Rogério Junqueira Prado

Exchange coupling behavior in bimagnetic CoFe2O4/CoFe2 nanocomposite

Abstract In this work we report a study of the magnetic behavior of ferrimagnetic oxide CoFe 2 O 4 and ferrimagnetic oxide/ferromagnetic metal CoFe 2 O 4 /CoFe 2 nanocomposite. The latter compound is a good system to study hard ferrimagnet/soft ferromagnet exchange coupled. Two steps were followed to synthesize the bimagnetic CoFe 2 O 4 /CoFe 2 nanocomposite: (i) first, preparation of CoFe 2 O 4 nanoparticles using a simple hydrothermal method, and (ii) second, reduction reaction of cobalt ferrite nanoparticles using activated charcoal in inert atmosphere and high temperature. The phase structures, particle sizes, morphology, and magnetic properties of CoFe 2 O 4 nanoparticles were investigated by X-Ray diffraction (XRD), Mossbauer spectroscopy (MS), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) with applied field up to 3.0xa0kOe at room temperature and 50xa0K. The mean diameter of CoFe 2 O 4 particles is about 16xa0nm. Mossbauer spectra revealed two sites for Fe 3+ . One site is related to Fe in an octahedral coordination and the other one to the Fe 3+ in a tetrahedral coordination, as expected for a spinel crystal structure of CoFe 2 O 4 . TEM measurements of nanocomposite showed the formation of a thin shell of CoFe 2 on the cobalt ferrite and indicate that the nanoparticles increase to about 100xa0nm. The magnetization of the nanocomposite showed a hysteresis loop that is characteristic of exchange coupled systems. A maximum energy product ( BH ) max of 1.22xa0MGOe was achieved at room temperature for CoFe 2 O 4 /CoFe 2 nanocomposites, which is about 115% higher than the value obtained for CoFe 2 O 4 precursor. The exchange coupling interaction and the enhancement of product ( BH ) max in nanocomposite CoFe 2 O 4 /CoFe 2 are discussed.

Local structure of the metal-oxygen bond in compositionally homogeneous, nanocrystalline zirconia-ceria solid solutions synthesized by a gel-combustion process

Compositionally homogeneous ZrO2–CeO2 nanopowders have been characterized by Raman and extended x-ray absorption fine structure (EXAFS) spectroscopies. These techniques revealed a tetragonal-to-cubic phase transition as a function of CeO2 content, as observed in a previous synchrotron x-ray diffraction study. The tetragonal–cubic phase boundary was found to be at (85 ± 5) mol% CeO2. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the Zr–O first neighbour coordination sphere, while the Ce–O coordination sphere preserves its cubic symmetry over the whole composition range.

Local atomic structure in tetragonal pure ZrO2 nanopowders

The local atomic structures around the Zr atom of pure (undoped) ZrO2 nanopowders with different average crystallite sizes, ranging from 7 to 40u2005nm, have been investigated. The nanopowders were synthesized by different wet-chemical routes, but all exhibit the high-temperature tetragonal phase stabilized at room temperature, as established by synchrotron radiation X-ray diffraction. The extended X-ray absorption fine structure (EXAFS) technique was applied to analyze the local structure around the Zr atoms. Several authors have studied this system using the EXAFS technique without obtaining a good agreement between crystallographic and EXAFS data. In this work, it is shown that the local structure of ZrO2 nanopowders can be described by a model consisting of two oxygen subshells (4 + 4 atoms) with different Zr—O distances, in agreement with those independently determined by X-ray diffraction. However, the EXAFS study shows that the second oxygen subshell exhibits a Debye–Waller (DW) parameter much higher than that of the first oxygen subshell, a result that cannot be explained by the crystallographic model accepted for the tetragonal phase of zirconia-based materials. However, as proposed by other authors, the difference in the DW parameters between the two oxygen subshells around the Zr atoms can be explained by the existence of oxygen displacements perpendicular to the z direction; these mainly affect the second oxygen subshell because of the directional character of the EXAFS DW parameter, in contradiction to the crystallographic value. It is also established that this model is similar to another model having three oxygen subshells, with a 4 + 2 + 2 distribution of atoms, with only one DW parameter for all oxygen subshells. Both models are in good agreement with the crystal structure determined by X-ray diffraction experiments.

Synchrotron X-ray powder diffraction and extended X-ray absorption fine structure spectroscopy studies on nanocrystalline ZrO2–CaO solid solutions

The crystal structure and the local atomic order of a series of nanocrystalline ZrO2–CaO solid solutions with varying CaO content were studied by synchrotron radiation X-ray powder diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy. These samples were synthesized by a pH-controlled nitrate–glycine gel-combustion process. For CaO contents up to 8u2005mol%, the t′ form of the tetragonal phase (c/a > 1) was identified, whereas for 10 and 12u2005mol% CaO, the t′′ form (c/a = 1; oxygen anions displaced from their ideal positions in the cubic phase) was detected. Finally, the cubic phase was observed for solid solutions with CaO content of 14u2005mol% CaO or higher. The t′/t′′ and t′′/cubic compositional boundaries were determined to be at 9u2005(1) and 13u2005(1)u2005mol% CaO, respectively. The EXAFS study demonstrated that this transition is related to a tetragonal-to-cubic symmetry change of the first oxygen coordination shell around the Zr atoms.

Crescimento de cristais de rubi e safira pelo método do fluxo

This work reports the growth of corundum crystals by the flux method. The main objective was the evaluation of versatility, effectiveness and real possibilities of the flux method to the synthesis and doping of monocrystals with impurities of particular interest. In this work the chosen impurities were i) Cr and ii) Fe and Ti, aiming the synthesis of rubies and sapphires, respectively. The crystals were grown by heating a mixture of Al2O3:Cr or Al2O3:Fe:Ti and flux (MoO3). The maximum crystal size obtained was 1.0 mm, all transparent, presenting well developed faces, bipiramidal hexagonal shape, and showing a typical red (ruby) and/or light blue (sapphire) color. EDX and XPD experiments were performed in order to characterize some of the synthesized crystals. All crystallized specimens presented the α-alumina atomic structure.

Caracterização do material particulado suspenso no ar de Cuiabá-MT no período de queimadas

The chemical and morphological characterization of the particulate matter found into the air of Cuiaba Municipality - Mato Grosso State - Brazil, during the forest burning period of 2012, was performed by Energy Dispersive X-Ray Fluorescence (EDXRF) and Scanning Electron Microscopy with X-ray Micro-analysis (SEM/EDS) techniques. It should be noted that there are few studies on the subject, particularly for Cuiaba Municipality and region. Using SEM/EDS techniques was possible the classification of the particulate matter in four different groups, defined by morphological similarity and chemical composition: agglomerates, plates, fibers and spheroids. Ten chemical elements (Al, S, Cl, K, Ca, Fe, Cu, Zn, Sr e Ba) were identified and quantified by EDXRF, mainly showing the existence of re-suspension of soil particles by mechanical processes and fires (Al, Fe, Ca, K e Cl), and in minor extent vehicle and industrial emissions (Zn, S, Cu). Concentrations of particulate matter found in air, after long drought, attained 306 μg/m3 of air, exceeding the physical limit established by the Brazilian environmental national council (CONAMA) and showing the poor quality of the local air in the period. Furthermore, the fibrous particles identified in this study have not been previously reported, being apparently characteristic of the region, and presenting chemical composition consistent with a strong influence of fire on their production.