Oxide ion conductors of the BIMEVOX family: Synthesis, structure, and conductivity

The synthesis, stable existence regions, structure, and migration characteristics of complex oxides of the BIMEVOX family were studied experimentally and theoretically. The temperature, composition, and oxygen partial pressure dependences of the conductivity of the samples obtained were determined. The contribution of oxide ion transport decreased as the temperature and oxygen partial pressure lowered. Quantum-chemical calculations of β-Bi₄V₂O₁₁ and γ-Bi₄V₂O₁₁ were performed to analyze the relative stabilities of the phases and the influence of dopants.     

Synthesis, structure, and conductivity of BINBVOX ceramics

The preparation and the structure and transport characteristics of Bi₄V_{2 ? x} Nb _x O₁₁ (BINBVOX) were studied. A comparative analysis of the synthesis of solid solutions was performed. The sintering of ceramics and the electrical conductivity as a function of temperature, composition and partial oxygen pressure were studied.     

Synthesis,region of existence,structural characteristics,and conductivity of BI(CR,FE)VOX solid solutions

The results of our studies of solid solutions of the general composition Bi₄V_{2 − x} Fe _x/2Cr _x/2O_{11 − δ} are presented. The crystal-chemical parameters of different polymorphic modifications of BIMEVOX were determined. The particle size distribution was determined by laser diffraction and optical microscopy. The surface of sintered preforms was studied by scanning electron microscopy. The conductivity of sintered poly-crystalline samples as a function of temperature and composition was studied by impedance spectroscopy. The conductivity was depends on the procedure for the synthesis of solid solutions. The most promising compositions were revealed.     

Oxide-ion conductivity of the oxygen deficient perovskite solid-solution system, (Ba0.5-xSrxLa0.5)2(In1-yMy)2O5.5 (M=Y or Ga)

In order to elucidate the mechanism of oxide-ion conductivity for (Ba_0.5-xSr_xLa_0.5)(In_1-yM_y)₂O_5.5 (M=Y or Ga, 0<x<0.2, 0<y<0.2) solid-solution system, the electrical conductivity was measured as a function of oxygen partial pressure and temperature, and the results were investigated in terms of a dopant content and unit cell free volume. The system was confirmed to be an oxide-ion conductor from the oxygen partial pressure dependence on electrical conductivity. The oxide-ion conductivity increased with increasing the unit cell free volume at first. However, it showed a maximum at a value of free volume, and then decreased. The decreasing conductivity vs. the volume would be related to the crystal symmetry change. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetics of the oxygen transfer reaction at the (U0.5Sc0.5)O2±x/YSZ interface by impedance spectroscopy

The complex impedance dispersion analysis technique was used to study the electrode kinetics of (U_0.5Sc_0.5)O_2±x, a fluorite type solid solution material potentially suitable as electrode for low temperature oxygen sensors. Variables included the temperature and oxygen partial pressure. The effect of heat treatment on the interfacial contact resistance and the electrode morphology was also investigated. A single are for the electrode reaction was observed over most of the experimental ranges of temperature and oxygen partial pressure. The angle of depression of the electrode are was small (8–18°) compared with platinum electrodes (20–45°). The activation energy for the overall electrode reaction was between 170 and 180 kJ mol^?1. The average value for the pressure exponent, determined from the oxygen partial pressure dependence of the electrode resistance, was 0.16. A mechanism for the oxygen transfer reaction is proposed. Materials of this type show promise for future use in low temperature oxygen sensors.     

Pr<Subscript>5</Subscript>Mo<Subscript>3</Subscript>O<Subscript>16 + δ</Subscript>: A New Anode Material for Solid Oxide Fuel Cells

The thermomechanical and electrical conductivity properties of praseodymium molybdate Pr₅Mo₃O_{16 + δ} prepared by a solid-phase method were studied. The electrical conductivity of praseodymium molybdate samples measured at temperatures in the range 373–1173 K with the oxygen partial pressure in the gas of 10^–3 to 0.21 atm was found to increase from ~10^–7 to ~10^–2 S/cm and to be almost independent of oxygen pressure. It is for the first time that electrical conductivity a reductive atmosphere (Ar/H₂ 5%) was found to increase from 0.1 to 1.2 S/cm in the same temperature range. Studies of the chemical stability of Pr₅Mo₃O_{16 + δ} with respect to solid electrolytes showed the absence of chemical reactions with GDC at 1273 K and with YSZ at 1223 K. The combination of these properties evidences for the potential of praseodymium molybdate for use as an anode material for solid oxide fuel cells (SOFCs).     

Nombre de transport et conductivité cationique dans l'oxyde de nickel: NiO

Ionic transport number measurements have been made on single crystals of NiO over the temperature range 900–1300°C and for oxygen partial pressures varying from 10^?8 to 1 atm. At 1000°C and in air, t_Ni ～ 2 × 10^?7. The variation in cationic conductivity as a function of oxygen partial pressure suggests that V′_Ni is the dominant defect at high temperature and low oxygen pressure and that V″_Ni is the dominant defect at low temperature and high oxygen pressure.     

Defect chemistry and physical properties of Ln0.5Sr0.5FeO3 (Ln: La,Pr)

A defect chemical model for the behavior of acceptor and donor-doped LaFeO₃ as a function of oxygen pressure is proposed. The nonstoichiometric deviation is calculated as a function of oxygen partial pressure, pO₂, at different temperatures. The defect structure of complex oxides is described within the point defects approach.The results show that the conductivity is dependent on pO₂ and temperature. Thermoelectric power values indicate clearly the nature of the dominant specie in charge transport. Stability regimes and compensation mechanisms at various oxygen partial pressures and temperatures are proposed. This model also examines the charge compensation mechanisms that dominate under the different regimes.From equilibrium constants, partial molar enthalpy and partial molar entropy of oxygen in Ln_0.5Sr_0.5Fe O_3?δ (Ln: La, Pr) are estimated.     

Transport and Electrochemical Properties of SrFe(Al,Mo)O<Subscript>3–δ</Subscript>

In this work, effects of molybdenum doping on the crystal structure, stability, electrical conductivity, oxygen permeability and thermomechanical properties of Sr(Fe,Al)O_3–δ-based perovskites, were studied. The electrochemical performance of model anodes of solid oxide fuel cells (SOFCs), made of SrFe_0.7Mo_0.3O_3–δ, was assessed. Whilst the introduction of Mo cations improves structural stability with respect to the oxygen vacancy ordering processes, excessive molybdenum content leads to a worse phase and mechanical stability under oxidizing conditions. Mo-doping was shown to decrease the thermal and chemical expansivity, to reduce p-type electronic conductivity and to increase n-type electronic conduction. The oxygen permeation fluxes through gas-tight Sr_0.97Fe_0.75Al_0.2Mo_0.05O_3–δ membranes are determined by both the bulk oxygen diffusion and surface exchange kinetics. The role of the latter factor increases on decreasing temperature and reducing oxygen partial pressure. Due to a relatively high electrical conductivity and moderate thermal expansion coefficients in reducing conditions, SrFe_0.7Mo_0.3O_3–δ-based anodes show a substantially high electrochemical activity.     

A study of TiO system between Ti3O5 and TiO2 at high temperature by means of electrical resistivity

The TiO system between the compositions TiO₂ and Ti₃O₅ has been studied in following the electrical conductivity against the oxygen partial pressure. Several features are discussed : the dependence of electrical conductivity versus oxygen pressure, the kinetics of approach to equilibrium, and reversibility during oxidation and reduction paths. The results suggest an important contribution of point defects for small departures from the composition TiO₂ at high temperature.     

Thermodynamic and Transport Properties of Two-temperature Oxygen Plasmas

Thermodynamic and transport properties of two-temperature oxygen plasmas are presented. Variation of species densities, mass densities, specific heat, enthalpy, viscosity, thermal conductivity, collision frequency and electrical conductivity as a function of temperature, pressure and different degree of temperature non-equilibrium are computed. Reactional, electronic and heavy particle components of the total thermal conductivity are discussed. To meet practical needs of fluid-dynamic simulations, temperatures included in the computation range from 300 K to 45,000 K, the ratio of electron temperature (T _e) to the heavy particle temperature (T _h) ranges from 1 to 30 and the pressure ranges from 0.1 to 7 atmospheres. Results obtained for thermodynamic equilibrium (T _e = T _h) under atmospheric pressure are compared with published results obtained for similar conditions. Observed overall agreement is reasonable. Slight deviations in some properties may be attributed to the values used for collision integral data and for the two temperature formulations used. An approach for computing properties under chemical non-equilibrium and associated deviations from two-temperature results under similar conditions are discussed.     

The diffusion of oxygen and ion transport in lanthanum cobaltite LaCoO<Subscript>3-δ</Subscript>

The chemical diffusion coefficient of oxygen vacancies and oxygen ion conductivity in lanthanum cobaltite LaCoO₃ were determined by the polarization method as functions of oxygen partial pressure \(p_{O_2 } \) (atm) and temperature T(K) over the ranges ?4 ≤ log \(p_{O_2 } \) ≤ 0 and 1173 K ≤ T ≤ 1323 K. The mobilities (cm²/(V s)) of oxygen vacancies calculated over the temperature range studied satisfy the inequalities 1.8 × 10^?5 ≤ \(v_{v_0 } \) ≤ 3.4 × 10^?5. The transfer numbers of oxygen vacancies were calculated. These numbers change depending on oxygen partial pressure over the range 5 × 10^?7 ≤ t ₀ ≤ 1 × 10^?5. The activation energy of self-diffusion of oxygen vacancies was found to be E _a= 104 ± 10 kJ/mol (1.1 ± 0.1 eV).     

Synthesis and electrical properties of dense Bi2Al4O9

Bi₂Al₄O₉ ceramics are difficult to sinter to greater than 80% theoretical density due to peritectic decomposition at 1,070 °C. A novel processing method is discussed where a high-bismuth oxide-based liquid is used as a sintering aid. After sintering, the high bismuth oxide phase is removed by leaching with 40% acetic acid. The resulting samples are phase pure and ∼91% dense. The grain size varies in a wide range with the average grain size of ∼1 μm. The electrical properties of these ceramics were measured as functions of temperature (550–850 °C) and oxygen partial pressure (6×10⁻⁶–1 atm). The total conductivity was separated into electronic and ionic contributions. The low ionic conductivity indicates that the material is not an ‘intrinsically defective fast ion conductor’. The ionic conductivity is due almost exclusively to compensating oxygen vacancies related to impurities. With increasing temperature and decreasing oxygen partial pressure, the electronic conduction dominates over the ionic conduction.     

Electric properties of oxyfluorides Ba2In2O5−0.5x F x with brownmillerite structure

Synthesis of fluoro-substituted substances based on brownmillerite Ba₂In₂O₅ is carried out. The width of the homogeneity region of the Ba₂In₂O_5?0.5x F _x (0 < x ≤ 0.25) solid solution was established using X-ray analysis. Measurement of temperature dependences of conductivity in atmospheres with different partial pressure of water vapor (pH₂O = 3.3 and 2 × 10³ Pa) showed an increase in conductivity at T ≤ 550°C in a humid atmosphere, which is due to appearance of proton transport. The dependence of conductivity on partial oxygen pressure (pO₂ = 0.21 × 10⁵ to 10^?15 Pa) is studied in the temperature range of 500–1000°C; ion transport numbers are calculated. The method of polarization measurements was used to determine transport numbers of fluoride. Total conductivity is divided into ion (proton, oxygen, and fluoride ion) and electron components. Analysis of concentration dependences of conductivities showed that low concentrations of fluoride allow increasing both the total and partial conductivities (oxygen-ion and proton) and, besides, allow shifting the “order-disorder” phase transition by 100°C to the low temperature range.     

Structure and conductivity in CeNb3O9-δ

Abstract  

In the search of new materials for solid oxide fuel cells, a study of the structure and electrical conductivity of Mg-doped and nominally pure CeNb₃O_9-δ was undertaken. This material exhibits an orthorhombic crystal structure as determined by Rietveld refinement. Through a combined study of 4-point DC and AC impedance spectroscopy, it was determined that the material presents oxygen ion conductivity, electron conductivity and electron-hole conductivity according to the partial pressure of oxygen and temperature in agreement with a simple defect chemistry model. Finally, some experiments seem to indicate the presence of proton conduction.     

The oxygen nonstoichiometry,defect structure,and thermodynamic characteristics of disordering of nickel-and iron-substituted lanthanum cobaltites

The oxygen nonstoichiometry of nickel-and iron-substituted lanthanum cobaltites of the compositions LaCo_1?x Ni _x O_3?δ (x = 0.1, 0.3) and LaCo_0.9Fe_0.1O_3?δ was studied by high-temperature thermogravimetric analysis over the temperature and oxygen partial pressure ranges 1223–1473 K and 10^?3–0.21 atm. The partial replacement of cobalt with nickel (an acceptor impurity) in lanthanum cobaltite was found to increase the number of defects in the oxygen sublattice, whereas the replacement with iron (a donor impurity) decreased this number. Correlations between the experimental \(\log p_{O_2 } = f(\delta )\) dependences and the suggested models of formation of point defects were analyzed taking into account the formation of Schottky defects. Interrelation between the defect structure, partial molar thermodynamic characteristics of oxygen release from the crystal lattices of the oxides studied, and the nature of substituting impurities (Ni and Fe) in lanthanum cobaltite was demonstrated.     

Synthesis,Crystal Structure Determination,and Physical Properties of Ag5IO6

The silver iodate(VII), Ag₅IO₆, was obtained by reacting a stoichiometric mixture of Ag₂O and KIO₃, at elevated oxygen pressure, adding a small portion of distilled water. The synthesis was done at 673 K and 270 MPa of oxygen pressure. The crystal structure was solved by direct methods based on single crystal diffraction data ( , Z = 6, a = 5.9366(1), c = 32.1471(6) Å, 323 independent reflections, R₁ = 2.31 %). According to conductivity measurements, Ag₅IO₆ is semiconducting with a specific resistance of 0.08 Ωcm at 300 K. The activation energy was determined as 7.4(1) meV in the temperature range of 220 – 300 K, and 4.3(1) meV in the temperature range of 90 – 180 K. The optical band gap for Ag₅IO₆ is 1.4 eV. Ag₅IO₆ is diamagnetic with a magnetic susceptibility of ?4.4×10^?4 emu/mol.     

Electrical properties of Ba2(In1 − x Al x )2O5 solid solutions

The electric conductivity of perovskite-like Ba₂(In_{1 ? x} Al _x )₂O₅ solid solutions (0 < x ≤ 0.20) characterized by structural disordering in the oxygen sublattice was studied as a function of temperature and partial pressure of oxygen in an atmosphere with a low content of water vapors ( $p_{H_2 O}$ = 3 × 10^?5 atm). When In³⁺ was partially replaced by Al³⁺, the oxygen ion conductivity increased because of the disordering of oxygen structural vacancies, leading to a significant increase in the total electric conductivity of the samples.     

Relationship between the ionic and electronic partial conductivities of co-doped LSGM ceramics from oxygen partial pressure dependence of the total conductivity

La_0.8Sr_0.2Ga_0.85-x Mg_0.15Co _x O_3±δ-materials (further cobalt-doped LSGM), where x varied from 0 to 0.20, were synthesized by means of the conventional powder route. The total conductivity of the La_0.8Sr_0.2Ga_0.85-x Mg_0.15Co _x O_3±δ samples was measured as a function of temperature (400–900 °C) and oxygen partial pressure by means of the impedance technique. The values of the oxygen ionic and the hole conductivities were determined from non-linear regression of the oxygen partial pressure dependence of the total conductivity. It was shown that the substitution of gallium by cobalt in the LSGM results in increasing either the oxygen ionic or the hole conductivity, although the increase of the hole conductivity due to the doping by cobalt is more significant than the increase of the oxygen ionic conductivity. The hole conductivity of the selected compositions was studied by oxygen permeation- and Hebb–Wagner-polarization measurements.     

Phase transitions, electrical conductivity and chemical stability of BiFeO3 at high temperatures

The multiferroic perovskite BiFeO₃ is reported to display two first order structural phase transitions. The structural phase transition at 925±5 °C is demonstrated to be first order by calorimetry and dilatometry. Electrical conductivity measurements revealed that the high temperature phase above 925±5 °C is semiconducting, in disagreement with recent reports. The sign and magnitude of the volumes of transition reflect the sign and magnitude of the discontinuities in electrical conductivity across the two first order phase transitions. A high partial pressure of oxygen was demonstrated to stabilise BiFeO₃ towards peritectic decomposition. Finally, the origins of the commonly observed decomposition of BiFeO₃ at high temperatures are discussed.