JPH02254709A - Manufacture of magnetic composite material of excellent magnetic characteristics - Google Patents

Abstract

PURPOSE:To make it possible to increase the specific resistance of the title magnetic composite material while necessary permeability is being secured by a method wherein specific spherical magnetic powder and solid-state particulate resin binder are dry-mixed, they are filled in a die, a direct powder-molding is conducted in the cold work applying specific surface pressure of 2t/cm<2>. CONSTITUTION:Spherical magnetic powder, having the ratio of building volume to lot of 55 to 95% against the entire magnetic composite material and average grain diameter of 50 to 300mum, and a resin binder are dry-mixed, and after the mixed powder has been filled in a die, a direct powder molding operation is conducted in the cold work applying the surface pressure higher than 2t/cm<2>, and the mixed powder is compacted. As a result, a magnetic composite material, with which a large-sized magnetic composite material tabular plate body, having a permeability (mu) of 50 or higher, specific resistance of 0.001cm or higher, tensile strength of 2kg/cm<2> or higher and area of 250cm<2> or wider, is formed, can be obtained. As a result, specific resistance can be increased while necessary permeability is being secured.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a method for manufacturing a magnetic composite material made by molding magnetic powder, such as magnetic iron powder, using a resin binder, and in particular, to a method for manufacturing a magnetic composite material that secures the necessary magnetic permeability. However, it also relates to magnetic composite materials that can increase resistivity. The present invention is a permanent magnet type MHI (Magantic Resonance
Since this method is most commonly used in magnetic pole plates used to support permanent magnets in imaging devices, the case where it is applied to this will be described below as an example.

[Conventional technology]

In recent years, MRI systems that utilize nuclear magnetic resonance phenomena are rapidly becoming out of favor as imaging devices that obtain tomographic images of the human body. This MHI device is
Compared to the T device, there is no radiation exposure, and it has the advantage of being able to obtain tomographic images in any direction, such as transverse images (sagittal images, coronal images, etc.). A permanent magnet system is employed as an example of a means for generating a static magnetic field in such MR equipment.

FIG. 1 and FIG. 2 show the schematic configuration of an MHI device using the permanent magnet. In the figure, the MRI apparatus 1 has a pair of rectangular plate-shaped yokes 2, 2 facing each other with a space S into which a human body can be inserted, and the yokes 2, 2 are connected to four pillars 3 arranged at the four corners. The yoke 2.2 is connected and supported by the yoke 2.2, and permanent magnets 4 are arranged on the inner side of the facing surface of the yoke 2.2.

Here, the upper and lower permanent magnets 4 need to have a large area in order to generate a -like magnetic field over a wide range, and their combined weight is about 1 ton.
Since it is difficult to integrally form such a large and heavy permanent magnet, conventionally, a large number of segmented magnets are combined to form one. However, when such a permanent magnet consisting of a large number of small pieces is installed, a problem arises in that the magnetic field tends to be non-uniform due to variations in the magnetic force of each permanent magnet piece. Moreover, since a structure for fixing and holding each permanent magnet piece to the yoke 2 is required, the permanent magnet 4 is held by a magnetic pole plate 5 made of pure iron, for example. This magnetic pole plate 5 can absorb variations in the magnetic force of each magnet piece to make the magnetic field uniform, and the permanent magnet 4
can be held.

By the way, the above-mentioned magnetic pole plate 5 has a high magnetic permeability (p) that does not attenuate the magnetic force of the permanent magnet (in order to transmit it), and ■ a magnetic field line detection curve for sensitively detecting changes in the magnetic field. In order to ensure that the current changes clearly in the form of a pulse wave and that so-called blurring does not occur, characteristics such as high specific resistance are required.However, when using the conventional pure iron magnetic pole plates mentioned above, the magnetic permeability Although the resistivity is high at around 2000, the resistivity is low at 11X10-''Qcm, and as a result, changes in the magnetic field cannot be detected sensitively and sharp tomographic images cannot be obtained, which is an obstacle to the practical application of MHI. ing.

The inventors of the present invention have focused on the use of a magnetic composite material formed by molding magnetic iron powder with a resin binder as an alternative to the conventional pure iron magnetic pole plate. In other words,
It is considered that by appropriately selecting the mixing amount of the magnetic iron powder and resin binder of this composite material, it is possible to increase both the magnetic permeability and the specific resistance. When manufacturing this type of composite material, an injection molding method has conventionally been generally employed.

[Problem that the invention seeks to solve]

However, when obtaining the above-mentioned magnetic pole plate by the above-mentioned injection molding, the fluidity of the material becomes a primary problem.

That is, in order to satisfy the characteristics required of the above magnetic pole plate,
In the injection molding method, it is necessary to increase the content of magnetic iron powder, and therefore the content of resin binder must be reduced accordingly.As a result, the content of resin binder must be increased to ensure fluidity. Injection molding, which cannot be practically adopted as a manufacturing method for magnetic pole plates, requires the content of iron powder to be 60% or less. In addition, in the above injection molding, a liquid film is formed on the surface of the iron powder during mixing, which makes it difficult for the iron powder to bond with each other after compression molding, resulting in the problem of lower magnetic permeability. As it turns out, the injection molding method cannot be adopted from this point as well.

By the way, when the above-mentioned magnetic composite material is employed, for example, as a magnetic pole plate of an MRI apparatus, it is required to improve the uniformity of the magnetic field by increasing the size of the magnetic pole plate as much as possible. In other words, as mentioned above, there is a limit to increasing the uniformity of the magnetic field itself due to the structure in which many small-area permanent magnets are bonded together, so errors are absorbed by magnetic pole plates. This is because in this case, the fewer the number of magnetic pole plates bonded together, the more advantageous it becomes. Therefore, it is necessary to increase the size and area of the magnetic pole plate.

The present invention was made in view of the above-mentioned conventional situation, and it is possible to increase specific resistance while ensuring the magnetic permeability necessary to transmit the magnetic force of a permanent magnet without attenuating it, thereby contributing to the practical application of MHI. It is an object of the present invention to provide a method for manufacturing a magnetic composite material that is suitable as a method for manufacturing a magnetic pole plate.

Means for Solving Problem C] Therefore, the present invention has a volume ratio of 5 to the entire magnetic composite material.
Dry mix spherical magnetic powder with an average particle size of 5 to 95% and an average particle size of 50 to 300 μm and a resin binder, fill the mixed powder in a mold, and then cool it to produce a powder with a particle size of more than 2 t/cm2. Direct powder compaction with surface pressure is performed to compact the powder, and the magnetic permeability (μ
) is 50 or more, specific resistance is 0.001Ω1 or more, tensile strength is 2-/■ Fuji, and area is 250- or more. It's a method.

Here, the reasons for setting various conditions in the present invention will be explained.

(2) The magnetic powder is made into a spherical shape so that the magnetic field can easily flow in the direction of the magnetic field.

For example, when used as the above-mentioned magnetic pole plate, the external magnetic field acts perpendicularly to the magnetic pole plate due to its structure, so if flat magnetic powder is used, the magnetization direction of each magnetic powder will be different from the external magnetic field. They are perpendicular to each other, which increases the so-called demagnetizing field, but by using spherical magnetic powder,
This problem can be suppressed.

■ Also, the reason why the volume percent of the magnetic powder is set to 55 to 95 is that if it is less than 55%, the absolute amount of magnetic powder is insufficient and the desired magnetic permeability cannot be obtained, and if it exceeds 95%, it is almost This is because molding becomes difficult because it consists only of magnetic powder, and furthermore, the strength decreases and machining becomes difficult.

(2) The dry method was adopted for mixing the magnetic powder and the resin binder because it is possible to increase the amount of magnetic powder contained in the entire i-magnetic composite material and achieve the maximum volume ratio of 95%. Furthermore, when a wet mixing method is employed, a resin film is formed on the surface of the magnetic powder particles, making it difficult to bond the particles to each other and increasing the demagnetizing field. On the other hand, since the dry method does not use a liquid, the magnetic powders can be bonded together by compaction, and the necessary magnetic permeability can be ensured. Furthermore, by employing dry mixing, it is possible to improve the uniform dispersibility of the magnetic powder and the uniformity of the transmitted magnetic force, and it is also possible to employ direct powder compaction using a mold, which allows for mass production.

■ Also, the surface pressure of the mixed powder filled in the above mold is 2t/aj.
The purpose of direct powder molding in the above process is to obtain a large magnetic composite material plate with a size of 250 mm or more, and this allows for an increase in the size of the magnetic pole plate when it is used, for example, in the magnetic pole plate of the above-mentioned MRIvt. This is because it can meet the demand for larger area. In this case, specifically, it is possible to use a large press machine with a large compressive force.

■ Furthermore, the above magnetic permeability is 50 or more, and the specific resistance is 0°QQ.
The reason for specifying IQcm or more is to satisfy the necessary characteristics when employing the magnetic composite material of the present invention for the magnetic pole plate of the above-mentioned MHI device.

■Furthermore, the reason why the tensile strength is set to 2-/■ or more is to provide strength against a certain degree of machining. Strength can be ensured.

Note that it is preferable to employ an epoxy resin as the resin binder, since this can improve strength, bondability with magnetic powder, or magnetic properties. In addition, when applying the magnetic composite material of the present invention to magnetic pole plates placed at 1 meter intervals in MRI, for example, depending on the size of the device, a plurality of segmented magnetic pole plates may be formed by direct powder molding, and the circumferential surfaces of each magnetic pole plate may be brought into contact with each other. However, if a gap is created between these abutting parts, the magnetic field tends to become non-uniform.Therefore, the circumferential side and corner parts of each segment are cut or machined by cutting. It is desirable to perform processing and remove the R portion.

[Effect]

According to the method for manufacturing a magnetic composite material with excellent magnetic properties according to the present invention, after dry mixing spherical magnetic powder having a predetermined volume ratio and a resin binder, and filling the mixed powder into a mold, The magnetic composite material manufactured by the present invention has a magnetic permeability of 50 or more and a specific resistance of 0.001Ω because it is compacted by direct powder compaction in a cold state with a surface pressure of 2t/el1 or more.
1 or more, and both magnetic permeability and specific resistance can be improved. Accordingly, for example, MRIl! When used as a permanent magnetic pole plate, it does not attenuate the magnetic force of the permanent magnet (and changes in the magnetic field can be detected sensitively, allowing sharp tomographic images to be obtained).

Furthermore, the magnetic composite material of the present invention has a tensile strength of 2-/-
Because of the above, when holding a small piece of permanent magnet on a yoke, bolt holes can be formed in the composite material and the permanent magnet can be attached to the yoke via bolts.

Furthermore, since the magnetic powder and resin binder are dry mixed, the content of magnetic powder can be increased to 95%.
Moreover, since no liquid is used, iron powder can be bonded together by compacting. Furthermore, since a large plate-like body of 250 mm or more can be obtained, it is possible to meet the demands for increasing the size and area of the magnetic pole plate of the MRIll apparatus, and improving the uniformity of the magnetic field.

〔Example〕

Hereinafter, a method for manufacturing a magnetic composite material according to an embodiment of the present invention will be described.

First, spherical magnetic iron powder with an average particle size of 50 to 300 μ- and solid particulate epoxy resin powder are prepared, and the volume fraction of the magnetic iron powder is within the range of 55 to 95% of the entire magnetic composite material. Then, dry mix to form a mixed powder.

Next, fill the above mixed powder into a mold, cool it,
The powder is compacted by direct powder compaction with a surface pressure of 2 t/- or more using a large press machine. Here, the projected area of the upper rudder mold is 250
- If no droplets are used, machining costs will increase, which is uneconomical, and the number of joints between magnetic pole plates will increase, making it difficult to obtain a uniform magnetic field strength distribution.

Then, the compacted mixed powder was heated and cured at 200° C., thereby producing the magnetic composite material of this example.

The magnetic composite material manufactured according to this example has a magnetic permeability of 5
0 or more, specific resistance 0.0010m or more, tensile strength 2
kr/m" or more, and has a surface area of 250 mm or more. This plate is used, for example, as the polar plate 5 of the MRIvi device 1 shown in FIGS. 1 and 2. When applied to the yoke 2, a large number of permanent magnet pieces 4 are held by fixing it to the yoke 2 with bolts.

Incidentally, the permanent magnet piece 4 may of course be composed of a single piece having the necessary area.

By the way, when the MHI device 1 becomes larger, it is necessary to further increase the magnet area, and the number of pieces per stone increases accordingly. It is difficult to manufacture a single magnetic pole plate that supports such large-area (many) magnet pieces using the magnetic composite material of the present invention. In this case, as shown in Fig. 3, a plurality of flat plates 6 are manufactured, laid out in a flat shape, and the abutting surfaces of the flat plates 6 are machined to remove the rounded portions and eliminate gaps. do. In addition, each of these flat plates 66
In order to ensure ease of assembling the magnet pieces 4, a pure iron plate 7 with high magnetic permeability is interposed between the stones.In addition to the pure iron plate 7, a steel plate can also be used, but in this case, the plate thickness is low due to the low magnetic permeability. This will have to be adjusted.

As described above, according to the manufacturing method of this example, spherical magnetic iron powder and epoxy resin powder are dry mixed, the mixed powder is filled into a mold, and then the surface pressure is 2t/- in the cold. Since the powder was compacted by the above direct powder compaction, when it is adopted as the magnetic pole plate 5 of the above MR1M 1, since the 'jsm ratio is 50 or more, the magnetic force of the permanent magnet 4 is transmitted without being attenuated. Moreover, since it has a specific resistance of 0.001 Ω1 or more, changes in the magnetic field can be detected sensitively.
A clear tomographic image of the human body can be obtained.

In addition, since the tensile strength is 2 kg/m" or more, the above-mentioned machining is possible, and this can be bolted and fixed to the yoke, and the small piece of permanent magnet can be held sufficiently. As a result, the MRIvt It can greatly contribute to the practical application of

Furthermore, since it is possible to obtain a large magnetic pole plate of 250 mm or more, a large number of permanent magnet pieces 4 of the MHI device can be supported by one magnetic pole plate 5 or by a small number of magnetic pole plates 6. The number of joints between them is reduced, and the uniformity of the magnetic field can be improved accordingly.

Furthermore, since we adopted a dry method for mixing magnetic iron powder and epoxy resin powder, the content of the iron powder can be increased to 95%, and since no liquid is used, it is possible to bond the iron powder together. I can do it.

The table shows the results of experiments conducted to confirm the effects of the present invention.

In this experiment, the volume ratio of magnetic iron powder to the entire magnetic composite material was varied in the range of 50 to 95% in the manufacturing method of the above example, and the molding pressure by the mold was further increased from 2 to 8 t/min.
Eight magnetic pole plates were manufactured by changing the magnetic pole plates in the range of -, and the maximum magnetic permeability, specific resistance, tensile strength, and tightening of each magnetic pole plate were determined by measuring the torque. Note that the projected area of the mold employed in this experiment was 1800-.

As is clear from the table, in each case, the tensile strength was 4.2 to 7.5 b/m'' and the tightening torque was 2.
A satisfactory value of 4 to 4.5-.- is obtained.

However, when 50vo 1% of magnetic iron powder is contained (
111iI, outside the invention), the absolute amount of iron powder is small, so although the specific resistance is high at 10, the magnetic permeability is low at 30. In addition, when the forming pressure is less than 2t/- (column 5), the specific resistance is high at 10, but the magnetic permeability is low at 20. This is because the iron powder is bonded together due to the low forming pressure. This is thought to be due to the fact that the On the other hand, the νo1% of magnetic iron powder is 60 to 95%, and the molding pressure is 4t/-
In the above cases (columns 2 to 4, 6 to 8 @ invention examples), the magnetic permeability is 50 to 200, the specific resistance &! 1.0 =
It can be seen that a satisfactory TEL value of 5 x 10-" was obtained. As a result, by employing the magnetic composite material of this example, it is possible to secure a predetermined magnetic permeability and improve specific resistance. I understand.

In the above embodiments, the magnetic pole plate for an MHI device was used as an example, but the scope of application of the magnetic composite material of the present invention is not limited to this magnetic pole plate. Any magnetic member that requires this can be used.

〔Effect of the invention〕

As described above, according to the method for producing a magnetic composite material with excellent magnetic properties according to the present invention, the entire magnetic composite material has a
Contains ~95νOX% and has an average particle size of 50~300μ
Dry mix Sono's spherical magnetic powder and resin binder,
After filling the mixed powder into the mold, it is heated to 2t/cm in the cold.
Since the powder was compacted by direct powder compaction with a surface pressure of more than 2, the permeability of the magnetic composite material was 50 or more, and the specific resistance was 0.0.
It is possible to obtain a large magnetic composite material plate with a tensile strength of 0.01 Ωcm or more and a tensile strength of 2 kg/ms or more, and a large specific resistance while ensuring the necessary magnetic permeability. This has the effect of being able to accommodate larger sizes and larger areas.

[Brief explanation of drawings]

Figure 1 shows a typical M to which the magnetic composite material of the present invention is applied.
FIG. 2 is a front view of the HI device, FIG. 2 is a plan view thereof, and FIG. 3 is a sectional side view showing a modification of the magnetic pole plate.

Claims (4)

[Claims] (1) Volume ratio of the entire magnetic composite material is 55-95%
A spherical magnetic powder with an average particle diameter of 50 to 300 μm and a resin binder in the form of solid particles are dry-mixed, the mixed powder is filled into a mold, and then cooled to 2t/cm^2.
The product is compacted by direct powder compaction with a surface pressure exceeding
A method for producing a magnetic composite material with excellent magnetic properties, characterized by obtaining a large-sized magnetic composite material plate having a diameter of m^2 or more. (2) The method for producing a magnetic composite material with excellent magnetic properties according to claim 1, wherein the resin binder is an epoxy resin. (3) The method for producing a magnetic composite material with excellent magnetic properties as set forth in claim 1 or 2, wherein the magnetic powders of the magnetic composite material are substantially bonded to each other. (4) The above-mentioned magnetic composite material is formed into a flat plate constituting a magnetic pole plate for an MRI apparatus, and the contact surface of each flat plate is cut or cut so that there is no gap between the contact surfaces of adjacent flat plates. A method for manufacturing a magnetic composite material having excellent magnetic properties according to any one of claims 1 to 3, characterized in that the material is subjected to a machining process such as the following.