۱- Introduction

۲- Experiments

۳- Results and discussion

۴- Conclusion

References

Abstract

Currently, metallic soft magnetic composites, due to their high saturation magnetization, have attracted much attention for application in high frequency mobile devices. These composites must approach a full densification to achieve the high permeability and magnetic flux density necessary for mobile device applications. This study was carried out to investigate the effects of Fe nano-powders on the compaction behaviors and magnetic properties of Fe-Si alloy composites. Fe-Si micro-powders were fabricated using a gas-atomizer; then, SiO2 was coated on them to suppress eddy current loss. Fe-nano-powders were obtained using spray-dried Fe2O3 powders. These nano-powders were homogeneously mixed with Fe-Si micro-powders to increase the density of the compacts. As a result, the composite with 80 wt% micro-powders showed excellent high frequency properties due to high compaction density. Microstructural analysis shows that changes in the magnetic properties of the composites are dependent on the fill factor and electrical path-ways of non-insulated nano-powders in pores between insulated micro-powders.

Introduction

Due to their high magnetic saturation value, good relative permeability, and low magnetic core loss, iron powders have been widely used as a magnetic core material of soft magnetic composites (SMCs). To take full advantage of these excellent properties, full densification of powders should be achieved because the magnetic permeability and magnetic flux density of SMCs are dependent on the SMC density [1]. Due to their high compressibility, pure iron powders with size of several hundred micrometers were reported to reach a compact density of 95% or more via a simple die compaction process [2]. With increasing operation frequency of magnetic devices, there is strong demand to reduce core losses (particularly eddy-current loss) by decreasing the particle size, adding other alloying elements to pure iron, and coating an insulation layer on the powder surface [1]. Unfortunately, achieving high densification using such pre-treated iron powders is very difficult. For example, due to their excellent soft magnetic properties and high electrical resistivity, Fe-Si alloy powders have been widely used as a magnetic core material. However, the addition of Si increases the mechanical strength of the Fe-Si alloy, which severely reduces the compaction density [3,4]. Furthermore, when the average particle size is reduced to less than 20 μm, inter-particle friction and particle bridging become critical problems [5,6]. This indicates that a simple compaction process is not suitable for the densification of Fe-Si powders applied to high frequency SMCs. To achieve high densification of hard and fine powders, dynamic compaction methods such as magnetic pulsed consolidation (MPC), or pressure-assisted sintering processes such as hot isostatic pressing (HIP) and spark plasma sintering (SPS), have been widely investigated to increase the compact density [7–۱۱]. These methods include a sintering process, and that is not suitable for the fabrication of SMCs, unlike powders for structural applications or magnetic ferrite powders, which can be sintered. This is because the electrical insulation layer on metal powders can decompose and the metal surface can be exposed during the sintering process [1,2]. Thus, it is required to develop a new method to increase the density of SMCs during the simple compaction process. It is well known that increasing initial packing density of powders increases the density of compacts. The packing density is dependent on several factors such as particle shape, inter-particle friction, surface chemistry, agglomeration, and size distribution. Among these, the particle size distribution has a strong effect on the packing density [12]. According to previous studies, bimodal powders can increase the packing density by more 10% compared to monomodal powders [13].