An intriguing mystery about tryptophan 2, 3-dioxygenase is its hydrogen peroxide-triggered enzyme reactivation from the resting ferric oxidation state to the catalytically active ferrous form. In this study, we found that such an odd Fe(III) reduction by an oxidant depends on the presence of L-Trp, which ultimately serves as the reductant for the enzyme. In the peroxide reaction with tryptophan 2, 3-dioxygenase, a previously unknown catalase-like activity was detected. A ferryl species (δ = 0.055 mm/s and ΔEQ = 1.755 mm/s) and a protein-based free radical (g = 2.0028 and 1.72 millitesla linewidth) were characterized by Mössbauer and EPR spectroscopy, respectively. This is the first compound ES-type of ferryl intermediate from a heme-based dioxygenase characterized by EPR and Mössbauer spectroscopy. Density functional theory calculations revealed the contribution of secondary ligand sphere to the spectroscopic properties of the ferryl species. A Trp-Trp dimer and a monooxygenated L-Trp were both observed as the enzyme reactivation by-products by mass spectrometry. Together, these results lead to the unraveling of an over 60-year old mystery of peroxide reactivation mechanism.

The synthesis, structure and physical properties of a series of Mnx(Co, Mg)₁ˍxFe₂O₄, (Mg, Sr)₀.₂ Mn₀.₁Co₀.₇Fe₂O₄ and Mg₀.₅Mn₀.₅(RE)₀.₁Fe₁.₉O₄ (where RE are rare earth elements) nanoferrites have been studied. These compounds were synthesized at low reaction temperature of about 200 ⁰C using the glycol-thermal method. The starting materials were high-purity metal chlorides or nitrates which were precipitated by NH₄OH and KOH respectively. In addition, MnxCo₁₋xFe₂O₄ (x = 0, 0.5 and 1) samples were produced directly from high-purity metal oxides by high-energy ball milling technique. Single-phase cubic spinel structure and nanoparticle structure of the synthesized samples were confirmed by X-ray diffraction (XRD) and transmission electron microscope (TEM). The results show that the produced powders of the asprepared samples have average grain sizes ranging from 7 to 16 nm. Filtering the precipitates by Whatman glass microfiber filters (GF/F) appears to be important in obtaining the small particle sizes. We suspect higher stability of the MnxCo₁₋xFe₂O₄ at x = 0 and 0.5 where complete symmetry in the proportion of the atoms on tetrahedral (A) and octahedral (B) sites would tend to favour larger nanoparticles. The evolutions of the magnetic properties as a function of composition, annealing temperature under air and argon atmospheres or measuring temperature have been investigated by ⁵⁷Fe Mössbauer spectroscopy, vibration sample magnetometer (VSM) and superconducting quantum interference device (SQUID). Significant changes in magnetic properties are observed across the composition ranges studied. The Mössbauer spectra indicate ferrimagnetic, superparamagnetic and paramagnetic behaviours of the compounds. The results show evidence of transformation from single-domain to multi-domain structure with thermal annealing in our samples. Temperature dependence of magnetization shows differences between field cooling (FC) and zero field cooling (ZFC) which we attribute to spin-freezing and thermal relaxation for typical nanoparticles. Significant increase in coercive field with reduction in measuring temperature is obtained in Co- based compounds. Mn₀.₅Co₀.₅Fe₂O₄, Sr₀.₂Mn₀.₁Co₀.₇Fe₂O₄ and Mg₀.₂Mn₀.₁Co₀.₇Fe₂O₄ have large coercive fields of 1.45, 3.02 and 10.70 kOe at 4 K compared to 0.17, 0.05 and 0.05 kOe at room temperature respectively. Variation of coercive fields (Hc) with measuri ing temperature for MnxCo₁₋xFe₂O₄ (x = 0.1 and 0.05), (Mg, Sr)₀.₂Mn₀.₁Co₀.₇Fe₂O₄ nanoferrites follow the Kneller's law for uniaxial non-interacting single domain particles of the form Hc(T) = Hc(0)[1-( T/Tβ)α]. The observed temperature dependences are consistent with α = 1/2. We also find evidence of the departure from this law at lower temperature. The temperature dependence of the saturation magnetizations were observed to vary with temperature according to the modified Bloch's law Ms(T) = Ms(0)[1 - ( T/T₀)ᵝ] where β is at least 1.5. This is attributed to the confinement effects of the spin-wave spectrum for magnetic clusters. The equation appears to fit the saturation magnetization data over the entire temperature range with values of β from 2.1 to 2.4 for the samples studied. These results are consistent with the nanoparticle nature of the compounds. In Mg₀.₅Mn₀.₅(RE)₀.₁Fe₁.₉O₄ nanoferrites, the grain sizes, lattice parameters and saturation magnetizations increase with RE substitution which we attribute to larger RE ions substituting smaller Fe ions. The results show evidence of superparamagnetic behaviour of the nanoparticles. The highest grain size and magnetizations are obtained for the Gd substituted sample. We find strong correlation between the saturation magnetizations, grain sizes and microstrains with de Gennes factor G. The correlation with grain sizes and microstrains appear to be unique and characteristic of the nanoparticle nature of the compounds. Bulk samples in the form of pellets were also produced from the as-prepared samples of MnxCo₁₋xFe₂O₄ for resistivity measurements. The temperature dependence of the electrical resistivity for samples sintered from 600 - 1100 ⁰C under argon atmosphere were studied using the four-probe method from room temperature to about 110 ⁰C. Two possible mechanisms for resistivity involving Tˉ¹ and Tˉ¹/² dependences were investigated which we associated with semiconducting and inter-grain conductivity respectively. The Tˉ¹/² dependence is found to fit the data better and predicts higher activation energies. The resistivity was observed to be sensitive to the surface of the pellet being probed and the annealing temperature. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2012.

Iron is essential in nearly all organisms. It is a cofactor in many proteins and enzymes. This transition metal can also be toxic because it participates in reactions which produce reactive oxygen species. To avoid these toxic effects while still being used for essential processes, the cell must regulate tightly iron import, metabolism, trafficking, and homeostasis. These processes were studied using biophysical methods centered on Mossbauer spectroscopy supplemented by electron paramagnetic resonance, electronic absorption spectroscopy, and inductively coupled plasma mass spectrometry. This integrated biophysical approach was applied to yeast cells, isolated yeast mitochondria, and mouse brains. We determined the concentration of Fe, and the proportion of that Fe present as iron-sulfur clusters, heme centers, mononuclear nonheme centers, and as Fe3+ oxyhydroxide (phosphate) nanoparticles for each system. In yeast, the dependence of metabolic mode of growth and iron in the growth medium on this distribution was studied. Approximately three-quarters of the iron in fermenting cells was located in vacuoles, where it was present as high-spin mononuclear Fe3+ species with rhombic symmetry. The remaining quarter was present in the mitochondria. In fermenting mitochondria 4 distinct species of iron were observed, including [Fe4S4]2+ clusters and low-spin Fe2+ hemes arising from respiratory complexes, non-heme high spin (NHHS) Fe2+ species, high spin nonheme Fe3+ species, and nanoparticles. These distributions (in both the cells and mitochondria) change when the cells are grown on iron deficient medium but remained relatively unaltered as iron in the growth medium was increased. Respiring cells had less Fe associated with vacuoles, and more Fe present as HS Fe2+. Respiring mitochondria contain more [Fe4S4]2+ clusters and low-spin Fe2+ hemes, more S = 1/2 [Fe2S2]1+ clusters, and less NHHS Fe2+, HS Fe3+ species and Fe3+ nanoparticles. These changes were rationalized by assuming that the NHHS Fe2+ and Fe3+ species, and the nanoparticles were in equilibrium within the matrix of the mitochondria, and that the Fe2+ species served as feedstock for the synthesis of iron-sulfur clusters and heme centers. The iron in the mouse brain consisted mostly of [Fe4S4]2+ clusters and Fe2+ hemes from mitochondria respiratory complexes, and of ferritin, an Fe storage protein complex. NHHS Fe2+ and Fe3+ species were also observed. The ratio of stored Fe to mitochondrial Fe was sensitive to age. The brains of prenatal animals were dominated by ferritin. Following birth up to the first 4 weeks of life, there was an increase in mitochondrial Fe and a decline of ferritin Fe. Beyond 4 weeks up to 58 weeks, levels of ferritin increased and mitochondrial Fe remained constant. The brains of mice fed an Fe-deficient diet were also studied; most of the Fe in these brains was present as mitochondrial Fe, with little stored as ferritin. A model was developed to explain these changes.

TEIXEIRA, João Damasceno. Estudos espectroscópicos de ferritas de níquel preparadas com água de coco in natura (cocus nucifera). 2007. 55 f. Dissertação (Mestrado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2007. / Submitted by francisco lima (admir@ufc.br) on 2014-03-17T13:43:32Z No. of bitstreams: 1 2007_dis_jdteixeira.pdf: 979699 bytes, checksum: 581e7af5023eefce377ade5a9496ba6b (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2014-05-19T22:09:44Z (GMT) No. of bitstreams: 1 2007_dis_jdteixeira.pdf: 979699 bytes, checksum: 581e7af5023eefce377ade5a9496ba6b (MD5) / Made available in DSpace on 2014-05-19T22:09:44Z (GMT). No. of bitstreams: 1 2007_dis_jdteixeira.pdf: 979699 bytes, checksum: 581e7af5023eefce377ade5a9496ba6b (MD5) Previous issue date: 2007 / The present work is dedicated to the studies of preparation and characterization of nickel ferrites. These ferrites were synthesized by the sun-gel process in suspension of coconut natural water (cocus nucifera) ltered, prepared for Dr. Marcelo A. Macedo team, from the Department of Physics of UFSE. This new synthesis route allowed to ob- tain nanoparticles of nickel ferrites, that are prepared with the technique of formation of mixed micelles, obtained by the coconut natural water. The molecules of that suspension are called tensoatives, they are molecules associates spontaneously in aqueous solution starting from a certain concentration denominated concentration critical micellar (CMC). Above this concentration, the tensoative molecules form great molecular of colloidal di- mensions. About analytic point of view, one of the most important properties of those organized structures is their capacity of solubilize solutes of di erent characteristics. The samples were roasted for 4 hours the since 400 C up to 1200 C in a fast cooling to the ambient temperature and 1200 C to the temperature of the liquid nitrogen. The di rac- tion of ray-X (XRD), the spectroscopic Raman, Infra-red and Electronic Microscopy of Sweeping (SEM), were applied to study the dependence of the sizes of the nanoparticles of nickel ferrite, with the temperature of the thermal treatments and their correlation with their magnetic properties. The sizes of the crystallites increased of 5 nm (in the tempera- ture of the thermal treatment of 400 C) until approximately 67 nm (in the temperature of the thermal treatment of 1200 C caught a cold to the temperature of the liquid nitrogen). The crystallinity of the nanoparticles increases with the increase of the temperature of the thermal treatment. The results of the microscopy M ossbauer show that the ferrites roasted to 1200 C and caught a cold to the temperature of the liquid nitrogen to present the phase mixed spinel / O presente trabalho é dedicado aos estudos de preparação e caracterização de ferritas de níquel Estas ferritas foram sintetizadas pelo processo de sol-gel em suspensão de água de como natural (cocos nucifera) filtrada preparada pela equipe do Dr. Marcelo A Macedo do Departamento de Física da UFSE Esta nova rota de síntese permitiu obtermos nanopartículas de ferritas de níquel que são preparadas pela técnica de formação de micelas mistas obtidas da água de coco natural As moléculas dessa suspensão são chamadas de tensoativos são moléculas anfifílicas caracterizadas por possuírem ambas as regiões estruturais hidrofílica e hidrofóbica que dinamicamente se associam espontaneamente em solução aquosa a partir de uma determinada concentração denominada concentração micelar crítica (CMC) Acima dessa concentração as moléculas do tensoativo formam grandes agregados moleculares de dimensões coloidais Do ponto de vista analítico uma das mais importantes propriedades dessas estruturas organizadas é sua capacidade de solubilizar solutos de diferentes características As amostras foram calcinadas durante 4 horas desde 400°C até 1200°C seguida de um esfriamento rápido até a temperatura ambiente e a 1200°C até a temperatura de nitrogênio líquido A difração de raios-X (XRD) as espectroscopias Raman Infra-Vermelho e a Microscopia Eletrônica de Varredura (SEM) foram aplicadas para estudarmos a dependência dos tamanhos das nanopartículas de ferrita de níquel com a temperatura dos tratamentos térmicos e sua correlação com as propriedades magnéticas Os tamanhos dos cristalitos aumentaram de 5nm (na temperatura de tratamento térmico de 400°C) até aproximadamente 67nm (na temperatura de tratamento térmico de 1200°C resfriada à temperatura do nitrogênio líquido) A cristalinidade das nanopartículas aumenta com o aumento da temperatura de tratamento térmico Os resultados da microscopia Mössbauer mostram que as ferritas calcinadas a 1200°C e resfriadas à temperatura do nitrogênio líquido apresentam a fase de spinel misto.

GOUVEIA, Daniel Xavier. Estudo de compostos lamelares contendo Fe usando Espectroscopia Mössbauer de 57Fe e técnicas complementares. 2006. 119 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2006. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-05-05T20:52:57Z No. of bitstreams: 1 2006_tese_dxgouveia.pdf: 8355694 bytes, checksum: 26ff8e3ed6ee26b0d1972a7024528620 (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-05-07T14:58:29Z (GMT) No. of bitstreams: 1 2006_tese_dxgouveia.pdf: 8355694 bytes, checksum: 26ff8e3ed6ee26b0d1972a7024528620 (MD5) / Made available in DSpace on 2015-05-07T14:58:29Z (GMT). No. of bitstreams: 1 2006_tese_dxgouveia.pdf: 8355694 bytes, checksum: 26ff8e3ed6ee26b0d1972a7024528620 (MD5) Previous issue date: 2006 / The structural and thermal decomposition properties of Mg-Fe and Co-Cu-Fe hy- drotalcites (HT) have been studied through thermogravimetric analysis, X ray powder difraction (XRD), Fourier transform infrared spectroscopy and 57Fe Mössbauer spec- troscopy. In the Mg-Fe system, the destruction of the layered structure took place at about 300 oC. The broad peaks observed in the X ray difractograms suggests that the resultant oxides constitute a solid solution. For samples treated at temperatures higher than 500 oC the formation of the MgO and MgFe2O4 formation of the MgO and MgFe2O4 spinel phases is observed. 57Fe Mössbauer spectroscopy was employed to monitor the Fe chemical environment for the samples annealed at diferent temperatures (100-900 oC). In situ XRD experiments revealed that the HTs start an interlayer contraction at about 180 oC. This phenomenon is identified as being due to a grafting process for which the interlamellar anions attach to the layers through a covalent bond. The reconstruction of the Mg-Fe HTs was also investigated and its eficiency depends on the thermal anneal- ing temperature and on the Mg/Fe ratio. The structure of the reconstructed samples was found to be exactly the same as the parent structure. The in situ 57Fe Mössbauer experiments were performed in the 100-500 oC temperature range confirm an increasing structural disorder in this temperature range. The quadrupolar splitting indicates that the maximum disorder occurs at 300 oC. Regarding the Co-Cu-Fe ternary system we have observed that due to the strong Jahn-Teller effect the Cu-Fe layered system is stabilized only in the presence of Co2+. At low Co2+ contents, additional phases are segregated in the solids. X ray patterns diffraction show the presence of Cu(OH)2 and CuO. The decomposition process was investigated by in situ X ray, in situ Mössbauer and FTIR experiments. By increasing the temperature from 25 oC up to 180 oC we observed that the structural disorder increases. This effect has been likely attributed to the Co2+ to Co3+ oxidation since thermal decomposition was carried out under static air atmosphere. Part of the Co3+ cations could migrate to the interlayer region, thus forming a metastable compound that still has a layered structure. Collapse of the layered structure was ob- served at about 200 oC. By further increasing the temperature the system becomes more crystalline and the formation of Co3O4 is observed in the X ray patterns. In Cu-rich HT, some of the carbonate anions are released at temperatures higher than 550 oC and this phenomenon is attributed to the formation of a carbonate-rich phase. The specific surface area data present its highest values in the temperature range where the collapse of the layered structure takes place. / As propriedades estruturais e de decomposição térmica das hidrotalcitas Mg-Fe e Co-Cu-Fe foram estudadas através de espectroscopia Mössbauer de 57Fe, análise termo- gravimétrica, difração de raios X, e espectroscopia de absorção no infravermelho (FTIR). No sistema Mg-Fe a destruição da estrutura lamelar ocorre em torno de 300 oC. O alargamento dos picos de difração de raios X observados nos difratogramas sugerem que os óxidos resultantes constituem uma solução sólida. Para as amostras tratadas em temperaturas maiores do que 500 oC a formação de fases do tipo MgO e MgFe2O4 é observada. A espectroscopia Mössbauer de 57Fe foi empregada para monitorar o ambiente químico do Fe na faixa 100-900 oC de temperatura. As medidas in situ de difração de raios X revelam que em 180 oC inicia-se uma contração interlamelar. Este fenômeno é atribuído ao processo de “grafting" no qual os ânions interlamelares ligam-se nas camadas através de uma ligação covalente. A reconstrução estrutural da hidrotalcita Mg-Fe também foi investigada. A eficiência da reconstrução estrutural depende da temperatura de tratamento e da razão molar Mg/Fe. A estrutura das amostras reconstruídas são as mesmas da amostra inicial. As medidas in situ de espectroscopia Mössbauer de 57Fe foram realizadas na faixa 100-500 oC confirmaram uma desordem estrutural crescente nesta região de temperaturas. Os valores do desdobramento quadrupolar indicam que o máximo de desordem ocorre em 300 oC. Com relação ao sistema ternário Co-Cu-Fe observamos que devido ao efeito Jahn-Teller o sistema Cu-Fe somente é estabilizado na presença de Co2+. Para baixas concentrações Co2+ fases adicionais segregadas são observadas nos sólidos. Os padrões de difração de raios X indicam a presença de Cu(OH)2 e CuO. O processo de decomposição térmica foi investigado através de difração de raios X, espectroscopia Mössbauer de 57Fe in situ e de espectroscopia de absorção no infravermelho (FTIR). Aumentando a temperatura de tratamento térmico das amostras de 25 oC a 180 oC observamos um aumento da desordem estrutural. Este efeito tem sido atribuído a oxidação Co2+ para Co3+ uma vez que a decomposição foi realizada ao ar. Parte dos cátions Co3+ migram para a região interlamelar formando um composto metastável que ainda possui uma estrutura lamelar. O colapso da estrutura lamelar é observado a 300 oC. Com o aumento posterior da temperatura o sistema torna-se mais cristalino e a formação de Co3O4 é observada através do ensaio de raios X. Nas hidrotalcitas com maior teor de Cu, alguns dos ânions carbonato são liberados somente acima de 550 oC sendo este fenômeno atribuído a formação de uma fase rica em carbonato. Os valores de área superficial específica apresentam um máximo na faixa de temperatura onde ocorre o colapso da estrutura lamelar.

Made available in DSpace on 2016-06-02T20:16:55Z (GMT). No. of bitstreams: 1 DissMSVK.pdf: 2016332 bytes, checksum: c7d2567be46c9a21abce63c045485207 (MD5) Previous issue date: 2005-03-18 / Financiadora de Estudos e Projetos / We presented in this work a study of the influence of the thickness of Al thin film deposited in substratum of low carbon steel. Alloys of steel low carbon and aluminum are used broadly in automobile and aerospace industries. We studied the nitrides formation, starting from a process of pre-heat-treatment of the samples in the nitriding camera followed by plasma nitriding, in the temperatures of 300°C and 400°C. The following techniques: Conversion Electrons Mössbauer Spectroscopy (CEMS), X-ray diffraction (XRD), Scan Eletron Microscopy (SEM) and Vickers microhardness test were used for the characterization of those samples. The Aluminium purity for the films was 99,99%. The steel low carbon substrate was supplied by IPT-SP (Institute of Researches Technological-SP) and after the samples be cut and polished, they received the film of aluminum. Soon after they were pre-heat-treated (section 4.1) and nitreded in the temperatures of 300oC and 400oC for 4 hours in composed atmosphere for 80% of H2 and 20%N2. After the nitriding the samples were characterized by the techniques referred above. For the temperature of 300oC, the X-ray Diffraction results showed the formation of the phases α-Fe, Al and AlN. The Mössbauer spectroscopy showed α- Fe and formation of solid solution of Fe in aluminum. For the temperature of 400oC, we still observed the formation of the phase Fe4Al13, for the two techniques. The Mössbauer Spectroscopy also showed that the increase of the percentage of the phase Fe4Al13 has a possible dependence with the nitriding temperature and with the thickness of the film of aluminum. The images of MEV and the Vickers microhardness tests, just revealed the presence of nitrides of iron in the borders of the samples. / Apresentamos neste trabalho um estudo da influência da espessura do filme de Al depositado em substrato de aço baixo carbono. Ligas de aço baixo carbono e alumínio são largamente empregadas em indústrias automobilísticas e aeroespaciais. Estudamos a formação de nitretos, a partir de um processo de prétratamento das amostras por aquecimento na câmara de nitretação seguido de nitretação a plasma, nas temperaturas de 300°C e 400°C. Para a caracterização dessas amostras foram utilizadas as técnicas de espectroscopia Mössbauer de elétrons de conversão (CEMS), difratometria de raios-x, microscopia eletrônica de varredura, além de ensaios de microdureza Vickers. A pureza do alumínio utilizado na produção dos filmes é de 99,99%. A matriz de aço baixo carbono foi fornecida pelo IPT-SP (Instituto de Pesquisas Tecnológicas-SP) e após as amostras serem cortadas e polidas, receberam o filme de alumínio. Em seguida foram pré-tratadas por aquecimento (seção 4.1) e nitretadas nas temperaturas de 300oC e 400oC durante 4 horas em atmosfera composta por 80% de H2 e 20%N2. Após a nitretação as amostras foram caracterizadas pelas técnicas referidas acima. Para a temperatura de 300 oC, os resultados obtidos com a técnica de difração de raios-x mostraram a formação das fases α-Fe, Al e AlN. A espectroscopia Mössbauer; por outro lado, mostrou α-Fe e formação de solução sólida de Fe em alumínio. Já para a temperatura de 400 oC, observamos ainda a formação da fase Fe4Al13, pelas duas técnicas. A Espectroscopia Mössbauer também mostrou que o aumento da porcentagem da fase Fe4Al13 tem uma possível dependência com a temperatura de nitretação e com a espessura do filme de alumínio. As imagens de MEV e as medidas de microdureza Vickers, revelaram a presença de nitretos de ferro apenas nas bordas das amostras.

M.Sc. (Physics) / Please refer to full text to view abstract

Presenta un estudio del orden estructural y microestructural de las aleaciones AlXFe(1−X) con x = 0,25; 0,5 y 0,75. Primeramente las muestras son sintetizadas por la técnica de horno de arco (HA) y tratadas térmicamente a temperaturas de 600, 950 y 10000C por los tiempos de 48, 290 y 48 horas respectivamente. Posterior al tratamiento térmico correspondiente a la temperatura de 6000C, las muestras son nanoestructuradas por la técnica de molienda mecánica (MM). La estructura y microestructura de las muestras sintetizadas y nanoestructuradas son estudiados por las técnicas de difracción de rayos X (DRX) y espectroscopía Mossbauer (EM). / Tesis

Estudia la estabilidad térmica de una aleación Fe-Cu obtenida por mecano-síntesis que consiste en una primera etapa en donde se estudio la preparación de la aleación Fe0,50Cu0,50 empleando la técnica conocida como mecano-síntesis, y una segunda etapa en donde se estudia la estabilidad térmica de este sistema. Con la finalidad de comparar las propiedades de la aleación obtenida por mecano-síntesis (MS) se preparó este mismo sistema en forma de películas delgadas, obtenidas por la técnica de congelamiento de vapor (VQ). Emplea ambas técnicas para extender los límites de solubilidad del sistema a través de soluciones sólidas cristalinas desordenadas metastables o amorfas. Comprende la correlación de una serie de análisis basados en técnicas como difracción de rayos X (XRD), espectroscopia por dispersión de energía (EDS), microscopía electrónica de barrido (SEM), espectroscopia por fotoelectrones de rayos X (XPS), espectroscopia Mössbauer por transmisión (TMS) y espectroscopia Mössbauer por electrones de conversión (CEMS). Los resultados presentan la formación de una solución sólida cristalina homogénea fcc-FeCu para las muestras preparadas por MS, mientras que para las muestras preparadas por VQ, una solución sólida altamente desordenada. La muestra preparada por MS se obtiene después de 16 hs de tratamiento mecánico sobre los polvos metálicos iniciales, notándose también una drástica reducción en el tamaño de grano hasta ordenes nanométricos para la solución sólida final. Los resultados obtenidos del tratamiento térmico muestran que la aleación obtenida fcc-FeCu es inestable, observándose la descomposición de esta arriba de 300°C en las fases iniciales del Fe (bcc) y Cu (fcc). Se nota también la posible presencia de y-Fe en las muestras tratadas térmicamente. / Tesis

Realiza un estudio sobre la síntesis de nanopartículas de magnetita haciendo uso de la técnica de molienda mecánica en una atmósfera controlada de gas argón. El interés actual por estas nanopartículas proviene principalmente por las propiedades físicas de su núcleo magnético, que le brinda potenciales aplicaciones en el campo de la medicina, así como en la remediación de aguas residuales, encontrando importantes aplicaciones biomédicas como catálisis, ferrofluídos, agente de contraste para imágenes de resonancia magnética, transporte de fármacos y en la remediación de aguas como absorvedor en la retención de metales pesados. El proceso de molienda mecánica se realizó durante un tiempo de 90 h, en el cual se fueron extrayendo pequeñas cantidades de muestra, de aproximadamente 100 mg, en intervalos regulares de tiempo. Estas muestras fueron caracterizadas estructural, magnética y elementalmente mediante las técnicas de difracción de rayos X (DRX), espectroscopia Mössbauer (EM), magnetometría de muestra vibrante (MMV), magnetometría a bajas temperaturas con campo (FC) y sin campo magnético (ZFC) y fluorescencia de rayos X (FRX). Se estudió el proceso de síntesis en función al tiempo de molienda. Los resultados de difracción de rayos X permitieron identificar las fases presentes en el proceso de síntesis, donde se verifica la disminución de la fase inicial de hierro y el aumento de una fase asociada a la magnetita, con respecto al tiempo de molienda. Se realizó el refinamiento Rietveld, el cual permitió cuantificar la variación de fases, teniendo solamente la presencia de magnetita a partir de las 85 h de molienda. El tamaño calculado de la fase de magnetita para las 90 h de molienda fue de aproximadamente 22 nm. Para complementar este estudio se determinó el cambio de la estructura a partir de la variación del parámetro de red. En el análisis de los espectros Mössbauer se reconoció estos cambios estructurales en la variación de los parámetros hiperfinos para ambas fases. Además se cuantificó el porcentaje de área para las fases presentes y se comparó con los porcentajes de fases, obtenidos mediante el refinamiento Rietveld. Estos cálculos permitieron cuantificar la estequiometria de la magnetita en función al tiempo de molienda. Los resultados mostraron que a partir de las 90 horas de molienda se alcanzó una estequiometria próxima a la ideal de la magnetita. En las medidas magnéticas se verificó la coexistencia de fases (hierro y magnetita) durante el proceso de molienda, notándose una disminución en la magnetización de saturación de 208 emu/g a 74 emu/g, desde 10 h hasta 90 h de molienda, respectivamente. De las curvas de magnetización a bajas temperaturas sin campo magnético (ZFC), se observó un cambio en su magnetización específica a una temperatura alrededor de 118 K, correspondiente a la denominada transición de Verwey, característico en la magnetita. Considerando que para una magnetita estequiométrica debería de obtenerse un valor de 122 K, acorde a la literatura. Por otro lado se realizó el análisis elemental por fluorescencia de rayos X, para determinar la presencia de contaminantes en la muestra. / Tesis