Ferrite powder and resin composition containing same

Abstract

A spherical ferrite powder according to the present invention has a median diameter D50 of 10 to 200 μm. The maximum height Rz on the particle surfaces of the ferrite powder is 1.5 μm or less, and the Heywood diameter of the crystal particle is in a range of 2.0 to 8.0 μm. For a 1-mm thick plate obtained by kneading and curing a resin and 60 vol% of the ferrite powder, the dielectric constant measured at 50 MHz is 1 to 20.

Description

Ferrite powder and resin composition containing same

[0001] The present invention relates to a ferrite powder and a resin composition containing the same.

[0002] Compacts containing resin and ferrite powder, a filler dispersed within the resin, are used for electronic components such as inductors, magnetic cores, and magnetic heads, as well as magnetic components such as magnetic shielding materials. These compacts, which have shapes such as sheets, are obtained by kneading the resin and ferrite powder and molding them into the desired shape. When the particles constituting the ferrite powder are nearly spherical, the fluidity during molding increases, improving moldability. This increases the filling rate of the ferrite powder within the resin, resulting in improved magnetic properties. Therefore, several techniques for forming ferrite powder into a spherical shape have been proposed. For example, Patent Document 1 describes a thermal spraying method in which a kneaded mixture of ferrite raw materials is pre-fired and granulated, resulting in granules that are then passed through a combustion flame. During thermal spraying, a ferritization reaction occurs, and a portion of the granules melts to form spherical ferrite particles.

[0003] Japanese Patent Application Laid-Open No. 2021-66648

[0004] The present inventors have found that while the technology of Patent Document 1 can produce ferrite powder with high sphericity, it has the problem of increasing the dielectric constant of the ferrite powder and causing the resonance frequency to appear in a low frequency band. Furthermore, they have found that this problem occurs because the ferrite structure is destroyed when the granulated material is passed through a high-temperature combustion flame of approximately 2000°C during thermal spraying, causing some of the metal elements to separate and precipitate from the ferrite structure.

[0005] An object of the present invention is to provide a ferrite powder or the like that has high fluidity when kneaded with a resin and that can keep the dielectric constant low.

[0006] In order to solve the above problems, the present invention provides the following techniques: [1] A median diameter D of 10 μm to 200 μm ５０[2] A spherical ferrite powder having the formula: wherein the maximum height Rz of the particle surface of the ferrite powder is 1.5 μm or less, and the Heywood diameter of the crystal grains is in the range of 2.0 μm to 8.0 μm, and a plate obtained by kneading 60 vol% of the ferrite powder with a resin and curing the mixture has a dielectric constant of less than 20 when measured at 50 MHz. ２ O ４ wherein A is at least one element selected from the group consisting of Mg, Mn, Co, Ni, Cu, and Zn. [3] A ferrite powder having a true density of 5.10 g / cm ３ [4] The ferrite powder according to [1] above, having a pore volume of 0.025 mL / g or less. [5] A resin composition containing the ferrite powder according to [1] above.

[0007] According to the present invention, the fluidity when kneaded with a resin is high, and the dielectric constant can be kept low.

[0008] 1 is a SEM photograph of the ferrite powder of Example 2 at 1,000x magnification. 2 is a SEM photograph of the ferrite powder of Example 2 at 5,000x magnification. 3 is a SEM photograph of the ferrite powder of Comparative Example 1 at 1,000x magnification. 4 is a SEM photograph of the ferrite powder of Comparative Example 1 at 5,000x magnification. 5 is a SEM photograph of the ferrite powder of Comparative Example 2 at 1,000x magnification. 6 is a SEM photograph of the ferrite powder of Comparative Example 2 at 5,000x magnification. 7 is a SEM photograph of the ferrite powder of Comparative Example 3 at 1,000x magnification. 8 is a SEM photograph of the ferrite powder of Comparative Example 3 at 5,000x magnification. 9 is a SEM photograph of the ferrite powder of Comparative Example 4 at 1,000x magnification. 10 is a SEM photograph of the ferrite powder of Comparative Example 4 at 5,000x magnification. 11 is a SEM photograph of the ferrite powder of Comparative Example 5 at 1,000x magnification. 12 is a SEM photograph of the ferrite powder of Comparative Example 5 at 5,000x magnification. 1 is a diagram schematically showing crystal grains on the surface of particles in ferrite powder.

[0009] <Ferrite Powder> As shown in FIG. 13, the ferrite powder 1 of this embodiment contains spherical ferrite particles 2 (hereinafter, may be simply referred to as "particles"). "Spherical" means that the two-dimensional projection image of the particle, which can be observed under a microscope or the like, is a shape that is close to a circle. For example, the circularity coefficient of the particle 2 is preferably 0.8 or more or 0.9 or more. The circularity coefficient is a value calculated based on the two-dimensional projection image of the particle 2, and is calculated as (4π × area inside the periphery of the particle) / (peripheral length of the periphery of the particle) ２ The circularity coefficient is calculated as the average value of the values ​​obtained for 50 particles 2 in the ferrite powder 1.

[0010] Median diameter D of ferrite powder 1 ５０ The median diameter D in the above range is 10 μm to 200 μm. ５０ The ferrite powder 1 having the median diameter D can be uniformly dispersed when kneaded into a resin or the like, and a resin composition highly filled with the ferrite powder 1 can be obtained. ５０ When the median diameter D is 10 μm or more, the particles 2 can be easily dispersed in the resin. ５０ Ferrite powder with a median diameter D of 50 μm or less is suitable for use in small electronic components. ５０ If the median diameter D is 200 μm or less, it is suitable for use as a sheet-shaped magnetic shielding material. ５０ is preferably 10 μm to 150 μm, more preferably 20 μm to 110 μm. The particle size distribution is measured by a laser particle size distribution meter in a dry state, more specifically, by a spray-type dry state. The particle size distribution is measured on a volume basis.

[0011] The maximum height Rz on the surface of particle 2 is 1.5 μm or less. The maximum height Rz is calculated by the following procedure: the particle surface is photographed with a microscope, and the obtained image is analyzed to determine a roughness curve. This roughness curve is sampled within a range of a reference length, and the sum of the highest part (maximum peak height) and the deepest part (maximum valley depth) within this range is the maximum height Rz. The maximum height Rz is preferably 0.05 μm to 1.0 μm, and more preferably 0.1 μm to 0.5 μm.

[0012] Each particle constituting the ferrite powder 1 is formed by a collection of multiple crystal grains 3 (FIG. 13). Between the crystal grains 3, there exists a boundary called a crystal grain boundary 4. The Heywood diameter (diameter equivalent to a circle with a projected area) of the crystal grains 3 on the surface of the particle 2 is 2.0 μm to 8.0 μm. The Heywood diameter is determined from an SEM image of the surface of the particle 2 using image analysis software, and is calculated as the average value of 50 crystal grains 3. The Heywood diameter of the crystal grains 3 is preferably 2.2 μm to 6.0 μm, more preferably 2.5 to 5.0 μm.

[0013] When the maximum height Rz on the surface of the particle 2 and the Heywood diameter of the crystal grain 3 are within the above ranges, it can be said that the surface of the particle 2 is smooth. When the ferrite powder 1 containing the spherical particles 2 with such smooth surfaces is used, the viscosity of the kneaded product obtained by kneading the resin and the ferrite powder 1 dispersed therein is low and the fluidity is high, so that high moldability can be obtained when a compact is formed from this kneaded product. Furthermore, the ferrite powder 1 is easily dispersed uniformly in the resin and easily packed at high density.

[0014] Furthermore, a 1 mm thick plate obtained by kneading 60 vol% ferrite powder 1 with a resin and curing the mixture has a dielectric constant of less than 20 when measured at 50 MHz. With a dielectric constant within this range, ferrite powder 1 can exhibit properties such as electromagnetic wave shielding performance over a wide frequency band. Furthermore, the fact that the dielectric constant is within this range means that ferrite powder 1 has a well-ordered ferrite structure, and the metal elements that make up ferrite powder 1 are contained within this structure.

[0015] A mixture of 30 vol% of the ferrite powder 1 and an epoxy resin having a viscosity of 1820 mPa·s was kneaded at 25°C at a shear rate of 200 S. －１ The viscosity of the epoxy resin alone is measured under the same conditions as those for the viscosity of the mixture kneaded with the ferrite powder.

[0016] The ferrite in the ferrite powder 1 is preferably a spinel ferrite. The spinel ferrite is AFe ２ O ４ where A is exemplified by at least one element selected from the group consisting of Mg, Mn, Co, Ni, Cu, and Zn. More specifically, Ni-Zn-Cu ferrite and Mn-Zn ferrite are exemplified. Ni-Zn-Cu ferrite has a lower dielectric constant than Mn-Zn ferrite, and is therefore often used in electronic components for high-frequency applications.

[0017] The true density of ferrite powder 1 is 5.10 g / cm ３ The true density is preferably 5.10 g / cm or more. The true density is measured by dry density measurement (gas displacement method). ３ With a true density of 5.15 g / cm or more, the magnetic permeability is increased, improving the performance as an electronic component or a magnetic shielding material. In addition, a true density in this range means that there are few voids in the particles constituting the ferrite powder 1. The true density of the ferrite powder 1 is more preferably 5.15 g / cm. ３ The theoretical upper limit of true density is 5.50 g / cm ３ is.

[0018] The pore volume of the ferrite powder 1 is preferably 0.025 mL / g or less. The pore volume is measured by mercury intrusion porosimetry. A pore volume of 0.025 mL / g or less can suppress an increase in viscosity of the kneaded product when the ferrite powder is kneaded with a resin. The pore volume of the ferrite powder 1 is more preferably 0.001 mL / g to 0.020 mL / g, and more preferably 0.002 mL / g to 0.010 mL / g.

[0019] <Resin Composition> The resin composition of this embodiment contains a resin and the above-described ferrite powder 1 dispersed in the resin.

[0020] Examples of the resin include epoxy resin, urethane resin, acrylic resin, silicone resin, various modified silicone resins (acrylic-modified, urethane-modified, epoxy-modified, fluorine-modified), polyamide resin, polyimide resin, polyamideimide resin, polyvinyl chloride, polyvinyl acetate, phenol resin, melamine resin, water-soluble polyester, fluorine resin, and the like, and one or more selected from these may be used in combination.

[0021] The resin composition is obtained by kneading (or mixing) a resin and the ferrite powder 1. The resin composition is molded into a shape according to the intended use and cured as necessary to form a molded article.

[0022] The content of the ferrite powder 1 in the resin composition can be changed depending on the purpose of use, the target properties, the shape of the molded body, etc. The content of the ferrite powder 1 is, for example, 50 mass % or more and 95 mass % or less.

[0023] <Method for producing ferrite powder> The above-mentioned ferrite powder 1 can be produced by a method comprising the following steps: (a) a step of obtaining a mixed powder by mixing raw materials in amounts corresponding to a target composition; (b) a step of obtaining a primary fired powder by primarily firing the mixed powder obtained in (a) above at 700°C to 850°C; (c) a step of obtaining spherical granules from the primary fired powder obtained in (b) above by spray drying; and (d) a step of secondary firing the granules obtained in (c) above at 900°C to 1000°C.

[0024] The mixing of the raw materials in the step (a) may be either dry or wet. When the step (a) includes a step of obtaining a slurry by wet mixing, the step (a) may further include a step of filtering the slurry and further drying it.

[0025] The temperature in the primary firing in step (b) is preferably 800° C. or lower.

[0026] Between the steps (b) and (c), a step of pulverizing the primary fired powder to make it finer and homogenous may be provided.

[0027] Furthermore, the above-mentioned step (c) may include a step of preparing a slurry by adding a solvent to the primary fired powder, and a step of spray-drying the slurry to obtain spherical granules. Water is a suitable solvent. In addition to the raw materials, the solvent may contain binders, dispersants, etc., as needed. Polyvinyl alcohol, for example, is a suitable binder. The binder content in the slurry is preferably about 0.1% to 2% by mass. Furthermore, suitable dispersants include ammonium polycarboxylate dispersants and methacrylic acid polymers. The dispersant content in the slurry is preferably about 0.1% to 2% by mass. Other additives that may be added include a pH adjuster such as ammonia, a lubricant, and a sintering accelerator. The solids concentration of the slurry is preferably in the range of 60% to 90% by mass. A solids concentration of 60% by mass or higher results in fewer intraparticle pores in the granules, preventing insufficient sintering during firing.

[0028] The spray dryer used in the spray drying may be either a nozzle type or a disk type. The ambient temperature during spray drying is preferably in the range of 100°C to 300°C. If the ambient temperature during spray drying is in the range of 100°C to 300°C, there will be few intra-particle pores in the granulated product, and insufficient sintering during firing can be prevented.

[0029] By setting the secondary firing temperature in the step (d) to 1000°C or less, a ferrite powder having a low dielectric constant can be obtained. This is because the ferrite structure is maintained even after firing due to the low firing temperature. By maintaining the ferrite structure, some of the metal elements, such as Fe, ３ O ４ , α-Fe ２ O ３ Alternatively, metal oxides and the like are not precipitated but are retained within the ferrite structure.

[0030] Furthermore, secondary firing at 1000°C or less produces spherical ferrite powder with high surface smoothness. Specifically, secondary firing at a low temperature reduces the growth rate of the ferrite structure, allowing the crystal grains to grow slowly and uniformly. As a result, the crystal grains can grow to a size that satisfies the above-mentioned Heywood diameter range while keeping the maximum height Rz of the particle surface small.

[0031] The reason why a spherical ferrite powder with a smooth surface and high density can be obtained even by such a low-temperature secondary firing is that the above-mentioned primary firing powder is fired at 700° C. to 850° C., is sufficiently ferritized, and is a powder that can be sintered even at a low secondary firing temperature of 1000° C. or less. The powder that can be sintered at a low temperature is, for example, a powder obtained by mixing 15 g of the primary firing powder with 1.5 mL of a 6.5% diluted PVA aqueous solution, which is placed in a mold with an outer diameter of 20 mm and an inner diameter of 10 mm, and pressed at 1 ton / cm using a press. ２ When the compact compressed with the above method was sintered at 900°C for 2 hours in air, the sintered density was 5.00g / cm ３ This is the powder.

[0032] Furthermore, by setting the secondary firing temperature to 900°C or higher, the density of the granulated material can be increased.

[0033] The secondary firing time may be set within a range in which sintering proceeds to obtain ferrite powder having the desired true density, and may be maintained for, for example, about 1 to 10 hours.

[0034] The method for producing a ferrite powder may further include a step of crushing the fired product after the step (d). The crushing may be performed in any manner that can disperse aggregated particles.

[0035] <Production of Ferrite Powder> [Example 1] Oxides of Fe, Ni, Zn, and Cu were weighed according to the composition in Table 1 and wet-mixed to obtain a slurry. The slurry was filtered and dried to obtain a mixed powder of raw materials. This mixed powder was subjected to primary firing at 760°C for 3 hours. The resulting fired product was pulverized in a vibration mill to obtain a primary fired powder of Ni-Zn-Cu ferrite. 15 g of the resulting primary fired powder was mixed with 1.5 mL of a 6.5% diluted PVA aqueous solution, and the mixture was placed in a mold with an outer diameter of 20 mmφ and an inner diameter of 10 mmφ, and pressed to a pressure of 1 ton / cm using a press. ２ The compact compressed at 900°C for 2 hours was sintered to obtain a sintered body with a density of 5.21 g / cm ３ This powder can be sintered at low temperatures.

[0036] An ammonium polycarboxylate dispersant was used at 1.0% by mass relative to the primary fired powder, and 25% by mass ammonia water was used as a pH adjuster at 0.5% by mass relative to the primary fired powder. The primary fired powder was added to water containing the ammonium polycarboxylate dispersant and ammonia water, stirred, and wet-pulverized to prepare a slurry with a concentration of 67% by mass. The resulting slurry was sprayed into hot air at 145°C using a spray dryer, yielding spherical granules.

[0037] The obtained granules were placed in an electric furnace and sintered at 950° C. for 3 hours. The sintered product was then crushed. The resulting powder was used as the ferrite powder of Example 1.

[0038] Example 2 Ferrite powder of Example 2 was prepared in the same manner as in Example 1, except that the particle size of the dried granules in the spray drying step was changed.

[0039] Example 3 A ferrite powder of Example 3 was prepared in the same manner as in Example 1, except that the particle size of the dried granules in the spray drying step was changed.

[0040] Example 4 A ferrite powder of Example 4 was prepared in the same manner as in Example 1, except that the particle size of the dried granules in the spray drying step was changed by spraying into hot air at 150°C using a spray dryer.

[0041] Example 5 A ferrite powder of Example 5 was prepared in the same manner as in Example 1, except that the particle size of the dried granules in the spray drying step was changed by spraying into hot air at 150°C using a spray dryer.

[0042] Comparative Example 1 Ferrite powder of Comparative Example 1 was prepared in the same manner as in Example 2, except that the electric furnace temperature in the secondary firing step was changed to 1200°C.

[0043] Comparative Example 2: Oxides of Fe, Ni, Zn, and Cu were weighed according to the composition in Table 1 and wet-mixed to obtain a slurry. The slurry was filtered and dried to obtain a mixed powder of raw materials. This mixed powder was subjected to primary firing at 1300°C for 3 hours. The resulting fired product was pulverized in a vibration mill to obtain a primary fired powder of Ni-Zn-Cu ferrite. The ferrite powder of Comparative Example 2 was prepared in the same manner as in Example 2, except that the primary fired powder, which was the raw material used, was changed and the amount of the polycarboxylate ammonium dispersant was changed to 1.5% by mass.

[0044] [Comparative Example 3] Oxides of Fe, Ni, Zn, and Cu were weighed according to the composition in Table 1 and wet-mixed to obtain a slurry. The slurry was filtered and dried. It was then crushed in a hammer mill and used for spray drying without primary firing. The ferrite powder of Comparative Example 3 was obtained in the same manner as in Example 2, except that the raw materials used were changed.

[0045] Comparative Example 4 The ferrite powder obtained in Example 5 was heated in a refrigerated oven with LP gas (flow rate 10 m ３ / hr) and oxygen (flow rate 45m ３ The treated sample was recovered to obtain the magnetic powder of Comparative Example 4.

[0046] Comparative Example 5 Oxides of Fe, Ni, Zn, and Cu were weighed according to the composition in Table 1 and wet-mixed to obtain a slurry. The slurry was filtered and dried to obtain a mixed powder of raw materials. This mixed powder of raw materials was subjected to primary firing at 1300°C for 3 hours. The obtained fired product was pulverized in a vibration mill to obtain a primary fired powder. The primary fired powder was then sintered in a liquefied petroleum gas (flow rate 10 m ３ / hr) and oxygen (flow rate 45m ３The powder after the flame treatment was used as the ferrite powder of Comparative Example 5.

[0047] <Evaluation> [Particle size distribution] The particle size distribution was measured by dispersing the ferrite powder at a pressure of 4 bar using a dry dispersion unit of a particle size distribution measuring device LMS-3000 (manufactured by Seishin Enterprise Co., Ltd.).

[0048] [Circularity Coefficient of Particles] The circularity coefficient was measured by taking an SEM image with a scanning electron microscope S-4800 (manufactured by Hitachi High-Technologies Corporation) and then performing image analysis using Mac-View Ver. 4 (manufactured by Mountec Co., Ltd.).

[0049] The circularity coefficient is (4π × area) / (perimeter) ２ The calculation was performed using the formula: Here, "area" means the area within the outline (periphery) of a particle constituting the ferrite powder when viewed from a predetermined direction, and "perimeter" means the perimeter (length of the periphery) of the outline. Specifically, the "area" in the formula for calculating the circularity coefficient was calculated as the area within the periphery obtained from the coordinates of the pixels forming the periphery of the particle, and the "perimeter" was calculated as the sum of the distances between pixels obtained from the coordinates of the pixels forming the periphery of the particle. 50 particles were randomly selected from the SEM image for use in the analysis, and the average of the measured circularity coefficient values ​​obtained from these particles was shown as the result.

[0050] [Maximum height Rz] Median diameter D of ferrite powder 1 measured by a laser microscope ５０ 0.8D ５０ ~1.2D ５０Particles with an acicular ratio of less than 1.5 were extracted, and their three-dimensional shapes were captured to determine Rz. Here, the acicular ratio is a parameter calculated from the ratio of the particle's maximum length to its diagonal width, and the diagonal width represents the shortest distance between two lines parallel to the maximum length when the particle is sandwiched between them. A laser microscope with an ultra-deep color 3D shape measurement microscope ("VK-X3000" manufactured by Keyence Corporation) was used. Measurements were performed using a 100x objective lens, a high-speed RPD scan pitch, and an RPDI Z-axis RPD algorithm. Surface observation and the three-dimensional shape were captured. Rz was measured using particle roughness inspection software ("WinROOF2023" manufactured by Mitani Shoji Co., Ltd.). From the obtained three-dimensional shapes, 21 roughness curves were obtained by drawing parallel lines across a 15 μm x 15 μm area on the particle surface. Surface correction was then performed, and a low-pass filter was applied at a 1.5 μm intensity, with a cutoff value λ of 80 μm. However, the median diameter D of the ferrite powder 1 ５０ When the particle surface area was less than 25 μm, Rz was determined by assuming the particle surface area to be 5 μm×5 μm.

[0051] Rz was calculated as the sum of the height of the highest peak and the depth of the deepest valley in the roughness curve. To calculate Rz, the average value of each parameter was used for 30 or more particles. The Rz measurement described above was performed in accordance with JIS B0601 (2001 edition).

[0052] [Heywood Diameter] The Heywood diameter of the crystal grains was calculated from the SEM image of the particle surface using image analysis software Mac-View (manufactured by Mountec Co., Ltd.) to obtain the average value of 50 crystal grains.

[0053] [True Density] The true density was measured using a dry automatic density meter Accupyc II 1340 (manufactured by Shimadzu Corporation).

[0054] [Pore Volume] The evaluation device used was a POREMASTER-60GT manufactured by Quantachrome. Specific measurement conditions were: Cell Stem Volume: 0.5 cm ３ Head pressure: 20 PSIA Surface tension of mercury: 485.00 erg / cm ２Contact angle of mercury: 130.00 degrees High pressure measurement mode: Fixed Rate Motor Speed: 1 High pressure measurement range: 20.00 to 10000.00 PSI 1.500 g of sample was weighed and measured at 0.5 cm ３ The volume B (cm) at 10,000 PSI was measured. ３ / g) to volume A (cm) at 60 PSI ３ The value obtained by subtracting the pore volume (1 / g) was taken as the pore volume.

[0055] [Specific surface area (BET)] The specific surface area was measured by the BET method using a Macsorb HM Model-1201 (manufactured by Mountec Co., Ltd.). The pretreatment conditions were heating at 120°C for 20 minutes while passing nitrogen gas through, followed by degassing. The results are shown in Table 2.

[0056] [Viscosity] The ferrite powder was kneaded with an epoxy resin (WE-2025, manufactured by Pelnox Co., Ltd.) so that the content of the ferrite powder was 30 vol %, and the mixture was stirred at 2000 rpm for 1 minute using a planetary centrifugal mixer "Awatori Rentaro ARE-310" (manufactured by Thinky Corporation).

[0057] Using an E-type viscometer (Toki Sangyo Co., Ltd. "TVE-35H" with a 3° × R9.7 cone rotor (0.2 mL of measurement sample)), the viscosity was measured at room temperature (25°C) at a rotation speed of 100 rpm (shear rate of 200 s －１ The viscosity of the paste after stirring was measured. When the viscosity of the epoxy resin alone was measured under the same conditions, it was 1820 mPa·s.

[0058] [Magnetic Permeability] The ferrite powder was kneaded with epoxy resin (WE-1264 / HV-127, manufactured by Pelnox) so that the content of the ferrite powder was 60 vol%, and the mixture was stirred at 2000 rpm for 1 minute using a planetary centrifugal mixer "Awatori Rentaro" ARE-310 (manufactured by Thinky Corporation). The resulting resin composition was sandwiched between glass plates to a thickness of 1 mm and cured at 120°C for 1 hour. The resulting plate was laser processed into a toroidal ring with an outer diameter of 20 mmφ and an inner diameter of 9 mmφ, which was used as a measurement sample.

[0059] The magnetic permeabilities μ′ and μ″ were measured using an impedance / material analyzer E4991A (manufactured by Agilent Technologies).

[0060] [Dielectric Constant] Under the same conditions as those for measuring the magnetic permeability, an epoxy resin and ferrite powder were kneaded and cured to obtain a plate with a thickness of 1 mm. The obtained plate was processed into a disk with a diameter of 20 mm by laser processing. The dielectric constants ε' and ε'' of this disk were measured using an impedance / material analyzer E4991A (manufactured by Agilent Technologies). The measurement results of viscosity, magnetic permeability, and dielectric constant are shown in Table 3.

[0061] [Observation by Scanning Electron Microscope (SEM)] The ferrite powders of Example 2 and Comparative Examples 1 to 5 were observed at 1000x and 5000x magnifications using an SEM S-4800 (manufactured by Hitachi High-Technologies Corporation). The results are shown in Figures 1 to 12.

[0062] <Results> As shown in Table 2, the ferrite powders of Examples 1 to 5 had a true density of 5.1 g / cm ３ The results are as above, and high magnetic permeability was observed.

[0063] In Examples 1 to 5, the specific surface area was 0.05 m ２ / g, which means that the surface is extremely smooth. In contrast, the specific surface area of ​​the comparative example was 0.05 m ２ / g or more.

[0064] Furthermore, when the ferrite powders were kneaded with a resin for viscosity measurement, the ferrite powders of Examples 1 to 5 exhibited lower viscosities than the ferrite powders of Comparative Examples 1 to 5. This means that the powder had high fluidity in the resin. Comparative Examples 2 and 3, which had particularly large pore volumes, exhibited particularly high viscosities.

[0065] In the measurements of magnetic permeability and dielectric constant, the viscosities of Comparative Examples 1 to 3 were too high to be kneaded with resin. Examples 1 to 5 and Comparative Examples 4 and 5 could be kneaded with resin and molded into plates, but Examples 1 to 5 exhibited high magnetic permeability and low dielectric constant, while Comparative Examples 4 and 5 exhibited low magnetic permeability and high dielectric constant.

[0066] As a result of SEM observation, the particles of Example 2 had a high circularity coefficient as shown in FIG. 1 and a smooth surface as shown in FIG.

[0067] In Comparative Example 1, the powder was granulated by spray drying and then fired at a high temperature, and many irregularly shaped particles were observed, with many irregularities on the particle surfaces (FIGS. 3 and 4).

[0068] Comparative Example 2 was primarily fired at a high temperature and had a high circularity coefficient, but many small holes (irregularities) were observed on the particle surfaces (FIGS. 5 and 6).

[0069] In Comparative Example 3, primary firing was not performed and firing was performed at a relatively low temperature after granulation. The circularity coefficient was high, but the crystal grain size was small and many pores (irregularities) were observed on the particle surfaces (FIGS. 7 and 8).

[0070] In Comparative Example 4, the powder of Example 5 was further exposed to high temperatures by a flame method. Although the circularity coefficient was high, fine irregularities were observed on the surface, and precipitation of metal elements was observed (FIGS. 9 and 10).

[0071] The powder in Comparative Example 5 was primarily fired at a high temperature, and then pulverized rather than spray-dried and exposed to high temperatures by the flame method. Although Comparative Example 5 had a high circularity coefficient (FIG. 11), fine irregularities were observed on the surface, and precipitation of metal elements (white particles in FIG. 12) was observed.

[0072] Furthermore, in Comparative Examples 4 and 5, the boundaries of the crystal grains (grain boundaries) could not be identified, and therefore the Heywood diameter could not be measured. This is thought to be because when secondary firing is performed by the flame method, the surfaces of the particles melt and then solidify.

[0073] As described above, in Examples 1 to 5, by performing primary firing at a relatively low temperature followed by spray drying and secondary firing at a relatively low temperature, it was possible to obtain particles with a high circularity coefficient, a small maximum surface height Rz, and a Heywood diameter within a preferred range, thereby obtaining fluidity suitable for molding. In addition, the molded bodies obtained from the ferrite powder and resin of Examples 1 to 5 exhibited low dielectric constants.

[0074]

[0075]

[0076]

[0077] The ferrite powder of the present invention can be used for electronic components such as inductors, magnetic cores, and magnetic heads, as well as magnetic members such as magnetic shielding materials.

[0078] 1 Ferrite powder 2 Particle 3 Crystal grain 4 Crystal grain boundary

Claims

1. Median diameter D of 10 μm to 200 μm ５０ A spherical ferrite powder having the formula: wherein the particle surface of the ferrite powder has a maximum height Rz of 1.5 μm or less and a Heywood diameter of the crystal grains in the range of 2.0 μm to 8.0 μm; and a plate obtained by kneading 60 vol % of the ferrite powder with a resin and curing the mixture has a dielectric constant of less than 20 when measured at 50 MHz.

2. In the ferrite powder, the ferrite is AFe ２ O ４ 2. The ferrite powder according to claim 1, wherein A is at least one element selected from the group consisting of Mg, Mn, Co, Ni, Cu, and Zn.

3. True density is 5.10 g / cm ３ The ferrite powder according to claim 1 , wherein the above-mentioned 4. The ferrite powder according to claim 1, having a pore volume of 0.025 mL / g or less.

5. A resin composition containing the ferrite powder according to any one of claims 1 to 4.

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