Zirconia metastability

Zirconia prepared by the thermal decomposition of zirconium salts is often metastable tetragonal, or metastable cubic, and reverts to the stable monoclinic form upon heating to 800°C. These metastable forms apparently occur because of the presence of other ions during the hydrolysis of the zirconium their stabiUty has been ascribed both to crystaUite size and surface energy (152—153) as well as strain energy and the formation of domains (154). 

The hydroxides as precipitated are amorphous, but if they are refluxed ia a neutral or slightly acidic solution they convert to a mixture of cubic and monoclinic hydrous zirconia crystaUites on continued refluxing, only the monoclinic form persists (196). If the refluxing is conducted in an alkaline solution, metastable cubic zirconia is formed (197). 

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. 

It should be noted that it is possible to produce fully stabilized bodies with much higher fracture strengths than listed here but this requires the use of fine particle size, chemically prepared powders (3). The use of this type of material involves a number of penalties both in cost and processability that may be prohibitive for a high volume automotive application. In addition to the type of partially stabilized body described here, two other basic types of partially stabilized bodies have been reported (4, ). Both are classified as transformation toughened partially stabilized zirconias and involve different processing techniques to obtain a body with various amounts of a metastable tetragonal phase. While the mechanical properties of these materials have been studied extensively, little has been reported about their electrical properties or their stability under the thermal, mechanical and chemical conditions of an automotive exhaust system. 

A detailed investigation of the surface properties of the tetragonal phase of Zr02 by HRTEM and IR techniques has also been reported (605). The tetragonal phase of zirconia is metastable at low temperatures. Therefore, yttria-stabilized samples were prepared and characterized (606-608). The structural, morphological, and surface hydration features of tetragonal ZrC>2 were investigated by XRD, HRTEM, and IR techniques (606), and the interactions of the surfaces with CO (607) and C02 (608) were reported. 

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] ... 

The pyrocholre-bascd cubic metastable phases can be obtained by the hydrogen reduction and subsequent reoxidation of tetragonal ceria-zirconia mixed oxides. In addition, these solid solutions can be synthesized by means of the thennal... 

Garvie RC (1965) The occtrrrence of metastable tetragonal zirconia as a crystalhte size effect. J Phys Chem 69 1238-1243... 

Figure 17. Phase diagram for Ce-doped zirconia. Dashed lines outline stability fields for monoehnie, tetragonal, and eubie phases. Solid lines demareate metastable phase fields. Modified from Figure 6 in Yashima et al. (1994).

T.K. Gupta, F.F. Lange, and J.H. Bechtold, Effect of stress-induced phase transformation on the properties of polycrystalline zirconia containing metastable tetragonal phase, J. Mater. Sci. 13(7), 1464-1470 (1978). 

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