Zirconia polymorphs

IR spectra clearly distinguish the three zirconia polymorphs [57]. The high-temperature cubic phase has only one IR active skeletal mode, found near 550 cm for powders, while tetragonal zirconia (the medium-temperature polymorph) has... 

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. 

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... 

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). 

Various ratio and distribution among the toughening zirconia polymorphs as result of the different sintering techniques could partially explain the obtained mechanical properties (Vickers hardness and fracture toughness by indentation). This assessment would be sustained by microstructural features analysis, XRD, far infrared and Raman spectra as follows. 

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

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 ... 

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