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Introduction
Starting from the difficult problem of synergistic regulation of high-frequency loss mechanisms, this paper analyzes how high entropy ceramics achieve synergistic enhancement of loss mechanisms through local chemical order regulation, directional freezing technology through anisotropic pore architecture, and high entropy M-type ferrite through iron defect strategy. The article explains the improvement mechanism of impedance matching and broadband absorption through multi-scale structural design, and provides a selection and judgment framework for evaluating the capabilities of high-frequency absorbing material suppliers at the microstructure level.
In the high frequency range above 10 GHz, a single loss mechanism is no longer sufficient to meet the demand for wideband efficient absorption. The natural resonance frequency of magnetic loss materials limits their high-frequency response capability, while dielectric loss materials face the inherent problem of impedance mismatch. In the past two years, the research focus in the field of materials science has shifted from "adjusting formula ratios" to "designing microstructures" - achieving synergistic work of multiple loss mechanisms in a single material system through atomic scale chemical sequence control and microscale pore architecture design.
High entropy ceramics: how local chemical order replaces atomic vacancies as the dominant factor in loss
The concept of high entropy ceramics originates from high entropy alloys, whose core feature is to incorporate multiple principal elements into the same lattice, utilizing severe lattice distortion and adjustable electronic structure to enhance electromagnetic wave absorption. But early research on high entropy ceramics faced a mechanistic confusion: which type of defect dominates the electromagnetic wave absorption performance between complex lattice distortions, atomic vacancies, and localized chemical orders?
A study published in Nature Communications provides a clear answer. The research team set a new record for single-phase high entropy ceramics by regulating the local chemical order in high entropy carbides through a simple heat treatment process, with an effective absorption bandwidth of 11.2 GHz. Atomic level chemical mapping combined with theoretical simulations reveals that local chemical orders, rather than atomic vacancies, are the core mechanisms that dominate electromagnetic wave absorption.
The practical engineering significance of this discovery is that the performance optimization of high entropy ceramics no longer relies on introducing vacancy defects to enhance polarization loss, but can precisely regulate the arrangement and orderliness of elements in the lattice through heat treatment processes. The change in local chemical order affects the electronic structure and local polarization strength, thereby regulating the frequency range and intensity of dielectric loss. This method has been validated in different high entropy ceramic families (borides, oxides, sulfides, silicides, and selenides) and has universality.
For engineers of absorbing materials, this mechanistic understanding means that when selecting high entropy ceramic absorbing materials, the heat treatment conditions in the preparation process have a greater impact on the final performance than the raw material ratio. The heat treatment process control capability of suppliers is a key dimension for evaluating the consistency of their product batches.
Directional freezing: How anisotropic pore architecture optimizes both impedance and mechanical properties simultaneously
If high entropy ceramics solve the problem of synergistic loss mechanisms at the atomic scale, then directional freezing technology solves the problem of the relationship between structure and electromagnetic properties at the micrometer scale.
The core of directional freeze-drying is to control the growth direction of ice crystals, so that sheet-like or two-dimensional fillers form parallel arrangements in the gaps between ice crystals. In recent years, significant progress has been made in the application of this technology in high-frequency absorbing materials. The team of the Chinese Academy of Sciences Hefei Institute of Physical Sciences realized the regulation of the structure evolution induced microwave absorption performance through the strong magnetic field inducing the orientation structure of Ti ₂ C ₂ T ₓ MXene nanosheets. By adjusting the magnetic field strength (0-6 T), the horizontal orientation of the MXene/EP composite system can be continuously adjusted to 0.74, establishing a direct quantitative correlation between magnetic field strength and structural orientation. Compared to the non oriented sample, the optimized oriented sample achieves a 5-fold improvement in microwave absorption performance at a thickness of 2 mm.
Higher frequency applications require finer structural control. The multilayered polyimide/MXene/Ni aero gel prepared by the team of Hong Kong University of Science and Technology has constructed three pore structures, namely, random, parallel and vertical, through directional freezing. The vertically oriented PI/MXene/Ni aerogel achieves a minimum reflection loss of − 72.42 dB at a thickness of 2.8 mm. The three-layer assembly structure optimized by genetic algorithm further extends the effective absorption bandwidth to 8.4 GHz while maintaining a peak absorption intensity of -61.69 dB. The performance improvement comes from the synergistic effect of dielectric magnetic structural triple losses, including interface polarization, Ohmic/eddy current dissipation, and gradient impedance matching.
High entropy M-type ferrite: iron defect strategy for wideband absorption in millimeter wave windows
At the chemical design level of high-frequency absorbing materials, the iron defect strategy of high entropy M-type ferrites provides a path different from traditional doping modification.
M-type hexagonal ferrite has attracted attention due to its excellent magnetic resonance and dielectric polarization properties, but precise control of its electromagnetic parameters has always faced challenges - the presence of intermediate phase impurities leads to insufficient configuration entropy. The team from Nanjing University of Aeronautics and Astronautics proposed an iron defect strategy to synthesize single-phase Zr ⁴⁺ doped high entropy M-type ferrites. Research has found that iron defects eliminate impurity phases through enhanced entropy effect and provide more adaptive local structures. The synergistic effect of high entropy and iron defects amplifies the dielectric loss ability, while Zr ⁴⁺ doping broadens the magnetic loss range by regulating natural resonance.
The material achieved an ultra wide absorption bandwidth of 12.18 GHz and an ultra-thin matching thickness of less than 1 mm in the millimeter wave atmospheric window near 35 GHz. This performance data indicates that the combination of high entropy design and iron defect strategy can achieve both wideband and thinning in the millimeter wave frequency band - two goals that are difficult to achieve in traditional ferrite materials.
The following table summarizes the core features of three microstructure design strategies:
A judgment framework for evaluating supplier capabilities from a microstructural perspective
For the selection of high-frequency absorbing materials, traditional evaluation dimensions such as peak reflection loss, effective bandwidth, and thickness are no longer sufficient to distinguish the technical capabilities of different suppliers. The following three dimensions of microstructural judgment can provide more discriminative references.
Firstly, does the supplier have the ability to regulate the microstructure of the material
Require suppliers to provide material microstructure characterization data, such as SEM/TEM morphology photos, XRD phase purity analysis, element distribution mapping, etc. If the supplier can only provide macroscopic performance curves but cannot provide microstructural characterization, it indicates that their control over material properties may remain at the formulation level, and batch consistency is difficult to guarantee. For high entropy ceramic materials, special attention should be paid to whether suppliers can provide corresponding relationship data between heat treatment process parameters and local chemical sequences.
Secondly, the quantitative control ability of orientation degree in directional freezing process
If the material adopts directional freezing or magnetic field orientation process, the quantitative control range and data of its orientation degree should be confirmed with the supplier. The aforementioned research indicates that the orientation degree can be continuously adjusted by adjusting the magnetic field strength, and a quantitative relationship between orientation degree and absorption performance has been established. If the supplier can provide similar structural performance quantitative data, it indicates that their process control capability has reached an engineering level. For materials that need to be applied in the millimeter wave frequency band, the influence of orientation on performance is more significant, and higher weight should be given when selecting.
Thirdly, has the multi mechanism collaborative material system been systematically validated
The performance advantages of high-frequency absorbing materials increasingly rely on the collaborative work of multiple loss mechanisms. When selecting, it should be confirmed whether the supplier has provided data on the dielectric loss tangent and magnetic loss tangent in the target frequency band, and whether the trends of the two with frequency are complementary. If there is a significant "frequency band mismatch" between the two loss mechanisms in the target frequency band, such as rapid attenuation of dielectric loss above 10 GHz and insufficient establishment of magnetic loss, the material may not achieve the performance specified in the specification sheet in practical applications.
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