Article provided with radio wave absorbent body, and radio wave absorbent body

Abstract

Provided is an article comprising: a radio wave transmission source; and a radio wave absorbent body. The radio wave absorbent body contains a binder and one or more types of particles that are selected from the group consisting of magnetic particles and dielectric particles, and the ten-point average roughness Rz of at least one of a pair of opposing main surfaces of the radio wave absorbent body is 25-Y μm. Y is a value calculated by Y=λ×1000 / 10, wherein λ mm is the wavelength of a radio wave transmitted by the radio wave transmission source. Also provided is a radio wave absorbent body comprising a binder and one or more types of particles that are selected from the group consisting of magnetic particles and dielectric particles, wherein the ten-point average roughness Rz of at least one of a pair of opposing main surfaces is 25-500 μm.

Description

Articles equipped with radio wave absorbers and radio wave absorbers

[0001] The present invention relates to an article provided with a radio wave absorber and to a radio wave absorber.

[0002] In recent years, radio wave absorbers to be incorporated into various articles have been studied (see, for example, Patent Document 1).

[0003] JP 2019-145534 A

[0004] As an article equipped with a radio wave absorber, for example, radar that recognizes an object by transmitting and receiving radio waves has attracted attention. As an example, an on-board radar transmits radio waves and receives the reflected waves from the object (pedestrian, vehicle, etc.), thereby measuring the direction of the object, the distance to the object, etc. Based on the radar measurement results, an automatic driving control system for an automobile can automatically brake the automobile to a stop as necessary to prevent a collision with the object, or automatically control the speed to maintain a distance from the object.

[0005] In order to increase the reliability of systems that perform various controls based on the results of radar detection as described above, it is desirable to improve the performance of the radar. To this end, in recent years, studies have begun to be conducted on improving the detection accuracy, for example, by installing a radio wave absorber on the front side of the radar's radio wave transmitting and receiving unit (the side on which radio waves enter from outside).

[0006] For example, from the viewpoint of improving the recognition accuracy of radar, it is desirable that the radio wave absorber incorporated in the radar can exhibit excellent radio wave absorbing properties. Also, in various other applications where radio wave absorbers are used, it is desirable that the radio wave absorber can exhibit excellent radio wave absorbing properties.

[0007] On the other hand, for example, in a quality inspection before an article including a radio wave absorber is shipped, an inspector may inspect the article for scratches. Also, when an article including a radio wave absorber is used, the radio wave absorber may be installed in a position where it can be seen. In such a case, for example, it is desirable that scratches on the radio wave absorber are difficult to see.

[0008] In view of the above, an object of one aspect of the present invention is to provide an article having a radio wave absorber that can exhibit excellent radio wave absorbing properties and in which scratches are difficult to see.

[0009] One aspect of the present invention is as follows. [1] An article including a radio wave transmission source and a radio wave absorber, wherein the radio wave absorber comprises one or more types of particles selected from the group consisting of magnetic particles and dielectric particles, and a binder, and at least one of a pair of opposing main surfaces of the radio wave absorber has a ten-point mean roughness Rz (hereinafter also simply referred to as "ten-point mean roughness Rz" or "Rz") of 25 μm to Y μm, wherein Y is a value calculated by Y = λ × 1000 / 10, where λ mm is the wavelength of the radio waves transmitted by the radio wave transmission source. [2] The article according to [1], wherein Y is 500 or less. [3] The article according to [1] or [2], wherein the particles are magnetic particles, and the magnetic particles are hexagonal ferrite particles. [4] The article according to [3], wherein the hexagonal ferrite is a substituted hexagonal ferrite. [5] The article according to [4], wherein the substitutional hexagonal ferrite is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.00. [6] The article according to [5], wherein the atom represented by A in formula 1 includes Sr. [7] The article according to formula 1, wherein Y is 500 or less, the particles are magnetic particles, the magnetic particles are particles of hexagonal ferrite, the hexagonal ferrite is a substituted hexagonal ferrite, and the substituted hexagonal ferrite is a substituted hexagonal ferrite represented by the following formula 1: The article according to [1], wherein the magnetoplumbite-type hexagonal ferrite has a composition represented by the formula: wherein in formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.00, and wherein in formula 1, the atom represented by A includes Sr. [8] A radio wave absorber comprising one or more particles selected from the group consisting of magnetic particles and dielectric particles, and a binder, wherein at least one of a pair of opposing main surfaces has a ten-point average roughness Rz of 25 μm or more and 500 μm or less. [9] The radio wave absorber according to [8], wherein the particles are magnetic particles, and the magnetic particles are particles of hexagonal ferrite.

[10] The radio wave absorber according to [9], wherein the hexagonal ferrite is a substituted hexagonal ferrite.

[11] The radio wave absorber according to

[10] , wherein the substituted hexagonal ferrite is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.00.

[12] The radio wave absorber according to

[11] , wherein the atom represented by A in formula 1 includes Sr.

[13] The particles are magnetic particles, wherein the magnetic particles are particles of hexagonal ferrite, and the hexagonal ferrite is a substituted hexagonal ferrite, and the substituted hexagonal ferrite is a ferrite represented by the following formula 1: [9] The radio wave absorber according to [8], which is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: wherein A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.00, and wherein the atom represented by A in formula 1 includes Sr.

[0010] According to one aspect of the present invention, it is possible to provide an article including a radio wave absorber that can exhibit excellent radio wave absorbing properties and whose scratches are difficult to visually detect. Also, according to another aspect of the present invention, it is possible to provide a radio wave absorber that can exhibit excellent radio wave absorbing properties and whose scratches are difficult to visually detect.

[0011] [Article with Radio Wave Absorber] One aspect of the present invention relates to an article with a radio wave source and a radio wave absorber.

[0012] <Specific example of the article> An example of the article is a radar. A radar is a device that includes a radio wave transmission source and receives reflected waves of radio waves transmitted from the radio wave transmission source to measure the direction of an object's location, the distance to the object, and the like. Specific examples of radar include automotive radar, marine radar, and radar for aircraft traffic control. Specific examples of the article also include wireless devices such as internal sensors in mobile phones and biometric sensors. The radio wave transmission source may be disposed, for example, in a radio wave transmitting / receiving unit. Regarding the structure of the article, details of the radio wave transmission source, and the like, known techniques related to articles including a radio wave transmission source and a radio wave absorber can be applied.

[0013] <Radio Wave Absorber> In the present invention and this specification, "radio waves" refers to electromagnetic waves with a frequency of 3 terahertz (THz) or less. A "radio wave absorber" has radio wave absorption properties for radio waves of one or more frequencies.

[0014] <Ten-point mean roughness Rz> The radio wave absorber provided in the above-mentioned article contains one or more types of particles selected from the group consisting of magnetic particles and dielectric particles, and a binder, and the ten-point mean roughness Rz of at least one of a pair of opposing main surfaces of the radio wave absorber is 25 μm or more and Y μm or less. Here, the above Y is a value calculated by "Y = λ × 1000 / 10", where λ mm is the wavelength of the radio wave emitted by the radio wave source. In other words, Y is 1 / 10 of the wavelength λ converted to a value expressed in μm. The wavelength λ can be calculated from the speed of light and the frequency of the radio wave using the following formula. The speed of light is 3 × 10 ８ m / sec. λ (unit: mm) = 1000 × speed of light / frequency (unit: Hz) λ is expressed in units of mm to one decimal place, and if the calculated value includes two or more decimal places, the second decimal place is rounded up and the third decimal place is discarded. For example, in the case of a frequency of 60 GHz, λ (mm) = 1000 × 3 × 10 ８ / (60 x 10 ９ ) = 5. Therefore, λ is 5.0 mm. Calculating in the same way, for example, when the frequency is 77 GHz, λ is 3.9 mm, and when the frequency is 150 GHz, λ is 2.0 mm.

[0015] The shape of the radio wave absorber is not particularly limited, and may be a planar shape, a three-dimensional shape, or a combination of a planar portion and a three-dimensional portion. Examples of planar shapes include a plate shape. Plate shapes can also be referred to as a sheet shape, a film shape, etc. Three-dimensional shapes include a tubular shape (cylinder, square tube, etc.), a horn shape, etc. Regardless of the shape, the surface with the largest area and the surface opposite to this surface of the radio wave absorber are referred to as the "main surface." The areas of the two main surfaces may be the same or different. The main surface may be flat or curved, or may be a surface including both a flat portion and a curved portion. Furthermore, the positional relationship between the two main surfaces may be parallel or non-parallel. For example, a plate-shaped radio wave absorber may have an upper surface, a lower surface, and four side surfaces located between the upper and lower surfaces. For example, if the areas of the upper and lower surfaces are larger than the areas of the four side surfaces, the upper and lower surfaces are the "main surfaces." A cylindrical or horn-shaped radio wave absorber may have an inner surface, an outer surface, and two annular side surfaces located between these surfaces. For example, if the area of ​​each of the inner surface and the outer surface is larger than the area of ​​each of the two side surfaces, the inner surface and the outer surface are the "main surfaces."

[0016] In a radio wave absorber, one of the main surfaces can be irradiated with radio waves emitted from a radio wave source. The radio waves can be irradiated in various ways, such as when radio waves emitted from the radio wave source directly strike the main surface of the radio wave absorber, or when reflected waves of the radio waves emitted from the radio wave source strike the main surface of the radio wave absorber. Specific examples include when radio waves emitted from the radio wave source directly strike the main surface of the radio wave absorber, when reflected waves emitted from the radio wave source and reflected by an object strike the main surface of the radio wave absorber, and when reflected waves reflected by the object strike the main surface of the radio wave absorber after being reflected by a metal plate or the like (e.g., a vehicle body structure, a frame, etc.) located behind the article. Therefore, it is preferable for a radio wave absorber to have excellent radio wave absorption characteristics measured on one of the main surfaces that can be irradiated with radio waves, from the perspective of improving the performance of an article equipped with the radio wave absorber (for example, improving the recognition accuracy of radar). Regarding this point, the present inventors have conducted extensive research and have newly discovered that if the ten-point mean roughness Rz of at least one of the main surfaces of a radio wave absorber is equal to or less than Y (μm), calculated by "Y = λ × 1000 / 10," where λ mm is the wavelength of the radio waves emitted from a radio wave transmission source provided in an article together with the radio wave absorber, the radio wave absorber can exhibit excellent radio wave absorption characteristics on such main surface. The present inventors speculate that this is because if Rz is equal to or less than Y μm, diffuse reflection of the radio waves emitted from the radio wave transmission source on the main surface can be suppressed. As illustrated above, for a frequency of 60 GHz, λ is 5.0 mm. In this case, Y = 500, so Rz is equal to or less than 500 μm. Similarly, for example, for a frequency of 77 GHz, λ is 3.9 mm and Y is 390, so Rz is equal to or less than 390 μm. For a frequency of 150 GHz, λ is 2.0 mm and Y is 200, so Rz is equal to or less than 200 μm. On the other hand, as a result of intensive studies by the present inventors, it has become clear that the higher the surface smoothness of the main surface of the radio wave absorber, the more easily scratches become visible on this main surface. Therefore, from the viewpoint of making scratches less visible on the main surface, Rz of the radio wave absorber is 25 μm or more, preferably 26 μm or more, and more preferably 27 μm or more, 28 μm or more, 29 μm or more, and 30 μm or more in that order.

[0017] In the above-mentioned radio wave absorber, the principal surface having an Rz of 25 μm or more and Y μm or less is at least one of a pair of opposing principal surfaces, and it is sufficient that the Rz of either one of the principal surfaces is 25 μm or more and Y μm or less. It is not essential that the Rz of both principal surfaces is 25 μm or more and Y μm or less. When the Rz of both principal surfaces is 25 μm or more and Y μm or less, the Rz of one principal surface and the Rz of the other principal surface may be the same value or different values.

[0018] The "ten-point average roughness Rz" in this specification and the present invention is the Rz specified in JIS B0601:1994 and is generally referred to as "Rzjis." Rz is measured using a three-dimensional shape measuring instrument. For example, in the Rz measurement described in the Examples section below, a Keyence VR-3200 three-dimensional shape measuring instrument was used. Rz was measured in a randomly selected area on the main surface of the object to be measured, with a measurement range of 18 mm x 6 mm, a measurement line length of 18 mm, and measurements on three parallel lines (hence, the number of measurements N = 3), with a 3 mm spacing between two adjacent measurement lines. The arithmetic mean of the measurement results for N = 3 is taken as the Rz on the main surface of the object to be measured. With regard to the three parallel lines, "parallel" does not necessarily mean "parallel" in the strict sense, but rather includes the range of error normally acceptable in the technical field to which this invention pertains. The range of error can mean, for example, a range of less than ±10° from strict parallelism.

[0019] A method for controlling Rz of the main surface of the radio wave absorber will be described later.

[0020] The above-mentioned wave absorber will be described in more detail below.

[0021] <Particles> The radio wave absorber includes one or more types of particles selected from the group consisting of magnetic particles and dielectric particles. The radio wave absorber may also include different types of particles selected from the group consisting of magnetic particles and dielectric particles in any ratio. In the present invention and this specification, "magnetic" means ferromagnetic property, and "dielectric" means a material having a volume resistivity (ρ) at 20°C that is an intrinsic value of the material. ｖ ) is 1 x 10 ５ This refers to materials with a resistance of Ω·m or more.

[0022] (Dielectric Particles) As the dielectric particles that can be contained in the radio wave absorber, particles of various dielectrics can be used. For example, the dielectric particles can be carbon-based particles. Here, "based" is used to mean "including." As the carbon-based particles, particles of various carbon-based materials such as carbon black and graphite can be used.

[0023] (Magnetic Particles) Examples of magnetic particles that can be contained in the radio wave absorber include particles of various magnetic materials such as ferrite, iron oxide, cobalt, and chromium oxide. From the viewpoint of radio wave absorption, hexagonal ferrite particles are preferred as the magnetic particles. The type of magnetic material constituting the magnetic particles can be confirmed by analysis using X-ray diffraction. For example, magnetic particle powder can be extracted from the radio wave absorber-producing composition or the radio wave absorber using a known method, and the extracted powder can be analyzed using X-ray diffraction. Alternatively, a portion of the magnetic particle powder used to prepare the radio wave absorber-producing composition can be sampled, and the sampled powder can be analyzed using X-ray diffraction. Alternatively, confirmation can be performed using, for example, the following method. Part or all of the radio wave absorber is finely chopped, immersed in a solvent (e.g., hexafluoroisopropanol) for 1 to 2 days, and then dried. The dried radio wave absorber is further ground into fine powder and analyzed using X-ray diffraction.

[0024] Hexagonal Ferrite Particles In one embodiment, the radio wave absorber may contain hexagonal ferrite particles as magnetic particles. In the present invention and this specification, "hexagonal ferrite particles" refers to magnetic particles in which a hexagonal ferrite crystal structure is detected as the main phase by analysis using X-ray diffraction. The main phase refers to the structure to which the highest diffraction peak belongs in the X-ray diffraction spectrum obtained by X-ray diffraction. For example, if the highest diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction is attributed to the hexagonal ferrite crystal structure, it is determined that the hexagonal ferrite crystal structure has been detected as the main phase. If only a single structure is detected by X-ray diffraction, this detected structure is considered to be the main phase. The hexagonal ferrite crystal structure contains at least iron atoms, divalent metal atoms, and oxygen atoms as constituent atoms. In unsubstituted hexagonal ferrite, the atoms that constitute the hexagonal ferrite crystal structure are iron atoms, divalent metal atoms, and oxygen atoms only. In contrast, substitutional hexagonal ferrite contains one or more other atoms in addition to iron atoms, divalent metal atoms, and oxygen atoms as atoms constituting the crystalline structure of hexagonal ferrite. These one or more other atoms are usually atoms that replace part of the iron in the crystalline structure of hexagonal ferrite. The divalent metal atoms are metal atoms that can become divalent cations as ions, and examples thereof include alkaline earth metal atoms such as strontium atoms, barium atoms, and calcium atoms, and lead atoms. In the present invention and this specification, hexagonal strontium ferrite particles refer to particles in which the main divalent metal atom contained therein is a strontium atom, and hexagonal barium ferrite particles refer to particles in which the main divalent metal atom contained therein is a barium atom. The same applies to particles of other types of hexagonal ferrite. The main divalent metal atom refers to the divalent metal atom that accounts for the largest proportion of the divalent metal atoms contained in the particle on an atomic percentage basis. However, the above divalent metal atoms do not include rare earth atoms. A "rare earth atom" in this invention and specification is selected from the group consisting of scandium atom (Sc), yttrium atom (Y), and lanthanide atom.The lanthanoid atom is selected from the group consisting of lanthanum atom (La), cerium atom (Ce), praseodymium atom (Pr), neodymium atom (Nd), promethium atom (Pm), samarium atom (Sm), europium atom (Eu), gadolinium atom (Gd), terbium atom (Tb), dysprosium atom (Dy), holmium atom (Ho), erbium atom (Er), thulium atom (Tm), ytterbium atom (Yb), and lutetium atom (Lu).

[0025] Regarding the hexagonal ferrite particles, the hexagonal ferrite constituting the particles can be one or more selected from the group consisting of hexagonal strontium ferrite, hexagonal barium ferrite, hexagonal calcium ferrite and hexagonal lead ferrite. From the viewpoint of radio wave absorption in the high frequency band, one or more selected from the group consisting of hexagonal strontium ferrite, hexagonal barium ferrite and hexagonal calcium ferrite are preferred, hexagonal strontium ferrite and / or hexagonal barium ferrite are more preferred, and hexagonal strontium ferrite is even more preferred.

[0026] In one embodiment, the hexagonal ferrite particles can be particles of magnetoplumbite-type (commonly referred to as "M-type") hexagonal ferrite. When magnetoplumbite-type hexagonal ferrite is an unsubstituted type that does not contain atoms substituting for iron, it has the composition formula: AFe １２ O １９ Here, A can represent one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and also includes an embodiment in which two or more of these atoms are contained in any ratio.

[0027] A preferred hexagonal ferrite from the viewpoint of radio wave absorption is a substitutional hexagonal ferrite. A specific example of the substitutional hexagonal ferrite is a substitutional magnetoplumbite hexagonal ferrite in which some of the iron atoms of the magnetoplumbite hexagonal ferrite are substituted with aluminum atoms. One form of such hexagonal ferrite is a hexagonal ferrite having a composition represented by the following formula 1:

[0028]

[0029] In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb (hereinafter also referred to as "A atoms"), and may be only one type, or may contain two or more types in any ratio, and from the viewpoint of improving the uniformity of the composition between particles contained in the radio wave absorber, it is preferable that only one type be present. From the viewpoint of radio wave absorption in the high frequency band, A in formula 1 is preferably one or more atoms selected from the group consisting of Sr, Ba, and Ca, more preferably one or more atoms selected from the group consisting of Sr and Ba, still more preferably containing Sr, and even more preferably being Sr.

[0030] In formula 1, x satisfies 0.50≦x≦8.00.

[0031] In one embodiment, x satisfies 1.50≦x≦8.00. From the viewpoint of radio wave absorption in the high frequency band, x is preferably 1.50 or more, more preferably greater than 1.50, even more preferably 2.00 or more, and even more preferably greater than 2.00. From the viewpoint of magnetic properties, x is 8.00 or less, preferably less than 8.00, more preferably 6.00 or less, and even more preferably less than 6.00.

[0032] A specific example of the magnetoplumbite-type substitutional hexagonal ferrite represented by formula 1, in which x satisfies 1.50≦x≦8.00, is 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 （６．２５） Al （５．７５） 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 and the like. Specific examples also include the substituted hexagonal strontium ferrite described in the Examples section below. The composition of the magnetic particles can be confirmed, for example, by subjecting a solution in which the magnetic particles have been dissolved to high-frequency inductively coupled plasma atomic emission spectroscopy. Specific examples of confirmation methods include the methods described in the Examples section below. Alternatively, the composition of the magnetic particles contained in the radio wave absorber can be confirmed by exposing a cross section by cutting the radio wave absorber, for example, and then subjecting the exposed cross section to energy dispersive X-ray analysis, for example.

[0033] In one embodiment, x may satisfy the condition 0.50≦x<1.50. From the viewpoint of radio wave absorption in the high frequency band, x may be 0.50 or more, and preferably 0.70 or more. From the viewpoint of magnetic properties, x is preferably less than 1.50, and more preferably 1.20 or less. An example of a magnetoplumbite-type substitutional hexagonal ferrite represented by Formula 1 in which x satisfies 0.50≦x<1.50 is the substitutional hexagonal strontium ferrite described in the Examples section below.

[0034] In one embodiment, the substitutional hexagonal ferrite may have a single crystalline phase or may contain multiple crystalline phases. A single crystalline phase is preferred, and a magnetoplumbite-type substitutional hexagonal ferrite having a single crystalline phase is more preferred. The term "single crystalline phase" refers to a case where only one diffraction pattern indicating a given crystalline structure is observed in an analysis performed by X-ray diffraction. For example, X-ray diffraction analysis can be performed using the method described in the Examples section below. When multiple crystalline phases are included, two or more diffraction patterns indicating a given crystalline structure are observed in the X-ray diffraction analysis. For the attribution of diffraction patterns, for example, the database of the International Centre for Diffraction Data (ICDD) (registered trademark) can be referenced. For example, for the diffraction pattern of a magnetoplumbite-type hexagonal ferrite containing Sr, reference can be made to "00-033-1340" of the International Centre for Diffraction Data (ICDD). However, if some of the iron atoms are substituted with substituting atoms such as aluminum atoms, the peak position shifts from the peak position when no substituting atoms are included.

[0035] Method for producing hexagonal ferrite particles There are various methods for producing hexagonal ferrite particles, and when the radio wave absorber contains hexagonal ferrite particles, the method for producing the hexagonal ferrite particles is not particularly limited. Below, a solid-phase method will be described as an example of a method for producing hexagonal ferrite particles. The solid-phase method is a method for producing hexagonal ferrite particles by mixing a plurality of solid raw materials and firing the resulting mixture.

[0036] The solid raw materials used in the solid-phase method include compounds of iron atoms and compounds of A atoms. When producing a powder of substitutional hexagonal ferrite, compounds of substitutional atoms can also be used. These compounds can be oxides, carbonates, etc. The A atoms and substitutional atoms are as described above. The mixing ratio of multiple solid raw materials can be determined depending on the desired hexagonal ferrite composition. A raw material mixture can be obtained by mixing multiple solid raw materials simultaneously or sequentially in any order. Furthermore, mixing of the solid raw materials is not limited to dry conditions. Mixing can also be performed under wet conditions, for example, by adding a solvent such as water to form a slurry. Mixing of the solid raw materials can be performed using a commercially available mixer or a mixer with a known configuration. An example of a mixer is a paint shaker. Mixing can be performed, for example, in an atmospheric atmosphere at room temperature.

[0037] After the above mixing, the obtained raw material mixture can be fired. During this firing, crystallization of the raw material mixture can be promoted, thereby forming a hexagonal ferrite crystal structure. As 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 ambient temperature within the firing device (for example, the temperature within a heating furnace). The firing time can be in the range of 1 hour to 6 hours. However, the above range is merely an example, and firing can be performed under conditions that allow the formation of a hexagonal ferrite crystal structure. Firing can be performed, for example, in an air atmosphere.

[0038] In the firing, a flux can be added to the powder of the raw material mixture before firing. As the flux, various fluxes can be used, for example, SrCl ２ , SrCl ２ ・6H ２ O, CaCl ２ ・2H ２ O, MgCl ２ , KCl, NaCl, BaCl ２ ・2H ２ O, Na ２ B ４ O ７ The amount added is preferably 0.1 to 10 parts by mass, and more preferably 0.1 to 8.0 parts by mass, relative to 100 parts by mass of the powder of the raw material mixture.

[0039] The raw material mixture before firing can be subjected to a dry or wet grinding process, and / or the fired product after firing can be subjected to a dry or wet grinding process. By performing the grinding process, the size of the particles constituting the hexagonal ferrite powder can be adjusted. Grinding can be performed using known grinding means such as a mortar and pestle, a grinder (cutter mill, ball mill, bead mill, roller mill, jet mill, hammer mill, attritor, etc.), etc.

[0040] The fired product may be subjected to a process such as pulverization if necessary, and then annealed. The annealing may be carried out, for example, in a heating furnace at a furnace temperature of 500 to 1100°C for about 1 to 10 hours.

[0041] Before and / or after the various steps described above, one or more known steps such as washing and drying may be optionally carried out.

[0042] Volume Filling Rate The wave absorber includes the particles and a binder. The particle filling rate of the wave absorber is not particularly limited. In one embodiment, the volume filling rate can be 35% by volume or less, for example, in the range of 15 to 35% by volume. In another embodiment, the volume filling rate can be 35% by volume or more. In this case, the volume filling rate can be, for example, in the range of 35 to 60% by volume, or even in the range of 35 to 50% by volume. The volume filling rate refers to the volumetric content when the total volume of the wave absorber is 100% by volume. The volume filling rate of the particles in the wave absorber-preparing composition can also be within the above range. With respect to the wave absorber-preparing composition, the volume filling rate refers to the volumetric content when the total volume of the wave absorber-preparing composition (excluding the solvent, if any; the same applies hereinafter) is 100% by volume.

[0043] The particle volume filling factor for a composition for producing a radio wave absorber can be determined from the composition of the composition. For a radio wave absorber, for example, the particles are collected from the radio wave absorber by a known method, and the particle volume filling factor can be determined by calculating "(volume of collected particles / total volume of radio wave absorber) x 100." Here, the total volume of the radio wave absorber and the volume of the particles can be determined by a known method. Alternatively, if the composition of the composition used to produce the radio wave absorber is known, the particle volume filling factor in the radio wave absorber can also be determined from this known composition. The particle volume filling factor in the radio wave absorber can also be determined by the following method using a cross-sectional SEM image obtained with a scanning electron microscope (SEM). A measurement sample having a square plane with sides of 5 mm is cut out from a randomly determined position on the radio wave absorber to be measured. A cross-sectional observation sample is prepared from the cut-out sample. The cross-sectional observation sample is prepared by FIB (Focused Ion Beam) processing. The prepared cross-sectional observation sample is observed using an SEM, and a cross-sectional image (cross-sectional SEM image) is captured. A field emission scanning electron microscope (FE-SEM) is used as the SEM. Using the FE-SEM, the cross-sectional observation sample is set on a stage with the FIB-processed cross section facing upward, and a cross-sectional SEM image with a field of view of 30 μm × 40 μm is obtained under conditions of an acceleration voltage of 15 kV and an observation magnification of 3000x. The obtained cross-sectional SEM image is then binarized to calculate the proportion (area basis) occupied by the particles. The above operation is performed on five measurement samples cut from different positions on the radio wave absorber to be measured, and the volume filling factor of the particles can be calculated as the arithmetic average of the five obtained values. If necessary, the portion of the particles in the cross-sectional SEM image can also be identified by performing elemental analysis of the cross-sectional observation sample. The volume filling rates of the other components described in this specification can be determined in the same manner as above.

[0044] <Binder> The radio wave absorber includes the particles and further includes a binder. The binder can be, for example, a resin, and examples of the resin include thermoplastic resins and thermosetting resins. Examples of the thermoplastic resin include acrylic resin, polyacetal, polyamide, 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 copolymerization of acrylonitrile, butadiene, and styrene, and AS (acrylonitrile styrene) resin obtained by copolymerization of acrylonitrile and styrene. Examples of the thermosetting resin include phenol resin, epoxy resin, melamine resin, urea resin, unsaturated polyester, diallyl phthalate resin, urethane resin, and silicone resin.

[0045] The binder may also be rubber. Examples of rubber that can be used, from the viewpoint of being able to produce a radio wave absorber that has good mixability with magnetic particles and is excellent in durability, weather resistance, and impact resistance, include butadiene rubber, isoprene rubber, chloroprene rubber, halogenated butyl rubber, fluororubber, urethane rubber, acrylic rubber (abbreviated as ACM) obtained by copolymerizing an acrylic ester (e.g., ethyl acrylate, butyl acrylate, and 2-ethylhexyl acrylate) with another monomer, ethylene-propylene rubber obtained by coordination polymerization of ethylene and propylene using a Ziegler catalyst, butyl rubber (abbreviated as IIR) obtained by copolymerizing isobutylene and isoprene, styrene-butadiene rubber (abbreviated as SBR) obtained by copolymerizing butadiene and styrene, acrylonitrile-butadiene rubber (abbreviated as NBR) obtained by copolymerizing acrylonitrile and butadiene, and silicone rubber.

[0046] When producing a radio wave absorber containing rubber as a binder, the composition may contain, in addition to rubber, various additives such as a vulcanizing agent, a vulcanization aid, a softener, a plasticizer, etc. Examples of vulcanizing agents include sulfur, an organic sulfur compound, and a metal oxide.

[0047] Examples of the binder include thermoplastic elastomers (TPE), such as olefin thermoplastic elastomers (TPO), styrene thermoplastic elastomers (TPS), amide thermoplastic elastomers (TPA), and polyester thermoplastic elastomers (TPC).

[0048] The wave absorber may contain only one type of binder, or may contain two or more types. The volume filling rate of the binder in the wave absorber is not particularly limited, and is, for example, 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 wave absorber contains two or more types of binders, the volume filling rate refers to the total volume filling rate of the two or more binders. This also applies to the volume filling rates of other components. The volume filling rate of the binder in the composition for producing a wave absorber can also be within the above range.

[0049] <Additives> The wave absorber may optionally contain one or more additives in any proportion. Examples of additives include antioxidants, light stabilizers, dispersants, dispersion aids, antifungal agents, antistatic agents, plasticizers, impact modifiers, crystal nucleating agents, lubricants, surfactants, pigments, dyes, fillers, mold release agents (fatty acids, fatty acid metal salts, oxyfatty acids, fatty acid esters, partially saponified aliphatic esters, paraffins, low-molecular-weight polyolefins, fatty acid amides, alkylene bisfatty 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, anti-drip agents, antibacterial agents, etc. One additive component may have two or more functions.

[0050] (Antioxidant) In one embodiment, a preferred additive can be an antioxidant. The antioxidant is not particularly limited, and known antioxidants can be used. Examples of antioxidants are described in, for example, "Comprehensive Technology of Polymer Stabilization - Mechanism and Application Development -" edited by Yasukazu Ohkatsu, published by CMC. This description is incorporated herein by reference. Types of antioxidants include phenolic antioxidants, amine-based antioxidants, phosphorus-based antioxidants, sulfur-based antioxidants, etc. As the antioxidant, it is preferable to use a phenolic antioxidant and / or an amine-based antioxidant in combination with a phosphorus-based antioxidant and / or a sulfur-based antioxidant.

[0051] Examples of the phenolic antioxidant include ADK STAB AO-20, ADK STAB AO-30, ADK STAB AO-40, ADK STAB AO-50, ADK STAB AO-60, ADK STAB AO-80, and ADK STAB AO-330 manufactured by ADEKA Corporation, and IRGANOX 1010, IRGANOX 1035, IRGANOX 1076, IRGANOX 1098, IRGANOX 1135, IRGANOX 1330, IRGANOX 1726, IRGANOX 245, IRGANOX 259, IRGANOX 3114, and IRGANOX 565 manufactured by BASF Japan. The above "ADK STAB" and "IRGANOX" are both registered trademarks.

[0052] Examples of amine-based antioxidants include Sanol LS-770, Sanol LS-765, and Sanol LS-2626 from Sankyo Lifetech Co., Ltd.; 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, and Adeka STAB LA-72 from ADEKA Corporation; and TINUVIN 123, TINUVIN 144, TINUVIN 622, TINUVIN 765, and TINUVIN 944 from BASF Japan. Note that "ADK STAB" and "TINUVIN" are both registered trademarks. Furthermore, amine-based compounds capable of quenching radicals can also be used as antioxidants. Examples of such amine compounds include polyethylene glycol bis-TEMPO (Sigma-Aldrich), sebacate bis-TEMPO, etc. "TEMPO" is an abbreviation for tetramethylpiperidine-1-oxyl.

[0053] Examples of phosphorus-based antioxidants include ADK STAB PEP-8, ADK STAB PEP-36, ADK STAB HP-10, and ADK STAB 2112 manufactured by ADEKA Corporation, and IRGAFOS 168 manufactured by BASF Japan Ltd. Both "ADK STAB" and "IRGAFOS" are registered trademarks.

[0054] Examples of sulfur-based antioxidants include Adekastab AO-412S and Adekastab AO-503S manufactured by ADEKA Corporation. Note that "Adekastab" is a registered trademark.

[0055] Among the above, the phenol-based antioxidant is preferably one or more selected from the group consisting of ADK STAB AO-20, ADK STAB AO-60, ADK STAB AO-80 and IRGANOX 1010, the amine-based antioxidant is preferably ADK STAB LA-52, the phosphorus-based antioxidant is preferably ADK STAB PEP-36, and the sulfur-based antioxidant is preferably ADK STAB AO-412S. When the radio wave absorber contains an antioxidant, it may contain only one type of antioxidant or two or more types.

[0056] When the wave absorber contains an antioxidant, the content of the antioxidant in the wave absorber is not particularly limited, and from the viewpoint of simultaneously suppressing decomposition of the binder and suppressing bleeding of the antioxidant, for example, the content is preferably 0.1 parts by mass to 10 parts by mass, and more preferably 0.5 parts by mass to 5 parts by mass, relative to 100 parts by mass of the binder.

[0057] (Light Stabilizer) In one embodiment, a preferred additive can be a light stabilizer. Examples of the light stabilizer include HALS (i.e., hindered amine light stabilizer), ultraviolet absorber, singlet oxygen quencher, 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.

[0058] When the wave absorber contains a light stabilizer, it may contain only one type of light stabilizer or may contain two or more types of light stabilizers.

[0059] High Molecular Weight HALS In the present invention and this specification, "high molecular weight HALS" refers to a hindered amine-based light stabilizer having a weight-average molecular weight of more than 1,000. Examples of high molecular weight HALS include oligomer-type HALS such as 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] and polycondensation product of dimethyl succinate-1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine. Examples of commercially available high molecular weight HALS include CHIMASSORB 944LD and TINUVIN 622LD manufactured by BASF Japan Ltd. It should be noted that "CHIMASSORB" and "TINUVIN" are both registered trademarks.

[0060] The weight average molecular weight (Mw) in the present invention and this specification is a value measured by gel permeation chromatography (GPC). In measurements using gel permeation chromatography (GPC), an HLC (registered trademark)-8220GPC (manufactured by Tosoh Corporation) can be used as a measuring device, and a series connection of one of the following columns can be used: TSKgel (registered trademark) Super HZM-M (4.6 mm ID x 15 cm, manufactured by Tosoh Corporation), Super HZ4000 (4.6 mm ID x 15 cm, manufactured by Tosoh Corporation), Super HZ3000 (4.6 mm ID x 15 cm, manufactured by Tosoh Corporation), and Super HZ2000 (4.6 mm ID x 15 cm, manufactured by Tosoh Corporation), and THF (tetrahydrofuran) can be used as the eluent. Measurement conditions can be set to a sample concentration of 0.2% by mass, a flow rate of 0.35 mL / min, a sample injection volume of 10 μL, and a measurement temperature of 40° C. A refractive index (RI) detector can be used as the detector. Calibration curves can be created using standard samples manufactured by Tosoh Corporation: "TSK standard, polystyrene," "F-40," "F-20," "F-4," "F-1," "A-5000," "A-2500," and "A-1000."

[0061] When the wave absorber contains a high-molecular-weight HALS, the content of the high-molecular-weight HALS in the composition and the wave absorber is not particularly limited, and is preferably, for example, 0.2% by mass to 10% by mass relative to the total mass of the wave absorber. From the viewpoint of improving weather resistance, it is preferable that the content of the high-molecular-weight HALS in the wave absorber be 0.2% by mass or more relative to the total mass of the wave absorber. When the content of the high-molecular-weight HALS in the wave absorber is 10% by mass or less relative to the total mass of the wave absorber, a decrease in mechanical strength and the occurrence of blooming tend to be suppressed.

[0062] Low Molecular Weight HALS In the present invention and this specification, the term "low molecular weight HALS" refers to a hindered amine light stabilizer having a molecular weight of 1,000 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, and tris(2,2,6,6-tetramethyl-4-piperidyl)benzene-1,3,5-tricarboxylate. Examples of commercially available low molecular weight HALS include 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, and bis(1,2,2,6,6-pentamethyl-4-piperidyl)2-(3,5-di-t-butyl-4-hydroxybenzyl)-2-n-butylmalonate. Examples of commercially available low molecular weight HALS include Adeka STAB LA-57 and Adeka STAB LA-52 manufactured by ADEKA Corporation, and TINUVIN 144 manufactured by BASF Japan. The above "ADK STAB" and "TINUVIN" are both registered trademarks.

[0063] When the wave absorber contains a low molecular weight HALS, the content of the low molecular weight HALS in the wave absorber is not particularly limited, and is preferably, for example, 0.2 mass % to 10 mass % relative to the total mass of the composition or wave absorber. It is preferable from the viewpoint of improving weather resistance that the content of the low molecular weight HALS in the wave absorber be 0.2 mass % or more relative to the total mass of the wave absorber. When the content of the low molecular weight HALS in the wave absorber is 10 mass % or less relative to the total mass of the wave absorber, it tends to be possible to suppress a decrease in mechanical strength and the occurrence of blooming.

[0064] Ultraviolet absorbers Examples of ultraviolet absorbers include 2-(2'-hydroxy-3',5'-di-t-butylphenyl)benzotriazole, 2-(3,5-di-t-amyl-2-hydroxyphenyl)benzotriazole, 2-(2'-hydroxy-5'-methyl-phenyl)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-( benzotriazole-based ultraviolet absorbers such as 2H-benzotriazol-2-yl)-4-methyl-6-(3,4,5,6-tetrahydrophthalimidylmethyl)phenol, 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, benzophenone-based ultraviolet absorbers such as hydroxybenzophenone, 4-dodecyloxy-2-hydroxybenzophenone, 3,5-di-t-butyl-4-hydroxybenzoylbenzoic acid n-hexadecyl ester, 1,4-bis(4-benzoyl-3-hydroxyphenoxy)butane, and 1,6-bis(4-benzoyl-3-hydroxyphenoxy)hexane; and cyanoacrylate-based ultraviolet absorbers typified by ethyl-2-cyano-3,3-diphenylacrylate.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., ADK STAB LA31 from ADEKA Corporation, and SEESORB 102, SEESORB 103, and SEESORB 501 from Shipro Kasei Co., Ltd. Note that the above-mentioned "TINUVIN," "IRGANOX," "ADK STAB," and "SEESORB" are all registered trademarks.

[0065] When the wave absorber contains an ultraviolet absorber, the content of the ultraviolet absorber in the composition and the wave absorber is not particularly limited, and is preferably, for example, 0.2% by mass to 10% by mass relative to the total mass of the wave absorber. From the viewpoint of improving weather resistance, it is preferable that the content of the ultraviolet absorber in the wave absorber be 0.2% by mass or more relative to the total mass of the wave absorber. When the content of the ultraviolet absorber in the wave absorber is 10% by mass or less relative to the total mass of the wave absorber, a decrease in mechanical strength and the occurrence of blooming tend to be suppressed.

[0066] Singlet oxygen quencher When the wave absorber contains a singlet oxygen quencher, the content of the singlet oxygen quencher in the wave absorber is not particularly limited, and is preferably, for example, 0.2% by mass to 10% by mass relative to the total mass of the wave absorber. From the viewpoint of improving weather resistance, it is preferable that the content of the singlet oxygen quencher in the wave absorber be 0.2% by mass or more relative to the total mass of the wave absorber. When the content of the singlet oxygen quencher in the wave absorber is 10% by mass or less relative to the total mass of the wave absorber, a decrease in mechanical strength and the occurrence of blooming tend to be suppressed.

[0067] When the wave absorber contains a light stabilizer, it may contain only one type of light stabilizer or may contain two or more types of light stabilizers.

[0068] The composition for producing a radio wave absorber may or may not contain a solvent. When the 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 organic solvents 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, ketones are preferred, and cyclohexanone is more preferred, from the viewpoint of drying speed. When the composition contains a solvent, the content of the solvent in the composition is not particularly limited, and may be determined appropriately.

[0069] The composition for producing a radio wave absorber 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. Examples of the stirring device include mixers such as a paddle mixer and an impeller mixer. The stirring time may be set depending on the type of stirring device, the composition of the composition, etc.

[0070] For example, the composition can be prepared as a kneaded product by heating and kneading a mixture of the particles and a binder, and, if necessary, a solvent, an additive, etc. The kneaded product can be obtained in any shape, such as a block, a pellet, or a filament that can be used for a 3D (three-dimensional) printer. A radio wave absorber (molded product) can be obtained 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, or 3D printing. The shape of the radio wave absorber is as described above. The radio wave absorber can be formed from only one type of composition, or can be a combination of two or more parts with different compositions (for example, a laminate).

[0071] Examples of methods for controlling the Rz of the main surface of the radio wave absorber include the following: A method of providing irregularities by etching the surface of a mold (e.g., a metal mold) used to mold a radio wave absorber-producing composition to obtain a molded article; A method of producing a three-dimensional structure (molded article) on which irregularities are formed using a 3D printer; A method of forming irregularities by sandblasting a molded article obtained by molding a radio wave absorber-producing composition.

[0072] For example, from the viewpoint of ease of handling, the thickness of the radio wave absorber is preferably 20.0 mm or less, more preferably 10.0 mm or less, and even more preferably 5.0 mm or less. From the viewpoint of mechanical properties, the thickness is preferably 1.0 mm or more, and more preferably 2.0 mm or more. 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 can be measured using a digital length measuring device, and can be, for example, the arithmetic average of measurements taken at nine randomly selected points.

[0073] The above-mentioned radio wave absorber can be incorporated into various articles to which it is desired to impart radio wave absorbing properties. For example, a plate-shaped radio wave absorber can be incorporated into an article in any desired form, either as is or by curving any desired portion. Furthermore, a radio wave absorber molded into a desired shape by injection molding or the like can also be incorporated into an article.

[0074] Meanwhile, automotive radar, which has been attracting attention in recent years, is a radar that uses radio waves in the millimeter wave frequency band. Millimeter waves are electromagnetic waves with frequencies between 30 GHz and 300 GHz. The radio wave absorber described above is suitable as a radio wave absorber to be incorporated into the front side (the side where radio waves enter from outside) of a radio wave transmitting / receiving unit in an automotive radar, for example, in order to reduce the side lobes of the automotive millimeter wave radar.

[0075] The radio wave absorber can also be incorporated into radio wave absorbing articles used in the field of wireless technology, such as motion sensors, etc. The radio wave absorber is suitable as a radio wave absorber for improving recognition accuracy by removing unnecessary radio waves in wireless devices, such as internal sensors in mobile phones and biometric information sensors.

[0076] An article comprising the radio wave absorber and the radio wave transmission source can be called a radio wave absorbing article. A radio wave absorbing article is an article that has radio wave absorption properties for radio waves of one or more frequencies, and by being provided with a radio wave absorber, radio wave absorption properties for radio waves of one or more frequencies can be achieved.

[0077] [Radio wave absorber] One aspect of the present invention relates to a radio wave absorber comprising one or more types of particles selected from the group consisting of magnetic particles and dielectric particles and a binder, and in which at least one of a pair of opposing main surfaces has a ten-point average roughness Rz of 25 μm or more and 500 μm or less.

[0078] The radio wave absorber has an Rz of at least one main surface of 500 μm or less, preferably 455 μm or less, and more preferably 450 μm or less, 400 μm or less, 350 μm or less, 300 μm or less, 250 μm or less, 200 μm or less, 150 μm or less, and 100 μm or less in that order. Such a radio wave absorber exhibits excellent radio wave absorption properties for radio waves in the frequency band of 60 GHz or more (hence, wavelength λ of 5.0 mm or less), and is therefore preferred as a radio wave absorber to be incorporated into a radio wave absorbing article for the frequency band of 60 GHz or more. A radio wave absorbing article for the frequency band of 60 GHz or more is an article that has radio wave absorption properties for radio waves of one or more frequencies in the frequency band of 60 GHz or more. This also applies to radio wave absorbing articles for the frequency bands described below. The radio wave absorbing article for the frequency band of 60 GHz or more can be, for example, a radio wave absorbing article for the frequency band of 60 GHz or more and 150 GHz or less, or can also be a radio wave absorbing article for the frequency band of 60 GHz or more and 77 GHz or less.

[0079] The Rz of the radio wave absorber is 25 μm or more for the reasons described above. The preferred range of Rz is as described above for the radio wave absorber included in the article. Other details of the radio wave absorber are also as described above for the radio wave absorber included in the article.

[0080] The present invention will be described below 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 unless otherwise specified. Furthermore, the steps and evaluations described below were performed at an ambient temperature of 23°C ± 1°C unless otherwise specified.

[0081] [Magnetic Particles] The magnetic particles used to prepare the samples described below are as follows: Magnetic Particle 1: In Formula 1, A = Sr, x = 0.75 Magnetic Particle 2: In Formula 1, A = Sr, x = 1.65 Magnetic Particle 3: In Formula 1, A = Sr, x = 3.52

[0082] <Confirmation of Crystal Structure> The crystal structure of the magnetic material constituting each of magnetic particles 1 to 3 was confirmed by X-ray diffraction analysis. The measurement device used was a powder X-ray diffractometer, X'Pert Pro, manufactured by PANalytical. The measurement conditions are as follows: - Measurement Conditions - X-ray source: CuKα radiation [wavelength: 1.54 Å (0.154 nm), output: 40 mA, 45 kV] Scan range: 20°<2θ<70° Scan interval: 0.05° Scan speed: 0.33° / min

[0083] As a result of the above X-ray diffraction analysis, it was confirmed that magnetic particles 1 to 3 have a magnetoplumbite-type crystal structure and are single-phase magnetoplumbite-type hexagonal ferrite particles that do not contain any crystal structure other than the magnetoplumbite-type.

[0084] <Confirmation of Composition> The composition of the magnetic material constituting each of magnetic particles 1 to 3 was confirmed by inductively coupled plasma atomic emission spectroscopy. Specifically, confirmation was performed by the following method. A container (beaker) containing 12 mg of magnetic particles and 10 mL of a 4 mol / L hydrochloric acid aqueous solution was held on a hot plate set at 120°C for 3 hours to obtain a solution. 30 mL of pure water was added to the obtained solution, and then the solution was filtered using a membrane filter with a pore size of 0.1 μm. Elemental analysis of the filtrate thus obtained was performed using an inductively coupled plasma atomic emission spectroscopy analyzer (Shimadzu Corporation, ICPS-8100). Based on the results of the elemental analysis, the content of each atom relative to 100 atomic % of iron atoms was calculated. The composition of the magnetic material was then confirmed based on the obtained content. As a result, it was confirmed that the composition of each of magnetic particles 1 to 3 was a composition in which A in Formula 1 was Sr and x was the value described above.

[0085] [Preparation of Molds] Seven molds (Molds 1 to 7) were prepared in which a plurality of recesses and projections were provided by chemical etching on only one of the two molding surfaces, the molding surface used to mold one of the main surfaces of the molded article. The depths of the recesses were as follows: Mold 1: approximately 1 μm, Mold 2: approximately 10 μm, Mold 3: approximately 30 μm, Mold 4: approximately 100 μm, Mold 5: approximately 300 μm, Mold 6: approximately 500 μm, and Mold 7: approximately 750 μm. Each sample in the table below is a radio wave absorber molded by injection molding using the mold described below. Sample 1-1, Sample 2-1 and Sample 3-1: radio wave absorbers molded by injection molding using mold 1; Sample 1-2, Sample 2-2 and Sample 3-2: radio wave absorbers molded by injection molding using mold 2; Sample 1-3, Sample 2-3 and Sample 3-3: radio wave absorbers molded by injection molding using mold 3; Sample 1-4, Sample 2-4 and Sample 3-4: radio wave absorbers molded by injection molding using mold 4; Sample 1-5, Sample 2-5 and Sample 3-5: radio wave absorbers molded by injection molding using mold 5; Sample 1-6, Sample 2-6 and Sample 3-6: radio wave absorbers molded by injection molding using mold 6; Sample 1-7, Sample 2-7 and Sample 3-7: radio wave absorbers molded by injection molding using mold 7.

[0086] [Preparation of Radio Wave Absorber] Each sample (radio wave absorber) shown in the table below was prepared by the following method. A powder of magnetic particles shown in the table below was added to a binder (polypropylene resin (Novatec PP MA3H manufactured by Japan Polypropylene Corporation)) in an amount such that the volume filling rate of the magnetic particle powder in the produced radio wave absorber was 20 volume %, and the mixture was kneaded using a twin-screw kneader, and then cut using a pelletizer to obtain a composition for preparing a radio wave absorber (pellets for injection molding). The obtained pellets were fed from the hopper of the injection molding machine to perform injection molding, and a molded product (radio wave absorber) was obtained in the form of an A5 size flat plate (148 mm x 210 mm). The molding die described above was used as the injection molding die. For each of the produced radio wave absorbers, the thickness was determined as the arithmetic average of measurements taken at nine randomly selected points using a digital length measuring machine (Litematic (registered trademark) VL-50A manufactured by Mitutoyo Corporation). The thickness of each wave absorber was 2.5 mm.

[0087] Except for the fact that a mold was used in which neither of the two molding surfaces had been chemically etched, molded articles were produced in the same manner as the samples shown in each of Tables 1 to 3 below. The molded articles produced in this manner were used as reference samples for the samples shown in Tables 1 to 3 respectively in the evaluation of radio wave absorption properties described below.

[0088] For each of the samples shown in the table below, 12 molded articles (wave absorbers) were prepared, one was used for measuring Rz and the above-mentioned thickness, another was used for evaluating the visibility of scratches, and the remaining 10 were used for evaluating the wave absorption properties.

[0089] [Measurement of Rz] For each sample shown in the table below, a three-dimensional shape measuring instrument, VR-3200 manufactured by Keyence Corporation, was used as the three-dimensional shape measuring instrument to measure the Rz of the main surface formed by the molding surface on the side where the irregularities were provided of each molding die, using the method described above.

[0090] [Evaluation Method] <Radio Wave Absorption Characteristics> The return loss (unit: dB) of each sample shown in the table below was measured using the following method. The return loss at a frequency of 60 GHz or 77 GHz was determined using the following method. For each sample, S parameters were measured using a Keysight vector network analyzer (product name: N5225B) and a Keycom horn antenna (product name: RH12S23) using the free space method, with an incident angle of 0° and a sweep frequency of 60 GHz to 90 GHz, with the main surface formed by the molding surface on the side with the irregularities of each mold facing the incident side, and the S parameter S11 was determined at the frequencies (60 GHz, 77 GHz) listed in each table below. The return loss at a frequency of 150 GHz was determined using the same method as above, except that a Keycom horn antenna (product name: RH06S10) was used and the sweep frequency was 110 GHz to 170 GHz. For each frequency (60 GHz, 77 GHz, 150 GHz), the arithmetic mean of the S11 values ​​(number of measurements N=10) found for 10 wave absorbers was taken as the return loss of each sample. The return loss was also found for N=10 reference samples for each sample. Regarding the evaluation criteria, when the change in return loss relative to the return loss of the reference sample was 0 dB or more and less than 1.5 dB, it was given an "A", when it was 1.5 dB or more and less than 3.0 dB, it was given a "B", and when it was 3.0 dB or more, it was given a "C".

[0091] <Visibility of Scratches> For each sample shown in the table below, the main surface molded by the molding surface on the side where the irregularities were provided of each molding mold was scratched using a pencil with a hardness of 4H and a scratch tester (HEIDON 14 model manufactured by Shinto Scientific Co., Ltd.) at a speed of 4 mm / sec under a load of 750 g. Regarding the evaluation criteria, the scratched surface was visually observed, and if scratches were confirmed visually, it was rated as "C", and if scratches were not confirmed visually, it was rated as "A". The same operator evaluated all samples, and the distance between the operator and the main surface to be observed was the same in the evaluation of all samples.

[0092] The results are shown in the table below.

[0093]

[0094]

[0095]

[0096] From the results shown in each table, it can be seen that samples having a Rz of 25 μm or more and Y μm or less on the main surface as determined by the above method had scratches that were difficult to see and exhibited excellent radio wave absorption properties (the evaluation result of the radio wave absorption properties was A). Furthermore, from the results shown in Tables 1 and 2, it can be seen that samples having a Rz of 500 μm or less on the main surface as determined by the above method exhibited good radio wave absorption properties in the frequency band of 60 GHz or more and 77 GHz or less (the evaluation result of the radio wave absorption properties was A or B).

[0097] One aspect of the present invention is useful, for example, in technical fields such as performing various automatic driving controls, such as automatic driving control of automobiles, and wireless technical fields such as motion sensors.

Claims

1. An article comprising a radio wave transmission source and a radio wave absorber, wherein the radio wave absorber comprises one or more types of particles selected from the group consisting of magnetic particles and dielectric particles, and a binder, and at least one of a pair of opposing main surfaces of the radio wave absorber has a ten-point average roughness Rz of 25 μm or more and Y μm or less, wherein Y is a value calculated by Y = λ x 1000 / 10, where λ mm is the wavelength of the radio waves transmitted by the radio wave transmission source.

2. The article of claim 1, wherein Y is 500 or less.

3. The article of claim 1, wherein said particles are magnetic particles, and said magnetic particles are particles of hexagonal ferrite.

4. The article of claim 3, wherein the hexagonal ferrite is a substitutional hexagonal ferrite.

5. The article according to claim 4, wherein the substitutional hexagonal ferrite is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.

00.

6. The article of claim 5, wherein the atom represented by A in formula 1 includes Sr.

7. The Y is 500 or less, the particles are magnetic particles, the magnetic particles are particles of hexagonal ferrite, the hexagonal ferrite is a substituted hexagonal ferrite, and the substituted hexagonal ferrite is represented by the following formula 1: The article according to claim 1, which is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula: wherein in formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca and Pb, and x satisfies 0.50≦x≦8.00, and in formula 1, the atom represented by A includes Sr.

8. A radio wave absorber comprising one or more types of particles selected from the group consisting of magnetic particles and dielectric particles, and a binder, and wherein the ten-point average roughness Rz of at least one of a pair of opposing main surfaces is 25 μm or more and 500 μm or less.

9. The radio wave absorber according to claim 8, wherein the particles are magnetic particles, and the magnetic particles are particles of hexagonal ferrite.

10. The radio wave absorber according to claim 9, wherein the hexagonal ferrite is a substitutional hexagonal ferrite.

11. The radio wave absorber according to claim 10, wherein the substitutional hexagonal ferrite is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: In formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 0.50≦x≦8.

00.

12. The radio wave absorber according to claim 11, wherein the atom represented by A in formula 1 includes Sr.

13. The particles are magnetic particles, the magnetic particles are particles of hexagonal ferrite, the hexagonal ferrite is a substituted hexagonal ferrite, and the substituted hexagonal ferrite is represented by the following formula 1:

9. The radio wave absorber according to claim 8, which is a magnetoplumbite-type hexagonal ferrite having a composition represented by the following formula 1: wherein in formula 1, A represents one or more atoms selected from the group consisting of Sr, Ba, Ca and Pb, and x satisfies 0.50≦x≦8.00, and in formula 1, the atom represented by A includes Sr.

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