The main goal of the present work was the synthesis of alkaline-earth nitridomanganates (AExMnyNz) with extended anionic structures and the characterization of their electronic and magnetic properties. Up to now, only compounds with isolated nitridomanganate anions have been reported in the discussed ternary systems. A systematic exploratory synthesis, employing high-temperature treatment of AE nitrides and Mn under controlled N2 pressure, yielded more than ten new nitridomanganates. Their crystal structures contain anionic building blocks of different dimensionalities, ranging from isolated species to three-dimensional frameworks. In general, the formation of Mn-rich compositions was found to be driven by the emergence of Mn-Mn interactions, which creates a link between nitridometalates and transition-metal-rich binary nitrides. The obtained nitridomanganates display a plethora of interesting phenomena, such as large spin-orbit coupling, magnetic frustration, quenching of magnetism due to Mn-Mn interactions, and metal-insulator transition.

As a part of the production of nitride nuclear fuel for use in fast nuclear reactors, zirconium is hydrided followed by nitriding and mixing with uranium nitride. This work concludes a study of unwanted metallic particles present in a powder that is supposed to be a zirconium hydride. Sponge zirconium was hydrided at different temperatures and different time intervals, and the resulting hydride was milled into a powder. The powders were analyzed using SEM and XRD after which the powders were pressed into pellets for light optical microscopic study. The primary goals were determination of the structure of the particles and thereafter elimination of them. It was seen that hydriding at 500 C results in less metal particles but more experiments need to be conducted to confirm this.

Gallium nitride (GaN) is a semiconductor material highly regarded for visible light generation since it provides the most efficient platform for compact violet, blue, and green light emitters, and in turn, high-quality and ubiquitous white lighting. Despite this fact, the potential of the GaN platform has not been fully exploited. This potential must enable the precise control in the various properties of light, realizing functions beyond the conventional. Simultaneously, the field of the telecommunications is looking for candidate technologies fit for wireless transmission in the next generations of communication. Visible light communication (VLC) may play a significant role in the future of the last mile of the network by providing both a fast internet connection and a high-quality illumination. Hence, a variety of optoelectronic platforms, including distributed-feedback (DFB) lasers, superluminescent diodes (SLDs), and multi-section lasers, can be used to exploit the full potential of GaN while offering unprecedented solutions for VLC and other applications, such as atomic clocks, high-resolution fluorescence microscopy, and on-chip nonlinear processing at visible wavelengths. This dissertation demonstrates green and sky-blue DFB lasers based on GaN, with resolution-limited single-mode emission at wavelengths around 514 nm and 480 nm, side-mode suppression ratio as large as 42.4 dB, and application to up to 10.5 Gbit/s data transmission. Preliminary observations of DFB lasers with emission close to the Fraunhofer lines are presented, offering a pathway for low-background noise applications. Blue-emitting SLDs are used to demonstrate a 3.8 Gbit/s transmitter while achieving spectral efficiency of up 118.2 (mW・nm)/(kA/cm2) in continuous-wave operation. Visual quality is confirmed by coherence length and white light generation. Short-wavelength SLDs have the potential for higher resolution and fluorescence excitation in classical optical coherence tomography and fiber gyroscopes. The demonstration of a two-section green laser diode is presented, achieving coupled-cavity lasing at wavelengths of 514 nm based on an integrated green laser–absorber in self-colliding pulse configuration, operated in continuous-wave electrical injection. The integrated laser offer potential for mode- locked and Q-switched lasing. The integrated laser is suitable for reconfiguration where laser–modulator, laser–absorber, and laser–amplifier are proposed and investigated at green wavelengths.

The Council for Scientific and Industrial Research (CSIR) in South Africa is developing a process to produce titanium tetrachloride from a low-grade material such as ilmenite. Titanium tetrachloride can then be used as feed material for titanium metal or pigment-grade titanium dioxide production. Titanium tetrachloride is commercially produced by chlorinating synthetic rutile (<92% TiO2) or titanium dioxide slag (<85% TiO2) at ~900 ˚C. A drawback of chlorination at this temperature is that any constituents other than TiO2 will end up as hazardous waste material. A characteristic step in the CSIR’s proposed process is to nitride titanium dioxide contained in the feed material before it is sent for chlorination. The chlorination of the resulting titanium nitride is achieved at a much lower temperature (~200 ˚C) than that of the existing titanium dioxide chlorination reaction. An added advantage of the low-temperature chlorination reaction is that chlorine is selective mostly towards titanium nitride and metallic iron, which means that any other constituents present are not likely to react with the chlorine. The result is reduced chlorine consumption and less hazardous waste produced. The nitrided ilmenite must, however, be upgraded by removing all iron before it can be sent for chlorination. Commercial ilmenite upgrading processes, called synthetic rutile production, also require the removal of iron and other transition metals before chlorination. A literature review of existing ilmenite upgrading processes revealed four possible process options that could remove iron from nitrided ilmenite. Two of these process options, the Becher and Austpac ERMS SR processes, are proven process routes. The other two are novel ideas – one to passivate iron contained in the nitrided ilmenite against chlorination and the other to use ammonium chloride (as used in the Becher process) as a stoichiometric reactant to produce a ferrous chloride solution. A preliminary experimental evaluation of these process options indicated that the Austpac ERMS SR process is the most viable option for removing iron from nitrided ilmenite. The Austpac ERMS SR process was therefore selected as a template for further process development. A detailed Austpac ERMS SR process review found that two process units in the Austpac ERMS SR process could be used in a process that separates iron from nitrided ilmenite. These are the Enhanced Acid Regeneration System and the Direct Reduced Iron process units. The review also concluded that another leach unit would have to be developed. It was therefore necessary to further investigate the dissolution of nitrided ilmenite in hydrochloric acid. A detailed experimental evaluation of nitrided ilmenite dissolution in hydrochloric acid found that hydrochloric acid could be used as the lixiviant to selectively remove iron from nitrided ilmenite. The dissolution of metallic iron in 90 ˚C hydrochloric acid reached levels of at least 96% after only 60 minutes. An average “combined resistance” rate law was found that could be used to describe this dissolution reaction. The observed activation energy and Arrhenius pre-exponential factor were found to be equal to 9.45 kJ.mol-1 and 30.8 s-1 respectively. The Austpac ERMS SR process review and experimental results described above were then combined and used to propose a process that could be employed to remove iron from nitrided ilmenite. The proposed process was modelled using the Flowsheet Simulation module in HSC Chemistry 7.0 / Dissertation (MEng (Chemical Engineering))--University of Pretoria, 2010. / Chemical Engineering / MEng (Chemical Engineering) / unrestricted

Die molekularen Tantal-Komplexe [TaCl4(NMe2)(OEt2)], [TaCl4(NEt2)(OEt2)], [TaCl3(NEt2)(OiPr)]2, [TaCl4(NEt2)]2, [TaCl3(NEt2)2]2, sowie die Pentachloroetherate [MCl5(OEt2)] (M=Nb, Ta) wurden synthetisiert und ihre Kristallstruktur aufgeklärt. Bei der Zersetzung der Chloridetherate in die korrespondierenden Oxidtrichloride, konnte die vermutete Abspaltung von Ethylchlorid bestätigt werden. Bei den Untersuchungen zur Leistungsfähigkeit der Polyol-Route wurden binäre und ternäre Sulfide, welche sich nicht im wäßrigen System fällen lassen, synthetisiert. Die Sulfide der Gruppe 10 konnten erstmals unter den milden Reaktionsbedingungen der Polyol-Route erhalten werden. Im Falle der untersuchten ternären Cuprosulfide von Indium, Zinn und Antimon, konnte gezeigt werden, daß diese durch die Präparation mittels Polyol-Route zunächst kubisch mit ungeordnetem Kationenteilgitter kristallisieren. Durch eine anschließende thermische Behandlung bei 623 K wurden diese in die tetragonal geordneten Modifikationen überführt. Bei der polyol-vermittelten Synthese binärer und ternärer Selenide wurde erstmals Selenoharnstoff erfolgreich eingesetzt. Hiermit konnte im Zuge der Synthese eine homogene, molekulare Lösung erhalten werden, bei der eine optimale Durchmischung der anionischen und kationischen Edukte gewährleistet ist. Das ternäre Mischoxid LaTaO4 wurde erfolgreich mit Hilfe der Polyol-Route hergestellt und in einem Wirbelschichtreaktor zu La2Ta2O5N2 umgesetzt. Dafür wurde eine neue Anlage entwickelt. Es konnte gezeigt werden, daß die Präparation von Tantaloxidnitriden im Wirbelschichtreaktor mit einem geringeren Ammoniak-Verbrauch als bei der bisherigen Prozeßführung möglich ist.

La fonctionnalisation de la surface du verre par des molécules organiques permet de modifier son énergie de surface ou d’améliorer l’adhésion d’un revêtement. La méthode classique de fonctionnalisation directe du verre repose sur une réaction de silanisation, via la formation de ponts siloxanes Si O Si. Ces ponts ont tendance à s’hydrolyser en milieu salin ou alcalin, entrainant la perte de la fonctionnalité du verre. Une solution envisagée consiste à déposer une couche de nitrure de silicium (SixN4) sur le verre, permettant de greffer des molécules organiques via des liaisons covalentes robustes : Si C ou N C. Le nitrure de silicium présente l’avantage d’être un matériau très souvent utilisé dans l’industrie verrière en raison de sa capacité à bloquer la diffusion des ions sodium et de protéger ainsi le verre de la corrosion.L’objectif de ce travail de thèse est de caractériser et contrôler la surface du nitrure de silicium, puis d’optimiser et de comprendre la modification de sa surface par le greffage covalent de molécules organiques.Lorsque le nitrure de silicium est exposé à l’air, une couche d’oxynitrure est formée en surface. L’optimisation et la compréhension du décapage de cette couche d’oxynitrure natif en milieu liquide est l’objet de la première phase de ce travail. La composition chimique de la surface est finement caractérisée et quantifiée en combinant des mesures de spectroscopie infrarouge en mode de réflexion totale atténuée (IR-ATR), de spectroscopie de photoélectrons X (XPS) et des dosages chimiques de surface. Le décapage dans des solutions fluorées (HF et NH4F) permet de retirer efficacement la couche d’oxynitrure et laisse majoritairement en surface des liaisons Si-F et dans une moindre mesure des liaisons N H et Si OH. La composition chimique de la surface peut toutefois être modifiée pour former des groupements Si H, soit en enrichissant la couche du SixN4 en silicium, soit en soumettant la surface à un traitement par plasma d’hydrogène à l’issue du décapage. A partir des observations expérimentales, une proposition décrivant les mécanismes mis en jeu lors du décapage est présentée.Dans la seconde partie de la thèse, la surface du nitrure de silicium est modifiée par l’immobilisation de molécules organiques, plus spécifiquement par la réaction d’un 1 alcène sous activation thermique ou photochimique. La composition chimique de la surface et les conditions d’activation de la réaction modifient la réaction de greffage et la densité des couches organiques. En particulier, la présence de liaisons Si-H et l’enrichissement de la couche en silicium sont étudiés en détail. Dans une dernière partie, dans une visée plus applicative, des couches denses fluorées présentant un caractère hydrophobe naturel sont greffées sur la surface du nitrure de silicium. / Covalent grafting of organic molecules on glass can modify its surface physico-chemical properties or improve the adhesion of a coating. Such a functionalization usually relies on a silanisation reaction, bonding molecules to the surface through Si-O-Si bonds. Unfortunately, the resulting molecular layers do not exhibit long-term stability due to the hydrolysis of siloxane groups. One solution would consist in depositing a silicon nitride layer on glass, allowing the glass surface to be functionalized through more stable bonds N-C or Si-C. Silicon nitride layers are frequently used in glass industry. They are well-known for their durability properties and are often used as a protective layer against glass corrosion.The aim of this project is to characterize and control the non-oxidized silicon nitride surface, then to optimize and understand the surface modification by covalent grafting of organic molecules.When silicon nitride is exposed to atmosphere, an oxynitride layer is formed on its surface. Several efficient ways to remove this native oxynitride are first studied and optimized. The quantitative characterization and control of the surface chemical composition provide a reliable starting point for the functionalization step. The surface chemical composition is quantitatively investigated by combining Attenuated Total Reflection InfraRed spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS) and chemical dosing. The etching in HF-based solutions efficiently removes the oxynitride layer and leads to a surface mainly covered with Si-F bonds and smaller amounts of Si-OH and N-H bonds. The surface composition can be modified by a H2 plasma treatment performed after the wet etching or by changing the silicon nitride layer composition (silicon enrichment), leading in either case to the formation of Si-H bonds on surface. An etching mechanism is suggested from these experimental observations.The second part of this work is focused on the grafting of the alkyl chains on the silicon nitride surface. The surface is reacted with a 1-alkene, using photochemical or thermal activation. The grafting efficiency depends on the surface composition and the activation conditions. The presence of surface Si-H bonds and the effect of Si enrichment are considered in details. In a final part, in an applicative view, functional hydrophobic molecules are grafted on the silicon nitride surface.

Le nitrure de bore hexagonal (hBN) est un semi-conducteur à grand gap (>6 eV) appartenant à la nouvelle famille des cristaux 2D. Ses propriétés isolantes et sa structure cristalline font de lui un matériau stratégique dans la réalisation d’hétérostructures 2D à base de graphène. L’objectif de cette thèse a été d’étudier les propriétés optiques et structurales des feuillets de hBN.Après une description des méthodes expérimentales, les propriétés du matériau massif - loin des interfaces - sont étudiées sur le cristal de référence synthétisé par croissance haute-pression haute-température au Japon. L’étude en microscopie électronique à transmission a permis d’identifier l’empilement AA’, caractéristique du hBN. Les 3 principales régions d’émission de luminescence du hBN sont identifiées et analysées dans le détail : excitons libres, excitons piégés et défauts profonds. L’efficacité radiative excitonique a été analysée sur des cristaux issus de différentes voies de synthèse mettant en évidence des qualités dispersées. L’origine des processus de luminescence est discutée en regard des différentes interprétations actuelles, théoriques et expérimentales.Le cœur de la thèse porte sur les propriétés des cristaux 2D de faibles épaisseurs obtenus par clivage mécanique, ceci jusqu’à la monocouche atomique. Les expériences réalisées en spectroscopie Raman basse fréquence, en spectroscopie de pertes d’énergie et en cathodoluminescence ont mis en évidence une série d’effets de basse dimensionnalité sur les propriétés vibrationnelles, diélectriques et excitoniques du hBN. L’étude des défauts introduits lors de l’étape d’exfoliation et leur impact sur les émissions de luminescence ont permis d’isoler les propriétés intrinsèques des cristaux 2D de hBN. Les premiers résultats obtenus sur des feuillets suspendus dans le vide sont présentés et les effets de déformation élastique et plastique sur la luminescence de hBN discutés.La dernière partie de cette thèse porte sur des cristaux de nitrure de bore rhomboédrique (rBN) où les feuillets atomiques forment un empilement ABC. Ces cristaux ont permis d’aborder l’effet de l’empilement des plans atomiques sur la luminescence du BN en hybridation sp². / Hexagonal boron nitride (hBN) is a wide bandgap semi-conductor (>6 eV) which belongs to the 2D crystals family. Its structure and insulating properties make him as a strategic component towards the conception of graphene-based 2D heterostructures. This thesis focuses on the structural and optical properties of hBN layers.After a brief description of experimental methods, bulk material properties have been investigated on the reference HPHT-grown crystal fabricated in Japan. The characteristic stacking AA’ sequence of the hexagonal BN phase has been identified by transmission electron microscopy. Characteristics features of the 3 main luminescence regions have been identified and analyzed into details: free excitons, bound excitons and deep defects. The radiative efficiency of excitons recombinations in hBN has been studied on crystals obtained through various synthesis routes. The origin of hBN luminescence processes is discussed on the basis of current theoretical and experimental interpretations.The main part of the thesis is dedicated to the study of nanometer-thick hBN crystals obtained by mechanical cleavage, down to the monolayer. Experiments carried out by low-frequency Raman spectroscopy, energy loss spectroscopy and cathodoluminescence demonstrated a series of low-dimensionality effects on the vibrational, dielectric and excitonic properties of hBN. Defects introduced during the exfoliation step have been studied, their impact on luminescence emissions allowed us to isolate the intrinsic properties of 2D hBN flakes. Preliminary results obtained on hBN layers suspended in vacuum are presented and the effects of elastic and plastic deformation on BN luminescence are discussed.The last part of the work focuses on rhombohedral boron nitride (rBN) crystals where the BN stacking sequence follows the ABC type. Studying these crystals made possible the investigation of the influence of the stacking sequence on sp² BN luminescence.

abstract: Silicon photovoltaics is the dominant contribution to the global solar energy production. As increasing conversion efficiency has become one of the most important factors to lower the cost of photovoltaic systems, the idea of making a multijunction solar cell based on a silicon bottom cell has attracted broad interest. Here the potential of using dilute nitride GaNPAs alloys for a lattice-matched 3-terminal 2-junction Si-based tandem solar cell through multiscale modeling is investigated. To calculate the electronic band structure of dilute nitride alloys with relatively low computational cost, the sp^3 d^5 s^* s_N tight-binding model is chosen, as it has been demonstrated to obtain quantitatively correct trends for the lowest conduction band near Γ, L, and X for dilute-N GaNAs. A genetic algorithm is used to optimize the sp^3 d^5 s^* tight-binding model for pure GaP and GaAs for their optical properties. Then the optimized sp^3 d^5 s^* s_N parametrizations are obtained for GaNP and GaNAs by fitting to experimental bandgap values. After that, a virtual crystal approach gives the Hamiltonian for GaNPAs alloys. From their tight-binding Hamiltonian, the first-order optical response functions of dilute nitride GaNAs, GaNP, and GaNPAs are calculated. As the N mole fraction varies, the calculated critical optical features vary with the correct trends, and agree well with experiment. The calculated optical properties are then used as input for the solar device simulations based on Silvaco ATLAS. For device simulation, a bottom cell model is first constructed to generate performance results that agree well with a demonstrated high-efficiency Si heterojunction interdigitated back contact (IBC) solar cell reported by Kaneka. The front a-Si/c-Si interface is then replaced by a GaP/Si interface for the investigation of the sensitivity of the GaP/Si interface to interface defects in terms of degradation of the IBC cell performance, where we find that an electric field that induces strong band bending can significantly mitigate the impact of the interfacial traps. Finally, a lattice-matched 3-terminal 2-junction tandem model is built for performance simulation by stacking a dilute nitride GaNP(As) cell on the Si IBC cell connected through a GaP/Si interface. The two subcells operate quasi-independently. In this 3-terminal tandem model, traps at the GaP/Si interface still significantly impact the performance of the Si subcell, but their effects on the GaNP subcell are relatively small. Assuming the interfacial traps are well passivated, the tandem efficiency surpasses that of a single-junction Si cell, with values close to 33% based on realistic parameters. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

This manuscript describes the design, development, and implementation of a linear high efficiency power amplifier. The symmetrical Doherty power amplifier utilizes TriQuint's 2nd Generation Gallium Nitride (GaN) on Silicon Carbide (SiC) High Electron Mobility Transistor (HEMT) devices (T1G6001032-SM) for a specified design frequency of 3.6 GHz and saturated output power of 40 dBm. Advanced Design Systems (ADS) simulation software, in conjunction with Modelithic's active and passive device models, were used during the design process and will be evaluated against the final measured results. The use of these device models demonstrate a successful first-pass design, putting less dependence on classical load pull analysis, thereby decreasing the design-cycle time. The Doherty power amplifier is a load modulated amplifier containing two individual amplifiers and a combiner network which provides an impedance inversion on the path between the two amplifiers. The carrier amplifier is biased for Class-AB operation and works as a conventional linear amplifier. The second amplifier is biased for Class-C operation, and acts as the peaking amplifier that turns on after a certain instantaneous power has been reached. When this power transition is met the carrier amplifier's drain voltage is already approaching saturation. If the input power is further increased, the peaking amplifier modulates the load seen by the carrier amplifier, such that the output power can increase while maintaining a constant drain voltage on the carrier amplifier. The Doherty power amplifier can improve the efficiency of a power amplifier when the input power is backed-off, making this architecture particularly attractive for high peak-to-average ratio (PAR) environments. The design presented in this manuscript is tuned to achieve maximum linearity at the compromise of the 6dB back-off efficiency in order to maintain a carrier-to- intermodulation ratio greater than 30 dB under a two-tone intermodulation distortion test with 5 MHz tone spacing. Other key figures of merit (FOM) used to evaluate the performance of this design include the power added efficiency (PAE), transducer power gain, scattering parameters, and stability. The final design is tested with a 20 MHz LTE waveform without digital pre-distortion (DPD) to evaluate its linearity reported by its adjacent channel leakage ratio (ACLR). The dielectric substrate selected for this design is 15 mil Taconic RF35A2 and was selected based on its low losses and performance at microwave frequencies. The dielectric substrate and printed circuit board (PCB) design were also modeled using ADS simulation software, to accurately predict the performance of the Doherty power amplifier. The PCB layout was designed so that it can be mounted to an existing 4" x 4" aluminum heat sink to dissipate the heat generated by the transistors while the part is being driven. The performance of the 3.6 GHz symmetrical Doherty power amplifier was measured in the lab and reported a maximum PAE of 55.1%, and a PAE of 48.5% with the input power backed-off by 6dB. These measured results closely match those reported by design simulations and demonstrate the models' effectiveness for creating a first-pass functional design.

Due to the multi-tasking and miniaturization of electronic devices, faster heat transfer is required from the device to avoid the thermal failure. Die-attached polymer adhesives are used to bond the chips in electronic packaging. These adhesives have to hold strong mechanical, thermal, dielectric, and moisture resistant properties. As polymers are insulators, heat conductive particles are inserted in it to enhance the thermal flow with an attention that there would be no electrical conductivity as well as no reduction in dielectric strength. This thesis focuses on the characterization of polymer nanocomposites for thermal and electrical properties with experimental and computational tools. Platelet geometry of hexagonal boron nitride offers highly anisotropic properties. Therefore, their alignment and degree of orientation offers tunable properties in polymer nanocomposites for thermal, electrical, and mechanical properties. This thesis intends to model the anisotropic behavior of thermal and dielectric properties using finite element and molecular dynamics simulations as well as experimental validation.