268 research outputs found
Experimental study of honeycomb SiCSi under highly concentrated solar flux: Evolution of its thermo-radiative properties
The material that is used in solar receivers is subjected to intense cyclic thermal stresses and extreme temperatures, which are directly dependent on the intermittence of the solar resource. These factors accelerate the ageing mechanisms and reduce the durability of the receivers because of a reduction of their thermal performance. This study presents guidelines to study the thermo-radiative properties of an absorber material that is subjected to a highly concentrated solar flux. The material was a square honeycomb SiCSi structure that is typically used in volumetric air receivers. Accelerated ageing tests were performed by means of crashing thermal treatments, in which the modulus and period of the incident flux and the boundary conditions of the material were varied. The reflectivity and absorptivity of the material were experimentally characterized before and after the thermal treatments. The measurements were performed using two different reflectometers, one monochromatic and one in the solar band; the latter can measure at ambient temperature or high temperature that is representative of the operational conditions (400-700 degrees C). However, only the solar band reflectometer working at high temperature was able to detect the evolution of the thermo-radiative properties of the material, which highlights the important role of the temperature and the wavelength. Furthermore, the thermal treatments in which the samples were water-cooled and in which the solar flux was medial more quickly accelerated the ageing mechanism of the material and reduced its absorptivity.The authors acknowledge the financial support of the research programme of the Universidad Carlos III de Madrid, 2015, which made this study possible through a mobility grant. Moreover, this work was supported by the French "Investments for the future" programme, which is managed by the National Agency for Research under contract ANR-10-LABX-22-01 by Labex SOLSTICE and by the Spanish government under the project ENE2012-34255
Integrating The Five Practices of Exemplary Leadership Model into Entrepreneurship Education
Entrepreneurship educators can assess their students by focusing on leadership self-efficacy dimensions that align with desirable entrepreneurship behaviors. To support this claim, we used the Student Leadership Practices Inventory (S-LPI) to survey a group of 46 undergraduate students in Mexico and 49 undergraduate students in Spain that were involved in entrepreneurship education programs. Independent samples t-tests show statistically significant differences between the two groups. We also compared the whole sample in terms of gender and found no differences. We propose that educators integrate Five Practices of Exemplary Leadership model into de design and assessment of their curricular and co-curricular entrepreneurship development programs. Specifically, the model serves educators from different countries, in this case Mexico and Spain, by identifying the leadership behaviors that their students enact. By integrating the five practices of exemplary leadership model, educators can account for variables like nation of origin and gender, and identify differences between groups
Optical and near-infrared spectroscopy of the black hole transient 4U 1543-47 during its 2021 ultra-luminous state
We present simultaneous optical and near-infrared spectra obtained during the
2021 outburst of the black hole transient 4U 1543-47. The X-ray
hardness-intensity diagram and the comparison with similar systems reveal a
luminous outburst, probably reaching the Eddington luminosity, as well as a
long-lasting excursion to the so-called ultra-luminous state. VLT/X-shooter
spectra were taken in two epochs 14 days apart during the early and brightest
part of the outburst, while the source was in this ultra-luminous accretion
state. The data show strong H and HeI emission lines, as well as
high-excitation HeII and OIII transitions. Most lines are single-peaked in both
spectra, except for the OIII lines that exhibit evident double-peaked profiles
during the second epoch. The Balmer lines are embedded in broad absorption
wings that we believe are mainly produced by the contribution of the A2V donor
to the optical flux, which we estimate to be in the range of 11 to 14 per cent
in the band during our observations. Although no conspicuous outflow
features are found, we observe some wind-related line profiles, particularly in
the near-infrared. Such lines include broad emission line wings and skewed red
profiles, suggesting the presence of a cold (i.e. low ionisation) outflow with
similar observational properties to those found in other low-inclination black
hole transients.Comment: Accepted for publication in A&
Discovery of optical outflows and inflows in the black hole candidate GRS 1716-249
We present optical spectroscopy obtained with the GTC, VLT and SALT
telescopes during the decline of the 2016-2017 outburst of the black hole
candidate GRS 1716-249 (Nova Oph 1993). Our 18-epoch data set spans 6 months
and reveals that the observational properties of the main emission lines are
very variable, even on time scales of a few hours. Several epochs are
characterised by P-Cyg (as well as flat-top and asymmetric) profiles in the
H, H and He II (4686) emission lines, implying the
presence of an accretion disc wind, which is likely hot and dense. The wind's
terminal velocity (2000 km s) is similar to that observed in other
black hole X-ray transients. These lines also show transient and sharp
red-shifted absorptions, taking the form of inverted P-Cyg profiles. We argue
that these profiles can be explained by the presence of infalling material at
1300 km s. We propose a failed wind scenario to explain this
inflow and discuss other alternatives, such as obscuration produced by an
accretion-related structure (e.g. the gas stream) in a high inclination system.Comment: Published in MNRAS main journa
Understanding the High Ionic Conductivity in Nanostructured Ytterbium Stabilized Zirconia Thin Films
Recently, high ionic conduction has been reported in nanostructured materials. This increase in conductivity can be important in technological applications, including micro-Solid Oxide Fuel Cells, so the understanding of this phenomenon is essential. In this work, XRD, Raman spectroscopy, SEM, EDS maps, and UV-Visible spectroscopy measurements are used to have an insight into the relationship between structural and electrical properties in nanostructured ytterbium stabilized zirconia (YbSZ) thin films prepared by ultrasonic spray pyrolysis. Raman measurements allowed the identification of a mixture of tetragonal and cubic phases at 4% of Yb doping, which cannot be detected by XRD, while the compositional maps suggest that Yb can be located preferentially in the grain boundaries. Changes in the activation energy values in bulk and grain boundaries are related to the small grain sizes (≤10 nm). UV measurements support the ionic nature of the charge transport. These results indicate that the high conductivity is a consequence of different physical parameters in the films such as stress in the materials, different crystalline phases, impurities diffusion to the grain boundaries, and the presence or absence of electronic conduction. A model that explains the increase of conductivity in nanostructured materials must include all these aspects
Evaluation of Human Cerebrospinal Fluid Malate Dehydrogenase 1 as a Marker in Genetic Prion Disease Patients
The exploration of accurate diagnostic markers for differential diagnosis of neurodegenerative diseases is an ongoing topic. A previous study on cerebrospinal fluid (CSF)-mitochondrial malate dehydrogenase 1 (MDH1) in sporadic Creutzfeldt-Jakob disease (sCJD) patients revealed a highly significant upregulation of MDH1. Here, we measured the CSF levels of MDH1 via enzyme-linked immunosorbent assay in a cohort of rare genetic prion disease cases, such as genetic CJD (gCJD) cases, exhibiting the E200K, V210I, P102L (Gerstmann-Straussler-Scheinker syndrome (GSS)), or D178N (fatal familial insomnia (FFI)) mutations in the PRNP. Interestingly, we observed enhanced levels of CSF-MDH1 in all genetic prion disease patients compared to neurological controls (without neurodegeneration). While E200K and V210I carriers showed highest levels of MDH1 with diagnostic discrimination from controls of 0.87 and 0.85 area under the curve (AUC), FFI and GSS patients exhibited only moderately higher CSF-MDH1 levels than controls. An impact of the PRNP codon 129 methionine/valine (MV) genotype on the amount of MDH1 could be excluded. A correlation study of MDH1 levels with other neurodegenerative marker proteins revealed a significant positive correlation between CSF-MDH1 concentration with total tau (tau) but not with 14-3-3 in E200K, as well as in V210I patients. In conclusion, our study indicated the potential use of MDH1 as marker for gCJD patients which may complement the current panel of diagnostic biomarkers
The dynamical mass of the white dwarf in XY Ari questions intermediate polar X-ray spectral models
We present a dynamical study of the eclipsing intermediate polar XY Ari based
on time-resolved near-infrared spectroscopy obtained with the EMIR spectrograph
on the 10.4-m Gran Telescopio Canarias. Using main sequence template spectra
taken with the same instrument setup as the target spectra, we measure a radial
velocity amplitude of the late K-type donor star km s.
We also obtain the rotational broadening of its photospheric lines
km s. From these and the eclipse
geometry, we derive a donor-to-white dwarf mass ratio , an orbital inclination and dynamical
masses and (). This result places the white dwarf in XY
Ari as one of the three most massive known in a cataclysmic variable.
Comparison with a number of white dwarf mass estimates from X-ray spectral
modelling indicates the necessity of a major revision of the cooling models
currently assumed for magnetic accretion in intermediate polars, as most of the
X-ray white dwarf masses lie significantly below the dynamical mass value.Comment: 9 pages, 4 figures, 5 tables, submitted for publication in MNRA
Application of Tensor Neural Networks to Pricing Bermudan Swaptions
The Cheyette model is a quasi-Gaussian volatility interest rate model widely
used to price interest rate derivatives such as European and Bermudan Swaptions
for which Monte Carlo simulation has become the industry standard. In low
dimensions, these approaches provide accurate and robust prices for European
Swaptions but, even in this computationally simple setting, they are known to
underestimate the value of Bermudan Swaptions when using the state variables as
regressors. This is mainly due to the use of a finite number of predetermined
basis functions in the regression. Moreover, in high-dimensional settings,
these approaches succumb to the Curse of Dimensionality. To address these
issues, Deep-learning techniques have been used to solve the backward
Stochastic Differential Equation associated with the value process for European
and Bermudan Swaptions; however, these methods are constrained by training time
and memory. To overcome these limitations, we propose leveraging Tensor Neural
Networks as they can provide significant parameter savings while attaining the
same accuracy as classical Dense Neural Networks. In this paper we rigorously
benchmark the performance of Tensor Neural Networks and Dense Neural Networks
for pricing European and Bermudan Swaptions, and we show that Tensor Neural
Networks can be trained faster than Dense Neural Networks and provide more
accurate and robust prices than their Dense counterparts.Comment: 15 pages, 9 figures, 2 table
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