Español
Français
Português
In the specification book of flexible ferrite absorbing wave plates, the magnetic permeability usually appears as a single number, such as μ '=35 @ 13.56 MHz, or μ'=150 @ 13.56 MHz. When engineers see this number, they naturally compare it with the nominal magnetic permeability of sintered ferrite in the same table and conclude that the performance of flexible sheets is only a fraction of that of sintered bodies.
This comparison holds numerically, but it can easily lead people in the wrong direction in engineering logic. The common range of magnetic permeability of sintered ferrite at 13.56 MHz is 65-240, while that of flexible magnetic sheets is about 35 at the same frequency. There is indeed a gap, but this gap is not simply a "dilution" effect, but rather a triple fundamental reconstruction of the magnetic flux path by flexible composite structures. Understanding these triple reconstructions is more effective in predicting the behavior of materials in specific scenarios than memorizing a single μ 'value.
First reconstruction: indirect coupling from continuous grain boundaries to "particle matrix particle"
The magnetic flux path of sintered ferrite advances along continuous grain boundaries, with direct contact between grains, and the transfer of magnetic moment does not require crossing heterogeneous interfaces. Flexible ferrite sheets disperse ferrite particles into polymer matrices such as epoxy, rubber, or PVDF. The particles are separated by insulating polymers, and magnetic flux cannot be directly transmitted along a continuous path. It must be indirectly coupled at the particle contact point or through the magnetic dilution effect of the matrix.
The direct consequence of this change is that the effective magnetic permeability is no longer determined solely by the intrinsic magnetic permeability of ferrite. The study of NiCuZn ferrite epoxy composite system shows that the dependence of macroscopic effective magnetic permeability on the initial magnetic permeability of ferrite is actually "weak", and the aggregation state and volume fraction of filler particles are the more dominant variables. In other words, a sintered ferrite formula with intrinsic μ'200, when used to make flexible composite sheets, may not result in a much higher final μ 'than a sheet made with an intrinsic μ'100 formula. The nominal value of the sintered body has lost its reference significance here.
Second reconstruction: The percolation threshold transforms the "filling amount" into a nonlinear switch
If magnetic flux can only be transmitted through contact between particles, then the filler content needs to reach a critical value for the particle network to form a continuous magnetic path. For the NiZn ferrite/YSZ composite system, the magnetic permeability threshold is approximately around 0.6 volume fraction of NiZn ferrite - below 0.6, the magnetic permeability slowly increases with the increase of filler; Beyond 0.6, the magnetic permeability sharply increases.
The existence of percolation threshold poses a structural dilemma for the design of flexible ferrite sheets. In order for the effective μ 'to reach a usable level, the volume fraction of the filler must be close to or exceed the percolation threshold; But the same set of data shows that the Vickers hardness of the composite material begins to "significantly decrease" when the NiZn ferrite content exceeds 0.5. High filling results in magnetic permeability, but simultaneously weakens mechanical integrity. There is a narrow window defined by the percolation threshold between the required deformability of flexible sheets during bending and assembly and the tendency towards brittleness caused by high filling.
This contradiction is most acute in the low frequency range (such as 13.56 MHz). Low frequency magnetic flux guidance requires a sufficiently high μ 'to constrain magnetic field lines, and the acquisition of μ' depends on high filling; The absorption in the GHz frequency band relies more on dielectric loss and resonance loss, with relatively lower dependence on absolute μ ', and a wider range of filler choices.
Third reconstruction: Bending is not just a geometric deformation, it directly rewrites the magnetic state of ferrite
Flexible ferrite sheets almost inevitably undergo bending in practical use. The usual engineering intuition is that bending may lead to microcracks, thereby reducing magnetic permeability. But the magnetostriction effect (magnetoelastic coupling) of ferrite means that the stress introduced by bending itself will directly change the magnetic anisotropy and resonance conditions of the material, even without any visible cracks.
Flexible Mn ₀ ₅Zn₀. The study of ₅ Fe ₂ O ₄ epitaxial thin films provides a clear quantitative reference: at a bending radius of 5 mm, the out of plane ferromagnetic resonance field (Hr) shifts to 704 Oe, and the ferromagnetic resonance linewidth (δ H) widens from 310 Oe to 1267 Oe, which is about four times that of the unfolded state. After stress release, Hr and δ H almost completely returned to their initial state, indicating that this change is a reversible magnetoelastic effect rather than permanent damage.
The engineering significance of this data is that the magnetic properties of flexible ferrite sheets in a bent state are a "state quantity" rather than a "material constant". The measured μ 'and resonance frequency in a planar state cannot be directly used to predict the behavior after fitting to a curved surface. Bending stress changes the anisotropic field through magnetoelastic coupling, thereby shifting the resonance frequency; If the target operating frequency happens to fall at the edge of the resonant absorption band, a change in curvature radius of a few millimeters is enough to cause the absorption peak to deviate from the design frequency point.
Why does the 'sintered reference frame' fail simultaneously in three frequency bands
The performance of the above triple reconstruction varies in different frequency bands, but the error of "extrapolating from sintered body data" occurs in each frequency band.
In the NFC frequency band of 13.56 MHz, the difference between the sintered body's μ'65-240 and the flexible sheet's approximately 35 is mainly due to the first (indirect coupling) and second (percolation threshold) reconstructions. The design goal of flexible sheets is not to pursue high μ ', but to maintain low μ' and high Q values at the available μ 'level to avoid consuming the limited magnetic field energy of NFC antennas.
In the EMI absorption scenario in the GHz frequency band, the weight of the third reconstruction increases. The peak return loss and bandwidth of the flexible ferrite composite system are highly sensitive to thickness. The NiCuZn ferrite epoxy system reaches -48 dB at 4.6 GHz at an F/P volume ratio of 0.55 and a thickness of 8 mm, but this thickness is almost unusable inside mobile phones. At an actual thickness of 0.1-0.5 mm, the absorption depth of the material in the GHz frequency band depends on the synergy of dielectric loss and resonance loss, rather than the nominal magnetic permeability of the sintered body.
In the high-power scenario of wireless charging, flexible ferrite sheets face saturation constraints that are the same as those of sintered bodies, but the situation is more complex. The saturation magnetization of ferrite is relatively low, and it is prone to saturation and even local fragmentation and overheating under strong alternating magnetic fields of high-power wireless charging; The polymer matrix in flexible composite structures provides mechanical flexibility, but does not improve the saturation characteristics of ferrite particles themselves. In high-power scenarios, the ability of flexible sheets to perform depends on the relationship between the working magnetic flux density at the target power and the saturation magnetic induction of ferrite particles, rather than the property of "flexibility" itself.
Selection judgment based on "finished magnetic spectrum"
Firstly, starting from the complex magnetic permeability data of the finished sheet at the target frequency and actual thickness, rather than the nominal μ 'of the sintered body
Require suppliers to provide the μ '(f) and μ "(f) curves of the flexible sheet itself, with frequency points covering the target operating frequency band, and indicate the test thickness and filler volume fraction. If the supplier can only provide the magnetic permeability of the sintered body or the μ 'of a single frequency point, their ability to predict actual behavior is limited. The effective μ 'of flexible sheets is less dependent on the intrinsic μ' of ferrites, and the filler volume fraction and aggregation state are more direct variables.
Secondly, evaluate the trade-off between "permeability flexibility" near the percolation threshold, rather than assuming that both can be independently optimized
When the volume fraction of the filler exceeds about 0.6, the magnetic permeability sharply increases, but the mechanical properties begin to decline when it exceeds 0.5. When selecting, the corresponding relationship between the volume fraction of filler and the flexural durability should be confirmed with the supplier. For applications that require fitting small curvature radius surfaces, it may be necessary to accept lower μ 'values in exchange for available flexibility, rather than pursuing the highest magnetic permeability values specified in the specifications.
Thirdly, treat the change in magnetic state after bending as an independent verification item, rather than just checking for appearance cracks
Under the target bending radius, the resonance frequency and loss of flexible ferrite sheets may undergo reversible or partially reversible shifts. For NFC scenarios, it should be confirmed whether the μ 'and Q values at 13.56 MHz after bending are still within the matching window; For GHz absorption scenarios, it should be confirmed whether the absorption peak still covers the target frequency band after bending. The magnetic spectrum in a planar state cannot replace the actual measurement in a bent state.
Advanced Institute (Shenzhen) Technology Co., LtdThe flexible ferrite absorbing plate product line covers 13.56 MHz NFC/RFID magnetic plates and GHz band EMI absorbing plates, providing data on the complex magnetic permeability curve, filler volume fraction, and magnetic performance retention rate of finished sheets at target frequencies.
The NFC series mainly consists of thin ferrite polymer composite structures, which can be matched according to the curvature radius of the mounting material system; The GHz absorption series supports customized adjustment of packing system and thickness. For more technical information, please visithttp://www.avanzado.cn/.