Effective Q Criterion for Disk Stability in an External Potential

The standard Q criterion (with Q > 1) describes the stability against local, axisymmetric perturbations in a disk supported by rotation and random motion. Most astrophysical disks, however, are under the influence of an external gravitational potential, which can significantly affect their stability. A typical example is a galactic disk embedded in a dark matter halo. Here, we do a linear perturbation analysis for a disk in an external potential and obtain a generalized dispersion relation and the effective stability criterion. An external potential, such as that due to the dark matter halo concentric with the disk, contributes to the unperturbed rotational field and significantly increases its stability. We obtain the values for the effective Q parameter for the Milky Way and for a low surface brightness galaxy, UGC 7321. We find that in each case the stellar disk by itself is barely stable and it is the dark matter halo that stabilizes the disk against local, axisymmetric gravitational instabilities. Thus, the dark matter halo is necessary to ensure local disk stability. This result has been largely missed so far because in practice the Q parameter for a galactic disk is obtained using the observed rotational field that already includes the effect of the halo

EFFECTIVE Q CRITERION FOR DISK STABILITY IN AN EXTERNAL POTENTIAL

standard Q criterion (with Q > 1) describes the stability against local, axisymmetric perturbations in a disk supported by rotation and random motion. Most astrophysical disks, however, are under the influence of an external gravitational potential, which can significantly affect their stability. A typical example is a galactic disk embedded in a dark matter halo. Here, we do a linear perturbation analysis for a disk in an external potential and obtain a generalized dispersion relation and the effective stability criterion. An external potential, such as that due to the dark matter halo concentric with the disk, contributes to the unperturbed rotational field and significantly increases its stability. We obtain the values for the effective Q parameter for the Milky Way and for a low surface brightness galaxy, UGC 7321. We find that in each case the stellar disk by itself is barely stable and it is the dark matter halo that stabilizes the disk against local, axisymmetric gravitational instabilities. Thus, the dark matter halo is necessary to ensure local disk stability. This result has been largely missed so far because in practice the Q parameter for a galactic disk is obtained using the observed rotational field that already includes the effect of the halo.

Analytical test functions for free vibration analysis of rotating non-homogeneous Timoshenko beams

In this paper, the governing equations for free vibration of a non-homogeneous rotating Timoshenko beam, having uniform cross-section, is studied using an inverse problem approach, for both cantilever and pinned-free boundary conditions. The bending displacement and the rotation due to bending are assumed to be simple polynomials which satisfy all four boundary conditions. It is found that for certain polynomial variations of the material mass density, elastic modulus and shear modulus, along the length of the beam, the assumed polynomials serve as simple closed form solutions to the coupled second order governing differential equations with variable coefficients. It is found that there are an infinite number of analytical polynomial functions possible for material mass density, shear modulus and elastic modulus distributions, which share the same frequency and mode shape for a particular mode. The derived results are intended to serve as benchmark solutions for testing approximate or numerical methods used for the vibration analysis of rotating non-homogeneous Timoshenko beams.

Voltammetric detection of arsenic on a bismuth modified exfoliated graphite electrode

We present the application of a bismuth modified exfoliated graphite electrode in the detection of arsenic in water. Bismuth film was electrodeposited onto an exfoliated graphite (EG) electrode at a potential of -600 mV. The modification of EG resulted in an increase in the electroactive surface area of the electrode and consequently peak current enhancement in Ru(NH3)(6)(2+/13+) redox probe. Square wave anodic stripping voltammetry was performed with the modified electrode (EG-Bi) in As (III) solutions at the optimum conditions of pH 6, deposition potential of -600 mV and pre-concentration time of 180s. The EG-Bi was able to detect As (III) to the limit of 5 mu g L-1 and was not susceptible to many interfering cations except Cu (II). The EG-Bi is low cost and easy to prepare. (C) 2013 Elsevier Ltd. All rights reserved.

Coconut kernel-derived activated carbon as electrode material for electrical double-layer capacitors

Carbonization of milk-free coconut kernel pulp is carried out at low temperatures. The carbon samples are activated using KOH, and electrical double-layer capacitor (EDLC) properties are studied. Among the several samples prepared, activated carbon prepared at 600 A degrees C has a large surface area (1,200 m(2) g(-1)). There is a decrease in surface area with increasing temperature of preparation. Cyclic voltammetry and galvanostatic charge-discharge studies suggest that activated carbons derived from coconut kernel pulp are appropriate materials for EDLC studies in acidic, alkaline, and non-aqueous electrolytes. Specific capacitance of 173 F g(-1) is obtained in 1 M H2SO4 electrolyte for the activated carbon prepared at 600 A degrees C. The supercapacitor properties of activated carbon sample prepared at 600 A degrees C are superior to the samples prepared at higher temperatures.

High specific surface area alpha-Fe2O3 nanostructures as high performance electrode material for supercapacitors

Porous alpha-Fe2O3 nanostructures have been synthesized by a simple sol-gel route. The alpha-Fe2O3 nanostructures are poorly crystalline and porous with BET surface area of 386 m(2) g(-1). The high discharge capacitance of alpha-Fe2O3 electrodes is 300 F g(-1) when the electrodes are cycled in 0.5 M Na2SO3 at a current density of 1 A g(-1). The capacitance retention after 1000 cycles is about 73% of the initial capacitance at a current density of 2 A g(-1). The high discharge capacitance of alpha-Fe2O3 in comparison with the literature reports are attributed to high surface area and porosity of the iron oxide prepared in the present study. As the iron oxides are inexpensive, the capacity of alpha-Fe2O3 is expected to be of potential use for supercapacitor application. (C) 2014 Elsevier B.V. All rights reserved.

A 3.8-V earth-abundant sodium battery electrode

Rechargeable lithium batteries have ushered the wireless revolution over last two decades and are now matured to enable green automobiles. However, the growing concern on scarcity and large-scale applications of lithium resources have steered effort to realize sustainable sodium-ion batteries, Na and Fe being abundant and low-cost charge carrier and redox centre, respectively. However, their performance is limited owing to low operating voltage and sluggish kinetics. Here we report a hitherto-unknown material with entirely new composition and structure with the first alluaudite-type sulphate framework, Na2Fe2(SO4)(3), registering the highest-ever Fe3+/ Fe2+ redox potential at 3.8V (versus Na, and hence 4.1V versus Li) along with fast rate kinetics. Rare-metal-free Na-ion rechargeable battery system compatible with the present Li-ion battery is now in realistic scope without sacrificing high energy density and high power, and paves way for discovery of new earth-abundant sustainable cathodes for large-scale batteries.

Iminodiacetic acid functionalized polypyrrole modified electrode as Pb(II) sensor: Synthesis and DPASV studies

An electrochemical lead ion sensor has been developed by modification of carbon paste electrode (CPE) using polypyrrole functionalized with iminodiacetic acid (IDA-PPy) containing carboxyl group. The electrochemical response of Pb2+ ion on the IDA-PPy modified CPE has been evaluated and the controling parameters have been optimized using differential pulse anodic stripping voltammetry (DPASV). The IDA-PPy modified CPE shows a linear correlation for Pb2+ concentrations in the range of 1 x 10(-6) to 5 x 10(-9) M and the lower detection limit of Pb2+ has been found to be 9.6 x 10(-9) M concentration. Other tested metal ions, namely Cu2+, Cd2+, Co2+, Hg2+, Ni2+ and Zn2+, do not exhibit any voltammetric stripping response below 1 x 10(-7) M concentration. However, the Pb2+ response is affected in the presence of molar equivalents or higher concentrations of Cu2+, Cd2+ and Co2+ ions in binary systems with Pb2+, consequent to their ability to bind with iminodiacetic acid, while Hg2+, Ni2+ and Zn2+ do not interfere at all. A good correlation has been observed between the lead concentrations as analyzed by DPASV using IDA-PPy modified CPE and atomic absorption spectrophotometry for a lead containing industrial effluent sample. (C) 2014 Elsevier Ltd. All rights reserved.

Nanostructured electrode materials for electrochemical energy storage and conversion

Manipulation of matter at the nanoscale is a way forward to move beyond our current choices in electrochemical energy storage and conversion technologies with promise of higher efficiency, environmental benignity, and cost-effectiveness. Electrochemical processes being basically surface phenomena, tailored multifunctional nanoarchitecturing can lead to improvements in terms of electronic and ionic conductivities, diffusion and mass transport, and electron transfer and electrocatalysis. The nanoscale is also a domain in which queer properties surface: those associated with conversion electrodes, ceramic particles enhancing the conductivity of polymer electrolytes, and transition metal oxide powders catalyzing fuel cell reactions, to cite a few. Although this review attempts to present a bird's eye view of the vast literature that has accumulated in this rather infant field, it also lists a few representative studies that establish the beneficial effects of going `nano'. Investigations on nanostructuring and use of nanoparticles and nanoarchitectures related to lithium-ion batteries (active materials and electrolytes), supercapacitors (electrical double-layer capacitors, supercapacitors based on pseudo-capacitance, and hybrid supercapacitors), and fuel cells (electrocatalysts, membranes and hydrogen storage materials) are highlighted. (C) 2012 John Wiley & Sons, Ltd.

Porous lithium rich Li1.2Mn0.54Ni0.22Fe0.04O2 prepared by microemulsion route as a high capacity and high rate capability positive electrode material

A porous layered composite of Li2MnO3 and LiMn0.35Ni0.55Fe0.1O2 (composition:Li1.2Mn0.54Ni0.22Fe0.04O2) is prepared by inverse microemulsion method and studied as a positive electrode material. The precursor is heated at several temperatures between 500 and 900 degrees C. The X-ray diffraction, scanning electron microscopy, and transmission electron microscopy studies suggested that well crystalline submicronsized particles are obtained. The product samples possess mesoporosity with broadly distributed pores around 10 similar to 50 nm diameter. Pore volume and surface area decrease by increasing the temperature of preparation. However, the electrochemical activity of the composite samples increases with an increase in temperature. The discharge capacity values of the samples prepared at 900 degrees C are about 186 mAh g(-1) at a specific current of 25 mA g(-1) with an excellent cycling stability. The composite sample also possesses high rate capability. The high rate capability is attributed to the porous nature of the material. (C) 2014 Elsevier Ltd. All rights reserved.

Modal tailoring and closed-form solutions for rotating non-uniform Euler-Bernoulli beams

In this paper, the free vibration of a rotating Euler-Bernoulli beam is studied using an inverse problem approach. We assume a polynomial mode shape function for a particular mode, which satisfies all the four boundary conditions of a rotating beam, along with the internal nodes. Using this assumed mode shape function, we determine the linear mass and fifth order stiffness variations of the beam which are typical of helicopter blades. Thus, it is found that an infinite number of such beams exist whose fourth order governing differential equation possess a closed form solution for certain polynomial variations of the mass and stiffness, for both cantilever and pinned-free boundary conditions corresponding to hingeless and articulated rotors, respectively. A detailed study is conducted for the first, second and third modes of a rotating cantilever beam and the first and second elastic modes of a rotating pinned-free beam, and on how to pre-select the internal nodes such that the closed-form solutions exist for these cases. The derived results can be used as benchmark solutions for the validation of rotating beam numerical methods and may also guide nodal tailoring. (C) 2014 Elsevier Ltd. All rights reserved.

Analysis of Entropy Generation During Mixed Convective Heat Transfer of Nanofluids Past a Rotating Circular Cylinder

The entropy generation due to mixed convective heat transfer of nanofluids past a rotating circular cylinder placed in a uniform cross stream is investigated via streamline upwind Petrov-Galerkin based finite element method. Nanosized copper (Cu) particles suspended in water are used with Prandtl number (Pr)=6.9. The computations are carried out at a representative Reynolds number (Re) of 100. The dimensionless cylinder rotation rate, a, is varied between 0 and 2. The range of nanoparticle volume fractions (phi) considered is 0 <= phi <= 5%. Effect of aiding buoyancy is brought about by considering two fixed values of the Richardson number (Ri) as 0.5 and 1.0. A new model for predicting the effective viscosity and thermal conductivity of dilute suspensions of nanoscale colloidal particles is presented. The model addresses the details of the agglomeration-deagglomeration in tune with the pertinent variations in the effective particulate dimensions, volume fractions, as well as the aggregate structure of the particulate system. The total entropy generation is found to decrease sharply with cylinder rotation rates and nanoparticle volume fractions. Increase in nanoparticle agglomeration shows decrease in heat transfer irreversibility. The Bejan number falls sharply with increase in alpha and phi.

Si-mediated fabrication of reduced graphene oxide and its hybrids for electrode materials

Here, we demonstrate a Si-mediated environmentally friendly reduction of graphene oxide (GO) and the fabrication of its hybrids with multiwall carbon nanotubes and nanofibers. The reduction of GO is facilitated by nascent hydrogen generated by the reaction between Si and KOH at similar to 60 degrees C. The overall process takes 5 to 7 minutes and 10 to 15 mu m of Si is consumed each time. We show that Si can be used multiple times and the rGO based hybrids can be used for electrode materials.

Three centered hydrogen bonds of the type C=O...H(N)...X-C in diphenyloxamide derivatives involving halogens and a rotating CF3 group: NMR, QTAIM, NCI and NBO studies

The existence of three centered C=O...H(N)...X-C hydrogen bonds (H-bonds) involving organic fluorine and other halogens in diphenyloxamide derivatives has been explored by NMR spectroscopy and quantum theoretical studies. The three centered H-bond with the participation of a rotating CF3 group and the F...H-N intramolecular hydrogen bonds, a rare observation of its kind in organofluorine compounds, has been detected. It is also unambiguously established by a number of one and two dimensional NMR experiments, such as temperature perturbation, solvent titration, N-15-H-1 HSQC, and F-19-H-1 HOESY, and is also confirmed by theoretical calculations, such as quantum theory of atoms in molecules (QTAIM), natural bond orbital (NBO) and non-covalent interaction (NCI).

Mixed Mode Analysis of Obliquely Oriented Edge Cracked Semicircular Disk Under Compression

A new mixed-mode compression fracture specimen, obliquely oriented edge cracked semicircular disk (OECSD) is analyzed by extending pure opening mode configuration of edge cracked semicircular disk (ECSD) under Hertzian compression. Photoelastic experiments are conducted on two different specimens of OECSD of same size and different crack lengths and inclinations. Finite element method (FEM) is used to solve a number of cases of the problem varying crack length and crack inclination. FE results show a good match with experiments. Inclination of edge crack in OECSD can be so made as to obtain any mode-mixity ratio between zero and one and beyond for any crack length. The new specimen can be used for fracture testing under compression more conveniently than the existing ones in several ways.