JP2005223259A - Dust core and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a dust core which increases the specific resistivity which makes insulation satisfactory to reduce the eddy current loss and further enhance the magnetic flux density and also has high strength, and to provide a method for manufacturing the same. <P>SOLUTION: The dust core consists of only iron powder particles and phosphate and the iron powder particles are separated apart from each another, at a distance of 200 to 600 nm via a phosphate phase containing 0.048 to 0.12 mass % phosphorus. In the method of manufacturing the dust core, the iron powder particles having surfaces coated with phosphate layers which contain 0.048 to 0.12 mass % phosphorus and are 160 to 400 nm in thickness are used as raw material powder and are compressed into a desired shape under a compacting pressure of 800 to 1,500 MPa. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a dust core suitable for high frequency applications such as a transformer, a reactor, a thyristor valve, a noise filter, and a choke coil, having low iron loss and excellent soft magnetic properties and high mechanical strength, and a method for manufacturing the same.

  With recent miniaturization and higher density of electrical and electronic equipment, there is an increasing demand for high-frequency cores that are small in size and have high magnetic flux density, magnetic permeability, and low iron loss for the magnetic core materials used in them. . Of these requirements, iron loss is the sum of eddy current loss, which has a large relationship with the core resistivity, and hysteresis loss, which is affected by the iron powder manufacturing process and subsequent distortions in the iron powder. In particular, since the eddy current loss increases in proportion to the square of the frequency, in order to reduce the iron loss in the high frequency region, it is necessary to increase the specific resistance to reduce the eddy current loss. .

  From this point of view, conventionally, ferrite cores that have low apparent eddy current loss even in the high frequency range due to their large apparent resistivity, and alloy powders such as Sendust / Permalloy are bonded with an insulating binder such as phenol resin or epoxy resin. A dust core has been used. However, all of these materials have low magnetic flux density and magnetic permeability, and it is difficult to reduce the size of the dust core.

  On the other hand, as a magnetic particle, a powder magnetic core using iron powder, which has a high magnetic flux density and is excellent in compression moldability and has a high cost performance, is also compared with the above-mentioned ferrite core in the high frequency region. It has a high magnetic flux density and has an advantage that it is inexpensive in price. However, in powder magnetic cores using iron powder with low specific resistance, loss due to eddy current generation between iron powders in an AC magnetic field increases, so a coating and insulating resin binder that electrically insulates the iron powder surface. It is necessary to coat.

  Under such circumstances, Patent Document 1 uses a powder obtained by coating particles of atomized iron powder or sponge iron powder with phosphoric acid and a layer containing insulating phosphorus, and optionally with a thermosetting resin. After mixing and compression molding, a dust core heated at 350 to 550 ° C. has been proposed. It is described that the phosphorus content is 0.005 to 0.03% by weight for atomized iron powder and 0.02 to 0.06% by weight for sponge iron powder. This Patent Document 1 describes that a very thin phosphate insulating layer is formed on the surface of iron particles and the P content is very low.

  Patent Document 2 discloses that a metal magnetic powder having an insulating film having a film thickness of 10 to 100 nm formed on the surface by a phosphate chemical conversion treatment solution is 84% or more by volume ratio and 1% by weight or more of thermosetting resin powder. A dust core having a specific resistance of 2 Ωcm or more is disclosed.

JP-T 9-512388 JP-A-11-251131

  The thin phosphate insulating layer as described above is attached to the surface of the iron powder particles by an anchoring effect or a binding force such as van der Waals force, so in the mixing process with the binder resin or compression molding Peeling and dropping off of the phosphate layer is likely to occur due to the sliding between the iron powder particles and the iron powder particles and the mold in the process. By the way, according to our experiments so far, the thinner the phosphate layer on the iron powder particles, the more the tendency of the separation and dropping is observed, and the electrical insulation of the individual iron powder particles is broken. As a result, eddy current generation between the iron powder particles in the AC magnetic field becomes significant, and as a result, a phenomenon in which the AC characteristics are deteriorated is observed. Therefore, in a conventional dust core insulated with phosphate, addition of a resin is essential. In Patent Document 2, it is essential to add 1% by weight or more of resin, and in Patent Document 1, all the resins having insulating properties are added in Examples, and this is considered to supplement the insulating properties.

  As a result of studies by the present inventors, it has been found that the addition of a resin increases the insulation, but does not contribute to the strength of the material, but rather causes a decrease in strength. For this reason, in Patent Documents 1 and 2, the iron loss is reduced, but it has been found that the addition of resin causes a decrease in density, a decrease in magnetic flux density associated therewith, and a decrease in mechanical strength.

  An object of the present invention is to provide a dust core having high insulation strength and high specific resistance to reduce eddy current loss and high magnetic flux density, and a method for manufacturing the same. For this issue, considering the required characteristics of the market, the target value is a specific resistance (ρ) of 100 μΩm or more, the magnetic flux density (B10k) at an applied magnetic field of 10,000 A / m is 1.6 T or more, and the mechanical strength. As a guideline, the crushing strength is 150 MPa or more.

  In order to solve the above-described problems and achieve the target value, the dust core of the present invention is composed of only iron powder particles and phosphate, and the iron powder particles contain phosphorus in a range of 0.048 to 0.00. It is characterized by being separated by a distance of 200 to 600 nm through a phosphate phase containing 12% by mass.

  Moreover, the manufacturing method of the powder magnetic core of the present invention comprises iron powder particles containing 0.048 to 0.12% by mass of phosphorus and having a surface coated with a phosphate layer having a thickness of 160 to 400 nm. It is used as a raw material powder and is compression-molded into a desired shape at a molding pressure of 800 to 1500 MPa, and a molded body obtained as desired is heated at a temperature of 300 to 500 ° C.

  The dust core of the present invention is composed only of iron powder particles and phosphate, and the iron powder particles are 200 to 200 through a phosphate phase containing 0.048 to 0.12% by mass of phosphorus. Because the phosphate phase is separated by a distance of 600 nm and the phosphate phase has a strong chemical bond, a powder magnetic core with low iron loss and high magnetic flux density is obtained compared to a conventional powder magnetic core, In addition to contributing to higher performance of soft magnetic materials for various magnetic cores, the range of application of conventional dust cores can be greatly expanded by improving the strength.

  In the dust core of the present invention, individual iron powder particles are separated by a phosphate phase at a distance of 200 to 600 nm, ensuring electrical insulation and generating vortices between iron powder particles in an alternating magnetic field. Current generation is suppressed. Further, the addition of a resin that causes a decrease in mechanical strength is abolished, and the mechanical strength of the material is increased by the chemical bonding force of the phosphate phase. Furthermore, the abolition of the resin increases the density of the iron powder particles, and as a result, the magnetic flux density is also improved.

  When the distance between the iron powder particles separated by the phosphate phase in the dust core is small, that is, when the phosphate layer coated on the surface of the raw iron powder particles is thin, the iron powder The mechanical entanglement between the two is dominant. However, by increasing the thickness of the phosphate layer coated on the surface of the iron powder particles as a raw material and increasing the distance between the iron powder particles separated by the phosphate phase of the dust core, In addition to the mechanical bond between the iron powders, the chemical bond between the phosphate phases by heating and firing is also added to further improve the strength. This increase in mechanical strength is achieved by coating the surface of the iron powder particles with a phosphate layer having a thickness of 160 to 400 nm, and the iron powder particles are placed at a distance of 200 to 600 nm via the phosphate phase. Shows a peak when separated. However, when the distance between the iron powder particles separated by the phosphate phase of the powder magnetic core is too large, that is, when the thickness of the phosphate layer coated on the surface of the raw iron powder particles is too thick. Since the mechanical bonding between the iron powders is not obtained and the strength is only the phosphate phase, the strength is remarkably lowered as the film thickness is increased.

  In the dust core of the present invention, since the phosphate phase between the iron powder particles is as thick as 200 to 600 nm, electrical insulation between the iron powder particles is sufficiently secured, and the specific resistance is high. The increase in iron loss is suppressed and the decrease in AC characteristics is small. Furthermore, since the phosphate layer formed on the surface of the iron powder particles is as thick as 160 to 400 nm, the phosphate layer is dropped and destroyed during compression molding, and further, the phosphate phase is deteriorated during heating. A powder magnetic core can be manufactured without causing a decrease in specific resistance.

  The reason for suppressing the separation and destruction of the phosphate phase on the iron powder surface has not yet been fully analyzed. In the case of a thick phosphoric acid coating according to the present invention, peeling / breaking occurs by suppressing the propagation of cleavage and slip by randomly laminating multiple crystals with different cleavage directions. Is considered to be suppressed. In combination with the thickness of the thick phosphoric acid coating, it is presumed that the contact between the iron powders accompanying the peeling of the phosphate phase is prevented, and the decrease in the specific resistance is suppressed.

  For this reason, in this invention, it was not necessary to add the resin for insulation, and the resin component used as the fall factor of mechanical strength was able to be excluded. Moreover, the density of the iron powder particles contributing to the magnetic flux density can be improved by eliminating the resin component, and the magnetic flux density can be improved. In the dust core of the present invention, it is preferable that the density ratio is 98% or more because the magnetic flux density is increased.

  The range of the phosphorus amount of the phosphate phase is selected from target values of specific resistance and magnetic flux density. When phosphorus is less than 0.048% by mass, the specific resistance of the material after compression molding is 100 μΩm or less, and generation of eddy currents generated between magnetic powders in an alternating magnetic field is increased. On the other hand, if it exceeds 0.12% by mass, the magnetic flux density becomes less than 1.6T, and desired magnetic properties cannot be obtained. From this, the phosphorus amount of the phosphate was set to 0.048 to 0.12% by mass with respect to the weight of the iron powder.

  As described above, the dust core of the present invention contains iron powder particles containing 0.048 to 0.12% by mass of phosphorus and having a surface coated with a phosphate layer having a thickness of 160 to 400 nm. Although used as a raw material powder, in order to obtain a high magnetic flux density and mechanical strength, the molding pressure needs to be a molding pressure of 800 to 1500 MPa in a region higher than a molding pressure of 400 to 700 MPa for mechanical parts by a normal powder metallurgy method. By using such molding conditions, even if iron powder particles having a thick phosphate layer of the present invention are used, the phosphate layers are closely adhered to each other, the distance between the iron powder particles is reduced, and a high magnetic flux density is obtained. It becomes possible to obtain mechanical strength. If the molding pressure is less than 800 MPa, the adhesion of the phosphate layer becomes insufficient, the mechanical strength becomes insufficient, the distance between the iron powder particles becomes large, and the desired magnetic flux density cannot be obtained. . On the other hand, if it exceeds 1500 MPa, molding becomes difficult from the viewpoint of the strength and structure of the mold. The upper limit of the molding pressure is preferably 1200 MPa, more preferably 1000 MPa. Even with such a high molding pressure, if the phosphate layer has the above-mentioned thickness, peeling does not easily occur. The iron powder particles having the acid salt layer face each other and come into close contact with each other, thereby preventing contact between the iron powders and suppressing a decrease in specific resistance.

  In addition, in the above molding, if the molding lubricant remains inside the powder magnetic core, it is considered to affect the magnetic properties and mechanical properties, so than the internal lubrication method in which the molding lubricant is added to the raw material powder, An external lubrication method in which a molding lubricant is applied to the inner wall surface of the mold is preferable. The molding lubricant is applied to the inner wall surface of the mold by applying a lubricant in which a predetermined amount of the molding lubricant is uniformly dispersed in an organic solvent to the inner wall surface of the mold or charged by electrostatic spraying. The lubricant may be sprayed on the inner wall surface of the mold and adsorbed by electric charges.

  The molded body obtained by the above compression molding has a phosphate layer in close contact and has a certain level of mechanical strength. A powder core having a specific resistance is obtained. However, when it is desired to improve the mechanical strength, the iron powder particles in which the phosphate phase is formed are compression molded at the above molding pressure, and then heated at a temperature of 300 to 500 ° C. A phosphate phase in which the phosphate layers formed on the surface are chemically bonded to each other can be formed, and a dust core having high mechanical strength can be obtained. If the heating temperature is less than 300 ° C, the chemical bonding between the phosphate layers will be insufficient, and if it exceeds 500 ° C, the phosphate phase will deteriorate, and in any case the mechanical strength will deteriorate. It becomes. Moreover, the furnace atmosphere of a heating process should just be performed in the atmosphere in which the phosphate coating | cover on the iron powder surface is not destroyed or deteriorated, and shall be in inert gas other than reducing atmosphere, or in air | atmosphere.

  The iron powder particles used in the present invention can be variously selected depending on the frequency region used, the magnetic flux density or the application. There are various types of iron powders by various production methods such as an atomization method and a reduction method, but atomized iron powders having excellent purity and compressibility are suitable.

  The particle diameter of the iron powder needs to be selected according to the frequency region to be used, and is preferably 150 μm or less, and more preferably 100 μm or less to reduce eddy current loss. In addition, since the eddy current loss is reduced and the AC characteristics are improved as the particle size is reduced, the lower limit of the particle size is not particularly set, but is usually preferably 10 μm or more. If the particle diameter is too small, the compression molding property and fluidity of the metal powder will deteriorate, and a high-density soft magnetic material cannot be obtained.

  Hereinafter, the present invention will be specifically described by way of examples. In the examples, atomized iron powder having an average particle diameter of 75 μm was used as the iron powder. This atomized iron powder was put in a container together with a phosphate aqueous solution in which 20 g of phosphoric acid was dissolved in 1 liter of water, mixed for a fixed time, and then dried at 180 ° C. for 60 minutes using a thermostatic bath. The thickness of the phosphate layer was adjusted by changing the amount of the phosphate aqueous solution added to the iron powder, and a plurality of layers having different film thicknesses and phosphorus contents were prepared. For comparison, a phosphate layer-forming iron powder having a thickness of 50 nm obtained as described above was mixed with 2% by mass of a polyamide resin and prepared as a conventional example. .

  The prepared phosphate layer-forming iron powder was compression-molded into a ring shape of φ23 × φ10 × 5 t at the molding pressure shown in Table 1, and heated under the heating conditions shown in Table 1 to prepare a ring test piece. The molding was performed by a mold lubrication method in which a predetermined amount of zinc stearate as a mold lubricant dispersed uniformly in an organic solvent was applied to the inner wall surface of the mold. In addition, heating was performed for 1 hour in a predetermined atmosphere and temperature.

  About the obtained ring test piece, each characteristic of a density, a specific resistance, a magnetic flux density, and a crushing strength was measured. About the thickness of the phosphate layer, the theoretical film thickness value calculated from the surface area of the iron powder was used. The density was measured by the Archimedes method, and the density ratio with respect to the theoretical density was determined. The distance between the iron powder particles was determined by observing the cut surface of the sample cut and polished with an electron microscope. The specific resistance was measured by polishing the surface of the test piece with # 800 polishing paper and measuring the polished surface by the four-end needle method. The magnetic flux density was measured by measuring the magnetic flux density B10k by applying a winding line of 120 times of the primary side coil and 20 times of the secondary side coil to the ring test piece and applying a magnetic field of 10 kA / m. The crushing strength was calculated from the maximum breaking load by performing a compression test at a crosshead speed of 0.5 mm / min using another ring specimen (φ23 × φ10 × 10 t). These results are shown in Table 2.

From the above embodiment, the following can be understood.
(1) By comparing the samples Nos. 01 to 04, 10, and 16 to 19 in Tables 1 and 2, the influence of the thickness of the phosphate layer formed on the surface of the iron powder can be examined. FIG. 1 shows a graph obtained by removing the sample number 19 from the graph. As shown in FIG. 1, the density ratio and magnetic flux density tend to decrease as the thickness of the phosphate layer increases. However, since the molding pressure is large, the density ratio is high, and the target is 400 nm or less. A high magnetic flux density of 6T or more is shown. When the distance between the iron powder particles of the sample at this time was observed with an electron microscope, it was 600 nm. The specific resistance increases as the thickness of the phosphate layer increases, and shows a high specific resistance of 100 μΩm or more, which is a target when the thickness is 160 nm or more. The distance between the iron powder particles of the sample at this time was similarly 200 nm. The crushing strength tends to increase as the thickness of the phosphate layer increases, but the rate of increase decreases as the thickness of the phosphate layer increases and tends to decrease with a peak at 300 nm. ing. In addition, the conventional resin-added sample (Sample No. 19) has a high specific resistance due to the insulating properties of the resin, but it is confirmed that the magnetic flux density is low and the mechanical strength is also low because the resin is added. It was.

  From the above, it was confirmed that the dust core of the present invention in which the thickness of the phosphate layer formed on the surface of the iron powder was increased to 160 to 400 nm and the resin was excluded has high magnetic properties and high mechanical strength. It was. Further, the distance between the iron powder particles of the dust core at this time is 200 to 600 nm.

(2) By comparing samples Nos. 05, 06, 10, 14, and 15 in Tables 1 and 2, the influence of the molding pressure can be examined. A graph of this is shown in FIG. From FIG. 2, when the molding pressure is 500 MPa of sample number 05, a sufficient density cannot be obtained and the magnetic flux density is lower than the target value of 1.6 T, but the molding pressure is 800, 1000, 1200, 1500 MPa (sample number). As the density increases to 06, 10, 14, 15), the density ratio is improved and the magnetic flux density is improved. In order to obtain the target magnetic flux density of 1.6 T or more, it was confirmed that the molding pressure should be 800 MPa or more. However, the rate of increase tends to decrease as the molding pressure increases. Although the specific resistance tends to decrease as the molding pressure increases, a target value of 100 μΩm or more is obtained even at a molding pressure of 1500 MPa. Further, the crushing strength tends to improve as the molding pressure increases, but similarly, the rate of increase tends to decrease as the molding pressure increases. From the above, it has been found that the targets of magnetic properties and mechanical strength can be achieved in the range of 800 to 1500 MPa as the molding pressure. However, on the high pressure side, the specific resistance is reduced due to the breakdown of the insulating film although the width of the improvement in the magnetic flux density and the crushing strength is narrow. Therefore, the molding pressure is preferably 1200 MPa or less.

(3) By comparing the samples Nos. 07 to 10, 12 and 13 in Tables 1 and 2, the presence or absence of the heat treatment and the influence of the heating temperature when performing the heat treatment can be examined. A graph of this is shown in FIG. According to FIG. 3 and Table 1, in the case of Sample No. 07 where heat treatment is not performed, although the crushing strength is lower than that in the case where heat treatment is performed, from a dust core (Sample No. 19) using a conventional resin The crushing strength is about twice as high. Moreover, since the specific resistance shows a very high value and the magnetic flux density exceeds the target 1.6T, it can be seen that the specific resistance is sufficiently practical in applications where strength is not particularly required. However, in applications where mechanical strength is required, heat treatment is effective, and the target temperature of 1500 MPa or more is achieved in the range of 300 to 500 ° C. From this, it can be seen that the phosphate layers are chemically bonded to each other by heat treatment to form a phosphate phase and contribute to the improvement of mechanical strength. However, heating above 500 ° C. degrades the phosphate phase and the specific resistance falls below the target 100 μΩm, so 500 ° C. should be the upper limit.

(4) By comparing the samples Nos. 10 and 11 in Tables 1 and 2, it can be seen that the influence of the atmosphere during the heat treatment is examined. That is, when the samples of sample numbers 10 and 11 are compared, even if the atmosphere at the time of the heat treatment is air, there is no difference in magnetic flux density and mechanical strength, and the values are equivalent. The specific resistance is slightly improved when the atmosphere is air. This is because the iron oxide formed by oxidizing the iron powder particles in the surface layer portion of the dust core worked as an insulating coating. However, the powder magnetic core when heated in a nitrogen atmosphere exhibits a metallic luster, whereas the powder magnetic core when heated in an air atmosphere feels lost. A nitrogen atmosphere is suitable.

  As an example of utilizing the powder magnetic core of the present invention, the specific resistance is increased to suppress the iron loss on the high frequency side, the magnetic flux density is increased to improve the magnetic characteristics, and the mechanical strength is also improved. It can be applied to high-performance transformers, reactors, thyristor valves, noise filters, choke coils, etc.

It is a graph which shows the relationship between the thickness of the phosphate layer formed in the iron powder surface, and each evaluation item. It is a graph which shows the relationship between a molding pressure and each evaluation item. It is a graph which shows the relationship between heating temperature and each evaluation item.

Claims (3)

  The iron powder particles are composed only of iron powder particles and phosphate, and the iron powder particles are separated by a distance of 200 to 600 nm through a phosphate phase containing 0.048 to 0.12 mass% of phosphorus. A powder magnetic core characterized by   Iron powder particles containing 0.048 to 0.12% by mass of phosphorus and having a surface coated with a phosphate layer having a thickness of 160 to 400 nm are used as raw material powder, and the desired shape is 800 to 1500 MPa. A method for producing a powder magnetic core, comprising compression molding at a molding pressure. The method for producing a powder magnetic core according to claim 2, wherein the compact obtained by the compression molding is heated at a temperature of 300 to 500 ° C.

Images