The high-temperature electrical conductivity of a model catalyst containing conducting and insulating oxides

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Abstract
DC conductivity of catalysts comprising insulating high-alumina cement and semi-metallic Fe3O4 was studied between 300 and 800K in an inert atmosphere. The activation energy of the thermally promoted conductivity decreased rapidly with increasing content of the conducting component. The high-temperature activation energy falls exponentially with increasing Fe3O4 content between about 0.1 and 0.7 volume fraction, approaching the value for pure Fe3O4 at 0.7. The change of conductivity with Fe3O4 content can be up to ten orders in magnitude. For reproducibility the percolation threshold at about 0.15 volume fraction must be exceeded. For the recommended fraction between 0.25 and 0.3 oxidation to Fe2O3 can cause a fall in conductivity by a factor of 100 whilst reduction to Fe can cause a rise of similar magnitude. As with thin-film cermets, the volumetric compositions and not the nature of support, additive or preparation conditions appears to be the decisive factor in determining the conductivity of the composite.

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