Masafumi Kobune

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.

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.

EFFECTS OF MANGANESE ADDITION ON PYROELECTRIC PROPERTIES OF (Bi0.5Na0.5 TiO3)0.94(BaTiO3)0.06 CERAMICS

ABSTRACT (Bi0.5Na0.5TiO3)0.94(BaTiO3)0.06 (BNBT-6) ceramics without additives and BNBT-6 ceramics with 0–0.8 mass% MnO2 were produced by a conventional ceramic fabrication technique. The effect of MnO2 addition on the pyroelectric coefficient, P, is investigated in detail. The figure of merit, F D, for specific detectivity of pyroelectric infrared sensors is calculated using the specific heat obtained and other pyroelectric parameters, and the optimal MnO2 content is discussed. The measurement revealed the detailed temperature dependence of the volume specific heat, and the room-temperature volume specific heat of BNBT-6 ceramics (2.8 JK−1 cm−3) is lower than that of PZT (3.1 JK−1 cm−3). The maximum P value, obtained at 0.2 mass% MnO2, was 3.5 × 10−8 C/cm2 K. The present BNBT-6 ceramics with 0.2 mass% MnO2 exhibited an excellent F D that was 4.7 × 10−9 Ccm/J. The BNBT-6 ceramics with 0.2 mass% MnO2 are highly promising as lead-free pyroelectric materials for pyroelectric infrared sensors.

Preparation and pyroelectric properties of (Pb, La)(Zr, Ti)O3 (PLZT) thin films

PLZT films around 3 μm thick with compositions of ((Pb 0.925 La 0.075 )(Zr y Ti 1-y ) 0.981 +0.5 mass%MnO 2 } where y = 0-0.10, which have an appropriately small relative dielectric constant (∼200) were deposited on Pt(100)/MgO (100) by rf-magnetron sputtering. The pyroelectric properties of PLZT films as element materials for pyroelectric IR sensors were investigated in detail. Assuming that the volume specific heat C v of the present films is 3.2 J/cm 3 .K, the PLZT films exhibited the highest F.M. of 1.4 X 10 -10 C.cm/J and F.M.D* of 1.6 X 10 -8 C cm/J, which were around 1.8 times and 1.5 times larger than those of PLT films, respectively. Consequently, the figures of merit for the pyroelectric IR sensors could be improved by adding a small amount of Zr to PLT.

Bulk photovoltaic effect in a BiFeO3 thin film on a SrTiO3 substrate

Epitaxial BiFeO3 (BFO) thin films with striped- and single-domain structures have been grown on SrTiO3 (STO) (103) and (113) substrates by radio-frequency planar magnetron sputtering. The domain structure of BFO was controlled by the orientation of the STO substrate. Piezoelectric force microscopy revealed that BFO thin films on STO (103) and STO (113) had a striped-domain structure with 71° domain walls running along 〈010〉STO, and a single-domain structure, respectively. To confirm the photovoltaic property, rectangular Pt electrodes with widths of 150–200 µm were deposited on BFO surfaces with interelectrodes distances of 200–250 µm. I–V characteristics were measured under an illumination of a collimated violet laser (λ = 405 nm) with a power density of 380 W/cm2. In the striped-domain-structure BFO film with Pt electrodes fabricated along domain walls, above-band-gap open-circuit voltage (VOC) of 29 V was observed. In addition, single-domain-structured BFO thin film with Pt electrodes fabricated along also showed above-band-gap Voc of 26 V despite the absence of domain walls. It is considered that these large Voc values originated from the photovoltaic effect not at the domain walls but in bulk BFO.

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.