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Acoustic streaming generated by a plane standing wave between two infinite plates or inside a cylindrical tube is considered, under the isentropic flow assumption. A two-dimensional analysis is performed in the linear case of slow streaming motion, based on analytical formal solutions of separate problems, each associated with a specific source term (Reynolds stress term). In order to obtain these analytical solutions, a necessary geometrical hypothesis is that
are the guide half-width (or radius) and length. The effect of the two source terms classically taken into account is quantified in order to derive the dependence of the maximum axial streaming velocity on the axis as a function of the ratio
is the acoustic boundary layer thickness. The effect of two other source terms that are usually neglected, is then analysed. It is found that one of these terms can generate a counter-rotating streaming flow. While negligible for very narrow guides, this term can become important for some values of the aspect ratio
Carbon nanofibers were well aligned in polymer composite magnetically at moderate fields up to 3 T. Due to the NiO-coating, carbon nanofibers exhibited strong magnetic moments that lead to alignment. Both TEM and SEM results showed the well-aligned nano-fibers in a polymer matrix. Mechanical testing showed a pronounced anisotropy in tensile strength in directions normal (12.1MPa) and parallel (22MPa) to the applied field, resulting from the well-aligned nanofibers in the polymer matrix. The mechanism of magnetic alignment due to coating of NiO on the nano fiber surface is discussed.
Carbon nanotubes and nanofibres are typically synthesised under substrate temperatures above 600°C. Here we investigate the influence of the substrate temperature and the plasma conditions on the growth of vertically aligned carbon nanostructures using Direct Current plasma Chemical Vapour Deposition, at temperatures below 550°C. These nanostructures are produced using a C2H2 based plasma and nickel thin film as the catalyst. We found that preferential deposition of amorphous carbon takes place as the synthesis temperature is lowered below 500°C. However, lowering the carbon concentration in the gas feedstock (<2% conc.) allows for the nucleation of nanofibre-like structures, whilst balancing the buildup of amorphous carbon. This method allows for the synthesis of vertically aligned structures at low temperatures (around 230°C) without intentional heating, while still achieving reasonable average growth rates up to 27 nm/min. The only heating was provided by the plasma, which typically consumes ∼ 4 W/cm2. It was found that by varying the applied plasma bias during high temperature synthesis, we increased the growth rate up to 165 nm/min. Based on the observations of experimental process variations and the morphology of the synthesised structures, we propose a growth mechanism for such low temperature growth and examine the resulting morphology changes.
Atomic vibrations are partially screened by electrons. In a metal this screening can change rapidly for vibrations associated to certain points of the Brillouin zone, entirely determined by the shape of the Fermi surface. The consequent anomalous behaviour of the phonon dispersion is called Kohn anomaly. Graphite is a semimetal. Nanotubes can be metals or semiconductors. We demonstrate that two Kohn anomalies are present in the phonon dispersion of graphite and that their slope is proportional to the square of the electron-phonon coupling. Metallic nanotubes have much stronger anomalies than graphite, due to their reduced dimensionality. Semiconducting nanotubes have no Kohn anomalies.
We report the synthesis of organized colloidal semiconductor nanocrystal / carbon singlewalled nanotube hybrid nanostructures. The synthetic protocol described here avoids the need for covalent chemical modification of carbon nanotube (CNT) surfaces. Specifically, InP quantum dots (QDs) and CdSe QDs were found to strongly adsorb onto the surfaces of carbon single-walled nanotubes (SWNTs) by gentle heating in organic solvents. Transmission electron microscopy (TEM) was used to characterize the semiconductor nanocrystal (NC) / SWNT assemblies, and revealed that the surfaces of the SWNT bundles template the adsorption of the NCs from solution. Small QDs were found to randomly absorb onto SWNTs, while larger QDs self-assembled into long linear chains. The nature of binding and ordering was investigated by simply considering van der Waals (vdW) forces for both NC-SWNT and NC-NC interactions. Quantum rods (QRs) were also found to adsorb along the nanotube surfaces. These findings have important implications for the synthesis of NC / SWNT hybrid nanostructures.
We have synthesized multi-walled carbon nanotubes (MWNTs) on nickel-deposited n-type silicon substrates by thermal chemical vapor deposition (Thermal CVD). The electroless plating method was adopted in the deposition of the Ni catalytic layer. Sulfuric acid solution was used as a buffer to adjust and maintain a pH value of 4.6. Both the deposition time of the nickel catalyst layer and the growth time of MWNTs were adopted to control the length and density of MWNTs. Scanning electron microscopy (SEM) images showed that the density of carbon nanotubes increased as the deposition time of nickel catalyst layer increased. The formation of nickel nucleation becomes rich as the immersion time of the substrate in electroless plating solution was longer, and this benefited the growth of carbon nanotubes. In addition, the Raman spectrum demonstrated that the ID/IG ratio of MWNTs decreases as the deposition and growth time increase, indicating that more graphene MWNT structures were formed.
Surface modification of carbon nanotubes with polyvinyl alcohol (PVOH) showed detection for humidity variation implying a possible application as a nanoscale humidity sensor. Wettability studies on single-wall, Y junction single wall and multiwall carbon nanotubes revealed that these nanotubes tend to become highly hydrophilic surfaces by this functionalisation. From the raman analysis it was determined that the majority of the nanotubes in our Y-junction single wall nanotubes were semiconducting in nature. The conductivity studies revealed that the Y junction nanotubes with PVOH functionalisation show large change in conductivity for varying relative humidity (RH). We propose a possible charge transport mechanism in these functionalized nanotubes.
We find that functionalized SWCNT and DWCNT's (mainly double wall carbon nanotubes) in composites, DWCNTs under hydrostatic pressure and blue illuminated DWCNTs in methanol show the same up shift of the Raman G band and the appearance of a new band at 1455cm−1. This is attributed to the interaction of the CH3 group of the amphiphilic molecule in composites or the CH3 group of alcohol with the outer tube of DWCNT's and indicates that laser heating of DWCNT's in methanol can induce the chemical adsorption of CH3 onto the CNT (carbon nanotube) surface.
Single walled carbon nanotubes (SWNT) were synthesized by methane CVD on a supported mixed transition metal (Fe/Mo) catalyst. Gas feed composition and reaction temperature were varied to identify the threshold conditions for the growth of SWNT. These reaction conditions closely approximate pseudo-equilibrium conditions with some active reaction intermediate (likely chemisorbed carbon atoms) that proceeds to nucleate and grow SWNT. This value also serves as an estimated upper limit of the free energy of formation ΔG*(T)SWNT since the active intermediate proceeds to form SWNT through a process that is thought to be essentially irreversible. The difference relative to graphite is in good agreement with literature values predicted from simulations for SWNT nuclei containing approximately 80 atoms, while considerably larger than that predicted for bulk 5, 5 SWNT. Our estimate over the range 700 to 1000 °C of 16.1 to 13.9 kJ/mol is considerably greater than the free energy of formation for diamond (between 5.8 and 6.9 kJ/mol from 700 to 925 °C).
Multi-walled carbon nanotube (MWNT) filled polystyrene (PS) composites were synthesized for electromagnetic interference (EMI) shielding applications. SEM images of composites showed the formation of the conducting networks through MWNTs within the PS matrix. The measured DC conductivity of composites increased with increasing MWNT loading, showing a typical percolation behavior. EMI shielding characteristics of MWNT-PS composites were investigated in the frequency range of 8.2–12.4 GHz (X-band). It was observed that the shielding effectiveness (SE) of such composite increased with the increase of MWNT loading. The SE of the composite containing 7 wt% MWNTs could reach more than 26 dB in the measured frequency region.
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