Magnetic and Microwave-absorption Properties of Graphite-coated (Fe, Ni) Nanocapsules

J. Mater. Sci. Technol., 2011, 27(7), 607-614.

Magnetic and Microwave-absorption Properties of Graphite-coated (Fe, Ni) Nanocapsules Zhigao Xie† , Dianyu Geng, Xianguo Liu, Song Ma and Zhidong Zhang Shenyang National Laboratory for Materials Science, Institute of Metal Research and International Centre for Material Physics, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China [Manuscript received July 6, 2010]

The structure, magnetic and microwave-absorption properties of graphite-coated (Fe, Ni) alloy nanocapsules, synthesized by the arc-discharge method, have been studied. High-resolution transmission electron microscopy shows that the nanocapsules have a core/shell structure with (Fe, Ni) alloy as the core and graphite as the shell. All (Fe, Ni) alloy nanocapsules/paraffin composites show good microwave-absorption properties. The optimal reflection loss (RL) was found for (Fe70 Ni30 )/C nanocapsules/paraffin composites, being −47.84 dB at 14.6 GHz for an absorber thickness of 1.99 mm, while the RL values exceeding −10 dB were found in the 12.4– 17.4 GHz range, which almost covers the Ku band (12.4–18 GHz). For (Fe70 Ni30 )/C nanocapsules/paraffin composites, RL values can exceed −10 dB in the 11.4–18 GHz range with an absorber thickness of 1.91 mm, which cover the whole Ku band. KEY WORDS: Magnetic; Nanocapsules; Microwave absorption

1. Introduction Due to the rapid development of wireless communications, the electromagnetic interference (EMI) pollution has become much serious. Therefore it is necessary to exploit new types of microwave-absorption materials with excellent properties, such as wide frequency range, strong absorption, low density, high resistivity, etc.. Soft magnetic materials may be potential candidates for microwave-absorption at high frequencies in the gigahertz range, because of their large saturation magnetization, high permeability and higher Snoek s limit[1] . However, the relative complex permeability of magnetic metallic materials may be decreased by eddy-current phenomena induced by electromagnetic (EM) wave. Therefore, it is better to use isolated metallic particles with a small size less than the skin-depth for suppressing the eddy current phenomenon, in order to enhance an effective incidence to EM wave absorbers. Magnetic nanocapsules † Corresponding author. Ph.D.; Tel.: +86 24 83978751; E-mail address: [email protected] (Z.G. Xie).

with core/shell structure (i.e. magnetic nanoparticles coated with a non-magnetic insulator) have attracted considerable attention because of their wide range of applications, including catalysis, clinical drug delivery, magnetic-storage media and EM-wave absorbing material[2–4] . The EM-wave absorption properties of magnetic nanocapsules with different core/shell struc[7] tures, including Ni/C[5] , Fe/ZnO[6] , FeCo/Al2 O3 , [9] [10] α-Fe/SmO[8] , Fe/SiO2 , Co/Y2 O3 etc., have been studied extensively. (Fe, Ni) alloys are soft magnetic with large saturation magnetization, high permeability and low energy losses. Recently, the EM-wave absorption properties of (Fe, Ni)/C nanocapsules prepared with starting powders of composition Fe90 Ni10 have been reported[11] . In the present investigation, a series of graphite-coated (Fe, Ni) alloy nanocapsules have been synthesized by the arc-discharge technique, with the purpose of finding optimal EM-wave absorption properties by adjusting (Fe, Ni) alloy components. The structure, magnetic and EM-wave absorption properties of these nanocapsules have been investigated systematically.

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608 2. Experimental

Graphite-coated (Fe, Ni) alloy nanocapsules were synthesized by the arc-discharge method with modified strategies[12,13] . Master Fe100−x Nix alloys with different Ni contents (x = 20, 30, 40, 50, 70 and 80) were prepared by arc-melting Fe and Ni bulk pieces with purity of 99.9% in a high-purity argon atmosphere. In the arc-discharge process, the Fe100−x Nix alloy ingots served as the anode, while the cathode was a graphite rod. The distance between the anode and the cathode was about 3 mm. A gas mixture of Ar (10 kPa) and H2 (4 kPa), together with 20 ml liquid ethanol were introduced into an evacuated chamber (6×10−3 Pa). During the arcdischarge process, the current was maintained at 80 A for 20 min and the voltage at 20 V. After being passivated in argon atmosphere (0.02 MPa) for 8 h, the products were collected from the surface at the top of the water-cooled chamber. Six samples of the nanocapsules (Fe100−x Nix )/C (x = 20, 30, 40, 50, 70 and 80) were marked as samples A, B, C, D, E and F, respectively. Phase analysis of the products was performed by powder X-ray diffraction (XRD) on a Rigaku D/max-2000 diffractometer with graphite monochromatized CuKα (λ=0.154056 nm) radiation. The morphology and microstructure of the nanocapsules were observed by high-resolution transmission electron microscopy (HRTEM JEOL-2010). The compositions of the nanocapsules were determined by scanning electron microscopy (SEM, Philips SSX-550) with energy dispersive spectrometry (EDS). The surface compositions were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB-250). The magnetic properties were measured by means of superconducting quantum interference device (SQUID, Quantum Design MPMS-7). Nanocapsule-paraffin composites were prepared by homogeneously mixing the nanocapsules with paraffin and pressing into cylinder-shaped compacts. Then the compacts were cut into a toroidal shape with outer diameter of 7.00 mm and inner diameter of 3.04 mm. The corresponding paraffin composites containing 50 wt% of the samples A, B, C, D, E and F were denoted as PA, PB, PC, PD, PE and PF, respectively. The scattering parameters (S11 and S21 ) were measured by an Agilent 8722ES network analyzer. The relative permeability (μr = μ + iμ ) and permittivity (εr = ε + iε ) values of the composites were determined from the scattering parameters as measured in the frequency range of 2–18 GHz. The RL curves were calculated from the relative permeability and permittivity for a given frequency and absorber thickness by means of the following equations[14] : Zin = Z0 (μr /εr )1/2 tanh[j(2πf d/c)(μr εr )1/2 ] RL = 20 log |(Zin − Z0 )/(Zin + Z0 )|

Fig. 1 XRD patterns of nanocapsules A to F

where f is the frequency of the EM wave, d the thickness of the absorber, c the velocity of light, Z0 the impedance of air, and Zin the input impedance of the absorber. According to Eqs. (1) and (2), an RL value of −20 dB corresponds to 99% EM-wave absorption, which can be considered as effective absorbance in practical applications[6,15] . 3. Results and Discussion The XRD patterns of the A-F nanocapsules are shown in Fig. 1. All sharp reflection peaks could be indexed to the (Fe, Ni) alloy. The main phase in the nanocapsules crystallizes in the fcc structure with space group Fm¯3m. No oxide peaks are detectable in the XRD patterns, indicating that the cores of the nanocapsules are almost free from oxidation, due to the protection of the carbon shell. Since carbon is mainly in the shell of the nanocapsules, it is difficult to detect its XRD pattern because of breaking down of the periodic boundary condition (translation symmetry) along radial direction. All diffraction peaks shift gradually to higher angle with increasing Ni content. The morphology, size distribution and core/shell structure of the five nanocapsules A-E were studied by TEM and exhibit similar behavior. As an example, the TEM results for the nanocapsule B are shown in Fig. 2. The nanocapsules have irregular spherical shape, which are about 10–50 nm in diameter (Fig. 2(a)). The HRTEM image in Fig. 2(b) and (c) show the core/shell structure with a crystalline core and an onion-like graphite shell with thickness of 3– 4 nm. In the core, the d-spacing of 0.207 nm corresponds to the lattice fringe {111} of (Fe, Ni) alloy. The lattice plane spacing of the shell is about 0.34 nm, which corresponds to the (002) planes of graphite. From the HRTEM image, it can be noticed that the lattice fringe outside is slightly larger than that inside. This means that the graphite is mainly located at the surface of the nanoparticle and that a little graphite enters into the (Fe, Ni) crystal lattice near the surface. These results are consistent with the

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Fig. 2 (a) Morphology, (b) and (c) HRTEM image of the nanocapsule B

Table 1 Compositions of the master alloy and A–F nanocapsules (by EDS) Master alloy/at.% Nanocapsules/at.% (wt%) Samples Fe Ni Fe Ni C A 80 20 37.36 (63.40) 9.69 (17.28) 52.95 (19.32) B 70 30 40.64 (58.83) 18.76 (28.53) 40.60 (12.64) C 60 40 34.86 (51.94) 21.84 (34.19) 43.30 (13.87) D 50 50 28.79 (42.48) 28.32 (43.91) 42.89 (13.61) E 30 70 16.53 (24.88) 38.23 (60.47) 45.24 (14.65) F 20 80 11.69(16.94) 45.84 (69.82) 42.47 (13.24)

Fig. 3 XPS survey patterns for C1s at the surface for different etching depths and the fitting curves of nanocapsules B. The inset is for etching times 0 and 15 s

XPS results. Figure 3 shows the binding energy of 1s electrons of graphite in the nanocapsules B and the fitting curves. The peak at 284.6 eV belongs to 1s electrons of the graphite[16] at the surface and the peak at 285.6 eV to 1s electrons of graphite in the (Fe, Ni) alloy. The compositions of the as-prepared nanocapsules have been determined by EDS. In Table 1, the compositions of the master alloys and the nanocapsules are displayed. The ratio of Fe and Ni in the as-prepared nanocapsules is almost the same as that in the master alloys. These slight differences are ascribed to different evaporating pressures, melting points of iron and nickel during the arc-discharge process.

Figure 4 shows the temperature dependence of the magnetization for the nanocapsules A-F at an applied field of 200 Oe between 5 and 300 K, after zerofield-cooling (ZFC) and field-cooling (FC) processes. The bifurcation between ZFC and FC curves indicates irreversible magnetic behavior. The maximums in the ZFC curves of nanocapsules A and B appear at 75 K and 175 K, respectively, which indicate that the nanocapsules show superparamagnetic from 75 K and 175 K to 300 K and the ZFC and FC curves of nanocapsules C-F suggest that the nanocapsules are ferromagnetic at/below 300 K. The different shapes of the ZFC and FC curves are ascribed to the different sizes of nanocapsules in six samples. The broad peak in the ZFC curve of nanocapsules B indicates a broad size distribution of the nanocapsules, which is consistent with the HRTEM result. Figure 5(a) shows the real part (ε ) and the imaginary part (ε ) of the relative permittivity (εr ) of the composites PA-PF in the 2–18 GHz range. Both ε and ε exhibit a decreasing trend between 2 and 18 GHz, and vary with the composition of different samples, the minimum being from sample PB. ε is relative to the polarization and ε implies the dielectric loss. In the present nanocapsules, the dipole polarization is dominant at a higher frequency and the weak space charge polarization mainly works at a lower frequency. According to free-electron theory[17] , ε ≈ 1/2πε0 ρf , where ρ is the resistivity. The small values of ε indicate a higher resistivity of the present composites compared with Fe/graphite nanotubes[18] and Fe/C[19] . This is ascribed to the effective disper-

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Fig. 4 ZFC and FC magnetization of the nanocapsules A–F between 5 and 300 K at an applied field of 200 Oe

Fig. 5 (a) Permittivity and (b) permeability of graphite-coated (Fe, Ni) nanocapsule-paraffin wax samples as a function of frequency in the 2–18 GHz range

sion and the protective shells, which play a role as insulator. In general, a high electrical resistivity is good for improving the EM-wave absorption proper-

ties. The small fluctuations that ε and ε exhibit in the 2–18 GHz range are ascribed to the displacementcurrent lag at the core/shell interface, which is similar

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Fig. 6 Typical Cole-Cole semicircles for samples PA–PF

to what has been found for Fe/C nanocapsules[19] . The real part (μ ) and the imaginary part (μ ) of the relative permeability (μr ) of the composites PA–PF are shown in Fig. 5(b). All μ values of PA– PF decrease in the 2–11 GHz range, but almost unchanged for the frequencies larger than 11 GHz. It is interesting to find that the values of μ are proportional to the Ni content, but this trend becomes opposite above about 11 GHz. The μ values vary with the compositions and exhibit broad multi-resonance peaks at 2–18 GHz. The imaginary part of permeability reflects the magnetic loss, which mainly comes from the magnetic hysteresis, domain-wall displacement, eddy current loss, and natural resonance. In the present nanoparticles, the multi-resonance peaks are due to the natural resonance, which is the main magnetic loss. According to the natural-resonance equa[20] tion 2πfr = rHeff , where fr is the natural-resonance frequency, γ the gyromagnetic ratio, and Heff the effective anisotropy field. Heff is proportional to the effective anisotropy constant Keff which includes the

volume Kv and surface Ks [21] . The Keff of small particles, especially in nanometer scale, will be remarkably increased by the small size effect[22] . So the naturalresonance frequency fr of nanoparticles shifts to a higher frequency, compared with bulk. A higher fr is important for application as EM-wave absorption materials in the microwave region. With increasing the Ni content, the μ values increase. The reason could be ascribed to the variation in the magnetocrystalline anisotropies according to the expression μ ∝ Ms2 /|K1 |[23] , where Ms is the saturated magnetization and K1 is the first-order magnetocrystalline anisotropy constant, and Fe has higher magnetocrystalline anisotropy than Ni. Figure 6 shows the characteristics of ε vs ε curve for the six PA-PF samples. According to the equation (ε − ε∞ )2 + (ε )2 = (εs − ε∞ )2 , where εs and ε∞ are the stationary dielectric constant and optical dielectric constant[24] , the plot of ε vs ε would be a single semicircle, which is usually defined as the Cole-Cole semicircle[25] . It can be noted that all samples PA–PF

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Fig. 7 Dielectric loss factor and magnetic loss factor of the nanocapsule-paraffin wax samples vs frequency

Fig. 8 Frequency dependences of RL of the graphite-coated (Fe, Ni) alloy nanocapsule-paraffin wax composites PA-PF for different thicknesses in the 2–18 GHz range

Z.G. Xie et al.: J. Mater. Sci. Technol., 2011, 27(7), 607–614

present clear segment of four overlapped semicircles, which suggests that there are dual dielectric relaxation processes, and each semicircle corresponds to a Debye dipolar relaxation[24] . The dielectric loss factor (tan(δε ))=ε /ε and the magnetic loss factor (tan(δμ ) = μ /μ ) of the nanocapsule-paraffin composites are shown in Fig. 7. A higher value of tan(δ) indicates a higher loss. The dielectric loss factor tan(δε ) values show the similar dramatic fluctuation in the 2–18 GHz range. The fluctuation is ascribed to relaxation process. For PA and PB, the dielectric loss factor is much larger than the magnetic loss factor. Thus the main contribution of the microwave absorption should come from the dielectric loss. To further reveal the EM-wave absorption properties, the RL values of the composites PA-PF were calculated. Figure 8 shows the EM-wave RL of graphitecoated (Fe, Ni) nanocapsule-paraffin wax samples PAPF as a function of frequency. For the sample PA, an optimal RL value of −30.13 dB is found at 15 GHz (fm ) for an absorber thickness of 1.80 mm. When the thickness is varied from 1.49 to 5.52 mm, the RL values can exceed −20 dB between 4–18 GHz. For sample PB, RL values exceeding −20 dB are found in the frequency range of 6.2– 18 GHz for absorber thicknesses ranging from 1.59– 4.07 mm, and an optimal RL value of −47.84 dB is found at 14.6 GHz for an absorber thickness of 1.99 mm. It is worth noting that, for a 1.99 mm thick absorber of sample PB, RL values exceeding −10 dB are obtained in the range 12.4-17.4 GHz, which almost cover the whole Ku band (12.4-18 GHz). This is good for practical application. The optimal RL values of samples PA and PB are larger than those of (Fe90 Ni10 )/C[11] . For the samples PC-PF, the RL is relatively poor, but it can still exceed −10 dB with absorber thickness above 1.41 mm, 1.36 mm, 1.33 mm and 1.38 mm in the 2–18 GHz range. Especially, for PC with an absorber thickness of 1.91 mm, RL values exceeding −10 dB are obtained in the 11.4–18 GHz range, which cover the whole Ku band. In addition, the maximum RL shifts to lower frequency with increasing the thickness of absorber. This phenomenon is consistent with the equation d=c/2πμ f where d, c, μ , and f are sample thickness, velocity of light, imaginary part of permeability and matching frequency, respectively[26] . The good EM-wave absorption properties are ascribed to magnetic and dielectric losses in the core/shell structure of the (Fe, Ni) alloy nanocapsules with ferromagnetic (Fe, Ni) cores and graphite shells, a strong natural resonance, and the match of dielectric loss and magnetic loss, etc. 4. Conclusion The structure, magnetic and EM-wave absorption properties of graphite-coated (Fe, Ni) alloy nanocapsules, with different Fe-Ni ratio and synthesized by

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arc-discharge, have been studied. The results show that the nanocapsules have a core/shell structure with (Fe, Ni) alloy as the core and onion-like graphite as the shell. All (Fe, Ni) alloy nanocapsules/paraffin composites show good EM-wave absorption performance. An optimal RL was found in sample PB, which can reach −47.84 dB at 14.6 GHz for an absorber thickness of 1.99 mm. For a 1.99 mm thick absorber of sample PB, RL values exceeding −10 dB are obtained in the range 12.4–17.4 GHz, which almost cover the whole Ku band. For sample PC with an absorber thickness of 1.91 mm, RL values can exceed −10 dB in the 11.4–18 GHz range, which cover the whole Ku band.

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