Oxides with Other Structures

Goodenough et al. [93] have reported the high oxide ion conductivity in several brownmillerite oxides. A listing [94] is given in Table 4.5. All these brownmillerites exhibit oxide ion conductivity and the conductivities are rather 

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Also, other metal oxide coatings are possible, for example, electrochemically deposited manganese dioxide. Moreover, further electrocatalytically active oxides are research objectives, for example, oxides with spinel structure such as CoMu204 [36]. 

La2Cu04, Sr2Cu04. As we show in chapter 6, when a perovskite forms a composite or intergrowth with other structures, new compounds of interest in catalysis can be formed (such as in high-temperature superconducting copper oxides) and EM is used to determine the structures and properties of these complex compounds. The merits of using perovskites in steam reforming, membrane catalysis and fuel cells are discussed in chapter 6. 

Mobile electronic defects may be understood as more or less localized reducing or oxidizing agents. They can react with other structure elements S as follows... 

B. Other Alkaline Earth Oxides with Cubic Structure... 

Of central importance for reaching attractive efficiencies is the development of a suitable oxide material. A thermochemical analysis of candidate metal oxides which, from their structural properties are suitable for cycling, reveals that without stabilisation of the FeO stoichiometry the achievable conversion of producer gas is not high enough. Stabilisation can be achieved by alloying the oxides with other transition metals. 

We have begun a study of the stabilization of seml-conductlng and metallic oxides with other metal cations that will font covalent metal-O-Cu bonds and a two level electronic band structure. These materials will be essentially semiconductors where the conductivity arises from doping to produce mixed-valence compounds. Ue chose to begin our study with cations that adopt tetrahedral coordination and focus on how to create structures that Incorporate distorted octahedral, square pyramidal and square planar coordination of copper compatible with still other electropositive (Ionic) cations. The mixed valency Introduced by doping can then be accommodated on the copper metal and adjacent oxygen atom sites by an accompanying bond polarization around the cation with tetrahedral coordination. 

Elastomers Another major class of plastic ablators is elastomeric-base materials and particularly sihcones. During ablation, they thermally decompose by such processes as depolymerization, pyrolysis, and vaporization. The silicone elastomers provide low thermal conductivity, high thermal efficiency at low to moderate heat fluxes, low temperature properties, elongation of several hundred percent at failure, oxidative resistance, low density, and compatibility with other structural substrate materials. Elastomeric materials are generally limited by the amount or structural quality of the char formed during ablation, which restricts their use to hyperthermal environments of relatively low mechanical forces. 

The cyanide ion, [ C=N ] , is closely similar to a halide ion in its properties. By oxidation it can be converted into cyanogen, C2N2 ( N=C—C=N ), which is analogous to the halogen molecules Fg, Cl, etc. By suitable procedures three anions can be made that are similar in structure to the carbon dioxide molecule 0=C=0 and the nitrous oxide molecule N=N=0 (these structures are hybridized with other structures, such as 0=C—O and its analogues). These anions are... 

All these oxide phases can be included in the oxides with framework structure. These are oxides (Ma/bO) with composition a/h<0.5 and effective molar volumes bigger than those for the oxides of compact structures (MO2) and the smallest metal content. In these oxides the coordination polyhedra can be identified, connected by corners, edges, etc. Most of these structures can be derived from the structure of the oxide ReQs where the MO octahedrons form a regular lattice. Other cations can easily be included in the structure of this type of oxides. This leads to the stabilization of the structure, and as a result, one can obtain the perovskite-type structures or their variations. 

In this section, we will focus on metal oxides with binary structures (M Oy). Moreover, composites containing other metal oxides and/or catalysts (e.g., Pt or Au) will also be considered. 

Again, as with pyridopyrimidines, the main reaction is oxidation of di- or poly-hydro derivatives to fully aromatic structures, often merely by air or oxygen. In some cases the reagent of choice is mercury(II) oxide, whilst other reagents used include sulfur, bromine, chloranil, chromium trioxide-acetic acid, hydrogen peroxide, and potassium ferricyanide, which also caused oxidative removal of a benzyl group in the transformation (306) (307)... 

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