Marlu César Steil

BICOVOX: Sintering and grain size dependence of the electrical properties

The sintering conditions and the effects of the mean grain size on the electrical properties of cobalt-substituted bismuth vanadate (Bi2V0·9Co0·1O5·35), referred to as BICOVOX, have been studied. The influence of the experimental conditions (sintering temperature and duration, oxygen partial pressure) on the resulting density of the pellet was evaluated by dilatometric measurements. The grain size dependence of the electrical properties was studied by impedance spectroscopy. It is shown that the BIMEVOX characteristic hysteresis, which results in a significant decrease of the ionic conductivity below about 450°C, is suppressed provided that the grain size is small enough.

Grain-sized influence on the phase transition of Bi26Mo9WO69: an X-ray diffraction and impedance spectroscopy study

Abstract In order to investigate the microstructure dependence of the monoclinic–triclinic phase transition of Bi 26 Mo 9 WO 69 , dense samples with different grain sizes were produced by isostatic pressing and controlling sintering temperature and time. The structural and electrical properties were studied by X-ray diffraction (at room and at high temperature) and impedance spectroscopy, respectively. The influence of the microstructural properties on the triclinic–monoclinic phase transition and on the electrical properties of the Bi 26 Mo 9 WO 69 were described by the following parameters derived from the fitting of impedance diagrams: depression angle, relaxation frequency and total electrical conductivity. Impedance spectroscopy results are in agreement with the high-temperature X-ray diffraction data; both techniques show that phase transition kinetics depends on the microstructural properties, remarkably on the grain size.

Evolution of microstructure and impedance upon the sintering of a Bi-Pr-V-based fluorite-type oxide conductor

Abstract Samples of Bi0.85Pr0.105V0.045O1.545, a fluorite-type oxide conductor, with varying porosities and different average grain sizes determined by the sintering temperature (500–900 °C), were investigated using scanning electron microscopy (SEM) and impedance spectroscopy. Sintering treatment leads to increased density of ceramic materials between 500 and 800 °C and predominantly to grain growth at higher temperatures, in dense samples (porosity The impedance diagram for these materials is composed of a high-frequency domain characteristic of the electrical properties of the ceramic itself and a low-frequency domain attributed strictly to electrode response. The impedance diagram of the ceramic is a single semicircle. Grain boundaries and porosity do not lead to the appearance of a separate semicircle. The influence of sintering temperature on electrical properties may be parameterized by total electrical resistance, depression angle and relaxation frequency as seen by curve fitting of the impedance diagram. The variation of electrical conductivity and dielectric constant as a function of sample porosity are reported and discussed.

Catalytic dense membranes of doped Bi4V2O11 (BIMEVOX) for selective partial oxidation: chemistry of defects versus catalysis

A catalytic dense membrane reactor (CDMR) is used to physically separate the reaction step from the reoxidation of the catalyst. By decoupling the redox mechanism prevailing in mild oxidation of hydrocarbons, the operating conditions may be optimized resulting in an increase of selectivity. The membranes are made up of BIMEVOX oxides, obtained by partial substitution of V in γ-Bi4V2O11 by ME (Co, Cu, Ta). Experiments performed on BIMEVOX dense membranes using propene and propane are described in terms of, (i) active sites on polished or unpolished surfaces, (ii) operating conditions (T, pO2 in the high oxygen partial pressure compartment), which determine the selectivity, either to mild oxidation products (acrolein, hexadiene, CO), or to partial oxidation products (CO, H2), and, (iii) nature of ME cations and relative properties. The discussion deals with the respective role of electronic versus oxide ion conductivities which depend on defects in the structure as well as on the redox properties of cations.