Janio Venturini

BICUVOX.1-matrix composite electrolyte with yttria-stabilized zirconia as an inert phase: SEM evaluation of the chemical stability under hydrogen atmosphere

The purpose of this study is to evaluate the efficacy of yttria-stabilized zirconia (3Y-TZP) as an inert phase to prevent the decomposition of Bi2 V0.9 Cu0.1 O5.5 -δ (BICUVOX.1) electrolyte under reducing atmosphere. A post-mortem scanning electron microscopy (SEM) study was performed after chemical stability tests under hydrogen-rich atmosphere using a Sieverts-type apparatus. SEM results showed that BICUVOX.1 decomposition starts under a hydrogen pressure of 19.7u2009atm at 300°C, even in the case of the composite containing 3Y-TZP. The microstructure of BICUVOX.1 after decomposition was observed to be composed of microspheres ranging from 10 to 100u2009µm formed primarily of metallic bismuth. In the composite, in addition to microspheres, the microstructure contained bismuth fibers growth from the grain area of the BICUVOX.1 matrix. Despite significant surface morphological modifications, the grain-boundary-arranged 3Y-TZP particles in a BICUVOX.1-matrix composite did not result in enhanced chemical stability.

Conductivity dynamics of metallic-to-insulator transition near room temperature in normal spinel CoFe2O4 nanoparticles

Herein, we report the preparation of minute CoFe2O4 nanoparticles which exhibit an unusually normal spinel characteristic and a metallic behavior at room temperature that has not yet been reported in the literature for such materials. A study on the synthesis and characterization of these nanoparticles and the effect of cation distribution on electrical properties was conducted. A xerogel was treated at 300 °C for 20 hours and the as-synthesized powder was structurally and electrically characterized. X-ray diffraction studies indicate that the synthesis pathway was successful in producing crystalline nanoparticles under such mild conditions. TEM images demonstrate that CoFe2O4 consists of monodispersed nanocrystalline particle aggregates with a particle size of ∼10 nm. Mossbauer spectroscopy investigation shows a normal spinel configuration, which is highly unusual for cobalt ferrite. Impedance analyses confirm that the material has a metallic characteristic at temperatures close to room temperature which arises as a consequence of octahedral site occupancy. Above 100 °C, the material displays a metallic-to-insulator transition, which has been related to the predominant OLPT conduction mechanism. An endothermic peak in the DSC curve indicates the occurrence of a structural transition which was associated with the Verwey-type reordering of charged metallic atoms.

Microestrutura, fases cristalinas e propriedades elétricas de porcelanas aluminosas contendo diferentes concentrações de Fe2O3sinterizadas em atmosfera redutora e oxidante

Porcelanas aluminosas contendo diferentes concentracoes de Fe2O3 foram sinterizadas em atmosfera oxidante e redutora. A microestrutura e fases formadas foram investigadas com o auxilio de difracao de raios X e microscopia eletronica de varredura. Espectroscopia de impedância foi utilizada para estudar as propriedades eletricas. Os resultados indicaram que todas as amostras apresentaram mulita e corindon como fases majoritarias e pequenas fracoes de quartzo. A adicao de Fe2O3 provocou o surgimento de hematita nas amostras sinterizadas em atmosfera oxidante e, ferro metalico nas amostras contendo > 3%p sinterizadas em atmosfera redutora. Os espectros de impedância indicam que as caracteristicas da fase vitrea e interface entre fase vitrea /fases cristalinas regem o comportamento eletrico global das amostras. Nao obstante, diferentes atmosferas de queima provocam efeitos opostos na resistividade eletrica das porcelanas contendo Fe2O3. A presenca de hematita foi considerada a responsavel pela diminuicao da resistividade eletrica nas amostras sinterizadas em atmosfera oxidante, enquanto que o aumento na resistividade eletrica das amostras sinterizadas em atmosfera redutora foi atribuido a elevada quantidade de fase vitrea e porosidade. Estes resultados indicam que atmosfera redutora durante a queima pode favorecer o uso de materias-primas com maiores concentracoes de Fe2O3, desde que todo oxido de ferro tenha se dissolvido na fase liquida durante a queima.