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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Electrical and optical characterization of beta-Ga2O3

Fiedler, Andreas 03 January 2020 (has links)
Diese Arbeit beschäftigt sich mit der Bewertung des Breitband-Halbleiters ß-Ga2O3 für die Hochleistungselektronik. Daher sind Schichten, die mit metallorganischer Gasphasenepitaxie (MOVPE) gewachsen sind, und Volumenkristalle, die mit der Czochralski-Methode gewachsen sind, elektrisch und optisch charakterisiert. Dabei werden die grundlegenden Eigenschaften des Materials untersucht und mit den theoretischen Vorhersagen verglichen. Der Einfluss und die Bildung von Defekten werden untersucht. Zu Beginn zeigten die MOVPE-gewachsenen Schichten ungünstige elektrische Eigenschaften, da sie bei niedrigeren Dotierungskonzentrationen vollständig kompensiert wurden und bei höheren Ladungsträgerkonzentrationen eine geringere Ladungsträgerbeweglichkeit aufwiesen. Ein quantitatives Modell des schädlichen Einflusses inkohärenter Zwillingsgrenzen auf elektrische Eigenschaften wird entwickelt, das zeigt, dass die Verhinderung der Bildung von diesen der Schlüssel zur Verbesserung des Materials ist. Die Dichte der inkohärenten Zwillingsgrenzen wurde um 4 Größenordnungen reduziert, was zu einer verbesserten Ladungsträgerbeweglichkeit führte. Dies bietet eine vielversprechende Perspektive für den Einsatz von ß-Ga2O3 in zukünftiger Leistungselektronik. Ramanspektroskopische Untersuchungen an hoch n-dotierten Kristallen zeigen die Bildung eines Störstellenbandes, geben Einblicke in die effektivmasseartige Donatornatur von Si und Sn und zeigen zusätzliche Raman-verbotene, longitudinale Phononen-Plasmonmoden durch Streuung durch Fluktuationen der freien Ladungsträgerdichte. Die relative statische Dielektrizitätskonstante von ß-Ga2O3 senkrecht zu den Ebenen (100), (010) und (001) wird auf 10,2, 10,87 bzw. 12,4 bestimmt, die eine zuverlässige Grundlage für die Simulation und Konstruktion von Bauelementen bilden. Die Erzeugung von heller, roter Elektrolumineszenz (EL) in Sperrrichtung betriebenen Schottky-Barrieredioden auf der Basis von mit Cr und Si co-dotierten Kristallen wird gezeigt. Die EL von Cr ist repräsentativ für die Fähigkeit, die lumineszierenden Zustände anderer Übergangsmetalle anzuregen. Solche lichtemittierenden Schottky-Barrieredioden können ein neues Anwendungsgebiet von ß-Ga2O3 eröffnen. / This thesis deals with the evaluation of the wide band gap semiconductor ß-Ga2O3 for high power electronics. Therefore, layers grown with metal-organic vapor phase epitaxy (MOVPE) and bulk crystals grown by Czochralski method are electrically and optically characterized. Hereby, the fundamental properties of the material are investigated and compared with the theoretical predictions. The influence and formation of defects are investigated. At the beginning the MOVPE grown layers showed unfavorable electrical properties as they were fully compensated at lower doping concentrations and showed lowered mobility at higher charge carrier concentrations. A quantitative model of the detrimental influence of incoherent twin boundaries on electrical properties is developed showing that the prevention of the formation of these is the key to improve the material. The density of incoherent twin boundaries was reduced by 4 orders of magnitude resulting in improved charge carrier mobility. This provides a promising outlook for the use of ß-Ga2O3 in future power electronics. Raman spectroscopic investigations of highly n-type doped crystals reveal the formation of an impurity band, give insight in the effective-mass like donor nature of Si and Sn, and show additional Raman forbidden, longitudinal phonon plasmon modes due to free-electronic-charge density fluctuations scattering. The relative static dielectric constant of ß-Ga2O3 perpendicular to the planes (100), (010), and (001) is determined to 10.2, 10.87, and 12.4, respectively, which give a reliable basis for the simulation and design of devices. The generation of bright, red electroluminescence (EL) in reverse biased Schottky barrier diodes based on crystals co-doped with Cr and Si is shown. The EL of Cr is representative of the ability to excite the luminescent states of other transition metals. Such light emitting Schottky barrier diodes may open up a new application field of ß-Ga2O3.
32

MODULAZIONE DELL'AROUSAL MEDIANTE LA STIMOLAZIONE ELETTRICA TRANSCRANICA A FREQUENZE RANDOM / AROUSAL MODULATION BY RANDOM NOISE TRANSCRANIAL ELECTRICAL STIMULATION

MAURI, PIERCARLO 17 March 2016 (has links)
Il lavoro di tesi si è focalizzato sullo studio dell’arousal come indice psicofisiologico di attivazione e sull’applicazione della metodica di stimolazione elettrica transcranica (tES) non invasiva con lo scopo di modulare tale indice. L’obiettivo è stato quello di indagare se, applicando la tES, fosse possibile migliorare la performance di soggetti giovani sani in compiti di tipo cognitivo. Il progetto di ricerca si è sviluppato in 2 studi principali per un totale di 4 esperimenti. Tali studi hanno previsto l’acquisizione e la successiva analisi sia di dati comportamentali (tempi di reazione, accuratezza), che di indici psicofisiologici (conduttanza cutanea, diametro pupillare). I risultati hanno evidenziato che è possibile modulare l’arousal con dei “bursts” di stimolazione elettrica transcranica, somministrati in concomitanza di stimoli salienti per il soggetto. Tale modulazione si è manifestata con una riduzione dei tempi di reazione ed un contemporaneo aumento della risposta di conduttanza cutanea. Questi dati supportano la possibilità di utilizzare questo protocollo in pazienti con difficoltà di attenzione o altri problemi cognitivi per aumentare l’efficacia di interventi di riabilitazione. / The thesis analyzed the role of the arousal as a psychophysiological index of activation, and the application of non-invasive transcranial electrical stimulation (tES) technique with the aim to modulate this index. In this work we investigated if the application of tES could increase the performance of healthy young subjects during cognitive tasks. The thesis is based on 2 main studies for a total of 4 experiments with the recording of behavioural (reaction times, accuracy) and psychophysiological (skin conductance, pupil diameter) indeces. The results showed that it is possible to modulate arousal with bursts of tES, administered during the presentation of salient stimuli for the subject. This modulation resulted in a reduction of reaction times and an increase of the skin conductance response. These data support the possibility to use this protocol of stimulation with patients with attentional and other cognitive deficits in a rehabilitative context.
33

In situ real-time investigation of Organic Ultra-Thin-Film transistors: growth, electrical properties and biosensing applications

Quiroga, Santiago David <1977> 04 June 2010 (has links)
Organic electronics has grown enormously during the last decades driven by the encouraging results and the potentiality of these materials for allowing innovative applications, such as flexible-large-area displays, low-cost printable circuits, plastic solar cells and lab-on-a-chip devices. Moreover, their possible field of applications reaches from medicine, biotechnology, process control and environmental monitoring to defense and security requirements. However, a large number of questions regarding the mechanism of device operation remain unanswered. Along the most significant is the charge carrier transport in organic semiconductors, which is not yet well understood. Other example is the correlation between the morphology and the electrical response. Even if it is recognized that growth mode plays a crucial role into the performance of devices, it has not been exhaustively investigated. The main goal of this thesis was the finding of a correlation between growth modes, electrical properties and morphology in organic thin-film transistors (OTFTs). In order to study the thickness dependence of electrical performance in organic ultra-thin-film transistors, we have designed and developed a home-built experimental setup for performing real-time electrical monitoring and post-growth in situ electrical characterization techniques. We have grown pentacene TFTs under high vacuum conditions, varying systematically the deposition rate at a fixed room temperature. The drain source current IDS and the gate source current IGS were monitored in real-time; while a complete post-growth in situ electrical characterization was carried out. At the end, an ex situ morphological investigation was performed by using the atomic force microscope (AFM). In this work, we present the correlation for pentacene TFTs between growth conditions, Debye length and morphology (through the correlation length parameter). We have demonstrated that there is a layered charge carriers distribution, which is strongly dependent of the growth mode (i.e. rate deposition for a fixed temperature), leading to a variation of the conduction channel from 2 to 7 monolayers (MLs). We conciliate earlier reported results that were apparently contradictory. Our results made evident the necessity of reconsidering the concept of Debye length in a layered low-dimensional device. Additionally, we introduce by the first time a breakthrough technique. This technique makes evident the percolation of the first MLs on pentacene TFTs by monitoring the IGS in real-time, correlating morphological phenomena with the device electrical response. The present thesis is organized in the following five chapters. Chapter 1 makes an introduction to the organic electronics, illustrating the operation principle of TFTs. Chapter 2 presents the organic growth from theoretical and experimental points of view. The second part of this chapter presents the electrical characterization of OTFTs and the typical performance of pentacene devices is shown. In addition, we introduce a correcting technique for the reconstruction of measurements hampered by leakage current. In chapter 3, we describe in details the design and operation of our innovative home-built experimental setup for performing real-time and in situ electrical measurements. Some preliminary results and the breakthrough technique for correlating morphological and electrical changes are presented. Chapter 4 meets the most important results obtained in real-time and in situ conditions, which correlate growth conditions, electrical properties and morphology of pentacene TFTs. In chapter 5 we describe applicative experiments where the electrical performance of pentacene TFTs has been investigated in ambient conditions, in contact to water or aqueous solutions and, finally, in the detection of DNA concentration as label-free sensor, within the biosensing framework. / Negli ultimi decenni l’organica elettronica ha subito un’importante crescita spinta da diversi risultati alquanto incoraggianti e dalle potenziali nuove applicazioni che possono dare luogo agli innumerevoli materiali organici esistenti, tra cui gli schermi flessibili e di grande superficie, i circuiti stampabili a basso costo, le celle solari plastiche e i dispositivi di tipo “lab-on-a-chip”. Inoltre, i campi di applicazione sono così vasti da comprendere medicina, biotecnologia, processi di automazione e monitoraggio dei parametri ambientali, nonché la difesa e la sicurezza. Tuttavia un considerevole numero di domande deve ancora trovare risposta. Tra queste il meccanismo di operazione, che rimane senza essere completamente compreso e la correlazione tra la morfologia e la risposta elettrica dei dispositivi. Nonostante si sia ampiamente riconosciuto l’importante ruolo che il modo di crescita ha sulla prestazione dei dispositivi, non è stata realizzata un’investigazione esaustiva dell’argomento. Il principale obbiettivo di questa tesi è quindi quello di trovare una correlazione tra i modi di crescita e le proprietà elettriche in transistor a film sottile (TFTs) di pentacene. Al fine di studiare la dipendenza della prestazione elettrica dei TFTs di pentacene al variare dello spessore, si è proceduto all’ideazione e alla costruzione di una strumentazione sperimentale ad hoc, che permettesse di realizzare misurazioni elettriche in tempo reale e caratterizzazioni in situ alla fine della deposizione. Abbiamo provveduto alla crescita di dispositivi TFTs di pentacene in condizione di alto vuoto, variando metodologicamente la velocità di deposizione a temperatura ambiente prefissata. Durante la crescita, la corrente di drain IDS e la corrente di gate IGS sono state monitorate in tempo reale e al termine di ogni deposizione si è proceduto alla caratterizzazione elettrica in situ. Alla fine si è investigato sulla morfologia ex situ con l’utilizzo di un microscopio di forza atomica (AFM). Nel presente lavoro si riporta la correlazione valida per TFTs di pentacene tra le condizioni di crescita, la lunghezza di Debye e la morfologia (quantificata attraverso la lunghezza di correlazione). Abbiamo dimostrato che i portatori di carica vengono distribuiti nei layers a seconda del modo di crestita eseguito dal film (che dipende dalla velocità di deposizione per una data temperatura), il quale porta a una variazione del canale attivo dai 2 ai 7 monolayers (MLs). I nostri risultati hanno conciliato altri riportati in precedenza, che sembravano alquanto contradittori e hanno evidenziato la necessità di rielaborare il concetto di lunghezza di Debye in dispositivi a strati a bassa dimensione. Inoltre, per la prima volta, si presenta un’innovativa tecnica che indica l’avvenimento della percolazione attraverso il monitoraggio elettrico della IGS durante la deposizione del film sottile, correlando i fenomeni morfologici con quelli elettrici. Il capitolo 1 di questa tesi ci introduce nel mondo dell’elettronica organica e ci spiega il funzionamento del TFT. Il capitolo 2, ci spiega la crescita dei materiali organici, offrendo prima un fondamento teorico per poi passare alla sperimentazione; dopodiché si esibisce la prestazione elettrica tipica dei dispositivi di pentacene e si presenta la suddetta tecnica correttiva sugli effetti della corrente di fuga. Nel capitolo 3 si presenta la strumentazione costruita ad hoc per il monitoraggio elettrico in tempo reale e in situ; si illustrano inoltre alcuni risultati preliminari, assieme alla suddetta tecnica “breakthrough”. Intanto, nel capitolo 4, si riportano i più rilevanti risultati ottenuti in tempo reale e in situ, che correlano il modo di crescita, le proprietà elettriche e la morfologia dei TFTs di pentacene. Infine, il capitolo 5, si concentra nello studio sulla risposta elettrica dei dispositivi TFTs di pentacene in contatto con l’ambiente, con acqua e altre soluzioni acquose e, principalmente, della sua applicazione nella biosensoristica come sensore di concentrazione di DNA.
34

Electrical control of quantum dot spin qubits

Laird, E. A. January 2009 (has links)
No description available.
35

The control of a multi-variable industrial process, by means of intelligent technology

Naidoo, Puramanathan January 2001 (has links)
Conventional control systems express control solutions by means of expressions, usually mathematically based. In order to completely express the control solution, a vast amount of data is required. In contrast, knowledge-based solutions require far less plant data and mathematical expression. This reduces development time proportionally. In addition, because this type of processing does not require involved calculations, processing speed is increased, since rule process is separate and all processes can be performed simultaneously. These results in improved product quality, better plant efficiency, simplified process, etc. Within this project, conventional PID control has already been implemented, with the control parameter adjustment and loop tuning being problematic. This is mainly due to a number of external parameters that affects the stability of the process. In maintaining a consistent temperature, for example, the steam flow rate varies, the hot well temperature varies, the ambient may temperature vary. Another contributing factor, the time delay, also affects the optimization of the system, due to the fact that temperature measurement is based on principle of absorption. The normal practice in industry to avoid an unstable control condition is to have an experienced operator to switch the controller to manual, and make adjustments. After obtaining the desired PV, the controller is switched back to automatic. This research project focuses on eliminating this time loss, by implementing a knowledge-based controller, for intelligent decision-making. A FLC design tool, which allows full interaction, whilst designing the control algorithm, was used to optimize the control system. The design tool executed on a PC is connected to a PLC, which in turn is successfully integrated into the process plant.
36

Design and implementation of robotic control for industrial applications

Will, Desmond Jeffrey January 2004 (has links)
Background: With the pressing need for increased productivity and delivery of end products of uniform quality, industry is turning more and more to computer-based automation. At the present time, most of industrial automated manufacturing is carried out by specialpurpose machines, designed to perform specific functions in a manufacturing process. The inflexibility and generally high cost of these machines often referred to as hard automation systems, have led to a broad-based interest in the use of robots capable of performing a variety of manufacturing functions in a more flexible working environment and at lower production costs. A robot is a reprogrammable general-purpose manipulator with external sensors that can perform various assembly tasks. A robot may possess intelligence, which is normally due to computer algorithms associated with its controls and sensing systems. Industrial robots are general-purpose, computer-controlled manipulators consisting of several rigid links connected in series by revolute or prismatic joints. Most of today’s industrial robots, though controlled by mini and microcomputers are basically simple positional machines. They execute a given task by playing back a prerecorded or preprogrammed sequence of motion that has been previously guided or taught by the hand-held control teach box. Moreover, these robots are equipped with little or no external sensors for obtaining the information vital to its working environment. As a result robots are used mainly for relatively simple, repetitive tasks. More research effort has been directed in sensory feedback systems, which has resulted in improving the overall performance of the manipulator system. An example of a sensory feedback system would be: a vision Charge-Coupled Device (CCD) system. This can be utilized to manipulate the robot position dependant on the surrounding robot environment (various object profile sizes). This vision system can only be used within the robot movement envelope
37

Intelligent monitoring and control system for a friction stir welding process

Kruger, Grant January 2003 (has links)
A Friction Stir Welding machine is proposed and built to allow future research into the process and to provide a framework from which the application of intelligent manufacturing to industrial processes can be investigated. Initially a literature survey was conducted upon which the design of the machine could be based. The conversion of a conventional milling machine into a Friction Stir Welding machine by applying modern monitoring and control systems is then presented. Complete digital control was used to drive actuators and monitor sensors. A wireless chuck mounted monitoring system was implemented, enabling forces, torques, temperature and speed of the tool to be obtained directly from the process. Software based on a hierarchical Open Systems Architectural design, incorporating modularity, interoperability, portability and extensibility is implemented. This experimental setup is used to analyze the Friction Stir Welding process by performing data analysis using statistical methods. Three independent variables (weld speed, spindle speed and plunge depth) were varied and the independent variables (forces, torques, power, temperature, speed, etc) recorded using the implemented software. The statistical analysis includes the analysis of variants, regression analysis and the creation of surface plots. Using these results, certain linguistic rules for process control are proposed. An intelligent controller is designed and discussed, using the derived rules to improve and optimize certain aspects of the process encountered during the experimental phase of the research.
38

Sensorless speed estimation in three phase induction motors

Govindsamy, Matthew January 2002 (has links)
This thesis proposes a technique to determine and improve the performance of a sensorless speed estimator for an induction motor based on Motor Current Signature Analysis (MCSA). The theoretical concepts underlying the parameter based observer are developed first and then the model of the observer is built using Simulink. The observer is developed based on Model Reference Adaptive System (MRAS). The dynamic performance of the observer and its behavior due to variation of machine parameters is studied. The error in speed estimated using this observer is shown and the ability of MCSA to retune the rotor speed from the stator current spectrum. The spectrum estimation technique has been implemented using a software routine in Matlab. Both the observer and MCSA techniques were implemented practically on an induction motor. The performance of the combined sensorless speed estimation system was tested and verified.
39

Design and development of a remote reconfigurable internet embedded I/O controller

Phillips, Grant January 2003 (has links)
The use of embedded Internet systems is growing rapidly in the manufacturing sector. These systems allow the monitoring and controlling of plant machinery and manufactured items from a remote location via a standard Web interface. In a manufacturing environment, it is inevitable that long running processes will require support for dynamic reconfiguration because, for example, machines may fail, services may be moved or withdrawn and user requirements may change. In such an environment it is essential that the operation and architecture of such processes can be modified to reflect such changes. This research project will present methods and ideas for establishing a reconfigurable remote system by using standard 8-bit microcontrollers and reconfigurable hardware. It will allow a manufacturing process to be modified and changed within minutes without even having to be physically present at the location where the process is running.
40

Hybrid field generator controller for optimised perfomance

Moleli, Christopher Teboho January 2003 (has links)
Battery charging wind turbines like, Hybrid Field Generator, have become more popular in the growing renewable energy market. With wind energy, voltage and current control is generally provided by means of power electronics. The paper describes the analytical investigation in to control aspects of a hybrid field generator controller for optimized performance. The project objective is about maintaining the generated voltage at 28V through out a generator speed range, between 149 rpm and 598 rpm. The over voltage load, known as dump load, is connected to the control circuit to reduce stress on the bypass transistor for speeds above 598 rpm. Maintaining a stable voltage through out the speed range, between 149rpm and 598rpm, is achieved by employing power electronics techniques. This is done by using power converters and inverters to vary the generator armature excitation levels hence varying its air gap flux density. All these take place during each of the three modes of generator operation, which are: buck, boost and permanent magnet modes. Although the generator controller is power electronics based, it also uses software to optimize its performance. In this case, a PIC16F877 microcontroller development system has been used to test the controller function blocks.

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