Preparation and characterisation of novel “sea-cucumber”-like structures containing carbon and boron

Spray-pyrolysis of a ferrocene-xylene-triethylborane mixture at 900 °C results in a novel “sea-cucumber”-like structure containing carbon and boron. SEM studies show that these structures are hollow with diameter between 100 and 500 nm and lengths varying from 30 to 40 μm. HRTEM and EELS studies reveal that the hollow structures are partly filled with iron. They are formed by a more graphitic internal core, which hardly contains any B. This core is coated by a more disordered B-containing C material. The amount of B in this coating is around 3 at.%. In addition, at this pyrolysis temperature, the growth of open-ended tubular nanostructures (30-40 nm diameter; 100-200 nm length) on the surface of the sea-cucumber-like structure is observed. These tubular nanostructures are not very graphitic and exhibit a similar composition to the coating of the sea-cucumber-like structure (around 3 at.% of B). The study of material produced at different pyrolysis temperatures show that the growth of these tubular nanostructures and the boron content are temperature dependent. Finally, preliminary results corresponding to oxidation resistance studies are presented.     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes. Part 2. Texture and structure

In a previous study we showed both the formation of genuine vapor grown carbon fibers (VGCFs) and of new and peculiar carbon nanotube-supported morphologies using a chemical vapor deposition process. Briefly, the latter are an association of beads (or fiber segments) with a more or less rough surface and more or less extended cone-based sub-morphologies with a smooth surface. The investigation of these materials regarding their texture, nanotexture and structure by transmission electron microscopy is reported, as a first step to understanding the formation mechanisms. It is shown that VGCFs exhibit a concentric texture, however with a variable microporous character and nanotexture quality. On the other hand, beads and related morphologies have a coarsely concentric microporous texture, as opposed to the cones and related morphologies, which exhibit a perfectly concentric and dense texture similar to that of perfect multiwall carbon nanotubes. Cross-sections performed with ultra-microtomy have revealed the spatial and textural relationship between cones and beads.     

The peculiarities of carbon interaction with catalysts during the synthesis of carbon nanomaterials

Catalysts based on metals of the iron group (Fe, Ni, Co) and their alloys are used in different methods of carbon nanomaterial production. The selection of optimal regimes for these processes calls for fundamental consideration of processes on the catalyst. An investigation of the distribution and thermally activated redistribution of interstitial carbon atoms in the volume and surface of crystalline films has been carried out. These crystals have a b.c.c. lattice and various types of free facets. The kinetic curves of interstitial atom redistribution under changes of temperature have been studied. Correlation has been made between theoretical calculations and experimental data for the graphitization of Fe-C, Ni-C, Co-C alloys and Fe-C alloy with V, Cu, Ti, Co or Ni impurities. The energetic conditions and possibility of macroscopic surface segregation of interstitial atoms have been established.     

Integration of carbon nanotubes with controllable inclination angle into microsystems

The possibility of growing carbon nanotubes in the immediate proximity of microstructures on a surface in a controllable way, with a high degree of control over the inclination angle, is demonstrated. Carbon nanotubes synthesised in a plasma-enhanced chemical vapour deposition process are known to grow in the direction of the electrical field. Geometrical features of the conductive substrate holder are used to distort the electrical field, thereby controlling the inclination angle of the carbon nanotubes locally. It is shown that the geometrical features of the microstructures on the silicon wafer do not interfere substantially with the resulting inclination angle. Finite element simulations show good agreement with the experimental observations, thus this is a route towards integrating carbon nanotubes with a special inclination angle on microstructures.     

Synthesis of carbon nanotubes on metal alloy substrates with voltage bias in methane inverse diffusion flames

Vertically well-aligned multi-walled carbon nanotubes (MWNTs) with uniform diameters (∼15 nm) were grown on catalytic probes at high yield rates in an inverse diffusion flame (IDF) of a co-flow jet configuration using methane as fuel. Varied parameters investigated included: alloy composition (e.g. Fe, Ni/Cu, Ni/Cr/Fe), sampling positions within the flame structure, and voltage bias applied to the probe substrate. Spontaneous Raman spectroscopy was utilized to determine the local gas-phase temperature, as well as the concentrations of carbon-based precursor species (e.g. CO, C₂H₂) within the flame structure at specific locations of carbon nanotube (CNT) growth during synthesis. The variation of the aforementioned parameters strongly affects CNT formation, diameter, growth rate, and morphology.     

Chemical vapor deposition of pyrolytic carbon on carbon nanotubes: Part 1. Synthesis and morphology

The chemical vapor deposition of the pyrocarbon from a CH₄+H₂ mixture is investigated using nanofilamentous substrates. The process consists of growing carbon nanotubes via a catalytic process, which then are thickened by pyrolytic carbon deposition to reach diameters in the nanometer to micrometer range. A key characteristic of the experimental reactor used was the long length of its isothermal zone, preceded (and followed) by a low thermal gradient zone. This allowed us to investigate the role of the variation of the local gas phase composition, which depends on the post-cracking secondary reactions, and on the quantity and quality of the deposited carbon. The ‘time of flight’ of the reactive species was found to be a leading parameter in the pyrolytic carbon deposition process. Various nanometric and micrometric morphologies, several of which are new, were synthesised and found constituted with an association of different sub-morphologies. The various morphologies, that can be sorted following a factor of morphological complexity, were investigated by scanning electron microscopy.     

Characterization of carbon nanofiber composites synthesized by shaping process

Catalytically grown carbon nanofibers (CNFs) are shaped into pellets in desired size and configuration by a conventional molding process so as to extend the potential applications of CNFs in industrial heterogeneous catalysis. After shaping, a novel carbon nanofiber composite with sufficient mechanical strength is produced, in which isolated CNFs are connected by a carbon network formed through polymer binder carbonization. Characterization of the synthesized CNF composite is performed by using HRTEM, XRD, Raman, N₂ physisorption, TPD and TGA. A comparison of the textural and structural properties, as well as the surface chemistry is made amongst the CNFs, the CNF composite, and a commercial coal-based activated carbon, in order to attain a comprehensive understanding of the CNF composite. The results show that the CNF composite preserves the mesoporous texture of the CNFs which will be beneficial to those reactions of mass transfer control. The modification effect of oxidative treatments on physico-chemical properties of the CNF composite is also investigated. More surface oxygen-containing groups are introduced to the composite by treating the material either in boiling HNO₃ solution or in static air at 400 °C.     

Large-scale synthesis and characterization of carbon spheres prepared by direct pyrolysis of hydrocarbons

Large-scale production of pure carbon spheres, with diameters from 50 nm to 1 μm, has been achieved via direct pyrolysis of a wide range of hydrocarbons, including styrene, toluene, benzene, hexane, cyclohexane and ethene, in the absence of catalyst. Specific systematic studies using styrene as the feedstock indicate that the sizes of the resulting of carbon nanospheres can be controlled quite well by adjusting the experimental conditions. The resulting materials have been fully characterized using SEM, TEM, AFM, HRTEM, EDX, elemental analysis, density measurement, XPS, FTIR, XRD, Raman, and TGA. The results show that the spheres, which are 99% carbon, consist of concentric incompletely closed graphitic shells. The dangling bonds on the edges of the shells result in high chemical reactivity.     

Flame synthesis of Ni-catalyzed nanofibers

Flame synthesis of Ni-catalyzed nanofibers is illustrated. Ni nanoparticles are formed by thermal decomposition of a nebulized Ni nitrate solution entrained into a reactive fuel mixture. Although at the earliest stages of growth, the Ni nanoparticles are sufficiently small to catalyze single-wall nanotubes (SWNTs), only the larger particles appear catalytically active yielding only nanofibers. Using different reactive gas mixtures consisting of CO or C₂H₂ or their combination, Ni nanoparticles exhibited a high preferential reactivity towards C₂H₂ to form nanofibers. These results are interpreted as a result of CO enhancing the dissociative adsorption of C₂H₂ through electronic charge donation mediated by the Ni nanoparticle and physically protecting active sites against blockage by aromatic molecules.     

Nano-sized carbon hollow spheres for abatement of ethylene

In this paper, the ethylene adsorption capacities of the nano-sized carbon hollow spheres (CNB) and active carbon (AC), the Pd (PdCl₂) impregnated CNB or AC (Pd/CNB, Pd/AC) and heat treatment under various conditions, were studied at different ethylene concentrations from 64 to 1060 ppm. The results indicated that AC had a good ethylene adsorption capacity at high ethylene concentration. Pd impregnation decreased the ethylene adsorption capacity of AC. Heat treatment and H₂ activation could increase the ethylene adsorption capacity, but also lowered than AC itself. CNB had lower ethylene adsorption capacity than AC, but heat treatment and H₂ activation could increase its ethylene adsorption capacity markedly. With activating condition from heat treatment in N₂ at 300 °C to activation in H₂/N₂ at 100 °C, to activation in H₂ at 200 °C, and to activation in H₂ at 300 °C, the ethylene adsorption capacity of Pd/CNB was increased regularly. At low ethylene concentration, viz., 64 ppm, the ethylene adsorption quantities (q _a) by Pd/CNB activated in H₂ at 200 or 300 °C were higher than any other adsorbents. So, activated in H₂ atmosphere at higher than 100 °C, Pd/CNB is particularly advantaged for adsorbing low concentration of ethylene. Amongst all the adsorbents used, Pd/CNB activated in H₂ atmosphere at 300 °C for 2 h has the highest ethylene adsorption capacity at lower concentration than 125 ppm. In addition, all the CNB, Pd/CNB, AC, and Pd/AC samples can be easily regenerated in airflow for more than 3 h.     

Solid-liquid-solid growth mechanism of single-wall carbon nanotubes

Reasons are presented which suggest that the liquefaction of the catalytic particles is a decisive condition for formation of single wall carbon nanotubes (SWNTs) by physical synthesis techniques. It is argued that the SWNT growth mechanism is a kind of solid-liquid-solid graphitization of amorphous carbon or other imperfect carbon forms catalyzed by molten supersaturated carbon-metal nanoparticles. The assumption of low temperature melting of these nanoparticles in contact with amorphous carbon followed by its precipitation in the form of SWNTs allows to explain qualitatively the experimentally observed SWNT growth rates and temperature dependence of the SWNT yield. Guidelines for increasing SWNT yield are proposed.     

Hydrogen storage capacity of carbon nanotubes, filaments, and vapor-grown fibers

We have studied the sorption of hydrogen by nine different carbon materials at pressures up to 11 MPa (1600 psi) and temperatures from −80 to +500°C. Our samples include graphite particles, activated carbon, graphitized PYROGRAF vapor-grown carbon fibers (VGCF), CO₂ and air-etched PYROGRAF fibers, Showa-Denko VGCF, carbon filaments grown from a FeNiCu alloy, and nanotubes from MER Corp. and Rice University. We have measured hydrogen sorption in two pieces of equipment, one up to 3.5 MPa, and one to 11 MPa. The results so far have been remarkably similar: very little hydrogen sorption. In fact, the sorption is so small that we must pay careful attention to calibration to get reliable answers. The largest sorption observed is less than 0.1 wt.% hydrogen at room temperature and 3.5 MPa. Furthermore, our efforts to activate these materials by reduction at high temperatures and pressures were also futile. These results cast serious doubts on any claims so far for room temperature hydrogen sorption in carbon materials larger than a 1 wt.%.     

Preparation of carbon hollow fiber membranes by pyrolysis of polyetherimide

The preparation of carbon membranes by pyrolysis of polyetherimide hollow fibers and the influence of process variables on the final membrane morphology using a statistical experimental design are described in this work. The characterization of polymers and membranes was carried out by thermal analysis and scanning electron microscopy (SEM). The carbonization process was accompanied by mass spectroscopy to monitor the products formed. Similar to carbonization of others polymers, H₂O, CO₂ and CO evolution from 420 to 680 °C, and hydrogen evolution from 450 to 800 °C, indicate the formation of crosslinking of polymeric chains and formation of a graphite-like structure. These experiments permitted the production of thermostable carbon hollow fibers and selection of best treatment conditions. The extent of membrane exposure under oxidizing atmosphere and the maximum temperature of stabilization were decisive in the final membrane morphologic characteristics and properties. When the stabilization temperature was above 500 °C an intensive degradation of the fiber was observed. An initial exposure to an oxidizing atmosphere seems to be fundamental in order to control the final membrane properties. In this atmosphere, heating rates as low as 1 °C min⁻¹ during stabilization reduce cracks in the surface of final membranes.     

Performance improvement of micron-sized fibrous metal filters by direct growth of carbon nanotubes

Carbon nanotubes were synthesized directly by thermal chemical vapor deposition onto the surface of micron-sized metallic fibers to improve the filtration performance of a conventional metal filter. Depending on the synthesis conditions, carbon nanotubes grew up to consist of microstructures like bushes surrounding the metal fibers or like webs crossing between the fibers. The carbon nanotubes grown around the fibers collected more particulate pollutants, so that the filtration efficiency increased without significant increase of pressure drop. Especially, the filtration performance was improved when the CNTs formed the microstructures having the morphology such as naps or weeds on the micron-fibers.