High permeability and low loss magnetic separator: Where does the loss advantage of Co based amorphous and Fe based nanocrystalline come from

In the design of wireless charging receivers, the "high permeability" and "low loss" of the magnetic separator are often considered as two parameters that can be independently optimized. But there is a fundamental constraint in materials physics: high magnetic permeability usually comes with high loss, and low loss often means sacrificing magnetic permeability.

To understand the source of this contradiction, we need to start from the microscopic process of material magnetization. The total loss of soft magnetic materials in an alternating magnetic field consists of three parts: hysteresis loss, eddy current loss, and residual loss. Hysteresis loss is proportional to frequency and originates from the irreversible movement of magnetic domain walls during magnetization; Eddy current losses are proportional to the square of frequency and originate from eddy currents induced by alternating magnetic fields inside conductive materials; The remaining losses include magnetic aftereffects and resonance losses. High magnetic permeability materials mean that magnetic domain walls are easy to move, but the interaction with lattice defects during the movement of magnetic domain walls can cause hysteresis losses. The smaller the grain size and the more grain boundaries, the stronger the pinning effect and the greater the hysteresis loss.

The core task of designing magnetic barrier materials is to improve magnetic permeability while reducing the resistance of magnetic domain wall movement through microstructure design, thereby controlling losses.

Co based amorphous: How to achieve low loss through low magnetostriction

Co based amorphous alloys (such as CoFeSiB) have unique advantages in high-frequency and low loss scenarios, and their loss control mechanism is different from that of Fe based materials.

The magnetostriction coefficient of Co based amorphous is close to zero. Magnetostriction refers to the phenomenon of size changes in materials during magnetization. Non zero magnetostriction generates stress inside the material, which in turn pinning magnetic domain walls through magnetoelastic coupling effects, increasing hysteresis losses. The magnetostriction coefficient of Co based amorphous is only on the order of about 1 × 10 ⁻⁷, much lower than the 20-40 × 10 ⁻⁶ order of Fe based amorphous. When magnetic domain walls move in this low stress environment, they experience less resistance and significantly reduce hysteresis losses.

At the same time, the high resistivity of Co based amorphous materials (approximately 100-150 μ Ω· cm) suppresses eddy current losses at high frequencies. Compared with Fe based amorphous, Co based amorphous has lower loss levels in the frequency range above 100 kHz. This makes its comprehensive performance of magnetic permeability and loss superior to Fe based amorphous and ferrite separators in the Qi standard frequency band for wireless charging (approximately 87-205 kHz).

Fe based nanocrystals: How to balance high magnetic permeability and low loss in 10-20 nm grains

Fe based nanocrystalline alloys (such as FeSiBNbCu, also known as Finemet type alloys) precipitate 10-20 nm sized nanocrystals in the amorphous matrix through a special crystallization annealing process. This microstructure design simultaneously solves the contradiction between high magnetic permeability and low loss.

In nanocrystalline alloys, the grain size is much smaller than the width of the magnetic domain wall (about 100 nm), and the magnetic domain wall passes through multiple grains and grain boundaries simultaneously as it moves, resulting in an average magnetic anisotropy of the grains. This "exchange coupling" effect significantly reduces the effective magnetic anisotropy of the material, resulting in an increase in magnetic permeability to the level of 30000-150000, while hysteresis loss does not significantly increase with the increase of magnetic permeability.

In terms of eddy current loss control, nanocrystalline alloys require magnetic fragmentation to divide continuous strips into small, mutually insulating fragments. The electrical resistivity of the air gap between fragments is much higher than that of the alloy itself, cutting off the closed path of eddy currents over a large area. But if the gap between the fragments is completely filled by the adhesive layer, the viscosity of the adhesive layer will generate stress on the fragments, causing the orientation of the fragments to change and the gap to expand, which in turn reduces the magnetic permeability. A better solution is to fill at least part of the debris gaps with air and use a coating to prevent colloids from entering the gaps. Research has shown that the average loss factor of Fe based nanocrystals is only about 2.4 × 10 ⁻⁷, which is lower than that of Co based amorphous materials, indicating that nanocrystalline structures have significant advantages in reducing intrinsic magnetization noise.

Application boundary and saturation anomaly problem of ferrite

MnZn and NiZn ferrites are the most mature materials for magnetic separators, with naturally high electrical resistivity and negligible eddy current losses. Hysteresis losses are the main contradiction in the low-frequency range. But the saturation magnetization Bs of ferrite is only 0.3-0.5 T, and the Curie temperature is about 100-250 ° C.

In wireless charging applications above 15W, the working magnetic flux density of ferrite separators may be close to their saturation magnetization. Once the material enters the saturation zone, the magnetic permeability drops sharply, and the magnetic field penetrates the magnetic barrier into the metal layer, causing both eddy current temperature rise and deterioration of charging efficiency. This saturation anomaly is one of the main reasons for the failure of magnetic separators in high-power wireless charging. In contrast, the Bs of nanocrystalline magnetic separators are about 1.2-1.3 T, which is 2.5-4 times that of ferrites, and the operating point is far away from the saturation region at the same magnetic flux density.

The following table summarizes the typical loss related parameters of three types of magnetic separators:

In the Qi standard frequency range of 110-205 kHz, the magnetic permeability of nanocrystalline separators is significantly higher than that of iron-based amorphous and ferrite, while the loss is the opposite. In the range of -40 ° C to 120 ° C, the magnetic permeability stability of nanocrystalline separators is also better than that of ferrite separators. When the ambient temperature is above 40 ° C or below -20 ° C, the magnetic permeability of ferrite materials fluctuates greatly, which may lead to abnormal operation, while nanocrystalline materials can still provide stable soft magnetic properties.

Three engineering judgments in selection

For the selection of high permeability and low loss magnetic separators, simply focusing on the permeability value is no longer sufficient to distinguish the actual performance of the material. The validation data from the following three dimensions can provide more discriminative references.