JPS63161602A - Dust core having excellent high-frequency magnetic characteristic - Google Patents

Abstract

PURPOSE:To obtain a core having high threshold frequency fcr without executing special insulating coating treatment by fining iron powder grains while adding a resin as a binder and a lubricant and molding the iron powder grains. CONSTITUTION:When the mean grain size of iron powder is represented by dmum, the content of a lubricant by x wt. % and the sectional area of a magnetic core vertical to a magnetic path by Acm<2>, these factors satisfy the relationship of d<=70mum and 10(A/d<2>)<0.3=x<=2.5. The mean grain size of iron powder is brought to 70mum or less in order to reduce transgranular eddy currents as one of conditions required for increasing fcr. Accordingly, a particular intergranular insulating material is not needed, thus easily acquiring a dust core having excellent high-frequency characteristics at low cost.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a powder magnetic core made of iron powder suitable for use in noise filters, smooth chokes, etc., and in particular aims to further improve high frequency magnetic properties. It is something.

(Prior Art) With the recent diversification of electrical and electronic equipment, various characteristics are required of magnetic materials.

Among these, powder magnetic cores made by molding ferromagnetic powder such as iron powder together with resin etc. have excellent DC superposition characteristics because the magnetic flux density is difficult to saturate with an increase in applied magnetic field, and also have excellent eddy current characteristics. It is used in power supply noise filters, smoothing chokes, etc. because it does not flow easily and has excellent frequency characteristics. However, even with regard to such powder magnetic cores, further improvement in high frequency characteristics is required due to stricter noise regulations and higher frequency equipment, and furthermore, the cost upper bound due to such characteristic improvements is at a minimum. We are now in a situation where we have to suppress it to the maximum extent possible.

Generally, the characteristics of a powder magnetic core can be evaluated using DC magnetic permeability: μDC and limit frequency: fcr.

To explain the evaluation method here, first, as shown in FIG. 1, the frequency dependence of magnetic permeability (initial magnetic permeability) is measured. Here, when the frequency becomes sufficiently small, the magnetic permeability μ
approaches a constant value, and this value is usually set to μ. . Expressed as on the other hand,
As the frequency increases, the magnetic permeability decreases. This is caused by eddy currents flowing in the magnetic core, but the magnetic permeability is μ8.
．． Letting fcr be the frequency when the frequency drops to 80% of the fcr, a magnetic core with excellent frequency characteristics is one in which fcr is as large as possible while μ and C are maintained at a certain level or higher.

The most common dust core made of iron powder has an average particle size of 80 μm.
Electrolytic iron powder, atomized iron powder or reduced iron powder of
It is molded and hardened with a binder such as resin or water glass. This type of product is manufactured using a hour wheel, so it has the advantage of low cost, and its characteristic level is μ. .

is about 60 to 80, and fcr is about 0.5 to 1.2 hillz.

By the way, what determines the frequency characteristics of the powder magnetic core is the eddy current flowing through the magnetic core. Eddy current includes intragranular eddy current, which flows closed inside the iron powder particles, and intergranular eddy current, which flows between particles. There is. In order to suppress intragranular eddy currents, it is necessary to use iron powder as an alloy powder to increase the electrical resistance specific to the particles, or to reduce the size of the particles. On the other hand, in order to suppress intergranular eddy currents, it is necessary to improve electrical insulation between particles.

Only by suppressing both of these eddy currents can fcr
can be made larger.

Therefore, efforts have been made to make the particle size of iron powder finer and to improve the insulation between the particles.

For example, “Industrial Heating M
ay, 1982 + page 14'', the above object is achieved by coating the particles with an electrical insulator.

However, this method has the disadvantage that coating is time-consuming and leads to a significant increase in cost.

The present invention solves the above-mentioned problems, and aims to propose an advantageous manufacturing method that is simple, low-cost, and capable of producing a powder magnetic core with excellent high-frequency characteristics.

(Means for solving the problem) As a result of intensive studies to solve the above problem, the inventors made the iron powder particles finer and added a resin as a binder and a lubricant. By molding with FCR, no special insulation coating treatment is required.
It was discovered that a high magnetic core can be obtained.

This invention is based on the above knowledge, and the most important point in this invention is the novelty that when iron powder particles are fine, the lubricant acts extremely effectively to increase the insulation between the particles. It is a fact.

That is, the present invention provides a powder magnetic core formed by molding a mixture of iron powder, resin, and lubricant, wherein the average particle diameter of the iron powder is d.
(μff1), the lubricant content is X (wtχ (hereinafter simply expressed as %)), and the cross-sectional area of the magnetic core perpendicular to the magnetic path is A (cn
When expressed as:
10 (A/d'') 0・'≦X≦2.5
- A powder magnetic core with excellent high-frequency magnetic properties, characterized by satisfying the relationship 12)).

Here, the average particle size is expressed as the median diameter of the weight particle size distribution measured by the particle size distribution of iron powder using the sieve weight or microtrack method. Also, the cross-sectional area of the magnetic core is the outer diameter = D0
, inner diameter: D, height = H, if it is a toroidal magnetic core, A
=H(DO-DI)/2.

This invention will be specifically explained below.

First, in this invention, the average particle size of iron powder is 70A! m or less, in order to reduce intragranular eddy current, which is one of the essential conditions for increasing the limit frequency fcr.
Assuming that the insulation between particles is sufficient, f cr increases as the average particle size decreases.

FIG. 2 shows the results of an investigation into the influence of the average grain size of iron powder on the limit frequency. The experiment was conducted as follows.

The mill scale reduced iron powder was sieved to obtain iron powders a to e having the following average particle diameters. Note that the average particle size was determined as the median value of the weight particle size distribution measured by the one-microtrack method.

a: 23 μm, b: 51 μm, c: 6
7μ product.

d: 84 μm, e: 111 μm
Add 1.4% zinc stearate and 0 epoxy resin powder (-250 mesh) to each iron powder a''-e as a lubricant.
．． 5% mixed, 7t/cI+! At the pressure of φ13×φ8×
It was molded into a ring shape of H111. This molded body was then cured in the atmosphere at 150° C. for 1 hour to obtain a powder magnetic core.

A formal copper wire with a diameter of 0.5 fin was wound 20 times around the thus obtained magnetic core, and the frequency dependence of the initial magnetic permeability was measured in the frequency range of 10 kHz to 10 MHz using an impedance meter to determine the limit frequency fer.

As is clear from the figure, when iron powder with an average particle size of 70 μm or less was used, excellent frequency characteristics with an fcr of 1.5 MHz or more were exhibited.

In this case, A is 0.125 C, and the lower limit of the amount of lubricant added in the invention method for each iron powder: 10 (A
The values of /d2)' and 3 are a: 0.82% and
b: o, si%, c: o, 43%.

d: o, 38%, e: 0.32%.

So, with this invention, the upper limit of the average particle size of iron powder is 70.
It was determined to be μm.

The iron powder is then molded together with a resin and a lubricant to form a magnetic core. Here, the resin has the role of a binder to harden the iron powder molded body and also contributes to electrical insulation between particles, and for example, epoxy resin powder is suitable. Although liquid resins cannot be used, they are more difficult to handle when added to iron powder than powders.

In addition, lubricants originally reduce friction between iron powder particles or between particles and a mold when molding iron powder, increase molding density, increase magnetic permeability, and suppress mold wear. For example, powders such as zinc stearate are advantageously suitable.

By the way, in the past, lubricants were not expected to have an electrical insulation effect between iron powder particles, but according to the inventors' experiments, when iron powder particles are fine, the lubricant has a larger electrical insulation effect than resin. It was discovered that it has an insulating effect.

This fact has extremely important significance in realizing a magnetic core with high characteristics at low cost. This is because when trying to achieve interparticle insulation by increasing the amount of resin, if the amount of resin exceeds a certain limit, it becomes difficult to mold and causes problems in manufacturing, but by increasing the amount of lubricant, This is because such problems can be eliminated by providing insulation, and moreover, a high effect can be obtained with a small amount.

The reason why the lubricant has such a high insulating effect is that when mixed with iron powder, lubricant powder easily adheres to the surface of the iron powder particles and covers the iron powder particles more uniformly than resin powder. Conceivable. For this reason, it is presumed that especially when the iron powder particles are fine, the effect is even greater than in the case of resin.

The appropriate amount of the lubricant to be added is determined depending on the particle size of the iron powder and the size of the magnetic core.

Therefore, next, we will discuss the DC permeability μoc and the critical frequency fcr.
The effect of the amount of lubricant and resin added on the amount of lubricant and resin added was investigated. The experiment was conducted as follows.

The amount of lubricant is 0.5 to 3% to mill scale reduced iron powder with an average particle size of 23 μm (at this time, the amount of resin is constant at 0.5%).
The results shown in Figure 2 above were obtained by adding various amounts of resin within the range of 0.5 to 3% (at this time, the amount of lubricant was constant at 0.5%). A ring-shaped powder magnetic core was prepared in the same manner as in the previous experiment.

The results of investigating the relationship between the DC magnetic permeability μDC of the powder magnetic core obtained in this way and the amounts of lubricant and resin added are summarized in the third section.
Figure 4 also shows the results of an investigation into the relationship between the limit frequency and the amount added.

As is clear from Figure 3.4, as the amount of lubricant or resin added increases, the insulation between particles improves, so generally f
Although cr improves, μ and C decrease. Here, lubricant has a greater insulating effect than resin, and rc
It can be seen that r is much higher and the value of μnc is also superior. In the example in Figure 4,
When the lubricant content is 0.8% or more, the superiority of the insulating effect of the lubricant to the resin is obvious. In particular, when the lubricant content is 1.4%, fcr is large and μ. . Since the decrease in the amount of lubricant is small, the optimum amount of lubricant added in this example can be said to be approximately 1.4%. On the other hand, the upper limit of the amount of added resin in the examples shown in FIGS. 3 and 4 is the point at which μIlc begins to decrease rapidly as the amount added increases2,5
It can be considered as %.

Thus, the upper limit for additive lubricant is μ. . , the lower limit is defined by fcr. μ9 here. is hardly affected by the particle size of the iron powder or the size of the magnetic core, so the upper limit of the lubricant amount of 2.5% shown above can be said to be universal.

However, since fcr depends on the particle size of the iron powder and the size of the magnetic core, it is essentially a lubricant! (X) It is not possible to set a lower limit. Lower limit value 0 found in Figures 3 and 4
．． 8% is applied only when the average particle diameter is 23 μm and the magnetic core size is φ13 (outer diameter) x φ8 (inner diameter) x 5 (height).

Generally, in order to find the lower limit of X, it is necessary to estimate an appropriate value of X as a function of the given average particle diameter of iron powder and magnetic core size. This can be obtained by simplifying the cylinder as shown below.

In the example shown in Figure 4, the optimal amount of lubricant (X) is 1.4%.
It is. This amount is the amount of lubricant when fcr reaches saturation even if X is increased. When X exceeds 1.4%, even if the interparticle insulation is further improved, the intragranular eddy current remains unchanged, so fcr hardly increases. On the other hand, when X is smaller than 1.4%, f due to insulation between particles.

is determined, so as X decreases, fo decreases. Thinking in this way, it can be seen that the optimum X corresponds to when the eddy currents within and between grains are of exactly the same magnitude.

The size effect of intragranular eddy currents is determined by the size of the particles, and the size effect of intergranular eddy currents is determined by the size of the magnetic core.

Therefore, if the ratio between the size of the particle and the size of the magnetic core is constant, it is considered that the ratio between the size of the eddy current inside the grain and that between the grains will also be constant. Here, if we take the magnetic core cross-sectional area A perpendicular to the magnetic path as the size of the magnetic core, then this A is a two-dimensional quantity of length, so the particle size can be expressed as the square of the average particle diameter d2. Therefore, it can be considered that the optimum X is expressed by the ratio A/d2.

That is, the optimal X can be said to be a function of A/d2.

The results of investigating the relationship between X and A/d'' will be described below.

A powder magnetic core was prepared in the same manner as in the experiment that yielded the results shown in Figures 1 and 2 above, but at this time, the average particle size of the iron powder was 23 μm, and the amount of resin added was set at 0.5%. . Further, fCr was measured using six types of magnetic core sizes shown below and varying the amount of lubricant added.

14　φ13×φ8×H3111゜ 2.　　　　　φ　13　×　φ8　×　H51 condition.

3. φ13 x φ8 x H81m.

4. φ20×φ12×118 fins.

5, φ38×φ25XH6,5 fins.

6, φ38×φ25Xtl 10+u.

The cross-sectional area A corresponding to each size is as follows.

1.0.075crA, 2.0.125cal,
3.0.20 cry.

4. 0.32 (J!, 5. 0.42
cII! , 6°0.65 cI11 Figure 5 summarizes the results of investigating the relationship between the amount of lubricant added and the limit frequency in each magnetic core.

Note that the marks in the figure represent the optimal amount of lubricant added for each size of magnetic core.

Next, FIG. 6 shows the results of an investigation into the relationship between the magnetic core cross-sectional area (A) and the optimum X based on the results shown in FIG.

According to this result, the optimal X is proportional to A to the 0.3 power. This means that the optimal X is a function of A/d'', so it is ultimately proportional to (A/d'')0.1.

The optimal X obtained in this way is x = 20 (A/
d")"'.

Here, the allowable lower limit value of X is Con5t, X (A/d
In the example shown in Figure 4, A=0.125 crA, d=23. cz
Since x=0.8% when m, Con5t, #
10, therefore, the lower limit of the optimal amount of X is expressed by the following formula: X ≧ 10 (8/a2) + 1-3.

(Example) Mixtures A, B, and C shown in Table 1 below were prepared. Here, two types of water atomized pure iron powder with different particle sizes were used as the iron powder, and the particle size was expressed as the median average particle size measured by a Microtrac particle size analyzer. Further, a powdered epoxy resin (average particle size: 6 μm) was used as the resin, and zinc stearate was used as the lubricant.

Table 1 B 36 0.6 0.5 These mixtures A-C were molded into a ring shape with an outer diameter of 13 ml, an inner diameter of 3 n+, and a height of 8 fl at a molding pressure of 7.1 t/ad. The molded body was then held at 150°C in the atmosphere for 1 hour to cure the epoxy resin.

In these examples, the value of 10(A/d")'・3 is 0.72 for A and B, and 0.44 for C. Therefore, A uses finer iron powder than usual, A large amount of lubricant is added compared to the amount normally required for lubrication, which satisfies the appropriate range of the present invention.

Point B has a comparative example in which the iron powder particle size is fine but the amount of lubricant added is insufficient, and point C has a comparative example in which the particle size is as coarse as normal iron powder.

For these molded bodies, μ. . and fcr are measured,
The results shown in Table 2 below were obtained.

Table 2 A 67.3 4.35 B 70.2 0.79 C68,20,77 As is clear from the table, Example A has a higher μ than Comparative Examples B and C. Although the decrease in , was slight (1 to 4%), it was confirmed that fcF was dramatically improved to more than 5 times.

(Effects of the Invention) Thus, according to the present invention, there is no need for a special interparticle insulating agent, and therefore a powder magnetic core with excellent high frequency characteristics can be easily obtained at low cost. It has a great effect on expanding the range of applications.

[Brief explanation of the drawing]

Figure 1 is a graph showing the frequency characteristics of the powder magnetic core, Figure 2 is a graph showing the relationship between the average particle diameter d of iron powder and the critical frequency fcr, and Figure 3 is a graph showing the relationship between the average particle diameter d of iron powder and the critical frequency fcr. Addition amount and DC permeability μ,
Figure 4 is a graph showing the relationship between the amount of lubricant added and the limit frequency fct, and Figure 5 is a graph showing the relationship between the amount of lubricant added and the limit frequency fcr using the magnetic core size as a parameter. The graph shown in FIG. 6 is a graph showing the relationship between the magnetic core size and the appropriate amount of lubricant added.

Claims (1)

[Claims] 1. A powder magnetic core formed by molding a mixture of iron powder, resin, and lubricant, wherein the average particle size of the iron powder is d (μm), and the content of the lubricant is x ( wt%), the cross-sectional area of the magnetic core perpendicular to the magnetic path is A
(cm^2), these are the following equations (1) and (2
) d≦70 (μm)…(1) 10 (A/d^2)^0^. ^3≦x≦2.5…(2)
A powder magnetic core with excellent high-frequency magnetic properties that satisfies the following relationship.