5,772 research outputs found

    Precession of the Isolated Neutron Star PSR B1828-11

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    Stairs, Lyne & Shemar have found that arrival time residuals from PSR B1828-11 vary periodically with a period of 500 days. This behavior can be accounted for by precession of the radiopulsar, an interpretation that is reinforced by the detection of variations in its pulse profile on the same timescale. Here, we model the period residuals from PSR B1828-11 in terms of precession of a triaxial rigid body. We include two contributions to the residuals: (i) the geometric effect, which arises because the times at which the pulsar emission beam points toward the observer varies with precession phase; (ii) the spindown contribution, which arises from any dependence of the spindown torque acting on the pulsar on the angle between its spin and magnetic axes. We use the data to probe numerous properties of the pulsar, most notably its shape, and the dependence of its spindown torque on the angle between its spin and magnetic axes, for which we assume a sum of a spin-aligned component (with a weight 1-a) and a dipolar component perpendicular to the magnetic beam axis (weight a), rather than the vacuum dipole torque (a=1). We find that a variety of shapes are consistent with the residuals, with a slight statistical preference for a prolate star. Moreover, a range of torque possibilities fit the data equally well, with no strong preference for the vacuum model. In the case of a prolate star we find evidence for an angle-dependent spindown torque. Our results show that the combination of geometrical and spin-down effects associated with precession can account for the principal features of PSR B1828-11's timing behavior, without fine tuning of the parameters.Comment: 22 pages, 14 figures, submitted to MNRAS; added references, corrected typo

    Surface-decorated ZnO nanoparticles and ZnO nanocoating on electrospun polymeric nanofibers by atomic layer deposition for flexible photocatalytic nanofibrous membranes

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    Cataloged from PDF version of article.Electrospun polymeric nanofibers were either surface-decorated with zinc oxide (ZnO) nanoparticles or coated with a continuous ZnO thin film with a precise thickness (similar to 27 nm) via atomic layer deposition (ALD) for the fabrication of flexible photocatalytic nanofibrous membranes

    Polymer-inorganic core-shell nanofibers by electrospinning and atomic layer deposition: flexible nylon-znO core-shell nanofiber mats and their photocatalytic activity

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    Cataloged from PDF version of article.Polymer-inorganic core-shell nanofibers were produced by two-step approach; electrospinning and atomic layer deposition (ALD). First, nylon 6,6 (polymeric core) nanofibers were obtained by electrospinning, and then zinc oxide (ZnO) (inorganic shell) with precise thickness control was deposited onto electrospun nylon 6,6 nanofibers using ALD technique. The bead-free and uniform nylon 6,6 nanofibers having different average fiber diameters (Ôł╝80, Ôł╝240 and Ôł╝650 nm) were achieved by using two different solvent systems and polymer concentrations. ZnO layer about 90 nm, having uniform thickness around the fiber structure, was successfully deposited onto the nylon 6,6 nanofibers. Because of the low deposition temperature utilized (200 ┬░C), ALD process did not deform the polymeric fiber structure, and highly conformal ZnO layer with precise thickness and composition over a large scale were accomplished regardless of the differences in fiber diameters. ZnO shell layer was found to have a polycrystalline nature with hexagonal wurtzite structure. The core-shell nylon 6,6-ZnO nanofiber mats were flexible because of the polymeric core component. Photocatalytic activity of the core-shell nylon 6,6-ZnO nanofiber mats were tested by following the photocatalytic decomposition of rhodamine-B dye. The nylon 6,6-ZnO nanofiber mat, having thinner fiber diameter, has shown better photocatalytic efficiency due to higher surface area of this sample. These nylon 6,6-ZnO nanofiber mats have also shown structural stability and kept their photocatalytic activity for the second cycle test. Our findings suggest that core-shell nylon 6,6-ZnO nanofiber mat can be a very good candidate as a filter material for water purification and organic waste treatment because of their photocatalytic properties along with structural flexibility and stability. ┬ę 2012 American Chemical Society

    Template-based synthesis of aluminum nitride hollow Nanofibers via plasma-enhanced atomic layer deposition

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    Cataloged from PDF version of article.Aluminum nitride (AlN) hollow nanofibers were synthesized via plasma-enhanced atomic layer deposition using sacrificial electrospun polymeric nanofiber templates having different average fiber diameters (~70, ~330, and ~740 nm). Depositions were carried out at 200┬░C using trimethylaluminum and ammonia precursors. AlN-coated nanofibers were calcined subsequently at 500┬░C for 2 h to remove the sacrificial polymeric nanofiber template. SEM studies have shown that there is a critical wall thickness value depending on the template's average fiber diameter for AlN hollow nanofibers to preserve their shapes after the template has been removed by calcination. Best morphologies were observed for AlN hollow nanofibers prepared by depositing 800 cycles (corresponding to ~69 nm) on nanofiber templates having ~330 nm average fiber diameter. TEM images indicated uniform wall thicknesses of ~65 nm along the fiber axes for samples prepared using templates having ~70 and ~330 nm average fiber diameters. Synthesized AlN hollow nanofibers were polycrystalline with a hexagonal crystal structure as determined by high-resolution TEM and selected area electron diffraction. Chemical compositions of coated and calcined samples were studied using X-ray photoelectron spectroscopy (XPS). High-resolution XPS spectra confirmed the presence of AlN. ┬ę 2012 The American Ceramic Societ

    Fabrication of AlN/BN bishell hollow nanofibers by electrospinning and atomic layer deposition

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    Cataloged from PDF version of article.Aluminum nitride (AlN)/boron nitride (BN) bishell hollow nanofibers (HNFs) have been fabricated by successive atomic layer deposition (ALD) of AlN and sequential chemical vapor deposition (CVD) of BN on electrospun polymeric nanofibrous template. A four-step fabrication process was utilized: (i) fabrication of polymeric (nylon 6,6) nanofibers via electrospinning, (ii) hollow cathode plasma-assisted ALD of AlN at 100 degrees C onto electrospun polymeric nanofibers, (iii) calcination at 500 degrees C for 2 h in order to remove the polymeric template, and (iv) sequential CVD growth of BN at 450 degrees C. AlN/BN HNFs have been characterized for their chemical composition, surface morphology, crystal structure, and internal nanostructure using X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, and selected area electron diffraction. Measurements confirmed the presence of crystalline hexagonal BN and AlN within the three dimensional (3D) network of bishell HNFs with relatively low impurity content. In contrast to the smooth surface of the inner AlN layer, outer BN coating showed a highly rough 3D morphology in the form of BN nano-needle crystallites. It is shown that the combination of electrospinning and plasma-assisted low-temperature ALD/CVD can produce highly controlled multi-layered bishell nitride ceramic hollow nanostructures. While electrospinning enables easy fabrication of nanofibrous template, self-limiting reactions of plasma-assisted ALD and sequential CVD provide control over the wall thicknesses of AlN and BN layers with sub-nanometer accuracy. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License

    Wind Speed Analysis Using Signal Processing Technique

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    In this study, wind energy, the formation of this energy, and the necessary stages for the production of electrical energy will be discussed. Then, the countriesÔÇÖ investments in wind energy will be mentioned. In the mathematical background, statistical methods and signal processing methods are used in the calculation of wind energy efficiency. In this chapter, a detailed analysis of the use of wind speed data with signal processing techniques will be made using the hourly wind speed data of Istanbul for the last 10 years. This data will be analyzed by the Fourier method. Afterward, analyses will be made with short-time Fourier transform (STFT) and bi-spectrum analysis method, and these results will be compared. The data obtained from the study can be considered as a framework for the wind farms to be constructed

    Structural, optical and electrical characteristics BaSrTiOx thin films: Effect of deposition pressure and annealing

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    Among perovskite oxide materials, BaSrTiOx (BST) has attracted great attention due to its potential applications in oxide-based electronics. However, reliability and efficiency of BST thin films strongly depend on the precise knowledge of the film microstructure, as well as optical and electrical properties. In the present work, BST films were deposited at room temperature using radio frequency magnetron sputtering technique. The impact of deposition pressure, partial oxygen flow, and post-deposition annealing treatment on film microstructure, surface morphology, refractive index, and dielectric constants were studied by X-ray diffraction, scanning electron microscopy, spectrophotometry, ellipsometry, photoluminescence, as well as capacitance-voltage measurements. Well-adhered and uniform amorphous films were obtained at room temperature. For all as-deposited films, the average optical transmission was ~ 85% in the VIS-NIR spectrum. The refractive indices of BST films were in the range of 1.90ÔÇô2.07 (╬╗ = 550 nm). Post-deposition annealing at 800 ┬░C for 1 h resulted in polycrystalline thin films with increased refractive indices and dielectric constants, however reduced optical transmission values. Frequency dependent dielectric constants were found to be in the range of 46ÔÇô72. However, the observed leakage current was relatively small, about 1 ╬╝A. The highest FOM values were obtained for films deposited at 0.67 Pa pressures, while charge storage capacity values increased with increased deposition pressure. Results show that room-temperature grown BST films have potential for device applications. ┬ę 2017 Elsevier B.V
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