Compositions – Magnetic – Iron-oxygen compound containing
Patent
1987-06-01
1989-02-28
Dixon, Jr., William R.
Compositions
Magnetic
Iron-oxygen compound containing
252 6257, 252 6258, 252 6259, 252 6260, 252 6261, 252 6262, 252 6263, 252 6264, C04B 3526
Patent
active
048083275
DESCRIPTION:
BRIEF SUMMARY
The object of this invention is new compositions of particulate magnetic oxides with a defect spinel structure, preparation thereof and application thereof.
It is known that in the industry relating to materials for high-density magnetic recording, powdered oxides of ferromagnetic materials are used in the form of submicron acicular particles with a high coercive field and high saturation and remanent magnetization values.
These magnetic powders can be prepared starting with organic salts, in particular, starting with mixed oxalates.
In French Patent Application No. 72.14215 (Publication No. 2,180,575), a process is proposed for preparing magnetic oxides of the ferrite type, i.e., compounds based on iron sesquioxide in which some of the ferric ions and some of the defects are replaced by one or more bivalent cations.
The bivalent cations which are compatible with a cubic spinel structure are those of cobalt, iron, zinc, copper, magnesium, nickel, manganese and cadmium.
The process according to the above-mentioned French patent consists, starting from a mixed oxalate, of decomposing this oxalate by heating it in air to a moderate temperature to produce a superparamagnetic oxide (because of the very small dimensions of the crystals, on the order of a few tens of Angstroms), reducing this oxide with hydrogen in the presence of water vapor to steer the crystallization of the oxide, which is still poorly organized, towards a cubic spinel lattice and produce a magnetite substituted with one or more bivalent metals, and finally oxidizing the magnetite thus obtained to convert all of the iron to the trivalent state. This results in solid solutions of gamma-Fe.sub.2 O.sub.3 and ferrites of the type MOFe.sub.2 O.sub.3, where M represents one or more bivalent metals selected from those previously mentioned.
The process for converting oxalates into oxides which has just been described makes it possible to produce particles of magnetic oxides most of which have retained the shape and dimensions of the initial oxalate particles. However, this process is not entirely satisfactory, for reasons which will be indicated.
It is worth mentioning here that when oxide particles are prepared, well-formed domains of matter (crystallites) form within the particles, separated by transition zones (crystallite boundaries) and micropores.
In this application, the term texture will be used to designate these arrangements of matter within a particle, the term internal sintering will be used to indicate improvement in this texture under the influence of heat treatments, and external sintering will be understood in its usual sense of leading to welding among particles with the appearance of dendrites. Internal sintering makes it possible to improve magnetic performance, while external sintering, in contrast, leads to a significant degradation therein.
Thus, the process which has just been described is not satisfactory in terms of the texture of the resulting oxide particles. The particles have a granular structure and exhibit crystallite boundaries with the presence of dendrites.
The presence of crystallite boundaries has an unfavorable influence not only on the magnetic properties but also on the mechanical strength characteristics of the particles. Specifically, crystallite boundaries constitute breakage point nuclei for the grains which, during subsequent handling of the powders, have a tendency to shatter into granules which are often essentially spherical in shape and are not compatible with good magnetic properties.
The powders obtained after such handling are therefore relatively heterogeneous.
In summary, these texture defects do not make it possible to obtain oxide powders meeting current needs in the area of magnetic recording.
The object of this invention is compositions of particulate magnetic oxides containing new adjuvant or substituent elements.
The adjuvants promote internal sintering and make it possible to significantly improve the texture of the particles and prevent external sintering. The particles produced have a homo
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Gougeon Michel
Mollard Paul
Rousset Abel
Salvaing Christiane B.
Tailhades Philippe
Brunsman David M.
Centre National de la Recherche Scientifique "CNRS"
Dixon Jr. William R.
Universite Paul Sabatier (Toulouse III)
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