Carbon Nanotube Bridges Research Articles

Controlling the growth morphology of carbon nanotubes: from suspended bridges to upright forests

We have developed two strategies to produce carbon nanotubes (CNTs) from low-density surface growth to high-density forest growth. We have demonstrated that by introducing a C(2)H(2) pulse at the beginning of the growth, where methane is still used as the main carbon feeding gas, the growth tendency of CNTs can be changed and the resulting growth morphology will vary from surface growth to forest growth. Similarly, the growth morphology can be changed when the growth temperature is raised. The further characterization via Raman spectroscopy indicates that an increasing C(2)H(2) pulse time will lead to a rise of the D peak for as-grown CNTs, due to the formation of more multi-walled CNTs and the amorphous carbon contamination introduced by extra C(2)H(2), while a high growth temperature tends to produce high-quality CNTs and to reduce the amorphous carbon contamination. Furthermore, by appropriately adjusting the growth temperature and controlling the C(2)H(2) pulse time, we have managed to produce both suspended CNT bridges and upright forests within a single growth procedure and to form suspended pristine CNT transistors with a relatively high yield. In addition, the electrical properties of these CNT nanostructures have been investigated by electrical transport and scanning photocurrent measurements.

Synthesis of nanocomposites based on nanotubes and silicates

In situ synthesis of nanocomposites based on carbon nanotubes and zeolite/montmorillonite was carried out in a hot filament CVD reactor where the precursors (methane and hydrogen) are activated by carbonized tungsten filaments heated up to 2200°C. In nanocomposites formed both on zeolite and montmorillonite we observed cross-linking of the catalytic particles by nanotubes and creation of carbon nanotube bridges and three-dimensional networks. The length of nanotube bridges was in a range from several nm to nearly 10μm. A high density of carbon nanotubes was observed in the whole volume of zeolite. The high catalytic efficiency of zeolite is most likely caused by its structure that allows anchoring of Fe3+ catalytic particles in the pores and prevents their migration from the sample. At the ends of the nanotubes grown on zeolite we observed particles of the catalyst. In montmorillonite, the particles catalyzing the growth of carbon nanotubes may be present not only on the external surface but also in the interlayer voids of the mineral. Its catalytic efficiency is enhanced as proved by the higher amount of CNTs and their bundles. In the course of CNTs synthesis probably also clumps of Fe3+ catalytic particles arise, which may be the reason for formation of bundles of nanotubes.

Wear Behavior of Plasma-Sprayed Carbon Nanotube-Reinforced Aluminum Oxide Coating in Marine and High-Temperature Environments

Wear behavior of plasma-sprayed carbon nanotube (CNT)-reinforced aluminum oxide (Al2O3) composite coatings are investigated at room temperature (298 K), elevated temperature (873 K), and in sea water. Lowest wear volume loss was observed in the sea water as compared to dry sliding at 298 and 873 K. Relative improvement in the wear resistance of Al2O3-8 wt.% CNT coating compared to Al2O3 was 72% at 298 K, 76% at 873 K, and 66% in sea water. The improvement in the wear resistance of Al2O3-CNT coatings is attributed to (i) larger area coverage by protective film on the wear surface at room temperature and in sea water, (ii) higher fracture toughness of Al2O3-CNT coatings due to CNT bridging between splats, and (iii) anti-friction effect of sea water. The average coefficient of friction (COF) was the lowest (0.55) in sea water and the highest (0.83) at 873 K for Al2O3-8 wt.% CNT coating.

Self-Aligned Growth of Single-Walled Carbon Nanotube Bridging Two Electrodes

An integrated method for the fabrication of electrical contacts to the single-walled carbon nanotubes (SWNTs) by gas-flowing techniques is reported. Bridge structures containing single SWNT channels are successfully fabricated through the CVD process. Raman spectra of the grown nanowires exhibit typical radial breathing modes which can be associated with the vibrational features of SWNT. Atomic force micrograph (AFM) and scanning electronic microscopy (SEM) images demonstrate that the as-growth SWNTs are indeed bridging two electrodes. I-V curves of such a structure confirm that the SWNTs are electronically linked with the Mo electrodes. This research provides a parallel method for the large-scale integration of SWNTs into electronic, optoelectronic, and sensing systems.

Novel architecture of carbon nanotube decorated poly(methyl methacrylate) microbead vapour sensors assembled by spray layer by layer

For the first time vapour sensors were made by assembling multi-wall carbon nanotube (CNT) decorated poly(methyl methacrylate) microbeads (PMMAµB) by spray layer by layer (sLbL). This combination of materials and technique resulted in an original hierarchical architecture with a segregated network of CNT bridging PMMAµB. The chemo-resistive behaviour of these conductive polymer nanocomposite (CPC) sensors was studied in terms of sensitivity and selectivity towards standard volatile organic compounds (VOC), as well as quantitativity and reproducibility of responses Ar to methanol, water, toluene and chloroform. Results show that 3D sLbL assembly allows boosting CNT network sensitivity by a factor 2 and selectivity for methanol vapour by a factor of 5. Additionally CNT-PMMAµB sensors gave responses proportional to vapour molecules content that could easily be fitted by the Langmuir–Henry-clustering model. Such sensors are thus expected to be good candidates for implementation in electronic noses.

Mechanics of carbon nanotube networks: microstructural evolution and optimal design

Carbon nanotube networks feature outstanding mechanical performance, and also hierarchical structures and network topologies. In this paper we investigate their structure–property relationship through mesoscale molecular dynamics simulations. We find that their microstructures undergo remarkable evolution under mechanical loads. The correlation between applied strain, microstructural evolution and failure mechanism, especially the bundling process and evolution of bridging carbon nanotubes, is discussed based on the simulation results. Based on the insights of the underlying mechanisms, further engineering approaches on the carbon nanotube networks towards enhanced mechanical properties are proposed and validated, e.g., by including intertube cross-links that resist shear, maintain the network topology and improve strain affinity.

Modelling of the carbon nanotube bridging effect on the toughening of polymers and experimental verification

The effect of aligned and randomly oriented carbon nanotube (CNT), with respect to the crack growth plane, on the fracture toughness of polymers is modelled in this paper using the Elastic Plastic Fracture Mechanics. According to a critical length, two dominant toughening mechanisms for CNT-modified polymers are presented, i.e. CNT pull-out and CNT rupture. The model is then used to identify the effect of CNTs geometrical and mechanical properties on the enhancement of fracture toughness in CNT-modified polymers. The key CNT properties are the CNT radius, average length, ultimate strength, elongation before failure, interfacial shear strength between CNTs and the polymer and nanotube volume fraction. Finally, experimental results are compared with the model predictions. The correlation shows that processing of long, aligned CNTs remains the main barrier in achieving major fracture toughness enhancement.

Growth and characterization of horizontally suspended CNTs across TiN electrode gaps

A technique is proposed to grow horizontal carbon nanotubes (CNTs) bridgingmetal electrodes and to assess their electrical properties. A test structure wasutilized that allows for selective electrochemical sidewall catalyst placement. Theselectivity of the technique is based on the connection of the desired metal electrodesto the silicon substrate where the potential for electrochemical depositionwas applied. Control over the Ni catalyst size (15–30 nm) and density (up to3 × 1011 particles cm − 2) is demonstrated. Horizontal CNTs with controlled diameter and densitywere obtained by CVD growth perpendicular to the sidewalls of patternedTiN electrode structures. Electrode gaps with spacings from 200 nm up to5 µm could be bridged by both direct CNT–electrode contact and CNT–CNT entanglement. TheTiN–CNT–TiN and TiN–CNT–CNT–TiN bridges were electrically characterizedwithout any further post-growth contacting. Resistance values as low as40 Ω were measured for the smallest gap spacing and depended mainly on the number andconfiguration of the CNT bridges. The proposed method could be implemented forCNT-based horizontal interconnections and be a route to make different nanoelectronicdevices such as chemical and electromechanical sensors.

Carbon Nanotube Bridge for DNA Transport

Carbon Nanotube Bridge for DNA Transport

Intermediate temperature tribological behavior of carbon nanotube reinforced plasma sprayed aluminum oxide coating

Tribological behavior of plasma sprayed carbon nanotube (CNT) reinforced aluminum oxide (Al 2O 3) composite coatings was examined at room temperature, 573 K and 873 K using tungsten carbide (WC) ball-on-disk tribometer. The weight loss due to wear of Al 2O 3 coating was found to be increasing with the temperature while Al 2O 3-CNT coating showed a decreasing trend in the weight loss with the temperature. Relative improvement in the wear resistance of Al 2O 3-CNT coating compared to Al 2O 3 coating was found to be 12% at room temperature which gradually increased to ∼ 56% at 573 K and ∼ 82% at 873 K. Protective layer as a result of tribo-chemical reaction was observed on the wear track of both of the coatings. The improvement in the wear resistance of Al 2O 3-CNT coating was attributed to three phenomena viz. (i) higher hardness at the elevated temperature as compared to Al 2O 3 coating, (ii) larger area coverage by protective film on the wear surface at the elevated temperature and (iii) CNT bridging between splats. The coefficient of friction (COF) of Al 2O 3 coating was nearly constant at room and elevated temperature whereas COF for Al 2O 3-CNT coating decreased at the elevated temperature (873 K).

Deterministic fabrication of carbon nanotube probes using the dielectrophoretic assembly and electrical detection

We report the controlled dielectrophoretic assembly for the deterministic fabrication of carbon nanotube (CNT) probes. Electrical detection was applied to the dielectrophoretic assembly of CNT probes. Dielectrophoretic manipulation with an ac electric field of 5 MHz was used to form the CNT bridge across oppositely aligned tungsten tips (W-tips). A dc electric field was simultaneously applied to monitor the direct current flowing through the gap. The detected nanocurrent reveals that the CNT bridge is formed between W-tips in real time. We compared current data with bundle diameter of CNT probes in field emission scanning electron microscopy (FE-SEM) images. As the number of assembled CNTs increased, current was increased. With the obtained linear relationship, the number of the attached CNTs can be estimated without confirmation of the FE-SEM image. This combined use of the current detection method with dielectrophoretic manipulation will provide a reliable process for the fabrication of CNT probes.

Synthesis of aluminum oxide coating with carbon nanotube reinforcement produced by chemical vapor deposition for improved fracture and wear resistance

Chemical vapor deposition (CVD) was used to achieve a homogeneous dispersion of carbon nanotubes (CNTs) on aluminum oxide (Al 2O 3) powder. This powder was plasma sprayed onto a steel substrate to produce a 96% dense Al 2O 3 coating with CNT reinforcement. Addition of 1.5 wt.% CNTs showed a 24% increase in the relative fracture toughness of the composite coating. The improvement in the fracture toughness is attributed to uniform dispersion of CNTs and toughening mechanism such as CNT bridging, crack deflection and strong interaction between CNT/Al 2O 3 interfaces. Wear and friction behavior of the CNT reinforced Al 2O 3 coating under dry sliding condition was investigated by ball-on-disk tribometer. With the increasing normal loads from 10 to 50 N, the wear volume loss and coefficient of friction of the coating increased, owing to transition from the mild to severe wear. Wear resistance of the Al 2O 3–CNT composite coating improved by ∼27% at 50 N. Coefficient of friction at 50 N was dependent on the competing phenomena of wear debris generation and graphitization due to pressure.

Continuum Modeling and Analysis of the Frictional Interaction Between a CNT and a Substrate During Dragging

A simple method to determine the frictional interaction between a carbon nanotube (CNT) and a substrate is analyzed for feasibility. In this technique an atomic force microscope (AFM) tip is used to drag a CNT along a substrate. Then the deformed shape of the CNT can be viewed either with the AFM or in a scanning electron microscope. An analysis of the steady-state deformed shape allows the determination of the frictional interactions, which occurred during dragging. It is important to quantify these interactions in a variety of potential applications of nanotechnology. In one such example, a CNT based nanoswitch consists of a CNT bridging over a trench. Actuation of the CNT causes it to stretch and can lead to partial slip at the interface. This slip causes hysteresis, which has been observed in the mechanical actuation of a CNT bridge. In this paper continuum level modeling of the frictional interaction is used to determine the relationship between the steady-state deformed shape of the CNT and the frictional interaction, which occurred between the CNT and substrate during dragging. The model and analysis indicate that this method should be feasible for CNTs with aspect ratios approximately in the 100–250 range.

Improvement of flexure strength and fracture toughness in alumina matrix composites reinforced with carbon nanotubes

In this study, alumina matrix composites reinforced with carbon nanotubes (CNTs) were fabricated by CNT purification, mixing, compaction, and sintering processes, and their relative density, electrical resistance, hardness, flexure strength, and fracture toughness were evaluated. 0–3 vol.% of CNTs was relatively homogeneously distributed in the composites, although some pores existed. The three-point bending test results indicated that the flexure strength increased with increasing volume fraction of CNTs, and reached the maximum when the CNT fraction was 1.5 vol.%. The fracture toughness increased as the CNT fraction increased, and the fracture toughness of the composite containing 3 vol.% of CNTs was higher by 40% than that of the monolithic alumina. According to the observation of the crack propagation path after the indentation fracture test, a new toughening mechanism of grain interface bridging-induced CNT bridging was suggested to explain the improvement of fracture toughness.

Electrical and optical properties of carbon nanotube/polypyrrole addressable intra-connects

Carbon nanotube (CNT) intra-connects (bridges spanning across in-plane electrodes) were electroplated with polypyrrole (PPy), an electrically conductive polymer (ECP). Sharp metal electrodes initiated the CNT growth at pre-selected locations. The CNT bridge was then used as an electrode for conductive polymer electro-deposition. The samples were characterized by Raman spectroscopy and current–voltage measurements. We found that current–gate voltage ( I ds– V gs) characteristics changed dramatically for the electroplated structures when the polymer exceeded a threshold thickness, in the order of 80 nm. In addition, the CNT/PPy structures exhibited large sensitivity to UV radiation: the current substantially reduced upon irradiation with moderate UV intensity values.

Three-dimensional architecture of carbon nanotube-anchored polymer nanofiber composite

Carbon nanotube (CNT)-anchored polymer nanofiber mats were designed by in situ spraying of carbon nanotubes while simultaneously electrospinning polymer nanofibers. The improved conductivity of the composite mat, high catalytic current (3400 mA/cm2/mg Pt) and long term stability of the composite mat was attributed to the efficient formation of CNT bridges between nanofibers.

Multiscale wear of plasma-sprayed carbon-nanotube-reinforced aluminum oxide nanocomposite coating

A multiple length scale wear study of plasma-sprayed Al 2O 3–carbon nanotube (CNT) nanocomposite coating is described. CNT content and dispersion have been shown to enhance the macro-wear resistance (pin-on-disk) by more than 49 times, and nano-wear (scratch) resistance up to 18 times. CNTs have been shown to reduce the wear of Al 2O 3 matrix by (i) increasing densification, (ii) CNT bridging and (iii) CNT lubrication. A strong dependence of wear volume loss on the microstructural features is described at different length scales, and the disparity of material loss observed in the macro- and nano-wear tests is explained. A wear model has been established to numerically quantify wear loss dependence in terms of bulk material properties and correlating these with wear parameters from nano-scratch testing.

Relation between the viscoelastic and flammability properties of polymer nanocomposites

Previous work has shown that the formation of a network structure of nanoparticles within a polymer matrix can significantly reduce nanocomposite flammability and that viscoelastic properties could be utilized to predict their flammability reduction. The present work extends this type of investigation to the study of clay and carbon nanotube nanocomposites. In particular, we study PS/clay, PS/MWNT, PMMA/clay, and PMMA/SWNT nanocomposites. At a clay level of about 10% by mass, the network structure is formed for the PS and the PMMA clay nanocomposites; it requires a level of about 0.5% with the SWNT and 2% with the MWNT. These samples showed significantly reduced mass loss rates of PS and PMMA. However, the solid residues collected from radiative gasification tests of PS/clay and PMMA/clay showed many small cracks, despite the network formation within the initial sample. This is in contrast to the smooth, continuous residues (no cracks or openings) for PS/MWNT and PMMA/SWNT nanocomposites. The cracks in the clay samples are probably formed due to weaker network at elevated temperatures due to weaker bridging interaction between clay platelets as compared to stronger network resulting from dense entanglement and bridging of carbon nanotubes.

Joining prepreg composite interfaces with aligned carbon nanotubes

An interlaminar reinforcement using aligned carbon nanotubes (CNTs) is demonstrated for prepreg unidirectional carbon tape composites. Aligned CNTs are grown at high temperature and then transfer-printed to prepreg at room temperature, maintaining CNT alignment in the through-thickness direction. In initial testing, the CNT-modified interface is observed to increase fracture toughness 1.5–2.5X in Mode I, and 3X in Mode II. Both compliant interlayer and bridging are considered as mechanisms of toughening, with evidence of CNT bridging observed in fracture micrographs. Fabrication methods are compatible with existing manufacturing processes and have the potential to enhance the structural and multifunctional properties of advanced composite laminates.

Wetting of carbon nanotubes by aluminum oxide

Most ceramic–carbon nanotube (CNT) composite processing utilizessolid state sintering, hence the concept of wetting of CNTs by moltenceramic is absolutely new. In the present work on a plasma sprayedAl2O3–CNT nanocomposite, it is observed that moltenAl2O3 spreads uniformly on the CNT surface by forming a thin(∼20–25 nm) ceramic layer without any cracks. The wettability of theAl2O3–CNT system is associated with the surface tension and capillary forces as captured from theevolution of microstructure. The dynamic equilibrium between melting and solidification ofAl2O3 was deduced from the meniscus height, curvature, contact perimeter and projection area of solidifiedAl2O3 onthe CNT surface. This interfacial phenomenon illuminates the mechanisms of microstructure evolutionfrom Al2O3-coated CNT bridge structures to CNT mesh formation. Consequent ab initio modeling depicteddistorted iso-surface contours at the interface, suggesting partial bonding and good wettability ofAl2O3 on the CNT surface.