Patent Application
20100079015
The present invention relates to a dust core, a method for producing the same, and an electric motor and a reactor each having a core member composed of the dust core.
In view of reducing environmental burdens, the development of hybrid vehicles and electric vehicles has been conducted day by day in the automobile industry. In particular, one urgent development objective is to realize a high-performance and downsized motor or reactor, which is a main apparatus mounted on vehicles.
A stator core or a rotor core, which constitutes a motor, and a reactor core, which constitutes a reactor, are each composed of a steel sheet laminate in which silicon steel sheets are laminated or of a dust core obtained via press molding of a resin-coated iron-based soft magnetic powder. A variety of cores formed with dust cores are advantageous in terms of magnetic properties that result in lower high-frequency iron loss than in the case in which laminated steel sheets are used, a variety of shapes that can result from press molding in a flexible manner at low costs.
In the case of a soft magnetic metal powder for a dust core, an insulating coat is formed on the surface of a soft magnetic metal powder particle such that not only powder insulation properties but also insulation properties of a dust core itself can be secured, resulting in inhibition of the occurrence of iron loss. Specifically, iron powder particles are covered with a silicone resin or an epoxy resin. In such case, in order to prevent film destruction upon press molding and secure insulation between iron powder particles, the amount of resin added to an iron powder is increased, for example.
a to 11c show experimental results obtained by the present inventors for the relationships between the amount of resin added and specific resistance, the relationship between the same and strength, and the relationship between the same and density, respectively. In the above experiments, a flake iron powder containing, as a main component, iron and Si (1% by weight) and having an aspect ratio of 6 was used. As is apparent from
In addition, there is a method for producing a dust core that comprises press-molding a magnetic powder comprising a silicone resin preliminarily condensed on the surfaces of iron powder particles. However, in this method, gaps tend to be generated between magnetic powder particles, resulting in reduction in dust core strength. Also, there is a method for producing a dust core that comprises press-molding a magnetic powder comprising a silica film preliminarily formed on the surfaces of iron powder particles. In this method, since a silica film is an inorganic insulating material, magnetic powder particles are merely interlocked with each other for binding therebetween, which inevitably results in reduction in the dust core strength.
Therefore, it is an urgent object to produce and develop a dust core having excellent insulating properties, high strength, and high density.
For example, Patent Documents 1 to 3 disclose conventional methods for producing a dust core. Patent Document 1 discloses a method for producing a dust core wherein the surfaces of iron powder particles are treated with a dispersant, and a silicone resin or the like is mixed therewith, followed by press molding and heat treatment. Patent Documents 2 and 3 disclose methods for producing a dust core wherein a pure iron powder or a pure iron powder comprising particles each having a phosphate film on the surface thereof is mixed with poly(phenylene sulfide) (PPS) or thermoplastic polyimide (PI), followed by press molding and heat treatment.
When the production method in Patent Document 1 is used to produce a dust core, it is impossible to solve the above problem of reduction in dust core density. When the production methods in Patent Documents 2 and 3 are used, PPS or PI softened by heat treatment is unlikely to fill gaps between powder particles, and thus it is impossible to solve the above problem of reduction in dust core density.
JP Patent Publication (Kokai) No. 11-126721 A (1999)
JP Patent Publication (Kokai) No. 2002-246219 A
JP Patent Publication (Kokai) No. 2006-310873 A
The present invention has been made in view of the above problems. It is an object of the present invention to provide a dust core having excellent insulating properties, high strength, and high density (high magnetic flux density), a method for producing the same, and an electric motor or reactor having a core member composed of the dust core.
In order to achieve the above object, the method for producing a dust core of the present invention comprises at least the following steps: a 1St step of preparing a resin powder and a magnetic powder comprising soft magnetic metal powder particles each having an insulating film preliminarily formed on the surface thereof; a 2nd step of obtaining a powder mixture by mixing the magnetic powder and the resin powder; and a 3rd step of allowing the resin powder to gel in an atmosphere at a certain temperature and press-molding the powder mixture so as to produce a dust core that is obtained as a press molded body.
Herein, examples of a soft magnetic metal powder that can be used include powders made from pure iron, iron-silicone based alloys, iron-nitrogen based alloys, iron-nickel based alloys, iron-carbon based alloys, iron-boron based alloys, iron-cobalt based alloys, iron-phosphorus based alloys, iron-nickel-cobalt based alloys, and iron-aluminium-silicone based alloys. In addition, examples of an insulating film that can be used include films comprising silica (SiO2), inorganic materials such as nitride film (Si3N4), and ceramic materials. However, the present invention is not limited by such examples as long as the material used has a melting point exceeding the temperature upon warm molding and does not gel upon warm molding.
Further, examples of a resin powder that can be used include a silicone resin, an epoxy resin, a phenol resin, a polyester resin, a polyamide resin, and a polyimide resin each in a powder form.
In the method for producing a dust core of the present invention, an insulating film is preliminarily formed on the surfaces of the above soft magnetic metal powder particles. A magnetic powder comprising particles coated with the insulating film is prepared. Herein, an example of a method for forming such an insulating film is a method wherein the surfaces of particles of a soft magnetic metal powder comprising pure iron or the like are siliconized with Si at a high concentration by use of a decarbonization/reduction reaction, followed by oxidization (corresponding to the 1st step).
Next, a powder mixture is prepared by mixing the thus formed magnetic powder and the above resin powder. The obtained powder mixture is placed in a certain high-temperature atmosphere such that the resin powder alone is allowed to gel. The powder mixture comprising the resin powder in a gel form is press-molded in a molding die having a certain shape such that gaps between magnetic powder particles coated with a hard insulating film are filled with gel-like resin particles.
According to the above production method, it is possible to increase the density of a produced dust core to a greater extent than that obtained by a conventional production method wherein a soft magnetic metal powder comprising a relatively large amount of resin formed on the surfaces of soft magnetic metal powder particles is press-molded. The realization of such high density leads to the improvement of the magnetic flux density of the dust core. Herein, a dust core with a high density can be obtained for the reasons described below. That is, the object of a conventional method is to form an insulating layer with resin particles. Therefore, in order to secure excellent insulating properties, large amounts of resin particles are used such that the resin particle proportion in a dust core increases, resulting in reduction in the density of the dust core. Meanwhile, according to the production method of the present invention, an insulating film is preliminarily formed on the surfaces of soft magnetic metal powder particles. Therefore, resin particles are mixed with magnetic powder particles to function as binders for binding the magnetic powder particles and thus not to be used for securing insulating properties. Accordingly, the necessary resin amount corresponds to an amount sufficient to fill gaps between magnetic powder particles.
In addition, the strength of a produced dust core can be improved as a result of binding of magnetic powder particles via a resin binder. The present inventors verified the following facts. According to the above conventional production method, the dust core strength deteriorates due to gaps generated between magnetic powder particles upon press molding. However, according to the production method of the present invention, the entire portion of a magnetic powder is press-molded under a condition in which gaps between magnetic powder particles are filled with gel-like resin particles. Thus, strong binding is achieved via a high binding force to which the adhesion force exhibited by a resin binder is added in addition to the interlocking force between magnetic powder particles. In addition, the dust core strength can be defined based on bending strength, tensile strength, radial crushing strength, or the like.
Herein, a condition in which resin particles are allowed to gel refers to a condition in which resin particles have viscosity characteristics that result in viscosity lower than a viscosity of 10000 Pa·s (Pascal second), at which the glass flow temperature is defined. In general, the resin particle viscosity is approximately 5000 Pa·s or lower.
Consequently, according to the method for producing a dust core of the present invention, it has become possible to produce a dust core having excellent strength properties and magnetic properties while securing insulating properties.
In addition, in one preferable embodiment of the method for producing a dust core of the present invention, the above press molded body is preferably annealed in the 3rd step. In such case, a silica film is formed with a resin added as a binder such that insulating properties are secured. Further, annealing results in elimination of processing strains generated in the dust core as a result of press molding. Thus, reduction in magnetic properties due to press molding can be prevented.
In another embodiment of the method for producing a dust core of the present invention, the above 3rd step is characterized by warm molding involving filling of a molding die with a powder mixture and press molding of the powder mixture in an atmosphere at a temperature at which the resin powder is not condensation-polymerized.
Warm molding refers to a molding method wherein a powder and a molding die (mold) are heated in an atmosphere at a temperature of approximately 100° C. to 150° C. and subjected to press molding during heating. In such temperature range, a silicone resin is not condensation-polymerized, for example.
Resin particles are formed into a gel in an atmosphere at a temperature for the above warm molding, that is to say, a temperature at which a resin is not condensation-polymerized or a temperature that is lower than the temperature for condensation polymerization of the resin. As described above, gaps between magnetic powder particles can be filled with the gel-like resin particles.
In addition, when the resin particles used are silicone resin particles that are specified as those commercially available such as YR3370 (produced by GE Toshiba Silicones Co., Ltd.) and the KR series (KR221, 240, 220L, etc.) (produced by Shin-Etsu Chemical Co., Ltd.), the temperature at the above 3rd step (i.e., the temperature for warm molding) is preferably set to approximately 120° C. to 145° C. Such commercially available silicone resins (powders) can be purchased at popular prices and thus dust cores can be produced at lower costs.
In addition, the dust core of the present invention is a dust core obtained in a manner such that a resin is used to fill gaps between magnetic powder particles comprising soft magnetic metal powder particles each having an insulating film preliminarily formed on the surface thereof, followed by curing. It is characterized in that the proportion of the resin mixed is 0.3% by weight or less, the magnetic flux density (B50) is 1.4 T (tesla) or more, and the radial crushing strength is 70 MPa or more.
In the cases of dust cores produced by conventional production methods, when it is attempted to improve insulating properties, it is inevitable to increase the amount of a resin. When the resin proportion in a dust core is increased, the dust core density decreases. Such a decrease in the dust core density directly causes a decrease in the magnetic flux density. On the other hand, when it is attempted to increase the dust core magnetic flux density, it is necessary to decrease the amount of a resin. As a result, sufficient adhesion force cannot be obtained using a decreased amount of a resin binder. Thus, dust core strength properties such as radial crushing strength are reduced. Therefore, dust cores produced by conventional production methods do not have excellent strength properties and excellent magnetic properties (e.g., magnetic flux density). In addition, the present inventors have demonstrated the following facts by experiments. In the cases of conventional dust core production methods, the radial crushing strength obtained is approximately 30 MPa at maximum when it is attempted to increase the magnetic flux density (B50) to 1.4 T or more, while on the other hand, the radial crushing strength obtained is approximately 50 MPa at maximum when it is attempted to suppress the magnetic flux density (B50) to approximately 1.2 T.
Unlike the above dust cores produced by conventional methods, a dust core obtained by the production method of the present invention described above has properties expressed by a magnetic flux density (B50) of 1.4 T or more and a radial crushing strength of 70 MPa or more and thus it has excellent strength properties and excellent magnetic properties.
Herein, it is preferable to use silica (SiO2) for an insulating film that constitutes a dust core having the above properties and to use a silicone resin as the above resin in view of production costs and the like.
Further, the amount of resin added when a dust core having the above properties is formed is adjusted to approximately 0.3% by weight or less. Experiments conducted by the present inventors demonstrated that the highest radial crushing strength can be obtained at a proportion of resin added of approximately 0.2% by weight, and that the magnetic flux density gradually decreases as a result of an increase in the proportion of resin added. In view of the experimental results, it is reasonable to set the proportion of resin added to approximately 0.3% by weight or less as described above and preferably 0.1% to 0.3% by weight in order to obtain a dust core having a magnetic flux density (B50) of 1.4 T or more and a radial crushing strength of 70 MPa or more. In addition, the aspect ratio of a soft magnetic metal powder to be used can be set to approximately 1 to 10, and the average particle size of the powder can be set to approximately 150 to 200 μm.
The above dust core having high strength and high magnetic flux density is used for a stator core and/or a rotor core for production of an electric motor. The thus obtained electric motor is preferably used for hybrid vehicles, electric vehicles, and the like, which require a driving electric motor having excellent magnetic properties and excellent strength properties.
Similarly, when the above dust core of the present invention is used for a reactor core, such a reactor core is preferably used for a reactor that is installed in hybrid vehicles, electric vehicles, and the like.
As is understood based on the above descriptions, a dust core having high strength and high magnetic flux density while securing insulating properties can be produced by the method for producing a dust core of the present invention. In addition, the dust core of the present invention has excellent strength properties and magnetic properties represented by a magnetic flux density (B50) of 1.4 T or more and a radial crushing strength of a 70 MPa, respectively.
a to 2e each show an explanatory drawing of a step of the method for producing a dust core of the present invention.
In the drawings, the numerical reference 1 denotes a magnetic powder, the numerical reference 11 denotes a pure iron powder (soft magnetic metal powder), the numerical reference 12 denotes a silica film (insulating film), the numerical reference 2 denotes a silicone resin powder (resin powder), the numerical reference 2A denotes a gel-like resin, the numerical reference 10 denotes a press-molded body, and the numerical reference 20 denotes a dust core.
Hereinafter, embodiments of the present invention are described with reference to the drawings.
First, the method for producing a dust core of the present invention is described in detail with reference to
a to 2e each show an explanatory drawing of a step of the method for producing a dust core of the present invention.
Referring back to
Herein, the steps in
When the silicone resin powder 2 in the powder mixture contained in the molding die is formed into a gel-like resin 2A, press molding is carried out as shown in
At the end, the press molded body 10 is annealed in an atmosphere at a temperature of approximately 600° C. to 750° C., which corresponds to the temperature (t5) in
[Experiments for strength properties and magnetic properties of the dust core (Example) of the present invention and the dust cores obtained in the Comparative Examples and the experimental results]
The present inventors used a pure iron powder as a soft magnetic metal powder. A magnetic powder was prepared by forming a silica film (an oxide of a silicone resin (YR3370)) over the surfaces of particles of the pure iron powder. The magnetic powder was mixed with a silicone resin in a manner such that the resulting mixture contained 0.2% by weight of the silicone resin added. Thus, a powder mixture was formed. Then, the silicone resin was allowed to gel in accordance with the above method, followed by press molding and annealing. Accordingly, a dust core was molded (Example). Meanwhile, two dust cores were molded by a conventional production method in the Comparative Examples. One of them (Comparative Example 1) was obtained by simply press-molding a magnetic powder comprising pure iron particles each having a silica thin film preliminarily formed on the surface thereof. The other one (Comparative Example 2) was obtained by press-molding a pure iron powder comprising particles coated with a relatively large amount of an Si resin. Table 1 below lists measurement values in terms of density, eddy loss, strength (radial crushing strength), and magnetic flux density B50 in the Example and Comparative Examples 1 and 2. In addition,
In addition, in the method for measuring the radial crushing strength, a ring-shaped dust core test piece with a thickness of 5 mm, an outside diameter of 39 mm, and an inside diameter of 30 mm was produced. The radial crushing strength was determined with an applied pressure at which cracks were generated in the test piece as a result of pressurization with a compressor.
As shown in table 1, in the case of Comparative Example 2, the amount of silicone resin increased and thus the resin film thickness on the surface of a pure iron powder particle increased. As a result, the density decreased to a greater extent than that in the Example and that in Comparative Example 1. Also, the magnetic flux density decreased.
In the case of Comparative Example 1, magnetic flux density comparable to that in the Example was obtained. However, the radial crushing strength significantly decreased to a level corresponding to 20% of that in the Example. The reason why the strength in Comparative Example 1 decreased to a greater extent than that in Comparative Example 2 is that an adhesion force was additionally exhibited by a resin binder upon binding between magnetic powder particles in Comparative Example 2.
In the case of the Example, the magnetic flux density (B50) was observed to reach a level as high as 1.4 T or higher compared with Comparative Examples 1 and 2. Also, the radial crushing strength was observed to reach a level as high as 70 MPa or higher. Thus, it is understood that the dust core obtained in the Example has excellent strength properties and excellent magnetic properties.
In addition, based on the results for the relationship between the radial crushing strength and the amount of resin added in the Example (line P1 in the graph) and in Comparative Example 2 (line Q1 in the graph) shown in
In addition, based on the results for the relationship between the magnetic flux density B50 and the amount of silicone resin added in the Example (line P2 in the graph) and in Comparative Example 2 (line Q2 in the graph) shown in
Based on the experimental results shown in
Based on
The dust core of the present invention described above has excellent strength properties and excellent magnetic properties. Thus, the dust core of the present invention is particularly preferably used for a stator core, a rotor core, or a reactor core for a reactor used in electric motors for vehicles such as hybrid vehicles that need to be durable in significantly changing environments and downsized while achieving high performance.
Embodiments of the present invention are described above with reference to the drawings. However, the specific constitution of the present invention is not limited to the embodiments. Therefore, the present invention encompasses any design changes or the like that do not depart from the spirit of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-111739 | Apr 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/058000 | 4/18/2008 | WO | 00 | 9/23/2009 |
