Single-phase oxide solid-solutions

Composites which form Single Phase Oxide Solid Solutions. 379... 

While few examples of this phenomenon exist and little work has been done in this area, it is possible to affect the oxidation behavior of materials which form a single phase oxide solid solution. For example, upon oxidation, a HfC/ZrC composite would form a solution of Hf02/Zr02. Use of this type of composite, and choosing the HfC to ZrC ratio might be useful to control the monoclinic to tetragonal transformation temperature of the resulting scale or other properties of the matrix or scale. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

One implication that has been adequately validated by experiment is that eomplex oxides with low particularly mullite, have creep strengths superior to single-phase oxides, such as alumina, yttria, and magnesia. However, the findings that solid solutions and nanopartieles profoundly affeet ereep rates in bulk polycrystalline oxides was not obvious from... 

Stabilization of Ru based oxides by valve metal oxides has not been studied in such detail using photoelectron spectroscopy. The most common compositions, however, with relatively high valve metal content, are not in favor of formation of a solid solution. Studies of the phase formation in Ru/Ti mixed oxides has shown [49] that homogeneous solutions are formed for compositions with Ru < 2% or Ru > 98% (see Section 3.1.1). Therefore electrodes with other compositions are better described as physical mixtures and the electrochemical behaviour is most likely that of a linear superposition of the single components. It has to be considered, however, that the investigations performed by Triggs [49] concern thermodynamic equilibrium conditions. If, by means of the preparation procedure, thermodynamic equilibrium is... 

Powder XR diffraction spectra confirm that all materials are single phase solid solutions with a cubic fluorite structure. Even when 10 mol% of the cations is substituted with dopant the original structure is retained. We used Kim s formula (28) and the corresponding ion radii (29) to estimate the concentration of dopant in the cerium oxide lattice. The calculated lattice parameters show that less dopant is present in the bulk than expected. As no other phases are present in the spectrum, we expect dopant-enriched crystal surfaces, and possibly some interstitial dopant cations. However, this kind of surface enrichment cannot be determined by XR diffraction owing to the lower ordering at the surface. 

For mixed metal oxides obtained from their hydroxide or carbonate precursors after calcination, it is generally difficult to determine whether the as-prepared precursor is a single-phase or multiphase solid solution [35]. Non-aqueous solvents appear superior for achieving two dissimilar metal oxides such as MM Oz or MM 04 precipitates such reactions cannot be carried out simultaneously in aqueous solution due to the large variations in pH necessary to induce precipitations [41,42]. Table 6.1 summarizes some of the nanoparticulate semiconducting metal oxides and mixed metal oxides prepared via co-precipitation techniques. The general procedure of achieving metal loaded nanoparticles on an oxide support is shown in Fig.6.5. 

The hosts for ACT and REE immobilization are phases with a fluorite-derived structure (cubic zirconia-based solid solutions, pyrochlore, zirco-nolite, murataite), and zircon. The REEs and minor ACTs may be incorporated in perovskite, monazite, apatite-britholite, and titanite. Perovskite and titanite are also hosts for Sr, whereas hollandite is a host phase for Cs and corrosion products. None of these ceramics is truly a single-phase material, and other phases such as silicates (pyroxene, nepheliiie, plagioclase), oxides (spinel, hibonite/loveringite, crichtonite), or phosphates may be present and incorporate some radionuclides and process contaminants. A brief description of the most important phases suitable for immobilization of ACTs and REEs is given below. 

Table 2.3 lists ternaries that have been deposited, together with indication of when clear single compounds formation was verified. While solid solution formation is usually the goal of these smdies, it should be kept in mind that separate phases, either as a composite or as separate layers, may be required for some purposes. For example, bilayers of CdS/ZnO and CdS/ZnS have been deposited from single solutions. These depositions depend on the preferential deposition of CdS over ZnS and, in the case of the former, the often-encountered greater ease of formation of the oxide (hydroxide) than the sulphide of Zn. 

The ternary iron oxides, as exemplified by the iron-niobium system, offer an opportunity to obtain single-phase, conducting n-type iron oxides in which the conductivity can be controlled by means of chemical substitution. At first glance, FeNbO and FeNb Og might appear to be very different materials. Yet as MM O and MM Og they merely represent superstructures of the basic a-PbO. structure obtained under the conditions of preparation (7 ). Consequently, they form a solid solution in which the two valence states of iron are uniformly distributed throughout a single homogeneous phase (j3). 

The products of co-precipitation reactions are usually amorphous at or near room temperature. It is difficult to determine experimentally whether the as-prepared precursor is a single-phase solid solution or a multi-phase, nearly homogeneous mixture of the constituent metal hydroxides, carbonates and oxides that react to form a single phase mixed metal oxide when heated. 

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