Showing papers by "Pulickel M. Ajayan published in 2005"

Super-Compressible Foamlike Carbon Nanotube Films

Abstract: We report that freestanding films of vertically aligned carbon nanotubes exhibit super-compressible foamlike behavior. Under compression, the nanotubes collectively form zigzag buckles that can fully unfold to their original length upon load release. Compared with conventional low-density flexible foams, the nanotube films show much higher compressive strength, recovery rate, and sag factor, and the open-cell nature of the nanotube arrays gives excellent breathability. The nanotube films present a class of open-cell foam structures, consisting of well-arranged one-dimensional units (nanotube struts). The lightweight, highly resilient nanotube films may be useful as compliant and energy-absorbing coatings.

Viscoelasticity in carbon nanotube composites.

Abstract: Polymer composites reinforced by carbon nanotubes have been extensively researched for their strength and stiffness properties. Unless the interface is carefully engineered, poor load transfer between nanotubes (in bundles) and between nanotubes and surrounding polymer chains may result in interfacial slippage and reduced performance. Interfacial shear, although detrimental to high stiffness and strength, could result in very high mechanical damping, which is an important attribute in many commercial applications. We previously reported evidence of damping in nanocomposites by measuring the modal response (at resonance) of cantilevered beams with embedded nanocomposite films. Here we carry out direct shear testing of epoxy thin films containing dense packing of multiwalled carbon nanotube fillers and report strong viscoelastic behaviour with up to 1,400% increase in loss factor (damping ratio) of the baseline epoxy. The great improvement in damping was achieved without sacrificing the mechanical strength and stiffness of the polymer, and with minimal weight penalty. Based on the interfacial shear stress (approximately 0.5 MPa) at which the loss modulus increases sharply for our system, we conclude that the damping is related to frictional energy dissipation during interfacial sliding at the large, spatially distributed, nanotube-nanotube interfaces.

Collagen-carbon nanotube composite materials as scaffolds in tissue engineering.

Abstract: Carbon nanotubes (CNT) are attractive for use in fiber-reinforced composite materials due to their very high aspect ratio, combined with outstanding mechanical and electrical properties. Composite materials comprising a collagen matrix with embedded CNT were prepared by mixing solubilized Type I collagen with solutions of carboxylated single-walled carbon nanotubes (SWNT) at concentrations of 0, 0.2, 0.4, 0.8, and 2.0 weight percent. Living smooth muscle cells were incorporated at the time of collagen gelation to produce cell-seeded collagen-CNT composite matrices. Constructs containing 2.0 wt % CNT exhibited delayed gel compaction, relative to lower concentrations that compacted at the same rate as pure collagen controls. Cell viability in all constructs was consistently above 85% at both Day 3 and Day 7, whereas cell number in CNT-containing constructs was lower than in control constructs at Day 3, though statistically unchanged by Day 7. Scanning electron microscopy showed physical interactions between CNT and collagen matrix. Raman spectroscopy confirmed the presence of CNT at the expected diameter (0.85-1.30 nm), but did not indicate strong molecular interactions between the collagen and CNT components. Such collagen-CNT composite matrices may have utility as scaffolds in tissue engineering, or as components of biosensors or other medical devices.

Multifunctional brushes made from carbon nanotubes.

Abstract: Brushes are common tools for use in industry and our daily life, performing a variety of tasks such as cleaning, scraping, applying and electrical contacts. Typical materials for constructing brush bristles include animal hairs, synthetic polymer fibres and metal wires (see, for example, ref. 1). The performance of these bristles has been limited by the oxidation and degradation of metal wires, poor strength of natural hairs, and low thermal stability of synthetic fibres. Carbon nanotubes2,3, having a typical one-dimensional nanostructure, have excellent mechanical properties, such as high modulus and strength4,5,6, high elasticity and resilience7, thermal conductivity8 and large surface area (50–200 m2 g−1)9. Here we construct multifunctional, conductive brushes with carbon nanotube bristles grafted on fibre handles, and demonstrate their several unique tasks such as cleaning of nanoparticles from narrow spaces, coating of the inside of holes, selective chemical adsorption, and as movable electromechanical brush contacts and switches. The nanotube bristles can also be chemically functionalized for selective removal of heavy metal ions.

Characterizing energy dissipation in single-walled carbon nanotube polycarbonate composites

Abstract: In this study, single-walled carbon nanotube and bisphenol-A-polycarbonate composite beams were fabricated by a solution mixing process and dynamic (cyclic) load tests were performed to characterize energy dissipation. We report up to an order of magnitude (>1000%) increase in loss modulus of the polycarbonate system with the addition of 2% weight fraction of oxidized single-walled nanotube fillers. We show that the increase in damping is derived from frictional sliding at the nanotube-polymer interfaces. The nanoscale dimensions of the tubes not only result in large interfacial contact area, thereby generating high damping efficiency, but also enable seamless integration of the filler materials into the composite structure.

Anisotropic thermal diffusivity of aligned multiwall carbon nanotube arrays

Abstract: A photothermoelectric technique was employed to determine the anisotropic thermal diffusivity of thick arrays of multiwalled carbon nanotubes grown by chemical-vapor deposition. The thermal diffusivity along the alignment direction was also determined using a self-heating 3ω method. The agreement between the measured thermal diffusivities with the two techniques is between 2% and 13% in the tested temperature range. The thermal diffusivity along the alignment direction decreases slightly with temperature in the 80–300‐K temperature range and is ∼ two orders of magnitude smaller than the thermal diffusivity along the planes of graphite. The thermal diffusivity across the alignment direction is ∼25 times smaller than along the alignment direction and is between 50% and five times smaller than the thermal diffusivity across the planes of graphite in the measured temperature range.

Ordered Ni nanowire tip arrays sticking out of the anodic aluminum oxide template

Abstract: We present a method for making highly ordered arrays of Ni nanowire (NW) tips fully exposed over the surface of anodic aluminum oxide (AAO) templates with uniform exposed lengths. Ni NWs are electrochemically deposited in the nanochannels of the AAO templates, and the templates surface is selectively etched to expose Ni NW tips of uniform lengths, which can be tuned by adjusting the etching time. The magnetic domain structure and magnetic hysteresis of the Ni NW tips were studied at room temperature, and the results indicate strong magnetic anisotropy for the NW arrays and magnetic coercivities significantly larger than that of bulk Ni. The ordered NW tips fully exposed over the surface of the AAO templates with uniform lengths could have various practical applications.

Embedded carbon-nanotube-stiffened polymer surfaces.

Abstract: Conducting surface coatings are useful for antistatic applications, whereas surface hardening of materials is useful for improving the wear and abrasion resistance. 3] For polymer materials, surface conductivity and stiffness may be improved by applying coatings or adding fillers to the polymer matrix. For example, polymers can be made scratch resistant by the addition of hard fillers. However, for the case of polymers, achieving excellent mechanical and electrical properties only at the surface is a challenge. Conventional hard fillers, such as alumina or silica, improve the scratch resistance of the polymer, but do not help improve the conductivity. On the other hand, conducting fillers such as micrometer-scale graphite particles do not considerably improve the mechanical properties of the polymer. Thus, there is a need to develop a surface engineering approach to alter the mechanical and electrical properties of polymer coatings. Multiwalled carbon nanotubes (MWNTs) are stiff macromolecular structures having outer diameters of 30 nm, and lengths on the order of a few tens of micrometers. The MWNTs also have a very high conductivity ( 10 Scm ), high modulus ( 1 TPa) along their length direction, as well as a high bending modulus (0.9 to 1.24 TPa). Possibilities of improving bulk mechanical and electrical properties of composites by nanotube-reinforcement have been discussed in the literature. 8–12] For example, the polymer-intercalated nanotube sheets have shown significant improvement in the modulus of the film. Our approach is to incorporate the excellent properties of nanotubes at a polymer surface in a well-ordered and distributed fashion resulting in the improvement in the electrical as well as mechanical properties of the polymer. This would enable multifunctional surface characteristics for polymer coatings. This paper describes the first report of the generation of such surface-engineered polymer coatings with nanotubes. In the present work, a thickness-aligned MWNT/polymer disc was prepared, where the MWNTs were reinforced into one of the surfaces of the disc, and were aligned in the thickness direction. The discs were made from two different polymers: polymethyl methacrylate (PMMA) and polydimethyl siloxane (PDMS). 14] Both PMMA and PDMS are insulating. However, PMMA is a glassy, rigid polymer at room temperature, whereas PDMS is a soft elastomer at room temperature. The synthesis of the composite disc was performed as follows: First, the aligned arrays of MWNTs ( 30 mm in length) were grown on a quartz substrate by chemical vapor deposition. 15] Subsequently, the quartz substrate with aligned nanotube arrays was gently immersed, with the nanotube side facing the top, into the excess monomer (or uncured resin) solution in a vial. By using the excess quantity, the resulting polymer not only occupied the inter-nanotube gaps in the MWNT arrays, but also formed a thick layer above the surface of the MWNT arrays. A portion of the same monomer solution was taken in a separate vial to make pure polymer as a control sample. After the in situ polymerization was complete, polymer discs were taken out of the quartz substrate. In order to make MWNT/PMMA discs, the monomer (methyl methacrylate (MMA)), the initiator (2,2’-azobisisobutyronitrile (AIBN)), and the chain-transfer agent (1-decanethiol) were mixed together in a given proportion (60 mL MMA: 0.17 g AIBN: 30 mL 1-decanethiol) in a quartz vial. The polymerization was carried out in a water bath at 55 8C, for 24 h. The weight fraction of MWNTs in the MWNT/PMMA composite films was estimated to be approximately 4%. Schematics of the synthesis process and the cross-sectional SEM micrographs of the thickness-aligned MWNT/PMMA discs are shown in Figure 1. Similarly, PDMS as well as MWNT/ PDMS films were prepared by infiltration of a mixture of silane resin and a curing agent (in a proportion of 10:1 by weight) into aligned MWNT arrays, followed by typical thermal cure cycles. The surface resistivity of the MWNT side of the polymer disc was compared with that of the pure PMMA side by measurement with a four-probe setup with a probe spacing of 500 mm. A dc current (I), on the order of a few hundred microamps, was applied through the sample, and the voltage (V) was measured in millivolts. The MWNT-reinforced side of the PMMA disc showed a dc conductivity of 0.60 0.07 Scm , whereas the pure PMMA side was not conducting. The reported conductivity of PMMA is 5 10 11 Scm . Thus, the addition of MWNTs increases the surface conductivity of PMMA significantly. The MWNTs used in the present analysis are macroscopically aligned but show less overall alignment owing to the waviness of the nanotubes. Thus, the percolation threshold is expected to be drastically lower than that which is expected for perfectly aligned fibers in a matrix. Therefore, the nanotube loading ( 4% by weight or 2% by volume) in the present composites is expected to be above percolation threshold, as indicated by the conductivity of 0.60 0.07 Scm . This value is higher than that reported in the literature for similar loadings of pure, non-aligned MWNTs (10 3 to 10 2 Scm ), but lower than for similar loadings of Fe-containing MWNTs in PMMA (>1 Scm ). The conductivity of the aligned MWNT/PMMA surfaces is large enough for poten[*] Dr. N. R. Raravikar, A. S. Vijayaraghavan, Prof. P. Keblinski, Prof. L. S. Schadler, Prof. P. M. Ajayan Departments of Materials Science and Engineering Rensselaer Polytechnic Institute, Troy, NY 12180 (USA) Fax: (+1)518-276-8554 E-mail: schadl@rpi.edu ajayan@rpi.edu [] These authors contributed equally to the work

Room-temperature resonant tunneling of electrons in carbon nanotube junction quantum wells

Abstract: Resonant tunneling structures [M. Bockrath, W. Liang, D. Bozovic, J. H. Hafner, C. B. Lieber, M. Tinkham, and H. Park, Science 291, 283 (2001)], formed between the junction of two single walled nanotubes and the conductive atomic force microscopy tip contact were investigated using current sensing atomic force microscopy. Oscillations in the current voltage characteristics were measured at several positions of the investigated nanotube. The oscillatory behavior is shown to follow a simple quantum mechanical model, dependent on the energy separation in the quantum well formed within the two junctions. Our model shows that these observations seen over several hundreds of nanometers, are possible only if the scattering cross section at defects is small resulting in long phase coherence length, and if the effective mass of the carrier electrons is small. We have calculated the approximate mass of the conduction electrons to be 0.003me.

Carbon nanotube filter

Abstract: Monolithic, macroscopic, nanoporous nanotube filters are fabricated having radially aligned carbon nanotube walls. The freestanding filters have diameters and lengths up to several centimeters. A single-step filtering process was demonstrated in two important settings: the elimination of multiple components of heavy hydrocarbons from petroleum, a crucial step in post-distillation of crude oil, and the elimination of bacterial contaminants such as Escherichia coli or the nanometer-sized poliovirus from drinking water. All the filtration processes were repeated several times with completely reproducible results. These nanotube filters can be cleaned repeatedly after each filtration process to regain their full filtering efficiency.

TerraByte flash memory with carbon nanotubes

Abstract: Using the large maximal current density and the small diameter of carbon nanotubes, a flash memory arrangement is proposed. It makes use of the spin sensitive transport through hybrid conductor junctions. This memory contains no moving mechanical part and a single layer’s theoretical information density can reach beyond 40Gbit∕cm2. It is easy to build a three-dimensional memory structure. Then, the theoretical capacity can reach beyond 1015bit∕cm3 (1000Terrabit∕cm3), which means that a memory with 1cm area and 1mm thickness could have about 10TerraByte capacity.

Recovered Bandgap Absorption of Single-Walled Carbon Nanotubes in Acetone and Alcohols†

Abstract: The authors acknowledge funding from the Focus Center New York for Electronic Interconnects at Rensselaer Polytechnic Institute, and NSF-NSEC at RPI for directed assembly of nanostructures. Angel Rubio was supported by the European Community Research Training Networks NANOPHASE (HPRN-CT-2000-00167), COMELCAN (HPRN-CT-2000-00128), M-DNA (IST-2001-38051), Spanish MCyT (MAT2001-0946), and the University of the Basque Country (9/UPV 00206.215-13639/2001). Peter Persans thanks the Department of Energy, Office of Basic Energy Sciences (grant DE-FG0297ER455662).

Comparing Damping Properties of Singlewalled and Multiwalled Carbon Nanotube Polymer Composites

Abstract: In this paper we compare the damping properties of polymer nano-composites filled with singlewalled and multiwalled carbon nanotube fillers. The polymer material chosen for this study is polycarbonate (Lexan 121, General Electric). The nanotube fillers are dispersed in the matrix using a novel solution mixing technique with tetra-hydro-furan as the solvent. Both the multiwalled and the singlewalled nanotube filled samples show higher damping level compared to the pristine (or unfilled) polycarbonate. However, the loss modulus of nano-composite samples with singlewalled nanotubes is significantly greater than with multiwalled tubes. This suggests that since the inner shells of the multiwalled tubes are not in contact with the polymer, they do not contribute to interfacial frictional sliding; this reduces the damping efficiency of the multiwalled tubes relative to the singlewalled tubes.

The Role Played by Strain Fields, Dislocation Arrays, and Domain Boundaries During the Catalytic Synthesis of Carbon Nanotubes

Abstract: Ex-situ transmission electron microscopy (TEM) was performed on catalytically-grown multi-wall carbon nanotubes (MWCNTs), leading to the identification of two types of catalyst-nanotube wall interfaces – respectively characterized by a quasi-spherical, low aspect ratio particle closer to the nanotube root and by a tapered, high aspect ratio particle farther away from it. The nanotubes exhibit two distinct types of boundaries between crystalline domains with different orientations – twist and twin boundaries in correspondence with quasi-spherical particles and tilt boundaries in correspondence with the tapered particles. TEM evidence suggests that the domain boundaries maintain a rather steady position coupled to the catalytic particles, while the carbon atoms diffuse along the nanotube axis away from the particles. From these considerations, it is possible to conclude that the relative movement of the carbon atoms with respect to the dislocation lines comprising the nanotube domain boundary located at the catalyst-wall interface is a significant mechanism for nanotube crystal growth mainly driven by surface diffusion. The results are interpreted in light of the concurrence of base- and tip- growth for the catalytic synthesis of nanotubes dominated by surface diffusion.