Alloys, intermetallic compounds and multi-metal oxides are generally made by traditional solid-state methods that often require melting or grinding/pressing powders followed by high temperature annealing (> 1000 degrees C) for days or weeks. The research presented here takes advantage of the fact that nanoparticles have a large fraction of their atoms on the surface making them highly reactive and their small size virtually eliminates the solid-solid diffusion process as the rate limiting step. Materials that normally require high temperatures and long annealing times become more accessible at relatively low-temperatures because of the increased interfacial contact between the nanoparticle reactants. Metal nanoparticles, formed via reduction of metal salts in an aqueous solution and stabilized by PVP (polyvinylpyrrolidone), were mixed into nanoparticle composites in stoichometric proportions. The composite mixtures were then annealed at relatively low temperatures to form alloy and intermetallic compounds at or below 600 degrees C. This method was further extended to synthesizing multi-metal oxide systems by annealing metal oxide nanoparticle composites hundreds of degrees lower than more traditional methods. Nanoparticles of Pt (supported or unsupported) were added to a metal salt solution of tetraethylene glycol and heated to obtain alloy and intermetallic nanoparticles. The supported intermetallic nanoparticles were tested as catalysts and PtPb/Vulcan XC-72 showed enhanced catalytic activity for formic acid oxidation while Pt3Sn/Vulcan XC-72 and Cu3Pt/y-Al2O3 catalyzed CO oxidiation at lower temperatures than supported Pt. Intermetallic nanoparticles of Pd were synthesized by conversion chemistry methods previously mentioned and were supported on carbon and alumina. These nanoparticles were tested for Suzuki cross-coupling reactions. However; the homocoupled product was generally favored. The catalytic activity of Pd3Pb/y-Al2O3 was tested for the Heck reaction and gave results comparable to Pd/y-Al2O3 with a slightly better selectivity. Conversion chemistry techniques were used to convert Pt nanocubes into Ptbased intermetallic nanocrystals in solution. It was discovered that aggregated clusters of Pt nanoparticles were capable of converting to FePt3; however, when Pt nanocubes were used the intermetallic phase did not form. Alternatively, it was possible to form PtSn nanocubes by a conversion reaction with SnCl2.

The objective of this work is to evaluate the reaction initiation characteristics of quasi-statically compressed intermetallic-forming aluminum-based reactive materials upon impact initiation, consisting of equi-volumetric tantalum-aluminum, tungsten-aluminum, nickel-aluminum, and pure aluminum. A modified Taylor rod-on-anvil setup was employed to determine the reaction initiation threshold kinetic energy and actual energy for plastic deformation and subsequent reaction. Experimental sample remnants were recovered and examined through X-ray diffraction to determine reaction products.The overall results indicate that of the various intermetallic-forming systems investigated, Ta+Al was the most reactive and was the only system where any reaction products were retrieved. While all of the intermetallic systems reacted in air, only Ta+Al and W+Al reacted in vacuum environment suggesting differences in reaction mechanisms influencing the reactivity of intermetallic mixtures. Based on the threshold energy for onset of reaction it appears that the Ta-Al compacts show reaction conditions below those required for reaction of Al in air. This combined with the fact that Ta+Al compacts also react in vacuum implies that the Ta+Al undergoes anaerobic intermetallic reaction while the other systems react with the oxidation of Al. The effect of compact packing density on the kinetic energy threshold for reaction initiation were also evaluated. It was observed more densely packed Ta+Al and Ni+Al powder compacts react more easily than less densely packed samples. While the effect of packing density is not as obvious in the case of pure Al and W+Al powder compacts. Finally, a particle size effect is seen on Ni+Al on samples of < 92% density where coarser (+325 -200 mesh) equal-volumetric powder mixtures were observed to be more reactive than finer Ni+Al (-325 mesh).

Sn-Pb solder has long been used in the Electronics industry. But, due to its toxic nature and environmental effects, certain restrictions are made on its use and therefore many researchers are looking to replace it. Sn-3.0Ag-0.5Cu (SAC) solders are suggested as lead-free replacements but their coarse microstructure and formation of hard and brittle Inter-Metallic Compounds (IMCs) like Ag₃Sn and Cu₆Sn₅ have limited their use in high temperature applications. In this research work, RE elements, mostly lanthanum (La), are used as potential additives to SAC alloys. They reduce the surface free energy, refine the grain size and improve the mechanical and wetting properties of SAC alloys. An extensive experimental work has been performed on the microstructure evolution, bulk mechanical properties, individual phase (matrix and IMCs) mechanical properties, creep behavior and wettability performance of the SAC and SAC-La alloys, with different (La) doping. SEM and EDS have been used to follow the continuous growth of the IMCs at 150°C and 200°C and thus provide a quantitative measure in terms of their size, spacing and volume fraction. Grain size is measured at regular intervals starting from 10 hours up to 200 hours of thermal aging using Optical Microscope with cross polarized light. Bulk mechanical properties are evaluated using tensile tests at low strain rates. Individual phase mechanical properties like Young's modulus, hardness, strain rate sensitivity index and bulge effects are characterized with nanoindentation from 100 µN up to 5000 µN loadings at different temperatures of 25°C, 45°C, 65°C and 85°C. Creep experiments are performed at elevated temperatures with good fitting of Dorn creep and back-stress creep models. Activation energy measurements are made at 40°C, 80°C and 120°C. Wettability testing on copper substrates is used for surface tension, wetting force and contact angle measurements of SAC and SAC-La doped alloys at 250°C and 260°C.

19 Dec 2004. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 1936" Jongik Park. 12/19/2004. Report is also available in paper and microfiche from NTIS.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / SOUSA, Vinícius da Silva Ramos de. O efeito magnetocalórico anisotrópico nos compostos RAl2 (R = Dy, Er, Ho, Nd e Tb). 2008. 99f. Dissertação (Mestrado em Física) - Instituto de Física Armando Dias Tavares, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, 2008. O efeito magnetocalórico é a base da refrigeração magnética. O potencial magnetocalórico é caracterizado por duas quantidades termodinâmicas: a variação isotérmica da entropia (&#916;Siso) e a variação adiabática da temperatura (&#916;Tad), as quais são calculadas sob uma variação na intensidade do campo magnético aplicado ao sistema. Em sistemas magnéticos que apresentam uma anisotropia magnética é observada uma mudança no efeito magnetocalórico, isto porque este potencial torna-se fortemente dependente da direção de aplicação do campo magnético. A anisotropia em sistemas magnéticos pode levar a um efeito magnetocalórico inverso, assim como à definição de um efeito magnetocalórico anisotrópico, o qual por definição é calculado para um campo cuja intensidade é mantida constante e cuja orientação variamos de uma direção difícil de magnetização para a direção fácil de magnetização. O efeito magnetocalórico anisotrópico foi estudado para os compostos intermetálicos de terras raras do tipo RAl2 considerando-se um modelo microscópico que leva em conta as interações de troca (na aproximação de campo médio), de Zeeman e a interação de campo elétrico cristalino, que é a responsável pela anisotropia nos compostos RAl2. O efeito magnetocalórico anisotrópico foi investigado para a série RAl2 e comparado com o efeito magnetocalórico usual. / The magnetic refrigeration is based on the magnetocaloric effect. The magnetocaloric potential is characterized by the two thermodynamics quantities: the isothermal entropy change (&#916;Siso) and the adiabatic temperature change (&#916;Tad), which are calculated upon a change in the intensity of the applied magnetic field. In anisotropic magnetic systems it is observed a change in the magnetocaloric effect, since this potential becomes strongly dependent on the direction in which the external magnetic field is applied. The anisotropy in such magnetic systems can lead to an inverse magnetocaloric effect, as well as to the definition of an anisotropic magnetocaloric effect, that by definition is calculated upon a magnetic field which intensity is kept fixed and which orientation is changed from a hard direction of magnetization to the easy direction of magnetization. This anisotropic magnetocaloric effect was performed for the RAl2 intermetallic compounds considering a microscopic model Hamiltonian that includes the Zeeman interaction, the exchange interaction (taken in the mean field approximation) and the crystalline electrical field, that is responsible for the anisotropy in the RAl2 compounds. The anisotropic magnetocaloric was fully investigated for the serie RAl2 and compared with the usual magnetocaloric effect and several curves of (&#916;Siso) and (&#916;Tad) were obtained.

Notre étude à caractère pluridisciplinaire comprend l'élaboration de composés intermétalliques ainsi que la caractérisation de leurs propriétés tant structurales que magnétiques. Nos travaux ont porté sur des borures RT12B6 où R est un élément de terre rare ou l'yttrium et T un métal de transition 3d ainsi que des phases de Laves (Hf,Ta)Fe2. Pour appréhender les propriétés physiques de ces composés, nous avons mis en œuvre diverses variables externes (température, champ magnétique, pression) mais aussi internes telle que la pression chimique liée à la substitution d'un élément par un autre. Nous apportons une contribution à l'étude des propriétés magnétiques des composés RCo12B6. Les propriétés magnétiques de ces composés sont caractérisées à la fois par une température d'ordre qui varie peu avec l'élément de terre rare R et un moment magnétique de Co remarquablement faible. Nous montrons que les interactions d'échange R-Co sont de plus d'un ordre de grandeur plus faibles que les interactions Co-Co existant dans ces composés. La substitution du fer au cobalt dans les composés RCo12B6 est possible et donne lieu à une localisation préférentielle. Grâce à la spectroscopie Mössbauer et à la diffraction neutronique, nous avons démontré l'extrême sensibilité de l'orientation des moments magnétiques à la substitution Fe/Co. Le composé LaFe12B6 présente des propriétés magnétiques remarquables avec un état fondamental antiferromagnétique (AFM) et une transition vers un état ferromagnétique (FM) qui peut être induite par le champ appliqué ou par la température. A basse température la transition métamagnétique AFM-FM est accompagnée d'une hystérésis très large et est caractérisée par des sauts spectaculaires comme l'illustre nos mesures magnétiques, de magnétostriction ou de transport. La transition métamagnétique s'avère également fort sensible à la pression appliquée. Le composé intermétallique LaFe12B6 est caractérisé par une forte expansion thermique linéaire, un large effet magnétovolumique et présente à la fois des effets magnétocaloriques inverse et normal. L'effet de la substitution du cobalt ou du manganèse au fer ou du cérium au lanthane sur les propriétés structurales et magnétiques a été étudié de façon détaillée. La substitution Co/Fe ou Mn/Fe entraine dans les deux cas une forte augmentation du champ critique de la transition métamagnétique. Inversement la substitution Ce/La, quant à elle, réduit fortement le champ de transition. L'étude de l'alliage amorphe LaFe12B6, préparé par hypertrempe, montre des propriétés magnétiques radicalement différentes puisque la phase amorphe devient alors ferromagnétique avec une haute température de Curie. Enfin nous avons étudié les propriétés magnétiques intrinsèques du système intermétallique Hf1-xTaxFe2 pour lequel la solution solide est complète. L'analyse de l'ensemble des mesures a mis en lumière des comportements originaux du magnétisme du fer et ceci tant dans l'état ordonné que dans l'état paramagnétique. Le caractère inhabituel du magnétisme de ces composés est attribué au comportement d'électrons itinérants, lequel est à l'origine de la transition métamagnétique entre l'état AFM et l'état FM. / Our multidisciplinary study includes the synthesis of intermetallic compounds and the characterization of their structural and magnetic properties. Our work has focused on RT12B6 borides where R is a rare earth element or yttrium and T a 3d transition metal as well as (Hf, Ta)Fe2 Laves phases. In order to understand the physical properties of these compounds, we have implemented various external variables (temperature, magnetic field, pressure) as well as internal variables such as the chemical pressure due to the substitution of one element with another. Through this experimental work, we investigated the magnetic properties of RCo12B6 compounds. The magnetic properties of these compounds present both an ordering temperature which is quasi independent of the rare earth element R and a remarkably small magnetic moment of Co. We show that the R-Co exchange interactions are more than an order of magnitude smaller that the Co-Co occurring in these compounds. We demonstrated that the iron for cobalt substitution in RCo12B6 compounds gives rise to a preferential substitution scheme. Combining Mössbauer spectroscopy and neutron diffraction, we have found that the magnetic ordering direction is extremely sensitive to Fe/Co substitution. LaFe12B6 compound presents remarkable magnetic properties with an antiferromagnetic (AFM) ground state but it can be transformed into a ferromagnetic (FM) state by the applied magnetic field or by the temperature. At low temperature, the field-induced AFM-FM metamagnetic transition has a large hysteresis and exhibits ultra sharp jumps as shown in our magnetic, magnetostriction and transport measurements. The metamagnetic transition is also very sensitive to the applied pressure. LaFe12B6 intermetallic compound shows a large linear thermal expansion, a huge volume magnetostriction and both normal and inverse magnetocaloric effects. The effect of cobalt or manganese for iron substitution or cerium for lanthanum substitution on the structural and magnetic properties was deeply investigated. Co/Fe or Mn/Fe substitution in both cases leads to a strong increase of the critical field of the metamagnetic transition. However Ce/La substitution reduces strongly the transition field. The investigation of LaFe12B6 amorphous alloy, prepared by melt spinning, shows radically different magnetic properties since the amorphous phase becomes ferromagnetic with a high Curie temperature. Finally we studied the intrinsic magnetic properties of the Hf1-xTaxFe2 system for which the solid solution is complete. The analysis of all the measurements highlighted original behaviours of the iron magnetism and this both in the ordered state and in the paramagnetic state. These remarkable properties are attributed to the itinerant character of the Fe 3d band magnetism, which gives rise to the metamagnetic transition between the AFM and FM states.

Made available in DSpace on 2016-12-08T17:19:30Z (GMT). No. of bitstreams: 1 elementos pre-textuaus.pdf: 86552 bytes, checksum: b4f9afc7367b0456da6227d06b49d099 (MD5) Previous issue date: 2007-06-29 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Blackeart malleable iron samples were galvanized using twenty different zinc bath compositions. Each bath was made with different concentration and combinations of bismuth, nickel, tin and aluminium. These additions aim not only to substitute lead, a hazardous element to the environment, but also to reduce coating thickness, usually higher than established by standards due to the great reactivity between cast iron and zinc bath. Additional studies were made to check the corrosion resistance and to check the maintenance or intensity of the coating s brightness. When using just bismuth and nickel additions to the zinc bath, it wasn t observed a considerable coating thickness reduction, but bismuth influenced bath fluidity, favoring better zinc draining and formation of more compacted and defined zinc-iron compounds. It was observed that bismuth decreased the coating s corrosion resistance, while nickel can increase the corrosion resistance when the bath has small bismuth concentrations. Tin addictions reduced the coating thickness when used together with bismuth and nickel addictions. However, tin did not only reduce the corrosion resistance, but also decreased the coating s brightness. Highest aluminium concentrations reduced the coating s thickness considerably when compared to the coating s thickness of the samples galvanized in the other baths. Aluminium also increased corrosion resistance when compared to the coatings of the samples galvanized in baths containing bismuth, nickel and tin. However excessive coating thickness reduction, caused by highest aluminium addictions to the bath, reduced corrosion resistance. Highest aluminium addictions were totally favorable to the coating s brightness. The combination of bismuth, nickel, tin and aluminium were effective on coating thickness reduction and some of these combinations also provided greater rust resistance and shinier coatings. Hence, the chemical elements added weren t detrimental to the environment and they are good alternatives to substitute lead in the hot-dip galvanizing process. / Amostras de ferro fundido maleável preto foram galvanizadas em vinte diferentes banhos de zinco, cada qual composto por concentrações e combinações variadas de bismuto, níquel, estanho e alumínio. Além de substituir o chumbo, um elemento tóxico e nocivo ao meio ambiente, objetiva-se com estas adições a redução da espessura do revestimento galvanizado, normalmente em excesso ao estabelecido em norma devido à grande reatividade dos ferros fundidos com o banho de zinco. Estudos adicionais foram realizados para a verificação da resistência à corrosão e para a verificação da manutenção ou intensificação do brilho dos revestimentos. Utilizando adições somente de bismuto e níquel ao banho de zinco não foi observada uma redução considerável da espessura do revestimento, mas o bismuto influenciou na fluidez do banho favorecendo o melhor escorrimento do zinco e a formação de fases zinco-ferro mais compactas e definidas. Observou-se que o bismuto diminuiu a resistência à corrosão dos revestimentos, mas o níquel pode aumentar a resistência à corrosão quando se têm adições menores de bismuto ao banho. Adições de estanho reduziram a espessura de camada quando utilizadas em conjunto com adições de bismuto e níquel, mas o estanho foi prejudicial à resistência à corrosão além de diminuir o brilho dos revestimentos. O alumínio em concentrações mais elevadas tornou as espessuras dos revestimentos consideravelmente menores quando comparadas às espessuras dos revestimentos galvanizados nos outros banhos estudados. O alumínio ainda aumentou a resistência à corrosão dos revestimentos em relação aos revestimentos galvanizados nos banhos contendo bismuto, níquel e estanho. Contudo a redução excessiva da espessura da camada, proporcionada por adições maiores de alumínio, fez com que a resistência à corrosão diminuísse novamente. Adições de maiores teores de alumínio foram totalmente favoráveis ao aumento do brilho dos revestimentos. A combinação de elementos químicos como o bismuto, níquel, estanho e alumínio mostrou-se efetiva na redução da espessura de camada, sendo que algumas destas combinações também proporcionaram maiores valores de resistência à corrosão e revestimentos com brilhos mais intensos. Além disso, os elementos químicos adicionados não são nocivos ao meio ambiente, sendo boas alternativas para substituir o chumbo na galvanização por imersão a quente. Palavras-chave: galvanização por imersão a quente, espessura de camada, resistência à corrosão, compostos intermetálicos zinco-ferro.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / SOUSA, Vinícius da Silva Ramos de. O efeito magnetocalórico anisotrópico nos compostos RAl2 (R = Dy, Er, Ho, Nd e Tb). 2008. 99f. Dissertação (Mestrado em Física) - Instituto de Física Armando Dias Tavares, Universidade do Estado do Rio de Janeiro, Rio de Janeiro, 2008. O efeito magnetocalórico é a base da refrigeração magnética. O potencial magnetocalórico é caracterizado por duas quantidades termodinâmicas: a variação isotérmica da entropia (&#916;Siso) e a variação adiabática da temperatura (&#916;Tad), as quais são calculadas sob uma variação na intensidade do campo magnético aplicado ao sistema. Em sistemas magnéticos que apresentam uma anisotropia magnética é observada uma mudança no efeito magnetocalórico, isto porque este potencial torna-se fortemente dependente da direção de aplicação do campo magnético. A anisotropia em sistemas magnéticos pode levar a um efeito magnetocalórico inverso, assim como à definição de um efeito magnetocalórico anisotrópico, o qual por definição é calculado para um campo cuja intensidade é mantida constante e cuja orientação variamos de uma direção difícil de magnetização para a direção fácil de magnetização. O efeito magnetocalórico anisotrópico foi estudado para os compostos intermetálicos de terras raras do tipo RAl2 considerando-se um modelo microscópico que leva em conta as interações de troca (na aproximação de campo médio), de Zeeman e a interação de campo elétrico cristalino, que é a responsável pela anisotropia nos compostos RAl2. O efeito magnetocalórico anisotrópico foi investigado para a série RAl2 e comparado com o efeito magnetocalórico usual. / The magnetic refrigeration is based on the magnetocaloric effect. The magnetocaloric potential is characterized by the two thermodynamics quantities: the isothermal entropy change (&#916;Siso) and the adiabatic temperature change (&#916;Tad), which are calculated upon a change in the intensity of the applied magnetic field. In anisotropic magnetic systems it is observed a change in the magnetocaloric effect, since this potential becomes strongly dependent on the direction in which the external magnetic field is applied. The anisotropy in such magnetic systems can lead to an inverse magnetocaloric effect, as well as to the definition of an anisotropic magnetocaloric effect, that by definition is calculated upon a magnetic field which intensity is kept fixed and which orientation is changed from a hard direction of magnetization to the easy direction of magnetization. This anisotropic magnetocaloric effect was performed for the RAl2 intermetallic compounds considering a microscopic model Hamiltonian that includes the Zeeman interaction, the exchange interaction (taken in the mean field approximation) and the crystalline electrical field, that is responsible for the anisotropy in the RAl2 compounds. The anisotropic magnetocaloric was fully investigated for the serie RAl2 and compared with the usual magnetocaloric effect and several curves of (&#916;Siso) and (&#916;Tad) were obtained.

Orientadores: Carlos Manuel Giles Antunez de Mayolo, Pascoal Jose Giglio Pagliuso / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-08T18:22:30Z (GMT). No. of bitstreams: 1 Serrano_RaimundoLora_D.pdf: 2642603 bytes, checksum: 743f3ac1f8dffb9e6358f1a7958794e6 (MD5) Previous issue date: 2006 / Resumo: Neste trabalho investigamos de forma sistemática as propriedades magnéticas macroscópicas (Susceptibilidade magnética, calor específico, resistividade elétrica) e as estruturas magnéticas de uma nova série de compostos tetragonais isoestruturais Rm Mn In3m+2n (R = Gd, Tb, Sm; M = Rh, Ir; m = 1, 2; n = 0, 1) e exploramos suas relações com as interessantes propriedades físicas encontradas em outros compostos desta família, especialmente quando R = Ce, onde tem sido observado um comportamento do tipo férmions pesados com supercondutividade não convencional (USC). Foram determinadas as estruturas magnéticas dos compostos Gd2IrIn8, GdRhIn5, GdIn3, TbRhIn5, Tb2RhIn8, Smn2IrIn8 em amostras monocristalinas de alta qualidade através da técnica de Difração Magnética de Raios-x (DMRX), e encontramos que todos se ordenam antiferromagneticamente com estruturas comensuráveis abaixo da temperatura de ordenamento (TN) com vetores de propagação (1/2,0,0), (0,1/2,1/2),(1/2,1/2,0), (1/2,0,1/2), (1/2,1/2,1/2) e (1/2,0,0), respectivamente. Os momentos magnéticos dos íons de terra rara se orientam no plano ab tetragonal no caso dos compostos com R = Gd e Sm2IrIn8 enquanto que no TbRhIn5 a orientação tem lugar ao longo do eixo c. Os compostos tetragonais inéditos a base de Tb foram todos sintetizados e caracterizados magnética e estruturalmente, pela primeira vez, dentro deste trabalho. Eles apresentam maior TN em relação ao composto cúbico TbIn3 (Tb1-0-3, TN ¿32 K), similar ao comportamento apresentado pelos compostos tetragonais de R = Nd. Com relação à direção dos momentos magnéticos no estado ordenado e à evolução de TN ao longo da série, os nossos resultados estão de acordo com um novo modelo teórico de campo médio, desenvolvido por colaboradores, que considera uma interação de primeiros vizinhos Ruderman-Kittel-Kasuya-Yosida (RKKY) isotrópica e efeitos de campo cristalino (CEF) tetragonal aplicado aos compostos com R = Ce, Nd e Tb. A idéia básica de interpretação dos nossos resultados, extraída dos resultados do modelo, é que as diferentes direções de ordenamento encontrados para diferentes R são determinadas por efeitos de CEF e que a variação dos parâmetros de CEF tetragonal determina a evolulção de TN . De acordo com isto, nos compostos com R = Gd, onde o momento angular orbital L = 0, os efeitos CEF não são importantes, TN é aproximadamente igual quando se comparam os compostos tetragonais com o cúbico GdIn3. Nos outros compostos tetragonais, cuja direção dos momentos tem lugar no plano ab (R = Ce e Sm), TN diminui, e aumenta quando a ordem ocorre ao longo do eixo c. O mecanismo de controle, por efeitos de campo cristalino, do comportamento da orientação dos momentos magnéticos e de TN pode, em particular, ser extrapolado aos compostos tetragonais de Ce já que a supressão de TN , combinada com efeitos de hibridização e efeito Kondo, muito importantes nestes casos, podem provocar fortes flutuações magnéticas no plano ab que, pela sua vez, podem ser relevantes no mecanismo de supercondutividade não convencional quase-2D / Abstract: In this work we present a systematic study of the physical properties (magnetic susceptibility, specific heat and electrical resistivity) and the determination of magnetic structures of a new series of isostructural tetragonal compounds RmMnIn3m+2n(R = Gd, Tb, Sm; M = Rh, Ir; m = 1,2; n = 0,1) exploring their relationships with the interesting physical properties found in other compounds of this family, specially when R = Ce compounds, for whose a heavy fermion behavior with unconventional superconductivity (USC) has been reported. The magnetic structures have been determined using high quality single crystalline samples of Gd2IrIn8, GdRhIn5, GdIn3, TbRhIn5, Tb2RhIn8 and Sm2IrIn8 through the resonant x-ray magnetic scattering (RXMS) technique. Our results show that all these systems order antiferromagnetically in commensurate structures below the ordering temperature (TN) with propagation vectors (1/2,0,0),(0,1/2,1/2),(1/2,1/2,0),(1/2,0,1/2),(1/2,1/2,1/2) and (1/2,0,0), respectively. The magnetic moments of rare earth ions are oriented in the tetragonal ab-plane for R = Gd and Sm2IrIn8 compounds, while for the TbRhIn5 they order along the c-axis direction. The tetragonal Tb-based compounds were synthesized and characterized for the first time during this work. In these cases TN is increased when compared to the TN of the cubic TbIn3 (Tb1-0-3, TN¿32 K), as has been found for tetragonal Nd-based compounds. Regarding the magnetic moment directions in the ordered phase and the TN evolution along the series our results agree with those obtained from a mean field theoretical model, developed by collaborators, which considers an isotropic first-neighbors Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and tetragonal crystal field effects (CEF) applied to Ce-, Nd- and Tb-based compounds. We can conclude from our results that the CEF effects are responsible in determining the magnetic moment directions for different R ions and varying the tetragonal CEF parameters we can also determine the TN evolution along the series. According to this idea, for Gd-based compounds, where the orbital angular momentum L = 0 and CEF effects are not important, TN is approximately the same for the tetragonal compounds when compared with cubic GdIn3. For those cases with ordered moments along any in-plane direction (R = Ce and Sm) TN decreases while it is increased when the moments orientation take place along the c-axis. The CEF effects-driven mechanism to determine the behavior of magnetic moment directions and TN, well explained by the mean field model, could be extrapolated to Ce-based compounds where combined with hybridization and Kondo effects, whose are very important, may create strong in-plane magnetic uctuations that can mediate the quasi-2D unconventional superconductivity in these systems / Doutorado / Física da Matéria Condensada / Doutor em Ciências

No presente trabalho quatro ligas Al-Si hipereutéticas, três das quais foram produzidas por conformação por \"spray\" e a outra por fundição, foram caracterizadas quanto a textura, dureza, microestrutura e resistência à corrosão em meio de condensado sintético automotivo (CSA). Duas das ligas conformadas por \"spray\" foram retiradas de camisas de cilindro e a outra de um pré-formado obtido em laboratório. A conformação por \"spray\" envolve a atomização de uma liga e a deposição de gotículas em um substrato, antes mesmo que todas se encontrem no estado sólido. Este processo permite a obtenção de materiais que se caracterizam por uma microestrutura livre de macrossegregações e bastante refinada, implicando melhor trabalhabilidade a quente. A caracterização da microestrutura das quatro ligas hipereutéticas revelou a presença de porosidades na liga obtida em laboratório, e microestrutura com distribuição homogênea de precipitados primários nas três ligas conformadas por \"spray\". A microestrutura de uma das ligas apresentou-se muito diferenciada, com a presença de eutético, sugerindo que esta foi fabricada por fundição. Nas camisas de cilindro foram feitas medidas de rugosidade, e em todas as ligas foram realizados ensaios de microdureza. A liga conformada por \"spray\" e obtida em laboratório foi laminada a quente e a frio. Foram realizados também estudos de textura, para tentar estabelecer uma correlação entre todas as ligas quanto ao processo de fabricação. A avaliação da textura indicou que a presença de fases de silício primário, finamente distribuídas impedem o aparecimento de texturas típicas de deformação de ligas de alumínio, mesmo após severas deformações, como as necessárias para a transformação de pré-formados em tubos que originam as camisas de cilindro. As medidas de rugosidade indicaram características próprias do acabamento superficial usado para a produção das camisas, por brunimento ou por ataque químico. Os ensaios de microdureza apresentaram variações de acordo com as rotas de fabricação sendo que a liga eutética apresentou os maiores valores de microdureza em comparação às ligas conformadas por \"spray\". Todas as ligas foram avaliadas quanto a resistência à corrosão por ensaios de espectroscopia de impedância eletroquímica em dois meios, CSA com pH 3,3 e CSA com pH 11. As quatro ligas hipereutéticas estudadas apresentaram mecanismo de corrosão similar em condensado sintético automotivo (CSA) pH 3,3. Em todas ocorreu o ataque intenso da matriz de alumínio e as partículas de silício primário atuaram como regiões catódicas. A liga 2 apresentou maior resistência à corrosão entre todas as ligas ensaiadas, tanto em CSA pH 3,3 como em pH 11. Neste último meio, uma camada de produtos de corrosão formou-se sobre todas as ligas, e os resultados indicaram mecanismos diferentes para o ataque da matriz de Al e para o crescimento da camada depositada na superfície das ligas estudadas. O ataque da matriz da liga 2 neste meio foi aparentemente mais lento do que sobre as demais ligas, com a formação de uma camada mais compacta de produtos de corrosão, estabelecendo um controle por difusão dos processos interfaciais para maiores períodos de ensaio. A camada formada sobre as demais ligas neste meio apresentou-se mais defeituosa, e nestas ocorreu também menor controle da velocidade das reações interfaciais por processos difusionais. / In the present study four hypereutectic Al-Si alloys, three produced by spray forming and one by casting, were characterized for microhardness, roughness, microstructure, texture and corrosion resistance in a synthetic automotive condensed solution (SACS). Two of the spray formed alloys tested were obtained from cylinder liners and the other was laboratory made. Spray forming involves alloy atomization and droplets deposition on a substrate, previous to the solidification of all of the droplets. This process favours the production of materials with a fine microstructure free of macrosegregation that is related to improved hot workability. The microstructure characterization of the four alloys revealed the presence of porosities in the laboratory made alloy. All the three alloys produced by spray forming showed a homogeneous distribution of primary precipitates. The microstructure of one of the alloys showed eutectic microstructure, indicating that this alloy was fabricated by casting. In the cylinder liners, the surface roughness was measured and the microhardness of all the alloys was also evaluated. Furthermore, the laboratory made alloy was hot and cold rolled. Texture determinations were carried out to investigate the correlation between the alloy type and their fabrication process. The texture investigation indicated that the fine distribution of primary silicon phase in the alloy hindered the development of texture typical of aluminium alloys deformation, even after severe mechanical work, such as those used in the conversion of pre-formed in cylinder liners. The surface roughness results indicated typical characteristics of the surface finishing used, honing or chemical etching. The microhardness results were dependent on the fabrication process used, with higher microhardness associated to the eutectic alloy comparatively to the spray formed ones. All hypereutectic alloys were tested for corrosion resistance using electrochemical impedance spectroscopy in two electrolytes, SACS with pH 3.3 and SACS with pH 11. The four tested alloys showed similar corrosion mechanisms in the acid electrolyte (SACS pH 3.3). The intense attack of the aluminium matrix occurred in ali tested alloys and the primary silicon phase was unattacked and appeared as emerging from the surface after corrosion immersion test. The primary silicon particles acted as cathodic sites. The alloy 2 showed the highest corrosion resistance among the tested alloys in both electrolytes, SACS pH 3.3 and SACS pH 11. In this last medium, a layer of corrosion products formed on all the alloys, and the results indicated different mechanisms for the aluminium matrix corrosive attack and growth of the deposited layer on the alloys surface. The kinetics of aluminium matrix attack was apparently slower in the alloy 2 than for the other alloys, resulting in the formation of a more compact layer of corrosion products, leading to diffusion controlled interfacial processes for longer test periods. The layer of corrosion products on the other three alloys (1, 3, and 4) had more defects and for these alloys diffusional controlled interfacial processes were not as significant as for alloy 2.