Intercalating Hybrids of Sandwich-like Fe3O4âGraphite: Synthesis and

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Intercalating hybrids of sandwich-like Fe3O4-graphite: Synthesis and their synergistic enhancement of microwave absorption Fuxi Peng, Fanbin Meng, Yifan Guo, Huagao Wang, Fei Huang, and Zuowan Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04021 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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ACS Sustainable Chemistry & Engineering

Intercalating hybrids of sandwich-like Fe3O4-graphite: Synthesis and their synergistic enhancement of microwave absorption Fuxi Peng, Fanbin Meng*, Yifan Guo, Huagao Wang, Fei Huang, Zuowan Zhou* Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, 111, North Section I, No 2 Ring Road, Jinniu District, Chengdu 610031, Sichuan , P. R. China * E-mail: [email protected], [email protected]. ABSTRACT Rational design on the components and microstructures of microwave-absorbing materials can be paving the way for upgrading their performances in electromagnetic pollution prevention. In this study, a Fe3O4-graphite intercalation hybrids (Fe3O4-GIH) with unique sandwich-like microstructure are fabricated by a molten salt route and subsequent temperature reduction. It is found that the gaseous FeCl3 molecular in the high temperature can diffuse into the graphite interlayer plane to obtain FeCl3-GIH, and the intercalated FeCl3 is then transferred into Fe3O4 nanoparticles under high temperature reduction , which can prop open the graphite interlayer, thus achieving sandwich-like Fe3O4-GIH. Therefore one step can give perfect features: transformation of graphite into graphene sheets, introduction of magnetic component and construction of multiple interfaces, which are benefit to the microwave absorption (MA). As a result, the maximum reflection loss of the as-obtained Fe3O4-GIH can be up to -51dB at 4.3 GHz with a matching thickness of 4.8 mm. Furthermore, the MA performances can be tuned by regulating the interlayer

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spacing of Fe3O4-GIH. The excellent microwave absorption performance may attribute to the synergistic effect between Fe3O4 nanoparticles with magnetic loss and graphite with dielectric loss and novel interfacial polarization originating from the sandwich-like Fe3O4-GIH. Additionally, it can be supposed that this sandwich structures are more beneficial to scatter the incident electromagnetic wave due to their large spacing and porous features. Keywords: Intercalation hybrid; sandwich-like microstructure; interfacial polarization; synergistic effect; microwave absorption

INTRODUCTION Recently, ultra-light graphene-based materials with excellent microwave absorption (MA) are becoming more and more attractive to address the increasingly electromagnetic interference pollution.1–4 Firstly, graphene and its derivatives promise to be a superior building block for constructing the MA materials, owing to its remarkable physical properties including lightweight, large surface area, high conductivity, good thermal conductivity and easy process ability.5–6 The graphene aerogels have attracted wide attention, applying for the microwave absorption performance. Owing to the effective dielectric loss and multiple scattering in the 3D network structure.7–8 Taking the advantage of graphene and introducing other loss materials, are not only beneficial to increase the interface polarization and electromagnetic loss ability, but also for impedance matching behaviour.9–12 Among them, through the introduction of Fe3O4 with low toxicity, high compatibility, and strong spin polarization at room temperature, the magnetic loss and electromagnetic attenuation of graphene can be significantly improved, which is benefit to enhance the MA. The interface introduced by Fe3O4 generates resonance in complex permittivity and permeability as well as enhanced magnetic loss,resulting in the enhanced MA capacity and


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widened effective absorption bandwidth.13−14 Which makes them ideal candidates as MA materials. To date, a series of graphene/Fe3O4 nanocomposites have been reported and applied to the field of MA. Many synthesis methods such as hydrothermal method, in situ precipitation and atomic layer deposition-assisted synthesis have been proposed to anchor or adsorb Fe3O4 nanoparticles on the surface of graphene.15−17 Furthermore, the porous, hollow or core−shell structures are introduced to further enhance the polarization loss to improve the impedance matching.18–19 Thus given their perfect impedance matching, microstructure as well as the multiple polarization can be superior to the MA performance. Moreover, in order to address the problem of distribution inhomogeneity of Fe3O4 nanoparticles on the graphene, the strategy of growing Fe3O4 nanosheets/or nanorods on the surfaces of graphene is developed.20−21 The special 3D architecture, synergy of dielectric loss and magnetic loss, as well as the multiple interfaces can further improve their MA performance. Lately, assembly into 3D graphene/Fe3O4 aerogels can open up a new way to achieve a high-performance MA, due to their 3D interconnected porous structure which can increase multiple reflection and dissipate microwave energy.22−23 Although pursuing complex structure and multiple interfaces can enhance the MA performance, it is inevitable to meet the problems of complex process, difficult to optimize the structure and tune the electromagnetic properties, limiting their practical applications. Therefore, it is still a challenge to develop a facile and universal strategy to effectively enhance the MA performance of graphene/Fe3O4 nanocomposites. The current graphene/Fe3O4 nanocomposites are mainly derived from the graphene nanosheets or their derivatives (graphene oxide or reduced graphene oxide), why not try to achieve the graphene/Fe3O4 hybrids by one step from the raw material of natural graphite? Herein, we propose


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a novel approach to constructing a sandwich-like Fe3O4-GIH by intercalation of natural graphite flakes with FeCl3 and subsequent high temperature reduction. The as-prepared sandwich-like Fe3O4-graphite hybrids can exhibit significant enhancement in microwave absorption with a minimum reflection loss value of −51 dB at 4.3 GHz. More importantly, the MA properties of Fe3O4-GIH can be tuned by regulating the intercalation degree of Fe3O4-GIH. EXPERIMENTAL SECTION Materials. Pristine graphite fakes (200 mesh) were provided by Shandong Graphite Factory (China). FeCl3 (A.R), hydrochloric acid (HCl) and ethanol (99.8%) were both bought from Chengdu Chron Chemical Factory (China). Preparation of Fe3O4-graphite hybrids. The FeCl3-GIH were fabricated by the molten salt method.24 The procedure is as follows. Firstly, FeCl3 and graphite (w/w, 3/1) were mixed uniformly and the mixture was then placed in an autoclave (50 ml) under vacuum at 400℃ for 12 h. To eliminate the possible influence of formed iron oxides on surface, the as-prepared FeCl3GIHs were rinsed with HCl, deionized water and ethanol until pH is 7, and then dried at 70℃ overnight. Later, FeCl3-GIH was placed in a tube furnace at 650℃ in Ar atmosphere to obtain Fe3O4-graphite intercalation hybrids, and the samples with various high-temperature treatment time (0.5, 1.0, 2.0 and 3.0 h) are denoted as 0.5-Fe3O4-GIH, 1.0-Fe3O4-GIH, 2.0-Fe3O4-GIH and 3.0-Fe3O4-GIH, respectively. Then, keep it cool down naturally. As a contrast, microwave irradiation was used to treat the as-prepared 2.0-Fe3O4-GIH, achieving Fe3O4-graphene nanosheets, which was denoted MWI-GIH. Characterization. The morphology and structure of the Fe3O4-GIH were characterized on field emission scanning electron microscopy with corresponding elemental mapping (FE-SEM, JEOL,


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JSM-7001F), transmission electron microscope (TEM, JEOL, JEM-2100F), and X-ray diffraction (XRD, Philips X'Pert PRO X-ray diffract to meter with using monochromatic Cu Kα1 (λ = 0.154178 nm) radiation (40 kV, 40 mA) in the range of 10–80° with a step scan of 0.01° per step). Raman spectra were on a Laser Raman spectroscopy (InVia, RENISHAW) using a 514 nm argon ion laser. X-ray photoelectron spectroscopy (XPS, VG Microtech, ESCA 2000) analysis was used to measure the chemical composition of the Fe3O4-GIH. The complex permeability and permittivity of the materials were measured using a vector network analyser (AV3618, CETC) in 2-18 GHz. The mixtures were prepared by uniformly mixing 5.0 wt% of samples with waxes and prepared in the toroidal shape with an outer diameter of 7.0 mm and an inner diameter of 3.04 mm. RESULTS AND DISCUSSION During the high temperature intercalation process, the gaseous FeCl3 molecules diffuse into the interlayers of graphite. resulting in obtaining FeCl3-GIH.25 Taken 2.0-Fe3O4-GIH as an example (Fig. 1), the multi-layers structure of natural graphite (Fig. 1a) can be broken after FeCl3 intercalation and the interlayer spacing is obviously enlarged (Fig. 1b), and the interlayer distance of graphite, FeCl3-GIH gradually increased from 0.34 nm to 0.77 nm, calculated from the XRD patterns as shown in the following Fig. 4a. After follow-up calcination, the intercalated FeCl3 is transferred into Fe3O4 nanoparticles, which can further prop open the interlayer spacing and the interlayer spacing is up to 0.8 μm as shown in Fig. 1c. Moreover, the formed Fe3O4 is homogeneously dispersed in the graphite interlayers, indicated by the element mappings of C, O, and Fe elements from Fig. 1d. From the TEM images of 2.0-Fe3O4-GIH (Fig. 2), the generated Fe3O4 nanoparticles featured a size of 30−100 nm are decorated uniformly on the graphite interlayer. The enlarged TEM image further exhibits Fe3O4 nanoparticles are closely attached to


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the surfaces of graphite interlayers (Fig. 2b). The high-resolution TEM image shows a typical Fe3O4 nanoparticle with a well crystalline structure, and the lattice spacing of 0.25 nm corresponds to the (311) plane of bulk Fe3O4 (Fig. 2c).

Fig. 1 SEM images of (a) natural graphite and (b) 2.0-FeCl3-GIH, (c) Fe3O4-GIH with corresponding element mapping (d). The formed sandwich-like Fe3O4-graphite hybrids can be tunable by controlling the calcination conditions. As shown in Fig. 3a-3c, Fe3O4 desorbed and aggregated with the form of nanoparticles at the edge of GIH when the calcination time is relatively short. As the calcination time increases to 2 h, Fe3O4 nanoparticles continued to grow and generate the flake-like structure (Fig. 3c). Further, obvious flake-like Fe3O4 can be observed in Fig. 3d when the calcination time is up to 3 h. Furthermore, the interlayer spacing of Fe3O4-GIH increases from 0.4 μm to 0.8 μm with the calcination time increasing from 0.5 h to 2 h (more detail images in Fig. S9), which is benefit to


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MA properties. This is consistent with the reported work of graphene oxide/ copper phthalocyanine composite multilayer films.26 Further, more Fe3O4 nanoparticle will be desorption and aggregate at the edge of GIH, which is due to FeCl3-GIH is unstable in the high temperature.27 Besides, the average size of Fe3O4 nanoparticles on the interlayers increase gradually from 51 nm to 230 nm (Fig. 3e-3h and Fig. S10).

Fig. 2 TEM images of 2.0-Fe3O4-GIH (a), enlarged image (b) and corresponding HRTEM (c).

Fig. 3 SEM images of Fe3O4-GIH at different calcination time. (a) 0.5, (b) 1.0, (c) 2.0, (d) 3.0 h. and corresponding TEM images (e-h). The crystal structures of the as-prepared Fe3O4-GIH are identified by XRD measurement (Fig. 4a). Obviously, the XRD peaks of natural graphite sheets in FeCl3-GIH disappear. This is because the FeCl3 diffuse into the graphite interlayer, leading to the increase of interlayer spacing. Sharp diffraction peaks at 11.5°, 22.5°, 27.9°, 46.2° and 57.6° match well with the XRD patterns of


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FeCl3-GIH,28 suggesting the presence of FeCl3 crystallites in the graphite interlayers. After calcination, the peaks at 30.1°, 35.4° and 62.5° are assigned to Fe3O4 (JCPDS 19-0629), indicating the transformation of FeCl3 to Fe3O4 within the graphite interlayers. Further, one prominent peak rises at 26.5 °, which should be corresponded to the (002) crystal plane of graphite. This is because partly FeCl3 is desorbed with the calcination procedure. And the other peak is located at 27.2 °, resulted from the generation of Fe3O4-GIH which are transformed from the FeCl3-GIH located at 27.9 °. This is further indicated the interlayer distance of Fe3O4-GIH increased, which is agreed with the SEM image of Fig. 1c. And with the calcination time increased (0.5-2h), the interlayer distance also increased (Fig. S1). Therefore, the intercalation hybrids can cause separation of graphite layer into graphene sheets. From the Raman characterization (Fig. 4b), two peaks are obviously existed at about 1350 cm−1 and 1600 cm−1, corresponding to the D band and G band from graphite, respectively. From graphite, FeCl3-GIH to Fe3O4-GIH, the intensity ratio of ID/IG increase gradually, suggesting the graphite sheets is more disordered. Besides, the value of ID/IG increases with the prolongation of calcination time, suggesting more defects in Fe3O4-GIH (Fig. S2). However, the value of ID/IG drops suddenly when the calcination time is 3 h, owing to the FeCl3 desorption from the graphite interlayers. Furthermore, compared to the graphite, blue-shift of the peaks for D and G bands can be found, indicating a vital charge transfer between the graphite and Fe3O4. It supports the Fe−O−C bonds formed between Fe3O4 and graphite.

2, 23

The charge

transfer between graphite and Fe3O4 is benefit to enhance its MA performance. The surface composition of Fe3O4-GIH are characterized by XPS analysis. As shown in Fig.4c, from the Fe3O4GIH, only C 1s (284.8 eV), O 1s (532.5 eV) and Fe 2p (711.1 eV) peaks can be observed. Two peaks Fe 2p3/2 and Fe 2p1/2 can be founded in the high-resolution Fe 2p XPS spectra (Fig. S3) at 711.8 and 725.3 eV, in accordance with the reported values for Fe3O4.29 Furthermore, the


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formation of Fe3O4 rather than Fe2O3 because there is no satellite peak at about 719 eV. Three peaks at 530.7, 532.0, 532.8 eV can be de-convoluted in the high-resolution O 1s XPS spectrum (Fig. 4d). Oxygen in the lattice of Fe3O4

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assigned to the peak at 530.7 eV and oxygen in the

lattice (C−O)31 attributed to the peak at 532.8 eV. Finally, the peak at 532.0 eV is assigned to the formation of Fe−O−C bond between Fe3O4 and graphite,23 which is beneficial for MA properties.

Fig. 4 XRD patterns (a) of the samples (natural graphite, FeCl3-GIH, Fe3O4-GIH) Raman (b) and XPS (c) spectra of the Fe3O4-GIH, and O 1s spectrum (d) of Fe3O4-GIH. To investigate the MA properties of the samples, 3D plots of reflection loss (RL) values were calculated according to the transmission line theory.25 Fig.5 shows the EM wave absorption performances of the specimens over the frequency and respective thicknesses.

RL  20 log

( Z in  Z 0 ) ( Z in  Z 0 )

(1)


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Z in  Z 0 r  r tanh  j (2 fd c  r r ) 

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(2)

here, εr is the relative complex permittivity and µr is permeability. And d, ƒ and c represent the absorber thickness, frequency and velocity of light, respectively. The input impedance is represented by Zin, finally, Z0 is the impedance in the free space. The calculated 3D RL plots of graphite, FeCl3-GIH, 2.0-Fe3O4-GIH and microwave irradiation (MWI)/GIH as functions of frequency and sample thickness are shown in Fig. 5. The minimum RL of FeCl3-GIH and 2.0Fe3O4-GIH are −20.0 dB at 4.0 GHz with a thickness of 4.8 mm and −51.0 dB at 4.3 GHz with a thickness of 4.8 mm, respectively (Fig. 5b and Fig. 5c). Thus, the strategy of FeCl3 intercalation of natural graphite to form sandwich-like Fe3O4-GIH and subsequent temperature reduction can achieve high-performance MA. As a contrast, the 2.0-Fe3O4-GIH is further treated by microwave irradiation, aiming to obtain Fe3O4-graphene hybrids with few layers (MWI-GIH) opened up due to the heat energy generated by electromagnetic wave (Fig. S4).32 Interesting, the as-obtained MWI-GIH exhibits decreased MA ability compared with Fe3O4-GIH (Fig.5 d), and the minimum RL of MWI-GIH is only –7.5 dB at 3.0 GHz. This is because the treatment of microwave irradiation leads to the existence of only graphene sheet-Fe3O4 interface, and less interface polarization result in weak MA ability.33 Furthermore, the MA performance of Fe3O4-GIH can also be tuneable by controlling the calcination time. As shown in Fig.6, the value of minimum RL gradually increases from −10 dB at 3.8 GHz, −22 dB at 4.0 GHz to −51.0 dB at 4.3 GHz with the calcination time increasing from 0.5 h to 2 h, but suddenly drops to −11 dB at 3.8 GHz for 3.0Fe3O4-GIH with a thickness of 4.8 mm, respectively (Fig. 6a-6d). Therefore, the size and distribution of Fe3O4 within the graphite interlayer and the interlayer spacing at different calcination time can significantly affect the MA performance of Fe3O4-GIH.


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Fig. 5 3D plots of the calculated reflection loss of NG (a), (b) FeCl3-GIH, (d) Fe3O4-GIH and (e) MWI-GIH. The comparison of minimum reflection loss of NG, FeCl3-GIH, Fe3O4-GIH and MWIGIH for a layer thickness of 4.8 mm (c).reflection loss of Fe3O4-GIH for different thickness.

Fig. 6 3D plots of the calculated reflection loss of Fe3O4-GIH with different calcination time. 0.5 (a), (b) 1, (c) 2 and (d) 3h.


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Fig. 7 Electromagnetic parameters of all samples mixed with wax: (a) real permittivity, (b) imaginary permittivity, (c) real permeability and (d) imaginary permeability. In order to further understand the MA performance of the prepared Fe3O4-GIH, the complex permittivity and permeability are investigated and shown in Fig.7. As shown in Fig. 7a and 7b, both ε′ and ε″ permittivity are found to decrease with the increase of frequency, due to the polarization lagging.34 Between the samples, the ε′ of NG are larger and Fe3O4-GIH shows relatively low values of ε′ and ε″, respectively. This is because the conductivity net of NG is broken when NG transferred to Fe3O4-GIH according to Electron-Hopping model35 and conductivenetwork equation.36 While the larger ε′ and ε″ aren’t suitable for MA properties, which lead to a shielding effect,37 suggesting a moderate value of ε′ and ε″ is more appropriate to the MA performance. That is, Fe3O4-GIH is suitable for MA application. Moreover, the ε′ and ε″ of Fe3O4GIH decrease initially with the calcination time from 0.5 h to 2 h and then increase (Fig. S5). This is because the conductivity net of Fe3O4-GIH is broke heavily. While if the calcination time is too long (about 3 h), the FeCl3 would be desorption lead to the conductivity net of Fe3O4-GIH is better.


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Finally, dielectric loss of Fe3O4-GIH may be originate from the multiple interfacial polarization coming from interfaces (such as graphite-Fe3O4-graphite, Fe3O4-Fe3O4 and graphite-graphite).38 Fig. 8 shows the ε′-ε″ curves of GIH, noted Cole–Cole semicircle, which further show the existence of multi-relaxation in Fe3O4-GIH samples. Generally, each semicircle corresponding a Debye dipolar relaxation.39 Clearly, from FeCl3-GIH and Fe3O4-GIH (Fig. 8b and c), the irregular Cole−Cole semicircles are found, respectively, indicating the interfacial polarizations produced by relaxation polarization procedure. Extra, the Cole–Cole semicircle of Fe3O4-GIH calcined at different time shows largely different (Fig. S6). Four irregular semicircles are observed in 2.0Fe3O4-GIH, indicating the presence of several Debye relaxation processes due to the chemical bonds, defects and interfaces in 2.0-Fe3O4-GIH. Furthermore, it should notice that the conductivity loss also worked because in the high frequency region no obvious semicircles appeared, which probably results from the enhanced conductive network in 2.0-Fe3O4-GIH.23 Therefore, besides the interfacial relaxations of Fe3O4-GIH, extra conductivity loss and sandwich-like structure also contribute to the dielectric losses. Fig. 7c and 7d illustrate the μ′ and μ″ of the samples, the μ′ of all samples tends to decline in the frequency range. The μ′ values decrease with some fluctuations from 1.03 to 0.74, 1.01 to 0.43, 1.02 to 0.57, and 1.00 to 0.78 for NG, FeCl3-GIH, Fe3O4-GIH and MWI-GIH, respectively, in detail. μ″ demonstrate the magnetic resonance peak, when NG transferred to Fe3O4-GIH. This is due to the magnetic particles introduced into the interlayer of graphite. In detail, two resonance peaks of the μ″ can be obviously observed in the 6−8 GHz and 10−17 GHz ranges for Fe3O4-GIH. In general, the magnetic resonance in low frequency is related to nature resonance and usually in high frequency (>10GHz) are part of exchange resonance.40 Thus the magnetic loss in the present Fe3O4-GIH is caused from a consequence of natural and exchange resonances. Besides, with the


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calcination time increased, both μ′ and μ″ of the samples are increased (Fig. S5), due to form more magnetic particles. While the calcination time too long (about 3 h), the μ′ and μ″ go down, because the FeCl3 desorption lead to less magnetic particles formed.

Fig. 8 Cole–Cole plots for Natural graphite (a), FeCl3-GIH (b), 2.0- Fe3O4-GIH, (c) and MWIGIH (d). Based on the aboved discussion, the MA properties of Fe3O4-GIH is better than other species, because the magnetic loss, dielectric losses and interfacial polarizations loss. Further, with the calcination time increased, which lead to interlayer distance increased. Thus, there are more interfaces produced in sandwich-like Fe3O4-GIH (such as graphite-Fe3O4-graphite, Fe3O4–Fe3O4 and graphite–graphite) from 0.5-2h, because the number of cole-cole semicircle in Fig. S6 is increased. And the tail ratio of cole-cole semicircle is decreased in Fig.S6 from 0.5-2h, indicating the interfacial polarizations loss is primary than conductivity loss. If the calcination time increased to 3h, the interlayer distance further increased, leading some interfaces disappeared (such as graphite-Fe3O4-graphite). Thus the conductivity loss is primary, because the number of cole-cole


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semicircle is decreased and the tail ratio is increased in Fig. S6. Finally, the tail ratio of cole-cole plots for MWI-GIH in Fig. 8 is further increased than 3.0-Fe3O4-GIH, indicating interfacial polarizations loss is more weakened. Because the GIH has been exfoliated so that more interfaces are disappeared. Hence, an appropriate interlayer distance of sandwich-like Fe3O4-GIH is important for MA properties. Generally, the 2.0-Fe3O4-GIH has an excellent EM wave absorption should be originated from the synergistic effects between the magnetic Fe3O4 and the dielectric graphite components, which a strong EM wave attenuation and good impedance matching in the inside is formed. A deltafunction method has been proposed to evaluate the EM impedance matching degree by means of an equation.41

  sinh 2  Kfd   M

(3) here the relative complex permittivity and complex permeability determined K and M as shown below.

K=

    m    2 

4  r' r'  sin 

c  cos   cos  m

(4)

    m  4  r' r' cos   cos  m  r' r'  sin    2  M= 2      m   2 2      r ' cos r ' cos m tan           r' cos    r' cos  m  2   

(5) The good impedance matching means a larger area close to zero and smaller delta value

(|Δ|

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