Chemical Vapor Deposition Of Carbon Nanotubes Research Articles

Mechanisms for Chemical Vapor Deposition Carbon Nanotube Growth by Surface Modification of 316L Stainless Steel.

Mechanisms for Chemical Vapor Deposition Carbon Nanotube Growth by Surface Modification of 316L Stainless Steel.

Study on Bimetallic Catalysts for Carbon Nanotube Growth on the Surface of Continuous Carbon Fibres

We investigated the influence of bimetallic catalysts with various compositions on the growth of carbon nanotubes (CNTs) on the surface of continuous carbon fibers (CFs). The samples were analyzed and characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectrometry. The catalyst plays an essential role in the catalytic chemical vapor deposition of CNT. The performance of these catalysts was estimated referenced to both catalyst and CNT morphologies, while the influences of each catalytic components were summarized according to the results. This work prepared and studied six different bimetallic catalysts composed by Cu, Fe, and Ni, and pure Fe catalyst was employed as a control group. Fe and Ni have been widely studied, while the reports of catalysts containing Cu are rare. We found that pure Fe catalyst leads to banded and clustered CNT distribution, and the addition of Cu can provide uniform and slender CNTs. The efficiency of catalyst can be significantly increased by adding Ni, producing CNTs as well as superfluous amorphous carbon. We determined that bimetallic catalyst can utilize multiple catalytic abilities of various metals by adjusting its composition, which can contribute to obtaining high-quality CNTs on CF surface.

Initial competing chemical pathways during floating catalyst chemical vapor deposition carbon nanotube growth

Ferrocene (Fc) is an effective precursor for the direct synthesis of high quality single-walled carbon nanotubes (SWCNTs) via floating catalyst chemical vapor deposition (FCCVD). However, the formation mechanism of the Fe floating catalyst and the SWNCT growth precursors, such as carbon chains, during Fc decomposition are not well understood. Here, we report first principles nonequilibrium quantum chemical molecular dynamics simulations that investigate the decomposition of Fc during FCCVD. We examine the influence of additional growth precursors including ethylene, methane, CO, and CO2 on the Fc decomposition mechanism and show that the dissociation of these species into C2Hx radicals and C atoms provides the key growth agents for the nucleation of carbon chains from Fc-derived species such as cyclopentadienyl rings. Without an additional growth precursor, Fc decomposes via the spontaneous cleavage of Fe–C and C–H bonds, thereby enabling Fe atoms to cluster and form the floating catalyst. On the basis of these simulations, we detail the two competing chemical pathways present during the initial stages of FCCVD: Fe catalyst nanoparticle growth and carbon chain growth. The latter is accelerated in the presence of the additional growth precursors, with the identity of the precursor determining the nature of the balance between these competing pathways.

CVD Conditions for MWCNTs Production and Their Effects on the Optical and Electrical Properties of PPy/MWCNTs, PANI/MWCNTs Nanocomposites by In Situ Electropolymerization

In this work, the optimal conditions of synthesizing and purifying carbon nanotubes (CNTs) from ferrocene were selected at the first stage, where decomposition time, argon fluxes, precursor amounts, decomposition temperature (at 1023 K and 1123 K), and purification process (HNO3 + H2SO4 or HCl + H2O2), were modulated through chemical vapor deposition (CVD) and compared to commercial CNTs. The processing temperature at 1123 K and the treatment with HCl + H2O2 were key parameters influencing the purity, crystallinity, stability, and optical/electrical properties of bamboo-like morphology CNTs. Selected multiwalled CNTs (MWCNTs), from 1 to 20 wt%, were electropolymerized through in-situ polarization with conductive polymers (CPs), poly(aniline) (PANI) and poly(pyrrole) (PPy), for obtaining composites. In terms of structural stability and electrical properties, MWCNTs obtained by CVD were found to be better than commercial ones for producing CPs composites. The CNTs addition in both polymeric matrixes was of 6.5 wt%. In both systems, crystallinity degree, related to the alignment of PC chains on MWCNTs surface, was improved. Electrical conductivity, in terms of the carrier density and mobility, was adequately enhanced with CVD CNTs, which were even better than the evaluated commercial CNTs. The findings of this study demonstrate that synergistic effects among the hydrogen bonds, stability, and conductivity are better in PANI/MWCNTs than in PPy/MWCNTs composites, which open a promissory route to prepare materials for different technological applications.

High-Performance and Lightweight Thermal Management Devices by 3D Printing and Assembly of Continuous Carbon Nanotube Sheets.

Free-standing carbon nanotube films or buckypaper can provide a significant platform to develop practical applications of nanocarbon materials. For this research, buckypaper with high thermal conductivity (20 W/m K) and large surface area (350 m2/g) was mass produced in-house to investigate for use in lightweight thermal management devices. Floating catalyst chemical vapor deposition carbon nanotube sheets were also studied in this work. We introduced two manufacturing techniques to use the sheets for heat dissipation: (1) printing conductive composite ink on the sheets to make lightweight thermal devices, such as heat sinks and (2) assembling the sheets directly into 3D structures that were mounted on the back of heat-generating devices. These manufacturing techniques resulted in extremely lightweight, high-performance heat dissipation devices compared with other heat sink materials.

Mechanical enhancement effect of the interlayer hybrid CNT film/carbon fiber/epoxy composite

Floating catalyst chemical vapor deposition carbon nanotube (CNT) film was intercalated into carbon fiber (CF) prepregs to fabricate hybrid composites. The effect of CNT film thickness was studied by using a 25 μm thick film and an ultrathin 2 μm film respectively. The results showed that the ultrathin CNT film interlayer had dramatically improved the compression strength of hybrid composite by 34% compared with CF/epoxy control composite owing to the altered failure modes. The damping ratio of ultrathin CNT film hybridized composite was substantially increased by two orders of magnitude, due to the energy dissipation of numerous nanoscale interconnections and interfaces. Moreover, the interlaminar properties and water resistance of the hybrid composites were all improved. Since a small amount of CNT film can significantly enhance the mechanical properties of CF/epoxy composite, this type of hybrid composite has potential application in multifunctional lightweight structures.

Chemical Vapor-Deposited Carbon Nanotubes as Electrode Material for Supercapacitor Applications

ABSTRACTWe report the capacitance of entangled carbon nanotubes (CNTs) synthesized on flexible carbon fabric via water-assisted chemical vapor deposition. The CNTs were grown at atmospheric pressure with iron (Fe) as the catalyst, ethylene (C2H4) and 5%/95% H2/Ar as precursor gasses, and aluminum oxide as a buffer/barrier layer. The effect of the catalyst thickness (5 and 10 nm) on the specific capacitance was studied. The capacitance behavior of CNTs was evaluated by cyclic voltammetry measurements via a three-electrode system. The highest specific capacitance, approximately 56 F/g, was obtained for electrodes with 5nm Fe thickness. A nearly rectangular shaped cyclic voltammogram was exhibited for the CNTs grown on the carbon fabric. A specific power density of 0.012 KW/Kg and specific energy density 0.15 Wh/Kg were calculated from the galvanic charge/discharge (CD) curves. In addition, electrochemical impedance spectroscopy (EIS) revealed a characteristic supercapacitive behavior with a low equivalent series resistance of 7 Ωcm2.

Modification of Hierarchical CNT Honeycomb Structures with Fe2O3 for Li-Ion Battery Anodes

Carbon nanotubes (CNTs) are attractive additives for battery electrodes because of their excellent electrical and thermal conductivity. However, these properties typically degrade by an order of magnitude in assemblies of CNTs compared to the properties measured in individual CNTs. This discrepancy can be bridged by aligning CNTs into yarns, sheets, or vertically aligned forests. In our work, we study aligned CNTs organized in micro-scale honeycomb structures which are desirable due to their mechanical robustness, conductivity, and high surface area. Because of their mechanical robustness, their surface chemistry can be modified to design battery electrodes. Here we focus on developing Li-ion battery anodes with a well-defined honeycomb morphology. These honeycombs are fabricated using chemical vapour deposition of CNTs from lithographically patterned catalyst particles. The grown CNTs are subsequently compacted using an elasto-capillary self-aggregation process and thus shaped into honeycomb patterns with varying cell diameters. These electrodes are tested and compared against unaligned CNT powders decorated with the same active nanoparticles, i.e. Iron(III)-Oxide (Fe2O3). The influence of the cell sizes and aspect ratios on the electrode performance is measured after the decoration with Fe2O3 as an active material. Figure 1

Molybdenum-rhenium alloy based high-Q superconducting microwave resonators

Superconducting microwave resonators (SMRs) with high quality factors have become an important technology in a wide range of applications. Molybdenum-Rhenium (MoRe) is a disordered superconducting alloy with a noble surface chemistry and a relatively high transition temperature. These properties make it attractive for SMR applications, but characterization of MoRe SMR has not yet been reported. Here, we present the fabrication and characterization of SMR fabricated with a MoRe 60–40 alloy. At low drive powers, we observe internal quality-factors as high as 700 000. Temperature and power dependence of the internal quality-factors suggest the presence of the two level systems from the dielectric substrate dominating the internal loss at low temperatures. We further test the compatibility of these resonators with high temperature processes, such as for carbon nanotube chemical vapor deposition growth, and their performance in the magnetic field, an important characterization for hybrid systems.

Effect of Catalyst Pretreatment on Chirality-Selective Growth of Single-Walled Carbon Nanotubes

We show that catalyst pretreatment conditions can have a profound effect on the chiral distribution in single-walled carbon nanotube chemical vapor deposition. Using a SiO2-supported cobalt model catalyst and pretreatment in NH3, we obtain a comparably narrowed chiral distribution with a downshifted tube diameter range, independent of the hydrocarbon source. Our findings demonstrate that the state of the catalyst at the point of carbon nanotube nucleation is of fundamental importance for chiral control, thus identifying the pretreatment atmosphere as a key parameter for control of diameter and chirality distributions.

Effects of Potassium Hydroxide Post-Treatments on the Field-Emission Properties of Thermal Chemical Vapor Deposited Carbon Nanotubes

In this study, a simple potassium hydroxide treatment was applied to functionalize the surface and to modify the structure of multi-walled carbon nanotubes grown on silicon substrates by thermal chemical vapor deposition. Scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and energy dispersive spectrometry were employed to investigate the mechanism causing the modified field-emission properties of carbon nanotubes. From our experimental data, the emitted currents of carbon nanotubes after potassium hydroxide treatment are enhanced by more than one order of magnitude compared with those of untreated carbon nanotubes. The emitted current density of carbon nanotubes increases from 0.44 mA/cm2 to 7.92 mA/cm2 after 30 minutes KOH treatment. This technique provides a simple, economical, and effective way to enhance the field-emission properties of carbon nanotubes.

Elongational and shear rheology of carbon nanotube suspensions

Rheological behavior of concentrated suspensions of chemical vapor deposition carbon nanotubes in uniaxial elongation and simple shear is studied experimentally and theoretically. Nanotubes are suspended in viscous host liquids—castor oil or its blends with n-decane. The elongational measurements are performed by analyzing self-thinning (due to surface tension effect) liquid threads of nanotube suspensions. A quasi-one-dimensional model is used to describe the self-thinning process, whereas corrections accounting for thread nonuniformity and necking are introduced a posteriori. The effects of nanotube concentration and aspect ratio, viscosity of the suspending liquid, and initial diameter of the self-thinning thread in uniaxial elongation are elucidated. The results for uniaxial elongation are compared with those for simple shear. The correspondence in the results of the shear and elongational measurements is addressed and interpreted. The results conform to the Herschel–Bulkley rheological constitutive equation (i.e., power law fluids with yield stress). However, the yield stress in elongation is about 40% higher than in simple shear flow, which suggests that the original Herschel–Bulkley model need modification with the yield stress being a function of the second invariant of the deviatoric stress tensor. The present effort is the first to study capillary self-thinning of Herschel–Bulkley liquids, which are exemplified here by suspensions of carbon nanotubes.

The role of surface species in chemical vapor deposited carbon nanotubes

Chemical vapor deposition (CVD) of carbon nanotubes (CNTs) has been investigated usinga coupled gas phase and surface chemistry model. This model successfully bridgedthe gap between the reactor and molecular length scales and allowed individualsurface kinetic processes to be identified as growth limiting directly from reactorscale parameters. Carbon nanotube growth rate is a function of the reactor walltemperature such that deposition would occur in the transition region between thehydrogen abstraction and hydrocarbon adsorption limited regimes. Deposition waslimited under low reactor temperatures or rich methane conditions by hydrogenabstraction from surface bound hydrocarbons due to the availability of gaseousH1. At high reactor temperatures and rich hydrogen conditions, the deposition reaction wasshown to be limited by hydrocarbon adsorption onto the nanoparticle surface. Optimalprocess conditions for efficient CNT production are discussed, as well as identifying thelimiting reaction steps for the surface chemistry.

In-Situ Sample Rotation as a Tool to Understand Chemical Vapor Deposition Growth of Long Aligned Carbon Nanotubes

A new tool for studying the process of carbon nanotube chemical vapor deposition (CVD) synthesis is described. By rotating the substrate in situ during the CVD process, the orientation of floating nanotubes with respect to the substrate is changed by interaction with the gas stream. Nanotubes lying on the surface of the substrate, however, will maintain their relative orientation. As a result different nanotube alignment angles are observed. By defining a time window through multiple rotation steps it is possible to study carbon nanotube behavior during CVD growth in a time-resolved manner. As an example, the settling process (i.e., the sinking of the nanotube to the substrate) is investigated. The analysis of forces acting on a floating nanotube shows that a vertical gas stream due to thermal buoyancy over the sample can keep long nanotubes floating for extended times. A stochastic process, indicated by a constant settling rate over time, forces the nanotube to make contact with the substrate, and this process is attributed to flow induced instability. Additional information on the floating and settling process are revealed from our study. The settling velocity could be inferred from curved nanotubes. The clearance between a floating nanotube and the substrate was found to be several hunded micrometers.

Amorphous carbon contamination monitoring and process optimization for single-walledcarbon nanotube integration

We detail the monitoring of amorphous carbon deposition during thermal chemical vapourdeposition of carbon nanotubes and propose a contamination-less process to integratehigh-quality single-walled carbon nanotubes into micro-electromechanical systems. Theamorphous content is evaluated by confocal micro-Raman spectroscopy and byscanning/transmission electron microscopy. We show how properly chosen processparameters can lead to successful integration of single-walled nanotubes, enablingnano-electromechanical system synthesis.

Indentation Response of Nanostructured Turfs

AbstractWhen grown via chemical vapor deposition carbon nanotubes (CNTs) may take on the form of a “turf”, consisting of many CNTs with a complex interconnectedness attached to an inflexible substrate. These turfs can be formed over large areas and with a range of heights (between 1 to 100 μm), and grown on photolithographically patterned catalysts to form different aspect ratios. This study focuses on the indentation and permanent deformation of CNT assemblages under applied contact loading. Nanoindentation was conducted on CNT turfs and the properties, nominally the turf's elastic modulus and hardness, were 14.9 MPa ± 5.7 MPa and 2 MPa respectively. The onset of permanent deformation during indentation occurred at applied stresses of 2.5 MPa. The turf's collective permanent deformation under applied compressive loading was also studied. A model predicting the buckling stress of CNT turfs is also described.

Metal catalyst-free low-temperature carbon nanotube growth on SiGe islands

A metal-catalyst-free growth method of carbon nanotubes (CNTs) has been developed using chemical vapor deposition of CNTs on carbon-implanted SiGe islands on Si substrates. From scanning electron microscopy and Raman measurements, the fabricated CNTs are identified as single-walled CNTs with a diameter ranging from 1.2 to 1.6 nm. Essential parts of the substrate preparation after CVD SiGe growth and carbon implant are a chemical oxidization by hydrogen peroxide solution and a heat treatment at 1000 °C prior to CNT growth. We believe that these processes enhance surface decomposition and assist the formation of carbon clusters, which play a role in seeding CNT growth. The growth technique is a practical method of growing metal-free CNTs for a variety of applications, while at the same time opening up the prospect of merging CNT devices into silicon very-large-scale-integration technology.

An investigation of plasma chemistry for dc plasma enhanced chemical vapour deposition ofcarbon nanotubes and nanofibres

The role of plasma in plasma enhanced chemical vapour deposition of carbonnanotubes and nanofibres is investigated with both experimental and computationaldiagnostic techniques. A residual gas analysis (RGA) of a 12 mbar dc discharge with aC2H2/NH3 gas mixture is conducted near the Ni catalyst surface employed for carbon nanofibre growth.The results are corroborated with a 1D dc discharge model that solves for species densities,ion momentum, and ion, electron and neutral gas thermal energies. The effect ofvarying the plasma power from 0 to 200 W on the gas composition is studied. Thedissociation efficiency of the plasma is demonstrated where over 50% of the feedstock isconverted to a mixture of hydrogen, nitrogen and hydrogen cyanide at 200 W.Finally, the important role that endothermic ion–molecule reactions play in thisconversion is, for the first time, established. Of these reactions, dissociative protonabstraction and collision-induced dissociation are of the greatest significance.

Engineered self-organization of neural networks using carbon nanotube clusters

A novel approach was developed to form engineered, electrically viable, neuronal networks, consisting of ganglion-like clusters of neurons. In the present method, the clusters are formed as the cells migrate on low affinity substrate towards high affinity, lithographically defined carbon nanotube templates on which they adhere and assemble. Subsequently, the gangliated neurons send neurites to form interconnected networks with pre-designed geometry and graph connectivity. This process is distinct from previously reported formation of clusterized neural networks in which a network of linked neurons collapses via neuronal migration along the inter-neuron links. The template preparation method is based on photo-lithography, micro-contact printing and carbon nanotube chemical vapor deposition techniques. The present work provides a new approach to form complex, engineered, interconnected neuronal network with pre-designed geometry via engineering the self-assembly process of neurons.

Single-walled Carbon Nanotubes Are a New Class of Ion Channel Blockers

Here we identify a novel class of biological membrane ion channel blockers called single-walled carbon nanotubes (SWNTs). SWNTs with diameter distributions peaked at approximately 0.9 and 1.3 nm, C60 fullerenes, multi wall nanotubes (MWNTs), and hyperfullerenes (nano-"onions") were synthesized by several techniques and applied to diverse channel types heterologously expressed in mammalian cells. External as-fabricated and purified SWNTs blocked K+ channel subunits in a dose-dependent manner. Blockage was dependent on the shape and dimensions of the nanoparticles used and did not require any electrochemical interaction. SWNTs were more effective than the spherical fullerenes and, for both, diameter was the determining factor. These findings postulate new uses for SWNTs in biological applications and provide unexpected insights into the current view of mechanisms governing the interaction of ion channels with blocking molecules.