568 research outputs found
Apatite germanates doped with tungsten: Synthesis, structure, and conductivity
et al.High oxygen content apatite germanates, La10Ge 6-xWxO27+x, have been prepared by doping on the Ge site with W. In addition to increasing the oxygen content, this doping strategy is shown to result in stabilisation of the hexagonal lattice, and yield high conductivities. Structural studies of La10Ge 5.5W0.5O27.5 show that the interstitial oxygen sites are associated to a different degree with the Ge/WO4 tetrahedra, leading to five coordinate Ge/W and significant disorder for the oxygen sites associated with these units. Raman spectroscopy studies suggest that in the case of the WO5 units, the interstitial oxygen is more tightly bonded and therefore not as mobile as in the case of the GeO5 units, thus not contributing significantly to the conduction process. © 2011 The Royal Society of Chemistry.Financial support from Spanish project MAT2007-64486-C07-02 is acknowledged.Peer Reviewe
Efecto de la granulometría de los áridos en la permeabilidad al aire del hormigón
Great durability problems are being found in concrete structures related to the penetrability of aggressive agents through the concrete (ie. chloride penetration, sulphate attack, carbonation, freezing and thawing, and so on). Air permeability coefficient is used as an effective tool to estimate the potential durability of concrete structures due to its direct relation with the microstructure and the moisture content.
This paper discusses the effect of the aggregate grading and water/cement ratio on the air permeability coefficient. An aggregate grading with more sand than coarse aggregates has resulted more beneficial from the point of view of concrete air permeability. This fact can be attributed to a denser skeleton formed by the finer aggregates. With fine aggregates, the higher water/cement ratio, the lower air permeability. However, the contrary was found with coarse aggregates. Overall, a temperature increase from 20 °C to 60 °C during preconditioning led to a Dair increase of 40–80%.Se han encontrado una gran cantidad de problemas de durabilidad de estructuras de hormigón relacionados con la penetración de agentes agresivos externos (es decir, penetración de cloruros, ataque por sulfatos, carbonatación, hielo-deshielo, etc.). El coeficiente de permeabilidad al aire se utiliza como una herramienta eficaz para estimar la durabilidad potencial de las estructuras de hormigón debido a su relación directa con su microestructura y contenido de humedad.
Se discute el efecto de la gradación de los áridos y relación agua/cemento en el coeficiente de permeabilidad al aire. Con áridos más finos que gruesos, el resultado es más beneficioso, lo que se atribuye a que la arena forma un esqueleto más denso. Con áridos más finos, al aumentar la relación agua/cemento, disminuye la permeabilidad al aire; pero con áridos más gruesos se ha observado lo contrario. Cuando se pre-acondiciona de 20 °C a 60 °C, se produce un aumento del Dair del 40–80%
Period-doubling bifurcations and islets of stability in two-degree-of-freedom Hamiltonian systems
In this paper, we show that the destruction of the main KAM islands in
two-degree-of-freedom Hamiltonian systems occurs through a cascade of
period-doubling bifurcations. We calculate the corresponding Feigenbaum
constant and the accumulation point of the period-doubling sequence. By means
of a systematic grid search on exit basin diagrams, we find the existence of
numerous very small KAM islands ('islets') for values below and above the
aforementioned accumulation point. We study the bifurcations involving the
formation of islets and we classify them in three different types. Finally, we
show that the same types of islets appear in generic two-degree-of-freedom
Hamiltonian systems and in area-preserving maps
Energy-based stochastic resetting can avoid noise-enhanced stability
The theory of stochastic resetting asserts that restarting a stochastic
process can expedite its completion. In this paper, we study the escape process
of a Brownian particle in an open Hamiltonian system that suffers
noise-enhanced stability. This phenomenon implies that under specific noise
amplitudes the escape process is delayed. Here, we propose a new protocol for
stochastic resetting that can avoid the noise-enhanced stability effect. In our
approach, instead of resetting the trajectories at certain time intervals, a
trajectory is reset when a predefined energy threshold is reached. The
trajectories that delay the escape process are the ones that lower their energy
due to the stochastic fluctuations. Our resetting approach leverages this fact
and avoids long transients by resetting trajectories before they reach low
energy levels. Finally, we show that the chaotic dynamics (i.e., the sensitive
dependence on initial conditions) catalyzes the effectiveness of the resetting
strategy
Interstitial oxide ion distribution and transport mechanism in aluminum-doped neodymium silicate apatite electrolytes
et al.Rare earth silicate apatites are one-dimensional channel structures that show potential as electrolytes for solid oxide fuel cells (SOFC) due to their high ionic conductivity at intermediate temperatures (500-700 °C). This advantageous property can be attributed to the presence of both interstitial oxygen and cation vacancies, that create diffusion paths which computational studies suggest are less tortuous and have lower activation energies for migration than in stoichiometric compounds. In this work, neutron diffraction of NdAlSiO (0 ≤ x ≤ 1.5) single crystals identified the locations of oxygen interstitials, and allowed the deduction of a dual-path conduction mechanism that is a natural extension of the single-path sinusoidal channel trajectory arrived at through computation. This discovery provides the most thorough understanding of the O transport mechanism along the channels to date, clarifies the mode of interchannel motion, and presents a complete picture of O percolation through apatite. Previously reported crystallographic and conductivity measurements are re-examined in the light of these new findings.We are pleased to acknowledge the Agency for Science, Technology and Research (A*STAR) PSF grant 082 101 0021 “Optimization of Oxygen Sublattices in Solid Oxide Fuel Cell Apatite Electrolytes” for funding the work and the Ministry of Education (MOE) Tier 2 grant T208B1212 for enabling the purchase of a single crystal X-ray diffractometer.Peer Reviewe
Neural network interpolation of the magnetic field for the LISA Pathfinder Diagnostics Subsystem
LISA Pathfinder is a science and technology demonstrator of the European
Space Agency within the framework of its LISA mission, which aims to be the
first space-borne gravitational wave observatory. The payload of LISA
Pathfinder is the so-called LISA Technology Package, which is designed to
measure relative accelerations between two test masses in nominal free fall.
Its disturbances are monitored and dealt by the diagnostics subsystem. This
subsystem consists of several modules, and one of these is the magnetic
diagnostics system, which includes a set of four tri-axial fluxgate
magnetometers, intended to measure with high precision the magnetic field at
the positions of the test masses. However, since the magnetometers are located
far from the positions of the test masses, the magnetic field at their
positions must be interpolated. It has been recently shown that because there
are not enough magnetic channels, classical interpolation methods fail to
derive reliable measurements at the positions of the test masses, while neural
network interpolation can provide the required measurements at the desired
accuracy. In this paper we expand these studies and we assess the reliability
and robustness of the neural network interpolation scheme for variations of the
locations and possible offsets of the magnetometers, as well as for changes in
environmental conditions. We find that neural networks are robust enough to
derive accurate measurements of the magnetic field at the positions of the test
masses in most circumstances
May anomalous X chromosome methylation be responsible for the spontaneous abortion of a male foetus?
Pregnancy loss is an important reproductive problem which appears to be highly associated with genetic factors. A spontaneous abortion occurred before prenatal diagnosis could be performed, in a woman who carried a fragile X full mutation. DNA extracted from formalin-fixed para ffin-embedded chorionic villi preparations indicated that it was a male embryo with an apparently methylated X chromosome. The previous analysis of the family showed that her daughter, who
also carried a full mutation, exhibited an extremely skewed X inactivation of the normal allele (100%) and a severe fragile X phenotype. Thus, we speculate that the aberrant pattern of X chromosome methylation in this family may provoke the spontaneous miscarriage of this pregnancy that could be explained by at least partial inactivation of the unique X chromosome in a male foetus. Spontaneous abortion occurs quite frequently in humans, and recurrent pregnancy loss is a significant problem in women’s health (Christiansen 2006). Many cases of spontaneous abortion defy diagnosis and genetic factors have been proposed as a major contribution (Lanasa and Hogge 2000; Sierra and Stephenson 2006). The X chromosome inactiva
tion (XCI) is the process in which one of the two X chromosomes present in each cell of female mammals is inactivated during early embryogenesis, to achieve dosage compensation with males (Avner and Heard 2001; Heard 2004). Initial steps of XCI involve a ‘counting process’, which senses the X chromosome/autosome ratio that restricts XCI to female embryos and, thereafter, the choice of which chromosome is inactivated (Morey et al . 2004).Skewed XCI leads to an expression of X-linked recessive disorders in females (Plenge et al. 2002; Mart´ınez et al.
2005; Renault et al. 2007). Another form of X-inactivation called meiotic sex chromosome inactivation (MSCI), takes place in males, during spermatogenesis and is a manifestation of the general meiotic-silencing mechanism (Turner 2007). In this study, we report the spontaneous abortion of a male foetus with an apparently methylated X chromosome, and propose that anomalous inactivation of its uniqueX chromosome could explain nonviability of the embryo and pregnancy loss. Therefore, we strongly suggest, as a follow up, that the methylation status of the X chromosome be tested in early spontaneous abortion of males in the absence of any other known genetic or nongenetic cause
On-ground tests of LISA PathFinder thermal diagnostics system
Thermal conditions in the LTP, the LISA Technology Package, are required to
be very stable, and in such environment precision temperature measurements are
also required for various diagnostics objectives. A sensitive temperature
gauging system for the LTP is being developed at IEEC, which includes a set of
thermistors and associated electronics. In this paper we discuss the derived
requirements applying to the temperature sensing system, and address the
problem of how to create in the laboratory a thermally quiet environment,
suitable to perform meaningful on-ground tests of the system. The concept is a
two layer spherical body, with a central aluminium core for sensor implantation
surrounded by a layer of polyurethane. We construct the insulator transfer
function, which relates the temperature at the core with the laboratory ambient
temperature, and evaluate the losses caused by heat leakage through connecting
wires. The results of the analysis indicate that, in spite of the very
demanding stability conditions, a sphere of outer diameter of the order one
metre is sufficient. We provide experimental evidence confirming the model
predictions.Comment: 18 pages, 5 figures, LaTeX2e (compile with pdflatex), sumbitted to
CQG. This paper is a significant extension of gr-qc/060109
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