Zirconia Y2O3-stabilized

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. 

Figure 7.8 Zirconia-yttria binary system. The introduction of yttria into zir-conia ( — 15-51% Y2O3) stabilizes the structure into the cubic form throughout the usable temperature range of the refractory material [11],...

Aerosol-assisted chemical vapor deposition is illustrated by work published by Siadati et al. (2004). An aerosol is a vapor suspension of finely divided particles. The particles can be solid, as in smoke, or liquid, as in fog. A toluene solution of Zr(tfac)4, Y(hfac)3, and Ce(tmhd)4, converted to an aerosol, was used to deliver the metal in a carrier gas of O2. The film deposited was CeC>2-doped Y2O3-stabilized zirconia. The ligands used in this work to produce volatile metal compounds are frequently used in chemical vapor deposition. Trifluoroacety-lacetonate (tfac), hexafluoroacetylacetonate (hfac), and tetramethylheptanedionate (tmhd) are all 3-diketonates, and their structures are shown in Figure 3.22. Like the diethyl dithiocarbamates mentioned above, these precursors could potentially be the source of both the metal and the oxygen in the chemically deposited film. However, to ensure that the metal remains at its highest oxidation state and to avoid a film with mixed valencies on the metal, oxygen is used as the carrier gas. 

The first implementation of such a sensor device is reported in Rbder-Roith et al. (2009), in which 8 mol% Y2O3 stabilized zirconia (YSZ) was used for the gas sensitive material. The sensor set-up was similar to the sensors described above however, an additional Pt-cermet was applied to get a high... 

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 Y2O3. 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. 2.72 The influence of the temperature, T (diagrams a, c, e) at V h = 0.5 mm/min and of the speed V<-h, (diagrams b, d, f) at T = 1200 °C on the appearance of the load (P) versus deflection <5 a, b SN-1 c, d A-1 e, f Y-PSZ-3. SN-1 is a silicon nitride-based ceramic with additions of Y2O3 and AI2O3 A-1 is an alumina-based ceramic with an addition of MgO and Y-PSZ-3 is a Y2O3 stabilized zirconia based single crystal [18]. With kind permission of Elsevier...

The transport kinetics of Mg in cubic 10mol%Y2O3-stabilized zirconia were studied. 

The SOFC is a complete solid-state device that uses an oxide ion-conducting ceramic material as the electrolyte. The electrolyte is a nonporous solid, such as Y2O3 stabilized Zr02 with conductivity-based oxygen ions [122, 128, 142-144]. Yttria-stabilized zirconia (YSZ) is the most commonly used material for the electrolyte. It was first used as a fuel cell electrolyte by Baur and Preis in 1937 [145]. The anode is usually made of a C0-Z1O2 or Ni-Zr02 cement [13, 95, 146, 147], while the cathode is made of Sr-doped LaMnOs (LSM) [13,148-150],... 

A schematic of the basic materials and electrochemical reactions of the SOFC is given in Figure 7.1. In the SOFC system, yttria- (Y2O3) stabilized zirconia (Z1O2) is most often nsed as the electrolyte, althongh many other combinations are continually evaluated [14]. In this solid-state electrolyte, Cf ions are passed from the cathode to the anode via oxygen vacancies in the electrolyte as described in Chapter 5. Other cell components such as interconnects and bipolar plates are typically doped ceramic, cermet, or metallic componnds. 

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). 

Solid oxide fuel cell (SOFC) working between 700 and 1000 °C with a solid oxide electrolyte, such as yttria-stabilized zirconia (Zr02-8% Y2O3), conducting by the... 

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. 

Arranged in layered fashion on the alumina substrate are the zirconia underlayer, the platinum reference electrode, the zirconia solid electrolyte stabilized with 5.1 mole % Y2O3, the platinum measurement electrode, and finally,the protective spinel (A203 Mg0) layer. The zirconia layer is Umm long, 1+mm wide and 30pm thick. 

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