Superfícies seletivas de cobalto para a conversão fototérmica da energia solar

Orientador: Milton AbramovichDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Uma eficiente conversão fototérmica da energia solar requer superfícies com uma alta absortância para comprimentos de onda l 2,5 mm. As superfícies que possuem estas características são conhecidas como "superfícies seletivas". Nós apresentamos neste trabalho um processo experimental, bem como uma análise teórica de filmes seletivos obtidos por evaporação de Co em presença de Oxigênio e Hélio. As amostras produzidas passaram pelo seguinte processo experimental: medida da transmitância espectral entre 0,3 mm e 20 mm, determinação da massa e espessura do filme, análise da estrutura e composição química por microscopia eletrônica de transmissão. Para uma das amostras foi determinada a distribuição estatística dos diâmetros das partículas produzidas, como também a influência de um tratamento térmico em ar sobre suas propriedades óticas, estruturais e químicas. A transmitância espectral teórica dos filmes foi calculada com base na teoria de meio efetivo de Maxwell Garnett. Considerando uma interação do tipo dipolo-dipolo entre as partículas e uma estrutura composta por esferas isoladas, cadeias e aglomerados, uma concordância satisfatória entre a transmitância espectral experimental e os cálculos teóricos foi obtidaAbstract: An efficient phototherrnal solar energy conversion requires surfaces with a high absorptance for wavelengths l 2.5 mm. The surfaces that have these characteristics are known by "selective surfaces". We present in this work an experimental procedure as well as a theoretical analysis of selective films produced by evaporation of Co in a gas mixture of Oxygen and Helium. The produced samples followed the experimental process below: measurements of the spectral transmittance from 0.38 mm to 20 mm, determination of the mass and the thickness of the film, structural and chemical composition analysis by transmission electron microscopy. For one of the samples it was determined the statistical distribution of the particles diameter and also the influence of a heat treatment in air over its optical, structural and chemical properties. The calculation of the theoretical spectral transmittance of the films was based on the effective medium theory of Maxwell Garnett. Considering a dipole-dipole interaction between the particles and a combined structure of isolated spheres, chains and clusters, a satisfactory agreement between the experimental spectral transmittance and the theoretical results was obtained.MestradoFísicaMestre em Físic

Estudo das propriedades óticas, elétricas e estruturais de filmes vaporizados de Sn O2 : F

Orientador: Iris C. L. de TorrianiTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb WataghinResumo: Filmes finos de dióxido de estanho dopado com flúor foram produzidos pelo método convencional de vaporização sobre substrato de vidro Corning 7059. As propriedades óticas elétricas dos filmes foram determinadas como função da temperatura de substrato (200 - 490ºC) e da concentração de dopante na solução básica de vaporização. A transmitância no visível e infravermelho próximo, como também a resistividade elétrica dos filmes decresceu com o aumento da concentração do flúor. As melhores propriedades eletro-óticas (transmitância média de 75% resistividade elétrica de 10-3 Wcm) foram obtidas para 250ºC £ Ts £ 350ºC e concentrações de flúor na solução de vaporização em torno de 2 at.% F: Sn. A textura superficial dos filmes foi investigada por microscopia ótica e eletrônica de varredura. A concentração de flúor nas amostras foi investigada por SIMS (Secondary Ions Mass Spectroscopy), AES (Auger Electron Spectroscopy) and ESCA (Electron Spectroscopy for Chemical Analysis). A análise por difratometria de raios X mostrou que os filmes depositados com Ts ³ 250ºC são policristalinos. Observou-se uma mudança sistemática das linhas de difração de raios X como função da concentração de flúor. Foram realizados cálculos teóricos dos fatores de estrutura associados à rede do SnO2 introduzindo-se átomos de flúor em posições substitucionais e intersticiais. A presença do dopante não intencional Cl foi também investigada e seu papel nos filmes de SnO2 : F sugeridoAbstract: Thin films of tin dioxide doped with fluorine were produced by the conventional spraying method using 7059 Corning Glass substrates. The electrical and optical properties of these films were determined as a function of the substrate temperature(200 - 450ºC) and dopant concentration in the basic spraying solution. The visible and near infrared transmittance, as well as the electrical resistivity of the films decreased with the increase in fluorine concentration. The best electro-optical properties (an average transmittance of 75% and an electrical resistivity of 10-3 Wcm) were achieved for 250ºC £ Ts £ 350ºC and fluorine concentration in the spraying solution of around 2 at.% F:Sn. The films surface texture was investigated by scanning electron microscopy and optical microscopy. Fluorine content of the samples was investigated. by SIMS (Secondary Ions Mass Spectroscopy), AES (Auger Electron Spectroscopy and ESCA (Electron Spectroscopy for Chemical Analysis). The x-ray diffractometric analyses showed that the films deposited at Ts ³ 250ºC are polycrystalline. A systematic chance in the intensity of the diffraction lines as a function of the fluorine content was observed. Theoretical calculations of the structure associated to the SnO2 lattice introducing fluorine atoms in substitutional and interstitial positions were performed. The presence of the non intentional dopant Cl was also investigated and its role in SnO2: F films suggested.DoutoradoFísicaDoutor em Ciência

Incorporation of proteins in large pore SBA-15.

Ordered mesoporous silica (OMS) SBA-15 has morphological properties useful for many applications, including high surface area, hydrothermal mechanical stability and selective adjustable pore size. These properties make the SBA-15 interesting for many applications such as catalysis, big molecules selection and protein encapsulation. Usually, a pore diameter of 10 to 12 nm is obtained in a conventional synthesis process with Pluronic 123 template

Structural and textural studies of NiO impregnation in mesoporous ZrO2-CeO2 for\ud catalysis applications.

The synthesis of zirconia-based ordered mesoporous structures for catalytic applications is a\ud research area under development. These systems are also potential candidates as anodes in\ud intermediate temperature solid oxide fuel cells (it-SOFC) due to an enhancement on their surface area\ud [1-4]. The structural features of mesoporous zirconia-ceria materials in combination with oxygen\ud storage/release capacity (OSC) are crucial for various catalytic reactions. The direct use of\ud hydrocarbons as fuel for the SOFC (instead of pure H2), without the necessity of reforming and\ud purification reactors can improve global efficiency of these systems [4].\ud The X-ray diffraction data showed that ZrO2-x%CeO2 samples with x>50 are formed by a larger\ud fraction of the cubic phase (spatial group Fm3m), while for x<50 the major crystalline structure is the\ud tetragonal phase (spatial group P42/nmc). The crystallite size of the cubic phase increases with\ud increase in ceria content. The tetragonal crystallite size decreases when ceria content increases. After\ud impregnation, the Rietveld analysis showed a NiO content around 60wt.% for all samples. The lattice\ud parameters for the ZrO2 tetragonal phase are lower for higher ZrO2 contents, while for all samples the\ud cubic NiO and CeO2 parameters do not present changes. The calculated densities are higher for higher\ud ceria content, as expected. The crystallite size of NiO are similar (~20nm) for all samples and 55nm\ud for the NiO standard.\ud Nitrogen adsorption experiments revealed a broader particle size distribution for higher CeO2\ud content. The superficial area values were around 35m2/g for all samples, the average pore diameter and\ud pore volumes were higher when increasing ceria content. After NiO impregnation the particle size\ud distribution was the same for all samples, with two pore sizes, the first around 3nm and a broader peak\ud around 10nm. The superficial area increased to approximately 45m2/g for all samples, and the pore\ud volume was also higher after impregnation and increased when ceria content increased.\ud These results point up that the impregnation of NiO improves the textural characteristics of the\ud pristine material. The complementary TEM/EDS images present a homogeneous coating of NiO\ud particles over the ZrO2-x%CeO2 support, showing that these samples are excellent for catalysis\ud applications.\ud [1] D. Y. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D. Stucky, Science\ud 279, 548-552 (1998).\ud [2] C. Yu, Y. Yu, D. Zhao, Chem. Comm. 575-576 (2000).\ud [3] A. Trovarelli, M. Boaro, E. Rocchini, C. de Leitenburg, G. Dolcetti, J. Alloys Compd. 323-324\ud (2001) 584-591.\ud [4] S. Larrondo, M. A. Vidal, B. Irigoyen, A. F. Craievich, D. G. Lamas, I. O. Fábregas, et al. Catal.\ud Today 107–108 (2005) 53-59.CNPqFAPES

X-ray absorption spectroscopy study on La0.6Sr0.4CoO3 and La0.6Sr0.4Co1¡yFeyO3 nanotubes and nanorods for IT-SOFC cathodes.

In the last years, extensive research has been devoted to develop novel materials\ud and structures with high electrochemical performance for intermediate-temperatures\ud solid-oxide fuel cells (IT-SOFCs) electrodes. In recent works, we have investigated the structural and electrochemical properties of La0:6Sr0:4CoO3 (LSCO) and\ud La0:6Sr0:4Co1¡yFeyO3 (LSCFO) nanostructured cathodes, finding that they exhibit\ud excellent electrocatalytic properties for the oxygen reduction reaction [1,2]. These materials were prepared by a pore-wetting technique using polycarbonate porous membranes as templates. Two average pore sizes were used: 200 nm and 800 nm. Our scanning electronic microscopy (SEM) study showed that the lower pore size yielded nanorods, while nanotubes were obtained with the bigger pore size. All the samples\ud were calcined at 1000oC in order to produce materials with the desired perovskite-type\ud crystal structure.\ud In this work, we analyze the oxidation states of Co and Fe and the local atomic order\ud of LSCO and LSCFO nanotubes and nanowires for various compositions. For this pur-\ud pose we performed XANES and EXAFS studies on both Co and Fe K edges. These\ud measurements were carried out at the D08B-XAFS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). XANES spectroscopy showed that Co and Fe only\ud change slightly their oxidation state upon Fe addition. Surprisingly, XANES results\ud indicated that the content of oxygen vacancies is low, even though it is well-known that\ud these materials are mixed ionic-electronic conductors. EXAFS results were consistent\ud with those expected according to the rhombohedral crystal structure determined in\ud previous X-ray powder dffraction investigations.\ud [1] M.G. Bellino et al, J. Am. Chem. Soc. 129 (2007) 3066\ud [2] J.G. Sacanell et al., J. Power Sources 195 (2010) 1786CNPqCAPE

Structural and reduction studies of ZrO2-CeO2:Ni for application in SOFC anodes.

Zirconia-ceria solid-solutions are extensively used as promoters for three-way catalysts, which are applied in the control of NOx, CO and hydrocarbons emission from automotive exhausts. In addition, thesematerials can be used as anodes in solid oxide fuel cells (SOFCs) operated with hydrocarbons. There areonly few works on ZrO2-CeO2 ordered mesoporous materials for catalytic applications and for anodes inSOFCs. The interest in these anodes relies on the fact that ZrO2-CeO2materials are mixed ionic/electronic conductors in reducing atmosphere and, therefore, fuel oxidation is produced on its entire surface, while it only occurs in the [anode/electrolyte/gas] interface (triple-phase boundaries) for electronic conductors.\ud In this work, a synthesis method was developed usingZr and Ce chloride precursors, HCl aqueous solution, Pluronic P123 as the structure directing agent, NH4OH to adjust the pH (3-4) and a Teflon autoclave to perform hydrothermal treatment (80ºC/48 hours). The samples were dried and calcined, until 540ºC in N2and 4 hours in air. The X-ray diffraction data showed that powders with higher CeO2 content are formed by a larger fraction of the cubic CeO2 phase, while for a lower CeO2content the major crystalline structure is the tetragonal ZrO2 phase. The NiO impregnation was made with an ethanol dispersion of Ni(NO3)×6H2O. The resulting powder was calcinated in air until 350ºC for 2 hours. Temperature-programmed reduction (TPR) data were collected in order to evaluate the reduction profiles of ZrO2-x%CeO2:Ni samples in H2/Ar atmosphere. Results showed lower reduction temperatures for all ceria content in samples comparing to a NiO standard

Properties of a new poly-ether-glycol copolymer.

Triblock copolymers are made of monomer segments, being the central part usually hydrophobic and the outer parts hydrophilic. By varying sizes, molecular weights and monomer types of the segments one obtains different final molecules, with different physico-chemical properties, which are directly related to the performance of the final product. Looking for new products to be used, among other possibilities, in biological applications, a new polymer (Figure 1) was synthesized by the Dow Chemical and studied by Size Exclusion Chromatography, Fourier Transformed Infrared Spectrometry, Small-angle X-ray Scattering (SAXS) and its cloud point was determined by measuring light transmittance. The studies showed low molecular polydispersivety, but different polarities in the macromolecules fractions. Due to the low solubility of Diol in water, a mixture of water/butyl diglycol was used as solvent. An extensive analysis by SAXS was performed for concentrations from 50 wt% to 80 wt% of Diol in solution. Small concentrations showed very low signal to noise ratio, making it impossible to be analysed. The scattering intensity including the form factor of polydisperse non-homogeneous spheres, and the structure factor of interacting hard spheres was fitted to the curves. As the polymer concentration is high, the fitting of form factors of direct and reverse micelles were compared. The results for direct micelles were better up to 80 wt%, whereas at 90 wt% and 95 wt% the curves were better fitted by reverse micelles. It might seem odd that direct micelles are present up to such high concentrations, but it might have been caused by the presence of butyl diglycol, which increases the solubility of Diol in water. The inner and outer radius of the micelles, electron density distribution, and interaction radius of the micelles were obtained. The polydispersivety increases with Diol concentration. Besides, the interaction radius increases with solvent concentration, even when reversed micelles are present. In the last case, accompanied by an increase of inner radius (water content), as there are fewer Diol molecules to involve the water nuclei, which become larger, further apart, and in less number

Retention at room temperature of the tetragonal t ``-form in Sc(2)O(3)-doped ZrO(2) nanopowders

Synchrotron X-ray powder diffraction was applied to the study of the effect of crystallite size on the crystal structure of ZrO(2)-10 mol% Sc(2)O(3) nanopowders synthesized by a nitrate-lysine gel-combustion route Nanopowders with different average crystallite sizes were obtained by calcination at several temperatures, ranging from 650 to 1200 degrees C The metastable t""-form of the tetragonal phase, exhibiting a cubic unit cell and tetragonal P4(2)/nmc spatial symmetry, was retained at room temperature in fine nanocrystalline powders, completely avoiding the presence of the stable rhombohedral beta phase. Differently, this phase was identified in samples calcined at high temperatures and its content increased with increasing crystallite size The critical maximum crystallite size for the retention of the mestastable t""-form resulted of about 35 nm (C) 2009 Elsevier B.V All rights reserve