Perovskite structured mixed metal oxides

Various strategies were developed in the past for the synthesis of perovskite-structured oxides (Table 3.1). Of these, the choice of a particular method depends on the type of application expected. For catalytic applications, specific surfece area and crystal structure play crucial roles. Hence, the synthesis of these materials for catalytic applications always focused on obtaining crystalline materials with high values of specific surface area. The oldest method for the synthesis of perovskite-structured mixed metal oxides is the ceramic method. In this method, thoroughly mixed precursors (oxides, hydroxides, or carbonates) of the metals are calcined at elevated temperatures (>800 °C) for several hours. The surfece area of thus synthesized perovskites was, however, found to be less than 5m /g [5,30]. The high temperature used in solid-state reactions, for perovskite crystallization, results in the sintering of particles, which in turn leads to a large... 

Table 3.1 Some general methods used for the synthesis of perovskite-structured mixed metal oxides.

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

How can we be sure that the U +(Q2-) complex in a mixed metal oxide is present as the UO octahedron This can be done by studying solid solution series between tungstates (tellurates, etc.) and uranates which are isomorphous and whose crystal structure is known. Illustrative examples are solid solution series with ordered perovskite structure A2BWi aUa 06 and A2BTei-a Ua 06 91). Here A and B are alkahne-earth ions. The hexavalent ions occupy octahedral positions as can be shown by infrared and Raman analysis 92, 93). Usually no accurate determinations of the crystallographic anion parameters are available, because this can only be done by neutron diffraction [see however Ref. (P4)]. Vibrational spectroscopy is then a simple tool to determine the site symmetry of the uranate complex in the lattice, if these groups do not have oxygen ions in common. In the perovskite structure this requirement is fulfilled. 

Numerous ceramics are deposited via chemical vapor deposition. Oxide, carbide, nitride, and boride films can all be produced from gas phase precursors. This section gives details on the production-scale reactions for materials that are widely produced. In addition, a survey of the latest research including novel precursors and chemical reactions is provided. The discussion begins with the mature technologies of silicon dioxide, aluminum oxide, and silicon nitride CVD. Then the focus turns to the deposition of thin films having characteristics that are attractive for future applications in microelectronics, micromachinery, and hard coatings for tools and parts. These materials include aluminum nitride, boron nitride, titanium nitride, titanium dioxide, silicon carbide, and mixed-metal oxides such as those of the perovskite structure and those used as high To superconductors. 

Compounds formally containing [Fe03] are actually mixed metal oxides CaFe03, SrFe03 and BaFe03 crystallize with the perovskite structure Figure 5.23). 

A considerable number of materials called titanates are known, some of which are of technical importance. Nearly all of them have one of the three major mixed metal oxide structures (page 54), and indeed the names of two of the structures are those of the titanium compounds that were the first found to possess them, namely, FeTi03, Hmenite, and CaTi03, perovskite. Other titanites with the ilmenite structure are MgTiOa, MnTi03, CoTi03... 

Mixed Metal Oxides with the Perovskite Structure... 

A. Kalendova, D. Vesely, and P. Kalenda, Pigments with Ti -Zn, Ca, Mg -based on mixed metal oxides with spinel and perovskite structures for organic coatings, Pigment and Resin Technology 36(1) 3-17, 2007. 

One attempt to solve these problems has been the development of S5mgas technology based on oxygen ion selective membranes [95]. The membrane materials are non-porous and are composed by mixed metal oxides (perovskite, etc.) that conduct oxygen ions and electrons through the oxygen-deficient lattice structure. At high temperature, 700-1100 C,... 

For many oxides, including mixed metal oxides, of catalytic interest, preparing a high-surface-area solid with thermally stable porous structure was extremely difficult until recentiy. The hard templating procedure provides a systematic solution to this problem. Nanocasting that makes use of a mesostructured solid template is a special case. Both mesostructured silica and mesostructured carbon have been demonstrated so fer as hard templates. Rather precise replicas of these nanomolds have been obtained for a variety of oxide-type perovskites. Unprecedented specific surface areas in the 150-200 m /g were reached. 

Here we summarized methods to produce 3DOM perovskite mixed metal oxides with pore sizes in the submicrometer range. There are many methods to produce 3DOM perovskites with many kinds of metals. The 3DOM structure ensures access of molecules and ions to the surfe.ce of perovskite mixed metal oxides, and many advantages of the 3DOM structure have been reported. It is expected that the introduction of 3DOM structure is one important way to enhance the performance of perovskite mixed oxides. 

The perovskite structure, which is adopted by a large number of mixed-metal oxides [7], has ABX3 stoichiometry, where A and B are cations and X is an anion. The ideal perovskite structure, shown in Fig. 1, has the A-site cation occupying a 12-coordinate site within a framework of comer-connected [BXg/2] octahedra. The tolerance factor is a geometrical relationship based on a hard sphere model of the atom that gives a measure of the fit of the A-site cation to the octahedral framework. Mathematically the tolerance factor ( ) is given by the following expression, t = (1/V5) [( a + rx)/ rv, + x)]. where Ta, r, and rx are the radii of the A, B, and X ions, respectively. When f = 1 the A-site cation is a... 

Smith AJ, Welch AJE (1960) Some mixed metal oxides of perovskite structure. Acta Crystallogr 13 653-656... 

Complex Base-Metal Oxides Complex oxide systems include the mixed oxides of some metals which have perovskite or spinel structure. Both the perovskites and the spinels exhibit catalytic activity toward cathodic oxygen reduction, but important differences exist in the behavior of these systems. 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

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