Atsushi Kaimai

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.

A Solar-Pumped cw 18 W Nd:YAG Laser

The development of a solar-pumped Nd:YAG laser is reported. A solar energy collector consisting of a paraboloidal mirror 10 m in aperture and 3.2 m in focal length is used to pump a water-cooled Nd:YAG laser rod 4 mm in diameter and 75 mm in length. The maximum output of the laser is 18 W in multi-mode, the highest output power reported so far in solar-pumped lasers. Further improvements designed to increase the power are also discussed.

Protonic-Electronic Mixed Conduction and Hydrogen Permeation in BaCe0.9 − x Y 0.1Ru x O 3 − α

The protonic-electronic mixed conductors are of great interest for their potential applications particularly for the hydrogen separation that is essential for hydrogen production from hydrocarbons. This paper deals with the mixed conduction properties of BaCe 0 . 9 - x Y 0 . 1 Ru x O 3 - α (x = 0-0.1) in which Ru is partially substituted for Ce in the high-temperature proton conductor, BaCe 0 . 9 Y 0 . 1 O 3 - α . Appreciable hydrogen permeation through the Ru-doped materials was observed and is attributed to ambipolar diffusion. The mixed conducting mechanism is discussed in terms of the defect chemistry and electronic structures revealed by the electrochemical and spectroscopic measurements.

Electrochemical Behaviors of Mixed Conducting Oxide Anodes for Solid Oxide Fuel Cell

To clarify guidelines for a high-performance mixed conducting oxide anode, electrochemical behaviors of mixed conducting oxide anodes were studied on the oxides, La 0.9 Ca 0.1 Cro 0.8 Al 0.2 O 3 , La 0.9 Ca 0.1 Cr 0.2 Al 0.3 O 3 , Sr 0.9 La 0.1 TiO 3 , Sr 0.8 La 0.2 TiO 3 , SrTi 0.97 Nb 0.03 O 3 , Ce 0.9 Gd 0.1 O 1.95 , and Ce 0.992 Nb 0.008 O 2 . The hydrogen oxidation on the oxide anodes is studied by ac impedance and steady-state polarization measurements. In the ac impedance measurements, a slow relaxation process of the order of 10 -2 Hz was observed with the CeO 2 -based and La 0.9 Ca 0.1 Cr 0.2 Al 0.8 O 3 anodes. The corresponding pseudocapacitances are 10 4 -10 6 μF cm -2 . These pseudocapacitances are identified as the chemical capacitance due to the variation of the nonstoichiometric oxygen content of the electrode material. At the same electrode potential, the CeO 2 -based anodes showed a far higher steady-state current than the LaCrO 3 - and the SrTiO 3 -based anodes. The extension of the reaction zone beyond the three phase boundary is estimated from our experimental results. The reaction zone of the Ce 0.9 Gd 0.1 O 1.95 anode extends from the three-phase boundary to the electrode/gas interface. To estimate what determines the electrode performance of the oxide anodes, the effect of the material property and the electrode microstructure was studied. The effect of the material property is much larger than that of the electrode microstructure. High ionic conductivity and catalytic activity with a certain level of electronic conductivity is required for a high-performance oxide anode.

Determination of the Reaction Zone in Gadolinia-Doped Ceria Anode for Solid Oxide Fuel Cell

In order to elucidate the reaction zone in porous Ce 0.9 Gd 0.1 O 1.95-δ anodes on yttria-stabilized zirconia (YSZ), ac impedance and steady-state polarization measurements are carried out in H 2 -H 2 O-Ar gas mixtures with different electrode thicknesses, 7, 20, and 45 μm anodes. Steady-state polarization current becomes larger as the electrode becomes thicker, while the current per unit surface area shows similar value. Therefore, the current seems to be a function of the electrode surface area. In ac impedance measurements, an extremely large pseudo-capacitance is observed to be as large as 10 5 -10 6 μF cm -2 . The measured pseudo-capacitance is caused by the nonstoichiometric perturbation of the oxygen content in the anode material. Both steady-state polarization and ac impedance measurements suggest that the reaction zone extends from the interface of YSZ/porous Ce 0.9 Gd 0.1 O 1.95-δ anodes toward the outer gas phase. The distribution of electrochemically active zone is semiquantitatively estimated. The activity for the surface reaction gradually decreases with increasing the distance from the electrode/electrolyte interface, and the reduction rate of the activity becomes high as the electrode thickness increases.

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.

Electrical conductivities of CaO doped ZrO2–CeO2 solid solution system

Abstract Electrical conductivities, σ , of (Zr 1- x Ce x O 2 ) 0.9 (CaO) 0.1 ( x =0.1–1.0) were investigated by a D.C. four terminal technique at 1173–1573 K as a function of the oxygen partial pressure from 10 -15 to 1 atm. In the region of x =0.4–1.0, the electrical conductivity is separated into the ionic and electronic conductivity by using the relation of σ = σ ion + σ e ∘ P O 2 -1/4 . The electrical conduction at P O 2 =1 atm is essentially ionic. The composition dependence of the electronic conductivity shows opposite tendency to that of the ionic conductivity, that is, the electronic conductivities increase with the increase in CeO 2 content ( x ) and show a maximum at x ≃0.6, then decrease with x while the ionic conductivity behaves vice versa. In the CaO doped ZrO 2 –CeO 2 system, the formation of the solid solution results in the low ionic and high electronic conduction.