Article pubs.acs.org/JPCC
Synergistic Effects of Carbon Nanotubes on Negative Dielectric Properties of Graphene-Phenolic Resin Composites Haikun Wu, Rui Yin, Yan Zhang, Zhongyang Wang, Peitao Xie, and Lei Qian* Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, 17923 Jingshi Road, Jinan 250061, China S Supporting Information *
ABSTRACT: Carbon nanotubes-graphene-phenolic resin (CNTs-GR-PR) composites are first prepared to investigate the effects of CNTs on negative dielectric properties of GR-PR composites. The results show that microstructures are easily adjusted by the mass ratio of CNTs to GR and total carbon content, which have an impact on the negative dielectric properties of the composites. The negative permittivity from the CNT1.6-GR5-PR is observed with the total carbon content of 6.6 wt %, attributed to the formation of conductive pathways. Yet the permittivity of the GR6.6-PR still remains positive, indicating that the addition of CNTs is beneficial to the appearance of negative permittivity and shows the synergistic effect. When the total carbon content exceeds 6.6 wt %, the addition of an appropriate amount of CNTs (the mass ratio of CNTs to GR is 0.33:1 and 1:1) improves the alternating conductivity (σac) and imaginary permittivity (ε″), resulting from the connectivity improvement in conductive phases. As compared to the dielectric loss tangent (tan δ) from the GR8-PR (below 35), the tan δ of CNT4-GR4-PR is higher over the whole frequency and above 100 before 400 MHz, indicating the appropriate addition of CNTs enhances the dielectric loss.
1. INTRODUCTION Graphene (GR) exhibits two-dimensional (2D) sp2-hybridized honeycomb structure, large surface area, and high electron mobility,1,2 so it is often used as an additive to improve dielectric properties of composites. For instance, Bansala et al.3 prepared GR-based polyurethane nanocomposites with enhanced electrical conductivity and superior electromagnetic interference (EMI) shielding effectiveness (SE) in the Ku band frequency region. The addition of GR changed the microstructures of the polyurethane nanocomposites, and electrical conduction pathways were formed. Xia et al.4 have designed GR-based nanocomposites with increased electrical conductivity and decreased dielectric permittivity in the low frequency region, attributed to the formation of imperfect interface, electron tunneling, and nanocapacitor from GR. Fan et al.5 mixed the poly(methyl methacrylate) with GR nanoplatelets to prepare composites and achieved increased dielectric permittivity near percolation threshold, resulting from the formation of interpenetrated network after GR entered into the polymeric chains. It also has been reported that the dielectric loss is increased by incorporation of GR into the epoxy resin.6 However, GR is easily aggregated in the matrix and forms the multilayer structure, resulting in high interfacial contact electrical resistance and low active sites. This obviously limits its applications in electronics, catalysis, and energy. As a result, carbon nanotubes (CNTs) are usually added to the GR composites to solve the above problems and improve characteristics of the composites. CNTs with one-dimensional structure play the role of wire, and prevent the GR aggregation. For example, Kong et al.7 have constructed CNTs-GR© XXXX American Chemical Society
poly(dimethylsiloxane) composites with stronger electromagnetic absorption capability and lower electromagnetic reflection by in situ growth method, which was attributed to the better dispersion of GR and lower interfacial contact electrical resistance due to the covalent C−C bonding between CNTs and GR. Singal et al.8 have reported that an electrodeposited CNTs-GR-poly(pyrrole-co-pyrrolepropylic acid) composite was used as a transducer for biointerfacial impedance sensing. It was found that the GR surface was functionalized by CNTs, resulting in fast electron transfer and high surface area. Liu et al.9 recently have prepared CNTs-GR-polyurethane nanocomposites with enhanced electrical conductivity used as sensors, due to the better dispersion of CNTs and the synergistic effect between CNTs and GR. In our previous work, the GR-phenolic resin (PR) composites with tunable and weakly negative permittivity were obtained.10 The resulted GR-PR composites belong to random composites with distribution of nanofillers,11−17 which are different from the traditional metamaterials with periodic structures. The PR with low density, high mechanical and thermal strength, low tendency to degeneration, good mechanical properties, and heat resistance is a good insulating material as compared to other thermosetting resins.18−25 The negative epsilon is due to the formation of continuous conductive pathways. Considering the advantages of CNTs, it was speculated that the addition of CNTs in GR-PR Received: March 26, 2017 Revised: May 21, 2017 Published: May 22, 2017 A
DOI: 10.1021/acs.jpcc.7b02858 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C composites could improve the dielectric properties. Adding CNTs can connect the interlayer GR in the composites and is beneficial to the formation of continuous conductive pathways. So in theory, adding CNTs can lead to the negative epsilon at lower total carbon content compared with pure GR-PR composites. To verify our guess and observe whether there are other effects of CNTs on negative dielectric properties of GR-PR composites after adding CNTs, the CNTs-GR-PR composites were prepared by the mechanical mixed method. Field emission scanning electron microscopy (FESEM) was used to characterize the CNTs-GR-PR composites. The effects of the mass ratio of CNTs to GR and total carbon content on negative dielectric properties, including alternating current conductivity (σac), complex permittivity (ε′, ε″) spectra, and dielectric loss tangent (tan δ), were investigated in detail.
Table 1. Produced Samples with Different Total Carbon Content and Mass Ratios of CNTs to GR
2. EXPERIMENTAL SECTION 2.1. Reagents. Graphene sheets (purity 98%, thickness 95 wt %, length 0.5−2 μm, inside diameter 3−5 nm, outside diameter 8−15 nm) were obtained from Nanjing JCNano Tech. Co. Ltd. 2.2. Procedure. GR (0.0079 g), CNTs (0.0026 g), and PR (0.7395 g) powder were weighed and mixed together with the total carbon mass fraction of 1.4 wt %. After 50 μL of 2.5 wt % PVA solution was dropped, the mixture was milled in the mortar for 5 min. The mixture then was pressed by the hydraulic press with the pressure of 35 MPa for 3 min, and the round-shape sample with diameter of 20 mm and thickness of 2 mm was obtained. The CNTs-GR-PR composite with 1.05 wt % GR and 0.35 wt % CNT was prepared and denoted as CNT0.35-GR1.05-PR. With the same total carbon content, the CNT0.7-GR0.7-PR, CNT0.9-GR0.5-PR, CNT1.1-GR0.3-PR, and GR1.4-PR were also produced. Other samples according to Table 1 were prepared with the same method. The microstructures of CNTs-GR-PR composites were observed by FESEM (Hitachi, SU-70, Tokyo, Japan). X-ray diffractmeter (XRD, Rigaku, Dmax-rc, Tokyo, Japan) was used to analyze the phase composition of samples. X-ray photoelectron spectroscopy (XPS, ThermoFisher, ESLAB 250) was employed to analyze the surface functional groups of GR and CNTs. To measure the dielectric parameters, the Agilent E4991A precision impedance analyzer (Agilent Technologies) was used, with 16453A dielectric test fixture under AC voltage 100 mV. The ε′ and ε″ of permittivity as well as σac were determined according to the following formula: Cd ε′ = ε0A ε″ =
σac =
(2)
d RA
(3)
GR content [wt %]
GR1.4-PR CNT0.35-GR1.05-PR CNT0.7-GR0.7-PR CNT0.9-GR0.5-PR CNT1.1-GR0.3-PR GR2.6-PR CNT0.65-GR1.95-PR CNT1.3-GR1.3-PR CNT1.7-GR0.9-PR CNT2.1-GR0.5-PR GR6.6-PR CNT1.6-GR5-PR CNT3.3-GR3.3-PR CNT4.4-GR2.2-PR CNT5.3-GR1.3-PR GR8-PR CNT2-GR6-PR CNT4-GR4-PR CNT5.3-GR2.7-PR CNT6.4-GR1.6-PR GR10-PR CNT2.5-GR7.5-PR CNT5-GR5-PR CNT6.7-GR3.3-PR CNT8-GR2-PR GR12-PR CNT3-GR9-PR CNT6-GR6-PR CNT8-GR4-PR CNT9.6-GR2.4-PR GR24-PR CNT6-GR18-PR CNT12-GR12-PR CNT16-GR8-PR CNT19.2-GR4.8-PR GR36-PR CNT9-GR27-PR CNT18-GR18-PR CNT24-GR12-PR CNT28.8-GR7.2-PR
0 0.35 0.7 0.9 1.1 0 0.65 1.3 1.7 2.1 0 1.6 3.3 4.4 5.3 0 2 4 5.3 6.4 0 2.5 5 6.7 8 0 3 6 8 9.6 0 6 12 16 19.2 0 9 18 24 28.8
1.4 1.05 0.7 0.5 0.3 2.6 1.95 1.3 0.9 0.5 6.6 5 3.3 2.2 1.3 8 6 4 2.7 1.6 10 7.5 5 3.3 2 12 9 6 4 2.4 24 18 12 8 4.8 36 27 18 12 7.2
mass ratio of CNTs to GR (CNTs:GR) 0.33:1 1:1 2:1 4:1 0.33:1 1:1 2:1 4:1 0.33:1 1:1 2:1 4:1 0.33:1 1:1 2:1 4:1 0.33:1 1:1 2:1 4:1 0.33:1 1:1 2:1 4:1 0.33:1 1:1 2:1 4:1 0.33:1 1:1 2:1 4:1
total content of carbon materials [wt %]
1.4
2.6
6.6
8
10
12
24
36
frequency, ε′ was the real part of permittivity, and ε″ was the imaginary part of permittivity, tan δ = ε″/|ε′|.
3. RESULTS AND DISCUSSION 3.1. Morphologies of the CNTs-GR-PR. The surface functional groups of GR and CNTs were analyzed by XPS. As shown in Figure 1a, the high resolution and deconvoluted XPS spectrum of the C1s signal from GR consisted of four peaks. The peak at 284.5 eV contributed 72.54% to the total C1s integrated intensity and was ascribed to the CC bonds. The peak at 285.4 eV corresponding to the C−OH bonds contributed 14.63% to the total integrated intensity. The binding energy from the C−O−C and O−CO bonds contributed 3.25% and 9.58% to the total intensity, respectively. These results showed that some functional groups existed on the GR surface.26−29 Figure 1b gives the C1s spectrum of CNTs,
(1)
d RA 2πfε0
sample
CNTs content [wt %]
where ε0 was the absolute permittivity of free space (8.85 × 10−12 F/m), C was the capacitance, d was the sample thickness, A was the electrode plate area, R was the resistance, f was the B
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Figure 2. FESEM images of the CNTs-GR-PR composites with different total carbon content and different mass ratios of CNTs to GR. (a and b) CNT4-GR4-PR, (c and d) CNT6.4-GR1.6-PR, (e and f) CNT9.6-GR2.4-PR. (b), (d), and (f) were the corresponding magnification images.
content. When the total carbon content was 8 wt %, for the CNT4-GR4-PR (Figure 2a and b), it was observed that CNTs, GR, and PR were uniformly mixed together. Besides, CNTs similar to wires connected the GR sheets, forming the interconnected network structures. It was found that the microstructures were influenced by the mass ratio of CNTs to GR. The result indicated that the addition of a small amount of CNTs was beneficial to form continuous conductive pathways. However, it was observed that the addition of lots of CNTs was not conductive to form continuous conductive pathways, such as CNT6.4-GR1.6-PR (4:1) (Figure 2c and d). With the decrease of GR, the CNTs tended to aggregate and the GR sheets were difficult to be connected by CNTs. As a result, the continuous conductive pathways were not formed. The microstructures were also affected by the total carbon content. When the total carbon content was improved to 12 wt % (such as CNT9.6GR2.4-PR), it was found that large amounts of conductive pathways were observed, and these conductive pathways were connected by CNTs (Figure 2e and f). These results showed that the microstructures were obviously influenced by the mass ratio of CNTs to GR and the total carbon content, and easily adjusted by the above two parameters. 3.2. Alternating Current Conductivity of the CNTs-GRPR. The relationship between the alternating conductivity (σac)
Figure 1. (a) XPS C1s spectrum of the GR powder. (b) XPS C1s spectrum of the CNTs powder. (c) XRD patterns of the GR and CNTs powder.
and four characteristic peaks at 284.4 (CC), 285.4 (C−OH), 286.7 (C−O−C), and 288.8 eV (O−CO) were observed, indicating that the surface of CNTs was functionalized by some oxygen-containing groups.30−32 The structures of the GR and CNTs powder were further analyzed by XRD. As shown in Figure 1c, an obvious diffraction peak at 2θ = 26.83° appeared, resulting from the (002) crystal plane of GR.33 The interplanar spacing of (002) was calculated to be about 0.33 nm from the Bragg equation. CNTs had an XRD structure similar to that of GR, and the interplanar spacing of (002) was about 0.34 nm.34,35 Figure 2 shows morphologies of the CNTs-GR-PR with different mass ratios of CNTs to GR and the total carbon C
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Figure 3. Frequency dependences of ac conductivity (σac) from the composites with different mass ratios of CNTs to GR. Total carbon content was 1.4 wt % (a), 6.6 wt % (b), 10 wt % (c), and 12 wt % (d).
phenomenon was also observed when the total carbon content was 8 wt % (Figure S1b). When the total carbon content was 10 wt % (Figure 3c), it was observed that the σac values of CNT2.5-GR7.5-PR and CNT5-GR5-PR were higher than that from the GR10-PR. However, the σac values of CNT6.7-GR3.3-PR and CNT8-GR2-PR were lower than that of GR10-PR. When the total carbon content exceeded 12 wt % (Figure 3d), it was found that the conductive mechanism of all samples showed skin effect. These results reflected that, as compared to the GRPR composites, the addition of an appropriate amount of CNTs improved the σac, and the excess CNTs reduced the σac when the total carbon content exceeded 2.6 wt %. 3.3. Dielectric Properties of the CNTs-GR-PR. Frequency dispersions of the real permittivity (ε′) from the CNTsGR-PR with different mass ratios of CNTs to GR and total carbon content are given in Figure 4. From Figure 4a, it revealed the relationship between ε′ with frequency when the total carbon content was 1.4 wt %, and it was found that the ε′ of all samples was positive. Under an alternating electric field, there is a polarization electric field Ep, induced electric field ΣEi, and original electric field Ef in the composites. When the total carbon content was low and the continuous conductive pathways were not formed, the ΣEi was small resulting in ΣEi + Ep < Ef, so the ε′ was positive.38 The permittivity was weak and the ε′ reduced with the frequency due to the enhanced electric leakage,39,40 but the dispersion phenomenon of the composites was not obvious. For example, the change of ε′ from GR1.4-PR did not exceed 11.32% over the whole frequency. As compared to GR1.4-PR, the ε′ of the CNTsGR-PR composites increased after CNTs were added to the GR-PR composite, and the ε′ increased with increase of the CNTs. When the total carbon content was low, GR sheets isolated by the PR matrix were equal to microcapacitor
with the frequency from the CNTs-GR-PR with different mass ratios of CNTs to GR and total carbon content is given in Figure 3. When the total carbon content was 1.4 wt % (Figure 3a), it was found that the σac of all samples linearly increased with the frequency, indicating that the hopping conduction played an important role in the conductive mechanism analyzed by the Jonscher power law.36,37 As compared to GR1.4-PR, the σac of the CNTs-GR-PR declined after adding CNTs, and the σac decreased with increase of the CNTs content. The total carbon content in the composites was only 1.4 wt %, so the conductive phases were isolated in the insulating matrix and continuous conductive pathways were not formed. Because of the relatively poor conductivity of CNTs, the σac of the CNTsGR-PR was reduced with the increase of CNTs. A similar phenomenon was also observed for the total carbon content of 2.6 wt % (Figure S1a). Figure 3b shows the relationship between the σac with the frequency when the total carbon content was 6.6 wt %. As compared to other samples (Figure 3a), it was found that the σac of CNT1.6-GR5-PR declined with the frequency, indicating the different conductive mechanisms. The phenomenon was explained by the skin effect.10 A 3D interconnected conductive network was formed in CNT1.6GR5-PR, so the electronic conduction played the role in the conductive mechanism. The σac values of CNT1.6-GR5-PR, CNT3.3-GR3.3-PR, and CNT4.4-GR2.2-PR were higher than that from the GR6.6-PR. However, the σac of CNT5.3-GR1.3-PR was lower than that of GR6.6-PR. These results indicated that the addition of an appropriate amount of CNTs improved the σac. The σac was significantly improved when CNTs were only added to 1.6 wt %. CNTs acted like micro wires and connected the GR sheets, resulting in the enhancement of conductivity. However, CNTs had greater resistance than GR, resulting in the decrease of conductivity for the excess CNTs. A similar D
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Figure 4. Frequency dependences of the real permittivity (ε′) from the composites with different mass ratios of CNTs to GR. Total carbon content was 1.4 wt % (a), 6.6 wt % (b), 8 wt % (c), 10 wt % (d), and 12 wt % (e).
positive. The appearance of negative permittivity was ascribed to the formation of continuous conductive pathways in the composite. It was interesting to note that the negative permittivity declined with the frequency, which was not consistent with the Drude model.42 It was speculated that the ε′ of CNT1.6-GR5-PR experienced a positive to negative transition below 40 MHz, which was not in the test frequency range. These results showed that the addition of an appropriate amount of CNTs (the mass ratio of CNTs to GR was 0.33:1) induced the appearance of negative permittivity when the total carbon content was 6.6 wt %. When the total carbon content was improved to 8 wt % (Figure 4c), it was found that negative permittivity appeared not only in CNT2-GR6-PR but also in CNT4-GR4-PR. However, the permittivity of GR8-PR was still positive in the frequency range. The negative permittivity of CNT2-GR6-PR increased with frequency, which was well fitted
electrodes. According to the results from XPS, CNTs surface contained some oxygen-containing groups, which reduced the direct contact between the adjacent CNTs and was beneficial to form more microcapacitors in the composites. In addition, the charge carriers in the interface between GR and CNTs, GR and PR, and CNTs and PR were entrapped due to interfacial polarization (Maxwell−Wagner−Sillars effect).41 As a result, the permittivity was enhanced. A similar phenomenon was also observed when the total carbon content was 2.6 wt % (Figure S2a). The results showed that the addition of CNTs improved the ε′ when the total carbon content was low (1.4 and 2.6 wt %). Figure 4b shows the frequency dispersions of the ε′ from CNTs-GR-PR with the total carbon content of 6.6 wt %. It was found that the permittivity of CNT1.6-GR5-PR, with the mass ratio of CNTs to GR of 0.33:1, was negative in the whole frequency. However, the permittivity of other samples was E
DOI: 10.1021/acs.jpcc.7b02858 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 5. Frequency dependences of the imaginary permittivity (ε″) from the composites with different mass ratios of CNTs to GR. Total carbon content was 1.4 wt % (a), 2.6 wt % (b), and 6.6 wt % (c).
that negative permittivity appeared in GR10-PR, attributed to the formation of continuous conductive pathways. Besides, the ε′ of GR10-PR showed a positive value at low frequencies and decreased to a negative value at about 700 MHz. For CNT2.5GR7.5-PR and CNT5-GR5-PR, the addition of an appropriate amount of CNTs in GR-PR composites led to the negative permittivity in the whole frequency and enhanced the absolute values of negative permittivity. The solid lines were fitted by the Drude model. The R-factor of the CNT2.5-GR7.5-PR fitted curve was 0.64, and the R-factor of the CNT5-GR5-PR fitted curve was 0.99. When the total carbon content was 12 wt % (Figure 4e), it was found that the ε′ of all samples appeared negative. The negative permittivities of CNT3-GR9-PR and CNT6-GR6-PR were higher than that of GR12-PR; however, the negative permittivities of CNT8-GR4-PR and CNT9.6-GR2.4-PR were lower than that of GR12-PR. The results indicated that the addition of an appropriate amount of CNTs in GR-PR composites enhanced the absolute values of negative permittivity. In addition, it was observed that the ε′ of CNT3-GR9-PR increased with frequency, and transferred from negative to positive at about 850 MHz. Also, the ε′ of CNT6GR6-PR was discovered to transition from negative to positive at about 840 MHz. A similar phenomenon was also observed when the total carbon content was 24 and 36 wt % (Figure S2b,c). 3.4. Dielectric Loss of the CNTs-GR-PR. Frequency dispersions of the imaginary permittivity (ε″) from the composites with different mass ratios of CNTs to GR and total carbon content are given in Figure 5. The ε″ is the signal of energy loss, and the dielectric loss was mainly the result of the interfacial polarization and conduction loss of the
by the Drude model. The Drude model gave the relationships between the ε′ and frequency:43 εr′(ω) = 1 −
εr″(ω) =
ωΡ =
ω Ρ2 ω 2 + ωτ 2
(4)
ω Ρ2ωτ ω3 + ωωτ 2
neff e 2 meff ε0
(5)
(6)
where ωp (ωp = 2πf p) was the angular plasma frequency, f p was the plasma frequency, ω (ω = 2πf) was the angular frequency of the applied electromagnetic field, ωτ (ωτ = 2πfτ) was the damping parameter, ε0 was permittivity of vacuum (8.85 × 10−12 F/m), neff was the effective concentration of conduction electrons, meff was the effective weight of electron, and e was electron charge (1.6 × 10−19 C). The solid line was fitted by the Drude model and R-factor was 0.93, indicating that the fitted curve was well consistent with the experimental results. Besides, for CNT4-GR4-PR, it was interesting that the ε′ decreased with the frequency and the ε′ turned from positive to negative at about 260 MHz, which was also not explained by the Drude model. The negative permittivity might be attributed to the appearance of a dielectric resonance, which was described as the Lorentz type.44,45 The Lorentz resonance derived from the intrinsic property, such as the induced electric dipole in the GR and CNTs. When the frequency of the electric field reached the resonance frequency, negative permittivity might appear. When the total carbon content was 10 wt % (Figure 4d), it was found F
DOI: 10.1021/acs.jpcc.7b02858 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 6. Frequency dependences of dielectric loss tangent (tan δ) from the composites with different mass ratios of CNTs to GR. Total carbon content was 1.4 wt % (a), 6.6 wt % (b), 8 wt % (c), and 24 wt % (d).
composite.46−51 From Figure 5a, it was found that the ε″ of all samples decreased with frequency. According to eq 2, the imaginary permittivity showed a ε″ ∝ (Rf)−1 behavior. As a result, the ε″ decreased with frequency and the falling slope was ascribed to the resistance (R). As compared to GR1.4-PR, the ε″ of the CNTs-GR-PR was higher after CNTs were added to the GR-PR composites. With the CNTs content increased, more microcapacitors were formed in the composites, and the interfacial polarization was enhanced. Therefore, the ε″ was improved. A similar phenomenon was also observed when the total carbon content was 2.6 wt % (Figure 5b). The results showed that the addition of CNTs improved the interfacial polarization loss when the total carbon content was low (1.4 and 2.6 wt %). However, when the total carbon content was 6.6 wt % (Figure 5c), it was observed that the ε″ values of CNT1.6GR5-PR, CNT3.3-GR3.3-PR, and CNT4.4-GR2.2-PR were higher than that of GR6.6-PR. Yet the ε″ of CNT5.3-GR1.3-PR was lower than that of GR6.6-PR. It possibly resulted from that the conduction loss played a major role in the composites when the total carbon content reached 6.6 wt %. The conductivity of the composites was increased after the addition of an appropriate amount of CNTs, resulting in the enhancement of eddy generated from the electromagnetic induction, so the conduction loss was enhanced. Yet the addition of excess CNTs led to the decrease of conductivity and lower conduction loss. A similar phenomenon was also observed when the total carbon content was 8−36 wt % (Figure S3). These results reflected that the addition of an appropriate amount of CNTs improved the conduction loss, but the excess CNTs decreased the conduction loss when the total carbon content was from 6.6 to 36 wt %.
As shown in Figure 6, it gives the relationship between the dielectric loss tangent (tan δ) with frequency from the composites with different mass ratios of CNTs to GR and total carbon content. From Figure 6a, when the total carbon content was 1.4 wt %, the tan δ of all samples decreased with frequency, attributed to the migration decrease from weakly bound conductive phases.52 Besides, the tan δ increased with CNTs content increasing, which was similar to the result of the ε″ in Figure 5a. So the addition of CNTs could enhance the dielectric loss before the continuous conductive network was formed. A similar phenomenon was also observed when the total carbon content was 2.6 wt % (Figure S4a). Also, the values of tan δ were very low when the total carbon content was 1.4 wt %. However, when the total carbon content was improved to 6.6 wt % (Figure 6b), it was found that the tan δ of CNT1.6GR5-PR was above 5 over the whole frequency and higher than 50 before 200 MHz. The tan δ of the CNTs-GR-PR decreased with CNTs content increasing. Interestingly, when the total carbon content was improved to 8 wt % (Figure 6c), it was observed that the tan δ of the CNT4-GR4-PR was above 100 before 400 MHz and reached a maximum value about 250 MHz. It was observed that the maximum value of tan δ occurred near the switching frequency of the ε′ from positive to negative in Figure 4c. The possible reason was that the current flew completely into the capacitor without flowing into the inductor near the maximum value of tan δ. Also, the jump of current needed a lot of energy, so a large amount of dielectric loss emerged.51 As compared to GR8-PR (tan δ was below 35), the tan δ of CNT4-GR4-PR was higher over the whole frequency and above 100 before 400 MHz, indicating the addition of CNTs could lead to the appearance of high dielectric loss. The maximum values of tan δ from GR10-PR, G
DOI: 10.1021/acs.jpcc.7b02858 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
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CNT 3-GR 9 -PR, and CNT 6-GR 6 -PR near the switching frequency of the ε′ were also observed (Figure S4b,c). However, when the total carbon content was 24 wt % (Figure 6d), it was found that the tan δ of GR24-PR reached a small peak of 18 at about 700 MHz, while there was not the transition of permittivity from negative to positive for GR24-PR. The same phenomenon was also observed in CNT28.8-GR7.2-PR with the total carbon content of 36 wt % (Figure S4d). Further experiments need be performed to clearly explain the phenomenon.
4. CONCLUSIONS The CNTs-GR-PR composites were prepared to observe the effects of CNTs on negative dielectric properties of GR-PR composites. It was observed that the microstructures of GR-PR composites were changed by the addition of CNTs, and easily adjusted by the mass ratio of CNTs to GR and the total carbon content. The permittivities of the CNT1.6-GR5-PR and CNT2GR6-PR were negative, resulting from the formation of continuous conductive pathways. However, the permittivity of the GR-PR composites with the total carbon content of 6.6 and 8 wt % was positive, indicating that the addition of CNTs induced the appearance of negative permittivity and exhibited the synergistic effects. When the total carbon content exceeded 6.6 wt %, the addition of an appropriate amount of CNTs (the mass ratio of CNTs to GR is 0.33:1 and 1:1) improved σac and ε″. CNTs addition in GR-PR composites improved and adjusted the negative dielectric properties by the mass ratio of CNTs to GR. The resulting CNTs-GR-PR composites with tunable negative dielectric properties show potential applications in electromagnetic interference absorbing and shielding, sensors, machine intelligence, and other fields.53−55
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b02858. Frequency dependences of ac conductivity, real and imaginary permittivities, and dielectric loss tangent from other CNTs-GR-PR composites (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-531-88393396. E-mail:
[email protected]. ORCID
Lei Qian: 0000-0001-5476-4426 Notes
The authors declare no competing financial interest.
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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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DOI: 10.1021/acs.jpcc.7b02858 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.7b02858 J. Phys. Chem. C XXXX, XXX, XXX−XXX
