Mixed electronic and ionic conductivity of LaCo(M)O3 (M=Ga, Cr, Fe or Ni).: V. Oxygen permeability of Mg-doped La(Ga, Co)O3−δ perovskites

The LaGa_1−x−yCo_xMg_yO_3−δ solid solutions with rhombohedrally-distorted perovskite structure were ascertained to form in the concentration range of 0≤y≤0.10 at x=0.60 and 0≤y≤0.20 at x=0.35–0.40. Increasing cobalt content results in increasing electrical conductivity and thermal expansion of the perovskites. Thermal expansion coefficients of the LaGa_1−x−yCo_xMg_yO_3−δ ceramics were calculated from the dilatometric data to vary in the range of 12.4–19.8×10⁻⁶ K⁻¹ at 300–1100 K. Doping La(Ga,Co)O_3−δ solid solutions with magnesium leads to increasing oxygen nonstoichiometry, electronic and oxygen ionic conductivity. Oxygen permeation fluxes through LaGa_1−x−yCo_xMg_yO_3−δ membranes were found to be limited by the bulk ionic conduction and to increase with magnesium concentration, being essentially independent of cobalt content.     

Perovskite-like system (Sr,La)(Fe,Ga)O3−δ: structure and ionic transport under oxidizing conditions

The maximum solid solubility of gallium in the perovskite-type La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.40–0.80; y=0–0.60) was found to vary in the approximate range y=0.25–0.45, decreasing when x increases. Crystal lattice of the perovskite phases, formed in atmospheric air, was studied by X-ray diffraction (XRD) and neutron diffraction and identified as cubic. Doping with Ga results in increasing unit cell volume, while the thermal expansion and total conductivity of (La,Sr)(Fe,Ga)O_3−δ in air decrease with gallium additions. The average thermal expansion coefficients (TECs) are in the range (11.7–16.0)×10⁻⁶ K⁻¹ at 300–800 K and (19.3–26.7)×10⁻⁶ K⁻¹ at 800–1100 K. At oxygen partial pressures close to atmospheric air, the oxygen permeation fluxes through La_1−xSr_xFe_1−yGa_yO_3−δ (x=0.7–0.8; y=0.2–0.4) membranes are determined by the bulk ambipolar conductivity; the limiting effect of the oxygen surface exchange was found negligible. Decreasing strontium and gallium concentrations leads to a greater role of the exchange processes. As for many other perovskite systems, the oxygen ionic conductivity of La_1−xSr_xFe_1−yGa_yO_3−δ increases with strontium content up to x=0.70 and decreases on further doping, probably due to association of oxygen vacancies. Incorporation of moderate amounts of gallium into the B sublattice results in increasing structural disorder, higher ionic conductivity at temperatures below 1170 K, and lower activation energy for the ionic transport.     

Luminescence properties of β-(Ga1−xInx)2O3 solid solutions

In this work, luminescence properties of β-(Ga_1−xIn_x)₂O₃ solid solutions were investigated with the purpose of making the new thermoluminophors for ultraviolet (UV) dosimetry. The doping of aliovalent cation admixture (Mg, Mo) in the β-Ga₂O₃ ceramic sample brings about the appearance of high-temperature thermoluminescence glow peaks with a maximum at 395 and 435 K. The maximum of the thermoluminophor photosensitivity shifts when the composition of solid solution changes.     

Oxygen nonstoichiometry of Sr(Co,Fe)O3−δ-based perovskites: I. Coulometric titration of SrCo0.85Fe0.10Cr0.05O3−δ by the two-electrode technique

The p(O₂)–T–δ diagram of perovskite-type SrCo_0.85Fe_0.10Cr_0.05O_3−δ was determined by the coulometric titration technique in the temperature range 770–1250 K at oxygen partial pressures from 8 10⁻¹⁰ to 0.5 atm. Stability of the cubic perovskite phase of SrCo_0.85Fe_0.10Cr_0.05O_3−δ, existing down to the oxygen pressures of 10⁻³–10⁻⁵ atm, was found to be slightly higher than that of SrCo_0.80Fe_0.20O_3−δ, probably due to stabilization of oxygen octahedra neighboring Cr⁴⁺ cations. When the oxygen nonstoichiometry of the Cr-containing perovskite decreases from 0.47 to 0.38, the partial molar enthalpy and entropy for overall oxygen incorporation reaction vary in the ranges −165 to −60 kJ mol⁻¹ and 90 to 150 J mol⁻¹ K⁻¹, respectively. Within the stability limits of the single perovskite phase, the p(O₂)–T–δ diagram can be adequately described by equilibrium processes of oxygen incorporation, cobalt disproportionation and interaction of cobalt and iron cations, with the thermodynamic functions independent of defect concentrations. Increasing grain size in SrCo_0.85Fe_0.10Cr_0.05O_3−δ ceramics from submicron size to 100–200 μm has no effect on the oxygen thermodynamics. The two-electrode coulometric titration technique, based on the alternate use of electrodes for oxygen pumping and e.m.f. measurements, is described and verified by studying oxygen nonstoichiometry of La_0.3Sr_0.7CoO_3−δ and PrO_x.     

Defect chemistry and oxygen transport of (La0.6Sr0.4 − xMx)0.99Co0.2Fe0.8O3 − δ, M = Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2), Sr: Part I: Defect chemistry

This paper is the first part of a two part series, where the effects of varying the A-site dopant on the defect chemistry, the diffusion coefficient and the surface catalytic properties of the materials (La_0.6Sr_0.4 − xM_x)_0.99Co_0.2Fe_0.8O_3 − δ, M = Sr, Ca (x = 0.05, 0.1), Ba (x = 0.1, 0.2) (LSMFC) have been investigated. In part I, the findings on the defect chemistry are reported, while the transport properties are reported in part II. Substitution of Sr²⁺ ions with Ca²⁺ ions (smaller ionic radius) and Ba²⁺ ions (larger ionic radius) strains the crystal structure differently for each composition while keeping the average valence of the cations constant. The Ba²⁺ containing materials show the largest oxygen loss at elevated temperatures, while the purely Sr²⁺ doped material showed the smallest oxygen loss. This was reflected in the partial oxidation entropy of the materials. The measured oxygen loss was modelled with point defect chemistry models. Measurements at very low p_O2 showed several phase transitions.     

Intercalation thermodynamics and chemical diffusion of oxygen in the solid solution YBa2Cu3−xCoxO6+δ

The equilibrium oxygen content as a function of the temperature and oxygen pressure was measured for the solid solution YBa₂Cu_3−xCo_xO_6+δ, where x=0, 0.2, 0.4, 0.6, 0.8, by using coulometric titration in the temperature range 600–850°C and oxygen pressures between 10⁻⁵ and 1.0 atm. The change in the partial molar enthalpy and entropy of the intercalated oxygen was determined at different oxygen and cobalt contents. The oxygen chemical diffusion was studied by thermogravimetric relaxation in the oxygen-controlled atmosphere. The thermodynamic data were employed to determine how the chemical diffusion coefficient, the thermodynamic factor and the random-diffusion coefficient depend on oxygen content in specimens with different cobalt concentration. The oxygen intercalation thermodynamics and diffusivity results provide evidence of ordering phenomena on a microscopic scale in the basal plane of the tetragonal solid solution YBa₂Cu_3−xCo_xO_6+δ. A model for the oxygen diffusion is suggested to explain the large difference between the random and tracer diffusion coefficients in YBa₂Cu₃O_6+δ     

Oxygen tracer diffusion in La1 − xSrxCoO3

Tracer diffusion of ¹⁸O in dense, polycrystalline La_1−xSr_xCoO₃ for x = 0.1 has been measured in the temperature range 400 to 600 °C and at 500 °C for x = 0.2 at an oxygen partial pressure of 1 × 10⁵ Pa. Depth profiles were obtained by secondary ion mass spectrometry. The diffusion coefficient for La_0.9Sr_0.1CoO₃ is given by D = (17–247) exp[(−232 ± 8 kJ/mole)/RT] cm²/s. This value is several orders of magnitude lower than D extrapolated from the results for x = 0.2 measured in the 700–900 °C temperature range. One possible explanation for the discrepancy is that the two measurements reflect different diffusion paths. As expected, La_0.8Sr_0.2CoO₃ exhibits a higher diffusivity at 500 °C than does La_0.9Sr_0.1CoO₃.     

Current–voltage characteristic of BaTiO3−δ in its mixed n/p regime under oxygen potential gradients

Current (I)–voltage (V) characteristic under oxygen potential gradients was experimentally examined on single crystal BaTiO_3−δ in its mixed ion/electron/hole regime at 1000 °C. The variation of I vs. V appears similar to that of an n/p junction, but with the limiting slope (dI/dV) approaching the maximum and minimum possible equilibrium conductances in the given oxygen potential gradient as increasing forward and reverse bias, respectively. This characteristic has been precisely traced theoretically by using the partial ionic and electronic conductivities of BaTiO_3−δ as measured against uniform oxygen chemical potential in equilibrium state. The nonlinear characteristic is attributed to the redistribution of oxygen chemical potential that is caused by a non-vanishing gradient of the ionic transference number of the oxide under the given oxygen potential gradient. It is demonstrated that the bulk transport properties of a mixed conductor may be tailored by terminal voltage in a chemical potential gradient.     

Electrical properties of perovskite-type La(Cr1−xMnx)O3+δ

Perovskite-type La(Cr_1−xMn_x)O_3+δ (0.0x1.0) was synthesized using a sol–gel process. The crystal structure of La(Cr_1−xMn_x)O_3+δ changes from orthorhombic to rhombohedral at x=0.6. The Mn⁴⁺ ion content increases monotonically in the range 0.2x1.0. The magnetic measurement of La(Cr_1−xMn_x)O_3+δ indicates that a Mn³⁺ ion is a high-spin state with (d)³(dγ)¹. The variation of the average (Cr, Mn)-O distance is explained by ionic radii of the Cr³⁺, the Mn³⁺, the Mn⁴⁺ ions. Since the log σT–1/T curve is linear and the Seebeck coefficient (α) is independent of temperature, it is considered that La(Cr_1−xMn_x)O_3+δ is a p-type semiconductor and exhibits the hopping conductivity.     

Transport properties of SrCe0.95Y0.05O3−δ and its application for hydrogen separation

Transport properties of SrCe_0.95Y_0.05O_3−δ were studied by impedance spectroscopy and by measuring open-cell voltage (OCV) and gas permeation. Ionic transference numbers were determined by measuring the OCV of concentration cells and water vapor evolution of an O₂/H₂ fuel cell. We observed interfacial polarization on the basis of the I–V curves obtained by discharging a hydrogen concentration cell or an O₂/H₂ fuel cell. The observed high protonic conductivity (high proton and low oxide ion transference numbers) makes SrCe_0.95Y_0.05O_3−δ a potential material for hydrogen separation. From proton conductivity measurements, under a given hydrogen partial pressure difference of 4%/0.488%, the hydrogen permeation rate (of a dense membrane with 0.11 cm in thickness) was calculated to be ≈0.072 cm³ (STP) cm⁻² min⁻¹ at 800°C, whereas the permeation rate calculated from short-circuit current measurements was ≈0.023 cm³ (STP) cm⁻² min⁻¹ at 800°C. The difference between calculated and observed permeation rates is probably due to interfacial polarization.     

Oxygen diffusion and surface exchange in La1−xSrxFe0.8Cr0.2O3−δ (x=0.2, 0.4 and 0.6)

Oxygen tracer diffusion (D*) and surface exchange rate constant (k*) have been measured, using isotopic exchange and depth profiling by secondary ion mass spectrometry (SIMS), in La_1−xSr_xFe_0.8Cr_0.2O_3−δ (x=0.2, 0.4 and 0.6). Measurements were made as a function of temperature (700–1000 °C) and oxygen partial pressure (0.21–10⁻²¹ atm) in dry oxygen, water vapour and water vapour/hydrogen/nitrogen mixtures. At high oxygen activity, D* was found to increase with increasing temperature and Sr content. The activation energies for D* in air are 2.13 eV (x=0.2), 1.53 eV (x=0.4) and 1.21 eV (x=0.6). As the oxygen activity decreases, D* increases as expected qualitatively from the increase in oxygen vacancy concentration. Under strongly reducing conditions, the measured values of D* at 1000 °C range from 10⁻⁸ cm² s⁻¹ for x=0.2 to 10⁻⁷ cm² s⁻¹ for x=0.4 and 0.6. The activation energies determined at constant H₂O/H₂ ratio are 1.21 eV (x=0.2), 1.59 eV (x=0.4) and 0.82 eV (x=0.6).

The surface exchange rate constant of oxygen for the H₂O molecule is similar in magnitude to that for the O₂ molecule and both increase with increasing Sr concentration.     

Superprotonic solid solutions between CsHSO4 and CsH2PO4

Solid solutions of (CsHSO₄)_1 − x(CsH₂PO₄)_x (x = 0.25-0.75) were synthesized by mechanical milling method over a wide range of compositions. Superprotonic cubic phase was confirmed for all these samples between 293 and 420 K depending on its composition. These superprotonic phases have primitive cubic structure similar to that of CsH₂PO₄. The kinetic stability of the supercooled cubic phase depends both on the composition x and the humidity of surrounding atmosphere. The most stable composition of the cubic phase was found around x = 0.67 and could be maintained for several days even under ambient atmosphere. The ionic conductivities of these superprotonic phases reached 10^− 2–10^− 3 S∙cm^− 1 at 450 K. With increasing x the ionic conductivity at the superprotonic phase decreased continuously associated with the increase of the activation energy. These findings suggest that the average number of the hydrogen bonds between XO₄ (X = S, P) units plays an important role on the stability of the cubic phase and also on the conductivity.     

Germanium and iron co-substituted SrCoO2.5+δ as oxygen permeable membrane

Germanium and iron co-doped SrCoO_2.5+δ was investigated in terms of phase stability, oxygen permeability and electrical conductivity. The favorable high-temperature cubic structure of SrCoO_2.5+δ was stabilized to lower temperatures by co-doping Ge (10 mol%) and Fe (10 mol%) that substituted for Co, which however could not be achieved by doping Ge (20 mol%) alone. In contrast to SrCo_0.8Ge_0.2O_2.5+δ sample which showed a sharp decrease in oxygen permeability at temperature of 875 °C upon cooling, SrGe_0.1Co_0.8Fe_0.1O_3−δ sample remained well-permeable to oxygen at lower temperatures down to at least 820 °C; an abrupt change in electrical conductivity in SrCo_0.8Ge_0.2O_2.5+δ also occurred accompanying the phase transition. The oxygen permeation flux for SrGe_0.1Co_0.8Fe_0.1O_3−δ increased significantly with the decrease of the membrane thickness, indicating the transport of oxygen ions in the bulk of the membrane as the rate-limiting step.     

Electrochemical behaviour of (La1 − xSrx)sCo1 − yNiyO3 − δ as porous SOFC cathodes

This paper shows that storage at room temperature and ambient atmosphere appears to affect the polarisation resistance significantly and R_P decreased initially with storage time. This improvement was more pronounced when stored in moisturised air. The performance was, however, also found to decrease when stored for sufficiently long periods. The absolute values of the electrode response of these materials are very difficult to reproduce and the performance appears to be largely dependent on the manufacturing process of the powder or the electrode itself. In spite of this, the electrode behaviours exhibited similar patterns with respect to the dependence on T and P_O2. An increase in polarisation resistance with time at SOFC operating conditions was observed, which was related exclusively to the electrode reaction kinetics and not to oxygen concentration polarisation. It was also found to be higher when measured in moisturised air.     

Surface reaction kinetics in oxygen nonstoichiometry re-equilibration of BaTiO3−δ

We report the (bare) surface redox-reaction rate constant that was determined, along with the chemical diffusivity , by a conductivity relaxation technique on Al-doped single crystal and undoped polycrystal BaTiO_3−δ as a function of oxygen activity in its range of −16≤log a_O2≤0 at elevated temperatures of 800–1100 °C. It takes a value in the range of −4<log( /cm s⁻¹)≤−1, which is even larger than that of the oxides that are considered best as oxygen membranes. It has been found that the surface reaction step grows more rate controlling as the electronic transference number gets smaller or the electronic stoichiometric composition (δ≈0) is approached. The oxygen potential drop due to the surface reaction was estimated by an oxygen concentration cell technique. The oxygen potential drop grows larger as the stoichiometric composition is approached, that is in accord with the variation of against oxygen activity.     

Transport properties and defect analysis of La1.9Sr0.1NiO4 + δ

The oxygen flux through La_1.9Sr_0.1NiO_4 + δ has been measured as a function of oxygen activity gradient and temperature (750–1000 °C). The oxygen nonstoichiometry was determined by thermogravimetry in the temperature range of 400–1000 °C and oxygen partial pressures of 0.0002–1 atm. The total conductivity was measured over a similar range of conditions. The oxide ion partial conductivity derived from the oxygen flux data is approximately 4 orders of magnitude lower than the total, mainly p-type electronic conductivity. The defect structure was derived based on the data. Combining the oxygen flux and oxygen nonstoichiometry, the self diffusion coefficient of oxygen interstitials was evaluated.     

Peak effect and oxygen ordering in YBa2Cu3O7−δ single crystals

We have investigated the second magnetization peak in pure YBa₂Cu₃O_7−δ single crystals with various oxygen contents (6.91<7−δ<6.97) and degrees of oxygen vacancy ordering, as achieved by low (1 bar) and high (100 bar) oxygen pressure annealing. Although the position of the peak changes drastically with oxygen stoichiometry, no dependence on the distribution of oxygen vacancies has been found for temperatures below 70 K. For T>70 K, however, ordering effects become important as demonstrated by the disappearance of the peak for the high pressure annealed samples. These results suggest that while at low temperatures, pinning of the vortex system by clusters or a more homogeneous distribution of oxygen vacancies is similar, at elevated T, the former are much stronger pinning sites leading to larger hysteresis and the presence of the peak.     

Oxygen nonstoichiometry and electrical conductivity of the solid solutions La2−xSrxNiOy (0≤x≤0.5)

Oxygen nonstoichiometry and electrical resistance of a series La_2−xSr_xNiO_y solid solutions, where x=0.0, 0.2 and 0.5 in argon flows at oxygen partial pressures 1.5, 10.2, 49.2, 100 and 286 Pa within the temperature range of 20–1050°C were studied. Nickelate oxygen desorption/sorption spectra when heating–cooling at constant rate demonstrated strong dependence of cation composition of the samples. Unlike La_1.5Sr_0.5NiO_y compounds those of La₂NiO_y and La_1.8Sr_0.2NiO_y have weakly bonded oxygen, capable to exchange reversibly with the gas phase at the temperatures higher than 250°C. The equilibrium values of oxygen nonstoichiometry and specific resistance for the these nickelates were determined at 300–1050°C and p_O2=1.5–286 Pa as a functions of temperature versus oxygen partial pressure. All nickelate studied appear to be p-type conductors with metal electric conductivity at equilibrium states.