Magnetic powder for radio wave absorber, method for producing the same, radio wave absorber, radio wave absorbing article, and radio wave absorbing composition

Abstract

Provided are: a magnetic powder for radio rave absorbers, which is a powder of hexagonal ferrite having a composition represented by formula 1, in which a region B is present on each of the surfaces of particles of the powder and the relational formulae represented by formulae 2 and 3 are satisfied; and a method for producing the magnetic powder. The region B is observed as a light region having a longer diameter of 0.1 μm to 0.6 μm inclusive in a binary-processed image which is produced by subjecting an image produced by imaging the surfaces of the particles with a scanning electron microscope to a binary processing. Also provided is a radio wave absorber comprising the magnetic powder for radio wave absorbers. Also provided is a radio wave absorbing article containing the radio wave absorber. Also provided is a radio wave absorbing composition containing the magnetic powder for radio wave absorbers. (In formula 1, A represents at least one atom selected from the group consisting of Sr, Ba, Ca and Pb (also referred to as an "A atom" in the present invention and the present description), and x satisfies the formula: 0.10 ≤ x ≤ 5.00.) (Formula 2) 0.3 ≤ ((the content of A atom in the region B) / (the content of Al atom in the region B)) ≤ 23.0 (Formula 3) 1.2 ≤ (the total of the content of A atom and the content of Al atom in the region B) / (the total of the content of A atom and the content of Al atom in the whole of the powder) ≤ 2.5

Description

Technical Field

[0001] The present invention relates to magnetic powder for a radio wave absorber, a method for manufacturing the same, a radio wave absorber, a radio wave absorbing article, and a radio wave absorbing composition.

Background Art

[0002] As a radio wave absorber, those containing magnetic powder as a radio wave absorbing material are known (see Patent Document 1 and Patent Document 2).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] In recent years, as an electronic device that uses radio waves, a radar for recognizing an object by transmitting and receiving radio waves has attracted attention. For example, an in-vehicle radar can recognize the presence of an object, the distance from the object, etc. by transmitting radio waves and receiving the radio waves reflected by the object (pedestrian, vehicle, etc.). An automatic driving control system of an automobile can automatically apply brakes to stop the automobile or automatically control the speed to maintain the distance from the object as needed based on the result of the radar recognizing the object in order to prevent a collision with the object.

[0005] In order to improve the reliability of a system that performs various controls based on the result recognized by the radar as described above, an improvement in the performance of the radar is desired. Therefore, in recent years, it has been started to be studied to install a radio wave absorber on the front side (the incident side of radio waves incident from the outside) of the radio wave transmitting and receiving unit of the radar to improve the recognition accuracy.

[0006] The radio wave absorber is desired to have excellent radio wave absorption properties. From the viewpoint of improving radio wave absorption properties, it is desirable to enhance the transmission attenuation characteristics of the radio wave absorber. As an index of the transmission attenuation characteristics of the radio wave absorber, the transmission attenuation amount can be cited. Using a radio wave absorber that exhibits a high transmission attenuation amount at the frequency to be absorbed is desirable from the viewpoint of improving the recognition accuracy of the radar. Also, in various other applications where the radio wave absorber is used, a radio wave absorber that exhibits a high transmission attenuation amount at the frequency to be absorbed is desirable. In this regard, further improvement is desired for conventional radio wave absorbers.

[0007] In view of the above, one aspect of the present invention aims to provide a new magnetic powder that enables the production of a radio wave absorber having excellent transmission attenuation characteristics.

Means for Solving the Problems

[0008] One aspect of the present invention is the following formula 1:

Chemical formula

[0009] In one form, the peak particle size of the magnetic powder for the radio wave absorber can be 4.5 μm or more and less than 12.0 μm.

[0010] In one form, in Formula 1, the A atom can be one or two atoms selected from the group consisting of Sr and Ba.

[0011] In one form, the magnetic powder for the radio wave absorber can further satisfy the following formula 4. The following content ratio is the content ratio with the total of A atoms, Fe atoms, and Al atoms being 100 atomic %, and the unit of the following content ratio is atomic %.

[0012] (Formula 4) 1.5 ≤ Content ratio of A atoms in region B / Content ratio of Al atoms in region B ≤ 10.0

[0013] In one form, the magnetic powder for the radio wave absorber has a ratio (σs / β) of the saturation magnetization σs to the half-value width β of the diffraction peak of the (107) plane obtained by X-ray diffraction analysis of 240 emu·g -1 ·degree -1 or more and can be a powder of hexagonal ferrite.

[0014] One aspect of the present invention relates to a radio wave absorber containing the above magnetic powder for the radio wave absorber.

[0015] In one form, the radio wave absorber can further contain a binder.

[0016] One aspect of the present invention relates to a radio wave absorbing article containing the above radio wave absorber.

[0017] One aspect of the present invention is a method for producing the magnetic powder for a radio wave absorber, which includes adding, to a mixture obtained by mixing raw materials of hexagonal ferrite, at least one chloride selected from the group consisting of strontium chloride, barium chloride, and hydrates thereof, in an addition amount of 3.0% by mass or more, with the total mass of the above raw materials being 100% by mass. relates to.

[0018] In one form, the addition amount of the chloride can be 5.0% by mass or more and 15.0% by mass or less.

[0019] In one form, the raw materials can include an Al compound having an average particle size of 100 μm or less.

[0020] One aspect of the present invention relates to a radio wave absorbing composition containing the magnetic powder for a radio wave absorber.

[0021] In one form, the radio wave absorbing composition can further include a binder.

[0022] In one form, the radio wave absorbing composition can be a filament for a 3D (three-dimensional) printer.

Advantages of the Invention

[0023] According to one aspect of the present invention, it is possible to provide a new magnetic powder (magnetic powder for a radio wave absorber) capable of manufacturing a radio wave absorber having excellent transmission attenuation characteristics and a method for manufacturing the same. Further, according to one aspect of the present invention, it is possible to provide a radio wave absorber and a radio wave absorbing composition containing the magnetic powder for a radio wave absorber, and a radio wave absorbing article containing the radio wave absorber.

Modes for Carrying Out the Invention

[0024] [Magnetic Powder for Radio Wave Absorber] Hereinafter, the magnetic powder for a radio wave absorber (hereinafter, also simply referred to as "magnetic powder") will be described.

[0025] In the present invention and this specification, "radio wave" refers to an electromagnetic wave having a frequency of 3 terahertz (THz) or less. A "radio wave absorber" has radio wave absorption properties. The radio wave absorption properties can be evaluated, for example, by the amount of transmission attenuation described in detail later. The higher the value of the amount of transmission attenuation, the higher the transmission attenuation characteristics and the better the radio wave absorption properties. The "magnetic powder for radio wave absorber" is a magnetic powder used for manufacturing a radio wave absorber and contained in the manufactured radio wave absorber.

[0026] In the present invention and this specification, "powder" means a collection of a plurality of particles. The "collection" is not limited to a form in which the particles constituting the collection are in direct contact, and also includes a form in which a binder or the like described later is interposed between the particles.

[0027] That the magnetic powder is a powder of hexagonal ferrite can be confirmed by X-ray diffraction analysis, as described later. The composition of the magnetic powder can be confirmed by performing high-frequency inductively coupled plasma optical emission spectrometry on a solution in which the magnetic powder is dissolved. Specific examples of the confirmation method include the methods described in the examples below. Alternatively, after exposing a cross section by cutting the radio wave absorber or the like, the composition of the magnetic powder contained in the radio wave absorber can also be confirmed by performing, for example, energy-dispersive X-ray analysis on the exposed cross section. In this way, it can be confirmed that the magnetic powder is a hexagonal ferrite having a composition represented by Formula 1.

[0028] Region B exists on the particle surface of the magnetic powder. In the present invention and this specification, the fact that "region B exists" on the particle surface of the magnetic powder is specified by the following method by SEM (Scanning Electron Microscope) measurement.

[0029] (Acquisition of SEM image and creation of binary-processed image) After placing the magnetic powder to be measured on the sample stage for SEM, platinum (Pt) is vapor-deposited on the magnetic powder on the sample stage. The Pt vapor deposition can be performed using an ion coater. As the ion coater, for example, the ion coater EIKO 1B-5 manufactured by Eiko Co., Ltd. can be used. The sample stage with the Pt-vapor-deposited magnetic powder is attached to the SEM, the acceleration voltage is set to 5 kV, the magnification is set to 10,000 times, and an SEM image of the magnetic powder is taken in a randomly selected area. As the SEM, a field emission scanning electron microscope (FE-SEM) can be used. As the FE-SEM, for example, the FE-SEM SU8220 manufactured by Hitachi High-Technologies Corporation can be used. The SEM image is acquired as a secondary electron image. Thereafter, the obtained SEM image is imported into image processing software and binarization processing is performed. The binarization processing can be performed using ImageJ, free software, as the image processing software. The binarization processing can be performed with the binarization processing conditions of ImageJ set to 8-bit, the default condition of the threshold value, AUTO. In this way, a binarized image in which the SEM image is divided into a bright region (white part) and a dark region (black part) is obtained.

[0030] (Identification of the existence of region B) For each bright region (white part) contained in the randomly selected binarized region with a size of "10 μm × 8 μm" obtained above, a rectangle is defined that has a size that can accommodate the entire bright region and that is in contact with the contour part of the bright region at each of its four sides. Such a rectangle can be determined visually by an operator, and in the examples described below, it was determined visually by the operator. The length of the long side of this rectangle is referred to as the major axis diameter of the bright region. When the determined rectangle is a square with the length of the long side equal to the length of the short side, the length of one side of such a square is taken as the major axis diameter. In the region with a size of "10 μm × 8 μm" described above, when 5 or more bright regions with a major axis diameter of 0.1 μm or more and 0.6 μm or less are confirmed, it is determined that region B exists on the particle surface. The reason for setting 5 as the threshold value is to reduce or eliminate the influence of error factors. Region B is presumed to be minute protrusions or minute attachments on the surface of the magnetic powder particles. However, the present invention is not limited to the presumption described in this specification. For a plurality of bright regions with a major axis diameter of 0.1 μm or more and 0.6 μm or less contained in the region with a size of "10 μm × 8 μm" described above, the arithmetic mean of the major axis diameters can be 0.1 μm or more and 0.6 μm or less, or can also be 0.2 μm or more and 0.5 μm or less.

[0031] The above magnetic powder satisfies the relational expressions of Formula 2 and Formula 3. The compositional analysis for determining that these relational expressions are satisfied is performed by the following method.

[0032] (Compositional Analysis) The composition of region B and the composition of the entire powder regarding Formula 2 and Formula 3 are specified by performing EDS (Energy Dispersive X-ray Spectroscopy) measurement on the same region as the region where the existence of region B was specified above. Specifically, for the region with a size of "10 μm × 8 μm" where the presence of the above-mentioned region B is specified, an image is obtained using an EDS device. For the obtained image, quantitative operations of A atoms, Fe atoms, and Al atoms are performed on the entire image and the rectangular portion surrounding the above-mentioned bright region. The amounts of various atoms in region B are determined as the total amount of a plurality of rectangular portions. From the quantitative results thus obtained, the "content ratio of A atoms in region B" regarding Formula 2 and Formula 3 is calculated as the content ratio of A atoms quantified for region B with respect to the total of A atoms, Fe atoms, and Al atoms (100 atomic %) quantified for region B. The "content ratio of Al atoms in region B" regarding Formula 2 and Formula 3 is calculated as the content ratio of Al atoms quantified for region B with respect to the total of A atoms, Fe atoms, and Al atoms (100 atomic %) quantified for region B. The "content ratio of A atoms in the entire powder" regarding Formula 3 is calculated as the content ratio of A atoms quantified for the entire image with respect to the total of A atoms, Fe atoms, and Al atoms (100 atomic %) quantified for the entire image (i.e., the image obtained for the region with a size of "10 μm × 8 μm" mentioned above). The "content ratio of Al atoms in the entire powder" regarding Formula 3 is calculated as the content ratio of Al atoms quantified for the entire image with respect to the total of A atoms, Fe atoms, and Al atoms (100 atomic %) quantified for the entire image.

[0033] The inventor believes that the fact that the magnetic powder having the composition represented by Formula 1 satisfies the relational expression of Formula 2 above indicates that the abundance of A atoms relative to Al atoms in region B is relatively large, and the fact that it satisfies the relational expression of Formula 3 above indicates that the proportions occupied by A atoms and Al atoms in region B are larger compared to the proportion in the entire powder. As a result of intensive studies, the inventor newly found that the presence of region B on the particle surface of the magnetic powder having the composition represented by Formula 1 and the satisfaction of the relational expressions of Formula 2 and Formula 3 regarding the compositions of region B and the entire powder can contribute to the excellent transmission attenuation characteristics of the electromagnetic wave absorber containing this magnetic powder.

[0034] Hereinafter, the above magnetic powder will be described in more detail.

[0035] <Powder of hexagonal ferrite> The magnetic powder is a powder of hexagonal ferrite having a composition represented by Formula 1. The type of magnetic material constituting the magnetic powder contained in the radio wave absorber can be confirmed by taking out the magnetic powder from the radio wave absorber by a known method and analyzing the taken-out magnetic powder by X-ray diffraction method. The extraction of the magnetic powder from the radio wave absorber can be carried out, for example, by finely cutting a part or all of the radio wave absorber, immersing it in a solvent (for example, hexafluoroisopropanol) for 1 to 2 days, then filtering out the magnetic powder portion and drying it. In addition, instead of or in addition to the immersion in the solvent, stirring and / or heating in the solvent can be appropriately carried out to promote the dissolution of components such as the binder in the solvent. For example, the magnetic powder after the above drying can be further finely ground and analyzed by X-ray diffraction method to confirm the type of magnetic material constituting the magnetic powder.

[0036] In the present invention and this specification, the "powder of hexagonal ferrite" refers to a magnetic powder in which a hexagonal ferrite-type crystal structure is detected as the main phase by analysis performed by X-ray diffraction method. The main phase refers to the structure to which the diffraction peak with the highest intensity in the X-ray diffraction spectrum obtained by X-ray diffraction method belongs. For example, when the diffraction peak with the highest intensity in the X-ray diffraction spectrum obtained by X-ray diffraction method belongs to a hexagonal ferrite-type crystal structure, it is determined that the hexagonal ferrite-type crystal structure has been detected as the main phase. When only a single structure is detected by X-ray diffraction method, this detected structure is taken as the main phase.

[0037] <Formula 1> The so-called unsubstituted hexagonal ferrite that does not contain a substituting atom for substituting an iron atom contains, as constituent atoms of the hexagonal ferrite, an iron atom, a divalent metal atom, and an oxygen atom. The divalent metal atom is a metal atom that can become a divalent cation as an ion, and examples thereof include alkaline earth metal atoms such as a strontium atom, a barium atom, and a calcium atom, and a lead atom. On the other hand, the hexagonal ferrite having the composition represented by Formula 1 can be referred to as a substituted magnetoplumbite-type hexagonal ferrite in which a part of the iron atoms of the magnetoplumbite-type hexagonal ferrite is substituted with aluminum atoms.

[0038] [Chemical formula] (In Formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.10 ≤ x ≤ 5.00.)

[0039] Hereinafter, Formula 1 will be described in more detail.

[0040] In Formula 1, A represents one or more atoms (A atoms) selected from the group consisting of Sr, Ba, Ca, and Pb. It may be only one kind, or two or more kinds may be contained in an arbitrary ratio. From the viewpoint of improving the uniformity of the composition between the particles constituting the powder, it is preferably only one kind or only two kinds, and more preferably only one kind.

[0041] From the perspective of the transmission attenuation characteristics in the high-frequency band, A in Formula 1 is preferably at least one atom selected from the group consisting of Sr, Ba, and Ca, more preferably at least one atom selected from the group consisting of Sr and Ba, and even more preferably only Sr or only Ba. In the present invention and this specification, when A in Formula 1 is only Sr, in the composition confirmed by the method described above, with the total of A atoms (i.e., the total of Sr, Ba, Ca, and Pb) being 100 atomic%, the content of Sr is 95 atomic% or more. When A in Formula 1 is only Ba, in the composition confirmed by the method described above, with the total of A atoms (i.e., the total of Sr, Ba, Ca, and Pb) being 100 atomic%, the content of Ba is 95 atomic% or more.

[0042] In Formula 1, x satisfies 0.10 ≦ x ≦ 5.00.

[0043] x is 0.10 or more from the perspective of improving the transmission attenuation characteristics, and more preferably 0.40 or more from the perspective of further improving the transmission attenuation characteristics. Also, x is 5.00 or less from the perspective of magnetic properties, and preferably 4.50 or less, more preferably 4.00 or less, still more preferably 3.50 or less, and even more preferably 3.00 or less from the perspective of further improving the transmission attenuation characteristics.

[0044] Specific examples of the composition represented by Formula 1 include SrFe (9.58) Al (2.42) O 19 、SrFe (9.37) Al (2.63) O 19 、SrFe (9.27) Al (2.73) O 19 、SrFe (9.85) Al (2.15) O 19 、SrFe (10.00) Al (2.00) O 19 、SrFe (9.74) Al (2.26) O 19 、SrFe (10.44) Al(1.56) O 19 、 SrFe (9.79) Al (2.21) O 19 、 SrFe (9.33) Al (2.67) O 19 、 SrFe (7.88) Al (4.12) O 19 、 SrFe (7.04) Al (4.96) O 19 、 SrFe (7.37) Al (4.63) O 19 、 SrFe (7.71) Al (4.29) O 19 、 Sr (0.80) Ba (0.10) Ca (0.10) Fe (9.83) Al (2.17) O 19 、 BaFe (9.50) Al (2.50) O 19 、 CaFe (10.00) Al (2.00) O 19 、 PbFe (9.00) Al (3.00) O 19 Examples include O, SrFeAlO, SrFeAlO, SrFeAlO, SrFeAlO, SrFeAlO, SrFeAlO, SrFeAlO, SrBaCaFeAlO, BaFeAlO, CaFeAlO, PbFeAlO, etc. Specific examples also include the compositions shown in Table 1 below.

[0045] In one form, the hexagonal ferrite powder can have a single crystal phase or can contain multiple crystal phases. It is preferably a single crystal phase, and more preferably a magnetoplumbite-type hexagonal ferrite powder with a single crystal phase. The case where "the crystal phase is a single phase" refers to the case where only one type of diffraction pattern indicating an arbitrary crystal structure is observed in the analysis performed by the X-ray diffraction method. For example, the analysis by the X-ray diffraction method can be performed by the method described in the examples below. When multiple crystal phases are included, two or more types of diffraction patterns indicating an arbitrary crystal structure are observed in the analysis by the X-ray diffraction method. For the assignment of the diffraction pattern, for example, the database of the International Centre for Diffraction Data (ICDD: International Centre for Diffraction Data (registered trademark)) can be referred to. For example, for the diffraction pattern of a magnetoplumbite-type hexagonal ferrite containing Sr, "00-033-1340" of the International Centre for Diffraction Data (ICDD) can be referred to. However, when a part of the iron atoms is substituted by substitution atoms such as aluminum atoms, the peak position shifts from the peak position in the case where no substitution atoms are included.

[0046] <Formulas 2 and 3> The above magnetic powder satisfies the relational expressions of the following Formulas 2 and 3. The present inventors consider that this is the reason why the electromagnetic wave absorber containing the above magnetic powder can exhibit excellent transmission attenuation characteristics.

[0047] (Formula 2) 0.3 ≤ Content ratio of A atoms in region B / Content ratio of Al atoms in region B ≤ 23.0

[0048] (Formula 3) 1.2 ≤ Total of content ratio of A atoms and content ratio of Al atoms in region B / Total of content ratio of A atoms and content ratio of Al atoms in the entire powder ≤ 2.5

[0049] From the viewpoint of further improving the transmission attenuation characteristics, a preferable form of Formula 2 can be the following Formula 4.

[0050] (Formula 4) 1.5 ≤ Content ratio of A atoms in region B / Content ratio of Al atoms in region B ≤ 10.0

[0051] A specific example of a method for manufacturing a magnetic powder having a composition represented by Formula 1, wherein Region B exists on the particle surface of the powder and the relational expressions of Formulas 2 and 3 are satisfied for the composition of Region B and the entire powder will be described later.

[0052] <Peak particle size> Regarding the particle size of the above magnetic powder, in the present invention and this specification, in the volume-based particle size distribution measured by the laser diffraction scattering method, the mode diameter, which is the most frequent value, shall be referred to as the "peak particle size". The peak particle size of the above magnetic powder is preferably 4.5 μm or more. Since a magnetic powder with a peak particle size of 4.5 μm or more has relatively few fine particles, there is a tendency that a radio wave absorber with more excellent radio wave absorption performance can be manufactured by using such a magnetic powder. From this point, the peak particle size of the above magnetic powder is preferably 4.8 μm or more, and more preferably 5.0 μm or more. On the other hand, the peak particle size of the above magnetic powder is preferably less than 12.0 μm. Since a magnetic powder with a peak particle size less than 12.0 μm has relatively few coarse particles, there is a tendency that a radio wave absorber with more excellent strength can be manufactured by using such a magnetic powder. From this point, the peak particle size of the above magnetic powder is preferably 11.5 μm or less, more preferably 11.0 μm or less, still more preferably 10.0 μm or less, and even more preferably 9.0 μm or less.

[0053] The peak particle size of the magnetic powder can be controlled by classification using a sieve, a centrifuge, etc., grinding using a mortar and pestle, an ultrasonic disperser, etc. For example, when controlling the particle size of the magnetic powder by grinding, the particle size can be adjusted by selecting grinding means, grinding time, the material of the media, the media diameter, etc. For example, the longer the grinding time, the smaller the particle size of the magnetic powder tends to be. Also, for example, the smaller the media diameter, the smaller the particle size of the magnetic powder tends to be.

[0054] The peak particle size (mode diameter) of the magnetic powder is a value determined based on the volume-based particle size distribution measured by the laser diffraction scattering method. The measurement of such a particle size distribution can be performed by the dry laser diffraction scattering method, and in the examples described later, it was performed by the following method. For the method of taking out the magnetic powder from the radio wave absorber, reference can be made to the previous description. Using a laser diffraction / scattering type particle size distribution measuring device (Partica LA-960) manufactured by Horiba, Ltd. as the measuring device, the magnetic powder is put into the measurement holder so that the transmittance displayed on the measurement monitor of the device is 95 to 98%, and the particle size distribution is measured by the laser diffraction scattering method under the condition of a compressed air pressure of 0.40 MPa.

[0055] The shape of the particles constituting the magnetic powder is not particularly limited, and examples thereof include spherical, rod-shaped, needle-shaped, plate-shaped, and irregular shapes.

[0056] <Ratio (σs / β)> In one form, the magnetic powder has a ratio (σs / β) of the saturation magnetization σs to the half-value width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis of 240 emu·g -1 ·degree -1 or more, and can be a powder of hexagonal ferrite. The ratio (σs / β) being 240 emu·g -1 ·degree -1 or more is preferable from the viewpoint of enabling the provision of a radio wave absorber that can exhibit more excellent radio wave absorption performance.

[0057] The saturation magnetization σs is also called the mass magnetization, and the unit is emu / g. 1 emu / g = 1 A·m 2 / kg. The saturation magnetization σs of the magnetic powder is a value measured using a vibrating sample magnetometer under the conditions of an atmospheric atmosphere at an ambient temperature of 23°C, a maximum applied magnetic field of 50 kOe, and a magnetic field sweep rate of 25 Oe / s. 1 [kOe] = 10 6 / 4π [A / m].

[0058] The above β is the half-value width of the diffraction peak of the (107) plane obtained by X-ray diffraction analysis of the hexagonal ferrite powder. The half-value width is the full width at half maximum (FWHM). In the present invention and this specification, the X-ray diffraction analysis for obtaining the ratio (σs / β) shall be performed under the following measurement conditions using a powder X-ray diffractometer. The X-ray diffraction spectrum is obtained as a spectrum with the vertical axis: intensity (unit: counts) and the horizontal axis: diffraction angle (unit: degree (°)). In the X-ray diffraction spectrum, the diffraction peak of the (107) plane is detected as a peak having the apex of the peak at a position where the diffraction angle 2θ is in the range of 32 to 33 degrees (usually around 32.5 degrees). The half-value width of the diffraction peak of the (107) plane can be obtained by the analysis software installed in the powder X-ray diffractometer or by a known calculation method. -Measurement Conditions- X-ray source: CuKα ray 〔Wavelength: 1.54 Å (0.154 nm), Output: 40 mA, 45 kV〕 Scan range: 25 degrees < 2θ < 35 degrees Scan interval: 0.05 degrees Scan speed: 0.33 degrees / min

[0059] The saturation magnetization σs is a kind of magnetic property of the magnetic powder. On the other hand, the present inventor speculates that the above β can be reduced by reducing the variation in the ferrite composition between the particles constituting the hexagonal ferrite powder. As one of the means for increasing this value of the ratio (σs / β), mention may be made of reducing the value of the half-value width β of the diffraction peak of the (107) plane obtained by X-ray diffraction analysis of the hexagonal ferrite powder. Also, as one of the means for increasing the value of σs / β, mention may be made of increasing the σs of the above hexagonal ferrite powder.

[0060] In one form, the above ratio (σs / β) is 242 emu·g -1 ·degree -1It is preferably the above, 245 emu·g -1 ·degree -1 More preferably, it is the above, 247 emu·g -1 ·degree -1 Even more preferably, it is the above, 250 emu·g -1 ·degree -1 Even more preferably, it is the above, 255 emu·g -1 ·degree -1 Even more preferably, it is the above, 260 emu·g -1 ·degree -1 Even more preferably, it is the above. Further, the above ratio (σs / β) is, for example, 320 emu·g -1 ·degree -1 or less, 315 emu·g -1 ·degree -1 or less or 310 emu·g -1 ·degree -1 or less. Or, the above ratio (σs / β) may be a value exceeding the values exemplified above.

[0061] Further, in one embodiment, σs / β of the powder of the above hexagonal ferrite is preferably 300 emu·g -1 ·degree -1 or more. In this embodiment, the above ratio (σs / β) is preferably 300 emu·g -1 ·degree -1 or more and 400 emu·g -1 ·degree -1 or less.

[0062] <Method for producing powder of hexagonal ferrite> As methods for manufacturing hexagonal ferrite powder, a solid-phase method and a liquid-phase method can be mentioned. The solid-phase method is a method for manufacturing hexagonal ferrite powder by firing a mixture obtained by mixing a plurality of solid raw materials. In contrast, the liquid-phase method includes a step of using a solution. The above-mentioned hexagonal ferrite powder can be manufactured by the solid-phase method or the liquid-phase method. The hexagonal ferrite powder manufactured by the solid-phase method and the hexagonal ferrite powder manufactured by the liquid-phase method can be easily distinguished. For example, the hexagonal ferrite powder manufactured by the liquid-phase method usually has deposits of alkali metal salts on the surface of the particles constituting the powder confirmed by SEM-EDX analysis (Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy) due to its manufacturing method. Also, for example, when the hexagonal ferrite powder manufactured by the solid-phase method is subjected to morphological observation of particles using FE-SEM (Field Emission-Scanning Electron Microscope), so-called amorphous particles can usually be confirmed. For example, in this way, the hexagonal ferrite powder manufactured by the solid-phase method and the hexagonal ferrite powder manufactured by the liquid-phase method can be easily distinguished. In one form, from the viewpoint of mass productivity and the like, as the hexagonal ferrite powder, the hexagonal ferrite powder manufactured by the solid-phase method is preferable.

[0063] As raw materials for hexagonal ferrite having the composition represented by Formula 1 used in the solid-phase method, an Fe compound, a compound of A atom, and an Al compound can be mentioned. These compounds can be oxides, carbonates, etc.

[0064] As for the Al (aluminum) compound used as the above raw material, the present inventors consider that using a powder of an Al compound with a small average particle size can contribute to the presence of region B on the particle surface of the powder and satisfy the relational expressions represented by Formula 2 and Formula 3. From this point of view, as the powder of the Al compound, those with an average particle size of 100 μm or less are preferable, and the average particle size is more preferably 80 μm or less, still more preferably 50 μm or less, even more preferably 10 μm or less, yet even more preferably 2 μm or less, and still even more preferably 100 nm or less. Further, the above average particle size can be, for example, 10 nm or more or 20 nm or more.

[0065] In the present invention and this specification, the average particle size of the Al compound is the median diameter D50 determined based on the volume-based particle size distribution measured by the laser diffraction scattering method. D50 is the particle size at which the cumulative volume is 50%. The measurement of the above particle size distribution can be carried out by the dry laser diffraction scattering method, and in the examples described later, it was carried out by the following method. Using a laser diffraction / scattering type particle size distribution measuring device (Partica LA-960) manufactured by Horiba, Ltd. as the measuring device, the powder of the Al compound is put into the measurement holder so that the transmittance displayed on the measurement monitor of the above device is 95 to 98%, and the particle size distribution is measured by the laser diffraction scattering method under the condition of a compressed air pressure of 0.40 MPa.

[0066] The mixing ratio of the plurality of raw materials may be determined according to the desired hexagonal ferrite composition. The plurality of raw materials can be mixed simultaneously or sequentially in any order and stirred to obtain a raw material mixture. The stirring of the raw materials can be performed by a commercially available stirring device or a stirring device with a known configuration. As an example, the rotation speed during stirring can be in the range of 300 to 3000 rpm (revolutions per minute), and the stirring time can be in the range of 10 minutes to 90 minutes. However, the rotation speed and stirring time during stirring may be set according to the configuration of the stirring device used and are not limited to the ranges exemplified above. Also, the mixing and / or stirring of the raw materials are not limited to being performed only under dry conditions. Under wet conditions, for example, a solvent such as water can be added and mixing and / or stirring can be performed in a slurry state. The above mixing and stirring can be performed, for example, under an atmospheric atmosphere at room temperature. In the present invention and this specification, "room temperature" shall refer to a temperature in the range of 20 to 27°C unless otherwise specified.

[0067] After the above stirring, the obtained raw material mixture can be fired. In this firing, the crystallization of the raw material mixture can be advanced, and thereby the crystal structure of hexagonal ferrite can be formed. As the firing conditions, for example, the firing temperature can be in the range of 1000°C to 1500°C. The firing temperature can be, for example, the atmospheric temperature in the device in which the firing is performed (for example, the furnace temperature in a heating furnace). The firing time can be in the range of 1 hour to 6 hours. However, the above ranges are exemplary, and firing may be performed under conditions capable of forming the crystal structure of hexagonal ferrite. The firing can be performed, for example, under an atmospheric atmosphere.

[0068] It is also possible to perform firing after adding a component that can function as a flux (a melting agent; hereinafter referred to as "flux") to the raw material mixture before firing. Examples of the flux include SrCl2, BaCl2, CaCl2, MgCl2, KCl, NaCl, BaCl2·2H2O, Na2B4O7, and their hydrates. Examples of the hydrates include SrCl2·6H2O, BaCl2·2H2O, CaCl2·2H2O, etc. Among them, from the viewpoint of easily obtaining the above magnetic powder in which region B exists on the particle surface of the powder and the relational expressions represented by Formula 2 and Formula 3 are satisfied, one or more chlorides selected from the group consisting of strontium chloride (SrCl2), barium chloride (BaCl2), and their hydrates (for example, SrCl2·6H2O, BaCl2·2H2O, etc.) are preferable. More preferably, the above chloride can be added to the mixture obtained by mixing the raw materials of the hexagonal ferrite in an amount of 3.0% by mass or more based on the total mass of the raw materials being 100% by mass. The above addition amount can be, for example, 5.0% by mass or more or 10.0% by mass or more. Also, the above addition amount can be, for example, 30.0% by mass or less, preferably 25.0% by mass or less, more preferably 20.0% by mass or less, and still more preferably 15.0% by mass or less. For the hydrates, the above addition amount shall be calculated based on the mass of the hydrate (including the mass of the water of hydration). In one form, from the viewpoint of obtaining a magnetic powder with higher compositional homogeneity, it is preferable to use a flux containing the same type of A atom as the compound of the A atom used as the raw material of the hexagonal ferrite. A high compositional homogeneity of the magnetic powder is preferable from the viewpoint of further improving the radio wave absorption performance. For example, when the compound of the A atom is a compound of the Sr atom, a flux selected from the group consisting of strontium chloride and its hydrates can be used as the flux, and when the compound of the A atom is a compound of the Ba atom, a flux selected from the group consisting of barium chloride and its hydrates can be used as the flux.

[0069] The raw material mixture before firing can be subjected to a pulverization process, and / or the fired product after firing can be subjected to a pulverization process. By performing the pulverization process, the size of the particles constituting the hexagonal ferrite powder can be adjusted. Pulverization can be carried out by known pulverization means such as a mortar and pestle, a pulverizer (cutter mill, ball mill, bead mill, roller mill, jet mill, hammer mill, attritor, etc.).

[0070] Before and / or after each of the various processes described above, known processes can also be optionally carried out. Such processes can include, for example, various known processes such as washing, drying, etc.

[0071] [Radio wave absorber, radio wave absorbing composition] One aspect of the present invention relates to a radio wave absorber containing the above magnetic powder.

[0072] Also, one aspect of the present invention relates to a radio wave absorbing composition containing the above magnetic powder.

[0073] [Magnetic powder] Details of the magnetic powder contained in the above radio wave absorber and the above radio wave absorbing composition are as described above.

[0074] (Volume filling ratio of magnetic powder) In the above radio wave absorber and the above radio wave absorbing composition, the filling ratio of the magnetic powder is not particularly limited. For example, in one form, the filling ratio can be 35% by volume or less as a volume filling ratio, and can also be in the range of 15 - 35% by volume. In another form, the volume filling ratio can also be 35% by volume or more. In this case, the volume filling ratio can be, for example, in the range of 35 - 60% by volume, and can also be in the range of 35 - 50% by volume. The volume filling ratio means, for the radio wave absorber, the volume-based content ratio with respect to the total volume (100% by volume) of the radio wave absorber. For the radio wave absorbing composition, the volume filling ratio means the volume-based content ratio with respect to the total volume (100% by volume) of the solid content (i.e., the components excluding the solvent).

[0075] The volume filling ratio of the magnetic powder in the radio wave absorber can be determined, for example, by collecting the magnetic powder from the radio wave absorber by a known method and calculating it as "(the volume of the collected magnetic powder / the total volume of the radio wave absorber) × 100". Here, the total volume of the radio wave absorber and the volume of the magnetic powder can be determined by known methods. Alternatively, when the composition of the radio wave absorbing composition used for manufacturing the radio wave absorber is known, the volume filling ratio of the magnetic powder in the radio wave absorber can also be determined from this known composition. In addition, the volume filling ratio of the magnetic powder in the radio wave absorber can also be determined by the following method using a cross-sectional SEM image obtained by a scanning electron microscope (SEM). A measurement sample having a square plane with a side length of 5 mm is cut out from a randomly determined position of the radio wave absorber to be measured. A sample for cross-sectional observation is prepared from the cut-out sample. The sample for cross-sectional observation is prepared by FIB (Focused Ion Beam) processing. The prepared sample for cross-sectional observation is observed by SEM, and a cross-sectional image (SEM image) is taken. As the SEM, a field emission scanning electron microscope (FE-SEM) is used. Using the FE-SEM, the sample for cross-sectional observation is set on the stage so that the FIB-processed cross-section faces upward, and a cross-sectional SEM image with a field of view of 30 μm × 40 μm is obtained under the conditions of an acceleration voltage of 15 kV and an observation magnification of 3,000 times. The obtained cross-sectional SEM image is binarized, and the ratio (area basis) occupied by the magnetic powder is calculated. The above operations are performed on five measurement samples cut out from different positions of the radio wave absorber to be measured, and the volume filling ratio of the magnetic powder can be determined as the arithmetic mean of the five obtained values. In addition, by performing elemental analysis of the sample for cross-sectional observation as necessary, the portion of the magnetic powder in the cross-sectional SEM image can also be specified. The volume filling ratios of the other components described in this specification can also be determined in the same manner as above.

[0076] <Binder> The above-described radio wave absorber and the above-described radio wave absorbing composition contain the above magnetic powder, and may further contain a binder. The binder can be, for example, a resin. Examples of the resin include thermoplastic resins and thermosetting resins. Examples of the thermoplastic resin include acrylic resins, polyacetals, polyamides, polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polyethylene terephthalate-1,4-cyclohexanedimethanol terephthalate copolymer, polylactic acid, polycarbonate, polystyrene, polyphenylene sulfide, polyvinyl chloride, ABS (acrylonitrile butadiene styrene) resin obtained by copolymerizing acrylonitrile, butadiene, and styrene; AS (acrylonitrile styrene) resin 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, urethane resins, silicone resins, and the like.

[0077] The binder can also be a rubber. From the viewpoint of, for example, good miscibility with the magnetic powder and the ability to produce a radio wave absorber with excellent durability, weather resistance, and impact resistance, examples of the rubber include butadiene rubber, isoprene rubber, chloroprene rubber, halogenated butyl rubber, fluororubber, urethane rubber, acrylic rubber (abbreviation: ACM) obtained by copolymerizing an acrylate ester (e.g., ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate) and another monomer, ethylene-propylene rubber obtained by coordination polymerization of ethylene and propylene using a Ziegler catalyst, butyl rubber (abbreviation: IIR) obtained by copolymerizing isobutylene and isoprene, styrene-butadiene rubber (abbreviation: SBR) obtained by copolymerizing butadiene and styrene, acrylonitrile-butadiene rubber (abbreviation: NBR) obtained by copolymerizing acrylonitrile and butadiene, silicone rubber, and the like.

[0078] When the radio wave absorber contains rubber as a binder, it may contain various additives such as vulcanizing agents, vulcanization aids, softening agents, and plasticizers in addition to the rubber. Examples of vulcanizing agents include sulfur, organic sulfur compounds, and metal oxides.

[0079] Examples of binders include thermoplastic elastomers (TPE). Examples of thermoplastic elastomers include olefin-based thermoplastic elastomers (TPO), styrene-based thermoplastic elastomers (TPS), amide-based thermoplastic elastomers (TPA), and polyester-based thermoplastic elastomers (TPC).

[0080] The above radio wave absorber and the above radio wave absorbing composition may contain only one type of binder or two or more types of binders. The volume filling ratio of the binder in the radio wave absorber and the radio wave absorbing composition is not particularly limited. For example, it is preferably 65% by volume or more, more preferably 65% by volume or more and 92% by volume or less, and even more preferably 65% by volume or more and 85% by volume or less. When the above radio wave absorber and the above radio wave absorbing composition contain two or more types of binders, the volume filling ratio refers to the total volume filling ratio of the two or more types of binders. This also applies to the volume filling ratio of other components.

[0081] <Additive> The above-described radio wave absorber and the above-described radio wave absorbing composition may or may not optionally contain one or more additives in an arbitrary ratio. Examples of the additives include antioxidants, light stabilizers, dispersants, dispersion aids, fungicides, antistatic agents, plasticizers, impact improvers, crystal nucleating agents, lubricants, surfactants, pigments, dyes, fillers, mold release agents (fatty acids, fatty acid metal salts, oxyfatty acids, fatty acid esters, aliphatic partial saponified esters, paraffins, low molecular weight polyolefins, fatty acid amides, alkylene bis fatty acid amides, aliphatic ketones, fatty acid lower alcohol esters, fatty acid polyhydric alcohol esters, fatty acid polyglycol esters, modified silicones, etc.), processing aids, antifogging agents, drip inhibitors, antibacterial agents, and the like. The additive may be a component that undertakes two or more functions.

[0082] (Antioxidant) In one form, preferred additives may include antioxidants. The antioxidant is not particularly limited, and known antioxidants can be used. Examples of the antioxidant are described, for example, in "Comprehensive Technology of Polymer Stabilization - Mechanism and Application Development -" supervised by Seiichi Oshiro, published by CMC. This description is incorporated herein by reference. Examples of the types of antioxidants include phenolic antioxidants, amine antioxidants, phosphorus antioxidants, sulfur antioxidants, and the like. As the antioxidant, it is preferable to use a phenolic antioxidant and / or an amine antioxidant in combination with a phosphorus antioxidant and / or a sulfur antioxidant.

[0083] Examples of phenolic antioxidants include Adeka's Adeka Stab AO-20, Adeka Stab AO-30, Adeka Stab AO-40, Adeka Stab AO-50, Adeka Stab AO-60, Adeka Stab AO-80, Adeka Stab AO-330, and BASF Japan's Irganox 1010, Irganox 1035, Irganox 1076, Irganox 1098, Irganox 1135, Irganox 1330, Irganox 1726, Irganox 245, Irganox 259, Irganox 3114, Irganox 565, etc. Note that both "Adeka Stab" and "Irganox" are registered trademarks.

[0084] Examples of amine antioxidants include Sankyo Lifetech's Sanol LS-770, Sanol LS-765, Sanol LS-2626, Adeka's Adeka Stab LA-77, Adeka Stab LA-57, Adeka Stab LA-52, Adeka Stab LA-62, Adeka Stab LA-63, Adeka Stab LA-67, Adeka Stab LA-68, Adeka Stab LA-72, and BASF Japan's Tinuvin 123, Tinuvin 144, Tinuvin 622, Tinuvin 765, Tinuvin 944, etc. Note that both "Adeka Stab" and "Tinuvin" are registered trademarks. In addition, as antioxidants, amine compounds capable of quenching radicals can also be used. Examples of such amine compounds include polyethylene glycol bis-TEMPO [Sigma-Aldrich] and sebacic acid bis-TEMPO. Note that "TEMPO" is an abbreviation for tetramethylpiperidine-1-oxyl.

[0085] Examples of phosphorus antioxidants include Adeka's Adeka Stab PEP-8, Adeka Stab PEP-36, Adeka Stab HP-10, Adeka Stab 2112, and BASF Japan's Irgafos 168, etc. Note that both "Adeka Stab" and "Irgafos" are registered trademarks.

[0086] Examples of sulfur-based antioxidants include Adeka's AdekaStab AO-412S, AdekaStab AO-503S, etc. Note that the above "AdekaStab" is a registered trademark.

[0087] Among the above, as the phenolic antioxidant, at least one selected from the group consisting of AdekaStab AO-20, AdekaStab AO-60, AdekaStab AO-80 and IRGANOX 1010 is preferable. As the amine-based antioxidant, AdekaStab LA-52 is preferable. As the phosphorus-based antioxidant, AdekaStab PEP-36 is preferable. As the sulfur-based antioxidant, AdekaStab AO-412S is preferable. When the above-described radio wave absorber and the above-described radio wave absorbing composition contain an antioxidant, they may contain only one kind of antioxidant or two or more kinds of antioxidants.

[0088] When the above-described radio wave absorber and the above-described radio wave absorbing composition contain an antioxidant, the content of the antioxidant in the radio wave absorber and the radio wave absorbing composition is not particularly limited. For example, from the viewpoint of achieving both decomposition suppression of the binder and bleed suppression of the antioxidant, it is preferably 0.1 part by mass to 10 parts by mass, more preferably 0.5 part by mass to 5 parts by mass, based on 100 parts by mass of the binder.

[0089] (Light stabilizer) In one form, preferable additives may include light stabilizers. Examples of light stabilizers include HALS (i.e., Hindered Amine Light Stabilizer), ultraviolet absorbers, singlet oxygen quenchers, etc. The HALS may be a high molecular weight HALS, a low molecular weight HALS, or a combination of a high molecular weight HALS and a low molecular weight HALS.

[0090] When the above-described radio wave absorber and the above-described radio wave absorbing composition contain a light stabilizer, they may contain only one kind of light stabilizer or two or more kinds of light stabilizers.

[0091] - High molecular weight HALS - In the present invention and this specification, "high molecular weight HALS" means a hindered amine light stabilizer having a weight average molecular weight exceeding 1000. Examples of high molecular weight HALS include poly[6-(1,1,3,3-tetramethylbutyl)imino-1,3,5-triazine-2,4-diyl][(2,2,6,6-tetramethyl-4-piperidyl)imino]hexamethylene[(2,2,6,6-tetramethyl-4-piperidyl)imino], which is an oligomeric type of HALS, and dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine polycondensate, and the like. Examples of commercially available products of high molecular weight HALS include CHIMASSORB 944LD and TINUVIN 622LD manufactured by BASF Japan Ltd. Note that both "CHIMASSORB" and "TINUVIN" are registered trademarks.

[0092] The weight average molecular weight (Mw) in the present invention and this specification is a value measured by gel permeation chromatography (GPC). For the measurement using gel permeation chromatography (GPC), as a measuring device, HLC (registered trademark)-8220GPC [manufactured by Tosoh Corporation] can be used. As columns, TSKgel (registered trademark) Super HZM-M [4.6 mm ID × 15 cm, manufactured by Tosoh Corporation], Super HZ4000 [4.6 mm ID × 15 cm, manufactured by Tosoh Corporation], Super HZ3000 [4.6 mm ID × 15 cm, manufactured by Tosoh Corporation], and Super HZ2000 [4.6 mm ID × 15 cm, manufactured by Tosoh Corporation] are each connected in series, and THF (tetrahydrofuran) can be used as an eluent. As measurement conditions, the sample concentration can be 0.2% by mass, the flow rate can be 0.35 mL / min, the sample injection volume can be 10 μL, and the measurement temperature can be 40°C. As the detector, a differential refractive index (RI) detector can be used. The calibration curve can be prepared using "Standard Sample TSK standard, polystyrene" manufactured by Tosoh Corporation: "F-40", "F-20", "F-4", "F-1", "A-5000", "A-2500", and "A-1000".

[0093] When the above radio wave absorber contains high molecular weight HALS, the content rate of high molecular weight HALS in the radio wave absorber is not particularly limited. For example, it is preferably 0.2% by mass to 10% by mass based on the total mass of the radio wave absorber. Setting the content rate of high molecular weight HALS in the above radio wave absorber to 0.2% by mass or more based on the total mass of the radio wave absorber is preferable from the viewpoint of improving weather resistance. When the content rate of high molecular weight HALS in the above radio wave absorber is 10% by mass or less based on the total mass of the radio wave absorber, there is a tendency to suppress a decrease in mechanical strength and the occurrence of blooming.

[0094] - Low molecular weight HALS - In the present invention and this specification, "low molecular weight HALS" means a hindered amine light stabilizer having a molecular weight of 1000 or less (preferably 900 or less, more preferably 600 to 900). Examples of low molecular weight HALS include tris(2,2,6,6-tetramethyl-4-piperidyl) benzene-1,3,5-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl)-2-acetoxypropane-1,2,3-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl)-2-hydroxypropane-1,2,3-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl) triazine-2,4,6-tricarboxylate, tris(2,2,6,6-tetramethyl-4-piperidyl) butane-1,2,3-tricarboxylate, tetrakis(2,2,6,6-tetramethyl-4-piperidyl) propane-1,1,2,3-tetracarboxylate, tetrakis(2,2,6,6-tetramethyl-4-piperidyl) 1,2,3,4-butanetetracarboxylate, tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl) 1,2,3,4-butanetetracarboxylate, 2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-n-butyl malonic acid bis(1,2,2,6,6-pentamethyl-4-piperidyl), and the like. Examples of commercially available products of low molecular weight HALS include ADEKA's AdekaStab LA-57, AdekaStab LA-52, BASF Japan's Tinuvin 144, and the like. Note that both "AdekaStab" and "Tinuvin" are registered trademarks.

[0095] When the above radio wave absorber contains low molecular weight HALS, the content of low molecular weight HALS in the radio wave absorber is not particularly limited. For example, it is preferably 0.2% by mass to 10% by mass based on the total mass of the radio wave absorber. Setting the content of low molecular weight HALS in the above radio wave absorber to 0.2% by mass or more based on the total mass of the radio wave absorber is preferable from the viewpoint of improving weather resistance. When the content of low molecular weight HALS in the above radio wave absorber is 10% by mass or less based on the total mass of the radio wave absorber, it tends to suppress a decrease in mechanical strength and the occurrence of blooming.

[0096] -Ultraviolet Absorbent- Examples of ultraviolet absorbers include benzotriazole-based ultraviolet absorbers such as 2-(2'-hydroxy-3',5'-di-t-butylphenyl)benzotriazole, 2-(3,5-di-t-amyl-2-hydroxyphenyl)benzotriazole, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, 2-(2'-hydroxy-5'-t-octylphenyl)benzotriazole, 2-(2'-hydroxy-3',5'-di-t-amylphenyl)benzotriazole, 2-[2'-hydroxy-3'-(3'',4'',5'',6''-tetrahydro-phthalimidomethyl)-5'-methylphenyl]benzotriazole, 2,2'-methylenebis[4-(1,1,3,3-tetramethylbutyl)-6-(2H-benzotriazol-2-yl)phenol], 2-[2-hydroxy-3,5-bis(α,α-dimethylbenzyl)phenyl]-2H-benzotriazole, 2-(2-hydroxy-4-octyloxyphenyl)-2H-benzotriazole, 2-(2H-benzotriazol-2-yl)-4-methyl-6-(3,4,5,6-tetrahydrophthalimidylmethyl)phenol; benzophenone-based ultraviolet absorbers such as 2-hydroxy-4-methoxybenzophenone, 2,4-dihydroxybenzophenone, 2,2'-dihydroxy-4-methoxybenzophenone, 2,2'-dihydroxy-4,4'-dimethoxybenzophenone, 2-hydroxy-4-n-octoxybenzophenone, 2,2',4,4'-tetrahydroxybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone, 3,5-di-t-butyl-4-hydroxybenzoyl benzoic acid n-hexadecyl ester, 1,4-bis(4-benzoyl-3-hydroxyphenoxy)butane, 1,6-bis(4-benzoyl-3-hydroxyphenoxy)hexane; and cyanoacrylate-based ultraviolet absorbers represented by ethyl-2-cyano-3,3-diphenylacrylate, etc. Examples of commercially available ultraviolet absorbers include TINUVIN 320, TINUVIN 328, TINUVIN 234, TINUVIN 1577, TINUVIN 622, and the IRGANOX series from BASF Japan Ltd., Adeka Stub LA31 from ADEKA Corporation, and SEESORB 102, SEESORB 103, SEESORB 501 from Shipro Kasei Co., Ltd. Note that "TINUVIN", "IRGANOX", "Adeka Stub", and "SEESORB" are all registered trademarks.

[0097] When the above radio wave absorber contains an ultraviolet absorber, the content rate of the ultraviolet absorber in the radio wave absorber is not particularly limited. For example, it is preferably 0.2% by mass to 10% by mass based on the total mass of the radio wave absorber. Setting the content rate of the ultraviolet absorber in the above radio wave absorber to 0.2% by mass or more based on the total mass of the radio wave absorber is preferable from the viewpoint of improving weather resistance. When the content rate of the ultraviolet absorber in the above radio wave absorber is 10% by mass or less based on the total mass of the radio wave absorber, there is a tendency to suppress a decrease in mechanical strength and the occurrence of blooming.

[0098] -Singlet oxygen quencher- When the above radio wave absorber contains a singlet oxygen quencher, the content rate of the singlet oxygen quencher in the radio wave absorber is not particularly limited. For example, it is preferably 0.2% by mass to 10% by mass based on the total mass of the radio wave absorber. Setting the content rate of the singlet oxygen quencher in the above radio wave absorber to 0.2% by mass or more based on the total mass of the radio wave absorber is preferable from the viewpoint of improving weather resistance. When the content rate of the singlet oxygen quencher in the above radio wave absorber is 10% by mass or less based on the total mass of the radio wave absorber, there is a tendency to suppress a decrease in mechanical strength and the occurrence of blooming.

[0099] When the above radio wave absorber contains a light stabilizer, it may contain only one kind of light stabilizer or two or more kinds of light stabilizers.

[0100] <Radio wave absorbing composition and method for manufacturing radio wave absorber> The above radio wave absorbing composition and the method for manufacturing the above radio wave absorber are not particularly limited. The above radio wave absorbing composition can be manufactured by a known method using, for example, the above magnetic powder, a binder, and, if necessary, a solvent, an additive, etc. For example, the above radio wave absorber can be a molded product obtained by molding the above radio wave absorbing composition. The above radio wave absorbing composition can be prepared, for example, by kneading a mixture of the above magnetic powder and binder, and further, if necessary, a solvent, an additive, etc. while heating to obtain a kneaded product. The kneaded product can be obtained in any shape such as a lump, a pellet, a filament usable for 3D (three-dimensional) printers, etc. By molding the kneaded product into a desired shape by a known molding method such as extrusion molding, press molding, injection molding, in-mold molding, 3D printing, etc., a radio wave absorber (molded product) can be obtained. The shape of the radio wave absorber is not particularly limited and can be any shape such as a plate shape, a linear shape, etc. "Plate shape" includes sheet shape and film shape. The plate-shaped radio wave absorber can also be called a radio wave absorbing plate, a radio wave absorbing sheet, a radio wave absorbing film, etc. The above radio wave absorber may be a radio wave absorber of a single composition (for example, a single-layer radio wave absorbing plate), or may be a combination of two or more parts having different compositions (for example, a laminate). Further, the above radio wave absorber may have a planar shape, may have a three-dimensional shape, or may be a combination of a part having a planar shape and a part having a three-dimensional shape. Examples of the planar shape include shapes such as a sheet shape and a film shape. Examples of the three-dimensional shape include a cylindrical shape (circular cylindrical shape, square cylindrical shape, etc.), a horn shape, a box shape (for example, at least one of the surfaces is open), etc.

[0101] For example, from the viewpoint of ease of handling, the thickness of the radio wave absorber is preferably 20 mm or less, more preferably 10 mm or less, and even more preferably 5 mm or less. From the viewpoint of mechanical properties, the thickness is preferably 1 mm or more, and more preferably 2 mm or more. By adjusting the thickness of the radio wave absorber, for example, the transmission attenuation amount described later can be adjusted. When the radio wave absorber is a laminate, the thickness refers to the total thickness of the radio wave absorbers constituting the laminate. The thickness of the radio wave absorber is a value measured using a digital length measuring instrument, specifically, the arithmetic mean of the measured values measured at 9 randomly selected locations.

[0102] The radio wave absorbing composition may or may not contain a solvent. When the radio wave absorbing composition contains a solvent, the solvent is not particularly limited, and examples thereof include water, an organic solvent, or a mixed solvent of water and an organic solvent. Examples of the organic solvent include alcohols such as methanol, ethanol, n-propanol, i-propanol, and methoxypropanol, ketones such as acetone, methyl ethyl ketone, and cyclohexanone, tetrahydrofuran, acetonitrile, ethyl acetate, and toluene. Among these, from the viewpoint of drying speed, ketones are preferred as the solvent, and cyclohexanone is more preferred. When the radio wave absorbing composition contains a solvent, the content of the solvent in the composition is not particularly limited and may be determined according to the manufacturing method of the radio wave absorber.

[0103] The above radio wave absorbing composition can be prepared by mixing the above components. The mixing method is not particularly limited, and examples thereof include a method of mixing by stirring. As the stirring means, a known stirring device can be used. For example, examples of the stirring device include mixers such as paddle mixers and impeller mixers. The stirring time may be set according to the type of the stirring device, the composition of the radio wave absorbing composition, and the like.

[0104] As one form of the method for manufacturing the above-described radio wave absorber, a method of molding the above-described radio wave absorbing composition into a desired shape by a known molding method as exemplified above can be mentioned. Also, as another form of the method for manufacturing the above-described radio wave absorber, a method of manufacturing a radio wave absorber as a radio wave absorption layer by applying the above-described radio wave absorbing composition to a support can be mentioned. The support used here may be removed before the radio wave absorber is incorporated into an article to which radio wave absorption properties are to be imparted, or may be incorporated into the article together with the radio wave absorber without being removed.

[0105] The support is not particularly limited, and a known support can be used. Examples of the support include a metal plate (a plate of a metal such as aluminum, zinc, copper, etc.), a glass plate, a plastic sheet [polyester (such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, etc.), polyethylene (such as linear low-density polyethylene, low-density polyethylene, high-density polyethylene, etc.), polypropylene, polystyrene, polycarbonate, polyimide, polyamide, polyamideimide, polysulfone, polyvinyl chloride, polyacrylonitrile, polyphenylene sulfide, polyetherimide, polyethersulfone, polyvinyl acetal, an acrylic resin sheet, etc.], a plastic sheet laminated or vapor-deposited with the metal exemplified above for the metal plate, etc. The plastic sheet is preferably biaxially stretched. The shape, structure, size, etc. of the support can be appropriately selected. Examples of the shape of the support include a plate shape. The structure of the support may be a single-layer structure or a laminated structure of two or more layers. The size of the support can be appropriately selected according to the size of the radio wave absorber, etc. The thickness of the support is usually about 0.01 mm to 10 mm. For example, from the viewpoint of handleability, it is preferably 0.02 mm to 3 mm, and more preferably 0.05 mm to 1 mm.

[0106] The method of applying the above radio wave absorbing composition on the support is not particularly limited, and examples thereof include methods using a die coater, a knife coater, an applicator, etc. The method of drying the coating film formed by applying the above radio wave absorbing composition is not particularly limited, and examples thereof include methods using a known heating device such as an oven. The drying temperature and drying time are not particularly limited. As an example, the drying temperature can be in the range of 70°C to 90°C, and the drying time can be in the range of 1 hour to 3 hours.

[0107] The above radio wave absorber can be incorporated into various articles that are desired to have radio wave absorption properties. For example, a plate-shaped radio wave absorber can be incorporated into an article in any form as it is, or by bending it at an arbitrary part. Also, it can be adjusted to a desired shape by injection molding or the like and incorporated into an article.

[0108] A radio wave absorber showing excellent transmission attenuation characteristics is useful for improving the recognition accuracy of a radar. As an index of the transmission attenuation characteristics, the transmission attenuation amount can be mentioned. In order to improve the recognition accuracy of the radar, it is desirable to enhance the directivity of the radar. A high transmission attenuation amount can contribute to the improvement of the directivity of the radar. From the viewpoint of improving the directivity of the radar, the transmission attenuation amount of the above radio wave absorber is preferably 5.0 dB or more, more preferably 8.0 dB or more, and still more preferably 10.0 dB or more. Also, the transmission attenuation amount of the above radio wave absorber can be, for example, 15.0 dB or less, 14.5 dB or less, 14.0 dB or less, 13.5 dB or less, 13.0 dB or less, or 12.5 dB or less. However, from the viewpoint of improving the directivity of the radar, it is preferable that the transmission attenuation amount of the radio wave absorber is high. Therefore, the transmission attenuation amount of the above radio wave absorber may exceed the values exemplified above. The above transmission attenuation amount can be, for example, a value measured for a radio wave absorber having a thickness of 2 mm. In one form, the above radio wave absorber can show the transmission attenuation amount in the above range at the frequency of the peak top obtained by the method described in the examples below.

[0109] By the way, an in-vehicle radar, which has recently attracted attention, is a radar that uses radio waves in the millimeter-wave frequency band. Millimeter waves are electromagnetic waves with a frequency of 30.0 GHz to 300.0 GHz. It is preferable that the radio wave absorber exhibits the above-described transmission attenuation amount for one or more frequencies within the frequency band of radio waves, that is, within the frequency band of 3 terahertz (THz) or less. From the viewpoint of the usefulness for improving the recognition accuracy of the in-vehicle radar, the frequencies at which the radio wave absorber exhibits the above-described transmission attenuation amount are preferably one or more frequencies within the millimeter-wave frequency band, that is, within the frequency band of 30.0 GHz to 300.0 GHz, more preferably one or more frequencies within the frequency band of 60.0 GHz to 90.0 GHz, and even more preferably one or more frequencies within the frequency band of 75.0 GHz to 85.0 GHz. Such a radio wave absorber is suitable as a radio wave absorber incorporated on the front side (the incident side of radio waves incident from the outside) of the radio wave transmission / reception unit in the in-vehicle radar for reducing the side lobe of the in-vehicle millimeter-wave radar.

[0110] In addition, from the perspective of the usefulness for improving the recognition accuracy of radio wave absorption articles used in the field of wireless technology such as motion sensors, the frequency at which the radio wave absorber exhibits the transmission attenuation amount within the above range is preferably one or more frequencies in the millimeter wave frequency band, that is, the frequency band of 30.0 GHz to 300.0 GHz, more preferably one or more frequencies in the frequency band of 50.0 GHz to 90.0 GHz, and even more preferably one or more frequencies in the frequency band of 55.0 GHz to 66.0 GHz. Such a radio wave absorber is suitable as a radio wave absorber for improving the recognition accuracy by removing unnecessary radio waves in wireless devices such as mobile phone internal sensors and biometric information sensors. Such a radio wave absorber can be preferably used, for example, in radio wave absorption articles in the 55.0 to 66.0 GHz band. A radio wave absorption article is an article having radio wave absorption properties for radio waves of one or more frequencies, and the radio wave absorption properties can be brought about by incorporating a radio wave absorber as at least a part of this article. A radio wave absorption article in the 55.0 to 66.0 GHz band is an article having radio wave absorption properties for radio waves of one or more frequencies in the frequency band of 55.0 to 66.0 GHz. Examples of such articles include the above various wireless devices. By incorporating the above radio wave absorber into such a radio wave absorption article, unnecessary radio waves can be removed and the recognition accuracy can be improved.

[0111] The "transmission attenuation amount" in the present invention and this specification is a value obtained as S21 of the S parameter by measuring the S parameter in a measurement environment with an ambient temperature of 15 to 35 °C with an incident angle of 0° by the free space method. The measurement can be performed using a known vector network analyzer and horn antenna. Specific examples of the measurement method include the method described in the examples below.

[0112] Depending on the type of radar in which the radio wave absorber is used, the bandwidth may be wideband. For example, a 60 GHz band radar may be used with a bandwidth of 7.0 GHz in the range of 57.0 to 64.0 GHz. For such wideband radar applications, a radio wave absorber capable of corresponding to widebanding can be produced by mixing multiple types of magnetic powders, and / or a radio wave absorber capable of corresponding to widebanding can also be produced by mixing multiple types of radio wave absorbers.

[0113] [Radio wave absorbing article] One aspect of the present invention relates to a radio wave absorbing article including the above radio wave absorber. Specific examples of the radio wave absorbing article can include in-vehicle radars. Specific examples can also include wireless devices such as in-mobile-phone sensors and biometric sensors. Further, in one form, the radio wave absorbing article can be a radio wave absorbing article in the 55.0 GHz to 66.0 GHz band. The radio wave absorbing article only needs to include the radio wave absorber according to one aspect of the present invention, and there are no particular limitations on other configurations, and known techniques related to radio wave absorbing articles can be applied.

Example

[0114] Hereinafter, the present invention will be described based on examples. However, the present invention is not limited to the embodiments shown in the examples. The steps and evaluations described below were performed in an air atmosphere at an ambient temperature of 23°C ± 1°C unless otherwise specified.

[0115] [Example 1] [Production of magnetic powder] Strontium carbonate (SrCO3; Fujifilm Wako Pure Chemical Industries, Ltd.) (46.3 g), α-iron(III) oxide (α-Fe2O3; Fujifilm Wako Pure Chemical Industries, Ltd.) (255.1 g), and aluminum oxide (Al2O3; Fujifilm Wako Pure Chemical Industries, Ltd., average particle size: 40 nm) (14.8 g) were mixed for 2 minutes using a Wonder Crush / Mill (Model WDL-1; Osaka Chemical Co., Ltd.). 300 g of water and flux (strontium chloride hexahydrate (SrCl2·6H2O; Fujifilm Wako Pure Chemical Industries, Ltd.)) were added to the mixture, which was then mixed for 30 minutes using a Waring blender (Model 7011HSJ; Waring Co., Ltd.) and dried in a drying apparatus with an internal atmosphere temperature of 95°C. The amount of flux added was the amount shown in the column for flux amount in Table 1, with the total of the above raw materials (strontium carbonate, α-iron (III) oxide, and aluminum oxide) being 100 mass %. Next, the dried mixture was stirred and pulverized for 2 minutes using the Wonder Crush / Mill to obtain a magnetic powder precursor. The obtained precursor was placed in a muffle furnace, and the temperature inside the furnace was set to 1200° C. in an air atmosphere, and the precursor was fired for 4 hours to obtain a fired body. The obtained sintered body was stirred and pulverized for 2 minutes using the Wonder Crush / Mill, washed repeatedly with water, and then dried in a drying device with an internal atmosphere temperature of 95° C. Thereafter, the sintered body was stirred and pulverized for 2 minutes using the Wonder Crush / Mill to obtain a magnetic powder.

[0116] <Preparation of radio wave absorber> The above magnetic powder was introduced into a kneader (Labo Plastomill manufactured by Toyo Seiki Seisakusho) together with a binder (olefin-based thermoplastic elastomer (TPO) [Milastomer (registered trademark) 7030NS manufactured by Mitsui Chemicals, Inc.]) in an amount such that the volume filling rate of the magnetic powder in the radio wave absorbing composition was 30 volume %, and the kneading was carried out for 20 minutes with the kneader set at a temperature of 200°C, to obtain a composition for forming a radio wave absorber (lumpy kneaded material). The obtained composition for forming a radio wave absorber was press-molded using a hot press machine to obtain a radio wave absorber (radio wave absorbing sheet) as a plate-shaped molded product having a square flat surface with a side length of 100 mm.

[0117] For each of the electromagnetic wave absorbers of Example 1 and Examples 2 to 14 and Comparative Examples 1 to 5 described below, the thickness was determined as the arithmetic mean of the measured values measured at 9 randomly selected locations using a digital length measuring instrument [Litematic (registered trademark) VL-50A manufactured by Mitutoyo Corporation]. The thickness of each of the above electromagnetic wave absorbers was 2 mm.

[0118] [Examples 2 to 14, Comparative Examples 1 to 5] Magnetic powder and electromagnetic wave absorbers were produced in the same manner as in Example 1, except that the various items shown in Table 1 were changed as shown in Table 1. In Examples and Comparative Examples where x in Formula 1 was different from that in Example 1, the various raw materials were mixed at a ratio such that hexagonal ferrite having a composition with the value of x shown in Table 1 was obtained. In Examples and Comparative Examples where "Sr" was described in the column of A atom in Formula 1 in Table 1, magnetic powder was produced using strontium carbonate [SrCO3; manufactured by Fujifilm Wako Pure Chemical Corporation] as the compound of A atom. In the Examples where "Ba" was described in the column of A atom in Formula 1 in Table 1, strontium carbonate [SrCO3; manufactured by Fujifilm Wako Pure Chemical Corporation] was 3; changed to barium carbonate [BaCO manufactured by Fujifilm Wako Pure Chemical Corporation] to produce magnetic powder. In Examples and Comparative Examples where "BaCl2·2H2O" was described in the column of flux in Table 1, magnetic powder was produced using barium chloride dihydrate [BaCl2·2H2O; manufactured by Fujifilm Wako Pure Chemical Corporation]. For the Examples where "40 nm" was described in the column of raw material Al size in Table 1, aluminum oxide [product model: aluminum oxide, 40 to 50 nm] manufactured by Fujifilm Wako Pure Chemical Corporation was used as the aluminum oxide. In the examples where "2 μm" is described in the column of the raw material Al size in Table 1, aluminum oxide manufactured by Fujifilm Wako Pure Chemical Corporation [Model number: α-aluminum oxide, 1 - 2 μm] was used. In the examples and comparative examples where "75 μm" is described in the column of the raw material Al size in Table 1, as the aluminum oxide, aluminum oxide manufactured by Fujifilm Wako Pure Chemical Corporation [Model number: Particle Size (Pass75μm)] was used. In the comparative example where "150 μm" is described in the column of the raw material Al size in Table 1, as the aluminum oxide, aluminum oxide manufactured by Nippon Steel Chemical & Material Co., Ltd. [Model number: AZ75 - 150] was used. The average particle size of the aluminum oxide described in the column of the raw material Al size in Table 1 is the D50 obtained from the volume-based particle size distribution of the aluminum oxide powder sampled as a measurement sample powder from the above-mentioned commercially available products, measured by the laser diffraction scattering method according to the method described above.

[0119] [Evaluation of Magnetic Powder] [Confirmation of the Existence of Region B, Measurement Regarding Formula 2 and Formula 3] Regarding each magnetic powder prepared above, the presence or absence of Region B and the evaluation regarding Formula 2 and Formula 3 were carried out by the method described above. As the ion coater for Pt deposition, the ion coater EIKO 1B - 5 manufactured by Eiko Co., Ltd. was used, as the SEM, the FE-SEM SU8220 manufactured by Hitachi High-Technologies Corporation was used, and as the image processing software, the free software ImageJ was used. The binarization process was carried out with the binarization process conditions of ImageJ set to 8-bit, the default conditions of the threshold value, AUTO. The value of "Content ratio of A atoms in Region B / Content ratio of Al atoms in Region B" calculated from the measurement results is shown in the column of "Formula 2" in Table 1. The value of "Total of the content ratio of A atoms and the content ratio of Al atoms in Region B / Total of the content ratio of A atoms and the content ratio of Al atoms in the whole powder" calculated from the measurement results is shown in the column of "Formula 3" in Table 1. The major axis diameter of Region B shown in Table 1 is the arithmetic mean of the major axis diameters of a plurality of bright regions as described above.

[0120] <Peak particle size> The volume-based particle size distribution of each magnetic powder prepared above was measured by the laser diffraction scattering method by the method described above, and the mode value (mode diameter) was obtained from the measured particle size distribution. The mode diameter thus obtained is shown as "Peak particle size" in Table 1.

[0121] <Confirmation of crystal structure> The crystal structure of the magnetic material constituting each magnetic powder prepared above was confirmed by X-ray diffraction analysis. As the measuring apparatus, X'Pert Pro of PANalytical, a powder X-ray diffractometer, was used. The measurement conditions are shown below. -Measurement conditions- X-ray source: CuKα ray 〔Wavelength: 1.54 Å (0.154 nm), Output: 40 mA, 45 kV〕 Scan range: 20 degree < 2θ < 70 degree Scan interval: 0.05 degree Scan speed: 0.75 degree / min

[0122] As a result of the above X-ray diffraction analysis, it was confirmed that all the magnetic powders have a magnetoplumbite-type crystal structure and are powders of single-phase magnetoplumbite-type hexagonal ferrite that do not contain crystal structures other than the magnetoplumbite type.

[0123] <Confirmation of composition> The composition of the magnetic material constituting each magnetic powder prepared above was confirmed by high-frequency inductively coupled plasma optical emission spectrometry. Specifically, it was confirmed by the following method. A container (beaker) containing 12 mg of magnetic powder and 10 mL of an aqueous hydrochloric acid solution with a concentration of 4 mol / L was held on a hot plate at a set temperature of 120 °C for 3 hours to obtain a dissolved solution. After adding 30 mL of pure water to the obtained dissolved solution, it was filtered using a membrane filter with a pore size of 0.1 μm. Elemental analysis of the filtrate thus obtained was performed using a high-frequency inductively coupled plasma optical emission spectrometer [ICPS-8100 manufactured by Shimadzu Corporation]. Based on the results of the obtained elemental analysis, the content rate of each atom with respect to 100 atomic % of iron atoms was determined. And based on the obtained content rate, the composition of the magnetic material was confirmed. As a result, it was confirmed that the composition of each magnetic powder was such that A in Formula 1 was the atom shown in the column of "A atom" in Table 1 and x was the composition of the value shown in Table 1. In Table 1, for Example 3, Comparative Example 3, and Comparative Example 4 in which "Sr / Ba" is described in the column of A atom in Formula 1, Sr and Ba were detected as A atoms in the above elemental analysis. Taking the total of the detected A atoms as 100 atomic %, the content rate of Sr was 75 atomic % in Example 2 and 89 atomic % in Comparative Example 3 and Comparative Example 4. On the other hand, as a result of the above elemental analysis, in the examples and comparative examples in which "Sr" is described in the column of A atom in Formula 1 in Table 1, it was confirmed that A in Formula 1 was only Sr, and in the examples in which "Ba" is described in the column of A atom in Formula 1, it was confirmed that A in Formula 1 was only Ba. As described above, that A in Formula 1 is only Sr means that the content rate of Sr is 95 atomic % or more with the total of A atoms (that is, the total of Sr, Ba, Ca, and Pb) being 100 atomic %. That A in Formula 1 is only Ba means that the content rate of Ba is 95 atomic % or more with the total of A atoms (that is, the total of Sr, Ba, Ca, and Pb) being 100 atomic %.

[0124] <Ratio (σs / β)> Regarding the magnetic powder produced in Example 1 and the magnetic powder produced in Example 4, when the ratio (σs / β) was calculated from σs and β measured by the following method, both were 240 emu·g -1 ·degree -1 or more. The saturation magnetization σs was measured by the following method. As a measuring device, a vibrating sample magnetometer (model number: TM-TRVSM5050-SMSL type) manufactured by Tamagawa Seisakusho Co., Ltd. was used. Under an air atmosphere at an ambient temperature of 23 °C, under the conditions of a maximum applied magnetic field of 50 kOe and a magnetic field sweep rate of 25 Oe / s, for each of the above magnetic powders, the magnetization intensity of the magnetic powder with respect to the applied magnetic field was measured. From the measurement results, the magnetic field (H)-magnetization (M) curve of the magnetic powder was obtained. Based on the obtained magnetic field (H)-magnetization (M) curve, the saturation magnetization σs (unit: emu / g) was determined. β was measured by the following method. As a measuring device, X’Pert Pro of PANalytical, a powder X-ray diffractometer, was used. Under the following measurement conditions, X-ray diffraction spectra were obtained for each of the above magnetic powders. In the X-ray diffraction spectra obtained for each magnetic powder, a diffraction peak of the (107) plane was confirmed as a peak having the apex of the peak at a position of about 32.5 degrees. For each magnetic powder, the half-value width β of the diffraction peak of the (107) plane was determined by the analysis software (HighScore Plus of PANalytical) installed in the above powder X-ray diffractometer. -Measurement conditions- X-ray source: CuKα ray [Wavelength: 1.54 Å (0.154 nm), Output: 40 mA, 45 kV] Scan range: 25 degrees < 2θ < 35 degrees Scan interval: 0.05 degrees Scan speed: 0.33 degrees / min

[0125] [Evaluation of radio wave absorber] [Transmission attenuation amount] By the following method, the frequency (in Table 1, "peak top") at which the absorption peak of the transmission attenuation of each of the above radio wave absorbers exists was measured. Here, the frequency at which the absorption peak of the transmission attenuation exists means the frequency at which the transmission attenuation amount becomes the maximum value in the swept frequency band. As a measuring device, a vector network analyzer (product name: N5225B) manufactured by Keysight Technologies and horn antennas (product names: RH12S23, RH06S10) manufactured by Keycom were used. By the free space method, with the incident angle set to 0°, and the sweep frequency bands set to 55.0 GHz to 95.0 GHz and 110.0 GHz to 170.0 GHz, one plane of each of the above radio wave absorbers was oriented towards the incident side, and the S-parameters were measured every 0.1 GHz. Taking S21 of the S-parameters as the transmission attenuation amount, the frequency at which the transmission attenuation amount reaches the maximum value in the above sweep frequency band was taken as the peak top and shown in Table 1, and the maximum value of the transmission attenuation amount in the above sweep frequency band was taken as the transmission attenuation amount and shown in Table 1. Based on the transmission attenuation amounts shown in Table 1, the transmission attenuation characteristics were evaluated according to the following evaluation criteria. A: The transmission attenuation amount is 10.0 dB or more B: The transmission attenuation amount is 8.0 dB or more and less than 10.0 dB C: The transmission attenuation amount is 5.0 dB or more and less than 8.0 dB D: The transmission attenuation amount is less than 5.0 dB

[0126] The above results are shown in Table 1. From the results shown in Table 1, it can be confirmed that the radio wave absorber of the example is superior in transmission attenuation characteristics compared to the radio wave absorber of the comparative example.

[0127]

Table 1

[0128] [Example 15] As a specific example of a radio wave absorber capable of coping with broadband, the radio wave absorber of Example 15 was manufactured by the method described for Example 1, except that half of the magnetic powder (by mass) was replaced with the magnetic powder prepared by the method described for Example 8. The transmission attenuation amount of the manufactured radio wave absorber was evaluated by the method described above. As a result, it was confirmed that the radio wave absorber of Example 15 showed a transmission attenuation amount of 5.0 dB or more throughout the range of 57.0 to 64.0 GHz and is a radio wave absorber suitable for broadband radar applications.

[0129] [Example 16] <Preparation of Radio Wave Absorbing Composition (Filament for 3D Printer)> A mixture with the following composition was prepared, and a 1.75 mm diameter filament for a 3D printer was produced using a compounding tester manufactured by Technovel Corporation. Magnetic powder: A magnetic powder mixture (723 g) obtained by mixing the magnetic powder prepared by the method described in Example 1 and the magnetic powder prepared by the method described in Example 8 at a mixing ratio of 1:1 (by mass). Resin: PETG (Glycol-modified PolyEthylene Terephthalate; RS PRO Clear 1.75 mm filament for 3D printer manufactured by RS Components) (278 g) Antioxidant: AO-60 manufactured by ADEKA (2.8 g)

[0130] <Preparation of Radio Wave Absorber (3D Printed Object)> The above-obtained filament for a 3D printer was attached to a 3D printer (Value 3D Magix MF-2500EP2 manufactured by Mutoh), and 3D printing was performed under the conditions of a nozzle temperature of 243°C and a stage temperature of 70°C to obtain a radio wave absorber (3D printed object) as a flat plate with a thickness of 1.9 mm and a size of 110 mm × 110 mm. The transmission attenuation amount of the obtained flat plate was evaluated by the method described above. As a result, it was confirmed that the radio wave absorber of Example 16 exhibited a transmission attenuation amount of 5.0 dB or more over the entire range of 57.0 to 64.0 GHz. From the above results, it was confirmed that a radio wave absorber suitable for radar applications with broad bandwidth can also be formed by 3D printing.

Industrial Applicability

[0131] One aspect of the present invention is useful in the technical field of performing various automatic driving controls such as automatic driving control of automobiles and the wireless technical field such as motion sensors.

Claims

1. The following formula (1): 【Chemical 1】 In formula (1), A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.10 ≦ x ≦ 5.00, A hexagonal ferrite powder having a composition represented by the formula, Region B exists on the particle surface of the powder, and The following formula (2) and formula (3): (Formula (2)) 0.3 ≦ Content ratio of A atoms in region B / Content ratio of Al atoms in region B ≦ 23.0 (Formula (3)) 1.2 ≦ Total of content ratio of A atoms and content ratio of Al atoms in region B / Total of content ratio of A atoms and content ratio of Al atoms in the whole powder ≦ 2.5 Satisfying the relational expression, the content ratio is a content ratio with the total of A atoms, Fe atoms, and Al atoms being 100 atomic%, and the unit of the content ratio is atomic%, The region B is a region observed as a bright region having a major axis diameter of 0.1 μm or more and 0.6 μm or less in a binarized image obtained by binarizing an image obtained by imaging the particle surface with a scanning electron microscope, a magnetic powder for a radio wave absorber.

2. The magnetic powder for a radio wave absorber according to claim 1, having a peak particle size of 4.5 μm or more and less than 12.0 μm.

3. In formula (1), the A atoms are one or two atoms selected from the group consisting of Sr and Ba, the magnetic powder for a radio wave absorber according to claim 1 or 2.

4. The following formula (4): (Formula (4)) 1.5 ≦ Content ratio of A atoms in region B / Content ratio of Al atoms in region B ≦ 10.0 Further satisfying the relational expression, the content ratio is a content ratio with the total of A atoms, Fe atoms, and Al atoms being 100 atomic%, and the unit of the content ratio is atomic%, the magnetic powder for a radio wave absorber according to any one of claims 1 to 3.

5. The ratio of the saturation magnetization σs to the half-value width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis, σs / β, is 240 emu·g －１ ·degree －１ The magnetic powder for a radio wave absorber according to any one of claims 1 to 4, which is the hexagonal ferrite powder as described above.

6. A radio wave absorber containing the magnetic powder for a radio wave absorber according to any one of claims 1 to 5.

7. The radio wave absorber according to claim 6, further containing a binder.

8. A radio wave absorbing article containing the radio wave absorber according to claim 6 or 7.

9. A method for producing the magnetic powder for a radio wave absorber according to any one of claims 1 to 5, The method includes adding, to a mixture obtained by mixing raw materials of hexagonal ferrite, one or more chlorides selected from the group consisting of strontium chloride, barium chloride, and their hydrates, in an addition amount of 3.0% by mass or more, with the total mass of the raw materials being 100% by mass.

10. The production method according to claim 9, wherein the addition amount of the chloride is 5.0% by mass or more and 15.0% by mass or less.

11. The manufacturing method according to claim 9 or 10, wherein the raw material contains an Al compound having an average particle size of 100 μm or less.

12. An electromagnetic wave absorbing composition containing the magnetic powder for an electromagnetic wave absorber according to any one of claims 1 to 5.

13. The electromagnetic wave absorbing composition according to claim 12, further containing a binder.

14. The electromagnetic wave absorbing composition according to claim 12 or 13, which is a filament for a 3D printer.

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