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expanded graphite nanocomposites with high conductivity for EMI shielding application
Materials Chemistry and Physics 142 (2013) 195e198
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Cost-efficient high performance polyetheretherketone/expanded graphite nanocomposites with high conductivity for EMI shielding application R.K. Goyal* Department of Metallurgy and Materials Science, College of Engineering, Shivaji Nagar, Pune 411 005, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A sharp increase in conductivity was observed at 1.5 wt% EG content. The conductivity of 10 wt% nanocomposites is about 12.3 S cm1. This conductivity is the highest among reported value in literature.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2012 Received in revised form 22 June 2013 Accepted 7 July 2013
The cost efficient expanded graphite (EG) filled polyetheretherketone (PEEK) nanocomposites were prepared by hot pressing, which exhibited an electrical conductivity percolation threshold of 1.5 wt%. The electrical conductivity of the 1.5 wt% nanocomposite increased approximately eleven orders of magnitude than that of pure PEEK. The conductivities of 5 wt% and 10 wt% nanocomposites were increased to about 3.24 S cm1 and 12.3 S cm1, respectively. Scanning electron microscope showed 3dimensional conductive network of EG across the PEEK matrix. The significant increase in electrical conductivity of the nanocomposites leads to the tremendous increase in electromagnetic interference shielding effectiveness. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Composite materials Electronic materials Electrical properties Electron microscopy
1. Introduction Due to the fast growth in electronic industries, electromagnetic interference (EMI) has become a serious concern in modern society. The protection of the electronic devices and circuits against EMI with shielded materials has become an essential issue [1e3]. The best shielded materials must possess a good electrical conductivity.
* Tel.: þ91 20 25507275; fax: þ91 20 25507299. E-mail address: [email protected]. 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.07.005
Metals are well suited for many EMI shielding applications, but they have shortcomings like heavy weight, susceptibility to corrosion, wear, physical rigidity etc. The conductive particles such as carbon fiber, carbon nanofiber and carbon nanotubes (CNTs) filled polymer-matrix composites score over metals due to their easy processing, flexibility and low density [4e6]. The high aspect ratio of CNTs attains a low percolation threshold value. The highest electrical conductivity achieved for 15 wt% CNT filled polycarbonate nanocomposites was 0.1 S cm1 [7]. However, CNTs have certain drawbacks such as its difficulties in dispersion and alignment in the matrix, poor compatibility with polymers, and high cost (w500
196
R.K. Goyal / Materials Chemistry and Physics 142 (2013) 195e198
times) compared to graphite. Furthermore, its breakage during processing or treatment with acids results in decreased aspect ratio [8] and hence, increased percolation threshold. Expanded graphite (EG) which exhibits electrical conductivity of w104 S cm1 and EMI shielding of w130 dB (for compressed sheet) has been widely studied as conductive filler for polymer matrices. The percolation threshold for the EG filled polymer nanocomposites ranged between 0.05 vol% and 2.5 vol% [9e12]. The high aspect ratio, honeycomb structure, and the larger surface area of EG are responsible for the lower percolation threshold and excellent mechanical properties [13]. In addition, EG contains multi-pores (2e10 nm), functional acids, and OH groups which promote a good affinity of EG to polymer and provides significant improvement in mechanical properties [14]. Polyetheretherketone (PEEK) is a high performance semi-crystalline polymer which exhibits a glass transition temperature of 143 C, a crystalline melting point of 335 C, and continuous use temperature of 250 C. Its melt processing temperature is about 350e380 C. Its excellent thermal stability (up to 585 C), fire retardancy, solvent resistance, chemical resistance, and electrical insulation properties have led to PEEK replacing some of the older thermosetting polymers in electrical/ electronics applications. To the best of our knowledge, the electrical properties of PEEK/ EG nanocomposites have not been studied yet. The aim of this work is to increase the electrical conductivity of PEEK/EG nanocomposites prepared by hot pressing and also to correlate the experimental data to a power law. These nanocomposites showed a dramatic increase in electric conductivity with a percolation threshold of 1.5 wt% untreated-EG. Most importantly, the relatively low cost of EG makes these nanocomposites cost efficient EMIshielded materials. 2. Experimental procedure
Fig. 2. SEM images of PEEK/EG (3 wt%) nanocomposite at (a) 600 and (b) 3000.
The commercial PEEK powder (Grade 5300 PF) obtained from Gharda Chemicals Ltd., Gujarat, India was used as received as polymer matrix. It has a reported inherent viscosity of 0.87 dl g1 measured at a concentration of 0.5 g dl1 in concentrated H2SO4. The morphology of PEEK powder observed by SEM is shown in Fig. 1a. The PEEK powder is in the form of rods of length 10e50 mm. The EG was obtained from Defence Research and Development Organisation, Hyderabad. Fig. 1b shows SEM image of EG powder. It can be seen that EG has honeycomb (or multi-pores) like structure. Its width is more than 100 mm and the thickness is less than 100 nm, i.e., the aspect ratio of EG is about 1000. An absolute ethanol (AR grade) purchased from commercial source was used as a solvent medium for mixing EG and PEEK powders using ultrasonic bath. Pure PEEK and EG powders were dried in an oven for 3 h at 150 C. The dried EG powder was first suspended in an absolute
ethanol using ultrasonic bath for 30 min. Then, the PEEK powder was added slowly into the EG/ethanol suspension with concurrent stirring and heating at 80 C till dried powder was obtained. The resultant powder was further dried at 120 C for 10 h in a vacuum oven. Then, the PEEK/EG nanocomposites were fabricated using the hot pressing technique. The dried powder was filled in a tool steel die having diameter 13 mm. The powder was heated at an average heating rate of 10 C min1 under pressure of 45 MPa to a maximum temperature of 380 C. After soaking period of 30 min, the samples were cooled to a temperature of 100 C and then compacted samples were ejected. The same procedure was used for making nanocomposites containing 0, 1, 1.5, 2, 3, 5 and 10 wt% EG. Volume fraction (Vf) of EG particles for a given weight fraction was determined using, Vf ¼ Wf/[Wf þ Wm(rf/rm)], where Wf is the weight fraction, and rf is the density of EG (2.2 g cc1) particles. Wm
Fig. 1. SEM of (a) PEEK and (b) EG powder. Inset shows SEM image of EG at 5000.
R.K. Goyal / Materials Chemistry and Physics 142 (2013) 195e198
and rm are the weight fraction and density of PEEK matrix, respectively. The density of PEEK was considered 1.3 g cc1. The corresponding vol% of EG of these nanocomposites were 0, 0.58, 0.87, 1.16, 1.75, 2.95 and 6.03, respectively. SEM (JEOL JSM 6360A) was used to investigate the distribution of EG in the PEEK matrix. The fractured surface of sample was coated with a thin layer of platinum using sputter coater to minimize charging effects. For the measurement of electrical conductivity, samples were coated with a thin coating of silver paste to avoid contact resistance. The volume resistance of samples was determined using high resistance meter (Keithley 6517B). However, when the volume resistance is below 106 U, a 6½ digit digital multi meter (Agilent 34401A) was used. Then, the volume resistivity was measured by the relation r ¼ R(A/L), r is the resistivity, R is resistance, L is the sample thickness and A is the cross sectional area of the sample. The electrical conductivity was reported as the reciprocal of the volume resistivity.
3. Results and discussion Fig. 2a shows the SEM image of the PEEK/EG nanocomposites containing 3 wt% EG. It can be seen that the EG particles are uniformly dispersion with a 3-dimensional network in the matrix. This is probably formed due to the penetration of molten PEEK in pores of EG during processing. This special microstructure is ascribed primarily to ultrasonic dispersion followed by stirring and heating which allows solvent to evaporate quickly and reduces possibility of sedimentation of the EG in the matrix. As shown in Fig. 2b, network consists of segregated EG particles. Each particle has several fine sheets of EG confirming a very high aspect ratio of EG. Fig. 3 shows the electrical conductivity of nanocomposites as a function of EG content. The conductivity of pure PEEK is about 1013 S cm1, which is slightly lower than reported elsewhere [15]. The conductivity of the nanocomposites increases with increasing content of EG in the matrix. At a low loading of EG (<1.5 wt%), the EG particles are probably isolated from each other in the insulating PEEK matrix. Hence, EG particles do not contribute significantly to the improvement in the conductivity of nanocomposites. This result is probably because of higher inter-particle distances than the tunneling distances (wtens of angstroms) [16]. However, a sharp increase in conductivity of nanocomposites (approximately eleven orders of magnitude) was observed between 1.5 wt% (0.87 vol%) and 2 wt% (1.16 vol%) EG content, suggesting the
1.E-03
log (σc)
Conductivity (S/cm)
1.E+00
1.E-06 1.E-09
3 2 1 0 -1 -2 -3 -4
y = 1.9392x + 8.4492 R2 = 0.9933 -8
-6
-4
-2
0
1.E-12
log (V f-Vc) 1.E-15 0
1
2
3
4
5
6
7
Volume % EG in PEEK matrix Fig. 3. Electrical conductivity as a function of volume % EG in PEEK matrix. The solid line is the guide to eyes. The inset shows a logelog plot of the conductivity versus (Vf Vc). A logelog plot has a linear relationship with a correlation factor of 0.999.
197
percolation threshold of the nanocomposites is about 1.5 wt%. This clearly indicates an insulatoreconductor transition as the EG particles form a conductive network in the matrix. The conductivities of 2 wt% and 5 wt% nanocomposites are 0.05 S cm1 and 3.24 S cm1, respectively, which surpassed the antistatic criterion for space applications (>108 S cm1) and EMI shielding (>0.1 S cm1). In contrast, an increase of about nine orders of magnitude in conductivity was reported, at percolation threshold, for multi walled carbon nanotube (MWNT) filled PEEK [15]. With further addition of EG to the PEEK matrix, conductivity was increased to 12.3 S cm1 for 10 wt% EG nanocomposite which is better than the highest conductivity of about 0.1 S cm1 reported for 15 wt% CNT filled polycarbonate nanocomposites [7] and of about 0.02 S cm1 for 20 wt% purified HiPCO EMI shielding grade SWCNT nanocomposite [17]. Interestingly, the conductivity of 5 wt % EG nanocomposites is much better than the reported conductivities of 0.07 S cm1 for 7 wt% functionalized-SWCNT filled PS nanocomposites [6] and of w0.01 S cm1 for 2.5 vol% phenyl isocyanate treated-EG filled polystyrene. However, Stankovich et al. reported a very low (w0.1 vol%) percolation threshold for treatedEG filled PS nanocomposites [18]. This comparison indicates that not only percolation threshold but also value of conductivity is very important for the application point of view. We did not observe a saturation in conductivity as in other studies [19]. Compared to other nanocomposites such as polymethyl methacrylate (PMMA)/ EG [20], Nylon 6/EG [21] and PS/EG [22], better conductivity of PEEK/EG may be attributed to higher aspect ratio of EG and better mixing procedure which results in formation of 3-dimensional network of EG in the PEEK matrix as shown in SEM image. Moreover, this may be attributed to the special morphology of PEEK (see Fig. 1a) particles and honeycomb like structure of EG (see Fig. 1b) which are mechanically locked together and forms a conductive interconnected network during processing [4]. In addition, EG possessing abundant pores allows easily intercalation of PEEK chains by adsorption mechanisms during processing. The measured electrical conductivity of two phase conductive polymer nanocomposites can be described by a power law [23e26],
t
sc ¼ s0 Vf Vc ;
(1)
where, sc is the conductivity of the nanocomposite, s0 is the conductivity of the EG, Vf is the volume fraction of the EG, Vc is the critical volume fraction of EG at percolation threshold, and t is the conductivity critical exponent. The experimental results are fitted by plotting log sc versus log (Vf Vc) and shown in the inset of Fig. 3. A linear relationship with a correlation factor of 0.999 was obtained. This correlation yields a percolation threshold value of approximately 0.8 vol%. This percolation threshold is, to the best of our knowledge, about twoethree times lower than the values reported for any other two dimensional filler with high aspect ratio [27]. Such a low percolation threshold may be attributed to the high aspect ratio of EG and its excellent dispersion in the matrix. The value of t for present system is found to be 1.94 which is in excellent agreement with the theoretical universal value (t ¼ 2) of three dimensional systems. Similar results were reported for coal tar pitch/EG (t ¼ 1.9) nanocomposites [28]. The low value of the critical volume fraction (i.e., 0.008 or 0.8 vol%) for the untreated expanded graphite would be partially beneficial for industries as higher volume fraction leads to the poor processability and mechanical properties [29]. In addition, for the first time, we report highest achieved electrical conductivity of 12.3 S cm1 for the nanocomposite containing 10 wt% (6 vol%) EG. Based on the conductivity, it can be concluded that these nanocomposites would be potential candidates for antistatic and EMI shielding applications.
198
R.K. Goyal / Materials Chemistry and Physics 142 (2013) 195e198
60
the experimental EMI-SE of PEEK/EG nanocomposites would be higher than those of calculated since multiple reflections are ignored in the above empirical equation.
50
4. Conclusions
40
The electrical properties of the high performance PEEK/EG nanocomposites fabricated by a novel method have been discussed. A percolation threshold of about 1.5 wt% (w0.8 vol%) EG was obtained. The electrical conductivity increased significantly with increasing content of EG in the matrix. The significant improvement in electrical conductivity was attributed to the high aspect ratio, special morphology and better dispersion of EG particles in the PEEK matrix. The EMI-SE of the PEEK/EG nanocomposites can be obtained up to 58 dB depending upon the wt% of EG particles in the matrix. Based on the results, it may be concluded that these nanocomposites with low cost may prove to be the potential high performance materials for the antistatic/EMI shielding applications.
EMI-SE (dB)
70
30 20
1MHz 10 MHz
10
15 MHz
0 0
2
4
6
8
Volume % EG in PEEK matrix Fig. 4. Theoretical EMI-SE of the nanocomposites as a function of EG content in the matrix.
Acknowledgement Table 1 Correlation between the electrical conductivity and EMI shielding.
Miss S. V. Gune and Miss B. J. Deshpande are acknowledged for preparing composite samples.
Compositions
Electrical conductivity (S cm1)
EMI-SE (dB) (approx.)
Ref.
PS/10 wt% CNF PS/10 wt% CNF/1 wt% CNT PS/10 wt% CNF/3 wt% CNT PMMA/10 wt% SWCNT Copolymer/20 wt% MWCNT PEEK/3 wt% EG PEEK/5 wt% EG PEEK/10 wt% EG
0.001 0.00156 0.00215 0.01 0.22 0.525 3.24 12.3
12.9 20.3 21.9 28 w30 16.1a 36.5a 57.9a
[26] [26] [26] [28] [28] e e e
References
a EMI-SE calculated at 10 GHz using Equation (2). Table 1 clearly shows that PEEK/ EG nanocomposites exhibit good electrical conductivity and EMI-SE.
The EMI shielding effective (SE) of the nanocomposites can be calculated from the empirical relation shown in Equation (2) [2], 1
SE ¼ 50 þ 10 logðsc =f Þ þ 1:7tðsc f Þ2
(2)
where, sc is the electrical conductivity (S cm1) of the sample, f is the frequency (MHz) and t is the thickness (cm) of the sample. The first two terms estimate the SE due to the reflection while the last term represents absorption contribution. It is to be noted that this empirical equation ignores the effect of multiple reflections of the SE, whereas in real applications multiple reflections significantly contribute to the SE of the conductive polymer nanocomposites [28]. Fig. 4 shows the variation of calculated SE of the PEEK/EG nanocomposites as a function of content of EG in the PEEK matrix at various frequencies. As expected, SE of the nanocomposites increases with increasing content of EG in the PEEK matrix. This result is due to the higher probability of forming a conductive network in the PEEK matrix with increasing EG content. As shown in Table 1, the SE of the nanocomposites increases with increasing electrical conductivity. The SE of 5 wt% and 10 wt% EG filled PEEK nanocomposites calculated at 10 GHz is 36.5 dB and 58 dB, respectively. In contrast, the SE of the 10 wt% SWCNT filled PMMA or 20 wt% MWCNT filled copolymer nanocomposites lies approximately between 28 dB and 30 dB [30]. The higher SE of our samples may be due to the higher conductivity. Moreover, it is expected that
[1] J. Joo, A.J. Epstein, Appl. Phys. Lett. 65 (1994) 2278e2280. [2] C.-S. Zhang, Q.-Q. Ni, S.-Y. Fu, K. Kurashiki, Compos. Sci. Technol. 67 (2007) 2973e2980. [3] Y. Yang, M.C. Gupta, D. Dudley, Micro Nano Lett. 2 (2007) 85e89. [4] Y. Ishigure, S. Iijima, H. Ito, T. Ota, H. Unuma, M. Takahashi, et al., J. Mater. Sci. 34 (1999) 2979e2985. [5] J. Sandler, M.S.P. Shaffer, T. Prasse, W. Bauhofer, K. Schulte, A.H. Windle, Polymer 40 (1999) 5967e5971. [6] R. Ramasubramaniam, J. Chen, H. Liu, Appl. Phys. Lett. 83 (2003) 2928e2930. [7] P. Pötschke, T.D. Fornes, D.R. Paul, Polymer 43 (2002) 3247e3255. [8] F.J. Owens, J.R.P. Jayakody, S.G. Greenbaum, Compos. Sci. Technol. 66 (2006) 1280e1284. [9] B.-K. Zhu, S.-H. Xie, Z.-K. Xu, Y.-Y. Xu, Compos. Sci. Technol. 66 (2006) 548e554. [10] M. Alvarez, V. Poblete, J. Appl. Polym. Sci. 99 (2006) 3005e3008. [11] I. Krupa, I. Chodák, Eur. Polym. J. 37 (2001) 2159e2168. [12] R.K. Goyal, S.D. Samant, A.K. Thakar, A. Kadam, J. Phys. D Appl. Phys. 43 (2010) 365404e365410. [13] W. Zheng, X. Lu, S. Wong, J. Appl. Polym. Sci. 91 (2004) 2781e2788. [14] V. Panwar, B. Kang, J.-O. Park, S. Park, R.M. Mehra, Eur. Polym. J. 45 (2009) 1777e1784. [15] D. Bangarusampath, H. Ruckdlaschel, V. Altstadt, J. Sandler, D. Garray, M. Shaffer, Polymer 50 (2009) 5803e5811. [16] I. Balberg, Phys. Rev. Lett. 59 (1987) 1305e1308. [17] N.C. Das, Y. Liu, K. Yang, W. Peng, S. Maiti, H. Wang, Polym. Eng. Sci. 49 (2009) 1627e1634. [18] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhas, et al., Nature 442 (2006) 282e286. [19] P. Pötschke, A.R. Bhattacharyya, A. Janke, H. Goering, Compos. Interfaces 10 (2003) 389e392. [20] Y.-X. Pan, Z.-Z. Yu, Y.-C. Ou, G.-H. Hu, J. Polym. Sci. B 38 (2000) 1626e1633. [21] S.C. Tjong, G.D. Liang, S.P. Bao, Polym. Eng. Sci. 48 (2008) 177e183. [22] K.-M. Jäger, D.H. McQueen, J. Vil cáková, J. Phys. D Appl. Phys. 35 (2002) 1068e 1075. [23] D. Stauffer, Introduction to Percolation Theory, Taylor and Francis, London and Philadelphia, 1985. [24] S. Kirkpatrick, Rev. Mod. Phys. 45 (1973) 574e588. [25] G. Chen, W. Weng, D. Wu, C. Wu, Eur. Polym. J. 39 (2003) 2329e2335. [26] T. Wang, J. Liu, J. Electron. Manuf. 10 (2000) 181e189. [27] Y. Yang, M.C. Gupta, K.L. Dudley, Nanotechnology 18 (2007) 345701. [28] I.M. Afanasov, V.A. Morozov, A.V. Kepman, S.G. Ionov, A.N. Seleznew, G.V. Tendeloo, V.V. Avdeev, Carbon 47 (2009) 263e270. [29] A. Yasmin, I.M. Daniel, Polymer 45 (2004) 8211e8219. [30] Y. Li, C. Chen, J.-T. Li, S. Zhang, Y. Ni, S. Cai, J. Huang, Nanoscale Res. Lett. 5 (2010) 1170e1176.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Cost-efficient high performance polyetheretherketone/expanded graphite nanocomposites with high conductivity for EMI shielding application R.K. Goyal* Department of Metallurgy and Materials Science, College of Engineering, Shivaji Nagar, Pune 411 005, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A sharp increase in conductivity was observed at 1.5 wt% EG content. The conductivity of 10 wt% nanocomposites is about 12.3 S cm1. This conductivity is the highest among reported value in literature.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 October 2012 Received in revised form 22 June 2013 Accepted 7 July 2013
The cost efficient expanded graphite (EG) filled polyetheretherketone (PEEK) nanocomposites were prepared by hot pressing, which exhibited an electrical conductivity percolation threshold of 1.5 wt%. The electrical conductivity of the 1.5 wt% nanocomposite increased approximately eleven orders of magnitude than that of pure PEEK. The conductivities of 5 wt% and 10 wt% nanocomposites were increased to about 3.24 S cm1 and 12.3 S cm1, respectively. Scanning electron microscope showed 3dimensional conductive network of EG across the PEEK matrix. The significant increase in electrical conductivity of the nanocomposites leads to the tremendous increase in electromagnetic interference shielding effectiveness. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Composite materials Electronic materials Electrical properties Electron microscopy
1. Introduction Due to the fast growth in electronic industries, electromagnetic interference (EMI) has become a serious concern in modern society. The protection of the electronic devices and circuits against EMI with shielded materials has become an essential issue [1e3]. The best shielded materials must possess a good electrical conductivity.
* Tel.: þ91 20 25507275; fax: þ91 20 25507299. E-mail address: [email protected]. 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.07.005
Metals are well suited for many EMI shielding applications, but they have shortcomings like heavy weight, susceptibility to corrosion, wear, physical rigidity etc. The conductive particles such as carbon fiber, carbon nanofiber and carbon nanotubes (CNTs) filled polymer-matrix composites score over metals due to their easy processing, flexibility and low density [4e6]. The high aspect ratio of CNTs attains a low percolation threshold value. The highest electrical conductivity achieved for 15 wt% CNT filled polycarbonate nanocomposites was 0.1 S cm1 [7]. However, CNTs have certain drawbacks such as its difficulties in dispersion and alignment in the matrix, poor compatibility with polymers, and high cost (w500
196
R.K. Goyal / Materials Chemistry and Physics 142 (2013) 195e198
times) compared to graphite. Furthermore, its breakage during processing or treatment with acids results in decreased aspect ratio [8] and hence, increased percolation threshold. Expanded graphite (EG) which exhibits electrical conductivity of w104 S cm1 and EMI shielding of w130 dB (for compressed sheet) has been widely studied as conductive filler for polymer matrices. The percolation threshold for the EG filled polymer nanocomposites ranged between 0.05 vol% and 2.5 vol% [9e12]. The high aspect ratio, honeycomb structure, and the larger surface area of EG are responsible for the lower percolation threshold and excellent mechanical properties [13]. In addition, EG contains multi-pores (2e10 nm), functional acids, and OH groups which promote a good affinity of EG to polymer and provides significant improvement in mechanical properties [14]. Polyetheretherketone (PEEK) is a high performance semi-crystalline polymer which exhibits a glass transition temperature of 143 C, a crystalline melting point of 335 C, and continuous use temperature of 250 C. Its melt processing temperature is about 350e380 C. Its excellent thermal stability (up to 585 C), fire retardancy, solvent resistance, chemical resistance, and electrical insulation properties have led to PEEK replacing some of the older thermosetting polymers in electrical/ electronics applications. To the best of our knowledge, the electrical properties of PEEK/ EG nanocomposites have not been studied yet. The aim of this work is to increase the electrical conductivity of PEEK/EG nanocomposites prepared by hot pressing and also to correlate the experimental data to a power law. These nanocomposites showed a dramatic increase in electric conductivity with a percolation threshold of 1.5 wt% untreated-EG. Most importantly, the relatively low cost of EG makes these nanocomposites cost efficient EMIshielded materials. 2. Experimental procedure
Fig. 2. SEM images of PEEK/EG (3 wt%) nanocomposite at (a) 600 and (b) 3000.
The commercial PEEK powder (Grade 5300 PF) obtained from Gharda Chemicals Ltd., Gujarat, India was used as received as polymer matrix. It has a reported inherent viscosity of 0.87 dl g1 measured at a concentration of 0.5 g dl1 in concentrated H2SO4. The morphology of PEEK powder observed by SEM is shown in Fig. 1a. The PEEK powder is in the form of rods of length 10e50 mm. The EG was obtained from Defence Research and Development Organisation, Hyderabad. Fig. 1b shows SEM image of EG powder. It can be seen that EG has honeycomb (or multi-pores) like structure. Its width is more than 100 mm and the thickness is less than 100 nm, i.e., the aspect ratio of EG is about 1000. An absolute ethanol (AR grade) purchased from commercial source was used as a solvent medium for mixing EG and PEEK powders using ultrasonic bath. Pure PEEK and EG powders were dried in an oven for 3 h at 150 C. The dried EG powder was first suspended in an absolute
ethanol using ultrasonic bath for 30 min. Then, the PEEK powder was added slowly into the EG/ethanol suspension with concurrent stirring and heating at 80 C till dried powder was obtained. The resultant powder was further dried at 120 C for 10 h in a vacuum oven. Then, the PEEK/EG nanocomposites were fabricated using the hot pressing technique. The dried powder was filled in a tool steel die having diameter 13 mm. The powder was heated at an average heating rate of 10 C min1 under pressure of 45 MPa to a maximum temperature of 380 C. After soaking period of 30 min, the samples were cooled to a temperature of 100 C and then compacted samples were ejected. The same procedure was used for making nanocomposites containing 0, 1, 1.5, 2, 3, 5 and 10 wt% EG. Volume fraction (Vf) of EG particles for a given weight fraction was determined using, Vf ¼ Wf/[Wf þ Wm(rf/rm)], where Wf is the weight fraction, and rf is the density of EG (2.2 g cc1) particles. Wm
Fig. 1. SEM of (a) PEEK and (b) EG powder. Inset shows SEM image of EG at 5000.
R.K. Goyal / Materials Chemistry and Physics 142 (2013) 195e198
and rm are the weight fraction and density of PEEK matrix, respectively. The density of PEEK was considered 1.3 g cc1. The corresponding vol% of EG of these nanocomposites were 0, 0.58, 0.87, 1.16, 1.75, 2.95 and 6.03, respectively. SEM (JEOL JSM 6360A) was used to investigate the distribution of EG in the PEEK matrix. The fractured surface of sample was coated with a thin layer of platinum using sputter coater to minimize charging effects. For the measurement of electrical conductivity, samples were coated with a thin coating of silver paste to avoid contact resistance. The volume resistance of samples was determined using high resistance meter (Keithley 6517B). However, when the volume resistance is below 106 U, a 6½ digit digital multi meter (Agilent 34401A) was used. Then, the volume resistivity was measured by the relation r ¼ R(A/L), r is the resistivity, R is resistance, L is the sample thickness and A is the cross sectional area of the sample. The electrical conductivity was reported as the reciprocal of the volume resistivity.
3. Results and discussion Fig. 2a shows the SEM image of the PEEK/EG nanocomposites containing 3 wt% EG. It can be seen that the EG particles are uniformly dispersion with a 3-dimensional network in the matrix. This is probably formed due to the penetration of molten PEEK in pores of EG during processing. This special microstructure is ascribed primarily to ultrasonic dispersion followed by stirring and heating which allows solvent to evaporate quickly and reduces possibility of sedimentation of the EG in the matrix. As shown in Fig. 2b, network consists of segregated EG particles. Each particle has several fine sheets of EG confirming a very high aspect ratio of EG. Fig. 3 shows the electrical conductivity of nanocomposites as a function of EG content. The conductivity of pure PEEK is about 1013 S cm1, which is slightly lower than reported elsewhere [15]. The conductivity of the nanocomposites increases with increasing content of EG in the matrix. At a low loading of EG (<1.5 wt%), the EG particles are probably isolated from each other in the insulating PEEK matrix. Hence, EG particles do not contribute significantly to the improvement in the conductivity of nanocomposites. This result is probably because of higher inter-particle distances than the tunneling distances (wtens of angstroms) [16]. However, a sharp increase in conductivity of nanocomposites (approximately eleven orders of magnitude) was observed between 1.5 wt% (0.87 vol%) and 2 wt% (1.16 vol%) EG content, suggesting the
1.E-03
log (σc)
Conductivity (S/cm)
1.E+00
1.E-06 1.E-09
3 2 1 0 -1 -2 -3 -4
y = 1.9392x + 8.4492 R2 = 0.9933 -8
-6
-4
-2
0
1.E-12
log (V f-Vc) 1.E-15 0
1
2
3
4
5
6
7
Volume % EG in PEEK matrix Fig. 3. Electrical conductivity as a function of volume % EG in PEEK matrix. The solid line is the guide to eyes. The inset shows a logelog plot of the conductivity versus (Vf Vc). A logelog plot has a linear relationship with a correlation factor of 0.999.
197
percolation threshold of the nanocomposites is about 1.5 wt%. This clearly indicates an insulatoreconductor transition as the EG particles form a conductive network in the matrix. The conductivities of 2 wt% and 5 wt% nanocomposites are 0.05 S cm1 and 3.24 S cm1, respectively, which surpassed the antistatic criterion for space applications (>108 S cm1) and EMI shielding (>0.1 S cm1). In contrast, an increase of about nine orders of magnitude in conductivity was reported, at percolation threshold, for multi walled carbon nanotube (MWNT) filled PEEK [15]. With further addition of EG to the PEEK matrix, conductivity was increased to 12.3 S cm1 for 10 wt% EG nanocomposite which is better than the highest conductivity of about 0.1 S cm1 reported for 15 wt% CNT filled polycarbonate nanocomposites [7] and of about 0.02 S cm1 for 20 wt% purified HiPCO EMI shielding grade SWCNT nanocomposite [17]. Interestingly, the conductivity of 5 wt % EG nanocomposites is much better than the reported conductivities of 0.07 S cm1 for 7 wt% functionalized-SWCNT filled PS nanocomposites [6] and of w0.01 S cm1 for 2.5 vol% phenyl isocyanate treated-EG filled polystyrene. However, Stankovich et al. reported a very low (w0.1 vol%) percolation threshold for treatedEG filled PS nanocomposites [18]. This comparison indicates that not only percolation threshold but also value of conductivity is very important for the application point of view. We did not observe a saturation in conductivity as in other studies [19]. Compared to other nanocomposites such as polymethyl methacrylate (PMMA)/ EG [20], Nylon 6/EG [21] and PS/EG [22], better conductivity of PEEK/EG may be attributed to higher aspect ratio of EG and better mixing procedure which results in formation of 3-dimensional network of EG in the PEEK matrix as shown in SEM image. Moreover, this may be attributed to the special morphology of PEEK (see Fig. 1a) particles and honeycomb like structure of EG (see Fig. 1b) which are mechanically locked together and forms a conductive interconnected network during processing [4]. In addition, EG possessing abundant pores allows easily intercalation of PEEK chains by adsorption mechanisms during processing. The measured electrical conductivity of two phase conductive polymer nanocomposites can be described by a power law [23e26],
t
sc ¼ s0 Vf Vc ;
(1)
where, sc is the conductivity of the nanocomposite, s0 is the conductivity of the EG, Vf is the volume fraction of the EG, Vc is the critical volume fraction of EG at percolation threshold, and t is the conductivity critical exponent. The experimental results are fitted by plotting log sc versus log (Vf Vc) and shown in the inset of Fig. 3. A linear relationship with a correlation factor of 0.999 was obtained. This correlation yields a percolation threshold value of approximately 0.8 vol%. This percolation threshold is, to the best of our knowledge, about twoethree times lower than the values reported for any other two dimensional filler with high aspect ratio [27]. Such a low percolation threshold may be attributed to the high aspect ratio of EG and its excellent dispersion in the matrix. The value of t for present system is found to be 1.94 which is in excellent agreement with the theoretical universal value (t ¼ 2) of three dimensional systems. Similar results were reported for coal tar pitch/EG (t ¼ 1.9) nanocomposites [28]. The low value of the critical volume fraction (i.e., 0.008 or 0.8 vol%) for the untreated expanded graphite would be partially beneficial for industries as higher volume fraction leads to the poor processability and mechanical properties [29]. In addition, for the first time, we report highest achieved electrical conductivity of 12.3 S cm1 for the nanocomposite containing 10 wt% (6 vol%) EG. Based on the conductivity, it can be concluded that these nanocomposites would be potential candidates for antistatic and EMI shielding applications.
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R.K. Goyal / Materials Chemistry and Physics 142 (2013) 195e198
60
the experimental EMI-SE of PEEK/EG nanocomposites would be higher than those of calculated since multiple reflections are ignored in the above empirical equation.
50
4. Conclusions
40
The electrical properties of the high performance PEEK/EG nanocomposites fabricated by a novel method have been discussed. A percolation threshold of about 1.5 wt% (w0.8 vol%) EG was obtained. The electrical conductivity increased significantly with increasing content of EG in the matrix. The significant improvement in electrical conductivity was attributed to the high aspect ratio, special morphology and better dispersion of EG particles in the PEEK matrix. The EMI-SE of the PEEK/EG nanocomposites can be obtained up to 58 dB depending upon the wt% of EG particles in the matrix. Based on the results, it may be concluded that these nanocomposites with low cost may prove to be the potential high performance materials for the antistatic/EMI shielding applications.
EMI-SE (dB)
70
30 20
1MHz 10 MHz
10
15 MHz
0 0
2
4
6
8
Volume % EG in PEEK matrix Fig. 4. Theoretical EMI-SE of the nanocomposites as a function of EG content in the matrix.
Acknowledgement Table 1 Correlation between the electrical conductivity and EMI shielding.
Miss S. V. Gune and Miss B. J. Deshpande are acknowledged for preparing composite samples.
Compositions
Electrical conductivity (S cm1)
EMI-SE (dB) (approx.)
Ref.
PS/10 wt% CNF PS/10 wt% CNF/1 wt% CNT PS/10 wt% CNF/3 wt% CNT PMMA/10 wt% SWCNT Copolymer/20 wt% MWCNT PEEK/3 wt% EG PEEK/5 wt% EG PEEK/10 wt% EG
0.001 0.00156 0.00215 0.01 0.22 0.525 3.24 12.3
12.9 20.3 21.9 28 w30 16.1a 36.5a 57.9a
[26] [26] [26] [28] [28] e e e
References
a EMI-SE calculated at 10 GHz using Equation (2). Table 1 clearly shows that PEEK/ EG nanocomposites exhibit good electrical conductivity and EMI-SE.
The EMI shielding effective (SE) of the nanocomposites can be calculated from the empirical relation shown in Equation (2) [2], 1
SE ¼ 50 þ 10 logðsc =f Þ þ 1:7tðsc f Þ2
(2)
where, sc is the electrical conductivity (S cm1) of the sample, f is the frequency (MHz) and t is the thickness (cm) of the sample. The first two terms estimate the SE due to the reflection while the last term represents absorption contribution. It is to be noted that this empirical equation ignores the effect of multiple reflections of the SE, whereas in real applications multiple reflections significantly contribute to the SE of the conductive polymer nanocomposites [28]. Fig. 4 shows the variation of calculated SE of the PEEK/EG nanocomposites as a function of content of EG in the PEEK matrix at various frequencies. As expected, SE of the nanocomposites increases with increasing content of EG in the PEEK matrix. This result is due to the higher probability of forming a conductive network in the PEEK matrix with increasing EG content. As shown in Table 1, the SE of the nanocomposites increases with increasing electrical conductivity. The SE of 5 wt% and 10 wt% EG filled PEEK nanocomposites calculated at 10 GHz is 36.5 dB and 58 dB, respectively. In contrast, the SE of the 10 wt% SWCNT filled PMMA or 20 wt% MWCNT filled copolymer nanocomposites lies approximately between 28 dB and 30 dB [30]. The higher SE of our samples may be due to the higher conductivity. Moreover, it is expected that
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