1,722 research outputs found
The scaling of X-ray variability with luminosity in Ultra-luminous X-ray sources
We investigated the relationship between the X-ray variability amplitude and
X-ray luminosity for a sample of 14 bright Ultra-luminous X-ray sources (ULXs)
with XMM-Newton/EPIC data, and compare it with the well established similar
relationship for Active Galactic Nuclei (AGN). We computed the normalised
excess variance in the 2-10 keV light curves of these objects and their 2-10
keV band intrinsic luminosity. We also determined model
"variability-luminosity" relationships for AGN, under several assumptions
regarding their power-spectral shape. We compared these model predictions at
low luminosities with the ULX data. The variability amplitude of the ULXs is
significantly smaller than that expected from a simple extrapolation of the AGN
"variability-luminosity" relationship at low luminosities. We also find
evidence for an anti-correlation between the variability amplitude and L(2-10
keV) for ULXs. The shape of this relationship is consistent with the AGN data
but only if the ULXs data are shifted by four orders of magnitudes in
luminosity. Most (but not all) of the ULXs could be "scaled-down" version of
AGN if we assume that: i) their black hole mass and accretion rate are of the
order of ~(2.5-30)x 10E+03 Msolar and ~ 1-80 % of the Eddington limit, and ii)
their Power Spectral Density has a doubly broken power-law shape. This PDS
shape and accretion rate is consistent with Galactic black hole systems
operating in their so-called "low-hard" and "very-high" states.Comment: 10 pages, 5 figures, 2 tables, accepted for publication in A&
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On the time-temperature-transformation behaviour of a new dual-superlattice nickel-base superalloy
Recent research has identified compositions of nickel-based superalloys with microstructures containing appreciable and comparable volume fractions of γ′ and γ″ precipitates. In this work, an alloy capable of forming such a dual-superlattice microstructure was subjected to a range of thermal exposures between 873 and 1173 K (600 and 900 ˚C) for durations of 1 to 1000 hours. The microstructures and nature of the precipitating phases were characterised using synchrotron X-ray diffraction and electron microscopy. These data have enabled the construction of a T-T-T diagram for the precipitating phases. Hardness measurements following each thermal exposure have identified the age-hardening behaviour of this alloy and allowed preliminary mechanical properties to be assessed.The authors would like to thank K. Roberts and S. Rhodes for experimental assistance, and acknowledge funding through the EPSRC/Rolls-Royce strategic partnership EP/M005607/1 and EP/H022309/1 as well as from the Diamond Light Source for the provision of beam time (EE9270)
The influence of Al: Nb ratio on the microstructure and mechanical response of quaternary Ni-Cr-Al-Nb alloys
The influence of Al:Nb ratio on the microstructure and properties of Ni–Cr–Al–Nb alloys has been investigated following long-term exposure at elevated temperatures. The γ′ volume fraction, size and lattice misfit were seen to increase with a larger Al:Nb ratio, although these changes resulted in reduced hardness. The change in the critical resolved shear stress (CRSS) associated with strong dislocation coupling was determined to be the dominant strengthening mechanism and increased with decreasing Al:Nb ratio. A distribution of tertiary γ′ was observed to be necessary in maximising the mechanical properties of these alloys.This work was supported by the EPSRC/Rolls-Royce Strategic Partnership (EP/H022309/1 and EP/H500375/1).This is the final published version, which can also be found on the Elsevier website at: http://www.sciencedirect.com/science/article/pii/S0921509314007369
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Microstructural evolution of a delta containing nickel-base superalloy during heat treatment and isothermal forging
The next generation of aerospace gas turbine engines need to operate at higher temperatures and stresses to improve their efficiency and reduce emissions. These operating conditions are beyond the capability of existing nickel-base superalloys requiring the development of new high temperature materials. Controlling the microstructures of these new materials is key to obtaining the required properties and therefore, it is critical to understand how these alloys respond to processing and heat treatment. Here, the microstructural evolution of V207M, a new δ containing, nickel-base superalloy, has been investigated following heat treatment and forging. The solvus temperatures of the γ′ and δ phases, determined by differential scanning calorimetry and microscopy, were found to be ~ 985 and ~ 1060 ˚C respectively. Isothermal forging of the alloy was conducted at 1000, 1050 and 1100 ˚C, corresponding to different volume fractions of retained δ. Considerable softening was observed prior to steady state flow when forging at 1000 ˚C, whilst only steady state flow occurred at 1050 and 1100 ˚C. The steady state flow process was believed to be dominated by dynamic recovery in the γ phase, with an activation energy of 407 kJ.mol-1. Samples that exhibited flow softening also showed a significant change in the orientation of the δ precipitates, preferentially aligning normal to the forging axis, and this reorientation was thought to be the cause of the observed flow softening.The authors would like to acknowledge M. Shakib for assistance with the forging and the EPSRC/Rolls-Royce Strategic Partnership for supporting this work through EP/H022309/1 and EP/H500375/1.This is the final published version. It first appeared at http://www.sciencedirect.com/science/article/pii/S0921509314013252#
Emergence of stationary multimodality under two-timescaled dichotomic noise
We study a linear Langevin dynamics driven by an additive non-Markovian symmetrical dichotomic noise. It is shown that when the statistics of the time intervals between noise transitions is characterized by two well differentiated timescales, the stationary distribution may develop multimodality (bi- and trimodality). The underlying effects that lead to a probability concentration in different points include intermittence and also a dynamical locking of realizations. Our results are supported by numerical simulations as well as by an exact treatment obtained from a Markovian embedding of the full dynamics, which leads to a third-order differential equation for the stationary distribution.Fil: Budini, Adrian Adolfo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Tecnológica Nacional; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Mc Hardy, Isaias. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; ArgentinaFil: Nizama Mendoza, Marco Alfredo. Universidad Nacional del Comahue; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Caceres Garcia Faure, Manuel Osvaldo. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin
The effect of phase chemistry on the extent of strengthening mechanisms in model Ni-Cr-Al-Ti-Mo based superalloys
The exceptional mechanical properties of polycrystalline nickel-based superalloys arise through various concurrent strengthening mechanisms. Whilst these mechanisms are generally understood, consensus has yet to be established on the precise contribution of each to the overall alloy strength. Furthermore, changes in alloy chemistry influence several different mechanisms, making the assessment of individual alloying elements complex. In this study, a series of model quinary Ni-based superalloys has been investigated to systematically study the effect of varying Mo content on the contributing strengthening mechanisms. Using microstructural data, the yield strength was modelled by summing the individual effects of solid solution in both the γ and γ ' phases, coherency, grain boundary and precipitation strengthening. The total predicted yield stress increased with Mo content despite the diminishing contribution of precipitation strengthening. It is shown that solid solution strengthening of the ordered γ' precipitate phase is a key contributor to the overall strength, and that variations in composition between the tertiary and secondary γ ' lead to significant changes in mechanical properties that should be accounted for in models of alloy strength.Funding was provided by the EPSRC/Rolls-Royce Strategic Partnership under EP/M005607/1 and EP/H022309/1. The Oxford Atom Probe facility was funded by the EPSRC under EP/M022803/1. E. I. Galindo-Nava would like to acknowledge the Royal Academy of Engineering for his fellowship funding. Neutron diffraction beam time was supported through the Canadian Neutron Beam Centre under Experiment number 1258
Quartic Parameters for Acoustic Applications of Lattice Boltzmann Scheme
Using the Taylor expansion method, we show that it is possible to improve the
lattice Boltzmann method for acoustic applications. We derive a formal
expansion of the eigenvalues of the discrete approximation and fit the
parameters of the scheme to enforce fourth order accuracy. The corresponding
discrete equations are solved with the help of symbolic manipulation. The
solutions are explicited in the case of D3Q27 lattice Boltzmann scheme. Various
numerical tests support the coherence of this approach.Comment: 23 page
Debris Disks: Probing Planet Formation
Debris disks are the dust disks found around ~20% of nearby main sequence
stars in far-IR surveys. They can be considered as descendants of
protoplanetary disks or components of planetary systems, providing valuable
information on circumstellar disk evolution and the outcome of planet
formation. The debris disk population can be explained by the steady
collisional erosion of planetesimal belts; population models constrain where
(10-100au) and in what quantity (>1Mearth) planetesimals (>10km in size)
typically form in protoplanetary disks. Gas is now seen long into the debris
disk phase. Some of this is secondary implying planetesimals have a Solar
System comet-like composition, but some systems may retain primordial gas.
Ongoing planet formation processes are invoked for some debris disks, such as
the continued growth of dwarf planets in an unstirred disk, or the growth of
terrestrial planets through giant impacts. Planets imprint structure on debris
disks in many ways; images of gaps, clumps, warps, eccentricities and other
disk asymmetries, are readily explained by planets at >>5au. Hot dust in the
region planets are commonly found (<5au) is seen for a growing number of stars.
This dust usually originates in an outer belt (e.g., from exocomets), although
an asteroid belt or recent collision is sometimes inferred.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018
A Study on the Impact of the Internet Finance On The Traditional Banking--A Study Based on The Case of Alibaba Internet Finance
随着大数据时代的降临,互联网金融崛地而起,发展势不可挡,互联网“开放、平等、协作、分享”的精髓通过互联网、移动互联网等工具,出现了阿里巴巴互联网金融模式、第三方支付抢占银行地盘、P2P平台迅速发展、众筹平台开始崭露头角等现象。当然,互联网金融与传统金融的区别不仅仅在于其所用的通道不同,更加重要的是,它能够让传统金融业务透明度更强、参与度更高、协作性更好、中间成本更低、操作更便捷。互联网金融的出现与发展,将加快金融脱媒、利率市场化进程,促使存款理财化、贷款债券化,更多的金融行为将从线下转到线上,商业银行的传统经营模式已受到冲击,特别是第三方支付、网络借贷、网络理财已经逐渐侵蚀传统银行业的核心业务...Along with the coming age of Big Data, the internet finance is arising with inundate tendency, and the internet spirit of “Open, Equal, Collaboration, Share” spreads through the internet, mobile internet and other channels. There are different internet finance model appeared, such as Alibaba Internet Financial Model, Third-Party Payment Platform, P2P, Kickstarter and so on, which are developing ra...学位:工商管理硕士院系专业:管理学院工商管理教育中心(MBA中心)_工商管理硕士(MBA)学号:1792011115066
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