This thesis presents the work of adding support for a communications protocol in a space application. The work is a part of KTHs MIST (MIniature STudent Satellite) project which aims at sending an experimental satellite into space. Each experiment on the satellite is designed as a subsystem. These subsystems need to be able to communicate with the main computer on the satellite in order to transfer the results of the experiments down to earth. In efforts prior to the current thesis, a special communications protocol has been specified to solve this problem. That protocol is called MSP (MIST Space Protocol). This paper describes the efforts to add support for MSP to two of the satellite’s experiments. These two experiments are called SiC in Space and Piezo-LEGS. However, since Piezo-LEGS is incompatible with the I2C bus in which MSP runs on top of, it must communicate through the SiC experiment. Which parts of the protocol that need to be supported by each experiment are defined. The result of the work is that the experiments can communicate with the main computer through the MSP protocol. / Denna kandidatuppsats beskriver arbetet med att implementera MSP protokollet för de två experimenten SiC och Piezo-LEGS. Syftet med MIST projektet är att skicka upp en experimentsatellit i omloppsbanan runt jorden. I satelliten finns ett antal experiment. De två experiment som arbetet ar fokuserat på är ”SiC in Space” och ”Piezo-LEGS”. SiC-experimentets syfte är att man ska göra mätningar på en kiselkarbid (SiC) transistor i rymdens vacuum. Syftet med Piezo-LEGS experimentet är att man vill mäta hur prestandan för en piezoelektrisk motor påverkas i rymden. Inom MIST-projektet har ett kommunikationsprotokoll som kallas MSP utvecklats för kommunikation mellan satellitens huvuddator och experimenten. I detta arbete har MSP protokollet implementerats för experimenten SiC och Piezo-LEGS Eftersom Piezo-LEGS experimentet är inte kompatibel med I2C bussen som används av MSP protokollet, utan istället använder sig utav ASCII-kommandon via RS-485, måste MSP kommandona översättas till ASCII-kommandon utav SiC.

M. Tech. (Department of Metallurgical Engineering, Faculty of Engineering Technology), Vaal University of Technology. / This work is an investigation in the development and characterisation of porous Ti-Sn-SiC and Ti-Nb-SiC composites. Pure Titanium (Ti), Tin (Sn), Niobium (Nb) and Silicon carbide (SiC) powders were used as starting materials. The Ti-Sn-SiC and Ti-Nb-SiC composites were produced by powder metallurgy (PM) press-and-sinter route. The Sn is an α-phase stabilizer while Nb is a β-phase stabilizer in Ti alloys. A systematic study of binary Ti-Sn and Ti-Nb alloys was conducted with the addition of SiC particles. The addition of Sn influences the microstructure of the titanium alloy. With increasing the percentage of Sn content, the density of the samples decreases on the Ti-Sn alloys. An increase in the Sn content from 10 to 25 wt. % content resulted in decreased hardness. The Ti-Sn binary revealed stability of the HCP phase with increasing composition of the Sn content. The porous structures of the Ti-Sn-SiC composites were evenly distributed throughout the materials. The sintered densities increase from 94.69% to 96.38%. XRD analysis detected the HCP crystal lattice structure for the Ti5.4Sn3.8SiC and Ti5.6-Sn3.8-SiC composites. XRD pattern of the Ti5.8-Sn3.8-SiC reveals both the HCP and FCC crystal structures. The HCP phase has lattice parameters a= 2.920 Å; c=4.620 Å with smaller c/a ratio of 1.589. Additionally, FCC lattice parameter a=5.620 Å Fm-3m # 225 was obtained both for Ti5.8Sn3.8SiC and Ti6.0Sn3.8SiC XRD patterns. On the other hand, Optical microscopy analysis of the Ti-Nb alloys revealed the equiaxed grains composed of the light β-phase segregating on the grain boundaries. The Ti9Nb1 has low Vickers hardness of all alloys while Ti8Nb2 and Ti7.5Nb2.5 alloys are harder due to high amount of Nb content. Generally, the densities of the Ti–Nb alloys increased with increasing Nb content. HCP and BCC phases have the lattice parameters a = 2.951 Å, c = 4.683 Å and 3.268 Å, respectively. An HCP (α′) phase was detected in the Ti8.5Nb1.5 alloy with lattice parameters a = 5.130 Å, c = 9.840 Å while a BCC phase had a = 3.287 Å. The sintered Ti8Nb2 alloy also had the α′-phase with a = 5.141 Å, c = 9.533 Å and BCC phase with a = 3.280 Å lattice parameters. On the contrary, the Ti7.5Nb2.5 alloy formed the α′-phase of a = 5.141 Å, c = 9.533 Å and BCC with a = 3.280 Å lattice parameters. For the 10 and 15 wt.% Nb alloys, very porous structures were observed. The pores appear spherical and widely distributed. As the Nb content is increased to 20 wt.% (Ti7Nb2SiC) and 25 wt.% (Ti7Nb2.5SiC), porosity was minimized. The sintered densities of the Ti-Sn alloys are decreasing from 95.90% to 92.80% with increased amount of Sn in the Ti, while the sintered densities of Ti-Sn-SiC are increasing from 94.69% to 96.38%. The high porosity, which developed in Ti7Nb1SiC and Ti7Nb2.5SiC, affected the densities of these composites. The sintered densities of Ti-Nb alloys are increasing from 92.08% to 97.65% with increased amount of Nb in the Ti. In terms of hardness Ti7Nb1SiC and Ti7Nb2.5SiC resulted in the lowest while Ti7Nb1.5SiC and Ti7Nb2SiC composites were 511.74 HV and 527.678 HV. The porosity levels were increased by the addition of SiC in the Ti-Sn-SiC and Ti-Nb-SiC composites. The XRD analysis revealed phase transformation on the Ti-Nb alloys and Ti-Nb-SiC composites.

De flesta nuvarande elfordon drivs med en systemspänning på 400V. En ökning av denna spänning skulle dock bland annat kunna möjliggöra en kortare laddningstid för fordonets batteri. Den högre systemspänningen medför ett antal utmaningar, varav en är att öka spänningsblockeringsförmågan hos spänningsomvandlarens transistorer. Det finns olika sätt att lösa detta problem. Om den vanligast förekommande tvånivåstopologin ska användas måste spänningskapaciteten för dess transistorer ökas. Alternativt kan man använda en topologi med tre nivåer med neutralpunktsanslutning, som inte kräver transistorer med så hög spänningsblockeringsförmåga. Syftet med denna rapport är att jämföra verkningsgraden hos dessa två topologier. Därför estimeras med hjälp av simulering spänningsomvandlarnas förluster och utgångseffekt i stationärt tillstånd över hela driftområdet för en elektrisk maskin som är lämplig för fordonsapplikationer. Olika sätt att estimera förlusterna utvärderades och transistorerna i de två topologierna valdes på ett sätt så att en rättvis jämförelse mellan topologierna kunde säkerställas. Ett antal nyckelparametrar som kan påverka effektiviteten hos spänningsomriktarna utvärderades i simuleringar. De erhållna resultaten tyder på att verkningsgraden hos både två nivå- och tre nivå-topologin med neutralpunktsanslutning är jämförbara för större delen av driftområdet. / Most current electric vehicles operate on a system voltage of 400V. However, increasing this voltage would come with, among other benefits, a reduced charging time for the vehicle’s battery. The higher system voltage does pose a number of challenges, with the increase in voltage blocking capability of the power switches of the voltage source inverter being one of them. There are different ways of solving this problem. If the currently preferred two-level topology is to be used, the voltage rating of its switches needs to be increased. Alternatively a three-level neutral point clamped topology could be used which does not require switches with such a high blocking voltage rating. The purpose of this thesis is to compare the efficiency of these two solutions. Therefore the steady state losses and output power of the voltage source inverters throughout the operating range of an electric machine suitable for automotive application were estimated through simulation. Different ways of estimating the losses were evaluated and the power switches of the two topologies were chosen in such a way that a fair comparison between the topologies could be ensured. A number of key parameters that could impact the efficiency of the voltage source inverters were evaluated in simulations. The obtained results suggest that for the majority of the operating range the efficiency of the two-level and the three-level neutral point clamped topology are closely matched.

GaN-HEMTs (Gallium Nitride-based High Electron Mobility Transistors) have, thanks to the large band gap of GaN, electrical properties that are suitable for applications of high electrical voltages, high currents, and fast switching. The large band gap also gives GaN-HEMTs a high resistance to radiation. In this degree project, the effects of 2 MeV proton irradiation of GaN-HEMTs constructed on both silicon carbide and silicon substrates are investigated. 20 transistors per substrate were irradiated in the particle accelerator 5 MV NEC Pelletron in the Ångström laboratory at Uppsala University. These transistors were exposed to radiation doses in the range of 10^11 to 10^15 protons/cm^2. The analysis shows that both transistors on silicon, as well as silicon carbide, are unaffected by proton irradiation up to a dose of 10^14 protons/cm^2. GaN-on-Si transistors show less influence of radiation than GaN-on-SiC transistors. The capacitances between gate and drain as well as drain and source for both GaN-on-SiC and GaN-on-Si HEMTs show hysteresis as a function of forward and backward gate voltage sweeps for the radiation dose of 10^15 protons/cm^2. / GaN-HEMTs (Galliumnitridbaserade High Electron Mobility Transistors) har tack vare det stora bandgapet i GaN goda elektriska egenskaper som lämpar sig för höga elektriska spänningar, höga strömmar och snabb växling mellan av- och på-tillstånd. Det stora bandgapet ger även GaN-HEMTs ett stort motstånd mot strålning.I detta examensarbete undersöks effekterna av 2 MeV protonbestrålning av GaN-HEMTs. Dessa HEMTs är konstruerade på både kiselkarbid- och kiselsubstrat.20 transistorer per transistorsubstrat bestrålades i partikelacceleratorn 5 MV NEC Pelletron i Ångströmslaboratoriet vid Uppsala Universitet. Dessa transistorer utsattes för strålningsdoser inom intervallet 10^11 till 10^15 protoner/cm^2. Resultaten visar att både tranisistorer på kisel såsom kiselkarbid är opåverkade av strålning upp till en dos av 10^14 protoner/cm^2. GaN-på-Si-transistorer visar en mindre påverkan av protonstrålning än GaN-på-SiC-transistorer. Ytterligare uppstod hysteresis för kapacitanser mellan gate och drain och mellan gate och source som en funktion av fram- och bakriktad gate-spänning efter en strålningsdos av 10^15 protoner/cm^2.

No description available.

In dieser Arbeit wird die elektroakustische Kopplung zwischen Ladungsträgern in Graphen und akustischen Oberflächenwellen (engl. surface acoustic waves, SAWs) untersucht. Es werden elektroakustische Bauelemente aus epitaktischem Graphen auf einem SiC-Substraten demonstriert, auf denen eine piezoelektrische ZnO-Schicht abgeschieden wurde, um die Erzeugung und Ausbreitung von SAWs zu verbessern. Eine dünne MgO-Schicht dient zum Schutz des Graphens während der ZnO Sputterbeschichtung. Bei zwei SAW-Moden mit Frequenzen um 2 GHz können wir in Graphen-Baulementen, welche sich im SAW-Ausbreitungsweg befinden, elektroakustische Ströme messen. Ein klassisches Relaxationsmodell der Wechselwirkung zwischen SAWs und den Ladungsträgern eines zweidimensionalen Elektronengases wird zur Erklärung der Ergebnisse herangezogen. Um die akusto-elektrischen Ströme in unseren Graphen-Bauelementen zu erhöhen, verwenden wir zwei Methoden: (1) Verbesserung der elektronischen Eigenschaften von Graphen und (2) Erzeugung starker Spannungsfelder in den Graphen-Bauelementen. Um die elektronischen Eigenschaften von Graphen zu verbessern, verwenden wir eine Methode namens Hydrierung, welche die Grenzfläche zwischen Graphen und dem SiC-Substrat modifiziert. Durch Raman-Charakterisierung belegen wir die Entkopplung von Pufferschicht und SiC-Substrat während des Hydrierungsprozesses, wodurch quasi-freistehendes zweilagiges Graphen mit verbesserten elektronischen Raumtemperatureigenschaften entsteht. Im Hinblick auf die Verstärkung der SAW-Spannungsfelder untersuchen wir die Leistungfähigkeit von interdigitalen Schallwandlern (engl. interdigital transducers, IDTs) mit unterschiedlichen Strukturformen, wie z. B. Splitfinger-IDTs mit Doppelfingern, fokussierende IDTs und tapered-IDTs. Die Oberflächenverschiebung entlang der SAW-Ausbreitungsstrecke wird gemessen, um die Fähigkeit dieser IDT-Strukturformen zu demonstrieren, starke Spannungsfelder in einem begrenzten Bereich anzuregen. / This thesis investigates the acousto-electric coupling between charge carriers in graphene and surface acoustic waves (SAWs). Acousto-electric devices based on epitaxial graphene on a SiC substrate are demonstrated, where a piezoelectric ZnO layer is deposited to enhance the SAW generation and propagation. A thin MgO layer is used to protect the graphene during the sputtering of the ZnO layer. By Raman spectroscopy and electronic characterization, we show that the structural and electrical properties of graphene are well preserved after the layer deposition. For two SAW modes with frequencies around 2 GHz, we measure acousto-electric currents in graphene devices placed at the SAW propagation path. A classical relaxation model of the interaction between SAWs and charge carriers in a two-dimensional electron gas is used to explain the results. In order to enhance the acousto-electric currents in our graphene devices, we take approaches in two directions: (1) improving the electronic properties of graphene and (2) exciting strong SAW fields in graphene devices. To improve the electronic properties of graphene, we use a method called hydrogenation to modify the interface between graphene and the SiC substrate. By Raman characterization, we confirm the decoupling of the buffer layer after the hydrogenation process, obtaining quasi-free-standing bilayer graphene with improved electronic properties at room temperature. Regarding the enhancement of the SAW field intensity, we investigate the performance of several types of interdigital transducers (IDTs) such as split-finger IDTs, focusing IDTs and tapered IDTs. Surface displacement along the SAW propagation path is measured to demonstrate the capability of these IDT designs to excite strong SAW fields in a confined area.

Graphene is a novel two dimensional material that is revolutionizing many areas of science and it is no surprise that a significant amount of effort is dedicated to its investigation. One of the major areas of graphene research is the development of procedures for large scale production. Among many recently developed methodologies, graphene oxide reduction stands out as a straightforward and scalable procedure for producing final material with properties similar to those of graphene. Laser reduction of graphite oxide is one of the novel approaches for producing multilayer graphene, and this work describes a viable approach in detail. It is determined that a material which is comprised of a combination of laser reduced graphite oxide-coupled to an unreduced graphite oxide layers beneath it, produces a broadband photosensitive material. The efficiency of light conversion into electrical current is greatly dependent upon the oxygen content of the underlying graphite oxide. Developing novel ways for reducing graphite oxide is an ongoing effort. This work also presents a new method for achieving complete reduction of graphite oxide for producing predominantly sp2 hybridized material. This approach is based on the irradiation of graphite oxide with a high flux 3 keV Ar ion beam in vacuum. It is determined that the angle of irradiation greatly influences the final surface morphology of reduced graphite oxide. Also, multilayer epitaxial graphene growth on silicon carbide in ultra-high vacuum was investigated with quadrupole mass spectrometry (QMS). Subliming molecular and atomic species were monitored as a function of temperature and heating time. The grown films were characterized with X-ray photoelectron spectroscopy coupled with Ar ion depth profiling.

First-principles calculations have been carried out to investigate structural and electronic properties of graphene on SiC and diamond substrates and for a study of doping of fluorographene with various surface adsorbates. New insight is given into the problem of the decoupling of the graphene layers from SiC substrates after epitaxial growth. Mechanisms of hydrogen penetration between graphene and SiC(0001) surface, and properties of hydrogen and fluorine intercalated structures have been studied. Energy barriers for diffusion of atomic and molecular hydrogen through the interface graphene layer with no defects and graphene layers containing Stone-Wales defect or two- and four-vacancy clusters have been calculated. It is argued that diffusion of hydrogen towards the SiC surface occurs through the hollow defects in the interface graphene layer. It is further shown that hydrogen easily migrates between the graphene layer and the SiC substrate and passivates the surface Si bonds, thus causing the graphene layer decoupling. According to the band structure calculations the graphene layer decoupled from the SiC(0001) surface by hydrogen intercalation is undoped, while that obtained by the fluorine intercalation is p-type doped. Further, structure and the electronic properties of single and double layer graphene on H-, OH-, and F- passivated (111) diamond surface have been studied. It is shown that graphene only weakly interacts with the underlying substrates and the linear dispersion of graphene pi-bands is preserved. For graphene on the hydrogenated diamond surfaces the charge transfer results in n-type doping of graphene layers and the splitting of conduction and valence bands in bilayer graphene. For the F- and OH-terminated surfaces, charge transfer and doping of graphene do not occur. Finally, the possibility of doping fluorographene by surface adsorbates have been investigated. The structure and electronic properties of fluorographene with adsorbed K, Li, Au atoms, and F4-TCNQ molecule are described. It is shown that adsorption of K or Li atoms results in electron doping of fluorographene, while Au atoms and F4-TCNQ introduce deep levels inside the band gap. The calculated value of the fluorographene work function is extremely high, 7.3 eV, suggesting that p-type doping is difficult to achieve.

Flexible thermal protection systems are of interest due to their necessity for the success of future atmospheric entry vehicles. Current non-ablative flexible designs incorporate a two-dimensional woven fabric on the leading surface of the vehicle. The focus of this research investigation was to characterize the aerothermal performance of silicon carbide fabric using the 30 kW Inductively Coupled Plasma Torch located at the University of Vermont. Experimental results have shown that SiC fabric test coupons achieving surface temperatures between 1000°C and 1500°C formed an amorphous silicon dioxide layer within seconds after insertion into air plasmas. The transient morphological changes that occurred during oxidation caused a time dependence in the gas / surface interactions which may detrimentally affect the in-flight performance. Room temperature tensile tests of the SiC coupons have shown a rapid strength loss for durations less than 240 seconds due to oxidation. Catastrophic failure and temperature spikes were observed on almost all SiC coupons when exposed to air plasmas at heat fluxes above 80 W/cm2. Interestingly, simulation of entry into the Mars atmosphere using a carbon dioxide plasma caused a material response that was vastly different than the predictable silica layer observed during air plasma exposure.

Silicon Carbide (SiC) is increasingly gaining attention as a potential fuel cladding material, on account of its favorable thermo-mechanical and neutronic properties. The major limitations of such a cladding is currently associated with joining and hermetic sealing. The work presented here investigated the use of Al, Cr and Fe metals and a specialized alloy (FeCrAl) to achieve hermetic sealing of SiC tubes as well as a joining technology of SiC. Major part of solving this issue requires addressing joining of ceramic and metallic components, which are largely dissimilar in both thermal and mechanical properties. Preliminary experiments to bond SiC with FeCrAl resulted in adverse separation partially attributed to the differences in thermal expansion mismatch. To alleviate these problems, thin and thick coatings of the metals and alloys were applied to SiC. Qualitative microstructural characterization of the final product indicated satisfactory bonding between the materials.