366 research outputs found
Optimization of the Process of Synthesis of Lignin – Phenol -Formaldehyde Organic Aerogels
Методом экспериментально-статистического анализа изучено влияние массового
соотношения фенола и лигнина (Х1) и массового соотношения их смеси с формальдегидом
(Х2) на значения параметров пористой структуры получаемых органических лигнин-
фенол-формальдегидный аэрогелей. На основании оценки полученной математической
модели установлено, что при соблюдении условий Х1 = 0,65-0,78, а Х2 = 1,7 прогнозируемые
максимальные значения удельной поверхности и объема мезопор получаемого органического
аэрогеля составляют 485 м2/г и 1,85 г/см3 соответственно. При оптимальных значениях
Х1 = 0,25, а Х2 = 1,25 рассчитанные значения объема макропор достигают 4,05 см3/г, а общего
объема пор – 4,67 см3/гThe method of experimental statistical analysis was applied to study the effect of mass ratio of
phenol and lignin (factor X1) and the weight ratio of thereof mixture with formaldehyde (factor
X2) on the values of specific parameters of the porous structure obtained organic lignin – phenolformaldehyde
aerogels. Based on the evaluation of the mathematical model was established that
under the observance terms of X1 = 0,65-0,78 and X2 = 1,7 the maximum predicted values of the
specific surface area and mesopore volume of obtained organic aerogels are 485 m2 /g and 1,83
cm3/g, accordingly. Under optimal values of X1 = 0.25, and X2 = 1.25 calculated values reach a
macropore volume of 4.05 cm3 /g and total pore volume – 4.67 cm 3 /
On the back reaction of gravitational and particle emission and absorption from straight thick cosmic strings: A toy model
The emission and absorption of gravitational waves and massless particles of
an infinitely long straight cosmic string with finite thickness are studied. It
is shown in a general term that the back reaction of the emission and
absorption {\em always} makes the symmetry axis of the string singular. The
singularity is a scalar singularity and cannot be removed.Comment: To appear in Gen. Relativ. Gra
Orbital ordering in transition-metal compounds: I. The 120-degree model
We study the classical version of the 120-degree model. This is an attractive
nearest-neighbor system in three dimensions with XY (rotor) spins and
interaction such that only a particular projection of the spins gets coupled in
each coordinate direction. Although the Hamiltonian has only discrete
symmetries, it turns out that every constant field is a ground state. Employing
a combination of spin-wave and contour arguments we establish the existence of
long-range order at low temperatures. This suggests a mechanism for a type of
ordering in certain models of transition-metal compounds where the very
existence of long-range order has heretofore been a matter of some controversy.Comment: 40 pages, 1 eps fig; a revised version correcting a bunch of small
error
Optical monitoring of the gravitationally lensed quasar Q2237+0305 from APO between June 1995 and January 1998
We present a data set of images of the gravitationally lensed quasar
Q2237+0305, that was obtained at the Apache Point Observatory (APO) between
June 1995 and January 1998. Although the images were taken under variable,
often poor seeing conditions and with coarse pixel sampling, photometry is
possible for the two brighter quasar images A and B with the help of exact
quasar image positions from HST observations. We obtain a light curve with 73
data points for each of the images A and B. There is evidence for a long (>~
100 day) brightness peak in image A in 1996 with an amplitude of about 0.4 to
0.5 mag (relative to 1995), which indicates that microlensing has been taking
place in the lensing galaxy. Image B does not vary much over the course of the
observation period. The long, smooth variation of the light curve is similar to
the results from the OGLE monitoring of the system (Wozniak et al. 2000a).Comment: 8 pages, 5 figures; accepted for publication in A&
Mapping the 3-D dark matter with weak lensing in COMBO-17
We present a 3-dimensional lensing analysis of the z=0.16 supercluster
A901/2, resulting in a 3-D map of the dark matter distribution within a 3 X
10^{5} [Mpc]^3 volume from the COMBO-17 survey. We perform a chi^2-fit of
isothermal spheres to the tangential shear pattern around each cluster as a
function of redshift to estimate the 3-D positions and masses of the main
clusters in the supercluster from lensing alone. We then present the first 3-D
map of the dark matter gravitational potential field, Phi, using the
Kaiser-Squires (1993) and Taylor (2001) inversion methods. These maps clearly
show the potential wells of the main supercluster components, including a new
cluster behind A902, and demonstrates the applicability of 3-D dark matter
mapping and projection free-mass-selected cluster finding to current data.
Finally, we develop the halo model of dark matter and galaxy clustering and
compare this with the auto-and cross-correlation functions of the 3-D
gravitational potential, galaxy number densities and galaxy luminosity
densities measured in the A901/2 field. We find significant anti-correlations
between the gravitational potential field and the galaxy number density and
luminosities, as expected due to baryonic infall into dark matter
concentrations. We find good agreement with the halo model for the number
densities and luminosity correlation functions.Comment: Submitted to MNRAS; 21 pages, 18 figure
PEN experiment: a precise measurement of the pi+ -> e+ nu decay branching fraction
A new measurement of , the decay
branching ratio, is currently under way at the Paul Scherrer Institute. The
present experimental result on constitutes the most accurate test
of lepton universality available. The accuracy, however, still lags behind the
theoretical precision by over an order of magnitude. Because of the large
helicity suppression of the decay, its branching ratio is
susceptible to significant contributions from new physics, making this decay a
particularly suitable subject of study.Comment: 4 pages, 3 figures, talk given at the Tenth Conference on the
Intersections of Particle and Nuclear Physics (CIPANP 2009), La Jolla/San
Diego, CA, 26-31 May 2009; to appear in Proceedings to be published by the
American Institute of Physic
Ab initio calculation of resonant X-ray scattering in Manganites
We study the origin of the resonant x-ray signal in manganites and generalize
the resonant cross-section to the band structure framework. With {\it ab
initio} LSDA and LSDA+U calculations we determine the resonant x-ray spectrum
of LaMnO. The calculated spectrum and azimuthal angle dependence at the Mn
-edge reproduce the measured data without adjustable parameters. The
intensity of this signal is directly related to the orthorhombicity of the
lattice. We also predict a resonant x-ray signal at the La -edge, caused by
the tilting of the MnO octahedra. This shows that the resonant x-ray signal
in the hard x-ray regime can be understood in terms of the band structure of a
material and is sensitive to the fine details of crystal structure.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev.
A Sharp Event in the Image a Light Curve of the Double Quasar 0957+561 and Prediction of the 1996 Image B Light Curve
CCD photometry of the gravitational lens system 0957+561A,B in the g and r
bands was obtained on alternate nights, weather permitting, from December 1994
through May 1995 using the Double Imaging Spectrograph (DIS) on the Apache
Point Observatory (APO) 3.5-meter telescope. The remote observing and fast
instrument change capabilities of this facility allowed accumulation of light
curves sampled frequently and consistently. The Honeycutt ensemble photometry
algorithm was applied to the data set and yielded typical relative photometric
errors of approximately 0.01 magnitudes. Image A exhibited a sharp drop of
about 0.1 magnitudes in late December 1994; no other strong features were
recorded in either image. This event displays none of the expected generic
features of a microlensing-induced flux variation and is likely to be intrinsic
to the quasar; if so, it should also be seen in the B image with the lensing
differential time delay. We give the expected 1996 image B light curves based
on two values of the time delay and brightness ratio which have been proposed
and debated in the literature. Continued monitoring of the system in the first
half of 1996 should easily detect the image B event and thus resolve the
time-delay controversy.Comment: submitted to ApJ Letters, 15 pages, uuencoded PostScript with figures
included; also available through WWW at
http://www.astro.princeton.edu/~library/prep.htm
Constraining the dark energy with galaxy clusters X-ray data
The equation of state characterizing the dark energy component is constrained
by combining Chandra observations of the X-ray luminosity of galaxy clusters
with independent measurements of the baryonic matter density and the latest
measurements of the Hubble parameter as given by the HST key project. By
assuming a spatially flat scenario driven by a "quintessence" component with an
equation of state we place the following limits on the
cosmological parameters and : (i) and (1) if the
equation of state of the dark energy is restricted to the interval (\emph{usual} quintessence) and (ii) and
() if violates the null energy condition and assume values (\emph{extended} quintessence or ``phantom'' energy). These results are in
good agreement with independent studies based on supernovae observations,
large-scale structure and the anisotropies of the cosmic background radiation.Comment: 6 pages, 4 figures, LaTe
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