CoordenaÃÃo de AperfeiÃoamento de Pessoal de NÃvel Superior / Devido ao rÃpido progresso no desenvolvimento de sistemas de comunicaÃÃo em microondas, as cerÃmicas dielÃtricas e magnÃticas (ferritas) se tornaram atrativas para o uso em dispositivos. Embora as ferritas do tipo espinel terem sido os primeiros materiais utilizados para microondas, as granadas possuem menores perdas dielÃtricas e, portanto, sÃo preferidas em muitas aplicaÃÃes. As elevadas exigÃncias que as aplicaÃÃes elÃtricas modernas requerem dos materiais magnÃticos fazem com que novas tÃcnicas e produtos estejam permanentemente sendo pesquisados, com o conseqÃente aparecimento de novas soluÃÃes para uma ampla sÃrie de aplicaÃÃes. Nesse sentido, a pesquisa sobre compÃsitos à empregada para a busca de novos materiais. Este trabalho apresenta a obtenÃÃo do compÃsito ferrimagnÃtico, constituÃdo pelas fases Y3Fe5O12 (YIG) e Gd3Fe5O12 (GdIG), atravÃs da rota sintÃtica no estado sÃlido com a utilizaÃÃo de moagem mecÃnica de alta energia. A DifraÃÃo de Raios-X, a Espectroscopia Raman, Infravermelho e MÃssbauer foram essenciais para caracterizaÃÃo estrutural dos compÃsitos. As peculiaridades da morfologia das amostras foram elucidadas pela Microscopia EletrÃnica de Varredura e Microdureza de Vickers. Adicionalmente, foram realizados experimentos para se avaliar o comportamento elÃtrico e magnÃtico dos compÃsitos na faixa de radio freqÃÃncia e de microondas para depois sugerir uma aplicaÃÃo tecnolÃgica cabÃvel. Os compÃsitos comportaram-se adequadamente como antenas ressoadoras de ferritas (FRAs) e de microlinha (filmes espessos depositados sobre a superfÃcie metalizada de um substrato de alumina, pela tÃcnica âscreen printingâ), na faixa de operaÃÃo das microondas. Os experimentos com os FRAs se mostraram satisfatÃrios devido ao controle das caracterÃsticas de radiaÃÃo das antenas e a sua sintonizaÃÃo pela aplicaÃÃo de um campo magnÃtico externo. As mesmas tiveram a caracterÃstica de antenas de banda larga. Os ressoadores provenientes dos compÃsitos projetados neste trabalho podem ser importantes para desenvolver uma antena de banda larga de terceira geraÃÃo (3G) para telefones celulares e outros produtos para redes sem fio. / Devido ao rÃpido progresso no desenvolvimento de sistemas de comunicaÃÃo em microondas, as cerÃmicas dielÃtricas e magnÃticas (ferritas) se tornaram atrativas para o uso em dispositivos. Embora as ferritas do tipo espinel terem sido os primeiros materiais utilizados para microondas, as granadas possuem menores perdas dielÃtricas e, portanto, sÃo preferidas em muitas aplicaÃÃes. As elevadas exigÃncias que as aplicaÃÃes elÃtricas modernas requerem dos materiais magnÃticos fazem com que novas tÃcnicas e produtos estejam permanentemente sendo pesquisados, com o conseqÃente aparecimento de novas soluÃÃes para uma ampla sÃrie de aplicaÃÃes. Nesse sentido, a pesquisa sobre compÃsitos à empregada para a busca de novos materiais. Este trabalho apresenta a obtenÃÃo do compÃsito ferrimagnÃtico, constituÃdo pelas fases Y3Fe5O12 (YIG) e Gd3Fe5O12 (GdIG), atravÃs da rota sintÃtica no estado sÃlido com a utilizaÃÃo de moagem mecÃnica de alta energia. A DifraÃÃo de Raios-X, a Espectroscopia Raman, Infravermelho e MÃssbauer foram essenciais para caracterizaÃÃo estrutural dos compÃsitos. As peculiaridades da morfologia das amostras foram elucidadas pela Microscopia EletrÃnica de Varredura e Microdureza de Vickers. Adicionalmente, foram realizados experimentos para se avaliar o comportamento elÃtrico e magnÃtico dos compÃsitos na faixa de radio freqÃÃncia e de microondas para depois sugerir uma aplicaÃÃo tecnolÃgica cabÃvel. Os compÃsitos comportaram-se adequadamente como antenas ressoadoras de ferritas (FRAs) e de microlinha (filmes espessos depositados sobre a superfÃcie metalizada de um substrato de alumina, pela tÃcnica âscreen printingâ), na faixa de operaÃÃo das microondas. Os experimentos com os FRAs se mostraram satisfatÃrios devido ao controle das caracterÃsticas de radiaÃÃo das antenas e a sua sintonizaÃÃo pela aplicaÃÃo de um campo magnÃtico externo. As mesmas tiveram a caracterÃstica de antenas de banda larga. Os ressoadores provenientes dos compÃsitos projetados neste trabalho podem ser importantes para desenvolver uma antena de banda larga de terceira geraÃÃo (3G) para telefones celulares e outros produtos para redes sem fio. / Due to a fast progress in the development of communication systems, the dielectric and magnetic ceramics (ferrites) have become attractive to be used in devices. Although the ferrites of the spinel type were the first material used in the microwave range, garnets have smaller dielectric losses and, therefore, are chosen for many applications. High demands for modern electric applications in magnetic materials results in new techniques and products being permanently studied and researched, with a consequent appearance of new solutions for a wide applications series. In this sense, the research in composites is employed to reach new materials. This work presents the study of the ferrimagnetic composite, constituted by Y3Fe5O12 (YIG) and Gd3Fe5O12 (GdIG) phases, through solid state synthetic route and submitted to high-energy mechanical milling. The X-Ray Diffraction and the Raman, Infrared and MÃssbauer Spectroscopy were essential for the structural characterization of the composites. The peculiarities in the morphology of the samples were elucidated by Scanning Electron Microscopy and Vickers Microhardness. Additionally, experiments were made in order to evaluate the electric and magnetic behavior of the composites at radio frequency and microwave range and then later suggest an adequate technological application. The composites were efficient as ferrite resonator antennas (FRAs) and microstrip antennas (thick films deposited on metalized surface alumina substrate by screen-printing technique), in the microwave frequency range. The experiments with FRAs showed satisfactory due to the control of the antennas radiation characteristics and their tuning by the use of an external magnetic field. They present a wideband antenna profile. The composite resonators studied in this work can be important to the development of a third generation (3G) wideband antennas to cell phones and other wireless products. / Due to a fast progress in the development of communication systems, the dielectric and magnetic ceramics (ferrites) have become attractive to be used in devices. Although the ferrites of the spinel type were the first material used in the microwave range, garnets have smaller dielectric losses and, therefore, are chosen for many applications. High demands for modern electric applications in magnetic materials results in new techniques and products being permanently studied and researched, with a consequent appearance of new solutions for a wide applications series. In this sense, the research in composites is employed to reach new materials. This work presents the study of the ferrimagnetic composite, constituted by Y3Fe5O12 (YIG) and Gd3Fe5O12 (GdIG) phases, through solid state synthetic route and submitted to high-energy mechanical milling. The X-Ray Diffraction and the Raman, Infrared and MÃssbauer Spectroscopy were essential for the structural characterization of the composites. The peculiarities in the morphology of the samples were elucidated by Scanning Electron Microscopy and Vickers Microhardness. Additionally, experiments were made in order to evaluate the electric and magnetic behavior of the composites at radio frequency and microwave range and then later suggest an adequate technological application. The composites were efficient as ferrite resonator antennas (FRAs) and microstrip antennas (thick films deposited on metalized surface alumina substrate by screenprinting technique), in the microwave frequency range. The experiments with FRAs showed satisfactory due to the control of the antennas radiation characteristics and their tuning by the use of an external magnetic field. They present a wideband antenna profile. The composite resonators studied in this work can be important to the development of a third generation (3G) wideband antennas to cell phones and other wireless products.

<p>A fundamental understanding of heterogeneous catalysis requires analysis of model catalytic surfaces in tandem with complex technical catalysts. This work was divided in three areas, 1- preparation and characterization of model surfaces synthesized by vapor deposition techniques, 2- kinetic evaluation of model catalysts for formic acid decomposition and dry methane reforming, 3- characterization and kinetic evaluation of technical catalysts for the water gas shift reaction.</p> <p>In the first project, model PdZn intermetallic surfaces, a relevant catalyst for propane dehydrogenation, were prepared using an ALD approach. In this work, model surfaces were synthesized by exposing Pd(111) and Pd(100) surfaces to diethylzinc at ca. 10^-6mbar. Several different surface structures were identified by careful control of the deposition temperature of the substrate. Modifications in the adsorption properties of these surfaces towards carbon monoxide and propylene coincided with the structure of the PdZn surface layer. </p> <p>In the second project, formic acid decomposition kinetics were evaluated on model Pt catalysts. Formic acid decomposition was found to be structure-insensitive on Pt(111), Pt(100), and a polycrystalline foil under standard reaction conditions. CO selectivity remained < 1% for conversions <10%. Additionally, inverse Pd-Zr model catalysts were prepared by ALD of zirconium-t-butoxide (ZTB). Depending on treatment conditions, either ZrO_xH_y or ZrO₂ overlayers or Zr as sub-nanometer clusters could be obtained. The activity of the model catalyst surface towards dry reforming of methane if the initial state of the zirconium is metallic. </p> <p>In the third project, Au/Fe₃O₄ heterodimer catalysts were characterized for their thermal stability. In-situ TEM and XPS characterization demonstrates that the gold nanoparticles transform into gold thin films that wet the Fe₃O₄ surface as the reduction of the oxide proceeds. DFT calculations show that the adhesion energy between the Au film is increased on a partially reduced Fe₃O₄ surface. Additionally, Pt/Nb₂CT_x catalysts were characterized and kinetics evaluated for the water gas shift reaction. XPS and TEM characterization indicates that a Pt-Nb surface alloy is formed under moderate reduction temperatures, 350^OC. Water-gas shift reaction kinetics reveal that the alloy-MXene interface exhibit high H₂O activation ability compared to a non-reducible support or bulk niobium carbide. </p>

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