GB682897A - Improved magnetic powders and method of making the same - Google Patents

Abstract

A process of preparing magnetic powder for magnetic cores having high resistivity and low power losses when operating at very high frequencies comprise impact milling a mixture of carbonyl iron powder, subject to excessive cluster formation, the particle size range of which does not substantially exceed a diameter of 5 microns, with a cluster - penetrating material which forms a solid, non-conductive, inert particle-separating deposit on the particles, the amount of the cluster-penetrating material being such as to yield the deposit in an amount from 0.2 to 5 per cent of the weight of the iron powder, and continuing impact milling of the mixture until the resulting powder has an apparent density when dry of at least 2.3 grams per c.c. and the deposit is uniformly distributed throughout the mixture. The carbonyl iron powder may be produced directly by thermal decomposition of iron carbonyl in such a manner as to have a weight average particle diameter of not more than 4 microns, e.g. from 2 to 4 microns. The cluster-penetrating material may be a powder, e.g. a silicate (e.g. silica dust or colloidal clay), or a metal oxide or sulfide, the particles of which have an average diameter less than 1/10th of the average diameter of the iron particles and less than 0.1 micron. Particle-separating materials also include glassy resins, e.g. solid silicones, which can be mixed with the carbonyl iron powder in the form of a solution in a volatile solvent, evaporated before impact milling or during the initial stage thereof to leave a solid glass film of resin on the particles which is pulverized by continuing the impact milling of the powder in the dry state. Normally solid fatty or waxy materials of high melting point, e.g. stearic acid, palmityl alcohol, may be used or use may be made of phosphoric acid or propionic acid especially in the presence of an organic solvent. The impact-milled powder is thoroughly mixed with a liquid particle insulating composition which forms an insulating coating on the individual particles. The composition may be a solution of 0.5 to 2.5 per cent of phosphoric acid in a volatile organic solvent, or an aqueous solution of an alkali metal silicate, the solvent being evaporated after mixing the solution with the impact-milled powder. The mixing can be caried out by convetional tumbling or by further impact milling. The resulting powder is mixed with 3 to 10 per cent of its weight of a binder, e.g. a resin or an alkali metal silicate, and moulded under pressure to the desired shape to form a core.