In the design of absorbing materials, there is a repeatedly verified rule: the greater the impedance difference between the material and free space, the more difficult it is for electromagnetic waves to enter the interior of the material.

Traditional absorbing materials often use high permeability ferrites or magnetic metal micro powders as absorbers - the higher the permeability, the stronger the ability to "capture" electromagnetic waves. This logic holds in the low frequency range (MHz level): high μ 'efficiently guides magnetic field lines into the interior of the material, and then consumes them through magnetic losses.

But in the high frequency range above GHz, things changed. The magnetic response of high permeability materials rapidly decays with increasing frequency, with a decrease in μ 'and an increase in μ' ', resulting in a sharp decrease in magnetic loss ability. More importantly, high magnetic permeability often means that the material's characteristic impedance is severely mismatched with free space (about 377 Ω). Electromagnetic waves are heavily reflected on the surface of materials and cannot enter at all. How can we talk about absorption?

2、 Impedance matching: the "first gate" of low permeability materials

Impedance matching is a prerequisite for electromagnetic waves to enter the interior of materials. According to the transmission line theory, the normalized input impedance of the material Zin=√ (μ r/ε r) · tanh [j (2 π ft/c) √ (μ r ε r)]. If the surface impedance is severely mismatched with free space, even if the material has strong loss ability, electromagnetic waves will be reflected back at the interface.

Low permeability materials naturally have better impedance matching potential - lower μ 'weakens the magnetic response of the material, equivalent to reducing the overall characteristic impedance of the material, making it closer to the impedance level of free space. But 'potential' does not equal 'reality'. If the dielectric constant is too high, the material impedance will still be mismatched.

Advanced Institute Technology achieves broadband impedance matching through dielectric constant gradient control. The real part of the surface dielectric constant is controlled between 3.5-6.0 (@ 10 GHz), and the imaginary part is controlled between 0.3-0.8, so that the surface reflection coefficient | Gamma |<0.15; The impedance gradient structure of "low dielectric layer high dielectric layer" is formed by increasing the bottom layer ε 'to 15-25 and ε "to 4-8. This gradient design is equivalent to a multi-layer quarter wavelength converter, allowing electromagnetic waves to gradually adapt to impedance changes as they enter the material layer by layer, rather than encountering sudden changes at a single interface. The proportion of frequency bands with vertical incident reflectance better than -12 dB in the range of 8-18 GHz measured by the material exceeds 85%.

3、 Dielectric loss: the "main engine" responsible for over 70% energy attenuation

In low permeability materials, the contribution of magnetic loss is relatively limited, and dielectric loss accounts for over 70% of the energy attenuation task. Advanced Institute Technology has activated four types of polarization mechanisms through multi-scale defect engineering to construct a "firepower network" of dielectric loss:

The four types of polarization mechanisms are interconnected in frequency and distributed in different layers of materials in space, collectively forming a dielectric loss system covering 2-40 GHz.

4、 The 'collaborative role' of magnetic loss: not dominant, but indispensable

Low magnetic permeability is not equal to zero magnetic permeability. In low permeability materials, although magnetic losses do not dominate, they provide critical supplementary losses in specific frequency bands and participate in fine-tuning impedance matching.

Introduction of Fe ∝ Si phaseIt is a typical case. In SiC/Fe ∝ Si@rGO In the composite aerogel, the Fe ≮ Si phase contributes to the magnetic loss mechanism, which forms a synergistic effect with the conductive loss and interface polarization enhanced by the three-dimensional conductive network of graphene. This multi loss synergy of "dielectric loss as the main and magnetic loss as the auxiliary" enables the material to achieve a minimum reflection loss of -46.68 dB at a thickness of 3.0 mm, and maintain effective absorption in the full frequency range of 4-18 GHz within a thickness range of 1.5-5.0 mm.

In the Ba ∝ Co ₂ Fe ₂₄ O ₄₁ @ CIP composite system, the composite of Co2Z and carbonyl iron powder significantly reduces magnetic and dielectric losses, while minimizing the weakening of magnetic polarization, thereby promoting impedance matching. The resin based patch with a Co2Z: CIP ratio of 1:1 achieved an effective absorption bandwidth of 5.6 GHz (7.4-13 GHz) at a thickness of 1.7 mm, covering the entire X-band.

5、 Material System: Lightweight Path from Carbon Based to Ceramic Based

The core goal of selecting a material system for low permeability absorbing materials is to reduce density while maintaining dielectric loss capability.

Carbon based materials (carbon nanotubes, graphene, carbon fibers)It is the "skeleton" of a low magnetic permeability system. They have high specific surface area, excellent conductivity, and extremely low density, and their dielectric constant can be controlled through defect engineering and surface functional groups. The high conductivity of pure carbon materials can lead to impedance mismatch, so it is necessary to composite with dielectric materials such as SiC to balance electromagnetic parameters.

Silicon carbide (SiC)It is the most widely used ceramic dielectric material in low permeability absorbing materials. SiC nanowires have excellent high-temperature stability and tunable dielectric properties, with carbon vacancies and nitrogen doped defects in their lattice providing effective dielectric losses from 12 GHz to 40 GHz. Advanced Institute Technology combines SiC with carbon materials, utilizing the dielectric loss capability of SiC to complement the conductive loss of carbon materials.

MXeneAs a two-dimensional transition metal carbide/nitride with high conductivity and abundant surface functional groups, it is an ideal material for constructing interface polarization and conductive loss networks. In low permeability systems, MXene is usually used in combination with other materials to regulate the dielectric constant by controlling the filling amount.

6、 Selection considerations: Engineering decisions from frequency to impedance matching

When engineers choose low permeability absorbing materials, the following dimensions deserve special attention:

• Matching target frequency with loss mechanism.The contribution frequency bands of different polarization mechanisms are different. Priority is given to dipole polarization in the C-band and some X-bands; Utilize interface polarization from X-band to K-band; The millimeter wave frequency band depends on defect polarization. Advanced Institute Technology's low permeability absorbing materials cover the full frequency range of 2-40 GHz through the collaborative design of four polarization mechanisms.
• Impedance matching verification.When selecting, it is not enough to only focus on the reflection loss value of the material, but also to confirm the impedance matching characteristics of the material in the target frequency band. Advanced Institute Technology regulates the surface reflection coefficient | Gamma |<0.15 through dielectric constant gradient control, ensuring that electromagnetic waves can effectively enter the interior of the material.
• The trade-off between thickness and density.One of the advantages of low magnetic permeability materials is the ability to achieve wideband absorption at thinner thicknesses. The effective absorption bandwidth of advanced technology products reaches ≥ 8 GHz in the Ku/K/Ka band, with a density as low as 0.9-1.6 g/cm ³, suitable for scenarios with strict constraints on weight and space.
• Environmental adaptability.SiC based and carbon based materials have excellent high-temperature stability and chemical inertness. Advanced Institute Technology's low permeability absorbing materials have a working temperature range of -40 ° C to 150 ° C, suitable for a wide temperature range from consumer electronics to aerospace environments.

7、 Advanced Institute Technology XJY-D series low permeability absorbing materials

The XJY-D series of low magnetic permeability absorbing materials from Advanced Institute Technology is mainly based on dielectric loss, with a small amount of magnetic loss synergistic as the technical route. The main parameters are as follows:

Data Source:Advanced Institute of Technology

The XJY-D series products support precision die-cutting and adhesive bonding, and can customize thickness, size, and surface treatment methods according to customer needs. The material is suitable for electromagnetic interference suppression in 5G communication equipment, millimeter wave radar, EMC stealth, and precision electronic devices.

8、 Conclusion

The essence of low permeability absorbing materials is an engineering choice made under the physical constraints of high-frequency electromagnetic absorption - not pursuing the maximization of the single indicator of "high permeability", but finding the optimal solution between impedance matching, dielectric loss, density, and thickness. It uses dielectric loss instead of magnetic loss as the main mechanism, uses gradient impedance matching to ensure that electromagnetic waves can enter, uses multi polarization synergy to ensure that electromagnetic waves can be retained and consumed, and uses lightweight material systems to ensure that they are usable.

The evolution of materials from ferrite to carbon based/ceramic based composite systems reflects the upgrade of absorption demand from "MHz level applicability" to "millimeter wave broadband stability". Understanding the engineering relationship between μ 'and impedance matching, the multi polarization synergistic mechanism of dielectric loss, and the trade-off between density and absorption performance - these are the key to upgrading low permeability absorbing materials from "functional materials" to "engineering decisions".