Research progress of flexible textile-based electromagnetic shielding composites
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摘要:
电子通讯的飞速发展与高频应用带来极大便利，但同时也导致电磁污染问题越发严重，因此开发电磁干扰屏蔽材料至关重要。在屏蔽材料日渐追求“薄、轻、宽、强”的今天，柔性电磁屏蔽材料以其轻质、柔韧及良好的加工性呈现出极大应用前景。目前从不同尺度对于柔性电磁屏蔽复合材料的系统性研究综述较少，因此本文首先从微观角度论述了几种常见基底复合纳米材料，发现大多研究是从多结构设计及构建多相材料复合体系角度提高电磁屏蔽效能，在此基础上进一步拓宽至宏观纺织基复合材料中，分析了不同形态复合纺织品在加工过程中的优化方法，主要有材料复合、结构设计及改性处理等。最后对相关研究工作进行总结及展望。本文可为柔性电磁屏蔽复合材料研究提供理论参考，为功能纺织品研发提供借鉴思路。
Abstract:The rapid development of electronic communication and high-frequency applications have brought great convenience, but at the same time, the problem of electromagnetic pollution has become more and more serious. Therefore, it is very important to develop electromagnetic interference shielding materials. Nowadays, shielding materials are increasingly pursuing 'thin, light, wide and strong', and the flexible electromagnetic shielding materials show great application prospects due to their light weight, flexibility and good processability. At present, there are few systematic studies on flexible electromagnetic shielding composites from different scales. Therefore, this paper first discusses several common substrate composite nanomaterials from the microscopic point of view, and finds that most of the research work is to improve the electromagnetic shielding effectiveness from the perspective of multi-structure design and construction of multi-phase material composite system. On this basis, it is further extended to macro textile-based composites. The optimization methods of different forms of composite textiles in the processing process were analyzed, including material composite, structural design and modification. Finally, the relevant research work is summarized and prospected. This paper can provide theoretical reference for the research of flexible electromagnetic shielding composite materials and provide reference ideas for the research and development of functional textiles.
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Keywords:
- flexibility /
- electromagnetic shielding /
- composite material /
- textile /
- structural design
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随着智能化设备和通讯网络的飞速发展，电磁污染对设备、人类和周围环境造成影响逐渐增大[1]。开发轻质、柔韧、高效的电磁屏蔽材料对于保证电子设备的稳定性和保护人体健康至关重要[2]，传统金属电磁屏蔽材料在实现高导电及电磁屏蔽性能的同时往往伴随密度大、刚度强及加工难的限制[3]，并且原料选择性较小，产品多样化困难，无法兼顾功能特性及应用要求。
柔性电磁屏蔽复合材料通常由柔性基体及电磁屏蔽功能体复合而成，具备一定的抗弯、抗扭、抗折叠变形能力同时保持较高的电磁屏蔽性能[4]。这对材料特质提出挑战，由于单一材料自身存在限制[5]，大多需与其他材料复合构建导电网络以实现柔性电磁屏蔽[6]。国内外对于柔性电磁屏蔽材料有所研究，但较少同时从微观及宏观两种视角进行系统性论述。
微观视角中，纳米材料以其大比表面积、高活性及独特光、电性能在电磁屏蔽材料中显现出独特优势，但由于纯纳米材料结构单一，多功能应用受限，在实际应用中往往需要将不同的纳米材料进行复合。宏观层面中，目前最为广泛应用的柔性电磁屏蔽材料为纺织品，纺织品因其易于获得、成本低、透气性和柔韧性好等被认为是屏蔽电磁干扰的良好基材[7]，在日常以及专业屏蔽领域被广泛应用，如军事、医疗领域、防护罩、服装和特殊壁纸等[8]。
然而，大多天然纺织品并不具备电磁屏
蔽功能，需引入导电填料或构建导电层创造导电特性而实现电磁屏蔽[9]。传统纺织品织造形态主要有纤维-纱线-织物，在加工中通过材料复合、表面改性、织造参数调控等可制备多种形态的电磁屏蔽织物；在此基础上，通过结构设计构建宏观多层级纺织基复合体系可进一步增强电磁波多重损耗，获得具备优异性能的纺织基电磁屏蔽复合材料[10]。
近几年来，柔性电磁屏蔽复合材料逐渐受到国内外研究学者广泛关注，图1 (源自web of science数据库)显示自2000年以来，柔性电磁屏蔽复合材料的论文发表数量和被引频次逐渐增多。基于国内外研究现状，本文以电磁屏蔽机制为理论基础，从微观尺度电磁屏蔽复合纳米材料、宏观纺织基电磁屏蔽材料双视角对柔性纺织基电磁屏蔽复合材料研究进展以及应用现状进行较为系统的论述。
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图 1 柔性电磁屏蔽复合材料发文量趋势图(自2000年，源自web of science数据库)Figure 1. Trend chart of publication volume for flexible electromagnetic shielding composite materials (since 2000, sourced from the Web of Science database)1. 电磁屏蔽机制
电磁屏蔽是指使用屏蔽材料部分或完全阻断电磁辐射干扰(EMI)以维持保护区域电磁稳定，通常用电磁屏蔽效能(SE)来定义[11]，以分贝(dB)表示。电磁屏蔽原理由三部分组成，如公式(1)所示 [12]。
SET＝SER＋SEA＋SEM (1) 其中SET、SER、SEA、 SEM分别为总损耗、反射损耗、吸收损耗以及多次反射损耗。
如图2所示，当电磁波接近屏蔽材料表面时，较大部分电磁波从屏蔽材料表面被反射出去，形成反射损耗，同时电磁波在屏蔽材料内部进行吸收或反射[13]，其中吸收损耗占据大部分，材料内部多次反射损耗次之，最后剩余电磁波透射出材料。电磁屏蔽效能的主要影响因素为电磁特性，因此，优质电磁屏蔽材料大多须具有高磁导率与导电性[14]。其次，频率范围、厚度以及接收机和发射机之间的距离等也会造成一定影响。目前电磁干扰屏蔽性能测试方法主要有屏蔽箱法、屏蔽室法及同轴传输线测试等，实验中常借助矢量网络分析仪(VNA)通过波导法进行测试。
2. 柔性电磁屏蔽纳米复合材料
纳米材料以其大比表面积、丰富界面结合力及可调节电磁性能引起了研究者极大兴趣[15]，但单一纳米材料往往因为某一方面的缺陷而限制其使用范围，难以满足多功能电磁屏蔽材料的需求，需与其他材料相结合来进一步提升电磁屏蔽、力学、传感及其他性能。从形态上可分为一维材料、二维薄膜、及三维气凝胶。本节重点介绍几种柔性薄膜形态的常见基底电磁屏蔽复合材料，包括导电聚合物基、碳基、MXene基、金属基等。
2.1 导电聚合物基
导电聚合物与传统金属电磁材料相比，具有柔韧性强、耐腐蚀、重量轻、强度高的优点，且具备一定电磁屏蔽功能，可分为本征导电聚合物(ICP)和导电聚合物基复合材料(CPC)[16]。ICP兼具类似金属的优异电磁性能和聚合物的轻质柔性，有“合成金属”之称[17]，包括聚吡咯(PPy)，聚乙烯(PE)，聚氨酯(PU)和聚苯胺(PANI)等种类。ICP内部分子主链单键和双键交错的结构，形成共轭π电子体系，使电子可远距离移动，因此表现出较高的电导率和介电常数，对电磁波有一定反射以及吸收衰减能力[18]。但由于ICP实际可加工性低，因此单一的ICP很少用作屏蔽材料，大多需掺杂小分子或与其他导电纳米材料复合，增加其导电性[19]，如Li D Y等[20]使用真空过滤辅助喷涂方法制备高导电聚苯胺 (PANI)/MXene/棉织物 (PMCF)，通过优化棉织物上 PANI/MXene 涂层结构，实现约 54 dB 的高 EMI 屏蔽效率。
除ICP外，传统聚合物基体如聚酰亚胺(PI)、聚二甲基硅氧烷(PDMS)、聚乳酸(PLA)、聚丙烯(PP)等具备优异柔韧性但并不具备导电性，需通过添加导电及磁性填料组成导电聚合物基复合材料(CPC)，聚合物基底赋予其高强度和刚度，可适用于结构应用，轻质特性使其适合如航空航天及交通运输的减重应用[21]。CPC往往具有高导电性、良好的力学性能和轻质性，通过调节导电网络可调控材料的导电性能和电磁屏蔽性能[22]。CPC的导电性主要取决于其填料电导率、磁导率、微观结构排列等，目前被广泛应用的主要有金属填料、MXene及碳基填料[23]，从材料形态上可分为零维粒子如银纳米粒子(AgNPs)、炭黑，一维银纳米线(AgNWs)、碳纳米管(CNTs)及二维石墨烯、MXene等。目前研究大多通过设置浓度梯度及构建多相填料体系赋予其优异性能，Chen J等[24]以MXene填料浓度为变量，通过逐层浇铸构建MXene/聚酰亚胺 (MXene/PI)复合薄膜，当MXene质量分数为29.7 wt%时，薄膜EMI SE为23.3 dB，增加为45.7 wt%时，最高可达40.7 dB；Yang S等[25]构建了多相填料体系聚偏氟乙烯(PVDF)-AgNW/MXene薄膜，显示出优异的导电性、在X波段高达47.8 dB的EMI屏蔽性能和机械稳定性，其中AgNW层作为导电网络，MXene片包裹于AgNW骨架上增强互连性并减少AgNW的暴露，多相填料协同增强屏蔽效能。
除此之外，由于导电填料和聚合物基体之间易存在巨大界面不相容性会使填料严重团聚，导致相对较低的EMI屏蔽效能，需通过构建结构排列模式如多孔结构[26]、多层结构[27]、分离结构[28]等制备复合材料，以增强导电性。Li H等[29]通过溶液共混与过滤工艺制备分离结构式高导电多壁纳米管(MWCNT)/水性聚氨酯(WPU)复合薄膜，样品在碳纳米管含量为21.1 wt%，薄膜厚度仅为0.1 mm的情况下SE可达到24.5 dB。
表 1 各种功能填料性能对比Table 1. Comparison of parameters of various functional fillersDimensions Fillers Advantages Disadvantages 0D Silver nanoparticles (AgNPs) Easy to prepare,
ultra-high conductivityPoor chemical stability and discontinuity Carbon black Abundant in resources and can be produced on a large scale Poor conductivity Ferrite Excellent magnetic loss properties, good dielectric properties Low electrical conductivity and properties 1D Silver nanowires (AgNWs) ultra-high conductivity, antibacterial, highly transparent Poor chemistry Metal nanomaterials Ultra-high conductivity, large specific surface area, and low cost, Low tensile properties, and low flexibility Carbon nanotubes (CNTs) Unique structure, high aspect ratio and conductivity,
good mechanical strengthHigh cost, poor dispersion, and stability in long-term storage 2D Graphene High conductivity,strength, thermal conductivity and transparency Easy to oxidize and high production cost MXene Adjustable layer spacing, ultra-high metal conductivity,
excellent mechanical stability and abundant polar terminal groups, high surface chemical activity and energy densityPoor environmental stability 2.2 碳基
碳材料具备质轻、高强、柔韧性好、良好导电性及耐用性等特点，在柔性电磁屏蔽产品中有巨大应用潜力[30]，类别有氧化石墨烯(GO)、CNTs 、碳纤维、炭黑、金属-有机框架衍生碳以及其他碳杂化物复合材料[31]。但这些材料在实际应用中存在一定限制，如石墨烯纳米片的团聚和堆叠作用导致力学性能减弱、CNTs 分散性差等缺陷[32]。目前研究工作主要通过构建层状结构或与其他电磁性材料复合来增强多重反射，提高电磁屏蔽性能。如Pang K等[33]将CuCl2插层嵌入石墨烯膜中以显著提高空穴密度，解决石墨烯膜导电性不稳定缺陷，获得超纯石墨烯膜的约10倍的电导率。在8.2 ~ 12.4 GHz范围内，厚度为35 mm的薄膜呈现出约126 dB的平均电磁干扰屏蔽效能，达到同等厚度膜材料的最高值。Lee S H等[34]使用微波方法直接合成多层互连的三维石墨烯碳纳米管氧化铁(3D G-CNT-Fe2O3)异质纳米结构薄膜，由于集成组分协同增强传导、磁滞耗及3D纳米结构表面和夹层多级反射、吸收和散射，使其兼具优异的柔性和极高的电磁干扰屏蔽性能，在8.0 ~ 12.0 GHz达到130 ~ 134 dB。
通过一定制备工艺的附加，如涂层法、沉积法、热压法等对薄膜进一步设计，也可提高电磁屏蔽性能。如Chao Z等[35]通过化学气相沉积工艺合成碳纳米管(CNT)/氧化铁(Fe3O4)复合膜，并通过水热还原工艺制备了Fe3O4涂层。CNT膜和Fe3O4颗粒分别作为导电骨架和强磁性颗粒，复合膜在X波段的电磁屏蔽效能达到91 dB。在提高电磁屏蔽效能的同时仍需保持或增强材料良好的柔性及力学性能，Kim J等[36]使用简单热压法将聚酰亚胺与炭黑复合，使硬质复合材料成为高柔韧性的EMI SE薄膜，拉伸强度增加了一倍，电磁屏蔽性能也得以增强，在航空航天和无线通信领域具有巨大的应用前景。
2.3 MXene基
近年来，MXene作为新型纳米材料，以其高比表面积、优异的导电性和亲水性等优点，成为EMI领域关注的焦点[37]。MXene基电磁屏蔽复合材料主要有纯MXene材料、金属或金属氧化物/MXene复合材料、碳纳米/MXene电磁屏蔽复合材料等[38]。但由于MXene本身过高导电性及较差的阻抗匹配，多以反射损耗为主要屏蔽机制，但过多电磁反射波对环境有一定污染性，需通过与其他材料复合或结构设计提高对电磁的吸收损耗。如Liang L等[39]利用磁性Ni链与MXene复合产生的协同效应获得了具备合适的阻抗匹配条件与良好的电磁波耗散能力的柔性薄膜，通过调整混合物中MXene的含量可获得优异的电磁波吸收和屏蔽性能。在11.9 GHz频点处，当MXene质量分数为10 wt%时，即可获得-49.9 dB的最小反射损耗。当MXene含量进一步增加到50 wt%时，SE可达到66.4 dB，其中SEA达到59.9 dB。Zhang H等[40]设计了双梯度结构 MXene 纳米片和 Fe3O4纳米颗粒的复合薄膜，实现以吸收为主的“吸收-反射-再吸收”相互作用过程，薄膜表现出 49.98 dB 的EMI屏蔽效能以及 0.51 的增强吸收系数。
2.4 金属基
作为典型的EMI屏蔽材料，金属材料在传统的EMI屏蔽产品中占据了很大的比例。但往往力学性能有一定局限性，随着纳米技术的发展，将传统金属材料加工成金属纳米粒子或纳米线是提高力学性能的重要方式，微纳米尺度的大比表面积特性与金属颗粒形成的导电网络可使材料兼具良好柔韧性、高透光性、高电磁屏蔽效能等。Zhu X等[41]通过简单涂覆方法制备基于AgNWs的透明EMI屏蔽膜，通过NaBH4处理及层压工艺有效提高AgNW基薄膜的电导率和光透过率，EMI SE平均为28 dB，可见光透过率为91.3％，优于大多数报道的透明的EMI屏蔽材料。除对金属进行微纳米尺度加工外，近年来镓基液态金属 (LM) 以高导电性和室温流动性，在电磁屏蔽领域引起研究者广泛兴趣，Ren N等[42]采用直接加工的方式制备了 LM 和纤维素纳米原纤维(CNF)的复合柔性薄膜，在8.2 ~ 18.0 GHz的宽频率范围内，平均 EMI 屏蔽效能值为 429 dB/mm，并表现出优异柔韧性及导热性，在先进EMI屏蔽产品开发方面具有较大的应用潜力。Sun Y团队[43]制备的液体注入光滑表面的还原氧化石墨烯(rGO)桥接LM层状异质结构纳米复合材料(S-rGO/LM)成功调节了高 EMI SE与低厚度之间的矛盾，在仅67 μm 的内部厚度下表现出100 dB的极高SE值，且在承受各种恶劣条件时表现出优异的 EMI屏蔽稳定性(EMI SE保持在70 dB以上)。金属基柔性电磁屏蔽材料应用逐渐朝向高性能、高稳定的多功能先进领域发展。
不同基材柔性电磁屏蔽纳米材料具备相应各异的特质，通过多相材料复合、多元结构设计及选择合适制备工艺可有效弥补单一纳米材料的缺陷。随着科技的不断进步，柔性电磁屏蔽纳米材料性能与应用逐渐趋向高性能、多元化领域发展。
3. 纺织基电磁屏蔽复合材料
纺织品作为柔性材料，具备优异的力学延展性，但导电性较差，因此一般并不具备EMI屏蔽能力。电磁屏蔽纺织品的开发需通过优化加工方式提高其导电性，以实现有效的电磁屏蔽性能[44]。本节主要从纤维-纱线-织物三种形态纺织品的宏观复合加工层面对纺织基电磁屏蔽复合材料进行论述。
3.1 复合纤维
纤维是纺织品的基本原料，是纺织品加工中不可或缺的一部分，但常规天然或合成纤维并不具备电磁屏蔽性能[45]，往往需要其它导电材料进行复合，方法主要有纺丝液掺杂、涂层包覆等[46]。利用静电纺丝技术，在纳米纤维中引入导电或磁性填料以制备高性能EMI屏蔽材料的有效方法，其中柔韧导电材料可有效规避物理刚性，改变填料类型与比例可织造不同性能复合微/纳米纤维[47]。Wei Z等[48]通过静电纺丝和原位聚合制备了具有良好导电性的聚醚砜PES/PDA/Ag纳米纤维。优化后的电阻率从2.1 × 109 Ω/cm降至仅202 Ω/cm。湿法纺丝也是制备复合纤维的重要方法，Wang Z等[49]使用液晶湿法纺丝制备了以吸收为主的高导电石墨烯纤维，制备示意图如图3(a)所示。特殊纤维结构促进了电磁波多重损耗，所得样品在X 波段EMI SE最高值达到81 dB，光透过率为91.3%，优于大多数报道的透明的EMI屏蔽材料，EMI屏蔽机制如图3(b)所示。
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除上述纺丝使纤维“内部”实现本征导电外，将导电物质涂层或包覆于纤维外部也是常见的电磁屏蔽复合纤维的制备方法，Lee J等[50]通过化学镀将FeCoNi涂覆在玻璃纤维上作为填料，制备的复合片材在5.3 GHz附近EMI SE最大为30 dB，与导电铜箔相当。Krishnasamy P等[51]利用化学镀镍磷涂层提高编织大麻纤维(HF)夹心碳纤维(CF)环氧复合材料的电磁干扰屏蔽效能，并发现随夹层数量叠加，EMI SE逐渐增加，两层涂层时，在8.0 ~ 12.0 GHz 频段表现出 92.77 dB优异电磁屏蔽性能，比原始复合材料提高51.66%。
在制备导电复合纤维基础上，研究者进一步探究不同结构或丝束排列的影响及新型功能性应用。Yin G等[52]通过湿纺技术将二维纳米材料(MXene、石墨烯)构筑于再生丝素蛋白(RSF)纤维上，丝素蛋白动态构象转变和核壳模量不匹配形成坚固可逆的皱纹结构，有效优化了组装界面，形成完整的导电通路，表现出约
1125 S/cm的高电导率，在自供电可穿戴电子产品和智能织物开发中具有巨大应用前景。Guan H等[53]在比较有和没有镍涂层的碳纤维性能的基础上，进一步探究了线圈与丝束排列对电磁屏蔽效能的影响，结果显示，对于平面线圈配置，镍涂层EMI SE增加较为明显，从 2 ~ 6 dB 增加到 13 ~ 26 dB (频率范围200 ~ 2000 MHz)，但对交叉/单向配置影响很小；在没有镍涂层的情况下，交叉层配置相对于单向配置的优势更大，结果与电磁理论基本一致。3.2 复合纱线
纱线是以各种纺织纤维为原料制备的连续条状物体，通过选择合适纤维原料、纺纱工艺以及后处理工艺，可制备高导电性能纱线，为电磁屏蔽织物开发奠定基础。目前常用的纺纱技术有环锭纺、气流纺、熔融纺和静电纺等[54]，在纺纱过程中，通过添加金属纤维、本征导电聚合物和碳基填料等，可混纺出多相电磁屏蔽复合纱线。Rathour R等[55]通过将粘胶短纤纱与不锈钢长丝纱线并合，并进一步探究了不同参数如导电丝直径、数量、纱线类型、支数、密度等对混纺纱导电性的影响规律，几种复合纱如图4所示。Liu X等[56]探究了不同混纺比的不锈钢短纤维和涤纶混纺纱的电磁屏蔽性能。Ning Li等[57]以不锈钢/棉(30/70)织造纬编针织物，分析了电磁波入射方向、针数、线圈长度和频率对电磁屏蔽性能的影响，其中纬镶嵌组织表现出最优异的屏蔽性能。
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涂覆法是对纱线改性的重要方法，M Lai等[58]利用聚丙烯/ TiO2涂层不锈钢缠绕纱线，在保持纱线柔性的条件下可增强导电性及电磁屏蔽效果。Lin Z I等[59]采用聚丙烯(PP)/多壁碳纳米管(MWCNT)包覆PET纱线制备导电机/针织物，在30 MHz ~ 3 GHz频段范围内，当含有质量分数为8 wt%的 MWCNT时，织物获得最佳的EMI SE。
3.3 复合织物
3.3.1 编织结构
织物是由纤维或纱线编织而成的二维或三维纺织品，不同织物类型对电磁屏蔽织物效能产生重要影响[60]。根据编织方式不同织物可分为机织、针织、编织和非织造等结构，机织物由经纬纱交织而成，是目前最常见的织物类型，改变其导电纱线参数及组织结构等可有效提升电磁屏蔽效能。Tugirumubano A 等[61]以预浸的碳纤维为纬纱，钢丝网为经纱生成平纹机织物，并与碳纤维层堆叠，其中不锈钢-铜-CFRP(碳纤维增强复合材料)复合材料的屏蔽效果最好，SE在0.5-1.5 GHz频率范围可达到131.6 dB
针织物具备良好延伸性、结构多样性、适形性，纱线屈曲程度较大而具备多孔、透气、弹性的特性[62]，拉伸变形能力对电磁屏蔽效能产生一定影响。Palanisamy S等[63]]研究不同的拉伸变形方式对针织物的影响，其中垂直拉伸对电磁屏蔽能力的积极影响最大，最大屏蔽灵敏度为12%。Lin Z I等[59]以 PP/MWCNT 涂层的 PET 纱线与普通 PET 纱线为纬纱和经纱，制备了平纹机织物，再以 PP/MWCNT 涂层的 PET 纱线为针织纱，制备纬编针织物，MWCNT有效加固了两种织物导电纱线，如图5所示。相比于平纹结构织物，纬编结构织物表现出较低的电磁屏蔽效能值。此外，非织造布[64]、三维立体结构织物[65]也多作为纺织基底结构实现电磁屏蔽效能。
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3.3.2 表面改性
对织物表层进行改性修饰以增强导电性是获得理想EMI屏蔽性能的重要方式。目前，织物表面改性方法主要有涂覆法、化学镀法以及原位聚合法几种。
涂覆法是指在织物表面进行浸涂或喷涂在其表面形成片状膜的方式。Sun Z等[66]以大间隙的纬编纺织品作为柔性基材，二维石墨烯纳米片(GNS)和一维 AgNWs 为导电填料制备复合纺织品，使用浸涂法对聚氨酯 (TPU) 纺织品进行处理得TPU/GNS/Ag NWs@textile(TAG@textile)复合体系，由于其具有良好吸波特性而获得高屏蔽效能，随着导电填料的添加，在X波段，SE增长到61.68 dB。L Qiu等[67]使用喷涂法制备了液态金属基导电纺织品，导电率最大值达到
5637 S/m。涂覆法工艺简单，织造成本较低，是最为常见的织物表面改性方式，但往往伴随均匀性较差的问题。化学镀法也称化学沉积法(ED)是一种化学还原过程，指在织物表面镀金属层以增强起导电性，达到提高电磁屏蔽性能的目的。在此过程中，离子被合适的还原剂还原并沉积在柔性基板的表面上。Lee J等[68]以活性碳纤维(ACF)织物作为主要框架，并在其表面化学镀制备均匀的 Cu 层，纤维内部多孔结构与表面镀层微结构为复合织物提供了多重内部反射界面和高效电子传输路径，所得Cu@ACF 复合织物在 30 MHz ~ 10 GHz 频段内的屏蔽效能高达 70 ~ 90 dB。化学镀法改善了导电层分布均匀性的问题，具备较高电磁屏蔽效率，但镀层易脱落。
原位聚合法是将柔性底物浸入单体混合物中，并加入引发剂引发聚合。HM Fayzan Shakir等[69]通过原位聚合将聚苯胺(PANI)涂层于聚酯织物上，在1 ~ 13 GHz频率范围复合织物的EMI SE可达到38 dB。由于织物和导电材料主要依靠分子间作用力结合，结构较为牢固，屏蔽效能高，但制备工艺复杂，生产力较差。
3.3.3 层级结构
相比于单层电磁屏蔽的织物来讲，多层结构在电磁屏蔽上展现出了巨大优势，通过构建宏观复合结构体系，可显著增加非均质界面数量，进而增加电磁损耗及多重反射作用[70]。
层接层(L-b-L)组装法是指借助分子间相互作用力，将基底织物与功能层复合的方法，可有效提高屏蔽性能，与单一功能层结构相比，构筑多种功能层交替结构效果更明显。如图6所示，Yin G等[71]通过MXenes片和PANI聚合物间的强界面相互作用交替组装在碳纤维(CF)织物上，获得了较高的电磁屏蔽性能。将组装法与其他改性方法结合可进一步提升屏蔽效能，Zhang Y[72]将层层组装技术与浸涂法结合，以棉织物为骨架，利用强静电相互作用和氢键效应，在织物表面形成聚乙烯亚胺/植酸(PEI/PA)层和 AgNWs导电网络，在X波段，EMI SE达到32.98 dB。
利用纺织加工的方法直接制备多层纺织结构复合材料，可有效保留织物基底的基本性能。如图7所示，Lin J H[73]将聚酯无纺布(L)、尼龙间隔布(N)和碳纤维机织布(C)按不同顺序层合，形成“三明治”结构的NLC、NLN、CLC、CLN复合材料。其中，在1 ~ 3 GHz的频率范围内两组NLC和CLN分别表现出−45~−65 dB以及−60 dB电磁屏蔽性能。为降低复合成本以及简化工艺，Y Zhang等[74]将疏水阻燃织物、碳化废棉毡和碳纤维毡简单缝合，制备了新型铝可燃性碳化废棉-碳毡(A-FCWCF)复合织物，在X 波段，EMI SE达到82.63 dB。Cheng H C等[75]使用了成本低廉的溶剂型聚四氟乙烯(PTFE)薄膜、棉织布和导电银浆，成功制备了具有成本效益的导电层压织物，通过改变层间结构参数，如层级数量、铺层角度、铺层方法等，可显著提高电磁屏蔽性能。Liang J[76]分别制备扁平、波纹和波纹夹层短切碳纤维毡/环氧树脂基复合材料，波纹夹层独特铺层方式引起的多次反射使复合材料在8.2 GHz频点处EMI SE大于70 dB。
除此之外，为实现电磁波特定频率可调，研究者通过丝网印刷、刺绣、3D打印等技术将频率选择表面(FSS)与织物基底结合。FSS是一种二维周期性结构，独特的结构设计使其可在特定的频率范围内对电磁波进行高效过滤，通过选择性地反射、透射或吸收电磁波，实现对电磁波传输的精确调控，在军事和民用领域都具有重要的应用价值和广阔的发展前景。Zhang等[77]将FSS、羰基铁涂层织物(CIFs)和镀铜镍导电机织织物(CWFs)组成三层柔性复合吸波织物，提高了
低频吸收强度和带宽，较单层结构在11.28 GHz处的反射率降低了8.06 dB，带宽增加了0.32 GHz。Y. Yang 等[78]将刺绣FSS、针织面料和金属化面料组成柔性刺绣基超材料吸收剂(MA)复合，如图8所示，在此基础上可进一步探究不同针刺密度的影响，以获得较高EMI SE。
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目前通过增大复合织物的层级数量可显著增强屏蔽性能，但同时会产生厚度过大，透气性较差等问题，这将在一定程度上限制了实际应用。
不同纺织基复合方法及处理方式对电磁屏蔽复合织物的性能产生不同影响。表2列出了代表性EMI屏蔽织物的类型及性能优缺点，现有的研究工作表明，在追求高效电磁屏蔽性能的同时往往伴随其他功能效应的下降，新型纺织基电磁屏蔽复合材料需朝向更加完善、高效、多功能应用等方向发展。
表 2 电磁屏蔽复合织物参数比较Table 2. Comparison of parameters of electromagnetic shielding composite fabricsEMI Shielding Fabric Advantages Disadvantages Ref PP/MWCNT coated knitting yarn forms a weft knitted fabric good extensibility, good conformability and porous air permeability Loose structure and easy to cause leakage [59] The prepreg carbon fiber and steel wire mesh warp stable morphology, dense structure, high shielding efficiency relatively poor flexibility [61] Carbon fiber waste processing nonwovens environmentally friendly, simple process, high economic benefits structural diversity is poor [64] Copper wire-based core sheath blended yarn is composed of three-dimensional orthogonal
woven hybrid conductive fabricstable structure, high overall performance, high shielding ability, easy to be affected by internal material limitations, and complex preparation [65] Preparation of liquid metal-based conductive textiles by spraying method The process is flexible and simple, the cost is low, The uniformity control is difficult [66] Carbon fiber fabric (ACF) surface plating Cu layer The coating layer is relatively uniform, the shielding efficiency is high, Easy to fall off, which is difficult to apply in practice [68] MXene/CF Fabric Surface Polymeric Aniline (PANI) Uniform dispersion, high binding The cost is high and the time is long [69] Polyester non-woven fabric (L), nylon spacer fabric (N) and carbon fiber woven fabric (C) lamination (three layers) The method is simple, the shielding capacity is high, High,thickness ,and the air permeability is poor [73] FSS, carbonyl iron-coated fabrics (CIFs) and copper-nickel-plated conductive woven fabrics (CWFs) form composite fabrics
(three layers)Various shapes, low loss, convenient for equipment miniaturization, adjustable frequency width, easy to follow the line The bandwidth is limited to a certain extent, and the air permeability is poor [77] 4. 结论
本文简述了电磁屏蔽机制，并以此为理论基础，分别从微观复合纳米材料、宏观复合纺织基电磁屏蔽复合材料两个层面对近年来相关的制备方法以及优化方式进行阐述，为后续柔性电磁屏蔽复合材料的研究提供理论参考。
(1)单一纳米材料具有一定缺陷以及功能局限性，可通过不同纳米材料的多相复合，构造多元结构如分离结构、层状结构、隔离结构等，并选择合适的制备方式，设计开发高性能、多功能的柔性复合材料体系。
(2)纺织基电磁屏蔽复合材料具有不同的结构形态，可选择合适的复合方法赋予其高效的电磁屏蔽性能，如纤维复合纺丝，纱线涂覆改性，织物成形方式、表面改性处理及多层结构构筑等。在提高电磁屏蔽效能同时，仍需保持纺织品本身柔性优势，使其应用于实际生产。
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图 1 柔性电磁屏蔽复合材料发文量趋势图(自2000年，源自web of science数据库)
Figure 1. Trend chart of publication volume for flexible electromagnetic shielding composite materials (since 2000, sourced from the Web of Science database)
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表 1 各种功能填料性能对比
Table 1 Comparison of parameters of various functional fillers
Dimensions Fillers Advantages Disadvantages 0D Silver nanoparticles (AgNPs) Easy to prepare,
ultra-high conductivityPoor chemical stability and discontinuity Carbon black Abundant in resources and can be produced on a large scale Poor conductivity Ferrite Excellent magnetic loss properties, good dielectric properties Low electrical conductivity and properties 1D Silver nanowires (AgNWs) ultra-high conductivity, antibacterial, highly transparent Poor chemistry Metal nanomaterials Ultra-high conductivity, large specific surface area, and low cost, Low tensile properties, and low flexibility Carbon nanotubes (CNTs) Unique structure, high aspect ratio and conductivity,
good mechanical strengthHigh cost, poor dispersion, and stability in long-term storage 2D Graphene High conductivity,strength, thermal conductivity and transparency Easy to oxidize and high production cost MXene Adjustable layer spacing, ultra-high metal conductivity,
excellent mechanical stability and abundant polar terminal groups, high surface chemical activity and energy densityPoor environmental stability 
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表 2 电磁屏蔽复合织物参数比较
Table 2 Comparison of parameters of electromagnetic shielding composite fabrics
EMI Shielding Fabric Advantages Disadvantages Ref PP/MWCNT coated knitting yarn forms a weft knitted fabric good extensibility, good conformability and porous air permeability Loose structure and easy to cause leakage [59] The prepreg carbon fiber and steel wire mesh warp stable morphology, dense structure, high shielding efficiency relatively poor flexibility [61] Carbon fiber waste processing nonwovens environmentally friendly, simple process, high economic benefits structural diversity is poor [64] Copper wire-based core sheath blended yarn is composed of three-dimensional orthogonal
woven hybrid conductive fabricstable structure, high overall performance, high shielding ability, easy to be affected by internal material limitations, and complex preparation [65] Preparation of liquid metal-based conductive textiles by spraying method The process is flexible and simple, the cost is low, The uniformity control is difficult [66] Carbon fiber fabric (ACF) surface plating Cu layer The coating layer is relatively uniform, the shielding efficiency is high, Easy to fall off, which is difficult to apply in practice [68] MXene/CF Fabric Surface Polymeric Aniline (PANI) Uniform dispersion, high binding The cost is high and the time is long [69] Polyester non-woven fabric (L), nylon spacer fabric (N) and carbon fiber woven fabric (C) lamination (three layers) The method is simple, the shielding capacity is high, High,thickness ,and the air permeability is poor [73] FSS, carbonyl iron-coated fabrics (CIFs) and copper-nickel-plated conductive woven fabrics (CWFs) form composite fabrics
(three layers)Various shapes, low loss, convenient for equipment miniaturization, adjustable frequency width, easy to follow the line The bandwidth is limited to a certain extent, and the air permeability is poor [77]
下载: 导出CSV
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目的
电子通讯的飞速发展与高频应用带来极大便利，但同时也导致电磁污染问题越发严重，开发高效的电磁干扰屏蔽性能的材料是解决电磁污染的有效途径。近年来，电磁屏蔽材料逐渐朝向实现“厚度薄、质量轻、频带宽、力学性能强”等目标发展，柔性电磁屏蔽材料以其轻质、柔韧及良好的加工性呈现出极大应用前景。但目前从不同尺度对于柔性电磁屏蔽复合材料的系统性研究综述较少，本论文系统性梳理了柔性电磁屏蔽复合材料研究进展，旨在为柔性电磁屏蔽复合材料研究提供理论参考，为功能纺织品研发提供借鉴思路。
方法本文首先从微观角度出发，论述了不同基底(导电聚合物基、碳基、MXene基、金属基)柔性复合纳米材料的屏蔽机制及研究现状，分析了不同复合材料的种类数量、内部结构、制备方式等对电磁屏蔽效能影响。在此基础上，将视角拓宽至宏观纺织基复合材料中，作为最典型的柔性电磁屏蔽复合材料，纺织基复合材料被广泛应用于各领域，因此，本综述进一步系统性分析了不同形态复合纺织品在加工过程中的优化方法，主要有材料复合、结构设计及改性处理等。最后对相关研究工作进行总结及展望。
结果微观角度，单一基底的纳米材料存在性能缺陷与应用局限性，但均有一定的改善方式。导电聚合物基复合材料可通过构建功能填料梯度以及排列结构提高电磁屏蔽效能；碳基材料可通过集成组分协同增强传导或附加制备工艺进一步设计；MXene基材料可构建层状结构或与其他磁性材料复合来增强磁导率，减少因过高导电性导致的二次反射污染现象；金属基材料可通过微纳米尺度加工形成导电网络，保持高电磁屏蔽效能的同时提高柔韧性。目前大多研究是从多结构设计及构建多相材料复合体系角度提高纳米复合材料电磁屏蔽效能，而随着科技的不断进步，柔性电磁屏蔽纳米材料性能与应用逐渐趋向高性能、多元化领域发展。宏观角度，复合纤维可通过纺丝液掺杂导电材料实现本征导电，也可将导电物质涂层或包覆于纤维外部制备高电磁屏蔽效能的复合纤维；通过选择合适纤维原料、纺纱工艺以及后处理工艺，可制备高导电、高电磁屏蔽效能性能纱线；复合织物可通过改变织物结构、表面改性方式及构建多层结构提高电磁屏蔽效能，织物结构方面，常见的机织物形态稳定、屏蔽效能较高，但柔性相对较差，针织物具备良好延伸性但结构松散易泄露电磁波；表面改性方式主要有涂覆法、化学镀法、原位聚合法等，涂覆法工艺简单但均匀性较差，镀层均匀但成本较高，原位聚合法分散均匀且结合力高，但成本也较高；层级结构方面，通过构建宏观多层复合结构体系，可显著增加非均质界面数量，进而增加电磁损耗及多重反射作用，但同时会产生厚度过大，透气性较差等问题，这将在一定程度上限制实际应用。
结论(1)单一纳米材料具有一定缺陷及功能局限性，可通过不同纳米材料的多相复合，构筑多元结构，并选择合适的制备方式，设计开发高性能、多功能的柔性复合材料体系；(2)纺织基电磁屏蔽复合材料具有不同的结构形态，可选择合适的复合方法赋予其高效的电磁屏蔽性能，如纤维复合纺丝，纱线涂覆改性，织物成形方式、表面改性处理及多层结构构筑等。在提高电磁屏蔽效能同时，仍需保持纺织品本身柔性优势，使其应用于实际生产。
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