Dissertations / Theses on the topic 'Hexagonal'
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Gardiner, Bryan. "Hexagonal image processing." Thesis, University of Ulster, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.535794.
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Kim, Jong-Woo. "Multiferroic hexagonal HoMnO3 films." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-26142.
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The fundamental properties of hexagonal multiferric HoMnO3 films have been thoroughly investigated. The films are grown by pulsed laser deposition on Y:ZrO2(111) substrates. High quality epitaxial HoMnO3 films of 25 { 1000 nm thickness were successfully prepared. The film properties are compared to those of single-crystals. The magnetization measurements revealed that the films show a deviating magnetic behavior from the single-crystals in several ways. For instance, the films have a weakened antiferromagnetic Ho3+ order confirmed from magnetic susceptibility. The difierences are likely to be related to the modified (mostly larger) lattice parameters of films. An approximate phase diagram in comparison with the single-crystal's one is constructed. For multiferroicity investigations, Second Harmonic Generation (SHG; in collaboration with the group of M. Fiebig) has been employed. By SHG, the ferroelectric polar order of the films is obviously confirmed. The ferroelectric switching at room temperature could be clearly demonstrated, whereas leakage of films requires generally a more sophisticated approachDie fundamentalen Eigenschaften von hexagonalen multiferroischen HoMnO3 Schichten werden eingehend untersucht. Die dünnen Schichten wurden mittels gepulster Laserdeposition auf Y:ZrO2(111)-Substraten gewachsen. Hochwertige epitaktische HoMnO3-Dünnschichten von 25 { 1000 nm Dicke wurden erfolgreich hergestellt. Die Dünnschichteigenschaften werden mit denen von Einkristallen verglichen. Die Magnitisierungsmessungen ergeben, dass die dünnen Schichten ein von den Einkristallen in verschiedener Weise abweichendes magnetischen Verhalten zeigen. Zum Beispiel haben die dünnen Schichten eine abgeschwächte antiferromagntetische Ho3+ Ordnung, die durch die magnetische Suszeptibilität bestätigt wird. Die Unterschiede sind wahrscheinlich auf die veränderten (meistens grösseren) Gitterparameter der dünnen Schichten zurückzuführen. Ein Phasendiagramm wird zum Vergleich mit Einkristallen konstruiert. Durch Second Harmonic Generation (SHG; in Zusammenarbeit mit der Gruppe von M. Fiebig) wird die ferroelektrische Ordnung der dünnen Schichten eindeutig bestätigt. Das ferroelektrische Umschalten bei Raumtemperatur kann eindeutig nachgewiesen werden, wobei durch den Leckstrom der dünnen Schichten allgemein eine detailliertere Vorgehensweise benötigt wird
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Kim, Jong-Woo. "Multiferroic hexagonal HoMnO3 films." Doctoral thesis, Göttingen Cuvillier, 2009. http://d-nb.info/99957177X/04.
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Liu, Chen. "Multilevel halftoning over hexagonal grids." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file 0.85 Mb., 47 p, 2006. http://wwwlib.umi.com/dissertations/fullcit/1435813.
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Venancio, Bruno Felipe. "Caminhadas quânticas na rede hexagonal." reponame:Repositório Institucional da UFPR, 2016. http://hdl.handle.net/1884/46351.
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Orientador: Prof. Dr. Marcos Gomes Eleutério da LuzTese (doutorado) - Universidade Federal do Paraná, Setor de Ciências Exatas, Curso de Pós-Graduação em Física. Defesa: Curitiba, 29/11/2016
Inclui referências : f. 158-170
Resumo: Nesta tese abordamos as caminhadas quânticas em tempo discreto no modelo de es-palhamento (CQEs) na rede hexagonal. Primeiro faremos uma revisão das CQEs em uma dimensão, dando um enfoque mais de propagação em rede do que em computação e informação quântica, que e o usual na área. Em seguida apresentaremos um formalismo geral para a implementação das CQEs em redes regulares e daremos o simples exemplo concreto da rede quadrada. Na sequência do trabalho, implementaremos as CQEs para a rede hexagonal e analisaremos suas propriedade no espaço de momentum. Além disso, realizamos uma breve discussão comparando as estruturas de bandas de energia obtidas para uma CQEs, caracterizada pela matriz de Grover, com as do grafeno. De maneira qualitativa, observa-se que esses dois sistemas possuem características comuns. Finalmente, através destes formalismos exploramos a estrutura topolágica da rede hexagonal para obter dez diferentes formulações, com características espaciais distintas. Além disso, exemplificamos a evolução temporal dessas diferentes formulações das CQEs considerando, por exemplo, as matrizes de espalhamento de Grover e da transformada discreta de Fourier. Nosso estudo ilustra que, combinações específicas dessas dez formulações com matrizes de espalhamento adequadas resultam em sistemas com as caracterásticas fenomenoláogicas distintas, que podem ser uma possível forma de controle de processos quântico nas CQEs na rede hexagonal.
Abstract: In this work we discuss the discrete time quantum walks in the scattering formulation (SQWs) for the honeycomb lattice. First we review the quantum walks in one dimension, giving a focus in lattice propagation than in quantum computation and quantum information, which is usual in the area. Then we present a general formalism for the implementation of SQWs in regular lattices and give a simple concrete example of the square lattice. Next, we implement the SQWs for the honeycomb lattice and analyze its properties in momentum space. In addition, we present a brief discussion comparing the energy bands obtained for a SQWs (characterized by Grover matrix) with those of the graphene. Qualitatively, we observe that the two energy structures have common features. Finally, from the proposed formalism we show that it is possible to explore the topological structure of the honeycomb lattice. We obtain ten different formulations, each one with spatial different features. Furthermore, we exemplify the time evolution of these different SQWs versions considering as scattering matrices, the Grover and discrete Fourier transform operators. Our study ilustrate that specific combinations of the ten formulations with adequate scattering matrices result in distinct phenomenological behavior, which therefore may be a possible way to control quantum processes in SQWs along the honeycomb lattice.
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Alharbi, Abdulaziz. "Deformation of hexagonal boron nitride." Thesis, University of Manchester, 2018. https://www.research.manchester.ac.uk/portal/en/theses/deformation-of-hexagonal-boron-nitride(6c6013c4-8c17-4dec-b250-ed3f0baea7ed).html.
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Boron nitride (BN) materials have unique properties, which has led to interest in them in the last few years. The deformation of boron nitride materials including hexagonal boron nitride, boron nitride nanosheets (BNNSs) and boron nitride nanotubes have been studied by Raman spectroscopy. Both mechanical and liquid exfoliations were employed to obtain boron nitride nanostructures. Boron nitride glass composites were synthesised and prepared in thin films to be deformed by bending test in-situ Raman spectroscopy. Hexagonal boron nitride in the form of an individual flake and as flakes dispersed in glass matrices has been deformed and Raman measurement shows its response to strain. The shift rates were, -4.2 cm-1/%, -6.5 cm-1/% for exfoliated h-BN flake with thick and thin regions and -7.0 cm-1/%, -2.8 cm-1/% for the h-BN flakes in the h-BN/ glass (I) and glass (II) composites. Boron nitride nanosheets (BNNSs) shows a G band Raman peak at 1367.5 cm-1, and the deformation process of BNNSs/ glass composites gives a shift rate of -7.65 cm-1/% for G band. Boron nitride nanotubes (BNNTs) have a Raman peak with position at 1368 cm-1, and their deformation individually and in composites gives Raman band shift rates of -25.7 cm-1/% and -23.6 cm-1/%. Glass matrices shows compressive stresses on boron nitride fillers and this was found as an upshift in the frequencies of G band peak of boron nitride materials. Grüneisen parameters of boron nitride (BN) were used to calculate the residual strains in glass matrices of BNNSs nanocomposites as well as to estimate the band shift rates which found to be in agreement with the experimental shift rate of bulk BN and BNNTs.
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Alevizos, Konstantinos. "Tube extrusion of hexagonal metals." Thesis, Imperial College London, 2010. http://hdl.handle.net/10044/1/6049.
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Zr-2.5 wt % Nb (Zr-2.5Nb) is the main alloy used in the pressure tubes of CANDU nuclear reactors, which are manufactured by hot extrusion. Pressure tubes are subjected to high irradiation fields and corrosion, in addition to the applied stress at operating temperatures of around 330°C, which leads to irradiation creep that is often life-limiting; re-tubing the reactors, is a source of significant through-life cost of the reactor system. However, significant variability in performance is observed between tubes and stations, which is felt to be due to variability in the fabrication and operation conditions. The performance of Zr-2.5Nb is sensitive to both microstructure and texture and therefore it is desirable to be able to understand the extrusion conditions more fully. In this thesis, the extrusion of Zr-2.5Nb is examined, along with commercially pure titanium (CP Ti), commercially pure magnesium (CP Mg) and AA2014. The effect of extrusion ratio, die geometry and rod versus tube conditions are examined. The resulting microstructures and textures are rationalised with the aid of a finite element model for the process. After the introduction and literature review (Chapters 1-2), the modelling procedure and extrusion theory are examined (Chapter 3). Constitutive data (including friction conditions) are gathered and a Norton-Hoff constitutive model is generated in Chapter 4. It is found that adiabatic heating can be important at high strain rates and low temperatures, particularly in CP Ti and CP Mg. Recrystallization during deformation can be observed in the flow curves, particularly in CP Mg and AA2014 at low strain rates and high temperatures. The extrusion of AA2014 tubes is examined in Chapter 5. It is found that satisfactory textures and microstructures can be obtained, and that the model can reproduce the observed load curves. Partially extruded gridded billets are also used to verify the flow conditions predicted by the model and to obtain textures and microstructures part-way through the extrusion process. The extrusion of CP Mg and CP Ti are examined in Chapters 6 and 7, respectively. It was found that CP Mg recrystallized very easily, dominating the microstructures and textures observed. The CP Ti extrusions were performed in the [alpha]+[beta] regime in order to match Zr-2.5Nb conditions. The high extrusion ratio rod textures were dominated by the [beta]->[alpha]transformation, while those in the tubes were more characteristic of deformation of the [alpha] phase. Zr-2.5Nb extrusion is examined in Chapter 8. Satisfactory microstructures with elongated grains surrounded by thin ligaments of [beta] were obtained in the tube extruded through a flat-faced die, with the expected texture for this ratio of wall to diametral reduction (paragraph 2.5.7.1, Figure 2.25). The microstructures obtained were found to be a product of the temperature in the die and the cooling rate of the material. Excessive cooling rates lead to the production of basket-weave microstructures, and breakup of the grain boundary [alpha] to very fine microstructures. Again, the extrusion modelling allowed the results obtained to be rationalised. Finally, the reader is referred in Chapter 9 for a discussion of the obtained result. Conclusions drawn and suggestions for further work can be found in Chapter 10, together with recommendations for the industrial modelling of tube extrusion and for industrial practice.
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Zamora, María Alejandra Sánchez [Verfasser]. "Hexagonal ice : Single crystalline hexagonal ice studied through surface-specific vibrational spectroscopy / María Alejandra Sánchez Zamora." Mainz : Universitätsbibliothek Mainz, 2018. http://d-nb.info/1161765298/34.
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Camurlu, Hasan Erdem. "Carbothermic Production Of Hexagonal Boron Nitride." Phd thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/3/12607808/index.pdf.
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Formation of hexagonal boron nitride (h-BN) by carbothermic reduction of B2O3 under nitrogen atmosphere at 1500oC was investigated. Reaction products were subjected to powder X-ray diffraction analysis, chemical analysis and were examined by SEM. B4C was found to exist in the reaction products of the experiments in which h-BN formation was not complete. One of the aims of this study was to investigate the role of B4C in the carbothermic production of h-BN. For this purpose, conversion reaction of B4C into h-BN was studied. B4C used in these experiments was produced in the same conditions that h-BN was formed, but under argon atmosphere. It was found that formation of h-BN from B4C&ndashB2O3 mixtures was slower than activated C&ndash
B2O3 mixtures. It was concluded that B4C is not a necessary intermediate product in the carbothermic production of h-BN. Some additives are known to catalytically affect the h-BN formation. The second aim of this study was to examine the catalytic effect of some alkaline earth metal oxides and carbonates, some transition metal oxides and cupric nitrate. It was found that addition of 10wt% CaCO3 into the B2O3+C mixture was optimum for increasing the rate and yield of h-BN formation and decreasing the B4C amount in the products and that the reaction was complete in 2 hours. CaCO3 was observed to be effective in increasing the rate and grain size of the formed h-BN. Addition of cupric nitrate together with CaCO3 provided a further increase in the size of the h-BN grains.
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Roussy, Marianne. "Studies of coarsening in hexagonal patterns." Thesis, McGill University, 2002. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=79118.
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A phase-field approach describing the formation and evolution of an hexagonally patterned surface is presented. We studied a free-energy and a time-dependent Ginzburg-Landau equation for which the order parameter is non-conserved. We give a review of the scaling phenomena in general and of hexagonal systems in particular, both from theoretical and experimental points of views. A squared shaped grid of varied sizes, with periodical boundary conditions, was used for the simulations. First, we studied the evolution of surfaces themselves. Then, we considered the evolution of the structure factor's maximum amplitude, and full width at half maximum. Scaling laws as a function of system size were found for these variables. We also made a study of surface energies. We monitored the evolution of the surface energies with time, and propose a scaling law for the energies. Finally, we studied the evolution of temporal correlation functions. We propose a further scaling law for the temporal correlation functions.
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Heppell, P. B. "Wave propagation in hexagonal elastic media." Thesis, University of East Anglia, 1986. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.372347.
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Jackson, Dominic. "Organic functionalisation of hexagonal mesoporous silica." Thesis, University of York, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.341112.
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Selezneva, Ekaterina. "Magnetoelectric composites based on hexagonal ferrites." Master's thesis, Universidade de Aveiro, 2008. http://hdl.handle.net/10773/2265.
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Mestrado em Ciência e Engenharia de MateriaisCompósitos Magnetoelectricos baseados em ferrites hexagonais BaFe12O19, SrFe12O19, Ba3Co2Fe24O41, e em perovsquites ferroelectricas BaTiO3, foram estudadas. Foram realizadas análises estruturais por difracção de Raios-X e microscopia de varrimento de electrões; ambas as fases, ferroelectrica e ferromagnética apresentam distorções apreciáveis dos parâmetros de rede cristalina. As propriedades ferroelectricas foram medidas por microscopia de força de resposta piezoelectrica, sendo detectado uma diminuição das propriedades ferroelectricas nos compósitos em comparação com o material BaTiO3 puro. Medidas magnéticas foram realizadas com magnetometria por vibração da amostra (VSM) e microscopia de força magnética (MFM). As medidas efectuadas por VSM confirmam a proporcionalidade entre a magnetização saturação e a respectiva fracção da fase ferromagnetica em todos os compósitos. Medidas locais do acoplamento magnetoelectrico foram efectuadas através de microscopia de força atómica, tendo sido detectadas efeitos localizados na magnetização de grãos ferrite contíguos a grãos de BaTiO3 sujeitos a uma inversão da polarização. ABSTRACT: Magnetoelectric composites based on the hexagonal ferrites BaFe12O19, SrFe12O19, Ba3Co2Fe24O41, and the ferroelectric perovskite BaTiO3, were studied. Structural and phase analysis of the samples was done by X-ray diffraction and scanning electron microscopy. Both ferroelectric and magnetic phases have structural distortion of the unit cell. Ferroelectic properties were studied by piezoresponse force microscopy. The results show significant worsening of ferroelectric and piezoelectric properties in the composite compared to the pure ferroelectric phase. Magnetic analyses were done by vibrating sample magnetometry (VSM) and magnetic force microscopy (MFM). The VSM measurements confirmed linear dependence of the magnetization saturation on the fraction of magnetic phase in all of the composites. The experiment on local electromagnetic coupling was performed by means of atomic force microscopy (AFM). It resulted in a weak effect of polarization switching on the local magnetic properties.
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Horák, Tomáš. "Terahertz nonreciprocal effects using hexagonal ferrites." Thesis, Lille 1, 2017. http://www.theses.fr/2017LIL10143/document.
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Des sources et des détecteurs ont été développés récemment pour les fréquences térahertz (THz) mais de nombreux composants passifs sont encore manquants. C’est le cas des composants non-réciproques (NR) qui sont indispensables pour la protection et la stabilisation des sources cohérentes. Dans cette thèse on étudie un nouveau concept d’isolateur THz, basé sur la combinaison d'une résonance plasmonique de surface (SP) avec un matériau à forte gyrotropie dans la gamme THz. Nous utilisons un matériau ferrite de type magnétoplumbite hexagonale, sa gyrotropie résulte d'effets gyromagnétiques dus à la précession de ses moments magnétiques à la fréquence de Larmor autour du champ magnétique interne. Il en résulte une perméabilité tensorielle dont les éléments non-diagonaux opposés induisent une réponse NR. Le fort champ interne d'une hexaferrite mène à une fréquence Larmor proche de la gamme millimétrique. Les permittivités et perméabilités diagonales ont été déterminées par analyseur de réseau vectoriel (VNA) avec un fenêtrage temporel. La gyrotropie et l’anisotropie THz ont été mesurées par spectroscopie dans le domaine temporel (TDS). Les mesures confirment les fortes valeurs attendues, un isolateur de Faraday THz a été conçu et mesuré. D’autre part, un miroir magnéto-plasmonique NR a été conçu, il est basé sur la combinaison du gyromagnétisme THz de l’hexaferrite avec les résonances SP à la surface d’un réseau métallique sub-longueur d'onde. Ces résonnances ont été démontrées expérimentalement et sont en accord avec les simulations. A proximité des résonances SP le dispositif se comporte comme un miroir unidirectionnelA key element to protect coherent sources and achieve desired power stability and spectral purity is an isolator, which in THz range has no effective solution. In this thesis, a novel design of THz isolating device based on a one-way reflecting surface is proposed. It combines gyrotropy with surface plasmon (SP) resonance phenomena and requires thus a sufficiently strong THz gyrotropic material. In the last decades a ferrite material with a hexagonal magnetoplumbite structure was created. Gyrotropy in this material is a result of gyromagnetic effects occurring when magnetic dipole moments precess nonreciprocally (NR) at Larmor frequency around an internal magnetic field. Permeability acquires a tensorial form and its unequal off-diagonal elements are responsible for NR behavior. The strong internal field in hexaferrites results in a Larmor frequency close to the mm-wave range. Foremost the diagonal permittivity and permeability elements have been characterized by time-windowed Vector Network Analyzer (VNA). Their strong gyrotropic and anisotropic properties in THz range are then investigated by a magneto-optical Time-domain spectrometry. The obtained strong gyrotropic spectra prove their unique potential for THz NR applications, as shown by an original Faraday isolation measurement using a VNA, and by first designs of a NR magnetoplasmonic mirror using the fitted material parameters. It combines strong hexaferrites gyrotropy with SPs resonances formed due to a metal grating present at the ferrite surface. The SPs excitation in THz is demonstrated numerically and experimentally with a strong agreement. Close to SPs resonance frequencies the device acts as a one-way mirror
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Plunkett, Brian W. "Plastic anisotropy of hexagonal closed packed metals." [Gainesville, Fla.] : University of Florida, 2005. http://purl.fcla.edu/fcla/etd/UFE0012500.
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Sarrazit, Franck. "Structure of grain boundaries in hexagonal materials." Thesis, University of Liverpool, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.367291.
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Perring, Toby George. "High energy magnetic excitations in hexagonal cobalt." Thesis, University of Cambridge, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.239595.
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Pujol, Patrick. "Mésophases massives de nitrure de bore hexagonal." Bordeaux 1, 2000. http://www.theses.fr/2000BOR12222.
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La réaction de condensation en masse d'un précurseur inorganique de nitrure de bore hexagonal, la borazine, a été développée dans le but d'élaborer des dérivés polycycliques de type mésophasique nommés borazines polycondensées (BPC). Les BPC ainsisynthétisées peuvent conduire à un état cristal liquide préfigurant la structure du nitrure de bore hexagonal,état obtenu après mise en solution dans un solvant adéquat. Elles ont été employées comme précurseur d'interphase BN dans des minicomposites Hi-Nicalon/BN/SiC, après dépôt par 'dip-coating' et pyrolyse du précurseur condensé. L'élaboration de monofilaments BN de courte longueur a aussi été examinée.
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Yellen, Duncan Howard. "Radiation damage in hexagonal-close-packed metals." Thesis, University of Liverpool, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.316660.
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Arzberger, Steven C. "Polymerizable amphiphiles for the inverted hexagonal phase." Diss., The University of Arizona, 2002. http://hdl.handle.net/10150/280149.
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Liquid crystals possess both order and mobility. Hydrated, natural and synthetic amphiphiles self-organize to form various liquid crystal phases as a function of molecular structure, temperature, concentration, and pressure. Self-organization is the ordering of molecules via non-covalent interactions, i.e. hydrogen bonding, van der Waals, pi-pi interactions, ionic interactions, hydrophobic short-range forces, and London dispersion forces. Amphiphiles contain both polar and non-polar moieties. In general, amphiphiles are composed of a polar, hydrophilic headgroup and one or more non-polar, hydrophobic tail(s). At equilibrium, the unfavorable enthalpic interaction of the polar water molecules with the non-polar amphiphile tails is minimized by the aggregation of the latter with the non-polar tails of other amphiphiles to form a water excluded hydrophobic block, while the hydrophilic headgroups line the interface of the phase-separated aqueous domains. Self-supported arrays of self-organized, hydrated amphiphile assemblies include lamellar/vesicles, various normal and inverted cubic phases, and normal and inverted hexagonal phases. The inverted hexagonal (HII) phase can be considered as aqueous columns patterned in a hexagonal fashion. The polar amphiphile headgroups are well ordered at the water-amphiphile interface, while their non-polar tails are disordered and fill the area between the aqueous water channels. In general, amphiphiles with two or more non-polar chains and a small, poorly hydrated headgroup favor the formation of the HII phase. Longer tails or the incorporation of bulky design elements, i.e. cis-double bonds or branching substituents, in the amphiphile tail(s) lowers the temperature associated with the formation of the HII phase. Several HII-forming amphiphiles have been designed and synthesized. Upon hydration, the phase behavior of these amphiphiles was evaluated by 31P-NMR assembly characterization. Radical polymerizations were used to stabilize the HII phase assemblies resulting in cross-linked polymer networks. The cross-linked materials displayed dramatically different physical properties, i.e. lowered solubility in common organic solvents. The polymer assembly phase behavior was evaluated via 31P-NMR after polymerization. A synthetic route to phosphoethanolamines via a novel di-protected glycerophosphoethanolamine has been designed and developed. A phosphoethanolamine lipid has been synthesized using this route. The route appears to be general to the synthesis of any phosphoethanolamine.
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Khan, Aamar Farooq. "The electrochemistry of 2D hexagonal boron nitride." Thesis, Manchester Metropolitan University, 2018. http://e-space.mmu.ac.uk/620319/.
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Since the discovery of the unique physical properties of graphene, research has intensified in the field of two-dimensional (2D) nanomaterial electrochemistry. Indeed, newly emerging 2D materials such as 2D-hexagonal boron nitride (2D-hBN) have the potential to transform the field of electrochemistry when implemented as a next generation electrode material. This thesis reports on the electrochemical applicability of utilising 2D-hBN, previously considered non-electroactive, as a novel electrode material. Also considered is the effect of the fabrication process of 2D-hBN when employed towards a range of electrochemical applications. Chapter 1 gives an overview of the general electrochemical concepts that concern this thesis. Chapter 2 offers an insight into recent 2D materials electrochemistry literature regarding, first, graphene and then 2D-hBN. From this, successive chapters follow the development and investigation of 2D-hBN, formed via differing synthesis techniques, thus enabling a truer reflection of 2D-hBN as an electrode material to be achieved. Chapter 3 details the relevant experimental information and the full physicochemical characterisation of the different 2D-hBN materials employed within this thesis. Chapters 4 and 5 utilise surfactant-free (pristine) 2D-hBN, where pristine 2D-hBN is ‘electrically wired’ upon a suitable electrode surface. Chapter 4 reveals for the first time that pristine 2D-hBN gives rise to beneficial electrochemical behaviour towards the oxygen reduction reaction (ORR) when immobilised upon a graphitic substrate. Chapter 5 explores pristine 2D-hBN towards a biological approach in the sensing of dopamine (DA) and its common interferents ascorbic acid (AA) and uric acid (UA). Pristine 2D-hBN exhibits a beneficial electrocatalytic effect towards the detection of dopamine when immobilised upon a graphitic substrate. This observed beneficial effect upon the utilisation of pristine 2D-hBN has not previously been reported in the literature when supported upon any electrode. Both chapters implement ‘mass coverage’ studies of 2D-hBN, an often overlooked parameter within the literature Chapters 6 and 7 utilise surfactant-exfoliated 2D-hBN and compare the effect of the fabrication process of 2D-hBN (pristine vs. surfactant-exfoliated) upon the observed electrochemistry towards the ORR, capacitance applications and the sensing of dopamine, via a dropcasting electrode modification approach. Chapter 6 explores surfactant-exfoliated 2D-hBN towards the ORR and capacitance applications for the first time. The surfactant-exfoliated 2D-hBN nanosheets are immobilised upon graphitic screen-printed electrodes (SPEs) with ‘mass coverage’ studies performed and the observed electrochemical response is compared to the surfactant-free pristine 2D-hBN approach. Chapter 7 explores surfactant-exfoliated 2D-hBN as a potential electrochemical sensing platform towards the electroanalytical sensing of dopamine (DA) in the presence of the common interferents, ascorbic acid (AA) and uric acid (UA) for the first time. Surfactant exfoliated 2D-hBN is electrically wired via a drop-casting modification process onto SPEs and the observed electrochemical response is compared to the surfactant-free (pristine) 2D-hBN approach. The performance of these surfactant-exfoliated 2D-hBN modified SPEs are critically evaluated upon the implementation of ‘mass coverage studies. Chapter 8 explores for the first time a low cost and reproducible approach for producing 2D Hexagonal Boron Nitride (2D-hBN) electrochemical screen-printed platforms (hBN-SPEs). These novel hBN-SPEs are explored as a potential electrocatalyst towards the ORR. This fabrication approach is compared to the drop casting technique of pristine and surfactant-exfoliated 2D-hBN utilised towards the ORR, thus offering an alternative approach. This thesis demonstrates for the first time that 2D-hBN is electroactive when immobilised upon a graphitic substrate towards a range of applications. It is also shown that fabrication process in the production of 2D-hBN can affect the observed electrochemistry, thus control experiments must be undertaken to truly understand the impact of this material.
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Karimpoor, Amir A. "Mechanical properties of bulk nanocrystalline hexagonal cobalt electrodeposits." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp05/MQ63139.pdf.
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Adkin, Josephine J. "The synthesis and characterisation of some hexagonal perovskites." Thesis, University of Oxford, 2008. http://ora.ox.ac.uk/objects/uuid:9b339008-23e6-432f-ad59-b90a018c9740.
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The structural chemistry and magnetic properties of a number of manganese containing hexagonal perovskites have been studied by X-ray diffraction, neutron diffraction and magnetometry. Trends in the magnetic properties are investigated as a function of the hexagonal stacking sequence and manganese oxidation state. The synthesis of the series of BaMnO3-x hexagonal perovskites is discussed. By varying both the partial pressure of oxygen and the firing temperature, stacking sequences with a range of hexagonal to cubic layer ratios can be synthesised. Factors which increase the structural tolerance factor are found to increase the proportion of hexagonal layers in the stacking sequence. The crystallographic properties of the BaMnO3-x compounds are discussed, and the oxide vacancies are found to be exclusively located in the face-sharing hexagonal (h) layers, particularly those which are adjacent to two apex-sharing cubic (c) layers. The preferential localisation of oxide vacancies in chc sites can be used to rationalise the observed stacking sequences, as well as the limiting stoichiometries of BaMnO3-x structures. The magnetic behaviour of the BaMnO3-x phases is investigated, both individually and as a function of the stacking sequence. A strong direct exchange interaction between face-sharing cations couples all magnetic moments antiferromagnetically above ambient temperature. A slightly weaker interaction (utilising the 180° superexchange pathways between apex-sharing MnOe octahedra) results in three-dimensional antiferromagnetic order at a temperature in the range 230 ≤ T (K) ≤ 280. The strength of this interaction is found to be dependent on the length of the face-sharing chains. A third interaction occurs at T ~ 45K, believed to represent a canting transition. Low temperature neutron diffraction data reveal that the magnetic moments order in a simple antiferromagnetic manner. The magnitude of the ordered moment shows a dependence on the length of the face sharing chains, where phases with shorter chains have a larger ordered moment. Long-range magnetic order is disrupted by charge disorder in the mixed Mn(III)/Mn(IV) system 4H-BaMnO2.6 5(1). The hexagonal BaMn1-zTiz O3- x and BaMn1-zZrz03-x systems were also studied. Synthesis under argon atmospheres allows a range of new phases to be accessed. These include a novel manganese-zirconium phase, 6H-BaMn0.8Zr0.2O2.81(1), and BaMno.55Ti0.45,/sub>O3-x, which adopts the rare 15R' stacking sequence. The BaMn1-z TizO3-x system confirms that although the tolerance factor controls the proportion of cubic layers, it does not control the arrangement of these layers. This is controlled by the size of the B cations and the proportion of oxide vacancies. Partial cation order occurs in 6H-BaMno.8Zr0.2O2.81(1), which can be rationalised on the basis of the size difference between manganese and zirconium ions. Topotactic reductions using binary hydrides are carried out in order to decouple the manganese oxidation state from the adopted stacking sequence. Using hydride reductions, 4H-Bao.5Sro.5MnO3-x can be topotactically reduced to 4H-Bao.5Sro.5MnO2.o2(1). This increases the strength of the 180° superexchange interaction, and antiferromagnetic order is achieved above ambient temperature. Using the same techniques, 4H-BaMnO3-x can be reduced to 4H-BaMnO2.47(1) or 4H-BaMnO2.o6(1). These phases undergo a distortion to an orthorhombic unit cell.
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Shatilla, Youssef A. (Youssef Abdel-Moneim). "A simple quadratic nodal model for hexagonal geometry." Thesis, Massachusetts Institute of Technology, 1992. http://hdl.handle.net/1721.1/34317.
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Willis, Scott A., Gary R. Dennis, Gang Zheng, and William S. Price. "Self-Diffusion of water in compressed hexagonal phases." Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-185711.
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Radzicki, Vincent R. "FMCW Subsurface Microwave Imaging with Hexagonal Antenna Arrays." International Foundation for Telemetering, 2014. http://hdl.handle.net/10150/578169.
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ITC/USA 2014 Conference Proceedings / The Fiftieth Annual International Telemetering Conference and Technical Exhibition / October 20-23, 2014 / Town and Country Resort & Convention Center, San Diego, CAGround-penetrating radar (GPR) imaging is typically conducted in the pulse-echo mono-static format with a simple CW pulse as the probing signal. Recently, the data-acquisition hardware has been extended to the use of linear multi-element arrays. This paper presents an advanced GPR imaging system with FMCW probing waveforms, with a seven-element hexagonal array and software-defined data-acquisition hardware. The use of FMCW probing signals is for the optimization of the information contents of the returned waveforms. The utilization of the hexagonal unit is to produce sub-images with direction-independent resolution capability. In this paper, mathematical analysis, system modeling, field experiments, and image reconstruction are included to illustrate the performance and capability of the engineering concepts.
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Asghar, Kaleem Abbas. "2D hexagonal and 3D gyroid structured platinum materials." Thesis, University of Reading, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.669697.
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Pullar, Robert Carlyle. "The synthesis and characterisation of hexagonal ferrite fibres." Thesis, University of Warwick, 1999. http://wrap.warwick.ac.uk/79313/.
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Hexagonal ferrites are a family of magnetic oxides which include M ferrite, the most commonly used magnetic material in the world today. They also have more specialist potential applications, such as microwave absorption and catalysis. They are highly complex crystal structures which usuaUy require high temperatures to form a well sintered product, at the expense of grain growth, which in tum affects the magnetic properties of the material. They all exhibit magneto crystalline anisotropy; depending upon the ferrite they can have a preferred direction of magnetisation along the c axis of the hexagonal crystal (uniaxial), or in the hexagonal plane or in a cone at an angle to the plane (ferroxplana). A material made in a fibrous form has many properties; firstly it can be incorporated into a matrix to form a composite material, imparting any special properties into the whole composite. Provided their microstructure is sufficiently fme, fibres tend to be stronger than the bulk ceramic form, and if fme enough they can even be woven into a cloth. It has also been suggested that some properties, such as magnetisation, will be enhanced when the material is an aligned fibre, although this specific case has never been proven previously. This thesis presents the manufacture of a range of hexagonal ferrite fme fibres from an inorganic aqueous sol-gel precursor, and their subsequent characterisation both physically and magnetically. The fibres were blow spun from a viscous solution using a modified commercial process, and they were collected in both random and aligned forms. The sols were made from the peptisation of a precipitated iron(III)hydroxide with a mineral acid, and various combinations of iron (III) salt and acid were investigated using PCS. The best results were found to come from iron(III)chloride precipitated by ammonia and peptised with HBr, with totally nitrate and chloride based systems proving inferior. Chloride was found to add structuring to the sol, aiding spinning but resulting in a thixotropic gel if too much was present. The stoichiometric ferrite sols were made by doping the iron (I II) sol with solution of metal salts, and these were found to be critical to the stability of the sol, with barium causing particular problems, and titania was added as a second colloidal species. Stoichiometric sols of BaM, SrM, BaCoxTixFel12-12xO19, SrCoxTixFe12 2x019, CozY, COzZ, COzW, CozX and CozU ferrites were made, among others, and aU were successfully spun as fibres. They typically had diameters of 4 - 6 um and an alignment of over 90% within +/. 20° of the axis of alignment, comparable to commercially developed fibres. The sol-gel process typically produces ceramic materials at lower temperatures and with smaller grain sizes than standard ceramic techniques, and this proved to be the case. SrM and BaM began to form at 800°C and had become the pure phase product by 1000 °C, being 98% sintered at this stage and with a submicron grain size. CozY ferrite was also the sole phase at a low temperature of 1000 °C, although it consisted of hexagonal platelets up to 3 um wide and 0.2 um thick. Substituted M fcrrites also formed the M phase at similar temperatures, but were more porous and did not have the greatly reduced grain size reported previously. None of these ferrites underwent the sudden, exaggerated grain growth seen in many hexagonal ferrites, even at 1200 °C. However, both the CozZ and CozW ferrite fibres underwent this process concurrently with the formation of these phases, and it appears to be an unavoidable step under normal conditions, although steam firing or doping with CaO enabled the formation of a majority-Z product without this growth. It was found that halide retained in the fibre inhibited the formation of the hexagonal ferrite phase, as it was retained up to 1000 °c. When the fibres were treated to remove halides or made from a halide free precursor they formed M ferrites -200°C lower, and with a reduced grain size below 0.1 !lm. The fibres were characterised by XRD, XRF, XPS and SEM. The samples were characterised magnetically by VSM, and while the M ferrites proved to have typical Ms values and high coercivities for polycrystalline samples, the ferroxplana ferrites all had Ms equal to or better than the previously reported maximum values. It was also found that there were defmite alignment effects on Ms in all fibres except those which had .. under gone the exaggerated grain growth, with an enhancement in Ms of up to +40% along the ilxis of alignment when parallel to the applied field compared to values for random fibre. This is the first'time that the effect of fibre alignment upon magnetic properties has been demonstrated.
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Ayoob, Raed. "Dielectric properties of hexagonal boron nitride polymer nanocomposites." Thesis, University of Southampton, 2017. https://eprints.soton.ac.uk/417272/.
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There is a growing research interest in polymer nanocomposite materials due to their potential in enhancing dielectric properties. However, a considerable amount of variability exists in the literature regarding the electrical performance of polymer nanocomposites, and therefore the underlying mechanisms underpinning their electrical properties are still far from fully understood. Possible reasons for the existing inconsistencies could be due to different material preparation techniques, different nanoparticle dispersion states, unknown filler content, inconsistent sample storage conditions, and unknown water level content in the samples. Determining the principal factors that dominate the electrical behaviour of polymer nanocomposites could allow engineers to tailor the electrical properties of dielectrics for their specific application. As a result, the work reported in this thesis was mainly set out to explore the factors governing the electrical properties of polymer nanocomposites such that the inconsistencies in the literature can be better understood, and consequently eliminated. This thesis investigated the performance of hexagonal boron nitride (hBN) nanocomposites based on two thermoplastic polymers: polystyrene and polyethylene. Prior to producing any nanocomposites, the hBN particles were characterised using different techniques. The characterisation primarily revealed that the boron nitride particles are in the hexagonal form and the surface of hBN contains a scarce amount of hydroxyl groups. Polystyrene nanocomposites were prepared containing identical amounts of hBN dispersed in different solvents in an attempt to obtain different dispersion states, as a result of different hBN/solvent interactions. The effect of solvent processing was negligible on the dispersion state of the hBN in the polystyrene; no observable difference in the dispersion and electrical properties was reported although the presence of hBN resulted in a slight increase in the breakdown strength relative to the unfilled polystyrene. A range of polyethylene nanocomposites were produced containing different amounts of hBN to understand the effect of the dispersion or aggregation state of the hBN on the breakdown strength. The results revealed that the nanocomposites, regardless of the morphology, exhibited a monotonic increase in breakdown strength with increasing hBN content from 2 wt % to 30 wt %, while maintaining the low dielectric losses of the unfilled polyethylene. While the hBN was found to have a strong nucleating effect on the polyethylene, it was determined that the local change in morphology was not the cause of the enhanced breakdown strength as both the polyethylene nanocomposites obtained by rapid crystallisation, where the development of spherulites was suppressed, and the amorphous polystyrene nanocomposites, also exhibited an improved breakdown strength. Further experiments indicated that the polyethylene nanocomposites did not absorb any moisture from the environment in ambient conditions, and absorbed a very small amount of water even in the 30 wt % polyethylene/hBN nanocomposite when completely immersed in water. Dielectric spectroscopy measurements revealed that the surface hydroxyl groups on the hBN are most likely located only on the edge surfaces of the hBN rather than basal surfaces. The water was most likely loosely bound to the hBN particles, where local water clusters formed. It was remarkable that a percolating water network was not formed in a nanocomposite consisting of an already percolating hBN network, which was largely attributed to the surface chemistry of hBN. Despite the presence of water in the system, the hBN nanocomposites continued to exhibit an enhanced breakdown strength in comparison to the unfilled polyethylene. Therefore, this thesis demonstrated that the electrical behaviour of polymer nanocomposites is most likely dominated by the surface state of the nanoparticles and how the particles interact with the charge carriers; any other effects due to local morphological changes or nanoparticle dispersion are considered to be secondary reasons for changes in the electrical properties.
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Grosser, Tobias. "A decoupled approach to high-level loop optimization : tile shapes, polyhedral building blocks and low-level compilers." Thesis, Paris 6, 2014. http://www.theses.fr/2014PA066270/document.
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Malgré des décennies de recherche sur l’optimisation de boucle auxhaut niveau et leur intégration réussie dans les compilateurs C/C++et FORTRAN, la plupart des systèmes de transformation de bouclene traitent que partiellement les défis posé par la complexité croissanteet la diversité du matériel d’aujourd’hui. L’exploitation de laconnaissance dédiée a un domaine d’application pour obtenir le codeoptimal pour cibles complexes, tels que des accélérateurs ou des microprocessorsmulti-coeur, pose des problèmes pour les formalismeset outils d’optimisation de boucle existants. En conséquence, de nouveauxschémas d’optimisation qui exploitent la connaissance dédiéea un domaine sont développées indépendamment sans profiter dela technologie d’optimisation de boucle existante. Cela conduit à despossiblités d’optimisation raté et ainsi qu’à une faible portabilité deces schémas d’optimisation entre des compilateurs différents. Un domainepour lequel on voit la nécessité d’améliorer les optimisationsest le calcul de pochoir itératifs, un probléme de calcul important quiest réguliérement optimisé par les compilateurs dédiées, mais pourlequel générer code efficace est difficile.Dans ce travail, nous présentons des nouvelles stratégies pour l’optimisationdédiée qui permettent la génération de code GPU haute performancepour des calculs de pochoir. À la différence de la façon dontla plupart des compilateurs existants sont mis en oeuvre, nous découplonsla stratégie d’optimisation de haut niveau de l’optimisationde bas niveau et la spécialisation nécessaire pour obtenir la performanceoptimale. Comme schéma d’optimisation de haut niveau, nousprésentons une nouvelle formulation de “split tiling”, une techniquequi permet la réutilisation de données dans la dimension du tempsainsi que le parallélisme équilibré à gros grain sans la nécessité derecourir à des calculs redondants. Avec le “split tiling”, nous montronscomment intégrer une optimisation dédiée dans un traducteurgénérique source-à-source, C vers CUDA, une approche qui nouspermet de réutiliser des optimisations existants non-dédiées. Nousprésentons ensuite notre technique appelée “hybrid hexagonal / parallelogramtiling", un schéma qui nous permet de générer du codeque cible directement les préoccupations spécifiques aux GPUs. Pourconclure notre travail sur le "loop tiling", nous étudions la rapport entre“diamond tiling” et “hexagonal tiling”. À partir d’une analyse de“diamond tiling” détailée, qui comprend les exigences qu’elle posesur la taille de tuile et les coefficients de front d’onde, nous fournissonsune formulation unifiée de l’“hexagonal tiling” et du “diamondtiling” qui nous permet de réaliser un “hexagonal tiling” pourvdes problèmes avec deux dimensions (un temps, un espace) dans lecadre d’un usage dans un optimiseur générique, comme “Pluto”. Enfin,nous utilisons cette formulation pour évaluer l’“hexagonal tiling”et le “diamond tiling” en terme de rapport de calcul-à-communicationet calcul-à-synchronisation.Dans la deuxième partie de ce travail, nous discutons nos contributionsaux composants de l’infrastructure les plus important, nos“building blocks”, qui nous permettent de découpler notre optimisationde haut niveau tant des optimisations nécessaires dàns la générationde code que de l’infrastructure de compilation générique. Nouscommençons par présenter le nouveau “polyhedral extractor” (pet),qui obtient une représentation polyédrique d’un morceau de code C.pet utilise l’arithmétique de Presburger en sa généralité pour élargirle fragment de code C supporté et porter une attention particulièreà la modélisation de la sémantique des langages même en présencede dépassement de capacité des entiersDespite decades of research on high-level loop optimizations and theirsuccessful integration in production C/C++/FORTRAN com- pilers, most compilerinternal loop transformation systems only partially address the challengesposed by the increased complexity and diversity of today’s hardware. Especiallywhen exploiting domain specific knowledge to obtain optimal code for complextargets such as accelerators or many-cores processors, many existing loopoptimization frameworks have difficulties exploiting this hardware. As aresult, new domain specific optimization schemes are developed independentlywithout taking advantage of existing loop optimization technology. This resultsboth in missed optimization opportunities as well as low portability of theseoptimization schemes to different compilers. One area where we see the need forbetter optimizations are iterative stencil computations, an importantcomputational problem that is regularly optimized by specialized, domainspecific compilers, but where generating efficient code is difficult.In this work we present new domain specific optimization strategies that enablethe generation of high-performance GPU code for stencil computations. Differentto how most existing domain specific compilers are implemented, we decouple thehigh-level optimization strategy from the low-level optimization andspecialization necessary to yield optimal performance. As high-leveloptimization scheme we present a new formulation of split tiling, a tilingtechnique that ensures reuse along the time dimension as well as balancedcoarse grained parallelism without the need for redundant computations. Usingsplit tiling we show how to integrate a domain specific optimization into ageneral purpose C-to-CUDA translator, an approach that allows us to reuseexisting non-domain specific optimizations. We then evolve split tiling into ahybrid hexagonal/parallelogram tiling scheme that allows us to generate codethat even better addresses GPU specific concerns. To conclude our work ontiling schemes we investigate the relation between diamond and hexagonaltiling. Starting with a detailed analysis of diamond tiling including therequirements it poses on tile sizes and wavefront coefficients, we provide aunified formulation of hexagonal and diamond tiling which enables us to performhexagonal tiling for two dimensional problems (one time, one space) in thecontext of a general purpose optimizer such as Pluto. Finally, we use thisformulation to evaluate hexagonal and diamond tiling in terms ofcompute-to-communication and compute-to-synchronization ratios.In the second part of this work, we discuss our contributions to importantinfrastructure components, our building blocks, that enviable us to decoupleour high-level optimizations from both the necessary code generationoptimizations as well as the compiler infrastructure we apply the optimizationto. We start with presenting a new polyhedral extractor that obtains apolyhedral representation from a piece of C code, widening the supported C codeto exploit the full generality of Presburger arithmetic and taking special careof modeling language semantics even in the presence of defined integerwrapping. As a next step, we present a new polyhedral AST generation approach,which extends AST generation beyond classical control flow generation byallowing the generation of user provided mappings. Providing a fine-grainedoption mechanism, we give the user fine grained control about AST generatordecisions and add extensive support for specialization e.g., with a newgeneralized form of polyhedral unrolling. To facilitate the implementation ofpolyhedral transformations, we present a new schedule representation, scheduletrees, which proposes to make the inherent tree structure of schedules explicitto simplify the work with complex polyhedral schedules.The last part of this work takes a look at our contributions to low-levelcompilers
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Connolly, Kevin John. "A coarse mesh radiation transport method for prismatic block thermal reactors in two dimensions." Thesis, Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/44823.
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In this paper, the coarse mesh transport method is extended to hexagonal geometry. This stochastic-deterministic hybrid transport method calculates the eigenvalue and explicit pin fission density profile of hexagonal reactor cores. It models the exact detail within complex heterogeneous cores without homogenizing regions or materials, and neither block-level nor core-level asymmetry poses any limitations to the method. It solves eigenvalue problems by first splitting the core into a set of coarse meshes, and then using Monte Carlo methods to create a library of response expansion coefficients, found by expanding the angular current in phase-space distribution using a set of polynomials orthogonal on the angular half-space defined by mesh boundaries. The coarse meshes are coupled by the angular current at their interfaces. A deterministic sweeping procedure is then used to iteratively construct the solution.
The method is evaluated using benchmark problems based on a gas-cooled, graphite-moderated high temperature reactor. The method quickly solves problems to any level of detail desired by the user. In this paper, it is used to explicitly calculate the fission density of individual fuel pins and determine the reactivity worth of individual control rods. In every case, results for the core multiplication factor and pin fission density distribution are found within several minutes. Results are highly accurate when compared to direct Monte Carlo reference solutions; errors in the eigenvalue calculations are on the order of 0.02%, and errors in the pin fission density average less than 0.1%.
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Krishnamurthy, Harikrishnan. "Square and hexagonal slotted spiral antennas for 3D- MMIC." Cincinnati, Ohio : University of Cincinnati, 2005. http://www.ohiolink.edu/etd/view.cgi?acc%5Fnum=ucin1132018374.
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Lundskog, Anders. "Controlled growth of hexagonal GaN pyramids and InGaN QDs." Doctoral thesis, Linköpings universitet, Halvledarmaterial, 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-79326.
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Gallium-nitride (GaN) and its related alloys are direct band gap semiconductors, with a wide variety of applications. The white light emitting diode (LED) is of particular importance as it is expected to replace energy inefficient light bulb and hazardous incandescent lamps used today. However, today’s planar hetero epitaxial grown LEDs structures contain an unavoidable number of dislocations, which serves as non-radiative recombination centers. The dislocations harm the luminous efficiency of the LEDs and generate additional heat. Pseudomorphically grown quantum dots (QDs) are expected to be dislocation free thus the injected carriers captured by the QDs essentially recombine radiatively since the dislocations remain outside the QD. Furthermore the continuous character of the density of states in bulk materials is redistributed when the size of the dot is reduced within the Bohr radius of the material. Fully discret energy levels are eventually reached, which offers additional control of the optical properties. The Coulomb interaction between the confined carriers also has influence on the emission energy of the recombining carriers, which opens up the possibility of manufacturing novel light sources such as the single photon emitter. Single photon emitters are essential building blocks for quantum cryptography and teleportation applications. The main contribution of the present work is the investigation of growth and characterization of sitecontrolled indium-gallium-nitride QDs embedded in GaN matrixes. The goal has been to demonstrate the ability to grow site-controlled InGaN QDs at the apex of hexagonal GaN pyramids in a controlled way using hot-wall metal organic chemical vapor deposition (MOCVD). Strong emphasis was set on the controlled growth of InGaN QDs. For example the growth of a single InGaN QD located at the apex of hexagonal GaN pyramids with tunable emission energy, the QD emission energy impact on the mask design, and a novel approach for the growth of InGaN QDs with polarization deterministic photon vectors were reported. The thesis is mainly based on experimental investigations by secondary electron microscope (SEM), micro photo-luminescence (μPL), and scanning transition electron microscopy ((S)TEM) characterization techniques. In Paper 1 and 2, we present the growth of symmetric GaN hexagonal pyramids which served as template for the InGaN QDs grown. In paper 1, it was concluded that the selective area growth (SAG) of hexagonal GaN pyramids by MOCVD through symmetric openings in a SIN mask roughly can be divided in two regimes where either the pyramid expands laterally or not. When the pyramid expanded laterally the resulting pyramid apex became (0001) truncated even after prolonged growth times. Lateral expansion also had major impact on the pyramid-to-pyramid uniformity. In paper 2, the MOCVD process parameter impact on the pyramid morphology was investigated. By tuning the growth temperature, the ammonia, and TMGa-flows a self limited pyramid structure with only {1101} facets visible was achieved. The presence of the {1101}, {1102}, and {1100} facets were discussed from surface stabilities under various growth conditions. Paper 3 and 4 concern the growth of InGaN QDs located at the apex of hexagonal GaN pyramids. In paper 3, we showed that it is possible to grow single QDs at the apex of hexagonal pyramids with emission line widths in the Ångström range. The QD emission energy was demonstrated to be tunable by the growth temperature. Basic spectroscopy data is also presented on a single QD in paper 3. In paper 4, the growth mechanisms of the QDs presented in paper 3 are presented. We concluded that (0001) truncated GaN pyramid base initiated the growth of InGaN QDs which gave rise to narrow luminescence peaks in the μPL spectra. In paper 5, the QD emission energy impact of the mask design was investigated. To our big surprise the QD emission energy increased with increasing pyramid pitch while the emission energy of the InGaN quantum wells located on the {1101} facets of the pyramids energetically shifted towards lower energies. The energy shift at the apex was found to be associated with the (0001) truncation diameter of the underlying GaN pyramid since no energy shift was observed for (0001) truncated pyramids with truncation diameters larger than 100 nm. In paper 6, the symmetry of the GaN pyramids were intentionally broken through the introduction of elongated openings in the SiN mask (symmetric openings was used in the previous five papers). The emission polarization vectors of the subsequently grown InGaN QDs were deterministically linked to the in-plane orientation of the pyramid it was nucleated upon, implying that the QDs inhibit an inplane anisotropy directly inherited from the pyramid template. Finally, paper 7 describes a hot-wall MOCVD reactor improvement by inserting insulating pyrolytic boron-nitride (PBN) stripes in the growth chamber. By doing this, we have completely eliminated the arcing problem between different susceptor parts. As a consequence, the reactor gained run-to-run reproducibility. Growth of state of the art advanced aluminum-gallium-nitride high electron mobility transistor structures on a 100 mm wafer with electron mobility above 2000 Vs/cm2 was demonstrated by the improved process.
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Rodriguez, Jan Bacca. "Blue noise methods for hexagonal grids and multitone dithering." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 124 p, 2007. http://proquest.umi.com/pqdweb?did=1246586631&sid=1&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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Rawal, Ritesh. "Synthesis and characterisation of B-site deficient hexagonal perovskites." Thesis, University of Sheffield, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.445124.
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McAuley, Nigel Anthony. "A study of interfacial defects in hexagonal based bicrystals." Thesis, University of Liverpool, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.235507.
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Diller, Peter Ray. "Wire wrapped fuel pin hexagonal arrays for PWR service." Thesis, Massachusetts Institute of Technology, 2005. http://hdl.handle.net/1721.1/42168.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, 2005.Includes bibliographical references (p. 231-234).
This work contributes to the Hydride Fuels Project, a collaborative effort between UC Berkeley and MIT aimed at investigating the potential benefits of hydride fuel use in light water reactors (LWRs). Core design is accomplished for both hydride and oxide-fueled cores over a range of geometries via steady-state and transient thermal hydraulic analyses, which yield the maximum power, and fuel performance and neutronics studies, which provide the achievable discharge burnup. The final optimization integrates the outputs from these separate studies into an economics model to identify geometries offering the lowest cost of electricity, and provide a fair basis for comparing the performance of hydride and oxide fuels. This work focuses on the steady-state and transient thermal hydraulic as well as economic analyses for PWR cores utilizing wire wraps in a hexagonal array with UZrH1.6 and UO2. It was previously verified that square and hexagonal arrays with matching rod diameters and H/HM ratio have the same thermal hydraulic performance. In this work, this equivalence is extended to hexagonal wire wrap arrays, and verified by comparing the thermal hydraulic performance of a single hexagonal wire wrap core with its equivalent square array core with grid spacers. A separate neutronics equivalence is developed, based on the assumption that arrays with matching rod diameters and H/HM ratios will have identical neutronic performance. Steady-state design limits were separated into hard limits, which must be satisfied, or soft limits, which serve to keep the design reasonable. Design limits were placed on the pressure drop, critical heat flux (CHF), vibrations, and fuel and cladding temperature. Vibrations limits on the wire wrap assemblies were imposed for flow induced vibrations (FIV) and thermal hydraulic vibrations (THV).
(cont.) An analysis of the fretting wear of wire wraps was performed, which outperformed the analogous fretting wear analysis for grid spacers. A CHF study found wire wraps to outperform grid spacers. LOCA and overpower transient analyses were performed for wire wraps. The overpower transient was analyzed over a range of geometries, and found to be more limiting than the steady-state analysis. The LOCA was analyzed for various powers at the reference geometry and another geometry of interest. Through all of these analyses, it was determined that the thermal hydraulic performance of UZrH1 .6 and UO2 are very similar. The optimal wire wrap designs were found to have significantly higher maximum powers than the reference core, allowing for uprates up to -54%. This is due to improved vibrations, pressure drop, and CHF. The steady-state and transient analyses were combined with fuel performance and neutronic studies into an economics model that determines the optimal geometries for incorporation into existing PWR's. The model also provides a basis for comparing the performance of UZrH1.6 to UO2 for a range of core geometries. Results presented herein show cost savings for oxide fuel with wire wraps over grid spacers of at least 0.8 mils/kWe-hr, or 4%, due to power increases predicted by the thermal hydraulic analyses. Wire wrap UZrHI.6 has a COE savings over U02 of 0.7 mils/kWe-hr, or 4%. Due to the large power uprates possible, a utility could achieve a cost savings of up to 10.9 mils/kWe-hr, or 40%, with a UZrH1.6 wire wrap uprate instead of building a new core.
by Peter Ray Diller.
S.M.
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Juan, Pierre-Alexandr. "Twinning in hexagonal materials: application to zirconium and magnesium." Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/53945.
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The main objective of this thesis is to investigate and quantify the influence of parent-twin and twin-twin interactions on the mechanical response of hexagonal close-packed metals. To study parent-twin interactions, a mean-field continuum mechanics approach has been developed based on a new twinning topology in which twins are embedded in twinned grains. A first model generalizing the Tanaka-Mori scheme to heterogeneous elastic media is applied to first and second generation twinning in magnesium. In the case of first generation twinning, the model is capable of reproducing the trends in the development of backstresses within the twin domain as observed experimentally. Applying the methodology to the case of second-generation twinning allows the identification, in exact agreement with experimental observations, of the most likely second-generation twin variants to grow in a primary twin domain. Because the elastic behavior assumption causes internal stress level magnitudes to be excessively high, the first model is extended to the case of elasto-plasticity. Using a self-consistent approximation, the model, referred to as the double inclusion elasto-plastic self-consistent (DI-EPSC) scheme, is applied to Mg alloy polycrystals. The comparison of results obtained from the DI-EPSC and EPSC schemes reveals that deformation system activities and plastic strain distributions within twins drastically depend on the interaction with parent domains.
The influence of twin-twin interactions on nucleation and growth of twins is being statistically studied from zirconium and magnesium electron back-scattered diffraction scans. A new twin recognition software relying on graph theory analysis has been developed to extract all microstructural and crystallographical data. It is capable of identifying all twinning modes and all twin-twin interaction types occurring in hexagonal close-packed materials. The first results obtained from high purity Zr electron back-scattered diffraction maps reveal that twin-twin interactions hinder subsequent twin nucleation. They also show that mechanisms involved in twin growth may differ significantly for each twinning mode. A second study performed on AZ31 Mg presents statistics about low Schmid factor {10-12} tensile twins and about {10-12}-{10-12} sequential double twins coupled with a simplified version of the Tanaka-Mori scheme generalized to heterogeneous elasticity with plastic incompatibilities.
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Purcell, M. D. "An investigation of CMOS sensing circuits using hexagonal lattices." Thesis, University of Edinburgh, 2002. http://hdl.handle.net/1842/12809.
Full textAbstract:
Traditionally, colour images have been captured by image sensors with a trichromatic checkerboard colour filter using the Bayer Pattern. This filter exhibits non-optimum colour aliasing. The following thesis introduces a novel way of processing an image using a hexagonal array with trichromatic or quadrochromatic filters. This form of processing ensure that all colours are equally sampled, and that the resolution in all directions for all colour channels is practically the same. This leads to a minimisation of aliasing if the lens is optimised for the array. A CMOS camera was manufactured with a standard OS process. Using raw data captured by the camera in association with colour filters, the trichromatic and quadrochromatic colour filter array options were evaluated. The implementation of the colour processor using switched capacitors is investigated alongside the more conservative digital route. A switched capacitor readout buffer is designed and simulated to test the overall performance of the circuit. Using this information, the design based on a switched capacitor implementation for the colour processor is evaluated and rejected. The digital resources required by the new system are investigated by programming an FPGA capable of processing raw data from the hexagonal CMOS camera into a colour picture in real-time. The resulting digital resources required to reconstruct an image from a camera with hexagonal pixels is compared to those needed by the Bayer pattern. Finally since the processing of the image is going to be performed on the same die as the sensor, the supplies are going to be affected by the switching elements, whether the colour processor is implemented in switched capacitor or digital form. This thesis also investigates how the pixels react to varying supplies, and how to render them more resistant to variations in the supplies, so as to deliver a better picture at a minimum cost. Various solutions to improve the power supply rejection of the camera are investigated and simulated. The more interesting options are laid out and tested.
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Fop, Sacha. "Novel oxide ion conductors in the hexagonal perovskite family." Thesis, University of Aberdeen, 2016. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=231087.
Full textAbstract:
Oxide ion conductors have received much attention in recent years due to their application as solid oxide fuel cells (SOFC) electrolytes. A strong correlation exists between the oxide ion conductivity and the crystal structure of an oxide ion conductor. Consequently, to develop novel electrolytes it is important to discover new structural families of oxide ion conducting materials. In the present study, the electronic properties and crystal structure of the hexagonal perovskite derivative Ba3MoNbO8.5 are reported. Ionic transport number and variable oxygen partial pressure conductivity measurements evidenced that Ba3MoNbO8.5 presents solid oxide ion conduction. A bulk conductivity of 2.2 x 10-3 S cm-1 at 600 C was observed, which is comparable to other leading oxide ion conductors. Ba3MoNbO8.5 is the first hexagonal perovskite derivative to exhibit fast solid oxide ion conductivity. The Ba3MoNbO8.5 structure was described by a hybrid structural model composed by a superimposition of the 9R hexagonal perovskite and palmierite structures. (Mo/Nb)O4 units coexist with (Mo/Nb)O6 units within the structure, forming a disordered arrangement of Mo/Nb tetrahedra and octahedra. Variable temperature neutron diffraction experiments allowed determination of the structural factors at the basis of the oxide ion conduction. In particular, the flexible coordination of the Mo/Nb cations and the distortion of the Mo(1)/Nb(1) polyhedra are thought to enhance the electrical properties so that a conductivity comparable to other leading solid oxide ion conductors is observed. Study of the electrical and structural features of the Ba3Mo1 xNb1+xO8.5-x/2 (x = 0.10, 0.20, 0.30) series also evidenced that the relative ratio of (Mo/Nb)O4 tetrahedra to (Mo/Nb)O6 octahedra and the disorder of the oxygen sub lattice are important for the conduction of the Ba3MoNbO8.5 system. Impedance spectroscopy measurements on the hexagonal perovskite derivatives Ba7MoNb4O20 and Ba3WNbO8.5 showed evidence of ionic contributions in these systems. In addition, neutron diffraction experiments revealed that both Ba7MoNb4O20 and Ba3WNbO8.5 exhibit structural characteristics analogues to Ba3MoNbO8.5. The results of the present study indicate the prospect of designing new oxide ion conductors with mixed tetrahedral and octahedral d-metal units in the hexagonal perovskite family.
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Nunes, Alécio Rodrigues. "Liberação controlada de curcumina ancorada em sílica hexagonal mesoporosa." reponame:Repositório Institucional da UnB, 2013. http://repositorio.unb.br/handle/10482/13796.
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Dissertação (mestrado)—Universidade de Brasília, Instituto de Química, 2013.Submitted by Albânia Cézar de Melo (albania@bce.unb.br) on 2013-07-29T15:20:06Z No. of bitstreams: 1 2013_AlecioRodriguesNunes.pdf: 5609146 bytes, checksum: 33c6207d871738859aa97317cd43a82f (MD5)
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Os materiais mesoporosos sempre tiveram um grande destaque no desenvolvimento tecnológico, principalmente após os estudos da Mobil. A sílica hexagonal mesoporosa desenvolvida nesse estudo foi caracterizada por 1 13espectroscopia na região do Infravermelho-FTIR, RMN-H, RMN- C, DRX, EDX, Área superficial, MEV, TG, evidenciando todas as suas propriedades e a sua capacidade em ser utilizada em sistemas de liberação controlada. Para o estudo de liberação foi realizado experimento de adsorção, através do qual foi possível determinar parâmetros termodinâmicos e, assim, obter dados como ∆H, ∆G e ∆S. Todos esses parâmetros descreveram que o processo de adsorção é entropicamente e entalpicamente favorável o que possibilita o uso do material em sistema de liberação controlada de curcumina, objetivo de avaliação desse estudo. A liberação controlada foi verificada em um espectrofotômetro de UV-vis e em HPLC possibilitando assim uma metodologia de estudo e de estratégia para validação de um método. ______________________________________________________________________________ ABSTRACT
The mesoporous materials have always had a big emphasis on technological development, especially after the studies of Mobil. The hexagonal mesoporous silica developed in this study was 1 13characterized by FTIR, H NMR, C NMR, XRD, EDX, surface area, SEM, TG, showing its properties and ability to be used in controlled release systems. For the release study, it was made an adsorption experiment in order to determine thermodynamic parameters and, thus, obtain data as H, G and S. All these parameters showed that the adsorption process is entropically and enthalpically favorable, allowing the use of this material in controlled release of curcumin systems objective of evaluation this study. The controlled release was observed in a UV-VIS spectrophotometer and HPLC, enabling a methodology to validate a method.
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エディ, タユフェール, and Eddy Taillefer. "Direction of arrival estimation using hexagonal-array signal processing." Thesis, https://doors.doshisha.ac.jp/opac/opac_link/bibid/BB10290094/?lang=0, 2008. https://doors.doshisha.ac.jp/opac/opac_link/bibid/BB10290094/?lang=0.
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Pelini, Thomas. "Optical properties of point defects in hexagonal boron nitride." Thesis, Montpellier, 2019. http://www.theses.fr/2019MONTS139.
Full textAbstract:
L’objectif de cette thèse était d’explorer et de caractériser optiquement les défauts ponctuels dans le nitrure de bore hexagonal. L’étude des défauts dans ce semiconducteur revêt un intérêt fondamental à la fois pour la science des matériaux dans laquelle il joue un rôle clé de part sa nature lamellaire (matériau 2D) et sa stabilité thermique et chimique très élevées, et également dans le domaine des technologies quantiques où son grand gap (~ 6 eV) permet d’exploiter les défauts ponctuels profonds comme «atome artificiel» dans la matrice cristalline. Au cours de cette thèse, des défauts appartenant à deux gammes spectrales ont été étudiés: une première classe émettant dans le visible, et une seconde émettant dans la gamme ultraviolette.Ainsi, dans un premier temps, nous avons exploité un microscope confocal à balayage fonctionnant à l’ambiante et dans les longueurs d’onde visibles. La réalisation de cartes spatiales de photoluminescence a permis de mettre en évidence l’existence de points chauds de photoluminescence localisés, sous la limite de diffraction du microscope, et émettant autour de 600 nm (2 eV). Des mesures de corrélations temporelles de photons montre d’une part qu’il s’agit d’émetteurs quantiques uniques, et permet d’autre part de sonder la photo-dynamique de ces systèmes, en particulier aux très longues échelles de temps. Différents régimes de photo-stabilité sont observés et discutés. Enfin, l’étude en puissance a aussi été effectuée et montre qu’une part des émetteurs (~ 5%) sont photo-stables à haute puissance d’excitation optique et saturent à un taux d’émission de plusieurs millions de coups par seconde: ces défauts ponctuels constituent une source de photons uniques parmi les plus brillantes à température ambiante dans un système à l’état solide.Dans un second temps, nous avons exploré les défauts émettant dans la gamme ultraviolette. Un préalable à la maitrise et l’utilisation des défauts dans les semiconducteurs à des fins technologiques est la connaissance de leur origine chimique. Dans cette optique nous avons tout d’abord étudié les niveaux énergétiques superficiels et profonds d’échantillons de nitrure de bore hexagonal enrichis en carbone en combinant des mesures de macro-photoluminescence et de réflectivité. L’existence de nouvelles transitions optiquement actives est révélée (autour de 300 nm), et l’implication du carbone comme origine de ces transitions est discutée. L’étude approfondie de ces nouvelles émissions a requiert la réalisation d’un microscope confocal à balayage fonctionnant dans l’ultraviolet à 266 nm et à température cryogénique. Le design du microscope est détaillé, les difficultés de sa mise en oeuvre expliquées, et ses performances démontrées. Ce nouvel outil expérimental nous permet d’examiner avec précision les défauts profonds. En particulier, une étude est faite sur la corrélation spatiale de ces nouvelles raies avec celle du défaut ponctuel bien connu à 4.1 eV. Ensuite, nous avons utilisé des nouveaux échantillons dopés en carbone isotopiquement purifié comme stratégie pour déterminer la nature chimique du défaut à 4.1 eV. À travers cette tentative, nous avons mis en lumière l’inhomogénéité spatiale des caractéristiques optiques de cet émetteur. Enfin, dans la dernière partie, on tente d’isoler l’émission provenant d’un défaut unique à 4.1 eV. Pour cela, on utilise des flocons fins pré-caractérisés en microscopie électronique et contenant une faible densité d’émetteurs. Leur photostabilité est étudiéeThe purpose of this thesis was to explore and caracterize optically the point defects in hexagonal boron nitride. The study of defects in this semiconductor is of fundamental importance firstly for the material science in which it plays a key role thanks to its lamellar structure (2D material) and its high thermal and chemical stability, and secondly for the quantum nanotechnology domain where its large bandgap (~ 6 eV) allows for exploiting deep levels point imperfections as «artificial atom» in the crystal lattice. During this thesis, defects in two spectral ranges have been studied: a first family emitting in the visible wavelengths, and a second one emitting in the ultraviolet range.Firstly, we made use of a scanning confocal microscope working in ambient conditions and at visible wavelengths. The recording of photoluminescence spatial maps permited to show the existence of localised hot spot of light, under the diffraction limit of the miscroscope, and emitting around 600 nm (2 eV). Time photon-correlation measurements revealed on one hand that we were dealing with single quantum emitters, and on the other hand allowed for probing the photodynamics of those systems, in particular at very long time-scale. Various photostability regimes are observed and discussed. Last but not least, power resolved study was also performed and demonstrated that a number of the emitters (~ 5%) are photo-stable at high excitation power and saturate at few millions counts per second: those point defects are one of the brightest single-photon source at room temperature in solid-state systems.Secondly, we explored the defects in the ultraviolet spectral range. A prerequisite to the engineering of defects in semiconductors for technological applications is the knowledge of their chemical origin. With this in mind, we studied shallow and deep levels in carbon-doped hBN samples by combining macro-photoluminescence and reflectance measurements. We showed the existence of new optically-active transitions (around 300 nm) and discussed the implication of carbon in these levels. The in-depth study of these levels have required the development of a new scanning micro-photoluminescence confocal microscope operating at 266 nm under cryogenic environment. The design and performances of the optical system are described, and the experimental challenges explained in details. Using this new setup, we went further into the examination of the deep levels. In particular, a study was carried out regarding the spatial correlation between these new spectral lines and the well-known point defect at 4.1 eV. Then, we used new crystals with isotopically-purified carbon doping as a strategy to investigate the long-standing question concerning the chemical origin of the 4.1 eV defect. Through this attempt, we brought to light the spatial dependence of the optical features for this specific emitter. Last but not least, we present our work dedicated to isolate the emission of a single 4.1 eV defect. We studied the photoluminescence of thin undoped flakes, pre-characterized with an electron microscope, that contain a low density of emitters, and inspected in particular their photostability in these thin crystals
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KRISHNAMURTHY, HARIKRISHNAN N. "SQUARE AND HEXAGONAL SLOTTED SPIRAL ANTENNAS FOR 3D-NMIC." University of Cincinnati / OhioLINK, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1132018374.
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González, Pintor Sebastián. "Approximation of the Neutron Diffusion Equation on Hexagonal Geometries." Doctoral thesis, Universitat Politècnica de València, 2012. http://hdl.handle.net/10251/17829.
Full textAbstract:
La ecuación de la difusión neutrónica describe la población de neutrones de un reactor nuclear. Este trabajo trata con este modelo para reactores nucleares con geometría hexagonal. En primer lugar se estudia la ecuación de la difusión neutrónica. Este es un problema diferencial de valores propios, llamado problema de los modos Lambda. Para resolver el problema de los modos Lambda se han comparado diferentes métodos en geometrías unidimensionales, resultando como el mejor el método de elementos espectrales. Usando este método discretizamos los operadores en geometrías bidimensiones y tridimensionales, resolviendo el problema algebraica de valores propios resultante con el método de Arnoldi.
La distribución de neutrones estado estacionario se utiliza como condición inicial para la integración de la ecuación de la difusión neutrónica dependiente del tiempo. Se utiliza un método de Euler implícito para integrar en el tiempo. Cuando un nodo está parcialmente insertado aparece un comportamiento no físico de la solución, el efecto ``rod cusping'', que se corrige mediante la ponderación de las secciones eficaces con el flujo del paso de tiempo anterior. Cuando la solución de los sistemas algebraicos que surgen en el método hacia atrás, un método de Krylov se utiliza para resolver los sistemas resultantes, y diferentes estrategias de precondicionamiento se evalúan se. La primera consiste en el uso de la estructura de bloque obtenido por los grupos de energía para resolver el sistema por bloques, y diferentes técnicas de aceleración para el esquema iterativo de bloques y un precondicionador utilizando esta estructura de bloque se proponen. Además se estudia un precondicionador espectral, que hace uso de la información en un subespacio de Krylov para precondicionar el siguiente sistema. También se proponen métodos exponenciales de segundo y cuarto orden integrar la ecuación de difusión neutrónica dependiente del tiempo, donde la exponencial de la matriz del sistema tiene quGonzález Pintor, S. (2012). Approximation of the Neutron Diffusion Equation on Hexagonal Geometries [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/17829
Palancia
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Piran, Asghar. "Numerical Study on Vortex- Induced Vibration of Hexagonal Cylinders." Thesis, Griffith University, 2018. http://hdl.handle.net/10072/382678.
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Vortex-Induced Vibration (VIV) is a major concern in design of slender
structures exposed to the wind flow, such as high-rise buildings and bridges, or water
currents, such as offshore structures, marine risers, mooring elements and free spanning
pipelines. Many researchers have investigated VIV of circular and rectangular crosssections.
However, flow characteristics and VIV of hexagonal cylinders have received
limited attention so far. Vortex-induced vibration of infinitely long hexagonal cylinders
with corner and face orientations are investigated herein using two-dimensional
Computational Fluid Dynamics (CFD). The numerical model is based on the solution of
two-dimensional Reynolds-Averaged Navier–Stokes (RANS) equations in ANSYS
Fluent. The RANS equations are solved in a moving mesh system using the Arbitrary-
Lagrangian–Eulerian (ALE) approach, which allows the cylinder to move as a rigid
body in the cross-flow direction. Turbulence is incorporated using the Shear Stress
Transport (𝑆𝑆𝑇) 𝑘– 𝜔 model. The numerical model and results are validated against
published experimental and numerical results on VIV of a circular cylinder for a
Reynolds number (𝑅𝑒) of 1000. Using the validated model, VIV of hexagonal cylinders
with corner and face oriented configurations are studied at reduced velocities (𝑉𝑟)
between 2 and 12. In all models the non-dimensional mass ratio (𝑚∗) of 2, and 𝑅𝑒 of
1000 are adopted. Results are presented in the form of drag and lift forces, vortex
shedding patterns, flow velocities, pressure distributions around the cylinders, frequency of vibrations, Strouhal numbers and amplitude of vibrations. Results show
that, the cross-flow amplitudes of vibration of both corner and face-oriented hexagonal
cylinders are much lower than that of a circular cylinder of the same diameter. The
vortex shedding frequencies of the hexagonal cylinders are smaller than the circular
cylinder.Thesis (Masters)
Master of Philosophy (MPhil)
School of Eng & Built Env
Science, Environment, Engineering and Technology
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Colatosti, Marco. "Numerical analysis of homogenized hexagonal-shaped composite as Cosserat continua." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020.
Find full textAbstract:
Composite materials are utilized in a several engineering fields, they can be studied
by modelling interactions among their constituents or by homogenizing an equivalent
continuum, the first approach requires an high computational cost to model
the particles and their interactions. The aim of this work is to study the behavior
of a structure with a regular microstructure: different textures are considered to
understand the effects of the geometry on the mechanical response and it is also
evaluated the in
uence of geometry's scale.
Furthermore, another purpose is to show how different materials, such as masonry
assemblies, can be described through micropolar continua theory, wherein the continuum
has additional degrees of freedom with respect to classical elasticity and it
is able to more accurately describe heterogeneous materials taking into account size
effects.
In this thesis various numerical examples are illustrated in which various structures
are loaded in different conditions and three different mathematical models are assumed:
a discrete model, where the microstructure is formed by rigid blocks with
elastic interfaces, a Cosserat continuum model and lastly a Cauchy continuum model.
The continuum models are obtained by a homogenization technique which is able to
carry out constitutive parameters using a principle of energetic equivalence: starting
from a representative volume element (RVE) is obtained the constitutive matrix. In
literature it's possible to find a numerical tests with a rectangular shape of blocks,
so it is interesting to prove that the procedure is expandable to other geometries.
The models are studied by numerical simulation using a MATLAB FEM code and
the FEM analysis software Abaqus through the use of Python code script.
In a second moment the dynamic behavior is studied as well, comparing the frequencies
and the deformed shape of the ways to vibrate for the different models.
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Jordan, Nicola A. "Synthesis and structural chemistry of hexagonal perovskites and related compounds." Thesis, University of Oxford, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.270636.
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Hashim, Dayang Maryani Awang. "Brillouin and neutron scattering study of hexagonal ABX3 ternary halides." Thesis, Loughborough University, 1995. https://dspace.lboro.ac.uk/2134/12085.
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The interest in one dimensional (1D) magnetism has been strongly renewed with the synthesis of many magnetic compounds which exhibit a quasi one dimensional magnetic behaviour. One of the peculiarities of this 1D system is the absence of a long range magnetic ordered phase at any finite temperature for the ideal 1D system with short range interaction. Tetramethylammonium manganese chloride (CH3)4NMnCl3(TMMC) exhibits the properties of an ideal one dimensional antiferromagnets for temperature above 1 K, the transition to a three dimensional (3D) long range ordered state only occurs at 0.84K. In addition to its magnetic transition, TMMC exhibits structural phase transition due to the ordering of the tetramethylammonium (TMA) ions which makes also this compound very attractive from a lattice dynamical point of view. Structural phase transitions of tetramethylammonium manganese chloride (TMMC), tetramethylammonium manganese bromide (TMMB) and tetramethylammonium manganese chloride doped with 8% Cu (TMMC:Cu) of the hexagonal type compounds are investigated using the Brillouin scattering method. These crystals show pronounced acoustic anomalies in the region of the structural phase transition. The acoustic anomalies were observed by measuring sound velocity and hence the elastic constant can be deduced. The phase transition temperatures were observed at 129.6K and 388.6K (TMMC), 114.6K and 377.6K (TMMB) and at 108.6K and 359.6K (TMMC:Cu). The elastic constant at room temperature were C11 = 2.10 (TMMC) and C11 = 1.59 (TMMB) in units of 1010 Nm-2. The phase transition of these compounds were further investigated macroscopically using the Differential Scanning Calorimetry (DSC) method. Activation energies of TMMC, TMMC:Cu, TMMB and deuterated TMMB at the phase transition were determined using this method. The values are 70.612 kJ/mol (TMMC), 49.224 kJ/mol (TMMC:Cu), 51.747 kJ/mol (TMMB) and 69.909 kJ/mol (d12-TMMB). The elastic constant of the linear chain antiferromagnet CsNiCl3 and RbNiCl3 was also determined using the Brillouin scattering method. The room temperature measurements give C11 = 3.77 (3.71) and C33 = 5.62 (5.42) in units of 1010 Nm·2 for CsNiCl3 and RbNiCl3 respectively. The phonon dispersion curves at room temperature in the hexagonal CsFeBr3 have been studied using the inelastic neutron scattering technique. From the initial slope of the dispersion curve, the sound velocity was deduced which enable us to calculate the elastic constant of CsFeBr3 at room temperature. The values obtained are C11 = 7.33, C66 = 1.01, C33 = 2.58 and C44 = 0.56 in units of 1010 Nm·2.
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McQueen, Andrew J. "Structure-property relationships in hexagonal perovskites for microwave dielectric applications." Thesis, University of Sheffield, 2011. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.574559.
Full textAbstract:
Structure-property relationships in various 12- and IS-layer hexagonal perovskites have been investigated for use in Microwave Dielectric (MD) applications. Specifically; (i) the structural origin(s) of a peak in Er at 465 K for the 12-layer hexagonal perovskite Ba3LaNb3012, BLN, and its effect on the Temperature Coefficient of resonant Frequency, TCF; (ii) synthesis and structure-property relationships of related 12-layer hexagonal perovskites based on Ba3RENb)012 and Sr)RENbJOI2 with RE ions smaller than La3+ ; (iii) a study of the IS-layer hexagonal perovskite solid solution phase (Bas-xSrx)Nb4015 to establish if similar properties to BLN can be obtained and therefore improve our understanding of how octahedral tilting in hexagonal perovskites influences their microwave dielectric properties. (i) A study of the BLN crystal structure using mode analysis of Neutron Powder Diffraction, NPD data showed the structure to go from one with a tilted NbOIi network (space group RI) at 300 K to un-tilted (space group RIm) at 500 K. On cooling, the change in Nb06 framework at - 500 K causes displacement of the A-site cations from their ideal positions within the AO .• layers at - 465 K; it is this movement of A-site cations in response to octahedral tilting that gives rise to the peak in Er. Raman Spectroscopy, RS showed the loss of a peak (- 425 cm -I) at - 400 K that is associated with the phase transition. (ii) Ba,RENbJOl2 with RE = Pr3+ or Nd3+ did not exhibit resonance at microwave frequencies. Instead a relaxor-type response was observed from radio frequency measurements with a maximum in I::r near or below room temperature. NPD and RS data showed this behaviour is linked to size mismatch (cation variance) between the A-site cations. Sr3PrNb3012 showed encouraging microwave dielectric properties with I::r - 35, Q.f - 17,000 GHz and TCF - 8 ppm K-'. (iii) The (Bas_xSrx)Nb4015 study found materials that do show similar properties to BLN. Sr-rich members of this study vi can be described has having a tilted NbOf, network (space group PJe!) whereas the Ba- rich members contain untilted Nb06 octahedra (space group PJmJ) at room temperature. This change from a tilted to untilted structure as the Ba content increases leads to a crossover from a tilt to untilt system at room temperature for Ba2:;Sr2.sNb4015 (B2.5S2SNO). This crossover at B2.5S2.sNO is reflected in the room temperature er and TCF (Er - 47, TCF - 343 ppm K-1) which have their highest values for this composition. This provides confirmation of the effect octahedral tilting can have on MD properties. The trend in Q.f is obscured by extrinsic effects, for example as a result of an irregular ceramic microstructure (SrsNb40ls) and the presence of impurities that exist as intergrowths within the main phase. The analysis of the FWHM of M" Debye peaks from IS data was useful to detect electrical heterogeneity within the grains and could be used to link with Q.f. vii