Bismuth oxide stabilizers

Bismuth oeeurs mainly as bismite (a-Bi203), bismuthinite (Bi2S3) and bismutite [(Bi0)2C03] very oeeasionally it oeeurs native, in assoeiation with Pb, Ag or Co ores. The main eommereial souree of the element is as a byproduet from Pb/Zn and Cu plants, from whieh it is obtained by special processes dependent on the nature of the main product. Sulfide ores are roasted to the oxide and then reduced by iron or charcoal. Because of its low mp, very low solubiUty in Fe, and fairly high oxidative stability in air, Bi can be melted and cast (like Pb) in iron and steel vessels. Like Sb, the metal is too brittle to roll, draw, or extrude at room temperature, but above 225°C Bi can be worked quite well. 

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). 

FIGURE 1.2 Composition dependence of conductivity for yttria-stabilized zirconia (YSZ) measured at 1000°C [7], yttria-doped bismuth oxide (YDB) at 600°C [6], and yttria-doped ceria (YDC) at 700°C [8],... 

A second class of membranes are described as dense membranes. They may consist of thin plates of metals (Pd and its alloys, Ag and some alloys) or oxides (stabilized zirconia or bismuth oxides, cerates). These membranes are permeable to atomic (for metals) or ionic (for oxides) forms of hydrogen or oxygen and have been studied, especially, in conjunction with chemical... 

Oxygen (O2-) anion conductors - stabilized zir-conia, stabilized - bismuth oxide, - BIMEVOX, doped cerium dioxide, numerous perovskite-type - solid solutions derived from Ln(A)B (B") 03 (A = Ca, Sr, Ba B = Ga, Al, In B" = Mg, Ni, Co, Fe), La2Mo207 and its derivatives, pyrochlores based on Ln2Ti07. 

Joshi, A.V. et al., Phase stability and oxygen transport characteristics of yttria- and niobia-stabilized bismuth oxide, J. Mater. Sci., 25, 1237-1245 (1990). 

Wachsman, E.D., Boyapati, S., Kaufman, M.J., and Jiang, N.X., Modeling of ordered structures of phase-stabilized cubic bismuth oxides. Journal of the American Ceramic Society, 2000, 83, 1964-1968. 

Boyapati, S., Wachsman, E.D., and Chakoumakos, B.C., Neutron diffraction study of occupancy and positional order of oxygen ions in phase stabilized cubic bismuth oxides. Solid State Ionics, 2001, 138, 293-304. 

Stabilized bismuthonium ylides have also been made from compounds having reactive methylene groups, CH2CXY, X, Y = RCO or RSO2, either by making their sodium salts and letting the latter react with dichlorotriphenylbismuth, or by reaction with triphenyl-bismuth oxide 1 181. 

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... 

Fig. 10.6. Data for the surface oxygen exchange rate, normalized to air, of 25 mol% erbia-stabilized bismuth oxide (BE25) from (a) isotopic exchange and (b) oxygen permeation measurements. Reprinted...

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,... 

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