MAGNETIC MATERIAL COMPRISING IRON, BORON AND A RARE EARTH METAL

Abstract: A magnetic material of the composition Fe 79-x-y B 21+x R y in which R is a rare earth element or a mixture of such elements and -5<x<5 and +1<y<+5. The preferred rare earth elements being neodymium and/or praseodymium.

Description: The invention relates to a magnetic material, comprising iron, boron and one or more rare earth elements. Magnetic materials based on the said elements are known; see, for example, Materials Letters 2 , pp . 411-5 (1984), Stadelmaier, Elmassy, Liu and Cheng, entitled: "The metallurgy of the Iron-Neodymium-Boron permanent magnet system". The known material consists mainly of tetragonal crystals of Nd₂Fe₁₄B embedded in a neodymium-rich second phase; the same applies to materials which comprise praseodymium as a rare earth element. Materials of this type poorly withstand corrosion as a result of the presence of a second phase which is rich in rare earth element. If a gross composition is chosen in such a manner that the second phase which is rich in rare earth element is not formed, the coercive force of the material is negligible (see page 415 of the said paper). US-A-4 402 770 discloses a magnetic material of rare earth, iron and boron, where the amount of rare earth is above the now claimed range. It is the object of the invention to provide magnetic materials of the said composition which have such a coercive force that they are technically useful and can better withstand corrosion than the said materials. The invention is based on the discovery that materials having approximately the gross composition Fe₃B which in themselves are soft magnetic and in the equilibrium condition at room temperature consist of α-Fe and Fe₂B (see, for example, GB 1,598,886) can obtain permanent magnetic properties by comparatively small additions of rare earth elements. The material according to the invention is characterized in that the gross composition satisfies the formula Fe 79-x-y B 21+x R y′ wherein R is a rare earth element and in which it holds that -5<x<+5 and +1<y<+5. As a result of the presence of a comparatively small quantity of rare earth element which in no case exceeds 5 at. %, the materials prove to have a coercive force H c of approximately 2 to 3.5 k Oe; for comparison: a material having a comparable gross composition of Fe₇₇B₂₃ provides a coercive force not higher than 800 A/m (= 0.01 k Oe), see "Behavior of glassy Fe₇₇B₂₃ upon anneal in the absence of externally applied fields" by Ramanan, Marti and Macur in J. Appl. Physics 52 (3), pp . 1874-6 (1981). When the boron content is increased or decreased beyond the indicated range of compositions, the compounds Fe₂B, Nd₁₁Fe₄B₄ and iron, respectively, prove to occur as contamination phases. When the rare earth element content increases, upon crystallisation, rare earth metal-rich crystalline second phases and iron are segregated as a result of which the material becomes sensitive to corrosion. X-ray examination has proved that the material comprises only one crystalline phase having the Fe₃B structure. If no rare earth element is present, said structure at room temperature is metastable, see, for eaxmple, Zts. f. Metallkunde 73 , p . 6246 (1982). "The phase Fe₃B" by Khan, Kneller and Sostarich. The materials according to the invention can be obtained as follows: The starting substances are melted in the desired quantities under a protective gas (for example, argon). The melt is then cooled rapidly, flakes of an amorphous material being formed, for eaxmple, by means of the so-called melt-spinning process. The flakes are then subjected to a thermal treatment to induce crystallisation. It was found that any composition in the specified range has its associated specific temperature treatment in which a maximum coercive force is obtained. This heat treatment can be determined by means of some simple experiments. Materials having the maximum possible coercive force proved to be single-phase materials on X-ray examination. When the heat treatment is continued, the coercive force decreases, which apparently is caused by the occurrence of a phase separation. The flakes may then be bonded with a synthetic resin to form a magnet or may be compressed as such at a higher temperature to form a magnet. The rare earth element in the composition according to the invention preferably is neodymium and/or praseodymium. The thermal treatment of the flakes may consist, for example, in that the flakes are heated to 720°C and are then cooled in a protective gas or , for example, are heated at 525°C in a vacuum for 20 hours and are then cooled in a vacuum. In this manner, technically useful synthetic resin-bonded magnets can be produced which, because of the low content of rare earth metal, for example, neodymium and/or praseodymium, are comparatively cheap. Generally, the materials have a remanence exceeding 0.5. In the table below, a number of magnetic materials which were manufactured in the above-specified manner with the measured coercive forces are indicated by way of example. Gross composition x y coercive force in k Oe heat treatment 1. Pr 3.8 Fe 77.0 B 19.2 -1.8 3.8 3 20 hrs at 525°C 2. Pr 4.1 Fe 77 B 18.9 -2.1 4.1 3 3. Nd 3.8 Fe 77.0 B 18.9 -1.8 3.8 2.6 heated to 720°C (20°C/min) 4. Nd 4.0 Fe 76.0 B 20 -1 4 2 Table 2 illustrates the effect of various heat treatments on the coercive force. Gross composition T. in °C duration in min. coercive force in k Oe Nd 3.8 Fe 77 B 19.2 615 30 2.9 x = -1.8 625 30 3.2 y = 3.8 635 30 3.0 Curie temp: 800°C 655 30 2.2 720 15 3.0 625 60 2.5 Nd₂Dy₂Fe 77.6 B 18.4 615 30 1.9 x = -2.6 620 30 2.8 y = 4 632 30 2.9 650 30 3.25 654 30 3.2 662 30 3.1 680 30 2.65

Claims: A magnetic material comprising iron, boron and one or more rare earth elements, characterized in that the magnetic material has the composition Fe 79-x-y B 21+x R y wherein R is a rare earth metal and wherein it holds that -5<x<+5 and +1<y<+5. A magnetic material as claimed in Claim 1, characterized in that R is Nd and/or Pr. A method of manufacturing a material as claimed in Claims 1 and 2, characterized in that the molten material is rapidly cooled, an amorphous material being formed, and is then subjected to a thermal treatment. Magnets formed from a material as claimed in Claims 1 and 2.

Inventors: BUSCHOW, KURT HEINZ JUERGEN, DE MOOY, DIRK BASTIAAN, Buschow, Kurt Heinz Jürgen
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H01F1
IPC codes: C22C38, C21D6, C22C33, H01F1