Process for preparing acicular magnetic single-crystal particles of alpha -ferric oxide and of magnetic particles modified with cobalt

摘要

The invention relates to a process for preparing single-crystal acicular (needle-shaped) alpha -ferric oxide particles, in which the particle size distribution is limited to a narrow range. The process includes heating an aqueous suspension of ferric hydroxide to 100 to 250 DEG C at an alkaline pH in the presence both of a water-soluble organic or inorganic compound which is suitable to form complexes with iron and for acting as a growth regulator, and of alpha -ferric oxide seed crystals in which the length of the secondary axes on average is no greater than 0.4 micrometres (microns), their amount, in terms of the iron content, being from 0.1 to 0.25 mol%, based on the ferric hydroxide. The gamma -ferric oxide obtained from the alpha -ferric oxide is modified with cobalt at from 30 to 50 DEG C in the presence of ferrous hydroxide and cobaltous hydroxide in amounts of, in each case, from 0.5 to 50 mol%, based on the iron content of the gamma -ferric oxide. IMAGE