JP7626063B2 - Magnetic core and method for manufacturing the same - Google Patents

Description

The present invention relates to a magnetic core and a method for manufacturing a magnetic core.

Coil components such as inductors, transformers and chokes have traditionally been used in a wide variety of applications, including home appliances, industrial equipment and vehicles. Coil components consist of a coil placed around a magnetic core, and ferrite, which has excellent magnetic properties, freedom of shape and cost, is widely used for such magnetic cores.

In recent years, power supply devices for electronic devices and the like have become smaller, and there is a demand for them to be usable in an even more diverse range of environments. As a result, there is a growing demand for coil components that are small and low-profile, and can be used in high-temperature environments exceeding 130°C, and with large currents. For such coil components, magnetic cores using metallic soft magnetic materials that have a higher Curie temperature and a higher saturation magnetic flux density than ferrite are increasingly being used. As metallic soft magnetic materials, magnetic particles such as Fe-Si, Fe-Ni, Fe-Si-Al, and Fe-Cr-Al alloys are used.

It is known that ferrite cores, which are generally manufactured by sintering, and powder cores using metallic soft magnetic materials tend to be porous with fine gaps and pores. Patent Document 1 describes providing a glass layer on the surface of the magnetic core to prevent adhesive from seeping into the fine gaps in the ferrite core. Patent Document 2 describes filling the pores in a powder core using a metallic soft magnetic material with a permeation prevention material to prevent the permeation of adhesives, etc.

JP 2011-014730 A JP 2013-045926 A

In Patent Documents 1 and 2, the penetration of adhesive is suppressed by performing a pore-sealing process on the porous magnetic core. However, the shape of the magnetic core is complex, and the amount of pores present is likely to be uneven due to density differences during molding, and the formation and filling conditions of the coating made of permeation prevention materials such as glass layers are also likely to vary. As a result, there are cases where the permeation prevention effect is not fully achieved. In the first place, it is not easy to determine whether a magnetic core after pore-sealing has sufficient permeation prevention performance, and the only option is to actually apply adhesive or the like to check the permeation state.

Therefore, the present invention aims to provide a magnetic core with excellent permeation prevention performance.

According to one embodiment of the present invention, there is provided a magnetic core having a coated surface, at least a part of which is coated with a siliceous film, and the arithmetic mean curve (Spc: ISO25178) of the peaks of the coated surface is 37000 mm ^-1 or less. This makes it possible to provide a magnetic core with excellent permeation suppression performance.

In one embodiment of the present invention, it is preferable that the magnetic core has a structure in which magnetic particles of an Fe-M (M is a metal that oxidizes more easily than Fe) alloy are bonded together via oxides derived from the magnetic particles.

In one embodiment of the present invention, it is preferable that the siliceous coating contains siloxane bonds (-Si-O-Si- bonds).

In one embodiment of the present invention, the oxide contains M, and it is preferable that the M contained in the oxide is at least one of Al and Cr.

In one embodiment of the present invention, it is preferable to have an electrode formed on the coated surface.

According to another aspect of the present invention, there is provided a method for manufacturing a magnetic core, comprising the steps of: coating at least a part of a magnetic core with a siliceous coating to form a coating surface having an arithmetic mean peak curvature (Spc: ISO25178) of 37000 mm ^-1 or less; and forming an electrode on the coating surface. This allows the electrode to be formed on a coating surface with excellent permeation suppression performance, and is expected to improve the reliability and yield of the magnetic core.

In another embodiment of the present invention, it is preferable that the magnetic core has a structure in which magnetic particles of an Fe-M (M is a metal that oxidizes more easily than Fe) alloy are bonded together via oxides derived from the magnetic particles.

In another embodiment of the present invention, it is preferable that the siliceous coating contains siloxane bonds (-Si-O-Si- bonds).

In another embodiment of the present invention, the oxide contains the M, and it is preferable that the M contained in the oxide is at least one of Al and Cr.

In another embodiment of the present invention, the siliceous coating is preferably formed by carrying out a process of applying a treatment liquid for forming a siliceous coating to the surface of the magnetic core and hardening the liquid at least once.

According to the present invention, it is possible to provide a magnetic core with excellent permeation prevention performance, which can contribute to improving the reliability and yield of the magnetic core. Furthermore, according to the present invention, it is possible to form an electrode on a coating surface with excellent permeation prevention performance, which can contribute to improving the reliability and yield of the magnetic core.

1 is a graph for explaining the relationship between the arithmetic mean curve (Spc: ISO25178) of the peaks of a surface coated with a siliceous film and the permeation prevention performance. 1 is an enlarged SEM image of the surface of a magnetic core that is not covered with a siliceous coating. 1 is an enlarged SEM image of the coating surface of a magnetic core coated with a siliceous film.

The following describes in detail an embodiment of the present invention, but the present invention is not limited to this.

The magnetic core of this embodiment is a porous core made of ceramic soft magnetic material such as Ni-based ferrite or magnetic particles of the above-mentioned metal-based soft magnetic material, and at least a part of the surface is coated with a siliceous film. By setting the arithmetic mean curve (Spc: ISO25178) of the peaks on the coated surface to 37000 mm ^-1 or less, the surface of the coated surface becomes a gentle shape, and accordingly, the permeation of adhesives and the like applied on the coated surface is suppressed, thereby improving the permeation suppression performance of the magnetic core. The arithmetic mean curve (Spc: ISO25178) of the peaks, which three-dimensionally evaluates the surface properties of the coated surface, is specified in ISO25178 of the International Organization for Standardization, and can be easily measured by measuring instruments such as a laser microscope described later.

ISO 25178 also includes other parameters of three-dimensional surface roughness, such as arithmetic mean height Sa, maximum height Sz, aspect ratio Str, and developed area ratio Sdr. Of these, it has been discovered that for magnetic cores having a coated surface, at least a portion of which is coated with a silica coating, by giving the coated surface a unique surface profile defined by the arithmetic mean curve Spc of the peaks, it is possible to provide a magnetic core with excellent permeation prevention performance.

The magnetic core of this embodiment may be porous with a porosity of 5 to 25%. The metallic soft magnetic material is preferably magnetic particles of a soft magnetic alloy containing, for example, Fe and an element M that is more easily oxidized than Fe. As a first aspect of the soft magnetic material of the magnetic core, it is preferable that the sum of Fe and M (Al and Cr) is 100 mass%, and that the soft magnetic alloy contains 3 mass% or more and 16.0 mass% or less of Al, 3 mass% or more and 7.0 mass% or less of Cr, and the remainder is Fe and unavoidable impurities.

In addition, a second embodiment of the soft magnetic material of the magnetic core is preferably a soft magnetic alloy containing 1.5% by mass or more and 8% by mass or less of M, more than 1% by mass or more and 7% by mass or less of Si, with the sum of Fe, M (Al or Cr) and Si being 100% by mass, and the remainder being Fe and unavoidable impurities.

In the first and second aspects described above, C, Mn, P, S, O, Ni, N, etc. may be contained as inevitable impurities. The contents of these inevitable impurities are preferably C≦0.05% by mass, Mn≦1% by mass, P≦0.02% by mass, S≦0.02% by mass, O≦0.5% by mass, Ni≦0.5% by mass, and N≦0.1% by mass. In the first aspect, Si may also be contained as an inevitable impurity at 0.5% by mass or less.

The shape of the magnetic particles of the soft magnetic alloy is not particularly limited, but from the standpoint of fluidity, for example, it is preferable to use magnetic particles produced by the atomization method, which makes it easier to obtain relatively spherical particles from molten metal adjusted to a specified composition. Atomization methods such as gas atomization and water atomization are suitable for producing magnetic particles of alloys that are highly malleable and ductile and difficult to crush.

The average particle size of the magnetic particles (here, the median diameter D50 in the cumulative particle size distribution is used) is not limited to this, but for example, those having an average particle size of 1 μm or more and 100 μm or less can be used. When magnetic particles are filled into a molding die, if particles with large particle sizes are adjacent to each other, large gaps are formed between the particles, and the filling rate does not increase, and the density of the molded body obtained by pressure molding tends not to increase. For this reason, it is preferable to classify the magnetic particles and remove particles with large particle sizes. As a classification method, dry classification such as sieving classification can be used, and it is preferable to obtain magnetic particles at least under 32 μm (i.e., passed through a sieve with an opening of 32 μm). By reducing the average particle size, the strength, core loss, and high frequency characteristics of the magnetic core are improved, so the average particle size (median diameter D50) is more preferably 15 μm or less. On the other hand, if the average particle size is small, the magnetic permeability decreases, so the average particle size (median diameter D50) is more preferably 5 μm or more.

It is preferable to mix the magnetic particles with a binder and form them into granules by granulation. This improves the fluidity and filling properties in the mold when forming the magnetic core, and also bonds the magnetic particles together during pressure molding, giving the molded body strength that can withstand handling after molding. There are no particular restrictions on the type of binder, but organic binders such as polyethylene, polyvinyl alcohol, and acrylic resins can be used. It is also possible to use inorganic binders that remain after heat treatment.

The amount of binder added should be sufficient so that the binder is distributed sufficiently between the magnetic particles and the strength of the molded body is sufficient, but if too much binder is added, the density and strength of the molded body tend to decrease. From this perspective, the amount of binder added is preferably 0.2 to 10 parts by weight, and more preferably 0.5 to 3.0 parts by weight, per 100 parts by weight of magnetic particles.

The method for mixing the magnetic particles and the binder is not particularly limited, and any conventionally known mixing method or mixer can be used. As the granulation method, for example, wet granulation methods such as rolling granulation and spray-drying granulation can be used. Among these, spray-drying granulation using a spray dryer is preferred, as this makes the shape of the granules closer to spherical, and the time of exposure to heated air is short, making it possible to obtain a large amount of granules.

The resulting granules preferably have a bulk density ^of 1.5 to 2.5 x ¹⁰ kg/m and an average particle size (D50) of 60 to 150 μm. Such granules have excellent flowability during molding, and the gaps between the magnetic particles are small, improving the filling ability into the mold, resulting in a compact with a high density and a magnetic core with high magnetic permeability. In order to obtain the desired granule size, classification using a vibrating sieve or the like can be used.

In addition, to reduce friction between the granules and the molding die during pressure molding, it is preferable to add a lubricant such as stearic acid or a stearate salt. The amount of lubricant added is preferably 0.1 to 2.0 parts by weight per 100 parts by weight of magnetic particles. The lubricant can also be applied to the die.

The magnetic particle granules are subjected to pressure molding. In the pressure molding, a press machine such as a hydraulic press or a servo press and a molding die are used to mold the mixed powder into a predetermined shape such as a toroidal shape or a rectangular parallelepiped shape. This pressure molding may be room temperature molding, or depending on the binder material, it may be warm molding in which the granules are heated to a temperature close to the glass transition temperature at which the binder softens, so long as the binder does not disappear. The magnetic particles in the compact obtained by pressure molding are in point contact or surface contact with each other and are adjacent to each other partially via gaps. The density of the compact is preferably 5.6 x ¹⁰³ kg/m3 or ^more .

Annealing is performed at a temperature of 650°C to 900°C as heat treatment for the compact. Annealing is performed in an oxygen-containing atmosphere, such as in air, a mixture of oxygen and an inert gas, or an atmosphere containing water vapor, among which heat treatment in air is simple and preferable. Oxides are obtained by the reaction between the magnetic particles and oxygen during heat treatment, and are produced by an oxidation reaction that exceeds the natural oxidation of the magnetic particles. The production of such oxides results in a high-strength magnetic core that has excellent insulation and corrosion resistance and is firmly bonded as grain boundaries between a large number of magnetic particles. In addition, the stress distortion introduced by pressure molding is alleviated, resulting in good magnetic properties.

The M elements Al and Cr that make up the soft magnetic alloy have a greater affinity with O (oxygen) than Fe and other non-ferrous metals. Therefore, during heat treatment, O in the air and O contained in the binder preferentially bind to Al and Cr near the surface of the magnetic particles, and are formed on the surface of the magnetic particles as chemically stable Al oxides, Cr oxides, or composite oxides with other non-ferrous metals. By forming oxides containing M elements on the surfaces of the magnetic particles, the corrosion resistance is excellent, the insulation of the magnetic particles is improved, and the eddy current loss can be reduced to improve the resistivity of the magnetic core. Since Al has a greater affinity with O than Cr, when Al and Cr are included as M elements, the degree of oxide formed tends to be dominated by Al oxides, and Cr oxides are suppressed. The surface of the magnetic core is then covered with oxides of the magnetic particles.

In the magnetic core obtained after heat treatment, it is preferable that the space factor (relative density) is in the range of 75 to 95%. The porosity of such a magnetic core is 5% to 25%. The space factor (relative density) is calculated by calculating the density of the magnetic core from its dimensions and mass, and dividing the density of the obtained magnetic core by the true density of the magnetic particles. The true density of the magnetic particles can be calculated by the density of an ingot produced by melting the same composition.

Next, a siliceous film is formed on the surface of the obtained magnetic core. The siliceous film has a molecular structure having a main chain in which three O and Si are siloxane-bonded (-Si-O-Si-). The siliceous film may have a bond represented by (-Si-O-Si-), and may have a continuous bond represented by (-Si-O-) repeated. A low-temperature method using a sol-gel method in which an alkoxysilane oligomer sol is gelled by hydrolysis through a condensation reaction is preferably used as a method for forming such a siliceous film on the surface of the magnetic core. The siliceous film is preferably formed by immersing the magnetic core in a siliceous film forming treatment liquid containing an alkoxysilane oligomer solution and colloidal silica, or by spraying the siliceous film forming treatment liquid on the magnetic core and then subjecting it to a hardening treatment. The solvent for the siliceous film forming treatment liquid is preferably a hydrophilic organic solvent such as an alcohol-based solvent or a glycol ether-based solvent. The siliceous film itself has excellent heat resistance and high strength, and can improve corrosion resistance by covering the entire surface of the magnetic core.

As mentioned above, the surface of the magnetic core is covered with an oxide film due to the oxides of the magnetic particles, and this film contains a state in which metal elements derived from the magnetic particles are bonded to O, and a state in which metal elements are bonded to hydroxyl groups (OH). The hydroxyl groups or oxides on the surface of the magnetic core are chemically bonded, such as by covalent bonds or ionic bonds, to the hydroxyl groups in the siliceous film, thereby increasing the adhesive strength between the magnetic particles and the siliceous film, thereby enabling the magnetic core and the siliceous film to adhere closely to each other.

In addition, a siliceous film is also formed on the voids inside the magnetic core that are reached by the siliceous film forming treatment liquid through the gaps between the magnetic particles on the surface of the magnetic core. The voids inside the magnetic core are filled with the siliceous film, and the surface (coated surface) of the magnetic core is given a gentle shape by the formation of the siliceous film. In other words, the state of the coated surface can be quantified by the arithmetic mean curve of the peaks (Spc: ISO25178) to easily check the sealing state of the magnetic core. If the arithmetic mean curve of the peaks (Spc: ISO25178) on the coated surface is 37000 mm ^-1 or less, the magnetic core can have excellent permeation suppression performance. In addition, by performing a process such as a vacuum impregnation method in which the magnetic core is immersed in the siliceous film forming treatment liquid and degassed, and the vacuum state is released, the magnetic core can be impregnated with the siliceous film forming treatment liquid in one layer.

The method of applying the siliceous film forming treatment liquid to the magnetic core may be performed according to a known method such as dip spin coating, dip coating, or spray coating, and is not particularly limited. Depending on the viscosity of the siliceous film forming treatment liquid, the temperature may be adjusted to 100 to 200 ° C. before application. The hardening treatment may be performed, for example, at room temperature, but hardening may be accelerated by placing the solution at a temperature of about 40 to 200 ° C. The thickness of the siliceous film formed on the surface of the magnetic core is preferably about 0.5 to 10 μm. The thickness of the siliceous film can be adjusted by repeatedly applying the siliceous film forming treatment liquid to the magnetic core surface and hardening it. As the film thickness increases, residual stress in the film makes it more likely that cracks will occur on the surface side of the film. Although the occurrence of cracks can be suppressed by slowing down the hardening speed, the film thickness is preferably 5 μm or less, more preferably 3 μm or less, in consideration of productivity.

An electrode can be formed on the coating layer of the magnetic core to form a magnetic core equipped with an electrode. A coating layer coated with a siliceous film and having an arithmetic mean curve of the peak (Spc: ISO25178) of 37000 mm ^-1 or less has excellent permeation suppression performance and can stably form an electrode. Conductive materials such as Ag, Cu, Al, etc. (including alloys thereof) can be used as the electrode material. In addition, as a method for forming an electrode, for example, a method of bonding and fixing a metal terminal such as SPCC, copper alloy, Ni alloy, stainless steel, etc., a sputtering method, an ion plating method, or a printing method, a transfer method, a dipping method, etc. using a conductive paste may be used to form an electrode in a film shape.

As magnetic particles, FeAlCr atomized powder (average particle size D50 ≒ 10 μm) containing 5% Al, 4% Cr, and the remainder Fe was prepared. A PVA (Poval PVA-205 manufactured by Kuraray Co., Ltd.) aqueous solution was used as the binder, and ion-exchanged water was used as the solvent. These were put into a stirring device and stirred and mixed to form a slurry. The blending ratio of the binder and ion-exchanged water was adjusted so that the PVA solid content was 0.75 parts by weight and the slurry concentration was 80 wt% for 100 parts by weight of the atomized powder. The slurry was spray-dried using a spray dryer device. The slurry was sprayed inside the spray dryer device, instantly dried with hot air at 240°C, and the granules that had become granular were collected from the bottom of the device. In order to remove coarse particles from the obtained granules, they were passed through a 100-mesh sieve, and the average particle size of the granules after sieving was set to be within the range of 60 μm to 80 μm. Zinc stearate was added to the granules obtained by each of the above granulation methods in a ratio of 0.4 parts by weight per 100 parts by weight of the granules, and mixed in a mixer to obtain granules for molding.

The granules obtained above were filled into a mold and molded at room temperature using a hydraulic press to obtain a molding density of 5.8 g/cm ³ and 6.3 g/cm ^3. The molded body had a cylindrical shape with an outer diameter of φ7.2 mm and a thickness of 2 mm.

The obtained compacts were heat-treated in air at 750°C for 1 hour to produce the magnetic cores of Sample A and Sample B. The density was then calculated from the dimensions and weight of the magnetic cores.

Next, the cylindrical magnetic core was placed in a stainless steel mesh cage and immersed in the treatment liquid for forming a siliceous film for one minute. The liquid temperature was 23°C. The treatment liquid for forming a siliceous film was a sealant from the Protector S series manufactured by Okuno Chemical Industries Co., Ltd. During immersion, the cage was swung to remove air bubbles attached to the magnetic core surface. The magnetic core was then placed in a centrifugal dehydrator, and the excess sealant attached at a peripheral speed of 8.4 m/s was removed, and the core was dried and hardened with hot air at 70°C. The coating process from immersion to drying was repeated 1 to 3 times. The condensation reaction also progresses in the hot air drying at 70°C, forming a siliceous film, but since unreacted portions remain, after the coating process, a hardening process was carried out again under the conditions of 120°C x 20 minutes, and the entire surface of the magnetic core was covered with a siliceous film.

The surface roughness of the obtained magnetic core was measured. The surface roughness was measured using a laser microscope VK-X120 manufactured by Keyence Corporation. The magnification was 100 times, the evaluation area was 145 μm × 109 μm, and the cross-sectional curve was leveled (tilt adjustment) and smoothed to obtain the arithmetic mean curve (Spc: ISO25178) of the average peak in five evaluation areas with different fields of view. The arithmetic mean curve (Spc: ISO25178) of the peak was also obtained in the same manner for a magnetic core without a siliceous coating. Figure 2 is an SEM image of the surface of sample A that was not coated at 1000 times, and Figure 3 is an SEM image of the surface of sample A that was coated twice at 1000 times.

An ink absorption test was used to confirm the pore sealing ability of the magnetic core. The test ink used was an arcotest surface energy evaluation pen (range 38 mN/m). The test ink was applied to the surface of the magnetic core, and the process was recorded as a video. A still image taken 2 seconds after application was binarized by computer for the test ink and other areas, and the pore sealing ability was judged from the proportion of ink absorbed on the observation surface. If the ink absorption was less than 30%, it was judged as good, and if it was more than 30%, it was judged as bad.

The measurement results of the arithmetic mean curve of the peaks (Spc: ISO25178) for the coating surface of the siliceous film on the magnetic core and the results of the ink absorption test are summarized in Table 1 and Figure 1. In the table, a good result in the ink absorption test is indicated by ○, and a bad result is indicated by ×.

As shown in Figure 1, when the arithmetic mean curvature (Spc: ISO25178) of the peaks on the surface of a magnetic core coated with a siliceous film is 37000 mm ^-1 or less, a good result is obtained in the ink absorption test, but when it exceeds 37000 mm ^-1 , a bad result is obtained, and it can be seen that the permeation suppression performance of the magnetic core can be easily evaluated. Therefore, by using a magnetic core in which the arithmetic mean curvature (Spc: ISO25178) of the peaks of the coating surface coated with a siliceous film is 37000 mm ^-1 or less, a magnetic core with excellent permeation suppression performance can be obtained.

Claims (12)

A coating surface is provided on at least a portion of the surface thereof, the coating surface being coated with a siliceous coating.
The arithmetic mean curvature (Spc: ISO25178) of the peaks of the coated surface is 37000 mm ^-1 or less,
The soft magnetic alloy has a structure in which magnetic particles of the soft magnetic alloy containing Fe and an element M which is more easily oxidized than Fe are bonded to each other via an oxide derived from the magnetic particles,
A magnetic core comprising voids between said magnetic particles, said voids being open to said surface and filled with said siliceous coating. The magnetic core according to claim 1, wherein the siliceous coating contains siloxane bonds (-Si-O-Si- bonds). The magnetic core according to claim 1 or 2, wherein the oxide contains the M, and the M contained in the oxide is at least one of Al and Cr. A magnetic core according to any one of claims 1 to 3, in which the siliceous coating covers the entire surface. A magnetic core according to any one of claims 1 to 4, wherein the thickness of the siliceous coating is 10 μm or less. A magnetic core according to any one of claims 1 to 5, having an electrode formed on the coating surface. A step of coating at least a portion of the magnetic core with a siliceous coating to form a coating surface having an arithmetic mean peak curvature (Spc: ISO25178) of 37000 mm ^-1 or less;
forming an electrode on the coated surface;
the magnetic core has a structure in which magnetic particles of a soft magnetic alloy containing Fe and an element M that is more easily oxidized than Fe are bonded together via an oxide derived from the magnetic particles,
A method for manufacturing a magnetic core comprising the steps of: forming a magnetic core having voids between the magnetic particles, the voids opening to the surface of the magnetic core and filled with the siliceous coating; The method for manufacturing a magnetic core according to claim 7, wherein the siliceous coating contains siloxane bonds (-Si-O-Si- bonds). The method for manufacturing a magnetic core according to claim 7 or 8, wherein the oxide contains the M, and the M contained in the oxide is at least one of Al and Cr. The method for manufacturing a magnetic core according to any one of claims 7 to 9, wherein the siliceous coating covers the entire surface of the magnetic core. The method for manufacturing a magnetic core according to any one of claims 7 to 10, wherein the thickness of the siliceous coating is 10 μm or less. The method for manufacturing a magnetic core according to any one of claims 7 to 11, wherein the siliceous coating is formed by applying a siliceous coating forming treatment liquid to the surface of the magnetic core and curing the liquid at least once.

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