Banca de DEFESA: ANA LUCIA GURGEL

Uma banca de DEFESA de DOUTORADO foi cadastrada pelo programa.
DISCENTE : ANA LUCIA GURGEL
DATA : 19/12/2016
HORA: 14:00
LOCAL: Auditório do Laboratório de Cimentos da UFRN
TÍTULO:

Synthesis and characterization of nanostructured cobalt ferrite by the microwave assisted hydrothermal method.

PALAVRAS-CHAVES:

CoFe₂O₄; CoFe₂; Core-Shell System; Hydrothermal Synthesis.

PÁGINAS: 100
GRANDE ÁREA: Engenharias
ÁREA: Engenharia de Materiais e Metalúrgica
SUBÁREA: Materiais Não-Metálicos
ESPECIALIDADE: Cerâmicos
RESUMO:

Cobalt ferrite is characterized by good magnetic moment saturation and considerable coercivity. The methods used in the production of CoFe2O4 that enable control of the particle size distributed are best suitable for different applications. The present study focused on the preparation and characterization, both structural and magnetic, of CoFe₂O₄ nanoparticles and CoFe₂O₄-CoFe₂ core-shell system. CoFe₂O₄ ferrite was produced via the Hydrothermal Microwave Method and, subsequently reduced in flowing hydrogen atmosphere for the formation of the CoFe₂O₄-CoFe₂ system. In the production of CoFe₂O₄, four series were produced individually using ammonia hydroxide, potassium hydroxide, sodium hydroxide and ethylene glycol. The structural characterization of the samples was performed by X-ray diffraction with Rietveld refinement and Transmission Electron Microscopy. Only the series using ammonia and potassium hydroxide depicted pure cobalt ferrite. Analyses of the diffractograms revealed that the average diameter of crystallites ranged from 5.2 to 21.3 nm. The potassium hydroxide series resulted in larger crystallites of mixed form (approximately spherical and cubic) whereas those of the ammonium hydroxide series were smaller and spherical. Samples of the sodium hydroxide and ethylene glycol series were characterized by the presence of hematite. The samples were also characterized by the magnetically vibrating sample method at room temperature and by physical property measurement system (PPMS) at low temperatures, in addition to Mössbauer spectroscopy. The results revealed the characteristic magnetically blocked behavior for samples with crystallites larger than 10 nm and superparamagnetic behavior for smaller crystallite sizes. The ratio MR:MH_max increased with increasing the crystallite size. In the production of CoFe₂O₄-CoFe₂ system, a sample of the potassium hydroxide  series was used after reduction in hydrogen atmosphere and stabilization at 350°C for 10 min. CoFe₂ was obtained from this procedure. The reduced sample was analyzed. The XRD pattern and TEM images showed the formation of the core-shell system, indicating that the average diameter of crystallites was 33.7 nm (6.3 nm for CoFe₂O₄ and 27.4 nm for CoFe₂). The magnetic properties, analyzed by the hysteresis plots and Mössbauer spectrum were also improved compared to the starting sample. The ratio MR:MH_max increased from 0.299 to 0.356. 

MEMBROS DA BANCA:
Presidente - 1298936 - ANTONIO EDUARDO MARTINELLI
Externo à Instituição - GEOVANI FERREIRA BARBOSA - UFERSA
Externo à Instituição - MARCOS TADEU DŽAZEREDO ORLANDO - UFES
Externo ao Programa - 1506706 - MILTON MORAIS XAVIER JUNIOR
Interno - 1350249 - RUBENS MARIBONDO DO NASCIMENTO