219 research outputs found
Self-doping processes between planes and chains in the metal-to-superconductor transition of YBa2Cu3O6.9
The interplay between the quasi 1-dimensional CuO-chains and the
2-dimensional CuO2 planes of YBa2Cu3O6+x (YBCO) has been in focus for a long
time. Although the CuO-chains are known to be important as charge reservoirs
that enable superconductivity for a range of oxygen doping levels in YBCO, the
understanding of the dynamics of its temperature-driven metal-superconductor
transition (MST) remains a challenge. We present a combined study using x-ray
absorption spectroscopy and resonant inelastic x-ray scattering (RIXS)
revealing how a reconstruction of the apical O(4)-derived interplanar orbitals
during the MST of optimally doped YBCO leads to substantial hole-transfer from
the chains into the planes, i.e. self-doping. Our ionic model calculations show
that localized divalent charge-transfer configurations are expected to be
abundant in the chains of YBCO. While these indeed appear in the RIXS spectra
from YBCO in the normal, metallic, state, they are largely suppressed in the
superconducting state and, instead, signatures of Cu trivalent charge-transfer
configurations in the planes become enhanced. In the quest for understanding
the fundamental mechanism for high-Tc-superconductivity (HTSC) in perovskite
cuprate materials, the observation of such an interplanar self-doping process
in YBCO opens a unique novel channel for studying the dynamics of HTSC.Comment: 9 pages, 4 Figure
Photoluminescence in SPS-processed transparent Ce:YAG ceramics
Ceramic phosphors display great promise for the realization of high-power lighting devices. Cerium-doped yttrium aluminum garnet (Ce:YAG) is commonly used as a phosphor in white light emitting diodes. Therefore, it was chosen as a case study to investigate photoluminescence of transparent ceramic phosphors fabricated by spark plasma sintering (SPS). In the present work, 0.5 at.% Ce:YAG nano-powder was synthesized by a co-precipitation method and subsequently consolidated by SPS into highly transparent ceramic samples. The effect of varying sintering parameters (temperature and pressure) and post-sintering treatments (hot isostatic pressing and air atmosphere thermal treatment) on optical properties was investigated. Correlations between in-line transmittance, photoluminescence (PL) and residual porosity characteristics (pore size and volume fraction) were established. It was also found that PL emission intensity and external quantum efficiency were significantly affected by intentionally created surface roughness.
Please click Additional Files below to see the full abstract
Microstructure evolution during high-pressure spark plasma sintering (HPSPS) of transparent alumina
Applying high-pressure during spark plasma sintering (so-called HPSPS) enables rapid densification of a ceramic powder at relatively lower temperatures, limiting grain growth and allowing fabrication of fine-grained highly transparent ceramics. The present work focuses on the fabrication of fine submicron transparent alumina from untreated commercial powder by HPSPS and the microstructure evolution during consolidation under high applied pressure. The sintering was conducted at relatively low temperatures (1000-1100°C) under high pressure varied from 250 to 800 MPa. We review unique sintering phenomena such as stress-enhanced grain growth and de-sintering, which are related to creep taking place during the final stage of pressure-assisted densification. In addition, optical and mechanical properties of obtained samples are discussed.
Please click Additional Files below to see the full abstract
Effect of Mn doping on densification and properties of transparent alumina by high- pressure SPS (HPSPS)
Please click Additional Files below to see the full abstract
ABOUT THE INELUENCE ОЕ WORKELOW AND OPERATIONAL MODE PARAMETERS ON THE ENERGY EEEICIENCY OE A SYSTEM OE SERIES CONNECTED ULTRALOW POWER DRIVES
Utilization of the excess pressure energy of natural gas in the system of series-connected turbo expanders is promising from the point of view of energy saving in the gas transportation system. Theoretical work on the optimization of the regime and geometric parameters of such systems in the early stages of design is absent. In this paper, parametric studies were carried out on the effect of the project's initial data on the energy efficiency of a system of two turbo-generators with a power of up to 50 kW. Rational ranges of thermodynamic and regime ratios were found, which allows to significantly increase the design efficiency of the system of axial small-sized turbo-generators in comparison with the case of its design without taking into account the obtained information
Correlation between Thermal and Electrical Properties of Spark Plasma Sintered (SPS) Porous Copper
Evidence for spin-triplet superconducting correlations in metal-oxide heterostructures with non-collinear magnetization
Heterostructures composed of ferromagnetic La0.7Sr0.3MnO3, ferromagnetic
SrRuO3, and superconducting YBa2Cu3Ox were studied experimentally. Structures
of composition Au/La0.7Sr0.3MnO3/SrRuO3/YBa2Cu3Ox were prepared by pulsed laser
deposition, and their high quality was confirmed by X-ray diffraction and
reflectometry. A non-collinear magnetic state of the heterostructures was
revealed by means of SQUID magnetometry and polarized neutron reflectometry. We
have further observed superconducting currents in mesa-structures fabricated by
deposition of a second superconducting Nb layer on top of the heterostructure,
followed by patterning with photolithography and ion-beam etching. Josephson
effects observed in these mesa-structures can be explained by the penetration
of a triplet component of the superconducting order parameter into the magnetic
layers.Comment: 10 pages, 6 figure
TECNIQUE ОЕ DETERMINATION OF NORMALIZING CONVERSION FACTORS ON THE NORMAL PARAMETERS VALUES OFA POWER PLANT
The necessity of determining the normal parameters values of a gas turbine power plant based on the test results in the prevailing external conditions and operational operating modes is justified. A technique for determining the ana¬lytical dependencies of the normalizing conversion factors is developed
- …