1 Introduction
The widespread application of the Internet of Things, microelectronic devices, smart wearable devices, and high-frequency electronic components has led to intensified electromagnetic radiation pollution
[1-3]. Although conventional electromagnetic shielding materials can provide stable shielding effectiveness, their structures cannot achieve real-time regulation in complex electromagnetic scenarios (such as coexistence of multi-band signals and coupling of electromagnetic interference between devices), resulting in insufficient electromagnetic compatibility
[4-8], making it difficult to adapt to the demands of dynamic electromagnetic environments
[9-11]. To address this issue, developing intelligent electromagnetic shielding materials has become one of the research hotspots
[12-13]; such materials can dynamically adjust the electromagnetic shielding performance of electromagnetic shielding materials through stimuli such as stress, temperature, humidity, or chemical agents, playing an important role in signal transmission control
[14-17], military stealth
[18-21], and human health protection
[22-26]and other fields.
The core of intelligent electromagnetic shielding materials lies in the integration of electromagnetic shielding units and intelligent modules: the former (such as conductive fillers, magnetic fillers) is responsible for providing electromagnetic shielding performance, while the latter (such as shape memory polymers, phase change materials) achieves functional switching by altering the material structure or carrier concentration. Common intelligent modes can be classified according to response types into stress response
[27-32], temperature response
[33-36], humidity response
[37-40], and chemical stimulus response
[41-42], etc. The working mechanisms mainly include: (1) altering the electrical conductivity or magnetic permeability of the material, regulated based on dielectric/magnetic loss (e.g., VO
2 phase change altering electrical conductivity); (2) reconstructing the conductive network through self-deformation (e.g., using deformation to cause contact/separation of conductive fillers); (3) utilizing the dielectric effect of polar molecules (e.g., humidity sensitivity).
Based on this, this paper first discusses the response mechanisms of intelligent electromagnetic shielding materials, focusing on analyzing the response mechanisms of stress, temperature, humidity, chemical stimuli, and other types of intelligent electromagnetic shielding materials; then, by comparing the preparation methods and application performance of different types of intelligent electromagnetic shielding materials, it analyzes the advantages and disadvantages of different response modes; finally, it provides an outlook on the development of intelligent electromagnetic shielding materials in terms of response time, response sensitivity, response mechanism, and shielding effectiveness.
2 Intelligent Electromagnetic Shielding Mechanism
The electromagnetic shielding process mainly refers to the reflection or absorption of electromagnetic waves by using electromagnetic shielding materials. Regarding the shielding mechanism of electromagnetic shielding materials, it is currently primarily explained by transmission line theory and can be divided into three main loss mechanisms: reflection loss, absorption loss, and multiple reflection loss. The shielding performance of electromagnetic shielding materials is typically related to sample thickness, electrical conductivity, magnetic permeability, and dielectric constant.
Intelligent electromagnetic shielding materials are developed by introducing intelligent materials into electromagnetic shielding materials to achieve tunable shielding properties. Intelligent materials can respond to external stimuli by changing their own characteristics (such as conductivity and dielectric constant) or shape (such as thickness and crossing angle), thereby altering shielding performance. Based on the type of stimulus, intelligent electromagnetic shielding materials can be classified into stress-responsive materials, temperature-responsive materials, humidity-responsive materials, chemically stimulated responsive materials, and other types of responsive materials.
2.1 Stress response mechanism
Compressing the material by applying stress
[43-45], stretching
[46-47], the shape of the material changes, its microstructure also changes accordingly, and the properties of the material are altered.
According to the Schelkunoff formula, electrical conductivity and thickness are key factors measuring a material's shielding performance; the higher the conductivity and the greater the thickness, the higher the shielding effectiveness. Stress can alter a material's shielding performance for two reasons: firstly, under external force, the material deforms, causing changes in the relative positions of internal conductive fillers and rearranging the conductive network. Since the integrity of this network affects the material's electrical conductivity—a more complete network yields higher conductivity, which in turn influences shielding effectiveness; secondly, external force changes the material's thickness, which directly impacts its shielding effectiveness.
The mechanism by which stress alters the shielding performance of electromagnetic shielding materials differs under low and high strain. Under low strain, there is no significant change in material thickness; at this stage, changes in shielding performance primarily depend on variations in the conductive network, where closer distances between conductive fillers result in a more complete network. Therefore, under low strain, the magnitude of electrical conductivity determines the level of shielding performance. Conversely, under high strain, material thickness plays a decisive role in shielding performance, because at this point, the integrity of the conductive network has already reached its maximum, and changes in material thickness become more pronounced; a reduction in thickness affects shielding effectiveness to a greater extent. Consequently, the impact of stress response on shielding performance is mainly manifested as a competitive effect between conductive network optimization and thickness reduction.
2.2 Temperature Response Mechanism
In addition to compressing or stretching the material, the material structure can also be changed through temperature stimulation methods, such as introducing thermally deformable materials
[48-50]or using thermally expandable microspheres
[51-52].
Among them, thermally deformable materials can change their shape upon temperature stimulation (shape memory polymers), thereby achieving the purpose of altering shielding effectiveness. The use of expandable microspheres involves the rapid vaporization and expansion of low-boiling-point liquids filled inside when heated, which squeezes surrounding conductive fillers to bring them closer together. Furthermore, as the microspheres expand, more internal interfaces are created, and the material thickness increases accordingly, leading to changes in shielding performance. This method of changing material shape or thickness through heating is almost consistent with the mechanism of stress response, primarily manifesting as a competition between the conductive network and thickness variation.
In addition, adding fillers with phase change characteristics to materials is also one of the methods to obtain temperature-responsive electromagnetic shielding materials. For example, VO
2is a metal oxide with phase change properties, and its phase change temperature is 68 °C
[53]. When the temperature is below 68 °C, VO
2is an insulating material and has no shielding effect on electromagnetic waves. However, when the temperature reaches 68 °C or above, VO
2undergoes a phase transition, changing from an insulator to a conductive material. The electrical conductivity increases rapidly, thereby enabling the reflection of electromagnetic waves. Therefore, a certain amount of VO can be added to the material
2, so that at room temperature, the material has no shielding effect on electromagnetic waves. When the temperature reaches the phase change temperature of 68 °C, VO
2transforms into a conductive material, enabling the construction of a complete conductive network within the material, allowing it to shield electromagnetic waves. In addition, VO can also be
2doped to regulate its phase change temperature
[54], thereby expanding its application range.
2.3 Humidity response mechanism
Adjusting the water content of materials is also one of the effective means for preparing intelligent electromagnetic shielding materials. According to classical electromagnetic theory, electric or magnetic dipoles in electromagnetic shielding materials interact with incident electromagnetic waves to convert electromagnetic wave energy into other forms (mainly thermal energy), thereby achieving loss. Specifically, under the action of electromagnetic waves (especially the electric field component), the bound charges inside the dielectric material are driven by electric field forces to undergo displacement or changes in orientation, forming macroscopic or microscopic dipole moments; this process is known as polarization. This dynamic polarization manifests as microscopic motion of molecules or atoms within the material. The relative movement between molecules, deformation within molecules, and frictional resistance encountered during charge migration convert part of the incident electromagnetic wave energy into thermal energy, thereby achieving the effect of shielding electromagnetic waves through dielectric loss.
[55].
Polar materials provide stronger polarization capabilities through the reorientation of intrinsic dipoles. As a highly polar molecule, the dynamic variation in water content within the material becomes a key factor in regulating the performance of intelligent electromagnetic shielding materials. As the water content increases, the introduced highly polar water molecules significantly enhance the overall polarity of the material. This enables more intense polarization oscillations and molecular friction under the same electric field, leading to increased polarization loss. This increase in loss contributes to enhancing the overall electromagnetic shielding effectiveness of the material. However, when the water content exceeds a certain critical value, the situation reverses. Excessive water molecules penetrate the interior of the material and disrupt its conductive network structure, causing a decline in shielding performance.
2.4 Chemical stimulus response mechanism
Conductive polymer composites can be classified into intrinsic conductive polymers and composite conductive polymers. Among them, intrinsic conductive polymers possess the characteristic that their electrical conductivity changes with the degree of doping, exhibiting intelligent properties. Therefore, by controlling the degree of material doping, it is possible to regulate their electrical conductivity to obtain intelligent electromagnetic shielding materials.
Therefore, chemical reagents can be used to dope conductive polymers. For instance, polyaniline exhibits a characteristic where its electrical conductivity changes with its doping level. The switching between conductive and insulating states can be influenced by acid/base doping and dedoping effects. After doping is completed, its electrical conductivity can increase by more than 10 orders of magnitude.
2.5 Other Response Mechanisms
In addition to the above four response mechanisms, some research groups have developed electrically responsive
[56-57]and cross-angle responsive
[39,58]intelligent electromagnetic shielding materials. Among them, electrical response works by generating heat through electricity to raise the material's temperature, and then, similar to the thermal response mechanism, alters the material's shape or filler properties to change its shielding performance.
The cross-angle response involves regulating the shielding performance of the material by changing the angle between the incident direction of the electromagnetic wave and the planar material. For electromagnetic shielding materials with fiber orientation, their conductive network is distributed along the fiber axis. The angle between the fiber axis and the incident direction of the electromagnetic wave is termed the cross-angle θ, and the angle between the conductive network direction and the electric field direction of the electromagnetic wave is ϕ, satisfying the following relationship:
$\mathrm{\varphi }=90°-\mathrm{\theta }$
When θ changes from 0° to 90°, the component of conductivity in the direction of the electromagnetic shielding electric field varies from zero to its maximum; therefore, the shielding performance of the material also changes from minimum to maximum, satisfying the following relationship:
$SE=S{E}_{\mathrm{m}\mathrm{a}\mathrm{x}}\cdot \mathrm{s}\mathrm{i}{\mathrm{n}}^{2}\mathrm{ }\mathrm{\theta }+S{E}_{\mathrm{m}\mathrm{i}\mathrm{n}}$
Therefore, for electromagnetic shielding materials with fiber orientation, changing the crossing angle can regulate the shielding performance of the material.
3 Intelligent Electromagnetic Shielding Materials
Based on the response mechanisms of electromagnetic shielding materials to external stimuli, intelligent electromagnetic shielding materials can be classified into stress-, temperature-, humidity-, chemical reagent-responsive, and other types. The shielding performance of these materials exhibits significant changes before and after stimulus response. Researchers have proposed a switching material based on intelligent electromagnetic shielding materials, whose core function is to achieve an "on-off" state transition through the controllable switching of the material's shielding performance. When the shielding effectiveness of the material exceeds a certain threshold or range, the system enters the shielding "on" state, effectively blocking electromagnetic waves in the target frequency band; conversely, when the shielding effectiveness falls below a certain threshold or range, the system switches to the shielding "off" state, allowing electromagnetic waves to pass through. This "on-off" characteristic can target specific frequencies or bands, such as the S, C, and X bands.
This reversible switching behavior between two characteristic values/intervals creates a significant switching effect, whose state transition can be triggered by external stimuli (such as temperature, electric field, or chemical environment). This design provides new ideas for developing adaptive electromagnetic control devices and has important application prospects in fields such as intelligent stealth and selective electromagnetic wave transparency, such as
Figure 1shows
[29].
3.1 Stress-responsive intelligent electromagnetic shielding materials
Stress-responsive intelligent electromagnetic shielding materials refer to electromagnetic shielding materials that undergo deformation (compression, stretching) under stress, causing changes in the relative positions of conductive fillers within the material, which affects the material's electrical conductivity and thereby alters its shielding effectiveness.
Due to their porous and lightweight nature, which facilitates the loading of nanofillers, aerogel materials serve as an excellent matrix for electromagnetic shielding. Using aerogel as a matrix allows for easy compression, thereby altering filler distribution or thickness to influence electromagnetic shielding performance. For example, as
shown in Figure 2a, Liu et al.
[29]subjected wood to high-temperature carbonization and absorbed acidified carbon nanoparticles XC-72; after freeze-drying and annealing, the carbonized wood/XC-72 sponge was transformed into a carbon aerogel. When compressive stress is applied to the material, the nano-carbon powder attached to adjacent upper and lower carbon layers moves closer during compression, constructing a conductive network that significantly enhances the material's electrical conductivity. The aerogel can rapidly respond to strain changes, achieving a reversible transition between absorption and shielding. Due to its compressibility, aerogel can serve as a smart, stress-responsive matrix for electromagnetic shielding; however, some aerogels struggle to recover their original shape after compression, exhibiting insufficient cyclic stability.
Elastomeric plastics possess excellent rebound properties, capable of withstanding repeated compression while maintaining their performance. Therefore, using elastomeric plastics as the substrate for stress-responsive intelligent electromagnetic shielding is a method to solve this problem. Yuan et al.
[30]prepared an intelligent asymmetric foam composed of chromium dioxide-modified graphene/thermoplastic polyurethane-polyvinyl alcohol foam and a eutectic liquid metal gallium layer through a "separated conductive modular design/assembly" strategy. Through the mechanism of regulating conductivity via compressive stress, the intelligent foam material can achieve the capture and attenuation of electromagnetic waves in an absorption-dominated or reflection-dominated manner by real-time adjustment according to compressive strain (
Figure 2b); meanwhile, the material also exhibits excellent rebound performance and cycling stability. After 1000 compression-release cycles at 50% strain, both the maximum stress and strain were fully recovered. As
shown in Figure 2c, Li et al.
[59]prepared polyurethane foam using a sacrificial template method, and then, using the microporous elastic polyurethane foam skeleton as the substrate and polydopamine as the binder, successfully fabricated MXene nanosheet-wrapped polyurethane@polydopamine/MXene foam (TPFM) via a vacuum-assisted impregnation method. After 100 cycles at 50% compressive strain, it still maintained good mechanical properties. Meanwhile, the SE of TPFM decreased from 72.2 dB to 48.4 dB; although the performance attenuated, it remained at a certain level. This may be because, under large strain, irreversible damage occurs to the internal conductive network of the material. Therefore, greater attention needs to be paid to the interfacial debonding and slippage between the filler and the matrix to ensure the integrity of the internal 3D conductive network of the material under compression or large strain.
In addition to compression, stretching is also one of the means to alter the stress-responsive electromagnetic shielding performance of shielding materials. Xia et al.
[60]fabricated a stress-responsive film by electrolessly depositing Cu nanoparticles on electrospun polyurethane elastomer films. When the tensile deformation reached 50%, the fabric could transition from conductive to insulating; upon release of the stress, the fabric could recover its conductivity. Tensile strain enables dynamic adjustment of shielding effectiveness, holding significant application potential in wearable smart devices.
General stretchable materials often suffer from a decline in shielding performance during stretching, which frequently limits their range of applications. To obtain smart electromagnetic shielding materials whose shielding performance increases with stretching, the selection of conductive fillers is a key factor. Liquid metal (LM) possesses both liquid fluidity and metallic conductivity. Incorporating LM into an elastomer matrix allows it to deform synchronously with the substrate's stretching, thereby avoiding the reduction in shielding performance caused by stretching. For example, Guo et al.
[61]employed an ultrasonic method to modify LM with carbon nanotubes (CNTs) and utilized UV in-situ polymerization to prepare a CNT@LM/polyacrylamide/gelatin double-network hydrogel elastomer (
Figure 3a). The deformable CNT@LM droplets elongate along with the film during stretching, thereby reducing the distance between liquid metal particles and exhibiting conductivity that enhances with stretching. However, controlling the distribution of LM within the elastomer is difficult; therefore, magnetic particles can be introduced to form magnetic liquid metal, which aligns under a magnetic field to achieve superior shielding and mechanical properties. As
shown in Figure 3b, Zhu et al.
[47]mixed magnetic nanoparticles with LM to obtain magnetic liquid metal and filled it into an elastomer matrix. This magnetic liquid metal was induced by a magnetic field to produce electromagnetic shielding films with both anisotropic and isotropic properties.
Although the addition of liquid metal can effectively address the issue of declining shielding performance often encountered during material stretching, liquid metal is relatively expensive, and this material requires excessively large strain values that are difficult to achieve in practical applications. To solve this problem, Bian et al.
[62]developed a switchable electromagnetic shielding elastomer by precisely dispersing added nickel microparticles within polydimethylsiloxane (PDMS). As
shown in Figure 3c, by controlling the stirring rate and time, nickel particles can form irregular short-chain clusters inside the matrix; these short chains are close to being connected but still maintain tiny gaps between them, presenting a unique pre-linked chain arrangement. Under only 20% tensile strain, the distance between these short pre-linked chains decreases due to pressure, forming a micro-scale conductive network that significantly improves the elastomer's electrical conductivity, enabling the elastomer to shield electromagnetic waves.
3.2 Temperature-responsive intelligent electromagnetic shielding materials
Temperature-responsive intelligent electromagnetic shielding materials are electromagnetic shielding materials that can change their shielding performance in response to temperature stimuli. This characteristic stems from the temperature-responsive materials within them, such as thermally deformable materials (shape memory materials) or phase change materials (VO2) etc. Meanwhile, heat dissipation often accompanies the operation of electronic devices, providing a source of thermal response stimuli; therefore, the development of temperature-responsive intelligent electromagnetic shielding materials has received significant attention.
Water undergoes phase transitions at 0 ℃ and 100 ℃, accompanied by volume changes, which endows it with the potential for preparing temperature-responsive electromagnetic shielding materials. For example, Singh et al.
[52]used polydimethylsiloxane/silver-doped zinc stannate/single-walled carbon nanotubes as the matrix material and water as the active fluid to prepare a temperature-responsive intelligent electromagnetic shielding material. When the composite material is cooled from room temperature to 0 ℃ or even lower, or heated to 100 ℃, the sealed water inside the material undergoes a phase transition. The resulting volume expansion increases the material thickness, thereby altering its shielding effectiveness (
Figure 4a). However, directly incorporating liquids into polymers can easily lead to leakage or reduced polymer strength. Therefore, phase change materials can be encapsulated within polymer shells to create expandable microspheres. These microspheres consist of an elastic shell and low-boiling-point hydrocarbon compounds encapsulated inside. When the temperature rises to the boiling point of the hydrocarbon compounds, they vaporize, causing the microspheres to expand. This alters the conductive network within the polymer, enabling the material to exhibit different shielding efficiencies. As
shown in Figure 4b, Cai et al.
[63]successfully obtained tunable electromagnetic shielding elastomer composites with temperature and strain responsiveness by introducing temperature-sensitive microspheres (TSM) into a polydimethylsiloxane/multi-walled carbon nanotube (PDMS/CNT) mixture. Upon heating the composite to the vaporization temperature, the expansion of the TSM alters the internal CNT distribution, leading to a rearrangement of the CNT conductive network and resulting in different electrical conductivity and electromagnetic shielding performance.
The process for preparing expanded microspheres with a core-shell structure is usually complex and difficult to achieve on a large scale. VO
2 is a material that can switch between insulating and conducting states. Therefore, using relatively inexpensive and readily available VO
2 is one of the methods to solve this problem. Liao et al.
[34] simply prepared temperature-responsive electromagnetic shielding materials by compounding vanadium dioxide/cellulose nanofibers, as
shown in Figure 4c, when the temperature reaches the VO
2 phase transition temperature, the electrical conductivity of the material increases rapidly, thereby changing its shielding performance.
3.3 Humidity-responsive intelligent electromagnetic shielding materials
Humidity-responsive intelligent electromagnetic shielding materials can respond to environmental humidity, spontaneously adjust their water content, and thereby regulate the level of electromagnetic shielding performance.
Humidity affects the electrical conductivity of materials. For instance, MXene materials are hydrophilic; in high-humidity environments, MXene absorbs water molecules from moist air, thereby influencing the formation of conductive networks. The relationship between shielding performance and air humidity involves spontaneous water absorption or desorption by the material under different humidity conditions, which affects the interlayer spacing between MXene sheets. As
shown in Figure 5a, Liu et al.
[64]prepared chitosan/MXene films with EMI shielding functionality via vacuum-assisted filtration. In low-humidity environments, water molecules intercalated within the chitosan/MXene films are released, causing the MXene layers to connect closely and promoting the formation of a perfect conductive network, thus enhancing conductivity. Due to this characteristic, the incorporation of MXene enables the fabrication of humidity-responsive smart electromagnetic shielding materials.
In addition to electrical conductivity, the dielectric constant of a material is also one of the key factors affecting its shielding effectiveness. According to effective medium theory, an increase in effective water molecules within the material will increase its dielectric constant. Therefore, directionally regulating shielding effectiveness by changing the water content of the material is an effective method for preparing humidity-responsive intelligent electromagnetic shielding materials. Wang et al.
[38]demonstrated a novel water-sensitive electromagnetic shielding sandwich structure with self-tuning capabilities by sandwiching conductive PG paper between polymer porous carbon-based spacers. As
shown in Figure 5b, this carbon-based sandwich structure, based on a composite of highly conductive pyrolytic graphite paper and porous polymer nonwoven fabric, serves as the conductive sandwich surface and the sandwich spacer layer, respectively. Water stimulation is utilized to regulate the electromagnetic response characteristics of the porous separator material. The presence of water molecules significantly improves the electromagnetic interference shielding performance of the spacer.
3.4 Chemical stimulus-responsive intelligent electromagnetic shielding materials
Chemically stimulus-responsive intelligent electromagnetic shielding materials are a class of intelligent electromagnetic shielding materials that can respond to chemical reagents. Typically, acid/base solutions or organic solvents are selected as environmental stimuli, and their conductivity is altered by regulating the doping level or conjugation degree of intrinsic conductive polymers, thereby changing the shielding effectiveness of the materials.
In intrinsic conductive polymers, polyaniline exhibits reversible acid/base doping/dedoping characteristics; acid or base treatment enables PANI to switch between a conductive doped state and an insulating dedoped state. Li et al.
[41]prepared polyaniline/MXene/cotton fabric using an efficient vacuum filtration-assisted spray method, alternately coating MXene sheets and polyaniline nanowires onto the cotton fabric substrate. Driven by stimulation from hydrogen chloride and ammonia vapors, this polyaniline/MXene/cotton fabric can adaptively act as an EMI shielding "switch" between effective strong shielding of 24 dB and ineffective weak shielding of 15 dB (
Figure 6a).
Compared to gas stimulus driving, liquid stimulus driving offers a safer operating environment and more precise control capabilities. Most acid-base gases are highly corrosive and toxic, requiring strict protection; whereas polar solvents (with few exceptions) generally have lower toxicity and are safer to handle. Meanwhile, selecting solvents with different polarities allows for controlling the conductivity of polyaniline within a more precise range, thereby flexibly adjusting the material's shielding effectiveness. As
shown in Figure 6b, Wang et al.
[65]designed this gel containing polyaniline and sodium alginate. Initially, in the aqueous system, sodium alginate acts as a proton reservoir for polyaniline, enhancing its doping level and increasing its conductivity. In this hydrogel system, the conjugation degree of polyaniline and sodium alginate polymer chains varies under different environmental conditions. By solvent exchange to replace the surrounding liquid or gas environment—such as water, acetonitrile, ethanol, n-hexane, and air—the conjugation degree of polyaniline in different conformations and the extent of electron migration along the conjugated chains are influenced. This process is reversible and does not damage the polymer chains, ensuring the durability and stability of the electromagnetic shielding switch. However, organic solvents typically possess pungent odors, and some can even be harmful to humans; furthermore, they may cause damage to devices during use, which often limits the application of chemically stimulus-responsive intelligent electromagnetic shielding materials in wearable electronic devices.
3.5 Other responsive mechanism-based intelligent electromagnetic shielding materials
In addition to the several smart materials mentioned above, smart electromagnetic shielding materials with other response mechanisms have also been developed, such as cross-angle response
[66-67]. Cross-angle response utilizes changes in the angle between the electric field direction of incident electromagnetic waves and the arrangement direction of the conductive network to achieve intelligent regulation of electromagnetic shielding performance.
Chen et al.
[66]selected commodity cotton pads with an oriented structure as carbon precursors and prepared a carbon network with an oriented structure via pyrolysis. By varying the angle between the electric field direction of the incident electromagnetic wave and the fiber orientation direction, the shielding effectiveness ranged from 28 to 36 dB, as
Figure 7shows. This carbon network with an oriented structure provides tunable shielding effectiveness without altering the material shape, offering a direction for constructing intelligent electromagnetic shielding materials.
In addition, multiple response mechanisms can be combined to develop various stimuli-responsive intelligent electromagnetic shielding materials. Multi-stimuli-responsive materials integrate the advantages of different responsive materials into a single material to achieve multifunctionality. Currently, cross-angle-humidity dual-responsive electromagnetic shielding fabrics have been successfully constructed
[39]and temperature-voltage dual-responsive electromagnetic shielding materials obtained using electrothermal materials
[68], while combinations of other response forms remain to be developed.
4 Conclusion and Outlook
This paper mainly introduces the shielding mechanism and response mechanism of intelligent electromagnetic shielding materials, and reviews the design strategies and research progress of intelligent electromagnetic shielding materials with different stimulus responses. Intelligent electromagnetic shielding materials are usually obtained by introducing intelligent materials into the shield. In the shield, intelligent materials use compressive/tensile strain, temperature, humidity, and chemical reagents as external stimuli to indirectly or directly disrupt the integrity of the material's conductive network, thereby achieving changes in shielding effectiveness. Strain-responsive electromagnetic shielding materials alter shielding effectiveness by applying compressive or tensile stress to change the material's shape; temperature-responsive electromagnetic shielding materials utilize the reversible phase transition characteristics of phase change materials to change the material's electrical conductivity and dielectric constant, thereby achieving reversibility in electromagnetic shielding; humidity-responsive electromagnetic shielding materials achieve changes in shielding effectiveness due to differences in polarity and dielectric constant under varying water content; chemically stimulus-responsive electromagnetic shielding materials are used to regulate shielding effectiveness by changing polarization and attenuation under different chemical environments. These intelligent electromagnetic shielding materials can adapt to increasingly complex usage environments.
Although intelligent electromagnetic shielding materials have developed significantly in recent years, there are still some issues that need to be overcome. First, explore more response mechanisms; in addition to the several response mechanisms mentioned above, more efficient response mechanisms can be used in future intelligent electromagnetic shielding materials. For example, using magnetic materials
[69-70], it is expected to obtain intelligent electromagnetic shielding materials with magnetic response characteristics. Magnetic response is a very promising response mechanism; it has the advantages of non-contact and remote control. Furthermore, adding magnetic materials can reduce pollution caused by secondary electromagnetic wave reflection, thus holding great potential in the field of intelligent electromagnetic shielding.
Subsequently, there is an urgent need to improve the response speed and sensitivity of intelligent electromagnetic shielding materials. Regarding currently developed intelligent electromagnetic shielding materials, their response sensitivity to external stimuli still requires significant enhancement to meet broader practical applications. Taking the strain response mechanism as an example, altering material efficacy typically necessitates relatively large shape changes; this macroscopic stimulus reflects insufficient material sensitivity. High sensitivity and rapid response enable materials to capture smaller external stimuli and provide more precise and faster response results. For instance, using liquid metals, a very significant change in shielding effectiveness occurs under small deformation, which will present more exciting and broader intelligent practical applications for intelligent electromagnetic shielding materials.
Finally, with increasing attention to environmental sustainability, "green" and environmentally friendly preparation processes are becoming increasingly attractive for the manufacturing of high-performance electromagnetic shielding materials. The future development direction for shielding materials is towards being environmentally friendly, sustainable, and renewable. To reduce secondary electromagnetic wave pollution, it is necessary to prepare low-reflection electromagnetic shielding materials. In smart electromagnetic shielding materials, replacing petroleum-based polymers with renewable and biomass-derived materials is also crucial for sustainable environmental development.
Natural Science Basic Research Program of Shaanxi(2024JC-YBMS-122)
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