Micropatterning of single-walled carbon nanotube forest

In this work, high-aligned single-walled carbon nanotube (SWCNT) forest have been grown using a high-density plasma chemical vapor deposition technique (at room temperature) and patterned into micro-structures by photolithographic techniques, that are commonly used for silicon integrated circuit fabrication. The SWCNTs were obtained using pure methane plasma and iron as precursor material (seed). For the growth carbon SWCNT forest the process pressure was 15 mTorr, the RF power was 250 W and the total time of the deposition process was 3 h. The micropatterning processes of the SWCNT forest included conventional photolithography and magnetron sputtering for growing an iron layer (precursor material). In this situation, the iron layer is patterned and high-aligned SWCNTs are grown in the where iron is present, and DLC is formed in the regions where the iron precursor is not present. The results can be proven by Scanning Electronic Microscopy and Raman Spectroscopy. Thus, it is possible to fabricate SWCNT forest-based electronic and optoelectronic devices.     

Size effect of X-shaped carbon nanotube junctions

Systematic calculations have been performed for X-shaped junctions formed between two crossed identical carbon nanotubes (armchair or zigzag tubes) with diameters ranging from 3.92 Å to 9.49 Å using an empirical potential method. The formation energy of X junctions is found to be a function of the diameter of the tubes. The formation temperature is dependent on the curvature of tubes. The calculations show that it is energetically favorable to form X junctions by the direct heating method if the tube diameter is less than 0.85 nm.     

Effect of agglomerate properties on agglomerate stability in fluidized beds

Fluidized bed agglomeration is used to stabilize particulate mixtures and reduce dust emissions. This technology is applied to a variety of production processes for the pharmaceutical, chemical, fertilizer and food industries. In most of these applications, agglomerate stability is an essential criterion. Agglomerates and granules that do not conform to size and shape specifications may create problems in downstream processes, such as tableting, thus compromising process efficiency and product quality. When an agglomerate is formed in a fluidized bed, it can grow by incorporating other bed particles, split into smaller fragments, or be eroded by fluidized bed solids. The objective of the present study is to determine the critical agglomerate liquid content at which the rates of agglomerate growth and shrinkage are balanced when artificial agglomerates made from glass beads and water are introduced into a fluidized bed. This study examined the effects of agglomerate size, agglomerate density, liquid viscosity, binder concentration, and fluidizing gas velocity on the critical initial liquid content. This study found that small agglomerates and low density agglomerates displayed higher critical initial moisture contents. When the viscosity was increased by using sugar solutions, agglomerates were very stable and had very low critical initial moisture contents. The study also found that as the superficial gas velocity increased, the agglomerates started to fragment, rather than erode.     

Multi-parameter optimisation of carbon nanotube synthesis in fluidised-beds

Multi-walled carbon nanotubes (MWCNTs) were synthesised on alumina supported Fe catalysts contained within a fluidised-bed, using ethylene gas as a carbon source. The influence of temperature, deposition time, feedstock concentration, fluidisation ratio, and their interactions was investigated using a statistical design methodology to give a static global optimisation of the multi-dimensional nanotube formation space. The optimum synthesis conditions, determined from these experiments, were highly variable. The maximum MWCNT yield for 10% ethylene was found to occur at a flowrate of 6 SLPM and temperature of 850 °C. For 25% ethylene it was between 3 and 4.5 SLPM at 650 °C. However, CNTs with a higher degree of graphitisation were favoured at higher temperatures and shorter deposition times. The fluidisation ratio played a significant role in determining the overall conversion rate.     

Tensile properties of ultrathin double-walled carbon nanotube membranes

Direct tensile tests of double walled carbon nanotube (DWCNT) membranes with thickness of 40–80 nm were performed using a micro-stress-strain puller. The tensile strength and Young’s modulus are 4.8E2–8.4E2 MPa and 4.4–8.8 GPa, respectively. The deformation and fracture processes were analyzed using the stress vs. strain curves, and SEM observations of the fracture surface of a membrane. The membrane experienced elastic strain and plastic strain during tensile-loading to fracture, and the plastic process is due to the real plastic deformation of the membrane and the slippage between the DWCNT bundles. Cracks occur and spread during the tensile test which causes the membrane to be mangled. With these excellent mechanical properties, the DWCNT membranes can be used in nanotube-reinforced composites.     

An experimental and theoretical investigation of the compressive properties of multi-walled carbon nanotube/poly(methyl methacrylate) nanocomposite foams

Polymer nanocomposite foams are promising low density substitutes for nanocomposites. Carbon nanotube/polymer nanocomposite foams possess high strength, low density, and can be made conductive. Good control of foam properties is of great importance in the application of such materials. In the current study, multi-walled carbon nanotubes (MWNTs) with controlled aspect ratio were used to alter the foam morphology in MWNT/poly(methyl methacrylate) (PMMA) nanocomposite foams produced by a supercritical carbon dioxide (CO₂) foaming process. It was found that with the addition of one weight percent of MWNTs, the Young’s modulus of polymer foams increased by as much as 82%, and the collapse strength increased by as much as 104%. The influence of MWNT aspect ratio on the compressive properties of nanocomposite foams was investigated. The addition of MWNTs influenced the foam properties in two ways: improving the compressive properties of the solid matrix, and reducing the bubble size of the nanocomposite foams. A modified constitutive model for predicting the compressive properties of high density closed-cell polymer foams was developed. The influence of the bubble size on the mechanical properties of polymer foams was discussed based on the new model.     

Failure investigation of carbon nanotube/3Y-TZP nanocomposites

3Y-TZP matrix composites containing 0.1–1 wt.% of multi-wall (MWCNT) and single-wall (SWCNT) carbon nanotubes were fabricated by spark plasma sintering technique. The sintered composites reached full density. Hardness and fracture toughness were measured using Vickers indention method. The hardness of the composite decreased with increasing weight content of the MWNT. The fracture toughness was 5.52 MPa m^0.5 when the amount of MWCNTs was 0.5 wt.%, however it decreased to 4.5 MPa m^0.5 when the content was raised to 1.0 wt.%. The composite containing 0.5 wt.% SWCNTs showed similar fracture toughness as that of matrix. The incorporation of CNTs into 3Y-TZP matrix led to no prominent improvement on the mechanical properties. The failure mechanism was analyzed finally.     

Fabrication and electrochemical properties of carbon nanotube film electrodes

Carbon nanotube (CNT) film electrodes were fabricated by a novel process involving the electrostatic spray deposition (ESD) of a CNT solution. Acid treated CNTs were dispersed in an aqueous solvent through sonication and then the CNT solution was electrostatically sprayed onto a metallic substrate by the ESD method. The CNT film electrodes showed well-entangled and interconnected porous structures with good adherence to the substrate. A specific capacitance of 108 F/g was achieved for the electrodes in 1 M H₂SO₄. In addition, the CNT film electrode showed good high rate capability.     

Experimental investigation of dispersion during flow of multi-walled carbon nanotube/polymer suspension in fibrous porous media

We prepared three types of multi-walled carbon nanotubes (MWNT)/vinyl ester resin suspensions with different degrees of initial MWNT dispersion. Each suspension was injected into a mold cavity to saturate a stationary random glass fiber preform. High shear rates were not encountered in the non-uniform porous media. The quality of dispersion of the MWNT caused by the interaction between the MWNT with glass fiber media was characterized by examining the sections of the cured composites using scanning electron microscope (SEM) and transmission electron microscope (TEM). The results revealed that the final MWNT/Glass fiber structure is a strong function of the initial state of the MWNT dispersion in the suspension and the porous media structure.     

Multiwall carbon nanotube elastomeric composites: A review

Nanostructured materials gained great importance in the past decade on account of their wide range of potential applications in many areas. A large interest is devoted to carbon nanotubes that exhibit exceptional electrical and mechanical properties and can therefore be used for the development of a new generation of composite materials. Nevertheless, poor dispersion and poor interfacial bonding limit the full utilization of carbon nanotubes for reinforcing polymeric media.In this paper, recent advances on carbon nanotubes and their composites will be presented through results of the author's research, essentially based on filled elastomeric networks. The intrinsic potential of carbon nanotubes as reinforcing filler in elastomeric materials will be demonstrated. It will be shown that, despite a poor dispersion, small filler loadings improve substantially the mechanical and electrical behaviors of the soft matrix. With the addition of 1 phr of multiwall carbon nanotubes in a styrene-butadiene copolymer, a 45% increase in modulus and a 70% increase in the tensile length are achieved. Straining effects investigated by atomic force microscopy and infrared and Raman spectroscopies, provide interesting results for the understanding of the mechanical behavior of these nanotube-based composites. All the experimental data lead to the belief that the orientation of the nanotubes plays a major role in the mechanical reinforcement. The strong restriction in equilibrium swelling in toluene with the MWNT content is not ascribed to filler-matrix interfacial interactions but to the occlusion of rubber into the aggregates. On the other hand, carbon nanotubes impart conductivity to the insulator matrix. Between 2 and 4 phr, the conductivity increases by five orders of magnitude reflecting the formation of a percolating network. Changes in resistivity under uniaxial extension completed by AFM observations of stretched composites bring new insights into the properties of these composites by highlighting the contribution of orientational effects.     

Structure and energetics of carbon nanotube ropes

High-resolution electron microscopy observation and energetic analysis have been performed on ropes formed from single-walled carbon nanotubes. When individual nanotubes are twisted, the nanotube ropes become energetically stable—a configuration that also offers better structural stability. Electron microscopic image simulations of an energetically-stable rope composed of seven single-walled carbon nanotubes have been carried out as well to elaborate the salient features that were observed experimentally.     

Simulation of carbon nanotube based p-n junction diodes

Taking advantage of the unique characteristics of an ambipolar carbon nanotube field effect transistor (CNTFET), a ‘p-n junction’ is simulated along the single-walled carbon nanotube channel using two separate gates close to the source and drain of the CNTFET, respectively. The current-voltage characteristics of the double-gated CNTFET are calculated using a semiclassical method based on the Schottky barrier field effect transistor mechanism. The calculation results show a good rectification performance of the p-n junction.     

On the mechanism of piezoresistivity of carbon nanotube polymer composites

Carbon nanotube (CNT) polymer composites exhibit strong nonlinear and asymmetric piezoresistivity about zero strain in tensile and compressive strain states. The existing models explain the characteristic qualitatively but not quantitatively. This paper attempts to understand the mechanisms of this piezoresistivity by developing a new 3-dimensional percolation CNT network model, where the effect of CNT deformation (wall indentation and tube bending) is considered for the first time. The predicted electrical conductivity and piezoresistivity agree with experiments quantitatively, which reveals that the CNT deformation is a dominant mechanism for the nonlinearity and asymmetry of piezoresistivity of CNT-polymer composites. Parametric studies have been conducted to show the effects of morphology and electrical properties of CNTs, work functions and Poisson's ratio of polymer on the piezoresistivity of CNT-polymer composites for future application in nanosensing composites.     

Stress transfer in polyacrylonitrile/carbon nanotube composite fibers

Gel spun polyacrylonitrile/carbon nanotube (PAN/CNT) composite fibers have been produced, and the stress-induced G′ Raman band shifts in the CNTs have been monitored to observe stress transfer during fiber strain. Improvements in CNT quality, CNT dispersion, and post-processing fiber drawing are shown to increase the stress transfer from the matrix to the CNT. Radial breathing mode (RBM) intensity of specific CNT chiralities confirms CNT debundling during fiber processing. During PAN/CNT fiber straining, there reaches a plateau in the CNT G′ downshift, signifying that the stress on the CNT is maintained despite continued straining of the PAN/CNT fiber. Correlating CNT strain with CNT modulus and volume fraction allows for the interfacial shear strength (τ_i) of the PAN-CNT interface to be determined. The as-spun and fully drawn PAN/CNT-A (99/1) nano composite fibers exhibit τ_i of 13.1 and 30.9 MPa, respectively, while an improved CNT dispersion (PAN/CNT-A (99.9/0.1)) results in τ_i equal to 44.3 MPa.     

Focused-ion-beam assisted fabrication of individual multiwall carbon nanotube field emitter

We report the fabrication of an individual carbon nanotube (CNT) electron field emitter using a focused-ion-beam (FIB) technique. The monolithic multiwall CNT with a graphitic shield is synthesized using chemical vapor deposition technique. The FIB technique is applied to attach the monolithic multiwall CNT on an etched tungsten tip. Field emission measurements are carried out in a vacuum of 10⁻⁷ Torr. Threshold voltage as low as 120 V has been obtained.