This experimental work aims to characterize the SiC layer of various Tri-Structural Isotropic (TRISO) coated fuel particles. In the first part of the work, Raman spectroscopy is used to qualitatively characterize the SiC TRISO layer and to identify the presence of silicon from peak positions. Free silicon poses a significant threat to the integrity of the SiC layer because it melts at 1414oC, significantly lower than the maximum operating temperature of 1550oC. Crystalline silicon is characterized with qualitative Raman spectroscopy by a 520 cm-1 peak. Silicon is found to be preferentially concentrated along the SiC layer close to the inner pyrolytic carbon (IPyC) layer. Samples that were only mounted and polished are compared with those that have in addition also been etched. Disordering of the crystals and peak splitting necessitated the use of peak de-convolution. The 3C, 6H and 15R polytypes of SiC were identified. The second part of the Raman spectroscopy work involves the development of calibration curves using peak areas from known binary mixtures (5%, 25%, 50% and 75% Si) to quantify the amount of silicon found relative to SiC. Initially the SiC polytypes used in these mixtures are 3C, 4H and 6H. Reasonably good logarithmic calibration fits were obtained with R2 values of 0.996, 0.966 and 0.988 respectively. However some error accompanied the calibration values and an average of ten analyses yielded a more reliable average. The calibration curve results made it possible to estimate the silicon content throughout the SiC layer for each sample, when combining the results of the qualitative and quantitative Raman spectroscopic study. Samples PO6 and PO8 revealed high peaks of crystalline silicon. When peak areas were quantified and related to the 3C calibration curve, as much as 60% silicon was calculated for both samples. Etching was found to slightly lower the silicon to SiC ratio. The calibration accuracy for the binary mixtures was checked by plotting calculated values against weighed-off values, yielding 3C, 4H and 6H straight-line fits with R2 values of 0.983, 0.941 and 0.981 respectively. These binary mixtures were analyzed with the SEM, which revealed variable particle size and segregation of silicon and SiC. Quantitative Raman spectroscopy is however known to be affected by a significant number of variables that are difficult to control. Attempts were made to decrease the scatter of the results from the calibration curve to yield more precise results. Two pure samples of silicon and SiC were studied separately, in attempts to better understand particle size and distortion effects. Distortion was found to have a greater impact on the scatter of peak area values than particle size. The scatter associated with pure sample peak areas casts doubt on the accuracy of the binary calibration curves. Rietveld analysis using X-ray powder diffraction is used to further support the Raman spectroscopy work by qualitatively and quantitatively characterizing the phases involved in each TRISO particle, to a greater degree of accuracy than the Raman spectroscopy. Refinement components include 2H graphite, quartz, SiC (3C, 6H, 8H and 15R), silicon and tetragonal ZrO2. Oxidized samples were compared with unoxidized samples. The outer pyrolytic carbon (OPyC) layer was oxidized (to improve the accuracy of quantitative measurements). Graphite percentages dominated the refinements with values ranging from 57% to 90% for unoxidized samples and 28% to 83% for oxidized samples. The 3C SiC polytype is the most abundant polytype and constitutes 78% to 83% of the SiC (unoxidized samples) and 82% to 90% (oxidized samples). Trace percentages of silicon were detected for PO6 (0.4%), PO8 (0.6%) and PO10 (0.1%) Quantitative XRD results are known to be accurate to around 1% at the 3ó level. Calibration curves were also subsequently constructed from the same samples as those used for quantitative Raman spectroscopy by comparing the weighed-off values to the measured ones. The 3C, 4H and 6H R2-fits are 0.991, 0.978 and 0.984 respectively. All the milled samples contained significant α-Fe which contaminated the samples from the grinding process. After dissolving the α-Fe in HCl a sample was tested to check the effect of the α-Fe specifically on microabsorption. Microabsorption was found to be an insignificant effect. The second part of the XRD work focused on the high-temperature stability of SiC up to 1400oC. Al2O3 was used as the standard and the instrument was calibrated using its two independent lattice parameter values along the a-axis and c-axis to make temperature corrections. Temperature corrected curves (of SiC and graphite) were constructed, which superimposed the theoretical Al2O3 curve along the a-axis and c-axis. The linear thermal expansion coefficients of SiC and graphite could then be determined from corrected lattice parameter values. The thermal expansion coefficients of G102 SiC had similar values to the literature values up to 800oC. Thereafter the experimental values had significantly higher thermal expansivity when compared to literature values. PO4 and PO9 thermal expansion coefficient values were higher below 500oC, but much closer as temperatures approached 1400oC. There was little correlation between G102, PO4 and PO9 graphite c-axis thermal expansion coefficient curves and literature values. The third section of the work involves the characterization of the SiC layers of three of the samples by transmission electron microscopy using their selected area electron diffraction patterns. This facilitates the unequivocal characterization of the SiC polytypes. The 3C and 6H polytypes were identified. There is substantial disorder in the crystals. Planar defects of differing periodicity are seen along the [111] direction of the 3C polytype. Copyright / Dissertation (MEng)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted

No description available.

No description available.

Depuis sa découverte, le graphène a attiré beaucoup d’intérêt et ses propriétés remarquables font de lui un matériau très étudié par la communauté scientifique. Ce travail de thèse porte non pas sur ces propriétés intrinsèques, mais sur les possibilités de dopage et de fonctionnalisation du graphène pour d’éventuelles applications futures. Le choix du graphène épitaxié sur SiC comme matériau de base nous a permis d’avoir des échantillons adaptés aux études spectroscopiques (XPS, ARPES, NEXAFS) effectuées au synchrotron SOLEIL. Ces études sont indispensables pour la caractérisation macroscopique du graphène dopé et fonctionnalisé. La croissance epitaxiale permet à la fois le dopage in-situ et ex-situ. Dans un premier temps nous avons étudié l’influence de l’azote, élément voisin du carbone. Nous avons opté pour une technique de dopage in-situ, ce qui nous a permis d’avoir du graphène dopé dans un seul et même processus de fabrication. De plus nous avons pu déterminer les conditions de croissance pour obtenir une couche de nitrure de silicium (Si3N4) entre le graphène et le substrat. D’autre part nous avons utilisé l’oxygène pour fonctionnaliser le graphène. En exposant le graphène vierge à l’oxygène atomique et moléculaire, on a pu étudier l’évolution des états vide du graphène en présence d’oxygène. Les bords des grains de graphène sont particulièrement adaptés pour la fonctionnalisation à cause de leur activité chimique. Nous avons ainsi synthétisé du graphène avec des grains de petites dimension (~100 nm) pour avoir une forte densité de bords dans l’échantillon. De cette manière nous avons pu détecter, par absorption des rayons X, la signature de ces états de bord. / Since its discovery, graphene has attracted tremendous interest and its remarkable properties make it a material intensively studied by the scientific community. This thesis is not directly concerned with its intrinsic properties, but the possibilities of doping and functionalization of graphene for future possible applications and devices. The choice of epitaxial graphene on SiC as basic material allowed us to have samples well adapted for spectroscopic studies (XPS, ARPES and NEXAFS) carried out on a synchrotron facility (SOLEIL). These studies are essential for the macroscopic characterization of doped graphene and its functionalization. Epitaxial growth provides us the possibility to dope graphene both in-situ and ex-situ. We first opted for an in-situ doping technique studying the influence of nitrogen as a chemical dopant on the growth process. This allowed us to fabricate doped graphene in a one-step process. By tuning the parameters for epitaxial growth the creation of a silicon nitride layer was also observed. We also used atomic and molecular oxygen for the functionalization of graphene. By exposing pristine graphene to oxygen in an ex-situ process, we were able to study the evolution of empty states of graphene and the consequences on the electronic structure. The edges of graphene crystallites are particularly adapted for functionalization because of their chemical activity. The epitaxial growth on a 3C-SiC substrate allowed us to synthesize graphene with a reduced lateral size (~100 nm) and to have a higher density of edges in our sample. In this way we were able to detect the signature of these edge states using non-local spectroscopic methods.

abstract: 4H-SiC has been widely used in many applications. All of these benefit from its extremely high critical electric field and good electron mobility. For example, 4H-SiC possesses a critical field ten times higher than that of Si, which allows high-voltage blocking layers composed of 4H-SiC to be approximately a tenth the thickness of a comparable Si device. This, in turn, reduces the device on-resistance and power losses while maintaining the same high blocking capability. Unfortunately, commercial TCAD tools like Sentaurus and Silvaco Atlas are based on the effective mass approximation, while most 4H-SiC devices are not operated under low electric field, so the parabolic-like band approximation does not hold anymore. Hence, to get more accurate and reliable simulation results, full-band analysis is needed. The first step in the development of a full-band device simulator is the calculation of the band structure. In this work, the empirical pseudopotential method (EPM) is adopted. The next task in the sequence is the calculation of the scattering rates. Acoustic, non-polar optical phonon, polar optical phonon and Coulomb scattering are considered. Coulomb scattering is treated in real space using the particle-particle-particle-mesh (P3M) approach. The third task is coupling the bulk full-band solver with a 3D Poisson equation solver to generate a full-band device simulator. For proof-of-concept of the methodology adopted here, a 3D resistor is simulated first. From the resistor simulations, the low-field electron mobility dependence upon Coulomb scattering in 4H-SiC devices is extracted. The simulated mobility results agree very well with available experimental data. Next, a 3D VDMOS is simulated. The nature of the physical processes occurring in both steady-state and transient conditions are revealed for the two generations of 3D VDMOS devices being considered in the study. Due to its comprehensive nature, the developed tool serves as a basis for future investigation of 4H-SiC power devices. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

Les convertisseurs de forte puissance sont des éléments critiques des futurs réseaux HVDC. À ce titre, leur fiabilité et leur endurance sont primordiales. La défaillance d’un composant se produit soit en circuit ouvert, ou en court-circuit. Le composant défaillant en circuit ouvert est inadmissible pour les convertisseurs utilisant une topologie de mise en série. En particulier, dans certaines applications HVDC, les modules doivent être conçus de telle sorte que lorsqu'une défaillance se produit, le module défaillant doit se comporter comme un court-circuit et supporter ainsi le courant nominal qui le traverse. Un tel comportement est appelé “défaillance en court-circuit” ou “failure-to-short-circuit”. Actuellement, tous les modules de puissance ayant un mode de défaillance en court-circuit disponibles dans le commerce utilisent des semi-conducteurs en silicium. Les potentialités des semi-conducteurs en carbure de silicium (SiC) poussent, aujourd’hui, les industriels et les chercheurs à mener des investigations pour développer des modules Fail-to-short à base des puces SiC. C’est dans ce contexte que se situe ce travail de thèse, visant à concevoir un module à base de puces SiC offrant un mode de défaillance de court-circuit. Pour cela nous présentons d’abord une étude de l’énergie de défaillance des puces SiC, afin de définir les plages d’activation du mécanisme Fail-to-short. Ensuite, nous démontrons la nécessité de remplacer les interconnexions classiques (fils de bonding) par des contacts massifs sur la puce. Enfin, une mise en œuvre est présentée au travers d’un module “demi pont” à deux transistors MOSFET. / The reliability and endurance of high power converters are paramount for future HVDC networks. Generally, module’s failure behavior can be classified as open-circuit failure and short-circuit failure. A module which fails to an open circuit is considered as fatal for applications requiring series connection. Especially, in some HVDC application, modules must be designed such that when a failure occurs, the failed module still able to carry the load current by the formation of a stable short circuit. Such operation is referred to as short circuit failure mode operation. Currently, all commercially available power modules which offer a short circuit failure mode use silicon semiconductors. The benefits of SiC semiconductors prompts today the manufacturers and researchers to carry out investigations to develop power modules with Fail-to-short-circuit capability based on SiC dies. This represents a real challenge to replace silicon power module for high voltage applications in the future. The work presented in this thesis aims to design a SiC power module with failure to short-circuit failure mode capability. The first challenge of the research work is to define the energy leading to the failure of the SiC dies in order to define the activation range of the Fail-to-short mechanism. Then, we demonstrate the need of replacing the conventional interconnections (wire bonds) by massive contacts. Finally, an implementation is presented through a "half bridge" module with two MOSFETs.

No description available.

The presented diploma thesis deals with the issue of wire electrical discharge machining of SiSiC ceramics. The first part explains the principles of electrical discharge machining, describes the WEDM technology and presents the properties of the advanced ceramics. The second part consists of a detailed analysis of the cutting process of eighteen samples obtained with systematically changing process parameters. Based on the obtained results from EDX analysis, SEM electron microscopy and topography there was performed an analysis of the influence of process parameters on the cutting speed, surface roughness, kerf width and number of wire breaks with usage of the selected brass cutting wire. From the evaluated results it was possible to select a combination of parameters that ensured a stable machining process.

Philosophiae Doctor - PhD / The framework agreement in the year 2000 by the international community to launch Generation IV program with 10 nations, to develop safe and reliable nuclear reactors gave rise to the increased interest in the studies of SiC and the effect of different irradiations on solids. Silicon carbide is a preferred candidate used in harsh environments due to its excellent properties such as high chemical stability and strong mechanical strength. The PBMR technology promises to be the safest of all nuclear technology that have been developed before. SiC has been considered one candidate material being used in the fabrication of pebble bed fuel cell. Its outstanding physical and chemical properties even at high temperatures render it a material of choice for the future nuclear industry as whole and PBMR in particular. Due to the hostile environment created during the normal reactor operation, some of these excellent properties are compromised. In order to use this material in such conditions, it should have at least a near perfect crystal lattice to prevent defects that could compromise its strength and performance. A proper knowledge of the behavior of radiation-induced defects in SiC is vital. During irradiation, a disordered crystal lattice occurs, resulting in the production of defects in the lattice. These defects lead to the degradation of these excellent properties of a particular material. This thesis investigates the effects of various radiation effects to 6H-SiC. We have investigated the effects of radiation induced damages to SiC, with a description of the beds and the importance of the stability of the SiC-C interface upon the effects of radiations (y-rays, hot neutrons). The irradiated samples of 6H-SiC have been studied with various spectroscopic and structural characterization methods. The surface sensitive techniques such as Raman spectroscopy, UV-Vis, Photoluminescence and Atomic Force Microscopy will be employed in several complimentary ways to probe the effect of irradiation on SiC. The obtained results are discussed in details.

No description available.