Navegando por Autor "Franco Júnior, Adolfo"

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    A comprehensive study of Co1−xNixFe2O4 nanoparticles fabricated via three different synthetic methods
    (2019) Pessoa, Marcio Solino; Moscon, Paulo Sérgio; Melo, R. S.; Franco Júnior, Adolfo; Morais, Paulo Cesar de
    Wereport on three different synthetic methods for producing Co1−xNixFe2O4 nanoparticles (x=0.0, 0.4, and 1.0): combustion (C), hydrothermal (HT), and forced hydrolysis (FH). Theoretical fittings of the ferromagnetic resonance (FMR) lines were achieved by considering a proposed four-fold symmetry model for randomly dispersed magnetic nanoparticles (NPs). The average grain size (D), between 10 and 80 nm, and magnetic properties are found to depend strongly on both the selectedsynthesis route and stoichiometry (x). Interestingly, while the HT- and FH-methods provide NPs with no systematic dependence of coercive field and remanence, upon x or D, the C-method showedmonotonic dependence. The magnetocrystalline anisotropy field (Ha), extracted from simulating theFMRspectra, systematically reduces with increasing x, regardless the synthetic route used. Worth mentioning that the largest (smallest) variation inHa is observed for the C-method (HTmethod) equals toΔHa=−3853 Oe (ΔHa=−1676 Oe).
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    Effect of particle shape and size on themorphology and optical properties of zinc oxide synthesized by the polyol method
    (2018) Alves, Thiago Eduardo Pereira; Kolodziej, Charles; Burda, Clemens; Franco Júnior, Adolfo
    In this study we prepared zinc oxide (ZnO) nanoparticles using the polyol method with zinc acetate and/or sodiumacetate in a propylene glycolmediumwith varied hydrolysis reaction time (10, 60 and 300min). Itwas observed that the hydrolysis reaction time and the concentration of acetate ions had a strong effect on the morphology and size of ZnO nanoparticles (NPs). X-ray diffraction (XRD) patterns revealed that all samples crystalized in the typical ZnO wurtzite structure and Rietveld refinement was used to characterize the structure and to suggest a preferential growth. Fourier transformed infrared spectroscopy (FTIR) was performed and showed the typical ZnO band, no impurity signals, as well as differences in A1 and E1 vibrationmodes due to different crystal growth in the structure. Raman scatteringwas carried out to determine the defects in the structure of ZnO, such as interstitial oxygen and oxygen vacancies. Transmission electron microscope (TEM) images revealed that ZnO NPs size and shape are strongly dependent on the hydrolysis reaction time and addition of acetate counter ions, respectively. The change in shape and size has also affected the optical properties, such as the optical bandgap (Eg) of ZnO NPs and their florescence.
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    Insights into the dielectric loss mechanism of bianisotropic FeSi/SiC composite materials
    (2020) Banerjee, Prasun; Kumar, Nagasamudram Suresh; Franco Júnior, Adolfo; Swain, Akshaya Kumar; Naidu, Kadiyala Chandra Babu
    High magnitudes of permittivity with the permeability of the materials help to absorb electromagnetic waves more efficiently. Snoek’s limit directly puts a constraint on the enhancement of the permeability of the material. However, the incorporation of the lossy material may help to enhance the permittivity abruptly. In this study, we prepared a FeSi/SiC composite material with the mechanical ball milling method and investigated its enhancement of the dielectric behavior. The bianisotropic nature was observed along with the phase purity in the morphological studies. The Cole−Cole relaxation mechanisms were observed to validate a complex relaxation mechanism with 104 times higher dielectric loss in the composite material. The detection of the Warburg capacitance using the impedance technique sheds new light on the ion diffusion mechanism in the metallic composite materials.
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    Intercalation of nanoscale multiferroic spacers between the two-dimensional interlayers of MXene
    (2022) Koneru, Bhargavi; Swapnalin, Jhilmil; Natarajan, Srinivasan; Franco Júnior, Adolfo; Banerjee, Prasun
    We report a facile synthesis of MXene/bismuth ferrite (Ti3C2Tx @ BiFeO3) nanocomposites using hydrazine hydrate induced with the hydrothermal method. Preparation of the composite with hydrazine hydrate prevents the formation of Bi25FeO39 inside the matrix. The 25 wt % loading of BiFeO3 acts as spacers for the multilayer restacking of MXene flakes by retaining the (002) plane. This promotes large dc conductivity with interfacial and defect dipole polarization mechanisms inside the composite material. The thinner M-H loop also indicates the minimal magnetic loss inside the composites. The facile synthesis strategy provides outstanding properties in Ti3C2Tx @ BiFeO3 composites as two-dimensional multiferroic materials.
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    On the correlation of the magnetic properties with the structural parameters in Fe-doped europium chromite
    (2021) Ratkovski, Danilo Roberto; Marin Ramirez, Juan Marcos; França, Emanoel Laurertan Tavares; Franco Júnior, Adolfo; Machado, Fernando L. A.
    In the present work, the dynamics of the spins and the structural parameters of thermal treated samples of Eu1−xFexCrO3 (x = 0, 0.1 and 0.2) were investigated. The ac-magnetic susceptibility (χac) was measured near TN for frequencies (f ) in the range 10 - 104 Hz, magnitude of the ac magnetic field of 10 Oe and for 5 ≤ T ≤ 300 K. X-ray diffraction data were used for determining the lattice parameters and the bonding angle θB (Cr– O(2) – Cr) for 100 ≤ T ≤ 300 K. The maximum in χac was found to shift to higher values of T for increasing values of f . The Vogel-Fulcher law was used for analyzing χac yielding values for the characteristic relaxation time τ0, activation energy Ea/kB and glassy temperature TG, respectively, in the ranges 2.10 - 3.96 ps, 46.5 - 47.2K and 169.9 -176.1 K. The super-exchange parameter J ∼ cos4[(180 − θB)/2]/d7, where d is the length of the bound Cr-O(2), was also obtained yielding a good correlation with the corresponding values of TN.
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    On the magnetic properties of the multiferroic ceramics Bi0:99Y0:01Fe1-xNixO3 Bi0:99Y0:01Fe1 xNixO3
    (2018) Ratkovski, Danilo Roberto; Ribeiro, P. R. T.; Machado, Fernando Luis de Araujo; Banerjee, Prasun; Franco Júnior, Adolfo
    Multiferroic ceramics of Bi0:99Y0:01Fe1 xNixO3 with 0:01 6 x 6 0:05 were synthesized by using a modified solid state reaction method. The crystalline structure and the morphology of the samples were investi gated by X-ray diffraction (XRD) and by scanning electron microscopy (SEM). The addition of Y and Ni to the bismuth ferrite (BiFeO3) was found to decrease the average grain size. Ac magnetic susceptibility and the zero-field-cooled (ZFC) and field-cooled (FC) magnetizations were measured for temperatures in the range 5 6 T 6 300 K. Hysteresis loops and an irreversible behavior in the temperature dependence of the magnetization not present in pure BiFeO3 were observed in the doped samples. However, the ferro magnetism was found more likely to be due to the presence of small amounts of magnetite. Nevertheless, the determination of the amount of Fe3O4 in these composite materials is important because it influences the magnetoelectric coupling which is important for some technological applications
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    On the optical and magnetic studies of YCrO3 perovskites
    (2018) Lazo Jara, Antonio Nolberto; Carvalho, Jesiel Freitas; Franco Júnior, Adolfo; Maia, Lauro June Queiroz; Santana, Ricardo Costa de
    In this paper we studied the Electron Paramagnetic Resonance (EPR) and Optical spectroscopies of mono disperse yttrium chromite (YCrO3) nanoparticles prepared by a modified polymeric precursor method. The particles crystallized in roughly spherical shape with size ranging from 100 to 200 nm. Diffuse reflectance spectrum exhibited very well defined bands related to the 4A2 → 4T2, 4A2 → 4T1, 4A2 → 2T1, 4A2 → 2 E transitions, characteristic of Cr3+ (3d3, 2D5/2) ions in nearly octahedral symmetry. The crystalline field Dq, Racah para meters (B and C) and Dq/B, could be determined and the results indicated that Cr3+ ions are in a strong ligand field site. The EPR spectra of YCrO3 shows a single broad resonance in the whole range of temperature studied, 100–300 K. Combining optical and EPR results the effective g-values of the paramagnetic Cr3+ were estimated to be 1.97 and 1.98, respectively, typical of Cr3+ in orthorhombic crystal field environment. The onset of the antiferro-paramagnetic transition, TÑ140 K, attributed to Cr3+-Cr3+ exchange interaction (J = −41.5 cm−1) was determinate by means of Magnetization and EPR measurements. The results were discussed in terms of the magnetic short-rang spin-spin exchange interaction between pairs of Cr3+.