7,742 research outputs found
Development of anion-selective membranes
Methods were studied of preparing anion-exchange membranes that would have low resistance, high selectivity, and physical and chemical stability when used in acidic media in a redox energy storage system. Of the twelve systems selected for study, only the system that was based on crosslinked poly-4-vinylpyridinium chloride produced physically strong membranes when equilibrated in l M HCl. The resistivity of the best membrane was 12 ohm-cm, and the transference number for chloride ions was 0.81
Commutators in the Two-Weight Setting
Let be the vector of Riesz transforms on , and let
be two weights on , . The
two-weight norm inequality for the commutator is shown to be equivalent to the function
being in a BMO space adapted to and . This is a common extension
of a result of Coifman-Rochberg-Weiss in the case of both and
being Lebesgue measure, and Bloom in the case of dimension one.Comment: v3: suggestions from two referees incorporate
The Static Dielectric Constant of a Colloidal Suspension
We derive an expression for the static dielectric constant of the colloidal
susp ensions based on the electrokinetic equations. The analysis assumes that
the ions have the same diffusivity, and that the double layer is much thinner
than the radius of curvature of the particles. It is shown that the dielectric
increment of the double layer polarization mechanism is originated from the
free energy stored in the salt concentration inhomogeniety. We also show that
the dominant polarization charges in the theory are at the electrodes, rather
than close to the particles.Comment: 15 pages, 1 figur
Analytical Models for the Energetics of Cosmic Accretion Shocks, their Cosmological Evolution, and the Effect of Environment
We present an analytical description of the energetics of the population of
cosmic accretion shocks, for a concordance cosmology. We calculate how the
shock-processed accretion power and mass current are distributed among
different shock Mach numbers, and how they evolve with cosmic time. We
calculate the cumulative energy input of cosmic accretion shocks of any Mach
number to the intergalactic medium as a function of redshift, and we compare it
with the energy output of supernova explosions as well as with the energy input
required to reionize the universe. In addition, we investigate and quantify the
effect of environmental factors, such as local clustering properties and
filament preheating on the statistical properties of these shocks. We find that
the energy processed by accretion shocks is higher than the supernova energy
output for z<3 and that it becomes more than an order of magnitude higher in
the local universe. The energy processed by accretion shocks alone becomes
comparable to the energy required to reionize the universe by z~3.5. Finally,
we establish both qualitative and quantitatively that both local clustering as
well as filament compression and preheating are important factors in
determining the statistical properties of the cosmic accretion shock
population.Comment: 13 pages, 5 figures, emulateap
The evolution of the star forming sequence in hierarchical galaxy formation models
It has been argued that the specific star formation rates of star forming
galaxies inferred from observational data decline more rapidly below z = 2 than
is predicted by hierarchical galaxy formation models. We present a detailed
analysis of this problem by comparing predictions from the GALFORM
semi-analytic model with an extensive compilation of data on the average star
formation rates of star-forming galaxies. We also use this data to infer the
form of the stellar mass assembly histories of star forming galaxies. Our
analysis reveals that the currently available data favour a scenario where the
stellar mass assembly histories of star forming galaxies rise at early times
and then fall towards the present day. In contrast, our model predicts stellar
mass assembly histories that are almost flat below z = 2 for star forming
galaxies, such that the predicted star formation rates can be offset with
respect to the observational data by factors of up to 2-3. This disagreement
can be explained by the level of coevolution between stellar and halo mass
assembly that exists in contemporary galaxy formation models. In turn, this
arises because the standard implementations of star formation and supernova
feedback used in the models result in the efficiencies of these process
remaining approximately constant over the lifetime of a given star forming
galaxy. We demonstrate how a modification to the timescale for gas ejected by
feedback to be reincorporated into galaxy haloes can help to reconcile the
model predictions with the data.Comment: 30 Pages, 16 Figures, MNRAS accepte
Multiparameter Riesz Commutators
It is shown that product BMO of Chang and Fefferman, defined on the product
of Euclidean spaces can be characterized by the multiparameter commutators of
Riesz transforms. This extends a classical one-parameter result of Coifman,
Rochberg, and Weiss, and at the same time extends the work of Lacey and
Ferguson and Lacey and Terwilleger on multiparameter commutators with Hilbert
transforms. The method of proof requires the real-variable methods throughout,
which is new in the multi-parameter context.Comment: 38 Pages. References updated. To appear in American J Mat
Distribution of the very first PopIII stars and their relation to bright z~6 quasars
We discuss the link between dark matter halos hosting the first PopIII stars
and the rare, massive, halos that are generally considered to host bright
quasars at high redshift z~6. The main question that we intend to answer is
whether the super-massive black holes powering these QSOs grew out from the
seeds planted by the first intermediate massive black holes created in the
universe. This question involves a dynamical range of 10^13 in mass and we
address it by combining N-body simulations of structure formation to identify
the most massive halos at z~6 with a Monte Carlo method based on linear theory
to obtain the location and formation times of the first light halos within the
whole simulation box. We show that the descendants of the first ~10^6 Msun
virialized halos do not, on average, end up in the most massive halos at z~6,
but rather live in a large variety of environments. The oldest PopIII
progenitors of the most massive halos at z~6, form instead from density peaks
that are on average one and a half standard deviations more common than the
first PopIII star formed in the volume occupied by one bright high-z QSO. The
intermediate mass black hole seeds planted by the very first PopIII stars at
z>40 can easily grow to masses m_BH>10^9.5 Msun by z=6 assuming Eddington
accretion with radiative efficiency \epsilon~0.1. Quenching of the black hole
accretion is therefore crucial to avoid an overabundance of supermassive black
holes at lower redshift. This can be obtained if the mass accretion is limited
to a fraction \eta~6*10^{-3} of the total baryon mass of the halo hosting the
black hole. The resulting high end slope of the black hole mass function at z=6
is \alpha ~ -3.7, a value within the 1\sigma error bar for the bright end slope
of the observed quasar luminosity function at z=6.Comment: 30 pages, 9 figures, ApJ accepte
Electron paramagnetic resonance in silicon
Electron Paramagnetic Resonance (E.P.R.) in silicon doped with the shallow Group V donors phosphorus and arsenic has been studied over the temperature range from 20 to 5O°K. The range of donor concentrations examined was sufficient for exchange to be possible in the more highly doped samples, all samples exhibiting a resonance characteristic of bound donor electrons at 20°K.
The observed reduction in the hyperfine splitting with increasing temperature is interpreted as being due to the phonon-induced transitions of electrons between the various 1s hydrogen-like states. For phosphorus donors an estimate of 10.7 +- 0.4 meV is obtained tor the valley-orbit splitting. The greater value of this splitting for arsenic donor results in an insignificant reduction in the hyperfine splitting for arsenic over the same temperature range.
Simultaneous with the reduction in the hyperfine splitting the linewidth and shape change. Although the above mechanism gives a calculated width of the correct order of magnitude, the increasing linewidth with temperature is better described by an activation energy of half the donor ionization energy. The exchange scattering of the bound donor electrons by conduction electrons is therefore thought to be the dominant
source of the linewidth, such that with the increasing exchange frequency with temperature the hyperfine lines will eventually be averaged out and replaced by a single line. For phosphorus and arsenic donor concentrations at approximately 5.10x17 cm-3 this narrowing single line was only observed for phosphoros. The difference in the donor ionisation energies is thought sufficient to prevent its observation in the case of arsenic.
A possible alternative explanation of the above effects, due to the phonon-induced hopping of electrons from electronically occupied to neighbouring unoccupied sites, was also examined. Calculation demon-states that the experimental results cannot be interpreted in this
fashion
Multi-Parameter Div-Curl Lemmas
We study the possible analogous of the Div-Curl Lemma in classical harmonic
analysis and partial differential equations, but from the point of view of the
multi-parameter setting. In this context we see two possible Div-Curl lemmas
that arise. Extensions to differential forms are also given.Comment: v1: 8 page
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