4,936 research outputs found
Preparation and spectroscopic studies of Hydroxyapatite embedded in Poly(N-Vinylpyrrolidone) matrix
Pure HAp powder was prepared by precipitation and HAp/PVP composites were prepared by casting technique. X-ray diffraction was performed to examine the crystallinity and the complexation between HAp and PVP when the amount of HAp increased. XRD and EDX analysis showed that the concentration 15 wt% is the most confirmation for the formation of HAp. SEM shows more than one form of HAp such as plates, needles and bundles in the morphological structure of pure HAp and 15 wt% of HAp. Two main weight loss regions and two peaks were observed in TGA and DSC thermograms that reveal reduction in the thermal stability of the prepared samples as HAp content increases. The activation energy of the films was decreased with increasing the concentration of HAp. DSC thermographs of the samples show that the values of the glass transition temperatures are decreased with increasing the content of HAp filler which suggest that HAp filler act as plasticizer. The conductivity–temperature plots show linear variation, which suggests an Arrhenius behavior. The conductivity increases with adding HAp content for low concentrations while decreased for the critical concentration (15 wt %) which confirms the XRD results
Wave Excitation in Disks Around Rotating Magnetic Stars
The accretion disk around a rotating magnetic star (neutron star, white dwarf
or T Tauri star) is subjected to periodic vertical magnetic forces from the
star, with the forcing frequency equal to the stellar spin frequency or twice
the spin frequency. This gives rise bending waves in the disk that may
influence the variabilities of the system. We study the excitation, propagation
and dissipation of these waves using a hydrodynamical model coupled with a
generic model description of the magnetic forces. The bending waves are
excited at the Lindblad/vertical resonance, and propagate either to larger
radii or inward toward the corotation resonance where dissipation takes place.
While the resonant torque is negligible compared to the accretion torque, the
wave nevertheless may reach appreciable amplitude and can cause or modulate
flux variabilities from the system. We discuss applications of our result to
the observed quasi-periodic oscillations from various systems, in particular
neutron star low-mass X-ray binaries.Comment: Small changes/clarifications. To be published in ApJ, Aug.20,2008
issu
MHD simulations of disk-star interaction
We discuss a number of topics relevant to disk-magnetosphere interaction and
how numerical simulations illuminate them. The topics include: (1)
disk-magnetosphere interaction and the problem of disk-locking; (2) the wind
problem; (3) structure of the magnetospheric flow, hot spots at the star's
surface, and the inner disk region; (4) modeling of spectra from 3D funnel
streams; (5) accretion to a star with a complex magnetic field; (6) accretion
through 3D instabilities; (7) magnetospheric gap and survival of protoplanets.
Results of both 2D and 3D simulations are discussed.Comment: 12 pages, 10 figures, Star-Disk Interaction in Young Stars,
Proceedings of the International Astronomical Union, IAU Symposium, Volume
243. See animations at http://astro.cornell.edu/~romanova/projects.htm and at
http://astro.cornell.edu/us-rus
"Propeller" Regime of Disk Accretion to Rapidly Rotating Stars
We present results of axisymmetic magnetohydrodynamic simulations of the
interaction of a rapidly-rotating, magnetized star with an accretion disk. The
disk is considered to have a finite viscosity and magnetic diffusivity. The
main parameters of the system are the star's angular velocity and magnetic
moment, and the disk's viscosity, diffusivity. We focus on the "propeller"
regime where the inner radius of the disk is larger than the corotation radius.
Two types of magnetohydrodynamic flows have been found as a result of
simulations: "weak" and "strong" propellers. The strong propeller is
characterized by a powerful disk wind and a collimated magnetically dominated
outflow or jet from the star. The weak propeller have only weak outflows. We
investigated the time-averaged characteristics of the interaction between the
main elements of the system, the star, the disk, the wind from the disk, and
the jet. Rates of exchange of mass and angular momentum between the elements of
the system are derived as a function of the main parameters. The propeller
mechanism may be responsible for the fast spinning-down of the classical T
Tauri stars in the initial stages of their evolution, and for the spinning-down
of accreting millisecond pulsars.Comment: 18 pages, 16 figures, ApJ (accepted), added references, corrected
typos; see animation at
http://astrosun2.astro.cornell.edu/us-rus/disk_prop.ht
Magnetically Torqued Thin Accretion Disks
We compute the properties of a geometrically thin, steady accretion disk
surrounding a central rotating, magnetized star. The magnetosphere is assumed
to entrain the disk over a wide range of radii. The model is simplified in that
we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal
component of the magnetic field as a function of radial distance. We find a
solution for the angular velocity profile tending to corotation close to the
central star, and smoothly matching a Keplerian curve at a radius where the
viscous stress vanishes. The value of this ''transition'' radius is nearly the
same for both of our adopted B-field models. We then solve analytically for the
torques on the central star and for the disk luminosity due to gravity and
magnetic torques. When expressed in a dimensionless form, the resulting
quantities depend on one parameter alone, the ratio of the transition radius to
the corotation radius. For rapid rotators, the accretion disk may be powered
mostly by spin-down of the central star. These results are independent of the
viscosity prescription in the disk. We also solve for the disk structure for
the special case of an optically thick alpha disk. Our results are applicable
to a range of astrophysical systems including accreting neutron stars,
intermediate polar cataclysmic variables, and T Tauri systems.Comment: 9 sharper figs, updated reference
The Propeller Regime of Disk Accretion to a Rapidly Rotating Magnetized Star
The propeller regime of disk accretion to a rapidly rotating magnetized star
is investigated here for the first time by axisymmetric 2.5D
magnetohydrodynamic simulations. An expanded, closed magnetosphere forms in
which the magnetic field is predominantly toroidal. A smaller fraction of the
star's poloidal magnetic flux inflates vertically, forming a magnetically
dominated tower. Matter accumulates in the equatorial region outside
magnetosphere and accretes to the star quasi-periodically through elongated
funnel streams which cause the magnetic field to reconnect. The star spins-down
owing to the interaction of the closed magnetosphere with the disk. For the
considered conditions, the spin-down torque varies with the angular velocity of
the star omega* as omega*^1.3 for fixed mass accretion rate. The propeller
stage may be important in the evolution of X-ray pulsars, cataclysmic variables
and young stars. In particular, it may explain the present slow rotation of the
classical T Tauri stars.Comment: 5 pages with 4 figures, LaTeX, macros: emulapj.sty, avi movies are
available at http://www.astro.cornell.edu/us-russia/disk_prop.ht
Windbreak-Grown Casuarina and Eucalyptus Trees for Unbleached Kraft Pulp
A laboratory-scale evaluation was conducted of juvenile windbreak-grown Casuarina and Eucalyptus trees for kraft pulp production. Test results of unscreened pulp yields, pulp chemical analyses, and handsheet physical properties indicated that windbreak-grown materials are suitable for unbleached kraft pulp. Casuarina gave the best pulp yield and had higher tear strength than Eucalyptus, but both species were superior to kraft pulps from agricultural raw materials such as rice straw and Thymelia, which are currently used in Egypt. For both species, the best kraft pulping schedule tested was a 4:1 liquor-to-wood ratio with 20% active alkali with additional conditions constant. Scanning electron micrographs of handsheets helped explain the observed differences in physical properties between the two species. Mixing of Casuarina and Eucalyptus raw material prior to pulping shows promise for unbleached kraft pulp production
Force-Free Models of Magnetically Linked Star-Disk Systems
Disk accretion onto a magnetized star occurs in a variety of astrophysical
contexts, from young stars to X-ray pulsars. The magnetohydrodynamic
interaction between the stellar field and the accreting matter can have a
strong effect on the disk structure, the transfer of mass and angular momentum
between the disk and the star, and the production of bipolar outflows, e.g.,
plasma jets. We study a key element of this interaction - the time evolution of
the magnetic field configuration brought about by the relative rotation between
the disk and the star - using simplified, largely semianalytic, models. We
first discuss the rapid inflation and opening up of the magnetic field lines in
the corona above the accretion disk, which is caused by the differential
rotation twisting. Then we consider additional physical effects that tend to
limit this expansion, such as the effect of plasma inertia and the possibility
of reconnection in the disk's corona, the latter possibly leading to repeated
cycles in the evolution. We also derive the condition for the existence of a
steady state for a resistive disk and conclude that a steady state
configuration is not realistically possible. Finally, we generalize our
analysis of the opening of magnetic field lines by using a non-self-similar
numerical model that applies to an arbitrarily rotating (e.g. keplerian) disk.Comment: 75 pages, 22 figures, 2 tables. Submitted to Astrophysical Journa
Izrada i značajke tankih polikristaliničnih hetero-spojeva n-Cd0.9Zn0.1S/p-CuGa0.3In0.7Se2
CuGa0.3In0.7Se2 polycrystalline thin films were prepared by thermal evaporation under vacuum of about 10-4Pa, with a deposition rate of about 200 nm/min. The selenization of these films at 723 K improves their properties. The activation energy as well as the optical energy gap of the investigated samples decreased with annealing and selenization. Polycrystalline thin film n-Cd0.9Zn0.1S/p-CuGa0.3In0.7Se2 heterojunctions were fabricated and the current density - voltage and capacitance - voltage characteristics of the junction were studied. The heterojunctions were exposed to light, and under illumination of 1000 mWcm-2, the open circuit voltage was 580 mV, the short circuit current density 4.8 mAcm-2, the fill factor 0.682 and the electrical conversion efficiency was 1.898% for cells of active area of 1 cm2.Tanke polikristalinične slojeve CuGa0.3In0.7Se2 pripremali smo naparavanjem u vakuumu oko 10−4 Pa, brzinom polaganja oko 200 nm/min. Seleniranje tih slojeva na 723 K poboljšava im svojstva. Aktivacijska energija i optički energijski procijep smanjuju se opuštanjem i seleniranjem. Pripremali smo i polikristalinične tankoslojne hetero-spojeve n-Cd0.9Zn0.1S/p-CuGa0.3In0.7Se2 i proučavali njihova značajke: gustoća struje – napon i kapacitet – napon. Ćelije ploštine 1 cm2 heterospojeva osvijetlili smo svjetlošću jakosti 1000 mW/cm2 i izmjerili napon otvorenog kruga od 580 mV, gustoću struje kratkog spoja 4.8 mAcm−2 , faktor punjenja 0.682 i učinkovitost pretvorbe 1.898%
Izrada i značajke tankih polikristaliničnih hetero-spojeva n-Cd0.9Zn0.1S/p-CuGa0.3In0.7Se2
CuGa0.3In0.7Se2 polycrystalline thin films were prepared by thermal evaporation under vacuum of about 10-4Pa, with a deposition rate of about 200 nm/min. The selenization of these films at 723 K improves their properties. The activation energy as well as the optical energy gap of the investigated samples decreased with annealing and selenization. Polycrystalline thin film n-Cd0.9Zn0.1S/p-CuGa0.3In0.7Se2 heterojunctions were fabricated and the current density - voltage and capacitance - voltage characteristics of the junction were studied. The heterojunctions were exposed to light, and under illumination of 1000 mWcm-2, the open circuit voltage was 580 mV, the short circuit current density 4.8 mAcm-2, the fill factor 0.682 and the electrical conversion efficiency was 1.898% for cells of active area of 1 cm2.Tanke polikristalinične slojeve CuGa0.3In0.7Se2 pripremali smo naparavanjem u vakuumu oko 10−4 Pa, brzinom polaganja oko 200 nm/min. Seleniranje tih slojeva na 723 K poboljšava im svojstva. Aktivacijska energija i optički energijski procijep smanjuju se opuštanjem i seleniranjem. Pripremali smo i polikristalinične tankoslojne hetero-spojeve n-Cd0.9Zn0.1S/p-CuGa0.3In0.7Se2 i proučavali njihova značajke: gustoća struje – napon i kapacitet – napon. Ćelije ploštine 1 cm2 heterospojeva osvijetlili smo svjetlošću jakosti 1000 mW/cm2 i izmjerili napon otvorenog kruga od 580 mV, gustoću struje kratkog spoja 4.8 mAcm−2 , faktor punjenja 0.682 i učinkovitost pretvorbe 1.898%
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