Enhanced Electrical Properties in Percolative Low ...

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 17, No. 3; June 2010

645

Enhanced Electrical Properties in Percolative Low-density Polyethylene/Carbon Nanotubes Nanocomposites Hong-Tao Song, Zhi-Min Dang State Key Laboratory of Chemical Resource Engineering Beijing University of Chemical Technology, Beijing 100029, China

Jing Lv, Sheng-Hong Yao, Jun-Wei Zha Key Laboratory of Beijing City for Preparation and Possessing of Novel Polymer Materials Beijing University of Chemical Technology, Beijing 100029, China

and Yi Yin Department of Electrical Engineering Shanghai Jiao Tong University, Shanghai 200030, China

ABSTRACT Enhanced dielectric permittivity and conductivity are observed in low-density polyethylene/multiwall carbon nanotubes (LDPE/MWCNT) nanocomposites prepared via a melt-blending process and subsequent a hot-molding procedure when the volume fraction of MWCNT is near the percolation threshold. In comparison to some reported results, the present percolation threshold is high. The high percolation threshold may be attributed to the MWCNT, which are fractured easily during the melt-blending process due to the strong shear rate. For the percolative LDPE/MWCNT nanocomposite, the dielectric permittivity and conductivity at 100 Hz are about 200 and 10-4 (S m-1), respectively. The results are also explained by employing the well-known percolation theory. This LDPE/MWCNT nanocomposite, with high dielectric permittivity and high conductivity, may be considered as a potential shield layer material for use with HVDC insulators. Index Terms — Dielectric permittivity, conductivity, percolation theory, cable, nanocomposite, MWCNT, LDPE.

1 INTRODUCTION WITH the rapid development of electrical engineering and electronic technology, a demand for flexible conducting polymer composites is attracting a broad attention [1-3]. In some applications, functional polymer-based composites with high dielectric permittivity and high electrical conductivity have a potential ability to meet some special needs [5, 6]. For instance, polyethylene (PE) is one of the excellent

Manuscript received on 30 December 2008, in final form 4 November 2009.

insulators in the field of high voltage direct current (HVDC) transmission [7-10]. A functional shield layer often consisting of PE-based composite with high dielectric permittivity and electrical conductivity is important in the HVDC cable field because this type of shield layer can prevent the formation of space charges [11]. Because of the space charges produced in the shield layer, they may also be released due to the high conductivity of the shield layer. In addition, the induced opposite electrical field can also force the high electric field at irregular surface of conductors in the cables to decrease. Therefore, the suitable inner shield layer materials with high dielectric permittivities and electrical

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H.-T. Song et al.: Enhanced Electrical Properties in Percolative Low-density Polyethylene/Carbon Nanotubes Nanocomposites

conductivities are a key factor to sustain the safe running of HVDC cables. To acquire the conducting polymer composites, the sphere-like metal particles and carbon blacks (CB) with dimensions on a micron scale are often used as the functional conducting fillers. However, due to the effect of environmental factors, the electrical properties of the present shield layer materials consisting of PE and the conducting particles degrades slowly with time because the sphere-like particles in the PE are easy to move. They can then form the clusters or accumulations of particles, which often leads to a significant decrease in the conductivity of such shield layers [12]. Finally, it may affect the safe running of HVDC cables. In practice, a high concentration of conducting particles in the polymer microcompsites is necessary in order to achieve the desired electrical properties because the critical concentration is relatively high when microscale conducting particles are employed. This often makes the composites have a high cost, poor flexibility and a high density [6]. It is well known that the electrical properties of the polymer composites are mainly governed by the type, size, shape and concentration of fillers, and the dispersion of fillers within polymeric matrix as well as the processing techniques [13-15]. Very recently, nanocomposites are attracting more attention due to their special characteristics as nanoscale fillers [16-19]. The use for nanoscale conducting fillers would overcome the disadvantages mentioned above for the microcomposites. We also notice that carbon nanotubes (CNT) [20-24] and carbon nanofibers (CNF) [13, 25] with large aspect ratios have attracted growing interest as reinforcement materials for polymers due to their unusual structural, chemical and physical characteristics. These one-dimensional nanofillers have been found to improve the electrical [20-24], mechanical and thermal properties [26] of polymers at very low filler content compared to conventional microcomposites because conducting networks can be formed easily. Therefore, an extremely low percolation threshold is often discovered in the one dimension nanofillers/polymer nanocomposites [5, 13, 27, 28]. For example, rapidly increased conductivity and/or dielectric permittivity are reported in the polystyrene/1.7 vol% CNF [13], polyvinylidene fluoride/1.6 vol% CNT [27], methylvinyl silicone/1.2 vol% CNT [5], and vinylidene fluoride-trifluoroethylene copolymers/2.6 wt% polyaniline nanofibers [28] nanocomposites. The obvious advantage is that the one dimensional nanofillers are hard to move in the nanocomposites, which ensures a stable microstructure. This paper reports enhanced electrical properties in percolative polyethylene/CNT nanocomposites prepared using a melt-blending process and subsequent hot-molding

technique. The change of electrical properties is discussed in terms of percolation theory.

2 EXPERIMENTAL 2.1 SAMPLE PREPARATION Multiwall carbon nanotubes (MWCNT) with a diameter about 20-30 nm and a length about 1-2 μm (aspect ratio: 33-100) were used as conducting fillers. The MWCNT morphology is also shown in Figure 1a. Low-density 3 polyethylene (LDPE) with a density about 0.92 × 10 (kg -3 m ) and a melting temperature about 106.7 °C was employed as matrix. The LDPE/MWCNT composites with different contents of MWCNT were prepared via a melt-blending process by using a Haaker mixing machine and subsequent a hot-molding procedure. To disperse MWCNT into the LDPE matrix uniformly, and to avoid thermal degradation of the polymer matrix, the mixing time was set to 15 min at 120 oC. The blended mixtures were then hot pressed at 140 oC under 10 MPa into plates of 1 mm thickness and 10 mm diameter.

2.2 MORPHOLOGICAL OBSERVATION The morphology of fracture surface of samples was observed using a scanning electron microscope (SEM, JSM 6360LV, Japan). The composite samples were fractured in liquid nitrogen. A thin layer of gold was then deposited on them prior to SEM observation.

2.3 DIFFERENTIAL SCANNING CALORIMETRY The melting temperatures of pure LDPE and its nanocomposites were determined using a differential scanning calorimetry (DSC, Netzcsh 204-F1, Germany). The specimens of about 5 mg were sealed in aluminum pans. They were heated from 25 oC to 130 oC at a rate of 10 o C/min.

2.4 ELECTRICAL PROPERTY MEASUREMENTS Samples for the dielectric permittivity and conductivity measurements were coated with silver paint prior to the tests. Two metallic electrodes were then connected to the samples. The dynamic alternating current (AC) dielectric permittivity (ε) and conductivity ( σ ) of samples were measured by employing an impedance analyzer (Agilent model 4294A, America) in the frequency range of 102 – 107 Hz and in a temperature range of –50 - 110 °C.

3 RESULTS AND DISCUSSIONS Figure 1b and 1c show the morphologies of fractured surfaces of the LDPE/MWCNT nanocomposites with the volume fraction of MWCNT (fMWCNT) at 0.02 and 0.08, respectively. It is obvious to see from two images that most MWCNT are uniformly dispersed within LDPE matrix and we can not find MWCNT agglomerate at the



Vol. 17, No. 3; June 2010

fractured cross-surfaces of the LDPE/MWCNT nanocomposites. For the LDPE/0.08 MWCNT composite, it is clear that the MWCNT almost connect each other and a network structure of MWCNT is observed. We also notice that although this fabrication process can successfully incorporate the MWCNT as individual nanotubes into the LDPE matrix, some MWCNT are broken due to the high shear rates experienced during the course of melt-blending process. Therefore, the effective aspect ratio of MWCNT might be reduced unexpectedly in this work.

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have a significant increment with increasing the fMWCNT at fMWCNT < 0.04, which suggests, in this case that the electrical conducting networks are not formed in the LDPE/MWCNT nanocomposites. Namely, at low MWCNT concentration, the MWCNT are individually dispersed within the polymeric matrix. This leads to the composites having a low electrical conductivity. Above fMWCNT > 0.04, the increase in the σ is very remarkable (from 10-9 (S m-1) at fMWCNT =0.04 to ~ 10-4 (S m-1) at fMWCNT = 0.085). Such a marked increase in the σvalues suggests there exists a critical concentration, known also as percolation threshold (fc), of fillers. Namely, when the fMWCNT is above the fc, the electrons can easily transfer

(a) -3

10

-4

-1

Conductivity ( S m )

10

(b)

-5

10

-6

10

fc=0.081 s'=0.873

-7

10

-8

10

-9

10

-10

10

0.00

0.04

0.08

0.12

fMWCNT Figure 2. Dependences of conductivity of the LDPE/MWCNT nanocomposites on fMWCNT at 102 Hz and room temperature.

(c)

between the MWCNT via a tunnel effect and an Ohmic contact because the distance between the MWCNT is very short. In this way, electrical conducting networks are formed in the LDPE/MWCNT nanocomposites as shown in Figure 1c. The result also demonstrates an insulator-conductor translation at about fMWCNT = 0.08. The change of electrical properties of the conducing polymer composites is frequently analyzed according to the well-known percolation theory [29,30]. The relationship between fMWCNT and σ can be described by the following power law equation [4,6,29-31]:

σ ∝ ( f c − f MWCNT )− s ' ,

Figure 1. (a) TEM image of original MWCNT and SEM images of the fractured cross-surface of the LDPE/MWCNT nanocomposites at (b) fMWCNT=0.02 and (c) fMWCNT =0.08, respectively.

Figure 2 shows the dependence of σ of the LDPE/MWCNT nanocomposites on fMWCNT at room temperature and 102 Hz. It is observed that the σdoes not

for f MWCNT < f c

(1)

where fc is the percolation threshold, fMWCNT is the volume fraction of the MWCNT, s ' is the critical exponent in the insulating region. The fits of theσdata to the log-log plot of the power law in equation (1) give fc=0.081 and s’=0.873. It is obvious that the percolation in the LDPE/MWCNT threshold (fc=0.081) nanocomposites is higher than those in other CNT/polymer composites [5, 27, 32, 33]. A larger value of percolation concentration of 5.0 vol% was recorded for the polypropylene/CNF composites [34]. These data suggest that the experimental fc values of CNT or CNF



ε ∝ ( f c − f MWCNT )− q ,

for

f MWCNT < f c

(2)

where q is also a critical exponent of about 1. The fits of the ε to the log-log plot of the power law in Equation (2) give fc=0.081 and q=0.831.

Dielectric permittivity

1000 800 600

= 0.08, the σ increase with frequency slowly. When the is 0.10, there is a remarkable fMWCNT frequency-independence of conductivity at low frequency ranges, indicating the conducting networks are formed well and the frequency has an only weak effect on conductivity at low frequency. In fact, according to the

0

-1


10

fMWCNT=0.02 0.08

-2

10

0.04

-4

-6

10

-8

10

(a)

-10

10

2

10

3

4

10

5

10

6

10

10

Frequency (Hz)

-3

10

(b)

Experimetal data Linear fit of data -4

10

-5

10

fMWCNT ~ fc = 0.081 u = 0.667

-6

fc=0.081 q =0.831

0.03 0.10

10

-1

filled polymers deviate from those predicted from theoretical models. We think that the high percolation threshold in this work would mainly ascribe to the fabrication process of the composites. Additionally, the conductivity and purity of MWCNT have also an effect on the final percolation threshold. If the conductivity is low and the purity is poor, they might lead to a high percolation threshold. In this study, the MWCNT as received is directly dispersed into the LDPE via the melt-blending process. The MWCNT with high aspect ratio would be broken down under the strong shear rate, as shown in Figure 1c. However, the value of s’ is close to 1, which supports the good dispersion of MWCNT in the composites. ε of the LDPE/MWCNT Dependence of nanocomposites on fMWCNT at room temperature and 10 2 Hz is shown in Figure 3. Similarly, the percolation phenomenon is also observed. Namely, the ε of the LDPE/MWCNT nanocomposites increases with increasing the fMWCNT at fMWCNT > 0.04. The ε at fMWCNT = 0.08 is about 100, which is about 50 times larger than that of pure LDPE ( ε=2.2). And the ε at fMWCNT = 0.085 is as high as 850. So large enhancements in the ε can also be explained according to the following power law [4, 6, 29-31]:


648

10

3

10

4

10

5

10

6

10

Frequency (Hz)

400

Figure 4. (a) Dependences of conductivity of the LDPE/MWCNT nanocomposites on frequency, (b) A best fit of the conductivity to

200

Eq.(3) for the LDPE/MWCNT nanocomposite at fMWCNT =0.081.

0 0.00

0.04

0.08

0.12

fMWCNT Figure 3. Dependences of dielectric permittivity of the LDPE/MWCNT nanocomposites on fMWCNT at 102 Hz and room temperature.

Figure 4 shows the variation of σ with frequency for the LDPE/MWCNT nanocomposites at room temperature. It is clear that the dependence of the conductivity on frequency can also be divided into three types. For the composites with fMWCNT < 0.04, the σ increases quickly with improving the frequency from 102 Hz to 106 Hz. At fMWCNT

percolation theory, as fMWCNT →fc [4, 6, 29-31], the dependences of conductivity on the measured frequency can be depicted as follow:

σ (ω , f c ) ∝ ω u

(3)

where ω = 2πv and u is a critical exponent. The data for the LDPE/MWCNT nanocomposite with fMWCNT = 0.081 gives u=0.667 (see Figure 4b), which is close to the normal value for the percolation theory. For the percolation LDPE/MWCNT nanocomposite, fMWCNT



Vol. 17, No. 3; June 2010

≈ 0.081, the measured data of the conductivity is close to a straight line in conductivity versus frequency log-log plots as shown in Figure 4b. When the concentration of MWCNT in the nanocomposites is higher than percolation threshold, the composites are electrically conductive, but when the concentration is lower than percolation threshold, the composites act as insulators. And the data points of the conductivity may be connected into the curves in conductivity versus frequency log-log plots (Figure 4a).

(a)

1000

fMWCNT=0.001 0.01 0.015 0.025 0.03 0.04 0.08

800

0.005 0.02 0.035 0.10

600

-2

400 200 0 2

3

4

10

5

10

10

6

10

Frequency (Hz) 8 7

Conductivity ( S m-1)

10

10


dominated by the inter-connected paths of percolating MWCNT clusters at low frequency ranges. But it is the capacitor effect between clusters at high frequency ranges. Thereof, the conductivity of the composites has weak frequency dependence at low frequency; the increase in conductivity often appears at high frequency. Figure 6 shows the dependence of σ of the LDPE/MWCNT nanocomposite with MWCNT at fMWCNT = 0.08 on temperature. It can be seen that the σ value has weak temperature dependence. A slight decrease in the σis be observed after the temperature is above 20 oC. The σ decrease at low frequency (103 Hz) is more visible that at high frequency (106 Hz) above 40 oC. The characteristic of the decrease conductivity with increasing the temperature

(b)

103 Hz 105 Hz

-3

10

-4

-5

10

-6

10

-7

-60 -40 -20 0

20 40 60 80 100 120

Temperature( oC)

6 Figure

5

6.

Temperature

dependence

of

conductivity 3

of

4

the 5

LDPE/MWCNT nanocomposite (fMWCNT =0.08) at 10 Hz, 10 Hz, 10 Hz

4

and 106 Hz, respectively.

3 10

104 Hz 106 Hz

10

10

80 3

10

4

10

5

10

6

Frequency (Hz) Figure 5. (a) Dependences of dielectric permittivity of the LDPE/MWCNT nanocomposites on frequency at room temperature. (b) A clear dependence of permittivity on frequency for the LDPE/MWCNT nanocomposites at fMWCNT =0.001~0.04, respectively.

Figure 5a shows the variation of ε with frequency for the LDPE/MWCNT nanocomposites at room temperature. At low volume fraction of MWCNT (fMWCNT < 0.04), a weak frequency-dependence characteristic in the ε is observed in Figure 5b. At fMWCNT =0.08, the ε decreases slowly from 102 Hz to 105 Hz. However, the remarkable decrease in the ε is seen in the fMWCNT=0.10 nanocomposite as shown in Figure 5a. In fact, the frequency dependences of conductivity and dielectric permittivity of the polymer composites can be interpreted in terms of the polarization between conductive clusters and anomalous diffusion of charge carries within each cluster [35] When the MWCNT concentration is beyond the percolation threshold significantly, the conductivity is mainly



1200

649

103 Hz 105 Hz

70 60

104 Hz 106 Hz

50 40 30 20 10 -60 -40 -20 0

20 40 60 80 100 120

Temperature( oC) Figure 7. Temperature dependence of dielectric permittivity of the LDPE/MWCNT nanocomposite (fMWCNT =0.08) at 103 Hz, 104 Hz, 105 Hz and 106 Hz, respectively.

is same with the conductors. The σ value of the LDPE/MWCNT nanocomposite is as high as 10-4 (S m-1), indicating the LDPE/MWCNT nanocomposite has



650

Heat flow Endo up (m W)

a high conductivity and it would be used as one of suitable shield layer materials in HVDC fields. Figure 7 shows the dependence of ε of the LDPE/MWCNT nanocomposite with MWCNT at fMWCNT = 0.08 on temperature. It can be observed though the ε value of the LDPE/MWCNT nanocomposite is as high as 60 at -50 oC, it decreases quickly with an increase in temperature at 10 3 Hz. The remarkable decrease in the ε is attributed to a destruction of the formed percolation structure in the LDPE/MWCNT nanocomposite with MWCNT at fMWCNT = 0.08 during the thermal expansion process with rising the temperature. However, the polarization time of the LDPE/MWCNT nanocomposite at high frequency (10 6 Hz) would be very slow so that the polarization process can not catch up with the frequency change of external AC field. As a reason, we observe that the ε of the LDPE/MWCNT nanocomposite show a negligible change at 10 6 Hz ( ε: 15→10) with increasing the temperature from -50 oC to 110 oC. The ε above 10 in the broad range of temperature suggests further that this kind of the LDPE/MWCNT nanocomposite would be one of the potential internal shield layer materials in the HVDC field.

1 2 3 4

1: fMWCNT=0 2: 0.01 3: 0.03 4: 0.08

20

40

60

80

100

120

Temperature ( oC)

would assume that there is not chemical bonding action between the MWCNT and the LDPE molecules. This also leads to the MWCNT fracturing easily as shown in Figure 1b and 1c during the melt-blending process due to the strong mechanical agitation. This result also supports the high percolation threshold (fMWCNT=0.081) in the LDPE/MWCNT nanocomposites due to the decrease in the aspect ratio of MWCNT. Table 1.

LDPE/MWCNT nanocomposites.

Tm (oC)

ΔH f (J·g-1)

0.00

106.7

117.1

0.01

106.9

113.0

0.03

106.9

104.7

0.08

106.3

92.7

f MWCNT

4 CONCLUSIONS Enhanced dielectric-permittivity and conductivity are observed in the LDPE/MWCNT nanocomposites. Conductivity and dielectric permittivity of the LDPE/MWCNT nanocomposites first increase gently with the increase of MWCNT concentration, and they increase significantly as the MWCNT volume fraction is near 0.081. The results are explained well by employing the percolation theory. A relative high percolation threshold (fc=0.081) is discovered because the MWCNT is broken easily during the melting mixture process due to the strong mechanical agitation. The LDPE/MWCNT nanocomposite at fMWCNT = 0.08 displays the high dielectric permittivity and the high conductivity, which would satisfy the need as a potential shield layer materials in the HVDC field.

Figure 8. DSC thermograms of pure LDPE and the LDPE/MWCNT nanocomposites at fMWCNT =0.01, 0.03, 0.08, respectively..

The melt-blending process in this work enables the MWCNT to be incorporated as individual nanotubes into the LDPE matrix as shown in Figure 1b and 1c. Results of DSC thermogram in Figure 8 show that the melting temperature (Tm) of the LDPE/MWCNT nanocomposites with different volume fractions of MWCNT (Tm=106.9 oC at fMWCNT = 0.01 and 0.03, Tm=106.3 oC at fMWCNT = 0.08) is almost same with that (Tm=106.7 oC) of pure LDPE. But the fusion enthalpy ( Δ Hf) of the LDPE/MWCNT nanocomposites above decreases dramatically, as shown in Table I, with the increase in the volume fraction of MWCNT. The same Tm values suggest that the use of MWCNT does not have an obvious effect on the crystallization temperature and molecule weight of LDPE during the simple melting mixture process. Therefore, we

The data of DSC thermograms of pure LDPE and the

ACKNOWLEDGEMENTS This work was financially supported by NSF of China (grant No. 50977001), State Key Laboratory of Power System (SKLD09KZ03), Ministry of Education of China through Doctor Project (grant No. 20050010010), the Ministry of Science and Technology of China through 863-project (grant No.2008AA03Z307), and Program for New Century Excellent Talents in University (NCET).

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H. T. Song received the B.S. degree in electrical engineering from Chongqin University and the M.S. degree in energy engineering from Xi’an Jiaotong University. He is currently a Ph.D. degree candidate in Materials science and engineering from Beijing University of Chemical Technology.

Z. M. Dang received the Ph.D. degree in electrical engineering from Xi’an Jiaotong University. He is currently a Professor in Materials Science and Engieering, Beijing University of Chemical Technology. He is also a member of IEEE and the Dielectric and Electrical Insulation Society. His present research interests are electrical functional materials. He has published more than 100 journal papers.


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H.-T. Song et al.: Enhanced Electrical Properties in Percolative Low-density Polyethylene/Carbon Nanotubes Nanocomposites J. Lv received the B.S. degree in materials science and engineering from Beijing University of Chemical Technology. She is currently a M.S. degree candidate in materials science and engineering from Beijing University of Chemical Technology

J. W. Zha received the B.S. degree in materials physics from Jinan University. He is currently a Ph.D. degree candidate in materials science and engineering from Beijing University of Chemical Technology. He has published 1 journal paper in Applied Physics Letters.

S. H. Yao received the B.S. degree in materials science and engineering from Beijing University of Chemical Technology. She is currently a Ph.D. degree candidate in materials science and engineering from Beijing University of Chemical Technology. She has published 4 journal papers in Applied Physics Letters.

Y. Yin received the B.S., M.S. and Ph.D. degrees in electrical engineering from Xi’an Jiaotong University. He is currently a Professor in the School of electronic, information and electrical engineering in Shanghai Jiaotong University.
