JP2008195986A - Powder of soft magnetic metal, green compact thereof, and method for manufacturing powder of soft magnetic metal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a powder of soft magnetic metal, which can be heat-treated at high temperature, shows low hysteresis loss and high electrical resistance, and to provide a method for manufacturing the same. <P>SOLUTION: The powder 1 of the soft magnetic metal has a composition expressed by the formula of Fe<SB>100-a-b</SB>Si<SB>a</SB>Cr<SB>b</SB>(0≤a≤8 and 0<b≤3, by wt.%), and has a higher Cr concentration 3 on the surface of the powder than that in the center part of the powder. It is preferable that all or one part of the surface of the powder of the soft magnetic metal is coated with an insulative oxide 2, and that an oxygen content in the whole powder of the soft magnetic metal including the insulative oxide is 10 mass% or less. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a soft magnetic metal powder and a green compact having a low iron loss and good electrical insulation when used as a powder magnetic core of an electric and electronic component such as a choke coil, a transformer, a reactor, and a motor, and a production thereof. Regarding the method.

  In recent years, electrical and electronic parts such as motor cores and transformer cores are required to be capable of high density and miniaturization and more precise control with low power. For this reason, development of soft magnetic materials that are used in these electric and electronic parts and that have excellent magnetic properties particularly in the mid-high frequency region has been underway. A dust core made of a soft magnetic metal powder has a higher saturation magnetic flux density than a conventionally used ferrite core, and is therefore advantageous for downsizing electronic components.

  However, a dust core made of soft magnetic metal powder has a low eddy current loss because it has a lower electrical resistivity than ferrite. Therefore, when a metal-based powder magnetic core is used as the core, the loss particularly in the mid-high frequency region is larger than that of the ferrite core. In addition, there was a problem of temperature rise due to heat generation of the core, and it was difficult to put it into practical use as an electronic component. Conventionally, as a means for solving this problem, in order to reduce the loss, the surface of the soft magnetic metal powder is made high in resistivity by coating with an insulator such as phosphate as in Patent Document 1, for example. Attempts have been made to suppress the generation of eddy currents. Non-Patent Document 1 proposes a method of increasing the resistance of the powder surface by coating the surface of the metal powder with an oxide such as MgO and reducing eddy current loss. All of these are intended to reduce eddy current loss and core loss by improving the electrical resistance of the powder surface.

By the way, the iron loss of the entire core is generally represented by the sum of hysteresis loss and eddy current loss. In general, the hysteresis loss changes in proportion to the first power of the frequency, and the eddy current loss changes in proportion to the second power of the frequency. Therefore, in the high frequency region, the eddy current loss contribution is larger than the hysteresis loss contribution, and reducing the eddy current loss is an effective means for reducing the core loss. On the other hand, for devices used at relatively low and medium frequencies such as motor cores and reactor cores, the contribution from hysteresis loss as well as eddy current loss cannot be ignored.
Therefore, in the low and medium frequency range, reducing not only eddy current loss but also hysteresis loss is effective in reducing core loss.

The hysteresis loss of a magnetic core is determined by the magnitude of hysteresis when a magnetic field is turned on / off in the core. In order to reduce the hysteresis loss, it is necessary to make the coercive force of the magnetic powder as small as possible. The coercive force of soft magnetic powder reflects the ease of movement of the domain wall when a magnetic field is applied, and includes distortions, dislocations, etc. due to plastic deformation generated during the formation of grain boundaries, impurity inclusions, and dust cores. This is a factor that prevents this. For this reason, in order to obtain a low hysteresis loss core, in the Fe-based magnetic powder, a low coercivity powder is originally molded at a low pressure,
It is desired to perform strain relief annealing at a high temperature of 700 ° C. or higher, preferably 900 ° C. or higher.

  However, in the prior art described in Patent Document 1, Non-Patent Document 1, etc., when heat treatment is performed at a high temperature of 800 ° C. or higher, the insulating material coated to suppress the generation of eddy current reacts with the magnetic powder, Insulating film deteriorates and deteriorates due to thermal decomposition, etc., resulting in inability to perform heat treatment at high temperatures, increasing crystal grain size and completely removing processing strain. Therefore, there is a problem that hysteresis loss cannot be sufficiently reduced.

Cr added to the raw material powder is an element effective for rust prevention and reduction of processing strain during molding during powder production. On the other hand, non-metallic inclusions such as carbides and oxides are formed in the grains to maintain the powder. It also has the disadvantage of increasing the magnetic force.
JP 2003-282316 A Gakuji Uozumi et al., Outline of the 2005 Fall Meeting of the Powder and Powder Metallurgy Association (2005), p136

  As described above, in order to reduce the core iron loss of a dust core, it is necessary to reduce both hysteresis loss and eddy current loss. For this reason, low coercive magnetic powder and insulation coating technology that can withstand high-temperature heat treatment are required. However, a soft magnetic metal powder with a high surface resistance that can withstand high-temperature heat treatment with a low hysteresis loss that realizes a low core loss has yet to be obtained.

  In order to solve the above problems, an object of the present invention is to provide a soft magnetic metal powder, a green compact, and a method for producing the same, which can be heat-treated at a high temperature, have a low hysteresis loss, and have a high electrical resistance.

  The present inventors attach a heat-resistant insulating oxide to a soft magnetic metal powder to which Cr is added, and preferably form a Cr-rich insulating film on the powder surface by annealing at a high temperature. It has been found that the above object can be achieved.

That is, the soft magnetic metal powder of the present invention is a soft magnetic powder represented by a composition formula of Fe _100-ab Si _a Cr _b (wt%, 0 ≦ a ≦ 8), The concentration is higher than the center of the powder.

  It is preferable that a part or the whole of the surface of the soft magnetic metal powder is covered with an insulating oxide, and the oxygen content of the entire soft magnetic metal powder containing the insulating oxide is 10% by mass or less.

  It is preferable that the thickness of the insulating oxide is 1 μm or less.

  It is preferable to heat-treat the oxide deposited from the alkoxide solution at 700 ° C. or higher.

  By using the soft magnetic metal powder produced according to the present invention, it is possible to significantly reduce both eddy current loss and hysteresis loss of the dust core (magnetic core).

  In addition to depositing the insulating oxide by the above manufacturing method, Cr, which is an additive element dissolved in the soft magnetic metal powder, is diffused back to the powder surface by heat treatment at high temperature, and Cr is rich on the powder surface. By forming the oxide, good electrical insulation can be obtained. Therefore, unlike the conventional insulating coating technology, the insulating coating is not destroyed even by high-temperature annealing at 700 ° C. or higher, preferably 900 ° C. or higher, but rather its formation is promoted.

  By performing the high temperature annealing, the distortion applied to the soft magnetic metal powder during core forming is sufficiently removed, the crystal grain size is increased, the domain wall movement in the powder is facilitated, and the hysteresis loss is greatly reduced. Further, as described above, Cr in the soft magnetic metal powder precipitates on the surface, so that the Cr concentration inside the powder becomes low. Therefore, the effect of reducing the loss derived from the raw material powder that has been considered inevitable among the hysteresis loss can be obtained.

The soft magnetic metal powder of the present invention and the production method thereof will be specifically described below.
The soft magnetic metal powder used in the present invention is preferably represented by a composition formula of Fe _100-ab Si _a Cr _b (by weight, 0 ≦ a ≦ 8, 0 <b ≦ 3). Si in the alloy composition may be omitted, but it is an effective element for reducing core loss, and its effect can be obtained by adding 1% by weight or more. On the other hand, if the amount of Si increases, the alloy powder becomes hard and brittle and the formability is lowered, so 8% is made the upper limit. Preferably they are 2 weight% or more and 7 weight% or less. In addition, Cr is an additive element that has a strong affinity for oxygen and is easily oxidized, so that it is effective for rust prevention of Fe in the powder manufacturing process and for reducing processing strain during molding. Although a small amount of additive may be used, a sufficient rust prevention effect is obtained if the added amount is 0.1% by weight or more. On the other hand, if the addition amount exceeds 3%, oxides and carbides are formed in the grains and the coercive force is increased, which is not preferable in terms of magnetic characteristics. Therefore, the addition amount is desirably 3% or less, more preferably 1.5% or less.

The electrical resistivity, the strength of the green compact, and the hysteresis loss vary depending on the amount of oxygen in the insulating oxide that covers part or all of the surface. In particular, in order to achieve both the soft magnetic characteristics such as residual magnetic flux density and the reduction of hysteresis loss, the oxygen amount of the entire soft magnetic metal powder including the insulating oxide is 10% by weight or less compared to the mass of the entire powder. It is preferable to become.
When using a spherical Fe6.5% Si powder (with an aspect ratio of 2 or less), a powder with high electrical resistivity, high strength, and low hysteresis loss when the oxygen content is in the range of 0.1 to 1.0 mass% It is possible to get a body. When the oxygen amount is 0.15 to 0.3% by mass, a more preferable green compact can be obtained.
When a soft magnetic metal powder having a flat shape (aspect ratio of more than 2) is used, a green compact having high electrical resistance can be obtained when the oxygen content is 1% by mass or more. More preferably 2% by mass or more.
This amount of oxygen is obtained by dissolving a soft magnetic metal powder coated with an insulating oxide in a graphite crucible in a He stream and measuring the generated CO gas by an infrared absorption method.

  The soft magnetic metal powder of the present invention may contain 1 wt% or less of (C, N, P, S, Ni, Mo, Mn, Cu, Al, etc.) as inevitable impurities.

FIG. 5 shows a schematic diagram of the soft magnetic metal powder produced according to the present invention.
In the figure, reference numeral 1 denotes an Fe—Si—Cr soft magnetic metal powder as a raw material. As a manufacturing method, any method such as a gas atomizing method, a water atomizing method, and other methods may be used.
In this schematic diagram, the powder shape is modeled as a sphere, but it does not actually depend on the powder shape or size. The effect of the present invention is effective in any shape such as a spherical shape, a flat shape, and an irregular shape.
In the figure, 2 is an insulating oxide, which covers the entire powder surface. However, the insulating effect is sufficient even if there is no insulating oxide partially depending on the shape, composition, surface state of the raw material powder, conditions of the production, and the like. Conversely, the thinner the oxide oxide layer and the higher the proportion of the metal powder component, the higher the density of the dust core. Accordingly, a core having a high magnetic flux density is obtained, and the processing conditions are simple and beneficial.

As a raw material of the insulating oxide, M is a metal element and has an alkyl chain length such as methoxide M (OCH3) n, ethoxide M (OC2H5) n, propoxide M (O.n-, i-C3H7) n. Use short alkoxides. The metal element M is at least one element selected from Mg, Al, Si, Ca, Ti, Sr, Ba, Li, Na, K, St, Ge, Bi, Cu, Y, Zr, and Ta. A metal alkoxide may be used individually by 1 type, and may be used in combination of 2 or more type. Si, Ti, Zr, and Al are particularly preferable as an element that forms a stable oxide.
The alkoxide solution in the production method of the present invention is a solution obtained by dissolving these metal alkoxides in an alcohol such as IPA or ethanol. Specifically, tetraethoxysilane or tetramethoxysilane which is an alkoxide solution of Si, or titanium alkoxide which is an alkoxide solution of Ti is preferable.

The metal alkoxide solution may contain ceramic particles such as alumina (Al ₂ O ₃ ), silica (SiO ₂ ), and magnesia (MgO). However, if these contents are large, the magnetic properties of the powder magnetic core deteriorate. Therefore, it is necessary to select and use as appropriate.
Further, an insulating viscous substance may be added to the metal alkoxide solution. The insulating viscous substance is added together with the ceramic particles to prevent its precipitation and uniformly disperse in the metal alkoxide solution. As a result, an effect that the insulating property of the soft magnetic powder is improved can be obtained. As the insulating viscous substance, any of a synthetic viscous substance, a semi-synthetic viscous substance, a natural viscous substance, and the like can be used as appropriate. As the synthetic viscous material, any of a cation system, an anion system, and a nonionic system can be used. As the semi-synthetic viscous material, for example, methylcellulose which is a cellulose dielectric system viscous material can be used. As the natural viscous substance, either plant mucilage or animal mucilage can be used, such as gum arabic or gelatin.

  These alkoxide solutions and the raw material powder are mixed. At the time of mixing, if the concentration of the alkoxide solution is extremely small, the oxide attached to the surface of the metal powder is not sufficient, and the effect of the present invention cannot be obtained. Therefore, the concentration of the alkoxide solution needs to be 10 mmol / L or more. There is no upper limit to the concentration, but if it is too high, the amount of waste that does not adhere to the metal powder increases, resulting in a problem in manufacturing cost. Further, in order to deposit a more uniform and dense oxide, it is practically preferable to be about 5 mol / L or less. The treatment time is the time for mixing the alkoxide solution and the raw material powder. However, if the treatment time is too short, the reaction hardly proceeds and the insulator cannot be deposited. There is no upper limit on the treatment time, but when the reaction reaches equilibrium, no further adhesion is expected, so there is no effect. Therefore, practically, a range of 30 minutes to 5 hours is preferable. The form and speed of the reaction of the alkoxide solution described above vary depending on the pH. For this purpose, it is more desirable that the pH is 6.0 to 12, preferably pH 8.3 to 10.5. A pH of 6 or less is not preferable because Fe elution occurs in the solution from the magnetic powder. On the other hand, if the pH is 10.5 or more, the amount of adhesion becomes excessive and the film thickness increases excessively. Since the adhesion amount is almost proportional to the oxygen amount of the soft magnetic metal powder, it is necessary to coat a preferable amount depending on the alloy composition and the powder shape.

  After removing the soft magnetic powder from the alkoxide solution, the powder is dried. When heat treatment is performed after molding, there is an effect of preventing a decrease in density due to evaporation of a solvent or water remaining in the metal powder. It is preferable to carry out at a temperature for about 10 minutes to 5 hours. The drying process is general and may be performed in the atmosphere, in a vacuum, or in an inert atmosphere, and may be performed at a temperature at which the solvent or water evaporates (50 ° C. to 300 ° C.) for 30 minutes to 10 hours.

At the time when the raw material powder and the alkoxide solution are mixed by the above process, no Cr concentration gradient occurs. Through the heat treatment step at 900 ° C. or higher, the soft magnetic metal powder of the present invention having a higher Cr concentration on the surface side than the center part can be obtained. An image of the Cr-concentrated site is indicated by reference numeral 3 in FIG. The heat treatment temperature is lower than the sintering temperature of the raw material powder and is more effective when it is 1000 ° C. or higher. Cr concentrated on the powder surface is oxidized to form an oxide. When this powder is used for a dust core, the core strength is improved. It is considered that the oxide deposited by the alkoxide solution and the aforementioned Cr are mixed with each other to form a composite oxide.
In addition, when using the soft magnetic metal powder of this invention as a powder magnetic core, it is possible to perform the heat processing process of 700 degreeC or more after shaping | molding. This makes it possible to perform strain relief annealing at a high temperature by making the heat treatment step for producing the soft magnetic metal powder of the present invention the same as the strain relief annealing step for the core. The heat treatment time may be about 30 minutes to 2 hours both in the powder state and after the core molding. The heat treatment atmosphere is preferably a non-oxidizing atmosphere.

(Example 1)
A composition composed of Fe-6.5% Si-1% Cr, 500 g of soft magnetic raw material powder having an average particle size of 40 μm, is mixed with 100 mL of tetraalkoxysilane (Kanto Chemical) / IPA solution, and using a propeller stirrer, Stir for 3 hours. Thereafter, the soft magnetic powder and the tetraalkoxysilane / IPA solution were separated and dried at 100 ° C. for 1 hour. The obtained soft magnetic powder was mixed with 0.5 wt% of 3% PVA aqueous solution as a binder and 0.3 wt% of zinc stearate as a lubricant. The soft magnetic powder thus obtained was compression molded at 1200 MPa at room temperature to produce a ring sample having an outer diameter of 14 mm, an inner diameter of 8 mm, and a height of 4.5 mm, and this was heat-treated at 1000 ° C. for 2 hours in nitrogen gas. went.
For comparison, the same raw material powder as in Example 1 was manufactured, and instead of the insulation treatment using the tetraalkoxysilane / IPA solution, 1.4 wt% of water glass, 1.3 wt% of kaolin, and stearin as a lubricant. A mixture of 0.3 wt% zinc oxide (Comparative Example 1) and a mixture of 1.5 wt% amorphous silica, 0.5 wt% kaolin and 0.3 wt% lubricant zinc stearate (Comparative Example 2) ) Was prepared. The soft magnetic powder thus obtained was subjected to compression molding and heat treatment in the same manner as in Example 1. The core loss of the dust core thus obtained was measured. The core loss was measured using a BH analyzer (product name SY8232) manufactured by Iwasaki Tsushinki Co., Ltd., and the values at an excitation magnetic flux density of 50 mT and a frequency of 50 kHz were measured. It is described in Table 1.

Moreover, the ring-shaped sample was embedded and polished in resin, and elemental analysis of the cross section was performed. The line analysis result by EPMA of Example 1 is shown in FIG. 1 (a), and the line analysis result of Comparative Example 1 is shown in FIG. 1 (b). In both FIG. 1 (a) and FIG. 1 (b), the portion where the Fe concentration is reduced is the end portion of the soft magnetic powder, and the measurement portion in between is the portion where the inside of the soft magnetic powder is measured. It is. Fe is decreased and a large amount of Cr is detected at the end of the soft magnetic powder. When the area between the Cr peaks, that is, the region showing the inside of the powder is enlarged, there is a slight gradient in the Cr concentration, and since O is detected at the position where the Cr peak is seen, it is concentrated on the surface side of the metal powder It is considered that Cr exists as an oxide.
On the other hand, as shown in FIG. 1B, the soft magnetic powder of Comparative Example 1 has a slightly reduced Cr content on the surface side of the metal powder.

  As shown in Table 1, despite the heat treatment at the same 1000 ° C., Example 1 has a lower core iron loss and a higher density than Comparative Example 1. This is due to the soft magnetic metal powder of the present invention and the production method thereof. Removal of molding distortion by high-temperature heat treatment and reduction of hysteresis loss due to reduction of Cr concentration inside the powder, and insulation oxide (silica in Example 1) deposited by alkoxide solution and Cr concentration on the surface of the powder increase. This is considered to be an effect of reducing eddy current loss due to the formation of.

(Examples 2 and 3)
In place of the tetraalkoxysilane alkoxide solution used in Example 1, experiments were performed using tetramethoxysilane and titanium isopropoxide alkoxide solutions. 500 g of the same raw material powder as in Example 1 was mixed with 100 mL of tetramethoxysilane (Kanto Chemical) / IPA solution and 100 mL of titanium isopropoxide (Kanto Chemical) / IPA solution, respectively, and stirred for 3 hours using a propeller stirrer. Thereafter, the soft magnetic powder and each solution were separated and dried at 100 ° C. for 1 hour. The obtained soft magnetic powder was mixed with 0.5 wt% of 3% PVA aqueous solution as a binder and 0.3 wt% of zinc stearate as a lubricant.
Thereafter, the dust core was compression molded in the same manner as in Example 1, and the core loss was measured. The measured results are also shown in Table 1. It can be seen that the core iron loss is low and the density is high as in Example 1.

(Examples 4 and 5)
The effect of hysteresis loss due to heat treatment temperature after molding was investigated. 500 g of the same raw material powder as in Example 1 was mixed with 100 mL of a tetraalkoxysilane / IPA solution and stirred for 3 hours using a propeller stirrer. Thereafter, the soft magnetic powder and the tetraalkoxysilane / IPA solution were separated and dried at 100 ° C. for 1 hour. The obtained soft magnetic powder was mixed with 0.5 wt% of 3% PVA aqueous solution as a binder and 0.3 wt% of zinc stearate as a lubricant. The soft magnetic powder thus obtained was compression-molded at 1200 MPa at room temperature to produce a ring sample having an outer diameter of 14 mm, an inner diameter of 8 mm, and a height of 4.5 mm, which was 800 ° C., 850 ° C., 900 ° C. in nitrogen gas. , 1000 ° C., 1100 ° C. for 2 hours. The results are shown in FIG.
When the heat treatment temperature is 900 ° C. or higher, the hysteresis loss becomes smaller, and when the temperature is 1000 ° C. or higher, the tendency is remarkable. Even when heat treatment is performed at 800 ° C., the hysteresis loss is a small value as compared with other powder processing methods.

(Example 6)
Table 2 shows the relationship between coating thickness and loss. The film thickness was changed by changing the alkoxide concentration of the alkoxide / IPA solution. A film having a film thickness of 0.2 μm is shown in FIG. 2A, and a film having a film thickness of 1.0 μm is shown in FIG. 2B. FIG. 3 is a schematic diagram of FIG. Normally, the thicker the coating, the better the insulation, so the eddy current loss can be suppressed. However, the soft magnetic powder of the present invention has a constant eddy current loss regardless of the coating thickness. The thin coating is preferable. In particular, when the thickness of the film is 1 μm or less, the effect is remarkably obtained. As the coating becomes thicker, the density decreases, with a tendency to increase hysteresis loss slightly.

(Example 7)
The relationship between the coating treatment of the insulating oxide with the alkoxide solution and the resulting oxygen amount was investigated. As performed in Example 1, 500 g of a spherical soft magnetic raw material powder having a composition of Fe-6.5% Si-1% Cr and an average particle size of 40 μm was added to a tetraalkoxysilane (Kanto Chemical) / IPA solution. It mixed with 100 mL and stirred for 3 hours using the propeller stirrer. The soft magnetic powder and the tetraalkoxysilane / IPA solution were separated and dried at 100 ° C. for 1 hour. The mixing and stirring of this soft magnetic raw material powder and tetraalkoxysilane were repeated, and the increase in oxygen content was measured. As shown in FIG. 6, the amount of oxygen increases almost in proportion to the number of treatments.
Further, when the relationship between the area ratio (coverage ratio) of the surface of the soft magnetic raw material powder covered with the insulating oxide and the amount of oxygen was examined, as shown in FIG. It was found that an insulating oxide covering the whole was obtained.

It is an EPMA line analysis result of a core section. It is a cross-sectional observation photograph of the soft magnetic metal powder of this invention. FIG. 3 is a schematic diagram of FIG. 2. It is a figure which shows the relationship between heat processing temperature and a hysteresis loss. It is a schematic diagram of the soft magnetic metal powder by this invention. It is a figure which shows the relationship between the covering state of an insulating oxide, and the oxygen amount of the whole soft-magnetic powder. It is a figure which shows the relationship between the covering state of an insulating oxide, and the oxygen amount of the whole soft-magnetic powder.

Explanation of symbols

  1 Raw material powder (soft magnetic powder), 2 insulating oxide, 3 Cr concentration site

Claims (5)

It is a soft magnetic powder represented by the composition formula Fe _100-ab Si _a Cr _b (weight%, 0 ≦ a ≦ 8, 0 <b ≦ 3), and the Cr concentration on the powder surface is the center of the powder Soft magnetic metal powder characterized by being higher. A part or the whole of the surface of the soft magnetic metal powder is covered with an insulating oxide, and the total amount of oxygen in the soft magnetic metal powder containing the insulating oxide is 10% by mass or less. Item 2. The soft magnetic metal powder according to Item 1. The soft magnetic metal powder according to claim 1 or 2, wherein a thickness of the insulating oxide is 1 µm or less. A green compact using the soft magnetic metal powder according to any one of claims 1 to 3. A method for producing a soft magnetic metal powder, wherein the insulating oxide is formed by heat-treating an oxide deposited from an alkoxide solution at 700 ° C. or higher.