Electrolytes stabilized bismuth oxide

With the conductivity of an aqueous electrolyte (e.g., IN KCl) serving as a reference, comparable conductivities can be achieved in solid electrolytes under certain conditions. Some of the best solid ionic conductors, commonly referred to as superionic conductors , have resistivities comparable to those of aqueous electrolytes at room temperature (e.g., RbAg4l5 and single crystal MgO-stabilized 6"-alumina). However, they are either in the form of single crystals, which is impractical for most applications, or composed of very expensive and relatively unstable materials. Resistivities comparable to those of aqueous electrolytes can be achieved in solid electrolytes at higher temperatures in both superionic conductors like 6"-alumina (i.e., 300°C) and normal ionic conductors such as stabilized zirconia (800-1000°C), stabilized cerium oxide (>800 C), and stabilized bismuth oxide (>600°C). Sodium ion conducting glasses are much less conductive than polycrystalline 8 -alumina. 

The ion conductivity of bismuth oxide is decreased with increasing concentration of Y2O3 dopant. Dopant concentrations of at least 25 mol% Y2O3 are necessary to stabilize the cubic structure at temperatures below 730°C. The higher conductivity of stabilized bismuth oxide compared to yttria-stabilized zirconia offers the possibility of its use as a solid electrolyte in the solid oxide fuel cell at reduced temperatures. However, the... 

As seen from Table 10.1 impressive oxygen fluxes have been reported through 25 mol% yttria-stabilized bismuth oxide (BY25) [110] and 25 mol% erbia-stabilized bismuth oxide (BE25) [111,112], which oxide electrolytes were rendered electronically conductive by dispersion with silver metal. A prerequisite is that both constituent phases in the composite membranes do form a continuous path for both ionic and electronic conduction, having their concentrations above the critical (percolation threshold) volume fraction <])(,. The latter quantity determines the minimum volume fraction in which conduction is possible and is a function of, for example, the relative dimensions and shape of the particles of both constituent phases [113]. In actual composite materials,... 

As seen from Table 14.1 impressive oxygen fluxes have been reported through 25 mol% yttria-stabilized bismuth oxide (BY25) and 25 mol% erbia-stabihzed bismuth oxide (BE25), 2 3° which oxide electrolytes were rendered electronically conductive by dispersion... 

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

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. 

Complex FCC oxides of the fluorite type represent oxygen-conduction solid electrolytes (SOE s). They comprise a typical class of materials for the manufacture of sensors of oxygen activity in complex gas mixtures, oxygen pumps, electrolyzers and high-temperature fuel elements. These materials are based on doped oxides of cerium and thorium, zirconium and hafnium, and bismuth oxide. Materials based on zirconium oxide, for example, yttrium stabilized zirconia (YSZ) are the most known and studied among them. This fact is explained both by their processibility and a wide spectrum of practical applications and by the possibility to conduct studies on single crystals, which have the commercial name "fianites" and are used in jewelry. 

Bismuth Oxide Based Electrolytes The cubic high temperature phase of bismuth oxide -Bi203 can be stabilized by adding of two- and three-valent ions as Sr or [5]. This is accompanied by a decrease in conductivity as compared to the pure 8-phase. Bismuth oxide-based electrolytes exhibit the highest ionic conductivity from all oxide systems, but they are not stable under reducing conditions and therefore not suitable for the most applications of oxide ion electrolytes. They can be used in cells for the oxygen enrichment in air. 

Jiang, Z.Y., Zhang, L., Cai, L.L. Xia, C.R. Bismuth oxide-coated (La,Sr)Mn03 cathodes for intermediate temperature solid oxide fuel-cells with yttria-stabilized zirconia electrolytes. Electrochim. Acta 54 (2009c), pp. 3059-3065. 

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