Particulate compositions of ferromagnetic metal oxalates in the

Details Particulate compositions of ferromagnetic metal oxalates in the Particulate compositions of ferromagnetic metal oxalates in the

1987-07-02

1989-02-07

Stallard, Wayland

Organic compounds -- part of the class 532-570 series

Organic compounds

Heavy metal containing

75 05AA, 252 6255, 252 6256, 252 6257, 252 6258, 252 6259, 252 626, 252 6261, 252 6262, 252 6263, 252 6264, 534 16, 556147, C07F 1100

Patent

active

048032913

DESCRIPTION:

BRIEF SUMMARY
The object of the present invention is particulate compositions of ferromagnetic metal oxalates in the form of submicronic acicular particles, the preparation of same and their application in particular as intermediate products in preparation of magnetic powders.
It is known that the industry for manufacturing magnetic recording media (magnetic tapes, tracks, discs, cards, etc.) uses magnetic powders of oxides, alloys or other materials derived from metals.
These powders can be obtained from organic salts and in particular from oxalates; see, for example, U.S. Pat. Nos. 3,317,574, and 3,994,819.
It is known that a particle can be characterized by its dimensions in the longitudinal (length) and transversal (diameter) directions and also by the acicular ratio, which is the ratio of the length to the diameter.
It is generally agreed that, in the above-mentioned field of use, the best results are obtained with particles whose length is between 0.05 and 0.5 micrometer and whose acicular ratio is approximately between 2 and 5.
The density of the recorded data in effect depends on the size of the particles, and the above-indicated values correspond to optimal values since, for lower values, superparamagnetism phenomena can occur and lower the performances.
In U.S. Pat. No. 3,317,574 a method is described which consists of preparing an aqueous solution of a mixture of a mineral salt, a ferromagnetic metal and a carboxylic acid, and then of adding to said solution an orgnic solvent which is capable of causing precipitation of the organic acid salts of the metals present.
Such a method, however, only provides particles having an acicular ratio of close to 2 for the submicronic particles, which is insufficient for certain applications. With the same method it is possible to obtain particles with a higher acicular ratio, but solely for particles whose average length is greater than 1 micrometer.
In U.S. Pat. No. 3,994,819, a method is described which provides, by slow addition of a hydroalcoholic solution of mineral salts of ferromagnetic metals to a hyroalcoholic solution of oxalic acid, particles of oxalates having an average length generally between 0.5 and 1 micrometer, with a relatively low acicular ratio. In the same U.S. patent, a method is described which enables the length of the particles to be decreased, said method consisting of very slowly adding the solution of salts to the solution of oxalic acid in a mixture of ethanol and petroleum ether and by carrying out the precipitation step at -20.degree. C. Particles with an average length equal to 0.4-0.5 micrometer are obtained and the average acicular ratio is approximately equal to 12, which is a little too high. Above all, however, the particles obtained are too heterogeneous for certain applications.
In addition, this method does not enable ferrous sulfate to be used as a starting material: it is necessary to use, at least in part, ferrous chloride, which is more soluble and more expensive, in order to avoid working with excessive amounts of solution.
The present invention resides in the discovery of a new method which, for the first time, provides single or mixed oxalate particles of ferromagnetic metals with dimensions and acicular ratios in accordance with the optimal norms indicated above. The method of the invention further has the advantage of not requiring cooling, of enabling the exclusive use of ferrous sulfate and of being quicker than the prior art methods.
The object of the present invention, therefore, is a particulate composition of single or mixed oxalates of ferromagnetic metals, characterized by the fact that it comprises acicular particles having a length of between 0.05 and 0.5 micrometer, with more than 60% of the particles having a length equal to the average length .+-.0.1 micrometer and an acicular ratio of greater than 3, with said average length being between 0.15 and 0.35 micrometer.
In certain cases, the method of the invention even allows particulate oxalate compositions to be prepared, more than 80% of the particles of which,

REFERENCES:
patent: 3317574 (1967-05-01), Morita et al.
patent: 3994819 (1976-11-01), Mollard et al.
patent: 4323596 (1982-04-01), Buxbaum et al.
patent: 4367214 (1983-01-01), Sarnecki et al.
patent: 4376714 (1983-03-01), Pingaud
patent: 4400432 (1983-08-01), Buxbaum et al.
patent: 4689265 (1987-08-01), Miyoshi

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