Number Of Carbon Nanotubes Walls Research Articles

Optimization of electrical and mechanical properties of a single-walled carbon nanotube composite using a three-roll milling method

In this study, the electrical and mechanical properties of carbon nanotube (CNT)/polydimethylsiloxane (PDMS) composites were optimized using a three-roll milling dispersion method. Depending on the number of CNT walls, such as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), the three-roll milling conditions required to optimize the electrical conductivity of CNT/PDMS composites are quite different. In addition, the mechanical properties of the CNT/PDMS composites depend on the dispersion time. These properties were improved under an appropriate three-roll milling time but gradually deteriorated after excessive processing time. To demonstrate and optimize this phenomenon, various mechanical, electrical, and thermal properties of the CNT/PDMS composites were measured with respect to the type of CNT, milling time, and filler weight fraction. Morphological and structural analyses were also conducted on the CNT fillers and composites after milling.

Facile tuning of carbon nanotube morphologies via residual carbon control during catalyst preparation stage

In this study, Fe–Mo–MgO catalysts for the synthesis of carbon nanotubes (CNTs) were prepared using the combustion method and CNTs were synthesized through catalytic chemical vapor deposition. The combustion time was controlled to 0.5, 1, 2, 3, 5, 10, and 24 h in the catalyst preparation stage. The residual carbon contents after the combustion stage and the morphologies of synthesized CNTs were also analyzed. The diameter, yield, and crystallinity of the synthesized CNTs were found to remarkably vary according to the combustion time in the catalyst preparation process. The amount of residual carbon in the catalyst considerably affects the purity, crystallinity, diameter and its distribution, and wall number of CNTs. Based on the yield and crystallinity, CNTs synthesized using the catalyst with a combustion time of 3 h were determined to be the most appropriate for application in field emitters

Interfacial characteristics of a carbon nanotube-polyimide nanocomposite by molecular dynamics simulation

Abstract With molecular dynamics simulations, nanocomposites were characterized that are comprised of a polyimide (PI) polymer and carbon nanotubes (CNTs) with the same outer diameter but with one, two, or three walls. The simulations indicate that the PI/CNT interaction is strong, regardless of the number of CNT walls, and that there is some degree of alignment of the PI chains near the CNT interface. As the number of CNT walls increased, the density of PI chains near the CNT interface also increased and the average radius of gyration of the PI chains decreased, and these observations were attributed to changes due to the intertube van der Waals interactions. From simulations of the constant force pullout process of the CNT from the PI matrix, the limiting pullout force was calculated to be higher for the triple-walled CNT than for the single-walled one. The interfacial shear strength of the nanocomposites was also calculated from the pullout energy, and the results indicate that increasing the number of walls is a critical factor for enhancing the interfacial stress transfer during tension.

Effect of Carbon Nanotube Addition on the Interfacial Adhesion between Graphene and Epoxy: A Molecular Dynamics Simulation.

The pullout process of graphene from an epoxy/graphene composite filled with a carbon nanotube (CNT) was simulated by molecular dynamics simulations. The interaction energy and the interfacial adhesion energy were calculated to analyze the effect of CNT addition on the interfacial adhesion between the graphene and the epoxy matrix, with varying CNT radii, distances between the CNT and the graphene sheet, CNT axial directions, and the number of CNT walls. Generally, the addition of a CNT strengthens the interfacial adhesion between the graphene and the polymer matrix. Firstly, a larger CNT radius induces a stronger interfacial adhesion of graphene with the matrix. Secondly, when the CNT is farther away from the graphene sheet, the interfacial adhesion of graphene with the matrix becomes weaker. Thirdly, the CNT axial direction has little effect on the interfacial adhesion of graphene in the equilibrium structure. However, it plays an important role in the graphene pullout process. Finally, compared with a single-walled CNT, the interfacial adhesion between graphene and the matrix is stronger when a double-walled CNT is added to the matrix.

Adhesion of carbon nanotubes on elastic substrates with finite thickness

How carbon nanotubes (CNTs) interact with substrates is fundamental for understanding their physical properties. In existing theoretical and modeling studies, the substrates are considered to be rigid with semi-infinite thickness. In this work, the effects of finite substrate thickness and elasticity are analyzed theoretically and numerically for free boundary conditions. Based on the energy-variational approach, considering the interfacial van der Waals interactions and bending strain energies stored in CNTs and substrates, the governing equations and boundary conditions are derived analytically. The theoretical predictions are in reasonable agreement with the results of molecular dynamics simulations. When the substrate is sufficiently thick, the results of the present theoretical model are entirely consistent with previous models for the infinite-thickness substrate. However, for relatively thin substrates, the effect of substrate thickness is significant due to the geometric large deformation. Three stable adhesive states (initial non-adhesive, partially adhesive, and fully wrapping states) can be achieved, dependent on the substrate thickness, the number of CNT walls, and the interfacial adhesion work. The stability of adhesive configurations is explored by analyzing the energy variations corresponding to the adhesive deformation. We show that there exist several modes of energy variations, depending on the adhesion work and the substrate-CNT bending stiffness ratio, which exhibit linear and nonlinear influences, respectively. Our results could serve as guidelines to design CNT-on-substrate systems.

Nanoscale analysis of multiwalled carbon nanotube by tip-enhanced Raman spectroscopy

The relationship between the 2D and G band intensities and the number of walls of a multiwalled carbon nanotube was investigated by tip-enhanced Raman spectroscopy, which enables spatial resolution of nanometer order. The 2D band to G band integrated intensity ratio was found to be sensitive to the number of carbon nanotube walls. Furthermore, the 2D band intensity was enhanced by the intertube interaction between carbon nanotubes. This phenomenon can be explained by quantum interference between Raman scattering paths resulting from the strong interaction between carbon nanotube walls.

Quantum dot conjugated S. cerevisiae as smart nanotoxicity indicators for screening the toxicity of nanomaterials.

In this study, we have evaluated the toxicity of different forms of carbon nanotubes (CNTs) using S. cerevisiae-QD (SQD) bioconjugates as a novel fluorescent biological nanotoxicity indicator. A CNT mediated effect in SQD bioconjugates was used as an indicator for the changes occurring at the cell-membrane interfaces that induced disruption of membrane bound QDs resulting in the loss of fluorescence. Single, double and multiwalled carbon nanotubes (SWCNTs, DWCNTs and MWCNTs) were tested for their toxicities imposed on SQD bioconjugates. Bioconjugates exposed to varying concentrations of different forms of CNTs exhibited different modes of toxicities on SQD bioconjugates. SQD bioconjugates were highly responsive in the 0.1-10 μg mL-1 CNT concentration range after 1 h of exposure. The toxicity of CNTs was linked to the number of CNT walls. These results were further confirmed by SEM analysis and cell-viability tests that were consistent with the toxicity assays using fluorescent bioconjugates with different types of CNTs. SWCNTs imposed more severe cellular toxicity followed by MWCNTs and DWCNTs and the order of increasing cellular-damage by CNTs followed DWCNTs < MWCNTs < SWCNTs. This study speculates that the cell-injury by CNTs depends on their physical properties, such as layers of walls, non-covalent forces and dispersion states. Our results demonstrated a facile optical strategy that enables rapid and real-time cytotoxicity screening with yeast as model living-cells for engineering nanomaterials.

Effects of surfactant on carbon nanotube assembly synthesized by direct spinning

A surfactant is used to reduce the agglomeration of iron catalysts during the synthesis of carbon nanotubes (CNTs) and their assembly by chemical vapor deposition. The agglomeration of iron catalysts during synthesis affects the CNT diameter and number of CNT walls. Also, the agglomerated catalysts within the CNT assembly indicate an inefficient reaction of catalysts during the CNT synthesis. In this study, the non-ionic surfactant, polysorbate, is added to the CNT synthesis solution in varying amounts from 0 to 3.0wt%. The effects of the surfactant are closely related to the crystalline perfection, diameter, and number of walls of CNTs synthesized under different concentrations of the surfactant. Here, 1.0wt% surfactant is the most favorable concentration to achieve crystalline perfection. When the surfactant is added at more than 1.0wt%, amorphous carbons increase, although the agglomeration of iron catalysts continues to decrease. The simple addition of this surfactant can facilitate the development of the CNT assemblies that have high mechanical and electrical properties.

The preparation of carbon nanotube (CNT)/copper composites and the effect of the number of CNT walls on their hardness, friction and wear properties

Carbon nanotubes with 2, 3, 8 and 20 walls are mixed with a copper powder (micrometer sized) and consolidated by spark plasma sintering. The microhardness of resulting composites is found to be over 50% higher than that for Cu and the friction coefficient against a steel ball is decreased by a factor of 3–4 while the wear and wear rate are reduced by a factor of 10–20. Raman maps of selected specimens outside and inside the worn surface show that double-wall carbon nanotubes remain intact. The reasons for the effect of the number of walls and carbon content are discussed.

Effects of tip-nanotube interactions on atomic force microscopy imaging of carbon nanotubes

We examine the effect of van der Waals (vdW) interactions between atomic force microscope (AFM) tips and individual carbon nanotubes (CNTs) supported on SiO2. Molecular dynamics (MD) simulations reveal how CNTs deform during AFM measurement, irrespective of the AFM tip material. The apparent height of a single- (double-) walled CNT can be used to estimate its diameter up to ~2 nm (~3 nm), but for larger diameters the CNT cross-section is no longer circular. Our simulations were compared against CNT dimensions obtained from AFM measurements and resonant Raman spectroscopy, with good agreement for the smaller CNT di-ameters. In general, AFM measurements of large-diameter CNTs must be interpreted with care, but the reliability of the approach is improved if knowledge of the number of CNT walls is avail-able, or if additional verification (e.g. by optical techniques) can be obtained.

Macroscopic Wall Number Analysis of Single-Walled, Double-Walled, and Few-Walled Carbon Nanotubes by X-ray Diffraction

The layer number is of great importance for nanocarbon materials, such as carbon nanotubes (CNTs) and graphene. While simple optical methods exist to evaluate few-layer graphene, equivalent analysis for CNTs is limited to transmission electron microscopy. We present a simple macroscopic method based on the (002) X-ray diffraction peak to evaluate the average wall number of CNTs in the range from single- to few-walled. The key was the finding that the (002) peak could be decomposed into two basic components: the intertube structure (outer-wall contacts) and the intratube structure (concentric shells). Decomposition of the peaks revealed a linear relationship between the average wall number and the ratio of the intertube and intratube contributions to the (002) peak. Good agreement with CNTs having average wall numbers ranging from 1 to ∼5 demonstrated this as a macroscopic method for average wall number analysis.

Morphology and structure of carbon nanotubes synthesized on iron catalyst in the presence of carbon monooxide

This paper is devoted to a survey of results obtained while investigating the growth processes of carbon nanotubes (CNTs) on different substrates using iron as the catalyst and CO as the carbon source. Three different approaches to the substrate coating by the catalyst are discussed: sputtering metallization and particle synthesis using the hot-wire generator and its ex situ and in situ deposition. The possibility of growing ultralong one- and double-walled thick nanotubes and CNT flow alignment are demonstrated. The influence of the diameter of single-wall CNTs on their deformation on the substrate surface is studied. The possibility of controlling the number of CNT walls depending on the experimental conditions is demonstrated. The role of etching agents during CNT synthesis is discussed.

Ethanol-Promoted High-Yield Growth of Few-Walled Carbon Nanotubes

We report the use of a small concentration of ethanol in addition to ethylene as the carbon source for growth of dense vertically aligned “forests” of few-walled carbon nanotubes (CNTs). Through a detailed comparison of CNTs grown with and without ethanol added to the C2H4/H2 feedstock, we quantify several important effects of the ethanol addition. We show that ethanol selectively reduces the number of CNT walls without changing the outer diameter, increases the catalyst lifetime more than 3-fold, and increases the rate of carbon conversion more than 5-fold. Online dewpoint and mass spectrometry measurements of the exhaust stream suggest that ethanol decomposes into active carbon species that enhance growth, and into H2O, which counteracts the accumulation of amorphous carbon and thus prolongs the catalyst lifetime. We performed a systematic study of the effect of the catalyst film thickness, and identify a set of conditions that provides growth of millimeter-tall double-walled CNT forests. Importantly, our study reveals that the chemistry of the CVD atmosphere alone plays a critical role in controlling the structure of CNTs, and that addition of ethanol results in few-walled CNTs over a broad range of growth conditions. These findings are an important step toward the ultimate goal of control of CNT chirality during synthesis as well as toward realization of important large-scale applications of aligned CNT films having high monodispersity and structural quality.

Incremental Variation in the Number of Carbon Nanotube Walls with Growth Temperature

Investigations of carbon nanotube (CNT) synthesis were carried out in a chemical vapor deposition reactor using CO as the carbon source, Fe as the catalyst material, and SiO2 as the catalyst support. This allowed us to synthesize CNTs in a wide range of diameters from 1.4 to 12 nm and lengths from 0.5 to 350 μm. An incremental variation in the number of CNT walls, increasing from one to four, was found in the temperature range of 590−1070 °C. The increase in CNT wall number and CNT length with temperature can be explained by enhancement of carbon solubility and diffusivity. CO2 was found to be a very important additive for the activation of catalyst particles and subsequently for the successful synthesis of CNTs. The prevention of cementite particle formation and etching amorphous carbon were also attributed to CO2.

Wall-Controlled Growth of Carbon Nanotubes Using Temperature Treatment

Large arrays of self-oriented, multiwalled carbon nanotubes were obtained by chemical vapor deposition using a metallic substrate as well as a Si substrate. We controlled the size of catalytic Fe nanoparticles in order to control the wall number of the carbon nanotubes (CNTs). The size and distribution of Fe nanoparticles varied according to the growth condition, heat exposure time, and the thickness of the catalytic metal layer, resulting in a change in the wall number of CNTs grown. A decrease in heat exposure time from 10 to 0 s decreased the average wall number of CNTs. A combination of atomic force microscopy, scanning electron microscopy and transmission electron microscopy images provided evidence of the crucial role in the surface distribution of metallic catalyst particles on the substrate during the growth of CNTs. We confirmed the vertically aligned growth of CNTs that have a small number of walls and synthesized CNTs on a stainless plate with a 5 mm diameter.

A cohesive law for interfaces between multi-wall carbon nanotubes and polymers due to the van der Waals interactions

We have established the cohesive law for interfaces between multi-wall carbon nanotubes (CNTs) and polymer that are not well bonded and are characterized by the van der Waals force. The cohesive law is obtained in terms of the area density of carbon atoms on CNT walls, number of CNT walls, volume density of CH 2 units in the polymer, and parameters in the van der Waals force. The simple, analytical expressions of the cohesive law are useful to study the interaction between multi-wall CNTs and polymer, such as in CNT-reinforced composites.

High-density and well-aligned carbon nanotubes formed by surface decomposition of SiC

The structure and diameter of the densely aligned carbon nanotubes (CNTs) by surface decomposition of SiC ( 0 0 0 1 ¯ ) C-face in vacuum were investigated by cross-sectional, plan-view transmission electron microscopy and electron diffraction methods. Zigzag-type CNTs were confirmed to be selectively formed and the formation mechanism was proposed from crystallographic analysis. Furthermore, the wall number of CNTs was found to be directly proportional to the diameter of CNTs. Comparing with the theoretical calculation, it was revealed that all of the carbon atoms remained on the surface after the selective evaporation of Si atoms by decomposition of each monolayer of SiC ( 0 0 0 1 ¯ ) , and then constructed the CNT walls with the minimum diffusion distance at the interface.

Growth process of close-packed aligned carbon nanotubes on SiC

Densely aligned carbon nanotubes (CNTs) by surface decomposition of SiC (0001¯) C-face in vacuum were observed by plan-view transmission electron microscopy. It was found that the wall number of CNTs was directly proportional to the diameter of CNTs. Comparing with the theoretical calculation, it was revealed that all of the carbon atoms remained on the surface after the selective evaporation of Si atoms by decomposition of each monolayer of SiC (0001¯), and then constructed the CNT walls with the minimum diffusion distance at the interface.

New continuous gas-phase synthesis of high purity carbon nanotubes by a thermal plasma jet

We have developed a new gas-phase synthesis technique to produce carbon nanotubes (CNTs) with a continuous process and at high temperature, by using a thermal plasma jet. A thermal plasma jet was generated by applying a direct current of 100–300 A, using Ar as the plasma gas with a flow rate of ∼6 ksccm. The temperature of the thermal plasma jet was very high (∼10 4 K) and the velocity was very fast (∼100 m/s). Fe(CO) 5 and CO were used as a catalyst precursor and carbon source, respectively. The yield of CNTs was dramatically increased by attaching a helical extension reactor at the end of the plasma nozzle. High purity (∼80%) CNTs were produced with a continuous process by using a thermal plasma jet with helical extension reactor equipment. The number of CNT walls produced was critically affected by the hydrogen gas injected as an auxiliary plasma gas. Without hydrogen gas, single-walled carbon nanotubes whose diameter was about 1 nm were mostly produced while with hydrogen gas double-walled carbon nanotubes (about 4 nm in diameter) were predominantly produced, with small amount of 3- and 4-walled carbon nanotubes.