802 research outputs found
Capillary Condensation, Freezing, and Melting in Silica Nanopores: A Sorption Isotherm and Scanning Calorimetry Study on Nitrogen in Mesoporous SBA-15
Condensation, melting and freezing of nitrogen in a powder of mesoporous
silica grains (SBA-15) has been studied by combined volumetric sorption
isotherm and scanning calorimetry measurements. Within the mean field model of
Saam and Cole for vapor condensation in cylindrical pores a liquid nitrogen
sorption isotherm is well described by a bimodal pore radius distribution. It
encompasses a narrow peak centered at 3.3 nm, typical of tubular mesopores, and
a significantly broader peak characteristic of micropores, located at 1 nm. The
material condensed in the micropores as well as the first two adsorbed
monolayers in the mesopores do not exhibit any caloric anomaly. The
solidification and melting transformation affects only the pore condensate
beyond approx. the second monolayer of the mesopores. Here, interfacial melting
leads to a single peak in the specific heat measurements. Homogeneous and
heterogeneous freezing along with a delayering transition for partial fillings
of the mesopores result in a caloric freezing anomaly similarly complex and
dependent on the thermal history as has been observed for argon in SBA-15. The
axial propagation of the crystallization in pore space is more effective in the
case of nitrogen than previously observed for argon, which we attribute to
differences in the crystalline textures of the pore solids.Comment: 10 pages, 7 figure
Hesitations in continuous tracking induced by a concurrent discrete task
Subjects performed a continuous visually-guided pursuit tracking task with the right hand. From time to time (intervals averaging 30 sec) an auditory tone appeared signaling the subjects to perform a discrete response with the left hand. The presence of this tone was frequently associated with a hesitation in right-hand tracking which lasted 1/3 sec or longer. The rate of occurrence of these hesitations was about the same when the left-hand response involved a choice between competing responses as when the left hand responded in a predetermined direction. Hesitations occurred for three different mechanical tracking manipulanda using different controlling muscles, and appeared to be due to freezing rather than to relaxation of muscular action. The rate of occurrence of hesitations declined with practice, and this improvement in right-hand performance was accompanied by an improvement in performance of the concurrent left-hand response. The presence of hesitations, and their reduction with practice, can be interpreted within several viewpoints
Crystal structures and freezing of dipolar fluids
We investigate the crystal structure of classical systems of spherical
particles with an embedded point dipole at T=0. The ferroelectric ground state
energy is calculated using generalizations of the Ewald summation technique.
Due to the reduced symmetry compared to the nonpolar case the crystals are
never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar)
interaction three phases are found upon increasing the dipole moment:
hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even
richer phase diagram arises for dipolar soft spheres with a purely repulsive
inverse power law potential . A crossover between qualitatively
different sequences of phases occurs near the exponent . The results are
applicable to electro- and magnetorheological fluids. In addition to the exact
ground state analysis we study freezing of the Stockmayer fluid by
density-functional theory.Comment: submitted to Phys. Rev.
Hesitation in tracking induced by a concurrent manual task
When people are required to track with one hand and perform occasional discrete responses with the other hand, there is a strong possibility that errors will be induced in tracking attributable to the simultaneous action by the other. This problem was investigated by pairing pursuit tracking (right hand) with a handle movement response (left hand) guided by an auditory stimulus. Tracking is assumed to represent flight control and the left hand response to represent other aspects of aircraft system management. The general goal of this research is to identify the types of errors induced into tracking by the requirement of a secondary response with the other hand. An attempt is reported to determine if hesitations can be reduced further by combining tracking emphasis with a higher degree of practice. It is concluded that there is a tendency to freeze the tracking response when a discrete simultaneous response is required of the other hand. By contrast, practice seems to reduce hesitations while also improving left hand reaction time. Thus there appears to be a mode of control which permits tracking and discrete simultaneous responses to occur together
Fluids of hard ellipsoids: Phase diagram including a nematic instability from Percus-Yevick theory
An important aspect of molecular fluids is the relation between orientation
and translation parts of the two-particle correlations. Especially the detailed
knowledge of the influence of orientation correlations is needed to explain and
calculate in detail the occurrence of a nematic phase.
The simplest model system which shows both orientation and translation
correlations is a system of hard ellipsoids. We investigate an isotropic fluid
formed of hard ellipsoids with Percus-Yevick theory.
Solving the Percus-Yevick equations self-consistently in the high density
regime gives a clear criterion for a nematic instability. We calculate in
detail the equilibrium phase diagram for a fluid of hard ellipsoids of
revolution. Our results compare well with Monte Carlo Simulations and density
functional theory.Comment: 7 pages including 4 figure
Density functional formalism in the canonical ensemble
Density functional theory, when applied to systems with , is based
on the grand canonical extension of the Hohenberg-Kohn-Sham theorem due to
Mermin (HKSM theorem). While a straightforward canonical ensemble
generalization fails, work in nanopore systems could certainly benefit from
such extension. We show that, if the asymptotic behaviour of the canonical
distribution functions is taken into account, the HKSM theorem can be extended
to the canonical ensemble. We generate -modified correlation and
distribution functions hierarchies and prove that, if they are employed, either
a modified external field or the density profiles can be indistinctly used as
independent variables. We also write down the % -modified free energy
functional and prove that its minimum is reached when the equilibrium values of
the new hierarchy are used. This completes the extension of the HKSM theorem.Comment: revtex, to be submitted to Phys. Rev. Let
First Stars. I. Evolution without mass loss
The first generation of stars was formed from primordial gas. Numerical
simulations suggest that the first stars were predominantly very massive, with
typical masses M > 100 Mo. These stars were responsible for the reionization of
the universe, the initial enrichment of the intergalactic medium with heavy
elements, and other cosmological consequences. In this work, we study the
structure of Zero Age Main Sequence stars for a wide mass and metallicity range
and the evolution of 100, 150, 200, 250 and 300 Mo galactic and pregalactic Pop
III very massive stars without mass loss, with metallicity Z=10E-6 and 10E-9,
respectively. Using a stellar evolution code, a system of 10 equations together
with boundary conditions are solved simultaneously. For the change of chemical
composition, which determines the evolution of a star, a diffusion treatment
for convection and semiconvection is used. A set of 30 nuclear reactions are
solved simultaneously with the stellar structure and evolution equations.
Several results on the main sequence, and during the hydrogen and helium
burning phases, are described. Low metallicity massive stars are hotter and
more compact and luminous than their metal enriched counterparts. Due to their
high temperatures, pregalactic stars activate sooner the triple alpha reaction
self-producing their own heavy elements. Both galactic and pregalactic stars
are radiation pressure dominated and evolve below the Eddington luminosity
limit with short lifetimes. The physical characteristics of the first stars
have an important influence in predictions of the ionizing photon yields from
the first luminous objects; also they develop large convective cores with
important helium core masses which are important for explosion calculations.Comment: 17 pages, 24 figures, 2 table
Inhomogeneous magnetization in dipolar ferromagnetic liquids
At high densities fluids of strongly dipolar spherical particles exhibit
spontaneous long-ranged orientational order. Typically, due to demagnetization
effects induced by the long range of the dipolar interactions, the
magnetization structure is spatially inhomogeneous and depends on the shape of
the sample. We determine this structure for a cubic sample by the free
minimization of an appropriate microscopic density functional using simulated
annealing. We find a vortex structure resembling four domains separated by four
domain walls whose thickness increases proportional to the system size L. There
are indications that for large L the whole configuration scales with the system
size. Near the axis of the mainly planar vortex structure the direction of the
magnetization escapes into the third dimension or, at higher temperatures, the
absolute value of the magnetization is strongly reduced. Thus the orientational
order is characterized by two point defects at the top and the bottom of the
sample, respectively. The equilibrium structure in an external field and the
transition to a homogeneous magnetization for strong fields are analyzed, too.Comment: 17 postscript figures included, submitted to Phys. Rev.
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
First Stars. II. Evolution with mass loss
The first stars are assumed to be predominantly massive. Although, due to the
low initial abundances of heavy elements the line-driven stellar winds are
supposed to be inefficient in the first stars, these stars may loose a
significant amount of their initial mass by other mechanisms.
In this work, we study the evolution with a prescribed mass loss rate of very
massive, galactic and pregalactic, Population III stars, with initial
metallicities and , respectively, and initial masses
100, 120, 150, 200, and 250 during the hydrogen and helium burning
phases.
The evolution of these stars depends on their initial mass, metallicity and
the mass loss rate. Low metallicity stars are hotter, compact and luminous, and
they are shifted to the blue upper part in the Hertzprung-Russell diagram. With
mass loss these stars provide an efficient mixing of nucleosynthetic products,
and depending on the He-core mass their final fate could be either
pair-instability supernovae or energetic hypernovae. These stars contributed to
the reionization of the universe and its enrichment with heavy elements, which
influences the subsequent star formation properties.Comment: Accepted for publication in Astrophysics & Space Science. 15 pages,
18 figure
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