Microwave Absorption Enhancement of Porous Carbon Fibers

Mar 22, 2012 - Porous carbon fibers (pores of: 0.1–3 μm in diameter) and carbon nanofibers (∼100 nm in diameter) were prepared from polyacrylonit...
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Microwave Absorption Enhancement of Porous Carbon Fibers Compared with Carbon Nanofibers Guang Li,* Tianshi Xie, Shenglin Yang, Junhong Jin, and Jianming Jiang State Key Laboratory for Modification of Chemical Fibers and Polymer Materials College of Material Science and Engineering, Donghua University, Shanghai 201620, P. R. China ABSTRACT: Porous carbon fibers (pores of: 0.1−3 μm in diameter) and carbon nanofibers (∼100 nm in diameter) were prepared from polyacrylonitrile/polymethyl methacrylate (PAN/PMMA) blend fibers with 70/30 and 30/70 weight ratio, respectively, as precursors. The composites containing 2−6 wt % porous carbon fibers or carbon nanofibers as microwave absorbents were fabricated. The complex permittivity of these composites was measured, and the microwave absorption properties were stimulated based on a model for a single-layer plane wave absorber. We found that composites filled with the porous carbon fibers exhibited a much better performance in microwave absorption than those containing the carbon nanofibers. Composites containing 6 wt % porous carbon fibers or carbon nanofibers showed the lowest reflection loss of −31 dB at 9.7 GHz and −12.2 dB at 10.7 GHz, respectively. The bandwidth with reflection loss below −5 dB covered the whole X band (4.2 GHz) in the former case, whereas it was only 2.6 GHz in the latter case, indicating the superior performances of the porous carbon fiber compared with carbon nanofiber in electromagnetic wave absorbing properties. We postulated that the combination of the dielectric-type absorption and the interference of multireflected microwaves could be responsible for the enhancement of microwave absorption in the porous carbon fibers.

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Ling15 found that polyethylene composites filled with 25 wt % short carbon fibers (5−7 μm in diameter, 60 μm in length) exhibited the lowest reflection loss of −11.29 dB at the thickness of 3 mm. CNTs with special morphologies and coatings have been paid more attention. Qi et al.16 prepared CNTs with high helicity and fabricated a composite by blending 30 wt % CNTs with paraffin, which showed a microwave absorption capability of −20 dB at 10.5 GHz, with a bandwidth of reflection loss below −5 dB ranging from 9.5 to 12.5 GHz. CNTs with or without defects displayed diversified electromagnetic behavior as a function of frequency. Watts et al.17 used boron-doped CNTs with defects and arc-made CNTs without defects as the fillers and incorporated them separately in polystyrene to make the composites. They found that the composites containing boron-doped CNTs exhibited a relatively high ε′ and ε″ compared with those containing arcmade CNTs.17 Fe-filled CNTs were also prepared, and the composite prepared with the addition of 20 wt % of the Fefilled CNTs showed the lowest reflection loss of −11.2 dB.18 In addition, the CNTs encapsulated with different forms of Fe also influenced the complex permittivity. When the Fe nanoparticles, Fe nanowires, and crystalline-Fe were encapsulated within CNTs and used as the microwave absorption fillers in the presence of epoxy resin with a weight ratio of 1:5, different values of reflection loss of the composites were observed, with the composites containing crystalline-Fe exhibiting the best

INTRODUCTION Microwave-absorbing materials have found promising applications in military and commodity markets.1 For example, microwave-absorbing materials could effectively reduce the radar cross-section of targets in the military and in this manner contributed to a stealth defense system. Similarly, microwave absorbents used in people’s daily lives could prevent human beings from harmful electromagnetic radiation. Therefore, there has been a great deal of interest in the preparation and characterization of the electromagnetic properties of various microwave absorbents. Typically, composite materials containing magnetic metal particles such as ferrite, nickel, zinc, and others have been used as magnetic losses of electromagnetic waves in conjuction with polymeric resins;2−4 because this method uses magnetic additives with a high specific mass, it has a serious weakness of generating a heavy piece of equipment. Currently, carbon materials, including carbon black,5,6 carbon fibers,7,8 and carbon nanotubes (CNTs)9,10 have played a leading role in the investigation and application of microwaveabsorbing materials due to their low specific mass. Among these carbon materials, carbon nanofibers (CNFs), CNTs, as well as carbon nanocoils become increasingly interesting because of their high aspect ratio, which results in high electromagnetic losses at low filler content along with a reduction of thickness and weight.11−13 Tang et al.14 synthesized twin carbon nanocoils and found that the composites containing 15 and 22 wt % twin carbon nanocoils exhibited the smallest reflection of −15 dB at 11 GHz and −27 dB at 13 GHz, respectively, which showed much higher electromagnetic losses as compared with that of short carbon fibers filled composites. For instance, © 2012 American Chemical Society

Received: January 3, 2012 Revised: March 18, 2012 Published: March 22, 2012 9196

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Figure 1. SEM images of the cross section of the PAN/PMMA = 30/70 (a) and PAN/PMMA = 70/30 (b) blend films; (c,d) SEM images of the PAN/PMMA = 30/70 and PAN/PMMA = 70/30 films treated with acetone.

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microwave absorption performance.19 Carbon black coated with polyaniline (PANI/CB), forming a core-sell structure, was synthesized by in situ polymerization with different amounts of carbon black (5−30 wt %) and then introduced into epoxy resin as a microwave absorber. In the frequency range of 18−40 GHz, PANI/CB containing 20 and 30 wt % carbon black demonstrated a reflection loss of −11 and −16 dB at 28 and 35 GHz, respectively.20 Despite a great deal of research on carbon material absorbents, until now there have been only a few reports on the microwave absorption performance of the composites filled with porous carbon fibers. In comparison with the use of porous carbon fibers in catalysis, sensors, and gas and liquid separation devices, almost no attempt has been made to use porous carbon fibers as microwave-absorbing fillers. In contrast with other carbon materials, porous carbon fibers exhibit superior properties, including a low specific gravity, multiform framework, and large surface area. Therefore, it could be expected that porous carbon fiber-filled composites could show the excellent microwave absorption properties. In this Article, two carbon materials, that is, porous carbon fibers (pores size: 0.1−3 μm in diameter) and CNFs (the diameter of ∼100 nm), were separately used as microwaveabsorbing fillers and added in epoxy resin to prepare a series of composites. The complex permittivity of these composites was measured by the free space method in the X-band frequency range (8.2−12.4 GHz), and the microwave absorption properties of the corresponding composites were stimulated based on a model for a single-layer plane wave absorber using the complex permittivity data. It was found that the microwaveabsorbing performance of the composites containing the porous carbon fibers is superior to that of the composites containing the CNFs.

EXPERIMENTAL SECTION Materials. All of the chemicals used are commercially available and were used as received without further purification. Poly(acrylonitrile) (PAN) was supplied by Shanghai Petroleum Chemical Engineering. Poly(methyl methacrylate) (PMMA), N,N′-dimethylformamide (DMF), epoxy-128, curing agent (593), and acetone were purchased from the National Chemical Reagent. Preparation of PAN/PMMA Blend Solutions and Blend Fibers. The PAN/PMMA (weight ratio: 70/30 and 30/70) blend solutions were prepared, using DMF as the solvent, with a total polymer concentration of 25 wt %. The corresponding blend fibers were obtained by means of typical wet spinning of polymer blend solutions. Carbonization of PAN/PMMA Blend Fibers. The PAN/ PMMA blend fibers were heated to 180−280 °C in air for a preoxidative stabilization process and then carbonized at 1200 °C under a nitrogen flow. Because of thermal decomposition, the PMMA component was completely removed from the fibers. As a result, porous carbon fibers and CNFs were fabricated from the PAN/PMMA (70/30) and (30/70) blend fibers, respectively. The principle and detailed explanation of this process have been described elsewhere.21−23 Fabrication of Composites. Separately, porous carbon fibers and CNFs were ground up to form micro- to millimetersized particles and then added in epoxy. A mixture of epoxy with porous carbon fibers or CNFs was diluted with acetone and was ultrasonically dispersed for 30 min. Subsequently, a curing agent (593) with a weight ratio of 1:4 (curing agent to epoxy) was added to the mixture, followed by stirring for 15 min. Finally, the mixture was injected into the mold with a size of 22.86 × 10.16 × 3.0 mm3 and cured in air for ∼24 h. Several composites containing 2, 4, and 6 wt % of the porous carbon fibers or CNFs were prepared. Characterizations. To study the cross-section morphologies of samples, PAN/PMMA blend films, and fibers, we 9197

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quenched porous carbon fibers in liquid nitrogen and examined them by scanning electron microscopy (SEM; JSM-5600LV and S-4800). The morphology of CNFs was directly observed by SEM. The electrical conductivities of these carbon materials were measured on a model 236 high-resistance meter (Keithley, USA) using rectangular samples that were prepared by coldpressing of the carbon materials under a pressure of 30 MPa at room temperature. The complex permittivity ε (ε = ε′ − jε″) of the composites was measured using a HP8722ES network analyzer.

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RESULTS AND DISCUSSION Morphology of the PAN/PMMA Blend Films. PAN is a highly polar polymer with a solubility parameter of 37.8 J/cm3, whereas PMMA is relatively nonpolar polymer with a solubility parameter of 54.6 J/cm3. Because of the fact that they are not compatible with one another, the phase separation of PAN/ PMMA blend films mainly depends on the weight ratio of the two components in the films. When a mixture has a low PAN ratio, that is, PAN/PMMA = 30/70, it forms a structure where PAN is the minority phase, as shown in Figure 1a. In contrast, when the mixture has a high PAN ratio, that is, PAN/PMMA = 70/30, it becomes the matrix phase, as shown in Figure 1b. Because PMMA is soluble in acetone, it is easy to distinguish the phase domains of the blend films after treatment with acetone. Figure 1c,d illustrates the change in the morphology of the blend films when PMMA is dissolved in the films. These results confirmed that PAN appears as the dispersed phase when its content is 30 wt %, seen in Figure 1a,c, and it remains in the matrix phase when its content reaches 70 wt %, as shown in Figure 1b,d. It was demonstrated that both the dispersed phase and the matrix material can be elongated along the drawing direction during the spinning of the blends,18 which allows a fibrillate structure of the matrix and dispersed phases to form upon spinning and drawing of the PAN/PMMA blends. Figure 2a,b showed the cross-section of the PAN/PMMA (70/30) fiber and the surface of the PAN/PMMA (30/70) fibers after treatment with acetone, respectively. Clearly, in the PAN/ PMMA (70/30) fiber, many pores are formed, producing a faveolate-like structure when PMMA is partially removed by acetone, seen in Figure 2a. In contrast, PAN micrometer fibrils are observed in the treated PAN/PMMA (30/70) blend fibers due to the removal of the PMMA matrix, as shown in Figure 2b. Characterization of As-Prepared Carbon Materials. It is easy to envisage what kinds of materials would form when PAN/PMMA blend fibers are oxidized and then carbonized at high temperatures. As shown in Figure 3, CNFs are obtained after the oxidation and carbonization of PAN/PMMA (30/70) blend fibers. The bundle appearance of the obtained CNFs is shown in Figure 3 a and appears similar to that of the nanofibers in Figure 2b. It could be expected that the PMMA component was completely removed during the carbonization process, and the CNFs were produced from the PAN precursor. The diameter of a characteristic carbon fiber, as revealed by a high-magnification TEM in Figure 3b, is ∼100 nm. As expected, for the PAN/PMMA (70/30) blend fiber precursor, wherein PAN is distributed in the matrix, porous carbon fibers were obtained, as shown in Figure 3c. Furthermore, the longitudinal section provided in Figure 3d demonstrates that the formed pores are oriented along the axis of fibers.

Figure 2. Cross section of the PAN/PMMA = 70/30 blend fiber (a) and the surface of PAN/PMMA = 30/70 blend fiber (b) treated with acetone.

Electromagnetic Properties and Microwave Absorption Performances of Composites Filled with AsPrepared Carbon Materials. The conductivity of the porous carbon fibers and CNFs used in the composites is approximately (2.3 to 3.1) × 102 S/cm, which is in favor of the absorption of electromagnetic waves according to our previous study,21 because low values of conductivities could not produce electromagnetic absorption efficiently and too high conductivities lead to reflections of electromagnetic waves. Figure 4 shows the spectra of the complex permittivity for CNFs/epoxy composites in a frequency range of 8.2−12.4 GHz. The ε′ of the composites containing 2, 4, and 6 wt % CNFs increases with increasing content of CNFs, but the ε′ values of the composites show insignificant variation in the measured frequency range. It is like the fact that the ε″ of the composites exhibits a similar trend as the ε′ with respect to the CNF content and frequency. In contrast with the features of ε′ and ε″, both μ′ and μ″ demonstrate little change as a function of frequency and content of CNFs. Like other nonmagnetic materials, μ′ is nearly equal to 1 and μ″ is equal to 0 in this system. When the porous carbon fibers act as absorbents for microwave absorption, the spectra of the complex permittivity for the prepared composites in the frequency range of 8.2−12.4 GHz is shown in Figure 5. The ε′ of the composites with different amounts of porous carbon fibers exhibits a fluctuation as the frequency increases; in particular, the ε′ of the composites with 4 and 6 wt % porous carbon fibers shows a dramatic decrease with increasing frequency. Clearly, the ε″ of these composites demonstrates a similar characteristic with respect to frequency as that of ε′, except for the composite containing 2 wt % porous carbon fibers, in which ε″ has no clear variation with increasing frequency. This characteristic behavior as a function of frequency, that is, the significant decrease in the ε′ and ε″ with increasing applied frequency, is called frequency 9198

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Figure 3. SEM images of a bundle of carbon nanofibers (a), several individual carbon nanofibers (b), the cross-section (c), and the longitudinal section (d) of a porous carbon fiber.

dispersion behavior, which is beneficial for the impedance matching of the incoming microwave.14 Typically, the measured values of ε′ and ε″ are used to determine the reflection loss of the prepared composites based

on a model for a single-layer plane-wave absorber. In this model, the wave impedance (Z) at the air-absorber interface is given by the equation Z = Z0(μr/εr)1/2 tanh[(−j2π/c)(μr/ εr)1/2fd], where μr = μ′ − jμ″ and εr = ε′ − jε″ are the relative

Figure 4. Permittivity spectra of the composites containing 2−6 wt % carbon nanofibers.

Figure 5. Complex permittivity of the composites containing 2−6 wt % porous carbon fibers. 9199

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complex permeability and permittivity of the absorber medium, respectively, while Z0 = 377 Ω and f are the wave impedance and frequency, respectively. In free space, c is the velocity of light and d is a thickness of a given sample. The reflection loss (RL) in decibels (dB) is then determined as RL = −20 log10[|(Z − Z0)/(Z + Z0)|]. The impedance matching condition, representing the properties of a perfect absorber, is given by Z = Z0. The microwave absorption properties of the prepared composites filled with CNFs are displayed in Figure 6a. The

the prepared composite at the same weight fraction with respect to epoxy and a larger thickness of 3.0 mm. Moreover, in the case of a composite filled with porous carbon fibers at a concentration of 4 and 6 wt %, the bandwidth with reflection loss below −5 dB covers the whole measured X-band (8.2−12.4 GHz), as shown in Figure 6b. Additionally, the porous carbon-fiber-filled composites show an additional minimum at a higher frequency band (∼11 GHz) in the measured microwave frequency range, which is favorable because it extends the absorption band. In comparison with other carbon material absorbents, such as carbon black and CNTs, our prepared porous carbon fibers are added at a relatively low amount (4−6 wt %), whereas much higher amounts are used in other carbon-material-filled composites in the range of 20−30 wt % of absorbents.18−20 Therefore, the fact that the incorporation of a relatively small amount of the porous carbon fibers in epoxy produces excellent microwave absorption properties is a testament to the outstanding performances of this system. Discussion of the Mechanism for Microwave Absorption of Porous Carbon Fibers. In general, the mechanism for microwave absorption in this kind of composite filled with conductive components is mainly due to a dielectric-type loss of electromagnetic waves. In other words, the absorption of microwaves is largely due to dielectric dispersion properties.14 Meanwhile, it has been reported that the presence of defects in conductive carbon nanocoils and multiwalled CNTs gives rise to electromagnetic wave absorption.14,17 Hsu and coworkers have shown that electronic spin due to point defects and polarized centers may have a profound effect on the response of CNTs.17 In comparison with the typical carbon materials, porous carbon fibers possess pores with different sizes and shapes within fibers, which could be considered to be “defects.” Furthermore, the presence of many pores in porous carbon fibers could provide another additional pathway for the absorption of electromagnetic waves. The aspect ratio of the milled porous carbon fibers after being ground into micro- to millimeter sizes is shown in Figure 7a, which demonstrates a microstructure that is similar to the cross sections and surfaces shown in Figure 3c,d. The presence of many pores in porous carbon fibers is equivalent to the occurrence of dihedral angles for microwaves, indicating that there is an appropriate space inside each carbon fiber that allows for multiple reflections of microwaves at dihedral angles, which increases the propagation paths of the microwaves in the absorbent,24 as shown in Figure 7 b. Therefore, in addition to the absorption of the carbon material and the profound effect due to defects, the multiple reflections of microwaves could also lead to other additional losses of electromagnetic energy based on the interference of varied reflected waves. Yusoff et al.25 once proposed that the electromagnetic waves can be absorbed via a so-called “geometrical effect,” which means that when the thickness of the object (d) satisfies the equation d = nλm/4 (n = 1, 2, 3, ...), the incident and reflection waves in the object are out of phase by 180°. As a result, the reflected waves in the air−object interface are completely canceled out. Therefore, when the porous carbon fibers are uniformly distributed in the composites as the absorbent, there are more chances to satisfy this equation. Therefore, the presence of two minima in the spectra of the reflection loss versus frequency (Figure 6 b) may be ascribed to the combination of absorption and interference of the microwaves.

Figure 6. Absorption properties of composites filled with carbon nanofibers with a thickness of 3 mm (a) and with porous carbon fibers with a thickness of 2.3 mm (b).

minima in the plots of reflection loss versus frequency are equivalent to the peak absorption of microwave power, where the condition is satisfied for a particular matching thickness (dm = 3 mm) and a matching frequency ( f m). It is clear that the reflection loss becomes lower with increasing CNF content in the composites. The composite with a thickness of 3 mm containing 6 wt % CNFs shows the minimum reflection loss of −12.2 dB at 10.7 GHz and a frequency band with a reflection loss below −5 dB ranges from 9.6 to 12.2 GHz (2.6 GHz). When the absorbing filler is replaced by the porous carbon fibers, the absorption properties of the composites (2.3 mm thickness) are shown in Figure 6b. A superior microwave absorption performance of the porous carbon fibers is clearly observed. Not only is the reflection loss lower for the porous carbon fibers but also the absorption frequency range where the reflection loss is lower than −5 dB becomes wider than it is in the composite containing CNFs as the filler. The lowest reflection loss of −32 dB is found at 9.7 GHz for the composite containing porous carbon fibers at 6 wt % and with a thickness of 2.3 mm, which shows a decrease of 19.8 dB compared with 9200

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (grant no. 50873023) and by the Innovation Fund of Shanghai Education Commission (no. 11zz62). We would like to thank Professor Junqing Hu, College of Material Science and Engineering, Donghua University, for helpful editing of the text.

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Figure 7. SEM micrograph of the ground porous carbon fibers (a). A possible electromagnetic wave absorption mechanism based on dihedral angles in porous carbon fibers (b).

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CONCLUSIONS Porous carbon fibers and CNFs were successfully prepared through carbonization of PAN/PMMA blend fibers with mass ratio of 70/30 and 30/70, respectively. The prepared porous carbon fibers and CNFs were used as microwave absorbents and added in epoxy to fabricate composites. The complex permittivity of the fabricated composites containing 2, 4, and 6 wt % of the absorbents was measured, and the microwave absorption properties were calculated using the complex permittivity. It was found that the microwave absorption properties of the composites were greatly improved by increasing the content of absorbents; moreover, the porous carbon fibers showed superior microwave absorption performances relative to that of the CNFs. With the addition of 6 wt % porous carbon fibers, the composite (thickness of 2.3 mm) exhibited the lowest reflection loss of −31 dB at 9.7 GHz, and the bandwidth corresponding to reflection loss below −5 dB covered the whole X band (4.2 GHz). In contrast, the composite with 6 wt % CNFs showed the lowest refection loss of −12.2 dB at 10.7 GHz, with the bandwidth corresponding refection loss below −5 dB of 2.6 GHz. It is believed that the enhanced microwave absorption from the porous carbon fibers is due to a combination of the dielectric-type absorption and the interference of multireflected microwaves.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-21-67792830. Fax: +86-2167792855. E-mail: lig@ dhu.edu.cn. Notes

The authors declare no competing financial interest. 9201

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