Thermal conductivity of an aligned carbon nanotube array

The thermal conductivity of vertically aligned CNT film arrays prepared using carbon vapor deposition (CVD) on a glass substrate was examined. A modern light flash device was first used to measure the effective thermal conductivity of the combined sample consisting of a sandwiched CNT film array thermal interface material between two glass plates. The results were used in an effective thermal resistance model to estimate the thermal conductivity of the CNT film. The value obtained was higher than that found in the literature and clearly demonstrated the advantage of using aligned CNTs and thus utilizing their maximum axial thermal conductivity. It was concluded that the quality of the CNT array produced using CVD directly contributed to the high thermal conductivity value obtained for the CNT film.     

Three dimensional solid-state supercapacitors from aligned single-walled carbon nanotube array templates

We demonstrate the fabrication of solid-state dielectric energy storage materials from self-assembled, aligned single-walled carbon nanotube arrays (VA-SWNTs). The arrays are transferred as intact structures to a conductive substrate and the nanotubes are conformally coated with a thin metal-oxide dielectric and a conductive counter-electrode layer using atomic layer deposition. Experimental results yield values in agreement with those obtained through capacitive modeling using Al₂O₃ dielectric coatings (C > 20 mF/cm³), and the solid-state dielectric architecture enables the operation of these devices at substantially higher frequencies than conventional electrolyte-based capacitor designs. Furthermore, modeling of supercapacitor architectures utilizing other dielectric layers suggests the ability to achieve energy densities above 10 W h/kg while still exhibiting power densities comparable to conventional solid-state capacitor devices. This device design efficiently converts the high surface area available in the conductive VA-SWNT electrode to space for energy storage while boasting a robust solid-state material framework that is versatile for use in a range of conditions not practical with current energy storage technology.     

Horizontally aligned single-walled carbon nanotube field emitters fabricated on vertically aligned multi-walled carbon nanotube electrode arrays

Vertically aligned multi-walled carbon nanotube (MWCNT) arrays were fabricated on an anodic aluminum oxide membrane bonded to a Si wafer. After obtaining a protruding tip for the MWCNTs by etching away some oxide, they were used as electrodes in the fabrication of carbon nanotube field emitters. Long single-walled carbon nanotubes (SWCNTs) were spin coated on the MWCNT arrays of uniform height. Clean SWCNTs were suspended by attaching them to the tips of the vertically aligned MWCNT arrays. The spin coated SWCNTs function as emitters, while the MWCNT arrays function as electrodes. The field emission was greatly improved by coating gold on the MWCNT arrays and annealing at 400 °C. Our field emitter exhibits good field emission properties such as a low turn-on field (1.4 V/μm), high current density (122 mA/cm²), and good stability (55 h for 10% degradation of current density from 400 μA/cm²).     

Dry densification of carbon nanotube bundles

A dry method for densifying vertically aligned carbon nanotube bundles is proposed and experimentally validated. The process uses the deposition of thin SiO₂ films to seal the porous CNT bundles at low pressure. When the CNT bundles are transferred back to ambient pressure they are densified by the pressure difference obtained between the inner and outer sides of the thin film. The effects of the densification have been studied for different thicknesses of SiO₂ films deposited by two different deposition techniques. The diameters of the narrowest densified sections are 26 ± 3% of their original sizes after dry densification by 50 nm thick SiO₂. The proposed dry densification method is also compared to existing liquid-based methods and its limitations are discussed.     

Vertically aligned carbon nanotube field-effect transistors

Vertically aligned carbon nanotube field-effect transistors (CNTFETs) have been developed using pure semiconducting carbon nanotubes. The source and drain were vertically stacked, separated by a dielectric, and the carbon nanotubes were placed on the sidewall of the stack to bridge the source and drain. Both the effective gate dielectric and gate electrode were normal to the substrate surface. The channel length is determined by the dielectric thickness between source and drain electrodes, making it easier to fabricate sub-micrometer transistors without using time-consuming electron beam lithography. The transistor area is much smaller than the planar CNTFET due to the vertical arrangement of source and drain and the reduced channel area.     

Phonons in single wall carbon nanotube bundles

We review recent and original results on the vibrational properties of single wall carbon nanotubes (SWNT). We especially focus on calculations and experiments performed on nanotube bundles. So far, the main technique for probing the dynamics has been Raman spectroscopy. Here, we discuss: (i) the relation between frequency of the A_1g radial breathing mode and nanotube diameter, (ii) the origin of resonance and the consequences on the profile and intensity of the Raman lines, and (iii) the assignment and resonant behaviour of the Raman lines between 700 and 1000 cm⁻¹. Recently, inelastic neutron scattering techniques (INS) were shown to be effective tools to probe the vibrational density of states of SWNT. We review the INS results and focus on the study of low frequency excitations, especially libration-twist modes and acoustic modes. Both Raman and INS results are analysed in the light of calculations performed in a valence force field model taking into account van der Waals intertubes interactions in the bundles.     

Morphology and processing of aligned carbon nanotube carbon matrix nanocomposites

Intrinsic and scale-dependent properties of carbon nanotubes (CNTs) have led aligned CNT architectures to emerge as promising candidates for next-generation multifunctional applications. Enhanced operating regimes motivate the study of CNT-based aligned nanofiber carbon matrix nanocomposites (CNT A-CMNCs). However, in order to tailor the material properties of CNT A-CMNCs, porosity control of the carbon matrix is required. Such control is usually achieved via multiple liquid precursor infusions and pyrolyzations. Here we report a model that allows the quantitative prediction of the CNT A-CMNC density and matrix porosity as a function of number of processing steps. The experimental results indicate that the matrix porosity of A-CMNCs comprised of ∼1% aligned CNTs decreased from ∼61% to ∼55% after a second polymer infusion and pyrolyzation. The model predicts that diminishing returns for porosity reduction will occur after 4 processing steps (matrix porosity of ∼51%), and that >10 processing steps are required for matrix porosity <50%. Using this model, prediction of the processing necessary for the fabrication of liquid precursor derived A-CMNC architectures, with possible application to other nanowire/nanofiber systems, is enabled for a variety of high value applications.     

Surface properties of vertically aligned carbon nanotube arrays

This study focuses on the structural changes of vertically aligned carbon nanotube (CNT) arrays while measuring their adhesive properties and wetting behaviour. CNT forests grown by chemical vapor deposition with a height of ~ 100 µm, an outer CNT diameter of ~ 10 nm and a density of the order of ~ 10¹⁰ CNTs/cm² show an average adhesion of 4 N/cm² when pressed against a glass surface. The applied forces lead to the collapse of the regular CNT arrays which limits their reusability as functional dry adhesives. Goniometric water contact angle (CA) measurements on CNT forests show a systematic decrease from an initial value of ~ 126° to a final CA similar to highly orientated graphite. Environmental scanning electron microscopy shows that this loss of hydrophobicity is due to an evaporation induced compaction of CNTs together with the loss of their vertical alignment. We observe the formation of cellular patterns for controlled drying.     

Frictional behaviour of vertically aligned carbon nanotube films

Vertically aligned CNT films were grown on polycrystalline β-SiC wafers by the surface decomposition method. Their frictional behaviours were investigated by AFM at the nanometer scales. Compared with DLC film and silicon wafers, they demonstrate an extremely low friction coefficient at the nanometer scale about 0.03-0.04. The effect of the surface topography on the friction coefficient is obvious for the aligned CNT film sliding at the nanometer scale. This implies that the excellent tribological properties of the vertically aligned CNT films, combined with their small dimensions and structural perfection, might lead to significant improvement of the performance of nano-devices.     

Effect of catalyst pattern geometry on the growth of vertically aligned carbon nanotube arrays

We report the effect of catalyst pattern geometry on the growth behaviour of carbon nanotube (CNT) vertical arrays. Larger patterns are seen to produce longer CNT arrays. We show that this is predominantly related to the pattern size dependence of the number of walls and relate this to the local availability of carbon feedstock species. In addition, the vertical alignment of CNT pillar arrays is seen to depend on the pattern design, in particular the relationship between the pillar dimension and the inter-pillar spacing.     

Tribological properties of vertically aligned carbon nanotube arrays

Carbon nanotubes (CNTs) are a promising material for the fabrication of biomimetic dry adhesives. The dimensions of single CNTs are in the range of those of terminal elements of biological dry hairy adhesion systems, such as the setal branches on the toe of the gecko. Here, the tribological properties of densely packed arrays of vertically aligned and up to 1.1 mm long multi-walled CNTs (VACNTs) synthesized by chemical vapor deposition are examined. The coefficient of friction μ is as high as 5–6 at the first sliding cycle, and decreases down to stable values between 2 and 3 at the fourth to fifth sliding cycles. Such high values of μ can only be explained by the strong contribution of adhesion induced by applied shear force. After the tests, wear-induced deformations of the VACNT surface are observed, which strongly depend on the amount of normal force applied during the friction experiments. Interestingly, the plastic deformation of the VACNTs does not significantly affect μ after a preconditioning by a few sliding cycles. However, a strong decrease of μ during the initial wear cycles has to be taken into account for the development of applications, such as non-slip surfaces and pick-and-place techniques for manufacturing.     

Densified aligned carbon nanotube films via vapor phase infiltration of carbon

We report a simple way to produce fully densified aligned carbon nanotube (ACNT) films. The simultaneous growth of nanotubes and densification of the ACNT films by carbon infiltration in the interstitial spaces between nanotubes are accomplished in a single step by the combination of the chemical vapor deposition and chemical vapor infiltration processes. Scanning electron microscope analysis and microbalance measurements showed that after infiltration, the diameters of nanotubes and bulk density of the ACNT films are increased by an order of magnitude (and hence the porosity of the ACNT films is decreased). Transmission electron microscope and Raman scattering analysis showed that after densification, the nanotubes are conformally coated by partially graphitized pyrolytic carbon. The compressive modulus of the densified ACNT films could be increased by three orders of magnitude compared to the pristine ACNT films. Electrical properties are also measured for the densified films showing marked differences with the ACNT films. The property enhanced densified ACNT films constitute a new form of carbon-carbon nanocomposites and could find applications as multifunctional nanocomposites.     

Comparison of vertically aligned carbon nanotube array intercalated production among vermiculites in fixed and fluidized bed reactors

Vertically aligned carbon nanotube (CNT) arrays with ordered nanostructure and extraordinary performance have become an important advanced material. CNTs were synchronously grown with high density to form aligned morphology, while the fine agglomerated structure was sensitive to the packing style of catalyst particles. Thus, synthesis of CNT arrays in a fixed bed or fluidized bed is an important issue. We reported that CNT arrays were grown from ethylene on a lamellar catalyst in a fixed bed reactor and a fluidized bed reactor, respectively. The reactor style affected the intercalated growth of CNT arrays greatly. The qualities of CNT array products in the fixed bed showed a distribution along the axis direction. The CNT arrays obtained at the top of the fixed bed were of good alignment and small diameter; while for the products obtained at the bottom of the reactor, CNT array blocks with higher densities, larger diameter, shorter length, and more defects, were formed. When CNT arrays were grown in a fluidized bed, they were of homogeneous structure, low densities, small uniform diameter, and had little defects, which can be attributed to available space, uniform temperature and reactant distribution in the fluidized bed reactor. These favored the mass production of CNT arrays with uniform properties in a fluidized bed reactor.     

Alcohol-assisted rapid growth of vertically aligned carbon nanotube arrays

Millimeter-to-centimeter scale vertically aligned carbon nanotube (VACNT) arrays are widely studied because of their immense potential in a range of applications. Catalyst control during chemical vapor deposition (CVD) is key to maintain the sustained growth of VACNT arrays. Herein, we achieved ultrafast growth of VACNT arrays using Fe/Al₂O₃ catalysts by ethanol-assisted two-zone CVD. One zone was set at temperatures above 850 °C to pyrolyze the carbon source and the other zone was set at 760 °C for VACNT deposition. By tuning synthesis parameters, up to 7 mm long VACNT arrays could be grown within 45 min, with a maximal growth rate of ∼280 μm/min. Our study indicates that the introduction of alcohol vapor and separation of growth zones from the carbon decomposition zone help reduce catalyst particle deactivation and accelerate the carbon source pyrolysis, leading to the promotion of VACNT array growth. We also observed that the catalyst film thickness did not significantly affect the CNT growth rate and microstructures under the conditions of our study. Additionally, the ultralong CNTs showed better processability with less structural deformation when exposed to solvent and polymer solutions. Our results demonstrate significant progress towards commercial production and application of VACNT arrays.     

Tensile properties of long aligned double-walled carbon nanotube strands

The mechanical properties of well-aligned double-walled carbon nanotube (DWNT) strands with diameters of 3-20 μm and lengths of ∼10 mm were measured using a stress-strain puller. The average tensile strength and Young’s modulus of the tested strands are 1.2 GPa and 16 GPa, respectively. Deformation and fracture processes of these samples are discussed. The tensile strength and Young’s modulus of an individual DWNT bundle were estimated, with values comparable to those of SWNT bundles. The superior mechanical strengths of our as-prepared DWNT strands are expected to give them potential as a high-strength material and a reinforcement in composites.