Wave absorber and wave absorbing composition

Abstract

To provide a radio wave absorber capable of contributing to recognition accuracy improvement of a radar.SOLUTION: A radio wave absorber contains a magnetic powder and a binder. In the radio wave absorber, the magnetic powder is a powder of hexagonal ferrite of which a ratio (σs / β) of saturation magnetization σs and a half value width β of a diffraction peak on a (107) face calculated by X-ray diffraction analysis is 240 emu g-1 degree-1 or more. A radio wave absorbable composition contains a magnetic powder and a binder. In the radio wave absorbable composition, the magnetic powder is a powder of hexagonal ferrite of which a ratio (σs / β) of saturation magnetization σs and a half value width β of a diffraction peak on a (107) surface calculated by X-ray diffraction analysis is 240 emu g-1 degree-1 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a radio wave absorber and a radio wave absorbing composition. [Background technology]

[0002] Known radio wave absorbers include those containing magnetic powder as a radio wave absorbing material. Examples of radio wave absorbers containing magnetic powder include a radio wave absorber in which magnetic powder is mixed with a binder (see Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 4674380 Summary of the Invention [Problem to be solved by the invention]

[0004] In recent years, radar, which recognizes objects by transmitting and receiving radio waves, has been attracting attention as an electronic device that uses radio waves. For example, automotive radar transmits radio waves and receives the radio waves reflected by the object (pedestrian, vehicle, etc.), thereby recognizing the presence of the object and the distance to the object. To prevent a collision with the object, an autonomous driving control system for an automobile can automatically brake the vehicle to a stop as necessary based on the results of the radar's recognition of the object, or automatically control the speed to maintain the distance from the object.

[0005] In order to increase the reliability of systems that perform various controls based on the results detected by radar as described above, it is desirable to improve the performance of the radar.To that end, in recent years, studies have begun to be conducted on 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) to improve recognition accuracy.

[0006] In view of the above, an object of one aspect of the present invention is to provide a radio wave absorber that can contribute to improving the recognition accuracy of radar. [Means for solving the problem]

[0007] One aspect of the present invention is A radio wave absorber comprising a magnetic powder and a binder, The ratio (σs / β) of the saturation magnetization σs to the half-width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis is 240 emu·g -1 degree -1 The electromagnetic wave absorber is a powder of the above hexagonal ferrite. Regarding.

[0008] Another aspect of the present invention is A radio wave absorbing composition comprising a magnetic powder and a binder, The ratio (σs / β) of the saturation magnetization σs to the half-width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis is 240 emu·g -1 degree -1 a radio wave absorbing composition which is a powder of the above hexagonal ferrite; Regarding.

[0009] In one embodiment, the radio wave absorber can be a molded article obtained by molding the radio wave absorbing composition.

[0010] In one embodiment, the hexagonal ferrite can be a substitutional hexagonal ferrite.

[0011] In one embodiment, the substitutional hexagonal ferrite can have a composition represented by the following formula 1: Formula 1:A 1 Fe (12-x) Al x O 19 In formula 1, A 1 represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 1.50≦x≦8.00.

[0012] In one embodiment, the ratio (σs / β) is 240 emu·g -1 degree -1 More than 310emu g -1 degree -1 It can be:

[0013] In one embodiment, the ratio (σs / β) is 245 emu·g -1 degree -1 More than 310emu g -1 degree -1 It can be:

[0014] In one embodiment, the hexagonal ferrite may be a substitutional hexagonal ferrite having a composition represented by the following formula 2: Formula 2:A 2 Fe (12-y) Al y O 19 In formula 2, A 2 represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and y satisfies 0.5≦y<1.5.

[0015] In one embodiment, in formula 2, y can satisfy 0.8≦y≦1.2.

[0016] In one embodiment, the ratio (σs / β) is 300 emu·g -1 degree -1 More than 400emu g -1 degree -1 It can be:

[0017] In one embodiment, the radio wave absorber can be used in a radio wave absorbing article for the 50 to 90 GHz band.

[0018] In one embodiment, the substituted hexagonal ferrite can be a substituted hexagonal strontium ferrite.

[0019] In one embodiment, the β can be 0.190 degrees or less.

[0020] In one embodiment, the volume filling rate of the magnetic powder in the radio wave absorber and / or the radio wave absorbing composition can be 35% by volume or less. [Effects of the Invention]

[0021] According to one aspect of the present invention, it is possible to provide a radio wave absorber that can contribute to improving the recognition accuracy of radar, and a radio wave absorbing composition that can be used to produce this radio wave absorber. DETAILED DESCRIPTION OF THE INVENTION

[0022] [Radio wave absorber, radio wave absorbing composition] One aspect of the present invention relates to a radio wave absorber comprising a magnetic powder and a binder, wherein the magnetic powder has a ratio (σs / β) of saturation magnetization σs to the half-width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis of 240 emu g -1 degree -1 The above is a powder of hexagonal ferrite.

[0023] Another aspect of the present invention relates to a radio wave absorbing composition comprising a magnetic powder and a binder, wherein the magnetic powder has a ratio (σs / β) of saturation magnetization σs to the half-width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis of 240 emu·g -1 degree -1 The above is a powder of hexagonal ferrite.

[0024] In the present invention and this specification, the term "radio waves" refers to electromagnetic waves with 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 transmission attenuation and / or return attenuation, which will be described in detail later. The higher the value of the transmission attenuation, the higher the value of the return attenuation, or the higher the values ​​of the transmission attenuation and return attenuation, the better the radio wave absorption properties can be said to be.

[0025] In the present invention and this specification, the term "powder" refers to an aggregate of a plurality of particles. The term "aggregate" is not limited to a form in which the particles constituting the aggregate are in direct contact with each other, but also includes a form in which a binder or the like is interposed between the particles.

[0026] In order to improve the radar's recognition accuracy, it is desirable to increase the radar's directivity. Furthermore, it is desirable to increase the selectivity with which the radar selectively receives radio waves from targets by removing or reducing unnecessary radio wave components. For the former, the higher the transmission attenuation of the radio wave absorber, the more preferable, and for the latter, the higher the return attenuation of the radio wave absorber, the more preferable. From the above perspectives, in order to improve the radar's recognition accuracy, it is desirable for the radio wave absorber installed on the front side of the radar's radio wave transmitting / receiving unit (the side where radio waves entering from outside are incident) to have both high transmission attenuation and return attenuation. However, with conventional radio wave absorbers containing magnetic powder and a binder, generally, attempts to increase the transmission attenuation tend to decrease the return attenuation, making it difficult to increase both the transmission attenuation and the return attenuation. In response to this, the inventors have conducted extensive research and found that in a radio wave absorber containing magnetic powder and a binder, the ratio σs / β of the magnetic powder, which will be described in detail later, is 240 emu·g -1 degree -1 It has been discovered that by using the above-mentioned hexagonal ferrite powder, it is possible to increase both the transmission loss and the return loss.

[0027] Incidentally, a radio wave absorber may have a metal layer laminated on the surface opposite to the surface onto which radio waves are incident (the so-called back surface). Such a radio wave absorber is called a matched radio wave absorber. A matched radio wave absorber can improve its return loss characteristics by providing a metal layer and utilizing phase difference absorption. In contrast, the above-mentioned radio wave absorber itself can have excellent return loss characteristics. Specifically, it can exhibit high return loss without relying on a metal layer. A radio wave absorber used without a metal layer laminated on the back surface is generally called a transmission-type radio wave absorber. In conventional transmission-type radio wave absorbers containing magnetic powder and a binder, generally, the return loss tends to decrease when an attempt is made to increase the transmission loss. In contrast, the above-mentioned radio wave absorber can exhibit high return loss and high transmission loss without relying on a metal layer. The term "metal layer" used herein refers to a layer containing metal and substantially reflecting radio waves. However, if the above-described radio wave absorber containing a magnetic powder and a binder contains a metal, such a radio wave absorber is not considered to be a metal layer. Here, "substantially reflecting radio waves" means, for example, that when a metal layer is laminated on the back surface of a radio wave absorber and radio waves are incident on the radio wave absorber, the radio wave absorber reflects 90% or more of the incident radio waves. Examples of the metal layer include a metal plate and a metal foil. For example, a metal layer formed by vapor deposition on the back surface of the radio wave absorber is included. The above-described radio wave absorber can be used without providing a metal layer on the back surface. Being able to use the radio wave absorber without a metal layer is preferable from the perspectives of recycling the radio wave absorber and cost. Furthermore, a radio wave absorber used with a metal layer laminated on the back surface may experience quality degradation due to deterioration of the metal layer, peeling between the metal layer and the radio wave absorber, etc. Being able to use the radio wave absorber without providing a metal layer on the back surface is also preferable because such quality degradation does not occur.

[0028] The radio wave absorber and the radio wave absorbing composition will be described in more detail below.

[0029] <Magnetic powder> (σs / β) The radio wave absorber and the radio wave absorbing composition are magnetic powders having a ratio (σs / β) of saturation magnetization σs to the half-width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis of 240 emu·g -1 degree -1 The above hexagonal ferrite powder is included.

[0030] The saturation magnetization σs is also called mass magnetization and has units of emu / g. 1 emu / g = 1 A m 2 / kg. The saturation magnetization σs of the magnetic powder is measured using a vibrating sample magnetometer under the conditions of an atmospheric 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].

[0031] The β is the half-width of the diffraction peak of the (107) plane determined by X-ray diffraction analysis of hexagonal ferrite powder. The half-width is the full width at half maximum (FWHM). As a result of the inventor's investigation, it has become clear that there is a correlation between the ratio (σs / β) determined for the half-width β of the diffraction peak of the (107) plane among the diffraction planes of hexagonal ferrite and the radio wave absorption performance. In the present invention and this specification, X-ray diffraction analysis is performed using a powder X-ray diffractometer under the following measurement conditions. The X-ray diffraction spectrum is obtained as a spectrum with the vertical axis representing intensity (unit: counts) and the horizontal axis representing the diffraction angle (unit: degree (°)). In the X-ray diffraction spectrum, the diffraction peak of the (107) plane is detected as a peak having an apex at a diffraction angle 2θ of 32 to 33 degrees (usually around 32.5 degrees). The half-value width of the diffraction peak of the (107) plane can be determined using 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: 25degree<2θ<35degree Scan interval: 0.05 degrees Scan speed: 0.33 degrees / min

[0032] Saturation magnetization σs is one of the magnetic properties of magnetic powder. On the other hand, the present inventors speculate that the above β can be reduced by reducing the variation in ferrite composition between particles that make up the hexagonal ferrite powder. The fact that the ratio of σs to β affects the radio wave absorption performance of the radio wave absorber is a new finding obtained as a result of the inventors' intensive studies. Based on this finding, the present inventors have conducted further studies and have found that the magnetic powder should have a σs / β of 240 emu·g -1 degree -1 It has been discovered that by using the above-mentioned hexagonal ferrite powder, it is possible to increase both the transmission loss and the reflection loss of a radio wave absorber containing magnetic powder and a binder.

[0033] As explained above, if both the transmission attenuation and return attenuation of the radio wave absorber can be increased, it can contribute to improving the recognition accuracy of the radar in which this radio wave absorber is installed. From the viewpoint of increasing both the transmission attenuation and return attenuation of the radio wave absorber, the σs / β of the hexagonal ferrite powder is set to 240 emu·g -1 degree -1 That's all.

[0034] In one embodiment, the σs / β of the hexagonal ferrite powder is 242 emu·g -1 degree -1 Preferably, it is 245 emu·g or more. -1 degree -1 More preferably, it is 247 emu·g or more. -1 degree -1 More preferably, it is 250 emu·g or more. -1 degree -1 More preferably, it is 255 emu·g or more. -1 degree -1 More preferably, it is 260 emu·g or more. -1 degree-1 It is even more preferable that the σs / β of the hexagonal ferrite powder is 320 emu·g or more. -1 degree -1 Below, 315emu g -1 degree -1 or less than 310 emu·g -1 degree -1 or less. Alternatively, the σs / β of the hexagonal ferrite powder may be a value exceeding the above-exemplified values. For example, a substitutional hexagonal ferrite powder having a composition represented by Formula 1, the details of which will be described later, is preferred as being capable of having a σs / β within the above range.

[0035] In one embodiment, the σs / β of the hexagonal ferrite powder is 300 emu·g -1 degree -1 For example, a powder of substitutional hexagonal ferrite having a composition represented by formula 2, which will be described in detail later, has a density of 300 emu·g -1 degree -1 In this embodiment, the σs / β of the hexagonal ferrite powder is preferably 300 emu·g -1 degree -1 More than 400emu g -1 degree -1 More preferably, it is:

[0036] One way to increase the value of σs / β is to reduce the value of the half-width β of the diffraction peak of the (107) plane, which is determined by X-ray diffraction analysis of the hexagonal ferrite powder. From this perspective, in one embodiment, β is preferably 0.190 degrees or less, more preferably 0.188 degrees or less, even more preferably 0.185 degrees or less, even more preferably 0.183 degrees or less, and even more preferably 0.180 degrees or less. β can be, for example, 0.130 degrees or more, 0.140 degrees or more, or 0.150 degrees or more. Alternatively, β may be a value lower than the values ​​exemplified above. However, as long as σs / β is within the range described above, the value of β is not particularly limited.

[0037] One way to increase the σ / β value is to increase the σ of the hexagonal ferrite powder. From this perspective, in one embodiment, σ is preferably 42.0 emu / g or more, more preferably 42.5 emu / g or more, and even more preferably 43.0 emu / g or more. σ can be, for example, 55.0 emu / g or less, 53.0 emu / g or less, 52.0 emu / g or less, or 50.0 emu / g or less. Alternatively, σ may be a value greater than the values ​​exemplified above. However, as long as σ / β is within the range described above, the σ value is not particularly limited.

[0038] The σs and β of the magnetic powder contained in the radio wave absorber can be determined, for example, by carrying out the above-mentioned measurements on the magnetic powder used in producing the radio wave absorber, or on magnetic powder from the same lot as the magnetic powder. Alternatively, for example, the σs and β of the magnetic powder contained in the radio wave absorber can be determined by extracting the magnetic powder from the radio wave absorber by a known method and then carrying out the above-mentioned measurements on the extracted magnetic powder. This also applies to the magnetic powder contained in the radio wave absorbing composition.

[0039] The hexagonal ferrite powder will now be described in more detail.

[0040] (Constituent atoms of hexagonal ferrite) In the present invention and this specification, the term "hexagonal ferrite powder" refers to a magnetic powder in which a hexagonal ferrite-type crystal structure is detected as the main phase by X-ray diffraction analysis. The main phase refers to the structure to which the most intense diffraction peak belongs in the X-ray diffraction spectrum obtained by X-ray diffraction analysis. For example, if the most intense diffraction peak in the X-ray diffraction spectrum obtained by X-ray diffraction analysis is assigned to a hexagonal ferrite-type crystal structure, it is determined that the hexagonal ferrite-type crystal structure has been detected as the main phase. If only a single structure is detected by X-ray diffraction analysis, this detected structure is considered to be the main phase. The hexagonal ferrite-type 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 only iron atoms, divalent metal atoms, and oxygen atoms. 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 hexagonal ferrite crystal structure. These one or more other atoms are usually atoms that replace part of the iron in the hexagonal ferrite crystal structure. 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 powder" refers to a powder in which the main divalent metal atom contained in the hexagonal ferrite crystal structure is strontium atoms. 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 hexagonal ferrite crystal structure 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 herein 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).

[0041] Substitutional hexagonal ferrite contains iron atoms, divalent metal atoms, and oxygen atoms as the atoms that constitute the crystal structure of hexagonal ferrite, as well as one or more other atoms. The saturation magnetization σs can be controlled by the type and content of the substitutional atoms. Such atoms include one or more trivalent metal atoms selected from the group consisting of Al, Ga, and In, and combinations of divalent and tetravalent metal atoms such as Mn and Ti, Co and Ti, and Zn and Ti.

[0042] In one embodiment, the magnetic powder can be a powder of magnetoplumbite-type (commonly called "M-type") hexagonal ferrite. When magnetoplumbite-type hexagonal ferrite is an unsubstituted type that does not contain atoms that substitute for iron, it has the composition formula: AFe 12 O 19 The substituted hexagonal ferrite has a composition represented by the following formula: where A represents at least one atom selected from the group consisting of Sr, Ba, Ca, and Pb, and also includes embodiments in which two or more of these atoms are contained in any ratio. The substituted hexagonal ferrite may preferably be a substituted hexagonal strontium ferrite.

[0043] A preferred hexagonal ferrite from the viewpoint of radio wave absorption performance is a substituted magnetoplumbite hexagonal ferrite in which some of the iron atoms of the magnetoplumbite hexagonal ferrite are substituted with aluminum atoms. One embodiment of such a hexagonal ferrite is a substituted hexagonal ferrite having a composition represented by the following formula 1:

[0044] Formula 1:A 1 Fe (12-x) Al x O 19

[0045] In formula 1, A 1 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 contain only one type or two or more types in any ratio, but from the viewpoint of improving the uniformity of the composition among the particles that make up the powder, it is preferable that only one type is contained. From the viewpoint of radio wave absorption performance in the high frequency band, A in Equation 1 1 is preferably one or more atoms selected from the group consisting of Sr, Ba and Ca, and more preferably Sr.

[0046] In formula 1, x satisfies 1.50≦x≦8.00. From the viewpoint of radio wave absorption performance in the high frequency band, x is 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. The larger the value of x, the smaller the value of σs tends to be. Furthermore, 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.

[0047] A specific example of the magnetoplumbite-type substitutional hexagonal ferrite represented by formula 1 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 include substitutional hexagonal strontium ferrites having the compositions shown in Table 1, which will be described later. The composition of the hexagonal ferrite can be confirmed by high-frequency inductively coupled plasma atomic emission spectrometry. Specific examples of confirmation methods include the methods described in the Examples, which will be described later. Alternatively, the composition of the magnetic powder contained in the radio wave absorber can be confirmed by exposing a cross section by cutting the radio wave absorber or the like, and then performing energy dispersive X-ray analysis on the exposed cross section, for example.

[0048] Furthermore, one embodiment of a substitutional magnetoplumbite hexagonal ferrite in which some of the iron atoms of magnetoplumbite hexagonal ferrite are substituted with aluminum atoms is a substitutional hexagonal ferrite having a composition represented by the following formula 2:

[0049] Formula 2:A 2 Fe (12-y) Al y O 19

[0050] In formula 2, A 2 represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb. 2 Regarding A in Eq. 1 As described above.

[0051] In formula 2, y satisfies 0.5≦y<1.5. From the viewpoint of radio wave absorption performance in a high frequency band around 60 GHz (for example, in the range of 55 to 66 GHz), y is 0.5 or more and preferably exceeds 0.8. The larger the value of y, the smaller the value of σs tends to be. Furthermore, from the viewpoint of magnetic properties, y is preferably less than 1.5 and 1.2 or less. Specific examples of magnetoplumbite-type substitutional hexagonal ferrites represented by formula 1 include, for example, substitutional hexagonal strontium ferrites having the compositions shown in Table 1 below.

[0052] In one embodiment, the substitutional hexagonal ferrite powder may have a single crystalline phase or may contain multiple crystalline phases, and is preferably a single crystalline phase, and more preferably a magnetoplumbite-type substitutional hexagonal ferrite powder having a single crystalline phase. The term "single-phase crystalline phase" refers to a case where only one diffraction pattern indicating a given crystalline structure is observed in X-ray diffraction analysis. X-ray diffraction analysis can be performed, for example, by the method described in the Examples below. When multiple crystalline phases are present, two or more diffraction patterns indicating the given crystalline structure are observed in X-ray diffraction analysis. For the attribution of diffraction patterns, the database of the International Centre for Diffraction Data (ICDD: International Centre for Diffraction Data (registered trademark)) can be referenced. For example, for the diffraction pattern of a magnetoplumbite-type hexagonal ferrite containing Sr, ICDD's "00-033-1340" can be referenced. However, if some of the iron atoms are replaced by substitutional atoms such as aluminum atoms, the peak position will shift from the peak position when no substitutional atoms are present.

[0053] (Method for producing hexagonal ferrite powder) Methods for producing hexagonal ferrite powder include a solid-phase method and a liquid-phase method. The solid-phase method is a method for producing hexagonal ferrite powder by dry-mixing multiple solid raw materials and firing the resulting mixture. In contrast, the liquid-phase method includes a step using a solution. The hexagonal ferrite powder can be produced by either a solid-phase method or a liquid-phase method. Hexagonal ferrite powder produced by a solid-phase method can be easily distinguished from hexagonal ferrite powder produced by a liquid-phase method. For example, hexagonal ferrite powder produced by a liquid-phase method can usually be confirmed to have alkali metal salt precipitates on the surfaces of particles constituting the powder by SEM-EDX analysis (Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy), due to the production method. Furthermore, for example, when hexagonal ferrite powder produced by the solid phase method is subjected to particle morphology observation using a field emission-scanning electron microscope (FE-SEM), so-called amorphous particles can usually be confirmed. For example, in this way, hexagonal ferrite powder produced by the solid phase method and hexagonal ferrite powder produced by the liquid phase method can be easily distinguished. In one embodiment, from the viewpoint of mass productivity, etc., hexagonal ferrite powder produced by the solid phase method is preferred.

[0054] The solid raw materials used in the solid phase method include compounds of iron atoms and compounds of A atoms, and 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 atom and the substitutional atom are as described above. The mixing ratio of the multiple solid raw materials may be determined according to the desired hexagonal ferrite composition. A raw material mixture can be obtained by simultaneously mixing the multiple solid raw materials or sequentially mixing them in any order and stirring them. The stirring of the solid raw materials can be performed using a commercially available stirring device or a stirring device with a known configuration. By adjusting the stirring conditions, the half-width β of the diffraction peak of the (107) plane of the hexagonal ferrite powder to be produced can be controlled. The value of β tends to decrease by applying a strong stirring force (e.g., increasing the rotation speed during stirring). Furthermore, the value of β tends to decrease by extending the stirring time. For example, the rotation speed during stirring can be set in the range of 300 to 3,000 rpm (rotations per minute), and the stirring time can be set in the range of 10 to 90 minutes. However, the rotation speed during stirring and the stirring time can be set depending on the configuration of the stirring device used and are not limited to the ranges exemplified above. The above mixing and stirring can be performed, for example, in an atmospheric atmosphere at room temperature. In the present invention and this specification, "room temperature" refers to a temperature in the range of 20 to 27°C.

[0055] After the stirring, the resulting 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. The firing can be performed, for example, in an air atmosphere.

[0056] During firing, a flux can be added to the raw material mixture powder before firing. Various fluxes can be used, including SrCl2·6H2O, CaCl2·2H2O, MgCl2, KCl, NaCl, BaCl2·2H2O, and Na2B4O7. The amount of flux added is preferably 0.1 to 10 parts by mass, and more preferably 0.1 to 8.0 parts by mass, per 100 parts by mass of the raw material mixture powder.

[0057] 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 performed using known pulverization means such as a mortar and pestle, or a pulverizer (cutter mill, ball mill, bead mill, roller mill, jet mill, hammer mill, attritor, etc.).

[0058] The obtained hexagonal ferrite powder may be subjected to a surface treatment with a known surface treatment agent as required, or may be used for preparing an electromagnetic wave absorbing composition without being subjected to a surface treatment.

[0059] Examples of types of surface treatments include oil treatments using hydrocarbon oils, ester oils, lanolin, etc.; silicone treatments using dimethylpolysiloxane, methylhydrogenpolysiloxane, methylphenylpolysiloxane, etc.; fluorine compound treatments using perfluoroalkyl group-containing esters, perfluoroalkylsilanes, perfluoropolyethers, and polymers having perfluoroalkyl groups, etc.; silane coupling agent treatments using 3-methacryloxypropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, etc.; titanium coupling agent treatments using isopropyl triisostearoyl titanate, isopropyl tris(dioctylpyrophosphate) titanate, etc.; metal soap treatments; amino acid treatments using acylglutamic acid, etc.; lecithin treatments using hydrogenated egg yolk lecithin, etc.; polyethylene treatments; mechanochemical treatments; and phosphate compound treatments using phosphoric acid, phosphorous acid, phosphates, phosphites, etc.

[0060] Among these, phosphate compound treatment is preferred as the surface treatment. Treating hexagonal ferrite powder with a phosphate compound allows the formation of a thick, highly polar layer on the surface of the particles constituting the powder. Formation of a highly polar layer on the surface of the particles can suppress aggregation due to hydrophobic interactions between the particles, thereby more effectively suppressing an increase in the viscosity of the radio wave absorbing composition. Therefore, in the case of a powder treated with a phosphate compound, the flowability of the radio wave absorbing composition is less likely to decrease due to a large powder content, and the handleability and processability tend to be less impaired. Furthermore, formation of a highly polar layer on the surface of the particles not only suppresses aggregation between the particles but also increases the affinity between the powder and the binder, allowing the powder to be more uniformly dispersed in the binder. Therefore, a radio wave absorber formed from a radio wave absorbing composition containing a powder treated with a phosphate compound tends to be less prone to variations in radio wave absorption performance and to have superior mechanical strength.

[0061] The phosphate compound includes, in addition to phosphoric acid, phosphorous acid, hypophosphorous acid, pyrophosphoric acid, linear polyphosphoric acid, cyclic metaphosphoric acid, and salts thereof. When the phosphate compound is in the form of a salt, the phosphate compound is preferably a metal salt. The metal salt is not particularly limited, and examples thereof include alkali metal salts and alkaline earth metal salts. The phosphate compound may also be an ammonium salt.

[0062] In the phosphoric acid compound treatment, only one type of phosphoric acid compound may be used, or two or more types of phosphoric acid compounds may be used.

[0063] In the treatment with a phosphate compound, the phosphate compound is usually mixed with a chelating agent, a neutralizing agent, etc. to form a surface treatment agent. In the phosphoric acid compound treatment, an aqueous solution containing a commercially available phosphoric acid compound can also be used as the surface treatment agent. The treatment of powder with a phosphate compound can be carried out, for example, by mixing the powder with a surface treatment agent containing a phosphate compound. Conditions such as mixing time and temperature may be appropriately set depending on the purpose. In the treatment with a phosphate compound, the dissociation (equilibrium) reaction of the phosphate compound is utilized to precipitate an insoluble phosphate compound on the surfaces of particles constituting the powder. For details of the phosphoric acid compound treatment, see, for example, "Hymen Gijutsu," Vol. 61, No. 3, p. 216, 2010, or "Hymen Gijutsu," Vol. 64, No. 12, p. 640, 2013.

[0064] As the surface treatment, a silane coupling agent treatment is also preferred. The silane coupling agent is preferably a silane coupling agent having a hydrolyzable group. In the silane coupling agent treatment using a silane coupling agent having a hydrolyzable group, the hydrolyzable group in the silane coupling agent is hydrolyzed by water to form a hydroxyl group, which undergoes a dehydration condensation reaction with the hydroxyl group on the surface of the silica particles, thereby modifying the surface of the particles. Examples of the hydrolyzable group include an alkoxy group, an acyloxy group, and a halogeno group.

[0065] The silane coupling agent may have a hydrophobic group as a functional group. Examples of silane coupling agents having a hydrophobic group as a functional group include alkoxysilanes such as methyltrimethoxysilane (MTMS), dimethyldimethoxysilane, phenyltrimethoxysilane, methyltriethoxysilane, dimethyldiethoxysilane, phenyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, hexyltrimethoxysilane, hexyltriethoxysilane, and decyltrimethoxysilane; chlorosilanes such as methyltrichlorosilane, dimethyldichlorosilane, trimethylchlorosilane, and phenyltrichlorosilane; and hexamethyldisilazane (HMDS). The silane coupling agent may also have a vinyl group as a functional group. Examples of silane coupling agents having a vinyl group as a functional group include alkoxysilanes such as methacryloxypropyltriethoxysilane, methacryloxypropyltrimethoxysilane, methacryloxypropylmethyldiethoxysilane, methacryloxypropylmethyldimethoxysilane, vinyltriethoxysilane, vinyltrimethoxysilane, and vinylmethyldimethoxysilane; chlorosilanes such as vinyltrichlorosilane and vinylmethyldichlorosilane; and divinyltetramethyldisilazane.

[0066] In the silane coupling agent treatment, only one type of silane coupling agent may be used, or two or more types may be used.

[0067] In addition to the above, examples of the surface treatment agent include the compounds described in paragraphs 0061 to 0063 of JP-A No. 2017-41624.

[0068] The method of surface treatment is not particularly limited, and known methods can be applied. Examples of the surface treatment method include a method of mixing the powder with a surface treatment agent or the like using a mixer such as a Henschel mixer, a method of spraying the surface treatment agent or the like onto particles constituting the powder, and a method of mixing the powder with a liquid containing the surface treatment agent or the like, in which the surface treatment agent or the like is dissolved or dispersed in a suitable solvent, and then removing the solvent.

[0069] (Magnetic powder volume filling rate) The radio wave absorber and the radio wave absorbing composition contain the above-described hexagonal ferrite powder as the magnetic powder. In the radio wave absorber and the radio wave absorbing composition, the filling rate of the hexagonal ferrite powder is not particularly limited. For example, in one embodiment, the filling rate can be 35% by volume or less, or can be in the range of 15 to 35% by volume. In another embodiment, the filling rate can be 35% by volume or more. In this case, the filling rate can be, for example, in the range of 35 to 60% by volume, or can be in the range of 35 to 50% by volume. In the case of a radio wave absorber, the volume filling rate means the volume-based content relative to the total volume (100% by volume) of the radio wave absorber. For the radio wave absorbing composition, the volume filling rate means the volume-based content of solids (i.e., components excluding solvents) relative to the total volume (100% by volume). Regarding the filling rate of the magnetic powder, according to the inventors' investigations, increasing the filling rate of the magnetic powder in the radio wave absorber tends to increase the transmission attenuation. On the other hand, according to the inventors' investigations, in a conventional radio wave absorber containing a magnetic powder and a binder, increasing the filling rate of the magnetic powder in the radio wave absorber tends to decrease the return attenuation. In contrast, by using the hexagonal ferrite powder described above as the magnetic powder, it is possible to increase both the transmission attenuation and the return attenuation of a radio wave absorber containing a magnetic powder and a binder.

[0070] The volume filling rate of the magnetic powder in the radio wave absorber can be determined, for example, by collecting magnetic powder from the radio wave absorber by a known method and calculating "(volume of collected magnetic powder / total volume of radio wave absorber) × 100." Here, the total volume of the radio wave absorber and the volume of the magnetic powder can be determined by a known method. Alternatively, when the composition of the radio wave absorbing composition used to produce the radio wave absorber is known, the volume filling rate of the magnetic powder in the radio wave absorber can also be determined from this known composition. The volume filling rate 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 with a square flat surface 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 using FIB (Focused Ion Beam) processing. The prepared cross-sectional observation sample is observed using an SEM, and a cross-sectional image (SEM image) is taken. A field emission scanning electron microscope (FE (Field Emission)-SEM) is used as the SEM. Using the FE-SEM, the cross-sectional observation sample is set on the stage with the FIB-processed cross section facing upward, and a cross-sectional SEM image with a field of view of 30 μm x 40 μm is obtained under conditions of an accelerating voltage of 15 kV and an observation magnification of 3,000x. The obtained cross-sectional SEM image is then binarized to calculate the proportion (area basis) occupied by the magnetic powder. The above operation is performed on five measurement samples cut out from different positions of the radio wave absorber to be measured, and the volume filling rate of the magnetic powder can be obtained as the arithmetic mean of the five obtained values. Note that, if necessary, elemental analysis of the cross-sectional observation sample can be performed to identify the magnetic powder portion in the cross-sectional SEM image. The volume filling rates of the other components described in this specification can be determined in the same manner as above.

[0071] <Binder> The radio wave absorber and the radio wave absorbing composition contain magnetic powder and a binder. The binder can be, for example, a resin, and examples of the resin include thermoplastic resins and thermosetting resins. Examples of thermoplastic resins include acrylic resin, polyacetal, polyamide, polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, 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.

[0072] The binder can also be rubber. Examples of rubber that can be used, from the viewpoint of being easily mixed with the magnetic powder and capable of producing a radio wave absorber that 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.

[0073] When the radio wave absorber contains rubber as a binder, it may contain, in addition to rubber, various additives such as a vulcanizing agent, a vulcanization aid, a softener, a plasticizer, etc. Examples of the vulcanizing agent include sulfur, an organic sulfur compound, and a metal oxide.

[0074] Examples of binders include thermoplastic elastomers (TPEs), such as olefin thermoplastic elastomers (TPOs), styrene thermoplastic elastomers (TPSs), amide thermoplastic elastomers (TPAs), and polyester thermoplastic elastomers (TPCs).

[0075] The radio wave absorber and the radio wave absorbing composition may contain only one type of binder, or may contain two or more types. The volume filling rate of the binder in the radio wave absorber and the radio wave absorbing composition is not particularly limited, and is, for example, preferably 65% ​​by volume or more, more preferably 65% ​​by volume to 92% by volume or less, and even more preferably 65% ​​by volume to 85% by volume or less. When the radio wave absorber and the radio wave absorbing composition contain 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.

[0076] <Additives> The radio wave absorber and radio wave absorbing composition contain magnetic powder and a binder, and 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, 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, and antibacterial agents. Other additives may be those that perform two or more functions.

[0077] (antioxidant) In one aspect, preferred additives include antioxidants. 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, the disclosure of which is incorporated herein by reference. Examples of the antioxidant include phenol-based antioxidants, amine-based antioxidants, phosphorus-based antioxidants, and sulfur-based antioxidants. As the antioxidant, it is preferable to use a phenol-based antioxidant and / or an amine-based antioxidant in combination with a phosphorus-based antioxidant and / or a sulfur-based antioxidant.

[0078] Examples of phenolic antioxidants 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 Ltd. Note that both "ADK STAB" and "IRGANOX" are registered trademarks.

[0079] Examples of amine antioxidants include Sanol LS-770, Sanol LS-765, and Sanol LS-2626 from Sankyo Lifetech Co., Ltd., Adeka Corporation's ADK STAB LA-77, ADK STAB LA-57, ADK STAB LA-52, ADK STAB LA-62, ADK STAB LA-63, ADK STAB LA-67, ADK STAB LA-68, and ADK STAB LA-72, and BASF Japan Ltd.'s TINUVIN 123, TINUVIN 144, TINUVIN 622, TINUVIN 765, and TINUVIN 944. Note that both "ADK STAB" and "TINUVIN" are registered trademarks. Furthermore, amine compounds capable of quenching radicals can also be used as antioxidants. Examples of such amine compounds include polyethylene glycol bis-TEMPO (Sigma-Aldrich) and sebacate bis-TEMPO. "TEMPO" is an abbreviation for tetramethylpiperidine-1-oxyl.

[0080] 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. Note that both "ADK STAB" and "IRGAFOS" are registered trademarks.

[0081] Examples of sulfur-based antioxidants include ADK STAB AO-412S and ADK STAB AO-503S manufactured by ADEKA Corporation. Note that "ADK STAB" is a registered trademark.

[0082] Among the above, the phenol-based antioxidant is preferably at least one 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 and the radio wave absorbing composition contain an antioxidant, they may contain only one type of antioxidant or two or more types of antioxidant.

[0083] When the radio wave absorber and the 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, and, for example, from the viewpoint of simultaneously suppressing decomposition of the binder and suppressing bleeding of the antioxidant, the content is preferably 0.1 to 10 parts by mass, and more preferably 0.5 to 5 parts by mass, relative to 100 parts by mass of the binder.

[0084] (light stabilizer) In one aspect, preferred additives include light stabilizers. Examples of light stabilizers include HALS (i.e., hindered amine light stabilizers), ultraviolet absorbers, and singlet oxygen quenchers. 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.

[0085] When the wave absorber and the wave absorbing composition contain a light stabilizer, they may contain only one type of light stabilizer or may contain two or more types of light stabilizers.

[0086] -High molecular weight HALS- In the present invention and this specification, the term "high molecular weight HALS" refers to a hindered amine light stabilizer having a weight average molecular weight of more than 1,000. Examples of high molecular weight HALS include oligomeric 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 products include CHIMASSORB 944LD and TINUVIN 622LD manufactured by BASF Japan Ltd. Both "CHIMASSORB" and "TINUVIN" are registered trademarks.

[0087] The weight-average molecular weight (Mw) in the present invention and this specification is a value measured by gel permeation chromatography (GPC). Measurements using gel permeation chromatography (GPC) are performed using an HLC (registered trademark)-8220GPC (manufactured by Tosoh Corporation) as a measuring device, 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) columns connected in series, and THF (tetrahydrofuran) as an eluent. The measurement conditions can be set as follows: sample concentration 0.2% by mass, flow rate 0.35 mL / min, sample injection amount 10 μL, and measurement temperature 40°C. A refractive index (RI) detector can be used as the detector. The calibration curve can be prepared using the "Standard Samples TSK Standard, Polystyrene" manufactured by Tosoh Corporation: "F-40", "F-20", "F-4", "F-1", "A-5000", "A-2500", and "A-1000".

[0088] When the wave absorber contains a high molecular weight HALS, the content of the high 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 wave absorber. It is preferable from the viewpoint of improving weather resistance that the content of high 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 high molecular weight HALS in the wave absorber is 10 mass % or less relative to the total mass of the wave absorber, the decrease in mechanical strength and the occurrence of blooming tend to be suppressed.

[0089] -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 1000 or less (preferably 900 or less, more preferably 600 to 900). 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)bromine. 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-butylmalonate bis(1,2,2,6,6-pentamethyl-4-piperidyl)malonate, and the like. Examples of commercially available low-molecular-weight HALS products include ADK STAB LA-57 and ADK STAB LA-52 from ADEKA Corporation, and TINUVIN 144 from BASF Japan Ltd. Note that both "ADK STAB" and "TINUVIN" are registered trademarks.

[0090] 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 wave absorber. It is preferable from the viewpoint of improving weather resistance that the content of the low molecular weight HALS in the radio wave absorber be 0.2 mass % or more relative to the total mass of the radio 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.

[0091] -Ultraviolet absorber- 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''-tetrahydrophthalimidomethyl)-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-(2 Benzotriazole-based ultraviolet absorbers such as H-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'-tetrahydro Examples of such ultraviolet absorbers include benzophenone-based ultraviolet absorbers such as 4-dodecyloxybenzophenone, 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 such as 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 Chemical Co., Ltd. Note that the above-mentioned "TINUVIN," "IRGANOX," "ADK STAB," and "SEESORB" are all registered trademarks.

[0092] When the wave absorber contains an ultraviolet absorber, the content of the ultraviolet absorber 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 wave absorber. It is preferable from the viewpoint of improving weather resistance that the content of the ultraviolet absorbing agent in the radio wave absorber be 0.2 mass % or more relative to the total mass of the radio wave absorber. When the content of the ultraviolet absorber 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.

[0093] -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. It is preferable from the viewpoint of improving weather resistance that the content of the singlet oxygen quencher in the radio wave absorber be 0.2 mass % or more relative to the total mass of the radio wave absorber. When the content of the singlet oxygen quencher in the radio wave absorber is 10% by mass or less with respect to the total mass of the radio wave absorber, it tends to be possible to suppress a decrease in mechanical strength and the occurrence of blooming.

[0094] 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.

[0095] <Method for producing radio wave absorbing composition and radio wave absorber> The methods for producing the radio wave absorbing composition and the radio wave absorber are not particularly limited. The radio wave absorbing composition can be produced by a known method using the magnetic powder, a binder, and, if necessary, a solvent, additives, etc. For example, the radio wave absorber can be a molded product obtained by molding the radio wave absorbing composition. The radio wave absorbing composition can be prepared, for example, by kneading a mixture of the magnetic powder and a binder, and, if necessary, a solvent, additives, etc., while heating. The kneaded product can be obtained in any shape, such as a block or pellet. The kneaded product can be molded into a desired shape by a known molding method, such as extrusion molding, press molding, injection molding, or in-mold molding, to obtain a radio wave absorber (molded product). The shape of the radio wave absorber is not particularly limited and can be any shape, such as a plate or a line. The term "plate" encompasses sheet and film shapes. A plate-shaped radio wave absorber can also be called a radio wave absorbing plate, radio wave absorbing sheet, radio wave absorbing film, etc. The 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 with different compositions (for example, a laminate). The radio wave absorber may have a planar shape, a three-dimensional shape, or a combination of a part having a planar shape and a part having a three-dimensional shape. Examples of the planar shape include a sheet shape, a film shape, etc. Examples of the three-dimensional shape include a tube shape (cylindrical shape, square tube shape, etc.), a horn shape, a box shape (for example, at least one of the faces is open), etc.

[0096] 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 below can be adjusted. Note that when the radio wave absorber is a laminate, the thickness refers to the total thickness of the radio wave absorbers that make up the laminate. The thickness of the radio wave absorber is a value measured using a digital length measuring device, and specifically, it is the arithmetic average of measurements taken at nine randomly selected points.

[0097] 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, and 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, 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 depending on the method for producing the radio wave absorber.

[0098] The radio wave absorbing composition can be prepared by mixing the components. The mixing method is not particularly limited, and examples thereof include a method of mixing by stirring. As a 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 can be set depending on the type of stirring device, the composition of the radio wave absorbing composition, etc.

[0099] One embodiment of the method for producing the radio wave absorber is a method in which the radio wave absorbing composition is molded into a desired shape by a known molding method such as those exemplified above. Another embodiment of the method for producing the radio wave absorber includes a method in which the radio wave absorbing composition is applied to a support and used as a radio wave absorbing layer to produce the radio wave absorber. The support used here may be removed before the radio wave absorber is incorporated into an article to which radio wave absorption properties should be imparted, or may be incorporated into the article together with the radio wave absorber without being removed.

[0100] The support is not particularly limited, and known supports can be used. Examples of the support include metal plates (plates made of metals such as aluminum, zinc, and copper), glass plates, plastic sheets (sheets of polyester (polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, etc.), polyethylene (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, acrylic resin, etc.), and plastic sheets laminated or vapor-deposited with the metals exemplified above for the metal plate. The plastic sheet is preferably biaxially stretched. The shape, structure, size, etc. of the support can be selected as appropriate. 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 selected as appropriate depending on the size, etc. of the radio wave absorber. The thickness of the support is usually about 0.01 mm to 10 mm, and from the viewpoint of ease of handling, for example, it is preferably 0.02 mm to 3 mm, and more preferably 0.05 mm to 1 mm.

[0101] The method for applying the radio wave absorbing composition onto a support is not particularly limited, and examples thereof include methods using a die coater, knife coater, applicator, etc. The method for drying the coating film formed by applying the 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. For 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.

[0102] 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 bending any desired portion. It can also be adjusted to a desired shape by injection molding or the like and then incorporated into an article.

[0103] A radio wave absorber having excellent radio wave absorbing performance is useful for improving the recognition accuracy of radar. One example of an index of radio wave absorbing performance is transmission attenuation. To improve the recognition accuracy of radar, it is desirable to increase the radar's directivity. A high transmission attenuation can contribute to improving the radar's directivity. From the viewpoint of improving the radar's directivity, the transmission attenuation of the radio wave absorber is preferably 8.0 dB or more, more preferably 8.5 dB or more, even more preferably 9.0 dB or more, and even more preferably 10.0 dB or more. The transmission attenuation of the radio wave absorber may 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, 12.5 dB or less, or 12.0 dB or less. However, from the viewpoint of improving the radar's directivity, a high transmission attenuation of the radio wave absorber is preferable. Therefore, the transmission attenuation of the radio wave absorber may exceed the values ​​exemplified above. Furthermore, to improve the radar's recognition accuracy, it is desirable to remove or reduce unnecessary radio wave components using a radio wave absorber, thereby increasing the selectivity with which the radar selectively receives radio waves from targets. A high return loss can contribute to the removal or reduction of unnecessary radio wave components. From this perspective, the return loss of the radio wave absorber is preferably 8.0 dB or more, more preferably 8.5 dB or more, even more preferably 9.0 dB or more, and even more preferably 10.0 dB or more. The return loss of the radio wave absorber may be, for example, 18.0 dB or less, 17.5 dB or less, 17.0 dB or less, 16.5 dB or less, 16.0 dB or less, 15.5 dB or less, or 15.0 dB or less. However, from the perspective of removing or reducing unnecessary radio wave components, a high return loss of the radio wave absorber is preferable. Therefore, the return loss of the radio wave absorber may exceed the values ​​exemplified above.

[0104] 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 a frequency of 30 GHz to 300 GHz. The radio wave absorber preferably exhibits transmission attenuation and return loss within the above ranges for one or more frequencies in the radio wave frequency band, i.e., a frequency band of 3 terahertz (THz) or less. From the viewpoint of usefulness for improving the recognition accuracy of the automotive radar, the frequencies at which the radio wave absorber exhibits transmission attenuation and return loss within the above ranges are preferably one or more frequencies in the millimeter-wave frequency band, i.e., a frequency band of 30 GHz to 300 GHz, more preferably one or more frequencies in the frequency band of 60 GHz to 90 GHz, and even more preferably one or more frequencies in the frequency band of 75 GHz to 85 GHz. As an example, the radio wave absorber may be one whose transmission attenuation and return loss at a frequency of 76.5 GHz are within the above ranges. Such a radio wave absorber 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 on-board millimeter wave radar in order to reduce the side lobes of the on-board millimeter wave radar.

[0105] Furthermore, from the viewpoint of usefulness for improving the recognition accuracy of radio wave absorbing articles used in wireless technology fields such as motion sensors, the frequencies at which the radio wave absorber exhibits transmission attenuation and return attenuation within the above ranges are preferably one or more frequencies in the millimeter wave frequency band, i.e., the 30 GHz to 300 GHz frequency band, more preferably one or more frequencies in the 50 GHz to 90 GHz frequency band, and even more preferably one or more frequencies in the 55 GHz to 66 GHz frequency band. For example, the radio wave absorber may have transmission attenuation and return attenuation within the above ranges at a frequency of 60.0 GHz. Such a radio wave absorber is suitable for improving the recognition accuracy of wireless devices such as internal sensors for mobile phones and biometric information sensors by removing unwanted radio waves. Such a radio wave absorber can be suitably used, for example, in radio wave absorbing articles for the 55 to 66 GHz band. A radio wave absorbing article is an article that has radio wave absorption properties for radio waves of one or more frequencies, and the above radio wave absorption properties can be achieved by incorporating a radio wave absorber as at least a part of the article. A 55-66 GHz band radio wave absorbing article is an article that has radio wave absorbing properties for radio waves of one or more frequencies in the 55-66 GHz frequency band. Examples of such articles include the various wireless devices mentioned above. By incorporating the radio wave absorber into such a radio wave absorbing article, unwanted radio waves can be removed, thereby improving recognition accuracy.

[0106] In the present invention and this specification, "transmission attenuation" refers to the value obtained as the S parameter S21 by measuring S parameters using the free space method at an incident angle of 0° in a measurement environment with an ambient temperature of 15 to 35°C. "Return attenuation" refers to the value obtained as the S parameter S11 by a similar measurement. Measurements can be performed using a known vector network analyzer and horn antenna. Specific examples of measurement methods include those described in the Examples below. [Example]

[0107] 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 carried out in an air atmosphere at an ambient temperature of 23°C ± 1°C unless otherwise specified.

[0108] [Preparation and evaluation of magnetic powders 1-12] <Magnetic powder production> Strontium carbonate (SrCO3), α-iron (III) oxide (α-Fe2O3), and aluminum oxide (Al2O3) were mixed in a ratio that would yield a hexagonal ferrite having a composition in which the value of x in formula 1 or the value of y in formula 2 is the value shown in Table 1, and the mixture was stirred using an Eirich intensive mixer (Eirich, model EL1) under the conditions shown in Table 1 to obtain a raw material mixture. For magnetic powders 1 to 7, the resulting raw material mixture was then crushed for 60 seconds using a cutter mill crusher, a Wonder Crusher WC-3 manufactured by Osaka Chemical Co., Ltd., with the variable speed dial of the crusher set to "3," to obtain a crushed product. The crushed product was placed in a muffle furnace, and the temperature inside the furnace was set to 1100°C in an air atmosphere, and fired for 4 hours to obtain magnetic powders 1 to 7. For magnetic powders 8 to 12, 5 mass% of strontium chloride hexahydrate [SrCl2·6H2O] was further added to the raw material mixture, and the furnace temperature was changed to 1200°C. Magnetic powders 8 to 12 were prepared in the same manner, except that the magnetic powders were obtained.

[0109] <Confirmation of crystal structure> The crystal structure of the magnetic material constituting each of the magnetic powders 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 ray [Wavelength: 1.54 Å (0.154 nm), Output: 40 mA, 45 kV] Scan range: 20 degrees < 2θ < 70 degrees Scan interval: 0.05 degrees Scan speed: 0.75 degrees / min

[0110] As a result of the above X-ray diffraction analysis, it was confirmed that magnetic powders 1 to 12 have a magnetoplumbite-type crystal structure and are single-phase magnetoplumbite-type hexagonal ferrite powders that do not contain any crystal structure other than magnetoplumbite-type.

[0111] <Confirmation of composition> The composition of the magnetic material constituting each of the magnetic powders was confirmed by high-frequency inductively coupled plasma atomic emission spectrometry. Specifically, it was confirmed by the following method. A container (beaker) containing 12 mg of magnetic powder and 10 mL of 4 mol / L hydrochloric acid solution was placed on a hot plate set at 120°C for 3 hours to obtain a solution. 30 mL of pure water was added to the resulting solution, which was then filtered through a membrane filter with a pore size of 0.1 μm. Elemental analysis of the filtrate was performed using a high-frequency inductively coupled plasma optical emission spectrometer (Shimadzu Corporation, ICPS-8100). Based on the results of the elemental analysis, the content of each atom relative to 100 atomic percent iron was calculated. The composition of the magnetic material was then confirmed based on the resulting content. As a result, it was confirmed that the compositions of magnetic powders 1 to 7 were as shown in Table 1, where A in Formula 1 is Sr and x is the value shown in Table 1.

[0112] <Measurement of saturation magnetization σs> Using a vibrating sample magnetometer (model: TM-TRVSM5050-SMSL) manufactured by Tamagawa Manufacturing Co., Ltd. as the measuring device, the magnetization strength of each of the magnetic powders described above was measured in response to the applied magnetic field under conditions of an ambient temperature of 23°C, a maximum applied magnetic field of 50 kOe, and a magnetic field sweep rate of 25 Oe / s. 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 calculated.

[0113] <Measurement of the half-width β of the diffraction peak of the (107) plane> The measurement equipment used was a powder X-ray diffractometer, X'Pert Pro from PANalytical, and the X-ray diffraction spectrum was obtained for each of the magnetic powders under the following measurement conditions. In the X-ray diffraction spectrum obtained for each magnetic powder, a diffraction peak of the (107) plane was confirmed, with its apex at approximately 32.5 degrees. For each magnetic powder, the half-width β of the diffraction peak of the (107) plane was determined using analysis software (PANalytical's HighScore Plus) installed in the 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: 25degree<2θ<35degree Scan interval: 0.05 degrees Scan speed: 0.33 degrees / min

[0114] <σs / β> For each of the magnetic powders, the ratio (σs / β) was calculated from σs and β determined by the above method.

[0115] [Examples 1 to 11, Comparative Examples 1 and 2] <Production of radio wave absorber> The magnetic powder shown in Table 1 was introduced into a kneader (Labo Plastomill manufactured by Toyo Seiki Seisaku-Sho, Ltd.) 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 would be the value shown in Table 1, and the kneading was carried out for 20 minutes with the temperature of the kneader set to 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 to obtain a radio wave absorber (radio wave absorbing sheet) in the form of a molded plate having a square flat surface with a side length of 100 mm. For each of the radio wave absorbers in the Examples and Comparative Examples, the thickness was determined as the arithmetic mean 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 of the above radio wave absorbers was 2 mm.

[0116] <Transmission loss and return loss> The transmission attenuation (unit: dB) and reflection attenuation (unit: dB) of each of the above-mentioned wave absorbers were measured by the following method.

[0117] (Examples 1 to 6, Comparative Examples 1 and 2) The measurement equipment used was a Keysight vector network analyzer (product name: N5225B) and a Keycom horn antenna (product name: RH12S23). Using the free space method, the incident angle was set to 0°, the sweep frequency was set to 60 GHz to 90 GHz, and one flat surface of each of the above-mentioned wave absorbers was oriented toward the incident side to measure the S parameters. The S parameter S21 at a frequency of 76.5 GHz was taken as the transmission loss, and the S parameter S11 at a frequency of 76.5 GHz was taken as the return loss.

[0118] Examples 7 to 11 The S parameters were measured in the same manner as above except that the sweep frequency was changed to 55 GHz to 90 GHz. The S parameter S21 at a frequency of 60.0 GHz was taken as the transmission loss, and the S parameter S11 at a frequency of 60.0 GHz was taken as the return loss.

[0119] The above results are shown in Table 1.

[0120] [Table 1]

[0121] The results shown in Table 1 confirm that the radio wave absorbers of Examples 1 to 11 have high transmission attenuation and reflection attenuation, and can contribute to improving the recognition accuracy of various radio wave absorbing articles such as radars and motion sensors. [Industrial Applicability]

[0122] 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 in the wireless technical field of motion sensors and the like.

Claims

1. A radio wave absorber comprising a magnetic powder and a binder, the binder is a resin, The volume filling rate of the magnetic powder is 15% by volume or more and 35% by volume or less, and The magnetic powder has a ratio of saturation magnetization σs to the half-width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis, σs / β, of 240 emu·g －１ ・degree －１ The radio wave absorber is a powder of the above hexagonal ferrite.

2. 2. The radio wave absorber according to claim 1, wherein the hexagonal ferrite is a substitutional hexagonal ferrite.

3. The radio wave absorber according to claim 2, wherein the substitutional hexagonal ferrite has a composition represented by the following formula 1: Formula 1: A １ Fe （１２－ｘ） Al ｘ O １９ In formula 1, A １ represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 1.50≦x≦8.

00.

4. 4. The radio wave absorber according to claim 2, wherein the substituted hexagonal ferrite is a substituted hexagonal strontium ferrite.

5. The ratio, σs / β, is 240 emu·g －１ ・degree －１ More than 310 emu・g －１ ・degree －１ 5. The radio wave absorber according to claim 1, wherein:

6. The ratio, σs / β, is 245 emu·g －１ ・degree －１ More than 310 emu・g －１ ・degree －１ 6. The radio wave absorber according to claim 1, wherein:

7. 2. The radio wave absorber according to claim 1, wherein the hexagonal ferrite is a substitutional hexagonal ferrite having a composition represented by the following formula 2: Formula 2: A ２ Fe （１２－ｙ） Al ｙ O １９ In formula 2, A ２ represents one or more atoms selected from the group consisting of Sr, Ba, Ca and Pb, and y satisfies 0.5≦y<1.

5.

8. 8. The radio wave absorber according to claim 7, wherein in formula 2, y satisfies 0.8≦y≦1.

2.

9. 9. The radio wave absorber according to claim 7, wherein the substituted hexagonal ferrite is a substituted hexagonal strontium ferrite.

10. The ratio, σs / β, is 300 emu·g －１ ・degree －１ More than 400 emu・g －１ ・degree －１ The radio wave absorber according to any one of claims 7 to 9, wherein:

11. The radio wave absorber according to any one of claims 7 to 10, which is used in a radio wave absorbing article for a 50 to 90 GHz band.

12. 12. The radio wave absorber according to claim 1, wherein said β is equal to or less than 0.190 degrees.

13. The radio wave absorber according to any one of claims 1 to 12, wherein the resin is a thermoplastic resin or a thermosetting resin.

14. A radio wave absorbing composition comprising a magnetic powder and a binder, the binder is a resin, The volume filling rate of the magnetic powder is 15% by volume or more and 35% by volume or less, and The magnetic powder has a ratio of saturation magnetization σs to the half-width β of the diffraction peak of the (107) plane determined by X-ray diffraction analysis, σs / β, of 240 emu·g －１ ・degree －１ The radio wave absorbing composition is a powder of the above hexagonal ferrite.

15. 15. The radio wave absorbing composition according to claim 14, wherein said hexagonal ferrite is a substitutional hexagonal ferrite.

16. The radio wave absorbing composition according to claim 15, wherein the substitutional hexagonal ferrite has a composition represented by the following formula 1: Formula 1: A １ Fe （１２－ｘ） Al ｘ O １９ In formula 1, A １ represents one or more atoms selected from the group consisting of Sr, Ba, Ca, and Pb, and x satisfies 1.50≦x≦8.

00.

17. 17. The radio wave absorbing composition according to claim 15, wherein said substituted hexagonal ferrite is a substituted hexagonal strontium ferrite.

18. The ratio, σs / β, is 240 emu·g －１ ・degree －１ More than 310 emu・g －１ ・degree －１ The radio wave absorbing composition according to any one of claims 14 to 17, wherein:

19. The ratio, σs / β, is 245 emu·g －１ ・degree －１ More than 310 emu・g －１ ・degree －１ The radio wave absorbing composition according to any one of claims 14 to 18, wherein:

20. 15. The radio wave absorbing composition according to claim 14, wherein the hexagonal ferrite is a substituted hexagonal ferrite having a composition represented by the following formula 2: Formula 2: A ２ Fe （１２－ｙ） Al ｙ O １９ In formula 2, A ２ represents one or more atoms selected from the group consisting of Sr, Ba, Ca and Pb, and y satisfies 0.5≦y<1.

5.

21. 21. The radio wave absorbing composition according to claim 20, wherein in formula 2, y satisfies 0.8≦y≦1.

2.

22. 22. The radio wave absorbing composition according to claim 20, wherein the substituted hexagonal ferrite is a substituted hexagonal strontium ferrite.

23. The ratio, σs / β, is 300 emu·g －１ ・degree －１ More than 400 emu・g －１ ・degree －１ The radio wave absorbing composition according to any one of claims 20 to 22, wherein:

24. 24. The radio wave absorbing composition according to claim 14, wherein said β is 0.190 degrees or less.

25. The radio wave absorbing composition according to any one of claims 14 to 24, wherein the resin is a thermoplastic resin or a thermosetting resin.

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