2,326 research outputs found
Fuzzy Logic Controlled Microturbine Generation System for Distributed Generation
AbstractThe microturbine based Distributed Generation (DG) system are becoming the popular source of power industries due to their fuel flexibility, reliability and power quality. The microturbine generation (MTG) system is a complicated thermodynamic electromechanical system with a high speed of rotation, frequency conversion and its control strategy. In spite of several techniques to control high speed of microturbine is not accurate and reliable due to their anti-interference problem. To resolve the anti-interfacing problem, this paper investigates the fuzzy logic based speed governor for a MTG system as an alternative to nominal PI or lead-lag based controller. The development of fuzzy logic based speed governor includes input and output membership function with their respective members. The load variation on MTG system is performed using conventional and fuzzy logic controller, implemented in Matlab/simulink and results are compared with each other. The simulation result shows that, the performance improvement of fuzzy logic governor over a nominal governor based MTG system
A treatise on first-principles studies of ZnO as diluted magnetic semiconductor
Diluted magnetic semiconductors (DMS) are important functional
materials that bring together two fundamental aspects of electron,
namely the charge and the spin, to derive new properties in
materials. Based on mean-field models it was shown that wide
band gap diluted semiconductors (with ZnO and GaN) could lead
to room temperature ferromagnetism. This triggered much interest
in the subject. However, these results are still under intensive
debate.
The scope of the present thesis is to study the properties of
ZnO doped with transition metal elements by the density functional
theory (DFT). The DFT is an accurate theory for modeling material
properties, which describe the fundamental interactions of electrons
with the nucleus and is in principle free from any parameterization.
In this thesis, there is extensive study of the electronic structure
of ZnO beyond the generalized gradient approximation (GGA) as
exchange-correlation functional (Exc), which is well
known to be associated with some drawback in predicting the
semiconducting properties. One of the method adopted for improvement
is by adding extra correlation-energy to GGA (GGA+U) in line with
the Hubbard model. We find that with adding U on Zn d-orbitals, the band gap improves with simultaneous shift of the Zn d-bands to lower energies. The band gap nevertheless is still far below the
experimental value. Another approach to correct for the Exc is by the treatment of screened hybrid-functionals proposed by Heyd-Scuseria-Ernzerhof (HSE), where the contribution to the exchange energy is through the blend of some percentage of screened Hartree-Fock exchange and rest from the GGA exchange. The correlation energy is derived from GGA. This method also has limitations in describing the electronic structure of ZnO. A simple proposition of mixing the GGA+U and the hybrid-functional treatment may be a solution, which has been investigated in this work. The band structure of ZnO has been compared
for various level of theory for Exc, namely, LDA, GGA,
GGA+U, and HSE functionals with different screening lengths (Ό).
A proposal for HSE+U functionals has been been put forward and
supported.
Using some of the treatments for Exc as stated above,
the magnetic properties of TM (Cr, Mn, Fe, Co, and Ni) doped ZnO are
studied. Firstly, the electronic structure calculations for TM doped
ZnO are done with the Korringa-Kohn-Rostoker (KKR) method with the
LDA and using the coherent potential approximation (CPA). The exchange
integrals are then calculated using the Liechtenstein's formalism,
which are then used in the Monte Carlo simulations to estimate the
critical temperature. A case study for Co doped ZnO is discussed,
where we have calculated the magnetic phase diagram of the system.
Apart from this, supercell calculations with Vienna ab-initio
simulation package (VASP) have been done to compare the role of
various treatment to the Exc. In case of GGA+U,
the U is separately treated on Zn d-orbitals and TM d-orbitals, and then together. From the total energy differences in magnetic states, it is observed that with incorporation of U, all the TM favor antiferromagnetic interactions. This may partially explain why some
experimental studies on TM doped ZnO do not show any ferromagnetic
correlation. Similar conclusion is also reached for most cases in HSE
and HSE+U approach.
One of the recent topics under discussion for polar semiconductor
materials like ZnO is d0 magnetism. There are several reports in experiments on magnetism due to intrinsic-defect and 2p-block elements doped extrinsic ZnO, the source of magnetism is due to
localization of holes and is a topic under active discussion. In present study based on GGA it is shown that Zn-vacancy (VZn) and C substitution on O site (CO) lead to spin-polarized solution. The magnetic energy is mostly below the room-temperature. Related to this, some perspectives of experimental situations which could lead to quenching of such
magnetization is also presented.
Furthermore, studies on pure ZnO clusters based on GGA and HSE
functionals are presented. A general tendency which is observed is
that the bond length predicted in HSE calculations are larger than
those predicted by GGA. Characteristic arrangement of magnetization
density in VZn and CO in clusters as
calculated from GGA is presented.
A direct comparison of the present results with experiments is
difficult because there is wide variety of experimental results
for TM doped ZnO which are mostly dependent on sample preparation
techniques and for the case of d0-magnetism there are very few element specific experimental characterization. However, the
studies presented in the thesis has up-to-date theoretical footing
and this is the strength of the work.VerdĂŒnnte magnetische Halbleiter (DMS) sind bedeutende funktionale
Materialien, die die zwei fundamentalen Aspekte des Elektrons
nÀmlich Ladung und Spin zusammen nutzen, um neue Materialeigenschaften
zu generieren. Im Rahmen von Mean-field Modellen wurde gezeigt,
dass fĂŒr verdĂŒnnte Halbleiter mit groĂer BandlĂŒcke (die auf ZnO und GaN basieren) Ferromagnetismus bei Raumtemperatur möglich ist.
Dies hat zu einem groĂen Interesse an diesen Systemen gefĂŒhrt. Allerdings sind diese Ergebnisse immer noch Gegenstand intensiver Diskussion. Das Ziel der vorliegenden Dissertation ist die Untersuchung von ZnO, das mit Ăbergangsmetallen dotiert wurde im Rahmen der Dichtefunktionaltheorie (DFT). Die DFT ist eine genaue, im Prinzip parameterfreie Methode
zur Modellierung von Materialeigenschaften, die die fundamentalen
Wechselwirkungen des Elektronensystems mit den Kernen beschreibt.
In dieser Arbeit werden intensive Studien von ZnO durchgefĂŒhrt,
die ĂŒber die verallgemeinerte Gradientenkorrektur (GGA) fĂŒr das Austausch-Korrelationsfunktional (Exc) hinausgehen, da die GGA den halbleitenden Charakter nicht hinreichend beschreiben kann. Eine Methode besteht darin der GGA im Rahmen des Hubbard-Modells eine extra Korrelationsenergie zuzufĂŒgen (GGA+U). Wir finden, dass U angewendet auf die Zn d-Orbitale zu einer besseren Beschreibung der BandlĂŒcke fĂŒhrt und gleichzeitig die Zn d-BĂ€nder zu tieferen Energien verschoben werden. Trotzdem ist die BandĂŒcke nach wie vor deutlich kleiner als der experimentelle Wert. Eine andere NĂ€herung, die verwendet wird, um Exc zu korrigieren, ist das von Heyd-Scuseria-Ernzerhof (HSE) vorgeschlagene abgeschirmte (screened) Hybridfunktional. Dabei wird die Austauschenergie anteilig aus screened Hatree-Fock- und GGA-Austauschenergie zusammengesetzt. Die Korrelationsernergie wird mittels GGA bestimmt. Auch diese Methode hat ihre Grenzen hinlĂ€nglich der Beschreibung der elektronischen Struktur von ZnO. Eine Lösung scheint die Kombination von GGA+U und Hybridfunktional zu sein, was im Rahmen dieser Arbeit untersucht wurde. Es wurden die Bandstrukturen von ZnO verglichen, die
sich im Rahmen verschiedener NĂ€herungen fĂŒr Exc
nÀmlich LDA, GGA, GGA+U und HSE mit unterschiedlichen
AbschirmlĂ€ngen (ÎŒ) ergaben. Die Untersuchungen lassen die Schlussfolgerung zu, dass die Verwendung von HSE+U die beste Beschreibung liefert.
Unter Verwendung der oben genannten NĂ€herungen fĂŒr Exc
wurden die magnetischen Eigenschaften von TM (Cr, Mn, Fe, Co und Ni)
dotiertem ZnO untersucht. Zuerst wurde die elektronische Struktur des
TM dotierten ZnO im Rahmen der Korringa-Kohn-Rostoker (KKR)-Methode
unter Verwendung der LDA und der Coherent Potential Approximation (CPA) bestimmt. Zur Berechnung der Austauschintegrale wurde der Formalismus von Liechtenstein verwendet. In anschlieĂenden Monte Carlo Simulationen wurden die damit bestimmten Austauschintegrale zur AbschĂ€tzung der kritischen Temperatur benutzt. In einer Fallstudie
wurde fĂŒr Co dotiertes ZnO die Berechnung des magnetischen
Phasendiagramms durchgefĂŒhrt.
Weiterhin wurden Superzellenrechnungen unter Verwendung des
Vienna Ab Initio Simulation Package (VASP) gemacht, um den Einfluss
verschiedener Behandlungen von Exc zu untersuchen. Im Fall der
GGA+U Approximation wurde U separat auf die d-Orbitale von Zn und TM angewendet sowie auch auf beide d-Orbitale gleichzeitig. Aus den Differenzen der Gesamtenergien folgt, dass in Anwesenheit von U alle TM Atome paramagnetisch sind. Dies könnte
teilweise erklÀren, warum in experientellen Untersuchungen TM dotiertes
ZnO keine ferromagnetischen Korrelationen gefunden werden. Ăhnliche SchluĂ folgerungen ergeben sich in den meisten FĂ€llen auch im Rahmen
der HSE und HSE+U NĂ€herung.
Ein aktuelles Thema bezĂŒglich polarer Halbleiter wie ZnO ist der
d0-Magnetismus. Obwohl bereits einige experimentelle Arbeiten fĂŒr ZnO vorliegen, die Magnetismus aufgrund von intrinsischen Defekten und extrinsischen Defekten aufgrund von Dotierung mit Hauptgruppenelementen diskutieren, ist der Ursprung des Magnetismus
nicht geklÀrt und immer noch Gegenstand aktueller Forschung. In der
vorliegenden auf der GGA basierenden Arbeit wird gezeigt, dass
fĂŒr Zn-Leerstellen (VZn) und C auf dem Sauerstoffplatz
(CO) eine endliche Spinpolarisation auftritt. The magnetische
Energie liegt in den meisten FĂ€llen unterhalb der Raumtemperatur.
In diesem Zusammenhang wird auch diskutiert welche Bedingungen im
Experiment zu einer Auslöschung der magnetischen Moments
fĂŒhren können.
Weiterhin wurden ZnO Cluster unter Verwendung von GGA und HSE Fuktionalen
untersucht. Dabei zeigt sich, dass die in HSE-Rechnungen vorhergesagte
BindungslĂ€nge gröĂer ist, als die mit GGA bestimmte BindungslĂ€nge.
Im Fall von GGA wurde auch die Magnetisierungsdichte aufgrund von
VZn und CO untersucht.
Ein direkter Vergleich der Ergebnisse mit experimentellen Daten ist
schwierig, da zwar eine Vielzahl von Ergebnissen fĂŒr TM dotiertes ZnO vorliegen, diese aber meist von der ProbenprĂ€paration abhĂ€ngen und fĂŒr den Fall von d0-Magnetismus nur wenige elementspezifische Daten verfĂŒgbar sind. Die StĂ€rke dieser Arbeit liegt daher in der Auffindung und Diskussion einer geeigneten theoretischen Beschreibung fĂŒr magnetische Halbleiter wie ZnO
Anisotropic ferromagnetism in carbon doped zinc oxide from first-principles studies
A density functional theory study of substitutional carbon impurities in ZnO
has been performed, using both the generalized gradient approximation (GGA) and
a hybrid functional (HSE06) as exchange-correlation functional. It is found
that the non-spinpolarized C impurity is under almost all
conditions thermodynamically more stable than the C impurity which
has a magnetic moment of , with the exception of very O-poor
and C-rich conditions. This explains the experimental difficulties in sample
preparation in order to realize -ferromagnetism in C-doped ZnO. From GGA
calculations with large 96-atom supercells, we conclude that two
C-C impurities in ZnO interact ferromagnetically, but
the interaction is found to be short-ranged and anisotropic, much stronger
within the hexagonal -plane of wurtzite ZnO than along the c-axis. This
layered ferromagnetism is attributed to the anisotropy of the dispersion of
carbon impurity bands near the Fermi level for C impurities in
ZnO. From the calculated results, we derive that a C
concentration between 2% and 6% should be optimal to achieve
-ferromagnetism in C-doped ZnO.Comment: 9 pages, 7 figure
First Principles Modeling of Topological Insulators: Structural Optimization and Exchange Correlation Functionals
Topological insulators (TIs) are materials that are insulating in the bulk
but have zero band gap surface states with linear dispersion and are protected
by time reversal symmetry. These unique characteristics could pave the way for
many promising applications that include spintronic devices and quantum
computations. It is important to understand and theoretically describe TIs as
accurately as possible in order to predict properties. Quantum mechanical
approaches, specifically first principles density functional theory (DFT) based
methods, have been used extensively to model electronic properties of TIs.
Here, we provide a comprehensive assessment of a variety of DFT formalisms and
how these capture the electronic structure of TIs. We concentrate on
BiSe and BiTe as examples of prototypical TI materials. We find
that the generalized gradient (GGA) and kinetic density functional (metaGGA)
produce displacements increasing the thickness of the TI slab, whereas we see
an opposite behavior in DFT computations using LDA. Accounting for van der
Waals (vdW) interactions overcomes the apparent over-relaxations and retraces
the atomic positions towards the bulk. Based on an intensive computational
study, we show that GGA with vdW treatment is the most appropriate method for
structural optimization. Electronic structures derived from GGA or metaGGA
employing experimental lattice parameters are also acceptable. In this regard,
we express a slight preference for metaGGA in terms of accuracy, but an overall
preference for GGA due to compensatory improvements in computability in
capturing TI behavior
Inducing Quantum Phase Transitions in Non-Topological Insulators Via Atomic Control of Sub-Structural Elements
Topological insulators (TIs) are an important family of quantum materials
that exhibit a Dirac point (DP) in the surface band structure but have a finite
band gap in bulk. A large degree of spin-orbit interaction and low bandgap is a
prerequisite for stabilizing DPs on selective atomically flat cleavage planes.
Tuning of the DP in these materials has been suggested via modifications to the
atomic structure of the entire system. Using the example of AsTe and
ZnTe, which are not TIs, we show that a quantum phase transition can be
induced in atomically flat and stepped surfaces, for AsTe and ZrTe,
respectively. This is achieved by establishing a framework for controlling
electronic properties that is focused on local perturbations at key locations
that we call sub-structural elements (SSEs). We exemplify this framework
through a novel method of isovalent sublayer anion doping and biaxial strain.Comment: 13 pages, 3 figures, 1 tabl
Comparison of the Effectiveness of Three Methods of Recanalization in a Model of the Middle Cerebral Artery: Thrombus Aspiration via a 4F Catheter, Thrombus Aspiration via the GP Thromboaspiration Device, and Mechanical Thrombectomy Using the Solitaire Thrombectomy Device
Introduction. This paper compares different approaches to recanalization in a model of the middle cerebral artery (MCA). Methods. An occlusive thrombus (lamb's blood) was introduced into the MCA of a model of the cerebral circulation perfused with Hartmann's solution (80 pulsations/min, mean pressure 90âmmâHg). Three methods of clot retrieval were tested: thrombus aspiration via a 4F catheter (n = 26), thrombus aspiration via the GP thrombus aspiration device (GPTAD) (n = 30), and mechanical thrombectomy via the Solitaire Device (n = 30). Results. Recanalization rate was similar for all 3 approaches (62%, 77%, and 85%). Time to recanalization was faster with aspiration devices (41 SD 42âs for 4F and 61 SD 21âs for GPTAD) than with the Solitaire (197 SD 64âs P < .05 Kruksal-Wallis). Clot fragmentation was the same in the Solitaire (23%) and the GPTAD (23%), but higher with the 4F (53%, P < .05). Conclusion. In this model, thrombus aspiration was faster than mechanical thrombectomy, and similarly effective at recanalization. These results should be confirmed in vivo
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