Atsushi Mineshige

Oxygen chemical potential variation in ceria-based solid oxide fuel cells determined by Raman spectroscopy

The profile of oxygen chemical potential in 20 mol% Sm-doped ceria Ce0.8Sm0.2O2−δ (SDC) at 1273 K under the open-circuit conditions of solid oxide fuel cells was determined using Raman spectroscopy. SDC pellets were annealed in various atmospheres, and a peak intensity of a Raman band of SDC in the range 540–600 cm−1, which has been assigned to a band that originates in the oxygen vacancies, increased with decreasing oxygen partial pressure [P(O2)] upon annealing. A clear relationship between the peak area of the oxygen-vacancy band and P(O2) upon annealing was obtained. This relationship was used to determine the profile of oxygen partial pressure, P(O2), in SDC located between fuel and air. The experimental profile was in good agreement with that obtained theoretically. The profile revealed that most part of SDC was mixed conductive and only a thin layer adjacent to the air side remained purely ionic conductive under the open-circuit conditions of SOFCs.

Growth rate of yttria-stabilized zirconia thin films formed by electrochemical vapour-deposition using NiO as an oxygen source II. Effect of the porosity of NiO substrate

Abstract Yttria-stabilized zirconia (YSZ) thin films were formed at 1000°C by a modified electrochemical vapour-deposition (EVD) using NiO as an oxygen source, and ZrCl4 and YCl3 as metal sources. Growth rate kinetics were examined using NiO pellet substrates with different pore structures. The thickness of YSZ film increased linearly with deposition time, and the growth rate increased with increasing the porosity of the substrate. The pore size as well as the porosity affected the growth rate. In addition, the observed growth rate was much slower than the theoretical one assuming that the electrochemical transportation of the charged species across the growing film is rate limiting. From these results, it was concluded that the rate-determining step is not the bulk electrochemical transport, but the mass transport of dissociated oxygen in the substrate pore.

Ionic and Electronic Conductivities and Fuel Cell Performance of Oxygen Excess-Type Lanthanum Silicates

Highly dense pellets of an oxygen excess-type lanthanum silicate (La 9.333+x Si 6 O 26+1.5x , x > ca. 0.3, OE-LSO) were successfully fabricated, and their electrical conducting properties were studied. The replacement of Si by Al enhanced its conductivity, and the slightly Al-doped OE-LSO specimen [La 9.62 (Si 5.79 Al 0.21 )O 26.33 ] had excellent features as a solid electrolyte; that is, it had high ionic conductivity and was highly stable under reducing as well as oxidizing conditions at 873-1073 K. In addition, the ionic transference number was higher than 0.99. In the fuel cell utilizing this electrolyte (0.72 mm thick), (La 0.6 Sr 0.4 )(Co 0.2 Fe 0.8 )O 3―δ cathode, and Ni―Ce 0.9 Gd 0.1 O 1.95―δ anode, good performance with the maximum power density of ca. 0.25 W cm ―2 was obtained at 1073 K. In addition, this electrolyte also had high compatibility with these conventional mixed conducting electrodes, according to an analysis near the electrode/electrolyte interfaces after the fuel cell test.

Effects of Pt/SrRuO3 Top Electrodes on Ferroelectric Properties of Epitaxial (Pb, La)(Zr, Ti)O3 Thin Films

Epitaxially c-axis oriented PLZT films with the composition (Pb0.925La0.075)(Zr0.4Ti0.6)O3 were deposited on Pt/MgO(100) substrate by rf-magnetron sputtering using a compacted powder target with the composition (0.8PLZT+0.2PbO). Pt/SrRuO3(SRO)/PLZT/Pt capacitors were successfully fabricated by forming Pt/SRO top electrodes onto PLZT films. The 206-nm-thick PLZT films with layered Pt/SRO top electrodes exhibited a slightly high leakage current at a low electric field, compared with PLZT films with the same thickness and a single Pt top electrode. The values of switchable polarization after 1010 cycles for Pt/PLZT/Pt capacitors decreased up to around 17% of their initial values, whereas the switchable polarization for Pt/SRO/PLZT/Pt capacitors hardly exhibited any fatigue degradation due to polarization reversal. The layered Pt/SRO is useful as a top-electrode material for fabricating the low leakage and high endurance ferroelectric capacitors.

Oxygen chemical potential and mixed conduction in doped ceria under influence of oxygen partial pressure gradient

Abstract The procedure to evaluate an oxygen chemical potential in Sm2O3-doped ceria, Ce0.8Sm0.2O2−δ (SDC), was developed using Raman spectroscopy. The oxygen chemical potentials of any positions in the SDC samples placed between fuel and air at 1273 K could be determined throughout the entire range of P(O2) by using relative areas and intensities of their Raman spectra. Using this method, P(O2) variation in the SDC disc-shaped sample of 4 mm in thickness and 15 mm in diameter was determined by evaluating P(O2) between fuel- and air-side faces, and along the radius inside the pellet under the open-circuit conditions of SOFCs. In addition, the variation of total (ionic and electronic) conduction in the sample under the influence of P(O2) gradient was discussed.

An X-ray absorption spectroscopic study on mixed conductive La0.6Sr0.4Co0.8Fe0.2O3−δ cathodes. I. Electrical conductivity and electronic structure

The electrical conduction mechanism of mixed conductive perovskite oxides, La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ), for cathode materials of solid oxide fuel cells has been investigated from electronic structural changes during oxygen vacancy formation. La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) was annealed under various oxygen partial pressures p(O(2))s at 1073 K and quenched. Iodometric titration indicated that the oxygen nonstoichiometry of La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) depended on the annealing p(O(2)), with more oxygen vacancies introduced at lower than at higher p(O(2))s. X-Ray absorption spectroscopic measurements were performed at the O K-, Co L-, Fe L-, Co K-, and Fe K-edges. The valence states of the Co and Fe ions were investigated by the X-ray absorption near edge structure (XANES) at the Co and Fe L(III)-edges. While the Fe average valence was almost constant, the valence of the Co ions decreased with oxygen vacancy introduction. The O K-edge XANES spectra indicated that electrons were injected into the Co 3d/O 2p hybridization state with oxygen vacancy introduction. Both absorption edges at the Co and Fe K-edge XANES shifted towards lower energies with oxygen vacancy introduction. The shift at the Co K-edge resulted from the decrease in the Co average valence and that at the Fe K-edge appeared to be caused by changes in the coordination environment around the Fe ions. The total conductivity of La(0.6)Sr(0.4)Co(0.8)Fe(0.2)O(3-δ) decreased with decreasing p(O(2)), due to a decreasing hole concentration.

Growth rate of yttria-stabilized zirconia thin films formed by electrochemical vapor-deposition using NiO as an oxygen source

Abstract Thin films of yttria-stabilized zirconia (YSZ) were formed onto nickel oxide substrates by an electrochemical vapor deposition process using the substrates as oxygen sources, and ZrCl 4 and YCl 4 as metal sources. After deposition at 1000 °C for 1–6 h, dense films of cubic YSZ were formed. The film thickness increased linearly with deposition time, and the linear growth rate constant was 1.1 × 10 −4 μ m s −1 . The deposition kinetics were discussed, and it was concluded that the rate-determining step of this process is not the electrochemical transportation of the oxide ions and electrons through the growing film, but the mass transport of oxygen gas, which is dissociated from the NiO substrate, through the substrate pores to the NiO/YSZ interface.

Relationship between Pyroelectric Properties and Electrode Sizes in (Pb, La)(Zr, Ti)O3 (PLZT) Thin Films

Highly c-axis-oriented PLZT films with compositions of (Pb0.925La0.075)(ZryTi1-y)0.981O3, where y=0.2–0.4, were deposited on Pt(100)/MgO(100) by the rf-magnetron sputtering, applying an intermittent deposition comprised of the repetition of deposition and nondeposition. It was confirmed that the highest figures of merit, F.M. and F.M.D* for the pyroelectric IR sensors were obtained by using powders with excess PbO of 20 mol% added to PLZT, as sputtering targets in the preliminary experiment. The PLZT film with y=0.2 exhibited the highest F.M. for voltage responsivity Rv of approximately 5.3×10-13 Cm/J among the three samples studied, which was around 1.8 times larger than that of PLZT ceramic with y=0.2. The top-electrode sizes of around 500 and 250 µm diameters were suitable for the measurement of pyroelectric properties, judging from the characteristics of the materials and the accuracy of the measuring instruments.

Characterization of (Bi3.25Nd0.75)Ti3O12 Thin Films with a- and b-Axis Orientations Deposited on Nb:TiO2 Substrates by High-Temperature Sputtering

a- and b-axis-oriented (Bi3.25Nd0.75)Ti3O12 (BNT-0.75) films, 3.0 µm thick, were fabricated on conductive Nb:TiO2(101) substrates with 0.001–0.79 mass % Nb at 650 °C by high-temperature sputtering. All the films had a mostly single-phase orthorhombic structure and a- and b-axis orientations. The degree of a- and b-axis orientations was high, with values of ≥96%. BNT-0.75 films grown heteroepitaxially on Nb:TiO2(101) substrates containing 0.79 mass % Nb were comprised of nanoplate-like crystals and exhibited the best hysteresis loop shapes, with a remanent polarization (2Pr) of 29 µC/cm2 and a coercive field (2Ec) of 297 kV/cm.

Porous Metal Tubular Support for Solid Oxide Fuel Cell Design

A porous nickel tubular support of 1 mm diam was successfully created by heat-treating a commercial nickel tube. The resulting tube contained uniform pores, whose diameters ranged from 0.5 to 2.5 μm, depending on treatment temperature. The porous metal tube obtained is a promising candidate for support materials for microfuel cells or gas separation membranes because it has the necessary gas permeability and mechanical strength. It was also confirmed that a solid oxide fuel cell design employing the porous nickel tube as a supporting anode could be operated.