Patent Application
20230130266
The present disclosure relates to a soft magnetic alloy powder, a dust core, and a magnetic device.
Magnetic devices such as inductors, transformers, and choke coils are widely used in power supply circuits of various electronic devices. In recent years, reduction of energy loss in power supply circuits and improvement of power supply efficiency have been emphasized for a low-carbon society, and higher efficiency and energy saving of magnetic devices are required.
In order to satisfy the above-mentioned requirements for magnetic devices, it is essential to improve the relative permeability of the magnetic core included in the magnetic device. In order to improve the relative permeability of the magnetic core, it is necessary to increase the packing rate of the magnetic powder contained in the magnetic core. Thus, in the field of magnetic devices, various attempts have been made with the aim of improving the packing rate of magnetic core. For example, Patent Document 1 discloses that the packing rate can be improved by increasing the circularity of the magnetic powder. Moreover, Patent Document 2 discloses a technique for increasing the packing rate of the magnetic powder by using a mixed powder of coarse powder and fine powder.
When the packing rate of the magnetic powder increases, however, the number of contact points between magnetic particles increases, and the withstand voltage of the magnetic core thus tends to decrease. That is, there is a trade-off relation between the packing rate (relative permeability) and the withstand voltage. In addition, the number of contact points for each particle differs as the packing rate increases, and the difference in the number of contact points increases the variation in withstand voltage, and the m value, which indicates the degree of variation, tends to decrease. Therefore, there is a demand for the development of a technique for obtaining a high withstand voltage and a high m value even when the packing rate of the magnetic powder is increased.
The present disclosure has been achieved under such circumstances. It is an object of the present disclosure to provide a soft magnetic alloy powder, a dust core, and a magnetic device capable of achieving a high withstand voltage and a high m value.
To achieve the above object, a soft magnetic alloy powder according to the present disclosure comprises:
a particle body comprising a soft magnetic alloy including Fe and Co; and
a surface layer located on a surface side of the particle body,
wherein
the surface layer includes one or more local maximum points of Si concentration and one or more local maximum points of Co concentration, and
DSi≤DCo is satisfied, in which
DSi is a distance from an interface between the particle body and the surface layer to a first Si local maximum point LSimax as a local maximum point located closest to a particle center among the one or more local maximum points of Si concentration, and
DCo is a distance from the interface to a first Co local maximum point LComax as a local maximum point located closest to the particle center among the one or more local maximum points of Co concentration.
When the soft magnetic alloy powder having the above-mentioned characteristics is used, the withstand voltage and the m value can be improved more than before with a high relative permeability.
Preferably, the surface layer satisfies DSi<DCo.
Preferably, the surface layer comprises an oxide phase.
Preferably, the surface layer comprises a Si oxide phase including a Si oxide, and the LSimax exists in the Si oxide phase.
Preferably, the surface layer comprises a Co oxide phase including a Co oxide, the LComax exists in the Co oxide phase, and a part of the Co oxide phase overlaps with a part of a surface side of the Si oxide phase. Instead, the Co oxide phase may be located closer to a surface side of the surface layer than the Si oxide phase.
The use of the soft magnetic alloy powder according to the present disclosure is not limited and can be applied to various magnetic devices. For example, the soft magnetic alloy powder according to the present disclosure can be favorably used as a dust core material in magnetic devices, such as inductors, transformers, and choke coils.
Hereinafter, the present disclosure is explained in detail based on an embodiment shown in the figures.
As shown in
The average particle size of the soft magnetic alloy powder 1 is not limited and can be, for example, 0.5 μm to 150 μm and is preferably 0.5 μm to 25 μm. When the soft magnetic alloy powder 1 includes other particles each including no surface layer 10, the average particle size of the first particles 1a is preferably 5 μm or more, and the average particle size of these other particles is preferably less than 5 μm.
Note that, the above-mentioned average particle size can be measured by various particle size analyzing methods, such as a laser diffraction method, but is preferably measured by a particle image analyzer Morphologi G3 (made by Malvern Panalytical Ltd). In Morphologi G3, the soft magnetic alloy powder is dispersed using air, and a projected area of the individual particles constituting the powder is measured so as to obtain a particle size distribution by circle equivalent diameters from the projected areas. Then, in the obtained particle size distribution, the average particle size is a particle size where a volume-based or number-based cumulative relative frequency is 50%. When the soft magnetic alloy powder 1 is included in the magnetic core, the average particle size is obtained by measuring the circle equivalent diameters of each particle included in the cross section of the magnetic core by cross-sectional observation using an electron microscope (SEM, STEM, or the like).
The particle body 2 is a base portion that occupies at least 90 vol % or more of the volume of the first particle 1a. Thus, the average composition of the first particle 1a can be regarded as the composition of the particle body 2, and the crystal structure of the first particle 1a can be regarded as the crystal structure of the particle body 2. The volume ratio of the particle body 2 can be substituted for the area ratio, and at least 90% or more of the cross-sectional area of the first particle 1a is the particle body 2.
The particle body 2 has a soft magnetic alloy composition including Fe and Co, and a specific alloy composition is not limited. For example, the particle body 2 can be a crystal type soft magnetic alloy of a Fe—Co based alloy, a Fe—Co—V based alloy, a Fe—Co—Si based alloy, a Fe—Co—Si—Al based alloy, or the like. Instead, from the point of lowering a coercivity, the particle body 2 is preferably constituted by an amorphous alloy composition or a nanocrystal alloy composition.
As an amorphous or nanocrystal soft magnetic alloy, a Fe—Co—P—C based alloy, a Fe—Co—B based alloy, a Fe—Co—B—Si based alloy, or the like may be mentioned. More specifically, the particle body 2 is preferably constituted by an alloy composition satisfying a compositional formula of (Fe(1-(α+β))CoαNiβ)(1-(a+b))X1aX2b When the particle body 2 is constituted by the alloy composition satisfying the above-compositional formula, a crystal structure made of amorphous, heteroamorphous, or nanocrystals tends to be obtained easily.
In the above-mentioned compositional formula, X1 is one or more elements selected from B, P, C, Si, and Al, and X2 is one or more elements selected from Ti, Zr, Hf, Nb, Ta, Mo, W, Cr, Ga, Ag, Zn, S, Ca, Mg, V, Sn, As, Sb, Bi, N, O, Au, Cu, rare earth elements, and platinum group elements. The rare earth elements include Sc, Y, and lanthanoids. The platinum group elements include Ru, Rh, Pd, Os, Ir, and Pt. Also, α, β, a, and b represent atomic ratios, and these atomic ratios preferably satisfy the following relations.
The Co content (α) with respect to Fe is within a range of 0.005≤α≤0.700, may be within a range of 0.010≤α≤0.600, may be within a range of 0.030≤α≤0.600, or may be within a range of 0.050≤α≤0.600. When the Co content (α) is within the above-mentioned range, the saturation magnetic flux density (Bs) and the corrosion resistance of the soft magnetic alloy powder 1 are improved. From the point of improving Bs, 0.050≤α≤0.500 is preferably satisfied. As the Co content (α) increases, the corrosion resistance tends to improve. When the Co content (α) is too large, however, Bs tends to decrease easily.
Also, the Ni content (β) with respect to Fe may be within a range of 0≤β≤0.200. That is, the soft magnetic alloy may not include Ni, and the Ni content (β) may be within a range of 0.005≤β≤0.200. From the point of improving Bs, the Ni content (β) may be within a range of 0≤β≤0.050, may be within a of 0.001≤β≤0.050, or may be within a range of 0.005≤β≤0.010. As the Ni content (β) increases, the corrosion resistance tends to improve. When the Ni content (β) is too large, however, Bs decreases.
When a sum of atomic ratios of elements constituting the soft magnetic alloy is 1, an atomic ratio (1−(a+b)) of a total amount of Fe, Co, and Ni is preferably within a range of 0.720≤(1−(a+b))≤0.950 and is more preferably within a range of 0.780≤(1−(a+b))≤0.890. When the above-mentioned relation is satisfied, Bs tends to improve easily. When 0.720≤(1−(a+b))≤0.890 is satisfied, amorphous is easily obtained, and the coercivity tends to decrease.
X1 may be included as impurities or may be added intentionally. The X1 content (a) may be within a range of 0≤a≤0.200. From the point of improving Bs, 0≤a≤0.150 is preferably satisfied.
X2 may be included as impurities or may be added intentionally. The X2 content (b) may be within a range of 0≤b≤0.200. From the point of improving Bs, 0≤b≤0.150 is preferably satisfied, and 0≤b≤0.100 is more preferably satisfied.
The composition of the above-mentioned particle body 2 (i.e., the composition of the first particle 1a) can be analyzed, for example, by inductively coupled plasma (ICP). Here, when it is difficult to determine an oxygen amount by ICP, an impulse heat melting extraction method can also be used. If it is difficult to determine a carbon amount and a sulfur amount by ICP, an infrared absorption method can also be used.
Except for ICP, a compositional analysis may be carried out by energy dispersive X-ray spectroscopy (EDX) or electron probe microanalyzer (EPMA) attached to an electron microscope. For example, regarding the soft magnetic alloy powder 1 included in a dust core containing a resin component, a compositional analysis by ICP may be difficult in some cases. In this case, the compositional analysis may be carried out using EDX or EPMA. If a detailed compositional analysis is difficult by any of the above-mentioned methods, the compositional analysis may be performed using three dimensional atom probe (3DAP). When 3DAP is used, the composition of the particle body 2 can be measured without the influence of the resin component, a surface oxidation, and the like in the area of analysis. This is because 3DAP can measure an average composition by determining a small area (e.g., an area of φ20 nm×100 nm) in the first particle 1a.
Note that, when a cross section near the surface of the first particle 1a is analyzed by a line analysis using EDX or electron energy loss spectroscopy (EELS), the particle body 2 can be recognized as an area having stable concentrations of Fe and Co (see
The crystal structure of the particle body 2 (i.e., the crystal structure of the first particle 1a) can be a crystalline structure, a nanocrystal structure, or an amorphous structure and is preferably a nanocrystal structure or an amorphous structure from the point of lowering the coercivity. For example, an amorphous degree X of the particle body 2 is preferably 85% or more. The crystal structure having an amorphous degree X of 85% or more is a structure that is mostly comprised of amorphous or heteroamorphous. The structure comprised of heteroamorphous is a structure in which crystals slightly exist inside amorphous. That is, in the present embodiment, an “amorphous structure” is a crystal structure having an amorphous degree X of 85% or more and means that crystals may be included in a range where this amorphous degree X is satisfied.
Note that, when the structure is heteroamorphous, the average crystal grain size of crystals existing in the amorphous structure is preferably within the range of 0.1 nm or more and 10 nm or less. In the present embodiment, “nanocrystal” means a crystal structure having an amorphous degree X of less than 85% and an average crystal grain size of 100 nm or less (preferably, 3 nm to 50 nm), and “crystalline” means a crystal structure having an amorphous degree X of less than 85% and an average crystal grain size of larger than 100 nm.
The amorphous degree X can be measured by X-ray crystallography using XRD. Specifically, 2θ/θ measurement is performed using XRD to the powder of the first particle 1a, and an X-ray diffraction chart is obtained. At this time, a measurement range of diffraction angle 2θ may be set so that an amorphous-derived halo pattern can be confirmed. For example, it is preferable to set 2θ in a range including 30° to 60°.
Next, the X-ray diffraction chart is profile-fitted using a Lorentz function represented by the following equation (2). In this profile fitting, a difference between the integrated intensities actually measured by XRD and the integrated intensities calculated using the Lorentz function is preferably determined within 1%. As a result of this profile fitting, a crystal scattering integrated intensity Ic and an amorphous scattering integrated intensity Ia are obtained. Then, the amorphous degree X is obtained by placing the crystal scattering integrated intensity Ic and the amorphous scattering integrated intensity Ia in the following equation (1).
X=100−(Ic/(Ic+Ia)×100) Equation (1)
Ic: crystal scattering integrated intensity
Ia: amorphous scattering integrated intensity
h: peak height
u: peak position
w: half bandwidth
b: background height
Note that, a method of measuring the amorphous degree X is not limited to the above-mentioned method using XRD, and the amorphous degree X may be measured by electron backscatter diffraction (EBSD) or electron diffraction.
The surface layer 10 is an area where the content rate of constituent elements of the soft magnetic alloy, such as Fe and Co, is different from that in the particle body 2. The surface layer 10 covers at least a part of periphery of the particle body 2. The coverage of the surface layer 10 with respect to the particle body 2 in the cross section of the first particle 1a is not limited and can be, for example, 50% or more, preferably 80% or more.
The surface layer 10 can be analyzed by observing a cross section near the surface of the first particle 1a with a scanning transmission electron microscope (STEM) or a transmission electron microscope (TEM) and performing a line analysis using EDX or EELS at that time. In the line analysis, as shown in
For example, the graphs shown in
As shown in
Specifically, a method for determining the change points CP and the interface 21 is described. First, a horizontal line AL corresponding to the average concentration in the particle body 2 is drawn in the concentration distribution of each constituent element. Then, an approximation straight line TL is drawn in a region where the concentration of the constituent element monotonically increases or decreases from the particle body 2 toward the particle surface side. The intersection between the horizontal line AL and the approximate straight line TL is defined as a change point CP in the concentration distribution of each constituent element. In
As shown in
Among one or more local maximum points of Si concentration, the local maximum point closest to the interface 21 (i.e., the local maximum point located closest to the center of the particle) is defined as a first Si local maximum point LSimax. In the graph of
In the concentration distribution near the surface as shown in
As described above, since the surface layer 10 includes LSimax and LComax and satisfies DSi≤DCo, the magnetic core including the soft magnetic alloy powder 1 of the present embodiment can improve the withstand voltage with a high relative permeability. In addition, variations in withstand voltage can be reduced (i.e., m value can be increased), and magnetic devices can be produced stably. In particular, since the surface layer 10 satisfies DSi<DCo, the withstand voltage and the m value can be further improved.
When the above-mentioned relation between DSi and DCo is represented by the formula “DCo−DSi”, “DCo−DSi” is 0 nm or more, preferably larger than 0 nm, more preferably 3 nm or more, and still more preferably 5 nm or more. The upper limit of “DCo−DSi” is not limited and can be, for example, 30 nm or less and may be 10 nm or less. The value of DSi and the value of DCo are not limited. For example, DSi is preferably 20 nm or less, and DCo is preferably 30 nm or less.
The surface layer 10 can be a metal phase, an oxide phase, a metal compound phase other than an oxide, or the like and preferably includes an oxide phase. When the surface layer 10 includes an oxide phase, a higher concentration of oxygen than in the particle body 2 is detected in the surface layer 10. For example, the graphs shown in
As shown in
The Si oxide phase 12 is a region where the Si concentration is higher than that in the particle body 2 and a convex peak related to the Si concentration exists. LSimax is located in the Si oxide phase 12. The Co oxide phase 14 is a region where a convex peak related to the Co concentration exists, and LComax is located in the Co oxide phase 14. In
Since the surface layer 10 includes the oxide phase (12, 14) structure as shown in
In addition to Si, Co, and O, each oxide phase (12, 14) may include elements constituting the average composition of the first particles 1a, such as Fe, Cr, Al, B, and P.
In the soft magnetic alloy powder 1 of the present embodiment, the thickness T of the surface layer 10 is not limited, but is, for example, preferably 1 nm or more and 30 nm or less, more preferably 5 nm or more and 20 nm or less. The thickness T of the surface layer 10 can be calculated as a distance from the interface 21 to an outer surface 10a of the surface layer 10. In the measurement of the thickness T, the interface 21 can be determined based on the change points CP as mentioned above, and the outer surface 10a can be determined by the following method.
For example, in the graph of
The first particle 1a may include an insulating layer covering the surface layer 10. The insulating layer is a coated layer formed by coating or the like after forming the surface layer 10 and has an average thickness of preferably 1 nm or more and 100 nm or less, more preferably 50 nm or less. In the TEM image or the STEM image, the insulating layer may be recognized as a region having a different contrast from that of the particle body 2 and the surface layer 10. In this case, the outer surface 10a of the surface layer 10 can be determined based on the contrast of the TEM image or the STEM image. Instead, the outer surface 10a of the surface layer 10 may be determined based on the concentration distribution of an element M specific to the insulating layer. According to the line analysis result, the concentration of the specific element M increases in the region where the surface layer 10 is switched to the insulating layer, and the change point where the specific element M increases may thus be defined as the outer surface 10a of the surface layer 10.
Hereinafter, a method of producing the soft magnetic alloy powder 1 according to the present embodiment is described. The soft magnetic alloy powder 1 according to the present embodiment can be produced by performing a surface modification treatment after producing a powder by a well-known method.
A method of producing the soft magnetic alloy powder before performing a surface modification treatment is not limited. For example, the soft magnetic alloy powder may be produced by an atomizing method, such as a water atomizing method and a gas atomizing method. Instead, the soft magnetic alloy powder may be produced by a synthesis method, such as a CVD method, using at least one or more of metal salt evaporation, reduction, and thermal decomposition. Instead, the soft magnetic alloy powder may be produced using an electrolysis method or a carbonyl method. Moreover, the soft magnetic alloy powder may be produced by pulverizing a starting alloy in the form of ribbon or thin plate. The produced powder may be appropriately classified so as to adjust the particle size of the soft magnetic alloy powder.
Next, the surface layer 10 is formed on the surface of the first particle 1a by subjecting the soft magnetic alloy powder to a surface modification treatment. The method for surface modification includes a CVD method, a mechanochemical method, and the like and is not limited. In the present embodiment, it is particularly preferable to carry out a surface modification treatment by the mechanochemical method in an atmosphere in which the oxygen partial pressure is controlled. Hereinafter, the mechanochemical method is described.
Conventionally, as a surface treatment method for the soft magnetic alloy powder, a method of forming an oxide layer on the surface of the particle by subjecting the powder to a heat treatment is known. In the conventional heat treatment, however, it is necessary to adjust conditions, such as temperature, according to the type of powder, and it is thus difficult to uniformly control the composition and the internal structure of the oxide layer.
On the other hand, the mechanochemical method is a method of applying a mechanofusion apparatus to a surface modification of the soft magnetic alloy powder. The mechanofusion apparatus is an apparatus that is conventionally used for a coating treatment of various powders. The inventors of the present disclosure have found that a desired surface layer 10 can be formed uniformly even for different types of powders by using a mechanofusion device to form the surface phase of powder by a different method from a conventional coating treatment.
In the mechanochemical method, first, the inside of the mechanofusion apparatus is made into a desired oxidizing atmosphere. For example, the oxygen partial pressure in the apparatus can be adjusted by using a mixed gas of Ar gas and air as the atmospheric gas to be filled in the apparatus and controlling the partial pressure of Ar gas and air in the mixed gas. The oxygen partial pressure in the apparatus is, for example, preferably 100 ppm to 3000 ppm, more preferably 500 ppm to 3000 ppm, and even more preferably 500 ppm to 1000 ppm. In the mixed gas, oxygen gas may be used instead of air, and inert gas, such as nitrogen gas and helium gas, may be used instead of Ar gas.
Next, the soft magnetic alloy powder is introduced into a rotating rotor of the mechanofusion apparatus, and the rotating rotor is rotated. A press head is installed inside the rotating rotor, and when the rotating rotor is rotated, the soft magnetic alloy powder is compressed in the gap between the inner wall surface of the rotating rotor and the press head. At this time, friction occurs between the soft magnetic alloy powder and the inner wall surface of the rotating rotor, and the soft magnetic alloy powder locally heats up. Due to this frictional heat, the surface layer 10 is formed on the surface of the particle body 2. In particular, the surface layer 10 including the oxide phases (12, 14) is easily formed by the above-mentioned mechanochemical method.
In the mechanochemical method, it is preferable to control the oxygen partial pressure within an appropriate range and to appropriately control the rotational speed of the rotating rotor and the gap between the inner wall surface of the rotating rotor and the press head. For example, the frictional heat generated with a low rotational speed is small, and the surface layer 10 is hard to be formed. On the other hand, if the rotational speed is too large, the compressive stress applied to the powder is large, and the surface layer 10 is likely to be formed. However, if the rotational speed is too large, the particle body 2 and the surface layer 10 are likely to be destroyed, and this may lead to deterioration of magnetic characteristics. In addition, if the gap between the inner wall surface of the rotating rotor and the press head is too large, the amount of frictional heat generated is small, and the surface layer 10 is hard to be formed. On the other hand, the smaller the gap between the inner wall surface of the rotating rotor and the press head is, the larger the compressive stress applied to the powder is, and the easier it is for the surface layer 10 to be formed, but the particle body 2 and the surface layer 10 are more likely to be destroyed.
After the surface modification by the mechanochemical method, a heat treatment may be performed in an atmosphere in which the surface structure does not change in order to remove the stress generated by the mechanochemical method.
When an insulating layer is formed on the surface layer 10, a coating treatment, such as a phosphate coating treatment, mechanical alloying, a silane coupling treatment, and hydrothermal synthesis, is performed after the surface modification treatment by the mechanochemical method. The material of the insulating layer to be formed includes phosphates, silicates, soda-lime glass, borosilicate glass, lead glass, aluminosilicate glass, borate glass, sulfate glass, or the like. For example, phosphates include magnesium phosphate, calcium phosphate, zinc phosphate, manganese phosphate, cadmium phosphate, and the like. Also, silicates include sodium silicate, and the like.
Through the above-mentioned steps, the soft magnetic alloy powder 1 including the surface layer 10 is obtained.
The use of the soft magnetic alloy powder 1 according to the present embodiment is not limited and can be applied to various magnetic devices. In particular, the soft magnetic alloy powder 1 can be favorably used as a dust core material in magnetic devices, such as inductors, transformers, and choke coils. Hereinafter, an example of a dust core and a magnetic device including the soft magnetic alloy powder 1 is described with reference to
A dust core 40 including the soft magnetic alloy powder 1 is formed to have a predetermined shape, and its outer dimensions and shape are not limited. As shown in the schematic cross-sectional view of
The soft magnetic alloy powder 1 of the dust core 40 may be composed only of the first particles 1a each including the surface layer 10, but is preferably composed by, as shown in
The ratio of the first particles 1a and the fine particles 1b in the dust core 40 is not limited. For example, the mass ratio indicated by “first particles 1a:fine particles 1b” can be in the range of 10:90 to 90:10, preferably in the range of 60:40 to 90:10.
The material of the resin 4 is not limited and may be, for example, a thermosetting resin, such as epoxy resin. The content rate of the resin 4 in the dust core 40 is not limited and is preferably, for example, 1.0 mass % to 2.5 mass %.
The packing rate of the soft magnetic alloy powder 1 in the dust core 40 can be controlled by the manufacturing conditions, such as compacting pressure, the content rate of the resin 4, or the like and can be, for example, 70 vol % to 90 vol %. From the point of increasing the relative permeability, the packing rate of the soft magnetic alloy powder 1 is preferably 80 vol % or more.
In conventional dust cores, when the packing rate of the magnetic powder is high, the relative permeability is high, but the withstand voltage is low, and it is thus difficult to achieve both high relative permeability and high withstand voltage. On the other hand, in the dust core 40 of the present embodiment, since the surface layer 10 having predetermined characteristics exists on each constituent particle (1a) of the soft magnetic alloy powder 1, it is possible to improve withstand voltage and m value even at a high packing rate of 80 vol % or more.
The method of manufacturing the dust core 40 is not limited. For example, the first particles 1a subjected to a surface modification treatment by a mechanochemical method and the fine particles 1b are mixed, and the resulting mixed powder and a thermosetting resin are thereafter kneaded to obtain a resin compound. Then, the resin compound is filled in a die and molded with pressure, and the thermosetting resin is thereafter cured to obtain the dust core 40 as shown in
In the magnetic device 100 shown in
Since the dust core 40 forming the element body has a good withstand voltage characteristic, the magnetic device 100 of the present embodiment is favorable for, for example, power inductors used in power supply circuits. The magnetic device including the soft magnetic alloy powder 1 is not limited to the mode as shown in
Hereinabove, an embodiment of the present disclosure is described, but the present disclosure is not limited to the above-mentioned embodiment and may be variously modified within the scope of the present disclosure.
Hereinafter, the present disclosure is described in further detail based on specific examples, but is not limited to the following examples. In the following tables, a sample number with “*” mark indicates a comparative example.
In Experiment 1, six types of soft magnetic alloy powders (Powder A to Powder F) shown in Table 1 were produced. All of Powder A to Powder F were prepared by the following procedure.
First, pure metal raw materials of Fe, Co, and other subcomponents were prepared and weighed so as to obtain a desired composition after melting. Then, the weighed pure metal raw materials were melted by high-frequency heating in an evacuated chamber to obtain a mother alloy. Next, the produced mother alloy was heated at 1500° C. and melted again, and a powder having a predetermined composition was thereafter obtained by a high-pressure water atomizing method. After the atomization, the obtained powder was classified by a predetermined method to adjust the particle size of the powder. The average particle size (D50) of Powder A to Powder F produced by the above-mentioned method was all within the range of 15 μm to 25 μm.
Next, each of Powder A to Powder F was divided into a plurality of samples, and each sample was subjected to a surface treatment under any of the conditions shown in Table 2.
In Conditions 1 to 5, the powder samples were subjected to a heat treatment while controlling the oxygen partial pressure within the range shown in Table 2. The temperature of the heat treatment was determined in an optimum range according to the composition of Powder A to Powder F.
In Conditions 6 to 10, the powder samples were subjected to a surface modification treatment by a mechanochemical method. At this time, the oxygen partial pressure in the rotating rotor was controlled within the range shown in Table 2 using AMS-Lab manufactured by Hosokawa Micron Corporation as a mechanofusion apparatus.
In Condition 11, a coating layer with a two-layer structure was formed on each of the surfaces of the particles constituting the powder sample by the following procedure. First, an aqueous solution of cobalt phosphate and the powder sample were put into a V-type mixer and sufficiently mixed, and the powder sample taken out of the mixer was thereafter sufficiently dried in the atmosphere. Next, the above-mentioned powder sample and a treatment liquid containing a phosphate and a silica source were put into a V-type mixer and sufficiently mixed, and the powder sample taken out of the mixer was thereafter sufficiently dried at 150-250° C. in the atmosphere.
The coating treatment of Condition 11 was performed only on the samples divided from Powder A. In the samples subjected to the coating treatment of Condition 11, a coating layer containing Co was formed on the contact side with the particle body, and a coating layer containing Si was formed on the coating layer containing Co. The total thickness of the coating layers formed by the coating treatment of Condition 11 (the sum of the thickness of the coating layer containing Co and the thickness of the coating layer containing Si) was within the range of 5 nm to 10 nm.
Next, dust cores were produced in the following procedure using the powder sample subjected to any of the surface treatment of Conditions 1 to 11. In Experiment 1, the powder sample subjected to any of Conditions 1 to 11 was a main powder, and a fine powder was mixed with the main powder to obtain a magnetic powder for the dust core. In all of the samples of Experiment 1, an Fe based soft magnetic alloy having an average particle size (D50) of 1 μm was used as the fine powder, and the mass ratio between the main powder and the fine powder was main powder:fine powder=80:20.
Then, the magnetic powder and an epoxy resin were kneaded to obtain a resin compound. In all of the samples of Experiment 1, the blending ratio between the magnetic powder and the epoxy resin was controlled so that the resin content rate in the dust core was 2.5 wt %. A toroidal green compact was obtained by filling the above-mentioned resin compound into a die and pressurizing it. At this time, the compacting pressure was within the range of 1 to 10 tons/cm2 and controlled so that the packing rate of the magnetic powders was at least 80 vol % in all of the samples of Experiment 1. Then, the green compact was heated at 180° C. for 60 minutes to cure the epoxy resin in the green compact, and a dust core having a toroidal shape (outer diameter: 11 mm, inner diameter: 6.5 mm, and thickness: 2.5 mm) was obtained.
In each sample of Experiment 1, the following evaluations were performed on the prepared powder sample (main powder) and dust core.
The surface structure of the soft magnetic alloy powders (Powder A to Powder F (main powders)) subjected to the predetermined surface treatment was analyzed by a line analysis using TEM-EDX. In the line analysis, the presence or absence of LSimax (local maximum point of Si concentration), the presence or absence of LComax (local maximum point of Co concentration), and “DCo−DSi” were determined.
The dimensions and mass of the produced dust core were measured, and the density p of the dust core was calculated from the dimensions and mass. Moreover, the theoretical density of the dust core was calculated from the specific gravity of the magnetic powder, assuming that the dust core was composed of only the magnetic powder. Then, the packing rate of the magnetic powder in the dust core was calculated by dividing the density p by the theoretical density.
A polyurethane copper wire (UEW wire) was wound around the toroidal dust core. Then, an inductance of the dust core at a frequency of 100 kHz was measured using an LCR meter (4284A manufactured by Agilent Technologies), and a relative permeability (no unit) of the dust core was calculated based on the inductance.
In the measurement of withstand voltage characteristic, a cylindrical test core was produced in the same manner as the toroidal core, and In—Ga electrodes were formed on both end surfaces of the test core. Next, a voltage was applied to the test core using a withstand voltage tester (THK-2011ADMPT manufactured by Tama Densoku Co., Ltd.), and a voltage value when an electric current of 1 mA flowed was measured. Then, a withstand voltage of the test core was measured by dividing the measured voltage value by the length of the test core (distance between the end surfaces).
The withstand voltages were measured on 20 test cores for each sample, and an average value of the 20 test cores was taken as the withstand voltage of each sample. Then, the withstand voltage of each sample was relatively evaluated using the withstand voltage of the reference sample. Specifically, a dust core was produced using a powder not subjected to the surface treatment shown in Table 2 and used as the reference sample. Then, a sample exhibiting a withstand voltage of less than 1.3 times with respect to the withstand voltage of the reference sample was considered to be “failed (F)”, a sample exhibiting a withstand voltage of 1.3 times or more and less than 1.5 times was considered to be “good (G)”, and a sample exhibiting a withstand voltage of 1.5 times or more was considered to be “very good (VG)”.
A Weibull plot was obtained using the withstand voltage data of the 20 test cores as a population, and a m value (no unit) of each sample was calculated from the Weibull plot. The m value is an index showing the degree of variation in withstand voltage. A m value of 3.0 or more was considered to be good, and a m value of 5.5 or more was considered to be very good.
The evaluation results of each sample in Experiment 1 are shown in Tables 3-8. Table 3 shows evaluation results of samples using Powder A as the main powder, Table 4 shows evaluation results of samples using Powder B as the main powder, Table 5 shows evaluation results of samples using Powder C as the main powder, Table 6 shows evaluation results of samples using Powder D as the main powder, Table 7 shows evaluation results of samples using Powder E as the main powder, and Table 8 shows evaluation results of samples using Powder F as the main powder. In each table, “-” in the column of surface treatment method means that the surface treatment shown in Table 2 was not performed. In addition, “-” in the column of DCo−DSi in each table means that DCo−DSi could not be measured because the surface layer of the main powder did not include LSimax and/or LComax.
As shown in Table 3, Table 5, and Table 7, the withstand voltage characteristic was not improved even when the surface modification treatment by the mechanochemical method was performed in the sample using the main powder (Powder A, Powder C, or Powder E) not containing Co. The withstand voltage characteristic was improved in Sample A-12 subjected to the coating treatment of Condition 11. However, in Sample A-12 satisfying 0>(DCo−DSi), the variation in withstand voltage was large, and the m value was not improved.
On the other hand, as shown in Table 4, Table 6, and Table 8, LSimax and LComax were formed on the particle surface layer by performing a surface modification treatment by a mechanochemical method in the sample using the main powder containing Co (Powder B, Powder D, or Powder F). Then, a high withstand voltage and a high m value were obtained in the sample satisfying 0≤(DCo−DSi). In addition, a relative permeability comparable to that of the reference sample was obtained in the sample satisfying 0≤(DCo−DSi). This result indicates that when the surface layer of the soft magnetic alloy powder includes LSimax and LComax and satisfies DSi≤DCo, the withstand voltage and the m value can be improved with a high relative permeability. In particular, it was found that when 3≤(DCo−DSi) is satisfied, the withstand voltage and the m value are further improved.
In the sample satisfying 0≤(DCo−DSi), the surface layer of the soft magnetic alloy powder includes an oxide phase containing Si and an oxide phase containing Co.
In Experiment 2, dust cores were produced using a fine powder different from that in Experiment 1 and a main powder (Powder B, Powder D, or Powder F). Specifically, in Experiment 2, the fine powder was an FeNi based soft magnetic alloy powder having an average particle size (D50) of 1 μm. In Experiment 2, the experimental conditions other than the type of fine powder were the same as those in Experiment 1, and the same evaluations as in Experiment 1 were performed. The evaluation results of Experiment 2 are shown in Tables 9-11. In addition to the results of Experiment 2, Tables 9-11 also show the evaluation results of Experiment 1 using the Fe based fine powder.
The results of Tables 9 to 11 indicate that the relative permeability may change by changing the type of fine powder. In the sample satisfying DSi≤DCo, even when the relative permeability was changed by the type of fine powder, the withstand voltage characteristic did not change, and a high withstand voltage and a high m value were obtained.
In each sample of Experiment 3, the resin content rate in the dust core was changed. Specifically, an epoxy resin and a magnetic powder containing a predetermined main powder (Powder B, Powder D, or Powder F) were kneaded so that the resin content rate was 2.5 wt %, 2.0 wt %, 1.5 wt %, or 1.0 wt %. In Experiment 3, the experimental conditions other than the resin content rate were the same as those in Experiment 1, and the same evaluations as in Experiment 1 were performed. The evaluation results of Experiment 3 are shown in Tables 12-14.
As shown in Tables 12-14, in each of the samples including no surface layer 10, the relative permeability was improved by reducing the resin content rate, but the withstand voltage and the m value were reduced. On the other hand, in each of the sample including the surface layer 10 satisfying DSi≤DCo, a high withstand voltage and a high m value were obtained even though the resin content rate was reduced. That is, the sample satisfying DSi≤DCo can achieve both a high relative permeability and a high withstand voltage characteristic even when the resin content rate is reduced.
In Experiment 4, an insulating layer composed of a phosphate based compound was formed on each particle surface of the main powder (Powder B, Powder D, or Powder F) by phosphate treatment. Specifically, samples each including only the insulating layer without performing a mechanochemical treatment and samples each including the insulating layer after performing a mechanochemical treatment were prepared. In all of the samples of Experiment 4, the average thickness of the insulating layers was within the range of 1 nm to 50 nm, and the resin content rate was 1.0 wt %. In Experiment 4, the experimental conditions other than the above were the same as those in Experiment 1, and the same evaluations as in Experiment 1 were performed. The evaluation results of Experiment 4 are shown in Table 15.
The results in Table 15 indicate that the withstand voltage characteristic was further improved by further forming an insulating layer on the outer surface of the surface layer 10 satisfying DSi≤DCo.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-172504 | Oct 2021 | JP | national |
