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Recently, B-site doping in the perovskite-type oxides has often been carried out to control various properties. One of the perovskite-type oxides, doped LaCrO 3 , is the candidate for the interconnector for Solid Oxide Fuel Cells (SOFC). For its stability under reducing atmospheres, B-site doped LaCrO 3 is also one of the candidates for the anode of SOFC. The oxygen nonstoichiometry, δ , in B-site doped LaCrO 3- δ was measured as a function of oxygen partial pressure at 973 K to 1373 K. The observed values of δ were different by the different B-site dopants, and also different from those of A-site doped LaCrO 3 . It was shown that only 10 mol% of dopant in B-site drastically affects the redox property of LaCrO 3 . Defect model for B-site controlled LaCrO 3 was interpreted by assuming the reaction, 2Cr Cr X + V O •• + 1/2O 2 (g)⇔2Cr Cr • + O O X .

The oxygen ion emission characteristics of CeO 2 were studied under electric field in a vacuum chamber to find a candidate material for a novel ion source, "solid oxide ion source (SOIS)". The emission current was observed from CeO 2 under a pressure of around 10 -3 Pa, at the temperature ranging from 973 K to 1173 K. It was found that the emission current increased with temperature and applied voltage. The ions emitted from CeO 2 were confirmed to be oxygen negative ions (O - ) by the use of quadrupole mass spectrometer. The emission current decreased with time as was observed in the earlier works with other oxide ion conductors such as stabilized zirconia [1,2] or other materials [3]. To enhance the emission current from CeO 2 , an introduction of donor into CeO 2 was tested using Ce 0.992 Nb 0.008 O 2 . For comparison, effect of acceptor doping was also tested using Ce 0.9 Gd 0.1 O 1.95 . The emission current from Ce 0.9 Gd 0.1 O 1.95 was smaller than that from donor-doped and pure CeO 2 . Clear enhancement of the emission current was not observed with Ce 0.992 Nb 0.008 O 2 .

The electrical properties of the surface layer formed on the Nb-doped SrTiO 3 single crystals after annealing in air were characterized by the measurements of current–voltage relationship on the junction, (−) Ga–In/bulk/surface layer/Ga–In (+), at low temperatures. Here, Ga–In alloy is an ohmic electrode material. In the forward bias, the tunneling process predominate the current below 130 K, while at high temperatures, the Poole–Frenkel effect determines the current. An anomalous decrease of current under reverse bias with increasing temperature was found. This phenomenon was attributed to the variation of the Fermi energy in the surface layer and the bulk with temperature.

The electrical conductivity of Ce0.992Nb0.008O2−δ was measured as a function of oxygen partial pressure between 10−28 and 1 bar in the temperature range of 873–1173 K. The activation energy for the conductivity at an oxygen partial pressure of 10−4 bar was 0.29 eV. This value was too small to be assigned for oxide ion conduction, indicating that, in oxidizing atmospheres, electronic conduction is dominant due to Nb doping. On the other hand, the oxygen partial pressure dependence of conductivity was complicated in reducing atmospheres. Defect chemical analysis was made to interpret the conductivity variation with oxygen partial pressure. The defect association model was introduced, which assumes that oxygen vacancies are easily formed around niobium ions and that one of the released electrons is trapped on Nb5+. The experimental data was found in accordance with that calculated by the proposed defect association model.

For LaMnO 3+ δ with oxygen-excess nonstoichiometry, in situ X-ray diffractometry was performed under high temperatures and precisely controlled oxygen partial pressures, P O 2 . A rhombohedral phase was observed under most experimental conditions. With temperature rising, the lattice volume increased linearly in a low temperature region; the coefficient of linear thermal expansion (CTE) was 1.06×10 −5 K −1 . In a high temperature region, the temperature dependence of the lattice volume upwardly deviated from the linearity, indicating that the unit cell expanded upon decrease in excess oxygen. The nonstoichiometry coefficient of the linear isothermal expansion was calculated at 5×10 −2 using literature data on δ ( T , P O 2 ). The rhombohedral distortion escalated with temperature and P O 2 decreasing. The dependence of cell volume and distortion on nonstoichiometry is well interpreted as the effect of variation in the ionic radius and tolerance factor.

EMF of the hydrogen isotope cell has been applied to the sensing of hydrogen isotopes. In this paper, the atomic hydrogen isotope composition and the total pressure of the hydrogen isotopes were determined simultaneously by use of two electrochemical cells with different kind of proton-conducting electrolytes. Among the several proton conductors, CaZr0.9In0.1O3−α and SrCe0.95Yb0.05O3−α were chosen for the sensor. The difference in EMFs of the two proton conductor cells was independent of the total pressure of hydrogen isotopes and, thus, served to determine the atomic hydrogen isotope compositions in gases.

The oxygen nonstoichiometry, δ , of La 1− x Ca x CrO 3− δ ( x =0.1–0.3) was measured by means of thermogravimetry as a function of oxygen partial pressure, P (O 2 ), temperature, T , between 1073 and 1373 K, and the content of calcium, x . In any compositions, δ becomes larger and is getting closer to δ = x /2 with decreasing P (O 2 ). At the same P (O 2 ), δ of La 1− x Ca x CrO 3− δ increases with increasing T and x . There is almost no difference in oxygen nonstoichiometry between Ca-doped lanthanum chromite and Sr-doped lanthanum chromite. Defect chemical analysis was performed taking defect interaction into consideration. The nonstoichiometry curves were reproduced well by the proposed defect model and the obtained thermodynamic parameters. The influence of defect interaction on defect formation energy can be mainly attributed to change in interatomic potential due to defect-induced lattice expansion.

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.

The far-infrared absorption of pure CeO 2 , CeO 2 :CaO and CeO 2 :ZrO 2 solid solutions has been measured at 11 and 297 K in the region 10–100 cm -1 . Two apparent absorption peaks are found at 11 K in the vicinity of 50–80 cm -1 only in the CeO 2 :CaO solid solution with oxygen vacancies, not in the CeO 2 :ZrO 2 solid solution without oxygen vacancies and are assigned to be due to oxygen vacancy modes or attempt modes. The continuous absorption showing a nearly quartic frequency dependence is observed in both the CeO 2 :CaO and CeO 2 :ZrO 2 solid solutions. This continuous absorption reflects the density of states of acoustic modes and exhibits non-glass-like temperature and frequency dependencies. It is suggested from the above result that the so-called boson peak in glasses does not possess the acoustic mode-like character.

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.

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.

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 1273 K in air.

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.

We have developed a new measurement technique combining electrochemical measurement with Raman microspectroscopy at high temperatures. In situ observation of carbon deposition on the anode for solid oxide fuel cells was attempted by Raman microspectroscopy. It was found that the form of deposited carbon was amorphous carbon in dry butane/Pt/8YSZ and graphite in humid methane/Ni/8YSZ, respectively. The deposited carbon on Ni surface could not be burned electrochemically.

Oxygen potential distribution profile was measured around (La,Sr)MnO 3 electrode/YSZ electrolyte interface. The local oxygen potential was measured by a newly developed microprobe that detects oxygen potential on conductive solids. A model electrode/electrolyte interface was fabricated by embedding a small piece of La 0.8 Sr 0.2 MnO 3 in YSZ. When the electrode was polarized, the oxygen potential on the electrode surface shifted out of equilibrium. The potential shift was larger and faster around the electrode/ electrolyte boundary and gradually faded with distance. The critical length was larger than 60 μm. A numerical calculation was made to simulate the relaxation behavior of the surface oxygen potential after an abrupt change in polarization voltage. The surface diffusion coefficient was roughly estimated to be about 5 x 10 -6 cm 2 s -1 .

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.

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.

The atomic hydrogen permeability, J H , in fluorite-type (CeO 2 ) 0.85 (CaO) 0.15 was measured to confirm the influence of CaO concentration in CeO 2 at 1050–1800 K. The decrease of J H with reducing temperature and the proportionality of log J H to inverse temperature are suggested. The estimated activation energies are 1.0 5 ±0.01, 1.3 5 ±0.02 and 0.2 2 ±0.04 eV at 1800–1400, 1400–1250 and 1250–1050 K, respectively. The J H of (CeO 2 ) 1− X (CaO) X decreases with increasing X and proportional to X 5/2 at 1800–1300 K under H 2 –H 2 O atmosphere. H 2 is decomposed to 2H + and 2e − on the surface of the specimen and they permeate it. They hop between oxygen ions and cerium ions, respectively. It would be difficult for H + to cross over Vo, which was produced by CaO, then J H decreased with increasing X . (CeO 2 ) 1− X (CaO) X would be the electron–proton mixed conductor under H 2 –H 2 O atmosphere at high temperatures.

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.

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.

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.

The electrochemical property of the galvanic cell, P(O2)I+P(CO), Au/YSZ/Pt, P(O2)II, has been studied for the applications of CO sensor. In order to elucidate the CO-sensing mechanism on Au/YSZ interface, steady state polarization measurements were carried out at 873-1073 K in O2-Ar, CO-CO2, CO-O2-Ar systems. The quantitative analysis of the I-V characteristics revealed the kinetics of the electrode reaction with O2, CO and CO2. The current density at Au/YSZ interface in CO-O2-Ar system can be formulated. As a result, it was confirmed that mixed potential was generated by a small amount of CO in oxygen-containing streams.

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 .

It was found that the sintered CeO{sub 2}, which was not perovskite-type but fluorite-type, permeated hydrogen under H{sub 2}-H{sub 2}O atmosphere at 1,050--1,800 K. Three tube-shaped specimens ({phi}0.287 x 0.894 cm) (SP I, SP II, SP), the lengths of which were 1.40, 0.37, and 1.45 cm, were prepared. The nonexistence of micropores in specimens was confirmed by supplement of He (11.2%) + N{sub 2} to the outside of SP at 1650--1820 K. H{sub 2} (7.90%) + H{sub 2}O (2.55%) + N{sub 2} was supplied to the outsides of SP I and SP II and Ar gas was allowed to flow to the inside of them. The flow rate to SP I was 21.2 cm{sub 3} min{sup {minus}1}, and the flow to SP II was controlled so that the hydrogen concentrations measured by a gas chromatograph were of the same value. After the calculation of H{sub 2}, H{sub 2}O, and O{sub 2} concentrations in an equilibrium state inside and outside SP I and SP II and the elimination of mechanically leaked H{sub 2} and permeated hydrogen through the supporting materials (Pt rings, alumina tube, and disk), the atomic hydrogen permeability, J{sub H}, was determined. The hydrogen partial pressures in the outsidemore » and inside were 7.90 x 10{sup {minus}2} and 7.43 x 10{sup {minus}5} to 1.53 x 10{sup {minus}3} atm at 1,050--1,800 K. Log(J{sub H} mol h{sup {minus}1} cm{sup {minus}1}) increases with rising temperature, T, while it is not proportional to 1/T. As the electronic conduction in CeO{sub 2} is considerably higher at low oxygen partial pressure, there is a possibility that CeO{sub 2} is an electron-proton mixed conductor under 2{sub 2}-H{sub 2}O atmosphere at high temperatures.« less

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.

High temperature current-voltage characteristics were investigated with a Nb doped SrTiO 3 (Nb–STO) single crystal. The conductivity of the 0·5 wt% 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 973 K 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 773 K. 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 873 K, and was frozen below 673 K. Those phenomena suggested that the Schottky barrier formation at the Pt/STO interface has a strong relation with the oxygen transport in Nb–STO.

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 constitutedmore » 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.« less

The rate processes 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 electrolyte. Direct-current polarization measurements and ac impedance spectroscopy were made with gas analysis before and after the reaction at 1073 K. We proposed an analytical method to determine the rates of electrochemical reactions taking place in parallel. When the ratio p(H 2 O)/p(CH 4 ) of the inlet gas was close to zero, the observed relationship between the polarization current and electrode potential was interpreted by the electrochemical oxidation processes of H 2 , CO, C, and CH 4 in parallel using the proposed method. For example, the ratio of the oxidation rates for C/CO/CH 4 /H 2 is 1/1.3 x 10/1.9 x 10 2 /2.8 x 10 3 at E = -600 mV vs. air. This result was obtained under very low CH 4 concentration. The estimated oxidation rates of H 2 and CO as functions of the electrode potential were described by the model proposed by Mizusaki et al. for the reaction of H 2 -H 2 O and CO-CO 2 .

The oxygen nonstoichiometry, the total conductivity, and the electronic conductivity of [(ZrO 2 ) 0.8 (CeO 2 ) 0.2 ] 0.9 (CaO) 0.1 were measured by using high temperature gravimetry, dc four-terminal method, and Hebb-Wagner's dc polarization technique, respectively. The change in the oxygen nonstoichiometry was explained by assuming Ce Zr ', Ca Zr , and V 0 ** as major defects. At 1173 K and P o = 1 ∼ 10 -11 atm, the total conductivity corresponded to the ionic conductivity is proportional to [V o ** ]. At the lower Po 2 , although the oxygen vacancy increased with the decreasing Po 2 , the ionic conductivity decreased. The relation between [V o **] and the ionic conductivity was similar to that in stabilized ZrO 2 with various CaO contents. It was shown that the electronic conductivity is proportional to [Ce Zr '], which is not in accordance with the hopping-type conduction between Ce 3+ and Ce 4+ . Assuming all Ce Zr ' release conduction electrons, the calculated mobility is in the order of 10 6 cm 2 V 1 s 1 at 1173 K and does not correspond to the band conduction model. It was considered that most of the electrons are trapped at the donor level formed by Ce Zr ' and only a small portion of the electrons is excited to conduction band as carriers.

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 S cm −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.

In order to elucidate the electrochemical reaction in a CH 4 -H 2 O system at the interface of a porous Pt electrode/Y 2 O 3 -stabilized ZrO 2 electrolyte, measurements were made on the gas composition before and after the reaction under directcurrent polarization and on electrochemical impedance spectra at 1073 K. The dominant electrochemical reaction was found to be the redox process of H 2 /H 2 O where H 2 and CO were generated by the steam reforming in a CH 4 -H 2 O system. The polarization curves showed good agreement with the model reported by Mizusaki et al. for a H 2 -H 2 O system.

A novel oxygen sensor was proposed. The sensor, an electrochemical cell, was composed of a metal as reference electrode, its oxide scale as electrolyte and Pt or other adequate materials as sample electrode. It is expected that the electrolyte is self-restorative because it can be restored by high temperature oxidation. The emf measurements were carried out at 873 K in cells using zirconium. At $${\text{P}}_{{\text{O}}_{\text{2}} } = 1 \sim 10^{ - 3}$$ atm, the emf vs. $${\text{log P}}_{{\text{O}}_{\text{2}} }$$ plot lies on a straight line and its gradient is 2.303RT/4F, suggesting t ion=1 at the surface of the scale. The emf steeply decreases with decreasing $${\text{P}}_{{\text{O}}_{\text{2}} }$$ at $${\text{P}}_{{\text{O}}_{\text{2}} } < 10^{ - 3}$$ atm which can not be explained by the increase in the electronic conductivity and is explained by a gas laminar film phase at the surface of the sample electrode.

To prepare a high-temperature oxygen permeation membrane, the enhancement of oxygen permeation rate in calucia-stabilized zirconia by CeO{sub 2} doping was studied. Two tube-shaped specimens ({phi}1.3 {times} 0.9 cm), 0.5 and 1.5 cm in length, were used to measure the bulk oxygen permeability from a N{sub 2} + O{sub 2} gas mixture into He, eliminating the oxygen permeation caused by the short-circuiting effect of Pt O-rings which were used to seal the interface of specimens and alumina tubes. Since the oxygen source was N{sub 21} + O{sub 2}, mechanical oxygen leakage was estimated from nitrogen leakage into He. From the measurement of oxygen permeations at 1,100--1,800 K in [(ZrO{sub 2}){sub 1{minus}x}(CeO{sub 2}){sub x}]{sub 0.9}(CaO){sub 0.1} (X = 0.1--1.0), it was shown that the oxygen permeability, Jr, which was normalized by the temperature and the oxygen partial pressures outside and inside the specimen, and the electronic conductivity which was at 1 atm of oxygen partial pressure and was calculated from Jr, increased exponentially with temperature and X for X {le} 0.5 while showing saturation tendency for X > 0.5. The oxygen separation membrane of X = 0.4--0.5 seems reasonable because of the remarkable permeability and lower content of CeO{sub 2} whichmore » has higher vapor pressure at high temperature.« less

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).

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.

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Thermionic emission of oxygen negative ions, O-, from a bare surface of yttria-stabilized zirconia (YSZ) was studied with a quadrupole mass spectrometer equipped with an energy filter, which is capable of detecting negative ions. Emission characteristics of O- ions were investigated at temperatures ranging from 725 to 925°C under pressures of 1.4×10-4 to 3.2×10-4 Pa. Mass spectra of emitted negatively charged particles showed that O- ions are emitted into a vacuum from the bare YSZ surface at elevated temperatures. By examining the energy distributions of O- ions, the number of emitted O- ions was found to increase with the temperature of YSZ.

The far-infrared (FIR) absorption of the oxygen-ion conductor (CeO 2 ) 0.9 (CaO) 0.1 and for comparison, pure CeO 2 ceramics, has been measured in the region 10–100 cm -1 at 11 K and 297 K. Continu...

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 constitutedmore » 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.« less

The effect of electrochemical oxygen pumping on the emission of oxygen negative ions, O - , from the surface of yttria-stabilized zirconia (YSZ) was studied with a quadrupole mass spectrometer capable of detecting negativeions. It was found that O - ions are emitted into a vacuum from the YSZ surface with a porous platinum electrode. With a potentiostat, the voltage between an air electrode and the platinum electrode on the emission surface was varied during the emission of O - ions at 925°C: deviation from the rest potential ranged from -750 to +150 mV. Negative ion mass analysis demonstrated that electrochemical polarization affected the emission rate of O - ions. The effect of electrochemical polarization increased with the magnitude of the polarization. The emission of O - ions was enhanced with cathodic polarization, whereas it was reduced with anodic polarization. Although oxygen pumping by anodic polarization enhanced the mass flow of oxygen from air into the vacuum chamber, it did not enhance the emission of O - ions. The variation in the emission rate of O - ions is due to the variation in the work function of the emission surface by electrochemical polarization.

A novel oxygen negative ion source named solid oxide ion source (SOIS) is proposed. To demonstrate its concept, thermionic emission of oxygen negative ions, O - , from a bare surface of yttria-stabilized zirconia (YSZ) was studied with a mass spectrometer capable of detecting negative ions. The emission current was investigated at the temperature range 750 to 950°C under pressures of about 2 x 10 3 to 4 x 10 4 Pa. O ions proved to he emitted into a vacuum from the hare YSZ surface at elevated temperatures. The emission current increased with temperature and applied voltage. From Richardson-Dushman plots, activation energy of the thermionic emission was estimated to be about 2 eV. In continuous operation, the emission current decreased with time, finally approaching zero; however, after intervals, the current recovered. On the basis of the results, a model for explaining the emission mechanism of O ions is proposed. The model is based on three key processes occurring on the surface of electrolytes: (1) formation of O ions on the surface by electron capture of oxygen atoms provided by the migration of oxide ions across electrolytes, (2) accumulation of O ions on the surface, and (3) emission of O ions from the surface.

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.

太陽エネルギーの変換貯蔵を目的として, 光作用の立場から太陽光励起レーザーの開発, また, 熱作用の立場からの高温水蒸気電気分解に関する基礎的実験研究を行ない, これらの方法が太陽エネルギーの貯蔵に有効であることを明らかにした。光作用の立場からは, 口径 10m, 焦点距離 3.2m の大型太陽集光器を用いて, Nd:YAG 結晶により太陽光を波長 1.06μm のレーザー光に変換すること成功し, レーザー光出力として 40W を得た。熱作用の立場からは, ZrO2+8mol% Y2O3を固体電解質として用い, 太陽加熱を模擬する電気炉で 1070～1670K の温度範囲における高温水蒸気電気分解の実験を行ない, 効率 92% で水素が得られることを確認した。

The direct thermal decomposition of carbon dioxide CO2 to produce carbon monoxide CO have been studied as a mean of conversion and storage of a thermal energy, such as the solar heat, into a gaseous fuel. The process under investigation consists to displace the dissociation equilibrium of carbon dioxide by a selective pumping of oxygen at high temperature through a semipermeable membrane. Large amounts of carbon monoxide have been produced after the extraction of the oxygen arising from the CO2 dissociation with the aid of a membrane made of calcia-stabilized zirconia. The variations of the flow of CO evolved from the reactor vs. temperature and oxygen partial pressure are in good agreement with the theoretical variations that may be deduced from the general equations of electrochemical transport in stabilized zirconia. Thereby, the efficiency of this process appears to be directly limited by the low oxygen permeation rate through the material used as oxygen selective membrane.

By using a large solar furnace, yttrium oxide crystals were made by a fusion process, and their reflectivity was measured in the spectral region 700 to 10 cm -1 , Several reststrahlen bands extending from 600 to 240 cm -1 were found. From the reflectivity curve, the parameters of seven lattice vibrations were analysed, and the optical constants in a wide spectral region were deduced. The transmittance measurement was made by using crystalline powder embedded in a polyethylene film and several weak bands were found in the far-infrared region. By measuring the transmittance of a thin plate of fused crystal, a number of summation bands were observed in the near-infrared region. In summarizing the bands, some considerations were given on the vibrational behaviour of yittrium oxide crystals.

The direct thermal decomposition of carbon dioxide CO2 to produce carbon monoxide CO have been studied as a mean of conversion and storage of a thermal energy, such as the solar heat, into a gaseous fuel. The process under investigation consists to displace the dissociation equilibrium of carbon dioxide by a selective pumping of oxygen at high temperature through a semipermeable membrane. Large amounts of carbon monoxide have been produced after the extraction of the oxygen arising from the CO2 dissociation with the aid of a membrane made of calcia-stabilized zirconia. The variations of the flow of CO evolved from the reactor vs. temperature and oxygen partial pressure are in good agreement with the theoretical variations that may be deduced from the general equations of electrochemical transport in stabilized zirconia. Thereby, the efficiency of this process appears to be directly limited by the low oxygen permeation rate through the material used as oxygen selective membrane.