results show that the as-made CF has a structure with cell sizes of 300-600 μm. X-ray diffraction studies show that iron-containing contaminates are present in the CLR. However, these species may act as a catalyst in the CCVD process as established in the literature. After the CCVD treatment, the cell walls of CF are covered by highly compacted CNFs that have external diameters of about 100 nm and lengths of several tens of micrometers. This work may open a new way for direct and effective utilization of the CLR.