Selective synthesis of metallic and semiconducting single-walled carbon nanotubes

As-prepared single-walled carbon nanotubes (SWNTs) are generally mixtures of semiconducting and metallic species, the proportion of the former being around 67%. Since most applications of SWNTs are best served by semiconducting or metallic nanotubes, rather than by mixtures of the two, methods which would directly yield semiconducting and metallic SWNTs in pure form are desirable. In this article, we present the available methods for the direct synthesis of such SWNTs along with the methods available to separate semiconducting and metallic SWNTs from mixtures. We also discuss the synthesis of Y-junction carbon nanotubes.

Unusual dependence of raman mode frequency on excitation wavelength in graphite and single walled carbon nanotubes

In this paper we report resonance Raman scattering from graphite covering excitation energies in the range 2.4 eV to 6 eV. The Raman excitation profile shows a maximum at 4.94 eV (lambda = 251nm) for the G - band (1582 cm(-1)). The D-band at similar to 1350 cm(-1), attributed to disorder activated Raman scattering, does not show up in Raman spectra recorded with excitation wavelengths smaller than 257.3 nm, revealing that the resonance enhancements of the G and D-modes are widely different. Earlier Raman measurements in carbon materials have also revealed a very large and unusual dependence of the D - mode frequency on excitation laser wavelength. This phenomenon is also observed in carbon nanotubes. In this paper we show for the first time that the above unusual dependence arises from the disorder - induced double resonance mechanism.

Non-resonant rf absorption evidence for reentrant melting of vortex lattice in Bi2Sr2CaCu2O8 single crystals

We have studied the magnetic field dependent rf (20 MHz) losses in Bi2Sr2CaCu2O8 single crystals in the low field and high temperature regime. Above HCl the dissipation begins to decrease as the field is increased and exhibits a minimum at HM>HCl. For H>HM the loss increases monotonically. We attribute the decrease in loss above HCl to the stiffening of the vortex lines due to the attractive electromagnetic interaction between the 2D vortices (that comprise the vortex line at low fields) in adjacent CuO bilayers. The minimum at HM implies that the vortex lines are stiffest and hence represents a transition into vortex solid state from the narrow vortex liquid in the vicinity of HCl. The increase in loss for H>HM marks the melting of the vortex lattice and hence a second transition into vortex liquid regime. We discuss our results in the light of recent theory of reentrant melting of the vortex lattice by G. Blatter et al. (Phys. Rev. B 54, 72 (1996)).

High emission currents and low threshold fields in multi-wall carbon nanotube-polymer composites in the vertical configuration

In this work, we present field emission characteristics of multi-wall carbon nanotube (MWCNT)-polystyrene composites at various weight fractions along the cross-section of sample. Scanning electron microscope images in cross-sectional view reveal that MWCNTs are homogeneously distributed across the thickness and the density of protruding tubes can be scaled with weight fraction of the composite film. Field emission from composites has been observed to vary considerably with density of MWCNTs in the polymer matrix. High current density of 100 mA/cm(2) was achieved at a field of 2.2 V/lm for 0.15 weight fraction. The field emission is observed to follow the Fowler-Nordheim tunneling mechanism, however, electrostatic screening is observed to play a role in limiting the current density at higher weight fractions. (C) 2012 American Institute of Physics. [doi:10.1063/1.3685754]

New Insights into Optimal Discrete Rate Adaptation for Average Power Constrained Single and Multi-Node Systems

The throughput-optimal discrete-rate adaptation policy, when nodes are subject to constraints on the average power and bit error rate, is governed by a power control parameter, for which a closed-form characterization has remained an open problem. The parameter is essential in determining the rate adaptation thresholds and the transmit rate and power at any time, and ensuring adherence to the power constraint. We derive novel insightful bounds and approximations that characterize the power control parameter and the throughput in closed-form. The results are comprehensive as they apply to the general class of Nakagami-m (m >= 1) fading channels, which includes Rayleigh fading, uncoded and coded modulation, and single and multi-node systems with selection. The results are appealing as they are provably tight in the asymptotic large average power regime, and are designed and verified to be accurate even for smaller average powers.

Molecular charge-transfer interaction with single-layer graphene

While the effect of electrochemical doping on single-layer graphene (SG) with holes and electrons has been investigated, the effect of charge-transfer doping on SG has not been examined hitherto. Effects of varying the concentration of electron donor and acceptor molecules such as tetrathiafulvalene (TTF) and tetracyanoethylene (TCNE) on SG produced by mechanical exfoliation as well as by the reduction of single-layer graphene oxide have been investigated. TTF softens the G-band in the Raman spectrum, whereas TCNE stiffens the G-band. The full-width-at-half-maximum of the G-band increases on interaction with both TTF and TCNE. These effects are similar to those found with few-layer graphene, but in contrast to those found with electrochemical doping. A common feature between the two types of doping is found in the case of the 2-D band, which shows softening and stiffening on electron and hole doping, respectively. The experimental results are explained on the basis of the frequency shifts, electron-phonon coupling and structural inhomogeneities that are relevant to molecule-graphene interaction.

Construction of Equivalent Circuit of a Single and Isolated Transformer Winding From FRA Data Using the ABC Algorithm

This paper reports the results of employing an artificial bee colony search algorithm for synthesizing a mutually coupled lumped-parameter ladder-network representation of a transformer winding, starting from its measured magnitude frequency response. The existing bee colony algorithm is suitably adopted by appropriately defining constraints, inequalities, and bounds to restrict the search space and thereby ensure synthesis of a nearly unique ladder network corresponding to each frequency response. Ensuring near-uniqueness while constructing the reference circuit (i.e., representation of healthy winding) is the objective. Furthermore, the synthesized circuits must exhibit physical realizability. The proposed method is easy to implement, time efficient, and problems associated with the supply of initial guess in existing methods are circumvented. Experimental results are reported on two types of actual, single, and isolated transformer windings (continuous disc and interleaved disc).

Loading of single-walled carbon nanotubes in cationic cholesterol suspensions significantly improves gene transfection efficiency in serum

We present here a series of cholesterol based cationic lipid suspensions that solubilize single-walled carbon nanotubes (SWCNT) efficiently in water. Each cationic lipid formulation was characterized in terms of their energy minimized molecular structures, bilayer widths of the aggregates based on X-ray diffraction. Then these aggregates were investigated pertaining to their DNA binding and release efficiency, effect of CNT inclusion on the stability of cationic cholesterol lipid-DNA complexes, Zeta potential values and changes in the chiro-optical property of DNA, effect on Raman spectral shift and changes in morphology by SEM and AFM. Each cationic lipid formulation was optimized for the amount of SWCNT solubilized in water, lipid-DNA ratio, amount of the plasmid DNA that can be transfected and the effect on the cellular toxicity. The resulting SWCNT-lipid formulations were then used for in vitro transfection of pEGFP-C3 in A549 (human alveolar basal epithelial) cells and HeLa (human cervical cancer) cells. Advantageously, the CNT-loaded formulations confer an excellent transfection efficiency even in high percentages of blood serum and showed significantly better gene transfer efficiency compared to one of the potent, well-known commercial transfection reagent, Lipofectamine2000.

A Five-Level Inverter Topology with Single-DC Supply by Cascading a Flying Capacitor Inverter and an H-Bridge

In this paper, a new three-phase, five-level inverter topology with a single-dc source is presented. The proposed topology is obtained by cascading a three-level flying capacitor inverter with a flying H-bridge power cell in each phase. This topology has redundant switching states for generating different pole voltages. By selecting appropriate switching states, the capacitor voltages can be balanced instantaneously (as compared to the fundamental) in any direction of the current, irrespective of the load power factor. Another important feature of this topology is that if any H-bridge fails, it can be bypassed and the configuration can still operate as a three-level inverter at its full power rating. This feature improves the reliability of the circuit. A 3-kW induction motor is run with the proposed topology for the full modulation range. The effectiveness of the capacitor balancing algorithm is tested for the full range of speed and during the sudden acceleration of the motor.

Study of Low Temperature Magnetic Properties of a Single Chain Magnet with Alternate Isotropic and Non-collinear Anisotropic Units

Here we study thermodynamic properties of an important class of single-chain magnets (SCMs), where alternate units are isotropic and anisotropic with anisotropy axes being non-collinear. This class of SCMs shows slow relaxation at low temperatures which results from the interplay of two different relaxation mechanisms, namely dynamical and thermal. Here anisotropy is assumed to be large and negative, as a result, anisotropic units behave like canted spins at low temperatures; but even then simple Ising-type model does not capture the essential physics of the system due to quantum mechanical nature of the isotropic units. We here show how statistical behavior of this class of SCMs can be studied using a transfer matrix (TM) method. We also, for the first time, discuss in detail how weak inter-chain interactions can be treated by a TM method. The finite size effect is also discussed which becomes important for low temperature dynamics. At the end of this paper, we apply this technique to study a real helical chain magnet.

Construction of Equivalent Circuit of a Single and Isolated Transformer Winding From FRA Data Using the ABC Algorithm

This paper reports the results of employing an artificial bee colony search algorithm for synthesizing a mutually coupled lumped-parameter ladder-network representation of a transformer winding, starting from its measured magnitude frequency response. The existing bee colony algorithm is suitably adopted by appropriately defining constraints, inequalities, and bounds to restrict the search space and thereby ensure synthesis of a nearly unique ladder network corresponding to each frequency response. Ensuring near-uniqueness while constructing the reference circuit (i.e., representation of healthy winding) is the objective. Furthermore, the synthesized circuits must exhibit physical realizability. The proposed method is easy to implement, time efficient, and problems associated with the supply of initial guess in existing methods are circumvented. Experimental results are reported on two types of actual, single, and isolated transformer windings (continuous disc and interleaved disc).

Fatigue de-bond growth in adhesively bonded single lap joints

The fatigue de-bond growth studies have been conducted on adhesively bonded lap joint specimens between aluminium and aluminium with Redux-319A adhesive with a pre-defined crack of 3 mm at the bond end. The correlations between fracture parameters and the de-bond growth data are established using both numerical and experimental techniques. In the numerical method, geometrically non-linear finite element analyses were carried out on adhesively bonded joint specimen for various de-bond lengths measured from the lap end along the mid-bond line of the adhesive. The finite element results were post processed to estimate the SERR components G (I) and G (II) using the Modified Virtual Crack Closure Integral (MVCCI) procedure. In experimental work, specimens were fabricated and fatigue de-bond growth tests were conducted at a stress ratio R = -1. The results obtained from both numerical analyses and testing have been used to generate de-bond growth curve and establish de-bond growth law in the Paris regime for such joints. The de-bond growth rate is primarily function of mode-I SERR component G (I) since the rate of growth in shear mode is relatively small. The value of Paris exponent m is found to be 6.55. The high value of de-bond growth exponent in Paris regime is expected, since the adhesive is less ductile than conventional metallic materials. This study is important for estimating the life of adhesively bonded joints under both constant and variable amplitude fatigue loads.

Quasi-static crack tip fields in rate-sensitive FCC single crystals

In this work, the effects of loading rate, material rate sensitivity and constraint level on quasi-static crack tip fields in a FCC single crystal are studied. Finite element simulations are performed within a mode I, plane strain modified boundary layer framework by prescribing the two term (K-T) elastic crack tip field as remote boundary conditions. The material is assumed to obey a rate-dependent crystal plasticity theory. The orientation of the single crystal is chosen so that the crack surface coincides with the crystallographic (010) plane and the crack front lies along 101] direction. Solutions corresponding to different stress intensity rates K., T-stress values and strain rate exponents m are obtained. The results show that the stress levels ahead of the crack tip increase with K. which is accompanied by gradual shrinking of the plastic zone size. However, the nature of the shear band patterns around the crack tip is not affected by the loading rate. Further, it is found that while positive T-stress enhances the opening and hydrostatic stress levels ahead of crack tip, they are considerably reduced with imposition of negative T-stress. Also, negative T-stress promotes formation of shear bands in the forward sector ahead of the crack tip and suppresses them behind the tip.

Longitudinal Magnetic Field Effect on Nonlocal Ultrasonic Vibration Analysis of Single-Walled Carbon Nanotubes Based on Wave Propagation Approach

This paper studies the effect of longitudinal magnetic field on ultrasonic vibration in single walled carbon nanotubes (CNTs) based on nonlocal continuum medium theory. Governing partial differential equations of CNTs are derived by considering the Lorentz magnetic forces applied on CNTs induced by a longitudinal magnetic field through Maxwell equations. The vibration characteristics of CNTs under a longitudinal magnetic field are obtained by solving the governing equations via wave propagation approach. The effects of longitudinal magnetic field on vibration of CNTs are discussed through numerical experiments. The present analysis show that vibration frequencies of CNTs drops dramatically in the presence of the magnetic field for various circumferential wavenumbers. Such effect is also observed for various boundary conditions of the CNT. New features for the effect of longitudinal magnetic field on ultrasonic vibration of CNTs, presented in this paper are useful in the design of nano-drive device, nano-oscillator and actuators and nano-electron technology, where carbon nanotubes act as basic elements.

Sivers effect and transverse single spin asymmetry in e+p(up arrow) -> e+J/psi+X

We discuss the possibility of using electroproduction of J/psi as a probe of gluon Sivers function by measuring single spin asymmetry (SSA) in experiments with transversely polarized protons and electron beams. We estimate SSA for JLab, HERMES, COMPASS, and eRHIC energies using the color evaporation model of charmonium production and find asymmetry up to 25% for certain choices of model parameters which have been used earlier for estimating SSA in the SIDIS and Drell-Yan processes.