Flow induced orientation of multiwalled carbon nanotubes in polycarbonate nanocomposites: Rheology, conductivity and mechanical properties

We investigated the effect of flow field and deformation rate on the nanotube alignment and on the properties of PC/multiwalled carbon nanotube nanocomposites. Samples of various MWCNT loadings were prepared by diluting a commercial masterbatch containing 15 wt% nanotubes using optimized melt mixing conditions. Different processing conditions were then used to systematically change the degree of nanotube alignment, from random orientation to highly aligned. Morphological studies and Raman spectroscopy analysis revealed that the nanotubes are preferentially aligned in the flow direction, particularly at large injection or compression rates. Rheological measurements corresponding to high shear rate conditions showed drastic changes in the viscoelastic behavior. The complex viscosity significantly decreased and percolation threshold notably rose. High degrees of nanotube alignment also resulted in a significant increase in the electrical percolation threshold. The mechanical properties of the nanocomposites for different nanotube loadings were also shown to depend on the processing conditions, and somehow improved when the material was processed at higher rates. Finally, we used a power-law type equation to correlate the percolation behavior and the nanotube alignment. The estimated percolation threshold and the power index, q, significantly increase with the degree of nanotube alignment as determined by Raman analysis.     

Magnetically processed carbon nanotube/epoxy nanocomposites: Morphology, thermal, and mechanical properties

The processing-structure-property relationships of multiwalled carbon nanotubes (MWNTs)/epoxy nanocomposites processed with a magnetic field have been studied. Samples were prepared by dispersing the nanotube in the epoxy and curing under an applied magnetic field. The nanocomposite morphology was characterized with Raman spectroscopy and wide angle X-ray scattering, and correlated with thermo-mechanical properties. The modulus parallel to the alignment direction, as measured by dynamic mechanical analysis, showed significant anisotropy, with a 72% increase over the neat resin, and a 24% increase over the sample tested perpendicular to the alignment direction. A modest enhancement in the coefficient of thermal expansion (CTE) parallel to the alignment direction was also observed. These enhancements were achieved even though the nanotubes were not fully aligned, as determined by Raman spectroscopy. The partial nanotube alignment is attributed to resin a gel time that is faster than the nanotube orientation dynamics. Thermal conductivity results are also presented.     

Understanding parameters affecting field emission properties of directly spinnable carbon nanotube webs

The electron field emission (FE) properties of highly aligned carbon nanotube webs (CNTWs) spun directly from carbon nanotube forests are elucidated in this study. By controlling the synthesis parameters, a series of CNTWs with different structural properties are synthesized and the effect of web areal density and the length of the constituent nanotubes on the field emission property studied. An empirical/analytical factor (Tip Factor, T) is developed which relates the structural properties of the web to their effect on the effective concentration of free nanotube ends, and hence on FE. The validity of T as a measure of tip concentration is further demonstrated by measuring the pull-off adhesive forces using an AFM-based technique. Both FE and mechanical adhesion are linearly related to T. These results suggest that in order to achieve the highest field emission or dry adhesion webs, features desired include short nanotubes, with dense and even coverage of the surface.     

Selective Deposition and Alignment of Single-Walled Carbon Nanotubes Assisted by Dielectrophoresis: From Thin Films to Individual Nanotubes

Dielectrophoresis has been used in the controlled deposition of single-walled carbon nanotubes (SWNTs) with the focus on the alignment of nanotube thin films and their applications in the last decade. In this paper, we extend the research from the selective deposition of SWNT thin films to the alignment of small nanotube bundles and individual nanotubes. Electrodes with “teeth”-like patterns are fabricated to study the influence of the electrode width on the deposition and alignment of SWNTs. The entire fabrication process is compatible with optical lithography-based techniques. Therefore, the fabrication cost is low, and the resulting devices are inexpensive. A series of SWNT solutions is prepared with concentrations ranging from 0.0125 to 0.2 mg/ml. The alignment of SWNT thin films, small bundles, and individual nanotubes is achieved under the optimized experimental conditions. The electrical properties of these samples are characterized; the linear current–voltage plots prove that the aligned SWNTs are mainly metallic nanotubes. The microscopy inspection of the samples demonstrates that the alignment of small nanotube bundles and individual nanotubes can only be achieved using narrow electrodes and low-concentration solutions. Our investigation shows that it is possible to deposit a controlled amount of SWNTs in desirable locations using dielectrophoresis.     

Films and fibers of oriented single wall nanotubes

As-produced nanotubes form a light, fragile and isotropic soot. Different efforts are made to process nanotubes into macroscopic forms of more practical use and more controlled properties. We briefly review in this paper two methods recently proposed to make films of magnetically aligned nanotubes and fibers by using an electrophoretic method. Preferential orientation of the nanotubes in the plane of the films or along the fiber axis is an important feature of the obtained materials. Then we describe in details a different, spinning like, process for making fibers out of single wall carbon nanotubes. This process consists of dispersing the nanotubes in a surfactant solution, re-condensing the nanotubes in the flow of a polymer solution to form a nanotube mesh, and then collating this mesh to a nanotube fiber. The behaviors of the surfactant-stabilized dispersions, which are also presented, are critical for this process. The degree of nanotubes alignment in dried fibers has been characterized by X-ray scattering. It is found to be smaller than the alignment obtained in the previous materials. However, the processing is simpler and faster and potentially scalable for large-scale production.     

The effect of carbon nanotube orientation on erosive wear resistance of CNT-epoxy based composites

Aligned carbon nanotube (CNT) polymer composites are envisioned as the next-generation composite materials for a wide range of applications. In this work, we investigate the erosive wear behavior of epoxy matrix composites reinforced with both randomly dispersed and aligned carbon nanotube (CNT) arrays. The aligned CNT composites are prepared in two different configurations, where the sidewalls and ends of nanotubes are exposed to the composite surface. Results have shown that the composite with vertically aligned CNT-arrays exhibits superior erosive wear resistance compared to any of the other types of composites, and the erosion rate reaches a similar performance level to that of carbon steel at 20° impingement angle. The erosive wear mechanism of this type of composite, at various impingement angles, is studied by Scanning Electron Microscopy (SEM). We report that the erosive wear performance shows strong dependence on the alignment geometries of CNTs within the epoxy matrix under identical nanotube loading fractions. Correlations between the eroded surface roughness and the erosion rates of the CNT composites are studied by surface profilometry. This work demonstrates methods to fabricate CNT based polymer composites with high loading fractions of the filler, alignment control of nanotubes and optimized erosive wear properties.     

A comparative study of melt spun polyamide-12 fibres reinforced with carbon nanotubes and nanofibres

A range of multi-wall carbon nanotubes and carbon nanofibres were mixed with a polyamide-12 matrix using a twin-screw microextruder, and the resulting blends spun to produce a series of reinforced polymer fibres. The aim was to compare the dispersion and resulting mechanical properties achieved for nanotubes produced by the electric arc and a variety of chemical vapour deposition techniques. A high quality of dispersion was achieved for all the catalytically-grown materials and the greatest improvements in stiffness were observed using aligned, substrate-grown, carbon nanotubes. The use of entangled multi-wall carbon nanotubes led to the most pronounced increase in yield stress, most likely as result of increased constraint of the polymer matrix due to their relatively high surface area. The degrees of polymer and nanofiller alignment and the morphology of the polymer matrix were assessed using X-ray diffraction and differential scanning calorimetry. The carbon nanotubes were found to act as nucleation sites under slow cooling conditions, the effect scaling with effective surface area. Nevertheless, no significant variations in polymer morphology as a function of nanoscale filler type and loading fraction were observed under the melt spinning conditions applied. A simple rule-of-mixture evaluation of the nanocomposite stiffness revealed a higher effective modulus for the multi-wall carbon nanotubes compared to the carbon nanofibres, as a result of improved graphitic crystallinity. In addition, this approach allowed a general comparison of the effective nanotube modulus with those of nanoclays as well as common short glass and carbon fibre fillers in melt-blended polyamide composites. The experimental results further highlight the fact that the intrinsic crystalline quality, as well as the straightness of the embedded nanotubes, are significant factors influencing the reinforcement capability.     

Thermo-electrical properties of PVA-nanotube composite fibers

We present in this work an experimental study of the resistivity of composite nanotube fibers made of polyvinyl alcohol and multiwalled carbon nanotubes. These fibers which exhibit exceptional mechanical properties could be used for new conductive and multifunctional textiles or composites. We report on their electrical properties and draw two main conclusions: (i) when the fibers contain a large fraction of amorphous polymer, a substantial decrease of the resistivity is observed in the vicinity of the glass transition temperature (T_g) of the pure PVA. On the basis of X-ray diffraction characterizations, we believe that this behavior results from the relaxation of stress in the polymer-nanotube composite. Slight structural modifications and partial loss of nanotube alignment at T_g could yield an increase of the density of intertube contacts and thereby to a decrease of the electrical resistivity. (ii) Annealing the fibers at high temperature reduces the fraction of amorphous PVA which becomes more crystalline. As a result, the conductivity becomes more stable and does not exhibit any abrupt variation at T_g. Instead the conductivity is non-metallic with an effective semi-conductor type behavior as observed in other nanotube composites or even in pure nanotube assemblies.     

Iron filled carbon nanotubes grown on substrates with thin metal layers and their magnetic properties

The thermal decomposition of ferrocene combined with a catalyst-assisted structuring of a Si-substrate surface is a favourable way to produce Fe-filled carbon nanotubes in good quality and in high yields. In this work we have studied the growth of such aligned filled nanotubes on iron and cobalt pre-coated Si-substrates and their dependence on the deposition time. The nanotube diameter depends on the used catalyst metal on the substrate surface. Magnetization measurements were carried out perpendicular (along tube axis) and parallel to the substrate and show excellent coercivities, a strong uniaxial anisotropy (ratios of H_c,per/H_c,par up to 6) and high saturation magnetization moments per substrate square. The magnetic behavior has been also interpreted as a function of deposition time and of the catalyst metal on the substrate. These investigations were complemented by X-ray diffraction, which revealed a majority fraction of α-Fe and a small amount of Fe₃C.     

定向碳纳米管填充橡胶复合材料的制备

采用传统的橡胶工艺制备了高取向碳纳米管(CNTs)填充5%和30%的橡胶复合材料。碳纳米管的取向可能是在球磨的优化过程中由拖曳剪切力引起的。与纯橡胶相比,碳纳米管的选择性取向使其弹性模量能都得到了提高。     

A method of mobilizing and aligning carbon nanotubes and its use in gel spinning of composite fibres

We report a method of improving the alignment of carbon nanotubes (CNTs) in a solid strand prepared from a CNT forest without dispersing and regrouping them. The use of a lubricant loosens the interconnections between the carbon nanotubes so that they can slide relative to each other and align themselves longitudinally. A highly stretchable polymer gel fibre is used as a carrier to apply the lubricant to a continuous length of the carbon nanotube strand so that the strand can be drawn uniformly and continuously by a series of drawing operations. The lubricant is then extracted to produce a highly aligned carbon nanotube reinforced polymer composite fibre that demonstrates a remarkable reinforcement effect. The CNT strength utilisation in the composite fibre is one order of magnitude greater than that in a pure carbon nanotube spun yarn.     

Electric field-induced aligned multi-wall carbon nanotube networks in epoxy composites

CVD-grown multi-wall carbon nanotubes were dispersed as an electrically conductive filler in an epoxy system based on a bisphenol-A resin and an amine hardener. The application of both AC and DC electric fields during nanocomposite curing was used to induce the formation of aligned conductive nanotube networks between the electrodes. The network formation process and resulting network structure were evaluated by in situ optical microscopy and current density measurements as a function of curing time. Parameters such as field strength and nanotube weight fraction were varied. The carbon nanotube agglomeration mechanism was dominated by the electric field-induced forces acting on the nanotubes, which have a negative surface charge after processing in the epoxy. The network structure formed in AC fields was more uniform and more aligned compared to that in DC fields. The specific bulk composite conductivity of fully processed composite samples reflected the differences in the nanotube network structure. Perhaps surprisingly, the network efficiency was not enhanced by this processing method, although the approach does offer the possibility of achieving bulk conductive nanotube-polymer composites with anisotropic electrical properties and a degree of optical transparency.     

Immobilization of pamidronic acids on the nanotube surface of titanium discs and their interaction with bone cells

Self-assembled layers of vertically aligned titanium nanotubes were fabricated on a Ti disc by anodization. Pamidronic acids (PDAs) were then immobilized on the nanotube surface to improve osseointegration. Wide-angle X-ray diffraction, X-ray photoelectron microscopy, and scanning electron microscopy were employed to characterize the structure and morphology of the PDA-immobilized TiO₂ nanotubes. The in vitro behavior of osteoblast and osteoclast cells cultured on an unmodified and surface-modified Ti disc was examined in terms of cell adhesion, proliferation, and differentiation. Osteoblast adhesion, proliferation, and differentiation were improved substantially by the topography of the TiO₂ nanotubes, producing an interlocked cell structure. PDA immobilized on the TiO₂ nanotube surface suppressed the viability of the osteoclasts and reduced their bone resorption activity.     

Production of controlled architectures of aligned carbon nanotubes by an injection chemical vapour deposition method

High purity, aligned multi-wall carbon nanotube films were grown on quartz substrates by injecting a solution of ferrocene in toluene into a suitable reaction furnace. The injection CVD method allows excellent control of the catalyst to carbon ratio. The detailed study presented here demonstrates how such a system can be used to control the nanotube diameter, length, alignment and yield by manipulating the experimental parameters. Primary growth was found to occur via a base growth mechanism, although overgrowths of single wall carbon nanotubes were obtained under certain conditions. Such a method also allows nanotubes of various packing densities to be produced which may be useful for specific applications such as electrodes.     

The effect of carbon nanotube properties on the degree of dispersion and reinforcement of high density polyethylene

The efficacy with which a range of nanotubes could reinforce a high density polyethylene (HDPE) matrix was investigated, in relation to nanotube diameter, purity, functionalization, alignment and nanotube bulk density. Composites were prepared by melt blending multiwall carbon nanotubes (MWNTs) with high density polyethylene (HDPE), followed by the injection molding of tensile specimens. At a 5 wt% loading, the most effective nanotubes were those of large diameter, received in an aligned form with low bulk density, producing a 66% increase in elastic modulus and a 69% improvement in yield stress. This was contradictory to theoretical mechanics calculations that predicted an increasing degree of reinforcement for nanotubes of reduced diameter. This difference was explained by the higher degree of dispersion observed in the composites with MWNTs of greater diameter.     

Growth of multiwalled carbon nanotubes during the initial stages of aerosol-assisted CCVD

We report a study of the initial stages of growth of aligned multiwalled carbon nanotubes (MWNT) synthesised by catalytic chemical vapour deposition (CCVD) of liquid aerosol obtained from toluene/ferrocene solution. A special experimental procedure has been developed to stop the process after short durations (30 s to 2 min). Two different pyrolysis temperatures are considered: 800 and 850 °C. Both scanning and transmission electron microscopy (SEM, TEM) coupled to energy-dispersive X-ray (EDX) analyses are used in order to determine the location of catalyst particles and to examine their chemical nature, morphology and size distribution when nanotubes start to grow. During the early stages (30 s), we observe the formation of a layer of catalyst particles on silicon substrates before the growth of nanotubes. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements indicate the occurrence of iron oxide (γ-Fe₂O₃ or Fe₃O₄). In addition, XPS analysis reveals the formation of graphite-like carbon, demonstrating that iron oxide particles catalyse the decomposition of toluene vapour. SEM and TEM observations show that these particles are most often located at the nanotube root, suggesting a base growth mechanism responsible for the formation of aligned nanotube when prolonging growth time (2 min).     

Carbon nanotube-nanoporous anodic alumina composite membranes with controllable inner diameters and surface chemistry: Influence on molecular transport and chemical selectivity

This work presents the fabrication of carbon nanotube composite membranes with controllable nanotube dimensions (inner diameters and lengths) and surface chemistry and explores their influence on the transport properties and chemical based transport selectivity. These membranes were prepared by growing of vertically aligned multiwalled carbon nanotubes (MWCNTs) inside nanoporous anodic alumina membranes (NAAMs) through a catalyst-free chemical vapour deposition (CVD) approach. The deposition time during CVD process and the length of NAAMs were used to control nanotube dimensions. The thermal annealing and wet and dry oxidation processes were used to control the surface chemistry of inner walls of nanotubes from highly graphitic-hydrophobic to oxygen rich and hydrophilic. The structural features and chemical composition of the prepared membranes are characterised by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The influence of the nanotube dimensions and surface chemistry on molecular transport properties of prepared membranes are assessed by analysing the transport of two models molecules with different hydrophilic–hydrophobic and charge properties. The obtained results reveal that the diffusional flux of model molecules through CNTs-NAAMs can be controlled by nanotube dimensions and surface chemistry of graphitic surface and these parameters can be used to tailor their chemical based molecular separation for specific applications.     

Shear-induced preferential alignment of carbon nanotubes resulted in anisotropic electrical conductivity of polymer composites

Multiwalled carbon nanotubes were used as filler to furan resin in the aim of producing an electrically conducting polymer composite that may be useful for electrode applications. The orientation of the nanotubes is controlled to prepare a composite with fillers unidirectionally oriented, which may result in higher electrical conductivity at one direction and at lower nanotube loading. Using the doctor blade technique, composite films were prepared and the alignment and its effect on the electrical conductivity of the composite were investigated. It was found that the doctor blade technique induced preferential alignment of the nanotubes in composite and a higher degree of alignment is achieved in composites with lower contents of nanotubes. Also, for low contents of nanotubes, the electrical conductivity of the composite with preferentially aligned nanotubes was up to a million times higher in the direction of alignment compared to that of the composite with randomly oriented nanotubes; however, at higher contents of nanotubes, this effect was diminished. The preferential alignment of the nanotubes also caused anisotropic electrical conductivity. The alignment and distribution is thought to create more junctions between nanotubes that resulted into the formation of more conducting channels in the polymer matrix parallel to blading direction.     

Polypropylene/carbon nanotube nanocomposite fibers: Process–morphology–property relationships

This study is focused on aligning carbon nanotubes in polypropylene matrix by melt spinning. Two different weight percentages (0.5% and 1.0%) of nanotubes were used for the synthesis of the nanocomposite fibers. The effect of the nanotubes on the crystallization and mechanical behavior of polypropylene as well as the effect of draw ratio on the nanocomposite morphology and properties is also discussed. Correlation of fiber morphology and nanotube alignment was done using differential scanning calorimetry, wide‐angle X‐ray diffraction, and transmission electron microscopy. Significant improvement in tensile modulus and tensile strength were observed, which is characteristic of a highly aligned nanotube system. A substantial vincrease in the onset of decomposition was observed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3844–3850, 2007     

Iron nanoparticles in aligned arrays of pure and nitrogen-doped carbon nanotubes

Arrays of aligned carbon nanotubes (CNTs) and nitrogen-doped carbon (CN_x) nanotubes have been grown on silicon substrates as the result of thermolysis of ferrocene/toluene and ferrocene/acetonitrile mixture. The microstructure of materials was studied by transmission and scanning electron microscopy, and X-ray diffraction was used to control the carbon and iron forms. The composition and properties of iron nanoparticles developed in the CNT and CN_x nanotube samples were determined from Mössbauer spectroscopy data. The total iron content in CN_x nanotubes was found to be considerably higher than that in CNTs. Three forms of iron nanoparticles α-Fe, γ-Fe, and Fe₃C were detected in CNTs and only two last of them in CN_x nanotubes. In the interior of CNT channels the α-Fe and Fe₃C nanoparticles were observed to be coupled by a strong exchange interaction and to exhibit magnetic behavior at room temperature.