Enhanced absorption performance of carbon nanostructure based

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Enhanced absorption performance of carbon nanostructure based metamaterials and tuning impedance matching behavior by an external AC electric field Reza Gholipur, Zahra Khorshidi, and Ali Bahari ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b02270 • Publication Date (Web): 27 Mar 2017 Downloaded from http://pubs.acs.org on March 28, 2017

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ACS Applied Materials & Interfaces

Enhanced Absorption Performance of Carbon Nanostructure Based Metamaterials and Tuning Impedance Matching Behavior by an External AC Electric Field Reza Gholipur*, Zahra Khorshidi, Ali Bahari Department of Solid State Physics, University of Mazandaran, Babolsar, 4741695447, Iran *Email: [email protected] Abstract Metamaterials have surprisingly broadened the range of available practical applications in new devices such as shielding, microwave absorbing, and novel antenna. More researches are related to the tuning DNG frequency bands of ordered or disordered metamaterials, and far less research has focused on the importance of impedance matching behavior, and is not effort and attention in adjusting the magnitude of negative permittivity values. This is particularly important if devices deal with low amplitude signals such as radio or TV antenna. The carbon/hafnium nickel oxide (C/Hf0.9Ni0.1Oy) nanocomposites with simultaneously negative permittivity and negative permeability, excellent metamaterial performance and good impedance matching could become an efficient alternative for the ordered metamaterials in wave-transparent, microwave absorbing, and solar energy harvesting fields. In this study, we prepared C/Hf0.9Ni0.1Oy nanocomposites by solvothermal method, and we clarified how the impedance matching and double negative (DNG) behaviors of C/Hf0.9Ni0.1Oy can be tuned by an external AC electric field created by electric quadrupole system. External electric field allows for the alignment of the well-dispersed nanoparticles of carbon with long-range orientations order. We believe that this finding broadens our understanding of moderate conductive material-based random metamaterials (MCMRMs), and provides a novel strategy for replacing high loss ordered or disordered metamaterials with MCMRMs.

Keywords: C/Hf0.9Ni0.1Oy nanocomposites; Impedance matching; Simultaneously negative permittivity and negative permeability; Disordered metamaterials; Electric quadrupole system.

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1. Introduction The respective significant properties of metamaterials can make them fascinating candidates for the development of exciting and impacting technology, and practical applications such as high resolution imaging,1,2 absorbers,3,4 perfect lens,5,6 invisibility cloaks,7 high precision lithography,8 optical analog simulators,9,10 quantum levitation,11 and small antennas.12 So far, many researches and efforts have been undertaken to develop practical applications and synthesis of DNG metamaterials.13-23 Overall, the preparation of metamaterials can be divided categories: (1) ordered (2) disordered metamaterials. Fabrication of exactly arranged periodic metamaterials requires both the reasonable design, and precise engineering of their structure and components.24,25 The effective complex permeability ( µ e = µ e' + iµe'' ) and permittivity ( ε e = ε e' + iε e'' ) of metamaterials can be controlled and tuned by changing the design of their structure which includes their arrangement, shape, size, geometry, and orientation that could affect electromagnetic wave in an unusual manner.26-29 One of the main advantages of disordered or random metamaterials are so-called “intrinsic metamaterials”, is their ability to provide DNG behavior using the chemical composition and microstructure of the material, and without arranged periodic blocks and the need to patterning techniques such as electron beam lithography to pattern the unit cells.30 These techniques are slow, expensive, and complicated, which prevent these structures for mass production. In order to overcome these weaknesses and make wider the application fields of disordered metamaterials, the studies it is becoming a hot topic. In recent years, the development of disordered metamaterials has primarily focused on using Ag, Ni, Fe, and Cu nanoparticles in dielectric medium that provides enormous negative permittivity values because to high carrier concentration (1022-1023 cm-3).31-34 The theoretically specific significance is adjusting the magnitude of simultaneously negative permittivity and permeability. This is particularly important if devices deal with low amplitude signals such as radio or TV antenna. At this time, more researches are related to the tuning DNG frequency bands and reducing losses of ordered or disordered metamaterials, and is not effort and attention in adjusting the magnitude of negative permittivity,35-42 and far less research has focused on the importance of MCMRMs with impedance matching behavior, which provide power efficiency of the light propagating inside metamaterial and device protection.43 2


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A mismatch between the permittivity and the permeability hampers the rational application in fields such as microwave attenuator, wave-transparent, microwave absorbing, and solar energy harvesting, and also causes reflected power of radio frequency (RF) circuit builds standing waves.44-46 Researches in carbon based composites with DNG property have developed slowly, although these composites can practically be synthesized with alternative techniques that allow mass production.47-52 It should be noted that these researches paid no attention to the impedance matching behavior. Nanocomposites based on graphene, carbon nanotube, carbon fiber, and carbon sources that provide weakly negative permittivity values can create a lower-loss medium than Ag and Au-based metamaterials.53-57 This purpose allows µ e / ε e ratio in the formula Z = Z 0 ( µ e / ε e )1/ 2 to approach unit, which produces an impedance matching behavior, gives a low-loss electric and magnetic response to incident electromagnetic waves.58,59 Therefore, MCMRMs with low or moderate carrier concentration will have been one of the promising composites in the evolution of metamaterial devices. In point of view percolation theory, when the concentration of carbon nanoparticles at semicontinuos metallic composites is more than the percolation threshold (pc), the network structure of carbon nanoparticles at Hf0.9Ni0.1Oy host causes continuous conducting paths are formed associated with increase electrical conductivity due to the number of new paths and metal-insulator transition in the composite. The purpose of this study is to describe the DNG and impedance matching behaviors of C/Hf0.9Ni0.1Oy nanocomposites with various mole values of H2BDC as carbon source by experimental descriptions of structural, electrical and magnetic properties. Then, we proposed an interesting simple method for tuning of DNG and impedance matching behaviors of C/Hf0.9Ni0.1Oy nanocomposite by an AC electric field created by an electric quadrupole system. It is currently widely known that the conductive network and moderate carrier concentration are responsible for the weakly negative permittivity, and the concentration of conduction electrons and thus metallic properties in C/Hf0.9Ni0.1Oy can be easily controlled by carbon content in the network. Here, we attribute negative permittivity and negative permeability of C/Hf0.9Ni0.1Oy nanocomposites to the plasma oscillation of the conduction electron, and eddy currents. The reason why magnetic properties depend on the alignment of the well-dispersed nanoparticles and AC electric field is that the current paths can be paralleled and number of ordered cell units 3



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increase and consequently longitudinal cross-sectional area of cell units and so its magnetic flux and moment increase. Further increase of the magnetic moment leads to increased diamagnetic response and the C/Hf0.9Ni0.1Oy nanocomposite will exhibit good impedance matching and DNG behaviors. This achievement in realizing DNG behavior of C/Hf0.9Ni0.1Oy nanocomposites with an impedance matching has not been explored until now, which can be compared with high conductive material-based metamaterials for new constructions and practical applications.

2. Experimental The experimental section consists of two steps: (1) synthesis of C/Hf0.9Ni0.1 samples (2) application of an AC electric field to CHN7 sample during synthesis. (1) C/Hf0.9Ni0.1 samples were prepared by solvothermal method. Hafnium(IV) chloride [HfCl4], and nickel(II) acetylacetonate [Ni(C5H7O2)2] were used as Hf and Ni sources, respectively, and 1,4-benzenedicarboxylic acid (H2BDC) [C8H6O4], and N,N-dimethylformamide (DMF) [C3H7ON] were used as organic ligand and solvent, respectively. In a typical procedure, 0.01 mol HfCl4, 0.001 mole Ni(C5H7O2)2, and 0.001 mole H2BDC were dissolved in 50 mL DMF, and the solution stirred for around 10 min using a magnetic stirrer at room temperature to get a transparent solution. Afterward, the solution was poured into a pyrex lab dish and sonicated for 60 min, which prevents the agglomeration. Then the final solution was placed in teflon-lined stainless steel autoclave, and heated at 120 °C for 72 h. The final product was cooled at temperature room, and washed with DMF, and then dried at 40 °C for about 30 h. In order to remove excess H2BDC and DMF impurities, the sample was heated at 300 °C for 24 h. Thereafter, dried product was heat treated at 600 °C for 5 h in N2 atmosphere with a heating rate of 5 °C.min-1 to obtain C/Hf0.9Ni0.1 nanocomposite, and then C/Hf0.9Ni0.1 powder was shaped as pellet and annular specimens.39 A series of C/Hf0.9Ni0.1 nanocomposites with various mole values of H2BDC (0.0003, 0.0005, 0.001, 0.0015, 0.002, 0.0025, and 0.003) were synthesized under the same condition, which were referred as CHN1, CHN2, CHN3, CHN4, CHN5, CHN6, and CHN7. (2) This step is performed in preparation of other three samples under equal conditions that the only difference is application time of the electric field. The AC voltage of 10 V and frequency of 1 MHz were selected according to findings by Fan et al.60 CHN7 solution sonicated for 60 min at 4


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the previous step was placed in the center of an electric quadrupole system (Fig. 1) under application times of 6, 9, and 12 s. These times were selected in practice experimentally. The continuation of the procedure is the same as the previous step. Samples subjected to an electric field with application times 6, 9, and 12 were named as CHN7-6, CHN7-9, and CHN7-12, respectively. Crystal structure analysis of C/Hf0.9Ni0.1 nanocomposites was performed by x-ray diffraction (XRD) using Cu Kα ( λ = 1.5408 Å) radiation (GBC-MMA007 (2000)). The surface morphology and microstructure of C/Hf0.9Ni0.1 nanocomposites were studied and analyzed by scanning electron microscope (SEM) (XL30-PHILIPS). Measurements of the complex permeability and permittivity were obtained using the “parallel plate capacitor” and “inductance” theories, respectively, at frequency range of 0.01-1 GHz. These measurements were made using an LCR meter, and complex permittivity and permeability are obtained according to findings by Yan et

al.42

Fig. 1. Image of an AC electric quadrupole system in the experiment.

3. Results and Discussion Fig. 2 shows the element maps for C/Hf0.9Ni0.1Oy matrix. Here, we show that there are no secondary phases, and all of the elements are well distributed in the matrix.

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Fig. 2. Elements mapping of surface morphology of C/Hf0.9Ni0.1Oy nanocomposite. (a) BSE image; (b)-(e) C, Hf, Ni, and O distributions, respectively; (f) distributions of C, Hf, Ni, and O.

Fig. 3. XRD patterns of: (a) C/Hf0.9Ni0.1Oy samples; (b) CHN7 sample subjected to external AC electric field. Fig. 3a shows the XRD patterns of the C/Hf0.9Ni0.1Oy samples. The six main diffraction peaks in the 2θ range of 20-60° originate from the (110), (111), (112), (113), (114), and (200) reflection planes of the Hf0.9Ni0.1Oy structure identified by JCPDS database (00-026-0733) are typical of this Hf3NiO phase. XRD results show a decrease in nanocrystalline size by the blunting and 6


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broadening of the diffraction peaks. Nanocrystalline size (D) is determined using DebyeScherrer’s formula ( 0.94λ / β cos θ ) where λ = 0.154056 nm and β is the high full width at half maximum (FWHM) in radian. Clearly, reflection planes FWHM of CHN7 sample are bigger than other samples. With increasing H2BDC concentration, some peaks somewhat shift toward higher 2 θ , and their FWHM increases that demonstrate carbon are successfully incorporated into the host lattice. It is therefore concluded that for CHN7 sample, “unit cell” volume decreases because amorphous properties of carbon causes “lattice constant” and consequently the “unit cell” volume decrease with increasing H2BDC concentration. The reduction of “unit cell” volume is associated with the shift of peak position towards larger 2θ. The “lattice constant” values for that for each peak of each C/Hf0.9Ni0.1Oy sample is calculated by using the formula: a0 = d(h + k + l)1/2 (where h, k and

l are miller indices of the crystal planes and d is interplanar spacing) for (112) reflection plane of CHN1, CHN2, CHN3, CHN4, CHN5, CHN6, and CHN7 samples are equal to 0.6246, 0.6240, 0.6238, 0.6232, 0.6229, 0.6226, and 0.6219 nm, respectively. As is clear from Fig. 3b, applying an AC electric field with application time to 9 s, increases somewhat intensity of planes which could be attributed to form intertwined structure, and then decrease for application time to 12 s. It is concluded that applying electric field decreases the average size of CHN7 nanocrystallites. The “lattice constant” values for CHN7, CHN7-6, CHN7-9, and CHN7-12 for (112) reflection plane are equal to 0.6219, 0.6209, 0.6194, and 0.6212 nm, respectively. The surface morphology of C/Hf0.9Ni0.1Oy samples with different H2BDC contents was characterized by SEM as shown in Fig. 4. The CHN1 (Fig. 4a) shows a porous morphology with nonuniform and rough surfaces that its pores are inhomogeneous with sizes in the range between 20-150 nm. The CHN3 sample (Fig. 4b) consists of lower pores than CHN1 sample with sizes in the range 20-100 nm that have relatively rough surfaces, which suggest CHN3 is unable to create the continuous electric current and percolation phenomena. It is shown that with increasing H2BDC content, the size of the nanocrystallites and pores becomes smaller, and they get a good contact. Amount of 0.003 mol of H2BDC (Fig. 4d) allows the development and creation of more uniform and interconnected surface morphology without pore, also suggesting good interfacial contacts have formed between surface nanocrystallites. SEM image of CHN7-9 sample in Fig. 4e

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shows that with the application an AC electric field structure reveals intertwined morphology of nanocrystallites that causes the crystalline-crystalline gap is extended.

Fig. 4. SEM images of: (a) CHN1; (b) CHN3; (c) CHN4; (d) CHN7; (e) CHN7-9 samples. . 8


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Fig. 5. Frequency dependence of: (a)-(c) real parts; (d)-(f) imaginary parts of the effective dielectric constant of C/Hf0.9Ni0.1Oy samples and CHN7 sample subjected to external AC electric field.

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The curves of the real and imaginary parts of the effective dielectric constant ( ε e ) with respect to frequency of C/Hf0.9Ni0.1Oy samples and CHN7 sample subjected to the external AC electric field are shown in Fig. 5. As shown in Fig. 5a, positive ε e' of CHN1-CHN3 samples slowly decrease with increasing frequency and increase with higher H2BDC concentration because of the localized electrons polarizations and the increase of carbon- Hf0.9Ni0.1Oy interface area. It is observed that a further increase of carbon content, leads to sign change of ε e' (above pc), and continuous conducting paths between the ends of a sample are created (Fig. 6). We observe that CHN7 sample shows low negative values of ε e' in the frequency range 0.011 GHz because moderate concentration of low conduction electrons in CHN7 sample which is important for good impedance matching behavior. More negative ε e' indicates less role of electric dipoles in CHN7 medium which signifies that the sample has more metal-like property. Negative ε e' occurs when the frequency of the applied electromagnetic wave is less than the plasma frequency ( ω p ). The behavior of the ε e' of CHN7 sample is well fitted to the frequency dispersion formula of the Drude model ( 1 − ω 2p /(ω 2 + iωγ )) where γ is related to energy losses (damping constant), suggesting ω p = 3.2 GHz. Figs. 5d-e exhibit the changes in the imaginary part of the ε e of samples below and above the percolation threshold on frequency, respectively. The ε e'' of samples below the percolation threshold increases as the H2BDC content increases because of increasing dielectric and leakage conductance losses. As Fig. 5e exhibits, at low frequency region below 0.2 GHz, ε e'' decreases with a much reduced slope which could be attributed to hopping, conductance, and leakage current losses. As carbon content increase, ε e'' of samples increases; i.e. interconnected network of structure and increased current paths lead to difficult movement of charged particles in the medium and so increased loss. Polarization disorders at the interface of the C/Hf0.9Ni0.1Oy crystallites can also play the role of the losses and energy barriers for the charge carriers. We are also concerned to study the effect of electric field on permittivity behavior that is shown in Fig. 5c. As application time of electric field increases to 9 s, real part of the ε e of CHN7 sample becomes more positive because more formed parallel rows which decrease the inductance 10


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behavior in a sample medium. In other words, more positive ε e' indicates more role of electric dipole in a medium which signifies that the sample has more capacitance-like property. As shown in Fig. 5c, increased application time of the electric field to 9 s leads to reduced plasma frequency. As application time increases to 12 s, the ε e' tends to positive region because the interconnected network and ordered structure of sample destroy, and electric dipole moments in the medium also increase. As is clear from Fig. 5f, with increasing application to 9 s, ε e'' of samples decreases and afterward increases.

Fig. 6. The schematic image of 2D continuous conducting path between the ends of CHN7 sample.

Fig. 7a-b shows the changes in real and imaginary parts of effective permeability ( µ e ) of C/Hf0.9Ni0.1Oy samples with respect to frequency. Carbon content has a very influential role on magnetization. Therefore, we should expect the magnetic properties of C/Hf0.9Ni0.1Oy samples to change considerably with increasing C content. The µ e' stays almost constant up to a certain frequency, afterward µ e' begins to decrease.

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Fig. 7. Plots of: (a) real parts; (b) imaginary parts of the effective magnetic permeability of C/Hf0.9Ni0.1Oy samples; (c) real parts; (d) imaginary parts of the effective magnetic permeability of CHN7 sample subjected to external AC electric field. As shown in Fig. 7a, CHN7 sample has a more negative permeability than other sample which could be attributed to its carbon content. CHN7 sample can include a network of unit cells, so that the flux density of electromagnetic wave passing from unit cells increases consequently causes increase of the current loops, and as a result inductive behavior, Eddy currents and diamagnetic response of CHN7 sample arising from Lenz’s law are higher than other samples. The tunneling process can play a role in the creation of magnetization in the CHN7 when the charge carriers pass through the interfaces of the CHN7 grains. Changes in the imaginary part of permeability in terms of frequency are presented in Fig. 7b. The peak of µe'' corresponding to the dispersion of µ e' shifts to higher frequency as the H2BDC content reaches 0.003 mol. Fig. 7c-d shows the plots of real and imaginary parts of µe of CHN7 sample subjected to external AC 12


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electric field as a function of frequency. As is clear from Fig. 7c, with increasing application time to 9 s, µ e' becomes more negative because of the alignment of carbon nanoparticles and improvement of their interconnection. When the sample is subjected to an AC electric field, an interconnected network of unit cells could be formed in the CHN7 sample so that amount of unit cells increases in CHN7-9 sample than other samples, and as a result, the flux density, inductance, and Eddy currents increases. As application time increases to 9 s, µ e'' peak shifts to the high frequency region, and then µe'' of CHN7-12 sample shows different behavior than other samples. It is observed that µe’ of CHN7-9 sample is close to its permittivity values that reveal CHN7-9 sample possesses good impedance matching.

Fig. 8. Room-temperature magnetic field dependence of the mass magnetization of: (a) C/Hf0.9Ni0.1Oy samples; (b) CHN7 sample subjected to external AC electric field. Fig. 8 shows room-temperature magnetic field dependence of the mass magnetization to investigate the magnetic properties and hysteretic behavior of the CHN4-CHN7 samples and CHN7 sample subjected to external AC electric field in the field range of ±8 kOe. S-shaped mass magnetization plots reveal that the C/Hf0.9Ni0.1Oy samples above the percolation threshold have ferromagnetic character (Fig. 8a). It is noted with increasing H2BDC content hysteresis loop, saturation magnetization, and the coercivity increase. Clearly CHN4-CHN6 samples do not achieve full saturation due to lack of resonance, and contrary to CHN7 sample, they display a negligible hysteresis. Fig. 8b shows the magnetic hysteresis loops of the CHN7 sample subjected 13



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to an external AC electric field that revealed saturation magnetization, coercivity, and remanence values of CHN7 increase to application time 9 s, and then decrease for application time 12 s.

Fig. 9. Region corresponding to the simultaneously negative permittivity and permeability of: (a) CHN7; (b) CHN7-6; (c) CHN7-9; (d) CHN7-12 samples.

Table 1. The frequency range of DNG regions for CHN7 sample and CHN7 sample subjected to external AC electric field. Sample Frequency range of DNG regions (MHz) CHN7 CHN7-6 CHN7-9 CHN7-

210 212 242 190 14


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Fig. 9 demonstrates DNG regions with simultaneous negative permittivity and negative permeability of CHN7 sample and CHN7 sample subjected to the external AC electric field. As Fig. 9 shows, regional area of simultaneously negative µe' and ε e' increases with increasing application time to 9 s. The frequency range of DNG regions is listed in Table 1. The results reveal that the application of an AC electric field has been successful in the improvement of DNG behavior.

Fig. 10. (µe/ԑe)1/2 values as function of frequency of DNG regions. Fig. 10 demonstrates the r = ( µ e / ε e ) 1 / 2 changes of CHN7 sample subjected to external AC electric field on the basis of application time of an AC electric field. Fig. 10 shows that r increases with increasing application time of the electric field effect to 9 s and then decreases for CHN7-12 sample. This could be explained as electric field caused the nanoparticles to be set in ordered and parallel rows and reach a regular arrangement. In this situation, the interface between the nanoparticles and the dielectric medium in these samples increases after applying the electric field up to application time 9 s. Increased interface surface creates more electric dipole moments in the medium which brings about reduced the plasma frequency of samples. As shown in Fig. 10, application time of the electric field effect to 9 s leads to approach r to unit, which the incident electromagnetic wave can enter this sample.

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Fig. 11 shows the imaginary part of impedance or reactance (Z") of C/Hf0.9Ni0.1Oy samples and CHN7 sample subjected to the external AC electric field with respect to the frequency that give us important comprehension with respect to structural, magnetic, and electrical properties. In general, Z" are dependent on the inductance and capacitive characteristics of the sample. For samples below the percolation threshold, Z" is negative in the whole frequency region, which these samples present capacitive characters, and phase of the voltage lags behind the phase of currents. It is observed that with increasing H2BDC content, the Z" value reduces at the whole frequency region. Decrement of Z" could be attributed to a decrease of capacitive properties since the interconnection of carbon nanoparticles at the network decrease capacitive properties. Sign Z" for CHN4-CHN7 samples in total frequency range is positive which manifests an inductive behavior. It other words, samples with Z"> 0 are above their percolation threshold, and current loops induced in the sample network by electromagnetic wave, lead to the inductive character. Positive sign of Z" shows that there is a lag of phase between the current inside and the applied voltage of the CHN4-CHN7 samples. As shown in Fig. 11b, with increment of application time to 9 s, the Z" value increases and then decreases in application time 12 s. This behavior could be attributed to inductance properties and decrease conductive so that with increasing application time the flux density increases, and average velocity of charges movement decreases.

Fig. 11. Frequency dependence of reactance for: (a) C/Hf0.9Ni0.1Oy samples; (b) CHN7 sample subjected to external AC electric field. 16


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Fig. 12. Frequency dispersions of ac conductivity for: (a)-(b) C/Hf0.9Ni0.1Oy samples; (c) CHN7 sample subjected to external AC electric field.

Fig. 12 shows frequency dispersions of AC conductivity (Gp) for C/Hf0.9Ni0.1Oy samples and CHN7 sample subjected to the external AC electric field with respect to frequency. As shown in Fig. 12a, Gp for CHN1 sample stays almost flat over the frequency range studied but at CHN2 and CHN3 samples the Gp value goes on increasing, and the frequency relation of these samples below the percolation threshold can change according to power law of Gp ≈ (2πν ) n characterizing hopping conduction behavior. As shown in Fig. 12b, at low frequency region, the Gp has an almost constant value and then it decreases with increase in frequency. With increasing H2BDC content, the Gp increases because metal-like conductivity (or resistivity) increases (or decreases) that increases the possibility of 17



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reaching electrons grain boundaries that consequently reduce damping constant and losses, and increases the displacement current. The Gp value of CHN7 sample, which the interconnectivity of carbon nanoparticles exceeds its percolation threshold, is the highest value as compared to other sample in the whole frequency region. It is observed, the Gp decreases with frequency characterizing metal-like conductivity. This conductivity is defined as Gp ∝ 1 / δ 2ωµ , where δ is penetration depth. Free charges of CHN7 in comparison with other sample above the percolation threshold are most affected by the medium as if their masses increased. A sufficiently low Gp is one of the indispensable requirements for magnetic resonance. As is clear from Fig. 12c, the Gp value of CHN7 sample subjected to external AC electric field decreases with an increment of application time to 9 s and then it increases for application time to 12 s. It could be concluded the charge carriers in the ordered structure is more under the effect of the sample medium as if their masses increased.

Fig. 13. Extinction coefficient of: (a) C/Hf0.9Ni0.1Oy samples; (b) CHN7 sample subjected to external AC electric field.

Fig. 13 plots the values of extinction coefficient (k) for C/Hf0.9Ni0.1Oy samples and CHN7 sample subjected to an external AC electric field that calculated by following formula:61

k=

µe′′ε e′ + µe′ε e′′ 2 ( ε e µe + µe′ε e′ − µe′′ε e′′)1/ 2

(1)

18


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From Fig. 13a one can observe the ݇ of the samples below percolation threshold increases with increasing frequency. As Fig. 13a shows with increasing H2BDC concentration k significantly increases, and k plot shifts towards higher frequency region that may be caused by metal-like properties of the sample and so increased absorption of electromagnetic waves by sample. The intensity of extinction coefficient for C/Hf0.9Ni0.1Oy samples above the percolation threshold also decreases with frequency. At lower frequencies, higher k and energy absorption can be observed because more energy is needed for surface plasmon resonance. As is clear from Fig. 13b, with increasing of application time to 9 s, k value decreases, and shifts towards the lower frequency region because capacitance-like behavior that shows reduce of electromagnetic waves scattering.

Fig. 14. Reflectance spectra of: (a) C/Hf0.9Ni0.1Oy samples; (b) CHN7 sample subjected to external AC electric field.

Fig. 14 shows reflectance spectra (R) of C/Hf0.9Ni0.1Oy samples and CHN7 sample subjected to external AC electric field calculated by formula as follows:62

(n'−1) 2 + k 2 R= (n'+1) 2 + k 2

(2)

where n´ calculated by the formula as follows:61

n′ =

±1 (ε µ + µe′ε e′ − ε e′′µe′′)1/ 2 2

(3)

It is obvious that the reflectance for C/Hf0.9Ni0.1Oy samples decreases with increasing frequency. As seen in Fig. 14a, the incorporation of carbon dopants with concentration > pc into the host 19



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Hf0.9Ni0.1Oy leads to the increase of the reflectance, this indicates that a strong interaction between electromagnetic waves and carbon dopants. As is clear from Fig. 14b, increment application time to 9 s causes R decreases, and the reflectance of CHN7-9 sample is least, indicating an impedance matching of the incident electromagnetic wave.

Fig. 15. Optical absorption coefficient of: (a) C/Hf0.9Ni0.1Oy samples; (b) CHN7 sample subjected to external AC electric field.

The calculated k values of C/Hf0.9Ni0.1Oy samples and CHN7 sample subjected to the external AC electric field are further utilized to calculate the absorption coefficient ( α = 4πkν / c ) where c is the speed of light in vacuum. As shown in Fig. 15a, α increases with increasing H2BDC concentration, and peak of α shifts towards higher frequency region. On the other hand, H2BDC content leads to a low penetration depth which could be attributed to grain size, carrier concentration, lattices train, size effect so that sample tends to absorb and save more energy. The

α for CHN7 sample has more value compared to other samples which is the reason for its metal-like property and reveals the reason for its less penetration depth. This improvement in the

α is believed to be due to the higher interconnection of the carbon nanoparticles compared to the other samples that as a result, intramolecular interactions in the CHN7 make it rather flexible structure. In the middle frequency region, higher α can be observed because more energy is required for polarization. But, at high frequency region, polarization in the grain boundaries with low energy absorption resonance occurs consequent energy losses or absorption is low in the 20


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higher frequency region. As is clear from Fig. 15b, increment application time to 9 s causes α decreases, and shifts towards lower frequencies.

4. Conclusions In this paper, we studied the structural, electrical, magnetical, and optical of C/Hf0.9Ni0.1Oy samples and CHN7 sample under the application an AC electric field that were synthesized by solvothermal process that offers a simple way to synthesize moderate conductive material-based random metamaterials. The pores size and crack density decreased with increase of H2BDC content and application an AC electric field. The simultaneously negative permittivity and negative permeability behaviors were observed in the CHN7 sample that this behavior was more evident with applying an AC electric field. The interconnectivity of structure nanocrystallites increased with increasing H2BDC content. Morphology of the CHN7 sample under the influence AC electric field acquired the most intertwined structure. Continous current paths were formed in the CHN7, leading to the negative permittivity, and could be easily controlled by application time of an AC electric field. The ε e' of the CHN7 sample was extracted by fitting the experimental data with the Drude model. The plasma frequency of CHN7 and CHN7-9 samples were less than other samples which could be due to an increment of the electron mass. The µ e' of CHN7 was smaller than zero near frequency 1 GHz which could be attributed to increase of the current loops, inductive behavior, Eddy currents, and the diamagnetic response of CHN7 sample arising from Lenz’s law, and could be easily controlled by application time. With increasing application time to 9 s, DNG region and plasma frequency increased. H2BDC content increased hopping conduction behavior to metal-like conduction behavior in the samples; i.e. carrier concentration of carbon in sample networks leaded to control of negative permittivity and inductive behavior in the testing frequency. Moreover, with increasing H2BDC content and application time, µ e / ε e ratio for CHN7 and CHN7-9 samples closed to unit which was the consequences of moderate conductive of carbon. Percolation theory could be responsible for the negative permittivity observed in this work. With increasing H2BDC content, carbon forms continuous paths in the host network and metal-like behavior prevails upon capacitive behavior. For CHN7 sample, skin depth was much smaller compared to the other samples. Overall, this good achievement will be helpful in the practical applications, and the current study underscores 21



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the importance of moderate conductive material-based random metamaterials in the evolution of metamaterial devices with good impedance matching and provides a new way toward the development of metamaterial technology.

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Table of Contents

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Enhanced absorption performance of carbon nanostructure based

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