Abstract:
A powder compressed magnetic core, which is produced by compressing iron powder or alloy powder of which the main component is iron. A fluoride compound layer of a fluoride of a rare earth element or a fluoride of an alkaline earth metal is formed on the surface of the powder, and an under layer is formed between the fluoride layer and the iron powder or the alloy powder.

Description:
CLAIM OF PRIORITY 
       [0001]    The present application claims priority from Japanese patent application serial No. 2007-317830, filed on Dec. 10, 2007, the content of which is hereby incorporated by reference into this application. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a compressed magnetic core, which is manufactured by compression-molding magnetic powder containing iron elements, and more particularly to a compressed magnetic core suitable for electric parts for electric rotating machines, reactors. 
       RELATED ART 
       [0003]    In recent years, electric automobiles have drawn attentions from the ecological points of view. Driving power mechanisms of the electric automobiles have electric rotating machines (motors) and inverter circuits have smooth transformers (reactors). An increase in efficiency of the electric parts is desired. The magnetic cores used for the electric parts should have a low iron loss and high magnetic flux density, and these magnetic characteristics should not deteriorate over a range of from low frequency to high frequency. 
         [0004]    The iron loss includes an eddy current loss, which has a close relation to a specific resistance of the magnetic core and a hysteresis loss, which is affected by strain in the iron powder occurred from a process history after iron powder production. The iron loss (W) is expressed by a sum of the eddy current (We) and the hysteresis loss (Wh) as shown in the equation 1, wherein f is a frequency, Bm magnetization magnetic flux density, ρ a specific resistance, t a thickness of material and k 1  and k 2  constants. 
         [0000]        W=We+Wh =( k   1   Bm   2   t   2 /ρ) f   2   +K   2   Bm   1.6   f   Equation 1 
         [0005]    According to the equation (1), the eddy current loss (We) increases in proportional to two powers of the frequency f. Therefore, in particular, in order to suppress deterioration of magnetic characteristics at high frequency zone, it is necessary to control the eddy current loss (We). In order to prevent the eddy current in the compressed magnetic iron core, a size of the magnetic powder should be optimized, and an insulating layer should be formed on the surface of each of the magnetic powder. Such the magnetic powder should be used for compression molding. 
         [0006]    If the insulation is insufficient in the compressed magnetic iron core, the specific resistance ρ decreases. 
         [0007]    In these compressed magnetic cores, if the insulation is insufficient, the specific resistance decreases to increase the eddy current loss (We). On the other hand, if a thickness of the insulating layer increases to secure sufficient insulation, the magnetic flux density decreases because a volume of a soft magnetic powder in the magnetic core decreases. In addition, if the density of the soft magnetic powder increases in conducting the compression of the soft magnetic powder under a high pressure, the strain of the magnetic core at the compression molding is not avoided thereby to increase the hysteresis loss (Wh). As a result, it is difficult to suppress the iron loss (W). Especially, since the eddy current loss (We) at low frequency zone is small, the hysteresis loss (Wh) in the iron loss (W) becomes large. 
         [0008]    Patent document No. 1 discloses a method wherein the soft magnetic powder and insulating powder are mixed to form an insulating layer on the surface of the soft magnetic powder. Patent document No. 2 discloses another method of manufacturing compressed powder magnetic core wherein a soft magnetic powder of Fe—Si groups having an insulating film such as an oxide film or phosphate film on the surface thereof is compression-molded. Patent document No. 3 discloses a method wherein a high resistive film is obtained by coating a liquid insulating material is coated and fixed by post-treatment. 
         [0009]    Patent document No. 4 discloses a method wherein magnesium oxide is used as an insulating film for the soft magnetic powder. In this method iron is oxidized, and magnesium powder is reacted on the iron oxide surface to substitute the iron oxide with magnesium oxide thereby to insulate the surface. However, it is impossible to make a MgO film thicker than the oxide film, and MgO film remarkably decreases its specific resistance after heat treatment for eliminating strain at 600° C. or higher. On the other hand, the fluoride coating material does not decrease its specific resistance even at a temperature as high as 600° C. or higher. However, the method requires controlling the shape of the iron powder to which the method is applied and requires making the thickness of the coating homogeneous, which increases a production cost. 
         [0010]    Patent document No. 1; Japanese patent laid-open 2003-332116 
         [0011]    Patent document No. 1; Japanese patent laid-open 2004-288983 
         [0012]    Patent document No. 1; Japanese patent No. 3475041 
         [0013]    Patent document No. 1; Japanese patent laid-open 2006-233325 
       SUMMARY OF THE INVENTION 
       [0014]    Although fluorides are excellent in heat resistance and hardly react with iron, and although the fluoride coating is a good method of eliminating the above-mentioned problems, it is necessary to use iron powder produced by gas atomizing or water atomizing method that has been subjected to treatment of removing projections, etc. Therefore, the above method was difficult to produce the magnetic cores for hybrid automobiles that require an extremely low price. Since the coating solution of the fluorides is an alcohol base solution, which hardly reacts with iron powder, it does not effectively cover the water atomizing powder having very complicated uneven surface. Since the phosphate treating solution is acidic, the surface of the iron powder is oxidized and coated with phosphate. Therefore, the water-atomized powder may become high resistivity. However, phosphate transforms at around 500° C., the heat resistance is around 500° C. 
         [0015]    The present invention relates to a method of coating fluorides on the iron powder to produce magnetic powder for cores, which are applicable to manufacturing even low price electric rotating machines. 
         [0016]    The present invention provides an inexpensive powder compressed magnetic core with high density, high resistivity and excellent magnetic characteristics and a magnetic powder suitable for manufacturing the same. 
         [0017]    The powder compressed magnetic core of the present invention comprises iron powder or alloy powder whose main ingredient is iron, the surface of the powder being covered with fluoride compound layer, wherein an under layer is formed between the powder of iron or the alloy and the fluoride layer. The under layer contains alkaline earth metal and alkaline earth metal oxide, such as Mg, Ca, Ba and/or Sr. 
         [0018]    The powder compressed magnetic core of the present invention comprises iron powder or alloy powder whose main ingredient is iron, the surface of the powder being covered with fluoride compound layer, wherein an under layer is formed between the powder of iron or the alloy and the fluoride layer. The under layer contains alkaline earth metal and alkaline earth metal oxide, and there is a layer containing oxygen between the fluoride layer and the under layer. 
         [0019]    Further, the present invention provides a compressed magnetic core having a combination of the fluoride compound coating film of rare earth element or alkaline earth metal is formed on the surface of the iron or iron alloy powder and an under layer. 
         [0020]    An average total thickness of the fluoride compound layer and the under layer is preferably 100 nm or less, and the under layer is preferably 50 nm or less. The fluoride compound is preferably magnesium fluoride MgF 2 . 
         [0021]    According to the present invention, provided is a compressed magnetic core having excellent heat resistance, large specific resistance and high density. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1  is a diagrammatic cross sectional view of a conventional fluoride compound coated iron powder. 
           [0023]      FIG. 2  is a diagrammatic cross sectional view of a fluoride compound coated iron powder of an embodiment according to the present invention. 
           [0024]      FIG. 3  is a graph showing a relationship between specific resistance and a total thickness of the coating of an example of the present invention. 
           [0025]      FIG. 4  is a graph showing a relationship between magnetic flux density and the total thickness of the fluoride coating of an embodiment of the present invention. 
           [0026]      FIG. 5  is a graph showing a relationship between a thickness of the under layer and the specific resistance. 
           [0027]      FIG. 6  is a graph showing a relationship between the thickness of the under layer and magnetic flux density. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]      FIG. 1  shows a diagrammatic cross sectional view of a conventional iron powder having a fluoride coating thereon. As the fluoride compound, MgF 2  was shown. Prior to forming the fluoride coating, there was an oxide layer having a thickness of 20 nm on the surface of the iron powder. The forming of the fluoride coating was conducted using alcohol as a solvent in which fluoride sol was dissolved. The solvent was removed at a temperature as low as 350° C., and the post heat treatment was carried out at most of 600 to 700° C. to thereby prepare a material for powder compressed iron core. The fluoride has a melting temperature of 1000° C., and there is no change in the interface structure at the post heat treatment temperature. Oxygen layer remained in the interface of the fluoride layer. The layer was an oxide of iron. 
         [0029]    From the energy phase diagram, any reaction does not take place because the fluoride is more stable than iron oxide, even if the iron powder is heat-treated. Since there is no reaction, there is pealing-off of the fluoride coating at the portions where unevenness of the iron powder is large whose radius of curvature is large. As a result, the specific resistance becomes large. 
         [0030]      FIG. 2  shows a concept of the present invention. A Ca layer is previously formed on the iron powder by vacuum vapor deposition, for example. The vacuum vapor deposition is conducted by bringing the iron powder and calcium in contact with each other in vacuum and heated at a temperature lower than a melting point or lower (550 to 600° C.). The resulting iron powder after being cooled has a CaO under layer and a Ca metal film on the surface thereof, the CaO under layer having a gradient composition. Thereafter, a fluoride coating is applied to the iron powder. 
         [0031]    Although a thin oxide layer is formed on the surface of calcium, the mixed interface of oxygen and fluorine is obtained by carrying out the fluoride coating at a temperature as high as 100° C. because calcium fluoride is more stable. As a result, the calcium fluoride hardly peels off even at the portions of the large radius of curvature of the iron powder because bonding force of calcium and fluorine is larger than that of fluoride and iron powder. Therefore, water atomized iron powder, which has large uneven surfaces and inexpensive can be used. When iron powder is heated at high temperature in air atmosphere prior to Ca vacuum vapor deposition, a thick oxide layer is obtained in accordance with temperature conditions. When the thus obtained iron powder is used, a thick under layer is obtained. 
         [0032]    Since an amount of calcium used is small, it does not increase a cost. In addition to calcium, low elements that form oxides such as Mg, Ba, Sr, etc can be effective. 
         [0033]    In the following, examples of the present invention will be explained in detail. The scope of the present invention is not limited by the examples. 
       EXAMPLE 1 
     A Method of Preparing a Treating Solution 
       [0034]    (1) 3 g of a high water soluble salt, i.e. magnesium acetate was added to 100 mL of water, and the water was stirred with a vibrator or a ultrasonic stirrer to thereby completely solve magnesium acetate. 
         [0035]    (2) Hydrofluoric acid solution of 10% was slowly added to the above solution in an amount equivalent to forming MgF 2 . 
         [0036]    (3) The solution containing sol precipitate of MgF 2  was stirred for at least 1 hour with the ultrasonic stirrer. 
         [0037]    (4) After the solution was centrifuged at a rotation number of 4000 to 6000 rpm, a supernatant was removed. Then, the same amount of methanol was added. 
         [0038]    (5) After the methanol solution containing sol state MgF 2  was stirred to make a complete suspension solution, the solution was stirred with the ultrasonic stirrer for at least 1 hour. 
         [0039]    (6) Stirring of (4) and (5) was repeated 3 to 10 times until acetate ions and fluorine ions were not detected. 
         [0040]    (7) Finally, a sol state MgF 2  was obtained. A solution for treating iron powder was diluted with methanol until a concentration of MgF 2  was 1 g/8 mL. 
         [0041]    (A Method of Coating a Fluoride) 
         [0042]    (1) 8 mL of the MgF 2  treating solution prepared was added to 40 g of gas atomized iron powder having a particle size of 100 μm, and the mixture was kneaded until the entire powder was wetted. 
         [0043]    (2) The MgF 2  treated iron powder in (1) was subjected to methanol removal under a reduced pressure of 2 to 5 torr. 
         [0044]    (3) The iron powder obtained in (2) was transferred to a quartz boat and subjected to heat treatment at 200° C. for 30 minutes and 350° C. for 30 minutes under a pressure of 5×10 −5  torr. 
         [0045]    (4) The iron powder obtained in (3) was subjected to pre-heat treatment at 600° C. 
         [0046]    (5) The iron powder obtained in (4) was pressure-molded with a super hard mold to prepare ring samples having an outer diameter of 18 mm and an inner diameter of 10 mm. The pressure was 10 t/cm 2 . 
         [0047]    (6) The iron powder obtained in (4) was pressure-molded with a mold of 10 mm×10 mm to prepare a cubic sample for measuring resistance. The pressure was 10 t/cm 2 . 
         [0048]    (7) The iron powder obtained in (4) was charged into a press-mold for manufacturing a stator, and was molded under a pressure of 10 t/cm 2 . The samples obtained in (5), (6) and (7) were subjected to heat treatment at 600° C. to release stress. 
         [0049]    (8) Winding was applied to the stator prepared in (7) to prepare a rotating machine. 
         [0050]      FIG. 3  shows a relationship between a total thickness of coating and a specific resistance of the molding sample in example 1, wherein the water atomized powder having a particle size of 70 μm was coated with calcium of 20 nm, and MgF 2  coating was applied on the calcium coating. The molding had a size of 10 mm×10 mm×2 mm (thickness), and the specific resistance was measured by a four terminal method. A molding pressure was 1.2 GPa. A strain relieving heat treatment was conducted at 600□. It is apparent from  FIG. 3  that the specific resistance was higher when the two-layer coating was employed, even if the total thickness of the coating was the same. The specific resistance should preferably be 2 mΩ·cm or more; the values of the comparative examples were not satisfactory in this sense. However, in the examples of the present invention, the values were all 2 mΩ·cm or more, when the total thickness was 50 nm or more. 
         [0051]      FIG. 4  shows magnetic flux density depending on the total coating thickness of a sample wherein the molding sample had an outer diameter of 50 mm, an inner diameter of 40 mm and a thickness of 5 mm, and a number of winding of a primary side was 200 turns and a number of a secondary side was 40 turns. A molding pressure was 1.2 GPa, and a temperature for relieving strain was 600° C. The measurement conditions were an exciting magnetic field strength of 10000 A/m in direct magnetic field. The necessary magnetic flux density was at least 1.65 T, the total thickness should preferably be 100 nm or less. 
       COMPARATIVE EXAMPLE 1 
       [0052]    Using water atomized iron powder having a particle size of 70 μm, which is coated with MgF 2 , the specific resistance and magnetic flux density were measured in the same manner as in the example 1. The results are shown in  FIGS. 3 and 4 . 
       COMPARATIVE EXAMPLE 2 
       [0053]    Using water atomized iron powder having a particle size of 70 μm, which is coated with Mg, the specific resistance and magnetic flux density were measured in the same manner as in the example 1. The results are shown in  FIGS. 3 and 4 . 
       EXAMPLE 2 
       [0054]    Using samples, which were prepared in the same manner as in example 1 wherein a thickness of MgF 2  was 60 nm and a thickness of calcium under layer was 20 to 60 nm, the specific resistance and magnetic flux density were measured. The results are shown in  FIGS. 5 and 6 .  FIG. 5  shows changes of specific resistance of 2 mΩ·cm or more within the range of the thickness of the calcium under layer. On the other hand,  FIG. 6  shows changes of magnetic flux density. The thickness of the calcium under layer of 50 nm or more gives a magnetic flux density of 1.65 T or more. Thus, the thickness of 50 nm or more is preferable. 
         [0055]    If the thickness of the coating layer is 60 nm or less, the calcium under layer should be thicker by a thickness of the reduced coating layer in order to keep the desired magnetic flux density. However, if the thickness of the calcium under layer is over 50 nm, the desired specific resistance was not kept because of insufficient heat resistance of CaO. Therefore, the thickness of the calcium under layer should be 50 nm or more.