Zirconia polymorphic structures

Ardizzone, S. and Bianchi, C.L., Electrochemical features of zirconia polymorphs. The interplay between structure and surface OH species, 7. Electroanal. Chem., 465, 136, 1999. 

The crystallographic data of the most important polymorphic structures of zirconia are listed in Table 7.6. 

Cubic stabilized zirconia (CSZ) Pure zirconia (Zr02) is either chemically extracted and purified from the mineral zircon (ZrSi04) or purified from baddeleyite. It occurs as three crystalline polymorphs with monoclinic, tetragonal and cubic structures. The monoclinic form is stable up to 1170°C... 

Figure 5. Schematic arrangement of the surface of a partly crystallized E-L TM amorphous alloy such as Pd-Zr. A matrix of zirconia consisting of the two polymorphs holds particles of the L transition metal (Pd) which are structured in a skin of solid solution with oxygen (white) and a nucleus of pure metal (black). The arrows indicate transport pathways for activated oxygen either through bulk diffusion or via the top surface. An intimate contact with a large metal-to-oxide interface volume with ill-defined defective crystal structures (shaded area) is essential for the good catalytic performance. The figure is compiled from the experimental data in the literature [26, 27].

The crystal structure of zirconia and the catalytic properties of SZ generally depend on the synthesis method and thermal treatment adopted. In particular zirconia crystallises in three different polymorphs characterised by monoclinic, tetragonal and cubic symmetry. Among them only the tetragonal SZ phase displays significant catalytic properties [5-7]. Unfortunately, the synthesis of the pure tetragonal polymorph is difiBcult and, in the absence of promoted oxides [8], it could be stabilised only through an accurate control of the synthesis parameters, with particular attention to the thermal treatments. 

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 ... 

Microstructural Aspects of Zirconia Ceramics. Zr02 is polymorphic and can have one of three crystal structures monoclinic (m) at ambient temperatures. 

A SOFC was proposed by Baur and Preis as far back as 1937 based upon an electrolyte of stabilised zirconia with metallic electrodes. Since then stabilised zirconia has been the electrolyte that has received most attention by fuel cell developers. Most zirconia electrolytes are based upon either yttria or scandia stabilisation of the tetragonal poly-morph, commonly referred to as YSZ and ScSZ, respectively, although a number of alternative dopants have been investigated (Tables 2.1 and 2.2). Conventionally the substitution level is between 3mol% and 8 mol% for the yttria-based materials and at 10-12 mol% for the scan-dia-based materials. The choice of the dopant level is dictated by a compromise between mechanical robustness and overall conductivity, as summarised in Table 2.1. Substitution of zirconia results in the stabilisation of either the tetragonal or cubic polymorphs adopting the fluorite type structure as shown in Figure 2.2. This substitution... 

Figure 2.2 Schematic representation of the (a) cubic and (b) tetragonal polymorphs of the fluorite structured yttria stabilised zirconia. Metal ions in grey, oxygen in black...

---