1,548 research outputs found
The influence of the precipitation rate on the properties of porous chromia
The properties were studied of heated (320°C) chromia samples, prepared by two precipitation methods: \ud
\ud
1. (1) addition of ammonia to chromium salt solutions,\ud
2. (2) OH− formation in chromium salt solutions through hydrolysis of urea.\ud
\ud
Samples formed by means of the first method are macro or mesoporous and have a lower specific surface area (~200 m2·g−1) than those formed by urea hydrolysis (~300 m2·g−1). Only in the case of a very slow addition of the ammonia solution these properties of the chromia's become equal. Experiments show that the micro porous type samples with high surface area are only formed if the pH range 5.1 to 5.7 is passed slowly. The formation of polychromium complexes of uniform size is suggested.\ud
\u
First order perturbations of the Einstein-Straus and Oppenheimer-Snyder models
We derive the linearly perturbed matching conditions between a Schwarzschild
spacetime region with stationary and axially symmetric perturbations and a FLRW
spacetime with arbitrary perturbations. The matching hypersurface is also
perturbed arbitrarily and, in all cases, the perturbations are decomposed into
scalars using the Hodge operator on the sphere. This allows us to write down
the matching conditions in a compact way. In particular, we find that the
existence of a perturbed (rotating, stationary and vacuum) Schwarzschild cavity
in a perturbed FLRW universe forces the cosmological perturbations to satisfy
constraints that link rotational and gravitational wave perturbations. We also
prove that if the perturbation on the FLRW side vanishes identically, then the
vacuole must be perturbatively static and hence Schwarzschild. By the dual
nature of the problem, the first result translates into links between
rotational and gravitational wave perturbations on a perturbed
Oppenheimer-Snyder model, where the perturbed FLRW dust collapses in a
perturbed Schwarzschild environment which rotates in equilibrium. The second
result implies in particular that no region described by FLRW can be a source
of the Kerr metric.Comment: LaTeX; 29 page
A counter-example to a recent version of the Penrose conjecture
By considering suitable axially symmetric slices on the Kruskal spacetime, we
construct counterexamples to a recent version of the Penrose inequality in
terms of so-called generalized apparent horizons.Comment: 12 pages. Appendix added with technical details. To appear in
Classical and Quantum Gravit
Gravitational radiation from dynamical black holes
An effective energy tensor for gravitational radiation is identified for
uniformly expanding flows of the Hawking mass-energy. It appears in an energy
conservation law expressing the change in mass due to the energy densities of
matter and gravitational radiation, with respect to a Killing-like vector
encoding a preferred flow of time outside a black hole. In a spin-coefficient
formulation, the components of the effective energy tensor can be understood as
the energy densities of ingoing and outgoing, transverse and longitudinal
gravitational radiation. By anchoring the flow to the trapping horizon of a
black hole in a given sequence of spatial hypersurfaces, there is a locally
unique flow and a measure of gravitational radiation in the strong-field
regime.Comment: 5 revtex4 pages. Additional comment
First and Second Order Perturbations of Hypersurfaces
In this paper we find the first and second order perturbations of the induced
metric and the extrinsic curvature of a non-degenerate hypersurface in
a spacetime , when the metric is perturbed arbitrarily to second
order and the hypersurface itself is allowed to change perturbatively (i.e. to
move within spacetime) also to second order. The results are fully general and
hold in arbitrary dimensions and signature. An application of these results for
the perturbed matching theory between spacetimes is presented.Comment: 31 pages, no figures. To be published in Classical and Quantum
Gravit
Searching for thermal signatures of persistent currents in normal metal rings
We introduce a calorimetric approach to probe persistent currents in normal
metal rings. The heat capacity of a large ensemble of silver rings is measured
by nanocalorimetry under a varying magnetic field at different temperatures (60
mK, 100 mK and 150 mK). Periodic oscillations versus magnetic field are
detected in the phase signal of the temperature oscillations, though not in the
amplitude (both of them directly linked to the heat capacity). The period of
these oscillations (, with the magnetic flux quantum)
and their evolution with temperature are in agreement with theoretical
predictions. In contrast, the amplitude of the corresponding heat capacity
oscillations (several ) is two orders of magnitude larger than
predicted by theory
G_2 Perfect-Fluid Cosmologies with a proper conformal Killing vector
We study the Einstein field equations for spacetimes admitting a maximal
two-dimensional abelian group of isometries acting orthogonally transitively on
spacelike surfaces and, in addition, with at least one conformal Killing
vector. The three-dimensional conformal group is restricted to the case when
the two-dimensional abelian isometry subalgebra is an ideal and it is also
assumed to act on non-null hypersurfaces (both, spacelike and timelike cases
are studied). We consider both, diagonal and non-diagonal metrics and find all
the perfect-fluid solutions under these assumptions (except those already
known). We find four families of solutions, each one containing arbitrary
parameters for which no differential equations remain to be integrated. We
write the line-elements in a simplified form and perform a detailed study for
each of these solutions, giving the kinematical quantities of the fluid
velocity vector, the energy-density and pressure, values of the parameters for
which the energy conditions are fulfilled everywhere, the Petrov type, the
singularities in the spacetimes and the Friedmann-Lemaitre-Robertson-Walker
metrics contained in each family.Comment: Latex, no figure
- …