Why does the absorption performance of flexible microwave absorbing sheets stabilize after bending

The reflection loss curve of flexible microwave absorbing sheets in laboratory flat state is usually very beautiful. But what engineers really care about is another issue: how much of that curve is left when this piece of material is attached to a curved shell, or after hundreds of bends.

In the past few years, there has been a clearer answer to this question. rGO/ CNT@Fe-MOF After 10000 compression rebound cycles, the minimum reflection loss of the gradient sponge changed from -54.0 dB to -52.1 dB, and the effective absorption bandwidth narrowed from 8.5 GHz to 6.4 GHz - the peak intensity of the loss remained almost unchanged, and the bandwidth contracted to some extent but still covered the main frequency band. The chain like metamaterial skin with a more structured design maintains its function completely after 10000 high curvature bends, with an absorption rate of over 90% in the range of 2-40 GHz. The liquid metal fiber membrane even exhibits enhanced absorption strength in a bent state.

The common point of these data is that bending does not necessarily lead to the collapse of absorption performance for structurally designed flexible absorbing materials. Understanding the physical mechanism behind it is more engineering meaningful than pursuing the ultimate absorption peak in a planar state.

How bending changes absorption behavior

The bending response of flexible wave absorbing plates depends on how the conductive network and magnetic filler distribution inside the material evolve with deformation.

Under tensile deformation, the fiber network is gradually pulled apart, the fiber spacing increases, and the interlayer contact changes. These changes will adjust the propagation path and scattering behavior of electromagnetic waves inside the material. If the material has a continuous conductive network inside, microcracks caused by stretching will directly cut off the conductive path, increase the impedance, and shift the impedance matching window. But if the material adopts a graded porous structure or fiber lap network, stretching is more manifested as the opening and closing of pores and the redistribution of contact points, rather than irreversible fracture.

Bending introduces another stress state. Bend the outer side to withstand tension and the inner side to withstand compression. For sheets composed of magnetic fillers and resin matrix, bending may cause microcracks in the resin matrix, but if sufficient dielectric coupling is still maintained between the magnetic fillers, impedance matching can continue to be maintained over a wide frequency range. Studies have observed that after multiple bending cycles, the attenuation constant of the material slightly decreases, but impedance matching remains good at thicknesses of 2.0 and 2.5 mm, thanks to the fact that the air filled portion in the porous structure can maintain impedance transition even after deformation.

Gradient design: making deformation a controllable variable

The performance of traditional wave absorbing plates decreases during bending, mainly due to the uniform distribution of conductive or magnetic fillers. Once the local structure is damaged, the overall electromagnetic parameters deviate from the design values. The idea of gradient design is to change the concentration or type of filler in the thickness direction, so that the material can maintain usable electromagnetic response after deformation.

rGO/ CNT@Fe-MOF The design of gradient sponge is a typical example. The proportion of metal ions and organic ligands in each layer is different, forming different MOF morphologies. The synergistic effect between layers creates complex transport pathways and multiple loss mechanisms. When the sponge is compressed or bent, interlayer sliding and pore deformation still allow electromagnetic waves to be reflected and absorbed multiple times between multiple layers of interfaces, rather than being reflected out at once by a single interface.

Matching frequency band selection with flexible implementation methods

The frequency range covered by flexible microwave absorbing sheets is very large, and the requirements for flexible substrates and filler systems are not the same for different frequency bands.

In the low frequency range of 10 MHz to 1 GHz, magnetic loss is the dominant mechanism, requiring a high amount of magnetic filler filling to obtain sufficient magnetic permeability. A high filler content usually means that the sheet is thicker and less flexible, so low-frequency flexible absorbing plates often need to accept thicker cross-sections and larger bending radii. XJY-L050 covers 10 MHz – 1 GHz at a thickness of 0.5 mm, with a peak reflection loss of -20 dB @ 300 MHz, and a bending radius controlled within ≤ 3 mm. It belongs to the scheme with good flexibility index in this frequency band.

1-6 GHz is the most mature range for the application of flexible absorbing wave plates. This frequency band covers Wi Fi (2.4/5 GHz), Sub-6 GHz 5G communication, and most consumer electronics switching noise harmonics. XJY-M020 achieves 1-6 GHz coverage at a thickness of 0.2 mm, with a peak reflection loss of -28 dB @ 2.4/5.8 GHz, a bending radius of ≤ 3 mm, and a bending resistance of over 100000 times. This combination means that it can adhere to the inside of the curved shell while withstanding repeated deformation during assembly.

The millimeter wave frequency band (24-40 GHz, 57-64 GHz) places higher demands on flexible materials. The higher the frequency, the shorter the wavelength, and the small fluctuations and local unevenness caused by bending on the material surface will change the phase relationship, resulting in absorption peak shift. XJY-MM030 covers 24-40 GHz at a thickness of 0.3 mm, with a peak reflection loss of -32 dB @ 35 GHz. The bending radius is also controlled within ≤ 3 mm, indicating that the millimeter wave flexible absorbing plate can balance high-frequency matching and deformation adaptability in structural design.

Three practical judgments in selection

For applications that require surface mounting or dynamic bending, it is recommended to prioritize the following three points when selecting.