Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 11927−11938
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Fibrous Composites with Double-Continuous Conductive Network for Strong Low-Frequency Microwave Absorption Li Huang,†,‡ Jianjun Li,*,§ Yibin Li,§ Xiaodong He,§ and Ye Yuan*,†,‡,§ †
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School of Materials Science and Technology, Tianjin Key Laboratory of Materials Laminating Fabrication and Interface Control Technology, and ‡School of Electronic and Information Engineering, Hebei University of Technology, Tianjin, 300401, People’s Republic of China § National Key Laboratory of Science and Technology on Advanced Composites in Special Environments, Harbin Institute of Technology, Harbin, 150080, People’s Republic of China ABSTRACT: Low-frequency electromagnetic absorption is an important issue that has troubled microwave absorbing materials for years. To solve these challenges, fibrous composites with a double-continuous conductive network sustainably derived from natural raw cotton fabrics, Fe3O4 nanoparticles (NPs), and polypyrrole (PPy) films were fabricated from bottom to top. The first level of magnetic fibrous structure is from the inherent raw cotton fabrics and Fe3O4 NPs, while the second level of the double-continuous conductive network is produced by the in situ polymerization of pyrrole monomers and FeCl3. The effect of fibrous pores and double-continuous conductive network endows the fibrous composites with excellent microwave absorption performance. The fibrous composites not only exhibit amazing reflection losses of −67.6 dB at 4.0 GHz and −54 dB at 3.5 GHz respectively, but also exhibit as being lightweight. The developed materials are promising for applications in high-performance stealth aircraft and electromagnetic pollution.
1. INTRODUCTION Microwave absorbing materials have been a hot research topic for years, not only for their ability to make aircraft invisible to advanced radar in military defense, but also for the rapid growth of electromagnetic (EM) pollution.1−5 Actually, much effort on microwave absorbing materials has been devoted for years. However, most of the research concerns high-frequency absorptions such as in the X-band; the low-frequency region (usually 99.7%, GC), and FeCl3 (>99.99%) were provided by Tianjin Chemical Co. All chemicals were of analytical grade and were used as received. All water used was deionized (DI) water. 2.2. Preparation of Carbon Hollow Fibers (CHF). First, 2 g of raw cotton fabrics was rinsed with hot water and cold water several times to rule out possible impurities and finally dried in a vacuum at 60 °C. After that, the cotton fabrics were heated to 300 °C slowly and maintained at this temperature for 0.5 h, and then pyrolyzed at 600 or 800 °C. Such heat-treated cotton fabrics were left to cool to room temperature. The whole pyrolysis process was under the protection of Ar atmosphere. Generally, the yield of this pyrolysis process was 69.2−71.4%. 2.3. Preparation of Fe3O4 Precursor Solution. A 0.01 mol sample of Fe(NO3)3·9H2O, 0.005 mol of FeSO4·7H2O, and 0.03 mol of citric acid were mixed in a reaction vessel containing 30 mL of distilled water under vigorous stirring. The resulting solution was evaporated to form a Fe3O4 precursor solution, which could be redissolved in ethanol to modify concentrations. 2.4. Preparation of CHF@Fe3O4@PPy Fibers. Carbon fibers were immersed into the Fe3O4 precursor. Concentration of the Fe3O4 precursor was controlled as 0.075 and 0.015 g/mL, respectively. The mixture was dried until the solvent was completely volatilized under ambient conditions. The residual mixture was sticky, and covered on the carbon fibers tightly. The mixture was heated at 600 °C under the protection of argon atmosphere, and finally CHF@Fe3O4 (or
[email protected]) fibers were obtained. Then, CHF@Fe3O4 fibers were directly mixed with pyrrole monomers. After 10 min, it was taken out and impregnated into a FeCl3 solution (0.2 M) for 20 min at room temperature. The obtained CHF@Fe3O4@PPy was taken out and rinsed with water followed by drying in air.
3. RESULTS AND DISCUSSION 3.1. Preparation of the CHF@Fe3O4@PPy Fibers. The preparation process of CHF@Fe3O4@PPy fibers is illustrated in Figure 1. Raw cotton fabrics were directly pyrolyzed in an argon atmosphere, while the color turned from white to black, along with a little volume shrinkage. The carbon hollow fibers were heated again to obtain magnetic carbon fibers after loading Fe3O4 precursor. The produced CHF@Fe3O4 fibers were chosen as the template and framework for the deposition of PPy through an in situ polymerization reaction. In this way, the CHF@Fe3O4@PPy composites with a double-continuous conductive network were fabricated. In the double-continuous conductive network, Fe3O4 nanoparticles (NPs) were anchored between carbon hollow fibers and PPy films. 3.2. Characterization of CHF@Fe3O4@PPy. SEM characterization in Figure 2 reveals the morphology of CHF fibers, CHF@Fe3O4 fibers, and CHF@Fe3O4@PPy fibers. In Figure 2a−c, the randomly distributed CHF fibers with different lengths are visible. Some of these fibers show spiral-shaped structure and rough surface. The diameters of these fibers range from 10 to 20 μm. They reveal a typical hollow structure after a closer observation on the cross section of a single fiber. This hollow structure inherits the natural structure of raw cotton fibers. It can be clearly seen in Figure 2d−f that a mass of Fe3O4 NPs arose on the surface of CHF fibers. From improved resolution of Figure 3a, the Fe3O4 NPs present a smooth surface with average diameters of ∼500 nm. The SEM images of the CHF@Fe3O4@ PPy fibers are shown in Figure 2g−i. It is clearly observed that Fe3O4 NPs are anchored on the surface of the carbon fibers. High-magnification SEM images of CHF@Fe3O4@PPy in Figure 3a,b reveal that the as-prepared Fe3O4 NPs exhibit a relative rough surface, which proves the formation of Fe3O4/PPy 11928
DOI: 10.1021/acs.iecr.9b01277 Ind. Eng. Chem. Res. 2019, 58, 11927−11938
Article
Industrial & Engineering Chemistry Research
Figure 3. (a) Morphologies of Fe3O4 NPs anchored on the surface of CHF fibers. (b) Morphologies of Fe3O4 NPs anchoring on the surface of CHF fibers after covering PPy skins. (c) Side view of the cross section of CHF@Fe3O4@PPy fibers. (d) Illustrations of hollow structure transforming to double-continuous conductive network.
speculated that the double-continuous conductive network is of great potential for designing low-frequency microwave absorbing materials. Besides, energy dispersived X-ray spectroscopic (EDS) mappings in Figure 2j−l reveal the element distribution (C, N, and Fe) of CHF@Fe3O4@PPy. Fourier transform infrared (FTIR) spectra in Figure 4a reveal characteristic peaks of raw cotton fabrics, CHF600 (pyrolyzed at 600 °C), CHF800, CHF800@PPy, and CHF800@Fe3O4@PPy. The spectrum of raw cotton shows characteristic peaks at 2917 (2845), 1740, 1367, and 1220 cm−1 attributable to −CH2−, CO, C−CH3, and C−O−C stretching, respectively. After pyrolysis at 600 and 800 °C, these peaks gradually vanished leaving the CC stretching peak (1639 cm−1), representing the removal of oxygen-containing groups during pyrolysis. Some new peaks (1544.4 cm−1) were found in the spectrum of CHF@ Fe3O4@PPy fibers due to the pyrrole ring vibration and C−N stretching vibration (1290.9 cm−1). The peaks at 1456.1, 1385.3, 896.6, and 783 cm−1 correspond to the =C−H in-plane vibration and the =C−H out-of-plane vibration of the pyrrole ring. Raman spectra of CHF600 and CHF800 fibers in Figure 4b show typical signal features of carbon materials. The peaks around 1320 and 1590 cm−1 are attributed to the D- and G-bands. The D/G intensity ratios of CHF600 and CHF800 were calculated as 0.93 and 0.75. The lower D/G intensity ratio of CHF800 shows the high degree of graphitization of CHF800. X-ray photoelectron spectroscopic (XPS) analysis in Figure 4c,d reveals changes in the nitrogen N 1s band for CHF600 and CHF800. The content of nitrogen was estimated to be 3.02% when carbonized at 600 °C, while this value decreased to 0.06% when carbonized at 800 °C. At the same time, the oxygen content decreased from 19.27 to 12.90%. This indicates the removal process of organics when increasing the pyrolysis temperature. The C 1s spectrum ranging from 282.5 to 289.5 eV is shown in Figure 4d. The C 1s peak can be divided into three obvious peaks centering at ca. 284.58, 285.48, and 286.4 eV after being carefully fitted. Among them, the binding energy at ca. 284.5 eV can be attributed to sp2-C bonds and that at ca. 285.48 eV corresponds to sp3-C bonds, whereas the ca. 286.4 eV binding energy is assignable to C−O bonds.58 In Figure 4e, powder X-ray diffraction (XRD) patterns in the 10−90° region of CHF600@Fe3O4@PPy and CHF800@ Fe3O4@PPy suggest that Fe3O4 NPs are the cubic inverse spinel structure. Carbon diffraction peaks are not observed because of the low crystalline nature of the sample. To
Figure 1. Schematic illustration of fabrication process of CHF@ Fe3O4@PPy. (a) Detailed structure evolution from CHF fibers to CHF@Fe3O4@PPy fibers. (b) Photos of fabrication of CHF@Fe3O4@ PPy fibers. (c) Corresponding SEM images from CHF fibers to CHF@ Fe3O4@PPy fibers.
Figure 2. (a−c) SEM images of CHF fibers, (d−f) SEM images of CHF@Fe3O4 fibers, (g−i) SEM images of CHF@Fe3O4@PPy fibers, and (j−l) EDS mappings of C, N, and Fe elements in CHF@Fe3O4@ PPy fibers, respectively.
interfaces. Also, as is shown in Figure 3c, the cutaway-view image proves a typical hollow double-continuous conductive network. A simplified hollow double-continuous conductive network is drawn in Figure 3d. This hollow double-continuous conductive network consists of C/Fe3O4 interfaces, Fe3O4/PPy interfaces, and C/PPy interfaces. It is known that conductive polymers are regarded as ideal microwave absorbing materials due to their light weight and effective EM absorbing ability.44 Thus, it can be 11929
DOI: 10.1021/acs.iecr.9b01277 Ind. Eng. Chem. Res. 2019, 58, 11927−11938
Article
Industrial & Engineering Chemistry Research
Figure 4. (a) FTIR spectra of raw cottons, CHF600, CHF800, and CHF800@PPy. (b) Raman spectra of CHF600 and CHF800. (c) XPS spectra of CHF600 and CHF800. (d) High-resolution XPS spectra and fitting curves of C 1s peak of CHF600 and CHF800. (e) XRD patterns of CHF600@ Fe3O4@PPy and CHF800@Fe3O4@PPy. (f) Magnetic properties of
[email protected]@PPy and CHF800@Fe3O4@PPy. Inset is the corresponding local enlargement.
investigate the magnetic properties of CHF800@Fe3O4@PPy with different Fe3O4 contents, magnetic hysteresis loops were recorded at 300 K using a Model 7404 vibrating sample magnetometer in Figure 4f. The M−H curves display typical Stype magnetic hysteresis loops, indicating their ferromagnetic behavior. The magnetization is saturated under an external magnetic field of 6000 Oe. As the initial Fe3O4 precursor concentration increases from 0.015 to 0.075 g/mL (0.2Fe3O4 to Fe3O4), saturation magnetization (Ms) approximately exhibits an increase from 18.87 to 34.43 emu·g−1. 3.3. EM Parameters and EM Wave-Absorption Properties. The frequency dependence of the real parts, imaginary parts, and loss tangents of CHF600, CHF800, CHF600@Fe3O4, and CHF800@Fe3O4 in 2−18 GHz are shown in Figure 5. It is known that permittivity and permeability stem from polarization and magnetic properties, which may be influenced by the composition and structural configuration of microwave
absorbing materials. As observed in Figure 5a, the real part (ε′) of all samples shows a typical frequency dispersion behavior. This dispersion behavior can be attributed to the increased lagging of polarization with respect to electric-field change at high frequency.33 Among these samples, ε′ values of CHF800 and CHF800@Fe3O4 are much larger than those of CHF600 and CHF600@Fe3O4. ε′ of CHF600@Fe3O4 keeps a steady low value in the whole frequency. Also, a similar variation trend of ε″ can be observed in Figure 5b. Differently, ε″ values of both CHF600 and CHF600@Fe3O4 are as low as zero. The expression of ε″ is as follows:25 ε″ =
εs − ε∞ 1 + ω 2τ 2
ωτ +
σ ωε0
(1)
Here, σ is electrical conductivity, ω is angular frequency, and τ is relaxation time. εs and ε∞ are the static permittivity and relative permittivity at the high-frequency limit, respectively. 11930
DOI: 10.1021/acs.iecr.9b01277 Ind. Eng. Chem. Res. 2019, 58, 11927−11938
Article
Industrial & Engineering Chemistry Research
Figure 5. (a) Real parts (ε′) and (b) imaginary parts (ε″) of complex permittivity of CHF600, CHF800, CHF600@Fe3O4, and CHF800@Fe3O4. (d) Real parts (μ′) and (e) imaginary parts (μ″) of complex permeability of corresponding samples. (c) Dielectric loss tangent and (f) magnetic loss tangent of corresponding samples.
It can be observed that the μ′ and μ″ values of CHF600 remain constant with slight fluctuations; other samples exhibit intensive fluctuations at 2−18 GHz. For example, the μ′ value of CHF800@Fe3O4 starts from 1.18 at 2 GHz and gradually drops to 0.8 at 9.8 GHz. Then, a notable rising trend of the μ′ value to 1.45 is observed from 9.8 to 13.3 GHz, and it finally fluctuates between 1.45 and 1.1 until 18 GHz. In regard to μ″ in Figure 5e, some similar peaks can be observed on μ″−frequency curves, which may be attributed to the magnetic loss mechanisms to multiple resonances and the eddy current effect at nanoscale. The four μ″−frequency curves vary steadily with the frequency until a sharp increase at 10 GHz. It should be noted that the eddy current effect is expressed by the equation37
The first term in eq 1 represents the contribution from polarization relaxation, while the second term is related to the conductive loss. Conductive loss is determined by the conductive current. Previous characterizations prove that CHF600 fibers possess more defects as well as lower graphitization degree compared with CHF800 fibers. This means the conductive current in CHF800 and CHF800@Fe3O4 fibers contribute a greater portion to conductive loss than CHF600 and CHF600@ Fe3O4 fibers. In general, electrical conductivity is related to the complex permittivity to some extent. When electrical conductivity is promoted (such as higher pyrolysis temperature or covering conductive PPy films), the ε″ values are also intensively unstable throughout the whole frequency range (Figure 5a,b). In addition to the conductive current, polarization relaxation also makes a large contribution to the ε″ values (first term in eq 1). The defects, oxygen-containing functional groups, and anchored Fe3O4 NPs in the double-continuous conductive network are considered as polarization centers that generate dipole polarization relaxations.59−62 Dielectric loss tangent values of CHF600, CHF800, CHF600@Fe3O4, and CHF800@Fe3O4 are shown in Figure 5c. Generally, dielectric loss is attributed to relaxation loss or conduction loss. Polarization loss is classified into electronic polarization, ionic polarization, and dipole orientation polarization, while conductivity loss comes from intrinsic and hopping conduction. Low tan δε values (
