Polymorphism of zirconia

Zirconium dioxide (or zirconia Z1O2) is found in natural state in the form of baddeleyite (primarily in South Africa), but is more frequently prepared from zirconium silicate sands (zircon ZrSi04) by high temperature heat treatments, accompanied by chemical treatments, which eliminate the siliceous fraction from the zircon. 

Among the uses of stabilized zirconia, denoted SZ , we can mention four main fields  

SZ has rather modest mechanical properties, significantly less remarkable compared to alumina which, associated with higher density, higher thermal expansion (consequently greater sensitivity to thermal shocks) and markedly increased costs explain why these stabilized zirconias a priori do not have a mechanical application. 

It is partially stabilized zirconias (PSZ) that have justified the resounding article ( Ceramic steel ) published in 1975 by Garvie et al. [GAR 75], The title suggests that a ceramic can exhibit the high mechanical performances associated with steel, but also that toughening mechanisms recall those used by steel manufacturers. The t- m transformation of zirconia is a martensitic transformation, in analogy with the transformation used to obtain martensite in tempered steels, and the role of microstructural parameters inZr02 is similar to what is observed in metals. 

The mechanical properties of zirconias with high mechanical performances - of which there are multiple varieties - constitute one of the themes that have inspired the greatest number of pubhcations in the field of ceramics. [HAN 00] constitutes an excellent study on the subject. 

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The possibility to stabilize the high-temperature polymorphs of zirconia, tetragonal or cubic, at ambient temperature by doping monoclinic TxOi is well-known. As a consequence of cationic substitutions for by Mg, Ca, or of anionic substitution anion vacancies are created, as required by charge neutrality. These... 

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].

Figure 6.11. A. O natural abundance MAS NMR spectra of zirconia polymorphs. The peak marked A is from oxygen in the alumina rotor. The asterisks denote spinning side bands. From Bastow and Stuart (1990) by permission of Elsevier Science. B. MAS NMR spectra of titania gel heated to various temperatures, showing the evolution of rutile at the expense of anatase. From Bastow et at. (1993) by permission of the Royal Society of Chemistry. C. Static and MAS O NMR spectra of cubic Y2O3. Adapted from Florian et at. (1995).

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. 

Ardizzone, S. et al.. Bulk surface and double layer properties of zirconia polymorphs subjected to mechanical treatments. Mater. Chem. Phys., 28, 399, 1991. 

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. 

Zirconia is known to exist as three, well-defined polymorphs, namely monoclinic, tetragonal, and cubic [173], although the existence of a high-pressure orthorhombic form has also been reported [174]. The lattice parameters of zirconia polymorphs are... 

One of the key features of zirconia lies in its polymorphism. Zirconia exhibits three polymorphs. The monoclinic phase is stable up to 1170°C where it transforms to the tetragonal phase, which is Itself stable up to 2370 C. Above this temperature, zirconia exists as a cubic Cap2 type phase. The reversible m- to t-Zr02 transformation is key to the use of zirconia in ceramics. First, it is reversible but occurs with a thermal hysteresis. Second, it is rapid and takes place by a diffusionless shear process similar to that of a martensitic transformation. Finally, it is dependent on particle size and occurs with a volume change (3 to 5%) [82]. 

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

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

As a result of such chemical modifications, the lifetime of a TBC has been maximized by varying the yttria content in zirconia. An yttria concentration of 6-8 mass% coincides with the maximum amount of the nonequUibrium, nontrans-formable tetragonal zirconia polymorph (t -zirconia) (see Section 7.2.S.3 see also Figure 7.27b). 

Until a decade ago, zirconia would not have been considered a candidate to be developed as an engineering ceramic because its polymorphism can lead to such large strains that it is self-fracturing. It has always been used in the refractories and glass industry in amounts approaching three-quarters of a million tonnes per year. A detailed study of the crystallography of zirconia, and in particular the mechanism of the phase changes, has... 

C.M. Phillippi, K.S.Mazdiyasni Infrared and Raman spectra of zirconia polymorphs, Joum. Amer.Ceram. Soc,. 54, (5), 254-256, (1971). 

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...

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