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The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption
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The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption Mingxu Suia,* , Xuliang Lüa,* , Aming Xieb,* , Weidong Xua , Xianhui Ronga , Guojing Wua a Key Laboratory of Science and Technology on Electromagnetic Environmental Effects and Electro-optical Engineering, PLA University of Science and Technology, Nanjing 210007, PR China b State Key Laboratory for Disaster Prevention & Mitigation of Explosion & Impact, PLA University of Science and Technology, Nanjing 210007, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 March 2015 Received in revised form 5 August 2015 Accepted 24 September 2015 Available online xxx
Novel 3D CIP@PPy aerogel composites were synthesized through a self-assembled polymerization method using dopamine as intermediary. The 3D CIP@PPy aerogel composites were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Fourier transform infrared spectrometer (FTIR). The absorption properties were investigated by a vector network analyzer (VNA) in the frequency range of 2–18 GHz. For the CIP@PPy 33 wt% aerogel, the RLs show two strong absorption peaks values of 38.9 and 39.5 dB at 12.2 GHz and 14.2 GHz under the thickness of 2.2 mm, respectively, and the bandwidth can reach 6.1 GHz. The CIP@PPy aerogel composites would be a promising candidate as microwave absorption materials (MAMs) with strong absorption, wide frequency band and lightweight. The improvement of microwave absorbing properties can be attributed to interfacial polarization and geometry effect. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Aerogel Composites Microwave absorption Self-assembled polymerization Dielectric loss
1. Introduction In the last decade, various microwave absorption materials (MAMs) have been widely investigated for electromagnetic interference (EMI) to protect human health and electronic equipments from electromagnetic (EM) pollution which is caused by the wide applications of high-power electronic devices and communication technology [1–4]. “Strong absorption, wide frequency band and low density” is a pursuing goal in the design of MAMs, and most current researches of EM absorption are focused on the range from 2 to 18 GHz [5–7]. Carbonyl iron powder (CIP), a kind of traditional MAM, has been extensively used in the field of EM absorption for the high saturated magnetization and magnetic loss properties. However, CIP has a high density which seriously limited its application [8]. Fortunately, a lot of researches show that inorganic-organic composites have lower density but preferable EM properties due to the synergetic or complementary behavior between organic and inorganic nanoparticles [7,9,10], such as a-MnO2/PVDF [7], BaFe12O19-PANI [11], TBT-PANI [12], Barium Ferrite-PEDOT [13],
* Corresponding authors. E-mail addresses: [email protected] (M. Sui), [email protected] (X. Lü), [email protected] (A. Xie).
Ni–Zn ferrite/polyethylene glycol [14] and MnFe2O4/Fe3O4/PTh [15]. Polypyrrole (PPy), one of the most extensively studied polymers, has attracted great interest in the construction of microwave absorber for its light weight, high electrical conductivity and favorable physicochemical properties [16,17]. For instance, Gao et al. synthesized MWCNT/PPy composite materials which exhibit both good EM wave absorption and infrared performance, and the microwave return losses can reach 19 dB [18]. Shen et al. [19] fabricated double core–shell (Z-BCF/SiO2) @PPy composites by in-situ chemical synthesis method and found that the microwave absorbing property is enhanced in the range of 2–18 GHz which can be ascribed to the PPy coating. The minimum reflection loss (RL) value of the (Z-BCF/SiO2)@PPy composite with 66.67 wt% PPy under the thickness of 2 mm is 19.65 dB. Liu et al. [20] synthesized GN/PPy/Fe3O4 composites by a co-precipitation method and the microwave absorbing performance is better than that of GN, GN/PPy and GN/Fe3O4. Various magnetic crystals have been introduced into PPy to obtain the composites with both electrical and magnetic properties such as Fe3O4 [21], ZnFe2O4 [4], Co [6], Ni [22], ZnLa0.02Fe1.98O4 [23], Zn0.6Cu0.4Cr0.5La0.04Fe1.46O4 [24] and Zn0.6Cu0.4Cr0.5Fe1.46Sm0.04O4 [25]. Furthermore, almost all PPymatrix composites possess preferable performance than the magnetic materials or PPy alone [26].
http://dx.doi.org/10.1016/j.synthmet.2015.09.025 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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Conducting polymer hydrogels (CPHs), combinations of conducting polymer and hydrogels, have stimulated considerable attention from research communities for the virtue of unique mechanical, electrical and optical properties, thus exhibit huge potential for application of energy storage devices [27], photovoltaic devices [28] and supercapacitors [29]. After drying out the solvents, hydrogels turn into light-weight and conductive 3Dreticulate aerogel materials, which may be useful composites, such as CPs/carbon nanotube [30] and polyaniline/graphene oxide sheets [31]. We herein fabricated novel three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites by a self-assembled polymerization in the H2O/ ethanol (1:1) solution using FeCl3 as oxidant. As it is well known, nano-structures and micromorphology of composite materials play a fundamental role in the microwave absorption performance [32,33]. Flaky carbonyl iron powder was chosen to enhance multiple reflections of microwave within the composites [34]. Dopamine was used as intermediary to connect CIPs with PPy. The aerogel composites were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Fourier transform infrared (FTIR). The absorption properties were investigated by a vector network analyzer in the frequency range of 2– 18 GHz. The density of CIP@PPy 50 wt% aerogel composite is only 89 mg/cm3 which is much lower than the CIPs, 671 mg/cm3. The result reveals that the composites prepared herein exhibit excellent microwave absorbing properties with the advantages of strong absorption, wide frequency band and low density. The improvement of microwave absorbing properties can be attributed to interfacial polarization and geometry effect. 2. Experimental 2.1. Materials All chemicals were used as received without further purification. Pyrrole (Py), anhydrous FeCl3, sodium 4-dodecyl benzene sulfonate (NaDBS), Tris (hydroxymethyl) aminomethane (Tris–Cl) and aqueous ethanol solution were purchased from GENERAL-
REAGENT, Titan Scientific Co., Ltd., Shanghai, China. Dopamine hydrochloride was purchased from Energy Chemistry, Saen Chemical Technology Co., Ltd., Shanghai, China. Distilled water was obtained from Direct-Q3 UV, Millipore. Flaky carbonyl iron powder was purchased from Nanjing University. 2.2. Synthesis of polydopamine (PDA)-functioned flaky carbonyl iron powder (CIP@PDA) CIP@PDA can be typically prepared as follows: The flaky carbonyl iron powder and 200 mg dopamine hydrochloride have been homogenized with 0.03 mol NaDBS as surfactant in 200 mL of 10 mM Tris–Cl solution (pH 8.5), the above solution was mechanical rabbled for 24 h to obtain CIP@PDA. The color of the solution turned black after 24 h due to the oxidation polymerization of dopamine. After the completion of the reaction, the CIP@PDA was filtered with a 0.2 mm membrane filter, then washed and dried at 50 C for 6 h. The dopamine was capped on the surface of CIPs via the oxidization of quinone from catechol groups when rabbling the solution in the weak alkaline solution, then a secondary processing can be carried out [35–39]. Besides, the ultrafine CIPs display poor monodispersion property, capped by polydopamine makes it possible to homodisperse in the aqueous solution by stirring under ultrasonic oscillation [38,39]. 2.3. Synthesis of three-dimensional PDA-functioned CIP@Polypyrrole (3D CIP@PPy) aerogel composites 3D CIP@PPy can be typically prepared as follows: 0.005 mol Py monomer and CIP@PDA powders were homogenized in the H2O/ ethanol (1:1) solution by stirring under ultrasonic oscillation and anhydrous FeCl3 (2.3 equiv.) solution was added into the above solution quickly. Py monomer was oxidized rapidly and the solution solidified into black hydrogel in several minutes, releasing plenty of heat. Obtained hydrogel was aged for 24 h, filtered, then washed and dialyzed for several times with plenty of distilled water and ethanol to wipe out impurities, then dried at 50 C for 12 h to form an aerogel. To study the influence of concentration of flaky CIPs in the composite, we set weight ratios of CIPs to Py
Fig. 1. Schematic representation of the formation of CIP@PPy aerogels and the interaction of the microwave within the composite.
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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monomer at 1:4, 1:2 and 1:1, marked as CIP@PPy aerogel 20, 33 and 50 wt%, respectively. Furthermore, PPy aerogel without CIPs has been synthesized by the same method for comparison. 2.4. Characterization and measurement The morphology analysis was described by a field emission scanning electron microscope (FE-SEM, S4800, Hitachi) at 15 kV, from samples obtained by dropping the thick suspension on a silicon slide. The crystal structure of samples was characterized by X-ray diffractometer (XRD, X’Pert Pro, Philips). The XRD patterns with Cu Ka radiation (l = 1.5406 Å) at 40 kV and 40 mA were recorded in the range of 2u = 10–80 . The XRD specimens were prepared by flattening the powder on the small slides. FT-IR spectroscopy was studied on a Nicolet iS10 FTIR instrument (Thermo Fisher Scientific, USA). All of the tests were carried out at room temperature, no special conditions. 2.5. Electromagnetic absorption property characterization The relative complex permittivity (er) and permeability (mr) were measured by a vector network analyzer (VNA, N5242A PNAX, Agilent) in the 2–18 GHz range using coaxial measurement method. The measured samples were compounded with paraffin to fabricate a cylindrical shaped compact (fout = 7.00 mm, fin = 3.04 mm) of filler loading with 25 wt% by a sample hot press method at 100 C. In a coaxial wire analysis, er of the dielectric material has been calculated from the experimental scattering parameters S11 (or S22) and S21 (or S12) using the standard Nicolson–Ross–Weir (NRW) algorithm [40,41]. Due to the frequency range is from 2–18 GHz, the source-toshield distance be greater than the free-space wavelength, so the measurements are considered under far field [42]. According to the
3
transmission line theory [43], the input impedance (Zin) of a single layer of absorber backed by metal and the reflection loss of samples can be expressed as: rffiffiffiffiffiffi mr 2pf dpffiffiffiffiffiffiffiffiffiffi tanh j Z in ¼ Z 0 er mr ð1Þ c er
RLðdBÞ ¼ 20lgj
Z in Z 0 j Z in þ Z 0
ð2Þ
where f is the frequency, mr is the complex permeability, mr = m0 jm00 , er is the complex permittivity, er = e0 je00 , d is the thickness and c is the velocity of light in the free space. When the RL is lower than –10 dB, 90% of the electromagnetic energy is absorbed. 3. Results and discussions Fig. 1 gives the schematic representation of polymerization of CIP@PPy and the interaction of microwave with the nanoparticles of composite. In the process of oxidation polymerization of Py monomers to fabricate a 3D network structure, CIPs were embedded by the PPy chains for the interaction of Py monomers with the polydopamine. It is not hard to understand the interaction of microwave with the nanoparticles displayed in Fig. 1, a large attenuation is carried out for the scattering and multiple reflecting which is enhanced by the flaky shape of CIPs in the composite. Therefore, the geometry effect is a crucial factor to reinforce the microwave absorption in this contribution. The SEM is taken to characterize the morphologies distribution of PPy, flaky CIPs and CIP@PPy 50 wt% aerogel composite in Fig. 2. From Fig. 2a, the PPy aerogel particles can be seen with homogeneous spherical morphology and the diameter is about
Fig. 2. SEM images of (a) PPy aerogel, (b) Flaky CIP, (c, d) CIP@PPy aerogel 50 wt%.
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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Fig. 3. X-ray diffraction (XRD) pattern of CIP@PPy aerogel 50 wt%.
100 nm. It can be obviously found that the CIPs reveal flaky shape with an average diameter of 350 nm in Fig. 3b. In Fig. 2c–d, PPy is oxidative polymerized on the surface of flaky CIPs generating to a core–shell structure and grow along the margin of flaky CIPs to form a 3D network structure, in addition to fasten the CIPs. The XRD pattern of CIP@PPy 50 wt% aerogel composites is displayed in Fig. 3. The broad diffraction peak at angel around 23 observed in Fig. 3 is corresponding to the amorphous structure of PPy. The diffraction peaks located at 2u = 22.89 , 33.72 , 36.72 , 42.97, 53.26 , 58.92 , 68.82 , 70.71 are associated with peaks of Fe2O3, and correspond to (0 1 2), (1 0 4), (11 0), (11 3), (0 2 4), (11 6), (2 1 4) and (3 0 0) crystal face (JCPDS NO. 84-0306), which is caused by the oxidation of excessive FeCl3 due to the drying process.
Fig. 4 shows the FTIR spectra of CIP@PPy 50 wt% aerogel composites (a) and PPy aerogel (b). From Fig. 4b, the characteristic peak observed for the fundamental vibration of pyrrole ring appears at 1516.74 cm1. The peak at 1429.96 cm1 can be assigned to the C¼C in-plane vibration of the pyrrole ring. The characteristic band for the in-plane deformation of C H and N H bonds of pyrrole ring locate at 1083.80 cm1. The peaks around 1273.75 and 1127.71 cm1 are related to the stretching vibration of C N and CC bonds, respectively. And the CH out-of-plane vibration peaks appear at 1002.80 and 959.41 cm1 [44,45]. The characteristic peaks of PPy and CIP@PPy aerogel are almost same in bulk range. This may demonstrate that PPy is the primary ingredient in the composites. However, the peaks of CIPs are weak because the CIPs
Fig. 4. FTIR spectra of (a) CIP@PPy aerogel 50 wt%. (b) PPy aerogel.
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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Fig. 5. Complex permittivity (a), complex permeability (b) and loss tangent (c) of CIP@PPy aerogels in frequency range of 2–18 GHz.
is well enwrapped by PPy. In addition, the peaks at 2992.01 and 2882.57 cm1 are also found in Fig. 4a corresponding to the NaDBS. The frequency dependence of e0 (real parts of permittivity), e00 (imaginary parts of permittivity), m0 (real parts of permeability) and m00 (imaginary parts of permeability) of the CIP@PPy aerogel composites are shown in Fig. 5. There is a steady decrease in all the
three e0 curves of samples in the 2–18 GHz range. And the e00 values of CIP@PPy 20 wt% decrease gradually from 13.7 to 6.3 with several weak fluctuations. However, the e00 values of CIP@PPy 33 wt% and 50 wt% maintain at around 4 in the 2–18 GHz range. Specially, the e00 curve of CIP@PPy 33 wt% has two peaks with the value of 3.92 and 4.13 at 10.5 and 16 GHz, respectively. We can notice that the
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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increase of the content of CIPs arouse to decrease the values of e0 and e00 , which is better for impedance matching and absorption [46,47]. As depicted in Fig. 5b, the m0 of the CIP@PPy 33 wt% are slightly larger than the others except for the minimal spot at 13.5 GHz. The m00 values are fluctuating within 0–0.1 in the range of 2–10 GHz, and almost approaching to zero in the range of 10– 18 GHz. Particularly, the m00 values of CIP@PPy 33 wt% have one peak of 0.18 at 12.9 GHz and a sharp decline even tune to negative from 12.9 to 18 GHz. As mentioned above, it can be speculated that the main microwave loss mechanism is the dielectric loss. The negative values of m00 derived from the magnetic energy radiation in strong motion of charges. To better understand the mechanism of absorption, we have calculated the dissipation factors (loss tangent) based on the data of the EM parameters. The frequency dependence of the dielectric loss tangent (tan de = e00 /e0 ) and the magnetic loss tangent (tan dm = m00 /m0 ) are shown in Fig. 5c. As can be observed from Fig. 5c, the tan de of the CIP@PPy aerogels decrease with the increasing of CIPs content. In general, the tan de values of each samples are increase in the range of 2–18 GHz with a few fluctuations, for example, tan de values of CIP@PPy 33 wt% change from 0.44 to 0.70 with two peaks of 0.56 and 0.67 at 10.5 and 16.0 GHz, respectively. The tan dm values of the composites are all fluctuating nearby zero line, and the tan dm of CIP@PPy 33 wt% have a maximum value of 0.18 at 13 GHz. It is reasonable to confirm the main mechanism of microwave attenuation is the dielectric loss rather than magnetic loss. The dielectric loss may originate from dipole polarization, interfacial polarization [19,25] and eddy current effect [6]. Specially, the core–shell structures of the CIP@PPy composites make a great benefit to enhance the interfacial polarization and the synergistic or complementary effect between CIPs and PPy particles should not be neglected [48]. Hence, the improvement of microwave absorbing properties should be attributed to both interfacial polarization and geometry effect. The theoretical RLs of the samples were calculated using the MATLAB soft based on Eqs. (1) and (2) represented in Figs. 6 and 7. Fig. 6 shows that RLs of the PPy aerogel and CIP@PPy aerogels with different mass ratios under the thickness of 2.2 mm in 2–18 GHz frequency range. As Fig. 6 displays, the microwave absorption
properties of CIP@PPy aerogel composites with 33 and 50 wt% CIPs are much better than PPy aerogel. The RLs curve of CIP@PPy 33 wt% appears two deep peaks of 38.9 and 39.5 dB at 12.2 and 14.2 GHz corresponding to the two peaks of tan de and the bandwidth below 10 dB is 6.1 GHz from 10 to 16.1 GHz. The RLs curve of CIP@PPy 50 wt% has one peak of 37.2 dB at 13.7 GHz, and the bandwidth below 10 dB is 4.8 GHz from 11.4 GHz to 16.2 GHz. The 3D presentations of theoretical RLs of the CIP@PPy aerogels are revealed in Fig. 7. The RLmin of the specimens all shift toward lower frequency bands with the increasing of thickness, which can be explained by the formula f = c/2pdm00 [49]. Therefore, the location of RLmin can be adjusted to the application in different frequency by manipulating the thickness of the composites. It is found that the microwave absorbing properties of the composites becomes better with the increase of CIPs. The strongest absorbing peak value of CIP@PPy aerogel 33 wt% is 41.6 dB located at (2.68 mm, 9.68 GHz). And it is noted worthy that bandwidth below 10 dB of the CIP@PPy aerogel 33 wt% is quite broad as the yellow region presented in Fig. 7b. The RLmin of CIP@PPy aerogel 50 wt% is 69.7 dB located at (3.72 mm, 7.24 GHz) which is marked in Fig. 7c. Besides, the density of CIP@PPy 50 wt% aerogel composite is only 89 mg/cm3 which is much lower than the CIPs, 671 mg/cm3. We believe that the CIP@PPy aerogel composites obtained herein are excellent candidates for strong absorption, wide frequency band and lightweight MAMs. In generally, the dissipation factors (loss tangent) are bound up with the absorption properties of MAMs. However, it is a surprise to notice that the tan de values of the CIP@PPy aerogel 20 wt% are higher than the rest, the microwave absorption performance of CIP@PPy aerogel 33 and 50 wt% both are preferable. Consider the pure dielectrics with m = 1 j0, e0 = 20, we set e00 increase homogeneously from 0.02 to 20, thus the tan de range homogeneously from 0.001 to 1. As for pure magnetic loss materials, we set e = 10 j0 and m0 = 30, m00 increase homogeneously from 0.03 to 30, thus tan me also range homogeneously from 0.001 to 1. The 3D RLs curve surfaces are depicted in Fig. 8 calculated by the data above. In Fig. 8a, there exists one peak in the area around tan de = 0.35. Dramatically, there are four concentric ellipses occurred in Fig. 8b. As a result, the RLs of the MAMs emerge oscillation
Fig. 6. Reflection loss of PPy aerogel and CIP@PPy aerogels with a sample thickness of 2.2 mm in the 2–18 GHz range.
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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Fig. 7. Three-dimensional representations of reflection loss and corresponding contour map of the CIP@PPy aerogels with different CIP weight ratios of (a) 20 wt%, (b) 33 wt%, (c) 50 wt%.
alteration with the homogeneously increasing of the loss tangent, thus an optimized value of dissipation factors can be expected to maximize the absorption, neither too large nor too small, which is satisfied with the conclusions reported by Qin and Brosseau [49]. We think that the more essential reason is the input impedance Zin possess a complex relationship with the EM parameters which leading to the oscillation variation of RLs.
4. Conclusions In summary, 3D CIP@PPy aerogel composites with different mass ratios of CIPs were synthesized by self-assembled polymerization in the H2O/ethanol (1:1) solution using FeCl3 solution as oxidant. Flaky CIPs were embedded in and wrapped by the PPy when fabricating the 3D network structure. Dopamine was used
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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Fig. 8. 3D representations of reflection loss for different values of loss tangent in the frequency range of 2–18 GHz.
as intermediary to connect CIPs with PPy. The aerogel composites were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Fourier transform infrared (FTIR). The absorption properties were investigated by a vector network analyzer in the frequency range of 2–18 GHz. The results reveal that the as-prepared composites exhibit excellent microwave absorbing properties. For CIP@PPy 33 wt% aerogel, the RLs show two strong absorption peaks of 38.9 and 39.5 dB at 12.2 GHz and 14.2 GHz, respectively, under the thickness of 2.2 mm, and the bandwidth (<10 dB) is 6.1 GHz from 10 to 16.1 GHz. The RLmin of CIP@PPy 50 wt% aerogel can reach 69.7 dB at 7.24 GHz when the sample thickness is 3.72 mm. The density of CIP@PPy 50 wt% aerogel composite is only 89 mg/cm3 which is much lower than the CIPs. Therefore it is reasonable to hold that the CIP@PPy aerogel composites can be regarded as a promising candidate as MAMs with the advantages of strong absorption, wide frequency band and lightweight. The improvement of microwave absorbing properties is considered to be attributed to interfacial polarization and geometry effect. Acknowledgments This project was financially supported by the Opening Foundation of Key Laboratory of Science and Technology on Electromagnetic Environmental Effects and Electro-optical Engineering (FD2015011) and graduate education funds of PLA University of Science and Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2015.09.025. References [1] Z. Chen, C. Xu, C. Ma, W. Ren, H. Cheng, Lightweight and flexible graphene foam composites for high-performance electromagnetic interference shielding, Adv. Mater. 25 (2013) 1296. [2] S. Varshney, A. Ohlan, V.K. Jain, V.P. Dutta, S.K. Dhawan, Synthesis of ferrofluid based nanoarchitectured polypyrrole composites and its application for electromagnetic shielding, Mater. Chem. Phys. 143 (2014) 806. [3] B.R. Kim, H.K. Lee, S.H. Park, H.K. Kim, Electromagnetic interference shielding characteristics and shielding effectiveness of polyaniline-coated films, Thin Solid Films 519 (2011) 3492–3496.
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The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption Mingxu Suia,* , Xuliang Lüa,* , Aming Xieb,* , Weidong Xua , Xianhui Ronga , Guojing Wua a Key Laboratory of Science and Technology on Electromagnetic Environmental Effects and Electro-optical Engineering, PLA University of Science and Technology, Nanjing 210007, PR China b State Key Laboratory for Disaster Prevention & Mitigation of Explosion & Impact, PLA University of Science and Technology, Nanjing 210007, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 12 March 2015 Received in revised form 5 August 2015 Accepted 24 September 2015 Available online xxx
Novel 3D CIP@PPy aerogel composites were synthesized through a self-assembled polymerization method using dopamine as intermediary. The 3D CIP@PPy aerogel composites were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Fourier transform infrared spectrometer (FTIR). The absorption properties were investigated by a vector network analyzer (VNA) in the frequency range of 2–18 GHz. For the CIP@PPy 33 wt% aerogel, the RLs show two strong absorption peaks values of 38.9 and 39.5 dB at 12.2 GHz and 14.2 GHz under the thickness of 2.2 mm, respectively, and the bandwidth can reach 6.1 GHz. The CIP@PPy aerogel composites would be a promising candidate as microwave absorption materials (MAMs) with strong absorption, wide frequency band and lightweight. The improvement of microwave absorbing properties can be attributed to interfacial polarization and geometry effect. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Aerogel Composites Microwave absorption Self-assembled polymerization Dielectric loss
1. Introduction In the last decade, various microwave absorption materials (MAMs) have been widely investigated for electromagnetic interference (EMI) to protect human health and electronic equipments from electromagnetic (EM) pollution which is caused by the wide applications of high-power electronic devices and communication technology [1–4]. “Strong absorption, wide frequency band and low density” is a pursuing goal in the design of MAMs, and most current researches of EM absorption are focused on the range from 2 to 18 GHz [5–7]. Carbonyl iron powder (CIP), a kind of traditional MAM, has been extensively used in the field of EM absorption for the high saturated magnetization and magnetic loss properties. However, CIP has a high density which seriously limited its application [8]. Fortunately, a lot of researches show that inorganic-organic composites have lower density but preferable EM properties due to the synergetic or complementary behavior between organic and inorganic nanoparticles [7,9,10], such as a-MnO2/PVDF [7], BaFe12O19-PANI [11], TBT-PANI [12], Barium Ferrite-PEDOT [13],
* Corresponding authors. E-mail addresses: [email protected] (M. Sui), [email protected] (X. Lü), [email protected] (A. Xie).
Ni–Zn ferrite/polyethylene glycol [14] and MnFe2O4/Fe3O4/PTh [15]. Polypyrrole (PPy), one of the most extensively studied polymers, has attracted great interest in the construction of microwave absorber for its light weight, high electrical conductivity and favorable physicochemical properties [16,17]. For instance, Gao et al. synthesized MWCNT/PPy composite materials which exhibit both good EM wave absorption and infrared performance, and the microwave return losses can reach 19 dB [18]. Shen et al. [19] fabricated double core–shell (Z-BCF/SiO2) @PPy composites by in-situ chemical synthesis method and found that the microwave absorbing property is enhanced in the range of 2–18 GHz which can be ascribed to the PPy coating. The minimum reflection loss (RL) value of the (Z-BCF/SiO2)@PPy composite with 66.67 wt% PPy under the thickness of 2 mm is 19.65 dB. Liu et al. [20] synthesized GN/PPy/Fe3O4 composites by a co-precipitation method and the microwave absorbing performance is better than that of GN, GN/PPy and GN/Fe3O4. Various magnetic crystals have been introduced into PPy to obtain the composites with both electrical and magnetic properties such as Fe3O4 [21], ZnFe2O4 [4], Co [6], Ni [22], ZnLa0.02Fe1.98O4 [23], Zn0.6Cu0.4Cr0.5La0.04Fe1.46O4 [24] and Zn0.6Cu0.4Cr0.5Fe1.46Sm0.04O4 [25]. Furthermore, almost all PPymatrix composites possess preferable performance than the magnetic materials or PPy alone [26].
http://dx.doi.org/10.1016/j.synthmet.2015.09.025 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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Conducting polymer hydrogels (CPHs), combinations of conducting polymer and hydrogels, have stimulated considerable attention from research communities for the virtue of unique mechanical, electrical and optical properties, thus exhibit huge potential for application of energy storage devices [27], photovoltaic devices [28] and supercapacitors [29]. After drying out the solvents, hydrogels turn into light-weight and conductive 3Dreticulate aerogel materials, which may be useful composites, such as CPs/carbon nanotube [30] and polyaniline/graphene oxide sheets [31]. We herein fabricated novel three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites by a self-assembled polymerization in the H2O/ ethanol (1:1) solution using FeCl3 as oxidant. As it is well known, nano-structures and micromorphology of composite materials play a fundamental role in the microwave absorption performance [32,33]. Flaky carbonyl iron powder was chosen to enhance multiple reflections of microwave within the composites [34]. Dopamine was used as intermediary to connect CIPs with PPy. The aerogel composites were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Fourier transform infrared (FTIR). The absorption properties were investigated by a vector network analyzer in the frequency range of 2– 18 GHz. The density of CIP@PPy 50 wt% aerogel composite is only 89 mg/cm3 which is much lower than the CIPs, 671 mg/cm3. The result reveals that the composites prepared herein exhibit excellent microwave absorbing properties with the advantages of strong absorption, wide frequency band and low density. The improvement of microwave absorbing properties can be attributed to interfacial polarization and geometry effect. 2. Experimental 2.1. Materials All chemicals were used as received without further purification. Pyrrole (Py), anhydrous FeCl3, sodium 4-dodecyl benzene sulfonate (NaDBS), Tris (hydroxymethyl) aminomethane (Tris–Cl) and aqueous ethanol solution were purchased from GENERAL-
REAGENT, Titan Scientific Co., Ltd., Shanghai, China. Dopamine hydrochloride was purchased from Energy Chemistry, Saen Chemical Technology Co., Ltd., Shanghai, China. Distilled water was obtained from Direct-Q3 UV, Millipore. Flaky carbonyl iron powder was purchased from Nanjing University. 2.2. Synthesis of polydopamine (PDA)-functioned flaky carbonyl iron powder (CIP@PDA) CIP@PDA can be typically prepared as follows: The flaky carbonyl iron powder and 200 mg dopamine hydrochloride have been homogenized with 0.03 mol NaDBS as surfactant in 200 mL of 10 mM Tris–Cl solution (pH 8.5), the above solution was mechanical rabbled for 24 h to obtain CIP@PDA. The color of the solution turned black after 24 h due to the oxidation polymerization of dopamine. After the completion of the reaction, the CIP@PDA was filtered with a 0.2 mm membrane filter, then washed and dried at 50 C for 6 h. The dopamine was capped on the surface of CIPs via the oxidization of quinone from catechol groups when rabbling the solution in the weak alkaline solution, then a secondary processing can be carried out [35–39]. Besides, the ultrafine CIPs display poor monodispersion property, capped by polydopamine makes it possible to homodisperse in the aqueous solution by stirring under ultrasonic oscillation [38,39]. 2.3. Synthesis of three-dimensional PDA-functioned CIP@Polypyrrole (3D CIP@PPy) aerogel composites 3D CIP@PPy can be typically prepared as follows: 0.005 mol Py monomer and CIP@PDA powders were homogenized in the H2O/ ethanol (1:1) solution by stirring under ultrasonic oscillation and anhydrous FeCl3 (2.3 equiv.) solution was added into the above solution quickly. Py monomer was oxidized rapidly and the solution solidified into black hydrogel in several minutes, releasing plenty of heat. Obtained hydrogel was aged for 24 h, filtered, then washed and dialyzed for several times with plenty of distilled water and ethanol to wipe out impurities, then dried at 50 C for 12 h to form an aerogel. To study the influence of concentration of flaky CIPs in the composite, we set weight ratios of CIPs to Py
Fig. 1. Schematic representation of the formation of CIP@PPy aerogels and the interaction of the microwave within the composite.
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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monomer at 1:4, 1:2 and 1:1, marked as CIP@PPy aerogel 20, 33 and 50 wt%, respectively. Furthermore, PPy aerogel without CIPs has been synthesized by the same method for comparison. 2.4. Characterization and measurement The morphology analysis was described by a field emission scanning electron microscope (FE-SEM, S4800, Hitachi) at 15 kV, from samples obtained by dropping the thick suspension on a silicon slide. The crystal structure of samples was characterized by X-ray diffractometer (XRD, X’Pert Pro, Philips). The XRD patterns with Cu Ka radiation (l = 1.5406 Å) at 40 kV and 40 mA were recorded in the range of 2u = 10–80 . The XRD specimens were prepared by flattening the powder on the small slides. FT-IR spectroscopy was studied on a Nicolet iS10 FTIR instrument (Thermo Fisher Scientific, USA). All of the tests were carried out at room temperature, no special conditions. 2.5. Electromagnetic absorption property characterization The relative complex permittivity (er) and permeability (mr) were measured by a vector network analyzer (VNA, N5242A PNAX, Agilent) in the 2–18 GHz range using coaxial measurement method. The measured samples were compounded with paraffin to fabricate a cylindrical shaped compact (fout = 7.00 mm, fin = 3.04 mm) of filler loading with 25 wt% by a sample hot press method at 100 C. In a coaxial wire analysis, er of the dielectric material has been calculated from the experimental scattering parameters S11 (or S22) and S21 (or S12) using the standard Nicolson–Ross–Weir (NRW) algorithm [40,41]. Due to the frequency range is from 2–18 GHz, the source-toshield distance be greater than the free-space wavelength, so the measurements are considered under far field [42]. According to the
3
transmission line theory [43], the input impedance (Zin) of a single layer of absorber backed by metal and the reflection loss of samples can be expressed as: rffiffiffiffiffiffi mr 2pf dpffiffiffiffiffiffiffiffiffiffi tanh j Z in ¼ Z 0 er mr ð1Þ c er
RLðdBÞ ¼ 20lgj
Z in Z 0 j Z in þ Z 0
ð2Þ
where f is the frequency, mr is the complex permeability, mr = m0 jm00 , er is the complex permittivity, er = e0 je00 , d is the thickness and c is the velocity of light in the free space. When the RL is lower than –10 dB, 90% of the electromagnetic energy is absorbed. 3. Results and discussions Fig. 1 gives the schematic representation of polymerization of CIP@PPy and the interaction of microwave with the nanoparticles of composite. In the process of oxidation polymerization of Py monomers to fabricate a 3D network structure, CIPs were embedded by the PPy chains for the interaction of Py monomers with the polydopamine. It is not hard to understand the interaction of microwave with the nanoparticles displayed in Fig. 1, a large attenuation is carried out for the scattering and multiple reflecting which is enhanced by the flaky shape of CIPs in the composite. Therefore, the geometry effect is a crucial factor to reinforce the microwave absorption in this contribution. The SEM is taken to characterize the morphologies distribution of PPy, flaky CIPs and CIP@PPy 50 wt% aerogel composite in Fig. 2. From Fig. 2a, the PPy aerogel particles can be seen with homogeneous spherical morphology and the diameter is about
Fig. 2. SEM images of (a) PPy aerogel, (b) Flaky CIP, (c, d) CIP@PPy aerogel 50 wt%.
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Fig. 3. X-ray diffraction (XRD) pattern of CIP@PPy aerogel 50 wt%.
100 nm. It can be obviously found that the CIPs reveal flaky shape with an average diameter of 350 nm in Fig. 3b. In Fig. 2c–d, PPy is oxidative polymerized on the surface of flaky CIPs generating to a core–shell structure and grow along the margin of flaky CIPs to form a 3D network structure, in addition to fasten the CIPs. The XRD pattern of CIP@PPy 50 wt% aerogel composites is displayed in Fig. 3. The broad diffraction peak at angel around 23 observed in Fig. 3 is corresponding to the amorphous structure of PPy. The diffraction peaks located at 2u = 22.89 , 33.72 , 36.72 , 42.97, 53.26 , 58.92 , 68.82 , 70.71 are associated with peaks of Fe2O3, and correspond to (0 1 2), (1 0 4), (11 0), (11 3), (0 2 4), (11 6), (2 1 4) and (3 0 0) crystal face (JCPDS NO. 84-0306), which is caused by the oxidation of excessive FeCl3 due to the drying process.
Fig. 4 shows the FTIR spectra of CIP@PPy 50 wt% aerogel composites (a) and PPy aerogel (b). From Fig. 4b, the characteristic peak observed for the fundamental vibration of pyrrole ring appears at 1516.74 cm1. The peak at 1429.96 cm1 can be assigned to the C¼C in-plane vibration of the pyrrole ring. The characteristic band for the in-plane deformation of C H and N H bonds of pyrrole ring locate at 1083.80 cm1. The peaks around 1273.75 and 1127.71 cm1 are related to the stretching vibration of C N and CC bonds, respectively. And the CH out-of-plane vibration peaks appear at 1002.80 and 959.41 cm1 [44,45]. The characteristic peaks of PPy and CIP@PPy aerogel are almost same in bulk range. This may demonstrate that PPy is the primary ingredient in the composites. However, the peaks of CIPs are weak because the CIPs
Fig. 4. FTIR spectra of (a) CIP@PPy aerogel 50 wt%. (b) PPy aerogel.
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Fig. 5. Complex permittivity (a), complex permeability (b) and loss tangent (c) of CIP@PPy aerogels in frequency range of 2–18 GHz.
is well enwrapped by PPy. In addition, the peaks at 2992.01 and 2882.57 cm1 are also found in Fig. 4a corresponding to the NaDBS. The frequency dependence of e0 (real parts of permittivity), e00 (imaginary parts of permittivity), m0 (real parts of permeability) and m00 (imaginary parts of permeability) of the CIP@PPy aerogel composites are shown in Fig. 5. There is a steady decrease in all the
three e0 curves of samples in the 2–18 GHz range. And the e00 values of CIP@PPy 20 wt% decrease gradually from 13.7 to 6.3 with several weak fluctuations. However, the e00 values of CIP@PPy 33 wt% and 50 wt% maintain at around 4 in the 2–18 GHz range. Specially, the e00 curve of CIP@PPy 33 wt% has two peaks with the value of 3.92 and 4.13 at 10.5 and 16 GHz, respectively. We can notice that the
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increase of the content of CIPs arouse to decrease the values of e0 and e00 , which is better for impedance matching and absorption [46,47]. As depicted in Fig. 5b, the m0 of the CIP@PPy 33 wt% are slightly larger than the others except for the minimal spot at 13.5 GHz. The m00 values are fluctuating within 0–0.1 in the range of 2–10 GHz, and almost approaching to zero in the range of 10– 18 GHz. Particularly, the m00 values of CIP@PPy 33 wt% have one peak of 0.18 at 12.9 GHz and a sharp decline even tune to negative from 12.9 to 18 GHz. As mentioned above, it can be speculated that the main microwave loss mechanism is the dielectric loss. The negative values of m00 derived from the magnetic energy radiation in strong motion of charges. To better understand the mechanism of absorption, we have calculated the dissipation factors (loss tangent) based on the data of the EM parameters. The frequency dependence of the dielectric loss tangent (tan de = e00 /e0 ) and the magnetic loss tangent (tan dm = m00 /m0 ) are shown in Fig. 5c. As can be observed from Fig. 5c, the tan de of the CIP@PPy aerogels decrease with the increasing of CIPs content. In general, the tan de values of each samples are increase in the range of 2–18 GHz with a few fluctuations, for example, tan de values of CIP@PPy 33 wt% change from 0.44 to 0.70 with two peaks of 0.56 and 0.67 at 10.5 and 16.0 GHz, respectively. The tan dm values of the composites are all fluctuating nearby zero line, and the tan dm of CIP@PPy 33 wt% have a maximum value of 0.18 at 13 GHz. It is reasonable to confirm the main mechanism of microwave attenuation is the dielectric loss rather than magnetic loss. The dielectric loss may originate from dipole polarization, interfacial polarization [19,25] and eddy current effect [6]. Specially, the core–shell structures of the CIP@PPy composites make a great benefit to enhance the interfacial polarization and the synergistic or complementary effect between CIPs and PPy particles should not be neglected [48]. Hence, the improvement of microwave absorbing properties should be attributed to both interfacial polarization and geometry effect. The theoretical RLs of the samples were calculated using the MATLAB soft based on Eqs. (1) and (2) represented in Figs. 6 and 7. Fig. 6 shows that RLs of the PPy aerogel and CIP@PPy aerogels with different mass ratios under the thickness of 2.2 mm in 2–18 GHz frequency range. As Fig. 6 displays, the microwave absorption
properties of CIP@PPy aerogel composites with 33 and 50 wt% CIPs are much better than PPy aerogel. The RLs curve of CIP@PPy 33 wt% appears two deep peaks of 38.9 and 39.5 dB at 12.2 and 14.2 GHz corresponding to the two peaks of tan de and the bandwidth below 10 dB is 6.1 GHz from 10 to 16.1 GHz. The RLs curve of CIP@PPy 50 wt% has one peak of 37.2 dB at 13.7 GHz, and the bandwidth below 10 dB is 4.8 GHz from 11.4 GHz to 16.2 GHz. The 3D presentations of theoretical RLs of the CIP@PPy aerogels are revealed in Fig. 7. The RLmin of the specimens all shift toward lower frequency bands with the increasing of thickness, which can be explained by the formula f = c/2pdm00 [49]. Therefore, the location of RLmin can be adjusted to the application in different frequency by manipulating the thickness of the composites. It is found that the microwave absorbing properties of the composites becomes better with the increase of CIPs. The strongest absorbing peak value of CIP@PPy aerogel 33 wt% is 41.6 dB located at (2.68 mm, 9.68 GHz). And it is noted worthy that bandwidth below 10 dB of the CIP@PPy aerogel 33 wt% is quite broad as the yellow region presented in Fig. 7b. The RLmin of CIP@PPy aerogel 50 wt% is 69.7 dB located at (3.72 mm, 7.24 GHz) which is marked in Fig. 7c. Besides, the density of CIP@PPy 50 wt% aerogel composite is only 89 mg/cm3 which is much lower than the CIPs, 671 mg/cm3. We believe that the CIP@PPy aerogel composites obtained herein are excellent candidates for strong absorption, wide frequency band and lightweight MAMs. In generally, the dissipation factors (loss tangent) are bound up with the absorption properties of MAMs. However, it is a surprise to notice that the tan de values of the CIP@PPy aerogel 20 wt% are higher than the rest, the microwave absorption performance of CIP@PPy aerogel 33 and 50 wt% both are preferable. Consider the pure dielectrics with m = 1 j0, e0 = 20, we set e00 increase homogeneously from 0.02 to 20, thus the tan de range homogeneously from 0.001 to 1. As for pure magnetic loss materials, we set e = 10 j0 and m0 = 30, m00 increase homogeneously from 0.03 to 30, thus tan me also range homogeneously from 0.001 to 1. The 3D RLs curve surfaces are depicted in Fig. 8 calculated by the data above. In Fig. 8a, there exists one peak in the area around tan de = 0.35. Dramatically, there are four concentric ellipses occurred in Fig. 8b. As a result, the RLs of the MAMs emerge oscillation
Fig. 6. Reflection loss of PPy aerogel and CIP@PPy aerogels with a sample thickness of 2.2 mm in the 2–18 GHz range.
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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Fig. 7. Three-dimensional representations of reflection loss and corresponding contour map of the CIP@PPy aerogels with different CIP weight ratios of (a) 20 wt%, (b) 33 wt%, (c) 50 wt%.
alteration with the homogeneously increasing of the loss tangent, thus an optimized value of dissipation factors can be expected to maximize the absorption, neither too large nor too small, which is satisfied with the conclusions reported by Qin and Brosseau [49]. We think that the more essential reason is the input impedance Zin possess a complex relationship with the EM parameters which leading to the oscillation variation of RLs.
4. Conclusions In summary, 3D CIP@PPy aerogel composites with different mass ratios of CIPs were synthesized by self-assembled polymerization in the H2O/ethanol (1:1) solution using FeCl3 solution as oxidant. Flaky CIPs were embedded in and wrapped by the PPy when fabricating the 3D network structure. Dopamine was used
Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025
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Fig. 8. 3D representations of reflection loss for different values of loss tangent in the frequency range of 2–18 GHz.
as intermediary to connect CIPs with PPy. The aerogel composites were characterized by scanning electron microscope (SEM), X-ray diffractometer (XRD) and Fourier transform infrared (FTIR). The absorption properties were investigated by a vector network analyzer in the frequency range of 2–18 GHz. The results reveal that the as-prepared composites exhibit excellent microwave absorbing properties. For CIP@PPy 33 wt% aerogel, the RLs show two strong absorption peaks of 38.9 and 39.5 dB at 12.2 GHz and 14.2 GHz, respectively, under the thickness of 2.2 mm, and the bandwidth (<10 dB) is 6.1 GHz from 10 to 16.1 GHz. The RLmin of CIP@PPy 50 wt% aerogel can reach 69.7 dB at 7.24 GHz when the sample thickness is 3.72 mm. The density of CIP@PPy 50 wt% aerogel composite is only 89 mg/cm3 which is much lower than the CIPs. Therefore it is reasonable to hold that the CIP@PPy aerogel composites can be regarded as a promising candidate as MAMs with the advantages of strong absorption, wide frequency band and lightweight. The improvement of microwave absorbing properties is considered to be attributed to interfacial polarization and geometry effect. Acknowledgments This project was financially supported by the Opening Foundation of Key Laboratory of Science and Technology on Electromagnetic Environmental Effects and Electro-optical Engineering (FD2015011) and graduate education funds of PLA University of Science and Technology. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. synthmet.2015.09.025. References [1] Z. Chen, C. Xu, C. Ma, W. Ren, H. Cheng, Lightweight and flexible graphene foam composites for high-performance electromagnetic interference shielding, Adv. Mater. 25 (2013) 1296. [2] S. Varshney, A. Ohlan, V.K. Jain, V.P. Dutta, S.K. Dhawan, Synthesis of ferrofluid based nanoarchitectured polypyrrole composites and its application for electromagnetic shielding, Mater. Chem. Phys. 143 (2014) 806. [3] B.R. Kim, H.K. Lee, S.H. Park, H.K. Kim, Electromagnetic interference shielding characteristics and shielding effectiveness of polyaniline-coated films, Thin Solid Films 519 (2011) 3492–3496.
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Please cite this article in press as: M. Sui, et al., The synthesis of three-dimensional (3D) polydopamine-functioned carbonyl iron powder@polypyrrole (CIP@PPy) aerogel composites for excellent microwave absorption, Synthetic Met. (2015), http://dx.doi.org/10.1016/j. synthmet.2015.09.025