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 2014-12-17T15:42:21Z (GMT). No. of bitstreams: 1 JoseEMS_TESE.pdf: 5374821 bytes, checksum: 56fe60cd3bbf0a42f7512c62fe5b976e (MD5) Previous issue date: 2013-03-08 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Were synthesized spinel-type ferrites with general formula Ni0,8Mg0.2-xMxFe2O4, where M represents the doping Mn, Co or Mn + Co simultaneously, x ranges for the values 0.02, 0.05 and 0.1. The value of x was divided by 2 in cases where M equals Mn and Co conjugates. We used the citrate precursor method and heat treatment to obtain the phases at 1100?C. The materials were characterized by XRD, TGA/ DTGA, SEM, MAV and reflectivity measurements by the method of waveguide. Powders to 350?C/3.5 h were crystalline and nanosized. According to the results this temperature all powders have a percentage of ferrite phase over 90%. The composition had the addition of Mn and Co simultaneously showed a higher percentage of secondary phase NiO, 5.8%. The TGA/DTGA curves indicate that this sample reached phase (s) crystalline (s) at lowest temperatures. The X-ray diffractograms of the samples calcined at 350?C and 1100?C were treated with the Rietveld refinament technique. The powders calcined at 1100 ?C/3h in air show to be 100% except spinel phase composition with 0.02 doping. The micrographs show clusters of particles with sizes smaller than 1 μm in calcination temperature of 1100?C which agreed with the result of Rietveld refinement. In the compositions doped with Mn were higher values of magnetization (45.90 and 53.20 Am2/kg), which did not cause high microwave absorption. The theoretical calculation of magnetization (MT) was consistent with the results, considering that there was agreement between the increase of magnetization experimental and theoretical. It was observed that there was the interrelation of the final effect of absorption with the thickness of MARE, the composition of ferrimagnetic materials and in particular the specific values of frequency. The analysis shows that the reflectivity increases in the concentration of cobalt increased the frequency range and also for absorption 10.17 GHz and 84%, respectively. The best result of chemical homogeneity and the value of 2.96 x 10-2 tesla coercive field were crucial for high performance ferrite absorber with 0.1 cobalt. The Cobalt has high magnetocrystalline anisotropy, it is associated with an increased coercive field, Hc. Therefore, this property improves the results of reflectivity of spinel ferrites / Foram sintetizadas ferritas do tipo espin?lio com f?rmula geral Ni0,8Mg0,2-xMxFe2O4, onde M representa os dopantes Mn, Co ou Mn + Co simultaneamente, x varia para os valores 0,02; 0,05 e 0,1. O valor de x foi dividido por 2 nos casos em que M equivale a Mn com Co conjugados. Foi utilizado o m?todo dos citratos precursores e tratamento t?rmico na obten??o das fases a 1100?C. Os materiais foram caracterizados por DRX, TGA/DTGA, MEV, MAV e medidas de refletividade pelo m?todo de guia de ondas. Os p?s a 350?C/3,5h foram cristalinos e nanom?tricos. De acordo com os resultados nesta temperatura todos os p?s apresentam um percentual de fase ferrita superior a 90%. A composi??o que teve a adi??o de Mn e Co simultaneamente apresentou maior percentual de fase secund?ria NiO, 5,8%. As curvas TGA/DTGA indicam que essa amostra alcan?ou fase(s) cristalina(s) em temperaturas mais baixas. Os difratogramas de raios X das amostras calcinadas a 350?C e 1100?C foram tratados pela t?cnica de refinamento Rietveld. Os p?s calcinados a 1100?C/3h em atmosfera ambiente mostram ser 100% de fase espin?lio exceto a composi??o com 0,02 de dopantes. As micrografias apresentam aglomerados de part?culas com tamanhos menores que 1 μm na temperatura de calcina??o de 1100?C, o que concordou com o resultado de refinamento Rietveld. Nas composi??es dopadas com Mn ocorreram os maiores valores de magnetiza??o (45,90 e 53,20 Am2/kg), o que n?o acarretou alta absor??o de microondas. O c?lculo te?rico da magnetiza??o (MT) foi coerente com os resultados, considerando-se que houve concord?ncia entre o aumento das magnetiza??es experimental e te?rica. Observa-se que houve interrela??o do efeito final de absor??o com a espessura do MARE, a composi??o do material ferrimagn?tico em particular e os valores espec?ficos de freq??ncia. A an?lise de refletividade indica que o aumento na concentra??o de cobalto aumentou a faixa de freq??ncia e a absor??o para 10,17 GHz e 84%, respectivamente. O resultado indica que a melhor homogeneidade qu?mica e o valor 2,96 x 10-2 tesla de campo coercitivo foram determinantes para o alto desempenho absorvedor da ferrita com 0,1 de cobalto. O Cobalto tem alta anisotropia magnetocristalina, isto est? associado ao aumento do campo coercitivo, Hc. Portanto, essa propriedade melhora os resultados de refletividade das ferritas espin?lio