Studies on the optimisation of unbalanced magnetron sputtering cathodes

The optimisation is reported on the design of unbalanced magnetron (UBM) sputtering cathodes. For the study, a planar circular cathode backed by a double-coil electromagnet (compatible for a 100 mm diameter target) was developed. The variation of the structure and strength of the magnetic field in front of the target was investigated for different current combinations in the electromagnetic coils, and its effect on the sputtering process was analysed. The observations on the magnetic field geometry revealed some interesting features, such as the balancing point of the fields along the axis (null-point), and the zero axial region over the target surface (B-z = 0 ring). The positions of both could be controlled by adjusting the ratio of the electric current in the coils. The magnetic field null-point could be used as a reference for the region of homogeneous film growth. The B-z = 0 ring was the location where the glow discharge concentrated (or where the maximum target erosion occurred). The diameter of the ring determined the area covered by the discharge and thus the sputtering efficiency. The optimum substrate position can be fixed according to the position of the null-point and optimisation of sputtering can be achieved by adjusting the diameter of the B-z = 0 ring. The results of this study should be helpful in the designing of an ideal UBM using permanent magnets as well as electromagnets. (C) 1999 Elsevier Science Ltd. All rights reserved.

Synthesis of single-walled carbon nanotubes using binary (Fe, Co, Ni) alloy nanoparticles prepared in situ by the reduction of oxide solid solutions

Passing a H-2-CH4 mixture over oxide spinels containing two transition elements as in Mg0.8MyMz'Al2O4 (M, M' = Fe, Co or Ni, y + z = 0.2) at 1070 degrees C produces small alloy nanoparticles which enable the formation of carbon nanotubes. Surface area measurements are found to be useful for assessing the yield and quality of the nanotubes. Good-quality single-walled nanotubes (SWNTs) have been obtained in high yields with the FeCo alloy nanoparticles, as evidenced by transmission electron microscope images and surface area measurements. The diameter of the SWNTs is in the 0.8-5 nm range, and the multiwalled nanotubes, found occasionally, possess very few graphite layers. (C) 1999 Elsevier Science B.V. All rights reserved.

Diaphragmless shock wave generators for industrial applications of shock waves

The prime focus of this study is to design a 50 mm internal diameter diaphragmless shock tube that can be used in an industrial facility for repeated loading of shock waves. The instantaneous rise in pressure and temperature of a medium can be used in a variety of industrial applications. We designed, fabricated and tested three different shock wave generators of which one system employs a highly elastic rubber membrane and the other systems use a fast acting pneumatic valve instead of conventional metal diaphragms. The valve opening speed is obtained with the help of a high speed camera. For shock generation systems with a pneumatic cylinder, it ranges from 0.325 to 1.15 m/s while it is around 8.3 m/s for the rubber membrane. Experiments are conducted using the three diaphragmless systems and the results obtained are analyzed carefully to obtain a relation between the opening speed of the valve and the amount of gas that is actually utilized in the generation of the shock wave for each system. The rubber membrane is not suitable for industrial applications because it needs to be replaced regularly and cannot withstand high driver pressures. The maximum shock Mach number obtained using the new diaphragmless system that uses the pneumatic valve is 2.125 +/- 0.2%. This system shows much promise for automation in an industrial environment.

Bubble size measurement in a gas-liquid ejector for a sodium chloride-air system

A high speed photographic technique has been employed to measure the Sauter mean diameter of bubbles experimentally in a gas liquid ejector using a sodium chloride-air system. The measured values are compared with the theoretically predicted maximum bubble size diameter using Sprow's correlation. Bubble size as a function of the liquid flow rate and also of its distance from the throat of the ejector has been reported in this paper. The results obtained for this non-reactive system are also compared with those obtained earlier for the air-water system.

Electrochemical co-deposition of bimetallic Pt-Ru nanoclusters dispersed on poly(3,4-ethylenedioxythiophene) and electrocatalytic behavior for methanol oxidation

Nanoclusters of bimetallic Pt-Ru are electrochemically deposited on conductive polymer, poly(3,4-ethylenedioxythiophene)(PEDOT), which is also electrochemically deposited on a carbon paper substrate. The bimetallic deposition is carried out in an acidic electrolyte consisting of chloroplatinic acid and ruthenium chloride at 0.0 V versus saturated calomel electrode (SCE) on PEDOT coated carbon paper. A thin layer PEDOT on a carbon paper substrate facilitates the formation of uniform, well-dispersed, nano clusters of Pt-Ru of mean diameter of 123 nm, which consist of nanosize particles. In the absence of PEDOT, the size of the clusters is about 251 nm, which are unevenly distributed on carbon paper substrate. Cyclic voltammetry studies suggest that peak currents of methanol oxidation are several times greater on PtRu-PEDOT electrode than on Pt-Ru electrode in the absence of PEDOT. (C) 2011 Elsevier B.V. All rights reserved.

Estimation of elasticity map of soft biological tissue mimicking phantom using laser speckle contrast analysis

Scattering of coherent light from scattering particles causes phase shift to the scattered light. The interference of unscattered and scattered light causes the formation of speckles. When the scattering particles, under the influence of an ultrasound (US) pressure wave, vibrate, the phase shift fluctuates, thereby causing fluctuation in speckle intensity. We use the laser speckle contrast analysis (LSCA) to reconstruct a map of the elastic property (Young's modulus) of soft tissue-mimicking phantom. The displacement of the scatters is inversely related to the Young's modulus of the medium. The elastic properties of soft biological tissues vary, many fold with malignancy. The experimental results show that laser speckle contrast (LSC) is very sensitive to the pathological changes in a soft tissue medium. The experiments are carried out on a phantom with two cylindrical inclusions of sizes 6 mm in diameter, separated by 8 mm between them. Three samples are made. One inclusion has Young's modulus E of 40 kPa. The second inclusion has either a Young's modulus E of 20 kPa, or scattering coefficient of mu'(s), = 3.00 mm(-1) or absorption coefficient of mu(a) = 0.03 mm(-1). The optical absorption (mu(a)), reduced scattering (mu'(s)) coefficient, and the Young's modulus of the background are mu(a) = 0.01 mm(-1), mu'(s) = 1.00 mm(-1) and 12kPa, respectively. The experiments are carried out on all three phantoms. On a phantom with two inclusions of Young's modulus of 20 and 40 kPa, the measured relative speckle image contrasts are 36.55% and 63.72%, respectively. Experiments are repeated on phantoms with inclusions of mu(a) = 0.03 mm-1, E = 40 kPa and mu'(s) = 3.00 mm(-1). The results show that it is possible to detect inclusions with contrasts in optical absorption, optical scattering, and Young's modulus. Studies of the variation of laser speckle contrast with ultrasound driving force for various values of mu(a), mu'(s), and Young's modulus of the tissue mimicking medium are also carried out. (C) 2011 American Institute of Physics. doi:10.1063/1.3592352]

Large eddy simulation of a premixed flame with approximate deconvolution modeling

Approximate deconvolution modeling is a very recent approach to large eddy simulation of turbulent flows. It has been applied to compressible flows with success. Here, a premixed flame which forms in the wake of a flameholder has been selected to examine the subgrid-scale modeling of reaction rate by this new method because a previous plane two-dimensional simulation of this wake flame, using a wrinkling function and artificial flame thickening, had revealed discrepancies when compared with experiment. The present simulation is of the temporal evolution of a round wakelike flow at two Reynolds numbers, Re = 2000 and 10,000, based on wake defect velocity and wake diameter. A Fourier-spectral code has been used. The reaction is single-step and irreversible, and the rate follows an Arrhenius law. The reference simulation at the lower Reynolds number is fully resolved. At Re = 10,000, subgrid-scale contributions are significant. It was found that subgrid-scale modeling in the present simulation agrees more closely with unresolved subgrid-scale effects observed in experiment. Specifically, the highest contributions appeared in thin folded regions created by vortex convection. The wrinkling function approach had not selected subgrid-scale effects in these regions.

Mean and fluctuating pressure in boat-tail separated flows at transonic speeds

Experiments were carried out investigating the features of mean and unsteady surface pressure fluctuations in boat-tail separated flows relevant to launch vehicle configurations at transonic speeds. The tests were performed on a generic axisymmetric body in the Mach-number range of 0.7-1.2, and the important geometrical parameters, namely, the boat-tail angle and diameter ratio, were varied systematically. The measurements made included primarily the mean and unsteady surface-pressure fluctuations on nine different model configurations. Flow-visualization studies employing a surface oil flow, and schlieren techniques were carried out to infer features like boundary-layer separation, reattachment, and shock waves in the flow. The features of mean and fluctuating surface pressures are discussed in detail including aspects of similarity. It has been observed that, on a generic configuration employed in the present study, the maximum levels of surface-pressure fluctuations in the reattachment zone are appreciably lower than those found on launch vehicle configurations having a bulbous or hammerhead nose shape. A simple correlation is suggested for the maximum value of rms pressure fluctuations in the reattachment zone at different freestream Mach numbers.

Role of length scales on microwave thawing dynamics in 2D cylinders

Microwave (MW) thawing of 2D frozen cylinders exposed to uniform plane waves from one face, is modeled using the effective heat capacity formulation with the MW power obtained from the electric field equations. Computations are illustrated for tylose (23% methyl cellulose gel) which melts over a range of temperatures giving rise to a mushy zone. Within the mushy region the dielectric properties are functions of the liquid volume fraction. The resulting coupled, time dependent non-linear equations are solved using the Galerkin finite element method with a fixed mesh. Our method efficiently captures the multiple connected thawed domains that arise due to the penetration of MWs in the sample. For a cylinder of diameter D, the two length scales that control the thawing dynamics are D/D-p and D/lambda(m), where D-p and lambda(m) are the penetration depth and wavelength of radiation in the sample respectively. For D/D-p, D/lambda(m) much less than 1 power absorption is uniform and thawing occurs almost simultaneously across the sample (Regime I). For D/D-p much greater than 1 thawing is seen to occur from the incident face, since the power decays exponentially into the sample (Regime III). At intermediate values, 0.2 < D/D-p, D/lambda(m) < 2.0 (Regime II) thawing occurs from the unexposed face at smaller diameters, from both faces at intermediate diameters and from the exposed and central regions at larger diameters. Average power absorption during thawing indicates a monotonic rise in Regime I and a monotonic decrease in Regime III. Local maxima in the average power observed for samples in Regime II are due to internal resonances within the sample. Thawing time increases monotonically with sample diameter and temperature gradients in the sample generally increase from Regime I to Regime III. (C) 2002 Elsevier Science Ltd. All rights reserved.

Tailoring the microstructure and mechanical properties of arrays of aligned multiwall carbon nanotubes by utilizing different hydrogen concentrations during synthesis

We synthesize vertically aligned arrays of carbon nanotubes (CNTs) in a chemical vapor deposition system with floating catalyst, using different concentrations of hydrogen in the gas feedstock. We report the effect of different hydrogen concentrations on the microstructure and mechanical properties of the resulting material. We show that a lower hydrogen concentration during synthesis results in the growth of stiffer CNT arrays with higher average bulk density. A lower hydrogen concentration also leads to the synthesis of CNT arrays that can reach higher peak stress at maximum compressive strain, and dissipate a larger amount of energy during compression. The individual CNTs in the arrays synthesized with a lower hydrogen concentration have, on average, larger outer diameters (associated with the growth of CNTs with a larger number of walls), but present a less uniform diameter distribution. The overall heights of the arrays and their strain recovery after compression have been found to be independent of the hydrogen concentration during growth. (C) 2011 Elsevier Ltd. All rights reserved.

General theories for the electrical transport properties of carbon nanotubes

We have shown that the general theories of metals and semiconductors can be employed to understand the diameter and voltage dependency of current through metallic and semiconducting carbon nanotubes, respectively. The current through a semiconducting multiwalled carbon nanotube (MWCNT) is associated with the energy gap that is different for different shells. The contribution of the outermost shell is larger as compared to the inner shells. The general theories can also explain the diameter dependency of maximum current through nanotubes. We have also compared the current carrying ability of a MWCNT and an array of the same diameter of single wall carbon nanotubes (SWCNTs) and found that MWCNTs are better suited and deserve further investigation for possible applications as interconnects.

Surfaces of Bounded Mean Curvature In Riemannian Manifolds

Consider a sequence of closed, orientable surfaces of fixed genus g in a Riemannian manifold M with uniform upper bounds on the norm of mean curvature and area. We show that on passing to a subsequence, we can choose parametrisations of the surfaces by inclusion maps from a fixed surface of the same genus so that the distance functions corresponding to the pullback metrics converge to a pseudo-metric and the inclusion maps converge to a Lipschitz map. We show further that the limiting pseudo-metric has fractal dimension two. As a corollary, we obtain a purely geometric result. Namely, we show that bounds on the mean curvature, area and genus of a surface F subset of M, together with bounds on the geometry of M, give an upper bound on the diameter of F. Our proof is modelled on Gromov's compactness theorem for J-holomorphic curves.

Role of spacer lengths of gemini surfactants in the synthesis of silver nanorods in micellar media

In this work, we have prepared Ag-nanorods using biscationic gemini surfactant micelles as the media by a seed-mediated wet synthesis method. Towards this end, we first synthesized Ag-nanoseeds of diameter similar to 7 nm stabilized by trisodium citrate (as the capping agent). Then these Ag-nanoseeds were used to synthesize Ag-nanorods of different aspect ratios. With decreasing Ag-nanoseed concentration, the aspect ratios of the Ag-nanorods stabilized by these gemini surfactants increased gradually. Various Ag-nanoseeds and Ag-nanospecies were characterized using UV-Vis spectroscopy (to know the surface plasmon bands), transmission electron microscopy (to find out their particle sizes and distribution), energy-dispersive X-ray spectroscopy and X-ray diffraction. When we used micelles derived from gemini surfactants of shorter spacer-(CH(2))(n)-(n = 2 or 4) to stabilize the Ag-nanorods, the lambda(max) of the longitudinal band shifted more towards the blue region compared to that of the gemini surfactant micelles with a longer spacer-(CH(2))(n)-(n = 5, 12) at a given amount of the Ag-nanoseed solution. So, the growth of Ag-nanorods in the gemini micellar solutions depends on the spacer-chain length of gemini surfactants employed.

A dynamical instability due to fluid–wall coupling lowers the transition Reynolds number in the flow through a flexible tube

A flow-induced instability in a tube with flexible walls is studied experimentally. Tubes of diameter 0.8 and 1.2 mm are cast in polydimethylsiloxane (PDMS) polymer gels, and the catalyst concentration in these gels is varied to obtain shear modulus in the range 17–550 kPa. A pressure drop between the inlet and outlet of the tube is used to drive fluid flow, and the friction factor $f$ is measured as a function of the Reynolds number $Re$. From these measurements, it is found that the laminar flow becomes unstable, and there is a transition to a more complicated flow profile, for Reynolds numbers as low as 500 for the softest gels used here. The nature of the $f$–$Re$ curves is also qualitatively different from that in the flow past rigid tubes; in contrast to the discontinuous increase in the friction factor at transition in a rigid tube, it is found that there is a continuous increase in the friction factor from the laminar value of $16\ensuremath{/} Re$ in a flexible tube. The onset of transition is also detected by a dye-stream method, where a stream of dye is injected into the centre of the tube. It is found that there is a continuous increase of the amplitude of perturbations at the onset of transition in a flexible tube, in contrast to the abrupt disruption of the dye stream at transition in a rigid tube. There are oscillations in the wall of the tube at the onset of transition, which is detected from the laser scattering off the walls of the tube. This indicates that the coupling between the fluid stresses and the elastic stresses in the wall results in an instability of the laminar flow.

Tailoring of Structural Morphology of Silver Nanowires in Electrochemical Growth

Noble metal such as Ag normally exists in an fcc crystal structure. However as the size of the material is decreased to nanometer lengthscales, a structural transformation from that of its bulk state can be expected with new atomic arrangements due to competition between internal packing and minimization of surface energy. In many previous studies, it has been shown that silver nanowires (AGNWs) grown inside anodic alumina (AAO) templates by ac or dc electrochemical deposition from silver salts or complexes, adopt fcc structure and below some critical diameter ∼ 20 nm they may acquire hcp structure at low temperature. This is, however, critically dependant on the nature of confinement, as AgNWs grown inside nanotube confinement with subnanometer diameter have been reported to have fcc structure. Hence the question of the crystal structure of metal nanowires under combined influence of confinement, temperature and deposition condition remains open. In this abstract we show that the alternative crystal structures of AGNWs at room temperature can be achieved with electrochemical growth processes under specific conditions determined by the deposition parameters and nature of confinement. We fabricated AgNWs of 4H hexagonal structure with diameters 30 – 80 nm inside polycarbonate (PC) templates with a modified dc electrodeposition technique, where the nanowires were grown at deposition potentials as low as 10 mV in 2 M silver nitrate solution[1]. We call this low-potential electrodeposition (LPED) since the electrodeposition process occurs at potential much less than the standard Nernst potential (770 mV) of silver. Two types of electrodes were used – stainless steel and sputtered thin Pt film, neither of which had any influence on the crystal structure of the nanowires. EDS elemental analysis showed the nanowires to consist only of silver. Although the precise atomic dynamics during the LPED process is unclear at present, we investigated this with HRTEM (high-resolution transmission electron microscopy) characterization of nanowires grown over various deposition times, as well as electrical conductivity measurements. These experiments indicate that nanowire growth does not occur through a three-dimensional diffusion controlled process, as proposed for conventional over-potential deposition, but follow a novel instantaneous linear growth mechanism. Further experiments showed that, (a) conventional electrochemical growth at a small over-potential in a 2 mM AgNO3 solution yields nanowires with expected fcc structure inside the same PC templates, and (2) no nanowire was observed under the LPED conditions inside hard AAO templates, indicating that LPED-growth process, and hcp structure of the corresponding nanowires depend on deposition parameters, as well as nature of confinement.