Materials Chemistry applied to bismuth cobaltite-rich nanocomposites with sillenite crystal structure

Abstract

The constant scientific advancement is contributing to multifunctional materials emergence, aiming to provide the growing worldwide technology demand. Thus, the study and development of materials is directly associated with the understanding of Materials Chemistry. In this scenario, this thesis aims to apply the four pillars of Materials Chemistry (synthesis, characterization, properties, and applications) in the production of an inorganic nanocomposite rich in bismuth cobaltite with a crystalline structure of sillenite (Co Sillenite - Co3O4) as a promising candidate for scientific-technological use. Initially, in a first work, two polycrystalline samples of the nanocomposite were prepared by combustion and sol-gel methods, with grain sizes ranging from 64.4 nm to 80 nm in all phases. The combustion sample displayed a diffractogram with slightly asymmetrical sillenite peaks, which indicated the presence of sillenite phases with some variation in its stoichiometry. From the magnetic susceptibility data, it was verified that the combustion sample has a degree of magnetic frustration twice as high as the sol-gel sample. Both samples presented a Néel temperature of 36 K for the secondary phase Co3O4 and a hysteresis at 6 K. However, in the MT measurements with a magnetic field of 200 Oe, above 148 K, only the combustion sample presented a superparamagnetic signal, this signal changes to 70 K when magnetic field of 600 Oe was applied. In a second work, directed to the application of the nanocomposite, the production process via combustion route and characterization of Sillenite Co – Co3O4 powder was reported. The dielectric characterization of the nanocomposite presented significant results in the X-band, with low values of conductivity and loss tangent from 6.0 GHz to 8.5 GHz, a characteristic that allows its application as a substrate for resonators. Therefore, the nanocomposite was applied as a substrate in a microstrip antenna. The reflection coefficient measurements displayed good agreement with the simulations, reaching only 0.39% error in the resonance frequency, validating the morphological and dielectric characterizations performed, according to the antenna design. In a third investigation, optimization of the nanocomposite synthesis was carried out based on a Box-Behnken Design of Experiments and the study of structural, chemical, and dielectric properties were performed to the samples synthetized by three different types of fuel: urea, glycine, and ethylene glycol. A total of 15 samples were prepared by combining combustion and sol-gel methods at low temperatures to evaluate the effects of fuel type, along with the percentage of fuel and initial pH of the reaction on the relative mass concentration of the Co Sillenite phase (% CoS) present in the nanocomposite. Validation experiments confirmed the good predictive potential of the proposed quadratic model between the response and the significant variables, with the ethylene glycol sample showing the highest % CoS, with 87.92%.