Ferrite powder, ferrite resin composite material, ferrite resin composite body, and electromagnetic wave absorber

Abstract

To provide ferrite powder exhibiting excellent electromagnetic wave absorption performance in a frequency band around 28 GHz, a ferrite resin composite material containing the ferrite powder, a ferrite resin composite body, and an electromagnetic wave absorber.SOLUTION: Ferrite powder includes a Li0.5Fe2.5O4 phase belonging to a space group Fd-3m, a LiFe5O8 phase belonging to a space group P4132, an Fe3O4 phase, and a LiMn1.5Fe0.5O4 phase. In the ferrite powder, the total content percentage of the Li0.5Fe2.5O4 phase and the LiFe5O8 phase is 65 mol% or more and 80 mol% or less, the content percentage of the Fe3O4 phase is 17 mol% or more and 27 mol% or less, and the content percentage of the LiMn1.5Fe0.5O4 phase is 4.4 mol% or more and 10 mol% or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] Rapid advances in wireless communication technology and increasingly sophisticated and diverse needs for wireless services are driving faster communication speeds and larger data volumes (high speed and large capacity). Amid this trend, commercial services for the fifth-generation mobile communication system (5G), the next-generation mobile communication system, began in the spring of 2020. 5G not only achieves high speed and large capacity, but also high reliability, low latency, and multiple simultaneous connections. As a result, it is attracting great expectations as an essential infrastructure for realizing an IoT society. For example, 5G will enable remote robot operation and instantaneous transmission and reception of information from multiple devices. Therefore, it is considered a key technology for advancing the practical application of telemedicine and autonomous driving.

[0003] Communication systems are plagued by the problem of electromagnetic interference (EMI). With the advancement of higher frequencies, 5G is becoming increasingly important, as the circuit length of electronic devices and the wavelength of electromagnetic waves are close to each other, making the effects of EMI particularly pronounced. The use of electromagnetic wave absorbers (radio wave absorbers) is effective in preventing problems such as equipment malfunctions and adverse effects on the human body caused by EMI. Ferrite powder is widely used as a material for electromagnetic wave absorbers.

[0004] For example, Patent Document 1 discloses Zn X Li (1-X)0.5 Fe (1-X)0.5 A Li-Zn ferrite powder for use in radio wave absorbers is disclosed, which is made of ferrite represented by the structural formula Fe2O4 (where X is 0.1 to 0.8), and it is stated that the ferrite powder significantly improves the absorption performance of radio waves in the 1 to 4 GHz band (claim 1 and

[0012] of Patent Document 1).

[0005] Furthermore, although not intended for use as an electromagnetic wave absorber, Patent Document 2 discloses a carrier core material for electrophotographic developers, which is made of Li ferrite, maghemite, and Fe3O4, some of which are substituted with Mn, has a Li content of 1 to 2.5 wt %, a Mn content of 2 to 7.5 wt %, and further contains 25 to 10,000 ppm of silicon (Claim 1 of Patent Document 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-268368 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-244571 Summary of the Invention [Problem to be solved by the invention]

[0007] Although the use of ferrite powder as an electromagnetic wave absorber has been proposed, conventional ferrite powders for electromagnetic wave absorbers have not performed well enough to be used as 5G-compatible electromagnetic wave absorbers. 5G will introduce a new wireless communication system using frequencies around 28 GHz in addition to the existing frequencies below 6 GHz. Therefore, it is desirable for the powder to exhibit excellent electromagnetic wave absorption performance in the frequency range around 28 GHz, specifically, to exhibit a deep electromagnetic wave absorption peak (sometimes simply referred to as the "absorption peak") in this frequency range.

[0008] Furthermore, the frequency range allocated for 5G varies by country and by telecommunications carrier. For example, focusing on frequencies around 28 GHz, Japan is allocated the 27-29.5 GHz frequency range, with different frequencies allocated to each carrier. The United States is allocated the 22.5-28.35 GHz frequency range, Europe the 24.25-27.5 GHz frequency range, China the 24.75-27.5 GHz frequency range, and South Korea the 26.5-29.5 GHz frequency range. Therefore, it is desirable to use an electromagnetic wave absorber that can be used across countries and carriers, i.e., has excellent versatility. To achieve this, it is necessary not only to have a deep absorption peak, but also to have a reasonably wide absorption frequency range, i.e., the width of the absorption peak. However, conventional ferrite powders have not been sufficient to meet these demands.

[0009] The present inventors have conducted studies in consideration of these problems. As a result, they have found that it is important to control the type and ratio of crystalline phases contained in ferrite powder. They have also found that by controlling the type and ratio of crystalline phases within a predetermined range, it is possible to obtain ferrite powder that exhibits excellent electromagnetic wave absorption performance in the vicinity of 28 GHz, particularly in the frequency range of 21 to 32 GHz.

[0010] The present invention was completed based on these findings, and aims to provide a ferrite powder that exhibits excellent electromagnetic wave absorption performance in a frequency range around 28 GHz. 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 this ferrite powder. [Means for solving the problem]

[0011] The present invention includes the following aspects (1) to (7). In this specification, the expression "to" includes both ends of the expression. In other words, "X to Y" is synonymous with "at least X and at most Y."

[0012] (1) Li, which belongs to the space group Fd-3m 0.5 Fe 2.5 O4 phase, LiFe5O8 phase and Fe3O4 phase belonging to the space group P4132, and LiMn1.5 Fe 0.5 Contains O4 phase, Li 0.5 Fe 2.5 The total content ratio of the O4 phase and the LiFe5O8 phase is 65 mol% or more and 80 mol% or less, The content of the Fe3O4 phase is 17 mol% or more and 27 mol% or less, LiMn 1.5 Fe 0.5 A ferrite powder having an O4 phase content of 4.4 mol % or more and 10 mol % or less.

[0013] (2) The ferrite powder of (1) above, containing lithium (Li) in an amount of 1.4% by mass or more and 1.6% by mass or less, manganese (Mn) in an amount of 1.4% by mass or more and 3.3% by mass or less, and iron (Fe) in an amount of 64.5% by mass or more and 67.0% by mass or less.

[0014] (3) The ferrite powder of (1) or (2) above, having a volume average particle size D50 of 5 μm or more and 15 μm or less.

[0015] (4) A ferrite resin composite material containing the ferrite powder according to any one of (1) to (3) above and a resin.

[0016] (5) A ferrite-resin composite, which is a molded body of the ferrite-resin composite material of (4) above.

[0017] (6) An electromagnetic wave absorber comprising the ferrite resin composite of (5) above. [Effects of the Invention]

[0018] The present invention provides a ferrite powder that exhibits excellent electromagnetic wave absorption performance in a frequency range around 28 GHz, as well as a ferrite resin composite material, a ferrite resin composite, and an electromagnetic wave absorber that contain the ferrite powder. DETAILED DESCRIPTION OF THE INVENTION

[0019] A specific embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described. Note that the present invention is not limited to the following embodiment, and various modifications are possible within the scope of the present invention. Furthermore, as long as technical consistency can be achieved, any combination of the preferred aspects disclosed in this specification can be adopted. For example, one preferred numerical range can be combined with the other preferred range.

[0020] <<1. Ferrite powder>> The ferrite powder of this embodiment is Li, which belongs to the space group Fd-3m. 0.5 Fe 2.5 O4 phase, LiFe5O8 phase and Fe3O4 phase belonging to the space group P4132, and LiMn 1.5 Fe 0.5 Contains O4 phase. 0.5 Fe 2.5 The total content of the O4 phase and the LiFe5O8 phase is 65 mol% or more and 80 mol% or less. The content of the Fe3O4 phase is 17 mol% or more and 27 mol% or less. 1.5 Fe 0.5 The content of the O4 phase is 4.4 mol% or more and 10 mol% or less. Ferrite is a ceramic whose main component is iron oxide. Therefore, the ferrite powder of this embodiment is made of an oxide containing iron (Fe), lithium (Li), manganese (Mn), and oxygen (O) as essential components.

[0021] The ferrite powder of this embodiment is Li, which belongs to the space group Fd-3m. 0.5 Fe 2.5 The LiFeO4 phase and the LiFe5O8 phase, which belong to the space group P4132, are the two phases that make up Li ferrite. Li ferrite is a type of soft magnetic ferrite with a cubic spinel crystal structure and is characterized by a high Curie temperature. Li ferrite consists of the α phase (LiFe5O8 phase), which belongs to the space group P4132, and the β phase (LiFe5O8 phase), which belongs to the space group Fd-3m. 0.5 Fe 2.5 O4 phase). The α phase is also called an ordered spinel phase, and is composed of iron ions (Fe 3+) occupy the octahedral 12d sites and the tetrahedral 8c sites of the cubic simple lattice, and lithium ions (Li + ) occupy the octahedral 4b sites. In contrast, the β phase has an irregular (disordered) structure. The β phase has an inverse spinel structure, and Fe 3+ ions occupy the tetrahedral 8a sites, and Li + ions and Fe 3+ ions randomly occupy the octahedral 16d sites. These two phases of Li ferrite, namely, the α phase (LiFe5O8 phase) and the β phase (Li 0.5 Fe 2.5 By incorporating coexisting ferrite powder with ZnO (O4 phase), excellent electromagnetic wave absorption performance can be imparted to the ferrite powder. Specifically, in the frequency range around 28 GHz, the product of the absorption peak depth and width due to natural resonance becomes high.

[0022] The content of Li ferrite, that is, Li 0.5 Fe 2.5 The total content of the O4 phase and the LiFe5O8 phase is 65 mol% or more and 80 mol% or less. This improves the electromagnetic wave absorption performance of the ferrite powder. On the other hand, if the Li ferrite content is less than 65 mol%, the phase state of the ferrite powder becomes Li 0.5 Fe 2.5 The state is close to one in which only one of the O4 phase and the LiFe5O8 phase exists. In other words, the crystal structure of the ferrite powder is aligned, so that the magnetic moment easily rotates in response to an externally applied magnetic field. As a result, combined with the influence of adjacent magnetic moments, the product of the depth and width of the absorption peak due to natural resonance becomes smaller. Also, when the Li ferrite ratio exceeds 80 mol%, the LiMn 1.5 Fe 0.5 Therefore, the crystal structure is easily aligned, and the product of the depth and width of the absorption peak becomes smaller. From the viewpoint of further improving the electromagnetic wave absorption performance, the content ratio of Li ferrite (Li 0.5 Fe 2.5 The total content of the LiFeO4 phase and the LiFe5O8 phase is preferably 67 mol % or more and 78 mol % or less, and more preferably 70 mol % or more and 77 mol % or less.

[0023] Assuming that the total content of Li ferrite is within the specified range, 0.5 Fe 2.5 The content of the O4 phase is preferably 46 mol% or more and 56 mol% or less, and more preferably 48 mol% or more and 56 mol% or less. 0.5 Fe 2.5 If the ratio of the O4 phase is increased appropriately, the ratio of the entire Li ferrite phase increases. Therefore, the product of the depth and width of the absorption peak becomes even higher. 0.5 Fe 2.5 When the ratio of the O4 phase is appropriately reduced, LiMn 1.5 Fe 0.5 This prevents the O4 phase from becoming extremely scarce, making it easier to control the absorption peak frequency within a desired range.

[0024] Assuming that the total content of Li ferrite is within a predetermined range, the content of the LiFe5O8 phase is preferably 14 mol% or more and 26 mol% or less, and more preferably 20 mol% or more and 24 mol% or less. If the proportion of the LiFe5O8 phase is increased appropriately, the proportion of the entire Li ferrite increases. Therefore, the product of the depth and width of the absorption peak becomes even higher. Also, if the proportion of the LiFe5O8 phase is decreased appropriately, the LiMn 1.5 Fe 0.5 This prevents the O4 phase from becoming extremely scarce, making it easier to control the absorption peak frequency within a desired range.

[0025] The ferrite powder of this embodiment contains an Fe3O4 phase (magnetite). The content of the Fe3O4 phase is 17 mol% or more and 27 mol% or less. When the content of the Fe3O4 phase is within the above-mentioned range, the depth of the absorption peak becomes appropriately large. On the other hand, when the content is less than 17 mol%, the total amount of Li ferrite becomes excessive, and the crystal structure becomes uniform. Therefore, there is a risk that the product of the depth and width of the absorption peak becomes small. When the content exceeds 27 mol%, the total amount of Li ferrite decreases relatively, and the Fe3O4 phase becomes LiMn 1.5 Fe 0.5In order to further improve the electromagnetic wave absorption performance, the content of the Fe3O4 phase is preferably 17.5 mol% or more and 25 mol% or less, and more preferably 18 mol% or more and 23 mol% or less.

[0026] The ferrite powder of this embodiment is LiMn 1.5 Fe 0.5 Contains O4 phase. LiMn 1.5 Fe 0.5 The content of the O4 phase is 4.4 mol% or more and 10 mol% or less. 1.5 Fe 0.5 The O4 phase is a paramagnetic material near room temperature. By incorporating this phase, it is possible to adjust the absorption peak frequency, although the exact mechanism is unknown. If the content ratio is within the range mentioned above, the absorption peak frequency will be within the desired range. On the other hand, if it is outside the range, the natural resonance frequency will be significantly shifted and the desired peak frequency will not be achieved. LiMn 1.5 Fe 0.5 The content of the O4 phase is preferably 5 mol % or more and 9.5 mol % or less, and more preferably 6 mol % or more and 9 mol % or less.

[0027] From the viewpoint of improving the electromagnetic wave absorption performance, the ferrite powder is preferably Li 0.5 Fe 2.5 O4 phase is 46 mol% or more and 56 mol% or less, LiFe5O8 phase is 14 mol% or more and 26 mol% or less, and Li ferrite (Li 0.5 Fe 2.5 O4 phase and LiFe5O8 phase) is 65 mol% or more and 80 mol% or less, Fe3O4 phase is 17 mol% or more and 27 mol% or less, and LiMn 1.5 Fe 0.5 The O4 phase is contained in a proportion of 4.4 mol% or more and 10 mol% or less. More preferably, Li 0.5 Fe 2.5 O4 phase is 46 mol% or more and 56 mol% or less, LiFe5O8 phase is 14 mol% or more and 26 mol% or less, and Li ferrite (Li 0.5 Fe 2.5The total of the LiFeO4 phase and the LiFe5O8 phase is 67 mol% or more and 78 mol% or less, the Fe3O4 phase is 17.5 mol% or more and 25 mol% or less, and LiMn 1.5 Fe 0.5 The O4 phase is contained in a proportion of 5 mol% to 9.5 mol%. More preferably, Li 0.5 Fe 2.5 O4 phase is 48 mol% or more and 56 mol% or less, LiFe5O8 phase is 20 mol% or more and 24 mol% or less, and Li ferrite (Li 0.5 Fe 2.5 O4 phase and LiFe5O8 phase) is 70 mol% or more and 77 mol% or less, Fe3O4 phase is 18 mol% or more and 23 mol% or less, and LiMn 1.5 Fe 0.5 The O4 phase is contained in a proportion of 6 mol% to 9 mol%. 0.5 Fe 2.5 O4 phase, LiFe5O8 phase, Fe3O4 phase, and LiMn 1.5 Fe 0.5 O4 phase) is less than 100 mol%.

[0028] The ferrite powder is Li 0.5 Fe 2.5 O4 phase, LiFe5O8 phase, Fe3O4 phase, and Li 0.5 Fe 2.5 The material may contain a crystalline phase (heterogeneous phase) other than the O4 phase. Examples of the heterogeneous phase include α-Fe2O3, Li oxide, and Mn oxide. However, reducing the amount of heterogeneous phase further improves the electromagnetic wave absorption performance. The total amount of heterogeneous phase is preferably 10 mol% or less, more preferably 5 mol% or less, even more preferably 3 mol% or less, and particularly preferably 1 mol% or less.

[0029] The ferrite powder preferably contains lithium (Li) in an amount of 1.4 to 1.6 mass%, manganese (Mn) in an amount of 1.4 to 3.3 mass%, and iron (Fe) in an amount of 64.5 to 67.0 mass%. If the ferrite powder composition is within this range, the proportion of each structure (crystalline phase) can be appropriately controlled. Therefore, it is possible to further improve the electromagnetic wave absorption performance.

[0030] Specifically, when the Li content is increased appropriately, the proportion of Li ferrite as a whole increases. Therefore, the product of the absorption peak depth and width becomes even higher. When the Li content is decreased appropriately, the Mn content becomes relatively larger. LiMn 1.5 Fe 0.5 This prevents the O4 phase from becoming too scarce, making it easier to control the absorption peak frequency within a desired range.

[0031] When the Mn content is increased appropriately, LiMn 1.5 Fe 0.5 This prevents the O4 phase from becoming too small. This makes it easier to control the absorption peak frequency within the desired range. When the Mn content is appropriately reduced, the Li content increases relatively. As the proportion of the entire Li ferrite increases, the product of the depth and width of the absorption peak becomes even higher.

[0032] When the amount of Fe is increased appropriately, LiMn 1.5 Fe 0.5 As a result, it is easy to control the absorption peak frequency within a desired range. By appropriately reducing the amount of Fe, it is possible to prevent the amount of Fe3O4 phase from becoming excessively large, and as a result, the absorption peak depth becomes even larger. Problems caused by an excessively small Li ferrite ratio and problems caused by LiMn 1.5 Fe 0.5 This is because the problem of the Fe3O4 phase inhibiting absorption based on the magnetic resonance of the O4 phase can be suppressed.

[0033] To further improve electromagnetic wave absorption performance, the ferrite powder preferably contains 1.4 to 1.6 mass% Li, 1.4 to 3.3 mass% Mn, and 64.5 to 67.0 mass% Fe. More preferably, the ferrite powder contains 1.4 to 1.5 mass% Li, 1.9 to 2.1 mass% Mn, and 66.0 to 66.5 mass% Fe. However, the proportion of each structure (crystalline phase) depends on the conditions during ferrite powder production, such as sintering conditions. Therefore, even if the ferrite powder composition is within the above-mentioned range, the proportion of each structure (crystalline phase) does not necessarily fall within the desired range.

[0034] The ferrite powder of this embodiment may contain components other than the above-described Li, Mn, Fe, and O. Examples of other components include, but are not limited to, strontium (Sr), calcium (Ca), copper (Cu), zinc (Zn), silicon (Si), and / or chlorine (Cl). However, to fully utilize the electromagnetic wave absorption performance based on the desired crystal phase, it is desirable that the amount of other components is not greater than necessary. The amount of other components is preferably 1.0 mass% or less, more preferably 0.7 mass% or less, and even more preferably 0.5 mass% or less. In particular, the amount of Si is preferably less than 25 ppm, less than 10 ppm, or less than 1 ppm. Furthermore, vanadium (V) and bismuth (Bi), which are elements that promote sintering, may increase the hardness of ferrite particles and make pulverization difficult, so it is preferable to minimize their inclusion. Specifically, the content of each of V and Bi is preferably 100 ppm or less.

[0035] Preferably, the volume average particle size D50 of the ferrite powder is 5 μm or more and 15 μm or less. By keeping the D50 within this range, it is possible to produce a ferrite resin composite (electromagnetic wave absorber) with excellent electromagnetic wave absorption performance. Specifically, by setting the D50 to 5 μm or more, it is possible to suppress an increase in material viscosity when kneading the ferrite powder with a resin to produce a composite. On the other hand, if the material viscosity increases, the ferrite powder will not disperse properly, making it difficult to adjust the absorption peak frequency within the desired range. While it is possible to reduce the amount of ferrite powder to lower the viscosity, this will result in a decrease in electromagnetic wave absorption performance. Furthermore, by setting the D50 to 15 μm or less, it is possible to reduce the voids between the particles (ferrite particles) constituting the ferrite powder contained in the composite, thereby further improving the electromagnetic wave absorption performance. On the other hand, if the voids between the particles are large, it will be difficult to adjust the absorption peak frequency within the desired range. More preferably, the D50 is 7 μm or more and 15 μm or less, and even more preferably 11 μm or more and 15 μm or less.

[0036] Preferably, the coercive force Hc of the ferrite powder is 55 Oe or more and 75 Oe or less. Hc is related to the ferrite particle size. Ferrite powder with an Hc of 55 Oe or more tends to have a moderately small particle size. The voids between the ferrite particles contained in the ferrite resin composite are reduced, further improving the electromagnetic wave absorption performance. Furthermore, ferrite powder with an excessively high Hc may contain heterogeneous phases (α-Fe2O3, Li oxide, and Mn oxide) that are non-ferritized nonmagnetic phases or antiferromagnetic phases. These heterogeneous phases may reduce the electromagnetic wave absorption performance. By keeping Hc at 75 Oe or less, the influence of the heterogeneous phases can be prevented, further improving the electromagnetic wave absorption performance. From the viewpoint of further improving the electromagnetic wave absorption performance, Hc is more preferably 57 Oe or more and 70 Oe or less, and even more preferably 60 Oe or more and 63 Oe or less. "Oe" is the unit of magnetic field strength in the cgs system, and 1 Oe is (1 / 4π) × 10 3 Equivalent to A / m.

[0037] Preferably, the remanence magnetization σr of the ferrite powder is 4.0 emu / g or more and 6.0 emu / g or less. σr is related to the ferrite particle size. Ferrite powder with a σr of 4.0 emu / g or more tends to have a moderately small particle size. The gaps between the ferrite particles contained in the ferrite resin composite are reduced, further improving the electromagnetic wave absorption performance. Furthermore, ferrite powder with an excessively large σr may contain heterogeneous phases that are not ferritized. By keeping σr at 6.0 emu / g or less, the influence of heterogeneous phases can be prevented, and the electromagnetic wave absorption performance can be further improved. σr is more preferably 4.3 emu / g or more and 5.5 emu / g or less, and even more preferably 4.6 emu / g or more and 5.0 emu / g or less. Note that "emu / g" is the unit of magnetic moment per unit mass in the CGS system, and 1 emu / g is 1 A·m 2 / kg.

[0038] Preferably, the shape factor SF-1 of the ferrite powder is 115 or more and 300 or less. SF-1 is an index of particle sphericity. For a perfect sphere, SF-1 is 100, and the more the particle is deviated from a spherical shape, the greater the SF-1 becomes. By appropriately distorting the shape of the ferrite particles, the electromagnetic wave absorption performance can be further improved. This is thought to be because the probability of particles contacting each other at multiple points rather than just one point in the electromagnetic wave absorber increases, and as a result, magnetic field components are less likely to leak outside the ferrite particles when electromagnetic waves pass through the electromagnetic wave absorber.

[0039] On the other hand, if SF-1 is too small, the particle sphericity becomes too high. Although the particle packing rate in the electromagnetic wave absorber increases, the probability of particle contact decreases, which may result in a decrease in electromagnetic wave absorption performance. Furthermore, ferrite powder with an excessively large SF-1 may contain extremely irregular particles, plate-like particles, or particles with concave surfaces. Powders containing such particles may make it difficult to increase the particle packing rate in the electromagnetic wave absorber. SF-1 is more preferably 120 or more and 280 or less.

[0040] Preferably, the shape factor SF-2 of the ferrite powder is 115 or more and 250 or less. SF-2 is an index of the roughness of the particle surface. For a perfect sphere, SF-2 is 100, and the more the particle is deviated from a spherical shape, the greater the roughness. A moderate amount of roughness on the ferrite particle surface allows the particles to interlock with each other, reducing the gaps between the particles and further improving the electromagnetic wave absorption performance. Note that if the particle size is excessively large, the gaps between the particles will be large. Therefore, even if SF-2 is within the above-mentioned range, the effect of improving the electromagnetic wave absorption performance may not be very significant. However, when the volume average particle size is 15 μm or less, a further improvement in the electromagnetic wave absorption capacity can be expected if SF-2 is within the above-mentioned range.

[0041] If SF-2 is too small, the particle surface roughness will be too small. This will result in insufficient particle interlocking within the electromagnetic wave absorber, resulting in larger interparticle gaps, which may result in reduced electromagnetic wave absorption performance. Furthermore, ferrite powders with excessively large SF-2 may contain particles with extremely irregular shapes, or particles that are plate-like or have depressions on their surfaces. Powders containing such particles may make it difficult to increase the particle packing rate within the electromagnetic wave absorber. SF-2 is preferably between 120 and 230.

[0042] Reflection coefficient (S 11 In the frequency characteristics of the ferrite powder, the peak is preferably in the frequency range of 23 GHz or more and 29 GHz or less, more preferably 24 GHz or more and 28.5 GHz or less, and even more preferably 25 GHz or more and 28.5 GHz or less. Ferrite powder having an absorption peak in this frequency range is highly useful in industry. 11 ) are S-parameters that describe the reflected signal and are measured using the free-space method.

[0043] Reflection coefficient (S 11) frequency characteristics, the absorption peak depth of the ferrite powder is preferably 7 dB or more, more preferably 8 dB or more, and even more preferably 10 dB or more. Ferrite powder with a large absorption peak depth has high industrial applicability. The absorption peak depth is determined by the reflection coefficient (S 11 ) is the absolute minimum value of

[0044] Reflection coefficient (S 11 In the frequency characteristics of ), the absorption peak width of the ferrite powder is preferably 3 GHz or more, more preferably 3.5 GHz or more, and even more preferably 4 GHz or more. Ferrite powder with a large absorption peak width has high industrial value. The absorption peak width is the width of the absorption peak at a depth of 5 dB.

[0045] Reflection coefficient (S 11 In the frequency characteristics of ), the product of the absorption peak depth and width of the ferrite powder is preferably 40 GHz dB or more, more preferably 60 GHz dB or more, and even more preferably 100 GHz dB or more. Ferrite powders with a large product of the absorption peak depth and width have high industrial utility. The product of the absorption peak depth and width is the product of the absorption peak depth and the absorption peak width.

[0046] The ferrite powder of this embodiment has the advantage of exhibiting a moderately broad and deep absorption peak in the frequency range around 28 GHz. Therefore, by using this ferrite powder, it is possible to obtain an electromagnetic wave absorber that has both excellent electromagnetic wave absorption performance and versatility for 5G.

[0047] <<2. Ferrite Powder Manufacturing Method>> The ferrite powder of this embodiment may be produced by any method as long as it satisfies the above-described requirements. However, it is preferable to produce the ferrite powder by the following method. A preferred production method includes the steps of: mixing an iron (Fe) raw material, a manganese (Mn) raw material, and a lithium (Li) raw material to obtain a raw material mixture (raw material mixing step); pre-firing the resulting mixture to obtain a pre-firing product (pre-firing step); pulverizing and granulating the pre-firing product to obtain a granulated product (granulation step); debindering the resulting granulated product to obtain a binder-free product (binder removal step); and firing the resulting binder-free product to obtain a fired product (firing step). If necessary, a post-processing step may be provided in which the fired product is crushed, classified, and / or pulverized. Details of each step are described below.

[0048] <Raw material mixing process> In the raw material mixing step, an iron (Fe) raw material, a manganese (Mn) raw material, and a lithium (Li) raw material are mixed to obtain a raw material mixture. Known ferrite raw materials such as oxides, carbonates, hydroxides, and chlorides of Fe, Mn, and / or Li may be used as raw materials. The raw materials may contain each component (Fe, Mn, Li) alone or in combination of two or more. The blending amounts of the Fe raw material, Mn raw material, and Li raw material may be determined so that the final ferrite powder has the desired composition and crystal phase. The raw material mixing may be performed using a known mixer such as a Henschel mixer, using either a dry method or a wet method, or both.

[0049] <Pre-firing process> In the calcination step, the resulting mixture is calcined to obtain a calcined product. Calcination promotes the ferritization reaction of the raw material mixture, allowing for the production of ferrite powder with a uniform composition. Calcination may be performed by a known method. It may be performed under known conditions using a furnace such as a rotary kiln, continuous furnace, or batch furnace. For example, the calcination may be performed under conditions such as maintaining the temperature at 700°C to 1300°C in an atmosphere such as air for 2 hours to 12 hours.

[0050] <Granulation process> In the granulation step, the resulting calcined product is pulverized and granulated to obtain a granulated product. The pulverization method is not particularly limited. It can be performed using a known pulverizer such as a vibration mill, ball mill, or bead mill, and can be either a dry or wet method, or both. The granulation method can also be a known method. For example, water and a binder, and optionally additives such as a dispersant and / or an antifoaming agent, can be added to the pulverized calcined product to adjust the viscosity, and then the product can be granulated using a granulator such as a spray dryer. Resin binders such as polyvinyl alcohol (PVA), polyvinylpyrrolidone, and / or acrylic resins can be used as the binder, and the amount added is, for example, 0.05% by mass to 1.0% by mass in terms of solid content relative to the raw material mixture.

[0051] <Debinding process> In the binder removal step, the resulting granules are subjected to a binder removal treatment to obtain a binder-removed product. In the binder removal treatment, the granules are heated to decompose and remove organic components such as the binder. The binder removal treatment may be carried out using a known heating furnace. Heating may be carried out under conditions that remove organic components, for example, at a temperature of 650°C to 1050°C for 0.5 hours to 12 hours. The binder removal treatment (heating) may be carried out, for example, in an atmosphere with an oxygen concentration of 18% by volume or less, although this is not limited thereto.

[0052] <Firing process> In the firing step, the debindered product is fired to obtain a fired product. Firing can be performed using a known firing furnace such as a rotary kiln, continuous furnace, or batch furnace. However, the temperature and atmosphere during firing are important for obtaining the ferrite powder of this embodiment. Specifically, the firing temperature is preferably 1140°C to 1220°C, more preferably 1190°C to 1200°C. The oxygen concentration in the atmosphere during firing is preferably 0% to 9% by volume, more preferably 0.5% to 1.5% by volume. Setting the firing temperature and atmosphere within the above-mentioned ranges makes it possible to appropriately control the amount of each crystalline phase in the ferrite powder and the D50. On the other hand, if the firing temperature is too low, sintering will not proceed sufficiently, resulting in excessive pulverization in the pulverization step, which will result in an excessively small D50 and make it difficult to control the absorption peak frequency within the desired range. If the firing temperature is too high, sintering will proceed excessively, resulting in insufficient pulverization in the pulverization process, resulting in an excessively large D50 and making it difficult to control the absorption peak frequency within the desired range. Also, if the oxygen concentration is too high, Li ferrite will not be sufficiently produced, which may result in a small product of the depth and width of the absorption peak.

[0053] <Post-processing process> If necessary, a post-processing step may be performed in which the obtained sintered product is crushed, classified, and / or pulverized. This allows the particle size of the sintered product (ferrite powder) to be adjusted. In particular, if the particle size of the ferrite powder is small, it may be difficult to manufacture a ferrite resin composite (electromagnetic wave absorber). That is, when manufacturing the ferrite resin composite, the ferrite powder is kneaded with a resin. If the particle size of the ferrite powder is excessively small, the viscosity of the material increases, making kneading difficult. Furthermore, if the particle size of the ferrite powder is excessively large, the voids between the ferrite particles contained in the composite may become large, which may result in a decrease in electromagnetic wave absorption performance. Therefore, in such cases, it is preferable to adjust the particle size by performing a post-processing on the sintered product.

[0054] The crushing may be carried out using a crusher such as a hammer crusher. The classification may be carried out by a method such as air classification or sieve classification. The pulverization may be carried out by either a dry method or a wet method, or both, using a known pulverizer such as a vibration mill, a ball mill, or a bead mill. However, as long as a ferrite powder having a desired particle size can be obtained after the sintering, the post-treatment step is not essential.

[0055] In this manner, the ferrite powder of this embodiment can be obtained.

[0056] <<3. Ferrite Resin Composite Materials>> The ferrite resin composite material of this embodiment contains the above-described ferrite powder and a resin. The ferrite resin composite material is a precursor material for a ferrite resin composite, which is a constituent member of an electromagnetic wave absorber. That is, the resin composite material is molded to produce a composite.

[0057] 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.

[0058] The composite material may contain components other than the ferrite powder and resin, such as solvents, fillers (organic fillers and inorganic fillers), plasticizers, antioxidants, dispersants, colorants such as pigments, and / or thermally conductive particles.

[0059] The proportion of ferrite powder relative 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 proportion of resin relative 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. Setting the proportions of ferrite powder and resin within the above ranges improves the dispersion stability of the ferrite powder in the composite material, as well as the storage stability and moldability of the composite material. Furthermore, the composite (resin molded product) obtained by molding the composite material has improved properties such as mechanical strength and magnetic properties.

[0060] <<4. Ferrite-resin composite>> The ferrite resin composite (resin molded article) of this embodiment is a molded article of the above-mentioned ferrite resin composition. That is, the composite is produced by molding the ferrite resin composition. 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.

[0061] <<5. Electromagnetic wave absorber>> The electromagnetic wave absorber of this embodiment includes the above-described ferrite resin composite. The electromagnetic wave absorber may be composed of only the composite, or may include other components. For example, an impedance matching layer or a surface protection layer may be provided on the surface. 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.

[0062] Reflection coefficient (S 11 In the frequency characteristics of the electromagnetic wave absorber, the electromagnetic wave absorber preferably has a peak in a frequency range of 23 GHz or more and 29 GHz or less, more preferably 24 GHz or more and 28.5 GHz or less, and even more preferably 25 GHz or more and 28.5 GHz or less. An electromagnetic wave absorber having an absorption peak in this frequency range is highly useful in industry.

[0063] Reflection coefficient (S 11 ) the absorption peak depth of the electromagnetic wave absorber is preferably 7 dB or more, more preferably 8 dB or more, and even more preferably 10 dB or more. An electromagnetic wave absorber with a large absorption peak depth has high industrial utility value.

[0064] Reflection coefficient (S 11 ) the absorption peak width of the electromagnetic wave absorber is preferably 3 GHz or more, more preferably 3.5 GHz or more, and even more preferably 4 GHz or more. An electromagnetic wave absorber with a large absorption peak width has high industrial utility value.

[0065] Reflection coefficient (S 11 ) the product of the absorption peak depth and width of the electromagnetic wave absorber is preferably 40 GHz dB or more, more preferably 60 GHz dB or more, and even more preferably 100 GHz dB or more. An electromagnetic wave absorber having a large product of the absorption peak depth and width is highly useful in industry.

[0066] 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 a moderately broad and deep absorption peak in a frequency range around 28 GHz, and is characterized by having both excellent electromagnetic wave absorption performance and versatility for 5G. Therefore, it is particularly suitable for 5G-compatible communication devices, and the transmission lines, high-frequency circuits, and / or electronic components used therein. [Example]

[0067] 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 the following examples.

[0068] (1) Preparation of ferrite powder [Examples 1-15] Alpha-iron oxide (alpha-Fe2O3), trimanganese tetroxide (Mn3O4), and lithium carbonate (Li2CO3) were used as raw materials, and these were weighed and mixed in the amounts shown in Table 1 below. Next, water was added to the resulting mixture so that the solid content concentration was 50 mass %, and the mixture was pulverized in a bead mill and then pre-granulated in a spray dryer. The resulting pre-granulated product was pre-fired in air at 1000°C for 2 hours.

[0069] Water was added to the obtained calcined product to a solids concentration of 50% by mass, and then PVA as a binder and an aliphatic polyhydric alcohol-based polyether polyol as an antifoaming agent were added, followed by pulverization in a bead mill to obtain a slurry. The amount of binder added was 0.6% by mass relative to the solids in the slurry. The amount of antifoaming agent added was 70 cc per 30 kg of solids in the slurry. The resulting pulverized powder slurry was then granulated using a spray dryer to obtain a granulated product.

[0070] The obtained granules were subjected to a binder removal treatment to remove organic components, and then subjected to sintering. The binder removal treatment and sintering were carried out under the conditions shown in Table 1 below. Next, the obtained sintered product was crushed using a hammer crusher and then dry-pulverized. In Examples 1 to 3 and Examples 5 to 15, dry pulverization was carried out using a planetary ball mill (Fritsche Japan Co., Ltd., P-5 Classic Line) at a rotation speed of 300 rpm for 15 minutes. On the other hand, in Example 4, dry pulverization was carried out using a dry continuous ultrafine pulverizer. In this way, ferrite powders of Examples 1 to 15 were obtained. Examples 1 to 10 are example samples, and Examples 11 to 15 are comparative examples.

[0071] [Table 1]

[0072] (2) Evaluation The ferrite powders produced in Examples 1 to 15 were evaluated for various properties as follows.

[0073] <Chemical analysis> The ferrite powder was subjected to chemical analysis to determine the metal content. First, 0.2 g of ferrite powder was weighed, and 60 ml of pure water, 20 ml of 1N hydrochloric acid, and 20 ml of 1N nitric acid were added to the weighed ferrite powder. The mixture was then heated to prepare an aqueous solution in which the sample was completely dissolved. The resulting aqueous solution was placed in an ICP analyzer (Shimadzu Corporation, ICPS-10001V) to measure the metal content.

[0074] <Particle size distribution> The particle size distribution of ferrite powder was measured as follows. First, 10 g of ferrite powder and 80 ml of water were placed in a 100 ml beaker, and two drops of sodium hexametaphosphate (dispersant) were added. The mixture was then dispersed using an ultrasonic homogenizer (UH-150, SMT Corporation). The output level of the ultrasonic homogenizer was set to 4, and the dispersion was carried out for 20 seconds. After that, bubbles formed on the surface of the homogenizer were removed, and the resulting dispersion was introduced into a laser diffraction particle size analyzer (SALD-7500nano, Shimadzu Corporation) for measurement. The measurement conditions were a pump speed of 7, an internal ultrasonic irradiation time of 30, and a refractive index of 1.70-050i. From this measurement, the 10% diameter (D10), 50% diameter (volume average particle size, D50), and 90% diameter (D90) in the volume particle size distribution were determined.

[0075] <Particle shape> The shape of particles in the ferrite powder was evaluated as follows. First, the ferrite powder was observed using a scanning electron microscope (SEM; Hitachi High-Technologies Corporation, SU-8020). During the observation, the magnification was set to 50,000 times. Then, an image was taken so that 1 to 30 particles, preferably 1 to 10 particles, were included in the field of view. At this time, 10 fields of view were photographed at random, and the particle shapes were examined.

[0076] <shape factor> The shape factors (SF-1 and SF-2) of the ferrite powder were determined using a particle image analyzer (Malvern Panalytical, Morphologi G3). First, the ferrite powder was analyzed using the particle image analyzer. During the analysis, image analysis was performed on each of 1,000 particles in the powder, and the circularity, perimeter, and equivalent circle diameter (CE diameter) were automatically measured. In this case, a 50x magnification objective lens was used for samples with an average particle size of less than 20 μm, and a 10x magnification objective lens was used for samples with an average particle size of 20 μm or more. The sample size was 3 mm. 3 The particles were dispersed on a glass slide using a dispersion tool attached to the device under a dispersion pressure of 5 bar.

[0077] Of the obtained data, the averages of the data for particles within ±5% of the volume average particle size were calculated as the average circularity, average perimeter, and average circle equivalent diameter (CE diameter), and these were used to calculate SF-1 and SF-2 according to the following equations (1) and (2).

[0078]

number

number

[0079] <xrd> The ferrite powder was analyzed by X-ray diffraction (XRD) under the following conditions.

[0080] - X-ray diffraction equipment: PANalytical X'pert MPD (including high-speed detector) - Source: Co-Kα -Tube voltage: 45kV -Tube current: 40mA - Entrance divergence slit: 0.04rad - Fixed divergence slit: 0.5° - Anti-scatter slit: 5.5mm - Receiving divergence slit: 0.04rad - Receiving slit: 0.15 mm

[0081] Next, the composition ratio of each crystal structure was calculated based on the obtained XRD profile. Specifically, the following crystal structure was assumed, and the following parameters were optimized using analysis software (RIETAN-FP u2.83). The profile function used was the pseudo-Voigt function of Thompson, Cox, and Hasting, and the composition ratio was calculated from the results of asymmetrization using Howard's method.

[0082] Lithium ferrite: LiFe5O8 Crystal structure: Space group P4132(No.213) <Atomic coordinates> Li:4a 3 / 8 3 / 8 3 / 8 Fe:8c xxx Fe:12d 1 / 8 y y+1 / 4 O: 8c xxx O:24e xyz

[0083] Lithium manganese ferrite: LiMn 1.5 Fe 0.5 O4 Crystal structure: Space group Fd-3m (No.227) <Atomic coordinates> Mn:8b 3 / 8 3 / 8 3 / 8 Fe:16c 0 0 0 O :32e xxx

[0084] The composition ratio of each structure was calculated from the metal component content and the composition ratio of each crystal structure. Specifically, the amount of Mn was A, the amount of Li was B, the amount of Fe was C, the composition ratio of the Fd-3m phase was X, the composition ratio of the P4132 phase was Y, and the lattice constant of the Fd-3m phase was Z. 1.5 Fe 0.5 Composition ratio D of O4(Fd-3m) phase, Li 0.5 Fe 2.5 The composition ratio E of the O4 (Fd-3m) phase, the composition ratio F of the LiFe5O8 (P4132) phase, and the composition ratio G of the Fe3O4 (Fd-3m) phase were calculated according to the following formulas (3) to (6).

[0085] D=A / 1.5 (3) E = (BD) 2X (4) F = (BD) Y (5) G=(C-0.5D-2.5E-5F) / 3 ···(6)

[0086] The theoretical lattice constant H of the Fd-3m phase and the lattice constant error rate of the Fd-3m phase were calculated using the following equations (7) and (8). As a result, it was confirmed that the lattice constant error rate was within 0.6% for all samples.

[0087] H=(8.412D+8.338E+8.395G) / (D+E+G) ···(7) Lattice constant measurement error rate (%)=|ZH| / Z×100 ···(8)

[0088] <Magnetic properties - saturation magnetization, remanence and coercive force> The magnetic properties (saturation magnetization, remanence, and coercivity) of the ferrite powder were measured as follows. First, 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 5 kOe, and then the applied magnetic field was decreased to plot a hysteresis curve. From the data of this curve, the saturation magnetization σs, remanence σr, and coercivity Hc of the ferrite powder were determined.

[0089] <Electromagnetic wave absorption performance> A ferrite resin composite (electromagnetic wave absorber) was prepared using ferrite powder as a filler, and its electromagnetic wave absorption performance was evaluated. Specifically, 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 obtained ferrite resin composite material was then filled into a mold and heated at 180°C for 2 hours while pressure molding using a press machine to prepare a composite. The dimensions of the obtained composite were 120 mm x 120 mm x 3 mm.

[0090] Next, the reflection coefficient (S 11 ) and the permeability coefficient (S 21 The frequency characteristics of the composite were determined and its electromagnetic wave absorption performance was evaluated. A network analyzer (Keysight Technologies, N5253E2) and a free space measurement device (EM Lab, FS-330) were used as measurement equipment. The incident angle was set to 0° and the sweep frequency was set to 17 GHz to 42 GHz, and the S parameters were measured. From the obtained S parameters, the peak frequency of electromagnetic wave absorption of the composite in the range of 17 to 42 GHz, the reflection coefficient (S 11 ) and the permeability coefficient (S 21 The S-parameter analysis was performed using the Keysight Materials Measurement Suite N1500A attached to the instrument.

[0091] (3) Evaluation results The evaluation results obtained for the ferrite powders of Examples 1 to 15 are summarized in the following Tables 2 and 3. Examples 1 to 10 are example samples, and Examples 11 to 15 are comparative samples.

[0092] The samples of Examples 1 to 10 are in the crystalline phase (LiFe5O8, Li 0.5 Fe 2.5 O4, Fe3O4, LiMn 1.5 Fe 0.5 All of the content ratios of 04) satisfied the range specified in this embodiment. When the electromagnetic wave absorption performance was evaluated, the samples of Examples 1 to 10 exhibited absorption peaks in the frequency range around 28 GHz (23.2 to 27.2 GHz). In addition, the product of the depth and width of the absorption peak was relatively large, at 41.0 GHzdB or more. In particular, the samples of Examples 2 and 3 exhibited a significantly large product of the depth and width of the absorption peak, at 102.9 GHzdB or more.

[0093] In contrast, in the samples of Examples 11 to 15, some of the content ratios of the crystalline phase did not satisfy the range specified in this embodiment. When the electromagnetic wave absorption performance was evaluated, the product of the depth and width of the absorption peak was relatively small, at 33.3 GHz dB or less, in Examples 11 to 13 and 15. In Example 14, the product of the depth and width of the absorption peak was relatively large, at 82.7 GHz dB, but the absorption peak frequency was high, at 29.4 GHz.

[0094] [Table 2]

[0095] [Table 3]

[0096] From the above results, it can be seen that the ferrite powder of this embodiment exhibits excellent electromagnetic wave absorption performance in the frequency range around 28 GHz.< / xrd>

Claims

1. Li belonging to the space group Fd-3m ０．５ Fe ２．５ O ４ phase, space group P4 １ LiFe belongs to 32 ５ O ８ phase, Fe ３ O ４ phase, and LiMn １．５ Fe ０．５ O ４ phase, The Li ０．５ Fe ２．５ O ４ phase and the LiFe ５ O ８ The total content of the phases is 65 mol% or more and 80 mol% or less, The Fe ３ O ４ The content of the phase is 17 mol% or more and 27 mol% or less, The LiMn １．５ Fe ０．５ O ４ The ferrite powder has a content of the phase of 4.4 mol % or more and 10 mol % or less.

2. 2. The ferrite powder according to claim 1, comprising lithium (Li) in an amount of 1.4 mass% or more and 1.6 mass% or less, manganese (Mn) in an amount of 1.4 mass% or more and 3.3 mass% or less, and iron (Fe) in an amount of 64.5 mass% or more and 67.0 mass% or less.

3. The ferrite powder according to claim 1 or 2, having a volume average particle size D50 of 5 μm or more and 15 μm or less.

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

5. A ferrite-resin composite, which is a molded product of the ferrite-resin composite material of claim 4.

6. An electromagnetic wave absorber comprising the ferrite resin composite according to claim 5 .

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