Differences between carbon nanofibers produced using Fe and Ni catalysts in a floating catalyst reactor

Carbon nanofibers were produced by the catalytic CVD process by the floating catalyst method, in semi-industrial systems at temperatures above 1350 K. Iron-derived carbon nanofibers were produced from natural gas and xylene, using ferrocene as catalyst source, yielding a thickened submicron vapor grown carbon fibers with a core of multi-wall nanotubes. For the production of Ni derived nanofibers, natural gas was used as the carbon feedstock, and the Ni was added in a nickel compound solution. When no sulfur is used, only soot was obtained, but when sulfur is added to the reactive feedstock, a highly graphitic and very nice stacked-cup-type nanofibers with no free-CVD thickened layer were produced. TEM-EDS analysis confirms that this type of stacked-cup carbon nanofiber is produced only with a partially molten catalyst and methane as hydrocarbon source. In fact, very few fibers have either a particle tip at the end or trapped metal particle inside the wide hollow core of this type of produced carbon material.     

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.     

Prevention of Si-contaminated nanocone formation during plasma enhanced CVD growth of carbon nanotubes

We investigated the growth behavior and morphology of vertically aligned carbon nanotubes (CNTs) on silicon (Si) substrates by direct current (DC) plasma enhanced chemical vapor deposition (PECVD). We found that plasma etching and precipitation of the Si substrate material significantly modified the morphology and chemistry of the synthesized CNTs, often resulting in the formation of tapered-diameter nanocones containing Si. Either low bias voltage (∼500 V) or deposition of a protective layer (tungsten or titanium film with 10-200 nm thickness) on the Si surface suppressed the unwanted Si etching during growth and enabled us to obtain cylindrical CNTs with minimal Si-related defects. We also demonstrated that a gate electrode, surrounding a CNT in a traditional field emitter structure, could be utilized as a protection layer to allow growth of a CNT with desirable high aspect ratio by preventing the nanocone formation.     

A reduced reaction model for carbon CVD/CVI processes

Carbon-carbon composites are produced by chemical vapor deposition/chemical vapor infiltration (CVD/CVI) processes. Models of carbon-carbon composite production processes will help reduce production costs. Reliable process models must, however, include details of the gas phase kinetics in order to identify optimal conditions. We have combined detailed gas phase kinetics, surface kinetics, and a pore closure model to predict pore geometry changes with respect to time. To determine the dominant gas phase kinetics, we reduced a large set of reactions to a minimal set using a sensitivity, rate, and dimensional analysis approach. These robust and relatively fast techniques can be used under a variety of conditions, including those within the pores of the composite. The process model shows that the deposition profile depends on the kinetic model chosen. Using the more realistic reaction model, conditions for uniform, or inside-out, densification can be suggested.     

Liquid reagent CVD of carbon. I. Processing and microstructure

The processing and microstructure of carbon coatings deposited using liquid reagent CVD were studied. High density pyrolytic carbon coatings were successfully deposited on graphite and molybdenum substrates from benzene and cyclohexane precursors. Very high deposition rates were obtained. Examination via transmission electron microscopy showed that the deposits were of the desired turbostratic nodular structure with low texture.     

Diameter and morphology dependence on experimental conditions of carbon nanotube arrays grown by spray pyrolysis

A systematic study on the controlled growth of large areas of aligned multi-wall carbon nanotube arrays, from ferrocene-benzene precursor, and of nanotube junctions from ferrocene-thiophene precursor, without hydrogen addition, using an injection CVD method easy to scale up for industrial production is reported.A detailed study is presented of how the synthesis parameters such as growth temperature, active solution flow rate, catalyst concentration or sulfur addition can control the properties and morphology of the grown nanotube mat. Nanotube junctions with considerable yield can be grown with our method by adding sulfur to the synthesis process. The sulfur addition also results in growth of carbon nanocones (CNC) in the lower temperature regime of the furnace. Observation of single-wall carbon nanotubes in our STM investigations provides further indication that under properly chosen conditions SWCNTs can be grown with similar continuous processes.     

Diameter control of carbon nanotubes by changing the concentration of catalytic metal ion solutions

We have developed a simple new method to control the diameter of carbon nanotubes (CNTs) using catalytic nanoparticle arrays fabricated by filling the pores of well-ordered porous anodic aluminum oxide (AAO) templates with a metal ion solution. Fe ion solution was used to fill the pores in which Co had been deposited electrochemically, and then the template was dried naturally on a magnet. After this process, the pores were widened in NaOH solution. Well-graphitized multi-walled CNTs were grown from almost all the pores and were very long in length and homogeneous in diameter. We were able to control the diameter of CNTs, simply, by changing the concentration of iron ion solution. For example, the average outer diameters of the CNTs are 7 ± 1.5, 13 ± 1, and 17 ± 1 nm when the concentrations of Fe ion in their mother solutions were 1.0 × 10⁻³, 3.0 × 10⁻³, and 6.0 × 10⁻³ M, respectively. The inner diameters of these CNTs corresponded to the calculated diameters of Fe nanoparticles by assuming that all Fe ions contained in each pore are reduced to a single nanoparticle. This means that homogeneous nanoparticles are made in each pore. Our new method could be used to fabricate homogeneous nanoparticles from most metal ion solutions.     

Characterizing herring bone structures in carbon nanofibers using selected area electron diffraction and dark field transmission electron microscopy

The use of selected area electron diffraction and centered dark field imaging using a transmission electron microscope is demonstrated for studying the herringbone structure of carbon nanofibers (CNFs). The experimental method is described and illustrated with CNFs that were grown via a chemical vapor deposition method with a nickel catalyst. It is demonstrated that this method gives the angle of the herringbone with great accuracy and gives insight into the uniformity of graphitic elements in the herringbone structure. It was found that the Ni catalyst could be removed from the fiber-tips by treatment in HNO₃, without affecting the carbon structure. These electron microscopy results were confirmed by XRD. The parameters that can be determined by application of this characterization method are expected to be useful to study and optimize growth conditions for carbon nanofibers grown by chemical vapor deposition.     

Field emission properties of carbon nanotubes synthesized by capillary type atmospheric pressure plasma enhanced chemical vapor deposition at low temperature

Carbon nanotubes (CNTs) were grown using a modified atmospheric pressure plasma with NH₃(210 sccm)/N₂(100 sccm)/C₂H₂(150 sccm)/He(8 slm) at low substrate temperatures (?500 °C) and their physical and electrical characteristics were investigated as the application to field emission devices. The grown CNTs were multi-wall CNTs (at 450 °C, 15-25 layers of carbon sheets, inner diameter: 10-15 nm, outer diameter: 30-50 nm) and the increase of substrate temperature increased the CNT length and decreased the CNT diameter. The length and diameter of the CNTs grown for 8 min at 500 °C were 8 μm and 40 ± 5 nm, respectively. Also, the defects in the grown CNTs were also decreased with increasing the substrate temperature (The ratio of defect to graphite (I_D/I_G) measured by FT-Raman at 500 °C was 0.882). The turn-on electric field of the CNTs grown at 450 °C was 2.6 V/μm and the electric field at 1 mA/cm² was 3.5 V/μm.     

Structural analysis of pyrolytic carbon deposits on a planar cordierite substrate

Pyrolytic carbon deposits were produced by chemical vapor deposition on a planar substrate of cordierite using methane as a source gas. The structure of the deposits was characterized by light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with electron-energy-loss spectroscopy (EELS). The surface morphology is characterized by a cell structure induced by grains elongated perpendicular to the substrate surface. Energetic shift and intensity fluctuation of plasmon peaks in EELS spectra taken from cell and interface regions between the cells correlate with an alteration of the SEM image contrast observed on freshly fractured surfaces. This correlation suggests the presence of a mixture of two materials exhibiting different crystallization degrees. The material located at the interface is more amorphous-like in comparison to the graphite-like material located within cells.     

Synthesis of carbon nanotubes on metallic substrates by a sequential combination of PECVD and thermal CVD

Carbon nanotubes were synthesized from acetylene and hydrogen gas mixture directly on stainless steel plates by sequential combination of rf PECVD and thermal CVD. PECVD was used for nucleation and initial growth of carbon nanotubes while thermal CVD was utilized for further growth of them. In this way decoupling of nucleation and growth of carbon nanotubes was realized, and growth of carbon nanotubes was enhanced compared to growth by PECVD. Synthesized carbon nanotubes were curly in shape, and proper pretreatment of the substrate surface was required for the satisfactory growth of carbon nanotubes. Carbon nanotubes could be fabricated into electrodes for electric double layer capacitors without any further treatment. With an electrolyte composed of lithium hexafluorophosphate, ethylene carbonate and diethyl carbonate, charge/discharge test showed specific capacitance in the range of 33-82 F/g.     

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.     

Microscopical study of carbon/carbon composites obtained by chemical vapor infiltration of 0°/0°/90°/90° carbon fiber preforms

The microstructure of carbon/carbon composites obtained by isothermal, isobaric chemical vapor infiltration (CVI) of carbon fiber preforms consisting of aligned fiber bundles separated by fiber fleeces was studied comparatively by polarized light microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with selected area electron diffraction (SAED). Deposition rate as well as matrix microstructure do not differ in the aligned fiber bundles and fiber fleeces exhibiting different local surface area/volume ratios. The matrices which are homogeneously textured according to PLM exhibit pronounced spatial texture gradients at the sub-μm-scale if investigated by SAED. The texture gradients appear to be independent on the infiltration time, distance between fibers but evidently depend on the total methane pressure. TEM and SEM observations show a thin high-textured layer between the fiber and the medium-textured transitional layer below the high-textured matrix layer containing columnar grains. This thin layer replicates the surface unevenness of the fiber surface while it is absent at the initial carbon fiber surface before infiltration.