- Email: [email protected]
carbon nanocomposites prepared by a CVD method
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 321 (2009) 1209–1213
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Electromagnetic wave absorption properties of Fe3C/carbon nanocomposites prepared by a CVD method Masao Terada, Masahiro Itoh, Jiu Rong Liu, Ken-ichi Machida Center for Advanced Science and Innovation, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
a r t i c l e in fo
abstract
Article history: Received 11 April 2008 Received in revised form 2 September 2008 Available online 17 November 2008
Dendrite-shaped iron nanowires with 50–200 nm in diameter and 10–20 mm in length were prepared by the CVD method from Fe(CO)5 as a starting source. Ethanol was cracked on the surface of the resultant iron nanowires to form the Fe3C/carbon nanocomposites, in which nanosized carbon beads covered the surface of Fe3C. Resin compact of the resultant Fe3C/carbon nanocomposites had excellent electromagnetic wave absorption ability in the range of 0.9–9.0 GHz, and such available absorption range more enhanced compared to that observed on the resin compact prepared from the original iron nanowires by the hybridization of magnetic (Fe3C) and dielectric (carbon) materials. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.50.Bb 75.30.Gw 75.75.+a Keywords: Fe3C Nanowires Carbon coating CVD Electromagnetic wave absorption
1. Introduction In recent years, with the spread of wireless communication devices using electromagnetic waves in the range around GHz, e.g. mobile phone (0.8–2.0 GHz), local area network system (2.4, 5.2–5.8, 19.0, 22.0 GHz), electronic toll collection in intelligent transport system (5.8 GHz), and so on, several problems such as an electromagnetic interference and information leakage have emerged [1]. Thus, the development of electromagnetic wave absorber is now strongly demanded especially in UHF and SHF bands. Iron nanowires are noted as one of the most promising materials for fabricating the electromagnetic wave absorber in the GHz range due to their high permeability and shape anisotropy [2–7]. For instance, it has been reported that the iron nanowires formed in carbon nano tubes showed the good electromagnetic wave absorption properties derived from their highly magnetic anisotropic field [8,9]. In our previous study, the dendrite-formed iron nanowires could be prepared by the simple CVD method to show the good electromagnetic wave absorption properties in a SHF band because of its high magnetic anisotropy of the iron nanowires [10]. The available absorption range with the high reflection loss
Corresponding author. Tel./fax: +81 6 6879 4209.
E-mail address: [email protected] (K.-i. Machida). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.11.001
less than 20 dB, however, existed in 6–14 GHz. Such frequency range does not meet the commonly used band in the customer products market such as mobile phone and wireless LAN as mentioned above. The complex permittivity and permeability of electromagnetic wave absorbers play an important role to control the reflection loss (electromagnetic wave absorption) properties. In this study, the Fe3C/carbon nanocomposites with an isotropic morphology were prepared by cracking ethanol on the iron nanowires as a catalyst. It is expected that the available microwave absorption range can be enhanced by hybridizing permittivity (carbon particles) and permeability (Fe3C nanowires) losses.
2. Experiment The dendrite-formed iron nanowires were prepared by the handmade CVD reactor [10]. An appropriate amount of Fe(CO)5 was vaporized by passing an Ar gas (150 mL/min) through the reservoir of Fe(CO)5 and introduced to a glass tube reactor (inner diameter: 15 mm, length: 300 mm) at 523 K. The Fe(CO)5 vapor was instantly decomposed and the dendrite-formed iron nanowires generated were collected at the downstream side of the reactor tube. Using the above iron nanowires as a catalyst, ethanol cracking was carried out at 673 K for 1 h with an Ar carrier gas (40 mL/min), in which 5 mL of ethanol was vaporized and supplied
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to the reactor tube in a similar manner as the case of Fe(CO)5. Simultaneous CVD reaction for Fe(CO)5 and ethanol was also performed at 673 K for 1 h by passing an Ar gas (60 mL/min) through the Fe(CO)5 ethanol solution reservoir. Part of the resultant Fe3C/carbon mixed particles was etched by hydrochloric acid to obtain the carbon particles. The phase identification of the samples obtained was performed by an X-ray diffractometer (Rigaku RINT2200) with Cu Ka radiation. Morphology of samples was checked by a scanning electron microscope (Hitachi S-3000H). The resin compacts were prepared by mixing thermosetting epoxy resin with 70 mass% of iron-based nano powders and pressing into toroidal shape (outer diameter: 7.00 mm, inner diameter: 3.04 mm, thickness: 1–2 mm) using a metal mold. Then the compacts were cured at 393 K for 1 h. The scattering parameters (S11, S21, S22, S12) of the resin compacts were measured by a vector network analyzer (Agilent E8363A) in the range of 0.05–18 GHz after a full two-port calibration (SHORT-OPEN-LOAD-THRU). Photographs of the measurement system are shown in Fig. 1. The relative complex permittivity (er ¼ e0 rje00 r) and permeability (mr ¼ m0 rjm00 r) values were determined from the scattering parameters using the Nicolson–Ross model for magnetic materials and the precision model (fixing m0 r ¼ 1, m00 r ¼ 0) for nonmagnetic materials, respectively [11]. The reflection loss (RL) was calculated
3 μm
Vector network analyzer 3 μm 2.4 mm coaxial cable Port 1
Port 2
2.4 mm/7 mm connector
Coaxial sample holder (7 mm)
2 μm
7.00 mm
Fig. 2. SEM images of the dendrite-formed iron nanowires (a) before and (b) after the ethanol cracking reaction, together with (c) the Fe3C/carbon mixed particles obtained by the simultaneous decomposition of FeCO5 and ethanol.
according to following equations based on an absorber model in which a metal sheet reflector is attached at the backside [12]. rffiffiffiffiffiffi mr 2pd pffiffiffiffiffiffiffiffiffi tanh j r mr (1) Z in ¼ Z 0
r
Sample 3.04 mm
Fig. 1. Photographs of (a) the present measuring apparatus and (b) the coaxial sample holder.
Z Z 0 RL ¼ 20 log in Z in þ Z 0
l
(2)
where l was the wavelength of the electromagnetic wave, d was the absorber thickness, Z0 was the impedance of air, and Zin was the input impedance of absorber. Namely, if Zin equals to Z0, desirable RL (N) can be attained.
ARTICLE IN PRESS M. Terada et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1209–1213
3. Results and discussion
301
212
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122
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Fe3C
110
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(b)
211 102 220 031 112
The scanning electron microscope (SEM) image of the dendrite-formed iron nanowires prepared from the CVD of Fe(CO)5 is shown in Fig. 2(a). They were formed by connecting numerous spherical iron particles with their average diameter of c.a. 150 nm, and these wires tangled in a net-like three-dimensional fine structure. The X-ray diffraction (XRD) pattern of the iron nanowires was assigned to a-Fe as shown in Fig. 3(curve (a)). The iron metal particles may act as an autocatalyst to grow a network structure. Moreover, the formation yield of such dendrite-formed iron nanowires was strongly dependent on the flow rate of Ar gas and reaction temperature. Decreasing the flow rate, the flake-like metal powders were preferably formed. The diameter of the iron nanowires enlarged with increasing the reaction temperature [10]. Fig. 2(b) shows the SEM image of the Fe3C/carbon nanocomposite powders obtained after cracking ethanol on the above iron nanowires. The nanosized carbon beads (50–100 nm) were generated on the surface of iron nanowires as a catalyst. Similar carbon beads formation on the LaNi5 catalyst was reported by Mi et al. [13]. The mean diameter of the present beads was smaller than that of the previous work [13], suggesting that the size of catalytic active site affects the particle size of carbon beads. The XRD pattern of iron nanowires after the cracking reaction showed that all peaks were indexed as orthorhombic Fe3C and no peak assigned to carbon was detected as shown in Fig. 3(curve (b)). This suggests the resultant carbon might be amorphous and a-Fe reacts with carbon to form Fe3C during the ethanol cracking. In order to discuss the formation mechanism of iron nanowires and carbon beads further, the simultaneous thermal decomposition of Fe(CO)5 and ethanol was carried out at 673 K. Fig. 2(c) shows the SEM image of the products obtained by the simultaneous decomposition. Fine spherical Fe3C and carbon mixed particles with less than 100 nm in diameter, not forming a network structure, have been obtained, even though this reaction condition is favorable for the formation of flake-like particles [10]. This result supports the above facts that the iron nanowires are grown by the autocatalysis of the generated fine iron particle itself and the carbon particle size obtained by the ethanol cracking depends on the iron particle size as a catalyst, because the aggregation of the iron and carbon particles was
α-Fe
(a) 40
45
50 2 (degree)
55
60
Fig. 3. XRD patterns of the dendrite-formed iron nanowires (a) before and (b) after the ethanol cracking reaction.
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suppressed by mutually working as a buffer layer during the simultaneous decomposition of iron carbonyl and ethanol. The complex er and mr values on the resin compacts including the respective iron nanowires and Fe3C/carbon nanocomposites were measured by a network analyzer and the results are shown in Fig. 4. The real (er0 ) and imaginary (e00 r) parts of relative permittivity for the iron nanowire resin compact were almost constant over the range of 0.05–18 GHz (e0 r ¼ ca. 9, e00 r ¼ ca. 0.5). The real part of relative permeability (m0 r) gradually declined from 3.0 to 1.0 as the frequency increased. The imaginary part of relative permeability (m00 r) possessed the maximum value (ca. 1.0) at 6 GHz. The RL curves calculated by the Eqs. (1) and (2) are shown in Fig. 5(a). The RL values less than 20 dB, as corresponding 99% absorption, were obtained in 6–14 GHz for the samples with respective thickness values from 3–1.5 mm. Natural resonance frequency (absorption frequency) of spherical iron metal particles is usually at around 1.5 GHz. The iron nanowires, however, provided the absorption at the higher frequency than the spherical iron powder. The absorption frequency is related to the magnetic anisotropy as below 2pf r ¼ gHA
(3)
where fr is the natural resonant frequency and HA the magnetic anisotropy field [14]. The iron nanowires have an anisotropic onedimensional structure. The absorption peak of the iron nanowires at the high frequency around 6–14 GHz is responsible for the large magnetic anisotropic field induced by the particle morphology with a high aspect ratio. The values for the relative permittivity of the resin compact made from the Fe3C/carbon nanocomposites were higher than that from the iron nanowires because the above composites contained conductive amorphous carbon with dielectric loss [15]. The e0 r and e00 r values changed from 30 to 17 and from 15 to 3 in the range of 0.05–18 GHz, respectively. The m0 r values lowered compared to the iron nanowires, only having the values from 2.2 to 1.4 in the range of 0.05–18 GHz. The maximum value of ca. 0.2 was obtained at around 5 GHz in the m00 r curve. The decrease of the relative permeability was due to the lower magnetization value of Fe3C than that of Fe and also the magnetic dilution with carbon. The resin compact prepared from the Fe3C/carbon nanocomposites had excellent electromagnetic wave absorption properties in a wide range of 0.9–9.0 GHz by changing the compact thickness from 13.5 to 1.7 mm as shown in Fig. 5(b). Since the frequency dependence of relative permeability of the Fe3C/carbon nanocomposite resin compact was more moderate than that of the iron nanowire one, the input impedance was well matched over the measuring frequency range. Also the complex permittivity for the Fe3C/carbon nanocomposites was higher than that for the iron nanowires, having effect on the electromagnetic wave absorption in a low frequency range due to the increase of electric length. Furthermore, the increase of imaginary part of permittivity also contributes to broadening the reflection loss curve. The dual losses of permeability and permittivity on the Fe3C/carbon nanocomposites consequently lead to enhance the available absorption range against an electromagnetic wave. The complex permittivity and permeability curves on the resin compact including the Fe3C/carbon mixed particles possessed intermediate characteristics between iron nanowire resin compact and Fe3C/carbon nanocomposite one as shown in Figs. 4(e) and (f). The e0 r and e00 r values were almost constant (e0 r ¼ ca. 13, e00 r ¼ ca. 1) in 0.05–18 GHz (not shown in fig). The mr0 values declined from 2.1 to 1.2 with increasing the frequency, whereas the m00 r values gave the almost constant value (ca. 0.6). The RL curves calculated for the resin compact including the Fe3C/carbon mixed particles are shown in Fig. 5(c). Its absorption property is
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a
b
15 εr '
μr '
3
εr "
μr "
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Permittivity, εr
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5
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0 0
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Fig. 4. Frequency dependences of complex ((a), (c), and (e)) permittivity and ((b), (d), and (f)) permeability on the resin compacts including the respective iron nanowires, Fe3C/carbon nanocomposites, and Fe3C/carbon mixed particles in 0.05–18 GHz.
similar to the Fe3C/carbon composites prepared by mechanical grinding a-Fe (80 vol%) and amorphous carbon (20 vol%) in our previous study [16]. In the Fe3C/carbon mixed particles and the Fe3C/carbon composites, Fe3C and carbon grains are homogeneously mixed in a sub-micrometer scale. Although they include the dielectric material of carbon, high permittivity is not observed due to electromagnetic insulating effect of Fe3C grains. On the
other hand, the fine carbon beads, which cover the surface of iron nanowires, efficiently gather the electromagnetic waves to yield high permittivity. Such high permittivity plays an important role to provide the good absorption property especially at the low frequency region around 1 GHz. In fact, the resin compact including the fine carbon particles, which were recovered from the Fe3C/carbon mixed particles by chemically etching the iron
ARTICLE IN PRESS M. Terada et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1209–1213
b
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-5
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12 mm
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10 Frequency (GHz)
Fig. 5. Electromagnetic wave absorption (reflection loss) properties of resin compacts prepared from (a) the dendrite-formed iron nanowires, (b) the Fe3C/carbon nanocomposite, (c) the Fe3C/carbon mixed particles, and (d) the carbon powders obtained by etching the Fe3C/carbon mixed particles.
component, gave the minimum reflection loss value at around 1 GHz with 12 mm in thickness as seen in Fig. 5(d). Its corresponding e0 r and e00 r values at 1 GHz evaluated from the precision model were 17.7 and 4.6, respectively (not shown in figure). The results obtained from the various resin compacts lead to the conclusion that the hybridization of permittivity and permeability is effective to broaden the available absorption range owing to the increase of electric length and dual magnetic and dielectric losses derived from the Fe3C and carbon powders.
4. Conclusion The dendrite-formed iron nanowires are obtained by the simple CVD method with an autocatalytic function of the iron nanoparticles. The resultant iron nanowires show high catalytic activity for cracking ethanol, and the Fe3C/carbon nanocomposites, in which carbon particles cover the surface of Fe3C particles, are resultantly produced after the cracking reaction. The resin compact prepared from the Fe3C/carbon nanocomposites provides excellent electromagnetic wave absorption properties in the wide range of 0.9–9 GHz with varying the absorber thickness. This is due to the hybridization of magnetic and dielectric materials, which contributes to the enhancement of electric length and dual electromagnetic wave attenuation by the magnetic and dielectric losses.
Acknowledgements This work was supported by Industrial Technology Research Grant Program from NEDO, and partly supported by Research for Promoting Technological Seeds from JST. References [1] S. Sugimoto, T. Maeda, D. Book, T. Kagotani, K. Inomata, M. Homma, H. Ota, Y. Houjou, R. Sato, J. Alloys Compd. 330–332 (2002) 301. [2] S. Yang, H. Zhu, D. Yu, Z. Jin, S. Tang, Y. Du, J. Magn. Magn. Mater. 222 (2000) 97. [3] H.R. Khan, K. Petrikowski, Mater. Sci. Eng. C 19 (2002) 345. [4] S. Ge, X. Ma, C. Li, W. Li, J. Magn. Magn. Mater. 226–230 (2001) 1867. [5] S. Valizadeh, J.M. George, P. Leisner, L. Hultman, Electrochim. Acta 47 (2001) 865. [6] J. Xu, X. Huang, G. Xie, Y. Fang, D. Liu, Mater. Lett. 59 (2005) 981. [7] E.L. Silva, W.C. Nunes, M. Knobel, J.C. Denardin, D. Zanchet, K. Pirota, D. Navas, M. Vazquez, Physica B 384 (2006) 22. [8] R. Che, L. Peng, X. Duan, Q. Chen, X. Liang, Adv. Mater. 16 (2004) 401. [9] H.M. Kim, K. Kim, C.Y. Lee, J. Joo, S.J. Cho, H.S. Yoon, D.A. Pejakovic, J.W. Yoo, A. Epstein, Appl. Phys. Lett. 84 (2004) 589. [10] J.R. Liu, M. Itoh, M. Terada, T. Horikawa, K. Machida, Appl. Phys. Lett. 91 (2007) 093101. [11] A.N. Yusoff, M.H. Abdulah, S.H. Ahmad, S.F. Jusoh, A.A. Mansor, S.A.A. Hamid, J. Appl. Phys. 92 (2002) 876. [12] S.S. Kim, S.B. Jo, K.I. Gueon, K.K. Choi, J.M. Kim, K.S. Churn, IEEE Trans. Magn. 27 (1991) 5462. [13] Y. Mi, Y. Liu, D. Yuan, J. Zhang, Chem. Lett. 34 (2005) 846. [14] S. Chikazumi, Physics of Ferromagnetism, second ed., Oxford University Press, New York, 1997. [15] Z. Fan, G. Luo, Z. Zhang, L. Zhou, F. Wei, Mater. Sci. Eng. B 132 (2006) 85. [16] J.R. Liu, M. Itoh, T. Horikawa, E. Taguchi, H. Mori, K. Machida, Appl. Phys. A 82 (2006) 509.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Electromagnetic wave absorption properties of Fe3C/carbon nanocomposites prepared by a CVD method Masao Terada, Masahiro Itoh, Jiu Rong Liu, Ken-ichi Machida Center for Advanced Science and Innovation, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
a r t i c l e in fo
abstract
Article history: Received 11 April 2008 Received in revised form 2 September 2008 Available online 17 November 2008
Dendrite-shaped iron nanowires with 50–200 nm in diameter and 10–20 mm in length were prepared by the CVD method from Fe(CO)5 as a starting source. Ethanol was cracked on the surface of the resultant iron nanowires to form the Fe3C/carbon nanocomposites, in which nanosized carbon beads covered the surface of Fe3C. Resin compact of the resultant Fe3C/carbon nanocomposites had excellent electromagnetic wave absorption ability in the range of 0.9–9.0 GHz, and such available absorption range more enhanced compared to that observed on the resin compact prepared from the original iron nanowires by the hybridization of magnetic (Fe3C) and dielectric (carbon) materials. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.50.Bb 75.30.Gw 75.75.+a Keywords: Fe3C Nanowires Carbon coating CVD Electromagnetic wave absorption
1. Introduction In recent years, with the spread of wireless communication devices using electromagnetic waves in the range around GHz, e.g. mobile phone (0.8–2.0 GHz), local area network system (2.4, 5.2–5.8, 19.0, 22.0 GHz), electronic toll collection in intelligent transport system (5.8 GHz), and so on, several problems such as an electromagnetic interference and information leakage have emerged [1]. Thus, the development of electromagnetic wave absorber is now strongly demanded especially in UHF and SHF bands. Iron nanowires are noted as one of the most promising materials for fabricating the electromagnetic wave absorber in the GHz range due to their high permeability and shape anisotropy [2–7]. For instance, it has been reported that the iron nanowires formed in carbon nano tubes showed the good electromagnetic wave absorption properties derived from their highly magnetic anisotropic field [8,9]. In our previous study, the dendrite-formed iron nanowires could be prepared by the simple CVD method to show the good electromagnetic wave absorption properties in a SHF band because of its high magnetic anisotropy of the iron nanowires [10]. The available absorption range with the high reflection loss
Corresponding author. Tel./fax: +81 6 6879 4209.
E-mail address: [email protected] (K.-i. Machida). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.11.001
less than 20 dB, however, existed in 6–14 GHz. Such frequency range does not meet the commonly used band in the customer products market such as mobile phone and wireless LAN as mentioned above. The complex permittivity and permeability of electromagnetic wave absorbers play an important role to control the reflection loss (electromagnetic wave absorption) properties. In this study, the Fe3C/carbon nanocomposites with an isotropic morphology were prepared by cracking ethanol on the iron nanowires as a catalyst. It is expected that the available microwave absorption range can be enhanced by hybridizing permittivity (carbon particles) and permeability (Fe3C nanowires) losses.
2. Experiment The dendrite-formed iron nanowires were prepared by the handmade CVD reactor [10]. An appropriate amount of Fe(CO)5 was vaporized by passing an Ar gas (150 mL/min) through the reservoir of Fe(CO)5 and introduced to a glass tube reactor (inner diameter: 15 mm, length: 300 mm) at 523 K. The Fe(CO)5 vapor was instantly decomposed and the dendrite-formed iron nanowires generated were collected at the downstream side of the reactor tube. Using the above iron nanowires as a catalyst, ethanol cracking was carried out at 673 K for 1 h with an Ar carrier gas (40 mL/min), in which 5 mL of ethanol was vaporized and supplied
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to the reactor tube in a similar manner as the case of Fe(CO)5. Simultaneous CVD reaction for Fe(CO)5 and ethanol was also performed at 673 K for 1 h by passing an Ar gas (60 mL/min) through the Fe(CO)5 ethanol solution reservoir. Part of the resultant Fe3C/carbon mixed particles was etched by hydrochloric acid to obtain the carbon particles. The phase identification of the samples obtained was performed by an X-ray diffractometer (Rigaku RINT2200) with Cu Ka radiation. Morphology of samples was checked by a scanning electron microscope (Hitachi S-3000H). The resin compacts were prepared by mixing thermosetting epoxy resin with 70 mass% of iron-based nano powders and pressing into toroidal shape (outer diameter: 7.00 mm, inner diameter: 3.04 mm, thickness: 1–2 mm) using a metal mold. Then the compacts were cured at 393 K for 1 h. The scattering parameters (S11, S21, S22, S12) of the resin compacts were measured by a vector network analyzer (Agilent E8363A) in the range of 0.05–18 GHz after a full two-port calibration (SHORT-OPEN-LOAD-THRU). Photographs of the measurement system are shown in Fig. 1. The relative complex permittivity (er ¼ e0 rje00 r) and permeability (mr ¼ m0 rjm00 r) values were determined from the scattering parameters using the Nicolson–Ross model for magnetic materials and the precision model (fixing m0 r ¼ 1, m00 r ¼ 0) for nonmagnetic materials, respectively [11]. The reflection loss (RL) was calculated
3 μm
Vector network analyzer 3 μm 2.4 mm coaxial cable Port 1
Port 2
2.4 mm/7 mm connector
Coaxial sample holder (7 mm)
2 μm
7.00 mm
Fig. 2. SEM images of the dendrite-formed iron nanowires (a) before and (b) after the ethanol cracking reaction, together with (c) the Fe3C/carbon mixed particles obtained by the simultaneous decomposition of FeCO5 and ethanol.
according to following equations based on an absorber model in which a metal sheet reflector is attached at the backside [12]. rffiffiffiffiffiffi mr 2pd pffiffiffiffiffiffiffiffiffi tanh j r mr (1) Z in ¼ Z 0
r
Sample 3.04 mm
Fig. 1. Photographs of (a) the present measuring apparatus and (b) the coaxial sample holder.
Z Z 0 RL ¼ 20 log in Z in þ Z 0
l
(2)
where l was the wavelength of the electromagnetic wave, d was the absorber thickness, Z0 was the impedance of air, and Zin was the input impedance of absorber. Namely, if Zin equals to Z0, desirable RL (N) can be attained.
ARTICLE IN PRESS M. Terada et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1209–1213
3. Results and discussion
301
212
040
122
131 221
Fe3C
110
Intensity (arb. unit.)
201
(b)
211 102 220 031 112
The scanning electron microscope (SEM) image of the dendrite-formed iron nanowires prepared from the CVD of Fe(CO)5 is shown in Fig. 2(a). They were formed by connecting numerous spherical iron particles with their average diameter of c.a. 150 nm, and these wires tangled in a net-like three-dimensional fine structure. The X-ray diffraction (XRD) pattern of the iron nanowires was assigned to a-Fe as shown in Fig. 3(curve (a)). The iron metal particles may act as an autocatalyst to grow a network structure. Moreover, the formation yield of such dendrite-formed iron nanowires was strongly dependent on the flow rate of Ar gas and reaction temperature. Decreasing the flow rate, the flake-like metal powders were preferably formed. The diameter of the iron nanowires enlarged with increasing the reaction temperature [10]. Fig. 2(b) shows the SEM image of the Fe3C/carbon nanocomposite powders obtained after cracking ethanol on the above iron nanowires. The nanosized carbon beads (50–100 nm) were generated on the surface of iron nanowires as a catalyst. Similar carbon beads formation on the LaNi5 catalyst was reported by Mi et al. [13]. The mean diameter of the present beads was smaller than that of the previous work [13], suggesting that the size of catalytic active site affects the particle size of carbon beads. The XRD pattern of iron nanowires after the cracking reaction showed that all peaks were indexed as orthorhombic Fe3C and no peak assigned to carbon was detected as shown in Fig. 3(curve (b)). This suggests the resultant carbon might be amorphous and a-Fe reacts with carbon to form Fe3C during the ethanol cracking. In order to discuss the formation mechanism of iron nanowires and carbon beads further, the simultaneous thermal decomposition of Fe(CO)5 and ethanol was carried out at 673 K. Fig. 2(c) shows the SEM image of the products obtained by the simultaneous decomposition. Fine spherical Fe3C and carbon mixed particles with less than 100 nm in diameter, not forming a network structure, have been obtained, even though this reaction condition is favorable for the formation of flake-like particles [10]. This result supports the above facts that the iron nanowires are grown by the autocatalysis of the generated fine iron particle itself and the carbon particle size obtained by the ethanol cracking depends on the iron particle size as a catalyst, because the aggregation of the iron and carbon particles was
α-Fe
(a) 40
45
50 2 (degree)
55
60
Fig. 3. XRD patterns of the dendrite-formed iron nanowires (a) before and (b) after the ethanol cracking reaction.
1211
suppressed by mutually working as a buffer layer during the simultaneous decomposition of iron carbonyl and ethanol. The complex er and mr values on the resin compacts including the respective iron nanowires and Fe3C/carbon nanocomposites were measured by a network analyzer and the results are shown in Fig. 4. The real (er0 ) and imaginary (e00 r) parts of relative permittivity for the iron nanowire resin compact were almost constant over the range of 0.05–18 GHz (e0 r ¼ ca. 9, e00 r ¼ ca. 0.5). The real part of relative permeability (m0 r) gradually declined from 3.0 to 1.0 as the frequency increased. The imaginary part of relative permeability (m00 r) possessed the maximum value (ca. 1.0) at 6 GHz. The RL curves calculated by the Eqs. (1) and (2) are shown in Fig. 5(a). The RL values less than 20 dB, as corresponding 99% absorption, were obtained in 6–14 GHz for the samples with respective thickness values from 3–1.5 mm. Natural resonance frequency (absorption frequency) of spherical iron metal particles is usually at around 1.5 GHz. The iron nanowires, however, provided the absorption at the higher frequency than the spherical iron powder. The absorption frequency is related to the magnetic anisotropy as below 2pf r ¼ gHA
(3)
where fr is the natural resonant frequency and HA the magnetic anisotropy field [14]. The iron nanowires have an anisotropic onedimensional structure. The absorption peak of the iron nanowires at the high frequency around 6–14 GHz is responsible for the large magnetic anisotropic field induced by the particle morphology with a high aspect ratio. The values for the relative permittivity of the resin compact made from the Fe3C/carbon nanocomposites were higher than that from the iron nanowires because the above composites contained conductive amorphous carbon with dielectric loss [15]. The e0 r and e00 r values changed from 30 to 17 and from 15 to 3 in the range of 0.05–18 GHz, respectively. The m0 r values lowered compared to the iron nanowires, only having the values from 2.2 to 1.4 in the range of 0.05–18 GHz. The maximum value of ca. 0.2 was obtained at around 5 GHz in the m00 r curve. The decrease of the relative permeability was due to the lower magnetization value of Fe3C than that of Fe and also the magnetic dilution with carbon. The resin compact prepared from the Fe3C/carbon nanocomposites had excellent electromagnetic wave absorption properties in a wide range of 0.9–9.0 GHz by changing the compact thickness from 13.5 to 1.7 mm as shown in Fig. 5(b). Since the frequency dependence of relative permeability of the Fe3C/carbon nanocomposite resin compact was more moderate than that of the iron nanowire one, the input impedance was well matched over the measuring frequency range. Also the complex permittivity for the Fe3C/carbon nanocomposites was higher than that for the iron nanowires, having effect on the electromagnetic wave absorption in a low frequency range due to the increase of electric length. Furthermore, the increase of imaginary part of permittivity also contributes to broadening the reflection loss curve. The dual losses of permeability and permittivity on the Fe3C/carbon nanocomposites consequently lead to enhance the available absorption range against an electromagnetic wave. The complex permittivity and permeability curves on the resin compact including the Fe3C/carbon mixed particles possessed intermediate characteristics between iron nanowire resin compact and Fe3C/carbon nanocomposite one as shown in Figs. 4(e) and (f). The e0 r and e00 r values were almost constant (e0 r ¼ ca. 13, e00 r ¼ ca. 1) in 0.05–18 GHz (not shown in fig). The mr0 values declined from 2.1 to 1.2 with increasing the frequency, whereas the m00 r values gave the almost constant value (ca. 0.6). The RL curves calculated for the resin compact including the Fe3C/carbon mixed particles are shown in Fig. 5(c). Its absorption property is
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Fig. 4. Frequency dependences of complex ((a), (c), and (e)) permittivity and ((b), (d), and (f)) permeability on the resin compacts including the respective iron nanowires, Fe3C/carbon nanocomposites, and Fe3C/carbon mixed particles in 0.05–18 GHz.
similar to the Fe3C/carbon composites prepared by mechanical grinding a-Fe (80 vol%) and amorphous carbon (20 vol%) in our previous study [16]. In the Fe3C/carbon mixed particles and the Fe3C/carbon composites, Fe3C and carbon grains are homogeneously mixed in a sub-micrometer scale. Although they include the dielectric material of carbon, high permittivity is not observed due to electromagnetic insulating effect of Fe3C grains. On the
other hand, the fine carbon beads, which cover the surface of iron nanowires, efficiently gather the electromagnetic waves to yield high permittivity. Such high permittivity plays an important role to provide the good absorption property especially at the low frequency region around 1 GHz. In fact, the resin compact including the fine carbon particles, which were recovered from the Fe3C/carbon mixed particles by chemically etching the iron
ARTICLE IN PRESS M. Terada et al. / Journal of Magnetism and Magnetic Materials 321 (2009) 1209–1213
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Fig. 5. Electromagnetic wave absorption (reflection loss) properties of resin compacts prepared from (a) the dendrite-formed iron nanowires, (b) the Fe3C/carbon nanocomposite, (c) the Fe3C/carbon mixed particles, and (d) the carbon powders obtained by etching the Fe3C/carbon mixed particles.
component, gave the minimum reflection loss value at around 1 GHz with 12 mm in thickness as seen in Fig. 5(d). Its corresponding e0 r and e00 r values at 1 GHz evaluated from the precision model were 17.7 and 4.6, respectively (not shown in figure). The results obtained from the various resin compacts lead to the conclusion that the hybridization of permittivity and permeability is effective to broaden the available absorption range owing to the increase of electric length and dual magnetic and dielectric losses derived from the Fe3C and carbon powders.
4. Conclusion The dendrite-formed iron nanowires are obtained by the simple CVD method with an autocatalytic function of the iron nanoparticles. The resultant iron nanowires show high catalytic activity for cracking ethanol, and the Fe3C/carbon nanocomposites, in which carbon particles cover the surface of Fe3C particles, are resultantly produced after the cracking reaction. The resin compact prepared from the Fe3C/carbon nanocomposites provides excellent electromagnetic wave absorption properties in the wide range of 0.9–9 GHz with varying the absorber thickness. This is due to the hybridization of magnetic and dielectric materials, which contributes to the enhancement of electric length and dual electromagnetic wave attenuation by the magnetic and dielectric losses.
Acknowledgements This work was supported by Industrial Technology Research Grant Program from NEDO, and partly supported by Research for Promoting Technological Seeds from JST. References [1] S. Sugimoto, T. Maeda, D. Book, T. Kagotani, K. Inomata, M. Homma, H. Ota, Y. Houjou, R. Sato, J. Alloys Compd. 330–332 (2002) 301. [2] S. Yang, H. Zhu, D. Yu, Z. Jin, S. Tang, Y. Du, J. Magn. Magn. Mater. 222 (2000) 97. [3] H.R. Khan, K. Petrikowski, Mater. Sci. Eng. C 19 (2002) 345. [4] S. Ge, X. Ma, C. Li, W. Li, J. Magn. Magn. Mater. 226–230 (2001) 1867. [5] S. Valizadeh, J.M. George, P. Leisner, L. Hultman, Electrochim. Acta 47 (2001) 865. [6] J. Xu, X. Huang, G. Xie, Y. Fang, D. Liu, Mater. Lett. 59 (2005) 981. [7] E.L. Silva, W.C. Nunes, M. Knobel, J.C. Denardin, D. Zanchet, K. Pirota, D. Navas, M. Vazquez, Physica B 384 (2006) 22. [8] R. Che, L. Peng, X. Duan, Q. Chen, X. Liang, Adv. Mater. 16 (2004) 401. [9] H.M. Kim, K. Kim, C.Y. Lee, J. Joo, S.J. Cho, H.S. Yoon, D.A. Pejakovic, J.W. Yoo, A. Epstein, Appl. Phys. Lett. 84 (2004) 589. [10] J.R. Liu, M. Itoh, M. Terada, T. Horikawa, K. Machida, Appl. Phys. Lett. 91 (2007) 093101. [11] A.N. Yusoff, M.H. Abdulah, S.H. Ahmad, S.F. Jusoh, A.A. Mansor, S.A.A. Hamid, J. Appl. Phys. 92 (2002) 876. [12] S.S. Kim, S.B. Jo, K.I. Gueon, K.K. Choi, J.M. Kim, K.S. Churn, IEEE Trans. Magn. 27 (1991) 5462. [13] Y. Mi, Y. Liu, D. Yuan, J. Zhang, Chem. Lett. 34 (2005) 846. [14] S. Chikazumi, Physics of Ferromagnetism, second ed., Oxford University Press, New York, 1997. [15] Z. Fan, G. Luo, Z. Zhang, L. Zhou, F. Wei, Mater. Sci. Eng. B 132 (2006) 85. [16] J.R. Liu, M. Itoh, T. Horikawa, E. Taguchi, H. Mori, K. Machida, Appl. Phys. A 82 (2006) 509.