Perovskite oxides conductivity

Double Substitution In such processes, two substitutions take place simultaneously. For example, in perovskite oxides, La may be replaced by Sr at the same time as Co is replaced by Fe to give solid solutions Lai Sr Coi yFey03 5. These materials exhibit mixed ionic and electronic conduction at high temperatures and have been used in a number of applications, including solid oxide fuel cells and oxygen separation. 

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

These results can be contrasted with results obtained using a less stringent equilibrium criterion of = 0.0005 S cm-1 min-1 as shown in the open circle data in Fig. 6. A large discrepancy is observed in the middle- o2 region. A difference between the measured values of Po2 by the sensors at the top and bottom of the cell of more than one order of magnitude in p0 was found under these conditions. In general, the p0 difference between the two sensors is slow to converge due to the slow equilibrium kinetics and consequently, both conductivity and log p0 difference criteria are needed to ensure that equilibrium is reached. The open circle data in Fig. 6 are similar to results in other ferrite perovskite oxides and reflect nonequilibrium behavior.14 17... 

Another class of important materials with oxygen ion conduction are perovskites (see Section 2.4.4). Perovskite phases have been, initially, considered as possible electrode materials for SOFCs. The first report of the existence of oxide conductivity in a perovskite material was made using a calcium-doped lanthanum aluminate [98], But the discovery of attractive oxygen conduction properties is rather recent [84],... 

Lanthanides as components in complex oxide systems In perovskites For conduction in electrocatalysis For oxidizing properties For hydrogenation properties For synthetic gas (CO-H2) reactions To provide sulfur oxides (SO, ) control In oxysulphides In other systems ... 

The intercoimecting material used in building a stack of fuel cells must have a high electrical conductivity and a coefficient of expansion similar to that of the electrolyte. The perovskite oxide LaCrOs meets these requirements nicely and is widely used although ferritic stainless steels have been used successfully in certain fuel cell geometries. 

Bockris and co-workers (317-320) conducted systematic studies on a variety of perovskite oxide catalysts in alkaline solutions and found the kinetics of the OER to have no functional dependence on the semiconductor-type properties of these oxides. The kinetics were found to improve with a decrease of magnetic moment, with a decrease of the enthalpy of formation of transition metal hydroxides, and with an increase in the number of d electrons in the transition metal ion. Thus, it has been suggested that, on the series of perovskites, there is a common slow step, OH desorption, with the differing —OH bond strength giving different isotherms and hence b values (i.e.. 

The mixed-conducting perovskite oxides have attracted particular interest for use as dense ceramic membrane to control partial oxidation of methane to C2 products or syngas. Such a process bypasses the use of costly oxygen since air can be used as oxidant on the oxygen-rich of the membrane. 

In our previous study, we found tliat Ti-based perovskite oxides, such as SrIiO, and ( a liO,. showed high corrosion resistance in a 50 wt % solution at 353 K. Since there are no d-electrons (Ti do system) and /-electrons in stoichiometric Ti-based perovskites (chemical formula A Ti Oa), no electrical conductivity of these materials is anticipated at the operating temperature. To obtain electrical conductivity, the fonnation of Tf in the perovskites is necessary. 

Reduction of NO with CO or H2 was found to be an interesting example of intrafacial catalytic process (30). If this reaction is conducted over a transition-metal oxide, the reaction rate appears to be related primarily to the thermodynamic stability of the oxygen vacancies adjacent to a transition metal ion. Associative as well as dissociative adsorption of NO have been reported on perovskite oxides (14, 22, 80, 174) (see also Section VI,B) the adsorption on the reduced oxides is stronger than in the oxidic compounds. Dissociative adsorption takes place at moderate temperatures as in NO reduction over Lao.gsBao.isCoOs at 100°C with the subsequent formation of N2 and N20 (73). 

Consequently, we are in the startup phase of our program. Our first task was to identify candidate perovskite oxide materials with high protonic conductivities. We have identified ytterbium doped strontium cerate and yttrium doped strontium zirconate materials as possible electrolyte materials. Barium cerate perovskites exhibit higher protonic conductivity, but the reactivity with carbon dioxide would require pretreatment of the steam. 

Semiconductor- Perovskite-type oxide Conductivity change by 10 -10 ppm 2-3 min. Air conditioner [29[, [31]... 

Key words Fuel Cell/Perovskite/Oxide Interlanthanoid/Electrical Conductivity Ceramics... 

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