Research progress on microwave absorption stealth technology of honeycomb sandwich structure composites
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摘要: 随着探测技术的快速发展与作战性能的需求,对雷达隐身技术提出了更高的要求,蜂窝夹层结构作为经典的结构型吸波材料近年来取得了长足的发展,本文旨在综合分析和总结国内外蜂窝夹层结构复合材料在吸波隐身技术领域的特点、研究现状及应用情况,重点探讨了影响蜂窝夹层结构吸波性能的关键因素,包括吸波剂性能、蜂窝结构设计和蒙皮的透波性能等,并就吸波带宽、吸收率等关键指标分析了不同蜂窝夹层结构吸波材料的优缺点。此外,总结了蜂窝夹层结构吸波隐身复合材料目前的发展趋势,归纳了发展现状,并对未来的发展方向进行了展望。Abstract: With the rapid development of detection technology and the demand for operational performance, higher requirements have been put forward for radar stealth technology, honeycomb sandwich structure as a classical structural wave-absorbing material has made great development in recent years, this paper aims to comprehensively analyse and summarize the characteristics, research status and application of honeycomb sandwich structure composites in the field of wave-absorbing stealth technology at home and abroad. This paper focuses on the key factors affecting the wave-absorbing performance of honeycomb sandwich structures, including the performance of absorbers, the design of honeycomb structures and the wave-transparent performance of skins, etc. It also analyses the advantages and disadvantages of different honeycomb sandwich structure wave-absorbing materials with respect to the key indexes of wave-absorbing bandwidth and absorption rate. In addition, this paper summarises the current development trend of honeycomb sandwich structure wave-absorbing stealth composites, summarises the current status of development, and gives an outlook on the future development direction.
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随着先进探测技术的飞速发展,为提高飞行器等航空装备的生存能力、突防能力和纵深打击能力,对隐身技术的发展提出了更高的要求。隐身技术可以分为光学隐身技术、红外隐身技术、雷达隐身技术等,由于雷达探测仍是现代军事战争中使用的最主要精确探测技术,减少雷达散射界面积(Radar cross section,RCS)在隐身设计中占有主导地位。减少RCS的方法主要包括外形设计和使用雷达吸波材料两大类[1],能够衰减或消除电磁波辐射的吸波材料也作为隐身材料中的一大分支广受关注。
雷达吸波材料按照材料的成型工艺与承载能力,可以分为涂覆型和结构型。涂覆型吸波材料多用于武器装备本身金属基体表面,具有制备工艺简单、成本低、吸波性能好、不受外形限制等特点[2-3];但由于吸波带宽较窄、涂层厚、质量较大、脱落风险高、耐环境性差等缺点[4],使其应用受到越来越大的挑战。为了满足对吸波材料“薄、轻、宽、强”的发展要求[5],结构型吸波材料进入了更多人的视野。结构型吸波材料与涂覆型吸波材料相比,其优点在于能够兼顾承载与吸波双重功能,不额外增重,还具有耐环境性能好、吸波频带宽、可设计性高、能成型为形状复杂部件等优点[6-8],可实现结构功能一体化的设计。
1. 结构型吸波复合材料
结构隐身材料根据其不同的结构形式,可以分为叠层型结构、尖锥型结构、层板型结构及夹层型结构[9-11],结构示意图如图1所示[12-13]。叠层型结构由介质材料与多层电阻膜按照一定间距交替堆叠构成,吸收性能主要由电阻膜方阻值、介质材料的电磁参数、吸波结构层数及电阻膜之间间距共同决定,通常存在中间介质层的选材要求苛刻、良好吸收性能与结构整体增厚之间协调难度大等缺点[14]。尖锥型结构的吸波材料大多数使用于微波暗室中,由于其本体材料是均匀的,制备工艺非常简单,制作成本低,易于大规模生产,但是其主要组成为聚氨酯泡沫与吸波炭黑等,力学性能较差,不能作为承载部件[12]。层板型结构通常是由电阻层(吸收层)和金属背板(反射层)组成的,采用共振型吸波结构,通过电磁波的干涉和共振来降低电磁波的反射损耗[15-16]。层板型结构存在吸波频点调控较困难、增加吸波带宽大多要通过增加结构厚度实现、增重与吸波带宽之间难以协调的矛盾[17]。而夹层结构的中空芯层可以在实现减重的同时通过填充吸波泡沫、吸波蜂窝实现良好的吸波性能,其因独特的孔洞结构还可以达到宽频带吸波效果[18]。
泡沫夹层结构的吸波材料通常采用预聚体与吸波剂混合制成的基体材料,再聚合发泡的制备方法[19-21]。夹层结构的吸波材料不仅对雷达波有较高的吸收率,且具有强度高、韧性好、质量轻、各向同性等特点[22-23],又由于其独特的发泡式成型特点,具有易加工成复杂型面的功能。
蜂窝夹层结构作为典型的结构型吸波材料,已经被广泛应用且经受了充足的考验,通常由透波蒙皮层和吸波蜂窝层构成,蜂窝层与蒙皮层通过胶膜粘接固化。其中,吸波蜂窝层通常由芳纶纸浸吸波剂制成,由于其仿蜂巢结构的特殊性质,是优异的质轻高强结构材料,且当雷达波进入六边形夹芯内部结构时,腔体对其进行多次散射与吸收,衰减雷达波能量,达到宽频、高强度的吸波效果[24-26]。与此同时,其电结构设计相对灵活、具有良好的宽频可设计性等特点也是吸引广大研究人员关注的主要因素。透波蒙皮层在其中起到调节阻抗匹配与增强力学性能的作用,阻抗匹配使电磁波尽可能多得进入蜂窝结构中,让电磁波在吸波蜂窝结构中经过多次反射吸收达到最大限度的宽频带吸收。
2. 吸波材料理论基础
吸波材料通过介电损耗或磁损耗将电磁波转化为热能或其他形式的能量进行耗散[27-28]。吸波材料的介电常数(ε)和磁导率(μ)是表征其吸波性能的重要电磁参数。介电常数通常用复数形式表示为
ε=ε′−jε″ (1) 其中, \varepsilon' 和 {\varepsilon'' } 为复介电常数的实部和虚部,虚部与实部的比值为介电损耗角正切值 \mathrm{tan}{\delta }_{\varepsilon } 。
磁导率也用复数形式表示为
\mu=\mu'-\mathrm{j}\mu'' (2) 其中, {\mu '} 和 {\mu'' } 为复磁导率的实部和虚部,虚部与实部的比值为磁损耗角正切值 \mathrm{tan}{\delta }_{\mu } 。
{\varepsilon '} 和 {\mu' } 表征材料的储能特性, {\varepsilon'' } 表征材料的电损耗特征, {\mu ''} 表征材料的磁损耗特征。损耗正切值越大,意味着可以将更多的电磁波转化为其他形式的能量从而耗散。不考虑阻抗匹配时损耗正切值越大吸波性能越好,但要想电磁波被吸收,首先要让其进入材料内部,这就不得不考虑阻抗匹配,也就是说材料的介电常数和磁导率要尽可能接近自由空间的介电常数和磁导率[29-30]。
以单层均质吸波体结构模型为例,理想的吸波材料要求材料的阻抗( {Z}_{{\mathrm{in}}} )与自由空间的阻抗( {Z}_{0} )相等,在以理想金属为背板的情况下,根据传输线理论,吸波材料的阻抗可以根据下式进行计算:
Z_{\mathrm{in}}=Z_0\left(\frac{\mu_{\mathrm{r}}}{\varepsilon_{\mathrm{r}}}\right)^{1/2}\tan\mathrm{h}\left[\frac{\mathrm{j}2\text{π}fd\left(\mu\mathrm{_r}\varepsilon_{\mathrm{r}}\right)^{1/2}}{c}\right] (3) 其中: \mu\mathrm{_r} 和 \varepsilon\mathrm{_r} 分别为吸波材料的相对复磁导率和相对复介电常数;f为频率;d为吸波材料的厚度;c为光速物理常数。因此,从材料的组分和结构进行设计,调控材料实现良好的阻抗匹配特性与强损耗能力是吸波材料设计的最根本诉求[31-32]。
3. 吸波蜂窝夹层结构研究进展
3.1 吸波剂相关研究
吸波剂是蜂窝夹层结构吸波材料实现吸波能力的重要组成部分,目前国内外学者大多采用损耗机制的吸收剂,即电阻型吸收剂来提高复合材料的吸波性能。电阻型吸波剂通常有炭黑、金属粉、石墨等[4, 33]。Rinaldi等[34]将石墨烯纳米片(Graphene nanoplatelet,GNP)分散在聚乙烯醇、水和1-丙醇的混合物中,制备了一种胶体悬浮液作为吸波剂,将这种基于GNP的聚合物涂料涂覆在酚醛芳纶蜂窝(Honeycomb,HC)表面,开发了一种新型宽带轻质雷达吸波蜂窝材料,并建立了GNP包覆HC的多尺度有效介质模型,完整的工艺流程如图2(a)所示。Li等[35]将芳纶蜂窝浸渍炭黑/环氧树脂吸波剂,制备了碳包覆蜂窝吸波材料(Honeycomb microwave absorbing materials,HMAMs),其中吸波剂的介电常数如图2(b)所示,实部 {\varepsilon }' 与虚部 {\varepsilon }'' 的曲线显示其储能特性与电损耗特征均较好;HMAMs材料的介电常数如图2(c)所示,有效介电常数的实部 {\varepsilon }_{{\mathrm{eff}}}^{\prime} 和虚部 {\varepsilon }_{{\mathrm{eff}}}^{\prime\prime} 分别为1.5和0.5左右,其相应介电损耗正切值 \mathrm{tan}\delta_{\mathrm{eff}} 如图中所示。在后续研究中发现,当材料阻抗与自由空间的阻抗基本相等时,材料具有3个吸收峰,如图2(d)所示,分别位于4 GHz、10 GHz和17 GHz,反射损耗(RL)分别为−10 dB、−20 dB和−25 dB,表现出完美的阻抗匹配和多阶干涉。巴金满等[36]使用分散在聚氨酯中的乙炔炭黑作为吸波剂多次浸渍蜂窝芯后,加入用碳纤维制作的频率选择表面(Frequency selective surface,FSS),在4~12 GHz范围内吸波性能得到提高,且不同尺寸的碳纤维FSS对其吸波性能的影响效果不同,本次试验的最佳尺寸为30 mm;除此之外,蜂窝芯的浸渍量也是影响吸波性能的重要因素,未添加FSS时蜂窝芯增重5%时性能最佳,加入FSS后最佳增重降低到3%,如图2(e)~2(g)所示,增重的降低意味着蜂窝浸渍工序的执行次数得以减少,这表明引入FSS可以提高生产效率、降低成本。Bi等[37]以碳纳米管/炭黑/还原氧化石墨烯(Carbon nanotubes/carbon black/reduced graphene oxide,CNTs/CB/RGO)和聚氨酯(Polyurethane,PU)树脂为浆料浸渍蜂窝芯(HC),调控CNTs与CB/RGO的比例,发现单层HC复合材料的RL曲线强度随着浸渍层内CNTs比例的增加先增大后减小(图3(a)),后又利用梯度介质原理设计了多层蜂窝复合材料,其中一组三层蜂窝的夹层吸波材料性能最佳,在2.2~18 GHz频率范围内RL < −10 dB,最大RL强度为−35 dB (图3(b))。此外,涂覆CNT/CB/RGO/PU复合材料的夹层结构吸波材料的平均压缩应力达到3.71 MPa (图3(c)),与铝HC板在2.2~18 GHz范围内的压缩应力相近。同样的原理,Bi等[38]采用改进的Hummers方法对CNTs和CB粉末进行改性并以CNTs/CB/PU为浆料,调控CNTs与CB的比例,图3(d)为不同比例浆料所得HC的吸波性能,基于此设计了多层蜂窝复合材料,其吸波性能如图3(e)、图3(f)所示,其中性能最好的两例对应的夹层结构复合材料吸波性能与力学性能如图3(g)~ 3(j)所示,表现最好的在2.2~17.7 GHz频率范围内实现了有效吸收(RL < −10 dB),最小RL值为−26.4 dB,压应力达到3.69 MPa。文献[34-38]中提到的蜂窝夹层吸波体各参数及吸波性能见表1,可以看到目前基于炭黑、石墨、碳纳米管的吸波剂研究均表现出较好的宽频吸波效果,且大多集中在2~18 GHz频段范围,对更低频频段的吸波性能不足。
图 2 (a) 石墨烯纳米片(GNP)包覆蜂窝(HC)材料的制备工艺流程[34];(b) 电介质涂层的复介电常数的实部( {\varepsilon' })和虚部( {\varepsilon'' } )及介电损耗角正切值( \mathrm{tan}{\delta }_{\varepsilon } );(c) 蜂窝吸波材料(HMAMs)的有效介电常数的实部( {\varepsilon }_{{\mathrm{eff}}}^\prime )和虚部( {\varepsilon }_{{\mathrm{eff}}} ^{\prime\prime})及相应的介电损耗正切值( \mathrm{tan}{\delta }_{{\mathrm{eff}}} );(d) 实际试验、有限元法和有效介质法得到的反射损耗(RL)曲线[35];(e) 蜂窝芯浸渍量对吸波性能的影响;(f) 碳纤维频率选择表面(FSS)尺寸对吸波性能的影响;(g) 加入碳纤维FSS后,蜂窝芯浸渍量对吸波性能的影响[36]Figure 2. (a) Preparation process of graphene nanosheet (GNP) coated honeycomb (HC) material[34]; (b) Real part (ε') and imaginary part (ε'') of relative complex permittivity and dielectric dissipation factor (tanδε) of the dielectric coating; (c) Real part ( {\varepsilon }_{{\mathrm{eff}}}^{\prime} ) and imaginary part ( {\varepsilon }_{{\mathrm{eff}}}^{\prime\prime} ) of the relative complex effective permittivity and effective dielectric dissipation factor ( \mathrm{tan}{\delta }_{{\mathrm{eff}}} ) of the honeycomb microwave absorbing materials (HMAMs); (d) Reflection loss (RL) curves of HMAMs obtained by the real test, finite element and effective medium methods[35];(e) Effect of impregnation amount of honeycomb core on absorbing performance; (f) Effect of size of carbon fiber frequency selective surface (FSS) on absorbing properties; (g) Effect of impregnation amount of honeycomb core on absorbing properties after adding carbon fiber FSS[36]WEGs—Wormlike expanded graphite; PVA—Polyvinyl-alcohol; GNP—Graphene nanoplatelet; HC—Honeycomb; PEC—Perfectly electric conducting; HMAM—Honeycomb microwave absorbing materials; FSS—Frequency selective surface; d—Coating thickness; r—Honeycomb pore radius; h—Honeycomb height; MLG—Multilayer graphene microsheets; RAM—Radar absorbing material; CST—Computer simulation technology图 3 (a) 单层HC复合材料的RL;(b)多层HC复合材料的RL;(c) 夹层结构HC吸波材料的力学性能[37];(d) 单层碳纳米管(CNTs)/炭黑(CB)/聚氨酯(PU) HC复合材料的RL;双层(e)和三层(f) HC复合材料的RL;双层(g)和三层(h)夹层结构HC复合材料的RL;双层(i)和三层(j)夹层结构HC复合材料的力学性能[38]Figure 3. (a) RL of single-layer HC composites; (b) RL of multi-layer HC composites; (c) Mechanical properties of sandwich HC absorber[37]; (d) RL of single-layer carbon nanotubes (CNTs)/carbon black (CB)/polyurethane (PU) HC composites; RL of double-layer (e) and three-layer (f) HC composites; RL of double-layer (g) and three-layer (h) HC sandwich composites; Mechanical properties of double-layer (i) and three-layer (j) HC sandwich composites[38]RGO—Reduced graphene oxide; H, h—HC; S, s—Sandwich HC表 1 不同吸波剂所制备的蜂窝夹层吸波体参数Table 1. Parameters of honeycomb sandwich absorbers prepared with different absorbersCategory Total thickness of
the absorber/mmAbsorbing
bandwidth/GHzAbsorptivity Extreme electromagnetic
loss/dBRef. PVA/GNP 32.6 8-12 ≥99% −36 [34] CB/epoxy resin 18.4 2.5-4.3; 7-18 ≥90% −25 [35] Acetylene CB/PU (Carbon fiber FSS is added
between the layers)≈10 6.8-11.5 ≥90% −14 [36] CNT/CB/RGO/PU 15 2.2-18 ≥90% −35 [37] CNTs/CB/PU 15 2.2-17.7 ≥90% −26.4 [38] 除吸波剂的成分研究外,在吸波剂的涂覆方式、涂覆图案设计等方面也有较丰富的研究成果,尤其是在周期阵列排布的超材料的引入方面[39-40]。Zhao等[41]提出了一种梯度蜂窝雷达吸波结构(Radar absorbing structure,RAS)设计,图4(a)为其结构示意图,具有宽频带、大角度吸波特性(图4(b)):入射角小于45°时反射率降低10 dB以上;入射角小于55°时反射率降低8 dB;入射角小于70°时反射率降低5 dB。Ghosh等[42]设计出一种超材料的晶胞拓扑结构如图4(c)所示,由六边形蜂窝状介电基板组成,优化了蜂窝结构的尺寸,并采用3D打印技术,在空心蜂窝结构上用丝网印刷技术将电阻涂料沿入射波的横向涂覆成周期性图案,配合底部金属背板,在正常入射角度下能在5.52~16.92 GHz频带内实现宽带吸收(图4(d)),吸收率大于90%。Shen等[43]通过构造长度渐变的蜿蜒线金属阵列和直线金属阵列混合结构(图4(e)),并将其印刷于二维介质格栅侧面,可以在5.0~31.6 GHz范围内实现连续的超宽带电磁吸波,如图4(f)所示,吸收效率超过90%。超材料的引入让吸波材料的发展具备了更多的可能性。
图 4 (a) 梯度蜂窝雷达吸波结构(RAS)设计结果;(b) 优化后的梯度蜂窝RAS的反射率[41];(c) 沉积电阻涂料拓扑结构的俯视图和侧视图;(d) 宽带吸收效果[42];(e) 多层弯折线等离子结构示意图;(f) 对于垂直入射电磁波的吸波性能[43]Figure 4. (a) Graded honeycomb radar absorbing structure (RAS) design result; (b) Reflectivity of the optimized graded honeycomb RAS[41]; (c) Top view and side view of the topological structure of the deposition resistance coating; (d) Wide-band absorption effect[42]; (e) Schematic diagram of multilayer curved line plasma structure; (f) Absorption properties for vertical incident electromagnetic waves[43]PLA—Polylactic acid; a, l1, l2, w1, w2, s, g, ta, tb—Optimized dimensions of the structure; PMA—Perfect metamaterial absorber; d—Height; p—Length; l'1, l'2—Total length of meandered wire; l'3—Length of straight wire; θ—Angle of incidence3.2 蜂窝结构相关研究
由于蜂窝的孔径、厚度、边长等结构参数也是影响蜂窝夹层结构吸波性能的重要因素,除传统的单层六边形芳纶蜂窝外,近年来对异形蜂窝、格栅结构等类蜂窝结构、多层蜂窝结构及泡沫填充蜂窝结构等进行了大量研究。Wang等[26]设计了一种嵌套锥形散射构型的异形蜂窝夹层结构(图5(a)),分析了几何参数和电磁参数对吸波性能的影响规律,结果表明异形蜂窝吸波结构可形成两个吸收峰,可以提高垂直和斜入射方向的反射率(图5(b)和图5(c))。Cheng等[44]通过在方形格栅壁上缠绕连续的碳纤维,制备出一种线性梯度碳纤维超材料集成的方形格栅吸波材料(Square honeycomb structure,SHS),如图5(d)所示,实现了宽频带(5~20 GHz)、多入射角(0°~70°)和高效(平均90%以上)的吸波性能(图5(e)、图5(f)),同时利用聚甲基丙烯酰亚胺(Polymethacrylimide,PMI)泡沫填充蜂窝芯,进一步提高了其力学性能。赵彦凯等[45]通过将导电炭黑分散至聚氨酯树脂中制备出一种复合吸波剂,采用浸渍法包覆在芳纶蜂窝表面形成双层蜂窝结构,并在增重量为20%、频率范围为4.5~48 GHz内表现出最大的吸波强度为−45.6 dB。孙启峰[46]通过引入导电炭黑对聚氨酯泡沫进行改性,设计出一种泡沫填充蜂窝结构吸波材料,制备工艺流程如图6(a)所示,对该材料进行反射率测试,结果如图6(c)所示,其中垂直极化(Vertical sending and vertical recieving,VV)时该改性泡沫的加入较原始蜂窝在7 GHz附近提升了2 dB,在10 GHz附近提升了4.6 dB,在15 GHz附近可提升17.3 dB;水平极化(Horizontal sending and horizontal recieving,HH)时该改性泡沫使蜂窝夹层结构在7 GHz附近提升1.8 dB,10 GHz附近反射率提升为3.1 dB,15 GHz附近反射率提升达到了8.1 dB,吸波性能增强的同时还改善了传统芳纶蜂窝结构的力学性能。Yan等[47]通过在芳纶蜂窝壁上添加吸波剂并在蜂窝孔中填充碳纤维增强聚合物(Carbon fiber reinforced plastics,CFRP)管,制备了一种新型吸波材料,图6(b)为实验使用的空蜂窝及管状增强蜂窝芯示意图及参数。吸波剂的加入使复合材料在7~18 GHz范围内平均吸收率大于85%,CFRP管的加入使蜂窝结构的归一化弹性模量、峰值应力和比能量吸收分别提高了55.74%、621.69%和327.86% (图6(d)、图6(e)),极大改善了蜂窝结构的力学性能。Luo等[48]制备了由两种不同孔径构成的双层蜂窝结构吸波材料(Two-layer honey-comb sandwich structure absorber,THSSA),其几何形状及设计参数如图6(f)所示,采用导电炭黑作为蜂窝吸波剂,表面蒙皮分别涂覆环氧树脂匹配层和片状羰基铁粉吸波材料,在THSSA厚度为8 mm时能在4.8~18 GHz频带内达到优于−10 dB的吸波效果(图6(g))。除结构方面的改进外,也有直接将微波损耗材料制备成蜂窝或类蜂窝结构的研究。Kwak等[49]通过化学镀的方法制备了镀镍的玻璃纤维织物,制备过程如图7(a)所示,将该织物作为蜂窝结构制备出的蜂窝夹芯复合材料在5.8~16.3 GHz范围内表现出−10 dB的吸波效果(图7(b)),该材料应用于机翼前缘后在水平和垂直极化下,从C波段到Ku波段的模拟回波RCS降低了10 dB (图7(c))。吕丽华等[50]用玄武岩纤维长丝纱和碳纤维长丝纱设计制备了蜂窝状三维整体机织结构吸波材料,图7(d)为其单胞的示意图,以羰基铁粉(Carbonyl iron powder,CIP)和CB为吸波剂,研究发现均具有优良的宽频吸波性能(图7(e));其吸波机制示意图如图7(f)所示,入射的电磁波一部分在表面发生反射,一部分通过透波层进入吸波层被吸收掉,其余未被吸收的部分在蜂窝结构内部经过多次反射被吸收,这种整体机织材料在提高吸波性能的同时还解决了芳纶蜂窝的易开裂和易分层问题。Wang等[51]采用造纸工艺制备了一种新型纸基复合材料(Paper-based composites,PBCs),其主要由短切碳纤维(CFs)、芳纶纤维(Aramid fibers,AFs)和芳纶原纤维(Aramid fibrils,AFB)组成,将CFs作为吸波剂集成到AFs与AFB制成的蜂窝框架中,经试验验证,由3wt%CFs组成的PBCs制成的蜂窝结构雷达吸波材料具有28 kg/m3的低密度,且在2~18 GHz范围内微波吸收性能优异。Zhang等[52]将石墨烯片自组装到聚丙烯织物中获得还原氧化石墨烯/聚丙烯(Reduced graphene oxide/polypropylene,RGO/PP)织物并制备了一种方形周期柔性结构,在2~40 GHz和75~110 GHz内具有优于−10 dB的吸波效果,在常用雷达频段实现了“全频段吸收”,其电磁吸收率大于97%,且在大角度(0°~60°)范围内对横向电极化波和横向磁极化波都能稳定吸收。文献[26, 44-51]中提到的不同结构蜂窝夹层吸波体各参数及吸波性能见表2,其中大多吸波体均表现出较好的宽频吸波效果,个别吸波体的吸波频带较窄,且在低频频段范围内没有表现出较好的吸波性能,但其中吸波体均表现出了优良的力学性能。
图 5 (a) 异形蜂窝结构设计模型;(b) 优化后垂直入射的反射率;(c) 优化后斜入射角的反射率[26];(d) 连续方形格栅吸波材料(SHS)原理图;(e) 连续SHS和聚甲基丙烯酰亚胺(PMI)泡沫填充的连续SHS在不同入射角θ时的吸收光谱;(f) 测量和模拟连续SHS和PMI泡沫填充连续SHS的电磁(EM)波吸收率[44]Figure 5. (a) Designing model for special-shaped honeycomb structure; (b) Reflectivity of perpendicularly incident angle for the optimized multi-absorbing structure; (c) Reflectivity of obliquely incident angle for the optimised multi-absorbing structure[26]; (d) Schematic diagram of interlocked squarehoneycomb structure (SHS); (e) Absorptivity spectra with different incident angle θ of interlocked SHS and polymethacrylimide (PMI) foam-filled interlocked SHS; (f) Measured and simulated electromagnetism (EM) wave absorptivity of interlocked SHS and PMI foam-filled interlocked SHS[44]EM—Electromagnetic; CF—Carbon fiber; CFRP—Carbon fiber reinforced plastics; φ—Incident direction; h, t—Height and thickness of honeycomb walls; w, s—Width of CF wire and the spacing between two adjacent CF wires; d1, d2—Thickness of FR-4 front face sheet and CFRP composite back face sheet; l1, ln—Shortest and longest of CF wire; FR-4—Glass fiber reinforced composite图 6 (a) 改性聚氨酯泡沫的制备流程图;(b) 空蜂窝及管状增强蜂窝芯示意图及参数;(c) 垂直极化(VV)与水平极化(HH)下的反射率曲线[46];(d) 垂直入射波反射率;(e) 空吸波蜂窝(E-A1-1)、CFRP管(T-1)、管增强吸波蜂窝(T-A1-1)及空吸波蜂窝与CFRP管之和在单轴压缩下的应力-应变曲线[47];(f) 两种不同孔径规格蜂窝的几何形状与设计参数;(g) 双层蜂窝结构吸波材料(THSSA)的宽带反射损耗[48]Figure 6. (a) Flow chart of preparation of modified polyurethane foam; (b) Schematic and parameters of empty honeycomb and tube-reinforced honeycomb core; (c) Reflectance curves under vertical sending and vertical recieving (VV) polarization and horizontal sending and horizontal recieving (HH) polarization[46]; (d) Experimental results of reflectivity for vertical incident waves; (e) Nominal stress-strain curves of empty absorbent honeycomb (E-A1-1), CFRP tube (T-1), tube-reinforced absorbent honeycomb (T-A1-1) and sum of empty absorbent honeycomb and CFRP tube under uniaxial compression[47]; (f) Geometry and design parameters of two kinds of honeycomb with different aperture specifications; (g) Measured broadband reflection loss of fabricated two-layer honey-comb sandwich structure absorber (THSSA)[48]φt—Outside diameter of CFRP tubes; Th—Wall thickness of honeycomb core; Lh—Hexagon side length; FCIP—Flaky carbonyl iron powder图 7 (a) 镀镍玻璃纤维蜂窝吸波夹层复合材料的完整制造过程;(b) 雷达吸波蜂窝夹层复合材料回波损耗的实测与仿真结果;(c) 在VV和HH极化下,铜板和所提出结构的单稳态雷达横截面(RCS)的仿真和测量结果[49];(d) 蜂窝状三维整体机织结构型吸波织物单胞示意图;(e) 蜂窝状三维整体机织结构型吸波复合材料的吸波损耗曲线;(f) 蜂窝状三维整体机织结构型吸波复合材料的吸波机制示意图[50]Figure 7. (a) Complete manufacturing process of the radar-absorbing honeycomb sandwich composite with a nickel-coated glass fabric; (b) Measured return loss of the fabricated radar-absorbing honeycomb sandwich composite along with the simulation results; (c) Simulation and measurement results of the monostatic radar cross-section (RCS) for a copper plate and for the proposed structure under VV and HH polarizations[49]; (d) Cell diagram of honeycomb 3D integral woven structure microwave absorbing fabric; (e) Reflection loss curves of honeycomb 3D woven structure microwave absorbing composites; (f) Schematic diagram of microwave absorbing mechanism of honeycomb 3D integral woven structure microwave absorbing composite[50]TE—Transverse electric; TM—Transverse magnetic; T—Temperature rate; \vec H —Perpendicular to glass fabric; \vec E—Lengthways; \vec k —Crosswise表 2 不同结构类型的蜂窝夹层吸波体参数Table 2. Parameters of honeycomb sandwich absorbers with different structure typesCategories Total thickness of
the absorber/mmAbsorbing
bandwidth/GHzAbsorptivity Ref. Structural design Nested cone-shaped scattering configuration ≈6 3-4 ≥90% [26] SHS 23.2 5-20 ≥90% [44] Double-layer honeycomb 10 4.5-48 ≥90% [45] Foam-filled honeycomb 30 5-7; 8.5-11; 12-20 ≥90% [46] CFRP tube-reinforced absorbent honeycomb 20 7-18 ≥90% [47] THSSA 9 4.8-18 ≥90% [48] Improvement of
raw materialsNickel-coated glass/epoxy honeycomb 4 5.8-16.3 ≥90% [49] Honeycomb 3D integral woven structure 7.5 14.34-18 ≥90% [50] PBCs 30 2-2.5; 4.4-18 ≥90% [51] Note: PBC—Paper-based composites. 4. 透波蒙皮层研究
透波蒙皮在蜂窝夹层结构吸波复合材料中担任着调节阻抗匹配的重要角色,近年来也有不少学者通过优化蒙皮结构、引入超材料单元[53-54]等方式针对透波蒙皮的选频透过性等方面进行了相关研究。由图8(a)可以看出,吸波蜂窝复合蒙皮后Ka波段反射率性能退化了5~30 dB,邢孟达等[55]针对此现象,对蒙皮结构进行了改良,改良后的复合吸波蜂窝结构如图8(b)所示,研究发现A型夹层结构蒙皮可有效改善蒙皮的高频段透波性能,0.3 mm石英布−2 mm透波蜂窝−0.3 mm石英布蒙皮结构的复合蜂窝在2~18 GHz及26.5~40 GHz的超宽频带内吸波性能优越(图8(c))。鹿海军等[56]采用与透波蒙皮相同树脂体系的碳纤维预浸料制备超材料结构单元,图8(d)给出了该超材料单元设计参数,并将该超材料单元复合在透波蒙皮中,将其制成蜂窝夹层结构后示意图如图8(e)所示,这种设计在保持原有力学性能的基础上提升了蜂窝夹层结构在1~2 GHz低频范围的吸波性能,L波段平均反射率降低4 dB以上(图8(f)、图8(g))。礼崇明等[57]采用可溶性聚醚醚酮薄膜镀铜后刻蚀的方法制备了金属超材料单元膜并复合于透波蒙皮中,透波蒙皮层厚度为1.0 mm、吸波蜂窝芯厚度为30 mm时,在低频1~2 GHz范围内平均吸波性能提升8 dB,同时材料质量相比同厚度蜂窝夹层结构材料降低40%。Wang等[58]设计了一种超表面/蜂窝复合结构的雷达吸波材料,使用碳涂层作为吸波剂浸渍蜂窝,并对透波蒙皮进行阻抗匹配设计,显著提高了低频范围的吸波性能。Indrusiak等[59]将添加了炭黑和石墨混合物介电材料的高密度聚乙烯(High density polyethylene/hybrid,HDPE/HB)作为吸收层面板,添加了花岗岩采石场废料作为磁性材料的高密度聚乙烯(HDPE/granitic quarry waste (GQW))作为匹配层面板与HC制成夹层结构,其中由4 mm厚的HC与1 mm厚的面板制成的吸波体在8.4~11 GHz范围内吸波效果优于−10 dB,在 8.75 GHz 时,反射损耗最小值为−30.43 dB。表3罗列了文献[42, 44, 56-57]中含超材料单元的蜂窝夹层结构复合吸波体的参数及其吸波性能,从中可以看出,超材料在蜂窝格栅上的添加有助于拓宽吸波体的吸波频宽,而在透波蒙皮中的添加则更倾向于调节阻抗匹配,提高吸波体在1~2 GHz低频范围内的吸波能力。
图 8 (a) 吸波蜂窝复合石英/环氧蒙皮前后的反射率;(b) A型蒙皮复合吸波蜂窝结构;(c) A型蒙皮复合吸波蜂窝的反射率测试值[55];(d) 含超材料蜂窝夹层结构吸波复合材料结构示意图;(e) 超材料单元结构示意图;(f) 含超材料单元与不含超材料单元的透波面板在1~18 GHz 频率范围内的透波率曲线;(g) 含碳纤维超材料单元与不含超材料单元的吸波蜂窝夹层结构的L波段反射率曲线[56]Figure 8. (a) Reflectivity of absorbing honeycomb with and without quartz/epoxy skin; (b) Structure of absorbing honeycomb with A-type skin; (c) Reflectance values of absorbing honeycomb with A-type skin[55]; (d) Schematic structure of the honeycomb sandwich composite containing metamaterial; (e) Schematic diagram of metamaterial unit structure; (f) Transmittivity curves of the wave-transmitting skin with/without metamaterial units in the frequency range of 1-18 GHz; (g) Reflectivity curves of the wave-absorbing honeycomb sandwich composites with/without carbon fiber metamaterials in the L-band[56]表 3 含超材料的蜂窝夹层吸波体参数Table 3. Parameters of honeycomb sandwich absorbers containing metamaterialsCycle size Dielectric substrate material Total thickness of the
absorber/mmAbsorbing
bandwidth/GHzAbsorptivity Ref. 16 mm×16 mm PLA perforated dielectric made from honeycomb 5.5 5.52-16.92 ≥90% [42] 22.5 mm×15 mm FR-4 honeycomb walls 23.2 5-20 ≥90% [44] 70 mm×70 mm Wave-permeable prepreg 52 1-1.96 ≥90% [56] Thickness
10-15 μmWave-permeable prepreg 31 1.1-1.9; 4.5-18 ≥90% [57] Note: PLA—Polylactic acid. 5. 结 语
蜂窝夹层结构吸波复合材料已经在多个方面取得了显著的成绩,总体而言,其发展趋势如下:
(1) 吸波隐身性能的不断提升。目前在吸波带宽和吸波效率方面的研究成果较丰富,而针对不同入射角度,尤其是大入射角度范围下吸波性能的研究还较少,具有更广阔的研究空间。随着探测技术的发展,米波隐身取代微波隐身成为研究重点,远程对空警戒雷达、太空雷达应用的日渐广泛,对低频范围的隐身性能应当更加重视,目前关于宽频吸收的研究大多集中在2~18 GHz频段范围,缺少L波段(1~2 GHz)范围内高效吸波材料的补充。学科发展不断深入,对于吸波隐身性能的追求也从“薄、轻、宽、强”转向了多频谱、多物理场、多应用范围结合的综合指标,未来,复杂应用场景、复杂环境要求、多项功能兼备的吸波材料将成为后续研究的重点、难点。
(2) 优化发展隐身/承载一体化设计。由于蜂窝本身结构及原材料的限制,一直以来,蜂窝夹层结构复合材料在力学性能方面表现的不尽如人意。现在更多学者在提升吸波性能的同时,也把改善蜂窝夹层结构作为结构承载材料的力学性能放在了同样重要的地位上。通过引入蜂窝填充物、机织蜂窝等方式,优化了蜂窝夹层隐身材料的抗压性能、剪切性能和弹性模量,对通常蜂窝夹层结构复合材料在服役过程中表现出来的易撕裂和易分层等问题进行了改善。
(3) 引入超材料。超材料的发展使可以通过调控电磁超材料结构单元的尺寸参数来灵活调控雷达波的幅度、相位、极化、色散等特性,且超材料基于一种更新颖的逆向设计模式,通过人工设计调控来突破自然规律的限制,从而弥补蜂窝夹层结构由于自身结构及材料因素而带来的某些吸波性能方面的劣势。未来的蜂窝夹层结构吸波材料更需要持续大力发展超材料技术,抓住突破吸波隐身技术瓶颈的有力手段。
蜂窝夹层结构吸波复合材料展现出优异的功能/结构一体化设计特征,具有可设计性强、自由度高、性能稳定、宽频吸收等优点。结合未来的发展趋势,应对探测手段的迅速发展和轻量化设计的需求,蜂窝夹层结构吸波复合材料仍具有广阔的研究前景。
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图 2 (a) 石墨烯纳米片(GNP)包覆蜂窝(HC)材料的制备工艺流程[34];(b) 电介质涂层的复介电常数的实部( {\varepsilon' })和虚部( {\varepsilon'' } )及介电损耗角正切值( \mathrm{tan}{\delta }_{\varepsilon } );(c) 蜂窝吸波材料(HMAMs)的有效介电常数的实部( {\varepsilon }_{{\mathrm{eff}}}^\prime )和虚部( {\varepsilon }_{{\mathrm{eff}}} ^{\prime\prime})及相应的介电损耗正切值( \mathrm{tan}{\delta }_{{\mathrm{eff}}} );(d) 实际试验、有限元法和有效介质法得到的反射损耗(RL)曲线[35];(e) 蜂窝芯浸渍量对吸波性能的影响;(f) 碳纤维频率选择表面(FSS)尺寸对吸波性能的影响;(g) 加入碳纤维FSS后,蜂窝芯浸渍量对吸波性能的影响[36]
Figure 2. (a) Preparation process of graphene nanosheet (GNP) coated honeycomb (HC) material[34]; (b) Real part (ε') and imaginary part (ε'') of relative complex permittivity and dielectric dissipation factor (tanδε) of the dielectric coating; (c) Real part ( {\varepsilon }_{{\mathrm{eff}}}^{\prime} ) and imaginary part ( {\varepsilon }_{{\mathrm{eff}}}^{\prime\prime} ) of the relative complex effective permittivity and effective dielectric dissipation factor ( \mathrm{tan}{\delta }_{{\mathrm{eff}}} ) of the honeycomb microwave absorbing materials (HMAMs); (d) Reflection loss (RL) curves of HMAMs obtained by the real test, finite element and effective medium methods[35];(e) Effect of impregnation amount of honeycomb core on absorbing performance; (f) Effect of size of carbon fiber frequency selective surface (FSS) on absorbing properties; (g) Effect of impregnation amount of honeycomb core on absorbing properties after adding carbon fiber FSS[36]
WEGs—Wormlike expanded graphite; PVA—Polyvinyl-alcohol; GNP—Graphene nanoplatelet; HC—Honeycomb; PEC—Perfectly electric conducting; HMAM—Honeycomb microwave absorbing materials; FSS—Frequency selective surface; d—Coating thickness; r—Honeycomb pore radius; h—Honeycomb height; MLG—Multilayer graphene microsheets; RAM—Radar absorbing material; CST—Computer simulation technology
图 3 (a) 单层HC复合材料的RL;(b)多层HC复合材料的RL;(c) 夹层结构HC吸波材料的力学性能[37];(d) 单层碳纳米管(CNTs)/炭黑(CB)/聚氨酯(PU) HC复合材料的RL;双层(e)和三层(f) HC复合材料的RL;双层(g)和三层(h)夹层结构HC复合材料的RL;双层(i)和三层(j)夹层结构HC复合材料的力学性能[38]
Figure 3. (a) RL of single-layer HC composites; (b) RL of multi-layer HC composites; (c) Mechanical properties of sandwich HC absorber[37]; (d) RL of single-layer carbon nanotubes (CNTs)/carbon black (CB)/polyurethane (PU) HC composites; RL of double-layer (e) and three-layer (f) HC composites; RL of double-layer (g) and three-layer (h) HC sandwich composites; Mechanical properties of double-layer (i) and three-layer (j) HC sandwich composites[38]
RGO—Reduced graphene oxide; H, h—HC; S, s—Sandwich HC
图 4 (a) 梯度蜂窝雷达吸波结构(RAS)设计结果;(b) 优化后的梯度蜂窝RAS的反射率[41];(c) 沉积电阻涂料拓扑结构的俯视图和侧视图;(d) 宽带吸收效果[42];(e) 多层弯折线等离子结构示意图;(f) 对于垂直入射电磁波的吸波性能[43]
Figure 4. (a) Graded honeycomb radar absorbing structure (RAS) design result; (b) Reflectivity of the optimized graded honeycomb RAS[41]; (c) Top view and side view of the topological structure of the deposition resistance coating; (d) Wide-band absorption effect[42]; (e) Schematic diagram of multilayer curved line plasma structure; (f) Absorption properties for vertical incident electromagnetic waves[43]
PLA—Polylactic acid; a, l1, l2, w1, w2, s, g, ta, tb—Optimized dimensions of the structure; PMA—Perfect metamaterial absorber; d—Height; p—Length; l'1, l'2—Total length of meandered wire; l'3—Length of straight wire; θ—Angle of incidence
图 5 (a) 异形蜂窝结构设计模型;(b) 优化后垂直入射的反射率;(c) 优化后斜入射角的反射率[26];(d) 连续方形格栅吸波材料(SHS)原理图;(e) 连续SHS和聚甲基丙烯酰亚胺(PMI)泡沫填充的连续SHS在不同入射角θ时的吸收光谱;(f) 测量和模拟连续SHS和PMI泡沫填充连续SHS的电磁(EM)波吸收率[44]
Figure 5. (a) Designing model for special-shaped honeycomb structure; (b) Reflectivity of perpendicularly incident angle for the optimized multi-absorbing structure; (c) Reflectivity of obliquely incident angle for the optimised multi-absorbing structure[26]; (d) Schematic diagram of interlocked squarehoneycomb structure (SHS); (e) Absorptivity spectra with different incident angle θ of interlocked SHS and polymethacrylimide (PMI) foam-filled interlocked SHS; (f) Measured and simulated electromagnetism (EM) wave absorptivity of interlocked SHS and PMI foam-filled interlocked SHS[44]
EM—Electromagnetic; CF—Carbon fiber; CFRP—Carbon fiber reinforced plastics; φ—Incident direction; h, t—Height and thickness of honeycomb walls; w, s—Width of CF wire and the spacing between two adjacent CF wires; d1, d2—Thickness of FR-4 front face sheet and CFRP composite back face sheet; l1, ln—Shortest and longest of CF wire; FR-4—Glass fiber reinforced composite
图 6 (a) 改性聚氨酯泡沫的制备流程图;(b) 空蜂窝及管状增强蜂窝芯示意图及参数;(c) 垂直极化(VV)与水平极化(HH)下的反射率曲线[46];(d) 垂直入射波反射率;(e) 空吸波蜂窝(E-A1-1)、CFRP管(T-1)、管增强吸波蜂窝(T-A1-1)及空吸波蜂窝与CFRP管之和在单轴压缩下的应力-应变曲线[47];(f) 两种不同孔径规格蜂窝的几何形状与设计参数;(g) 双层蜂窝结构吸波材料(THSSA)的宽带反射损耗[48]
Figure 6. (a) Flow chart of preparation of modified polyurethane foam; (b) Schematic and parameters of empty honeycomb and tube-reinforced honeycomb core; (c) Reflectance curves under vertical sending and vertical recieving (VV) polarization and horizontal sending and horizontal recieving (HH) polarization[46]; (d) Experimental results of reflectivity for vertical incident waves; (e) Nominal stress-strain curves of empty absorbent honeycomb (E-A1-1), CFRP tube (T-1), tube-reinforced absorbent honeycomb (T-A1-1) and sum of empty absorbent honeycomb and CFRP tube under uniaxial compression[47]; (f) Geometry and design parameters of two kinds of honeycomb with different aperture specifications; (g) Measured broadband reflection loss of fabricated two-layer honey-comb sandwich structure absorber (THSSA)[48]
φt—Outside diameter of CFRP tubes; Th—Wall thickness of honeycomb core; Lh—Hexagon side length; FCIP—Flaky carbonyl iron powder
图 7 (a) 镀镍玻璃纤维蜂窝吸波夹层复合材料的完整制造过程;(b) 雷达吸波蜂窝夹层复合材料回波损耗的实测与仿真结果;(c) 在VV和HH极化下,铜板和所提出结构的单稳态雷达横截面(RCS)的仿真和测量结果[49];(d) 蜂窝状三维整体机织结构型吸波织物单胞示意图;(e) 蜂窝状三维整体机织结构型吸波复合材料的吸波损耗曲线;(f) 蜂窝状三维整体机织结构型吸波复合材料的吸波机制示意图[50]
Figure 7. (a) Complete manufacturing process of the radar-absorbing honeycomb sandwich composite with a nickel-coated glass fabric; (b) Measured return loss of the fabricated radar-absorbing honeycomb sandwich composite along with the simulation results; (c) Simulation and measurement results of the monostatic radar cross-section (RCS) for a copper plate and for the proposed structure under VV and HH polarizations[49]; (d) Cell diagram of honeycomb 3D integral woven structure microwave absorbing fabric; (e) Reflection loss curves of honeycomb 3D woven structure microwave absorbing composites; (f) Schematic diagram of microwave absorbing mechanism of honeycomb 3D integral woven structure microwave absorbing composite[50]
TE—Transverse electric; TM—Transverse magnetic; T—Temperature rate; \vec H —Perpendicular to glass fabric; \vec E—Lengthways; \vec k —Crosswise
图 8 (a) 吸波蜂窝复合石英/环氧蒙皮前后的反射率;(b) A型蒙皮复合吸波蜂窝结构;(c) A型蒙皮复合吸波蜂窝的反射率测试值[55];(d) 含超材料蜂窝夹层结构吸波复合材料结构示意图;(e) 超材料单元结构示意图;(f) 含超材料单元与不含超材料单元的透波面板在1~18 GHz 频率范围内的透波率曲线;(g) 含碳纤维超材料单元与不含超材料单元的吸波蜂窝夹层结构的L波段反射率曲线[56]
Figure 8. (a) Reflectivity of absorbing honeycomb with and without quartz/epoxy skin; (b) Structure of absorbing honeycomb with A-type skin; (c) Reflectance values of absorbing honeycomb with A-type skin[55]; (d) Schematic structure of the honeycomb sandwich composite containing metamaterial; (e) Schematic diagram of metamaterial unit structure; (f) Transmittivity curves of the wave-transmitting skin with/without metamaterial units in the frequency range of 1-18 GHz; (g) Reflectivity curves of the wave-absorbing honeycomb sandwich composites with/without carbon fiber metamaterials in the L-band[56]
表 1 不同吸波剂所制备的蜂窝夹层吸波体参数
Table 1 Parameters of honeycomb sandwich absorbers prepared with different absorbers
Category Total thickness of
the absorber/mmAbsorbing
bandwidth/GHzAbsorptivity Extreme electromagnetic
loss/dBRef. PVA/GNP 32.6 8-12 ≥99% −36 [34] CB/epoxy resin 18.4 2.5-4.3; 7-18 ≥90% −25 [35] Acetylene CB/PU (Carbon fiber FSS is added
between the layers)≈10 6.8-11.5 ≥90% −14 [36] CNT/CB/RGO/PU 15 2.2-18 ≥90% −35 [37] CNTs/CB/PU 15 2.2-17.7 ≥90% −26.4 [38] 表 2 不同结构类型的蜂窝夹层吸波体参数
Table 2 Parameters of honeycomb sandwich absorbers with different structure types
Categories Total thickness of
the absorber/mmAbsorbing
bandwidth/GHzAbsorptivity Ref. Structural design Nested cone-shaped scattering configuration ≈6 3-4 ≥90% [26] SHS 23.2 5-20 ≥90% [44] Double-layer honeycomb 10 4.5-48 ≥90% [45] Foam-filled honeycomb 30 5-7; 8.5-11; 12-20 ≥90% [46] CFRP tube-reinforced absorbent honeycomb 20 7-18 ≥90% [47] THSSA 9 4.8-18 ≥90% [48] Improvement of
raw materialsNickel-coated glass/epoxy honeycomb 4 5.8-16.3 ≥90% [49] Honeycomb 3D integral woven structure 7.5 14.34-18 ≥90% [50] PBCs 30 2-2.5; 4.4-18 ≥90% [51] Note: PBC—Paper-based composites. 表 3 含超材料的蜂窝夹层吸波体参数
Table 3 Parameters of honeycomb sandwich absorbers containing metamaterials
Cycle size Dielectric substrate material Total thickness of the
absorber/mmAbsorbing
bandwidth/GHzAbsorptivity Ref. 16 mm×16 mm PLA perforated dielectric made from honeycomb 5.5 5.52-16.92 ≥90% [42] 22.5 mm×15 mm FR-4 honeycomb walls 23.2 5-20 ≥90% [44] 70 mm×70 mm Wave-permeable prepreg 52 1-1.96 ≥90% [56] Thickness
10-15 μmWave-permeable prepreg 31 1.1-1.9; 4.5-18 ≥90% [57] Note: PLA—Polylactic acid. -
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目的
随着探测技术的快速发展与提高航空装备生存率的需求,对雷达隐身技术提出了更高的要求,而由透波蒙皮层和吸波蜂窝层构成的蜂窝夹层结构因其优异的功能/结构一体化设计特征,受到了广泛的关注。本文从吸波剂性能、蜂窝结构设计和蒙皮的透波性能三个方面出发,概述了不同吸波器的制备方法、吸波性能、结构特征等内容,详细综述了不同类型吸波器的优缺点,总结了蜂窝夹层结构吸波隐身复合材料目前的发展趋势,对未来发展方向提出了建议。
方法本文从蜂窝夹层结构吸波材料的三个组成部分(吸波剂、蜂窝结构、蒙皮)出发,通过检索相关文献,在吸波剂的成分研究、吸波剂的涂覆方式、涂覆图案设计、蜂窝结构设计、蜂窝材料改进、优化蒙皮阻抗匹配、超材料的引入等角度概述了近年来在以上三个方面做出的研究,通过对比吸波带宽、吸收率等关键指标分析了不同吸波器的优缺点,总结归纳了目前研究的发展现状及不足之处。
结果通过总结相关文献发现,蜂窝夹层结构吸波材料由于其本身结构的特性,在不同研究方向均具有非常优异的可设计性与超高的自由度,其在面对承载、轻量化、隐身、耐环境、多入射角等多功能兼备的要求时具有更高的潜力。同时,超材料的引入让蜂窝夹层结构吸波材料多了一种逆向设计模式,从而弥补吸波器由材料、结构等原因带来的劣势。总之,蜂窝夹层结构吸波材料的未来发展前景还非常广阔。
结论蜂窝夹层结构吸波复合材料已经在多个方面取得了显著的成绩,总体而言,其发展趋势如下:(1)吸波隐身性能方面。目前在吸波带宽和吸波效率方面的研究成果较为丰富,而针对不同入射角度,尤其是大入射角度范围下吸波性能的研究还较少。随着探测技术的发展,米波隐身取代微波隐身成为研究重点,对低频范围的隐身性能应当更加重视,目前关于宽频吸收的研究大多集中在2~18GHz频段范围,缺少L波段范围内高效吸波材料的补充。未来,复杂应用场景、复杂环境要求、多项功能兼备的吸波材料将成为后续研究的重点、难点。(2)优化发展隐身/承载一体化设计。在不断提高吸波性能的同时兼顾了蜂窝夹层结构作为结构承载材料的力学性能提升,通过引入蜂窝填充物、机织蜂窝等方式优化了蜂窝夹层隐身材料的抗压性能、剪切性能、弹性模量等,改善了其易撕裂、易分层等问题。(3)超材料的引入。随着超材料的火热发展,通过对电磁超材料结构单元的尺寸参数调控实现对雷达波的幅度、相位、极化、色散等特性的灵活调控,来解决蜂窝夹层结构本身在吸波性能方面的劣势已经成为热门研究方向。蜂窝夹层结构吸波复合材料具有可设计性强、自由度高、性能稳定、宽频吸收等优点,结合未来的发展趋势,应对探测手段的迅速发展和轻量化设计的需求,蜂窝夹层结构吸波复合材料仍具有广阔的研究前景。