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graphene composites: A comparative insight
Journal Pre-proof Improved microwave absorption behavioral response of Ni/SiC and Ni/SiC/graphene composites: A comparative insight Samarjit Singh, Anil Kumar Maurya, Rajeev Gupta, Abhishek Kumar, Dharmendra Singh PII:
S0925-8388(20)30143-2
DOI:
https://doi.org/10.1016/j.jallcom.2020.153780
Reference:
JALCOM 153780
To appear in:
Journal of Alloys and Compounds
Received Date: 27 September 2019 Revised Date:
7 January 2020
Accepted Date: 9 January 2020
Please cite this article as: S. Singh, A.K. Maurya, R. Gupta, A. Kumar, D. Singh, Improved microwave absorption behavioral response of Ni/SiC and Ni/SiC/graphene composites: A comparative insight, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153780. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
CRediT author statement S No Authors Credits 1 Mr. Samarjit Singh Methodology, Validation, Conducting a research and investigation process Formal analysis 2 Mr. Anil Kumar Maurya Experimentation and Testing, Conducting a research and investigation process 3 Dr. Rajeev Gupta Supervision, Writing - Review & Editing 4 Dr. Abhishek Kumar Conceptualization, Funding acquisition, Supervision, Writing – Review, Analysis & Editing 5 Prof. Dharmendra Singh Testing Facility, Review and Analysis
Improved microwave absorption behavioral response of Ni/SiC and Ni/SiC/Graphene composites: A comparative insight Samarjit Singha, Anil Kumar Mauryaa, Rajeev Guptab, Abhishek Kumara* and Dharmendra Singhc a
Department of Applied Mechanics, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, India b
Department of Electronics and Communication Engineering, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, India
c
Department of Electronics and Computer Engineering, Indian Institute of Technology Roorkee, Roorkee, India
Abstract In the present work, composites of SiC and Ni in various proportions such as 4:1, 1:1 and 1:4 have been developed through the hydrothermal synthesis of Ni microspheres with and without the dispersion of 2.5 wt.% graphene nanoplatelets. The effect of concentration variation in SiC and Ni microspheres with and without the dispersion of graphene nanoplatelets on the microwave absorption behavior of the Ni/SiC and Ni/SiC/Graphene composites have been explored in the frequency range 2-18 GHz. A comparative insight into the absorption mechanisms for the developed composites has been discussed for achieving enhanced microwave absorption. It has been observed that the dispersion of graphene nanoplatelets in Ni/SiC composites have achieved enhanced absorption behavior. Graphene nanoplatelets dispersed Ni/SiC composites have achieved a minimum reflection loss (RL) value of -59.15 dB at 1.9 mm absorber thickness for SiC:Ni = 1:4. The corresponding -10 dB absorption bandwidth has been observed to be 4.48 GHz. The realization of improved microwave absorption characteristics for Ni/SiC and Ni/SiC/Graphene composites may be ascribed to the synergistic influence of various attenuation mechanisms. Keywords: SiC; Ni microspheres; Graphene nanoplatelets; Microwave absorption; Reflection loss.
1. Introduction The present era has been completely dominated by electronic devices in every sphere of modern life ranging from common household items, health care, and communication systems to military-based defence equipment [1,2]. This extensive usage of electronic items has exploited the microwave spectrum for its functioning which has led to the emergence of electromagnetic
pollution problems [3]. Apart from this, there arises a need to disguise military weapons from being detected by the enemy’s electronic tracking systems. This gives an extra edge to the army in war-like situations in hostile territories. The need to counter such problems have led the researchers around the globe to focus their research on the development of efficient and costeffective microwave absorbing materials [4,5]. The attenuation of microwave signals in absorbers depends on the dissipation of microwave radiations as heat energy. In order to realize high attenuation behavior, microwave absorbers require excellent magnetic and dielectric properties. The synergic role of magnetic and dielectric properties in absorption mechanisms has led to the development of composite materials exhibiting magnetic as well as dielectric characteristics. This has proven to be very instrumental in the development of efficient and lowcost microwave absorbers. Among the potential candidates, Ni exhibits as one of the potential material to absorb microwave radiation [6]. The realization of high microwave absorption has led to considerable effort to tailor the morphological structure of Ni by different synthesis routes along with the synthesis of various Ni-based dielectric composites. B. Zhao et al. [7] synthesized and inspected the attenuation performance of Ni/ZnO based microwave absorbers. The synthesized Ni/ZnO composite attained a minimum RL value of -48.6 dB at 2 mm thickness. B. Zhao et al. [8] successfully demonstrated the synthesis of Sn6O4(OH)4 coated Ni microspheres and achieved a minimum RL value of -32.4 dB at 13.2 GHz with 5 mm thickness. W. Zhang et al. [9] examined the microwave attenuation behavior of Ni@MnO2. The absorber achieved a minimum RL value of -37.55 dB at 11.2 GHz with 3.6 mm thickness. In the present work, as-received micron-sized SiC particles in various proportions (with respect to Ni) have been dispersed during the hydrothermal synthesis of Ni microspheres with and without the dispersion of 2.5 wt.% graphene nanoplatelets. An effort has been made to present a comparative insight on the microwave absorption behavior of Ni/SiC composites with and without the addition of graphene nanoplatelets in the frequency range of 2-18 GHz. The heterogeneous composites are expected to integrate the magnetic and dielectric properties of the mixed materials in order to tune the complex permittivity (εr) and permeability (µr) values for achieving enhanced microwave absorption with reasonably thin thickness.
2. Experimental methodology 2.1 Materials
Analytical grade materials have been used in the adopted synthesis methodology without any additional purification. Nickel chloride hexahydrate (NiCl2.6H2O), Cetyltrimethyl Ammonium Bromide (CTAB) were obtained from Sisco Research Laboratories (SRL), India and Hydrazine monohydrate (N2H4.H2O) solution and Acetone (CH3COCH3) were procured from Molychem, India. 2.2 Synthesis of Ni particles An appropriate amount of NiCl2.6H2O was dissolved in distilled water and stirred on a magnetic stirrer for 1 h at 1000 RPM till all the crystals were perfectly soluble. Another mixture was prepared by mixing CTAB in N2H4.H2O using glass rod stirring. The CTAB mixture was then poured slowly into the NiCl2.6H2O solution with continuous stirring. The resultant solution was then stirred for another 40 minutes and put into stainless steel hydrothermal jar with a teflon tube. The sealed hydrothermal jar was heated at 160 °C in a hot air oven for 6 hours. This was followed by natural cooling of the hydrothermal system to room temperature. Black solid products thus obtained was subjected to filtration and washing with ethanol and deionized water alternatively. Finally, the filtered product was dried in vacuum at 60 °C for 24 hours. 2.3 Synthesis of Ni/SiC and Ni/SiC/Graphene composites SiC powders 200-450 mess size acquired from Sigma-Aldrich, India was added in different proportions in the prepared solution for Ni synthesis and sealed in teflon tube covered with stainless steel hydrothermal jar. The entire solution was heated at 160 °C in a hot air oven for 6 hours for the formation of Ni/SiC composites. Another composition has been prepared with the addition of 2.5 wt.% graphene nanoplatelets in the above mixture to form Ni/SiC/Graphene composites. As graphene nanoplatelets have a strong tendency to agglomerate owing to their large surface area, the graphene nanoplatelets were ultrasonicated for 1 hour to minimize the agglomeration tendency before mixing into the mixture of Ni/SiC. Various proportions of SiC to Ni viz. 4:1, 1:1 and 1:4 were taken to synthesize the Ni/SiC composites. Apart from this, 2.5 wt.% graphene nanoplatelets (with respect to estimated SiC+Ni yield) were added to form Ni/SiC/Graphene composites with the above mentioned ratios of SiC and Ni. The nomenclature of the various synthesized composites has been presented in Table 1. 2.4 Instruments and measurements
Phase analysis of the synthesized composites has been analyzed by recording the diffracted beam intensities against diffraction angle (2θ) in 10-90° range using an X-ray diffractometer (Rigaku Smartlab). Investigation of the morphological features of the powder composites has been accomplished using the images obtained from Quanta 200 field emission scanning electron microscopy (FE-SEM). Measurement of the magnetic properties of the prepared composites was carried out at room temperature using Versa vibrating sample magnetometer (VSM) in -20000 to +20000 Oe applied field. Investigation of the absorption behavior of microwave signals has been accomplished by dielectric measurement of the prepared composites using Agilent N5222 PNA series vector network analyzer (VNA) in the frequency range of 2-18 GHz.
3. Results and Discussion 3.1 Phase analysis The X-ray diffraction patterns of the synthesized composites containing SiC, Ni and C diffraction peaks have been depicted in Figure 1. The crystal structure of SiC (ICSD: 27051) has been identified to be hexagonal. For pure Ni sample, the diffraction peaks appearing at 2θ = 44.5, 51.95 and 76.55° is indexed as (111), (002) and (022) crystal planes of Ni, respectively. The diffraction pattern of Ni depicts the face centre cubic (FCC) (ICSD: 646088) structure. No distinct diffraction peaks other than FCC-Ni were observed in the sample indicating the purity of the synthesized Ni powder. The diffraction patterns of the synthesized composites reveal the presence of phases corresponding to Ni and SiC in samples S1, S2, S3, S4, S5, and S6. Dispersion of graphene nanoplatelets in samples S4, S5 and S6 reveal the presence of carbon (C) (ICSD: 76767) peaks (except for sample S4) approximately at 2θ = 26.5° which is a clear indication of the presence of graphene nanoplatelets in the composite samples. The absence of C peak in sample S4 might be due to the very small amount of graphene nanoplatelets present in the selected powder sample for XRD characterization. Due to very minute amount, the C peak might not have been detected. It can also be observed that with the increase in the concentration of Ni; the intensity of the diffraction peaks corresponding to Ni increases. No distinct diffraction peaks other than Ni, SiC and C have been found in the XRD patterns which indicate the absence of any unwanted phase in the synthesized composites. 3.2 Morphological analysis
The FE-SEM micrographs corresponding to pure SiC, Ni and graphene nanoplatelets have been shown in Figure 2. As-received SiC particles having an average particle size of 52 µm display non-uniform morphological features in the FE-SEM micrographs as shown in Figure 2 (a). Non-uniform morphology with sharp edges is also a characteristic feature of SiC used which is distinctively evident from Figure 2 (a). The synthesized Ni particles display uniform spherical morphological features as depicted in Figure 2 (b). The average size of the Ni particles has been measured to be 1 µm and therefore, the synthesized Ni particles are termed as Ni microspheres. Figure 2 (c) indicates the FE-SEM micrograph of as-received graphene nanoplatelets. Graphene nanoplatelets have flake-like morphological features with 5 µm average size. The morphological analyses of all composites with and without graphene nanoplatelets addition have been shown in Figure 3. Ni/SiC composites containing Ni microspheres and nonuniform SiC particles for the samples S1, S2 and S3 are indicated in Figure 3 (a), (b) and (c), respectively. Figure 3 (b) and (c) show higher concentration of Ni microspheres as compared to Figure 3 (a) in the synthesized composites as per the experimental plan. Individual components viz. Ni, SiC and graphene nanoplatelets can be identified for the samples S4, S5 and S6 in Figure 3 (d), (e) and (f), respectively. The inset images show the enlarged view of a particular region in the synthesized samples. The Ni and graphene nanoplatelets in the FE-SEM micrographs have been represented by Ni and GP, respectively. The presence of graphene nanoplatelets in samples S4, S5 and S6 can be easily comprehended from the FE-SEM micrographs as shown in Figure 3 (d), (e) and (f). 3.3 Magnetic behavior The magnetic behaviors of the synthesized Ni/SiC and Ni/SiC/Graphene composites have been depicted in Figure 4. Figure 4 reflects that the value of saturation magnetization rises with the increase in Ni microsphere concentration in Ni/SiC and Ni/SiC/Graphene composites. Due to the increase in Ni microsphere concentration which is magnetic in nature, the value of saturation magnetization has increased considerably. The saturation magnetization of pure Ni microspheres has been observed to be 51.88 emu/g. It may be observed from the magnetization curves of Ni/SiC composites that the saturation magnetization has increased from 18.83 emu/g for sample S1 to 50.95 emu/g for sample S3. In the case of Ni/SiC/Graphene composites, the saturation magnetization has increased from 16.18 emu/g for sample S4 to 50.16 emu/g for sample S6. However, there is no noticeable change in the coercivity of the synthesized composites. The one-
to-one comparison of the synthesized composites with and without graphene nanoplatelet addition reveals that the magnetization properties are almost the same for similar variations in constituent concentration. This is in good agreement with the fact that the synthesized Ni/SiC composites with and without graphene nanoplatelets have almost similar concentration variations. 3.4 Dielectric analysis Figure 5 (a) reflects the real part of complex permittivity (εʹ) of the synthesized composite samples in 2-18 GHz frequency range. It is very clearly evident from Figure 5 (a) that with the increase in Ni microspheres concentration, the value of εʹ increases for samples S2 and S3 as compared to sample S1 which has a lower concentration of Ni microspheres. It may be further observed from Figure 5 (a) that with the dispersion of graphene nanoplatelets, the values of εʹ have increased considerably which can be visualized from the one-to-one comparison of the samples viz. S1 & S4; S2 & S5 and S3 & S6. The presence of Ni and graphene nanoplatelets may contribute to the enhancement in the conductivity of composite systems. This contributes to the enhancement in the values of εʹ. The presence of particles with different electronegativity viz. SiC, Ni and C generate heterogeneous interfaces that entrap charge carriers. These entrapped charge carriers result in interfacial polarization. An increase in the concentration of Ni particles and dispersion of graphene nanoplatelets create several heterogeneous interfaces in the entire heterogeneous composite system. The presence of a large number of heterogeneous interfaces in alternating electric field aids in the accumulation of charges at the sharp edges of these interfaces resulting in enhanced interfacial polarization [2,10]. This phenomenon significantly contributes to the enhancement in the values of εʹ for Ni/SiC and Ni/SiC/Graphene composites. The imaginary part of complex permittivity (εʺ) for Ni/SiC and Ni/SiC/Graphene composites in the investigated 2-18 GHz frequency range has been shown in Figure 5 (b). It is evident from Figure 5 (b) that with the increase in Ni concentration for Ni/SiC composites with and without graphene nanoplatelets, the values of εʺ increases. This increase conforms to the free-electron theory given by eqn. (1). According to this theory, εʺ increases with the increase in conductivity which increases the ohmic loses due to the nomadic charge transfer [10,11]. εʺ = 1 / (2πεoρf) where, ρ and εo represent electrical resistivity and free space permittivity, respectively.
(1)
The increase in Ni content may form conductive networks in the composite system for the hopping of free electrons from one particle to another. The conductive networks increase the conductivity of the Ni/SiC system and correspondingly the εʺ value increases. For the case of Ni/SiC/Graphene composites, apart from the hopping of electrons from one Ni particle to another Ni particle, there is hopping of ᴫ-ᴫ electrons due to the dispersion of graphene nanoplatelets in the heterogeneous composite system [12]. The presence of a large number of conducting particles also creates number of heterogeneous interfaces which increases the interfacial polarization and hence, εʺ of the system increases. The nature of the real part of complex permeability (µʹ) in 2-18 GHz frequency range has been presented in Figure 5 (c). The nature of µʹ reflects a complex trend with the increase in Ni concentration and graphene addition in the composites. It may be observed from Figure 5 (c) that µʹ has followed a decreasing nature with the increase in frequency for most of the samples. Figure 5 (d) represents the nature of imaginary part of complex permeability (µʺ) for the synthesized composites in 2-18 GHz frequency range. Ni/SiC and Ni/SiC/Graphene composites depict higher values of µʺ in 2-8 GHz frequency range as compared to the pure SiC system (except for S1). It may be observed that the trend for µʺ is almost the inverse of εʺ. Such resonance characteristics in the nature of complex permittivity and permeability curves may arise due to the resonance-antiresonance behaviour [13] owing to the presence of magnetic Ni particles, dielectric SiC and graphene nanoplatelets in the composite systems. A slight negative trend in µʺ may be observed for all of the composites at higher frequency range as depicted in Figure 5 (d). The negative behavior may be associated with the magnetic energy being radiated out as electric energy in the presence of alternating electromagnetic field [14]. The energy conservation principle dictates that the negative values of µʺ should be represented by a loss of electrical energy [13]. The higher value of εʺ for the composites at higher frequencies where µʺ is negative conforms to this principle. 3.5 Microwave absorption behavior The microwave absorption behavior of the synthesized composites has been analyzed in 2-18 GHz frequency range with the help of reflection loss (RL) curves. The nature of RL curves representing the absorption behavior has been calculated by the transmission line theory using eqns. (2) & (3) [15,16] and the microwave absorption measurement technique has been clearly explained in [17,18].
RL = 20 log |(Zin- Zo)/ (Zin+ Zo)|
(2)
Zin = Zo (µr /εr)1/2tanh {j.(2πft/c) (µr.εr)1/2}
(3)
where, Zin and Zo represent the characteristic impedance of the absorber and free space, respectively. f represents the frequency, t represents the thickness of the absorber, c represents the speed of light. Figure 6 represents the minimum RL behavior of Ni/SiC and Ni/SiC/Graphene composites at the matching absorber thickness in the frequency range of 2-18 GHz. The minimum RL values and the corresponding -10 dB absorption bandwidth values of the synthesized composites have been shown in the inset of Figure 6. The absorption behavior of Ni/SiC and Ni/SiC/Graphene composites have followed an increasing trend with the increase in the concentration of Ni microspheres. From the one-to-one comparison of RL behavior of samples S1 & S4; S2 & S5 and S3 & S6, it may be pointed out that with the little addition of graphene nanoplatelets, the microwave absorption performance of Ni/SiC/Graphene composites has enhanced significantly. The minimum reflection loss indicating maximum absorption has been realized for sample S6 at 15.28 GHz with 1.9 mm absorber thickness. The corresponding RL value and -10 dB bandwidth has been measured to be -59.15 dB and 4.48 GHz, respectively which is significantly higher as compared to pure SiC and pure Ni microspheres samples values of -14.57 dB and -17.9 dB [19], respectively. Table 2 shows the microwave absorption characteristics of all synthesized composite samples. The microwave absorption performance depends on the synergistic effect of many absorption mechanisms and factors viz. dielectric relaxation, interfacial polarization, defects induced dipole polarization, magnetic losses, multiple scatterings and reflections, quarter-wavelength phase cancellation, impedance matching, etc [10]. The dielectric relaxation processes constitute an instrumental role in the microwave absorption behavior of the synthesized composites. The dielectric relaxation process is represented by the Cole-Cole curves [20]. Each characteristic semi-circle in the Cole-Cole curve represents one debye relaxation process. Debye relaxation theory is utilized to investigate the major sources for dielectric losses within the composite system upon interaction with the alternation electromagnetic fields. The Cole-Cole plots for the synthesized samples have been shown in Figure 7 (a-g). For pure SiC, two semi-circles are evident which corresponds to two Debye relaxation processes as depicted in Figure 7 (a). The Debye relaxation in pure SiC may be attributed to the presence of defects in the crystal. These defects may act as dipoles in alternating
electromagnetic field converting electromagnetic energy into thermal energy due to orientation polarization processes. It is evident from Figure 7 (a-g) that samples with a lower concentration of Ni microspheres viz. S1 and S4 have two semi-circles representing two debye relaxation processes. The samples with a higher concentration of Ni microspheres viz. S2, S3, S5, and S6 have three semi-circles representing three debye relaxation processes. Hence the samples with higher Ni content have shown enhanced absorption behavior. Increased number of semi-circles indicate the presence of various relaxation processes such as interfacial polarization, dipole polarization, defects induced polarization, etc. These relaxation processes aids in realizing enhanced microwave attenuation. It may also be noted for the composite samples that a long tail exists in between two semi-circles which indicates that apart from dielectric losses, conduction loss also contributes to microwave absorption. In addition to dielectric losses, magnetic losses also constitute an important absorption mechanism in the case of composite systems with magnetic components. It has been reported in various literature that domain wall movement, hysteresis loss, eddy current loss, exchange resonance, and natural resonance are the major sources for magnetic loss. The contribution of domain wall movement and hysteresis loss may be neglected in the gigahertz frequency region [21]. Exchange resonance is applicable for materials with particle size less than 100 nm [22]. Therefore, in the current case, only the loses arising due to eddy current and natural resonance are applicable. Magnetic loss due to eddy current effect is dominant when the value and characteristic nature of µʺ(µʹ)-2f-1 curve do not vary with frequency [21,22]. Figure 8 shows that the values of µʺ(µʹ)-2f-1 varies with increasing frequency. This signifies that the eddy current effect is not the only dominant magnetic loss mechanism in the present composites. The variation in the curve indicates that natural resonance may be the major source for magnetic loss in the synthesized composites. The quarter wavelength phase cancellation model has also been investigated for microwave absorption due to the geometrical factor. According to this model, destructive interference of the microwaves results in phase cancellation at the air-absorber interface which accounts for enhanced microwave absorption. The quarter wavelength model comes into play when the thickness of the absorber satisfies eqn. (4) which is mathematically expressed as [10]: tcal = [n c / 4fm√(|εrµr|)] (n =1,3,5,7,…)
(4)
where, tcal represents the calculated thickness of the absorber, n is the order of reflection and other symbols represent their usual meaning. The quarter wavelength model for phase cancellation has been depicted in Figure 9 (i), (ii) & (iii). It can be observed that the phase cancellation effect is not so dominant for pure SiC and composites with a low concentration of Ni as the matching thicknesses and calculated thicknesses are not in good agreement. However, composites with higher Ni concentration reflect a good agreement between the matching thicknesses and calculated thicknesses. This suggests that for samples S3, S5 and S6, the phase cancellation effect also contributes to microwave attenuation. The most important factor for realizing better absorption behavior is dictated by impedance matching of the system which depends on εr and µr values. Impedance matching indicates the extent to which microwave signals may travel into the interior of the absorber [16]. The εr and µr values should be close for realizing better impedance matching. |Zin/Zo| represents the normalized impedance matching factor given in eqn (5). The characteristic value of |Zin/Zo| indicates the extent to which the incident EM wave signal traverses into the interior of the absorber material by assessing the degree of closeness between the εr and µr values of the composite system. |Zin/ Zo| = (µr /εr)1/2tanh {j.(2πft/c) (µr.εr)1/2}
(5)
where, each symbol represents their usual meaning. Normalized impedance matching factor |Zin/Zo| should be close to 1 for good microwave absorption characteristics. It is evident from Figure 10 (a) that the impedance matching for samples having a higher concentration of Ni microspheres is closer to 1 as compared to the samples having a lower concentration of Ni microspheres. Best microwave absorption behavior has been observed for sample S6 owing to its excellent impedance matching while poor absorption performance has been shown by sample S1 owing to its low impedance matching characteristic. Another factor that dictates the absorption characteristics of microwave absorbers is the attenuation constant (α) in Np/m which is given by eqn. (6) [23,24]: α=
2 πf c
(µ ′′ε′′ − µ ′ε′) + (µ ′′ε′′ − µ ′ε′) 2 + (µ ′ε′′ − µ ′′ε′) 2
where, each symbol represents their usual meaning.
(6)
It represents the materials’ intrinsic ability to attenuate the incident microwave signals in the interior of the absorber material. Figure 10 (b) depicts the attenuation constant curves for the synthesized composite samples. It can be noted from Figure 10 (b) that the value of attenuation constant for samples S5 and S6 is high along with good impedance matching and these favorable characteristics is also reflected in their enhanced microwave absorption behavior. This trend is the same for all the synthesized composites and ascertains the fact that impedance matching factor and attenuation constant work in tandem in realizing enhanced microwave absorption behavior. Apart from the discussed mechanisms, defects induced polarization and interfacial polarization also affects the absorption behavior. With the increase in Ni content and graphene nanoplatelet dispersion, the number of heterogeneous surfaces in the composite system increases; thereby increasing the interfacial polarization due to the trapping of the charges. The storage of charges due to the capacitive nature of the composite owing to the presence of conducting and dielectric system also contributes to microwave absorption. The increase in the electric path due to multiple scattering and reflections may also aid in microwave absorption. All these mechanisms work synergistically in enhancing the microwave absorption performance Ni/SiC and Ni/SiC/Graphene composites and have been illustrated in Figure 11. A comparison of the present work with Ni-based absorbers available in various literature based on microwave absorption characteristics has been presented in Table 3. Ni/SiC/Graphene composites show excellent microwave absorption characteristics as compared to the reported Ni-based absorbers.
4. Conclusions In summary, Ni/SiC and Ni/SiC/Graphene composites have been successfully synthesized through hydrothermal route for achieving enhanced microwave absorption. The investigation of absorption behavior reflects that the graphene addition in Ni/SiC composites exhibits significant enhancement in the attenuation performance. The investigation also reflects that the microwave absorption behavior is significantly enhanced with the increase in the concentration of Ni microspheres in the composite system. The minimum value of RL for composites without graphene nanoplatelets has been observed for composite sample with SiC:Ni = 1:4. The corresponding RL value and -10 dB bandwidth have been observed to be -30.63 dB at 1.9 mm absorber thickness and 3.36 GHz, respectively. With the addition of graphene nanoplatelets, a minimum RL value of -59.15 dB has been realized for the sample with SiC:Ni = 1:4 and 2.5
wt.% graphene nanoplatelets. The corresponding -10 dB absorption bandwidth has been observed to be 4.48 GHz at 1.9 mm. The results indicate that Ni/SiC/Graphene nanoplatelets, as well as Ni/SiC composites, may prove to be encouraging candidates for realizing efficient and low-cost microwave absorbers for practical applications.
Acknowledgments The authors, Samarjit Singh, and Anil Kumar Maurya express their sincere gratitude to the Ministry of Human Resources and Development (MHRD), India, for providing fellowships. The authors are also thankful to the Department of Electronics and Computer Science, Indian Institute of Technology Roorkee, India for extending the facility of Vector Network Analyzer for dielectric measurements.
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[26]
B. Lu, X.L. Dong, H. Huang, X.F. Zhang, X.G. Zhu, J.P. Lei, J.P. Sun, Microwave absorption properties of the core/shell-type iron and nickel nanoparticles, Journal of Magnetism and Magnetic Materials 320 (2008) 1106-1111.
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B. Zhao, G. Shao, B. Fan, W. Li, X. Pian, R. Zhang, Enhanced electromagnetic wave absorption properties of Ni–SnO2 core-shell composites synthesized by a simple hydrothermal method, Materials Letters 121 (2014) 118–121.
[29]
Y. Cao, Q. Su, R. Che, G. Du, and B. Xu, One-step chemical vapor synthesis of Ni/graphene nanocomposites with excellent electromagnetic and electrocatalytic properties, Synthetic Metals 162 (2012) 968–973.
[30]
T. Chen, F. Deng, J. Zhu, C. Chen, G. Sun, S. Ma, X. Yang, Hexagonal and cubic Ni nanocrystals grown on graphene : phase-controlled synthesis, characterization and their enhanced microwave absorption, Journal of Materials Chemistry 22 (2012) 15190– 15197.
[31]
B. Gao, L. Qiao, J. Wang, Q. Liu, F. Li, and J. Feng, Microwave absorption properties of the Ni nanowires composite, Journal of Physics D: Applied Physics 41 (2008) 235005.
[32]
J. Yuan, H. Yang, Z.L. Hou, W. Song, H. Xu, Y. Q. Kang, H.B. Jin, X. Y. Fang, M.S. Cao, Ni-decorated SiC powders: Enhanced high-temperature dielectric properties and microwave absorption performance, Powder Technology 237 (2013) 309–313.
Figure 1: X-ray diffraction patterns of synthesized samples. Figure 2: FE-SEM micrographs (a) Pure SiC; (b) Pure Ni and (c) Pure graphene nanoplatelets. Figure 3: FE-SEM micrographs (a) SiC:Ni = 4:1; (b) SiC:Ni = 1:1; (c) SiC:Ni = 1:4; (d) (SiC:Ni)/2.5 wt.% graphene = 4:1; (e) (SiC:Ni)/2.5 wt.% graphene = 1:1 and (f) (SiC:Ni)/2.5 wt.% graphene = 1:4 Figure 4: Magnetic hysteresis loops of the synthesized composite samples. Figure 5: Variation in (a) real part of complex permittivity; (b) imaginary part of complex permittivity; (c) real part of complex permeability; (d) imaginary part of complex permeability for the prepared samples in 2-18 GHz range. Figure 6: Microwave absorption behavior of the synthesized composite samples in 2-18 GHz range. Figure 7: Cole-Cole plots representing Debye relaxation processes for the synthesized composites. Figure 8: Eddy current effect plot representing dominant magnetic loss mechanism for the synthesized composites. Figure 9(i): Dependence of quarter wavelength (λ/4) thickness on frequency for the synthesized composite samples (a) Pure SiC; (b) S1and (c) S2. Figure 9(ii): Dependence of quarter wavelength (λ/4) thickness on frequency for the synthesized composite samples (d) S3 and (e) S4. Figure 9(iii): Dependence of quarter wavelength (λ/4) thickness on frequency for the synthesized composite samples (f) S5 and (g) S6. Figure 10: Variation of (a) Impedance matching at matching absorber thickness and (b) Attenuation constant curves for the synthesized composites in 2-18 GHz range. Figure 11: Schematic representation of electromagnetic wave interactions with the heterogeneous structure of Ni/SiC/Graphene nanoplatelets based composites illustrating various attenuation mechanisms.
Table 1: Nomenclature of synthesized composites Sample code
Composition
S1
SiC:Ni= 4:1
S2
SiC:Ni= 1:1
S3
SiC:Ni= 1:4
S4
SiC:Ni= 4:1 with 2.5 wt.% Graphene nanoplatelets
S5
SiC:Ni= 1:1 with 2.5 wt.% Graphene nanoplatelets
S6
SiC:Ni= 1:4 with 2.5 wt.% Graphene nanoplatelets
Table 2. Reflection loss measurements for Ni/SiC and Ni/SiC/Graphene nanoplatelets based composites.
Sample
Min.
Code
RL (dB)
Pure SiC
Matching
Frequency
Thickness
Bandwidth corresponding
(mm)
(GHz)
to -10 dB (GHz)
-14.57
2.1
17.5
2.28
S1
-10.15
2.2
17.44
0.64
S2
-20.36
2.0
16.4
3.12
S3
-30.63
1.9
15.84
3.36
S4
-20.33
1.9
17.68
2.08
S5
-38.78
1.9
14.8
4.32
S6
-59.15
1.9
15.28
4.48
Table 3. Microwave absorption characteristics of Ni-based absorbers.
Composition
Min. RL value (dB)
Matching thickness (mm)
Bandwidth corresponding to -10 dB
Reference
(GHz)
Ni sub-microsphere
-17.9
1.2
-
[19]
rGO–Ni ([email protected] wt%)
-23.3
3
1.8
[25]
Ni nanoparticles
-14.9
3
-
[26]
Ni(C)
-32.0
2
4.2
[27]
Ni/SnO2 microspheres
-18.6
7
-
[28]
Ni/graphene
-13.0
2
-
[29]
Hexagonal Ni/graphene
-17.8
5
-
[30]
Ni nanowires
-8.5
3
-
[31]
Ni/SiC
-28.54
-
3.57
[32]
SiC:Ni = 1:4
-30.63
1.9
3.36
Present Work
-38.78
1.9
4.32
Present Work
-59.15
1.9
4.48
Present Work
SiC:Ni =1:1 and 2.5 wt.% graphene nanoplatelets SiC:Ni =1:4 and 2.5 wt.% graphene nanoplatelets
Highlights: • • •
Effect of Ni microspheres concentration on microwave absorption behavior of Ni/SiC composites has been studied. Effect of graphene nanoplatelets dispersion on microwave absorption behavior of Ni/SiC composites has also been analyzed. Minimum RL of -59.15 dB at 15.28 GHz was obtained for Ni/SiC/Graphene nanoplatelets at 1.9 mm thickness with corresponding -10 dB bandwidth of 4.48 GHz.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Corresponding authors on behalf of all authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. (Abhishek Kumar)
S0925-8388(20)30143-2
DOI:
https://doi.org/10.1016/j.jallcom.2020.153780
Reference:
JALCOM 153780
To appear in:
Journal of Alloys and Compounds
Received Date: 27 September 2019 Revised Date:
7 January 2020
Accepted Date: 9 January 2020
Please cite this article as: S. Singh, A.K. Maurya, R. Gupta, A. Kumar, D. Singh, Improved microwave absorption behavioral response of Ni/SiC and Ni/SiC/graphene composites: A comparative insight, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153780. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
CRediT author statement S No Authors Credits 1 Mr. Samarjit Singh Methodology, Validation, Conducting a research and investigation process Formal analysis 2 Mr. Anil Kumar Maurya Experimentation and Testing, Conducting a research and investigation process 3 Dr. Rajeev Gupta Supervision, Writing - Review & Editing 4 Dr. Abhishek Kumar Conceptualization, Funding acquisition, Supervision, Writing – Review, Analysis & Editing 5 Prof. Dharmendra Singh Testing Facility, Review and Analysis
Improved microwave absorption behavioral response of Ni/SiC and Ni/SiC/Graphene composites: A comparative insight Samarjit Singha, Anil Kumar Mauryaa, Rajeev Guptab, Abhishek Kumara* and Dharmendra Singhc a
Department of Applied Mechanics, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, India b
Department of Electronics and Communication Engineering, Motilal Nehru National Institute of Technology Allahabad, Prayagraj, India
c
Department of Electronics and Computer Engineering, Indian Institute of Technology Roorkee, Roorkee, India
Abstract In the present work, composites of SiC and Ni in various proportions such as 4:1, 1:1 and 1:4 have been developed through the hydrothermal synthesis of Ni microspheres with and without the dispersion of 2.5 wt.% graphene nanoplatelets. The effect of concentration variation in SiC and Ni microspheres with and without the dispersion of graphene nanoplatelets on the microwave absorption behavior of the Ni/SiC and Ni/SiC/Graphene composites have been explored in the frequency range 2-18 GHz. A comparative insight into the absorption mechanisms for the developed composites has been discussed for achieving enhanced microwave absorption. It has been observed that the dispersion of graphene nanoplatelets in Ni/SiC composites have achieved enhanced absorption behavior. Graphene nanoplatelets dispersed Ni/SiC composites have achieved a minimum reflection loss (RL) value of -59.15 dB at 1.9 mm absorber thickness for SiC:Ni = 1:4. The corresponding -10 dB absorption bandwidth has been observed to be 4.48 GHz. The realization of improved microwave absorption characteristics for Ni/SiC and Ni/SiC/Graphene composites may be ascribed to the synergistic influence of various attenuation mechanisms. Keywords: SiC; Ni microspheres; Graphene nanoplatelets; Microwave absorption; Reflection loss.
1. Introduction The present era has been completely dominated by electronic devices in every sphere of modern life ranging from common household items, health care, and communication systems to military-based defence equipment [1,2]. This extensive usage of electronic items has exploited the microwave spectrum for its functioning which has led to the emergence of electromagnetic
pollution problems [3]. Apart from this, there arises a need to disguise military weapons from being detected by the enemy’s electronic tracking systems. This gives an extra edge to the army in war-like situations in hostile territories. The need to counter such problems have led the researchers around the globe to focus their research on the development of efficient and costeffective microwave absorbing materials [4,5]. The attenuation of microwave signals in absorbers depends on the dissipation of microwave radiations as heat energy. In order to realize high attenuation behavior, microwave absorbers require excellent magnetic and dielectric properties. The synergic role of magnetic and dielectric properties in absorption mechanisms has led to the development of composite materials exhibiting magnetic as well as dielectric characteristics. This has proven to be very instrumental in the development of efficient and lowcost microwave absorbers. Among the potential candidates, Ni exhibits as one of the potential material to absorb microwave radiation [6]. The realization of high microwave absorption has led to considerable effort to tailor the morphological structure of Ni by different synthesis routes along with the synthesis of various Ni-based dielectric composites. B. Zhao et al. [7] synthesized and inspected the attenuation performance of Ni/ZnO based microwave absorbers. The synthesized Ni/ZnO composite attained a minimum RL value of -48.6 dB at 2 mm thickness. B. Zhao et al. [8] successfully demonstrated the synthesis of Sn6O4(OH)4 coated Ni microspheres and achieved a minimum RL value of -32.4 dB at 13.2 GHz with 5 mm thickness. W. Zhang et al. [9] examined the microwave attenuation behavior of Ni@MnO2. The absorber achieved a minimum RL value of -37.55 dB at 11.2 GHz with 3.6 mm thickness. In the present work, as-received micron-sized SiC particles in various proportions (with respect to Ni) have been dispersed during the hydrothermal synthesis of Ni microspheres with and without the dispersion of 2.5 wt.% graphene nanoplatelets. An effort has been made to present a comparative insight on the microwave absorption behavior of Ni/SiC composites with and without the addition of graphene nanoplatelets in the frequency range of 2-18 GHz. The heterogeneous composites are expected to integrate the magnetic and dielectric properties of the mixed materials in order to tune the complex permittivity (εr) and permeability (µr) values for achieving enhanced microwave absorption with reasonably thin thickness.
2. Experimental methodology 2.1 Materials
Analytical grade materials have been used in the adopted synthesis methodology without any additional purification. Nickel chloride hexahydrate (NiCl2.6H2O), Cetyltrimethyl Ammonium Bromide (CTAB) were obtained from Sisco Research Laboratories (SRL), India and Hydrazine monohydrate (N2H4.H2O) solution and Acetone (CH3COCH3) were procured from Molychem, India. 2.2 Synthesis of Ni particles An appropriate amount of NiCl2.6H2O was dissolved in distilled water and stirred on a magnetic stirrer for 1 h at 1000 RPM till all the crystals were perfectly soluble. Another mixture was prepared by mixing CTAB in N2H4.H2O using glass rod stirring. The CTAB mixture was then poured slowly into the NiCl2.6H2O solution with continuous stirring. The resultant solution was then stirred for another 40 minutes and put into stainless steel hydrothermal jar with a teflon tube. The sealed hydrothermal jar was heated at 160 °C in a hot air oven for 6 hours. This was followed by natural cooling of the hydrothermal system to room temperature. Black solid products thus obtained was subjected to filtration and washing with ethanol and deionized water alternatively. Finally, the filtered product was dried in vacuum at 60 °C for 24 hours. 2.3 Synthesis of Ni/SiC and Ni/SiC/Graphene composites SiC powders 200-450 mess size acquired from Sigma-Aldrich, India was added in different proportions in the prepared solution for Ni synthesis and sealed in teflon tube covered with stainless steel hydrothermal jar. The entire solution was heated at 160 °C in a hot air oven for 6 hours for the formation of Ni/SiC composites. Another composition has been prepared with the addition of 2.5 wt.% graphene nanoplatelets in the above mixture to form Ni/SiC/Graphene composites. As graphene nanoplatelets have a strong tendency to agglomerate owing to their large surface area, the graphene nanoplatelets were ultrasonicated for 1 hour to minimize the agglomeration tendency before mixing into the mixture of Ni/SiC. Various proportions of SiC to Ni viz. 4:1, 1:1 and 1:4 were taken to synthesize the Ni/SiC composites. Apart from this, 2.5 wt.% graphene nanoplatelets (with respect to estimated SiC+Ni yield) were added to form Ni/SiC/Graphene composites with the above mentioned ratios of SiC and Ni. The nomenclature of the various synthesized composites has been presented in Table 1. 2.4 Instruments and measurements
Phase analysis of the synthesized composites has been analyzed by recording the diffracted beam intensities against diffraction angle (2θ) in 10-90° range using an X-ray diffractometer (Rigaku Smartlab). Investigation of the morphological features of the powder composites has been accomplished using the images obtained from Quanta 200 field emission scanning electron microscopy (FE-SEM). Measurement of the magnetic properties of the prepared composites was carried out at room temperature using Versa vibrating sample magnetometer (VSM) in -20000 to +20000 Oe applied field. Investigation of the absorption behavior of microwave signals has been accomplished by dielectric measurement of the prepared composites using Agilent N5222 PNA series vector network analyzer (VNA) in the frequency range of 2-18 GHz.
3. Results and Discussion 3.1 Phase analysis The X-ray diffraction patterns of the synthesized composites containing SiC, Ni and C diffraction peaks have been depicted in Figure 1. The crystal structure of SiC (ICSD: 27051) has been identified to be hexagonal. For pure Ni sample, the diffraction peaks appearing at 2θ = 44.5, 51.95 and 76.55° is indexed as (111), (002) and (022) crystal planes of Ni, respectively. The diffraction pattern of Ni depicts the face centre cubic (FCC) (ICSD: 646088) structure. No distinct diffraction peaks other than FCC-Ni were observed in the sample indicating the purity of the synthesized Ni powder. The diffraction patterns of the synthesized composites reveal the presence of phases corresponding to Ni and SiC in samples S1, S2, S3, S4, S5, and S6. Dispersion of graphene nanoplatelets in samples S4, S5 and S6 reveal the presence of carbon (C) (ICSD: 76767) peaks (except for sample S4) approximately at 2θ = 26.5° which is a clear indication of the presence of graphene nanoplatelets in the composite samples. The absence of C peak in sample S4 might be due to the very small amount of graphene nanoplatelets present in the selected powder sample for XRD characterization. Due to very minute amount, the C peak might not have been detected. It can also be observed that with the increase in the concentration of Ni; the intensity of the diffraction peaks corresponding to Ni increases. No distinct diffraction peaks other than Ni, SiC and C have been found in the XRD patterns which indicate the absence of any unwanted phase in the synthesized composites. 3.2 Morphological analysis
The FE-SEM micrographs corresponding to pure SiC, Ni and graphene nanoplatelets have been shown in Figure 2. As-received SiC particles having an average particle size of 52 µm display non-uniform morphological features in the FE-SEM micrographs as shown in Figure 2 (a). Non-uniform morphology with sharp edges is also a characteristic feature of SiC used which is distinctively evident from Figure 2 (a). The synthesized Ni particles display uniform spherical morphological features as depicted in Figure 2 (b). The average size of the Ni particles has been measured to be 1 µm and therefore, the synthesized Ni particles are termed as Ni microspheres. Figure 2 (c) indicates the FE-SEM micrograph of as-received graphene nanoplatelets. Graphene nanoplatelets have flake-like morphological features with 5 µm average size. The morphological analyses of all composites with and without graphene nanoplatelets addition have been shown in Figure 3. Ni/SiC composites containing Ni microspheres and nonuniform SiC particles for the samples S1, S2 and S3 are indicated in Figure 3 (a), (b) and (c), respectively. Figure 3 (b) and (c) show higher concentration of Ni microspheres as compared to Figure 3 (a) in the synthesized composites as per the experimental plan. Individual components viz. Ni, SiC and graphene nanoplatelets can be identified for the samples S4, S5 and S6 in Figure 3 (d), (e) and (f), respectively. The inset images show the enlarged view of a particular region in the synthesized samples. The Ni and graphene nanoplatelets in the FE-SEM micrographs have been represented by Ni and GP, respectively. The presence of graphene nanoplatelets in samples S4, S5 and S6 can be easily comprehended from the FE-SEM micrographs as shown in Figure 3 (d), (e) and (f). 3.3 Magnetic behavior The magnetic behaviors of the synthesized Ni/SiC and Ni/SiC/Graphene composites have been depicted in Figure 4. Figure 4 reflects that the value of saturation magnetization rises with the increase in Ni microsphere concentration in Ni/SiC and Ni/SiC/Graphene composites. Due to the increase in Ni microsphere concentration which is magnetic in nature, the value of saturation magnetization has increased considerably. The saturation magnetization of pure Ni microspheres has been observed to be 51.88 emu/g. It may be observed from the magnetization curves of Ni/SiC composites that the saturation magnetization has increased from 18.83 emu/g for sample S1 to 50.95 emu/g for sample S3. In the case of Ni/SiC/Graphene composites, the saturation magnetization has increased from 16.18 emu/g for sample S4 to 50.16 emu/g for sample S6. However, there is no noticeable change in the coercivity of the synthesized composites. The one-
to-one comparison of the synthesized composites with and without graphene nanoplatelet addition reveals that the magnetization properties are almost the same for similar variations in constituent concentration. This is in good agreement with the fact that the synthesized Ni/SiC composites with and without graphene nanoplatelets have almost similar concentration variations. 3.4 Dielectric analysis Figure 5 (a) reflects the real part of complex permittivity (εʹ) of the synthesized composite samples in 2-18 GHz frequency range. It is very clearly evident from Figure 5 (a) that with the increase in Ni microspheres concentration, the value of εʹ increases for samples S2 and S3 as compared to sample S1 which has a lower concentration of Ni microspheres. It may be further observed from Figure 5 (a) that with the dispersion of graphene nanoplatelets, the values of εʹ have increased considerably which can be visualized from the one-to-one comparison of the samples viz. S1 & S4; S2 & S5 and S3 & S6. The presence of Ni and graphene nanoplatelets may contribute to the enhancement in the conductivity of composite systems. This contributes to the enhancement in the values of εʹ. The presence of particles with different electronegativity viz. SiC, Ni and C generate heterogeneous interfaces that entrap charge carriers. These entrapped charge carriers result in interfacial polarization. An increase in the concentration of Ni particles and dispersion of graphene nanoplatelets create several heterogeneous interfaces in the entire heterogeneous composite system. The presence of a large number of heterogeneous interfaces in alternating electric field aids in the accumulation of charges at the sharp edges of these interfaces resulting in enhanced interfacial polarization [2,10]. This phenomenon significantly contributes to the enhancement in the values of εʹ for Ni/SiC and Ni/SiC/Graphene composites. The imaginary part of complex permittivity (εʺ) for Ni/SiC and Ni/SiC/Graphene composites in the investigated 2-18 GHz frequency range has been shown in Figure 5 (b). It is evident from Figure 5 (b) that with the increase in Ni concentration for Ni/SiC composites with and without graphene nanoplatelets, the values of εʺ increases. This increase conforms to the free-electron theory given by eqn. (1). According to this theory, εʺ increases with the increase in conductivity which increases the ohmic loses due to the nomadic charge transfer [10,11]. εʺ = 1 / (2πεoρf) where, ρ and εo represent electrical resistivity and free space permittivity, respectively.
(1)
The increase in Ni content may form conductive networks in the composite system for the hopping of free electrons from one particle to another. The conductive networks increase the conductivity of the Ni/SiC system and correspondingly the εʺ value increases. For the case of Ni/SiC/Graphene composites, apart from the hopping of electrons from one Ni particle to another Ni particle, there is hopping of ᴫ-ᴫ electrons due to the dispersion of graphene nanoplatelets in the heterogeneous composite system [12]. The presence of a large number of conducting particles also creates number of heterogeneous interfaces which increases the interfacial polarization and hence, εʺ of the system increases. The nature of the real part of complex permeability (µʹ) in 2-18 GHz frequency range has been presented in Figure 5 (c). The nature of µʹ reflects a complex trend with the increase in Ni concentration and graphene addition in the composites. It may be observed from Figure 5 (c) that µʹ has followed a decreasing nature with the increase in frequency for most of the samples. Figure 5 (d) represents the nature of imaginary part of complex permeability (µʺ) for the synthesized composites in 2-18 GHz frequency range. Ni/SiC and Ni/SiC/Graphene composites depict higher values of µʺ in 2-8 GHz frequency range as compared to the pure SiC system (except for S1). It may be observed that the trend for µʺ is almost the inverse of εʺ. Such resonance characteristics in the nature of complex permittivity and permeability curves may arise due to the resonance-antiresonance behaviour [13] owing to the presence of magnetic Ni particles, dielectric SiC and graphene nanoplatelets in the composite systems. A slight negative trend in µʺ may be observed for all of the composites at higher frequency range as depicted in Figure 5 (d). The negative behavior may be associated with the magnetic energy being radiated out as electric energy in the presence of alternating electromagnetic field [14]. The energy conservation principle dictates that the negative values of µʺ should be represented by a loss of electrical energy [13]. The higher value of εʺ for the composites at higher frequencies where µʺ is negative conforms to this principle. 3.5 Microwave absorption behavior The microwave absorption behavior of the synthesized composites has been analyzed in 2-18 GHz frequency range with the help of reflection loss (RL) curves. The nature of RL curves representing the absorption behavior has been calculated by the transmission line theory using eqns. (2) & (3) [15,16] and the microwave absorption measurement technique has been clearly explained in [17,18].
RL = 20 log |(Zin- Zo)/ (Zin+ Zo)|
(2)
Zin = Zo (µr /εr)1/2tanh {j.(2πft/c) (µr.εr)1/2}
(3)
where, Zin and Zo represent the characteristic impedance of the absorber and free space, respectively. f represents the frequency, t represents the thickness of the absorber, c represents the speed of light. Figure 6 represents the minimum RL behavior of Ni/SiC and Ni/SiC/Graphene composites at the matching absorber thickness in the frequency range of 2-18 GHz. The minimum RL values and the corresponding -10 dB absorption bandwidth values of the synthesized composites have been shown in the inset of Figure 6. The absorption behavior of Ni/SiC and Ni/SiC/Graphene composites have followed an increasing trend with the increase in the concentration of Ni microspheres. From the one-to-one comparison of RL behavior of samples S1 & S4; S2 & S5 and S3 & S6, it may be pointed out that with the little addition of graphene nanoplatelets, the microwave absorption performance of Ni/SiC/Graphene composites has enhanced significantly. The minimum reflection loss indicating maximum absorption has been realized for sample S6 at 15.28 GHz with 1.9 mm absorber thickness. The corresponding RL value and -10 dB bandwidth has been measured to be -59.15 dB and 4.48 GHz, respectively which is significantly higher as compared to pure SiC and pure Ni microspheres samples values of -14.57 dB and -17.9 dB [19], respectively. Table 2 shows the microwave absorption characteristics of all synthesized composite samples. The microwave absorption performance depends on the synergistic effect of many absorption mechanisms and factors viz. dielectric relaxation, interfacial polarization, defects induced dipole polarization, magnetic losses, multiple scatterings and reflections, quarter-wavelength phase cancellation, impedance matching, etc [10]. The dielectric relaxation processes constitute an instrumental role in the microwave absorption behavior of the synthesized composites. The dielectric relaxation process is represented by the Cole-Cole curves [20]. Each characteristic semi-circle in the Cole-Cole curve represents one debye relaxation process. Debye relaxation theory is utilized to investigate the major sources for dielectric losses within the composite system upon interaction with the alternation electromagnetic fields. The Cole-Cole plots for the synthesized samples have been shown in Figure 7 (a-g). For pure SiC, two semi-circles are evident which corresponds to two Debye relaxation processes as depicted in Figure 7 (a). The Debye relaxation in pure SiC may be attributed to the presence of defects in the crystal. These defects may act as dipoles in alternating
electromagnetic field converting electromagnetic energy into thermal energy due to orientation polarization processes. It is evident from Figure 7 (a-g) that samples with a lower concentration of Ni microspheres viz. S1 and S4 have two semi-circles representing two debye relaxation processes. The samples with a higher concentration of Ni microspheres viz. S2, S3, S5, and S6 have three semi-circles representing three debye relaxation processes. Hence the samples with higher Ni content have shown enhanced absorption behavior. Increased number of semi-circles indicate the presence of various relaxation processes such as interfacial polarization, dipole polarization, defects induced polarization, etc. These relaxation processes aids in realizing enhanced microwave attenuation. It may also be noted for the composite samples that a long tail exists in between two semi-circles which indicates that apart from dielectric losses, conduction loss also contributes to microwave absorption. In addition to dielectric losses, magnetic losses also constitute an important absorption mechanism in the case of composite systems with magnetic components. It has been reported in various literature that domain wall movement, hysteresis loss, eddy current loss, exchange resonance, and natural resonance are the major sources for magnetic loss. The contribution of domain wall movement and hysteresis loss may be neglected in the gigahertz frequency region [21]. Exchange resonance is applicable for materials with particle size less than 100 nm [22]. Therefore, in the current case, only the loses arising due to eddy current and natural resonance are applicable. Magnetic loss due to eddy current effect is dominant when the value and characteristic nature of µʺ(µʹ)-2f-1 curve do not vary with frequency [21,22]. Figure 8 shows that the values of µʺ(µʹ)-2f-1 varies with increasing frequency. This signifies that the eddy current effect is not the only dominant magnetic loss mechanism in the present composites. The variation in the curve indicates that natural resonance may be the major source for magnetic loss in the synthesized composites. The quarter wavelength phase cancellation model has also been investigated for microwave absorption due to the geometrical factor. According to this model, destructive interference of the microwaves results in phase cancellation at the air-absorber interface which accounts for enhanced microwave absorption. The quarter wavelength model comes into play when the thickness of the absorber satisfies eqn. (4) which is mathematically expressed as [10]: tcal = [n c / 4fm√(|εrµr|)] (n =1,3,5,7,…)
(4)
where, tcal represents the calculated thickness of the absorber, n is the order of reflection and other symbols represent their usual meaning. The quarter wavelength model for phase cancellation has been depicted in Figure 9 (i), (ii) & (iii). It can be observed that the phase cancellation effect is not so dominant for pure SiC and composites with a low concentration of Ni as the matching thicknesses and calculated thicknesses are not in good agreement. However, composites with higher Ni concentration reflect a good agreement between the matching thicknesses and calculated thicknesses. This suggests that for samples S3, S5 and S6, the phase cancellation effect also contributes to microwave attenuation. The most important factor for realizing better absorption behavior is dictated by impedance matching of the system which depends on εr and µr values. Impedance matching indicates the extent to which microwave signals may travel into the interior of the absorber [16]. The εr and µr values should be close for realizing better impedance matching. |Zin/Zo| represents the normalized impedance matching factor given in eqn (5). The characteristic value of |Zin/Zo| indicates the extent to which the incident EM wave signal traverses into the interior of the absorber material by assessing the degree of closeness between the εr and µr values of the composite system. |Zin/ Zo| = (µr /εr)1/2tanh {j.(2πft/c) (µr.εr)1/2}
(5)
where, each symbol represents their usual meaning. Normalized impedance matching factor |Zin/Zo| should be close to 1 for good microwave absorption characteristics. It is evident from Figure 10 (a) that the impedance matching for samples having a higher concentration of Ni microspheres is closer to 1 as compared to the samples having a lower concentration of Ni microspheres. Best microwave absorption behavior has been observed for sample S6 owing to its excellent impedance matching while poor absorption performance has been shown by sample S1 owing to its low impedance matching characteristic. Another factor that dictates the absorption characteristics of microwave absorbers is the attenuation constant (α) in Np/m which is given by eqn. (6) [23,24]: α=
2 πf c
(µ ′′ε′′ − µ ′ε′) + (µ ′′ε′′ − µ ′ε′) 2 + (µ ′ε′′ − µ ′′ε′) 2
where, each symbol represents their usual meaning.
(6)
It represents the materials’ intrinsic ability to attenuate the incident microwave signals in the interior of the absorber material. Figure 10 (b) depicts the attenuation constant curves for the synthesized composite samples. It can be noted from Figure 10 (b) that the value of attenuation constant for samples S5 and S6 is high along with good impedance matching and these favorable characteristics is also reflected in their enhanced microwave absorption behavior. This trend is the same for all the synthesized composites and ascertains the fact that impedance matching factor and attenuation constant work in tandem in realizing enhanced microwave absorption behavior. Apart from the discussed mechanisms, defects induced polarization and interfacial polarization also affects the absorption behavior. With the increase in Ni content and graphene nanoplatelet dispersion, the number of heterogeneous surfaces in the composite system increases; thereby increasing the interfacial polarization due to the trapping of the charges. The storage of charges due to the capacitive nature of the composite owing to the presence of conducting and dielectric system also contributes to microwave absorption. The increase in the electric path due to multiple scattering and reflections may also aid in microwave absorption. All these mechanisms work synergistically in enhancing the microwave absorption performance Ni/SiC and Ni/SiC/Graphene composites and have been illustrated in Figure 11. A comparison of the present work with Ni-based absorbers available in various literature based on microwave absorption characteristics has been presented in Table 3. Ni/SiC/Graphene composites show excellent microwave absorption characteristics as compared to the reported Ni-based absorbers.
4. Conclusions In summary, Ni/SiC and Ni/SiC/Graphene composites have been successfully synthesized through hydrothermal route for achieving enhanced microwave absorption. The investigation of absorption behavior reflects that the graphene addition in Ni/SiC composites exhibits significant enhancement in the attenuation performance. The investigation also reflects that the microwave absorption behavior is significantly enhanced with the increase in the concentration of Ni microspheres in the composite system. The minimum value of RL for composites without graphene nanoplatelets has been observed for composite sample with SiC:Ni = 1:4. The corresponding RL value and -10 dB bandwidth have been observed to be -30.63 dB at 1.9 mm absorber thickness and 3.36 GHz, respectively. With the addition of graphene nanoplatelets, a minimum RL value of -59.15 dB has been realized for the sample with SiC:Ni = 1:4 and 2.5
wt.% graphene nanoplatelets. The corresponding -10 dB absorption bandwidth has been observed to be 4.48 GHz at 1.9 mm. The results indicate that Ni/SiC/Graphene nanoplatelets, as well as Ni/SiC composites, may prove to be encouraging candidates for realizing efficient and low-cost microwave absorbers for practical applications.
Acknowledgments The authors, Samarjit Singh, and Anil Kumar Maurya express their sincere gratitude to the Ministry of Human Resources and Development (MHRD), India, for providing fellowships. The authors are also thankful to the Department of Electronics and Computer Science, Indian Institute of Technology Roorkee, India for extending the facility of Vector Network Analyzer for dielectric measurements.
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Figure 1: X-ray diffraction patterns of synthesized samples. Figure 2: FE-SEM micrographs (a) Pure SiC; (b) Pure Ni and (c) Pure graphene nanoplatelets. Figure 3: FE-SEM micrographs (a) SiC:Ni = 4:1; (b) SiC:Ni = 1:1; (c) SiC:Ni = 1:4; (d) (SiC:Ni)/2.5 wt.% graphene = 4:1; (e) (SiC:Ni)/2.5 wt.% graphene = 1:1 and (f) (SiC:Ni)/2.5 wt.% graphene = 1:4 Figure 4: Magnetic hysteresis loops of the synthesized composite samples. Figure 5: Variation in (a) real part of complex permittivity; (b) imaginary part of complex permittivity; (c) real part of complex permeability; (d) imaginary part of complex permeability for the prepared samples in 2-18 GHz range. Figure 6: Microwave absorption behavior of the synthesized composite samples in 2-18 GHz range. Figure 7: Cole-Cole plots representing Debye relaxation processes for the synthesized composites. Figure 8: Eddy current effect plot representing dominant magnetic loss mechanism for the synthesized composites. Figure 9(i): Dependence of quarter wavelength (λ/4) thickness on frequency for the synthesized composite samples (a) Pure SiC; (b) S1and (c) S2. Figure 9(ii): Dependence of quarter wavelength (λ/4) thickness on frequency for the synthesized composite samples (d) S3 and (e) S4. Figure 9(iii): Dependence of quarter wavelength (λ/4) thickness on frequency for the synthesized composite samples (f) S5 and (g) S6. Figure 10: Variation of (a) Impedance matching at matching absorber thickness and (b) Attenuation constant curves for the synthesized composites in 2-18 GHz range. Figure 11: Schematic representation of electromagnetic wave interactions with the heterogeneous structure of Ni/SiC/Graphene nanoplatelets based composites illustrating various attenuation mechanisms.
Table 1: Nomenclature of synthesized composites Sample code
Composition
S1
SiC:Ni= 4:1
S2
SiC:Ni= 1:1
S3
SiC:Ni= 1:4
S4
SiC:Ni= 4:1 with 2.5 wt.% Graphene nanoplatelets
S5
SiC:Ni= 1:1 with 2.5 wt.% Graphene nanoplatelets
S6
SiC:Ni= 1:4 with 2.5 wt.% Graphene nanoplatelets
Table 2. Reflection loss measurements for Ni/SiC and Ni/SiC/Graphene nanoplatelets based composites.
Sample
Min.
Code
RL (dB)
Pure SiC
Matching
Frequency
Thickness
Bandwidth corresponding
(mm)
(GHz)
to -10 dB (GHz)
-14.57
2.1
17.5
2.28
S1
-10.15
2.2
17.44
0.64
S2
-20.36
2.0
16.4
3.12
S3
-30.63
1.9
15.84
3.36
S4
-20.33
1.9
17.68
2.08
S5
-38.78
1.9
14.8
4.32
S6
-59.15
1.9
15.28
4.48
Table 3. Microwave absorption characteristics of Ni-based absorbers.
Composition
Min. RL value (dB)
Matching thickness (mm)
Bandwidth corresponding to -10 dB
Reference
(GHz)
Ni sub-microsphere
-17.9
1.2
-
[19]
rGO–Ni ([email protected] wt%)
-23.3
3
1.8
[25]
Ni nanoparticles
-14.9
3
-
[26]
Ni(C)
-32.0
2
4.2
[27]
Ni/SnO2 microspheres
-18.6
7
-
[28]
Ni/graphene
-13.0
2
-
[29]
Hexagonal Ni/graphene
-17.8
5
-
[30]
Ni nanowires
-8.5
3
-
[31]
Ni/SiC
-28.54
-
3.57
[32]
SiC:Ni = 1:4
-30.63
1.9
3.36
Present Work
-38.78
1.9
4.32
Present Work
-59.15
1.9
4.48
Present Work
SiC:Ni =1:1 and 2.5 wt.% graphene nanoplatelets SiC:Ni =1:4 and 2.5 wt.% graphene nanoplatelets
Highlights: • • •
Effect of Ni microspheres concentration on microwave absorption behavior of Ni/SiC composites has been studied. Effect of graphene nanoplatelets dispersion on microwave absorption behavior of Ni/SiC composites has also been analyzed. Minimum RL of -59.15 dB at 15.28 GHz was obtained for Ni/SiC/Graphene nanoplatelets at 1.9 mm thickness with corresponding -10 dB bandwidth of 4.48 GHz.
Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Corresponding authors on behalf of all authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. (Abhishek Kumar)