Constructing Repairable Meta-Structures of Ultra-Broad-Band

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 43179−43187

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Constructing Repairable Meta-Structures of Ultra-Broad-Band Electromagnetic Absorption from Three-Dimensional Printed Patterned Shells Wei-Li Song,*,†,‡ Zhili Zhou,†,‡ Li-Chen Wang,†,‡ Xiao-Dong Cheng,†,‡ Mingji Chen,*,†,‡ Rujie He,†,‡ Haosen Chen,*,†,‡ Yazheng Yang,†,‡ and Daining Fang†,‡ †

Institute of Advanced Structure Technology and ‡Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and Structures, Beijing Institute of Technology, Beijing 100081, P.R. China S Supporting Information *

ABSTRACT: Ultra-broad-band electromagnetic absorption materials and structures are increasingly attractive for their critical role in competing with the advanced broad-band electromagnetic detection systems. Mechanically soft and weak wax-based materials composites are known to be insufficient to serve in practical electromagnetic absorption applications. To break through such barriers, here we developed an innovative strategy to enable the wax-based composites to be robust and repairable meta-structures by employing a three-dimensional (3D) printed polymeric patterned shell. Because of the integrated merits from both the dielectric loss wax-based composites and mechanically robust 3D printed shells, the asfabricated meta-structures enable bear mechanical collision and compression, coupled with ultra-broad-band absorption (7−40 and 75−110 GHz, reflection loss smaller than −10 dB) approaching state-of-the-art electromagnetic absorption materials. With the assistance of experiment and simulation methods, the design advantages and mechanism of employing such 3D printed shells for substantially promoting the electromagnetic absorption performance have been demonstrated. Therefore, such universal strategy that could be widely extended to other categories of wax-based composites highlights a smart stage on which highperformance practical multifunction meta-structures with ultra-broad-band electromagnetic absorption could be envisaged. KEYWORDS: 3D printing, meta-structure, ultra broad band, electromagnetic absorption, repairable

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INTRODUCTION The worldwide demand for advanced electronics and devices has been growing rapidly, which attracts tremendous development in designing innovative materials and structures to meet the demand.1−4 In the communication, detection, and aerospace communities, stealth technology, which relies on electromagnetic absorption materials in the gigahertz (GHz) range, plays a significant role in manipulating information transportation, satellite communication, signal detection, and security.5−12 Because of great detection technique progress achieved in the frequency regions of 3∼30 GHz (wavelength on a centimeter scale) and 30−300 GHz (wavelength on a millimeter scale), materials with ultra-broad-band electromagnetic absorption capability (covering more than four bands) are highly pursued. Among recent strategies, several approaches, involving uniform porous electrical loss materials and artificial structures, have been proved to be effective for designing and fabricating ultra-broad-band absorption materials. Among uniform electromagnetic materials, one specific exploration is utilization of three-dimensional (3D) reduced graphene oxide foams as the electric loss medium to create multiple internal reflections and © 2017 American Chemical Society

attenuations, which deliver ultra-broad-band electromagnetic absorption.9 On the other hand, employment of artificial structures appears to be more universal with excellent designing and processing capability, which has been used for ultra-broadband electromagnetic absorption structures. On the basis of circuit analogue method, the frequency selective surface (FSS) structures would allow each resistive or conductive pattern to act as the electromagnetic attenuators toward certain frequency, and thus multilayer periodic FSS patterns (or Jaumann screen) would easily generate broad-band electromagnetic absorption via precise design.5,6 In addition, the periodic patterned structures generated by sufficient dielectric or magnetic loss materials would be responsible for creating multiple absorption peaks along with additional electromagnetic wave scattering based on structural influence, leading to broad-band absorption.7,8 In the evaluation of the intrinsic electromagnetic parameters, that is, complex permittivity and complex permeability, wax Received: October 10, 2017 Accepted: November 17, 2017 Published: November 17, 2017 43179

DOI: 10.1021/acsami.7b15367 ACS Appl. Mater. Interfaces 2017, 9, 43179−43187

Research Article

ACS Applied Materials & Interfaces

Figure 1. Meta-structure fabrication and ultra-broad-band absorption performance: (a) Schemes of preparing carbonized cotton (CC) monolith, CC/wax mixture, and meta-structures assembled with heated CC/wax mixture and 3D printed patterned shell, along with size of the unit cells (CC20). In the unit cell, the widths of each layer from top to bottom are a = 9, b = 11, c = 13, and d = 15 mm coupled with a period of w = 20 mm, and the height of each layer is 2 mm along with a shell thickness of 1 mm. Photographs of (b) commercial cotton, (c) CC upon carbonization, (d) mixture of CC/wax in a 400 mL beaker, (e) 3D printed patterned shell, and (f) the as-fabricated meta-structure based CC/wax and 3D printed patterned shell. (g) Experimentally measured and CST simulation performance of ultra-broad-band absorption of the as-fabricated meta-structure.

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matrixes have been widely used to incorporate with active fillers to form wax-based composite samples.13−22 Note that such composites are apparently not appropriate to directly serve as practical electromagnetic absorption materials because wax matrixes are known to be mechanically weak and soft. In contrast, wax is largely used in industries because of some unique advantages, including environmental friendliness, excellent chemical stability in corrosive conditions, and easy processing. For tackling the bottlenecks of enabling wax-based composites to be practical electromagnetic absorption materials, in the present contribution, we demonstrate a universal strategy of employing 3D printed polymeric patterned shells to serve as mechanical protective covers out of the wax-based composites. By structural design in CST, an integrated metastructure has been modeled, where wax-based composites were processed into the 3D printed polymeric patterned shells. The as-fabricated meta-structures enable offering of ultra-broadband absorption (reflection loss smaller than −10 dB) with effective bandwidth of 7−40 and 75−110 GHz (more than 7 bands), approaching state-of-the-art broad-band absorption materials. Mechanically, employment of the 3D printed shell as robust patterned architecture enables the wax-based composites to undergo mechanical collision and compression. Because of the presence of the polymeric shells, the incorporated waxbased composite fillers were able to be easily repaired based on the phase-transformation feature of wax matrixes. Indication of the results collectively suggests that such strategy is simple and universal, which promises a great plateau to achieve practical multifunctional meta-structures of ultra-broad-band electromagnetic absorption with wax-based composites.

RESULTS AND DISCUSSION In the typical design, the meta-structures were assembled by 3D printed patterned shells along with filling the carbonized cotton (CC) monolith/wax composites (Figure 1a). Briefly, the fabrication of the meta-structure involves carbonization of cotton, preparation of CC/wax mixture, determination of the unit cell size for the meta-structure, and incorporation of the CC/wax into the 3D printed polymer shell (Figure 1a). Initially, electromagnetic active CC was prepared by thermally annealing the commercial cotton (Figure 1b) in the inert atmosphere (nitrogen) at 800 °C (Figure 1c). Upon carbonization, the CC monolith was ground into powders and added into the melted wax to form uniform mixture (Figure 1d). Meanwhile, the complex permittivity of CC/wax with different filler loadings (10 wt %, CC-10; 20 wt %, CC-20; 30 wt %, CC-30) and 3D printing polymers (Verowhite) were measured by a coaxial method in the range of 2−18 GHz. On the other hand, novel meta-structures that are assembled with Verowhite shells and wax-based fillers (Figure 1a) are designed in the CST software. When the measured complex permittivity of all the materials are imported, the electromagnetic performance (S11, also reflection loss) of the designed unit cells with key size parameters (a, b, c, d, and period w) were simulated by sweeping the setting various value ranges. According to the sweeping results, the representative optimal sizes of the unit cells of the 300 × 300 × 11 mm3 metastructures (accounting the thickness as ∼1 mm in the Verowhite framework and height as ∼2 mm in each layer) are given as a = 9, b = 11, c = 13, d = 15, and w = 20 mm. Subsequently, the designed polymeric Verowhite shell (300 × 300 × 11 mm3, Figure S1 of the Supporting Information) was fabricated by 3D printing (Figure 1e), followed by filling with 43180


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Figure 2. Materials and characterizations: (a, b) SEM and (c, d) TEM images of the as-fabricated CC. (e) XRD, (f) XPS, and (g) XPS C 1s of the CC. (h) Tensile strength of the polymer Verowhite (used for 3D printing). (i) Real permittivity, (j) imaginary permittivity of CC/wax composites (10, 20, and 30 wt % filler loadings), and (k) complex permittivity of the polymer Verowhite (used for 3D printing) measured from coaxial method in the range of 2−18 GHz. (l) CST simulation of the designed meta-structures filled with various CC/wax filler, where complex permittivity of all the materials were from (i−k).

CC of micrometer-scale fibrous features possesses typical network features, which are responsible for creating 3D conductive pathways in terms of attenuating electromagnetic waves.23−26 Transmission electron microscopy (TEM) images of the ground CC are shown in Figure 2c,d, suggesting a nanoscaled stripe fashion with amorphous carbon characteristics. As confirmed by X-ray diffraction (XRD) spectrum (Figure 2e), the CC holds a broadened peak around 26°, consistent with the observation in TEM. According to the X-ray photoelectron spectroscopy (XPS) in Figure 2f, CC was measured to possess 81.2% C and 18.8% O. Also, the C 1s spectra imply that carbon−carbon bonding including sp3 and sp2 hybridization are the main components in the carbon configuration, along with −C−O− and −CO as typical oxygen-containing functional groups (Figure 2g).26−29 In addition, the tensile strength of the 3D printed Verowhite testing samples was measured on a mechanical analyzer, which indicates that the 3D printed polymer has strong tensile strength higher than 35 MPa. This result suggests that such 3D printed polymer shell enables offering of sufficient mechanical stability for fabricating the meta-structures. Complex permittivity of the samples (CC-10, CC-20, CC-30, and Verowhite) was obtained by measuring the as-fabricated toroidal bulks (thickness ∼2 mm) using the coaxial method in the range of 2−18 GHz (Figure 2i−k). As plotted in Figure 2i,j, the complex permittivity of the wax-based composites shows that both the real and imaginary parts increased with the enlarged filler loadings. Particularly, the CC-20 and CC-30 deliver moderate tangent loss (ratios of imaginary permittivity to real permittivity), which is favorable for delivering effective electromagnetic absorption performance. When the electromagnetic absorption is simulated based on the measured complex permittivity, Figure S2 shows that CC-20 has greater

the heated CC/wax (CC-20) mixture to form the metastructure (Figure 1f). With the arch method in the testing ranges of 4−40 and 75− 110 GHz (Figure 1g), the as-fabricated meta-structure presented effective electromagnetic absorption capability of reflection loss smaller than −10 dB over 7−40 and 75−110 GHz, almost in agreement with the CST simulated results. The error from the experimental and simulated results is understandable by the factor that the complex permittivity used for simulation in the frequency region of 18−110 GHz was automatically obtained by the extended values based on 2−18 GHz in the simulation process. Because of the large spans over seven bands (C, X, Ku, K, Ka, E, and F bands), the measured result indicates ultra-broad-band widths around 68 GHz (reflection loss smaller than −10 dB) and 67 GHz (reflection loss smaller than −15 dB), which are known to be the widest value among the previously reported samples.5−9 It is noted that such ultra-broad-band absorption should be linked with several mechanisms.7,8,23 (1) The periodic patterns in the metastructures collectively utilize the microwave absorption capability at various thicknesses, where the structure design based on dielectric materials is linked with the electromagnetic performance. (2) The presence of rectangular corners on the meta-structures is also responsible for nonuniform scattering, leading to additional mechanism for dissipating the electromagnetic waves. (3) The impedance matching conditions should be well realized by the integrated 3D printed shell metastructures because gradient materials and structures could be generated via specific structural design.23−26 For understanding the basic properties of component materials (CC and Verowhite), various techniques were applied to characterize the materials. As shown in Figure 2a,b, scanning electron microscopy (SEM) images show that the as-prepared 43181


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Figure 3. Effects of 3D printed patterned shell on the electromagnetic absorption performance: (a) Schemes of three types of structures used for electromagnetic absorption. (b) CST simulated 2D power loss distribution of the unit cells from the meta-structures without and with the 3D printed patterned shell. (c) Photographs of practical measurement. (d) Simulation of traditional single-layer cubic samples (based on CC-20) with various material thicknesses. (e) Simulation of the meta-structures with various 3D printed shell thicknesses, where d = 0 indicates the absence of the shell. (f) Experimental measured electromagnetic absorption performance of the as-fabricated meta-structure with various electromagnetic wave incident angles.

Figure 4. Waterproof properties and mechanical stability: Waterproof and stability of the as-fabricated meta-structure under rapid water flow and water shock (a) back side (under the tap water flow) and (b) front side (assembled on the wall); mechanical stability evaluation upon (c) human stamping, (d) moving bicycle compression, and (e) impacts as a flying object. (f) Experimentally measured electromagnetic absorption performance before and after water shock and mechanical impacts under the various above treatments.

opportunities to provide efficient absorption performance in the investigated region, where 2D diagrams have been plotted in Figure S2a−c to illustrate CC-10, CC-20, and CC-30. Meanwhile, the Verowhite polymer holds a typical polymeric dielectric feature, with stable real parts around 2.8 in the entire frequency range. When the measured complex permittivity values (CC-10, CC-20, CC-30, and Verowhite) are imported

into the meta-structure design (Figure 2l), the meta-structure filled with CC-20 should be the preferable design for achieving broad-band absorption. Furthermore, the influences of the 3D printed Verowhite shell were studied by CST simulation. As illustrated in Figure 3a, traditional single-layer absorber (CC-20), neat CC-20 patterned structure, and CC-20 integrated 3D printed shell 43182


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Figure 5. Repairable properties: (a) Schemes of the repairable features via filler damage, material refilling, and repairing processes. Photographs of the repairing processes: (b) original feature, (c) digging a hole in the filler material with a knife, (d) refilling the fillers back into the hole, (e) healing the wax-based fillers under heating, and (f) completion of repair. (g) Experimental measured electromagnetic absorption performance before and after repair.

meta-structures were modeled on a perfect electrical conductor (PEC) substrate. With the assistance of CST simulation (Figure 2b), 2D electromagnetic power loss distributions of the unit cells at three typical cross-section planes were displayed. As a result, the presence of 3D printed patterned shell in the meta-structures is more favorable to reach the ideal impedance matching conditions for dissipating the electromagnetic energy, compared to the model without the patterned shell. When the traditional single-layer absorber (CC-20) is simulated, the reflection loss indicates that the absorbers with uniform thickness (4−10 mm) would offer a narrowed effective bandwidth smaller than 3 GHz. In contrast, the simulated reflection loss performance using the 3D printed patterned shells shows that broad-band absorption (width >10 GHz) would be available, and would also be greater than the pattern without shell (Figure 3e). Because of similar improvements in broadening the effective absorption bandwidth, it is reasonable to utilize the 3D printed patterned shells with thickness of 1 mm with the consideration of structural mechanical stability. Thus, the meta-structures assembled by CC-20 filler and 3D printed shells of 1 mm in thickness were used for reflection loss measurement under different electromagnetic incident angles. As manifested in Figure 3f, the as-fabricated meta-structure enables offering of stable reflection loss when the angles of the incident electromagnetic waves were changed from 5° to 60°. Consequently, the design of the multilayer meta-structures is insensitive to the electromagnetic waves from different directions, implying excellent electromagnetic absorption within broad incident directions. Moreover, the pyramid structures with various heights were also used to investigate the effects on pyramid thickness on the microwave absorption. In the unit height of each layer, the values were set as 1, 1.5, and 2 mm, which corresponds to the total thicknesses of 6, 8.5, and 11 mm, respectively. As shown in Figure S3, increased thickness would substantially broaden the effective absorption

in the range of 2−18 GHz. Implication of the results suggests that the employment of 3D printed patterned shells would massively improve the broad-band electromagnetic absorption ability in comparison with uniform dielectric absorbing configuration. Generally, wax-based composites are known as mechanically soft and weak materials, but with advantages being easily processed and waterproof because of the small hydrophobic molecules. Hence, wax has been widely used as the matrix for hosting active fillers in complex permittivity parameters measurement via processing of small samples.13−22 To date, wax-based composites with electromagnetic attenuation properties have been rarely reported to be utilized in practical electromagnetic materials. In the present work, the presence of the 3D printed pattern shells endows the wax-based composites with more opportunities for potential applications in certain cases. In the typical evaluation of the advantages of incorporating wax-based composites into 3D printed shells, the mechanical stability of the as-fabricated meta-structures were characterized under various situations, including application of water flow (Figure 4a), water impact (Figure 4b), mechanical stamping (Figure 4c), compression (Figure 4d), and collision as a flying object (Figure 4e). According to the testing results, wax-based composites (back side) have been well endowed with hydrophobic features from wax matrixes, and the 3D printed patterned shell (front side) is sufficiently strong to bear the flow impact, as shown in Figure 4a,b. On the other hand, mechanical testing from Figure 4c−e indicates that integration of soft wax composites (as the fillers) and 3D printed polymer frameworks (as the shell) enable the as-fabricated metastructures to bear mechanical impact and compression under certain mechanical conditions (not applied to extreme conditions or impacts). More importantly, the measured electromagnetic absorption performance after various treat43183


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Figure 6. Typical features and perspective: (a) Typical experimentally measured ultra-broad-band absorption materials and structures: frequency selective surface (FSS) (refs 5 and 6), patterned design composites (refs 7 and 8), and compressible carbon foams (ref 9). (b−e) Representative features of the as-fabricated meta-structure along with other categories of solid dense electromagnetic absorption materials and structures. (f, g) Perspectives of the as-fabricated meta-structures in electromagnetic stealth applications.

ments (water flow, impacts, and compressions described in above conditions), the reflection loss curve is found to be well matched with the original one, indicating that these treatments have limited impacts on the mechanical and electromagnetic functional properties. Consequently, such features may allow the meta-structures to serve as electromagnetic functional bricks. It is interesting that wax is also a low-temperature phasetransformation material, and therefore wax-based composites could be reshaped via solid−liquid−solid processing. Because of this unique feature, the wax-based fillers in the metastructures could be easily repaired once the filling materials are damaged. For demonstration of the repairable characteristics (Figure 5a), a portion of the filled wax composites (CC-20) were initially damaged by digging a hole, as shown in Figure 5b,c. Subsequently, the removed CC-20 powders were refilled back into the hole, followed by heating the wax powders (Figure 5d,e). Simultaneously, the surface of the refilled hole was paved by flattening the heated (melted) CC-20 (Figure 5e), which allows the wax-based powders to be reshaped into the holes via a solid−liquid−solid process. Until the wax part was naturally cooled to temperature, the meta-structure with damaged wax-based composites was repaired. Also, the electromagnetic absorption performance of the repaired metastructure was remeasured, which implies that the repaired process has very limited influence on the performance (Figure 5g). Apparently, the integration based on 3D printing technology and wax composites establish a universal platform for enabling

the mechanically poor wax-based composites to act as practical electromagnetic structures with ultra-broad-band absorption capability (Figure 6a). Among the various types of ultra-broadband structures, (1) the wide broad band of FSS types is more likely to be linked with layers of the combined resistor surfaces and spacers, and thus broader bandwidth is associated with larger entire structure thickness.2,5,6 (2) The periodic patterns with uniform dielectric or magnetic composites is another typical category of the broad-band absorption structures.7,8 In these structures, the designed patterns and electrical/magnetic loss materials are responsible for scattering and attenuating the electromagnetic waves, both contributing to broadening the effective absorption band. (3) Another type of broad-band materials is compressible carbon foam,9 but it is required to substantially improve the mechanical stability for practical applications. (4) In addition to promotion in the mechanical stability, in this work, the coupling mechanisms are presented by electromagnetic wave scattering and loss from 3D patterned structures and CC-20 composites, respectively, which enables the meta-structures to deliver ultra-broad-band width approaching 67 GHz (7−40 and 75−110 GHz). Note that electromagnetic wave attenuation in U band (40−60 GHz) and V band (50−75 GHz) is stronger in the atmosphere in comparison with other bands, and thus few studies have been carried out on these two bands. Besides the electromagnetic absorption performance, the asfabricated meta-structures here also hold unique advantageous features in comparison with other typical electromagnetic absorption materials and structures. According to the schemes 43184


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was ground into carbon powders (or short fibers) and then slowly added into the melted wax. Note that the mixing process should ensure the homogeneity of CC and wax and slow addition was recommended to avoid unexpected aggregations. The oil bath stayed above 100 °C in the entire mixing process, and finally the CC/wax mixture was obtained. Specifically, 0.12 kg of CC and 0.48 kg of wax were used for fabricating a 20% CC/wax mixture. 3D Printed Patterned Shells. According to the patterned design from CST software, the parameters of the designed shells were used for creating 3D modeling in the Solidworks, followed by importing into the 3D printing machine (Stratasys Objet 350 Connex3). In the 3D printing of the patterned shells, Verowhite polymers (commercially available) were selected as the materials. Wax-Based Meta-Structures with 3D Printed Shell. Until the 3D printed frameworks (total size 300 × 300 × 11 mm3) (assembled by four pieces because of the upper-limit printing planar size of 200 × 200 mm2) with specific shell patterns were achieved, the heated CC/ wax mixture was then processed into the 3D printed patterned shells in a heated oven (∼70 °C) by filling and slight compression. When the 3D printed patterned shells were fully filled with heated CC/wax mixture, the samples were then transferred into ambient conditions for cooling. Practical Electromagnetic Absorption Performance from Arch Method Measurement. In the measurement of practical performance, the as-fabricated artificial structures (300 × 300 mm2 in planar size) were placed on the holder of the arch setup. In the investigation region from 4 to 110 GHz, the setup was performed on VNA (Agilent Technology NS5244A). Coaxial Method Measurement for Complex Permittivity. The CC/wax mixtures with various CC filler loadings (10, 20, and 30 wt %) were compacted into a toroidal shape (Φout, 7.03 mm; Φin, 3.00 mm) with thickness ∼2 mm. Then complex permittivity of the samples was measured on an Anritsu AV3672D vector network analyzer (VNA) with the coaxial method in the range of 2−18 GHz. Simulated Electromagnetic Absorption Performance from Complex Permittivity. Complex permittivity of the measured samples was used for simulating the electromagnetic absorption performance by considering different thicknesses (d) in the range of 2−18 GHz. Electromagnetic absorption performance was obtained by the reflection loss (RL)−complex permittivity relationship, which is generally known as eqs 1 and 2,24,25,31

illustrated in Figure 6b−e, the as-fabricated meta-structures not only possess the merits in electromagnetic absorption functions, mechanical robustness, and lightweight (assembled by all lightweight materials, including 3D printed polymer, carbon filler, and wax matrixes) but also are endowed with exclusive advantages in structure healing from the intrinsic characteristics of wax matrixes. Note that the repairable properties would be linked with the practical meta-materials, whose structure is damaged or is needed to update with other new materials for promoting materials performance. Because of the versatile merit of the strategy, there is still plenty of room for technology and material improvements, including use of the low-cost injection compression molding technology for scalable processing of the patterned shells, which would substitute for the 3D printing technology aimed at large-scale manufacturing. Alternatively, low-cost-effective fillers, commercial carbon black, for example, would be employed to further reduce the cost of the production. As a result, such a strategy could be widely extended to fabricate advanced practical multifunctional materials and structures with excellent ability,30 which would be utilized in specific constructions or buildings, where practical electromagnetic stealth technology is demanded.

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CONCLUSIONS In summary, a universal approach was proposed to address the critical issue of enabling the wax-based composites as practical broad-band electromagnetic absorption structures. When 3D printed patterned shells are employed, the as-fabricated metastructure based on CC/wax composites was able to offer ultrabroad-band absorption in the frequency of 7−40 and 75−110 GHz. When the merits from both 3D printed patterned shells and wax matrixes are integrated, the meta-structures of mechanical robustness hold a unique characteristic in repairable capability. The results suggest that such a strategy could be extendable to a range of wax-based electromagnetic absorption composites, which are able to serve as practical multifunctional meta-structures with exclusive properties and desirable performance.

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RL = 20 log

EXPERIMENTAL SECTION

Design of Meta-Structures with Artificial Patterns. The periodic structure design of composites (NCC-20) was performed by simulation through commercial software CST studio suite 2014. Initially, unit cell designs of various patterns based on different features were completed via modeling in the CST software. Until the experimentally measured complex permittivity of various samples was obtained, the parameters were imported into the established modeling, followed by sweeping of the S11 parameters. It should be noted that the complex permittivity that was used in CST simulation and design was all measured in the range of 2−18 GHz with the axial method. The complex permittivity values out of the 2−18 GHz frequency range (18−110 GHz) were generated by CST software via automatic extension based on the electromagnetic parameters from 2 to 18 GHz range. More details would be available in the information from CST software. Carbon Fillers and Wax Composites. In the scalable fabrication of carbon fillers, commercial cotton was used as the source for preparing carbonized cotton (CC) monolith. In the preparation, 0.5 kg of cotton was sealed in a nitrogen-protected oven, and then heated up to 800 °C for 1 h. In the entire heating process, the heating rate was set as 5 °C min−1. In the fabrication of CC/wax composites, dry wax powders were scaled and preheated in a large beaker (400 mL) under an oil bath (100−120 °C). Upon vigorous stirring with the magnetic stirring bar, viscous liquid melted wax was obtained. Meanwhile, a portion of CC monolith (according to the filler loading vs wax matrix)

Z in =

μr εr

|Z in − 1| |Z in + 1|

(1)

⎡ 2π ⎤ με tanh⎢j fd r r ⎥ ⎣ c ⎦

(2)

where Zin is the normalized input impedance, f the frequncy, c the light velocity, and μr the complex permeability. Material Characterizations. Field-emission scanning electron microscopy was applied on the ZEISS supra 55 systems. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM2010 scanning TEM system. X-ray powder diffraction characterization was performed on a PANalytical X′Pert PRO MPD diffraction system. X-ray photoelectron spectroscopy was captured on a PHI-5300 system. Tensile strength was acquired on a mechanical analyzer (TAXT Plus system, SMS). The 3D printed testing sample (material: Verowhite polymer) was fixed between stainless steel clamps with a tensile speed of 2 mm/min at room temperature.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15367. Modeling the 3D printed patterned shell in the Solidworks software, simulation of uniform materials with the CC/wax composites, and simulation of the 43185


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meta-structures with different total thicknesses at 6, 8.5, and 11 mm (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-68913302. E-mail: [email protected] (W.L.S.). *E-mail: [email protected] (M.C.). *E-mail: [email protected] (H.C.). ORCID

Wei-Li Song: 0000-0002-4328-8919 Mingji Chen: 0000-0002-8400-7159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from NSF of China (51302011), the Project of Beijing Municipal Science and Technology Commission (Z161100001416007), the Project of State Key Laboratory of Explosion Science and Technology (ZDKT17-02), and Beijing Natural Science Foundation (16L00001) is gratefully acknowledged.

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REFERENCES

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Constructing Repairable Meta-Structures of Ultra-Broad-Band

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