GB674719A - Improvements in magnetic compositions - Google Patents

Abstract

674,719. Magnetic compositions. CROWLEY-REPUBLIC STEEL CORPORATION. Dec. 14, 1950 [Feb. 18, 1950], No. 30540/50. Class 38 (ii). A magnetic composition is composed principally of ferromagnetic material containing a mixed ferrite having distributed through it metallic iron in crypto-crystalline form with, optionally, further powdered metallic iron. This composition may be produced by intimately mixing ferric oxide with oxides of metals which modify the electrical and magnetic properties of the composition, e.g. zinc, magnesium, copper and cobalt, nickel, manganese (and possibly chromium), respectively, subjecting the oxide mixture to prefiring treatment at a temperature between 800‹ and 1200‹ C., in a reducing atmosphere, grinding the reduced product, acidulating the powder with hydrochloric acid or ferrous chloride, heat-treating between 1100‹ and 1450‹ C. in an atmosphere less reducing then the first, and cooling. The final product contains comminuted iron distributed through it, as a result of dissociation of some ferrous oxide into iron and magnetit so the material reverts to magnetic form during cooling, and in addition powdered iron may be added, or produced by reduction of ferrous oxide. Between 50 and 90 per cent but usually not more than 70 per cent of ferric oxide may be used with zinc and nickel oxides in the ratio by weight of between 2: 1 and 1: 1, various proportions being stated. A carbonaceous material such as dextrin, soluble starch, or flour, may be included with the initial mix to reduce the Fe 2 O 3 to FeO. The firing may be done with commercial gas and air in such proportions as provide a suitable reducing atmosphere. In pre-firing, the material, in containers 15 X 10 X 2 inches is heated up for 20 minutes to two hours, retained, preferably at 1050- 1100‹ C. for 2-6 hours and cooled to room temperature during 3-6 hours. Grinding, completed or partly done before acidulation, may be to a fineness of 1 micron. After the acidulation which promotes rapid sintering and homogenizing, the material is washed until its pH value is about 3, dried, and optionally mixed with a carbonaceous binder, e.g. phenolformaldehyde resins with or without a solvent, which is burnt away at the second firing. The shaped body may be coated with a ceramic material, e.g. of 15 per cent talc and 85 per cent feldspar. The final heating, preferably up to 1400‹ C., is in an atmosphere slightly less reducing than the first, heating up taking about 4 hours, constant heating about 8 hours and cooling 4 hours. Figures are given showing the improved values of Q, Y and their temperature coefficients of the composition.