Синтез нового катодного материала для твердооксидных топливных элементов на основе кобальтита лантана стронция и исследование зависимости термодинамических функций кислородного обмена от нестехиометрии

The mechanical endurance of brittle components under operating conditions is extremely important to ensure the reliability of Solid Oxide Fuel Cell (SOFC). To prevent a decline in SOFC performance due to mechanical damages, it is essential to deal with the mechanical design which was considered the mechanical properties changing under the operating conditions. Electrochemical properties of the oxygen-ion conducting ceramics were revealed to be affected by not only the temperature but also the oxygen partial pressure. On the other hand, because of the limitation of technique, the mechanical properties were still unclear, have been revealed only under the air. Our group therefore developed the mechanical testing machine under the temperature and gas controlled conditions [1]. In this study, oxygen partial pressure dependence of fracture strength and the Young’s modulus of ceria doped with 10 mol.% Gadolinia and 10, 20 and 30 mol.% Yttria were investigated by means of small punch testing method under controlled the temperature (1073 K) and the oxygen partial pressure (log(p(O2)/atm) = -1.1, 17.4, -21.4, -22.0). Figure 1 shows the oxygen partial pressure dependence of (a) Young’s modulus and (b) fracture strength for 10GDC at 1073 K. For purpose of comparison, stoichiometric oxide for usual SOFC components, 8YSZ was also shown in Fig. 1. Regardless of p(O2), the Young’s modulus and fracture strength of 8YSZ at 1073 K remained almost constant, and hence there is no dependence of mechanical properties on the oxygen partial pressure. On the other hand, for 10GDC, high temperature mechanical properties were affected by the oxygen partial pressure, namely the oxygen nonstoichiometry. At log(p(O2)/atm) = -21.4, the young’s modulus decreased E SP =162.0 ± 4.1 to 127.7 ± 4.0 GPa, approximately 22 % degradation. This behavior is in good agreement with the by Amezawa et al[2].Furthermore, the fracture strength were increased monotonically with the decreasing the oxygen partial pressure. Under the low oxygen partial pressure, oxygen vacancies which were induced by the oxygen nonstoichiometry and the weakening bonding strength resulting from the chemical expansion can have a significant effect on the mechanical properties, however, this results suggested the presence of improving mechanisms and it exceeds the factors listed above. The oxygen partial pressure dependence of (a) the Young’s modulus of 10, 20 and 30YDC at 1073 K is shown in figure 2. The Young’s modulus under the low oxygen partial pressure decreased monotonically. Regardless of the amount of Y dopant, there is a similar decreasing tendency, and therefore changes in the Young’s modulus of doped ceria seems to depends on the generated oxygen vacancies, not initial ones introduced by doping the rare-earth oxides. Figure 3 shows normalized fracture strength of 10GDC, 10 and 20YDC vs relative reduction expansion of the lattice constant. The relative expansion values of the lattice constant for the rare-earth doped ceria were taken from S. Wang et al [3,4]. Around a/a 0 = 0.30 %, the fracture strength reached a maximum value and then somewhat decrease. Strength decreasing region could be attributed to the too much reduction expansion, and therefore it is suggested that the mechanical damage by the chemical expansion affected the decreasing the fracture strength. In relatively small reduction expansion region, it is expected that the homogenous structure by partial cation reduction of Ce4+ to Ce3+ plays a dominant role in improving a resistance for crack extension.

The results of studying high-temperature oxygen desorption from an YBaCo2O6-δ oxide in the quasi-equilibrium regime are presented. The solid-phase synthesis of the oxide, ways and methods of its characterisation are described. The data obtained by quasi-equilibrium oxygen release technique allowed us to provide a reliable description of oxygen desorption from the oxide and to calculate the change in oxygen content in the oxide. Oxygen content in YBaCo2O6-δ is determined as a continuous function of temperature and the partial pressure of oxygen within the ranges of 600-900 °C and 0.2-10-5 atm, respectively. The data obtained make it possible to identify the conditions of phase transitions in the oxide and determine the stability regions of the conductive phase, which is necessary for the development of cathode materials for solid oxide fuel cells.

Introduction Solid oxide fuel cells (SOFCs) are an electrochemical conversion system with high efficiency and low-emission of pollution. In order to develop a commercial SOFC system with high performance and long-term stability, it is important to understand temperature distribution in the cell and stack [1]. For this purpose, thermal properties of SOFC components should be evaluated particularly under SOFC operating conditions, e.g. at elevated temperatures and under various oxygen partial pressures. Although there exist several reports on the thermal properties of the SOFC components, most of the data were measured at room temperature or in some specific atmospheric condition. [2-3] Thus, we aimed to investigate thermal properties of SOFC components as functions of temperature and oxygen partial pressure. Perovskite-type oxides La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) are selected for this work. They are known to exhibit oxygen nonstoichiometry changes depending on temperature and oxygen potential. [4-7] The variation in oxygen vacancy concentration is known to influence electrochemical properties and mechanical properties. However, little is known about the effect on the thermal properties although. There exist several reports on the thermal properties of the LSCF. In this study, thermal diffusivities, heat capacities and thermal conductivities have been investigated by using the laser flash method and TG-DSC at high temperatures up to 1273 K under controlled oxygen atmospheres. Dependencies of the thermal properties of LSCF on temperature, oxygen partial pressure and the material composition will be discussed in terms of oxygen nonsotichiometry and crystal structures. Experimental Powders of La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) were prepared by a conventional Pechini method. Obtained powders were hydrostatically pressed at 150 MPa into compacts, and then sintered in air at 1573K-1473K for 6h. For the thermal diffusivity measurement, the sintered compacts were cut into rectangles (c.a. 10 x 10 x 1 mm). For the TG-DSC measurement, the sintered compacts were cut smaller than Pt crucible. The phase of sintered samples was characterized by XRD to investigate any phase change after sintering. Thermal diffusivities and heat capacities of the LSCF were measured by using the laser flash method (LFA 457 Micro Flash, NETZSCH) and TG-DSC (STA 447, NETZSCH) in the oxygen partial pressure range from 10-4 to 0.2 bar and in the temperature range from R.T. to 1173 K. In order to control the atmospheres, a gas mixing system and an oxygen sensor are additionally attached to the laser flash system. The thermal conductivities (λ) of LSCF were calculated by using Eq(1). α : thermal diffusivity of LSCF with respect to temperature, ρ : the density of sintered LSCF sample and Cp: heat capacity of the LSCF as a function of temperature. λ = α·ρ·Cp (1) Results and Discussion In this study, the thermal properties of perovskite-type oxides La0.6Sr0.4Co1-x Fe x O3-δ (LSCF) were investigated. The thermal diffusivities of LSCF were dependence of temperature as well as oxygen partial pressure and Co contents ratio in intermediate temperatures (873K to 1173K).[Fig.1] The thermal diffusivity of LSCF decreased gradually as p(O2) decreased at all investigated temperatures, and decreased as temperature increased in all investigated p(O2) range. The thermal diffusivity depended on oxygen partial pressure more significant in lower oxygen partial pressure and at higher temperature. The thermal diffusivity increases with Co contents ratio increases and each composition increases with temperature through a maximum, then decreases. Heat capacity and thermal conductivity of LSCF (x=0, 0.8, 1.0) were almost constant from R.T to 873K, however increasing with increasing temperature over 873K. However, measured heat capacity compare with the Neumann-Kopp rule-specific heat of LSCF (x=0, 0.8, 1.0) were not consistent. LSCF is known to show the oxygen nonstoichiometry changes at relatively higher temperatures. Therefore, we suggest these dependenciese could be interpreted by the change in the oxygen nonstoichiometry, meaning that the thermal properties of LSCF were significantly affected by the oxygen nonstoichiometry.

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

In the present work, the results of high-temperature oxygen desorption from oxide with mixed conductivity composed of SrCoO3 – δ obtained via original quasi-equilibrium oxygen release technique were shown. Measurements were carried out with a characterized powder sample in a tubular reactor. The equilibrium phase diagram of the oxide in the temperature range and partial pressure of oxygen: 600–850 оC and 0.2–6⋅10-5 atm, respectively, was obtained. With the help of literary data, correlation of phase diagram regions with their corresponding structures was carried out.

Effect of Oxygen Partial Pressure on Solid Oxide Electrolysis Cells

The electrical properties of doped and were studied as a function of temperature, and oxygen and water vapor partial pressure using electrochemical impedance spectroscopy. Rare-earth elements, such as Yb, Eu, and Sm, were used as cation dopants because of their multivalent oxidation state, low third ionization potential, and comparable ionic radius with cerium. Higher conductivity values were recorded doping and oxides with Yb. Partial conductivities and transport numbers were calculated from the conductivity measurements at different oxygen partial pressures using an established defect model. In an oxidizing condition, electron–hole conduction was dominant at high temperatures and decreased with decreasing the temperature and increasing the water vapor content in the gas atmosphere. presented the highest conductivity among the tested samples both in dry and wet oxygen atmospheres. Moreover, showed a mixed conduction of ions and holes at temperatures above , suggesting that this perovskite-type oxide could be a possible cathode material in a solid oxide fuel cell (SOFC) based on a proton conductor electrolyte.

La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) is a mixed ionic-electronic conductor and one of the candidates to be used for oxygen separation membrane and cathode material for solid oxide fuel cell. The ionic conductivity of LSCF, which is still by several orders of magnitude lower than its electronic conductivity, needs to be further improved for higher performance of those devices. LSCF has a cubic perovskite structure. The oxygen vacancies are formed at elevated temperatures above 873 K, while the transition metal ions are reduced for the charge neutrality, accompanied by a sudden volume expansion, i.e., an increase in lattice constant. This implies that the oxygen vacancies could be introduced by applying mechanical strain or stress, as has been discussed in recent reports. In the present study, the oxygen-nonstoichiometry in LSCF was examined under uniaxial compression using coulometric titration technique. The test cell consisted of polycrystalline LSCF placed between 8 mol%-yttria stabilized zirconia (YSZ) plates with the platinum paste on the interfaces and on the surfaces, i.e., Pt/YSZ/Pt/LSCF/Pt/YSZ/Pt. The side of LSCF was covered with borosilicate glass. A constant voltage (25 mV to 100 mV) was applied to YSZs using a galvanostat in order to control the oxygen partial pressure in the cell (from 0.0678 to 0.0026). After the measured current reached a constant value (e.g., zero in case there is no leakage), the circuit was opened and the decay current was subsequently measured. The change in the oxygen deficiency was calculated from the integration of the measured decay current. Then, the oxygen non-stoichiometries in LSCF under various oxygen partial pressures were examined. The measurement was conducted under uniaxial compressive stress ranging from 0 to 10 MPa, and all the tests were performed at 1073 K in air. In addition to the above experiment, the relationship between the uniaxial strain and the oxygen non-stoichiometry was estimated based on the mechanical property as well as thermo-chemical-mechanical property at 1073 K based on the literature data, assuming that the oxygen deficiency merely depends on the volume change. The oxygen deficiency increased as the oxygen partial pressure was decreased, and there was almost no leakage from the test cell. The obtained results showed a good agreement with the previous reports. There was no significant effect of the applied stress up to 10 MPa on the measured decay current or the oxygen non-stoichiometry, whereas the stress above 20 MPa caused the fracture of the test cell. According to our estimation based on the mechanical and thermo-chemical-mechanical properties, the oxygen deficiency could be decreased by ~0.006 when the uniaxial compressive stress of 100 MPa is applied, which is being examined using a modified experimental setup.

Defect Chemistry of La2−xSrxCuO4−δ: Oxygen Nonstoichiometry and Thermodynamic Stability

Surface tension of molten Si, Ag, and Fe–18Cr–8Ni alloy (Type-304 stainless steel) were measured as a function of the temperature and oxygen partial pressure of an ambient atmosphere by an oscillating drop method using electromagnetic levitation, which assures measurement over a wide range of temperatures below and above the melting point and of oxygen partial pressure. For molten silicon, the lower limit of measured temperature was determined by equilibrium oxygen partial pressure for SiO2 formation; as oxygen partial pressure increases, undercooled conditions necessary for taking measurements become smaller. This is attributed to the low solubility of oxygen in molten silicon. For Ag, however, surface tension can be measured in a wide range of temperature and oxygen partial pressures due to the high solubility of oxygen. A boomerang-shaped behavior of surface tension was observed for Ag in an atmosphere with high oxygen partial pressure; surface tension has a maximum value when plotted against temperature. This boomerang-shaped behavior is attributed to desorption of oxygen at high temperature; surface tension increases with increasing temperature. The boomerang-shaped behavior was also observed for Fe–18Cr–8Ni alloy.

Design and operation of a variable high-temperature oxygen partial-pressure probe device for solid-state NMR

A simple and reliable method of constructing an intracellular pH microelectrode is described. Antimony is used as the pH sensor. Antimony electrodes have been used in biological systems as early as 1927 (1). Since then several investigators have used different kinds of Sb electrodes. Roos and Boton (3) describe the latest methods of construction and application of pH sensitive micro-electrodes. In most cases the pH electrodes were fabricated by pulling glass capillaries filled with molten antimony and in a few cases by coating glass with antimony. The electrodes were of the open type.

New cathode materials with high performance are required to realize efficient solid oxide fuel cells (SOFCs) operating at intermediate temperatures (IT) of 500-700°C. Bismuth ruthenate is known to be catalytically active toward oxygen reduction and to have high electronic conductivity in the temperature range of interest, making it a prospective cathode material for IT-SOFCs. In this work, the performance of bismuth ruthenate pyrochlore electrodes was studied using impedance spectroscopy. Symmetrical cells were fabricated on gadolinium-doped ceria electrolytes and studied as a function of oxygen partial pressure in the temperature range 400-700°C. Bismuth ruthenate electrodes showed an area specific resistance (ASR) of at 700°C and at 500°C in air. The change in ASR with oxygen partial pressure suggests that the rate-limiting step for oxygen reduction is the surface diffusion of dissociatively adsorbed oxygen at the electrode surface to the triple-phase boundaries.

Heat capacity and thermodynamic properties of Co 3O 4 from 5 to 307 K Low-temperature transition

In this paper, the intrinsic point defects in ZnO crystal have been studied by the approach that integrates first-principles, thermodynamic calculations and the contributions of vibrational entropy. With temperature increasing and oxygen partial pressure decreasing, the formation energies of oxygen vacancy [Formula: see text], zinc interstitial [Formula: see text] and zinc anti-site [Formula: see text] are decreasing, while it increases for zinc vacancy [Formula: see text], oxygen interstitial [Formula: see text] and oxygen anti-site [Formula: see text]. They are more sensitive to temperature than oxygen partial pressure. There are two interesting phenomena. First, [Formula: see text] or [Formula: see text] have the lowest formation energies for whole Fermi level at special environment condition (such as at [Formula: see text], about [Formula: see text] or [Formula: see text], about [Formula: see text]) and intrinsic [Formula: see text]-type doping of ZnO is possible by [Formula: see text] at these special conditions. Second, [Formula: see text] as donors have lowest formation energy for all Fermi level at high temperature and low oxygen partial pressure [Formula: see text], [Formula: see text]. According to our analysis, the [Formula: see text] could produce [Formula: see text]-type doping in ZnO at these special conditions and change [Formula: see text]-type ZnO to [Formula: see text]-type ZnO at condition from low temperature and high oxygen partial pressure to high temperature and low oxygen partial pressure.