218,277 research outputs found
Temperature effect on space charge dynamics in XLPE insulation
This paper reports on space charge evolution in crosslinked polyethylene (XLPE) planar samples approximately 1.20 mm thick subjected to electric stress level of 30 kVdc/mm under four temperature 25 oC, 50 oC, 70 oC and 90 oC for 24 hours. Space charge profiles in both as-received and degassed samples were measured using the laser induced pressure pulse (LIPP) technique. The dc threshold stresses at which space charge initiates are greatly affected by testing temperatures. The results suggest that testing temperature has numerous effects on space charge dynamics such as enhancement of ionic dissociation of polar crosslinked by-products, charge injection, charge mobility and electrical conductivity. Space charge distributions of very different nature were seen at lower temperatures when comparing the results of as-received samples with degassed samples. However at higher temperature, the space charge distribution took the same form, although of lower concentration in degassed samples. Space charge distributions are dominated by positive charge when tested at high temperatures regardless of sample treatment and positive charge propagation enhances as testing temperature increases. This can be a major cause of concern as positive charge propagation has been reported to be related to insulation breakdown
The effect of degassing on morphology and space charge
It is believed that space charge buildup in cross-linked polyethylene (XLPE) insulation is the main cause for premature failure of underground power cables. The space charge activities in XLPE depend on many factors such as additives, material treatment, ambient temperature, insulator/electrode interface, etc. Degassing is one of the material treatment process commonly employ in cable manufacturing to improve insulation performance. In this paper, investigation on the effect of degassing period has on the morphology and space charge was carried out. Planar XLPE samples of the same composite were subjected to different degassing time. It is discovered that apart from removing volatile by-products, degassing also anneal XLPE material; changing the morphology as a result
The GTC exoplanet transit spectroscopy survey X. Stellar spots versus Rayleigh scattering: the case of HAT-P-11b
Rayleigh scattering in a hydrogen-dominated exoplanet atmosphere can be
detected from ground or space based telescopes, however, stellar activity in
the form of spots can mimic Rayleigh scattering in the observed transmission
spectrum. Quantifying this phenomena is key to our correct interpretation of
exoplanet atmospheric properties. We obtained long-slit optical spectroscopy of
two transits of HAT-P-11b with the Optical System for Imaging and
low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at Gran Telescopio
Canarias (GTC) on August 30 2016 and September 25 2017. We integrated the
spectrum of HAT-P-11 and one reference star in several spectroscopic channels
across the 400-785 nm region, creating numerous light curves of
the transits. We fit analytic transit curves to the data taking into account
the systematic effects and red noise present in the time series in an effort to
measure the change of the planet-to-star radius ratio
() across wavelength. By fitting both transits
together, we find a slope in the transmission spectrum showing an increase of
the planetary radius towards blue wavelengths. A closer inspection to the
transmission spectrum of the individual data sets reveals that the first
transit presents this slope while the transmission spectrum of the second data
set is flat. Additionally we detect hints of Na absorption in the first night,
but not in the second. We conclude that the transmission spectrum slope and Na
absorption excess found in the first transit observation are caused by
unocculted stellar spots. Modeling the contribution of unocculted spots to
reproduce the results of the first night we find a spot filling factor of
and a spot-to-photosphere temperature difference
of K.Comment: Accepted for publication in Astronomy & Astrophysics, 13 page
High Tc superconductors: The scaling of Tc with the number of bound holes associated with charge transfer neutralizing the multivalence cations
It is observed that for the known high-T(sub c) Cu-, Tl-, and Bi-based superconductors, T(sub c) scales consistently with the number of bound holes per unit cell which arise from charge transfer excitations of frequency approximately = 3 x 10(exp 13) that neutralized the multivalence cations into diamagnetic states. The resulting holes are established on the oxygens. Extrapolation of this empirical fit in the up-temperature direction suggests a T(sub c) of about 220-230 K at a value of 25 holes/unit cell (approximately the maximum that can be materials-engineered into a high-T(sub c) K2MnF4 or triple Perovskite structure). In the down-temperature direction, the extrapolation gives a T(sub c) in the vicinity of 235 K for the Y-Ba-Cu-O system as well as the known maximum temperature of 23 K for low-T(sub c) materials shown by Nb3Ge. The approach is also consistent with the experimental findings that only multivalence ions which are diamagnetic in their atomic state (Cu, Tl, Bi, Pb, and Sb) associate with high-T(sub c) compounds
NMR evidence of strong-correlated superconductivity in LiFeAs: tuning toward an SDW ordering
In this letter, we reported the results of NMR study on LiFeAs single
crystals. We find a strong evidence of the low temperature spin fluctuations;
by changing sample preparation conditions, the system can be tuned toward an
spin-density-wave (SDW) quantum-critical point. The detection of an
interstitial Li(2) ion, possibly locating in the tetrahedral hole, suggests
that the off-stoichiometry and/or lattice defect can probably account for the
absence of the SDW ordering in LiFeAs. These facts show that LiFeAs is a
strongly correlated system and the superconductivity is likely originated from
the SDW fluctuations.Comment: 5 pages, s figure
The GTC exoplanet transit spectroscopy survey. VII. Detection of sodium in WASP-52b's cloudy atmosphere
We report the first detection of sodium absorption in the atmosphere of the
hot Jupiter WASP-52b. We observed one transit of WASP-52b with the
low-resolution Optical System for Imaging and low-Intermediate-Resolution
Integrated Spectroscopy (OSIRIS) at the 10.4 m Gran Telescopio Canarias (GTC).
The resulting transmission spectrum, covering the wavelength range from 522 nm
to 903 nm, is flat and featureless, except for the significant narrow
absorption signature at the sodium doublet, which can be explained by an
atmosphere in solar composition with clouds at 1 mbar. A cloud-free atmosphere
is stringently ruled out. By assessing the absorption depths of sodium in
various bin widths, we find that temperature increases towards lower
atmospheric pressure levels, with a positive temperature gradient of 0.88 +/-
0.65 K/km, possibly indicative of upper atmospheric heating and a temperature
inversion.Comment: 6 pages, 5 figures, accepted for publication in A&A Lette
Numerical simulation of liquid sloshing in a partially filled container with inclusion of compressibility effects
A numerical scheme of study is developed to model compressible two-fluid flows simulating liquid sloshing in a partially filled tank. For a two-fluid system separated by an interface as in the case of sloshing, not only a Mach-uniform scheme is required, but also an effective way to eliminate unphysical numerical oscillations near the interface. By introducing a preconditioner, the governing equations expressed in terms of primitive variables are solved for both fluids (i.e. water, air, gas etc.) in a unified manner. In order to keep the interface sharp and to eliminate unphysical numerical oscillations in unsteady fluid flows, the non-conservative implicit Split Coefficient Matrix Method (SCMM) is modified to construct a flux difference splitting scheme in the dual time formulation. The proposed numerical model is evaluated by comparisons between numerical results and measured data for sloshing in an 80% filled rectangular tank excited at resonance frequency. Through similar comparisons, the investigation is further extended by examining sloshing flows excited by forced sway motions in two different rectangular tanks with 20% and 83% filling ratios. These examples demonstrate that the proposed method is suitable to capture induced free surface waves and to evaluate sloshing pressure loads acting on the tank walls and ceiling
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