Kenichi Kawamura

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.

Ce3+ concentration in ZrO2–CeO2–Y2O3 system studied by electronic Raman scattering

Ceria doped yttria stabilized zirconia (ZrO2–CeO2–Y2O3 system) shows electron–ion mixed conduction at high temperatures under a reduced atmosphere. In this system, the electronic conductivity is caused by electrons, which may diffuse by hopping conduction between Ce4+ and Ce3+. We applied the electronic Raman scattering technique on the system in order to measure the concentration of Ce3+ in samples. For the X=0.6 and 0.8 samples in [(ZrO2)1−X (CeO2)X]0.9(Y2O3)0.1, the oxygen partial pressure dependence of the electronic Raman intensities agrees well with that of the oxygen non-stoichiometry measured by thermogravimetry. From this result, it is confirmed that all of the introduced electrons at reduced condition are trapped at cerium centers in these materials.

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.

High temperature transport properties at Metal/SrTiO3 interfaces

Abstract High temperature current-voltage characteristics were investigated with a Nb doped SrTiO 3 (Nb–STO) single crystal. The conductivity of the 0·5xa0wt% Nb doped SrTiO 3 showed high n-type conductivity with a negative temperature coefficient. The Pt/Nb–STO interface freshly prepared by laser ablation at 973xa0K in high vacuum condition showed ohmic behavior. However, it turned to show a Schottky type non linearity when annealed in oxygen gas at temperatures higher than 773xa0K. The I–V curve in the forward direction was well fitted with the equation based on the thermionic emission model. At high temperatures, the I–V behavior was dependent on the oxygen partial pressure. The lower oxygen partial pressure resulted in a lower barrier height. The change in the I–V curve with oxygen potential was almost reversible at 873xa0K, and was frozen below 673xa0K. Those phenomena suggested that the Schottky barrier formation at the Pt/STO interface has a strong relation with the oxygen transport in Nb–STO.

High temperature transport properties in SrTiO3 under an oxygen potential gradient

Abstract Electrical transport properties were investigated in a single crystalline SrTiO 3 sample. The electronic conductivity was determined to be 4.0×10 −3 P 1/4 O 2 +3.0×10 −6 P −1/4 O 2 at 1173 K by a four terminal method. The sample was placed under an oxygen potential gradient, and the electromotive force and the current–voltage behaviour were investigated. Oxide ion conductivity was estimated to be 2.5×10 −5 Sxa0cm −2 comparing the emf data with the theoretical values. The average conductivity was explained by a calculation based on local thermodynamic equilibrium and local charge neutrality. Under high applied voltages, a nonlinear current–voltage relationship was observed. Attempts were made to explain the nonlinearity considering the redistribution of oxide ion vacancy in the bulk and oxygen blocking effect at the electrode. The oxygen potential at the polarized electrode was estimated and compared with the oxide ion current.

Phase stability of La1−xCaxCrO3−δ in oxidizing atmosphere

Abstract The chemical stability of perovskite-type La1−xCaxCrO3−δ (x=0.1, 0.2, 0.3) in high oxygen partial pressure, PO2, was investigated with three methods: thermogravimetry, XRD analysis, and thermodynamic calculation. The second phase, CaCrO4 was observed by XRD analysis on the powder equilibrated in high PO2. Thermogravimetry under fixed temperatures sensitively detected the segregation of the second phase in the form of oxygen incorporation, because oxidation of chromium ion accompanies the segregation. The second phase tended to appear in high PO2 and at low temperature. The single-phase regions of La1−xCaxCrO3−δ obtained from the two experimental methods well agreed with each other. The results of thermodynamic calculation on the assumption of ideality of the solid solution also agreed with the experimental results. These results suggested the sufficient chemical stability of La1−xCaxCrO3−δ in high PO2 concerning the application to an interconnector of high-temperature solid oxide fuel cells; for example, La0.7Ca0.3CrO3−δ is stable at 1273xa0K in air.

Influence of the coexisting gases on the electrochemical reaction rates between 873 and 1173 K in a CH4-H2O/Pt/YSZ system

Abstract The rates of electrochemical reactions were clarified in a CH 4 –H 2 O system at the interface of a porous Pt electrode/Y 2 O 3 -stabilized ZrO 2 (YSZ) electrolyte between 873 and 1173 K to elucidate the kinetics of the anode reaction of solid oxide fuel cells (SOFCs). The dominant electrochemical reaction was found to be the redox process of H 2 /H 2 O, where H 2 , C, CO, and CO 2 were formed without a current by the chemical reactions in a CH 4 –H 2 O system. The partial electrochemical reaction rates of H 2 , CO, C, and CH 4 were determined. The rate of the electrochemical reaction of CO/CO 2 in a CH 4 –H 2 O system is larger than that in a CO–CO 2 system under anodic polarization at 873 and 973 K. This means both the efficiency and the rate of the utilization of fuels on SOFCs are increased.

Hydrogen permeability in Ce0.8Yb0.2O1.9 at high temperatures

Abstract It was found that hydrogen permeated Ce 0.8 Yb 0.2 O 1.9 (fluorite-type) at 1800–1000 K, which was sintered at 1970 K in air. The tube shaped two specimens were used to measure the hydrogen permeability, J H . Log( J H /mol h −1 cm −1 ) decreases with reducing temperature and is proportional to reverse temperature at 1800–1500, 1500–1300 and 1200–1000/K. The activation energies were 0.92±0.06, 1.10±0.06 and 0.58±0.05/eV, respectively. Ce 0.8 Yb 0.2 O 1.9 would be an electron–proton mixed conductor under H 2 –H 2 O atmosphere at high temperature.

Oxygen permeability in ZrO2-CeO2-MgO at high temperatures

Abstract The oxygen permeability, Jo2, in CeO2 doped MSZ, [(ZrO2)1 − x(CeO2)x]0.9(MgO)0.1 (x = 0.1 ∼ 0.6), was measured at 1100–1750 K under the oxygen partial pressures, Po2, of 0.2 − 1 × 10−4atm. The specimens were predominantly oxide-ion conductors with n-type electronic conduction. The oxygen permeabilities normalized by temperature, T, and P o 2 , log Jr = log [ J o 2 (T(P o 2 II − 1 4 − P o 2 I − 1 4 ) ] , were proportional to 1 T and increased with temperature. At 1100–1500 K, Jr increased exponentially for x ≦ 0.4 and the electronic activation energy, E, was about 2 eV and was independent of x. At 1500–1750 K, Jr increased exponentially for x ≦ 0.4, while for x>0.4 it showed a saturation tendency and the E values were 1.86 ± 0.10 (x = 0.1), 1.67 ± 0.08 (x = 0.2), 1.50 ± 0.16 (x = 0.3), 1.39 ± 0.05 (x = 0.4), 1.22 ± 0.06 (x = 0.5) and 1.32 ± 0.04 (x = 0.6) eV. The log Jr of x = 0.6 was a little higher than that of x = 0.5 at 1100–1280 K (E = 2.10 ± 0.07 for x = 0.5 and 1.79 ± 0.06 for x = 0.6 eV), while at 1280–1750 K, the value of x = 0.6 was a little lower than that of x = 0.5 (E = 1.57 ± 0.06 for x = 0.5 and 1.49 ± 0.03 for x = 0.6 eV at 1280–1500 K).

An Oxygen Sensor Using a Process of High‐Temperature Oxidation of Metal

Oxygen sensors using stabilized ZrO{sub 2} as the electrolyte are used for steel making and heat engines to obtain high performance. An oxygen sensor is proposed which is represented by an electrochemical cell metal {vert_bar} oxide scale {vert_bar} sensing electrode, where the metal, its oxide scale, and sensing electrode work as reference electrode, electrolyte, and sample electrode, respectively. Here the oxide scale is required to be an oxide-ion conductor, and the sensing electrode is not to be reactive with the oxygen. It is expected that the electrolyte is self-restorative because it can be reformed by high-temperature oxidation. The electromotive force (EMF) measurements were carried out at 873 K with cells using zirconium as the metal electrode and Pt as the sensing electrode. At p{sub o{sub 2}} = 1--10{sup {minus}4} atm, the EMF vs. log p{sub o{sub 2}} plot lies on a straight line and its gradient is 2.303 RT/4F, suggesting unity of the oxide-ion transference number at the surface of the scale. The EMF steeply decreases with decreasing p{sub o{sub 2}} at p{sub o{sub 2}} < 10{sup {minus}4} atm, which cannot be explained by the increase in the electronic conductivity. The oxidation behaviors showed linear oxidation. Assuming repetition which constitutedmorexa0» of parabolic oxide film growth until a certain thickness and its crack formation, the linear rate constants were described as a function of the oxygen partial pressure. It was considered that the steep decrease in EMF is caused by the change of the rate-determining process to form the scale.«xa0less