C Nanofibers with

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Structural and carbonized design of 1D FeNi/C nanofibers with conductive network to optimize its electromagnetic parameters and absorption abilities Jing Lv, Xiaohui Liang, Bin Quan, Wei Liu, Guangbin Ji, and Youwei Du ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03807 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Sustainable Chemistry & Engineering

Structural and carbonized design of 1D FeNi/C nanofibers with conductive network to optimize electromagnetic parameters and absorption abilities Jing Lv a, Xiaohui Liang a, Guangbin Ji a,*, Bin Quan a, Wei Liu a, and Youwei Du b a

College of Material Science and Technology, Nanjing University of Aeronautics and Astroanutics, 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 The optimized electromagnetic (EM) parameters are highly indispensable for outstanding microwave absorbers. Generally speaking, it is very necessary to suitably improve permittivity and permeability of the materials. The combination of magnetic/dielectric materials is a good choice. Herein, the irregular shaped FeNi/C composites were synthesized in N2 atmosphere with unsatisfied EM parameters. To further optimize EM parameters and enhance microwave absorption abilities, constructing one-dimensional (1D) structure is also an excellent scheme. By electrospinning technology combined with heat treatment, 1D FeNi/C nanofibers were successfully obtained. Enhanced microwave absorption abilities can be fulfilled by conductive network structure, better dielectric loss and stronger interface polarization intensity. Moreover, carbonized time towards nanofibers plays a key role in microwave absorption, which could influence complex permittivity and dielectric loss of materials. It is found that FeNi/C nanofibers with highly carbonized degree display better microwave absorbing properties. The reflection loss (RL) values less than -10 dB can be observed in 12.8-17.2 GHz (a broad bandwidth of 4.4 GHz) with an absorber thickness of only 1.8 mm. The absorber with a thickness of 2.7 mm has the minimum RL value of -24.8 dB at 9.4 GHz. In this regard, these nanofibers are very likely to be used as EM-wave absorbers in practical application. Furthermore, this work provides a useful strategy to optimize electromagnetic parameters and absorption abilities of metal/carbon absorbers. It may promote the development of 1D metal/carbon composites in microwave absorption field.

Key words: one-dimensional nanofibers; FeNi/C; conductive network; microwave absorption; carbonized time


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INTRODUCTION One-dimensional structural materials like nanowires (NWs),1 nanorods (NRs)2 and nanotubes (NTs)3 have been widely researched because of their distinctive properties compared with their bulk nanomaterials.4-6 They can provide large aspect ratio and direct pathways for charge transport. Meanwhile, they also possess anisotropic structures and better dispersion characteristics.7-9 Based on the advantages mentioned above, many 1D nanomaterials have been investigated with enhanced microwave absorption properties. For example, Meng et al synthesized unique porous C@Fe3O4 hybrid nanotubes and got the minimum reflection loss (RL) of -45.0 dB at 6.18 GHz with a sample thickness of 3.4 mm.10 Our group has also obtained 1D Ni@C nanorods through self-template method.11 With core-shell structures and pore structures, the 1D nanorods (obtained at 500 oC) showed the RL value of -26.3 dB at 10.8 GHz with a thickness of 2.3 mm. Other 1D nanomaterials such as Fe/SiC nanofibers and Ni-Al2O3-ZnO nanowires about microwave absorption performance have also been reported in recent years.12,13 Carbon nanofibers, a kind of 1D nanomaterials, have attracted considerable attention because of their light weight, high strength, thermal and chemical stabilities and low cost in microwave absorption field.14,

15

Compared with carbon nanotubes, carbon nanofibers can be designed with applicable

electrical conductivity, which would contribute to optimized EM parameters. An excellent EM-wave absorber should hold the following advantages: thin-thickness, low-density, wide frequency bandwidth and strong-absorption.16 Lots of works on carbon nanofibers are tried to meet these characteristics. For instance, the Fe3O4/carbon nanofibers gained at 700 oC after 5 h showed the minimum reflect loss value of -45.0 dB at 8.0 GHz.17 Wang et al. successfully used natural collagen fibers as precursors to get ferromagnetic hierarchical carbon nanofiber bundles, which was a kind of light-weight and 3 ACS Paragon Plus Environment


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strong-absorption microwave absorption material.18 Xiang and co-workers prepared carbon nanofibers loading metal Fe/Co/Ni nanoparticles (NPs) by electrospinning technology.19 These nanofibers displayed superior EM-wave absorption performance with lower density and lower weight. Electrospinning technology is a quite simple, mature and cost-efficient way for large-scale production of 1D nanofiber.20 It is notable that carbon nanofibers combined with metal NPs can optimize EM parameters and promote interface polarization. Moreover, various kinds of treatment methods including improving carbonization temperature and changing nanostructures were adopted to enhance microwave absorption performance of metal materials/carbon nanofibers.21-23 Several works on carbonized time have already been done. Calcination time towards the microwave absorption of honeycomb-like FeCo/C, PAN cloth and BaZn2Fe16O27/C has also been discussed.24-26 Nevertheless, investigations about the relationship between carbonized time and microwave absorption of metal/carbon nanofibers are not enough. In this work, we used electrospinning technology and heat treatment to obtain FeNi/C nanofibers (FeNi/CNFs). This work was designed to optimize EM parameters and enhance microwave absorbing properties in the range of 2-18 GHz. Significantly, 1D conductive network makes the imaginary part of permittivity improved, suggesting that there is a higher dissipation of electric energy. Furthermore, with the increasing carbonized time, the FeNi/C nanofibers show better microwave absorbing performance due to the better dielectric loss and stronger interface polarization effect. The minimum RL value of -24.8 dB can be achieved at the frequency of 9.4 GHz with the thickness of 2.7 mm. The effective absorption band (RL values that below -10 dB) is 4.4 GHz (12.8-17.2 GHz) with a quite thin thickness of only 1.8 mm. In conclusion, this work can provide great help for developing microwave absorbers. Moreover, 1D FeNi/C nanofibers are very potential for application of EM-wave absorbers in the future. 4 ACS Paragon Plus Environment

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EXPERIMENTAL SECTION Chemicals All chemicals including ferric acetylacetonate (C15H21FeO6), nickel acetate (Ni(CH3COO)2.4H2O), polyacrylonitrile (PAN) and N, N-dimethylformamide (DMF) were chosen from commercial suppliers and there are no pretreatments to them. Synthesis of FeNi/Carbon composites Firstly, 0.025 mmol ferric acetylacetonate and 0.0125 mmol nickel acetate were added to 5 ml N, N-dimethylformamide with magnetic stirring. Then, 0.5 g of polyacrylonitrile (Mw=150,000) was introduced to this solution. After that, the mixture solution was heated in a water bath at 50 oC for 30 min with continuously stirring to get a homogeneous solution. Then, the precursor solution was dried at 60 oC for 24 h in a vacuum oven. The mixtures were calcined at 700 oC for 6 h under N2 atmosphere at a rate of 2 oC/min. The FeNi/C (C1) composites finally were obtained. Synthesis of FeNi/Carbon nanofibers FeNi/CNFs were prepared by electrospinning technique and carbonization process. The synthesis procedure of precursor solution was the same as the steps mentioned above. Then, the precursor solution was placed in a plastic syringe equipped with a 20-gauge stainless steel needle. The mixture solution feed rate was 0.45 mL/h. The tip-to-collector distance was 15 cm and the electrostatic voltage was at 17 kV. To make solvents volatilized, precursor nanofiber mats were dried at 60 oC for 24 h in a vacuum oven. In a typical pre-oxidation process of PAN nanofibers, the polymeric fibers were calcined at 250 oC for 2 h at a ramping rate of 2 oC/min in air. Finally, the nanofibers were calcined at 700 oC under N2 atmosphere at a ramping rate of 2 oC/min. The carbonized time is 1 h, 3 h, 4 h, 6 h, and 12 h, respectively. And the products are named as CNF-1, CNF-3, CNF-4, CNF-6 and CNF-12. 5 ACS Paragon Plus Environment


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Characterization The phase of the products was characterized by an X-ray diffractometer (XRD) with Cu Kα radiation (λ=1.5406 Å) between 2 θ =10 ° to 90 °. X-ray photoelectron spectroscopy (XPS) patterns were recorded on a PHI 5000 VersaProbe equipped with an Al Kα X-ray source operating at 150 W. Raman spectroscopy was carried out on a Renishaw inVia 2000 Raman Microscope. The morphology of the samples was examined by scanning electron microscopy (SEM, Hitachi-4800) and transmission electron microscope (TEM, Tecnai G2 F30). Electromagnetic parameters were measured by a vector network analyzer (Agilent, N5244A) in the frequency range of 2-18 GHz. The samples (the loading ratio is 30 %) were mixed with paraffin uniformly and were pressed into a cylindrical model with a material thickness of 2.0 mm (inner diameter of 3.0 mm and outer diameter of 7.0 mm).

RESULTS AND DISCUSION In this work, we firstly synthesized the irregular shaped FeNi/C composites with unsatisfied microwave absorption abilities. In order to optimize the electromagnetic parameters and microwave absorption abilities of the absorbers, the 1D FeNi/C nanofibers were obtained by electrospinning technique and subsequent carbonization process. Through structural and carbonized design towards FeNi/C composites, it is found that the microwave absorption performance can be greatly improved.


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Figure 1. (a) Schematic diagram of electrospinning equipment and the fabrication process of FeNi/CNFs; the physical photos of (b) PAN precursor nanofibers mats and (c) CNF-6 nanofibers networks; SEM images of (d) PAN precursor nanofibers, (e) CNF-1, (f) CNF-3, (g) CNF-4, (h) CNF-6, (i) CNF-12 and (j) C1; Insert: the magnified images. The schematic diagram of electrospinning equipment and the fabrication process of FeNi/CNFs can be seen in Figure 1 (a). After the electrospinning process, the solution of PAN and metallic origin salts 7 ACS Paragon Plus Environment


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becomes the faint yellow PAN precursor nanofibers. And the black nanofibers were obtained after carbonization at 700 oC. There are some blue balls that represent FeNi NPs inside the black nanofibers, which are shown in the cross-section drawn of FeNi/CNFs. Moreover, Figure 1 (b) and Figure 1 (c) are physical photos of the faint yellow PAN precursor nanofibers mats and the black CNF-6 nanofibers membranes. The morphologies of electrospun mats, obtained nanofibers and the irregular shaped composites are characterized through scanning electron microscope (SEM). Figure 1 (d) shows a typical SEM image of one-dimensional PAN precursor nanofibers by electrospinning and these nanofibers exhibit a smooth and uniform surface with average diameter of 125 nm. Significantly, 1D morphology is perfectly maintained after a carbonization process at 700 oC after 1 h to 6 h, as exhibited in Figure 1 (e-h). But the surface has become rougher with the increasing time. Regrettably, CNF-12 did not show the 1D feature in Figure 1 (i). Most interesting thing is that there are some short protrusions on the nanofiber surface after 3 h of carbonization, shown in Figure 1 (f). But they can’t be observed with longer carbonized time in Figure 1 (g and h). However, Figure 1 (j) displays the irregular shaped composites with 6 h of carbonized time.


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(a)♥

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Figure 2. (a) XRD patterns of C1, CNF-1, CNF-3, CNF-4, CNF-6 and CNF-12; C 1s XPS spectra of (b) CNF-1 (c) CNF-3 (d) CNF-4 (e) CNF-6 and (f) CNF-12. The XRD patterns of C1, CNF-3, CNF-4, CNF-6 and CNF-12 are similar (Figure 2 (a)). Three diffraction peaks locate in 2 θ = 43.5, 50.7 and 74.5 °could be indexed from (111), (200) and (220) planes of FeNi (JCPDS No:. 47-1417) with faced-centered cubic structure and Fm3m space group.27 No other impurity peaks can be observed. Obviously, no FeNi alloys are oxidized or corroded in air because of the protection of carbon. And the broad peaks located around 20-25 ° belong to the carbon. But the XRD pattern of CNF-1 provides another phases of Fe3O4 (JCPDS No:. 65-3107). According to the above results, we can infer that Fe3O4 is slowly reduced to FeNi alloy with the increasing carbonized time. Besides, it is extremely important and necessary to point out that the sharp and narrow diffraction peaks of nanofibers indicate that one-dimensional nanofibers are of high crystallinity, which is highly



related to the more long-range ordered crystal structural units inside the nanofibers. XPS technology can efficiently test the valence state of elements.28 Figure 2 (b-f) shows C 1s photoelectron spectra of 1D nanofiber with different carbonized time. As we can see, C 1s XPS spectrum is made up of three common signals. The binding energy of 284.6 eV are attributed to C-C/C=C groups and the ones of 285.6 eV, 285.8 eV and 288.3 eV are ascribed to the C-N groups and C=O groups, respectively.29 Obviously, the strongest signals which belong to C-C/C=C bonds suggest that the most parts of PAN precursors have already become into amorphous carbon after different carbonized time at 700 oC. After longer carbonized process, C-N bonds and C=O bonds become weak. These facts manifest that the more annealing time PAN nanofibers take, the better carbonized degree they are of, which is beneficial to improve complex permittivity. 800 0.99

CNF-1 CNF-3 CNF-4 CNF-6 CNF-12 C1

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400 1.01

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Figure 3. Raman spectra of C1, CNF-1, CNF-3, CNF-4, CNF-6 and CNF-12. To further make clear the carbonization degree of the irregular shaped composites and nanofibers, Raman spectroscopy technique was adopted. The Raman spectra (Figure 3) of C1, CNF-1, CNF-3, CNF-4, CNF-6 and CNF-12 show two main broad peaks located at about 1340 and 1590 cm-1, which are corresponding to the D and G bands, respectively. According to previous literatures,30 D band is 10 ACS Paragon Plus Environment

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associated with the vibration of sp3 carbon while G band is attributed to the vibration of sp2 carbon. The values of ID / IG represent the disorder structure of carbon. In this regard, the ID / IG values of CNF-1, CNF-3, CNF-4, CNF-6 and CNF-12 are 1.38, 1.04, 1.02, 1.01 and 0.99, respectively, testifying the increased graphitization degree. It demonstrates that most part of the nanofibers is amorphous carbon with little sp2 carbon, which is consistent with the XRD and XPS consequences. It is also found that the intensity of D band and G band becomes stronger with the increasing time of heat treatment, suggesting that PAN nanofibers are of better carbonized degree when extended annealing time. However, the weak signals of the irregular shaped composites show the poorly carbonized degree among these three products. This is maybe because that there is no pre-oxidation process of C1 sample. The pre-oxidation of PAN precursor nanofibers plays a significant role in the formation of the final products. In pre-oxidation, oxygen content of nanofibers increases and molecular structure dramatically changes.31 During high temperature heating in N2 atmosphere, deoxidization let the trapezoidal frame of PAN change into turbostratic graphite structure, which is critical to the carbonization strategy.



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Figure 4. (a-d) TEM images of CNF-1, CNF-3, CNF-4 and CNF-6; (e and f) TEM images of CNF-3 and CNF-6. Insert: The nanoparticles size distribution histogram of CNF-3 and CNF-6 as well as SAED of CNF-6; (g) TEM image of CNF-12; Corresponding EDS elemental mapping of C, Fe, and Ni for (h-k) CNF-3 (l-o) CNF-6. To further research the crystal structure of one-dimensional FeNi/C nanofibers, transmission electron microscope (TEM) is used to provide more details. TEM images of CNF-1, CNF-3, CNF-4 and CNF-6 are provided in Figure 4 (a-d). Obviously, lots of nanofibers overlap with each other and the conductive network can be built in Figure 4 (a-d). As shown in Figure 4 (e), short protrusions on the nanofiber 12 ACS Paragon Plus Environment

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surface could be FeNi alloys particles encapsulated at the tip of short carbon nanotubes. Interestingly enough, the FeNi NPs are uniformly embedded inside the carbon nanofibers after long carbonization (Figure 4 (f)). The nanoparticles size distribution histograms show the average size of FeNi NPs in CNF-3 and CNF-6 are 16.3 nm and 11.8 nm, respectively. And the selected area electron diffraction (SAED) is applied to verify the existence of the FeNi NPs, which displays the diffraction rings of (111), (200) and (220) of FeNi NPs. Nevertheless, Figure 4 (g) tells the damage of 1D structure after 12 h at 700 oC. In addition, the EDS spectra of CNF-3 and CNF-6 (Figure 4 (h-o)) demonstrate the homogeneous distribution of C, Fe and Ni elements while the C element exists in whole nanofibers. Figure 4 (h-o) also reveals FeNi alloys phase in nanofibers.

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Figure 5 shows the magnetic hysteresis loops of C1, CNF-3 and CNF-6 samples measured at room temperature. It is obvious that these three samples show representative soft ferromagnetic behavior. The saturation magnetization intensity is about 10 kOe and the Ms values of C1, CNF-3 and CNF-6 are 1.99 emu/g, 32.75 emu/g and 9.13 emu/g respectively. Compared with the irregular shaped composites, 1D FeNi/C nanofibers show better magnetic properties. Inside the polycrystalline nanomaterials like FeNi/C nanofibers, there are lots of single crystals carrying easy axes in each direction and these easy axes can be averaged in all direction, so crystal anisotropy of 1D FeNi/C nanofibers is not the main factor that influences the magnetization behavior. Thus, shape anisotropy powerfully determines the magnetization behavior of 1D FeNi/C nanofibers. In other words, these nanofibers are simpler to be magnetized along the long-axis direction,32 which are beneficial to optimize the magnetic parameters. However, the Ms value of CNF-6 is lower than that of CNF-3, which could be related to the decreasing nanosize of FeNi particles.33


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CNF-4, CNF-6 and CNF-12/paraffin composites in the range of 2-18 GHz. The electromagnetic parameters of the FeNi/C composites and FeNi/CNFs are mainly decided by their relative complex permittivity (εr=ε′-jε″) and relative complex permeability (µr=µ′-jµ″),34 which are measured using a vector network analyzer in the rage of 2-18 GHz at room temperature. In EM parameter measurements, the samples (the loading ratio is 30 wt%) were mixed with paraffin uniformly and they were pressed into a cylindrical model with a material thickness of about 2.0 mm. The real permittivity (ε′) and real permeability (µ′) represent the storage ability for electric and magnetic energy, 15 ACS Paragon Plus Environment


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while the imaginary permittivity (ε″) and imaginary permeability (µ″) stand for the dissipation of electric and magnetic energy.35, 36 Figure 6 (a-c) shows the real parts of permittivity, the imaginary parts of permittivity and relative complex permeability values of C1, CNF-1, CNF-3, CNF-4, CNF-6 and CNF-12/paraffin composites, respectively. It is observed that the 1D structure has an important impact on the electromagnetic parameters. The equation: ε″ = ωτ(εs-ε∞)/(1+ω2τ2) + σ/ωε0

(1)

where the ω, τ, εs, ε∞ and σ represent angular frequency, polarization relaxation time, static permittivity, the relative dielectric permittivity at the high-frequency limit and electrical conductivity, respectively. The electrical conductivity (σ) becomes the dominate factor that affects the value of ε″ due to the intrinsic parameters of nanomaterials including ω, τ, εs and ε∞. Importantly, 1D nanofiber can build conductive network, which brings about better electrical conductivity (higher σ value). Lee et al successfully achieved thin porous graphitic carbon membranes and these membranes showed a high electrical conductivity.37 Therefore, nanofibers are of higher ε″ values, which means higher dissipation of electric energy. Thus, the ε″ values of CNF-3 sample are higher than that of C1 sample in the rage of 2-18 GHz. Notably, the ε curves of nanofibers show a downward tendency due to an increasing lagging behind of the dipole-polarization with the increasing frequency.38 Compared with nanofibers samples, the ε″ curve of C1 sample shows an obvious peak over 12.2-15.0 GHz, indicating a dielectric resonance behavior. And the fluctuation of ε″ values of these samples is closely connected with the multiple resonance behavior. The extra slight peak of CNF-6 sample is mainly associated with the additional interfaces between FeNi and carbon.39,40 As we can see, complex permittivity becomes higher when carbonized time has been increased from 1 h to 6 h. Nevertheless, compared with CNF-4 and CNF-6 samples, it is found that CNF-12 sample exhibit lower complex permittivity and higher complex 16 ACS Paragon Plus Environment

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permeability, which could be resulted from the destruction of 1D carbon nanofibers structure.41 As can be seen in Figure 6 (c), these samples exhibit similar and lower relative complex permeability curves, which are related to the smaller magnetization. Moreover, it has well documented that nonmagnetic carbon component in nanofibers could mainly result to this phenomenon.19 It is well known that initial µi can be expressed as: 42 µi = Ms2/akHcMs+bλξ

(2)

where a and b are two constants determined by composition, k is proportion coefficient, λ is magnetostriction constant, ξ is an elastic strain parameter of the crystal. Different parameters of samples would contribute to different initial µi. In addition, the Snoek limit always make permeability drop at high frequency range (Figure 6 (c)). Enhancement of Ms could be the key point to increase magnetic loss. In generally, the magnetic loss is caused by the eddy current loss, natural resonance, and exchange-resonance.43 To the best of our knowledge, the eddy current loss can be expressed using the equation: 44 µ″(µ′) -2 f -1 = 2πµ0d2σ

(3)

where f is the frequency, µ0 is the vacuum permeability and σ is the electric conductivity. Furthermore, if the values of µ″(µ′) -2 f -1 are a constant, the magnetic loss is only resulted from the eddy current loss. In Figure 6 (d), it is observed that the values of µ″(µ′)

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and fluctuate a lot in the range of 2-18 GHz, suggesting that the eddy current loss is not the main loss mechanism that leads to the magnetic loss. In addition, the natural resonance of magnetic products always happens in the MHz range. Interestingly, the very-small-size effect of FeNi nanoparticles contributes to the increase of surface anisotropic field and effective anisotropic field, so the natural resonance begins to appear in the 2-18 GHz range.45 So the values of µ′ fluctuate a lot and go down to 1 17 ACS Paragon Plus Environment


in the frequency range of 2-18 GHz. More attention could be paid on the work of improving permeability of FeNi alloys. Based on previous research, permeability may be affected by component and nanosize of magnetic particles.46 Namely, both electrospinning liquid and heat treatment are adjustable to synthesize magnetic/carbon nanofibers with high magnetic loss.

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10

12

14

16

18

CNF-12

0 -5

-5

-20

4

Frequency(GHz)

CNF-6 (f)

(e) 0

-5

-15

-6

Frequency(GHz)

CNF-4

-10

1.8 mm 2.0 mm 2.2 mm 2.4 mm 2.7 mm

-4

-2.5

Frequency(GHz)

(d) 0

RL(dB)

RL(dB)

RL(dB)

-2

RL(dB)

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

Page 18 of 35

2

4

6

8

-10

1.8 mm 2.0 mm 2.2 mm 2.4 mm 2.7 mm

-15 -20 -25

10

12

14

16

18

Frequency(GHz)

-30 2

4

6

8

10

12

14

16

18

Frequency(GHz)

Figure 7. Reflection loss curves of (a) C1 (b) CNF-1 (c) CNF-3 (d) CNF-4 (e) CNF-6 and (f) CNF-12 /paraffin composites with different thickness; 3D RL plots of (g) C1 (h) CNF-1 (i) CNF-3 (j) CNF-4 (k) 18 ACS Paragon Plus Environment

Page 19 of 35 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


CNF-6 and (l) CNF-12/paraffin composites with filler loading ratio of 30 wt% in the frequency range of 2-18 GHz. According to the transmission line theory, the reflection loss (RL) values are calculated using the following equations.47-49 RL ( dB ) = 20 lg

Z in = Z 0

Z in − Z 0 Z in + Z 0

2π fd µ r ε r µr tanh( j ) εr c

(4)

(5)

where Z0, Zin, εr, µr, f, c and d stand for the impedance of free space, input characteristic impedance, the complex permittivity, the complex permeability, frequency, velocity of light and the thickness of composites, respectively. Thus, the RL values of these samples can be obtained via equation (4) and (5), respectively (shown in Figure 7 (a-l)). Apparently, compared with nonuniformly parent counterparts sample, samples including CNF-3, CNF-4, CNF-6 and CNF-12 have better microwave absorption properties when they are under the same loading ratio. Especially, for 1D nanofibers, CNF-6 sample shows the stronger microwave absorption properties because the all the RL values of CNF-6 sample with different thickness are up to -10 dB while the RL values of CNF-3 are not. This may be ascribed to the fact that CNF-6 has the highest permittivity. Besides, their microwave absorption abilities can be improved with the increased carbonized time from 1 h to 6 h. Figure 7 (a-l) shows that the peak of RL values is closely depended on the thickness of these samples and the position of strong peaks sits at lower frequency with the increasing thickness. The RL values less than -10 dB are observed in 12.8-17.2 GHz (4.4 GHz) with an absorber thickness of only 1.8 mm. The absorber with a thickness of 2.7 mm has the minimum RL value of -24.8 dB at 9.4 GHz. The point is that an RL value below -10 dB means



that 90 % microwave is attenuated and it is useful for a practical application.50-52

(a)

0.5 0.4


0.2

0.3

0.1

0.2

0.0

0.1 0.0


(b)

0.3

tandm

0.6

tande

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

Page 20 of 35

-0.1 2

4

6

8 10 12 14 16 18

2

Frequency(GHz)

4

6

8 10 12 14 16 18

Frequency(GHz)

Figure 8. Frequency dependence of (a) dielectric loss and (b) magnetic loss tangent of C1, CNF-1, CNF-3, CNF-4, CNF-6 and CNF-12/paraffin composites. Figure 8 (a) and (b) show frequency dependence of dielectric loss factor (tan δe=ε″/ε′) and magnetic loss factor (tan δm=µ″/µ′). High loss tangent can enhance microwave absorption abilities.53 As detected in Figure 8 (a), tan δe values of nanofibers trend to be higher when carbonized time increases from 1 h to 6 h. Nevertheless, C1 and CNF-12 present lower tan δe values as compared to CNF-3, CNF-4 and CNF-6. 1D nanofibers show enhanced EM-wave absorption properties with increasing tan δe values while lower tan δm values, suggesting that 1D structure is beneficial for EM-wave absorption abilities. The tangent dielectric loss of CNF-6 sample (average value is 0.35) is the highest among all samples. However, the tan δm value of CNF-6 sample is the lowest and fluctuates a lot in the frequency range of 2-18 GHz. Based on above analysis, CNF-6 sample is mainly acted as dielectric absorbers. It is worth noticing that tan δm values of nanofibers present negative values, which are related to the negative permeability. When FeNi/CNFs are irradiated by EM-wave, magnetic energy could increase along with electric energy decrease.54 This energy conversion can be reflected in the relational curves between


Page 21 of 35

increased dielectric loss and decreased magnetic loss, especially in the range of 9-18 GHz. In other words, increasing electric current can be dissipated while others can produce magnetic field. 1.0

2.0

(a)

C1

1.8

5.0

(b)

CNF-6

4.5

1.6

4.0

1.4

e'' 0.4

3.5

e''

0.6

1.2

3.0

1.0

2.5

0.8

0.2

2.0

0.6 4.0

(c)

CNF-3

0.8

e''

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


4.2

4.4

e'

4.6

4.8

5.0

3.5

4.0

4.5

5.0

5.5

e'

8

9

10

e'

11

12

13

Figure 9. Cole-Cole curves of (a) C1 (b) CNF-3 (C) CNF-6. Here, we can draw a conclusion that for 1D CNF-6 sample, the microwave absorption abilities were primarily caused by dielectric loss rather than the smaller magnetic loss. And the interface polarization intensity inevitably plays a significant role in dielectric loss. The relative complex permittivity can be described by the equation.55-57

εr = ε∞ +

εs − ε∞ = ε '− jε '' 1 + j 2π f τ

(6)

Where εs, ε∞, f and τ are static permittivity, the relative dielectric permittivity at high-frequency limit, frequency and polarization relaxation time, respectively. So, the ε′ and ε″ can be calculated according to the following equations.

ε ' = ε∞ + ε '' =

ε s − ε∞ 1 + (2π f )2τ 2

2π f τ (ε s − ε ∞ ) 1 + (2π f ) 2τ 2

(7)

(8)

Based on the equation (8) and (9), ε′ and ε″ can be described as:

(ε '− ε ∞ ) 2 + (ε '') 2 = (ε s − ε ∞ ) 2

(9) 21

ACS Paragon Plus Environment


The semicircles in Figure 9 (a-c) are called as Cole-Cole semicircle. Moreover, every semicircle means one Debye relaxation process related to interface polarization effect. In this case, interfacial polarization takes place on the interfaces between FeNi metal, carbon nanofibers, paraffin and air cavities.58 Compared with nanofibers, C1 sample exhibits unique Cole-Cole curves and evident resonance peak can be discovered in Figure 8 (a), indicating a relaxation dielectric property that supposed to mainly originate from interfacial polarizations of FeNi/ C hybrids. As shown in Figure 9 (a-c), it is found that more Cole-Cole semicircles exist in CNF-6/paraffin composites because of the stronger interface polarization effect, which does do great help for dielectric loss.

1.0

(a)

0.5

C1

CNF-3

0.8

s AC/S.cm-1

0.7

0.3

0.6

4

3.0

6

2

4

6

6

s AC/S.cm-1

X

Ku

2

4

6

0.5

8 10 12 14 16 18

Frequency(GHz)

0

1.0

8 10 12 14 16 18 20

Frequency(GHz)

0.0345 0.2552 0.6453

4

2.6468

3.2974 C

S 2

2.8373

0.5

1.5

3.898

e''

1.0

1.02105

0.8993 Ku

8 10 12 14 16 18

8 10 12 14 16 18 20

C1 CNF-3 CNF-6

2.0

CNF-6

1.24685

0.4842

(d)

2.5

6

Frequency(GHz)

2.5

4.4 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.8 2.6 2.4

1.5412

e''

0

CNF-6

1.5

4

Frequency(GHz)

3.0

2.0

C

S 2

0.1

8 10 12 14 16 18 20

Frequency(GHz)

(c)

0.6

2.2057

2

X

0.8

8 10 12 14 16 18

Frequency(GHz)

0

1.0

0.2

Ku

1.2

0.4035 0.7494

X

CNF-3

1.5731

6

1.4

0.3

C 4

0.4

1.6

0.2239 0.5661

2

1.8

1.0955

S

0.5

0.0985 0.4142

0.0

0.40375

e''

0.1

C1 0.29655

0.2

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

0.20855

s AC/S.cm-1

(b)

0.9

0.4

s AC/S.cm-1

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

Page 22 of 35

0.0

S

C

X

Ku

Figure 10. Frequency dependence of electrical conductivity of (a) C1 (b) CNF-3 and (c) CNF-6. Insert: 22 ACS Paragon Plus Environment

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The imaginary part of permittivity of samples in S, C, X and Ku band. (d) Average conductivity values of C1, CNF-3 and CNF-6 in different band. Electrical conductivity has been considered to be an important parameter to prove the formation of 1D conductive network. Based on the EM parameter, conductivity can be estimated: 59 σ AC = ε0ε″w = ε0ε″2πf and ε0 = 10^(-9)/36π

(10)

Figure 10 (a-c) gives the relationships between σ AC and f in S, C, X and Ku band respectively, where ε″ is the average values of S, C, X and Ku band. To clearly figure out the function of 1D nanofibers, average values of conductivity of different band were displayed in Figure 10 (d). In this case, 1D conductive network was fortunately built with the increasing electrical conductivity. In addition, it can be inferred that the electrical conductivity dramatically increases along with the enhanced ε″ values.

Figure 11. Schematic diagram of microwave attenuation in FeNi/carbon nanofibers. The schematic diagram of microwave attenuation in FeNi/C nanofibers is showed in Figure 11. Firstly, conductive network made up by nanofibers plays a vital role in dielectric loss because more conductive paths could promote exciting hopping electrons carries.60 Meanwhile, nanofibers can provide direct path



Page 24 of 35

ways for charge transport as well. Secondly, electromagnetic wave could also be attenuated due to the magnetic loss of FeNi alloys.61 Significantly, multiple interfaces between FeNi NPs and carbon would bring about dielectric dipole interactions and associated relaxation, making more electromagnetic wave dissipated.62 Table 1. The microwave absorption performance of FeNi/CNFs and some other similar materials. Sample

RLmix

Thickness

Filler

Effective

(dB)

(mm)

loadings

bandwidth

(wt%)

(

C Nanofibers with

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