529 research outputs found
Improving the phase stability and oxidation resistance of β-NiAl
This thesis is written in an alternate format. The thesis is composed of a general introduction, four original manuscripts, and a general conclusion. References cited within each chapter are located immediately after that section. In addition, figures and tables are numbered independently within each chapter.
The general introduction focuses on the driving force behind this research, and gives an overview of previous work done on nickel-based superalloys. Chapter 2 describes the preliminary experiments and how those experiments guided the rest of the thesis work. Chapter 3 deals specifically with the oxidation performance of platinum group metal (PGM) and hafnium modifications to β-NiAl intermetallic. Chapter 4 investigates the role of grain size on the oxidation resistance of NiAl based alloys. Chapter 5 focuses on the role of melting temperature on the oxidation resistance of NiAl based alloys. Chapter 6 summarizes the important results of this study
Group Analysis of Self-organizing Maps based on Functional MRI using Restricted Frechet Means
Studies of functional MRI data are increasingly concerned with the estimation
of differences in spatio-temporal networks across groups of subjects or
experimental conditions. Unsupervised clustering and independent component
analysis (ICA) have been used to identify such spatio-temporal networks. While
these approaches have been useful for estimating these networks at the
subject-level, comparisons over groups or experimental conditions require
further methodological development. In this paper, we tackle this problem by
showing how self-organizing maps (SOMs) can be compared within a Frechean
inferential framework. Here, we summarize the mean SOM in each group as a
Frechet mean with respect to a metric on the space of SOMs. We consider the use
of different metrics, and introduce two extensions of the classical sum of
minimum distance (SMD) between two SOMs, which take into account the
spatio-temporal pattern of the fMRI data. The validity of these methods is
illustrated on synthetic data. Through these simulations, we show that the
three metrics of interest behave as expected, in the sense that the ones
capturing temporal, spatial and spatio-temporal aspects of the SOMs are more
likely to reach significance under simulated scenarios characterized by
temporal, spatial and spatio-temporal differences, respectively. In addition, a
re-analysis of a classical experiment on visually-triggered emotions
demonstrates the usefulness of this methodology. In this study, the
multivariate functional patterns typical of the subjects exposed to pleasant
and unpleasant stimuli are found to be more similar than the ones of the
subjects exposed to emotionally neutral stimuli. Taken together, these results
indicate that our proposed methods can cast new light on existing data by
adopting a global analytical perspective on functional MRI paradigms.Comment: 23 pages, 5 figures, 4 tables. Submitted to Neuroimag
The role of the chair of the school governing body in England
The research reported here analysed the role of the chair of the school governing body in England, drawing on a national survey of governors and the study of governing in 30 schools. The role encompassed: being a governor; appointing and working with the head teacher; acting as a change agent; active participation in the school; organising the governing body; dealing with complaints; working with parents; and chairing meetings. We discuss the role and the way it is experienced and conclude that the position of chair is substantially under-played; given insufficient status; and is a significant educational and community leadership responsibility.17 page(s
The Number Density Evolution of Extreme Emission Line Galaxies in 3D-HST: Results from a Novel Automated Line Search Technique for Slitless Spectroscopy
The multiplexing capability of slitless spectroscopy is a powerful asset in
creating large spectroscopic datasets, but issues such as spectral confusion
make the interpretation of the data challenging. Here we present a new method
to search for emission lines in the slitless spectroscopic data from the 3D-HST
survey utilizing the Wide-Field Camera 3 on board the Hubble Space Telescope.
Using a novel statistical technique, we can detect compact (extended) emission
lines at 90% completeness down to fluxes of 1.5 (3.0) times 10^{-17}
erg/s/cm^2, close to the noise level of the grism exposures, for objects
detected in the deep ancillary photometric data. Unlike previous methods, the
Bayesian nature allows for probabilistic line identifications, namely redshift
estimates, based on secondary emission line detections and/or photometric
redshift priors. As a first application, we measure the comoving number density
of Extreme Emission Line Galaxies (restframe [O III] 5007 equivalent widths in
excess of 500 Angstroms). We find that these galaxies are nearly 10 times more
common above z~1.5 than at z<0.5. With upcoming large grism surveys such as
Euclid and WFIRST as well as grisms featuring prominently on the NIRISS and
NIRCam instruments on James Webb Space Telescope, methods like the one
presented here will be crucial for constructing emission line redshift catalogs
in an automated and well-understood manner.Comment: 16 pages, 14 Figures; Accepted to Ap
Ages of massive galaxies at from 3D-HST rest-frame optical spectroscopy
We present low-resolution near-infrared stacked spectra from the 3D-HST
survey up to and fit them with commonly used stellar population
synthesis models: BC03 (Bruzual & Charlot, 2003), FSPS10 (Flexible Stellar
Population Synthesis, Conroy & Gunn 2010), and FSPS-C3K (Conroy, Kurucz,
Cargile, Castelli, in prep). The accuracy of the grism redshifts allows the
unambiguous detection of many emission and absorption features, and thus a
first systematic exploration of the rest-frame optical spectra of galaxies up
to . We select massive galaxies (), we
divide them into quiescent and star-forming via a rest-frame color-color
technique, and we median-stack the samples in 3 redshift bins between
and . We find that stellar population models fit the observations well
at wavelengths below rest-frame, but show systematic residuals
at redder wavelengths. The FSPS-C3K model generally provides the best fits
(evaluated with a statistics) for quiescent galaxies, while BC03
performs the best for star-forming galaxies. The stellar ages of quiescent
galaxies implied by the models, assuming solar metallicity, vary from 4 Gyr at
to 1.5 Gyr at , with an uncertainty of a factor of 2
caused by the unknown metallicity. On average the stellar ages are half the age
of the Universe at these redshifts. We show that the inferred evolution of ages
of quiescent galaxies is in agreement with fundamental plane measurements,
assuming an 8 Gyr age for local galaxies. For star-forming galaxies the
inferred ages depend strongly on the stellar population model and the shape of
the assumed star-formation history.Comment: 13 pages, 15 figures, accepted for publication in Ap
JWST/NIRSpec Measurements of the Relationships Between Nebular Emission-line Ratios and Stellar Mass at z~3-6
We analyze the rest-optical emission-line ratios of star-forming galaxies at
2.7<=z<6.5 drawn from the Cosmic Evolution Early Release Science (CEERS)
Survey, and their relationships with stellar mass (M_*). Our analysis includes
both line ratios based on the [NII]6583 feature -- [NII]6583/Ha,
([OIII]5007/Hb)/([NII]6583/Ha) (O3N2), and [NII]6583/[OII]3727 -- and those
those featuring alpha elements -- [OIII]5007/Hb, [OIII]5007/[OII]3727 (O_32),
([OIII]4959,5007+[OII]3727)/Hb (R_23), and [NeIII]3869/[OII]3727. Given the
typical flux levels of [NII]6583 and [NeIII]3869, which are undetected in the
majority of individual CEERS galaxies at 2.7<=z<6.5, we construct composite
spectra in bins of M_* and redshift. Using these composite spectra, we compare
the relationships between emission-line ratios and M_* at 2.7<=z<6.5 with those
observed at lower redshift. While there is significant evolution towards higher
excitation (e.g., higher [OIII]5007/Hb, O_32, O3N2), and weaker nitrogen
emission (e.g., lower [NII]6583/Ha and [NII]6583/[OII]3727) between z~0 and
z~3, we find in most cases that there is no significant evolution in the
relationship between line ratio and M_* beyond z~3. The [NeIII]3869/[OII]3727
ratio is anomalous in showing evidence for significant elevation at 4.0<=z<6.5
at fixed mass, relative to z~3.3. Collectively, however, our empirical results
suggest that there is no significant evolution in the mass-metallicity
relationship at 2.7<=z<6.5. Representative galaxy samples and metallicity
calibrations based on existing and upcoming JWST/NIRSpec observations will be
required to translate these empirical scaling relations into ones tracing
chemical enrichment and gas cycling, and distinguish among the descriptions of
star-formation feedback in simulations of galaxy formation at z>3.Comment: 10 pages, 4 figures, ApJL, in pres
JWST/NIRSpec Balmer-line Measurements of Star Formation and Dust Attenuation at z~3-6
We present an analysis of the star-formation rates (SFRs) and dust
attenuation properties of star-forming galaxies at drawn from
the Cosmic Evolution Early Release Science (CEERS) Survey. Our analysis is
based on {\it JWST}/NIRSpec Micro-Shutter Assembly (MSA)
spectroscopic observations covering approximately m. Our primary
rest-frame optical spectroscopic measurements are H/H Balmer
decrements, which we use as an indicator of nebular dust attenuation. In turn,
we use Balmer decrements to obtain dust-corrected H-based SFRs (i.e.,
SFR(H)). We construct the relationship between SFR(H) and
stellar mass () in three bins of redshift (, , and ), which represents the first time the star-forming
main sequence has been traced at these redshifts using direct spectroscopic
measurements of Balmer emission as a proxy for SFR. In tracing the relationship
between SFR(H) and back to such early times (), it is
essential to use a conversion factor between H and SFR that accounts
for the subsolar metallicity prevalent among distant galaxies. We also use
measured Balmer decrements to investigate the relationship between dust
attenuation and stellar mass out to . The lack of significant redshift
evolution in attenuation at fixed stellar mass, previously confirmed using
Balmer decrements out to , appears to hold out to . Given
the rapidly evolving gas, dust, and metal content of star-forming galaxies at
fixed mass, this lack of significant evolution in attenuation provides an
ongoing challenge to explain.Comment: 9 pages, 4 figures, ApJ, in pres
Epigenetic mediation of AKT1 rs1130233's Effect on delta-9-tetrahydrocannabinol-induced medial temporal function during fear processing
High doses of delta-9-tetrahydrocannabinol (THC), the main psychoactive component of cannabis, have been shown to have anxiogenic effects. Additionally, THC effects have been shown to be modulated by genotype, including the single nucleotide polymorphism (SNP) rs1130233 at the protein kinase AKT1 gene, a key component of the dopamine signalling cascade. As such, it is likely that epigenetic methylation around this SNP may affect AKT gene expression, which may in turn impact on the acute effects of THC on brain function. We investigated the genetic (AKT1 rs1130233) and epigenetic modulation of brain function during fear processing in a 2-session, double-blind, cross-over, randomized placebo-controlled THC administration, in 36 healthy males. Fear processing was assessed using an emotion (fear processing) paradigm, under functional magnetic resonance imaging (fMRI). Complete genetic and fMRI data were available for 34 participants. THC caused an increase in anxiety and transient psychotomimetic symptoms and para-hippocampal gyrus/amygdala activation. Number of A alleles at the AKT1 rs1130233 SNP, and percentage methylation at the CpG11-12 site, were independently associated with a greater effect of THC on activation in a network of brain regions including left and right parahippocampal gyri, respectively. AKT1 rs1130233 moderation of the THC effect on left parahippocampal activation persisted after covarying for methylation percentage, and was partially mediated in sections of the left parahippocampal gyrus/hippocampus by methylation percentage. These results may offer an example of how genetic and epigenetic variations influence the psychotomimetic and neurofunctional effects of THC
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