Formation Of CNTs Research Articles

In-situ formation of carbon nanotubes in pyrolytic carbon–silicon nitride composite ceramics

Carbon nanotube reinforced pyrolytic carbon/silicon nitride (CNT-PyC/Si3N4) composite ceramics were fabricated through in-situ growth of CNTs in PyC–Si3N4 ceramics by precursor infiltration and pyrolysis. CNTs were in-situ formed by the addition of nano-sized Ni into phenolic resin and Si3N4 powder blend, and the content of CNTs increased with the increase of Ni content. Due to the formation of CNTs in the inter-particle pores of CNTs/PyC–Si3N4 ceramic, the PyC wrapped Si3N4 particles were bridged by CNTs, which led to the increase of electrical conductivity. Different from the previously reported PyC–Si3N4 ceramics fabricated by chemical vapor infiltration (CVI), the PyC–Si3N4 ceramics exhibited a shielding mechanism dominated by absorption. Compared to PyC–Si3N4 ceramics prepared with same procedure, the shielding effectiveness of CNTs-PyC/Si3N4 increased significantly with the formation of CNTs. With the increase of CNT content, the shielding effectiveness of CNTs/PyC–Si3N4 ceramics increased from 25.5 to 43.6 dB, in which the absorption shielding effectiveness increased from 15 to 30 dB over the frequency range of 8.2–12.4 GHz.

Effect of Magnetic Field on the Growth of Aligned Carbon Nanotubes Using a Metal Free Arc Discharge Method and their Purification

Multiwalled Carbon Nanotubes (MWCNTs) have been synthesized using a low cost arc discharge method without using metal catalyst and vacuum devices. Effect of magnetic field on the synthesis of MWCNTs and their purity has been scrutinized. A magnetic field of 310 gauss has been found to give better purity of carbon nanotubes as confirmed by Raman spectroscopy. However, the removal of amorphous carbon from the surface of so prepared multiwalled carbon nanotubes has been achieved by different oxidizing conditions. It has been observed that the maximum removal of amorphous carbon found by using the strong oxidizing agent HNO3/H2O2. This strong oxidizing agent HNO3/H2O2removes most of the carbonaceous impurities leading to thermal stability of carbon nanotubes suggested by thermo gravimetric analysis. X-ray diffraction show the formation of carbon nanotubes having a peak indexed at (002) as the fingerprint for multiwalled carbon nanotubes. Fourier Transform Infrared (FTIR) spectra confirmed the formation of the multiwalled carbon nanotubes showing a characteristic stretching band at 1615 cm-1corresponding to the C=C bonds of tubular carbon. Raman spectroscopy revealed invaluable insights into the purification of nanotubes. G-band (1577 cm-1) corresponds to the confirmation of MWCNTs. Defect induced D-band (1355 cm-1) has been minimized after purifying CNTs with HNO3/H2O2for 24 hrs. Transmission Electron microscopic (TEM) studies indicate the formation of CNTs with controlled alignment having diameter in the range 2-8 nm.

Dependence of the Coating Carbon Layer Configuration of Cordierite Monolith on Coating Reaction Conditions in Methane Decomposition

Dependence of the coating carbon layer configuration of cordierite monolith on reaction conditions in methane decomposition was investigated using N2 physisorption, SEM and TEM. The monolith surface area was increased after coating CNFs/CNTs. The microstructure of the coating layers in the coated monolith is dependent on the coating reaction conditions and the entanglement among CNFs/CNTs. High coating temperature and the presence of hydrogen in reaction gas for coating are advantageous to formation of CNTs. By comparison, coating layers of CNFs have better stability than that of CNTs in ultrasonic treatment.

Carbon nanotubes coated with a nitrogen-doped carbon layer and its enhanced electrochemical capacitance

Nitrogen-doped carbon layer coated carbon nanotubes (NC-CNTs) were prepared by using a simple two-step procedure of in situ polymerization of pyrroles onto CNTs and subsequent carbonization, without using template removal and activation treatment. Structure and composition analyses show that in situ polymerization results in the formation of CNTs coated with a polypyrrole layer (CNTs@PPy), by following carbonization process which is converted into the NC-CNTs with nitrogen content of 8.6 at%. N2 adsorption–desorption isotherms reveal that the NC-CNTs have not only much larger specific surface area (SSA) than that of the CNTs but also a hierarchical pore structure consisting of micropores, mesopores and macropores. Owing to the combination effect of abundant nitrogen containing groups and improved pore structure, the NC-CNTs electrode without conducting additives exhibits much higher capacitance (205 F g−1) than the CNTs electrode (25 F g−1) and excellent cycling stability in 6 M KOH electrolyte, although the NC-CNTs has only moderate SSA of 197 m2 g−1. The easy preparation and smart electrochemical performance of NC-CNTs makes it a promising electrode material for supercapacitors.

Dielectrophoretic Alignment and Pearl Chain Formation of Single-Walled Carbon Nanotubes in Deuterium Oxide Solution

Dielectrophoretic filtering and alignment of single-walled carbon nanotubes (SWCNTs) were tested using deuterium oxide as a solvent. A solution of deuterium oxide-SWCNTs was dropped on top of a silicon chip and an ac electric field was applied between pre-defined electrodes. Deuterium oxide was found to be a better solvent than hydrogen oxide for the dielectrophoresis process with higher efficiency of filtering. This was demonstrated by comparing Raman spectra measured on the initial solution with those measured on the filtered solution. We found that the aligned nanotubes along the electric field were not deposited on the substrate but suspended in solution, forming chain-like structures along the field lines. This so-called pearl chain formation of CNTs was verified by electrical measurements through the aligned tubes. The solution was frozen in liquid nitrogen prior to the electrical measurements to maintain the chain formation. The current-voltage characteristics for the sample demonstrate the existence of conduction channels in the solution, which are associated with the SWCNT chain structures.

The Catalytic Properties of the Anodic Aluminum Oxide Template in Synthesis of Carbon Nanotubes

Both the catalytic property of the pore walls and the catalytic property of the top surface of AAO templates were closely investigated by pyrolizing acetylene at 650°C for 5min and 1h respectively. The conclusion that the top surface of the AAO template with smaller diameters has a stronger catalytic property was drawn. And the effects of the reaction time, the reaction gas flow rate and the location where the AAO template was placed on the properties of AAO templates were discussed. Furthermore, this paper proposed that in the competitive between catalyzes of AAO templates and the transition metal catalysts, the transition metal catalysts will play a dominant role in the formation of CNTs by adjusting the quantity of the transition metal catalysts embedded in AAO templates

Synthesis of Graphene-CNT Hybrid Nanostructures

ABSTRACTUsing chemical vapor deposition technique, a novel 3D carbon nano-architecture called a pillared graphene nanostructure (PGN) is in situ synthesized. The fabricated novel carbon nanostructure consists of CNT pillars of variable length grown vertically from large-area graphene planes. The formation of CNTs and graphene occurs simultaneously in one CVD growth treatment. The detailed characterization of synthesized pillared graphene shows the cohesive structure and seamless contact between graphene and CNTs in the hybrid structure. The synthesized graphene-CNT hybrid has a tunable architecture and attractive material properties, as it is solely built from sp2 hybridized carbon atoms in form of graphene and CNT. Our methodology provides a pathway for fabricating novel 3D nanostructures which are envisioned for applications in hydrogen storage, nanoelectronics, and supercapacitors.

Synthesis of Carbon Nanomaterials in Flames

<p>Usage of combustion processes for production of target products is less common than the application of catalytic processes. However, there are known examples of production of carbon black, HCl, TiO<sub>2</sub> etc. Some research work was done at the Institute of Combustion Problems (Almaty, Kazakhstan) consider the synthesis in flame of carbon nanomaterials: fullerenes, nanotubes and soot nanobeads with superhydrophobic surface. An alternative of fullerenes and nanotubes synthesis in arc discharge of graphite is the method using stationary hydrocarbon flames. Flame is a self-sustaining system in which the hydrocarbons can be precursors of carbon nanomaterials, and the heat released during combustion, is a parameters of the process control. It is known that PAH are nucleation centers of forming soot i.e. PAH can be converted into either soot or fullerenes. The formation of CNTs occurs in diffusion flames from the fuel side and is initiated by transition metals particles. The paper presents data on the formation of fullerenes and carbon nanotubes as well as soot with the superhydrophobic surface, obtained on nickel and silicon supports in benzene-oxygen and propane-oxygen diffusion flames. New results regarding the synthesis of superhydrophobic surface with a contact angle 135-175° have great practical interest as anti-corrosion additives to various materials.</p>

Dynamics of catalyst particle formation and multi-walled carbon nanotube growth in aerosol-assisted catalytic chemical vapor deposition

In aerosol-assisted catalytic chemical vapor deposition (CCVD), the catalyst and carbon precursors are introduced simultaneously in the reactor. Catalyst particles are formed in situ and aligned multi-walled CNTs grow at a high rate. To scale-up the process, it is crucial to understand the chemical transformation of the precursors along the thermal gradient of the reactor, and to correlate nanotube growth with catalyst nanoparticle formation. The products synthesized along a cylindrical CVD reactor from an aerosol composed of ferrocene and toluene, as catalyst and carbon precursor, respectively, were studied. The product surface density and iron content are determined as a function of the location and the iron vapor pressure in the reactor. Samples are analyzed by electron microscopy, X-ray diffraction and Raman spectroscopy. We show the strong influence of the thermal gradient on location and rate of formation of both iron particles and CNTs, and demonstrate that catalyst particles are formed by gas phase homogeneous nucleation with a size which correlates with iron vapor pressure. They are gradually deposited on the reactor walls where nanotubes grow with an efficiency which is varying linearly with catalyst particle density. CNT crystallinity appears very high for a large range of temperature and iron content.

Thin-walled carbon nanotubes grown using a zirconium catalyst

Thin-walled carbon nanotubes (TWCNTs) were synthesized using a zirconium (Zr) plate by cold-wall chemical vapor deposition of a mixture gas consisting of C2H2 and H2 under a pressure of 1.20torr. The Zr plate was used for the formation of Zr nanocatalysts during the thermal process and used as substrate to eliminate the effects of interactions between catalyst and substrate during CNT synthesis. The TWCNTs were 4–9nm in diameter and a few micrometers long. Nanometer-sized defects on the Zr bulk material may have a low melting temperature, allowing them to form droplet-like catalysts and achieve high carbon solubility for the formation of CNTs.

Influence of Carbon Source and Fe-Catalyst Support on the Growth of Multi-Walled Carbon Nanotubes

Catalytic activity of iron based catalysts in the production of multi-walled carbon nanotubes (MWCNTs) has been investigated. The effect of the carbon source (ethane or isobutane), catalyst support (Al2O3 or SiO2), iron loading, catalyst reduction temperature and reaction temperature on yield and quality of carbon products has been examined. The structural and morphological properties of catalyst and carbon products obtained have been analyzed by means of scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy (RS), thermogravimetric analysis (TGA) and X-ray powder diffraction (XRD). The iron-based catalysts supported on alumina seem to be efficient systems for the production of carbon nanotubes from chemical vapor deposition (CVD) of isobutane with very interesting yields. The opportune calibration of reaction parameters, such as iron loading and reaction temperature, can in fact drive the synthesis toward the formation of high quality CNTs.

Carbon nanotube formation catalyzed by Ni nanoparticles in carbonaceous solid

AbstractThermal decomposition of a Ni‐containing organometallic compound or Ni salt in the presence of an excess amount of a multi(ethynyl)aromatic compound results in the formation of CNTs during the carbonization process. The uniqueness of the process is the formation of MWNTs in high yield within the solid carbonaceous domain upon heat treatment to elevated temperature under ambient pressure. The formation of MWNTs in a shaped, solid configuration readily occurred when the precursor mixture was heated above 600 °C. 1,2,4,5‐Tetrakis(phenylethynyl)benzene was reacted with varying amounts of a Ni‐containing compound in an organic solvent to produce the Ni adduct of the ethynyl unit. Other mixtures were formulated by physical mixing of 1,2,4,5‐tetrakis(phenylethynyl)benzene and a Ni salt. Heat treatment of the various mixtures to temperatures up to 1300 °C under atmospheric pressure resulted in the decomposition of the Ni‐containing compound or salt and the formation of Ni atoms, clusters, and/or nanoparticles in the polymeric‐to‐carbon nanoparticle‐to‐carbon nanotube compositions. The Ni atoms, clusters, and/or nanoparticles are the key to the formation of the carbon nanotubes. The resulting Ni nanoparticle–CNT compositions show magnetic properties. X‐ray diffraction, Raman spectroscopy, transmission electron microscopy, and scanning electron microscopy studies show the presence of a large quantity of MWNTs in the carbonaceous solid residue. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study on the stability of carbon nanotubes

When the a graphite crystal is crushed, it is breaks into micro-scale laminar flakes. There are many dangling bonds on the edge of an isolated graphite micro-sheet which elevates its energy and makes it unstable. When a graphite micro-sheet curles into a nano-tube, the number of dangling bonds decrease, and the system energy is reduced accordingly. Meanwhile, the curled sheet obtains deformation energy, which can more than counteract the energy reduction due to the diminished dangling bonds and make the CNT even less favorable energetically than the sheet, and becomes unstable. On the basis of advancing the mechanical model of the formation of CNTs, a thorough theoretical analysis was made, which lead to the conclusion that the CNTs can exist stably at a minimum diameter of approximate 0.32nm.

Carbon nanotubes with 2D and 3D multiple junctions

Multiple junction CNTs and 3D junction CNTs were obtained using a thermal chemical vapor deposition method without using a template substrate. The silicon substrates used were simply scratched using various types of abrasives prior to the growth of carbon nanotube. The hydrocarbon gas used was methane, which was balanced by hydrogen. A total of four catalyst precursors, Fe powders, Ni powders, FeS powders, and ferrocene, were used. It was found that only the use of ferrocene gave the formation of multiple junction CNTs. These multiple junction CNTs are found and suspended exclusively in the scratched striations. The mechanism of (multiple) junction CNT formation is discussed by addressing the role of the substrate and the catalyst.