5,416 research outputs found
Flaw imaging and ultrasonic techniques for characterizing sintered silicon carbide
The capabilities were investigated of projection microfocus x-radiography, ultrasonic velocity and attenuation, and reflection scanning acoustic microscopy for characterizing silicon carbide specimens. Silicon carbide batches covered a range of densities and different microstructural characteristics. Room temperature, four point flexural strength tests were conducted. Fractography was used to identify types, sizes, and locations of fracture origins. Fracture toughness values were calculated from fracture strength and flaw characterization data. Detection capabilities of radiography and acoustic microscopy for fracture-causing flaws were evaluated. Applicability of ultrasonics for verifying material strength and toughness was examined
Multi--dimensional Cosmological Radiative Transfer with a Variable Eddington Tensor Formalism
We present a new approach to numerically model continuum radiative transfer
based on the Optically Thin Variable Eddington Tensor (OTVET) approximation.
Our method insures the exact conservation of the photon number and flux (in the
explicit formulation) and automatically switches from the optically thick to
the optically thin regime. It scales as N logN with the number of hydrodynamic
resolution elements and is independent of the number of sources of ionizing
radiation (i.e. works equally fast for an arbitrary source function).
We also describe an implementation of the algorithm in a Soften Lagrangian
Hydrodynamic code (SLH) and a multi--frequency approach appropriate for
hydrogen and helium continuum opacities. We present extensive tests of our
method for single and multiple sources in homogeneous and inhomogeneous density
distributions, as well as a realistic simulation of cosmological reionization.Comment: Accepted for publication in New Astronomy. Color GIF versions of
figures 6, 7, 8, and 11 are available at
http://casa.colorado.edu/~gnedin/PAPERPAGES/rt.htm
Reionization Histories of Milky Way Mass Halos
We investigate the connection between the epoch of reionization and the
present day universe, by examining the extended mass reionization histories of
dark matter halos identified at z=0. We combine an N-body dark matter
simulation of a 600 Mpc volume with a three-dimensional, seminumerical
reionization model. This provides reionization redshifts for each particle,
which can then be connected with the properties of their halos at the present
time. We find that the vast majority of present-day halos with masses larger
than ~ few x 10^11 Msun reionize earlier than the rest of the universe. We also
find significant halo-to-halo diversity in mass reionization histories, and
find that in realistic inhomogenous models, the material within a given halo is
not expected to reionize at the same time. In particular, the scatter in
reionization times within individual halos is typically larger than the scatter
among halos. From our fiducial reionization model, we find that the typical 68%
scatter in reionization times within halos is ~ 115 Myr for 10^(12 \pm 0.25)
Msun halos, decreasing slightly to ~ 95 Myr for 10^(15 \pm 0.25) Msun halos. We
find a mild correlation between reionization history and environment: halos
with shorter reionization histories are typically in more clustered
environments, with the strongest trend on a scale of ~ 20 Mpc. Material in
Milky Way mass halos with short reionization histories is preferentially
reionized in relatively large HII regions, implying reionization mostly by
sources external to the progenitors of the present-day halo. We investigate the
impact on our results of varying the reionization model parameters, which span
a range of reionization scenarios with varying timing and morphology.Comment: 11 pages, 10 figures, 1 table. Submitted to Ap
Nondestructive evaluation of structural ceramics
A review is presented on research and development of techniques for nondestructive evaluation and characterization of advanced ceramics for heat engine applications. Highlighted in this review are Lewis Research Center efforts in microfocus radiography, scanning laser acoustic microscopy (SLAM), scanning acoustic microscopy (SAM), scanning electron acoustic microscopy (SEAM), and photoacoustic microscopy (PAM). The techniques were evaluated by applying them to research samples of green and sintered silicon nitride and silicon carbide in the form of modulus-of-rupture bars containing seeded voids. Probabilities of detection of voids were determined for diameters as small as 20 microns for microfucus radiography, SLAM, and SAM. Strengths and limitations of the techniques for ceramic applications are identified. Application of ultrasonics for characterizing ceramic microstructures is also discussed
Truancy and teenage pregnancy in English adolescent girls: Can we identify those at risk?
This is the final version. Available from Oxford University Press via the DOI in this record.Background Truancy has been linked to risky sexual behaviours in teenagers. However, no studies in England have examined the association between truancy and teenage pregnancy, and the use of truancy as a marker of teenagers at risk of pregnancy. Methods Using logistic regression, we investigated the association between truancy at age 15 and the likelihood of teenage pregnancy by age 19 among 3837 female teenagers who participated in the Longitudinal Study of Young People of England.We calculated the areas under the ROC curves of four models to determine how useful truancy would be as a marker of future teenage pregnancy. Results Truancy showed a dose-response association with teenage pregnancy after adjusting for ethnicity, educational intentions at age 16, parental socioeconomic status and family composition ('several days at a time' versus 'none', odds ratio 3.48 95% confidence interval 1.90-6.36, P, 0.001). Inclusion of risk behaviours improved the accuracy of predictive models only marginally (area under the ROC curve 0.76 full model versus 0.71 sociodemographic characteristics only). Conclusions Truancy is independently associated with teenage pregnancy among English adolescent girls. However, the discriminatory powers of models were low, suggesting that interventions addressing the whole population, rather than targeting high-risk individuals, might be more effective in reducing teenage pregnancy rates.Economic and Social Science Research Council (ESRC
X-ray Scattering Study of the spin-Peierls transition and soft phonon behavior in TiOCl
We have studied the S=1/2 quasi-one-dimensional antiferromagnet TiOCl using
single crystal x-ray diffraction and inelastic x-ray scattering techniques. The
Ti ions form staggered spin chains which dimerize below Tc1 = 66 K and have an
incommensurate lattice distortion between Tc1 and Tc2 = 92 K. Based on our
measurements of the intensities, wave vectors, and harmonics of the
incommensurate superlattice peaks, we construct a model for the incommensurate
modulation. The results are in good agreement with a soliton lattice model,
though some quantitative discrepancies exist near Tc2. The behavior of the
phonons has been studied using inelastic x-ray scattering with ~2 meV energy
resolution. For the first time, a zone boundary phonon which softens at the
spin-Peierls temperature Tsp has been observed. Our results show reasonably
good quantitative agreement with the Cross-Fisher theory for the phonon
dynamics at wave vectors near the zone boundary and temperatures near Tsp.
However, not all aspects of the data can be described, such as the strong
overdamping of the soft mode above Tsp. Overall, our results show that TiOCl is
a good realization of a spin-Peierls system, where the phonon softening allows
us to identify the transition temperature as Tsp=Tc2=92 KComment: 14 pages, 14 figure
Photoionized gas in hydrostatic equilibrium: the role of gravity
We present a method to include the effects of gravity in the plasma physics
code Cloudy. More precisely, a term is added to the desired gas pressure in
order to enforce hydrostatic equilibrium, accounting for both the self-gravity
of the gas and the presence of an optional external potential. As a test case,
a plane-parallel model of the vertical structure of the Milky Way disk near the
solar neighbourhood is considered. It is shown that the gravitational force
determines the scale height of the disk, and it plays a critical role in
setting its overall chemical composition. However, other variables, such as the
shape of incident continuum and the intensity of the Galactic magnetic field,
strongly affect the predicted structure.Comment: 9 pages, 8 figures, MNRAS in pres
Supersymmetry, local horizontal unification, and a solution to the flavor puzzle
Supersymmetric gauge models with local horizontal symmetries are known to
generate large flavor changing neutral current effects induced by supersymmetry
breaking D-terms. We show how the presence of a U(1) gauge symmetry solves this
problem. We then construct a realistic gauge model with SU(2)_H x U(1)_H as the
local horizontal symmetry and suggest that the U(1)_H factor may be identified
with the anomalous U(1) induced by string compactification. This model explains
the observed hierarchies among the quark masses and mixing angles, accommodates
naturally the solar and atmospheric neutrino data, and provides simultaneously
a solution to the supersymmetric flavor problem. The model can be excluded if
the rare decay \mu --> e \gamma is not observed in the current round of
experiments.Comment: 10 pages in RevTe
- …