The effect of magnesium, strontium, and other alkaline earths on the formation and persistence of metastable carbonates in the natural environment was investigated to determine the nature of the controlling mechanism. Barium and beryllium were studied to evaluate the effect of ionic radius; magnesium and strontium, in order to determine if the results correlate with the usual order of stability for complexes and adsorbed species. Known weights of aragonite were placed in contact with solutions of beryllium, magnesium, calcium, strontium, and barium. Samples were covered and periodically both pH and percent composition of aragonite determined; supernatant liquids and precipitates were analyzed for cation concentrations by atomic absorption spectroscopy and titrimetric methods. Results indicated that the order of effectiveness of alkaline earth metals in inhibiting recrystallization is : Be > Mg > Sr > Ba. This is the expected order of effectiveness for both surface and solution effects. A solution effect (i.e., sequestration of bicarbonate ions) is strongly suggested by the chemical behavior of each cation.

Multiferroics are currently intensely investigated because the coexistence and coupling of ferroic arrangements brings about new physical effects and, for the few room-temperature examples, interesting prospects for applications in various fields. This interest is illustrated by the recent publication of several articles on multiferroics in high impact reviews over the last five years. The main goal of the thesis was to look for new multiferroics by exploiting overlooked and original polar and magnetic arrangements. We more precisely investigated compounds based on lead iron tungsten PbFe2/3W1/3O3 (PFW) and lead zirconate PbZrO3 (PZO) oxides. PFW displays long- and short-range both polar and magnetic orders (ferroelectric-relaxor and antiferromagnetic-spin-glass) while PZO is antiferroelectric with antiferrodistorsivity (oxygen tilts) and existence of ferroelectric instabilities. Combining various techniques from synthesis to electric, magnetic and structural characterizations, we demonstrated that it is possible to get a multiferroic compound (50%PFW-50%PZO) with coexistence of multiple ferroic and structural arrangements with room temperature properties of practical interest. This work opens new prospects in this rich field of multiferroics in peculiar by using antiferroelectrics.

A reforma do metano é um processo de grande interesse industrial para a produção de hidrogênio e de gás de síntese. Entre as reações de reforma do metano, destacam-se as reações de reforma a vapor e a reação com dióxido de carbono. O catalisador comumente utilizado nos processos e Ni/Al2O3. Porém, durante este processo, ocorre uma indesejada formação de depósitos carbonáceos na superfície do catalisador, os quais levam a sua destruição mecânica e, consequentemente, sua desativação. Por isso, uma das propriedades mais importantes de um bom catalisador para as reações de reforma do metano é a sua resistência a desativação. Entre as propostas para melhorar o desempenho do catalisador encontra-se a incorporação do óxido de cério junto ao suporte alumina. Catalisadores Ni/Al2O3 incorporados em soluções sólidas formadas por CeO2-ZrO2, ZrO2-La2O3 e CeO2-La2O3 foram preparados, caracterizados e submetidos a ensaios catalíticos nas reações de reforma a vapor e com dióxido de carbono e oxidação parcial do metano com objetivo de avaliar o efeito da adição da solução sólida ao suporte. Os suportes foram preparados pelo método da co-precipitação e os catalisadores foram obtidos pelo método de impregnação e calcinados a 500°C. Estes compostos foram caracterizados por Fisissorção de Nitrogênio, Difração de Raios X (DRX), Espectroscopia dispersiva de raios X (EDX), espectroscopia de na região do ultra violeta e do visível (UV-vis-NIR), Redução à Temperatura Programada (RTP), Espectrocopia RAMAN, Espectroscopia fotoeletrônica de Raios X (XPS), Espectroscopia de absorção de Raios X (XAS) e Análise termogravimétrica. Os ensaios catalíticos mostraram que a adição de solução sólida melhorou o desempenho do catalisador Ni/Al2O3 e, dentre todos os catalisadores avaliados, os melhores desempenhos obtidos foram com os catalisadores suportados em Ni/CeO2-La2O3-Al2O3. / Nowadays, the methane reforming is large interest industrial for the take advantage of these gas in production the hydrogen and synthesis gas (syngas). Among in the reactions of methane stand of the reactions steam reforming and carbon dioxide reforming of methane. The main catalysts uses in the methane reforming is Ni/Al2O3. However, the supported-nickel catalyst is susceptible to the deactivation or the destruction by coke deposition. The carbon dissolves in the nickel crystallite and its diffuses through the nickel, leading for formation of the carbon whiskers, which results in fragmentation of the catalyst. Modification of such catalysts, like incorporation of suitable promoters, is desirable to achieve reduction of the methane hydrogenolysis and/or promotion of the carbon gasification. Catalysts 5% Ni/Al2O3 supported on solid solutions formed by ZrO2-CeO2, La2O3 and CeO2-ZrO2-La2O3 were prepared, characterized and evalueted in reactions steam and carbon dioxide refoming and partial oxidation of methane with objetive the value effect loading solution solid in support. The supports were prepared by co-precipitation method and catalysts were prepared by impregnation method and calcined at 500°C. The supports and catalysts were characterized by Nitrogen Adsorption, method -rays diffraction (XRD), X-rays dispersive spectroscopy (XDS), spectroscopy in the region of the ultraviolet and the visible (UV-vis NIR) to and temperature programmed reduction (TPR), RAMAN Spectrocopy, X-ray absorption spectroscopy and Termogravimetric Analysis. After all the catalytic reactions check which the addition of solid solution is beneficial for Ni/Al2O3 catalysts and the best catalysts are Ni/CeO2-La2O3-Al2O3.

[pt] Os óxidos mistos CeO2-ZrO2 são utilizados como suporte de catalisadores automotivos devido a sua capacidade de estocar e liberar oxigênio em condições pobres ou ricas de combustível, respectivamente, a fim de auxiliar nas reações redox que minimizam a emissão de poluentes oriunda da queima incompleta do combustível. Neste trabalho, os óxidos mistos CeO2-ZrO2 foram preparados pelo método de coprecipitação, utilizando-se como sais precursores o ZrO(NO3)2 e o (NH4)2Ce(NO3)6 ou Ce(NO3)3 com a finalidade de se observar a influência do sal precursor de cério no sistema CeO2-ZrO2. Além disso, investigou-se o efeito de algumas condições de preparação como o envelhecimento do precipitado, o controle de pH durante a síntese, a secagem e o envelhecimento térmico nos óxidos mistos obtidos. Os resultados mostram que a variável mais significante na preparação dos óxidos mistos CeO2-ZrO2 por coprecipitação é o sal precursor de cério. Em todas as amostras preparadas a partir do precursor de cério IV houve a formação de solução sólida, enquanto que no caso das amostras preparadas a partir do precursor de cério III observou-se a formação de duas fases segregadas, c-CeO2 e t-ZrO2. Além disso, concluiu-se que a redução do material obtido não ocorre somente na superfície, mas também na parte mássica e que os materiais formados por solução sólida são os que apresentam melhor capacidade de armazenamento de oxigênio o que é muito importante para o uso em catalisadores automotivos. O envelhecimento térmico do catalisador mostrou que há uma sinterização devido à alta temperatura e que também há uma segregação da solução sólida quando esta foi submetida às condições de envelhecimento utilizadas. / [en] The CeO2-ZrO2 mixed oxides are used as automotive catalyst support due to their capacity to storage and to release oxygen in lean and rich conditions, respectively, in order to assist redox reactions that minimize the emission of pollutants caused by fuel incomplete combustion. In this present work, the CeO2- ZrO2 mixed oxides had been prepared by the coprecipitation method using as precursor salts the ZrO(NO3)2 and the (NH4)2Ce(NO3)6 or the Ce(NO3)3 with the purpose of observing the influence of the cerium precursor salt in the CeO2-ZrO2 system. Moreover, the effect of some preparation conditions like drying, thermal aging, pH control and aging along precipitation was investigated. The results show that the most significant variable in the coprecipitation preparation of the CeO2-ZrO2 mixed oxides is the cerium salt precursor. There were formation of solid solutions in all samples prepared from the cerium IV precursor, while in the case of the samples prepared from the cerium III precursor, it was observed formation of two segregated phases, c-CeO2 and ZrO2. In addition, one concluded that the reduction of the obtained material does not only occur in the surface, but also in the bulk. The solid solution materials are the ones that present the best oxygen storage capacity what is very important for the use in the automotives catalysts. The thermal aging of the catalyst showed that there are a sintering due to the high temperature and a segregation of the solid solution when the material was submitted to the used aging conditions.

Borides and carbides of tantalum and hafnium are of great interest due to their ultrahigh temperature applications. Properties of these ceramics including oxidation resistance and mechanical properties might be further improved through solid solution/composite formation. Synthesis of single-phase TaxHf1-xC and TaxHf1-xB2 solid solution powders including nanopowders via carbothermal reduction (CTR) is complicated due to noticeable difference in reactivity of parent oxides with carbon, and also the low solubility of those oxides in each other. Moreover, for TaC-HfC system the solid solution may go through phase separation due to the presence of a miscibility gap at temperatures below 887°C.In this study, a method of low-cost aqueous solution processing followed by CTR was used to synthesize TaxHf1-xC and TaxHf1-xB2 solid solution powders. In fact, method was first used to synthesize boron carbide (B4C) powders as it paves the way for a detailed study on the synthesis of TaxHf1-xC and TaxHf1-xB2 solid solutions powders considering the fact that B4C contains both carbon and boron in its structure. Particular emphasis was given to investigate the influences of starting compositions and processing conditions on phase separation during the formation of both carbide and boride phase(s). It was found that individual TaC-HfC and TaB2-HfB2 phases always form quickly but separately during the CTR process (e.g., at 1600 °C within a few minutes). Those carbides and borides remain phase-separated unless heated to much higher temperatures for long time due to the slow inter-diffusion between them. It was also found that for TaxHf1-xC applying a DC electric field through the use of spark plasma sintering (SPS) system significantly accelerates the inter-diffusion of Ta and Hf leading to formation of a single-phase TaxHf1-xC solid solution at 1600 °C for 15 minutes. On the other hand, for borides alkali metal reduction reaction (AMR) method appears to be an excellent alternative to CTR-based method for formation of a single-phase TaxHf1-xB2 solid solution. In this method, chlorides of tantalum and hafnium are directly reduced using sodium borohydride (NaBH4) giving rise to formation of a single-phase Ta0.5Hf0.5B2 solid solution nanopowders in one step at much lower temperatures (e.g., 700 °C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR-based process.

The understanding of the strengthening mechanisms is crucialboth in the development of new materials with improvedmechanical properties and in the development of better materialmodels in the simulation of industrial processes. The aim ofthis work has been to study different strengthening mechanismsfrom a fundamental point of view that enables the developmentof a general model for the flow stress. Two differentmechanisms namely, solid solution strengthening and grain sizestrengthening have been examined in detail. Analytical modelsproposed in the literature have been critically evaluated withrespect to experimental data from the literature. Two differentexperimental surface techniques, atomic force microscopy (AFM)and electron backscattered diffraction (EBSD) were used tocharacterize the evolving deformation structure at grainboundaries, in an ultra low-carbon (ULC) steel. A numericalmodel was also developed to describe experimental featuresobserved locally at grain boundaries. For the case of solid solution strengthening, it is shownthat existing models for solid solution strengthening cannotexplain the observed experimental features in a satisfactoryway. In the case of grain size strengthening it is shown that asimple model seems to give a relatively good description of theexperimental data. Further, the strain hardening in materialsshowing a homogenous yielding, is controlled by grainboundaries at relatively small strains. The experimentalresults from AFM and EBSD, indicate more inhomogenousdeformation behaviour, when the grain size is larger. Bothtechniques, AFM and EBSD, correlate well with each other andcan be used to describe the deformation behaviour both on alocal and global scale. The results from the numerical modelshowed a good qualitative agreement with experimentalresults. Another part of this project was directed towards thedevelopment of continuum models that include relevantmicrostructural features. One of the results was the inclusionof the pearlite lamellae spacing in a micromechanically basedFEM-model for the flow stress of ferriticperlitic steels.Moreover a good agreement was achieved between experimentalresults from AFM and FEM calculations using a non-local crystalplasticity theory that incorporates strain gradients in thehardening moduli. The main philosophy behind this research has been to combinean evaluation of existing strengthening models, with newexperiments focused on studying the fundamental behaviour ofthe evolving dislocation structure. This combination can thenbe used to draw general conclusions on modelling thestrengthening mechanisms in metals. <b>Keywords:</b>strengthening mechanisms, flow stress, solidsolution strengthening, grain size strengthening,micromechanical modelling, AFM, EBSD

This thesis reports on synthesis, microstructure and properties of binary and ternary carbide thin films deposited by dc magnetron sputtering. These materials are interesting since they exhibit a wide range of useful properties, such as high hardness, resistance to wear and oxidation, and high electrical conductivity. Here, an early transition metal (M) and carbon (C) have been used as the basis, often with the addition of a second M-element or an A-group element (A). In these systems nanocomposites, metastable solid solutions, multilayers, or Mn+1AXn-phases have been deposited. The Mn+1AXn-phases are a group of nanolaminated compounds with a unique mixture of metallic and ceramic properties. In general X is carbon or nitrogen, although here only carbon has been used. Epitaxial MAX-phase thin films of Ti2AlC, Ti3AlC2 and V2GeC have been deposited for the first time. They have been studied with emphasis on phase stability, phase composition and nucleation characteristics to gain deeper insights into their growth. The microstructure of the films was characterized by electron microscopy and X-ray diffraction. In addition, bond strength characteristics have been studied by soft X-ray spectroscopy and complementary calculations within DFT. Their mechanical and electrical properties have been studied, and the results are discussed on the basis of their electronic structure. Furthermore, by interleaving the Ti3SiC2 MAX-phase with TiC0.67 a multilayer structure has been formed, for which a new intrusion-type deformation behaviour has been described. A new concept in the design of nanocomposite films has been developed, whereby a solid solution of a weak carbide-forming element in the carbide structure creates a driving force for surface segregation of C. This concept has been verified both theoretically and experimentally for the Ti-Al-C and Ti-Fe-C systems. It has been shown by pin-on-disc measurements that this surface segregation leads to graphitization and consequently a very low friction coefficient for these films. Finally, it has been demonstrated that low-friction films with tunable magnetic properties can be achieved in the Ti-Fe-C system.

Lithium iron phosphate (LiFePO4) is one of the most promising cathode materials for lithium ion rechargeable batteries. It has a high theoretical specific capacity (170 mAh/g) and operating potential (3.45 V vs. Li+/Li). Additionally, the material is extremely stable thermally and electrochemically at ambient conditions, which is very suitable to be used in electric vehicles. However, the electronic and ionic conductivities of the material are quite low, which limits the power performance of the batteries. In the last decade, extensive work was reported on various methods to improve the electronic conductivity extrinsically, for example carbon coating, metallic additives and molecular wiring. Nevertheless, energy density of the cells will be reduced because of non-electrochemically active nature of the additives. In principle, electronic and ionic conductivities can be boosted intrinsically. One of the methods is to increase the number of charge carriers in the material, for instance in two-phase solid solution system LiαFePO4/Li1-βFePO4 or single solid solution phase LixFePO4. Since the formation of solid solution has been reported to be size dependent, it is highly desired to know how to synthesize LiFePO4 particles with different sizes. In this study, we have used hydrothermal synthesis and polyol process to control the size of LiMPO4 (M: Fe, Mn, and Co) particles. We will present how we prepare particles with different sizes. Moreover, the solid solution properties of various sizes of LiMPO4 (M: Mn and Fe) were studied. The result will be presented. Part of the preliminary findings have been published in the peer-reviewed journals or conference presentations: 1) Journal of Materials Chemistry [Ellis B.; Kan W. H.; Makahnouk W. R. M.; Nazar L. F. J. Mater. Chem. 2007, 17 (30) 3248., 2) Journal of the American iv Chemical Society [Lee K. T.; Kan W. H.; Nazar L. F. J. Am. Chem. Soc. (submitted)], 3) Material Research Society Meeting [Kan W. H.; Maunders C.; Badi S.; Ellis B.; Botton G.; Nazar L. F. MRS Fall Meeting 2008 in Boston]

Lithium iron phosphate (LiFePO4) is one of the most promising cathode materials for lithium ion rechargeable batteries. It has a high theoretical specific capacity (170 mAh/g) and operating potential (3.45 V vs. Li+/Li). Additionally, the material is extremely stable thermally and electrochemically at ambient conditions, which is very suitable to be used in electric vehicles. However, the electronic and ionic conductivities of the material are quite low, which limits the power performance of the batteries. In the last decade, extensive work was reported on various methods to improve the electronic conductivity extrinsically, for example carbon coating, metallic additives and molecular wiring. Nevertheless, energy density of the cells will be reduced because of non-electrochemically active nature of the additives. In principle, electronic and ionic conductivities can be boosted intrinsically. One of the methods is to increase the number of charge carriers in the material, for instance in two-phase solid solution system LiαFePO4/Li1-βFePO4 or single solid solution phase LixFePO4. Since the formation of solid solution has been reported to be size dependent, it is highly desired to know how to synthesize LiFePO4 particles with different sizes. In this study, we have used hydrothermal synthesis and polyol process to control the size of LiMPO4 (M: Fe, Mn, and Co) particles. We will present how we prepare particles with different sizes. Moreover, the solid solution properties of various sizes of LiMPO4 (M: Mn and Fe) were studied. The result will be presented. Part of the preliminary findings have been published in the peer-reviewed journals or conference presentations: 1) Journal of Materials Chemistry [Ellis B.; Kan W. H.; Makahnouk W. R. M.; Nazar L. F. J. Mater. Chem. 2007, 17 (30) 3248., 2) Journal of the American iv Chemical Society [Lee K. T.; Kan W. H.; Nazar L. F. J. Am. Chem. Soc. (submitted)], 3) Material Research Society Meeting [Kan W. H.; Maunders C.; Badi S.; Ellis B.; Botton G.; Nazar L. F. MRS Fall Meeting 2008 in Boston]

This thesis is about the kinetics of anisotropic acidic/hydrothermal dissolution of apatite bulk single crystal vs. nanorods, and the kinetic phase change of dense nanocondensates of SnO2 vs. Ni-dissolved SnO2 prepared by laser ablation condensation technique. In the first regard, directional dissolution of a natural (OH,F,Cl)-bearing apatite has been studied at various solution pH values (0~3) and 30 oC. This apatite showed abnormally high O-H stretching frequencies due to the substitution of Cl for OH. The advance of dissolution front indicated that steady-state directional dissolution for pH = 0-2 followed an apparent rate law of rate(mole / m2h)¡×kaH+n, where the rate constants (k) are 2.15 and 1.61; and the rate orders (n) are 1.44 and 1.30 for [0001] and <11 0> directions, respectively. Previous study, however, indicated a smaller n value (n = 0.55~0.70) for fluorapatite powders at higher pHs. A nonlinear pH dependence of logarithmic dissolution rate at a wide pH range implied that the surface active sites and/or rate-determining steps have changed when the acidity of solution and/or the composition of the apatite were changed. The opening of etch pits on basal planes further indicated that the dissolution rates along the three principal directions have the following relationship: [0001] > <11-20> > <10-10> for pH=0-1, but the order was reversed for pH > 3. As a comparison, static immersion of needle-like hydroxyapatite nanoparticles in neutral hydrothermal solution at 100oC caused preferential dissolution along the crystallographic c-axis to form nanorods with a lower aspect ratio. The anisotropic dissolution behavior is due to diffusion-controlled rapid dissolution at the sharp tip, and interface-controlled dissolution at side surfaces in terms of active sites. Extensive dissolution was accompanied with amorphization via explosive generation of dislocations, forming corrugated surface with both negative and positive curvature regions. The amorphous residue was significantly Ca and OH depleted when treated in the hydrothermal solution at pH=3. The BET specific surface area of the apatite nanoparticles remained 45¡Ó1 m2/g after immersion in neutral solution at 100oC for 36 h, but drastically decreased to 24.5 m2/g in acidic (pH =3) solution at 100oC for 8 h due to coalescence of the partially amorphized apatite powders. The specific surface area and average pore size also remained nearly unchanged for the dry pressed powders subject to firing at 100oC, but decreased and increased, respectively when sintered shortly at 600oC in air. BJH measurements at 77 K indicated the N2 adsorption/desorption hysteresis loops shift toward high relative pressure for sintered/hydrothermally etched powders indicating a higher activation energy of forming overlain liquid-like nitrogen layers. This can be attributed to a lower surface energy of the powders due to their shape change and/or partial amorphization. Alternatively, desorption through cavitation via the small voids could occur, in particular for such treated samples with characteristic bimodal pore size distribution. In the second subject, dense SnO2 with fluorite-type related structures were synthesized via very energetic Nd-YAG laser pulse irradiation of oxygen-purged Sn target. Combined effects of rapid heating to very high temperatures, nanophase effect, and dense surfaces account for the condensation of fluorite-type structure which transformed martensitically to baddeleyite-type accompanied with twinning, commensurate shearing and shape change. Alternatively Pa-3-modified fluorite-type hardly survived transformation to a-PbO2 type and rutile type in the dynamic process analogous to the case of static decompression. In addition, the rutile-type SnO2 nanocondensates have {110}, {100} and {101} facets, which are beneficial for {~hkl} vicinal attachment to form edge dislocations, faults and twinned bicrystals. The {011}-interface relaxation, by shearing along <011> directions, accounts for a rather high density of edge dislocations near the twin boundary thus formed. The rutile-type SnO2 could be alternatively transformed from orthorhombic CaCl2-type structure (denoted as o) following parallel crystallographic relationship, (0 1)r//(0 1)o; [111]r//[111]o, and full of commensurate superstructures and twins parallel to (011) of both phases. Furthermore, SnO2-NiO solid solution (ss) condensates were fabricated by laser ablation on Ni-Sn target at 1.1 J/pulse and oxygen flow of 50 L/min. AEM observations indicated that the particles were more or less coalesced/agglomerated as nano chain aggregate or in close packed manner. The Ni-rich condensates have rock salt structure with defect clusters not in paracrystalline distribution as would otherwise develop into the spinel phase. The Sn-rich condensates are predominantly rutile-type with minor baddeleyite-type, which are vulnerable to martensitic transformation/relaxation to form {101} incommensuare faults as well as epitaxial twin variants of rutile upon rapid cooling and/or electron irradiation. Islands of metallic Ni-Sn-NiSn were partially oxidized/solidified when deposited on silica glass.