Aligned Nanotubes Research Articles

Time resolved strain dependent morphological study of electrically conducting nanocomposites

An efficient and reliable method is introduced to understand the network behaviour of nano-fillers in a polymeric matrix under uniaxial strain coupled with small angle x-ray scattering measurements. The nanoparticles (carbon nanotubes) are conductive and the particles form a percolating network that becomes apparent source of electrical conduction and consequently the samples behave as a bulk conductor. Polyurethane based nanocomposites containing 2% w/w multiwall carbon nanotubes are studied. The electrical conductivity of the nanocomposite was (3.28×10-5s/m).The sample was able to be extended to an extension ratio of 1.7 before fracture. A slight variation in the electrical conductivity is observed under uniaxial strain which we attribute to the disturbance of conductive pathways. Further, this work is coupled with in- situ time resolved small angle x-ray scattering measurements using a synchrotron beam line to enable its measurements to be made during the deformation cycle. We use a multiscale structure to model the small angle x-ray data. The results of the analysis are interpreted as the presence of aggregates which would also go some way towards understanding why there is no alignment of the carbon nanotubes.

Fabrication of superaligned carbon nanotubes reinforced copper matrix laminar composite by electrodeposition

A new kind of laminar metal matrix nanocomposite (MMC) was fabricated by an electrodeposition process with copper and superaligned carbon nanotubes film (SACNT film). The SACNT film was put on a titanium plate and then a layer of copper was electrodeposited on it. By repeating the above process, the laminar Cu/SACNT composite which contains dozens or hundreds of layers of copper and SACNT films was obtained. The thickness of a single copper layer was controlled by adjusting the process parameter easily and the thinnest layer is less than 2 μm. The microscopic observation shows that the directional alignment structure of SACNT is retained in the composite perfectly. The mechanical and electrical properties testing results show that the tensile and yield strengths of composites are improved obviously compared with those of pure copper, and the high conductivity is retained. This technology is a potential method to make applicable MMC which characterizes high volume fraction and directional alignment of carbon nanotubes.

Photo-Electrochemical Properties of Anodised Titania Nanotube Arrays Annealed in Nitrogen Atmosphere

Self organised aligned titania nanotubes were synthesised by electrochemical anodisation of titanium foil and the effect of annealing in nitrogen atmosphere was investigated. The diameter and length of nanotubes were 90–110 nm and 450–500 nm respectively giving high aspect ratio nanomaterial. Samples annealed in nitrogen atmosphere were found to retain the tubular structure with diameter 80–110 nm. Annealing of nanotubes in nitrogen atmosphere indicated substitution of nitrogen into the TiO2 lattice; however large amounts were still present as physisorbed, chemisorbed and possibly as O—Ti—N structure in TiO2 lattice. The photo-electrochemical properties of titania electrodes were investigated under solar simulated and visible illumination. The effect of methanol as hole acceptor was also investigated. The electrodes annealed in N2 atmosphere demonstrated higher solar photocurrent response than air annealed electrode. The sample annealed for 1 h in N2 resulted in optimum photocurrent response under solar simulated illumination. The photocurrent response obtained with the electrolyte containing methanol was found to be increased by a factor of 2.4 under solar simulated light and 1.3 under visible illumination, however the overall visible photocurrent response was very small compared to the solar simulated response. The additional small peaks at negative potentials may involve excitation from filled defect level.

Nanoscratch technique for aligning multiwalled carbon nanotubes synthesized by the arc discharge method in open air

Horizontally aligned and densely packed multiwalled carbon nanotubes (MWCNTs) were synthesized in an open air, without the need for a controlled atmosphere, using a rotating cathode arc discharge method with the help of a metal scraper. The physical force exerted by the scraper results in in-situ alignment of MWCNTs along the direction of scrape marks. This strategy, which enables the alignment of nanotubes in a controlled fashion to any length and direction of interest, was examined to determine the force required to align a nanotube. A model is developed to understand the alignment process. Using the nanoscratch technique to mimic this strategy, and incorporating the data obtained from the nanoscratch technique into the model developed, the minimum force required to align a MWCNT, as well as the energy required to align a gram of nanotubes, has been estimated. The method demonstrated represents an economical approach for large-scale synthesis of aligned MWCNTs at low costs.

Observation of impedance oscillations in single-walled carbon nanotube bundles excited by high-frequency signals

We report experimental observation of impedance oscillations in single-walled carbon nanotube bundles measured from 10 MHz to 65 GHz on coplanar waveguides and a power law dependence of the differential conductance with bias voltage. From the crossover of the real and imaginary parts of the complex impedance observed in the range of 10 GHz, we estimate a long lifetime of 15 ps that can support the claim of ballistic transport. By measuring the scattering parameters at high frequencies at low temperatures we show that this observation is strongly influenced by the number of aligned nanotube bundles present in the devices.

Optical reflection and absorption of carbon nanotube forest films on substrates

The correlation between the height and density of multi-walled carbon nanotube forests and their optical properties in the mid-infrared region was investigated using nanotube forests grown on Al/Si, Al/Nb/Si, and fused silica substrates. Measurements of the hemispherical reflectance and transmittance of carbon nanotube forests are presented. Analyses by an effective medium approximation and a circular waveguide model are compared. It is found that circular waveguides with graphite walls of reduced conductivity can generate similar spectra of the absorption coefficients as carbon nanotube forests do. Parameters from the waveguide model can describe qualitatively the density and alignment of carbon nanotubes in the forest. With a proper density, a randomly modulated forest of less than 20 μm in height can generate a hemispherical reflectance of less than 0.002 in the mid-infrared region.

Extraction of intercalated O2 from aligned carbon nanotubes: the breaking of intertube paths and exponential changes in resistance.

In carbon nanotube films, the alignment of carbon nanotubes creates Lennard-Jones potentials at intertube junctions and trapped O2 appears to oscillate at elevated temperatures. Electrical measurements reveal a low hopping barrier along the transverse direction and an underlying mechanism that involves intercalated molecules acting as charge carriers between tubes. Ab initio calculations support dynamic intercalation and charge transfer through O2 bouncing between tubes.

Geometrically constrained self-assembly and crystal packing of flattened and aligned carbon nanotubes

While the mechanical properties of highly aligned carbon nanotube (CNT) thin films and their nanocomposites have been widely studied, the load transfer mechanisms and failure modes of aligned CNT composites have not been sufficiently explored and understood. In this research, super-aligned CNT thin films with a measured alignment fraction of up to 0.93 are fabricated by mechanical stretching. High concentration (50–60wt% CNT) CNT reinforced bismaleimide (CNT/BMI) nanocomposites are fabricated from the aligned network to study mechanical properties and microstructures. Atomic resolution transmission electron microscopy (TEM) analysis reveal unusual CNT crystal packing and permit the observation of interesting structural features of the CNTs and their assemblages, including collapse, flattened packing, preferred stacking, folding and twisting phenomena, as well as CNT pullouts from bundles and the resin matrix. The large surface-to-surface contact areas between aligned and flattened nanotubes, driven by van der Waals interactions, give rise to a high density packing of the flattened CNTs in the nanocomposite, resembling a graphitic material. Molecular dynamics (MD) simulations are performed to model the packing structure and understand the dependence of density on the relative content of flattened nanotube and void space. The modeling results support the conclusions drawn from the experimental observations.

Electrokinetics of scalable, electric-field-assisted fabrication of vertically aligned carbon-nanotube/polymer composites

Composite thin films incorporating vertically aligned carbon nanotubes (VACNTs) offer promise for a variety of applications where the vertical alignment of the CNTs is critical to meet performance requirements, e.g., highly permeable membranes, thermal interfaces, dry adhesives, and films with anisotropic electrical conductivity. However, current VACNT fabrication techniques are complex and difficult to scale up. Here, we describe a solution-based, electric-field-assisted approach as a cost-effective and scalable method to produce large-area VACNT composites. Multiwall-carbon nanotubes are dispersed in a polymeric matrix, aligned with an alternating-current (AC) electric field, and electrophoretically concentrated to one side of the thin film with a direct-current (DC) component to the electric field. This approach enables the fabrication of highly concentrated, individually aligned nanotube composites from suspensions of very dilute (ϕ=4×10−4) volume fraction. We experimentally investigate the basic electrokinetics of nanotube alignment under AC electric fields, and show that simple models can adequately predict the rate and degree of nanotube alignment using classical expressions for the induced dipole moment, hydrodynamic drag, and the effects of Brownian motion. The composite AC + DC field also introduces complex fluid motion associated with AC electro-osmosis and the electrochemistry of the fluid/electrode interface. We experimentally probe the electric-field parameters behind these electrokinetic phenomena, and demonstrate, with suitable choices of processing parameters, the ability to scalably produce large-area composites containing VACNTs at number densities up to 1010 nanotubes/cm2. This VACNT number density exceeds that of previous electric-field-fabricated composites by an order of magnitude, and the surface-area coverage of the 40 nm VACNTs is comparable to that of chemical-vapor-deposition-grown arrays of smaller-diameter nanotubes.

Inside Cover: Solution‐Processed Ultrathin Aluminosilicate Nanotube–Poly(vinyl alcohol) Composite Membranes with Partial Alignment of Nanotubes (ChemNanoMat 2/2015)

Inside Cover: Solution‐Processed Ultrathin Aluminosilicate Nanotube–Poly(vinyl alcohol) Composite Membranes with Partial Alignment of Nanotubes (ChemNanoMat 2/2015)

Temperature-tuned transport in biomembrane pores

Fast transport of complex molecular fluids along a biomembrane composed of wavy-rough nanotubes was investigated by using the absolute-reaction approach together with a boundary perturbation method. The critical parameters related to the nearly frictionless transport of molecules were identified to be the possible phase-transition temperature after selecting specific activation energies and activation volumes for confined (cylindrical) geometry considering the dominated role of shear viscosity. Temperature-tuned activation energy as well as activation volume are crucial to the rapid transport of biomolecular fluids. Our numerical results can make the biomembrane composed of aligned wavy-rough nanotubes a promising mimic of protein channels for selective chemical sensing and transdermal drug delivery.

Solution‐Processed Ultrathin Aluminosilicate Nanotube–Poly(vinyl alcohol) Composite Membranes with Partial Alignment of Nanotubes

AbstractThe fabrication of ultrathin (300 nm) aluminosilicate nanotube–poly(vinyl alcohol) composite membranes with partial vertical alignment of the nanotubes (up to 33 %), by solution‐casting methods on porous polymeric substrates, is reported. A high loading (up to 60 vol %) of nanotubes is achieved. A comprehensive microstructural characterization of the membranes is performed by a combination of SEM, TEM, grazing‐incidence wide‐angle X‐ray scattering measurements, and simulations. This investigation shows that the nanotubes are individually dispersed and partially aligned in the polymeric matrix by the use of appropriate matrix and substrate materials. Permeation measurements of gas probe molecules (CO2 and CH4) on two different types of membranes, one containing bare nanotubes and the other containing amine‐functionalized nanotubes, also support the proposed microstructure of the thin nanocomposite membranes.

Structural modification for carbon nanotube film and the composite film by processing optimization

As a kind of 2D macroscopic assembly of carbon nanotube (CNT), CNT film fabricated from CNT array performs outstanding properties and potential applications in many areas. For CNT film fabricated from spinnable array, the nanotube's alignment and packing density are two important factors for improving the properties. In this work, inspired by the unique transferring process, we intended to adjust the CNT stacking status in the film via optimizing the winding process and assisting appropriate post-treatment. Array-based CNT films with different winding parameters were fabricated to study the effect of winding speed and winding thickness on the film's tensile properties. It is revealed that related with the drawing force, the winding speed plays an important role in tuning CNTs arrangement in nanotube sheet. Within a certain range, increasing speed is helpful to straighten and uniformly disperse nanotubes, and further improve the tensile properties of the film. While for winding thickness, appropriate reducing thickness is favorable for sheets compaction and decreasing defects, which further enhance the properties. Additionally, combining the low elongation feature of array-based CNT film, little-ratio post-stretching is applied on the films, which is also proved to be efficient for densifying and aligning, stimulating the film stronger and more conductive simultaneously.

Strong and Conductive Dry Carbon Nanotube Films by Microcombing.

In order to maximize the carbon nanotube (CNT) buckypaper properties, it is critical to improve their alignment and reduce their waviness. In this paper, a novel approach, microcombing, is reported to fabricate aligned CNT films with a uniform structure. High level of nanotube alignment and straightness was achieved using sharp surgical blades with microsized features at the blade edges to comb single layer of CNT sheet. These microcombs also reduced structural defects within the film and enhanced the nanotube packing density. Following the microcombing approach, the as-produced CNT films demonstrated a tensile strength of up to 3.2 GPa, Young's modulus of up to 172 GPa, and electrical conductivity of up to 1.8 × 10(5) S m(-1) , which are much superior to previously reported CNT films or buckypapers. More importantly, this novel technique requires less rigorous process control and can construct CNT films with reproducible properties.

(Invited) New Solar and Chemical Energy Conversion Mechanisms Enabled By Nanocarbon Systems

Our laboratory at MIT has been interested in how nanocarbon systems, such as carbon nanotubes and graphene, can enable new energy conversion mechanisms. This presentation will highlight two such conversion schemes: chemical energy conversion via reactivity waves and solar energy harvesting using nanotube based photovoltaics. Towards the former, spatially propagating reaction waves are already central to a variety of energy applications, such as high temperature solid phase or combustion synthesis, and thermopower waves. In this work, we identify and study a previously unreported property of such waves, specifically that they can generate temperatures far in excess of the adiabatic limit. We show that this superadiabaticity occurs when a reaction wave in either one dimension (1D) or two dimensions (2D) impinges upon an adiabatic boundary under specific reaction and heat transfer conditions. This property is studied analytically and computationally for a series of 1D and 2D example systems, producing an estimate of the upper bound for excess temperature rise as high as 1.8 times the adiabatic limit, translating to temperatures approaching 2000 K for some practical materials. We show that superadiabaticity may enable several new types of energy conversion mechanisms, including thermophotovoltaic wave harvesting, which we analyze for efficiency and power density. Towards the second application, photovoltaics using single-walled carbon nanotubes (SWNTs) as near-infrared photo-absorbers have seen rapid recent progress, offering promise for long-wavelength light harvesting devices. Despite this interest the fundamental design questions remain, such as optimal device thickness, nanotube orientation, density, and the impact of impurities. To address this challenge, we develop a deterministic model of SWNT PVs derived directly from SWNT photophysics using photon, exciton, and charge carrier population balances. The model accounts for arbitrary distributions of nanotube chiralities, lengths, orientations, defect types and concentrations, bundle fraction and size, and density. We show that feasible devices can achieve external quantum efficiencies above 60%. We reveal a sharply optimal device thickness that is a function of nanotube density, orientation, and quenching site concentration. This thickness stems from a tradeoff between exciton generation and diffusion to the electrodes, and is at a minimum at the limit of close-packed nanotube density. We show that this minimum characterizes a given device design and scales with mean nanotube length to exponent 0.4. The normalized difference between optimal thickness and this close-packed limit scales inversely with density to the 0.24 power. Practically, in-plane aligned nanotube configurations yield optimal thicknesses less than 10 nm, increasing to a range of 50 to 200 nm for vertical alignment. Due to weak inter-SWNT exciton transport relative to exceptional intra-SWNT diffusion, vertically-aligned films are unambiguously favored at densities above 3% of the close-packed limit; at lower densities however an optimum emerges at an intermediate angle to compensate for weaker light absorption of vertical nanotubes. Comparison to published experimental devices displays the model’s utility for device design.

Fracture resistance, thermal and electrical properties of epoxy composites containing aligned carbon nanotubes by low magnetic field

Abstract The alignment of carbon nanotubes (CNTs) with residual catalyst Ni particles in epoxy resin (EP) was achieved under a low magnetic field of 0.4 T. EP composites containing different loading of CNTs were prepared at 60 °C based on the rheology results of CNT/EP suspensions. The effects of CNT loading and magnetic field on the fracture toughness (KIC), glass transition temperature (Tg) and electrical properties of the composites were studied. The obtained results showed significant enhancements in KIC, independent of the orientation of the CNTs, compared to neat epoxy. The maximum increase (∼51%) was achieved by composites with 3 wt.% aligned CNTs transverse to crack growth. Additionally, composites with magnetically aligned CNTs displayed anisotropy in KIC. Compared to CNT/EP composites with no applied magnetic field, the KIC increase was less effective at high CNT loading. Finally, toughening mechanisms, Tg and electrical conductivity results were discussed.

Fluorinated liquid crystalline surfactants for dispersion and alignment of carbon nanotubes

An effective and promising approach for dispersion and alignment of carbon nanotubes (CNTs) with the help of fluorinated liquid crystalline surfactants (FLCSs), which possess excellent affinity with CNTs and liquid crystals (LCs), is presented here. A binary mode is designed to express the dispersion of CNTs with CNTs joining the nematic matrix of FLCSs. Ultraviolet–visible spectrum and fourier transform infrared (FTIR) spectrum are recorded to figure out the concentration of effectively dispersed CNTs by FLCSs, the concentration in chloroform reaches a maximum 1.481 mg mL−1. The miscibility between FLCSs and CNTs is analyzed by FTIR imaging system with P7 giving rise to the best miscibility. The homogeneously dispersed CNTs decrease the glass transition temperature of FLCSs by increasing the degree of freedom of the segment on FLCS to move, and enhance the thermal stability of FLCSs by increasing the decomposition temperature and the activation energy for thermal decomposition. The optical textures show that P7 increases the compatibility between LC host and CNTs. The chiral stability and electro-optical property of LC host are both optimized by CNT doping.

Guided Growth of Horizontal Single-Wall Carbon Nanotubes on M-Plane Sapphire

The alignment of carbon nanotubes has been the subject of extensive research. Among the different alignment methods, surface alignment, such as alignment along favorable directions on the anisotropic surface of single-crystal substrates, including sapphire and quartz, has stood out as very effective and robust. Recently, this method was shown to also be highly effective for the guided growth of semiconducting nanowires. Interestingly, while the same directions of alignment were found for the growth of nanotubes and nanowires on R-plane (11̅02) and A-plane (112̅0) sapphire (α-Al2O3), nanotubes, unlike nanowires, have been reported not to align on M-plane (101̅0) sapphire. Here we report the alignment of carbon nanotubes on M-plane sapphire, as well as on the annealed M-plane, which is periodically faceted, in directions that match the directions reported for aligned nanowires. We go on to discuss a possible common alignment mechanism for nanowires and nanotubes on sapphire surfaces.

Synthesis and photovoltaic application of platinum-modified conducting aligned nanotube fiber

Platinum-modified carbon nanotube (CNT) fibers with controlled platinum weight percentages were synthesized via an electrochemical deposition method. Platinum nanoparticles can be uniformly deposited on the surface of the aligned CNTs in the fiber, which possesses an efficient improved catalytic activity in the reduction of I3 −/I− compared with other fiber materials such as a carbon fiber without the aligned nanostructure. The hybrid CNT fiber was further used as a counter electrode to fabricate fiber-shaped dye-sensitized solar cell (DSSC). A maximal power conversion efficiency of 8.10% was achieved, compared with that of 4.91% for a bare aligned CNT fiber and 5.50% for a platinum-modified carbon fiber as the counter electrode under the same condition.

Large-area assembly of densely aligned single-walled carbon nanotubes using solution shearing and their application to field-effect transistors.

Dense alignment of single-walled carbon nanotubes over a large area is demonstrated using a novel solution-shearing technique. A density of 150-200 single-walled carbon nanotubes per micro-meter is achieved with a current density of 10.08 μA μm(-1) at VDS = -1 V. The on-current density is improved by a factor of 45 over that of random-network single-walled carbon nanotubes.