J. Cent. South Univ. (2014) 21: 3007−3012 DOI: 10.1007/s11771-014-2269-9
Synthesis of flowerlike nickel particles and their microwave absorbing properties QI Hai-ping(齐海萍)1, 2, CAO Hai-lin(曹海琳)2, 3, HUANG Yu-dong(黄玉东)3 1. Chemical Engineering and Technology Post-Doctorate Mobile Station, Harbin Institute of Technology, Harbin 150001, China; 2. Shenzhen Key Laboratory of Composite Materials, Shenzhen Academy of Aerospace Technology, Shenzhen 518057, China; 3. Department of Polymer Science and Engineering, Harbin Institute of Technology, Harbin 150001, China © Central South University Press and Springer-Verlag Berlin Heidelberg 2014 Abstract: Two kinds of nickel particles with flower-like structures assembled with a number of nano-flakes were synthesized and the relationship of their morphology and microwave absorbing properties was studied. The electromagnetic parameters of these flower-like Ni were measured with vector network analyzer at 2−18 GHz frequency and the reflection losses (RL) with different sample thicknesses were calculated. The results indicate that the flower-like nickel-wax composites with the sample thickness less than 2 mm show excellent absorbing ability. This result is expected to play a guiding role in the preparation of the highly efficient absorber. Key words: nickel; flower-like; microwave absorbing properties
1 Introduction With the development of radar technology, microwave communication technology and microwave darkroom, there has been considerable interest in the application of electromagnetic wave absorbing materials [1−6]. The higher requirements for highly efficient absorbing materials, such as thin thickness, light density, broad frequency band and strong absorption are proposed. Previous studies have illustrated that microwave absorption abilities of absorbing materials would strongly depend on their morphologies [7−12]. Good examples for flaky ferrite [13] and polycrystalline iron fiber [14] are widely used which exhibit excellent absorption properties. In particular, enhanced microwave absorption properties can be obtained from hierarchical materials with complicated geometrical morphologies. For instance, nanobelt CoO [15], dendritic ZnO [16], urchinlike Fe3O4 [17] and hollow Fe3O4 [18] were found to exhibit good microwave absorption. Therefore, synthesis of microwave absorbing materials with specific morphologies has attracted much attention in recent years. Nickel is a kind of widely used microwave absorbing material. Up to now, nickel materials with
different morphologies have been widely reported [19−21]. However, most of these works only exhibit absorbing property and are focused on stronger absorption more than other factors. Obviously, it is a challenge to develop absorbing materials with properties of “thin, light, broad and strong”. Herein, a simple synthesis of nickel particles with flower-like structures is introduced. It is interestingly found that there is a morphology-dependent electromagnetic property of these nickel structures. The flower-like nickel samples exhibit excellent microwave absorbing properties with thin thickness, which demonstrate their promising applicability in microwave absorption fields. The probable reason of morphologydependent absorbing properties is discussed.
2 Materials and methods All chemicals were of analytical grade and used without purification. There were two methods to prepare the samples. Firstly, a typical experiment was as follows: A 400 mL ethanol solution of dimethylglyoxime (dmgH, 0.53 mol) was added into 800 mL of ethylene glycol solution of NiCl2·6H2O (0.1 mol/L) to give a mass of red floccus. After stirring for several minutes, 120 mL N 2 H 4 ·H 2 O (80%) and 48 mL NaOH saturation water
Foundation item: Project(JC201006020838A) supported by the Basic Research Funds of Science and Technology Foundation of Shenzhen, China Received date: 2013−05−08; Accepted date: 2013−10−28 Corresponding author: CAO Hai-lin, Professor, PhD; Tel: +86−755−26996829; Email:
[email protected]
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solution were dropped into the suspending liquid. Then, the mixture was put in water-bath at 90 °C for 12 h. The resulting black solid precipitates were separated easily from the reaction system by applying a permanent magnet and rinsed with distilled water and ethanol for several times. The products were dried in a vacuum oven at 40 °C, named sample F1. Secondly, a 30 mL hydrazine hydrate (N2H4·H2O, 80%) was added into 250 mL water solution of NiSO4·6H2O (1 mol/L) under mechanical stirring. After stirring for several minutes, 25 mL NaOH water solution (10 mol/L) was added. Then, the mixture was stirred vigorously and refluxed at 60 °C for 2 h. The resulting black solid precipitates were separated easily from the reaction system by applying a permanent magnet and rinsed with distilled water and ethanol several times. the products were dried in a vacuum oven at 40 °C, named sample F2. The phases and morphologies of as-prepared products were characterized by X-ray powder diffractionmeter with Cu Kα radiation (λ=0.154178 nm), field emission scanning electron microscopy (FE-SEM, JEOL JSM-6700M), transmission electron microscope (TEM, JEOL-2010), respectively. The complex permittivity (εr=ε'− jε") and permeability (μr=μ'−jμ") of the Ni/paraffin mixtures in the 0.5−18 GHz frequency range were recorded on an HP8722ES vector network analyzer at Beijing Institute of Aeronautical Materials, China. The composite samples were prepared by dispersing the as-obtained Ni powders in paraffin wax with mass fraction of 60% and then pressing the mixture into toroidal shaped samples (φout=7 mm, φin=3 mm, and thickness of 2 mm). The reflection losses were
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calculated using parameters.
the
measured
electromagnetic
3 Results and discussion The phase and purity of the resulting materials were tested by XRD, as shown in Fig. 1. It can be clearly found that all the reflection peaks of samples F1 and F2 can be perfectly indexed as face-centered cubic (FCC) Ni (PDF standard cards, JCPDS 04-0850, space group F3 ¯m). No characteristic peaks of impurities are detected. The strong reflection peaks and flossy baseline indicate that all these Ni particles have well crystalline.
Fig. 1 XRD patterns of as-prepared samples
Figure 2 shows the typical FE-SEM and TEM pictures of as-prepared samples. It can be clearly found that there are two different flower-like Ni particles. As shown in Figs. 2(a) and (b), sample F1 shows a sphere-
Fig. 2 SEM pictures of as-prepared samples: (a) F1; (b) TEM picture of flower in (a); (c) F2; (d) TEM picture of flower in (c)
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like shape with the diameter of 400−700 nm. From the TEM picture in the top right inset of Fig. 2(b), it can be seen that every sphere of F1 is composited of number of nanoflakes with average thick of 10 nm and average diameter of 100 nm which assemble layer by layer along the normal direction of spherical surface. In Figs. 2(c) and (d), sample F2 show a flower shape with average diameter of 900 nm and every flower consists of dozens of nanoflakes with average thickness of 50 nm, and those nanoflakes cross each other in the center of flower. The complex permittivities (εr=ε'−jε") and complex permeabilities (μr=μ'−jμ") of Ni flowers composites with different morphologies were investigated by vector network analyzer in the frequency range of 0.5−18 GHz, as shown in Fig. 3. Samples were prepared by dispersing the Ni powders randomly in paraffin wax with mass fraction of 60%. Figures 3(a) and (b) show that the ε' and ε" curves of each sample exhibit an increase with increasing frequency. It can be seen that both ε' and ε" values of sample F2 are higher than those of F1. It is worthy to notice that the ε" of both flower samples exhibit abrupt increase after 15 GHz, in which the maximum ε" of samples F1 and F2 are 16.5 and 6.7 near 18 GHz, indicating an intensive resonance behavior. In
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Fig. 4(c), the μ' curves of both F1 and F2 decrease with the increase of frequency. The μ' value of F1 at 0.5 GHz is about 1.3, whereas the F2 is 1.4; the μ' values of both samples are 0.6 at 18 GHz. Figure 3(d) reveals that μ" curves show two peaks at 0.5−18 GHz. The first peaks of F1 and F2 are a broad peak at about 8 GHz (for F1) and 5 GHz (for F2), respectively; the second peaks are a sharp peak near 18 GHz. The maximum μ" value of F2 is 0.42 at 17.5 GHz which is higher than that of F1. The dielectric loss tangents (tanδε=ε"/ε') and magnetic loss tangents (tanδμ=μ"/μ') of Ni-wax composites are shown in Fig. 4. The tanδε curve and tanδμ curve are similar with the ε" curve and μ" values, respectively. For flower Ni, the magnetic loss is dominating loss style. Magnetic loss of sample F1 is bigger than dielectric loss before 14 GHz, and dielectric loss is bigger than magnetic loss later. For sample F2, magnetic loss is larger than dielectric loss before about 12.5 GHz; and magnetic and dielectric loss increase rapidly later, the maximum tanδε and tanδμ values are 0.79 and 0.72 near 18 GHz, respectively. According to transmission line theory, the reflection loss (LR) of absorbing material can be expressed as following equations:
Fig. 3 Electromagnetic parameters of sample/paraffin wax composites in frequency range of 2−18 GHz: (a) Real imaginary part of relative permittivity for sample wax composite; (b) Imaginary part of relative permittivity for paraffin wax composite; (c) Real imaginary part of relative permittivity for sample wax composite; (d) Imaginary part of relative permittivity for paraffin wax composite
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Fig. 4 Dielectric loss tangents (tanδε=ε"/ε') and magnetic loss tangents (tanδμ=μ"/μ') of sample/paraffin wax composites
LR 20 lg
Z0
Z in Z 0 Z in Z 0
0 0
Z in Z 0
(1)
(2)
tanh( j2πfd 0 0 )
(3)
where f is the frequency of the electromagnetic wave, d is the thickness of the absorber, c is the velocity of light, Z0 is the impedance of free space, and Zin is the input impedance of the absorber. Figure 5 shows the calculated LR curves for the Ni powders-paraffin wax composites with Ni mass fraction of about 60%. Obviously, the maximum LR shifts to low frequency range with increasing thickness layer. Samples F1 and F2 have the minimum LR with the sample thickness of 2 mm, the values of −18.7 dB (at 9.6 GHz) and −21.3 dB at 8.7 GHz, respectively. When with same sample thickness, sample F2 has less LR value than sample F2, meaning F2 has better absorption ability. The nickel with flower-like structure exhibit enhanced electromagnetic absorbing ability compared with the reported spheres [22], nanowires [23], branched nanowires [19], urchinlike [24], flower-like [25] nickel. It is worthy to notice that the LR values of F1 and F2 with the thickness of 1.5 mm reach −15.8 dB and −18.5 dB, respectively. More detailed LR values with thin thickness are listed in Table 1. When the thickness was changed from 2.5 mm to 1.3 mm, the LR values of F1 fluctuate between −15.2 dB and −19.3 dB; while the LR values of F2 fluctuate between −17.6 dB and −25.5 dB. The highly efficient absorbing materials require thin thickness as far as possible while ensuring the absorption efficiency. From the result, the flower Ni composites with thin thickness exhibit excellent absorbing ability. The morphologies of materials are considered to play a crucial role for improving absorbing ability. As
Fig. 5 Reflection loss curves of different composites with different thicknesses: (a) F1; (b) F2 Table 1 Reflection loss values at different thicknesses
Thickness/mm
F1
F2
LR/dB
f/GHz
LR/dB
f/GHz
2.5
−15.2
7.3
−22.9
6.8
2.4
−15.7
7.9
−23.8
7.2
2.3
−18.7
8.0
−25.5
7.5
2.2
−19.1
8.6
−25.0
7.9
2.1
−18.1
9.1
−23.8
8.4
2.0
−18.7
9.6
−21.3
8.7
1.9
−19.3
10.1
−21.6
9.4
1.8
−17.6
10.8
−21.3
10.0
1.7
−17.5
11.5
−20.1
10.5
1.6
−18.3
12.4
−17.6
11.4
1.5
−15.8
13.5
−18.5
12.2
1.4
−12.8
14.3
−20.3
13.3
1.3
−15.6
16.4
−25.5
14.1
shown in Fig. 2, samples F1 and F2 consist of dozens of nanoflakes. WALSER et al [26−27] reported that a susceptibility enhancement by a factor of 10−100 can be obtained in the 1−20 GHz frequency range by using oblate spheroids with aspect ratios between 10 and 1000.
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In this work, the excellent absorption performance could be attributed to the shape effect of flaky Ni, which resulted in increase of permeability. On the other hand, the complicated geometrical morphologies have an obvious effect on improving the absorption properties. The incident electromagnetic can be refracted and reflected on the interface of material, which will increase the chance of absorption of electromagnetic wave. This situation can be seen as the increase of thickness of the absorber, which will result in lower reflection loss. Here, the sample F1 can be seen as multilayer sphere, electromagnetic wave can be refracted and reflected many times inside sphere. For F2, the electromagnetic wave would be multi-scattered in the surface of flower. The diagrammatic illustration of absorbing electromagnetic wave is shown in Fig. 6.
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Fig. 6 Diagrammatic illustration of absorbing electromagnetic wave of flower-like nickel
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4 Conclusions [12]
In summary, we successfully synthesize two kinds of nickel particles with flower-like shapes. The two flower-like structures are assembled by a number of flakes. The calculative reflection losses (LR) values indicate that the flower Ni composites with thin thickness still exhibit excellent absorbing ability, the minimum LR value reaches −15 dB to −25 dB. The morphologies will increase the chance of absorption of electromagnetic wave. This result is expected to play a guiding role in the highly efficient absorber with thin thickness.
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