GB634504A - Improvements in or relating to magnetic cores for use at high frequencies - Google Patents

Abstract

Ferromagnetic cubic nickel-zinc ferrite for use in the cores of inductance and transformer coils operating at frequencies exceeding 1 mc/sec. is made by forming a finely-divided mixture of compounds of iron, nickel and zinc wherein the proportion of iron compound or compounds corresponds to a molecular percentage of from 40-50 per cent of ferric oxide and the proportion of iron, nickel and zinc compounds are such as to yield a ferrite with a Curie point of at least 250 DEG C., the said mixture being sintered in an atmosphere containing oxygen at a temperature not exceeding 1300 DEG C. The oxide mixture may be produced by deposition with sodium carbonate from a solution of nickel sulphate, zinc sulphate and iron nitrate, and then dried and heated in air for one hour at 900 DEG C. It may be moulded into a ring at 3 tons/cm.2, heated at 1000 DEG C. or 1300 DEG C. for 1 hour in oxygen and cooled at about 5 DEG C. a minute in oxygen. Alternatively, a mixture of the oxides may be ground in a ball-mill for four hours, heated for one hour to 800 DEG C. in oxygen, re-ground and compressed to a ring which is heated for 1 hour at 1050 DEG C. in oxygen and slowly cooled. To provide a correct dosing of oxygen, a stream of superheated water-vapour may be passed over the ferrite. This water-vapour is partly dissociated into H and O and the oxygen partial pressure decreases with decrease in temperature whereby the oxygen content is controlled. Alternatively the water-vapour may be replaced by a mixture of an inert gas with oxygen obtained from water by electrolysis by a current varying with the temperature of the ferrite. Specifications 626,946 and 645,056 are referred to. The product may be resintered several times; suitable proportions of oxides, and properties of the finished ferrites are stated.