Patent GB-888688-A (GB)
Title: Magnetisable materials comprising barium and ferric oxides in combined form and magnets produced therefrom
IPC: H01F1/36
Filed: 1958-05-20
Abstract: Amorphous particulate barium ferrite, the particles of which have an average diameter less than 1,000   and are spheroidal in shape, is prepared by injecting a solution in a mutual solvent of a barium salt and an iron salt into an oxidizing flame and rapidly cooling the resulting particulate product.  Subsequent heat treatment at above 600 DEG C. will increase the average diameter to 2,000 to 100,000  , develop crystallinity in the particles and make them hard magnetically.  The initial barium and iron salts may be in solution in an aqueous or an organic solvent and the combined solution may be atomized in the oxidizing flame either by p  employing liquid pressure or by using part of the oxidizing or the combustible gas. Examples of the latter are carbon monoxide, cyanogen, hydrogen, methane, ethane, propane, butane ethylene, propylene and petroleum hydrocarbons.  Flame temperatures of 600 DEG  to 1,600 DEG C. may be employed and the atomized particles are subjected to such temperatures for periods from 1 to 0,001 seconds.  The particles may be cooled in less than a second to a temperature below 350 DEG C. The amorphous particles so formed may be heat treated at a temperature above 600 DEG C. but below the sintering temperature in trays, rotary furnaces or in fluidized beds to convert the particles to a crystalline form.  Alternatively, the heating step may be a part of the initial ferrite formation step, this being effected by using a longer combustion tube or higher flame temperature, thus increasing the time at which the particles are exposed to high temperature.  The amorphous barium oxide-iron oxide ferrite product has a bulk density of 70 lbs. per cubic ft., but after heat treatment at 1,250 DEG C. for an hour the bulk density is increased to 103 lbs. per cubic ft., the colour changing from red to purple black.  The ferrite may be employed to form resin-bonded magnets by incorporating the ferrite in a resinous precursor or a coldsetting cement such as silicate cement. A polymerizable monomer may be employed and the polymerization effected in a magnetic field whereby a shaped magnet may be produced in a single step.  The magnet may contain from 30 to 95% by weight of the ferrite.  Suitable synthetic resins are polystyrene, phenolic resins, epoxy resins, polymerized vinyl resins such as polyvinyl chloride or acetate, polyethylene or butyl rubber.  Natural rubber may be used.  According to Examples, (1) a methanol solution of ferrous bromide and barium bromide was atomized by an oxygen jet in a central spray nozzle surrounded by a concentric set of jets of hydrogen; further air was supplied as an outer series of concentric jets.  A flame temperature of 1,350 DEG C. was attained and the particles leaving the oxidation zone passed through a cooling zone at 110 DEG  to 120 DEG C.  The final product had a formula BaO.6Fe2O3 and the particles were spheroidal in shape and had an average size of 650 Angstrom units. (2) An aqueous solution of barium chloride and ferrous chloride was atomized into a flame generated by streams of natural gas and oxygen.  The flame temperature was 1,100 DEG -1,200 DEG C. and the combustion products were cooled at 70 DEG -80 DEG C. whereby a barium ferrite of formula 1,1 BaO.6Fe2O3 in the form of amorphous powder having an average particle size of 750 Angstrom units was obtained.  (4) Barium ferrite of (2) above was heat-treated in an electric furnace in a toroidal former at 1,200 DEG C.  The product had a remanence of 2,250 gauss, a coercivity of 1,640 oersteds and was 98% crystalline.  (5) A barium ferrite was heated at 1,250 DEG C. and mixed to a putty-like consistency with an epoxy resin containing diethylenetriamine as accelerator.  The mixture was pressed into a disc-shaped mould and cured at room temperature in a strong electromagnetic field.  The resulting magnet had a marked directional field.  (6) A barium ferrite was mixed with monomeric styrene and polymerized in a magnetic field at 150 DEG C.  (7) A barium ferrite was mixed with polyvinyl chloride resin, dioctyl phthalate and a stabilizer and milled at 250 DEG C. after which the product was callendered to form a polyvinyl chloride sheet 0,020 inch thick.  This sheet could be magnetized and exhibited a directional magnetic field.  A similar tape could be prepared with polyethylene by extrusion.  (8) A magnet was prepared from an epoxy resin material consisting of the mixed dimer and trimer of bisphenol glycidyl ether and a minor proportion of diethylenetriamine, 40 to 70% by weight of the ferrite was incorporated and the mixture was pressed into moulds and cured at room temperature.  The polymerized forms were magnetized in a 20,000 gauss field to produce strongly oriented magnets.