Combined use of lightweight magnetic Fe3O4-coated hollow glass spheres and electrically conductive reduced graphene oxide in an epoxy matrix for microwave absorption

Author’s Accepted Manuscript Combined use of lightweight magnetic Fe3O4coated hollow glass spheres and electrically conductive reduced graphene oxide in an epoxy matrix for microwave absorption Junpeng Wang, Jun Wang, Bin Zhang, Yu Sun, Wei Chen, Tao Wang www.elsevier.com/locate/jmmm

PII: DOI: Reference:

S0304-8853(15)30635-1 http://dx.doi.org/10.1016/j.jmmm.2015.10.001 MAGMA60694

To appear in: Journal of Magnetism and Magnetic Materials Received date: 5 September 2015 Revised date: 30 September 2015 Accepted date: 1 October 2015 Cite this article as: Junpeng Wang, Jun Wang, Bin Zhang, Yu Sun, Wei Chen and Tao Wang, Combined use of lightweight magnetic Fe 3O4-coated hollow glass spheres and electrically conductive reduced graphene oxide in an epoxy matrix for microwave absorption, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.10.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Combined use of lightweight magnetic Fe3O4-coated hollow glass spheres and electrically conductive reduced graphene oxide in an epoxy matrix for microwave absorption Junpeng Wang, Jun Wang *, Bin Zhang, Yu Sun, Wei Chen, Tao Wang

School of material science and engineering, Wuhan University of Technology, Wuhan 430070, China Abstract: Epoxy resin based lightweight composites comprising Fe3O4-coated hollow glass spheres (HGS@Fe3O4) and reduced graphene oxide (RGO) were prepared. Impedance matching condition and electromagnetic wave attenuation characteristic are used for analysis of the reflection loss (RL) performance of the composites. Compared with pure HGS@Fe3O4 and RGO composite, the -10 dB absorption bandwidth and the minimum RL of the hybrid composites are enhanced. RL values less than -10 dB are obtained in a wide frequency range and the corresponding bandwidth can reach up to 3.6 GHz when an appropriate absorber thickness is chosen. The density of the hybrid composite is in the range of 0.57-0.72 g/cm3, which is attractive candidate for a new type of lightweight microwave absorber. Key words: Microwave absorption; Fe3O4-coated hollow glass spheres; RGO; Electromagnetic parameters; Polymer composite 1. Introduction With the rapid development of electronic devices and microwave communication equipment, electromagnetic radiation has become increasingly serious. As a result, protecting person’s health from electromagnetic radiation and protecting information security from electromagnetic wave leaking have become a serious concern[1,2]. A high-performance microwave absorber can eliminate such adverse electromagnetic radiation effectively. In addition to high absorption efficiency, being lightweight is also important for effective and practical application. _____________________________ *

Corresponding author.

Email address: [email protected]; [email protected] 1

A good microwave absorption material is required to meet two demands simultaneously: impedance matching and microwave attenuation[3]. Firstly, when the microwave is incident onto the absorber surface, the reflection of the microwave should be minimal. Secondly, the incident electromagnetic energy should be transformed into heat or other forms of energy effectively through the dielectric loss and (or) magnetic loss. Polymer composites containing different kinds of magnetic or dielectric fillers have been prepared during the past decades, including carbonyl iron powder[4], ferrites[5], carbon black[6], carbon nanotubes[7], graphene[8],etc. However, it is difficult for them to meet all of the requirements such as being lightweight, having good mechanical properties, wide effective absorption bandwidth and high absorption efficiency[9]. Ferrites are promising materials and extensively used for microwave absorption in higher gigahertz range because of their large tunable anisotropy field. However, the most obvious shortcomings of this kind of fillers are their high-density and strong inclination to form agglomeration. To alleviate the aggregation problem, coating hollow glass spheres (HGS) with magnetic films can be utilized. On one hand, the density of the absorber is reduced because of the low density of HGS. On the other hand, when the magnetic film is coated on the surface of HGS, agglomeration can be significantly alleviated owing to the increased specific surface area. Han et al. deposited amorphous FeCoNiB coatings on hollow glass microspheres by an electroless plating technique while they designed a double-layer structure to reduce the total weight of an absorber and enhance the microwave absorption performance[10]. Zhu et al. synthesized highly regulated Fe3O4-polyelectrolyte modified polyaniline hollow sphere nanocomposites using an electrostatic self-assembly approach and found that the hybrid nanocomposites exhibited excellent absorbing abilities and wide response bandwidths[11]. Graphene, the basic structural element of some carbon allotropes, is a two-dimensional, one-atom thick layer of graphite. It has attracted tremendous research interest due to its special characteristics: very high mechanical strength and electron mobility[12], room-temperature ferromagnetism[13], room-temperature quantum Hall effect[14], as well as superior thermal conductivity[15]. Besides, graphene can also act as 2

lightweight fillers for novel hybrid materials owning to its huge surface area and special surface properties[16]. Generally, magnetic materials are used to implement microwave absorber, since the absorbing bandwidth can be effectively enlarged. Such methods usually rely on a high weight ratio of the magnetic fillers for sufficient absorption performance. However, the absorbers usually have limitations in terms of application in the field that requiring lightweight. To the best of our knowledge, the microwave absorption properties of the composites consisting of RGO and HGS@Fe3O4 have not been reported. In this study, by combining HGS@Fe3O4 with a small quantity of RGO within the epoxy matrix, we have designed a novel lightweight absorber. An important feature of the present work is that the density of HGS@Fe3O4 is rather low, which is much less than traditional ferrite absorber. In other words, the density of absorber is mainly dominated by the matrix materials not by the fillers. The density of the epoxy matrix is ca. 1.2g/cm3, thus, the as-prepared absorbers have relatively low densities. In addition, the HGS@Fe3O4 in the composite can be considered as excluded volume in which RGO sheets cannot penetrate. The effective concentration of the RGO sheets thus increases with the addition of HGS@Fe3O4, which is in favor of microwave absorption. Thus, the as-prepared composites are promising to be used as lightweight and wideband microwave absorbers. 2. Material and methods The HGS@Fe3O4 was prepared using ferrite plating method. Details of the process were described elsewhere[17,18]. The density was about 0.37 g/cm3 based on the calculation of weight gain after plating. The resulting HGS@Fe3O4 was dried in a vacuum oven at 60 °C overnight. Graphene oxide (GO) was prepared by pressurized oxidation method because the as-obtained GO sheets can reserve large lateral dimension, HI-AcOH was selected as the reduction agent system for the high electrical conductivity[19,20]. The resulting RGO was freeze-dried. HGS@Fe3O4-RGO/epoxy samples with 1 wt. % of RGO while 30, 40, and 50 wt. % of HGS@Fe3O4 were prepared via solution processing. Different formulations of polymer composites having epoxy: HGS@Fe3O4: RGO weight ratios of 69:30:1 (RGO-SF (1)), 59:40:1 (RGO-SF (2)), 49:50:1 (RGO-SF 3

(3)), 70:30:0 (SF (1)), 60:40:0 (SF (2)), 50:50:0 (SF (3)) and 99:0:1 (RGO (1)) were prepared. In a typical procedure (RGO-SF (3)), 0.08 g dried RGO was dispersed in 80 ml DMF under ultrasonic treatment for 1.0 h. 2.9 g epoxy resin (EPIKOTE 862, Shell Inc) was added to the mixture and sonicated for another 1.0 h. The solvent was evaporated off by magnetically stirring at a temperature of 90 °C. After eliminating most of the DMF, the mixture was placed in a vacuum oven to ensure removing the residual solvent. 4.0 g HGS@Fe3O4 and 1.02 g amine hardener were proportionally added to the cool down mixture and was blended by a high speed shear mixer (SR-2000, RSIMAI Co., LTD) at a speed of 2000 rpm for 10 min. Then the final mixture was poured into the mold and cured at room temperature for 6 h. The composites were cut into samples by a high resolution engraving machine (SE-3230, Woodpecker Inc) with outer diameter of 7.0 mm, inner diameter of 3.04 mm, and height of about 2.0 mm. In control experiments, RGO/epoxy composites were also prepared in similar procedures while in the absence of HGS@Fe3O4. And for HGS@Fe3O4/epoxy composite, HGS@Fe3O4 was directly mixed with epoxy resin by the high speed shear mixer. The morphology and microstructure of samples were characterized by scanning electron microscope (SEM, JSM-5610LV). The structural characterization of RGO and HGS@Fe3O4 was conducted by X-ray diffraction (XRD, Bruker D8 Advance diffractometer using a Cu Kα source, λ=0.154056nm). Magnetic property of the HGS@Fe3O4 sample was characterized using a vibrating sample magnetometer (VSM, Riken Denshi, BHV-525) at room temperature. Electromagnetic parameters were measured by a vector network analyzer (VNA, Agilent N5247A) and Agilent coaxial airline type 85051-60007 via the reflection/transmission method in the frequency range of 1-18 GHz[21]. Calibration of VNA was in the same frequency range: a power of + 5 dBm was set to increase the dynamic range of measurements and an IF bandwidth of 200 Hz was set to increase the accuracy of the measurements. The real and imaginary parts of the complex permittivity and complex permeability were calculated based on Nicolson-Ross-Weir (NRW) method. To reduce uncertainties (the precision of engraving machine is ±0.01 mm), all values were obtained by averaging the data measured for five different samples. The 4

return loss curves of a metal-backed single absorbing layer were calculated according to the transmit line theory. It can be expressed as the following equation[2]:

RL=20log  Zin  Z0  /  Zin  Z0 



Zin  r /  r tanh j 2 fd r  r / c

(1)



(2)

Where Zin is the characteristic input impedance of the absorber, Z0 ≈377 ohm is the free space impedance, f is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of microwave in free space. In general, materials with reflection loss values less than -10 dB (90% absorption) are considered as efficient microwave absorbers. 3. Results and discussion Fig.1 shows the XRD patterns of GO, RGO and HGS@Fe3O4 powders. The diffraction peak at 2θ=11° in curve (a) is typical for GO and corresponding to the (001) reflection. The reduction of GO is conformed that except for a broad diffraction peak at 2θ=20°-30° (curve (b), reaggregation of graphene when it was dried), no other diffraction peak is found. In curve (c), eight peaks at 18.1 °, 30.2°, 35.6°, 37.1°, 43.1°, 53.3°, 57.2°, 62.5° are observed for HGS@Fe3O4, corresponding to the (111), (220), (311), (222), (400), (422), (511) and (440) planes of Fe3O4 (JCPDS No.65-3107), indicating the Fe3O4 film on the surface of hollow glass spheres is achieved.

5

Fig. 1. XRD patterns of (a) GO, (b) RGO and (c) HGS@Fe3O4

Fig. 2 shows the magnetic hysteresis loops (M-H loops) curve of HGS@Fe3O4 measured at room temperature. As can be seen, the sample exhibits typical soft ferromagnetism and its magnetization reaches saturation with the external field of about 7000 Oe. The saturation magnetization Ms of the sample is 49.8 emu/g, which is lower than that of pure Fe3O4 nanoparticles (75.4 emu/g) due to the presence of HGS[22]. As a result, it can be estimated that the mass percentage of Fe3O4 is about 66%. Furthermore, the coercivity of the sample is 28.4 Oe, which is much lower than that of bulk Fe3O4 (115-150 Oe). This might be due to the morphology-related shape anisotropy. 60 40

Hc=28.4 Oe Ms=49.8 emu/g

M(emu/g)

20

Mr=1.2 emu/g

0 -20 -40 -60 -8000 -6000 -4000 -2000

0

2000 4000 6000 8000

H(Oe)

Fig. 2. Magnetization hysteresis loop of the HGS@Fe3O4 powders.

The morphologies of uncoated, Fe3O4 coated HGS and HGS@Fe3O4-RGO/epoxy composites have been studied with scanning electron microscopy, as shown in Fig. 3. The raw HGS present spherical smooth surface 6

morphology (Fig. 3(a)) and the average diameter is determined to be 50 μm. After ferrite plating, the Fe3O4 magnetic films is observed evenly covering the entire surface of the HGS (Fig. 3(b)). The hollow structure of Fe3O4@HGS can be observed from the image of the broken parts (Fig. 3(c)). The thickness of the Fe3O4 films is within the range of 500 to 700 nm. In Fig. 3(d), the RGO sheets are uniform dispersed among HGS@Fe3O4. The effective concentration of RGO sheets increases when adding HGS@Fe3O4 into the composite.

FIG. 3. SEM images of (a) HGS; (b,c) HGS@Fe3O4; (d) HGS@Fe3O4-RGO/epoxy composite

Fig. 4 shows the frequency dependence of the complex permittivity (εr=ε′-jε″) and complex permeability (μr=μ′-jμ″) for RGO/epoxy, HGS@Fe3O4/epoxy, and HGS@Fe3O4-RGO/epoxy composites, respectively. The ε′ values of pure RGO/epoxy samples are decreasing from 6.2 to 4.4 while HGS@Fe3O4/epoxy samples show a nearly constant ε′ value of ~4.6, ~4.9 and ~5.6 for different HGS@Fe3O4 concentrations (Fig. 4 (a)). In contrast, the ε′ values for RGO-SF (1) lie from 7.7 at 1 GHz to 5.1 at 18 GHz, i.e. decreasing trend with increase in frequency. The ε′ values for RGO-SF composites increase with increasing HGS@Fe3O4 concentrations, with maximum ranging from 9.6 at 1 GHz to 6.3 at 18 GHz for 50 wt.% of HGS@Fe3O4 (Fig. 4 (b)). The decrease of ε′ as the increase of frequency may arise from the lags of induced charges in the material to follow the 7

reversing electromagnetic filed in the higher frequency range[23]. Meanwhile, the ε″ values of pure RGO/epoxy samples are decreasing from 1.7 to 1.1 while HGS@Fe3O4/epoxy samples show a nearly constant ε″ value of ~0.45, ~0.47 and ~0.51, respectively (Fig. 4 (c)). In contrast, the ε″ values for RGO-SF (1) lie from 1.9 at 1 GHz to 1.4 at 18 GHz. The ε″ values for RGO-SF (1), RGO-SF (2) and RGO-SF (3) composites also increase with increasing HGS@Fe3O4 concentration (Fig. 4 (d)). By combining HGS@Fe3O4 with a small quantity of RGO sheets, the ε′ and ε″ values both exhibit significant increase. The causes for the increment include interfacial polarization and electronic dipole polarization[24]. According to the Maxwell-Wagner interfacial polarization principle[25], interfacial polarizations increase due to the increased interfaces and decreased gap between fillers. Interfacial polarizations between HGS@Fe3O4/epoxy interfaces, RGO/epoxy interfaces and between HGS@Fe3O4/RGO interfaces all contribute to the significant increase of ε′. Furthermore, as epoxy is a strong dipole material and RGO is a new microwave absorbing material due to its desirable physical properties, more diploes present, which may result in electronic dipole polarization. In addition, the HGS@Fe3O4 in the composite can be considered as excluded volume in which RGO sheets cannot penetrate. The effective concentration of the RGO sheets thus increases with the addition of HGS@Fe3O4, rendering the formation of a more developed interconnected conductive network. According to the free electron theory[26], ε″=σ/ωε0, where ε0 is the permittivity of the free space, ω is the angular frequency, σ is the electrical conductivity, the conductivity of RGO-SF (1), RGO-SF (2), and RGO-SF (3) is higher than pure HGS@Fe3O4/epoxy or pure RGO/epoxy composites, thus leading higher ε″ values. Fig. 4 (e-h) shows the μ′ and the μ″ values of the composite. For RGO/epoxy samples, both μ′ and μ″ values are very low due to the weak magnetic characteristic. For HGS@Fe3O4/epoxy composite, the μ′ values obviously decreases from ~1.6 to ~1.1 with increasing frequency. By combining HGS@Fe3O4 with RGO sheets, the μ′ and μ″ values remained basically unchanged because of the weak magnetic characteristic of RGO sheets. The μ″ values first increase and then decrease for all HGS@Fe3O4 samples, exhibiting obvious resonance peaks 8

at about 2.5 GHz. In addition, μ″ values increase with the increase of HGS@Fe3O4 loadings because of the concentration of the magnetic filler. 6.5

10

(a)

(b)

RGO (1) SF (1) SF (2) SF (3)

6.0

RGO-SF (1) RGO-SF (2) RGO-SF (3)

9

8

ε'

ε'

5.5

5.0

7

4.5

6

5

4.0 2

4

6

8

10

12

14

16

18

2

4

6

8

12

14

16

18

Frequency (GHz)

Frequency (GHz) 1.8

2.7

(d)

RGO (1) SF (1) SF (2) SF (3)

(c) 1.5

RGO-SF (1) RGO-SF (2) RGO-SF (3)

2.4

1.2

2.1

ε''

ε''

10

0.9

1.8

0.6 1.5

0.3 1.2

2

4

6

8

10

12

14

16

2

18

4

6

Frequency (GHz)

10

12

14

16

18

Frequency (GHz)

1.7 1.6

8

1.6

(e)

(f)

RGO (1) SF (1) SF (2) SF (3)

1.5

RGO-SF (1) RGO-SF (2) RGO-SF (3)

1.5 1.4 1.3

1.3

u'

u'

1.4

1.2

1.2

1.1

1.1

1.0 1.0

0.9 2

4

6

8

10

12

14

16

18

Frequency (GHz)

2

4

6

8

10

12

Frequency (GHz)

9

14

16

18

0.4

0.3

(g)

(h)

RGO (1) SF (1) SF (2) SF (3)

0.3

0.2

0.2

0.1

u''

u''

RGO-SF (1) RGO-SF (2) RGO-SF (3)

0.1

0.0 0.0 -0.1 -0.1 -0.2 2

4

6

8

10

12

14

16

18

Frequency (GHz)

2

4

6

8

10

12

14

16

18

Frequency (GHz)

Fig. 4. (a,b) The real and (c,d) imaginary parts of the permittivity; (e,f) the real and (g,h) imaginary parts of the permeability of the composites

In general, the magnetic loss of a material originates mainly from magnetic hysteresis, domain wall resonance, eddy current effect, natural resonance and exchange resonance[27]. The magnetic hysteresis loss is caused mainly by the time lags of the magnetization vector behind external electromagnetic field vector and is negligible in weak applied field. The domain wall resonance occurs only in multidomain materials and usually at lower frequency range (<1GHz). So neither hysteresis loss nor domain wall resonance is the main contributor to magnetic loss of the absorber. Then if magnetic loss is only from the eddy current effect, μ″(μ′)-2f-1 should be a constant. Fig. 5 shows the dependence of μ″(μ′)-2f-1 values on frequency. The μ″(μ′)-2f-1 first increase and then decrease with the increasing frequency in the 1-10 GHz range and then remain constant in the frequency range of 10-18 GHz, illustrating that the magnetic loss in higher frequency range (10-18 GHz) is caused by eddy current effect. According to Aharoni’s theory[28], exchange resonance occurs at a higher resonance frequency than natural resonance. It may be reasonable to deduce that the resonance peak around 2.5 GHz is due to natural resonance, which is attributed to the incorporation of magnetocrystalline anisotropy and shape anisotropy of Fe3O4 magnetic particles.

10

0.06 RGO-SF (1)

0.04

RGO-SF (2) RGO-SF (3)

μ″(μ′)-2f-1

0.02 0.00 -0.02 -0.04 -0.06 -0.08 0

2

4

6

8

10

12

14

16

18

Frequency (GHz)

Fig. 5. μ″(μ′)-2f-1 versus frequency curves for RGO-HGS@Fe3O4/epoxy composite.

To reveal the microwave absorption performance of the composites, the RL values of composites were calculated according to the transmission line theory. The calculated RL curves of the composites with different thicknesses are shown in Fig. 6 and Fig. 7. The RL of HGS@Fe3O4 composites are relatively poor and there are no absorption bandwidth under -5 dB, as shown in Fig. 6 (a-c). In contrast, Fig. 6 (d) shows that the RL curves of RGO (1) are all below -5 dB for different layer thicknesses. According to the previous analysis of electromagnetic parameters in Fig. 4, the imaginary permittivity ε″ of RGO/epoxy composite is much higher than HGS@Fe3O4/epoxy composite and ε″ is directly related to the microwave absorption of the material. Furthermore, by combining HGS@Fe3O4 with RGO, the microwave absorption performance of the composite is further enhanced. In Fig. 7, the minimum RL for RGO-SF (1) is -11.6 dB for layer thickness of 4 mm. For RGO-SF (2) composite, the minimum RL is -13.3dB while the bandwidth less than -10 dB can reach up to 2.1 GHz(from 11.8 to 13.9 GHz). All minimum RL are less than -10 dB in the thicknesses range of 2.5-4 mm. Whereas for RGO-SF (3) composite, the minimum RL is -15.8 dB at 11.9 GHz when the layer thickness is 2.5 mm, the bandwidth less than -10 dB can reach up to 3.6 GHz(from 10.3 to 13.9 GHz). In fact, compared with the existing data of prior works, the results aren't necessarily better. However, the employment of lightweight HGS@Fe3O4 and RGO could lead to lightweight microwave absorber (0.57-0.72 g/cm3), so the performance of the as-prepared absorber can catch up with the rest of those "heavier" ones. Table 1 lists some representative 11

lightweight absorbers[29–34].

0

0

(a)

SF(1)

-4

-6

2.5 mm 3 mm 3.5 mm 4 mm

-8

(b)

-2

Reflection loss (dB)

Reflection loss (dB)

-2

-4

-6

2.5 mm 3 mm 3.5 mm 4 mm

-8

-10

SF(2)

-10

2

4

6

8

10

12

14

16

18

2

4

6

8

10

12

14

16

18

Frequency (GHz)

Frequency (GHz)

0

0

RGO (1) -2

-4

Reflection loss (dB)

Reflection loss (dB)

-2

(c)

SF(3)

-6

2.5 mm 3 mm 3.5 mm 4 mm

-8

(d)

-4

-6

2.5 mm 3 mm 3.5 mm 4 mm

-8

-10

-10

2

4

6

8

10

12

14

16

18

2

4

6

8

10

12

14

16

18

Frequency (GHz)

Frequency (GHz)

Fig. 6. Reflection loss curves of (a-c) Fe3O4@HGS and (d) RGO composites with different thicknesses. 0

0

RGO-SF (2)

RGO-SF (1) -2

(a) Reflection loss (dB)

Reflecton loss (dB)

-2 -4 -6

2.5 mm 3 mm 3.5 mm 4 mm

-8 -10

(b)

-4 -6

2.5mm 3 mm 3.5 mm 4 mm

-8 -10 -12

-12

-14 2

4

6

8

10

12

14

16

18

Frequency (GHz)

2

4

6

8

10

12

Frequency (GHz)

12

14

16

18

0

RGO-SF (3)

Reflection loss (dB)

-4

(c)

-8

-12

2.5 mm 3 mm 3.5 mm 4 mm

-16

-20 2

4

6

8

10

12

14

16

18

Frequency (GHz)

Fig. 7. Reflection loss curves of RGO-SF composites with different thicknesses

Table 1. Microwave absorption of some representative lightweight absorbers

Bandwidth

Frequency range

(GHz)

(GHz)

(RL<-10 dB)

(RL<-10 dB)

RGO-HGS@Ag

4.1

E-glass-CNTs

Density

Min RL

(g/cm3)

value (dB)

13.1-17.2

0.8

-18.6

29

3.6

8.4-12

1.98

-20

30

BFCMs

2.9

9.1-9.8,12.5-13.8,15.1-16

1.8

-31

31

CNTs-FeCoNi

5.6

12.4-18

1.18

-28.2

32

GS@CoFe2O4

None

None

0.46

-8.3

33

SiCNWs

4.2

29.1-33.3

1.3

-32.3

34

Our work

3.6

10.3-13.9

0.57

-15.8

/

Sample

Ref

As a good microwave absorber, two basic conditions are required: when the microwave is incident to the 13

surface of an absorber, the direct reflection of microwaves should be minimum, thus it is possible that most microwave propagate into the absorber. This requires the input impedance Zin equals free-space wave impedance Z0 (377 ohm). In addition, according to the previous study[35], for a single-layer absorber the strong RL peak originates from the cancellation of two microwaves at front interface of the absorber. The two waves are the reflection wave which is reflected at the front interface of the absorber and the emerging wave which is reflected by the metal reflector. The peak intensity of RL is directly affected by the intensity of the two waves. It is believed that at the minimal reflection point, the input impedance Zin equals free-space wave impedance Z0, which means the imaginary input impedance, Zin , approaches zero ohm while the corresponding real input impedance, Zin , approaches 377 ohm[36]. Furthermore, the microwave within the propagation medium should be quickly and completely attenuated through dielectric loss and magnetic loss. The absorbent properties are mainly given by the attenuation constant α and can be expressed by:





f 2 2 2 2  2                     c 





12



12

  

(3)

The attenuation constant determines the attenuation properties of an absorber. As a consequence, the improvement of microwave absorption mainly originates from two key factors: impedance matching and electromagnetic wave attenuation. Fig. 8 shows the complex impedance of RGO (1), SF (3) and RGO-SF (3) composite samples with the layer thickness of 2.5 mm. It can be seen that at 11.9 GHz, Zin for RGO-SF (3) composite is found to be -44.6 ohm, the corresponding Zin is 503.1 ohm, which is very close to the required values of zero ohm and 377 ohm. As for RGO (1) composite, Zin and Zin are found to be 19.3 ohm and 939.9 ohm respectively at 14.6 GHz. The Zin is very close to zero, however, Zin is far away from 377 ohm, 14

leading a worse microwave absorption performance compared to RGO-SF (3). Whereas for SF (3) composite, the Zin is 84.3 ohm at 12.4 GHz while the corresponding Zin is 2025 ohm, the highest real impedance among the samples. As a consequence, the microwave absorption performance is very poor. Fig. 9 shows the dependence of attenuation constant on frequency. The RGO-SF (3) composite has the maximum α value among the three samples, therefore, it exhibits better microwave absorption performance. In addition, due to the low density of RGO and HGS@Fe3O4, the density of RGO-HGS@Fe3O4/epoxy composites is in the range of 0.57-0.72 g/cm3, suggesting that such hybrid composite is more advantageous than traditional heavy inorganic absorbing materials in the field requiring lightweight. 2100

1200

(b) 1800

600 Real part of Zi (Ohm)

Imaginary part of Zi (Ohm)

900

(a)

300 0 -300

RGO-SF (3) RGO (1) SF (3)

-600

1500 1200 RGO-SF (3) RGO (1) SF (3)

900 600 300

-900

0

-1200 2

4

6

8

10

12

14

16

2

18

4

6

8

10

12

14

Frequency (GHz)

Frequency (GHz)

Fig. 8. Complex impedance of the composites: (a) imaginary impedance and (b) real impedance. 160 RGO-SF (3) RGO (1) SF (3)

Attenuation constant α (Np/m)

140 120 100 80 60 40 20 0 2

4

6

8

10

12

14

16

Frequency (GHz)

Fig. 9. Attenuation constant α of the composites.

4. Conclusions 15

18

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In summary, using a combination of lightweight HGS@Fe3O4 and electrically conductive RGO within the epoxy matrix, we have prepared a novel lightweight microwave absorber. The influence of the HGS@Fe3O4 and RGO contents on the electromagnetic properties and microwave absorptions of the resulting composites were demonstrated. Compared with HGS@Fe3O4/epoxy and RGO/epoxy composite, the hybrid composites showed enhanced values of complex permittivity. The density of the resulted composite is in the range of 0.57-0.72 g/cm3, the absorption bandwidth less than -10 dB can reach up to 3.6 GHz with layer thickness of 2.5 mm for 50 wt. % of HGS@Fe3O4 and 1 wt. % of RGO. The enhanced microwave absorption performance is due to the better impedance matching and enhanced microwave attenuation. The presented hybrid composite can be expected to find potential application in reduction of EMI in stealth technology. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51373129) [1] Zhang W, Zhang D. EM-wave absorption properties of hollow spiral iron particles. J Magn Magn Mater 2015;396:169-71. [2] Qin F, Brosseau C. A review and analysis of microwave absorption in polymer composites filled with carbonaceous particles. J Appl Phys 2012;111:061301. [3] Fannin PC, Marin CN, Malaescu I, Stefu N, Vlazan P, Novaconi S, et al. Microwave absorbent properties of nanosized cobalt ferrite powders prepared by coprecipitation and subjected to different thermal treatments. Mater Des 2011;32:1600-4. [4] Wang T, Han R, Tan G, Wei J, Qiao L, Li F. Reflection loss mechanism of single layer absorber for flake-shaped carbonyl-iron particle composite. J Appl Phys 2012;112:104903. [5] Dong C, Wang X, Zhou P, Liu T, Xie J, Deng L. Microwave magnetic and absorption properties of M-type ferrite BaCoxTixFe12−2xO19 in the Ka band. J Magn Magn Mater 2014;354:340-4. [6] Brosseau C, Boulic F, Queffelec P, Bourbigot C, Le Mest Y, Loaec J, et al. Dielectric and microstructure properties of polymer carbon black composites. J Appl Phys 1997;81:882. [7] Adohi BJ-P, Mdarhri A, Prunier C, Haidar B, Brosseau C. A comparison between physical properties of carbon black-polymer and carbon nanotubes-polymer composites. J Appl Phys 2010;108:074108. [8] Adohi BJP, Bychanok D, Haidar B, Brosseau C. Microwave and mechanical properties of quartz/graphene-based polymer nanocomposites. Appl Phys Lett 2013;102:072903. [9] Brosseau C, NDong W, Mdarhri A. Influence of uniaxial tension on the microwave absorption properties of filled polymers. J Appl Phys 2008;104:074907. [10] Han M, Ou Y, Deng L. Microwave absorption properties of double-layer absorbers made of NiCoZn ferrites and hollow glass microspheres electroless plated with FeCoNiB. J Magn Magn Mater 2009;321:1125-9. [11] Zhu Y-F, Ni Q-Q, Fu Y-Q, Natsuki T. Synthesis and microwave absorption properties of electromagnetic functionalized Fe3O4–polyaniline hollow sphere nanocomposites produced by electrostatic self-assembly. J Nanoparticle Res 2013;15:1988. [12] Lee C, Wei X, Kysar JW, Hone J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008;321:385-8. 16

[13] Wang Y, Hoang Y, Song Y, Zhang X, Ma Y, Liang J, et al. Room-temperature ferromagnetism of graphene. Nano Lett 2009;9:220-4. [14] Novoselov KS, Jiang Z, Zhang Y, Morozov S V., Stormer HL, Zeitler U, et al. Room-temperature quantum Hall effect in graphene. Science 2007;315:1379. [15] Balandin A a, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8:902-7. [16] Wen B, Wang XX, Cao WQ, Shi HL, Lu MM, Wang G, et al. Reduced graphene oxides: the thinnest and most lightweight materials with highly efficient microwave attenuation performances of the carbon world. Nanoscale 2014;6:5754-61. [17] Wei J, Liu J, Li S. Electromagnetic and microwave absorption properties of Fe3O4 magnetic films plated on hollow glass spheres. J Magn Magn Mater 2007;312:414-7. [18] Liu J, Wei J, Li S. Preparation and characteristics of Fe3O4 magnetic thin films plated on hollow glass spheres. Mater Lett 2007;61:1529-32. [19] Bao C, Song L, Xing W, Yuan B, Wilkie C a., Huang J, et al. Preparation of graphene by pressurized oxidation and multiplex reduction and its polymer nanocomposites by masterbatch-based melt blending. J Mater Chem 2012;22:6088-96. [20] Moon IK, Lee J, Ruoff RS, Lee H. Reduced Graphene Oxide by Chemical Graphitization. Nat Commun 2010;1:73. [21] Wei J, Zhao R, Liu X. Only Ku-band microwave absorption by Fe3O4/ferrocenyl-CuPc hybrid nanospheres. J Magn Magn Mater 2012;324:3323-7. [22] Ni S, Lin S, Pan Q, Yang F, Huang K, He D. Hydrothermal synthesis and microwave absorption properties of Fe3O4 nanocrystals. J Phys D Appl Phys 2009;42:055004. [23] Brosseau C, Quéffélec P, Talbot P. Microwave characterization of filled polymers. J Appl Phys 2001;89:4532-40. [24] Adohi BJP, Laur V, Haidar B, Brosseau C. Measurement of the microwave effective permittivity in tensile-strained polyvinylidene difluoride trifluoroethylene filled with graphene. Appl Phys Lett 2014;104:082902. [25] Tamura R, Lim E, Manaka T, Iwamoto M. Analysis of pentacene field effect transistor as a Maxwell-Wagner effect element. J Appl Phys 2006;100:114515. [26] Zhang XF, Dong XL, Huang H, Liu YY, Wang WN, Zhu XG, et al. Microwave absorption properties of the carbon-coated nickel nanocapsules. Appl Phys Lett 2006;89:053115. [27] Wang J, Wang J, Xu R, Sun Y, Zhang B, Chen W, et al. Enhanced microwave absorption properties of epoxy composites reinforced with Fe50Ni50-functionalized graphene. J Alloys Compd 2015;653:14-21. [28] Aharoni A. Exchange resonance modes in a ferromagnetic sphere. J Appl Phys 1991;69:7762. [29] Wang J, Sun Y, Chen W, Wang T, Xu R, Wang J. Enhanced microwave absorption performance of lightweight absorber based on reduced graphene oxide and Ag-coated hollow glass spheres/epoxy composite. J Appl Phys 2015;117:154903. [30] Choi I, Lee D, Lee DG. Radar absorbing composite structures dispersed with nano-conductive particles. Compos Struct 2015;122:23-30. [31] Mu G, Shen H, Qiu J, Gu M. Microwave absorption properties of composite powders with low density. Appl Surf Sci 2006;253:2278-81. [32] Lv R, Kang F, Gu J, Gui X, Wei J, Wang K, et al. Carbon nanotubes filled with ferromagnetic alloy nanowires: Lightweight and wide-band microwave absorber. Appl Phys Lett 2008;93:1-4. [33] Fu W, Liu S, Fan W, Yang H, Pang X, Xu J, et al. Hollow glass microspheres coated with CoFe2O4 and its microwave absorption property. J Magn Magn Mater 2007;316:54-8. [34] Chiu S-C, Yu H-C, Li Y-Y. High Electromagnetic Wave Absorption Performance of Silicon Carbide Nanowires in the Gigahertz Range. J Phys Chem C 2010;114:1947-52. [35] Wang T, Qiao L, Han R, Zhang Z. The origin of reflection loss peaks in the double-layer electromagnetic wave absorber. J Magn Magn Mater 2012;324:3209-12. [36] Gogoi JP, Bhattacharyya NS, Bhattacharyya S. Single layer microwave absorber based on expanded graphite–novolac phenolic resin composite for X-band applications. Compos Part B Eng 2014;58:518-23. 17

Highlights 1. Epoxy resin based lightweight composites comprising Fe3O4-coated hollow glass spheres (HGS@Fe3O4) and reduced graphene oxide (RGO) were prepared. 2. The absorption bandwidth less than -10 dB can reach up to 3.6 GHz with layer thickness of 2.5 mm. 3. The density of the composites is in the range of 0.57-0.72 g/cm3, which is attractive candidate for a new type of lightweight microwave absorber 4. The enhanced microwave absorption properties are attributed to the better impedance matching and enhanced microwave attenuation.

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