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The electromagnetic characteristics of carbon foams
Available online at www.sciencedirect.com
Carbon 45 (2007) 2873–2879 www.elsevier.com/locate/carbon
The electromagnetic characteristics of carbon foams Zhigang Fang, Chusen Li, Jiayan Sun, Hongtao Zhang, Jinsong Zhang
*
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Received 4 June 2007; accepted 11 October 2007 Available online 24 October 2007
Abstract Three different pore size carbon foams with variable electric conductivities were prepared by a polymer sponge replication method. The electromagnetic parameters of these carbon foams and their corresponding pulverized powders were measured by a resonant cavity perturbation technique at a frequency of 2450 MHz. The results show that carbon foams have smaller dielectric constants but several times larger dielectric loss compared with their corresponding pulverized powders. Moreover, carbon foams show magnetic loss while no magnetic loss can be observed from their corresponding pulverized powders. The magnetic loss of carbon foams is apparently a kind of extrinsically magnetic loss and believed to be able to maintain at high temperatures. The electromagnetic characteristics of carbon foams demonstrate that macrostructure modification is an effective way to modulate electromagnetic properties of such materials. Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction Radar absorbing materials (RAMs) are widely applied in order to achieve the objective of stealth for military and civil targets. Presently, RAMs can be classified into two categories: magnetic and dielectric absorbing materials. Magnetic absorbers depend on the magnetic hysteresis effect, which is obtained when magnetic materials such as ferrites are added to a matrix. However, densities of the magnetic materials are usually high and absorbing bandwidths for magnetic absorbers are usually narrow. Moreover, the magnetic absorbers will fail when the temperature is higher than their Curie temperatures (Tcs). While the dielectric absorbers depend on the ohmic loss, which can be achieved by adding conductive fillers such as carbon black, graphite, or metallic particles to a matrix. Dielectric absorbers have an advantage of light-weight but do not match up to the absorptivity of magnetic absorbers [1–3]. These two kinds of materials have different advantages and disadvantages when they are applied as absorbers. They can be used together as a composite, and the mag-
*
Corresponding author. Fax: +86 24 23906640. E-mail address: [email protected] (J. Zhang).
0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.10.013
netic loss material is usually the base one, but heavy weight of the material is still a concerning problem. Recently carbon foams found their way to act as RAMs, mainly due to their well matched impedance, high durability, light-weight, and effective EM wave absorbing capability [4]. Their characteristic structure and properties can be utilized to overcome some of the shortcomings of existing RAMs. However, to the best of our knowledge, little work been carried out to explore the radar absorbing properties of carbon foams. Numeric simulations in our research group have predicted that silicon carbide and carbon foams have excellent microwave absorbing properties and unique electromagnetic (EM) wave dissipation characteristics compared with the same composition bulk or particles [5–7]. However, these results were achieved only by numeric simulation, it is necessary to investigate and validate the electromagnetic characteristics of this novel RAM by detailed experiments. In the present paper, the investigation of the electromagnetic characteristics of carbon foams was carried out experimentally. The electromagnetic parameter estimation of carbon foams and their pulverized powders was carried out for comparison. Because the wide bandwidth electromagnetic characteristics of materials are usually more difficult to obtain, the electromagnetic parameter measurements
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of materials made at spot frequencies instead are frequently carried out [8]. While resonant cavity perturbation technique is the most applicable method for measuring electromagnetic parameters of materials on spot frequencies and the frequency of 2450 MHz is the most widely adopted one [9], so in this paper the electromagnetic parameters of carbon foams and their pulverized powders were made at a frequency of 2450 MHz by the resonant cavity perturbation technique.
electric conductivities were prepared by elaborately controlling their carbonization process. Carbon foams were also pulverized into powders and sifted with a 100-mesh sieve.
2.2. Electric conductivity measurements The electric conductivities of carbon foams were calculated according to Ohm’s law from the geometric dimension and the electrical resistance of carbon foams measured by a numeric double bridge circuit at room temperature.
2.3. Electromagnetic parameter measurements
2. Experimental 2.1. Sample preparation Carbon foams were fabricated by a polymer sponge replica method [10], as demonstrated in Fig. 1. Three different pore size polymer sponges, 0.3 mm, 1.0 mm and 2.0 mm, were chosen as templates, so carbon foams with the same three pore sizes were obtained and named as 0.3 mm foam, 1.0 mm foam and 2.0 mm foam, respectively. Carbon foams with variable
Preparing slurry with proper viscosity
Immersing polymer sponges into slurry several times
Remove excess slurry from sponges by pressing and blowing after every immersing
Gelling the coated sponges at 80ºC
Carbonizing the dried bodies at certain temperatures in inert atmosphere
Fig. 1. Flowchart for fabricating carbon foams.
The electromagnetic parameters were measured by the TE103 resonant cavity perturbation technique at 2450 MHz [9]. For dielectric measurements, samples were placed in a maximum electric field, and for magnetic measurements, in a maximum magnetic field. Carbon foams for dielectric and magnetic parameter measurements were machined to small cylinders with 8 mm in diameter. Pulverized powders of carbon foams were loaded and encapsulated into Teflon hollow pipes with 9 mm in diameter and 0.5 mm in wall thickness for measurements. Two kinds of pulverized powders of each carbon foam were prepared for comparison: one was tightly pressed to form a compacted cylinder; another was just slightly vibrated to form a loose cylinder. The two kinds of pulverized powders were named as compacted powder and loose powder, respectively, in the following section of this paper. A thin plastic pipe was used to carry samples to the perturbation location for measurements, and the perturbation effects by the Teflon hollow pipe and plastic pipe were first calibrated and subtracted from each measurement.
3. Results and discussion 3.1. Morphology and structure Fig. 2 shows the SEM morphology of carbon foams with different pore sizes. As can be seen from the picture, carbon foams have relatively evenly-distributed pore sizes. There are some closed cells in the carbon foam with pore
Fig. 2. SEM images of carbon foams with different pore sizes (a) pore size about 2.0 mm; (b) pore size about 1.0 mm; (c) pore size about 0.3 mm.
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the carbonization temperature when other carbonization process parameters are identical. The electric conductivity of carbon foams with pore size 2.0 mm and 0.3 mm follows the similar changing tendency with respect to the carbonization temperature. Fig. 4 indicates that the carbonization temperature has a direct effect on the electric conductivity of carbon foams. And the electric conductivity of carbon foams increases quickly when the carbonization temperature improves, so the electric conductivity of carbon foams is adjustable by controlling the carbonization temperatures. The carbonization temperature of carbon foams affects the graphitization degree of their amorphous structures and further their electric conductivity, which has also been found on other carbon-based materials [11].
Bragg angle (Degrees) Fig. 3. The XRD spectrum of one carbon foam sample.
-0.5
3.3.1. Dielectric constants The variation of the real parts of the complex permittivity e0r of three different pore size carbon foams and their corresponding pulverized powders at 2450 MHz with electric conductivity is plotted in Fig. 5. It can be seen that e0r of three different pore size carbon foams and their corresponding pulverized powders increase gradually with the increase of electric conductivity. The real part the complex permittivity of a material presents its polarization capability at a certain frequency of EM waves, so carbon foams and their corresponding pulverized powders have an increasing polarization capability at 2450 MHz as the electric conductivity improves, which maybe relate to their microstructure change. An important fact from Fig. 5 is that the e0r of carbon foams is quite lower, about 50%, than those of their corresponding compacted powders. For example, e0r for the carbon foam with pore size of 1.0 mm increases from 2.94 to 4.90 while that for its corresponding compacted powder ranging from 4.59 to 5.70 when the electric conductivity increases from 0.035 S/m to 5.64 S/m. The effective medium theory (EMT) can help in understanding the e0r difference between carbon foams and their corresponding compacted powders [12]. An incident wave is not sensitive to particles or structures that are smaller than a sensing wavelength. EMT uses this property of radiation to determine the effective mean values (eeff and leff) of e and l. The Maxwell–Garnett (M–G) theory is used widely for calculating effective medium which can be expressed as
-1.0
M–G eeff ¼ e1
size of 0.3 mm, which were produced during the fabrication process for the difficulty of removing excess slurry for such small pore size foams. Apart from the macro-pores in carbon foams, there also exist holes left by the elimination of the polymer sponge template in the center of each ligament of carbon foams. Fig. 3 gives the XRD spectrum for pulverized powders of one carbon foam sample. There are obviously no sharp but weak peaks of graphitic carbon at planes (0 0 2) and (1 0 1) [11]. There were no obvious difference between the XRD spectra of other carbon foams and the one presenting in Fig. 3, which indicates that carbon foams carbonized at low temperatures as in this paper are composed of amorphous structures. 3.2. Electric conductivity Fig. 4 shows the relationship between the electric conductivity of carbon foam with pore size of 1.0 mm and 1.0 0.5
lgσ (S/m)
3.3. Electromagnetic parameters
0.0
-1.5 700
710
720
730
740
750
760
Carbonization temperature/ºC Fig. 4. Relationship between the electric conductivity of carbon foams with pore size of 1.0 mm and carbonization temperature.
ðe2 þ 2e1 Þ þ 2f ðe2 e1 Þ ðe2 þ 2e1 Þ f ðe2 e1 Þ
where e1 and e2 are the dielectric permittivity of the host and guest, respectively, and f corresponds to the volume fraction of the guest in the effective medium [12]. The dimensions of the pore structure of carbon foams are much smaller than the wavelength, so carbon foams can also be treated as an effective medium as a mixture of the insulating air and the amorphous carbon in the ligament of
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6.0 0.3mm foam 1.0mm foam 2.0mm foam compacted powder of 0.3mm foam compacted powder of 1.0mm foam compacted powder of 2.0mm foam loose powder of 0.3mm foam loose powder of 1.0mm foam loose powder of 2.0mm foam
5.6 5.2
εr'
4.8 4.4 4.0 3.6 3.2 2.8 0
1
2
3
σ (S/m)
4
5
6
7
Fig. 5. The real parts of the complex permittivity for three different pore size carbon foams and their corresponding pulverized powders versus electric conductivity at 2450 MHz.
carbon foams. The same assumption can be applied to the compacted and loose powders of carbon foams. As can be seen from the above formula, higher volume fraction f of a guest medium will lead to higher eeff for the effective medium. Carbon foams and their corresponding compacted powders have the same host and guest mediums, but the host medium fractions, i.e., the volume fractions of amorphous carbon, in carbon foams are much lower than those in their corresponding compacted powders, so carbon foams have much lower effective dielectric constants accordingly. Fig. 5 also indicates that the loose powders of carbon foams have even smaller e0r than carbon foams. Besides the volume fraction difference of amorphous carbon between carbon foams and their corresponding loose powders, other factors affecting e0r of materials should be taken into consideration. The real part of complex dielectric constant is an expression of polarization ability of a material which mainly arises from dipolar polarization and interfacial polarization at microwave frequencies [13]. However, for the loose powders of carbon foams, the distance between powder particles is too large to act as charge carrier, so the interfacial polarization effect would be very slight and almost only the dipolar polarization can contribute to their overall polarization capability [14]. Therefore eeff of the loose powders of carbon foams are lower than those of carbon foams even if the volume fractions of amorphous carbon in the former are higher than those in the latter. The pore size has a minor effect on e0r of different pore size carbon foams and their corresponding pulverized powders, respectively. Compared with the large effect produced by macrostructure change, i.e., from foam to powder, on e0r of amorphous carbon, such a minor effect is neglectable. The real part of the complex permittivity of a medium is an important factor to determine the ratios of reflection for an incident EM wave upon the surface of medium. The impedance of a medium with lower e0r is much closer to
the impedance of free space, hence good conjugation with free space will get because the impedance varies gently at the interface. Therefore the EM wave can easily penetrate the medium with lower e0r and travel a long distance in it. So it is obvious that the low e0r of carbon foams is quite advantageous to their application as RAMs. 3.3.2. Dielectric loss The variation of the imaginary parts of the complex permittivity e00r and the dielectric loss tangents tg de for three different pore size carbon foams and their pulverized powders with electric conductivity at 2450 MHz are shown in Figs. 6 and 7, respectively. As can be seen from these two figures, e00r and tg de of the three pore size carbon foams and their corresponding pulverized powders do not change with the electric conductivity monotonically, they first increasing to a maximum and then decreasing gradually. This is because the direct reflection of incident EM waves from the surface of conductive dielectric RAMs becomes more and more prominent and less incident EM waves can enter into them for further attenuation when their electric conductivity improves gradually [15,16]. Therefore, e00r and tg de for conductive dielectric RAMs would reach a maximum at a certain electric conductivity although the ohmic loss improves monotonically with the increase of electric conductivity for conductive dielectric RAMs. By comparing the e00r and tg de of carbon foams and their corresponding pulverized powders in Figs. 6 and 7 it can be concluded that e00r and tg de of carbon foams are much larger than those of their corresponding pulverized powders, about two and four times, respectively, as large as those of the compacted powders and the loose powders. Similarly, the pore size does not have a significant effect on e00r and tg de of carbon foams with different pore sizes and their corresponding pulverized powders, respectively. As has mentioned above, the interfacial polarization effect in the loose powders of carbon foams is very weak, hence nearly only the dipolar polarization relaxation loss
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0.8 0.3mm foam 1.0mm foam 2.0mm foam compacted powder of 0.3mm foam compacted powder of 1.0mm foam compacted powder of 2.0mm foam loose powder of 0.3mm foam loose powder of 1.0mm foam loose powder of 2.0mm foam
0.7 0.6
εr''
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3
σ (S/m)
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Fig. 6. The imaginary parts of the complex permittivity for three different pore size carbon foams and their corresponding pulverized powders versus electric conductivity at 2450 MHz.
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0.3mm foam 1.0mm foam 2.0mm foam compacted powder of 0.3mm foam compacted powder of 1.0mm foam compacted powder of 2.0mm foam loose powder of 0.3mm foam loose powder of 1.0mm foam loose powder of 2.0mm foam
0.18 0.16
tgδ e
0.14 0.12 0.10 0.08 0.06 0.04 0
1
2
3
σ (S/m)
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Fig. 7. The dielectric loss tangents of three different pore size carbon foams and their corresponding pulverized powders versus electric conductivity at 2450 MHz.
of the amorphous carbon can contribute to the dielectric loss of the loose powders of carbon foams. Therefore the dielectric loss of the loose powders of carbon foams at a certain electric conductivity represents the intrinsically dielectric loss of the amorphous carbon at that electric conductivity. When the loose powders of carbon foams are pressed tightly to form the compacted powders, the distance between powder particles is greatly reduced and thus the interfacial polarization effect and partially electric conductive performance have been improved more or less. So the interfacial polarization relaxation loss and partially ohmic loss should be taken into account besides the intrinsically dielectric loss of amorphous carbon for the dielectric loss of the compacted powders of carbon foams. Just as can be seen from the above two graphs, the e00r and tg de curves for the compacted powders of carbon foams have raised upward a little compared to those of the loose powders. As for carbon foams, the electric conductivity is relatively high because the amorphous carbon has formed
three dimensionally continuous conductive paths, which will give rise to considerable ohmic loss for carbon foams [17,18]. Therefore the dielectric loss of carbon foams is greatly improved and the e00r and tg de curves of carbon foams have raised upward a large extent compared to those of their corresponding pulverized powders. The above results demonstrate that macrostructure modification has a significant effect on the dielectric properties of materials. Carbon foams possess relatively lower e0r but much larger dielectric loss compared with their corresponding pulverized powders just because of the macrostructure difference between them. It proves that macrostructure modification is possibly a new and effective way to modulate the dielectric properties of materials. 3.3.3. Magnetic loss Fig. 8 describes the variation of the imaginary parts of the complex permeability l00r and the magnetic loss tangents tg dm for three different pore size carbon foams and their
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corresponding pulverized powders with electric conductivity at 2450 MHz. The real parts of the complex permeability of all samples are 1.0, which is in agreement with the fact that all samples are nonmagnetic, so l00r equals to tg dm mathematically for all samples. However, the important thing is that carbon foams have incredible magnetic loss while both the corresponding compacted and the loose powders of carbon foams have no such magnetic loss. In Fig. 8, the magnetic loss curves of the compacted and the loose powders of carbon foams were combined into one group for simplicity. For cautiousness, the measurement process and the measurement results were checked again and again, after all carbon foams investigated here are nonmagnetic. Ideally the magnetic field perturbation measurement should be made where the magnetic field is maximal and the electric field is zero [10]. However, the practical resonant cavity may not strictly satisfy such ideal conditions, for instance, the electric field is not completely zero where the magnetic loss measurements were made. Thus, a thin plastic pipe and Teflon hollow pipes with small e0r were used as sample holders to minimize their perturbation effects on the field distribution in the resonant cavity. Actually there are no measurable resonant frequency f0 shift and measurable quality factor Q0 reduction when the plastic pipe and Teflon hollow pipe were located where the magnetic loss measurements were conducted, so it can be predicated that any measurable resonant frequency f0 shift and quality factor Q0 reduction were completely produced by samples carried by the plastic pipe and Teflon hollow pipe. There indeed exist obvious Q0 reductions but no resonant frequency f0 shifts when carbon foam samples were placed at the place for magnetic loss measurements. Therefore, the fact that carbon foams possess magnetic loss is undoubted. Numeric simulation was made simultaneously in our research group to explore how the magnetic loss of car0.25 0.20
tgδ m , μr''
0.15 0.3mm foam 1.0mm foam 2.0mm foam powder of 0.3mm foam powder of 1.0mm foam powder of 2.0mm foam
0.10 0.05 0.00 -0.05 -1
0
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σ (S/m) Fig. 8. The imaginary parts of the complex permeability or the magnetic loss tangents of three different pore size carbon foams and their corresponding pulverized powders versus electric conductivity at 2450 MHz.
bon foams come into being [5–7]. According to the numeric simulation results, the local electric field is scattered in a foam structure while the direction of local electric field in bulks is mostly the same as that of the external electric field. However, electric and magnetic fields are mutually concomitant as EM wave propagates, so a magnetic field changes its direction when an electric field changes its direction. The magnitude and direction of the fields vary with time when scattered, and the varying magnetic field will induce electric currents on conductive circular paths according to Lenz’s law. As has depicted in our previous published papers [5–7], the time-varying magnetic fields of EM waves can induce currents on adjacent struts of carbon foams and thus flowing currents in a circular path are created. The flowing currents decay in the conductive foam structure by converting into heat, which leads to partial energy attenuation of the scattered magnetic fields. Although the local electric field is also scattered in particle-stacked structures, flowing currents can not be produced since there are no conductive circular paths in particle-stacked structures [7], which can be explained why no magnetic loss were observed from the pulverized powders of carbon foams. Larger induced currents will produce for carbon foams having larger electric conductivity, which is why the magnetic loss of carbon foams increases with increasing electric conductivity. This mechanism allows for magnetic loss to take place in non-magnetic carbon foams. Similar results have been reported elsewhere [19,20]. The existence of magnetic loss of carbon foams also partially validate the correctness of numeric simulations previously made in our research group. The pore size of carbon foams has no obvious influence on their tg dm or the l00r except that the magnetic loss of the carbon foam with pore size of 0.3 mm is a little smaller than those of the other two. Such difference is possibly caused by the imperfection of foam structure for the carbon foam with pore size of 0.3 mm as illustrated in Fig. 2c. Because carbon foams are nonmagnetic, so the magnetic loss of carbon foams is apparently extrinsical. As we know, conventional magnetic RAMs have a deadly limitation: their magnetic loss will disappear above their Tcs, which greatly holds back their application as high temperature RAMs despite of their high microwave absorption efficiency. The extrinsically magnetic loss of carbon foams is completely different from conventional magnetic loss, and it is believed that the extrinsically magnetic loss of carbon foams has no Curie transition characteristics and thus can still exist at elevated temperatures [19,20]. The magnetic measurement results once again demonstrate the modulation effect of macrostructure modification on the EM properties of materials. With the macrostructure of amorphous carbon transforming from powders to foam structure, extrinsically magnetic loss has occurred on carbon foams. The results indicate a new way to modulate EM properties of materials and design new kinds of radar absorbing material and structure.
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4. Conclusion The electromagnetic characteristics of carbon foams were investigated at a frequency of 2450 MHz by the TE103 resonant cavity perturbation technique. Compared with their corresponding pulverized powders, carbon foams have relatively lower e0r but several times larger dielectric loss. The most important characteristic of carbon foams is the discovery of their extrinsically magnetic loss, which is usually called artificial magnetic loss and is believed to be able to maintain at high temperatures. The electromagnetic characteristics of carbon foams adequately prove that macrostructure modification is a novel and effective way to modulate electromagnetic properties of materials. Combining all above mentioned good features coupled with low density, carbon foams can be exploited as promising RAMs to satisfy the increasing requirements of stealthy technique. References [1] Seo S, Chin WS, Lee DG. Characterization of electromagnetic properties of polymeric composite materials with free space method. Compos Struct 2004;66(1–4):533–42. [2] Park KY, Lee SE, Kim CG, Han JH. Fabrication and electromagnetic characteristics of electromagnetic wave absorbing sandwich structures. Compos Sci Technol 2006;66(3–4):576–84. [3] Oh JH, Oh KS, Kim CG, Hong CS. Design of radar absorbing structures using glass/epoxy composite containing carbon black in Xband frequency ranges. Compos Part B–Eng 2004;35(1): 49–56. [4] Yang J, Shen ZM, Hao ZB. Microwave characteristics of sandwich composites with mesophase carbon foams as core. Carbon 2004;42(8– 9):1882–5. [5] Fang ZG, Cao XM, Li CS, Zhang JS, Zhang HT, Zhang HY. Investigation of carbon foams as microwave absorber: numerical prediction and experimental validation. Carbon 2006;44(15): 3368–70.
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[6] Zhang HT, Zhang JS, Zhang HY. Electromagnetic properties of silicon carbide foams and their composites with silicon dioxide as matrix in X-band. Compos Part A–Appl S 2007;38(2):602–8. [7] Zhang HT, Zhang JS, Zhang HY. Computation of radar absorbing silicon carbide foams and their silica matrix composites. Comput Mater Sci 2007;38(4):857–64. [8] William WB. Automatic measurement of complex dielectric constant and permeability at microwave frequencies. Proc IEEE 1974;62(1): 33–6. [9] Sheen J. Study of microwave dielectric properties measurements by various resonance techniques. Measurement 2005;37(2):123–30. [10] Inagaki M, Morishita T, Kuno A, Kito T, Hirano M, Suwa T, et al. Carbon foams prepared from polyimide using urethane foam template. Carbon 2004;42(3):497–502. [11] Hirose T, Fujino T, Fan TX. Effect of carbonization temperature on the structural changes of woodceramics impregnated with liquefied wood. Carbon 2002;40(5):761–5. [12] Levy O, Stroud D. Maxwell Garnett theory for mixtures of anisotropic inclusions: application to conducting polymers. Phys Rev B 1997;56(13):8035–46. [13] Burns G. Solid State Physics. Florida: Academic Press, Inc.; 1985. p. 454–61. [14] Penn SJ, Alford NM, Templeton A, Wang X, Xu M, Reece M, et al. Effect of porosity and grain size on the microwave dielectric properties of sintered alumina. J Am Ceram Soc 1997;80(7):1885–8. [15] Dishovskya N, Grigorovab M. On the correlation between electromagnetic waves absorption and electrical conductivity of carbon black filled polyethylenes. Mater Res Bull 2000;35(3):403–9. [16] Unsworth J, Kaynak A, Lunn BA. Microwave transmission, reflection and dielectric properties of conducting and semiconducting polypyrrole films and powders. J Mater Sci 1993;28(12):3307–12. [17] Grimes CA, Mungle C, Kousoudis D, Fang S, Eklund PC. The 500 MHz to 5.50 GHz complex permittivity spectra of single-wall carbon nanotube-loaded polymer composites. Chem Phys Lett 2000;319(5–6):460–4. [18] Yang Y, Zhang BS, Xu WD, Shi YB, Zhou NS, Lu HX. Preparation and electromagnetic characteristics of a novel iron-coated carbon fiber. J Alloy Compd 2004;365(1–2):300–2. [19] Baena JD, Marques R, Medina F. Artificial magnetic material design by using spiral resonators. Phys Rev B 2004;69(1):014402:1–5. [20] Gorkuonov M, Lapine M, Shamonina E. Effective magnetic properties of a composite material with circular conductive elements. Eur Phys J B 2002;28(2):263–9.
Carbon 45 (2007) 2873–2879 www.elsevier.com/locate/carbon
The electromagnetic characteristics of carbon foams Zhigang Fang, Chusen Li, Jiayan Sun, Hongtao Zhang, Jinsong Zhang
*
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China Received 4 June 2007; accepted 11 October 2007 Available online 24 October 2007
Abstract Three different pore size carbon foams with variable electric conductivities were prepared by a polymer sponge replication method. The electromagnetic parameters of these carbon foams and their corresponding pulverized powders were measured by a resonant cavity perturbation technique at a frequency of 2450 MHz. The results show that carbon foams have smaller dielectric constants but several times larger dielectric loss compared with their corresponding pulverized powders. Moreover, carbon foams show magnetic loss while no magnetic loss can be observed from their corresponding pulverized powders. The magnetic loss of carbon foams is apparently a kind of extrinsically magnetic loss and believed to be able to maintain at high temperatures. The electromagnetic characteristics of carbon foams demonstrate that macrostructure modification is an effective way to modulate electromagnetic properties of such materials. Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction Radar absorbing materials (RAMs) are widely applied in order to achieve the objective of stealth for military and civil targets. Presently, RAMs can be classified into two categories: magnetic and dielectric absorbing materials. Magnetic absorbers depend on the magnetic hysteresis effect, which is obtained when magnetic materials such as ferrites are added to a matrix. However, densities of the magnetic materials are usually high and absorbing bandwidths for magnetic absorbers are usually narrow. Moreover, the magnetic absorbers will fail when the temperature is higher than their Curie temperatures (Tcs). While the dielectric absorbers depend on the ohmic loss, which can be achieved by adding conductive fillers such as carbon black, graphite, or metallic particles to a matrix. Dielectric absorbers have an advantage of light-weight but do not match up to the absorptivity of magnetic absorbers [1–3]. These two kinds of materials have different advantages and disadvantages when they are applied as absorbers. They can be used together as a composite, and the mag-
*
Corresponding author. Fax: +86 24 23906640. E-mail address: [email protected] (J. Zhang).
0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.10.013
netic loss material is usually the base one, but heavy weight of the material is still a concerning problem. Recently carbon foams found their way to act as RAMs, mainly due to their well matched impedance, high durability, light-weight, and effective EM wave absorbing capability [4]. Their characteristic structure and properties can be utilized to overcome some of the shortcomings of existing RAMs. However, to the best of our knowledge, little work been carried out to explore the radar absorbing properties of carbon foams. Numeric simulations in our research group have predicted that silicon carbide and carbon foams have excellent microwave absorbing properties and unique electromagnetic (EM) wave dissipation characteristics compared with the same composition bulk or particles [5–7]. However, these results were achieved only by numeric simulation, it is necessary to investigate and validate the electromagnetic characteristics of this novel RAM by detailed experiments. In the present paper, the investigation of the electromagnetic characteristics of carbon foams was carried out experimentally. The electromagnetic parameter estimation of carbon foams and their pulverized powders was carried out for comparison. Because the wide bandwidth electromagnetic characteristics of materials are usually more difficult to obtain, the electromagnetic parameter measurements
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of materials made at spot frequencies instead are frequently carried out [8]. While resonant cavity perturbation technique is the most applicable method for measuring electromagnetic parameters of materials on spot frequencies and the frequency of 2450 MHz is the most widely adopted one [9], so in this paper the electromagnetic parameters of carbon foams and their pulverized powders were made at a frequency of 2450 MHz by the resonant cavity perturbation technique.
electric conductivities were prepared by elaborately controlling their carbonization process. Carbon foams were also pulverized into powders and sifted with a 100-mesh sieve.
2.2. Electric conductivity measurements The electric conductivities of carbon foams were calculated according to Ohm’s law from the geometric dimension and the electrical resistance of carbon foams measured by a numeric double bridge circuit at room temperature.
2.3. Electromagnetic parameter measurements
2. Experimental 2.1. Sample preparation Carbon foams were fabricated by a polymer sponge replica method [10], as demonstrated in Fig. 1. Three different pore size polymer sponges, 0.3 mm, 1.0 mm and 2.0 mm, were chosen as templates, so carbon foams with the same three pore sizes were obtained and named as 0.3 mm foam, 1.0 mm foam and 2.0 mm foam, respectively. Carbon foams with variable
Preparing slurry with proper viscosity
Immersing polymer sponges into slurry several times
Remove excess slurry from sponges by pressing and blowing after every immersing
Gelling the coated sponges at 80ºC
Carbonizing the dried bodies at certain temperatures in inert atmosphere
Fig. 1. Flowchart for fabricating carbon foams.
The electromagnetic parameters were measured by the TE103 resonant cavity perturbation technique at 2450 MHz [9]. For dielectric measurements, samples were placed in a maximum electric field, and for magnetic measurements, in a maximum magnetic field. Carbon foams for dielectric and magnetic parameter measurements were machined to small cylinders with 8 mm in diameter. Pulverized powders of carbon foams were loaded and encapsulated into Teflon hollow pipes with 9 mm in diameter and 0.5 mm in wall thickness for measurements. Two kinds of pulverized powders of each carbon foam were prepared for comparison: one was tightly pressed to form a compacted cylinder; another was just slightly vibrated to form a loose cylinder. The two kinds of pulverized powders were named as compacted powder and loose powder, respectively, in the following section of this paper. A thin plastic pipe was used to carry samples to the perturbation location for measurements, and the perturbation effects by the Teflon hollow pipe and plastic pipe were first calibrated and subtracted from each measurement.
3. Results and discussion 3.1. Morphology and structure Fig. 2 shows the SEM morphology of carbon foams with different pore sizes. As can be seen from the picture, carbon foams have relatively evenly-distributed pore sizes. There are some closed cells in the carbon foam with pore
Fig. 2. SEM images of carbon foams with different pore sizes (a) pore size about 2.0 mm; (b) pore size about 1.0 mm; (c) pore size about 0.3 mm.
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the carbonization temperature when other carbonization process parameters are identical. The electric conductivity of carbon foams with pore size 2.0 mm and 0.3 mm follows the similar changing tendency with respect to the carbonization temperature. Fig. 4 indicates that the carbonization temperature has a direct effect on the electric conductivity of carbon foams. And the electric conductivity of carbon foams increases quickly when the carbonization temperature improves, so the electric conductivity of carbon foams is adjustable by controlling the carbonization temperatures. The carbonization temperature of carbon foams affects the graphitization degree of their amorphous structures and further their electric conductivity, which has also been found on other carbon-based materials [11].
Bragg angle (Degrees) Fig. 3. The XRD spectrum of one carbon foam sample.
-0.5
3.3.1. Dielectric constants The variation of the real parts of the complex permittivity e0r of three different pore size carbon foams and their corresponding pulverized powders at 2450 MHz with electric conductivity is plotted in Fig. 5. It can be seen that e0r of three different pore size carbon foams and their corresponding pulverized powders increase gradually with the increase of electric conductivity. The real part the complex permittivity of a material presents its polarization capability at a certain frequency of EM waves, so carbon foams and their corresponding pulverized powders have an increasing polarization capability at 2450 MHz as the electric conductivity improves, which maybe relate to their microstructure change. An important fact from Fig. 5 is that the e0r of carbon foams is quite lower, about 50%, than those of their corresponding compacted powders. For example, e0r for the carbon foam with pore size of 1.0 mm increases from 2.94 to 4.90 while that for its corresponding compacted powder ranging from 4.59 to 5.70 when the electric conductivity increases from 0.035 S/m to 5.64 S/m. The effective medium theory (EMT) can help in understanding the e0r difference between carbon foams and their corresponding compacted powders [12]. An incident wave is not sensitive to particles or structures that are smaller than a sensing wavelength. EMT uses this property of radiation to determine the effective mean values (eeff and leff) of e and l. The Maxwell–Garnett (M–G) theory is used widely for calculating effective medium which can be expressed as
-1.0
M–G eeff ¼ e1
size of 0.3 mm, which were produced during the fabrication process for the difficulty of removing excess slurry for such small pore size foams. Apart from the macro-pores in carbon foams, there also exist holes left by the elimination of the polymer sponge template in the center of each ligament of carbon foams. Fig. 3 gives the XRD spectrum for pulverized powders of one carbon foam sample. There are obviously no sharp but weak peaks of graphitic carbon at planes (0 0 2) and (1 0 1) [11]. There were no obvious difference between the XRD spectra of other carbon foams and the one presenting in Fig. 3, which indicates that carbon foams carbonized at low temperatures as in this paper are composed of amorphous structures. 3.2. Electric conductivity Fig. 4 shows the relationship between the electric conductivity of carbon foam with pore size of 1.0 mm and 1.0 0.5
lgσ (S/m)
3.3. Electromagnetic parameters
0.0
-1.5 700
710
720
730
740
750
760
Carbonization temperature/ºC Fig. 4. Relationship between the electric conductivity of carbon foams with pore size of 1.0 mm and carbonization temperature.
ðe2 þ 2e1 Þ þ 2f ðe2 e1 Þ ðe2 þ 2e1 Þ f ðe2 e1 Þ
where e1 and e2 are the dielectric permittivity of the host and guest, respectively, and f corresponds to the volume fraction of the guest in the effective medium [12]. The dimensions of the pore structure of carbon foams are much smaller than the wavelength, so carbon foams can also be treated as an effective medium as a mixture of the insulating air and the amorphous carbon in the ligament of
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6.0 0.3mm foam 1.0mm foam 2.0mm foam compacted powder of 0.3mm foam compacted powder of 1.0mm foam compacted powder of 2.0mm foam loose powder of 0.3mm foam loose powder of 1.0mm foam loose powder of 2.0mm foam
5.6 5.2
εr'
4.8 4.4 4.0 3.6 3.2 2.8 0
1
2
3
σ (S/m)
4
5
6
7
Fig. 5. The real parts of the complex permittivity for three different pore size carbon foams and their corresponding pulverized powders versus electric conductivity at 2450 MHz.
carbon foams. The same assumption can be applied to the compacted and loose powders of carbon foams. As can be seen from the above formula, higher volume fraction f of a guest medium will lead to higher eeff for the effective medium. Carbon foams and their corresponding compacted powders have the same host and guest mediums, but the host medium fractions, i.e., the volume fractions of amorphous carbon, in carbon foams are much lower than those in their corresponding compacted powders, so carbon foams have much lower effective dielectric constants accordingly. Fig. 5 also indicates that the loose powders of carbon foams have even smaller e0r than carbon foams. Besides the volume fraction difference of amorphous carbon between carbon foams and their corresponding loose powders, other factors affecting e0r of materials should be taken into consideration. The real part of complex dielectric constant is an expression of polarization ability of a material which mainly arises from dipolar polarization and interfacial polarization at microwave frequencies [13]. However, for the loose powders of carbon foams, the distance between powder particles is too large to act as charge carrier, so the interfacial polarization effect would be very slight and almost only the dipolar polarization can contribute to their overall polarization capability [14]. Therefore eeff of the loose powders of carbon foams are lower than those of carbon foams even if the volume fractions of amorphous carbon in the former are higher than those in the latter. The pore size has a minor effect on e0r of different pore size carbon foams and their corresponding pulverized powders, respectively. Compared with the large effect produced by macrostructure change, i.e., from foam to powder, on e0r of amorphous carbon, such a minor effect is neglectable. The real part of the complex permittivity of a medium is an important factor to determine the ratios of reflection for an incident EM wave upon the surface of medium. The impedance of a medium with lower e0r is much closer to
the impedance of free space, hence good conjugation with free space will get because the impedance varies gently at the interface. Therefore the EM wave can easily penetrate the medium with lower e0r and travel a long distance in it. So it is obvious that the low e0r of carbon foams is quite advantageous to their application as RAMs. 3.3.2. Dielectric loss The variation of the imaginary parts of the complex permittivity e00r and the dielectric loss tangents tg de for three different pore size carbon foams and their pulverized powders with electric conductivity at 2450 MHz are shown in Figs. 6 and 7, respectively. As can be seen from these two figures, e00r and tg de of the three pore size carbon foams and their corresponding pulverized powders do not change with the electric conductivity monotonically, they first increasing to a maximum and then decreasing gradually. This is because the direct reflection of incident EM waves from the surface of conductive dielectric RAMs becomes more and more prominent and less incident EM waves can enter into them for further attenuation when their electric conductivity improves gradually [15,16]. Therefore, e00r and tg de for conductive dielectric RAMs would reach a maximum at a certain electric conductivity although the ohmic loss improves monotonically with the increase of electric conductivity for conductive dielectric RAMs. By comparing the e00r and tg de of carbon foams and their corresponding pulverized powders in Figs. 6 and 7 it can be concluded that e00r and tg de of carbon foams are much larger than those of their corresponding pulverized powders, about two and four times, respectively, as large as those of the compacted powders and the loose powders. Similarly, the pore size does not have a significant effect on e00r and tg de of carbon foams with different pore sizes and their corresponding pulverized powders, respectively. As has mentioned above, the interfacial polarization effect in the loose powders of carbon foams is very weak, hence nearly only the dipolar polarization relaxation loss
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0.8 0.3mm foam 1.0mm foam 2.0mm foam compacted powder of 0.3mm foam compacted powder of 1.0mm foam compacted powder of 2.0mm foam loose powder of 0.3mm foam loose powder of 1.0mm foam loose powder of 2.0mm foam
0.7 0.6
εr''
0.5 0.4 0.3 0.2 0.1 0
1
2
3
σ (S/m)
4
5
6
7
Fig. 6. The imaginary parts of the complex permittivity for three different pore size carbon foams and their corresponding pulverized powders versus electric conductivity at 2450 MHz.
0.20
0.3mm foam 1.0mm foam 2.0mm foam compacted powder of 0.3mm foam compacted powder of 1.0mm foam compacted powder of 2.0mm foam loose powder of 0.3mm foam loose powder of 1.0mm foam loose powder of 2.0mm foam
0.18 0.16
tgδ e
0.14 0.12 0.10 0.08 0.06 0.04 0
1
2
3
σ (S/m)
4
5
6
7
Fig. 7. The dielectric loss tangents of three different pore size carbon foams and their corresponding pulverized powders versus electric conductivity at 2450 MHz.
of the amorphous carbon can contribute to the dielectric loss of the loose powders of carbon foams. Therefore the dielectric loss of the loose powders of carbon foams at a certain electric conductivity represents the intrinsically dielectric loss of the amorphous carbon at that electric conductivity. When the loose powders of carbon foams are pressed tightly to form the compacted powders, the distance between powder particles is greatly reduced and thus the interfacial polarization effect and partially electric conductive performance have been improved more or less. So the interfacial polarization relaxation loss and partially ohmic loss should be taken into account besides the intrinsically dielectric loss of amorphous carbon for the dielectric loss of the compacted powders of carbon foams. Just as can be seen from the above two graphs, the e00r and tg de curves for the compacted powders of carbon foams have raised upward a little compared to those of the loose powders. As for carbon foams, the electric conductivity is relatively high because the amorphous carbon has formed
three dimensionally continuous conductive paths, which will give rise to considerable ohmic loss for carbon foams [17,18]. Therefore the dielectric loss of carbon foams is greatly improved and the e00r and tg de curves of carbon foams have raised upward a large extent compared to those of their corresponding pulverized powders. The above results demonstrate that macrostructure modification has a significant effect on the dielectric properties of materials. Carbon foams possess relatively lower e0r but much larger dielectric loss compared with their corresponding pulverized powders just because of the macrostructure difference between them. It proves that macrostructure modification is possibly a new and effective way to modulate the dielectric properties of materials. 3.3.3. Magnetic loss Fig. 8 describes the variation of the imaginary parts of the complex permeability l00r and the magnetic loss tangents tg dm for three different pore size carbon foams and their
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corresponding pulverized powders with electric conductivity at 2450 MHz. The real parts of the complex permeability of all samples are 1.0, which is in agreement with the fact that all samples are nonmagnetic, so l00r equals to tg dm mathematically for all samples. However, the important thing is that carbon foams have incredible magnetic loss while both the corresponding compacted and the loose powders of carbon foams have no such magnetic loss. In Fig. 8, the magnetic loss curves of the compacted and the loose powders of carbon foams were combined into one group for simplicity. For cautiousness, the measurement process and the measurement results were checked again and again, after all carbon foams investigated here are nonmagnetic. Ideally the magnetic field perturbation measurement should be made where the magnetic field is maximal and the electric field is zero [10]. However, the practical resonant cavity may not strictly satisfy such ideal conditions, for instance, the electric field is not completely zero where the magnetic loss measurements were made. Thus, a thin plastic pipe and Teflon hollow pipes with small e0r were used as sample holders to minimize their perturbation effects on the field distribution in the resonant cavity. Actually there are no measurable resonant frequency f0 shift and measurable quality factor Q0 reduction when the plastic pipe and Teflon hollow pipe were located where the magnetic loss measurements were conducted, so it can be predicated that any measurable resonant frequency f0 shift and quality factor Q0 reduction were completely produced by samples carried by the plastic pipe and Teflon hollow pipe. There indeed exist obvious Q0 reductions but no resonant frequency f0 shifts when carbon foam samples were placed at the place for magnetic loss measurements. Therefore, the fact that carbon foams possess magnetic loss is undoubted. Numeric simulation was made simultaneously in our research group to explore how the magnetic loss of car0.25 0.20
tgδ m , μr''
0.15 0.3mm foam 1.0mm foam 2.0mm foam powder of 0.3mm foam powder of 1.0mm foam powder of 2.0mm foam
0.10 0.05 0.00 -0.05 -1
0
1
2
3
4
5
6
7
8
9
10
σ (S/m) Fig. 8. The imaginary parts of the complex permeability or the magnetic loss tangents of three different pore size carbon foams and their corresponding pulverized powders versus electric conductivity at 2450 MHz.
bon foams come into being [5–7]. According to the numeric simulation results, the local electric field is scattered in a foam structure while the direction of local electric field in bulks is mostly the same as that of the external electric field. However, electric and magnetic fields are mutually concomitant as EM wave propagates, so a magnetic field changes its direction when an electric field changes its direction. The magnitude and direction of the fields vary with time when scattered, and the varying magnetic field will induce electric currents on conductive circular paths according to Lenz’s law. As has depicted in our previous published papers [5–7], the time-varying magnetic fields of EM waves can induce currents on adjacent struts of carbon foams and thus flowing currents in a circular path are created. The flowing currents decay in the conductive foam structure by converting into heat, which leads to partial energy attenuation of the scattered magnetic fields. Although the local electric field is also scattered in particle-stacked structures, flowing currents can not be produced since there are no conductive circular paths in particle-stacked structures [7], which can be explained why no magnetic loss were observed from the pulverized powders of carbon foams. Larger induced currents will produce for carbon foams having larger electric conductivity, which is why the magnetic loss of carbon foams increases with increasing electric conductivity. This mechanism allows for magnetic loss to take place in non-magnetic carbon foams. Similar results have been reported elsewhere [19,20]. The existence of magnetic loss of carbon foams also partially validate the correctness of numeric simulations previously made in our research group. The pore size of carbon foams has no obvious influence on their tg dm or the l00r except that the magnetic loss of the carbon foam with pore size of 0.3 mm is a little smaller than those of the other two. Such difference is possibly caused by the imperfection of foam structure for the carbon foam with pore size of 0.3 mm as illustrated in Fig. 2c. Because carbon foams are nonmagnetic, so the magnetic loss of carbon foams is apparently extrinsical. As we know, conventional magnetic RAMs have a deadly limitation: their magnetic loss will disappear above their Tcs, which greatly holds back their application as high temperature RAMs despite of their high microwave absorption efficiency. The extrinsically magnetic loss of carbon foams is completely different from conventional magnetic loss, and it is believed that the extrinsically magnetic loss of carbon foams has no Curie transition characteristics and thus can still exist at elevated temperatures [19,20]. The magnetic measurement results once again demonstrate the modulation effect of macrostructure modification on the EM properties of materials. With the macrostructure of amorphous carbon transforming from powders to foam structure, extrinsically magnetic loss has occurred on carbon foams. The results indicate a new way to modulate EM properties of materials and design new kinds of radar absorbing material and structure.
Z. Fang et al. / Carbon 45 (2007) 2873–2879
4. Conclusion The electromagnetic characteristics of carbon foams were investigated at a frequency of 2450 MHz by the TE103 resonant cavity perturbation technique. Compared with their corresponding pulverized powders, carbon foams have relatively lower e0r but several times larger dielectric loss. The most important characteristic of carbon foams is the discovery of their extrinsically magnetic loss, which is usually called artificial magnetic loss and is believed to be able to maintain at high temperatures. The electromagnetic characteristics of carbon foams adequately prove that macrostructure modification is a novel and effective way to modulate electromagnetic properties of materials. Combining all above mentioned good features coupled with low density, carbon foams can be exploited as promising RAMs to satisfy the increasing requirements of stealthy technique. References [1] Seo S, Chin WS, Lee DG. Characterization of electromagnetic properties of polymeric composite materials with free space method. Compos Struct 2004;66(1–4):533–42. [2] Park KY, Lee SE, Kim CG, Han JH. Fabrication and electromagnetic characteristics of electromagnetic wave absorbing sandwich structures. Compos Sci Technol 2006;66(3–4):576–84. [3] Oh JH, Oh KS, Kim CG, Hong CS. Design of radar absorbing structures using glass/epoxy composite containing carbon black in Xband frequency ranges. Compos Part B–Eng 2004;35(1): 49–56. [4] Yang J, Shen ZM, Hao ZB. Microwave characteristics of sandwich composites with mesophase carbon foams as core. Carbon 2004;42(8– 9):1882–5. [5] Fang ZG, Cao XM, Li CS, Zhang JS, Zhang HT, Zhang HY. Investigation of carbon foams as microwave absorber: numerical prediction and experimental validation. Carbon 2006;44(15): 3368–70.
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