JP7566482B2 - Molding material for radio wave absorbing components and radio wave absorber - Google Patents

Description

This disclosure relates to a molding material for radio wave absorbing components that absorbs radio waves, and a radio wave absorber made using this material, and in particular to a molding material for radio wave absorbing components and a radio wave absorber that have magnetic iron oxide powder as a radio wave absorbing component and are capable of absorbing high-frequency radio waves from several tens of gigahertz to the millimeter wave band.

Radio wave absorbers are used to absorb radio waves, in order to avoid the effects of leaked radio waves emitted to the outside from electric circuits, etc., and undesired reflected radio waves. Radio wave absorbers are formed by molding or coating a material with radio wave absorbing properties according to the application, such as into a block or sheet shape.

In recent years, research has been progressing on technologies that utilize centimeter waves with a frequency band of several gigahertz (GHz), millimeter waves with frequencies of 30 to 300 gigahertz, and even radio waves with a frequency band of 1 terahertz (THz) that exceeds the millimeter wave band, for mobile communications such as mobile phones, wireless LANs, and electronic toll collection systems (ETC).

In response to this technological trend toward using higher frequency radio waves, there is a growing demand for radio wave absorbers that can absorb high frequency radio waves from tens of gigahertz to the millimeter wave band, as a way to absorb unwanted radio waves.

As a radio wave absorber capable of absorbing such high frequency radio waves, a radio wave absorber has been proposed that uses a powder with a high proportion of particles close to primary particles by regulating the particle size distribution of the hexagonal ferrite powder, thereby enabling it to absorb radio waves of 1 GHz or higher with a thinner sheet thickness than conventional ones (see Patent Document 1).

In addition, a sintered body of hexagonal ferrite with limited orientation of ferrite crystal grains and a manufacturing method thereof have been proposed as a radio wave absorber that has high absorption characteristics for radio waves in the GHz band and has no anisotropy in radio wave absorption properties (see Patent Document 2).

JP 2012-216865 A JP 2018-154541 A

By including magnetic iron oxide as a radio wave absorber, high frequency radio waves can be absorbed by the magnetic resonance of the magnetic iron oxide. Hexagonal ferrite and epsilon iron oxide, which were also used in the above-mentioned conventional technology, are already in practical use as such magnetic iron oxides.

To produce a radio wave absorber that contains hexagonal ferrite as a radio wave absorbing material, a composition is prepared as a molding material by mixing a powder of sintered particles of hexagonal ferrite into an organic binder, and then molding the composition by injection molding, extrusion molding, or other molding process, or the composition is applied by a coater method or other method, dried, and then formed into a sheet of a desired thickness by an appropriate calendaring process, or other method.

The inventors have confirmed that when the composition that is the molding material is molded or when it undergoes a stirring process using a kneader or the like, the radio wave absorption characteristics of the processed radio wave absorber differ from the radio wave absorption characteristics that were previously understood at the time of the molding material, and more specifically, undesirable situations arise, such as a shift in the frequency of the absorbed radio waves or a decrease in the radio wave absorption capacity at a specific frequency. In this way, when the designed absorption frequency and the actual absorption frequency of the processed radio wave absorber differ, it is not possible to stably manufacture a radio wave absorber with the desired radio wave absorption characteristics.

The present disclosure aims to solve the above-mentioned conventional problems and to obtain a molding material for radio wave absorbing components that does not significantly change in the radio wave absorbing properties that it possesses as a molding material even after undergoing processes in which a high mechanical shear force is applied to the radio wave absorbing composition, such as molding and kneading processes, and a radio wave absorber produced using this molding material for radio wave absorbing components.

In order to solve the above problems, the molding member for radio wave absorbing material disclosed in this application is a molding material for radio wave absorbing material that contains magnetic iron oxide that undergoes magnetic resonance with radio waves having a frequency of 20 GHz to 300 GHz and an organic binder, and is molded into a predetermined shape, the magnetic iron oxide is hexagonal ferrite, and when the point A on the hysteresis curve where a positive magnetic field of +10 kOe or more and +15 k or less is applied and the point B where a negative magnetic field of -10 kOe or more and -15 kOe or less is applied by decreasing the applied magnetic field strength from point A and magnetizing the hysteresis curve where the applied magnetic field is in the range of 0 to -10 kOe, is taken as P1, the apex of the maximum peak in the positive direction in the curve obtained by differentiating the hysteresis curve from point A to point B is taken as P2, and the apex of the peak having the next extreme value in the positive direction is taken as P2, the value of P2/P1 is 0.15 or less.

The radio wave absorber disclosed in this application is characterized in that it is produced by molding any one of the molding materials for radio wave absorbing members disclosed in this application.

The molding material for radio wave absorbing components disclosed in this application has a P2/P1 value of 0.15 or less, where P1 and P2 are the apexes of two peaks in a curve obtained by differentiating a hysteresis curve twice. By using such a molding material for radio wave absorbing components, the change in radio wave absorption characteristics before and after molding is suppressed within a specified range, and a radio wave absorber suitable for use in applications where radio waves of a specific frequency are used can be produced.

1 is a cross-sectional view illustrating the configuration of a radio wave absorbing sheet, which is a radio wave absorber produced from a molding material for radio wave absorbing members according to this embodiment. FIG. 2 is a diagram illustrating a hysteresis loop of the first radio wave absorbing member molding material according to the present embodiment. FIG. 2 is a diagram showing a curve obtained by differentiating a hysteresis curve of a first molding material for a radio wave absorbing member according to the present embodiment. FIG. 2 is a diagram showing a curve obtained by differentiating twice the hysteresis curve of the first molding material for radio wave absorbing member according to the present embodiment. 3 is a diagram showing the radio wave absorption characteristics before and after molding of the first radio wave absorbing member molding material according to the present embodiment. FIG. FIG. 4 is a diagram showing a curve obtained by differentiating twice the hysteresis curve of the second radio wave absorbing member molding material according to the present embodiment. FIG. 4 is a diagram showing the radio wave absorption characteristics before and after molding of the second radio wave absorbing member molding material according to the present embodiment. FIG. 11 is a diagram illustrating a hysteresis loop of the fourth radio wave absorbing member molding material according to the present embodiment. FIG. 11 is a diagram showing a curve obtained by differentiating twice the hysteresis curve of the fourth molding material for radio wave absorbing members according to the present embodiment. FIG. 11 is a diagram showing the radio wave absorption characteristics before and after molding of the fourth radio wave absorbing member molding material according to the present embodiment. FIG. 4 is a diagram illustrating a hysteresis loop of the molding material for the radio wave absorbing member of the first comparative example. FIG. 2 is a diagram showing a curve obtained by differentiating twice the hysteresis curve of the molding material for radio wave absorbing members of the first comparative example. FIG. 2 is a diagram showing the radio wave absorption characteristics before and after molding of the molding material for radio wave absorbing members of the first comparative example. FIG. 13 is a diagram showing a curve obtained by differentiating twice the hysteresis curve of the molding material for radio wave absorbing members of the second comparative example. FIG. 11 is a diagram showing the radio wave absorption characteristics before and after molding of a molding material for radio wave absorbing members of a second comparative example.

The molding material for radio wave absorbing members disclosed in this application is a molding material for radio wave absorbing members that contains magnetic iron oxide that undergoes magnetic resonance with radio waves having a frequency of 20 GHz to 300 GHz and an organic binder, and is molded into a predetermined shape, in which the magnetic iron oxide is hexagonal ferrite, and in a hysteresis curve, when a point A is defined as a point magnetized by applying a positive magnetic field of +10 kOe or more and +15 k or less, and when a point B is defined as a point magnetized by reducing the applied magnetic field strength from point A to applying a negative magnetic field of -10 kOe or more and -15 kOe or less, the hysteresis curve from point A to point B is differentiated twice, and when the apex of the maximum peak in the positive direction in the curve in the range of the applied magnetic field from 0 to -10 kOe is defined as P1, and the apex of the peak having the next extreme value in the positive direction is defined as P2, the value of P2/P1 is 0.15 or less.

In this way, the molding material for radio wave absorbing components disclosed in this application can suppress the change in radio wave absorption characteristics before and after molding within a specified range. This makes it possible to produce a radio wave absorber that effectively absorbs undesired radio waves from equipment used in specific fields where the range of frequencies used is regulated.

In addition, in the molding material for radio wave absorbing members disclosed in this application, the magnetic iron oxide is preferably strontium ferrite.

In addition, the molding material for radio wave absorbing members disclosed in this application may be one in which some of the Fe sites of the magnetic iron oxide are replaced with trivalent metal atoms.

Furthermore, in the molding material for radio wave absorbing members disclosed in this application, the binder can be any one of thermosetting rubber, thermoplastic elastomer, and thermoplastic resin.

The radio wave absorber disclosed in this application is produced by molding any of the radio wave absorbing molding materials disclosed in this application.

By using the molding material for radio wave absorbing components disclosed in this application, the change in radio wave absorption characteristics before and after molding can be suppressed within a specified range, so the radio wave absorption characteristics of the molded radio wave absorber can be fully controlled at the molding material stage. As a result, there is no significant difference between the design absorption frequency and the absorption frequency of the molded body after processing, and it is possible to easily produce a radio wave absorber that can be used in electronic devices in specific fields where the range of frequencies used is regulated, and that can prevent unwanted radio waves from being emitted or entering the electronic device.

The molding material for radio wave absorbing components disclosed in this application and the radio wave absorber produced by molding this molding material will be described below with reference to the drawings.

(Embodiment)
[Radio wave absorber]
First, as one embodiment of the radio wave absorber disclosed in the present application, a radio wave absorbing sheet, which is a radio wave absorber, will be described, which is composed of a radio wave absorbing layer containing particulate magnetic iron oxide and an organic binder.

Figure 1 is a cross-sectional view showing the configuration of a radio wave absorbing sheet, which is a radio wave absorber described in this embodiment.

Figure 1 shows the electromagnetic wave absorbing composition applied to a resin sheet 2 as a substrate and then dried to form an electromagnetic wave absorbing sheet 1.

Note that Figure 1 is a diagram provided to facilitate understanding of the configuration of the electromagnetic wave absorbing sheet according to this embodiment, and the sizes and thicknesses of the components shown in the diagram are not depicted in accordance with reality.

The electromagnetic wave absorbing sheet 1 illustrated in this embodiment is formed as an electromagnetic wave absorbing layer containing strontium ferrite powder 1a as magnetic iron oxide powder and a resin binder 1b.

In FIG. 1, a radio wave absorbing sheet is shown as an example of the radio wave absorber disclosed in this application, but the radio wave absorber disclosed in this application can be produced in the form of a block or in any shape that corresponds to the shape of the location where it is to be placed, by injection molding or extrusion molding a molding material for radio wave absorbing components using a specified mold (die).

The radio wave absorbing sheet 1 illustrated in this embodiment contains strontium ferrite magnetic iron oxide 1a, which is magnetic iron oxide that magnetically resonates with radio waves of 20 GHz to 300 GHz, in an organic material binder 1b.

[Magnetic iron oxide]
In the radio wave absorbing sheet 1 according to this embodiment, strontium ferrite is used as the particulate magnetic iron oxide 1a.

Strontium ferrite can have a resonance frequency of 60 GHz to 80 GHz by adding Al to SrFe ₁₂ O ₁₉ , making it possible to produce a radio wave absorber compatible with 60 GHz wireless LAN. By adding Al, the frequency at which radio wave absorption is exhibited shifts to the high frequency side, which is thought to correspond to an increase in the value of the anisotropic magnetic field (H _A ).

In addition, barium ferrite magnetic powder can be used as the hexagonal ferrite magnetic powder used in the radio wave absorbing sheet disclosed in the present application. In M-type (magnetoplumbite type) ferrites containing barium ferrite, the imaginary part (μr″) of the complex permeability related to radio wave absorption becomes high at the frequency at which resonance occurs when the magnetic material is magnetized at high frequency, and therefore the magnetic resonance frequency f is proportional to the anisotropic magnetic field (H _A ) of the material, and therefore the higher the anisotropic magnetic field (H _A ) of the material, the higher the value of the magnetic resonance frequency f. The magnetic resonance frequency f of BaFe ₁₂ O ₁₉ , which is an M-type ferrite, is calculated to have a value of H _A ranging from 1.35 MA/m to 48 GHz, and can absorb radio waves in the high GHz band. In addition, by replacing a part of Fe ³⁺ with (TiMn) ³⁺ or Al ³⁺ , the value of the anisotropic magnetic field (H _A ) can be controlled to control the magnetic resonance frequency f in the range of 5 GHz to 150 GHz.

[binder]
The radio wave absorbing sheet 1 according to this embodiment has overall flexibility due to the magnetic iron oxide particles 1a being dispersed in the organic binder 1b.

Thermosetting plastics, thermoplastic plastics, and elastomers can be used as the organic binder 1b used in the molding material for the radio wave absorbing component to produce the radio wave absorbing sheet 1.

Thermosetting plastics that can be used include phenolic resin (PF), urea resin (UF), melamine resin (MF), unsaturated polyester (UP), epoxy resin (EP), silicone resin (SI), and polyurethane (PUR).

Thermoplastics include general-purpose plastics, engineering plastics, and super engineering plastics. General-purpose plastics include polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), polystyrene (PS), acrylonitrile butadiene styrene (ABS), acrylonitrile styrene (AS), polymethyl methacrylate (PMMA), polyvinyl alcohol (PVA), polyvinylidene chloride (PVDC), and polyethylene terephthalate (PET).

Engineering plastics that can be used include polyamide (PA), polyacetal (POM), polycarbonate (PC), polypropylene ether (PPE), polybutylene terephthalate (PBT), ultra-high molecular weight polyethylene (U-EP), and polyvinylidene fluoride (PVDF).

Polysulfone (PSU), polyethersulfone (PES), polyphenylene sulfide (PPS), polyarylate (PAR), polyamide-imide (PAI), polyetherimide (PEI), polyetheretherketone (PEEK), polyimide (PI), liquid crystal polymer (LCP), and polytetrafluoroethylene (PTFE) can be used as super engineering plastics.

Elastomers also include thermosetting rubber (vulcanized rubber) and thermoplastic elastomers. Examples of thermosetting rubber (vulcanized rubber) that can be used include styrene butadiene rubber (SBR), nitrile rubber (NBR), butadiene rubber (BR), chloroprene rubber (CR), ethylene propylene diene rubber (EPDM), butyl rubber (IIR), isobutylene rubber (IR), acrylic rubber (ACM), and silicone rubber (Q).

Thermoplastic elastomers that can be used include softened polyvinyl chloride, heat-resistant polyvinyl chloride, ethylene-vinyl acetate copolymer (EVA), ethylene propylene rubber (EPR), styrene-based TPE (TPS), olefin-based TPE (TPO), urethane-based TPE (TPU), polyester-based (TPPE), chlorinated polyethylene, etc.

From an environmental perspective, it is preferable to use organic materials that do not contain halogens as binders. These resin materials are commonly used as binder materials for resin sheets and are therefore easily available.

Here, in this specification, having flexibility refers to a state in which the radio wave absorbing sheet 1 can be bent to a certain degree, in other words, a state in which the radio wave absorbing sheet 1 returns to its original shape without undergoing plastic deformation such as breakage when it is bent and then returned to its original shape. Also, even if the radio wave absorber is not in a sheet shape but is a block body having a certain thickness or more, for example, when an external force is applied while holding both ends of the radio wave absorber, the radio wave absorber will bend, and when this external force is released, the radio wave absorber can easily return to its original shape.

As a molding material for radio wave absorbing members for forming the radio wave absorber 10, it is important to disperse the magnetic iron oxide powder well in the binder made of an organic material. For this reason, the binder 2 may contain, as a dispersant, arylsulfonic acids such as phenylphosphonic acid and phenylphosphonic acid dichloride, alkylphosphonic acids such as methylphosphonic acid, ethylphosphonic acid, octylphosphonic acid, and propylphosphonic acid, or polyfunctional phosphonic acids such as hydroxyethanediphosphonic acid and nitrotrismethylenephosphonic acid. These phosphoric acid compounds are flame retardant and function as dispersants for the magnetic iron oxide powder, allowing the magnetic iron oxide to be well dispersed in the binder.

More specifically, dispersants that can be used include phenylphosphonic acid (PPA) manufactured by Wako Pure Chemical Industries, Ltd. or Nissan Chemical Industries, Ltd., and oxidized phosphoric acid ester "JP-502" (product name) manufactured by Johoku Chemical Industry Co., Ltd.

As an example of the composition of the molding material for radio wave absorbing components, 100 parts of magnetic iron oxide powder can contain 2 to 50 parts of organic binder and 0.1 to 15 parts of phosphate compound. If the organic binder is less than 2 parts, the magnetic iron oxide cannot be dispersed well, and the specified shape of the radio wave absorber cannot be maintained. If it is more than 50 parts, the volume content of magnetic iron oxide in the radio wave absorber becomes small, and the magnetic permeability decreases, resulting in a smaller radio wave absorption effect.

If the content of the phosphate compound is less than 0.1 parts, the magnetic iron oxide cannot be dispersed well using a resin binder. If it is more than 15 parts, the effect of dispersing the magnetic iron oxide well becomes saturated. The volume content of the magnetic iron oxide in the radio wave absorbing layer becomes small, and the magnetic permeability becomes low, resulting in a small radio wave absorption effect.

[Method of manufacturing radio wave absorber]
Here, an example of a method for manufacturing the radio wave absorber according to this embodiment will be described.

The molding material for radio wave absorbing members according to this embodiment is produced as a magnetic compound containing magnetic iron oxide powder, a binder made of an organic material such as rubber, and an appropriate filler. The magnetic compound can be obtained by kneading magnetic iron oxide powder and a binder, mixing the resulting kneaded product with a crosslinking agent, and adjusting the viscosity.

As shown in Figure 1, the radio wave absorbing sheet 1 as a radio wave absorber can be produced by applying a magnetic compound to a predetermined thickness, drying it, and then calendering it.

When preparing the electromagnetic wave absorbing sheet 1, as shown in FIG. 1, the magnetic compound prepared above is applied onto a resin sheet 2. As an example of the resin sheet 2, a 38 μm-thick polyethylene terephthalate (PET) sheet with a surface treated for release by a silicone coat can be used. The magnetic compound is applied onto the resin sheet 2 using a coating method such as a table coater method or a bar coater method. The wet magnetic compound is then dried and further subjected to a calendaring process. The thickness of the electromagnetic wave absorbing sheet 1 can be controlled by the coating thickness and the conditions of the calendaring process. The electromagnetic wave absorbing sheet 1 after the calendaring process is peeled off from the resin sheet 2 to obtain an electromagnetic wave absorbing sheet 1 of a desired thickness. The calendaring process may be performed as necessary, and if the volume content of the magnetic iron oxide powder is within a predetermined range when the magnetic compound is dried, the calendaring process may not be performed.

If the thickness of the formed radio wave absorbing sheet 1 is too thin to obtain the required strength of the radio wave absorbing sheet 1, the resin sheet 2 used as the base material can be used as a base film, and the radio wave absorbing sheet 1 can be laminated to the resin base film 2. When the resin sheet 2 is used as the base film, in addition to the various resin films such as the PET film described above, rubber and paper materials such as Japanese paper can be used. Since the material and thickness of the base film do not affect the radio wave absorbing characteristics of the radio wave absorbing sheet 1, a base film made of an appropriate material and having an appropriate thickness can be selected from the practical viewpoint of the strength of the radio wave absorbing sheet and ease of handling.

Next, we will explain how to create a radio wave absorber with a desired shape.

As an example, when using a rubber binder, 80 parts by weight of strontium ferrite as the magnetic iron oxide powder and 18 parts by weight of silicone rubber KE-510-U (product name: manufactured by Shin-Etsu Chemical Co., Ltd.) as the rubber binder are kneaded in a pressurized batch kneader. 2 parts by weight of 2.5 dimethyl-2.5 bishexane C-8A (product name: manufactured by Shin-Etsu Chemical Co., Ltd.) as the crosslinking agent are mixed into this kneaded mixture.

The radio wave absorbing member molding material thus obtained is vulcanized and molded using a press molding machine into the desired shape of a sheet, O-ring (packing), etc. As described above, the radio wave absorber disclosed in this application is based on silicone rubber, and therefore has the deformability to curve as a whole and absorb some degree of unevenness. For example, by arranging it so as to cover a specific circuit component that is a noise source mounted on a circuit board, it is possible to tightly enclose the periphery of the noise source electronic component and improve the shielding of undesirable leaked radio waves.

When making a radio wave absorber in a block shape, such as a rectangular parallelepiped, the magnetic compound, which is the molding material for radio wave absorbing components, is cross-linked and molded into a sheet at a temperature of 150°C, for example, using a hydraulic press. After that, a secondary cross-linking process is performed in a thermostatic chamber at a temperature of, for example, 170°C, to produce a radio wave absorber of the desired shape. In this way, when a rubber-based binder is used, it can be produced by direct molding using heat vulcanization by pressing.

When a thermoplastic resin is used as the binder, 80 parts by weight of strontium ferrite as the magnetic iron oxide powder and 20 parts by weight of nylon 6 1013B (product name: manufactured by Ube Industries, Ltd.) as the thermoplastic resin are kneaded in a pressurized batch kneader.

The molding material for radio wave absorbing components thus obtained is injection molded using an injection molding machine to be molded into the desired shape of a bracket, cover, etc. As described above, the radio wave absorber disclosed in this application is based on nylon 6, and therefore has overall toughness and strength. For this reason, it is suitable for use as a radio wave absorber to be placed in, for example, an in-vehicle millimeter wave radar for collision prevention that is subject to vibration, and can absorb and suppress unwanted radio waves that are diffusely reflected from the radio waves irradiated from the radar.

[Adhesive layer]
Although not shown in Fig. 1, an adhesive layer can be formed on at least a part of the surface of the radio wave absorber so that the radio wave absorber according to this embodiment can be easily fixed to a predetermined position. In particular, in the case of a thin block-shaped radio wave absorber, by providing an adhesive layer on at least one of its main surfaces, the radio wave absorber can be easily attached to a desired position on the inner surface of a housing that houses an electric circuit or on the inner or outer surface of an electric device. In particular, since the radio wave absorber according to this embodiment has elasticity due to the use of a rubber binder, the adhesive layer 2 allows the radio wave absorber to be easily attached to a curved surface, improving the ease of handling of the radio wave absorber.

The adhesive layer may be a known material used as an adhesive layer for adhesive tapes, etc., an acrylic adhesive, a rubber adhesive, a silicone adhesive, etc. Also, a tackifier or a crosslinking agent may be used to adjust the adhesive strength to the adherend and reduce glue residue. The adhesive strength to the adherend is preferably 5N/10mm to 12N/10mm. If the adhesive strength is less than 5N/10mm, the radio wave absorber may easily peel off or shift from the adherend. If the adhesive strength is more than 12N/10mm, it will be difficult to peel the radio wave absorber off the adherend.

The thickness of the adhesive layer is also preferably a predetermined thickness, for example, about 20 μm to 1 mm or more, so that the radio wave absorber does not easily peel off. When reattaching the radio wave absorber as needed, it is necessary to consider the adhesive strength and thickness of the adhesive layer so that the adherend is not damaged when peeling it off, the radio wave absorber is not damaged, and glue residue is not left behind.

In addition, when adhering the radio wave absorber to a specified surface, even if the radio wave absorber does not have an adhesive layer, it is possible to attach the radio wave absorber by providing adhesiveness to the surface of the member on which the radio wave absorber is to be placed. Also, by using double-sided tape or adhesive, the radio wave absorber can be attached to a specified location. In this respect, it is clear that the adhesive layer is not an essential component of the radio wave absorber shown in this embodiment.

[Radio wave absorption characteristics]
The magnetic properties stipulated for the molding material for radio wave absorbing members disclosed in the present application will be specifically described below.

When measuring the magnetic properties, a radio wave absorber containing a specified magnetic iron oxide was cut to create a disk-shaped sample with a diameter of 8 mm and a thickness of 0.1 mm to 10 mm, and a vibration sample magnetometer VSM-P7 (product name) manufactured by Toei Kogyo Co., Ltd. was used to measure the applied magnetic field in the range of -15 kOe to +15 kOe. The measurement time constant Tc was 0.03 sec, the drawing step was 40 bits, and the wait time was 0.2 sec.

The average particle size of the magnetic iron oxide was measured as follows:

Using a Hitachi S-4800 (product name) scanning electron microscope (SEM), the maximum diameter of 100 particles of iron oxide powder in one field of view was measured from photographs taken at an acceleration voltage of 2 kV, magnification of 10,000 times, and observation conditions of U-LA100. The simple average of the measured particle diameters was used as the average particle diameter.

Figure 2 shows the magnetization curve for the first radio wave absorbing member molding material according to this embodiment.

The first molding material for radio wave absorbing members (hereinafter, appropriately referred to as "molding material of Example 1") used 79 parts by weight of strontium ferrite as magnetic iron oxide powder, 17 parts by weight of nylon 6 (1013B (product name) manufactured by Ube Industries, Ltd.) as a binder, and 4 parts by weight of a low-volatile carboxylic acid derivative as a dispersant. The average particle size of the strontium ferrite was 3.6 μm .

After stirring the magnetic iron oxide powder, binder material, and dispersant in a tumbler, they were continuously mixed in a twin-screw extruder (15 nmφ, L/D = 45) at a temperature of 235°C to 270°C and a screw rotation speed of 100 rpm. The strand-like mixture extruded from the nozzle was cooled with water and cut into pieces 3 mm long using a pelletizer to form molding material, and a molded body 120 mm square and 2 mm thick was produced using an injection molding machine.

A disk-shaped sample of Example 1 with a diameter of 8 mm and a thickness of 1 mm was made from this molded body.

As shown in Figure 2, the magnetization curve, which indicates the strength of magnetization remaining in magnetic iron oxide when an external magnetic field of varying strength is applied, forms a so-called hysteresis curve.

The relationship shown in the following formula (1) holds between the value of the anisotropic magnetic field (H _A ) and the magnetic resonance frequency fr of the magnetic material.

fr=ν/2π*H _A (1)
Here, ν is the gyromagnetic constant, and its value is determined depending on the type of magnetic material.

In this way, in a gyromagnetic resonance type magnetic body, a proportional relationship is established between the value of the anisotropic magnetic field (H _A ) and the magnetic resonance frequency fr.

For example, when a magnetic body contains magnetic materials with different anisotropic magnetic fields (H _A ), magnetic resonance occurs at different frequencies, converting radio waves of the frequencies into heat and attenuating them. As a result, radio waves of different predetermined frequencies can be absorbed. It is also known that the magnitude of the anisotropic magnetic field (H _A ) of magnetic iron oxide varies depending on its particle size. Therefore, by analyzing the hysteresis curve of magnetic iron oxide, the particle size distribution of magnetic iron oxide contained in a sample can be understood.

Figure 3 shows the curve (SFD: Switching Field Distribution) obtained by differentiating the hysteresis curve of the molding material of Example 1 shown in Figure 2 from point A, where the applied magnetic field is +15 kOe, to point B, where the applied magnetic field is -15 kOe. Note that Figure 3 shows the SFD curve for applied magnetic fields from 0 kOe to -10 kOe.

More specifically, in the measurement device described in this embodiment, since the hysteresis curve shown in Figure 2 is drawn from point A to point B using 637 pieces of data, a linear least-squares approximation was performed for the 11th to 627th measurement points, including 10 points before and after the 11th measurement point, for a total of 21 points, and the slope of the obtained approximation equation was taken as the differential value at that measurement point (the value obtained by first differentiation).

Figure 4 shows the hysteresis curve of the molding material of Example 1 shown in Figure 2, obtained by differentiating twice from point A, where the applied magnetic field is +15 kOe, to point B, where the applied magnetic field is -15 kOe. Figure 4 also shows a twice-differentiated curve from an applied magnetic field of 0 kOe to -10 kOe.

Specifically, the differential values at each measurement point from 11th to 627th points on the hysteresis curve shown in Figure 2 were again subjected to least squares approximation of the differential values at measurement points from 21st to 617th points, and the differential values at 10 points before and after each measurement point, for a total of 21 measurement points, to determine the slope of a straight line, and this value was taken as the second derivative value (the value obtained by differentiating twice) of the hysteresis curve at that measurement point.

In the molding material of Example 1 of this embodiment, as shown in Figure 4, a first positive peak P1 (3.12 x ^10-6 ) appears when the applied magnetic field is in the vicinity of -4000 kOe, and a second positive peak P2 (4.33 x ^10-7 ) appears when the applied magnetic field is in the vicinity of -9500 kOe. From these P1 and P2, it is found that P2/P1 is 0.14.

Figure 5 shows the radio wave absorption characteristics of the molding material of Example 1 and the radio wave absorption characteristics of the first radio wave absorber obtained by injection molding the molding material of Example 1.

The radio wave absorption characteristics were measured using the free space method to measure the amount of radio wave absorption (amount of radio wave attenuation). Specifically, using an Anritsu Corporation millimeter wave network analyzer ME7838A (product name), input waves (millimeter waves) of a specified frequency were irradiated from a transmitting antenna through a dielectric lens to either sample 1 or a radio wave absorber made from sample 1, and the transmitted radio waves were measured by a receiving antenna placed on the back side. The strength of the irradiated radio wave and the strength of the transmitted radio wave were each determined as voltage values, and the amount of radio wave attenuation was calculated in dB from the difference in strength.

As shown in Figure 5, the difference in absorption peak frequency between the radio wave absorption characteristics of the molding material of Example 1 (symbol 5a) and the radio wave absorption characteristics of the radio wave absorber molded using the molding material of Example 1 (symbol 5b) is small at 0.1 GHz, and the curves showing the two radio wave absorption characteristics almost overlap, confirming that the change in the radio wave absorption characteristics before and after molding was small.

Next, a molding material for radio wave absorbing components of Example 2 was prepared using strontium ferrite as the magnetic iron oxide, and a molding material for radio wave absorbing components of Example 3 was prepared using strontium ferrite as the magnetic iron oxide.

In addition, a molding material for radio wave absorbing members of Example 4 using barium ferrite as the magnetic iron oxide and a molding material for radio wave absorbing members of Example 5 using barium ferrite as the magnetic iron oxide were prepared. Details of the molding materials of Examples 2 to 5 are as follows.

[Example 2]
The procedure was the same as in Example 1, except that strontium ferrite having an average particle size of 4.1 μm was used as the magnetic iron oxide powder.

[Example 3]
The procedure was the same as in Example 1, except that strontium ferrite having an average particle size of 2.3 μm was used as the magnetic iron oxide powder.

[Example 4]
The procedure was the same as in Example 1, except that barium ferrite having an average particle size of 4.8 μm was used as the magnetic iron oxide powder.

[Example 5]
The procedure was the same as in Example 1, except that barium ferrite having an average particle size of 5.0 μm was used as the magnetic iron oxide powder.

Furthermore, as comparative examples, Comparative Example 1 and Comparative Example 2, which are molding materials for radio wave absorbing components using strontium ferrite as the magnetic iron oxide, and Comparative Example 3, which is a molding material for radio wave absorbing components using barium ferrite as the magnetic iron oxide, were prepared. Details of the molding materials for radio wave absorbing components of Comparative Example 1, Comparative Example 2, and Comparative Example 3 are as follows.

[Comparative Example 1]
As the magnetic iron oxide powder, strontium ferrite having an average particle size of 4.1 μm was used, the magnetic iron oxide powder was 82 parts by weight, nylon 6 was 18 parts by weight, and no low-volatile carboxylic acid derivative was used. The rest was the same as in Example 1.

[Comparative Example 2]
The procedure was the same as in Example 1, except that strontium ferrite having an average particle size of 49.4 μm was used as the magnetic iron oxide powder.

[Comparative Example 3]
The procedure was the same as in Example 1, except that barium ferrite having an average particle size of 5.5 μm was used as the magnetic iron oxide powder.

The magnetic properties of the molding materials of Examples 2 and 4, as well as those of Comparative Examples 1 and 2, as molding materials for radio wave absorbing components were measured in the same manner as in the molding material of Example 1 described above, and the obtained hysteresis curves were differentiated twice from point A to point B.

Figure 6 shows the curve resulting from twice differentiating the hysteresis curve of the molding material of Example 2.

Figure 7 shows the radio wave absorption characteristics of the molding material of Example 2 when used as a molding material for radio wave absorbing components, and the radio wave absorption characteristics of a radio wave absorber formed by injection molding the molding material.

6, the curve obtained by differentiating the hysteresis curve twice shows a first positive peak P1 (3.99× ^10-6 ) near an applied magnetic field of -4000 kOe and a second positive peak P2 (6.02× ^10-7 ) near an applied magnetic field of -9500 kOe. From P1 and P2, it is found that P2/P1 is 0.15.

In addition, when comparing the radio wave absorption characteristics of the molded member of Example 2 (reference symbol 7a) shown in Figure 7 with the radio wave absorption characteristics of a radio wave absorber molded using the molded member of Example 2 (reference symbol 7b), it can be seen that by molding into a radio wave absorber, the absorption curve shifts to the lower frequency side compared to before molding. However, the respective peak absorption frequencies are 77 GHz and 76 GHz, and the shift in the absorption peak frequency is only 0.9 GHz. In addition, it can be seen that the transmission attenuation, which indicates the absorption capacity, is approximately 14.5 dB, and almost the same absorption characteristics are obtained for both.

Figure 8 shows the hysteresis curve of the molded part of Example 4.

The molded member of Example 4 uses barium ferrite as the magnetic iron oxide, and therefore the shape of the hysteresis curve is different from that of the molding material for radio wave absorbing members of Example 1, which uses strontium ferrite as the magnetic iron oxide shown in Figure 2.

In this case too, as shown in Figure 8, a curve is obtained by differentiating twice from point A, where the applied magnetic field is +15 kOe, to point B, where the applied magnetic field is -15 kOe.

Figure 9 shows the curve obtained by twice differentiating the hysteresis curve from point A to point B of the molded member of Example 4. Note that, like Figures 4 and 6, Figure 9 also shows a twice-differentiated curve for an applied magnetic field from 0 kOe to -10 kOe.

9, the curve obtained by differentiating the hysteresis curve twice shows a first positive peak P1 (1.33× ^10-5 ) near an applied magnetic field of -1000 kOe and a second positive peak P2 (5.84× ^10-7 ) near an applied magnetic field of -9000 kOe. From P1 and P2, it is found that P2/P1 is 0.04.

In addition, when comparing the radio wave absorption characteristics of the molded member of Example 4 shown in Figure 10 (symbol 10a) with the radio wave absorption characteristics of the radio wave absorber molded using the molded member of Example 4 (symbol 10b), it is found that the shift in the absorption peak frequency before and after molding is approximately 0 GHz, and the radio wave absorption characteristics have hardly changed at all.

Next, we will consider the comparative example of a molding material for radio wave absorbing components.

Figure 11 shows the hysteresis curve of the molding material of Comparative Example 1, which is a molding material for radio wave absorbing components, a sample created as a comparative example.

Figure 12 shows the curve obtained by differentiating twice from point A to point B of the hysteresis curve of the molding material of Comparative Example 1. Note that, like Figures 4, 6, and 9, Figure 12 also shows a twice-differentiated curve from an applied magnetic field of 0 kOe to -10 kOe.

12, the curve obtained by differentiating the hysteresis curve of the molding material of Comparative Example 2 twice shows a first positive peak P1 (3.54× ^10-6 ) at an applied magnetic field of about -4000 kOe and a second positive peak P2 (6.14× ^10-7 ) at an applied magnetic field of about -9500 kOe. From P1 and P2, it is found that P2/P1 is 0.17.

Figure 13 shows the radio wave absorption characteristics of the molded member of Comparative Example 1 when it is used as a molding material for radio wave absorbing members, and the radio wave absorption characteristics of the radio wave absorber after injection molding.

When comparing the radio wave absorption characteristics (symbol 13a) of the molding material of Comparative Example 1 shown in Figure 13 with the radio wave absorption characteristics (symbol 13b) of the radio wave absorber after molding, it can be seen that by molding into a radio wave absorber, the absorption characteristics are shifted to the lower frequency side compared to before molding. As shown in Figure 13, the peak absorption frequency of the radio wave absorption characteristics (symbol 13a) of the molded member of Comparative Example 1 is about 77.3 GHz, while the peak absorption frequency of the radio wave absorption characteristics (symbol 13b) of the radio wave absorber molded using the molded member of Comparative Example 1 is about 76.1 GHz, showing a shift in the absorption peak frequency of about 1.2 GHz.

Figure 14 shows the curve obtained by differentiating twice from point A to point B of the hysteresis curve of the molding material of Comparative Example 2. Note that, like Figures 4, 6, 9, and 12, Figure 14 also shows a twice-differentiated curve from an applied magnetic field of 0 kOe to -10 kOe.

14, the curve obtained by differentiating the hysteresis curve twice shows a first positive peak P1 (3.44× ^10-6 ) near an applied magnetic field of -4000 kOe and a second positive peak P2 (11.16× ^10-6 ) near an applied magnetic field of -9500 kOe. From P1 and P2, P2/P1 is found to be 0.34.

Figure 15 shows the radio wave absorption characteristics of the molding material of Comparative Example 2 when used as a molding material for radio wave absorbing components, and the radio wave absorption characteristics of the radio wave absorber after injection molding.

When comparing the radio wave absorption characteristics (symbol 15a) of the molding material of Comparative Example 2 shown in Figure 15 with the radio wave absorption characteristics (symbol 15b) of the radio wave absorber after molding using Comparative Example 2, it can be seen that by molding into a radio wave absorber, the peak absorption frequency of the radio wave absorption characteristics (symbol 15a) of the molding material is about 77.5 GHz, while the peak absorption frequency of the radio wave absorption characteristics (symbol 15b) of the radio wave absorber molded using Comparative Example 2 is about 75.6 GHz, resulting in a shift of about 1.9 GHz in the peak absorption frequency.

The above measurement results, as well as the values of P1, P2, P2/P1, and the peak frequency deviation for the molded parts of Example 3, Example 5, and Comparative Example 3, are summarized in Table 1 below.

The reason why the shift in the absorption peak frequency was larger in the molding material of Comparative Example 3 compared to the molding materials of Examples 4 and 5, which also used barium ferrite magnetic powder as the magnetic iron oxide, is thought to be that, as with the molding material of Comparative Example 2 described above, the average particle diameter of the barium ferrite magnetic powder was large, so the particle diameter changed due to the shear force applied to the particles in the injection molding process, resulting in a difference in the effective average particle diameter before and after injection molding, and as a result, the anisotropic magnetic field (H _A ) of the magnetic iron oxide changed.

For example, the frequency of radio waves is regulated according to the intended use, such as 76.5 GHz ±0.5 GHz for vehicle-mounted radar. For this reason, if the peak absorption frequency that was determined at the stage of the molding material for radio wave absorbing components shifts significantly in the state of the radio wave absorber after molding, as in the molded components of Comparative Example 1 and Comparative Example 2, the performance of the radio wave absorber used in the originally intended technical field will be significantly reduced.

Therefore, if it is known at the stage of the molding material for radio wave absorbing components that the shift in the peak frequency of the absorbed radio waves before and after molding is small, there will be no large difference between the designed absorption frequency and the absorption frequency of the molded body after processing, and radio wave absorbers can be produced with peace of mind. In addition, by not using materials that cause a large shift, the yield of radio wave absorbers can be improved and costs can be reduced.

The molding material for radio wave absorbing members disclosed in this application is specified such that, when the point A on the hysteresis curve is the point magnetized by applying a positive magnetic field of +10 kOe or more and +15 kOe or less, and the point B is the point magnetized by reducing the applied magnetic field strength from point A and applying a negative magnetic field of -10 kOe or more and -15 kOe or less, the value of P2/P1 is 0.15 or less when the apex of the maximum peak in the positive direction in the curve in which the applied magnetic field ranges from 0 to -10 kOe obtained by differentiating the hysteresis curve from point A to point B is P1 and the apex of the peak having the next extreme value in the positive direction is P2. By specifying it in this way, the molding material for radio wave absorbing members disclosed in this application can suppress the change in the radio wave absorption characteristics before and after molding into a radio wave absorber, and realize a radio wave absorber that absorbs radio waves of a specified frequency well.

In the above measurements, the amount of deviation in the absorption peak frequency before and after injection molding was measured. However, when magnetic iron oxide powder was mixed with a resin binder to prepare an electromagnetic wave absorbing paste, and this was used to prepare a sheet-shaped electromagnetic wave absorbing sheet, it was confirmed that, as in the case of injection molding, if the value of P2/P1 was 0.15 or less, the amount of deviation between the absorption peak frequency in the molding material state and the absorption peak frequency after molding into a sheet could be suppressed to 1 GHz or less.

In addition to the strontium ferrite and barium ferrite exemplified in the above embodiment, other hexagonal ferrites such as Pb ferrite are also thought to exhibit similar radio wave absorption characteristics, but Pb ferrite is not suitable for practical use because it contains lead.

In addition, in the above embodiment, it was explained that the absorption frequency peak can be shifted by substituting some of the Fe sites of hexagonal ferrite with Al, but the metal to be substituted can be gallium, Ti-Cu-Zn, rhodium, etc., other than aluminum.

As explained above, the molding material for radio wave absorbing components disclosed in this application, and the radio wave absorber obtained by molding this material, have two peak values P1 and P2 in the curve obtained by differentiating the hysteresis curve of the molding material twice, and the P2/P1 value is 0.15 or less, so that the shift in the radio wave absorption characteristics, particularly the absorption peak frequency, before and after molding can be suppressed within a certain range (approximately 1 GHz).

The strength of the external magnetic field used to measure the hysteresis curve is set to 15 kOe to -15 kOe because applying an external magnetic field in at least this range will result in a good hysteresis curve. Therefore, there is no problem if the absolute value of the applied external magnetic field is greater than 15 kOe. Simply measure the hysteresis curve when the external magnetic field is in the range of 15 kOe to -15 kOe, and find the values of P1 and P2 from the second derivative curve.

The molding material for radio wave absorbing components disclosed in this application is useful as a molding material that effectively absorbs radio waves from 20 GHz to 300 GHz and suppresses changes in radio wave absorption characteristics before and after molding. In addition, radio wave absorbers molded using this molding material can be produced at low cost as radio wave absorbers in which fluctuations in the radio wave absorption characteristics are suppressed to a certain range, making them extremely useful in practical use.

1a Magnetic iron oxide particles 1b Binder 10 Radio wave absorber

Claims (5)

A molding material for a radio wave absorbing member, which comprises a magnetic iron oxide that undergoes magnetic resonance by radio waves having a frequency of 20 GHz to 300 GHz and an organic binder, and is molded into a predetermined shape,
The magnetic iron oxide is a hexagonal ferrite having the same composition ,
A molding material for radio wave absorbing members, characterized in that, in a hysteresis curve, a point A is a point magnetized by applying a positive magnetic field of +10 kOe or more and +15 kOe or less, and a point B is a point magnetized by reducing the applied magnetic field strength from point A to point B and applying a negative magnetic field of -10 kOe or more and -15 kOe or less, the applied magnetic field strength being reduced from point A, and when the apex of the maximum peak in the positive direction in a curve in which the applied magnetic field ranges from 0 to -10 kOe obtained by differentiating the hysteresis curve twice from point A to point B is P1 and the apex of the peak having the next extreme value in the positive direction is P2, the value of P2/P1 is 0.15 or less. The molding material for radio wave absorbing members according to claim 1, wherein the magnetic iron oxide is strontium ferrite. The molding material for radio wave absorbing members according to claim 1 or 2, in which a portion of the Fe sites of the magnetic iron oxide is substituted with trivalent metal atoms. The molding material for radio wave absorbing members according to any one of claims 1 to 3, wherein the binder is any one of thermosetting rubber, thermoplastic elastomer, and thermoplastic resin. A radio wave absorber produced by molding a molding material for radio wave absorbing members according to any one of claims 1 to 4.

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