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Materials Science and Engineering B 138 (2007) 101–104

Short communication

Microwave absorbing property of Fe-filled carbon nanotubessynthesized by a practical route

Hong Zhu ∗, Haiyan Lin, Hongfan Guo, Liufang YuSchool of Science, Beijing Jiaotong University, Beijing 100044, China

Received 22 July 2006; received in revised form 6 December 2006; accepted 23 December 2006

bstract

Fe-filled carbon nanotubes were prepared reliably and economically via pyrolyzing of ferrocene and activated carbon. The sample was charac-

erized by high-resolution transmission electron microscopy, electron diffraction and fast Fourier transforms. The complex relative permittivity andermeability parameters were measured in a frequency range of 2–18 GHz and microwave absorbing behavior was investigated. The sample exhibitshe maximum reflection loss and the widest bandwidth of below −5 dB is −11.29 dB and 4.13 GHz (matching thickness = 2.5 mm), respectively. 2007 Elsevier B.V. All rights reserved.

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eywords: Carbon nanotubes; Electromagnetic parameter; Microwave absorbe

. Introduction

Metal-filled carbon nanotubes have considerable potential forngineering applications because of the novel structure of encap-ulating second phase inside carbon shells can immunize thencapsulated species against environmental degradation effectshile retaining their intrinsic properties, which make the metal-lled carbon nanotubes exhibit unique electric, magnetic andonlinear optical properties, and can also offer an opportunityo investigate dimensionally confined system. And it has beenuggested that these striking properties might find their wideromising applications such as magnetic data storage, xerog-aphy and magnetic resonance imaging [1,2]. So far, variousynthesis methods has been employed to prepare metal-filledarbon nanotubes, including wet-chemical methods [3,4], arcechniques [5,6], catalytic chemical vapor deposition [7,8] andyrolysis of organometallic compound [9–11], etc.

Pyrolysis of metallocenes has been well proved to be aromising way in preparing not only multi-wall nanotubes,

ingle-wall nanotubes [12,13], but also carbon nanotubes encap-ulated metals [9–11]. This approach is based on the pyrolysisf metallocenes, which acts as the carbon resource and themportant catalysts. Also, it offers the possibility of controllinghe nanoscale and the microstructure of materials by changing

∗ Corresponding author. Tel.: +86 10 51688469; fax: +86 10 51688433.E-mail address: zhuho128@263.net (H. Zhu).

[aabotes

921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.mseb.2006.12.018

rolyzing synthesis

he precursors or the reaction temperature, and the process isrelatively simple way of preparing chemically homogenous,

igh-purity and phase-pure powders at a lower temperature. Aonsiderable amount of recent work has been devoted to syn-hesis and to probe new properties of composites prepared byyrolysis of metallocenes process. The interest of hybrid mate-ials carbon nanotubes encapsulation of metal thus simulatedesearchers to extensively enlarge the range of the carbon pre-ursors from acetylene [12], benzene [13], thiophene [14,15] to60 [16]. However, most procedures reported to date for car-on nanotubes encapsulated with nanoparticles have applied ateast two steps, namely, preparation of CNTs followed by fill-ng of the CNTs, and the carbon resource such as thiophene and

60 is so expensive. Therefore, the lack of a process by whichhese materials can be fabricated reliably and economically haseverely hindered their applications.

In this paper, Fe-filled CNTs were prepared reliably andconomically via a straightforward, one-step method by usingctivated carbon as carbon resource. CNTs modified by coating17,18] or filling [19] with metal is commonly use as a goodbsorber for microwave absorbing technology. So we focus ourttention on the microwave absorbing properties of Fe-filled car-on nanotubes. In addition, combining the promising properties

f the encapsulated metal and carbon nanotubes, we believe thathe Fe-filled CNTs composites are likely to be used in nano-lectronics devices, magnetic recording media and biologicalensors.

1 nd Engineering B 138 (2007) 101–104

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02 H. Zhu et al. / Materials Science a

. Experimental

The method for fabricating Fe-filled carbon nanotubes is asollows: activated carbon (C.P.) and ferrocene (A.R.) were usedithout further pre-treatment. In a typical synthesis, firstly, acti-ated carbon and ferrocene were mixed physically with desiredolar ratio and loaded into the cymbiform crucibles, then put

nto a conventional tube furnace. The reactor was heated at1 K/min to the desired temperature (1000 ◦C) and was keptt this temperature for 3.0 h in a N2 stream (∼20–40 ml/min).fter allowing it to cool naturally to ambient temperature,lack powder-like solid was obtained from inside the cymbi-orm crucibles. The morphology of the sample was observedsing a JEM-2010 (JEOL) high-resolution transmission elec-ron microscope (HRTEM). Fast Fourier transforms and electroniffraction were taken and analyzed. The mixing ratio of Fe-lled MWNTs-to-olefin was fixed at 20 wt.%. The testingpecimens have a toroidal shape with both thickness being 1 mmnd outer and inner diameters being 7.0 and 3.0 mm, respec-ively. ε′, ε′′, μ′ and μ′′ are measured versus frequency range inhe 2–18 GHz with a HP8722ES vector network analyzer. Thebsorbing properties with different thickness were calculatedith equation as follows [20]:

.L. (dB) = 20 log10

∣∣∣∣Zin − 1

Zin + 1

∣∣∣∣ (1)

in =(

μr

εr

)1/2

tanh

[j

(2πfd

c

)(μrεr)

1/2]

(2)

here Zin is the normalized input impedance relating to thempedance in free space, εr = ε′ − jε′′ and μr = μ′ − jμ′′ is theomplex relative permeability and permittivity of the material,is the thickness of the absorber, and C and f are the velocity of

ight and the frequency of microwave in free space, respectively.he impedance matching condition is given by Zin = 1 to repre-ent the perfect absorbing properties. The impedance matchingondition is determined by the combinations of six parameters′, ε′′, μ′, μ′′, f and d. Also, knowing εr and μr, the R.L. valueersus frequency can be evaluated at a specified thickness.

. Results and discussion

.1. Morphology observation

The obtained solid materials are black voluminous powder,xhibiting strong ferromagnetic property. Part filled and fulllled CNTs were found in the low magnification TEM imagess shown in Fig. 1(a) and (b), nanotubes with an average outeriameter and inner diameter between 60 and 80 nm and 30 and0 nm, respectively, length up to 1–2 �m, and the purity of CNTss more than 90%. Fig. 2 and the inset in Fig. 1(b) are high mag-ification image of the nanotubes shown by an arrow in Fig. 1(a)nd (b), respectively. It is worth noticing that both multiwalled

nd double-lay CNTs were found, and full filled situation pre-er to occur in the double-lay CNTs, as shown Fig. 1(b). Fillede nanowires are of similar diameters in cross section of car-on nanotubes, no obvious voids can be observed between the

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ig. 1. TEM images of Fe-filled CNTs. The insets in (b) shows the electroniffraction pattern recorded perpendicular to the nanotube long axis and the highagnification image of the nanotubes shown by an arrow in (b), respectively.

anowires and the shell. The tubular layers of both multiwallednd double-lay CNTs have the same spacing of lattice fringesabout 0.34 nm), which is close to that of the graphite (0 0 2)lanes. The encapsulated iron nanowires are crystal and have lat-ice fringe spacing of 0.20 nm (Fig. 2), correspond to the (1 1 0)lane of the bcc-Fe crystal (JCPDS 6-0696). Combining withhe electron diffraction pattern of the Fe-filled CNTs (Fig. 1(b)),onfirm the filled iron nanowires are �-Fe.

.2. XRD analysis

XRD characterization is performed to further validate the cor-esponding structure of the TEM data. Fig. 3 presents the profilef the as-synthesized product. The peaks at 44.9 and 65.7 ◦C cane identified as the (1 1 0) and (2 0 0) planes of bcc-Fe (JCPDS

H. Zhu et al. / Materials Science and Engineering B 138 (2007) 101–104 103

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ig. 2. High magnification image of the nanotubes shown by an arrow inig. 1(a). The inset is the corresponding fast Fourier transform.

-0696). No signals of iron carbide or iron oxide are observed,n agreement with Lu et al. [10]. Although it is difficult to com-letely rule out the presence of very small oxide and carbidearticles from the XRD pattern because the particles possess-ng a scattering region smaller than 2 nm cannot be registeredith the X-ray technique [11]. TEM observation shows that the

ize of the filled Fe-nanowires is larger than 5 nm, suggestinghe encapsulated Fe should be bcc-Fe. The diffraction peak atbout 26.2 ◦C can be assigned to the (0 0 2) planes of hexag-nal graphite structure with an interlayer spacing of 0.34 nm,orresponding to the encapsulating carbon shells. The peak isf high symmetry, indicating a good crystalline dimension andong-range order.

.3. Dielectric and magnetic parameters

Since the absorbing properties of RAM can be estimated from

heir magnetic and dielectric properties, the essence of research-ng microwave absorbers is to design the compound and structureorm of the materials, by adjusting and optimize the dielectricnd magnetic properties such as complex relative permittivity

ig. 3. XRD pattern of the products from the mixture of activated carbon anderrocene with the ratio of 1:2 calcined at 1000 ◦C.

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Fig. 4. Permittivity and permeability spectra Fe-filled CNTs composite.

εr = ε′ − jε′′) and permeability (μr = μ′ − jμ′′) of the materials21].

Fig. 4 manifest the real part and the imaginary part of theelative permittivity (ε′ and ε′′) and permeability (μ′ and μ′′)pectra of Fe-filled carbon nanotubes composites. The real partnd the imaginary part of the relative permeability (μ′ and μ′′)f the sample are almost constant. This is most likely to beaused by the intrinsically small magnetic loss tangent (tan δM)f the CNTs used in this study. The values of ε′ and ε′′ dependn the frequency of the Fe-filled carbon nanotubes compositesxhibit the same trends as shown in Fig. 4. The real part andmaginary part of permittivity present a fall from 11.2 to 8.1ε′) and from 3.1 to 1.6 (ε′′) in the frequency region from 2.0o 18.0 GHz, respectively. It indicates that the composite havegood and stable dielectric loss property. These characteristic

lectromagnetic properties, resulting from the nanometer sizeffect, can be applied to the microwave absorbing material.

.4. Microwave absorbing properties

In order to achieve the absorbing properties of the material,ccording to foregoing Eqs. (1) and (2), surface reflectance ofn absorber is a function of six characteristics parameters: ε′, ε′′,′, μ′′, f and d. Thus, if the parameters of the material are known,

he absorbing properties of the material for single layer coatingr for multi-layer coating can be calculated. The values of reflec-ion loss calculated by using Eqs. (1) and (2) for the measuredalues of ε′, ε′′, μ′, μ′′ of Fe-filled CNTs with the matchinghickness is 2.5 mm are shown in Fig. 5. It indicates that thes prepared absorber presents potential absorbing property in-band. The sample exhibits the largest reflection loss and theidest bandwidth of below −5 dB is −11.29 dB and 4.13 GHz,

espectively. The reflection loss is not so excellent as Peng etl. [19]. But the method used in the present paper is more fea-ible for materials engineering application. And absorber fromNTs-based substances is of light and good mechanical prop-rty. By coating Co, Ni or other magnetic alloys may be useful

o improve the absorbing property.

In conclusion, Fe-filled CNTs were fabricated reliably andconomically via pyrolyzing of ferrocene and activated carbon.RTEM image and electron diffraction pattern confirms the for-

104 H. Zhu et al. / Materials Science and En

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ig. 5. Microwave absorption behavior of Fe-filled CNTs composites (matchinghickness = 2.5 mm).

ation of Fe nanowire encapsulation in CNTs. Investigationf electromagnetic property and microwave absorbing behaviorndicates that the as synthesized Fe-filled CNTs has potentialpplication in the microwave absorbing materials fields.

cknowledgement

This work was supported by the National Natural Foundationf China (No. 50674006).

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