阅读说明：本技术 电波吸收体及复合物 (Radio wave absorber and composite ) 是由 桥本浩一 于 2020-03-18 设计创作，主要内容包括：本发明提供一种电波吸收体及复合物,所述电波吸收体不具有金属层,并且毫米波段中的透射衰减量及反射衰减量同为10dB以上。本发明提供一种电波吸收体及复合物,所述电波吸收体具有包含磁性粉体和粘合剂的电波吸收层,并且不具有金属层,其中,上述电波吸收层中的上述磁性粉体的填充率为35体积％以下,当将上述电波吸收层中的上述磁性粉体的填充率设为P体积％且将上述电波吸收层的厚度设为Qmm时,满足0.65≤(P/100)×Q的关系。(The invention provides a radio wave absorber and a composite, wherein the radio wave absorber does not have a metal layer, and the transmission attenuation and the reflection attenuation in a millimeter wave band are both more than 10 dB. The present invention provides a radio wave absorber and a composite, wherein the radio wave absorber has a radio wave absorbing layer containing magnetic powder and a binder, and does not have a metal layer, wherein the filling rate of the magnetic powder in the radio wave absorbing layer is less than 35% by volume, and when the filling rate of the magnetic powder in the radio wave absorbing layer is P% by volume and the thickness of the radio wave absorbing layer is Qmm, the relation of (P/100) multiplied by Q of 0.65 is not more than.)

1. A radio wave absorber having a radio wave absorbing layer containing magnetic powder and a binder and having no metal layer, wherein,

the filling rate of the magnetic powder in the electric wave absorption layer is 35 vol% or less,

the magnetic powder in the radio wave absorption layer satisfies a relation of 0.65 ≦ (P/100). times.Q, assuming that the filling rate of the magnetic powder in the radio wave absorption layer is P volume% and the thickness of the radio wave absorption layer is Qmm.

2. A wave absorber according to claim 1,

the thickness of the radio wave absorbing layer is 10mm or less, and the filling rate is 8 vol% or more and 35 vol% or less.

3. The electric wave absorber according to claim 1 or 2, wherein,

the thickness of the radio wave absorbing layer is 5mm or less, and the filling rate is 15 vol% or more and 35 vol% or less.

4. The electric wave absorber according to any of claims 1 to 3, wherein,

the magnetic powder contains a magnetoplumbite type hexagonal ferrite powder.

5. The electric wave absorber according to any of claims 1 to 4, wherein,

the magnetic powder contains a magnetoplumbite-type hexagonal ferrite powder represented by the following formula (1),

AFe_(12-x)Al_xO₁₉… … (formula 1)

In the formula (1), A represents at least one metal element selected from the group consisting of Sr, Ba, Ca and Pb, and x satisfies 1.5. ltoreq. x.ltoreq.8.0.

6. A wave absorber according to claim 5,

a in the formula (1) is Sr.

7. The electric wave absorber according to claim 5 or 6, wherein,

x in the formula (1) satisfies 1.5-6.0.

8. The electric wave absorber according to any of claims 1 to 7, wherein,

the magnetic powder has a number-based particle size distribution measured by a laser diffraction scattering method, wherein D represents a cumulative 10% diameter and a mode diameter₁₀And setting the cumulative 90% diameter as D₉₀When the mode diameter is 5 μm or more and less than 10 μm, and (D90-D10)/mode diameter is not more than 3.0.

9. The electric wave absorber according to any of claims 1 to 8, wherein the electric wave absorber has a planar shape.

10. The radiowave absorber according to any one of claims 1 to 8, wherein the radiowave absorber has a three-dimensional shape.

11. A composite for use in the production of the electric wave absorber as claimed in any one of claims 1 to 10, and,

the composite contains a magnetic powder and a binder, and the magnetic powder has a filling rate of 35 vol% or less.

Technical Field

The present invention relates to a radio wave absorber and a composite.

Background

In recent years, with diversification of use forms of radio waves in high frequency bands such as Electronic Toll Collection Systems (ETC), Advanced road navigation Systems (AHS), and satellite broadcasting, malfunction and failure of Electronic devices due to radio wave interference have become a problem. In order to reduce the influence of such radio wave interference on the electronic device, a process of preventing reflection of radio waves by absorbing unnecessary radio waves by a radio wave absorber is performed.

For example, patent document 1 discloses a radio wave absorption sheet including a flexible radio wave absorption layer containing a particulate radio wave absorption material and a resin binder, wherein the radio wave absorption material is a magnetic iron oxide that generates magnetic resonance in a frequency band of a millimeter wave band or more, and an axis of easy magnetization of the radio wave absorption material is oriented by a magnetic field in one direction in a plane of the radio wave absorption sheet.

Prior art documents

Patent document

Patent document 1: japanese patent laid-open publication No. 2018-56492

Disclosure of Invention

Technical problem to be solved by the invention

The radio wave absorber is required to have high reflection attenuation and high transmission attenuation, and specifically, is required to have a reflection attenuation and a transmission attenuation of 10dB or more.

As a method of increasing the reflection attenuation amount of the radio wave absorber, there is a method of providing a reflection layer that reflects radio waves on a side (so-called back side) opposite to a side of the radio wave absorption layer on which radio waves are incident. A layer containing a metal (so-called metal layer) is generally used as the reflective layer, but in recent years, the use of a metal layer tends to be avoided from the viewpoint of recycling. Further, in the radio wave absorber having the metal layer, there are problems that the radio wave absorbing layer and the metal layer are easily peeled off, the metal layer itself is easily deteriorated, and the cost is increased. Therefore, it is desirable to design a radio wave absorber without a metal layer.

However, in a dielectric wave absorber that is generally used, when a metal layer is not provided, the reflection attenuation amount is reduced when trying to secure the transmission attenuation amount.

In the conventional radio wave absorber without a metal layer, it has been difficult to increase both the reflection attenuation and the transmission attenuation.

Regarding the above points, patent document 1 does not pay attention to the problem in the case of having a metal layer, and does not mention that the reflection attenuation amount and the transmission attenuation amount are increased at the same time in the radio wave absorber having no metal layer.

An object to be solved by one embodiment of the present invention is to provide a radio wave absorber which does not have a metal layer and in which both the transmission attenuation and the reflection attenuation in the millimeter wave band are 10dB or more.

Another object of the present invention is to provide a composite for producing the radio wave absorber.

Means for solving the technical problem

The means for solving the above problems include the following means.

<1> an electric wave absorber having an electric wave absorbing layer containing a magnetic powder and a binder and having no metal layer, wherein,

the filling rate of the magnetic powder in the radio wave absorbing layer is 35 vol% or less,

The magnetic powder in the radio wave absorption layer has a filling rate of P volume% and a thickness of Qmm, and satisfies a relation of 0.65 ≦ (P/100). times.Q.

<2> the radio wave absorber according to <1>, wherein,

the thickness of the radio wave absorbing layer is 10mm or less, and the filling rate is 8 vol% or more and 35 vol% or less.

<3> the radio wave absorber according to <1> or <2>, wherein,

the thickness of the radio wave absorbing layer is 5mm or less, and the filling rate is 15 vol% or more and 35 vol% or less.

<4> the radio wave absorber according to any one of <1> to <3>, wherein,

the magnetic powder contains a magnetoplumbite type hexagonal ferrite powder.

<5> the radio wave absorber according to any one of <1> to <4>, wherein,

the magnetic powder contains a magnetoplumbite-type hexagonal ferrite powder represented by the following formula (1).

[ chemical formula 1]

AFe_(12-x)Al_xO₁₉… … (formula 1)

In the formula (1), A represents at least one metal element selected from the group consisting of Sr, Ba, Ca and Pb, and x satisfies 1.5. ltoreq. x.ltoreq.8.0.

<6> the radio wave absorber according to <5>, wherein,

a in the above formula (1) is Sr.

<7> the radio wave absorber according to <5> or <6>, wherein,

X in the formula (1) satisfies 1.5. ltoreq. x.ltoreq.6.0.

<8> the radio wave absorber according to any one of <1> to <7>, wherein,

in the above magnetic powder, in a number-based particle size distribution measured by a laser diffraction scattering method, when a mode value is a mode diameter and a cumulative 10% diameter is D₁₀And setting the cumulative 90% diameter as D₉₀When the mode diameter is 5 μm or more and less than 10 μm, and (D)₉₀-D₁₀) The mode diameter is less than or equal to 3.0.

<9> the radio wave absorber according to any one of <1> to <8>, which has a planar shape.

<10> the radio wave absorber according to any one of <1> to <8>, which has a three-dimensional shape.

<11> a composite for use in the production of the radio wave absorber of any one of <1> to <10>, wherein the composite comprises a magnetic powder and a binder, and the magnetic powder has a filling rate of 35% by volume or less.

Effects of the invention

According to one embodiment of the present invention, there is provided a radio wave absorber and a composite, the radio wave absorber having no metal layer and having a transmission attenuation and a reflection attenuation both of which are equal to or greater than 10dB in a millimeter wave band.

According to another embodiment of the present invention, there is provided a composite for manufacturing the above-described electric wave absorber.

Detailed Description

An example of an embodiment to which the radio wave absorber of the present invention is applied will be described below. However, the present invention is not limited to the following embodiments, and can be implemented by appropriately changing the embodiments within the scope of the object of the present invention.

In the present invention, the numerical range represented by "to" represents a range including numerical values before and after "to" as the minimum value and the maximum value, respectively.

In the numerical ranges recited in the present invention, the upper limit or the lower limit recited in a certain numerical range may be replaced with the upper limit or the lower limit recited in another numerical range recited in a stepwise manner. In addition, in the numerical ranges described in the present invention, the upper limit value or the lower limit value described in a certain numerical range may be replaced with the values shown in the examples.

In the present invention, a combination of two or more preferred embodiments is a more preferred embodiment.

In the present invention, when there are a plurality of substances corresponding to each component, the amount of each component represents the total amount of the plurality of substances unless otherwise specified.

In the present invention, the term "step" includes not only an independent step but also a step that can achieve the intended purpose of the step even when the step is not clearly distinguished from other steps.

In the present invention, the conversion coefficient from the non-SI unit "Oe" to the SI unit "A/m" is set to "10³And 4 pi'. Here, "pi" is set to 3.1416.

In the present invention, from non-SI units "emu" to SI units "A · m²"has a conversion coefficient of" 10^-3”。

[ electric wave absorber ]

The radio wave absorber of the present invention has a radio wave absorbing layer containing magnetic powder and a binder, and does not have a metal layer, the filling rate of the magnetic powder in the radio wave absorbing layer is 35 vol% or less, and when the filling rate of the magnetic powder in the radio wave absorbing layer is P vol% and the thickness of the radio wave absorbing layer is Qmm, the relation of 0.65 ≦ (P/100). times.Q is satisfied.

The radio wave absorber of the present invention has a radio wave absorbing layer containing magnetic powder and a binder, wherein the filling rate of the magnetic powder in the radio wave absorbing layer is 35 vol% or less, and the relationship of (P/100). times.Q is satisfied when the filling rate of the magnetic powder in the radio wave absorbing layer is P vol% and the thickness of the radio wave absorbing layer is Qmm, so that the transmission attenuation amount and the reflection attenuation amount in the millimeter wave band are both 10dB or more although the metal layer is not included.

A radio wave absorber having both a transmission attenuation and a reflection attenuation of 10dB or more can absorb at least 90% of radio waves.

Constitution of electric wave absorber

The radio wave absorber of the present invention has a radio wave absorbing layer without a metal layer.

In the present invention, the "metal layer" is a layer containing a metal and means a layer that substantially reflects radio waves. Here, "substantially reflecting the radio wave" means, for example, reflecting 90% or more of the incident radio wave.

The metal layer may be a metal plate, a metal foil, or the like, and may be a thin metal film formed by vapor deposition, for example. The metal layer is usually formed on the opposite side to the side of the radio wave absorption layer on which the radio wave is incident.

In addition, a layer corresponding to the radio wave absorbing layer is not included in the "metal layer" in the present invention.

The radio wave absorber of the present invention may have a layer other than the radio wave absorbing layer (so-called other layer) as necessary within a range not impairing the effect of the radio wave absorber of the present invention.

Examples of the other layer include a protective layer, an adhesive layer, and a release layer.

The radio wave absorber of the present invention may have a reflective layer containing no metal as another layer within a range not impairing the effect of the radio wave absorber of the present invention, but a mode not containing a reflective layer containing no metal is preferable.

Shape of electric wave absorber >

The radio wave absorber of the present invention may have a planar shape or a three-dimensional shape.

The planar shape is not particularly limited, and may be a sheet shape, a film shape, or the like.

The three-dimensional shape includes a tubular shape (a cylindrical shape, a square tubular shape, or the like), a horn shape, a box shape (one surface of which is open), and the like.

[ electric wave absorbing layer ]

The radio wave absorbing layer includes a magnetic powder and a binder.

The details of the components contained in the radio wave absorbing layer will be described later.

The filling rate of the magnetic powder in the radio wave absorbing layer is 35 vol% or less, preferably 8 vol% or more and 35 vol% or less, more preferably 15 vol% or more and 35 vol% or less, still more preferably 20 vol% or more and 35 vol% or less, and particularly preferably 25 vol% or more and 35 vol% or less.

When the filling rate of the magnetic powder in the radio wave absorbing layer is 35 vol% or less, the radio wave absorber can realize a reflection attenuation of 10dB or more even without the metal layer.

The filling factor of the magnetic powder in the radio wave absorbing layer was measured and calculated by the following method using a Scanning Electron Microscope (SEM).

The radio wave absorbing layer was cut into a size of 5mm × 5 mm. After the cut radio wave absorbing layer was attached to a stage, the cross section was processed in the thickness direction using a Focused Ion Beam (FIB). After the processed radio wave absorbing layer was set on a stage so that the cross section thereof was on the upper side, a cross-sectional SEM image with a field of view of 30 μm 40 μm was obtained using a field emission scanning electron microscope (FE-SEM) under a applied voltage of 15kV and an observation magnification of 3,000 times. The obtained cross-sectional SEM image was subjected to binarization processing to determine the proportion of the magnetic powder, and the filling ratio of the magnetic powder was calculated.

The above operation was performed 5 times by changing the cut portion of the radio wave absorbing layer, and the arithmetic average value of the calculated values was used as the filling rate of the magnetic powder in the radio wave absorbing layer. In addition, the arithmetic mean rounds the first digit after the decimal point.

As the Focused Ion Beam (FIB) device, for example, a high-performance Focused Ion Beam (FIB) device (product name: MI4050) of Hitachi, ltd. However, the Focused Ion Beam (FIB) device is not limited thereto.

As the field emission scanning electron microscope (FE-SEM), for example, a field emission scanning electron microscope (product name: SU-8220) of Hitachi, Lt d. can be preferably used. However, the field emission scanning electron microscope (FE-SEM) is not limited thereto.

In the radio wave absorbing layer, when the filling rate of the magnetic powder in the radio wave absorbing layer is P volume% and the thickness of the radio wave absorbing layer is Qmm, the relationship of 0.65. ltoreq. P/100. ltoreq.Q is satisfied, preferably the relationship of 0.65. ltoreq. P/100. ltoreq.Q 5.0 is satisfied, more preferably the relationship of 0.65. ltoreq. P/100. ltoreq.Q 3.5 is satisfied, still more preferably the relationship of 0.65. ltoreq. P/100. ltoreq.Q 1.75 is satisfied, and particularly preferably the relationship of 0.65. ltoreq. P/100. ltoreq. Q1.0 is satisfied.

When the relation of 0.65. ltoreq. P/100. times.Q is satisfied, the radio wave absorber can realize a transmission attenuation of 10dB or more.

The thickness of the radio wave absorbing layer is not particularly limited as long as the above-mentioned relationship of "0.65. ltoreq. (P/100). times.Q" is satisfied.

For example, from the viewpoint of the degree of freedom of the installation position, the thickness of the radio wave absorbing layer is preferably 20mm or less, more preferably 10mm or less, and still more preferably 5mm or less.

The lower limit of the thickness of the radio wave absorbing layer is not particularly limited, but is preferably 2mm or more, for example, from the viewpoint of mechanical properties.

The thickness of the electromagnetic wave absorbing layer is a value measured using a digital display length measuring machine, specifically an arithmetic mean value of measured values measured at arbitrarily selected 9 positions.

As the digital length measuring machine, for example, a digital length measuring machine of Mitutoyo Corporation [ product name: littematic (registered trademark) VL-50A ]. However, the digital display length measuring machine is not limited thereto.

As a mode satisfying the relationship of 0.65 ≦ (P/100) × Q, for example, a mode in which the thickness of the radio wave absorbing layer is 10mm or less and the filling rate of the magnetic powder in the radio wave absorbing layer is 8 vol% or more and 35 vol% or less is preferable, a mode in which the thickness of the radio wave absorbing layer is 5mm or less and the filling rate of the magnetic powder in the radio wave absorbing layer is 15 vol% or more and 35 vol% or less is more preferable, and a mode in which the thickness of the radio wave absorbing layer is 2mm or more and 5mm or less and the filling rate of the magnetic powder in the radio wave absorbing layer is 20 vol% or more and 35 vol% or less is more preferable.

< magnetic powder >

The radio wave absorbing layer contains magnetic powder.

The magnetic powder is not particularly limited, and examples thereof include powders of ferrite, iron oxide, cobalt, chromium oxide, and the like.

For example, from the viewpoint of radio wave absorption performance, the magnetic powder is preferably a powder containing magnetoplumbite-type hexagonal ferrite (hereinafter, also referred to as "magnetoplumbite-type hexagonal ferrite powder"), and more preferably a magnetoplumbite-type hexagonal ferrite powder.

The magnetoplumbite-type hexagonal ferrite is generally represented by the composition formula A¹Fe₁₂O₁₉(in the formula, A)¹Represents metal elements such as Sr, Ba, Ca, Pb, etc. ) The compound shown in the specification.

However, in the concept of "magnetoplumbite-type hexagonal ferrite" in the present invention, except for the composition formula A¹Fe₁₂O₁₉The magnetoplumbite-type hexagonal ferrite shown in the above description includes magnetoplumbite-type hexagonal ferrite represented by the following formula (1).

For example, from the viewpoint of operability and handleability, A¹Preferably at least 1 metal element selected from the group consisting of Sr, Ba, Ca and Pb.

For example, from the viewpoint of excellent magnetic properties and excellent radio wave absorption performance even in a high frequency band, the magnetic powder is preferably a powder containing magnetoplumbite-type hexagonal ferrite represented by the following formula (1), and is preferably a powder of magnetoplumbite-type hexagonal ferrite represented by the formula (1).

Hereinafter, the magnetoplumbite-type hexagonal ferrite represented by formula (1) is referred to as "specific magnetoplumbite-type hexagonal ferrite". The powder of the specific magnetoplumbite-type hexagonal ferrite is also referred to as "specific magnetoplumbite-type hexagonal ferrite powder".

[ chemical formula 2]

AFe_(12-x)Al_xO₁₉… … (formula 1)

In the formula (1), A represents at least one metal element selected from the group consisting of Sr, Ba, Ca and Pb, and x satisfies 1.5. ltoreq. x.ltoreq.8.0.

As long as a in formula (1) is at least one metal element selected from the group consisting of Sr, Ba, Ca and Pb, the kind and amount of the metal element are not particularly limited.

For example, a in formula (1) is preferably at least 1 metal element selected from the group consisting of Sr, Ba, and Ca from the viewpoint of handling and handling properties.

Further, for example, from the viewpoint of exhibiting excellent radio wave absorption performance in the vicinity of 79GHz, a in the formula (1) preferably contains Sr, more preferably Sr.

In the formula (1), x satisfies 1.5. ltoreq. x.ltoreq.8.0, preferably 1.5. ltoreq. x.ltoreq.6.0, more preferably 1.5. ltoreq. x.ltoreq.4.0, and further preferably 1.5. ltoreq. x.ltoreq.3.0.

When x in the formula (1) is 1.5 or more, radio waves in a frequency band higher than 60GHz can be absorbed.

When x in formula (1) is 8.0 or less, the magnetoplumbite-type hexagonal ferrite has magnetism.

Specific magnetoplumbite-type hexagonal ferrite includes SrFe_(10.44)Al_(1.56)O₁₉、SrFe_(10.00)Al_(2.00)O₁₉、SrFe_(9.95)Al_(2.05)O₁₉、SrFe_(9.85)Al_(2.15)O₁₉、SrFe_(9.79)Al_(2.21)O₁₉、SrFe_(9.74)Al_(2.26)O₁₉、SrFe_(9.70)Al_(2.30)O₁₉、SrFe_(9.58)Al_(2.42)O₁₉、SrFe_(9.37)Al₍₂.₆₃₎O₁₉、SrFe_(9.33)Al_(2.67)O₁₉、SrFe_(9.27)Al_(2.73)O₁₉、SrFe_(7.88)Al_(4.12)O₁₉、SrFe₍7.₇₁₎Al_(4.29)O₁₉、SrFe_(7.37)Al_(4.63)O₁₉、SrFe_(7.04)Al_(4.96)O₁₉、SrFe_(6.25)Al_(5.75)O₁₉、BaFe_(9.50)Al_(2.50)O₁₉、BaFe_(10.05)Al_(1.95)O₁₉、CaFe_(10.00)Al_(2.00)O₁₉、PbFe₍9.₀₀₎Al_(3.00)O₁₉、Sr_(0.80)Ba_(0.10)Ca_(0.10)Fe_(9.83)Al_(2.17)O₁₉、Sr_(0.80)Ba_(0.10)Ca_(0.10)Fe_(8.85)Al_(3.15)O₁₉And the like.

For example, SrFe_(10.00)Al_(2.00)O₁₉A specific magnetoplumbite-type hexagonal ferrite having a resonance frequency in the vicinity of 76.5GHz, SrFe_(9.70)Al_(2.30)O₁₉Is a specific magnetoplumbite-type hexagonal ferrite having a resonance frequency in the vicinity of 85.0 GHz.

The method for producing the specific magnetoplumbite-type hexagonal ferrite powder will be described later.

The composition of the magnetoplumbite-type hexagonal ferrite was confirmed by high-frequency inductively Coupled Plasma (ICP: inductively Coupled Plasma) emission spectrometry.

Specifically, a pressure-resistant container containing 12mg of the sample powder and 4mol/L (liter; hereinafter, the same) of a hydrochloric acid aqueous solution 10mL was held in an oven at a set temperature of 120 ℃ for 12 hours to obtain a solution. Then, 30mL of pure water was added to the resulting solution, followed by filtration using a 0.1 μm membrane filter. Elemental analysis of the filtrate thus obtained was performed using a high-frequency Inductively Coupled Plasma (ICP) emission spectrometer. From the obtained elemental analysis results, the content of each metal atom to 100 atomic% of iron atom was determined. The composition was confirmed from the obtained content.

As the ICP emission spectrometer, for example, IC PS-8100 (model) of Shimadzu Corporation can be suitably used. However, the ICP emission spectrometer is not limited to this.

The crystal phase of the magnetoplumbite-type hexagonal ferrite may or may not be a single phase, and when the magnetoplumbite-type hexagonal ferrite is a specific magnetoplumbite-type hexagonal ferrite, the crystal phase is preferably a single phase.

In the case where the content ratio of aluminum is the same, the specific magnetoplumbite-type hexagonal ferrite powder having a single-phase crystal phase tends to have a higher coercive force and more excellent magnetic properties than the specific magnetoplumbite-type hexagonal ferrite powder having a non-single-phase crystal phase (for example, a two-phase crystal phase).

In the present invention, the phrase "the crystal phase is a single phase" means that only one kind of Diffraction pattern showing the crystal structure of the magnetoplumbite-type hexagonal ferrite of an arbitrary composition is observed in the powder X-ray Diffraction (XRD: X-Ra y-Diffraction; the same applies hereinafter) measurement.

On the other hand, in the present invention, the case where "the crystal phase is not a single phase" refers to a case where a plurality of magnetoplumbite-type hexagonal ferrites of arbitrary compositions are mixed and two or more kinds of diffraction patterns are observed or a diffraction pattern of a crystal other than the magnetoplumbite-type hexagonal ferrites is observed.

In the case where the crystal phase is not a single phase, a diffraction pattern in which a main peak and other peaks are present can be obtained. Here, the "main peak" means a peak having the highest diffraction intensity value in the observed diffraction pattern.

When the radiowave absorption layer contains the magnetoplumbite-type hexagonal ferrite powder as the magnetic powder, for example, from the viewpoint of enabling the production of a radiowave absorber having more excellent radiowave absorption performance, the ratio (Is/Im) of the diffraction intensity value (hereinafter, referred to as "Is") of another peak to the diffraction intensity value (hereinafter, referred to as "Im") of the main peak obtained by powder X-ray diffraction (XRD) measurement of the magnetoplumbite-type hexagonal ferrite powder Is preferably 1/2 or less, and more preferably 1/5 or less.

When two or more types of diffraction patterns overlap each other and the peak of each diffraction pattern has a maximum value, each maximum value Is defined as Im and Is, and the ratio of the maximum values Is determined. When two or more diffraction patterns overlap and another peak Is observed in the shoulder portion as the main peak, the maximum intensity value of the shoulder portion Is defined as Is, and the ratio of these values Is obtained.

When two or more other peaks are present, the total value of the diffraction intensities Is defined as Is, and the ratio of the values Is determined.

For the distribution of the Diffraction patterns, for example, a database of International centre for Diffraction Data (ICDD: International centre for Diffraction Data, registered trademark) may be referred to.

For example, the diffraction pattern of a magnetoplumbite-type hexagonal ferrite containing Sr can be referred to "00-033-. However, if a part of iron is replaced with aluminum, the peak position shifts, as in the case of the specific magnetoplumbite-type hexagonal ferrite.

As described above, the case where the magnetoplumbite-type hexagonal ferrite has a single phase in the crystal phase was confirmed by powder X-ray diffraction (XRD) measurement.

Specifically, the measurement was performed under the following conditions using a powder X-ray diffraction (XRD) apparatus.

As the powder X-ray diffraction (XRD) device, for example, X' Pert Pro (product name) available from PANALYTICAL CORPORATION can be suitably used. However, the powder X-ray diffraction (XRD) device is not limited thereto.

-conditions-

An X-ray source: CuK alpha line

[ wavelength:(0.154nm), output: 40mA, 45 kV)

Scanning range: 20 DEG < 2 theta < 70 DEG

Scanning interval: 0.05 degree

Scanning speed: 0.75 degree/min

For example, it can be confirmed that the radio wave absorption layer contains a magnetoplumbite-type hexagonal ferrite powder by the following method.

After the radio wave absorbing layer is finely cut, the layer is immersed in a solvent (e.g., acetone) for 1 to 2 days and then dried. The structure can be confirmed by further finely grinding the dried radio wave absorbing layer and performing powder X-ray diffraction (XRD) measurement.

After the radio wave absorbing layer is cut out in cross section, the composition can be confirmed by using an energy dispersive X-ray analyzer, for example.

The shape of the particles constituting the magnetic powder is not particularly limited, and examples thereof include spherical, rod-like, needle-like, flat, irregular, and the like.

The shape of the particles constituting the magnetoplumbite-type hexagonal ferrite powder is, for example, a flat plate shape or an irregular shape.

The size of the particles constituting the magnetic powder is not particularly limited.

Magnetic powder (preferably magnetoplumbite-type hexagonal ferrite powder, more preferably specific magnetoplumbite-type hexagonal ferrite powder; the same applies hereinafter) has a mode value as a mode diameter and a cumulative 10% diameter as D in a number-based particle size distribution measured by a laser diffraction scattering method₁₀And setting the cumulative 90% diameter as D₉₀When the average molecular weight is larger than or equal to 5 μm and smaller than 10 μm, it is preferable that the average molecular weight is (D)₉₀-D₁₀) A mode diameter of 3.0 or less, more preferably a mode diameter of 5 μm or more and less than 10 μm, and is (D)₉₀-D₁₀) A mode diameter of 2.5 or less, more preferably 5 to less than 10 μm, and is (D)₉₀-D₁₀) A mode diameter of 2.0 or less, particularly preferably a mode diameter of not less than 5 μm and less than 10 μm, and is (D)₉₀-D₁₀) A mode diameter of 1.5 or less, most preferably a mode diameter of 5 μm or more and less than 10 μm, and is (D)₉₀-D₁₀) The mode diameter is less than or equal to 1.0.

According to a mode diameter of 5 μm or moreTo (D)₉₀-D₁₀) Magnetic powder having a mode diameter of 3.0 or less tends to produce a radio wave absorber having more excellent radio wave absorption performance because fine particles having poor magnetic characteristics are relatively small.

A diameter of less than 10 μm according to mode and (D) ₉₀-D₁₀) Magnetic powder having a/mode diameter of 3.0 or less tends to produce a radio wave absorber having a higher strength because coarse particles are relatively small.

Particle diameter (i.e., mode diameter, D) of magnetic powder₁₀And D₉₀) The control can be performed by classification with a sieve, a centrifuge, or the like, or pulverization with a mortar, pestle, ultrasonic disperser, or the like. For example, when the particle diameter of the magnetic powder is controlled by pulverization, the particle diameter can be adjusted to a target value by selecting a pulverization mechanism, a pulverization time, a medium material, a medium diameter, and the like.

For example, the particle size of the magnetic powder tends to be smaller by pulverization using a medium. For example, the longer the pulverization time, the smaller the particle size of the magnetic powder tends to be. For example, the smaller the medium diameter, the smaller the particle diameter of the magnetic powder tends to be.

“(D₉₀-D₁₀) The value of the/mode diameter "can be adjusted to a target value by sorting particles after pulverization by, for example, classification with a sieve, a centrifuge, or the like.

The mode value, cumulative 10% diameter, and cumulative 90% diameter of the magnetic powder are values obtained from a number-based particle size distribution measured by a laser diffraction scattering method. Specifically, the value is measured by the following method.

500mL of cyclohexanone was added to 10mg of the magnetic powder to dilute the powder, and the resulting solution was stirred with a shaker for 30 seconds to obtain a sample for particle size distribution measurement. Next, the particle size distribution was measured by a laser diffraction scattering method using the particle size distribution measuring sample. A laser diffraction/scattering particle size distribution measuring apparatus was used as the measuring apparatus.

As the laser diffraction/scattering type particle size distribution measuring apparatus, for example, particle LA-960 (product name) of HORIBA, ltd. However, the laser diffraction/scattering particle size distribution measuring apparatus is not limited to this.

The particle diameter of the magnetic powder contained in the radio wave absorbing layer can be confirmed, for example, by the following method.

After the electric wave absorption layer is finely cut, ultrasonic waves are dispersed in a solvent (e.g., acetone). The particle size of the magnetic powder can be confirmed by measuring the obtained dispersion as a sample by a laser diffraction scattering method.

The coercive force (Hc) of the magnetic powder is not particularly limited, and is, for example, preferably 2.5kOe or more, more preferably 4.0kOe or more, and further preferably 5.0kOe or more.

When the coercive force (Hc) of the magnetic powder is 2.5kOe or more, an excellent radio wave absorber can be produced by the radio wave absorption performance.

The upper limit of the coercive force (Hc) of the magnetic powder is not particularly limited, and is preferably 18kOe or less, for example.

The saturation magnetization (δ s) per unit mass of the magnetic powder is not particularly limited, and is, for example, preferably 10emu/g or more, more preferably 20emu/g or more, and still more preferably 30emu/g or more.

When the saturation magnetization (δ s) per unit mass of the magnetic powder is 10emu/g or more, an excellent radio wave absorber can be produced by the radio wave absorption performance.

The upper limit of the saturation magnetization (δ s) per unit mass of the magnetic powder is not particularly limited, and is preferably 60emu/g or less, for example.

The coercive force (Hc) and the saturation magnetization per unit mass (δ s) of the magnetic powder were measured using a vibration sample type magnetometer under the environment of an ambient temperature of 23 ℃ and under the conditions of a maximum applied magnetic field of 50kOe and a magnetic field scanning speed of 25Oe/s (sec).

As the vibrating sample type magnetometer, for example, TAMAKAWA CO., LTD TM-TRVSM5050-SMSL type (model) can be suitably used. However, the vibrating sample magnetometer is not limited to this.

The radio wave absorbing layer may contain only one type of magnetic powder, or may contain two or more types of binders.

The content of the magnetic powder in the radio wave absorbing layer based on the mass is not particularly limited as long as the filling rate of the magnetic powder in the radio wave absorbing layer is 35 vol% or less.

Process for producing specific magnetoplumbite-type hexagonal ferrite powder

The method for producing the specific magnetoplumbite-type hexagonal ferrite powder is not particularly limited.

The specific magnetoplumbite-type hexagonal ferrite powder can also be produced by either a solid-phase method or a liquid-phase method.

As a method for producing a specific magnetoplumbite-type hexagonal ferrite powder by a solid phase method, for example, SrCO can be used₃、Al₂O₃、α-Fe₂O₃And the like are used as the raw material. For a general production method by a solid phase method of a specific magnetoplumbite-type hexagonal ferrite powder, reference can be made to [0023 ] of Japanese patent No. 4674380]～[0025]And (4) section.

As a method for producing the specific magnetoplumbite-type hexagonal ferrite powder, a method described below (hereinafter referred to as "production method a") is preferred from the viewpoint of easily obtaining a specific magnetoplumbite-type hexagonal ferrite powder having more excellent magnetic properties.

The manufacturing method A comprises: a step (A) of obtaining a reaction product containing Fe, Al, and at least one metal element selected from the group consisting of Sr, Ba, Ca, and Pb (hereinafter, also referred to as "specific metal element") by a liquid phase method; a step B of drying the reaction product obtained in the step A to obtain a dried product; and a step C of either calcining the dried product obtained in the step B to obtain a calcined product, and then pulverizing the calcined product obtained in the step B (hereinafter, also referred to as "C1 step"), or pulverizing the dried product obtained in the step B to obtain a pulverized product, and then calcining the pulverized product obtained (hereinafter, also referred to as "C2 step").

The steps A, B and C can be divided into 2 or more stages.

The production method a may include steps other than the steps a, B, and C as necessary.

Hereinafter, each step will be described in detail.

(Process A)

The step a is a step of obtaining a reaction product containing Fe, Al, and at least 1 metal element (i.e., a specific metal element) selected from the group consisting of Sr, Ba, Ca, and Pb by a liquid phase method.

In the step a, a reaction product which becomes a precursor of the specific magnetoplumbite-type hexagonal ferrite powder can be obtained. The reaction product obtained in step a is presumed to be iron hydroxide, aluminum hydroxide, a composite hydroxide of iron and aluminum with a specific metal element, or the like.

The step a preferably includes a step (hereinafter, also referred to as "step a 1") of mixing an aqueous solution containing an Fe salt, an Al salt, and a salt of a specific metal element (hereinafter, also referred to as "raw material aqueous solution") with an aqueous alkali solution to obtain a reaction product.

In step a1, the aqueous raw material solution and the aqueous alkali solution are mixed to generate a precipitate of the reaction product. In step a1, a liquid containing a reaction product (so-called precursor-containing liquid) that is a precursor of the specific magnetoplumbite-type hexagonal ferrite powder can be obtained.

Preferably, the step a includes a step of solid-liquid separating the reaction product obtained in the step a1 (hereinafter, also referred to as "step a 2").

In step a2, a reaction product that becomes a precursor of the specific magnetoplumbite-type hexagonal ferrite powder (i.e., the reaction product in step a) can be obtained.

Procedure A1-

Step a1 is a step of mixing an aqueous solution (i.e., a raw material aqueous solution) containing a Fe salt, an Al salt, and a salt of a specific metal element with an aqueous alkali solution to obtain a reaction product.

The salt of Fe salt, Al salt, or salt of a specific metal element is not particularly limited, and for example, from the viewpoint of easy availability and cost, a water-soluble inorganic acid salt such as nitrate, sulfate, or chloride is preferable.

Specific examples of the Fe salt include iron (III) chloride hexahydrate [ FeCl ]₃·6H₂O, iron (III) nitrate nonahydrate [ Fe (NO)₃)₃·9H₂O, etc.

Specific examples of the Al salt include aluminum chloride hexahydrate [ AlCl ]₃·6H₂O, aluminum nitrate nonahydrate [ Al (NO)₃)₃·9H₂O, etc.

Specific examples of the Sr salt include strontium chloride hexahydrate [ SrCl ]₂·6H₂O ] and strontium nitrate [ Sr (NO)₃)₂Strontium acetate 0.5 hydrate [ Sr (CH) ]₃COO)₂·0.5H₂O, etc.

Specific examples of the Ba salt include barium chloride dihydrate [ BaCl ] ₂·2H₂O ] and barium nitrate [ Ba (NO)₃)₂Barium acetate [ (CH ]₃COO)₂Ba, etc.

Specific examples of the Ca salt include calcium chloride dihydrate [ CaCl [ ]₂·2H₂O, calcium nitrate tetrahydrate [ Ca (NO)₃)₂·4H₂O ], calcium acetate monohydrate [ (CH)₃COO)₂Ca·H₂O, etc.

Specific examples of the Pb salt include lead (II) chloride [ PbCl ]₂Lead (II) nitrate [ Pb (NO)₃)₂And the like.

The aqueous alkali solution is not particularly limited, and examples thereof include an aqueous sodium hydroxide solution and an aqueous potassium hydroxide solution.

The concentration of the aqueous alkaline solution is not particularly limited, and may be, for example, 0.1 to 10 mol/L.

The raw material aqueous solution and the alkali aqueous solution may be mixed.

The total amount of the raw material aqueous solution and the aqueous alkali solution may be mixed at once, or the raw material aqueous solution and the aqueous alkali solution may be slowly mixed little by little. Alternatively, one of the aqueous solution of the raw material and the aqueous alkali solution may be added little by little and mixed.

For example, from the viewpoint of reproducibility of the radio wave absorption performance, it is preferable to slowly mix the raw material aqueous solution and the aqueous alkali solution little by little.

The method for mixing the aqueous raw material solution and the aqueous alkali solution is not particularly limited, and examples thereof include a method of mixing by stirring.

The stirring mechanism is not particularly limited, and a general stirrer or a stirring device can be used.

The stirring time is not particularly limited as long as the reaction of the mixed components is completed, and can be appropriately set according to the composition of the raw material aqueous solution, the kind of the stirrer or the stirring apparatus, and the like.

The temperature at the time of mixing the aqueous raw material solution and the aqueous alkaline solution is preferably 100 ℃ or lower, for example, from the viewpoint of preventing bumping, more preferably 95 ℃ or lower, and still more preferably 15 ℃ or higher and 92 ℃ or lower, from the viewpoint of obtaining a reaction product well.

The mechanism for adjusting the temperature is not particularly limited, and a general heating device, cooling device, or the like can be used.

The pH of the aqueous solution obtained by mixing the raw material aqueous solution and the aqueous alkali solution at 25 ℃ is preferably 5 to 13, more preferably 6 to 12, for example, from the viewpoint of easier availability of the reaction product.

The mixing ratio of the raw material aqueous solution and the alkaline aqueous solution is not particularly limited, and the alkaline aqueous solution can be set to 0.1 to 10.0 parts by mass with respect to 1 part by mass of the raw material aqueous solution, for example.

Procedure A2-

Step a2 is a step of subjecting the reaction product obtained in step a1 to solid-liquid separation.

The method of solid-liquid separation is not particularly limited, and examples thereof include decantation, centrifugation, and filtration (such as suction filtration and pressure filtration).

When the method of solid-liquid separation is centrifugal separation, the conditions of centrifugal separation are not particularly limited. For example, it is preferable to perform centrifugal separation at a rotation speed of 2000rpm or more for 3 to 30 minutes. Also, the centrifugal separation may be performed a plurality of times.

(Process B)

The step B is a step of drying the reaction product obtained in the step a to obtain a dried product (powder of precursor).

By drying the reaction product obtained in the step a before firing, the radio wave absorber produced has good reproducibility of the radio wave absorption performance. Further, the reaction product obtained in the drying step a before pulverization makes it easy to control the particle size distribution of the specific magnetoplumbite-type hexagonal ferrite powder by pulverization.

The drying mechanism is not particularly limited, and examples thereof include a dryer such as an oven.

The drying temperature is not particularly limited, and is, for example, preferably 50 to 200 ℃ and more preferably 70 to 150 ℃.

The drying time is not particularly limited, and is, for example, preferably 2 to 50 hours, and more preferably 5 to 30 hours.

(Process C)

The step C is any one of a step (i.e., the step C1) of calcining the dried product obtained in the step B to obtain a calcined product and then pulverizing the calcined product obtained in the step B, and a step (i.e., the step C2) of pulverizing the dried product obtained in the step B to obtain a pulverized product and then calcining the pulverized product obtained in the step B

The specific magnetoplumbite-type hexagonal ferrite powder having a target particle diameter can be obtained by calcining the dried product obtained in step B to obtain a calcined product, and then pulverizing the calcined product obtained, or by pulverizing the dried product obtained in step B to obtain a pulverized product, and then calcining the pulverized product obtained.

The step C may be a step C1 or a step C2.

For example, the step C is preferably a step C2 in order to make the magnetic properties more uniform after firing.

In the case where the step C is the step C2, the calcined pulverized material may be further pulverized.

Calcination may be performed using a heating device.

The heating device is not particularly limited as long as it can heat to a target temperature, and any known heating device can be used. As the heating device, for example, a calcination device separately manufactured in accordance with a production line may be used in addition to the electric furnace.

The calcination is preferably carried out in an atmospheric environment.

The calcination temperature is not particularly limited, and is, for example, preferably 900 ℃ or higher, more preferably 900 to 1400 ℃, and still more preferably 1000 to 1200 ℃.

The calcination time is not particularly limited, and is, for example, preferably 1 hour to 10 hours, and more preferably 2 hours to 6 hours.

The pulverizing mechanism is not particularly limited as long as a specific magnetoplumbite-type hexagonal ferrite powder having a target particle diameter can be obtained.

Examples of the pulverizing mechanism include a mortar and pestle, a pulverizer (a chopper, a ball mill, a bead mill, a roll mill, a jet mill, a hammer mill, an attritor, etc.), and the like.

When the pulverization is carried out using a medium, the particle diameter of the medium (so-called medium diameter) is not particularly limited, and is preferably 0.1mm to 5.0mm, for example. More preferably 0.5mm to 3.0 mm.

In the present invention, the "medium diameter" represents the diameter of a medium (for example, beads) when the medium is a spherical medium (for example, spherical beads), and represents the diameter obtained by measuring the circle-equivalent diameters of a plurality of media (for example, beads) from an observation image of a Transmission Electron Microscope (TEM) or a Scanning Electron Microscope (SEM) and arithmetically averaging the measured values, when the medium is an aspherical medium (for example, aspherical beads).

The material of the dielectric is not particularly limited, and for example, a dielectric made of glass, alumina, steel, zirconia, ceramic, or the like can be preferably used.

< adhesive agent >

The electric wave absorbing layer contains a binder.

Examples of the binder include thermoplastic resins and thermosetting resins.

As the thermoplastic resin, there can be mentioned: acrylic resin; a polyacetal; a polyamide; polyethylene; polypropylene; polyethylene terephthalate; polybutylene terephthalate; a polycarbonate; polystyrene; polyphenylene sulfide; polyvinyl chloride; ABS (acrylonitrile butadiene styrene) resin obtained by copolymerizing acrylonitrile, butadiene and styrene; AS (acrylonitrile-styrene) resins obtained by copolymerizing acrylonitrile and styrene, and the like.

Examples of the thermosetting resin include phenol resins, epoxy resins, melamine resins, urea resins, unsaturated polyesters, diallyl phthalate resins, polyurethane resins, and silicone resins.

Examples of the adhesive include rubber.

As the rubber, for example, from the viewpoint of being able to produce a radio wave absorber having good mixing properties with the magnetic powder and further excellent durability, weather resistance and impact resistance, it is preferable that: butadiene rubber; isoprene rubber; chloroprene rubber; halogenated butyl rubber; a fluororubber; a urethane rubber; acrylic rubber (ACM) obtained by copolymerizing acrylic esters (e.g., ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate) and other monomers; ethylene-propylene rubbers obtained by coordination polymerization of ethylene and propylene using a ziegler catalyst; butyl rubber (IIR) obtained by copolymerizing isobutylene and isoprene; styrene Butadiene Rubber (SBR) obtained by copolymerizing polybutadiene and styrene; acrylonitrile butadiene rubber (NBR) obtained by copolymerizing acrylonitrile and butadiene; synthetic rubbers such as silicone rubber.

Examples of the binder include thermoplastic elastomers (TPE).

Examples of the thermoplastic elastomer include olefinic thermoplastic elastomer (TPO), styrenic thermoplastic elastomer (TPS), amide thermoplastic elastomer (TPA), and polyester thermoplastic elastomer (TPC).

When the radio wave absorbing layer contains rubber as a binder, various additives such as a vulcanizing agent, a vulcanization aid, a softening agent, and a plasticizer may be contained in addition to the rubber.

Examples of the vulcanizing agent include sulfur, an organic sulfur compound, and a metal oxide.

The melt flow rate (hereinafter also referred to as "MFR") of the binder is not particularly limited, and is, for example, preferably 1g/10min to 200g/10min, more preferably 3g/10min to 100g/10min, still more preferably 5g/10min to 80g/10min, and particularly preferably 10g/10min to 50g/10 min.

When the MFR of the adhesive is 1g/10min or more, the fluidity is sufficiently high and appearance defects are less likely to occur.

When the MFR of the binder is 200g/10min or less, the mechanical properties such as strength of the molded article can be more easily improved.

The MFR of the adhesive is according to JIS K7210: 1999 the value measured under the conditions of the measurement temperature of 230 ℃ and the load of 10 kg.

The hardness of the binder is not particularly limited, and is, for example, preferably 5g to 150g, more preferably 10g to 120g, still more preferably 30g to 100g, and particularly preferably 40g to 90g, from the viewpoint of moldability and applicability.

The hardness of the adhesive was measured according to JIS K6253-3: 2012 measured instantaneous value.

The density of the binder is not particularly limited, and is preferably 600kg/m, for example, from the viewpoint of moldability³～1100kg/m³More preferably 700kg/m³～1000kg/m³More preferably 750kg/m³～1050kg/m³Particularly preferably 800kg/m³～950kg/m³。

The density of the adhesive was according to JIS K0061: 2001, the measured value.

The 100% tensile stress of the binder is not particularly limited, and is, for example, preferably 0.2 to 20MPa, more preferably 0.5 to 10MPa, still more preferably 1 to 5MPa, and particularly preferably 1.5 to 3MPa, from the viewpoint of molding applicability.

The tensile strength of the binder is not particularly limited, and is, for example, preferably 1 to 20MPa, more preferably 2 to 15MPa, further preferably 3 to 10MPa, and particularly preferably 5 to 8MPa, from the viewpoint of molding applicability.

The elongation at cutting of the binder is not particularly limited, and is, for example, preferably 110% to 1500%, more preferably 150% to 1000%, further preferably 200% to 900%, and particularly preferably 400% to 800% from the viewpoint of molding applicability.

The above tensile properties were measured according to JIS K6251: 2010 measured value. The measurement was carried out under the condition of a tensile rate of 500mm/min using JIS No. 3 dumbbell as a test piece.

The radio wave absorbing layer may contain only one kind of binder, or may contain two or more kinds of binders.

The filling rate of the binder in the radio wave absorbing layer is not particularly limited, but is, for example, preferably 65 vol% or more, more preferably 65 vol% or more and 92 vol% or less, and still more preferably 65 vol% or more and 85 vol% or less.

< other additives >

The radio wave absorbing layer may contain various additives (so-called other additives) as necessary within a range not impairing the effect of the radio wave absorber of the present invention, in addition to the magnetic powder and the binder.

Examples of the other additives include a dispersant, a dispersing aid, a fungicide, an antistatic agent, and an antioxidant. The other additives may be additives in which one component plays two or more roles.

[ method for producing an electromagnetic wave absorber ]

The method for producing the radio wave absorber of the present invention is not particularly limited.

The radio wave absorber of the present invention can be produced by a known method using a magnetic powder, a binder, a solvent, other additives, and the like as needed.

The radio wave absorber of the present invention can be produced by, for example, the following method X.

The mixture containing the magnetic powder, the binder, and if necessary, other additives is kneaded by a kneader to obtain a composite. Next, the obtained composite is subjected to a molding process, whereby a radio wave absorber including a radio wave absorbing layer can be manufactured.

The magnetic powder in method X has the same meaning as the magnetic powder described in the term "radio wave absorber" and is preferably the same as that described in the term "radio wave absorber", and therefore, the description thereof is omitted here.

The content of the magnetic powder in the mixture may be adjusted so that the filling rate of the magnetic powder in the finally obtained radio wave absorbing layer becomes 35 vol% or less.

The pressure-sensitive adhesive in method X is the same as the pressure-sensitive adhesive described in the "radio wave absorber" section, and the preferred embodiment is the same, and therefore, the description thereof is omitted here.

The content of the binder in the mixed product is not particularly limited, and for example, the filling rate of the binder in the radio wave absorbing layer to be finally obtained is preferably adjusted to the filling rate of the binder in the radio wave absorbing layer.

The other additives in the method X are the same as those described in the "radio wave absorber" section, and the preferred embodiments are also the same, and therefore, the description thereof is omitted here.

In the mixture, the magnetic powder and the binder may be mixed.

The method of mixing the magnetic powder and the binder is not particularly limited, and examples thereof include a method of mixing by stirring.

The stirring mechanism is not particularly limited, and a general stirring device can be used.

Examples of the stirring device include a paddle stirrer, an impeller stirrer, and the like.

The stirring time is not particularly limited, and can be appropriately set according to, for example, the type of the stirring device, the composition of the mixture, and the like.

The heating temperature of the mixture is not particularly limited, and can be appropriately set according to the type of the binder, for example.

The heating temperature is preferably a temperature at which the binder can be melted, and can be, for example, 170 to 300 ℃.

The kneading mechanism is not particularly limited, and a general kneading apparatus can be used.

Examples of the kneading apparatus include a stirrer, a twin roll, and a kneader.

The kneading conditions are not particularly limited, and may be appropriately set depending on, for example, the type of kneading apparatus and the composition of the mixture.

Examples of the molding process include processes such as press molding, extrusion molding, injection molding, in-mold molding, and molding using a three-dimensional molding machine.

The molding conditions are not particularly limited, and may be appropriately set according to, for example, the type of molding apparatus, the composition of the mixture, the thickness of the radio wave absorbing layer, and the like.

When the filling rate of the magnetic powder in the radio wave absorption layer is P vol% and the thickness of the radio wave absorption layer is Qmm, the thickness of the radio wave absorption layer is not particularly limited as long as the relationship of 0.65 ≦ (P/100) × Q is satisfied.

The thickness of the radio wave absorbing layer can be adjusted by, for example, molding the composite.

The radio wave absorber of the present invention can be produced, for example, by the following method Y.

A coating film of the radio wave absorbing layer forming composition is formed by coating the radio wave absorbing layer forming composition containing the magnetic powder, the binder, and if necessary, the solvent, other additives, and the like on the dummy substrate. Next, the radio wave absorbing layer is formed by drying the coating film of the radio wave absorbing layer forming composition. Next, the radio wave absorber including the radio wave absorbing layer can be manufactured by peeling the dummy support from the radio wave absorbing layer.

The magnetic powder in method Y has the same meaning as the magnetic powder described in the term "radio wave absorber" and is preferably the same as that described in the term "radio wave absorber", and therefore, the description thereof is omitted here.

The content of the magnetic powder in the composition for forming a radio wave absorbing layer is adjusted so that the filling rate of the magnetic powder in the finally obtained radio wave absorbing layer becomes 35 vol% or less.

The pressure-sensitive adhesive in method Y has the same meaning as the pressure-sensitive adhesive described in the term "radio wave absorber", and the preferred embodiment is the same, and therefore, the description thereof is omitted here.

The content of the binder in the radio wave absorbing layer-forming composition is not particularly limited, and for example, the filling rate of the binder in the radio wave absorbing layer to be finally obtained is preferably adjusted to the filling rate of the binder in the radio wave absorbing layer.

The solvent in the method Y is not particularly limited, and examples thereof include water, an organic solvent, and a mixed solvent of water and an organic solvent.

Examples of the organic solvent include alcohols such as methanol, ethanol, n-propanol, isopropanol and methoxypropanol, ketones such as acetone, methyl ethyl ketone, cyclohexane and cyclohexanone, tetrahydrofuran, acetonitrile, ethyl acetate and toluene.

When the radio wave absorbing layer-forming composition contains a solvent, the content of the solvent in the radio wave absorbing layer-forming composition is not particularly limited, and can be appropriately set according to the kind, amount, and the like of the components to be blended in the radio wave absorbing layer-forming composition, for example.

The other additives in the method Y have the same meanings as those of the other additives described in the term "radio wave absorber", and the preferred embodiments are also the same, and therefore, the description thereof is omitted here.

In the composition for forming a radio wave absorbing layer, the magnetic powder and the binder may be mixed.

The method of mixing the magnetic powder and the binder is not particularly limited, and examples thereof include a method of mixing by stirring.

The stirring mechanism is not particularly limited, and a general stirring device can be used.

Examples of the stirring device include a paddle stirrer, an impeller stirrer, and the like.

The stirring time is not particularly limited, and may be appropriately set according to, for example, the type of the stirring device, the composition of the radio wave absorbing layer-forming composition, and the like.

The dummy support is not particularly limited.

Examples of the dummy substrate include a metal plate (a metal plate such as aluminum, zinc, and copper), a glass plate, a plastic sheet [ polyester (polyethylene terephthalate, polyethylene naphthalate, and polybutylene terephthalate), polyethylene (linear low-density polyethylene, and high-density polyethylene), polypropylene, polystyrene, polycarbonate, polyimide, polyamide, polyamideimide, polysulfone, polyvinyl chloride, polyacrylonitrile, polyphenylene sulfide, polyetherimide, polyether sulfone, polyvinyl acetal, and a sheet such as acrylic resin ], and the like.

The plastic sheet is preferably subjected to a mold release treatment on the surface.

The size of the dummy support is not particularly limited, and can be set appropriately according to the size of the radio wave absorbing layer, for example.

The thickness of the dummy support is not particularly limited, but is usually about 0.01mm to 0.5mm, and for example, preferably 0.05mm to 0.2mm from the viewpoint of workability.

The method of applying the radio wave absorbing layer forming composition to the dummy support is not particularly limited, and examples thereof include a method using a film coater, a knife coater, an applicator, and the like.

The method for drying the coating film of the radio wave absorption layer forming composition is not particularly limited, and examples thereof include a method using a heating device such as an oven.

The drying temperature and the drying time are not particularly limited as long as the solvent contained in the coating film of the radio wave absorption layer forming composition can be volatilized.

For example, the heating is carried out at 70 ℃ to 90 ℃ for 1 hour to 3 hours.

When the filling rate of the magnetic powder in the radio wave absorption layer is P vol% and the thickness of the radio wave absorption layer is Qmm, the thickness of the radio wave absorption layer is not particularly limited as long as the relationship of 0.65 ≦ (P/100) × Q is satisfied.

The thickness of the radio wave absorbing layer can be adjusted by, for example, the amount of the radio wave absorbing layer-forming composition applied.

[ Complex ]

The composite of the present invention is a composite for producing the radio wave absorber of the present invention, and comprises a magnetic powder and a binder, and the filling rate of the magnetic powder is 35% by volume or less.

The magnetic powder in the composite of the present invention has the same meaning as the magnetic powder described in the term "radio wave absorber" and is preferably the same as the magnetic powder described in the term "radio wave absorber", and therefore, the description thereof is omitted here.

The filling rate of the magnetic powder in the composite of the present invention is 35 vol% or less, preferably 8 vol% or more and 35 vol% or less, more preferably 15 vol% or more and 35 vol% or less, and further preferably 20 vol% or more and 35 vol% or less.

The filling ratio of the magnetic powder in the composite of the present invention is a value measured by the following method.

The composite of the present invention is used to produce an electric wave absorbing layer. The radio wave absorbing layer thus produced was measured by the same method as the method for filling the magnetic powder in the radio wave absorbing layer described in the section "radio wave absorber".

The binder in the composite of the present invention has the same meaning as the binder described in the term "radio wave absorber" and is preferably the same as the binder, and therefore, the description thereof is omitted here.

The filling rate of the binder in the composite of the present invention is not particularly limited, but is, for example, preferably 65 vol% or more, more preferably 65 vol% or more and 92 vol% or less, and still more preferably 65 vol% or more and 85 vol% or less.

The composite of the present invention may be in the form of particles.

The size (diameter) of the particulate composite is not particularly limited, but is, for example, preferably 0.5mm to 20mm, more preferably 1mm to 10mm, still more preferably 2mm to 8mm, and particularly preferably 3mm to 6 mm.

The density of the particulate composite is not particularly limited, and is preferably 500kg/m, for example³～5000kg/m³More preferably 800kg/m³～4000kg/m³More preferably 1000kg/m³～3500kg/m³Particularly preferably 1200kg/m³～3000kg/m³。

The density of the composite was according to JIS K0061: 2001, the measured value.

The composite of the present invention may contain various additives (so-called other additives) as necessary in addition to the magnetic powder and the binder.

The other additives in the composite of the present invention have the same meanings as those of the other additives described in the section "radio wave absorber", and the preferred embodiments thereof are also the same, and therefore, the description thereof is omitted here.

Examples

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to the following examples as long as the gist thereof is not exceeded.

[ production of magnetic powder ]

[ magnetic powder 1 ]

400.0g of water kept at 35 ℃ under stirring, and the total amount of iron (III) chloride hexahydrate [ FeCl ] was added to the stirring water at the same addition timing at a flow rate of 10mL/min₃·6H₂57.0g of O, strontium chloride hexahydrate [ SrCl ]₂·6H₂O27.8 g and aluminum chloride hexahydrate₃·6H₂O10.2 g of an aqueous raw material solution prepared by dissolving the above components in 216.0g of water and a solution prepared by adding 113.0g of water to 181.3g of a 5mol/L aqueous sodium hydroxide solution were added to obtain the 1 st liquid.

Next, the temperature of the 1 st liquid was changed to 25 ℃, and then 39.8g of a 1mol/L aqueous sodium hydroxide solution was added while maintaining the temperature, to obtain a 2 nd liquid. The pH of the obtained 2 nd liquid was 10.5. Further, the pH of the 2 nd liquid was measured using a benchtop pH meter F-71 (product name) of HORIBA, ltd. (the same applies hereinafter).

Subsequently, the 2 nd liquid was stirred for 15 minutes to complete the reaction, thereby obtaining a liquid containing a reaction product which becomes a precursor of the magnetoplumbite-type hexagonal ferrite powder (i.e., a precursor-containing liquid).

Subsequently, the precursor-containing liquid was centrifuged three times (rotation speed: 3000rpm, rotation time: 10 minutes), and the obtained precipitate was collected.

Next, the collected precipitate was dried in an oven at an internal ambient temperature of 95 ℃ for 12 hours to obtain an aggregate of particles composed of the precursor (i.e., precursor powder).

Next, the precursor powder was placed in a muffle furnace, and the furnace was calcined for 4 hours under an atmospheric environment at a temperature of 1100 ℃.

Next, using Wonder Crusher WC-3 (product name) of osaka magnetic co., ltd., magnetic powder 1 was obtained by setting the variable speed dial to "5" and pulverizing the obtained calcined body for 90 seconds.

[ magnetic powder 2 ]

400.0g of water kept at 35 ℃ under stirring, and the total amount of iron (III) chloride hexahydrate [ FeCl ] was added to the stirring water at the same addition timing at a flow rate of 10mL/min₃·6H₂57.0g of O, strontium chloride hexahydrate [ SrCl ]₂·6H₂O27.8 g and aluminum chloride hexahydrate₃·6H₂O10.2 g of an aqueous raw material solution prepared by dissolving the above components in 216.0g of water and a solution prepared by adding 113.0g of water to 181.3g of a 5mol/L aqueous sodium hydroxide solution were added to obtain the 1 st liquid.

Next, the temperature of the 1 st liquid was changed to 25 ℃, and 30.2g of a 1mol/L aqueous solution of sodium hydroxide was added while maintaining the temperature, to obtain a 2 nd liquid. The pH of the obtained 2 nd liquid was measured to be 9.5.

Subsequently, the 2 nd liquid was stirred for 15 minutes to complete the reaction, thereby obtaining a liquid containing a reaction product which becomes a precursor of the magnetoplumbite-type hexagonal ferrite powder (i.e., a precursor-containing liquid).

Subsequently, the precursor-containing liquid was centrifuged three times (rotation speed: 3000rpm, rotation time: 10 minutes), and the obtained precipitate was collected.

Next, the collected precipitate was dried in an oven at an internal ambient temperature of 95 ℃ for 12 hours to obtain an aggregate of particles composed of the precursor (i.e., precursor powder).

Next, the precursor powder was placed in a muffle furnace, and the furnace was calcined for 4 hours under an atmospheric environment at a temperature of 1100 ℃.

Next, using Wonder Crusher WC-3 (product name) of OSAKA CHEMICAL co., ltd., magnetic powder 2 was obtained by setting the variable speed dial to "5" and pulverizing the obtained calcined body for 60 seconds.

< confirmation of Crystal Structure >

The crystal structure of the magnetic material (hereinafter, also referred to as "magnetic material 1 and magnetic material 2", respectively) forming each of the magnetic powder 1 and the magnetic powder 2 was confirmed by the X-ray diffraction (XRD) method.

Specifically, it was confirmed whether the crystal had a magnetoplumbite-type crystal structure and whether the crystal had a single phase or two or more different crystal phases.

The measurement apparatus used X' Pert Pro (product name) from PANALYTICAL CORPORATION as a powder X-ray diffraction apparatus. The measurement conditions are shown below.

Determination of conditions

An X-ray source: CuK alpha line

[ wavelength:(0.154nm), output: 40mA, 45 kV)

Scanning range: 20 DEG < 2 theta < 70 DEG

Scanning interval: 0.05 degree

Scanning speed: 0.75 degree/min

As a result, it was confirmed that each of the magnetic bodies 1 and 2 had a magnetoplumbite type crystal structure and was a single-phase magnetoplumbite type hexagonal ferrite including no crystal structure other than the magnetoplumbite type.

< confirmation of composition >

The composition of each of the magnetic bodies 1 and 2 was confirmed by high-frequency Inductively Coupled Plasma (ICP) emission spectroscopy.

Specifically, the confirmation was performed by the following method.

A beaker (pressure-resistant vessel) containing 12mg of magnetic powder and 10mL of 4mol/L aqueous hydrochloric acid was kept in an oven set at 120 ℃ for 12 hours to obtain a solution. 30mL of pure water was added to the resulting solution, and the mixture was filtered through a 0.1 μm membrane filter. Using a high-frequency Inductively Coupled Plasma (ICP) luminescence spectroscopic analyzer [ model: ICPS-8100, Shimadzu Corporation ] elemental analysis of the filtrate thus obtained was carried out.

From the obtained elemental analysis results, the content of each metal atom to 100 atomic% of iron atom was determined. Then, the composition of the magnetic material was confirmed from the obtained content. The composition of each magnetic material is shown below.

Magnetic body 1: SrFe_(10.00)Al_(2.00)O₁₉

Magnetic material 2: SrFe_(9.70)Al_(2.30)O₁₉

As a result, it was confirmed that both the magnetic powder 1 and the magnetic powder 2 were powders of magnetoplumbite-type hexagonal ferrite represented by formula (1) (i.e., specific magnetoplumbite-type hexagonal ferrite powders).

< measurement of particle size distribution >

The particle size distribution of each of the magnetic powders 1 and 2 was measured by the laser diffraction scattering method based on the number of the magnetic powders, and a mode value (so-called mode diameter), a cumulative 10% diameter, and a cumulative 90% diameter were obtained.

Specifically, 500mL of cyclohexanone was added to 10mg of the magnetic powder to dilute the powder, and the resulting solution was stirred with a shaker for 30 seconds to obtain a sample for particle size distribution measurement.

Next, a laser diffraction/scattering particle size distribution measuring apparatus [ product name: particle size distribution of the samples for particle size distribution measurement were measured using a particle size distribution analyzer, particle LA-960, HORIBA, Ltd.

Then, from the obtained number-based particle size distribution, a mode diameter (unit: μm) as a mode value and D as a cumulative 10% diameter were obtained ₁₀(unit: μm) and D as cumulative 90% diameter₉₀(unit: μm). And calculate "(D)₉₀-D₁₀) Value of/mode diameter ".

As a result, in the magnetic powder 1, the mode diameter was 6.7 μm and D was₁₀Is 4.1 μm, D₉₀Was 9.5 μm, "(D)₉₀-D₁₀) The value of/mode diameter "is 0.81. In the magnetic powder 2, the mode diameter was 8.8 μm,D₁₀5.5 μm, D₉₀12.5 μm, "(D)₉₀-D₁₀) The value of/mode diameter "is 0.80.

< measurement of magnetic Properties >

The coercive force (Hc) and saturation magnetization (δ s) were measured as magnetic properties for each of the magnetic powder 1 and the magnetic powder 2.

Specifically, the measurement was performed by the following method.

As the measurement device, a vibration sample type magnetometer [ model: TM-TRVSM5050-SMSL type, TAMAKAWA co., LTD ], the intensity of magnetization of the magnetic powder with respect to the applied magnetic field was measured under the conditions of a maximum applied magnetic field of 50kOe and a magnetic field scanning speed of 25Oe/s (sec) in an environment at an ambient temperature of 23 ℃. From the measurement results, a magnetic field (H) -magnetization (M) curve of the magnetic powder was obtained. The coercive force (Hc) (unit: kOe) and saturation magnetization (δ s) (unit: emu/g) of the magnetic powder were obtained from the obtained magnetic field (H) -magnetization (M) curve.

As a result, in magnetic powder 1, the coercive force (Hc) was 10.5kOe, and the saturation magnetization (. delta.s) was 43.2 emu/g. In magnetic powder 2, the coercive force (Hc) was 10.0kOe, and the saturation magnetization (δ s) was 40.2 emu/g.

[ production of radio wave absorber (1) ]

[ example 1 ]

1. Preparation of the composite

Magnetic powder 1 and a binder [ trade name: a compound was prepared from Mirastomer (registered trademark) 7030NS, olefin thermoplastic elastomer (TPO), and Mitsui Chemicals, inc. Specifically, the following procedure was carried out.

The magnetic powder 1 and the binder were mixed to obtain a mixture. The amount of the magnetic powder 1 and the binder added is such that the filling ratio of the magnetic powder 1 in the finally obtained composite becomes 15 vol%. Next, the obtained mixture was kneaded at a set temperature of 200 ℃ and a rotation speed of 50rpm for 20 minutes using LABO PLASTOMILL [ product name, Toyo Seiki Seisaku-sho, Lt d. ], to obtain a composite.

2. Production of radio wave absorber

By using a hot press apparatus, at a forming pressure: 20MPa, pressing temperature: 200 ℃ and pressing time: the obtained composite was molded under a condition of 10 minutes to obtain a sheet-like radio wave absorber (size: 100 mm. times.100 mm). The amount of the composite used was such that the thickness of the finally obtained radio wave absorber became 4.5 mm.

[ example 2]

A sheet-shaped radio wave absorber (size: 100mm × 100mm) was obtained by performing the same operations as in example 1, except that the loading of the magnetic powder 1 and the binder was changed to an amount such that the filling ratio of the magnetic powder 1 in the finally obtained composite became 30 vol%, and the usage of the composite was changed to an amount such that the thickness of the finally obtained radio wave absorber became 3 mm.

Comparative example 1

A sheet-like radio wave absorber (size: 100 mm. times.100 mm) was obtained in the same manner as in example 1, except that the amount of the composite used was changed to an amount such that the thickness of the finally obtained radio wave absorber became 3 mm.

Comparative example 2

A sheet-shaped radio wave absorber (size: 100mm × 100mm) was obtained by performing the same operations as in example 1, except that the loading of the magnetic powder 1 and the binder was changed to an amount such that the filling ratio of the magnetic powder 1 in the finally obtained composite became 30 vol%, and the usage of the composite was changed to an amount such that the thickness of the finally obtained radio wave absorber became 2 mm.

[ comparative example 3 ]

A sheet-shaped radio wave absorber (size: 100mm × 100mm) was obtained by performing the same operations as in example 1, except that the loading of the magnetic powder 1 and the binder was changed to an amount such that the filling ratio of the magnetic powder 1 in the finally obtained composite became 40% by volume, and the usage of the composite was changed to an amount such that the thickness of the finally obtained radio wave absorber became 2 mm.

[ comparative example 4 ]

A sheet-shaped radio wave absorber (size: 100mm × 100mm) was obtained by performing the same operations as in example 1, except that the loading of the magnetic powder 1 and the binder was changed to an amount such that the filling ratio of the magnetic powder 1 in the finally obtained composite became 40% by volume, and the usage of the composite was changed to an amount such that the thickness of the finally obtained radio wave absorber became 3 mm.

Comparative example 5

A sheet-shaped radio wave absorber (size: 100mm × 100mm) was obtained by performing the same operations as in example 1, except that the loading of the magnetic powder 1 and the binder was changed to an amount such that the filling ratio of the magnetic powder 1 in the finally obtained composite became 50% by volume, and the usage of the composite was changed to an amount such that the thickness of the finally obtained radio wave absorber became 2 mm.

Table 1 shows "filling rate (unit: volume%) of magnetic powder in the radio wave absorbing layer", thickness (unit: mm) of the radio wave absorbing layer ", and" (P/100) × Q "of each radio wave absorber of example 1, example 2, and comparative examples 1 to 5.

[ production of radio wave absorber (2) ]

[ example 3 ]

1. Preparation of the composite

Magnetic powder 2 and a binder [ trade name: a compound was prepared from Mirastomer (registered trademark) 7030NS, olefin thermoplastic elastomer (TPO), and Mitsui Chemicals, inc. Specifically, the following procedure was carried out.

The magnetic powder 2 and the binder were mixed to obtain a mixture. The amount of the magnetic powder 2 and the binder added is such that the filling ratio of the magnetic powder 2 in the finally obtained composite becomes 15 vol%. Next, the obtained mixture was kneaded at a set temperature of 200 ℃ and a rotation speed of 50rpm for 20 minutes using LABO PLASTOMILL [ product name, Toyo Seiki Seisaku-sho, Lt d. ], to obtain a composite.

2. Production of radio wave absorber

By using a hot press apparatus, at a forming pressure: 20MPa, pressing temperature: 200 ℃ and pressing time: the obtained composite was molded under a condition of 10 minutes to obtain a sheet-like radio wave absorber (size: 100 mm. times.100 mm). The amount of the composite used was such that the thickness of the finally obtained radio wave absorber became 4.5 mm.

[ example 4 ]

A sheet-shaped radio wave absorber (size: 100mm × 100mm) was obtained by performing the same operations as in example 3, except that the loading of the magnetic powder 2 and the binder was changed to an amount such that the filling ratio of the magnetic powder 2 in the finally obtained composite became 30 vol%, and the usage of the composite was changed to an amount such that the thickness of the finally obtained radio wave absorber became 3 mm.

Comparative example 6

A sheet-shaped radio wave absorber (size: 100mm × 100mm) was obtained by performing the same operations as in example 3, except that the loading of the magnetic powder 2 and the binder was changed to an amount such that the filling ratio of the magnetic powder 2 in the finally obtained composite became 40% by volume, and the usage of the composite was changed to an amount such that the thickness of the finally obtained radio wave absorber became 2 mm.

Table 2 shows "filling rate of magnetic powder in radio wave absorbing layer (unit: volume%)", "thickness of radio wave absorbing layer (unit: mm)" and "(P/100). times.Q" of each of the radio wave absorbers of examples 3 and 4 and comparative example 6.

[ measurement ]

1. Measurement of Transmission attenuation and reflection attenuation (1)

The transmission attenuation (unit: dB) and the reflection attenuation (unit: dB) of each of the radio wave absorbers of examples 1 to 4 and comparative examples 1 to 6 were measured.

Specifically, the measurement was performed as follows.

The measurement device used was a vector network analyzer (product name: N5225B) manufactured by keysight Corporation and a horn antenna (product name: RH12S23) manufactured by KEYCOM Corporation, and the transmission attenuation amount and the reflection attenuation amount at 76.5GHz were determined for the radio wave absorbers of example 1, example 2, and comparative examples 1 to 5, and the transmission attenuation amount and the reflection attenuation amount at 85.0GHz were determined for the radio wave absorbers of example 3, example 4, and comparative example 6, by measuring the S parameter with the incident angle set to 0 ° and the scanning frequency set to 60GHz to 90GHz by the free space method. The results are shown in tables 1 and 2, respectively.

[ Table 1]

[ Table 2]

As shown in tables 1 and 2, it was confirmed that the radio wave absorbers of examples 1 to 4 all had the transmission attenuation and the reflection attenuation of 10dB or more in the millimeter wave band.

On the other hand, it was confirmed that at least one of the transmission attenuation amount and the reflection attenuation amount of the radio wave absorbers of comparative examples 1 to 6 was less than 10 dB.

[ production of radio wave absorber (3) ]

[ example 5 ]

1. Preparation of the composite

Magnetic powder 1 and a binder [ trade name: a compound was prepared from Mirastomer (registered trademark) 7030NS, olefin thermoplastic elastomer (TPO), and Mitsui Chemicals, inc. Specifically, the following procedure was carried out.

The magnetic powder 1 and the binder were mixed to obtain a mixture. The amount of the magnetic powder 1 and the binder is set so that the filling ratio of the magnetic powder 1 in the finally obtained composite becomes 30 vol%. Next, the obtained mixture was kneaded at a set temperature of 200 ℃ and a rotation speed of 50rpm for 20 minutes using LABO PLASTOMILL [ product name, Toyo Seiki Seisaku-sho, Lt d. ], to obtain a composite.

2. Production of radio wave absorber

The compound obtained was extruded using a two-shaft mixing extruder [ model: KZW15TW, TECHNOVE L CORPORATION ], a molten composite was extruded from a die into a cylindrical shape with the screw temperature set at 200 ℃ to obtain a cylindrical radio wave absorber (inner diameter: 100mm, height: 150mm, thickness: 3 mm).