Binary oxide materials perovskites

In a much later stage of development, other ceramics that were not clay-or silicate-based depended on much more sophisticated raw materials, such as binary oxides, carbides, perovskites, and even completely synthetic materials for which there are no natural equivalents. The microstructures of these modern ceramics were at least an order of magnitude finer and more homogeneous and much less porous than those of their traditional counterparts. It is the latter — the modern or technical ceramics — with which this book is mainly concerned. 

Our work has applied these techniques to the study of the binary insulating materials including the fluorites, alkali halides, alkaline earth oxides, and perovskites. Many of these are simple materials that are commonly used as models for all solid state defect equilibria. Our work has had the goal of determining at the microscopic level the defect equilibria and dynamics that are important in understanding solid state chemistry as well as developing new tools for the studies of solid materials. 

In catalysis, oxides with well defined acidic and basic properties are used in different forms that have found application in numerous catalytic applications in the gas-solid and liquid-solid heterogeneous catalysis [3, 46, 47], Among the most used oxide materials in catalysis, we And (i) bulk oxides (one component metal oxides) (ii) doped and moditied oxides (iii) supported metal oxides (dispersed active oxide component onto a support oxide component) (iv) bulk and supported binary metal oxides to quaternary metal oxides (mixed oxide compositions) (v) complex oxides (e.g., spinels, perovskites, hexa-aluminates, bulk and supported hydrotalcites, pillared clays, bulk and supported heteropolyacids, layered silicas, etc.). 

Moving to strontium-based perovskites, we find the same trend as for the barium members. However, the conductivity is smaller, peaking between 10 and 10 S/cm for the best ones (SrZrOs and SrCeOs). SrCe03 is one of the best studied proton conductors, but the tolerance factor is low and the material is on the verge of decomposition into the binary oxides. It is therefore very vulnerable to reaction with CO2, for example. 

Many of the perovskite compounds can be prepared by high-temperature solid-state reaction of binary oxides or peroxides. However, some of these tend to be refractory and hence unreactive. Others have a tendency to hydrate or oxidize and are inconvenient to use. Therefore, it is preferable to use carbonates, oxalates, or other easily decomposable compounds, assuming they can be obtained with suitable purity. These materials are usually in the form of fine powders and decompose in the initial stages of the reaction so that faster reaction rates are obtained. In some cases fine powders may be obtained by... 

Several methods have been deseribed for the preparation of perovskite-type oxides. Many of them are analyzed in detail in several references [11, 12] and for this reason only a brief account on this aspect will be given here. Practically all the standard methods known for the synthesis of solid materials [13] can be applied to the preparation of perovskite-type oxides. The classieal route is the reaction between intimate mixtures of the constituent binary oxides or their preeursors (nitrates, carbonates, etc) in the solid state at high temperatures. These reactions are slow as a result of large diffusion distances, and repeated grinding and heating are required in order to obtain pure and fine powders. In some cases, speeial precautions must be taken. For example, compounds containing Pb (II) in the A-site of the perovskite stmcture... 

Another issue for SOFC material research is to increase the available thermodynamic data for rare-earth containing complex oxides. Although there has been intensive researches on the binary oxides, there is a paucity of data for complex oxides such as the perovskites, and the fluorite oxides with some substitutions. Most of the SOFC materials are complex oxides with several metal components which have been reported in the past twenty years. However, it is a matter of regret that there are only a few reports concerning calorimetric studies of these materials. Fundamental research on these advanced materials is necessary especially from the viewpoint of evaluating chemical stabihty and durabihty of the SOFC components. 

The most important catalyst systems involving rare earth elements are the oxides and intermetallics. Catalytic properties of rare earth oxides are described in section 4 and those of intermetallic compounds in section 6. Reports on surface reactivities of other binary rare earth compounds are only sparse, and this is mentioned in section 5. A very interesting class of catalyst systems comprises the mixed oxides of the perovskite structure type. As catalysis on these oxides is mainly determined by the d transition metal component and the rare earth cations can be regarded essentially as spectator cations from the catalytic viewpoint, these materials have not been included in this chapter. Instead, we refer the interested reader to a review by Voorhoeve (1977). Catalytic properties of rare earth containing zeolites are, in our opinion, more adequately treated in the general context of zeolite catalysis (see e.g. Rabo, 1976 Katzer, 1977 Haynes, 1978) and have therefore been omitted here. 

However, current devices exploiting SHG effects are usually based on inorganic materials, typically binary and ternary metal oxides, such as quartz, and certain perovskites e.g. LiNb03, PbTi03). Quartz itself exhibits SHG properties, but is taken, in the examples below, as the reference for inorganic materials (Eq = 1). For comparison purposes, quartz is referenced to urea, so that Eu = 400 Eq, and we can see... 

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