Theoretical studies on the charge-induced failure of single-walled carbon nanotubes

Electromechanical coupling in single-walled carbon nanotubes has been studied theoretically. The charge distribution on single-walled carbon nanotubes in an electric field is obtained by an atomistic moment method based on classical electrostatics theory. The electrostatic interactions between charged carbon atoms are calculated using the Coulomb law. The charge-induced deformations of single-walled carbon nanotubes in axial and radial directions are obtained by using the molecular structural mechanics method and considering the electrostatic interactions as external loads acting on carbon atoms. The electrical failure of charged carbon nanotubes is found to be controlled by the charge level and also affected by the caps on the nanotube ends. The results indicate that the bond breaking first appears at the tube ends and the end-caps can enhance the stability of the nanotubes.     

In situ transmission electron microscopy of electrochemical lithiation,delithiation and deformation of individual graphene nanoribbons

We report an in situ transmission electron microscopy study of the electrochemical behavior of few-layer graphene nanoribbons (GNRs) synthesized by longitudinal splitting the multi-walled carbon nanotubes (MWCNTs). Upon lithiation, the GNRs were covered by a nanocrystalline lithium oxide layer attached to the surfaces and edges of the GNRs, most of which were removed upon delithiation, indicating that the lithiation/delithiation processes occurred predominantly at the surfaces of GNRs. The lithiated GNRs were mechanically robust during the tension and compression tests, in sharp contrast to the easy and brittle fracture of the lithiated MWCNTs. This difference is attributed to the unconfined stacking of planar carbon layers in GNRs leading to a weak coupling between the intralayer and interlayer deformations, as opposed to the cylindrically confined carbon nanotubes where the interlayer lithium produces large tensile hoop stresses within the circumferentially-closed carbon layers, causing the ease of brittle fracture. These results suggest substantial promise of graphene for building durable batteries.     

Boron Nitride with Packets of Nanotubes for Microcomposite Ceramics

The possibility of creating microcomposite technical ceramics based on boron pyronitride (BPN) with nanotubes and packets of nanotubes providing a high microplasticity is considered. It is shown that the irradiation by nanosecond proton-ion beams with a high powder density promotes the growth of wurtzite-like BPN based on self-organizing mesoscopic (SOM) particles, SOM-blocks, and nanocrystalline particles. The sizes of SOM-particles and SOM-blocks of the initial and irradiated BPN are determined. The self-organization of nanotubes outside the zone of irradiation has a spiral cyclic nature and is caused by shear deformations of the layers composed of SOM-blocks. Tubes inclined to the surface with symmetry axes of the fifth order and packets of nanotubes of a disk shape are observed. The obtained micrographs are compared with model forms of manifestation of spiral cyclic structures.     

Alignment of graphene nanoribbons by an electric field

In this paper, we develop an analytical approach to predict the field-induced alignment of cantilevered graphene nanoribbons. This approach is validated through molecular simulations using a constitutive atomic electrostatic model. Our results reveal that graphene’s field-oriented bending angle is roughly proportional to the square of field strength or to the graphene length for small deformations, while is roughly independent of graphene width. The effective bending stiffness and the longitudinal polarizability are found to be approximately proportional to the square of graphene length. Compared with carbon nanotubes, graphene nanoribbons are found to be more mechanically sensitive to an external electric field.     

Stress simulation of carbon nanotubes in tension and compression

Based on a continuum shell model, a structural mechanics approach is presented to simulate stress-strain behavior of carbon nanotubes (CNTs). The nanoscale continuum theory is established to directly incorporate the Morse potential function into the constitutive model of CNTs. According to the present model, the mechanical properties of both zigzag and armchair tubes are investigated. The result shows that the atomic structures of CNTs have a significant influence on the stress-strain behavior. The armchair zigzag tube exhibits larger stress-strain response than the zigzag tube under tensile loading, but its relationship turns over between the tension and compression deformations. The theoretical approach supplies a set of very simple formulas and able be serve as a good approximation on the mechanical properties for CNTs.     

Micro-drawing of glassy polybenzoxazole rigid rods and the molecular interactions with carbon nanotubes

The high-temperature (T_g > 650°) wholly aromatic polybenzoxazoles (PBO) polymer chains in thin films underwent elastic energy release via local deformation of crazing when stretched beyond a critical strain around 0.5%. The strain localization in the ultra-rigid polymer was quickly superseded by craze fibril breakdown, triggering catastrophic fracture at low extensions below ～3%. Although the drawing stress of craze fibrillation, determined to be ～3 GPa, was insufficient to separate chains in PBO crystallites, it forced the chains in the amorphous regions to flow into large molecular deformations (～300% strain) at room temperature. The poor craze fibril stability of the rigid-rod chains was enhanced dramatically when surface-functionalized single-walled carbon nanotubes (SWCNTs) were dispersed into the polymer. No toughening effects were observed, however, for multi-walled carbon nanotubes (MWCNTs) although the elastic enhancement leading to increase of strain delocalization was still operative. The toughening selectivity was attributed to the PBO/CNT load transfer coupling during nanoplastic flows in which only the CNTs of compatible bending moments permitting fibril drawing were allowed to participate.     

Rheological behavior of multiwalled carbon nanotube/polycarbonate composites

The rheological behavior of compression molded mixtures of polycarbonate containing between 0.5 and 15 wt% carbon nanotubes was investigated using oscillatory rheometry at 260 °C. The nanotubes have diameters between 10 and 15 nm and lengths ranging from 1 to 10 μm. The composites were obtained by diluting a masterbatch containing 15 wt% nanotubes using a twin-screw extruder. The increase in viscosity associated with the addition of nanotubes is much higher than viscosity changes reported for carbon nanofibers having larger diameters and for carbon black composites; this can be explained by the higher aspect ratio of the nanotubes. The viscosity increase is accompanied by an increase in the elastic melt properties, represented by the storage modulus G′, which is much higher than the increase in the loss modulus G″. The viscosity curves above 2 wt% nanotubes exhibit a larger decrease with frequency than samples containing lower nanotube loadings. Composites containing more than 2 wt% nanotubes exhibit non-Newtonian behavior at lower frequencies. A step increase at approximately 2 wt% nanotubes was observed in the viscosity-composition curves at low frequencies. This step change may be regarded as a rheological threshold. Ultimately, the rheological threshold coincides with the electrical conductivity percolation threshold which was found to be between 1 and 2 wt% nanotubes.     

Gas sensing mechanism of carbon nanotubes: From single tubes to high-density networks

Gas sensing in carbon nanotubes is still poorly understood. Possible mechanisms are charge transfer between adsorbed gas molecules and the nanotubes or gas-induced changes at the interface between the nanotubes and their metal contacts. For carbon nanotube networks, it is also important to understand how defects and junctions formed by crossing nanotubes affect adsorption. Previous work demonstrates that for devices made with a single carbon nanotube, the response was mainly due to modifications at the nanotube/metal contact interfaces rather than molecular adsorption on the nanotube. Here it is shown that in networks of carbon nanotubes the gas sensitivity is due to both the nanotube/metal contacts and the carbon nanotube network. The network effects are dominated by gas-induced changes in nanotube/nanotube junctions, rather than gas adsorption on regions of the nanotubes away from the junctions.     

Synthesis and characterisation of medium surface area silicon carbide nanotubes

Silicon carbide nanotubes with medium surface area (30-60 m²/g) were successfully prepared by reaction between carbon nanotubes and SiO vapor according to the shape memory synthesis (SMS). The gross morphology of the carbon nanotubes was maintained during the carburization process. A calcination in air at 600 °C was performed to remove unreacted carbon domains in order to obtain pure carbon-free SiC nanotubes. The synthesized SiC nanotubes had a mean outer diameter of 100 nm and lengths up to several tens of micrometres.     

Analysis of the vibration characteristics of double-walled carbon nanotubes

The induced electric-field has been applied to measure the elastic modulus of carbon nanotubes. However, the vibrational modes of the multi-walled carbon nanotubes are quite different from those of the single-walled carbon nanotubes. Analysis of the vibration characteristics of double-walled carbon nanotubes (DWCNTs) with simply supported boundary condition are carried out based on Euler–Bernoulli beam theory. The DWCNTs are considered as two nanotube shells coupled through the van der Waals interaction between them. It is found that the vibrational modes of DWCNTs are noncoaxial intertube vibrations, and the deflections of the inner and outer nanotubes can occur in the same or in opposite deflections. In the same vibrational mode, the resonant frequencies of DWCNTs with deflections between the inner and outer nanotubes in the same direction are smaller than those of DWCNTs with the opposite deflections.     

Elucidation of the origin of grown-in defects in carbon nanotubes

The origin of grown-in defects in carbon nanotubes (CNTs) is elucidated by in situ atomic-scale environmental transmission electron microscope (ETEM) observations of the chemical vapor deposition growth of CNTs. Our high-resolution ETEM observations clearly demonstrate that the deformation of nanoparticle catalysts (NPCs) during the growth of CNTs triggers the formation of various defects in CNTs. The small deformation of NPCs at the interface with CNTs gives rise to the formation of bends and disorder of the interlayer spacing in CNTs. The changes in the diameter and number of graphitic layers in CNTs are caused by the large protrusion on and shrink deformations of NPCs. This study provides insightful strategies to control the grown-in defects of CNTs.     

Comparison of structural changes in nitrogen and boron-doped multi-walled carbon nanotubes

We investigated the effect of the reaction parameters on the structure of multi-walled carbon nanotubes containing different concentrations of nitrogen and boron. The nanotubes were produced using a ‘standard’ aerosol chemical vapour deposition technique in conjunction with benzylamine, triethylborane, hexane and toluene mixtures. These precursors were thermally decomposed between 800 and 1100 °C under argon at atmospheric pressure. By varying the precursor concentrations, the nitrogen and boron content of the nanotubes could be altered between 0-2.2 and 0-0.5 at.% respectively. Using a typical laboratory-sized 50 cm long tube furnace, yields between 0.3 and 1.5 g of nanotubes/10 min were relatively easily achieved. Moreover, we show that doping carbon nanotubes with heteroatoms, such as B and N, can be used to control nanotube diameters, change their defect density, and manipulate their oxidation resistance within a range of ca. 170 °C. Hence, we show that it is possible to tune nanotube properties within a certain interval and to produce nanotubes with relatively well defined properties in quantities usable for further characterisation and for studying their viability in applications such as composite materials, gas sensors, capacitors, and electronic components.     

Rheology,curing and time-dependent behavior of epoxy/carbon nanoparticles systems

This study delved into the characterization of epoxy nanocomposites containing diglycidyl ether of bisphenol-A (DGBEA), graphene nanoplates (GN), and carbon nanotubes (CNT) across a range of weight percentages (0.05% to 2%). The nanocomposites were produced through a process involving mechanical stirring and ultrasonication. To assess compatibility, three-dimensional solubility parameters (3DSP) were employed. CNT demonstrated superior compatibility with epoxy and triethylenetetramine (TETA), due to its higher amount of oxygenated species in nanoparticle surface compared to GN. A rheological percolation phenomenon occurred in CNT systems at concentrations above 0.2%, while GN did not display percolation even at 2% concentration. Incorporating nanoparticles led to increased curing enthalpy due to surface functional groups. As the percolation network formed, viscosity rose, and a reduced glass transition temperature (T_g) indicated restricted molecular mobility. Surprisingly, T_g consistently increased by approximately 27°C during composite annealing, regardless of nanoparticle type or concentration. This was attributed to forming a three-dimensional network structure potentially originating from reactions between nanoparticle-oxygenated groups and the epoxy matrix. This phenomenon was crucial in heightened creep and irreversible deformations, setting these nanocomposites apart from pure resin behavior.     

Electrical and mechanical properties of polyimide-carbon nanotubes composites fabricated by in situ polymerization

Polyimide (PI)-carbon nanotubes composites were fabricated by in situ polymerization using multi wall carbon nanotubes (MWNT) as fillers. It suggested that in situ polymerization is an ideal technique to make a perfect dispersion of carbon nanotubes into matrixes. Besides it, the pre-treatment of carbon nanotubes in solvent to make the networks untied enough and to let solvent percolated into the networks is very important for forming uniform entanglements between carbon nanotubes and polymer molecular chains. The results imply that the percolation threshold for the electric conductivity of the resultant PI-MWNT composites was ca. 0.15 vol%. The electrical conductivity has been increased by more than 11 orders of magnitude to 10⁻⁴ S/cm at the percolation threshold. The mechanical properties of the polyimide composite were not improved significantly by addition of carbon nanotubes.     

Selective destruction of individual single walled carbon nanotubes by laser irradiation

We demonstrate selective burnout of individual carbon nanotubes that are electronically resonant with the incident laser energy. Raman spectroscopy and atomic force microscopy are used to quantify the burnout of nanotubes. The threshold laser power for rapid burnout is found to occur between 0.4 and 0.9 W/μm². At lower laser powers of 80 mW/μm², the burnout depends linearly on time, over tens of minutes. Non-resonant nanotubes could not be burned out even with high laser power or long exposure times. This preferential burnout of resonant nanotubes demonstrates the possibility of selective removal of metallic nanotubes from an inhomogeneous sample.     

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.     

Binding of pulmonary surfactant proteins to carbon nanotubes; potential for damage to lung immune defense mechanisms

Potential pulmonary toxicity of carbon nanotubes is a research area that has received considerable attention. Surfactant proteins A and D (SP-A and SP-D) are collectin proteins that are secreted by airway epithelial cells in the lung. They play an important role in first-line defense against infection within the lung. The aim of this study was to investigate the interaction between carbon nanotubes and proteins contained in lung surfactant. By using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE), Western Blotting, a novel technique of affinity chromatography based on carbon nanotube-Sepharose matrix [1] and electron microscopy data it was shown that SP-A and SP-D selectively bind to carbon nanotubes. The binding was Ca²⁺-ion dependent, and was variable between batches of nanotubes. It was therefore likely to be mediated by surface impurities or chemical modifications of the nanotubes. Chronic level exposure to carbon nanotubes may result in sequestration of SP-D and SP-A. Absence of these proteins in knockout mice leads to susceptibility to lung infection and emphysema.     

Synthesis of ceramic nanotubes using AAO templates

In this paper, the metal aluminum oxide ceramic nanotubes (M–Al–O, M = Zn, Mg and Ba) were synthesized by the use of porous anodic aluminum oxide (AAO) template. Metal oxide nanotubes were firstly formed from the thermal decomposition of respective nitrates within the pores of AAO templates. Then M–Al–O nanotubes were synthesized through the solid-state chemical reaction between the metal oxide nanotubes and AAO. TEM observation shows that the as-prepared samples are nanotubes and XRD measurement reveals that M–Al–O nanotubes (M = Zn, Mg and Ba) are crystalline in nature with spinel structure.     

Polydimethylsiloxane/silica composites incorporating pyrite powders for actuation elements

BACKGROUND: Among polymers, elastomers can achieve very large deformations and deliver relatively high forces when an electric field is applied to them. In addition, a number of active components, such as carbon black, multi‐walled nanotubes or metallic powders, can be used to obtain increased effects. One elastomer used as a host for these active fillers is polydimethylsiloxane. RESULTS: Pyrite powders were physically incorporated in a polymeric matrix consisting of interconnected polydimethylsiloxane/silica networks. The mixtures were processed as films by casting on a Teflon substrate before crosslinking. The films thus formed were investigated from the point of view of their mechanical behaviour and were tested as active elements in an actuation system. CONCLUSION: Electromechanical tests of the polymeric membranes under investigation revealed a very good behaviour for applications in nano‐ and micro‐actuation. Some of the prepared films showed significant displacements at reasonable voltages (i.e. 56 000 nm at 460 V or 1000 nm at 6 V). Sensitivities between 2.608 and 333.33 nm V^?1 were obtained in inverse dependence on the Young's modulus values. Copyright © 2009 Society of Chemical Industry