Composite magnetic material and method for manufacturing same

Abstract

Provided is a composite magnetic material in which low electrical conductivity and high magnetic permeability are achieved, and in which a frequency band in which decoupling is caused encompasses higher frequencies. The composite magnetic material comprises a flat soft magnetic metal powder; insulating particles which are smaller than an average thickness of the soft magnetic metal powder and which are disposed on a surface of the soft magnetic metal powder; and an organic binder material which retains the soft magnetic metal powder and the insulating particles in a dispersed manner. In a cross section in a thickness direction of the soft magnetic metal powder, there is at least one insulating particle per a length of 0.2 μm of the soft magnetic metal powder surface.

Claims

1. A composite magnetic material comprising: soft magnetic metal powder having a flat shape; insulating particles distributed on a surface of the soft magnetic metal powder by drying an aqueous solvent, the insulating particles each having a size smaller than an average thickness of the soft magnetic metal powder; and an organic binder configured to retain the soft magnetic metal powder and the insulating particles in a dispersed manner, wherein the insulating particles and the organic binder are distributed between layers of the soft magnetic metal powder so that a number of the insulating particles is at least one per a length of 0.2 μm of the surface of the soft magnetic metal powder in a cross section of the soft magnetic metal powder in a thickness direction and so that the surface of the soft magnetic metal powder is completely and uniformly coated with the insulating particles and the binder, wherein (Sz/2)/Sp=n is satisfied, where Sp represents a specific surface area of the soft magnetic metal powder, Sz represents a specific surface area of the insulating particles, n represents a number of laminated layers of the insulating particles, and n is one, and wherein the soft magnetic metal powder has a 10% cumulative particle diameter D.sub.10 of at least 2 μm and at most 6 μm and a 90% cumulative particle diameter D.sub.90 of at least 8 μm and at most 27 μm in a volume-based particle size distribution.

2. The composite magnetic material according to claim 1, wherein the insulating particles each comprise at least one kind of alumina, silica, magnesia, titania, or zirconia.

3. The composite magnetic material according to claim 1, wherein the insulating particles each have a volume resistivity of at least 1×10.sup.13 Ω.Math.cm.

4. The composite magnetic material according to claim 1, wherein the insulating particles have a median diameter D.sub.50 of at least 10 nm and at most 70 nm in a volume-based particle size distribution.

5. A method of producing a composite magnetic material, the method comprising: immersing soft magnetic metal powder having a flat shape in a sol which is obtained by dispersing insulating particles each having a size smaller than an average thickness of the soft magnetic metal powder in an aqueous solvent and has a pH of more than 7.0, followed by drying the aqueous solvent to coat a surface of the soft magnetic metal powder with the insulating particles; kneading the soft magnetic metal powder with an organic binder to cause the soft magnetic metal powder to be dispersed therein; and forming the resultant into a sheet shape such that the insulating particles and the organic binder are distributed between layers of the soft magnetic metal powder so that a number of the insulating particles is at least one per a length of 0.2 μm of the surface of the soft magnetic metal powder in a cross section of the soft magnetic metal powder in a thickness direction and so that the surface of the soft magnetic metal powder is completely and uniformly coated with the insulating particles and the binder, wherein a blending ratio of the soft magnetic metal powder and the insulating particles is set such that (Sz/2)/Sp=n is satisfied, where Sp represents a specific surface area of the soft magnetic metal powder, Sz represents a specific surface area of the insulating particles, n represents the number of laminated layers of the insulating particles, and n is one, and wherein the soft magnetic metal powder has a 10% cumulative particle diameter D.sub.10 of at least 2 μm and at most 6 μm and a 90% cumulative particle diameter D.sub.90 of at least 8 μm and at most 27 μm in a volume-based particle size distribution.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 is a schematic view for illustrating a sectional structure of a composite magnetic material according to this invention.

(2) FIGS. 2A-2D include SEM observation photographs of soft magnetic metal powder coated with insulating particles in a composite magnetic material according to this invention. FIG. 2A is a photograph of soft magnetic metal powder coated with zirconia particles having a D.sub.50 of 70 nm, and FIG. 2B is an enlarged photograph of a surface shown in FIG. 2A. FIG. 2C is a photograph of soft magnetic metal powder coated with zirconia particles having a D.sub.50 of 12 nm, and FIG. 2D is an enlarged photograph of a surface shown in FIG. 2C.

(3) FIGS. 3A and 3B include graphs for showing frequency response of a dielectric constant of a composite magnetic material. FIG. 3A is a graph for showing frequency response of a dielectric constant of a composite magnetic material of Example 1, and FIG. 3B is a graph for showing frequency response of a dielectric constant of a composite magnetic material of Comparative Example 1.

(4) FIG. 4 is a graph for showing frequency response of an Rda of the composite magnetic material according to this invention.

(5) FIGS. 5A and 5B include schematic views for illustrating an application example of the composite magnetic material of Example 1 or Comparative Example 1. FIG. 5A is a schematic view for illustrating a cross section of an internal structure of a communication device 100, and FIG. 5B is a schematic view for illustrating an evaluation system 200 in which the communication device 100 is modeled.

(6) FIG. 6 is a graph for showing frequency response of decoupling in Example 1 and Comparative Example 1.

MODES FOR EMBODYING THE INVENTION

(7) Now, an embodiment of this invention is described in detail.

Embodiment

(8) FIG. 1 is a schematic view for illustrating a sectional structure of a composite magnetic material according to this invention. As illustrated in FIG. 1, a composite magnetic material according to an embodiment of this invention has the following configuration: the composite magnetic material includes: soft magnetic metal powder 1 having a flat shape; insulating particles 2 distributed on the surface of the soft magnetic metal powder, the insulating particles 2 each having a size smaller than the average thickness of the soft magnetic metal powder; and an organic binder 3 configured to retain the soft magnetic metal powder 1 and the insulating particles 2 in a dispersed manner.

(9) In order to reduce the electrical conductivity of the composite magnetic material, the insulating particles 2 are distributed between layers of the soft magnetic metal powders 1 so that the number of the insulating particles 2 is one or more per a length of 0.2 μm of the surface of the soft magnetic metal powder 1 in a cross section of the soft magnetic metal powder 1 in a thickness direction, to thereby achieve desired characteristics. FIGS. 2A-2D are SEM observation photographs of the soft magnetic metal powder 1 of this invention. The surface of the soft magnetic metal powder 1 is almost completely coated with the insulating particles 2.

(10) With regard to the soft magnetic metal powder, for example, in order to obtain a large magnetic loss μ″ in a frequency band of 1 GHz or more, magnetic resonance frequency needs to be extended and a large μ is required. Further, a material for the soft magnetic metal powder also needs to have such ductility as to enable its processing into powder having a flat shape. Therefore, it is preferred to use, as a soft magnetic metal material having a saturated magnetic flux density Bs of 1 T or more, Fe or an alloy thereof, specifically, Fe, Fe—Si, Fe—Al, Fe—Cr, Fe—Si—Al, or the like.

(11) The material for the soft magnetic metal powder may appropriately be selected and used in combination depending on magnetic characteristics to be required and a target frequency band. For example, a Ni-based alloy having a lower saturated magnetic flux density than an Fe-based alloy, such as Ni—Fe, may be used.

(12) With regard to the particle diameter of the soft magnetic metal powder, in order to achieve the magnetic characteristics to be required without reducing the density of the soft magnetic metal powder in the composite magnetic material, the soft magnetic metal powder preferably has a 10% cumulative particle diameter D.sub.10 of 2 μm or more and 6 μm or less and a 90% cumulative particle diameter D.sub.90 of 8 μm or more and 27 μm or less in a volume-based particle size distribution.

(13) It is required for the insulating particles to have high volume resistivity and maintain a distance between the particles of the soft magnetic metal powder without being deformed even through pressurization at the time of forming, and hence an oxide-based insulating material is suitable. In particular, alumina, silica, magnesia, titania, zirconia, or a material including at least one kind thereof is suitable for the composite magnetic material of this invention. Those materials are more suitable when having a volume resistivity of 1×10.sup.13 Ω.Math.m or more and having a median diameter D.sub.50 of 10 nm or more and 70 nm or less in a volume-based particle size distribution. When the D.sub.50 is less than 10 nm, insulation properties between the soft magnetic metal powders become insufficient, and the electrical conductivity of the composite magnetic material is increased. In addition, when the D.sub.50 is more than 70 nm, the distance between the soft magnetic metal powders is increased to 140 nm or more, and their magnetic bonding is weakened, with the result that the magnetic permeability of the composite magnetic material is reduced.

(14) As a method of forming the insulating particles on the surface of the soft magnetic metal powder, a method involving immersing the soft magnetic metal powder in a sol which is obtained by dispersing the insulating particles in an aqueous solvent and has a pH of more than 7.0, more preferably a pH of more than 7.0 and 8.2 or less, which is a weak alkaline pH, followed by heating to dryness, is preferred. When the specific surface area of the soft magnetic metal powder having a flat shape and the specific surface area of each of the insulating particles are defined as Sp and Sz, respectively, and the number of laminated layers of the insulating particles is defined as n, the blending ratio of each component is adjusted so that the equation (Sz/2)/Sp=n is established. In order to uniformly form the insulating particles on the surface of the soft magnetic metal powder, n is preferably set to 1 or more. In addition, when the number of laminated layers of the insulating particles is too large, the distance between the soft magnetic metal powders is increased, and their magnetic bonding is weakened, with the result that the magnetic permeability of the composite magnetic material is reduced. Therefore, n is preferably set to 2 or less.

(15) As the organic binder, any material which is capable of dispersing and retaining the soft magnetic metal powder in which the insulating particles are distributed on the surface thereof, and has easy formability may be used with no particular limitation. For example, an acrylic rubber, a urethane resin, a silicone resin, or an epoxy resin may be used. In addition, when it is required to improve bonding properties between the soft magnetic metal powder and the organic binder or impart flame retardancy, any known coupling agent or flame retardant may be added.

(16) When the soft magnetic metal powder in which the insulating particles are formed on the surface thereof is kneaded with the organic binder so that the soft magnetic metal powder is dispersed therein, and the resultant is formed into a sheet shape, the composite magnetic material of this invention is obtained.

EXAMPLES

(17) Now, Example of this invention is described.

Example 1

(18) The composite magnetic material according to the embodiment of this invention was produced through use of carbonyl iron powder as the soft magnetic metal powder and zirconia particles as the insulating particles.

(19) Commercially available carbonyl iron powder was subjected to pulverization treatment with a wet-type attritor, and a slutty after the pulverization is dried in a vacuum drier, followed by classification with a mesh. Thus, soft magnetic metal powder having a flat shape having a specific surface area Sp of 2.2 m.sup.2/g was obtained.

(20) As the insulating particles, zirconia particles having a particle diameter D.sub.50 of 12 nm and each having a specific surface area Sz of 41.7 m.sup.2/g were mixed with a dispersant and water so as to achieve a solid content of 10 wt %, and the resultant was subjected to dispersion treatment with an ultrasonic disperser. Thus, a zirconia sol having a pH of 7.4 was obtained.

(21) The addition amount of the zirconia sol was determined so that one layer of zirconia nanoparticles was formed on the surface of one particle of the soft magnetic metal powder having a flat shape, that is, the equation (Sz/2)/Sp=1 was established. The zirconia sol in the determined amount and the soft magnetic metal powder were mixed with each other, and then, the mixture was dried in an oven in the atmosphere. Thus, the soft magnetic metal powder whose surface was coated with the zirconia particles was obtained. As shown in FIGS. 2C and 2D, from scanning electron microscope (SEM) images of a composite magnetic material to be obtained, it was able to be confirmed that the zirconia particles were almost uniformly fixed to the surface of the soft magnetic metal powder.

(22) 50 vol % of the flat carbonyl iron powder coated with the zirconia nanoparticles, 40 vol % of an acrylic rubber, and 10 vol % of a silane coupling agent were blended, and mixed with a planetary centrifugal mixer AR-100 (manufactured by THINKY Corporation) for 12 minutes to produce an application liquid.

(23) The application liquid was applied onto an polyester sheet and formed into a film with a baker applicator, and was dried to produce a green sheet. Next, the green sheets each having formed into a film were laminated on one another and subjected to thermal pressure bonding. Thus, a composite magnetic material having a sheet shape having a thickness of 100 μm was obtained. From the scanning electron microscope (SEM) images of the resultant composite magnetic material after being cut, it was confirmed that the zirconia particles serving as the insulating particles were distributed so that the number of the zirconia particles was one or more per a length of 0.2 μm of the surface of the soft magnetic metal powder in a cross section of the soft magnetic metal powder in a thickness direction. The composite magnetic material having a sheet shape had a surface resistance of 1.5×10.sup.5Ω and a sheet density of 3.8 g/cc.

Comparative Example 1

(24) A composite magnetic material having a sheet shape having a thickness of 100 μm was obtained in the same manner as in Example 1 except that the soft magnetic metal powder having a flat shape was not coated with the insulating particles. The composite magnetic material having a sheet shape had a surface resistance of 2.5×10.sup.1Ω and a sheet density of 4.3 g/cc.

(25) The composite magnetic materials having a sheet shape of Example 1 and Comparative Example 1 were each punched into a toroidal shape measuring 7.9 mm in outer diameter, 3.05 mm in inner diameter, and 100 μm in thickness, and measured for in-plane magnetic permeability with an impedance/material analyzer E4991A (Agilent Technologies) and a magnetic material measurement fixture 16454A (Agilent Technologies). A flat μ′ value with respect to frequency was 18 in Example 1 and 24 in Comparative Example 1. Comparative Example 1 had a configuration in which a distance between the soft magnetic metal powders was smaller than that in Example 1, and hence their magnetic bonding was strengthened, with the result that a value for the magnetic permeability was high in Comparative Example 1. However, the surface resistance was as extremely low as 2.5×10.sup.1Ω, resulting in insufficient insulation properties. Meanwhile, in Example 1, the surface resistance was as high as 1.5×10.sup.5Ω, and a reduction in magnetic permeability was able to be suppressed to about 18.

(26) The composite magnetic materials having a sheet shape of Example 1 and Comparative Example 1 were each punched into a toroidal shape measuring 7.00 mm in outer diameter, 3.05 mm in inner diameter, and 100 μm in thickness, and measured for in-plane dielectric constant with a network analyzer ENA E5080A (Keysight Technologies) and a transmission line method coaxial sample holder CSH2-APC7 (Kanto Electronic Application and Development Inc.). The results are shown in FIGS. 3A and 3B. In Example 1, a value for ε′10 and a value for ε″11 at 0.1 GHz were 137 and 17, respectively. In Comparative Example 1, a value for ε′12 and a value for ε″13 at 0.1 GHz were 320 and 116, respectively. It was confirmed that both ε′ and ε″ in Example 1 were lower than those in Comparative Example 1.

(27) The composite magnetic materials having a sheet shape of Example 1 and Comparative Example 1 were each cut into a sheet measuring 40 mm in length, 40 mm in width, and 100 μm in thickness, and measured by a method specified in International Electrotechnical Commission Standard IEC 62333-2. The results are shown in FIG. 4. In Example 1, an Rda (Intra-decoupling ratio) was more than 0 up to 1.1 GHz, and in Comparative Example 1, an Rda was more than 0 up to 0.7 GHz. It was confirmed that a frequency band in which decoupling occurred was extended in the case of the composite magnetic material of Example 1 as compared to the case of the composite magnetic material of Comparative Example 1.

(28) Next, the characteristics of the composite magnetic materials having a sheet shape of Example 1 and Comparative Example 1 are described in detail.

(29) FIG. 5A is a schematic view for illustrating a cross section of an internal structure of a communication device 100, such as a cellular phone, to which each of the composite magnetic materials having a sheet shape of Example 1 and Comparative Example 1 is to be applied. As illustrated in FIG. 5A, the communication device 100 includes an electronic component 21, an antenna 22, and a metal housing 23.

(30) Herein, in the communication device 100, the electronic component 21 is a noise source which generates noise. In this case, the noise generated from the electronic component 21 interferes with the antenna 22 through intermediation of the metal housing 23 in a reflection region 110. As a method of suppressing such noise, a method involving bonding a magnetic sheet to the metal housing 23 in an inside of the communication device 100, to thereby suppress noise, has been known.

(31) FIG. 5B is a schematic view for illustrating an evaluation system 200 in which the internal structure of the communication device 100 illustrated in FIG. 5A is modeled. The evaluation system 200 includes a noise source Tx, an antenna Rx, a metal plate 24, and a magnetic sheet 25. Herein, the noise source Tx corresponds to the electronic component 21 in FIG. 5A. The antenna Rx corresponds to the antenna 22 in FIG. 5A. The metal plate 24 corresponds to the metal housing 23 in FIG. 5A.

(32) The magnetic sheet 25 is bonded to the surface of the metal plate 24. In this case, noise generated from the noise source Tx interferes with the antenna Rx through intermediation of the magnetic sheet 25 bonded onto the metal plate 24.

(33) The frequency responses of decoupling in the cases of using the above-mentioned composite magnetic materials having a sheet shape of Example 1 and Comparative Example 1 as the magnetic sheet 25 in the evaluation system 200 were compared to each other. Specifically, in a frequency region of from 0 GHz to 9 GHz, a coupling attenuation amount between the noise source Tx and the antenna Rx was confirmed.

(34) FIG. 6 is a graph for showing frequency response of decoupling in Example 1 and Comparative Example 1 in the evaluation system 200. Specifically, in FIG. 6, the abscissa represents frequency (GHz) and the ordinate represents coupling (dB) between the noise source Tx and the antenna Rx. In FIG. 6, first frequency response 31 is frequency response in Example 1, and second frequency response 32 is frequency response in Comparative Example 1.

(35) With reference to FIG. 6, as shown by the first frequency response 31, it is revealed that, in Example 1, decoupling occurs in the entire frequency region of from 0 GHz to 9 GHz. Meanwhile, as shown by the second frequency response 32, in Comparative Example 1, coupling of noise occurs in a frequency region of from 3.6 GHz to 6.2 GHz.

(36) Specifically, coupling in Example 1 in the evaluation system 200 was less than 0 in the measured range of up to 9 GHz, but coupling in Comparative Example 1 in the evaluation system 200 was less than 0 up to 3.5 GHz. That is, in the evaluation system 200, the composite magnetic material of Example 1 allows a frequency band in which decoupling occurs to cover higher frequency than the composite magnetic material of Comparative Example 1.

(37) Accordingly, as shown in FIG. 6, it was confirmed that the composite magnetic material of Example 1 was able to satisfactorily cause decoupling as compared to the composite magnetic material of Comparative Example 1.

REFERENCE SIGNS LIST

(38) 1 soft magnetic metal powder 2 insulating particle 3 organic binder 10, 12 ε′ 11, 13 ε″ 21 electronic component 22 antenna 23 metal housing 24 metal plate 25 magnetic sheet 31 first frequency response 32 second frequency response 100 communication device 110 reflection region 200 evaluation system