Made available in DSpace on 2014-12-17T15:41:43Z (GMT). No. of bitstreams: 1 JoseEMS.pdf: 2230919 bytes, checksum: f0f4e57c89f3eae4282c6daae0fe0aba (MD5) Previous issue date: 2008-04-18 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / It was synthesized different Ni1-xMgxFe2O4 (0,2 ≤ x ≤ 0,7) compositions by use of citrate precursor method. Initially, the precursory citrates of iron, nickel and magnesium were mixed and homogenized. The stoichiometric compositions were calcined from 350?C to 1200?C at ambient atmosphere or in argon atmosphere. The calcined powders were characterized by XRD, TGA/DTG, FTIR, magnetic measures and reflectivity using the wave guide method. I was observed pure magnetic phase formation between 350?C and 500?C, with formation of ferrite and hematite after 600?C at ambient atmosphere. The calcined powder at argon atmosphere formed pure ferromagnetic phase at 1100?C and 1200?C. The Rietveld analyses calculated the cations level occupation and the crystallite size. The analyses obtained nanometric crystals (11-66 nm), that at 900?C/3h presents micrometric sizes (0,45 - 0,70 Om). The better magnetization results were 54 Am2/Kg for x= 0,2 composition, calcined at 350?C/3h and 30 min, and 55,6 Am2/Kg for x= 0,2 1200?C, calcined in argon. The hysteresis shows characteristics of soft magnetic material. Two magnetization processes were considered, superparamagnetism at low temperature and the magnetic domains formation at high temperatures. The materials presented absorption less or equal the 50 % in ranges specific frequency. As for the 2,0 and 3,0 thickness (in 11,0 - 11,8 GHz), the reflectivity of the x= 0,3, 0,5 and 0,4 compositions, all calcined at 900?C/3h showed agreement with MS and O. Various factors contribute for the final radiation absortion effect, such as, the particle size, the magnetization and the polymer characteristics in the MARE composition. The samples that presented better magnetization does not obtaining high radiation absorption. It is not clear the interrelaction between the magnetization and the radiation absorption in the strip of frequencies studied (8,2 - 12,4 GHz) / Foram sintetizadas diferentes composi??es da ferrita Ni1-xMgxFe2O4 com (0,2 ≤ x ≤ 0,7) pelo uso do m?todo dos citratos precursores. Para se obter a fase estequiom?trica Ni1-xMgxFe2O4, inicialmente foram misturados e homogeneizados os citratos precursores de ferro, n?quel e magn?sio. As composi??es estequiom?tricas foram calcinadas entre as temperaturas de 350?C e 1200?C, em atmosfera ambiente ou de arg?nio. Os p?s calcinados foram caracterizados por DRX, TGA/DTG, FTIR, medidas magn?ticas e refletividade pelo m?todo de guia de ondas. Foi observada a forma??o de fase pura ferrimagn?tica entre 350?C e 500?C, formando ferrita e hematita ap?s 600?C, em atmosfera ambiente. O p? calcinado em atmosfera de arg?nio formou fase ferrimagn?tica pura em 1100?C e 1200?C. A an?lise pelo m?todo de Rietveld calculou o n?vel de ocupa??o dos c?tions e o tamanho de cristalito. A an?lise obteve tamanhos de cristais nanom?tricos, (11 - 66 nm), que a 900?C/3h apresentam tamanhos microm?tricos (0,45 0,70 Om). Os melhores resultados de magnetiza??o foram 54 Am2/Kg para a composi??o x= 0,2, calcinada a 350?C/3h e 30 min, e 55,6 Am2/Kg para x= 0,2 a 1200?C, calcinada em arg?nio. As histereses mostraram um perfil de materiais magn?ticos moles. Dois processos de magnetiza??o foram considerados, o superparamagnetismo a baixa temperatura e a forma??o de dom?nios magn?ticos em altas temperaturas. Os materiais apresentaram absor??o igual ou inferior a 50 % em faixas de freq??ncias espec?ficas. Para as espessuras 3,0 e 2,0 mm (em 11-11,8 GHz), as refletividades das composi??es x= 0,3, x= 0,5 e x= 0,4, todas a 900?C/3h demonstraram concord?ncia com MS e O. V?rios fatores contribuem para o efeito final de absor??o de radia??o, tais como, o tamanho de part?culas, a magnetiza??o, e as caracter?sticas do pol?mero na composi??o do MARE. As amostras que apresentaram maiores magnetiza??es n?o atingiram alta absor??o de radia??o. N?o ficou esclarecido a interrela??o entre a magnetiza??o e a absor??o de radia??o na faixa de freq??ncia estudada (8,2 12,4 GHz)

Made available in DSpace on 2014-12-17T15:42:27Z (GMT). No. of bitstreams: 1 MariaJOSB.pdf: 5434037 bytes, checksum: 1a000204d534c8f41a3acbfce7dfaef5 (MD5) Previous issue date: 2007-09-17 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / It was synthesized MnZn ferrite with general formulae Mn1-xZnxFe2O4 (mol%), 0,3 &#8804; x &#8804; 0,7 by using the citrate precursor method. The precursors decomposition was studied by thermogravimetric analysis (TGA), differential thermogravimetric analysis (DTG), differential thermal analysis (DTA) and Fourier transform infrared (FTIR) of powder calcined at 350?C/3,5h. X-ray diffraction pattern (XRD) of samples was done from 350 to 1200?C/2h using various atmospheres. The power calcined at 350?C/3,5h formed spinel phase. It is necessary atmosphere control to avoid secondary phase such as hematite. From 900 to 1200?C was obtained 90,66 and 100% of MnZn spinel ferrite phase, respectively. Analysis by dispersive energy scanning (EDS) at 350?C shows high Mn and Zn dispersion, indicating that the diffusion process was homogeneous. Semi-quantitative analysis by EDS verified that despite the atmosphere control during calcinations at high temperatures (< 800?C) occurred ZnO evaporation causing stoichiometric deviation. Vibrating sample magnetometer (VSM) measures show soft ferrite material characteristics with Hc from 6,5 x 10-3 to 11,1 x 10-2 T. Saturation magnetization (Ms) and initial permeability (?i) of MnZn spinel phase obtained, respectively, from 14,3 to 83,8 Am2/kg and 14,1 to 62,7 (Am2/kg)T / Foi sintetizada ferrita de MnZn com f?rmula geral Mn1-xZnxFe2O4 (mol%) para 0,3 &#8804; x &#8804; 0,6 com uso do m?todo dos citratos precursores. A decomposi??o dos precursores foi estudada por an?lise termogravim?trica (TGA), termogravim?trica diferencial (DTG), an?lise t?rmica derivada (DTA) e espectroscopia na regi?o do infravermelho (FTIR) dos p?s calcinados em 350?C/3,5h. Foram feitas difra??es de raios X (DRX) entre 350 e 1200?C/2h usando v?rias atmosferas. O p? calcinado a 350?C/3,5h formou fase espin?lio. ? necess?rio o controle da atmosfera para evitar fases secund?rias como a hematita. Em 900 e 1200?C foram obtidos respectivamente, 90,7 e 100% de fase ferrita espin?lio. An?lises por espectroscopia de energia dispersiva (EDS) em 350?C mostrou alta dispers?o de Mn e Zn, indicando que o processo de difus?o foi homog?neo. A an?lise semi-quantitativa por EDS verificou que apesar do controle da atmosfera durante a calcina??o em altas temperaturas (> 800?C), ocorreu vaporiza??o de ZnO causando desvio estequiom?trico. Medidas por magnet?metro de amostra vibrante (MAV) mostraram caracter?sticas de materiais magneticamente macios com Hc de 6,5 x 10-3 at? 11,1 x 10-2 T. A magnetiza??o de satura??o e a permeabilidade inicial da fase espin?lio de MnZn foi obtida entre 14,3 a 83,8 Am2/kg e 14,1 a 62,7 (Am2/kg)T, respectivamente

Submitted by Celso Magalhaes (celsomagalhaes@ufrrj.br) on 2017-05-15T11:16:01Z No. of bitstreams: 1 2016 - Maria Clara Adum de paiva.pdf: 2279180 bytes, checksum: d1de128b3d5f5984c844a68538cd255e (MD5) / Made available in DSpace on 2017-05-15T11:16:01Z (GMT). No. of bitstreams: 1 2016 - Maria Clara Adum de paiva.pdf: 2279180 bytes, checksum: d1de128b3d5f5984c844a68538cd255e (MD5) Previous issue date: 2016-08-04 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior - CAPES / The demand for electricity production, and the need for thermoelectric use as power generating units, makes it necessary to search for less polluting matrices. In this search the catalytic combustion of methane, both from natural gas and other sources (residue disposal, for example) have shown promising. It is known the use of cobalt oxides is an alternative to the use of catalysts based on noble metals which has shown high performance. In order to potentiate the activity of the cobalt based catalyst, thus reducing both the ignition temperature and the temperature of total burning of methane, this work proposes the production of cobalt-based precursors using different types of layered double hydroxides (LDHs). Cobalt based catalysts partially substituted in layered double hydroxides (LDH Mg, Al and CO32-) were prepared by co-precipitation and impregnation with a nominal content of 9% CoII in LDH, or by impregnating a commercial hydrotalcite (HT). The LDH precursors were characterized by X-ray powder diffraction (XRD) and infrared absorption spectroscopy (FTIR). The XRD patterns indicated a LDH of 3R polytype. XRD showed the presence of Gibbisite in the HT precursor. The infrared spectra of precursor LDHs presented bands related to ?1, ?2 and ?3 vibrations of the carbonate anion, and interlayer water characteristic bands, thus in line with the XRD data. Analysis by XRD of the catalysts after calcination at 800 ?C showed the presence of periclase and spinel phases. Infrared spectra showed bands ascribed to the Mg-O and Co-O stretching in tetrahedral and octahedral sites as well as bands attributed to the Mg-O-Al bond and the cobalt spinel. The catalytic activity of these materials was investigated in the catalytic combustion of methane under kinetic regime, using predetermined reaction conditions to avoid diffusional limitations, resulting in a significant decrease in the combustion temperature, with the higher activity observed for the catalyst prepared by impregnating a commercial HT. Scanning electron microscopy (SEM) and quantitative chemical analysis (EDS) of catalysts with improved performance show both the homogeneous dispersion of the components in the sample surface and the higher aluminum content of the sample supported on HT. / A demanda de produ??o de energia el?trica, e a necessidade do uso de termel?tricas como unidades geradoras de energia, fazem com que seja necess?ria a busca por matrizes menos poluentes. Nessa busca a combust?o catal?tica do metano, tanto proveniente do g?s natural como de outras fontes (res?duo do tratamento de lixo, por exemplo), tem se mostrado promissora. Sabidamente o uso de ?xidos de cobalto ? uma alternativa, que vem mostrando alto desempenho, ao uso de catalisadores a base de metais nobres. Com o intuito de potencializar a atividade do catalisador a base de cobalto, reduzindo assim tanto a temperatura de igni??o quanto a de queima total do metano, esse trabalho prop?e sua produ??o utilizando como precursor diferentes tipos de hidr?xidos duplos lamelares (HDLs). Catalisadores ? base de cobalto parcialmente substitu?dos em hidr?xidos duplos lamelares (HDL de Mg, Al e CO32-) foram preparados por co-precipita??o e impregna??o num teor nominal de 9% de CoII em HDL, ou por impregna??o em hidrotalcita comercial (HT). Os HDLs precursores foram caracterizados por difra??o de raios?X pelo m?todo de p? (DRX) e por espectroscopia de absor??o no infravermelho (FTIR). Os difratogramas indicaram a obten??o de um HDL de politipo 3R. Por meio do DRX foi identificada a presen?a de Gibbisita no suporte de HT. Os espectros de infravermelho dos HDLs precursores presentaram bandas referentes ?s vibra??es ?1, ?2 e ?3 do ?nion carbonato, al?m de bandas caracter?sticas de ?gua interlamelar, estando portanto de acordo com os dados de DRX. A an?lise por difra??o de raios-X dos catalisadores ap?s calcina??o a 800?C mostrou apenas as fases espin?lio e pericl?sio. Os espectros de infravermelho apresentaram bandas atribu?das aos estiramentos Mg?O e Co?O em s?tios tetra?dricos e octa?dricos, assim como bandas caracter?sticas de Mg?O?Al e de Co3O4. A atividade catal?tica desses materiais foi investigada na combust?o catal?tica do metano, em regime cin?tico, empregando-se condi??es reacionais preestabelecidas de forma a evitar limita??es difusionais, obtendo-se uma significativa diminui??o na temperatura de combust?o, sendo que a maior atividade foi observada para o catalisador preparado por impregna??o em HT comercial. Foi realizada microscopia eletr?nica de varredura (MEV) e an?lise qu?mica quantitativa (EDS) para os catalisadores com melhor desempenho, mostrando tanto a dispers?o homog?nea dos componentes na superf?cie das amostras como o maior teor de alum?nio presente na amostra suportada em HT.

The Fe-C-Cr-Mn-Ni-O system is of fundamental importance when describing the influence of oxygen on high alloyed steels. Both solid and liquid phases are of great interest: The solid phases regarding oxidation processes like the formation of oxide layers, inner oxidation, sintering processes and high temperature corrosion. The liquid phase is of interest concerning the interaction between steel and its slag in a metallurgical context. In this thesis the thermodynamic properties of this system is described using the Calphad technique. The main idea of the Calphad technique is to describe the Gibbs energy of all phases in the system as a function of temperature, pressure and composition using appropriate thermodynamic models. When thermodynamic descriptions of all phases taking part in the system are modelled and described in a database, the equilibrium state could be calculated with a software that minimizes the total Gibbs energy. Models within the compound energy formalism are used for all solution phases, among them the ionic two-sublattice liquid model, to describe both the metallic and oxide melts. All simple spinels (Cr3O4, FeCr2O4, Fe3O4, FeMn2O4, Mn3O4, MnCr2O4, NiCr2O4, NiFe2O4, NiMn2O4) within this system are described using a four-sublattice model. In this thesis several binary and ternary systems have been assessed or partly reassessed. The Fe-C-Cr-Mn-Ni-O database achieved can be used with an appropriate thermodynamic software to calculate thermodynamic properties, equilibrium states and phase diagrams. In general, the agreement between calculated and experimental values is good. / QC 20100723

Understanding of the interrelationship between structure, thermodynamic properties and phase diagrams is very useful for rationalizing the behavior of materials and development of predictive models, which can be used to optimize the composition of materials and their fabrication processes. The properties of materials are governed by its electronic and crystallographic structure. Chemical bonding determines the electronic structure of materials. Furthermore, the electronic structure plays a predominant role in determining the physical, electrical, magnetic, thermal and optical properties of materials. Crystal structure also influences most properties of materials. Since changes in thermodynamic variables such as temperature, pressure, and composition dramatically alter the physical properties of materials and its structure, it is desirable to study the thermodynamic stability of materials in conjunction with phase relations. Phase diagrams can indicate the ranges of pressure, temperature and chemical composition where specific phases and mixtures of phases are stable. If the Gibbs energies of all the phases involved are known, phase diagram can be computed using Gibbs energy minimization algorithms. In recent times, one of the important uses of thermodynamics in materials science has been in the computation of phase diagrams. To materials scientists phase diagrams are like maps to travelers. They guide the path through the composition space to find phases, fulfilling specific materials performance requirements. As phase diagrams are the graphic representations of minimizations of Gibbs energy under given constraints, computational thermodynamics significantly expands our capability to walk in the multi-component space of engineering materials. High-temperature phase-equilibrium studies, thermodynamics and materials processing have had a close relationship over a number of decades. Successful utilization of ceramic materials under different environmental conditions at high temperatures requires accurate thermodynamic data. Focus of the present investigation is to obtain correct phase relations and accurate thermodynamic data in selected technologically important ceramic oxide systems in which the data are either not available or are inconsistent. Based on the experimental data, different types of phase diagrams are computed for the systems of contemporary relevance. After a brief introduction, Chapter 1 discusses the brief overview of the experimental techniques available for determining the phase relations and thermodynamic properties at high temperatures and the methods used in this study. The chapter reviews the possible sources of errors in experimental techniques and tests for correct functioning. In Chapter 2, systematic studies on high-temperature phase equilibria and thermodynamic properties of compounds in the ternary systems Ln-Pd-O (Ln = La, Pr, Eu, Gd, Tb, Dy, Ho and Er) are presented. Some of the ternary oxides on the Ln-Pd-O systems have potential application in catalysis and electrochemistry. To optimize the parameters for the synthesis and to understand the behavior of the catalysts, it is useful to have information on the thermodynamic stability domain of each compound. Quantitative information on the stability of the ternary oxides is also useful for assessing the interaction of metal Pd with ceramic compounds containing rare-earth elements under different environments. Furthermore, the thermodynamic data are beneficial for the design of processes for the recovery of rare earth and precious metals from scrap. There is very little thermodynamic and phase diagram information on the Ln-Pd-O systems. Isothermal sections of phase diagram for the ternary system La-Pd-O at 1200 K and for the systems Ln-Pd-O (Ln = Pr, Eu, Gd, Tb, Dy, Ho and Er) at 1223 K, were established by the isothermal equilibration technique at high temperatures. Phases were identified after quenching by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS). Based on the phase relations, the thermodynamic properties of ternary interoxide compounds were determined by the solid-state galvanic cell technique over a range of temperature between 925 - 1400 K. An advanced version of the solid-state cell incorporating a buffer electrode was used for high temperature thermodynamic measurements. The function of the buffer electrode, placed between reference and working electrodes, was to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. These novel features enhanced the accuracy of thermodynamic data. From electrochemical measurements, the standard enthalpies of formation of these oxides from elements and their standard entropies at 298.15 K were also evaluated. The variation of the lattice parameters and unit cell volume as a function of rare earth atomic number for the three ternary compounds Ln4PdO7, Ln2PdO4 (Ln = La, Pr, Nd, Sm, Eu, Gd) and Ln2Pd2O5 (Ln = La to Er) are discussed. The systematic variations of thermodynamic properties of all the ternary compounds as a function of rare earth atomic number are presented and correlated with structural features. Thermodynamic and structural parameters of uninvestigated Ln-Pd-O systems (Ln = Ce, Pm) can be obtained by interpolation. Based on the thermodynamic information obtained in this study and auxiliary data on binary compounds available in the literature, different types of phase diagrams, isothermal oxygen potential diagrams, isobaric phase diagrams, isothermal two dimensional and three-dimensional chemical potential diagrams for the systems Ln-Pd-O (Ln = La, Pr, Eu, Gd, Tb, Dy, Ho and Er) are constructed. Chapter 3 contains the studies on partial phase diagrams of the systems M-Ru-O (M = Ca and Sr) at 1300 K and determination of Gibbs energies of formation of calcium and stronsium ruthenates in the temperature range from 925 to 1350 K using solid-state cells with yttria-stabilized zirconia as the electrolyte and Ru + RuO2 as the reference electrode. Gibbs energies, enthalpies and entropies of formation of calcium and strontium ruthenates from their component binary oxides were deduced. The standard enthalpies of formation of these oxides from elements and their standard entropies at 298.15 K were also evaluated. Based on the thermodynamic data obtained in this study and auxiliary information from the literature, the three dimensional representation of oxygen potential diagram for the M-Ru-O systems (M = Ca and Sr) as a function of composition and temperature are computed. The purpose of this chapter is to determine the thermodynamic stability of alkaline earth metal ruthenates in the perovskite related layered system Mn+1RunO3n+1 (n = 1, 2, and ¥ for Ca-Ru-O system and n = 1, 2, 3 and µ for Sr-Ru-O system) since these calcium and stronsium ruthenates have interesting magnetic and electronic device applications. Moreover, there is no literature available for thermodynamic properties on first and second members of the Ruddelsdon-Popper (R-P) series in Ca-Ru-O system, Ca2RuO4, Ca3Ru2O7 and third member of R-P series in Sr-Ru-O system, Sr4Ru3O10. Some of the available literature information on thermodynamic properties for other compounds of R-P series in Mn+1RunO3n+1 (M = Ca, Sr) are found to be based on incorrect assumptions and erroneous calculation. Thus, this chapter provides the complete thermodynamic information for all the electronically and magnetically applicable alkaline earth metal ruthenates for optimizing the deposition condition in device fabrications. Chapter 4 gives the structure-properties correlations of 2-3 spinel compounds and spinel-corundum equilibria for the system NiO-Al2O3-Cr2O3 at 1373 K. Nickel, aluminum and chromium are important base-constituent elements of high-temperature oxidation-resistant alloys. A spinel phase is usually found in the protective scale formed on the surface of the alloys. There is no thermodynamic data on spinel solid solution NiAl2O4-NiCr2O4. Thus, the phase relations and mixing properties of the spinel solid solution have been determined in this chapter. The inter-crystalline ion-exchange equilibrium between NiAl2+2xO4+3x-NiCr2O4 spinel solid solution and Al2O3-Cr2O3 solid solution with corundum structure in pseudo-ternary system NiO-Al2O3-Cr2O3 have been determined by the conventional tie-line rotation method at 1373 K. The nonstoichiometry of NiAl2+2xO4+3x has been taken into consideration. Lattice parameters were used to obtain the compositions of the corundum and spinel solid solutions at equilibrium. Formation of homogeneous solid solutions and attainment of equilibrium were confirmed by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). From the experimental tie-line information and thermodynamic data on Al2O3-Cr2O3 solid solution available in the literature, the activities in the spinel solid solution were derived by using a modified Gibbs-Duhem integration technique. Gibbs energy of mixing of the spinel solid solution has been calculated from the derived activity data. Since high temperature data generation is expensive and time consuming, it is useful to develop models, which relate thermodynamic properties to electronic and crystallographic structure, leading to predictive modeling of mixing properties. By comparing the results from models with experimental information, one can evolve methodologies for the prediction of the properties of uninvestigated system. A model can be used to discriminate among conflicting experimental data and extrapolate the data into regions where direct measurements are lacking or difficult to perform. In this chapter, a model approach has also been considered to analyze the activity-composition relationship in the NiAl2O4-NiCr2O4 spinel solid solution in terms of the intra-crystalline exchange of cations between the tetrahedral and octahedral sites of the spinel structure governed by site preference energies of the cations. Since Ni2+ and Cr3+ ion in tetrahedral coordination exhibits Jahn-Teller distortion, an entropy corresponding to randomization of the distortion in the cubic phase has been incorporated in the cation distribution model. The thermodynamic mixing properties of stoichiometric spinel solid solution NiAl2O4-NiCr2O4 in terms of one mole of mixing species were computed at 1373 K. The strain energy caused by size mismatch was added as a separate term to the Gibbs energy of mixing using empirical relationship between enthalpy of mixing for a pair of ions and the difference in their ionic volumes. Madelung constant and electrostatic contribution of energy of mixing of the spinel solid solution have also been computed. Comparison of Gibbs energy of mixing calculated using the cation mixing model for the stoichiometric spinel solid solution NiAl2O4-NiCr2O4 with that of the experimental tie-line data for nonstoichiometric spinel solid solution NiAl2+2xO4+3x-NiCr2O4 were included in this chapter. The thermodynamic mixing properties obtained in this study would be helpful in understanding the formation of complex spinel protective layers on alloys containing nickel, aluminium and chromium in high-temperature applications. The summary of the important finding and the conclusions arrived at on the basis of results obtained from the present investigations are presented in Chapter 5.

This dissertation reports on the synthesis, structural and thermal characterization and electrical and thermal transport properties of a variety of strongly correlated spinels. General structure property relationships for electrical and thermal transport are discussed. However, the relationship between thermopower and features of the crystal structure such as spin, crystal field, anti-site disorder, and structural distortions are explored in depth. The experimental findings are reported in the context of improving existing oxide thermoelectric materials, screening for new materials or using thermopower as a unique characterization tool to determine the cation distribution in spinels. The need for improved n-type oxide thermoelectric materials has led researchers to consider mixed valence \((+3/+4)\) manganese oxides. Contrary to previous findings we report herein that the \(LiMn_2O_4\) compound reaches the relatively large n-type thermopower of \(-73 \mu V/K\) which is three times larger than the value observed in other manganese oxides, \(-25 \mu V/K\). The cause of this increase in thermopower is shown to be the absence of a Jahn-Teller distortion on the \(Mn^{3+}\) ions in \(LiMn_2O_4\). By avoiding this structural distortion the orbital degeneracy is doubled and the Koshibae et al.’s modified Heikes formula predicts a thermopower of \(-79 \mu V/K\) in good agreement with the experiment. Altering the \(Mn^{3+/4+}\) ratio via aliovalent doping did not affect the thermopower and is a second evidence of universal charge transport first reported by Kobayashi et al. The role of anti-site disorder was further examined in \(Fe_xMn_{1-x}NiCrO_4\) x=0, ½, ¾, 1 spinels but the effect on thermopower was inconclusive due to the presence of impurity phases. Next, the thermopower as a function of temperature in \(Co_3O_4\) was investigated as a means whereby the Wu and Mason’s 30 year old model for using thermopower to calculate cation distribution in spinels could be revisited. We report evidence that Wu and Mason’s original model using the standard Heikes formula and considering octahedral sites alone leads to a stoichiometrically inconsistent result at high temperatures. Alternate models are evaluated considering Koshibae et al.’s modified Heikes formula and accounting for tetrahedral site contributions. Furthermore, the effect of a possible spin state transition is considered. / Engineering and Applied Sciences

Made available in DSpace on 2014-12-17T14:06:58Z (GMT). No. of bitstreams: 1 WalquiriaJS_DISSERT_PACIAL.pdf: 740304 bytes, checksum: 3d1d601a3873230ed2af2ce67496fdf5 (MD5) Previous issue date: 2011-07-21 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / The development and study of detectors sensitive to flammable combustible and toxic gases at low cost is a crucial technology challenge to enable marketable versions to the market in general. Solid state sensors are attractive for commercial purposes by the strength and lifetime, because it isn t consumed in the reaction with the gas. In parallel, the use of synthesis techniques more viable for the applicability on an industrial scale are more attractive to produce commercial products. In this context ceramics with spinel structure were obtained by microwave-assisted combustion for application to flammable fuel gas detectors. Additionally, alternatives organic-reducers were employed to study the influence of those in the synthesis process and the differences in performance and properties of the powders obtained. The organic- reducers were characterized by Thermogravimetry (TG) and Derivative Thermogravimetry (DTG). After synthesis, the samples were heat treated and characterized by Fourier Transform Infrared Spectroscopy (FTIR), X-ray Diffraction (XRD), analysis by specific area by BET Method and Scanning Electron Microscopy (SEM). Quantification of phases and structural parameters were carried through Rietveld method. The methodology was effective to obtain Ni-Mn mixed oxides. The fuels influenced in obtaining spinel phase and morphology of the samples, however samples calcined at 950 ?C there is just the spinel phase in the material regardless of the organic-reducer. Therefore, differences in performance are expected in technological applications when sample equal in phase but with different morphologies are tested / O desenvolvimento e estudo de detectores sens?veis a gases combust?veis inflam?veis e t?xicos de baixo custo ? um desafio tecnol?gico determinante para possibilitar vers?es comercializ?veis ao mercado em geral. Sensores no estado s?lido s?o atrativos para fins comerciais pela robustez e tempo de vida, uma vez que n?o s?o consumidos durante o processo de rea??o com o g?s. Paralelamente, o uso de t?cnicas de s?ntese mais vi?veis pela aplicabilidade em escala industrial s?o mais atrativas para obten??o de produtos comerciais. Dentro desse contexto foram obtidos materiais cer?micos com estrutura espin?lio via combust?o assistida por microondas, visando aplica??o em detectores de gases combust?veis inflam?veis. Adicionalmente foram empregados direcionadores-org?nicos alternativos para estudo da influ?ncia dos mesmos no processo de s?ntese e nas diferen?as nas propriedades e desempenho dos p?s obtidos. Os direcionadores-org?nicos foram caracterizados por Termogravimetria (TG). Ap?s a s?ntese, as amostras foram tratadas termicamente e caracterizadas por Espectroscopia de absor??o na regi?o do infravermelho com Transformada de Fourier (FTIR), Difra??o de Raios X (DRX), Medidas de ?rea espec?fica por BET e Microscopia Eletr?nica de Varredura (MEV). A quantifica??o de fases e a determina??o de par?metros estruturais foram determinados por refinamento atrav?s do M?todo Rietveld. A metodologia empregada foi eficaz para obten??o de ?xidos mistos Ni-Mn. Os combust?veis utilizados influenciaram na obten??o da fase espin?lio e na morfologia das amostras, sendo que as amostras calcinadas a 950?C s?o monof?sicas independentemente do direcionador. Logo, diferen?as de desempenho s?o esperadas em aplica??es tecnol?gicas quando amostras iguais em fase mas com diferentes morfologias s?o testadas

Made available in DSpace on 2014-12-17T14:07:12Z (GMT). No. of bitstreams: 1 RaimisonBA_DISSERT.pdf: 2147079 bytes, checksum: db6881c15458f1e487de74465b7088a0 (MD5) Previous issue date: 2013-07-04 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Magnetic ceramics have been widely investigated, especially with respect to intrinsic and extrinsic characteristics of these materials. Among the magnetic ceramic materials of technological interest, there are the ferrites. On the other hand, the thermal treatment of ceramic materials by microwave energy has offered various advantages such as: optimization of production processes, high heat control, low consumption of time and energy among others. In this work were synthesized powders of Ni-Zn ferrite with compositions Ni1- xZnxFe2O4 (0.25 &#8804; x &#8804; 0.75 mols) by the polymeric precursor route in two heat treatment conditions, conventional oven and microwave energy at 500, 650, 800 and 950?C and its structural, and morphological imaging. The materials were characterized by thermal analysis (TG/ DSC), X-ray diffraction (XRD), absorption spectroscopy in the infrared (FTIR), scanning electron microscopy (SEM), X-ray spectroscopy and energy dispersive (EDS) and vibrating sample magnetometry (VSM). The results of X-ray diffraction confirmed the formation of ferrite with spinel-type cubic structure. The extrinsic characteristics of the powders obtained by microwave calcination and influence significantly the magnetic behavior of ferrites, showing particles ferrimagn?ticas characterized as soft magnetic materials (soft), is of great technological interest. The results obtained led the potential application of microwave energy for calcining powders of Ni-Zn ferrite / Cer?micas magn?ticas t?m sido amplamente investigadas, principalmente com rela??o as caracter?sticas intr?nsecas e extr?nsecas destes materiais. Dentre os materiais cer?micos magn?ticos de interesse tecnol?gico, destacam-se as ferritas. Por outro lado, o tratamento t?rmico de materiais cer?micos por energia de microondas tem oferecido diversas vantagens, tais como: otimiza??o dos processos de obten??o, alto controle de aquecimento, menor consumo de tempo e energia entre outras. Neste trabalho, foram sintetizados p?s de ferritas Ni-Zn com composi??es Ni1-xZnxFe2O4 (0,25 &#8804; x &#8804; 0,75 mols) pela rota dos precursores polim?ricos em duas condi??es de tratamento t?rmico, em forno convencional e por energia de microondas a 500, 650, 800 e 950?C, e sua caracteriza??o estrutural, morfol?gica e magn?tica. Os materiais obtidos foram caracterizados por an?lises t?rmicas (TG/DSC), difra??o de raios x (DRX), espectroscopia de absor??o na regi?o do infravermelho (FTIR), microscopia eletr?nica de varredura (MEV), espectroscopia de raios X por dispers?o de energia (EDS) e magnetometria de amostra vibrante (VSM). Os resultados da difra??o de raios X confirmaram a forma??o da ferrita com estrutura c?bica do tipo espin?lio. As caracter?sticas extr?nsecas dos p?s obtidos e a calcina??o por microondas influenciaram de forma significativa o comportamento magn?tico das ferritas, apresentando part?culas ferrimagn?ticas, caracterizadas como materiais magn?ticos moles (macias), sendo de grande interesse tecnol?gico. Os resultados obtidos induziram o potencial de aplica??o de energia de microondas para a calcina??o de p?s de ferritas Ni-Zn

Ces travaux de thèse portent sur la synthèse et la caractérisation de nanoparticules d’oxyde spinelle à base de cobalt, déposée à la surface de feuillets d’oxyde de graphène réduit. Ces nanocomposites ont pour but de servir de catalyseurs bifonctionnels intégrés à l’électrode à air des batteries lithium-air. La particularité des synthèses présentées repose sur le chauffage micro-onde des précurseurs en milieu aqueux entre 100 et 200 °C. Cette approche originale de chimie douce permet en outre la création de l’interface entre les nanoparticules d’oxyde et les feuillets de graphène directement pendant la synthèse des particules, au lieu de nécessiter une étape supplémentaire de greffage. L’oxyde de cobalt Co3O4 a servi de matériau modèle pour optimiser les différents paramètres de synthèse puis le protocole a été adapté à une série d’oxydes binaires en substituant une partie des atomes de cobalt par du nickel, du manganèse ou encore du fer. La co-précipitation des cations de métaux de transition au sein d’une même phase a été rendue possible grâce à la compréhension des diagrammes de Pourbaix pour ajuster les conditions redox et acido-basiques.Enfin, les propriétés électrocatalytiques des catalyseurs composites vis-à-vis des deux réactions inverses d’ORR et OER ont été testées. L’écart entre les potentiels d’OER et ORR a été diminué de manière significative grâce à la présence des nanoparticules de Co3O4 sur feuillets d’oxyde de graphène réduit, montrant ainsi la réversibilité du catalyseur.Des caractérisations structurales et morphologiques qui ont permis d’établir certains liens entre synthèse, nanostructure et activité électrocatalytique complètent ce travail. / This research work is focused on the design and characterisation of cobalt based oxides nanospinels anchored onto the surface of reduced graphene oxides (RGO) nanosheet, which will serve as bifunctional catalysts for the new generation lithium-air batteries. Whereas nanospinels are relatively simple to synthesize by conventional colloidal routes as nanoparticles dispersed into an aqueous solution, the synthesis we developed relies on a hydrothermal microwave treatment in the 100°C-200°C range. The main challenges of this nano-heterostructures synthesis was to create the interface between the nanoparticles and the RGO directly during the nanoparticles nucleation. RGO are very efficient microwave absorbers and could then convert microwave irradiation into heat in order to trigger precipitation of the spinel at the surface of the RGO sheet.Starting from Co3O4 as proof-of-concept material, the synthesis protocol has been successfully adapted to address binary oxides by substituting cobalt with first row transition metals such as nickel, manganese or iron. The precipitation of binary and ternary oxides was achievable thanks to a good understanding of the Pourbaix diagrams of all cations to adjust the acido-basic and redox conditions.Finally, the electrocatalytic activity of these supported spinel oxides was measured for both the oxygen reduction and oxygen evolution reactions (ORR and OER). The gap between the ORR and the OER potentials was significantly lowered by the presence of Co3O4 nanoparticles on the RGO, thus assuring the reversibility of this catalytic system, which is to be integrated in future scale-up test.

Submitted by ANA KARLA PEREIRA RODRIGUES (anakarla_@hotmail.com) on 2017-08-07T12:48:06Z No. of bitstreams: 1 arquivototal.pdf: 4672568 bytes, checksum: 30655ee26e612b43221335d08bddc500 (MD5) / Made available in DSpace on 2017-08-07T12:48:06Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 4672568 bytes, checksum: 30655ee26e612b43221335d08bddc500 (MD5) Previous issue date: 2016-02-29 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / Magnesium stannate (Mg2SnO4) and titanate (Mg2TiO4) are inverse spinel-type oxides, applied as humidity sensors, hot resistor, dieletric, temperature compensation capacitor, electronic ceramic and refractory material. In the present work, these two materials were combined in order to obtain a solid solution, Mg2(Ti1-xSnx)O4, (x = 0; 0,25; 0,50; 0,75; 1,0) by the modified-Pechini method, in order to investigate the influence of Sn4+ substitution for Ti4+ in the spinel lattice for application as photocatalyst for discoloration of Gold yellow remazol and to avoid micro-organism adhesion into surfaces. The synthesis of the spinels was optimized. Catalysts were characterized by X-ray diffraction, UV-visibe spectroscopy, Raman spectroscopy, infrared spectroscopy, surface area measurement using BET method. Mg2TiO4 is a metastable compound, which decomposes into ilmenite (MgTiO3) above 800 °C. On the other hand, reaches long-range order at lower temperatures, while single phase Mg2SnO4 is only obtained from 900 °C. For these reasons, different temperatures are necessary to obtain the single phase materials. Infrared and Raman spectra confirmed the presence of [MgO6]-10, [TiO6]-8, [SnO6]-8 octahedra and (MgO4)-6 tetrahedra. The photocatalytic tests were carried out in a reactor comprising of UVC lamp (λ = 254 nm). Mg2SnO4 presented the best photocatalytic result, with 79% of discoloration at pH 6 and 87% at pH 3, while Mg2TiO4 showed 7% of conversion at pH 6, without increasing efficiency at pH 3. The obtained spinels were effective in inhibiting bacterial and fungal growth as indicated by fluorescence analysis for Gram positive bacteria (S. aureus and S. mutans), Gram negative bacteria (P. aeruginosa and E. coli) and fungus (Candida albicans) showing the potential for microbial anti-adhesion material. Tests with higher titanium content showed the best results except for E. coli. / O estanato (Mg2SnO4) e o titanato de magnésio (Mg2TiO4) são óxidos do tipo espinélio inverso, aplicados como sensores de umidade, resistor de calor, dielétrico, capacitor para compensação de temperatura, cerâmica eletrônica e material refratário. Nesse trabalho, os dois materiais foram combinados com a finalidade de obter uma solução sólida, Mg2(Ti1-xSnx)O4, (x = 0; 0,25; 0,50; 0,75; 1,0), utilizando o método Pechini modificado, de modo a investigar a influência da substituição dos íons Sn4+ por íons Ti4+ na rede do espinélio para aplicação como catalisadores na descoloração do corante remazol amarelo ouro e na antiadesão microbiana. A síntese dos espinélios foi otimizada. Os catalisadores foram caracterizados pelas técnicas de difração de raios-X, espectroscopia na região do ultravioleta e do visível, e espectroscopia Raman, espectroscopia na região do infravermelho e medida de área superficial por BET. O Mg2TiO4 é um composto metaestável, se decompondo em ilmenita (MgTiO3) acima de 800 °C. Por outro lado, o Mg2TiO4 se organiza a longo alcance em temperaturas mais baixas enquanto que o Mg2SnO4 monofásico é obtido apenas a partir de 900°C. Com isso, observa-se que diferentes temperaturas são necessárias para se obter os materiais monofásicos. Os espectros de IV e Raman confirmaram a presença dos octaedros [MgO6]-10, [TiO6]-8, [SnO6]-8 e tetraedro (MgO4)-6. Os testes fotocatalíticos foram realizados em um reator composto por lâmpadas UVC (λ = 254 nm). O Mg2SnO4 apresentou o melhor resultado, com descoloração de 79 % em pH 6 e 87 % em pH 3, enquanto o Mg2TiO4 apresentou conversão de 7 % em pH 6, sem aumento de eficiência em pH 3. Os espinélios obtidos mostraram-se eficazes na inibição do crescimento bacteriano e fúngico como indicado pela análise de fluorescência para bactérias Gram positivas (S. aureus e S. mutans), Gram negativas (P. aeruginosa e E. coli) e para o fungo (Candida albicans) mostrando o potencial de antiadesão microbiana dos materiais. Os testes com maior teor de titânio apresentaram os melhores resultados exceto para a E. coli.