Yutaka Nigara

Determination of Oxygen Vacancy Concentration in a Thin Film of La0.6Sr0.4CoO3 − δ by an Electrochemical Method

Equivalent circuit analysis was undertaken on a mixed conductor electrode/solid oxide electrolyte system. In a limited case where surface reaction is the predominant rate-controlling process, the equivalent circuit was simplified to parallel connection of a resistor (surface reaction resistance) and a capacitor (chemical capacitance due to oxygen nonstoichiometry). Equilibrium oxygen vacancy concentration was correlated with the chemical capacitance. The model was applied to a dense film of La 0.6 Sr 0.4 CoO 3-δ deposited on a sintered plate of Ce 0.9 Gd 0.1 O 1.95 by a laser ablation method. Frequency response of the electrochemical impedance was measured under dc bias at 873-1073 K in O 2 -Ar gas mixtures. The observed capacitance was extremely large, e.g., around 0.1 to I F cm -2 for the 1.5 μm thick film. The oxygen vacancy concentration in the film was calculated from the capacitance and compared with the literature data measured by thermogravimetry. The film was found to show smaller oxygen nonstoichiometry. The enthalpy for oxygen vacancy formation in the film was about 40 kJ mol -1 larger than the bulk.

Oxygen isotope exchange with a dense La0.6Sr0.4CoO3−δ electrode on a Ce0.9Ca0.1O1.9 electrolyte

Abstract Oxygen isotope exchange experiments were carried out with a dense La0.6Sr0.4CoO3−δ film (0.5 μm thick) deposited on a Ce0.9Ca0.1O1.9 substrate by a laser ablation method. The isotope exchange profile was measured from the surface into the electrolyte by a secondary ion mass spectrometer (SIMS). The oxygen diffusion through the La0.6Sr0.4CoO3−δ film was fast enough not to make any observable gradient in oxygen isotope concentration inside the film. The surface isotope exchange rate, k*, was calculated from the diffusion profile into the electrolyte layer. The electrochemical impedance, σE, was compared with k*. The oxygen partial pressure dependence of those two parameters were quite similar. The absolute value of k* was larger than expected from σE by a factor of 2 or higher.

Lattice expansion upon reduction of perovskite-type LaMnO3 with oxygen-deficit nonstoichiometry

Abstract Volume variation of dense LaMnO 3+ δ specimens was measured at elevated temperatures and in stepwise changes of oxygen partial pressure. The specimen expanded upon reduction at fixed temperatures, and its dependence on temperature and oxygen partial pressure conformed to that of oxygen-deficit nonstoichiometry, which was formulated on the basis of a defect model by Mizusaki et al. [Solid State Ionics 129 (2000) 163]. The measured expansion was in almost linear relationship with the oxygen nonstoichiometry. In consideration of reported behavior that the unit cell shrinks upon Sr-doping and upon formation of cation vacancy, the origin of the expansion upon reduction is regarded as the increase in the size of B-site cation, which accompanies the charge compensation for the formation of oxide ion vacancies. The expansion rate that was estimated from the variation in averaged ionic radius of Mn was higher than the experimentally observed one; it is supposed that the effect of the increase in B-site radius is partly offset by the increase in the crystallographic distortion.

Mass transport properties of Ce0.9Gd0.1O2−δ at the surface and in the bulk

Abstract The isothermal thermogravimetry was performed on the system of Ce 0.9 Gd 0.1 O 2− δ (CGO10) to determine the oxygen nonstoichiometry, δ , in the temperature range between 1073 and 1173 K and the oxygen partial pressure range of 10 −22 –1 bar. In addition, chemical diffusion coefficients, D chem , and surface reaction rate constants, k , were obtained in CO–CO 2 and H 2 –H 2 O atmospheres through the analysis of a weight relaxation process. The values of obtained chemical diffusion coefficients were consistent with those estimated from partial conductivity and oxygen nonstoichiometry through the P O 2 regions from predominant ionic conduction to predominant electronic conduction. Surface reaction rate constants are not directly related with oxygen partial pressure but depend on the gas species and the composition. The isotope exchange depth profiling with secondary ion mass spectrometry (SIMS) was done to examine the relationship between k and surface exchange coefficients, k s . It was shown that the hydrogen partial pressure dependence of k s converted from k was the same, although the absolute value was about 1 over 30 of those from SIMS analysis.

Nonstoichiometry of Ce1−XYXO2−0.5X−δ (X=0.1, 0.2) ☆

Abstract Nonstoichiometry of the fluorite-type oxide solid solutions Ce 1− X Y X O 2−0.5 X − δ ( X =0.1, 0.2) was measured as a function of temperature ( T =973–1373 K) and oxygen partial pressure ( P (O 2 )=10 −2 –10 −24 atm) by means of thermogravimetry. The result shows that the nonstoichiometry of Ce 1− X Y X O 2−0.5 X − δ ( X =0.1, 0.2) is not explained by a simple point defect model, therefore, defect association models are suggested. From the calculation, it is found that (Ce Ce ′V O Ce Ce ′) x is the major defect association not only in Ce 1− X Y X O 2−0.5 X − δ ( X =0.1, 0.2), but also in pure CeO 2 for nonstoichiometries less than 0.10. It is also found that the defect association (Ce Ce ′V O Ce Ce ′) x is dominating at lower temperature and smaller X composition.

Determination of hydrogen solubility in oxide ceramics by using SIMS analyses

Abstract Hydrogen solubility in oxide ceramics was evaluated by annealing in D 2 O-containing atmosphere and subsequent secondary ion mass spectrometry (SIMS). A good proportional relation was obtained between the relative intensity of deuterium ion in SIMS and literature data of hydrogen solubility for YSZ and Sr(Yb)CeO 3− δ . The hydrogen solubility in CeO 2 and rare-earth doped ceria (Ce 0.8 M 0.2 O 1.9 , M=Y, La, Nd, Sm, Gd, Yb) was evaluated. The hydrogen solubility in ceria dramatically increased by rare earth substitution, and it depended on the lattice parameter of Ce 0.8 M 0.2 O 1.9 .

Measurement of oxygen permeability in CeO2 doped CSZ

The oxygen permeability in [(ZrO2)1 − x(CeO2)x]0.9(CaO)0.1 (x = 0.1−0.4) was measured at 1100–1800 K under the gradient of oxygen partial pressure created between air and inert gas. Tube shaped specimens were used with 13 and 9 mm outer and inner diameters respectively and lengths of 5 and 15 mm. One end of each specimen was fixed to an alumina tube and the other end to an alumina disk with Pt-O-rings at 1800 K, and were set in a SiC furnace. Air and helium were supplied to the outsides and insides of specimens respectively. The oxygen permeation rate was calculated from the increase in oxygen concentration and the flow rate of helium. The oxygen permeability was obtained from the oxygen permeations of two specimens different in length to eliminate the short-circuiting effect of Pt-O-rings. It was shown that the oxygen permeability was determined by the electronic conductivity of the solid solution, which was n-type, and increased as the Ce concentration and the temperature were increased.

Lattice creation and annihilation of LaMnO3+δ caused by nonstoichiometry change

Abstract In order to identify the majority ionic defect of La 1− x Sr x MnO 3+ δ in the oxygen-excess region, a dilatometry was made on La 0.96 MnO 2.94+ δ at a fixed temperature (1273 K) by varying the ambient oxygen partial pressure. The specimen of La 0.96 MnO 2.94+ δ expanded with increasing P O 2 in the region of δ >0, while the unit cell volume was known to decrease with increasing P O 2 . The morphological evolution was also observed on the surface of the La 0.96 MnO 2.94+ δ specimen after annealing in an oxidizing atmosphere, indicating that the new lattices were created. From these results, it is concluded that the majority ionic defect of the perovskite oxide, La 1− x Sr x MnO 3+ δ , is cation vacancies in the oxygen-excess region.

Hydrogen permeability in (CeO2)0.9(CaO)0.1 at high temperatures

The atomic hydrogen (1/2H2) permeability, JH, in (CeO2)0.9(GdO1.5)0.1 (fluorite-type) was measured at 1800–800 K. Two tubular specimens (SP(L), SP(S)) of different lengths sintered at 1970 K in air were used to eliminate the permeation through supporting materials (Pt rings, alumina tube and alumina disk). The JH was 2.81×10−6–1.50×10−8/mol h−1 cm−1, when it was assumed that PH2=7.34×103 Pa and PH2O=2.28×103 Pa outside of both specimens and that the Ar flow rates to the inside of SP(L) and SP(S) were 20.9 and 9.4–16.0 cm3 min−1, respectively. LogJH decreased with decreasing temperature and was proportional to the inverse temperature at 1800–1100 and 1100–800 K, and the activation energies were 1.12±0.01 and 0.18±0.01 eV, respectively. (CeO2)0.9(GdO1.5)0.1 would be an electron–proton mixed conductor under H2–H2O atmosphere at high temperatures. The protonic conductivity was roughly calculated from the JH and from the hydrogen partial pressures inside and outside the specimens, its values being 1.4×10−4–7.2×10−7/S cm−1 and 1.2×10−6/S cm−1 at 1070 K.

Hydrogen permeability of YSZ single crystals at high temperatures

Abstract The atomic hydrogen (H) permeability, J H , of 10YSZ single crystal (fluorite-type) was measured at 1800–1050 K using the Nigara-method . Two tubular specimens (φ 1.30×0.93 cm; length: 3.35 cm (Specimen(L)) and 1.00 cm (Specimen(S))), annealed at 1570 K for 6 h in air, were used to eliminate the permeations of supporting materials (Pt rings, alumina tube, and alumina disk). Measurement was carried out after the specimen had been kept at 1800 K for 20 h. The J H was 3.97×10 −7 –2.79×10 −9 /mol h −1 cm −1 , when it was assumed that P H 2 =7.36×10 3 Pa and P H 2 O =2.30×10 3 Pa outside of both specimens and that the Ar flow rates to the inside of Specimen(L) and Specimen(S) were 21.5 and 16.9–19.9 cm 3 min −1 , respectively. Log J H decreased with the decrease of temperature and was proportional to the inverse temperature. The activation energies were 0.61±0.09, 1.60±0.05, and 0.79±0.02 eV at 1800–1680, 1680–1330, and 1330–1050 K, respectively. The protonic conductivity, which was roughly calculated from the J H and the hydrogen partial pressures inside and outside of the specimens, was 2.4×10 −6 –1.5×10 −8 /S cm −1 at 1800–1050 K. The J H of other 10YSZ single crystal specimens, which were not annealed and which were measured after being kept at 1800 K for 1 h, was lower than that of the above specimens, especially at high temperatures, and the activation energies were 1.27±0.03, 0.15±0.03, and 1.49±0.09 eV at 1800–1425, 1425–1150, and 1150–1050 K, respectively. The 10YSZ single crystal would be an electron–proton mixed conductor under H 2 –H 2 O atmosphere at high temperatures. Since the J H of 12YSZ single crystal specimens, which were annealed at 1870 K for 24 h in air and were measured after being kept at 1800 K for 18 h, was less than 1×10 −9 /mol h −1 cm −1 at 1800 K, it was difficult to determine the relation between its hydrogen permeability and temperature.