Navegando por Autor "Conceição, Ozivam Lopes de Aquino"
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Artigo Critical dimension for magnetic exchange-spring coupled core/shell CoFe2O4=CoFe2 nanoparticles(Elsevier, 2013-01) Araújo, José Humberto de; Soares, João Maria; Galdino, V.B.; Conceição, Ozivam Lopes de Aquino; Torres, Marco Antonio Morales; Machado, Fernando L. A.Core/shell nanoparticles of CoFe2O4=CoFe2 were prepared by reducing nanoparticles of CoFe2O4 under hydrogen atmospheres. The structure, morphology and room temperature magnetization of the core/ shell nanoparticles were analyzed by X-ray diffraction, transmission electron microscopy and magnetometry, respectively. The sample preparation procedure allowed the diameter of the CoFe2O4 core to be varied from 67.7 to 2.8 nm. From the magnetic data it was found that the core couples to the shell through the exchange-spring mechanism while the critical thickness of the soft phase (shell) was estimated using a current theoretical model to be 8.0 nm. This value is very close to the one (7.8 nm) obtained for nanoparticles with same composition but obtained using different synthesis route and having an average particle size (32 nm) less than half of the value estimated (73 nm) for the samples investigated in the present workArtigo Influence of reaction temperature, proportions of iron, cobalt and KOH on the CoFe2O4 synthesis by hydrothermal method assisted by microwave heating(Springer, 2017-06-19) Paskocimas, Carlos Alberto; Conceição, Ozivam Lopes de Aquino; Garcia, Laurênia Martins Pereira; Lovisa, Laura Ximena; Torres, M. M.; Soares, J. M.; Delmonte, Maurício Roberto Bomio; Motta, Fabiana Villela daIn this study, several factors such as reaction temperature, proportions of iron, cobalt and KOH were considered in the synthesis of cobalt ferrite nanoparticles by hydrothermal method assisted by microwave radiation. These parameters were studied using the full factorial experiment design. The effect of these factors on crystallite size was investigated using surface response methodology. The structure, crystallite size, magnetic properties, degree of inversion, as well as the chemical occupation of Fe ions were investigated in this study. The results indicated that all factors were statistically significant by analysis of variance (F-test), with a confidence level of 95%. The factors that influenced the growth of the crystallite were: KOH and iron concentration, precursors of cobalt salts and the effect of the interaction between the concentration of KOH and the concentration of precursors. The analysis of variance at a significance level of 95% indicates that the model adjusted to the experimental points is significantArtigo Microwave-assisted hydrothermal synthesis and magnetic properties of nanostructured cobalt ferrite(Elsevier BV, 2019-08) Martinelli, Antonio Eduardo; Gurgel, Ana Lúcia; Conceição, Ozivam Lopes de Aquino; Xavier Jr, Milton Moraes; Torres, Marco Antonio Morales; Melo, Dulce Maria de AraújoCobalt ferrite is an important magnetic material due to its high saturation magnetization and high energy product BH. CoFe2O4 nanoparticles can be synthesized by a variety of methods that often demand long synthesis times. Using the microwave-assisted hydrothermal method, single-phase CoFe2O4 with controlled crystallite size was rapidly obtained. The effect of a preheating step and mineralizing agent, i.e., NH4OH and KOH, was investigated. Exposure to microwave radiation ranged from 0.5 to 2 h. The formation of CoFe2O4 was confirmed by X-ray diffraction. The average crystallite size of CoFe2O4 nanoparticles was estimated between 5.2 nm and 21.3 nm by Rietveld refinement. The crystallite size depended on the preheating step and mineralizing agent used. Transmission electron microscopy images revealed that CoFe2O4 nanoparticles were nearly spherical when prepared with NH4OH but cubic and spherical when prepared with KOH. Both, hysteresis curves and Mossbauer spectra at room temperature, € showed superparamagnetic behavior for samples with the smaller crystallite size. Samples with larger crystallites consisted of blocked particles and depicted an increase in the MR/MHmax ratio and coercive field, Hc. Distinct magnetic properties resulting from variations in the synthesis parameters yield different applications for nanostructured CoFe2O4