Uso de metodo sol-gel e de coprecipitação na preparação de oxidos metalicos
AUTOR(ES)
Eduardo Antonio Pinheiro
DATA DE PUBLICAÇÃO
1992
RESUMO
This thesis describes the preparation of two crystalline solids obtained by thermal decomposition of poorly crystallized solids obtained by precipitation techniques. Magnetite a ferrimagnetic crystalline solid, was obtained by heating non-crystalline iron (III) hidroxoacetate (HAFe), under nitrogen. Mixed oxides of La-Ba-Cu and Y-Ba-Cu, which are superconductors, were obtained by thermal decomposition of the respective oxalates, prepared by coprecipitation. HAFe was prepared by hydrolysis of ferric nitrate with ammonia, in the presence of acetic acid. Mixed oxalates of La-Ba-Cu and Y-Ba-Cu were obtained by addition of ammonium oxalate to solutions of the mixed nitrates, under different conditions of pH, temperature and concentration of reactants, using a batch-type reactor and a flow reactor. HAFe thermal transformations were studied by ESR, determination of iron (II) and total iron, density, TGA and DSC. Ammonia, carbon dioxide and acetic acid were released under HAFe heating. The effect of some preparative variables on the magnetic properties of the final product was determined, following a factorial design. The experimental results given in this work demonstrate that there is a temperature threshold at which solid ions are sufficiently mobile to allow mass transfer within the solid bulk; chemical reactions and ion reorganization are thus allowed. This change occurs at temperatures in which X-ray diffractograms show no diffraction lines and a low iron (II) content is observed. The magnetic parameters were obtained from magnetization curves and showed a strong dependence with time and heating temperature of HAFe, under nitrogen. Chemical analysis of La-Ba-Cu and Y-Ba-Cu oxalates, by X-ray fluorescence, showed that the best coprecipitation yield is obtained between pH4 and 5, in a flow reactor. The formation of the superconductor phase from oxalates begins of 910°C. Some intermediate products were identified. La-Ba-Cu oxides obtained in this work show a large resistive transition beginning at 30 K but which is still incomplete around the liquid helium temperature but the Y-Ba-Cu oxides showed narrow transitions with Tc in the 90-95 K range.
ASSUNTO(S)
ACESSO AO ARTIGO
http://libdigi.unicamp.br/document/?code=vtls000045209Documentos Relacionados
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