Binary oxide materials

Figure 25.4 The different strategies to improve molecular homogeneity in the final binary oxide material.

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

The binary representation is applicable to various other oxide materials. However, an important distinction can be made between borates and other main group element oxide systems, such as aluminates and silicates. In the latter systems cations predominantly reside at sites created by the demands of rigid anionic oxide frameworks. Although some degree of structural control may be obtained by varying cations or by use of template synthesis, the oxide frameworks of these systems tend to be relatively inflexible in comparison with... 

Gloom for Oxide Superconductors Dismayed at the progress through the years, even with the most promising room-temperature metallic, binary oxides, many scientists abandoned the search for new high temperature oxide superconductors. Also, it should be mentioned that a deep-rooted prejudice had developed which claimed that the BCS theory had imposed a maximum transition temperature limit of 25 K for all superconducting materials, and that this temperature had already been achieved in certain alloys of niobium. Some scientists, however, were steadfast in their determination to break this barrier, optimistic in their outlook, and they continued their search for this unusual phenomenon in other metallic oxide systems. 

A second-order structural transformation in La uC at 233 5°C was reported (118) in a thermal expansion study. The LajCu04 product was prepared at high temperature (1100°C) by the solid-state reaction of the corresponding binary oxides. The material was found to decompose above 1200°C with the loss of oxygen. Samples of La uC, prepared at 1200°C, then maintained at 750°C in vacuum, yielded products having the general composition La2Cu04.x, or... 

While a binary oxide can be used as a starting material, some of these are reactive compounds that are difficult to handle. For example, BaO reacts rapidly with C02 in the air to produce barium carbonate. Other oxides, including BaO, are hygroscopic. Thus, compounds which are unreactive towards C02 and HzO at room temperature, but decompose to oxides plus volatile gases at elevated temperatures, are often used as oxide precursors. In the case of barium, BaCOs is a stable compound under ambient conditions but decomposes upon heating to 950°C according to the reaction... 

Knowledge of the factors that determine the sign and magnitude of ATG° for reaction (15.33) contributes to an understanding of the range of stability of ternary oxides relative to binary oxides. In some cases, favorable enthalpy effects may tend to stabilize a material, while in other instances, a favorable entropy effect may dominate stability considerations. ... 

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. 

All of the hydrotaleite-derived magnesia supports were prepared by first coprecipitating magnesium aluminum hydroxycarbonate in the presence of Mg and Al nitrates, KOH, and K2C03 according to procedures already described (8,9). Hydrotaleites were then decomposed by calcination at 873 K for 12-15 h to yield the binary oxide to be used for a catalyst support. The specific surface area of the hydrotaleites determined by nitrogen adsorption was typically about 220 m2g 1 after calcination. X-ray powder diffraction patterns of the materials were recorded on a Scintag X-ray diffractometer. 

Table 6.5 XPS binding energies of the Ml phase MoVTeNb materials under reaction conditions (see Figure 6.12) in comparison with reference binary oxides measured under oxygen at elevated temperature (623-673 K). At these conditions charging was practically not observed. Experiments used low kinetic energy electrons, and hence were sensitive to the topmost layers.

Figure 6.14 Synchrotron based valence band spectra (hv = ISOeV) of clean reference binary oxides (A) and catalytic materials under reaction conditions (B). The reference oxides were recorded at elevated temperature (623-673 K) in 0.2-1 mbar oxygen to remove carbon contamination and minimize reduction. The Ml catalysts (1 Ml l, 2 Ml 2) are under propane oxidation conditions with addition of steam to the feed. 3 M050,4-type MoVW-...

There has been a strong effort to rationalise and elucidate a structural principle which will account for all the anion-deficienl, fluorite-related, mixed-valent binary oxides of cerium, praseodymium and terbium. This is a key step not only for the solid-state chemistry of these materials but also for a large class of fluorite-related materials involved in applications such as fast oxygen conductors and as catalysts. The two main theoretical approaches to the problem were developed by Martin and by Kang and Eyring, and will be illustrated in the following sections. 

A number of factors can influence the behavior of ferroelectric thin films and multilayer stmctures with layer thickness at nanometer scale. One of the major factors is strain in epitaxial structures [15]. Recent demonstrations of huge strain effect on ferroelectric properties include changes in the phase diagram [16-22], dramatic enhancement of ferroelectric polarization, and increase of the ferroelectric phase transition temperature [23-27], induced ferroelectricity in non-ferroelectric materials like SrTi03 or KTa03 [28-33], or even simple rocksalt binary oxides like BaO ([34], theoretically predicted). 

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