Facile synthesis of cobalt-zinc ferrite microspheres decorated nitrogen-doped multi-walled carbon nanotubes hybrid composites with excellent microwave absorption in the X-band

Facile synthesis of cobalt-zinc ferrite microspheres decorated nitrogen-doped multi-walled carbon nanotubes hybrid composites with excellent microwave absorption in the X-band

Composites Science and Technology 184 (2019) 107839 Contents lists available at ScienceDirect Composites Science and Technology journal homepage: ht...

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Composites Science and Technology 184 (2019) 107839

Contents lists available at ScienceDirect

Composites Science and Technology journal homepage: http://www.elsevier.com/locate/compscitech

Facile synthesis of cobalt-zinc ferrite microspheres decorated nitrogen-doped multi-walled carbon nanotubes hybrid composites with excellent microwave absorption in the X-band Ruiwen Shu a, b, *, Yue Wu a, Zhenyin Li a, Jiabin Zhang a, Zongli Wan a, Yin Liu c, Mingdong Zheng a, ** a

School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China School of Earth and Environment, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China c School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China b

A R T I C L E I N F O

A B S T R A C T

Keywords: Carbon nanotubes Hybrid composites Functional composites Interface Magnetic properties

Herein, nitrogen-doped multi-walled carbon nanotubes/cobalt-zinc ferrite (NMWCNTs/Co0⋅5Zn0⋅5Fe2O4) hybrid composites were synthesized through a facile one-step solvothermal route. Results of morphology observations revealed that Co0⋅5Zn0⋅5Fe2O4 microspheres were uniformly loaded on the surface of NMWCNTs and threedimensional (3D) conductive networks were in-situ constructed by the entanglement of NMWCNTs in the asprepared hybrid composites. Moreover, the influence of contents of NMWCNTs on the electromagnetic param­ eters and microwave absorption properties of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4/paraffin wax composites were elab­ orately investigated. It was found that the obtained hybrid composites demonstrated superior microwave absorption performance in the X-band. Remarkably, the minimum reflection loss reached 64.7 dB with a matching thickness of 3.1 mm and effective absorption bandwidth achieved 4.3 GHz (11.7–16.0 GHz) with a thickness of merely 2.1 mm. Furthermore, a dual-band (C and Ku bands) microwave absorption characteristic was observed in the obtained hybrid composites. Besides, the microwave absorption properties of as-prepared hybrid composites could be facilely tuned by changing the matching thicknesses and contents of NMWCNTs. The superior microwave absorption properties of obtained hybrid composites mainly originated from the syn­ ergistic effects of magnetic loss, conduction loss and dielectric loss, and optimized impedance matching. It was believed that our results could be helpful for the structural design and facile fabrication of 3D MWCNTs-based hybrid composites as high-efficient microwave absorbers.

1. Introduction With the increasingly serious problems of electromagnetic interfer­ ence (EMI) and electromagnetic pollution originated from the extensive use of electronic equipment and devices, EMI shielding materials have drawn considerable interests in the field of functional materials [1,2]. In the past few decades, tremendous efforts have been devoted to con­ structing high-efficient EMI shielding materials [2–5]. Among them, conductive polymer composites (CPCs) consist of conductive fillers and polymer matrix, which have attracted much attention owing to their facile processability, low density and high EMI shielding effectiveness (SE) [3–5]. For example, Gu et al. fabricated a series of epoxy

resins-based CPCs such as reduced graphene oxide with honeycomb structure (rGH)/epoxy composites [3], three-dimensional Fe3O4 deco­ rated carbon nanotubes/reduced graphene oxide foam/epoxy (3D Fe3O4-CNTs/rGF/EP) nanocomposites [4], 3D porous graphene nano­ platelets/reduced graphene oxide foam/epoxy (GNPs/rGO/EP) nano­ composites as high-performance EMI shielding materials [5]. In order to effectively eliminate secondary electromagnetic pollution, microwave absorbing materials (MAMs) with low reflection and high absorption have received extensive attention in recent years due to their promising functions for regulating electromagnetic pollution [6-13]. Generally, ideal MAMs should simultaneously meet these re­ quirements such as strong microwave absorption intensity, thin

* Corresponding author. School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China. ** Corresponding author. E-mail addresses: [email protected] (R. Shu), [email protected] (M. Zheng). https://doi.org/10.1016/j.compscitech.2019.107839 Received 23 July 2019; Received in revised form 20 September 2019; Accepted 27 September 2019 Available online 28 September 2019 0266-3538/© 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. Schematic illustration of the synthesis process of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites.

Fig. 2. (a) XRD patterns of S1, S2 and S3, (b) FT-IR spectra and (c) Raman spectra of NMWCNTs, S1, S2 and S3, and (d) TGA curves of S2 and S3.

matching thickness, light weight and wide absorption bandwidth [14–19]. Recently, multi-walled carbon nanotubes (MWCNTs) have been considered as potential candidates for microwave absorption due to these advantages such as one-dimensional (1D) tubular structure, low density, good chemical stability and notable conduction loss [20–22]. However, single MWCNTs used as MAMs suffer from inferior impedance matching and poor attenuation loss, which greatly restrict their practical applications [20–22]. Thus, it is very necessary to optimize the imped­ ance matching and further enhance the microwave absorption perfor­ mance of MWCNTs for dealing with the growing problem of electromagnetic pollution. According to the electromagnetic theories, both impedance match­ ing and attenuation loss conditions should be well satisfied for achieving superior microwave absorption properties [22]. Recently, numerous investigations demonstrated that the complexation of magnetic spinel ferrites MFe2O4 (M ¼ Fe, Co, Ni, Mn, Zn, etc.) with MWCNTs to fabricate MWCNTs-based hybrid composites could be an effective strategy for enhancing the microwave absorption performance of MWCNTs [23–35]. For instance, Xing et al. fabricated the Zn ferrite/MWCNTs composites through a one-pot hydrothermal method and found that the Zn ferri­ te/MWCNTs/paraffin wax composites showed the minimum reflection loss (RLmin) of 42.6 dB [30]. Duan et al. prepared the MWCNTs/MnFe2O4 composites by a hydrothermal reaction and further

explored the microwave absorption properties of MWCNTs/MnFe2O4/­ paraffin wax composites. It was observed that the obtained composites displayed the RLmin of 41.0 dB and effective absorption bandwidth (EAB, RL less than 10 dB) of 3.59 GHz [33]. In our recent work, we fabricated MWCNTs/NiFe2O4 hybrid composites by a facile sol­ vothermal route. The obtained MWCNTs/NiFe2O4/paraffin wax com­ posites exhibited the RLmin of 42.3 dB and EAB of 3.8 GHz [35]. However, most of the reported MWCNTs/spinel ferrites hybrid com­ posites used as MAMs suffer from the drawbacks of unsatisfactory mi­ crowave absorption intensity (RLmin � 55 dB) and narrow absorption bandwidth (EAB � 4.0 GHz). Furthermore, the relationship between structure and microwave absorption properties, and underlying micro­ wave absorption mechanisms of MWCNTs/spinel ferrites hybrid com­ posites have not been clearly clarified. Therefore, it is valuable for fabricating the high-performance MWCNTs/spinel ferrites hybrid com­ posites and clarifying their underlying microwave absorption mechanisms. In view of the merits of cobalt-zinc ferrite (Co0⋅5Zn0⋅5Fe2O4) with moderate magnetic loss, good chemical stability, facile synthesis and low cost [36], MWCNTs with large aspect ratio, low density and notable conduction loss [20–22,29,30,34,35], nitrogen doping enhanced defects/dipole polarization and dielectric loss of MWCNTs [37–40], it is believed that superior microwave absorption properties could be 2

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Fig. 3. XPS spectra of (a) wide scan, (b) C 1s, (c) O 1s, (d) Fe 2p, (e) Co 2p and (f) Zn 2p of the sample of S3.

paraffin wax composites was systematically investigated in the fre­ quency range of 2–18 GHz. Results demonstrated that the as-prepared hybrid composites showed superior microwave absorption properties in the X-band, which could be used as potential candidates for electro­ magnetic absorption or shielding. Besides, the possible microwave ab­ sorption mechanisms of obtained hybrid composites were clarified.

Table 1 The atomic percentages of different elements for the sample of S3 from the XPS analysis. Elements

Atomic percentages (%)

C N O Co Zn Fe

40.41 0.59 39.74 3.22 2.94 13.1

2. Experimental 2.1. Materials Nitrogen-doped multi-walled carbon nanotubes (NMWCNTs, N content: 2.98 wt%, electric conductivity > 100 S/cm) were provided by Nanjing XFNANO Materials Tech Co., Ltd (Nanjing, China) with outer diameter of 30–50 nm and length of 10–30 μm. Zinc chloride (ZnCl2), cobalt chloride (CoCl2⋅6H2O), ferric chloride (FeCl3⋅6H2O), nitric acid (HNO3, 65–68 wt%), sodium acetate (NaAc), ethylene glycol (EG), polyethylene glycol (PEG, Mw ¼ 6000 g mol 1) and anhydrous ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the chemical reagents were analytical grade and used without further purification. Deionized water was produced in our laboratory (electrical resistivity ~ 18.2 MΩ cm).

achieved through fabricating the nitrogen-doped multi-walled carbon nanotubes/cobalt-zinc ferrite (NMWCNTs/Co0⋅5Zn0⋅5Fe2O4) hybrid composites. To the best of our knowledge, the investigations of facile synthesis and contents of NMWCNTs on the electromagnetic parameters and microwave absorption properties of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites have been rarely reported. Herein, we fabricated the NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid com­ posites with well-designed 3D net-like structure by a facile one-step solvothermal strategy. Various technique was adopted to explore the relationship between structure and microwave absorption properties of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites. Moreover, the influ­ ence of contents of NMWCNTs on the electromagnetic parameters and microwave absorption properties of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4/ 3

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Fig. 4. SEM images with different magnifications: (a)‒(c) S1, (d)‒(f) S2 and (g)‒(i) S3.

2.2. Preparation of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites

containing functional groups such as –COOH and –OH, and defects could be generated on the surface of NMWCNTs during the acid-treated pro­ cess [22,29,35]. These functional groups and defects provided abundant locations for the deposition of metal cations (Fe3þ, Co2þ and Zn2þ, etc.) [29,35]. Then, these oxygen-containing groups existing on the surface of NMWCNTs were beneficial to binding the metal cations by the electro­ static attraction interactions under the alkaline conditions [29,35]. Lastly, numerous Co0⋅5Zn0⋅5Fe2O4 nanocrystals were in-situ formed and anchored on the surface of NMWCNTs, and further assembled into mi­ crospheres during the solvothermal process [29,35,41]. As a result, the NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites were obtained. The detailed characterization sections of NMWCNTs/ Co0⋅5Zn0⋅5Fe2O4 hybrid composites can be found in the electronic sup­ plementary materials.

In order to purify and improve the dispersion of NMWCNTs in a polar solvent of EG, pristine NMWCNTs were treated by reflux in concentrated HNO3, as described in our previous works [22,29,35]. NMWCNTs/Co0.5Zn0.5Fe2O4 hybrid composites were synthesized by a facile solvothermal method. Briefly, the acid-treated NMWCNTs with different additive amounts (0, 10 and 20 mg) were firstly dispersed into 30 mL of EG by ultrasonic treatment, respectively. Then, 0.5 mmol of CoCl2⋅6H2O, 0.5 mmol of ZnCl2 and 2 mmol of FeCl3⋅6H2O were completely dissolved into the above NMWCNTs/EG dispersions by vigorous stirring. Next, 2.7 g of NaAc and 0.75 g PEG were fully dis­ solved into the mixture dispersions under vigorous stirring, respectively. Afterward, the mixture dispersions were poured into a Teflon-lined stainless-steel autoclave (50 mL) and reacted at 200 � C for 8 h. The ob­ tained products were collected by magnetic separation, and then puri­ fied by repeated washing with deionized water and anhydrous ethanol for several times. Finally, the powder-like samples were obtained by grinding dried solid products, which were dried at 55 � C for 24 h in a vacuum oven. For simplicity, the as-prepared NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites with different additive amounts of NMWCNTs were labeled as S1 (0 mg), S2 (10 mg) and S3 (20 mg), respectively. The schematic synthesis procedures of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites were described in Fig. 1. Firstly, numerously oxygen-

3. Results and discussion 3.1. Structural analysis As depicted in Fig. 2(a), the diffraction peaks from X-ray diffraction (XRD) patterns of the samples of S1, S2 and S3 appearing at 2θ ¼ 30.2, 35.6, 43.1, 57.0 and 62.6� are in good accordance with the (220), (311), (400), (511) and (440) crystal planes of Co0⋅5Zn0⋅5Fe2O4 (JCPDS 22–1086), respectively [36]. However, it is difficult to distinguish the characteristic diffraction peak around 26� of NMWCNTs in the XRD 4

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Fig. 5. TEM images with different magnifications: (a)‒(c) and HRTEM image (d) of the sample of S3.

two obvious scattering peaks located at 1580 cm 1 (G band) and 1344 cm 1 (D band). The D and G bands signify the sp3 defects or dis­ order, and sp2 hybridization, respectively [22,29,35]. Generally, ID/IG is often used to reflect the degree of disorder [22,29,35]. It can be seen that the values of ID/IG for NMWCNTs, S2 and S3 are 0.97, 1.04 and 1.12, respectively. Thus, the as-prepared hybrid composites (S2 and S3) exhibit obviously enhanced ID/IG compared with NMWCNTs, which suggests a higher degree of defects. Notably, the S3 shows the biggest degree of defects, which are helpful for microwave attenuation under the alternating electromagnetic fields. Furthermore, the D and G bands of hybrid composites present a slight blue shift compared with pure NMWCNTs, which indicates there are interfacial interactions between NMWCNTs and Co0⋅5Zn0⋅5Fe2O4 in the hybrid composites [29]. Besides, the characteristic Raman scattering peaks of Co0⋅5Zn0⋅5Fe2O4 in the low wavenumbers range (400–700 cm 1) [29,35] can be observed in the S1, S2 and S3. Thermal gravimetric analysis (TGA) measurements were conducted for determining the contents of NMWCNTs in the S2 and S3. As shown in Fig. 2(d), the thermal decomposition process could be divided into two stages. Firstly, a small weight loss (~5.0 wt%) occurs below 220 � C, which is mainly caused by the loss of some oxygen-containing groups such as –COOH and –OH on the surface of NMWCNTs [42]. Secondly, an obvious weight loss between 220 � C and 550 � C, which could be ascribed to the degradation of NMWCNTs [43]. Besides, the residual products are deduced to the constituent of Co0⋅5Zn0⋅5Fe2O4. Thus, the contents of NMWCNTs in the S2 and S3 are estimated as 17.2 wt% and 19.7 wt%, respectively. The surface chemical compositions and valence states of S3 were investigated by X-ray photoelectron spectroscopy (XPS) analysis. Fig. 3 (a) shows the typical spectrum of wide scan, which confirms that the S3

Fig. 6. Magnetic hysteresis loops of the samples of S1, S2 and S3 at room temperature. Inset: the magnified magnetization curves at the low field.

patterns of S2 and S3. This result could be explained by the fact that the surface of NMWCNTs is fully covered by Co0⋅5Zn0⋅5Fe2O4 particles and relatively low diffraction strength of NMWCNTs compared with Co0⋅5Zn0⋅5Fe2O4 in the hybrid composites [29,35]. Fig. 2(b) shows the typical Fourier transform infrared (FT-IR) spectra of NMWCNTs, S1, S2 and S3. The peaks appearing at 3446, 1640 and –C 1040 cm 1 could be attributed to the stretching vibrations of –OH, C– and C–O, respectively [35]. Besides, the peak at around 600 cm 1 can be assigned to the characteristic absorption of Fe–O [35,36]. Degree of graphitization of obtained samples could be evaluated by Raman spectroscopy. From Fig. 2(c), the NMWCNTs, S2 and S3 exhibit 5

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Fig. 7. Frequency dependence of reflection loss with different thicknesses and 3D plots: (a) and (a’) S1, (b) and (b’) S2, and (c) and (c’) S3.

contains the elements of Zn, Co, Fe, O and C. As shown in Fig. 3(b), the peaks of C 1s at 288.8, 286.3, 285.2 and 284.6 eV can be assigned to – O, C–N and C–C/C– – C bonds, respectively [22,29]. From C–OH, C– Fig. 3(c), the O 1s spectra can be fitted into three peaks of Fe–O–C, – O and Fe–O [22,29]. From Fig. 3(d), the peaks at 711.3 and O–C– 724.7 eV could be assigned to Fe 2p3/2 and Fe 2p1/2, respectively [22, 29]. Besides, the satellite peak appears at around 719.7 eV, which sug­ gests the existence of Fe3þ in the hybrid composite [7]. As displayed in Fig. 3(e), the peaks at 781.0 and 796.8 eV could be assigned to Co 2p3/2 and Co 2p1/2, respectively [17,39]. Besides, the two satellite peaks appear at around 786.6 and 803.4 eV, which indicate the existence of Co2þ in the hybrid composite [44]. From Fig. 3(f), the peaks at 1021.9 and 1045.2 eV could be ascribed to Zn 2p3/2 and Zn 2p1/2, respectively [22,29]. As shown in Table 1, the atomic percentages of elements of Zn, Co, Fe, O, N and C for the sample of S3 are 2.94%, 3.22%, 13.1%, 39.74%, 0.59% and 40.41%, respectively. Thus, the molar ratio of Co: Zn: Fe approximates to 1:1:4, which suggests that the Co0⋅5Zn0⋅5Fe2O4 has been in-situ formed in the S3. Furthermore, a little amount of N atoms are detected in the S3, which suggests the N atoms have been doped into MWCNTs. Therefore, the results of XPS analysis demonstrate that the NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composite has been successfully

prepared in this work. 3.2. Morphological analysis Scanning electron microscopy (SEM) was used to observe the micromorphology of the samples of S1, S2 and S3, as shown in Fig. 4. From Fig. 4(a)–(c), it is obvious that the pure Co0⋅5Zn0⋅5Fe2O4 particles (S1) exhibit a spherical morphology with a size in the range of 50–160 nm. Furthermore, all the Co0⋅5Zn0⋅5Fe2O4 microspheres present the coarse surfaces, which mainly derived from the self-assembly pro­ cess of in-situ formed small primary nanocrystals during the sol­ vothermal reactions [29,35,41]. Similar results have been also observed in our previous works [29,35]. From Fig. 4(d)–(f), it can be observed that numerously micro-sized Co0⋅5Zn0⋅5Fe2O4 particles with rough sur­ faces were uniformly loaded on the surface of NMWCNTs in the S2. As displayed in Fig. 4(g)–(i), it is clear that more NMWCNTs appear in the S3 and the surface of NMWCNTs is almost completely covered by spherical Co0⋅5Zn0⋅5Fe2O4 particles. Due to the 1D tubular structure, large aspect ratio and high conductivity of NMWCNTs, locally 3D conductive networks could be constructed in the hybrid composite of S3 (as marked by the dashed box in Fig. 4(g)), which are helpful for the electrons hopping and migration [22,29,35]. According to the formula 6

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Fig. 8. Frequency dependence of (a) ε0 , (b) ε’’, (c) μ0 , (d) μ’’, (e) tanδe and (f) tanδm for the samples of S1, S2 and S3.

of micro-current networks reported by Cao et al. [23], when the surface of NMWCNTs is loaded with Co0⋅5Zn0⋅5Fe2O4 microspheres and twined each other, the Co0⋅5Zn0⋅5Fe2O4 can act as the electrons hopping bridges among neighboring NMWCNTs, which contributes to the formation of micro-current networks, resulting in enhanced electric conductivity and conduction loss compared with pure Co0⋅5Zn0⋅5Fe2O4 [23,29,35]. Furthermore, it should be noted that abundantly heterogeneous in­ terfaces between Co0⋅5Zn0⋅5Fe2O4 microspheres and NMWCNTs could be formed in the hybrid composites of S2 and S3, which will notably enhance the interfacial polarization and dielectric loss of S2 and S3 under the alternating electromagnetic fields [29,35]. The micromorphology and structure of S3 was further characterized by transmission electron microscopy (TEM), as displayed in Fig. 5. It is clear that the Co0⋅5Zn0⋅5Fe2O4 particles present a regularly spherical morphology and the surface of NMWCNTs is almost completely covered by Co0⋅5Zn0⋅5Fe2O4 microspheres with a slight aggregation (Fig. 5(a)– (c)). Furthermore, it can be found that almost none of Co0⋅5Zn0⋅5Fe2O4 microspheres are dropped off from the surface of NMWCNTs under powerful ultrasound treatment. Therefore, it is believed that the

Co0⋅5Zn0⋅5Fe2O4 microspheres are tightly anchored on the surface of NMWCNTs [29,35]. As shown in Fig. 5(d), the high-resolution trans­ mission electron microscopy (HRTEM) image reveals the inter-plane distances of 0.302, 0.245 and 0.208 nm, which correspond to the (220), (311) and (400) crystal planes of Co0⋅5Zn0⋅5Fe2O4, respectively. 3.3. Magnetic properties According to the electromagnetic theories, the magnetic loss is vitally important for microwave attenuation derived from the inherent magnetic properties [22,29,35]. Fig. 6 depicts the magnetic hysteresis loops of the samples of S1, S2 and S3 at room temperature. As shown in the inset of Fig. 6, the magnified magnetization curves at the low field clearly demonstrate that all the samples display typically ferromagnetic behaviors [22,29,35]. Notably, the values of saturation magnetization (Ms) of S1, S2 and S3 are 64.4, 61.4 and 54.8 emu/g, respectively. The decreasing of Ms from S1 to S3 mainly comes from the increasing con­ tents of non-magnetic NMWCNTs in the hybrid composites [22,29,35].

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Fig. 9. (a) Frequency dependence of C0 for the samples of S1, S2 and S3, Cole-Cole semicircle (ε’’ versus ε0 ) curves: (b) S1, (c) S2 and (d) S3.

Fig. 10. Frequency dependence of (a) attenuation constant (α) and (b) normalized impedance matching (Z) with a thickness of 3.1 mm for the samples of S1, S2 and S3.

3.4. Microwave absorption properties

MAMs [22,29,35]. The real permittivity (ε0 ) and real permeability (μ0 ) represent the storage ability of electric and magnetic field energies, whereas the imaginary permittivity (ε’’) and imaginary permeability (μ’’) indicate the dissipation capacity of electric and magnetic field energies, respectively [22,29,35]. Thus, the frequency dependence of electromagnetic parameters of S1, S2 and S3 was carefully investigated, as depicted in Fig. 8(a)–(d). From Fig. 8(a), the ε0 of all the samples shows a decline trend with the increasing of frequency and reveals a frequency dispersion effect, which is helpful for microwave absorption [45]. Specifically, the values of ε0 decrease from 6.4 to 5.5, 6.8 to 6.0, 9.8 to 8.1 for S1, S2 and S3, respectively. Moreover, the ε0 enhances with the increasing of contents of NMWCNTs and the S3 exhibits the biggest ε0 among all the samples. As shown in Fig. 8(b), the ε’’ shows a similar increasing trend as ε0 with the increasing of contents of NMWCNTs. It can be found that the ε’’ enhances in sequence of S1, S2 and S3 in the frequency range of 7–18 GHz. On the basis of free electron theory, it can be deduced that the ε’’ enhances with the increasing of electric con­ ductivity [22,46,47]. Owing to the good electric conductivity of

As described in Fig. 7(a)‒(c), the RLmin of S1, S2 and S3 achieves 9.8, 19.2 and 64.7 dB, respectively. Thus, the microwave absorp­ tion strength of the samples obviously enhances with the increasing of contents of NMWCNTs. Significantly, the S3 presents the RLmin of 64.7 dB at 8.0 GHz with a matching thickness of 3.1 mm and maximum EAB of 4.3 GHz (11.7–16.0 GHz) with a thickness of merely 2.1 mm. Furthermore, the EAB reaches 3.6 GHz (8.1–11.7 GHz) with a thickness of 2.5 mm, which almost covers the whole X-band (8.0–12.0 GHz). Fig. 7 (a’)‒(c’) display the 3D plots of reflection loss for S1, S2 and S3, respectively. Remarkably, the RLmin corresponding to the maximum microwave absorption could locate at various frequencies by modu­ lating the matching thicknesses of absorbers [22,29,35]. Besides, a notable dual-band (C and Ku bands) absorption characteristic could be observed in the S3 with a matching thickness of 5.0 mm. Generally, the electromagnetic parameters (ε0 , ε’’, μ0 , μ’’) are very important for determining the microwave absorption properties of

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both dielectric loss and magnetic loss contribute to the microwave attenuation for all the samples. Recent investigations revealed that the magnetic loss was primarily derived from the eddy current loss, exchange resonance and natural resonance in the microwave frequency range of 2–18 GHz [22,35, 55–58]. The size of Co0⋅5Zn0⋅5Fe2O4 microspheres for all the samples is much larger than the exchange length of 10 nm [22,56,57], which in­ dicates the exchange resonance should not be responsible for magnetic loss. The eddy current loss could be calculated by the following equation [22,35,55–58]: . μ’’ � 2πμ0 ðμ’ Þ2 σ d2 f 3 (1) Herein σ (S/cm) and μ0 (H/m) are the vacuum conductivity and permeability, respectively [22,35,55–58]. Eddy current coefficient C0 (μ’’(μ0 ) 2f 1) should keep constant as the frequency increasing if the eddy current effect is mainly responsible for magnetic loss [22,35, 55–58]. As depicted in Fig. 9(a), the C0 of all the samples almost de­ creases with some fluctuations as the frequency increasing. This finding suggests that the eddy current loss should not be the primary mechanism of magnetic loss. It should be noted that an obvious peak in the fre­ quency range of 2–5 GHz of C0 ~ f curves could be ascribed to the nat­ ural resonance [22,35,55–58]. As shown in the supplementary materials, the Cole-Cole semicircle theory was deduced. Fig. 9(b)–(d) show the Cole-Cole plots of S1, S2 and S3. All the samples exhibit at least three semicircles, which indicate the existing of multiple Debye dipolar relaxation processes [29,57,59,60]. Furthermore, it can be observed that the semicircles are distorted in some degree, which suggests that the Debye relaxation is not the only mechanism for dielectric loss but other mechanisms such as conduction loss and interfacial polarization could be responsible for microwave absorption [29,57,59,60]. In general, an ideal microwave absorber should satisfy the two re­ quirements, i.e. impedance matching and maximum attenuation [22,29, 35]. Normalized impedance matching (Z) is often described as follows [22,61–64]: � � �rffiffiffiffi �� � �� �Zin � � μr 2πfd pffiffiffiffiffiffiffi �� Z ¼ �� �� ¼ ​ �� tanh j μr εr � (2) c Z0 εr

Fig. 11. (a) Frequency-dependent reflection loss, (b) simulations of tm versus fm under the λ/4 and 3λ/4 models, and (c) normalized impedance matching (Z) as a function of frequency for the sample of S3 with different thicknesses.

NMWCNTs, it is believed that the obtained hybrid composites (S2 and S3) exhibit larger electric conductivity than pure Co0⋅5Zn0⋅5Fe2O4 mi­ crospheres (S1). Furthermore, the well-constructed 3D conductive net­ works in the S3 (as shown in Fig. 4(g)) could notably enhance the electric conductivity. As a consequence, the electric conductivity of the hybrid composites enhances with the increasing of contents of NMWCNTs, leading to the enhancement of ε’’ [35,48–52]. Besides, all the samples show obvious relaxation peaks of ε’’ at around 17.3 GHz (as marked by the dashed box in Fig. 8(b)), which could be ascribed to the interfacial polarization [35,49]. Numerous reports demonstrated that ε0 kept pace with the change in electric conductivity and there existed identical change rules about ε0 and ε’’ [45,53,54]. Therefore, both ε0 and ε’’ enhance with the increasing of contents of conductive NMWCNTs, indicating the enhanced storage capability of electric energy as well as dielectric loss [45,48]. From Fig. 8(c), the μ0 of S1, S2 and S3 displays a decline trend with the increasing of frequency and the values of μ0 are in the range of 0.9–1.81. As described in Fig. 8(d), the μ’’ of S1, S2 and S3 generally presents a decline trend as the frequency increasing. However, there are obvious resonance peaks of μ’’ for all the samples in the frequency range of 3–5 and 13–16 GHz (as marked by the dashed box in Fig. 8(d)) [35, 49]. Besides, both μ0 and μ’’ demonstrate a decline trend with the increasing of contents of NMWCNTs in the frequency range of 8–18 GHz, which could be ascribed to the decreasing of Ms from S1 to S3. The microwave attenuation capacity of MAMs is basically deter­ mined by the dielectric loss and magnetic loss [22,29,35]. Thus, the frequency dependence of dielectric loss tangent (tanδe ¼ ε’’/ε0 ) and magnetic loss tangent (tanδm ¼ μ’’/μ0 ) of S1, S2 and S3 was investigated, as shown in Fig. 8(e) and (f). From Fig. 8(e), the S3 exhibits obviously enhanced tanδe than S1 and S2, suggesting the improved dielectric loss against the incident microwaves. It should be noted that the obviously enhanced dielectric loss could be attributed to the notably increased ε0 and ε’’ of S3. However, there is no distinguished difference of tanδe between S1 and S2 in the frequency range of 8–18 GHz. As depicted in Fig. 8(f), the tanδm of S1, S2 and S3 displays a similar decline trend as μ’’ with the increasing of frequency. However, the S3 presents the biggest μ’’ of 0.52 at 3.5 GHz, which indicates the strongest magnetic loss ca­ pacity of S3 in the low frequency region. Besides, it should be noted that

Herein Zin and Z0 are the input impedance and free space, respectively. Electromagnetic attenuation capacity is often reflected by the attenuation constant (α), which can be expressed as follows [22,29,35, 65,66]: ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiq pffiffi 2π f ð � (3) α¼ μ’’ε’’ μ’ε’Þ þ ðμ’’ε’’ μ’ε’Þ2 þ ðε’μ’’ þ ε’’μ’Þ2 c Fig. 10 shows the frequency-dependent α and Z for S1, S2 and S3. From Fig. 10(a), it is clear that the values of α enhance from S1 to S3. Remarkably, the S3 displays the biggest value of α (139.5), suggesting the strongest electromagnetic attenuation capacity. As described in Fig. 10(b), the values of Z for S1 and S2 are far from 1, which suggest inferior impedance matching. However, the values of Z for S3 are obviously lower than S1 and S2 and the S3 achieves the optimal impedance matching at around 8 GHz. As the optimal impedance matching is achieved, most of the incident microwaves can enter into the specimen; meanwhile, the strongest electromagnetic attenuation ca­ pacity could effectively transform the electromagnetic energies into thermal energies [22,35]. Therefore, the S3 demonstrates the best mi­ crowave absorption performance among the three samples. The relationship between absorption peak frequency (fm) and matching thickness (tm) can be well clarified by the quarter-wavelength (λ/4) matching theory, which is usually described as follows [22,29,35, 65,66]:

9

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Composites Science and Technology 184 (2019) 107839

Fig. 12. Schematic illustration of the possible microwave absorption mechanisms of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites.

tm ¼

nλ nc ¼ pffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 3; 5; ::::Þ 2 4fm jεr μr j

dipoles could rotate toward the alternating electromagnetic field, con­ verting the electromagnetic energies into thermal energies due to the relaxation loss [43,67–69]. Secondly, numerously heterogeneous in­ terfaces among paraffin matrix, NMWCNTs and Co0⋅5Zn0⋅5Fe2O4 could be considered as the capacitor-like structure [22,23,29,43]. According to the model proposed by Cao et al. [23], the capacitor-like structure at the interfaces could attenuate the power of incident microwaves by aligning the polar bonds or charges under the alternating electromag­ netic fields [23]. Thirdly, the nitrogen doping in MWCNTs could not only enhance the defects and dipole polarization loss capacity, but also optimize the impedance matching [15,17,22,37–40]. Fourthly, accord­ ing to the Cao’s electrons hopping model [50,51], the electrons can absorb electromagnetic energies to migrate on the surface of NMWCNTs, and then convert the electromagnetic energies into thermal energies by colliding with the lattice [69]. Besides, the electrons could absorb more electromagnetic energies to jump across the potential barriers at the contact sites of neighboring NMWCNTs and Co0⋅5Zn0⋅5Fe2O4 micro­ spheres. Thus, more electrons hop in the 3D conductive networks and the conductivity of conductive networks is enhanced, which convert more electromagnetic energies into thermal energies [43,67–69]. Lastly, the synergistic effects of dielectric loss derived from interfacial polari­ zation, defect polarization and dipole polarization, magnetic loss

(4)

If tm and fm meet the above equation, a phase cancellation effect could effectively attenuate the incident microwaves [22,29,35,65,66]. As depicted in Fig. 11(a), it can be found that the RL peaks of S3 shift to lower frequency with the increasing of tm. Fig. 11(b) shows the sim­ ulations of tm versus fm under the λ/4 and 3λ/4 models. The pentagram exp signifies the experimental tm (denoted as texp m ). Significantly, all the tm are exactly located at the λ/4 and 3λ/4 curves. Furthermore, the dual RL peaks with the thickness larger than 4.5 mm can be well clarified by the λ/4 and 3λ/4 models. These findings indicate that the nλ/4 matching theory essentially determines the relationship between tm and fm [22,29, 35]. Therefore, it is valuable for designing the thickness of absorbers according to the quarter-wavelength matching theory. Besides, the strongest RL peak ( 64.7 dB at 8.0 GHz and 3.1 mm) corresponds well with the optimal impedance matching of Z equals to 1 (Fig. 11(c)). Fig. 12 describes the possible microwave absorption mechanisms of NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites. Firstly, the residual oxygen-containing groups such as –COOH and –OH, and structure de­ fects on the surface of NMWCNTs could cause the charge asymmetric distributions, inducing the formation of dipoles [43,67–69]. These 10

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Composites Science and Technology 184 (2019) 107839

originated from ferromagnetic Co0⋅5Zn0⋅5Fe2O4 microspheres, and con­ duction loss coming from the electrons migrating and hopping in the 3D conductive networks of NMWCNTs, which significantly improve the microwave attenuation capacity and optimize the impedance matching.

[10] [11]

4. Conclusions

[12]

In summary, NMWCNTs/Co0⋅5Zn0⋅5Fe2O4 hybrid composites were successfully prepared by a facile solvothermal strategy. Results revealed that numerously ferromagnetic Co0⋅5Zn0⋅5Fe2O4 microspheres were uniformly loaded on the surface of NMWCNTs. Moreover, well-designed 3D conductive networks were in-situ constructed by the entanglement of NMWCNTs in the hybrid composite of S3. Furthermore, the contents of NMWCNTs had remarkable influence on the electromagnetic parame­ ters and microwave absorption properties of NMWCNTs/ Co0⋅5Zn0⋅5Fe2O4/paraffin wax composites. Significantly, the as-prepared hybrid composites showed superior microwave absorption properies with the RLmin of 64.7 dB in the X-band for a matching thickness of 3.1 mm and EAB of 4.3 GHz for a thickness of merely 2.1 mm. Besides, the possible microwave absorption mechanisms of as-prepared hybrid composites were proposed, which could be ascribed to the synergistic effects of magnetic loss, conduction loss and dielectric loss, and opti­ mized impedance matching. Therefore, the obtained NMWCNTs-based hybrid composites could be used as high-efficiency microwave ab­ sorbers in the fields of military stealth and electromagnetic protection.

[13] [14] [15]

[16]

[17] [18]

[19]

Acknowledgments

[20]

This work was financially supported by the Foundation of Provincial Natural Science Research Project of Anhui Colleges (Grant No. KJ2019A0119), China Postdoctoral Science Foundation (Grant No. 2019M652160), National Natural Science Foundation of China (Grant No. 51507003), Lift Engineering of Young Talents and Doctor’s Start-up Research Foundation of Anhui University of Science and Technology (Grant No. ZY537).

[21] [22]

[23]

Appendix A. Supplementary data

[24]

Supplementary data to this article can be found online at https://doi. org/10.1016/j.compscitech.2019.107839.

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