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Doped calcium manganite perovskites CaMnO 3− δ which constitute some of the most promising materials for all-oxide thermoelectric generators for conversion of heat to electricity operating in air over a wide temperature range have been modified by Sr–Pb–Ho co-doping. The Ca 0.5 Sr 0.45− x Pb 0.05 Ho x MnO 3− δ samples (x = 0.05, 0.10, 0.15 and 0.20) synthesized by the citrate-nitrate method of precursor preparation are shown to have an orthorhombic crystal structure (space group Pbnm) at room temperature. The relationship between temperature, concentration of donor ions, Jahn-Teller distortions and energy parameters of carrier formation and mobility that affect the thermoelectric properties of electron-doped manganites has been studied. The temperature-activated nature of the electrical conductivity (σ) was revealed to be consistent with the adiabatic mechanism of small-radius polaron transport. The increase in the absolute values of the Seebeck coefficient (S) with rising temperature and the decrease in σ above 500 K are attributed to the decrease in the concentration of Mn3+ ions due to their disproportionation into Mn2+ and Mn4+ ions. The equilibrium constants of the disproportionation reaction, carrier concentration and mobility have been derived from the temperature dependencies of S (T) and σ (T) in the conditions when δ ≈ 0. The best values of power factor (S 2 σ) and dimensionless thermoelectric figure-of-merit (ZT = S 2 σT / κ) have been found at the substitution level x = 0.15. [ABSTRACT FROM AUTHOR]

Magnesium orthovanadate Mg 3 V 2 O 8 provides an excellent model for studying the transport of magnesium cations in a three-dimensional matrix with two types of Mg(1)O 6 and Mg(2)O 6 octahedra connected by edges and chains of VO 4 tetrahedra isolated from each other. In the present paper, the magnesium cations have been found to be the ionic charge carriers in this system using the Tubandt method. The electrical conductivity (σ) has been studied by the impedance spectroscopy in the range 770–1270 K. The activation energy of σ (E σ) has been demonstrated to be 1.25 eV in the range 923–1023 K. For the first time, the 51V NMR spectra have been obtained and analysed in the range 295‒900 K. The activation energy (E NMR) for the diffusive jumps of Mg2+ cations has been identified to be 1.03 eV by analyzing the temperature dependence of the 51V spin-lattice relaxation. The smaller E NMR value is due to the rapid movement of magnesium along the chains of Mg(2)O 6 octahedra. Large E σ value and low ionic conductivity indicate that the limiting step in Mg 3 V 2 O 8 is the cation hopping between the chains of Mg(2)O 6 octahedra through the intermediate Mg(1) positions. [Display omitted] [ABSTRACT FROM AUTHOR]

The tubular ceramic membrane of La 0.5 Sr 0.5 FeO 3– δ was tested in a condition of simultaneous partial oxidation of methane (POM), occurring in space adjacent to the outer surface of the membrane, and water splitting (WS) accompanied by hydrogen generation taking place in the internal membrane space. It was shown that a membrane with a surface area of more than 10 cm2 could successfully operate without carrier gases. A detected effect of CO selectivity in POM on the thermal condition of the process was another result using a membrane of notable size. The heat power released/absorbed on the membrane, associated with partial oxidation of methane and water splitting, was calculated using experimental data. The absorbed heat power of the overall process was found to be sensitive to CO selectivity, especially if selectivity exceeded 80%. The combined POM and WS process with the hydrogen production rate of ∼2 mL min−1cm−2 during 300 h with stable methane conversion of about 99% and CO selectivity around 96% was successfully carried out. • Hydrogen production by water splitting on the tubular ceramic membrane was performed. • Partial oxidation of methane was the bound process on the other side of the membrane. • No carrier gases were used owing to sufficient membrane surface area (11.3 cm2). • Effect of selectivity variation on the thermal condition of the membrane was recorded. [ABSTRACT FROM AUTHOR]

An attempt of fluorine and chlorine substitution for oxygen in strontium ferrite SrFeO 3– δ was undertaken in search of a new means of influence on the oxide properties. An uncontrollable removal of halogen-ions from oxides during synthesis was detected. Nevertheless, residual halogens were found to provide a notable impact on ion conductivity and oxide stability under reducing conditions. More than twice higher ion conductivity relative to SrFeO 3– δ was recorded in materials with nominal compositions SrFeO 2.8– δ F 0.2 , SrFeO 2.6– δ Cl 0.4 , and SrFeO 2.4– δ Cl 0.6 . The strong effect of halogen addition on ceramic properties of oxides was revealed. While SrFeO 3– δ was known for its poor sinterability, adding 0.2 mol of fluorine or chlorine during the synthesis of SrFeO 3– δ was shown to result in successful obtaining gas-tight ceramics. [ABSTRACT FROM AUTHOR]

The Sr 0.75 Ca 0.25 Mn 1− x V x O 3− δ (x = 0.05 and 0.10) oxides with a cubic perovskite-type structure (space group P m 3 ‾ m) have been synthesized via citrate-nitrate route in air. The electron donor vanadium (V5+) strongly affects on electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (κ) and thermoelectrical characteristics in the range of 340–770 K, in which the oxygen content does not change with temperature (T). Temperature activated character of conductivity is consistent with the adiabatic small polaron transport mechanism. The higher n-type electrical conductivity in Sr 0.75 Ca 0.25 Mn 1−x V x O 3− δ than in other cubic SrMnO 3 - based oxides is explained by the decrease in the distortion of MnO 6 octahedra. The metallic-like increase of the absolute S values with T is explained by the decrease in the concentration of Mn3+ ions due to their disproportionation to Mn2+ and Mn4+ ions. The temperature dependent variations of the charge disproportionation equilibrium constant, concentration and mobility of charge carries are derived from the experimental data for S (T) and σ (T). The formation of Mn3+ ions as a result of the partial removal of the oxygen from the samples influences on the thermoelectrical characteristics at high temperatures. The highest values of dimensionless figure-of-merit are achieved in the composition Sr 0.75 Ca 0.25 Mn 0.95 V 0.05 O 3− δ. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The conductivity is measured in the series of solid solutions SrFe1− x Nb x O3− δ , where x =0.05, 0.1, 0.2, 0.3, 0.4, within the oxygen partial pressure limits 10−18–0.5atm and temperature range 650–950°C. The contributions to the total conductivity from oxygen ions, electrons and electron holes are obtained based on their different pressure dependences. The doped derivative with x =0.1 is found to be a singular composition where ion conductivity attains a maximal value while activation energy for ion transport is minimal. This peculiar behavior is attributed to formation of favorable microstructure in the oxide. The deeper doping results in deterioration of ion transport, which is explained by oxygen vacancy filling. It is shown that replacement of iron for niobium favors enhanced thermodynamic stability towards reduction. The oxygen permeability is evaluated from the conductivity data, and it achieves rather high values in the doped derivatives. These oxides can, therefore, be recommended for further evaluation as oxygen separating membrane materials for partial oxidation of natural gas. [Copyright &y& Elsevier]