JP2009249739A - Metal magnetic particulate, method for producing the same and powder magnetic core - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide metal magnetic particulates having high saturation magnetization and also having excellent corrosion resistance, and to provide a method for producing the same. <P>SOLUTION: Oxide powder of Fe, compound powder containing an element(s) X (X denotes at least one selected from Al, Co, Ni and Si) and compound powder containing carbon are mixed, and the obtained powder is subjected to heat treatment (first heat treatment) within the range of 800 to 1,600°C in a nonoxidizing atmosphere, so as to form metal magnetic particulates including nuclear particles (comprising Fe and X) and a carbon film, and, after the first heat treatment, the metal magnetic particulates are subjected to heat treatment (second heat treatment) within the range of 400 to 750°C. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a magnetic recording medium such as a magnetic tape or a magnetic recording disk, a radio wave absorber, an electronic device such as an inductor or a printed board (soft magnetic body such as a yoke), a dust core, a photocatalyst, a magnetic bead for nucleic acid extraction, The present invention relates to metal magnetic fine particles used for medical microspheres and the like, and a method for producing the same.

  As electronic devices become smaller and lighter and have higher performance, materials used for electronic devices constituting electronic devices are also required to have higher performance and nano-size. For example, magnetic fine particles applied to a magnetic tape are required to be nanosized and to improve saturation magnetization simultaneously for the purpose of improving magnetic recording density.

  In addition, magnetic separation methods have been widely used as a method for recovering proteins such as antigens for diagnosing diseases such as allergies. Nano-sized magnetic beads having high saturation magnetization and excellent corrosion resistance have been widely used. There is an increasing demand. Patent Document 1 proposes a magnetic bead using metallic iron fine particles having higher saturation magnetization than iron oxide. Further, as the driving frequency of electronic equipment is increased, a high-performance magnetic material capable of realizing a highly efficient magnetic element even under a high frequency is required. For this reason, Patent Document 2 proposes an iron-based metal powder having a high saturation magnetic flux density.

Since metal magnetic fine particles have a larger saturation magnetization than oxide fine particles such as ferrite, they are highly expected for industrial use. For example, the saturation magnetization of metallic Fe is 218 Am ² / kg, which is very large compared to iron oxide, and therefore has an advantage of excellent magnetic field response and high signal intensity. However, metal fine particles such as metal Fe readily oxidize. For example, if they are formed into fine particles of 10 μm or less, they violently oxidize and burn in the atmosphere due to an increase in specific surface area, so it is difficult to handle them in the air in a dry state. Therefore, chemically stable oxide fine particles such as ferrite and magnetite are widely used.

  In order to prevent oxidation of metal magnetic fine particles, Patent Document 3 discloses a compound containing at least one of boron, nitrogen, and carbon as a main element by solid-phase reduction of an oxide of Fe with boron or carbon. It is coated with fine particles. However, in this method, the coating of the metal magnetic fine particles is not sufficient. For example, when used in an aqueous solution, the coating is corroded from an incomplete portion, and is not suitable for applications requiring high corrosion resistance such as magnetic beads.

  Moreover, in the metal Fe fine particles coated with carbon, the paramagnetic γ- (Fe, C) phase called austenite phase is stabilized by the solid solution of carbon in the core particles, or iron carbide is formed. There is a problem that the saturation magnetization is lowered. In particular, when the particle size of the metal fine particles is 10 μm or less, and further 1 μm or less, the γ- (Fe, C) phase is remarkably stabilized, and it is difficult to obtain fine particles with high saturation magnetization. As described above, a method excellent in industrial productivity that can produce metal particles having high saturation magnetization and high corrosion resistance at low cost is desired.

JP 2006-97123 A JP 2007-146259 A Japanese Patent Laid-Open No. 2005-89796

  Accordingly, an object of the present invention is to provide a metal magnetic fine particle having high saturation magnetization and excellent corrosion resistance, and a method for producing the same.

As a result of intensive studies in view of the above object, the present invention has been achieved.
The method for producing metal magnetic fine particles of the present invention includes a compound powder containing Fe oxide powder and element X (X is at least one selected from Al, Co, Ni and Si), and a compound powder containing carbon. And mix
The obtained powder is heat-treated in a non-oxidizing atmosphere within a range of 800 to 1600 ° C. (first heat treatment), whereby a core particle (the core particle contains Fe and X) and a metal having a carbon film Forming magnetic particles,
After the first heat treatment, the metal magnetic fine particles are heat-treated within a range of 400 to 750 ° C. (second heat treatment).

  In the first heat treatment, the Fe oxide is reduced. In the second heat treatment, the reaction of decomposing the γ- (Fe, C) phase or iron carbide generated during the first heat treatment into the α-Fe phase and carbon is promoted. This decomposition reaction is promoted by the addition of the element X, and the decomposition is insufficient when the element X is not added. For this reason, in order to produce the metal magnetic fine particles of the present invention, it is preferable to carry out a two-stage heat treatment (the first heat treatment and the second heat treatment).

Through the second heat treatment of the production method of the present invention, the core particles have an α-Fe phase and a ferromagnetic phase different from the α-Fe phase, and the ferromagnetic phase is obtained by Mossbauer spectroscopy. The obtained magnetic phase is a phase in which the value of the internal magnetic field is in the range of 25 to 32 T, and the core particles obtain metal magnetic particles containing 20 to 55 vol% of the ferromagnetic phase. When the element X is not added, 10 vol% or more of iron carbide having a low internal magnetic field (the internal magnetic field of Fe ₃ C is about 20 T) is precipitated, thereby reducing the overall saturation magnetization. By adding the element X, 20 to 55 vol% of a ferromagnetic phase having an internal magnetic field value in the range of 25 to 32 T is included, so that high saturation magnetization can be maintained.

  In the metal magnetic fine particle, the core particle preferably contains 1 to 10 mass% of at least one element selected from Al, Co, Ni, and Si. These elements have the effect of precipitating carbon in Fe, the core particle, as graphite, so they not only promote the decomposition of the paramagnetic γ-Fe phase and low saturation magnetization iron carbide, but also convert the carbon into a coating layer. Precipitates to improve coverage and improve corrosion resistance.

  According to the present invention, metal magnetic fine particles having both high saturation magnetization and high corrosion resistance can be obtained.

[1] Method for Producing Metal Magnetic Fine Particle The metal magnetic fine particle of the present invention is a solid-phase reduction reaction in which Fe oxide is thermally reduced with carbon (C). In order to avoid the formation of the Fe—C phase, element X that promotes C precipitation is added to the raw material while performing a two-step heat treatment.

(1) Fe Oxide Powder The particle size of the Fe oxide powder can be selected according to the target particle size of the metal magnetic fine particles, but is preferably in the range of 0.001 to 5 μm. When the particle size is less than 0.001 μm, secondary aggregation occurs remarkably, so that it is difficult to handle in the following manufacturing process. If it exceeds 5 μm, the specific surface area of the metal oxide powder is too small, so that the reduction reaction proceeds slowly. The practical particle size of the metal oxide powder is more preferably 0.005 to 1 μm. Examples of the Fe oxide include Fe ₂ O ₃ , Fe ₃ O ₄ , FeO, and the like, and Fe ₂ O ₃ is preferable in that it is inexpensive.

(2) Compound powder containing element X (X is at least one selected from Al, Co, Ni and Si) Element X is preferably at least one selected from Al, Co, Ni and Si In particular, Si is suitable because it has a large C precipitation promoting effect. The element X compound powder is preferably element X alone, carbide, nitride, or oxide. However, among the oxides of the element X, those which are thermodynamically more stable than iron oxide are not suitable because it is difficult to react with the thermally reduced Fe. Specific examples of the X compound include Al, AlC, AlN, Co, Co ₃ O ₄ , Ni, NiO, Ni ₃ N, Si, SiC, Si ₃ N ₄ , and alloys thereof. Al ₂ O ₃ and SiO ₂ are not suitable because they are thermodynamically stable. In consideration of solid phase reactivity, the particle size of the element X compound powder is preferably in the range of 0.001 to 5 μm. If the particle size is less than 0.001 μm, the specific surface area is too large and it is easily oxidized and difficult to handle. If it exceeds 5 μm, the specific surface area is too small, so the reactivity with Fe is low and the effect of addition cannot be expected. More preferably, 0.001-1 micrometer is preferable.

(3) Compound powder containing carbon As a raw material powder to be a carbon supply source, carbon powder (graphite, carbon black, natural graphite or the like) is suitable, but a compound containing carbon may also be used. That is, it may be various carbides including coal, activated carbon, coke, fatty acid, polymer such as polyvinyl alcohol, BC compound, and metal. However, carbon powder may be used to increase the carbon purity of the coating.

(4) Mixing of raw material powders In order to mix an oxide powder of Fe and a compound powder containing element X (X is at least one selected from Al, Co, Ni and Si) and a compound powder containing carbon For this, it is preferable to use a ball mill, a bead mill, a V-type mixer, a mortar, a reiki machine, or various mixers. In order to mix more uniformly, it is preferable to perform wet mixing in an organic solvent such as water or alcohol.

  The mixing ratio of the carbon compound powder to the Fe oxide powder is preferably at least a stoichiometric ratio necessary for the reduction reaction. When it is less than the stoichiometric ratio, sintered grains grow during the heat treatment and fine particles cannot be obtained. The compounding ratio of the compound powder containing the element X (X is at least one selected from Al, Co, Ni, and Si) is such that the concentration of the element X is 1 to 10 mass% in comparison with Fe. Is preferred. When the Fe contrast concentration of the element X is less than 1 mass%, the C precipitation promoting effect cannot be sufficiently obtained, which is not preferable. On the other hand, when the Fe contrast concentration of the element X exceeds 10 mass%, the content of the ferromagnetic phase exhibiting high saturation magnetization is lowered, and the saturation magnetization is lowered. Moreover, in order to mix the said raw material powder uniformly, it is preferable to add a dispersing agent as needed. The Fe contrast concentration represents the ratio of X when the total mass of Fe and X contained in each raw material powder is 100 mass%, and is represented by X / (Fe + X) × 100 (mass%) in the general formula.

(5) Heat treatment The mixed powder of the raw materials is preferably subjected to the two-stage heat treatment described below. First, the first heat treatment is performed in the range of 800 to 1600 ° C., and the Fe oxide is reduced to metal Fe by carbon. If the temperature is less than 800 ° C., the progress of the reduction reaction is insufficient, which is not preferable. A high temperature exceeding 1600 ° C. is not preferable because particles in the powder grow into sintered grains. Further, the furnace used for the heat treatment is also required to have heat resistance, which increases the manufacturing cost. In the first heat treatment, the reduction reaction proceeds and the element X (X is at least one selected from Al, Co, Ni and Si) and Fe are alloyed to lower the C solid solubility limit in Fe. There is also an effect. Furthermore, it is preferable to heat-process in the range of 400-750 degreeC as 2nd heat processing. This temperature range is a temperature region in which the α-Fe phase is stable in the Fe-C binary phase diagram. By applying this second heat treatment, the residual γ-Fe phase, which is a high temperature phase, can be suppressed. Since the γ-Fe phase is a paramagnetic phase, it is preferable to suppress the residual in order to maintain high saturation magnetization. If the temperature of the second heat treatment is less than 400 ° C., the phase transition from the γ-Fe phase to the α-Fe phase is insufficient, which is not preferable. Further, if it exceeds 750 ° C., it reaches the stable region of the γ-Fe phase, which is not preferable. In order to sufficiently suppress the residual γ-Fe phase, the temperature range is more preferably 600 to 750 ° C.
In order to allow the reduction reaction to proceed sufficiently in the heat treatment described above, a non-oxidizing atmosphere is preferable, and an inert gas such as Ar or He, hydrogen, nitrogen, carbon dioxide gas, or the like is selected. In particular, a nitrogen atmosphere is more preferable in terms of safety and low cost.

(6) Purification / Magnetic Separation The metal magnetic fine particles of the present invention are produced by the heat treatment, and include a surplus of a compound powder containing carbon used as a raw material as a nonmagnetic component. Therefore, in order to obtain metal magnetic fine particles having higher saturation magnetization, it is preferable to purify only metal magnetic fine particles by magnetic separation operation. In this refining treatment, the powder immediately after the heat treatment is sufficiently dispersed in alcohol such as isopropyl alcohol or ethanol or acetone, and then the magnetic particles are fixed with a permanent magnet, and other nonmagnetic components are discharged and removed. preferable. For dispersion in alcohol, it is preferable to use a surfactant that exhibits a dispersion function, or mechanically impart a dispersion force.

[2] Structure and properties of metal magnetic fine particles
(1) Structure of metal magnetic fine particles The metal magnetic fine particles of the present invention are coated with carbon, and the core particles are metallic iron. In addition to the α-Fe phase, the core particles preferably contain 20 to 55 vol% of a ferromagnetic phase in which the value of the internal magnetic field obtained by Mossbauer analysis is in the range of 25 to 31 T. If the value of the internal magnetic field is less than 25T, the magnetism becomes weak and the saturation magnetization becomes small. If the internal magnetic field exceeds 31T, iron oxide Fe ₃ O ₄ is contained in addition to the α-Fe phase. The ferromagnetic phase in which the internal magnetic field is in the range of 25 to 31 T is preferably contained in a volume ratio of 20 to 55 vol%. When the content of the ferromagnetic phase is less than 20 vol%, the effect of adding the element X (X is at least one selected from Al, Co, Ni and Si) cannot be sufficiently obtained, and carbon is contained in Fe. Not only does the saturation magnetization decrease, but also the coating with carbon becomes insufficient and the corrosion resistance decreases. On the other hand, if the content of the ferromagnetic phase exceeds 55 vol%, the content of the α-Fe phase, which is highly saturated, is reduced, resulting in a decrease in saturation magnetization, which is not preferable.

  The coating layer covering the core particles preferably has an average film thickness of 30 to 40 nm. If the average film thickness is less than 30 nm, the core particles are not sufficiently protected by the coating, and the corrosion resistance is lowered. On the other hand, if the average film thickness is more than 40 nm, the volume of the coating layer is so large that the volume ratio of the magnetic component is lowered and the saturation magnetization is lowered. In order to sufficiently protect the core particles and to maintain the volume ratio of the magnetic component to some extent, it is preferable that the average film thickness of the coating layer is in the above range. More preferably, the ratio of the average particle diameter d50 to the average film thickness t (= t / d50) is in the range of 0.016 to 0.020. Here, the average film thickness of the coating layer can be measured using an electron micrograph of metal magnetic fine particles. 30 or more grains are measured per sample, and the average value is defined as the average film thickness. The average particle diameter d50 is a volume-based median diameter, and is a particle diameter value at an integrated value of 50% in an integrated distribution curve obtained from a particle diameter distribution (volume basis).

  Moreover, it is preferable that d50 of the metal magnetic fine particle of this invention is 0.1-5 micrometers. If d50 is less than 0.1 μm, the saturation magnetization per particle will be small. On the other hand, if it exceeds 5 μm, the specific surface area becomes small. For example, when used as medical magnetic beads, the reactivity with the target substance is lowered. Further, when used for high-frequency components, eddy current loss increases, which is not preferable. Even a metal magnetic fine particle of 0.1 to 5 μm exhibits little saturation of γ-Fe phase and exhibits high saturation magnetization.

(2) Magnetic properties The saturation magnetization of the metal magnetic fine particles of the present invention is preferably 100 to 195 Am ² / kg. If the saturation magnetization is less than 100 Am ² / kg, it will be equivalent to the value of iron oxide. On the other hand, if it is 195 Am ² / kg or more, the volume ratio of the coating layer is too small and the coating becomes insufficient, so that the corrosion resistance is lowered. More preferably, it is 150-195 Am < ² > / kg, More preferably, it is 180-195 Am < ² > / kg. Further, the holding force of the metal magnetic fine particles of the present invention is preferably 1.3 kA / m. If it is 1.3 kA / m or more, the remanent magnetization becomes large and the particles are magnetically aggregated, which is not preferable for practical use and is not suitable for use as a soft magnetic material.

(3) Evaluation of corrosion resistance It is preferable that the Fe elution amount after immersing 25 mg of the metal magnetic fine particles of the present invention in 1 ml of PBS buffer at 37 ° C. for 168 h is less than 0.8 mg / l. If it is 0.8 mg / l or more, coating of core particles mainly composed of metallic Fe is insufficient, corrosion in an aqueous solution is unavoidable, and stability of the magnetic material is lowered, which is not preferable. Further, the Fe elution amount after immersion of 25 mg of the metal fine particles of the present invention at room temperature in 1 ml of an aqueous solution of chaotropic salt (guanidine hydrochloride, guanidine nitrate, guanidine carbonate, guanidine thiocyanate, urea) is less than 170 mg / l. Is preferred. If it is 170 mg / l or more, the core particles mainly composed of metal Fe are significantly corroded, and the quality as a magnetic material is deteriorated. The metal magnetic fine particles of the present invention can sufficiently coat the core particles (Fe core) by sufficiently depositing carbon dissolved in Fe into the coating layer and exhibit high corrosion resistance. it can.

  According to the present invention, metal magnetic fine particles having high saturation magnetization and excellent corrosion resistance can be obtained inexpensively and easily. By precipitating C dissolved in Fe on the outer surface, the core particles can be composed only of a magnetic phase having a high internal magnetic field, and sufficient coating is realized. Therefore, it is suitable for a magnetic core for biomedical use used in an environment where high magnetic permeability powder cores and high corrosion resistance are required.

(4) High-frequency characteristics After mixing the metal magnetic fine particles of the present invention and an organic resin, compression molding is performed, and if necessary, the resin is heat-cured to provide excellent magnetic characteristics in a frequency band of 5 to 10 MHz ( A dust core exhibiting a permeability μ = 8-9) can be obtained. Here, for example, a phenol resin, an acrylic resin, a silicone resin, or an epoxy resin can be used as the resin.

(Examples 1-4)
Table 1 shows the mixing ratio of α-Fe ₂ O ₃ powder having an average particle diameter of 0.05 μm, SiC powder having an average particle diameter of 0.04 μm, and carbon black powder (CB) having an average particle diameter of 0.02 μm. And were wet mixed in a ball mill for 20 hours. After drying the obtained mixed slurry, an appropriate amount of this mixed powder is filled into an alumina boat and placed in a furnace, and the flow rate is from room temperature to 3 ° C./min in a nitrogen gas stream with a flow rate of 2 (l / min). After the temperature was raised, the furnace was cooled to room temperature at 1400 ° C. for 2 hours, further heated to 700 ° C., held for 1 hour, and then cooled to room temperature. The saturation magnetization of the sample immediately after the heat treatment was measured with a vibrating sample magnetometer (VSM) with a maximum applied magnetic field of 1.6 MA / m. The results are shown in Table 1. The sample powder immediately after the heat treatment was subjected to X-ray diffraction measurement (ray source: Cu-Kα ray, measurement range: 20 to 120 °) to identify the phase.

(Comparative Example 1)
A sample was prepared in the same manner as in Example 1 except that no SiC powder was added, and the saturation magnetization of the sample was measured. The results are shown in Table 1. X-ray diffraction measurement was also performed in the same manner as in Example 1.

(Comparative Example 2)
A sample was prepared in the same manner as in Comparative Example 1 except that the heat treatment was performed at 1400 ° C. for 1 hour for 2 hours. Table 1 shows the saturation magnetization immediately after the heat treatment.

In Examples 1 to 4, the saturation magnetization is 148 Am ² / kg or more, but the saturation magnetizations of Comparative Examples 1 and ² are relatively low, 141 and 130 Am ² / kg. This is because the volume ratio of the γ-Fe phase contained in the sample powders of Examples 1 to 4 is reduced by the effect of the two-stage heat treatment and the effect of addition of Si. FIG. 1 shows the relationship between the volume ratio of the γ-Fe phase and the Si concentration (= Si / (Fe + Si)). As the Si concentration increases, the volume ratio of the γ-Fe phase decreases. The volume ratio of the γ-Fe phase (described as C _γ ) is the diffraction peak of the γ phase ((111) _γ , (200) _γ , (220) _γ ) and the diffraction peak of the α phase ((110) _α , ( 200) _α and (211) _α ) were calculated from the respective intensities and R values using the following [Equation 1]. The R value was calculated using the following [Equation 2]. θ represents the diffraction angle of each diffraction peak, v represents the volume of the unit cell calculated from the lattice constant, and can be obtained by actual measurement. For each of the other parameters, the values described in the reference (“New edition: X-ray diffraction theory” by Karity (Gentaro Matsumura, translation) Agne, Inc.) were used.

  From the above, when a γ-Fe phase that is paramagnetic is included in the sample, not only the saturation magnetization is lowered, but even if only the metal magnetic fine particles can be magnetically separated, the separation and recovery rate is lowered, which is not preferable. Adding Si as shown in Examples 1 to 4 and performing a two-step heat treatment in a region where the α-Fe phase is stable is effective in suppressing the precipitation of the γ-Fe phase. In Table 1, “Si concentration” corresponds to (Si / (Fe + Si)) × 100.

(Examples 5 to 8)
In order to extract only the metal magnetic fine particles from the sample powders obtained in Examples 1 to 4, the following purification and magnetic separation treatment was performed. First, 2 g of the sample powders of Examples 1 to 4 and 100 ml of isopropyl alcohol are put into a glass bottle, and ultrasonic waves (25 kHz) are irradiated for 5 minutes with a bathtub-type ultrasonic cleaner. Next, the metal magnetic fine particles are fixed to the inner wall of the glass bottle with a permanent magnet, and the supernatant is removed. This operation was repeated until the supernatant became transparent, and magnetic powders of Examples 5 to 8 were obtained.

  Mossbauer spectroscopy was performed on the obtained magnetic powder, and the magnetic phase contained in each magnetic powder was analyzed. The results are shown in Table 2. In the Mossbauer spectroscopic analysis, Co-57 (Rh matrix) was used as a radiation source, and the sample powders of Examples 5 to 8 were each fixed with an adhesive tape and measured. The driving speed of the radiation source was calibrated with an α-Fe standard sample and measured in the range of −10 mm / s to 10 mm / s. The detected Mossbauer spectrum was fitted with a Lorentz function, and the internal magnetic field was calculated from the peak interval of the spectrum. When a plurality of magnetic phases other than α-Fe were detected, the spectrum area of each magnetic phase was obtained and the ratio was taken as the volume ratio of each magnetic phase. In the magnetic powder of Example 5, the α-Fe phase with an internal magnetic field of 33.0 T and the Fe—Si alloy phase with an internal magnetic field of 30.6 T (only one of the eight nearest atoms of Fe in the bcc structure was replaced with Si). Structure, hereinafter referred to as Fe-Si (I)). In the magnetic powders of Examples 6 to 8, in addition to the α-Fe phase and Fe—Si (I) phase detected in Example 5, an Fe—Si alloy phase having an internal magnetic field of about 27 T (the nearest atom of Fe in the bcc structure) It has been found that a structure in which only two of the eight are replaced by Si (hereinafter referred to as Fe-Si (II)) is also included. Table 2 shows the volume ratio of each phase. In Examples 5 to 8, the α-Fe phase is contained in 46.4 to 76.3 vol%, and the Fe—Si alloy phase having an internal magnetic field of 27 to 31 T is contained in 23.7 to 53.7 vol%.

  The magnetic characteristics (saturation magnetization and coercive force) of the magnetic powders of Examples 5 to 8 were measured in the same manner as in Example 1. The results are shown in Table 2.

  Moreover, the average particle diameter (volume-based median diameter) d50 of each magnetic powder was measured using a laser diffraction measuring apparatus (HORIBA, LA-920). The results are shown in Table 3.

  Further, when each of the magnetic powders of Examples 5 to 8 was observed with a transmission electron microscope (Hitachi FE-TEM, HF-2100), Fe core particles coated with a carbon (C) film could be observed. A representative photograph is shown in FIG. FIG. 2 shows 10 photographs, corresponding to Comparative Example 3 and Examples 5 to 8 from the left side of the upper stage, and the lower stage is an enlarged photograph of the vicinity of the upper particle coating (background / carbon coating / Fe core particles). is there. It can be seen that the C coating film becomes thicker as the Si concentration increases. For each sample, 40 electron micrographs were randomly taken, the thickness of the C coating film was measured, and the average coating film thickness t was calculated. t also increases as the Si concentration increases. Table 3 shows the coating thickness t and the ratio of t to the average particle diameter d50 (= t / d50). Since the average particle size does not change greatly even when Si is added, t / d50 is 0.015 to 0.022, which is 1.5 times or more of the value obtained without addition.

  Next, 25 mg of each sample powder was immersed in 1 ml of PBS buffer (GIBCO, Phosphate Buffered Saline, pH = 7.4, Cat. No. 10010-023) at 37 ° C. for 168 h, and the Fe ion concentration eluted in the PBS buffer was determined by ICP. It was measured by analysis (Inductively Couple Plasma, manufactured by SII Nano Technology, Inc .: SPS3100H). The measurement results are shown in Table 3 as “Fe elution amount”. Table 3 also shows the Fe ion concentration when the aqueous solution of chaotropic salt (dissolution / adsorption solution attached to TOYOBO, MagExtractor-Genome-) is used instead of the PBS buffer and the immersion conditions are 24 h at room temperature. It can be seen that the Fe elution amount is reduced by the addition of Si.

  From the above, it has been found that the C coating film thickness is increased without greatly changing the average particle diameter by the addition of Si. As a result, it can be seen that the Fe ion concentration in the aqueous solution is greatly reduced and the corrosion resistance is greatly improved. That is, the C coverage to Fe core particles was improved by adding Si.

(Comparative Example 3)
The magnetic powder was purified and magnetically separated from the sample powder of Comparative Example 1 in the same manner as in Examples 5 to 8, and the obtained magnetic powder was measured for Mossbauer spectroscopy and magnetic properties in the same manner as in Examples 5 to 8. . The results are shown in Table 2. Moreover, the average particle diameter d50, the average coating film thickness, and the Fe elution amount to each aqueous solution (PBS buffer, chaotropic salt aqueous solution) were measured in the same manner as in Examples 5-8. The results are shown in Table 3.

From the above, it was found that Comparative Example 3 contained 10.6 vol% of an iron carbide (Fe ₃ C) phase having an internal magnetic field of 20.7 T. This iron carbide phase has a smaller internal magnetic field than the α-Fe and Fe—Si alloy phases detected in Examples 5 to 8, and causes a decrease in saturation magnetization. The addition of Si has the effect of improving the saturation magnetization of the magnetic powder. The reason is that the iron carbide phase having weak magnetism disappears, and instead, 23.7 to 53.7 vol% of a Fe—Si phase having a relatively large internal magnetic field (27 to 31 T) is precipitated.

(Examples 9 to 11)
A sample powder was prepared in the same manner as in Examples 1 to 3 except that AlN powder having an average particle diameter of 1 μm (Soekawa Riken Co., Ltd.) was used instead of SiC. A magnetic powder was obtained by performing a magnetic separation operation. Table 4 summarizes the amount of Fe elution from the obtained magnetic powder into each aqueous solution.

(Examples 12 to 14)
Sample powders were prepared in the same manner as in Examples 1 to 3 except that Co ₃ O ₄ powder (OMG, 72/73) having an average particle size of 0.5 μm was used instead of SiC. Similarly, purification and magnetic separation operations were performed to obtain a magnetic powder. Table 4 summarizes the amount of Fe elution from the obtained magnetic powder into each aqueous solution.

(Examples 15 to 17)
A sample powder was prepared in the same manner as in Examples 1 to 3 except that NiO powder (Soekawa Rika) having an average particle size of 0.4 μm was used instead of SiC, and purification and magnetic properties were performed in the same manner as in Examples 5 to 7. Separation operation was performed to obtain a magnetic powder. Table 4 summarizes the amount of Fe elution from the obtained magnetic powder into each aqueous solution.

  From the above, it can be seen that the addition of any of Al, Co, and Ni, which are known as graphitization promoting elements, is effective in reducing the amount of Fe elution as with Si.

(Examples 18 to 22)
The sample powder was prepared in the same manner as in Example 1 except that the compounding ratio was adjusted so that the Si concentration (= Si / (Fe + Si)) was the value shown in Table 5 and the first heat treatment temperature was 1200 ° C. Further, purification and magnetic separation operations were performed in the same manner as in Example 5 to obtain magnetic powders of Examples 18-22. The magnetic characteristics of each magnetic powder were measured in the same manner as in Example 1, and the results are shown in Table 5.

Using the magnetic powders of Examples 18 to 22, green compacts were produced according to the following procedure. First, 4 parts by weight (compared to magnetic powder) of epoxy resin powder (Somal Corporation, EPX-6136) was dissolved in an appropriate amount of acetone, and then 100 parts by weight of magnetic powder was added. While stirring and mixing the mixture at room temperature, only acetone was evaporated, and then granulated powder was obtained through a sieve having an opening of 500 μm. This granulated powder was molded at a molding pressure of 6 ton / cm ² (588 MPa) to obtain a ring-shaped molded body having an outer diameter of 7 mm, an inner diameter of 4 mm, and a height of 3 mm. This molded body was kept in the atmosphere at 200 ° C. for 1 hour, and the epoxy resin was thermally cured to obtain a ring-shaped sample. The frequency dependence of the magnetic permeability μ ′ of this ring-shaped sample was measured in the range of 1 MHz to 1.8 GHz with an impedance analyzer (Agilent Technology 4291B) to obtain the magnetic permeability μ ′ at 5 MHz and 10 MHz (Table). 5).

From Table 5, the magnetic permeability μ is improved as the amount of Si added is increased compared to the case where Si is not added (Comparative Example 4), and the Si concentration (= Si / (Fe + Si)) is saturated at 8 mass% or more. I understand. This is because the coercive force decreases to less than 0.8 kA / m with the addition of Si.

It is a graph which shows the relationship between the volume ratio of a (gamma) -Fe phase, and Si density | concentration (= Si / (Fe + Si)). It is a transmission electron micrograph of each magnetic powder of Examples 5-8.

Claims (11)

Fe oxide powder and element X (where X is at least one selected from Al, Co, Ni and Si)
It is a seed. ) Containing compound powder and carbon containing compound powder,
The obtained powder is heat-treated in a non-oxidizing atmosphere within a range of 800 to 1600 ° C. (first heat treatment), whereby a core particle (the core particle contains Fe and X) and a metal having a carbon film Forming magnetic particles,
After the first heat treatment, the metal magnetic fine particles are heat-treated within a range of 400 ° C. to 750 ° C. (second heat treatment). By passing through the second heat treatment, the core particles have an α-Fe phase and a ferromagnetic phase different from the α-Fe phase,
The ferromagnetic phase is a phase in which the value of the internal magnetic field obtained by Mossbauer spectroscopy is in the range of 25 to 32 T.
The method for producing metal magnetic fine particles according to claim 1, wherein the core particles contain 20 to 55 vol% of the ferromagnetic phase. It is a metal magnetic fine particle in which the core particle is coated with carbon,
The core particles have an α-Fe phase and a ferromagnetic phase different from the α-Fe phase,
The ferromagnetic phase has an internal magnetic field value obtained by Mossbauer analysis in the range of 25 to 32 T,
The metal magnetic fine particles, wherein the core particles contain 20 to 55 vol% of the ferromagnetic phase.   The metal magnetic fine particles according to claim 3, wherein the volume-based median diameter d50 is 0.1 to 5 µm.   The metal magnetic fine particles according to claim 3 or 4, wherein the coating layer has an average film thickness of 30 to 40 nm.   The metal magnetic fine particles according to claim 3, wherein the core particles contain 1 to 10 mass% of at least one element selected from Al, Co, Ni, and Si. The metal magnetic fine particles according to claim 3, wherein the saturation magnetization is 180 to 195 Am ² / kg.   The metal magnetic fine particles according to any one of claims 3 to 7, wherein the coercive force is less than 1.3 kA / m.   In the metal magnetic fine particles according to any one of claims 3 to 8, the amount of Fe elution after being immersed in PBS buffer at a particle concentration of 25 mg / ml at 37 ° C for 168 hours is less than 0.8 mg / l. Metallic magnetic fine particles characterized by   The metal magnetic fine particles according to any one of claims 3 to 9, wherein the Fe elution amount after being immersed in an aqueous chaotropic salt solution at a particle concentration of 25 mg / ml at room temperature for 24 hours is less than 170 mg / l. Metal magnetic fine particles.   A dust core comprising the metal magnetic fine particles according to claim 3 and an organic resin.