GB403368A - Improvements in or relating to magnetic cores for high frequency inductance coils and transformers - Google Patents

Abstract

A magnetic core material for radio frequency inductance coils and transformers comprises a compressed mass of finely divided magnetic material bound together by an insulating binder, the magnetic particles being fine enough to pass through a 400-mesh sieve and insulated from one another by a solid insulating material in addition to the binder. The magnetic material may be prepared by heating finely-divided pure iron oxides in a stream of hydrogen. The iron particles so prepared should be about 5 microns in size. The iron powder is mixed with an insulating material which does not chemically unite with the iron such as China wood oil varnish or a solution of phenol resin in acetone, dried in air, and subsequently baked so that the particles remain loose, each particle being individually coated. The insulated particles are then mixed with an insulating binder such as phenol resin, natural resin, or synthetic gums, either in dry powdered state or in solution, the amount of binder being such that the compressed core will contain about 10 per cent of inert material. The material is placed in moulds heated to 100 DEG C. and subjected to a pressure of about 5 to 25 tons per sq. in. so as to reduce it to about 0.4 of its initial volume, and is then treated so as to harden the resinous binding material. The moulding pressure may be released prior to the hardening treatment in order to allow slight expansion of the material and separation of the magnetic particles. After removing the mould, the continuity of the outer skin of the core is broken by mechanical or chemical means such as sand-blasting or pickling with acids. Specification 366,475, [Group XL], is referred to.