Aleksandra Mielewczyk-Gryń

Influence of Sb-substitution on ionic transport in lanthanum orthoniobates

The results of ionic transport measurements for the lanthanum orthoniobate substituted with 10 and 30 mol% of antimony (LaNb0.9Sb0.1O4 and LaNb0.7Sb0.3O4) are presented and discussed. The influence of calcium co-doping on these properties has also been analysed. It has been shown that for the investigated material protonic conductivity predominates at temperatures up to 800 °C in oxidizing atmospheres under wet conditions. The maximum observed protonic conductivity reaches ∼10−4 S cm−1 at 800 °C (in humidified air); under dry conditions, the increasing influence of oxygen vacancies and holes is detected. Oxygen self-diffusivity has also been analysed by isotopic exchange to investigate the possible diffusion paths.

Characterization of CaTi0.9Fe0.1O3/La0.98Mg0.02NbO4 composite

It has been recently shown that suitable asymmetric microstructures can be used to achieve microorganism separation. Here we study numerically how the separation process depends on the specific motility strategies of the microorganisms involved. Crucial properties such as the separation efficiency and the separation time for two bacterial strains are precisely defined and evaluated. In particular, the sorting of two bacterial populations inoculated in a box consisting of a series of chambers separated by columns of asymmetric obstacles is investigated and compared to that occurring in a box free of geometrical constraints.A composite of CaTi0.9Fe0.1O3 and electrolyte material, i.e. magnesium doped La0.98Mg0.02NbO4 was prepared and studied. The phase content and the sample microstructure was examined by an X-ray diffraction method and scanning electron microscopy. EDS measurements were done both for composite samples and the diffusion couple. The electrical properties were studied by four terminal DC method. The high-temperature interaction between the two components of the composite has been observed. It has been suggested that lanthanum diffused into the perovskite phase and substituted for calcium whereas calcium and niobium formed the Ca2Nb2O7 pyrochlore phase. At 1500°C very large crystallites of the pyrochlore were observed. Regardless of strong interaction between the composite components, its total conductivity was weakly dependent on the sintering temperature.

Perovskites in Solid Oxide Fuel Cells

Perovskite oxides comprise large families among the structures of oxide compounds, and several perovskite-related structures are also known. Because of their diversity in chemical composition, properties and high chemical stability, perovskite oxides are widely used for preparing solid oxide fuel cell (SOFC) components. In this work a few examples of perovskite cathode and anode materials and their necessary modifications were shortly reviewed. In particular, nickel-substituted lanthanum ferrite and iron-substituted strontium titanate as cathode materials as well as niobium-doped strontium titanate, as anode material, are described. Electrodes based on the modified perovskite oxides are very promising SOFC components.

Thermochemistry of rare earth perovskites Na3xRE0.67−xTiO3 (RE = La, Ce)

Abstract High-temperature oxide melt solution calorimetry using sodium molybdate (3Na2O·4MoO3) solvent at 973 K was performed for the Na3xRE0.67–xTiO3 (RE = La, Ce) perovskite series. The enthalpies of formation of lanthanum perovskites from oxides (La2O3, Na2O, TiO2), are –107.25 ± 2.56, –93.83 ± 6.06, –80.68 ± 5.93, and –33.49 ± 4.26 kJ/mol and enthalpies of formation from elements are –1614.05 ± 5.37, –1596.44 ± 7.68, –1594.03 ± 7.58, and –1577.56 ± 6.36 kJ/mol for Na0.459La0.522Ti0.999O3, Na0.454La0.523Ti0.994O3, Na0.380La0.567Ti0.980O3, and La0.692Ti0.979O3, respectively. The enthalpies of formation of cerium perovskites are –99.98 ± 5.78 and –45.78 ± 3.30 kJ/mol from oxides (Ce2O3, Na2O, TiO2), and –1611.34 ± 6.90 and –1602.06 ± 2.72 kJ/mol from elements for Na0.442Ce0.547Ti0.980O3 and Ce0.72Ti0.96O3. The A-site defect perovskites become more stable relative to oxide components as sodium contents increase. Na0.5Ce0.5TiO3 and Na0.5La0.5TiO3 could be considered as thermodynamically stable end-members in natural loparite minerals, in which these end-members are in solid solution with CaTiO3 and other components.

Tailoring structural properties of lanthanum orthoniobates through an isovalent substitution on the Nb-site

A tetragonal polymorph of lanthanum orthoniobate can be stabilized to room temperature by the substitution of Nb with an isovalent element. LaNb1−xAsxO4 (0 < x ≤ 0.3), where As is an element stabilizing the tetragonal structure, were successfully synthesized using a combined co-precipitation and solid-state reaction method. The phase transition temperature, above which the material has a tetragonal structure, decreases linearly with increasing As content, and LaNb0.7As0.3O4 is tetragonal at room temperature. The analysis of the influence of different isovalent substituents, namely V, Sb and Ta, has shown that there is a relation between the properties of the chemical element and its effect on the structure. It was proposed that the electronegativity of the substituent determines the type of stabilization – the tetragonal/monoclinic structure is stabilized by chemical elements with electronegativity higher/lower than that of niobium. The slope of the phase transition temperature dependence on the substituent content has been proposed as a parameter determining the “quality” of the stabilization, since a steeper function leads to a larger decrease of transition temperature for the same content of different substituents. The analysis has shown that apart from the electronegativity, the stabilization of the tetragonal structure depends also on the ionic radius of a substituent. Arsenic has been found to be a better stabilizer of the tetragonal polymorph of lanthanum orthoniobate than Sb, but worse than V.