Abstract
The mechanism of the high shrinkage that occurs in chromic oxide in reducing conditions has been investigated by micrographic, analytical, electrical resistance, and other means. The effect of the reducing conditions is to modify the surface of the oxide particles, the core remaining normal oxide (i.e. oxygen excess). The modification is predominantly a change to a lower oxide or oxides and/or metal, although transitional formation of oxygen-deficient oxide is implied. In oxidising conditions (which cause fritting, but not shrinkage), modification to the extent that surfaces become of approximately stoichiometric proportions also occurs, but this change is apparently caused by loss of trioxide and results in unreactive oxide.
Types of electrical resistance test employed include (i) constant low-temperature, variable-oxygen, p/n tests on compacts and powders, and (ii) variable-temperature tests across a compact surface, across a fracture, and across chromium trioxide undergoing decomposition.
Additionally, chromium trioxide has been decomposed to chromic oxide over the range 400 to 1400° C, in argon and in oxygen. The apparent O/Cr ratio decreases on decomposition at 1400° C, particularly markedly in an oxygen atmosphere, and is accompanied by a drastic reduction in reactivity.
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Stone, H.E.N., Lockington, N.A. The morphology of oxide reduction: Chromic oxide. J Mater Sci 2, 112–117 (1967). https://doi.org/10.1007/BF00549569
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DOI: https://doi.org/10.1007/BF00549569