JP2021138583A - Magnetoplumbite-type hexagonal ferrite magnetic powder and method for producing the same - Google Patents

Abstract

To provide: magnetoplumbite-type hexagonal ferrite magnetic powder which is suitable for use as a material for radio wave absorber having excellent performance of absorbing radio wave in a 60-85 GHz zone; and a method for producing the same.SOLUTION: A method for producing magnetoplumbite-type hexagonal ferrite magnetic powder comprises: mixing SrCO3 powder, Fe2O3 powder, and Al2O3 powder as raw material powder of magnetoplumbite-type hexagonal ferrite magnetic powder represented by a composition formula SrFe(12-x)AlxO19 (where x=1.0 to 2.2); firing the mixture in a furnace; and pulverizing the fired product. At the time of firing, a mass ratio of CO2 in the raw material powder per volume inside the furnace is configured to be 1 g/L or more.SELECTED DRAWING: None

Description

The present invention relates to a magnetoplumbite-type hexagonal ferrite magnetic powder and a method for producing the same, and more particularly to a magnetoplumbite-type hexagonal ferrite magnetic powder suitable for use as a material such as a radio wave absorber and a method for producing the same.

In recent years, with the advancement of information and communication technology, GHz band radio waves are used in various applications such as mobile phones, wireless LAN, satellite broadcasting, intelligent transportation systems, automatic toll collection systems (ETC), and driving support road systems (AHS). Is used. When the usage patterns of radio waves are diversified in such a high frequency band, there are concerns about failures, malfunctions, malfunctions, etc. due to interference between electronic components, and as one of the countermeasures, an unnecessary radio wave is absorbed by using a radio wave absorber. However, it prevents the reflection and intrusion of radio waves.

In particular, in recent years, research on autonomous driving support systems has become active, and the development of in-vehicle radar that detects information such as inter-vehicle distance using radio waves (millimeter waves) in the 60 to 85 GHz band (such as the 76 GHz band) has been promoted. Along with this, there is a demand for a material that exhibits excellent radio wave absorption ability at 60 to 85 GHz (such as around 76 GHz).

As a material exhibiting such radio wave absorption ability, composition formula AFe _(12-x) Al _x O ₁₉ (however, A is one or more of Sr, Ba, Ca and Pb, x = 1.0 to 2.2. ), A magnetic powder for a radio wave absorber having a peak particle size of 10 μm or more in the laser diffraction scattering particle size distribution has been proposed as a powder of a magnetoplumbite type hexagonal ferrite represented by (see, for example, Patent Document 1). ).

JP-A-2007-250823 (paragraph number 0011)

However, if the usage patterns of radio waves (millimeter waves) in the 60 to 85 GHz band are diversified in the future, even a radio wave absorber using the magnetic powder for a radio wave absorber of Patent Document 1 as a material may not have sufficient radio wave absorption capacity. Further, a magnetoplumbite-type hexagonal ferrite magnetic powder suitable for use as a material for a radio wave absorber having excellent radio wave absorbing ability is desired.

Therefore, in view of such conventional problems, the present invention is a magnetic powder of a magnetoplumbite type hexagonal ferrite, which is suitable for use as a material of a radio wave absorber having excellent radio wave absorption capacity in the 60 to 85 GHz band, and a magnetic powder thereof. It is an object of the present invention to provide a manufacturing method.

As a result of diligent research to solve the above problems, the present inventors have made _{a magnetoplumbite type represented by the composition formula SrFe (12-x)} Al _x O ₁₉ (however, x = 1.0 to 2.2). _{SrCO 3} powder, Fe ₂ O ₃ powder and Al ₂ O ₃ powder are mixed as raw materials for hexagonal ferrite magnetic powder, fired in a furnace, and the obtained fired body is crushed to form a magnetoplumbite type. In the method for producing hexagonal ferrite magnetic powder, by setting _{the ratio of the mass of CO 2} in the _{SrCO 3} powder to the volume in the furnace to 1 g / L or more at the time of firing, the radio wave absorption capacity in the 60 to 85 GHz band is achieved. We have found that it is possible to produce a magnetoplumbite-type hexagonal ferrite magnetic powder suitable for use as a material for an excellent radio wave absorber, and have completed the present invention.

That is, the method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder according to the present invention is a magnetoplumbite represented by the composition formula SrFe _(12-x) Al _x O ₁₉ (where x = 1.0 to 2.2). _{SrCO 3} powder, Fe ₂ O ₃ powder and Al ₂ O ₃ powder are mixed as raw materials for the type hexagonal ferrite magnetic powder, fired in a furnace, and the obtained fired body is crushed to obtain a magnet plumbite. A method for producing a type hexagonal ferrite magnetic powder is characterized in that the ratio of the mass of _{CO 2} in the _{SrCO 3 powder to the volume in the furnace at the time of firing is 1 g / L or more.}

In this method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder, the firing temperature is preferably 1150 to 1400 ° C. Further, it is preferable that the fired body is pulverized by coarse pulverization and then wet pulverization.

Further, the magnetoplumbite-type hexagonal ferrite magnetic powder according to the present invention is represented by the composition formula SrFe _(12-x) Al _x O ₁₉ (where x = 1.0 to 2.2), and has a laser diffraction type particle size distribution. The cumulative 50% particle size (D ₅₀ ) based on the volume measured by the measuring device is 6 μm or less, the crystallite diameter Dx determined by X-ray diffraction measurement is 90 nm or more, and the Ba content is 0.40 mass% or less. It is characterized by being.

The BET specific surface area of this magnetoplumbite-type hexagonal ferrite magnetic powder ^{is preferably 2 m 2} / g or less. The product of the BET specific surface area and the volume-based cumulative 50% particle size (D ₅₀ ) is preferably 6 μm · m ² / g or less.

Further, the radio wave absorber according to the present invention is characterized by containing the above-mentioned magnetoplumbite-type hexagonal ferrite magnetic powder and a resin.

According to the present invention, it is possible to produce a magnetoplumbite-type hexagonal ferrite magnetic powder suitable for use as a material for a radio wave absorber having excellent radio wave absorption capacity in the 60 to 85 GHz band.

An embodiment of the method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder according to the present invention is a composition formula SrFe _(12-x) Al _x O ₁₉ (provided that x = 1.0 to 2.2 (preferably 1. _{SrCO 3} powder, Fe ₂ O ₃ powder and Al ₂ O ₃ powder are mixed as the raw material of the magnetoplumbite type hexagonal ferrite magnetic powder shown in 3 to 2.0)) (preferably granulated). After filling the molded product such as pellets obtained by molding into a firing container, the opening on the upper surface of the container is closed with a lid and fired in a firing furnace (preferably at 1150 to 1400 ° C.) to obtain the obtained product. In a method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder by crushing the fired body (preferably coarsely crushed by impact crushing with a hammer mill or the like and then wet crushing), during firing (from outside the furnace). _{The ratio of the mass (g) of CO 2} in the _{SrCO 3} powder to the volume (L) in the furnace (without introducing gas) is 1 g / L or more (preferably 1 to 1000 g / L).

The mass of CO ₂ in the SrCO ₃ powder is the product of the number of moles of _{SrCO 3 and the mass of CO 2} per mole (44.0 (g / mol)). Further, of the volume in the furnace, the volume effective for firing is the volume obtained by subtracting the volume of the firing container to be put into the furnace after filling the molded body, but the volume of the firing container is the volume in the furnace. Since it is very small compared to the above, the volume of the firing container can be ignored. When the firing furnace is a closed type firing furnace, the volume inside the furnace is the volume of the furnace chamber in which the firing container filled with the molded body is arranged.

In this way, if the ratio of the mass of CO _{2 in the SrCO 3} powder as the raw material powder per volume in the furnace is _{increased during firing, the concentration of CO 2} gas will increase _{when SrCO 3} is decomposed to some extent during firing. The higher the value, the slower the rate of decomposition of _{SrCO 3 into SrO.} The initial firing, (SrCO ₃ was formed by decomposition) is considered to nuclei from SrO hexagonal ferrite crystals is produced, due to the slow rate of producing the (SrCO ₃ is decomposed) SrO, It is possible to suppress the formation of a large amount of hexagonal ferrite crystal nuclei at the initial stage of calcination. Therefore, _{it is considered that SrO (produced by decomposition of SrCO 3} ) is used more for crystal growth instead of nucleation of hexagonal ferrite crystals, which facilitates crystal growth and increases crystallite diameter. Be done.

By the embodiment of the above-described method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder, an embodiment of a magnetoplumbite-type hexagonal ferrite magnetic powder according to the present invention can be produced.

Moreover, the embodiment of the magnetoplumbite type hexagonal ferrite magnetic powder according to the present invention is represented by the composition formula SrFe _(12-x) Al _x O ₁₉ (where x = 1.0 to 2.2), and is represented by a laser. A volume-based cumulative 50% particle size (D ₅₀ ) measured by a diffraction type particle size distribution measuring device is 6 μm or less (preferably 1 to 6 μm, more preferably 2 to 5.5 μm), and a crystal determined by X-ray diffraction measurement. The child diameter Dx is 90 nm or more (preferably 90 to 180 nm, more preferably 100 to 150 nm), and the Ba content (content of Ba as an impurity) is 0.40% by mass or less (preferably 0.01 to 0). .30% by mass, more preferably 0.05 to 0.20% by mass). As described above, the _{magnetoplumbite type hexagonal ferrite magnetic powder represented by the composition formula SrFe (12-x)} Al _x O ₁₉ (however, x = 1.0 to 2.2) is measured by a laser diffraction type particle size distribution measuring device. If the cumulative 50% particle size (D ₅₀ ) based on the volume is set to 6 μm or less and the crystallite diameter Dx determined by X-ray diffraction measurement is set to 90 nm or more, the radio wave absorption ability of millimeter waves in the 60 to 85 GHz band is excellent. It is possible to produce a magnetoplumbite-type hexagonal ferrite magnetic powder suitable for use as a material for a radio wave absorber. _{Further, if the cumulative 50% particle size (D 50} ) based on the volume measured by the laser diffraction type particle size distribution measuring device of the magnetic powder of the magnetoplumbite type hexagonal ferrite is set to 6 μm or less, radio wave absorption using the magnetic powder is performed. It is also possible to make the body sheet thinner. Further, in consideration of overseas environmental regulations, it is preferable that the Ba content in the magnetoplumbite-type hexagonal ferrite magnetic powder is low.

The BET specific surface area of this magnetoplumbite-type hexagonal ferrite magnetic powder ^{is preferably 2 m 2} / g or less, and more preferably 0.5 to ² m 2 / g or less. Moreover, the product of the cumulative 50% particle diameter of the volume-based and BET specific surface area _{(D 50),} but preferably not more than 6μm · ^m 2 / g, in the range of 1~6μm · ^m 2 / g more It is preferably 3.5 to 5.5 μm · m ² / g, most preferably 3.5 to 5.5 μm · m 2 / g. When this product (BET × D ₅₀ ) is 6 μm · m ² / g or less, the transmission attenuation of the radio wave absorber sheet using the magnetic powder is high (radio wave absorption) while maintaining the coercive force Hc of the magnetic powder high. The ability can be increased).

Further, a radio wave absorber can be manufactured by kneading the magnetoplumbite-type hexagonal ferrite magnetic powder of the above-described embodiment with a resin. This radio wave absorber can be made into various shapes depending on the application, but when a sheet-shaped radio wave absorber (radio wave absorber sheet) is produced, a magnetic powder of a magnetoplobite type hexagonal ferrite is used as a resin. The radio wave absorber material (kneaded product) obtained by kneading with the above material may be rolled to a desired thickness (preferably 0.1 to 4 mm, more preferably 0.2 to 2.5 mm) by a rolling roll or the like. Further, the content of the magnetoplumbite-type hexagonal ferrite magnetic powder in the radio wave absorber material (kneaded material) is 70 to 95% by mass in order to obtain a radio wave absorber having excellent radio wave absorbing ability in the 76 GHz band. Is preferable. The content of the resin in the radio wave absorber material (kneaded product) is 5 to 30% by mass in order to sufficiently disperse the magnetoplumbite-type hexagonal ferrite magnetic powder in the radio wave absorber material (kneaded product). Is preferable. Further, the total content of the magnetoplumbite-type hexagonal ferrite magnetic powder and the resin in the radio wave absorber material (kneaded product) is preferably 99% by mass or more.

From the results of X-ray diffraction measurement by the powder X-ray diffraction method (XRD), it was confirmed that the embodiment of the magnetoplumbite-type hexagonal ferrite magnetic powder according to the present invention has a magnetoplumbite-type hexagonal ferrite structure. And it is determined that it is the basic structure of _{SrFe 12} O _19. In the magnetoplumbite-type hexagonal ferrite structure, Al enters the Fe site, so the composition formula can be represented _{by rFe (12-x)} Al _x O _19. In this magnetoplumbite-type hexagonal ferrite magnetic powder, Sr × 12 / (Fe + Al) = 1.0 (0.95 to 1.04) is obtained from the result of composition analysis. Therefore, the composition formula SrFe _(12-x) It can be represented by Al _x O ₁₉ (where x = 1.0 to 2.2).

Hereinafter, examples of the magnetoplumbite-type hexagonal ferrite magnetic powder according to the present invention and a method for producing the same will be described in detail.

[Example 1]
First, 1325 g of SrCO ₃ (purity 99% by mass), 730 g of Al ₂ O ₃ (purity 99.9% by mass), and 6946 g of Fe ₂ O ₃ (purity 99% by mass) were weighed as raw material powders, and the raw material powder was weighed. Was mixed by a Henschel mixer and then further mixed by a dry method using a vibration mill. The molar ratio of Sr, Fe, and Al in this raw material powder is Sr: Fe: Al = 1.06: 10.29: 1.71, Sr × 12 / (Fe + Al) = 1.06, Al. / (Fe + Al) = 0.143. The mixed powder thus obtained was granulated and molded into pellets to obtain a molded body, and then (width 300 mm, depth 300 mm, height 60 mm, internal volume 300 mm × 300 mm × 60 mm = 5400 cm ³ calcined sheath ( A firing container) is prepared, 9 kg of the obtained molded body is filled in the firing sheath, the opening on the upper surface of the firing sheath is closed with a lid, and the atmosphere is at room temperature (width 500 mm, depth 530 mm, height 720 mm). After putting it in a box-type firing furnace (with an internal volume of 191 L) and closing the door of this combustion furnace, the temperature inside the furnace is raised from room temperature to 800 ° C. at a heating rate of 3.2 ° C./min, and from 800 ° C. to 1279. The temperature was raised to ° C. (firing temperature) at a rate of temperature rise of 2.1 ° C./min, and the calcined product was held at this firing temperature for 4 hours before firing. However, it was 38.4%. After the calcined product was coarsely pulverized with a hammer mill, the obtained crude powder was wet pulverized with an attritor (using water as a solvent) for 10 minutes, and the obtained slurry was obtained. Was separated into solid and liquid, and the obtained cake was dried and crushed to obtain a magnetic powder. In this example and Example 2 described below, CO _{2 in the compact packed in the calcined sheath was used. The mass (g) of CO 2 in} the compact is 2.1 g / L with respect to the internal volume (L) of the firing furnace.

The composition of the magnetic powder thus obtained was analyzed to determine the BET specific surface area and particle size distribution, and X-ray diffraction (XRD) measurement was performed to determine the crystallite diameter Dx.

In the composition analysis of the magnetic powder, the quantification of Sr, Ba and Al is performed using the high frequency inductively coupled plasma emission spectrometer ICP (720-ES) manufactured by Agilent Technologies Co., Ltd., and the quantification of Fe is performed by Hiranuma Sangyo Co., Ltd. Hiranuma automatic titrator (CONTIME-980 type) was used. As a result, the molar ratio of Sr, Ba, Fe and Al in the magnetic powder was Sr: Ba: Fe: Al = 1.01: 0.01: 10.21: 1.79, and Sr × 12 / ( Fe + Al) = 1.01. The Ba content in the magnetic powder was 0.13% by mass (1300 ppm).

The BET specific surface area of the magnetic powder was measured by the BET 1-point method using a specific surface area measuring device (Macsorb model-1210 manufactured by Mountech Co., Ltd.). As a result, the BET specific surface area of the magnetic powder was 1.11 m ² / g.

The particle size distribution of the magnetic powder is measured by dry dispersion at a dispersion pressure of 1.7 bar using a laser diffraction type particle size distribution measuring device (Heros particle size distribution measuring device (HELOS & RODOS) manufactured by Nippon Denshi Co., Ltd.), and average. When the cumulative 50% particle size (D ₅₀ ) based on the volume was determined as the particle size, it was 3.52 μm. The product of the BET specific surface area and the volume-based cumulative 50% particle size (D ₅₀ ) was 3.91 μm · m ² / g.

For X-ray diffraction measurement of magnetic powder, a powder X-ray diffractometer (horizontal multipurpose X-ray diffractometer Ultra IV manufactured by Rigaku Co., Ltd.) is used, the source is CuKα ray, the tube voltage is 40 kV, and the tube current is 40 mA. , The measurement range was set to 2θ = 10 ° to 75 °, and the measurement was performed by powder X-ray diffraction method (XRD). As a result of this X-ray diffraction measurement, it was confirmed that the obtained magnetic powder was a magnetoplumbite-type hexagonal ferrite. _{The magnetoplumbite-type hexagonal ferrite can be represented by the composition formula SrFe (12-x)} Al _x O ₁₉ (x = 1.79) from the result of the composition analysis.

The crystallite diameter Dx of the magnetic powder was determined by Scherrer's formula (Dx = Kλ / βcosθ). In this equation, Dx is the size of the crystallite diameter (Angstrom), λ is the wavelength of the measured X-ray (Angstrom), and β is the spread of the diffraction line according to the size of the crystallite (rad) (half-value width). ), θ is the Bragg angle (rad) of the diffraction angle, and K is the Scherrer constant (K = 0.94). The peak data of the (114) plane (diffraction angle 2θ = 34.0 to 34.8 °) was used for the calculation. As a result, the crystallite diameter Dx on the (114) plane of the magnetic powder was 110.7 nm.

As the magnetic characteristics of the magnetic powder, a vibrating sample magnetometer (VSM) (VSM-7P manufactured by Toei Kogyo Co., Ltd.) was used to measure the BH curve at an applied magnetic field of 1193 kA / m (15 kOe). , Coercive force Hc, saturation magnetization σs, and square ratio SQ were evaluated. As a result, the coercive force Hc was 3424 Oe (272 kA / m), the saturation magnetization σs was 33.9 emu / g, and the square ratio SQ was 0.638.

Further, a mixed powder (powder content 30.0% by mass) obtained by mixing 0.36 g of the obtained magnetic powder and 0.84 g of microcrystalline cellulose was pressure-molded at 28 MPa (20 kN) to have a diameter. A green compact having a thickness of 13 mm and a thickness of 5 mm was obtained. This green compact (sample) is placed on a sample holder (diameter φ10 mm) of the Terrahertz spectroscopic system (TAS7400SL manufactured by Advantest Co., Ltd.), and the measurement mode is transmitted, the frequency resolution is 1.9 GHz, and the vertical axis is the absorption amount. With the horizontal axis as frequency (THz) and cumulative number as 2048, the transmission attenuation amount is measured by terahertz wave time region spectroscopy, and the transmission attenuation amount of the blank is measured by the same method without placing the sample on the sample holder. The signal waveform of the measured sample and the reference waveform of the blank are extended to 2112 ps and Fourier transformed, and the Fourier spectrum ratio (Sig / Sff) (Sig is the amplitude of the Fourier spectrum of the sample, Sref is the Fourier spectrum of the blank. The amplitude) was calculated, and the permeation attenuation of the green compact (sample) was calculated. As a result, the peak frequency of the green compact was 77.2 GHz, and the transmission attenuation was 12.0 dB.

[Example 2]
A magnetic powder was produced by the same method as in Example 1 except that the firing time was set to 8 hours. The porosity of the fired body obtained by this firing was measured by the same method as in Example 1 and found to be 25.5%.

The composition of the magnetic powder thus obtained is analyzed by the same method as in Example 1, the BET specific surface area and particle size distribution are obtained, and X-ray diffraction (XRD) measurement is performed to determine the crystallite diameter Dx. I asked. As a result, the molar ratios of Sr, Ba, Fe and Al in the magnetic powder were Sr: Ba: Fe: Al = 1.04: 0.01: 10.23: 1.77, Sr × 12 / (Fe + Al). = 1.04, and the Ba content in the magnetic powder was 0.13% by mass (1300 ppm). The BET specific surface area of the magnetic powder is 1.08 m ² / g, the volume-based cumulative 50% particle size (D ₅₀ ) is 5.01 μm, and the BET specific surface area and volume-based cumulative 50% particle size (D ₅₀ ). The product was 5.41 μm · m ² / g, and the crystallite diameter Dx was 106.9 nm. Further, it was confirmed that the obtained magnetic powder was composed of a magnetoplumbite-type hexagonal ferrite by the same method as in Example 1, and from the result of the composition analysis, the composition formula SrFe _(12-x) Al _x O ₁₉ It can be represented by (x = 1.77). Further, when the magnetic properties of the magnetic powder were evaluated by the same method as in Example 1, the coercive force Hc was 2836 Oe (226 kA / m), the saturation magnetization σs was 33.6 emu / g, and the square ratio SQ was 0.630. there were.

Further, using this magnetic powder, a green compact was prepared by the same method as in Example 1, and the peak frequency and the transmission attenuation of the green compact were determined. The peak frequency was 77.7 GHz, and the green powder was transmitted. The amount of attenuation was 11.5 dB.

[Comparative Example 1]
The magnetic powder was prepared by the same method as in Example 1 except that the amount of the molded body filled in the calcined sheath was 2 kg, the opening on the upper surface of the calcined sheath was not closed with a lid, and the firing temperature was set to 1240 ° C. Made. The porosity of the fired body obtained by this firing was measured by the same method as in Example 1 and found to be 27.04%. In Comparative Example 2 is described in this comparative example and the following, the mass of CO ₂ in the filled firing sheath molded body is 88 g, the mass of CO ₂ in the shaped body with respect to the internal volume of the sintering furnace (L) (G) is 0.5 g / L.

The composition of the magnetic powder thus obtained is analyzed by the same method as in Example 1, the BET specific surface area and particle size distribution are obtained, and X-ray diffraction (XRD) measurement is performed to determine the crystallite diameter Dx. I asked. As a result, the molar ratios of Sr, Ba, Fe and Al in the magnetic powder were Sr: Ba: Fe: Al = 1.02: 0.01: 10.25: 1.75, Sr × 12 / (Fe + Al). = 1.02, and the Ba content in the magnetic powder was 0.13% by mass (1300 ppm). The BET specific surface area of the magnetic powder is 1.54 m ² / g, the volume-based cumulative 50% particle size (D ₅₀ ) is 5.17 μm, and the BET specific surface area and volume-based cumulative 50% particle size (D ₅₀ ). The product was 7.97 μm · m ² / g, and the crystallite diameter Dx was 82.7 nm. Further, it was confirmed that the obtained magnetic powder was composed of a magnetoplumbite-type hexagonal ferrite by the same method as in Example 1, and from the result of the composition analysis, the composition formula SrFe _(12-x) Al _x O ₁₉ It can be represented by (x = 1.75). Further, when the magnetic properties of the magnetic powder were evaluated by the same method as in Example 1, the coercive force Hc was 3812 Oe (303 kA / m), the saturation magnetization σs was 34.0 emu / g, and the square ratio SQ was 0.642. there were.

Further, using this magnetic powder, a green compact was prepared by the same method as in Example 1, and the peak frequency and the transmission attenuation of the green compact were determined. The peak frequency was 77.2 GHz, and the green powder was transmitted. The amount of attenuation was 9.2 dB.

[Comparative Example 2]
A magnetic powder was produced by the same method as in Comparative Example 1 except that the firing temperature was set to 1279 ° C. The porosity of the fired body obtained by this firing was measured by the same method as in Example 1 and found to be 1.12%.

The composition of the magnetic powder thus obtained is analyzed by the same method as in Example 1, the BET specific surface area and particle size distribution are obtained, and X-ray diffraction (XRD) measurement is performed to determine the crystallite diameter Dx. I asked. As a result, the molar ratios of Sr, Ba, Fe and Al in the magnetic powder were Sr: Ba: Fe: Al = 1.01: 0.01: 10.20: 1.80, Sr × 12 / (Fe + Al). = 1.01, and the Ba content in the magnetic powder was 0.14% by mass (1400 ppm). The BET specific surface area of the magnetic powder is 0.97 m ² / g, the volume-based cumulative 50% particle size (D ₅₀ ) is 12.42 μm, and the BET specific surface area and volume-based cumulative 50% particle size (D ₅₀ ). The product was 12.00 μm · m ² / g, and the crystallite diameter Dx was 90.4 nm. Further, it was confirmed that the obtained magnetic powder was composed of a magnetoplumbite-type hexagonal ferrite by the same method as in Example 1, and from the result of the composition analysis, the composition formula SrFe _(12-x) Al _x O ₁₉ It can be represented by (x = 1.80). Further, when the magnetic properties of the magnetic powder were evaluated by the same method as in Example 1, the coercive force Hc was 2131Oe (170 kA / m), the saturation magnetization σs was 33.8 emu / g, and the square ratio SQ was 0.631. there were.

Further, using this magnetic powder, a green compact was prepared by the same method as in Example 1, and the peak frequency and the transmission attenuation of the green compact were determined. The peak frequency was 80.7 GHz, and the green powder was transmitted. The amount of attenuation was 10.4 dB.

Tables 1 to 4 show the production conditions and characteristics of the magnetic powders obtained in these Examples and Comparative Examples and the characteristics of the green compact.

The magnetoplumbite-type hexagonal ferrite magnetic powder according to the present invention can be used for producing a radio wave absorber sheet having excellent radio wave absorption ability in the 76 GHz band.

Claims (7)

Composition formula SrFe _(12-x) Al _x O _{19 (However, x = 1.0 to 2.2) SrCO 3} powder and Fe ₂ O are the raw material powders of the magnetoplumbite type hexagonal ferrite magnetic powder. _{In a method} of mixing 3 powder and Al ₂ O ₃ powder, firing in a furnace, and crushing the obtained fired body to produce a magnetoplumbite type hexagonal ferrite magnetic powder, in the furnace at the time of firing. A method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder, wherein the ratio of the mass of CO ₂ in the SrCO _{3 powder per volume is 1 g / L or more.} The method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder according to claim 1, wherein the firing temperature is 1150 to 1400 ° C. The method for producing a magnetoplumbite-type hexagonal ferrite magnetic powder according to claim 1 or 2, wherein the fired body is pulverized by coarse pulverization and then wet pulverization. Composition formula SrFe _(12-x) Al _x O ₁₉ (where x = 1.0 to 2.2), and the cumulative 50% particle size (D) based on the volume measured by the laser diffraction type particle size distribution measuring device. ₅₀ ) is 6 μm or less, the crystallite diameter Dx determined by X-ray diffraction measurement is 90 nm or more, and the Ba content is 0.40 mass% or less. .. The magnetoplumbite-type hexagonal ferrite magnetic powder according to claim 4, wherein the magnetoplumbite-type hexagonal ferrite magnetic powder has a BET specific surface area of 2 m ² / g or less. The magnetoplumbite-type hexagon according to claim 4 or 5, wherein the product of the BET specific surface area and the volume-based cumulative 50% particle size (D ₅₀ ) is 6 μm · m ^{2 / g or less.} Crystalline ferrite magnetic powder. A radio wave absorber comprising the magnetoplumbite-type hexagonal ferrite magnetic powder according to any one of claims 4 to 6 and a resin.