211,494 research outputs found
The long-term optical behavior of MRK421
All data available in B band for the BL Lac object MRK421 from 22
publications are used to construct a historical light curve, dating back to
1900. It is found that the light curve is very complicated and consists of a
set of outbursts with very large duration. The brightness of MRK421 varies from
11.6 magnitude to more than 16 magnitude. Analyses with Jurkevich method of
computing period of cyclic phenomena reveal in the light curve two kinds of
behaviors. The first one is non-periodic with rapid, violent variations in
intensity on time scales of hours to days. The second one is periodic with a
possible period of years. Another possible period of years is not very significant. We have tested the robustness of the
Jurkevich method. The period of about one year found in the light curves of
MRK421 and of other objects is a spurious period due to the method and the
observing window. We try to explain the period of years under the
thermal instability of a slim accretion disk around a massive black hole of
mass of .Comment: Tex, 14 pages, 5 Postscript figures. Accepted for publication in A&A
Supplement Serie
Method of increasing minority carrier lifetime in silicon web or the like
A silicon dendrite is grown as a ribbon forming two silicon crystal layers which are separated by an interface layer which contains a large number of defects. Significant increase of minority carrier lifetime with homogeneous distribution at the outer surfaces of the two silicon crystal layers is achieved by processing the web in an atmosphere of a selected gas, e.g., oxygen, nitrogen or an inert gas, for about 30 minutes to several hours at a temperature preferably on the order of 900 to 1200 C
A New Spin Gapless Semiconductors Family: Quaternary Heusler Compounds
Using first-principles calculations, we investigate the band structures of a
series of quaternary LiMgPdSn-type Heusler compounds. Our calculation results
show that five compounds CoFeMnSi, CoFeCrAl, CoMnCrSi, CoFeVSi and FeMnCrSb
possess unique electronic structures characterized by a half-metallic gap in
one spin direction while a zero-width gap in the other spin direction showing
spin gapless semiconducting behavior. We further analysis the electronic and
magnetic properties of all quaternary Heusler alloys involved, and reveal a
semi-empirical general rule (total valence electrons number being 26 or 28) for
indentifying spin gapless semiconductors in Heusler compounds. The influences
of lattice distortion and main-group element change have also been discussed.Comment: 20 pages, 5 figures, 1 supplementary file, submitted for publicatio
Soil, grain and water chemistry and human selenium imbalances in Enshi district, Hubei Province, China
Many elements which are essential to human and other animal health in small doses can
be toxic if ingested in excess. Selenium (Se), a naturally occurring metalloid element is
found in all natural materials on earth including rocks, soils, waters, air, plant and
animal tissues. Since the early 1930ās, it has been recognised that Se toxicity causes
hoof disorders and hair loss in livestock. Se was also identified as an essential trace
element to humans and other animals in the late 1950ās. It forms a vital constituent of
the biologically important enzyme glutathione peroxidase which acts as an anti-oxidant
preventing cell degeneration. Se deficiency has been implicated in the aetiology of
several diseases including cancer, muscular dystrophy, muscular sclerosis and cystic
fibrosis. Se can be assimilated in humans through several pathways including food,
drinking water and inhalation of Se-bearing particles from the atmosphere. In the
majority of situations, food is the most important source of Se, as levels in water are
very low. The narrow range between deficiency levels (<40 pg per day) and toxic
levels in susceptible people (> 900 pg per day) makes it necessary to carefully control
the amount of Se in the diet.
In China, Se deficiency has been linked to an endemic degenerative heart disease
known as Keshan Disease (KD) and an endemic osteoarthropathy which causes
deformity of affected joints, known as Kaschin-Beck Disease. These diseases occur in
a geographic belt stretching from Heilongjiang Province in north-east China to Yunnan
Province in the south-west. In the period between 1959 and 1970, peak KD incidence
rates exceeded 40 per 100 000 (approximately 8500 cases per annum) with 1400 - 3000
deaths recorded each year. Incidence rates have since fallen to less than 5 per 100 000
with approximately 1000 new cases reported annually (Levander, 1986). Se toxicity
(selenosis) resulting in hair and nail loss and disorders of the nervous system in the
human population, has also been recorded in Enshi District, Hubei Province and in
Ziyang County, Shanxi Province. China possesses one of the best epidemiological
databases in the world on Se-related diseases which has been used in conjunction with
geochemical data to demonstrate a significant geochemical control on human Se
exposure. However, the precise geographical areas at risk and the geochemical controls
on selenium availability have yet to be established
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P2-type Na2/3Ni1/3Mn2/3O2 Cathode Material with Excellent Rate and Cycling Performance for Sodium-Ion Batteries
P2-type Na2/3Ni1/3Mn2/3O2 is an air-stable cathode material for sodium-ion batteries. However, it suffers irreversible P2-O2 phase transition in 4.2-V plateau and shows poor cycling stability and rate capability within this plateau. To evaluate the practicability of this material in 2.3ā4.1 V voltage range, single-crystal micro-sized P2-type Na2/3Ni1/3Mn2/3O2 with high rate capability and cycling stability is synthesized via polyvinylpyrrolidone (PVP)-combustion method. The electrochemical performance is evaluated by galvanostatic charge-discharge tests. The kinetics of Na+ intercalation/deintercalation is studied detailly with potential intermittent titration technique (PITT), galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV). The discharge capacity at 0.1 C in 2.3ā4.1 V is 87.6 mAh gā1. It can deliver 91.5% capacity at 40 C rate and keep 89% after 650 cycles at 5C. The calculated theoretical energy density of full cell with hard carbon anode is 210 Wh kgā1. The moderate energy density associated with high power density and long cycle life is acceptable for load adjustment of new-energy power, showing the prospect of practical application
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The polymeric conformational effect on capacitive deionization performance of graphene oxide/polypyrrole composite electrode
Exploitation of novel faradic materials is an alternative implementation for solving the problem of poor specific electrosorption capacity that conventional carbon materials are encountered in capacitive deionization. Particularly, composite electrode is just a suitable choice because of its potentially high ion-storage ability. Herein, a cyclic voltammetric treatment method with different low limit of potential window was used to manipulate the polymeric conformation and doping level of graphene oxide/polypyrrole (GO/PPy) composite electrode. Based on it, the effect of polymeric structure on the electrosorption performance was systematically studied. When the low limit of potential window is shifted negatively enough, the irreversible polymeric conformational shrinks of GO/PPy are promoted, which not only hinders the insertion process of ions, but also decreases the doping level of polymer due to the intensive interchain-action produced by more entangled polymeric chain. Thus, the number of intercalated ions should decrease, which is expressed by electrochemical impedance spectroscopy (EIS) results and is proportional to the electrosorption capacity of GO/PPy composite electrode in membrane capacitive deionization (MCDI) process. Our work suggests that the less packing density, higher doping level and more charge delocalization on PPy backbone in electrode are beneficial to enhance its capacitive deionization performance
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Robust filtering for stochastic genetic regulatory networks with time-varying delay
This is the post print version of the article. The official published version can be obtained from the link - Copyright 2009 Elsevier LtdThis paper addresses the robust filtering problem for a class of linear genetic regulatory networks (GRNs) with stochastic disturbances, parameter uncertainties and time delays. The parameter uncertainties are assumed to reside in a polytopic region, the stochastic disturbance is state-dependent described by a scalar Brownian motion, and the time-varying delays enter into both the translation process and the feedback regulation process. We aim to estimate the true concentrations of mRNA and protein by designing a linear filter such that, for all admissible time delays, stochastic disturbances as well as polytopic uncertainties, the augmented state estimation dynamics is exponentially mean square stable with an expected decay rate. A delay-dependent linear matrix inequality (LMI) approach is first developed to derive sufficient conditions that guarantee the exponential stability of the augmented dynamics, and then the filter gains are parameterized in terms of the solution to a set of LMIs. Note that LMIs can be easily solved by using standard software packages. A simulation example is exploited in order to illustrate the effectiveness of the proposed design procedures.This work was supported in part by the Biotechnology and Biological Sciences Research Council (BBSRC) of the U.K. under Grants BB/C506264/1 and 100/EGM17735, an International Joint Project sponsored by the Royal Society of the U.K., the Research Grants Council of Hong Kong under Grant HKU 7031/06P, the National Natural Science Foundation of China under Grant 60804028, and the Alexander von Humboldt Foundation of Germany
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