Thesis (M.S.)--Brigham Young University. Dept. of Physics and Astronomy, 2006. / Includes bibliographical references (p. 83-84).

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This dissertation is concerned with the behaviour of square-on-square double-layer space grids (DLSG's) and in particular their collapse mechanisms after initial buckling. A detailed review of the methods used for collapse analysis of these structures is conducted, which shows that their collapse can be of progressive nature. An outline of the remedies that have been suggested to make safe their collapse behaviour is given, and it traces the line for further study. The present investigation starts by describing the redundancy in these regular pin-jointed assemblies in terms of unit states of self-stress. This enables us to understand, by means of the extended Maxwell rule, how it is that mechanisms of collapse can occur when relatively few bars in the assembly have failed. The influence of boundary conditions on the mechanisms of collapse and its progressive nature is also investigated. This method of analysis is verified by detailed examination of several previously reported experimental results, and some conclusions can be drawn. The prestressing of statically indeterminate assemblies by means of lack-of-fit is investigated in the hope of changing their collapse characteristics. The unit state of self-stress has been used to explain a method for selecting the bars that can be shortened/lengthened in order to impose a prescribed state of self-stress, and the scope for achievable patterns of bar tensions has been described. A by-product of the investigation is an algorithm which calculates the required lack-of-fit to impose an initial state of self-stress which maximizes the load-carrying capacity of indeterminate trusses. Using the formulae derived for prestress, a statistical analysis is developed to estimate initial bar tensions due to random lack-of-fit in order to obtain a realistic estimate of the load-factor of an assembly before initial bar failure. Furthermore, we have argued and demonstrated how backlash at the bar-joint connection may have a beneficial effect in reducing substantially initial bar tensions due to the inevitable lack-of-fit on account of manufacturing errors of bars. An extensive experimental program has been conducted to verify the assumptions adopted and formulae derived in this dissertation.

Page 190 was not included in the thesis. / <p> A research program is presented for assessing the plastic collapse load and shake-down load of reinforced concrete continuous beams. This investigation attempts to establish a range of validity of simple plastic theory when applied to the under-reinforced concrete beams and to determine the sensitivity of such structures to variable repeated loading. In attempt for more accurate prediction of the behaviour of reinforced concrete beams when subjected to variable repeated loading, the numerical beam analysis was developed.</p> <p> An experimental program was conducted on 10 reinforced concrete continuous beams. Deflections and strains of these specimens of nearly prototype size were measured and compared with predicted values at critical cross-sections. Resulting conclusions and recommendations for further research are made.</p> / Thesis / Master of Engineering (MEngr)

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Ce manuscrit présente les résultats d’une analyse exhaustive des mécanismes physiques qui limitent les performances et la fiabilité des transistors à haute mobilité d’électrons (HEMT) sur nitrure de gallium (GaN). En particulier : • Les phénomènes de dégradation à fort champ électrique des HEMT sur GaN sont analysés en comparant les données expérimentales avec les résultats de simulations physiques. Des stresses DC de 150 heures ont été effectués en conditions de canal ouvert et de pincement. Les effets des dégradations qui ont caractérisé ces deux types de stresses sont les suivants: une chute de courant DC de drain, une amplification des effets de gate-lag, et une diminution du courant inverse de grille. Les simulations physiques indiquent que la génération simultanée de piéges de surface (et/ou barrière) et de volume peut expliquer tous les modes de dégradation décrits plus haut. Les mesures expérimentales ont également montré que le stress en canal ouvert a causé une chute de la transconductance seulement pour de fortes valeurs de la tension VGS, alors que le stress au pincement a provoqué une chute de transconductance uniforme pour toutes les valeurs de VGS. Ce comportement peut être reproduit par la simulation physique pourvu que, dans le cas de stress a canal ouvert, on considère que les piéges s’accumulent au long d’une vaste région qui s’étend latéralement du bord de la grille vers le contact de drain, tandis que, dans le cas du stress au pincement, on considère que la génération des pièges ait lieu dans une portion plus petite de la zone d’accès à proximité de la grille et qu’elle soit accompagnée par une grande dégradation des paramètres de transport du canal. Enfin on propose que les électrons chauds et l’augmentation de la contrainte par le champ électrique soient à l’origine des dégradations observées après les stresses a canal ouvert et au pincement respectivement. • Les piéges dans les HEMT sur GaN ont été caractérisés en utilisant les techniques de DLTS et leur comportement associé de charge/décharge est interprété à l’aide des simulations physiques. Sous certaines conditions de polarisation, les piéges du buffer peuvent produire de faux signaux de piéges de surface, c'est-à-dire, le même type de signaux I-DLTS et ICTS attribués généralement aux piéges de surface. Clarifier cet aspect est très important à la fois pour les tests de fiabilité et pour l’optimisation des dispositifs, car il peut provoquer une identification erronée du mécanisme de dégradation, et par conséquent induire une mauvaise correction des procédés technologiques. • Les mécanismes physiques qui provoquent l’effondrement du courant RF dans les HEMT sur GaN sont analysés par le biais de mesures expérimentales et de simulations physiques. Ce travail propose les conditions suivantes : i) les piéges du buffer aussi bien que ceux de surface peuvent contribuer à l’effondrement du courant RF à travers un mécanisme identique qui impliquerait la capture et l’émission des électrons provenant de la grille; ii) la passivation de la surface diminue considérablement l’effondrement du courant RF par la réduction du champ électrique en surface et la diminution qui en découle de l’injection d’ électrons de la grille vers les pièges ; iii) pour des densités de piéges de surface inférieures à 9 × 1012 cm-2 , des barrières de potentiel superficiels dans l’ordre de 1-2 eV peuvent coexister avec des piéges de surface ayant des énergies plus faibles et qui causent l’effondrement du courant RF caractérisé par des constantes de temps relativement courtes. • Les effets de l’effondrement du courant dans les HEMT sur GaN sont étudiés en utilisant les résultats de mesures expérimentales et de simulations physiques. D’après les mesures pulsées, les dispositifs employés montrent un gate-lag considérable et un drain-lag négligeable qui peuvent être attribués à la présence de piéges de surface et de buffer respectivement. / This thesis reports the results of an extensive analysis of the physical mechanisms that limit the performance and reliability of gallium nitride (GaN) based High Electron Mobility Transistors (HEMT). In particular: • High electric field degradation phenomena are investigated in GaN-capped AlGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate–source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters. Channel hot electrons and electric-field-induced strain-enhancement are finally suggested to play major roles in power-state and off-state degradation, respectively. • Traps are characterized in AlGaN-GaN HEMTs by means of DLTS techniques and the associated charge/discharge behavior is interpreted with the aid of numerical device simulations. Under specific bias conditions, buffer traps can produce ‘‘false’’ surface-trap signals, i.e. the same type of current-mode DLTS (I DLTS) or gate-lag signals that are generally attributed to surface traps. Clarifying this aspect is important for both reliability testing and device optimization, as it can lead to erroneous identification of the degradation mechanism, thus resulting in wrong correction actions on the technological process. • The physical mechanisms underlying RF current collapse effects in AlGaN-GaN high electron mobility transistors are studied by means of measurements and numerical device simulations. This work suggests the following conclusions: i) both surface and buffer traps can contribute to RF current collapse through a similar physical mechanism involving capture and emission of electrons tunneling from the gate; ii) surface passivation strongly mitigates RF current collapse by reducing the surface electric field and inhibiting electron injection into traps; iii) for surface-trap densities lower than 9 × 1012 cm-2, surface-potential barriers in the 1–2 eV range can coexist with surface traps having much a shallower energy and, therefore, inducing RF current-collapse effects characterized by relatively short time constants. • Current collapse effects are investigated in AlGaN/GaN HEMTs by means of measurements and numerical device simulations. According to pulsed measurements, the adopted devices exhibit a significant gate-lag and a negligible drain-lag ascribed to the presence of surface and buffer traps, respectively. Furthermore, illumination of the devices with two specific wavelengths can result in either a recovering of current collapse or a decrease in the gate current. On the other hand, numerical device simulations suggest that the kink effect can be explained by electron trapping into barrier traps and the subsequent electron emission after a critical electric-field value is reached. / Questa tesi riporta i risultati ottenuti da un’ampia analisi dei meccanismi fisici che limitano le prestazioni e l’affidabilità dei transistor ad alta mobilità di elettroni (HEMT) al nitruro di gallio (GaN). In particolare: • I fenomeni di degradazione ad alto campo elettrico nei GaN/AlGaN/GaN HEMT sono analizzati confrontando i dati sperimentali con i risultati delle simulazioni numeriche. Sono stati effettuati stress DC di 150 ore in condizioni di canale aperto e chiuso. Gli effetti di degradazione che hanno caratterizzato entrambi i tipi di stress sono i seguenti: una caduta nella corrente DC di drain, un’amplificazione degli effetti di gate lag, e una diminuzione della corrente inversa di gate. Le simulazioni numeriche indicano che la generazione simultanea di trappole in superficie (e/o barriera) e buffer può spiegare tutti i suddetti modi di degradazione. Le misure sperimentali hanno mostrato inoltre che lo stress a canale aperto ha causato una caduta della tranconduttanza solo ad alte tensioni VGS, mentre lo stress a canale chiuso ha provocato una caduta della transconduttanza uniforme a tutte le tensioni VGS. Questo comportamento può essere riprodotto con le simulazioni se, nel caso di stress a canale aperto, si assume che le trappole si accumulano lungo un’ampia regione che si estende lateralmente dal bordo di gate verso il contatto di drain, mentre, nel caso di stress a canale chiuso, si suppone che la generazione delle trappole abbia luogo in una più stretta porzione della zona di accesso vicino al bordo di gate e che sia accompagnata da una degradazione significativa dei parametri di trasporto del canale. Infine si propone che gli elettroni caldi del canale e l’aumento di strain indotto dal campo elettrico siano alla base delle degradazioni osservate dopo gli stress a canale aperto e chiuso rispettivamente. • Le trappole in AlGaN-GaN HEMTs sono caratterizzate usando le tecniche di DLTS e il relativo comportamento di carica/scarica é interpretato con l’aiuto delle simulazioni numeriche. Sotto particolari condizioni di polarizzazione, le trappole di buffer possono produrre falsi segnali da trappole di superficie, ossia lo stesso tipo di segnali I-DLTS e forma d’onda di gate lag attribuiti generalmente alle trappole di superficie. Chiarire questo aspetto è molto importante sia per le prove di affidabilità che per l’ottimizzazione dei dispositivi, in quanto può provocare una errata identificazione del meccanismo di degradazione, portando ad azioni correttive sbagliate nell’ottimizzazione del processo tecnologico. • I meccanismi fisici che originano il collasso di corrente RF negli HEMT AlGaN-GaN sono analizzati usando misure sperimentali e simulazioni numeriche. Questo lavoro suggerisce le seguenti condizioni: i) sia le trappole di superficie che quelle di buffer possono contribuire al collasso di corrente RF tramite un simile meccanismo fisico che coinvolge la cattura e l’emissione di elettroni provenienti dal gate; ii) la passivazione della superficie diminuisce fortemente il collasso della corrente RF tramite la riduzione del campo elettrico in superficie e la conseguente diminuzione dell’iniezione di elettroni dal gate alle trappole; iii) per densità di trappole di superficie minori di 9 × 1012 cm-2 , barriere di potenziale superficiale di 1-2 eV possono coesistere con trappole di superficie aventi energie relativamente basse e che provocano effetti di collasso di corrente RF caratterizzati da costanti di tempo relativamente corte. • Gli effetti di collasso di corrente negli HEMT AlGaN-GaN sono studiati usando i risultati delle misure sperimentali e delle simulazioni numeriche. Basandosi sulle misure delle caratteristiche d’uscita impulsate, i dispositivi utilizzati mostrano un evidente gate-lag e un trascurabile drain-lag, attribuiti alla presenza di trappole di superficie e buffer rispettivamente.

A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy in the Faculty of Science School of Computational and Applied Mathematics. May 2015. / In this thesis we present numerical simulations of the gravitational collapse of isothermal clouds of one solar mass at a temperature of 10K. We will consider two types of initial conditions – initially uniform spheres and perturbed Bonnor-Ebert spheres. The aim of the performed numerical simulations is to investigate the core bounce described by Hayashi and Nakano [1]. They reported that if strong enough, the shock wave would be capable of ionizing the gas in the collapsing cloud. The simulations are performed using two numerical methods: the TVD MUSCL scheme of van Leer using a Roe flux on a uniform grid and the TVD Runge-Kutta time-stepping using a Marquina flux on a non-uniform grid. These two particular methods are used because of their differences in numerical structure. Which allows us to confidently make statements about the nature of the collapse, particularly with regards to the core bounce. The convergence properties of the two methods are investigated to validate the solutions obtained from the simulations. The numerical simulations have been performed only in the isothermal regime by using the Truelove criterion [2] to terminate the simulation before central densities become large enough to cause artificial fragmentation. In addition to the numerical simulations presented in this thesis, we also introduce new, analytical solutions for the steady-state accretion of an isothermal gas onto a spherical core as well as infinite cylinders and sheets. We present the solutions and their properties in terms of the Lambert function with two parameters, γ and m. In the case of spherical accretion we show that the solution for the velocity perfectly matched the solutions of Bondi [3]. We also show that the analytical solutions for the density – in the spherical case – match the numerical solutions obtained from the simulations. From the agreement of these solutions we propose that the analytical solution can provide information about the protostellar core (in the early stages of its formation) such as the mass.

Magnetic X-point collapse is investigated using a 2.5D fully relativistic particle-in-cell simulation, with varying strengths of guide-field as well as open and closed boundary conditions. In the zero guide-field case we discover a new signature of Hall-reconnection in the out-of-plane magnetic field, namely an octupolar pattern, as opposed to the wellstudied quadrupolar out-of-plane field of reconnection. The emergence of the octupolar components was found to be caused by ion currents and is a general feature of X-point collapse. The effect was shown to be independent of system size and ion mass and confined to a few ion inertial lengths from the reconnection current sheet. In a comparative study of tearing-mode reconnection, signatures of octupolar components are found only in the out-flow region. It is argued that space-craft observations of magnetic fields at reconnection sites may be used accordingly to identify the type of reconnection. Further, initial oscillatory reconnection is observed, prior to reconnection onset, generating electromagnetic waves at the upper-hybrid frequency, matching solar flare progenitor emission. When applying a guide-field, in both open and closed boundary conditions, thinner dissipation regions are obtained and the onset of reconnection is increasingly delayed. Investigations with open boundary conditions show that, for guide-fields close to the strength of the in-plane field, shear flows emerge, leading to the formation of electron flow vortices and magnetic islands. Asymmetries in the components of the generalised Ohm's law across the dissipation region are observed and inertial components are shown to play a role at the X-point. Extended in 3D geometry, it is shown that locations of magnetic islands and vortices are not constant along the height of the current-sheet. Vortices formed on opposite sides of the current-sheet travel in opposite directions along it, leading to a criss-cross vortex pattern. Similarly to oblique current sheets previously observed in 3D guide-field reconnection studies, vortex-tubes are inclined at the same angle as the magnetic field.

This research focuses on improvements and application of 1D finite elements for nonlinear dynamic analysis of reinforced concrete frames under extreme loadings. The concept of force interpolation is adopted for the element formulation and a solution scheme developed based on a total secant stiffness approach that provides good convergence characteristics. The geometrical nonlinearities including 2nd order P-Delta effects as well as catenary action are considered in the element formulation. It is shown that geometrical nonlinearities may have a significant effect on member (structure) response within extreme loading scenarios. In the analysis of structures subjected to extreme loadings, accurately modelling of the post peak response is vital and, in this respect, the objectivity of the solution with softening must be maintained. The softening of concrete under compression is taken into account, and the objectivity preserved, by adopting a nonlocal damage model for the compressive concrete. The capability of nonlocal flexibility-based formulation for capturing the post-peak response of reinforced concrete beam-columns is demonstrated by numerical examples. The 1D frame element model is extended for the modelling of 3D framed structures using a simplified torque-twist model that is developed to take account of interaction between normal and tangential forces at the section level. This simplified model can capture the variation of element torsional stiffness due to presence of axial force, bending moment and shear and is efficient and is shown to provide a reasonable degree of accuracy for the analysis of 3D reinforced concrete frames. The formulations and solution algorithms developed are tested for static and dynamic analysis of reinforced concrete framed structures with examples on impact analysis of beams, dynamic analysis of frames and progressive collapse assessment of frames taken from the literature. The verification shows that the formulation is very efficient and is capable of modelling of large scale framed structures, under extreme loads, quickly and with accuracy.