JP5182863B2 - Radio wave absorber and manufacturing method thereof - Google Patents

Description

  The present invention relates to a radio wave absorber for a wide band and a manufacturing method thereof, and more particularly, to a radio wave absorber and a manufacturing method thereof in which the packing density of magnetic powder blended in the radio wave absorber increases with an inclination in the traveling direction of the radio wave.

  In recent years, devices using radio waves such as mobile phones, car navigation systems, satellite broadcasts, etc. have been widely used, and accordingly, in order to prevent false images in television and radar, malfunction of electronic devices, etc. Development of industrial materials that absorb radio waves (hereinafter referred to as “radio wave absorbers”) is advancing.

  The principle of absorbing radio waves in a conventional radio wave absorber is that the electrical length of the radio wave absorber (the physical thickness of the absorber multiplied by the square root of the product of its dielectric constant and permeability) is 1 of the wavelength of the radio wave. / 4, thereby canceling the incident wave on the surface and the reflected wave from the back surface by interference.

  However, in this case, the wavelength of the radio wave absorbed by the interference is limited to a value that is four times the electrical length of the radio wave absorber or in the vicinity thereof, and the frequency band that can be absorbed by the radio wave absorber is very narrow. Become.

  In order to widen the absorption band, a technique related to a laminated radio wave absorber in which materials having different dielectric constants and magnetic permeability, that is, different electrical lengths, are laminated and reflected (multiple reflection) at the interface of each layer has been proposed. (Non-Patent Document 1). At this time, in such a laminated wave absorber, the material constant is greatly different from that of air, so that the radio wave is largely reflected on the surface (incident surface) of the wave absorber, thereby preventing this. Therefore, it is desirable to increase the material constants (dielectric constant, magnetic permeability, etc.) of each absorber layer stepwise as it proceeds from the incident direction of the radio wave to the inside of the radio wave absorber.

  In addition, the electromagnetic wave absorber has a shape in which a large number of wedges are arranged in a plane (hereinafter referred to as “wedge shape”) with the direction in which radio waves are incident (hereinafter referred to as “wedge shape”) inside the electromagnetic wave absorbing portion consisting of the electromagnetic wave absorber and air. A technique has also been proposed in which the material constant of the portion increases in a slope as the progress proceeds, and thereby radio wave absorption in a wide band (Non-Patent Document 2).

  In addition, the fiber with conductive carbon attached to the surface is combined to form a sheet called lock, and the bulk density is increased toward the inside of the sheet to increase the material constant in an inclined manner. There is also proposed a technique for performing radio wave absorption in Japanese Patent Application Laid-Open No. 2003-228561.

Radio wave absorbers have also been developed that use ferrite (non-conductive magnetic ceramics mainly composed of iron oxide) instead of conductive carbon, use glass instead of fibers, and have a multilayered structure. (Patent Document 2).
Tohoku Kako Co., Ltd., “Radio wave absorber for anechoic chamber in product information column, flat plate type radio wave absorber”, [online], [February 6, 2008 search], Internet <URL: http: // www1k. mesh. ne. jp / tci / kasei / denpa / anshitsu / uf. html> Tohoku Kako Co., Ltd., “Radio wave absorber for anechoic chamber in product information column, pyramid type wave absorber”, [online], [searched February 6, 2008], Internet <URL: http: // www1k. mesh. ne. jp / tci / kasei / denpa / anshitsu / up. html> Japanese Patent Application Laid-Open No. 2004-179459 JP 2000-353893 A

  However, the electromagnetic wave absorber in which the electrical length is simply ¼ of the wavelength of the radio wave described in the beginning is not only very narrow in the frequency band of the radio wave that can be absorbed, but also progresses if the incident angle of the radio wave changes. Since the electrical length in the direction also changes, the dependence on the incident angle also increases.

  In addition, the multilayered wave absorber made by laminating materials with different electrical lengths, the wider the frequency band that can be absorbed, the greater the thickness required for the entire laminated wave absorber, making it difficult to manufacture. There is also a cost increase.

  In addition, the wedge-shaped electromagnetic wave absorber is not only difficult to manufacture, but also the gradient of the material constant is performed by dilution with air (both the relative permeability and the relative permittivity are both about 1) (the volume occupied by the air as it goes into the interior) Therefore, the wave absorber is inevitably thick, and the thickness of the space for storing the wave absorber is increased. For example, if an attempt is made to obtain a reflection loss of -30 dB (decibel) or less for radio waves of 3 GHz or more, a thickness of 100 mm or more is required.

  Also, the radio wave absorber called “lock” uses dilution by air in the same way as the wedge-shaped radio wave absorber, so that the radio wave absorber becomes thicker if not as much as the wedge-shaped radio wave absorber. That is, if an attempt is made to obtain a reflection loss of -30 dB or less for radio waves of 3 GHz or more, a thickness of 40 mm is necessary.

  In addition, the multilayered wave absorber using ferrite and glass is not only thick because it uses a low dielectric constant hollow shirasu balloon, but also uses a glass with a high density as the ferrite matrix, so the whole The density of this will increase, and it will inevitably become heavier from this aspect.

  For this reason, it is desirable to develop a radio wave absorber that has excellent absorption characteristics for radio waves of a wide frequency range, requires a small thickness, has high tolerance for changes in incident angle, and is easy to manufacture and inexpensive. It was.

The present invention has been made in order to solve the above problems, the magnetic powder, in which the density of the magnetic powder in the resin were dispersed in the manner continuously increases along the traveling direction of the radio wave It is a radio wave absorber having a resin composite. The invention of each claim will be described below.

The first technique related to the present invention is:
Radio wave absorption characterized by having a magnetic powder / resin composite in which magnetic powder is dispersed in a resin so that the packing density of the magnetic powder continuously increases along the traveling direction of the radio wave Is the body.

Hereinafter, the principle of the operation of the radio wave absorber according to the first technique will be theoretically described using mathematical expressions.

  In the conventional wedge-shaped wave absorber and the wave absorber in which the bulk density is pseudo-continuously changed, the dielectric constant and permeability are diluted with air having a small relative dielectric constant (relative dielectric constant: about 1). However, as shown in Equation 1 below, in order to match the impedance, it is necessary to bring the term of the characteristic impedance (Equation 2) close to the characteristic impedance of the air. Therefore, the ratio between the relative permittivity and the relative permeability is set to 1. It is necessary to approach. Therefore, for example, a radio wave absorber using a dielectric material cannot use a material having a high dielectric constant, and the electrical length (Equation 3) becomes small. For this reason, the electric wave absorber naturally has a long electric length, and consequently becomes thick.

In Equation (1), Z _in is the incident surface impedance. Z ₀ is the characteristic impedance of air, approximately 377Ω. μ _r is the complex relative permeability. ε _r is a complex relative dielectric constant. d is the thickness of the wave absorber. RL is a reflection loss.

  Also, in the conventional wave absorber, the magnetic permeability increases when the magnetic material is used, so the impedance matching is better than the former, but dilution with air, that is, a wedge-shaped shape is used as a method of inclination. You can't earn an electrical length because you use it. As a result, the radio wave absorber becomes thick.

On the other hand, the electromagnetic wave absorber of the first technology uses a magnetic material, and unlike the conventional wedge-shaped or sheet-like electromagnetic wave absorber, the relative permittivity is larger than air but does not affect impedance matching so much. Since the magnetic material, that is, the permeability, is diluted with an organic resin having a dielectric constant, the electrical length can be increased, and as a result, the radio wave absorber can be made thinner.

Furthermore, in the radio wave absorber of the first technology , the magnetic powder exhibits the maximum density in the vicinity of the back surface where the magnetic field component is maximum, and the concentration of the low dielectric constant resin is increased on the radio wave absorber surface where the electric field is maximum. Therefore, as a material having a low dielectric constant and a high magnetic permeability, the ratio between the dielectric constant and the magnetic permeability approaches 1 in terms of characteristic impedance. As a result, impedance matching becomes easy, and combined with the effect of multiple reflection by infinite lamination, it is possible to absorb radio waves in a wide band.

Based on the above theory, the configuration of the first technique will be described.
In the first technique , the magnetic powder in the magnetic powder / resin composite of the radio wave absorber has a packing density (hereinafter referred to as “density” in principle to avoid complication) along the traveling direction of the radio wave. Dispersed in such a way as to increase continuously, resulting in a completely continuous concentration distribution, so that the radio wave has a low density of magnetic powder or impedance mismatch from zero incident surface regardless of the wavelength. Intrusion into the magnetic powder / resin composite without large reflection due to, and is absorbed by multiple reflection that repeats smaller reflections.

  As a result, radio waves are reflected and absorbed regardless of the wavelength, so that the absorption efficiency is good for wide-band radio waves. It becomes a radio wave absorber that efficiently absorbs radio waves.

  Furthermore, the tolerance for changes in the incident angle is improved. Further, since the material is thin, the material cost can be reduced, and the manufacturing cost is reduced from this aspect.

The distribution of the magnetic powder in the magnetic powder / resin composite may be zero on the surface (incident surface) on which radio waves are incident, and the other side is 100% magnetic powder (no resin). Also good.

  Furthermore, when the magnetic powder density is 100%, a radio wave reflector such as an aluminum foil may be disposed (added or adhered) on the surface (back surface) opposite to the incident surface.

  The “magnetic powder” is preferably a high-permeability conductive magnetic powder, particularly a metal powder, and more preferably a supermalloy, particularly when the target radio wave is GHz, especially when the frequency is 1 to 30 GHz. .

  Furthermore, the “resin” is not limited to petroleum-based synthetic resins such as polyethylene, epoxy resin, styrene resin, and also includes rubber such as silicon rubber, and further improves the dispersibility of the magnetic material in the resin, An auxiliary agent for enhancing the fluidity of the resin during production, for example, a surfactant or the like may be contained. That is, even if it is a resin having a low dielectric constant and suitable for a matrix, it is non-ionic, cationic or anionic even if it is difficult to use from the viewpoint of dispersion of magnetic powder due to its high viscosity (fluidity). It becomes possible to reduce the viscosity using a dispersing agent such as.

  In addition, if a dispersant is used, the probability that the magnetic powders are in contact with each other is reduced, which contributes to the reduction of the dielectric constant, and as a result, the characteristic impedance increases, which is preferable in terms of good matching with the incident surface impedance. . In particular, it is preferable to use an electrostatic neutral nonionic surfactant in order to enhance the effect of the degree of dispersion. In addition, as a means to disperse | distribute, industrial stirring can be considered.

From the viewpoint of strength, an epoxy resin is preferable.
Further, from the viewpoint of production, a thermosetting resin is preferable, but from the viewpoint of preventing oxidation of the magnetic material during production, the curing temperature is preferably 200 ° C. or less, particularly 150 ° C. or less. Is preferred.

The second technique related to the present invention is:
The magnetic powder is dispersed in a resin having a relative dielectric constant of 5 or less so that the packing density of the magnetic powder continuously increases along the traveling direction of the radio wave, and the impedance of the incident surface of the radio wave is 300Ω to 377Ω. It is a radio wave absorber characterized by having a magnetic powder / resin composite.

In the second technique , since the relative dielectric constant of the resin is 5 or less, the adverse effect on impedance matching is reduced, and the impedance of the incident surface of the radio wave is 300Ω to 377Ω, which is the same as or close to air, There is little reflection of radio waves on the surface, which results in an excellent radio wave absorber.

  The relative dielectric constant of the resin is preferably 2 or more in order to increase the electrical length.

A third technique related to the present invention is the above-described radio wave absorber,
A radio wave absorber comprising a layer made of a low dielectric constant material on a radio wave incident surface of the magnetic powder / resin composite.

In the third technology , since the magnetic powder / resin composite has a layer made of a low dielectric constant material on the incident surface of the electric wave, the reflection of the electric wave on the incident surface is reduced, resulting in excellent absorption of the electric wave Become a body.

  Here, “dielectric constant” refers to an apparent dielectric constant in the case of a composite material such as polyurethane foam made of urethane resin (the dielectric constant of the resin itself is about 7) and air or chlorofluorocarbon gas. “Low dielectric constant” refers to a relative dielectric constant of 1 to 2. Adopting a foamed polyurethane layer as a layer made of a low dielectric constant material is similar to the characteristic impedance of air due to the dielectric constant, so that reflection of radio waves at the incident surface is prevented, and since the density is small and lightweight, This is also preferable from the viewpoint of preventing an increase in weight.

The 4th technique relevant to the present invention is the above-mentioned wave absorber,
The resin is an electromagnetic wave absorber characterized in that it is either epoxy resin or rubber.

In the fourth technology , since either epoxy resin or rubber is used as a matrix resin for dispersing the magnetic substance of the magnetic powder / resin composite, not only impedance matching, electrical length, etc., but also manufacturing, strength In addition, the electromagnetic wave absorber is excellent in terms of cost and the like.

The 5th technique relevant to the present invention is the above-mentioned wave absorber,
The magnetic powder / resin composite is a radio wave absorber characterized by having a laminated structure.

In the fifth technique , since the magnetic powder / resin composite has a laminated structure, it becomes easy to give flexibility and diversity to the dispersion or density gradient and inclination of the magnetic powder, and the thickness of the radio wave absorber. As a result, it is possible to provide a radio wave absorber suitable for various wavelengths and applications.

A sixth technique related to the present invention is the above-described radio wave absorber,
The magnetic powder is a radio wave absorber characterized in that at least one of shape and material is a plurality of types of magnetic powder.

In the sixth technique , since at least one of the shape or material of the magnetic powder is a plurality of types, the dispersibility of the magnetic powder in the magnetic powder / resin composite can be obtained by combining magnetic powders with appropriate characteristics. It is possible to produce magnetic powder / resin composites having various dispersion modes, excellent radio wave absorption, and providing a radio wave absorber having special characteristics according to various uses It becomes possible

  In addition, as means for improving the dispersibility of the magnetic powder, other means such as adjusting the particle size distribution of the magnetic powder and utilizing the difference in the sedimentation speed in the resin depending on the size of the particle diameter may be used in combination.

A seventh technique related to the present invention is the method of manufacturing a radio wave absorber according to any one of the first technique to the sixth technique ,
Centrifugal force is used as means for dispersing the magnetic powder in the resin in the magnetic powder / resin composite so that the packing density of the magnetic powder continuously increases along the traveling direction of radio waves. A method of manufacturing a radio wave absorber.

In the invention of this claim, as a means for dispersing the magnetic powder in the resin in the magnetic powder / resin composite so that the packing density continuously increases along the traveling direction of the radio wave, a viscous liquid is used. Since centrifugal force is used for the mixed fluid in which the magnetic powder is dispersed in the resin, the manufacture becomes easy.

In addition, the following measures can also be taken.
A radio wave absorber using a high magnetic permeability alloy such as permalloy powder or sendust powder is manufactured as magnetic powder. Thereby, since the magnetic permeability is high, the radio wave absorbability tends to be improved, and as a result, the thickness of the radio wave absorber can be reduced. In addition, a flat or rod-shaped magnetic powder that is not easily affected by the demagnetizing field effect is used to increase the permeability. To absorb radio waves in the MHz band, a ferrite-based material having a high permeability in this frequency region is used. It is also possible to adopt means such as use.

Moreover, it is set as the electromagnetic wave absorber using curable rubber as resin.
Further, a magnetic powder / resin composite is produced by using a flat permalloy powder and a spherical carbonyl powder in combination as magnetic powder. In this case, since the dispersibility of both magnetic powders in the resin is different, the particle size distribution of the magnetic powder is increased, the packing density near the reflector is increased, and excellent radio wave absorption characteristics are exhibited. It becomes.

  The present invention is based on the above technique, and the invention of each claim is as follows.

  The invention described in claim 1
  A magnetic powder / resin composite in which the magnetic powder is dispersed in the resin so that the packing density of the magnetic powder continuously increases along the traveling direction of the radio wave;
  A magnetic wave absorber, wherein a magnetic powder having a large particle diameter and a magnetic powder having a small particle diameter are used in combination as the magnetic powder, and the materials of the magnetic powder used in combination are the same. .

  The invention described in claim 2
  The magnetic powder is dispersed in a resin having a relative dielectric constant of 5 or less so that the packing density of the magnetic powder continuously increases along the traveling direction of the radio wave, and the impedance of the incident surface of the radio wave is 300Ω to 377Ω. The electromagnetic wave absorber according to claim 1, comprising a magnetic powder / resin composite.

  The invention according to claim 3
  3. The radio wave absorber according to claim 1, wherein a layer made of a low dielectric constant material is provided on a radio wave incident surface of the magnetic powder / resin composite.

  The invention according to claim 4
  The radio wave absorber according to any one of claims 1 to 3, wherein the resin is either an epoxy resin or rubber.

  The invention described in claim 5
  The magnetic powder / resin composite has a laminated structure,
  Each laminated plate constituting the laminated structure has a magnetic powder density gradient,
  The laminated structure according to any one of claims 1 to 4, wherein the laminated structure is formed by laminating the laminated plates in an order in which the density gradient of the magnetic powder continuously changes as a whole. The electromagnetic wave absorber described.

  The invention described in claim 6
  The electromagnetic wave absorber according to any one of claims 1 to 5, wherein the magnetic powder is a plurality of types of magnetic powder.

  The invention described in claim 7
  A method of manufacturing a radio wave absorber according to any one of claims 1 to 6,
  A mixing step of mixing the magnetic powder and the resin;
  A centrifugal treatment step of dispersing the magnetic powder in the resin in the magnetic powder / resin composite by using centrifugal force so that a packing density of the magnetic powder continuously increases along a traveling direction of radio waves;
  And a heating step of heating and curing the magnetic powder / resin composite that has been subjected to the centrifugal treatment.

In the present invention, the magnetic powder is a radio wave absorber having a magnetic powder / resin composite in which the magnetic powder is dispersed in the resin so that the packing density of the magnetic powder continuously increases along the traveling direction of the radio wave. Therefore, it is possible to provide a radio wave absorber that has excellent absorption characteristics for radio waves of a wide frequency range, requires a small thickness, has high tolerance for changes in incident angle, is easy to manufacture, and is inexpensive. Become.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the present invention is not limited to the following embodiments. Various modifications can be made to the following embodiments within the same and equivalent scope as the present invention.

Example 1
The form of Example 1 uses spherical silicon steel magnetic powder as magnetic powder.
0.5 g of commercially available spherical silicon steel magnetic powder was weighed, and 0.34 g of a commercially available epoxy resin main ingredient and 0.16 g of a curing agent were added thereto, and mixed until uniform in an agate mortar. The mixed fluid of the magnetic powder and resin (however, the viscosity is high) was poured into the mold, and the whole mold was centrifuged at 4000 rpm for 10 minutes. The acceleration is about 3000G. However, the acceleration at this time depends on the particle size and blending ratio of the magnetic powder, the type of resin (the blending ratio of the main agent and curing agent and the manufacturer may vary slightly), and the viscosity of the resin at the temperature at which acceleration is applied (summer Since the preferred value varies somewhat according to the difference in the winter season, etc., it is preferable to carry out a test with varying acceleration three or four times to obtain an appropriate value.

Subsequently, the resin was cured by heating at 120 ° C. for 1 hour to obtain a magnetic powder / resin composite in which the magnetic powder was dispersed in the cured epoxy resin as a matrix. Here, the heating temperature was set to 120 ° C. in order to prevent oxidation of the magnetic powder. Further, as Comparative Example 1, a sample having the same conditions except that no magnetic powder density gradient was provided was also prepared.

Furthermore, for magnetic powder / resin composites with a density gradient, on the back side of the magnetic powder with a high packing density, for magnetic powder / resin composites without a concentration gradient, either side can be used as a reflector. Aluminum foil was affixed to complete the radio wave absorbers of Example 1 and Comparative Example 1.

In FIG. 1, the cross section cut | disconnected along the advancing direction of an electromagnetic wave of the electromagnetic wave absorber of Example 1 is shown notionally. In FIG. 1, arrows indicate the direction of incidence of radio waves, 10 is a spherical silicon steel magnetic powder having a small particle diameter, 11 is a spherical silicon steel magnetic powder having a large particle diameter, and 20 is an epoxy resin cured by heating. , 30 is an aluminum foil, and 50 is a magnetic powder / resin composite. Also, the vertical axis indicated by the dotted line indicates the electric field strength, the vertical axis indicated by the broken line indicates the magnetic field strength, and 91 indicates the electric field strength at each part of the incident direction of the radio wave in the magnetic powder / resin composite of the radio wave absorber. 92 is also a broken line indicating the magnetic field strength. The thickness of the magnetic powder / resin composite is 10 mm.

As shown in FIG. 1, it can be seen that a good density distribution is obtained by centrifugal force in the magnetic powder / resin composite 50 of the radio wave absorber. That is, the spherical silicon steel magnetic powders 10 and 11 are more densely distributed toward the inner side (the traveling direction of radio waves), and the spherical silicon steel magnetic powder 11 having a larger particle diameter is more than the spherical silicon steel magnetic powder 10 having a smaller particle diameter. It turns out that the tendency is great. For this reason, the density of the magnetic powder / resin composite 50 increases toward the inner side, and the (filling) density of the magnetic powder increases. The distribution of magnetic field strength and electric field strength will be described later.

  Next, the results of the performance test of the radio wave absorber of Example 1 and Comparative Example 1 will be described while comparing both results. The amount of reflection loss was measured by making a radio wave incident on these two types of radio wave absorbers and comparing the energy of the radio wave (reflected wave) reflected from the radio wave absorber with the energy of the incident wave. The measurement results are shown in FIG. In FIG. 2, the solid line is the measurement result of the radio wave absorber (with density gradient) in Example 1, and the dotted line is the measurement result of the radio wave absorber (without density gradient) of Comparative Example 1.

  As shown in FIG. 2, in the radio wave absorber of Example 1 provided with the density gradient, the reflection loss amount (dB) increases in almost the entire region below 18 GHz as compared with the radio wave absorber of Comparative Example 1, that is, the radio wave In particular, it was confirmed that the absorption characteristics were even better for radio waves of 10 GHz or higher. This is probably because multiple reflections in the radio wave absorber are good.

  Further, in the radio wave absorber of Example 1, a peculiar phenomenon in which the amount of reflection loss increases as the frequency increases is observed. This is because a low dielectric constant material is disposed on the radio wave absorber surface where the electric field is maximum. In addition, the distribution of spherical silicon steel magnetic powders 10 and 11 having magnetism is increased on the surface of the aluminum foil 30 where the magnetic field is maximum, and the magnetic permeability is increased, so that the incident surface impedance of the radio wave absorber is matched to the characteristic impedance of the air. It seems that it became easy.

(Example 2)
In the second embodiment, a urethane foam layer is formed on the radio wave incident surface side of the radio wave absorber of the first embodiment.

In the radio wave absorber of Example 2, a urethane foam layer close to the dielectric constant of air is arranged on the incident surface side of the radio wave absorber (magnetic powder / resin composite 50) of Example 1, and the radio wave absorber of Example 1 is used. Compared to a radio wave absorber, the incident impedance of the surface on which radio waves are incident is made closer to the characteristic impedance of air, and the reflection of radio waves from the surface is reduced accordingly.

  In FIG. 3, the measurement result of the electromagnetic wave absorber of Example 2 is shown in comparison with the measured value of the electromagnetic wave absorber of Example 1. In FIG. 3, the dotted line is the measurement result of the radio wave absorber of Example 1 (graded sample: with density gradient), and the solid line is the measurement result of the radio wave absorber of Example 2 (graded sample + urethane foam). .

As can be seen from FIG. 3, the radio wave absorber of Example 2 has a foamed urethane layer disposed on the radio wave incident surface side of the magnetic powder / resin composite 50 of the radio wave absorber of Example 1; The base of the line is further lowered as compared with the radio wave absorber of the first embodiment. In particular, radio waves with a frequency of 10 GHz or more show better reflection loss characteristics of −20 dB or less compared to the radio wave absorber of Example 1. The reason for this improvement is that the resonance mode of the radio wave in the magnetic powder / resin composite of the radio wave absorber is an electric field at the radio wave entrance surface (the side facing the air of the radio wave absorber) as shown in FIG. Therefore, it is considered that an effective lowering of the dielectric constant occurs when a foamable urethane resin having a dielectric constant close to air is introduced into the front surface.

(Example 3)
Example 3 uses carbonyl iron powder (iron powder having a spherical particle size distribution of 0.3 μm to 10 μm) as a magnetic powder, and further uses the dispersant. Carbonyl iron powder, various ones are commercially available, manufactured by Wako Pure Chemical Industries, Ltd. of carbonyl iron powder were _used: (D ₅₀ to 3 micrometers). 0.4 g of this carbonyl iron powder was weighed, 0.26 g of the same epoxy resin main ingredient as used in Example 1 and 0.13 g of a curing agent were added, and mixed until uniform in an agate mortar, and further obtained. The entire mixed fluid of magnetic powder and resin was poured into a mold, and the whole mold was centrifuged at 4000 rpm for 10 minutes. Thereafter, the resin was cured by heating at 120 ° C. for 1 hour.

The thickness of the magnetic powder / resin composite is 7 mm. In Example 3, as in Comparative Example 3, a sample without a density gradient was prepared as Comparative Example 3.

  In the radio wave absorber of Example 3, the formation of a magnetic powder distribution gradient in the cured resin was easily observed with the naked eye.

  Next, samples are prepared by cutting the cured resin layer to a thickness of 1 mm in a plane direction perpendicular to the incident direction of radio waves, and the weight (density) of each sample, that is, the inside of each sample is measured. The state in which the distribution of the magnetic powder was changed was measured. FIG. 4 shows the measurement results.

The density of the resin used is about 1.2 g / cm 3 and the density of iron is about 7.9 g / cm 3. For this reason, in the magnetic powder / resin composite, there is almost no magnetic powder near the surface of the wave absorber where the radio wave is incident, almost no resin is present on the opposite side, and there is magnetic powder in the middle of both surfaces. It can be seen that the resin ratio changes almost linearly, that is, the density (distribution) of the magnetic powder is appropriately graded.

  Next, the results of the performance tests of the radio wave absorbers of Example 3 and Comparative Example 3 will be described with reference to FIG. 5 while comparing the measurement results of both (with density gradient and without density gradient). In FIG. 5, the solid line is the measurement result of the radio wave absorber of Example 3 (with density gradient), and the dotted line is the measurement result of the radio wave absorber of Comparative Example 3 (without density gradient). Similar to the first embodiment, the radio wave absorber of the third embodiment has a tendency that the amount of reflection loss increases as the frequency increases as compared with the radio wave absorber of the third comparative example, and absorbs radio waves having a wider wavelength range. Is recognized.

In Example 3, when mixing the magnetic powder and the resin, in order to increase the dispersibility of the magnetic powder in the resin, polyethylene (10) octylphenyl ether (non-ionic surfactant) 1% by weight was added to the magnetic powder. As a result, the viscosity of the mixed fluid composed of the magnetic powder and the resin after mixing is reduced as compared with the mixed fluid of Example 1 in which no addition is made, and not only filling into the mold becomes easy, but also curing by heating. Since the dispersibility of the magnetic powder in the magnetic powder / resin composite was improved and the dielectric constant decreased, the baseline of the radio wave absorber of the finally obtained radio wave absorber was further lowered.

In addition, when compared with the radio wave absorber of Example 1, the radio wave absorber of Example 3 is shifted to the high frequency side when compared with the radio wave absorber of Example 1. This is because the thickness of the magnetic powder / resin composite is thin. It depends. That is, in the radio wave absorber of the present invention, as in the radio wave absorber of the prior art, the corresponding absorption region can be adjusted by changing the thickness of the magnetic powder / resin composite. It was.

Example 4
Example 4 relates to confirming the effect of continuously changing the (filling) density of the magnetic powder.
Weigh 0.45 g of carbonyl iron powder, add 0.2 g of epoxy resin main agent and 0.1 g of curing agent to this, mix until uniform in an agate mortar, and further obtain a mixed fluid of magnetic powder and resin Was poured into a mold, and the whole mold was centrifuged for 30 minutes at a rotational speed of 4000 rpm. Thereafter, the resin was cured by heating at 120 ° C. for 1 hour. The obtained magnetic powder / resin composite has a thickness of about 7 mm.

In order to produce Comparative Example 4 in which the distribution (concentration) of the magnetic substance in the magnetic powder / resin composite was changed in three steps, the upper, middle, and lower parts of the magnetic powder / resin composite in the mold were cut out. Their average density was measured by the Archimedes method.

Next, three types of magnetic powder / resin composites were produced in which the distribution of the magnetic powder in the magnetic powder / resin composite was uniform and the packing density was different. On top of that, these three magnetic powder-resin composite is cut to a suitable thickness (slice with) to produce a magnetic powder-resin composite slice, further magnetic powder-resin composite slices prepared The pieces are laminated (bonded) in ascending order of density from the incident surface side, and as a whole, the average density is the same as the average density of the magnetic powder / resin composite in which the distribution of the internal magnetic powder is continuously changing, and Thick magnetic powder / resin composite was prepared. In addition, the weight% of the magnetic powder of the slice pieces of these three kinds of magnetic powder / resin composites is 80 wt%, 30 wt%, and 20 wt% in descending order of density.

FIG. 6 shows the results of measuring the radio wave absorption characteristics of the radio wave absorber in which the distribution of the magnetic powder in the magnetic powder / resin composite continuously changes and the radio wave absorber in three stages. In FIG. 6, the solid line is the measurement result of the radio wave absorber in which the distribution of the magnetic powder in the magnetic powder / resin composite continuously changes, and the dotted line is the measurement result of Comparative Example 4 that changes in three stages.

In the radio wave absorber of Comparative Example 4, slices of magnetic powder / resin composite were laminated in order from the incident surface side of the radio wave in order of increasing magnetic powder distribution (increase in packing density), and as a whole, thick Although the density of the sheath was the same, the characteristics seen in the radio wave absorbers of Examples 1 to 3 were not observed. That is, in the radio wave absorber of Comparative Example 4, it can be seen that the radio wave absorptivity does not increase with an increase in frequency, and that the radio wave absorptivity is reduced at a point exceeding 20 GHz.

Furthermore, the filling rate of the magnetic powder is 0 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90 wt%, In addition, magnetic powder / resin composites in which magnetic powder is uniformly dispersed are produced, and further sliced, and slices of these magnetic powder / resin composites produced on the magnetic powder / resin composites are arranged in the order of decreasing density. Various wave absorbers were produced by laminating, and the absorption characteristics of these waves were measured. However, a phenomenon peculiar to the radio wave absorber of the present invention in which the distribution (density) of the magnetic powder was graded (changed continuously) inside the magnetic powder / resin composite was not observed.

As described above, in the radio wave absorber of the present invention, since the density of the magnetic powder is continuously increased inside the magnetic powder / resin composite, near multiple reflection of incident radio waves occurs, resulting in simple lamination. It can be seen that a wide-band electromagnetic wave absorption characteristic that cannot be obtained by the conversion is obtained.

(Example 5)
Example 5 compares the difference by the presence or absence of a foamed urethane layer, after increasing the usage-amount of magnetic powder by the magnetic powder filling rate of Example 4. FIG.
0.6g of carbonyl iron powder was weighed, 0.26g of epoxy resin main agent and 0.13g of curing agent were added thereto, and mixed in an agate mortar until uniform. The mixed fluid of the magnetic powder and the resin was poured into a mold, and the whole mold was centrifuged for 20 minutes at a rotation speed of 4000 rpm. Thereafter, the resin was cured by heating to obtain a magnetic powder / resin composite having a thickness of 10 mm.

Next, to produce even magnetic powder-resin composite in which a layer of urethane foam and was applied stack thickness 2mm on the incident surface side of the radio wave of the magnetic powder-resin composite of the thickness is about 10 mm. Next, to produce a wave absorber using a magnetic powder-resin composite having a thickness of 10 mm, the wave absorber using foamed adding a layer of urethane (laminated) with magnetic powder-resin composite having a thickness of 2mm The absorption characteristics of both radio waves were measured.

The measurement results are shown in FIG. In FIG. 7, the solid line indicates the measured value of the radio wave absorber to which the urethane foam layer is added, and the dotted line indicates the measured value of the radio wave absorber that is not added.
Although the density of the magnetic powder is large and the gradient is steep compared to the radio wave absorbers of Example 1 and Example 2, the radio wave absorbers of either radio wave absorber are the same as those of Example 1 and Example 2. There is a phenomenon in which the ability to absorb radio waves increases with increasing frequency.

  In addition, as in Example 2, the radio wave absorber with the urethane foam layer added has a lower baseline for absorbing radio waves than the non-added radio wave absorber, and the absorption capacity is improved. Is recognized. The reason is the same as in the second embodiment, that is, when the ratio of dielectric constant to magnetic permeability in the term of characteristic impedance approaches 1, the matching with respect to the characteristic impedance of air becomes good.

In the radio wave absorber of Example 5, compared with Examples 1 and 2, the thickness of the magnetic powder / resin composite of the radio wave absorber is increased, and the resin blending rate (amount used) is further reduced. Therefore, the electrical length is efficiently increased, and as a result, it is used in UWB (a radio wave from 3.1 to 10.6 GHz is used in a wide band for data communication), which is expected as the next generation communication standard. A radio wave absorber capable of absorbing 90% of the radio wave having a bandwidth even though it is as thin as about 10 mm is obtained.

(Example 6)
In Example 6, a magnetic powder / resin composite of a radio wave absorber has a laminated structure.
In FIG. 8, the cross section which cut | disconnected the electromagnetic wave absorber of Example 6 along the advancing direction of an electromagnetic wave is shown notionally. In FIG. 8, 21 is a first magnetic powder / resin composite slice piece layer, 22 is a second magnetic powder / resin composite slice piece layer, and 23 is a third magnetic powder / resin composite slice piece. It is a single layer, 40 is a urethane foam layer, 71 and 72 are the boundary surfaces of the respective layers, 81 is a line indicating the density distribution in the traveling direction of radio waves of the magnetic powder / resin composite, and the vertical axis is Indicates density.

As shown in FIG. 8, this radio wave absorber has a three-layer structure of magnetic powder / resin composite, but the density distribution inside thereof is continuous, and a line 81 indicating the density distribution is shown for each magnetic powder. In the resin composite slice piece layers 21, 22, 23, the gradient (inclination) decreases in order.

Production of the magnetic powder / resin composite 50 of the radio wave absorber of Example 6 is performed by individually manufacturing and laminating the urethane foam layer 40 and each magnetic powder / resin composite slice piece layer 21, 22, 23. It was.

The radio wave absorber of Example 6 also showed excellent radio wave absorption characteristics like the radio wave absorbers of the above Examples.
Further, even if some air was present on the boundary surfaces 71 and 72 between the magnetic powder / resin composite slice pieces 21, 22, and 23, the performance was not adversely affected.

It is a figure which shows notionally the cross section cut | disconnected along the advancing direction of the electromagnetic wave of the electromagnetic wave absorber of Example 1 of this invention. It is a figure which shows the result of the performance test of the electromagnetic wave absorber of Example 1 of this invention and Comparative Example 1. It is a figure which shows the result of the performance test of the electromagnetic wave absorber of Example 2 and the electromagnetic wave absorber of Example 1 of this invention. In the electromagnetic wave absorber of Example 3 of this invention, it is a figure which shows a mode that the density of magnetic powder and a resin composite, ie, distribution (filling density) of an internal magnetic powder, increases along the advancing direction of an electromagnetic wave. . It is a figure which shows the result of the performance test of the electromagnetic wave absorber of Example 3 and Comparative Example 3 of this invention. It is a figure which compares and shows the electromagnetic wave absorption characteristic of the electromagnetic wave absorber in which distribution of the magnetic powder in a magnetic powder and resin composite body changes continuously, and the electromagnetic wave absorber which changes in three steps. It is a figure which compares and shows the electromagnetic wave absorption characteristic of the electromagnetic wave absorber which added the foaming urethane layer in Example 5 of this invention, and the electromagnetic wave absorber which does not add the foaming urethane layer. It is a figure which shows notionally the cross section orthogonal to the advancing direction of an electromagnetic wave of the electromagnetic wave absorber of Example 6 of this invention.

10 spherical silicon steel magnetic powder with small particle diameter 11 spherical silicon steel magnetic powder with large particle diameter 20 cured epoxy resin 21 first magnetic powder / resin composite slice piece layer 22 second magnetic powder / resin composite slice piece Layer 23 Third magnetic powder / resin composite slice piece layer 30 Aluminum foil 40 Urethane foam layer 50 Magnetic powder / resin composite 71, 72 Interface 81 Line 91 showing density distribution 91 Dotted line 92 showing electric field strength Magnetic field strength Broken line

Claims (7)

A magnetic powder / resin composite in which the magnetic powder is dispersed in the resin so that the packing density of the magnetic powder continuously increases along the traveling direction of the radio wave ;
An electromagnetic wave absorber , wherein a magnetic powder having a large particle diameter and a magnetic powder having a small particle diameter are used in combination as the magnetic powder, and the materials of the magnetic powder used in combination are the same . The magnetic powder is dispersed in a resin having a relative dielectric constant of 5 or less so that the packing density of the magnetic powder continuously increases along the traveling direction of the radio wave, and the impedance of the incident surface of the radio wave is 300Ω to 377Ω. The electromagnetic wave absorber according to claim 1, comprising a magnetic powder / resin composite. The radio wave absorber according to claim 1 or 2, further comprising a layer made of a low dielectric constant material on a radio wave incident surface of the magnetic powder / resin composite.   The radio wave absorber according to any one of claims 1 to 3, wherein the resin is an epoxy resin or rubber. The magnetic powder-resin composite, Ri laminated structure der,
Each laminated plate constituting the laminated structure has a magnetic powder density gradient,
The laminated structure according to any one of claims 1 to 4, wherein the laminated structure is formed by laminating the laminated plates in an order in which the density gradient of the magnetic powder continuously changes as a whole. The electromagnetic wave absorber described. The electromagnetic wave absorber according to any one of claims 1 to 5, wherein the magnetic powder is a plurality of types of magnetic powder . A method of manufacturing a radio wave absorber according to any one of claims 1 to 6,
A mixing step of mixing the magnetic powder and the resin;
A centrifugal treatment step of dispersing the magnetic powder in the resin in the magnetic powder / resin composite by using centrifugal force so that a packing density of the magnetic powder continuously increases along a traveling direction of radio waves ;
And a heating step of heating and curing the magnetic powder / resin composite that has been subjected to the centrifugal treatment .

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