CoFe2O4 composites

Author’s Accepted Manuscript Synthesis and microwave absorption properties of coiled carbon nanotubes/CoFe2O4 composites Jiantao Feng, Yechen Wang, Yanhui Hou, Juanbi Li, Liangchao Li www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(16)31418-3 http://dx.doi.org/10.1016/j.ceramint.2016.08.110 CERI13562

To appear in: Ceramics International Received date: 1 August 2016 Revised date: 10 August 2016 Accepted date: 18 August 2016 Cite this article as: Jiantao Feng, Yechen Wang, Yanhui Hou, Juanbi Li and Liangchao Li, Synthesis and microwave absorption properties of coiled carbon nanotubes/CoFe2O4 composites, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.08.110 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.

Synthesis and microwave absorption properties of coiled carbon nanotubes/CoFe2O4 composites Jiantao Feng, Yechen Wang, Yanhui Hou, Juanbi Li, Liangchao Li* College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua, Zhejiang 321004, China *

Corresponding author.

Tel: +86 579 82282384; fax: +86 579 82282489. [email protected]

Abstract Coiled carbon nanotubes (CCNTs) were coated by CoFe2O4 (CFO) nanoparticles to form the CCNTs/CFO composites. The phase composition, morphology, electrical and magnetic performances of the composites were characterized by means of modern analytical techniques. The results indicated that the conductivity and magnetic performances of the samples were related to their compositions. The CCNTs/CFO composite with ωCCNT/ωCFO of 0.4 exhibited the best microwave absorption property in the prepared samples, whose minimum reflection loss was about -14 dB in the frequency range of 2-18 GHz for a layer of 2.5-3.0 mm thickness, and the available bandwidth corresponding to RL less than -10 dB can reach 4.0 GHz. Hence, the obtained composite could be used as a new candidate for selective frequency shielding and lightweight microwave absorbing material. Keywords CCNTs; CoFe2O4; electrical and magnetic properties; microwave absorption;

1 Introduction

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In recent years, with the constant development of telecommunications and digital systems, there is a growing demand for the exploration of high-efficiency microwave absorption materials with light weight, broad bandwidth and strong adsorption [1,2], not only for their great potential in civilian application including protecting people from electromagnetic radiation, but also for military purposes, such as radar invisible aircrafs, bombs and tanks [3-5]. An excellent microwave absorption material usually consists of two types of components: magnetic loss component and dielectric loss component, which are elaborately combined with each other to achieve impedance matching conditions [6,7]. So it is important to choose the optimal combination between them to obtain the desired microwave absorption materials. Spinel ferrites, MFe2O4 (M: Co2+, Ni2+,Cu2+, Zn2+ and so on), are a kind of extremely important magnetic semiconductors [8], among which CoFe2O4 has attracted much attention due to its ultra high resistivity, high frequency permeability, large coercive force and anisotropy constants [9,10]. As an electrical and magnetic medium, CoFe2O4 not only lies in the electromagnetic equipment as an excellent component, but also has been widely applied to the microwave absorption field because of the relative high real part and imaginary part of complex permeability in certain microwave frequency range [11]. However, the application of CoFe2O4 as an ideal absorbing material are limited due to its high density, easy agglomeration, low magnetic loss in the high frequency and other shortcomings [12,13], and cannot meet all the requirements. In order to improve this, dielectric loss fillers such as conducting polymers [14], carbon materials [15] and woods [16] are always introduced into the system, which can obtain higher absorption and broaden the frequency band. Typically, carbon materials have been the most widely

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studied and lots of novel works have been reported, such as CoFe2O4/r-GO [17], carbon fibers/CoFe2O4 [18] and r-GO@PPy@CoFe2O4 [19]. CCNTs possess a unique coiled structure [20] (shown in Fig. 1) except for the advantages of stability, light weight and good conductance that ordinary carbon materials have. Ihara and Dunlap [21,22] have predicted the coiled structure of pure carbon materials by theoretical calculation, and proved that there exist five, six and seven membered carbon rings with simulation means and the whole structure is stable in hermodynamics. Outside the obvious tubular structure, CCNTs also possess mesoscopic sized coiled morphology [23] which makes the material present chiral features [24] and enhanced dielectric loss characteristic [25]. It is generally recognized that their coiled character roots in the non six membered carbon rings [26] will be able to urge electromagnetic waves entered into the CCNTs to be decayed by multiple reflections and refractions in the tubes [27,28]. Chen et al have prepared a kind of carbon coil-carbon fiber hybrid materials, and it was found that the special coiled morphology of the as-prepared composites do be beneficial to improve its microwave absorption ability. Its’ optimal reflection loss (RL) value was -17.3 dB and maximum bandwidth of absorption exceeding -10 dB was up to 5.8 GHz [29]. So if CCNTs is introduced into the ferrite, the obtained composites will combine the magnetism of ferrite and the electrical conductivity of CCNTs to realize the triple absorption (electrical loss, magnetic loss, chiral control) on electromagnetic wave. Based on the above assumptions, the CCNTs/CoFe2O4 composites were prepared by using the oxidized CCNTs as chiral substrate. The structure, morphology, electrical conductivity, electrical and magnetic loss performances were characterized and the

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microwave absorbing properties were also investigated for the prepared samples.

Fig. 1 The structure diagram types of coiled carbon nanotubes

2 Experimental section 2.1 Materials Iron (Ⅲ) nitrate nonahydrate (Fe(NO3)3·9H2O), cobalt (Ⅱ) nitrate hexahydrate (Co(NO3)2·6H2O), sodium hydroxide (NaOH, 25-28 wt%), concentrated sulfuric acid (H2SO4, 98 wt%) and concentrated nitric acid (HNO3) were purchased from Sinopharm Chemical Reagent Co. Ltd.(China). Coiled carbon nanotubes (inside diameter =120 nm) were received from Beijing Boyu Gaoke New Material Technology co., Ltd. (China). All reagents were of analytical grade and used without any further purification. 2.2 Oxidization of CCNTs A certain amount of CCNTs were dissolved in 80 mL mixed acid consists of concentrated sulfuric acid and nitric acid ( the volume ratio of 3:1 ), and sonicated for about 10 min , followed by refluxing for 2 h at 90 oC. The products were then filtered and washed with deionized water until the supernatant was neutral. The oxidized CCNTs will

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be obtained by ball-milling [30]. 2.3 Synthesis of CoFe2O4 (CFO) nanoparticles Fe(NO3)3·9H2O and Co(NO3)2·6H2O with a mole ratio of 2:1were dissolved into distilled water to form a transparent solution under vigorous stirring. 1 mol·L-1 of NaOH solution was added slowly to adjust pH of the solution to 10, and stirred for 2 h at 80 oC, then cooled to room temperature. The powders obtained by magnetic separation were washed alternately with distilled water and ethanol for several times until the supernatant was neutral, dried to constant weight at 85 °C in vacuum. After sintered at 500 oC for 2 h, the CFO nanoparticles were obtained. 2.4 Synthesis of CCNTs/CoFe2O4 (CCNTs/CFO) composites The CCNTs/CFO composites were prepared by chemical co-precipitation method. Fe(NO3)3·9H2O and Co(NO3)2·6H2O weighed according to the stoichiometric ratio of CoFe2O4 were dissolved in 40 mL of distilled water to form a transparent solution. Subsequently, a certain quality of oxidized CCNTs was added into the above solution. After being sonicated for 1 h, 1 mol·L-1 of NaOH solution was added dropwise to the above solution until the pH value was up to 10, and stirred for 2 h at 80 °C, then cooled to room temperature. The solid phase collected by magnetic separation was washed alternately with distilled water and ethanol until the supernatant was neutral, dried to constant weight at 85 °C under vacuum, and calcined at 500 oC for 2 h under N2. Finally, the CCNTs/CFO composites with different mass ratios of CCNTs to CFO (ωCCNT/ωCFO) were obtained. 2.5 Characterization and measurement The X-ray diffraction (XRD) patterns of the samples were obtained by using a

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Philps-PW3040/60 diffractometer with Cu Kα radiation (2θ = 20° - 80°, λ = 0.154056 nm). Thermogravimetric analysis (TG-SDTA) was carried out on a Netzsch SDTA581 thermal analyzer from room temperature to 800 oC at a heating rate of 10 oC/ min in air atmosphere. The morphology and the microstructure of the synthesized samples were observed by a scanning electron microscopy (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, JEOL2010), respectively. FT-IR spectra of samples were recorded by a Fourier transform infrared spectrometer (FTIR, NEXUS-670) between 400 and 4000 cm-1. The electrical conductivities were carried out on a four-probe resistivity instrument (SDY-4) at room temperature. A Lakeshore 7404 vibrating sample magnetometer was used to measure the magnetization of the samples in applied magnetic fields over the range of -15 to +15 kOe at room temperature. The transmission/reflection method was applied to determine the relative complex permeability and permittivity of the sample/wax composites through an Agilent N5230 vector network analyzer system. The cylindrical samples, with a 3.0 mm inner diameter, 7.0 mm outer diameter and a thickness of 1.5-3.5 mm, were fabricated by uniformly mixing wax with the absorbents in mass ratios of 1:2 and then pressed into cylindrical compacts. Based on the above relative permeability and permittivity at the given frequency (f) and thickness (d), the reflection loss (RL) values of absorbing materials were calculated according to the transmission line theory [31]

3 Results and discussion 3.1 XRD analysis The XRD patterns of CCNTs, CFO and CCNTs/CFO composites are presented in Fig. 2. From Fig. 2a, the diffraction peaks presented at 25.61o, 43.53o correspond to (002)

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and (100) crystal face of CCNTs [32]. The peaks at 18.37o, 30.34°, 35.52°, 43.17°, 53.70°, 57.14° and 62.76° in the Fig. 2b can be indexed to (111), (220), (311), (400), (422), (511) and (440) crystal planes of CFO nanoparticles (JCPDS-ICDD22-1086), respectively. The peaks of CCNTs/CFO composites are shown at 2θ of 26.16°, 30.34°, 35.52°, 43.17°, 43.96°, 53.70° and 57.14° in Fig. 2(c-e), in which the peaks at 26.16° and 43.96° are attributed to CCNTs, while others correspond to CFO. It is also found that the characteristic peaks of CFO in the CCNTs/CFO composites move in the direction of increasing 2θ due to the interactions between CCNTs and CFO. In additional, the relative intensities of the characteristic peaks of CCNTs increase as the contents of CCNTs in the composites increasing, while those of CFO decrease gradually.

Intensity(a.u.)

e d c

311 111

220

(002)

10

20

30

400

422 511 440

b a

(100)

40

50

60

70

80

90

2Thetadegree

Fig. 2 XRD patterns of CCNTs (a), CFO (b) and composites with ωCCNT/ωCFO of 0.2 (c), 0.4 (d) and 0.6 (e)

3.2 Thermal stability Fig. 3 shows the TG curves of CFO (a), CCNTs (b) and CCNTs/CFO composite with ωCCNT/ωCFO of 0.6 (c). It can be known from Fig. 3a that the mass loss of CFO is very little, which may be caused by adsorbed water and small amount of hydroxide. And for CCNTs (Fig. 3b), it is oxidized completely until 630 oC, and the residual mass about 7

1.13% is related to the deposition of carbon. A two-step pattern for mass loss can be observed for CCNTs/CFO composite in Fig. 3(c). The first-step mass loss below 120 oC is probably due to the removal of surface adsorbed water. The maximum mass loss occurs in the second step between 122 and 720 oC and is mainly ascribed to oxidation of carbon nanotubes and decomposition of a few hydroxide. As carbon components in the composite can be completely burned in air, the final product can be composed of CFO and the a small amount of CCNTs. The residual content (61.72 %) is basically agreed with the theoretical content (62.5 %), which further confirms that the composite is composed of CCNTs and CFO. Furthermore, the final decomposition temperature of CCNTs is 634 oC, while that of CCNTs/CFO is 720 oC, illustrating the thermal stability of CCNTs/CFO composites is better than CCNTs. This is because the CFO coating enhances its interaction with the CCNTs and weakens the heat conduction effect of CCNTs at the same time.

a

100

Mass (%)

80

c 60

40

20

b 0 0

100

200

300

400

500

600

700

800

Temperature (oC)

Fig. 3 TG cruves of CFO (a), CCNTs (b) and composite with ωCCNT/ωCFO of 0.6 (c)

3.3 FTIR spectra Fig. 4 shows the FTIR spectra of CFO (a), CCNTs (b) and CCNTs/CFO composite

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with ωCCNT/ωCFO of 0.6 (c). The characteristic peaks of O-H stretching vibration [33,34] observed at 3430 cm-1 and 1630 cm-1 illustrate that the products contain a certain amount of adsorbed water and there are some hydroxyl groups on the surface of CCNTs. In Fig. 4(a), absorption peaks appear at 570 cm-1 and 416 cm-1 demonstrate that the CFO has a spinel structure [35]. In Fig. 4(b), absorption peak near 1720 cm-1 shows that there exist oxidized -COOH in the surface of CCNTs , while the absorption band at 1250 - 1150 cm-1 is caused by the stretching vibration of C-C bond. All the above absorption peaks appear in the Fig. 4(c), illustrating that the both CFO and CCNTs exist in the composite simultaneously. It also can be observed that the peak at 583 cm-1 related to the M-O bond stretching vibration of the ferrite tetrahedral site in the Fig. 4(c) has some deviation to varying degrees compared with pure CFO (Fig. 4(a)), revealing that there exists some interactions between CCNTs and CFO in the composite.

Transmittance(%)

a

c

b

3500

3000

2500

2000

1500

1000

500

Wavenumber(cm-1)

Fig. 4 FT-IR spectra of CFO (a), CCNTs (b), composite with ωCCNT/ωCFO of 0.6 (c)

3.4 The morphology Fig. 5 shows the SEM and TEM images of CCNTs, CFO and CCNTs/CFO composite, respectively. As shown in Fig. 5(a), the carbon nanotubes appear to be a

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helical structure with small screw pitch and several micrometers length, and their external diameter is about 100 - 200 nm ( Fig. 5(b)). It is clearly seen from Fig. 5(c) that CFO particles are spherical with an average size of ~50 nm and have apparent agglomeration. It can be observed from Fig. 5(c, d) that CFO nanoparticles have completely coated on the surface of CCNTs, and have agglomeration to some extent, indicating that CCNTs/CFO composites have been prepared successfully.

Fig. 5 SEM images of CCNTs (a), CFO (c), composite with ωCCNT/ωCFO of 0.6 (d) and TEM image of CCNTs (b)

3.5 Electrical conductivity The electrical conductivities of CCNTs and CCNTs/CFO composites are listed in the table 1. As is known that the resistivity of CFO is very high and its order of magnitude reaches up to 107, while the CCNTs is a kind of excellent conductive materials. It is clearly seen from table 1 that the electrical conductivities of composites increase with 10

increasing of ωCCNT/ωCFO attributing to the conductive CCNTs in the composites. Tabel 1The electrical conductivities of CCNTs and CCNTs/CFO composites Samples

Conductivity (S/cm)

ωCCNT/ωCFO = 0.2

0.2780

ωCCNT/ωCFO = 0.4

6.371

ωCCNT/ωCFO = 0.6

13.75

CCNTs

56.61

3.6 Magnetic properties The hysteresis loops of CFO and CCNTs/CFO composites are shown in Fig. 6 and their saturation magnetization and coercivity are listed in the table 2. As is shown in Fig. 6, the Ms values of CCNTs/CFO composites are obviously smaller than that of pure CFO nanoparticle because the CCNTs are nonmagnetic. It can be observed from table 2 that the Ms value of composites decreases from 13.9165 emu/g to 4.37744 emu/g with the content of CCNTs increasing, while the Hc does not have apparent changes. And according to Ms = φms [36] (Ms: saturation magnetization of the composites; φ: the volume fraction of magnetic particles; ms: saturation magnetization of a single magnetic particle), the Ms of CCNTs/CFO composites mainly depends on the content of magnetic CFO, the greater the φ of CFO is, the higher the Ms of CCNTs/CFO is. The Hc of the CCNTs/CFO composite with ωCCNT/ωCFO of 0.2 is the maximum among all the composites. This may be because that the synthetic effects between CFO particles and CCNTs increase the surface anisotropy of the CCNTs/CFO, then leading to the Hc increasing. And the Hc of the composites is influenced by many factors, such as the morphology, grain shape, components, anisotropy (magnetocrystallinity, stress, shape) and magnetostriction [37, 38].

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MassMoment(emu/g)

60

a

40 20

b c d

0 -20 -40 -60 -15000 -10000 -5000

0

5000

10000 15000

Field(G)

Fig. 6 Hysteresis loops of CFO (a) and CCNTs/CFO composites with ωCCNT/ωCFO of 0.2 (b), 0.4 (c) and 0.6 (d) Table 2 Saturation magnetization and coercivity of CFO and CCNTs/CFO composites Smaples ωCCNT/ωCFO = 0.2

Saturation magnetization (Ms/emu/g) 13.9165

Coercivity (Hc/Oe) 149.333

ωCCNT/ωCFO = 0.4

8.66852

87.066

ωCCNT/ωCFO = 0.6

4.37744

141.602

CFO

55.7813

210.667

3.7 Electromagnetic loss It is well known that the real parts of complex permittivity (ε′) and permeability (μ′) represent the storage of electric and magnetic energy, respectively, while the imaginary parts of complex permittivity (ε") and permeability (μ") symbolize the loss of electric and magnetic energy, respectively. The dielectric loss tangent (tanδε = ε"/ε′) and magnetic loss tangent (tanδμ = μ"/μ′) show the dielectri loss and magnetic loss, respectively. An excellent absorber should have higher tanδε and tanδμ, which means that the ε" and μ" are as large as possible based on the biggish values of ε′ and μ′. 3.7.1 Dielectric loss The ε′, ε" and tanδε of CFO and CCNTs/CFO composites with different values of

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ωCCNT/ωCFO values varying from frequency are given in the Fig. 7, respectivity. Generally, the larger the tanδε is, the better the dielectric loss performance of the sample will be in the same frequency range. It is seen that the ε′ and ε" values are small, leading to small tanδε value. In the frequency range of 2 - 18 GHz, the ε′ values of composites are bigger than those of CFO, while the ε" and tanδε values have the opposite trend besides sample of ωCCNT/ωCFO = 0.6 in the range of 2 - 12 GHz. The dielectric loss of CCNTs/CFO composites, depends on the electric dipole polarization and interfacial polarization in the high frequency section [39, 40], is mainly derived from the polarization and relaxation effect caused by the local bound charges of CCNTs and polarization between the CCNTs and CFO. And it is the synthetic effects of ε′ and ε". 4.4 CFO CCNTs/CFO = 0.2 CCNTs/CFO = 0.4 CCNTs/CFO = 0.6

0.4

4.0



3.6 0.0 CFO CCNTs/CFO = 0.2 CCNTs/CFO = 0.4 CCNTs/CFO = 0.6

3.2

-0.2

2.8 2

4

6

8

10

12

14

16

18

2

4

6

8

f (GHz)

10

f(GHz)

0.15 CFO CCNTs/CFO = 0.2 CCNTs/CFO = 0.4 CCNTs/CFO = 0.6

0.10

tan



0.2

0.05 0.00 -0.05 -0.10 2

4

6

8

10

f(GHz)

13

12

14

16

18

12

14

16

18

Fig. 7 Frequency dependence of ε′, ε" and tanδε of CFO and CCNTs/CFO composites

3.7.2 Magnetic loss The μ′, μ" and tanδμ values of CFO and CCNTs/CFO composites with different ωCCNT/ωCFO varying from the frequency are presented in Fig. 8. It is usually thought that the tanδμ value has a positive correlation with the magnetic loss performance of samples. The magnetic loss of the ferrite is attributed to the hysteresis, domain wall resonance, eddy current effect, natural resonance and exchange resonance of magnetic particles [41, 42]. It can be observed from the Fig. 8 that the magnetic loss performance of the composites is poor and seems to be almost negligible behind 10 GHz. This may be because two reasons. First, CFO content in the composites is smaller than that of pure CFO, and the smaller magnetic content, the smaller magnetic loss; Second, the CFO nanoparticles load on the surface of CCNTs, which can weaken the intergranular exchange coupling interaction of the adjacent ferrite particles, resulting in the decrease of magnetisability and saturation magnetization of the samples. 1.6

0.8 CFO CCNTs/CFO = 0.2 CCNTs/CFO = 0.4 CCNTs/CFO =0.6

1.4

CFO CCNTs/CFO = 0.2 CCNTs/CFO = 0.4 CCNTs/CFO = 0.6

0.6

0.4





1.2

0.2

1.0

0.0 0.8 2

4

6

8

10

12

14

16

18

2

f (GHz)

4

6

8

10

f (GHz)

14

12

14

16

18

0.8 CFO CCNTs/CFO = 0.2 CCNTs/CFO = 0.4 CCNTs/CFO = 0.6

0.6

tan

0.4

0.2

0.0

2

4

6

8

10

12

14

16

18

f (GHZ) Fig. 8 Frequency dependence of μ′, μ" and tanδμ of CFO and CCNTs/CFO composites

It is expected that the efficient cooperation between the dielectric loss and magnetic loss enhance greatly the microwave absorption and broaden the microwave absorption bandwidth. Hence, it can make a deduction from the above tanδε and tanδμ values that the prepared samples have better microwave absorbing properties in the range of 10-16 GHz. 3.8 Microwave absorbing properties The microwave absorbing properties of the materials can be defined by the reflection loss. The reflection loss is calculated based on the relative complex permeability (r =j ) and permittivity (r =j ) at a given frequency and absorber medium thickness by the following equations [43]: RL(dB)  20 log

Zin  Z 0

Zin  Z0 Z in  Z 0

(1)

r tanhj (2fd / c)(  r  r ) 1/ 2  r

(2)

Where Z0   0 /  0 is the characteristic impedance of the free space. Zin is the normalized input impedance of absorber, c is the speed of light in the free space, and d is the thickness of the absorber coating. An excellent magnetic absorber depends on the

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impedance matching condition determined by the combination of six parameters, ε′, ε″, μ′, μ″, f (the frequency of electromagnetic wave) and d. Therefore, to design an EMW absorber, the control of the frequency dependencies of the relative complex permeability

r and the relative complex permittivity r are of significance.

Fig. 9 Schematic diagram of measuring reflection coefficient on electromagnetic wave

The reflection loss of the CFO and CCNTs/CFO composites with thickness of 3 mm are presented in Fig. 10. It can be seen that the CFO has obvious effect on microwave absorption properties and its minimum reflection loss (RL) is -13.57 dB at 12.22 GHz. The CCNTs/CFO composite with ωCCNT/ωCFO of 0.4 possess enhanced microwave absorption performance. A available bandwidths below -10 dB is obtained at 11 ~ 15.4 GHz and the minimum RL is -14.4 dB at 13.36 GHz, which coincides with data of tanδε and tanδμ. But, it is strange that microwave absorption performance of the composite with ωCCNT/ωCFO of 0.6 is weaker than pure CFO, which is related to its conductivity. Because the conductivity of sample with ωCCNT/ωCFO of 0.6 is the biggest among composites. It is similar to the metal that the bigger the conductivity of the sample is, the greater its surface reflection on the microwave, and the reflection loss (dielectric loss and conduction loss) will decrease. It also can be observed that the frequency corresponding to the minimum RL of the composites move in the direction of low frequency with the

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increase of CCNTs content in the composites. This phenomenon can be explained by spin-rotating mechanism. The resonant frequency ( f ) of spin-rotating mechanism can be denoted as Snoek model, f = (γ/2π) (HA+HD) [44,45], where HA is the anisotropy field; HD is the demagnetizing field; γ is the gyromagnetic ratio. With the increase of the relative content of CCNTs in the composites, the interactions among the CFO particles weaken gradually, which brings about the decreasing of the anisotropy field and increasing of the demagnetizing field and results in the value of (HA+HD) decrease. So the resonant frequency of the composites shifts to lower frequencies.

Reflection Loss (dB)

0 -2 -4 -6 -8 CFO CCNTs/CFO = 0.2 CCNTs/CFO = 0.4 CCNTs/CFO = 0.6

-10 -12 -14 -16 2

4

6

8

10

12

14

16

18

f (GHz) Fig. 10 Microwave reflection loss of CFO and CCNTs/CFO composites

Fig. 11 shows the reflection loss of the CCNTs/CFO composite of ωCCNT/ωCFO of 0.4 with various layer thicknesses. The samples with matching thickness of 1.5, 2, 2.5, 3 and 3.5 mm exhibit minimum RL values (corresponding frequency/GHz) of -8.01 (15.13), -10.33 (15.08), -14.39 (13.90), -14.44 (13.20) and -11.64 dB (12.55), respectively, and the corresponding available bandwidths below -10 dB exceed 4.0 GHz with a thickness of 2.5~3.0 mm. This suggests that the samples can be used as a kind of candidate material

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of high-frequency absorbing medium. It also can be found that the peaks value corresponding to minimum RL shift lower frequency with increase of the specimen thickness. And this can be explained by formula which calculated as f  c / 2 r'' d , where

 r'' is imaginary part of relative permeability, and d is the thickness of the samples. The volume fraction of magnetic components in the samples with various thickness remains constant, which means that the  r'' almost is invariant. And the resonant frequency is inversely proportional to the thickness of the samples. Therefore, the reflection loss of the samples can be improved by controlling the coating thickness. As is known that CCNTs are dielectric substrate with conduction loss, dielectric loss and interface relaxation loss for absorbing microwave, while CFO belongs to magnetic substrate and absorbs electromagnetic wave by magnetic hysteresis, eddy current effect, domain-wall resonance and natural resonance. In this case, the hysteresis loss is weak applied magnetic field. The domain wall motion loss which usually takes place in the MHz range rather than GHz can also be excluded. Therefore, the microwave absorption performance of the composites is summation of the conductive loss and dielectric loss caused by CCNTs and the magnetic loss resulting from CFO with the exchange resonance, natural resonance and eddy current loss, as well as the interfacial polarization and relaxation effects between these two components. In addition, the spiral morphology of CCNTs can also enhance microwave absorption performance of CCNTs/CFO [30].

18

0

Reflection Loss (dB)

-3 -6 1.5 mm 2.0 mm 2.5 mm 3.0 mm 3.5 mm

-9 -12 -15 2

4

6

8

10

12

14

16

18

f (GHz) Fig. 11 The effect of samples thicknesses on microwave reflection loss of CCNTs/CFO composite with ωCCNT/ωCFO of 0.6

4 Conclusion In summary, CCNTs/CFO composites were synthesized by a co-precipitation method. The composition and microstructure were analyzed by modern means. Owing to the interactions between the two components, the CCNTs/CFO composite with ωCCNT/ωCFO of 0.4 displays the best microwave-absorption performance, among which the sample with thickness of 2.5 - 3 mm obtains optimal reflection loss value of about -14 dB in the frequency range of 2 - 18 GHz and its available bandwidth below -10 dB exceeds 4 GHz throughout the test frequency range. Therefore, the prepared CCNTs/CFO composite with ωCCNT/ωCFO of 0.4 may have potential application in microwave shielding and absorption.

Acknowledgments This author was grateful to the National Nature Science Foundation of China (No 21071125) for financial support for conducting this research work.

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