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

Abstract

To provide a ferrite powder that exhibits excellent electromagnetic wave absorption performance in the frequency range near 28 GHz. Also, to provide a ferrite resin composite material, a ferrite resin composite, and an electromagnetic wave absorber containing this ferrite powder. [Solution] A ferrite powder containing lithium (Li) in a proportion of 0.6% to 2.5% by mass, manganese (Mn) in a proportion of 0.2% to 0.9% by mass, and iron (Fe) in a proportion of 61.0% to 67.7% by mass, wherein the ratio (I / I0) of the intensity of the (210) diffraction peak based on Li ferrite (I) to the intensity of the (311) diffraction peak based on Li ferrite (I0) in the X-ray diffraction profile is 0.01 to 0.12.

Description

[Technical Field]

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

[0002] With the rapid advancement of wireless communication technology and the increasing sophistication and diversification of needs for wireless services, communication speeds are becoming faster and data capacity is increasing (high-speed, high-capacity). In this context, commercial services for the fifth-generation mobile communication system (5G) began in the spring of 2020. 5G enables not only high speed and high capacity, but also high reliability, low latency, and massive simultaneous connections. Therefore, it is attracting great attention as an indispensable infrastructure for realizing an IoT society. For example, using 5G will enable remote robot operation and instantaneous transmission and reception of information from numerous devices. Consequently, it is considered a key technology for advancing the practical application of telemedicine and autonomous driving.

[0003] Incidentally, electromagnetic interference (EMI) is a persistent problem in communication systems. Especially with the increasing frequency of 5G, the circuit length in electronic devices and the wavelength of electromagnetic waves are in close proximity, making the effects of EMI particularly pronounced. To prevent problems such as equipment malfunction or adverse effects on the human body due to EMI, the use of electromagnetic wave absorbers (radio wave absorbers) is effective. Ferrite powder is widely used as a material for electromagnetic wave absorbers.

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

[0012] of Patent Document 1).

[0005] Furthermore, although not used for electromagnetic wave absorption applications, Patent Document 2 discloses a carrier core material for electrophotographic developers, which consists of Li ferrite, magnetite, and Fe3O4, with a portion of which is substituted with Mn, having a Li content of 1 to 2.5% by weight, a Mn content of 2 to 7.5% by weight, and further containing 25 to 10,000 ppm of silicon (Claim 1 of Patent Document 2). [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2005-268368 [Patent Document 2] Japanese Patent Publication No. 2009-244571 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] While the application of ferrite powder as an electromagnetic wave absorber has been proposed for some time, conventional ferrite powders for electromagnetic wave absorbers have been insufficient in performance for use in 5G-compatible electromagnetic wave absorbers. Specifically, 5G introduces a new wireless communication method using frequencies near 28 GHz, in addition to existing frequencies below 6 GHz. Therefore, it is desirable to exhibit excellent electromagnetic wave absorption performance in the frequency range near 28 GHz, and more specifically, to show a deep electromagnetic wave absorption peak (sometimes simply called an "absorption peak") in this frequency range.

[0008] In addition, the frequency bands allocated for 5G differ by country or by telecommunications carrier. For example, focusing on frequencies around 28 GHz, in Japan, the frequency band of 27 to 29.5 GHz is allocated, and different frequencies within this range are assigned to each carrier. In the United States, the frequency band of 22.5 to 28.35 GHz is allocated, in Europe, the frequency band of 24.25 to 27.5 GHz is allocated, in China, the frequency band of 24.75 to 27.5 GHz is allocated, and in South Korea, the frequency band of 26.5 to 29.5 GHz is allocated. Therefore, it is desirable to use an electromagnetic wave absorber that can be used across countries and carriers, that is, an absorber with excellent versatility. For this purpose, it is desired not only that the absorption peak is deep, but also that the absorption frequency band, that is, the width of the absorption peak is appropriately wide. However, conventional ferrite powders have not been sufficient in meeting these requirements.

[0009] In view of such problems, the inventors of the present invention conducted studies. As a result, they obtained the knowledge that it is important to control the types and ratios of crystal phases contained in the ferrite powder. And by controlling the types and ratios of crystal phases within a predetermined range, they obtained the knowledge that a ferrite powder showing excellent electromagnetic wave absorption performance in the frequency band around 28 GHz, particularly in the frequency band of 20 to 30 GHz, can be obtained.

[0010] The present invention has been completed based on such knowledge, and an object thereof is to provide a ferrite powder showing excellent electromagnetic wave absorption performance in the frequency band around 28 GHz. Another object of the present invention is also to provide a ferrite resin composite material, a ferrite resin composite, and an electromagnetic wave absorber containing this ferrite powder.

Means for Solving the Problems

[0011] The present invention includes the following aspects (1) to (5). In this specification, the expression "~" includes the values at both ends. That is, "X~Y" is synonymous with "X or more and Y or less".

[0012] (1) It contains lithium (Li) at a ratio of 0.6% by mass or more and 2.5% by mass or less, manganese (Mn) at a ratio of 0.2% by mass or more and 0.9% by mass or less, and iron (Fe) at a ratio of 61.0% by mass or more and 67.7% by mass or less. In the X-ray diffraction profile, the ratio (I / I0) of the (210) diffraction peak intensity (I) of lithium (Li) ferrite to the (311) diffraction peak intensity (I0) of Li ferrite is 0.01 or more and 0.12 or less. Ferrite powder.

[0013] (2) The ferrite powder according to (1) above, having a volume average particle diameter (D50) of 1 μm or more and 20 μm or less.

[0014] (3) A ferrite resin composite material containing the ferrite powder according to (1) or (2) above and a resin.

[0015] (4) A ferrite resin composite, which is a molded body of the ferrite resin composite material according to (3) above.

[0016] (5) An electromagnetic wave absorber including the ferrite resin composite according to (4) above.

Advantages of the Invention

[0017] According to the present invention, ferrite powder showing excellent electromagnetic wave absorption performance in the frequency range near 28 GHz is provided. Further, a ferrite resin composite material, a ferrite resin composite, and an electromagnetic wave absorber containing this ferrite powder are provided.

Embodiments for Carrying Out the Invention

[0018] Specific embodiments 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 embodiments, and various modifications are possible without changing the gist of the present invention. Also, as long as technical consistency can be achieved, any combination of the preferred embodiments disclosed in this specification can be adopted. For example, any combination of one and the other of the preferred numerical ranges can be made.

[0019] <<1. Ferrite Powder>> The ferrite powder of this embodiment contains lithium (Li) ferrite as the main component (main crystal phase). Li ferrite has a general formula: Li 0.5 Fe 2.5 O4, and is a type of soft magnetic ferrite with a cubic spinel crystal structure having a basic composition represented by it, and is characterized by a high Curie temperature. In Li ferrite, there are two different crystal phases, namely, an α-phase (hereinafter referred to as the "P4132 phase") belonging to the space group P4132 and a β-phase (hereinafter referred to as the "Fd-3m phase") belonging to the space group Fd-3m. It is known that the α-phase (P4132 phase) of Li ferrite is also called an ordered spinel phase, where iron ions (Fe 3+ ) occupy the octahedral 12d sites and tetrahedral 8c sites of the cubic simple lattice, and lithium ions (Li + ) occupy the octahedral 4b sites. In contrast, the β-phase of Li ferrite has an irregular (disordered) structure. The β-phase (Fd-3m phase) has an inverse spinel structure, where Fe 3+ ions occupy the tetrahedral 8a sites, and Li + ions and Fe 3+ ions randomly occupy the octahedral 16d sites. By including these two phases of Li ferrite, namely, the α-phase (P4132 phase) and the β-phase (Fd-3m phase), in a predetermined ratio, excellent electromagnetic wave absorption performance can be imparted to the ferrite powder. Specifically, in the frequency range near 28 GHz, the product of the absorption peak depth and width based on natural resonance becomes high. In this specification, the main component means the component with the largest ratio based on mass. That is, the content ratio of the main component (Li ferrite) in the ferrite powder is 50% by mass or more.

[0020] In this embodiment, the ferrite powder exhibits an X-ray diffraction profile in which the ratio (I / I0) of the (210) diffraction peak intensity (I) of Li ferrite to the (311) diffraction peak intensity (I0) of Li ferrite is between 0.01 and 0.12. Here, the (210) diffraction peak is a peak based on the P4132 phase of Li ferrite. The (311) diffraction peak is a peak based on two phases of Li ferrite: the P4132 phase and the Fd-3m phase. By using the ratio (I / I0) of the (210) diffraction peak intensity I to the (311) diffraction peak intensity I0 as an indicator, the proportion of the P4132 phase in Li ferrite can be estimated.

[0021] If the ratio (I / I0) is within the aforementioned range (0.01 to 0.12), the P4132 phase exists in the Li ferrite in an optimal state (proportion), resulting in a desired absorption peak depth. In other words, ferrite powder with a moderate amount of P4132 phase contains a moderate amount of the secondary Fd-3m phase along with the main P4132 phase, making it difficult for the magnetic moment to rotate in response to an externally applied magnetic field. As a result, combined with the influence of adjacent magnetic moments, the electromagnetic wave absorption performance based on natural resonance is enhanced. Conversely, if the ratio (I / I0) is less than 0.01, the P4132 phase is excessively low, resulting in a smaller absorption peak depth. Also, if the ratio (I / I0) is greater than 0.12, the P4132 phase is excessively high, resulting in a structure (LiMn) due to the Mn composition. 1.5 Fe 0.5 The amount of O4 phase, MnFe2O4 phase, etc. decreases. As a result, the absorption peak depth also decreases. From the viewpoint of increasing the absorption peak depth, a ratio (I / I0) of 0.03 to 0.10 is more preferable, and 0.06 to 0.09 is even more preferable.

[0022] The peak intensities (I, I0) are the heights of the diffraction peaks determined by X-ray diffraction analysis using Co-Kα as the source. Specifically, they can be determined by the method described in the examples below or a similar method. In the X-ray diffraction profile using Co-Kα as the source, the (210) diffraction peak of Li ferrite appears in the region 2θ = 27 to 29°. The (311) diffraction peak appears in the region 2θ = 40 to 42°.

[0023] The ferrite powder of this embodiment mainly contains Li ferrite (P4132 phase, Fd-3m phase). However, this does not exclude components other than Li ferrite (crystalline phases). Such crystalline phases include magnetite (Fe3O4) phase and LiMn 1.5 Fe 0.5 Examples include the O4 phase. However, from the viewpoint of fully utilizing the excellent electromagnetic wave absorption performance based on Li ferrite, a high proportion of Li ferrite is desirable. The content of the P4132 phase (α phase) of Li ferrite is preferably 5% by mass or more, more preferably 40% by mass or more and 70% by mass or less, even more preferably 50% by mass or more and 70% by mass or less, and particularly preferably 50% by mass or more and 60% by mass or less. Note that magnetite phase (Fe3O4) phase and LiMn 1.5 Fe 0.5 The O4 phase and the MnFe2O4 phase can form the Fd-3m phase together with the β phase of Li ferrite. In other words, the Fd-3m phase consists of the β phase of Li ferrite, as well as small amounts of magnetite phase (Fe3O4) and LiMn 1.5 Fe 0.5 It may contain an O4 phase and a MnFe2O4 phase.

[0024] The ferrite powder of this embodiment contains lithium (Li) in a ratio of 0.6% to 2.5% by mass, manganese (Mn) in a ratio of 0.2% to 0.9% by mass, and iron (Fe) in a ratio of 61.0% to 67.7% by mass. If the ferrite powder composition is within this range, the proportions of each structure (P4132 phase, Fd-3m phase) can be appropriately controlled. Therefore, it becomes possible to further improve electromagnetic wave absorption performance.

[0025] If the Li content is less than 0.6 mass%, the formation of the P4132 phase (Li ferrite) is suppressed, which may result in a smaller absorption peak depth. If the Li content is greater than 2.5 mass%, the Fe content is relatively low, so the formation of the P4132 phase does not progress, which may also result in a smaller absorption peak depth. If the Mn content is less than 0.2 mass%, the formation of the P4132 phase proceeds excessively, and the structure due to the Mn composition is reduced, which may result in a smaller absorption peak depth. If the Mn content is greater than 0.9 mass%, the Li and Fe content are relatively low, so the formation of the P4132 phase does not progress, resulting in a smaller absorption peak depth. If the Fe content is less than 61.0 mass%, the formation of the P4132 phase is suppressed, resulting in a smaller absorption peak depth. If the Fe content is greater than 67.7 mass%, the Li content is relatively low, so the formation of the P4132 phase does not progress, which may also result in a smaller absorption peak depth.

[0026] From the viewpoint of further improving electromagnetic wave absorption performance, the ferrite powder preferably contains Li in a proportion of 1.0% to 2.5% by mass, Mn in a proportion of 0.5% to 0.9% by mass, and Fe in a proportion of 61.5% to 67.0% by mass. More preferably, it contains Li in a proportion of 1.5% to 2.5% by mass, Mn in a proportion of 0.7% to 0.9% by mass, and Fe in a proportion of 62.0% to 66.0% by mass. Note that ferrite is a ceramic mainly composed of iron oxide. Therefore, the ferrite powder of this embodiment consists of an oxide containing iron (Fe), lithium (Li), manganese (Mn), and oxygen (O) as essential components.

[0027] The ferrite powder of this embodiment may contain other components besides the Li, Mn, Fe, and O mentioned above. Other components, though not limited to them, include strontium (Sr), calcium (Ca), copper (Cu), zinc (Zn), silicon (Si), and / or chlorine (Cl). However, in order to fully utilize the electromagnetic wave absorption performance based on the desired crystalline phase, it is desirable that the amount of other components not be excessively high. The amount of other components is preferably 1.0% by mass or less, more preferably 0.7% by mass or less, and even more preferably 0.5% by mass or less. In particular, the Si content 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, can increase the hardness of ferrite particles and make grinding difficult, so it is preferable to include them as little as possible. Specifically, the content of V and Bi is preferably 100 ppm or less.

[0028] Preferably, the volume-average particle size (D50) of the ferrite powder is 1 μm or more and 20 μm or less. If D50 is within this range, it becomes possible to produce a ferrite resin composite (electromagnetic wave absorber) with even better electromagnetic wave absorption performance. Specifically, if D50 is 1 μm or more, the increase in material viscosity when kneading the ferrite powder with the resin to produce the composite can be suppressed. 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. It is possible to reduce the amount of ferrite powder to lower the viscosity, but in that case, the electromagnetic wave absorption performance will decrease. Also, if D50 is 20 μm or less, the voids between the particles (ferrite particles) constituting the ferrite powder contained in the composite become smaller, so the electromagnetic wave absorption performance is further improved. Conversely, if the voids between particles are large, it may be difficult to adjust the absorption peak frequency within the desired range. D50 is more preferably 2 μm or more and 18 μm or less, and even more preferably 4 μm or more and 15 μm or less.

[0029] Preferably, the bulk density of the ferrite powder is 1.15 g / cm³. 3That concludes the explanation. When the bulk density is outside this range, the particles become porous and contain internal voids and pores, which may reduce their electromagnetic wave absorption performance. The bulk density referred to here is the initial bulk density value measured in accordance with JIS R 1628:1997.

[0030] Preferably, the saturation magnetization (σs) of the ferrite powder is 61 emu / g or more and 66 emu / g or less. Also preferably, the remanent magnetization (σr) of the ferrite powder is 3 emu / g or more and 10 emu / g or less. Ferrite powder with an excessively small σs may contain other phases, such as non-magnetic or antiferromagnetic phases that have not been ferriteized. Such other phases may reduce the electromagnetic wave absorption performance. By setting σs to 61 emu / g or higher, the influence of other phases can be prevented, and the electromagnetic wave absorption performance can be further improved. Furthermore, σs is related to the particle size of the ferrite powder. Ferrite powder with a σs of 66 emu / g or less tends to have a moderately small particle size. In ferrite resin composites made from ferrite powder, the interparticle voids become smaller, which further improves the electromagnetic wave absorption performance. From the viewpoint of further improving electromagnetic wave absorption performance, σs is more preferably 61 emu / g or more and 65 emu / g or less, and even more preferably 61 emu / g or more and 63 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 equal to 1 A·m 2 This corresponds to / kg.

[0031] Preferably, the coercivity Hc of the ferrite powder is between 40Oe and 75Oe. Hc is related to the particle size of the ferrite powder. Ferrite powder with an Hc of 40Oe or more tends to have a moderately small particle size. This reduces the voids between ferrite particles contained in the ferrite resin composite, further improving electromagnetic wave absorption performance. Also, ferrite powder with an excessively high Hc may contain non-ferrite phases. By keeping Hc below 75Oe, further improvement in electromagnetic wave absorption performance can be achieved. From the viewpoint of further improving electromagnetic wave absorption performance, an Hc of 50Oe to 75Oe is more preferable, and 55Oe to 75Oe is even more preferable. Note that "Oe" is the unit of magnetic field strength in the cgs system, and 1Oe is (1 / 4π) × 10 3 This corresponds to A / m.

[0032] Reflectance coefficient (S 11 In the frequency characteristics of the ferrite powder, the ferrite powder preferably has a peak in the frequency range of 20 GHz to 30 GHz, more preferably 24 GHz to 28.5 GHz, and even more preferably 25 GHz to 28.5 GHz. Ferrite powder having an absorption peak in this frequency range has high industrial value. Note that the reflection coefficient (S 11 ) is an S-parameter representing the reflected signal, and is measured using the free-space method.

[0033] Reflectance coefficient (S 11 In the frequency characteristics of the ferrite powder, the absorption peak depth is preferably 5 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 value. The absorption peak depth is determined by the reflection coefficient (S) at the absorption peak. 11 This is the absolute value of the minimum (peak) value of ).

[0034] Reflectance coefficient (S 11In the frequency characteristics of the ferrite powder, the absorption peak width is preferably 3 GHz or higher, more preferably 3.5 GHz or higher, and even more preferably 4 GHz or higher. Ferrite powder with a large absorption peak width has high industrial value. Note that the absorption peak width is the width of the absorption peak at a depth of 5 dB.

[0035] Reflectance coefficient (S 11 In the frequency characteristics of the ferrite powder, the product of the absorption peak depth and width is preferably 20 GHz dB or higher, more preferably 40 GHz dB or higher, and even more preferably 60 GHz dB or higher. Ferrite powders with a large product of absorption peak depth and width have high industrial value. Note that the product of absorption peak depth and width is the product of absorption peak depth and absorption peak width.

[0036] The ferrite powder of this embodiment has the characteristic of exhibiting a moderately broad and deep absorption peak in the frequency range near 28 GHz. Therefore, by using this ferrite powder, an electromagnetic wave absorber can be obtained that has both excellent electromagnetic wave absorption performance and versatility in 5G.

[0037] <<2. Method for producing ferrite powder>> The ferrite powder of this embodiment is not limited in its manufacturing method as long as it satisfies the requirements described above. However, it is preferable to manufacture the ferrite powder by the following method. The preferred manufacturing method comprises the steps of: mixing iron (Fe) raw material, manganese (Mn) raw material, and lithium (Li) raw material to obtain a raw material mixture (raw material mixing step); calcining the obtained mixture to obtain a calcined product (calcination step); crushing and granulating the obtained calcined product to obtain granules (granulation step); debinding the obtained granules to obtain a debinding product (debinding step); and firing the obtained debinding product to obtain a calcined product (calcination step). In addition, if necessary, a step of crushing, classifying and / or pulverizing the calcined product (post-processing step) may be provided. Details of each step are described below.

[0038] <Raw material mixing process> In the raw material mixing process, iron (Fe), manganese (Mn), and lithium (Li) raw materials 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) individually or in combination of two or more. The proportions of Fe, Mn, and Li raw materials should be determined so that the final ferrite powder has the desired composition and crystalline phase. Raw material mixing may be carried out using a known mixer such as a Henschel mixer, either dry or wet, or both.

[0039] <Calibration process> In the calcination process, the obtained mixture is calcined to obtain a calcined product. Calcination promotes the ferrite reaction of the raw material mixture, allowing for the production of a ferrite powder with a uniform composition. Calcination can be carried out by known methods. It can be done using a furnace such as a rotary kiln, continuous furnace, or batch furnace, under known conditions. For example, one possible condition is to hold the mixture at a temperature of 700°C to 1300°C for 2 to 12 hours in an atmosphere such as air.

[0040] <Granulation process> In the granulation process, the calcined material is crushed and granulated to obtain granules. The crushing method is not particularly limited. Known crushing machines such as vibratory mills, ball mills, or bead mills may be used, and the process may be carried out either dry or wet, or both. The granulation method may also be a known method. For example, water and a binder, along with additives such as a dispersant and / or defoamer as needed, may be added to the crushed calcined material to adjust the viscosity, and then granulation may be carried out using a granulator such as a spray dryer. As the binder, polyvinyl alcohol (PVA), polyvinylpyrrolidone, and / or resin binders such as acrylic resins may be used, and the amount added is, for example, 0.05% by mass or more and 1.0% by mass or less in terms of solid content relative to the raw material mixture.

[0041] <Binder removal process> In the debindering process, the obtained granules are debindered to obtain a debindered product. In the debindering process, the granules are heated to decompose and remove organic components such as binders. The debindering process can be carried out using a known heating furnace. The heating should 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. Furthermore, the debindering process (heating) is not limited to, but should be carried out in an atmosphere with an oxygen concentration of 21% by volume or less.

[0042] <Firing Process> In the firing process, the debindered material is fired to obtain a fired product. The firing can be carried out using a known firing furnace such as a rotary kiln, a continuous furnace, or a 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 1125°C to 1250°C, and more preferably 1190°C to 1210°C. The oxygen concentration in the atmosphere during firing is preferably 0% to 5% by volume, and more preferably 0% to 2% by volume.

[0043] By setting the firing temperature and atmosphere within the above-mentioned range, it becomes possible to appropriately generate the P4132 phase of Li ferrite in the ferrite powder. If the firing temperature is too low, the generation of the P4132 phase will be insufficient, which may result in a small absorption peak depth. If the firing temperature is too high, the generation of the P4132 phase will proceed excessively, reducing the structure due to the Mn composition, which may also result in a small absorption peak depth. Similarly, if the oxygen concentration is too high, the generation of the P4132 phase may proceed excessively.

[0044] <Post-processing steps> If necessary, a post-processing step may be provided in which the resulting calcined material is crushed, classified, and / or pulverized. This makes it possible to adjust the particle size of the calcined material (ferrite powder). In particular, if the particle size of the ferrite powder is too small, it may be difficult to manufacture the ferrite resin composite (electromagnetic wave absorber). That is, when manufacturing the ferrite resin composite, the ferrite powder is kneaded with the resin. If the particle size of the ferrite powder is too small, the viscosity of the material increases, making kneading difficult. Also, if the particle size of the ferrite powder is too large, the voids between the ferrite particles in the composite become larger, which may reduce the electromagnetic wave absorption performance. Therefore, in such cases, it is preferable to adjust the particle size by post-processing the calcined material.

[0045] Crushing can be carried out using a crusher such as a hammer crusher. Classification can be carried out by methods such as air flow classification or sieving classification. Grinding can be carried out using known grinders such as vibratory mills, ball mills, or bead mills, either dry or wet, or both. However, if ferrite powder with the desired particle size can be obtained after the main calcination, the post-processing step is not essential.

[0046] In this way, the ferrite powder of this embodiment can be obtained.

[0047] <<3. Ferrite resin composite materials>> The ferrite resin composite material of this embodiment includes the ferrite powder and resin described above. The ferrite resin composite material is a precursor material for the ferrite resin composite that is a component of the electromagnetic wave absorber. That is, the composite is made by molding the resin composite material.

[0048] Examples of resins constituting the composite material include epoxy resins, urethane resins, acrylic resins, silicone resins, polyamide resins, polyimide resins, polyamide-imide resins, fluororesins, or combinations thereof. The silicone resin may be a modified silicone resin modified with acrylic, urethane, epoxy, and / or fluorine.

[0049] The composite material may contain components other than ferrite powder and resin. Examples of such components include solvents, fillers (organic fillers, inorganic fillers), plasticizers, antioxidants, dispersants, colorants such as pigments, and / or thermally conductive particles.

[0050] The proportion of ferrite powder to the total solid content in the composite material is preferably 50% to 95% by mass, and more preferably 80% to 95% by mass. Similarly, the proportion of resin to the total solid content in the composite material is preferably 5% to 50% by mass, and more preferably 5% to 20% by mass. By setting the proportions of ferrite powder and resin within the above ranges, the dispersion stability of the ferrite powder in the composite material, as well as the storage stability and moldability of the composite material, are improved, and the properties such as mechanical strength and magnetic properties of the composite (resin molded body) obtained by molding the composite material are also improved.

[0051] <<4. Ferrite resin composite>> The ferrite resin composite (resin molded body) of this embodiment is a molded body of the ferrite resin composition described above. That is, the composite is made by molding the ferrite resin composition. The molding method is not particularly limited and includes, for example, compression molding, extrusion molding, injection molding, blow molding, or calendering. Alternatively, a method of forming a coating film of the composite material on a substrate may also be used.

[0052] <<5. Electromagnetic wave absorber>> The electromagnetic wave absorber of this embodiment comprises the ferrite resin composite described above. The electromagnetic wave absorber may consist only of the composite, or it may include other components. For example, it may have an impedance matching layer or a surface protection layer on its surface. It may also have a reflective member on its back surface. An example of an impedance matching layer is a layer in which magnetic powder or dielectric powder is dispersed in a resin. An example of a surface protection layer is a layer made of resin or glass. Examples of reflective members include film-like, foil-like, or mesh-like metal members.

[0053] Reflectance coefficient (S 11In terms of frequency characteristics, the electromagnetic wave absorber preferably has a peak within a frequency range of 20 GHz to 30 GHz, more preferably 24 GHz to 28.5 GHz, and even more preferably 25 GHz to 28.5 GHz. Electromagnetic wave absorbers having an absorption peak within this frequency range have high industrial value.

[0054] Reflectance coefficient (S 11 In the frequency characteristics of the electromagnetic wave absorber, the absorption peak depth is preferably 5 dB or more, more preferably 8 dB or more, and even more preferably 10 dB or more. Electromagnetic wave absorbers with a large absorption peak depth have high industrial value.

[0055] Reflectance coefficient (S 11 In the frequency characteristics of the electromagnetic wave absorber, the absorption peak width is preferably 3 GHz or higher, more preferably 3.5 GHz or higher, and even more preferably 4 GHz or higher. Electromagnetic wave absorbers with a large absorption peak width have high industrial value. Note that the absorption peak width is the width of the absorption peak at a depth of 5 dB.

[0056] Reflectance coefficient (S 11 In the frequency characteristics of the electromagnetic wave absorber, the product of the absorption peak depth and width is preferably 20 GHz dB or higher, more preferably 40 GHz dB or higher, and even more preferably 60 GHz dB or higher. Electromagnetic wave absorbers with a large product of absorption peak depth and width have high industrial value.

[0057] The applications of electromagnetic wave absorbers are not limited as long as their purpose is electromagnetic wave absorption. For example, they can be 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 the frequency range near 28 GHz, and has the advantage of combining excellent electromagnetic wave absorption performance and versatility in 5G. Therefore, it is particularly suitable for 5G-compatible communication equipment and the transmission lines, high-frequency circuits, and / or electronic components used therein. [Examples]

[0058] The present invention will be described in more detail using the following examples and comparative examples. However, the present invention is not limited to the following examples.

[0059] (1) Preparation of ferrite powder [Examples 1-13] Using α-iron oxide (α-Fe2O3), trimanganese tetroxide (Mn3O4), and lithium carbonate (Li2CO3) as raw materials, these were weighed and mixed according to the amounts shown in Table 1 below. Next, water was added to the resulting mixture to a solid content of 50% by mass, and after grinding in a bead mill, the mixture was pre-granulated using a spray dryer. The resulting pre-granulated material was then calcined in air at 1000°C for 2 hours.

[0060] Water was added to the calcined product to achieve a solid content concentration of 50% by mass. Then, PVA as a binder and an aliphatic polyhydric alcohol-based polyether polyol as an antifoaming agent were added and the mixture was ground in a bead mill to obtain a slurry. The amount of binder added was 0.6% by mass relative to the solid content in the slurry. The amount of antifoaming agent added was 70 cc per 30 kg of solid content in the slurry. Next, the resulting pulverized slurry was granulated using a spray dryer to obtain granules.

[0061] The obtained granules were subjected to a debinder treatment to remove organic components, and then calcined. The debinder treatment and calcination were carried out under the conditions shown in Table 1 below. Next, the obtained calcined material was crushed with a hammer crusher and then dry-milled. In this process, dry milling was performed using a planetary ball mill (Fritsche Japan Co., Ltd., P-5 Classic Line) at a rotation speed of 300 rpm for 15 minutes. In this way, ferrite powders of Examples 1 to 13 were obtained. Examples 1 to 6 are example samples, and Examples 7 to 13 are comparative example samples.

[0062] [Table 1]

[0063] (2) Evaluation The ferrite powders prepared in Examples 1-13 were evaluated for various properties as follows.

[0064] <Chemical analysis> The metal content of ferrite powder was determined by chemical analysis. 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 it. 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.

[0065] <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 2 drops of sodium hexametaphosphate were added as a dispersant. Next, the mixture was dispersed using an ultrasonic homogenizer (SMT Corporation, UH-150 model). The output level of the ultrasonic homogenizer was set to 4, and dispersion was performed for 20 seconds. After that, the bubbles formed on the surface of the beaker were removed, and the resulting dispersion was introduced into a laser diffraction particle size distribution analyzer (Shimadzu Corporation, SALD-7500nano) 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 50% diameter (volume average particle size (D50)) in the volume particle size distribution was determined.

[0066] <xrd> The ferrite powder was analyzed by X-ray diffraction (XRD). The analysis was performed under the following conditions.

[0067] - X-ray diffractometer: Panalytical X'pertMPD (including high-speed detector) - Source: Co-Kα - Tube voltage: 45kV -Tube current: 40mA - Incident divergence slit: 0.04 rad - Fixed divergence slit: 0.5° - Scatter prevention slit: 5.5 mm - Light-receiving divergence slit: 0.04 rad - Light-receiving slit: 0.15mm

[0068] Next, the composition ratios of each crystal structure were calculated based on the obtained XRD profiles. Specifically, the following crystal structures were assumed, and the following parameters were optimized using the analysis software (RIETAN-FP u2.83). The profile function was a pseudo-Voigt function of Thompson, Cox, and Hasting, and the composition ratios were determined from the results of asymmetric analysis using Howard's method.

[0069] Lithium iron rite: 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

[0070] 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 Li:16c 0 0 0 O :32e xxx

[0071] <Magnetic Properties - Saturation Magnetization, Remanent Magnetization, and Coercivity> The magnetic properties (saturation magnetization, remanent magnetization, and coercivity) of ferrite powder were measured as follows. First, 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 type magnetic measuring device (Toei Kogyo Co., Ltd., VSM-C7-10A). An applied magnetic field was swept up to 5 kOe, and then the applied magnetic field was decreased to generate a hysteresis curve. From the data of this curve, the saturation magnetization σs, remanent magnetization σr, and coercivity Hc of the ferrite powder were determined.

[0072] <Electromagnetic wave absorption performance> A ferrite resin composite (electromagnetic wave absorber) was fabricated 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. Powdered fluororesin was used as the binder resin. The mixing was performed using a sample mill. Next, the obtained ferrite resin composite material was filled into a mold, and a composite (resin molded body) was produced by heating at 180°C for 2 hours while press-molding with a press machine. The dimensions of the obtained composite were 120 mm × 120 mm × 3 mm.

[0073] Next, the reflection coefficient (S) is calculated from the incident and reflected energy of electromagnetic waves measured by the free-space method (FS method). 11 ) and transmission coefficient (S 21 The frequency characteristics of the composite (resin molded body) were determined, and the electromagnetic wave absorption performance was evaluated. For the measurement, a 1 mm thick aluminum plate was attached to one side of the fabricated composite (resin molded body), and the amount of attenuation when the incident electromagnetic wave passed through the composite, reflected off the aluminum plate, and returned after passing through the composite again was used to determine the S-parameter (S 11 ,S 21 The following was determined: A network analyzer (Keysight Technologies, N5253E2) and a free-space measuring device (EM Lab Co., Ltd., FS-330) were used as the measuring devices. The S-parameters were measured with an incident angle of 0° and a sweep frequency of 17 GHz to 42 GHz. The S-parameters were analyzed using the Keysight material measurement suite N1500A attached to the device.

[0074] And the reflection coefficient (S 11 The presence or absence of absorption peaks was examined from the frequency characteristics of the signal, and if absorption peaks were observed, the depth (in dB) and width (in GHz) of the absorption peaks were determined.

[0075] (3) Evaluation results The evaluation results obtained for the ferrite powders of Examples 1 to 13 are summarized in Tables 2 and 3 below. Examples 1 to 6 are example samples, and Examples 7 to 13 are comparative example samples.

[0076] The example samples (Examples 1-6) satisfied the range specified in this embodiment (0.01 to 0.12) for composition (Fe content, Mn content, Li content) and XRD peak intensity ratio (I / I0). When the electromagnetic wave absorption performance was evaluated, absorption peaks were observed in the frequency range of 20-30 GHz. Furthermore, the product of the depth and width of the absorption peak was relatively large, at 22.4 GHz dB or more. In particular, the sample in Example 4 had an extremely large product of the depth and width of the absorption peak, at 329.4 GHz dB or more.

[0077] In contrast, the comparative examples (Examples 7-13) did not satisfy the range of XRD peak intensity ratio (I / I0) specified in this embodiment. When the electromagnetic wave absorption performance was evaluated, the absorption peak depth was small, at a maximum of 4.7 dB. Therefore, it was not possible to determine the width of the absorption peak (width of the absorption peak at a depth of 5 dB) or the product of the depth and width of the absorption peak.

[0078] [Table 2]

[0079] [Table 3]

[0080] From the above results, it is understood that the ferrite powder of this embodiment exhibits excellent electromagnetic wave absorption performance in the frequency range near 28 GHz.< / xrd>

Claims

1. A ferrite powder containing lithium (Li) in a proportion of 0.6% to 2.5% by mass, manganese (Mn) in a proportion of 0.2% to 0.9% by mass, and iron (Fe) in a proportion of 61.0% to 67.7% by mass, wherein the ratio (I / I0) of the (210) diffraction peak intensity (I) of Li ferrite to the (311) diffraction peak intensity (I0) of Li ferrite in the X-ray diffraction profile is 0.01 to 0.

12.

2. The ferrite powder according to claim 1, wherein the volume-average particle size (D50) is 1 μm or more and 20 μm or less.

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

4. A ferrite resin composite, which is a molded body of the ferrite resin composite material according to claim 3.

5. An electromagnetic wave absorber comprising the ferrite resin composite described in claim 4.

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