2,938 research outputs found
Development of a Flame Resistant Silicone Rubber
Flame resistant silicone rubber using aluminum silicates and aromatic bromide
Flame resistant elastomeric polymer development
Elastomeric products were developed for use in the space shuttle program, and investigations were conducted to improve the properties of elastomers developed in previous programs, and to evaluate the possibility of using lower-cost general purpose polymers. Products were fabricated and processed on conventional processing equipment; these products include: foams based on fluorinated rubber flame-retarded compounds with a density of 20-30 pounds/cubic foot for use as padding and in helmets; foams based on urethane for use in instrument packaging in the space shuttle; flexible and semi-rigid films of fluorinated rubber and neoprene compounds that would not burn in a 70% nitrogen, 30% oxygen atmosphere, and in a 30% nitrogen, 70% oxygen atmosphere, respectively for use in packaging or in laminates; coated fabrics which used both nylon and Kelvar fabric substrates, coated with either fluorinated or neoprene polymer compositions to meet specific levels of flame retardancy; and other flame-resistant materials
Non-flammable elastomeric fiber from a fluorinated elastomer and containing an halogenated flame retardant
Flame retardant elastomeric compositions are described comprised of either spandex type polyurethane having incorporated into the polymer chain halogen containing polyols, conventional spandex type polyurethanes in physical admixture with flame retardant additives, or fluoroelastomeric resins in physical admixture with flame retardant additives. Methods are described for preparing fibers of the flame retardant elastomeric materials and articles of manufacture comprised of the flame retardant clastomeric materials and non elastic materials such as polybenzimidazoles, fiberglass, nylons, etc
Development of a flameproof elastic elastomeric fiber
Various flexible polyurethane structures containing halogen were synthesized from polyesters derived from aliphatic or aromatic polyols and dibasic acids. Aliphatic halide structures could not be used because they are unstable at the required reaction temperatures, giving of hydrogen halide which hydrolyzes the ester linkages. In contract, halogen-containing aromatic polyols were stable and satisfactory products were made. The most promising composition, a brominated neopentyl glycol capped with toluene disocyanate, was used as a conventional diisocyanate, in conjunction with hydroxy-terminated polyethers or polyesters to form elastomeric urethanes containing about 10% bromine with weight. Products made in this manner will not burn in air, have an oxygen index value of about 25, and have tensile strength values of about 5,000 psi at 450% elongation. The most efficient additives for imparting flame retardancy to Spandex urethanes are aromatic halides and the most effective of these are the bromide compounds. Various levels of flame retardancy have been achieved depending on the levels of additives used
Using Bars As Signposts of Galaxy Evolution at High and Low Redshifts
An analysis of the NICMOS Deep Field shows that there is no evidence of a
decline in the bar fraction beyond z~0.7, as previously claimed; both
bandshifting and spatial resolution must be taken into account when evaluating
the evolution of the bar fraction. Two main caveats of this study were a lack
of a proper comparison sample at low redshifts and a larger number of galaxies
at high redshifts. We address these caveats using two new studies. For a proper
local sample, we have analyzed 134 spirals in the near-infrared using 2MASS
(main results presented by Menendez-Delmestre in this volume) which serves as
an ideal anchor for the low-redshift Universe. In addition to measuring the
mean bar properties, we find that bar size is correlated with galaxy size and
brightness, but the bar ellipticity is not correlated with these galaxy
properties. The bar length is not correlated with the bar ellipticity. For
larger high redshift samples we analyze the bar fraction from the 2-square
degree COSMOS ACS survey. We find that the bar fraction at z~0.7 is ~50%,
consistent with our earlier finding of no decline in bar fraction at high
redshifts.Comment: In the proceedings of "Penetrating Bars through Masks of Cosmic Dust:
The Hubble Tuning Fork strikes a New Note
RNAi Codex: a portal/database for short-hairpin RNA (shRNA) gene-silencing constructs
Use of RNA interference (RNAi) in forward genetic screens is proliferating. Currently, short-interfering RNAs (siRNAs) and short-hairpin RNAs (shRNAs) are being used to silence genes to tease out functional information. It is becoming easier to harness RNAi to silence specific genes, owing to the development of libraries of readymade shRNA and siRNA gene-silencing constructs by using a variety of sources. RNAi Codex, which consists of a database of shRNA related information and an associated website, has been developed as a portal for publicly available shRNA resources and is accessible at . RNAi Codex currently holds data from the Hannon–Elledge shRNA library and allows the use of biologist-friendly gene names to access information on shRNA constructs that can silence the gene of interest. It is designed to hold user-contributed annotations and publications for each construct, as and when such data become available. We will describe features of RNAi Codex and explain the use of the tool
The fractal distribution of haloes
We examine the proposal that a model of the large-scale matter distribution
consisting of randomly placed haloes with power-law profile, as opposed to a
fractal model, can account for the observed power-law galaxy-galaxy
correlations. We conclude that such model, which can actually be considered as
a degenerate multifractal model, is not realistic but suggests a new picture of
multifractal models, namely, as sets of fractal distributions of haloes. We
analyse, according to this picture, the properties of the matter distribution
produced in cosmological N-body simulations, with affirmative results; namely,
haloes of similar mass have a fractal distribution with a given dimension,
which grows as the mass diminishes.Comment: 7 pages, 1 figure (3 EPS files), accepted in Europhysics Letter
Constraints on the small-scale power spectrum of density fluctuations from high-redshift gamma-ray bursts
Cosmological models that include suppression of the power spectrum of density
fluctuations on small scales exhibit an exponential reduction of high-redshift,
non-linear structures, including a reduction in the rate of gamma ray bursts
(GRBs). Here we quantify the constraints that the detection of distant GRBs
would place on structure formation models with reduced small-scale power. We
compute the number of GRBs that could be detectable by the Swift satellite at
high redshifts (z > 6), assuming that the GRBs trace the cosmic star formation
history, which itself traces the formation of non-linear structures. We
calibrate simple models of the intrinsic luminosity function of the bursts to
the number and flux distribution of GRBs observed by the Burst And Transient
Source Experiment (BATSE). We find that a discovery of high-z GRBs would imply
strong constraints on models with reduced small-scale power. For example, a
single GRB at z > 10, or 10 GRBs at z > 5, discovered by Swift during its
scheduled two-year mission, would rule out an exponential suppression of the
power spectrum on scales below R_c=0.09 Mpc (exemplified by warm dark matter
models with a particle mass of m_x=2 keV). Models with a less sharp suppression
of small-scale power, such as those with a red tilt or a running scalar index,
n_s, are more difficult to constrain, because they are more degenerate with an
increase in the power spectrum normalization, sigma_8, and with models in which
star-formation is allowed in low-mass minihalos. We find that a tilt of \delta
n_s ~ 0.1 is difficult to detect; however, an observed rate of 1 GRB/yr at z >
12 would yield an upper limit on the running of the spectral index, alpha =
d(n_s)/d(ln k) > -0.05.Comment: 10 pages, 6 figures; Minor changes to match version published in Ap
Conditional Mass Functions and Merger Rates of Dark Matter Halos in the Ellipsoidal Collapse Model
Analytic models based on spherical and ellipsoidal gravitational collapse
have been used to derive the mass functions of dark matter halos and their
progenitors (the conditional mass function). The ellipsoidal model generally
provides a better match to simulation results, but there has been no simple
analytic expression in this model for the conditional mass function that is
accurate for small time steps, a limit that is important for generating halo
merger trees and computing halo merger rates. We remedy the situation by
deriving accurate analytic formulae for the first-crossing distribution, the
conditional mass function, and the halo merger rate in the ellipsoidal collapse
model in the limit of small look-back times. We show that our formulae provide
a closer match to the Millennium simulation results than those in the spherical
collapse model and the ellipsoidal model of Sheth & Tormen (2002).Comment: 5 pages, 3 figures, accepted by MNRAS letter
Redshift space correlations and scale-dependent stochastic biasing of density peaks
We calculate the redshift space correlation function and the power spectrum
of density peaks of a Gaussian random field. In the linear regime k < 0.1
h/Mpc, the redshift space power spectrum is
P^s_{pk}(k,u) = exp(-f^2 s_{vel}^2 k^2 u^2) * [b_{pk}(k) + b_{vel}(k) f
u^2]^2 * P_m(k), where u is the angle with respect to the line of sight,
s_{vel} is the one-dimensional velocity dispersion, f is the growth rate, and
b_{pk}(k) and b_{vel}(k) are k-dependent linear spatial and velocity bias
factors. For peaks, the value of s_{vel} depends upon the functional form of
b_{vel}. The peaks model is remarkable because it has unbiased velocities --
peak motions are driven by dark matter flows -- but, in order to achieve this,
b_{vel} is k-dependent. We speculate that this is true in general: k-dependence
of the spatial bias will lead to k-dependence of b_{vel} even if the biased
tracers flow with the dark matter. Because of the k-dependence of the linear
bias parameters, standard manipulations applied to the peak model will lead to
k-dependent estimates of the growth factor that could erroneously be
interpreted as a signature of modified dark energy or gravity. We use the
Fisher formalism to show that the constraint on the growth rate f is degraded
by a factor of two if one allows for a k-dependent velocity bias of the peak
type. We discuss a simple estimate of nonlinear evolution and illustrate the
effect of the peak bias on the redshift space multipoles. For k < 0.1 h/Mpc,
the peak bias is deterministic but k-dependent, so the configuration space bias
is stochastic and scale dependent, both in real and redshift space. We provide
expressions for this stochasticity and its evolution (abridged).Comment: 24 pages, 7 figures (v3): references added (v4): added
figure+appendix. In press in PR
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