Fully stabilized zirconia

The following table contrasts the properties of a typical partially stabilized zirconia (PSZ) body as used in this application with a typical fully stabilized (Y2O3) body. 

Figure 11. Optical micrograph of high silica content (> 1%) yttria fully stabilized zirconia body (bar = 37 microns)...

As can be seen, the mechanical strength of the partially stabilized body is approximately twice that of the fully stabilized and the thermal expansion is approximately 30% lower. Because of this, the thermal shock resistance of the PSZ body is greatly improved. The ionic conductivity of the PSZ body is lower but is still adequate for automotive applications. Figure 10 compares the conductivity of a yttria partially stabilized zirconia body with several fully stabilized bodies. 

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. 

The issue of mismatch of thermal expansion coefficients similar to that for a composite membrane is also very critical for fuel cells. In the fuel cells, electrodes are attached to solid electrolyte membranes. Significant temperature variations during applications, pretreatments or regeneration of the membranes (e.g., decoking) can cause serious mechanical problems associated with incompatible thermal expansions of different components. A possible partial solution to the above problem is to use partially stabilized instead of fully stabilized zirconia. The former has a significantly lower thermal expansion coefficient than the latter. 

For those dense solid electrolyte membranes using metal oxides, the degree of stabilization can make a difference in the resulting thermal shock resistance. For example, the fully stabilized zirconia has poor thermal shock resistance compared to the partially stabilized zirconia. 

Historically, stabilized (and partially stabilized) zirconia ceramics were prepared from powders in which the component oxides are mechanically blended prior to forming and sintering. Because solid state diffusion is sluggish, firing temperatures in excess of 1800°C are normally required. Furthermore, the dopant was nonuniformly distributed, leading to inferior electrical properties. Trace impurities in the raw materials can also lead to enhancement of electronic conductivity in certain temperature ranges, which is also undesirable. To overcome these problems, several procedures have been developed to prepare reactive (small particle size) and chemically pure and homogeneous precursor powders for both fully stabilized and partially stabilized material. Two of these are alkoxide synthesis and hydroxide coprecipitation. 

Fully stabilized zirconias doped with Y2O3 (8-9 mol%) or CaO (15 mol%), or partially stabilized zirconia doped with MgO (3 mol%), are the most widely used electrolytes in devices operating at temperatures higher than 800-850 °C. 

At higher Y levels (e. g., 9-10 mol%) the material becomes completely cubic (FSZ, fully stabilized zirconia), which gives the highest ionic conductivity but weak mechanical properties. 

In particular, the evolution of radiation-induced defects in fully stabilized cubic zirconia (FSZ) (Y, Ca, and Er dopants acting as stabilizing agents) and a pure unstabilized monoclinic zirconia was investigated [60],... 

The addition of sufficient alloying component to facilitate a partial stabilization of the cubic phase leads to partially stabilized zirconia (PSZ), which exhibits an improved thermal shock resistance in comparison to fully stabilized zirconia, as well as an excellent fracture toughness. Today, four oxides - CaO, MgO, Y2O3, and Ce02 - are commonly used to produce PSZ which, in fact, is a mixture of cubic and tetragonal/ monodinic phases that can be prepared by heat treatment of the cubic phase. This process is aimed at the development of a two-phase ceramic when the concentration of stabilizing agents is insufficient to produce full stabilization of the cubic structure. 

Fig. 2.76 Temperature dependence of flexural strength of partially and fully stabilized zirconia single crystals, polycrystalline PSZ, and hot-pressed Si3N4 (HPSN). PSZ is partially stabilized Zr02 with Y2O3 [24], With kind permission of Wiley and Sons...

Commercial zirconia grades. Usually unstabilized zirconia (i.e., fully monoclinic), partially stabilized zirconia, and stabilized zirconia (i.e., completely cubic) grades exist commercially and are available among advanced ceramic producers worldwide (e.g., Zircoa, Vesuvius, and Degussa), and they are briefly described below. 

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