Perovskite-type oxides resistivity

Figure 7. Dependence of the resistivity of perovskite-type oxides on the X value.

IMPROVING THE S02 RESISTANCE OF PEROVSKITE TYPE OXIDATION CATALYST... 

In recent years, much attention has been focused on hydrocarbons total oxidation over mixed oxides. It was reported that perovskite type oxides remarkably oxidise carbon monoxide, light alkanes and also methane at low temperatures [1]. However, the major obstacles to the successful application of these materials in a large scale are both then-low resistance to sulphur poisoning and also their scarce BET surface area which is often linked to the catalytic activity. For this, development of more active catalysts has become a challenge to be overcome. Many attempts have been made to develop new preparation methods to improve... 

The catalytic activity of cation-doped hexaaluminate was not as high as the Pd catalyst or some perovskite-type oxides. But the thermal stability is superior for hexaaluminate to the Pd or perovskite-based catalysts. Pd/cordierite is the popular combustion catalyst due to its very high thermal shock resistance resulting from the low thermal expansion coefficient of the support material. However, the thermal stabilities are not high enough due to its low melting point. 

For perovskite-type oxides, the proton solubility decreases, but the stability to CO2 increases as the basicity of B-site ions decreases in the order of Ce Zr Sn Nb Ti [156]. The best compromise for high proton conductivity and chemical stability is found for Y-doped BaZr03, but unfortunately, this material exhibits large grain boundary resistance, and it stiU reacts slightly with CO2 under certain conditions [157]. Chemical stability against CO2 has been demonstrated recently with BaCeo,3Zro,5Yo.203 8 by Fabbri et al. [158]. 

Similarly to flame-made titanates, the precise control of the stoichiometry and material purity has an influence on the electronic properties of perovskite-type oxides such as LSC, LSCF, and BSCF. While conductivity is comparable with the highest reported in the literature (Figure 4.5), other unique properties are documented for these flame-made compositions. LSC, LSCF, and BSCF from a FSS process feature a pronounced shift of the temperature, at which the maximum conductivity is observed. The FSS process resulted in materials with an exceptional electronic conductivity, which may better match to a SOFC operated at intermediate or low temperatures. For example, LSCF cathodes based on flame-made nanopowders have shown polarization resistances in the range of 0.7 Q. cm at 592 °C [59], which are among the lowest reported for thick film layers of this material stoichiometry. Similar conclusions were drawn with respect to LSC-based cathodes, for which very low overpotentials were documented [60]. 

Hexaaluminates represent a class of materials that are highly resistant to sintering at high temperature. Therefore, these materials have attracted the attention of researchers who are involved in developing catalysts for high temperature applications. Hexaaluminates have been used as supports as well as the active material in catalytic combustion reactions [121-142]. Hexaaluminates can be represented by the formula AAI12O19 where A is an alkaline or alkaline-earth metal. They consist of a lamellar structure and both A cation and A1 can be partly substituted by other cations. Incorporation of other cations drastically modify the catalytic activity of these materials. However, such modifications are rather limited compared to the possibilities existing in perovskite type oxides. 

A large group of chemical compositions has been investigated as potential candidates for IT SOFC cathode materials. A recent article by Skinner has provided an overview of the progress of perovskite type oxides for the SOFC cathode, with an emphasis on the role of chemical compositions [41]. On the contrary, microstructure plays a major role in the cathode function as well. This is particularly true when the composite cathode, which shows a better performance compared to a single composition cathode, is used. Several authors have shown that electrode microstructure and transport properties have a profound effect on polarization. Tanner et al. [42] have shown that polarization resistance (Rp) depends upon the grain size, d, of the ionic conductor in the composite electrode and the volume fraction porosity, which was further derived as in (1) by considering the monolayer gas adsorption. A similar relation has been proposed as [43] ... 

In the present study, the long-term oxidation resistance of some of these FeCrMn(La/Ti) steels in both air and simulated anode gas has been studied and compared with the behaviour of a number of commercially available ferritic steels. Main emphasis was put on the growth and adherence of the oxide scales formed during exposure, their contact resistance at service temperature as well as their interaction with various perovskite type contact materials. 

Perovskite-type metal oxides are promising candidates as deep oxidation catalysts due to their robustness and good resistance to sulphur poisoning. Three sorts of reaction mechanism have been proposed for methane combustion over different perovskites, i.e. 

---