Characterization of pore evolution in ceramics during creep failure and densification. Final report, April 15, 1984--April 14, 1995

This research program was divided into two phases, one involving creep cavitation, the other cavity evolution during sintering. In the former, work was aimed at determining the effect of microstructure and stress state upon creep cavitation, while in the latter, the principal objective was the characterization of pore evolution during sintering. In order to meet these objectives, the creep cavitation portion of the program was centered around small-angle neutron scattering, supplemented by electron microscopy and precision density measurements. The neutron scattering measurements yielded cavity nucleation and growth rates, and average pore, size, distribution, and morphology. These data were used to evaluate current cavitation models, and to implement improved modelling efforts. Additionally, stereoimaging analysis was used to determine grain boundary sliding displacements, which appear to be the critical driving force responsible for cavity nucleation and early growth. Effort in the pore sintering phase focussed on characterization of pore evolution during intermediate and final stage sintering of alumina using both single and multiple scattering techniques. Electron microscopy, density measurements, and mercury intrusion porosimetry measurements complemented the scattering results. The effects of sintering trajectory, green state, powder morphology, and additives were evaluated. These results were compared to current sintering models

Weyl Group Invariance and p-brane Multiplets

In this paper, we study the actions of the Weyl groups of the U duality groups for type IIA string theory toroidally compactified to all dimensions $D\ge 3$. We show how these Weyl groups implement permutations of the field strengths, and we discuss the Weyl group multiplets of all supersymmetric $p$-brane solitons.Comment: 31 pages, Late

Model fluid in a porous medium: results for a Bethe lattice

We consider a lattice gas with quenched impurities or `quenched-annealed binary mixture' on the Bethe lattice. The quenched part represents a porous matrix in which the (annealed) lattice gas resides. This model features the 3 main factors of fluids in random porous media: wetting, randomness and confinement. The recursive character of the Bethe lattice enables an exact treatment, whose key ingredient is an integral equation yielding the one-particle effective field distribution. Our analysis shows that this distribution consists of two essentially different parts. The first one is a continuous spectrum and corresponds to the macroscopic volume accessible to the fluid, the second is discrete and comes from finite closed cavities in the porous medium. Those closed cavities are in equilibrium with the bulk fluid within the grand canonical ensemble we use, but are inaccessible in real experimental situations. Fortunately, we are able to isolate their contributions. Separation of the discrete spectrum facilitates also the numerical solution of the main equation. The numerical calculations show that the continuous spectrum becomes more and more rough as the temperature decreases, and this limits the accuracy of the solution at low temperatures.Comment: 13 pages, 12 figure

Structure of pair winds from compact objects with application to emission from bare strange stars

We present the results of numerical simulations of stationary, spherically outflowing, electron-positron pair winds, with total luminosities in the range 10^{34}- 10^{42} ergs/s. In the concrete example described here, the wind injection source is a hot, bare, strange star, predicted to be a powerful source of electron-positron pairs created by the Coulomb barrier at the quark surface. We find that photons dominate in the emerging emission, and the emerging photon spectrum is rather hard and differs substantially from the thermal spectrum expected from a neutron star with the same luminosity. This might help distinguish the putative bare strange stars from neutron stars.Comment: 4 pages, 6 figures, 1 table, added references, to appear in the proceedings of the conference "Isolated Neutron Stars: from the Surface to the Interior", London, UK, 24-28 April 200

Black holes as mirrors: quantum information in random subsystems

We study information retrieval from evaporating black holes, assuming that the internal dynamics of a black hole is unitary and rapidly mixing, and assuming that the retriever has unlimited control over the emitted Hawking radiation. If the evaporation of the black hole has already proceeded past the "half-way" point, where half of the initial entropy has been radiated away, then additional quantum information deposited in the black hole is revealed in the Hawking radiation very rapidly. Information deposited prior to the half-way point remains concealed until the half-way point, and then emerges quickly. These conclusions hold because typical local quantum circuits are efficient encoders for quantum error-correcting codes that nearly achieve the capacity of the quantum erasure channel. Our estimate of a black hole's information retention time, based on speculative dynamical assumptions, is just barely compatible with the black hole complementarity hypothesis.Comment: 18 pages, 2 figures. (v2): discussion of decoding complexity clarifie

Slow dynamics of a confined supercooled binary mixture II: Q space analysis

We report the analysis in the wavevector space of the density correlator of a Lennard Jones binary mixture confined in a disordered matrix of soft spheres upon supercooling. In spite of the strong confining medium the behavior of the mixture is consistent with the Mode Coupling Theory predictions for bulk supercooled liquids. The relaxation times extracted from the fit of the density correlator to the stretched exponential function follow a unique power law behavior as a function of wavevector and temperature. The von Schweidler scaling properties are valid for an extended wavevector range around the peak of the structure factor. The parameters extracted in the present work are compared with the bulk values obtained in literature.Comment: 8 pages with 8 figures. RevTeX. Accepted for publication in Phys. Rev.

Particle motion in the field of a five-dimensional charged black hole

In this paper, we have investigated the geodesics of neutral particles near a five-dimensional charged black hole using a comparative approach. The effective potential method is used to determine the location of the horizons and to study radial and circular trajectories. This also helps us to analyze the stability of radial and circular orbits. The radius of the innermost stable circular orbits have also been determined. Contrary to the case of massive particles for which, the circular orbits may have up to eight possible values of specific radius, we find that the photons will only have two distinct values for the specific radii of circular trajectories. Finally we have used the dynamical systems analysis to determine the critical points and the nature of the trajectories for the timelike and null geodesics.Comment: 15 pages, accepted for publication in Astrophysics and Space Scienc

Instanton Cosmology and Domain Walls from M-theory and String Theory

The recent proposal by Hawking and Turok for obtaining an open inflationary universe from singular instantons makes use of low-energy effective Lagrangians describing gravity coupled to scalars and non-propagating antisymmetric tensors. In this paper we derive some exact results for Lagrangians of this type, obtained from spherical compactifications of M-theory and string theory. In the case of the S^7 compactification of M-theory, we give a detailed discussion of the cosmological solutions. We also show that the lower-dimensional Lagrangians admit domain-wall solutions, which preserve one half of the supersymmetry, and which approach AdS spacetimes near their horizons.Comment: 51 pages, Latex (3 times). Discussion and references adde

M-theory/heterotic Duality: a Kaluza-Klein Perspective

We study the duality relationship between M-theory and heterotic string theory at the classical level, emphasising the transformations between the Kaluza-Klein reductions of these two theories on the K3 and T^3 manifolds. Particular attention is devoted to the corresponding structures of sigma-model cosets and the correspondence between the p-brane charge lattices. We also present simple and detailed derivations of the global symmetries and coset structures of the toroidally-compactified heterotic theory in all dimensions D \ge 3, making use of the formalism of solvable Lie algebras.Comment: Latex, 60 pages, Latex 3 time

Constraints on diffuse neutrino background from primordial black holes

We calculated the energy spectra and the fluxes of electron neutrino emitted in the process of evaporation of primordial black holes (PBHs) in the early universe. It was assumed that PBHs are formed by a blue power-law spectrum of primordial density fluctuations. We obtained the bounds on the spectral index of density fluctuations assuming validity of the standard picture of gravitational collapse and using the available data of several experiments with atmospheric and solar neutrinos. The comparison of our results with the previous constraints (which had been obtained using diffuse photon background data) shows that such bounds are quite sensitive to an assumed form of the initial PBH mass function.Comment: 18 pages,(with 7 figures