Doutorado em Física (IF)
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Navegando Doutorado em Física (IF) por Por Orientador "Franco Júnior, Adolfo"
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Item Síntese e caracterização estrutural, dielétrica e magnética dos compósitos cerâmicos multifuncionais (1-x)K0,5Na0,5Nb0,7Ta0,3O3- xCoFe2O4 (0≤x≤0,3)(Universidade Federal de Goiás, 2016-12-22) Carvalho, Fernando Marques; Franco Júnior, Adolfo; http://lattes.cnpq.br/0187547195548392; Franco Júnior, Adolfo; http://lattes.cnpq.br/0187547195548392; Valverde, Clodoaldo; Santana, Ricardo Costa deLead-free multifunction composites are of great interest to the scientific community as they have several potential applications. Such composites were produced from the ferroelectric compound free of lead niobate and potassium sodium tantalate (KNNT) and the magnetic nanoparticles of cobalt ferrite (CFO). A particulate composite was then produced with the ferroelectric matrix and magnetic nanoparticles. First, the influence of the sintering temperature (950 and 1100 °C) on the ceramic densification process was investigated. Ceramics sintered at 950 °C showed very low density, unlike those sintered at 1100 °C, which have a density of 90%. The microstructure of the composites and how the magnetic particles are distributed in the matrix were investigated. The composites presented the perovskite, spinel and tungsten bronze structural phases; The lattice constant of all crystalline phases remained constant, so there was no migration of cations between the distinct phases. The value of the relative dielectric constant of the pure KNNT is 700, which is comparable to the values found in the literature. In the composites, the dielectric constant value decreases coherently with the amount of cobalt ferrite. The coercivity of the nanoparticles before and after the sintering is 1494 and 675 Oe, respectively, in the composites the value of coercivity remains constant. On the other hand, the saturation magnetization of nanoparticles before and after sintering is 63 and 68 emu/g, respectively. The value of the saturation magnetization reduces coherently with the amount of ferrite present in the other composites. In this way, it was shown that the electrical and magnetic properties coexist in the composite, since the magnetic properties depend exclusively on the cobalt ferrite.Item Cerâmicas multifuncionais de oxido de zinco dopadas com cobalto: propriedades ópticas, dielétricas e magnéticas(Universidade Federal de Goiás, 2019-10-23) Silva, Hermínia Veridiana dos Santos Pessoni e; Franco Júnior, Adolfo; http://lattes.cnpq.br/0187547195548392; Franco Júnior, Adolfo; Machado, Fernando Luis de Araujo; Silva, Sebastião Willian da; Bufaiçal, Leandro Felix de Sousa; Rosa, Andreia Luisa daIn this work, Zinc oxide Cobalt-doped ceramics (Zn1−xCoxO) were produced with 0,00≤x≤0,05 molar concentrations to obtain multifunctional materials. The Solid State Reaction with slow pressing step and maximum sintering temperature of T=1150ºC was used. Structural characterization by Apparent Density (ρap), Energy Dispersion Spectroscopy (EDS), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) indicated formation of typical ZnO hexagonal wurtizite phase for all samples (space group - P63mc (C6v)), without formation of spurious phases and good stoichiometric measured coherence. The results also indicated good densification for the ceramics (ρrel=97−91%). The optical characterizations performed by Infrared (FTIR), UV-Vis and Raman allowed the observation of the characteristic vibrational modes of ZnO and the confirmation of the total introduction of Co ions in the tetrahedral sites in the lattice, it also allows the study of evolution in the defects’ amount of VO and Zni type and systematic reduction of optical bandgap Eg due to doping. Electrical characterizations performed by Dielectric Permittivity Spectroscopy (EPD) and Impedance revealed Colossal Dielectric Permittivity (PDC) behavior in all samples (ϵ’~105) and, doping intensifies the effect. The overall dielectric behavior of ceramics is consistent with the Maxwell-Wagner (MW) dielectric relaxation model, which was also used to explain the PDC phenomenon. Magnetic characterizations performed using Magnetic Hysteresis, Zero Field / Field Cooling (ZFC / FC) and Electronic Paramagnetic Resonance (EPR) measurements indicated diamagnetic ordering (DM) in the pure sample and predominant paramagnetic (PM) ordering in the doped samples, as well antiferromagnetic ordering (AFM) in doped samples due to the approximation of Co ions in the lattice (sample x=0,05 - 62%PM×38%AFM). Weak Co-ion spin-orbit coupling has been identified in the presence of the ZnO crystalline field. Non-occurrence of ferromagnetism (FM) was discussed in terms of the defects’ placement in ZnO samples.