Papers by Author: Débora A. Vieira

Abstract: This work involved a study of the reproducibility of the process of combustion synthesis to produce Ni-Zn ferrites. The structural, morphological and magnetic characteristics of the samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and magnetometry using an alternating gradient magnetometer (AGM). The XRD diffractograms of the samples indicated that they are monophasic, crystalline, with crystallite sizes ranging from 21 to 38 nm, and have a homogeneous morphology consisting of agglomerates of spherical particles. The samples behaved as soft magnetic materials, with magnetization levels ranging from 37 to 47 emug^-1. The combustion synthesis was found to be efficient in producing Ni-Zn nanoferrites, yielding reproducible results.

Abstract: This study proposes to evaluate the influence of the variation of sintering temperature on microstructural characteristics and magnetic ferrite Ni_0,5Zn_0,5Fe₂O₄ sintered by microwave energy. The samples were sintered at 900, 1000, 1100 and 1200°C for exposure time of 10 minutes, with rate 50°C/minutes and characterized by density and porosity, X-ray diffraction, scanning electron microscopy and magnetic measurements. The results indicate that the values of density and apparent porosity were 4.2, 4.5, 4.4 and 4.5 g/cm³ and 3.4, 2.1, 2.2 and 2.4% for the sintering temperatures of 900, 1000, 1100 and 1200°C respectively. The formation of the ferrite phase Ni_0,5Zn_0,5Fe₂O₄ been identified for all conditions of sintering, with grain sizes of 52, 62, 71 and 58nm and saturation magnetization values of 63, 68, 69 and 27 emu/g to temperatures sintering 900, 1000, 1100 and 1200°C respectively.

Abstract: This paper reports the preparation of ferrite Ni_0,5Zn0,5Fe₂O₄ by combustion reaction in a microwave oven, and its structural, morphological and magnetic characterization. The influence of microwave power and the fuel type was investigated. The samples were characterized by: XRD, BET, SEM and AGM. The results showed the formation of phase ferrite Ni_{0, 5}Zn_0,5Fe₂O₄ in all conditions evaluated. The presence of secondary phase hematite and nickel were observed only in samples with glycine. The microwave oven power and the fuel type altered the structure, morphology and magnetic behavior of the samples. In general, the samples synthesized with urea are promising for applications in catalysis, ferrofluids, magnetic sensors and the samples synthesized with glycine are promising for use as absorber electromagnetic radiation, due to the large particle size and good magnetic characteristics observed.

Abstract: This paper proposes to assess the sintering of Ni-Zn ferrites synthesized by combustion reaction in advance to procure materials for applications as soft magnetic devices. The samples used for sintering have different morphological characteristics and were previously synthesized by microwave energy. The samples were uniaxially pressed and sintered in a microwave oven at 1200°C/2h with a heating rate of 5°C/min, and characterized by XRD, SEM and magnetic measurements. The results show that for all samples have the formation of Ni-Zn phase and traces hematite as secondary phase. The resulting microstructure after sintering was different and was influenced by previous morphological characteristics of the synthesized samples. As for the magnetic parameters, all samples were characteristic of soft magnetic material with saturation magnetization between 57 and 62 emu.g^-1, indicating are promising materials for the fabrication of soft magnetic devices.

Abstract: This paper assesses the sintering in a conventional oven and microwave oven of Ni-Zn ferrite. The samples were previously synthesized by combustion reaction in the microwave, and then sintered in a conventional oven and microwave oven 1200°C/2h with a heating rate of 5°C/min. The samples were characterized by: XRD, SEM and magnetic measurements by AGM. The results indicate the formation of inverse spinel crystalline phase of Ni-Zn ferrite, with inter-and intergranular porosity and average grain size of 0.15 µm and 0.30 µm, respectively for the samples sintered in a conventional oven and microwave oven. The sample sintered in conventional furnace showed a pattern characteristic of superparamagnetic materials with saturation magnetization of 7.6 emu.g^-1, while in the microwave sintered sample showed a typical behavior of soft magnetic materials with saturation magnetization of 70 emu.g^-1.

Abstract: This paper describes the synthesis of Ni-Zn nanoferrites by combustion reaction using microwave energy as a heating source, and evaluates the performance of these materials as absorbers of electromagnetic energy at frequencies between 4 - 12 GHz. The influence of the synthesis conditions on the structure, morphology and absorption characteristics was investigated. The powders were characterized by DRX, BET, AGM and reflectivity measurements in the frequency bands of 8 to 12 GHz. The XRD results show the formation of Ni-Zn ferrite phase and Fe2O3 and Ni as secondary phases. The crystallite sizes ranged from 32 to 42 nm. The parameters of exposure time and power of the microwave oven changed the final characteristics of the resulting powders. The morphology of all the powders consisted of soft nanoparticle agglomerates. The best saturation magnetization and attenuation results were 70 emu/g and -4.1 dB in the frequency of 10 GHZ.

Abstract: CoFe2O4 powders were synthesized by combustion reaction using glycine as fuel, aiming obtaining nanosized and monophase powders. Thus, different conditions of external heating during the synthesis were investigated. The powders were prepared according to the propellants and explosives theory, using glycine as fuel in the stoichiometric proportion (Φe = 1). During the synthesis the flame temperature and time were measured. The resulting powders were characterized by X-rays diffraction and scanning electronic microscopy (SEM). The results show that the condition in which the synthesis was done it influences in the combustion flame temperature and time and contributes for the obtainment of powders with majority phase without secondary phases. Crystallite size varied of 33 to 50 nm. All powders presented morphology constituted by soft agglomerated formed by nanoparticles.

Abstract: Diluted magnetic semiconductors (DMS), which have both semiconducting and magnetic properties, are those in which transitions metal ions substitute cations of host semiconductor materials [1]. There is a great interest for DMS for use as the material of spintronics. In this study is reported the structural and morphologic characterization of Zn1.95Co0.05O nanoparticles obtained by Pechini method and combustion reaction. The powders resulting were characterized by X-ray diffraction (XRD) for determination of the phases, crystalline phase and lattice parameter; nitrogen adsorption by BET for determination of the specific superficial area and calculation the particle size from the superficial area and scanning electron microscopy (SEM) for morphologic analysis. The XRD results demonstrated the viability of obtaining crystalline and nanosize powders by the both synthesis routes. For all samples the average crystallite sizes was nanosized, but the powders obtained by reaction combustion is smaller. The SEM micrographs shows that the powders obtained for both syntheses are constituted of soft agglomerates.