Xiuchao Yaoa,
Xuechen Koua,
Jun Qiu*ab and
Mark Moloneyc
aSchool of Materials Science and Engineering, Tongji University, Shanghai 201804, PR China. E-mail: qiujun@tongji.edu.cn
bKey Laboratory of Advanced Civil Engineering Materials (Tongji University), Education of Ministry, Shanghai 201804, PR China
cDepartment of Chemistry, University of Oxford, 12 Mansfield Road, Oxford, OX1 3TA, England, UK
First published on 5th April 2016
Conductive polymers filled with carbon nanotubes have been proved to have negative permittivity but the mechanism of its strange properties has not been intensively studied. Thus we need to study the mechanism of negative permittivity of conductive polymer metamaterials and reveal the relationship between the permittivity and fillers. Polyaniline (PANI) filled with multi-walled carbon nanotubes (MWCNTs) is chosen as the target to explore the generation mechanism of the negative dielectric properties. PANI with low resistivity (CPA0) and MWCNTs/CPA0 composites synthesized are found to possess negative permittivity. A structure model of “double nano wires” is initially constructed after some structure and properties analysis, and the amplification mechanism of MWCNTs on negative permittivity of CPA0 and the transition phenomenon of permittivity from negative to positive value are correlated with this model. In order to verify the model, PANI with high resistivity (CHK0) and MWCNTs/CHK0 composites are prepared. The surface of MWCNTs is then coated with polyvinylpyrrolidone (PVP). These results further demonstrate the validity of the structure model of “double nano wires” to account for the negative permittivity of MWCNTs/PANI composites. This study provides an experimental basis on further exploration of metamaterials of carbon nanotube/conductive polymer composites.
The special properties of structural metameterial are derived from the arrangement of ordered structures. However, doped conductive polymers without obvious periodic array were found possessing such negative permittivity in recent years. Negative permittivity of doped conductive polymers obviously depended on compositions, types and distributions of constitutive materials. These metamaterials are so-called intrinsic metamaterials. These results have greatly expanded the scope of metamaterials. Epstein reported that polyaniline doped with camphor benzene sulfonic acid showed negative permittivity in the microwave frequency range in 2001.8 Gu et al. also found negative permittivity in graphene/polyaniline composites or multi-walled carbon nanotube (MWCNTs)/polyaniline composites after 2011.9–12 MWCNTs/polypropylene composites prepared by Qian and MWCNTs/phenolic resin composites prepared by Zhang also showed negative permittivity.13,14 Quasi 3D variable range hopping behavior of electrons in composite was used to explain the mechanism in above papers. The addition of carbon materials enhanced the transfer capability of electrons and weakened immobilization of charged particles so that the permittivity of the composites was reduced.
At present, the generation of negative permittivity in structural metamaterials and intrinsic metamaterials is explained by different mechanisms. Whether the internal mechanism of property of these two types of metamaterials is similar deserves to intensively study. In fact, Zhong et al. considered CNFs in CNFs/PI composites similar with the fine wires in the study of Pendry, which was essential for the realization of negative permittivity of nanocomposites, but there is no sufficient evidence to support more.15 In our study, MWCNTs/PANI composites was prepared to study the mechanism of the generation and improvement of their negative permittivity and the structure model of “double nano wires” of PANI and MWCNTs was firstly constructed. Through comparing the permittivity of MWCNTs/CPA0 (PANI with low resistivity) composites, MWCNTs/CHK0 (PANI with high resistivity) composites and PVP coated MWCNTs/CHK0 composites, the validity of the structure model of “double nano wires” is demonstrated.
Hydrochloric acid (3.55 g), potassium dichromate (7.05 g) and different contents of MWCNTs–PVP (10 wt%, 15 wt%, according to mass of MWCNTs) were dispersed in 300 ml distilled water with ultrasonic for 40 min (power 320 W). The resultant dispersion was placed into a crystallizing dish containing a mixture of ice and water. Next, aniline (An, 3.35 g) in 60 ml water was added dropwise into the dispersion under a magnetic stirring with 1000 rpm speed for 10 min when the temperature of the dispersion solution was kept lower than 10 °C. The dispersion with An was treated with ultrasonic for 60 min (320 W) and reacted for 6 h in the ice/water mixture. Finally, the product solution was filtered in a sand core funnel and washed with deionized water and ethanol until the supernatant was transparent. MWCNTs/PANI composites were obtained after drying for 6 hours in an air blast oven at 80 °C. These samples are designated as CPHK n (n = 10, 15).
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Fig. 1 Permittivity of CPA0 (PANI) filled with different content of MWCNTs and transition curves of negative permittivity to positive permittivity of PANI and MWCNTs/PANI composites. |
What are the reasons for the magnification of negative permittivity of MWCNTs/CPA0 composites? In order to answer this question, the structure of MWCNTs/CPA0 composites required Fig. 2(b) gives the XRD curves of MWCNTs, CPA0 and MWCNTs/CPA0 composites. PANI has two weak and broad crystallization peaks in 20.38° and 25.08°, one represents periodic vertical to main chains, and the other is periodic parallel to main chains.17,18 which indicate PANI possesses the crystalline region. The results are consistent with the other studies in literature.16,19,20 Two peaks of 25.82, 42.88 can be clearly seen in the curve for the MWCNTs, indicating (002) and (100) crystal plane characteristic peaks respectively, while there is no characteristic peak of MWCNTs in the MWCNTs/CPA0 composites perhaps due to their small content. Crystallization peaks in the MWCNTs/CPA0 composite indicate existence of some crystalline regions in the PANI matrix. The full width at half maxima (FWHM) of the composites added with MWCNTs is found to increase compared with CPA0, which indicates the grain size of the MWCNTs/PANI composite decreases with the addition of MWCNTs.
SEM images of CPA1, CPA3, CPA5 and CPA10 are shown in Fig. 2(c–f). The distribution of MWCNTs is uniform in all of the MWCNTs/CPA0 composites. MWCNTs in the images look denser with the increase of MWCNTs content. The materials with a certain length–diameter ratio can be clearly seen in the images, while the white core structures should be MWCNTs and the transparent outer layer is PANI. Such images indicate that MWCNTs covered with PANI is uniformly distributed in the PANI matrix, which corresponds with the results of XRD and ATR. Some long and interconnected MWCNTs can be found in Fig. 2(e and f) because of the high content of MWCNTs, which can help electron delocalization in a broad range.
The resistivity of CPA0, CPA1, CPA3, CPA5, CPA10 is shown in Fig. 3. It can be seen that the difference of resistivity is not large, respectively 1.54 ohm cm, 1.35 ohm cm, 1.24 ohm cm, 0.89 ohm cm, 0.76 ohm cm at 20 Hz. All the resistivity is in the edge of the semiconductor resistivity (close to the conductor) and decreases with the increase of MWCNTs content. This is because the resistivity of MWCNTs is lower than that of PANI and the content of MWCNTs gradually reaches the percolation threshold of MWCNTs when it increases. The “skin effect” will occur on the sample surface with frequency increasing, so the resistivity of the samples increases.
Pendry found that the structure of an array with parallel metal wires in certain frequency band could show characteristics of negative permittivity in 1996.2,3 Then Maslovski et al. used a thin metal rod with the unit length as an inductor for studying the negative permittivity through equivalent circuit methods.21 The equivalent circuit method can also be used to analysis MWCNTs/PANI composites. Long chains in PANI have two states of existence. One state is an ordered arrangement of neat chains in tiny crystalline regions, and another state is disordered arrangement of long chains existing between crystalline regions as shown in Fig. 4(a). These long chains of PANI are similar to metal thin wires because of their electrical conductivity, and here should be “nano wires” because of their dimension. Such MWCNTs are also nano wires with good electrical conductivity and regular shape, as shown in Fig. 4(b). Therefore, there are two kinds of nano wires in MWCNTs/PANI composites (MWCNTs and PANI), so we establish a structure model of “double nano wires” to investigate the generation mechanism of the negative permittivity of MWCNTs/PANI composites.
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Fig. 4 Structure models of CPA0 (PANI) and MWCNTs/PANI composites (a) “nano wires” model, (b) “double nano wires” model. |
The real situation in the materials is very complex so it is very difficult to present an accurate description only through Maxwell's equation groups. We can simplify the internal condition of the composite for analysis although we cannot provide a full accurate description of the internal condition of the composite. So how can “nano wires” generate negative permittivity? First, PANI long chains and MWCNTs with high length–diameter ratio can be equivalent to inductors with certain resistance. Under an alternative electric field, the phase of the electric field is ahead of the phase of current for π/2, as shown in Fig. 5(a). Alternating current is equivalent to an oscillating electric dipole, the phase of current is ahead of the phase of oscillating electric dipole for π/2, as shown in Fig. 5(b). Therefore, the phase of the electric field is ahead of the phase of oscillating electric dipole for π. That means the direction of the electric field strength Ef (free of charge) is always opposite to that of the induced electric field strength Ei (induced charge). These alternating dipoles have about 1–2 cycle of oscillation and overlap with new Ei to form ∑Ei by multiple effects. When the sum of the induced electric field ∑Ei and polarization electric field Ep (polarization charge) is more than the original electric field Ef, the negative permittivity of the composite will appear, as shown in Fig. 5(c).
So how do “double nano wires” in MWCNTs/PANI composites exert their effect? MWCNTs/PANI composites have two kinds of inductors, PANI and MWCNTs. Similar to the generating mechanism of the negative permittivity in the PANI above, the two inductors in MWCNTs/PANI composites are equivalent to double nano wires and result in alternating currents under an external alternating electric field. These alternating currents also generate alternating current dipoles, leading to the sum of Ei and Ep being more than Ef. However, there is a difference in the permittivity of MWCNTs/CPA0 composites and that of CPA0, for example, the permittivity of CPA1 at 20 Hz (−2.42 × 105) and the permittivity of CPA3 at 20 Hz (−2.39 × 105) are 10 times less than the permittivity of CPA0 (−2.3 × 104). This is because the two kinds of nano wires in the composites produce a synergistic effect. Nano wires of such MWCNTs seem better than those of the CPA0 due to their more regular shape and lower resistance (shown in Fig. 3), so MWCNTs themselves can produce a larger alternating electric field Ei under an alternating electric field with the help of CPA0, and at same time, they can also help CPA0 produce a larger alternating electric field Ei through improving polarization of alternating current dipoles in CPA0. Therefore, Ei (MWCNTs) + Ei (PANI) + Ep > Ei (PANI) +Ep > Ef, MWCNTs/PANI composites accordingly possess a much larger negative permittivity. This is the structure model of “double nano wires” as shown in the Fig. 4(b). It is worth mentioning that only the MWCNTs parallel to or less than 45° the direction of the electric field can be equivalent to inductors to generate alternating currents and alternating current dipoles.
It can be seen from Fig. 1(a) that CPA3 possesses the largest negative permittivity (−2.52 × 105) for all the MWCNTs/CPA0 composites, exceeding the maximum value of CPA1 because the content of MWCNTs in CPA3 is more. It also can be seen that negative permittivity of CPA5 increases while negative permittivity of CPA10 decreases. There are some MWCNTs coated with long chains of PANI in CPA5, which intertwine and form structures similar to a parallel ones so as to reduce the resistance of the long chains, result in the decrease of the inductor effect (as shown in Fig. 2(e)). The decrease in negative permittivity of CPA10 is because the better arranged MWCNTs in the MWCNTs/CPA0 composite can compensate for above problem (as shown in Fig. 2(f)), but the negative permittivity of CPA10 is greatly increased compared to that of CPA1 and CPA3. It can be observed in Fig. 1(a) that negative permittivity always appears at low frequency, and even an initial frequency of 20 Hz. This is because MWCNTs/CPA0 composites are still semiconductors and their resistance value is relatively high. Also the composite material inside belongs to a micro-nano structure so the resistance to an external electric field change is weaker compared to a large metal inductor. When the frequency is above 20 Hz, inductive reactance has become an obstacle to current.
The resistivity of PANI and its composites increase with the increasing frequency leading to the weakness of the ability of electrons to follow the external electric field. Electrons are difficult to hop between chains so that Ei is decreased, as shown in Fig. 5(c). Therefore, the sum of Ei and Ep will gradually reduce and the permittivity will gradually increase. The permittivity will translate from negative to positive value when the sum of Ei and Ep is less than Ef, and so negative permittivity phenomenon disappears.
Fig. 1(b) shows the transition curves of negative permittivity to positive permittivity of PANI and MWCNTs/PANI composites. When the content of MWCNTs in the MWCNTs/PANI composites is small, PANI may play a main role to the negative dielectric phenomena. The decrease of the grain size of PANI perhaps is due to that the addition MWCNTs destroys the grain structure of PANI, so the local resistivity of the MWCNTs/PANI composites will increase, the effect of the inductor is lost in advance and the transition frequency of the permittivity from negative to positive value will accordingly decrease. Although MWCNTs may compensate for the decrease of the resistivity in theory, it is not enough because of small content. With the increase of the content of MWCNTs in the composites, a large number of long MWCNTs can be seen (as shown in Fig. 2(e) and (f)). Because the resistivity of MWCNTs is lower than that of CPA0, MWCNTs can adapt more easily on influence of the external electric field so that the transition frequency of the permittivity from negative to positive value increases. It can also be seen from Fig. 1(b) that the transition frequency of the permittivity from negative to positive value decreases and then increases with the increase of the content of MWCNTs (0–10 wt%). In the study of WO3/PANI composite,9 the transition points can also be observed.
Curves of tanδ versus frequency of PANI with different contents of MWCNTs are shown in Fig. 6(a). It can be seen that tan
δ of all samples increases first and then decreases with frequency increasing. Though with lower negative permittivity at lower frequency, tan
δ of CPA0 and CPA5 is far higher than that of the other three samples (as shown in Fig. 6(b)). Combined with the variation tendency of the negative permittivity of MWCNTs/PANI composites in Fig. 1(a), it can be indicated from another point of view that the magnification of negative permittivity of MWCNTs/PANI composites is not caused by high dielectric loss. In the study of J. Zhu,9 the dielectric loss of pure PANI at low frequency is also higher than that of its composites.
It can be known that the values of tanδ are 6587.96, 808.341, 392.714, 3542.43 and 1504.83 with the increase of the content of MWCNTs at 20 Hz. So the addition of MWCNTs can basically reduce the dielectric loss. The peaks of tan
δ appear at 2.16 × 104 Hz, 3.1 × 103 Hz, 1 × 104 Hz, 3.9 × 104 Hz and 1.36 × 105 Hz, with values of 9.3 × 104, 4.4 × 104, 6.6 × 103, 2.6 × 104 and 4.9 × 104, respectively. It is not difficult to find that the maximum peak of tan
δ appears near the transition frequency of permittivity from negative to positive value shown in Fig. 1(b). This is because the current may fully flow into the capacitor without flowing through the inductor near the loss peak. At such frequency, the current will jump from one turn to another, which requires a large amount of energy, resulting in a large amount of dielectric loss. After crossing the peaks, tan
δ decreases with the increase of frequency and is always lower than that at low frequencies. For example, the values of tan
δ at 2 × 106 Hz are 113.857, 33.3805, 38.0846, 37.9785, 98.8486. Compared to the low frequency range, tan
δ of the MWCNTs/PANI composite at higher frequency shows a large reduction because the long range migration of some weakly bound conductive particles decreases, so the low frequency leakage and dielectric loss decrease with increase of frequency. At the same time, “the skin effect” will occur on the sample surface with frequency increasing, and so resistivity of the samples increases as shown in Fig. 3. In addition to loss peak range, the loss values of samples with MWCNTs are much lower than that of PANI in the range of 20 to 2 × 106 Hz, as shown in Fig. 6(a). The tan
δ of MWCNTs/CPA0 composites is higher than that of composites in the study of Gu.12
Fig. 7(a) provides curves of permittivity versus frequency of the CHK0 and the CHK0 composites with 1 wt%, 3 wt%, 5 wt%, 10 wt% content of MWCNTs (samples of CHK1, CHK3, CHK5, CHK10). It can be shown that the permittivity of CHK0, CHK1, CHK3 and CHK5 changes in the range of positive value in the frequency range of 20 to 2 × 106 Hz at room temperature. In study of Gu, PANI synthesis by potassium dichromate as oxidizing agent also has positive permittivity.12 The permittivities of CHK10 and CHK15 have high values at low frequency. The permittivity of CHK10 reaches 1.56 × 105 at 20 Hz, and then it decreases to the negative dielectric region at 2 × 103 Hz, and it can reach the extreme negative value of −189.78 at 1.2 × 104 Hz. Similarly, the permittivity of CHK15 reaches 1.53 × 105 at 21.6 Hz and decreases to the negative dielectric region at 2.8 × 103 Hz, and it can reach the extreme negative value of −183.021 at 3.8 × 103 Hz. The permittivities of CHK10 and CHK15 go up gradually in the negative region from the frequency of the extreme negative permittivity to 2 × 106 Hz.
Such a permittivity trend in MWCNTs/CHK0 composites is due to the change of oxidizing agent and protonic acid, resulting in a substantial increase in the resistivity of the PANI matrix, as shown in Fig. 7(b). The resistivity of CHK0 is close to the insulator while the resistivity of CPA0 is close to the conductor. The resistivity of CPA0, CPA1, CPA3, CPA5, CPA10 is respectively 3.8 × 107 ohm cm, 2.02 × 107 ohm cm, 2.19 × 107 ohm cm, 1.06 × 107 ohm cm, 15 ohm cm, 0.12 ohm cm at 20 Hz. Such a high resistivity is caused by fewer free electrons on the long chains of CHK0. The CHK0 synthesized with this approach is close to the fully oxidized state and the delocalization of electrons on the chains to be difficult due to a large proportional difference of phenylenediamine and quinonediimine in the doped polyaniline, so the function of “nano wire” of PANI is weakened or eliminated in the MWCNTs/CHK0 composites. While CHK10 and CHK15 with low resistivity is due to the increased content of MWCNTs is more than the percolation threshold of MWCNTs. The MWCNTs inside the composites is overlapped with each other. High resistivity CHK0 possesses low positive permittivity, similar with other ordinary polymer dielectrics. As one of the “double nano wires” mentioned above, MWCNTs has not lost their ability to conduct electrons. When the content of MWCNTs is low in the MWCNTs/CHK0 composites, MWCNTs are coated with high resistivity of polymer chains (as shown in Fig. 7(c)) and no conductive network is formed. The resistivity of CHK0 is high and so the alternating current dipoles cannot be conducted, although the MWCNTs still play the role of “nano wires”. When MWCNTs reach certain content, where the percolation threshold is exceeded, the resistivity of the MWCNTs/CHK0 composites rapidly decreases to a very low value, as shown in Fig. 7(b). Some of the MWCNTs have been able to transfer charges to form a conductive network when the content is up to 10 wt% in the MWCNTs/CHK0 composites. The alternating currents generated by the nano wires-MWCNTs can be passed in the conductive network, and so generate the negative permittivity. The negative permittivity of CHK10 is achieved at frequencies of more than 2.0 × 103 Hz and the negative permittivity of CHK15 is achieved at the frequencies of more than 2.8 × 103 Hz. In conclusion, this result shows that the conductive network generated by nano wires is the primary condition for negative permittivity generation.
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