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Influence of a graphite shell on the thermal, magnetic and electromagnetic characteristics of Fe nanoparticles
Journal of Alloys and Compounds 548 (2013) 239–244
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Influence of a graphite shell on the thermal, magnetic and electromagnetic characteristics of Fe nanoparticles Xianguo Liu a,b,⇑, Siu Wing Or b,⇑, Yuping Sun c, Weihuo Li a, Yizhu He a, Guohui Zhu a, Chuangui Jin a, Qun Yan c, Yaohui Lv a, Siu Lau Ho b, Shengsheng Zhao d a
School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, PR China Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Center for Engineering Practice and Innovation Education, Anhui University of Technology, Ma’anshan 243002, PR China d School of Mechanical and Electrical Engineering, Shenzhen Polytechnic, Shenzhen 518055, PR China b c
a r t i c l e
i n f o
Article history: Received 10 June 2012 Received in revised form 1 September 2012 Accepted 3 September 2012 Available online 14 September 2012 Keywords: Graphite Nanoparticles Magnetic Complex permeability Complex permittivity
a b s t r a c t Graphite-coated Fe nanocapsules were prepared by arc discharge method. Fe nanocapsules have a core/ shell microstructure, in which Fe nanoparticles as cores and graphite layers as shells. Compared with Fe nanoparticles with an oxide shell, the graphite shell can restrain the growth of Fe nanoparticles, which leads to lower saturation magnetization and higher natural resonance frequency. The Fe nanocapsules are stable securely below 220 °C in the air, due to the enhancement of thermal stability and the anti-oxidation by graphite layer shell. Dielectric relaxation of graphite shell and the interfacial relaxation between graphite shell and Fe core lead to the dual nonlinear dielectric resonance. Graphite shells can dramatically improve the magnetic/dielectric loss and the attenuation constant in the 2–18 GHz though the dual dielectric resonance and protection of Fe cores, leading to enhanced microwave absorption properties of Fe nanocapsules at 2–18 GHz. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Much attention has been paid to magnetic metallic nanoparticle by virtue of their potential for applications in fields, such as magnetic recording media, catalysis, drug delivery and microwave absorbents [1,2]. However, applications have been limited because these particles suffer from rapid environmental degradation and oxidation. Nanocapsules are developed to overcome this disadvantage, which are the encapsulation of nanoparticles inside protective outer shells. In particular, the outer shells can be a component, consisting of nanocomposites through microstructures of ‘‘core/shell’’ type. These excellent properties would provide opportunities for practical applications in many fields. Recently, increasing scientific interest has been focused on the electromagnetic (EM) characteristics of metal-based nanocomposites. Magnetic metals can remain high EM parameters in a high frequency range due to its large saturation magnetization and higher Snoek’s limit [3,4]. Nonetheless, the weak magnetocrystalline anisotropy and attenuated permeability due to the eddy current phenomenon may limit its applications at higher frequencies. Increased surface anisotropies and reduced eddy current can be realized by dimin⇑ Corresponding authors. Tel./fax: +86 555 2311570. E-mail addresses: [email protected] (X. Liu), [email protected] (S.W. Or). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.09.006
ishing the metallic particles’ size and these become the main advantages of nanosized metallic particles used as an EM wave absorber. Graphite layers are ideal shells because they can be easily absorbed by the surface of magnetic nanoparticles and can protect the nanoparticles effectively against oxidation. In addition, the studies on materials based on graphite have been intensive, which can be used as EM shielding and absorption materials, due to their lower density and rich electric transport properties [5,6]. Zhang and co-workers have reported that graphite-coated magnetic nanocapsules exhibit improved EM absorption properties because of their proper EM matching between the dielectric and the magnetic losses [7,8]. Recently, we have prepared a series of graphite-coated FeNi alloy nanocapsules by arc discharging the FeNi alloy ingot with different composition and investigated their EM properties in detail [5,9–11]. In the present work, the effect of the graphite shell has been systematically investigated. Graphite-coated Fe nanocapsules and Fe nanoparticles are studied in detail, in particular concerning the influence of the graphite shell on the thermal and magnetic properties and on the EM characteristics of the Fe nanoparticles. Compared with Fe nanoparticles, graphite shells effectively improve thermal stability of Fe nanocapsules and permeability of Fe nanocapsules at GHz range by means of the protection of Fe nanoparticles cores, restrict the growth of the Fe nanoparticles, which leads to lower saturation
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magnetization, and reduces the electrical resistivity. Dielectric relaxation of graphite shells and the interfacial relaxation between graphite shells and Fe nanoparticle cores lead to the dual nonlinear dielectric resonance. As a result of the enhanced magnetic/dielectric loss and the attenuation constant, the Fe nanocapsules exhibit good EM-absorption properties in the 2–18 GHz range.
Table 1 The average diameter, average shell thickness and saturation magnetization MS of Fe nanocapsules/nanoparticles. Sample
Average diameter (nm)
Average shell thickness (nm)
MS at 295 K (Am2/kg)
Fe nanocapsules Fe nanoparticles
27.8 59.6
2.4 4.8
45.2 63.4
2. Experimental The used arc-discharge method has been described in detail elsewhere [3–5, 7–11]. In brief, Fe ingot on a water-cooled copper crucible has been used as the anode. The cathode was a carbon needle. After the chamber was evacuated, 1.4 104 Pa of pure argon and 0.6 104 Pa of hydrogen were introduced into the chamber. The arc-discharge current was maintained at 100 A for 1 h for sufficient evaporation of the anode material. For the preparation of Fe nanoparticles, a tungsten needle was introduced as the cathode under the same experimental conditions. Phase analysis of the products was performed by powder X-ray diffraction (XRD), acquired by a Bruker D8 Advance X-ray diffractometer equipped with a monochromatized Cu Ka radiation. The morphology of the nanoparticles was stdudied in detail in a high-resolution transmission electron microscope (HRTEM JEOL2010) with an emission voltage of 200 kV. The magnetic properties were measured in a superconducting quantum interference device (SQUID, Quantum Design MPMS XL-7).The oxidation behavior was investigated by thermal gravimetric analysis (TGA) and scanning differential thermal analysis (SDTA) in air atmosphere at a heating rate of 10 °C min1 from 20 to 800 °C. The Ni nanocapsules-paraffin composite was prepared by uniformly mixing Ni nanocapsules with paraffin, as described in detail elsewhere [3–5,7–11], by homogeneously mixing the nanocapsule with paraffin and pressing them into cylinder-shaped compacts. Then the compact was cut into toroidal shape with 7.00 mm outer diameter and 3.04 mm inner diameter. The EM parameters are measured for paraffin-Ni nanocapsules composite containing 40 wt.% Ni nanocapsules, using an Agilent 8722ES network analyzer. Coaxial method was used to determine the EM parameters of the toroidal samples in a frequency range of 2–18 GHz with a transverse EM mode. The VNA was calibrated for the full two-port measurement of reflection and transmission at each port. The complex permittivity and complex permeability were calculated from S-parameters tested by the VNA, using the simulation program of Reflection/Transmission Nicolson-Ross model.
3. Results and discussion Fig. 1 shows XRD patterns and HRTEM images of graphitecoated Fe nanocapsules and Fe nanoparticles. In Fig. 1(a), all sharp reflection peaks could be indexed to the Fe with a body-centeredcubic structure. No reflection for oxides could be found, indicating that Fe nanoparticles are almost without oxidation, due to the protective graphite shell. In the XRD patterns, there are no detectable peaks for pure C, indicating its small amount (less than 3% in the samples). As a comparison, the XRD pattern of Fe nanoparticles is presented in Fig. 1(b). It is note worthy that the spectrum is not smooth, indicating the existence of some oxides in the as-prepared Fe nanoparticles. An HRTEM image of a part of graphite-coated Fe nanocapsules shows the clear core/shell microstructure, in which the surface of Fe nanoparticle was coated by a multi-layered
Fig. 1. XRD patterns and HRTEM images of (a) graphite-coated Fe nanocapsules and (b) Fe nanoparticles.
graphitic shell around 2 nm in thickness. The lattice spacing of the coating is close to that of the bulk graphite (0 0 2) planes, which is accordance with the previous results [8–11]. Nevertheless, a mass of lattice imperfections can be seen in the outer shells as a consequence of the serious bending and collapsing of the graphite atom layer, which is similar with the FeNi/C nanocapsules [9–11]. As shown in Fig. 1(a), since graphite is on the shell of the nanocapsules, it is difficulty to detect its XRD pattern because of breaking down of the periodic boundary condition (translation symmetry) along radial direction. A typical HRTEM image of a part of Fe nanoparticle, as shown in Fig. 1(b), clearly indicates that an amorphous oxide shell about 2 nm in thickness formed in the passivation process on the surface of the nanoparticle, which further proves the high surface energy and chemical activity for Fe nanoparticles. The average diameter and shell thickness of Fe nanocapsules/ nanoparticles, as estimated by measuring above 300 nanocapsules/nanoparticles, are shown in Table 1. In addition, the saturation magnetization MS at 295 K is also listed in Table 1. It is observed that the average diameter and shell thickness of the Fe/ C nanocapsules is lower than that of the Fe nanoparticles, ascribed to that the graphite shell can restrain the growth of Fe nanoparticles in the arc-discharge process. In addition, the MS of the nanocapsules is lower than that of nanoparticles, which can be explained by the fact that the MS drops monotonically with the size of the particles at ambient temperature region [12]. The relation between the MS and the size of particles measured at room temperature agrees with the previous calculations [12]. Metallic nanoparticles are easily oxidized or self-ignited, due to the large ratio of surface/volume [13,14]. In order to investigate the influence of graphite shell on thermal stability and anti-oxidation behavior, SDTA and TGA were done for the Fe nanocapsules/nanoparticles at a heating rate of 10 °C min1 in a flowing air atmosphere. Thermal analysis was used to estimate the nature and stability, as well as the density of defects in the carbon layers [15].As shown in the TGA and SDTA curves of Fe/C nanocapsules (Fig. 2(a)), in the segment from 220 to 320 °C, the TGA–SDTA curves exhibit a gradual weight gain, associated with an exothermic peak at 330 °C. This weight gain implies that the Fe cores are being slowly oxidized. It also indicates that graphite-coated Fe nanocapsules are stable securely below 220 °C in the air. Above 320 °C, it is thought that the Fe cores may undergo a serious oxidation and/or self ignition corresponding to an exothermic peak at about 550 °C, and the weight gain is neglected by the weight loss due to oxidation of carbon to CO2 [5,15]. That is, the weight loss between 320 and 800 °C is recognized as an active chemical reaction in which the graphite phase is oxidized to CO2 gas. The oxidation temperature of the graphite shell in the Fe/C nanocapsules is significantly lower than that of CNTs (527–727 °C) [16] and bulk graphite crystals (846 °C) [17], which is ascribed to a mass of lattice defects resulting from serious bending of the graphite atomic layers, such as vacancies and interstitial bonding, etc. The results of HRTEM image also confirm the existence of a mass of graphitic layer defects, which are active sites for O2 attack. In comparison, the oxidation of Fe nanoparticles takes place at about 90 °C accompanying a broad exothermic peak near the temperature range from 190 to 260 °C (as shown in Fig. 2(b)), which is different with the previous reports on Cu/C nanocapsules and FeNi/C nanocapsules
X. Liu et al. / Journal of Alloys and Compounds 548 (2013) 239–244
Fig. 2. SDTA and TGA curves of (a) graphite-coated Fe nanocapsules and (b) Fe nanoparticles.
[5,15]. The TGA-SDTA curves of Fe nanoparticles in the range of 90– 190 °C exhibit a gradual weight gain, associated with the broad exothermic peak at 190 °C, implying that the oxidation process and crystal process of amorphous oxide shell. The broad exothermic peak from 190 to 260 °C results from the phase transformation from the low temperature oxide of Fe3O4 into high temperature oxide of Fe2O3 on the surface of nanoparticles due to the high surface energy and from the phase transformation from Fe to FeO and Fe3O4 in Fe cores at the same time. As the heating temperature increases, an exothermic peak occurs at 538 °C, associated with a gradual weight gain. This implies that the oxidation of Fe cores to Fe2O3 happens, representing the extent of oxidization from unstable to stable. On the TGA curves of Fe nanocapsules/nanoparticles, it is worthy noted that a weight loss of about 1 wt.% occurs in Fe nanocapsules/nanoparticles below 50 °C, which is attributed to the release of moisture on the surface of the nanocapsules/nanoparticles [14]. As we known, the reduction associates with a rapid weight loss and
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the oxidization associates with the weight gain in the TGA curve. From TGA analysis, the carbon-coated Fe nanocapsules appear to be stable below 220 °C in air. O2 and other gases can be absorbed by the defects on the graphite shell, which lead to the increase of weight from 50 to 150 °C. With the increase of temperature, the release of gases which can not react with Fe leads to the slight weight loss. The Fe cores are slowly oxidized with the further increase of temperature. The weight loss between 320 and 800 °C is due to oxidation of carbon to CO2. As shown in Fig. 2(b), the weight gain of Fe nanoparticles gradually increase from 50 to 800 °C, which proves that Fe nanoparticles can be easily oxidized. The graphite shell can effectively improve the thermal stability of Fe nanocapsules. Fig. 3 shows the frequency dependence of relative permittivity (e) and relative permeability (l) of samples-paraffin composites. As shown in Fig. 3(a), it can be found that there exist frequencies intervals in which the permittivity presents resonant characteristics. The maximum/minimum values can be found before/after the resonant frequency on the real part (e0) curve; a peak can also be observed near the resonant frequencies on the imaginary part (e00) curve. These phenomena are typical characteristics of nonlinear resonant behaviors [4]. For Fe nanoparticles in Fig. 3(a), the e0 and e00 is almost independent of frequency. However, e0 and e00 of the Fe/C nanocapsules are higher than those of the Fe nanoparticles in the 2–18 GHz, indicating a lower resistivity of the former, according to the free electron theory [3]. The measured electric resistivity 55 X m of the Fe/C nanocapsules is lower than 75 X m of Fe nanoparticles, which is well consistent with the results of e Although the electric resistivity of the bulk graphite is about 7.5 106 X m [18], close to that of metallic magnets (106– 108 X m), it is expected that the electrical resistivity of curving graphite shells, several nanometers in thickness, is increased, because of the special microstructure and defect properties [8]. According to the Debye dipolar relaxation expression, ðe0 e1 Þ2 þ ðe00 Þ2 ¼ ðes e1 Þ2 , where es and e1 are stationary dielectric constant and optical dielectric constant, respectively, and the plot of e0 versus e00 would be a single semicircle, which is usually defined as the Cole–Cole semicircle [19]. It is worthy to note that the composite containing Fe/C nanocapsules presents a clear segment of two semicircles in Fig. 4(a), suggesting the existence of dual dielectric relaxation processes, while each semicircle corresponds to a Debye dipolar relaxation. During the activation of an EM wave, a redistribution process of the charges occurs periodically in Fe cores and C shells [4,19]. As a result, apart from the dielectric relaxation of C shells, an additional interfacial relaxation between Fe cores and C shells is present because a complete core/ shell interface is constructed [4]. The combined dual dielectric losses in Fe/C nanocapsules are the origins of the enhanced EM
Fig. 3. (a) Relative permittivity and (b) relative permeability of samples-paraffin composites as a function of frequency.
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Fig. 4. (a) Typical Cole–Cole semicircles and (b) values of l00 ðl0 Þ2 f 1 and (c) dielectric loss factor and magnetic loss factor and (d) attenuation constant of samples-paraffin composites versus frequency.
absorption abilities. As shown in Fig. 4(a), the composite containing Fe nanoparticles just exhibits a clear semicircle from the interfacial relaxation between Fe core and oxides shell, due to the fact that a redistribution process of the charges is absent in the oxides shell with giant resistance. Fig. 3(b) shows the real part (l0) and imaginary part (l00) of l at the 2–18 GHz. As can be seen in Fig. 3(b), the real part l0 decreases with increasing frequency in Fe nanocapsules/nanoparticles in the 2–18 GHz range. The values of l0 and l00 in the nanocapsules is higher than that in nanoparticles over 2–18 GHz, which is attributed to the protection of graphite shell on the magnetic properties at high frequency. It is note worthy that the maximum value of the l00 in Fe nanocapsules appears at 12.2 GHz, while the l00 in Fe nanoparticles exhibits no clear peaks at 2–18 GHz, which implies that the natural resonance occurs in the Fe nanocapsules and natural resonance in Fe nanoparticles may occur below 2 GHz. According to the natural resonance equation 2pfr ¼ rHa [20], where r = 2.8 GHz kOe1 is the gyromagnetic ratio and Ha ¼ 4jK 1 j=3l0 M s the anisotropy coefficient (K1) for bulk a-Fe which is about 4.81 104 Jm3, the natural-resonance frequency (fr) should be around several tens of megahertz. The anisotropy energy of small size particles, especially on nanometer scale, may be remarkably increased due to the surface anisotropic field affected by the very-small-size effect [5,10]. The maximum of the curve for the Fe nanocapsules has shifted to a higher frequency value (12.2 GHz), which is important for their use as EM-wave-absorption materials in the higher-frequency region. The contributors to magnetic loss, such as magnetic hysteresis loss, domain-wall displacement loss and eddy current loss, can be excluded in the Fe nanocapsules/nanoparticles. The hysteresis loss is mainly caused by the time lags of the magnetization vector behind the external EM-field vector, which is negligible in a weak
applied field [4,21]. Because domain-wall displacement loss at GHz frequency range can make l0 to be zero, the contribution of the domain-wall displacement loss can be excluded. If the magnetic loss results from eddy current loss, the values of l00ðl0Þ2 f 1 should be constant when frequency is varied [22]. We can call this the skineffect criterion. As shown in Fig. 4(b), the values of l00ðl0Þ2 f 1 remarkably decrease with increasing frequency. Therefore, the magnetic loss in the present nanocapsules/nanoparticles is caused mainly by the natural resonance. There are two possible contributions for microwave absorption, namely, dielectric loss and magnetic loss [10]. The dielectric loss factor (tan(de) = l00=l0) and the magnetic loss factor (tan(dl) = l00=l0) are plotted as a function of frequency in Fig. 4(c). As is shown in Fig. 4(c), the tan(de) of Fe/C nanocapsules exhibit the dramatic fluctuation, due to the relaxation process, however, the tan(de) of Fe nanoparticles approximatively remains a constant. The average values of tan(de) of the nanocapsules is bigger than that of the nanoparticles over 2–18 GHz, ascribed to the dual dielectric resonance induced by graphite shells. The tan(dl) of Fe/ C nanocapsules is bigger than that of Fe nanoparticles at the frequency range of 2–18 GHz, due to the natural resonance. In addition, EM-wave attenuation in the interior of absorber is one of key factors for an excellent absorber. The attenuation constant a, which determines the attenuation properties of materials, can be determined as:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf ðl00e00 l0e0Þ þ ðl00e00 l0e0Þ2 þ ðe0l00 þ e00l0Þ2 ; a¼ c where f is the frequency of the EM-wave and c is the velocity of light [23]. In Fig. 4(d), the frequency dependence of a shows that graphite-coated Fe nanocapsules has bigger a at 2–18 GHz,
X. Liu et al. / Journal of Alloys and Compounds 548 (2013) 239–244
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Fig. 5. Frequency dependence of the RL of composite containing 40 wt.% (a) graphite-coated Fe nanocapsules and (b) Fe nanoparticles, for layers of different thickness.
Table 2 The optimal RL and absorption width (RL < 10 dB and RL < 20 dB) of Fe nanocapsules/nanoparticles at the same thickness. Thickness (mm)
Optimal RL (nanocapsules/ particles) (dB)
Absorption width (GHz) (nanocapsules/ particles) (RL < 10 dB)
Absorption width (GHz) (nanocapsules/ particles) (RL < 20 dB)
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
3.6/2.4 15.4/9.8 28.3/24.2 33.1/19.2 20.7/16.8 18.5/14.6 17.1/12.9 17.0/11.7 16.0/11.8
0/0 1.6/0 7/4.4 6/4.2 5.8/3.2 2.6/2.4 2.4/1.8 2.0/1.2 1.8/1.0
0/0 0/0 1.4/1.4 1.8/0 0.4/0 0/0 0/0 0/0 0/0
indicating the excellent attenuation or EM-wave absorption at 2– 18 GHz. From above equation, higher values of l00 would result in higher a. Accordingly, it is predicted that the Fe/C nanocapsules will exhibit better EM-absorption properties compared with Fe nanoparticles at 2–18 GHz. According to the transmission-line model, the RL of a metalbacked microwave absorbing layer is
rffiffiffiffiffi jZ tanhðkdÞ 1 lr 2pf pffiffiffiffiffiffiffiffiffi RL ¼ 20lg lr er with Z ¼ and k ¼ jZ tanhðkdÞ þ 1 c er Here, the materials constants lr ¼ l0 jl00 and er ¼ e0 je00 are the complex permeability and complex permittivity, respectively, d is the absorption layer thickness, and f is the frequency of incident wave. The dip in RL indicates the occurrence of absorption or optimal reflection of the microwave power. To further prove the enhanced EM-wave absorption properties at 2–18 GHz, the RL (dB) were calculated, as shown in Fig. 5. The RL below 10 dB and 20 dB indicate that more than 90% and 99% of the incident EMwave absorbed, respectively. As shown in Table 2, microwave absorption ability of graphite-coated Fe nanocapsules is enhanced over 2–18 GHz, as seen from not only the high optimal RL but also the wide effective absorption band width (RL < 20 dB and RL < 10 dB) at the same thickness. Enhanced EM-wave absorption properties are attributed to the improved magnetic/dielectric loss and attenuation constant by graphite layer shells. 4. Conclusions The Fe/C nanocapsules with graphite layers as shells and Fe nanoparticles as cores and the Fe nanoparticles have been prepared
by arc-discharge technique. The influence of graphite layers on the microstructure, magnetic, thermal and EM-characteristic of Fe nanoparticles have been investigated in detail. Compared with Fe nanoparticles, the lower MS of Fe/C nanocapsules is attributed to the smaller size, due to the fact that the graphite shells can restrain the growth of Fe nanoparticles. TGA and SDTA show that graphite layers can help Fe/C nanocapsules to be stable securely below 220 °C in the air, while Fe nanoparticles are oxidized at about 90 °C. HRTEM and TGA analysis indicate that there are a mass of defects in the graphite shells, which leads to lower oxidization temperature in graphite shell compared with CNTs and bulk graphite crystals. Graphite layer can protect the soft-magnetic properties of Fe cores at the high frequency range as well as keep the l higher than that of Fe nanoparticles. Graphite layer shell is helpful for increasing the magnetic/dielectric loss and attenuation constant, which leads to enhanced EM-wave absorption properties in 2–18 GHz. Acknowledgments This study has been supported partly by the National Natural Science Foundation of China (Grant Nos. 51071001, 51201002 and 21071003), by the Hong Kong Polytechnic University Postdoctoral Fellowships Scheme (G-YX3V), and by the Research Grants Council of the HK SAR Government (PolyU 5236/12E), and by the Hong Kong Polytechnic University (G-YK59, 4-ZE 7L, and G-YX3V). References [1] Z.D. Zhang, Nanocapsules, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, American Scientific Inc., California, 2004, p. 77. [2] M.E. McHenry, D.E. Laughlin, Acta Mater. 48 (2000) 223. [3] X.G. Liu, D.Y. Geng, H. Meng, P.J. Shang, Z.D. Zhang, Appl. Phys. Lett. 92 (2008) 173117. [4] X.G. Liu, J.J. Jiang, D.Y. Geng, B.Q. Li, Z. Han, W. Liu, Z.D. Zhang, Appl. Phys. Lett. 94 (2009) 053119. [5] X.G. Liu, Z.Q. Ou, D.Y. Geng, Z. Han, J.J. Jiang, W. Liu, Z.D. Zhang, Carbon 48 (2010) 891. [6] R.T. Lv, F.Y. Kang, W.X. Wang, J.Q. Wei, J.L. Gu, K.L. Wang, Carbon 45 (2007) 1433. [7] X.F. Zhang, X.L. Dong, H. Huang, Y.Y. Liu, W.N. Wang, X.G. Zhu, Appl. Phys. Lett. 89 (2006) 053115. [8] X.F. Zhang, X.L. Dong, H. Huang, Y.Y. Liu, B. Lv, J.P. Lei, J. Phys. D: Appl. Phys. 40 (2007) 5383. [9] X.G. Liu, Z.Q. Ou, D.Y. Geng, Z. Han, Z.G. Xie, Z.D. Zhang, J. Phys. D: Appl. Phys. 42 (2009) 155004. [10] X.G. Liu, B. Li, D.Y. Geng, W.B. Cui, F. Yang, Z.G. Xie, D.J. Kang, Z.D. Zhang, Carbon 47 (2009) 470. [11] Z.G. Xie, D.Y. Geng, X.G. Liu, S. Ma, Z.D. Zhang, J. Mater. Sci. Tech. 27 (2011) 607. [12] C.Q. Sun, Prog. Solid State Chem. 35 (2007) 1. [13] M. Respaud, J.M. Broto, H. Rakoto, A.R. Fert, L. Thomas, B. Barbara, M. Verelst, E. Snoeck, P. Lecante, A. Mosset, Phys. Rev. B 57 (1998) 2925. [14] Z.H. Wang, C.J. Choi, B.K. Kim, J.C. Kim, Carbon 41 (2003) 1751.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Influence of a graphite shell on the thermal, magnetic and electromagnetic characteristics of Fe nanoparticles Xianguo Liu a,b,⇑, Siu Wing Or b,⇑, Yuping Sun c, Weihuo Li a, Yizhu He a, Guohui Zhu a, Chuangui Jin a, Qun Yan c, Yaohui Lv a, Siu Lau Ho b, Shengsheng Zhao d a
School of Materials Science and Engineering, Anhui University of Technology, Ma’anshan 243002, PR China Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong Center for Engineering Practice and Innovation Education, Anhui University of Technology, Ma’anshan 243002, PR China d School of Mechanical and Electrical Engineering, Shenzhen Polytechnic, Shenzhen 518055, PR China b c
a r t i c l e
i n f o
Article history: Received 10 June 2012 Received in revised form 1 September 2012 Accepted 3 September 2012 Available online 14 September 2012 Keywords: Graphite Nanoparticles Magnetic Complex permeability Complex permittivity
a b s t r a c t Graphite-coated Fe nanocapsules were prepared by arc discharge method. Fe nanocapsules have a core/ shell microstructure, in which Fe nanoparticles as cores and graphite layers as shells. Compared with Fe nanoparticles with an oxide shell, the graphite shell can restrain the growth of Fe nanoparticles, which leads to lower saturation magnetization and higher natural resonance frequency. The Fe nanocapsules are stable securely below 220 °C in the air, due to the enhancement of thermal stability and the anti-oxidation by graphite layer shell. Dielectric relaxation of graphite shell and the interfacial relaxation between graphite shell and Fe core lead to the dual nonlinear dielectric resonance. Graphite shells can dramatically improve the magnetic/dielectric loss and the attenuation constant in the 2–18 GHz though the dual dielectric resonance and protection of Fe cores, leading to enhanced microwave absorption properties of Fe nanocapsules at 2–18 GHz. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Much attention has been paid to magnetic metallic nanoparticle by virtue of their potential for applications in fields, such as magnetic recording media, catalysis, drug delivery and microwave absorbents [1,2]. However, applications have been limited because these particles suffer from rapid environmental degradation and oxidation. Nanocapsules are developed to overcome this disadvantage, which are the encapsulation of nanoparticles inside protective outer shells. In particular, the outer shells can be a component, consisting of nanocomposites through microstructures of ‘‘core/shell’’ type. These excellent properties would provide opportunities for practical applications in many fields. Recently, increasing scientific interest has been focused on the electromagnetic (EM) characteristics of metal-based nanocomposites. Magnetic metals can remain high EM parameters in a high frequency range due to its large saturation magnetization and higher Snoek’s limit [3,4]. Nonetheless, the weak magnetocrystalline anisotropy and attenuated permeability due to the eddy current phenomenon may limit its applications at higher frequencies. Increased surface anisotropies and reduced eddy current can be realized by dimin⇑ Corresponding authors. Tel./fax: +86 555 2311570. E-mail addresses: [email protected] (X. Liu), [email protected] (S.W. Or). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.09.006
ishing the metallic particles’ size and these become the main advantages of nanosized metallic particles used as an EM wave absorber. Graphite layers are ideal shells because they can be easily absorbed by the surface of magnetic nanoparticles and can protect the nanoparticles effectively against oxidation. In addition, the studies on materials based on graphite have been intensive, which can be used as EM shielding and absorption materials, due to their lower density and rich electric transport properties [5,6]. Zhang and co-workers have reported that graphite-coated magnetic nanocapsules exhibit improved EM absorption properties because of their proper EM matching between the dielectric and the magnetic losses [7,8]. Recently, we have prepared a series of graphite-coated FeNi alloy nanocapsules by arc discharging the FeNi alloy ingot with different composition and investigated their EM properties in detail [5,9–11]. In the present work, the effect of the graphite shell has been systematically investigated. Graphite-coated Fe nanocapsules and Fe nanoparticles are studied in detail, in particular concerning the influence of the graphite shell on the thermal and magnetic properties and on the EM characteristics of the Fe nanoparticles. Compared with Fe nanoparticles, graphite shells effectively improve thermal stability of Fe nanocapsules and permeability of Fe nanocapsules at GHz range by means of the protection of Fe nanoparticles cores, restrict the growth of the Fe nanoparticles, which leads to lower saturation
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magnetization, and reduces the electrical resistivity. Dielectric relaxation of graphite shells and the interfacial relaxation between graphite shells and Fe nanoparticle cores lead to the dual nonlinear dielectric resonance. As a result of the enhanced magnetic/dielectric loss and the attenuation constant, the Fe nanocapsules exhibit good EM-absorption properties in the 2–18 GHz range.
Table 1 The average diameter, average shell thickness and saturation magnetization MS of Fe nanocapsules/nanoparticles. Sample
Average diameter (nm)
Average shell thickness (nm)
MS at 295 K (Am2/kg)
Fe nanocapsules Fe nanoparticles
27.8 59.6
2.4 4.8
45.2 63.4
2. Experimental The used arc-discharge method has been described in detail elsewhere [3–5, 7–11]. In brief, Fe ingot on a water-cooled copper crucible has been used as the anode. The cathode was a carbon needle. After the chamber was evacuated, 1.4 104 Pa of pure argon and 0.6 104 Pa of hydrogen were introduced into the chamber. The arc-discharge current was maintained at 100 A for 1 h for sufficient evaporation of the anode material. For the preparation of Fe nanoparticles, a tungsten needle was introduced as the cathode under the same experimental conditions. Phase analysis of the products was performed by powder X-ray diffraction (XRD), acquired by a Bruker D8 Advance X-ray diffractometer equipped with a monochromatized Cu Ka radiation. The morphology of the nanoparticles was stdudied in detail in a high-resolution transmission electron microscope (HRTEM JEOL2010) with an emission voltage of 200 kV. The magnetic properties were measured in a superconducting quantum interference device (SQUID, Quantum Design MPMS XL-7).The oxidation behavior was investigated by thermal gravimetric analysis (TGA) and scanning differential thermal analysis (SDTA) in air atmosphere at a heating rate of 10 °C min1 from 20 to 800 °C. The Ni nanocapsules-paraffin composite was prepared by uniformly mixing Ni nanocapsules with paraffin, as described in detail elsewhere [3–5,7–11], by homogeneously mixing the nanocapsule with paraffin and pressing them into cylinder-shaped compacts. Then the compact was cut into toroidal shape with 7.00 mm outer diameter and 3.04 mm inner diameter. The EM parameters are measured for paraffin-Ni nanocapsules composite containing 40 wt.% Ni nanocapsules, using an Agilent 8722ES network analyzer. Coaxial method was used to determine the EM parameters of the toroidal samples in a frequency range of 2–18 GHz with a transverse EM mode. The VNA was calibrated for the full two-port measurement of reflection and transmission at each port. The complex permittivity and complex permeability were calculated from S-parameters tested by the VNA, using the simulation program of Reflection/Transmission Nicolson-Ross model.
3. Results and discussion Fig. 1 shows XRD patterns and HRTEM images of graphitecoated Fe nanocapsules and Fe nanoparticles. In Fig. 1(a), all sharp reflection peaks could be indexed to the Fe with a body-centeredcubic structure. No reflection for oxides could be found, indicating that Fe nanoparticles are almost without oxidation, due to the protective graphite shell. In the XRD patterns, there are no detectable peaks for pure C, indicating its small amount (less than 3% in the samples). As a comparison, the XRD pattern of Fe nanoparticles is presented in Fig. 1(b). It is note worthy that the spectrum is not smooth, indicating the existence of some oxides in the as-prepared Fe nanoparticles. An HRTEM image of a part of graphite-coated Fe nanocapsules shows the clear core/shell microstructure, in which the surface of Fe nanoparticle was coated by a multi-layered
Fig. 1. XRD patterns and HRTEM images of (a) graphite-coated Fe nanocapsules and (b) Fe nanoparticles.
graphitic shell around 2 nm in thickness. The lattice spacing of the coating is close to that of the bulk graphite (0 0 2) planes, which is accordance with the previous results [8–11]. Nevertheless, a mass of lattice imperfections can be seen in the outer shells as a consequence of the serious bending and collapsing of the graphite atom layer, which is similar with the FeNi/C nanocapsules [9–11]. As shown in Fig. 1(a), since graphite is on the shell of the nanocapsules, it is difficulty to detect its XRD pattern because of breaking down of the periodic boundary condition (translation symmetry) along radial direction. A typical HRTEM image of a part of Fe nanoparticle, as shown in Fig. 1(b), clearly indicates that an amorphous oxide shell about 2 nm in thickness formed in the passivation process on the surface of the nanoparticle, which further proves the high surface energy and chemical activity for Fe nanoparticles. The average diameter and shell thickness of Fe nanocapsules/ nanoparticles, as estimated by measuring above 300 nanocapsules/nanoparticles, are shown in Table 1. In addition, the saturation magnetization MS at 295 K is also listed in Table 1. It is observed that the average diameter and shell thickness of the Fe/ C nanocapsules is lower than that of the Fe nanoparticles, ascribed to that the graphite shell can restrain the growth of Fe nanoparticles in the arc-discharge process. In addition, the MS of the nanocapsules is lower than that of nanoparticles, which can be explained by the fact that the MS drops monotonically with the size of the particles at ambient temperature region [12]. The relation between the MS and the size of particles measured at room temperature agrees with the previous calculations [12]. Metallic nanoparticles are easily oxidized or self-ignited, due to the large ratio of surface/volume [13,14]. In order to investigate the influence of graphite shell on thermal stability and anti-oxidation behavior, SDTA and TGA were done for the Fe nanocapsules/nanoparticles at a heating rate of 10 °C min1 in a flowing air atmosphere. Thermal analysis was used to estimate the nature and stability, as well as the density of defects in the carbon layers [15].As shown in the TGA and SDTA curves of Fe/C nanocapsules (Fig. 2(a)), in the segment from 220 to 320 °C, the TGA–SDTA curves exhibit a gradual weight gain, associated with an exothermic peak at 330 °C. This weight gain implies that the Fe cores are being slowly oxidized. It also indicates that graphite-coated Fe nanocapsules are stable securely below 220 °C in the air. Above 320 °C, it is thought that the Fe cores may undergo a serious oxidation and/or self ignition corresponding to an exothermic peak at about 550 °C, and the weight gain is neglected by the weight loss due to oxidation of carbon to CO2 [5,15]. That is, the weight loss between 320 and 800 °C is recognized as an active chemical reaction in which the graphite phase is oxidized to CO2 gas. The oxidation temperature of the graphite shell in the Fe/C nanocapsules is significantly lower than that of CNTs (527–727 °C) [16] and bulk graphite crystals (846 °C) [17], which is ascribed to a mass of lattice defects resulting from serious bending of the graphite atomic layers, such as vacancies and interstitial bonding, etc. The results of HRTEM image also confirm the existence of a mass of graphitic layer defects, which are active sites for O2 attack. In comparison, the oxidation of Fe nanoparticles takes place at about 90 °C accompanying a broad exothermic peak near the temperature range from 190 to 260 °C (as shown in Fig. 2(b)), which is different with the previous reports on Cu/C nanocapsules and FeNi/C nanocapsules
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Fig. 2. SDTA and TGA curves of (a) graphite-coated Fe nanocapsules and (b) Fe nanoparticles.
[5,15]. The TGA-SDTA curves of Fe nanoparticles in the range of 90– 190 °C exhibit a gradual weight gain, associated with the broad exothermic peak at 190 °C, implying that the oxidation process and crystal process of amorphous oxide shell. The broad exothermic peak from 190 to 260 °C results from the phase transformation from the low temperature oxide of Fe3O4 into high temperature oxide of Fe2O3 on the surface of nanoparticles due to the high surface energy and from the phase transformation from Fe to FeO and Fe3O4 in Fe cores at the same time. As the heating temperature increases, an exothermic peak occurs at 538 °C, associated with a gradual weight gain. This implies that the oxidation of Fe cores to Fe2O3 happens, representing the extent of oxidization from unstable to stable. On the TGA curves of Fe nanocapsules/nanoparticles, it is worthy noted that a weight loss of about 1 wt.% occurs in Fe nanocapsules/nanoparticles below 50 °C, which is attributed to the release of moisture on the surface of the nanocapsules/nanoparticles [14]. As we known, the reduction associates with a rapid weight loss and
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the oxidization associates with the weight gain in the TGA curve. From TGA analysis, the carbon-coated Fe nanocapsules appear to be stable below 220 °C in air. O2 and other gases can be absorbed by the defects on the graphite shell, which lead to the increase of weight from 50 to 150 °C. With the increase of temperature, the release of gases which can not react with Fe leads to the slight weight loss. The Fe cores are slowly oxidized with the further increase of temperature. The weight loss between 320 and 800 °C is due to oxidation of carbon to CO2. As shown in Fig. 2(b), the weight gain of Fe nanoparticles gradually increase from 50 to 800 °C, which proves that Fe nanoparticles can be easily oxidized. The graphite shell can effectively improve the thermal stability of Fe nanocapsules. Fig. 3 shows the frequency dependence of relative permittivity (e) and relative permeability (l) of samples-paraffin composites. As shown in Fig. 3(a), it can be found that there exist frequencies intervals in which the permittivity presents resonant characteristics. The maximum/minimum values can be found before/after the resonant frequency on the real part (e0) curve; a peak can also be observed near the resonant frequencies on the imaginary part (e00) curve. These phenomena are typical characteristics of nonlinear resonant behaviors [4]. For Fe nanoparticles in Fig. 3(a), the e0 and e00 is almost independent of frequency. However, e0 and e00 of the Fe/C nanocapsules are higher than those of the Fe nanoparticles in the 2–18 GHz, indicating a lower resistivity of the former, according to the free electron theory [3]. The measured electric resistivity 55 X m of the Fe/C nanocapsules is lower than 75 X m of Fe nanoparticles, which is well consistent with the results of e Although the electric resistivity of the bulk graphite is about 7.5 106 X m [18], close to that of metallic magnets (106– 108 X m), it is expected that the electrical resistivity of curving graphite shells, several nanometers in thickness, is increased, because of the special microstructure and defect properties [8]. According to the Debye dipolar relaxation expression, ðe0 e1 Þ2 þ ðe00 Þ2 ¼ ðes e1 Þ2 , where es and e1 are stationary dielectric constant and optical dielectric constant, respectively, and the plot of e0 versus e00 would be a single semicircle, which is usually defined as the Cole–Cole semicircle [19]. It is worthy to note that the composite containing Fe/C nanocapsules presents a clear segment of two semicircles in Fig. 4(a), suggesting the existence of dual dielectric relaxation processes, while each semicircle corresponds to a Debye dipolar relaxation. During the activation of an EM wave, a redistribution process of the charges occurs periodically in Fe cores and C shells [4,19]. As a result, apart from the dielectric relaxation of C shells, an additional interfacial relaxation between Fe cores and C shells is present because a complete core/ shell interface is constructed [4]. The combined dual dielectric losses in Fe/C nanocapsules are the origins of the enhanced EM
Fig. 3. (a) Relative permittivity and (b) relative permeability of samples-paraffin composites as a function of frequency.
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Fig. 4. (a) Typical Cole–Cole semicircles and (b) values of l00 ðl0 Þ2 f 1 and (c) dielectric loss factor and magnetic loss factor and (d) attenuation constant of samples-paraffin composites versus frequency.
absorption abilities. As shown in Fig. 4(a), the composite containing Fe nanoparticles just exhibits a clear semicircle from the interfacial relaxation between Fe core and oxides shell, due to the fact that a redistribution process of the charges is absent in the oxides shell with giant resistance. Fig. 3(b) shows the real part (l0) and imaginary part (l00) of l at the 2–18 GHz. As can be seen in Fig. 3(b), the real part l0 decreases with increasing frequency in Fe nanocapsules/nanoparticles in the 2–18 GHz range. The values of l0 and l00 in the nanocapsules is higher than that in nanoparticles over 2–18 GHz, which is attributed to the protection of graphite shell on the magnetic properties at high frequency. It is note worthy that the maximum value of the l00 in Fe nanocapsules appears at 12.2 GHz, while the l00 in Fe nanoparticles exhibits no clear peaks at 2–18 GHz, which implies that the natural resonance occurs in the Fe nanocapsules and natural resonance in Fe nanoparticles may occur below 2 GHz. According to the natural resonance equation 2pfr ¼ rHa [20], where r = 2.8 GHz kOe1 is the gyromagnetic ratio and Ha ¼ 4jK 1 j=3l0 M s the anisotropy coefficient (K1) for bulk a-Fe which is about 4.81 104 Jm3, the natural-resonance frequency (fr) should be around several tens of megahertz. The anisotropy energy of small size particles, especially on nanometer scale, may be remarkably increased due to the surface anisotropic field affected by the very-small-size effect [5,10]. The maximum of the curve for the Fe nanocapsules has shifted to a higher frequency value (12.2 GHz), which is important for their use as EM-wave-absorption materials in the higher-frequency region. The contributors to magnetic loss, such as magnetic hysteresis loss, domain-wall displacement loss and eddy current loss, can be excluded in the Fe nanocapsules/nanoparticles. The hysteresis loss is mainly caused by the time lags of the magnetization vector behind the external EM-field vector, which is negligible in a weak
applied field [4,21]. Because domain-wall displacement loss at GHz frequency range can make l0 to be zero, the contribution of the domain-wall displacement loss can be excluded. If the magnetic loss results from eddy current loss, the values of l00ðl0Þ2 f 1 should be constant when frequency is varied [22]. We can call this the skineffect criterion. As shown in Fig. 4(b), the values of l00ðl0Þ2 f 1 remarkably decrease with increasing frequency. Therefore, the magnetic loss in the present nanocapsules/nanoparticles is caused mainly by the natural resonance. There are two possible contributions for microwave absorption, namely, dielectric loss and magnetic loss [10]. The dielectric loss factor (tan(de) = l00=l0) and the magnetic loss factor (tan(dl) = l00=l0) are plotted as a function of frequency in Fig. 4(c). As is shown in Fig. 4(c), the tan(de) of Fe/C nanocapsules exhibit the dramatic fluctuation, due to the relaxation process, however, the tan(de) of Fe nanoparticles approximatively remains a constant. The average values of tan(de) of the nanocapsules is bigger than that of the nanoparticles over 2–18 GHz, ascribed to the dual dielectric resonance induced by graphite shells. The tan(dl) of Fe/ C nanocapsules is bigger than that of Fe nanoparticles at the frequency range of 2–18 GHz, due to the natural resonance. In addition, EM-wave attenuation in the interior of absorber is one of key factors for an excellent absorber. The attenuation constant a, which determines the attenuation properties of materials, can be determined as:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf ðl00e00 l0e0Þ þ ðl00e00 l0e0Þ2 þ ðe0l00 þ e00l0Þ2 ; a¼ c where f is the frequency of the EM-wave and c is the velocity of light [23]. In Fig. 4(d), the frequency dependence of a shows that graphite-coated Fe nanocapsules has bigger a at 2–18 GHz,
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Fig. 5. Frequency dependence of the RL of composite containing 40 wt.% (a) graphite-coated Fe nanocapsules and (b) Fe nanoparticles, for layers of different thickness.
Table 2 The optimal RL and absorption width (RL < 10 dB and RL < 20 dB) of Fe nanocapsules/nanoparticles at the same thickness. Thickness (mm)
Optimal RL (nanocapsules/ particles) (dB)
Absorption width (GHz) (nanocapsules/ particles) (RL < 10 dB)
Absorption width (GHz) (nanocapsules/ particles) (RL < 20 dB)
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
3.6/2.4 15.4/9.8 28.3/24.2 33.1/19.2 20.7/16.8 18.5/14.6 17.1/12.9 17.0/11.7 16.0/11.8
0/0 1.6/0 7/4.4 6/4.2 5.8/3.2 2.6/2.4 2.4/1.8 2.0/1.2 1.8/1.0
0/0 0/0 1.4/1.4 1.8/0 0.4/0 0/0 0/0 0/0 0/0
indicating the excellent attenuation or EM-wave absorption at 2– 18 GHz. From above equation, higher values of l00 would result in higher a. Accordingly, it is predicted that the Fe/C nanocapsules will exhibit better EM-absorption properties compared with Fe nanoparticles at 2–18 GHz. According to the transmission-line model, the RL of a metalbacked microwave absorbing layer is
rffiffiffiffiffi jZ tanhðkdÞ 1 lr 2pf pffiffiffiffiffiffiffiffiffi RL ¼ 20lg lr er with Z ¼ and k ¼ jZ tanhðkdÞ þ 1 c er Here, the materials constants lr ¼ l0 jl00 and er ¼ e0 je00 are the complex permeability and complex permittivity, respectively, d is the absorption layer thickness, and f is the frequency of incident wave. The dip in RL indicates the occurrence of absorption or optimal reflection of the microwave power. To further prove the enhanced EM-wave absorption properties at 2–18 GHz, the RL (dB) were calculated, as shown in Fig. 5. The RL below 10 dB and 20 dB indicate that more than 90% and 99% of the incident EMwave absorbed, respectively. As shown in Table 2, microwave absorption ability of graphite-coated Fe nanocapsules is enhanced over 2–18 GHz, as seen from not only the high optimal RL but also the wide effective absorption band width (RL < 20 dB and RL < 10 dB) at the same thickness. Enhanced EM-wave absorption properties are attributed to the improved magnetic/dielectric loss and attenuation constant by graphite layer shells. 4. Conclusions The Fe/C nanocapsules with graphite layers as shells and Fe nanoparticles as cores and the Fe nanoparticles have been prepared
by arc-discharge technique. The influence of graphite layers on the microstructure, magnetic, thermal and EM-characteristic of Fe nanoparticles have been investigated in detail. Compared with Fe nanoparticles, the lower MS of Fe/C nanocapsules is attributed to the smaller size, due to the fact that the graphite shells can restrain the growth of Fe nanoparticles. TGA and SDTA show that graphite layers can help Fe/C nanocapsules to be stable securely below 220 °C in the air, while Fe nanoparticles are oxidized at about 90 °C. HRTEM and TGA analysis indicate that there are a mass of defects in the graphite shells, which leads to lower oxidization temperature in graphite shell compared with CNTs and bulk graphite crystals. Graphite layer can protect the soft-magnetic properties of Fe cores at the high frequency range as well as keep the l higher than that of Fe nanoparticles. Graphite layer shell is helpful for increasing the magnetic/dielectric loss and attenuation constant, which leads to enhanced EM-wave absorption properties in 2–18 GHz. Acknowledgments This study has been supported partly by the National Natural Science Foundation of China (Grant Nos. 51071001, 51201002 and 21071003), by the Hong Kong Polytechnic University Postdoctoral Fellowships Scheme (G-YX3V), and by the Research Grants Council of the HK SAR Government (PolyU 5236/12E), and by the Hong Kong Polytechnic University (G-YK59, 4-ZE 7L, and G-YX3V). References [1] Z.D. Zhang, Nanocapsules, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, American Scientific Inc., California, 2004, p. 77. [2] M.E. McHenry, D.E. Laughlin, Acta Mater. 48 (2000) 223. [3] X.G. Liu, D.Y. Geng, H. Meng, P.J. Shang, Z.D. Zhang, Appl. Phys. Lett. 92 (2008) 173117. [4] X.G. Liu, J.J. Jiang, D.Y. Geng, B.Q. Li, Z. Han, W. Liu, Z.D. Zhang, Appl. Phys. Lett. 94 (2009) 053119. [5] X.G. Liu, Z.Q. Ou, D.Y. Geng, Z. Han, J.J. Jiang, W. Liu, Z.D. Zhang, Carbon 48 (2010) 891. [6] R.T. Lv, F.Y. Kang, W.X. Wang, J.Q. Wei, J.L. Gu, K.L. Wang, Carbon 45 (2007) 1433. [7] X.F. Zhang, X.L. Dong, H. Huang, Y.Y. Liu, W.N. Wang, X.G. Zhu, Appl. Phys. Lett. 89 (2006) 053115. [8] X.F. Zhang, X.L. Dong, H. Huang, Y.Y. Liu, B. Lv, J.P. Lei, J. Phys. D: Appl. Phys. 40 (2007) 5383. [9] X.G. Liu, Z.Q. Ou, D.Y. Geng, Z. Han, Z.G. Xie, Z.D. Zhang, J. Phys. D: Appl. Phys. 42 (2009) 155004. [10] X.G. Liu, B. Li, D.Y. Geng, W.B. Cui, F. Yang, Z.G. Xie, D.J. Kang, Z.D. Zhang, Carbon 47 (2009) 470. [11] Z.G. Xie, D.Y. Geng, X.G. Liu, S. Ma, Z.D. Zhang, J. Mater. Sci. Tech. 27 (2011) 607. [12] C.Q. Sun, Prog. Solid State Chem. 35 (2007) 1. [13] M. Respaud, J.M. Broto, H. Rakoto, A.R. Fert, L. Thomas, B. Barbara, M. Verelst, E. Snoeck, P. Lecante, A. Mosset, Phys. Rev. B 57 (1998) 2925. [14] Z.H. Wang, C.J. Choi, B.K. Kim, J.C. Kim, Carbon 41 (2003) 1751.
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