Fluorite structured electrolytes

V represents the oxygen vacancy created for every 2M atoms incorporated into the lattice. The vacancy concentration follows the electrical neutrality condition  

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Properties and fabrication of two of the most common fluorite structured electrolyte materials, zirconia based and ceria based, are discussed below. 

In addition to fluorite structure electrolytes such as stabilised zirconia and ceria, there are many non-fluorite structure oxides which are potentially attractive for SOFC electrolyte application. These include perovskites like lanthanum gallate and to a lesser degree calcium titanate. Alternative oxides are the pyrochlores such as yttrium zirconate (YZr207)and gadolinium titanate (Gd2Ti207) [49,50], but these are only suitable in very limited oxygen pressure ranges. Therefore, the main discussion here focuses on the perovskites. 

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). 

The fluorite oxides are the classical oxygen ion conducting oxide materials the study of these materials as electrolytes derives from the early investigations of Walther Nemst 1900. The fluorite structure illustrated in Figure 13 is best described for the purposes of the present discussion as a primitive cubic array of anions (O ) with half the cube centers occupied by cations the latter form a face-centered-cubic (fee) arrangement of the cation sublattice. The unoccupied cube centers play a central role in the defect physics and ionic conductivity of fluorites, because they... 

The ionic transport properties of fluorite-type oxide phases (see Chapter 2), another important family of solid electrolytes, are also discussed in subsequent chapters. Briefly, for the well-known zirconia electrolytes, Zr itself is too small to sustain the fluorite structure at moderate temperatures doping with divalent (Ca + ) or trivalent (e.g., Y " ", St " ", Yb " ") cations stabilizes the high-temperature polymorph with the cubic fluorite-type structure. Due to the electroneutrahty condition, anion vacancies are formed ... 

Several fluorides form the cubic fluorite structure (including CaF2 for which it is named), and are excellent fluoride ion conductors [50] (see Chapter 2). With the introduction of an oxide phase, either during fabrication or in situ during operation, fluoride ion conductors can be used as electrolytes in oxygen sensors [51]. One ofthe advantages of using fluoride electrolytes is that they tend to remain pure ionic... 

The highly conducting Zr02 crystalhzes in the fluorite structure, which stabilizes by doping with Y2O3 or CaO. Zr02 is used in several electrochemical developments, e.g., as electrolyte in solid oxide fuel cells (SOFCs) or for an electrochemical oxygen sensor in the car industry (A,-probe). 

Rare-earth elements are vital constituents of several prominent high-temperature solid electrolytes ranging from oxygen- or fluoride-ion conductors in the fluorite structures to protonic conductors in the doped perovskite phases and trivalent-ion conduction in Sc2(W04)3 and 3-alumina-type compounds. Solid electrolytes are considered as important for scientific studies and technological applications in vital areas such as fuel cells, batteries, sensors, process control and environmental protection. 

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