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In the selection of nanocrystalline magnetic separators for wireless charging receivers, magnetic permeability is usually the first parameter to be considered. A nanocrystalline strip with an initial magnetic permeability exceeding 80000 appears to be an ideal magnetic flux guiding material. But when it is made into a finished magnetic isolation sheet and attached to the back of the receiving coil, the effective magnetic permeability is often only 800-1500. This gap is not a process defect, but an inevitable result of the demagnetization process.
Magnetic fragmentation is a key process in the transformation of nanocrystalline magnetic separators from "strip" to "sheet", and it is also the starting point for understanding the contradiction between their loss and reliability.
Why Broken Magnetism Cannot be Omitted: Physical Constraints of Eddy Current Loss
Nanocrystalline alloys are metallic materials with a resistivity of only about 130 Ω· cm, much lower than ferrites. In the alternating magnetic field of wireless charging, low resistivity means that significant eddy currents will be induced inside the large-area continuous strip, and eddy current losses dominate the total losses. If no treatment is taken, the larger the strip area, the longer the eddy current closure path, the more severe the loss, and the charging efficiency is directly lowered, resulting in an increase in temperature rise.
The function of demagnetization is to divide continuous nanocrystalline strips into small, mutually insulating fragments, cutting off the closed path of eddy currents over a large area. The electrical resistivity of the air gap between fragments is much higher than that of the alloy itself, and eddy currents can only form small loops inside individual fragments, greatly suppressing losses. The fragment size is usually in the range of 0.005-20 mm, and the common fragmentation spacing in industry is about 0.5-2 mm.
Without broken magnets, the high magnetic permeability of nanocrystals cannot be effectively utilized in alternating magnetic fields below GHz. Magnetic fragmentation is a prerequisite for obtaining engineering usability of nanocrystalline magnetic separators.
The first cost of magnetic fragmentation: a systematic decrease in effective magnetic permeability
The effective magnetic permeability of the fragmented magnetic separator is much lower than the initial value of the strip. The initial magnetic permeability of the original nanocrystalline strip can reach over 80000, but after fragmentation and lamination, the effective magnetic permeability of the finished product usually falls within the range of 800-1500. The root cause of the decline lies in the "air gap" formed by the debris gap in the magnetic circuit. The magnetic field lines need to jump between fragments when penetrating the magnetic separator, and each jump is equivalent to an increase in magnetic resistance. The smaller the fragment and the larger the total gap area, the more significant the loss of effective magnetic permeability.
This cost is directly transmitted as a decrease in coupling efficiency in the wireless charging receiver. The core function of a magnetic separator is to constrain magnetic flux and reduce leakage. Insufficient effective magnetic permeability means that some magnetic lines cannot be guided to the coupling area of the coil, resulting in increased leakage and deviation of the inductance value of the receiving coil from the design value. For the operating frequency range of 110-205 kHz according to the Qi standard, the LC resonance matching at the receiving end is sensitive to changes in inductance, and the batch fluctuations in effective magnetic permeability will be directly converted into a shift in resonance frequency.
But the decrease in magnetic permeability caused by broken magnetic tape is not without compensation space. The high saturation magnetization Bs (about 1.2-1.25 T) of nanocrystalline materials is much higher than that of ferrites (0.4-0.5 T), which allows nanocrystalline magnetic separators to achieve magnetic flux guidance effects comparable to ferrites at thinner magnetic layer thicknesses. When the magnetic permeability is the same as 600, nanocrystals only need 0.11 mm to achieve the equivalent magnetic conductivity of ferrite 0.27 mm, with a charging efficiency about 2.5 percentage points higher and a temperature rise 7-8 ℃ lower. High Bs serve as a 'buffer pad' for the cost of magnetic fragmentation, allowing engineers to adjust thickness and efficiency even after a decrease in magnetic permeability.
The second cost of magnetic fragmentation: μ 'drift under bending stress
The way in which fragmented magnetism endows the magnetic separator with flexibility is by transferring bending strain to the gaps between the fragments. The fragmented unit undergoes relative sliding during bending, and the adhesive layer and air gap absorb deformation. Macroscopically, the sheet can adhere to the curved surface without breaking. But this mechanism also introduces a reliability hazard: the length of the fragment gap is sensitive to bending stress.
Nanocrystalline alloys themselves are mechanically very brittle, and the crack state inside the fragments will change under bending stress. Existing microcracks may propagate, and the gap length between fragments may also change due to deformation of the adhesive layer. Any change in gap length will alter the equivalent air gap in the magnetic circuit, thereby changing the real part of the magnetic permeability μ '. The drift of μ 'does not decrease unidirectionally, but depends on the stress direction and fragment orientation, which may increase or decrease. This uncertainty makes it difficult to predict the resonance matching after assembly.
For the wireless charging receiver, the drift of μ 'is equivalent to the drift of the inductance L of the receiving coil, and the resonant frequency f=1/(2 π√ LC) shifts accordingly. A slight offset is manifested as a decrease in charging efficiency or a slightly higher temperature rise, while a larger offset may cause the receiving end to fail to vibrate normally under specific coil alignment or load conditions. This type of failure is not visible in incoming inspection, and is only exposed after assembly bending and stacking of the entire machine, where the magnetic piece has already been encapsulated inside the module.
Annealing process: finding a window between soft magnetic properties and bending toughness
The cost of demagnetization largely depends on the annealing state of the nanocrystalline strip before demagnetization. Annealing determines grain size, coercivity, and residual stress, which simultaneously affect soft magnetic properties and mechanical brittleness.
The nanocrystalline strip obtained a uniform grain structure of 10-20 nm through rapid solidification and crystallization annealing, with a coercivity Hc reduced to below 0.8 A/m and losses controlled within 20 W/kg at 100 kHz/0.1 T. Low coercivity directly reduces hysteresis loss, allowing the material to maintain low residual loss even after demagnetization.
But the annealing process has a narrow window. Research on Fe based nanocrystalline alloys shows that rapid annealing can maintain fracture strain at a usable level while achieving high Bs and low Hc; If the annealing time is too long, it will intensify the element gradient at the amorphous/crystalline interface, suppress the formation of shear bands, and increase macroscopic brittleness. This means that the precision of the annealing process control by the supplier not only determines the initial magnetic permeability and loss of the magnetic separator, but also determines its μ 'stability after assembly and bending.
The following table summarizes the cost and adjustable space of the demagnetization process in two dimensions:
Three engineering judgments in selection
For the selection of nanocrystalline magnetic separators, focusing solely on the initial magnetic permeability of the strip or the nominal magnetic permeability of the finished product is no longer sufficient to predict actual performance. The following three dimensions of data can better reflect the availability of materials under real working conditions.
Firstly, confirm the effective magnetic permeability after demagnetization, rather than the initial value of the strip
The magnetic flux guiding ability of the finished magnetic separator is determined by the effective magnetic permeability after fragmentation and lamination. When selecting, the supplier should be required to provide effective magnetic permeability data in the finished product state and confirm that the test frequency covers the target operating frequency band (Qi standard 110-205 kHz or AirFuel 6.78 MHz). If the supplier only provides the initial magnetic permeability of the strip, their ability to predict the actual coupling efficiency is limited.
Secondly, verify the stability of the bent μ '
For scenarios that require surface mounting or assembly bending, suppliers should be required to provide μ 'data before and after bending at a specific curvature radius. A reference criterion is that before and after bending with a curvature radius of 5 mm, the μ '(5)/μ' (0) at 145 kHz should fall within the range of 0.9-1.1. Exceeding this window means that the drift of the resonant frequency may exceed the matching tolerance of the receiving IC.
Thirdly, distinguish between "low loss" and "controllable loss composition"
After magnetic fragmentation, the eddy current losses of the nanocrystalline magnetic separator have been significantly suppressed, and the proportion of hysteresis losses and interface losses related to fragment gaps in the remaining losses has increased. When selecting, it should be confirmed whether the supplier has provided data on the variation of losses with frequency and magnetic flux density, as well as whether the debris gap is filled with air or adhesive layer. Adhesive layer filling may introduce greater stress drift after long-term thermal cycling and bending, while air gap filling is usually more advantageous in terms of stability.
Advanced Institute (Shenzhen) Technology Co., LtdThe nanocrystalline magnetic separator adopts iron-based nanocrystalline tape as the functional layer, which is made through multi-layer compression and insulation coating process. The effective magnetic permeability of the finished product is 150-2000, which can be customized. The saturation magnetic inductance is about 1.2 T, the coercivity is ≤ 1.0 A/m, and the loss is ≤ 10 W/kg @ 20kHz/0.5T.
The product has a thickness range of 0.03-0.3 mm, a temperature resistance of -40 ° C to 150 ° C, and a Curie temperature of ≥ 500 ° C. It supports roll, sheet, shaped die-cutting, and multi-layer composite, and can provide stability data and multiple batches of magnetic performance sampling reports before and after bending. Parameters can be customized according to specific frequency, power, and assembly space. For more technical information, please visithttp://www.avanzado.cn/.