462 research outputs found
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SOFC chromite sintering and electrolyte/air-electrode interface reactions
Air sintering of chromites was investigated in La(Sr)CrO[sub 3], La(Ca)CrO[sub 3], and Y(Ca)CrO[sub 3]. Effects of alkaline earth dopant level and chromium enrichment/depletion on chromite sintered densities and microstructures are discussed. Ac impedance spectroscopy and dc polarization coupled with an unbonded interface cell were used to examine SOFC (solid oxide fuel cells) electrochemical reactions at solid-solid-gas interfaces, particularly for La[sub 1-x]Sr[sub x]MnO[sub 3]. 5 refs
Tests of Transfer Reaction Determinations of Astrophysical S-Factors
The reaction has been used to determine
asymptotic normalization coefficients for transitions to the ground and first
excited states of . The coefficients provide the normalization for
the tails of the overlap functions for and allow us
to calculate the S-factors for at astrophysical
energies. The calculated S-factors are compared to measurements and found to be
in very good agreement. This provides the first test of this indirect method to
determine astrophysical direct capture rates using transfer reactions. In
addition, our results yield S(0) for capture to the ground and first excited
states in , without the uncertainty associated with extrapolation from
higher energies.Comment: 6 pages, 2 figure
Optical model potentials involving loosely bound p-shell nuclei around 10 MeV/A
We present the results of a search for optical model potentials for use in
the description of elastic scattering and transfer reactions involving stable
and radioactive p-shell nuclei. This was done in connection with our program to
use transfer reactions to obtain data for nuclear astrophysics, in particular
for the determination of the astrophysical S_17 factor for 7Be(p,\gamma)8B
using two (7Be,8B) proton transfer reactions. Elastic scattering was measured
using 7Li, 10B, 13C and 14N projectiles on 9Be and 13C targets at or about
E/A=10 MeV/nucleon. Woods-Saxon type optical model potentials were extracted
and are compared with potentials obtained from a microscopic double folding
model. We use these results to find optical model potentials for unstable
nuclei with emphasis on the reliability of the description they provide for
peripheral proton transfer reactions. We discuss the uncertainty introduced by
the procedure in the prediction of the DWBA cross sections for the (7Be,8B)
reactions used in extracting the astrophysical factor S_17(0).Comment: 16 pages, LaTEX file, 9 figures (PostScript files
Thermodynamics of Mesoscopic Vortex Systems in 1+1 Dimensions
The thermodynamics of a disordered planar vortex array is studied numerically
using a new polynomial algorithm which circumvents slow glassy dynamics. Close
to the glass transition, the anomalous vortex displacement is found to agree
well with the prediction of the renormalization-group theory. Interesting
behaviors such as the universal statistics of magnetic susceptibility
variations are observed in both the dense and dilute regimes of this mesoscopic
vortex system.Comment: 4 pages, REVTEX, 6 figures included. Comments and suggestions can be
sent to [email protected]
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Advanced materials and electrochemical processes in high-temperature solid electrolytes
Fuel cells for the direct conversion of fossil fuels to electric energy necessitates the use of high-temperature solid electrodes. This study has included: (1) determination of electrical transport, thermal and electrical properties to illucidate the effects of microstructure, phase equilibria, oxygen partial pressure, additives, synthesis and fabrication on these properties; (2) investigation of synthesis and fabrication of advanced oxide materials, such as La{sub 0.9}Sn{sub 0.1}MnO{sub 3}; and (3) application of new analytical techniques using complex impedance coupled with conventional electrochemical methods to study the electrochemical processes and behavior of materials for solid oxide fuel cells and other high-temperature electrolyte electrochemical process. 15 refs., 10 figs., 2 tabs. (BM
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Effects of neutron irradiation on thermal conductivity of SiC-based composites and monolithic ceramics
A variety of SiC-based composites and monolithic ceramics were characterized by measuring their thermal diffusivity in the unirradiated, thermal annealed, and irradiated conditions over the temperature range 400 to 1,000 C. The irradiation was conducted in the EBR-II to doses of 33 and 43 dpa-SiC (185 EFPD) at a nominal temperature of 1,000 C. The annealed specimens were held at 1,010 C for 165 days to approximately duplicate the thermal exposure of the irradiated specimens. Thermal diffusivity was measured using the laser flash method, and was converted to thermal conductivity using density data and calculated specific heat values. Exposure to the 165 day anneal did not appreciably degrade the conductivity of the monolithic or particulate-reinforced composites, but the conductivity of the fiber-reinforced composites was slightly degraded. The crystalline SiC-based materials tested in this study exhibited thermal conductivity degradation after irradiation, presumably caused by the presence of irradiation-induced defects. Irradiation-induced conductivity degradation was greater at lower temperatures, and was typically more pronounced for materials with higher unirradiated conductivity. Annealing the irradiated specimens for one hour at 150 C above the irradiation temperature produced an increase in thermal conductivity, which is likely the result of interstitial-vacancy pair recombination. Multiple post-irradiation anneals on CVD {beta}-SiC indicated that a portion of the irradiation-induced damage was permanent. A possible explanation for this phenomenon was the formation of stable dislocation loops at the high irradiation temperature and/or high dose that prevented subsequent interstitial/vacancy recombination
Recommended from our members
Effects of neutron irradiation on thermal conductivity of SiC-based composites and monolithic ceramics
A variety of SiC-based composites and monolithic ceramics were characterized by measuring their thermal diffusivity in the unirradiated, thermal annealed, and irradiated conditions over the temperature range 400 to 1,000 C. The irradiation was conducted in the EBR-II to doses of 33 and 43 dpa-SiC (185 EFPD) at a nominal temperature of 1,000 C. The annealed specimens were held at 1,010 C for 165 days to approximately duplicate the thermal exposure of the irradiated specimens. Thermal diffusivity was measured using the laser flash method, and was converted to thermal conductivity using density data and calculated specific heat values. Exposure to the 165 day anneal did not appreciably degrade the conductivity of the monolithic or particulate-reinforced composites, but the conductivity of the fiber-reinforced composites was slightly degraded. The crystalline SiC-based materials tested in this study exhibited thermal conductivity degradation of irradiation, presumably caused by the presence of irradiation-induced defects. Irradiation-induced conductivity degradation was greater at lower temperatures, and was typically more pronounced for materials with higher unirradiated conductivity. Annealing the irradiated specimens for one hour at 150 C above the irradiation temperature produced an increase in thermal conductivity, which is likely the result of interstitial-vacancy pair recombination. Multiple post-irradiation anneals on CVD {beta}-SiC indicated that a portion of the irradiation-induced damage was permanent. A possible explanation for this phenomenon was the formation of stable dislocation loops at the high irradiation temperature and/or high dose that prevented subsequent interstitial/vacancy recombination
Three dimensional resonating valence bond liquids and their excitations
We show that there are two types of RVB liquid phases present in
three-dimensional quantum dimer models, corresponding to the deconfining phases
of U(1) and Z_2 gauge theories in d=3+1. The former is found on the bipartite
cubic lattice and is the generalization of the critical point in the square
lattice quantum dimer model found originally by Rokhsar and Kivelson. The
latter exists on the non-bipartite face-centred cubic lattice and generalizes
the RVB phase found earlier by us on the triangular lattice. We discuss the
excitation spectrum and the nature of the ordering in both cases. Both phases
exhibit gapped spinons. In the U(1) case we find a collective, linearly
dispersing, transverse excitation, which is the photon of the low energy
Maxwell Lagrangian and we identify the ordering as quantum order in Wen's
sense. In the Z_2 case all collective excitations are gapped and, as in d=2,
the low energy description of this topologically ordered state is the purely
topological BF action. As a byproduct of this analysis, we unearth a further
gapless excitation, the pi0n, in the square lattice quantum dimer model at its
critical point.Comment: 9 pages, 2 figure
Relaxation time for a dimer covering with height representation
This paper considers the Monte Carlo dynamics of random dimer coverings of
the square lattice, which can be mapped to a rough interface model. Two kinds
of slow modes are identified, associated respectively with long-wavelength
fluctuations of the interface height, and with slow drift (in time) of the
system-wide mean height. Within a continuum theory, the longest relaxation time
for either kind of mode scales as the system size N. For the real, discrete
model, an exact lower bound of O(N) is placed on the relaxation time, using
variational eigenfunctions corresponding to the two kinds of continuum modes.Comment: 12 pages, LaTeX; 1 figure in PostScript file; to appear, J. Stat.
Phys. Sections and subsections were reshuffled to improve presentation, some
text added on quantum-dimer model, fully-frustrated Ising model, and
application to general class of "height" model
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