Preparation and Oxygen Permeability of Gd-Doped Ceria and Spinel-Type Ferrite Composites
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Preparation and Oxygen Permeability of Gd-Doped Ceria and Spinel-Type Ferrite Composites Hitoshi Takamura, Masashi Kawai, Katsutoshi Okumura, Atsunori Kamegawa, and Masuo Okada Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, JAPAN. ABSTRACT The preparation and oxygen permeability of composites of Ce0.8Gd0.2O2-δ (GDC) and spinel-type ferrites, MFe2O4 (M = Co and Mn) have been investigated. The composites of GDC x vol% MFe2O4, where x ranged from 5 to 65, were prepared by a citrate-based liquid-mix technique. The composites were found to be almost fully densified by sintering at 1300 °C for 2 h. From TEM observations, the grain size of GDC and spinel-type phases was found to be less than 0.5 µm. In the case of M = Co, GDC - 25 vol% CoFe2O4 with a membrane thickness of 1.0 mm exhibited an oxygen flux density of 0.21 µmol·cm-2·s-1 under the P(O2) difference between He (20 sccm) and air at 1000 °C. Under reducing atmosphere of Ar-5%H2, the oxygen flux density of this composite increased up to 1.3 µmol·cm-2·s-1. Moreover, under Ar-10%CH4 gas flow, GDC - 15 vol% MnFe2O4 with a membrane thickness of 0.24 mm exhibited the oxygen flux density of 2 and 7 µmol·cm-2· s-1 at 800 and 1000 °C, respectively. INTRODUCTION Oxygen separation membranes based on mixed oxygen-ion and electronic conductors have been extensively studied because of their promising applications such as oxygen and syngas production [1]. Perovskite-type oxides in La-Sr-Co-Fe and La-Sr-Ga-Fe systems are well known to exhibit a high oxygen flux density at elevated temperatures of 750 ~ 1000 °C [2, 3]. To date, the highest value of 8.2 µmol·cm-2·s-1 has been reported for La0.7Sr0.3Ga0.6Fe0.4O3-δ with a membrane thickness of 0.3 mm at 1000 °C [4]. In addition to these single-phase mixed conductors, dual-phase-type ones comprising of an ionic conductor such as yttria-stabilized zirconia (YSZ) and an electronic conductor, for example, a precious metal of Pd, have been developed as well [5]. For the composite-type mixed conductor, it is possible to choose good ionic and electronic conductors as components, and to control the mixed conductivity by adjusting the volume fraction under the restriction of the percolation theory. At the same time, since the surface oxygen exchange takes place at the three-phase-boundary (TPB) regions, fine microstructures are essential to obtain high oxygen flux densities. In addition to this, in the case of using oxide-based electronic conductors, the combination of ionic and electronic conductive phases, which does not form insulating bi-product phases, has to be carefully selected. As such ceramics-based composites, the combinations of Gd-doped CeO2 (GDC) as the ionic conductor and Sr-doped LaMnO3 (LSM) or Ca-doped GdCoO3 (GCC) as the electronic conductor have been reported [6, 7]. GDC is a well-known oxygen-ion conductor and recently receiving much attention as a model material to investigate nano-scale effects [8-10]. As another class of composite-type mixed conductors base
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