JP2001152211A - Method for producing flat metal powder for electromagnetic wave absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing metal powder low in an oxygen content at a low cost by which the working environment is satisfactorily retained, and the problem of waste treatment is reduced in a method for producing the flat powder of soft magnetic metal composing an electromagnetic wave absorber obtained by dispersing the powder of soft magnetic metal into a matrix of rubber or plastics and having a high aspect ratio by wet grinding treatment. SOLUTION: The powder of an Fe-Cr alloy containing, by weight, 5 to 25% Cr, and the balance substantial Fe is subjected to grinding treatment for a suitable time, ordinarily, for 3 to 10 hr with water instead of an organic solvent as a medium by using an attoritor. At the time of the treatment, a lubricant such as zinc stearate may be added thereto.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a flat metal powder which is used as a material for an electromagnetic wave absorber obtained by dispersing a soft magnetic metal powder in a rubber or plastic matrix.

[0002]

2. Description of the Related Art In order to prevent electromagnetic waves generated from various electronic devices from absorbing and leaking to the outside, to prevent electromagnetic waves from entering the device from obstructing operation, and to prevent various elements inside the device from being operated. Materials that absorb electromagnetic waves are used to shield them from interfering with each other and hindering each other. Most of them are obtained by dispersing a soft magnetic metal powder in a rubber or plastic matrix, and usually, a sheet is preferred because it is convenient for processing and use. There are also products made according to the requirements. The applicant has also developed various grades of the electromagnetic wave absorbing sheet with the name "DPR" and has put it to practical use.

[0003] As a metal of the soft magnetic powder constituting this kind of electromagnetic wave absorber, Fe-Si represented by Sendust is used.
-Al alloy, Fe-Cr-Si alloy, F
The e-Cr-Al alloy has been selected. Chlorinated polyethylene is preferably used as the matrix material.

As is well known, an electromagnetic wave absorbing sheet has frequency characteristics. The main factors that determine the peak frequency of absorption are the particle size and shape of the soft magnetic metal powder, the amount of dispersion in the matrix and the thickness of the sheet. Regarding the shape of the powder, for a very high frequency region, that is, an electromagnetic wave having a frequency of several GHz or more, a powder having a nearly spherical shape gives a high-performance electromagnetic wave absorbing sheet. For a frequency range of the order, a flat flake-like (5 or more, preferably 7 or more in aspect ratio) powder is suitable.

[0005] In order to obtain a flat soft magnetic metal powder, the powder is usually ground by using an attritor or the like to grind it. Performing this operation in a wet type rather than a dry type, that is, in a liquid medium has been achieved for the purpose of flattening. It is convenient if the liquid serving as the medium is sufficient with water, but there is a problem that the metal powder is oxidized by oxygen dissolved in the water during the grinding. The soft magnetic metal powder is desired to be dispersed in the matrix material at a high rate as much as possible, but if the metal powder is oxidized, the metal component that can be dispersed in the matrix decreases, The performance of the resulting electromagnetic wave absorbing sheet will be poor. In addition, when milling is performed in water, flattening is also performed, but at the same time, pulverization proceeds,
The aspect ratio of the product powder usually did not increase very much.

For this reason, conventionally, when a powder of a soft magnetic metal is to be crushed to obtain a flat material, the crushing is carried out not by water but by an organic solvent, for example, a naphthenic hydrocarbon. I was forced to do that. However, when an organic solvent is used, first, there is a problem that it is not easy to maintain a good working environment by preventing odors and effects on the skin. Even if it can be solved, the problem of waste oil treatment remains, and in any case, the production cost is unavoidable.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to use a soft magnetic metal powder constituting an electromagnetic wave absorber, which is sufficiently flat, using water as a medium instead of a problematic organic solvent, Therefore, the present invention provides a method for producing a flat metal powder capable of supplying a flat powder having a low aspect ratio, low oxidation, low oxidation of a metal powder, and a high aspect ratio, while maintaining a good working environment and reducing the problem of waste disposal. Is to do.

[0008]

According to the present invention, there is provided a method of manufacturing a flat metal powder for achieving the above object, wherein the flat metal powder is dispersed in a rubber or plastic matrix and used in an electromagnetic wave absorber. A method for producing a fine metal powder, characterized in that a powder of a Fe-Cr alloy containing 5 to 25% by weight of Cr and the balance substantially consisting of Fe is ground in water.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As a means for the grinding treatment, a conventional attritor may be used. The processing conditions vary depending on the characteristics of the equipment used, the alloy composition of the metal to be processed, the particle size of the powder, and the like, and a combination of the number of rotations and the processing time that achieves a desired flatness may be selected. However,
If the treatment time is insufficient, the flattening becomes insufficient. However, even if the milling is performed for an excessively long time, fineness occurs after the flattening and the aspect ratio is rather lowered.

In the milling treatment by an attritor, a higher fatty acid or a metal salt thereof, for example, stearic acid or a salt of zinc or calcium, is added as a lubricant in an amount of 2% by weight or less, usually about 1% by weight of the raw metal powder. Processing may be performed. Thereby, a high aspect ratio can be realized in the product powder.

In the present invention, as a soft magnetic alloy powder to be subjected to the grinding treatment, F containing 5 to 25% by weight of Cr is used.
The reason for selecting the e-Cr alloy powder is that Cr is 5% by weight.
If it is contained as described above, as shown in Examples described below,
This is because even if the grinding treatment is performed in water, the increase in the oxygen amount due to the flattening is not much different from the case where the grinding treatment is performed in an organic solvent. When the upper limit of 25% by weight of Cr exceeds that,
This is because the magnetic properties of the alloy are not suitable as a material for the electromagnetic wave absorber. The reason that the balance was substantially composed of Fe and that the composition containing no other components was selected was selected from those commonly used as the soft magnetic alloys described above, for example, Fe-Si-Al alloy, Fe-Cr-Si alloy, This is because a material containing Si or Al, such as a -Cr-Al alloy, has low ductility and is liable to be pulverized due to the flattening described above, and is not suitable for grinding in water. In contrast, Fe-Cr alloy has high ductility,
There is almost no concern about pulverization due to grinding in water.

[0012]

[Example 1 and Comparative Example 1] Fe-12Cr
The molten alloy was sprayed with water to produce a powder having an average particle size D50 = 8 μm. This metal powder was ground by an attritor under the following conditions to flatten it. Ball used: SUS440C 18 kg Metal powder: 1.8 kg Medium: Vacuum degassed water (Example 1) or naphthenic hydrocarbon "naphthesol" manufactured by Nippon Petrochemical (Comparative Example 1) 20 liters Both rotation speed: 250 rpm Processing time : Up to 15 hours Lubricant: None (Average particle size "D50" is the particle size at which the cumulative weight becomes 50% as measured by a laser-type particle size distribution meter.) Separated and dried in a vacuum, the amount of oxygen and the average particle size D50 were measured. FIG. 1 shows the increasing tendency of the oxygen amount up to the treatment time of 9 hours, and FIG. 2 shows the change of D50 up to 15 hours.

From the results shown in FIG. 1, it can be seen that even when flattening is performed using water as a medium, there is no significant difference between the case where an organic solvent is used as a medium and the amount of oxygen contained in the metal powder. The data in FIG. 2 shows that the D50 increases or flattened rapidly until the treatment time is 5 to 6 hours, then keeps a substantially constant value, and gradually decreases after about 10 hours. It shows that there is almost no difference between the case where the medium is water and the case where the medium is an organic solvent.

[Examples 2 to 4 and Comparative Examples 2 and 3] Fe-12Cr alloy (Example 2), Fe-17Cr alloy (Example 3), Fe-17Cr alloy produced by spraying molten metal
-20Cr alloy (Example 4) and Fe-9Si-5A
1 alloy (Comparative Example 2, Sendust alloy) and Fe powder (Comparative Example 3) produced by the carbonyl method were used as raw materials, and each powder was placed in the small attritor used in Example 1, and water was used as a medium for 6 hours. Was carried out. Only in Example 2, 1% by weight of metal powder zinc stearate was added as a lubricant. For each powder, the oxygen content before and after the treatment, the average particle size D50, the average thickness of the powder, and the aspect ratio were measured. The results are shown in Table 1 below.

[0015] Each of the obtained powders and the powder of Example 1 (having a grinding treatment for 6 hours) are kneaded with chlorinated polyethylene,
An electromagnetic wave absorbing sheet containing 65% by volume of soft magnetic metal powder was manufactured. For this sheet, the AC magnetic permeability μ ′ at 50 MHz was measured. Table 1 shows the results.
Listed in

Table 1 Example 1 Example 2 Example 3 Example 4 Comparative Example 2 Comparative Example 3 Soft magnetic metal Fe-12Cr Fe-12Cr Fe-17Cr Fe-20Cr Fe-9Si-5Al Fe Powder manufacturing method Water spray Water Spray Water spray Water spray Water spray Carbonyl lubricant -1%----Oxygen content (% by weight) Raw powder 0.25 0.25 0.27 0.32 0.21 0.25 Treated powder 0.71 0. 69 0.75 0.80 3.1 2.9 Average particle size D50 (μm) Raw powder 8 8 12 13 11 10 Processed powder 14 18 20 22 12 16 Average thickness (μm) 2.0 2.2 2. 8 3.0 2.9 2.4 Aspect ratio 7.0 8.2 7.1 7.3 4.1 6.7 Permeability μ '20 21 21 20 11 13 The average thickness is from the microphotograph of the powder. It was measured and averaged.

The aspect ratio is a value obtained by dividing the average particle diameter D50 by the above average thickness.

The sendust alloy of Comparative Example 2 has a low effect of flattening because the amount of oxygen is significantly increased by the treatment and the aspect ratio is low. Pure Fe of Comparative Example 3 is flattened because it is soft, but the amount of oxygen is also greatly increased, and as a result, the magnetic permeability μ ′ of the electromagnetic wave absorbing sheet remains at a low value. On the other hand, the soft magnetic metal powders manufactured by the method of the present invention all have a low oxygen content and a high aspect ratio, and the electromagnetic wave absorbing sheet has a high magnetic permeability μ ′.

[0019]

When a flat soft magnetic metal powder for an electromagnetic wave absorber is produced by the method of the present invention, water is used as a grinding medium instead of an organic solvent, so that a good working environment is maintained. It is easy to carry out the grinding treatment without any problem of waste treatment. In the flattened powder, the content of oxygen is not much different from the case where an organic solvent is used, and it is possible to avoid the pulverization that is likely to occur as the flattening progresses. In this way, it has become possible to produce a flat soft magnetic metal powder providing an absorber having a high electromagnetic wave absorbing ability, while maintaining favorable conditions in terms of cost and environmental impact.

[Brief description of the drawings]

FIG. 1 is a graph showing data of Example 1 and Comparative Example 1 of the present invention, showing a tendency of an increase in the oxygen content of a metal powder with the lapse of time of a grinding treatment.

FIG. 2 is also data of Example 1 and Comparative Example 1 of the present invention, showing the average particle diameter D with the lapse of time of the grinding treatment.
50 is a graph showing 50 changes.

Claims (3)

[Claims] 1. A method for producing a flat metal powder used for an electromagnetic wave absorber obtained by dispersing a soft magnetic metal powder in a rubber or plastic matrix, comprising 5 to 25% by weight of Cr, Fe whose balance is substantially Fe
-A method for producing a flat metal powder, wherein a powder of a Cr alloy is ground in water. 2. The method according to claim 1, wherein an attritor is used as a means for the grinding treatment. [3] In the attrition processing by the attritor,
3. The method according to claim 2, wherein the higher fatty acid or its metal salt is added as a lubricant in an amount of not more than 2% by weight of the raw metal powder.