Boron-free hard magnetic comprising a magnetic tetragonal phase

Description

[0001] The invention relates to a boron-free hard magnetic material on the basis of neodymium, iron and carbon, which comprises a hard magnetic tetragonal phase. The invention also relates to a method of manufacturing such a material.

[0002] A known material of this type comprises Nd₂Fe₁₄B having a tetragonal crystal structure as a hard magnetic phase. It is known that in this compound a partial substitution of boron by carbon leads to an increased magnetic anisotropy (see, for example, Journal de Physique Colloque C6, supplément au no. 9, T. 46, Sept. 1985, page C6-305/308: " Magnetic Anisotropy of Carbon Doped Nd₂Fe₁₄B" by Bolzoni, Leccabue, Pareti and Sanchez). In this article it is stated on page 306 that when boron is completely replaced by carbon it is impossible to obtain a tetragonal phase which is required to obtain the desired hard magnetic properties.

[0003] It is an object of the invention to provide a hard magnetic material having a high crystal anisotropy, which comprises iron, neodymium and only carbon instead of boron.

[0004] It was found that this object can be achieved by a material having the following overall composition, 11.2 to 15.2 at.% of Nd, 74.8 to 84.8 at.% of Fe and 4 to 10 at.% of C, a finely crystalline tetragonal phase having a grain size of 0.2 µm or smaller and having the Nd₂Fe₁₄C structure being present in the material. This composition range comprises material compositions having coercive forces µ_oH_c of 0.6 T or larger. It should be noted that materials having the highest coercive force are obtained if the composition of the materials is selected as follows: Nd: 12 - 15 at.%, Fe: 75 - 84 at.% and C: 4.8 - 10 at.%. The magnetic properties of the material improve, however, if the composition of the material is selected as follows, Nd: 12.0 - 14.2 at.%, Fe: 76.6 - 83.2 at.% and C: 4.8 - 9.2 at.%, the Curie temperature of these materials being in general 260°C or higher. However, sometimes the material also contains a second soft magnetic phase. Substantially single-phase materials in general have the following composition, Nd: 12 - 14 at.%, Fe: 78 - 81 at.% and C: 6 - 8 at.%. The compositions of the materials of the present invention possess the known property of the Nd₂Fe₁₄B material that substitution of part of the iron by cobalt increases the Curie temperature.

[0005] Finely crystalline is to be understood to mean herein that at least 90% of the crystallites in the hard magnetic phase have a dimension of 0.2 µm or smaller. The hard magnetic material according to the invention can be obtained by melting together the starting elements, followed by melt spinning on a chill surface, for example the surface of a rapidly rotating copper disc, which moves rapidly relative to the orifice.

[0006] A method of manufacturing a hard magnetic material in the form of a ribbon is known from European Patent Application EP-A 108 474. In the known method, the moving chill surface is formed by the circumferential surface of a disc of a material having a high thermal conductivity such as copper coated with chromium.

[0007] This European Patent Application does not give any indication that the method can also be used for the manufacture of boron-­free finely crystalline hard magnetic materials on the basis of neodymium and iron, provided that the method is suitably adapted. For it has been found in the investigation leading to the invention that on rapidly cooling carbon-containing, boron-free neodymium-iron alloys no phase having the desired tetragonal crystal structure is formed. A suffiently rapid cooling for the desired effect is obtained if, by a suitable choice of the spraying pressure, the slot dimensions, the disc slot spacing and the speed of the disc, the spraying process can be carried out such that ribbons or flakes having a thickness of 30 µm or smaller are obtained. The ribbons or flakes formed comprise a microcrystalline phase having a structure which resembles that of Nd₂Fe₁₇; the material is not hard magnetic. It must be assumed that part of the carbon present is dissolved in this phase. In this respect, the material according to the invention clearly differs from boron-­containing materials. Boron is substantially insoluble at the temperatures used in the Nd₂Fe₁₇ phase.

[0008] According to another aspect of the invention, the desired hard magnetic properties can be obtained if immediately after melt spinning the material is subjected to a recrystallizing annealing treatment of short duration at a relatively low temperature.

[0009] Surprisingly, it has been found that in this annealing treatment a microcrystalline phase having a tetragonal structure and substantially the composition Nd₂Fe₁₄C is formed in the material, in which phase some extra carbon may possibly still be dissolved.

[0010] The annealing treatment is carried out at a temperature between 675 and 750°C, preferably at a temperature between 685°C and 730°C. On heating to a temperature of 720°C the desired finely crystalline tetragonal phase Nd₂Fe₁₄C is formed within a minute after attaining the said temperature. The annealing duration necessary for the crystallization can be established readily in an experimental way at any annealing temperature in the indicated temperature range. A suitable annealing treatment consists in that the material is maintained in a furnace for 1 to 5 minutes, the furnace being at a temperature of 720°C. In practice, a dwell time of 2 to 3 minutes was found to be suitable. The material thus obtained has suitable permanent magnetic properties. A saturation magnetization µ_oM_sat of > 1.2 T, a remanence µ_oM_r of > 0.50 T and a coercive force µ_oH_c of > 1.0 T were measured. The particular thing about the new magnetic material is that also after annealing for 6 days at 680°C it still has a relatively high coercive force of µ_oH_c = 0.75 T. In the material according to the invention a marked grain growth occurs when it is heated for a long time (360 hours) at 800°C, the grains being formed having a dimension between 0.2 and 0.6 µm. In this process the coercive force decreases to 0.2 T. A further increase of the dimensions of the grains was observed when they were heated at 850°C and 900°C for 15 days. Then, the most important phase is still the Nd₂Fe₁₄C phase.

[0011] During the investigation leading to the invention, it was established that when material compositions containing less than 11.2 at.% of neodymium are subjected to the annealing treatment at 720°C a phase of more than 10% is formed having the α-Fe structure, in which some carbon may possibly be dissolved; in which case roentgenographically no traces of neodymium can be found. If the material contains more than 15.2 at.% of neodymium, the tetragonal Nd₂Fe₁₄C phase is not formed or only to a small extent, and the material is predominantly in a Nd₂Fe₁₇ phase having a rhombohedral structure, in which the carbon is dissolved. Of course, also other techniques of rapidly cooling a molten alloy can be used, which have the same result.

[0012] For the manufacture of magnets, the material in the form of flakes is compressed in a hot condition, for example at 650°C, if necessary, after it has been powdered or ground. The material in the form of flakes may alternatively be bound with a synthetic resin, if necessary after it has been powdered or ground, and formed into a magnet.

[0013] An advantage of the new magnetic material is that during its manufacture it cannot form toxic boron-containing compounds when it contacts water and/or hydrogen.

Exemplary embodiment 1:

[0014] 20 grams of an alloy having the following composition, neodymium 13.5 at.%, iron 79.6 at.% and carbon 6.9 at.% were melted by heating to a temperature of 1300°C. The liquid alloy was sprayed onto a copper disc through an aperture of 0.4 x 10 mms. The distance between the orifice and the disc was 0.2 mm, the spraying pressure was 2.10⁴ Pa and the disc had a speed of 28 m/s relative to the orifice. The flowing rate was 5 cm³/s, and spraying was carried out in an argon atmosphere. In this way, ribbons having a width of approximately 10 mm, an average length of 10 mm and a thickness of 20 µm were obtained. Subsequently, the flakes obtained were introduced into a furnace which was heated to a temperature of 720°C, and they were maintained therein for 3 minutes. The material thus obtained had, predominantly, a finely crystalline tetragonal structure (Nd₂Fe₁₄C). Crystallite dimension (more than 90%): 0.2 µm.

The following magnetic properties were measured:
Coercive force µoHc)	:	1.02 T
Saturation magnetization µoMsat)	:	1.34 T
Remanence µoMr	:	0.72 T
Curie temperature	:	269°C

Exemplary embodiment 2:

[0015] The material compositions listed in the Table below were treated in a completely analogous manner. The Table below lists the properties measured on the crystallized material:

Material composition	Coercive force	Magnetization	Remanence
	µoHc in T	µoMsat in T	µoMr in T
Nd13.3Fe80.9C5.8	0.65	1.31	0.55
Nd13.5Fe77.7C8.8	0.60	1.33	0.64
Nd14.8Fe76.4C8.8	1.16	1.32	0.70

Claims

1. A boron-free hard magnetic material on the basis of neodymium, iron and carbon, which comprises a hard magnetic, tetragonal phase, characterized in that the material has the following overall composition, neodymium 11.2 - 15.2 at.%, iron 74.8 - 84.8 at.% and carbon 4 - 10 at.%, and that a finely crystalline tetragonal phase having a grain size of 0.2 µm or smaller and having the Nd₂Fe₁₄C structure is present in the material.

2. A boron-free hard magnetic material as claimed in Claim 1, characterized in that the material has the following overall composition neodymium 12 - 15 at.%, iron 75 - 84 at.% and carbon 4.8 - 10 at.%.

3. A boron-free hard magnetic material as claimed in Claim 2, characterized in that the material has the following overall composition, neodymium 12.0 - 14.2 at.%, iron 76.6 - 83.2 at.% and carbon 4.8 - 9.2 at.%.

4. A boron-free hard magnetic material as claimed in Claim 3, characterized in that the material has the following overall composition, neodymium 12 - 14 at.%, iron 78 - 81 at.% and carbon 6 - 8 at.%.

5. A boron-free hard magnetic material as claimed in Claim 4, characterized in that the material has the following overall composition, neodymium 13.5 at.%, iron 79.6 at.% and carbon 6.9 at.%.

6. A boron-free hardmagnetic material as claimed in any one of the preceding Claims, characterized in that in the overall composition part of the iron is substituted by cobalt in an amount sufficient to increase the Curie termperature.

7. A method of manufacturing a hard magnetic material on the basis of neodymium, iron and carbon by spraying a molten alloy onto a rapidly moving chill surface and further processing of the ribbons or flakes thus obtained, characterized in that the method comprising the following steps,
ribbons or flakes having a thickness of 30 µm or less are manufactured from an alloy having the following overall composition, neodymium 11.2 - 15.2 at.%, iron 74.8 - 84.8 at.% and carbon 4 - 10 at.%, the material obtained being annealed at a temperature between 675 and 750°C.

8. A method as claimed in Claim 7, characterized in that the material is annealed at 685 - 730°C.

9. A method as claimed in Claim 8, characterized in that the material is annealed at 720°C.

10. A method as claimed in Claim 7, characterized in that an alloy having the following overall composition is used, neodymium 12 - 15.2 at.%, iron 76.6 - 83.2 at.% and carbon 4.8 - 9.2 at.%.

11. A method as claimed in Claim 7, characterized in that an alloy having the following overall composition is used, neodymium 12.0 -14.2 at.%, iron 76.6 - 83.2 at.% and carbon 4.8 - 9.2 at.%.

12. A method as claimed in Claim 11, characterized in that an alloy having the following overall composition is used, neodymium 12 - 14 at.%, iron 78 - 81 at.% and carbon 6 - 8 at.%.

13. A method as claimed in Claim 12, characterized in that an alloy having the following overall composition is used, neodymium 13.5 at.%, iron 79.6 at.% and carbon 6.9 at.%.

14. A magnet of a material having the composition as claimed in Claims 1 - 6.

15. A magnet of a material obtained by using a material manufactured as claimed in Claims 7 - 13.