THE INFLUENCE OF SYNTHESIS PARAMETERS ON THE PRODUCTION OF MULTIWALLED CARBON NANOTUBES BY THE FERROCENE CATALYZED PYROLYSIS OF TOLUENE

A high yield (∼32 wt.%) of multiwalled carbon nanotubes was obtained in an iron catalyzed reaction. This was achieved in the temperature range 800-1000°C under an atmosphere of H₂/Ar by an improved solution injection method in a horizontal reactor using toluene as carbon source and ferrocene as catalyst precursor. The pyrolysis temperature, ferrocene concentration, solution feeding rate and carrier gas flow rate all influenced the yield of carbon nanotubes and the thickness of the aligned carbon nanotube films. The carbon nanotubes was prepared in high purity using optimized pyrolysis conditions.     

Chirality-enriched semiconducting carbon nanotubes synthesized on high surface area MgO-supported catalyst

Single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution were synthesized by radio frequency-Catalytic Chemical Vapor Deposition (RF-CCVD) through the pyrolysis of CH4. Fe-Co bimetallic catalytic nanoclusters were supported on high-surface area MgO nanopowders and used in the nanotube synthesis process. Nanolog absorption fluorescence analysis was used to characterize the chiralities of the as-produced SWCNTs over this nanostructural catalyst. In the final SWCNT sample, the (7,5) semiconducting carbon nanotube species were found to be dominant, with a low chirality variation.     

THE INFLUENCE OF SYNTHESIS PARAMETERS ON THE PRODUCTION OF MULTIWALLED CARBON NANOTUBES BY THE FERROCENE CATALYZED PYROLYSIS OF TOLUENE

ABSTRACT

A high yield (～32?wt.%) of multiwalled carbon nanotubes was obtained in an iron catalyzed reaction. This was achieved in the temperature range 800–1000°C under an atmosphere of H₂/Ar by an improved solution injection method in a horizontal reactor using toluene as carbon source and ferrocene as catalyst precursor. The pyrolysis temperature, ferrocene concentration, solution feeding rate and carrier gas flow rate all influenced the yield of carbon nanotubes and the thickness of the aligned carbon nanotube films. The carbon nanotubes was prepared in high purity using optimized pyrolysis conditions.     

Relation between Growth Parameters and Morphology of Vertically Aligned Fe-filled Carbon Nanotubes

Fe-filled multi-wall carbon nanotubes (MWNTs) were produced by pyrolysis of ferrocene in a dual furnace system. They grew vertically aligned on oxidized silicon substrates placed inside the reaction zone of a chemical vapor deposition reactor. A variation of the growth parameters has been performed in order to evaluate the possibility to control the Fe-filled nanotube growth process and thereby the nanotube- and the filling length, diameter and yield, and also the nanotube alignment. Electron microscopy studies show nanotubes with quite different morphologies. The relation between the aligned Fe-filled MWNTs growth and the most important growth parameters is discussed.     

Ni-Fe Competition in the Catalysis of Carbon Nanotube Growth

The paper presents the results of competitive catalysis investigation of the carbon nanotube growth in situ of the partial oxidation process of methane. The competition between Ni and Fe results in suppression of Ni catalytic activity and the growth of Fe-capped carbon nanotubes. The discrimination is so strong that iron is segregated from Ni-Fe based stainless steel alloy leaving characteristic Ni-enriched corrosion caverns. The process strongly depends on temperature. Depending on particular catalyst bed composition, the nanotubes of various morphology may occur. In particular, the use of perovskite-type catalyst leads to formation of “olive-branch”-like peculiar carbon nanostructures.     

Scalable preparation of carbon nanotubes by thermal decomposition of phenol with high carbon utilizing rate

Practical application of carbon nanotube would have to be determined by a matter of its economical and large-scale preparation. In this study, phenol was used as carbon resource to fabricate carbon nanotubes with large scale in home-made chemical vapor deposition setup. The as-prepared carbon nanotubes are of multi-walls with bamboo compartments, and of having a limited distribution in diameter of 10-15 nm and length of several decades μm. In particular, in this preparation the carbon transformation rate from the carbon content in used phenol to carbon nanotubes is 75%, much higher than the previous reports. Hence this work would be of significance for the industrial preparation of carbon nanotubes even their practical applications.     

用空气氧化法高效纯化炭纳米管

使用碱对炭纳米管进行了预处理,由于碱是一种分散剂,可以加强炭纳米颗粒和氧化剂的反应,使炭纳米管和其它形式的炭相分离。本法和Ａｊａｙａｎ所采用的空气氧化法相比,具有纯化效率高、炭纳米管烧损少等优点。     

Studies on mechanism of single-walled carbon nanotube formation

We presented detailed studies of the formation of single-walled carbon nanotubes by an aerosol method based on the introduction of pre-formed catalyst particles into conditions leading to carbon nanotube synthesis. Carbon monoxide and iron nanoparticles were used as a carbon source and a catalyst, respectively. The vital role of etching agents such as CO2 and H2O in CNT formation was demonstrated on the basis of on-line Fourier-transform infrared spectroscopy measurements. Hydrogen was shown to participate in the reaction of carbon release and to prevent the oxidation of the catalyst particles and the hot wire. The addition of H2 and small amounts of CO2 and H2O led to an increase in the carbon nanotube lengths. The catalyst particle evaporation process inside the reactor was found to become significant at temperatures higher than 1100 degrees C. The carbon nanotube growth was found to occur at a temperature of around 900 degrees C in the heating section of the reactor by in situ sampling and the growth rate was calculated to exceed 1.1 microm/s. A detailed analysis of possible processes during carbon nanotube formation revealed heptagon transformation as a limiting stage. A mechanism for carbon nanotube formation was proposed.     

Modeling of the HiPco process for carbon nanotube production. II. Reactor-scale analysis

The high-pressure carbon monoxide (HiPco) process, developed at Rice University, has been reported to produce single-walled carbon nanotubes from gas-phase reactions of iron carbonyl in carbon monoxide at high pressures (10-100 atm). Computational modeling is used here to develop an understanding of the HiPco process. A detailed kinetic model of the HiPco process that includes of the precursor, decomposition metal cluster formation and growth, and carbon nanotube growth was developed in the previous article (Part I). Decomposition of precursor molecules is necessary to initiate metal cluster formation. The metal clusters serve as catalysts for carbon nanotube growth. The diameter of metal clusters and number of atoms in these clusters are some of the essential information for predicting carbon nanotube formation and growth, which is then modeled by the Boudouard reaction with metal catalysts. Based on the detailed model simulations, a reduced kinetic model was also developed in Part I for use in reactor-scale flowfield calculations. Here this reduced kinetic model is integrated with a two-dimensional axisymmetric reactor flow model to predict reactor performance. Carbon nanotube growth is examined with respect to several process variables (peripheral jet temperature, reactor pressure, and Fe(CO)5 concentration) with the use of the axisymmetric model, and the computed results are compared with existing experimental data. The model yields most of the qualitative trends observed in the experiments and helps to understanding the fundamental processes in HiPco carbon nanotube production.     

Continuous synthesis of carbon nanotubes using a metal-free catalyst by CVD

This study demonstrates the first example of the use of a metal-free catalyst for the continuous synthesis of carbon nanotubes (CNTs) by chemical vapor deposition (CVD). In this paper silica nanoparticles produced from the thermal decomposition of PSS-(2-(trans-3,4-Cyclohexanediol)ethyl)-Heptaisobutyl substituted (POSS) were used as catalyst and ethanol was served as both the solvent and the carbon source for nanotube growth. The POSS/ethanol solution was nebulized by an ultrasonic beam. The tiny mists were continuously introduced into the CVD reactor for the growth of CNTs. The morphology and structure of the CNTs have been investigated by scanning electron microscopy, high-resolution transmission electron microscopy, and Raman spectroscopy. The obtained CNTs have a multi-walled structure with diameters mainly in the size range from 13 to 16 nm. Detailed investigations on the growth conditions indicate that the growth temperature and POSS concentration are important for achieving high-quality nanotubes, and that the existing of small amount of water in ethanol is effective to remove amorphous carbon species during the formation of CNTs. The mass production of CNTs without any metal contaminant will provide a chance for investing and understanding the intrinsic properties of CNTs and applications particularly in nanoelectronics and biomedicines.     

Iron catalyst chemistry in modeling a high-pressure carbon monoxide nanotube reactor

The high-pressure carbon monoxide (HiPco) technique for producing single-wall carbon nanotubes (SWNTs) is analyzed with the use of a chemical reaction model coupled with flow properties calculated along streamlines, calculated by the FLUENT code for pure carbon monoxide. Cold iron pentacarbonyl, diluted in CO at about 30 atmospheres, is injected into a conical mixing zone, where hot CO is also introduced via three jets at 30 degrees with respect to the axis. Hot CO decomposes the Fe(CO)5 to release atomic Fe. Then iron nucleates and forms clusters that catalyze the formation of SWNTs by a disproportionation reaction (Boudouard) of CO on Fe-containing clusters. Alternative nucleation rates are estimated from the theory of hard sphere collision dynamics with an activation energy barrier. The rate coefficient for carbon nanotube growth is estimated from activation energies in the literature. The calculated growth was found be about an order of magnitude greater than measured, regardless of the nucleation rate. A study of cluster formation in an incubation zone prior to injection into the reactor shows that direct dimer formation from Fe atoms is not as important as formation via an exchange reaction of Fe with CO in FeCO.     

碳纳米管负载Fe2O3催化剂的制备及其乙苯脱氢催化活性

采用FeSO4-H2O2体系对碳纳米管氧化修饰的同时,氢氧化铁被吸附在碳纳米管管壁上,然后分别通过氢气、氮气、空气在723K下处理2h,制备了碳纳米管负载的γ-Fe2O3催化剂、γ-Fe2O3和α-Fe2O3复合催化剂和非晶态Fe2O3催化剂。采用XRD、TEM和TG-DSC表征了催化剂结构,采用连续流动乙苯气相脱氢生成苯乙烯反应对催化剂性能进行评价,结果表明：热处理条件对催化剂乙苯脱氢的催化性能影响明显,碳纳米管负载的晶态Fe2O3纳米催化剂对乙苯脱氢具有高的活性与选择性。     

Molecular dynamic simulations on tensile mechanical properties of single-walled carbon nanotubes with and without hydrogen storage

Using a bond order potential, molecular dynamics (MD) simulations have been performed to study the mechanical properties of single-walled carbon nanotubes (SWNTs) under tensile loading with and without hydrogen storage. (10,10) armchair and (17,0) zigzag carbon nanotubes have been studied. Up to the necking point of the armchair carbon nanotube, two deformation stages were identified. In the first stage, the elongation of the nanotube was primarily due to the altering of angles between two neighbor carbon bonds. Young's Modulus observed in this stage was comparable with experiments. In the second stage, the lengths of carbon bonds are extended up to the point of fracture. The tensile strength in this stage was higher than that observed in the first stage. Similar results were also found for the zigzag carbon nanotube with a lower tensile strength. Hydrogen molecules stored in the nanotubes reduced the maximum tensile strength of the carbon nanotubes, especially for the armchair type. The effect may be attributed to the competitive formation between the hydrogen–carbon and the carbon–carbon bonds.     

Synthesis of fibrous carbon nanomaterials from ethanol vapor on a nickel catalyst

The processes underlying the synthesis of fibrous carbon nanomaterials via nickel-catalyzed pyrolysis of ethanol vapor are investigated. The IR spectra of the gas released from the reactor during synthesis indicate that, in the initial stages of the process, ethanol molecules decompose into simpler species: CH₄, CO, and H₂. In the temperature range 400–600°C, carbon monoxide disproportionation plays a key role in carbon deposition. Varying the process conditions, one can obtain carbon nanofibers of various thicknesses or carbon nanotubes.     

纳米碳管电化学储氢的研究进展

纳米碳管的储氢是近年来纳米碳管领域研究的一个热点。纳米碳管储氢研究有两种方法，一种是气相法，另一种是电化学法。本文对纳米碳管电化学储氢的基本原理、纳米碳管电化学储氢的理论计算以及氢与纳米碳管的相互作用机制，特别是目前单壁和多壁纳米碳管电化学储氢的实验研究进展进行了综述，展望了利用其电化学储氢特性作为高性能电池的可能性。     

Elastic Vibration Behaviors Oof Carbon Nanotubes Based on Micropolar Mechanics

The concept of the micropolar theory is employed to investigate vibration behaviors of carbon nanotubes. The constitutive relation has been deduced from the two-dimensional analysis of the microstructure of the carbon nanotube. Van der Waals interactions are simulated by a weak spring model. Hamilton's principle is employed to obtain dynamics equations of the multi-walled carbon nanotube. Numerical examples for both single-walled and double-walled carbon nanotubes are presented and the significant difference in vibration behaviors between them has been distinguished. Numerical results show that fundamental frequencies for the cantilever single-walled carbon nanotube decreases with increase of the aspect ratio of them, and the fundamental frequencies of the double-walled carbon nanotube are lower than those of the single-walled carbon nanotube with the same inner diameter and length. The first four natural frequencies for the double-walled carbon are coaxial.     

A study of carbon-nanotube-based nanoelectromechanical resonators tuned by shear strain

This paper has investigated, using a classical molecular dynamics simulation method based on the Tersoff-Brenner potential, the resonance-frequency changes of single-walled carbon-nanotube resonators originating from the purely mechanical response of single-walled carbon nanotubes. The tension decreased with increasing rotation angle, so the resonance frequencies could be changed by controlling the rotation angles of the single-walled carbon nanotubes. The resonance frequencies decreased with increasing angle, and when the rotation angle was greater than 60°, the changes were marked. For nanotubes of similar length, the bandwidth for the (3, 3) single-walled carbon nanotube was higher than for the (5, 0) single-walled carbon nanotube. Because properties arising from the shear-strain-induced tension response can affect the electromechanical behavior of carbon nanotubes, the shear-strain-induced tension response should be given serious consideration in the application of embedded carbon nanotubes in nanoelectromechanical systems.     

Coral-like amorphous carbon nanotubes synthesized by a modified arc discharge

A coral-like amorphous carbon nanotube was prepared by a modified arc discharging furnace in hydrogen atmosphere with a mixture of Mo-Co₂O₃-Mg powders as catalyst at 600°C. This carbon nanotube presented a microscopic coral-like by SEM observation and amorphous structure of nanotubes by HRTEM observation. The XRD diffraction and Raman pattern presented noncrystal characteristics compared to the normal graphite structure. We believed that these results may be affected by the “synergistic” effect of catalyst, atmosphere, and temperature in the synthesis process. The possible explanations to the formation mechanism of this novel carbon nanotube have also been proposed.     

Modeling of the HiPco process for carbon nanotube production. I. Chemical kinetics

A chemical kinetic model is developed to help understand and optimize the production of single-walled carbon nanotubes via the high-pressure carbon monoxide (HiPco) process, which employs iron pentacarbonyl as the catalyst precursor and carbon monoxide as the carbon feedstock. The model separates the HiPco process into three steps, precursor decomposition, catalyst growth and evaporation, and carbon nanotube production resulting from the catalyst-enhanced disproportionation of carbon monoxide, known as the Boudouard reaction: 2 CO(g)-->C(s) + CO2(g). The resulting detailed model contains 971 species and 1948 chemical reactions. A second model with a reduced reaction set containing 14 species and 22 chemical reactions is developed on the basis of the detailed model and reproduces the chemistry of the major species. Results showing the parametric dependence of temperature, total pressure, and initial precursor partial pressures are presented, with comparison between the two models. The reduced model is more amenable to coupled reacting flow-field simulations, presented in the following article.     

碳纳米管储氢

近年来,碳纳米管由于其独特的力学、电学等性能以及在众多方面的潜在应用,越来越受到世界各国科学家的关注.最近,碳纳米管由于其大表面积和中空的结构,被应用于氢气储存.本文介绍了该领域最新的一些研究结果