method for the electrical insulation of the surface of metallic sheets for laminated magnetic cores

摘要

In a method of producing a thin electrically insulating coating on metallic sheets for laminated magnetic cores a solution of one or more salts selected from magnesium, nickel, zinc, copper and chromium salts which will produce free oxygen and a divalent or trivalent oxide when heated, is applied to the surface of the metallic sheet and then dried, so that a continuous crystal layer is obtained, which by a subsequent heating is disintegrated, so that the oxygen liberated from the salt reacts with the surface of the metallic sheet to form a divalent or trivalent oxide, the divalent oxide thus formed combining with the trivalent oxide formed from the disintegrated salt or salt mixture, or the trivalent oxide thus formed combining with the divalent oxide formed from the disintegrated salt or salt mixture, in order that a coating of a spinellike chemical compound shall be formed on the surface of the metallic sheet. A solution of a nitrate to which has been added a small amount of an organic salt, e.g. magnesium acetate or formate, is preferably used; the solvent may be water or ethyl alcohol. The method may be combined with the usual annealing process for the sheets of laminated magnetic cores. In this case the sheets are immersed in the salt solution and then removed and heated, so that the salt will be dried to form a continuous crystal layer, the salt layer then being disintegrated by the succeeding heating to a high temperature, the desired layer subsequently being formed at the still higher temperature at which annealing takes place. By way of example is described the formation of a spinel-like compound, composed of ferric oxide and magnesium oxide, on iron sheets, by immersion for 5 secs. in an aqueous solution of magnesium nitrate and magnesium acetate, drying at 200 DEG C., and annealing for 5 minutes at 800 DEG C.