Hydrogen-Permeable Perovskites

The extensive variety of properties that these compounds show is derived from the fact that around 90% of the metallic natural elements of the periodic table are known to be stable in a perovskite-type oxide structure [74], Besides, the possibility of synthesizing multicomponent perovskites by partial substitution of cations in positions A and B gives rise to substituted compounds with a formula A, A B,. B 03 ft. The resulting materials can be catalysts, insulators, semiconductors, superconductors, or ionic conductors. 

FIGURE 2.18 AB03 ideal perovskite structure, (a) The center of the cube is occupied by the A cation and (b) the B cation placed in the center of the cube. 

In the ideal structure, where the atoms are in contact, the B-0 distance is equal to all, where a is the cubic unit-cell parameter likewise, the A-0 distance is a/yjl, then the following correlation between the ionic radii holds [76]  

To measure the departure from the ideal condition, Goldschmidt introduced a tolerance factor, t, defined by the equation  

However, a large number of perovskite structures are distorted to orthorhombic, rhombohedral, or tetragonal, which can be approximated as cubic with t deviated from 1. In most cases, t varies between 0.75 and 1 [77], 

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Adding carbon to a FesAl aluminum alloy was studied using electrochemical permeation technique [96]. The addition of 1 wt.% carbon to Fe-16 wt.% causes a perovskite FesAlCo.s carbide phase to precipitate in the Fe3Al-based matrix, resulting in a 50% hydrogen permeability and difEtsivity decrease. 

The perovskite structure, ABO3, is a well-known class of solid oxide hydrogen permeable materials. The hydrogen is absorbed in the material as a hydroxide via the reaction... 

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