Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide

Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide

Author’s Accepted Manuscript Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide Yaru Lai, Suyu...

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Author’s Accepted Manuscript Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide Yaru Lai, Suyun Wang, Danlin Qian, Suting Zhong, Yanping Wang, Sujuan Han, Wei Jiang www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)31402-5 http://dx.doi.org/10.1016/j.ceramint.2017.06.188 CERI15710

To appear in: Ceramics International Received date: 8 June 2017 Revised date: 27 June 2017 Accepted date: 29 June 2017 Cite this article as: Yaru Lai, Suyun Wang, Danlin Qian, Suting Zhong, Yanping Wang, Sujuan Han and Wei Jiang, Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.06.188 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Tunable electromagnetic wave absorption properties of nickel microspheres decorated reduced graphene oxide Yaru Lai, Suyun Wang, Danlin Qian, Suting Zhong, Yanping Wang, Sujuan Han, and Wei Jiang * National Special Superfine Powder Engineering Technology Research Center, Nanjing University of Science and Technology, 210094, Nanjing, China. * Corresponding author. E-mail: [email protected]; Tel: (86)-25-8431-5942.

Abstract The composites of nickel microspheres decorated reduced graphene oxide (NG) with extraordinary electromagnetic absorption properties were fabricated via a facile one-step solvothermal reduction approach. The morphology characteristics suggest that nickel microspheres loaded on both reduced graphene oxide (RGO) sheet sides can prevent sheets from restacking, in turn, RGO network can also inhibit the agglomeration of nickel particles. The electromagnetic parameters reveal that the samples exhibit tunable electromagnetic absorption properties through adjusting the initial concentrations of nickel salt. With the matching thickness of 1.2 mm, the reflection loss (RL) of NG-1 achieves –31.4 dB at 14.5 GHz, and the effective absorption band (RL<–10 dB) ranges from 13.3 to 16.0 GHz. Besides, the optimal RL of NG-2 and NG-3 can reach –35.4 dB at 8.2 GHz and –49.0 dB at 5.3 GHz, respectively. Furthermore, the excellent electromagnetic wave absorption capability can be reasonably interpreted by the impedance matching characteristics and the 1

quarter-wavelength attenuation model. The hybrids of nickel microspheres and RGO flakes in this work are hopeful to serve as attractive electromagnetic absorption materials, which can realize strong and broad effective absorbing at thin matching thickness. Keywords:

Composites;

Nickel

microspheres;

Reduced

graphene

oxide;

Electromagnetic wave absorption

1. Introduction With the rapidly expanding application of electromagnetic (EM) wave circuit devices, EM wave irradiation has caused the serious EM pollution problem, not only creating disturbances on the operation of electronic devices but also being harmful to human health [1, 2]. To resolve the problem of EM interference pollution, one efficient solution is the application of EM wave absorption materials to attenuate the unexpected EM energies [3, 4]. The ideal EM wave absorption materials need to meet the requirements of thin thickness, wide absorption frequency bandwidth, light weight and strong absorption. Recently, great efforts have been made on the research of EM wave absorption materials, such as the various carbon materials [5, 6], conducting polymers [7], dielectric or magnetic fillers [8, 9], as well as their hybrids [10-12]. Graphene, composed of a single layer of sp2-bonded carbon atoms, has become a focus in the searching of the lightweight and efficient EM wave absorbers [13]. The EM wave absorption performance is primarily determined by the impedance matching characteristic at the absorber-space interface and the attenuation capability for EM 2

energies [14, 15]. The entered EM waves are considered to be attenuated by the dielectric loss and magnetic loss mechanisms [16]. The properties of large interface, low density and high conductivity make graphene become a promissing candidate as the EM wave absorber. However, the sole graphene material exhibits poor EM absorption performance on account of the limited dielectric loss mechanism and interfacial impedance mismatching [17-19]. It has been confirmed that the folded structures, residual defects and oxygen-containing groups existed in the chemically reduced graphene oxide (RGO) could generate polarization relaxation to enhance the EM wave absorbing capability [17, 20, 21]. Meanwhile, RGO flakes dispersed in specific solvents with ultrasonic assistance can be easily functionalized and decorated with various kinds of particles [2, 22, 23]. It is an effective method to improve the magnetic loss of EM energies via decorating RGO with magnetic particles. There are plenty of research results demonstrating that the synergistic effect of dielectric loss and magnetic loss of the magnetic RGO composite could enhance the EM absorbing performance [23-25]. The optimal reflection loss (RL) of NiFe2O4/RGO could reach – 42 dB with a broad effective EM wave absorption bandwidth (RL ≤ –10 dB) of 5.3 GHz and a corresponding thickness of 5 mm [25]. The porous Fe3O4 decorated RGO showed the minimal RL of –20 dB at 17.2 GHz with the thickness of 2 mm [26]. The RGO-CoFe2O4 composite displayed the strong RL of –44.1 dB at 15.6 GHz with a thickness of 1.6 mm, and the effective EM wave absorption bandwidth was up to 4.7 GHz (from 13.3 to 18.0 GHz) with a thickness of 1.5 mm [27]. Among various magnetic absorbers, nickel as a typical magnetic metal exhibits 3

the extraordinary potential for application in the field of EM absorption materials owing to its low cost, facile preparation and unique electromagnetic property [28-31]. Wang et al. [32] found that the prepared graphene/nickel composite displayed much better characteristics on EM wave absorption comparing with pure Ni particles and graphene. Liu et al. [33] fabricated the urchinlike Ni nanoparticles/reduced graphene oxide composites, and the optimal RL reached –32.1 dB at 13.8 GHz when the matching thickness is 2 mm. Zhu et al. [34] synthesized the RGO-Ni composite by the EM wave-assisted heating approach, exhibiting the minimum RL of –42 dB at 17.6 GHz corresponding to the thickness of 2 mm. Regrettably, when the thickness is blew 2 mm, the reported Ni/RGO composites can hardly achieve the effective EM wave absorption. Herein, the composites of Ni microspheres decorated RGO were successfully synthesized via a facile one-step solvothermal reduction method. The microstructural features, magnetic properties and electromagnetic parameters were systematically investigated. The obtained results exhibit that the EM wave absorption properties at the specific thickness and frequency regions can be effectively tuned by adjusting the initial nickel salt concentrations. The as-prepared samples could achieve outstanding EM wave absorbing even with the thickness as thin as 1 mm.

2. Experimental 2.1. Materials and characterization Nickel

chloride

hexahydrate

(NiCl2∙6H2O), 4

anhydrous

sodium acetate

(CH3COONa), trisodium citrate dehydrate, Graphite oxide (GO), 1, 2-propanediol, ethylene glycol (EG), hydrazine hydrate solution (N2H4·H2O, 80 wt%). The commercial graphite oxide with the average thickness of 1.5 nm and the sheet size range of 1–50 μm was purchased from Nanjing JCNANO Technology Co. Ltd. The distilled water was purified through a Millipore System. All the chemicals were of analytical grade and used without further purification. The crystalline structures of the samples were investigated by X-ray diffraction (XRD) on a Bruker D8 Advance system with Cu Kα radiation (λ=1.5418 Å). The microscopic morphology was observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800 Japan) and transmission electron microscopy (TEM, Philips Tecnai 12). Quantitative information of the elements existed in the sample was recorded by energy-dispersive X-ray spectrometry (EDX) attached to the FE-SEM. The magnetic properties of the samples were measured by a vibrating sample

magnetometer

(VSM,

Lake-shore-735)

at

room

temperature.

The

measurements of electromagnetic parameters including the complex permeability and permittivity were carried out using an Agilent N5244A vector network analyzer in the frequency range of 2–18 GHz. The measured samples were prepared by uniformly mixing 30 wt% of products with paraffin wax, and then mixtures were pressed into a toroidal shape with an inner diameter of 3.04 mm and outer diameter of 7.00 mm.

2.2. Synthesis method The composites of nickel microspheres decorated reduced graphene oxide (NG) were synthesized by a simple one-step solvothermal method. Briefly, quantities of 40 5

mg GO, 0.12 g NiCl2∙6H2O, 0.2 g trisodium citrate dehydrate and 1.5 g anhydrous sodium acetate were dissolved in the mixed solution of 30 mL 1, 2-propanediol and 30 mL ethylene glycol. The mixture was underwent ultrasonic treatment for 90 min. Subsequently, 5 mL N2H4·H2O (80 wt%) was added to the suspension with vigorous stirring. Following stirred for 30 min, the mixture was transferred into a Teflon-lined stainless-steel autoclave and maintained at 140 ºC for 15h. After cooled to room temperature naturally, the product was separated with the help of an external magnetic field, and washed with distilled water and anhydrous ethanol several times. The obtained sample denoted as NG-1 was finally dried at 50 ºC in a vacuum for 8 h. Keep the mass of GO constant and other experimental conditions unchanged, the other ratios of Ni against RGO were regulated by increasing the amounts of NiCl2∙6H2O to 0.20 g and 0.28 g, respectively. The obtained samples of above experiments were denoted as NG-2 and NG-3.

3. Results and discussion 3.1. Formation and morphology analysis The schematic representation of the formation of NG was illustrated in Fig. 1. The graphite oxide dispersed in the mixed alcohol solution was firstly flaked to disordered sheets with plenty of oxygen functional groups under ultrasonic treatment [26]. The large surface of GO provides many nucleation sites so that the Ni2+ cations were absorbed on the position by electrostatic attraction, later reduced and growing to large-size Ni micropheres. Meanwhile, GO could be partially reduced to graphene 6

under the effect of high temperature, high pressure and reducing reagent. The microstructures of three samples observed by SEM and TEM are shown in Fig. 2(a-c) and Fig 3(a-c). Obviously, the large amounts of Ni microspheres are nearly evenly distributed on the RGO surface, except a small number of particles tend to agglomerated. The RGO sheets are well decorated by Ni microspheres without large vacancies on surface. Combing the SEM and TEM images of RGO-based composites, the particular folded structures and rippled textures of RGO sheets are exhibited in the images of all samples. From the SEM image of NG-2, it can be observed that Ni particles coated both sheet sides can prevent RGO sheets from restacking, and RGO network contributes to the separation of Ni particles [26]. Moreover, the NG composite demonstrates a sandwich structure as observed, and the partial particles are wrapped by RGO sheets [35]. In addition, comparing the SEM images of three samples, the distribution densities and the average particle sizes of Ni microspheres are found trended to increase with the increasing amounts of NiCl2∙6H2O in raw materials, which is in accordance with the observed result from TEM images. The particle size distribution histograms of Ni particles supported on RGO, obtained according to the TEM images in Fig. 3(a-c), are clearly given in Fig. 3(d-f). The Ni particles of NG-1 display an average particle diameter of about 421 nm, and the size distribution is more uniform compared with the samples of NG-2 and NG-3. When the initial masses of NiCl2∙6H2O are increased to 0.20 g and 0.28 g, the average particle sizes increase to 541 nm and 983 nm, respectively. Thus, the initial Ni2+ concentrations make a great effect on the morphology of the NG composites. The 7

higher initial Ni2+ concentration results in the more metal ions being reduced and agglomerated into larger-size Ni microspheres [35]. The elements composition of each sample was determined by EDX measurement. Fig. 2(d-f) give the detected elements distributions of three samples. The Au element derives from the Au film coated on the sample before SEM measurement to enhance its conductivity. The carbon and oxygen should come from the RGO sheets with residual oxygen functional groups existed. The peak intensities difference of C and O reflects that the content of C is higher than O, revealing the successfully reduced of GO. It is further confirmed by Raman spectra to estimate the bonding structure of carbon atoms of RGO.

3.2. Structural characterization The crystal structure and phase composition of the products were determined by means of power XRD. As shown in Fig. 4(a), the diffraction peaks at 2θ = 44.5°, 52.0°, and 76.5° are assigned to (111), (200), and (220) planes of face-centered cubic (fcc) nickel

(JCPDS 04-0850), respectively.

The iconic peak at 2θ =

10.2°corresponding to (001) plate reflection of GO is disappearing in the XRD curves of the NG samples, confirming the reduction of oxygen groups in GO. Furthermore, there is no distinguishable diffraction peak of graphite existed in plots, which suggests that the RGO flakes still remain a disordered stacking state [36]. In addition, with the increase of the Ni2+ molar concentration in the raw materials, the Ni characteristic diffraction peak intensity enhances accordingly. Raman spectra is efficiently used to discern the bonding state of carbon atoms 8

and the changes of carbon framework during the oxidation and reduction process of various carbon materials. As illustrated in Fig. 4(b), there are two distinguishable peaks at approximately 1345cm-1 and 1580cm-1, which attributed to the D-band and G-band of carbon materials, respectively. The D-band stems from the breathing stretching mode of K-boundary phonons of A1g, caused by the defects and disorder in the graphene structure. While the G-band corresponds to a first-order scattering of the E2g mode of sp2-bonded carbon domains [37]. The intensity ratio of D-band to G-band (ID/IG) is related to the degree of crystallinity of carbon materials [33]. Compared to GO, the increase in ID/IG of NG indicates the formation of more defects and smaller sp2-bonded carbon domains as a result of the hydrazine reduction and introduction of Ni particles [38]. It has been confirmed that the proper content of defects can promote the attenuation of incident EM waves, which is owing to the improvement of defect dipole polarization relaxation to create the dielectric loss of EM energies. Moreover, the generation of nanocrystalline graphite causes the ID/IG value increased, signifying the enhanced graphitization degree of NG samples after hydrazine reduction [39].

3.3. Magnetic studies The field-dependent magnetization of the sample was recorded by VSM with an applied field of -10 kOe to 10 kOe at room temperature, which is exhibited in Fig. 5 (the inset is the magnified view of the hysteresis loops at low applied fields). It is striking to note that the M-H curves display a similar S-type shape, and demonstrate obvious hysteresis loops, indicating that all the samples possess the ferromagnetic behavior. The primary magnetic parameters including the saturated magnetizations 9

(Ms), magnetic coercivity (Hc) and remanent magnetization (Mr) are listed in Table 1. Comparing the Ms values of three samples, NG-3 shows the highest Ms value of 36.2 emu/g. The Ms value increases as the increasing initial Ni2+ concentration. Moreover, the relatively higher Hc values than bulk Ni are likely ascribed by high anisotropy and the effect of particle size, stimulating the high frequency resonance which can be observed in the electromagnetic parameters curves [10]. Obviously, the initial concentration of the nickel salt plays a crucial role in determining the magnetic properties of composites. The introduction of Ni into RGO matrix could improve the impedance matching characteristic of the composite and promote attenuation of EM wave by the adjunction of magnetic loss.

3.4. EM wave absorption properties The EM absorption properties of the samples are evaluated by the values of reflection loss (RL), calculated using the relative EM parameters. The complex permittivity (𝜀r ) and permeability (𝜇r ) of wax-based composites were measured by an Agilent E8363A vector network analyzer in the frequency range of 2–18 GHz. According to the transmit line theory [40], the RL values are calculated at a given frequency and thickness layer as follows: RL(dB) = 20 log | (𝑍in − 1)⁄(𝑍in + 1) |

(1)

𝑍in is the normalized input impedance of the absorber, which can be summarized by the following formula: 𝑍in = √𝜇r ⁄𝜀r tanh [ 𝑗(2𝜋𝑓𝑑 ⁄𝑐 )√𝜇r 𝜀r ]

(2)

Where 𝜀r is the complex permittivity ( 𝜀r = 𝜀 ′ − 𝑗𝜀 ″ ), 𝜇r is the complex 10

permeability (𝜇r = 𝜇 ′ − 𝑗𝜇 ″ ), 𝑓 is the frequency of EM wave, 𝑐 is the velocity of EM wave in free space (same as the speed of light), and 𝑑 is the thickness of the absorber, respectively. Figure 6 presents the plots of the three dimensional calculated RL values versus frequencies with various thicknesses of three samples. It is identified that 90% of the EM wave energy is absorbed when RL is less than –10 dB. The corresponding frequency region is considered as the effective EM wave absorption bandwidth suitable to practical applications. Three samples all achieve effective absorbing at different specific frequency regions within 1.0–5.5 mm, indicating the excellent EM wave absorption performances for composites. It is well accepted that the reflection loss, matching thickness and effective absorption bandwidth are critical parameters used to evaluate the eligible EM wave absorber property. NG-1 demonstrates outstanding high-frequency EM wave absorption capability at thin thicknesses, as revealed in Fig. 6(a). The minimum RL of –20.5 dB is observed at 11.5 GHz with the effective absorption bandwidth of 2.5 GHz (10.4–12.9 GHz) when the absorber thickness is 1.5 mm. The optimal RL of NG-2 reaches –35.4 dB at 8.2 GHz and the effective absorption bandwidth is 2.2 GHz (7.2–9.4 GHz) with a thickness of 2.5 mm. NG-3 shows a strongest reflection loss of –49.0 dB at 5.3 GHz and the effective absorption bandwidth is 1.5 GHz (4.6-6.1 GHz) corresponding to a thickness of 4.0 mm. Additionally, NG-2 and NG-3 could remain RL values less than –10.0 dB within 1.5–5.5 mm, exhibiting a broad effective response bandwidth from 3.0 to 16.0 GHz and 3.2 to 18.0 GHz, respectively. Therefore, NG-2 and NG-3 possess good EM 11

wave absorption capacity both in low and high frequency regions. As we know, the excellent EM wave absorber needs to meet the requirement including the low reflection loss, thin thickness, light weight and wide absorption frequency bandwidth. When the matching thickness is as thin as 1.0 mm, NG-1 could realize the effective absorption within 15.6–18 GHz, which is not found in the plots of other samples. According to the above discussions, the NG-1 hybrid generated with the lowest initial Ni2+ concentration, exhibits extraordinary high-frequency EM wave absorption performance with the thin thickness and light weight. To further investigate the strongest reflection loss for the NG-1 composite, the varying reflection loss values within the thickness range from 0.9 mm to 1.6 mm are plotted in Fig. 7. The curves demonstrate that with the absorber thickness of 1.2 mm, the strongest RL reaches – 31.4 dB at 14.5 GHz and the effective absorption bandwidth achieves 2.7 GHz (13.3– 16.0 GHz). On the basis of eq. (1) and eq. (2), the complex permittivity and the complex permeability determine the electromagnetic wave absorption capacity of the sample. The real parts of complex permittivity (𝜀 ′ ) and permeability (𝜇 ′ ) symbolize the storage capability of electric and magnetic energy, and the imaginary parts of complex permittivity (𝜀 ″ ) and permeability (𝜇 ″ ) represent the loss capability of electric and magnetic energy, respectively [41]. Fig. 8 illustrates the complex permittivity and the complex permeability of NG samples at the same mass fraction of 30 wt% mixed with paraffin within a range of 2–18 GHz. As shown in Fig. 8(a), the 𝜀 ′ values of three testing NG composites decrease gradually as increasing frequencies on the whole, 12

consistent with most dielectric materials [42]. It is noteworthy that 𝜀 ′ values of three samples at the same frequency are decreased with the increasing concentration of Ni2+ ions in the original reactants. The 𝜀 ′ values of NG-1, NG-2 and NG-3 decline from 27.8 to 18.7, 18.4 to 13.0 and 15.4 to 11.1, respectively, over 2–18 GHz. Meanwhile, the similar variation trend can be observed in the curves of the imaginary parts of complex permittivity varying with the increased frequencies, as observed in Fig. 8(b). In the range of 2–7 GHz, the decline rates of 𝜀 ″ values are getting slower. For the NG-1 sample, the curve of varied 𝜀 ″ values presents a slow rising tendency in the high frequency region (13–18 GHz). And the 𝜀 ″ values of other samples exhibit slight fluctuation after 8 GHz. The complex permittivity is mainly affected by the conductivity and polarizations including dipolar, interfacial, electronic, and ionic polarizations [8]. Since both electronic and ionic polarizations usually occur in much higher frequency area (103–106 GHz), these two types of polarizations could be excluded [40]. The fluctuation resonance peaks of 𝜀 ″ values are most likely attributed to the interfacial polarization caused by the multilayer interfaces of Ni-RGO, Ni-Ni and the folded structure in the composite, as well as the dipole polarization probably arising from defects of the Ni and RGO materials. According to the free electron theory [39], 𝜀 ″ ≈ 1/2𝜋𝜌𝑓𝜀0 , where 𝜌 is the resistivity, 𝑓 is the frequency, and 𝜀0 is the dielectric constant of a vacuum. It can be interpreted that the high conductivity results in the high 𝜀 ″ value. The introduction of Ni particles causes a certain impact on the connected conductive network and residual defects of RGO, which lead to the decline of 𝜀 ′ and 𝜀 ″ values with the increasing initial Ni2+ 13

concentration, suggesting the efficiencies in storage and attenuation of electric energy become lower [43]. Additionally, the existence of magnetic particles can improve the magnetic loss of the EM wave absorber. Figure 8(c) and 8(d) show the real part (𝜇 ′ ) and imaginary part ( 𝜇 ″ ) of the complex permeability ( 𝜇r ) of the wax-based composites in the measured frequency range. The 𝜇 ′ values of all samples exhibit a rapid downward trend at 2–8 GHz and notable increasing trend at 13–16 GHz. Meanwhile, the resonance peaks over 2–8 GHz and 13–16 GHz are also emerged in the 𝜇 ″ curves. In this study, NG samples generated with different initial Ni2+ concentrations show the similar 𝜇 ′ values in the frequency region before 9 GHz, indicating the similar storage capability of magnetic energy within 2–9 GHz for three samples. However, the 𝜇 ′ values exhibit enhancement with decreasing Ni2+ concentration after 12.4 GHz, which is mainly influenced by the particle sizes and amounts of magnetic microspheres. As illustrated in Fig. 8(d), the higher Ni2+ concentration lead to the higher 𝜇 ″ value before 9 GHz, suggesting the enhanced magnetic loss capability in the low-frequency region. After that, NG-2 gives highest 𝜇 ″ values, revealing the most efficiency in loss capacity of magnetic energy in high-frequency region. The negative values observed in the 𝜇 ″ curves always mean the radiation of magnetic energy from materials to space, which may be a result of the induction magnetic field arising from the eddy currents [44]. Besides, the experimental devices and noise are the significant effect factors on the negative permeability as well [33]. The strong fluctuations of 𝜇 ′ and 𝜇 ″ are related to the eddy currents provoked by electromagnetic waves, which is resulted from the high 14

conductivity of Ni and RGO. Generally, the Cole-Cole semicircle can be used to analyze the dielectric behavior of the EM wave absorber based on Debye theory [40, 45]. According to the theory of Debye dipolar relaxation, the relative complex permittivity (𝜀r ) can be expressed as follows: 𝜀r = 𝜀 ′ − 𝑗𝜀 ″ = 𝜀∞ + (𝜀s − 𝜀∞ )⁄(1 + 𝑗2𝜋𝑓𝜏)

(3)

𝜀 ′ = 𝜀∞ + (𝜀s − 𝜀∞ )⁄[1 + (2𝜋𝑓)2 𝜏 2 ]

(4)

𝜀 ″ = 2𝜋𝑓𝜏(𝜀s − 𝜀∞ )⁄[1 + (2𝜋𝑓)2 𝜏 2 ]

(5)

where 𝜀s and 𝜀∞ represent the static permittivity and dielectric permittivity at infinite frequency, respectively, 𝜏 corresponds to the polarization relaxation time. Both 𝜀s and 𝜀∞ are relative with 2𝜋𝑓𝜏. Therefore, the relationship of 𝜀 ′ and 𝜀 ″ can be further described as [𝜀 ′ − (𝜀s + 𝜀∞ )⁄2]2 + (𝜀 ″ )2 = [(𝜀s − 𝜀∞ )⁄2]2

(6)

On the basis of above equation, there may be a semicircle existed in the curve of 𝜀 ″ values against 𝜀 ′ values, which is called a Cole-Cole semicircle. As shown in Fig. 9, there are several semicircles can be observed in the plots of 𝜀 ″ values versus 𝜀 ′ values, in which one semicircle corresponds to one polarization relaxation process. It can be clearly seen that the curve of NG-1 displays two small semicircles. NG-2 and NG-3 give at least three conspicuous semicircles demonstrated in 𝜀 ″ against 𝜀 ′ curves, suggesting that there are multiple dielectric relaxations emerging in the frequency range of 2–18 GHz. The multi-interfaces between the Ni microspheres, RGO folded structures and paraffin matrix are beneficial to absorb the 15

electromagnetic wave, which is ascribed to the interaction of EM wave radiation with charged multi-poles, as well as the interfacial polarization relaxation induced by the migration and accumulation of charge carriers at the interfaces [46]. As revealed in the frequency dependence curves of the complex permeability of three samples, there is no significant difference in variation tendencies of the complex permeability against frequency on the whole, suggesting that their magnetic loss mechanisms are quite similar. The magnetic loss mainly stems from hysteresis, domain wall resonance, eddy current effect, natural resonance and exchange resonance [11]. In general, the natural resonance occurs at lower frequencies than exchange resonance. The hysteresis loss resulted from irreversible magnetization usually occurs in the strong magnetic field, and the domain wall resonance existed in multi-domain materials only occurs in the much lower frequency region(<2 GHz) [40]. Therefore, the hysteresis loss and domain wall resonance can be negligible in the measured frequency range. Taking account of the influence of the eddy current effect, 𝜇 ″ can be expressed as follows equation [30]: 𝜇 ″ = 2𝜋𝜇0 (𝜇 ′ )2 𝜎 ∙ 𝑑 2 𝑓⁄3

(7)

in which 𝜎 is the electrical conductivity and 𝜇0 is the vacuum permeability. The values of C0 (C0 = 𝜇 ″ (𝜇 ′ )−2 𝑓 −1 ) would be constant with varied frequencies if the magnetic loss is only originated from the eddy current effect. As observed in Figure 9(d), the varying range of C0 values is about 0.025, meaning the C0 values are approximately constant. Especially for NG-2, the C0 values in the high-frequency region almost remain unchanged. The nearly constant C0 values imply that the eddy 16

current loss plays a main role in the magnetic loss mechanism of the NG composites. However, the C0 curves of three samples still show slight fluctuations on the whole, conforming that there are other mechanisms including natural resonance and exchange resonance to improve the magnetic loss. The dielectric loss tangent values (tan𝛿e = 𝜀 ″ ⁄𝜀 ′ ) and the magnetic loss tangent values (tan𝛿m = 𝜇 ″ ⁄𝜇 ′ ) stand for the dissipation ability of electric energy and magnetic energy. The loss tangent of the samples are illustrated in Fig. 10. It is clear to see that NG-1 possesses much higher tan𝛿e than others, implying its strong dielectric loss capability. Moreover, the values of tan𝛿e for NG-2 and NG-3 display similar variation within 2–11 GHz, while NG-3 exhibits higher tan𝛿e within 11–17 GHz. As shown in Fig. 10(b), tan𝛿m exhibit the same variation tendency as 𝜇 ″ of the composites. The multiple resonance peaks are mainly induced by the natural resonance, exchange resonance, as well as the eddy current effect. Compared to tan𝛿e values, tan𝛿m of NG samples show much lower values, indicating the dielectric loss is the major contribution to the reflection loss of EM wave. The electromagnetic parameter matching condition (tan𝛿e = tan𝛿m ) has been verified to promote the EM wave absorbing [47]. However, the ratio between tan𝛿e and tan𝛿m of the NG samples is far away from unity as clearly implied in Fig. 10, indicating the outstanding properties of EM wave absorption for the NG composites in this work are different from the traditional electromagnetic parameter matching condition. This phenomenon has occurred in the other magnetic RGO composite [48]. Although the NG material demonstrates a certain magnetic performance, the magnetic loss is very 17

small, or even negligible. According to the above analysis on C0 values, the magnetic loss can be considered mainly caused by the eddy current effect. Moreover, as the electromagnetic wave interred with the helix, the magnetic polarization will be induced along the axis. With the electromagnetic wave flowing in the direction parallel to the axis, it will produce electric polarization to attenuate EM wave [49]. Therefore, the efficient absorption of EM wave mainly depends on the dielectric loss of the NG composite. The difference of dielectric loss and magnetic loss among the samples can be mainly attributed to the difference of the interfaces between Ni and RGO, the particle sizes and structural defects in the composites. The initial nickel salt concentration makes a critical influence on the electromagnetic parameters and EM absorption capabilities. Furthermore, the impedance matching characteristic determines the capability for the absorber to make EM wave entered. 𝑍in is the normalized input impedance of the absorber, which has been calculated by eq. (2) and revealed in Fig. 11(a-c). When the 𝑍in value is equal or close to 1, it is effective to reduce the EM wave reflection at the absorber surface and make incident EM wave converted into other types of energy or dissipated by interference. Therefore, the strongest EM wave absorption peaks would be obtained at the corresponding frequency where 𝑍in value is close to 1. As observed, the impedance matching characteristics of NG samples get improved with increasing nickel salt. Recently, the quarter-wavelength criteria has been proposed to interpret the physical phenomena of electromagnetic wave absorption. The relationship of matching thickness (

m)

and corresponding peak frequency (𝑓m ) of the 18

minimum RL value with matched impedance is in accordance with the phenomena of quarter-wavelength attenuation [11], which can be expressed by the equation: m

in which

=

⁄ = 𝑐 ⁄( 𝑓m √|𝜇r ||𝜀r |)

( =13

)

(8)

is the wavelength of EM wave, |𝜇r | and |𝜀r | stand for the modulus

of 𝜇r and 𝜀r at matching 𝑓m , respectively. Fig. 11(d-f) exhibit the dependence curves of matching thicknesses and peak frequencies of the minimum RL value according to the quarter-wavelength criteria for three samples, in which the black line is denoted as

calculated by eq. (8), and the red dots are denoted as

e

representing the matching thickness corresponding to the optimal RL value of the experimental sample. Fig. 11(g-i) display the RL values curves with different frequencies and thicknesses. Obviously, the experimental data of the matching thickness at a specific frequency fits well with the above model. When the frequency and thickness satisfy the quarter-wavelength attenuation simulation, it induces the electromagnetic cancellation effect to make EM wave extinct at interface between the EM wave absorber and air space [50]. Thus, the excellent EM wave absorption performance can be reasonably interpreted by the impedance matching characteristics and the quarter-wavelength attenuation model. The outstanding EM absorption properties of as-synthesized NG composites could be explained by the following several reasons. Firstly, the introduction of magnetic Ni particles to RGO matrix could adjust the permittivity and permeability to improve the impedance matching characteristic, which is in favor of EM wave absorption. Secondly, the defects and of RGO and Ni particles formatted from the 19

reduction process of GO and metal ions can induce the electronic dipole polarization and related relaxation to increase the dielectric loss. Thirdly, the multilayer interfaces existed between Ni-Ni and Ni-RGO can produce the interfacial polarizations and make EM wave multiple scattering to enhance the EM energy attenuation. Finally, the conductive network constructed by Ni particles and RGO flakes could incite the eddy current effect, prompting incident EM wave dissipated through dielectric loss and magnetic loss mechanisms. [49, 51-52]

4. Conclusion In conclusion, Ni microspheres decorated reduced graphene oxide (NG) with excellent EM wave absorption properties were successfully synthesized by a facile one-step solvothermal method. The research suggests that the initial concentrations of nickel salt make significant effects on the particle sizes, magnetic properties and electromagnetic parameters of the as-prepared samples. The optimal RL of the NG-1 sample reaches –31.4 dB at 14.5 GHz and the effective absorption bandwidth achieves 2.7 GHz (13.3–16.0 GHz) with a thickness of only 1.2 mm. The NG-2 and NG-3 samples can achieve the minimum reflection loss of –35.4 dB at 8.2 GHz and –49.0 dB at 5.3 GHz, respectively. The extraordinary and tunable EM wave absorption capabilities obtained by adjusting the initial nickel salt concentrations, are mainly ascribed to the improved impedance matching characteristics and attenuation mechanisms. The composites can be considered as promising candidates for lightweight and efficient EM wave absorbers. 20

Acknowledgments This work was financially supported by the Qing Lan Project, Environmental Protection Scientific Research Project of Jiangsu Province (2016056),the Shanghai Aerospace Science and Technology Innovation Fund (SAST2015020) and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Table 1 Magnetic parameters of NG samples.

Parameters Samples

Ms

Hc

Mr

(emu/g)

(Oe)

(emu/g)

NG-1

27.7

117.8

2.9

NG-2

31.4

137.6

4.1

NG-3

36.2

117.9

3.7

28

Fig. 1 Schematic illustration of the formation of NG.

Fig. 2 SEM images of the samples: (a) NG-1, (b) NG-2, (c) NG-3; and EDX spectrums of (d) NG-1, (e) NG-2, (f) NG-3.

Fig. 3 TEM images of the samples: (a) NG-1, (b) NG-2, (c) NG-3; and particle size distributions of Ni particles in the sample of: (d) NG-1, (e) NG-2, (f) NG-3.

Fig. 4 (a) XRD patterns and (b) Raman spectra of the GO and NG samples

Fig. 5 Hysteresis loops of NG samples at room temperature.

Fig. 6 Three-dimensional representation of RL values of the samples: (a) NG-1, (b) NG-2, and (c) NG-3.

Fig. 7 Frequency dependence of RL values for NG-1 sample within 0.9-1.6 mm.

Fig. 8 Frequency dependence of electromagnetic parameters of the samples: (a) The real part (𝜀 ′ ) and (b) imaginary part (𝜀 ″ ) of the complex permittivity, (c) the real part (𝜇 ′ ) and (d) imaginary part (𝜇 ″ ) of the complex permeability.

Fig. 9 Typical Cole-Cole semicircles for (a) NG-1, (b) NG-2, (c) NG-3, and (d) the eddy current curves of three samples.

Fig. 10 Frequency dependence of (a) dielectric loss tangent and (b) magnetic loss tangent of the samples.

Fig. 11 The relationship between the normalized input impedance (𝑍in ) and the frequency of (a) NG-1, (b) NG-2, (c) NG-3; comparison of the calculated matching thickness ( ) to the

obtained from RL values of (d) NG-1, (e) NG-2, (f) NG-3;

frequency dependence curves of RL values for (g) NG-1, (h) NG-2 and (i) NG-3.