High Aspect Ratio Of Carbon Nanotubes Research Articles

Fowler Nordheim theory of carbon nanotube based field emitters

Field emission (FE) phenomena are generally explained in the frame-work of Fowler Nordheim (FN) theory which was given for flat metal surfaces. In this work, an effort has been made to present the field emission mechanism in carbon nanotubes (CNTs) which have tip type geometry at nanoscale. High aspect ratio of CNTs leads to large field enhancement factor and lower operating voltages because the electric field strength in the vicinity of the nanotubes tip can be enhanced by thousand times. The work function of nanostructure by using FN plot has been calculated with reverse engineering. With the help of modified FN equation, an important formula for effective emitting area (active area for emission of electrons) has been derived and employed to calculate the active emitting area for CNT field emitters. Therefore, it is of great interest to present a state of art study on the complete solution of FN equation for CNTs based field emitter displays. This manuscript will also provide a better understanding of calculation of different FE parameters of CNTs field emitters using FN equation.

Outstanding adsorption performance of high aspect ratio and super-hydrophobic carbon nanotubes for oil removal

Oil removal from water is a highly important area due to the large production rate of emulsified oil in water, which is considered one of the major pollutants, having a negative effect on human health, environment and wildlife. In this study, we have reported the application of high quality carbon nanotube bundles produced by an injected vertical chemical vapor deposition (IV-CVD) reactor for oil removal. High quality, bundles, super hydrophobic, and high aspect ratio carbon nanotubes were produced. The average diameters of the produced CNTs ranged from 20 to 50 nm while their lengths ranged from 300 to 500 μm. Two types of CNTs namely, P-CNTs and C-CNTs, (Produced CNTs from the IV-CVD reactor and commercial CNTs) were used for oil removal from water. For the first time, thermogravimetric analysis (TGA) was conducted to measure maximum oil uptake using CNT and it was found that P-CNT can take oil up to 17 times their weight. The effect of adsorbent dosage, contact time, and agitation speed were examined on the oil spill clean-up efficiency using batch sorption experiments. Higher efficiency with almost 97% removal was achieved using P-CNTs compared to 87% removal using C-CNTs.

Flexible thin SiC fiber fabrics using carbon nanotube modification for improving electromagnetic shielding properties

In order to obtain a flexible electromagnetic shielding materials with thin thickness and low electromagnetic wave reflection, pyrolytic carbon (PyC) and carbon nanotube (CNT) were in situ formed on the surface of SiC fiber fabrics (SiCf, with the thickness of 0.34mm) through chemical vapor deposition and catalytic chemical vapor deposition to form PyC-SiCf and CNT-SiCf fabrics, separately. CNT-SiCf can maintain the flexibility of SiC fiber fabrics because of the high aspect ratio of CNT, but PyC-SiCf fabrics can not. CNT-SiCf had much lower electrical conductivity than PyC-SiCf because of their higher contact electrical resistance. At the same loading, CNT-SiCf and PyC-SiCf could obtain higher EMI SE (≥15dB), but CNT-SiCf had lower reflection loss (≤6dB) than PyC-SiCf (≥8dB) because CNT-SiCf had much lower electrical conductivity and larger interface area than PyC-SiCf.

Vapor–solid preparation of densely distributed and small-sized graphene nanoflakes on one-dimensional nanomaterials for low-field and highly stable field emission

Differently shaped two-dimensional (2-D) graphene nanoflakes (GNFs) were prepared on one-dimensional (1-D) carbon nanotubes (CNTs) and silicon nanowires (SiNWs) forming 2-D–1-D composites by using microwave plasma enhanced chemical vapor deposition. The GNFs are vertically aligned on 1-D nanomaterials with their sharp edges (less than 10 layers) unfolded outside and are rich in defects. GNF–CNT (or SiNW) composites with densely distributed and small-sized GNFs and not obviously thickened CNTs (or SiNWs) are found to have superior FE properties. The optimal FE performance obtained from the GNF–CNT composites shows a low threshold field of 1.54 V/μm and an extremely large maximum emission current density of 75.46 mA/cm2, far better than 1.77 V/μm and 27.90 mA/cm2 for the pristine CNTs, respectively. The FE improvement is ascribed to the significant increase of active emission sites and also the preservation of the high aspect ratio of CNTs. Furthermore, longtime (30 h) stable FE is achieved from GNF–CNT composites with optimal shapes at a high mean emission current density of 45.47 mA/cm2 and a low operation field of 1.766 V/μm, showing promising prospects in high-performance vacuum electronic device applications.

Carcinogenic Potential of High Aspect Ratio Carbon Nanomaterials.

Engineered nanomaterials, including high aspect ratio carbon nanomaterials, are often commercialized without a complete human risk assessment and safety evaluation. A health concern has been raised that high aspect ratio nanomaterials such as carbon nanotubes may cause unintended health consequences, such as asbestos-like lung cancer and mesothelioma, when chronically inhaled. Considering the widespread industrial and clinical applications and the increasing incidence of human exposure to nanomaterials, it is important to address the issue of nanomaterial carcinogenicity in a timely manner. This review summarizes recent advances in nanomaterial genotoxicity and carcinogenicity with a focus on high aspect ratio carbon nanotubes, and discusses current knowledge gaps and future research directions.

High Aspect Ratio Carbon Nanotube Membranes Decorated with Pt Nanoparticle Urchins for Micro Underwater Vehicle Propulsion via H2O2 Decomposition.

The utility of unmanned micro underwater vehicles (MUVs) is paramount for exploring confined spaces, but their spatial agility is often impaired when maneuvers require burst-propulsion. Herein we develop high-aspect ratio (150:1), multiwalled carbon nanotube microarray membranes (CNT-MMs) for propulsive, MUV thrust generation by the decomposition of hydrogen peroxide (H2O2). The CNT-MMs are grown via chemical vapor deposition with diamond shaped pores (nominal diagonal dimensions of 4.5 × 9.0 μm) and subsequently decorated with urchin-like, platinum (Pt) nanoparticles via a facile, electroless, chemical deposition process. The Pt-CNT-MMs display robust, high catalytic ability with an effective activation energy of 26.96 kJ mol(-1) capable of producing a thrust of 0.209 ± 0.049 N from 50% [w/w] H2O2 decomposition within a compact reaction chamber of eight Pt-CNT-MMs in series.

High performance carbon nanotube--polymer nanofiber hybrid fabrics.

Stable nanoscale hybrid fabrics containing both polymer nanofibers and separate and distinct carbon nanotubes (CNTs) are highly desirable but very challenging to produce. Here, we report the first instance of such a hybrid fabric, which can be easily tailored to contain 0-100% millimeter long CNTs. The novel CNT - polymer hybrid nonwoven fabrics were created by simultaneously electrospinning nanofibers onto aligned CNT sheets which were drawn and collected on a grounded, rotating mandrel. Due to the unique properties of the CNTs, the hybrids show very high tensile strength, very small pore size, high specific surface area and electrical conductivity. In order to further examine the hybrid fabric properties, they were consolidated under pressure, and also calendered at 70 °C. After calendering, the fabric's strength increased by an order of magnitude due to increased interactions and intermingling with the CNTs. The hybrids are highly efficient as aerosol filters; consolidated hybrid fabrics with a thickness of 20 microns and areal density of only 8 g m(-2) exhibited ultra low particulate (ULPA) filter performance. The flexibility of this nanofabrication method allows for the use of many different polymer systems which provides the opportunity for engineering a wide range of nanoscale hybrid materials with desired functionalities.

Novel vertical spinning preparation of free-standing carbon nanotube–polyaniline composite films with high electrical conductivity

A novel vertical spinning (VS) process is reported for fabricating free-standing carbon nanotube (CNT)–polyaniline (PANI) composite films with high electrical conductivity (EC). In the VS process, a high centrifugal force is exerted on the PANI solution containing CNTs to achieve a uniform dispersion of CNTs in the composites; meanwhile, the high aspect ratio of CNTs is greatly maintained to a high degree (75%). These contribute to the high EC of the CNT–PANI films since the EC benefits from both the good dispersion and the high aspect ratio of CNTs. The EC of the as-prepared composite film with the 10 wt% CNTs has a value above 150 S cm−1, which is much higher than that at the same CNT content reported previously in the literature. For the purpose of comparison, the CNT–PANI composite films were prepared via the commonly used plane casting process. Moreover, the sandwich-structured PANI–CNT–PANI composite films were synthesized via the VS process to illustrate another advantage of this process in manipulating the distribution of fillers in the composites. The CNT–PANI composite films prepared by the VS process show the highest EC. The sandwiched structure shows a reasonably high conductivity and a relatively low percolation threshold among the three composite films.

Influences of high aspect ratio carbon nanotube network on normal stress difference measurements and extrusion behaviors for isotactic polypropylene nanocomposite melts

We have investigated the role that representative nonequilibrium carbon nanotube (CNT) network structures play in the flow-induced property changes of relatively high aspect ratio CNT/isotactic polypropylene (iPP) nanocomposites. Stable CNT network can form in CNT/iPP nanocomposites at relatively low CNT contents due to the employment of high aspect ratio CNTs. As experimentally demonstrated and theoretically predicted by Kharchenko et al. (Nat. Mater., 2004), large apparent negative normal stress differences (ΔN) might occur in long CNT systems under some particular conditions, for example, which satisfy the condition that the CNT network is strong enough to accommodate appreciable rotational distortion. In this study, we focused on the measurements of normal stress differences to explore the particular conditions for appearance of negative ΔN. We find that the values of ΔN are immensely influenced by initial normal stresses induced by squeezing deformation during the sample loading. Large negative ΔN can be observed under the subsequent shear flow with zeroing the initial normal stresses in our high aspect ratio CNT/iPP nanocomposites only if the CNT content is above the CNT gelation concentration, which is consistent with the previous studies of Kharchenko et al. This interesting result about the measurements of ΔN is further confirmed by a die-shrinkage extrusion behavior and at the same time the effect of shape-distortion instability in the extrusion of neat iPP can be diminished in the CNT/iPP nanocomposites.

Design of multi-functional dual hole patterned carbon nanotube composites with superhydrophobicity and durability

Most current research on nanocomposites has focused on their bulk attributes, i.e., electrical, microwave, thermal, and mechanical properties. In practical applications, surface properties such as robustness against environmental contamination are critical design considerations if intrinsic properties are to be maintained. The aim of this research is to combine the bulk properties of nanocomposites with the superhydrophobic surface properties provided by imprinting techniques to create a single multi-functional system with enhanced bulk properties. We report the development of a highly conductive superhydrophobic nanotube composite, which is directly superimposed with a durable dual hole pattern through imprinting techniques. The dual hole pattern avoids the use of high slenderness ratio structures resulting in a surface which is robust against physical damage. Its stable superhydrophobic properties were characterized both theoretically and experimentally. By incorporating high aspect ratio carbon nanotubes (CNTs), the dual patterned composites can also be effectively used for anti-icing and deicing applications where their superhydrophobic surface suppresses ice formation and their quick electric heating response at low voltage eliminates remaining frost. In addition, superior electromagnetic interference (EMI) shielding effectiveness (SE) was attained, with one of the highest values ever reported in the literature. Open image in new window

Clothing polymer fibers with well-aligned and high-aspect ratio carbon nanotubes

It is believed that the crucial step towards preparation of electrical conductive polymer-carbon nanotube (CNT) composites is dispersing CNTs with a high length-to-diameter aspect ratio in a well-aligned manner. However, this process is extremely challenging when dealing with long and entangled CNTs. Here in this study, a new approach is demonstrated to fabricate conductive polymer-CNT composite fibers without involving any dispersion process. Well-aligned CNT films were firstly drawn from CNT arrays, and then directly coated on polycaprolactone fibers to form polymer-CNT composite fibers. The conductivity of these composite fibers can be as high as 285 S m(-1) with only 2.5 wt% CNT loading, and reach 1549 S m(-1) when CNT loading is 13.4 wt%. As-prepared composite fibers also exhibit 82% retention of conductivity at a strain of 7%, and have improved mechanical properties.

Size and synergy effects of nanofiller hybrids including graphene nanoplatelets and carbon nanotubes in mechanical properties of epoxy composites

We study mechanical reinforcement in a widely used epoxy matrix with the addition of graphene nanoplatelets (GnPs) and various mixture ratios of carbon nanotubes (CNTs) with GnPs. Two different dimensions of GnPs were used with flake sizes of 5μm and 25μm to investigate the influence of nanofiller size on composite properties. In GnP reinforced composites, bigger flakes showed greater reinforcement at all GnP concentrations as they actively control the failure mechanisms in the composite. In the mixture samples, highest CNT content (9:1) showed marked improvement in fracture toughness of 76%. The CNT:GnP ratio is an interesting factor significantly influencing the properties of the epoxy based nanocomposites. The combination of high aspect ratio of CNTs and larger surface area of GnPs contribute to the synergistic effect of the hybrid samples. Thermal conductivity consistently increases with incorporation of GnPs in the matrix. Transmission electron microscopy (TEM) images confirm the uniform nanofiller dispersion achieved in the composites. For the hybrid samples CNTs are seen to align themselves on the GnP flakes creating an inter-connected strong nanofiller network in the matrix. The homogeneous nanofiller dispersions have been achieved by high shear calendaring which is a method capable of being industrially scaled up.

High luminance organic light-emitting diodes with efficient multi-walled carbon nanotube hole injectors

We report on high luminance organic light-emitting diodes using acid functionalized multi-walled carbon nanotube (o-MWCNTs) as efficient hole injector electrodes, using a simple, solution processable device structure. At only 10V, the luminance approaches 50,000cd/m2 with an external quantum efficiency over 2% and a current efficiency greater than 21cd/A. The investigation of hole-only devices shows that the mechanism for hole injection changes from injection limited to bulk limited because of the higher effective work function of the anode modified by the o-MWCNTs. We expect the enhancement of the local electric field, brought about by both the dielectric inhomogeneities within the o-MWCNT containing anode and the high aspect ratio carbon nanotubes, improves hole injection from the anode to the organic active layer at much lower applied voltage.

Comparison of Polyimide Composites with Non-covalent Modified and Acid Modified Multi-wall Carbon Nanotube

In this study, it was attempted to increase the electrical conductivity of polyimide (PI) by adding multi-wall carbon nanotubes (MWNT), mainly because the high aspect ratio carbon nanotubes can reduce resistance and enhance antistatic ability to meet electrostatic charge (ESC) criteria of polymer hybrid films. MWNT were modified successfully with a sodium salt of 6-aminohexanoic acid (Na-AHA), which has been verified by FTIR and TEM observations. During the blending process, the PI/MWNT composites thus produced were compared with unmodified MWNT to study the miscibility and interaction among the components. The homogeneous dispersion of the non-covalently modified MWNT (Na-AHA-CNT) promoted the thermal and electrical conductivity properties of PI composite films. The results are compared with those of PI composites with acid-modified and unmodified MWNT.

Catalytic synthesis of a high aspect ratio carbon nanotubes bridging carbon felt composite with improved electrical conductivity and effective surface area

High aspect ratio (>5000) composite with macroscopic shape consisted of multi-walled carbon nanotubes bridging a 3D network of the micrometers carbon felt host structure was synthesized by means of the chemical vapour deposition technique. Such carbon-based nano-macro composite material displays a high mechanical strength, high electrical conductivity along with a high and fully accessible effective surface area (>100m2g−1), which allows it to be efficiently employed as structured catalyst support, in fixed-bed catalytic reactions, without the problem of pressure drop as encountered with the nanoscopic carbon support materials. The nanoscopic structure also provides a high degree of mixing of the reactant mixture which strongly increases the effective surface contact between the reactants and the active sites. In addition, the total effective surface area of the composite can be finely tuned, depending to the downstream application, as a function of the carbon nanotubes intake.

Patterned growth of ultra long carbon nanotubes. Properties and systematic investigation into their growth process

High aspect ratio carbon nanotube arrays were grown using a hydrogen/water assisted catalytic chemical vapor deposition method using ethylene as precursor which achieves a growth rate of 25 to 30 μm carbon nanotube length per minute. The influence of the growth parameters determining the growth rate and the growth height of the carbon nanotubes arrays was studied in detail. Water and hydrogen concentration were varied systematically and it was found that they have to be in a precise ratio in order to establish ultralong growth together with high quality (no contamination with carbonaceous side products) of the arranged carbon nanotubes. For given growth conditions, the hydrogen content in the feed gas stream controls the carbon nanotube length, the purity level and the number of graphitic walls of carbon nanotubes grown. It was found that water acts as a weak oxidant keeping the mixed aluminium/iron catalyst active for ultrahigh length growth. Its concentration should be established very precisely in the process to achieve a maximum growth rate of carbon nanotubes. Furthermore a straightforward, non-costly mask technique is presented to achieve highly structured carbon nanotube growth. Finally the field emission characteristics of such ultrahigh, well arranged and structured carbon nanotube blocks was studied and this revealed low turn-on electric and threshold field values of 0.34 V μm−1 and 0.71 V μm−1 respectively.

Electrically conductive alumina–carbon nanocomposites prepared by Spark Plasma Sintering

Carbon nanotubes (CNTs) and carbon black were added to alumina to convert it into a good electrical conductor. Alumina–CNT and alumina–carbon black nanocomposites were fabricated by Spark Plasma Sintering (SPS). The electrical conductivity of alumina–CNT nanocomposites was found to be four times higher as compared to alumina–carbon black nanocomposites due to the fibrous nature and high aspect ratio of CNTs. The electrical conductivity of alumina–CNT nanocomposite increased with increasing grain size due to increasing density of CNTs at the grain boundaries. This effect was not observed for alumina–carbon black nanocomposite due to the particulate geometry of the carbon black.

The fabrication of high aspect ratio carbon nanotube arrays by direct laser interferencepatterning

High aspect ratio periodic carbon nanotube arrays were fabricated using direct laser interferencepatterning with a frequency tripled Nd:YAG (YAG: yttrium aluminium garnet) laser (λ = 355 nm) emitting 10 ns laser pulses. The depth of the fabricated arrays could be adjusted bycontrolling the number of laser pulses. Application of successive laser pulses (10–20)induced pattern distortion, resulting in the formation of conical organized arrays.The number of laser pulses necessary to produce this distortion is proportionalto the spatial period of the interference pattern. Raman spectroscopy analysesconfirmed that the chemical form of the carbon nanotubes was preserved afterpatterning.

Synthesis of High Aspect-Ratio Carbon Nanotube “Flying Carpets” from Nanostructured Flake Substrates

We present a robust method for synthesis of aligned, single-walled carbon nanotube (CNT) "flying carpets" from nanostructured alumina flakes. Roll-to-roll e-beam deposition is utilized to produce the flakes, and hot filament chemical vapor deposition is utilized to grow dense, aligned carbon nanotubes from the flakes with remarkably high CNT yields. The flakes are captured inside a mesh cage and freely suspended in the gas flow during growth. Optical characterization indicates the presence of high quality, small diameter single-walled carbon nanotubes.

Ultrasharp and high aspect ratio carbon nanotube atomic force microscopy probes forenhanced surface potential imaging

The resolution of scanning surface potential microscopy (SSPM) is mainly limited bynon-local electrostatic interactions due to the finite probe size. Here we present highresolution surface potential imaging with ultrasharp and high aspect ratio carbon nanotube(CNT) atomic force microscopy (AFM) probes fabricated via dielectrophoresis.Enhancement of surface potential contrast by several factors is reported for integratedcircuit structures and purple membrane fragments for these CNT AFM probesas compared to conventional probes. In particular, ultrahigh lateral resolution(∼2 nm) surface potential images of self-assembled bacteriorhodopsin proteins are reported atambient conditions, with the implication of label-free protein detection by SSPMtechniques.