Zirconia cubic phase

The transition temperature from the low-temperature tetragonal phase to the monoclinic phase occurs at a lower temperature (6(X)-700 C) for the more acidic gels (pH =1.5 and 6.0) than the more basic gels (pH = 12) which transforms at 800 C [Colomban and Bruneton, 1992]. Like in other ceramic systems, a small addition of other oxide can change the nature of the phase(s) formed at a given temperature. For example, with 10% molar Ce02 present, the zirconia cubic phase can exist with tetragonal which is stable up to 13(X) C [Colomban and Bruneton, 1992]. 

The YSZ films are amorphous, after deposition, and a heat treatment was required in order to stabilize the zirconia cubic phase. This heat treatment was performed at 700 °C for two hours in a furnace with a constant heating rate of 2 °C.min-i and slow cooling. 

The heat treatment at 700 °C allowed the stabilization of the zirconia cubic phase, which is the phase of interest for application as electrolyte in solid oxide fuel cells, as indicated by x-ray diffraction analysis and confirmed by the Fourier transform infrared spectroscopy analysis. 

To avoid this phase change, zirconia is stabilized in the cubic phase by the addition of a small amount of a divalent or trivalent oxide of cubic symmetry, such as MgO, CaO, or Y2O3. The additive oxide cation enters the crystal lattice and increases the ionic character of the metal-oxygen bonds. The cubic phase is not thermodynamically stable below approximately 1400°C for MgO additions, 1140°C for CaO additions, and below 750°C for Y2O3 additions. However, the diffusion rates for the cations are so low at Xhtstsubsolidus temperatures that the cubic phase can easily be quenched and retained as a metastable phase. Zirconia is commercially applied by thermal spray. It is also readily produced by CVD, mostly on an experimental basis. Its characteristics and properties are summarized in Table 11.8. 

Tetragonal zirconia was partly transformed into the cubic phase during the sintering process of the UAP (uniaxially pressed) and CIP-prepared green compacts, and was accompanied by the formation of a-TPC. The transformation tends to accelerate as the PSZ quantity increases. 

As an attempt to solve this problem, zirconia is "stabilized" in the cubic phase by alloying it with an appropriate amount of di-or tri-valent oxide of cubic symmetry such as CaO, MgO or Y203. This results in a lowering of the temperature for the two lowest temperature transitions. These alloys are called partially stabilized zirconia, PSZ and they are a mixture of cubic and monoclinic or tetragonal phases and fully stabilized zirconia (all cubic phase) depending upon the concentration of the "dopant" or added metal oxide. 

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... 

Finally, interest in zirconia membranes has increased in recent years. Zirconia exhibits three well-defined phases in the order of increasing temperature the monoclinic, tetragonal and cubic phases. However, it has been suggested that a low-temperature metastable tetragonal phase exists in contrast to the high-temperature tetragonal phase [Cot, 1991 Colomban and Bruneton, 1992]. For pure zirconia membranes, it appears that the following phase uansitions occur [Stevens, 1986 Colomban and Bruneton, 1992] ... 

Moderate doping with rare earth cations such as or 80 + stabilizes cubic zirconia at relatively low temperatures and increases conductivity, the maximum of this quantity being reached when the concentration of acceptor-type dopants is close to the minimum necessary for stabilizing the cubic phase (Kharton et al., 2004). For YSZ,... 

The conclusion that cubic zirconia yields active catalysts after sulfation finds further proof in the comparison of the properties of ZrOz samples without any alkaline earth metal dope. Table 2 shows that sample lZS-0 prepared by precipitation under neutral conditions and having some cubic phase has much higher activity than purely monoclinic sample 2ZS-0. However, the anion stabilization of the cubic phase does not seem to present any practical interest since catalyst lZS-0 is subjected to a relatively fast deactivation. Its activity after the first isomerization-regeneration cycle is several times lower than its initial... 

FTIRS data of lattice modes (not shown for brevity) revealed pronounced distortion of the local coordination sphere of cations in samples synthesized via OPCM route. In ceria samples, along with main band at 400 cm, a strong shoulder at 510 cm is observed as well, which is not expected for symmetric cubic phase. This feature can be assigned to the coordination sphere distortion due to the presence of residual hydroxyls. In ceria-zirconia samples the intensity of this high-frequency shoulder is increased, while the band is broaden. Since in cubic Ca-ZrOz sample prepared via OPCM route (Table 1), a band at -505 cm with a strong shoulder at 590 cm is observed, it implies that a similar type of Zr cations coordination exists in CeOz-ZrOa samples, which agrees with the conclusions of Liu et al [8]. 

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