<p> Promoted and supported catalysts were studied with an electron probe microanalyzer. Investigations we re made on promoted fused iron ammonia synthesis catalysts regardirlg the general morphology of the catalysts, the distribution of promoters, and the reduction and poisoning processes . The effect of the promoters on the reduction process , the effect of the impurity silica on the promoter distributions and the nature of the poisoning are discussed. Supported catalysts were prepared by impregnating porous y-alumina spheres with solutions of chromium and/or copper compounds. Concentration profiles for chromium and/or copper , i.e., concentration as a function of distance from the center, were determined on sectioned spheres. Different distributions of these elements on the alumina support were obtained by varying the chemical used, the concentration and the amount of the solution impregnated. Explanations are given for the physical and chemical processes involved during the impregnation . The amount of chromium or copper impregnated is compared with the amount present in each particle. </p> / Thesis / Doctor of Philosophy (PhD)

Silicon carbide is a very promising semiconductor material for high-power, highrequency, and high-temperature applications. SiC distinguishes from traditional narrow bandgap semiconductors, such as silicon, in that common doping impurities in SiC have activation energies larger than the thermal energy kT even at room temperature. This causes incomplete ionization of such impurities, which leads to strong temperature and frequency dependence of the semiconductor junction differential admittance and, if carrier freeze-out effects are not taken into account, errors in doping profiles calculated from capacitance-voltage data. Approaches commonly used to study the influence of incomplete impurity ionization on the junction admittance are based on the truncated space charge approximation and/or the small-signal approximation. The former leads to impurity ionization time constant and occupation number errors, while the latter fails if the measurement ac signal amplitude is larger than kT/q. In this work, a new reverse bias Schottky junction admittance model valid for the general case of an arbitrary temperature, measurement signal frequency and amplitude, and doping occupation number and time constant distributions is developed. Results of junction admittance calculations using the developed model are compared with the results of traditional models. Based on the general model, a new method of admittance spectroscopy data analysis is created and used to determine impurity parameters more accurately than allowed by traditional approaches. Incomplete impurity ionization is investigated for the case of nitrogen donors and aluminum and boron acceptors in 4H- and 6H-SiC. It is shown that the degree of carrier freeze-out is significant in heavily N-doped 6H-SiC and in Al- and B-doped SiC. Frequency dispersion of the junction admittance is shown to be significant at room temperature in N- and B-doped SiC. Junction capacitance calculations as a function of applied dc bias show that calculated doping profiles deviate from the actual impurity concentration profiles if the impurity ionization time constant is comparable with the ac signal period. This is the case for N- and B-doped SiC with certain values of the impurity activation energy and capture cross-section. Validity of the new model and its predictions are successfully tested on experimental admittance data for N- and B-doped SiC Schottky diodes.

No description available.

Ag thin film dewetting upon high temperature annealing is a non-trivial problem for its application in the semiconductor industry as an ohmic contact metal. Thus, preventing Ag thin film from dewetting is of great importance. Typically, adhesion-promoting layers of chromium are deposited to prevent dewetting, but this deposition has its own process optimization parameters. In this thesis, we introduce an alternative, novel strategy for dewetting prevention via Si or In ion implantation. Electron microscopy including SEM, AFM and AES are conducted to characterize changes in film morphology after ion implantation. Thermodynamic simulation is established to better understand the mechanism of this anti-dewetting approach as well as to predict the performance of doped Ag thin films. It is found that Ag films implanted by a trace amount of Si dopants remain intact after 24 h annealing at 530℃ rather than break down into isolated particles as pure Ag film did. Furthermore, Ag grains in doped samples are much smaller than that in non-doped samples and higher Si or In doses contribute to smaller grains, indicating that a retarding force against film grain growth is introduced by the implanting species. Fortunately, electrical conductivity and optical reflectivity of doped films change trivially, suggesting an insignificant influence of external species on the film performance. The retarding force suppressing film grain growth is demonstrated to be solute drag, which will introduce a size limit towards Ag grain growth. A grain growth model including the solute drag effect is established here to describe the grain growth process. Combining our thermodynamic simulation with our grain growth model in the presence of the solute drag effect, the critical grain diameter to initiate agglomeration of 100 nm thick Ag thin film is calculated to be 350 nm and the critical Si dose to prevent 100 nm thick Ag thin film from dewetting is predicted to be 2.0×1013 per cm2. Finally, we successfully synthesize ultrathin Si thin films via ion implantation and pattern as-deposited Si films by implanting through a hard mask in order to identify some steps towards synthesizing 2-D silicon, or silicene. The as-achieved pattern has an identical shape as that of mask, suggesting Si atoms only diffuse within the implanting regions during thermal annealing. Even though only amorphous Si films are prepared at present, this novel strategy possesses potential to fabricated CMOS-compatible 2-D silicon films for semiconductor industry. / Thesis / Master of Applied Science (MASc)

The subject of the present thesis are some aspects of impurities affecting mesoscopic systems with regard to their topological properties and related effects like Majorana fermions and quantized conductance. A focus is on two-dimensional systems including both topological insulators and superconductors. First, the question of whether individual nonmagnetic impurities can induce zero-energy states in time-reversal invariant superconductors from Altland-Zirnbauer (AZ) symmetry class DIII is addressed, and a class of symmetries which guarantee the existence of such states for a specific value of the impurity strength is defined. These general results are applied to the time-reversal invariant p-wave phase of the doped Kitaev-Heisenberg model, where it is also demonstrated how a lattice of impurities can drive a topologically trivial system into the nontrivial phase. Second, the result about the existence of zero-energy impurity states is generalized to all AZ symmetry classes. This is achieved by considering, for general Hamiltonians H from the respective symmetry classes, the “generalized roots of det H”, which subsequently are used to further explore the opportunities that lattices of nonmagnetic impurities provide for the realization of topologically nontrivial phases. The 1d Kitaev chain model, the 2d px + ipy superconductor, and the 2d Chern insulator are considered to show that impurity lattices generically enable topological phase transitions and, in the case of the 2d models, even provide access to a number of phases with large Chern numbers. Third, elastic backscattering in helical edge modes caused by a magnetic impurity with spin S and random Rashba spin-orbit coupling is investigated. In a finite bias steady state, the impurity induced resistance is found to slightly increase with decreasing temperature for S > 1/2. Since the underlying backscattering mechanism is elastic, interference between different scatterers can explain reproducible conductance fluctuations. Thus, the model is in agreement with central experimental results on edge transport in 2d topological insulators.

Cette thèse porte sur l’étude de l’émissivité et de l’opacité dans les plasmas de fusion inertielle (FCI), et des processus atomiques dans les plasmas de fusion magnétique (FCM).En FCI, nous avons étudié les spectres d’émissivité du hohlraum de l’approche indirecte et d’opacité des dopants de l’ablateur. Leur connaissance permet d’améliorer la compression de la cible de D-T et ainsi favoriser les réactions de fusion. Nous avons caractérisé les spectres lié-lié de l’or, du carbone et du germanium au moyen de méthodes détaillées (code PPP) mais qui s’avèrent coûteuses en temps de calculs et limitées quand le nombre de niveaux/ions augmente. Pour optimiser le temps de calcul sans perdre en précision, une méthode hybride détaillée/statistique (code SCO-RCG) a fait l’objet de comparaison/validation avec le code PPP sur des cas tests. Cette approche a ensuite été appliquée au calcul de l’opacité totale (lié-lié, lié-libre, libre-libre) pour tous les états d’ionisation du germanium et du silicium. Les spectres obtenus sont ensuite comparés dans une large gamme d’énergies en vue d’optimiser la compression de la cible.En FCM, à partir d’expériences effectuées sur les tokamaks Tore Supra (CEA Cadarache) et ASDEX Upgrade (Max Planck Institut, Garching), nous nous proposons de développer une nouvelle base de données de physique atomique (sections efficaces, taux des processus,…) à l’aide du code HULLAC pour analyser des coefficients de transport. Le but de cette étude est la sensibilité aux données atomiques de la reconstruction des coefficients de transport par le code ITC. Pour le cas de l’argon, les sections efficaces de quelques processus sont présentées et les coefficients de taux sont comparés à ceux provenant du consortium ADAS. / The thesis is devoted on the study of emissivity and opacity in inertial confinement fusion plasmas (ICF), and on atomic processes in magnetic fusion plasmas (MCF).In ICF, we have studied the emissivity of the hohlraum and the opacity of ablator’s dopants in the indirect drive scheme. The knowledge of these quantities allows the improvement of the target compression and, as a consequence, the fusion reactions. We have characterized the bound-bound spectrum of gold, carbon and germanium with detailed line calculation (PPP code). Such calculations are time consuming and thus restricted to small numbers of levels/ions. To optimize the time calculation without lack of precision, a hybrid approach statistical/detailed (SCO-RCG code) was compared with the PPP code for test cases. Then the hybrid approach was applied to total opacity calculations (bound-bound, bound-free and free-free transitions) for each ionization state of germanium and silicon. The spectra are compared in a large temperature range in order to optimize the target compression.In MCF, from experiences carried out on Tore Supra (CEA Cadarache) and ASDEX Upgrade (Max Planck Institut, Garching) tokomak, we have provided new atomic data (cross sections, rates of processes) with the HULLAC code in order to analyze the transport coefficients. The aim of this study is the sensitivity of atomic data on the reconstruction of transport coefficients by the ITC code. For the argon case, the cross sections of some processes are presented and the rate coefficients are compared to those of ADAS consortium.

Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2014-08-15T14:25:56Z No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Francisco Manoel Bezerra e Rocha.pdf: 738392 bytes, checksum: a00ec5827bbe61c7fe8f36e3b2be9973 (MD5) / Made available in DSpace on 2014-08-15T14:25:56Z (GMT). No. of bitstreams: 2 license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) Francisco Manoel Bezerra e Rocha.pdf: 738392 bytes, checksum: a00ec5827bbe61c7fe8f36e3b2be9973 (MD5) Previous issue date: 2012-06-21 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / We apply the perturbative eld-theoretical renormalization group (RG) implemented within an approach which considers the calculation for the e ective couplings up to one loop and the computation of the self-energy up to two loops of the single-impurity Anderson model with particle-hole symmetry. To this end, we follow Feynman&#039;s diagrammatic method applied to the model and we begin our analysis by calculating the so-called vertex corrections up to one loop. The e ect of correlations on the single-particle excitations is viewed most clearly by means of the computation of the self-energy and its closely-related quantity: the quasiparticle weight. Moreover, to determine the nature of the ground state of the model, we also perform the RG calculation of the so-called uniform spin susceptibility. Then we apply the RG technique, adapting it conveniently to our problem at hand. The next step consists of deriving analytically and solving numerically the coupled di erential RG ow equations for the e ective couplings, the quasiparticle weight and the uniform spin susceptibility. We show that our results agree qualitatively with other analytical works available in the literature, such as, e.g., the functional RG. To benchmark our method, we compare our results with Wilson&#039;s numerical RG data. This latter method provides highly accurate numerical results for the quantities analyzed here and, for this reason, it will be an important check for our analytical method. Since the eldtheoretical RG turns out to be a exible technique and also simpler to be implemented at higher orders if compared to some versions of the functional RG method, we argue here that the present methodology could potentially o er a possible alternative to other analytic RG methods to describe eletronic correlations within the single-impurity Anderson model. / Nesta dissertação, aplicamos o método do grupo de renormalização (GR) perturbativo construído a partir de uma abordagem mista que mescla um cálculo até um loop para os acoplamentos efetivos e um cálculo até dois loops para a auto-energia do modelo de Anderson de uma impureza com simetria partícula-buraco. Para isso, utilizamos o método diagramático de Feynman aplicado ao modelo e iniciamos nossa análise calculando todos os chamados diagramas de correção ao vértice até um loop. Os efeitos de correlação nas excitações de uma partícula são vistos mais claramente por meio do cálculo da auto-energia e de uma quantidade física diretamente relacionada que é o peso da quasipartícula. Além disso, para determinarmos a natureza do estado fundamental desse modelo, efetuamos também o cálculo de GR da chamada susceptibilidade uniforme de spin. Desenvolveremos, em seguida, a técnica do GR, adaptando-a convenientemente ao nosso problema de interesse. O próximo passo consistiu em derivar analiticamente e então resolver numericamente as equações diferenciais acopladas para os acoplamentos efetivos, o peso da quasipartículas e a susceptibilidade uniforme de spin. Mostramos que os nossos resultados concordam qualitativamente com outros trabalhos analíticos disponíveis na literatura como, por exemplo, o método do grupo de renormalizalização funcional. Procuramos também comparar nossos resultados com o chamado método do GR numérico de Wilson. Esse é o último método fornece resultados numéricos altamente precisos para as grandezas aqui calculadas em nosso modelo, de modo que essa técnica serviria como uma referência-padrão para nossa abordagem analítica. Nesse sentido, como o método do GR de teoria de campos se revela uma técnica flexível e mais simples de ser utilizada em cálculos perturbativos de ordens superiores se comparada com algumas implementações do método do grupo de renormalização funcional, vamos argumentar que a presente metodologia do GR pode oferecer uma abordagem analítica alternativa para descrever as correlações eletrônicas contidas no modelo de Anderson de uma impureza

La motivation principale de cette thèse a été de déterminer les avantages et les inconvénients de l'utilisation de la spectroscopie de photoémission résolue en angle dans le domaine des rayons X mous.<br />L'étude d'un système bien connu, Ag(001) nous permet de discuter plusieurs questions telles que le rôle de la quantité de mouvement du photon, la pertinence de l'approximation d'électron libre à l'état final, et le rôle des phonons. La polarisation de la lumière incidente a aussi été exploitée. En choisissant un tel système, nous avons aussi voulu comparer les résultats expérimentaux avec des spectres calculés de photoémission résolue en angle dans cette gamme d'énergie.<br />Le comportement à basse température atypique des composés de Cérium est généralement imputé à l'effet Kondo. Des résultats originaux ont été obtenus en étudiant la bande de valence de trois composés monocristallins iso-structuraux de Cérium, CeCu2Ge2, CeNi2Ge2 et CeCo2Ge2. La position du seuil d'absorption M5 du Cérium dans la bande d'énergie des rayons X mous est exploitée pour isoler la contribution 4f à ces spectres. De plus, l'utilisation de photons incidents d'énergie relativement élevée permet de minimiser les effets de surface. Les spectres de photoémission présentés dans cette thèse incluent des études de dépendance en température, des spectres à la résonance, des spectres résolus en angle ou bien intégrés angulairement. Les premiers sont en accord avec le modèle d'impureté unique d'Anderson, alors que les derniers suggèrent qu'il est important de prendre en compte le réseau cristallin.

Issues of major relevance to the n-type conductivity of Al0.77Ga0.23N associated with Si and O incorporation, their shallow donor or deep donor level behavior, and carrier compensation are elucidated by allying (i) study of Si and O incorporation kinetics at high process temperature and low growth rate, and (ii) electron paramagnetic resonance measurements. The Al0.77Ga0.23N composition correlates to that Al content for which a drastic reduction of the conductivity of AlxGa1−xN is commonly reported. We note the incorporation of carbon, the role of which for the transport properties of AlxGa1−xN has not been widely discussed.

The Wide-View Charge Exchange Recombination Spectroscopy (CXRS) diagnostic at Alcator C-Mod, originally designed for measurement of boron, has been modified to fit several different roles. By measuring the He¹⁺ (n = 4 [rightwards arrow] 3) emission line at 4686Å and surrounding spectra, we can measure ⁴He and ³He density, temperature, and velocity profiles and use this information to study turbulent impurity transport. The transport is characterized using a standard ansatz for the radial particle flux: [mathematical equation]. This effort is designated He CXRS. Also, direct measurement of ³He are used to test models of Ion Cyclotron Resonance Heating (ICRH). We look for evidence of fast ion production and the effect of the minority ion profile on fast wave heating. Several modifications were made to the hardware. Light is collected via two optical arrays: poloidal and toroidal. The toroidal array has been upgraded to increase throughput and spatial resolution, increasing the number of toroidal channels from 10 to 22. A new protective shroud was installed on the poloidal array. Additional diagnostics (a 11 channel beam duct view, neutralizer view, duct pressure monitor) were added to the Diagnostic Neutral Beam to improve DNB modeling for CXRS. This work includes investigation of plasmas where helium is at low concentration (<1%), acting passively, as well as scenarios with a large fraction (>~20%). Using the STRAHL code, time-dependent helium density profiles are used to obtain anomalous transport parameters. Thermodiffusion and curvature pinch terms are also estimated from experimental scaling studies. Results are compared with neoclassical results from the NCLASS code and calculations by the GENE gyrokinetic code. Another focus is verification of power deposition models which are crucially dependent on minority ion density, for which ³He is used. At low ³He fraction, direct absorption by ³He generates fast ions with anisotropic velocity-space distribution functions. At high ³He fraction, mode conversion heating of electrons is dominant. The minority distribution function and predicted wave deposition are simulated using AORSA and CQL3D. This work provides the first measurements of helium transport on C-Mod and expands our understanding of helium transport and fast wave heating. / text