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Effects of particle size on the magnetic and microwave absorption properties of carbon-coated nickel nanocapsules
Journal of Alloys and Compounds 656 (2016) 628e634
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of particle size on the magnetic and microwave absorption properties of carbon-coated nickel nanocapsules Niandu Wu, Xianguo Liu*, Chengyun Zhao, Caiyun Cui, Ailin Xia School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2015 Received in revised form 1 October 2015 Accepted 3 October 2015 Available online 8 October 2015
To study the effects of particle size on magnetic and microwave absorption properties, carbon-coated nickel (Ni/C) nanocapsules were prepared by an arc discharge method, where their particles size was adjusted by varying the arc current value from 40 to 100 A. The average particle size of the Ni/C nanocapsules increases from 25 to 53 nm while the thickness of carbon shells keeps independence with increasing the arc current value. The saturation magnetization and coercivity increase with increasing the particle size, due to the small size effect. The complex permittivity, dielectric loss and attenuation constant of paraffin-Ni/C composites are in inverse proportion to the particle size of Ni/C nanocapsules, ascribed to the larger interface area and more defects in the smaller nanocapsules. With the reduction of particle size of Ni/C nanocapsules, the peak of reflection loss (RL) at fixed absorber shifts to lower frequency and the bandwidth becomes broader, while the maximum RL of the composites can be achieved at thinner absorber layer. The phenomenon results from the bigger dielectric loss, larger attenuation constant and better impedance matching degree. The control of particle size is an effective way for adjusting the microwave absorption properties. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic Microwave absorption Nickel Nanocapsules
1. Introduction In recent years, microwave absorption materials have attracted increasing research interests due to the serious problems of electromagnetic (EM) interference and radiation that arise from the rapid advancements of mobile telephones, wireless network, microwave devices, and satellite broadcast systems [1,2]. The performance of EM wave absorption depends strongly on the combination of dielectric loss and magnetic loss of materials. Moreover, optimal absorption materials are expected to have broad absorbing bandwidth, strong absorption capability, low density, and thin absorber thickness [3]. However, traditional absorption materials still have some disadvantages. Traditional ferrite materials are required to have thick absorbent layers because of their weakened EM loss in gigahertz frequency range [4]. Nanosized carbon materials possess high conductivity and dielectric losses, but the materials alone cannot obtain effective absorption due to the absence of magnetic loss [5]. Metallic magnetic nanoparticles possess high permeability at high frequency over gigahertz, and the
* Corresponding author. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.jallcom.2015.10.027 0925-8388/© 2015 Elsevier B.V. All rights reserved.
eddy current effect can be restrained due to the nanometer size lower than skin depth [6,7]. However, metallic nanoparticles are easily oxidized or self-ignited in the air [8]. Recently, the nanocomposites composed of dielectric materials and magnetic materials have been considered as an effective strategy to improve the microwave absorption properties. These nanocomposites can obtain not only the complementarity between dielectric and magnetic loss but also additional physical and chemical properties. Among the candidates for microwave absorption, the nanocapsules have attracted wide interest because of their heterogeneous interface and special magnetic-core/dielectric-shell structure [2]. Many kinds of nanocapsules, including carbon-coated nickel (Ni/C) [9,10], Co/C [11], Fe/ZnO [12], Ni/Polypyrrole [13], Fe3O4/C [14,15], CoNi/C [16] have been investigated extensively, and all of them exhibit outstanding microwave absorption properties. Ni/C nanocapsules, which have the advantages of simple preparation process, cheap compositions, and good environmental stability, is an attractive kind of absorption materials [10]. In previous studies, Zhang et al. [9] prepared Ni/C nanocapsules by an arc discharge method, and the Ni/C exhibited extraordinary microwave absorption properties. The maximum reflection loss (RL) of Ni/C nanocapsules reached 32 dB at 13 GHz with 2 mm thickness layer, and the excellent EM properties were attributed to the good EM
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matching, the strong natural resonance, and the lags of polarization. Wang et al. [10] investigated Ni/C nanocapsules with different graphite shell thickness. The dielectric and the microwave absorption properties were enhanced by increasing the thickness of graphite shells. However, the maximum RL was achieved under a relatively thick absorber layer (7.8 nm). As with the shell, the particle size is also a significant parameter, which can affect the EM and the microwave absorption properties. Many studies showed that the particle size could be controlled by varying the experimental condition [17e19]. However, to the best of our knowledge, the effects of the particle size of nanocapsules on the microwave absorption properties are rarely investigated. In this paper, we present a series of Ni/C nanocapsules with different particle size prepared by an arc discharge method. The particle size varied with the arc current value. In order to avoid the influence from the shell thickness, the thickness of carbon shell was kept almost constant via introducing same amount of carbon source (ethanol). The magnetic loop, EM parameters, EM loss, attenuation constant, the calculated reflection loss and impedance matching degree were investigated to reveal the effect of particle size on microwave absorption properties. 2. Experiments The Ni/C nanocapsules were prepared by the modified arc discharge method [12,20]. Nickel ingots placed in a water-cooled cooper crucible were used as the anode and a tungsten electrode with 5 mm diameter as the cathode. After the chamber was evacuated in a vacuum of 5.0 103 Pa, argon (20 kPa) and hydrogen (10 kPa) were introduced into the chamber. Meanwhile, liquid ethanol was introduced as the carbon source. During the experiments, the arc discharge was maintained for about 30 min. Then the products on the inner surface of the chamber were collected after passivation for about 10 h in argon atmosphere. For studying the effect of particle size, four samples were prepared at different arc discharge currents of 40, 60, 80, and 100 A, respectively, with other factors fixed. The four as-prepared samples with the arc current values of 40, 60, 80, and 100 A were marked as samples AeD, respectively. The microstructure and size distribution were confirmed by a transmission electron microscope (TEM, JEOL-2010). The phase of the products was examined by X-ray powder diffraction (XRD, Brucker D8 Advance) with Cu-Ka radiation. The magnetic properties at room temperature were measured using a vibrating sample magnetometer (VSM). In order to measure the EM parameters (the complex permittivity and the complex permeability), the nanocapsules were mixed with 50 wt.% paraffin (transparent for EM wave) and then pressed into a toroid with 7.00 mm outer diameter and 3.04 mm inner diameter. The EM properties of the composites were measured from 2 to 18 GHz by using a vector network analyzer (VNA, Anritsu 37269D). For convenience, the paraffinbonded composites containing 50 wt.% samples AeD were marked as samples PAePD, respectively. 3. Results and discussion 3.1. Structure and phase characterization The morphology and microstructure of the Ni/C nanocapsules were investigated by TEM and HRTEM studies. As shown in the TEM images of Fig. 1(aed), all the nanocapsules are irregular spheres and have the core/shell structure. Table 1 presents the measured particle sizes of samples AeD by analyzing more than 100 nanoparticles. It is obviously seen that the average particle size increases from sample A to sample D. That is, the particle size increases with
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the increase of current value. More information about the microstructure of the as-prepared nanocapsules is shown in HRTEM images of Fig. 1(eeh). It is clearly indicated that all samples have the crystalline core and the onion-like shell, which is similar with that in previous works [9,10]. The lattice plane spacing of 0.203 nm in the core is consistent with the lattice distance of (111) planes in face-center cubic (fcc) nickel, and the d-spacing of about 0.34 nm in the shell corresponds to the (002) planes of graphite. The carbon shells exhibit no difference among the four samples, and their average thickness is 1.8e2.1 nm, due to introducing the same amount ethanol. In addition, the planes bending and collapsing can be observed in the carbon shells, arising from the non-equilibrium thermodynamic process. The amount of defects in sample A is more than that in sample D, indicating more defects in smaller nanoparticles. The above phenomena can be ascribed to the formation mechanism of the nanocapsules during the arc discharge process [21,22]. The arc discharge can provide extremely high temperature to evaporate the bulk metal and dissociate the ethanol, and the high temperature just exists in a narrow zone between the arc and molten pool. For this reason, the formation of the Ni/C nanocapsules here was a non-equilibrium process. At extremely high temperature, nickel bulk evaporated and formed a region of metal vapor above the molten pool. At the region, nickel atoms collided and absorbed the carbon atoms that were from decomposed ethanol to form aggregates. When the vapor was supersaturated, the aggregates nucleated and rapidly condensed to liquid droplets. As they were away from the arc zone, temperature further decreased, and the carbon atoms were separated out from the droplets and formed a shell at the surface. When the arc current increased, the arc temperature increased and more nickel was evaporated, which were helpful for increasing the particle size [17]. However, due to the insufficient carbon source and low solubility of carbon in nickel, the thickness of carbon shell would not change significantly. The XRD patterns of samples AeD are shown in Fig. 2. All diffraction peaks can be indexed to the (111), (200), and (220) planes of fcc-Ni (JCPDS No. 65-2865), indicating Ni is the main phases for the four samples. There is no clear peak about carbon due to the onion-like structure that can be observed from the HRTEM images. Furthermore, from samples A to D, the relative intensity of the diffraction peaks of Ni increases and the peaks gradually become narrow, implying that the crystallinity and crystal size of nickel core increases with increasing the arc current value. 3.2. Magnetic properties The magnetic hysteresis loops at room temperature of the four samples are presented in Fig. 3. They are similar and magnetization are all saturated at 10 kOe. The measured saturation magnetization (MS) and coercivity (HC) are listed in Table 1. It can be found that both MS and HC gradually increase from sample A to sample D. The increase of MS can be interpreted by the weakened thermal fluctuation near the surface and less magnetically disordered surface formed due to the reduction of surface-to-volume ration with increase in particle size [23]. In addition, the MS values of the four samples are less than that of bulk nickel (55 emu/g) [24], which may be attributed to the existence of nonmagnetic carbon and the surface disorder or spin canting [12,25]. The HC increases with the increase of particle size, which can be explained by the monodomain behavior [26]. The particle sizes are less than the calculated critical single-domain size of bulk nickel (~55 nm) [27], indicating that the magnetic nickel core is of the single-domain structure. When the nanoparticle is the single-domain structure,
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Fig. 1. (aed) TEM images and (eeh) HRTEM images of the samples A, B, C, and D.
Table 1 Structural and magnetic parameters of samples AeD. Sample
Average particle size (nm)
Average shell thickness (nm)
MS (emu/g)
Hc (Oe)
A B C D
25 38 46 53
1.9 2.1 1.8 2.0
30.2 36.5 41.9 44.6
86.8 103.7 112.4 118.3
Fig. 2. XRD patterns for samples A, B, C, and D.
the HC increases with increasing the particle size. 3.3. Electromagnetic properties The effective absorption performance is associated with the good EM impedance matching under a certain thickness of absorber layers, which depends on the complex permittivity (εr ¼ ε0 jε00 ) and the complex permeability (mr ¼ m0 jm00 ) of the microwave absorption materials [18]. Fig. 4(a) and (b) show the frequency dependence of real part (ε0 ) and imaginary part (ε00 ) of the complex permittivity of the four samples. For samples PAePD, the ε0 value decreases from 9.95, 8.89, 6.75, and 6.02 to 5.76, 5.08, 4.82 and 5.25, and the ε00 value decreases from 4.50, 4.76, 2.85, and 2.05 to 3.95, 2.41, 1.78, and 1.53, respectively, in 2e18 GHz with a little fluctuation. The tendency of decreasing with increasing
Fig. 3. Magnetic hysteresis loops of samples A, B, C, and D at room temperature.
frequency for ε0 and ε00 can be ascribed to the delayed response of dipole polarization with respect to the electric-field change at higher frequencies [10]. The downward tendency for samples PA and PB is more obvious than that for samples PC and PD, indicating more electric dipoles exist in samples PA and PB. For sample PA, it is worthy noted that the ε00 decreases from 4.50 to 3.00 in the 2e7.5 GHz range and then slightly increases in the 7.5e18 GHz range, which reveals the enhanced effect of dielectric polarization in high frequency range for sample PA. Both ε0 and ε00 decrease with increasing particle size from samples PA to PD, indicating that reducing particle size is beneficial to enhance the permeability. This phenomenon may be interpreted as follows: the interfacial polarization is dominant in the heterogeneous coreeshell structures, which contributes to the high permittivity [28]. The nanocapsules with smaller particle size can obtain larger interface area between core and shell, resulting in an enhanced heterogeneous interfacial
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Fig. 4. (a) Real part (ε0 ) and (b) imaginary part (ε0 0 ) of the relative complex permittivity, and (c) real part (m0 ) and (d) imaginary part (m0 0 ) of the relative complex permeability of samples PA, PB, PC, and PD in the frequency range of 2e18 GHz.
polarization. In addition, some defects existing in carbon shells and nickel cores act as the dipolar polarization centers, leading to various dielectric relaxations in different frequency ranges [11]. With decreasing the particle size of nanocapsules, larger interface area and more defects exist in the nanocapsules, which enhance the permittivity of the nanocapsules. The frequency dependence of real part (m0 ) and imaginary part (m00 ) of the complex permeability of samples PAePD are presented in Fig. 4(c) and (d). The m0 value of samples PA, PB, PC, and PD decreases rapidly from 1.25, 1.28, 1.29, and 1.20 to 1.01, 0.98, 0.97, and 0.94, respectively, in the frequency range of 2e7 GHz, and then are almost constant. It can be observed that m0 of the samples with smaller size are higher than those of the larger ones in high frequency. For sample PD, there is an apparently decline tendency for m0 observed in the 15e18 GHz, indicating the magnetic properties of Ni nanoparticles with the large size cannot be effectively protected by the carbon shell in high gigahertz frequency range [29]. From Fig. 4(d), broad multi-resonance peaks of m00 are observed at about 2e6 GHz for all the samples, which is corresponding to multimagnetic resonance, like in CoNi@C nanocapsules [16]. However, the m00 values have no clear difference among the four samples. Compared with εr, the variation of mr among the four samples is not obvious, revealing the particle size has little effect on mr. In previous studies on Ni/C nanocapsules [9,10], the mr of Ni/C nanocapsules with different diameters (25e30 nm in Ref. [9] and average 50 nm in Ref. [10], respectively) is mainly determined by the magnetic nickel core. The mr of Ni/C nanocapsules in the two previous studies have similar trends and values (m0 ¼ 1.26e0.93; m00 ¼ 0.22 ~ 0.8), which are also similar to that in this work. Those similar mr for Ni/C nanocapsules with different size further confirm that the effect of the particle size is too weak on mr. The dielectric loss tangent (tande ¼ ε00 /ε0 ) and the magnetic loss tangent (tandm ¼ m00 /m0 ) are used to quantify the EM loss capacity [30]. The calculated dielectric and magnetic loss tangents are shown in Fig. 5(a) and (b), respectively. For the four samples, the dielectric loss is always larger than the magnetic loss. Furthermore, it can be found that tande of samples PA and PB are higher than
Fig. 5. (a)The dielectric loss tangent (tande) and (b) the magnetic loss tangent (tandm) of samples PA, PB, PC, and PD.
those of samples PC and PD. The tande of sample PB is larger than that of sample PA in 2e9 GHz, while the former is lower than the latter in 9e18 GHz. Compared with the variation of tande, the tandm values have almost no difference among the four samples. This
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result suggests that reducing the particle size of Ni/C nanocapsules can enhance the dielectric loss and have no apparent influence on the magnetic loss. 3.4. Microwave absorption properties The microwave absorption materials can absorb the energy of EM wave and convert them to heat by the complementarity between dielectric and magnetic losses. The attenuation constant (a) can stand for the effectiveness of absorber materials. Based on measured complex permittivity and permeability, the attenuation constant (a) can be calculated using the following equation [31]:
a¼
pffiffiffi 2pf =c rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðm00 ε00 m0 ε0 Þ þ
ðm00 ε00 m0 ε0 Þ2 þ ðε0 m00 þ ε00 m0 Þ2
(1)
where f is microwave frequency, and c is the velocity of light in free space. The frequency dependences of a value for samples PAePD are shown in Fig. 6. It can be observed that the sample with smaller size can get higher a value, indicating the more effective attenuation. From the above equation, it is noted that higher values of ε0 and ε00 would result in higher a when the values of m0 and m00 keep unchanged. The RL of samples PAePD can be calculated by the formula of the transmission-line theory [32]:
RL ¼ 20lgjðZin Z0 Þ=ðZin þ Z0 Þj
(2)
i h Zin ¼ Z0 ðmr =εr Þ1=2 tanh ið2pfd=cÞðmr εr Þ1=2
(3)
where Zin is the normalized input impedance at the air-absorber interface, Z0 is the free space impedance, and d is the absorber layer thickness. The calculated RL values of samples PAePD with the selected absorber layer thickness of 1.0e5.5 nm are shown in Fig. 7, in which all the samples exhibit good microwave absorption properties. The maximum RL reaches 32 dB at 11.7 GHz with absorber thickness of 2.5 mm for sample PA, 34 dB at 8.9 GHz with 3.5 mm thickness for sample PB, 25 dB at 5.8 GHz with 5.5 mm thickness for sample PC, and 17 dB at 6.3 GHz with 5.5 mm thickness for sample PD, respectively. By altering the layer thickness from 1.0 to 5.5 mm, the absorption bandwidth with the RL under 20 dB (99% absorption) can reach 10.3 GHz (5.8e16.1 GHz)
Fig. 7. Reflection loss for (a)e(d) samples PAePD with selected absorber thickness in the frequency range of 2e18 GHz, respectively.
Fig. 6. Attenuation constant (a) of samples PA, PB, PC, and PD in the frequency range of 2e18 GHz.
for sample PA, 7.0 GHz (6.2e13.2 GHz) for sample PB, and 1.9 GHz (5.5e7.4 GHz) for sample PC, respectively. The bandwidth decreases with the increase of particles size from samples PA to PC, and sample PD even cannot achieve the RL value under 20 dB by selecting the thickness from 1.0 to 5.5 nm. The above results show that decreasing the particle size of Ni/C nanocapsules not only reduces the absorber thickness for maximum RL, but also extends the absorption bandwidth. Fig. 8 shows the RL peak position of Ni/C nanocapsules at the same absorber thickness shifts from low to high frequency with increasing the size of nanocapsules, which may be due to the shift of EM matching range. In Table 2, the
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Fig. 8. The curves of the RL peak positions vs. the average diameter of Ni/C nanocapsules at different absorber thickness.
microwave absorption properties of Ni/C nanocapsules in this work are also compared with the results of other Ni/C nanocapsules systems. It can be observed that the Ni/C nanocapsules with small particle size can obtain the stronger microwave absorption abilities at a thinner absorber layer, revealing that reducing the particle size of nanocapsules is favorable for the improvement of microwave absorption properties. For dielectric/magnetic-loss composites, the good microwave absorption properties mainly arise from proper EM impedance matching. The matching degree can be evaluated by a deltafunction method [33], which is described as:
D ¼ sinh2 ðKfdÞ M
(4)
where K and M are determined by εr and mr. The smaller delta value indicates better EM impedance matching. Fig. 9 presents the calculated delta value maps of samples PAePD. The dark blue area (in the web version) of sample PA in Fig. 9(a) is largest among the four samples, indicating the optimal EM impedance matching for the Ni/C nanocapsules achieved at the smallest size. In addition, the dark blue area (in the web version) moves upward gradually with increasing size from samples PA to PD, revealing the nanoparticles with larger size are required to have thicker absorber layers to reach good EM impedance matching. This result is consistent with the variation of RL and their corresponding thickness, indicating that the enhanced microwave absorption properties with decreasing particles size is caused by the increase of EM impedance matching degree.
Fig. 9. Calculated delta value maps of (a)e(d) samples PAePD, respectively.
4. Conclusion The Ni/C nanocapsules were prepared by the arc discharge method. By changing the arc current from 40 to 100 A, the average particle size of the Ni/C nanocapsules varied from 25 to 53 nm and the thickness of carbon shell kept the independence. Due to the small size effect, the saturation magnetization and coercivity are proportional to the particle size. With the reduction of particle size, the larger interface area between core and shell results in the enhanced heterogeneous interfacial polarization and more defects existing in carbon shells and nickel cores leads to various dielectric relaxations in different frequency ranges, which are favorable for the improvement of complex permittivity, dielectric loss and attenuation constant. Sample PA, mixing Ni/C nanocapsules with the average particle size of 25 nm, exhibits a maximum RL value of 32 dB at 11.6 GHz with a thin absorber of 2.5 nm, and the bandwidth of the RL values exceeding 20 dB reaches 10.3 GHz (5.8e16.1 GHz) by selecting proper absorber layer thickness. Paraffin-bonded composites mixing the Ni/C nanocapsules with smaller particle size can obtain maximum RL values at thinner absorber layers with broader bandwidth, ascribed to the bigger dielectric loss, larger attenuation constant and better EM impedance matching degree. They thus shed light on the control of particle size to adjust the microwave absorption performance.
Table 2 Reported microwave absorption properties of Ni/C nanocapsules. Sample
Particle size/shell thickness (nm)
Ni/C PA PB PC PD Ni/C (S1) Ni/C (S3)
25e30/5e6 25/1.9 38/2.1 46/1.8 53/2.0 50/2 50/9
Optimal RL
Ref.
RL value (dB)
fm (GHz)
Absorber thickness (mm)
32 32 34 25 17 >20 40
13 11.7 8.9 5.8 6.3 e 3.2
2 2.5 3.5 5.5 5.5 e 7.8
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properties of Fe3O4/C self-assemblies, Mater. Lett. 122 (2014) 103e105. [16] H. Wang, Y.Y. Dai, W.J. Gong, D.Y. Geng, S. Ma, D. Li, W. Liu, Z.D. Zhang, Broadband microwave absorption of CoNi@C nanocapsules enhanced by dual dielectric relaxation and multiple magnetic resonance, Appl. Phys. Lett. 102 (2013) 223113. [17] Z.Q. Wei, T.D. Xia, L.F. Bai, J. Wang, Z.G. Wu, P.X. Yan, Efficient preparation for Ni nanopowders by anodic arc plasma, Mater. Lett. 60 (2006) 766e770. [18] K.M. Lim, K.A. Lee, M.C. Kim, C.G. Park, Complex permeability and electromagnetic wave absorption properties of amorphous alloy-epoxy composites, J. Non-Cryst. Solids 351 (2005) 75e83. [19] S.S. Kim, S.T. Kim, Y.C. Yoon, K.S. Lee, Magnetic, dielectric, and microwave absorbing properties of iron particles dispersed in rubber matrix in gigahertz frequencies, J. Appl. Phys. 97 (2005) 10F905. [20] X.G. Liu, J.J. Jiang, D.Y. Geng, B.Q. Li, Z. Han, W. Liu, Z.D. Zhang, Dual nonlinear dielectric resonance and strong natural resonance in Ni/ZnO nanocapsules, Appl. Phys. Lett. 94 (2009) 053119. [21] X.F. Zhang, H. Huang, X.L. Dong, Core/shell metal/heterogeneous oxide nanocapsules: the empirical formation law and tunable electromagnetic losses, J. Phys. Chem. C 117 (2013) 8563e8569. [22] H.X. Qiao, Z.Q. Wei, H. Yang, L. Zhu, X.Y. Yan, Preparation and characterization of NiO nanoparticles by anodic arc plasma method, J. Nanomater. 2009 (2009) 795928. [23] H. Iida, K. Takayanagi, T. Nakanishi, T. Osaka, Synthesis of Fe3O4 nanoparticles with various sizes and magnetic properties by controlled hydrolysis, J. Colloid Interface Sci. 314 (2007) 274e280. [24] J.H. Hwang, V.P. Dravid, M.H. Teng, J.J. Host, B.R. Elliott, D.L. Johnson, T.O. Mason, Magnetic properties of graphitically encapsulated nickel nanocrystals, J. Mater. Res. 12 (1997) 1076e1082. [25] A.K. Gaharwar, J.E. Wong, D. Müller-Schulte, D. Bahadur, W. Richtering, Magnetic nanoparticles encapsulated within a thermoresponsive polymer, J. Nanosci. Nanotechnol. 9 (2009) 5355e5361. [26] C.R. Lin, Y.M. Chu, S.C. Wang, Magnetic properties of magnetite nanoparticles prepared by mechanochemical reaction, Mater. Lett. 60 (2006) 447e450. [27] J. Frenkel, J. Dorfman, Spontaneous and induced magnetisation in ferromagnetic bodies, Nature 126 (1930) 274e275. [28] N. Bowler, Designing dielectric loss at microwave frequencies using multilayered filler particles in a composite, IEEE Trans. Dielectr. Electr. Insul. 13 (2006) 703e711. [29] X.G. Liu, B. Li, D.Y. Geng, W.B. Cui, F. Yang, Z.G. Xie, D.J. Kang, Z.D. Zhang, (Fe, Ni)/C nanocapsules for electromagnetic-wave-absorber in the whole Ku-band, Carbon 47 (2009) 470e474. [30] P.F. Guan, X.F. Zhang, J.J. Guo, Assembled Fe3O4 nanoparticles on graphene for enhanced electromagnetic wave losses, Appl. Phys. Lett. 101 (2012) 153108. [31] V. Sunny, P. Kurian, P. Mohanan, P.A. Joy, M.R. Anantharaman, A flexible microwave absorber based on nickel ferrite nanocomposite, J. Alloys Compd. 489 (2010) 297e303. [32] M. Matsumoto, Y. Miyata, Thin electromagnetic wave absorber for quasimicrowave band containing aligned thin magnetic metal particles, IEEE Trans. Magn. 33 (1997) 4459e4464. [33] Z. Ma, C.T. Cao, Q.F. Liu, J.B. Wang, A new method to calculate the degree of electromagnetic impedance matching in one-layer microwave absorbers, Chin. Phys. Lett. 29 (2012) 038401.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of particle size on the magnetic and microwave absorption properties of carbon-coated nickel nanocapsules Niandu Wu, Xianguo Liu*, Chengyun Zhao, Caiyun Cui, Ailin Xia School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2015 Received in revised form 1 October 2015 Accepted 3 October 2015 Available online 8 October 2015
To study the effects of particle size on magnetic and microwave absorption properties, carbon-coated nickel (Ni/C) nanocapsules were prepared by an arc discharge method, where their particles size was adjusted by varying the arc current value from 40 to 100 A. The average particle size of the Ni/C nanocapsules increases from 25 to 53 nm while the thickness of carbon shells keeps independence with increasing the arc current value. The saturation magnetization and coercivity increase with increasing the particle size, due to the small size effect. The complex permittivity, dielectric loss and attenuation constant of paraffin-Ni/C composites are in inverse proportion to the particle size of Ni/C nanocapsules, ascribed to the larger interface area and more defects in the smaller nanocapsules. With the reduction of particle size of Ni/C nanocapsules, the peak of reflection loss (RL) at fixed absorber shifts to lower frequency and the bandwidth becomes broader, while the maximum RL of the composites can be achieved at thinner absorber layer. The phenomenon results from the bigger dielectric loss, larger attenuation constant and better impedance matching degree. The control of particle size is an effective way for adjusting the microwave absorption properties. © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic Microwave absorption Nickel Nanocapsules
1. Introduction In recent years, microwave absorption materials have attracted increasing research interests due to the serious problems of electromagnetic (EM) interference and radiation that arise from the rapid advancements of mobile telephones, wireless network, microwave devices, and satellite broadcast systems [1,2]. The performance of EM wave absorption depends strongly on the combination of dielectric loss and magnetic loss of materials. Moreover, optimal absorption materials are expected to have broad absorbing bandwidth, strong absorption capability, low density, and thin absorber thickness [3]. However, traditional absorption materials still have some disadvantages. Traditional ferrite materials are required to have thick absorbent layers because of their weakened EM loss in gigahertz frequency range [4]. Nanosized carbon materials possess high conductivity and dielectric losses, but the materials alone cannot obtain effective absorption due to the absence of magnetic loss [5]. Metallic magnetic nanoparticles possess high permeability at high frequency over gigahertz, and the
* Corresponding author. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.jallcom.2015.10.027 0925-8388/© 2015 Elsevier B.V. All rights reserved.
eddy current effect can be restrained due to the nanometer size lower than skin depth [6,7]. However, metallic nanoparticles are easily oxidized or self-ignited in the air [8]. Recently, the nanocomposites composed of dielectric materials and magnetic materials have been considered as an effective strategy to improve the microwave absorption properties. These nanocomposites can obtain not only the complementarity between dielectric and magnetic loss but also additional physical and chemical properties. Among the candidates for microwave absorption, the nanocapsules have attracted wide interest because of their heterogeneous interface and special magnetic-core/dielectric-shell structure [2]. Many kinds of nanocapsules, including carbon-coated nickel (Ni/C) [9,10], Co/C [11], Fe/ZnO [12], Ni/Polypyrrole [13], Fe3O4/C [14,15], CoNi/C [16] have been investigated extensively, and all of them exhibit outstanding microwave absorption properties. Ni/C nanocapsules, which have the advantages of simple preparation process, cheap compositions, and good environmental stability, is an attractive kind of absorption materials [10]. In previous studies, Zhang et al. [9] prepared Ni/C nanocapsules by an arc discharge method, and the Ni/C exhibited extraordinary microwave absorption properties. The maximum reflection loss (RL) of Ni/C nanocapsules reached 32 dB at 13 GHz with 2 mm thickness layer, and the excellent EM properties were attributed to the good EM
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matching, the strong natural resonance, and the lags of polarization. Wang et al. [10] investigated Ni/C nanocapsules with different graphite shell thickness. The dielectric and the microwave absorption properties were enhanced by increasing the thickness of graphite shells. However, the maximum RL was achieved under a relatively thick absorber layer (7.8 nm). As with the shell, the particle size is also a significant parameter, which can affect the EM and the microwave absorption properties. Many studies showed that the particle size could be controlled by varying the experimental condition [17e19]. However, to the best of our knowledge, the effects of the particle size of nanocapsules on the microwave absorption properties are rarely investigated. In this paper, we present a series of Ni/C nanocapsules with different particle size prepared by an arc discharge method. The particle size varied with the arc current value. In order to avoid the influence from the shell thickness, the thickness of carbon shell was kept almost constant via introducing same amount of carbon source (ethanol). The magnetic loop, EM parameters, EM loss, attenuation constant, the calculated reflection loss and impedance matching degree were investigated to reveal the effect of particle size on microwave absorption properties. 2. Experiments The Ni/C nanocapsules were prepared by the modified arc discharge method [12,20]. Nickel ingots placed in a water-cooled cooper crucible were used as the anode and a tungsten electrode with 5 mm diameter as the cathode. After the chamber was evacuated in a vacuum of 5.0 103 Pa, argon (20 kPa) and hydrogen (10 kPa) were introduced into the chamber. Meanwhile, liquid ethanol was introduced as the carbon source. During the experiments, the arc discharge was maintained for about 30 min. Then the products on the inner surface of the chamber were collected after passivation for about 10 h in argon atmosphere. For studying the effect of particle size, four samples were prepared at different arc discharge currents of 40, 60, 80, and 100 A, respectively, with other factors fixed. The four as-prepared samples with the arc current values of 40, 60, 80, and 100 A were marked as samples AeD, respectively. The microstructure and size distribution were confirmed by a transmission electron microscope (TEM, JEOL-2010). The phase of the products was examined by X-ray powder diffraction (XRD, Brucker D8 Advance) with Cu-Ka radiation. The magnetic properties at room temperature were measured using a vibrating sample magnetometer (VSM). In order to measure the EM parameters (the complex permittivity and the complex permeability), the nanocapsules were mixed with 50 wt.% paraffin (transparent for EM wave) and then pressed into a toroid with 7.00 mm outer diameter and 3.04 mm inner diameter. The EM properties of the composites were measured from 2 to 18 GHz by using a vector network analyzer (VNA, Anritsu 37269D). For convenience, the paraffinbonded composites containing 50 wt.% samples AeD were marked as samples PAePD, respectively. 3. Results and discussion 3.1. Structure and phase characterization The morphology and microstructure of the Ni/C nanocapsules were investigated by TEM and HRTEM studies. As shown in the TEM images of Fig. 1(aed), all the nanocapsules are irregular spheres and have the core/shell structure. Table 1 presents the measured particle sizes of samples AeD by analyzing more than 100 nanoparticles. It is obviously seen that the average particle size increases from sample A to sample D. That is, the particle size increases with
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the increase of current value. More information about the microstructure of the as-prepared nanocapsules is shown in HRTEM images of Fig. 1(eeh). It is clearly indicated that all samples have the crystalline core and the onion-like shell, which is similar with that in previous works [9,10]. The lattice plane spacing of 0.203 nm in the core is consistent with the lattice distance of (111) planes in face-center cubic (fcc) nickel, and the d-spacing of about 0.34 nm in the shell corresponds to the (002) planes of graphite. The carbon shells exhibit no difference among the four samples, and their average thickness is 1.8e2.1 nm, due to introducing the same amount ethanol. In addition, the planes bending and collapsing can be observed in the carbon shells, arising from the non-equilibrium thermodynamic process. The amount of defects in sample A is more than that in sample D, indicating more defects in smaller nanoparticles. The above phenomena can be ascribed to the formation mechanism of the nanocapsules during the arc discharge process [21,22]. The arc discharge can provide extremely high temperature to evaporate the bulk metal and dissociate the ethanol, and the high temperature just exists in a narrow zone between the arc and molten pool. For this reason, the formation of the Ni/C nanocapsules here was a non-equilibrium process. At extremely high temperature, nickel bulk evaporated and formed a region of metal vapor above the molten pool. At the region, nickel atoms collided and absorbed the carbon atoms that were from decomposed ethanol to form aggregates. When the vapor was supersaturated, the aggregates nucleated and rapidly condensed to liquid droplets. As they were away from the arc zone, temperature further decreased, and the carbon atoms were separated out from the droplets and formed a shell at the surface. When the arc current increased, the arc temperature increased and more nickel was evaporated, which were helpful for increasing the particle size [17]. However, due to the insufficient carbon source and low solubility of carbon in nickel, the thickness of carbon shell would not change significantly. The XRD patterns of samples AeD are shown in Fig. 2. All diffraction peaks can be indexed to the (111), (200), and (220) planes of fcc-Ni (JCPDS No. 65-2865), indicating Ni is the main phases for the four samples. There is no clear peak about carbon due to the onion-like structure that can be observed from the HRTEM images. Furthermore, from samples A to D, the relative intensity of the diffraction peaks of Ni increases and the peaks gradually become narrow, implying that the crystallinity and crystal size of nickel core increases with increasing the arc current value. 3.2. Magnetic properties The magnetic hysteresis loops at room temperature of the four samples are presented in Fig. 3. They are similar and magnetization are all saturated at 10 kOe. The measured saturation magnetization (MS) and coercivity (HC) are listed in Table 1. It can be found that both MS and HC gradually increase from sample A to sample D. The increase of MS can be interpreted by the weakened thermal fluctuation near the surface and less magnetically disordered surface formed due to the reduction of surface-to-volume ration with increase in particle size [23]. In addition, the MS values of the four samples are less than that of bulk nickel (55 emu/g) [24], which may be attributed to the existence of nonmagnetic carbon and the surface disorder or spin canting [12,25]. The HC increases with the increase of particle size, which can be explained by the monodomain behavior [26]. The particle sizes are less than the calculated critical single-domain size of bulk nickel (~55 nm) [27], indicating that the magnetic nickel core is of the single-domain structure. When the nanoparticle is the single-domain structure,
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Fig. 1. (aed) TEM images and (eeh) HRTEM images of the samples A, B, C, and D.
Table 1 Structural and magnetic parameters of samples AeD. Sample
Average particle size (nm)
Average shell thickness (nm)
MS (emu/g)
Hc (Oe)
A B C D
25 38 46 53
1.9 2.1 1.8 2.0
30.2 36.5 41.9 44.6
86.8 103.7 112.4 118.3
Fig. 2. XRD patterns for samples A, B, C, and D.
the HC increases with increasing the particle size. 3.3. Electromagnetic properties The effective absorption performance is associated with the good EM impedance matching under a certain thickness of absorber layers, which depends on the complex permittivity (εr ¼ ε0 jε00 ) and the complex permeability (mr ¼ m0 jm00 ) of the microwave absorption materials [18]. Fig. 4(a) and (b) show the frequency dependence of real part (ε0 ) and imaginary part (ε00 ) of the complex permittivity of the four samples. For samples PAePD, the ε0 value decreases from 9.95, 8.89, 6.75, and 6.02 to 5.76, 5.08, 4.82 and 5.25, and the ε00 value decreases from 4.50, 4.76, 2.85, and 2.05 to 3.95, 2.41, 1.78, and 1.53, respectively, in 2e18 GHz with a little fluctuation. The tendency of decreasing with increasing
Fig. 3. Magnetic hysteresis loops of samples A, B, C, and D at room temperature.
frequency for ε0 and ε00 can be ascribed to the delayed response of dipole polarization with respect to the electric-field change at higher frequencies [10]. The downward tendency for samples PA and PB is more obvious than that for samples PC and PD, indicating more electric dipoles exist in samples PA and PB. For sample PA, it is worthy noted that the ε00 decreases from 4.50 to 3.00 in the 2e7.5 GHz range and then slightly increases in the 7.5e18 GHz range, which reveals the enhanced effect of dielectric polarization in high frequency range for sample PA. Both ε0 and ε00 decrease with increasing particle size from samples PA to PD, indicating that reducing particle size is beneficial to enhance the permeability. This phenomenon may be interpreted as follows: the interfacial polarization is dominant in the heterogeneous coreeshell structures, which contributes to the high permittivity [28]. The nanocapsules with smaller particle size can obtain larger interface area between core and shell, resulting in an enhanced heterogeneous interfacial
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Fig. 4. (a) Real part (ε0 ) and (b) imaginary part (ε0 0 ) of the relative complex permittivity, and (c) real part (m0 ) and (d) imaginary part (m0 0 ) of the relative complex permeability of samples PA, PB, PC, and PD in the frequency range of 2e18 GHz.
polarization. In addition, some defects existing in carbon shells and nickel cores act as the dipolar polarization centers, leading to various dielectric relaxations in different frequency ranges [11]. With decreasing the particle size of nanocapsules, larger interface area and more defects exist in the nanocapsules, which enhance the permittivity of the nanocapsules. The frequency dependence of real part (m0 ) and imaginary part (m00 ) of the complex permeability of samples PAePD are presented in Fig. 4(c) and (d). The m0 value of samples PA, PB, PC, and PD decreases rapidly from 1.25, 1.28, 1.29, and 1.20 to 1.01, 0.98, 0.97, and 0.94, respectively, in the frequency range of 2e7 GHz, and then are almost constant. It can be observed that m0 of the samples with smaller size are higher than those of the larger ones in high frequency. For sample PD, there is an apparently decline tendency for m0 observed in the 15e18 GHz, indicating the magnetic properties of Ni nanoparticles with the large size cannot be effectively protected by the carbon shell in high gigahertz frequency range [29]. From Fig. 4(d), broad multi-resonance peaks of m00 are observed at about 2e6 GHz for all the samples, which is corresponding to multimagnetic resonance, like in CoNi@C nanocapsules [16]. However, the m00 values have no clear difference among the four samples. Compared with εr, the variation of mr among the four samples is not obvious, revealing the particle size has little effect on mr. In previous studies on Ni/C nanocapsules [9,10], the mr of Ni/C nanocapsules with different diameters (25e30 nm in Ref. [9] and average 50 nm in Ref. [10], respectively) is mainly determined by the magnetic nickel core. The mr of Ni/C nanocapsules in the two previous studies have similar trends and values (m0 ¼ 1.26e0.93; m00 ¼ 0.22 ~ 0.8), which are also similar to that in this work. Those similar mr for Ni/C nanocapsules with different size further confirm that the effect of the particle size is too weak on mr. The dielectric loss tangent (tande ¼ ε00 /ε0 ) and the magnetic loss tangent (tandm ¼ m00 /m0 ) are used to quantify the EM loss capacity [30]. The calculated dielectric and magnetic loss tangents are shown in Fig. 5(a) and (b), respectively. For the four samples, the dielectric loss is always larger than the magnetic loss. Furthermore, it can be found that tande of samples PA and PB are higher than
Fig. 5. (a)The dielectric loss tangent (tande) and (b) the magnetic loss tangent (tandm) of samples PA, PB, PC, and PD.
those of samples PC and PD. The tande of sample PB is larger than that of sample PA in 2e9 GHz, while the former is lower than the latter in 9e18 GHz. Compared with the variation of tande, the tandm values have almost no difference among the four samples. This
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result suggests that reducing the particle size of Ni/C nanocapsules can enhance the dielectric loss and have no apparent influence on the magnetic loss. 3.4. Microwave absorption properties The microwave absorption materials can absorb the energy of EM wave and convert them to heat by the complementarity between dielectric and magnetic losses. The attenuation constant (a) can stand for the effectiveness of absorber materials. Based on measured complex permittivity and permeability, the attenuation constant (a) can be calculated using the following equation [31]:
a¼
pffiffiffi 2pf =c rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ðm00 ε00 m0 ε0 Þ þ
ðm00 ε00 m0 ε0 Þ2 þ ðε0 m00 þ ε00 m0 Þ2
(1)
where f is microwave frequency, and c is the velocity of light in free space. The frequency dependences of a value for samples PAePD are shown in Fig. 6. It can be observed that the sample with smaller size can get higher a value, indicating the more effective attenuation. From the above equation, it is noted that higher values of ε0 and ε00 would result in higher a when the values of m0 and m00 keep unchanged. The RL of samples PAePD can be calculated by the formula of the transmission-line theory [32]:
RL ¼ 20lgjðZin Z0 Þ=ðZin þ Z0 Þj
(2)
i h Zin ¼ Z0 ðmr =εr Þ1=2 tanh ið2pfd=cÞðmr εr Þ1=2
(3)
where Zin is the normalized input impedance at the air-absorber interface, Z0 is the free space impedance, and d is the absorber layer thickness. The calculated RL values of samples PAePD with the selected absorber layer thickness of 1.0e5.5 nm are shown in Fig. 7, in which all the samples exhibit good microwave absorption properties. The maximum RL reaches 32 dB at 11.7 GHz with absorber thickness of 2.5 mm for sample PA, 34 dB at 8.9 GHz with 3.5 mm thickness for sample PB, 25 dB at 5.8 GHz with 5.5 mm thickness for sample PC, and 17 dB at 6.3 GHz with 5.5 mm thickness for sample PD, respectively. By altering the layer thickness from 1.0 to 5.5 mm, the absorption bandwidth with the RL under 20 dB (99% absorption) can reach 10.3 GHz (5.8e16.1 GHz)
Fig. 7. Reflection loss for (a)e(d) samples PAePD with selected absorber thickness in the frequency range of 2e18 GHz, respectively.
Fig. 6. Attenuation constant (a) of samples PA, PB, PC, and PD in the frequency range of 2e18 GHz.
for sample PA, 7.0 GHz (6.2e13.2 GHz) for sample PB, and 1.9 GHz (5.5e7.4 GHz) for sample PC, respectively. The bandwidth decreases with the increase of particles size from samples PA to PC, and sample PD even cannot achieve the RL value under 20 dB by selecting the thickness from 1.0 to 5.5 nm. The above results show that decreasing the particle size of Ni/C nanocapsules not only reduces the absorber thickness for maximum RL, but also extends the absorption bandwidth. Fig. 8 shows the RL peak position of Ni/C nanocapsules at the same absorber thickness shifts from low to high frequency with increasing the size of nanocapsules, which may be due to the shift of EM matching range. In Table 2, the
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Fig. 8. The curves of the RL peak positions vs. the average diameter of Ni/C nanocapsules at different absorber thickness.
microwave absorption properties of Ni/C nanocapsules in this work are also compared with the results of other Ni/C nanocapsules systems. It can be observed that the Ni/C nanocapsules with small particle size can obtain the stronger microwave absorption abilities at a thinner absorber layer, revealing that reducing the particle size of nanocapsules is favorable for the improvement of microwave absorption properties. For dielectric/magnetic-loss composites, the good microwave absorption properties mainly arise from proper EM impedance matching. The matching degree can be evaluated by a deltafunction method [33], which is described as:
D ¼ sinh2 ðKfdÞ M
(4)
where K and M are determined by εr and mr. The smaller delta value indicates better EM impedance matching. Fig. 9 presents the calculated delta value maps of samples PAePD. The dark blue area (in the web version) of sample PA in Fig. 9(a) is largest among the four samples, indicating the optimal EM impedance matching for the Ni/C nanocapsules achieved at the smallest size. In addition, the dark blue area (in the web version) moves upward gradually with increasing size from samples PA to PD, revealing the nanoparticles with larger size are required to have thicker absorber layers to reach good EM impedance matching. This result is consistent with the variation of RL and their corresponding thickness, indicating that the enhanced microwave absorption properties with decreasing particles size is caused by the increase of EM impedance matching degree.
Fig. 9. Calculated delta value maps of (a)e(d) samples PAePD, respectively.
4. Conclusion The Ni/C nanocapsules were prepared by the arc discharge method. By changing the arc current from 40 to 100 A, the average particle size of the Ni/C nanocapsules varied from 25 to 53 nm and the thickness of carbon shell kept the independence. Due to the small size effect, the saturation magnetization and coercivity are proportional to the particle size. With the reduction of particle size, the larger interface area between core and shell results in the enhanced heterogeneous interfacial polarization and more defects existing in carbon shells and nickel cores leads to various dielectric relaxations in different frequency ranges, which are favorable for the improvement of complex permittivity, dielectric loss and attenuation constant. Sample PA, mixing Ni/C nanocapsules with the average particle size of 25 nm, exhibits a maximum RL value of 32 dB at 11.6 GHz with a thin absorber of 2.5 nm, and the bandwidth of the RL values exceeding 20 dB reaches 10.3 GHz (5.8e16.1 GHz) by selecting proper absorber layer thickness. Paraffin-bonded composites mixing the Ni/C nanocapsules with smaller particle size can obtain maximum RL values at thinner absorber layers with broader bandwidth, ascribed to the bigger dielectric loss, larger attenuation constant and better EM impedance matching degree. They thus shed light on the control of particle size to adjust the microwave absorption performance.
Table 2 Reported microwave absorption properties of Ni/C nanocapsules. Sample
Particle size/shell thickness (nm)
Ni/C PA PB PC PD Ni/C (S1) Ni/C (S3)
25e30/5e6 25/1.9 38/2.1 46/1.8 53/2.0 50/2 50/9
Optimal RL
Ref.
RL value (dB)
fm (GHz)
Absorber thickness (mm)
32 32 34 25 17 >20 40
13 11.7 8.9 5.8 6.3 e 3.2
2 2.5 3.5 5.5 5.5 e 7.8
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