Tetragonal stabilized zirconia

Four solid oxide electrolyte systems have been studied in detail and used as oxygen sensors. These are based on the oxides zirconia, thoria, ceria and bismuth oxide. In all of these oxides a high oxide ion conductivity could be obtained by the dissolution of aliovalent cations, accompanied by the introduction of oxide ion vacancies. The addition of CaO or Y2O3 to zirconia not only increases the electrical conductivity, but also stabilizes the fluorite structure, which is unstable with respect to the tetragonal structure at temperatures below 1660 K. The tetragonal structure transforms to the low temperature monoclinic structure below about 1400 K and it is because of this transformation that the pure oxide is mechanically unstable, and usually shatters on cooling. The addition of CaO stabilizes the fluorite structure at all temperatures, and because this removes the mechanical instability the material is described as stabilized zirconia (Figure 7.2). 

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. 

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. 

Fig. 17.2. Results of polarized Raman spectroscopic analyses on the surface of yttria-stabilized zirconia in the presence of an indentation print (a) optical micrograph of the indentation print, (b) map of transformed monoclinic fraction, (c) map of 6-axis orientation of the monoclinic phase, and (d) map of the angle of c-axis orientation of the residual tetragonal phase. In both (c) and (d), the represented angle gives the inclination of the respective axes with respect to the sampling plane...

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... 

Schelling, P.K., Phillpot, S.R., Wolf, D. Mechanism of the cubic-to-tetragonal phase transition in zirconia and yttria-stabilized zirconia by molecular-dynamics simulation. J. Am. Ceram. Soc. 2001, 84,1609-19. 

As an example of the application of the WH method. Figure 13.7 shows the diffraction pattern of a ceria stabilized-zirconia powder sample [with a 20 wt.% addition of standard silicon (SRM 640b distributed by the NIST)], together with the corresponding WH plot for the tetragonal zirconia phase. The WH plot points out the presence of both size and strain effects (respectively, non-zero intercept and slope), and the best fit of Equation (14) gives < L >v = 18(l)nm and e = 0.0024(3). ... 

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