Magnetoplumbite-type hexagonal ferrite powder, ferrite resin composite material, ferrite resin composite, and electromagnetic wave absorber

Abstract

A ferrite powder comprising, as a main component compound, a magnetoplumbite-type hexagonal ferrite having a composition represented by the formula Sr(FexAlyZrz)12O19+δ (where x, y, z, and δ satisfy 0.400≤x<1.000, 0.030≤y≤0.350, 0.001≤z≤0.040, 0.750≤x+y+z<1.000, and -5.00≤δ≤-0.200), and having a volume resistance of 1.0×109 Ω·cm or less. A ferrite resin composite material, ferrite resin composite, and electromagnetic wave absorber containing said ferrite powder.

Description

Hexagonal magnetoplumbite-type ferrite powder, ferrite-resin composite material, ferrite-resin composite, and electromagnetic wave absorber

[0001] The present invention relates to a hexagonal magnetoplumbite ferrite powder, a ferrite-resin composite material, a ferrite-resin composite, and an electromagnetic wave absorber.

[0002] Recent advances in information and communication technology have led to the development and use of a variety of communication devices. The frequency bands used are also shifting from microwaves to high-frequency bands, including millimeter waves. For example, the introduction of systems using electromagnetic waves (radio waves) in the millimeter wave band, such as high-speed wireless LAN (65 GHz) and collision prevention radar (76.5 GHz), is progressing. Millimeter waves are electromagnetic waves with a frequency band between 30 GHz and 300 GHz, and have the advantage of being suitable for high-speed, large-capacity communications.

[0003] On the other hand, communication devices also generate unwanted electromagnetic waves unintentionally. These unwanted electromagnetic waves may cause electromagnetic interference (EMI). Specifically, they may cause malfunctions or problems in other devices, or may have adverse effects on the human body. For this reason, the importance of electromagnetic compatibility (EMC) measures to improve the electromagnetic environment is strongly recognized. As a specific means of EMC measures, a method is used in which electromagnetic wave absorbers are used to absorb unwanted electromagnetic waves and suppress their effects.

[0004] Magnetic materials, dielectric materials, and / or conductive materials are used as electromagnetic wave absorbers. Magnetic materials undergo magnetic resonance with electromagnetic waves of a specific frequency. During this process, magnetic materials exhibit large magnetic losses, and much of the electromagnetic wave energy is absorbed by the material and converted into thermal energy. Therefore, electromagnetic wave absorption is possible by utilizing magnetic losses. Furthermore, dielectric materials experience dielectric losses due to a phase shift between the electromagnetic wave and the dielectric polarization, and electromagnetic wave absorption is possible by utilizing this dielectric loss.

[0005] Ferrite has been proposed as a magnetic material for electromagnetic wave absorbers, and among them, sintered bodies and magnetic composites such as spinel-type ferrite are widely used. ２ O ３ A radio wave absorber has been proposed which is characterized by comprising a spinel main phase containing 40.0 to 49.9 mol% of MnO, 4.0 to 26.5 mol% of ZnO, and the remainder MnO, and a subphase containing CaO as a main component, and the mass ratio of the spinel main phase is 50 to 99 mass% with respect to the total mass of the spinel main phase and the subphase (claim 1 of Patent Document 1).

[0006] Japanese Patent Application Publication No. 2004-247603

[0007] As described above, although electromagnetic wave absorbers containing ferrite have been proposed in the past, it has been difficult with conventional techniques to obtain excellent electromagnetic wave absorption performance in high frequency bands such as the millimeter wave band.

[0008] According to a theory known as Snoek's limit, the magnetic properties of magnetic materials such as ferrite are limited in the high frequency range. It is particularly difficult to achieve high permeability in the high frequency range with materials with small magnetic anisotropy, such as spinel ferrite. Therefore, conventional technology has limitations in achieving excellent electromagnetic wave absorption performance in the high frequency range.

[0009] The present inventors have conducted extensive research in light of these problems. As a result, they have discovered that a hexagonal magnetoplumbite-type ferrite powder having a predetermined composition and exhibiting a specific electrical resistivity has not only high magnetic properties but also dielectric properties in the millimeter wave band. They have also discovered that by using this ferrite powder, it is possible to obtain an electromagnetic wave absorber that exhibits excellent electromagnetic wave absorbing performance in the millimeter wave band, despite having a thickness equivalent to that of conventional products.

[0010] The present invention was completed based on these findings, and aims to provide a hexagonal magnetoplumbite-type ferrite powder that can be used to obtain an electromagnetic wave absorber that exhibits excellent electromagnetic wave absorption performance in the millimeter wave band despite having a thickness equivalent to that of conventional ferrite powders. Another aim of the present invention is to provide a ferrite-resin composite material, a ferrite-resin composite, and an electromagnetic wave absorber that contain the ferrite powder.

[0011] The present invention encompasses the following aspects (1) to (7). In this specification, the expression "to" includes both the numerical values ​​at both ends. In other words, "X to Y" is synonymous with "X or more and Y or less." In addition, in this specification, any combination of suitable aspects can be adopted as long as technical consistency can be achieved. For example, one of the suitable numerical ranges can be combined with the other.

[0012] (1) Formula: Sr(Fe ｘ Al ｙ Zr ｚ ) １２ O １９＋δ (where x, y, z, and δ satisfy 0.400≦x<1.000, 0.030≦y≦0.350, 0.001≦z≦0.040, 0.750≦x+y+z<1.000, and −5.00≦δ≦−0.200), and the volume resistivity is 1.0×10 ９ Ferrite powder having a resistivity of Ω·cm or less.

[0013] (2) The ferrite powder of (1) above, having an average particle size of 3 μm or more and 35 μm or less.

[0014] (3) The ferrite powder of (1) or (2) above, wherein the respective contents of strontium (Sr), aluminum (Al), and zirconium (Zr) are: Sr: 8.0 mass% or more and 12 mass% or less; Al: 1.0 mass% or more and 15 mass% or less; and Zr: 0.10 mass% or more and 4.5 mass% or less.

[0015] (4) Divalent iron ion (Fe ２＋ The ferrite powder according to any one of (1) to (3), wherein the content of (I) is 0.05 mass% or more and 4.0 mass% or less.

[0016] (5) A ferrite resin composite material containing the ferrite powder according to any one of (1) to (4) above and a resin.

[0017] (6) A ferrite-resin composite, which is a molded body of the ferrite-resin composite material of (5) above.

[0018] (7) An electromagnetic wave absorber comprising the ferrite resin composite of (6) above.

[0019] According to the present invention, there is provided a hexagonal magnetoplumbite-type ferrite powder that can be used to obtain an electromagnetic wave absorber that exhibits excellent electromagnetic wave absorption performance in the millimeter wave band despite having a thickness equivalent to that of conventional ferrite powders. Also provided are a ferrite-resin composite material, a ferrite-resin composite, and an electromagnetic wave absorber that contain the ferrite powder.

[0020] Specific embodiments of the present invention (hereinafter referred to as "present embodiments") are described below. However, the present invention is not limited to the following embodiments, and various modifications are possible within the scope of the present invention. In the present invention, a combination of two or more preferred embodiments is a more preferred embodiment.

[0021] <<1. Ferrite Powder>> The ferrite powder of this embodiment is a ferrite powder having the formula: Sr(Fe ｘ Al ｙ Zr ｚ ) １２ O １９＋δ (where x, y, z, and δ satisfy 0.400≦x<1.000, 0.030≦y≦0.350, 0.001≦z≦0.040, 0.750≦x+y+z<1.000, and −5.00≦δ≦−0.200) as a main component compound. The volume resistivity of this ferrite powder is 1.0×10 ９ The resistivity is Ω cm or less. Such ferrite powder enables improvement of electromagnetic wave absorption performance by utilizing not only magnetic loss but also dielectric loss. It is suitable for use in electromagnetic wave absorption (shielding) in the high frequency band, particularly in the millimeter wave band.

[0022] M-type ferrite has the general formula: MFe １２ O １９ (where M is a metal element such as strontium (Sr) or barium (Ba)) and has a hexagonal crystal structure (space group: P63 / mmc). M (Sr, Ba, etc.) is usually a divalent ion (M ２＋ Iron (Fe) is usually contained in M-type ferrite in the form of trivalent ions (Fe ３＋ ) and iron ions (Fe ３＋ ) may be substituted with other transition metal ions of similar size.

[0023] Using M-type ferrite as the main component compound makes it possible to impart excellent electromagnetic wave absorption performance to ferrite powder in the high frequency band. That is, ferromagnetic materials exhibit high magnetic loss at a specific frequency (resonance frequency) due to ferromagnetic resonance (FMR). It is also known that the resonant frequency (FMR frequency) at which magnetic loss occurs is proportional to the magnitude of the material's magnetic anisotropy (magnetic anisotropy constant). In this respect, M-type ferrite is a ferromagnetic material. Furthermore, unlike spinel-type ferrite, M-type ferrite has uniaxial magnetic anisotropy and a large magnetic anisotropy. Therefore, using such M-type ferrite as the main component makes it possible to obtain ferrite powder with a high FMR frequency and excellent electromagnetic wave absorption performance (EMI shielding performance) in the high frequency band. Specifically, FMR can be induced in the high frequency band of 45 GHz or higher.

[0024] In this specification, the term "main component" refers to the component with the largest mass in the ferrite powder, i.e., the ferrite powder contains M-type ferrite in an amount of 50% by mass to 100% by mass.

[0025] The ferrite powder may contain other components in addition to M-type ferrite. The other components may include raw material components (Fe, ２ O ３ , Al ２ O ３ , ZrO ２However, from the viewpoint of utilizing the excellent properties of M-type ferrite, the content of M-type ferrite is preferably high. The content of M-type ferrite is preferably 70% by mass or more and 100% by mass or less, more preferably 80% by mass or more and 100% by mass or less, and even more preferably 90% by mass or more and 100% by mass or less. In addition, other components, such as iron oxide (Fe ２ O ３ The content of ) is preferably 0% by mass or more and 30% by mass or less, more preferably 0% by mass or more and 20% by mass or less, and even more preferably 0% by mass or more and 10% by mass or less.

[0026] M-type ferrite and other components (Fe ２ O ３ By setting the content of the components (e.g., ferrite, saturation magnetization, coercive force) within the above-mentioned range, it is possible to easily obtain absorption performance suitable for the desired application. On the other hand, if the content of M-type ferrite is too small, the magnetic properties (saturation magnetization, coercive force) decrease, making it difficult to obtain excellent absorption performance. The content of M-type ferrite and other components can be determined by analyzing the ferrite powder by X-ray diffraction and performing Rietveld analysis on the obtained X-ray diffraction pattern. Specifically, it can be determined by the method of the examples described below or a method similar thereto.

[0027] The ferrite powder of this embodiment contains aluminum (Al) in M-type ferrite. This increases the resonant frequency (FMR frequency) that indicates magnetic loss, thereby improving the electromagnetic wave absorption performance in a desired frequency band. Although this should not be interpreted in a limiting manner, the following mechanism may be cited as the reason for this.

[0028] As described above, by using M-type ferrite, it is possible to obtain ferrite powder having electromagnetic wave absorption performance (EMI shielding performance) in the high frequency band. However, since ferromagnetic resonance (FMR) occurs in a specific frequency band, it is difficult to obtain broadband or multi-band absorption performance using pure M-type ferrite alone.

[0029] On the other hand, aluminum (Al) is usually present as a trivalent ion (Al ３＋ ) is stable. ３＋) is a trivalent iron ion (Fe ３＋ ) is close to Al in M-type ferrite. ３＋ Becoming Fe ３＋ It is thought that Fe in M-type ferrite is substituted. ３＋ Al ３＋ It has been reported that the magnitude of magnetic anisotropy (magnetic anisotropy constant) changes when Al is substituted. Since the FMR frequency is proportional to the magnitude of the magnetic anisotropy of the material, it is possible to control the FMR frequency by substituting Al.

[0030] Furthermore, the ferrite powder of this embodiment also contains zirconium (Zr), which can further improve the electromagnetic wave absorption performance. Although this should not be interpreted as being limited, the following mechanism can be cited as the reason for this.

[0031] Zirconium (Zr) is usually found in oxides as a tetravalent ion (Zr ４＋ ) is stable. ４＋ ) is a trivalent iron ion (Fe ３＋ ) is close to Zr in M-type ferrite. ４＋ Becoming Fe ３＋ However, Zr ４＋ and Fe ３＋ The valences are different. ４＋ With the substitution, Fe ３＋ A part of the iron ions (Fe ２＋ ) or oxygen vacancies (VO) are generated. ２＋ This is because the generation of oxygen vacancies satisfies the charge neutrality condition, thereby stabilizing the crystal structure.

[0032] On the other hand, divalent iron ions (Fe ２＋When oxygen vacancies (VO) are generated, a charge imbalance occurs in the crystal, which induces electric dipoles and dielectric polarization. The induction of dielectric polarization increases the dielectric properties and the resulting loss (dielectric loss), and it is believed that this dielectric loss can be utilized for electromagnetic wave absorption. In fact, the inventors have confirmed that incorporating zirconium (Zr) into ferrite powder increases the relative permittivity (εr) and reduces the electrical resistance (volume resistance). Both of these are due to the fact that Fe ２＋ This confirms the generation of oxygen vacancies.

[0033] The composition formula of M-type ferrite, which is the main component compound of the ferrite powder, is (Sr(Fe ｘ Al ｙ Zr ｚ ) １２ O １９＋δ The Fe content (x) in the above formula (1) is 0.400 or more and less than 1.000 (0.400≦x<1.000). If x is less than 0.400, the M-type ferrite will have an Fe-deficient composition. Magnetic properties (saturation magnetization, coercive force) may be reduced, and as a result, electromagnetic wave absorption performance may be reduced. On the other hand, if x is 1.000 or more, the composition will have an Fe-excess. Solid solution of Al and Zr in the M-type ferrite may be inhibited, making it difficult to obtain the effects of adding Al and Zr. x is preferably 0.500 or more and 0.900 or less (0.500≦x≦0.900), and more preferably 0.600 or more and 0.800 or less (0.600≦x≦0.800). In a preferred embodiment, the Fe content (x) is preferably 0.400 or more and 0.900 or less, and more preferably 0.400 or more and 0.800 or less. In a preferred embodiment, the Fe content (x) is preferably 0.500 or more and less than 1.000, more preferably 0.500 or more and 0.900 or less, and even more preferably 0.500% or more and 0.800 or less. In a preferred embodiment, the Fe content (x) is preferably 0.600 or more and less than 1.000, more preferably 0.600 or more and 0.900 or less, and even more preferably 0.600 or more and 0.800 or less.

[0034] The Al content (y) in the composition formula of M-type ferrite is 0.030 or more and 0.350 or less (0.030≦y≦0.350). If y is less than 0.030, the composition will be Al-deficient. This will change the resonant frequency, making the ferrite powder unsuitable for the intended application. On the other hand, if y exceeds 0.350, the composition will be Al-excessive. The non-ferromagnetic component, Al-based oxide (SrAl ２ O ４ , Al ２ O ３ The content of y is preferably 0.030 or more and 0.300 or less (0.030≦y≦0.300), and more preferably 0.030 or more and 0.250 or less (0.030≦y≦0.250).

[0035] The Zr content (z) in the composition formula of M-type ferrite is 0.001 or more and 0.040 or less (0.001≦z≦0.040). If z is less than 0.001, the composition will be Zr-deficient. The effect of improving dielectric properties by adding Zr will not be obtained, and there is a risk of a decrease in electromagnetic wave absorption performance. On the other hand, if z exceeds 0.040, the composition will be Zr-excessive. The amount of oxygen vacancies will be insufficient, and there is a risk of a decrease in electromagnetic wave absorption performance. z is preferably 0.020 or more and 0.040 or less (0.020≦z≦0.040), and more preferably 0.030 or more and 0.040 or less (0.030≦z≦0.040).

[0036] The sum of the Fe content, Al content, and Zr content (x+y+z) in the composition formula of M-type ferrite is 0.750 or more and less than 1.000 (0.750≦x+y+z<1.000). If x+y+z is less than 0.750, the content of Fe site ions in the M-type ferrite is small. This may result in a decrease in magnetic properties (saturation magnetization, coercive force), and therefore in a decrease in electromagnetic wave absorption performance. In addition, the composition may become excessive in Sr, Al, and Zr, resulting in the SrAl that was not ferriteized during sintering. ２ O ４ , Al ２ O ３Compounds such as ZnO and / or ZrO may remain in the ferrite powder as heterophases. If these heterophases remain, it may cause a deterioration in magnetic properties. On the other hand, if x + y + z is 1.000 or more, the solid solution of Al and Zr in M-type ferrite may be inhibited, and it may become difficult to obtain the effect of adding Al and Zr. In addition, since the composition becomes Fe-excessive, Fe may be removed during sintering. ３ O ４ may remain in the ferrite powder. ３ O ４ is a ferrimagnetic material, and when this material is produced, it becomes difficult to selectively absorb electromagnetic waves in the desired frequency range (around 50 to 75 GHz). x + y + z is preferably 0.800 or greater but less than 1.000 (0.800≦x + y + z < 1.000), and more preferably 0.850 or greater but less than 1.000 (0.850≦x + y + z < 1.000).

[0037] The absolute value of δ in the oxygen (O) content (19+δ) in the composition formula of M-type ferrite means the amount of oxygen vacancies (deficiencies). δ is −5.00 or more and −0.200 or less (−5.00≦δ≦−0.200). As mentioned above, pure M-type ferrite that does not contain substitution elements has the general formula: MFe １２ O １９ Therefore, the amount of oxygen (O) is 19. On the other hand, since the ferrite powder of this embodiment contains a substitution element (Zr, etc.), the amount of divalent iron ions (Fe ２＋) and oxygen vacancies (VO). Compared to pure M-type ferrite, this composition is oxygen (O) deficient, so δ takes a negative value. By setting δ within the desired range, a desired amount of oxygen vacancies and other elements, which are effective in improving electromagnetic wave absorption performance, are generated. When δ is less than -5.00 (δ<-5.00), the lattice defects in M-type ferrite become excessive, resulting in insufficient contribution to the dielectric constant. This results in reduced electromagnetic wave absorption performance. Even when δ exceeds -0.200 (-0.200<δ), the contribution to the dielectric constant is small, resulting in reduced electromagnetic wave absorption performance. δ is preferably -4.00 or more and -1.00 or less (-4.00≦δ≦-1.00), and more preferably -3.00 or more and -2.00 or less (-3.00≦δ≦-2.00). In a preferred embodiment, δ in the oxygen (O) amount (19 + δ) is preferably −5.00 or more and −1.00 or less, and more preferably −5.00 or more and −2.00 or less. In a preferred embodiment, δ in the oxygen (O) amount (19 + δ) is preferably −4.00 or more and −0.200 or less, more preferably −4.00 or more and −1.00 or less, and even more preferably −4.00 or more and −2.00 or less. In a preferred embodiment, δ in the oxygen (O) amount (19 + δ) is preferably −3.00 or more and −0.200 or less, more preferably −3.00 or more and −1.00 or less, and even more preferably −3.00 or more and −2.00 or less.

[0038] x, y, z, x + y + z, and δ are preferably 0.500≦x≦0.900, 0.030≦y≦0.300, 0.020≦z≦0.040, 0.800≦x + y + z < 1.000, and −4.00≦δ≦−1.00, and more preferably 0.600≦x≦0.800, 0.030≦y≦0.250, 0.030≦z≦0.040, 0.850≦x + y + z < 1.000, and −3.00≦δ≦−2.00.

[0039] The ferrite powder of this embodiment has a volume resistivity of 1.0×10 ９Ω·cm or less. By maintaining the volume resistivity (electrical resistance) at a low level, it is possible to further improve the electromagnetic wave absorption performance in the high frequency band. That is, as described above, the ferrite powder of this embodiment has oxygen vacancies. It is also believed that the presence of these oxygen vacancies reduces the electrical resistance (volume resistance). Furthermore, it is believed that the presence of oxygen vacancies causes dielectric loss, which contributes to improving the electromagnetic wave absorption performance. If the electrical resistance (volume resistance) is excessively high, it is not possible to obtain the excellent absorption performance resulting from the oxygen vacancies. From the viewpoint of improving the electromagnetic wave absorption performance, it is desirable that the volume resistance is low. Specifically, the volume resistance is 1.0×10 ８ Ω cm or less is preferable, and 1.0 × 10 ７ The volume resistivity may be measured by the method described in the Examples below or a method based thereon. If the volume resistivity is too low, the transmission characteristics in the electromagnetic wave absorption performance are thought to deteriorate. Therefore, the volume resistivity is preferably 1.0×10 6 It is preferably Ω·cm or more.

[0040] The average particle size of the ferrite powder is preferably 3 μm or more and 35 μm or less, more preferably 3 μm or more and 25 μm or less, and even more preferably 3 μm or more and 15 μm or less. In many cases, the ferrite powder is used as a filler and mixed with a resin to prepare a composite material, which is then molded to obtain a composite (such as an electromagnetic wave absorber). If the average particle size is within the above-mentioned range, the flowability and measuring and handling properties of the ferrite powder and the composite material are improved. Therefore, the filler filling rate of the composite material is improved, making it possible to obtain a composite (such as an electromagnetic wave absorber) with excellent mechanical strength.

[0041] On the other hand, if the average particle size of the ferrite powder is too small, the fluidity of the ferrite powder and the composite material decreases, and the particles (ferrite particles) constituting the ferrite powder tend to aggregate. This makes it difficult to obtain a composite with a high filler loading. Furthermore, if the average particle size is too large, the voids between the ferrite particles become large. This also makes it difficult to obtain a composite with a high filler loading. The average particle size can be measured using a particle size analyzer. Specifically, it can be measured using the method described in the Examples below or a method similar thereto.

[0042] The respective contents of strontium (Sr), aluminum (Al), and zirconium (Zr) in the ferrite powder are preferably Sr: 8.0 mass% to 12 mass%, Al: 1.0 mass% to 15 mass%, and Zr: 0.10 mass% to 4.5 mass%. This allows the ferrite powder to have even better electromagnetic wave absorption performance. Specifically, if the Sr content is within the above-mentioned range, a ferrite powder exhibiting electromagnetic wave absorption performance suitable for the desired application can be obtained. If the Al content is within the above-mentioned range, the resonant frequency can be optimized. For example, it is possible to obtain an electromagnetic wave absorber that functions in the 60 GHz band (60 to 65 GHz). If the Zr content is within the above-mentioned range, oxygen vacancies are induced, resulting in further improvement in absorption performance.

[0043] On the other hand, if the Sr content is too small, the magnetic properties (saturation magnetization, coercive force) of the ferrite powder will decrease, which may result in a decrease in electromagnetic wave absorption performance. On the other hand, if the Sr content is too large, the generation of Sr-based oxides (Sr-Fe oxide, SrO, etc.), which are non-ferromagnetic components, will proceed, which may lead to a decrease in absorption performance. If the Al content is too small, the effect of increasing the resonance frequency (FMR frequency) based on Al substitution will be insufficient. As a result, the FMR frequency will shift to the low frequency side, which may be inappropriate for some applications. If the Al content is too large, the FMR frequency will shift to the high frequency side, which may also be inappropriate for some applications. In addition, the generation of Al-based oxides (SrAl), which are non-ferromagnetic components, will proceed. ２ O ４ , Al ２ O ３ If the Zr content is too small, the number of oxygen vacancies that contribute to the occurrence of dielectric loss will decrease, which may result in a decrease in absorption performance.If the Zr content is too large, the generation of Zr-based components (Zr-Fe oxide, ZrO), which are non-ferromagnetic components, will proceed, which may result in a decrease in absorption performance.

[0044] From the viewpoint of further improving the electromagnetic wave absorption performance, the respective contents of strontium (Sr), aluminum (Al), and zirconium (Zr) are preferably Sr: 8.0 mass% to 11 mass%, Al: 1.0 mass% to 11 mass%, and Zr: 2.0 mass% to 4.5 mass%, and even more preferably Sr: 8.0 mass% to 10 mass%, Al: 1.0 mass% to 7.0 mass%, and Zr: 3.0 mass% to 4.5 mass%. The contents of Sr, Al, and Zr can be determined by chemical analysis of the ferrite powder. Specifically, analysis can be performed using the method of the examples described below or a method similar thereto.

[0045] Divalent iron ions (Fe) in ferrite powder ２＋ The content of Fe is preferably 0.05% by mass or more and 4.0% by mass or less, more preferably 2.0% by mass or more and 4.0% by mass or less, and even more preferably 3.0% by mass or more and 4.0% by mass or less. ２＋ If the amount is within the above range, the effect of improving the electromagnetic wave absorption performance by utilizing the dielectric properties (dielectric loss) can be significantly achieved. ２＋ If the amount is too small, there is a limit to the improvement of the dielectric properties. ２＋ If the amount is too large, the ferrite powder may be excessively oxidized, and the magnetic properties (saturation magnetization, coercive force) may be reduced. ２＋ is caused by Zr in the grains. ２＋ By evaluating the amount of Zr, the presence or absence of Zr can be confirmed. ２＋ The amount can be determined by oxidation-reduction titration, specifically by the method described in the Examples below or a method based thereon.

[0046] Sr, Al, Zr, and Fe ２＋ The respective contents of Sr are 8.0 mass% or more and 11 mass% or less, Al is 1.0 mass% or more and 11 mass% or less, Zr is 2.0 mass% or more and 4.5 mass% or less, and Fe is ２＋ : 2.0 mass% or more and 4.0 mass% or less, Sr: 8.0 mass% or more and 10 mass% or less, Al: 1.0 mass% or more and 7.0 mass% or less, Zr: 3.0 mass% or more and 4.5 mass% or less, and Fe２＋ : 3.0 mass % or more and 4.0 mass % or less is more preferable.

[0047] Regarding the dielectric properties of the ferrite powder, the relative permittivity (εr) is preferably 10 or more, more preferably 12 or more, and even more preferably 14 or more. A sufficiently high relative permittivity allows for more effective penetration of electromagnetic waves into an electromagnetic wave absorber containing the ferrite powder and for electromagnetic wave absorption utilizing the dielectric properties (dielectric loss). This further improves absorption performance. On the other hand, an excessively low relative permittivity may prevent smooth penetration of electromagnetic waves. The relative permittivity (εr) is measured using the free-space method in the frequency band of 50 to 75 GHz. Specifically, it may be measured using the method described in the Examples below or a method similar thereto. Since a too high relative permittivity is thought to deteriorate the transmission characteristics in the electromagnetic wave absorption performance, a relative permittivity of 30 or less is preferred.

[0048] Regarding the magnetic properties of the ferrite powder, the saturation magnetization (Ms) is preferably 10 emu / g or more and 50 emu / g or less, more preferably 10 emu / g or more and 40 emu / g or less, and even more preferably 10 emu / g or more and 30 emu / g or less. The remanence (Mr) is preferably 15 emu / g or more and 35 emu / g or less, more preferably 15 emu / g or more and 30 emu / g or less, and even more preferably 15 emu / g or more and 25 emu / g or less. The coercive force (Hc) is preferably 6000 Oe or less, more preferably 5000 Oe or less, and even more preferably 4000 Oe or less. By setting Ms, Mr, and Hc within the above-mentioned ranges, magnetization rotation in the high frequency band is smooth, and the real part of magnetic permeability (μ') is high. Therefore, it is possible to obtain electromagnetic wave absorption performance suitable for the desired application. On the other hand, if Ms is too large and Hc is too small, the FMR frequency may shift excessively toward the low frequency side, making it difficult to obtain excellent absorption performance in the desired frequency band. Also, if Ms is too small and Hc is too large, the FMR frequency may shift excessively toward the high frequency side. Furthermore, if the coercive force is too low, the frequency in the electromagnetic wave absorption performance is thought to decrease, so a coercive force of 1000 Oe is preferable. The magnetic properties may be measured using the method of the examples described below or a method similar thereto.

[0049] <<2. Method for Producing Ferrite Powder>> The method for producing the ferrite powder of this embodiment is not limited as long as it satisfies the above-mentioned requirements. However, the method preferably includes the following steps: a step of mixing a strontium (Sr) source, an iron (Fe) source, an aluminum (Al) source, a zirconium (Zr) source, water, and, if necessary, a binder, a dispersant, an antifoaming agent, and / or a pH adjuster to prepare a slurry (raw material mixing step), a step of spray-granulating the obtained slurry to prepare a granulated product (granulation step), and a step of firing the obtained granulated product to prepare a fired product (firing step). If necessary, a step of post-treating the obtained fired product (post-treatment step) may also be included. Details of each step are described below.

[0050] <Raw Material Mixing Step> In the mixing step, a strontium (Sr) source, an iron (Fe) source, an aluminum (Al) source, a zirconium (Zr) source, water, and, if necessary, a binder, a dispersant, an antifoaming agent, and / or a pH adjuster are prepared as raw materials, and these raw materials are mixed to prepare a slurry. Known ferrite raw materials such as oxides, carbonates, and hydroxides may be used as the Sr source, Fe source, Al source, and Zr source. Preferably, strontium carbonate (SrCO ３ ), iron oxide (Fe ２ O ３ ), aluminum oxide (Al ２ O ３ ), and zirconium oxide (ZrO ２ These are inexpensive, stable at room temperature and humidity, and easy to handle.

[0051] Additives such as binders, dispersants, antifoaming agents, and pH adjusters may be added as needed. The binder is added to improve the strength of the granules obtained after spray granulation. Resin compounds such as polyvinyl alcohol (PVA) and / or polyvinylpyrrolidone (PVP) may be used as the binder.

[0052] The raw materials (Sr source, Fe source, Al source, Zr source, water, etc.) may be mixed by a known method. For example, the raw materials may be mixed using a mixer / pulverizer such as a wet bead mill to form a slurry. The solid content concentration of the resulting slurry is preferably 30% by mass or more and 75% by mass or less, more preferably 45% by mass or more and 65% by mass or less, and even more preferably 50% by mass or more and 60% by mass or less. In a preferred embodiment, the solid content concentration of the slurry is preferably 30% by mass or more and 65% by mass or less, and even more preferably 30% by mass or more and 60% by mass or less. In a preferred embodiment, the solid content concentration of the slurry is preferably 45% by mass or more and 75% by mass or less, more preferably 45% by mass or more and 65% by mass or less, and even more preferably 45% by mass or more and 60% by mass or less. In a preferred embodiment, the solid content of the slurry is preferably 50% by mass or more and 75% by mass or less, more preferably 50% by mass or more and 65% by mass or less, and even more preferably 50% by mass or more and 60% by mass or less.

[0053] <Granulation Step> In the granulation step, the obtained slurry is spray-granulated to produce a granulated product. Spray granulation may be performed using a spray dryer. Granulation may be performed under known conditions. For example, the conditions include a slurry discharge rate of 200 g / min to 500 g / min, an atomizer disk rotation speed of 1000 rpm to 12000 rpm, and a drying temperature of 100°C to 500°C. The average particle size of the granulated product formed in the granulation step is preferably 20 μm to 120 μm.

[0054] In the granulation step, the resulting granules may be subjected to a binder removal treatment. In the binder removal treatment, the granules are heated to decompose and remove organic components such as the binder, thereby obtaining a defatted product. The binder removal treatment may be performed using a known heating furnace. Heating may be performed under conditions that remove organic components, for example, at a temperature of 650°C to 1050°C for 0.5 hours to 12 hours. Furthermore, resin binders such as polyvinyl alcohol (PVA), polyvinylpyrrolidone, and / or acrylic binders may be used as the binder, and the amount added is 0.05% by mass to 1.0% by mass in terms of solid content relative to the raw material mixture.

[0055] <Sintering Step> In the sintering step, the obtained granulated or defatted material is sintered (main sintering) to obtain a sintered product. The sintering temperature is preferably 1100°C or higher and 1300°C or lower, more preferably 1200°C or higher and 1300°C or lower, and even more preferably 1220°C or higher and 1260°C or lower. When the sintering temperature is within the above-mentioned range, a ferrite powder exhibiting electromagnetic wave absorption performance suitable for the desired application can be obtained. On the other hand, if the sintering temperature is too low, the FMR frequency may be excessively shifted to the low frequency side, resulting in a decrease in absorption performance. If the sintering temperature is too high, the FMR frequency may be excessively shifted to the high frequency side, resulting in a decrease in absorption performance. In a preferred embodiment, the sintering temperature is preferably 1100°C or higher and 1260°C or lower. In a preferred embodiment, the sintering temperature is preferably 1200°C or higher and 1300°C or lower, and more preferably 1200°C or higher and 1260°C or lower. In a preferred embodiment, the firing temperature is preferably 1220° C. or higher and 1300° C. or lower, more preferably 1220° C. or higher and 1260° C. The firing temperature may be maintained for 3 hours or higher and 6 hours or lower.

[0056] The firing can be carried out in a nitrogen atmosphere. ２ The concentration of O in the atmosphere is preferably 10% by volume or less, more preferably 5.0% by volume or less, and even more preferably 1.0% by volume or less. ２ By suppressing the concentration, the Fe in the obtained ferrite powder ２＋ Therefore, it is possible to fully utilize the effect of the dielectric properties (dielectric loss) caused by the oxygen vacancies. ２ If the concentration is too high, Fe ２＋ and oxygen vacancies may decrease, resulting in a decrease in absorption performance.

[0057] <Post-treatment step> If necessary, the fired product may be subjected to post-treatment. Examples of post-treatment include pulverization, heat treatment, and classification. The post-treatments may be performed alone or in combination. In this manner, the ferrite powder of the present embodiment can be obtained.

[0058] <<3. Ferrite Resin Composite Material>> The ferrite resin composite material of this embodiment (hereinafter may be referred to as the "composite material") contains the above-described ferrite powder and a resin. The composite material is a precursor material of a ferrite resin composite, which is a constituent member of an electromagnetic wave absorber. In other words, the composite material is molded to produce a composite.

[0059] Examples of resins constituting the composite material include epoxy resins, urethane resins, acrylic resins, silicone resins, polyamide resins, polyimide resins, polyamideimide resins, fluororesins, and combinations thereof. The silicone resin may be a modified silicone resin modified with acrylic, urethane, epoxy, and / or fluorine.

[0060] The composite material may contain components other than the ferrite powder and the resin, such as a solvent, a filler (organic filler or inorganic filler), a plasticizer, an antioxidant, a dispersant, a colorant such as a pigment, and / or thermally conductive particles.

[0061] The ratio of the ferrite powder to the total solid content in the composite material is preferably 50% by mass to 95% by mass, more preferably 80% by mass to 95% by mass. The ratio of the resin to the total solid content in the composite material is preferably 5% by mass to 50% by mass, more preferably 5% by mass to 20% by mass. By setting the ratios of the ferrite powder and the resin within the above-mentioned ranges, the dispersion stability of the ferrite powder and the storage stability and moldability of the composite material are improved, and the properties such as the mechanical strength and magnetic properties of the composite obtained by molding the composite material are improved.

[0062] <<4. Ferrite Resin Composite>> The ferrite resin composite of this embodiment (hereinafter may be referred to as "composite") is a molded product of the above-described ferrite resin composite material. That is, the composite is produced by molding the composite material. The molding method is not particularly limited, and examples thereof include compression molding, extrusion molding, injection molding, blow molding, and calendar molding. Alternatively, a method of forming a coating film of the composite material on a substrate may be used.

[0063] The filler filling rate of the composite is preferably 70% by volume or more, more preferably 80% by volume or more, and even more preferably 90% by volume or more. The filler filling rate is a measure of the proportion of ferrite particles (particles constituting ferrite powder) in the composite. By setting the filler filling rate within the above-mentioned range, the dispersion stability of the ferrite particles and the storage stability and moldability of the composite are excellent. On the other hand, if the filler filling rate is excessively low, the strength of the composite may decrease. Since an excessively high filler filling rate is thought to result in a decrease in the density of the compact, a filler filling rate of 99% by volume or less is preferable. The filler filling rate can be determined by observing the cross section of the composite using an SEM and determining the proportion of ferrite particles in the obtained cross-sectional SEM image. Specifically, it can be measured using the method described in the Examples below or a method similar thereto. However, the magnification during SEM observation can be selected depending on the particle size of the ferrite powder and the thickness of the composite.

[0064] The flexural strength of the composite is preferably 1000 MPa or more, more preferably 2000 MPa or more, and even more preferably 3000 MPa or more. This makes it possible to obtain a composite with excellent mechanical strength. On the other hand, if the flexural strength is excessively low, defects such as chipping and cracking of the composite are more likely to occur. The flexural strength is the biaxial flexural strength measured in a biaxial bending test. Specifically, it may be measured by the method of the examples described below or a method similar thereto.

[0065] <<5. Electromagnetic Wave Absorber>> The electromagnetic wave absorber of this embodiment includes the ferrite resin composite described above. The electromagnetic wave absorber may be composed of only the composite, or other components may be provided. For example, an impedance matching layer or a surface protection layer may be provided on the surface of the composite. A reflective member may also be provided on the back surface. An example of the impedance matching layer is a layer in which magnetic powder or dielectric powder is dispersed in resin. An example of the surface protection layer is a layer made of resin or glass. An example of the reflective member is a film-like, foil-like, or mesh-like metal member.

[0066] The electromagnetic wave absorber desirably has a peak in the frequency characteristics of return loss. The peak frequency of return loss is preferably 50 GHz or more and 75 GHz or less, more preferably 55 GHz or more and 70 GHz or less, and even more preferably 60 GHz or more and 65 GHz or less. In a preferred embodiment, the peak is preferably 50 GHz or more and 70 GHz or less, and even more preferably 50 GHz or more and 65 GHz or less. In a preferred embodiment, the peak is preferably 55 GHz or more and 75 GHz or less, more preferably 55 GHz or more and 70 GHz or less, and even more preferably 55 GHz or more and 65 GHz or less. In a preferred embodiment, the peak is preferably 60 GHz or more and 75 GHz or less, more preferably 60 GHz or more and 70 GHz or less, and even more preferably 60 GHz or more and 65 GHz or less. Furthermore, the return loss at the peak frequency (return loss peak amount) is preferably 10 dB or more, more preferably 20 dB or more, and even more preferably 30 dB or more. If the peak frequency and the return loss peak amount are within the above-mentioned ranges, absorption performance suitable for the desired application can be obtained.

[0067] The electromagnetic wave absorber desirably has a peak in the frequency characteristics of transmission attenuation. The peak frequency of the transmission attenuation is preferably 50 GHz or more and 75 GHz or less, more preferably 55 GHz or more and 70 GHz or less, and even more preferably 60 GHz or more and 65 GHz or less. In a preferred embodiment, the peak is preferably 50 GHz or more and 70 GHz or less, and even more preferably 50 GHz or more and 65 GHz or less. In a preferred embodiment, the peak is preferably 55 GHz or more and 75 GHz or less, more preferably 55 GHz or more and 70 GHz or less, and even more preferably 55 GHz or more and 65 GHz or less. In a preferred embodiment, the peak is preferably 60 GHz or more and 75 GHz or less, more preferably 60 GHz or more and 70 GHz or less, and even more preferably 60 GHz or more and 65 GHz or less. Furthermore, the transmission attenuation at the peak frequency (transmission attenuation peak amount) is preferably 10 dB or more, more preferably 20 dB or more, and even more preferably 30 dB or more. As long as the peak frequency and transmission attenuation peak amount are within the above-mentioned ranges, absorption performance suitable for the desired application can be obtained. The reflection attenuation and transmission attenuation amounts are measured by the free space method. Specifically, they may be measured by the method of the examples described below or a method equivalent thereto.

[0068] The use of the electromagnetic wave absorber is not limited as long as it is intended to absorb electromagnetic waves. For example, it is applied to transmission lines, high-frequency circuits, electronic components, and / or electronic devices. The electromagnetic wave absorber of this embodiment exhibits excellent electromagnetic wave absorption performance in the millimeter wave band (30 GHz or more and 300 GHz or less). Therefore, it is particularly suitable for electromagnetic wave absorption applications in the millimeter wave band, such as high-speed wireless LANs and collision prevention radars.

[0069] The present invention will be described in more detail with reference to the following examples and comparative examples, but the present invention is not limited to these examples.

[0070] (1) Preparation of ferrite powder and ferrite resin composite [Example 1] <Mixing of raw materials> Iron oxide (Fe ２ O ３ ), strontium carbonate (SrCO３ ), zirconium oxide (ZrO ２ ), and aluminum oxide (Al ２ O ３ ) and these were mixed with Fe ２ O ３ :60.4mol%, SrCO ３ : 15.3 mol%, ZrO ２ : 6.7 mol%, and Al ２ O ３ Water was then added to the weighed raw materials, and the mixture was finely pulverized using a wet bead mill equipped with zirconia beads of 0.65 mmφ to obtain a slurry.

[0071] <Granulation> To the obtained slurry, polyvinyl alcohol (PVA, 15% aqueous solution) as a binder, acrylic copolymer ammonium salt (BASF, AA-4040, 40% aqueous solution) as a dispersant, and aqueous ammonia solution (25% aqueous solution) as a pH adjuster were added. The amount of binder (PVA) added was 1% by mass in terms of solid content, and the amount of dispersant (acrylic copolymer ammonium salt) added was 0.3% by mass in terms of solid content. The concentration (solid content) of the obtained slurry was 55% by mass. The slurry containing the binder, dispersant, and pH adjuster was then spray-granulated using a spray dryer to obtain a granulated product. The average particle size of the obtained granulated product was 60 μm.

[0072] <Firing> Next, the obtained granules were fired (main firing) to obtain a fired product. ２ The firing was carried out at 1260°C for 5 hours in an atmosphere containing 0.0% by volume of ammonium hydroxide. The fired product was then pulverized using a hammer mill to obtain a fired powder with an average particle size of 14.1 μm. Ferrite powder was thus produced. The production conditions for the ferrite powder are summarized in Table 1 below.

[0073] <Preparation of Composite> A ferrite resin composite material was prepared by mixing and kneading 90 parts by mass of filler (ferrite powder) and 10 parts by mass of binder resin. A powdered fluororesin was used as the binder resin. Mixing was performed using a sample mill. The resulting ferrite resin composite material was then filled into a mold and heated at 180°C for 2 hours while being pressure-molded using a press machine to prepare a resin molded body. The resulting sheet (resin molded body) had a thickness of 3 mm. The resulting sheet was used as a ferrite molded body (ferrite resin composite) for evaluation.

[0074] [Examples 2 to 6 and Comparative Examples 1 to 6] Raw materials (Fe ２ O ３ , SrCO ３ , ZrO ２ and Al ２ O ３ The compounding ratio of the above and the firing conditions were changed as shown in Table 1 below. Other than that, the same procedure as in Example 1 was carried out to prepare a ferrite powder and a composite.

[0075]

[0076] (2) Evaluation The ferrite powders and composites obtained in Examples 1 to 6 and Comparative Examples 1 to 6 were evaluated for various properties as follows.

[0077] <Chemical Analysis> The contents of Fe, Sr, Al, and Zr in the ferrite powder were measured by ICP atomic emission spectrometry. Specifically, 0.2 g of the ferrite powder to be measured was weighed out. 20 ml of 1N hydrochloric acid and 20 ml of 1N nitric acid were added to 60 ml of pure water and heated, and the weighed ferrite powder was added thereto and completely dissolved to prepare an aqueous solution. The prepared aqueous solution was used as a measurement sample, and the sample was placed in an ICP atomic emission spectrometer (Shimadzu Corporation, ICPS-1000IV) to measure the contents of Fe, Sr, Al, and Zr.

[0078] <Oxidation-reduction titration> Fe in ferrite powder ２＋ The amount of HCl was determined by oxidation-reduction titration using a potassium permanganate solution in accordance with JIS M 8213-1995, except that a potassium permanganate solution was used instead of a potassium dichromate solution.

[0079] <XRD> A powder X-ray diffraction pattern is obtained using ferrite powder as a sample, and the obtained pattern is subjected to Rietveld analysis to identify each crystalline phase (M-type ferrite phase, Fe ２ O ３ Although it can be difficult to identify and quantify each crystalline phase using waveform separation of powder X-ray diffraction patterns, Rietveld analysis based on a crystal structure model makes it possible to identify and quantify each phase.

[0080] The X-ray diffraction device used was an "X'PertPRO MPD" manufactured by PANalytical. A Co tube (CoKα rays) was used as the X-ray source. A focused optical system and a high-speed detector, "X'Celarator," were used as the optical system. The measurement conditions were as follows:

[0081] - Scan speed: 0.08° / sec - Divergence slit: 1.0° - Scattering slit: 1.0° - Receiving slit: 0.15 mm - Voltage and current of sealed tube: 40 kV / 40 mA - Measurement range: 2θ = 15° to 90° - Number of accumulations: 5 times

[0082] Based on the measurement results, the crystal structure was identified as follows: Magnetoplumbite ferrite (M-type ferrite) Space group: P63 / mmc (No. 194) Iron oxide (Fe ２ O ３ ) Space group: R-3c (No.167)

[0083] Next, the following parameters were optimized using the analysis software "RIETAN-FP v2.83", and based on the optimization results, M-type ferrite and Fe in the ferrite powder were analyzed. ２ O ３ The content of was determined. During optimization, the profile function used was the pseudo-Voigt function of Thompson, Cox, and Hastings, and asymmetrization was performed using Howard's method. In addition, the following parameters were optimized so that the Rwp value and S value, which represent the accuracy of fitting, were Rwp: 2% or less and S value: 1.5 or less, respectively.

[0084] (Parameters to be optimized) - Shift factor - Scale factor - Background parameter - Gaussian function U, V, W - Lorentz function X, Y - Asymmetry parameter As - Lattice constant - Atomic coordinates

[0085] <Composition formula> The contents of Fe, Sr, Al, and Zr determined by chemical analysis, and the content of divalent iron ions (Fe ２＋ ) content, and the content of each crystalline phase (M-type ferrite, Fe ２ O ３ ) content, the composition formula of M-type ferrite contained in the ferrite powder (Sr(Fe ｘ Al ｙ Zr ｚ ) １２ O １９＋δ ) was calculated according to the following procedure.

[0086] The mass fractions of Fe (A mass %), Sr (B mass %), Al (C mass %), and Zr (D mass %) determined by chemical analysis can be converted to molar fractions to determine the corresponding A' (mol %), B' (mol %), C' (mol %), and D' (mol %). Specifically, the molar fractions of Fe (55.845 g / mol), Sr (87.62 g / mol), Al (26.9815 g / mol), and Zr (91.224 g / mol) can be used to determine the Fe (mol %), Sr (mol %), Al (mol %), and Zr (mol %) content, A' (mol %), B' (mol %), C' (mol %), and D' (mol %) content, respectively, according to the following formulas (1) to (4).

[0087] A'=(A / 55.845) / (A / 55.845+B / 87.62+C / 26.9815+D / 91.224)×100...(1) B'=(B / 87.62) / (A / 55.845+B / 87.62+C / 26.9815+D / 91.224)×100...(2) C'=(C / 26.9815) / (A / 55.845+B / 87.62+C / 26.9815+D / 91.224)×100...(3) D'=(D / 91.224) / (A / 55.845+B / 87.62+C / 26.9815+D / 91.224)×100...(4)

[0088] Magnetoplumbite (M) type ferrite and Fe ２ O ３ The amount (mass fraction) of the two phases is converted into a molar fraction, and the content (mol %) of Fe constituting the M-type ferrite can be calculated. For example, the amount of M-type ferrite is α (mass %), Fe ２ O ３ The amount of ferrite is β (mass %). An example of M-type ferrite is SrFe １２ O １９ When SrFe is used, １２ O １９ Molar mass (1061.7486 g / mol) and Fe ２ O ３ Using the molar mass (159.6882 g / mol), the M-type ferrite amount α' (mol%) and Fe ２ O ３ The amount β' (mol %) is calculated according to the following formulas (5) and (6).

[0089] α'=(α / 1061.7486) / (α / 1061.7486+β / 159.6882)×100...(5) β'=(β / 159.6882) / (α / 1061.7486+β / 159.6882)×100...(6)

[0090] Using these values, the amount of Fe γ (mol %) constituting the M-type ferrite can be calculated according to the following formula (7).

[0091] γ=A'-A'×(β'×2 / (α'×12+β'×2))...(7)

[0092] Next, the composition formula Sr(Fe ｘ Al ｙ Zr ｚ ) １２ O １９＋δ x, y, z, and x+y+z can be calculated according to the following equations (8) to (11).

[0093] x=γ / (B'×12)...(8) y=C' / (B'×12)...(9) z=D' / (B'×12)...(10) x+y+z=γ / (B'×12)+ C' / (B'×12)+ D' / (B'×12)...(11)

[0094] Fe ２＋ When the content of is σ and the total number of cations in the M-type ferrite is ζ, σ and ξ satisfy the relationship of the following formula (12).

[0095] ζ=3×12×(x×(1-σ))+2×12×(x×σ)+3×12×y+4×12×z+1×2...(12)

[0096] Fe ２＋ The content σ of "Fe" represents the content (mass ratio) when the total amount of the ferrite powder is 1. For example, in Example 1, ２＋ The "content of Fe" is 3.52 mass% (Table 2). ２＋ The content σ is 3.52 / 100.

[0097] Here, since oxygen is deficient in the composition formula to satisfy the electrical neutrality condition of M-type ferrite, the amount of oxygen deficiency δ can be calculated according to the following formula (13).

[0098] δ=(ζ / 2)-19...(13)

[0099] In this way, the composition formula Sr(Fe ｘ Al ｙ Zr ｚ ) １２ O １９＋δ The values ​​of x, y, z, x+y+z, and δ in

[0100] <Volume Average Particle Size> The volume average particle size (D50) of the ferrite powder was measured using a Microtrac particle size analyzer (Nikkiso Co., Ltd., Model 9320-X100). Specifically, 10 g of ferrite powder and 80 ml of water were placed in a 100 ml beaker, and 2 to 3 drops of a dispersant (sodium hexametaphosphate) were added. The resulting mixture was then subjected to a dispersion treatment using an ultrasonic homogenizer (SMT. Co. LTD., UH-150 type). The output level of the ultrasonic homogenizer was set to 4. The dispersion treatment was carried out for 20 seconds. After the dispersion treatment, bubbles formed on the surface of the beaker were removed to prepare a measurement sample. The resulting measurement sample was subjected to the above-mentioned particle size analyzer to measure D50.

[0101] <Volume Resistivity> The volume resistivity of the ferrite powder was measured. ２ A measurement sample was prepared by filling a fluororesin cylinder with ferrite powder to a height of 4 mm. Next, electrodes were attached to both ends (top and bottom) of the measurement sample, and a 1 kg weight was placed on top of them to measure the electrical resistance. An electrometer (KEITHLEY, insulation resistance meter, model 6517A) was used to measure the resistance at an applied voltage of 100 V. The volume resistance was calculated from the obtained resistance value. The volume resistance was measured at room temperature in the atmosphere (300 K).

[0102] <Relative permittivity> The relative permittivity (εr) of the composite was measured by the free space method. The measurement was performed using a network analyzer (Keysight Technologies, Inc., E5253E2) and a free space measurement device (EM Lab, Inc., FS-330). Specifically, using one port, an input wave of a predetermined frequency (50 to 75 GHz) was irradiated onto the composite from a transmitting / receiving antenna via a dielectric lens. The reflection and transmission characteristics (S parameters) of the electromagnetic wave were then measured, and the relative permittivity was calculated from the obtained characteristics. In this case, the relative permittivity was analyzed using the Keysight Material Measurement Suite N1500A included with the device. The analysis was performed using the Nicholson-Ross-Weir (NRW method) model.

[0103] <Filler filling rate> The filler filling rate in the ferrite compact was measured by observing the cross section of the ferrite compact with a scanning electron microscope (SEM). Specifically, the ferrite compact was processed to have dimensions of 5 mm x 5 mm and a thickness of 3 mm using an ion milling device (Hitachi High-Tech Corporation, IM-4000). The processed ferrite compact was then set on a stage with its cross section facing up to serve as a measurement sample. The ion milling process was performed under the following conditions.

[0104] - DISCHARGE VOLTAGE (discharge voltage): 1.5 kV - ACCELERATION VOLTAGE (acceleration voltage): 6 kV - STAGE CONTROL (processing mode): C3 - DISCHARGE CURRENT (discharge current inside the ion gun): 380 to 450 μA - ION BEAM CURRENT (ion beam current): 110 to 140 μA - GAS FLOW (argon gas flow rate): 0.07 to 0.10 cm ３ / min - Processing time: 60 minutes

[0105] Next, the prepared measurement sample was observed using an SEM (SU-8020 manufactured by Hitachi High-Technologies Corporation). SEM observation was performed under conditions of an acceleration voltage of 1 kV and a magnification of 500 to 10,000 times (a magnification at which approximately 30 particles fit in one field of view), and cross-sectional SEM images of three fields of view were taken in LA mode. The obtained cross-sectional SEM images were then binarized to determine the proportion of ferrite powder, and the filler filling rate was calculated. The filler filling rate calculated as the average of the three fields of view was rounded to one decimal place.

[0106] <Magnetic Properties> The saturation magnetization (Ms), remanence (Mr), and coercive force (Hc) of the ferrite powder were measured. Specifically, the ferrite powder was packed into a cell with an inner diameter of 5 mm and a height of 2 mm, and set in a vibrating sample magnetometer (Toei Kogyo Co., Ltd., VSM-C7-10A). A magnetic field was applied and swept up to 10 kOe, and then the applied magnetic field was reduced to draw a hysteresis curve. Ms, Mr, and Hc were read from the data of the obtained curve.

[0107] <Reflection and Transmission Characteristics> Using the composite as a sample, the reflection and transmission characteristics were measured by the free-space method. A network analyzer (Keysight Technologies, Inc., E5253E2) and a free-space measurement device (EM Lab Co., Ltd., FS-330) were used as measurement devices. S parameters were measured with an incident angle of 0° and a sweep frequency of 50 GHz to 75 GHz. From the obtained S parameters, the peak frequency of electromagnetic wave absorption, transmission attenuation, and return attenuation of the composite in the range of 50 to 75 GHz were calculated. The S parameter analysis was performed using the Keysight Material Measurement Suite N1500A attached to the device.

[0108] <Flexural Strength> The flexural strength (biaxial flexural strength) of the composite was measured by a method conforming to ISO / DIS 6872. For the measurement, a sheet-like composite measuring 15 mm x 15 mm and 3 mm thick was used. The measurement was performed three times for composites produced under the same conditions, and the average value was taken as the biaxial flexural strength.

[0109] (3) Evaluation Results The results obtained for the ferrite powder and the composite are summarized in Tables 2 and 3 below.

[0110] The ferrite powders of Examples 1 to 6 had compositions and volume resistivities that satisfied the ranges specified in this embodiment. Therefore, the dielectric constants were relatively high, at 10.5 or more, and the return loss in the millimeter wave band was relatively large, at 11.8 dB or more. In particular, Examples 1 and 2 had a large return loss of 28.9 dB or more, and the flexural strength of the composite was relatively high, at 3,367 MPa or more.

[0111] In contrast, Comparative Examples 1 to 5, in which at least one of the values ​​of the composition and the volume resistivity did not satisfy the range specified in this embodiment, had a low relative dielectric constant of 9.8 or less and a low return loss of 8.1 or less.

[0112] In addition, although the relative permittivity and return loss of Comparative Example 6 were within the preferred ranges, the composition and firing temperature values ​​did not satisfy the specified ranges, and therefore the peak frequency of the return loss was outside the desired frequency range of 50 to 75 GHz required for this embodiment.

[0113]

[0114]

[0115] From the above results, it can be seen that the present embodiment provides a hexagonal magnetoplumbite ferrite powder that can produce an electromagnetic wave absorber that exhibits excellent electromagnetic wave absorption performance in the millimeter wave band, despite having a thickness equivalent to that of conventional ones.

[0116] According to the present invention, there is provided a hexagonal magnetoplumbite-type ferrite powder that can be used to obtain an electromagnetic wave absorber that exhibits excellent electromagnetic wave absorption performance in the millimeter wave band despite having a thickness equivalent to that of conventional ferrite powders. Also provided are a ferrite-resin composite material, a ferrite-resin composite, and an electromagnetic wave absorber that contain the ferrite powder.

[0117] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2023-221337) filed on December 27, 2023, the contents of which are incorporated herein by reference.

Claims

1. The formula: Sr(Fe ｘ Al ｙ Zr ｚ )( １２ O １９＋δ ), where x, y, z, and δ satisfy 0.400 ≤ x < 1.000, 0.030 ≤ y ≤ 0.350, 0.001 ≤ z ≤ 0.040, 0.750 ≤ x + y + z < 1.000, and -5.00 ≤ δ ≤ -0.

200. The ferrite powder contains a hexagonal magnetoplumbite-type ferrite having the above composition as the main component compound, and has a volume resistivity of 1.0 × 10 ９ Ω·cm or less.

2. The ferrite powder according to claim 1, having an average particle size of 3 μm or more and 35 μm or less.

3. The ferrite powder according to claim 1 or 2, wherein the respective contents of strontium (Sr), aluminum (Al), and zirconium (Zr) are:

4. The content of divalent iron ions (Fe ２＋ ) is 0.05% by mass or more and 4.0% by mass or less, and the ferrite powder according to claim 1 or 2. Sr: 8.0% by mass or more and 12% by mass or less, Al: 1.0% by mass or more and 15% by mass or less, Zr: 0.10% by mass or more and 4.5% by mass or less.

5. A ferrite resin composite material comprising the ferrite powder according to claim 1 or 2 and a resin.

6. A ferrite resin composite, which is a molded body of the ferrite resin composite material according to claim 5.

7. An electromagnetic wave absorber comprising the ferrite resin composite according to claim 6.

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