Effect of Packing Fraction on Magnetic Properties of the Fe-Si-Al Powder Cores by Coarse Powder and Fine Powder Mixing

Abstract

Fe-Si-Al of soft magnetic materials are known as high permeability alloys, and these are used as dust cores. In general, Fe-Si-Al powders, what is called “Sendust”, have poor compressibility because of their hardness, therefore it is difficult to increase the packing fraction of their core. We researched the effect of packing fraction on magnetic properties in order to mix the powders which have different mean particle size. In case of mixing the coarse powders and fine powders, it is assumed that fine particles penetrate into the opening gaps which are located in close packing of coarse particles. They are called 6-, 4- and 3-configuration. The sizes of opening gap in close packing are 0.414, 0.225 and 0.154 of coarse particle size, respectively. Fe-9.5Si-5.5Al alloy powders were prepared by gas atomization process. Mean particle size of coarse powders are fixed about 40 μm and fine powders are prepared the size by classifier. Coarse powders and fine powders were mixed to the optional ratio. Relative permeability and core loss up to 1 MHz were measured. Packing fraction of core was improved more than 80 % by optimization of the mixing ratio of fine powders. As a result, relative permeability was increased with the packing fraction.

1 Introduction

Fe-9.5Si-5.5Al alloy called “SENDUST” developed by Yamamoto and Masumoto is known as high-permeability soft magnetic material¹⁾. The Fe-Si-Al alloy powders are widely used in various inductors for electronic parts as dust cores. In general, the magnetic properties of the dust core of the inductor require higher magnetic permeability and lower core loss. In order to increase the permeability of the dust core, it is necessary to increase the packing fraction of the magnetic powders. However, Fe-Si-Al alloy powders are not be able to increase the packing fraction of the powders sufficiently by the molding process due to their hardness. In this study, we attempted to increase the packing fraction of the molded cores by filling the fine powders into the gaps of coarse powders packing. Mixing the coarse powders and the fine powders, it was investigated the influence of the mixing ratio of fine powders on the magnetic properties.

In case of closed packing of spheres, there are 3 kinds of gap among the spheres shown in Fig. 1. They are called octahedral 6-coordination, tetrahedral 4-coordination and triangle 3-coordination, respectively. As shown in Table 1, the sizes of the gap, are 0.414, 0.225, 0.154 to the size of the closed packing of spheres, respectively. In the case of filling the small spheres having the same size of the gap into the gaps of large spheres’ packing, the ratio of both spheres are also shown. When the size of large spheres are fixed and the sizes of small spheres are same size of the gaps, the ratio of the small spheres which are placed at the each gap are shown in Table 2. According to this table, the ratio of small spheres to be placed at the gaps are estimated about 10 %. Mixing the various size of fine powders to the coarse powder which were regarded as spheres by gas atomization, we considered the effect of the mixing to the packing fraction of cores and improved the magnetic properties for dust cores.

Fig. 1

Scheme of each configuration: (a) 6-coordination, (b) 4-coordination and (c) 3-coordination.

Table 1 Size of the gap in closed packing of spheres.

	Size	Number of Site
		Large Sphere	Small Sphere
Octahedral 6-configuration	0.414	4	4
Tetrahedral 4-configuration	0.225	4	8
Triangular 3-configuration	0.154	4	32

Table 2 Calculation of ratio of small spheres.

	Size of Small Sphere
	0.414	0.225	0.154
Octahedral 6-configuration	6.6 %	8.4 %	5.2 %
Tetrahedral 4-configuration	—	2.2 %	1.4 %
Triangular 3-configuration	—	—	2.8 %
	6.6 %	10.6 %	9.5 %

2 Experiment

Fe-9.5Si-5.5Al alloy powders were prepared by gas atomization. It was performed classification to sharp distribution. Because it was reduce influence of particle size distribution. Particle size of coarse powder was 40 μm in mean particle size and fixed. Fine powders were classified into 16 μm in mean particle size, 9 μm, 6 μm and 3 μm, respectively. Particle size distribution were measured by laser diffractometry. Coarse powders and fine powders were mixed to the optional ratio. 3 mass% of silicone resin was added to Fe-Si-Al alloy powders as an insulation and a binder. 0.5 mass% of zinc stearate was added to all powders as a lubricant. And then, these were molded into toroidal shape cores with the size of 21 mm in outer diameter, 13.5 mm in inner diameter and 4.5 mm in thickness, at 1470 MPa. The cores were heat-treated at 150 °C for 1 h in air atmosphere to harden the silicone resin. And these cores were heat-treated in molded state at 700, 750 and 800 °C for 1 h in Ar atmosphere.

Relative permeability of the core μ_r was measured up to 1 MHz in frequency. Relative permeability μ_r was calculated by equation (1) from measured inductance of the core by LCR meter.

  

$${\mathrm{\mu }}_{\text{r}}=\frac{\text{Ll}}{{\mathrm{\mu }}_0{\text{n}}^2\text{A}},$$(1)

where L is inductance of core, l is magnetic pass length, μ₀ is permeability in vacuum, n is turn of coil and A is cross sectional area of core.

Core loss of the core for 0.1 T in the magnetic flux density P_c was measured up to 150 kHz in frequency by B-H analyzer.

3 Results

3.1 Effect of fine powder addition on packing fraction and magnetic properties

The composition of the powders are shown in Table 3. The particle size distributions of coarse powders and fine powders are shown in Fig. 2. Values of D₁₀, D₅₀ and D₉₀ of each powders are shown in Table 4. The effect of addition of each fine powders on packing fraction is shown in Fig. 3. The packing fractions were increased up to 35~40 mass% with increasing fine powder ratio, and decreased over the ratio. The effect of addition of each fine powders on relative permeability at 100 kHz is shown in Fig. 4. The relative permeability were increased up to 40~45 mass% with increasing fine powder ratio, and decreased over the ratio. The effects of addition of each fine powders on the core loss for 0.1 T at 100 kHz are shown in Fig. 5. The core loss were decreased up to 30~40 mass% with increasing fine powder ratio, and increased.

Table 3 Composition of Fe-Si-Al powder by gas atomization.

Composition	Si [mass%]	Al [mass%]	Fe [mass%]
Fe-Si-Al	9.67	5.65	Bal.

Fig. 2

Particle size distribution of coarse and fine powders: (a) Corse powder (40 μm), (b) Fine powder (16 μm), (c) Fine powder (9 μm), (d) Fine powder (6 μm) and (e) Fine powder (3 μm).

Table 4 Value of D₁₀, D₅₀ and D₉₀ of each powder.

	D10 [μm]	D50 [μm]	D90 [μm]
Fe-Si-Al 40 μm	28.7	40.3	58.6
Fe-Si-Al 16 μm	10.6	16.1	28.7
Fe-Si-Al 9 μm	5.3	8.5	13.4
Fe-Si-Al 6 μm	3.2	5.9	10.2
Fe-Si-Al 3 μm	1.7	3.3	5.2

Fig. 3

Effect of addition of each fine powder on packing fraction.

Fig. 4

Effect of addition of each fine powder on relative permeability.

Fig. 5

Effect of addition of each fine powder on core loss.

The magnetic properties were improved by addition of fine powders into coarse powder. The effect of particle size of fine powder on the relative permeability for suitable fine powder ratio is shown in Fig. 6. The relative permeability of the addition of 6 μm-powder was highest. The effect of particle size of fine powder on the core loss for suitable fine powder ratio is shown in Fig. 7. The core loss of the addition of 3 μm-powder was lowest. From these results, we considered that the most suitable particle size of fine powder was 6 μm or 3 μm.

Fig. 6

Effect of particle size of fine powder on relative permeability for suitable fine powder ratio.

Fig. 7

Effect of particle size of fine powder on core loss for suitable fine powder ratio.

3.2 Effect of heat treatment

The cores were heat-treated in order to improve the magnetic properties. The effects of each heat treatment temperature for the addition of 6 μm-powder on the relative permeability are shown in Fig. 8. The relative permeability were increased by the heat treatment. In the case of 40 mass% of the fine powder ratio and 750 °C in the heat-treatment, the relative permeability indicated the highest value 55. The effect of each heat treatment temperature for addition of 6 μm-powder on the core loss is shown in Fig. 9. The core loss were drastically reduced and were not depend on the addition the 6 μm-powder by the heat-treatment except 800 °C.

Fig. 8

Effect of heat-treatment for 6 μm-powder on permeability.

Fig. 9

Effect of heat-treatment for 6 μm-powder on core loss.

Frequency characteristics of relative permeability and the core loss for 40 mass% in fine power ratio of 6 μm-powder and only coarse powder are shown in Fig. 10 and Fig. 11, respectively.

Fig. 10

Frequency characteristics of relative permeability.

Fig. 11

Frequency characteristics of core loss.

4 Discussion

As shown in Fig. 3, the packing fraction were increased with increasing fine powder addition. Photo images of only coarse powder (A) and 40 mass% in fine powder ratio of 6 μm-powder (B) are shown in Fig. 12 (a) and (b), respectively. These results were significantly different from expected packing state shown in Fig. 1. The tendency of the relative permeability was similar to one of the packing fraction. Relationship of the packing fraction and the relative permeability for resin hardened and heat treatment are shown in Fig. 13. The relative permeability tended to increase with the packing fraction. It was confirmed that the relative permeability was correlated with the packing fraction for both of resin hardened and heat-treated core.

Fig. 12

Photo images: (a) only coarse powder (b) 40 mass% in fine powder ratio of 6 μm-powder.

Fig. 13

Relationship of packing fraction and relative permeability.

The core loss P_c was separated into a hysteresis loss P_h and an eddy current loss P_e by equation (2).

  

$$\begin{array}{l}{\text{P}}_{\text{c}}={\text{P}}_{\text{h}}+{\text{P}}_{\text{e}}\\ {\text{P}}_{\text{c}}={\text{k}}_1·\text{f}+{\text{k}}_2·{\text{f}}^2\end{array}$$(2)

k₁ is hysteresis loss coefficient, k₂ is eddy current loss coefficient and f is frequency.

The separated core loss for resin hardened and heat-treated cores of powder (A) and (B) are shown in Fig. 14. The hysteresis loss was dominant in the core loss. The hysteresis loss of 40 mass% in fine powder ratio of 6 μm-powder (B) was lower than only coarse powder (A). The hysteresis loss of powder (A) and (B) were reduced by heat-treatment.

Fig. 14

The separated core loss of powder (A) and (B) for resin hardened and heat-treated.

Effects of molding pressure for powder (A) and (B) on the core loss are shown in Fig. 15. The core loss for powder (A) was reduced with decreasing molding pressure. But the core loss for mixed powder (B) was slightly reduced with decreasing molding pressure. Therefore, it was considered that the core loss for powder (A) occurred by molding strain. It was thought that reduction of molding strain was due to the fine powder getting into the gaps of coarse powder packing. It was considered that the fine powder acted as lubricant.

Fig. 15

Effect of molding pressure on core loss for powder (A) and (B).

5 Conclusion

In this work, Fe-Si-Al alloy powder was investigated the effects of packing fraction and the magnetic properties, mixing the various size of fine powders to the coarse powders. We found out as below.

1. 1.   The packing fraction and the relative permeability were maximum for 30 mass%~40 mass% in fine powder ratio.
2. 2.   The packing fraction and the permeability were maximum for the addition of 6 μm-powder.
3. 3.   The core loss was decreased by fine powder addition into coarse powder.
4. 4.   The core loss was drastically reduced by heat treatment.
5. 5.   The relative permeability was 55 at 100 kHz, and the core loss was 341 kW/m₃ for 0.1 T at 100 kHz.

Reference

• 1)   H.  Masumoto,  T.  Yamamoto: “On a New Alloy “Sendust” and its Relative and Electric Properties”, Nippon Kinzoku Gakkai-shi, 3 (1937) 127–135.