- Email: [email protected]
epoxy–silicone composites
Composites: Part B 43 (2012) 2980–2984
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Dielectric and microwave-absorption properties of the partially carbonized PAN cloth/epoxy–silicone composites Zhibin Huang ⇑, Wancheng Zhou, Wenbo Kang, Xiufeng Tang, Fa Luo, Dongmei Zhu, Ting Sha State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e
i n f o
Article history: Received 25 April 2011 Received in revised form 10 September 2011 Accepted 29 May 2012 Available online 5 June 2012 Keywords: A. Fabrics/textiles A. Polymer–matrix composites (PMCs) B. Electrical properties E. Cure
a b s t r a c t Partially carbonized PAN cloth/epoxy–silicone composites were prepared as microwave-absorbing materials in this paper. Their dielectric properties were studied in X-band, and the results showed that the complex permittivity of the composites was closely correlated with the carbonization degree of PAN cloth. The microwave wave absorption ability of the composites could be primarily attributed to the growth of graphite sheets in partially carbonized PAN cloth, which could be optimized by means of controlling carbonization temperature and time of the stabilized PAN cloth. The results demonstrated the possible applications of partially carbonized PAN cloth/epoxy–silicone composites as lightweight electromagnetic wave absorbers. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, carbon materials such as carbon nanotubes [1–4], carbon black [5], graphite flakes [6,7], carbon fibers (CFs) [8–10] and filaments [11], have attracted great interests as microwave absorbers and shielding materials in the gigahertz frequency range due to their excellent properties i.e. lightweight, flexibility, broadband and low-cost industrial processing. Among these carbon materials, carbon fibers and their composites have been found to be fascinating candidates for promising absorbers owing to their excellent mechanical and electrical properties. Usually, the CFs used for microwave-absorbing materials are completely carbonized fabricated from PAN, pitch, etc. Although the composites added with such completely carbonized CFs exhibit high absorbing effectiveness over a broad range of frequency, there are still some problems such as difficulty of homogeneous distribution of CFs in the matrix and complex fabrication process, which have been challenged in the practical application. Partially carbonized PAN cloth (PCPC), which is characterized by the simple fabrication process, no distribution procedure of the absorbent into matrix as the PCPC cloth is braided beforehand, and the adjustable complex permittivity, is also promising to be employed as microwave-absorbing materials. High-resolution transmission electron microscopy studies [12] of the completely carbonized CFs showed that the fibers were composed of basic graphite sheets imbedded in uncyclized materials, which can be ⇑ Corresponding author. E-mail address: [email protected] (Z. Huang). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.05.045
viewed as conductive graphite units imbedded in the insulating matrix. For PCPC, the size and the number of these conductive graphite units can be adjusted by means of controlling the carbonization degree of carbon cloth with the purpose of getting proper microwave absorbing ability. In this paper, we prepared PCPC/epoxy–silicone composites and studied their dielectric properties. The epoxy–silicone was chosen as matrix because its properties such as good solidity and thermal stability. The mechanism on the microwave attenuation of the composites in X band was also investigated.
2. Experimental In this work, the stabilized PAN cloth (Jiyan Carbon Co. Ltd., Jilin, China) was 2D orthogonally braided with mutually perpendicular tows (as shown in Fig. 1). A single tow contained 6000 monofilaments in 10 lm diameter. Before carbonization, the stabilized PAN cloth was ultrasonically rinsed in distilled water and then cut into the size of 80 mm 40 mm 0.4 mm. The carbonization of stabilized PAN cloth was carried out in a conventional vacuum sintering furnace at 0.2 Pa nitrogen pressure with the strictly controlled heating rate of 10 °C/min from the room temperature to different carbonization temperatures (650 °C and 700 °C), and then the temperatures were maintained at that level for different time (10 min, 2 h, 5 h, 8 h and 12 h). After that, the PAN cloth was impregnated with a mixture of resin, hardener and ethanol. At last, the samples were stored in vacuum, and pre-cured at 90 °C for 30 min, then post-cured at 120 °C for 2 h under a 0.3 MPa load.
Z. Huang et al. / Composites: Part B 43 (2012) 2980–2984
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Fig. 1. Optical micrograph of SPC.
The dc electric conductivity of PCPC was calculated by dividing the resistance of 1 cm of fiber yarn by the resistance and the cross sectional area of the fiber yarn. Two equations as the below are applied to calculate the electric conductivity.
R ¼ SL=A ¼ SLðpD2 =4Þ
ð1Þ
K ¼ 1=S
ð2Þ
where R is the resistance (O), L the length (m), A is the cross-section-al area (m), D the diameter (m), S the specific resistance (O m), and K is the conductivity (O1 m1). The complex permittivities of the composites were measured by using the waveguide method. The test samples were pressed into a rectangle plate with the dimension of 22.86 mm 10.16 mm 2 mm to fit the waveguide sample holder. The complex relative permittivities e = e0 je00 were measured by Agilent technologies E8362B vector network analyzer working at the X band. The reflection loss (RL) curves were calculated from the relative permeability and permittivity at a given frequency and the absorber thickness, according to the following equations:
RLðdBÞ ¼ 20 log jðZ in Z 0 Þ=ðZ in þ Z 0 Þj Z in ¼ Z 0
rffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi lr 2p lr er fd tanh j er c
Fig. 2. XRD patterns of the composites filled with 650 °C-carbonized PCPC and stabilized PAN cloth.
Fig. 3 shows the SEM images of the fractured cross section of PCPC/epoxy–silicone composites. It is observed that the PCPC is well impregnated with the epoxy–silicone and no significant porosity is noticed. These figures suggest that the load applied on the samples and vacuum environment effectively make the epoxy–silicone soak into PCPC. Also, the structure of the PCPC is integrated after impregnating epoxy–silicone.
ð3Þ ð4Þ
where f is the frequency of the EM wave, t the thickness of the absorber, c the velocity of the light, Z0 the impedance of free space, and Zin the input impedance of the absorber. 3. Results and discussion Fig. 2 shows the XRD patterns of PCPC/epoxy–silicone composites. All samples exhibit a broad peak of (0 0 2) carbon reflection at about 26°, which represents the crystal sizes perpendicular to graphitic plane [13,14]. It can be clearly seen that the (0 0 2) carbon reflection peak becomes narrower and stronger with the increasing carbonization time, indicating that the graphite sheets grow up due to the aromatic growth and polymerization taking place in the PAN cloth at 650 °C. In addition, the (10) carbon reflection peak at about 45° also appears when the carbonization time is 12 h, which is absent in the other samples. As we know, in the carbonization process, the insulating ladder structures gradually combine to form conducting graphite sheet structures in the stabilized PAN cloth, which lead to the appearance of the carbon reflection peak. Actually, the aim is to control the growth of these graphite sheet structures in order that the PCPC filled composites can possess ideal microwave-absorbing properties in this research.
Fig. 3. SEM images of PCPC/epoxy–silicone composites (a) transverse direction (b) longitudinal direction.
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Figs. 4 and 5 exhibits the complex permittivities of the PCPC/ epoxy–silicone composites. A visible increase of both the real part (e0 ) and the imaginary part (e00 ) of permittivities with the increasing carbonization time of PCPC is observed in these composites. It is also worth to note from Fig. 4 that the e0 of the composite filled with stabilized PAN cloth (without carbonization) is only about 3, far less than the composite filled with PCPC. As we know, e0 is the expression of polarization ability of materials, which can be mainly attributed to the free electron formed in PCPC in this case. As electrons are very light, they respond rapidly to an alternating electromagnetic [4]. When carbonized at high temperature, the graphite sheets gradually form in PCPC (which is proved by XRD patterns in Fig. 2). Each graphite sheet is equivalent to an electrical dipole, in which the confined free electrons and/or weakly bound electron in individual graphite sheet respond rapidly to alternating electromagnetic field, which contribute to the increase of e0 . With the increasing carbonization time, more graphite sheets form in PCPC, so the e0 of composite increase. As for the e00 , which represents capacity of dielectric loss in the microwave frequency, is also associated with the carbonization time of PCPC. Dielectric losses usually consist of polarization losses and electric conductance loss in the GHz range [15]. In this case, the losses consist mainly the relaxation polarization loss and electric conductance loss. The reasons are as follows: the graphite sheets imbedded in the insulating matrix of short ladder chains can be viewed as basic conductive units, or basic absorber units.
Fig. 5. (a) The e0 and (b) the e00 of the composites filled with 700 °C-carbonized PCPC.
In these carbon units, each carbon atom is bonded trigonally to three others in a plane composed of fused hexagonal rings. Because of the delocalization of one of the outer electrons of each atom to form a p-cloud, graphite conducts electricity in the plane of each covalently bonded sheet. When subjected to alternating electrical field, the free electrons migrate within individual graphite sheet (migrating electrons) and/or hopping among the graphite sheets (hopping electrons), as illustrated in the insets of Fig. 6. In order to overcome the electrical resistance for the electrons, the microwave energy simultaneously converts to thermal energy. From the discussion above, the dielectric loss, namely e00 can be expressed as follows [15]:
e00 ¼ e00I þ e00C
Fig. 4. (a) The e0 and (b) the e00 of the composites filled with 650 °C-carbonized PCPC.
ð5Þ
The e00I is due to the migrating electrons within graphite sheets (relaxation polarization), and the e00C is due to hopping electrons resistance among graphite sheets (electric conductance loss). Obviously, With the increasing carbonization time, both the e00I and e00C increase: (i) e00I increases because more graphite sheets form in PCPC with the increasing carbonization time. (ii) The dc electrical conductivity of PCPC, as shown in Table 1, continuously increase with the increasing carbonization time, namely the electric conductance loss ðe00C Þ will also increase. It can be easily imaged that the average distance between graphite sheets will decrease with the increasing number of graphite sheets. As a result, the electron is easier to jump between graphite sheets.
Z. Huang et al. / Composites: Part B 43 (2012) 2980–2984
Fig. 6. Frequency dependent of the reflection loss of composites filled with (a) PCPC carbonized at 650 °C for 10 min or (b) PCPC carbonized at 700 °C for 12 h.
Table 1 Electric conductivity of PCPC. Sample codes
Electric conductivity of individual fiber r (S/m)
650 °C–10 min 650 °C–2 h 650 °C–5 h 650 °C–12 h 700 °C–10 min 700 °C–2 h 700 °C–5 h 700 °C–12 h
<0.1 3.2 ± 1.1 15.6 ± 3.3 167.5 ± 16.5 12.1 ± 3.7 115.3 ± 15.3 183.1 ± 39.8 547.4 ± 115.6
From the discussion above, we know that the dielectric properties of the composites are closely correlated with the growth of these graphite sheets. By means of optimizing the carbonization processing, it is possible for the composites to get proper permittivity as microwave-absorbing materials. On the other hand, the carbonization temperature of PCPC cloth also exhibits great effects on the complex permittivity of the composites. As the carbonization temperature is 650 °C, the e00 of the samples increases from 10 to 40 at 8.2 GHz when the carbonization time rises from 10 min to 12 h. While, the corresponding value increases from 22 to 100 as the carbonization temperature is 700 °C. As we know, carbonization of the PAN cloth is an aromatic growth and polymerization processing in which PAN undergoes heating processes to remove the non-carbon elements in the forms of the gases of H2, HCN, H2O and N2, etc. [16,17]. At the temperature higher than 600 °C, the graphitization process is accelerated when
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N2 is expelled [18]. The permittivity results in this paper indicate that the growth of the basic graphite units at 700 °C is greatly faster than that at 650 °C in PCPC. Obviously, carbonization temperature is also an important factor affecting the complex permittivity of the PCPC/polymer composites. Fig. 6 shows the calculated reflection loss of the composites with different thickness. The reflection loss below 7 dB with a minimum value of 16 dB can be obtained as shown in Fig. 6a. It also can be observed that the microwave-absorbing ability of the composites become worse when the PCPC is over-carbonized as shown in Fig. 6b. Generally, the excellent microwave absorbing properties of the absorber requires properly incorporating the following two keys (i) the impedance matching characteristic and (ii) the attenuation characteristic of the composites. The failure of either of the two keys will result in the decrease of the microwave absorption [19]. When the carbonization temperature is 650 °C, the basic crystalline units grow slowly in the PCPC. The free electrons shift in the graphite sheet rather than hop between the graphite units because the average distance between the graphite units is longer than the electron hopping gap width (shown in the inset of Fig. 6a). As for the PCPC carbonized at 700 °C, the basic graphite units grow fast and the average distance between the graphite sheets is easy to reach the electron hopping gap width. The free electrons hop and shift between more and more graphite sheets (shown in the inset of Fig. 6b), which leads to the increase of DC conductivity and permittivity as discussed before. So we can know from Eqs. (3) and (4) that the mismatch of the intrinsic impedance of the composites (Zin) with the impedance of free space (Z0 = 377 X) become worse because the Zin will decrease with the increasing permittivity. That is to say more reflection can be caused on the PCPC/polymer interfaces and the microwave absorption is decreased. Fig. 7 shows the reflection loss of the composites filled with PCPC carbonized at 650 °C for different temperature. It can be seen that the PCPC without carbonization has no microwave-absorbing ability because its imaginary part of permittivity is almost zero. When the carbonization time is 10 min, the composites shows the lowest reflection loss, then the reflection loss gradually increase with the increasing carbonization time. This increase of the reflection loss, similarly to the discussion on Fig. 6, can be attributed to the samples with too large complex permittivity, which cause additional reflective wave on the coatings surface. Thus, it can be found that controlling the growth of the graphite units is significant in the use of the PCPC to design a microwaveabsorbing composite. Actually, the carbonization temperature is
Fig. 7. Frequency dependent of the reflection loss of the composites filled with PCPC carbonized at 650 °C for different temperature.
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the dominating factor affecting the graphite unit growth in the PCPC. The results show that when the carbonization temperature is 650 °C, it is appropriate for the PCPC to possess good waveabsorbing ability. 4. Conclusions In summary, the complex permittivity of the composites was closely correlated with the carbonization degree of PAN cloth. The microwave-absorbing property of PCPC/epoxy–silicone composites can be primarily attributed to the growth of the basic graphite units in the PCPC, and it is possible to be optimized by means of controlling carbonization temperature and time. Our study on PCPC has demonstrated their possible applications as lightweight electromagnetic wave absorbers. Acknowledgements This work was financially supported by the fund of the National Nature Science Foundation of China (No. 51072165) and States Key Laboratory of the Solidification Processing in NWPU (No. KP200901).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
Xiang C, Pan Y, Liu X, Sun X, Shi X, Guo J. Appl Phys Lett 2005;87(12):123103. Li N, Huang Y, Du F, He X, Lin X, Gao H, et al. Nano Lett 2006;6(6):1141–5. Che RC, Peng L-M, Duan XF, Chen Q, Liang XL. Adv Mater 2004;16(5):401–5. Song W-l, Cao M-S, Hou Z-L, Yuan J, Fang X-Y. Scripta Mater 2009;61(2):201–4. Liu X, Zhang Z, Wu Y. Compos Part B-Eng 2011;42(2):326–9. Fan Y, Yang H, Li M, Zou G. Mater Chem Phys 2009;115(2–3):696–8. Gogoi JP, Bhattacharyya NS, James Raju KC. Compos Part B-Eng 2011;42(5):1291–7. Cao M-S, Song W-L, Hou Z-L, Wen B, Yuan J. Carbon 2010;48(3):788–96. Wang X, Luo F, Yu X, Zhu D, Zhou W. Scripta Mater 2007;57(4):309–12. Rosa IMD, Dinescu A, Sarasini F, Sarto MS, Tamburrano A. Compos Sci Technol 2010;70(1):102–9. Chung DDL. Carbon 2001;39(2):279–85. Guigon M, Oberlin A, Desarmot G. Fibre Sci Tech 1984;20(1):55–72. Tse-Hao K, Tzy-Chin D, Jeng-An P, Ming-Fong L. Carbon 1993;31(5):765–71. Hashisho Z, Rood MJ, Barot S, Bernhard J. Carbon 2009;47(7):1814–23. Zhang B, Li J, Sun J, Zhang S, Zhai H, Du Z. J Eur Ceram Soc 2002;22(1):93–9. Edie DD. Carbon 1998;36(4):345–62. Zhu D, Xu C, Nakura N, Matsuo M. Carbon 2002;40(3):363–73. Fitzer E, Frohs W, Heine M. Carbon 1986;24(4):387–95. Singh P, Babbar VK, Razdan A, Srivastava SL, Goel TC. Mat Sci Eng B-Solid 2000;78(2–3):70–4.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Dielectric and microwave-absorption properties of the partially carbonized PAN cloth/epoxy–silicone composites Zhibin Huang ⇑, Wancheng Zhou, Wenbo Kang, Xiufeng Tang, Fa Luo, Dongmei Zhu, Ting Sha State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
a r t i c l e
i n f o
Article history: Received 25 April 2011 Received in revised form 10 September 2011 Accepted 29 May 2012 Available online 5 June 2012 Keywords: A. Fabrics/textiles A. Polymer–matrix composites (PMCs) B. Electrical properties E. Cure
a b s t r a c t Partially carbonized PAN cloth/epoxy–silicone composites were prepared as microwave-absorbing materials in this paper. Their dielectric properties were studied in X-band, and the results showed that the complex permittivity of the composites was closely correlated with the carbonization degree of PAN cloth. The microwave wave absorption ability of the composites could be primarily attributed to the growth of graphite sheets in partially carbonized PAN cloth, which could be optimized by means of controlling carbonization temperature and time of the stabilized PAN cloth. The results demonstrated the possible applications of partially carbonized PAN cloth/epoxy–silicone composites as lightweight electromagnetic wave absorbers. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, carbon materials such as carbon nanotubes [1–4], carbon black [5], graphite flakes [6,7], carbon fibers (CFs) [8–10] and filaments [11], have attracted great interests as microwave absorbers and shielding materials in the gigahertz frequency range due to their excellent properties i.e. lightweight, flexibility, broadband and low-cost industrial processing. Among these carbon materials, carbon fibers and their composites have been found to be fascinating candidates for promising absorbers owing to their excellent mechanical and electrical properties. Usually, the CFs used for microwave-absorbing materials are completely carbonized fabricated from PAN, pitch, etc. Although the composites added with such completely carbonized CFs exhibit high absorbing effectiveness over a broad range of frequency, there are still some problems such as difficulty of homogeneous distribution of CFs in the matrix and complex fabrication process, which have been challenged in the practical application. Partially carbonized PAN cloth (PCPC), which is characterized by the simple fabrication process, no distribution procedure of the absorbent into matrix as the PCPC cloth is braided beforehand, and the adjustable complex permittivity, is also promising to be employed as microwave-absorbing materials. High-resolution transmission electron microscopy studies [12] of the completely carbonized CFs showed that the fibers were composed of basic graphite sheets imbedded in uncyclized materials, which can be ⇑ Corresponding author. E-mail address: [email protected] (Z. Huang). 1359-8368/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2012.05.045
viewed as conductive graphite units imbedded in the insulating matrix. For PCPC, the size and the number of these conductive graphite units can be adjusted by means of controlling the carbonization degree of carbon cloth with the purpose of getting proper microwave absorbing ability. In this paper, we prepared PCPC/epoxy–silicone composites and studied their dielectric properties. The epoxy–silicone was chosen as matrix because its properties such as good solidity and thermal stability. The mechanism on the microwave attenuation of the composites in X band was also investigated.
2. Experimental In this work, the stabilized PAN cloth (Jiyan Carbon Co. Ltd., Jilin, China) was 2D orthogonally braided with mutually perpendicular tows (as shown in Fig. 1). A single tow contained 6000 monofilaments in 10 lm diameter. Before carbonization, the stabilized PAN cloth was ultrasonically rinsed in distilled water and then cut into the size of 80 mm 40 mm 0.4 mm. The carbonization of stabilized PAN cloth was carried out in a conventional vacuum sintering furnace at 0.2 Pa nitrogen pressure with the strictly controlled heating rate of 10 °C/min from the room temperature to different carbonization temperatures (650 °C and 700 °C), and then the temperatures were maintained at that level for different time (10 min, 2 h, 5 h, 8 h and 12 h). After that, the PAN cloth was impregnated with a mixture of resin, hardener and ethanol. At last, the samples were stored in vacuum, and pre-cured at 90 °C for 30 min, then post-cured at 120 °C for 2 h under a 0.3 MPa load.
Z. Huang et al. / Composites: Part B 43 (2012) 2980–2984
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Fig. 1. Optical micrograph of SPC.
The dc electric conductivity of PCPC was calculated by dividing the resistance of 1 cm of fiber yarn by the resistance and the cross sectional area of the fiber yarn. Two equations as the below are applied to calculate the electric conductivity.
R ¼ SL=A ¼ SLðpD2 =4Þ
ð1Þ
K ¼ 1=S
ð2Þ
where R is the resistance (O), L the length (m), A is the cross-section-al area (m), D the diameter (m), S the specific resistance (O m), and K is the conductivity (O1 m1). The complex permittivities of the composites were measured by using the waveguide method. The test samples were pressed into a rectangle plate with the dimension of 22.86 mm 10.16 mm 2 mm to fit the waveguide sample holder. The complex relative permittivities e = e0 je00 were measured by Agilent technologies E8362B vector network analyzer working at the X band. The reflection loss (RL) curves were calculated from the relative permeability and permittivity at a given frequency and the absorber thickness, according to the following equations:
RLðdBÞ ¼ 20 log jðZ in Z 0 Þ=ðZ in þ Z 0 Þj Z in ¼ Z 0
rffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi lr 2p lr er fd tanh j er c
Fig. 2. XRD patterns of the composites filled with 650 °C-carbonized PCPC and stabilized PAN cloth.
Fig. 3 shows the SEM images of the fractured cross section of PCPC/epoxy–silicone composites. It is observed that the PCPC is well impregnated with the epoxy–silicone and no significant porosity is noticed. These figures suggest that the load applied on the samples and vacuum environment effectively make the epoxy–silicone soak into PCPC. Also, the structure of the PCPC is integrated after impregnating epoxy–silicone.
ð3Þ ð4Þ
where f is the frequency of the EM wave, t the thickness of the absorber, c the velocity of the light, Z0 the impedance of free space, and Zin the input impedance of the absorber. 3. Results and discussion Fig. 2 shows the XRD patterns of PCPC/epoxy–silicone composites. All samples exhibit a broad peak of (0 0 2) carbon reflection at about 26°, which represents the crystal sizes perpendicular to graphitic plane [13,14]. It can be clearly seen that the (0 0 2) carbon reflection peak becomes narrower and stronger with the increasing carbonization time, indicating that the graphite sheets grow up due to the aromatic growth and polymerization taking place in the PAN cloth at 650 °C. In addition, the (10) carbon reflection peak at about 45° also appears when the carbonization time is 12 h, which is absent in the other samples. As we know, in the carbonization process, the insulating ladder structures gradually combine to form conducting graphite sheet structures in the stabilized PAN cloth, which lead to the appearance of the carbon reflection peak. Actually, the aim is to control the growth of these graphite sheet structures in order that the PCPC filled composites can possess ideal microwave-absorbing properties in this research.
Fig. 3. SEM images of PCPC/epoxy–silicone composites (a) transverse direction (b) longitudinal direction.
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Figs. 4 and 5 exhibits the complex permittivities of the PCPC/ epoxy–silicone composites. A visible increase of both the real part (e0 ) and the imaginary part (e00 ) of permittivities with the increasing carbonization time of PCPC is observed in these composites. It is also worth to note from Fig. 4 that the e0 of the composite filled with stabilized PAN cloth (without carbonization) is only about 3, far less than the composite filled with PCPC. As we know, e0 is the expression of polarization ability of materials, which can be mainly attributed to the free electron formed in PCPC in this case. As electrons are very light, they respond rapidly to an alternating electromagnetic [4]. When carbonized at high temperature, the graphite sheets gradually form in PCPC (which is proved by XRD patterns in Fig. 2). Each graphite sheet is equivalent to an electrical dipole, in which the confined free electrons and/or weakly bound electron in individual graphite sheet respond rapidly to alternating electromagnetic field, which contribute to the increase of e0 . With the increasing carbonization time, more graphite sheets form in PCPC, so the e0 of composite increase. As for the e00 , which represents capacity of dielectric loss in the microwave frequency, is also associated with the carbonization time of PCPC. Dielectric losses usually consist of polarization losses and electric conductance loss in the GHz range [15]. In this case, the losses consist mainly the relaxation polarization loss and electric conductance loss. The reasons are as follows: the graphite sheets imbedded in the insulating matrix of short ladder chains can be viewed as basic conductive units, or basic absorber units.
Fig. 5. (a) The e0 and (b) the e00 of the composites filled with 700 °C-carbonized PCPC.
In these carbon units, each carbon atom is bonded trigonally to three others in a plane composed of fused hexagonal rings. Because of the delocalization of one of the outer electrons of each atom to form a p-cloud, graphite conducts electricity in the plane of each covalently bonded sheet. When subjected to alternating electrical field, the free electrons migrate within individual graphite sheet (migrating electrons) and/or hopping among the graphite sheets (hopping electrons), as illustrated in the insets of Fig. 6. In order to overcome the electrical resistance for the electrons, the microwave energy simultaneously converts to thermal energy. From the discussion above, the dielectric loss, namely e00 can be expressed as follows [15]:
e00 ¼ e00I þ e00C
Fig. 4. (a) The e0 and (b) the e00 of the composites filled with 650 °C-carbonized PCPC.
ð5Þ
The e00I is due to the migrating electrons within graphite sheets (relaxation polarization), and the e00C is due to hopping electrons resistance among graphite sheets (electric conductance loss). Obviously, With the increasing carbonization time, both the e00I and e00C increase: (i) e00I increases because more graphite sheets form in PCPC with the increasing carbonization time. (ii) The dc electrical conductivity of PCPC, as shown in Table 1, continuously increase with the increasing carbonization time, namely the electric conductance loss ðe00C Þ will also increase. It can be easily imaged that the average distance between graphite sheets will decrease with the increasing number of graphite sheets. As a result, the electron is easier to jump between graphite sheets.
Z. Huang et al. / Composites: Part B 43 (2012) 2980–2984
Fig. 6. Frequency dependent of the reflection loss of composites filled with (a) PCPC carbonized at 650 °C for 10 min or (b) PCPC carbonized at 700 °C for 12 h.
Table 1 Electric conductivity of PCPC. Sample codes
Electric conductivity of individual fiber r (S/m)
650 °C–10 min 650 °C–2 h 650 °C–5 h 650 °C–12 h 700 °C–10 min 700 °C–2 h 700 °C–5 h 700 °C–12 h
<0.1 3.2 ± 1.1 15.6 ± 3.3 167.5 ± 16.5 12.1 ± 3.7 115.3 ± 15.3 183.1 ± 39.8 547.4 ± 115.6
From the discussion above, we know that the dielectric properties of the composites are closely correlated with the growth of these graphite sheets. By means of optimizing the carbonization processing, it is possible for the composites to get proper permittivity as microwave-absorbing materials. On the other hand, the carbonization temperature of PCPC cloth also exhibits great effects on the complex permittivity of the composites. As the carbonization temperature is 650 °C, the e00 of the samples increases from 10 to 40 at 8.2 GHz when the carbonization time rises from 10 min to 12 h. While, the corresponding value increases from 22 to 100 as the carbonization temperature is 700 °C. As we know, carbonization of the PAN cloth is an aromatic growth and polymerization processing in which PAN undergoes heating processes to remove the non-carbon elements in the forms of the gases of H2, HCN, H2O and N2, etc. [16,17]. At the temperature higher than 600 °C, the graphitization process is accelerated when
2983
N2 is expelled [18]. The permittivity results in this paper indicate that the growth of the basic graphite units at 700 °C is greatly faster than that at 650 °C in PCPC. Obviously, carbonization temperature is also an important factor affecting the complex permittivity of the PCPC/polymer composites. Fig. 6 shows the calculated reflection loss of the composites with different thickness. The reflection loss below 7 dB with a minimum value of 16 dB can be obtained as shown in Fig. 6a. It also can be observed that the microwave-absorbing ability of the composites become worse when the PCPC is over-carbonized as shown in Fig. 6b. Generally, the excellent microwave absorbing properties of the absorber requires properly incorporating the following two keys (i) the impedance matching characteristic and (ii) the attenuation characteristic of the composites. The failure of either of the two keys will result in the decrease of the microwave absorption [19]. When the carbonization temperature is 650 °C, the basic crystalline units grow slowly in the PCPC. The free electrons shift in the graphite sheet rather than hop between the graphite units because the average distance between the graphite units is longer than the electron hopping gap width (shown in the inset of Fig. 6a). As for the PCPC carbonized at 700 °C, the basic graphite units grow fast and the average distance between the graphite sheets is easy to reach the electron hopping gap width. The free electrons hop and shift between more and more graphite sheets (shown in the inset of Fig. 6b), which leads to the increase of DC conductivity and permittivity as discussed before. So we can know from Eqs. (3) and (4) that the mismatch of the intrinsic impedance of the composites (Zin) with the impedance of free space (Z0 = 377 X) become worse because the Zin will decrease with the increasing permittivity. That is to say more reflection can be caused on the PCPC/polymer interfaces and the microwave absorption is decreased. Fig. 7 shows the reflection loss of the composites filled with PCPC carbonized at 650 °C for different temperature. It can be seen that the PCPC without carbonization has no microwave-absorbing ability because its imaginary part of permittivity is almost zero. When the carbonization time is 10 min, the composites shows the lowest reflection loss, then the reflection loss gradually increase with the increasing carbonization time. This increase of the reflection loss, similarly to the discussion on Fig. 6, can be attributed to the samples with too large complex permittivity, which cause additional reflective wave on the coatings surface. Thus, it can be found that controlling the growth of the graphite units is significant in the use of the PCPC to design a microwaveabsorbing composite. Actually, the carbonization temperature is
Fig. 7. Frequency dependent of the reflection loss of the composites filled with PCPC carbonized at 650 °C for different temperature.
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the dominating factor affecting the graphite unit growth in the PCPC. The results show that when the carbonization temperature is 650 °C, it is appropriate for the PCPC to possess good waveabsorbing ability. 4. Conclusions In summary, the complex permittivity of the composites was closely correlated with the carbonization degree of PAN cloth. The microwave-absorbing property of PCPC/epoxy–silicone composites can be primarily attributed to the growth of the basic graphite units in the PCPC, and it is possible to be optimized by means of controlling carbonization temperature and time. Our study on PCPC has demonstrated their possible applications as lightweight electromagnetic wave absorbers. Acknowledgements This work was financially supported by the fund of the National Nature Science Foundation of China (No. 51072165) and States Key Laboratory of the Solidification Processing in NWPU (No. KP200901).
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