909 research outputs found
Low-energy, planar magnetic defects in BaFe2As2: nanotwins, twins, antiphase and domain boundaries
In BaFe2As2, structural and magnetic planar defects begin to proliferate
below the structural phase transition, affecting descriptions of magnetism and
superconductivity. We study using density-functional theory the stability and
magnetic properties of competing antiphase and domain boundaries, twins and
isolated twins (twin nuclei) - spin excitations proposed and/or observed.
These nanoscale defects have very low surface energy (-~Jm),
with twins favorable to the mesoscale. Defects exhibit smaller moments confined
near their boundaries -- making a uniform-moment picture inappropriate for
long-range magnetic order in real samples. {\it{Nano}}twins explain features in
measured pair distribution functions, so should be considered when analyzing
scattering data. All these defects can be weakly mobile and/or have
fluctuations that lower assessed "ordered" moments from longer spatial and/or
time averaging, and should be considered directly.Comment: 6 pages, 6 figures, 1 tabl
Renal and visceral protection in thoracoabdominal aortic surgery
ObjectivesOpen thoracoabdominal aortic aneurysm (TAAA) repair traditionally carries substantial perioperative morbidity and mortality, primarily from distal aortic ischemia. Advances in surgical techniques, adjuncts, and strategies have greatly improved outcomes.MethodsWe analyzed outcomes of 1267 open consecutive TAAA repairs between January 2005 and September 2013. We provided cold crystalloid renal perfusion whenever the renal ostia were accessible; according to extent of repair, we selectively used left heart bypass and provided isothermic blood to the celiac axis and superior mesenteric artery. Repair was extensive (Crawford extent I and II) in 717 cases (57%). Left heart bypass was used in 645 (51%) cases, cold crystalloid renal perfusion in 987 (78%), and isothermic visceral perfusion in 318 (25%). Additional patient-specific surgical adjuncts included endarterectomy of renal or visceral vessels, open stent placement within these vessels, or use of both techniques; at least one was used in 447 repairs (35%).ResultsThirty-day survival was 95% (1198/1267); overall operative mortality was 8% (104/1267). Acute renal dysfunction occurred in 155 (12%), renal failure requiring hemodialysis at hospital discharge in 84 (7%), and bowel ischemia in 9 (<1%). Extent II and III TAAA repairs carried the highest risks of postoperative renal dysfunction and renal failure requiring hemodialysis at hospital discharge.ConclusionsContemporary protective strategies allow open TAAA repair with substantially fewer renal and visceral ischemic complications. Although bowel ischemia is uncommon, renal failure remains a concern, especially in extent II and extent III TAAA repairs. Additional studies are needed to identify and improve renal protection strategies
Lattice thermal conductivity of disordered binary alloys : a formulation
We present here a formulation for the calculation of the configuration
averaged lattice thermal conductivity in random alloys. Our formulation is
based on the augmented-space theorem, introduced by one of us, combined with a
generalized diagrammatic technique. The diagrammatic approach simplifies the
problem of including effects of disorder corrections to a great extent. The
approach allows us to obtain an expression for the effective heat current in
case of disordered alloys, which in turn is used in a Kubo-Greenwood type
formula for the thermal conductivity. We show that disorder scattering
renormalizes the phonon propagators as well as the heat currents. The
corrections to the current terms have been shown to be related to the
self-energy of the propagators. We also study the effect of vertex corrections
in a simplified ladder diagram approximation. A mode dependent diffusivity
and then a total thermal diffusivity averaged over different modes
are defined. Schemes for implementing the said formalism are discussed. A few
initial numerical results on the frequency and temperature dependence of
lattice thermal conductivity are presented for NiPd alloy and are also compared
with experiment. We also display numerical results on the frequency dependence
of thermal diffusivity averaged over modes.Comment: 16 pages, 17 figure
Inelastic neutron scattering in random binary alloys : an augmented space approach
Combining the augmented space representation for phonons with a generalized
version of Yonezawa-Matsubara diagrammatic technique, we have set up a
formalism to seperate the coherent and incoherent part of the total intensity
of thermal neutron scattering from disordered alloys. This is done exacly
without taking any recourse to mean-field like approximation (as done
previously). The formalism includes disorder in masses, force constants and
scattering lengths. Implementation of the formalism to realistic situations is
performed by an augmented space Block recursion which calculates entire Green
matrix and self energy matrix which in turn is needed to evaluate the coherent
and incoherent intensities. we apply the formalism to NiPd and NiPt alloys.
Numerical results on coherent and incoherent scattering cross sections are
presented along the highest symmetry directions. Finally the incoherent
intensities are compared with the CPA and also with experiments.Comment: 18 pages, 13 figure
Energy Conservation Techniques to Mitigate the Power Shortage Problem in Pakistan (Case Studies)
The main objective of this research paper is to show benefits of different energy conservation techniques. As a first case study, I performed analysis on University of Gujrat, electrical power system. This case study involves analysis of motors and tube lights installed at the pumping stations and in Engineering Block of UOG respectively, with the help of energy analyzer before and after the installation of required rating capacitors. Power system analysis also done which includes power distribution system losses for example line losses and copper losses of different rating transformers of UOG. Cost and payback period calculation had been done. Second case study is performed on 11 KV Ali Park and Rachna Town feeders to show fruitful results obtained by implementing rehabilitation techniques on the above said feeders. The results showed by adopting energy conservation techniques not only energy is conserved, it also brings other benefits
Integration of Renewable Energy Resources in Microgrid
Microgrid is a new concept in power generation. The Microgrid concept assumes a cluster of loads
and micro sources operating as a single controllable system that provides both power and heat to
its local area. Not much is known about Microgrid behavior as a whole system. Some models exist
which describe the components of a Microgrid. In this paper, model of Microgrids with steady state
and their transient responses to changing inputs are presented. Current models of a fuel cell, microturbines, wind turbine and solar cell have been discussed. Finally a complete model built of
Microgrid including the power sources, their power electronics, and a load and mains model in
MATLAB/Simulink is presented
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