Ceria-zirconia-yttria

There is a relation between material properties and morphology. In order to investigate this relation, Zhang et prepared nanorodlike, micro-spherical, micro-bowknot-like and micro-octahedral shaped ceria-zirconia-yttria (CZY) solid solutions vtith the surfactant-assisted hydrothermal method using a triblock copolymer (Pluronic P123) or cetyltrimethylammonium bromide (CTAB) surfactant. The formation mechanism of these materials by the surfactant-assisted hydrothermal method is shown schematically in Fig. 8.6. 

Figure 8.6 Formation of ceria-zirconia-yttria precursors and solid solutions under surfactant-assisted hydrothermal conditions. Reprinted with permission from Zhang et al Copyright 2009 the American Chemical Society.

Since columbite is a very complex mineral indeed, containing niobic, tantalic, titanic, and tungstic acids, zirconia, thoria, ceria, and yttria, Hatchett must have possessed great analytical ability in order to discover in it the new element, columbium. Although the greatest chem-... 

SOFCs employ a ceramic oxide (ceria- or yttria-doped zirconia, electrolyte... 

The present work was focused on the synthesis of nanocrystalline components for electrochemical cells via the cellulose-precursor technique. This method was used to prepare nanostructured intermediate-temperature (IT) SOFC anodes made of a series of cermets comprising gadolinia-doped ceria (CGO), yttria-stabilized zirconia (YSZ), Gdi.86Cao.i4Ti207.5 (GCTO) pyrochlore, metallic nickel and copper. Perovskite-type SrFcojAlo.sOs.s (SFA) powder, also obtained via the cellulose precursor, was applied onto membranes of the same composition to enhance specific surface area and electrocatalytic activity in the reactors for methane conversion [3]. 

The Glass and Ceramics Institute of the Spanish CSIC carried out an analytical study of new materials used in structural and electrical ceramics, the stoichiometries and impurity contents of which were highly interesting because of their influence on the physico-chemical properties and service response of the pieces [24]. The specific materials studied were tetragonal zirconia stabilized with ceria and yttria (Y-TZP/Ce) lead titanate modified with gadolinium and doped with manganese (Gd-PT) and lead titanate-zirconate modified with lanthanum (PLZT). The first is a structural ceramic and the other two are electrical ceramics. 

Recent precise structural analyses of ceria-zirconia and ceria-rare-earth-oxides have demonstrated the positional disorder and diffusional pathway of oxide ions at high temperatures. In a ceria-yttria material, the oxide ions diffuse along the <100> directions (Fig. 1. 32). We found that the diffusion coefficient increases with... 

Other refractory oxides that can be deposited by CVD have excellent thermal stability and oxidation resistance. Some, like alumina and yttria, are also good barriers to oxygen diffusion providing that they are free of pores and cracks. Many however are not, such as zirconia, hafnia, thoria, and ceria. These oxides have a fluorite structure, which is a simple open cubic structure and is particularly susceptible to oxygen diffusion through ionic conductivity. The diffusion rate of oxygen in these materials can be considerable. 

FIGURE 1.2 Composition dependence of conductivity for yttria-stabilized zirconia (YSZ) measured at 1000°C [7], yttria-doped bismuth oxide (YDB) at 600°C [6], and yttria-doped ceria (YDC) at 700°C [8],... 

Another way to decrease the anodic overpotential is to intercalate a mixed conductor between the yttria stabilized zirconia electrolyte and the metallic anode. Such a combination enlarges the reaction area which theoretically lowers the anodic overpotential. Tedmon et al. [93] pointed out a significant decrease of polarization when ceria-based solid solutions like (Ce02)o.6 (LaO, 5)04 are used as anode materials for SOFCs. This effect is generally attributed to the mixed conductivity resulting from the partial reduction of Ce4+ to Ce3+ in the reducing fuel atmosphere. A similar behaviour was observed in water vapor electrolysis at high temperature when the surface zirconia electrolyte is doped with ceria [94, 95]. 

There is an obvious overlap among various applications categories. An example of the overlap is alumina which is both a structural refractory ceramic as well as a catalyst support. The additives modify the interconversion of various AI2O3 phases and the high surface area of y-Al203 is maintained by the added 3 wt% ceria or lanthana. Additives like yttria stabilize zirconia with respect to inertness and mechanical stability. Addition of yttrium or lanthanide to Fe-Cr-Al alloys reduces the spallation of oxide film. 

Mixed ion and electronic conducting ceramic membranes (e.g. yttria-stabilized zirconia doped with titania or ceria) can be slip cast into a tubular form from the pastes containing the constituent oxides in an appropriate proportion and other ingredients and the cast tubes are then subject to sintering at 1,200 to 1,500X to render them gas impervious [Hazbun, 1988]. 

The electrolyte in an SOFC must consist of a good ion conductor, which has essentially no electronic conductivity. Otherwise the cell will be internally short-circuited. An often-used electrolyte material is yttria-stabilised zirconia (YSZ). The electrodes must pos.scss good electron conductivity in order to facilitate the electrochemical reaction and to collect the current from the cell. The fuel electrode usually contains metallic nickel for this purpose. The anodic oxidation of the fuel (H or CO) can only take place in the vicinity of the so-called three-phase boundary (TPB), where all reactants (oxide ions, gas molecules and electrons) are present. Thus, it is advantageous to extend the length and width of the TPB zone as much as possible. One way to do this is by making a composite of Ni and YSZ called a Ni-YSZ-cermet. Another way is to use a mixed ionic and electronic conductor, which in principle can support the electrochemical reaction all over the surface as illustrated in Fig. 15.1. Partially reduced ceria is a mixed ionic and electronic... 

Figure 13.19 Outputs of mixed potential CO sensors with gold and platinum electrodes and yttria-stabilized zirconia (YSZ), ceria gadolinium oxide, and p alumina electrolytes [228-235],...

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