282 research outputs found
Electronic properties of ion-implanted yttria-stabilized zirconia
Ion implantation of iron and titanium has been applied to modify the surface properties of polycrystalline yttria-stabilized zirconia ((ZrO2)0.87(YO1.5)0.13 (YSZ)) discs in an attempt to prepare surfaces with a mixed conductivity and by this an enhanced surface oxygen exchange kinetics. Surface-sensitive spectroscopic techniques were applied to investigate the implanted layers as a function of different pretreatments such as oxidation, reduction and annealing. Depth profiles were recorded by Rutherford Backscattering Spectroscopy (RBS) and X-ray Photoelectron Spectroscopy (XPS) in combination with sputtering. Ion Scattering Spectroscopy (ISS) and XPS were used to investigate the surface composition and valency of implanted ions. Electronic properties like the band gap, the work function and the energy difference between the Fermi level and valence band edge (EF-EV) were obtained from Ultraviolet Photoelectron Spectroscopy (UPS) and Electron Energy Loss Spectroscopy (EELS). Overlayers of Fe2O3 or TiO2 are formed during oxidation of as-implanted samples. The Fe- and Ti-oxides could be reduced in hydrogen to the oxidation states Fe2+, Fe0 or Ti3+. Annealing of the samples leads to decreased surface concentrations of the implanted ions due to in-diffusion. At the surface of the annealed iron-implanted samples, Fe2+ and metallic Fe could be generated after further reduction whereas at the surface of the annealed Ti-implanted samples only Ti4+ was detectable.\u
Scanning Electron Microscope Studies on the Breakdown of Passivity of a Nickel-Chromium-Molybdenum Dental Alloy
The breakdown of passivity and localized corrosion of a Ni-20Cr-10Mo alloy was investigated. The methods employed were potentiodynamic polarization and SEM, and AES and EDX after potentiostatic polarization over a period of 20 hours in the passive and transpassive regions. The 1 ÎŒm finished as-cast specimens were polarized in aerated 0.1 M NaCl. The cyclic polarization curves revealed a critical pitting potential of 470 mv (SCE), while the protection potential was 300 mV (SCE). Using the potentiostatic polarization technique, nearly constant corrosion currents appeared, indicating that the whole surface was corroded uniformly. SEM pictures of samples, corroded at 650 mv, showed little pits under the oxide layer and a thinning down of the outer oxide layer. This lead to the opinion that the penetration as well as the adsorption mechanism determine the breakdown of passivity. EDX analysis and AES depth profiles showed an enrichment of Cr and Mo in the oxide. In contrast to oxidized samples, no second layer of Ni was found in the outer oxide region. In the transpassive region the relative amount of Cr and Mo in the oxide layer was higher than the one found in corresponding samples polarized in the passive region. The oxide thickness found was about 5 nm in the passive region (300 mV SCE) and about 250 nm in the transpassive region (650 mV SCE)
Nonlinear Advantage: Trained Networks Might Not Be As Complex as You Think
We perform an empirical study of the behaviour of deep networks when fully
linearizing some of its feature channels through a sparsity prior on the
overall number of nonlinear units in the network. In experiments on image
classification and machine translation tasks, we investigate how much we can
simplify the network function towards linearity before performance collapses.
First, we observe a significant performance gap when reducing nonlinearity in
the network function early on as opposed to late in training, in-line with
recent observations on the time-evolution of the data-dependent NTK. Second, we
find that after training, we are able to linearize a significant number of
nonlinear units while maintaining a high performance, indicating that much of a
network's expressivity remains unused but helps gradient descent in early
stages of training. To characterize the depth of the resulting partially
linearized network, we introduce a measure called average path length,
representing the average number of active nonlinearities encountered along a
path in the network graph. Under sparsity pressure, we find that the remaining
nonlinear units organize into distinct structures, forming core-networks of
near constant effective depth and width, which in turn depend on task
difficulty
Spreads in Effective Learning Rates: The Perils of Batch Normalization During Early Training
Excursions in gradient magnitude pose a persistent challenge when training
deep networks. In this paper, we study the early training phases of deep
normalized ReLU networks, accounting for the induced scale invariance by
examining effective learning rates (LRs). Starting with the well-known fact
that batch normalization (BN) leads to exponentially exploding gradients at
initialization, we develop an ODE-based model to describe early training
dynamics. Our model predicts that in the gradient flow, effective LRs will
eventually equalize, aligning with empirical findings on warm-up training.
Using large LRs is analogous to applying an explicit solver to a stiff
non-linear ODE, causing overshooting and vanishing gradients in lower layers
after the first step. Achieving overall balance demands careful tuning of LRs,
depth, and (optionally) momentum. Our model predicts the formation of spreads
in effective LRs, consistent with empirical measurements. Moreover, we observe
that large spreads in effective LRs result in training issues concerning
accuracy, indicating the importance of controlling these dynamics. To further
support a causal relationship, we implement a simple scheduling scheme
prescribing uniform effective LRs across layers and confirm accuracy benefits
General relativistic gravitational field of a rigidly rotating disk of dust: Solution in terms of ultraelliptic functions
In a recent paper we presented analytic expressions for the axis potential,
the disk metric, and the surface mass density of the global solution to
Einstein's field equations describing a rigidly rotating disk of dust. Here we
add the complete solution in terms of ultraelliptic functions and quadratures.Comment: 5 pages, published in 1995 [Phys. Rev. Lett. 75 (1995) 3046
Thermal history modeling of the H chondrite parent body
The cooling histories of individual meteorites can be empirically
reconstructed by using ages from different radioisotopic chronometers with
distinct closure temperatures. For a group of meteorites derived from a single
parent body such data permit the reconstruction of the cooling history and
properties of that body. Particularly suited are H chondrites because precise
radiometric ages over a wide range of closure temperatures are available. A
thermal evolution model for the H chondrite parent body is constructed by using
all H chondrites for which at least three different radiometric ages are
available. Several key parameters determining the thermal evolution of the H
chondrite parent body and the unknown burial depths of the H chondrites are
varied until an optimal fit is obtained. The fit is performed by an 'evolution
algorithm'. Empirical data for eight samples are used for which radiometric
ages are available for at least three different closure temperatures. A set of
parameters for the H chondrite parent body is found that yields excellent
agreement (within error bounds) between the thermal evolution model and
empirical data of six of the examined eight chondrites. The new thermal model
constrains the radius and formation time of the H chondrite parent body
(possibly (6) Hebe), the initial burial depths of the individual H chondrites,
the average surface temperature of the body, the average initial porosity of
the material the body accreted from, and the initial 60Fe content of the H
chondrite parent body.Comment: 16 pages, 7 figure
Analysis of the noise-induced bursting-spiking transition in a pancreatic beta-cell model
A stochastic model of the electrophysiological behavior of the pancreatic
ÎČ
cell is studied, as a paradigmatic example of a bursting biological cell embedded in a noisy environment. The analysis is focused on the distortion that a growing noise causes to the basic properties of the membrane potential signals, such as their periodic or chaotic nature, and their bursting or spiking behavior. We present effective computational tools to obtain as much information as possible from these signals, and we suggest that the methods could be applied to real time series. Finally, a universal dependence of the main characteristics of the membrane potential on the size of the considered cell cluster is presented.This work has been supported by the Spanish Ministry of Science and Technology under Project Nos. BFM2000-0967 and BFM2003-03081 by a scholarship from the Spanish Ministry of Foreign Affaires (2001), and by Universidad Rey Juan Carlos under Project Nos. PGRAL-2001-02, PIGE-02-04, and GCO-2003â16. J.A. acknowledges support from the Danish Natural Science Foundation.Peer reviewe
Frame dragging and bending of Light in Kerr and Kerr-(anti) de Sitter spacetimes
The equations of general relativity in the form of timelike and null
geodesics that describe motion of test particles and photons in Kerr spacetime
are solved exactly including the contribution from the cosmological constant.
We then perform a systematic application of the exact solutions obtained to the
following cases. The exact solutions derived for null, spherical, polar and
non-polar orbits are applied for the calculation of frame dragging
(Lense-Thirring effect) for the orbit of a photon around the galactic centre,
assuming that the latter is a Kerr black hole for various values of the Kerr
parameter including those supported by recent observations. Unbound null polar
orbits are investigated, and an analytical expression for the deviation angle
of a polar photon orbit from the gravitational Kerr field is derived. In
addition, we present the exact solution for timelike and null equatorial
orbits. In the former case, we derive an analytical expression for the
precession of the point of closest approach (perihelion, periastron) for the
orbit of a test particle around a rotating mass whose surrounding curved
spacetime geometry is described by the Kerr field. In the latter case, we
calculate an exact expression for the deflection angle for a light ray in the
gravitational field of a rotating mass (the Kerr field). We apply this
calculation for the bending of light from the gravitational field of the
galactic centre for various values of the Kerr parameter and the impact factor.Comment: LaTeX file, 45 pages 1 figure, typos fixed, v3 published in Classical
and Quantum Gravity 22 (2005) 4391-442
Single-nucleotide polymorphism associations with preterm delivery: a case-control replication study and meta-analysis
BackgroundThe aim of this study was to replicate single-nucleotide polymorphism (SNP) associations with preterm birth (PTB; birth at MethodsSpontaneous PTB cases and controls were selected from an existing cohort. Candidate SNPs were taken from an existing genotype panel. A systematic review was conducted for each SNP in the panel to determine suitability as a PTB candidate. Those with significant associations previously reported in Caucasians were selected for replication. Candidate SNPs were already genotyped in cases and controls and clinical data were accessed from state perinatal and cerebral palsy databases. Association analysis was conducted between each SNP and PTB, and meta-analysis was conducted if there were â„ 3 studies in the literature. Maternal and fetal SNPs were considered as separate candidates.ResultsA cohort of 170 cases and 583 controls was formed. Eight SNPs from the original panel of genotyped SNPs were selected as PTB candidates and for replication on the basis of systematic literature review results. In our cohort, fetal factor V Leiden (FVL) was significantly associated with PTB (odds ratio (OR): 2.6, 95% confidence interval (CI): 1.31-5.17), and meta-analysis confirmed this association (OR: 2.71, 95% CI: 1.15-6.4).ConclusionReplication and meta-analysis support an increased risk of PTB in Caucasians with the fetal FVL mutation.Michael E. OâCallaghan, Alastair H. MacLennan, Gai L. McMichael, Eric A. Haan and Gustaaf A. Dekke
Electron Beam-Induced Writing of Nanoscale Iron Wires on a Functional Metal Oxide
Electron beam-induced surface activation (EBISA) has been used to grow wires of iron on rutile TiO2(110)-(1 Ă 1) in ultrahigh vacuum. The wires have a width down to âŒ20 nm and hence have potential utility as interconnects on this dielectric substrate. Wire formation was achieved using an electron beam from a scanning electron microscope to activate the surface, which was subsequently exposed to Fe(CO)5. On the basis of scanning tunneling microscopy and Auger electron spectroscopy measurements, the activation mechanism involves electron beam-induced surface reduction and restructuring
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