- Email: [email protected]
polymer composites for charge storage applications
Composites Science and Technology 78 (2013) 24–29
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
An innovative method to reduce the energy loss of conductive filler/polymer composites for charge storage applications Mohammad Arjmand a, Mehdi Mahmoodi b, Simon Park b, Uttandaraman Sundararaj a,⇑ a b
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Canada Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Canada
a r t i c l e
i n f o
Article history: Received 24 August 2012 Received in revised form 14 January 2013 Accepted 23 January 2013 Available online 1 February 2013 Keywords: A. Carbon nanotubes A. Nanocomposites B. Electrical properties E. Injection moulding D. Transmission electron microscopy (TEM)
a b s t r a c t In this study, we present conductive filler alignment as a novel approach to reduce the dissipation factor of conductive filler/polymer composites and to widen the typically narrow concentration window near the percolation threshold, which is used to tune the dielectric properties, i.e., real permittivity and imaginary permittivity. The effects of multi-walled carbon nanotube (MWCNT) alignment on the dielectric properties for MWCNT/polystyrene composites in the X-band (8.2–12.4 GHz) were investigated by comparing the dielectric properties of injection moulded samples, where MWCNTs were aligned, versus compression moulded samples, where MWCNTs were randomly distributed. Raman spectroscopy technique was employed to verify partial alignment of MWCNTs in the injection moulded samples. The compression moulded samples showed an insulator–conductor transition window at 0.50–2.00 wt% of MWCNT, whereas the injection moulded samples showed a significantly wider transition window at 3.50– 10.00 wt% of MWCNT. Broader insulator–conductor transition window reduces challenges and risks in manipulating conductive filler/polymer composites around the percolation threshold to regulate the dielectric properties. Moreover, it was observed that MWCNT alignment improved the dielectric properties by reducing the dissipation factor. For instance, at MWCNT concentrations of 0.50 and 2.00 wt%, the compression moulded samples showed dissipation factors of 0.06 and 0.59, respectively, while the injection moulded samples presented the dissipation factors considerably lower and equal to 0.01 and 0.18, respectively. This study shows that injection moulding process, as an industrial technique, can be employed to improve significantly the dielectric properties of conductive filler/polymer composites for charge storage applications. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Consumers are demanding lighter weight and smaller electronic devices in today’s marketplace; therefore, printed circuit board (PCB) space is becoming a scarce resource. Accordingly, industry is now moving toward replacing large surface mounted capacitors with miniature capacitors embedded into PCBs [1,2]. The material requirements for embedded capacitors include high real permittivity, low leakage current (imaginary permittivity) and process compatibility with PCBs. Recently, conductive filler/ polymer composites (CPCs) have been proposed as candidates for embedded capacitors, due to their high real permittivity, low cost, light weight and process compatibility with PCBs [3–5]. According to the percolation theory, a high real permittivity with a low leakage current in CPCs can only be achieved at filler loadings very close to the percolation threshold [6]. This poses a challenge to use CPCs as charge storage materials, because of the typically nar⇑ Corresponding author. Tel.: +1 403 220 5751; fax: +1 403 284 4852. E-mail address: [email protected] (U. Sundararaj). 0266-3538/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.01.019
row insulator–conductor transition window around the percolation threshold. Two strategies are usually used to avoid the direct contact between conductive fillers and thereby obstruct the insulator–conductor transition: (1) covering the surface of conductive fillers with an insulative layer [3,7]; and, (2) introducing secondary particles as insulating barriers between conductive fillers [8,9]. Both of these methods require additional processing steps to obtain the final composite and may also adversely affect the real permittivity. Herein we present conductive filler alignment as a novel approach to reduce imaginary permittivity and to hinder the sharp insulator–conductor transition in CPCs. In this study, a multiwalled carbon nanotube/polystyrene (MWCNT/PS) composite, as a typical CPC, was employed to investigate the effects of conductive filler alignment on dielectric properties, i.e., real permittivity and imaginary permittivity. MWCNTs were chosen as the conductive fillers, due to their unique electronic structure and growing industrial usage. The alignment of MWCNTs was induced by applying a high shear/drag force using an injection moulding machine. The results showed that the MWCNT alignment led to a tremendous
25
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
decrease in the dissipation factor of the moulded samples arising from lower probability of the MWCNTs neighboring or contacting each other. The alignment of the MWCNTs also impeded the sharp increase in the imaginary permittivity near the percolation threshold. This feature of the MWCNT-aligned samples broadens the narrow filler concentration window near the percolation threshold in CPCs, which is used to adjust the dielectric properties.
dielectric properties of the aligned injection moulded samples were compared with those of the compression moulded samples, where MWCNTs were randomly distributed. A Carver compression moulder (Carver Inc., Wabash, IN) was employed to fabricate the compression moulded samples with the same dimensions as the injection moulded samples. The compression moulding process was carried out at 210 °C for 10 min under 38 MPa pressure.
2. Materials and methods
2.3. Morphological analysis
2.1. Materials
In order to investigate the morphology of the compression and injection moulded samples, transmission electron microscopy (TEM) was employed. The TEM analysis of the nanocomposites was carried out on ultramicrotomed sample sections using a Tecnai TF20 G2 FEG-TEM (FEI, Hillsboro, Oregon, USA) at a 200 kV acceleration voltage with the standard single-tilt holder. The samples were ultramicrotomed to sections of 70 nm at room temperature using a Leica EM UC6. The images were captured by a Gatan UltraScan 4000 CCD (Gatan, Pleasanton, California, USA) at 2048 2048 pixels.
A masterbatch of 20.0 wt% MWCNT in PS was obtained from Hyperion Catalysis International, Cambridge, MA, USA. According to the supplier, the MWCNTs were vapor grown and typically had an outer diameter of 10–15 nm wrapped around a hollow core with a diameter of 5 nm. The lengths ranged between 1 and 10 lm, while their density was approximately 1.75 g/cm3. The masterbatch was diluted with a pure PS (StyronÒ 610), with a density of 1.06 g/cm3, kindly provided by Americas Styrenics LLC, in order to prepare nanocomposite samples with different MWCNT concentrations. Prior to mixing, all the materials were dried at 50 °C for at least 4 h under vacuum. The composites with different concentrations of MWCNT were prepared employing a 25 mm Coperion ZSK co-rotating intermeshing twin-screw extruder operated at a barrel temperature, extruder speed and residence time of 200 °C, 150 rpm and 2 min, respectively. Considering the densities of neat PS and MWCNTs, the concentrations of prepared nanocomposites in terms of weight percent and volume percent are presented in Table 1. 2.2. Composite moulding Our previous investigations showed that there is a direct relationship between MWCNT alignment and volume resistivity [10,11]. This relationship and also the inverse relationship between volume resistivity and imaginary permittivity were the inspirations for the investigation of the effects of MWCNT alignment on the dielectric properties. Our previous studies showed that the melt temperature followed by the injection velocity had the greatest impact on the MWCNT alignment of the injection moulded MWCNT/PS nanocomposites [12]. On the other hand, the mould temperature and injection/holding pressure did not significantly affect the MWCNT alignment. Given the remarkable effects of the melt temperature and injection velocity on the MWCNT alignment in the injection moulded MWCNT/PS nanocomposites, the injection moulding process was carried out at the lowest possible melt temperature, i.e., 215 °C and the highest possible injection velocity, i.e., 240 mm s1 to obtain the MWCNT/PS nanocomposites with the greatest MWCNT alignment. The mould temperature and injection/ holding pressure employed were 60 °C and 100 bar, respectively. The injection moulded samples were used to investigate the effect of MWCNT alignment on the dielectric properties. An injection moulding machine (Boy 12A) was used to inject the MWCNT/PS nanocomposite melt into a rectangular cavity. The cavity was fed with an edge gate and had dimensions of 22.86 10.16 2.0 mm. A detailed description of the designed mould and injection moulding machine can be found in our previous studies [11,12]. To achieve a more comprehensive picture of the effects of MWCNT alignment on the dielectric properties, the
2.4. Determination of carbon nanotube length distribution The investigations of carbon nanotube length distribution using a TEM procedure was developed by Krause et al. [13].The evaluation of nanotube length distribution was conducted for the asextruded, compression moulded and injection moulded composites containing 2.00 and 10.0 wt% of MWCNT to examine the effects of both the processing and the MWCNT concentration (viscosity) on length distribution. In order to assess the nanotube length distribution in the composites, chloroform was used to dissolve the PS matrix at room temperature for 4 h without any additional treatment, until only MWCNTs remained. All dispersions were treated with a low energy ultrasonic equipment for 3 min, and then one drop of dispersion was placed on a copper grid and dried at air. A transmission electron microscope was used to take images of the collected MWCNTs. Measurement of the length of the MWCNTs was carried out for 500 individual MWCNTs using the ImageJ software. In order to measure the length of very long nanotubes, several images were stiched together. 2.5. Raman spectroscopy Raman spectroscopy was employed to verify the alignment of MWCNTs in the injection moulded samples. A Renishaw spectrometer equipped with an inVia Raman microscope was used to obtain the Raman spectra from the moulded samples. The samples were excited by a near-infrared (NIR) laser beam in regular mode. The Raman intensity measurements were performed at two normal orientations of the laser beam with respect to the flow direction, i.e., parallel and perpendicular, to obtain information about the MWCNT alignment. 2.6. Electrical and dielectric properties measurements The in-flow volume resistivity measurements were performed using two different setups. For the samples with a volume resistivity of less than 104 X cm, the measurements were conducted according to the ASTM 257-75 standards, using a Loresta GP
Table 1 The concentrations of the prepared nanocomposites in terms of weight percent and volume percent. MWCNT concentration (wt%) MWCNT concentration (vol%)
0.1 0.06
0.30 0.18
0.50 0.30
1.0 0.60
2.00 1.22
3.50 2.15
5.00 3.09
10.0 6.30
20.0 13.2
26
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
resistivity meter (MCP-T610 model, Mitsubishi Chemical Co., Japan) connected with a four-pin probe. For a volume resistivity of more than 104 X cm, the measurements were performed using a Hiresta-UP resistivity meter (MCP-HT450 model, Mitsubishi Chemical Co., Japan) connected with a piece of URS probe. The thickness volume resistivity measurements were conducted employing two different systems. For the samples with a volume resistivity of less than 104 X cm, the volume resistivity measurements were conducted using a Keithley 2400 sourcemeter, while for a volume resistivity more than 104 X cm, a Keithley 6517A electrometer was used. Both types of Keithleys were connected to a Keithley 8009 test fixture (Keithley Instruments, USA). The applied voltage for all the resistivity measurements was 10 V. For each datum, the resistivity of at least three specimens was measured. To evaluate the potential of CPCs as charge storage materials for high-frequency range applications, it is essential to characterize the high-frequency dielectric properties of CPCs. Accordingly, the dielectric properties in the X-band (8.2–12.4 GHz) frequency range were investigated in this study. The dielectric properties in the X-band are important for many military and commercial applications, e.g., Doppler, weather radars and TV picture transmitters [14]. The complex permittivity measurements in the X-band were carried out in a WR-90 rectangular waveguide using an Agilent programmable network analyzer (Model E8364B). The S-parameters of each sample were recorded one at a time and used to calculate the complex permittivity with the Nicolson–Ross–Weir method [15,16]. For each datum, the S-parameters of at least three specimens was measured. It should be mentioned that in the dielectric spectroscopy, the electromagnetic wave interacted with the samples in the thickness direction; therefore, all the dielectric properties reported in this article belong to the thickness direction.
3. Results and discussion
TEM micrographs of an injection moulded sample and a compression moulded sample of 5.00 wt% MWCNT/PS composite are shown in Fig. 1a and b, respectively. In these images, individual MWCNTs are clearly observable without any significant agglomeration, indicating that the MWCNTs were disentangled and dispersed well during mixing via twin-screw extrusion. The aligned segments of MWCNTs can be easily seen in Fig. 1a, where the white arrow shows the direction of MWCNT alignment. Due to the curved structure of MWCNTs, the MWCNTs were not ideally aligned in the flow direction; however, Fig. 1a confirms partial alignment of MWCNTs in the injection moulded samples. Fig. 1b shows that MWCNTs were randomly distributed in the compression moulded samples, and no distinct direction can be observed for the alignment of MWCNTs. In order to obtain more detailed information about the alignment of MWCNTs, a Raman spectroscopy technique was employed. The Raman spectra of MWCNT/polymer composites provide two important features: the D band (disorder band) and G band (graphite band). The D band, correlating to disorder in the sp2 hybridized graphitic structure, is more responsive to the alignment of MWCNTs than the G band, which corresponds to the in-plane vibration of the graphitic wall [17] Higher intensity ratios of Djj =D? and Gjj =G? (parallel/perpendicular to the flow direction) correspond to higher MWCNT alignment. The injection moulded samples showed Djj =D? and Gjj =G? ratios equal to 1.66 and 1.51, respectively, whereas the compression moulded samples exhibited Djj =D? and Gjj =G? of 1.01 and 1.01, respectively. From the Raman spectroscopy results, it can be claimed that the MWCNTs were
Fig. 1. TEM micrographs of (a) an injection moulded sample, and (b) a compression moulded sample. Both are 5.00 wt% MWCNT/PS composite. The white arrow in (a) indicates the flow direction.
Lave (nm)
Normalized Frequency [-]
3.1. Morphological analysis and Raman spectroscopy
417 411
As-extruded Compression Molding
363
Injection Molding
122 132 470
300
113
75 113 243 281
246
555
100 nm
0
400
800
1200
1600
2000
2400
Nanotube Length (nm) Fig. 2. Effects of moulding on length distribution of MWCNTs in 2.00 wt% MWCNT/ PS composites.
randomly distributed in the compression moulded samples, while they were partially aligned in the injection moulded samples. 3.2. The effects of processing and moulding on MWCNT length distribution Fig. 2 presents the length distribution of MWCNTs for the asextruded, injection moulded and compression moulded MWCNT/
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
PS composites with 2.00 wt% MWCNT loading. The MWCNT length distribution for the as-extruded composites ranged from less than 100 nm to 2400 nm. Considering the length distribution of MWCNTs in the masterbatch, i.e. 1–10 lm, it is obvious that twin-screw extrusion significantly shortened the length of MWCNTs by applying a high shear rate. Investigating the MWCNT length distribution of the composites showed that the as-extruded, compression moulded and injection moulded composites presented average MWCNT lengths of 417, 411 and 363 nm, respectively. All the composites showed standard deviations equal to 230 nm. These results show that the compression moulding process did not affect the MWCNT length while the injection moulding process led to a 12% reduction in the MWCNT length. This reduction can be attributed to the shear rate applied in the injection moulding process. To investigate the effect of MWCNT concentration, which correlates to melt viscosity, on the MWCNT length distribution, MWCNT length distributions of 2.00 and 10.0 wt% MWCNT/PS composites were compared with each other. It was observed that 10.0 wt% MWCNT/PS composites showed a very similar length distribution to that of 2.00 wt% MWCNT/PS composites, indicating that MWCNT concentration did not impact the MWCNT length distribution. 3.3. The effects of MWCNT alignment and length on the dielectric properties In addition to a high dielectric permittivity, CPCs used as capacitors must show a low leakage current. In general, the leakage current of a CPC has an inverse relationship with its volume resistivity; therefore, investigating the effect of the MWCNT alignment on the volume resistivity will aid us in comprehending the effect of the MWCNT alignment on the leakage current. Fig. 3 depicts the volume resistivity of the compression moulded and injection moulded samples in the flow and thickness directions as a function of MWCNT concentration. Despite the MWCNT alignment, the volume resistivity of the injection moulded samples showed very similar trends in both the in-flow and thickness directions. This fact can be related to comparable MWCNT network formation in both directions, arising from the curved structure of the MWCNTs. For both injection moulded and compression moulded samples, the volume resistivity showed a steep decline at a particular concentration (percolation threshold) and a decaying trend at higher MWCNT concentrations which can be ascribed to the formation of additional conductive networks in the composites. As shown in Fig. 3, the volume resistivity and percolation threshold in the injection moulded samples were higher than those
Fig. 3. Volume resistivity for the compression moulded and injection moulded samples of the MWCNT/PS composites as a function of MWCNT concentration.
27
in the compression moulded samples. The percolation threshold in the compression moulded samples obtained from the percolation theory was 0.70 wt%; however, it was interesting to observe that the percolation threshold in the injection moulded samples was about six times greater than the percolation threshold in the compression moulded samples. The higher volume resistivity and percolation threshold in the injection moulded samples can be attributed to the lower probability of MWCNTs neighboring or contacting each other due to MWCNT alignment [10,18]. Lower aspect ratio of the MWCNTs in the injection moulded samples can also be considered as another reason for higher volume resistivity of the injection moulded samples. Other studies showed that decrease in MWCNT aspect ratio can lead to lower conductivity and higher percolation threshold, due to significant decrease in chance of MWCNTs contacting each other [19,20]. Another important characteristic depicted in Fig. 3 is that the steep decline in the volume resistivity of the injection moulded samples around the percolation threshold was muted in comparison to that of the compression moulded samples. The logarithm of the volume resistivity for the compression moulded samples at 0.50, 1.00 and 2.00 wt% of MWCNT are 13.3, 8.7 and 5.5, respectively, showing an insulator–conductor transition at the concentration window of 0.50–2.00 wt%. The logarithm of the volume resistivity for the injection moulded samples at the MWCNT concentrations of 3.50, 5.00 and 10.00 wt% are 13.0, 10.2 and 3.6, respectively, roughly indicating an insulator–conductor transition at the concentration window of 3.50–10.0 wt%. The broader concentration window of the insulator–conductor transition in the injection moulded samples can be attributed to MWCNT alignment and a lower MWCNT aspect ratio in the injection moulded samples. This provides a significant advantage for aligned samples for use as capacitors. As mentioned previously, a high real permittivity with a low leakage current in CPCs can only be achieved at filler loadings very close to the percolation threshold. The increased real permittivity observed in CPCs near the percolation threshold results from the formation of a large number of nanocapacitors, i.e., conducting clusters isolated by thin layers of polymer [14]. These nanocapacitors enable CPCs to store large amount of charges. The insulator– conductor transition that occurs in CPCs at the percolation threshold leads to a drastic variation in the volume resistivity and imaginary permittivity; thereby it prohibits using CPCs as charge storage materials above the percolation threshold. Accordingly, there is a very narrow concentration window near the percolation threshold for high aspect ratio fillers, such as MWCNTs, to adjust the dielectric properties. In contrast, for samples with significant MWCNT alignment, there is a moderate descending trend of the volume resistivity around the percolation threshold; thus it can be claimed that the MWCNT alignment can provide a wider concentration window around the percolation threshold to regulate the dielectric properties. Fig. 4a and b shows the imaginary and real permittivities, respectively, of the compression moulded and injection moulded samples as a function of MWCNT concentration in the frequency range of the X-band. The absolute values of the complex permittivities of the compression moulded samples, shown in Fig. 4, are of the same order of magnitude as those reported previously in the X-band [21,22]. As can be observed in Fig. 4a, the imaginary permittivity increased with increase in the MWCNT concentration for both types of samples. In general, the imaginary permittivity of CPCs can result from the polarization loss, e.g., distortional and interfacial, and/or Ohmic loss. Increase in the MWCNT concentration is equivalent to an increase in the amount of mobile charge carriers (Ohmic loss) and the number of nanocapacitors (polarization loss), both of which can account for an enhancement in the imaginary permittivity.
28
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
Fig. 4. (a) Imaginary permittivity and (b) real permittivity, as a function of MWCNT concentration, for the compression moulded and injection moulded samples of the MWCNT/PS composites in the X-band.
Fig. 4a, however, shows that the imaginary permittivities of the injection moulded samples were significantly lower than those of the compression moulded samples. The imaginary permittivities for the compression moulded samples at the MWCNT concentrations of 0.50, 1.00 and 2.00 wt% (compression moulding transition window) were 0.21, 1.16 and 6.14, respectively; whereas, the imaginary permittivities for the injection moulded samples at these concentrations were significantly lower and equal to 0.02, 0.12 and 0.35, respectively. Lower imaginary permittivity of the injection moulded samples relative to the compression moulded samples can be related to inferior network formation arising from MWCNT alignment and lower MWCNT aspect ratio. The differences between the imaginary permittivities of the compression moulded and injection moulded samples were even greater at higher MWCNT concentrations. The compression moulded samples at MWCNT concentrations of 3.50, 5.00 and 10.00 wt% (the injection moulding transition window) showed the imaginary permittivities equal to 17.52, 36.24 and 189.85, respectively, while the injection moulded samples showed the imaginary permittivities equal to 1.48, 5.01 and 42.63, respectively. By increasing the MWCNT concentration, the imaginary permittivities of the compression moulded samples grew more than those of the injection moulded samples; therefore, it can be claimed that the movement scales of the electrons in each half cycle of the alternating field in the compression moulded samples, because of the greater network formation, must have grown considerably more than those of the injection moulded samples. This is what led to a very large difference between the imaginary permittivities for the two types of samples at high MWCNT concentrations. Moreover, the higher applied field between the conductive fillers in the compression moulded samples provided more chances for the electrons to pass through the polymer layer in the form of conduction current. This fact originated from higher probability of the conductive fillers neighboring each other and thus lower thickness of the insulative gaps. This resulted in more energy loss for the compression moulded samples. Fig. 4b shows that the real permittivity increased with increases in the MWCNT concentration. The enhancement of the real permittivity with an increased MWCNT concentration is well-established, and is attributed to an increase in number of nanocapacitors and a decrease in the thickness of insulative polymer gaps (nanodielectrics), both of which contributed to greater charge polarization.
Moreover, it is believed that the real permittivity of MWCNT/polymer composites is influenced by the polarization within the MWCNTs, and this also contributes to the greater real permittivity at higher MWCNT concentrations [2,14]. As presented in Fig. 4b, it was surprising to observe that the MWCNT alignment had an adverse influence on the real permittivity. At MWCNT concentrations of 0.50, 1.00 and 2.00 wt% (the compression moulding transition window), the compression moulded samples showed the real permittivities equal to 3.58 and 5.06 and 10.37, respectively, while the real permittivities of the injection moulded samples were 3.21, 3.70 and 5.24, respectively. At the MWCNT concentrations of 3.50, 5.00 and 10.00 wt% (the injection moulding transition window), the compression moulded samples showed the real permittivities equal to 15.20, 21.25 and 41.30, respectively; whereas the injection moulded samples exhibited the real permittivities equal to 8.22, 12.29, and 15.52, respectively. The difference between the real permittivities of the compression moulded and injection moulded samples can be ascribed to the greater probability that MWCNTs are in close proximity to each other in the compression moulded samples. In the narrow insulative gaps between the conductive fillers, there may be a buildup of very high field strength, which is higher than the macroscopic field strength by a factor of M (i.e., the ratio of the average size of the conducting MWCNT aggregates to the average gaps width) [23,24]. This high field strength significantly contributed to the electronic polarization of the PS matrix. In the compression moulded samples, the insulative gaps of the polymer were thinner leading to a higher applied field and greater electronic polarization of the PS matrix. Therefore, the compression moulded samples showed greater real permittivity than the injection moulded samples. Chin et al. [25] measured the dielectric properties of MWCNTs in three distinct arrangements with respect to incident electromagnetic wave, namely parallel, perpendicular and random distributions. Their results showed that dielectric properties were very high when the electromagnetic wave oscillated along the axis of the nanotubes and dropped significantly when it oscillated normal to the axis of MWCNTs. Random distribution of MWCNTs showed an intermediate value due to the combination of vertical and horizontal arrangements of MWCNTs from the electromagnetic wave. Higher dielectric properties in the longitudinal direction were related to field induced intra-band transition. The data presented in this article are in a very good agreement with the data reported by Chin et al. As verified by the TEM images, the MWCNTs in the injection moulded samples showed mostly perpendicular arrangements with respect to the incident electromagnetic wave. However, the compression moulded samples, due to the random distribution of MWCNTs, showed a combination of vertical and horizontal arrangements of MWCNTs. Therefore, the inferior dielectric properties of the injection moulded samples, relative to the compression moulded samples, can also be justified considering different MWCNT arrangements. MWCNT alignment reduced both the real permittivity and imaginary permittivity; therefore, it is necessary to evaluate the overall impact of MWCNT alignment on the dielectric properties. Hence, the dissipation factors (imaginary permittivity/real permittivity) of the compression moulded and injection moulded samples at different MWCNT concentrations were compared with each other. As can be observed in Fig. 5, the MWCNT alignment decreased the dissipation factors at all the MWCNT concentrations, demonstrating its positive effect on the dielectric properties. For instance, at the MWCNT concentrations of 0.50, 1.00 and 2.00 wt%, the dissipation factors of the compression moulded samples were 0.06, 0.23 and 0.59, respectively, while the injection moulded samples presented the dissipation factors significantly lower and equal to 0.01, 0.03 and 0.07, respectively. These results
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
Fig. 5. Dissipation factors for the compression moulded and injection moulded samples of the MWCNT/PS composites as a function of MWCNT concentration in the X-band.
prove that the positive effect of the MWCNT alignment on reducing the dissipative energy dominated its adverse effect on decreasing the capacitive energy. 4. Conclusions In conclusion, it was shown MWCNT alignment, induced by an injection moulding machine, in the MWCNT/PS composites positively influenced the dielectric properties. MWCNT alignment widened the typically narrow concentration window near the percolation threshold, which is used to tune the dielectric properties, thereby reducing challenges and risks in manipulating CPCs as charge storage materials. It was also shown that the MWCNT alignment reduced both the real permittivity and imaginary permittivity; nonetheless, the positive effect of the MWCNT alignment on reducing the imaginary permittivity overshadowed its negative effect of reducing the real permittivity. The positive impact of MWCNT alignment on the dielectric properties presented in this article is industrially significant because injection moulding is one of the most common fabrication methods for polymer nanocomposites and it can be used to control and tune dielectric properties. Acknowledgements The authors acknowledge support for this project from Natural Sciences and Engineering Research Council of Canada (NSERC). We would like to gratefully acknowledge Thomas Apperley and Dr. Michal Okoniewski for assistance with the dielectric properties measurements. We are grateful to Americas Styrenics LLC, for providing the neat polystyrene. We would like to thank Dr. Samaneh Abbasi of Ecole Polytechnique (Montreal, Canada) for assistance with Raman spectroscopy. References [1] Dang ZM, Wang L, Yin Y, Zhang Q, Lei QQ. Giant dielectric permittivities in functionalized carbon-nanotube/electroactive-polymer nanocomposites. Adv Mater 2007;19:852–7.
29
[2] Yuan JK, Yao SH, Dang ZM, Sylvestre A, Genestoux M, Bai JB. Giant dielectric permittivity nanocomposites: realizing true potential of pristine carbon nanotubes in polyvinylidene fluoride matrix through an enhanced interfacial interaction. J Phys Chem C 2011;115:5515–21. [3] Kim JB, Yi JW, Nam JE. Dielectric polymer matrix composite films of CNT coated with anatase TiO(2). Thin Solid Films 2011;519:5050–5. [4] Lei YJ, Zhao R, Zhan YQ, Meng FB, Zhong JC, Yang XL, et al. Generation of multiwalled carbon nanotubes from iron–phthalocyanine polymer and their novel dielectric properties. Chem Phys Lett 2010;496:139–42. [5] Xu JW, Wong M, Wong CP. Super high dielectric constant carbon black-filled polymer composites as integral capacitor dielectrics. In: Proc of ECTC. Las Vegas, USA; 2004. p. 536–41. [6] Dang ZM, Yao SH, Yuan JK, Bai JB. Tailored dielectric properties based on microstructure change in BaTiO(3)–carbon nanotube/polyvinylidene fluoride three-phase nanocomposites. J Phys Chem C 2010;114:13204–9. [7] Czerw R, Guo ZX, Ajayan PM, Sun YP, Carroll DL. Organization of polymers onto carbon nanotubes: a route to nanoscale assembly. NanoLett 2001;1:423–7. [8] Yao SH, Yuan JK, Dang ZM, Bai JB. High dielectric performance of threecomponent nanocomposites induced by a synergetic effect. Mater Lett 2010;64:2682–4. [9] Shen Y, Lin YH, Li M, Nan CW. High dielectric performance of polymer composite films induced by a percolating interparticle barrier layer. Adv Mater 2007;19:1418–22. [10] Arjmand M, Mahmoodi M, Gelves GA, Park S, Sundararaj U. Electrical and electromagnetic interference shielding properties of flow-induced oriented carbon nanotubes in polycarbonate. Carbon 2011;49:3430–40. [11] Arjmand M, Apperley T, Okoniewski M, Sundararaj U. Comparative study of electromagnetic interference shielding properties of injection moulded versus compression moulded multi-walled carbon nanotube/polystyrene composites. Carbon 2012;50:5126–34. [12] Mahmoodi M, Arjmand M, Sundararaj U, Park S. The electrical conductivity and electromagnetic interference shielding of injection moulded multi-walled carbon nanotube/polystyrene composites. Carbon 2012;50:1455–64. [13] Krause B, Villmow T, Boldt R, Mende M, Petzold G, Potschke P. Influence of dry grinding in a ball mill on the length of multiwalled carbon nanotubes and their dispersion and percolation behavior in melt mixed polycarbonate composites. Compos Sci Technol 2011;71:1145–53. [14] Jiang MJ, Dang ZM, Bozlar M, Miomandre F, Bai JB. Broad-frequency dielectric behaviors in multiwalled carbon nanotube/rubber nanocomposites. J Appl Phys 2009;106:084902–84906. [15] Nicolson AM, Ross GF. Measurement of intrinsic properties of materials by time-domain techniques. IEEE Trans Instrum Meas 1970;IM19:377–82. [16] Weir WB. Automatic measurement of complex dielectric-constant and permeability at microwave-frequencies. Proc IEEE 1974;62:33–6. [17] Abbasi S, Carreau PJ, Derdouri A. Flow induced orientation of multiwalled carbon nanotubes in polycarbonate nanocomposites: rheology, conductivity and mechanical properties. Polymer 2010;51:922–35. [18] Behnam A, Guo J, Ural A. Effects of nanotube alignment and measurement direction on percolation resistivity in single-walled carbon nanotube films. J Appl Phys 2007;102:044313–44317. [19] Li J, Ma PC, Chow WS, To CK, Tang BZ, Kim J-K. Correlations between percolation threshold, dispersion state, and aspect ratio of carbon nanotubes. Adv Funct Mater 2007;17:3207–15. [20] Kim D, Yun Y, Bak H, Cho S, Jin H-J. Aspect ratio control of acid modified multiwalled carbon nanotubes. Curr Appl Phys 2010;10:1046–52. [21] Watts PCP, Ponnampalam DR, Hsu WK, Barnes A, Chambers B. The complex permittivity of multi-walled carbon nanotube–polystyrene composite films in X-band. Chem Phys Lett 2003;378:609–14. [22] Che RC, Peng LM, Duan XF, Chen Q, Liang XL. Microwave absorption enhancement and complex permittivity and permeability of Fe encapsulated within carbon nanotubes. Adv Mater 2004;16:401–5. [23] Sichel EK, Gittleman JI, Sheng P. Transport properties of the compositematerial carbon–poly(vinyl chloride). Phys Rev B 1978;18:5712–6. [24] Gusev AA, Guseva OA. Voltage breakdown in random composites. Adv Eng Mater 2003;5:713–5. [25] Chin W, Lu C-L, Hsu W-K. A radiofrequency induced intra-band transition in carbon nanotubes. Carbon 2011;49:2648–52.
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
An innovative method to reduce the energy loss of conductive filler/polymer composites for charge storage applications Mohammad Arjmand a, Mehdi Mahmoodi b, Simon Park b, Uttandaraman Sundararaj a,⇑ a b
Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Canada Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Canada
a r t i c l e
i n f o
Article history: Received 24 August 2012 Received in revised form 14 January 2013 Accepted 23 January 2013 Available online 1 February 2013 Keywords: A. Carbon nanotubes A. Nanocomposites B. Electrical properties E. Injection moulding D. Transmission electron microscopy (TEM)
a b s t r a c t In this study, we present conductive filler alignment as a novel approach to reduce the dissipation factor of conductive filler/polymer composites and to widen the typically narrow concentration window near the percolation threshold, which is used to tune the dielectric properties, i.e., real permittivity and imaginary permittivity. The effects of multi-walled carbon nanotube (MWCNT) alignment on the dielectric properties for MWCNT/polystyrene composites in the X-band (8.2–12.4 GHz) were investigated by comparing the dielectric properties of injection moulded samples, where MWCNTs were aligned, versus compression moulded samples, where MWCNTs were randomly distributed. Raman spectroscopy technique was employed to verify partial alignment of MWCNTs in the injection moulded samples. The compression moulded samples showed an insulator–conductor transition window at 0.50–2.00 wt% of MWCNT, whereas the injection moulded samples showed a significantly wider transition window at 3.50– 10.00 wt% of MWCNT. Broader insulator–conductor transition window reduces challenges and risks in manipulating conductive filler/polymer composites around the percolation threshold to regulate the dielectric properties. Moreover, it was observed that MWCNT alignment improved the dielectric properties by reducing the dissipation factor. For instance, at MWCNT concentrations of 0.50 and 2.00 wt%, the compression moulded samples showed dissipation factors of 0.06 and 0.59, respectively, while the injection moulded samples presented the dissipation factors considerably lower and equal to 0.01 and 0.18, respectively. This study shows that injection moulding process, as an industrial technique, can be employed to improve significantly the dielectric properties of conductive filler/polymer composites for charge storage applications. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Consumers are demanding lighter weight and smaller electronic devices in today’s marketplace; therefore, printed circuit board (PCB) space is becoming a scarce resource. Accordingly, industry is now moving toward replacing large surface mounted capacitors with miniature capacitors embedded into PCBs [1,2]. The material requirements for embedded capacitors include high real permittivity, low leakage current (imaginary permittivity) and process compatibility with PCBs. Recently, conductive filler/ polymer composites (CPCs) have been proposed as candidates for embedded capacitors, due to their high real permittivity, low cost, light weight and process compatibility with PCBs [3–5]. According to the percolation theory, a high real permittivity with a low leakage current in CPCs can only be achieved at filler loadings very close to the percolation threshold [6]. This poses a challenge to use CPCs as charge storage materials, because of the typically nar⇑ Corresponding author. Tel.: +1 403 220 5751; fax: +1 403 284 4852. E-mail address: [email protected] (U. Sundararaj). 0266-3538/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.01.019
row insulator–conductor transition window around the percolation threshold. Two strategies are usually used to avoid the direct contact between conductive fillers and thereby obstruct the insulator–conductor transition: (1) covering the surface of conductive fillers with an insulative layer [3,7]; and, (2) introducing secondary particles as insulating barriers between conductive fillers [8,9]. Both of these methods require additional processing steps to obtain the final composite and may also adversely affect the real permittivity. Herein we present conductive filler alignment as a novel approach to reduce imaginary permittivity and to hinder the sharp insulator–conductor transition in CPCs. In this study, a multiwalled carbon nanotube/polystyrene (MWCNT/PS) composite, as a typical CPC, was employed to investigate the effects of conductive filler alignment on dielectric properties, i.e., real permittivity and imaginary permittivity. MWCNTs were chosen as the conductive fillers, due to their unique electronic structure and growing industrial usage. The alignment of MWCNTs was induced by applying a high shear/drag force using an injection moulding machine. The results showed that the MWCNT alignment led to a tremendous
25
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
decrease in the dissipation factor of the moulded samples arising from lower probability of the MWCNTs neighboring or contacting each other. The alignment of the MWCNTs also impeded the sharp increase in the imaginary permittivity near the percolation threshold. This feature of the MWCNT-aligned samples broadens the narrow filler concentration window near the percolation threshold in CPCs, which is used to adjust the dielectric properties.
dielectric properties of the aligned injection moulded samples were compared with those of the compression moulded samples, where MWCNTs were randomly distributed. A Carver compression moulder (Carver Inc., Wabash, IN) was employed to fabricate the compression moulded samples with the same dimensions as the injection moulded samples. The compression moulding process was carried out at 210 °C for 10 min under 38 MPa pressure.
2. Materials and methods
2.3. Morphological analysis
2.1. Materials
In order to investigate the morphology of the compression and injection moulded samples, transmission electron microscopy (TEM) was employed. The TEM analysis of the nanocomposites was carried out on ultramicrotomed sample sections using a Tecnai TF20 G2 FEG-TEM (FEI, Hillsboro, Oregon, USA) at a 200 kV acceleration voltage with the standard single-tilt holder. The samples were ultramicrotomed to sections of 70 nm at room temperature using a Leica EM UC6. The images were captured by a Gatan UltraScan 4000 CCD (Gatan, Pleasanton, California, USA) at 2048 2048 pixels.
A masterbatch of 20.0 wt% MWCNT in PS was obtained from Hyperion Catalysis International, Cambridge, MA, USA. According to the supplier, the MWCNTs were vapor grown and typically had an outer diameter of 10–15 nm wrapped around a hollow core with a diameter of 5 nm. The lengths ranged between 1 and 10 lm, while their density was approximately 1.75 g/cm3. The masterbatch was diluted with a pure PS (StyronÒ 610), with a density of 1.06 g/cm3, kindly provided by Americas Styrenics LLC, in order to prepare nanocomposite samples with different MWCNT concentrations. Prior to mixing, all the materials were dried at 50 °C for at least 4 h under vacuum. The composites with different concentrations of MWCNT were prepared employing a 25 mm Coperion ZSK co-rotating intermeshing twin-screw extruder operated at a barrel temperature, extruder speed and residence time of 200 °C, 150 rpm and 2 min, respectively. Considering the densities of neat PS and MWCNTs, the concentrations of prepared nanocomposites in terms of weight percent and volume percent are presented in Table 1. 2.2. Composite moulding Our previous investigations showed that there is a direct relationship between MWCNT alignment and volume resistivity [10,11]. This relationship and also the inverse relationship between volume resistivity and imaginary permittivity were the inspirations for the investigation of the effects of MWCNT alignment on the dielectric properties. Our previous studies showed that the melt temperature followed by the injection velocity had the greatest impact on the MWCNT alignment of the injection moulded MWCNT/PS nanocomposites [12]. On the other hand, the mould temperature and injection/holding pressure did not significantly affect the MWCNT alignment. Given the remarkable effects of the melt temperature and injection velocity on the MWCNT alignment in the injection moulded MWCNT/PS nanocomposites, the injection moulding process was carried out at the lowest possible melt temperature, i.e., 215 °C and the highest possible injection velocity, i.e., 240 mm s1 to obtain the MWCNT/PS nanocomposites with the greatest MWCNT alignment. The mould temperature and injection/ holding pressure employed were 60 °C and 100 bar, respectively. The injection moulded samples were used to investigate the effect of MWCNT alignment on the dielectric properties. An injection moulding machine (Boy 12A) was used to inject the MWCNT/PS nanocomposite melt into a rectangular cavity. The cavity was fed with an edge gate and had dimensions of 22.86 10.16 2.0 mm. A detailed description of the designed mould and injection moulding machine can be found in our previous studies [11,12]. To achieve a more comprehensive picture of the effects of MWCNT alignment on the dielectric properties, the
2.4. Determination of carbon nanotube length distribution The investigations of carbon nanotube length distribution using a TEM procedure was developed by Krause et al. [13].The evaluation of nanotube length distribution was conducted for the asextruded, compression moulded and injection moulded composites containing 2.00 and 10.0 wt% of MWCNT to examine the effects of both the processing and the MWCNT concentration (viscosity) on length distribution. In order to assess the nanotube length distribution in the composites, chloroform was used to dissolve the PS matrix at room temperature for 4 h without any additional treatment, until only MWCNTs remained. All dispersions were treated with a low energy ultrasonic equipment for 3 min, and then one drop of dispersion was placed on a copper grid and dried at air. A transmission electron microscope was used to take images of the collected MWCNTs. Measurement of the length of the MWCNTs was carried out for 500 individual MWCNTs using the ImageJ software. In order to measure the length of very long nanotubes, several images were stiched together. 2.5. Raman spectroscopy Raman spectroscopy was employed to verify the alignment of MWCNTs in the injection moulded samples. A Renishaw spectrometer equipped with an inVia Raman microscope was used to obtain the Raman spectra from the moulded samples. The samples were excited by a near-infrared (NIR) laser beam in regular mode. The Raman intensity measurements were performed at two normal orientations of the laser beam with respect to the flow direction, i.e., parallel and perpendicular, to obtain information about the MWCNT alignment. 2.6. Electrical and dielectric properties measurements The in-flow volume resistivity measurements were performed using two different setups. For the samples with a volume resistivity of less than 104 X cm, the measurements were conducted according to the ASTM 257-75 standards, using a Loresta GP
Table 1 The concentrations of the prepared nanocomposites in terms of weight percent and volume percent. MWCNT concentration (wt%) MWCNT concentration (vol%)
0.1 0.06
0.30 0.18
0.50 0.30
1.0 0.60
2.00 1.22
3.50 2.15
5.00 3.09
10.0 6.30
20.0 13.2
26
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
resistivity meter (MCP-T610 model, Mitsubishi Chemical Co., Japan) connected with a four-pin probe. For a volume resistivity of more than 104 X cm, the measurements were performed using a Hiresta-UP resistivity meter (MCP-HT450 model, Mitsubishi Chemical Co., Japan) connected with a piece of URS probe. The thickness volume resistivity measurements were conducted employing two different systems. For the samples with a volume resistivity of less than 104 X cm, the volume resistivity measurements were conducted using a Keithley 2400 sourcemeter, while for a volume resistivity more than 104 X cm, a Keithley 6517A electrometer was used. Both types of Keithleys were connected to a Keithley 8009 test fixture (Keithley Instruments, USA). The applied voltage for all the resistivity measurements was 10 V. For each datum, the resistivity of at least three specimens was measured. To evaluate the potential of CPCs as charge storage materials for high-frequency range applications, it is essential to characterize the high-frequency dielectric properties of CPCs. Accordingly, the dielectric properties in the X-band (8.2–12.4 GHz) frequency range were investigated in this study. The dielectric properties in the X-band are important for many military and commercial applications, e.g., Doppler, weather radars and TV picture transmitters [14]. The complex permittivity measurements in the X-band were carried out in a WR-90 rectangular waveguide using an Agilent programmable network analyzer (Model E8364B). The S-parameters of each sample were recorded one at a time and used to calculate the complex permittivity with the Nicolson–Ross–Weir method [15,16]. For each datum, the S-parameters of at least three specimens was measured. It should be mentioned that in the dielectric spectroscopy, the electromagnetic wave interacted with the samples in the thickness direction; therefore, all the dielectric properties reported in this article belong to the thickness direction.
3. Results and discussion
TEM micrographs of an injection moulded sample and a compression moulded sample of 5.00 wt% MWCNT/PS composite are shown in Fig. 1a and b, respectively. In these images, individual MWCNTs are clearly observable without any significant agglomeration, indicating that the MWCNTs were disentangled and dispersed well during mixing via twin-screw extrusion. The aligned segments of MWCNTs can be easily seen in Fig. 1a, where the white arrow shows the direction of MWCNT alignment. Due to the curved structure of MWCNTs, the MWCNTs were not ideally aligned in the flow direction; however, Fig. 1a confirms partial alignment of MWCNTs in the injection moulded samples. Fig. 1b shows that MWCNTs were randomly distributed in the compression moulded samples, and no distinct direction can be observed for the alignment of MWCNTs. In order to obtain more detailed information about the alignment of MWCNTs, a Raman spectroscopy technique was employed. The Raman spectra of MWCNT/polymer composites provide two important features: the D band (disorder band) and G band (graphite band). The D band, correlating to disorder in the sp2 hybridized graphitic structure, is more responsive to the alignment of MWCNTs than the G band, which corresponds to the in-plane vibration of the graphitic wall [17] Higher intensity ratios of Djj =D? and Gjj =G? (parallel/perpendicular to the flow direction) correspond to higher MWCNT alignment. The injection moulded samples showed Djj =D? and Gjj =G? ratios equal to 1.66 and 1.51, respectively, whereas the compression moulded samples exhibited Djj =D? and Gjj =G? of 1.01 and 1.01, respectively. From the Raman spectroscopy results, it can be claimed that the MWCNTs were
Fig. 1. TEM micrographs of (a) an injection moulded sample, and (b) a compression moulded sample. Both are 5.00 wt% MWCNT/PS composite. The white arrow in (a) indicates the flow direction.
Lave (nm)
Normalized Frequency [-]
3.1. Morphological analysis and Raman spectroscopy
417 411
As-extruded Compression Molding
363
Injection Molding
122 132 470
300
113
75 113 243 281
246
555
100 nm
0
400
800
1200
1600
2000
2400
Nanotube Length (nm) Fig. 2. Effects of moulding on length distribution of MWCNTs in 2.00 wt% MWCNT/ PS composites.
randomly distributed in the compression moulded samples, while they were partially aligned in the injection moulded samples. 3.2. The effects of processing and moulding on MWCNT length distribution Fig. 2 presents the length distribution of MWCNTs for the asextruded, injection moulded and compression moulded MWCNT/
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
PS composites with 2.00 wt% MWCNT loading. The MWCNT length distribution for the as-extruded composites ranged from less than 100 nm to 2400 nm. Considering the length distribution of MWCNTs in the masterbatch, i.e. 1–10 lm, it is obvious that twin-screw extrusion significantly shortened the length of MWCNTs by applying a high shear rate. Investigating the MWCNT length distribution of the composites showed that the as-extruded, compression moulded and injection moulded composites presented average MWCNT lengths of 417, 411 and 363 nm, respectively. All the composites showed standard deviations equal to 230 nm. These results show that the compression moulding process did not affect the MWCNT length while the injection moulding process led to a 12% reduction in the MWCNT length. This reduction can be attributed to the shear rate applied in the injection moulding process. To investigate the effect of MWCNT concentration, which correlates to melt viscosity, on the MWCNT length distribution, MWCNT length distributions of 2.00 and 10.0 wt% MWCNT/PS composites were compared with each other. It was observed that 10.0 wt% MWCNT/PS composites showed a very similar length distribution to that of 2.00 wt% MWCNT/PS composites, indicating that MWCNT concentration did not impact the MWCNT length distribution. 3.3. The effects of MWCNT alignment and length on the dielectric properties In addition to a high dielectric permittivity, CPCs used as capacitors must show a low leakage current. In general, the leakage current of a CPC has an inverse relationship with its volume resistivity; therefore, investigating the effect of the MWCNT alignment on the volume resistivity will aid us in comprehending the effect of the MWCNT alignment on the leakage current. Fig. 3 depicts the volume resistivity of the compression moulded and injection moulded samples in the flow and thickness directions as a function of MWCNT concentration. Despite the MWCNT alignment, the volume resistivity of the injection moulded samples showed very similar trends in both the in-flow and thickness directions. This fact can be related to comparable MWCNT network formation in both directions, arising from the curved structure of the MWCNTs. For both injection moulded and compression moulded samples, the volume resistivity showed a steep decline at a particular concentration (percolation threshold) and a decaying trend at higher MWCNT concentrations which can be ascribed to the formation of additional conductive networks in the composites. As shown in Fig. 3, the volume resistivity and percolation threshold in the injection moulded samples were higher than those
Fig. 3. Volume resistivity for the compression moulded and injection moulded samples of the MWCNT/PS composites as a function of MWCNT concentration.
27
in the compression moulded samples. The percolation threshold in the compression moulded samples obtained from the percolation theory was 0.70 wt%; however, it was interesting to observe that the percolation threshold in the injection moulded samples was about six times greater than the percolation threshold in the compression moulded samples. The higher volume resistivity and percolation threshold in the injection moulded samples can be attributed to the lower probability of MWCNTs neighboring or contacting each other due to MWCNT alignment [10,18]. Lower aspect ratio of the MWCNTs in the injection moulded samples can also be considered as another reason for higher volume resistivity of the injection moulded samples. Other studies showed that decrease in MWCNT aspect ratio can lead to lower conductivity and higher percolation threshold, due to significant decrease in chance of MWCNTs contacting each other [19,20]. Another important characteristic depicted in Fig. 3 is that the steep decline in the volume resistivity of the injection moulded samples around the percolation threshold was muted in comparison to that of the compression moulded samples. The logarithm of the volume resistivity for the compression moulded samples at 0.50, 1.00 and 2.00 wt% of MWCNT are 13.3, 8.7 and 5.5, respectively, showing an insulator–conductor transition at the concentration window of 0.50–2.00 wt%. The logarithm of the volume resistivity for the injection moulded samples at the MWCNT concentrations of 3.50, 5.00 and 10.00 wt% are 13.0, 10.2 and 3.6, respectively, roughly indicating an insulator–conductor transition at the concentration window of 3.50–10.0 wt%. The broader concentration window of the insulator–conductor transition in the injection moulded samples can be attributed to MWCNT alignment and a lower MWCNT aspect ratio in the injection moulded samples. This provides a significant advantage for aligned samples for use as capacitors. As mentioned previously, a high real permittivity with a low leakage current in CPCs can only be achieved at filler loadings very close to the percolation threshold. The increased real permittivity observed in CPCs near the percolation threshold results from the formation of a large number of nanocapacitors, i.e., conducting clusters isolated by thin layers of polymer [14]. These nanocapacitors enable CPCs to store large amount of charges. The insulator– conductor transition that occurs in CPCs at the percolation threshold leads to a drastic variation in the volume resistivity and imaginary permittivity; thereby it prohibits using CPCs as charge storage materials above the percolation threshold. Accordingly, there is a very narrow concentration window near the percolation threshold for high aspect ratio fillers, such as MWCNTs, to adjust the dielectric properties. In contrast, for samples with significant MWCNT alignment, there is a moderate descending trend of the volume resistivity around the percolation threshold; thus it can be claimed that the MWCNT alignment can provide a wider concentration window around the percolation threshold to regulate the dielectric properties. Fig. 4a and b shows the imaginary and real permittivities, respectively, of the compression moulded and injection moulded samples as a function of MWCNT concentration in the frequency range of the X-band. The absolute values of the complex permittivities of the compression moulded samples, shown in Fig. 4, are of the same order of magnitude as those reported previously in the X-band [21,22]. As can be observed in Fig. 4a, the imaginary permittivity increased with increase in the MWCNT concentration for both types of samples. In general, the imaginary permittivity of CPCs can result from the polarization loss, e.g., distortional and interfacial, and/or Ohmic loss. Increase in the MWCNT concentration is equivalent to an increase in the amount of mobile charge carriers (Ohmic loss) and the number of nanocapacitors (polarization loss), both of which can account for an enhancement in the imaginary permittivity.
28
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
Fig. 4. (a) Imaginary permittivity and (b) real permittivity, as a function of MWCNT concentration, for the compression moulded and injection moulded samples of the MWCNT/PS composites in the X-band.
Fig. 4a, however, shows that the imaginary permittivities of the injection moulded samples were significantly lower than those of the compression moulded samples. The imaginary permittivities for the compression moulded samples at the MWCNT concentrations of 0.50, 1.00 and 2.00 wt% (compression moulding transition window) were 0.21, 1.16 and 6.14, respectively; whereas, the imaginary permittivities for the injection moulded samples at these concentrations were significantly lower and equal to 0.02, 0.12 and 0.35, respectively. Lower imaginary permittivity of the injection moulded samples relative to the compression moulded samples can be related to inferior network formation arising from MWCNT alignment and lower MWCNT aspect ratio. The differences between the imaginary permittivities of the compression moulded and injection moulded samples were even greater at higher MWCNT concentrations. The compression moulded samples at MWCNT concentrations of 3.50, 5.00 and 10.00 wt% (the injection moulding transition window) showed the imaginary permittivities equal to 17.52, 36.24 and 189.85, respectively, while the injection moulded samples showed the imaginary permittivities equal to 1.48, 5.01 and 42.63, respectively. By increasing the MWCNT concentration, the imaginary permittivities of the compression moulded samples grew more than those of the injection moulded samples; therefore, it can be claimed that the movement scales of the electrons in each half cycle of the alternating field in the compression moulded samples, because of the greater network formation, must have grown considerably more than those of the injection moulded samples. This is what led to a very large difference between the imaginary permittivities for the two types of samples at high MWCNT concentrations. Moreover, the higher applied field between the conductive fillers in the compression moulded samples provided more chances for the electrons to pass through the polymer layer in the form of conduction current. This fact originated from higher probability of the conductive fillers neighboring each other and thus lower thickness of the insulative gaps. This resulted in more energy loss for the compression moulded samples. Fig. 4b shows that the real permittivity increased with increases in the MWCNT concentration. The enhancement of the real permittivity with an increased MWCNT concentration is well-established, and is attributed to an increase in number of nanocapacitors and a decrease in the thickness of insulative polymer gaps (nanodielectrics), both of which contributed to greater charge polarization.
Moreover, it is believed that the real permittivity of MWCNT/polymer composites is influenced by the polarization within the MWCNTs, and this also contributes to the greater real permittivity at higher MWCNT concentrations [2,14]. As presented in Fig. 4b, it was surprising to observe that the MWCNT alignment had an adverse influence on the real permittivity. At MWCNT concentrations of 0.50, 1.00 and 2.00 wt% (the compression moulding transition window), the compression moulded samples showed the real permittivities equal to 3.58 and 5.06 and 10.37, respectively, while the real permittivities of the injection moulded samples were 3.21, 3.70 and 5.24, respectively. At the MWCNT concentrations of 3.50, 5.00 and 10.00 wt% (the injection moulding transition window), the compression moulded samples showed the real permittivities equal to 15.20, 21.25 and 41.30, respectively; whereas the injection moulded samples exhibited the real permittivities equal to 8.22, 12.29, and 15.52, respectively. The difference between the real permittivities of the compression moulded and injection moulded samples can be ascribed to the greater probability that MWCNTs are in close proximity to each other in the compression moulded samples. In the narrow insulative gaps between the conductive fillers, there may be a buildup of very high field strength, which is higher than the macroscopic field strength by a factor of M (i.e., the ratio of the average size of the conducting MWCNT aggregates to the average gaps width) [23,24]. This high field strength significantly contributed to the electronic polarization of the PS matrix. In the compression moulded samples, the insulative gaps of the polymer were thinner leading to a higher applied field and greater electronic polarization of the PS matrix. Therefore, the compression moulded samples showed greater real permittivity than the injection moulded samples. Chin et al. [25] measured the dielectric properties of MWCNTs in three distinct arrangements with respect to incident electromagnetic wave, namely parallel, perpendicular and random distributions. Their results showed that dielectric properties were very high when the electromagnetic wave oscillated along the axis of the nanotubes and dropped significantly when it oscillated normal to the axis of MWCNTs. Random distribution of MWCNTs showed an intermediate value due to the combination of vertical and horizontal arrangements of MWCNTs from the electromagnetic wave. Higher dielectric properties in the longitudinal direction were related to field induced intra-band transition. The data presented in this article are in a very good agreement with the data reported by Chin et al. As verified by the TEM images, the MWCNTs in the injection moulded samples showed mostly perpendicular arrangements with respect to the incident electromagnetic wave. However, the compression moulded samples, due to the random distribution of MWCNTs, showed a combination of vertical and horizontal arrangements of MWCNTs. Therefore, the inferior dielectric properties of the injection moulded samples, relative to the compression moulded samples, can also be justified considering different MWCNT arrangements. MWCNT alignment reduced both the real permittivity and imaginary permittivity; therefore, it is necessary to evaluate the overall impact of MWCNT alignment on the dielectric properties. Hence, the dissipation factors (imaginary permittivity/real permittivity) of the compression moulded and injection moulded samples at different MWCNT concentrations were compared with each other. As can be observed in Fig. 5, the MWCNT alignment decreased the dissipation factors at all the MWCNT concentrations, demonstrating its positive effect on the dielectric properties. For instance, at the MWCNT concentrations of 0.50, 1.00 and 2.00 wt%, the dissipation factors of the compression moulded samples were 0.06, 0.23 and 0.59, respectively, while the injection moulded samples presented the dissipation factors significantly lower and equal to 0.01, 0.03 and 0.07, respectively. These results
M. Arjmand et al. / Composites Science and Technology 78 (2013) 24–29
Fig. 5. Dissipation factors for the compression moulded and injection moulded samples of the MWCNT/PS composites as a function of MWCNT concentration in the X-band.
prove that the positive effect of the MWCNT alignment on reducing the dissipative energy dominated its adverse effect on decreasing the capacitive energy. 4. Conclusions In conclusion, it was shown MWCNT alignment, induced by an injection moulding machine, in the MWCNT/PS composites positively influenced the dielectric properties. MWCNT alignment widened the typically narrow concentration window near the percolation threshold, which is used to tune the dielectric properties, thereby reducing challenges and risks in manipulating CPCs as charge storage materials. It was also shown that the MWCNT alignment reduced both the real permittivity and imaginary permittivity; nonetheless, the positive effect of the MWCNT alignment on reducing the imaginary permittivity overshadowed its negative effect of reducing the real permittivity. The positive impact of MWCNT alignment on the dielectric properties presented in this article is industrially significant because injection moulding is one of the most common fabrication methods for polymer nanocomposites and it can be used to control and tune dielectric properties. Acknowledgements The authors acknowledge support for this project from Natural Sciences and Engineering Research Council of Canada (NSERC). We would like to gratefully acknowledge Thomas Apperley and Dr. Michal Okoniewski for assistance with the dielectric properties measurements. We are grateful to Americas Styrenics LLC, for providing the neat polystyrene. We would like to thank Dr. Samaneh Abbasi of Ecole Polytechnique (Montreal, Canada) for assistance with Raman spectroscopy. References [1] Dang ZM, Wang L, Yin Y, Zhang Q, Lei QQ. Giant dielectric permittivities in functionalized carbon-nanotube/electroactive-polymer nanocomposites. Adv Mater 2007;19:852–7.
29
[2] Yuan JK, Yao SH, Dang ZM, Sylvestre A, Genestoux M, Bai JB. Giant dielectric permittivity nanocomposites: realizing true potential of pristine carbon nanotubes in polyvinylidene fluoride matrix through an enhanced interfacial interaction. J Phys Chem C 2011;115:5515–21. [3] Kim JB, Yi JW, Nam JE. Dielectric polymer matrix composite films of CNT coated with anatase TiO(2). Thin Solid Films 2011;519:5050–5. [4] Lei YJ, Zhao R, Zhan YQ, Meng FB, Zhong JC, Yang XL, et al. Generation of multiwalled carbon nanotubes from iron–phthalocyanine polymer and their novel dielectric properties. Chem Phys Lett 2010;496:139–42. [5] Xu JW, Wong M, Wong CP. Super high dielectric constant carbon black-filled polymer composites as integral capacitor dielectrics. In: Proc of ECTC. Las Vegas, USA; 2004. p. 536–41. [6] Dang ZM, Yao SH, Yuan JK, Bai JB. Tailored dielectric properties based on microstructure change in BaTiO(3)–carbon nanotube/polyvinylidene fluoride three-phase nanocomposites. J Phys Chem C 2010;114:13204–9. [7] Czerw R, Guo ZX, Ajayan PM, Sun YP, Carroll DL. Organization of polymers onto carbon nanotubes: a route to nanoscale assembly. NanoLett 2001;1:423–7. [8] Yao SH, Yuan JK, Dang ZM, Bai JB. High dielectric performance of threecomponent nanocomposites induced by a synergetic effect. Mater Lett 2010;64:2682–4. [9] Shen Y, Lin YH, Li M, Nan CW. High dielectric performance of polymer composite films induced by a percolating interparticle barrier layer. Adv Mater 2007;19:1418–22. [10] Arjmand M, Mahmoodi M, Gelves GA, Park S, Sundararaj U. Electrical and electromagnetic interference shielding properties of flow-induced oriented carbon nanotubes in polycarbonate. Carbon 2011;49:3430–40. [11] Arjmand M, Apperley T, Okoniewski M, Sundararaj U. Comparative study of electromagnetic interference shielding properties of injection moulded versus compression moulded multi-walled carbon nanotube/polystyrene composites. Carbon 2012;50:5126–34. [12] Mahmoodi M, Arjmand M, Sundararaj U, Park S. The electrical conductivity and electromagnetic interference shielding of injection moulded multi-walled carbon nanotube/polystyrene composites. Carbon 2012;50:1455–64. [13] Krause B, Villmow T, Boldt R, Mende M, Petzold G, Potschke P. Influence of dry grinding in a ball mill on the length of multiwalled carbon nanotubes and their dispersion and percolation behavior in melt mixed polycarbonate composites. Compos Sci Technol 2011;71:1145–53. [14] Jiang MJ, Dang ZM, Bozlar M, Miomandre F, Bai JB. Broad-frequency dielectric behaviors in multiwalled carbon nanotube/rubber nanocomposites. J Appl Phys 2009;106:084902–84906. [15] Nicolson AM, Ross GF. Measurement of intrinsic properties of materials by time-domain techniques. IEEE Trans Instrum Meas 1970;IM19:377–82. [16] Weir WB. Automatic measurement of complex dielectric-constant and permeability at microwave-frequencies. Proc IEEE 1974;62:33–6. [17] Abbasi S, Carreau PJ, Derdouri A. Flow induced orientation of multiwalled carbon nanotubes in polycarbonate nanocomposites: rheology, conductivity and mechanical properties. Polymer 2010;51:922–35. [18] Behnam A, Guo J, Ural A. Effects of nanotube alignment and measurement direction on percolation resistivity in single-walled carbon nanotube films. J Appl Phys 2007;102:044313–44317. [19] Li J, Ma PC, Chow WS, To CK, Tang BZ, Kim J-K. Correlations between percolation threshold, dispersion state, and aspect ratio of carbon nanotubes. Adv Funct Mater 2007;17:3207–15. [20] Kim D, Yun Y, Bak H, Cho S, Jin H-J. Aspect ratio control of acid modified multiwalled carbon nanotubes. Curr Appl Phys 2010;10:1046–52. [21] Watts PCP, Ponnampalam DR, Hsu WK, Barnes A, Chambers B. The complex permittivity of multi-walled carbon nanotube–polystyrene composite films in X-band. Chem Phys Lett 2003;378:609–14. [22] Che RC, Peng LM, Duan XF, Chen Q, Liang XL. Microwave absorption enhancement and complex permittivity and permeability of Fe encapsulated within carbon nanotubes. Adv Mater 2004;16:401–5. [23] Sichel EK, Gittleman JI, Sheng P. Transport properties of the compositematerial carbon–poly(vinyl chloride). Phys Rev B 1978;18:5712–6. [24] Gusev AA, Guseva OA. Voltage breakdown in random composites. Adv Eng Mater 2003;5:713–5. [25] Chin W, Lu C-L, Hsu W-K. A radiofrequency induced intra-band transition in carbon nanotubes. Carbon 2011;49:2648–52.