Nanocrystalline materials are polycrystalline solids with grain size in the nanometer range (< 100nm), which have been found to exhibit superior properties such as high magnetic permeability and corrosion resistance, as well as a considerably increase of strength when compared with their coarse grain counterparts. All those improved properties are attributed to the high volume fraction of grain boundaries (GBs). However, the high density of GBs brings a large amount of excess enthalpy to the whole system, making the nanostructures unstable and suffer from severe thermal or mechanical grain growth. In order to maintain the advantageous properties of nanocrystalline materials, it is necessary to stabilize GB and inhibit grain growth. While alloying has been found to be an effective way of achieving stabilized nanocrystalline metal alloys experimentally, the direct quantification of solute effects on GB motion still poses great challenge for investigating thermal stability of general nanocrystalline materials. In this research, impurity segregation and solute drag effects on GB motion were investigated by extending the interface random-walk method in direct molecular dynamics simulations. It was found that the GB motion was controlled by the solute diffusion perpendicular to the boundary plane. Based on the simulation results at different temperatures and impurity concentrations, the solute drag effects can be well modeled by the theory proposed by Cahn, Lücke and Stüwe (CLS model) more than fifty years ago. However, a correction to the original CLS model needs to be made in order to quantitatively predict the solute drag effects on a moving GB.

The Kim and Chan experiment in 2004 gave the first experimental evidence of a possible supersolid state. Even though the origin of this state is not clear yet, several experimental and theoretical investigations suggest defects are responsible for this curious phase. We have used heat pulses and thermal quenching to study pressure gradients and annealing mechanisms in solid 4He crystals. Large pressure gradients exist in crystals grown at constant volume. These can be enhanced by phase transitions, thermal quenching or by partial melting. Annealing reduces defect densities and hence pressure gradients in crystals. Our measurements show that the pressure at different points in a crystal can behave differently, even if there is little change in the crystals average pressure. We measured the activation energy that is associated with the annealing process.

Les travaux effectués ont consisté en une étude théorique du matériau BaSnO3 en tant que matériau conducteur protonique pour électrolytes de piles à combustible. Ces matériaux sont obtenus après un dopage aliovalent préalable qui génère des lacunes d'oxygène sur le sous-réseau d'oxygène du matériau. Ce matériau, placé en milieu humide va s'hydrater, c'est à dire que des molécules d'eau vont se dissocier au sein du matériau. La propriété principale souhaitée pour de tels matériaux est la conductivité protonique. Celle-ci dépend du nombre de porteurs de charges (les hydrogènes ou protons apportés par les molécules d'eau) et de leur mobilité. Ces deux paramètres sont quantifiés par des grandeurs thermodynamiques (l'enthalpies d'hydratation) et cinétiques (énergies d'activation) qui peuvent dépendre très fortement des dopants et de leur concentration. Une étude systématique a donc été entreprise sur ce matériau dopé par Ga, In, Y, Gd, Sm et La sur le site du Sn. Les objectifs étaient, d'une part de déterminer les paramètres clés de la conduction protonique et de les comparer aux données expérimentales, et d'autre part de corréler ces informations énergétiques aux effets structuraux imputables aux dopants, dans le but de comprendre comment ces derniers influencent la conduction. Pour remonter à ces paramètres, des calculs basés sur la Théorie de la Fonctionnelle de la Densité ont été réalisés dans l'approximation GGA-PBE, par l'intermédiaire de deux codes de calculs différents: ABINIT et SIESTA. Les calculs ont été menés à la fois à des concentrations de 12,5% et de 3,7% de dopants et le matériau BaTiO3 a également été étudié. D'intéressants résultats ont étés obtenus, notamment d'un point de vue structural, avec l'analyse des déformations locales aux alentours des dopants. Ont été mis en évidence: i. La stabilisation préférentielle de certaines positions des défauts due aux interactions électrostatiques. ii. L'effet de la concentration des dopants sur les énergies d'interaction entre dopant et défauts (lacune d'oxygène et proton) et iii. Un effet de taille de dopant, perceptible notamment dans le cas des gros dopants, et qui stabilise préférentiellement une autre position que celle favorisée d'un point de vue électrostatique. / The present work consist in a theoretical study of the BaSnO3 compound as a protonic conductor for fuel cell electrolytes. These materials are obtained after an aliovalent doping stage that will create oxygen vacancies on the oxygen sublattice of the compound. Then, in a moist atmosphere, this lacunar material is going to hydrate: water molecule will be dissociated, creating protonic defects inside of the compound. The main desired property is the protonic conduction, which is due to two major contributions: number of charge careers (hydrogen or proton coming from the hydration reaction) and their mobility, at a given temperature. These two parameters are quantified by a thermodynamic quantity (hydration enthalpy) and a kinetic parameter (activation energy), which are known to be dependant on the dopant concentration. Thus, a systematic study has been done for the material doped Ga, In, Y, Gd, Sm and La on the Sn site. The objectives of this study were, first, to compute the key parameters of the protonic conduction and to compare them to the experimental data, and, in second, to correlate the calculated results to structural effect due to the dopants, in order to understand how they influence the conduction parameters. To determine these parameters, calculations based on the Density Functional Theory in the GGA-PBE form were carried out, using two different codes: ABINIT and SIESTA. Computations were done for dopant concentrations going from 12.5% to 3.7%, the BaTiO3 compound were also studied. Interesting results were also obtained, from a structural point of view, and concerning dopant local environment. Were evidenced: i. Prefential stabilization of defects, relatively to electrostatic interaction considerations. ii. The dopant concentration effect on dopant-defect (oxygen vacancy and proton) interactions. iii. A dopant size effect which acts in particular in the case of big dopants and which stabilize an other defect position than the one favoured by electrostatic considerations.

The cores of early UK graphite moderated research and production nuclear fission reactors operated at temperatures below 150°C. Due to this low temperature their core graphite contains significant amounts of stored (Wigner) energy that may be released by heating the graphite above the irradiation temperature. This exothermic behavior has lead to a number of decommissioning issues which are related to long term "safe-storage", reactor core dismantling, graphite waste packaging and the final disposal of this irradiated graphite waste. The release of stored energy can be modeled using kinetic models. These models rely on empirical data obtained either from graphite samples irradiated in Material Test Reactors (MTR) or data obtained from small samples obtained from the reactors themselves. Data from these experiments is used to derive activation energies and characteristic functions used in kinetic models. This present research involved the development of an understanding of the different grades of graphite, relating the accumulation of stored energy to reactor irradiation history and an investigation of historic stored energy data. The release of stored energy under various conditions applicable to decommissioning has been conducted using thermal analysis techniques such as Differential Scanning Calorimetry (DSC). Kinetic models were developed, validated and applied, suitable for the study of stored energy release in irradiated graphite components. A potentially valid method was developed, for determining the stored energy content of graphite components and the kinetics of energy release. Another parameter investigated in this study was dedicated in the simulation of irradiation damage using ion irradiation. Ion bombardment of small graphite samples is a convenient method of simulating fast neutron irradiation damage. In order to gain confidence that irradiation damage due to ion irradiation is a good model for neutron irradiation damage the properties and microstructure of various grades of ion irradiated nuclear graphite were also investigated. Raman Spectroscopy was employed to compare the effects of ion bombardment with the reported effects of neutron irradiation on the content of the defects. The changes of the of defect content with thermal annealing of the ion irradiated graphite have been compared with the annealing of neutron irradiated nuclear graphite.

Release of uranium from Na-autunite, an artificial mineral created as a result of polyphosphate injection in the subsurface at the DOE Hanford Site, takes place during slow dissolution of the mineral structure. Stability information of the uranyl-phosphate phases is limited to conditions involving pH, temperature, and a few aqueous organic materials. The carbonate ion, which creates very strong complexes with uranium, is the predominant ion in the groundwater composition. The polyphosphate technology with the formation of autunite was identified as the most feasible remediation strategy to sequester uranium in contaminated groundwater and soil in situ. The objectives of the experimental work were (i) to quantify the effect of bicarbonate on the stability of synthetic sodium meta-autunite created as a result of uranium stabilization through polyphosphate injection, (ii) calculate the kinetic rate law parameters of the uranium release from Na-autunite during dissolution, and (iii) to compare the process parameters with those obtained for natural calcium meta-autunite. Experiments were conducted using SPTF apparatus, which consists of syringe pumps for controlling flow rate, Teflon reactors and a heating/cooling system. 0.25 grams of synthetic Na-autunite was placed in the reactor and buffer solutions with varying bicarbonate concentrations (0.0005 to 0.003 M) at different pH (6 - 11) were pumped through the reactors. Experiments were conducted at four different temperatures in the range of 5 - 60oC. It was concluded that the rate of release of uranium from synthetic Na-autunite is directly correlated to the bicarbonate concentration. The rate of release of uranium increased from 1.90 x 10-12 at pH 6 to 2.64 x 10-10 (mol m-2 s-1) at pH 11 at 23oC over the bicarbonate concentration range tested. The activation energy values were invariant with the change in the bicarbonate concentration; however, pH is shown to influence the activation energy values. Uranyl hydroxides and uranyl carbonates complexes helped accelerate the dissolution of autunite mineral.

The master´s thesis is focused on the characterization of degradation process of polypropylene and polypropylene with statistic copolymer into which manganese (II) stearate and cobalt (II) stearates were added at 0.05, 0.10 and 0.20 wt.% loadings. The aim of the thesis was to prepare the controlled life-time polypropylene. The degradation was studied at varying temperatures and prodegradant loadings. The prodegradants were synthetized and characterized using the FTIR and DSC techniques. Thermooxidation of the tested samples induced the changes in crystallinity, melting points and melt-flow indexes. Tensile strength and other mechanical properties were determined by means of the tensile test. The carbonyl index was determined using FTIR, the thermooxidation stability test was used to determine the activation energies of reactions. The changes in morphology of degraded samples were observed by SEM analysis.

>Magister Scientiae - MSc / This study presents the successful characterization and hydrolysis of magnesium hydride (MgH2) for hydrogen generation. The as-received MgH2 served as a precursor in most of the hydrolysis experiments for H2 generation. The phase-structural and morphological characteristics of the as-received MgH2 were evaluated using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray dispersive diffraction (XRD) characterization techniques. The hydrogen storage performance of the as-received MgH2 was analysed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and thermal desorption spectroscopy (TDS) techniques. The hydrolysis of MgH2 was performed in a hydrogen generation reactor operated in a batch mode where the temperature and H2 flow rate were logged.

The oxidation of thiols in petroleum is a subject of ongoing research, discussion and removal of sulfur is a topic of ongoing legislation. The Merox® process requires high pressures and temperatures. Novel catalysts and methods innovations are of interest. This work examines the synthesis, purification and use of sodium cobalt(II) tetrasulfophthalocyanine to oxidize ethanethiol to diethyl disulfide. Many systems using phthalocyanines carry out the oxidation reaction under basic conditions. This work oxidized ethanethiol to diethyl disulfide in dimethylformamide using cobalt tetrasulfophthalocyanine (CoTSPc) under alkali free conditions and was compared to cobalt sulfate heptahydrate, cobalt phthalocyanine (CoPc), FeTSPc and CuTSPc. The reaction was carried out in oxygen saturated DMF while stirring at 15, 25 and 40.00 °C. The amount of ethanethiol remaining over time was determined using Ellman's reagent. A simple GC method quantified the amount of diethyl disulfide. The reaction proceeded to completion within 10 min at 40.00 °C. A turn over number of 72 and frequency of 8.1 min−1 is obtained. The activation energy was approximately 32 kJ/mol. The prepared CoTSPc catalyst was most catalytic toward oxidation of ethanethiol followed by cobalt sulfate heptahydrate, CoPc, FeTSPc and CuTSPc was non-catalytic.

This work demonstrates how local, randomized tailoring of bond stiffness can affect the activation energy of diffusion in model alloys using density functional theory-based computations. This work is organized into two parts. The first part deals with the vacancy diffusion mechanism, and it compares the in–plane (IP) vs out-of-plane (OOP) diffusion paths in prototypical binary Mg-X (Ca, Y, and Gd) and ternary Mg-X (Ca, Y, and Gd)-Zn alloys. We examine how vacancy formation, migration, and solute vacancy binding energies in binary Mg-X alloys influence diffusion activation and correlated them with conventional diffusion model based solely on the solute sizes. Next, we explore how Zn addition to binary Mg-X (Ca, Y, and Gd) alloys influences the OOP activation energy barrier is discussed in terms of detailed energetic computations and bond characterization in the present work. Our results indicate that Zn addition further enhances the OOP activation energy barrier compared to corresponding activation energies in Mg binaries. This work concludes that engineering stiffer directional bonds via micro-alloying additions in Mg is a promising route to dramatically improve their high temperature creep response. The second part of my work investigates the effects of Si, P, and S solutes on H interstitial diffusion mechanism in Ni. It examines how H interacts with vacancy, impurity atom, and vacancy-impurity atom defect pair by performing binding energy calculations. Results indicate that vacancy-impurity atom defect pair strongly traps the H atom compared to isolated defects. Finally, the effect of impurities on activation energy barrier of H diffusing in Ni is discussed by correlating migration energetics with bonding characteristics by performing charge density and electron density calculations. Our study validates experimental hypothesis of Berkowitz and Kane which postulates that P enhances the H diffusion in Ni. The present work also shows that H diffusion speeds up in Ni in the presence of Si and S solutes. In conclusion, we show that micro-alloying additions induce local lattice level pockets with covalent character, which substantially enhances the local bond stiffness. This will increase activation energy for vacancy diffusion mechanism while it reduces activation energy for interstitial H diffusion.

Fluorinated polymers of substantial high performance such as perfluorocyclobutyl (PFCB) and fluorinated aryl vinyl ether (FAVE) polymers can readily be synthesized by thermal [2+2] cyclopolymerization as a melt or by classical polycondensation. These fluoropolymers naturally possess high thermal and chemical resistance, low conductivity properties, and other mechanical properties. In this work, a method using 0th order kinetics is proposed and thermal degradation studies were conducted on six different aromatic ether-based polymers to gauge trends in activation energy barrier and differences in thermal stability by 0th order degradation kinetics. The activation barrier (E_a) obtained can give accurate insight into the stability of the polymer based only on structure for external applications. Activation energies ranging from 17 to 41 kcal/mol were obtained for the various polymers. Overall, this study provides an established method using TGA for thermal stability studies through 0th order kinetics that can be potentially used for future lab applications.