Investigation on Microwave Absorption Properties for Multiwalled

ARTICLE pubs.acs.org/JPCC

Investigation on Microwave Absorption Properties for Multiwalled Carbon Nanotubes/Fe/Co/Ni Nanopowders as Lightweight Absorbers Fusheng Wen,* Fang Zhang, and Zhongyuan Liu State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinghuangdao 066004, People’s Republic of China ABSTRACT: Multiwalled carbon nanotubes (MWCNTs)/Fe, MWCNTs/Co, and MWCNTs/Ni nanopowders have been prepared conveniently by a simple chemical method. The excellent microwave absorption properties in S-band have been obtained due to proper combination of the complex permeability and permittivity resulting from the magnetic nanoparticles and lightweight MWCNTs. The frequency of microwave absorption obeys the quarter-wavelength matching model. For the most excellent microwave absorption properties in S-band, it is found that the reflection loss exceeds
20 dB from 2.04 to 3.47 GHz for the absorber thickness between 3.36 and 5.57 mm, and a minimum reflection loss value of
39 dB was observed at 2.68 GHz on a specimen with a matching thickness of 4.27 mm for MWCNTs/Fe. The MWCNTs/Fe/Co/Ni nanopowders all can be a promising candidate for lightweight microwave absorption materials in S-band.

’ INTRODUCTION Recently, various electronic and communication devices using the electromagnetic (EM) wave range of 1
40 GHz are being developed rapidly because of the advantage of large data transmission. For the normal operation of devices, the human health, and the application in microwave darkrooms and national defense industry, the corresponding EM wave absorbers received increasingly considerable attention.1
3 Especially, the absorption properties in microwave frenquency S-band (2
4 GHz) are important in theory and actual application now, for example, Wi-Fi network at 2.4 GHz, microwave ovens at 2.45 GHz, and fixed satellite at 2.4, 2.6 GHz. Otherwise, as a microwave absorber, the lightweight demands have been met for low concentration and small matching thickness. Complex permittivity (εr = εr0
jεr00 ) and permeability (μr = μr0
jμr00 ) of the microwave absorbers play key roles in microwave absorption properties. For corresponding microwave absorbing materials, first, as a microwave absorber for dielectric loss, ZnO,4 MnO2,5 BaTiO3,6 SiC,7 TiO2,8 et al. particles have been intensively researched for their microwave absorption properties. Second, as a microwave absorber for magnetic loss, magnetic particles have been employed such as Fe50Ni50,9 Co3Fe7
Co,10 carbonyl iron,11 FeCuNbSiB,12 and so on. It is hard to attain impedance match condition Zm = Z0(μr/εr)1/2, for unilateral dielectric loss or magnetic loss.13 For attaining the impedance match, the magnetic loss materials have been coated by the dielectric loss materials, such as Fe/SmO,14 Fe/Y2O3,15 Fe/Fe3B/Y2O3,16 Ni/C,17 Fe/ZnO,18 Fe3Al/Al2O3,19 porous C/Co,20 and FeCo/Al2O3,21 which have excellent microwave absorption properties. However, the lightweight and small matching thickness is hard to be obtained in S-band. The minimal reflections can be gained at given frequencies if the r 2011 American Chemical Society

thickness of the absorber (tm) satisfies22 tm ¼

nλ nc pffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 4 4fm jεr jjμr j

ðn ¼ 1, 3, 5, :::Þ

ð1Þ

where λ, μr, and εr are the wavelength in the materials, complex permeability at fm, and complex permittivity at fm, fm and tm are the peak frequency and the matching thickness of maximum microwave absorptions, and c is the velocity of light. According to the eq 1, the enhanced μr and εr of the materials are necessary to obtain the small matching thickness tm in lower frequency (S-band). As a lightweight microwave absorber, the multiwalled carbon nanotubes (MWCNTs) have been considerably researched for their good compatibility and small density.23
25 Recently, the absorbing coating with a loading of 15 wt % MWCNTs/epoxy resin, whose thickness is 3 mm, has been found to exhibit an absorbing peak of 10.5 dB at 3.85 GHz which has excellent microwave absorption properties in S-band.26 Moreover, several metal nanoparticles such as Fe,27,28 Ni,29 Co,30 and CoFe2O431 have been filled into MWCNTs, and their microwave absorbing properties have been investigated. However, most of the research works have focused on the frequency range 8
18 GHz. According to eq 1, it is difficult to obtain good reflection loss for small matching thickness in S-band with low permeability and unmatching permittivity. In this work, for attaining the impedance matching to achieve good reflection loss in which high permeability and moderate Received: March 4, 2011 Revised: June 16, 2011 Published: June 16, 2011 14025

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Figure 2. XRD patterns of MWCNTs/Fe, MWCNTs/Co, and MWCNTs/Ni nanopowders.

Figure 1. TEM and HETEM image of nanopowders (a), SEM images of nanopowders (b, c), and the surface SEM image of resin composites with 60 wt % (d).

permittivity are needed,32 the MWCNTs/Fe/Co/Ni nanopowders have been prepared by a simple chemical method. The complex permittivity and permeability of resin composites with 60 wt % are analyzed, and the microwave absorbing characteristics are evaluated. The peak frequency dependency of the minimum reflection loss on the sample thickness and electromagnetic parameters has been studied by comparing both the formulas.

’ EXPERIMENT In a typical synthesis, the MWCNTs were first ultrasonic in vitriol and nitric acid (3:1) for 5 h to remove trace impurity and to open the cap of the tubes. Centrifuging, the remainder was dried in a vacuum oven at 60 °C for 12 h. The purified MWCNTs were dispersed in Fe(NO3)3 3 9H2O, or Co(NO3)2 3 9H2O, or Ni(NO3)2 3 6H2O solution with the help of an ultrasonic bath. After draining excess water on a rotary evaporator with a vacuum pump and washing with distilled water, the resulting materials were reduced using H2 at 900 °C for 3 h. The phase was examined using X-ray diffraction (XRD) using Cu KR radiation on a Philips X0 perts diffractometer. The image of MWCNTs was taken by a transimission electronic microscope (TEM) (Hitachi S-480), a high-resolution TE microscope (HETEM) (JEM-2010, JEOL), and a scanning electron microscope (SEM) (Hitachi S-4800). The hysteresis loop was recorded on a vibrating sample magnetometer (Lake Shore 7304). Raman study of randomly oriented samples was performed with a Renishaw inVia Raman Microscope (wavelength = 514.5 nm). The epoxy resin composites for high-frequency magnetic properties measurement were prepared by epoxy resin with 60 wt % MWCNTs/Fe/Co/Ni and pressing into toroidal shape

Figure 3. Hysteresis loop of MWCNTs/Fe, MWCNTs/Co, and MWCNTs/Ni at room temperature.

(jout, 7.00 mm; jin, 3.04 mm). The surface microstructures of the composites were analyzed using a scanning electron microscope (SEM) (Hitachi S-4800). The scatting parameters (S11, S21) were measured on the toroidal-shape samples by a network analyzer (Agilent Technologies E8363B) in the range of 0.1
18 GHz. The relative complex permeability (μr) and permittivity (εr) values were determined from the scattering parameters to evaluate the electromagnetic wave absorption properties.

’ RESULTS AND DISCUSSION Fe/Co/Ni nanoparticles embedded in MWCNTs are shown in Figure 1a
c. The homogeneous nanoparticles have been obtained inside the inner cavities of MWCNTs. The outer diameter of MWCNTs is about 60 nm. Figure 1d shows the surface SEM image of resin composites with 60 wt % MWCNTs/ Fe/Co/Ni. We can see that the MWCNTs are well dispersed in the resin composites. 14026

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Figure 4. Normal Raman spectra of MWCNTs, MWCNTs/Fe, MWCNTs/Co, and MWCNTs/Ni nanopowders at room temperature.

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Figure 6. Frequency dependence of the real part (μr0 ) and imaginary part (μr00 ) of relative complex permeability of the composites with 60 wt %.

Figure 7. Values of μr00 (μr0 )
2f
1 for the composites vs frequency.

Figure 5. Frequency dependence of the real part (εr0 ) and imaginary part (εr00 ) of relative complex permittivity of the composites with 60 wt %.

Figure 2 shows the XRD patterns of MWCNTs/Fe, MWCNTs/ Co, and MWCNTs/Ni. For MWCNTs/Fe, the coexistence of graphite, bcc-Fe, and n-diamond phase in the samples is evident. No signals of iron carbide or iron oxide are observed. For MWCNTs/Co, the fcc-Co has been found which is attributed to the lower surface energy of fcc-Co phase compared with hexagonal close-packed Co.33 For MWCNTs/Ni, the fcc-Ni has been observed. Different from the MWCNTs/Co and MWCNTs/Ni, the n-diamond phase has been obtained in the MWCNTs/Fe nanopowders, which is reaction product.34 Hysteresis loops of MWCNTs/Fe, MWCNTs/Co, and MWCNTs/Ni at room temperature are shown in Figure 3. The saturation magnetizations of MWCNTs/Fe, MWCNTs/Co, and MWCNTs/Ni were measured to be 76.77, 60.50, and 27.70 emu/g, respectively. The MWCNTs/Fe has the largest saturation magnetization which is a benefit for high permeability. Figure 4 gives typical Raman spectra of randomly oriented MWCNTs, MWCNTs/Fe, MWCNTs/Co, and MWCNTs/Ni nanopowders at room temperature. The Raman spectra of all

samples exhibit two main broad peaks at ∼1347 cm
1 marked as D peak for “disordered” and ∼1572 cm
1 marked as G peak for “graphite carbon”.35,36 Comparing with the MWCNTs, the increase of the intensity of the D peak indicates the increase of disorders for MWCNTs/Fe, MWCNTs/Co, and MWCNTs/Ni nanopowders. Figure 5 shows the frequency dependence of the real part (εr0 ) and imaginary part (εr00 ) of relative complex permittivity of the composites with 60 wt %. It can be found that the value of εr0 almost remains constant, and εr00 increases from 0.5 to 9.3, 5.7, and 7.3 for MWCNTs/Fe, MWCNTs/Co, and MWCNTs/Ni, respectively, over the 0.1
18 GHz frequency range. According to the free electron theory,37 εr00 ≈ 1/2πε0Ff, where F is the resistivity. It can be speculated that the lower εr00 values indicate a higher electric resistivity. The higher electric resistivity may result from the deaggregated magnetic nanoparticles in the cavities of MWCNTs. Figure 6 shows the frequency dependence of the real part (μr0 ) and imaginary part (μr00 ) of relative complex permeability of the composites with 60 wt %. The real part of relative complex permeability μr0 declines from 1.77 to 0.42 for MWCNTs/Fe, 1.47 to 0.35 for MWCNTs/Co, and 1.76 to 0.48 for MWCNTs/ Ni with increasing frequency. For the imaginary part of relative 14027

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Figure 8. Frequency dependence of the loss tangent of dielectric/ magnetic of resin composites with 60 wt %.

Figure 10. Color map of the reflection loss (RL) values calculated from the measured electromagnetic parameters of composites (a), (c), and (e); dependence of λ/4 thickness on frequency for composites (b), (d), and (f).

Figure 9. Attenuation constant of resin composites versus frequency.

complex permeability μr00 , two obvious resonance peak have been observed. Let us understand the physical origin of the resonance peak. In general, the magnetic loss of magnetic materials originates mainly from natural resonance, exchange resonance, and eddy current loss in the microwave region. The contribution from eddy current loss related to thickness (d) and the electric conductivity (σ) of the composites can be expressed by38,39 μr 00 ðμr 0 Þ
2 f
1 ¼ 2πμ0 d2 σ

ð2Þ

where μ0 is the permeability of vacuum. Figure 7 shows the calculated evolution μr00 (μr0 )
2f
1 with frequency. If the observed magnetic loss only results from eddy current loss, the values μr00 (μr0 )
2f
1 should be constant with increasing frequency. It is obvious that the difference of μr00 (μr0 )
2f
1 values is 0.08 ns for MWCNTs/Fe, 0.03 ns for MWCNTs/Co, and 0.07 ns for MWCNTs/Ni. Therefore, the eddy current loss could also be precluded. For the magnetocrystalline anisotropy field of bcc Fe, fcc Co, and fcc Ni is 600,40 500,41 and 130 Oe,42 the natural resonance frequency can be calculated by the equation: f = γHk/ 2π,43 where γ is the gyromagnetic ratio, Hk the anisotropy field, and f the resonance frequency. The calculated natural resonance frequency is 1.8, 1.5, and 0.39 GHz, respectively, which is smaller

than the experimental resonance frequency. Therefore, it may be attributed to the shape anisotropy of magnetic nanoparticles as pointed out by Kittle.43 Moreover, the resonance at higher frequency may be due to the exchange resonance according to Aharoni’s theory44 which has been proven in ferromagnetic nanoparticles.45,46 In order to investigate the intrinsic reasons for microwave absorption of the composites, the loss tangent of the dielectric/ magnetic can be expressed as tan δE = εr00 /εr0 and tan δM = μr00 /μr0 , respectively. Figure 8 shows the frequency dependence of the loss tangent of dielectric/magnetic of composites. According to previous research work, the microwave absorption enhancement of MWCNTs resulting from dielectric loss rather than magnetic loss has been observed.47 For the three samples, it suggests that microwave absorption enhancement of composites results mainly from magnetic loss rather than dielectric loss. Depending on transmission line theory and electromagnetic wave propagation constant, attenuation constant a determines the attenuation properties of materials, which is defined as48,49 pffiffiffi ! 2πf pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðμr 00 εr 00
μr 0 εr 0 Þ a¼ c +

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðμr 00 εr 00
μr 0 εr 0 Þ2 + ðεr 0 μr 00
εr 00 μr 0 Þ2

ð3Þ

where f is the microwave frequency and c is the velocity of light. As shown in Figure 9, from 0.1 to 18 GHz, the MWCNTs/Fe have the biggest attenuation constant a in all frequency ranges, indicating the excellent attenuation or microwave absorption. 14028

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Table 1. Microwave Absorption of Some Representative Materials minimum sample

frequency dm (mm)

RL

range (GHz) percentage

Value (dB) (RL <
20 dB) (RL <
20 dB)

(wt %)

ref

Fe/SmO


52

7.9
13.1

0.73
1.3

80

14

Fe/Y2O3


36

3
5

2
3.5

80

15

Fe/Fe3B/Y2O3


33

3
6

2.7
6.6

80

16

Ni/C


32

2

10.2
18

50

17

Fe/ZnO


57.1

1.5
5

6.1
15.7

40

18

the complex permeability and permittivity resulting from the magnetic nanoparticles and lightweight MWCNTs. The frequency of microwave absorption obeys the quarter-wavelength matching model. For the most excellent microwave absorption properties in S-band, it is found that the RL exceeds
20 dB from 2.04 to 3.47 GHz for the absorber thickness between 3.36 and 5.57 mm for MWCNTs/Fe. The nanopowders prepared by simple chemical method can be a promising candidate for lightweight microwave absorption materials in S-band.

’ AUTHOR INFORMATION

Fe3Al/Al2O3


45

1.5
2.5

7.2
17.4

75

19

Corresponding Author

porous C/Co


40

5

4.2

30

20

FeCo/Al2O3 MWCNTs/Fe


30.8
39

1.2
2.0 3.36
5.57

10.9
15.6 2.04
3.47

40 60

21

*Tel. +86 335 8074631; fax +86 335 8074545; e-mail address [email protected].

MWCNTs/Co


37

4.18
6.82

2.35
3.51

60

MWCNTs/Ni


37

3.77
6.56

1.83
3.07

60

The frequency dependence of reflection loss (RL) values was estimated from the complex permittivity (εr = εr0
jεr00 ) and permeability (μr = μr0
jμr00 ) according to the following equations50   1=2   μ 2πfd Zin ¼ Z0 r tanh j ðμr εr Þ1=2 ð4Þ c εr   Z
Z   in 0 RL ¼ 20 log    Zin + Z0 

ð5Þ

where f is the frequency of the microwave, d is the thickness of an absorber, Z0 is the impedance of air, Zin is the input impedance of absorber, and c is the velocity of the light. Figure 10 shows the color map of the reflection loss (RL) values calculated from the measured electromagnetic parameters of composites (a), (c), and (e) and dependence of λ/4 thickness on frequency for composites (b), (d), and (f) according to eq 1. It is obvious that the minimum reflection loss was found to move forward the lower frequency region with increasing thickness, and the frequency of microwave absorption obeys the quarterwavelength (λ/4) matching model.51 A minimum RL value of
39,
37,
37 dB was observed at 2.68, 2.77, 2.14 GHz on a specimen with a matching thickness (dm) of 4.27, 5.25, 5.19 mm, respectively. Compared with the aforementioned composites, as shown in Table 1, in the S-band (2
4 GHz), MWCNTs/Fe/ Co/Ni nanopowders can be used as excellent lightweight microwave absorbers with small thickness. Moreover, MWCNTs/Fe has the highest permeability which results from the largest saturation magnetization, so MWCNTs/Fe has the most excellent microwave absorption properties with the smallest matching thickness in S-band. The excellent microwave absorption properties in S-band may be ascribed to the proper combination of the complex permeability and permittivity resulting from the magnetic nanoparticles and lightweight MWCNTs.

’ CONCLUSIONS In summary, MWCNTs/Fe, MWCNTs/Co, and MWCNTs/ Ni nanopowders have been prepared conveniently by simple chemical method. The excellent microwave absorption properties in S-band have been obtained due to proper combination of

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