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As technology develops at a rapid pace, electromagnetic and radiation pollution have become significant issues. These forms of pollution can cause many important environmental issues. If they are not properly managed and addressed, they will be everywhere in the global biosphere, and they will have devastating impacts on human health. In addition to minimizing sources of electromagnetic radiation, the development of lightweight composite shielding materials to address interference from radiation has become an important area of research. A suitable shielding material can effectively reduce the harm caused by electromagnetic interference/radiation. However, membrane shielding materials with general functions cannot effectively exert their shielding performance in all fields, and membrane shielding materials used in different fields must have specific functions under their use conditions. The aim of this review was to provide a comprehensive review of these issues. Firstly, the causes of electromagnetic/radiation pollution were briefly introduced and comprehensively identified and analyzed. Secondly, the strategic solutions offered by membrane shielding materials to address electromagnetic/radiation problems were discussed. Then, the design concept, technical innovation, and related mechanisms of the existing membrane shielding materials were expounded, the treatment methods adopted by scholars to study the environment and performance change laws were introduced, and the main difficulties encountered in this area of research were summarized. Finally, on the basis of a comprehensive analysis of the protection provided by membrane shielding materials against electromagnetic/radiation pollution, the action mechanism of membrane shielding materials was expounded in detail, and the research progress, structural design and performance characterization techniques for these materials were summarized. In addition, the future challenges were prospected. This review will help universities, research institutes, as well as scientific and technological enterprises engaged in related fields to fully understand the design concept and research progress of electromagnetic/radiation-contaminated membrane shielding materials. In addition, it is hoped that this review will facilitate efforts to accelerate the research and development of membrane shielding materials and offer potential applications in areas such as electronics, nuclear medicine, agriculture, and other areas of industry.
Keywords:
electromagnetic radiation, radiation, membrane shielding materials, shielding mechanisms, composite materials
With the rapid development of electronic technology, the widespread application of high-end precision electronic components and 5G communication systems in aerospace, military engineering, electrical electronics, wireless computers, mobile phones, wearable smart devices, and other fields has greatly enriched our daily lives and changed our lifestyles [74,82]. In recent years, with the rapid growth of electronic communication equipment, the use of electronic products by the eight billion people across the world has increased exponentially. The resulting electromagnetic pollution problem was very prominent, which not only affected the normal work of high-end precision electronic components but also posed a significant threat to human health [82], as shown in . According to research reports, economic loss due to electronic equipment failures caused by electromagnetic interference around the world is as high as $500 million USD every year [83]. At the same time, if people all over the world were exposed to radiation for prolonged durations, the health hazards would inevitably become overwhelming. For example, electromagnetic waves can interfere with aircraft navigation [85], cause damage to electronic equipment, and lead to information leakage [84]. In addition, electromagnetic radiation interferes with the normal operation of equipment, causing electronic equipment failure, which leads to serious losses in military and civil applications. Besides, electromagnetic waves can affect the human body, causing different degrees of damage to various organs and tissues [86]. Recent studies have pointed out that these forms of radiation can lead to depression, suicidal tendencies, children&#;s ADHD, and neuropsychiatric disorders, as well as abnormal births [86]. More importantly, with the development of 5G technology in recent years, people who are often accompanied by mobile phones and computers have become increasingly worried about the health implications of electromagnetic radiation. Compared with the traditional 4G network, which mainly works at around 2.4 GHz, the emerging 5G (6 GHz) network operates at a higher frequency, so it will produce higher energy electromagnetic radiation, which will cause great harm to people&#;s health and the operation of electronic equipment [87]. In order to protect human health and ensure the normal operation of precision electronic equipment, there is an urgent need for efficient electromagnetic interference membrane shielding materials to eliminate electromagnetic radiation. New electromagnetic interference membrane shielding materials should be light weight, inexpensive, porous, highly efficient, have high thermal conductivity, have wide absorption bands, and offer controllable comprehensive performance.
Open in a separate windowWith the rapid development of the nuclear energy industry, α, β, x, γ, neutrons and other rays were widely used in medical detection, aerospace, nuclear submarines, nuclear power generation, nondestructive testing, the military, as well as in agriculture and industry [88,89,90]. Although the application of radiation plays a great role in promoting human development and is becoming increasingly important, the existing radiation hazards can not be ignored, as shown in . Alpha rays are comprised of mainly helium nuclei, and some radioactive heavy elements will emit alpha particles through alpha decay, thus becoming photons. Once alpha particles are inhaled or injected into the human body, they can destroy the cells of internal organs [90]. Beta rays are a type of charged particle that moves at high speed and is released during the radioactive decay of a nuclide. Originating from either man-made or natural sources, beta rays are a more harmful form of radiation than alpha rays, and they can penetrate more deeply into materials and tissues, including skin [91]. X/γ rays have high photon energy as well as strong penetrating capabilities [92], and they can ionize substances, so that they not only damage human tissues and organs [93], but also pollute the environment, and have thus been classified as Class I carcinogens [94]. A neutron is one of the nuclei that make up the nucleus of an atom. Neutron radiation has a strong penetrating power, and it is more dangerous to the human body than the same dose of X/γ rays, which is 2~14 times that of X/γ rays, and it is also included in the list of Class I carcinogens [95]. After the human body is exposed to radiation, the digestive system and male gonads will become seriously damaged, potentially inducing the formation of tumors, which will easily lead to early death. At the same time, the damaged body was susceptible to severe infection. Therefore, providing effective protection against radiation has a critical role in protecting human health and environmental safety [88,89,90,91,92,93,94,95]. In order to minimize the risk of potential adverse effects arising from excessive radiation exposure, appropriate and effective radiation shielding materials must be utilized in all facilities with radiation to reduce the radiation damage of the target site, especially for the health protection of operators. Therefore, there is an urgent need for a new type of efficient, convenient, nontoxic, and more environmentally friendly membrane shielding material to provide protection against radiation.
Open in a separate windowIn view of the increasingly serious electromagnetic/radiation pollution, the research, development, and application of membrane shielding materials have garnered significant attention in various fields. Electromagnetic/radiation shielding materials refer to materials that attenuate electromagnetic waves through reflection, multiple reflection, and absorption, and cut off or reduce the transmission of electromagnetic/radiation waves [54]. Their mechanism of action was different from that of wave absorbing materials [54]. As can be seen from , when the incident electromagnetic wave reaches the surface of the shielding material from the emission source, due to the impedance change of the propagation medium, part of the electromagnetic wave is reflected back to the space on the same side of the emission source, and the attenuation of the electromagnetic wave caused by this is called reflection loss. The reflection loss value was proportional to the interface impedance difference. Subsequently, the remaining electromagnetic waves within the shielding body were further absorbed by the shielding material through dielectric loss or magnetic loss (absorption loss) or attenuated through multiple reflections (multiple reflection loss), and finally a small amount of electromagnetic waves passed through the shielding material to reach the reflection source [167]. The ability of a material to reflect, absorb, and attenuate radiation was closely related to its own electronic and magnetic properties. For conductive shielding materials, increasing conductivity can enhance absorption and reflection loss at the same time [63]. Generally, the shielding effectiveness or electromagnetic wave attenuation rate can be used to evaluate the shielding performance of materials. Shielding efficiency was the result of the joint action of three attenuation modes, and this value is expressed in decibel (dB) units. The larger the shielding value, the better the electromagnetic wave blocking effect. However, the electromagnetic waves reflected back to the same side of the emitting source or transmitted through the shielding material will continue to endanger human health, interfere with the operation of equipment, and cause secondary pollution. At present, a growing amount of research is devoted to reducing the proportion of reflection loss, improving the absorption efficiency of shielding materials, and reducing the transmission coefficient of shielding materials.
Open in a separate windowShielding efficiency was the most intuitive index to measure the performance of shielding materials [54]. If a material&#;s shielding efficiency is less than 10 dB, it can be considered to have no shielding efficiency. If the shielding efficiency was below 30 dB, the material was considered to have poor shielding performance. A material with a shielding efficiency in the range of 30&#;60 dB was considered to have moderate performance which can meet the requirements of civil, general commercial, or industrial electronic equipment. At 60&#;90 dB, the shielding material was considered to have high shielding performance and could be used for military and aerospace applications. When a material&#;s shielding efficiency was greater than 90 dB, it was considered to have excellent shielding performance, and was suitable for demanding scenarios such as the shielding of high-precision equipment [63]. According to their shielding effectiveness, shielding materials can be divided into different categories of shielding levels, as shown in .
Open in a separate windowAccording to the mechanism of electromagnetic shielding materials, the total electromagnetic shielding effectiveness (SETotal) can be divided into three parts as expressed in Equation (1) [54,63,169,170]. The first of these parts is a reflection loss (SER) and is shown in Equation (2), which refers to the loss caused by the impedance mismatch of electromagnetic waves on the surface of electromagnetic shielding materials. The second part is the absorption loss (SEA) as shown in Equation (3), which refers to the loss caused by the absorption of the energy of electromagnetic waves by electromagnetic shielding materials during transmission within these materials after the electromagnetic waves enter the materials. SEA is further sub-divided into electrical loss tanδμ which is expressed by Equation (4) and magnetic loss tanδε which is expressed by Equation (5). The third category is multiple reflection loss (SEMR), which is expressed by (6). This loss is caused by multiple reflections between the inner walls of electromagnetic shielding materials, and it should be noted that SEMR can be omitted when the electromagnetic shielding effectiveness is greater than 15 dB.
SETotal=SER+SEA+SEMR
(1)
SER=20logη0ηs=39.5+10logσ2πfμ=50+10logσf=10log11&#;R
(2)
SEA=20logedδ=8.7logπfμσ=10log11&#;R
(3)
tanδμ=μ&#;μ&#;
(4)
tanδε=ε&#;ε&#;
(5)
SEMR=20log(1&#;e2dδ)
(6)
In the above equations, η0 and ηs represent the inherent impedance of the propagation medium and material, respectively. Meanwhile, ε&#; (the real part of the dielectric constant) and μ&#; (the real part of permeability), respectively, represent the material&#;s ability to store electromagnetic waves, while ε&#; (the imaginary part of dielectric constant) and μ&#; (the imaginary part of permeability) represent the material&#;s ability to lose electromagnetic waves. In addition, δ denotes the skin depth, d is the material thickness, σ represents the electrical conductivity, f denotes the electromagnetic wave frequency, μ is the magnetic permeability, while R and T denote the reflection coefficient and transmission coefficient, respectively.
However, in the face of a complex electromagnetic/radiation environment, the state of knowledge regarding this which impedes efforts to develop membrane shielding materials for future applications. With the continuous efforts by researchers, deeper insight was gradually being gained about the mechanism of shielding materials. Jin et al. [170] have proposed that a unique alternating multilayer structure could have an important role in shielding radiation. When the incident electromagnetic microwave (EMW) was eliminated, it was absorbed or dissipated in the material in the form of heat, and the internal transmission times are increased to form multiple reflections to weaken the EMW. In addition, an MXene layer provides a continuous thermal conduction network in the whole membrane, thus greatly enhancing the in-plane thermal conductivity of multilayer membrane materials ( a). Cheng et al. [171] have proposed that the inhomogeneity of a medium leads to multiple scattering phenomena and reflection of EMW in multi-channels. The macro sandwich cavity structure formed by the prepared AN@MXene/TW material greatly prolongs the transmission path of the electron beam, resulting in more absorption attenuation, which has better shielding performance than a single-sided coating ( b). Zhu et al. [172] have postulated that the electromagnetic synergistic network composed of reasonably designed conductive network and confined magnetic particles was the main reason why composite aerogels have excellent EMI performance while maintaining ultra-low reflectivity. The aligned layered structure of this type of aerogel delays the transmission of microwave by nearly infinite internal reflection and scattering, which provides space for effective attenuation of the electromagnetic cooperative network ( c). Shahzad et al. [173] had suggested that shielding was mainly attributable to multiple internal reflections generated by MXene structure. EMW can be reflected back and forth between layers (I, II, III, etc.) until it was completely absorbed by the structure ( d). Huang et al. [174] established a perfect double permeation structure to explain its mechanism ( e). On the one hand, the double-permeability structure produced more interfaces, and its resonance characteristics [175] absorbed numerous electromagnetic waves due to multiple reflections in the structure. On the other hand, the interaction between electric dipole and electromagnetic wave [176] strengthened the absorption of electromagnetic waves. Zhang et al. [175] have suggested that wave interference occured during the course of multiple reflections, and its possible resonance characteristics would promote the absorption of specific electromagnetic waves ( f). Wang et al. [177] prepared a shielding material with a layered structure of conductive pearls. When an electromagnetic wave reached the surfaces of the conductive pearls, it would interact with the carrier wave on the surface of MXene and thus become partially reflected. In addition, it would enter the inside of the conductive pearl layer, and the layered structure would reflect and scatter many times, resulting in the absorption and attenuation of electromagnetic energy. In addition, the existence of functional groups (-O, -OH, -F) and N atoms on the surface of MXene may lead to polarization under the action of an alternating electric field, resulting in polarization loss, which comprehensively enhanced the shielding effect ( g). Therefore, with the deepening of the research on shielding mechanism, the development of lightweight and efficient three-dimensional porous membrane shielding materials will become an important focus of future research.
Open in a separate windowBy combining different components, researchers have prepared a wide variety of composite materials with excellent electromagnetic/radiation shielding properties. The properties of some reported composite electromagnetic/radiation shielding materials are listed in . The SE value of most cellulose composite electromagnetic interference shielding materials was in the range of 25&#;70 dB in the frequency range of 8.2&#;12.4 GHz (X-band), and the SE values of some other membrane shielding materials can reach up to 91 dB. However, the properties of membrane shielding materials prepared with different composite materials are very different.
The thickness and density of a shielding material greatly affect its performance and applicability. Therefore, various parameters and properties, as well as the correlation among many factors and the performance response mechanism, should be comprehensively considered when developing membrane shielding materials. Cellulose was anticipated to have a very important role in the preparation of shielding materials because the world was turning toward sustainable and renewable materials, and cellulose and its derivatives have important characteristics such as biodegradability, biocompatibility, non-toxicity, high surface area, molecular polarization, switchable hydrogen bonding, and low cost [51,52,53,54]. The introduction of different types of nanomaterials improves the properties of cellulose and its derivatives. Combining a material such as cellulose with graphene or carbon nanotubes yields materials with good electromagnetic interference shielding performance, light weight, and low density. However, both graphene and carbon nanotubes are expensive, so the compromise between SE and cost should be considered when designing materials for practical applications. When cellulose is combined with metal or metal oxide, the resulting material has high electromagnetic interference SE, but its high weight and poor corrosion resistance limit its applicability. However, the EMI SE of the material prepared by combining cellulose with conductive polymer was lower than that of the above two materials. In addition, significant progress has been made through the combination of cellulose with MXene, and the obtained materials show excellent EMI shielding performance. Moreover, the use of 3D printing technology to design electromagnetic interference was a very promising preparation method, and combining theoretical simulation with experimental data will have an important role in optimizing manufacturing methods.
In addition, compared with dense thin membranes, three-dimensional porous materials can trap electromagnetic waves in pores and increase the number of reflections, which is beneficial to enhancing electromagnetic absorption loss. This is mainly due to the unique multi-reflection mechanism of porous structures, which can not only reduce the material&#;s density and improve the impedance matching characteristics of the absorbent, but also facilitate the adsorption, and recombination of powder or wave-absorbing nanomaterials, thus meeting the requirements of &#;thin, light, wide, and strong&#; electromagnetic shielding materials. However, in order to realize industrial application, it was necessary to find ways to further reduce the manufacturing costs and simplify the preparation methods. If low-density and ultra-light biomass porous carbon materials are used, it will be easier to construct binary, ternary, or even more composite absorbing materials and thus achieve stronger electromagnetic shielding performance, which will have broader applicability. However, to overcome the contradiction between impedance matching and attenuation characteristics and to achieve a synergistic enhancement of electromagnetic loss by organic coupling of various mechanisms, further research will be needed on the synergistic loss mechanism of multi-element three-dimensional porous composite absorbing materials. In addition, it was necessary to continue to investigate ways to construct multi-component composite three-dimensional porous electromagnetic shielding materials with stronger electromagnetic loss capabilities and a higher impedance matching level through microstructure design and to optimize the preparation processes leading to three-dimensional porous electromagnetic shielding materials. Finally, three-dimensional porous material-based electromagnetic shielding materials with high temperature resistance, corrosion resistance, compressibility, and flexibility can be developed to further improve the practicability and applicability of electromagnetic shielding materials.
In order to obtain a good shielding effect, it is necessary to modify filler materials via approaches such as morphology control, coating modification, and blending modification. Achieving this goal will enable the development of inexpensive new conductive fillers with high conductivity, good mechanical properties, and good compatibility with the shielding materials. If these types of fillers become available, it will be easier to form three-dimensional network frameworks, thereby improving the absorption rate of electromagnetic waves/rays and reducing the reflectivity and transmittance of membrane shielding materials. In addition, combined with advanced material characterization methods, various related factors that affect the shielding effectiveness of electromagnetic shielding materials are sorted out, the internal relationship of each influencing factor is revealed, and the electromagnetic shielding mechanism related to multiple factors in complex composite materials is analyzed. Therefore, it will be highly desirable to develop a new type of low-cost, and high-performance composite membrane shielding materials. The modification methods, the selection of modified materials and the preparation process all affect the performance of the composite membrane shielding materials together, and successfully taking these considerations into account will enable the construction of an efficient conductive network and a green shielding material that can be readily dispersed in its matrix. In particular, when designing composite membrane shielding materials, the shielding mechanism should be considered in a comprehensive manner.
In summary, electromagnetic interference and radiation pollution seriously interfere with the normal work of precision electronic components and directly endanger information security as well as human health. Therefore, it is particularly important to develop efficient electromagnetic interference shielding materials to prevent the failure of high-precision electronic instruments and protect human health. The rapid pace of scientific and technological development has led to more demanding requirements for shielding materials, such as high absorption capacity, low density, wide frequency range, good thermal stability, good mechanical properties, light weight, flexibility, and low cost.
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This paper mainly explores the recent progress that has been achieved with single structure design, multiple composite structure design, preparation technology, and the electromagnetic loss mechanism of membrane shielding materials designed to counteract electromagnetic/radiation pollution. It has been found that by introducing porous structures, constructing heterogeneous structures, designing multilayer structures, as well as utilizing 3D structures, simulations, fillers, and magnetic materials, the electromagnetic shielding performance of membrane shielding composites can be effectively improved. In addition, a shielding material can be endowed with other functional characteristics to realize the integration of structure and function.
In the future, the development direction of membrane shielding materials will be mainly as follows:
(1) Simulation. WinXCOM, Auto-Zeff software, EGS software, MCNP software, CST software, COMSOL and other software programs are used for simulation so as to optimize the preparation process leading to membrane shielding materials. As computing technology continues to progress, the development of new and more accurate simulation software will likely provide further benefits in this area; (2) Multiple compounding of functional materials. The use of carbon-based functional materials such as conductive polymers and graphene or metals, fibers, or fabrics are promising ways to enhance the properties of shielding materials, and these strategies are likely to garner significant attention in the future; (3) Functional integration. With good shielding performance, multiple functions can be achieved, such as good wave absorbing performance, flame retardant performance, antibacterial performance, and radiation resistance; (4) Intelligence. By exhibiting a timely response to the surrounding environment, a smart or intelligent shielding material could adjust its internal structure and electromagnetic characteristics according to the changes in the surrounding environment; (5) Green and environmental protection. Researchers should make efforts to develop promising green electromagnetic interference shielding materials with low reflectivity and superior dynamic performance adjustment function that help reduce secondary electromagnetic pollution and can be applied in complex situations; (6) Self-repairing materials. The design of supramolecular networks to repair the damaged surface independently can be employed to impart membrane shielding materials with self-repairing capabilities, which can prolong the service lifetimes of these materials, reduce the costs associated with repairing or replacing the damaged equipment, and improve the safety for all users and the public.
In conclusion, optimizing the design of shielding systems from the perspective of theoretical simulation, developing new shielding materials with multiple functions, light weight, high efficiency, and high strength materials, and organically combining them to enhance the integration degree for the structure/function of the electromagnetic/radiation shielding systems are key aspects of current and future research in this field. Most importantly, researchers in the future should give full consideration to the unique properties of membrane shielding materials, such as porosity, multilayers, magnetism, and conductivity, which will enable the development of a wider range of applications for these materials (especially in harsh environments) and also help to address challenges encountered during production.
We thank Ian Wyman for proofreading this paper.
The authors thank the National Natural Science Foundation of China () and the Guangdong Natural Science Foundation (A) for supporting this research.
H.Z.: conceptualization, investigation, writing-original draft, writing-review and editing. S.L.: writing-review and editing, supervision, project administration, funding acquisition. All authors have read and agreed to the published version of the manuscript.
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The authors declare no conflict of interest.
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