RARE EARTH METAL-FREE HARD MAGNETS

Abstract

The invention relates to materials with permanent magnetic propertiesalso known as hard magnetshaving the formula (Fe.sub.1-y Co.sub.y).sub.2P.sub.1-xZ.sub.x with Z=Si, Ge, B, As; and 0.5?x?0.5, and 0.05?y?0.3. The invention further relates to the hard magnet itself and a process for making the hard magnets.

Claims

1. A hard magnetic material having the formula:
(Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x with Z=Si, Ge, B, As; and 0.05?x?0.5, and 0.05?y?0.3.

2. The hard magnetic material according to claim 1, wherein Z is Si.

3. The hard magnetic material according to claim 1, wherein 0.08?x?0.25 and 0.08?y?0.15.

4. The hard magnetic material according to claim 1, having the formula (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.89Si.sub.0.11, (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.86Si.sub.0.14, (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.81Si.sub.0.19 or (Fe.sub.0.92Co.sub.0.08).sub.2P.sub.0.78Si.sub.0.22.

5. The hard magnetic material according to claim 1, having a saturation magnetization ?.sub.0M.sub.s along a c axis at 300 K of ?0.4 T.

6. The hard magnetic material according to claim 1, having a magnetocrystalline anisotropy K.sub.1 at 300 K of ?0.4 MJm.sup.?3.

7. The hard magnetic material according to claim 1, having a Curie temperature of ?350 K.

8. The hard magnetic material according to claim 1, having a magnetic hardness parameter K of ?1 at 300 K.

9. The hard magnetic material according to claim 1, exhibiting a compositional change of less than 1 wt.-% after being exposed to the HCl (wt.-18%) for a week.

10. The hard magnetic material according to claim 1, exhibiting no first order transition ?1000 K.

11. A hard magnet comprising a hard magnetic material according to claim 1 which has been magnetized with a permanent magnet or an electromagnet.

12. A hard magnet comprising a hard magnetic material according to claim 1 and a binder material.

13. The hard magnet according to claim 12, wherein the binder is selected from one or more members of the group consisting of nylon, polyamide, polyphenylene sulfide (PPS) and nitrile butadiene rubber (NBR).

14. A method of making a hard magnetic material according to claim 1, comprising the steps of sealing a mixture of elements Fe, Co and Z with Z=Si, Ge, B, As of a desired composition and then heating and cooling the sealed mixture according to a stepwise temperature/time profile.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 shows the composition of (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.89Si.sub.0.11 measured by energy-dispersive X-ray spectroscopy (EDX). The inserts show the images of single crystals.

[0027] FIG. 2 shows the magnetization curves at 2 and 300 K along both c and a axes.

[0028] FIG. 3 shows the magnetization versus temperature curves under applied magnetic fields of 0.01 and 1 T.

[0029] FIG. 4 shows the magnetization curves at 300 K along both c and a axes before and after corrosion with 18 wt.-% HCl for one (1) week.

[0030] FIG. 5 shows the XRD curve at 300 K for powders of (Fe.sub.0.88Co.sub.0.12).sub.2P.sub.0.90Si.sub.0.10 produced by the melting and etching method. The observed intensity, the calculated intensity and the corresponding peak position are shown.

DETAILED DESCRIPTION OF THE INVENTION

[0031] Hard magnetic compounds which meet the objects of the present invention are selected from the group consisting of compounds with the formula: (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x (Z=Si, Ge, B, As), 0.05?x?0.5 and 0.05?y?0.3. Independently from one another preferably Z is Si; and/or x is 0.06?x?0.30 and/or 0.06?y?0.2. Independently from one another most preferred Z is Si; and/or x is 0.08?x?0.25 and/or 0.08?y?0.15.

[0032] This co-doped Fe.sub.2P phase crystallizes in hexagonal space-group p62m (189). The Wykoff positions of Fe and P are replaced by Co and the Z element respectively according to the atomic fractions in the formula (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x.

[0033] A Co content of lower than 0.05, leads to an unfavorable decrease in Curie temperature, which is e.g. at least 50 K lower for the same x and Z. If the Co content is higher than 0.3, the formation of an orthorhombic structure increases which is no longer hexagonal. Hard magnets need a uniaxial anisotropy, such as in hexagonal or tetragonal structures. Therefore, the higher the orthorhombic portion, the lesser the hard magnetic property. Moreover, when too much Co is in the lattice, the moments of Co and Fe become non-collinear. If the Z element content is below the lower limit of 0.05 this again leads to an unfavorable decrease in Curie temperature, which is e.g. at least 30 K lower for the same y. If the Z element content is higher than 0.5, again the formation of an orthorhombic structure increases, resulting in a reduction of hard magnetic property.

[0034] Crystals of (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x can be magnetized along the c axis e.g. with a Nd.sub.2Fe.sub.14B magnet at room temperature.

[0035] At 300 K the saturation magnetization ?.sub.0M.sub.s along the c axis of the compounds of the formula (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x is ?0.4 T, preferably ?0.6 T, more preferred ?0.7 T.

[0036] The magnetocrystalline anisotropy at 300 K of (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x is ?0.4 MJm.sup.?3, preferably ?0.6 MJm.sup.?3, more preferred ?0.8 MJm.sup.?3.

[0037] The Curie temperature of (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x is ?350 K, preferably ?400 K, more preferred ?500 K.

[0038] The magnetic hardness parameter ? of (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x is ?1, preferably .fwdarw.1.2, more preferred ?1.4 at 300 K.

[0039] The anisotropic field B.sub.a (the saturation field along the hard axis) is ?1.5 T, preferably ?2 T, more preferred ?2.8 T at 300 K.

[0040] The crystals are highly corrosion resistant. After treatment with a mineral acid, e.g. HCl, for a week, the magnetic properties remain unchanged (see FIG. 4). The composition change within the accuracy of the detection (<0.1 wt.-%) by the Wavelength-dispersive X-ray spectroscopy is less than 1 wt.-%, preferably less than 0.5 wt.-%, and more preferred less than 0.1 wt.-% (based on the mass of the pure untreated compound) after being exposed to the HCl (18 wt.-%). This pronounced chemical stability is important for the commercial use of the compounds as hard magnets. Due to this stability there is no need for an additional coating to protect the magnet from corrosion.

[0041] Moreover the compounds of the present invention show very high thermo-stability. There is no first order phase transition ?1000 K, preferably ?1200 K, more preferred ?1500 K, which indicates that these compounds can be formed into bulk magnets by sintering the orientated (preferably, parallelly aligned in crystal growth direction) powders with high density.

Manufacturing Methods

[0042] Multiple methods can be used to manufacture the compounds of the present invention. Non-limiting examples are: the sputtering method, the Sn-flux method and the melting and etching method. Preferably, the starting materials are highly pure elements (>99.9 atomic %).

[0043] The sputtering technique allows the manufacture of thin layers (films) of the compounds. For this purpose elemental metals and/or alloys of two metals are used as targets in sputtering. The base pressure of the vacuum receiver is preferably ?10.sup.?6 mbar, more preferably ?10.sup.?7 mbar and most preferred ?10.sup.?8 mbar and the deposition preferably takes place at 0.1?10.sup.?3 mbar to 10?10.sup.?3 mbar, more preferred at 1?10.sup.?3 mbar to 5?10.sup.?3 mbar, and most preferred at 3?10.sup.?3 mbar within a preferred temperature range of 100? C. to 500? C., more preferred 150? C. to 450? C. and most preferred 200? C. to 400? C. The growth rate of the thin layers is about 0.03 to ?0.04 nm/s. After deposition, the thin layers on the substrate within the recipient are preferably vacuum annealed for preferably 5 to 25 minutes, more preferred 10 to 20 minutes and most preferred for about 15 minutes and then slowly cooled to room temperature.

[0044] In the Sn-flux method single crystals of (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x can be grown in a Sn matrix. Compared to phosphorus, the Z-element does not enter the Fe.sub.2P-crystal as easy; therefore the Z-element has to be added to the starting element mixture in excess to the aimed composition. Cobalt and iron have almost the same electronegativity; which is why the Co/Fe ratio in the final compound is about the same as in the starting material. Preferably the Z element excess over the aimed (true) content is in the range of 100-300 atomic-%, preferably 120-250 atomic-%, more preferred 130-180 atomic-% in the starting mixture.

[0045] In the Sn-flux method the mixture of the elements is sealed in a crucible, e.g. in an alumina tube which in turn is sealed in another tube under reduced pressure, e.g. a quartz tube in vacuum. This tube assembly is then heated to a maximum temperature of about 1500 K according to a defined stepwise temperature/time profile: [0046] Preferably at first it is heated to 570-580 K, preferably about 575 K within 2-4 hours, preferably about 3 hours, [0047] it is then maintained at this temperature over 8-12 hours, preferably about 10 hours, [0048] then heated up to its maximum temperature of 1300-1600 K, preferably about 1500 K within 20-30 hours, preferably about 24 h, [0049] then maintained at this temperature over 20-30 hours, preferably about 24 h, [0050] and then cooled to 750-800 K, preferably about 773 K with a cooling rate of 2-5 K/h, preferably about 3 K/h [0051] and preferably finally centrifuged upon reaching the cooling temperature.

[0052] Since the compounds according to the present invention do not react with acid, crystalline powders can also be produced by the melting and etching method. Compared to the flux-method, the composition of the starting materials is almost the same as the composition of the desired compound and only very little material is wasted. The powders of the elements are mixed in a crucible, e.g. a BN crucible, which is then sealed under vacuum e.g. in a Ta-tube. The Ta-tube is slowly heated in vacuum to a temperature above the melting temperature of (Fe.sub.1-yCo.sub.y).sub.2P.sub.1-xZ.sub.x, e.g. to 1750-1850 K, preferably about 1800 K within 20-30 hours, preferably 24 h. This temperature is maintained for 20-30 hours, preferably 24 h in order to achieve a high degree of homogeneity and then cooled to room temperature, preferably by simply turning off the power of the furnace. The thus obtained ingot is ground into powder and then transferred into a mineral acid like HCl (e.g. 15-20 wt.-%) for 20-30 hours, preferably 24 h to remove eventually present secondary phase(s), which typically is/are present in the initial ingot in an amount from about 2-4 vol.-%.

[0053] The single crystals are shining needle-like crystals (see FIG. 1 or FIG. 4). The direction parallel to the needle direction is the crystallographic c axis. The composition of the single crystals can be verified by EDX (see FIG. 1).

[0054] The crystal structure of the polycrystalline sample prepared by the melting and etching method can be verified by XRD (see FIG. 5).

[0055] The magnetic properties, including the magnetization, the saturation field (see FIG. 2) and the Curie temperature (see FIG. 3), along both c and a axes are measured with a Vibrating Sample Magnetometer. The magnetocrystalline anisotropy K.sub.1 is ??.sub.0M.sub.sH.sub.a, where H.sub.a is the saturation magnetic field along the a axis.

Manufacture of Magnets for Use

[0056] For use as a magnet the compounds of the present invention can e.g. be sintered as raw material or bonded with an appropriate binder material. Sintered magnets are usually stronger and anisotropic but shapes are limited. They are made by pressure forming the raw materials followed by a heating process. Bonded magnets are less strong as sintered ones but less expensive and can be made into almost any size and shape. For bonded magnets the compounds according to the invention are mixed with 5 to 90 wt.-%, preferably 10 to 60 wt.-%, more preferably 20-40 wt.-% binder, compacted and cured at elevated temperature (e.g. at 50-350? C., preferably at 80-280? C., more preferably at 100-200? C.; depending on the binder used). They are isotropic, i.e. they can be magnetized in any direction. The molding process can e.g. be an injection molding or a compression bonding process. Typical binder types are Nylon, Polyamide, Polyphenylene sulfide (PPS) and Nitrile Butadiene Rubber (NBR).

EXAMPLES

[0057] The invention is explained in more detail with reference to the following examples.

Example 1

[0058] Manufacture of Single Crystals of (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.89Si.sub.0.11

[0059] The initial atomic ratio before crystal growth is Fe:Co:P:Si:Sn=1.8:0.2:0.8:0.3:20, the final product has the composition with an atomic ratio of Fe:Co:P:Si=1.82:0.18:0.89:0.11.

[0060] The saturation magnetization along c axis at 300 K is ?.sub.0M.sub.s=0.68 T. The saturation field along the a axis is B.sub.a=?.sub.0H.sub.a=2.3 T. The magnetocrystalline anisotropy is K.sub.1=0.63 MJm.sup.?3. The Curie temperature Tc=414 K.

[0061] After sinking into the 18% (mass) HCl for a week, the magnetic properties remain unchanged.

[0062] FIG. 1 shows the composition measured by EDX. The inserts show the images of needle-shaped single crystals. The crystals can be magnetized along the c axis with a Nd.sub.2Fe.sub.14B magnet at room temperature.

[0063] FIG. 2 shows the magnetization curves at 2 and 300 K along both c and a axes.

[0064] FIG. 3 shows the magnetization versus temperature curves under applied magnetic fields of 0.01 and 1 T. The Curie temperature deduced by the 0.01 T curve is 414 K.

[0065] FIG. 4 shows the magnetization curves at 300 K along both c and a axes before and after corrosion in 18 wt.-% HCl for one week. The insert shows the shining surface after corrosion. The composition was not changed within the accuracy of the detection (<0.1%) by the Wavelength-dispersive X-ray spectroscopy.

[0066] FIG. 5 shows the XRD result of the polycrystalline powder produced by the melting and etching method. There is only a single phase of the Fe.sub.2P-type hexagonal structure.

[0067] The melting temperature of (Fe.sub.0.91Co.sub.0.09).sub.2(P.sub.0.89Si.sub.0.11) is 1520 K. Below this temperature, no first order transition exists.

Example 2

[0068] Manufacture of Single Crystals of (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.86Si.sub.0.14

[0069] (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.86Si.sub.0.14 was prepared in the same way as described in Example 1 but using an initial atomic ratio before crystal growth of Fe:Co:P:Si:Sn=1.8:0.2:0.78:0.22:20

Example 3

[0070] Manufacture of Single Crystals of (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.81Si.sub.0.19

[0071] (Fe.sub.0.91Co.sub.0.09).sub.2P.sub.0.81Si.sub.0.19 was prepared in the same way as described in Example 1 but using an initial atomic ratio before crystal growth of Fe:Co:P:Si:Sn=1.8:0.2:0.73:0.27:20

Example 4

[0072] Manufacture of Single Crystals of (Fe.sub.0.92Co.sub.0.08).sub.2P.sub.0.78Si.sub.0.22

[0073] (Fe.sub.0.92Co.sub.0.08).sub.2P.sub.0.78Si.sub.0.22 was prepared in the same way as described in Example 1 but using an initial atomic ratio before crystal growth of Fe:Co:P:Si:Sn=1.8:0.2:0.67:0.33:20

Example 5

[0074] Manufacture of (Fe.sub.0.88Co.sub.0.12).sub.2P.sub.0.90Si.sub.0.10 Powder

[0075] The initial atomic ratio before reaction is Fe:Co:P:Si=1.78:0.22:0.89:0.11, the final product has the composition with an atomic ratio of Fe:Co:P:Si=1.76:0.24:0.90:0.10.

[0076] Table 1 shows the properties of the compounds according to the examples compared to Fe.sub.2P, MnAl, MnBi, Mn.sub.2Ga and BaFe.sub.12O.sub.19. The properties of (Fe.sub.0.88Co.sub.0.12).sub.2P.sub.0.90Si.sub.0.10 are not included since the sample is a powder where properties along the crystallographic axes could not be determined.

TABLE-US-00001 TABLE 1 ?.sub.0M.sub.s ?.sub.0M.sub.s K.sub.1 K.sub.1 (BH).sub.max [T] @ [T] @ [MJm.sup.?3] B.sub.a @ [MJm.sup.?3] B.sub.a @ T.sub.c ? @ in theory 2K 300K @ 2K 2K @ 300K 300K [K] 300K [kJm.sup.?3] (Fe.sub.0.91Co.sub.0.09).sub.2 0.89 0.68 2.17 6.2 0.63 2.3 414 1.3 92.5 (P.sub.0.89Si.sub.0.11) (Fe.sub.0.91Co.sub.0.09).sub.2 1.11 0.90 2.17 4.9 0.89 2.5 451 1.2 162 (P.sub.0.86Si.sub.0.14) (Fe.sub.0.91Co.sub.0.09).sub.2 1.09 0.92 1.97 4.5 0.86 2.4 480 1.1 170 (P.sub.0.81Si.sub.0.19) (Fe.sub.0.92Co.sub.0.08).sub.2 0.82 0.71 1.49 4.6 0.81 2.8 506 1.4 101 (P.sub.0.78Si.sub.0.22) Fe.sub.2P 1.03 2.36 6.5 214 BaFe.sub.12O.sub.19 0.72 0.48 0.45 1.7 0.33 1.7 740 1.3 46.0 MnAl 0.75 1.7 5.7 650 2 113 MnBi 0.73 0.90 3.1 633 1.5 107 Mn.sub.2Ga 0.59 2.35 10 770 2.4 69.6

[0077] The error bar for the Curie temperature is ?5 K.