Chromium dioxide for magnetic tape - mfd. by oxidn. of hydrated chromic oxide with an oxyacid of the halogens

摘要

Pure CrO2 is mfd. by oxidising hydrated chromic oxide Cr2O3.xH2O, (where x is not 1). Oxidn. occurs at min. 350 degree C and min. 2 kilobars, using an oxidant which is solid or in aq. soln. The oxidant is HXOy.zH2O (where X is Cl or I; y = 1-4 and z is the water of crystallisation in the solid or the dilution of the liq.). The Cr2O3 is pref. made by adding NH3 to a soln. of CrCl3 and washing to remove NH4Cl4, then drying. The oxidants are pref. HClO4.zH2O, HIO3 or HIO4.2H2O, used at 400-500 degrees C and 2.5-4 kilobars. Prod. is used for industrial synthesis of ferromagnetic CrO2, esp. black acicular microcrystals with a rutile structure and min. Curie temp. is 120 degrees C.