Consolidation of carbon nanofibers with pyrolytic carbon

A strategy of industrial-scale manufacture for a wide range of carbon materials based on carbon nanofibers is proposed. It was shown that porous materials with a high sorption capacity can be obtained with the use of carbon nanofibers by means of conventional manufacturing operations. The results of studying of consolidation of carbon nanofibers with pyrolytic carbon are reported. It was found that the nature of carbon material has a substantial effect on the rate of deposition of pyrolytic carbon. The most appropriate temperature range in which carbon nanofibers should be consolidated for the preparation of materials with a high catalytic activity was determined.     

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.     

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.     

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.     

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.     

Mechanical enhancement of C/C composites via the formation of a machinable carbon nanofiber interphase

C/C composites with improved mechanical strength were synthesized using a filler constituted by a carbon felt covered with catalytically grown carbon nanofibers (CNFs) and a carbonaceous matrix generated by the pyrolysis of a phenolic resin. First, the synthesis method of the filler allows the homogeneous deposition and anchorage of CNFs on the host microfilaments at a rapid densification rate. Carbon nanofibers grown this way lead to the formation of numerous micro- and nanobridges between the microfilaments, conferring a significant improvement of the mechanical resistance of the CNF/C system allowing one to tailor its dimensions and shape. Thus, further fabrication of C/C composites can be achieved: the CNF/microfilament structure was infiltrated with a phenolic resin and carbonized at 650 °C to generate a carbonaceous matrix by thermal decomposition. Similar experiments on the microfilaments carried out at the same synthesis time, without catalyst and at higher reaction temperatures led to the deposition of a pyrolytic carbon sheath and to poor mechanical enhancements. This clearly indicates the advantage of using CNF growth as an efficient densification process before infiltration. Such C/C composites exhibit high-quality bonding between the two carbon phases, the matrix and the CNF/microfilament filler, via the formation of a considerable amount of CNF interphase.     

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。     

Hydrogen diffusion and solubility in pyrolytic carbon

Tritium diffusion coefficients and deuterium solubilities have been measured for laminar pyrolytic carbon in the temperature range 900–1500°C. The tritium diffusion coefficients were much lower than those for metals at equivalent temperatures, but the activation energy for the diffusion was much higher (～-100 kcal/mole). Tritium diffusion coefficients measured for silicon-doped pyrolytic carbon were over an order of magnitude higher than the values for the undoped laminar pyrolytic carbon. The solubility of deuterium in laminar pyrolytic carbon was found to decrease with increasing temperature and exhibited a pressure dependence of $\text{p.}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$     

Field emission and structure of aligned carbon nanofibers deposited by ECR-CVD plasma method

Aligned carbon nanofibers and hollow carbon nanofibers were grown by MW ECR-CVD method using methane and argon mixture gas at a temperature of 550°C. The carbon nanofibers and the hollow carbon nanofibers were deposited perpendicularly on Si substrates and on Si substrates coated with Ni catalyst, respectively. From TEM analysis the diameter and length of the nanofibers are approximately 60 nm and 15 μm, respectively. Raman spectra of these aligned carbon nanofibers showed new bands of 1340 and 1612 cm⁻¹ of the first-order Raman scattering and 2660, 2940 and 3220 cm⁻¹ of the second-order Raman scattering. The second-order Raman scattering bands were assigned to two overtone and one combination bands on the basis of a similar assignment of micro-crystal graphite by Nemanich and Solin. By the measurement of XPS C1s band energies of 284.6 eV for the carbon nanofiber and 284.7 eV for the hollow carbon nanofiber indicate mainly sp² carbon component in the inclusion of a small amount (<5%) of oxygen in a high binding energy region (∼288 eV). Field emission characteristics of the well-aligned carbon nanofibers and hollow carbon nanofibers were investigated and the current densities were 7.25 and 0.69 mA/cm² at 12.5 V/μm, respectively.     

Formation of pyrolytic carbon in a continuous reactor for coal hydrogenation

Microscopic examination of blockage material from the CSIRO laboratory-scale continuous reactor for coal hydrogenation has indicated the presence of pyrolytic carbon. Both the morphology and abundance of pyrolytic carbon were variable. In some samples pyrolytic carbon spheres (≈ 1 μm) formed large deposits with clear zonal texture. Spherical agglomerates (up to 70 μm) of these particles, and large particles were also observed. In some cases spherulitic pyrolytic carbon was embedded in vitroplast and/or mesophase. The occurrence of pyrolytic carbon in continuous reactors for coal hydrogenation is significant as it can provide, in the absence of other information, a posteriori indication of undesirable reaction conditions, i.e. large gas hold-ups and localized overheating.     

Improved graphitization and electrical conductivity of suspended carbon nanofibers derived from carbon nanotube/polyacrylonitrile composites by directed electrospinning

Single suspended carbon nanofibers on carbon micro-structures were fabricated by directed electrospinning and subsequent pyrolysis at 900 °C of carbon nanotube/polyacrylonitrile (CNT/PAN) composite material. The electrical conductivity of the nanofibers was measured at different weight fractions of CNTs. It was found that the conductivity increased almost two orders of magnitude upon adding 0.5 wt.% CNTs. The correlation between the extent of graphitization and electrical properties of the composite nanofiber was examined by various structural characterization techniques, and the presence of graphitic regions in pyrolyzed CNT/PAN nanofibers was observed that were not present in pure PAN-derived carbon. The influence of fabrication technique on the ordering of carbon sheets in electrospun nanofibers was examined and a templating effect by CNTs that leads to enhanced graphitization is suggested.     

Electrocatalytic properties of Pt/carbon composite nanofibers

Pt/carbon composite nanofibers were prepared by electrodepositing Pt nanoparticles directly onto electrospun carbon nanofibers. The morphology and size of Pt nanoparticles were controlled by the electrodeposition time. The resulting Pt/carbon composite nanofibers were characterized by running cyclic voltammograms in 0.20 M H₂SO₄ and 5.0 mM K₄[Fe(CN)₆] + 0.10 M KCl solutions. The electrocatalytic activities of Pt/carbon composite nanofibers were measured by the oxidation of methanol. Results show that Pt/carbon composite nanofibers possess the properties of high active surface area and fast electron transfer rate, which lead to a good performance towards the electrocatalytic oxidation of methanol. It is also found that the Pt/carbon nanofiber electrode with a Pt loading of 0.170 mg cm⁻² has the highest activity.     

Influence of the textual properties of activated carbon nanofibers on the performance of electric double-layer capacitors

To investigate the relationship between textural properties and electrochemical properties, activated carbon nanofibers were manufactured using an electrospinning process followed by chemical activation using KOH or NaOH. The specific surface area of the KOH-activated carbon nanofibers was higher than that of NaOH-activated carbon nanofibers; however, the total pore volume and mesopore volume of the NaOH-activated carbon nanofibers were greater than those of the KOH-activated carbon nanofibers when the same number of moles of KOH and NaOH were used. The specific capacitances increased as the specific surface area and pore volume of the activated carbon nanofibers were increased. However, the specific capacitance obtained at a high scan rate (50 mV/s) and the retained capacitance of the activated carbon nanofibers increased with increasing total pore and mesopore volume, especially for mesopores with diameters of 2–4 nm.     

Fabrication and characterization of carbon nanofibers with a multiple tubular porous structure via electrospinning

Carbon nanofibers with a multiple tubular porous structure were prepared via electrospinning from a polymer blend solution of polyacrylonitrile (PAN) and polylactide (PLA) followed by carbonization. The electrospun composite nanofibers underwent pre-oxidization and carbonization, which selectively eliminated PLA phases and transformed the continuous PAN phase into carbon, thereby porous structure formed in the carbon nanofibers. The morphologies of as-spun, pre-oxidized and carbonized nanofibers were studied by scanning electron microscope (SEM) and transmission electron microscopy (TEM). It was found that carbon nanofibers with an average diameter about 250 nm and a multiple tubular porous structure were obtained. The chemical changes during thermal treatment were studied by Fourier transform infrared spectrometer (FTIR), Raman spectra, differential thermal analysis (DTA) and thermogravimetric analysis (TG). The results showed that PLA phases were effectively removed and the continuous PAN phase was completely carbonized. The obtained carbon nanofibers had more disordered non-graphitized structures than non-porous nanofibers.     

The chemical vapor infiltration of exfoliated graphite to produce carbon/carbon composites

Chemical vapor infiltration was used for the production of carbon/carbon composites based on exfoliated graphite and pyrolytic carbon. Two different exfoliated graphites compacted to densities of 0.05–0.4 g/cm³ were used as a preform. The influence of the synthesis conditions (temperature, pressure, time etc.) on the degree of infiltration, the pyrolytic carbon morphology and the C/C composite characteristics was examined using Raman spectroscopy, scanning electron microscopy and low-temperature nitrogen adsorption.     

The production of porous carbon nanofibers from cross-linked polyphosphazene nanofibers

Uniform porous carbon nanofibers with an average diameter of 90 nm were fabricated by forming polyphosphazene nanofibers and carbonizing them, without the need for any activation step. The structure and morphology of the carbon nanofibers were characterized by SEM, TEM, EDX, XRD, Raman spectrum and N₂ adsorption. Results showed that the carbon nanofibers have a BET surface area of about 540 m² g⁻¹, a total pore volume of about 0.37 m³ g⁻¹, and a narrow pore size distribution in the micropore range.     

废轮胎热解炭黑的研究进展

热解发黑作为废轮胎热解的关键产物,其品质较差,达不到使用要求,而且会对环境造成二次污染.主要介绍了废轮胎热解炭黑常用的分析表征方法,对热解炭黑的活化改性及再利用途径进行综述,旨在提高其利用价值.     

Fabrication and electrical conductivity of suspended carbon nanofiber arrays

We demonstrate a simple, efficient and novel self-assembly based method to fabricate arrays of suspended polymeric nanofibers of polyacrylonitrile and SU-8 negative photoresist by electrospinning on micro-fabricated posts of resorcinol–formaldehyde (RF) gel. The suspended electrospun nanofibers together with the RF gel posts were subsequently pyrolyzed in an inert atmosphere to yield large area monolithic structures of suspended glassy carbon nanofibers (CNF) integrated on RF gel derived carbon posts. The electrospun nanofibers self-assemble to connect the posts owing to a stronger electric field on their tips, obviating the need for positioning and integration of carbon nanowires with the underlying microstructures and paving the way for fabricating novel carbon based micro and nanoscale devices. The fabrication technique also allowed measurements of electrical conductivity of a single suspended CNF between carbon electrodes using I–V characteristics and comparison of the carbon nanowire conductivities for the CNF derived from different polymer precursors.     

Carbon nanotubes and carbon nanofibers fabricated on tubular porous Al2O3 substrates

Carbon nanotubes and carbon nanofibers were grown at different temperatures on porous ceramic Al₂O₃ substrates with single channel geometry by means of a chemical vapor deposition technique using methane as carbon source and palladium as catalyst. Time-resolved in-situ Fourier transformed infrared spectroscopy was used for the investigation of methane decomposition for characterizing the catalyst’s performance. With increasing synthesis temperature, a structural transition from carbon nanofibers to carbon nanotubes was observed. At a synthesis temperature of 700 °C, solely carbon nanofibers were found, whereas at 800 °C a mixture of two types of bamboo-shaped carbon nanofibers were obtained, suggesting a structural transition. A synthesis temperature to 850 °C results in bamboo-shaped multi-walled carbon nanofibers and multi-walled carbon nanotubes. The carbon products and the observed structural transition were characterized by means of field emission scanning electron microscopy, high-resolution transmission electron microscopy, thermal gravimetric analysis, and Raman spectroscopy.     

Synthesis of carbon nanofibers and measurements of hydrogen storage

The synthesis of carbon nanofibers was carried out by catalytic decomposition of ethylene in presence of hydrogen. Bimetallic catalysts, e.g. Fe-Cu or Ni-Cu, were synthesized by coprecipitation, reduction-precipitation and reverse microemulsion techniques and were proven to have a strong influence on the morphology of the nanofibers. The best results in terms of synthesis homogeneity were obtained by supporting the bimetallic catalyst on a high surface area silica support by the “incipient wetness” method. The hydrogen storage capacity of carbon nanofibers was tested in a custom made Sievert apparatus operating up to 160 bar and 450 °C. Several “in situ” activation procedures were experimented, however according to our data carbon nanofibers do not seem a suitable candidate for hydrogen storage. With the purpose of promoting a “spillover” function, 2 wt.% Pd-doped nanofibers were prepared. After loading at 77 bar, a hydrogen storage of 1.38 ± 0.30 wt.% was measured at room temperature.