Tunable Dielectric Performance Derived from the MetalâOrganic

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Tunable dielectric performance derived from the MOFs/RGO hybrid with broadband absorption Xiaohui Liang, Bin Quan, Guangbin Ji, Wei Liu, Huanqin Zhao, Sisi Dai, Jing Lv, and Youwei Du ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02565 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 13, 2017

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ACS Sustainable Chemistry & Engineering

Tunable dielectric performance derived from the MOFs/RGO hybrid with broadband absorption Xiaohui Liang†, Bin Quan†, Guangbin Ji *,†, Wei Liu†, Huanqin Zhao†, Sisi Dai†, Jing Lv†, Youwei Du‡ †

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, No.

29 Yudao Street, Nanjing 210016, P.R. China ‡

Laboratory of Solid State Microstructures, Nanjing University, No. 22 Hankou Road, Nanjing 210093,

P.R. China *To whom correspondence should be addressed: E-mail: [email protected]; Tel: +86-25-52112902

ACS Paragon Plus Environment

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ABSTRACT In terms of microwave absorption, dielectric performance acts vital but negative characters in attenuation and impedance matching. In this study, ZnO/NPC (nanoporous carbon)/RGO materials have been fabricated through a simple and valid hydrothermal method derived from the Zn-MOFs (metal-organic frameworks). By changing the molar ratio of the precursors, the permittivity of the ZnO/NPC/RGO can be calculated and the greatest balance between energy conservation and impedance matching eventually emerged with the GO addition of 4 mL. It could be found that at 14 GHz a thin sample consisting of 40 wt% ZnO/NPC/RGO in the wax matrix exhibited RLmin (minimum reflection loss) of -50.5 dB with the thickness of 2.4 mm and with a thickness of 2.6 mm, the effective microwave absorption bandwidth is coverage from 9.6 to 17 GHz. What is worth mentioning is that we have also interpreted the relationships between highest reflection loss values and matching thicknesses. This work not only proposal the ZnO/NPC/RGO samples are able to play as perfect absorbent with broad frequency bandwidth and strong absorption, but also provide better candidates in designing other lightweight microwave absorbents. Key words: dielectric performance; ZIF-8; effective electromagnetic wave absorption bandwidth; matching thickness; lightweight


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INTRODUCTION Electromagnetic wave absorbing materials have drawn increasing attention around the world with swift evolution of wireless communications and high frequency equipment. An ideal electromagnetic wave absorber should be lightweight with high absorption efficiency in a broad frequency band at a low filler loading ratio. 1 As the evolution of nanotechnology, nanosized magnetic or dielectric materials, for instance, ZnO (cagelike-structure, 2 nanorod 3), CuS, 4 α-MnO2, 5 dendrite-like Fe3O4, γ-Fe2O3, and Fe 6 have been fabricated to improve the electromagnetic wave absorption performance. Due to the dielectric constant and high conductivity, the traditional absorbers, for instance, metals have strong absorption performances, but the drawbacks like bad anticorrosion, valuableness, and high density confine their practical application. 7-8 In recent years, significant efforts have already been devoted to graphene and reduced graphene oxide (RGO) due to their impressive electromagnetic wave absorbing ability.

9-11

However, sole graphene

suffers from poor dispersion in the matrix, 12-13 interfacial impedance mismatching, 14-15 and limited loss mechanism.

16-17

Therefore, incorporations of other lossy materials has been widely studied as the

imperative solution to improve its microwave absorption performance. 18 ZnO hollow spheres enwound by reduced grapheme oxide sheets exhibited -45.05 dB with maximum reflection loss and 3.3 GHz of effective EM absorption bandwidth. 19 RGO/α-Fe2O3 composite with absorption bandwidth of 6.4 GHz and the RLmin of -33.5 dB reported by Tian et al.

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The defects of thick thickness (> 2.0 mm) and

two-by-four effective frequency bandwidth (< 4.0 GHz) in spite of their RLmin values are less than -20 dB seriously restrict their application. In fact, combining diverse ingredients together and give consideration to both energy conservation and impedance matching are an excellent absorber should be done.

21-22

RGO possess high dielectric

dissipation capacity, which is originated from its strong dielectric behavior.

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Nevertheless, poor

impedance matching exactly results from the strong dielectric constant. Therefore, selecting a low dielectric candidate to combine with RGO is critical. Due to the complex permittivity and permeability, ZnO-based nanomaterials can be employed as super effective microwave-absorbing materials. ACS Paragon Plus Environment

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Thus, 3


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our target in this study is to realize ZnO/NPC/RGO derived from ZIF-8/GO. Our study exhibited the dielectric constant of ZnO/NPC/RGO samples that could be modulated via regulating combination ratio. Too high or too low permittivity can hardly satisfy an ideal absorbers, the optimal electromagnetic (EM) wave absorbing property appears on the composite with a broadband effective frequency. The composites with the filler loading as 40 wt% in a wax matrix and the thicknesses of 2.4 mm showed the RLmin of -50.5 dB and at the thickness of 2.6 mm the effective EM wave absorption bandwidth is 7.4 GHz. Thus, the ZnO/NPC/RGO hybrids maybe promise electromagnetic wave absorbing materials with broad absorption at low filler loading.

EXPERIMENTAL SECTION Preparation of ZIF-8, ZnO/NPC and ZnO/NPC/RGO In a typical synthesis of ZIF-8, 40 mL methanolic solutions including 810 mg zinc nitrate hexahydrate and 40 mL methanolic solutions including 526 mg MeIm were mixed under stirring for 4 h. Then it was placed for 12 h. Collecting and washing white powder by centrifugation and methanol. In addition, we dried it at 80 °C. ZIF-8 crystals were thermally converted to ZnO/NPC materials through carbonization under a N2 flow at 700 oC keeping for 2 h. For synthesis of ZnO/NPC/RGO hybrid, 80 mg ZnO/NPC and GO (2 mg/mL, 3, 4 and 5 mL, respectively) was added to 15 mL H2O and 15 mL ethanol. With the different adding of GO, we named the ZnO/NPC/RGO-x as S1 (x=3 mL), S2 (x=4 mL) and S3 (x=5 mL). The mixed solution was sonicated for 1 h to obtain a homogenous dispersion, which was then kept at 180 oC for 12 h. Collecting and washing the powder by centrifugation and deionized water, drying at 60 oC. Scheme 1 summarized this synthetic process. Characterization and Measurement The morphology of ZnO/NPC/RGO hybrid was characterized with a FE-SEM (field emission scanning electron microscope), S-4800, Hitachi and a FE-TEM (field emission transmission electron microscope), Tecnai G2 F20UTwin, FEI. Powder diffraction data were collected from 10° to 90° in 2θ using an X-ray diffractometer (XRD). By Raman Microscope equipped with a 514 nm laser Raman spectroscopy was carried out on a Renishaw. Recording X-ray photoelectron spectra used a Thermo 4 ACS Paragon Plus Environment

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Scientific ESCALAB 250 Xi X-ray photoelectron spectrometer. The relative complex permittivity (εr) and permeability (µr) were measured by a vector network analyzer (VNA, N5244A PNA-X, Agilent) in the frequency range 2-18 GHz. εr of the dielectric material has been calculated from the experimental scattering parameters S11 (or S22) and S21 (or S12) in a coaxial wire analysis. 25-26

Scheme 1. Scheme for the synthesis of ZIF-8 crystal, ZnO/NPC and ZnO/NPC/RGO.

RESULTS AND DISCUSION The microstructures of ZnO/NPC nanoparticles decorated graphene were recorded by SEM and TEM. Fig. 1a and 1b present the SEM images of ZIF-8 crystals and ZnO/NPC composites with size of about 1.5 µm. From Fig. 1c, one can find the diameter size of ZnO/NPC is corresponding to ZIF-8. Typical SEM images of the as-prepared ZnO/NPC/RGO hybrid are presented in Figure 1d-f with different volume of GO. Both flexible two-dimensional sheet-like structures and three-dimensional granular particles are found in the hybrid. The flexible sheets with size several micrometers are supposed to be RGO, whereas the granular particles are thought to be the agglomerate of ZnO/NPC nanoparticles. This assumption is confirmed by TEM characterizations. As shown in Figure 1g-h, electron beam transparent two-dimensional flexible sheets are together with small dark particles attached to them. Clearly, the


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ZnO/NPC particles are wrapped by RGO sheets, indicating the granular structure of ZnO/NPC and sheets of the RGO are formed. The flexible sheets of RGO in Fig. 1g-1h are good for multiple reflection of electromagnetic wave.

Figure 1. SEM images of the ZIF-8 (a), ZnO/NPC (b) and TEM image of ZnO/NPC (c); (d-f) SEM images of S1, S2 and S3; TEM images of S2 (g) and magnification TEM image of S2 (h).


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ZnO/NPC

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Figure 2. Pore size distribution plots (the insert) and N2 adsorption-desorption isotherms of ZnO/NPC. To demonstrate the nanoporous structure of the ZnO/NPC, the nitrogen adsorption isotherms obtained are shown in Fig. 2. The ZnO/NPC showed a surface area (30.40 m2·g-1). The micropore-size distribution curves of the ZnO/NPC samples demonstrate a pore size distribution from 1 to 10 nm and a great deal of disordered nanopores, which could benefit from the collapse of the well-defined microporous structure of ZIF-8. Nevertheless, ZnO/NPC still possesses high surface area and total pore volume, which provide more active sites for the effective reflection and scattering of incident electromagnetic waves, promoting the multiple absorption processes of electromagnetic wave more efficiently. X-ray powder diffraction pattern of ZIF-8, ZnO/NPC and ZnO/NPC/RGO hybrid is given in Figure 3a-3c. The positions of diffraction peaks of the obtained ZIF-8 (Fig. 3a) crystals are corresponding to the PXRD patterns simulated from single crystal structures of ZIF-8.27 In addition, the wide-angle XRD patterns for ZnO/NPC (Fig. 3b) exhibited diffraction peaks at 41.2o and 47.5o that are identical to the diffraction of carbon and ZnO. Little diffraction peaks belong to ZnO, suggesting that the ZnO aggregated on nanoporous carbon may be amorphous. After hydrothermal treatment, the peaks of ZnO appeared and the sharp peaks are corresponding to ZnO proving the crystallinity of the ZnO. For Fig. 3c, pure phase of zincite ZnO diffraction peaks can be indexed to a relatively with no additional characteristic peaks. Moreover, we can confirm ZnO is highly crystallized with sharp and strong


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diffraction peaks. Diffraction peaks 2θ = 31.671, 34.311, 36.141, 47.401, 56.521, 62.731, 67.911 and 69.031 are to (100), (002), (101), (102), (110), (103), (112) and (201) lattice planes of ZnO.

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Consistency between them confirms the existence of ZnO in the hybrid. Signals of graphene are invisible, which is caused by the weak scattering power and low crystallinity of RGO. Nevertheless, RGO in the hybrid can be readily identified by Raman spectroscopy (Figure 3d). The Raman peaks at around1590 cm-1 and 1352 cm-1 are G-band and D-band. Because of vibrations of ordered sp2 carbon atoms the G-band appears in the hexagonal lattice of GO, while from edges, defects and disordered carbon the D-band evolves.

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ID/IG is a testing method of the defects in a carbon material, and is

increased gradually with the adding of GO in this experiments. For S2, a set of Raman peaks appeared in the spectrum. The weak one consists of two Raman peaks, which locate at ~438 cm-1 and ~574 cm-1, respectively, which can be attributed to the Raman-active vibration modes of ZnO 30. This proves again the presentation of ZnO/NPC as a separate phrase in ZnO/NPC/RGO.

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Figure 3. Wide-angle XRD patterns of ZIF-8 (a), ZnO/NPC (b) and ZnO/NPC/RGO (c); Raman spectrum of the ZnO/NPC/RGO (d).


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a

C1s

b

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O1s

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Figure 4. XPS survey spectrum of S2 (a); (b) C 1s of core level spectrum in S2; (c) O 1s of core level spectrum in S2 and (d) Zn 2p of core level spectrum in S2. Composition analysis of the composites was conducted by XPS. The results acquired for the S2 sample are presented in Fig. 4. Figure 4a shows the wide spectra of S2 composites. Fig. 4b shows that the C1s spectra of the composite are divided into three fitting peaks, corresponding to C-C emerged at 284.6 eV, C-O appeared at 285.9 eV and C=O (286.7 eV) respectively

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. Compared to the

corresponding peak of GO, the peak at 286.7 eV is diminished in intensity in the C-1s spectrum of S2, which indicated the reduction of GO to RGO. Fig. 4c is the O-1s spectrum, which emerged two peaks at 531.6 and 532.5 eV. The 531.6 eV peak is related to lattice O of Zn and RGO. In Fig. 4d, peaks in the spectrum of Zn 2p are loaded at 1045.5 eV and 1022.4 eV, which are differentiating confirmed to Zn 2p1/2 and Zn 2p3/2.


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b 20

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S1 S2 S3

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Figure 5. (a) Real and (b) imaginary part of permittivity and(c) permeability of ZnO/NPC/RGO-wax composites with different volume of the GO. Figure 5 shows complex permittivity of the as-fabricated ZnO/NPC/RGO samples. The samples reveal representative frequency dependent permittivity, with the frequency in the tested region values of ε′ and ε″ decrease (Figure 5a-b). As shown in Figure 5c, the complex permeability with real part (µ') and imaginary part (µ'') were independent on the frequency, which indicated that ZnO/NPC and RGO flakes are high-dielectric material but weak-magnetic. Hence, we pay attention to the variation of complex permittivity. On basis of the Debye theory, ε′ and ε″ can be described as 24 ε′ = ε∞ + (εs-ε∞)/(1+ω2τ2)

(1)

ε″ = (εs-ε∞) ωτ/(1+ω2τ2) + σac/ωε0

(2)

where εs represents the static permittivity, ε∞ represents the relative dielectric permittivity, ω represents angular frequency, τ represents polarization relaxation time, σac represents the alternative conductivity, and ε0 represents the dielectric constant in vacuum. According to Eq 1, the decline in ε′ is benefit from increase in ω. In the lower frequency, this phenomenon can be regarded as the polarization relaxation. ACS Paragon Plus Environment

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With increased of RGO, both ε′ and ε″ achieved significant enhancement (Figure 5a and 5b). Generally, ε' is on behalf of storage ability of microwave energy, however, ε'' symbolizes dissipation ability of microwave energy. For ZnO/NPC nanoparticles, the value of ε' and ε'' are lower.

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In our study, fine

impedance match can results from relatively low complex permittivity of the nonmagnetic ZnO/NPC and ZnO/NPC/RGO composites, nevertheless, in the transmission process, ε'' is so low that due to little attenuation occurs the electromagnetic wave entering into interior voids is transmitted and reflected outside absorbent. It’s important that note with the increase of frequency, ε' of all composites decreases and frequency dispersion effect that is good for microwave absorption has been revealed. 32 For ZnO/NPC/RGO samples, with the increasing of GO, values of complex permittivity are higher than ZnO/NPC and ascend. But higher permittivity in the surface of absorber could cause more reflection and is not beneficial for the impedance match. Moreover, ε'' and δ 34 presented as: ε'' = 1/Πε0ρf

(3)

δ = 1/(Πfµσ)1/2

(4)

where ρ represents electrical resistivity, σ represents electrical conductivity, δ represents skin depth33-34. The higher ε'' signify the low resistivity of ZnO/NPC/RGO, the high conductivity would increase worth skin depth effect, resulting in undesired reflection of microwave. Moreover, the ZnO/NPC/RGO exhibits some narrow resonance peaks originating from ZnO/NPC/RGO interfacial polarization and dipole polarization. The δε (tangent of dielectric loss) of the material can be shown as 25 tan δε = ε″/ε′

(5)

Figure 6a shows tan δε of ZnO/NPC/RGO versus frequency at different adding of GO. In general, the values of ε″ (Figure 5b) and tan δε both increase with the increasing of RGO, and several relaxation peaks can be found for each curve in the testing frequency range.


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Figure 6. (a) Dielectric loss tangents and (b-d) Cole-Cole plots of ZnO/NPC/RGO composites with different volume of GO. When the second part of the Eq. 2 is ignored, the relationship between ε′ and ε″ can be written: (ε′ - (εs + ε∞)/2)2 + (ε″)2 = ((εs-ε∞)/2)2

(6)

It corresponds to a circle centered at ((εs + ε∞)/2, 0). As shown in Figure 6b-d, each Cole-Cole curve of the ZnO/NPC/RGO composite is all containing individual semicircles, due to the multirelaxations dielectric properties. These multirelaxations can be well explained by the mechanism proposed by Cao et al. 35 They are supposed to originate from the defect polarization of the oxygen-containing groups and the imperfect carbon structures in RGO as well as the multiple interfacial polarizations in ZnO/NPC/RGO hybrids (Figure 7).


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Figure 7. Illustrations of dipole polarization in the ZnO/NPC/RGO hybrid: (a) Defect dipole

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Attennuation constant α

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Figure 8. Frequency dependence of α and Zr of S1, S2 and S3. For further probe the in-depth microwave absorption mechanism behind the EM parameters, the (Zr) and (α) of ZnO/NPC/RGO at 2-18 GHz were counted, which are shown in Figure 8. In general, two factors should be considered for a distinguished absorbent. One is energy conservation, which demanded the strong EM propagation loss in the interior of absorbers; the other is impedance match, which required the values of εr and µr tend to be equal. Hence, Zr proposed by Lv et al.

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is used to

reveal impedance matching of the ZnO/NPC/RGO presented via the following equations: Zr = Z/Z0

(7)

Z = Z0 / (µr/εr)1/2

(8)


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εr = ε′- jε″

(9)

µr = µ′- jµ″

(10)

|εr| = (ε′2+ε″2)1/2

(11)

|µr| = (µ′2+µ″2)1/2

(12)

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where Zr represents the impedance matching ratio value, Z0 represents the impedance of the free space, Z represents the impedance value of the absorbent, εr and µr are complex permittivity and permeability values. Clearly, absorber shows superior impedance matching if εr equals to µr and Z = Z0. That is, if Zr = 1, at the front surface of the absorbers incident microwave can gain zero-reflection. Higher Zr presents better impedance matching. At the same time, based on transmission line theory EM propagation loss is contributed by α expressing in equation (13): α = 21/2 Πf ((µ″ε″-µ′ε′)+((µ″ε″-µ′ε′)2+(µ′ε″+µ″ε′)2)1/2)1/2/c

(13)

where f represents the frequency of the EM wave, c represents the velocity of light. Obviously, high values of µ' and ε' could bring out high. Fig. 6 shows the overall tend of Zr value is ZS1 > ZS2 > ZS3 and α is αS3 > αS2 > αS1 in the overall frequency. S1 owns best impedance matching capability and worst attenuation ability by reason of relatively lowwe complex permittivity, on the contrary, S3 owns poorest impedance matching activity and superior attenuation property. Besides, both Zr and α of sample S2 is in the middle sequences among three ZnO/NPC/RGO samples. An outstanding absorbent should consider both energy conservation and impedance matching meanwhile. Therefore, we conclude that S2 have the superior electromagnetic wave absorption properties with attenuation ability and moderate impedance matching characters. To more study microwave absorption properties of ZnO/NPC/RGO composites, ZnO, ZnO/NPC and RGO, based on the transmit-line theory the reflection loss (RL) values were simulated.

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Zin of EM

wave absorption presented as equation (14) Zin = Z0 (µr/εr)1/2 tan h [j (2πfd (µr εr)1/2/c)]

(14)

Where Z0 represents the impedance of free space. The RL of incident microwave expressed as equation (15) ACS Paragon Plus Environment

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RL (dB) = 20 log |(Zin - Z0)/(Zin + Z0)|

(15)

Permittivity, permeability and reflection loss values are presented in Fig. 9. Clearly, the ZnO and ZnO/NPC have a lower storage ability and dissipation ability of microwave energy. Nevertheless, the RGO possess higher electrical conductivity and dissipation ability of microwave energy. Owing to its high dielectric loss RGO could be an superior candidate of microwave absorption material. However, RL of all the samples are exceed -10 dB, this is because the poor impedance matching and constant α. This is corresponding to our previous statements. RL value of ZnO/NPC/RGO-x samples with various coating thicknesses are shown in Fig. 10a. Compared with S1 and S3, the S2 appeared best microwave absorption performance. The microwave absorption results demonstrate the inference in Figure 10a, S2 has superior microwave absorption properties with coating thickness of 2.4 mm for its attenuation loss ability and moderate impedance matching character. At surface of absorbers, outstanding impedance matching can minish the reflection of incidence microwave while if attenuation ability of materials is weak microwaves entering into nanoporous carbon would be transmitted and reflected out of absorber. On the contrary, if impedance matching ability is weak, strong attenuation loss ability is insignificance for little entered microwave. Therefore, optimal absorber should take both energy conservation and impedance matching into account. Both S1 and S3 present weak microwave absorption ability, and maximum the RL value of S1 is -19.9 dB with the thickness of 2.3 mm and S3 is -13 dB with the thickness of 2.3 and 2.6 mm. Fig. 10b shows the minimum RL value (RLmin) and effective frequency bandwidth (fE) of ZnO/NPC/RGO composites with thickness of 2.4 mm. For S2 with the thicknesses of 2.4 mm from 10 GHz to 16.5 GHz, superior effective absorption bandwidth of 6.5 GHz is obtained. For the S3 and S1, the fE with the thickness of 2.4 mm are 2.8 and 6.2 GHz from 8.6-11.4 GHz and 11.2-17.4 GHz, respectively.


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8

10

12

14

Frequency (GHz)

16

100

Frequency (GHz)

ε' ε'' µ' µ''

80 60 40 20 0 2

18

f

4

6

8

10

12

14

18

0

-2

-4

2

4

6

8

10

12

14

16

16

18

Frequency (GHz) 2

RGO

Relection loss (dB)

Relection loss (dB)

-4

6

16

120

ZnO/NPC

-2

4

14

2

ZnO

2

Permittivity and permeability

b 3.0



a

Relection loss/dB

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 27

18

0

-2

-4

2

Frequency (GHz)

4

6

8

10

12

14

16

18

Frequency (GHz)

Figure 9. Electromagnetic parameters of ZnO (a), ZnO/NPC (b) and RGO (c) with 60 wt% paraffin; RL values of ZnO (d), ZnO/NPC (e) and RGO (f), respectively. Based on the fundamental mechanism of electromagnetic wave absorption, the most effective absorption is exhibited while impedance match between free space and absorbers is obtained.

37-38

RL

values of S2-wax composites at thickness from 1.8 to 3.6 mm are shown in Figure 11a. The maximum reflection loss of -50.5 dB is observed at 14 GHz for a S2-wax composite with a thickness of 2.4 mm (Figure 11b). The highest effective absorption bandwidth of 7.4 GHz is achieved for the composite at the thicknesses of 2.6 mm from 9.6 GHz to17 GHz (Figure 11c). Fig. 11d shows the fE of S2 with different coating thickness. We can see that it only can reach highest effective bandwidth frequency at coating thickness of 2.6 mm. It is corresponding with the results of Fig. 11c.


16

Page 17 of 27 -60

a Reflection loss/dB

-50

S1 S2 S3

-40 -30 -20 -10 0

2.3mm

2.4mm

2.5mm

2.6mm

b

20

20

10

10

0

0

-10

-10

-20

-20

-30

-30

-40

-40 RLmin fE

-50 -60

5mL

Effective frequency (GHz)

Coating thickness

Reflection loss (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-50 3mL

4mL

-60

Different addition of GO Figure 10. Comparison of the reflection loss of ZnO/NPC/RGO with different coating thickness (a); RLmin and fE with different addition of GO (b). In contrast to the high performance demonstrated by S2-wax composites, only weak electromagnetic wave absorption was exhibited by the composites of wax and S1 and S3 (Fig. 10a). This implies that the amount of RGO nanosheets into the ZnO/NPC/RGO-wax composites has played a critical role in improving the electromagnetic wave absorption performance. It was found that RGO nanosheets resulted in a significant increased of the imaginary permittivity of the composites (Figure 5), indicating the tapering of conductivity. 39 We also noted that no characteristic of Debye relaxation was observed in ACS Paragon Plus Environment

17


the Cole-Cole plot of the S1 and S3 (Fig. 6b and 6d). This is in sharp contrast to that observed in the case of S2 wax composite (Figure 6c). Clearly, it is the multi-relaxation that improves microwave absorption performance of S2-wax composites. a

b

0

1.8mm 2.0mm 2.2mm 2.4mm 2.6mm 2.8mm 3.0mm 3.2mm 3.4mm 3.6mm

-20 -30 -40 -50 2

4

RL (dB)

RL (dB)

2.4mm

0

-10

-10

-20 -30 -40 -50

6

8

10

12

14

16

2

18

4

6

c

0

d

2.6mm

10

12

14

16

18

8 7

7.4 GHz

-10

8

Frequency (GHz)

Frequency (GHz) S2

6

-20

fE (GHz)

RL (dB)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 27

-30 -40

5 4 3 2 1

-50

0 2

4

6

8

10

12

14

16

18

Frequency (GHz)

2.0

2.2

2.4

2.6

2.8

Coating thickness (mm)

Figure 11. (a) RL curves of S2 at different thickness; a RL curve of S2 at thickness of 2.4 mm (b), 2.6 mm (c) and fE of S2 with different coating thickness. Normally, the EM wave are dissipated.40 If thickness of absorber (tm) at peak frequency (fm) meets formula: tm = nc/(4fm(|µr||εr|)1/2) (n = 1, 3, 5…)

(16)

where c reprensents the velocity of light in the free, |µr| and |εr| are the moduli of µr and εr,. To demonstrate maximum RL value appear at 2.4 mm, as shown in Figure 12, we conduct the simulations of absorber tm at the minimum RL values versus peak frequency (fm) for the S2 under λ/4 conditions41. Black curve is on behalf of the simulation thickness (tfitm) at 2-18 GHz via violet dots and quarter wavelength principles are the experimental matching thickness (texpm) at fm. We found that value of texpm at 2.4 mm is well fitting with the simulation tfitm, while the texpm deviate from the tfitm to variable extent. Hence, superior microwave absorption activity presents at 2.4 mm illustrated via the geometrical effect. 18 ACS Paragon Plus Environment

Page 19 of 27

The optimal microwave absorption property is derived from both attenuation loss ability and moderate impedance matching character. Besides, exception magnetic and dielectric loss, interference hardening loss is another significant dissipation factor and the quarter-wave principle is effective method for offering important direct in thickness designing of the EM wave absorbent.

Reflection loss (dB)

0

-10

-20

-30

2.2 mm 2.4 mm 2.6 mm 2.8 mm

-40

-50 12

1/4 exp

10

• t m

8

tm (mm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

4

• 2 2

4

6

8

10

• •

12

14

• 16

18

Frequency (GHz)

Figure 12. Comparison of tm for S2 under λ/4 conditions at fm. Electromagnetic wave absorption properties of ZnO/NPC/RGO hybrid absorber together with other RGO-based materials reported recently were summarized in Table 1. In comparison with the recently reported RGO-based materials, ZnO/NPC/RGO-wax composites exhibited superior performance at a rather broadband effective frequency, demonstrating the promising perspective of ZnO/NPC/RGO hybrid absorber in the development of light weighted and obvious electromagnetic wave absorption. Table 1. Microwave absorption performance of the similar materials. Filler

RL

Thickness

Frequency range


Effective bandwith

Ref.

19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(dB)

(mm)

(GHz)

(

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