16,694 research outputs found
Use of combinatorial analysis for the study of new material for solar cells applications
This paper presents a combinatorial method for the deposition and
characterization of new metallic precursors for photovoltaic materials.
Onedimensional thin film alloy “libraries” were electrodeposited on Mo-coated
glass. The library elements were deposited in two consecutive baths and then
heated in a reducing atmosphere to promote interdiffusion of the elements. At
the end of this process, the libraries possessed a composition gradient along
their lengths, with single elements at their two opposite ends and one or more
alloys and/or a solid state solution in between. This continuous range of
compositions can therefore be considered a collection of specific precursors
that can be interrogated by examining their corresponding locations, with the
crystallographic structure along the library changing in accordance with the
phase diagram for the metals. The libraries were then sulphurised or selenised
by heating in a sulphur-rich or selenium rich atmosphere; this converted the
metallic precursors in a continuous range of materials, candidates for potential
solar cells absorbers. The libraries were analysed by X-ray diffraction and
energy dispersive X-ray spectrometry. The X-ray diffraction results show phase
changes across the libraries, which can be correlated with the original
precursor concentration at that particular p
Forces between functionalized silica nanoparticles in solution
To prevent the flocculation and phase separation of nanoparticles in
solution, nanoparticles are often functionalized with short chain surfactants.
Here we present fully-atomistic molecular dynamics simulations which
characterize how these functional coatings affect the interactions between
nanoparticles and with the surrounding solvent. For 5 nm diameter silica
nanoparticles coated with poly(ethylene oxide) (PEO) oligomers in water, we
determined the hydrodynamic drag on two approaching nanoparticles moving
through solvent and on a single nanoparticle as it approaches a planar surface.
In most circumstances, acroscale fluid theory accurately predicts the drag on
these nano-scale particles. Good agreement is seen with Brenner's analytical
solutions for wall separations larger than the soft nanoparticle radius. For
two approaching coated nanoparticles, the solvent-mediated
(velocity-independent) and lubrication (velocity-dependent) forces are purely
repulsive and do not exhibit force oscillations that are typical of uncoated
rigid spheres.Comment: 4 pages, 3 fig
Inverse monoids and immersions of 2-complexes
It is well known that under mild conditions on a connected topological space
, connected covers of may be classified via conjugacy
classes of subgroups of the fundamental group of . In this paper,
we extend these results to the study of immersions into 2-dimensional
CW-complexes. An immersion between
CW-complexes is a cellular map such that each point has a
neighborhood that is mapped homeomorphically onto by . In order
to classify immersions into a 2-dimensional CW-complex , we need to
replace the fundamental group of by an appropriate inverse monoid.
We show how conjugacy classes of the closed inverse submonoids of this inverse
monoid may be used to classify connected immersions into the complex
Symmetry and Surface Symmetry Energies in Finite Nuclei
A study of properties of the symmetry energy of nuclei is presented based on
density functional theory. Calculations for finite nuclei are given so that the
study includes isospin dependent surface symmetry considerations as well as
isospin independent surface effects. Calculations are done at both zero and
non-zero temperature. It is shown that the surface symmetry energy term is the
most sensitive to the temperature while the bulk energy term is the least
sensitive. It is also shown that the temperature dependence terms are
insensitive to the force used and even more insensitive to the existence of
neutron skin. Results for a symmetry energy with both volume and surface terms
are compared with a symmetry energy with only volume terms along the line of
stability. Differences of several MeV are shown over a good fraction of
the total mass range in . Also given are calculations for the bulk, surface
and Coulomb terms.Comment: 11 pages, 2 figures, Added a new tabl
Anchors for the Cosmic Distance Scale: the Cepheids U Sgr, CF Cas and CEab Cas
New and existing X-ray, UBVJHKsW(1-4), and spectroscopic observations were
analyzed to constrain fundamental parameters for M25, NGC 7790, and dust along
their sight-lines. The star clusters are of particular importance given they
host the classical Cepheids U Sgr, CF Cas, and the visual binary Cepheids CEa
and CEb Cas. Precise results from the multiband analysis, in tandem with a
comprehensive determination of the Cepheids' period evolution (dP/dt) from ~140
years of observations, helped resolve concerns raised regarding the clusters
and their key Cepheid constituents. Specifically, distances derived for members
of M25 and NGC 7790 are 630+-25 pc and 3.40+-0.15 kpc, respectively.Comment: To appear in Astronomy and Astrophysic
Climatology of Mid-latitude Ionospheric Disturbances from the Very Large Array Low-frequency Sky Survey
The results of a climatological study of ionospheric disturbances derived
from observations of cosmic sources from the Very Large Array (VLA)
Low-frequency Sky Survey (VLSS) are presented. We have used the ionospheric
corrections applied to the 74 MHz interferometric data within the VLSS imaging
process to obtain fluctuation spectra for the total electron content (TEC)
gradient on spatial scales from a few to hundreds of kilometers and temporal
scales from less than one minute to nearly an hour. The observations sample
nearly all times of day and all seasons. They also span latitudes and
longitudes from 28 deg. N to 40 deg. N and 95 deg. W to 114 deg. W,
respectively. We have binned and averaged the fluctuation spectra according to
time of day, season, and geomagnetic (Kp index) and solar (F10.7) activity.
These spectra provide a detailed, multi-scale account of seasonal and intraday
variations in ionospheric activity with wavelike structures detected at
wavelengths between about 35 and 250 km. In some cases, trends between spectral
power and Kp index and/or F10.7 are also apparent. In addition, the VLSS
observations allow for measurements of the turbulent power spectrum down to
periods of 40 seconds (scales of ~0.4 km at the height of the E-region). While
the level of turbulent activity does not appear to have a strong dependence on
either Kp index or F10.7, it does appear to be more pronounced during the
winter daytime, summer nighttime, and near dusk during the spring.Comment: accepted for publication in Radio Scienc
Anchors for the Cosmic Distance Scale: the Cepheid QZ Normae in the Open Cluster NGC 6067
Cepheids are key to establishing the cosmic distance scale. Therefore it's
important to assess the viability of QZ Nor, V340 Nor, and GU Nor as
calibrators for Leavitt's law via their purported membership in the open
cluster NGC 6067. The following suite of evidence confirms that QZ Nor and V340
Nor are members of NGC 6067, whereas GU Nor likely lies in the foreground: (i)
existing radial velocities for QZ Nor and V340 Nor agree with that established
for the cluster (-39.4+-1.2 km/s) to within 1 km/s, whereas GU Nor exhibits a
markedly smaller value; (ii) a steep velocity-distance gradient characterizes
the sight-line toward NGC 6067, thus implying that objects sharing common
velocities are nearly equidistant; (iii) a radial profile constructed for NGC
6067 indicates that QZ Nor is within the cluster bounds, despite being 20' from
the cluster center; (iv) new BVJH photometry for NGC 6067 confirms the cluster
lies d=1.75+-0.10 kpc distant, a result that matches Wesenheit distances
computed for QZ Nor/V340 Nor using the Benedict et al. (2007, HST parallaxes)
calibration. QZ Nor is a cluster Cepheid that should be employed as a
calibrator for the cosmic distance scale.Comment: To appear in ApS
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