2016 International Conference on Mechanical, Control, Electric, Mechatronics, Information and Computer (MCEMIC 2016) ISBN: 978-1-60595-352-6 Microwave Absorption Enhancement of RGO/ Ni Nanocomposites F.L. Yang, X.Z. Hou, Y.J. Li, M. Yu and P.A. Yang ABSTRACT RGO/Ni nanocomposites were prepared by in situ method. The microstructure and electromagnetic properties of the samples were investigated by X-ray diffraction, scanning electron microscopy and a vector network analyzer. Graphene oxide and Ni2+ at the same time reducing the synthesis RGO/Ni nanocomposite have wide reflection loss band can reach 1.6GHz. Nickel is deposited directly on graphene synthesis RGO/Ni nanocomposite have relatively sharp absorption peaks can reach - 21.46 at 14.46GHz. The efficient complementarities between magnetic loss and dielectric loss can also contribute to the microwave absorption. KEYWORD: In situ; microwave absorption; nanocomposites; reflection loss. INTRODUCTION In recent years, the pollution of electromagnetic waves on the environment is growing, there has been an increasing interest in how to reduce the adverse effects of electromagnetic waves on the environment.[1,2] Microwave absorbing material can absorb electromagnetic waves within a certain frequency range, so that the electromagnetic waves of incident on the material surface converted into other energies, thus reducing the reflection of electromagnetic waves greatly. The absorbing properties of the magnetic materials are considered to be strongly associated with their sizes, shapes and structures, etc.[3] Graphene has excellent performance in terms of energy applications, and it is the absorbing material which have been widely studied. This is mainly due to its low density, high specific surface area, high conductivity, good thermal stability and good mechanical properties.[4-7] Soft magnetic metal material has a large saturation magnetization, higher Snoek's limit and low coercivity, so that it may be a potential candidate for microwave absorption at high frequency.[8-10] Magnetic metallic material has a high electrical conductivity, F.L. Yang, X.Z. Hou. Electric Power Research Institute, Chongqing Electric Power Corporation, State Grid, Chongqing. Y.J. Li, M. Yu, P.A. Yang. College of Optoelectronic Engineering, Chongqing University, Chongqing. 141
it will cause eddy current losses in the electromagnetic environment, and reduce magnetic permeability, which is not conducive to absorbing. Thin insulating oxidation shell would greatly decrease the complex permittivity and improve the impedance matching condition and it could enhance the microwave absorbing properties.[11,12] Graphene has received much attention because of its application in super capacitors,[13] superconductor materials,[14] hydrogen storage materials,[15] biosensors. It studies in absorbing electromagnetic field is still in its infancy. The study of graphene electromagnetic interference and response characteristics indicates that, graphene has a good electromagnetic interference effect in the X-band, and its electronic has a nonlinear response characteristics respected to frequency.[16] However, low magnetic loss limits the microwave absorption properties of graphene. Combined magnetic loss material with electric loss material may serve two loss mechanisms well and get better absorbing properties.[17] For composite materials, through adding some functional ingredients to achieve its functional characteristics an important way to achieve the integration of composite structure and function, commonly used functional ingredients have graphene.[18] Graphene reduces the density of the composite material, and increasing relevant properties of composite materials to provide the possibility for the preparation of high-performance microwave absorbing materials. The residual defects and groups in RGO prepared by utilizing a redox reaction not only improve impedance matching characteristics, but also introduce the transition from contiguous states to Fermi level, defect polarization relaxation, and groups electronic dipole polarization relaxation, which are all in favor of electromagnetic wave penetration and absorption.[19] EXPERIMENTAL Preparation of Graphene Oxide and Graphene The GO was synthesized from natural graphite with the modified Hummers method. Take half of the GO, reduce it with zinc powder to get RGO. The obtained materials was used for all further studies. Synthesis of RGO/Ni nanocomposites 80 mg GO was added into 20 ml water, and then the mixture was subjected to sonication for 30 min to obtain a homogeneous solution. 9.07 g NaBH4 dissolved in 200 ml water, 0.8 g NaOH dissolved in 40 ml water, these two solutions were mixed and stirred for 10 min to obtain a solution A. NiSO4 6H2O was dissolved in 400mL water, and mixed with GO, stirred for 10 minutes to obtain a solution B. Solution A and solution B was dripped into a beaker slowly and uniformly. After the dropping, let it react for 6 h under stationary condition. The final product was vacuum filtration, and then dried at 60 under vacuum, finally obtained RGO/Ni-1 nanocomposite. Similar to the synthesis of RGO/Ni-1 nanocomposite, 60 mg RGO was added into 20mL water, and then the mixture was subjected to sonication for 30 min. Repeat the above procedure to get the RGO/Ni-2 nanocomposite. 142
Characterization The structural characterization of nanocomposites was carried out by X-ray powder diffraction (XRD) performed by using CuKα1 (λ = 0.154 nm) radiation. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used respectively to observe the particle morphology and elemental composition of the RGO/Ni-1 and RGO/Ni-2 nanocomposite. The electromagnetic parameters of the samples were measured in a vector network analyzer in the range of 2 18 GHz. The measured samples were prepared by uniformly mixing 40 wt% of the sample with a paraffin matrix. RESULTS AND DISCUSSION As shown in Figure 1, the obtained composites do not have the crystal structure, which indicates that the nickel is present in the form of amorphous or microcrystalline. It can be clearly seen that, RGO / Ni composite 2θ wide angle diffraction peaks are 45, corresponding to 111 crystal planes of nickel shown in Figure 1. This XRD result indicates the successful compound of graphene and nickel nanoparticles. Figure 1. XRD patterns of RGO / Ni. To investigate the morphology of as-prepared composites, scanning electron microscope (SEM) reveals a planar morphology of test sample. Figure 2a is RGO/Ni-1 FESEM image, we can see that the nickel particles are deposited on the surface of graphene uniformly, without large uncovered areas or agglomeration, indicating the nickel nanoparticles well dispersed on graphene and the particle size is about 20-30nm. The nickel particles of deposited on the graphene surface are dispersed also relatively uniform, agglomeration is not significant, as shown in Figure 2b. The nickel particles deposited graphene more intensive compared to deposited on the graphene oxide, and nickel particles size about 30-40nm. Figure 2 shows graphene of nickel plating is not easy to assemble, as a result of graphene surface tension will decrease after nickel plating. To further investigate the composition of the material, we measured the energy dispersive spectroscopy (EDS) of RGO/Ni nanocomposites. The peaks of the C and Ni elements are presented in the energy dispersive spectroscopy spectrum from Figure 3, indicate that graphene and nickel composite effectiveil 143
together. The nickel content of RGO/Ni-2 nanocomposite more than RGO/Ni-1 nanocomposite, it is also consistent with the results observed in the FESEM. The nanocomposites tested ground to a powder, 40 wt% nanocomposites is mixed with paraffin, then melted into the standard mold which has a thickness of 2.68mm, and make RGO/Ni/paraffin a annular to test electromagnetic parameters. Natural graphite which is used in the experiment has a high purity, and graphite does not contain the magnetic material. The electromagnetic loss of graphene which has no nickel deposited on surface is very small, so the electromagnetic parameters of graphene have not been studied. Figure 2. a, b FESEM images of RGO/Ni-1 and RGO/Ni-2. Figure 3. a and b energy dispersive spectroscopy (EDS) graph of RGO/Ni-1 and RGO/Ni-2. The nanocomposites tested ground to a powder, 40 wt% nanocomposites is mixed with paraffin, then melted into the standard mold which has a thickness of 2.68mm, and make RGO/Ni/paraffin a annular to test electromagnetic parameters. Natural graphite which is used in the experiment has a high purity, and graphite does not contain the magnetic material. The electromagnetic loss of graphene which has no nickel deposited on surface is very small, so the electromagnetic parameters of graphene have not been studied. Permittivity ε and permeability μ are two basic parameters to assess absorb microwave property of absorbing materials in the electromagnetic field, the general plural form is expressed as: ε=ε'-jε" and μ=μ'-jμ". Figure 4(a-d) are the real and imaginary parts of the complex relative permittivity and permeability of the two materials. The real part (ε') of complex permittivity of RGO/Ni-1 nanocomposite first increases and then decreases with the increasing frequency. In general, the real (ε') and imaginary (ε") parts of the complex permittivity of RGO/Ni-1 nanocomposite fluctuate strongly in the frequency range of 2-18GHz, as presented in Figure 4(a and b). The real (ε') and imaginary (ε") parts of the complex permittivity of RGO/Ni-2 nanocomposite fluctuate strongly in the range of 2.9-4.5GHz 13.4-14.5GHz and 144
16.6-17.3GHz, relatively stable in the other frequency ranges. The imaginary (μ") of the complex relative permeability of RGO/Ni-1 nanocomposite similar first increases and then decreases with the increasing frequency in the range of 3.1-5.9GHz, other frequency range relatively stable. The imaginary (μ") of the complex permeability of RGO/Ni-2 appeared greater fluctuations in the range of 2-4.8GHz and 13.6-17.4GHz. The magnetic loss tangents (tanδμ=μ"/μ') and dielectric loss tangent (tanδε=ε"/ε') of RGO/Ni are shown in Figure 4 (e and f). The dielectric loss tangent of RGO/Ni-1 presented a peak in the range of 7.4-11.3GHz, and the magnetic loss tangent is the same as that in the 3.8-5.4GHz range. The dielectric loss tangent and magnetic loss tangent of RGO/Ni-2 appear sharp peaks in the range of 13.5-14.5GHz. Figure 4. Frequency dependence on a real and b imaginary parts of the complex permittivity of RGO/Ni, c real and d imaginary parts of the complex permeability, and the corresponding e dielectric and f magnetic loss tangents of RGO/Ni composites. Figure 5. Calculated reflection loss of the two as-obtained samples. Figure 6. The value C 0 (μ"(μ') -2 f -1 ) of Ni/SnO2 paraffin composites as a function of frequency. 145
In order to evaluate and compare the microwave absorbing properties which synthesized by two different methods, the reflection loss (RL) values of RGO/Ni nanocomposites was calculated from complex permittivity and permeability with the change of frequency and thickness according to the transmit line theory by the following equations:[20-22] Z in Z 0 2 fd r r r tanh j c r Z Z Z Z RL db =20l og in in (2) 10 0 0 where Z0 is the impedance of free space, Zin is the input impedance of the composites, εr is the complex permittivity, μr is the complex permeability, f is the microwave frequency, c is the velocity of light, and d is the thickness of the composites. The relative complex permeability and permittivity were tested on a network analyzer with a frequency in the 2 18 GHz range. Figure 5 shows the RL of nanocomposites are synthesized by two methods. We can see that the nickel is deposited directly on the surface of the graphene synthesized RGO/Ni-2 nanocomposite have higher peaks, and it has the maximum absorption at 14.46GHz, about -26.46dB. But simultaneously reducing obtained the RGO/Ni-1 nanocomposite have wider absorption band, and the reflection attenuation are -10dB or more from 8.48-10.08GHz. It is noteworthy that, RGO/Ni-1 loss at 3.5-4.6GHz and 8.5-10.1GHz exactly correspond with the magnetic loss tangents and dielectric loss tangents, respectively. It is shown that the main causes of loss in these two frequency ranges are magnetic loss and dielectric loss. The absorption loss of RGO/Ni-2 at 13.4-14.5GHz is also coincident with the magnetic loss tangents and dielectric loss tangents in the frequency range of 13.5-14.5GHz, indicate the absorption loss of RGO/Ni-2 in 13.4-14.5GHz is the results of the collective effect of dielectric loss and magnetic loss. The magnetic loss of magnetic particles originates from hysteresis loss, domain wall resonance loss, eddy current effect loss, natural resonance and exchange resonance loss. The eddy current loss is related to the diameter of particles and the electric conductivity. Eddy current loss can be evaluated by C0=μ"(μ') -2 f -1 =2πμ0σ, where σ is conductivity, f is the applied frequency, d is the depth of particle and μ0 is the vacuum permeability. If C0 is a constant with the change of frequency, the magnetic loss results from the Eddy current loss effect. As shown in Figure 6, C0 has been fluctuating in the frequency range of 2-8GHz. The value of C0 is almost constant in the frequency range of 8 18GHz, indicating that eddy current effect is the main loss mechanism in this range for RGO/Ni nanocomposites. Additionally, the hysteresis loss is negligible in weak applied field, the domain wall displacement loss occurs only in the MHz range rather than GHz, so the magnetic loss should result from the nature resonance and exchange resonance in the range of 2-8GHz. CONCLUSION On the graphene surface was prepared nickel nanoparticles obtain RGO/Ni nanocomposites by in situ method. Graphene oxide and Ni 2+ reduction at the same time prepared the RGO/Ni-1 nanocomposite reflection loss above -10dB frequency band reached 1.6GHz. Nickel particles deposited on graphene prepared RGO/Ni-2 nanocomposite reflection loss peak reached -26.46. The efficient complementarities (1) 146
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