1,064 research outputs found
Glass formation, properties, and structure of soda-yttria-silicate glasses
The glass formation region of the soda yttria silicate system was determined. The glasses within this region were measured to have a density of 2.4 to 3.1 g/cu cm, a refractive index of 1.50 to 1.60, a coefficient of thermal expansion of 7 x 10(exp -6)/C, softening temperatures between 500 and 780 C, and Vickers hardness values of 3.7 to 5.8 GPa. Aqueous chemical durability measurements were made on select glass compositions while infrared transmission spectra were used to study the glass structure and its effect on glass properties. A compositional region was identified which exhibited high thermal expansion, high softening temperatures, and good chemical durability
Near-Net-Shape Processing of Sintered Fibrous Ceramics Achieved
A variety of sintered fibrous ceramic (SFC) materials have been developed over the last 50 years as thermal barrier materials for reentry applications. SFC materials typically exhibit very low thermal conductivities combined with low densities and good thermal stability up to 2500 F. These materials have flown successfully on the space shuttle orbiters since the 1960's. More recently, the McDonnell Douglas Corporation successfully used SFC tiles as a heat shield on the underside of its DC X test vehicle. For both of these applications, tiles are machined from blocks of a specific type of SFC called an alumina-enhanced thermal barrier (AETB). The sizes of these blocks have been limited by the manufacturing process. In addition, as much as 80 to 90 percent of the material can be lost during the machining of tiles with significant amounts of curvature. To address these problems, the NASA Glenn Research Center at Lewis Field entered a cooperative contract with the Boeing Company to develop a vacuum-assisted forming process that can produce large (approximately 4 square feet), severely contoured panels of AETB while saving costs in comparison to the conventional cast-and-machine billet process. For shuttle use, AETB is slurry cast, drained, and fired to form square billets conforming to the shape of the filtration box. The billets are then cut into tiles of the appropriate size for thermally protecting the space shuttle. Processing techniques have limited the maximum size of AETB billets to 21.5 square inches by 6.5-in. thick, but the space shuttles use discrete heat shield tiles no more than 8 to 12 square inches. However, in other applications, large, complex shapes are needed, and the tiling approach is undesirable. For such applications, vacuum-assisted forming can produce large parts with complex shapes while reducing machining waste and eliminating cemented joints between bonded billets. Because it allows contoured shapes to be formed, material utilization is inherently high. Initial estimates show that the amount of material lost during machining can be reduced by 50 percent or more. In addition, a fiber alignment favorable for minimum heat transfer is maintained for all panel shapes since the fibers are aligned parallel to the contoured surface of the forming tool or mold. The vacuum-assisted forming process can complete the entire forming operation in a matter of minutes and can produce multiple parts whose size is limited only by the size of the forming tool. To date, panels as large as 2 square feet have been demonstrated The vacuum-assisted forming process starts with the fabrication of a permeable forming tool, or mold, with the proper part contour. This reusable tool is mounted over an internal rib support structure, as depicted in the diagram, such that a vacuum can be pulled on the bottom portion of the tool. AETB slurry is then poured over and around the tool, liquid is drawn from the slurry, and the part forms over the tool surface. The part is then dried, fired, and finished machined. Future plans include an evaluation of the need for additional coatings and surface-toughness treatments to extend the durability and performance of this material
Testable two-loop radiative neutrino mass model based on an effective operator
A new two-loop radiative Majorana neutrino mass model is constructed from the
gauge-invariant effective operator that violates lepton number conservation by two units. The
ultraviolet completion features two scalar leptoquark flavors and a color-octet
Majorana fermion. We show that there exists a region of parameter space where
the neutrino oscillation data can be fitted while simultaneously meeting
flavor-violation and collider bounds. The model is testable through lepton
flavor-violating processes such as , , and
conversion, as well as collider searches for the scalar
leptoquarks and color-octet fermion. We computed and compiled a list of
necessary Passarino-Veltman integrals up to boxes in the approximation of
vanishing external momenta and made them available as a Mathematica package,
denoted as ANT.Comment: 42 pages, 11 figures, typo in Eq. (4.9) as well as wrong chirality
structures in Secs. 4.5 and 5.2 corrected, final results unchange
Dark-ages Reionization & Galaxy Formation Simulation VIII. Suppressed growth of dark matter halos during the Epoch of Reionization
We investigate how the hydrostatic suppression of baryonic accretion affects
the growth rate of dark matter halos during the Epoch of Reionization. By
comparing halo properties in a simplistic hydrodynamic simulation in which gas
only cools adiabatically, with its collisionless equivalent, we find that halo
growth is slowed as hydrostatic forces prevent gas from collapsing. In our
simulations, at the high redshifts relevant for reionization (between
and ), halos that host dwarf galaxies () can be reduced by up to a factor of 2 in mass due to the
hydrostatic pressure of baryons. Consequently, the inclusion of baryonic
effects reduces the amplitude of the low mass tail of the halo mass function by
factors of 2 to 4. In addition, we find that the fraction of baryons in dark
matter halos hosting dwarf galaxies at high redshift never exceeds
of the cosmic baryon fraction. When implementing baryonic processes, including
cooling, star formation, supernova feedback and reionization, the suppression
effects become more significant with further reductions of to
60\%. Although convergence tests suggest that the suppression may become weaker
in higher resolution simulations, this suppressed growth will be important for
semi-analytic models of galaxy formation, in which the halo mass inherited from
an underlying N-body simulation directly determines galaxy properties. Based on
the adiabatic simulation, we provide tables to account for these effects in
N-body simulations, and present a modification of the halo mass function along
with explanatory analytic calculations.Comment: 17 pages, 11 figures; Updated to match the published version. Two
changes in Figures 1 and 3 in order to 1) correct bin sizes of the 10^8 and
10^8.5 Msol bins for NOSN_NOZCOOL_NoRe (was 0.5, should be 0.25); 2) include
stellar mass in baryon fraction (was missed in Fig. 3). Quantitative
description of Fig. 3 changed slightly in Section 2.2. All other results and
conclusions remain unchange
Dark-ages reionization and galaxy formation simulation--VII. The sizes of high-redshift galaxies
We investigate high-redshift galaxy sizes using a semi-analytic model
constructed for the Dark-ages Reionization And Galaxy-formation Observables
from Numerical Simulation project. Our fiducial model, including strong
feedback from supernovae and photoionization background, accurately reproduces
the evolution of the stellar mass function and UV luminosity function. Using
this model, we study the size--luminosity relation of galaxies and find that
the effective radius scales with UV luminosity as at --. We show that recently discovered very luminous
galaxies at (Bowler et al. 2016) and (Oesch et al. 2016)
lie on our predicted size--luminosity relations. We find that a significant
fraction of galaxies at will not be resolved by JWST, but GMT will have
the ability to resolve all galaxies in haloes above the atomic cooling limit.
We show that our fiducial model successfully reproduces the redshift evolution
of average galaxy sizes at . We also explore galaxy sizes in models
without supernova feedback. The no-supernova feedback models produce galaxy
sizes that are smaller than observations. We therefore confirm that supernova
feedback plays an important role in determining the size--luminosity relation
of galaxies and its redshift evolution during reionization.Comment: 10 pages, 4 figures, Accepted for publication in MNRA
Dark-ages Reionization and Galaxy Formation Simulation - X. The small contribution of quasars to reionization
Motivated by recent measurements of the number density of faint AGN at high
redshift, we investigate the contribution of quasars to reionization by
tracking the growth of central supermassive black holes in an update of the
Meraxes semi-analytic model. The model is calibrated against the observed
stellar mass function at , the black hole mass function at
, the global ionizing emissivity at and the Thomson
scattering optical depth. The model reproduces a Magorrian relation in
agreement with observations at and predicts a decreasing black hole
mass towards higher redshifts at fixed total stellar mass. With the
implementation of an opening angle of 80 deg for quasar radiation,
corresponding to an observable fraction of per cent due to
obscuration by dust, the model is able to reproduce the observed quasar
luminosity function at . The stellar light from galaxies hosting
faint AGN contributes a significant or dominant fraction of the UV flux. At
high redshift, the model is consistent with the bright end quasar luminosity
function and suggests that the recent faint AGN sample compiled by
Giallongo et al. (2015) includes a significant fraction of stellar light.
Direct application of this luminosity function to the calculation of AGN
ionizing emissivity consequently overestimates the number of ionizing photons
produced by quasars by a factor of 3 at . We conclude that quasars are
unlikely to make a significant contribution to reionization.Comment: 21 pages, 12 figures; Updated to match the published version. All
results and conclusions remain unchange
Electrical Conductivity, Relaxation and the Glass Transition: A New Look at a Familiar Phenomenon
Annealed samples from a single melt of a 10 mol% K2O-90SiO2 glass were reheated to temperatures ranging from 450 to 800 C, held isothermally for 20 min, and then quenched in either air or a silicon oil bath. The complex impedance of both the annealed and quenched samples was measured as a function of temperature from 120 to 250 C using ac impedance spectroscopy from 1 Hz to 1 MHz. The dc conductivity, sigma(sub dc), was measured from the low frequency intercept of depressed semicircle fits to the complex impedance data. When the sigma(sub dc) at 150 C was plotted against soak temperature, the results fell into three separate regions that are explained in terms of the glass structural relaxation time, tau(sub S). This sigma(sub dc) plot provides a new way to look the glass transition range, Delta T(sub r). In addition, sigma(sub dc) was measured for different soak times at 550 C, from which an average relaxation time of 7.3 min was calculated. It was found that the size and position of the Delta T(sub r) is controlled by both the soak time and cooling rate
Dark-ages reionization & galaxy formation simulation IV: UV luminosity functions of high-redshift galaxies
In this paper we present calculations of the UV luminosity function from the
Dark-ages Reionization And Galaxy-formation Observables from Numerical
Simulations (DRAGONS) project, which combines N-body, semi-analytic and
semi-numerical modelling designed to study galaxy formation during the Epoch of
Reionization. Using galaxy formation physics including supernova feedback, the
model naturally reproduces the UV LFs for high-redshift star-forming galaxies
from through to . We investigate the luminosity--star
formation rate (SFR) relation, finding that variable SFR histories of galaxies
result in a scatter around the median relation of -- dex depending on
UV luminosity. We find close agreement between the model and observationally
derived SFR functions. We use our calculated luminosities to investigate the
luminosity function below current detection limits, and the ionizing photon
budget for reionization. We predict that the slope of the UV LF remains steep
below current detection limits and becomes flat at
. We find that () per cent of the total
UV flux at () has been detected above an observational limit of
, and that galaxies fainter than
are the main source of ionizing photons for
reionization. We investigate the luminosity--stellar mass relation, and find a
correlation for galaxies with that has the form
, in good agreement with observations, but
which flattens for fainter galaxies. We determine the luminosity--halo mass
relation to be , finding that
galaxies with reside in host dark matter haloes of
at , and that this mass decreases
towards high redshift.Comment: 17 pages, 14 figures, Accepted for publication in MNRA
- …