Constructing large interconnect conductive networks: an effective

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Constructing large interconnect conductive networks: an effective approach for excellent electromagnetic wave absorption at gigahertz Huanqin Zhao, Yan Cheng, Xiaohui Liang, Youwei Du, and Guangbin Ji Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05141 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 1, 2018

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Industrial & Engineering Chemistry Research

Constructing large interconnect conductive networks: an effective approach for excellent electromagnetic wave absorption at gigahertz Huanqin Zhaoa, Yan Chenga, Xiaohui Lianga, Youwei Dub, Guangbin Jia, * a

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics,

Nanjing 210016, China b

Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

*Corresponding Author: Prof. Dr. Guangbin Ji. Tel: +86-25-52112902; Fax: +86-25-52112626 E-mail: [email protected] Address: 29# Yudao Street, Nanjing 210016, P.R China

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ABSTRACT Recent years, with the development of electronic equipment, fabricating lightweight and effective absorber to prevent electromagnetic (EM) wave pollution has been an imperative mission. Among diverse innovative strategies, constructing large interconnect conductive network has been an effective approach to dissipate EM wave. Herein, Co/C hybrids with twisty carbon nanotubes (CNTs) were prepared from melamine-formaldehyde resin through a polymerization method together with subsequent calcination process. The resultant Co/C hybrids were strung into an interconnected framework by long and twist CNTs, which exhibited remarkable microwave characteristic at low loading content of 20 wt% in wax matrix. The reflection loss (RL) intensity of -43 dB was achieved with an effective frequency bandwidth (RL< -10 dB) of 4.45 GHz at 1.85 mm. Such excellent absorbing properties at lower filler loading and thin thickness endow the Co/C hybrids with significant potential for application in attenuation EM wave energy. Key words: conductive network, carbon nanotubes, lightweight, dielectric loss, electromagnetic absorption

1. INTRODUCTION In order to prevent harm and obsession from the EM radiation for human beings, exploring and finding of the superior absorber have been an essential mission.

1, 2

It is well known that ideal electromagnetic

wave absorption (EMA) materials need to satisfy five criterions, that is “broad”, “lightweight”, “thin”, “strong”, and “low cost”. Based on the above-mentioned standards, there are countless efforts to fabricate remarkable absorber.

3-5

These absorbers mainly involve dielectric materials such as MnO2, 6 , 7 TiO2, 8, 9

ZnO, 10-12 SnO2, 13, 14 CuS, 15 and BaTiO3 16 et al., and magnetic media relating to Fe, 17 Co, 18 Ni, 19 FeCo, 20, 21

and Fe3O4, 22 and so on. Generally, the above mentioned materials own high permittivity (εr) or high

permeability (µr), which can efficiently attenuate EM wave through internal electric and/or magnetic dipoles interacting with incoming alternating electromagnetic field.


23

However, εr and µr values of their

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wax/epoxy composites usually are low at gigahertz region.

24

For solving the fatal drawback, it is needed

to increase the filler content (50-70 wt%) of mediums in composites, which not only result in the increase of expense and weight for composites, but also damage their mechanical characteristics.

25-27

In recent

years, conductive materials such as graphene,28, 29 CNTs, 30-32 conductive polymers, 33 mesoporous carbon, 34

etc. have attracted tremendous attention because of their low density and well conductive characteristic.

Especially, the CNTs with one-dimensional cylinder structure can form interconnected conductive network which provides high-efficiency electronic transmission paths. Moreover, the induced current along with CNTs at alternating EM field is contributed to the reinforced interface polarization, being beneficial to EM wave dissipation. By virtue of these unique advantages, CNTs has been taken as an optional media to design superior absorber. For example, Cao’ group synthetized 3D Fe3O4-MWCNTs nanostructure through a co-precipitation method and investigated its EMA properties at filler content of 20 wt%. The enhanced imaginary part of permittivity was put down to the electrically conductive network built by CNT aggregations, which played a dominant role in final EMA performance.

35

Chen and his

coworker reported the synthesis of ultralight multi-walled carbon nanotube (MWCNT)/graphene foams (CGFs) composites. The formation of interlaced network of MWCNT endows the CGFs with high bulk electrical conductivity and polarization loss, leading to impressive EMA properties in C-band and X-band. 36

Yin et al. designed Co−C/MWCNTs composite by pyrolysis of a ZIF-67/MWCNTs precursor

exploiting the MWCNT as templates, and the RL of the composites could reach -47.5 dB at filler content of 15 wt%. This study emphasized the significant effect of a conductive meshwork on enhancing the EMA properties.

37

But these CNTs materials are expensive and usually require complicated

functionalization process, constraining the practical application of CNTs composites. Hence, it is highly desirable to develop other strategies to construct CNTs-based absorber with excellent EMA properties on a large scale and low-cost. So far, Liu et al. synthesized a porous CNTs/Co composite by in situ pyrolysis of ZIF-67 under Ar/H2 atmosphere and obtained the strong EMA capability with RL values of -60.4 dB.


38


the CNTs derived from ZIF-67 are short with a diameter of 10~40 nm. Herein, we synthesized the Co/C hybrids with twist spiral-like CNTs by heating treatment of Co2+/melamine-formaldehyde resin precursor prepared by polymerization process. This synthetic method is facile, inexpensive well as highly-productive. Moreover, the obtained long and distorted CNTs were intertwined into large interconnect conductive network. More interestingly, as Co/C hybrids was mixed with paraffin at a mass percentage of 20%, a RL value of -43 dB with effective bandwidth of 4.45 GHz was achieved at a coating thickness of 1.85 mm, and the strongest EMA peak can reach to -50.6 at 2.35 mm. These outstanding properties mean its underlying application as a lightweight and high-performance EMA material.

2. EXPERIMENTAL SECTION 2.1. Materials Melamine (C3H6N6, CP, Sinopharm Chemical Reagent Co.), Formaldehyde (CH3OH, 37wt%, Nanjing chemical reagent Co., LTD.), Cobaltous Nitrate Hexahydrate (Co(NO3)2·6H2O, 99%, Nanjing chemical reagent Co., LTD.), OP-10 (AR, Wuxi yatai united chemical engineering Co., LTD) and Hydrochloric acid (HCl, 36 wt%, Nanjing chemical reagent Co., LTD.) were used without further purification. 2.2. Synthesis of melamine-formaldehyde resin/Co2+ (MF/Co2+) composites In a typical synthesis, 75 mmol of melamine, 196 mmol of formaldehyde were mixed in 40mL deionized water and mechanically stirred for 30 min at 80 ºC to obtain a transparent solution, denoted as A solution. Meanwhile, 7.5 mL of OP-10 (10wt% aqueous solution) and Co(NO3)2·6H2O (3 mmol) were mixed in 40 mL deionized water and stirred at 50 ºC for 1 h (named as B solution). When the A solution was cooled down to 50 ºC, which was added dropwise into B solution whilst stirring at 50 ºC. Afterwards, the pH value of above mixtures was


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adjusted to 4.5 through 2 mol/L HCl solution, and the mixtures retain stir for 2 h. The resultant MF/Co2+ composites were collected by vacuum filtration, and dried at 60 ºC in vacuum oven. 2.3. Preparation of Co/C hybrids The Co/C hybrids was obtained by heating treatment of MF/Co2+ composites at 800 ºC for 2 h with heating rate of 2 ºC /min in Ar atmosphere, denoted as S1. For comparison, the S0 was synthesized through the same method without adding Co(NO3)2·6H2O. 2.4. Characterization The crystal structure of the obtained sample was identified by X-ray diffractometer (XRD, Bruker D8 ADVANCE) using Cu-Kα as the irradiation source (λ= 1.54 Å, 40 kV, 40 mA). The morphology and microscopy structure was observed by field emission scanning electron microscopy (FE-SEM, Hitachi S4800) and transmission electron microscopy (JEOL JSM-2010). Raman spectra were detected on a Raman spectrometer (Renishaw Invia). XPS analysis was performed in a PHI 5000 Versa Probe systems with an Al Kα X-ray source at 150 W. A vibrating sample magnetometer (VSM, Lakeshore, Model 7400 series) was employed to measure the magnetic behavior of sample under an applied magnetic field of 10 kOe. The content of Co was determined by thermal gravimetric analyzer (NETZSCH STA 449F3) under air atmosphere from 23 to 900 °C. The electromagnetic parameters of all samples were measured by Agilent PNA N5244A vector network analyzer ground on the coaxial-line method. The obtained samples were homogeneously dispersed in the paraffin matrix with different mass ratio and then pressed into toroidal-shaped composites (Φin: 3.04 mm, Φout: 7.0 mm) for microwave measurement.

3. RESULTS AND DISCUSION



Figure 1. Schematic diagram of the synthesis procedure for Co/C hybrids. The synthetic process of Co/C hybrids was displayed in Figure 1. The whole reaction process could be divided into three stages. Firstly, the hydroxymethylation reaction between melamine and formaldehyde, that is, the H atoms in the NH2 groups of the melamine was replaced by the methylol groups (–CH2OH) to synthesize the methylolmelamines.

39

Secondly, these methylolmelamines were cross-link via

methylene linkages and aggregated to form the small MF resin cluster around the surface of surfactant micelle.

40

Meanwhile, the Co2+ was incorporated into the cluster. At last, the Co/C hybrids could be

obtained by heating treatment of MF/Co2+ resin at 800 ºC with heating rate of 2 ºC /min. The formation of CNTs is attributed to the Co nanoparticles (NPs) catalyst effects. 38 The morphology and microstructure of the Co/C hybrids and its precursor were observed by FE-SEM. It can be observed clearly that the MF/Co2+ composites prepared by polymerization were composed of smooth-faced spherical particles with a diameter of 0.7-3.6 µm (Figure 2a). After thermal treatment process, high density of long and curved spiral-like CNTs distribute inhomogeneously on the spherical particles surface. These long CNTs are intertwined and constructed an interconnected network benefiting electron hopping and migration, as shown in Figure 2b, c. In addition, as can be seen in the Figure S1, the sample S0 consists of many fragments, which further reveals that the catalyst effect of metal on the


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forming of CNTs. The EDS spectrum demonstrates the presence of C, N, O and Co elements in the Co/C hybrids (Figure 2d).

Figure 2. SEM image of (a) precursor for S1 and (b, c) S1, and (d) EDS analysis of S1.

Figure 3. (a, b) TEM images, (c) HRTEM images, and (d) SAED patterns of S1. From TEM images of the typical Co/C hybrids (Figure 3a, b), it is demonstrated that Co NPs dispersed randomly in the inner or periphery of CNTs, and the thick diameter of CNTs are about 92 nm. In the Figure 3c, two lattice fringes with a distance of 0.344 nm and 0.205 nm are corresponded well to the



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graphitization carbon and the (111) planes of the cubic cobalt, respectively. The selected area electron diffraction (SAED) pattern of the Co/C hybrids was displayed in Figure 3d. The four diffraction rings could be well matched with the (111), (200), and (220) planes of fcc Co and the (002) plane of graphitic carbon, separately. These TEM images confirm the formation of CNTs-wrapped Co NPs composites. For identifying the crystalline structure of carbon and cobalt in S1 and S0 samples, the XRD measurement and Raman spectra were carried out. As shown in Figure 4a, the S0 exhibits two broad peaks at 2θ about 26.5° and 44°, which are ascribed to the amorphous state of carbon.

41

When the Co2+

was introduced into the reaction system, the new signals emerge after the heat treatment process. The peaks at 2θ = 44°, 51° are indexed to the (111) and (200) planes of cubic phase cobalt (JCPDS 01-1259), respectively, which is in accordance with the aforementioned results of TEM. Raman spectra (Figure 4b) further disclose the carbon crystal state. Clearly, the two samples display the two cognizable peaks. The signal centered at about 1350 cm-1 is taken as the disorder-induced D band, and the G band locates at 1590 cm-1, meaning the sp2 hexagonal graphitic lattice. 42 The relative intensity proportion of D band and G band (ID/IG) is used to evaluate the graphitization degree of carbon component. For S1 sample, the value of ID/IG is 1.01, lower than that of S0 (ID/IG =1.12), meaning that the S1 possess a higher degree of graphitization or less disorder sites. Moreover, the occurrence of 2D band centered at 2687 cm-1 for S1 further manifests that the carbon component has a high crystallinity because of the catalytic graphitization effect of metal cobalt on carbon at high calcination temperature. The improved graphitizing degree will help to enhance EMA properties. The content of cobalt in S1 sample was measured by TG analyzer, as presented in the Figure 4c. At the beginning stage (below 200 ºC), the mass loss of 10.05 wt % is the consequence of water evaporation. 43 Then, the burning of substantial carbon constituent under air leads to the rapid decline of weight with around 84.5 wt% (within a temperature zone of 200 ~ 730 ºC). At last, the residual was Co3O4 because the metal Co NPs could be oxidized at elevated temperature under air. 38 The content of Co element in S1, hence, is less than 5%, which is in favor of reducing the weight density of absorber.


♥

S0 S1

10

20

30

C100

40 50 60 2θ θ (degree)

70

80

Ms (emu/g)

60 ~84.5

20 0 200

400

2D

ID/IG=1.12

1000

1500 2000 2500 Raman shift (cm-1)

3000

D8

80

0

ID/IG=1.01

90

~ 10.05%

40

S0 S1

600

800

Temperature (℃ )

6 4 2 0 -2 -4 -6 -8

Ms = 8.05 emu/g

Hc = 252 Oe 4

Ms (emu/g)

Intensity (a.u.)

200

002

♥

♣

B

Co: PDF#01-1259

111

A

Weight (wt%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u.)

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2 0 -2 -4

-1

-10 -8 -6 -4 -2

0

2

0 Hc (kOe)

4

6

1

8 10

Magnetic Field (kOe)

Figure 4. XRD profiles (a) and Raman spectra (b) of S0 and S1, TG curve (c) and hysteresis loop of S1 (d). The hysteresis loop of S1 was exhibited in Figure 4d. The magnetic performance is clearly demonstrated by visible hysteresis loop. The saturation magnetization (Ms) and coercive force (Hc) values are 8.05 emu/g and 252 Oe, separately, which could lead to magnetic loss in gigahertz frequency region. 43

The elemental composition and chemical state of the S1 sample was analyzed by XPS spectra. Figure 5a displays the XPS total survey spectrum. One can observe that the S1 is mainly consist of C, N, O and Co elements, which agrees well with the analyses of EDS. Figure 5b shows the high resolution C 1s spectra, the C element presents three kinds of chemistry valence, the peak centered at 284.8, 285.6 and 288.1 eV are attributed to the C-C/C=C, C-O, and C=O species, respectively. 38, 44 The N 1s spectrum was deconvoluted into five signals (Figure 5c), recognized as pyridinic nitrogen (398.4 eV), pyrrolic nitrogen (399.9 eV), quaternary N (400.8 eV), oxidized nitrogen (401.89 eV) and chemisorbed nitrogen (404.58



eV). 45-47 Among them, the pyridinic nitrogen had a non-equivalent sp2-hybridization and linked with the two adjacent C atom. Pyrrolic nitrogen has an equivalent sp2-hybridization linked with two C atom and [48]

one H atom; Quaternary N replaced the C atoms in inside of the graphene.

The incorporating of

nitrogen in CNT results in the destruction of the perfect sp2 hybridization of the carbon atoms. In addition, the O element is present in the forms of oxygen functional groups such as hydroxyl and carbonyl groups. These defects and groups in the C matrix caused by the N and O atoms may result in defect polarization relaxation and electronic dipole polarization relaxation because of their different [49]

electronegativity, which is favorable to attenuating EM wave.

The binding energy peaks of cobalt

nanoparticle was presented in Figure 5d, the peaks at around 779.9 and 795.3 eV are the characteristic feature of Co2+ 2p3/2 and Co2+ 2p1/2, respectively, and the other signals are satellite peaks. It is clarified that the metal cobalt on the surface of S1 sample was oxidized in air atmosphere.

B

A

C-C/C=C C-O C=O

Co 2p

O 1s

200 300 400 500 600 700 800 900 Binding Energy (eV)

C

Pyridinic N Pyrrolic N Quaternary N Oxidezed N Chemisorbed N

396

399

402

405

408

Binding Energy (eV)

411

Intensity (a.u.)

N 1s

280 282 284 286 288 290 292 294 296 298 Binding Energy (eV)

D

Co 2p3/2

Intensity (a.u.)

Intensity (a.u.)

C 1s

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Co 2p1/2 Sat.

Sat.

780

790

800

810

Binding Energy (eV)

Figure 5. (a) XPS total survey spectrum of S1, (b) high resolution of C 1s XPS spectra, (c) high resolution of N 1s XPS spectra, and (d) the Co 2p XPS spectrum.


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The electromagnetic parameters including complex permittivity (εr =ε′ - jε″) and complex permeability (µr = µ′ – jµ″) play a dominate role in EMA properties of absorber. It is well known that the real permittivity (ε′) and the real permeability (µ′) stand for the storage capacities of electric and magnetic energy, respectively, and the imaginary permittivity (ε″) and the imaginary permeability (µ″) represent the competencies for dissipating electric and magnetic energy.

50

Figure 6 displays the measured EM

parameters of four wax composites, including the sample S1 uniformly mixed with wax with different mass percentage of 10, 20 and 30 wt% (denoted as 10% S1, 20% S1, 30% S1,), as well as the composite with a loading of 20 wt% S0 (denoted as 20 % S0). For 20 % S0, the ε′ value was almost constant about 7.0 in the range of 2 - 18 GHz. Such low value means the poor storage ability for electric energy (Figure 6a). Compared with 20% S0, the ε′ of S1-wax composites increases obviously. As the mass percentage of S1 increases from 10 to 30 wt% in the paraffin matrix, the ε′ values change from 7.0 to 23, illustrating the enhanced storage ability of electric energy in order of 10% S1, 20% S1 and 30% S1. The enhanced tendency is complied with the effective media theory. 5 While the ε″ values vary from 0.62 to 1.16, 0.14 to 0.87, 3.2 to 7.9, 4.7 to 16.6 for 20 % S0, 10% S1, 20% S1 and 30% S1, respectively (Figure 6b). According to the anterior theory, it can be inferred that the attenuation ability to electric energy for the 20 % S0 and 10% S1 composites are inappreciable, while stronger for 20% S1and 30% S1 appears. Meanwhile, the obvious fluctuation can be found in the curves of ε′ and ε″, especially for 20% S1 and 30% S1. This phenomenon is mainly accounted for the electric polarization and electrical conductive. The complex permeability of all composites is presented in Figure 6c, d. the values of µ′ vary in the range of 0.93 - 1.08, 0.94 - 1.05, 0.94 - 1.18, 0.96 - 1.15 for 20% S0, 10% S1, 20% S1, 30% S1, respectively. It can be observed that the permeability values of the four composite were no significant differences, which is mainly attributed to the low content of Co for S1, and even no Co for S0. The similar results could be found in the µ″ curve. Additionally, for 20% S1 and 30% S1, the obvious negative µ″ can be observed, this can be considered as the induced magnetic energy going out of the absorber as explained by Zhao et al. 24



10% S1 20% S1 30% S1 20% S0

2

4

6

C1.25 Real part of permeability

B 16 Imaginary part of permittivity

24 22 20 18 16 14 12 10 8 6 4 8 10 12 14 Frequency (GHz)

16

18

1.15 1.10 1.05 1.00 0.95 2

4

6

8

10

12

14

8 4 1.2 0.8 0.4 0.0 4

6

8 10 12 14 Frequency (GHz)

D0.15

10% S1 20% S1 30% S1 20% S0

1.20

10% S1 20% S1 30% S1 20% S0

12

2

Imaginary part of permeability

Real part of permittivity

A

16

18

0.10 0.05 0.00 -0.05 -0.10 -0.15 -0.20 -0.25 -0.30

16

18

16

18

10% S1 20% S1 30% S1 20% S0

2

4

6

Frequency (GHz)

8

10

12

14

Frequency (GHz)

Figure 6. Frequency dependence of real permittivity (a), imaginary permittivity (b), real permeability (c), and imaginary permeability of four composites (d).

0.8

B 0.15

0.7

0.10

0.6

0.05

0.5

tan δµ

A tan δε

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 10% S1 20% S1 30% S1 20% S0

0.3 0.2 0.1

0.00 -0.05 -0.10

10% S1 20% S1 30% S1 20% S0

-0.15 -0.20

0.0

-0.25 2

4

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8

10

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18

2

4

Frequency (GHz)

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8

10

12

14

16

18

Frequency (GHz)

Figure 7. The tangent dielectric loss (a) and tangent magnetic loss (b) of four composites. In order to know the EMA mechanism, the dielectric loss and magnetic loss of all composites were further assessed by the loss tangent, i.e., the dielectric loss tangent (tan δε = ε″/ε′) and the magnetic loss tangent (tan δµ = µ″/µ′). Figure 7 depicts the frequency dependence of tan δε and tan


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δµ for all the composites. As a whole, the magnetic loss tangent of all composites is much lower than that dielectric loss tangent, demonstrating that the dielectric loss plays a dominate role in the EMA properties. Moreover, the tan δε values are great difference among them. In detail, 10% S1 and 20% S0 both have low tan δε value (below 0.15), while it is interesting that the 20% S1 and 30% S1 composites possess high tan δε value, fluctuated from 0.33 to 0.63 and 0.39 to 0.77, respectively. It is concluded that the 20% S1 and 30% S1 have a better capacity for dielectric loss.

Figure 8. The plots of ε″ versus ε′ for (a) 10% S1, (b) 20% S1, (c) 30% S1 and (d) 20 % S0. To further uncover the mechanism of dielectric loss for all composites, the real permittivity as functions of imaginary permittivity were presented in Figure 8. According to the Debye equation, the ε″ and ε′ can be written as: 51

ε' = ε∞ +

εs − ε ∞

1 + (2πf ) τ 2 2


(1)


ε' ' =

2πfτ (εs − ε∞ ) 1 + (2πf ) τ 2

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(2)

2

Where f is the frequency, εs is the static permittivity, ε∞ is the relative dielectric permittivity at the high-frequency limit, and τ is polarization relaxation time.

5

Combined the Eqns (1) and (2), the

association between ε′ and ε″ can be inferred. 2

  ε − ε∞  ε + ε∞  2  ε' - s  + (ε' ' ) =  s  2 2    

2

(3)

Hence, based on the Eqn (3), It can be inferred that the curve of ε′ versus ε″ for absorber should be a hemicycle during the Debye relaxation process, and the hemicycle is generally named as the Cole-Cole semicircle. 52 From Figure 8, it is clearly seen that the ε′ - ε″ plots of S1-wax composites show obvious overlapped semicircle, but invisible in S0 (Figure 8d), indicating that the contribution of Debye relaxation polar on the dielectric loss for S0 is negligible. In the meantime, one semicircle is found in the 10% S1, six in the 20% S1 and three in the 30% S1, respectively, (Figure 8a, b and c) which stands for the contribution of the Debye relaxation polar to the enhanced dielectric characteristics of S1-wax composites. In addition, the Cole-Cole semicircles are distorted, meaning that apart from the Debye relaxation, other kinds of relaxations, such as Maxwell-Wagner relaxation and electron polarization, may be occurring in the S1-wax composites.

13

For Co/C hybrids-wax composites, the existence of heterogeneous media,

containing Co-CNTs, CNTs-paraffin, Co-paraffin and CNTs-CNTs interfaces, could result in interfacial polarization owing to the amassing of electric charges at these interfaces. According to the analyses of XPS and Raman, there are small amounts of N, O atoms and defect carbon in the S1 sample, which can play the role of polarized center in the presence of external electromagnetic field, being beneficial for the dielectric properties of absorber. Excepted for the polarization relaxation induced dielectric loss, the conductive ability is also a critical factor to impact the dielectric loss. According to the Eqn (4):


ε' ' =

εs − ε∞ σ ωτ + 2 2 ωε 0 1+ωτ

(4)

where σ, ω, and τ is electrical conductivity, angular frequency and relaxation time, respectively. 38 Eqn (4) infers that the ε″ is in proportion to the conductivity of composites. The conductive values of all composites are shown in the Figure 9. It is seen that the electrical conductivity of 20 % S0 and 10 % S1 are smaller than that of 20 % S1 and 30 % S1 at different radar frequency bands. This result can be interpreted by the components and microstructure of composites. In detail, the low graphitization degree of S0, having been demonstrated by the previous Raman analysis, results in the weak conductivity. For the composite 10% S1, the linked conductive network has not been formed because of such low loading concentration, leading to its poor conductivity. The high-concentration of S1 with CNTs network

0.05

0.11

nd -ba u nd K ba Xnd ba Cd an S-b

0.18

20 20

10

%

%

30 % %

8 7 6 5 4 3 2 1 0

σ//S.cm −1

0.32

1.63

0.70

1.27

2.75

3.29

4.56

4.96

5.30

7.30

8.02

structure in paraffin endows 20% S1 and 30% S1 with high conductivity.

0.19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.43

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S1

S1

S0

S1

Figure 9. The electrical conductivity values of all composites at different radar frequency bands. (σ was calculated by the equation: σ = πε0ε″f, ( ε0 = 8.55 ×10 −12 F/m). 53 Based on the above EM parameters analyses, the microwave absorption performances of four composites were evaluated, and the results were displayed in Figure 10. As shown in Figure 10a, c, for 10% S1 and 20% S0, almost no RL value is less than –10 dB. This result indicates poor EM wave



properties. When the mass content of S1 is 20 wt %, the optimal microwave absorption is that the RL value can reach -43 dB with effective frequency bandwidth (fe) of 4.45 GHz at 1.85 mm. Further increased the amount of S1 to 30 wt% in the paraffin matrix, the optimal refection loss is -18.5 dB with fe of 3.8 GHz at the thickness of 1.45 mm. The change tendency of RL is different from that of the dielectric properties. In theory, high ε″ and tan δε predict strong EM energy dissipation in the wax composites. However, it does not mean that a superior reflection loss can be obtained. For example, further increasing the S1 proportion in paraffin to 30 wt%, ε″ and tan δε increase a lot in comparison with 20 wt% filler content, but the RL becomes even worse than the 20% S1 on the contrary. The reason is that unilateral excessive increase of εr brings out the impedance mismatch. Therefore, a moderate complex permittivity is in favor of impedance match, which provide the advantages for more EM wave entering into the interior of materials.

54

In Figure S2, the coefficient of standard input impedance (|Zin/Z0|) for all

composites is given. In general, the |Zin/Z0| value is near to 1 in a broad frequency range, which is needed for the ideal impedance match. It is evident that 20% S1 exhibits better impedance matching than other composites. Among all the composites, hence, the 20 % S1 composite exhibits strong RL properties benefiting from optimal impedance match together with medium loss abilities. Figure 11 shows the possible dissipate EM energy mechanism in Co/C hybrids. When the EM wave is radiated into the interior of composite, EM energy is transformed to the heat energy by enhanced dielectric loss and weak magnetic loss. The reason for the enhanced dielectric loss can be attributed the follow items. (i) The micro-current. It was resulted from the large conductive framework constructed by long and twist CNTs (Figure 11a); (ii) The interfacial polarization. It was arisen from the multicomponent presence of Co, C and paraffin in composite (Figure 11b); (iii) The dipole polarization. It was caused by the existence of defects, doping atom N and oxygenic functional groups (Figure 11c). In addition, the weak magnetic loss originates from the low content of metal Co (Figure 11b).


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RL (dB)

-10 1.45mm 1.65mm 1.85mm 2.05mm 2.35mm 2.65mm 2.95mm 3.15mm

-20 -30 -40 -50

1.85 1.65

6

D5 Thichness (mm)

2.05

4

8 10 12 14 Frequency (GHz)

4

1

2 3 fe (GHz)

4

18

tm simλ/4 t expλ/4 m

3 2

1.45 0

16

♦

20% S1 30% S1

2

♦

C2.25

2.25

♦ ♦

1.65 1.85 2.05 Thickness (mm)

♦

RL (dB)

0

♦

-35 -30 -25 -20 -15 -10 -5 0

1.45

Thichness (mm)

B

10% S1 20% S1 30% S1 20% S0

♦

-45

A -40

♦

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


♦

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1 2

4

6

8

10

12

14

16

18

Frequency (GHz)

Figure 10. (a) RL values vs different thickness of all composites, (b) RL peak for 20 % S1 composite with different thickness, (c) effective frequency bandwidth at different thickness of 20 % S1and 30 % S1, and (d) dependence of matching thickness (tm) on matching frequency (fm) of 20 % S1 at the wavelengths of λ/4. Figure 10b exhibits the specific absorption peak value and effective frequency bandwidth of 20 % S1 with different thickness. It is worth mentioning that RL value of -50.6 dB is fulfilled with a film thickness of 2.35 mm. In addition, the maximum RL peak moves toward the low-frequency region with thickness increasing from 1.45 to 3.15 mm. This phenomenon can be interpreted by one-fourth wavelength equation, that is tm = nλ/4 = nc/(4fm((｜µr｜｜εr｜)1/2) (n = 1, 3, 5 ….), where tm and fm represent the matching thickness and the matching frequency relating to the maximum RL peak, separately.

28

When the

relationship between absorption frequency and thickness obeys the above criteria, the two reflected EM waves from the air-absorber interface and absorber-conductive background interface are out of phase by 180°, leading to a disappearance of them on the absorber-air interface. In this case, the RL reaches the maximum value. Figure 10d plots the simulation of tm (denoted as tmsim) vs fm for 20 % S1 composites, based on the above equation. Meanwhile, the experimental matching thickness (tmexp) corresponding to



the maximum RL value was extracted from the RL curve in Figure 10b, and marked as the blue rhombus. It is interesting to find that the tmsim value and tmexp value are almost equivalent at the same frequency, which concludes that the relationship between the matching thickness and peak frequency for the EM wave absorption of the 20% S1 obeys the quarter-wavelength matching conditions. Moreover, the satisfying absorption performance can be achieved through modulating the thickness of absorber.

Figure 11. Scheme of primary EM wave attenuation process in Co/C hybrids-paraffin composites (a) conductive network formed by CNTs (b) interfacial polarization (c) dipole polarization. 4. CONCLUSIONS In summary, we present a feasible method to prepare the Co/C hybrids with long and high density spiral-like CNTs. These CNTs construct a giant interconnect network, providing an effective pathway for electron migration and hopping. Benefiting from the large conductive framework, interfacial polarization as well as dipole polarization, the Co/C hybrids showed remarkable EMA performances because of the enhanced dielectric loss. When the Co/C hybrids were mixed uniformly with wax with a mass percentage of 20 wt%, the optimal microwave property is -43 dB with effective bandwidth of 4.45 GHz at 1.85 mm. At the same time, the


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maximum reflection loss of -50.6 dB can be obtained at 9.4 GHz with a matching thickness of 2.35 mm. This work provides an efficient method for designing new types of high-performance EMA materials. Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.xxx. Additional SEM image of S0 sample and plots from the｜Zin/Z0｜coefficients of the obtained composites.

ACKNOWLEDGMENT Financial supports from the National Nature Science Foundation of China (No. 11575085), the Qing Lan Project, Six talent peaks project in Jiangsu Province (No.XCL-035) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) are gratefully acknowledged. Conflicts of interest There are no conflicts to declare. REFERENCES (1) Han, M.; Yin, X.; Hou, Z.; Song, C.; Li, X.; Zhang, L.; Cheng, L. Flexible and Thermostable Graphene/SiC Nanowire Foam Composites with Tunable Electromagnetic Wave Absorption Properties. ACS Appl. Mater. Interfaces 2017, 9, 11803−11810. (2) Cheng, Y.; Meng, W.; Li, Z.; Zhao, H.; Cao, J.; Du, Y.; Ji, G. Towards outstanding dielectric consumption derived from designing one-dimensional mesoporous MoO 2/C hybrid heteronanowires. J. Mater. Chem. C 2017, 5, 8981−8987. (3) Qiang, R.; Du, Y. C.; Zhao, H. T.; Wang, Y.; Tian, C. H.; Li, Z. G.; Han, X. J.; Xu, P. Metal organic framework-derived Fe/C nanocubes toward efficient microwave absorption. J. Mater. Chem. A 2015, 3, 13426−13434.



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Table of Contents

Large interconnected conductive network was constructed for enhanced dielectric loss abilities.


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Constructing large interconnect conductive networks: an effective

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