Preparation of ferromagnetic chromium dioxide having granulometric and magnetic characteristics varying within wide limits as well as a new chromium dioxide of particular granulometric characteristics

摘要

A process is disclosed for obtaining a ferromagnetic chromium dioxide having granulometric and magnetic characteristics varying within a very wide range, starting from hydrated chromium (III) chromate or from mixtures of metal chromium and chromium oxide having a valency greater than 4, by heating up to between 300 DEG and 500 DEG C under an oxygen pressure of 30-1000 atm., characterized in that the variations of the granulometry and of the magnetic characteristics are obtained by regulating the heating, by means external to the reaction mass or by adding to the starting materials from 0.05 to 30% by weight (with respect to the chromium present) of compounds that decompose exothermically between 100 DEG and 270 DEG C under reaction conditions that lead to CrO2 by favoring the reduction to CrO2 in such a way that the reaction mass will cross the temperature interval of from 200 DEG to 300 DEG C in pre-established times, depending on the granulometry and coercive force desired in the product. Suitable exothermically decomposing compounds include ammonia, oxalic acid, lactic acid, citric acid, and tartaric acid. A novel ferromagnetic chromium dioxide is also disclosed.