Perovskites based materials

F. Dogan, T. Susuki, P. Jasinski and H.U. Anderson, in Synthesis Properties and Crystal Chemistry of Perovskite Based Materials, W. Wong-Ng, A. Goyal, R. Guo, and S. Bhalla (editors), Proceedings of the 106th Annual Meeting of the American Ceramic Society, ACS, Westerville, OH, 2005, p. 39. 

Bernardi, M.I.B., Andres, J., Varela, J.A., and Longo, E. (2009) Structural and optical properties of CaTiOs perovskite-based materials obtained by microwave-assisted hydrothermal synthesis an experimental and theoretical insight. 

Taking into account the Mars-van Krevelen mechanism, any enhanced oxygen vacancy densities can improve the oxidation activities of an oxide-based catalyst Perovskite-based materials can act as catalysts for NO SCR by H2 or hydrocarbons [72,73] or simultaneously reduce NO in the presence of PM under lean conditions [74-77]. The major drawback of such high-temperature crystal oxides is their low surface area, for example, <2-3 m /g. However, over the last years improved preparation methods and compositional control had a significant effect on materials features and performance of the perovskite-type catalysts [75,78-85] as illustrated in Figure 26.3. However, the performance of perovskite-based catalysts becomes remarkable when noble metal coexist, either as supported or as dopant or even in the form of a solid solution [15,17,81-86]. 

In all cases, perovskite-based materials can be utilized regarding their high thermal stability and with the possibility to accommodate trace amount of noble metals to improve their catalytic performance especially at low temperature toward the conversion of NO. 

Chemical compatibility of perovskite-based materials with the YSZ electrolyte is important due to the high operation and even higher fabrication temperatures of the SOFC. The reactivity between LaMnOs and YSZ has been extensively studied both experimentally and theoretically. 

Fig. 2 shows the temperature as a function of irradiation time of Cu based material under microwave irradiation. CuO reached 792 K, whereas La2Cu04, CuTa20e and Cu-MOR gave only 325, 299 and 312 K, respectively. The performances of the perovskite type oxides were not very significant compared to the expectation from the paper reported by Will et al. [5]. This is probably because we used a single mode microwave oven whereas Will et al. employed multi-mode one. The multi-mode microwave oven is sometimes not very sensitive to sample s physical properties, such as electronic conductivity, crystal sizes. From the results by electric fixmace heating in Fig. 1, at least 400 K is necessary for NH3 removal. So, CuO was employed in the further experiments although other materials still reserve the possibility as active catalysts when we employ a multi-mode microwave oven. 

Based on these considerations the development of perovskite-based catalysts could be an alternative due to their ability to stabilised well-dispersed active sites more resistant to deactivation and to their tolerance to poisoning effects. Consequently, those materials... 

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. 

In addition to being able to catalyze the dissociation of O2. the material used for the cathode must be electronically conductive in the presence of air at high temperature, a property found primarily in noble metals and electronically conductive oxides. Ionic conductivity is also desirable for extending the reaction zone well into the electrode since the ions must ultimately be transferred to the electrolyte. Since precious metals are prohibitively expensive when used in quantities sufficient for providing electronic conductivity, essentially all SOFC prototypes use perovskite-based cathodes, with the most common material being a Sr-doped LaMnOs (LSM). In most cases, the cathode is a composite of the electronically conductive ceramic and an ionically conductive oxide, often the same material used in the electrolyte. 

The discovery of high Tq superconductivity in La2-xMxCu04 (M = Ba, Sr) (Bednorz and Muller 1987) based on perovskite and rock-salt structures has led to an international effort in superconductivity research over the last decade. The principles that govern superconducting copper-oxide-based materials have enormous significance in the application of these oxides as potential catalysts... 

Burakov, B. E., Anderson, E. B., Knecht, D. A., Zamoryanskaya, M. A., Strykanova, E. E. Yagovkina, M. A. 1999. Synthesis of garnet/ perovskite-based ceramic for the immobilization of Pu-residue wastes. Materials Research Society Symposium Proceedings, 556, 55-62. 

Another group of materials that has displayed high oxide ion conductivity is based upon a layered bismuth perovskite-based structure, first reported by Aurivillius in 1949 [90-92], The so-called Aurivillius phases are chemically expressed normally as Bi2A B 03 +3 [82], where A is a large 12-coordinated cation and B a small 6-coordinated cation. The structure is formed by n perovskite-like layers, (A 1B 03n+1)2, sandwiched between bismuth-oxygen fluorite-type sheets, (Bi/) 2 [93,94],... 

Materials of particular interest are the perovskite oxides BaTiOs-SrTiOs (BST) and PbZrOs-PbTiOs (PZT) solid solutions as well as the layered perovskites based upon SrBi2Ta209 (SBT). Since the ferroelectric effect requires... 

Extensive studies have been carried out on the microwave dielectric materials with high K and thermal stability for the miniaturization of microwave passive component. In particular, CaTi03-based materials have attracted considerable interest due to their high K. These titanates can be easily combined with other perovskite compounds to form solid solutions. However, they have a large TCF for practical apphcations. Various attempts have been made to control the TCFs of CaTiOj-based materials. However, most of them are mainly empirical, such as the addition of the materials with negative TCF values. 

In noncubic perovskites, whilst some of the 0—0 distances become shorter, any continuous path must also include longer 0-0 distances. The cubic perovskites should, therefore, be better proton conductors, and a shortening of the 0—0 distances should be dynamic, due to thermal motion. The H — M" repulsion contributes to the activation energy of the proton transport. Indeed, A + + O3-based materials are much poorer proton conductors than A + M + 03-based phases. Consequently, the A + O3 perovskites appear to show promise ]15,21], although... 

Figure 9.10 Comparison of the oxygen ionic conductivity of several ferrite-based materials with layered, perovskite and garnet structures [68,133,134], and total conductivity of Ba2ln2O5-based ceramics [125, 127, 129]. Data for Ba2ln2O5 [125] correspond to...

Perovskite-like materials AgF and CuFz based catalysts. 

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