Fabrication of Graphene Network in Alumina

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Fabrication of Graphene Network in Alumina Ceramics with Adjustable Negative Permittivity by Spark Plasma Sintering Rui Yin, Haikun Wu, Kai Sun, Xiaomin Li, Chao Yan, Wen Zhao, Zhanhu Guo, and Lei Qian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11177 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on January 9, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Fabrication of Graphene Network in Alumina Ceramics with Adjustable Negative Permittivity by Spark Plasma Sintering

Rui Yina, Haikun Wua, Kai Sun,a,b Xiaomin Lia, Chao Yan,c Wen Zhaoa, Zhanhu Guob*, Lei Qiana*

a

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, 17923 Jingshi Road, Jinan 250061, China

b

Integrated Composites Laboratory (ICL), Department of Chemical & Biomolecular Engineering, University of Tennessee Knoxville, Knoxville, TN 37996

c

School of Material Science and Engineering, Jiangsu University of Science and Technology (JUST), No 2, Mengxi Rd, Zhenjiang, Jiangsu, China

*Corresponding author, E-mail: [email protected], [email protected]

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Abstract The graphene/alumina (GR/Al2O3) composites with adjustable negative permittivity were prepared by spark plasma sintering method. The microstructures of the composites with different GR contents were investigated by field emission scanning electron microscopy. With increasing the GR content, the grain size of Al2O3 tended to decrease and the grain shape transformed from granulous to ellipsoidal. The radio-frequency dielectric properties of the GR/Al2O3 composites including permittivity (ε′ and ε′′), dielectric loss tangent (tanδ) and reactance (Z′′) were investigated. The negative permittivity appeared when the GR content exceeded 15.38 wt%. The plasma oscillation of conduction electrons in the GR networks was considered to cause the negative permittivity. The maximum dielectric loss tangent for the GR/Al2O3 composites with the GR mass fraction of 15.38 wt% and 18.46 wt% appeared near 40 and 70 MHz, respectively, corresponding to the transition of ε′ from negative to positive, which was produced by the LC resonance. The impedance of the GR/Al2O3 composites with the equivalent circuit models was also discussed.

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1. Introduction As one of the most extensively used ceramics, alumina (Al2O3) is widely applied because of its high mechanical property, chemical inertness and electrical property.1-4 The good physical properties endow Al2O3 ceramics multiple applications in wear-resistant materials, high frequency insulating materials and so on. However, due to the insulating character of single Al2O3 ceramics, their applications still have some limitations, so diverse conductive materials have been introduced to the Al2O3 ceramics to improve their conductivity.5-8 Moreover, the addition of conductive fillers will make the composites further applicable in electromagnetic interference (EMI) shielding, microwave absorbing materials and novel electromagnetic functional materials.9-20 Carbon materials, especially the carbon nanomaterials, are very good candidates of fillers for the single-phase ceramics due to their high specific surface area and outstanding electrical conductivity. Carbon fibers and carbon nanotubes have been used to improve the composite performances.21-31 Owing to their outstanding physical properties such as high thermal conductivity (5300 W·m-1·K-1), electronic mobility (2×105 cm2 ·V-1·s-1), and specific resistance (10-6 Ω·cm), the two-dimension graphene (GR) has become more attractive compared with other carbon nanomaterials.32-40 The introduction of GR into the Al2O3 ceramics to improve their mechanical properties has been reported. For example, Liu et al.41 fabricated the GR platelets/Al2O3 composite ceramics by spark plasma sintering and analyzed the enhancement effects of the GR platelets on the mechanical properties of the composites. Wozniak et al.42 reported the addition of the modified graphene oxide nanoplatelets to the Al2O3 significantly 3



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improved the mechanical properties of the produced composites. Liu et al.43 prepared the GR platelet reinforced Al2O3 ceramic composites and investigated the mechanical properties. Zhang et al.44 investigated the enhanced effect of bending strength and fracture toughness of GR/Al2O3 composites. Metacomposites with negative electromagnetic parameters have been prepared by traditional composing methods in recent years. The negative permittivity has been found in many types of composite materials, such as polymer/ceramic composites45-47 and polymer/carbon composites.48 Compared with metamaterials obtained from periodic arrays, metacomposites have some advantages of plentiful raw materials, adjustable and uniform microstructures.7,8,27,49-50 The metacomposites have potential applications in the field of perfect absorbing, sensing, and magnetic attenuation. With high electrical and thermal conductivity, the GR has the potential to tune electromagnetic properties of metacomposites. However, there was little literature focused on the electromagnetic properties of the Al2O3 composite ceramics.49 In this work, the GR/Al2O3 composite ceramics with uniform dispersing of GR were fabricated by spark plasma sintering (SPS) and their dielectric properties were investigated. Different from common hot-pressing sintering, the SPS was chosen to produce the GR/Al2O3 composite ceramics, due to the advantages of rapid heating rate, short sintering time, energy saving and environment benign. The porosity of the samples was calculated by the Archimedes method. The microstructure and composition of the GR/Al2O3 composite ceramics were analyzed by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Raman spectroscopy. The dielectric properties of the GR/Al2O3 composite ceramics were tested at the frequency range from 10 MHz to 1 GHz. 4


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2. Experimental Section 2.1 Materials Graphene sheets (purity 98%, thickness < 1 nm, sheet size 20 × 20 µm) were purchased from Jinan Moxi New Materials Tech. Co. Ltd. The Al2O3 powders (purity 99.99%, particle size < 30 nm) were obtained from Hangzhou Wanjing New Material Co. Ltd. 2.2 Composite Samples Preparation The GR powders were mixed with Al2O3 powders with the GR content of 0, 0.40, 1.58, 4.74, 9.50, 11.54, 15.38 and 18.46 wt% respectively by dry grinding. In order to describe expediently, the composites with different GR contents were labeled as GR0, GR0.40, GR1.58, GR4.74, GR9.50, GR11.54, GR15.38, and GR18.46, respectively. The sintering system was controlled by holding at 1550 ℃ for 5 min with a heating rate of 150 ℃/min to prepare the dense GR/Al2O3 composite ceramics. The sintered composite ceramics were polished by grinding machine, and the final size of composite ceramic was Φ15 mm×2 mm. 2.3 Characterization The porosity of the GR/Al2O3 composite ceramics was measured by the Archimedes method, which was measured by hydrostatic method using liquid static balance with distilled water as the medium. The porosity of the GR/Al2O3 composite ceramics was calculated by the dry weight, wet weight and buoyant weight of the samples. The phase composition of the samples was characterized by X-ray diffraction (XRD) (Rigaku Dmax-rc, Tokyo, Japan) with Cu Kα radiation. The microstructure of the GR/Al2O3 composite ceramics was observed using a field emission scanning electron microscope (FESEM) (Hitachi SU-70, Tokyo, Japan) equipped with 5



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energy dispersive X-ray spectrometer (EDS). Raman spectrometer with an incident radiation of 532 nm was used to test the structure defects of the GR/Al2O3 composite ceramics. The electrical properties (permittivity and impedance) of the GR/Al2O3 composite ceramics were investigated by Precision Impedance Analyzer (Agilent E4991A) with 16453A dielectric test fixture with the frequency ranging from 10 MHz to 1 GHz. The capacitance (Cp), resistance (Rp) and impedance (Z′, Z′′) of the samples were measured under AC voltage of 100 mV at room temperature. The parameters of complex permittivity (ε′, ε′′) and the dielectric loss tangent (tanδ) were calculated by the following formula:27,49

ε' =

Cp d

ε 0S

(1)

where Cp is the capacitance; d is the sample thickness; ε0=8.85×10-12 F·m-1; and S is the electrode plate area.

d

ε '' =

2π f ε 0 SR p

(2)

where Rp is parallel resistance; f is the electric field frequency.

tan δ =

ε '' |ε' |

(3)

3. Results and Discussion 3.1 Microstructures of the GR/Al2O3 composite ceramics

Figure 1 shows the FESEM images of Al2O3 powders, Al2O3 ceramics and the GR/Al2O3 composite ceramics with low GR contents. The initial size of Al2O3 powders was less than 30 nm, Figure 1a. Figure 1b illustrates the fracture surface of dense Al2O3 ceramics fabricated by SPS, 6


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and the grain size is larger than 2 µm. Compared with pure Al2O3 ceramics with large and uneven grains, the GR/Al2O3 composite ceramics revealed more uniform microstructures with homogeneous grains. For the GR/Al2O3 composite ceramics with 1.58 wt% GR, as shown in the insert of Figure 1c, GR was inserted in the Al2O3 matrix on a large scale and GR was uniformly distributed in the Al2O3 matrix. Figure 1c shows no obvious difference for the grain size between the GR1.58 and pure Al2O3 ceramics. Meanwhile, when the content of GR was increased to 9.50 wt%, Figure 1d, the grain size became smaller than that of the composite ceramics with low GR contents. The GR was embedded in the Al2O3 matrix and wrapped the Al2O3 grains, then the layered structures were formed in the samples (insert of Figure 1d). In addition, the edges and corners of the Al2O3 grains became smooth compared with pure Al2O3 ceramics. The EDS analysis (Figure 1e) further demonstrates the existence of GR in the GR/Al2O3 composite ceramics. Figure 2 shows the FESEM images of GR/Al2O3 composite ceramics with high GR contents from 11.54 to 18.46 wt%. The fracture surface of the GR/Al2O3 composite ceramics manifested the stratified structures (insert of Figure 2a), which were ascribed to the axial pressure induced by SPS. And GR had a certain degree of stacks under the axial pressure. Besides, GR tended to be distributed between the boundaries of Al2O3 grains and prevented the migration of grain boundaries, leading to a refinement of microstructure with the relatively homogeneous and thin grains.41-44 As shown in Figure 2b&c, the intensive shrinking effect resulted in the decrease of grain size and the transformation of angular to smooth shape of Al2O3 grains. With increasing the GR content, the grain size of Al2O3 tended to decrease. The GR 7



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acting as a package impeded the growth of the Al2O3 grain during the sintering process. The adding of GR gave rise to a comparatively lower interface energy of the GR/Al2O3 composite ceramics, and thus reduced the driving force of grain growth.44 The porosity of the GR/Al2O3 composite ceramics with high GR contents was investigated by the Archimedes method. The test results show that the porosity of GR11.54, GR15.38, and GR18.46 is 1.56%, 3.15% and 4.36%, respectively. The porosity of the samples had a tendency of gradually increasing with increasing the GR content. The FESEM images indicate the wrinkled structure of GR covered on the surface of Al2O3 grains, which impeded the densification process compared with pure Al2O3 ceramics. Besides, some tiny GR structures embedded in the matrix broadened the distance between grains. Pores were tended to be formed when GR and Al2O3 matrix was not combined well, resulting from different shrinkages between the GR and Al2O3 matrix during a cooling process.41 Thus, the porosity of the GR/Al2O3 composite ceramics was related to the GR content. In addition, the low porosity of the GR/Al2O3 composite ceramics indicated that the spark plasma sintering improved the densification and thus resulted in better mechanical properties compared with normal sintering method. 3.2 Composition analysis of the GR/Al2O3 composite ceramics

Figure 3a shows the XRD patterns of the GR, Al2O3 powders and the GR/Al2O3 composite ceramics prepared by SPS. In the XRD pattern of pure GR powders, there was only one peak at 26.3° of (002).51-52 From the pattern of Al2O3 ceramics, it was found that the crystalline structure showed pure α-Al2O3 phase.52 As shown in Figure 3a, apart from peaks of α-Al2O3 and GR, no other phase was detected in the XRD patterns of the GR/Al2O3 composites. 8


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Further, the peak position of Al2O3 was invariable for the GR/Al2O3 composite ceramics, and the peak of GR was still clearly observed, illustrating that the GR was stable in the GR/Al2O3 composite ceramics during the sintering process. Moreover, the diffraction peaks of GR in the XRD patterns of GR0.40 ~ GR9.50 were stronger than those of pure GR sheets, indicating that the integrity of the crystal structure of GR was increased after the high temperature treatment. Figure 3b shows the Raman spectra of the GR/Al2O3 composite ceramics and the GR/Al2O3 composite powders with 9.50 wt% GR. Remarkable D (1350 cm-1) and G (1580 cm-1) bands are observed in Figure 3b. The Raman G-band is related to the E2g-vibration mode of sp2 hybridized GR, whereas the D-band is assigned to the structural defects/disorder of sp2 domains.53-56 Thus, the ID/IG peak intensity ratio is often used to determine the defects in the GR structure.57-60 In addition, the ID/IG of GR0.40, GR1.58, GR4.74 and GR9.50 was calculated to be 0.45, 0.29, 0.26 and 0.23, respectively. It was found that the ID/IG declined with increasing the GR content, which indicated an increase in the amount of six-membered ring ordered GR structures.61-62 The ID/IG of GR9.50 powders was 1.12, which was larger than that of the GR9.50 obviously, indicating that the defects of samples decreased significantly after SPS. Along with the sintering temperature as high as 1550 ℃, the functional groups of the fractured GR facilitated the formation of the six-membered ring structure.59-62 Thus, the integrity of the crystal structure of GR sheets was enhanced. The graphitization degree of the GR/Al2O3 composite ceramics was increased, which was consistent with the XRD results of the GR/Al2O3 composite ceramics. 3.3 Dielectric properties of the GR/Al2O3 composite ceramics

Figure 4 shows the frequency dependence of permittivity (real permittivity ε′ and 9



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imaginary permittivity ε′′) and dielectric loss tangent (tanδ) for the GR/Al2O3 composite ceramics with different GR contents. It was found that the ε′ of the GR/Al2O3 composite ceramics increased with increasing the GR content, Figure 4a, which was attributed to the enlarged GR-Al2O3 interface area among the samples.49 The neighboring GR sheets were isolated by the Al2O3 grains, which are equivalent to a series of tiny capacitors. Due to the increase of the GR content, more interfacial areas were produced in the composites, leading to the formation of more tiny capacitors in the GR/Al2O3 composite ceramics. The charge carriers are trapped in the interfaces between the GR and Al2O3, and thus a higher ε′ is observed. When the GR content was 4.74 wt%, the ε′ reached the maximum. Obviously, the ε′ had a tendency of decrease with further increasing the GR content to 9.50 wt%. It was attributed to the emergence of the connected phase among the GR/Al2O3 composite ceramics. Though there was more isolated phase than connected phase distributed in the GR/Al2O3 composite ceramics, the connected phase performed as electrical inductance27,50 and led to the decrease of tiny capacitors in the GR/Al2O3 composite ceramics, thus, the ε′ appeared to decrease. It was found that the ε′ of GR11.54 was higher than that of GR9.50 at higher test frequencies. The possible reason was that the ε′ of GR11.54 was negative at the lower frequency which was out of the tested frequency in this work, and the ε′ increased gradually, transferred from negative to positive, and exceeded the GR9.50. With increasing the GR content, a plasma-like negative ε′ was observed at low frequencies in the GR/Al2O3 composite ceramics such as GR15.38 and GR18.46. The appearance of negative ε′ was attributed to the formation of conductive GR networks. Moreover, the transition of ε′ from negative to positive for GR15.38 and GR18.46 was observed near 40 and 70 MHz, 10


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respectively. It was considered to be the LC resonances, which is expressed as f = 1/[2π(LC)1/2],8 giving rise to the emission of electromagnetic waves under certain frequencies, and the GR content will influence the value of L and C. And the frequency dispersion might occur when the emitted electromagnetic waves interfered with the external high frequency electric field.63-64 The frequency dispersion gradually become strong with the increase of the LC resonance frequency. These results indicate that the ε′ of the GR/Al2O3 composite ceramics was tuned by the content and distribution of GR sheets.49,61 In this work, the ε′ values of GR15.38 and GR18.46 were almost zero approached the plasma frequency, which was the oscillation frequency caused by the positive and negative charge separation in the plasma. Thus, the GR/Al2O3 composite ceramics were considered as the epsilon-near-zero materials, whose permittivity was near zero at the corresponding frequency band. These materials possess special properties and potential applications, such as cloaking, optical devices and antennas.49 As is well known, the ε′′ is the loss factor, and represents the dielectric loss of materials, includes the polarization loss and the conduction loss. The polarization loss is attributed to various relaxation polarization related to the changes of the turning-direction polarization within the materials. The conduction loss is related to the leakage current of the materials. However, the dielectric loss is concerned with the frequency and the content of conductive phase among the percolation system. And the dielectric loss usually contains the conduction loss and the polarization loss. The conduction loss caused by a leakage current among the conductive phases or the contact resistors in the equivalent circuit model, and the polarization loss derived from the polarization currents and the shift of dipoles with the constant change of the electric fields.61,63 11



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As shown in Figure 4b, the ε′′ of the GR/Al2O3 composite ceramics was enhanced with increasing the GR content, which is resulted from the improvement of the dielectric loss and leakage conductance loss.65 For the GR/Al2O3 composite ceramics of GR1.58, the ε′′ had a rising trend at the test frequency. The ε′′ had the linear decrease relationship while the GR content exceeded 4.74 wt%, and the ε′′ decreased with increasing the frequency, indicating the dominant conduction loss in the system.27 Figure 4c shows the frequency-dependent dielectric loss tangent, and higher GR contents led to the increased dielectric loss tangent, which was attributed to the high conductivity. Besides, the maximum values of dielectric loss tangent for GR15.38 and GR18.46 were observed near 40 and 70 MHz, respectively, which was consistent with the transition of ε′ from negative to positive. For GR15.38 and GR18.46, the dominant position of the dielectric loss transformed from conduction loss to polarization loss. With the frequency increased, the conduction loss decreased, and the polarization loss became a leading position at the high frequency.27,49 3.4 Impedance and equivalent circuit analysis

Figure 5 shows the frequency dependence of the reactance for the GR/Al2O3 composite ceramics with different GR contents and the impedance for GR11.54 and GR15.38 with the equivalent circuit models. The test frequency ranges form 10 MHz to 1 GHz. When an ac electric field was applied, the materials were considered as a circuit made up of resistors, capacitors and inductors.49,63,66-67 The capacitor and inductor will respond to the change of electric field in the circuit, which is called capacitive reactance (ZC=1/ωC) and inductive reactance (ZL=ωL), C is the capacitance, L is the inductance, and it was known that Z′′=ZL-ZC.49 When the content of GR 12


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was low, the reactance (Z′′) of the GR/Al2O3 composite ceramics was negative (Z′′ < 0) in the whole tested frequency band, manifesting the ZC was greater than the ZL.64-69 The inductance was very weak in the GR/Al2O3 composite ceramic with the low GR contents. Due to the distance between the GR sheets was relative large, it was difficult to form the conduction path in the composites. Thus, the GR/Al2O3 composite ceramic was regarded as a capacitor and assumed a capacitive behavior. And the capacitance increased with increasing the GR content due to the enlarged GR-Al2O3 interface area.49,63,71-74 For GR11.54, part of the connected and gathered GR was formed in the GR/Al2O3 composite ceramics, which gave rise to the formation of the conduction path.63,69,75-77 Thus, GR11.54 was imitated by the equivalent circuit consisted of resistor R and capacitor C. It was observed that the equivalent circuit model fitted the test data well. In the same way, the Z′′ of the GR/Al2O3 composite ceramics with high GR contents was positive (Z′′ > 0) in the whole tested frequency band, demonstrating that the inductive reactance was larger than the capacitive reactance.49,61 The GR/Al2O3 composite ceramics could be identified as an inductor and exhibited an inductive behavior, thus the inductors were introduced into the circuit simulation. For GR15.38, the GR formed the continuous network in the GR/Al2O3 composite ceramics, and it could be simulated by an equivalent circuit composed of resistor R, capacitor C and inductance L. Theoretically, the negative permittivity in the GR15.38 sample was possibly attributed to the action of its parallel inductances, which were caused by the formation of current loops resulting from the large number of interconnected GR sheets.63-70 The results showed that the GR/Al2O3 composite ceramics underwent the transition from capacitive to inductive behavior because of the formation of more conductive networks in the composite 13



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ceramics with the increase of GR content.

4. Conclusions The GR/Al2O3 composite ceramics with GR contents between 0 and 18.46 wt% were fabricated by the SPS. It was found that GR was uniformly dispersed in the Al2O3 matrix. The compositions of the samples were examined by XRD and Raman spectra. The microstructures of the GR/Al2O3 composite ceramics were investigated by FESEM, the fracture surface of Al2O3 ceramics manifested the stratified structures when the GR content was relatively high. And the radio-frequency dielectric properties including permittivity (ε′ and ε′′), dielectric loss tangent (tanδ) and reactance (Z′′) of composites were investigated in detail. The formation of continuous conducting GR network led to the negative permittivity behavior, which was observed in the GR/Al2O3 composite ceramics with the GR contents exceeded 15.38wt%. Two maximum of the dielectric loss tangent for GR15.38 and GR18.46 appeared near 40 and 70 MHz, respectively. And the impedance for GR11.54 and GR15.38 with the equivalent circuit models was discussed. The GR/Al2O3 composite ceramics underwent the transition from capacitive to inductive behavior with the increase of the GR contents. Our work indicates that GR can be a good candidate for achieving negative permittivity which can be easily tuned by adjusting the content of GR. In addition, the proposed approach of fabricating the composite ceramics with tuned permittivity can be possibly used for preparing other composite ceramics with negative permittivity. They have potential applications such as electromagnetic attenuation, microwave absorbing and wireless charging.

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Acknowledgments This work was supported by the National Nature Science Foundation of China (No. 51672162), Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (No. KF201606).

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Polyoxymethylene/Ethylene

Butylacrylate

Copolymer/Ethylene-Methyl

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Figure 1 FESEM images of (a) Al2O3 powders; (b) Al2O3 ceramic; (c) GR1.58; (d) GR9.50; and (e) EDS analysis of GR9.50 sample. Insert: the corresponding low magnification images.

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Figure 2 FESEM images of the GR/Al2O3 composite ceramics with high GR contents, (a) GR11.54; (b) GR15.38; (c) GR18.46. Insert: the corresponding low magnification image.

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Al O

a

2 3

GR

Intensity (a.u.)

GR9.50 GR4.74 GR1.58 GR0.40 Al O

2 3

GR

30

60

90

2θ (degrees)

b Intensity (a.u.)

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


GR 9.50 GR 4.74 GR 1.58 GR 0.40 GR powders 9.50

1000

1500

2000

Raman shift (cm-1) Figure 3 (a) XRD patterns of the GR powders and the GR/Al2O3 composite ceramics with different GR contents. (b) Raman spectra of the GR/Al2O3 composite ceramics with different GR contents.

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500

a

10000

GR 4.74

GR9.50

GR11.54

GR15.38

GR 18.46

100

GR0

150

GR0.40 GR1.58

100

-1000

10

GR4.74

50

-1500

GR1.58

ε″

-500

b

1000

0

ε′

GR9.50

1

GR11.54

0

GR15.38 100M

-2000

1G

100M

0.1

GR18.46

100M

1G

90

1G

Frequency (Hz)

Frequency (Hz)

c

GR0 GR0.40 GR1.58

60

tanδ

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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GR4.74 2

GR9.50 GR11.54 GR15.38 GR18.46

0

30 50M

100M

0 100M

1G

Frequency (Hz) Figure 4. Frequency dependence of (a) real permittivity, (b) imaginary permittivity and (c) dielectric loss tangent of the GR/Al2O3 composite ceramics with different GR contents.

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0

a

60

Re(Z) -Im(Z) Fitted

b

GR11.54 -500

GR0 GR0.40

0

GR1.58

-1000

Z

Z''

30

GR4.74 GR9.50

-50

GR11.54

-1500

GR15.38

30M

60M

GR18.46

100M

0

1G

100M

16

1G

Frequency (Hz)

Frequency (Hz)

c

Re(Z) -Im(Z) Fitted

GR 15.38 Z

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


8

0

100M

1G

Frequency (Hz)

Figure 5. (a) Frequency dependence of the reactance for the GR/Al2O3 composite ceramics with different GR contents. (b) The impedance for GR11.54. Insert: The equivalent circuit model. (c) The impedance for GR15.38. Insert: The equivalent circuit model.

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TOC Graphic

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Fabrication of Graphene Network in Alumina

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