25,065 research outputs found
High-temperature catalyst supports and ceramic membranes: Metastability and particle packing
Parameters and/or processes responsible for the stability of catalyst supports and ceramic membranes are discussed. Two major parameters/processes were identified which are responsible for the stability of sol-gel derived nanostructured oxides at elevated temperatures. They are metastable-to-stable phase transformation and structure and packing of primary particles within the aggregate. Based on these observations, strategies to develop thermostable nanostructured oxides for high-temperature membrane and catalyst applications are discussed by taking titania and titania-alumina nanocomposites as examples
The Isospin Asymmetry in Anomalous Fluid Dynamics
The dynamics of fluids in which the constituent particles carry nonabelian
charges can be described succinctly in terms of group-valued variables via a
generalization of the co-adjoint orbit action for particles. This formalism,
which is particularly suitable for incorporating anomalies, has previously been
used for the chiral magnetic and chiral vorticity effects. Here we consider the
similar effect for the isospin which corresponds to an angular asymmetry for
neutral pions.Comment: 12 page
Stability Properties of the Time Domain Electric Field Integral Equation Using a Separable Approximation for the Convolution with the Retarded Potential
The state of art of time domain integral equation (TDIE) solvers has grown by
leaps and bounds over the past decade. During this time, advances have been
made in (i) the development of accelerators that can be retrofitted with these
solvers and (ii) understanding the stability properties of the electric field
integral equation. As is well known, time domain electric field integral
equation solvers have been notoriously difficult to stabilize. Research into
methods for understanding and prescribing remedies have been on the uptick. The
most recent of these efforts are (i) Lubich quadrature and (ii) exact
integration. In this paper, we re-examine the solution to this equation using
(i) the undifferentiated form of the TD-EFIE and (ii) a separable approximation
to the spatio-temporal convolution. The proposed scheme can be constructed such
that the spatial integrand over the source and observer domains is smooth and
integrable. As several numerical results will demonstrate, the proposed scheme
yields stable results for long simulation times and a variety of targets, both
of which have proven extremely challenging in the past.Comment: 9 pages, 13 figures. To be published in IEEE Transactions on Antennas
and Propagatio
Textural evolution and phase transformation in titania membranes: Part 1. -unsupported membranes
Textural evolution in sol–gel derived nanostructured unsupported titania membranes has been studied using differential scanning calorimetry (DSC), differential thermal analysis (DTA), thermal gravimetry (TG), X-ray diffraction (XRD) and N2 adsorption. The anatase-to-rutile phase transformation kinetics were studied using the Avrami model. The precursor gel had a surface area of ca. 165 m2 g–1, which after heat treatment at 600 °C for 8 h reduced to zero. Undoped titania-gel layers transformed to more than 95% rutile after calcination at 600 °C for 8 h. The causes of surface-area reduction and pore growth were anatase crystallite growth and the enhanced sintering of rutile during transformation. Lanthanum oxide was identified as a suitable dopant for shifting the transformation temperature to ca. 850 °C. Lanthanum oxide doped titania showed an improved stability of porous texture compared to that of the undoped titania membranes
Distribution functions for hard thermal particles in QCD
We find a closed-form for the distribution function (defined in terms of a
Wigner operator) for hot coloured particles in a background gluon field, in the
hard thermal loop approximation. We verify that the current is the same as that
derived from the known effective action.Comment: 12 page
Biexciton recombination rates in self-assembled quantum dots
The radiative recombination rates of interacting electron-hole pairs in a
quantum dot are strongly affected by quantum correlations among electrons and
holes in the dot. Recent measurements of the biexciton recombination rate in
single self-assembled quantum dots have found values spanning from two times
the single exciton recombination rate to values well below the exciton decay
rate. In this paper, a Feynman path-integral formulation is developed to
calculate recombination rates including thermal and many-body effects. Using
real-space Monte Carlo integration, the path-integral expressions for realistic
three-dimensional models of InGaAs/GaAs, CdSe/ZnSe, and InP/InGaP dots are
evaluated, including anisotropic effective masses. Depending on size, radiative
rates of typical dots lie in the regime between strong and intermediate
confinement. The results compare favorably to recent experiments and
calculations on related dot systems. Configuration interaction calculations
using uncorrelated basis sets are found to be severely limited in calculating
decay rates.Comment: 11 pages, 4 figure
Effective Theory of Wilson Lines and Deconfinement
To study the deconfining phase transition at nonzero temperature, I outline
the perturbative construction of an effective theory for straight, thermal
Wilson lines. Certain large, time dependent gauge transformations play a
central role. They imply the existence of interfaces, which can be used to
determine the form of the effective theory as a gauged, nonlinear sigma model
of adjoint matrices. Especially near the transition, the Wilson line may
undergo a Higgs effect. As an adjoint field, this can generate eigenvalue
repulsion in the effective theory.Comment: 6 pages, LaTeX. Final, published version. Refs. 7, 39, and 40 added.
In Ref. 37, there is an expanded discussion of a "fuzzy" bag mode
- …