5,410 research outputs found
Tungsten wire-reinforced superalloys for 1093 C (2000 F) turbine blade applications
Various combinations of fiber and matrix materials were fabricated and evaluated for the purpose of selecting a specific combination that exhibited the best overall properties for a turbine blade application. A total of seven matrix alloys, including Hastelloy X, Nimonic 80A, Inconel 600, Inconel 625, IN-102, FeCrA1Y, were investigated reinforced with either 218CS tungsten, or W-Hf-C fibers. Based on preliminary screening studies, FeCrA1Y, Inconel 600 and Inconel 625 matrix composites systems were selected for extended thermal cycle tests and for property evaluations which included stress rupture, impact, and oxidation resistance. Of those investigated, the FeCrA1Y matrix composite system exhibited the best overall properties required for a turbine blade application. The W-Hf-C/FeCrA1Y system was selected for further property evaluation. Tensile strength values of up to 724 MPa (105,000 psi) were obtained for this material at 982 C and 607 MPa at 1093 C
Fracture of bio-cemented sands
Bio-chemical reactions enable the production of biomimetic materials such as
sandstones. In the present study, microbiologically-induced calcium carbonate
precipitation (MICP) is used to manufacture laboratory-scale specimens for
fracture toughness measurement. The mode I and mixed-mode fracture toughnesses
are measured as a function of cementation, and are correlated with strength,
permeability and porosity. A micromechanical model is developed to predict the
dependence of mode I fracture toughness upon the degree of cementation. In
addition, the role of the crack tip -stress in dictating kink angle and
toughness is determined for mixed mode loading. At a sufficiently low degree of
cementation, the zone of microcracking in the vicinity of the crack tip is
sufficiently large for a crack tip -field to cease to exist and for crack
kinking theory to not apply. The interplay between cementation and fracture
properties of sedimentary rocks is explained; this understanding underpins a
wide range of rock fracture phenomena including hydraulic fracture
One-dimensional metallic behavior of the stripe phase in LaSrCuO
Using an exact diagonalization method within the dynamical mean-field theory
we study stripe phases in the two-dimensional Hubbard model. We find a
crossover at doping from diagonal stripes to vertical
site-centered stripes with populated domain walls, stable in a broad range of
doping, . The calculated chemical potential shift and the doping dependence of the magnetic incommensurability are in
quantitative agreement with the experimental results for doped
LaSrCuO. The electronic structure shows one-dimensional
metallic behavior along the domain walls, and explains the suppression of
spectral weight along the Brillouin zone diagonal.Comment: 4 pages, 4 figure
New Algorithm for Mixmaster Dynamics
We present a new numerical algorithm for evolving the Mixmaster spacetimes.
By using symplectic integration techniques to take advantage of the exact Taub
solution for the scattering between asymptotic Kasner regimes, we evolve these
spacetimes with higher accuracy using much larger time steps than previously
possible. The longer Mixmaster evolution thus allowed enables detailed
comparison with the Belinskii, Khalatnikov, Lifshitz (BKL) approximate
Mixmaster dynamics. In particular, we show that errors between the BKL
prediction and the measured parameters early in the simulation can be
eliminated by relaxing the BKL assumptions to yield an improved map. The
improved map has different predictions for vacuum Bianchi Type IX and magnetic
Bianchi Type VI Mixmaster models which are clearly matched in the
simulation.Comment: 12 pages, Revtex, 4 eps figure
Partially Annealed Disorder and Collapse of Like-Charged Macroions
Charged systems with partially annealed charge disorder are investigated
using field-theoretic and replica methods. Charge disorder is assumed to be
confined to macroion surfaces surrounded by a cloud of mobile neutralizing
counterions in an aqueous solvent. A general formalism is developed by assuming
that the disorder is partially annealed (with purely annealed and purely
quenched disorder included as special cases), i.e., we assume in general that
the disorder undergoes a slow dynamics relative to fast-relaxing counterions
making it possible thus to study the stationary-state properties of the system
using methods similar to those available in equilibrium statistical mechanics.
By focusing on the specific case of two planar surfaces of equal mean surface
charge and disorder variance, it is shown that partial annealing of the
quenched disorder leads to renormalization of the mean surface charge density
and thus a reduction of the inter-plate repulsion on the mean-field or
weak-coupling level. In the strong-coupling limit, charge disorder induces a
long-range attraction resulting in a continuous disorder-driven collapse
transition for the two surfaces as the disorder variance exceeds a threshold
value. Disorder annealing further enhances the attraction and, in the limit of
low screening, leads to a global attractive instability in the system.Comment: 21 pages, 2 figure
Magnetic phases near the Van Hove singularity in s- and d-band Hubbard model
We investigate the magnetic instabilities of the nondegenerate (s-band) and a
degenerate (d-band) Hubbard model in two dimensions using many-body effects due
to the particle-particle diagrams and Hund's rule local correlations. The
density of states and the position of Van Hove singularity change depending on
the value of next-nearest neighbor hopping t'. The Stoner parameter is strongly
reduced in the s-band case, and ferromagnetism survives only if electron
density is small, and the band is almost flat at small momenta due to
next-nearest neighbor hopping. In contrast, for the d-band case the reduction
of the Stoner parameter which follows from particle-particle correlations is
much smaller and ferromagnetism survives to a large extent. Inclusion of local
spin-spin correlations has a limited destabilizing effect on the magnetic
states.Comment: 8 pages, 7 figure
Stability of metallic stripes in the extended one-band Hubbard model
Based on an unrestricted Gutzwiller approximation (GA) we investigate the
stripe orientation and periodicity in an extended one-band Hubbard model. A
negative ratio between next-nearest and nearest neighbor hopping t'/t, as
appropriate for cuprates, favors partially filled (metallic) stripes for both
vertical and diagonal configurations. At around optimal doping diagonal
stripes, site centered (SC) and bond centered (BC) vertical stripes become
degenerate suggesting strong lateral and orientational fluctuations. We find
that within the GA the resulting phase diagram is in agreement with experiment
whereas it is not in the Hartree-Fock approximation due to a strong
overestimation of the stripe filling. Results are in agreement with previous
calculations within the three-band Hubbard model but with the role of SC and BC
stripes interchanged.Comment: 10 pages, 8 figure
Dynamics of metallic stripes in cuprates
We study the dynamics of metallic vertical stripes in cuprates within the
three-band Hubbard model based on a recently developed time dependent
Gutzwiller approximation. As doping increases the optical conductivity shows
transfer of spectral weight from the charge transfer band towards i) an
incoherent band centered at 1.3eV, {ii} a Drude peak, mainly due to motion
along the stripe, {iii} a low energy collective mode which softens with doping
and merges with ii} at optimum doping in good agreement with experiment. The
softening is related to the quasidegeneracy between Cu centered and O centered
mean-field stripe solutions close to optimal doping.Comment: 4 pages, 5 figures, corrections to Fig.
Breakdown of time-dependent mean-field theory for a one-dimensional condensate of impenetrable bosons
We show that the time-dependent nonlinear Schrodinger equation of mean-field
theory has limited utility for a one-dimensional condensate of impenetrable
bosons. Mean-field theory with its associated order parameter predicts
interference between split condensates that are recombined, whereas an exact
many-body treatment shows minimal interference.Comment: 4 pages, 2 EPS figure
Spectral and transport properties of doped Mott-Hubbard systems with incommensurate magnetic order
We present spectral and optical properties of the Hubbard model on a
two-dimensional square lattice using a generalization of dynamical mean-field
theory to magnetic states in finite dimension. The self-energy includes the
effect of spin fluctuations and screening of the Coulomb interaction due to
particle-particle scattering. At half-filling the quasiparticles reduce the
width of the Mott-Hubbard `gap' and have dispersions and spectral weights that
agree remarkably well with quantum Monte Carlo and exact diagonalization
calculations. Away from half-filling we consider incommensurate magnetic order
with a varying local spin direction, and derive the photoemission and optical
spectra. The incommensurate magnetic order leads to a pseudogap which opens at
the Fermi energy and coexists with a large Mott-Hubbard gap. The quasiparticle
states survive in the doped systems, but their dispersion is modified with the
doping and a rigid band picture does not apply. Spectral weight in the optical
conductivity is transferred to lower energies and the Drude weight increases
linearly with increasing doping. We show that incommensurate magnetic order
leads also to mid-gap states in the optical spectra and to decreased scattering
rates in the transport processes, in qualitative agreement with the
experimental observations in doped systems. The gradual disappearence of the
spiral magnetic order and the vanishing pseudogap with increasing temperature
is found to be responsible for the linear resistivity. We discuss the possible
reasons why these results may only partially explain the features observed in
the optical spectra of high temperature superconductors.Comment: 22 pages, 18 figure
- …