Perovskite and Pyrochlore Oxides

Alternate Oxygen Ion Conducting Electrolytes 17.3.7.4.3. Perovskite and Pyrochlore Oxides. 

Metal oxides are usually prepared by calcinations of suitable precursors such as hydroxides, nitrates, carbonates, carboxylates, etc. This process usually gives oxides with pseudomorphs of the starting materials. When large amounts of thermal energy are applied for the decomposition of the precursors, it facilities sintering of the product particles and therefore aggregated particles are obtained. When mixed oxides such as spinel, perovskite, and pyrochlore are the desired products, heat treatment at higher temperatures is required. 

Other oxygen ion conductors that have potential use as solid electrolytes in electrochemical devices are stabilized bismuth and cerium oxides and oxide compounds with the perovskite and pyrochlore crystal structures. The ionic conductivity and related properties of these compounds in comparison with those of the standard yttria-stabilized zirconia (YSZ) electrolyte are briefly described in this section. Many of the powder preparation and ceramic fabrication techniques described above for zirconia-based electrolytes can be adapted to these alternative conductors and are not discussed further. 

Noble metals (e.g., platinum, gold, and silver), metal oxides, mixed oxides such as spinels, perovskites, and pyrochlores have been investigated, but by far the best catalytic material is highly dispersed platinum. The high cost of this metal stimulated research to look for less expensive alternative materials... 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

As described in Section 8.2.6, along with YSZ, mixed oxygen-ion, and electron-conducting oxides with a perovskite-type structure, the so-called Aurivillius phase and pyrochlore materials are fundamentally used for the production of a variety of high-temperature electrochemical devices [50-58],... 

Corrosion of the Pu-doped pyrochlore-based ceramics is by incongruent dissolution and segregation of secondary phases [40], Leach rates in 1-year PCT-B tests were found to be (in g/(m xday) Ca - 10, Gd - 10, Pu - 10, Zr - <10. The leach rate of Pu is reduced by one order of magnitude - from lO" g/(m day) in short tests (1 to 7 days) to 10 g/(m xday) after 112-324 days of interaction with water [105], Introduction of 8 to 15 wt.% of oxides of typical contaminants (F, Cl, Na, Mg, K, Na, Si, Al, Ga, Mo, W, etc.) yielded extra glass, perovskite, and Ca-Al-Ti phase instead of brannerite [117]. This does not affect or even improve the chemical durability with respect to actinides. 

Perovskite (ABO3 in which A is divalent and B is tetravalent) and pyrochlore (AaBaOi in which A is trivalent and B is tetravalent) oxide compounds have been proposed as oxygen ion conducting electrolytes for electrochemical devices. Some of the perovskite structures (e.g., BaCeO and SrCeOs) are generating interest because of... 

Additional attempts have been presented to render hosts with the fluorite and the related pyrochlore structure electronically conductive by doping with mixed-valence and/or shallow dopants. The list of dopant materials examined includes oxides of elements of, for example, Ti, Cr, Mn, Fe, Zn, Fe, Sn, Ce, Pr, Gd, Tb and U. In general, however, the extent of mixed conductivity that can be obtained in fluorite-type ceramics is rather limited, by comparison with the corresponding values found in some of the perovskite and perovskite-related oxides considered in the next section. 

As the anion-exchange membrane fuel cell is the alkaline-based system, we can use non-platinum-based catalyst. This is a big advantage to lower the cost of fuel cells. Especially perovskite-type and pyrochlore-type oxides have high performance to oxygen-electrocatalysts which could be applicable to the cathode materials. Some oxides have also bifunctional activities as oxygen electrode catalyst to produce a reversible fuel cell thus, future deployment is expected. While, the big problems are stability of the base... 

Oxides with close-packed oxygen lattices and only partially filled tetrahedral and octahedral sites may also facilitate diffusion of metal ions in the unoccupied, interstitial positions. Finally, even large anions may diffuse interstitially if the anion sublattice contains structurally empty sites in lines or planes which may serve as pathways for interstitial defects. Examples are rare earth sesquioxides (e.g. Y2O3) and pyrochlore-type oxides (e.g. La2Zr207) with fluorite-derived structures and brownmillerite-type oxides (e.g. Ca2Fe205) with perovskite-derived structure. 

A review paper by Neburchilov et al. (2001) discusses the composition, design, and fabrication methods of air cathodes for alkaline zinc-air cells, one of the few successfully commercialized semi-fuel cells. The more promising compositions for air cathodes are based on individual oxides, or mixtures of such, with a spinel, perovskite, or pyrochlore structure MnOa, C03O4, La203, LaNi03, NiCo204, LaMnOs, LaNi03, and so on. 

In alkaline solutions, bifunctional properties are exhibited by catalysts having the pyrochlore structure A2B2O7, where A = Pb, Bi and B = Ru, Ir (Horowitz et al., 1983), and by oxide catalysts having the perovskite structure (e.g., Lao,6Cao,4Co03) (Wu et al., 2003). The properties of bifunctional oxygen electrodes are discussed in greater detail in a paper by Jorissen (2006). 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

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