Dissertations / Theses on the topic 'High energy deposition'

The high-latitude ionosphere is a dynamic region, in which a variety of phenomena including particle precipitation, currents and waves contribute to the energy budget. In this thesis, statistical and case studies of ion frictional heating are presented, including investigations into the dependence of enhanced ion temperature on time and altitude. The relationship between parallel ion temperature and ion velocity is compared to simplified forms of the ion energy balance equation. In addition, the generation mechanisms of atmospheric gravity waves are studied by means of measurements made during the WAGS campaign of October 1985. The results indicate that auroral precipitation can influence frictional heating events to a greater extent than has previously been realised and that during frictional heating the molecular content of the lower ionosphere is enhanced, affecting the electron density. Any analysis which takes no account of the modified composition underestimates the parallel ion temperature, particularly between 200 and 300 km altitude. The relationship between ion velocity and parallel ion temperature is most easily explained by an anisotropic ion velocity distribution, consistent with resonant charge exchange collisions. The relationship varies with altitude, however, possibly due to ion-ion collisions. An experimental method is described by which the temperature anisotropy can be obtained directly and early results are discussed. For the investigation of atmospheric gravity waves and their sources, HF Doppler observations in the UK enabled wave speeds and azimuths to be deduced, whilst EISCAT simultaneously observed the possible source region. Although the study was characterised by moderate activity, more active days showed higher phase speeds and southerly azimuths. Some of these waves may have originated at high latitudes during positive bay activity, when both Joule heating and the Lorentz force contributed to wave generation.

Thesis (M.S.)--Missouri University of Science and Technology, 2010.
Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed Jan. 5, 2010). Includes bibliographical references (p. 52-53).

La filamentation laser est un régime de propagation optique spectaculaire atteint pour des impulsions dont la puissance crête excède quelques gigawatts dans l’air. Le filament se forme sous l’action de l’effet Kerr optique du milieu traversé qui tend à auto-focaliser le faisceau jusqu’à ce que l’intensité résultante atteigne le seuil d’ionisation du milieu par absorption multiphotonique. Une compétition dynamique complexe s’établit alors entre l’effet Kerr, d’une part, et la diffraction, l’absorption non-linéaire de l’énergie laser et l’effet défocalisant du plasma d’autre part. Il en résulte une réorganisation du profil du faisceau, caractérisée par un coeur mince (100 µm) et intense (10^18 W/m²) pouvant se maintenir sur une distance égale à plusieurs longueurs de Rayleigh. Lorsque la puissance initiale de l’impulsion dépasse largement le seuil de filamentation, on assiste à la formation de plusieurs filaments co-propagatifs au sein du même faisceau, chacun de ces multifilaments possédant des caractéristiques physiques proches de monofilaments isolés. Au cours de sa propagation dans l’air, le filament transfère une partie de l’énergie laser au milieu, principalement via l’excitation rotationnelle Raman des molécules d’air, l’ionisation de l’air et l’effet de Bremsstrahlung inverse au sein du plasma. Cette énergie est redistribuée au cours de la nanoseconde suivant le passage du laser, principalement sous forme d’énergie translationnelle des molécules d’air, c’est-à-dire de chaleur. Le milieu réagit à ce chauffage rapide par la formation d’une onde de pression cylindrique, qui ramène le système à l’équilibre de pression en éjectant de la matière du centre. Il en résulte la formation d’un canal d’air sous-dense et chaud, qui se résorbe par diffusion à des échelles de temps supérieures à la milliseconde. Ma thèse s’est en premier lieu focalisée sur l’étude et l’optimisation du dépôt d’énergie dans l’air par filamentation. J’ai ainsi étudié l’influence des différents paramètres laser, comme l’énergie de l’impulsion, la focalisation employée et la durée d’impulsion sur la densité d’énergie déposée. Pour ce faire, j’ai employé plusieurs diagnostics complémentaires : mesure des ondes de pression à l’aide de microphones, analyse du plasma de filament par spectroscopie et mesure résolue en temps des canaux sous-dense par interférométrie. J’ai ainsi montré en régime de monofilamentation qu’au-delà d’une certaine énergie laser initiale, le dépôt d’énergie devient si important qu’une onde de choc est générée en lieu et place d’une onde sonore, et que les canaux sous-denses résultant ont des durées de vie de l’ordre de 100 ms. J’ai également étudié et caractérisé le régime de multifilamentation à haute énergie, montrant qu’en focalisant modérément l’impulsion, les filaments se réorganisent dans la zone focale pour former des structures plus larges générant un plasma dix fois plus dense que les filaments. Les effets hydrodynamiques engendrés par filamentation entraînent un abaissement transitoire du seuil de claquage électrique de l’air le long du trajet de l’impulsion laser, permettant ainsi de déclencher et de guider des décharges électriques. La seconde partie de ma thèse avait pour objet l’étude et l’optimisation de telles décharges guidées pour la mise au point d’une antenne plasma radio-fréquence, de commutateurs haute tension sans contact ou encore d’un paratonnerre laser. Pour ce faire, j’ai développé et construit un diagnostic plasma interférométrique à deux couleurs permettant de caractériser la durée de vie des plasmas générés. J’ai également participé à une expérience de principe démontrant la possibilité de réaliser une antenne plasma RF à partir d’un filament laser. Enfin, j’ai participé à diverses études expérimentales prospectives dans l’optique du développement d’un paratonnerre laser
Laser filamentation is a spectacular optical propagation regime appearing for pulses of which peak power exceeds a few GW in air. Filament forms due to the optical Kerr effect, which tends to self-focus the beam until intensity reaches the medium ionization threshold by multiphoton absorption. A complex dynamic competition is then established between the Kerr effect on the one hand, and diffraction, nonlinear absorption and plasma defocusing effect on the other hand. This results in a reorganization of the beam profile, characterized by a thin (100 µm) and intense (10^18 W/m²) core able to propagate over a distance much longer than the Rayleigh length. When the initial pulse peak power largely exceeds filamentation threshold, several co-propagating filaments are formed in the same beam, with each of these multifilaments sharing physical properties of isolated single filaments. While propagating in air, filaments transfer a portion of the laser energy to the medium, mainly through Raman rotational excitation of air molecules, ionization and inverse Bremsstrahlung in the plasma. This energy is redistributed in one nanosecond and almost entirely converted into air molecule translational energy, that is heat. The medium reacts to this rapid heating by launching a cylindrical pressure wave that brings the system back to pressure equilibrium by ejecting matter from the center. This results in the formation of a hot underdense air channel, which slowly resorbs by diffusion at timescales > 1 ms. My work as a Ph. D. Student first focused on the study and the optimization of laser energy deposition in air by filamentation. Thus, I investigated the influence of laser parameters such as pulse energy, focusing strength or pulse duration on deposited energy. To this purpose, I used several complementary diagnostics: study of pressure waves using microphones, characterization of the filamentation plasma by means of spectroscopy and time resolved study of underdense air channels using interferometry. I demonstrated in the single filamentation regime that above a given pulse energy, energy deposition becomes so important that the medium generates a shock wave instead of a sound wave, and that underdense channels can last for more than 100 ms. I also studied and characterized the high energy multifilamentation regime, showing that moderately focusing the pulse leads to a reorganization of filaments in the focal zone, generating large structures with a resulting plasma ten times denser than filaments. Filamentation-induced hydrodynamic effects lead to a transient reduction of the air breakdown voltage along the path of the laser pulse, enabling one to trigger and guide electric discharges. The second part of my thesis focused on the study and the optimization of such guided discharges for the design of a radio-frequency plasma antenna, contactless high-voltage switches or a laser lightning rod. To this purpose I developed and built an interferometric plasma diagnostic, allowing to measure the lifetime of generated plasmas. I also contributed to the proof of principle for a filament induced plasma antenna emitting RF signal. Finally, I took part to prospective experimental studies for the development of a laser lightning rod

Technological progress is determined to a great extent by developments of novel materials from new combinations of known substances with different dimensionality and functionality. We investigate the development of 3D ‘hybrid’ nanomaterials by utilizing graphene based systems coupled with transition metal oxides (e.g. manganese oxides MnO2 and Mn3O4). This lays the groundwork for high performance electrochemical electrodes for alternative energy owing to their higher specific capacitance, wide operational window and stability through charge-discharge cycling, environmental benignity, cost effective, easily processed, and reproducible in a larger scale. Thus far, very few people have investigated the potential of combining carbon sheets that can function as a supercapacitor in certain systems with transition metals that have faradaic properties to create electrochemical capacitors. Previous work by Wang et al. has focused on the structural combination of Mn3O4 and graphene based materials,1 and research by Jafta et al. studied the electrochemical properties of MnO2 with GO.2 We find that both physical and chemical attachment of manganese oxide on graphene allows for electrical interplay of the materials as indicated in electrochemical analysis and Raman spectroscopy. Attachment of the two materials is also characterized by scanning electron microscopy and X-ray diffraction.

Increasing worldwide demand for petroleum motivates greater efficiency, safety, and environmental responsibility in upstream oil and gas processes. The objective of this research is to improve these areas with advanced control methods. This work develops the integration of optimal control methods including model predictive control, moving horizon estimation, high fidelity simulators, and switched control techniques applied to subsea riser slugging and managed pressure drilling. A subsea riser slugging model predictive controller eliminates persistent offset and decreases settling time by 5% compared to a traditional PID controller. A sensitivity analysis shows the effect of riser base pressure sensor location on controller response. A review of current crude oil pipeline wax deposition prevention, monitoring, and remediation techniques is given. Also, industrially relevant control model parameter estimation techniques are reviewed and heuristics are developed for gain and time constant estimates for single input/single output systems. The analysis indicates that overestimated controller gain and underestimated controller time constant leads to better controller performance under model parameter uncertainty. An online method for giving statistical significance to control model parameter estimates is presented. Additionally, basic and advanced switched model predictive control schemes are presented. Both algorithms use control models of varying fidelity: a high fidelity process model, a reduced order nonlinear model, and a linear empirical model. The basic switched structure introduces a method for bumpless switching between control models in a predetermined switching order. The advanced switched controller builds on the basic controller; however, instead of a predetermined switching sequence, the advanced algorithm uses the linear empirical controller when possible. When controller performance becomes unacceptable, the algorithm implements the low order model to control the process while the high fidelity model generates simulated data which is used to estimate the empirical model parameters. Once this online model identification process is complete, the controller reinstates the empirical model to control the process. This control framework allows the more accurate, yet computationally expensive, predictive capabilities of the high fidelity simulator to be incorporated into the locally accurate linear empirical model while still maintaining convergence guarantees.

The direct epitaxial growth of multilayer BN by atomic layer deposition is of critical significance forfo two-dimensional device applications. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) demonstrate layer-by-layer BN epitaxy on two different substrates. One substrate was a monolayer of RuO2(110) formed on a Ru(0001) substrate, the other was an atomically clean Ni(111) single crystal. Growth was accomplished atomic layer deposition (ALD) cycles of BCl3/NH3 at 600 K substrate temperature and subsequent annealing in ultrahigh vacuum (UHV). This yielded stoichiometric BN layers, and an average BN film thickness linearly proportional to the number of BCl3/NH3 cycles. The BN(0001)/RuO2(110) interface had negligible charge transfer or band bending as indicated by XPS and LEED data indicate a 30° rotation between the coincident BN and oxide lattices. The atomic layer epitaxy of BN on an oxide surface suggests new routes to the direct growth and integration of graphene and BN with industrially important substrates, including Si(100). XPS and LEED indicated epitaxial deposition of h-BN(0001) on the Ni(111) single crystal by ALD, and subsequent epitaxially aligned graphene was deposited by chemical vapor deposition (CVD) of ethylene at 1000 K. Direct multilayer, in situ growth of h-BN on magnetic substrates such as Ni is important for spintronic device applications. Solid-state electrolytes (SSEs) are of significant interest for their promise as lithium-ion conducting materials but are prone to degradation due to lithium carbonate formation on the surface upon exposure to atmosphere, adversely impacting Li ion conduction. In situ XPS monitored changes in the composition of the SSE Li garnet (Li6.5La3Zr1.5Ta0.5O12, LLZTaO) upon annealing in UHV and upon Ar+ ion sputtering. Trends in core level spectra demonstrate that binding energy (BE) calibration of the Li 1s at 56.4 eV, yields a more consistent interpretation of results than the more commonly used standard of the adventitious C 1s at 284.8 eV. Annealing one ambient-exposed sample to >1000 K in UHV effectively reduced surface carbonate and oxygen, leaving significant amounts of carbon in lower oxidation states. A second ambient-exposed sample was subjected to 3 keV Ar+ ion sputtering at 500 K in UHV, which eliminated all surface carbon, and reduced the O 1s intensity and BE. These methods present alternative approaches to lithium carbonate removal than heating or polishing in inert atmospheres and are compatible with fundamental surface science studies. In particular, the data show that sputtering at mildly elevated temperatures yields facile elimination of carbonate and other forms of surface carbon. This is in contrast to annealing in either UHV or in noble gas environments, which result in carbonate reduction, but with significant remnant coverages of other forms of carbon.

A significant effort has been made recently to develop second-generation HTSC tapes. In these tapes, ReBaCuO (rare-earth barium copper oxide - YBCO) thin films are produced on metallic substrates, such as textured Ni, NiW alloys and stainless steel. To prevent the interdiffusion of elements between metal substrate and superconducting material, and to match the YBCO lattice parameters with the substrate texture, different buffer layers were deposited on the substrate. In commercially available HTSC tapes, several buffer layers are typically used to obtain high-quality YBCO superconductor coatings (i.e. appropriate texture, defect-free and with a high critical current density, Jc,). Many existing HTS tape technologies use a variety of buffer layer architecture, which include YSZ, MgO, Y2O3 and CeO2 nm-thick layers and their combinations.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Engineering
Science, Environment, Engineering and Technology
Full Text

Etude de la cinetique et des mecanismes d'oxydation thermique de films dopes a 210**(20) cm**(-3) b "in situ" et par implantation post-depot. Les resultats prennent en compte les influences du mode de dopage et de la microstructure initiale des films (qui varie d'un etat quasi-amorphe a un etat nettement polycristallin). Leur analyse s'appuie sur a) un logiciel de modelisation de l'oxydation de si et la comparaison avec la cinetique d'oxydation de temoins monocristallins qui permettent de les exprimer en termes de constante de diffusion d et de la vitesse de reaction de surface k::(s) de l'oxydation, b) la comparaison entre depots non dopes, dopes a b par implantation ou "in situ" et c) le suivi des proprietes structurales (rugosite, diagrammes rheed, observations tem) et electroniques des films

碩士
國立清華大學
原子科學系
92
Recently, high energy radiation has been used for medical diagnosis and treatment, and the standard dosimetry is defined as air kerma and absorbed dose to water. Because human’s body contains 70% water, so it can be assumed that body is water equivalent. To measure the absorbed dose to water in medical radiation oncology, an ion chamber calibrated with colbat-60 irradiation is used together with a phantom to measure the electric charge after being exposed to high energy radiation, and then according to AAPM No.21 or No.51,the human’s body dose can be calculated. Many research workers have investigated the glow curve and dynamic of CVD diamond film fabricated with chemical vapour deposition methods(CVD method), and they have also used computer code for glow curve fitting and identification of the light source interference from radiation. In practice, the glow curve must be analyzed by computer, in order to obtain the dose. This work is to study CVD diamond as a tool for radiation dosimetry, and to find the reading modular similar to TLD which can then be used in laboratories. The main component of CVD diamond film is carbon, its atomic number is 6 which is close the mean atomic number of 7.4 for water, it can be used as a tissue equivalent material, and is convenient to measure absorbed dose to water. Results of experiment show that the difference between absolute dose measured and dose profile obtained is about ±10%. The CVD diamond is a practical material for high-energy radiation measurements.

High capacitance devices were developed using rapid electrophoretic deposition (EPD) of barium titanate (BaTiO3) on ultrasonically etched nickel (Ni) substrates. The microstructural and electrical properties of films with varying thicknesses, sintering temperatures and substrate etching times were investigated to study their effect on the capacitance. Although increasing the capacitance was the primary goal, decreasing manufacturing costs and reducing environmental impact was also considered. After confirming the tetragonality and particle size of a 0.2 µm hydrothermal powder, it was dispersed in an aqueous-organic mixture of ethanol, acetone and water. A zeta potential of 50 mV was observed at the EPD pH level (6.8). Flocculation or coagulation was not likely in this situation. Therefore, mixing water with an organic solution was an effective method for reducing environmental impact while maintaining deposition quality. The presence of BaCO3 in the films was proven using X-ray diffraction. Other impurities such as TiO2 and NiO were not detected. A significant variation in the average grain size was not observed for films with different thicknesses whereas films sintered at different temperatures displayed greater variation. A bimodal pore size distribution in the samples suggested that the powder was agglomerated after deposition due to a high deposition voltage (20 V). Rapid deposition times of 2 to 8 seconds offered a potential for cost reduction compared to longer deposition times implemented in industry. Therefore the increased porosity was accepted. The dielectric constant of the films increased from 2900 to 6730 as the thickness increased from 17.75 µm to 47.5 µm. The dissipation factor decreased from 0.27 to 0.06 with increasing thickness. This behavior was attributed to a low dielectric constant interfacial layer and a higher dielectric leakage current at smaller thicknesses. The dielectric constant increased from 1700 to 6350 and the dissipation factor decreased from 0.23 to 0.04 as the sintering temperature increased from 1150°C to 1300°C. This was attributed to an increase in tetragonality with grain size and a decrease in porosity with sintering temperature. Finally, etching a substrate for 60 seconds increased its capacitance by 27.47% and using ultrasonic agitation further increased the capacitance by 8.75%.
Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-10-12 00:54:53.915

University of Technology Sydney. Faculty of Engineering and Information Technology.
Energy storage systems (ESSs) play a critical role in plenty of applications including renewable energy systems, power systems for electric vehicles (EVs) and hybrid electric vehicles (HEVs), and electrical power grids for improving reliability and overall use of the entire system. Currently, there are several types ESSs dominated the energy storage. Each kind of ESSs has their own operation mechanism, energy efficiency, energy density, power density, cycle life, charge and discharge capability, cost efficiency, operating temperature. The common ESS is based on lead acid battery which stores electrical energy in the form of chemical energy. However, if the batteries are overdischarged or kept at a discharged state, its capability will be irreversibly undermined because the sulfate crystals become larger and more difficult to break up during recharge. Since the first NiCd battery was created by Waldemar Jungner in 1899, even though NiCd battery technologies have experienced a series of evolutionary developments, its demerits are obvious including 1) shorter life cycle; 2) memory effect; 3) toxicity of Cd; 4) lower energy density; and 5) limited negative temperature coefficient. Based on the development of NiCd battery technology, nickel metal hydride (NiMH) batteries was proposed by researchers which possess better performance than NiCd batteries in cycle life, energy density and charge&discharge rates. Lithium ion is the preferred chemistry, having a superior specific energy and power density to nickel metal hydride. More lithium per gram stored in the electrodes contributes to higher energy density and power density. In addition to chemical battery system, researchers recently proposed some new sorts of ESSs including flywheel, compressed air energy storage (CAES), superconductive magnetic energy storage (SMES), etc. All of them can provide super energy density and power density. But they are more or less blocked ether in complex mechanical construction or cooling device. Supercapacitor has emerged to be an exciting energy storage device, which is able to provide high specific power, charge and discharge up to million times, have long lifetime and broad range of working temperature. Even though supercapacitor has been widely seen as a promising energy storage candidate to replace the traditional chemical batteries, it also suffer its drawback that the low energy density (the energy stored in per unit of volume and weight), high equivalent series resistance (ESR) and its high cost associated with its performance. Therefore, this PHD thesis project aims to address these drawbacks of supercapacitor by designing different nanotechnologies and fabrication methods to synthesize advance materials with better performance than that of conventional supercapacitor. A Series of designed structures and materials were fabricated by designed methods. All the materials were also investigated by using X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) observation techniques, Brunauer–Emmett–Teller (BET) surface area measurement and electrochemical testing. A facile and effective hydrothermal treatment that is able to control the condensation speeds of precursors in the solution along the <010>, <100> and <001> directions was designed to fabricate vanadium oxide nanoribbon used for the electrode of supercapacitor. It was achieved by controlling the hydrothermal reaction time and the weight ratio to synthesize the ultralong vanadium oxide nanoribbon with controlled width. It has high specific capacitance of 453 F g⁻¹ at the scam rate of 2 mV s⁻¹ in 2 M NaCl electrolyte, and it still maintained a high capacitance of 201 F g⁻¹ at a higher scan rate of 50 mV s⁻¹, attributing to the easy ion insertion and electronic transport along the a-b plane rather through the layers of the c-axis. Vanadium oxide nanotubes were synthesized by a revised hydrothermal treatment with high-speed stirring. The preparation involved dissolution of V₂O₅ into H₂O₂ and high-speed stirring (10000 r/min) with hexadecylamine. The product was characterized by scanning electron microscopy, transmission electron microscope, X-ray diffraction and thermogravimetric analysis. The electrochemical properties of the materials as electrodes for electrochemical capacitors were evaluated by cyclic voltammetry in a three electrode system consisting of a saturated calomel electrode as reference electrode, platinum as a counter electrode and the active materials as the working electrode. A high capacitance of 148.5 F g⁻¹ was obtained at a scan rate of 2 mV s⁻¹ in 2M KCl. The electrode maintained a high capacitance of 105 F g⁻¹ at a higher scan rate of 50 mV s⁻¹ in 2M KCl electrolyte. 3D mesoporous hybrid NiCo₂O₄@graphene nanoarchitectures were successfully synthesized by a combination of freeze drying and hydrothermal reaction. Field-emission scanning electron microscopy (FESEM) and TEM analyses revealed that NiCo₂O₄@graphene nanostructures consist of a hierarchical mesoporous sheet-on-sheet nanoarchitecture with a high specific surface area of 194 m² g⁻¹. Ultrathin NiCo₂O₄ nanosheets, with a thickness of a few nanometers and mesopores ranging from 2 to 5 nm, were wrapped in graphene nanosheets and formed hybrid nanoarchitectures. When applied as electrode materials in supercapacitors, hybrid NiCo₂O₄@graphene nanosheets exhibited a high capacitance of 778 F g⁻¹ at the current density of 1 A g⁻¹, and an excellent cycling performance extending to 10000 cycles at the high current density of 10 A g⁻¹. We also presented a rational, large-scale and general method, called controllable freeze casting (CFC), to fabricate a high-densely assembled and aligned free-standing NiCo₂O₄@graphene 3D foam by vacuum filtration and air compress pressure assembly method. In the designed method, the amount of water is controllable, therefore controlling the size and the shape of the ice when the material was introduced into freeze drying system, finally achieving controllable pore size and aligned structure. This free-standing foam retains the intrinsic properties of graphene sheet, such as high surface area and high electrical conductivity. In the foam, the graphene sheets build the high conductive skeletons. And the skeletons with high surface areas support the uniform distribution of NiCo₂O₄ nanoparticles on the graphene sheets. By controlling the amount of water in the precursor, it is possible to fabricate 3D NiCo₂O₄@graphene foams with a wide range of thickness and pore size. This dense NiCo₂O₄@graphene material exhibited a high capacitance of 790 F g⁻¹ at a current density of 2 A g⁻¹, and an excellent cycling performance at a high current density of 10 A g⁻¹. The compression test revealed that the 3D NiCo₂O₄@graphene foam exhibited strong mechanical property which is able to support 20,000 times its own weight without structure collapsing. The novel synthesis method of such 3D foam with excellent properties paves the way to explore the application of lamellar materials like graphene in a self-supporting, metal oxide deposition and 3D foam.

To facilitate ferroelectric-based actuator integration with silicon electronics fabrication technology, we have developed a route to produce biaxially textured ferroelectrics on amorphous layers by using biaxially textured MgO templates.

Using a kinematical electron scattering model, we show that the RHEED pattern from a biaxially textured polycrystalline film can be calculated from an analytic solution to the electron scattering probability. We found that diffraction spot shapes are sensitive to out-of-plane orientation distributions and in-plane RHEED rocking curves are sensitive to the in-plane orientation distribution. Using information from the simulation, a RHEED-based experimental technique was developed for in situ measurement of MgO biaxial texture. The accuracy of this technique was confirmed by comparing RHEED measurements of in-plane and out-of-plane orientation distribution with synchrotron x-ray rocking curve measurements.

Biaxially textured MgO was grown on amorphous Si3N4 by ion beam-assisted deposition (IBAD). MgO was e-beam evaporated onto the amorphous substrate with a simultaneous 750-1200 eV Ar⁺ ion bombardment at 45° from normal incidence. We observed a previously unseen, dramatic texture evolution in IBAD MgO using transmission electron microscopy (TEM) and RHEED-based quantitative texture measurements of MgO. The first layers of IBAD MgO are diffraction amorphous until the film is about 3.5 nm thick. During the next 1 nm of additional growth, we observed rapid biaxial texture evolution. RHEED and TEM studies indicate that biaxially textured MgO film results from a solid phase crystallization of biaxially textured MgO crystals in an amorphous matrix.

Biaxially textured perovskite ferroelectrics were grown on biaxially textured MgO templates using sol-gel, metallorganic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). Through RHEED-based biaxial texture analysis we observed that the heteroepitaxial ferroelectric in-plane orientation distribution, deposited using ex situ techniques (not performed in the same high vacuum growth environment where the MgO was deposited), narrowed significantly with respect to the in-plane orientation distribution of its MgO template (from 11° to 6° FWHM). Evidence from cross section TEM and RHEED suggest that atmospheric moisture degrades the crystallinity of highly defective, misaligned MgO grains and that heteroepitaxially grown ferroelectrics preferentially nucleate on well-aligned grains and over grow misaligned regions of MgO.

Στη παρούσα Διατριβή διερευνήθηκε η χρήση υλικών υψηλής διηλεκτρικής σταθεράς (high-k) ως οξειδίων ελέγχου σε διατάξεις παγίδευσης φορτίου τύπου MONOS (Μetal-Οxide-Νitride-Οxide-Silicon). Τα οξείδια που εξετάστηκαν ήταν το HfO2, τo ZrO2 και το Al2O3. Η ανάπτυξή τους πραγματοποιήθηκε με χρήση της μεθόδου εναπόθεσης ατομικού στρώματος (ALD). Οι ιδιότητες των δομών μνήμης μελετήθηκαν συναρτήσει: (α) των πρόδρομων μορίων της εναπόθεσης για τα HfO2 και ZrO2, (β) του οξειδωτικού μέσου της εναπόθεσης για την περίπτωση του Al2O3 και (γ) της επακόλουθης ανόπτησης. Η ηλεκτρική συμπεριφορά των δομών εξετάστηκε με την κατασκευή πυκνωτών τύπου MOS. Τα υμένια του HfO2 αναπτύχθηκαν επί διστρωματικής στοίβας SiO2/Si3N4 με (α) αλκυλαμίδιο του χαφνίου (ΤΕΜΑΗ) και Ο3 στους 275 oC, και (β) κυκλοπενταδιενύλιο του χαφνίου (HfD-04) και Ο3 στους 350 οC. Ομοίως, τα υμένια του ZrO2 αναπτύχθηκαν επί διστρωματικής στοίβας SiO2/Si3N4 με: (α) αλκυλαμίδιο του ζιρκονίου (ΤΕΜΑΖ) και Ο3 στους 275 oC και (β) κυκλοπενταδιενύλιο του ζιρκονίου (ZrD-04) με Ο3 στους 350 oC. Ο δομικός χαρακτηρισμός, για το HfO2, φανέρωσε πως η ύπαρξη ή όχι κρυσταλλικού χαρακτήρα και η σύσταση του οξειδίου εξαρτάται τόσο από το πρόδρομο μόριο αλλά και από την ανόπτηση (600 οC, 2 min). Αντίθετα, το ZrO2 έχει σε κάθε περίπτωση κρυσταλλικότητα. Τα ηλεκτρικά χαρακτηριστικά των πυκνωτών Si/SiO2/Si3N4/high-k/Pt, δείχνουν ότι οι δομές έχουν ικανοποιητική συμπεριφορά ως στοιχεία μνήμης αφού όλες οι ιδιότητες πληρούν τις βασικές προϋποθέσεις ως στοιχεία μνήμης, παρά την ανυπαρξία ενεργειακού φραγμού μεταξύ στρώματος παγίδευσης και οξειδίου ελέγχου. Η ικανότητα παγίδευσης και η επίδοση των δομών με HfO2 και ZrO2 δεν διαφοροποιούνται σημαντικά με χρήση διαφορετικού πρόδρομου μορίου ή με την ανόπτηση. Ο έλεγχος όμως της αντοχής των δομών σε επαναλαμβανόμενους παλμούς εγγραφής/διαγραφής αναδεικνύει ότι αμφότερες οι δομές που ανεπτύχθησαν με βάση το κυκλοπενταδιενύλιο έχουν μειωμένη αντοχή ηλεκτρικής καταπόνησης. Τo Al2O3 αναπτύχθηκε χρησιμοποιώντας το μόριο ΤΜΑ και ως οξειδωτικό μέσο: (α) H2O, (β) O3 και (γ) Plasma Ο2 (μέθοδος PE-ALD) σε συνδυασμό με ΤΜΑ. Οι δομές στην αρχική κατάσταση, χωρίς ανόπτηση, χαρακτηρίζονται από ισχυρό ρεύμα έγχυσης ηλεκτρονίων από την πύλη (υπό αρνητικές τάσεις) περιορίζοντας την ικανότητα φόρτισης και την επίδοση διαγραφής. Η ανόπτηση σε φούρνο και αδρανές περιβάλλον (850 ή 1050 oC, 15 min) προκάλεσε σημαντική βελτίωση των ηλεκτρικών χαρακτηριστικών των δομών λόγω του σημαντικού περιορισμού του παραπάνω φαινομένου. Μετά το στάδιο της ανόπτησης οι συνδυασμοί ΤΜΑ/Η2Ο και ΤΜΑ/Plasma Ο2 έχουν καλύτερες χαρακτηριστικές σε σχέση με αυτές του συνδυασμού ΤΜΑ/Ο3. Το φαινόμενο της διαρροής ηλεκτρονίων από την πύλη αποδίδεται στη μεγάλη συγκέντρωση και χωρική κατανομή του υδρογόνου στο υμένιο υψηλής διηλεκτρικής σταθεράς. Τέλος, διερευνήθηκε η τροποποίηση των ιδιοτήτων μνήμης των δομών με εμφύτευση ιόντων αζώτου χαμηλής ενέργειας και υψηλής δόσης στο Al2O3 και επακόλουθη ανόπτηση υψηλής θερμοκρασίας. Η παρουσία αζώτου στο υμένιο καθώς και ο χημικός δεσμός του εμφυτευμένου αζώτου είναι συνάρτηση της θερμοκρασίας ανόπτησης. Επομένως, οι ιδιότητες μνήμης εξαρτώνται από τη μορφή σύνδεσης και την συγκέντρωση του εμφυτευμένου αζώτου στο τροποποιημένο Al2O3. Η υψηλή θερμοκρασία ανόπτησης (1050 οC, 15 min) φαίνεται να αποφέρει δομές με τις καλύτερες ιδιότητες μνήμης.
This thesis studies the functionality of high-k oxides as blocking oxide layers in SONOS type charge-trap memory devices. The oxide materials that were examined were the HfO2, the ZrO2 and the Al2O3. All these blocking oxide layers were deposited by atomic layer deposition technique (ALD). The electrical performance of the trilayer stacks was examined using Pt-gate MOS-type capacitors. The properties of the memory structures were examined as a function of: (a) precursor chemistry of HfO2 and ZrO2 deposition, (b) the deposition oxidizing agent in the case of Al2O3 and (c) subsequent high temperature annealing steps. The HfO2 films were deposited on SiO2/Si3N4 bilayer stacks using: (a) hafnium alkylamide (TEMAH) and O3 at 275 oC, and (b) hafnium cyclopentadienyl (HfD-04) and O3 at 350 oC. Similarly the ZrO2 films were deposited by (a) zirconium alkylamide (TEMAZ) and O3 at 275 oC, and (b) zirconium cyclopentadienyl (ZrD-04) and O3 at 350 oC The structural characterization of the HfO2 showed that the crystallinity of the deposited high-k material depends on the precursor choice and the post deposition annealing step (600 °C, 2 min). On the contrary ZrO2 is deposited in a crystalline phase independent of the deposition conditions and the choice of the precursors. The electrical characterization of Si/SiO2/Si3N4/high-k/Pt capacitors showed that all fabricated structures operate well as memory elements, despite the absence of an energy barrier between the trapping layer and control oxide. The trapping efficiency and the performance of structures with HfO2 or ZrO2 blocking layers do not revealed a dependence upon the precursor chemistry. However, endurance testing using continuous write/erase pulses showed that both structures deposited by cyclopentadienyl precursors cannot sustain the resulting electrical stress. The Al2O3 layers were deposited using the TMA molecule while three different oxidizing agents were used: (a) H2O, (b) O3 and (c) oxygen plasma. Electrical testing of the resulting Pt-gate trilayer capacitors showed that in the deposited condition all three samples were characterized by gate electrode induced electron leakage currents in the negative bias regime, which completely masked the substrate hole injection effects. This effect limits the performance and the functionality of the memory stacks. After a high temperature annealing step (850 or 1050 oC, 15 min) this leakage current is reduced significantly and the stacks can function as memory elements. The results point to suggest that after annealing the best performance is exhibited by the TMA/H2O and TMA/Plasma O2 samples. The effect of gate induced electron leakage current is attributed to hydrogen related contamination, which has been verified by ToF-ERDA in depth profile measurements, at least for the case of TMA/H2O samples. The modification of the memory properties of the SiO2/Si3N4/Al2O3 stacks was also investigated using low energy and high fluence nitrogen implantation into Al2O3 layer. The concentration and the chemical bonding of the implanted nitrogen is a function of annealing temperature. The memory properties of the stack depend therefore on the chemical bonding and the concentration of the remaining nitrogen in the modified Al2O3. The high temperature annealing (1050 oC, 15 min) appears to provide the structures with improved memory properties in terms of retention and fast erase performance.

The Large Hadron Collider (LHC) at CERN is designed to search for new physics by colliding protons with a center-of-mass energy of 13 TeV. The ATLAS detector is a multipurpose particle detector built to record these proton-proton collisions. In order to improve sensitivity to new physics at the LHC, luminosity increases are planned for 2018 and beyond. With this greater luminosity comes an increase in the number of simultaneous proton-proton collisions per bunch crossing (pile-up). This extra pile-up has adverse effects on algorithms for clustering the ATLAS detector's calorimeter cells. These adverse effects stem from overlapping energy deposits originating from distinct particles and could lead to difficulties in accurately reconstructing events. Machine learning algorithms provide a new tool that has potential to improve clustering performance. Recent developments in computer science have given rise to new set of machine learning algorithms that, in many circumstances, out-perform more conventional algorithms. One of these algorithms, convolutional neural networks, has been shown to have impressive performance when identifying objects in 2d or 3d arrays. This thesis will develop a convolutional neural network model for calorimeter cell clustering and compare it to the standard ATLAS clustering algorithm.
Graduate