Comparative study of structure, mechanical and electromagnetic interference shielding properties of carbon nanotube buckypapers prepared by different dispersion media

Materials & Design 184 (2019) 108175

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Comparative study of structure, mechanical and electromagnetic interference shielding properties of carbon nanotube buckypapers prepared by different dispersion media Yunping Hu , Dongcheng Li , Ping Tang , Yuezhen Bin *, Hai Wang * Department of Polymer Science and Engineering, College of Chemical Engineering, Dalian University of Technology, No.2 Linggong Rd, Dalian 116024, P. R. China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


MWCNT/water dispersions are far superior to MWCNT/ethanol dispersions in stability, but slightly inferior in monodispersity.
The BPs prepared from MWCNT/water dispersions showed remarkable high electrical conductivity and mechanical stren.
A notable absorption-dominated SE of 101.7 dB is achieved by BP (450 mm) prepared from MWCNT/water dispersions on 8.2-18 GHz.
The SE of BPs (450 mm) prepared from MWCNT/water dispersion is 26.0% higher than that prepared from MWCNT/ethanol dispersion.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 July 2019 Received in revised form 29 August 2019 Accepted 30 August 2019 Available online 31 August 2019

In order to prepare multi-wall carbon nanotube (MWCNT) buckypapers (BPs) with excellent comprehensive properties, an investigation was conducted regarding the effect of dispersing medium on CNT dispersions and resultant BPs. Three types of MWCNTs with different lengths and diameters, denoted as CNT-A, CNT-B and CNTeC, were dispersed into water, ethanol and 1-methyl-2-pyrrolidone, respectively. The dispersion stability was monitored by UVevis spectroscopy and conductivity meter. The results showed that all MWCNT/water dispersions were far superior to MWCNT/ethanol dispersion in stability, but slightly inferior in monodispersity. Subsequently, BPs prepared by vacuum filtration using the abovementioned dispersions were investigated to clarify the pore structure, electrical, mechanical and electromagnetic shielding properties. Compared with BPs prepared with CNT-A/ethanol dispersion (BP-A-E), the BP prepared with CNT-A/water dispersion (BP-A-W) showed smaller pore size, and exhibited significantly improvement in electrical conductivity (74%), strength (665%). A super high absorptiondominated electromagnetic interference shielding effectiveness (SE) of 101.7 dB was achieved by BP-AW of 450 mm thick in the range of 8.2e18 GHz, which was 26% higher than that of BP-A-E. A notable SE of 48.4 dB and a high specific SE of 29,300 dB$cm2/g were obtained by BP of 45 mm thick prepared with CNT-C/water dispersion. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Carbon nanotube Dispersion Buckypaper Mechanical property Electromagnetic interference shielding

* Corresponding authors. E-mail addresses: [email protected] (Y. Bin), [email protected] (H. Wang). https://doi.org/10.1016/j.matdes.2019.108175 0264-1275/© 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Y. Hu et al. / Materials & Design 184 (2019) 108175

1. Introduction All sorts of electronic equipment have brought so much convenience for industry and daily life of human beings. However, high-energy electromagnetic radiation generated from these electronic equipment has also resulted in negative effects to some extent, such as the malfunction, service life shortening and information leakage of the equipment, and even harm on our health [1]. Therefore, it is inevitable to develop high effective electromagnetic shielding materials for a better and long-term performance of these equipment [2,3]. Among the many candidates, carbon nanotubes (CNTs) have been widely used in electromagnetic shielding materials due to the extraordinary mechanical, conductive and electromagnetic absorbing properties [4,5]. But, there lies one major issue of using CNTs for electromagnetic shielding purpose, i.e. CNT dispersion. Lightweight CNT/polymer composites with electromagnetic interference (EMI) property could be prepared by mixing CNTs with resin directly using mechanical processing methods [6e10]. But a homogeneous dispersion of high content of CNTs in polymer matrix has always been difficult due to the aggregation of nano-sized fillers [11] and poor CNT-polymer interfacial interaction. Theoretically, EMI shielding effectiveness (SE) is correlated positively with the conductivity and thickness of the shielding material [12]. But it is impossible to increase infinitely the filler loading for a promoted conductivity or a better SE because of the filler aggregation. Instead, shielding materials with larger thickness were prepared. Subhadip et al. [6] reported that the SE of chlorinated polyethylene/MWCNT composites was 36 dB with the MWCNT content of 15 wt% and a large thickness of 2 mm. Unfortunately, electronic devices are becoming more compact and integrated at present, which makes the application of lightweight, thin and high performance shielding materials an irreversible tendency. Buckypaper (BP) is a lightweight, self-supporting and conductive film composed of entangled CNTs, which has presented great potential for the next generation of electromagnetic shielding materials due to its high flexibility and electrical conductivity. Chemical vapor deposition (CVD) [11,13], vacuum filtration [12,14e19] and spin coating [20] of carbon materials were the most popular methods of BP preparation. However, CVD and spin coating methods are inefficient, expensive and energy-consuming, making the largescale production of BPs substantially difficult, even if SEs with desirable properties can be obtained. The vacuum filtration method is widely used because of its simple process, convenient operation, short cycle, and low cost. Vacuum filtration has been frequently used in the preparation of BP with the purpose of interference shielding applications [12,14e17], sensors [21], actuators [22], biofuel cells [23], and supercapacitor [24]. Park et al. [12] showed that BP/polyethylene composites presented outstanding SE of 20e60 dB, depending on the BP conductivity within the frequency range of 2e18 GHz. The Fe3O4/MCMBs/MWCNTs composite paper prepared by Anisha et al. [16] exhibited an excellent absorption dominated EMI SE of 80 dB, which had a thickness of only 0.5 mm. But a critical problem of the BPs prepared by vacuum filtration is the very low tensile strength, which was not considered in the above mentioned several literatures. In order to enhance the mechanical strength of BPs, in situ cross-linking [25,26] and compression processes after filtration [27] were carried out. But the former would probably damage the electrical properties of BPs, the latter was still controversial so far. Hata et al. [27] reported that the tensile strength of BP increased linearly with the increasing mass density resulted from compression. Whereas Kumar et al. [28] found that the mechanical properties of compressed BPs and uncompressed BPs were almost the same. In contrast, it is easier and more feasible to optimize the length of CNTs [27], surfactant [29], dispersion method [30], and dispersing medium so as to improve the strength and retain the

intrinsic excellent electrical property of BPs. Among the many factors that related with the properties of BPs, the dispersion of CNTs has to be concerned firstly with no doubt. CNTs aggregate and form bundles very easily due to the strong van der Waals interaction, resulting in an extremely poor dispersion in both organic and aqueous media. Some literatures have reported the effects of surfactants, ultrasound and super plasticizer on the dispersion of CNTs [31e33]. However, the dispersion of high content of CNTs is still a challenge even for small scale experiment in lab, and many problems related with dispersion have not yet been solved. Meanwhile, very few reports have discussed the dispersity and dispersion stability of CNTs in various dispersion media. The correlation between the status of CNT dispersion and the properties of BPs after vacuum filtration has not been discussed to our best knowledge. As we know, few reports detailed the influence of dispersing medium on properties of BP, especially the mechanical and electromagnetic shielding properties. In this paper, we explored the effects of ultrasound and dispersion media (water, ethanol and 1Methyl-2-pyrrolidone (NMP)) on the dispersity and dispersion stability of MWCNTs by means of UVevisible spectroscopy and solution conductivity. Subsequently, BPs were prepared from different MWCNT dispersions, including MWCNT/water, MWCNT/ ethanol and MWCNT/NMP dispersions. The correlation between CNT dispersity and electrical, mechanical and electromagnetic shielding properties of BPs was investigated in details. 2. Experimental 2.1. Materials Four types of unfunctionalized MWCNTs were used in this research, including CNT-A from Chengdu Organic Chemicals Co., Ltd., and CNTeB, CNT-C1 and CNT-C2 from Showa Denko Co. Ltd. The parameters of different MWCNTs are detailed in Table 1. All CNTs were used as purchased without further functionalization. Triton X-100 and anhydrous ethanol (AR) were purchased from Tianjin Damao Chemical Reagent Factory. NMP was supplied by Liaodong Chemical Reagent Company, China. 2.2. Preparation of BP All CNTs were ground before dispersing. Take CNT-A dispersed in water as an example. CNT-A (400 mg) with 1 wt% of Triton X-100 was added into water (1000 mL), forming a uniform dispersion with the help of stirring and sonication. The obtained CNT-A/water dispersion was filtered through cellulose membrane with a pore size of 0.45 mm, and the filter cake was rinsed with a large amount of deionized water to get rid of the residual surfactant. Then the filter cake was immersed into acetone bath to separate BP with cellulose membrane filter. The obtained free-standing BP was named as BP-A-W, where W denoted water. CNT-A/ethanol dispersion and CNT-A/NMP dispersion were prepared in the same way, and the corresponding BPs was denoted as BP-A-E and BP-AN, respectively. The same procedure was conducted to prepare BPs using CNT-B and CNT-C (50 wt% CNT-C1 þ 50 wt% CNTeC2). The resultant BPs were denoted as BP-B-W, BP-B-E, BP-C-W, and BP-C-E. All the details of the different BP samples are listed in Table 1. 2.3. Characterization 2.3.1. Characterization of MWCNTs and MWCNT dispersions The morphology of MWCNTs was examined by a transmission electron microscope (TEM, Tecnai F30, FEI). The degree of graphitization was investigated by a Raman spectroscopy (DXR Microscope, Thermos Fisher). The UVevis absorbance of MWCNT

Y. Hu et al. / Materials & Design 184 (2019) 108175

dispersions in different media was monitored by a UVevis spectroscopy (SP-752PC, Shanghai Spectrum Instrument Co., Ltd.). The conductivity of MWCNT dispersions was measured using a conductivity meter (DDS-307A, Shanghai Yidian Scientific Instrument Co., Ltd.).

2.3.2. Characterization of BPs Surface morphology and chemical composition of BP were observed by a Quanta-450 scanning electron microscopy (FE-SEM, NOVA NanoSEM 450, FEI) equipped with an energy dispersive spectroscopy (EDS). Pore structure of the BP surface was analyzed by an image analysis software (Image-Pro Plus 6.0). The N2 adsorption-desorption isotherms of BP was tested by physical adsorption instrument (BelSorp-Mini, BEL JAPAN, INC). The specific surface area and pore size distribution of BPs were fitted by Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) models, respectively. A Raman spectroscopy (DXR Microscope, Thermos Fisher) was used to detect the defect degree of MWCNTs in BPs. The electrical conductivity of BP was recorded with a dual electro-testing four-point probes resistivity tester (model RST-9, PROBES TECH). The tensile strength and tensile modulus of BP were measured using an electronic single fiber strength tester (LLY06E, Laizhou Electronic Instrument Co., Ltd.). The contact angle of BPs was detected by a contact angle tester (JY-82, Chengde Testing Machine Factory). The EMI SE was characterized by a waveguide measurement system using a vector network analyzer (AV3629D, the 41st Institute of China Electronics Technology Group Corporation) in the frequency range of 8.2e18 GHz. The samples were cut into small pieces with a rectangular shape to fit the chamber of the sample holder. Four scattering parameters, denoted as S11, S12, S21, and S22, were measured. The incident electromagnetic wave passing through a piece of material undergoes four different processes: reflection, absorption, penetration, and multiple reflection [34]. The EMI SE should be a sum of its reflection and internal absorption energy. SER, SEA and EMI SE were analyzed based on the values of S11, S12, S21, and S22 using the following equations [35,36]:

R ¼ jS11 j2 ¼ jS22 j2

(1)

T ¼ jS12 j2 ¼ jS21 j2

(2)

A ¼ 1RT

(3)

SER ¼ 10logð1=ð1  RÞ Þ

(4)

SEA ¼ 10logðð1  RÞ=T Þ

(5)

SE ¼ SEA þ SER

(6)

where, R, T and A present the reflection coefficient, transmission coefficient and absorption coefficient, respectively.

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3. Results and discussion 3.1. Morphology of MWCNTs The typical TEM images of different MWCNTs are presented in Fig. 1. MWCNTs exhibited a dispersed morphology after ball milling and sonication. Amorphous carbon can be observed on the surface of the four types of MWCNTs. As shown in Fig. 1e, f and h, defects were observed on the walls of CNT-A, CNT-B and CNTeC2, which would directly affect the electrical properties of the CNTs. In contrast, CNT-C1 showed a relatively intact and regular wall structure (Fig. 1g). In addition, many granular and short tubular particles were observed in CNTeC1, which were suspected as catalysts and incompletely growing CNTs.

3.2. Monodispersity and stability of MWCNT dispersions 3.2.1. Ultrasonic time dependent MWCNT dispersity CNTs are easily agglomerated in organic and aqueous media. To improve the dispersity of CNTs, several common methods were combined in this research, including ball milling, ultrasonic treatment and adding surfactant. Since the structure of MWCNTs will be damaged by the excessive energy from long time ultrasonic treatment, sonication time have to be optimized. A series of time dependent sonication experiments of MWCNT dispersions were conducted, which were monitored using UVevis spectra as shown in Fig. 2. The results of CNT-A dispersions with different dispersing media were discussed as an example. As illustrated in Fig. 2(a), the spectra of CNT-A/water dispersions exhibit two maxima between 200 and 300 nm and a gradual attenuation from 200 nm to larger wavelength, which was similarly to the absorption spectra reported before [31,33]. This is partly due to scattering, especially in the lower wavelength range [33]. As the sonication time was expanded, the maximum absorbance at 223 and 275 nm continued to increase until the ultrasonic treatment was longer than 25 min at a constant power of 300 W (Fig. 2(d)). Monodisperse CNTs are active in the UVevis region and exhibit characteristic bands. The maximum achievable exfoliation was correspondent with the maximum UVevis absorbance of the CNT solution [33]. It was indicated from Fig. 2(a) and (d) that the CNT-A was maximally dispersed after a sonication treatment for 25 min at 300 W. It can be seen in Fig. 2(aeb), the two set of UVevis spectra of CNT-A/water dispersions and CNT-A/ethanol dispersions showed a similar manner. But CNT-A/ethanol dispersions exhibited a higher absorbance compared to CNT-A/water dispersions, which may be attributed to the better monodispersity of CNTs in ethanol. In term of NMP system (Fig. 2(c)), the region on 200 to 260 nm of the UVevis curves was not presented since there was the absorbance of NMP itself. As for CNT-B and CNT-C dispersed in water and ethanol, very similar tendency was observed on the corresponding UVevis spectra (Fig. S1). In general, the UVevis absorbance of MWCNT dispersions, either in water or ethanol, increased rapidly with the

Table 1 Information of BPs prepared by vacuum filtration method. Diameter/nm

Length/mm

Samples

MWCNT type

BP-A-W BP-A-E BP-A-N BP-B-W BP-B-E BP-C-W BP-C-E

CNT-A

10e20

10e30

CNT-B

8e15

1e100

CNT-C

20e30 (CNTeC1), 15 (CNT-C2)

>10 (CNTeC1), 3 (CNTeC2)

Dispersing medium Water Ethanol NMP Water Ethanol Water Ethanol

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extension of the ultrasonic time, which tended to be stable after about 20 min. CNT/ethanol dispersion presented a higher absorbance compared with CNT/water dispersion for both CNT-B and CNTeC. Time dependent sonication experiments were also conducted in such a way that the conductivity of the CNT dispersions was detected simultaneously. The monodispersed CNTs contribute to the formation and improvement of conductive pathways, thus a well dispersed CNT dispersion is expected to have a higher conductivity. As shown in Fig. 3, the conductivity of CNT-A/water and CNT-A/ethanol dispersions reached a plateau when the sonication time was longer than 25 min (energy inputs of ca. 450 kJ). The stabilized conductivity suggested the optimum dispersion of MWCNTs, which was consistent with the results of UVevis absorption spectra. In terms of CNT-A/NMP dispersions, the conductivity was kept at a low value even with the extension of sonication time. The conductivity of CNT dispersions was not only related to the dispersion state of CNTs, but also to the dispersion medium. Therefore, CNT dispersions exhibited different conductivities in water, ethanol and NMP. But there is no doubt that the monodisperse CNTs are good for improving the conductivity of dispersions. 3.2.2. Colloidal stability of MWCNT dispersions The colloidal stability was measured to analyze the dispersion stability of MWCNTs in different dispersion media. The optimum dispersion of MWCNTs was attained with the help of sonication and surfactant. The long-term stability of MWCNT dispersions was evaluated using UVevis absorption spectra and conductivity. As illustrated in Fig. 4(a), (c), and (d), CNT-A/water and CNT-A/NMP dispersions presented excellent stability in several days, judging from the almost constant UVevis absorbance and conductivity during the observation period. By contrast, CNT-A/water dispersion without surfactant exhibited a dramatically decrease in UVevis absorbance and conductivity after 24 h due to sedimentation (Fig. S2). The CNT-A/ethanol dispersion also presented significantly sedimentation in 36 h despite the addition of surfactant, indicating from the quickly declined UVevis absorbance and conductivity (Fig. 4(b) and (d)), which suggested a poor stability of CNT-A/ ethanol dispersions. 3.3. Morphology, mechanical strength and shielding performance of BPs 3.3.1. Impact of sonication on the structure of CNTs Sonication is able to disentangle MWCNT bundles to some extent, but an excessive sonication would damage the structure of

MWCNTs. The defected structure of CNTs was normally characterized by Raman spectroscopy [31,37,38]. As shown in Fig. 5, the spectra of CNT-A and BPs displayed two characteristic peaks between 800 and 2000 cm1. The D band located at ca. 1350 cm1 was attributed to defects in the nanotube lattice, including the sp3 hybridized carbon. The G band at ca. 1590 cm1 was associated with tangential CeC bond stretching motions [37]. The intensity ratio of D and G band, i.e. ID/IG represented the graphite degree and regularity of carbon-based materials [37,38]. It was seen from Fig. 5 and table S1 that there was no obvious change in ID/IG of BP-A-W, BP-AE and BP-A-N compared with original CNT-A. The same experiment and analysis were carried out for CNTeB, CNT-C and BPs made of these MWCNTs (Fig. S3). Similarly, BP-B-W, BP-B-E, BP-C-W, and BP-C-E did not have any distinct change on D and G bands. This indicates that ultrasound does not destroy the structure of the CNTs that undergone a sonication treatment of 25 min (energy inputs of ca. 450 kJ). The ID/IG of all CNTs and BPs were calculated and listed in Table S1. The BPs presented smaller ID/IG compared with MWCNTs due to that amorphous carbon on the surface of MWCNTs was peeled off during sonication. 3.3.2. Morphology, mechanical strength and conductivity of BPs The SEM images representing the morphology of BPs made of CNT-A were shown in Fig. 6(a)e(c). The pore structures of BPs were analyzed on the basis of these SEM images using Image-Pro software. The statistical data of the pore diameters was summarized in Fig. 6(d), and the average pore sizes were calculated and listed in Table 2. CNT-As were relatively discrete in diameter and had almost no catalyst impurities, judging from the SEM images and elemental analysis (Fig. 6(a-c) and Table S2). It can be seen in Fig. 6(a) that MWCNTs were uniformly distributed and tightly entangled in BPA-W to form conductive network. The MWCNTs with larger diameter tangled into trunk network for long-distance electronic and stress transmission. MWCNTs with relatively small diameter entrapped in the interconnected trunk network to serve as a bridge, making the network more unobstructed. By contrast, BP-A-E appeared to be looser on morphology (Fig. 6(b)). Thus BP-A-E presented a larger average pore size and a wider pore size distribution compared with BP-A-W (Fig. 6(d) and Table 2). BP-A-N exhibited an obvious “sea island structure” (Fig. 6(c)). MWCNTs with larger diameter tangled into the “sea”, those with smaller diameter closely intertwined to form the “island”. The SEM images of BPs made of CNT-B and CNT-C were shown in Fig. S4, and the pore structure estimation was shown in Fig. S5. In general, the BPs prepared from water system were denser than those prepared from ethanol system. The smaller pore size and thickness of BP-A-W, BP-

Fig. 1. TEM images of CNT-A (a & e), CNT-B (b & f), CNT-C1 (c & g), and CNT-C2 (d & h). Figure e-h are magnified images of Figure a-d, respectively.

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Fig. 2. Evolution of UVevis spectra of CNT-A/water (a), CNT-A/ethanol (b) and CNT-A/NMP dispersions (c) as a function of sonication time at the continuous power of 300 W. The insert in Figure (c) was UVevis spectra of NMP. Time dependent of the absorbance of CNT-A dispersions at 223 nm and 275 nm (d). All dispersions had a MWCNT concentration of 0.4 g/L (dispersions were diluted by a factor of 100 for the test).

B-W and BP-C-W were the most direct evidence (Table 2). For all types of BPs the porosity fell on the range of 75e84% as listed in Table 2. The specific surface area and pore size distribution were also evaluated by N2 adsorption-desorption isotherms. The results of BPs prepared from CNT-A were discussed as an example. As shown in Fig. 7(a), curves of BP-A-W, BP-A-E and BP-A-N had a significant hysteresis loop, indicating that BP-As had mesoporous structures. Subsequently, the pore size distribution was calculated by BJH model as shown in Fig. 7(b). The pore size was mainly distributed between 5 and 80 nm. The peaks at ca. 3 nm in the pore size distribution curve of the three BP-As were resulted from the inner diameter of MWCNTs and inter-tube channels between adjacent MWCNTs filled with nitrogen. BP-A-W showed a narrower pore size distribution and smaller peak value at 36 nm compared with BP-AE at 40 nm. BP-A-N presented two peaks between 5 and 80 nm due to the intra-bundle and inter-bundle pores, corresponding to its “sea island structure”. These results were consistent with the pore structure estimated from SEM images. Whitby et al. [39] also

Fig. 3. Evolution of the conductivity of CNT-A/water, CNT-A/ethanol and CNT-A/NMP dispersions as a function of sonication time at the continuous power of 300 W. All dispersions had a MWCNT concentration of 0.4 g/L.

reported that the distribution of mesopores and small macropores was dependent on the medium from which BP was cast. The differences in pore size distributions were caused by the effect of dispersing media on the packing order of MWCNTs in BPs. In addition, as illustrated in Fig. S6(b) and Table 2, BP-B-W had the smallest pore size (peak value at 29 nm) and largest specific surface area because of the small diameter and uniform distribution of CNTeB. As presented in Table 2 and Fig. 6(e), BP-A-W presented a higher conductivity (ca. 43 S/cm) and tensile strength (ca. 13 MPa) compared with BP-A-E (ca. 23 S/cm and 1.7 MPa). This can be attributed to the tighter entanglement and more uniform distribution of CNTs in BP-A-W (Fig. 6(a)). The conductivity and mechanical strength of BP-A-N lay between BP-A-W and BP-A-E on the basis of Table 2. Moreover, BP-B-W and BP-C-W also displayed much better electrical and mechanical strength compared with BPB-E and BP-C-E as compared in Table 2 and Fig. S (7). The tensile strength of BPs prepared from CNT/water dispersions were higher than the values of pristine BPs reported in previous literature (1e7 MPa) [29,40e42], and even higher than that of some commercial BPs (8 MPa) [43]. The much better comprehensive properties of BPs prepared from water dispersions can be put down to the good dispersion and standing stability of CNT/water dispersions. In general, the electrical conductivity and tensile strength of BPs made of the same CNTs decreased with increasing of the pore size and porosity, which was consistent with the result reported by Liu et al. [44]. In addition, it was surprised to find that BPs prepared from CNT/ water dispersions showed a larger water contact angle than BPs prepared from CNT/ethanol dispersions, especially for the case of CNT-B as listed in Table 2. BPs prepared from CNT/water dispersions were denser and shiny in appearance due to smaller pore size. The pore structures may affect the contact angle of BPs, and the sonication treatment in different dispersing media may change the hydrophilicity of CNTs. Because the test of contact angle was influenced by the porous structure of BPs and residual surfactant, the found value did not represent the contact angle that the single

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Fig. 4. Evolution of UVevis spectra and conductivity of CNT-A/water (a & d), CNT-A/ethanol (b & d) and CNT-A/NMP dispersions (c & d) as a function of standing time. The notes in Figure (a) and Figure (c) represent the standing time in days (d) and hours (h), respectively. All dispersions had a MWCNTs concentration of 0.4 g/L (dispersions are diluted by a factor of 100 for UVevis spectra).

nanotube form with water. The low contact angle of BPs has also been reported in the previous literature by Martinelli et al. [43].

integrating with both density and thickness of the BPs to evaluate the shielding performance more comprehensively. The specific SE of BP was calculated on the range of 8.2e18 GHz based on Eq. (7).

3.3.3. Electromagnetic shielding property of BPs The EMI shielding performance of BPs was measured from 8.2 to 18 GHz. Fig. 8(a) shows the EMI SE of BPs prepared with different dispersing media and MWCNTs. Obviously, the SE of BP-C-W was higher than that of BP-C-E, and similar results can be observed for BP-As and BP-Bs, such as BP-A-W > BP-A-E and BP-B-W > BP-B-E. Moreover, BP-C-W exhibited the highest SE (48.3 dB) due to its maximum conductivity and thickness, followed by BP-B-W (37.1 dB) and BP-A-W (33.7 dB). The combination of CNT-C1 and CNT-C2 made the electronic transmission of BP-C-W and BP-C-E more unobstructed, which led to a higher conductivity and a superior shielding performance. In addition, a concept of specific SE was introduced [45,46] by

 Specific SE ¼ SE ðr$tÞ

Fig. 5. Raman spectra of CNT-A and BPs made of CNT-A treated in different dispersing media. The MWCNT dispersions were sonicated for 25 min before they were used in the preparation of BPs.

(7)

where, r and t present the density and thickness of BP, respectively. The SE presents the average SE value on the range of 8.2e18 GHz, both of which are listed in Table 3. Similarly to the results of SE, the specific SE of BPs prepared from water system was higher than those from ethanol system as compared in Table 3. More importantly, a super high specific SE of 29,300 dB$cm2/g was obtained by BP-C-W due to its excellent SE, followed by BP-B-W (21,900 dB$cm2/g) and BP-A-W (19,900 dB$cm2/g). These values showed a great advantage compared with the reported polymer/ MWCNT composite (150e500 dB$cm2/g) [34,46], BPs (3000e14,000 dB$cm2/g) [14e17,30] and typical metals used in military and aerospace. BP-A-W and BP-A-E with various thickness were prepared to further investigate the influence of dispersing medium on SE of BPs. As shown in Fig. 8(b), BP-A-W with a thickness of only 30 mm presented an average EMI SE of 31.9 dB (99.9% attenuation [47]) on the range of 8.2e18 GHz. This SE value was 7% higher than that of BP-A-E (29.8 dB) with the same thickness. As the thickness of BPs was increased to 450 mm, the SE of BP-A-W increased to 101.7 dB coupled with an increment of 26% compared with 80.7 dB of BP-AE. In general, the SEs of BP-A-Ws were superior to BP-A-Es with the same thickness, and the difference between BP-A-W and BP-A-E increased with the increasing of thickness. It was worth noting that both BP-A-W and BP-A-E exhibited a great advantage over CNT composite films with larger thickness due to the considerably larger conductivity of BPs. As reported by Huang et al. [48], epoxy/ SWCNT composites present a SE of only 20e30 dB with a very high thickness of 2 mm and high filler loading of 15%. Moreover, the SE of BPs was almost independent of frequency in the test frequency band.

Y. Hu et al. / Materials & Design 184 (2019) 108175

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Fig. 6. The SEM images of BP-A-W (a), BP-A-E (b) and BP-A-N (c); pore size distributions of BP-A-W, BP-A-E and BP-A-N summarized from SEM images (d); typical stress-strain curves of BP-A-W and BP-A-E (e).

Table 2 Pore structure, mechanical strength and conductivity of various BPs. BP

N2 adsorption data Peak value (nm)

BP-A-W BP-A-E BP-A-N BP-B-W BP-B-E BP-C-W BP-C-E

36 40 12, 36 29 e 10, 36 e

BJH

Average pore size(nm) Specific surface area (m2/g) 113.2 105.5 116.7 209.8 e 179.7 e

SEM

Conductivity Strength Modulus (S/cm) (MPa) (MPa)

Thickness Porosity Contact (mm) (%) angle(o)

40 ± 2 23 ± 2 28 ± 2 42 ± 2 32 ± 2 54 ± 2 42 ± 2

27 ± 3 30 ± 3 29 ± 2 40 ± 2 41 ± 2 42 ± 4 47 ± 4

BET

Water Ethanol 32 ± 3 38 ± 5 30 ± 5 24 ± 3 29 ± 3 28 ± 3 32 ± 5

To further clarify the shielding mechanism of BPs, the absorption loss and reflection loss were analyzed using the S parameters (S11, S12, S21, S22) as shown in Fig. 8(c) and (d). BP had a porous network structure so that the incident microwaves could be

13 ± 1 1.7 ± 0.3 5.1 ± 0.5 17 ± 2 2.7 ± 0.5 10 ± 1 2.8 ± 0.5

830 ± 100 228 ± 30 600 ± 50 780 ± 100 236 ± 30 560 ± 40 150 ± 20

75.0 77.3 75.8 81.2 82.1 82.2 84.1

90 ± 5 68 ± 5 65 ± 5 80 ± 5 15 ± 5 78 ± 5 75 ± 5

0 0 0 0 0 0 0

reflected and scattered many times between MWCNT-air interfacial areas until being absorbed [7,36]. Besides, BP showed a greater reflection coefficient due to the large impedance difference between MWCNT and air interface. Take BP-A-W with a thickness of

Fig. 7. The N2 adsorption-desorption isotherms (a) and the pore size distribution calculated by BJH model (b) of BP-A-W, BP-A-E and BP-A-N.

Y. Hu et al. / Materials & Design 184 (2019) 108175

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Fig. 8. The SE of BPs with the same mass on the range of 8.2e18 GHz (a); the SE (b), SEA (c) and SER (d) of BPs (BP-A-W and BP-A-E) with the thickness of 30, 200 and 450 mm on the range of 8.2e18 GHz.

Table 3 Average and specific SE of BPs in 8.2e18 GHz. Sample

Average SE (dB)

Specific SE (dB$cm2/g)

BP-A-E BP-A-W BP-B-E BP-B-W BP-C-E BP-C-W

29.8 31.8 32.3 34.5 43.0 46.1

18,900 19,900 20,600 21,900 27,400 29,300

200 mm as an example. It showed an SEA of ca. 52.4 dB and SER of ca. 13.8 dB, which meant the contribution from SEA (79.2%) was much larger than that from SER (20.8%). This suggested BP-A-W exhibited an absorption-dominated shielding mechanism. In contrast, the SEA of BP-A-E (200 mm, 37.3 dB) was much lower than that of BP-AW (200 mm, 52.4 dB), but their SER values were almost the same. By comparing the SEA and SER of other pairs of BP-A-W and BP-A-E with the same thickness (Fig. 8(ced), Fig. S8(a)), it could be drawn that the difference of SEs between BP-A-W and BP-A-E was mainly caused by the SEA due to the difference in morphology and conductivity of BPs. Such a behavior was direct consequence of square root and logarithmic dependence of SEA and SER on conductivity, respectively [49,50]. In addition, the contribution of SEA increased with increasing of thickness for both BP-A-E and BP-A-W (Fig. S8(b)). In short, dispersion medium has a great influence on the dispersity of CNTs and properties of BPs. In view of the much better comprehensive properties of BPs, non-toxic and low-cost deionized water is the best dispersion medium for the preparation of CNT dispersions compared with ethanol and NMP.

4. Conclusion A series of MWCNT dispersions were prepared using water, ethanol and NMP as the dispersing media. MWCNTs exhibited

relatively good monodispersity and excellent dispersion stability in water, rather than in ethanol. BPs were prepared from the MWCNT/ water, MWCNT/ethanol and MWCNT/NMP dispersions by vacuum filtration following the same procedure. Compared with BPs prepared from ethanol dispersions, those obtained from water dispersions presented significantly improvements in conductivity and mechanical strength. All these was ascribed to the good dispersion and standing stability of MWCNT/water dispersions. More importantly, BP-A-W with the thickness of 200 and 450 mm exhibited the absorption-dominated SE values of 66.2 and 101.7 dB, respectively, which were 30.6% and 26% higher than that of BP-A-E with the same thickness. In addition, super high specific SE of 19,900, 21,900 and 29,300 dB$cm2/g were achieved by BP-A-W, BP-B-W and BP-CW, respectively. These specific SE values were more competitive than other kinds of BPs or BP composite films (3000e14,000 dB$cm2/g) [14e17,30]. The hybrid of CNT-C1 and CNT-C2 in the preparation of BPs showed a highly improved comprehensive performance compared to BPs from pure CNT-A or CNTeB. In future work, the modification aiming at improving the impedance matching and maximum attenuation over a high frequency range will be conducted, the results of which will be discussed in detail. Declaration of competing interest There are no conflicts of interest to declare.

CRediT authorship contribution statement Yunping Hu: Conceptualization, Methodology, Validation, Formal analysis, Writing - original draft, Writing - review & editing.Dongcheng Li: Formal analysis, Resources, Writing - review & editing.Ping Tang: Resources, Project administration.Yuezhen Bin: Resources, Supervision, Writing - review & editing. Hai Wang: Resources, Writing - review & editing.

Y. Hu et al. / Materials & Design 184 (2019) 108175

Acknowledgements The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21374014 and U1663226). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matdes.2019.108175.

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