Crystalline oxide materials, applications

Crystalline oxide materials with optical, electrical, magnetic, thermal, chemical, and electrochemical applications have been of great technological interest for many years. They often comprise no less than three components in no less than three sublattices, and tend to be even more complex in terms of the number of components and sublattices. The simplest examples may be BaTiOs, with a perovskite structure, and MnFe204 with a spinel structure. These are quintessential ingredients of dielectrics and soft ferrites, respectively, and currently take the lion s share in the worldwide electroceramics market. 

Porous oxide catalytic materials are commonly subdivided into microporous (pore diameter <2nm) and mesoporous (2-50 nm) materials. Zeolites are aluminosilicates with pore sizes in the range of 0.3-1.2 nm. Their high acidic strength, which is the consequence of the presence of aluminium atoms in the framework, combined with a high surface area and small pore-size distribution, has made them valuable in applications such as shape-selective catalysis and separation technology. The introduction of redox-active heteroatoms has broadened the applicability of crystalline microporous materials towards reactions other than acid-catalysed ones. 

Abstract Refractory oxides encompass a broad range of unary, binary, and ternary ceramic compounds that can be used in structural, insulating, and other applications. The chemical bonds that provide cohesive energy to the crystalline solids also influence properties such as thermal expansion coefficient, thermal conductivity, elastic modulus, and heat capacity. This chapter provides a historical perspective on the use of refractory oxide materials, reviews applications for refractory oxides, overviews fundamental structure-property relations, describes typical processing routes, and summarizes the properties of these materials. 

Since many properties of crystalline oxides, e.g., acidity, hydrothermal stability, etc., are the essential features exploited in commercial applications of these oxides, it is not unexpected that the ordered, mesoporous materials have not yet found much commerical use. The large void volumes, pore sizes and surface areas of the ordered, mesoporous materials provide advantages over microporous solids in certain areas of application but issues such as stability remain. Thus, if crystalline, extra-large pore solids could be prepared in the pore size and void volume ranges of the mesoporous materials, they would be immediately commercialized. The question remains as to why crystalline materials of this size range have not been synthesized. Navrotsky et al. have shown that pure silica, ordered, mesoporous silicas are energetically very close to pure silica, crystalline... 

The synthesis of ordered macroporous crystalline materials has been attracting much attention. Walls of macroporous materials are larger than those of mesoporous materials, and this macroporosity can be introduced into a wide variety of transition metal oxides. Potential applications of these materials include photonic materials, catalysts and electrode materials. The ordering scale is close to the wavelength of light, and interest has therefore been shown in photonic materials. In some cases, introduction of macroporosity increases the surface area, and these materials show better catalytic performance than that of nonporous materials. Similar to mesoporous materials, macropores are favoured for diffusion of reactants compared with nonporous materials and many applications, such as in a Li battery electrode, have been reported. 

In order to lower the deposition temperature and to minimize the interaction with the substrate materials, Yamada et al. (1996a, 1997a) prepared Y123 single-crystalline oxide coated fibers for power applications on the YSZ (100) and SrTi03 (100) substrate... 

There have been many instances of examination of the effect of additive product on the initiation of nucleation and growth processes. In early work on the dehydration of crystalline hydrates, reaction was initiated on all surfaces by rubbing with the anhydrous material [400]. An interesting application of the opposite effect was used by Franklin and Flanagan [62] to inhibit reaction at selected crystal faces of uranyl nitrate hexa-hydrate by coating with an impermeable material. In other reactions, the product does not so readily interact with reactant surfaces, e.g. nickel metal (having oxidized boundaries) does not detectably catalyze the decomposition of nickel formate [222],... 

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

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