Made available in DSpace on 2014-12-17T14:58:15Z (GMT). No. of bitstreams: 1 LaureniaMPG_DISSERT.pdf: 2747629 bytes, checksum: a8b9f0209bf9acabd46832a08265ed7f (MD5) Previous issue date: 2012-05-08 / The lanthanum strontium cobalt iron oxide (La1-xSrxCo1-yFeyO3 LSCF) is the most commonly used material for application as cathode in Solid Oxide Fuel Cells (SOFCs), mainly due to their high mixed ionic electronic conductivity between 600 and 800?C. In this study, LSCF powders with different compositions were synthesized via a combination between citrate and hydrothermal methods. As-prepared powders were calcined from 700 to 900?C and then characterized by X-ray fluorescence, X-ray diffraction, thermal analyses, particle size analyses, nitrogen adsorption (BET) and scanning electronic microscopy. Films of composition La0,6Sr0,4Co0,2Fe0,8O3 (LSCF6428), powders calcined at 900?C, were screen-printed on gadolinium doped ceria (CGO) substrates and sintered between 1150 and 1200?C. The effects of level of sintering on the microstructure and electrochemical performance of electrodes were evaluated by scanning electronic microscopy and impedance spectroscopy. Area specific resistance (ASR) exhibited strong relation with the microstructure of the electrodes. The best electrochemical performance (0.18 ohm.cm2 at 800?C) was obtained for the cathode sintered at 1200?C for 2 h. The electrochemical activity can be further improved through surface activation by impregnation with PrOx, in this case the electrode area specific resistance decreases to values as low as 0.12 ohm.cm2 (800?C), 0.17 ohm.cm2 (750?C) and 0.31 ohm.cm2 (700?C). The results indicate that the citrate-hydrothermal method is suitable for the attainment of LSCF particulates with potential application as cathode component in intermediate temperature solid oxide fuel cells (IT-SOFCs) / A cobaltita de lant?nio dopada com estr?ncio e ferro (La1-xSrxCo1-yFeyO3 LSCF) ? comumente o material mais utilizado para aplica??o como catodo em c?lula a combust?vel de ?xido s?lido (SOFC), principalmente devido a sua elevada condutividade mista i?nica e eletr?nica entre 600 e 800?C. Neste trabalho, p?s de LSCF de diferentes composi??es foram sintetizados via uma combina??o entre os m?todos citrato e hidrotermal. Os p?s como obtidos foram calcinados entre 700 e 900 ?C e caracterizados por fluoresc?ncia de raios X, difratometria de raios X, an?lises t?rmicas, distribui??o de tamanho de part?cula, adsor??o gasosa (BET) e microscopia eletr?nica de varredura. Filmes de composi??o La0,6Sr0,4Co0,2Fe0,8O3 (LSCF6428) foram obtidos por serigrafia de p?s calcinados a 900?C. Os filmes foram depositados sobre substratos de c?ria dopada com gadol?nia (CGO) e ent?o sinterizados entre 1150 e 1200?C. Os efeitos do patamar de sinteriza??o na microestrutura e no desempenho eletroqu?mico dos eletrodos foram avaliados por microscopia eletr?nica de varredura e espectroscopia de imped?ncia. A resist?ncia espec?fica por ?rea apresentou forte rela??o com a microestrutura dos eletrodos. O melhor desempenho eletroqu?mico (0,18 ohm.cm2 a 800?C) foi obtido para o catodo sinterizado a 1200?C por 2 horas. A atividade eletroqu?mica pode ainda ser melhorada mediante ativa??o superficial por impregna??o com PrOx, neste caso a resist?ncia espec?fica por ?rea do eletrodo diminui para valores t?o baixos como 0,12 ohm.cm2 (800?C), 0,17 ohm.cm2 (750?C) e 0,31 ohm.cm2 (700?C). Os resultados obtidos indicam que o m?todo citrato-hidrotermal ? adequado para a prepara??o de particulados de LSCF com potencial aplica??o como catodo em c?lulas a combust?vel de ?xido s?lido de temperatura intermedi?ria (600-800?C)

Made available in DSpace on 2014-12-17T15:01:34Z (GMT). No. of bitstreams: 1 IndiraAFM_DISSERT_Parcial.pdf: 1021016 bytes, checksum: 339ff82fc9edc8d5f827859010abd0dc (MD5) Previous issue date: 2013-12-17 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / In this work it was synthesized and characterized the cobalt ferrite (CoFe2O4) by two methods: complexation combining EDTA/Citrate and hydrothermal investigating the influence of the synthesis conditions on phase formation and on the crystallite size. The powders were mainly characterized by x-ray diffraction. In specific cases, it was also used scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), x-ray fluorescence (XRF) and isotherms of adsorption and desorption of nitrogen (BET method). The study of the crystallite size was based on the interpretation of x-ray diffractograms obtained and estimated by the method of Halder-Wagner-Scherrer and Langford. An experimental design was made in order to assist in quantifying the influence of synthesis conditions on the response variables. The synthesis parameters evaluated in this study were: pH of the reaction medium (8, 9 and 10), the calcination temperature (combined complexation method EDTA/Citrate 600?C, 800?C and 1000?C), synthesis temperature (hydrothermal method 120?C, 140?C and 160?C), calcination time (combined complexation method EDTA/Citrate - 2, 4 and 6 hours) and time of synthesis (hydrothermal method 6, 15 and 24 hours). By the hydrothermal method was possible to produce mesoporous powders with high purity, with an average crystallite size up to 7 nm, with a surface area of 113.44 m?/g in the form of pellets with irregular morphology. By using the method of combined complexation EDTA/Citrate, mesoporous powders were produced with greater purity, crystallite size up to 22nm and 27.95 m?/g of surface area in the form of pellets with a regular morphology of plaques. In the experimental design was found that the hydrothermal method to all the studied parameters (pH, temperature and time) have significant effect on the crystallite size, while to the combined complexation method EDTA/Citrate, only temperature and time were significant / Neste trabalho foi sintetizada e caracterizada a ferrita de cobalto (CoFe2O4) atrav?s dos m?todos complexa??o combinada EDTA/Citrato e hidrot?rmico, investigando a influ?ncia das condi??es de s?ntese na forma??o da fase e no tamanho m?dio de cristalito. Os p?s foram caracterizados principalmente por difra??o de raios-x. Em casos espec?ficos, tamb?m foi utilizado microscopia eletr?nica de varredura (MEV), espectroscopia de energia dispersiva (EDS), fluoresc?ncia de raios-x (FRX) e isotermas de adsor??o e dessor??o de nitrog?nio (M?todo BET). O estudo do tamanho de cristalito foi baseado na interpreta??o dos difratogramas de raios-x obtidos e estimado atrav?s do m?todo de Halder-Wagner-Langford (HWL) e de Scherrer. Um planejamento experimental foi realizado com a finalidade de auxiliar na quantifica??o da influ?ncia das condi??es de s?ntese nas vari?veis-resposta. Os par?metros de s?ntese avaliados neste trabalho foram: pH do meio reacional (8, 9 e 10), temperatura de calcina??o (m?todo de complexa??o combinando EDTA/Citrato 600?C, 800?C e 1000?C), temperatura de s?ntese (m?todo hidrot?rmico 120?C, 140?C e 160?C), tempo de calcina??o (m?todo de complexa??o combinando EDTA/Citrato 2, 4 e 6 hrs) e tempo de s?ntese (m?todo hidrot?rmico 6, 15 e 24 hrs). Pelo m?todo hidrot?rmico foi poss?vel produzir p?s mesoporosos com elevado grau de pureza, com tamanho m?dio de cristalito de at? 7nm, com ?rea superficial de 113,44m?/g e na forma de aglomerados com morfologia irregular. Ao se utilizar o m?todo de complexa??o combinando EDTA/Citrato foram produzidos p?s mesoporosos com maior pureza, cristalitos com at? 22nm de tamanho, 27,95m?/g de ?rea superficial e na forma de aglomerados com morfologia regular de placas. No planejamento experimental foi constatado que no caso do m?todo hidrot?rmico todos os par?metros estudados (pH, Temperatura e tempo) apresentam efeito significativo no tamanho de cristalito, enquanto que, para o m?todo de complexa??o combinando EDTA/Citrato, apenas a temperatura e o tempo foram significativos / 2020-01-01