Magnetic fluids and process for obtaining them

Claim 2

Ferrofluids consisting essentially of an aqueous solution free of surfactants of polyoxoanions of Fe(III) and at least one metal with an oxidation degree II selected from transition metals consisting of iron, cobalt, manganese, copper and nickel, with an associated cation selected from H + , N(CH 3 ) + 4 and N(C 2 H 5 ) + 4 .

Claim 3

Ferrofluids according to claim 1, wherein the polyoxoanions form particles having a mean diameter in the order of a hundred angstroms and a molecular weight in the order of 10 6 to 10 7 .

Claim 4

Ferrofluids according to claim 1 which has an absorption spectrum in the visible light showing a shoulder in the range of about 450 to 500 nm.

Claim 5

Ferrofluids according to claim 1, which comprise Fe(III)/Fe(II) polyoxoanions and have a maximum absorption at 1400 nm in the near IR.

Claim 6

Ferrofluids according to claim 1, which are dehydrated.

Claim 7

Ferrofluids according to claim 1, which have an X-ray powder diagram identical to that of bivalent metal ferrites.

Claim 8

A process for producing ferrofluids according to claim 1, which comprises adding to a base selected from NaOH, NH 3 in aqueous solution, tetramethyl ammonium hydroxide and tetraethyl ammonium hydroxide a suitable amount of the product of dissolution in water of the salts of the appropriate metals to form a gel; after optional separation of this gel, effecting on said gel a cation exchange by means of an aqueous solution of a suitable cation provided by means of a compound selected from HCl, HNO 3 , CH 3 COOH, tetramethyl ammonium hydroxide and d tetraethyl ammonium; and separating the gel so obtained, which is resolubilized to an aqueous solution with, optionally, adjustment of the pH by a base.

Claim 9

A process according to claim 7, wherein amount of the base to which there is added the product of dissolution in water of the metal salts shows an excess based on the stoichiometric amount necessary for the formation of hydroxides of the metals presents.

Claim 10

A process according to claim 7, wherein separation of the gel is dispensed with by using a base the cation of which is suited to constitute the associated cation.

Claim 11

A process according to claim 7, which comprises using, as sources of starting metals: for Fe(III): ferric alum, ferric chloride or ferric nitrate; for M(II): Mohr's salt, ferrous chloride, ferrous sulphate or a hydrosoluble salt of at least one metal of oxidation degree II of said transition metals.

Claim 12

Ferrofluids consisting essentially of an aqueous sol free of surfactants of polyoxoanions of Fe(III) and at least one metal with an oxidation degree II selected from transition metals consisting of iron, cobalt, manganese, copper and nickel, with an associated cation selected from H + , N(CH 3 ) + 4 and N(C 2 H 5 ) + 4 , wherein said polyoxoanion particles have a mean diameter in the order of a hundred angstroms and a molecular weight in the order of 10 6 to 10 7 , said ferrofluids having an absorption spectrum in the visible light with a shoulder in the range of about 450 to 500 nm and an X-ray powder diagram identical to that of bivalent metal ferrites.

Claim 13

Ferrofluids consisting essentially of an aqueous sol free of surfactants of polyoxoanions of Fe(III) and at least one metal with an oxidation degree II selected from transition metals consisting of iron, cobalt, manganese, copper and nickel, with an associated cation selected from H + , N(CH 3 ) + 4 and N(C 2 H 5 ) + 4 , wherein said polyoxoanion particles have a mean diameter in the order of a hundred angstroms and a molecular weight in the order of 10 6 to 10 7 , said ferrofluids having an absorption spectrum in the visible light with a shoulder in the range of about 450 to 500 nm and an X-ray powder diagram identical to that of bivalent metal ferrites.