867 research outputs found
Effect of distributed energy systems on the electricity grid
A feasibility study is being carried out at Ecotricity into a distributed
energy storage system comprising Energy stores (batteries) placed at
consumer level (in customer’s homes). The aim is to flatten consumer
demand and make better use of home-based generation. The Study
Group considered the mechanism of connecting batteries to the local
distribution system, the ability to meet engineering requirements for the
standard of the connection, and the potential impact of large numbers of
such connections on stability of the local distribution network. Network
and (DC-AC) invertor models were used to examine network connection
transients. A statistical model was proposed to estimate the distribution
of key electrical parameters to determine the likelihood of engineering
standards being exceeded. The Study Group also considered stochastic
methods of modelling wind speed, to better understand the requirements
for battery energy storage as a complement to wind power
Strong Pinning Enhancement in MgB2 Using Very Small Dy2O3 Additions
0.5 to 5.0 wt.% Dy2O3 was in-situ reacted with Mg + B to form pinned MgB2.
While Tc remained largely unchanged, Jc was strongly enhanced. The best sample
(only 0.5 wt.% Dy2O3) had a Jc of 6.5 x 10^5 A/cm^2 at 6K, 1T and 3.5 x 10^5
A/cm^2 at 20K, 1T, around a factor of 4 higher compared to the pure sample, and
equivalent to hot-pressed or nano-Si added MgB2 at below 1T. Even distributions
of nano-scale precipitates of DyB4 and MgO were observed within the grains. The
room temperature resistivity decreased with Dy2O3 indicative of improved grain
connectivity.Comment: 13 pages, 4 figures and 1 tabl
Enhanced critical current densities in MgB2 by mixing relatively impure boron powders.
Polycrystalline MgB2 samples were prepared from 99.98% purity Mg powder and different mixtures of relatively impure boron (99% pure crystalline boron and 95–97% amorphous boron) precursor powders. At both 6 and 20 K, for the mixed boron samples a doubling in Jc was observed over the highest values for single precursor samples. It is shown that the enhanced Jc results from the mixing effect of using different reaction rates of the different boron precursor powders. The work represents a cost-effective means of significantly improving current carrying Performance in MgB2 conductors
Improved Current Densities in MgB2 By Liquid-Assisted Sintering
Polycrystalline MgB2 samples with GaN additions were prepared by reaction of
Mg, B, and GaN powders. The presence of Ga leads to a low melting eutectic
phase which allowed liquid phase sintering and produces plate-like grains. For
low-level GaN additions (5% at. % or less), the critical transition
temperature, Tc, remained unchanged and in 1T magnetic field, the critical
current density, Jc was enhanced by a factor of 2 and 10, for temperatures of
\~5K and 20K, respectively. The values obtained are approaching those of hot
isostatically pressed samples.Comment: 12 pages, 1 table, 4 figures, accepted in Applied Physics Letter
The critical current density of polycrystalline MgB2 prepared by using boron mixture
In this study, boron powders with varying purity and form were mixed in different proportion to serve as precursors for reaction with Mg forming polycrystalline MgB2 bulks. The inductively measured superconducting transition temperature, Tc and the critical current density, J c were compared to that of samples prepared from the respective single boron. Overall, Tc remains largely unchanged for all samples. It was found that Jc at 6 K and 20 K did not degrade significantly up to 4.6 T as a result of adding impure boron as much as 10 wt.% indicating comparable Jc can be obtained without dependence of use of expensive high purity boron powder alone. The systematic decrease of Jc with increasing impure boron additions shows that a compromise between desired and cost reduction can be made by varying the boron powder proportion. Finally, samples prepared from the mixture of both impure crystalline and amorphous borons even show enhanced Jc up to 3 T at 20 K. The increase in Jc correlates with the retention of strain level in these samples probably resulted from the more similar reaction rate of the respective borons
Thin-Film Trilayer Manganate Junctions
Spin-dependent conductance across a manganate-barrier-manganate junction has
recently been demonstrated. The junction is a LaSrMnO%
-SrTiO-La SrMnO trilayer device supporting
current-perpendicular transport. Large magnetoresistance of up to a factor of
five change was observed in these junctions at 4.2K in a relatively low field
of the order of 100 Oe. Temperature and bias dependent studies revealed a
complex junction interface structure whose materials physics has yet to be
understood.Comment: 20 pages, 14 figures. To appear in Phil. Trans. R. Soc. Lond. A
vol.356 (1998
Fatalities due to intestinal obstruction following the ingestion of foreign bodies
Two fatalities due to an occlusive ileus following the ingestion of foreign bodies in patients with psychiatric disorders are described. A severely mentally handicapped young man developed a temperature and died 1 h after admission to a surgical ward. At autopsy, not, vert, similar 2000 cm3 of foreign material, including broken glass and porcelain, branches, buttons, parts of clothing and other material were found in the gastrointestinal tract, leading to a complete obstruction of the distal intestine and colon with resulting faecal vomiting. The other case was even more unusual as a hair fetishist had swallowed a thick strand of his own hair, 50 cm long, also resulting in mechanical obstruction of the distal intestine
Angular dependent vortex pinning mechanisms in YBCO coated conductors and thin films
We present a comparative study of the angular dependent critical current
density in YBa2Cu3O7 films deposited on IBAD MgO and on single crystal MgO and
SrTiO3 substrates. We identify three angular regimes where pinning is dominated
by different types of correlated and uncorrelated defects. We show that those
regimes are present in all cases, indicating that the pinning mechanisms are
the same, but their extension and characteristics are sample dependent,
reflecting the quantitative differences in texture and defect density. In
particular, the more defective nature of the films on IBAD turns into an
advantage as it results in stronger vortex pinning, demonstrating that the
critical current density of the films on single crystals is not an upper limit
for the performance of the IBAD coated conductors.Comment: 14 pages, 3 figures. Submitted to AP
Normal state properties of high angle grain boundaries in (Y,Ca)Ba2Cu3O7-delta
By lithographically fabricating an optimised Wheatstone bridge geometry, we
have been able to make accurate measurements of the resistance of grain
boundaries in Y1-xCaxBa2Cu3O7-d between the superconducting transition
temperature, Tc, and room temperature. Below Tc the normal state properties
were assessed by applying sufficiently high currents. The behaviour of the
grain boundary resistance versus temperature and of the conductance versus
voltage are discussed in the framework charge transport through a tunnel
barrier. The influence of misorientation angle, oxygen content, and calcium
doping on the normal state properties is related to changes of the height and
shape of the grain boundary potential barrier.Comment: 17 pages, 1 table, 5 figures, submitted to PR
Characteristics of shock-induced boundary layer separation on nacelles under windmilling diversion conditions
The boundary layer on the external cowl of an aero-engine nacelle under windmilling diversion conditions is subjected to a notable adverse pressure gradient due to the interaction with a near-normal shock wave. Within the context of Computational Fluid Dynamics (CFD) methods, the correct representation of the characteristics of the boundary layer is a major challenge to capture the onset of the separation. This is important for the aerodynamic design of the nacelle as it may assist in the characterization of candidate designs. This work uses experimental data obtained from a quasi-2D rig configuration to provide an assessment of the CFD methods typically used within an industrial context. A range of operating conditions is investigated to assess the sensitivity of the boundary layer to changes in inlet Mach number and mass flow through a notional windmilling engine. Fully turbulent and transitional boundary layer computations are used to determine the characteristics of the boundary layer and the interaction with the shock on the nacelle cowl. The correlation between the onset of shock induced boundary layer separation and pre-shock Mach number is assessed and the boundary layer integral characteristics ahead of the shock and the post-shock recovery evaluated and quantified. Overall, it was found that the CFD is able to discern the onset of boundary layer separation for a nacelle under windmilling conditions
- …