Continuous deposition of carbon nanotubes on a moving substrate by open-air laser-induced chemical vapor deposition

Continuous deposition of carbon nanotubes under open-air conditions on a moving fused quartz substrate is achieved by pyrolytic laser-induced chemical vapor deposition. A CO₂ laser is used to heat a traversing fused quartz rod covered with bimetallic nanoparticles. Pyrolysis of hydrocarbon precursor gas occurs and subsequently gives rise to rapid growth of a multi-wall carbon nanotube forest on the substrate surface. A “mushroom-like” nanotube pillar is observed, where a random orientation of carbon nanotubes is located at the top of the pillars while the growth is more aligned near the base. The typical carbon nanotube deposition rate achieved in this study is approximately 50 μm/s. At high power laser irradiation, various carbon microstructures are formed as a result of excessive formation of amorphous carbon on the substrate. High-resolution transmission and scanning electron microscopy, and X-ray energy-dispersive spectrometry are used to investigate the deposition rate, microstructure, and chemical composition of the deposited carbon nanotubes.     

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.     

Effect of hydrocarbons precursors on the formation of carbon nanotubes in chemical vapor deposition

High-temperature decomposition of hydrocarbons may lead to the formation of carbon deposits. However in our present studies, we found that the morphology of carbon deposits over MgO supported Fe catalyst during chemical vapor deposition (CVD) process was closely related to the thermodynamic properties and chemical structures of hydrocarbon precursors. Six kinds of hydrocarbons (methane, hexane, cyclohexane, benzene, naphthalene and anthracene) were used as carbon precursors in this study. Methane which has a pretty simple composition and is more chemically stable was favorable for the formation of high-purity single walled carbon nanotubes (SWNTs). For high-molecular weight hydrocarbons, it was found that the chemical structures rather than thermodynamic properties of carbon precursors would play an important role in nanotube formation. Specifically, the CVD processes of aromatic molecules such as benzene, naphthalene and anthracene inclined to the growth of SWNTs. While the cases of aliphatic and cyclic hydrocarbon molecules seemed a little more complicated. Based on different pyrolytic behaviors of carbon precursors and formation mechanism of SWNTs and multi-walled carbon nanotubes (MWNTs), a possible explanation of the difference in CVD products was also proposed.     

A study on the deposition of pyrolytic carbons from hydrocarbons

A new method for forming isotropic, laminar, and columnar pyrolytic carbons is proposed. For this, a low RPM (below 2.4 rpm) tumbling bed has been used to deposit pyrolytic carbons from hydrocarbon gases. All deposits were made on graphite substrates from propane and methane at a constant temperature of 1200°C. The microstructures of the pyrolytic carbons deposited were dependent on the flow pattern of the reactant gas, the rpm of the reactor, the hydrocarbon concentration, the nature of the hydrocarbon, and the geometry of the bed. Isotropic pyrolytic carbon is formed under deposition conditions where homogeneous nucleation occurs in the gas phase and at the gas flow conditions where the gas-borne droplets can collide on the substrate. Laminar carbon is formed under deposition conditions where homogeneous nucleation does not occur in the gas phase and at gas flow conditions where the carbon species existing in the bulk of the gas phase can collide on the substrate. Columnar carbon is formed when any carbon products existing in the bulk of the gas phase cannot collide on the substrate. The suggested deposition mechanism can also be applied to pyrolytic carbons deposited in a fluidized bed or in a stationary bed. In particular, isotropic carbon can be obtained even in a stationary bed if the requirements for the deposition of the isotropic carbon described above are satisfied.     

Polymer blend emulsion stabilization using carbon nanotubes interfacial confinement

The present study demonstrates the effect of unfunctionalized MultiWalled Carbon Nanotubes (MWNTs) interfacial confinement on coalescence suppression in an immiscible polymer blend exhibiting a sea-island morphology. The effect of carbon nanotubes on morphological stabilization in polyamide (PA)/ethylene-methyl acrylate random copolymer (EA) blends is studied using electronic microscopy techniques. Owing to their interfacial localization, MWNTs are shown to enhance both phase dispersion and stability of the dispersed phase for long mixing time (at least 60 min) and very low filler content (0.5 wt.-% MWNTs) compared to what was previously observed in literature. MWNTs also produce a more uniform distribution of droplets size. The main stabilization mechanism proposed is the formation of a deformable barrier network providing a mechanical barrier against coalescence. Blends stabilized by solid anisotropic nanoparticles, like MWNTs, could therefore offer an interesting alternative to blends compatibilized by block-copolymers.     

Models for fuel droplet heating and evaporation: Comparative analysis

The results of comparative analysis of liquid and gas phase models for fuel droplets heating and evaporation, suitable for implementation into computational fluid dynamics (CFD) codes, are presented. Among liquid phase models, the analysis is focused on the model based on the assumption that the liquid thermal conductivity is infinitely large, and the so-called effective thermal conductivity model. Seven gas phase models are compared. These are six semi-theoretical models based on various assumptions and a model based merely on the approximation of experimental data. It is pointed out that the gas phase model, taking into account the finite thickness of the thermal boundary layer around the droplet, predicts the evaporation time closest to the one based on the approximation of experimental data. In most cases, the droplet evaporation time depends strongly on the choice of the gas phase model. The droplet surface temperature at the initial stage of heating and evaporation does not practically depend on the choice of the gas phase model, while the dependence of this temperature on the choice of the liquid phase model is strong. The direct comparison of the predictions of various gas models, with available experimental data referring to droplet heating and evaporation without break-up, leads to inconclusive results. The comparison of predictions of various liquid and gas phase models with the experimentally observed total ignition delay of n-heptane droplets without break-up, has shown that this delay depends strongly on the choice of the liquid phase model, but practically does not depend on the choice of the gas phase model. In the presence of droplet break-up processes, the evaporation time and the total ignition delay depend strongly on the choice of both gas and liquid phase models.     

Deposition rate and temperature of carbon films during laser-induced CVD on a moving substrate

The feasibility of using pyrolytic laser-induced chemical vapor deposition (LCVD) to deposit carbon coatings on moving fused quartz substrates is investigated. This LCVD system uses a CO₂ laser to locally heat a substrate in open air to create a hot spot. Pyrolysis of hydrocarbon species occurs and subsequently deposits a layer of carbon film onto the substrate surface. The results of this study indicate that growth kinetics and the geometry of uniform carbon stripes deposited by pyrolytic LCVD strongly related to the laser power, the traverse velocity of the substrate, the type of hydrocarbon species used in deposition, and the diameter of the substrate. The deposition rate of carbon film increases exponentially with the laser power, while an increase in traverse velocity of the substrate will also increase the deposition rate until a maximum deposition rate is reached; further increases in the traverse velocity will decrease the deposition rate. We suspect that this optimal deposition rate is caused by substrate motion, which affects the substrate surface temperature, and consequently the effective surface area available for film deposition. The substrate temperature is observed to behave linearly with the deposition parameters considered in this study.     

Deposition mechanism of pyrolytic carbons at temperature between 800–1200 °C

The textures, growth features, microstructures and binding of carbon atoms of pyrolytic carbons prepared by chemical vapor deposition (CVD) at a temperature between 800–1200 °C on graphite substrate and carbon fibers were studied. The intermediate product phase of pyrolytic carbons was also investigated. Based on the present study a deposition model of viscous droplet was proposed in this paper. The viscous droplet here refers to all kinds of fine spheroids that are more or less viscous. The mechanism of the formation of three typical textures namely, smooth laminar, rough laminar and isotropic carbons can be satisfactorily explained by this model.     

Mass production of high-quality multi-walled carbon nanotube bundles on a Ni/Mo/MgO catalyst

A new catalyst (Ni/Mo/MgO) is reported, with which one can synthesize multi-walled carbon nanotube (MWNT) bundles with a yield of more than 45 times the amount of the pristine catalyst, using a methane-hydrogen mixture as precursor. Powder X-ray diffraction, Raman spectroscopy and thermal gravimetric analysis show that the purity of the as-prepared MWNTs is over 97%. The diameter of the carbon nanotubes is 9-20 nm, measured by high-resolution electron microscopy on 421 individual MWNTs. The high purity of the as-prepared MWNTs allows us to omit the usual complex purification process required for carbon nanotubes synthesized by chemical vapor deposition. Because of its durable high activity, the Ni/Mo/MgO catalyst in its pristine state is ideal for mass production of high-quality MWNTs. The synergism of nickel and molybdenum is considered the main reason for the high yield of carbon nanotubes.     

Liquid crystal behavior of single wall carbon nanotubes

Single wall carbon nanotubes are dispersed in water with the water-soluble polymer polyvinylpyrrolidone and the surfactant sodium dodecylbenzene sulfonate, and then deposited by evaporative deposition onto degeneratively-doped silicon wafer substrates. These deposits were examined by scanning electron microscopy, which revealed highly-ordered arrays of large single wall carbon nanotube bundles. Various solution concentrations were prepared and deposition conditions were varied to determine their affect on the single wall carbon nanotube arrays. These observations were related to existing lyotropic liquid crystal theory and theories explaining the behavior of carbon nanotubes in solution, which allowed for further development and interpretation of the phase diagram which describes the behavior of single wall carbon nanotubes in lyotropic liquid crystal systems, and how competing liquid crystal systems in the same solution directly affect the ordering of the single wall carbon nanotube arrays.     

The role of carbon nanotube structure in purification and hydrogen adsorption

Structural properties of carbon nanotubes were studied by using samples from various manufacturers synthesized by different processes. A two-stage purification method was applied to all samples. Relationships between synthesis techniques and carbon nanotube structure are discussed. The role of carbon nanotube structural features such as degree of crystallinity, tube diameter, tube wall structure, and bundling behavior in purification and hydrogen adsorption were investigated by a combination of transmission electron microscopy and magnetic resonance techniques. It is suggested that MWNTs with low crystallinity and SWNTs with large diameters and open tube ends yielded the highest hydrogen uptake capacities. Both MWNTs and SWNTs show low hydrogen storage capacities (less than 1 wt%) at hydrogen pressures up to 1480 kPa.     

Reduction of cathode carbon deposit by buffer gas outflow

The problem of the cathode carbon deposit arising from the carbon vapor developed by conventional arc discharge plasma during fullerene synthesis is addressed. This carbon deposit significantly reduces fullerene productivity, wastes valuable energy and graphite material, and presents a major obstacle in achieving desirable productivity and scalability. A study of the deposit microstructure and its mechanical and physical properties revealed that the deposit is of a three-dimensional fractal structure having a rigid and microporous structure.Analysis reveals that the graphite evaporation rate determines carbon deposit formation, soot content, fullerene yield, and vapor quality, as well as defines the deposit formation. The buffer gas outflow approach utilizes the injection of a buffer gas into the hot plasma zone, provides quick evacuation of the produced carbon vapor away from the plasma zone, allows usage of large currents and bigger electrodes without a significant reduction of fullerene yield, and reduces the carbon deposit by more then 99%. Based on this work, the elimination of the cathode deposit is a major step in the development of efficient, productive and scalable fullerene and nanotube technology.     

Formation of polymer/carbon nanotubes nano-hybrid shish-kebab via non-isothermal crystallization

Multi-walled carbon nanotubes (MWNTs) periodically decorated with polyethylene (PE) lamellar crystals had been prepared using the non-isothermal crystallization method. The morphology and structure of polyethylene attached to MWNTs were investigated by means of transmission electron microscopy (TEM). A nano-hybrid shish-kebab (NHSK) structure was observed wherein the average diameter of PE lamellar crystals varies from 30 to 150 nm with average periodicity of 35-80 nm. The TEM images of samples obtained at 125 °C showed that MWNTs were first wrapped by a homogeneous coating of PE with few subglobules, then PE chains epitaxially grew from the subglobule and formed lamellar crystals perpendicular to the carbon nanotube axis. It is suggested that the homogeneous coating plays a key role in the formation of NHSK structures. And the formation process was discussed based on the intermediate state images of samples obtained at 95 °C. While NHSK structures cannot be formed by using polypropylene (PP). This may attribute to the zigzagged conformation of PP chains on the surface of MWNTs, which hinders the formation of homogeneous coating of PP on it.     

Highly sensitive integrated pressure sensor with horizontally oriented carbon nanotube network

This paper presents a functionalized, horizontally oriented carbon nanotube network as a sensing element to enhance the sensitivity of a pressure sensor. The synthesis of horizontally oriented nanotubes from the AuFe catalyst and their deposition onto a mechanically flexible substrate via transfer printing are studied. Nanotube formation on thermally oxidized Si (100) substrates via plasma-enhanced chemical vapor deposition controls the nanotube coverage and orientation on the flexible substrate. These nanotubes can be simply transferred to the flexible substrate without changing their physical structure. When tested under a pressure range of 0 to 50 kPa, the performance of the fabricated pressure sensor reaches as high as approximately 1.68%/kPa, which indicates high sensitivity to a small change of pressure. Such sensitivity may be induced by the slight contact in isolated nanotubes. This nanotube formation, in turn, enhances the modification of the contact and tunneling distance of the nanotubes upon the deformation of the network. Therefore, the horizontally oriented carbon nanotube network has great potential as a sensing element for future transparent sensors.     

SiC-assisted growth of tubular graphenic cones with carbon nanotube tips

SiC-assisted growth of tubular graphenic cones (TGCs) with carbon nanotube tip is achieved with high yield in the microwave plasma chemical vapor deposition process. No pre-existing metal or semiconductor catalyst particles are required on the substrate prior to the deposition process. Instead, the in situ grown SiC crystallites serve as the catalyst for the growth of TGCs. Thanks to the easy formation of SiC on various substrates, the process is compatible with a wide range of substrates, i.e. Si, diamond, 2H–SiC, GaN, and SiO₂, but not only limited to them. In addition to developing the approach, the mechanism for the formation of TGCs is also proposed. When Si is used as the substrate, the 3C–SiC crystallites grow epitaxially on the surface, which further initiate the epitaxial growth of graphene with its basal plane parallel to the {111} planes of 3C–SiC. The further expansion of graphene is constrained by the 3C–SiC crystallites, leading to the formation of curved graphene layers, i.e. onion-like or bowl-shaped graphene-based carbon. These curved graphene layers are believed to be the possible nucleation sites for the growth of TGCs. Inclusion of boron in the gas phase promotes the growth rates and the final yields of TGCs.     

Critical role of gas phase diffusion and high efficiency in vertically aligned carbon nanotube forest growth

Here the growth kinetics of vertically aligned carbon nanotube forests depend on the size of the patterned catalyst films from which they grow. Forests are grown using chemical vapor deposition on thin film catalyst islands patterned at the 100 μm scale on silicon wafers. The smaller the pattern, the faster the forest grows and the earlier it stops growing. Furthermore, the shape and structure of the forests, in particular the concavity of their top surface, also depend on the size of the catalyst islands. This result can be understood as a consequence of the high efficiency by which the acetylene source gas is converted into carbon nanotubes (here ∼30%) and a varying local amount of acetylene source gas available for growth. A diffusion model can explain the observed shape and structure of the forests and their growth kinetics by using experimentally measured parameters. This model also gives insight into the density and growth rate of carbon nanotube forests and suggests a mechanism that coordinates growth rates across the sample and, under certain conditions, can limit the fraction of catalyst nanoparticles that produce nanotubes.     

Nucleation and growth of carbon black particles during thermal decomposition of benzene

The authors determined the mechanism of carbon black formation during the thermal decomposition of benzene diluted with a stream of nitrogen. The kinetics study of nucleation and growth of particles led them to the following conclusions: The initial hydrocarbon is transformed by a gas phase reaction into macromolecules. The partial pressure of the macromolecules increases with reaction time until a supersaturation is high enough to induce condensation of macromolecules into droplets. The formation of liquid nuclei eliminates the supersaturation, and the formation of additional liquid nuclei becomes impossible. The macromolecules which continue to be formed maintain the nuclei growth. The liquid droplets are pyrolysed into a solid material. A statistical study of the distribution curves of the particle diameters indicates that the growth rate of each particle is proportional to its diameter.     

Preparation and characterization of multiwalled carbon nanotubes/polyacrylonitrile nanofibers

Multiwalled carbon nanotube/Polyacrylonitrile (MWNT/PAN) composite nanofibers were prepared by electrospinning technique, whereby functionalized MWNTs (F-MWNTs) and pristine MWNTs (P-MWNTs) were used as reinforcing materials. The F-MWNTs were functionalized by Friedel-Crafts acylation, which introduced aromatic amine (COC₆H₄-NH₂) groups onto the sidewall. The diameter range of the PAN nanofibers was 400-100 ± 50 nm. The beads formation was also observed when the amounts of MWNTs were increased in the PAN solution. The bead formation in F-MWNT/PAN composite nanofibers was less as compared to P-MWNT/PAN. The MWNTs were embedded within nanofibers and were well oriented along the nanofiber axis, as confirmed by transmission electron microscopy. The mechanical and thermal properties of the PAN nanofibers were improved by the incorporation of MWNTs.     

Preparation and characterization of alkylated carbon nanotube/polyimide nanocomposites

BACKGROUND: The development of carbon nanotube‐reinforced composites has been impeded by the difficult dispersion of the nanotubes in polymers and the weak interaction between the nanofiller and matrices. Efficient dispersion of carbon nanotubes is essential for the formation of a functional nanotube network in a composite matrix. RESULTS: Multiwalled carbon nanotubes (MWNTs) were incorporated into a polyimide matrix to produce MWNT/polyimide nanocomposites. To disperse well the MWNTs in the matrix and thus improve the interfacial adhesion between the nanotubes and the polymer, ‘branches’ were grafted onto the surface of the nanotubes by reacting octadecyl isocyanate with carboxylated MWNTs. The functionalized MWNTs were suspended in a precursor solution, and the dispersion was cast, followed by drying and imidization to obtain MWNT/polyimide nanocomposites. CONCLUSION: The functionalized MWNTs appear as a homogeneous dispersion in the polymer matrix. The thermal stability and the mechanical properties are greatly improved, which is attributed to the strong interactions between the functionalized MWNTs and the polyimide matrix. Copyright © 2009 Society of Chemical Industry