JPH10233309A - Flat ferrite powder and their manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the magnetic field shielding ability of flat ferrite particles obtained by pulverizing a soft magnetic ferrite formed by a casting method in a high-frequency region by specifying the lengths of the powder in the longitudinal direction and aspect ratios of the particles. SOLUTION: Flat ferrite powder 2 is in a long foil form and has an aspect ratio P of P=b/a (where, a and b respectively represent the length of the particle in the longitudinal direction and the largest width of the particle). The length in the longitudinal direction and aspect ratio of the powder 2 are respectively adjusted to 1-100μm and 5-100. The flat ferrite powder 2 are obtained by pulverizing a soft magnetic ferrite ingot obtained by a prescribed casting method. The casting method includes a melting process for obtaining a molten ferrite and a casting process for obtaining the ferrite ingot by casting the molten ferrite. The raw material of the soft magnetic ferrite includes the oxide powder of Zn, Mn, Ni, Cu, Fe, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flat ferrite powder and a method for producing the same, and more particularly, to a flat ferrite powder capable of improving the magnetic field shielding characteristics of a sheet-shaped magnetic field shielding material and a method for easily producing the same. About.

[0002]

2. Description of the Related Art As the performance and size of electronic and communication equipment have become higher and smaller, parts and devices used in the equipment have been affected by external unnecessary magnetic fields or mutually generated by magnetic fields generated by each other. It may cause inconvenience such as interference. Therefore, in order to prevent these inconveniences, a magnetic field shielding material for protecting the components and devices from an external magnetic field is used.

The magnetic field shield material 1 is made of a material having a high magnetic permeability (soft magnetic material) itself or a composite material containing a material having a high magnetic permeability, and as shown in FIG. That can be concentrated,
Location to be protected from magnetic field (right side of shield material 1 in Fig. 1)
Does not reach the magnetic flux F, and serves to protect the location from an external magnetic field. In this way, the magnetic field shielding material 1 shields the magnetic field by not concentrating the magnetic flux F but by continuously concentrating the magnetic flux F inside the magnetic field shielding material. For this reason, the higher the magnetic permeability is, the better the ability to continuously concentrate the magnetic flux inside (hereinafter referred to as “easy flow of the magnetic flux”) is the ability to shield the magnetic field of the magnetic field shielding material (hereinafter referred to as the magnetic field shielding ability). ) Will improve.

As one of such magnetic field shielding materials,
2. Description of the Related Art A sheet-shaped magnetic field shielding material having flexibility (hereinafter, simply referred to as a “magnetic field shielding sheet”) is known. This magnetic field shielding sheet forms a resin composition by dispersing powder of a metal material having high magnetic permeability in, for example, a synthetic resin such as a thermoplastic resin or a synthetic rubber, and forms the resin composition into a sheet shape. It is obtained by doing. Examples of the metal material powder include permalloy-based metal material powder. Since this magnetic field shielding sheet has flexibility,
It can be arranged according to the shape of the component to be protected from the magnetic field. Therefore, the space occupied by the magnetic field shielding sheet in electronic and communication equipment can be reduced. Therefore, the sheet can shield a magnetic field without hindering downsizing of electronic and communication devices.

Here, in the magnetic field shielding sheet,
It is known that an elongated flat metal material powder oriented in a direction along a sheet surface is more excellent in magnetic field shielding ability than a spherical metal material powder uniformly dispersed. This is because the magnetic flux flows more easily in the sheet when the elongated flat metal material powder is oriented than when the spherical metal material powder is dispersed. For this reason, even if the content is the same, the magnetic field shielding sheet containing flat powder has better magnetic field shielding ability than the magnetic field shielding sheet containing spherical powder.

[0006]

Recently, as the operating frequency of electronic and communication equipment has shifted to a high frequency range, the magnetic field shielding material used therein also has a high magnetic field shielding ability in a high frequency range. There is a need to provide shielding material. For example, as an index indicating the degree of the magnetic field shielding ability, the following equation is used: S = ((H ₀ −H ₁ ) / H ₀ ) × 100 (1) (where H ₀ is a predetermined area formed of a magnetic field shielding material) The intensity of the magnetic field in the region in an unenclosed state, and H ₁ represents the intensity of the magnetic field in the region in a state in which the region is surrounded by a magnetic field shielding material.) %), A magnetic field shielding material having an attenuation rate of a magnetic field of 1000 MHz of 30% or more is desired.

Here, the metal material contained in the magnetic field shielding sheet exhibits a high magnetic permeability in a low frequency range of less than 1000 MHz, and thus exhibits a high magnetic field shielding ability against a low frequency magnetic field. However, in a high-frequency range of 1000 MHz or more, the value of the magnetic permeability sharply decreases. Therefore, for a high-frequency magnetic field, there is an inconvenience that the magnetic field attenuation rate decreases and the magnetic field shielding ability decreases. Here, for example, the attenuation rate of the magnetic field of 1000 MHz of the magnetic field shielding sheet containing permalloy powder is about 5%.

The present inventors have focused on soft magnetic ferrite having a high magnetic permeability in a high frequency range as a powder to be dispersed in a synthetic resin in order to produce a magnetic field shielding sheet capable of shielding a magnetic field in the high frequency range. Then, the present inventors have attempted to improve the magnetic field shielding ability by making the shape of the soft magnetic ferrite powder flat and orienting it in the direction along the sheet surface.

As described above, in order to employ soft magnetic ferrite, the present inventors have studied various methods for producing soft magnetic ferrite powder. Usually, the powder of soft magnetic ferrite is obtained by manufacturing a block of soft magnetic ferrite and pulverizing the block. Here, as a method of manufacturing a block of soft magnetic ferrite, a sintering method and a casting method are known.

First, in the sintering method, several kinds of predetermined oxide powders as raw materials of soft magnetic ferrite are produced by a dry method, a coprecipitation method, a spray pyrolysis method or the like. Then, the oxide powders are mixed at a predetermined ratio to prepare a mixture,
Then, the mixture is shaped into a desired shape, and then subjected to press sintering or HIP processing to obtain a sintered body (block).

The sintered body obtained as described above is pulverized by a pulverizer to obtain soft magnetic ferrite powder. However, in the sintering method, since the ferrite crystal grains are sphericalized by heating during sintering, the obtained powder is spherical. That is, it is difficult to obtain a flat ferrite powder from the block obtained by the sintering method. Therefore, it is difficult to orient the ferrite powder manufactured by employing the sintering method in the magnetic field shielding sheet. Therefore, the sintering method is not suitable for the method of producing a block for obtaining the flat ferrite powder contemplated in the present invention.

When a magnetic field shielding sheet is manufactured using a spherical soft magnetic ferrite powder obtained by employing the above-described sintering method, the attenuation rate S of a 1000 MHz magnetic field of the sheet is 7%. there were. From these results, the magnetic field shielding sheet containing ferrite powder obtained by adopting the sintering method is superior in the magnetic field shielding ability in a high frequency range as compared with the magnetic field shielding sheet containing metal material type powder. It can be said that it is insufficient as a magnetic field shielding sheet that meets the above demand.

On the other hand, in the casting method, a predetermined oxide powder as a raw material of soft magnetic ferrite is melted, and the obtained molten metal is cast into a mold and cooled to obtain an ingot (block). The ingot of soft magnetic ferrite thus obtained is pulverized by a pulverizer to obtain a soft magnetic ferrite powder. At this time, the present inventors have found that the aspect ratio of the obtained powder changes when the cooling condition of the molten metal is appropriately selected. That is, it has been found that if a casting method is employed as a method of manufacturing a ferrite block, a flat ferrite powder having a desired shape can be arbitrarily manufactured.

The present invention solves the inconvenience of employing the above-described sintering method by employing a casting method as a method of manufacturing a soft magnetic ferrite block based on the above findings. It is an object of the present invention to provide a flat ferrite powder capable of improving the ability of the sheet to shield a magnetic field in a high-frequency range of 1000 MHz or more when it is contained, and a method for producing the same.

[0015]

In order to meet the above-mentioned demands, the present inventor has set forth a relationship between the shape of ferrite powder and a magnetic field shielding ability and a casting process for obtaining a ferrite powder having a suitable shape excellent in magnetic field shielding ability. As a result of a detailed study of the conditions, the inventors have developed the flat ferrite powder of the present invention and a method for producing the same.

That is, the flat ferrite powder of the present invention is a powder obtained by pulverizing soft magnetic ferrite produced by a casting method, and has a longitudinal length of 1 to 100 μm.
m, the aspect ratio is 5 to 100, and the method of manufacturing the soft magnetic ferrite is carried out at a temperature not lower than the melting point of the raw material and not higher than 1750 ° C. in an atmosphere having an oxygen partial pressure of 80% or higher. After the molten metal obtained in the melting step of melting and the melting step is cast into a mold preheated to a temperature in the range of 1000 to 1500 ° C. in an atmosphere having an oxygen partial pressure of 20% or more, 1 to 100 ° C./hr A cooling step of cooling to at least a temperature of 700 ° C., and then cooling to room temperature with an atmosphere of 100% Ar to produce a soft magnetic ferrite ingot, and crushing the ingot obtained by the casting step. And a pulverizing step of pulverizing with.

In the present invention, when an ingot of soft magnetic ferrite is produced under the above conditions, the obtained ingot of ferrite becomes very brittle. Therefore, the ingot can be easily pulverized. In addition, each crystal grain constituting the ferrite ingot obtained under the above conditions is in a state where cleavage fracture easily occurs on a specific crystal plane. Therefore, when stress is applied to the crystal grains during the pulverization process, cleavage fracture is likely to occur at the specific crystal plane, and a crack propagates from the cleavage plane generated at that time, so that fracture in a specific direction increases. . For this reason, in the ferrite ingot according to the present invention, the obtained powder has a flat shape due to the relationship of the orientation of the crystal plane in which the cleavage fracture is likely to occur.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The flat ferrite powder of the present invention comprises:
It is a powder obtained by pulverizing an ingot of soft magnetic ferrite. As the soft magnetic ferrite, those having a magnetic permeability of 10 or more at 1000 MHz are preferable in order to exhibit a magnetic field shielding effect in a high frequency range.

As the soft magnetic ferrite as described above, for example, MO.Fe ₂ O ₃ (where M is Zn, M
and at least one of n, Ni, Cu, and Fe). Here, for example, when a Mn—Zn ferrite is adopted as the flat ferrite of the present invention, the ferrite is made of ZnO, Mn.
O, Fe ₂ O ₃ , and the composition ratio is ZnO: 5 to 3
0 mol%, MnO: 20 to 60 mol%, Fe ₂ O _3: 40
It is preferably about 70 mol%. If the composition ratio of each of these components is out of the above range, both the magnetocrystalline anisotropic energy constant and the magnetostriction greatly deviate from zero, and a high magnetic permeability cannot be obtained.

FIG. 2 schematically shows the shape of the flat ferrite powder 2 according to the present invention. That is, it is a thin piece having a long shape, and in the present invention, this shape is flat. Here, the aspect ratio P in the present invention is represented by the following formula P = b / a, where a is the longest width in a cross section orthogonal to the longitudinal direction and b is the length in the longitudinal direction. Value.

In the flat ferrite powder of the present invention,
The length in the longitudinal direction is 1 to 100 μm and the aspect ratio is 5
To 100. Here, the length in the longitudinal direction is 100 μm
When it exceeds, the size of the powder becomes too large, and it becomes difficult to knead the powder when the powder is charged into the synthetic resin. Moreover, the powder is less likely to be oriented in the direction along the sheet surface of the magnetic field shielding sheet. Conversely, the longitudinal dimension of the powder is 1μ
If it is less than m, the powder becomes too fine, and in this case, the powder is difficult to be oriented in the direction along the sheet surface of the magnetic shielding sheet. If the aspect ratio is less than 5,
When the powder shape approaches a sphere and is dispersed in the synthetic resin, and the synthetic resin is extruded into a sheet, the degree of orientation of the powder is reduced. For this reason, in the obtained magnetic field shielding sheet, the magnetic flux does not easily flow, and the ability to shield the magnetic field is reduced. Conversely, if the aspect ratio exceeds 100, kneading becomes difficult. Preferably, the length in the longitudinal direction is 1 to 100 μm, and the aspect ratio is 10 to 50.

The flat ferrite powder of the present invention is produced by crushing a soft magnetic ferrite ingot obtained by a predetermined casting method in a crushing step. The casting method includes a melting step of obtaining a molten ferrite, and a casting step of casting the molten metal to obtain an ingot of ferrite. In the melting step, first, a predetermined amount of a predetermined oxide powder as a raw material of the soft magnetic ferrite is weighed, and these are mixed to form a mixed powder. Here, as a raw material of the soft magnetic ferrite, for example, an oxide powder of Zn, Mn, Ni, Cu, Fe or the like can be mentioned.

Next, the mixed powder is set in a melting chamber of a melting apparatus, and heated to a temperature in the range of from the melting point of the mixed powder to 1750 ° C. to melt the whole and obtain a molten ferrite. At that time, the atmosphere in the melting chamber was 80% oxygen partial pressure.
Above. Here, if the melting temperature exceeds 1750 ° C., the magnetic permeability of the obtained ferrite in a high frequency range becomes low and the target value cannot be realized, so the melting temperature is set to 1750 ° C. or less. Further, when the oxygen partial pressure in the melting chamber is less than 80%, it becomes difficult for the ferrite to have a spinel-type crystal structure, and it is difficult to obtain high magnetic permeability. The atmosphere in the melting chamber may be composed of only oxygen gas, or may be composed of a mixture with an inert gas such as nitrogen gas or Ar. The atmosphere may be in a pressurized state up to about 1000 atm, or may be at normal pressure.

Next, in the casting step, the molten metal obtained in the melting step is cast into a mold in a casting chamber. At this time,
The casting chamber is set to an atmosphere with an oxygen partial pressure of 20% or more, and the mold is preheated to a temperature in the range of 1000 to 1500 ° C.,
It is necessary to cast the molten metal here.

Here, the oxygen partial pressure of the atmosphere in the casting chamber is 20
%, It becomes difficult to obtain a spinel-type crystal structure in the obtained ferrite, and it is difficult to obtain a high magnetic permeability. Preferably, the oxygen partial pressure in the casting chamber is at least 50%. The preheating temperature of the mold is 1500 ° C.
If higher, in the resulting ingot of ferrite,
Since the bonding of crystals becomes strong, it becomes difficult to grind. In addition, the value of the magnetic permeability of the obtained ferrite powder is saturated. Conversely, if the preheating temperature of the mold is lower than 1000 ° C., it is difficult to increase the magnetic permeability of the obtained ferrite powder, so the lower limit of the preheating temperature of the mold is set to 1000 ° C.

The mold used at this time is preferably a material having low wettability with the molten metal and relatively good thermal conductivity. For example, a crucible made of Pt-Rh, a chromite sand mold, or the like is used. . Next, the molten metal is cooled and solidified. At this time, the cooling rate is 1 to 100 ° C /
Set to hr.

If the cooling rate is less than 1 ° C./hr, the crystal grains become coarse and the bonds between the crystal grains become strong, making it difficult to pulverize the ingot. For this reason, the size of the obtained powder becomes large, and it becomes difficult to obtain a flat ferrite powder having a size (longitudinal dimension 1 to 100 μm, aspect ratio 5 to 100) specified in the present invention. Conversely, if the cooling rate exceeds 100 ° C./hr, the spinel-type crystal structure is difficult to be locally formed, and it becomes difficult to secure a predetermined magnetic permeability. Therefore, the upper limit of the cooling rate is set to 100 ° C./hr. Stipulate.

After cooling to at least 700 ° C. at the above-mentioned cooling rate, Ar gas is introduced into the casting chamber and Ar 10
The atmosphere is 0%, and the cooling condition does not need to be limited at all, and may be arbitrarily cooled to room temperature. Next, the ferrite ingot obtained as described above is pulverized by pulverizing means. Here, the crushing means is not particularly limited, and a crusher such as a commonly used ball mill or hammer crusher is used. At this time, the longitudinal dimension of the obtained powder is 1 to 1
The ingot is pulverized so as to have a thickness of 00 μm and an aspect ratio of 5 to 100.

Here, depending on the size of the ingot to be pulverized, for example, a length of 20 mm and a width of 2 mm cast under the above conditions are used.
When the ingot having a height of 0 mm and a height of 15 mm is pulverized, if the pulverization time is less than 1000 seconds, the lengthwise dimension of the obtained powder is 100 μm or more and the aspect ratio is less than 5, and
If it exceeds 000 seconds, the dimension in the longitudinal direction is less than 100 μm and the aspect ratio exceeds 100. Therefore, in this case, by setting the pulverization time to 1000 to 10000 seconds, the longitudinal dimension is 1 to 100 μm, and the aspect ratio is 5 to 5.
100 flat ferrite powders are obtained.

The flat ferrite powder obtained as described above is used by being dispersed in a magnetic field shielding sheet. The sheet using the flat ferrite powder is manufactured, for example, as follows. That is, a predetermined synthetic resin is prepared, and flat ferrite powder is charged into the resin. Then, the synthetic resin is kneaded to obtain a synthetic resin composition in which the flat ferrite powder is dispersed. Thereafter, the synthetic resin composition is extruded into a sheet by an extruder to obtain a magnetic field shielding sheet. At this time, the content of the flat ferrite powder in the synthetic resin is 30
If the content is less than 90% by volume, the flexibility of the sheet is impaired. For this reason, flat ferrite powder is
It is preferable to contain 30 to 90% by volume of the synthetic resin. More preferably, the content of the flat ferrite powder with respect to the synthetic resin is 30 to 70% by volume. Also,
In the magnetic field shielding sheet, when the flat ferrite powder having the length and the aspect ratio in the above-described ranges is used, the flat ferrite powder is oriented in a favorable state along the sheet surface. In this way, by orienting the flat ferrite powder in the direction along the sheet surface, the magnetic flux that has entered from one side of the sheet tends to concentrate and flow in the sheet, and transmission to the other side of the sheet is suppressed. . Therefore, the magnetic field shielding ability is improved.

The synthetic resin is not particularly limited as long as it can be formed into a flexible sheet, and examples thereof include a thermoplastic resin such as polyethylene and a synthetic rubber such as chloroprene rubber. .

[0032]

【Example】

Example 1 Each powder of ZnO, MnO, and Fe ₂ O ₃ was 1% by mole%.
2: 34.5: 53.5, and the mixed powder was placed in a Pt-Rh crucible. This crucible is set in the melting chamber of the melting apparatus, and the atmosphere in the melting chamber is set to an oxygen partial pressure of 1.
The atmosphere is set to about 1 atm at 00%, and from room temperature to 1700
After heating to 4 ° C. for 4 hours, the mixture was kept at that temperature for about 30 minutes to completely dissolve the mixed powder and obtain a molten ferrite.

On the other hand, a mold (manufactured by Pt-Rh) was set in the casting chamber, and the mold was heated from room temperature to 1500 ° C. over 3.5 hours in an oxygen-Ar mixed gas atmosphere having an oxygen partial pressure of 50%. It was kept at that temperature. The molten metal at the above temperature was cast into the mold in this state, and cooled at a cooling rate of 10 ° C./hr. After that, when the atmosphere is cooled to at least 700 ° C., the atmosphere is switched to Ar 100%.
Ingot of soft magnetic ferrite was produced.

The composition of the obtained ingot was 11 mol% of ZnO, 35 mol% of MnO, and 55 mol% of Fe ₂ O ₃ , and the crystal grains had a spinel type crystal structure. In addition, the said crystal structure measured the X-ray diffraction pattern of the obtained ferrite crystal using the X-ray diffractometer, and identified it from this diffraction pattern. Next, the obtained ingot was put into a ball mill and pulverized for 3600 seconds. As a result of microscopic observation of the shape of the obtained powder, the length in the longitudinal direction was 10 μm on average,
The aspect ratio was 10 on average. The magnetic permeability of the powder at 1000 MHz was 20.

Here, the magnetic permeability means that a standard sample is prepared by pressing the obtained ferrite powder into a cylindrical shape having an outer diameter of 5 mm, an inner diameter of 3 mm, and a height of 5 mm. Hewlett Packard 4
291A) refers to the magnetic permeability measured by setting it in the measuring section. Next, the obtained flat ferrite powder was added to the raw material of the chloroprene-based synthetic rubber melted by heating so as to be 60% by volume, and the synthetic rubber was kneaded. In this way, a chloroprene-based synthetic rubber composition in which the flat ferrite powder is uniformly dispersed is obtained, and then, the chloroprene-based synthetic rubber composition is extruded by a molding machine.
Extruded, flat ferrite powder oriented in the direction along the sheet surface, thickness 5 mm, length 200 mm, width 20
It was formed into a 0 mm sheet.

With respect to the obtained magnetic field shielding sheet, the attenuation rate with respect to the magnetic field was measured by the following procedure. That is, first, the Hall element magnetic sensor was arranged at the center of a plastic cylinder having a diameter of 50 mm and a height of 150 mm. And
An external magnetic field of 0.01 G was applied to the cylinder at 0.1 MHz, and the strength of the magnetic field at the center of the cylinder (magnetic field strength H ₀ without being surrounded by the magnetic field shield material) was measured. Thereafter, the sheet was wound around the outer periphery of the cylinder, and in this state, the strength of the magnetic field at the center of the cylinder (magnetic field strength H _{1 in} a state surrounded by the magnetic field shield material) was measured. Then, the result was substituted into the equation (1) to determine the attenuation rate of the magnetic field. The above operation was repeated while changing the intensity of the external magnetic field, and the result is shown in FIG. 3 as the relationship between the intensity of the external magnetic field and the attenuation factor. The higher the attenuation rate, the better the magnetic field shielding ability. Example 2 A magnetic field shielding sheet was manufactured in the same manner as in Example 1, except that the pulverizing time by a ball mill was 2000 seconds and the aspect ratio of the powder was 5. Next, the relationship between the strength of the external magnetic field and the attenuation rate was determined in the same manner as in Example 1 using the obtained sheet. The result is shown in FIG. Comparative Example 1 A commercially available ferrite having the same composition produced by HIP was pulverized,
A magnetic field shielding sheet was manufactured in the same manner as in Example 1 except that a spherical powder having a particle size of 10 μm was used, and the relationship between the intensity of the external magnetic field and the attenuation rate was determined using the sheet. The result is shown in FIG.

From the results shown in FIG. 3, it can be seen that the sheets of Examples 1 and 2 containing the flat ferrite powder have a lower attenuation rate against the external magnetic field than the sheet of Comparative Example 1 containing the spherical ferrite powder. It is clear that the magnetic field shielding ability is excellent. Further, using the sheets of Examples 1 and 2 and Comparative Example 1, the frequency of the external magnetic field was set to 0.1 MHz and 100 MHz.
z, 1000MHz, 2000MHz, 3000MHz
And the attenuation rate of the magnetic field when it was changed. The results are shown in FIG. For comparison, a magnetic field shielding sheet manufactured in the same manner as in Example 1 except that a powder of an Fe-Ni-based alloy (permalloy) containing 80% of Ni was used instead of the flat ferrite powder. Also, the attenuation rate of the magnetic field when the frequency was changed as described above was obtained, and the result is also shown in FIG.

Here, the attenuation rate of the magnetic field at a frequency of 0.1 MHz was measured by the same method as described in the first embodiment. Then, at a frequency of 100 MHz or more, the attenuation rate of the magnetic field was measured by the following method. That is, first, the obtained magnetic field shielding sheet is cut,
A measurement sample having a size of 10 mm, a width of 10 mm and a thickness of 5 mm is prepared.

A probe having a pair of coil antennas facing each other (TR1730 manufactured by Advantest)
Prepare 3). In the probe, when a current is passed through one coil antenna at a desired frequency to generate a magnetic field, an electromotive force is generated in the facing coil antenna by the magnetic field. At this time, the electromotive force generated in the other coil antenna is proportional to the strength of the magnetic field in the other coil antenna. Therefore, by measuring the electromotive force of the other coil antenna, the strength of the magnetic field at that portion can be known.

Here, in the probe, first, 2
An electromotive force in a state where there is nothing between the two coil antennas is obtained in advance. The electromotive force is simply referred to as the strength H ₀ of the magnetic field in the predetermined area in the equation (1) when the predetermined area is not surrounded by the magnetic field shield material. Next, the sample is sandwiched between the probes so that the sample exists between the two coil antennas. In the probe, the sample is sandwiched in the thickness direction. In this state, a magnetic field of a desired frequency is generated by one coil antenna, and the electromotive force of the other coil antenna at that time is measured. Its electromotive force in a simple manner, the intensity H ₁ of the magnetic field of the region in a state surrounded by a magnetic field shield material a predetermined area in the equation (1).

By substituting H ₀ and H ₁ obtained as described above into equation (1), the attenuation rate of the magnetic field at a predetermined frequency is obtained. From the results shown in FIG. 4, it can be seen that the sheets of Examples 1 and 2 maintain a relatively high attenuation rate even after shifting to a high frequency range of 1000 MHz or more (attenuation of 30% or more at 1000 MHz in both Examples 1 and 2). Rate is secured.) On the other hand, the sheet of Comparative Example 1 using spherical ferrite powder and the sheet containing permalloy powder have an attenuation factor of 10% or less at 1000 MHz or more, and it is difficult to shield a magnetic field at a high frequency range of 1000 MHz or more. It can be seen that it is. Examples 3 and 4, Comparative Examples 2 and 3 Magnetic field shielding sheets were produced in the same manner as in Example 1 except that the pulverization time by a ball mill was changed as shown in Table 1. Table 1 also shows the longitudinal dimension and aspect ratio of the obtained flat ferrite powder. Next, using the obtained sheet, the attenuation rate of the magnetic field of 2000 MHz was determined by the method using the above-described coil antenna, and the relationship between the attenuation rate and the aspect ratio was determined. The results are shown in FIG.

[0042]

[Table 1]

As shown in FIG. 5, when the aspect ratio of the flat ferrite powder is less than 5, the attenuation factor is lower than 20%, and it is difficult to exhibit a sufficient magnetic field shielding ability. Further, as the aspect ratio of the flat ferrite powder exceeds 5, the attenuation rate increases as the aspect ratio increases, and the magnetic field shielding ability improves. However, when the aspect ratio of the flat ferrite powder exceeds 100, the attenuation rate decreases. It can be seen that the degree of the rise is saturated, and accordingly, the magnetic field shielding ability is also saturated. Examples 5 to 9, Comparative Examples 4 to 7 Ferrite ingots were produced in the same manner as in Example 1 except that the oxygen partial pressure in the atmosphere of the casting chamber was changed as shown in Table 2.

Thereafter, the ingot was ground in the same manner as in Example 1, and the magnetic permeability of the powder at 1000 MHz was measured in the same manner as in Example 1. The results are shown in Table 2. Then, a magnetic field shielding sheet was manufactured in the same manner as in Example 1 using the obtained powder. With respect to the obtained sheet, the attenuation rate with respect to the magnetic field of 2000 MHz was obtained by the method using the above-described coil antenna. The results are shown in Table 2.

[0045]

[Table 2]

As is clear from Table 2, it was found that when the oxygen partial pressure during casting was increased, the magnetic permeability of the obtained ferrite powder was increased. Here, it can be seen that when a magnetic field shielding sheet is manufactured using a ferrite powder having a magnetic permeability of less than 1.1, the attenuation rate decreases to 0.8% or less. That is, it is understood that the magnetic field shielding ability is reduced when the oxygen partial pressure during casting is set to less than 20%. Examples 10 and 11 and Comparative Examples 8 to 11 Flat ferrite powder was produced in the same manner as in Example 1 except that casting was performed while controlling the preheating temperature of the mold during casting as shown in Table 3. did. The magnetic permeability of the obtained powder at 1000 MHz was measured in the same manner as in Example 1. The results are shown in Table 3. Next, a magnetic field shielding sheet was manufactured in the same manner as in Example 1 using the obtained powder. 2000 MHz for the obtained sheet
Was determined by the method using the coil antenna described above. The results are shown in Table 3.

[0047]

[Table 3]

As is clear from Table 3, the lower the preheating temperature of the mold, the lower the magnetic permeability. Accordingly, the attenuation rate of the magnetic field when the sheet is formed decreases, and the magnetic field shielding ability decreases. The preheating temperature of the mold is 1500
If the temperature exceeds ℃, the value of the magnetic permeability is saturated, and the attenuation rate of the magnetic field when contained in the sheet is also saturated. Examples 12 to 15 and Comparative Examples 12 and 13 Flat ferrite powders were produced in the same manner as in Example 1 except that the cooling rate to 700 ° C. was changed as shown in Table 4 during cooling of the mold. . For the obtained powder, the magnetic permeability at 1000 MHz was measured in the same manner as in Example 1, and the results are shown in Table 4. Then, a magnetic field shielding sheet was manufactured in the same manner as in Example 1 using the obtained powder. 2000 MHz for the obtained sheet
Was determined by the method using the coil antenna described above. The results are shown in Table 4.

[0049]

[Table 4]

As is clear from Table 4, the cooling rate was 10
If it exceeds 0 ° C./hr, it is impossible to secure a magnetic permeability of 10 or more in the obtained powder. Therefore, the attenuation rate of the magnetic field when the sheet is formed is reduced, and the magnetic field shielding ability is reduced (Comparative Example 12). The cooling rate is 1 ° C / hr.
If it is less than 1, the crystal grains become coarse, and it is difficult to obtain a powder having a longitudinal dimension of 100 μm or less. Therefore, when it is contained in the sheet, the ferrite powder becomes difficult to orient, and the magnetic field attenuation rate becomes a low value,
The magnetic field shielding ability was reduced (Comparative Example 13). When the cooling rate is set within the range of 1 to 100 ° C./hr specified in the present invention,
The magnetic permeability of the obtained flat ferrite powder is high, and the flat ferrite powder is oriented in a good state in the sheet. Therefore, the magnetic field shielding sheet containing the powder has a high magnetic field attenuation rate and is excellent in magnetic field shielding ability. You can see that. Examples 16 to 20 The flat ferrite powder obtained in the same manner as in Example 1 was used.
The chloroprene-based synthetic rubber was charged into the raw material so as to have the content shown in Table 5, and a magnetic field shielding sheet was manufactured in the same manner as in Example 1. With respect to the magnetic field shielding sheets obtained in this manner, in which the content of the flat ferrite powder was changed, the attenuation rate with respect to the magnetic field of 2000 MHz was obtained by the method using the above-described coil antenna. The results are shown in Table 5.

[0051]

[Table 5]

From the results shown in Table 5, it can be seen that the higher the content of the flat ferrite powder, the higher the magnetic field attenuation rate, and the higher the proportion of the flat ferrite powder in the magnetic field shielding sheet, the better the magnetic field shielding ability. 20 from Table 5
In order to obtain an attenuation rate of 20% or more with respect to a magnetic field of 00 MHz, flat ferrite powder should be 30
It turns out that it is necessary to contain it by volume% or more.
However, if the content exceeds 90% by volume, the flexibility of the sheet is impaired. Therefore, the content of the flat ferrite powder is preferably set to 90% by volume or less.

[0053]

As is clear from the above description, since the flat ferrite powder of the present invention has a high magnetic permeability and is flat, it is contained in the sheet-shaped magnetic field shield material while being oriented in the direction along the sheet surface. When this is done, it contributes to the improvement of the magnetic field shielding ability in the high frequency range of 1000 MHz or more. For this reason, the flat ferrite powder of the present invention is useful when manufacturing a sheet-shaped magnetic field shielding material that can cope with higher frequencies in electronic and communication devices.

Further, the method for producing a flat ferrite powder of the present invention comprises the steps of merely pulverizing an ingot of ferrite cast under the conditions specified in the present invention without a difficult operation such as pulverizing a spherical powder. Since flat ferrite powder can be easily produced, it contributes to simplification of the production process of ferrite powder for a sheet-like magnetic field shielding material, and its industrial value is great.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing a flow of a magnetic flux in a magnetic field shield material.

FIG. 2 is a perspective view schematically showing a flat ferrite powder.

FIG. 3 is a graph showing the relationship between the intensity of an external magnetic field and the attenuation factor.

FIG. 4 is a graph showing a relationship between a frequency of an external magnetic field and an attenuation factor.

FIG. 5 is a graph showing the relationship between the aspect ratio and the attenuation factor of the flat ferrite powder.

[Explanation of symbols]

 1 Magnetic field shield material 2 Flat ferrite powder F Magnetic flux

Claims (2)

[Claims] 1. A powder obtained by pulverizing soft magnetic ferrite produced by a casting method and having a longitudinal length of 1
A flat ferrite powder having a size of from 100 to 100 µm and an aspect ratio of from 5 to 100. 2. A soft magnetic ferrite raw material having an oxygen partial pressure of 80
% Of the raw material in an atmosphere of 1750% or more.
A melting step of melting at a temperature of not more than 10 ° C. and a melt obtained in the melting step in an atmosphere having an oxygen partial pressure of 20% or more.
After casting into a mold that has been preheated to a temperature in the range of 00 to 1500 ° C., it is cooled to a temperature of at least 700 ° C. at a cooling rate of 1 to 100 ° C./hr.
% Ferrite powder characterized by comprising a casting step of cooling to room temperature as a percentage to produce a soft magnetic ferrite ingot, and a pulverizing step of pulverizing the ingot obtained in the casting step by a pulverizing means. Manufacturing method.