Oxygen transport in Ce0.8Gd0.2O2 - δ-based composite membranes

Gadolinia-doped ceria electrolyte Ce0.8Gd0.2O2 - δ (CGO) and perovskite-type mixed conductor La0.8Sr0.2Fe0.8Co0.2O3 - δ (LSFC), having compatible thermal expansion coefficients (TECs), were combined in dual-phase ceramic membranes for oxygen separation. Oxygen permeability of both LSFC and composite LSFC/CGO membranes at 970-1220 K was found to be limited by the bulk ambipolar conductivity. LSFC exhibits a relatively low ionic conductivity and high activation energy for ionic transport (∼ 200 kJ/mol) in comparison with doped ceria. As a result, oxygen permeation through LSFC/CGO composite membranes, containing similar volume fractions of the phases, is determined by the ionic transport in CGO. The permeation fluxes through LSFC/CGO and La0.7Sr0.3MnO3 - δ/Ce0.8Gd0.2O2 - δ (LSM/CGO) composites have comparable values. An increase in the p-type electronic conductivity of ceria in oxidizing conditions, which can be achieved by co-doping with variable-valence metal cations, such as Pr, leads to a greater permeability. The oxygen ionic conductivity of the composites consisting of CGO and perovskite oxides depends strongly of processing conditions, decreasing with interdiffusion of the phase components, particularly lanthanum and strontium cations from the perovskite into the CGO phase

Exploring the high-temperature electrical performance of Ca3-xLaxCo4O9 thermoelectric ceramics for moderate and low substitution levels

Aliovalent substitutions in Ca3Co4O9 often result in complex effects on the electrical properties and the solubility, and impact of the substituting cation also depends largely on the preparation and processing method. It is also well-known that the monoclinic symmetry of this material’s composite crystal structure allows for a significant hole transfer from the rock salt-type Ca2CoO3 buffer layers to the hexagonal CoO2 ones, increasing the concentration of holes and breaking the electron-hole symmetry from the latter layers. This work explored the relevant effects of relatively low La-for-Ca substitutions, for samples prepared and processed through a conventional ceramic route, chosen for its simplicity. The obtained results show that the actual substitution level does not exceed 0.03 (x < 0.03) in Ca3-xLaxCo4O9 samples with x = 0.01, 0.03, 0.05 and 0.07 and that further introduction of lanthanum results in simultaneous Ca3Co4O9 phase decomposition and secondary Ca3Co2O6 and (La, Ca)CoO3 phase formation. The microstructural effects promoted by this phase evolution have a moderate influence on the electronic transport. The electrical measurements and determined average oxidation state of cobalt at room temperature suggest that the present La substitutions might only have a minor effect on the concentration of charge carriers and/or their mobility. The electrical resistivity values of the Ca3-xLaxCo4O9 samples with x = 0.01, 0.03 and 0.05 were found to be ~1.3 times (or 24%) lower (considering mean values) than those measured for the pristine Ca3Co4O9 samples, while the changes in Seebeck coefficient values were only moderate. The highest power factor value calculated for Ca2.99La0.01Co4O9 (~0.28 mW/K2m at 800 °C) is among the best found in the literature for similar materials. The obtained results suggest that low rare-earth substitutions in the rock salt-type layers can be a promising pathway in designing and improving these p-type thermoelectric oxides, provided by the strong interplay between the mobility of charge carriers and their concentration, capable of breaking the electron-hole symmetry from the conductive layers. © 2021 by the authors

Cellular MgAl2O4 spinels prepared by reactive sintering of emulsified suspensions

Emulsification of mixed precursor powder mixtures (Al2O3+MgCO3) and subsequent reactive firing were, for the first time, combined to prepare single phase cellular MgAl2O4 and corresponding MgxAl3−xO4 spinels with deviations from ideal stoichiometry. Suspensions with 50% v/v solid load were emulsified in melted paraffin, with collagen additions to assist consolidation by gelcasting. Early burnout stages and firing conditions were adjusted to avoid collapse of green bodies on heating and to induce favorable microstructural changes, with emphasis on porosity and percolation

A Model for Digital Topology

In the framework known as digital topology, two different adjacency relations are used for structuring the discrete space Z . In this paper, we propose a model for digital topology based on the notion of order and discrete topology. We "validate" our model by considering the two fundamental notions of surface and simple point. At last, we give the different possible configurations that may appear in 2- and 3dimensional surfaces in Z which correspond to our model

Catalysis of ceria incorporated magnesium hydride: a follow up study

In continuation with the results explored in our recent previous study (https://doi.org/10.1016/j.apsusc.2021.150062), the current work sheds more light regarding the active in situ catalytic species in ceria additive loaded hydrogen storage system, MgH2. For this study, two samples, MgH2+0.167CeO2 and MgH2+0.5CeO2 were processed through mechanical milling (5 h/200 rpm) and tests were conducted at various stages of hydrogenation/dehydrogenation cycles (cycles: 1, 5 and 10). Evidence for mild chemical interaction between MgH2 and CeO2 is observed at the time of mechanical milling, whereas strong redox type interaction is witnessed in the cycle tested samples. In-situ X ray diffraction study confirms that the thermally activated interaction between MgH2 and CeO2 does not produce MgO. In situ Raman spectra provide crucial evidence that reduced cerium oxides exist at all stages of interactions in the MgH2/CeO2 hydrogen storage system. Detailed advanced electron microscopic observations concur well with the in-situ X ray diffraction and Raman spectroscopy studies. Although Gibbs free energy calculations reveal the possible existence of cerium hydrides, owing to the structural similarities between CeHx and CeOx phases, the chemical identity of the most abundant catalytic product remains debatable. Regarding the catalytic mechanism, suppression of MgO rock salt formation is identified to be a key step where the role of CeHx/CeOx interfaces gains more importance.publishe

Redox stability and high-temperature electrical conductivity of magnesium-and aluminium-substituted magnetite

Spinel-type magnetite-based oxides, possessing relatively high electrical conductivity, are considered as promising consumable anode materials for high temperature pyroelectrolysis, a breakthrough low-CO2 steel technology to overcome the environmental impact of classical extractive metallurgy. The present work was focused on the analysis of phase stability, thermal expansion and high-temperature electrical conductivity in (Fe,Mg,Al)(3)O-4 system under oxidizing and mildly reducing conditions. Metastable, nearly single-phase at room temperature (Fe,Mg,Al)(3)O-4 ceramics was obtained by sintering at 1753-1773 K for 10 h in argon atmosphere. Thermal expansion and redox induced dimensional changes were studied on heating, using TG, XRD and dilatometry. The results revealed that magnesium improves the tolerance against oxidative decomposition and minimizes unfavorable dimensional changes in ceramic samples upon thermal cycling. Co-substitution of iron with aluminium and magnesium was proved to be a promising strategy for improvement of refractoriness and phase stability of Fe3O4-based spinels at elevated temperatures, without significant reduction in the electrical conductivity. (C) 2013 Elsevier Ltd. All rights reserved

Redox stability and electrical conductivity of Fe2.3Mg0.7O4 +/-delta spinel prepared by mechanochemical activation

This paper addresses the potential of mechanochemical activation of MgO and alpha-Fe2O3 precursor powders to obtain Fe2.3Mg0.7O4 ceramics with enhanced redox stability and electrical conductivity. X-ray diffraction (XRD) and Mossbauer spectroscopy suggest the initial formation of the spinel phase after 5 h of high-energy milling in inert gas, but after 10 h of mechanoactivation, the precursor still comprised hematite as a major phase with minor amounts of magnesiowustite as by-product. The activated mixtures can be nearly completely converted to spinel solid solution by heating to 1173 K, whereas single-phase, dense spinel ceramics can be prepared by sintering at 1773 K in inert atmosphere. These ceramics demonstrated redox stability under mildly reducing conditions (p(O-2) similar to 10 Pa), as confirmed by XRD, thermogravimetry and electrical measurements. The electrical conductivity of Fe2.3Mg0.7O4 at this oxygen partial pressure is lower compared to magnetite, but it is still as high as 60 S/cm at 1073 K and 15 S/cm at room temperature. Cooling below 1473 K in air results in a drop of conductivity due to segregation of hematite phase at the grain boundaries. However, the phase separation is kinetically stagnated at 1073 K, and, after slight initial degradation, the retained electrical conductivity is more than 3 orders of magnitude higher compared to hematite and MgFe2O4 spinel. (C) 2013 Elsevier Ltd. All rights reserved