Patent GB-311677-A (GB)
Title: Improvements in composite magnetic materials
IPC: H01F1/00
Filed: 1928-02-11
Abstract: 311,677. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). Feb. 11, 1928. Electromagnets.-A magnetic material for the cores of loading coils for telephone circuits &c. comprises finely-divided particles of magnetic material, such as a nickel-iron alloy containing 78¢ per cent of nickel, coated with a material which has been decomposed from a non-acid substance during a heat treatment subsequent to its mixture with the magnetic particles. The insulator may be prepared by dissolving mercuric nitrate in water, adding the magnetic particles, and boiling to dryness whilst stirring. The resulting dust is dried for from fifteen to thirty minutes, compressed into rings under a pressure of about 200,000 lbs. per sq. in., and annealed at about 500‹ C. Another insulator for the particles comprises magnesium borate and silica, which may be prepared by dissolving 150 grams of boric acid in 500 c.c. of water to which is added 16 grams of magnesium oxide, the mixture being stirred while being heated to 90‹ C. to dissolve the oxide. The magnetic dust of nickeliron alloy is mixed with the solution and about 2 per cent by volume of ground silica, the solution comprising about 4 per cent of the mixture. The mixture is dried in an open vessel at 100‹ C. and then in an evacuated vessel at 200‹ C. The resulting dry magnesium borate contains the silica dust. The coated particles are then compressed in a mould. Alternatively the insulating substance may consist of a coating of magnesium and zinc oxides formed by dissolving zinc oxide in ammonia and adding finely-divided magnesium oxide which is held in suspension. About 6 per cent by volume of the suspension is added to the magnetic particles and the mass is stirred and dried at 100‹ C. in air and at 200‹ C. in vacuo. Another insulating material comprises finelydivided hydrated silica, which may be prepared by causing silicon tetrachloride to react with steam, the resulting vapour being condensed to form a colloidal suspension of hydrated silica in hydrochloric acid, the acid being then chemically removed. Alternatively the hydrated silica may be prepared by treating sodium silicate with hydrochloric or other acid. The colloidal suspension is ground in a colloidal mill to separate the colloidal aggregates, the product being mixed with the nickel-iron alloy particles and the mixture evaporated to dryness. A coating of silica is thus produced on the particles, which are then compressed in a mould. In a further modification, higher oxides which readily break down on heat treatment may be employed to form the insulating material, suitable oxides being lead or nickel sesquioxide, manganese dioxide, antimony pentoxide, and bismuth oxide. The magnetic dust is mixed with the oxide to ensure a thorough coating and the mass is compressed into core rings and heated. Oxygen is liberated and forms an insulating coating of iron and nickel oxides on the particles, the lead or other oxide residue forming a secondary insulating coating. Specifications 269,510, [Class 37, Electricity, Measuring &c.], and 269,511 are referred to.