Fe3O4 Nanoflower-Carbon Nanotube Composites for

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Fe3O4 Nanoflower-Carbon Nanotube Composites for Microwave Shielding Xiaojun Zeng, Guangming Jiang, Lingyu Zhu, Chenyu Wang, Meng Chen, and Ronghai Yu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01076 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Nano Materials

Fe3O4 Nanoflower-Carbon Nanotube Composites for Microwave Shielding Xiaojun Zeng#,*, Guangming Jiang#, Lingyu Zhu, Chenyu Wang, Meng Chen, and Ronghai Yu* School of Materials Science and Engineering, Beihang University, Beijing 100083, China *Corresponding author: Xiaojun Zeng, E-mail: [email protected]; Ronghai Yu, E-mail: [email protected]. #These

authors contributed equally to this work.

ABSTRACT Unsatisfactory reflection loss (RL) and thick thickness of the absorber severely limit the practical application of advanced electromagnetic (EM) microwave absorbers. Herein, we report a heterogeneous architecture that consists of Fe3O4 nanoflower, which construct from ultrathin nanosheets, and carbon nanotubes (CNTs) backbone to enhance their RL in low-thickness regions. Thanks to the reasonable structural design and dielectric loss regulation, the optimized Fe3O4/CNTs composites exhibit superior microwave absorbing properties. The minimal RL is -58.6 dB at 15.28 GHz. Meanwhile, the thickness is only 1.52 mm. Furthermore, when RL is below -10 dB, the actual absorption bandwidth is as high as 15 GHz at absorber thickness below 5 mm. The effective permeability (μr) and permittivity (εr), and Z value indicate that the remarkable microwave absorption attributed

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to the coordination of the excellent polarization relaxation of CNTs, the good intrinsic magnetic properties of Fe3O4 nanoflower, and the abundant interfacial polarization between Fe3O4 and CNTs. Keywords: Fe3O4 nanoflower; Ultrathin nanosheets; CNTs backbone; Microwave absorbing properties; Microwave shielding.

1. INTRODUCTION High-performance electromagnetic (EM) microwave absorbers are rapidly developing to address the serious EM radiation and EM interference pollution problems.1−3 At present, composites of magnetic loss components and dielectric loss components are considered to be important EM microwave absorption materials because of their excellent impedance matching.4−6 For example, Ding et al. reported that the minimum reflection loss (RL) of the Fe3O4/Fe nanorings embedded reduced graphene oxide nanocomposite (FeNR@rGO) is 23.09 dB (d = 4 mm, f = 9.16 GHz).7 Zhang et al. showed that the minimum RL of RGO/MnFe2O4/PVDF composite was -29 dB at a frequency of 9.2 GHz (d = 3 mm).8 Zhao et al. reported a three-dimensional (3D) composite hydrogel consisting of α-Fe2O3 nanoparticles and reduced graphene oxide (rGO@α-Fe2O3) with a minimum RL of -33.5 dB (d = 5 mm, f = 7.12 GHz).9 The pFe3O4@ZnO spheres mixed with the graphene substrate exhibited good microwave absorption performances (RL = -37 dB, f = 11 GHz) at a thickness of 5 mm.10 Hu et al. prepared 3D graphene-Fe3O4 nanocomposites by direct hydrothermal grafting method.11 The resulting composites had a reflection loss of -27 dB 2


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at a thickness of 2 mm. The above results demonstrate that the carbon-based magnetic composites have enhanced EM absorbing performances and a relatively low overall density. However, most of the aforementioned carbon-based magnetic composites still exhibit unsatisfactory RL values. Particularly bad is that the thickness of the absorber is too thick, so that the density of material is too large to be suitable for practical application. Therefore, designing and fabricating carbon-based magnetic composites with excellent absorbing performance and lightweight character is extremely desirable. This work regulated the permittivity of the composites of Fe3O4 nanoflower and CNTs by adjusting the content of CNTs, resulting in a good impedance matching and an appreciable microwave absorption property with a minimum reflection loss of -58.6 dB and an ultra-thin absorber of 1.52 mm. Designing nanostructures with rich heterogeneous interface is one of the effective strategies to improve the EM absorbing properties. The interfaces involved in EM devices dominate device performance, such as strong interaction and abundant charge carrier channels at the interface between carbon and magnetic materials.3,12 In addition, the abundant interface will induce strong interface polarization to enhance the EM microwave absorbing performance. Furthermore, the flake-like structure is more favorable for overcoming the Snoek limit and achieving good microwave absorption.13-15 The introduction of a controllable complex effective permittivity (εr) is also an acceptable strategy to improve the microwave absorbing performance.16 A suitable εr will result in high dielectric loss.17 Furthermore, proper dielectric properties can be balanced with magnetic properties, resulting in good impedance matching.18-20 Recently, some studies 3



have focused on nanostructured EM absorber with heterogeneous interface and controllable εr, such as CoFe2O4/graphene oxide hybrids,21 ferromagnetic γ-Fe2O3-MWNTs-PBO composites,22 urchin-like Ni nanoparticles/reduced graphene oxide (u-Ni/RGO) composites.23 Unfortunately, carbon-based magnetic composites are still difficult to achieve achieve strong reflection loss in low-thickness regions without reasonable structural design and dielectric loss adjustment. Herein, we used a simple solvothermal process to combine uniform Fe3O4 nanoflower organized from ultrathin nanosheets with CNTs backbone (Fe3O4/CNTs) for advanced EM microwave absorption. The superior performances of the prepared Fe3O4/CNTs composites are attributed to the following aspects: (i) In the overall composites, the highly conductive CNTs are tightly bind the Fe3O4 nanoflower, which greatly improves the dielectric loss and can also be used as a lightweight absorber; (ii) The abundant interfaces between the Fe3O4 nanoflower and the CNTs backbone induce interfacial polarization and relaxation, thereby promoting the microwave absorbing properties; (iii) A tunable effective εr is obtained by a controllable CNTs amount, which balances the μr to achieve suitable impedance matching, thereby producing appreciable properties. Therefore, the Fe3O4/CNTs composites exhibits an unexpected EM microwave absorbing performance. The low absorber thickness, super reflection loss, and large absorption bandwidth of Fe3O4/CNTs composites hold great potential in practical applications.

2. EXPERIMENTAL SECTION 4


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2.1. Synthesis of flower-like Fe3O4 nanosphere As we have previously reported,24 a simple solvothermal process was applied to prepare the flower-like iron oxide nanosphere. The main process is as follows. First, ferric chloride hexahydrate (FeCl3·6H2O, 0.4 g), urea (CO(NH2)2, 1.35 g), and tetrabutylammonium bromide (TBAB, 3.6 g) were sequentially dissolved in ethylene glycol (EG, 60 mL). After magnetic stirring for 30 min and sonication for 20 min. The synthesis process was performed at 180 ℃ for 1 h in a Teflon-lined stainless-steel autoclave. Then, the product was collect by centrifugation, washed with deionized water, dried at 40 ℃, and calcined in a mixed atmosphere of 5% H2/Ar at 300 ℃. 2.2. Fabrication of the Fe3O4/CNTs composites In a typical procedure, urea (1.35 g), FeCl3·6H2O (0.4 g), and TBAB (3.6 g) were sequentially added to 60 mL EG. Various amount of CNTs was added to the above mixture. After stirring and ultrasonication, a uniform suspension was formed. The rest of the process is the same as that of the above-mentioned flower-like Fe3O4 nanosphere. The final samples were named S1, S2, and S3, and the corresponding CNTs mass fraction were 3 wt%, 5 wt%, and 7 wt%, respectively. 2.3. Sample characterization An X-ray diffraction (XRD, Rigaku D/max-2500) was applied to study the crystallinity of samples. Morphology was determined by scanning electron microscopy (SEM, JEOLJSM7500) and transmission electron microscopy (TEM, JEOL-2100). X-ray photoelectron spectroscopy (XPS) were recorded using ESCALAB 250 Xi spectrometer. Vibrating 5



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sample magnetometer (VSM) detected the hysteresis loops of samples. 2.4. Evaluation of electromagnetic properties The electromagnetic properties of samples were performed on Agilent N5230C network analyzer. Before measurement, the prepared samples were added to solid paraffin (70 wt%) at 45 ℃. The mixed powder was carefully compressed into cylindrical compacts. The inner diameter, outer diameter, and thickness of the cylinder were 3.04 m, 7 mm, and 3 mm, respectively. The data obtained from all tests were analyzed by transmission line theory to characterize the sample’s microwave absorbing properties, marked as reflection loss (RL).

3. RESULTS AND DISCUSSION 3.1. Characterization of the Fe3O4/CNTs composites Figure 1 is the schematic illustration of the synthesis process of heterogeneous architecture

Fe3O4

nanoflower/CNTs

composite.

The

growth

of

the

Fe3O4

nanoflower/CNTs composites mainly includes the following three steps:24 (i) The absorption of iron ions on CNTs via the electrostatic effects between iron ions and abundant oxygen-containing groups,16 accompanied by the formation of iron alkoxide in the EG system; (ii) Nucleation and growth into the primary nanoparticles; (iii) Aggregation of the particles to form nanosphere and growth into a flower-like structure. In this designed Fe3O4 nanoflower/CNTs structure, CNTs not only acted as a backbone matrix to uphold the structural integrity of the network, but also acted as a dielectric material to balance the magnetic loss of the composites.25 In addition, Fe3O4 tends to nucleate on the surface of 6


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CNTs and then anchor to the CNTs during hydrothermal process.26

Figure 1. Schematic illustration of the synthesis process of Fe3O4 nanoflower/CNTs composites. As shown in the SEM images of Figure 2a, the hydrothermally prepared Fe3O4 nanostructure is a very uniform flower-like nanosphere formed by a large number of ultrathin nanosheets. The thickness of the building block is ~50 nm. Even if CNTs are added to the Fe3O4 nanoflower (Figure 2b-d), the morphology of the nanoflower can be well maintained without being damaged. The suitable amount of CNTs can connect the entire composites, but excessive CNTs can cause CNTs agglomeration (Figure 2d). The XRD patterns (Figure 2e) clearly show that all the diffraction peaks of Fe3O4 nanoflower point to the Fe3O4 phase (JCPDS cards no. 79-0419), which fully indicates that the synthesized Fe3O4 has high purity and good crystallinity. The high background of Fe3O4/CNTs composites (S1, S2, and S3 samples) at low angle is caused by CNTs doping. The chemical state of Fe element in Fe3O4 nanoflower can also be determined by the XPS spectrum (Figure 2f). Obviously, the Fe 2p spectrum has a 2p3/2 peak at 709.9 eV, a 2p1/2 peak at 723.5 eV, and no satellite peak. This further indicates that a high purity Fe3O4 phase 7



is formed.

Figure 2. SEM images of (a) Fe3O4 nanoflower, (b) S1, (c) S2, and (d) S3 samples. (e) XRD patterns of all products. (f) Fe 2p spectrum of S2 sample. To further examine the detailed morphology of Fe3O4 nanoflower and Fe3O4/CNTs composites, TEM observations were carried out. Fe3O4 nanoflower are constructed from many ultrathin nanosheets (Figure 3a). In addition, the middle part of the nanoflower is hollow due to the Ostwald ripening occurring during hydrothermal process.24,27 Notably, the Fe3O4 nanoflower can remain the structure very well after adding CNTs, implying the high chemical stability of nanoflower (Figure 3b-d). Clearly, one-dimensional CNTs tightly envelop the nanoflower to form a good conductive network, which greatly improves the electrical conductivity, increases the interface, and reduces the density of the composites. Typically, the incident EM microwave can be dissipated and adsorbed due to scattering and reflection between the interfaces of Fe3O4 nanoflower and CNTs.15,28-30 This 8


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further promotes the Fe3O4/CNTs composite to become a high-performance EM microwave absorption material.

Figure 3. TEM images of (a) Fe3O4 nanoflower, (b) S1, (c) S2, and (d) S3 samples. 3.2. Magnetic and electromagnetic characteristics of the Fe3O4/CNTs composites To understand the magnetic properties of composites, the hysteresis loops are recorded (Figure 4). The composites display superparamagnetic characteristics. The saturation magnetization (Ms) value of S2 sample is lower than that of Fe3O4 nanoflower due to the the reduction of magnetic Fe3O4 phase. In addition, S2 sample presents good magnetic properties with high Ms and low coercivity (Hc), which can induce strong μr and magnetic loss, thereby enhancing electromagnetic properties, as described in the previous work.16

Figure 4. Magnetization curves of Fe3O4 nanoflower and S2 sample. 9



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The reflection loss depends on the measured complex effective permeability (μr) and permittivity (εr), and is calculated by transmission line theory as shown below:31 RL (dB) = 20 log|(Zin – Z0)/(Zin + Z0)|

(1)

Zin = (μr/εr)1/2tanh[j(2πfd/c)(μrεr)1/2]Z0

(2)

Where Z0 represents the impedance of the free space, Zin and d represents the input impedance and the thickness of absorber, f is the frequency of EM wave, c is the velocity of light. Figure 5 displays the 2D and 3D reflection loss of the Fe3O4 nanoflower and Fe3O4/CNTs composites calculated by transmission line theory. The RL value of pure Fe3O4 nanoflower is almost negligible (Figure 5a). Excitingly, the RL value of Fe3O4 nanoflower with CNTs added was greatly improved. Obviously, the increased interfacial polarization and improved dielectric properties is responsible for this phenomenon. When 5 wt% of CNTs were added, the RL value of composites reached an unexpected minimum of -58.6 dB. The frequency and thickness are 15.28 GHz and 1.52 mm, respectively. (Figure 5c). Such remarkable microwave absorbing properties far exceeds the most reported advanced materials reported in Table 1. It should be taken into account that the ultrathin absorber thickness offers the possibility of practical application of Fe3O4/CNTs composites. Introducing the thickness as the third factor, the corresponding 3D images ((Figure 5e-h) of RL values can more clearly show the RL values of composites, the effective absorption bandwidth (RL < -10 dB) and the absorber thickness, which provides a good guiding significance for the precise regulation of dielectric loss. Obviously, the S2 sample can reach 10


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a very high RL value between in the thickness range of 1-4 mm (Figure 5g). More importantly, the S2 sample has a big absorption bandwidth (>15 GHz) at an absorber thickness of less than 5 mm, including almost all the test frequencies. A slightly lower microwave absorbing properties can be obtained in the S1 and S3 (Figure 5f,h). A slight imbalance between dielectric loss and magnetic loss results in this phenomenon.

Figure 5. (a-d) 2D and (e-h) 3D reflection loss dependence of frequency for Fe3O4 nanoflower, S1, S2, and S3 samples. Table 1. Comparison and summary of the recently reported carbon-based magnetic composites and Fe3O4/5 wt% CNTs composites. Samples

RL (dB)

Frequency range (GHz)

NiO@GO

-59.6

12.48-16.72

4.24

1.7

25

-58.6

13.5-18.0

4.5

1.52

30

-54.0

27.24-38.52

11.28

1.0

50

33

-51.0

8.5-12.6

4.1

2.8

30

34

Fe3O4/5 wt% CNTs PANI/Fe3O4 NiFe@C nanocubes@GO

Effective bandwidth (GHz)

Thickness (mm)

Weight fraction (wt%)

Refs. 32

11


This work


Fe3O4/Al2O3/CN

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-40.3

-

-

3.1

25

1

-40.0

4.6-18.0

13.4

2.5

40

35

-38.4

5.2-8.2

3.0

5.0

20

36

FeCo-GNFs

-30.6

6.4-8.3

1.9

2.0

30

37

Fe3O4-PEDOT

-30.0

7.7-12.8

5.1

4.0

20

38

-29.58

12.42-118.0

5.58

2.0

20

39

-29.2

13.6-18.0

4.4

2.0

60

40

Cs FeCo@C G/Fe3O4@Fe/Zn O

Fe7S8/Ni17S18/C NTs NiFe2O4-GO

Complex effective permittivity and permeability were further analyzed to deeply investigate the superior microwave absorption of Fe3O4/CNTs composites. As discovered in Figure 6a,b, Fe3O4 nanoflower and Fe3O4/CNTs composites present analogous permeability. This is because trace amounts of CNTs doping have little effect on the magnetic properties of composites. The μ' values of Fe3O4/CNTs composites (S1, S2, and S3 samples) successively exhibit sharp decrease (2.8–5 GHz), slight fluctuation (5–15 GHz), and increase (15–18 GHz) with increasing frequency. This phenomenon is consistent with many other studies.6,41 The μ' value of the S3 sample increases significantly in the high frequency region, which may be because the overdoped CNTs in the S3 sample are easily agglomerated.6,22 The μ" curve has resonance peaks in two regions (high frequency and low frequency), which is caused by exchange resonance and natural resonance.42 At the same time, all samples exhibit similar magnetic loss tanδμ (μ"/μ') (Figure 6c), suggesting that these samples have similar ability to store and dissipate 12


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magnetic energy. In contrast, after adding a certain amount of CNTs, the ε' and ε" values are greatly improved (Figure 6d,e), which is accompanied by a large increase in dielectric loss and storage capability. This is because the addition of an appropriate amount of CNTs will induce an increase in electric polarization and conductivity. Furthermore, as the content of CNTs increase, permittivity improves from S1 to S2 but drop for S3, while all the composites have higher ε' and ε" values than Fe3O4 nanoflower. Actually, the coordination of CNTs and Fe3O4 nanoflower in different samples is different,16,43 resulting in the above results. Similar to other magnetic-carbon composites, the ε' value decreases as the frequency increase because of the increased polarization hysteresis relative to the electric-field variation in high frequency region.44 In addition, the ε" value obtains a maximum in the range of 14.6–18 GHz, disclosing that the resonance phenomenon may respond strongly to microwave absorption.22,41 Finally, S2 sample (Fe3O4/5 wt% CNTs) displays the highest dielectric loss tanδε (ε"/ε') (Figure 6f), mainly contributed by the large interfacial polarization between the nanoflower and the CNTs backbone.45 Furthermore, a controllable permittivity can be achieved by doping a certain amount of CNTs. The above results and analysis demonstrate that the dielectric loss is dominant in the overall reflection loss of Fe3O4/CNTs composites in microwave absorbing measurements.

13



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Figure 6. The measured μr (a, b) and εr (d, e) of Fe3O4 nanoflower, S1, S2, and S3 samples. The corresponding magnetic loss (c) and dielectric loss (f). The eddy current (C0 = μ"(μ')-2f-1 = 2πμ0d2ϭ) is also vital for magnetic loss.46 As can be seen from Figure 7a, as the frequency increases, the C0 values of all samples change drastically. This suggests that the magnetic loss of the sample is primarily derived from exchange resonance and natural ferromagnetic resonance.47 The attenuation constant (α) is the main parameter for studying the EM attenuation among the absorber, as shown below.48 α = {[(ε'μ" - ε"μ')2 + (μ"ε"- μ'ε')2]1/2 + (μ"ε"- μ'ε')}1/2[((2)1/2fπ)/c]

(3)

The Fe3O4/CNTs composites have strong EM attenuation capability over the whole frequency range (Figure 7b). Therefore, S2 sample shows the large attenuation capability. However, for efficient absorbent, impedance matching and energy conservation are essential.49

14


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Figure 7. The C0 (a) and α (b) values of Fe3O4 nanoflower, S1, S2, and S3 samples. It is widely accepted that the microwave absorption ability is the attenuation of EM waves caused by good impendence matching.22 As mentioned above, CNTs can increase the dielectric properties, thereby balancing the magnetic properties. The good impendence matching of the absorbing materials means that they have a Z (Zin/Z0) value close to 1. In other words, the reflection at the interface of the air and absorber is close to zero.50 Figure 8a reveals that the Z value of Fe3O4/5 wt% CNTs is closest to 1, indicating its optimal impendence matching, which is consistent with its maximum reflection loss result. Figure 8b shows that the Z value is closest to 1 at the matching thickness of 1.52 mm, indicating that the well-matched characteristic impedance of S2 sample. Undoubtedly, the strong dielectric loss including interfacial polarization and electrons hopping, and the magnetic loss caused by the exchange resonance and natural resonance of nanoflower are the main reasons for the above results (Figure 8c). In addition, the interfaces involved in EM devices 15



dominate device performance, such as strong interaction and abundant charge carrier channels at the interface between CNTs and Fe3O4 nanoflower.3,12 S1 and S3 samples have a Z value closer to 1 than pure Fe3O4 nanoflower, which fully demonstrates the effect of CNTs on balanced magnetic loss. Therefore, the remarkable microwave absorption of Fe3O4/CNTs are attributed to the coordination of the good magnetic properties of Fe3O4 nanoflower, the excellent polarization relaxation of CNTs, and the rich interfacial polarization between Fe3O4 nanoflower and CNTs backbone.

Figure 8. The Z values of Fe3O4 nanoflower, S1, S2, and S3 samples (a). The normalized impedance Z value for S2 sample (b). Illustrations of EM microwave attenuation included in Fe3O4/CNTs composites (c).

4. CPNCLUSIONS In summary, we demonstrated a heterogeneous architecture that consists of Fe3O4 nanoflower and CNTs backbone, which provides a well-defined ultrathin nanosheets and abundant interface. After the reasonable structural design and dielectric loss regulation, the obtained Fe3O4/CNTs composites show controllable dielectric loss that balances the magnetic loss, resulting in the superior microwave absorbing properties. The minimal RL 16


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is -58.6 dB at 15.28 GHz. Meanwhile, the thickness is only 1.52 mm. The remarkable properties are derived from the high-conductivity CNTs, numerous interfaces between Fe3O4 nanoflower and CNTs, and good impedance matching. The strong reflection loss and ultrathin thickness of the absorber contribute to its practical application in EM microwave absorption.

ACKNOWLEDGEMENTS This research was financed with the funds from the National Natural Science Foundation of China (Grant nos. 51671010 and 51101007).

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TOC graphic

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Figure 1. Schematic illustration of the synthesis process of Fe3O4 nanoflower/CNTs composites. 149x45mm (300 x 300 DPI)



Figure 2. SEM images of (a) Fe3O4 nanoflower, (b) S1, (c) S2, and (d) S3 samples. (e) XRD patterns of all products. (f) Fe 2p spectrum of S2 sample. 150x81mm (300 x 300 DPI)


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Figure 3. TEM images of (a) Fe3O4 nanoflower, (b) S1, (c) S2, and (d) S3 samples. 83x56mm (300 x 300 DPI)



Figure 4. Magnetization curves of Fe3O4 nanoflower and S2 sample. 54x41mm (300 x 300 DPI)


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Figure 5. (a-d) 2D and (e-h) 3D reflection loss dependence of frequency for Fe3O4 nanoflower, S1, S2, and S3 samples. 161x60mm (300 x 300 DPI)



Figure 6. The measured μr (a, b) and εr (d, e) of Fe3O4 nanoflower, S1, S2, and S3 samples. The corresponding magnetic loss (c) and dielectric loss (f). 150x75mm (300 x 300 DPI)


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Figure 7. The C0 (a) and α (b) values of Fe3O4 nanoflower, S1, S2, and S3 samples. 54x83mm (300 x 300 DPI)



Figure 8. The Z values of Fe3O4 nanoflower, S1, S2, and S3 samples (a). The normalized impedance Z value for S2 sample (b). Illustrations of EM microwave attenuation included in Fe3O4/CNTs composites (c). 141x37mm (300 x 300 DPI)


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Table of Contents (TOC) graphic 122x50mm (300 x 300 DPI)


Fe3O4 Nanoflower-Carbon Nanotube Composites for

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