The factors that influence the performance of soft magnetic composites are the focus of this article, which aims to help readers understand how to improve the performance of these materials through process enhancements.
Advantages of Soft Magnetic Composites
The wonderful performance of soft magnetic composites is an important reason for the widespread use of this material.
Soft magnetic composites exhibit low eddy current losses, high magnetic permeability, low coercivity, and high resistivity, while also integrating the benefits of both metal soft magnetic materials and soft magnetic ferrites. Compared to metal soft magnetic materials, the insulated soft magnetic composites have higher resistivity, effectively reducing high-frequency eddy current losses. In contrast to soft magnetic ferrites, SMCs have relatively higher flux density, better meeting the miniaturization requirements of electronic components.
Factors Affecting the Performance of Soft Magnetic Composites
Powder Composition
The composition of the powder largely determines the performance of soft magnetic composites, affecting parameters such as permeability and losses. Selecting suitable alloy compositions based on requirements is the most critical step in preparing soft magnetic composites. Mixing two types of amorphous powders makes it possible to create composite magnetic powder cores with high-quality factors and good inductive frequency characteristics.
The performance of soft magnetic composites changes with variations in the mixing ratio of the two materials. For example, adding permalloy can increase the packing density of the FeSiB magnetic powder core and improve its permeability, although it may decrease the temperature stability of that permeability.
Powder Preparation Methods
The method used to prepare the powder significantly impacts the performance of soft magnetic composites. Currently, commonly used methods include the atomization method and the mechanical grinding method, with the former further categorized into gas atomization and water atomization.
| Gas atomization involves using high-speed jets to disperse molten liquid alloy into small droplets, solidifying into powder. This method results in spherical alloy powders with smooth surfaces that facilitate uniform insulation coating.
| Water atomization produces powders that are often ellipsoidal and angular, which are favorable for pressing but do not coat well.
| The mechanical grinding method involves breaking metal blocks into powder through mechanical forces. This powder tends to be irregularly shaped, providing the best pressability among these methods, leading to higher initial magnetic performance, yet its coating effectiveness is relatively poorer.
Powder Particle Size
The size and distribution of the powder particles can influence the pressing effect and density of soft magnetic composites, thereby affecting their overall performance. Under the same resin encapsulation conditions, as the powder particle size decreases, the density of the soft magnetic composite increases, enhancing permeability and saturation magnetic induction strength.
Concurrently, eddy current losses also increase while hysteresis losses decrease. Therefore, it is essential to select an appropriate particle size based on the performance requirements of the soft magnetic composites to balance parameters like permeability and losses. Using alloy powders of different particle sizes in tandem can effectively reduce the demagnetizing fields in iron-based amorphous alloy magnetic powder cores, thus enhancing the performance of soft magnetic composites.
Powder Insulation Coatings
The insulation coating is also an important factor affecting the performance of soft magnetic composites.
The insulation coating during the preparation of magnetic powder cores is a critical step, as the quality of the coating greatly affects the losses and magnetic permeability of the cores. Existing insulation coatings can generally be divided into three categories: organic coatings, inorganic coatings, and organic-inorganic composite coatings.
| Organic coating materials soften at certain heating temperatures, making pressing easier and improving the density of the magnetic powder core with a good coating effect. However, organic materials have lower melting points, and high-temperature treatments can carbonize them, damaging the insulation coating layer and reducing the insulation between the powders. Therefore, they cannot eliminate internal stresses in soft magnetic composites through heat treatment, which is detrimental to the performance of soft magnetic composites.
| Inorganic coatings mainly include phosphoric acid and phosphates. When metal powders react with phosphoric acid or phosphates, phosphide films can form on the surface, providing insulation and reducing losses in the magnetic cores.
Inorganic coatings can effectively control costs and are widely used, but excessive passivator usage can create thick passivation layers on the surface of soft magnetic composites, leading to decreased magnetic performance of soft magnetic composites and potential delamination. Nowadays, many magnetic powder core production processes commonly involve coating after passivation treatment with additional insulation materials.
| Organic-inorganic composite coatings are generating interest as they combine the advantages of both organic and inorganic materials, such as high resistivity, good temperature stability, and ideal pressability. This method is likely to become a key focus for future research. However, organic-inorganic composite coatings generally have higher costs.
Heat Treatment
Heat treatment refers to the process of heating the pressed soft magnetic composites to a specific temperature and maintaining that temperature for a period to obtain the magnetic core products. The main roles of this step include increasing the mechanical strength of soft magnetic composites and reducing or eliminating internal stresses and dislocation defects generated during production. Key process parameters include protective atmosphere, heating rate, heat treatment temperature, holding time, and cooling rate.
A heating rate that is too slow or too fast is detrimental to the performance of soft magnetic composites and must be carefully controlled. When the holding time is extended from 60 minutes to 90 minutes, the performance of the magnetic cores improves. Further extending the holding time to 150 minutes shows minimal changes in the characteristics of the soft magnetic composites.
Before conducting heat treatment, it is essential to consider the material of the magnetic core, the application environment, and the frequency range to select the most suitable heat treatment conditions.
Soft Magnetic Composites Provider
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