In the Mars atmosphere, reactivity of trace components is as relevant as that of the major compounds if formation of complex molecules is considered. These are of great importance concerning the existence of a past or future life on Mars. In this context, the gas-phase chemistry of alkylcarbonate and alkyloxalate ions and radicals, ROCOO–/• and ROCOCOO–/•, is investigated for different alkyl substituents R (H, CH3, C2H5, i-C3H7, and t-C4H9). This study describes the structures, stability, and unimolecular dissociation behavior of these species and is achieved by means of mass spectrometric methods and ab initio calculations. Standard heats of formation of the ions and radicals are determined via computational methods, using atomization energies and bond-separation reactions. Vertical charge-transfer experiments are performed to provide evidence for the existence of the radicals under study and the NIDD (ion and neutral decomposition difference) method is employed to determine their reactivity. Typical processes observed involve direct bond cleavages leading to elimination of carbon dioxide. Concerning anionic compounds, classical metastable ion (MI) and collisional activation (CA) experiments enable the determination of their gas-phase behavior. This, in contrast to radicals, is not only constituted by direct bond cleavages, but also by hydride-transfer reaction or carbon monoxide expulsion involving formation of ion-neutral complexes as intermediates. Translational energy loss spectra are also employed to gain more insights concerning the dissociation energetics of CH3OCOO• and CH3OCOO+ formed by vertical charge-transfer of methylcarbonate. This rather unusual method for such study implies a careful evaluation of the error caused by the instrument which may otherwise generate dramatic deviations of the results compared to theory.

<p>Transport properties within the Boltzmann transport equation for metallic multi-layer structures as well as bulk materials, were the prime topic of this work. <i>Ab initio</i> total energy calculations for Hydrogen loaded metallic multi-layers were performed in order to shed some light onto problem of H depleted layers at the interfaces that have been experimentally observed. It was explained in connection with structural relaxation of the interface layers. </p><p>Further on conductivity behavior of Fe/V vs. Mo/V during Hydrogen load was discussed. The difference in, on first sight, rather similar multi-layer structures was explained by the magnitude of Hydrogen induced Vanadium expansion. Problem of variation of conductivity with changed c/a ratio of metals and semiconductors in general was addressed as well. The variations due to change of the Fermi surface of the corresponding materials were observed as well as some intriguing general patterns. The phenomenon could be regarded as piezoresistivity on electronic structure level. For the 3d transition metals variation of conductivity/resistivity through the period was studied.</p><p>A possible explanation for anomalous behavior of Manganese resistivity due to its much greater lattice constant in comparison to its neighbors in the period is presented. Field of disordered alloys and low dimensional magnetism was touched by discussing Mo/Ru formation energy as well as magnetic nano-wires grown on surfaces.</p><p>All total energy calculations as well as band structure calculations were performed by using Density Functional Theory based numerical computations. A short but comprehensive review of most common linear-response electron transport techniques is given.</p>

Transport properties within the Boltzmann transport equation for metallic multi-layer structures as well as bulk materials, were the prime topic of this work. Ab initio total energy calculations for Hydrogen loaded metallic multi-layers were performed in order to shed some light onto problem of H depleted layers at the interfaces that have been experimentally observed. It was explained in connection with structural relaxation of the interface layers. Further on conductivity behavior of Fe/V vs. Mo/V during Hydrogen load was discussed. The difference in, on first sight, rather similar multi-layer structures was explained by the magnitude of Hydrogen induced Vanadium expansion. Problem of variation of conductivity with changed c/a ratio of metals and semiconductors in general was addressed as well. The variations due to change of the Fermi surface of the corresponding materials were observed as well as some intriguing general patterns. The phenomenon could be regarded as piezoresistivity on electronic structure level. For the 3d transition metals variation of conductivity/resistivity through the period was studied. A possible explanation for anomalous behavior of Manganese resistivity due to its much greater lattice constant in comparison to its neighbors in the period is presented. Field of disordered alloys and low dimensional magnetism was touched by discussing Mo/Ru formation energy as well as magnetic nano-wires grown on surfaces. All total energy calculations as well as band structure calculations were performed by using Density Functional Theory based numerical computations. A short but comprehensive review of most common linear-response electron transport techniques is given.

First-principles calculations such as density functional theory (DFT) employ numerical approaches to solve the Schrodinger equation of a system. Standard functionals employed to determine the cohesive system energy, specifically the local density and generalized gradient approximations (LDA and GGA), underestimate the correlation of 5f electrons to their ions in AO₂ systems (A=U/Pu/Np). The standard correction, the "Hubbard +U," causes the multidimensional energy surface to develop a large number of local minima which do not correspond to the ground state (global minimum). Because all useful energy values derived from DFT calculations depend on small differences between relatively large cohesive energies, comparing systems wherein one or more of the samples are not in the ground state has the potential to introduce large errors. This work presents an analysis of the fundamental issues of metastable states in both pure and binary AO₂ systems, investigates novel methods of handling them, and describes why current literature approaches which appear to work well for the pure compounds are not well-suited for systems containing multiple actinide species.

This work is a thermodynamic and kinetic study of the Fe-Cr-Ni system as the core of stainless steels. The Fe-Cr, Fe-Ni and Cr-Ni systems were studied intensively using both computational and experimental techniques, including CALPHAD (CALculation of PHAse Diagrams), phase field simulation, ab initio modeling, calorimetry, and atom probe tomography. The purpose of this thesis is to reveal the complexity of the phase transformations in the Fe-Cr-Ni system via the integrated techniques. Due to the importance of the binary Fe-Cr system, it was fully reassessed using the CALPHAD technique by incorporating an updated description of the lattice stability for Fe down to zero kelvin. The improved thermodynamic description was later adopted in a phase field simulation for studying the spinodal decomposition in a series of Fe-Cr binary alloys. Using atom probe tomography and phase field simulation, a new approach to analyze the composition amplitude of the spinodal decomposition was proposed by constructing an amplitude density spectrum. The magnetic phase diagram of the Fe-Ni system was reconstructed according to the results from both ab initio calculations and reported experiments. Based on the Inden-Hillert-Jarl magnetic model, the thermodynamic reassessment of the Fe-Ni system demonstrated the importance of magnetism in thermodynamic and kinetic investigations. Following this, the current magnetic model adopted in the CALPHAD community was further improved. Case studies were performed showing the advantages of the improved magnetic model. Additionally, the phase equilibria of the Fe-Cr-Ni ternary were discussed briefly showing the need of thermodynamic and kinetic studies at low temperatures. The “low temperature CALPHAD” concept was proposed and elucidated in this work showing the importance of low temperature thermodynamics and kinetics for designing the new generation of stainless steels. / <p>QC 20120612</p> / Hero-m

Metallic fuels have an important role to play in "fast breeder" Gen-IV type nuclear reactors, and U-Pu-Zr is one of the prototypical systems. Because of the variability in fuel chemistry during burn-up, it is important to understand the effect of minor actinides and fission products on phase stability. Within this framework, we present a study on phase equilibria in the binary Np-Zr alloy system on which little work has been published. To resolve the contradictory reports on the ordering/ clustering trends of the bcc phase, a thermodynamic study is performed using the CALPHAD method. The calculated Np-Zr phase diagram is consistent with two sets of data: formation enthalpies of the bcc phase that are calculated with ab initio KKR-ASA-CPA electronic-structure method and lattice stabilities of solution phases obtained from first-principles technique. Another important feature in the Np-Zr alloy system is the non-stoichiometric delta-NpZr2 phase that forms in a hexagonal C32 structure similar to the delta-phase in the U-Zr system and the w-phase in pure Zr. An increase in the homogeneity range of the delta-phase when going from Pu to Np and to U is attributed to a lowering of its heat of formation that is caused by an increase in d-band occupation. Two different possibilities for the stability of the delta- and w- phases have been proposed in the present work. Additionally, calculated changes in enthalpy versus temperature are plotted for two alloy compositions of the Np-Zr system to guide future experimental work in resolving important issues in this system. Finally, an ab initio study, implemented with the L(S)DA U formalism, is performed for pure Np that reveals a transition from a non-magnetic to a magnetic state at a critical U parameter.

Computational chemists are concerned about two aspects when choosing between the myriad of theoretical methodologies: the accuracy (the "truth") and the computational cost (the tractability). Among the least expensive methods are the Hartree-Fock (HF), density functional theory (DFT), and second-order Moller-Plesset perturbation theory (MP2) methods. While each of these methods yield excellent results in many cases, the inadequate inclusion of certain types of electron correlation (either high-orders or nondynamical) can produce erroneous results. The compromise for the computation of noncovalent interactions often comes from empirically scaling DFT and/or MP2 methods to fit benchmark data sets. The DFT method with an empirically fit dispersion term (DFT-D) often yields semi-quantitative results. The spin-component scaled MP2 (SCS-MP2) method parameterizes the same- and opposite-spin correlation energies and often yields less than 20% error for prototype noncovalent systems compared to chemically accurate CCSD(T) results. There is no simple fix for cases with a large degree of nondynamical correlation (such as transition metal-salen complexes). While testing standard and new DFT functionals on the spin-state energy gaps of transition metal-salen complexes, no DFT method produced reliable results compared to very robust CASPT3 results. Therefore each metal-salen complex must be evaluated on a case-by-case basis to determine which methods are the most reliable. Utilizing a combination of DFT-D and SCS-MP2 methods, the reaction mechanism for the addition of cyanide to unsaturated imides catalyzed by the Al(Cl)-salen complex was performed. Various experimental observations are rationalized through this mechanism.

La nature et le rôle de défauts est de première importance pour la compréhension des propriétés physiques des nanotubes monoparoi (SWNT) de carbone et nitrure de bore. Lamicroscopie électronique en transmission (TEM) est un outil très puissant pour l'étude des défauts dans les matériaux mais dans le cas de SWNT les atomes peuvent aussi être éjectés par l'irradiation électronique. Cet effet peut changer la structure initiale du tube mais peut être également vu comme un outil potentiel pour "usiner" des structures nanométriques. <br />Nous avons développé un outil théorique pour la description du mécanisme d'irradiation. Dans un premier temps, nous avons dérivé, par des calculs basés sur la théorie de la fonctionnelle de densité, la carte des seuils d'énergie d'émission. Ensuite, nous avons dérivé numériquement la section efficace total de Mott pour différents sites d'émission dans des nanotubes de carbone et nitrure de bore. Utilisant un microscope STEM, nous avons été capables de contrôler la génération de défauts dans des systèmes nanotubulaires avec des conditions expérimentales optimisées sur la base de nos calculs de section efficace. Défauts ponctuels ou lignes de dislocation peuvent ainsi être obtenus avec une résolution spatiale de quelques nanomètres.<br />La structure, l'énergie et les propriétés électroniques des défauts ponctuels et des lignes de défauts ont été étudiées dans les systèmes de BN. L'énergie d'activation et les chemins réactionnels pour la diffusion de mono et de bi-lacunes dans du BN hexagonal ont été dérivés en utilisant le "nudged elastic band method" combiné avec les techniques basées sur la fonctionnelle de la densité. Nous avons aussi démontré que l'apparition de défauts étendus est plus favorable qu'une distribution aléatoire de défauts ponctuels et que cela est dû à l'existence de sites préférentiels pour l'émission d'atomes en présence de défauts préexistants plutôt qu'à des phénomènes de migration et nucléation thermique des lacunes.

Ce mémoire est consacré à l'étude théorique des systèmes SinMp(+) avec M=Li, Na, K. Les calculs ont principalement été effectués au niveau DFT/B3LYP. <br /> Nous avons déterminé les géométries, les énergies de liaison, les potentiels d'ionisation et les moments dipolaires. Pour la majorité des SinMp(+) étudiés, les Si conservent le même arrangement que dans Sin, les alcalins M étant adsorbé à la surface. <br /> L'électron de valence s de l'alcalin est transféré vers la LUMO de Sin de sorte que la structure électronique de SinMp correspond approximativement à celle de Sinp- + pM+. La charge transférée est partagée entre les Si premiers voisins de M. Ce transfert de charge crée un moment dipolaire. Nos résultats pour le potentiel d'ionisation sont en bon accord avec les valeurs expérimentales seulement disponibles pour SinNa.<br /> Nous avons également étudié la possibilité de stabiliser une cage Sin en introduisant un alcalin à l'intérieur.

Ce document décrit les résultats de plusieurs études des dislocations dans les semiconducteurs, effectuées au cours de la période 2000-2005. Le premier chapitre se rapporte à la nucléation des dislocations à partir de défauts de surface, le second chapitre à la détermination de diverses propriétés des dislocations en massif. Enfin, le dernier chapitre concerne les dislocations de misfit à l'interface entre Si(001) et SiC.