Patent Application
20080008897
A preferred embodiment of the inventive magnetic powder and the inventive soft magnetic composite formed by use of the magnetic powder will be described below with reference to the drawings.
As shown in
The magnetic powder obtained as described above is filled into a desired mold, and is compressed so that the density becomes 7.4 g/cm3 or more and 7.8 g/cm3 or less, more preferably 7.5 g/cm3; during the compression-molding process, the magnetic powder may be heated to a prescribed temperature, if necessary. In the soft magnetic composite formed by the compression-molding, as shown in
When the average particle diameter of the compressed powder particles 11a, 11b, . . . in the soft magnetic composite is more than 200 μm (that is, when the magnetic powder 10 including the powder 11 with an average particle diameter of more than 200 μm is used for compression-molding), unless the average thickness of the insulator 12 increases largely, the insulating property after-mentioned of the compression-molded soft magnetic composite is degraded, depending on the frequency range in which the soft magnetic composite is used. When the average particle diameter of the compressed powder particles 11a, 11b, . . . is less than 50 μm (that is, when the magnetic powder 10 including the powder 11 with an average particle diameter of less than 50 μm is used for compression-molding), the domain-wall pinning effect at the particle surface increases, increasing the coercive force and thus increasing the hysteresis loss. Accordingly, the average particle diameter of the powder 11 (average particle diameter of the powder particles 11a, 11b, . . . of the soft magnetic composite) is preferably 50 to 200 μm.
When the average thickness of the insulating layer 12a is less than 40 nm (that is, when the magnetic powder 10 including the insulator 12 with an average thickness of less than 20 nm is used for compression-molding), the tunnel current easily flows in the compression-molded soft magnetic composite, degrading the insulation characteristic. On the other hand, when the average thickness of the insulating layer 12a is more than 800 nm (that is, when the magnetic powder 10 including the insulator 12 with an average thickness of more than 400 nm is used for compression-molding), the spacing between neighbor magnetic powder particles in the soft magnetic composite spreads and the particles are made magnetically independent. A diamagnetic field generates at the particle surface and then demagnetizes the magnetic field of the interior of the particle, making it difficult for the magnetic field to saturate. As a result, a desired magnetic flux density of the soft magnetic composite cannot be obtained. Accordingly, the average thickness of the insulator 12 of the magnetic powder 10 is preferably 20 nm or more but less than 400 nm; the preferable average thickness of the insulating layer is thus 40 nm or more but less than 800 nm. The thickness of the insulator 12 can be controlled by changing a temperature in an insulator formation process or changing a composition in the treatment liquid.
High mechanical strength, if required for the soft magnetic composite, can be obtained more effectively by applying surface treatment to the powder 11 which has undergone phosphate treatment and then by executing a annealing process (heat treatment) after compression-molding. As the surface treatment, an inorganic binder (e.g., a water solution of Na2O/SiO2 based liquid glass or a phosphate/boric-acid/magnesia based water solution) can be used. When the annealing temperature (heat treatment temperature) is 600° C. or more, the inorganic binder softens during the annealing process, the surface of the powder 11 is extensively wetted by the inorganic binder material, and the inorganic binder solidifies upon the completion of the annealing process, thereby increasing the mechanical strength of the soft magnetic composite. In this case, the ratio of the volume of the solidified inorganic binder to the volume of the soft magnetic composite must be 3 vol. % or less in order to assure the magnetic characteristic.
In the soft magnetic composite 1 obtained as described above, the rare earth fluorides, alkaline metal fluorides, or alkaline earth metal fluorides do not diffuse into the iron particles during the heat treatment, suppressing the hysteresis loss. Furthermore, since the insulating layer 12a, with an approximately uniform thickness, comprising rare earth fluorides, alkaline metal fluorides, or alkaline earth metal fluorides is formed, the eddy current loss can be suppressed in a wide frequency band and the hysteresis loss due to compressed residual distortion in soft magnetic powder can also be suppressed.
Examples in the present invention will be given below.
Gas atomized powder, which comprises pure iron and has an average particle diameter of 100 μm, was prepared as spherical, soft powder.
[Preparing Treatment Liquid for Forming an Insulator]
Treatment liquid for forming an insulator on the surface of the gas atomized powder was prepared by a procedure described below.
(1) Four grams of Nd acetate was added to 100 ml of water, and completely dissolved in water by use of a shaker or ultrasonic agitator.
(2) Hydrofluoric acid diluted to 10% was gradually added by an amount equal to an equivalent in a chemical reaction by which NdF3 is produced.
(3) A solution in which gelled NdF3 precipitated was stirred with the ultrasonic agitator for one hour or more.
(4) The solution was centrifuged at a rotation of 4000 to 6000 rpm, after which the supernatant fluid was removed and methanol of the same volume was added.
(5) The methanol solution including gelled NdF3 was stirred to obtain complete suspension. The suspension was further stirred with the ultrasonic agitator for one hour or more.
(6) Operations in (4) and (5) were repeated three to ten times until anions such as acetate ions or nitrate ions were not detected.
(7) Finally, solated NdF3 was obtained. Methanol solution, in which 1 gram of NdF3 was dissolved per 4 ml, was used as treatment liquid.
[Insulator Forming Process and Compression-Molding]
A soft magnetic composite was formed as described below, by using the gas atomized powder and the above treatment liquid.
(1) In order to form magnetic powder, the surface of which is covered with an insulator, 8 ml of NdF3 treatment liquid was added to 40 g of gas atomized powder with an average particle diameter of 100 μm, and mixed until it was confirmed that the entire iron powder was wetted.
(2) The methanol solvent was removed from the iron powder treated with NdF3 added in (1) under a reduced pressure of 2 to 5 Torr.
(3) The magnetic powder, from which the solvent was removed in (2), was moved to a quartz boat, and heated at 200° C. for 30 minutes and then at 350° C. for 30 minutes under a reduced pressure of 5×10−5 Torr.
(4) When a large amount of the magnetic powder was necessary, the amount of treatment liquid was increased according to the necessary amount of the magnetic powder, and iron powder, which was the raw material, was treated up to the necessary amount.
(5) The magnetic powder resulting from the heat treatment performed in (3) was filled in a superhard mold. Then, a molding load of 18 tons was applied to the magnetic powder so that the density of the soft magnetic composite became 7.5 g/cm3. The molded soft magnetic composite for magnetism measurement was formed into a ring core with an outer diameter of 18 mm and an inner diameter of 10 mm.
(6) The magnetic powder resulting from the heat treatment performed in (3) was filled in another superhard mold. A molding load of 15 tons was applied to the magnetic powder so that the density of the soft magnetic composite became 7.5 g/cm3. The molded soft magnetic composite for resistivity measurement was formed into a rectangular parallelepiped core with a size of 10 mm×10 mm.
(7) The samples prepared in (5) and (6) were heated at 600° C. under a reduced pressure of 5×10−5 Torr. The relative densities of these samples were 95% or more.
<Evaluation Method>
[Observation of Cross Section Microstructure]
A cross section of the formed soft magnetic composite was observed using a microscope.
[Evaluation of Waviness]
Waviness was evaluated by a method described below. An exemplum of a cross section microstructure is shown in
[Measurement of Resistivity and Magnetic Flux Density]
The above-mentioned rectangular parallelepiped soft magnetic composite prepared for resistivity measurement was used to measure its resistivity and total loss by an ordinary method (e.g., four-probe resistive measurement and magnetic characteristics measurement). Table 1 also indicates the measurement results.
A soft magnetic composite was formed in the similar way as in Example 1. Example 2 differs from Example 1 in that, aqueous atomized powder rather than gas atomized powder was pulverized with a ball mill to obtain magnetic powder, the surface of the powder was smoothed by mechanical working, and the resulting powder was classified so that powder with an average diameter of 150 μm was used. The soft magnetic composite in Example 2 was evaluated in the same evaluation method as in Example 1. Table 1 shows the evaluation results.
A soft magnetic composite was formed in the similar way as in Example 1. Comparative example 1 differs from Example 1 in that the aqueous atomized powder without a smoothing process of the particle surface, which has been described above with reference to
(Result 1)
The observation of the cross section microstructure indicates that the compressed powder particles in the soft magnetic composites in Examples 1 and 2 are not complete sphere, but the boundary between the particle and the insulating layer is mostly flat and the thickness of the insulating layer is approximately uniform. There are no cracks, which are usually found in Fe—Si based cores. For the soft magnetic composites in Examples 1 and 2, an insulating layer, comprising rare earth fluorides, alkaline metal fluorides, or alkaline earth metal fluorides, with an approximately uniform thickness is formed along the surface of each compressed powder particle in such a way that the insulating layer covers the particle, as compared with that in Comparative example 1. The insulating layer is formed continuously and has a little waviness. For the soft magnetic composite in Comparative example 1, the thickness of the insulating layer is not uniform and there are no insulating layers between some compressed powder particles.
(Result 2)
As shown in Table 1, the soft magnetic composites in Examples 1 and 2 have a few compressed powder particles including crossing segments. On the other hand, for the soft magnetic composite in Comparative example 1, segments A-A, B-B, C-C and like, as shown in
(Result 3)
As indicated in Table 1, the resistivity of the soft magnetic composites in Examples 1 and 2 are higher than that in Comparative example 1, and the total loss in Comparative example 1 is higher than that in the other examples.
(Discussion 1)
The soft magnetic composites in Examples 1 and 2 were formed from spherical powder or powder the surface of which was smoothed, and thus had a little waviness. The soft magnetic composite in Comparative example 1 was formed from powder of indefinite shapes, such as non-worked aqueous atomized powder, so waviness was generated on surfaces and fracture regions of the insulating layer were also formed. It can be considered that these microstructural features of the soft magnetic composites lead to above Results 1 to 3. It can be also considered that the insulator before being compressed is locally present in concave parts of the particle in Comparative example 1. That is another possible reason why the thickness of the insulating layer is not uniform in Comparative example 1 (see
In order to obtain a desired magnetic characteristic reliably, the soft magnetic composite is preferably such that the density ratio of the iron in the soft magnetic composite is 95% or more and that the ratio of the volume occupied by the soft magnetic powder in the soft magnetic composite is 90% or more. Accordingly, a saturation flux density almost equal to 1.7 T (tesla), which is the saturation flux density of a silicon steel plate that has been widely used, is obtained. Here, the ratio of the volume occupied by the soft magnetic powder means the ratio of the volume of the soft magnetic powder itself, excluding the insulating layer.
Soft magnetic composites were formed in the similar way as in Example 1. However, as shown in
Soft magnetic composites were formed in the similar way as in Example 3. However, as shown in
(Result 4)
As shown in
(Discussion 2)
As described above, the coercive force increased with decreasing the average powder particle diameter (see
Soft magnetic composites were formed in the similar way as in Example 1. However, Example 4 differs from Example 1 in that soft magnetic powder with an average particle diameter of 96 μm was used and that insulators with an average thickness of 20 to 400 nm were formed as shown in
Soft magnetic composites were formed in the similar way as in Example 4. However, as shown in
(Result 5)
As shown in
(Discussion 3)
Since the resistivity and magnetic flux density of a soft magnetic composite used in a motor or another unit are respectively 2 μΩ·m or more and 1.7 T or more, it can be considered that a preferable average thickness of the insulator is 20 nm or more but less than 400 nm, as indicated in Example 4. When the thickness of the insulator is less than 20 nm, the tunnel current flows between the molded magnetic particles in the molded soft magnetic composite, lowering the insulation characteristic. On the other hand, when the thickness of the insulator is more than 400 nm, the spacing between neighbor magnetic particles in the molded soft magnetic composite is expanded and thus the particles are made magnetically independent. That leads a diamagnetic field to generate at the particle surface, and then the magnetic field of the interior of the particle is demagnetized, making it difficult for the magnetic field to saturate. As a result, it can be regarded that a desired magnetic flux density cannot be obtained and that the cause of the reduction in the magnetic flux density is not deterioration of particles due to, for example, oxidization.
Soft magnetic composites were formed in the similar way as in Example 1. However, as shown in
Soft magnetic composites were formed in the similar way as in Example 5. However, Comparative example 6 differs from Example 5 in that a phosphate layer is provided instead of the NdF3 insulator; Comparative example 7 differs from Example 5 in that only iron powder is used without an insulator being coated on the magnetic powder; Comparative example 8 differs from Example 5 in that aqueous atomized powder is used instead of gas atomized powder. The coercive force and resistivity of these soft magnetic composites were measured in the same way as in Example 5. The results are shown in
(Result 6)
The coercive force of soft magnetic composites in Example 5 is almost the same level as those of other soft magnetic composites over an entire temperature region, as shown in
(Discussion 4)
It can be regarded from the above results that heat treatment is preferably performed at 600° C. or more in order to lower the coercive force. The soft magnetic composites in Comparative example 6, in which phosphate is used, can be said to be inappropriate because their resistivity rapidly decrease during a heat treatment process at 500° C. or more. When an NdF3 layer is used as the insulating layer as in Example 5, a relatively high resistivity can be maintained at up to 700° C. Accordingly, it can be thought that the heat treatment process is preferably performed at 600 to 700° C.
The soft magnetic composite in Example 1 was used for a stator 102 of a motor 100 as shown in
Soft magnetic composites were formed in the similar way as in Example 6. However, Comparative example 8 differs from Example 6 in that aqueous atomized powder that was not worked for smoothing was used as magnetic powder. These soft magnetic composites were evaluated in the same way as in Example 6.
(Result 7)
With the soft magnetic composites in Example 6, the ratio of the volume occupied by the core is 80%, and the saturation flux density of the soft magnetic composites is 1.77 T. It was confirmed that when iron powder that had undergone NdF3 treatment was used for the stator 102, the efficiency of a motor increased as compared with a case in which a laminated structure of 0.15-mm-thick silicon steel plates was used. Since the saturate magnetic flux density when an NdF3 insulator is formed is the same level to that of the silicon steel plate, there was no problem on the magnetic saturation. The iron loss of the soft magnetic composites in Example 6 was half or less than that in Comparative example 8. For the motor in Example 6 in which the stator 102 was made of iron powder formed by applying NdF3 to gas atomized powder, an advantage by high resistivity was confirmed; the temperature of the heat generated at a rotation of 3000 rpm was lowered by 20° C., as compared with a case in which non-smoothed aqueous atomized powder was used.
(Discussion 5)
The reason why a soft magnetic composite is used for the stator 102 is that this type of motor has a plurality of poles and thus the eddy current generated by a rotating magnetic field must be low. The rotor is preferably formed by molding a powder material with a compression-molding means; the molded structure has a bond magnet portion mainly comprising a bonding material and magnet powder, and has a soft magnetic portion mainly comprising a bonding material and soft magnetic powder; at least one surface of the magnetic poles of the bond magnet portion is mechanically joined to the soft magnetic portion. The bond magnet is formed for each segment in a temporary molding process. Accordingly, the rotor is more preferably formed as a rotor for a motor by giving anisotropy during the temporary molding process, by forming a rotor with a plurality of poles from the temporary structure having the anisotropy in a final molding process, and by magnetizing the rotor in a magnetic field.
According to the preferred embodiments of the present invention, heat treatment for recovering the magnetic properties deteriorated during a molding process can be performed with an eddy current loss suppressed. The present invention is preferably applied to cores requiring a small hysteresis loss and/or a small eddy current loss, motor iron cores requiring a high magnetic flux density, solenoid cores (fixed iron cores) incorporated into electronically controlled fuel injectors of diesel engines and gasoline engines, and core parts for plunger and other actuators. The present invention is also suitable for motors in air-conditioners and other home electric appliances, powder generators for distributed power supplies, and motors for driving hybrid electric vehicles (HEVs).
Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-186860 | Jul 2006 | JP | national |
