Ultrathin Flexible Carbon Fiber Reinforced Hierarchical Metastructure

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 44731−44740

Ultrathin Flexible Carbon Fiber Reinforced Hierarchical Metastructure for Broadband Microwave Absorption with Nano Lossy Composite and Multiscale Optimization Yixing Huang,† Xujin Yuan,‡,§ Mingji Chen,*,‡,§ Wei-Li Song,*,‡,§ Jin Chen,∥ Qunfu Fan,∥,⊥ Liqun Tang,† and Daining Fang*,‡,§,∥ †

School of Civil Engineering and Transportation, South China University of Technology, Guangzhou 510641, P. R. China Beijing Key Laboratory of Lightweight Multi-functional Composite Materials and Structures, Beijing Institute of Technology, Beijing 100081, P. R. China § State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, P. R. China ∥ Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, P. R. China ⊥ School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China

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‡

ABSTRACT: The implementation of thin structure for broadband microwave absorption is challenging due to the requirement of impedance match across several frequency bands and poor mechanical properties. Herein, we demonstrate a carbon fiber (CF) reinforced flexible thin hierarchical metastructure (HM) composed of lossy materials including carbonyl iron (CI), multiwall carbon nanotube (MWCNT), and silicone rubber (SR) with thickness of 5 mm and optimal concentration selected from 12 formulas. Optimization for the periodical unit size is applied, and impacts of structural sizes on absorption performance are also investigated. An effective process combining the vacuum bag method and the hand lay-up technique is used to fabricate the HM. Experimental reflectivity of the absorber achieves broadband absorption below −10 dB in 2−4 GHz and 8−40 GHz. The full band in 2−40 GHz is covered below −8 dB. Yielding stress of the HM is increased to 24 MPa with attachment of CF, while the fracture strain of the composite reaches 550%. The soft HM is suitable to adhere to the curved surface of objects needed to be protected from microwave radiation detection and electromagnetic interference. Enhanced mechanical properties make it possible for further practical applications under harsh service environments such as the ocean and machines with constant vibration. KEYWORDS: hierarchical metastructure, broadband microwave absorption, mechanical properties, dielectric-magnetic lossy material, flexible

1. INTRODUCTION Microwave absorbers have been widely used in stealthy technologies and electromagnetic compatibility.1−3 A great numbers of studies on how to improve absorption bandwidth have been conducted in the past decade. A large variety of nanomaterials has been fabricated to different constitutions, shapes, and structures.4−7 Synthesis and addition of lossy materials such as MWCNTs,8−11 graphene,12−14 ceramics,15,16 and alloys17−19 has become a major trend and method to improve dielectric and magnetic polarization for larger electromagnetic loss. The concentration of lossy materials, which often appear to be micro- or nanopowders, is essential to manipulate permittivity−permeability combination, which affects position and number of absorption peaks and bandwidth severely due to percolation effects. To reduce the resonant thickness, magnetic elements such as ferrites20−24 are synthesized into the dielectric-loss materials such as MWCNTs25−27 and graphene.28−30 Previous works31 showed © 2018 American Chemical Society

that high complex permittivity may weaken absorption peaks, even though the material had high dielectric loss (imaginary permittivity). Thus, magnetic ingredients are added to increase wave impedance of the material.32−34 Magnetic loss (imaginary permeability) also helps attenuate electromagnetic energy.35−37 However, simple manipulation of lossy powder constitution and its concentration in the matrix has limited improvement for broadband absorption owing to the dependence of resonant thickness. Therefore, structural designs such as metasurface and circuit-analog laminate38−42 are introduced to broaden the absorption bandwidth. Nevertheless, structural dielectric loss was paid the most attention to, while structural magnetic loss had very few investigations. Similar to the crucial Received: September 28, 2018 Accepted: November 21, 2018 Published: November 21, 2018 44731

DOI: 10.1021/acsami.8b16938 ACS Appl. Mater. Interfaces 2018, 10, 44731−44740

Research Article

ACS Applied Materials & Interfaces

Figure 1. Fabrication process and final product of CF-reinforced HM. added into the mixed LSR immediately with continuous stirring for 5 min. The treatment on CI/MWCNT powder was given in our previous work.45 The mixture was then poured into the polytetrafluoroethylene molds to be cured in vacuum at 70 °C for 2 h. The specimens were cut into the sizes of 72.14 mm × 34.04 mm × 3 mm, 47.55 mm × 22.15 mm × 3 mm, 34.85 mm × 15.80 mm × 3 mm, 22.86 mm × 10.16 mm × 3 mm, and 15.80 mm × 7.90 mm × 3 mm, respectively, to fit the waveguide for electromagnetic parameters measurement from 2.6 to 18 GHz. LSR turns to solid SR after curing. 2.3. CF Reinforced Hierarchical Metastructure Fabrication. The liquid mixture processed with the method mentioned above was poured carefully into the 350 mm × 350 mm female acrylic mold engraved from a holistic thick acrylic plate to the reverse HM shape by computer numerical control milling (Figure 1). After curing, the back of the primitive HM was coated with LSR. Then a layer of CF cloth was covered on the top of the liquid LSR pressed by the brush. LSR immersed gradually into the interval of carbon fibers with the pressure. Another LSR layer was smeared on the other side of the CF layer. This procedure was repeated three more times until four layers of CF cloth were attached coherently with the primitive HM. Following the hand lay-up steps, the structure was put into the vacuum bag as soon as possible for the second curing of the LSR in vacuum. The plastic vacuum bag was attached on the aluminum plate with high-temperature glue. A hole was opened on the vacuum bag to insert a silicone pipe connected to vacuum pump. To ensure vacuum atmosphere, a check valve was used to prevent air from entering the vacuum bag again. Once the vacuum pump is on, the vacuum atmosphere will be created in the vacuum bag, and when the curing at 70 °C for 2 h is finished the CF cloth and LSR will be adhered to the primitive HM closely by the pressure of the atmosphere. Since there is little adhesion between SR and acrylic mold, the HM can be demolded directly. 2.4. Characterization. The cross-section morphologies and interfaces between CI/MWCNT/SR and the CF cloth layer were observed by a field emission scanning electron microscope (Quanta 450 FEG, FEI), at an accelerating voltage of 5.0 kV. The complex permittivity and permeability of CI/MWCNT/SR composites with different weight fraction of CI and MWCNT were measured with the two-port waveguide connected to vector network analyzer (VNA, Ceyear AV3672D). Five calibration kits for waveguide ports equipped in the VNA in 2.6−3.95 GHz, 3.95−5.99 GHz, 5.99−8.2 GHz, 8.2− 12.4 GHz, and 12.4−18 GHz are used for measurement setup, respectively. The five sample holders have the same sizes as the five specimens cut in section 2.2. The reflectivity of the shaped 300 mm*300 mm HM in normal and oblique incidence between 2 and 40

status of magnetic polarization in coating absorbers, structural absorbers also need materials with large permeability to achieve broadband absorption with small thickness. Some previous works have successfully integrated magneticloss materials into the structural design of absorbers.25,43−46 However, dielectric loss is also needed to deepen absorption peaks and further improve absorption bandwidth. Moreover, microwave absorbers will stop on the laboratory stage until their mechanical properties are also considered and enhanced for practical applications under harsh service environments. In this paper, we demonstrate a flexible thin hierarchical metastructure (HM) for broadband microwave absorption with mechanical enhancement by means of carbon fiber (CF) layers. The lossy material used to fabricate the absorber is composed of spherical carbonyl iron (CI), multiwall carbon nanotube (MWCNT), and silicone rubber (SR) to introduce dielectric and magnetic loss with large extension. Twelve concentrations of the materials were fabricated to investigate how CI and MWCNT affect electromagnetic parameter combination. Multiscale optimization on the material level and structural level is applied to achieve improved microwaveabsorbing and mechanical performance. The product exhibits both broadband absorption in 2−40 GHz and satisfying mechanical strength. A useful method is also presented to fabricate the CF-reinforced HM.

2. EXPERIMENTAL SECTION 2.1. Materials. The MWCNT of purity 97% was purchased from Shenzhen Nanotech Port Co. Ltd., China, with outer diameter ranging from 60 to 100 nm and average length ranging from 5 to 15 μm. The spherical CI particles with average diameters of 3−4 μm were supplied by Shanxi Xinghua Co. Ltd., China. The platinumcatalyzed liquid silicone rubber (LSR) Ecoflex 0030 with mixed viscosity of 3000 cps was purchased from Smooth-on Inc. USA. The plain weave CF cloth with 3k carbon fibers per sheaf was supplied by Weihai Guangwei Composites Co. Ltd. 2.2. MWCNT/CI/SR Composite Fabrication. In order to measure the electromagnetic parameters, the composite was fabricated in rectangular shapes following the standard waveguide method. The platinum-catalyzed LSR matrix and curing agent were equally mixed and vigorously stirred at room temperature. Different weight fractions of processed CI/MWCNT powder mixture were 44732


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ACS Applied Materials & Interfaces GHz was measured with the NRL arch method according to national standard GJB2038A-2011 using the same VNA connected to two horn antennas for emitting and receiving microwave signals. The mechanical properties of (1) CI/MWCNT/SR composite with different concentrations of CI and MWCNT, (2) CF reinforced pure SR (CFRSR), and (3) CF reinforced CI/MWCNT/SR composite were characterized with the universal testing machine (Instron Legend 2367) to obtain stress−strain curves, Young’s modulus, and fracture strength. The microwave field distribution analysis was simulated with CST Microwave Studio.

3. RESULTS AND DISCUSSION The key procedures of fabricating HM are presented in Figure 1. On the right of Figure 1, the top and bottom of the final

Figure 3. (a) Real permittivity, (b) imaginary permittivity, (c) real permeability, and (d) imaginary permeability of Material #1 to Material #12.

absorption without incising it into small pieces to cover the reflective area. Observation of the cross section of the proposed HM suggests that the HM attached well to the CF layer bonded with SR (the rightmost inset in Figure 1). In order to realize how the CI and MWCNT distributed and adhered with the SR matrix, SEM pictures were taken in different scales. From Figure 2(a), we find that the matrix is cured to compact linkage without any evidently visible voids, which is attributed to the vacuum atmosphere in the curing stage. Besides, dense wrinkles can be found on the matrix, showing the composite has potentials for large extension. As can be seen in Figure 2(b), the CI particles are paired with a large amount of MWCNTs to construct an internal conductive network inside the insulated SR. In this way, an exuberant amount of spatial resistances, capacitances, and inductances are generated to affect the imaginary permittivity of the composites. MWCNTs as bridges to connect different parts of the matrix are shown in Figure 2(c). Moreover, CI particles and MWCNTs are covered with the matrix, indicating they are immersed well in the matrix. Although three or five CI particles cluster together, most of them are dispersed evenly in the matrix (Figure 2(d)). Since different amounts of CI particles and MWCNT added in the matrix lead to varying electromagnetic parameters, which will affect the comprehensive microwave absorption performance of the HM, it is necessary to conduct researches on how the concentration of CI and MWCNT affects the

Figure 2. SEM image of the CI/MWCNT/SR composite in different scales. (a) Cross-sectional morphology of CI/MWCNT/SR composite with wrinkles. (b) CI particles and MWCNTs are inserted in the matrix to form a spatial conductive network. (c) MWCNT connects different parts of the insulated matrix. (d) CI particles are dispersed uniformly in the SR matrix.

Table 1. Different Weight Fraction of CI and MWCNT Dispersed in the SR Matrix number

component

number

component

#1

pure SR

#7

#2

75.69 wt % CI

#8

#3

72.57 wt % CI + 0.174 wt % MWCNT 68.58 wt % CI + 0.397 wt % MWCNT 63.32 wt % CI + 0.690 wt % MWCNT 56.05 wt % CI + 1.096 wt % MWCNT

#9

45.36 wt % CI + 1.690 wt % MWCNT 28.08 wt % CI + 2.657 wt % MWCNT 72.69 wt % CI

#10

68.86 wt % CI

#11

63.54 wt % CI + 0.347 wt % MWCNT 63.76 wt % CI

#4 #5 #6

#12

product are demonstrated. The bottom CF layers of the flexible HM can be bent easily to 180° to reach the top hierarchical structure, indicating that the metastructure can be paved on curved surfaces conformally for microwave 44733


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Figure 4. Reflectivity distribution of single-layered coatings composed of (a) Material #1−(l) Material #12 with respect to coating thickness and frequency range between 2.6 and 18 GHz.

rubber.47 Enhancing the concentration of MWCNT made the conductive network denser in the matrix. Thus, more interfaces between MWCNT/rubber and CI/rubber were formed, and the real permittivity and dielectric loss were increased subsequently. Besides, higher volume fraction of MWCNT improved the conductivity of the material, and the microcircuit network was constructed by linking CI particles with MWCNT. More electromagnetic energy was dissipated by motion of more electrons, which increased the imaginary permittivity as well. As CI concentration is higher than 63.32 wt %, real permeability at 2 GHz remains close to 2.75. However, when CI concentration is lower than the percolation threshold, the real permeability drops rapidly, such that it even shows no significant magnetic features as CI concentration turns to 28.08 wt % (μ′ =1.25 at 2 GHz, Figure 3(c)) . When the volume fraction of conductive particles in the insulated matrix surpasses a particular value, the conductivity of the material experiences a sudden increase up to several orders of magnitude. The particular volume fraction is often termed percolation threshold.48 A similar percolation phenomenon occurs in the imaginary permeability diagram (Figure 3(d)). Thus, there exists a percolation threshold that determines whether the lossy composite exhibits effective magnetic

electromagnetic parameters. The concentrations of the 12 formulas are listed in Table 1. Material #2 to Material #8 have identical total volume fractions of MWCNT and CI (52.15 vol %). Materials #9, #10, #11, and #12 are control groups compared with Materials #3, #4, and #5. The complex permittivity and complex permeability of the 12 formulas were measured to characterize the electromagnetic properties of the lossy composite, as shown in Figure 3. The measured data are processed and fitted with the Debye model by using the parameter fitting module in CST Microwave Studio. The real permittivity of Material #2 to Material #8 fluctuates around 10.5 when total volume fractions of CI and MWCNT are identical (Figure 3(a)). (For example, Material #2 to Material #8 have the same total volume fraction of MWCNT and CI.) Increasing concentration of MWCNT has significant impacts on enhancement of imaginary permittivity, which is due to more complex conductive linkage in the matrix and more severe dielectric polarization loss (Figure 3(b)). In the heterogeneous structure of the MWCNT/CI/silicone rubber material, the real permittivity originated not only from the intrinsic polarization of CI and MWCNT but also from the structural interfacial polarization at the interfaces of conductive CI particles or MWCNT and insulated silicone 44734


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Figure 5. (a) Structure and detailed sizes of the periodical unit in the HM. Optimization of reflectivity in the frequency range from 2 to 40 GHz to select the unit size of HM by means of varying (b) L1, (c) L2, (d) X1, (e) X2, and (f) X3.

#12 was calculated using the classical reflectivity equation as follows

Table 2. Optimized Size Combination for Periodical Unit of the Proposed HM size

L1

L2

L3

X1

X2

X3

total thickness

value (mm)

10

12

15

2

2

1

5

RL(f ) = 20log10|(Zin − Z0)/(Zin + Z0)|

(1)

Zin = Z0(μr /εr )1/2 tanh[i(2πfd /c)(με )1/2 ] r r

(2)

where Z0 is the free space impedance; Zin is the input impedance at the air-coating interface; c is the velocity of electromagnetic wave in vacuum; f is the frequency of microwave; and d is the coating thickness. As can be seen in Figure 4(a), pure SR is not capable of absorbing microwave within the 2.6−18 GHz range. Material #2 to Material #8 have similar reflectivity tendency from 2.6 to 18 GHz due to identical total volume fractions of MWCNT and CI (all of them are 52.15 vol %). Increasing CI concentration can reduce resonant thickness in lower bands (Figure 4(b)−(d)). Although adding more MWCNT helps generate more absorption peaks with variable coating thickness, the lowerband resonant thickness for absorption is also increased (Figure 4(e)−(h)). Control groups like Material #12 suggest that when total volume fraction of CI and MWCNT is smaller than percolation threshold of 52.15 vol % absorption performance is worsened severely. After comparison, Material #3 was selected as the fundamental material for further structural design and optimization owing to its comparatively low real permittivity, high complex permeability, and severe absorption at low frequency ranges as coatings with comparatively small thickness. Since it is difficult to create absorption peaks in both low and high frequency ranges simultaneously, we propose a thin

Figure 6. Reflectivity of the proposed HM composed of Material #2 to Material #12, respectively, listed in Table 1 with the same size set given in Table 2.

polarization and magnetic loss. Reflectivity distribution of single-layered coatings composed of Material #1 to Material 44735


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Figure 7. (a) Experimental and simulated reflectivity of HM with representative frequency points of absorption marked out. Experimental reflectivity of HM in oblique incidence from 0° to 60° with steps of 15° is measured in both (b) TE polarization and (c) TM polarization.

(Figure 5(c)). Results show that X1 and X2 have minor influences on the tendency of reflectivity, but enhancing X1 and X2 broadens lower border below 5 GHz (Figure 5(d) and (e)). It is surprising that increasing thickness of the third layer (X3) worsens the absorption performance in the low frequency range (Figure 5(f)) without affecting that in the high frequency range. With the parameter decoupled optimization discussed above, the final size set is listed in Table 2. The influences of different concentrations of CI and MWCNT on HM absorption performance were investigated. Material #2 to Material #11 were applied to the given structure of HM in Table 2, as shown in Figure 6. Reflectivities of the 11 formulas have similar tendency, all of which have absorption peaks, both at around 5 and 27.5 GHz. Nevertheless, different concentrations of materials have essential impacts on bandwidth extension at the lower frequency range between 3 and 5 GHz. It is rather effective for HM to achieve excellent absorption in the high frequency range, especially larger than 18 GHz with total volume fraction percolation threshold of CI and MWCNT to be 52.15 vol %. Moreover, it should be noticed that material concentration optimization is a useful way to broaden the absorption bandwidth at the low frequency range. With the optimization, Material #3 is the final candidate to fabricate the CF-reinforced HM. The reflectivity of the proposed HM was measured and compared with the simulated results, as shown in Figure 7(a). It can be found that the tendency of experimental and simulated reflectivity is matched in the high frequency range. Although a small shift of the absorption peak at 4 GHz due to fabrication errors and slight size control inaccuracy occurs, the experimental result shows that the designed absorption peak still exists but is moved below 2 GHz, which indicates that the proposed HM can even achieve absorption in the MHz range. Except for the C band, experimental reflectivity achieves −10 dB absorption from 2 to 40 GHz and −15 dB absorption above 10 GHz, which proves the effectiveness of the structural design, size optimization, and material optimization mentioned previously. The full band (2−40 GHz) is covered below −8 dB. Measured reflectivity of oblique incidence in TE and TM polarization is also investigated. In TE polarization, as the incident angle increases, absorption in the C band is weakened. When the incident angle approaches 45° and up to 60°, HM loses absorption capability, as shown in Figure 7(b). Conversely in TM polarization, as the incident angle increases

Figure 8. Complex electric field intensity time-average amplitude distribution, complex magnetic field intensity time-average amplitude distribution, and power loss distribution of the proposed HM at frequency points marked in Figure 7(a) including (a) 4.43 GHz, (b) 8.57 GHz, (c) 12.22 GHz, (d) 20.24 GHz, (e) 27 GHz, and (f) 33.73 GHz.

HM to realize the goal. The structure and size of the periodical unit are shown in Figure 5(a). The shape of the unit is a hierarchical structure with the side length of the cuboid decreased from the top to the bottom. The spatial absorber is composed of the periodical arrangement of the unit in two dimensions. Below the structure is the CF layer as a flexible reflective substrate served like a perfect conductor. In order to optimize the sizes of the HM, the decoupled optimization method is utilized to select the best size combination. As shown in Figure 5(b), with L1 increased, the lower border of the −10 dB absorption bandwidth is moved from 6 to 3 GHz. However, absorption in the high frequency range will be affected negatively if L1 becomes larger than 10 mm. When L1 = 14 mm, the proposed HM loses absorption. Similar to L1 variation, enhancing L2 can also broaden the lower border of the absorption bandwidth from 6 to 3 GHz, but the absorption bandwidth can be held below −10 dB unlike the cases of L1 44736


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Figure 9. (a) Stress−strain relationships of Material #1 to Material #12. (b) The initial Young’s modulus of Material #1 to Material #12 in the first stage with respect to concentration of CI and MWCNT. (c) The nominal stress−nominal strain relationships of CFRSR, CF layer attached to CI/ MWCNT/SR composite (composite-CFRSR), and average equivalent stress (equivalent stress) of the CF reinforced CI/MWCNT/SR composite with respect to the whole composite section compared with the mechanical behaviors of Material #1 and Material #3. (d) The hardening and softening Young’s modulus of Material #1 to Material #12 in the second stage with respect to concentration of CI and MWCNT.

over 45°, the proposed HM is able to achieve −10 dB absorption bandwidth at 2−40 GHz (Figure 7(c)). The complex electric and magnetic field intensity timeaverage amplitude distribution and power loss distribution of designated frequency points marked in Figure 7(a) are simulated by CST Microwave Studio as shown in Figure 8. From lower frequency at 4.43 GHz (Figure 8(a)) to higher frequency at 33.73 GHz (Figure 8(f)), the majority of electric field is adhered from the top layer to the external space, and only a weak electric field remains in the internal structure. At frequencies higher than 20.24 GHz (Figure 8(d)), the electric field begins to distort to an elliptic shape outside the periodical unit. Conversely, except for the magnetic field stored deep inside the structure at 4.43 GHz, the magnetic field at other frequency points expands from the top layer to the external space at 8.57 GHz (Figure 8(b)) in the first place, and then it shrinks gradually to the corners of the top layer (Figure 8(d), (e), (f)). Interestingly, the power loss distribution coincides well with the magnetic field distribution, indicating that magnetic loss plays an essential role in microwave absorption, which is mainly caused by intergranular domain wall motion and hysteresis loss.49 Electric polarization loss plays assistant roles in reducing reflection. Electromagnetic field concen-

tration is caused by spatial periodical design of the HM by generating a standing wave inside the structure and in the adjacent space to improve impedance match with air. Combining material loss features and hierarchical design, conflicts on maintaining broadband absorption in both low and high frequency ranges can be solved. For practical applications, mechanical properties of HM should be strengthened to adapt to harsh service conditions. Tensile properties of Material #1 to Material #12 and CF reinforced CI/MWCNT/SR composite are measured. Stress− strain relationships of the 12 formulas are given in Figure 9(a). Material #1 to Material #12 without CF exhibit two-stage tensile behaviors including the initial stage and hardening or softening stage. Pure SR (Material #1) experiences the initial stage with low Young’s modulus of 47.8 kPa first and then enters the hardening stage with Young’s modulus increased up to six times (315 kPa) (Figure 9(b),(d)). Compared with pure SR, addition of CI and MWCNT increases the initial Young’s modulus severely, but the second stage turns into softening ones. The initial and hardening Young’s moduli of Material #1 to Material #12 with respect to concentration of CI and MWCNT are compared in Figure 9(b) and (d). Addition of CI increases the initial Young’s modulus, while addition of 44737


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4. CONCLUSIONS A thin flexible HM for broadband microwave absorption has been demonstrated via combination of vacuum bag techniques and hand lay-up methods. The proposed absorber can achieve −10 dB absorption bandwidth within S, X, Ku, K, and Ka bands covering from 2 to 4 GHz and from 8 to 40 GHz, with total thickness of only 5 mm. Optimization is successfully applied to choose the best concentrations of CI and MWCNT in SR and seek out the optimum size set for the periodical unit. Both material concentration and size variation have significant impacts on absorption performance in the low frequency range between 2 and 6 GHz, while they have minor influences on higher frequency ranges above 18 GHz. Mechanical properties of the metastructure are greatly improved via attaching the CF layer, with yielding stress increased from 1 to 24 MPa, 24 times larger than before. Extremely large fracture strain of the composite up to 550% is also maintained. Enhanced stealthy and mechanical features of the proposed metastructure shed light on practical applications for radar invisibility and electromagnetic compatibility in 5G communication.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Mingji Chen). *E-mail: [email protected] (Wei-Li Song). *E-mail: [email protected] (Daining Fang).

Figure 10. (a) SEM image of the interface between Material #3 and the CF layer. (b) SEM image of details pointed in (a). (c) SEM image of CF encapsulated in SR. (d) SEM image of Material #3 and CF buried in SR in the interface.

ORCID

Xujin Yuan: 0000-0003-1630-3180 Mingji Chen: 0000-0003-4337-9519 Wei-Li Song: 0000-0002-4328-8919 Jin Chen: 0000-0001-9554-0445

MWCNTs enhances hardening of the Young’s modulus. Furthermore, as shown in Figure 9(c) the yielding stress of CFRSR is approaching 18 MPa. After attaching Material #3 to CF, the nominal yielding stress of CF attached to the CI/ MWCNT/SR composite (Composite-CFRSR) can even be increased to 24 MPa (fracture stress of nitrile-butadiene rubber is typically 11 MPa). The average equivalent nominal yielding stress with respect to the whole section of CR-reinforced CI/ MWCNT/SR composite approaches 6 MPa, more than six times higher than that of Material #3. Thus, addition of CF has a great improvement on the strength of the composite. Besides, after fracture of CF, the extremely large fracture strain of the composite up to 550% avoids brittle failure of the soft metastructure. The proposed CF reinforced composite has comparatively large yielding strength and large fracture strain simultaneously to satisfy harsh service environments. The SEM image of the interface between Material #3 and the CF layer shows that Material #3 serves as a thick soft foundation for the CF layer to be based on, and the soft foundation as a constraint protects SR immersed between CFs from quick fracture (Figure 10(a)). Since longitudinal and traverse CF are immersed well with SR, the stress of CF can be transferred to SR (Figure 10(b)). As CF is encapsulated inside the bags composed of pure SR, a sudden brittle fracture of CF can be avoided as much as possible (Figure 10(c)). Material #3 is adhered tightly to pure SR which protects CF due to similar molecular constitution, and no evident cracks and gaps in the adhesion can be found (Figure 10(d)). With these synthetic effects, the CF layer with attachment of Material #3 bears more than 6 MPa in yielding strength than that of CFRSR without Material #3.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (11872113) and the Project of State Key Laboratory of Explosion Science and Technology.

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REFERENCES

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