Ultra-high temperature ceramics (UHTCs), such as ZrB2-based ceramic composites, have been identified as next generation candidate materials for leading edges and nose cones in hypersonic air breathing vehicles. Mechanical performance of ceramic composites play an important role in the ultra-high temperature applications, therefore SiC is added to ZrB2 as a strengthening phase to enhance its mechanical performance. The high melting temperatures of both ZrB2 and SiC, as well as the ability of SiC to form SiO2 refractory oxide layers upon oxidation make ZrB2-SiC ceramics very suitable for aerospace applications. Thermal residual stresses appearing during processing are unavoidable in sintered ZrB2-SiC ceramic composites. Residual microstresses appear at the microstructural level (intergranular microstresses) or at the crystal structure level (intragranular microstresses). These microstresses are of enormous importance for the failure mechanisms in ZrB2-SiC ceramics, such as ratio of the trans- and intergranular fracture; crack branching or bridging, microcracking, subcritical crack growth and others, as they govern crack propagation–induced energy dissipation and affect the toughness and strength of the ceramic material. Therefore, understanding the evolution of residual stress state in processed ZrB2-SiC ceramic composites and accurate measurements of these stresses are of high priority. In the present research the ZrB2-17vol%SiC, ZrB2-32vol%SiC, and ZrB2-45vol%SiC ultra-high temperature particulate ceramic composites were sintered using both Hot Pressing (HP) and Spark Plasma Sintering (SPS) techniques. The mechanical performance of the ZrB2-SiC composites was investigated using 3- and 4-point bending techniques for measurements of instantaneous fracture strength and fracture toughness. Resonant Ultrasound Spectroscopy was used for measurement of Young's, shear, and bulk moduli as well as Poisson's ratio of the composites. The distribution of thermal residual stresses and the effect of the applied external load on their re-distribution was studied using micro-Raman spectroscopy. Piezospectroscopic coefficients were determined for all compositions of ZrB2-SiC ceramic under study and their experimentally obtained values were compared with the piezospectroscopic coefficients both published in the literature and calculated using theoretical approach. Finally, the novel ZrB2-IrB2-SiC ceramic composites were also produced using Spark Plasma Sintering (SPS), where IrB2 powder was synthesized using mechanochemical route. It is expected that the IrB2 additive phase might contribute to the improved overall oxidation resistance of ZrB2 based ultra-high temperature ceramic composites.

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Continuous Fiber Reinforced Ceramic Composites (CFCC's) are being currently investigated as potential materials for high temperature applications such as combustor liners in stationary gas turbines. The creep behavior of woven Enhanced SiC/SiC composites was studied at temperatures from 600 to 1200 °C and at 140 to 220 MPa stress levels. Most researchers studying the creep behavior of ceramic matrix composites (CMCs) use the time hardening model and rate equations for expressing the dependence of creep strain on time, temperature and stress. Such laws, although simple and easy to use, are inadequate to represent the creep behavior over a range of stress levels and temperatures and cannot be used to quantify the pest phenomenon commonly observed in CMCs. Hence, these laws were modified to include the pest phenomenon and an empirical equation was developed that can be used to represent the creep behavior at various stresses and temperatures. The modified equation was used in the finite element analysis and the results were compared with the time and strain hardening models. Microscopic observations on the fractured surfaces revealed the pseudo-ductile behavior of the material at high temperatures. A quasi-static test was conducted at 1200 °C to determine the unloading response of the material. The stress-strain response of the composite demonstrates a hysterisis loop and a small amount of permanent strain, which are characteristic of the CMC's [3]. Finally, a test was conducted at 1200 oC to investigate the recovery behavior of the material. The material exhibits a tendency to recover the accumulated creep strain as well as the small permanent strain upon unloading, if sufficient time is allowed for recovery. The creep data were also modeled using the representations such as Monkmon-Grant and Larsen-Miller equations. A modified Monkman-Grant equation was used to model the stratification of the creep strain rate data with temperature. A finite element model based on the plasticity theory was developed to simulate the quasi-static cyclic behavior of the material. Though the loading behavior of CMCs can be modeled using the bilinear or multilinear kinematic hardening plasticity models, the unloading behavior as predicted by the models is entirely different from the experimentally observed behavior. Hence, these models were modified to correctly predict the stress-strain behavior. The model, which was input via a user defined subroutine into the ANSYS finite element program uses the concept of state or internal variables to define the unloading portion of the stress-strain curve. The results were compared with the test data and they show very good agreement. The model was then used to predict the stress-strain response of a plate with a notch. The results from the analysis were compared with the experimental data and they show good agreement if average values of strains are considered. / Master of Science

To support the study of potential utilization of the emerging silicon carbide (SiC) devices, two SiC active switches, namely 1.2 kV, 5 A SiC JFET manufactured by SiCED, and 1.2 kV, 20 A SiC MOSFET by CREE, have been investigated systematically in this thesis. The static and switching characteristics of the two switches have firstly been characterized to get the basic device information. Specific issues in the respective characterization process have been explored and discussed. Many of the characterization procedures presented are generic, so that they can be applied to the study of any future SiC unipolar active switches. Based on the characterization data, different modeling procedures have also been introduced for the two SiC devices. Considerations and measures about model improvement have been investigated and discussed, such as predicting the MOSFET transfer characteristics under high drain-source bias from switching waveforms. Both models have been verified by comparing simulation waveforms with the experimental results. imitations of each model have been explained as well. In order to capture the parasitic ringing in the very fast switching transients, a modeling methodology has also been proposed considering the circuit parasitics, with which a device-package combined simulation can be conducted to reproduce the detailed switching waveforms during the commutation process. This simulation, however, is inadequate to provide deep insights into the physics behind the ringing. Therefore a parametric study has also been conducted about the influence of parasitic impedances on the device's high-speed switching behavior. The main contributors to the parasitic oscillations have been identified to be the switching loop inductance and the device output junction capacitances. The effects of different parasitic components on the device stresses, switching energies, as well as electromagnetic interference (EMI) have all been thoroughly analyzed, whose results exhibit that the parasitic ringing fundamentally does not increase the switching loss but worsens the device stresses and EMI radiation. Based on the parametric study results, this thesis finally compares the difference of SiC JFET and MOSFET in their respective switching behavior, comes up with the concept of device switching speed limit under circuit parasitics, and establishes a general design guideline for high-speed switching circuits on device selection and layout optimization. / Master of Science

In this thesis work several manufacturing methods for graphene is discussed followed by an indepth study of graphene grown by a high temperature sublimation method (sublimation of siliconcarbide). The graphene surfaces studied have been grown by Graphensic AB, both graphenegrown on the Si-face and the C-face of the silicon carbide were studied. Six graphene samplesgrown 4H-SiC substrates were examined for homogeneity and surface morphology as well assome surface roughness parameters using Atomic Force Microscopy (AFM). The graphene wasstudied to get a better understanding of the surfaces and the growth mechanisms to improvemanufacturing parameters while also being informative for graphene sample customers. Anadditional graphene sample grown on 6H-SiC epitaxial layer was also studied to get a betterunderstanding of the sublimation mechanism. If graphene could be manufactured in a cheaprepeatable way the applications are endless and a new era of technology could emerge muchlike the silicon era that began several decades ago. In this thesis work the results are presentedas topography images as well as tables and histograms in the results section. The growth onthe Si-face is found to be well ordered when compared to the C-face which shows signs of alargely complex growth. The graphene on the Si-face lies on top of silicon carbide steps like acarpet with a buer layer interface against the silicon carbide. On the C-face this buer layeris not present but the graphene is deformed by buckling which is suspected to originate fromdierences in thermal properties between the graphene and the C-face. The in uence of AFMsettings for characterization of graphene while using intermittent mode have been evaluated andrecommendations are given. Finally a method for evaluating the homogeneity of the graphenelm is proposed but is in need of further verication.

This thesis is about to introduce the development of switched power supply with output voltage of 5,5 kV and power of 4,3 kW. This thesis directly follows the outputs of two previous semester projects. Main task is about to finish the development of remaining printed circuit boards and perform launching of all parts of developed power supply. Next necessary task is about to assemble a functional prototype of the supply using the developed parts, performing a series of measurement supply’s parameters and creating of technical documentation of mechanical part. There are all stages of supply’s development included in this thesis. From the first part, that describes various types of converters, through the design of all the supply’s parts, to final implementation of functional prototype. There are the results of measurement supply’s parameters at the end of this thesis.

In dieser Arbeit wurden winkelaufgeloeste Photoemissionsuntersuchungen (ARUPS)und Messungen der Reflexionsanisotropie (RAS) durchgefuehrt. Fuer die Experimente standen p-3C-SiC(001)/n-3C-SiC(001)/n-Si-Proben zur Verfuegung. Den Mittelpunkt bildeten sowohl die Praeparation verschiedener Oberflaechen- rekonstruktionen als auch die oben genannten Untersuchungsmethoden. Fuer die Praeparation wurden zwei unterschiedliche Heizverfahren verwendet - die Elektronenstoßheizung und die direkte Heizung. Die Charakterisierung der Oberflaechenrekonstruktion erfolgte mittels LEED. Die Untersuchungen wurden an den Hauptrekonstruktionen der (001)-Oberflaeche des kubischen Siliziumkarbids - der (3x2)-, der (2x1)- und der c(2x2)- Rekonstruktion - durchgefuehrt, welche mit der direkten Heizmethode praepa- riert worden waren. Aus den winkelaufgeloesten Photoemissionsspektren konnten die jeweiligen Oberflaechenbandstrukturen ermittelt werden. Daraus ließen sich Schlußfolgerungen ueber die Ausbildung von Oberflaechenzustaenden bzw. Oberflaechenresonanzen treffen. Die RAS-Spektren zeigten für alle Rekonstruk- tionen deutlich unterschiedliche Verlaeufe.

SiC has become an attractive wide bandgap semiconductor due to its unique physical and electronic properties and is widely used in high temperature, high frequency, high power and radiation resistant applications. SiC has been used as an alternative to Si in harsh environments such as in the oil industry, nuclear power systems, aeronautical, and space applications. SiC is also known for its polytypism and among them 3C-SiC, 4H-SiC and 6H-SiC are the most common polytypes used for research purposes. Among these polytypes 4H-SiC is gaining importance due to its easy commercial availability with a large bandgap of 3.26 eV at room temperature. Controlled creation of defects in materials is an approach to modify the electronic properties in a way that new functionality may result. SiC is a promising candidate for defect-induced magnetism on which spintronic devices could be developed. The defects considered are of room temperature stable vacancy types, eliminating the need for magnetic impurities, which easily diffuse at room temperature. Impurity free vacancy type defects can be created by implanting the host atoms of silicon or carbon. The implantation fluence determines the defect density, which is a critical parameter for defect induced magnetism. Therefore, we have studied the influence of low fluence low energy silicon and carbon implantation on the creation of defects in n-type 4H-SiC. The characterization of the defects in these implanted samples was performed using the techniques, RBS-channeling and Raman spectroscopy. We have also utilized these characterization techniques to analyze defects created in much deeper layers of the SiC due to implantation of high energy nitrogen ions. The experimentally determined depths of the Si damage peaks due to low energy (60 keV) Si and C ions with low fluences (< 1015 cm-2) are consistent with the SRIM-2011 simulations. From RBS-C Si sub-lattice measurements for different fluences (1.1×1014 cm-2 to 3.2×1014 cm-2) of Si implantation in 4H-SiC, the Si vacancy density is estimated to range from 1.29×1022 cm-3 to 4.57×1022 cm-2, corresponding to average vacancy distances of 4.26 Å to 2.79 Å at the damage peak (50±5 nm). Similarly, for C implanted fluences (1.85×1014 cm-2 to 1×1015 cm-2), the Si vacancy density varies from 1.37×1022 cm-3 to 4.22×1022 cm-3 with the average vacancy distances from 4.17 Å to 2.87 Å at the damage peak (110±10 nm). From the Raman spectroscopy, the implantation-induced lattice disorders calculated along the c-axis (LO mode) and perpendicular to c-axis (TO mode) in 4H-SiC are found to be similar. Furthermore, the results obtained from SQUID measurements in C implanted n-type 4H-SiC sample with fluences ranging from 1×1012 to 1.7×1016 ions/cm2 have been discussed. The implanted samples showed diamagnetism similar to the unimplanted sample. To date, to our best of knowledge, no experimental work has been reported on investigating defect induced magnetism for self-ion implantation in n-type 4H-SiC. These first reports of experimental results can provide useful information in future studies for a better understanding of self-ion implantation in SiC-based DMS.

Les composites SiC/SiC pr´esentent d’excellentes propri´et´es thermom´ecaniques `a hautes temp´eratures. Ils apparaissent donc comme des candidats cr´edibles pour remplacer les alliages m´etalliques dans les zones chaudes de moteurs a´eronautiques civils afin d’en r´eduire l’impact environnemental. Comprendre et pr´evoir l’apparition des premiers endommagements constitue donc un enjeu industriel majeur. La d´emarche multi-´echelle propos´ee permet d’int´egrer dans des mod`eles num´eriques les sp´ecificit´es du mat´eriau. Elle s’articule autour d’une phase exp´erimentale de caract´erisation des endommagements et d’une phase de mod´elisation par ´el´ements finis aux ´echelles microscopique et m´esoscopique. Des essais in situ sous microscopes et sous micro-tomographie X (μCT) sont e↵ectu´es pour visualiser et quantifier les m´ecanismes d’endommagement `a des ´echelles compatibles avec les mod`eles num´eriques. Sur la base des observations exp´erimentales, des calculs d’endommagement sont r´ealis´es `a l’´echelle microscopique afin de simuler la fissuration transverse des torons. Des essais virtuels permettent alors d’identifier des lois d’endommagement `a l’´echelle sup´erieure et de mod´eliser l’apparition des premi`eres fissures dans des textures tiss´ees 3D `a l’´echelle m´esoscopique. Cela permet de mettre en ´evidence les liens entre l’organisation du mat´eriau aux di↵´erentes ´echelles et l’initiation des premiers endommagements. Des confrontations essais/calculs sont finalement propos´ees, en comparant notamment les sites d’amor¸cage des endommagements observ´es exp´erimentalement lors des essais in situ sous μCT avec ceux pr´edits par les simulations. / SiC/SiC composites display excellent thermomechanical properties at high temperatures. They appear as promising candidates to replace metallic alloys in hot parts of aircraft engines to reduce their environmental impact. Thus, to understand and to predict the onset of damage in such materials is critical. An integrated multi-scale approach is developed to construct numerical models that integrate the specificities of the material at the di↵erent relevant scales. This work is twofold : an experimental characterization of the damage, and finite element modeling at the microscopic and mesoscopic scales. In situ tensile tests are carried out under microscopes and X-ray micro-tomography (μCT). Images are analyzed to visualize and quantify the damage mechanisms at scales consistent with the numerical models. Based on these observations, damage calculations are performed at the microscopic scale to simulate the transverse yarns cracking. Virtual tests are then used to identify damage laws at the upper scale and to simulate the first cracks in 3D woven composites at the mesoscopic scale. Through these simulations, we highlight the links between the organization of the material at di↵erent scales and the initiation of the damages. Comparisons between experiments and calculations are finally performed. In particular, the predicted damage events are compared to those obtained experimentally on the same specimen during in situ μCT tensile tests.