Carbon-Encapsulated Fe Nanoparticles Embedded in Organic

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Carbon-Encapsulated Fe Nanoparticles Embedded in Organic Polypyrrole Polymer as a High Performance Microwave-Absorber Linwen Jiang, Zhenhua Wang, Dianyu Geng, Yu Wang, Jing An, Jun He, Da Li, Wei Liu, and Zhidong Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09445 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 22, 2016

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The Journal of Physical Chemistry

Carbon-Encapsulated Fe Nanoparticles Embedded in Organic Polypyrrole Polymer as a High Performance Microwave-Absorber Linwen Jiang†, Zhenhua Wang†,*, DianyuGeng†, Yu Wang‡, Jing An‡, Jun He‡, Da Li†, Wei Liu†, and Zhidong Zhang† †

Shenyang National Laboratory for Materials Science, Institute of Metal Research and

International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China ‡

Division of Functional Material Research, Central Iron & Steel Research Institute,

Beijing 100081, People’s Republic of China

ABSTRACT In this study, high-effective organic-inorganic composites Fe/C/PPy (Fe/C nanocapsules embedded in polypyrrole) were prepared by combining arc-discharge process and in-situ oxidative polymerization method. It was found that Fe/C/PPy-paraffin composites have higher dielectric loss in comparison with both Fe/C-paraffin and PPy-paraffin composites, implying enhanced dielectric properties by this embedded structure. The RL (reflection loss) exceeding-10 dB is obtained in the 11.2–16.9 GHz for a given thickness of 2.2 mm, which covers most of Ku-band (12-18 GHz). The RL exceeding-10 dB is obtained in the 9.6–14.1 GHz for a given thickness of 2.5 mm, which covers most of X-band (8-12 GHz). The illustration of physical model indicated thatthe special embedded structure with abundant heterogeneous interfaces is responsible for the excellent microwave-absorption performances.

1. INTRODUCTION Microwave absorber is a kind of functional material that can absorb microwave effectively and converts microwave energy into thermal energy or make microwave dissipate by interference. Over the past decade, increasing demands of a variety of materials used as microwave absorbers for military and commercial applications

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result in the extensive studies for the development of novel high-efficient microwave absorbers with lightweight, thin thickness, wide absorption bandwidth, and strong absorption characteristics.1-5Recently, organic-inorganic composites have attracted considerable attention in the microwave-absorption field, as these systems with an organic structure usually provide a new functional hybrid, with synergetic or complementary behaviors between organic and inorganic materials.6 In particular, polypyrrole (PPy) is known as one of the most important organic polymers due to its high conductivity, ease of preparation and good environmental stability. This kind of organic-inorganic composites under high-temperature conditions is very unstable. We can embed these composites into thermostable methylsilicone resin matrix materials, which can prevent oxidizing and corroding of the composites, and this kind of composites can bear high temperature above 1000 ℃.The embedment of inorganic nanoparticles inside the conducting PPy polymer is interesting due to the strong electronic interaction between inorganic nanoparticles and polymer matrices.7,8 The wide study of organic-inorganic composites in microwave absorption field can be attributed to their special heterogeneous interfaces. The existence of heterogeneous interfaces can enhance microwave-absorption properties greatly. For example, the interfaces can increase dielectric loss as a result of multi-interfacial polarizations.9,10 Moreover, the interfaces also can provide more active sites for reflection and scattering of microwave. The microwave will be reflected and absorbed repeatedly inside the composites.11-14Theinorganic-organic composites were expected to have a significant breakthrough in microwave-absorption properties. The structural types of organic-inorganic composites contain embedded structure,6multi-layered structure (or sandwich structure),15,16and core-shelled structure.17-20Among these structures, the study of core-shell structured nanocapsules, composed of an inorganic core (Ni, FeNi, Co, etc.) and an organic shell(polyaniline, C, PPy, etc.) in nanometer size, is very active in the microwave-absorption field. The core-shell structure can induce many beneficial physical effects on microwave absorption, including reﬂecting and scattering inside materials,11-14cooperative effects of dielectric properties and


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magnetic properties,17,21and multi-polarizations at heterogeneous interfaces.9,19The magnetic properties are related with the component and microstructure. If we keep both the component and microstructure stable, the magnetic properties of composites will be stable. The embedded type structure can keep magnetic materials isolated from ambient environment, which is favorable to keep the magnetic properties stable. In this study, high-effective organic-inorganic composites Fe/C/PPy (Fe/C nanocapsules embedded in polypyrrole) were prepared by combining the arc-discharge process and in-situ oxidative polymerization method. Here, organic PPy polymer as an effective microwave-absorption material not only can reduce the density of the absorber, but also can increase the heterogeneous interfacial areas of composites, enhancing greatly microwave-absorption properties. It was found that the dielectric loss of Fe/C/PPy-paraffin composites is higher compared with that of both Fe/C-paraffin and PPy-paraffin composites, implying enhanced dielectric properties by this embedded structure. The microwave-absorption results reveal clearly that the Fe/C/PPy-paraffin composites present excellent absorption performances for their high absorption intensity and wide absorption bandwidth at low thicknesses.

2. EXPERIMENTALSECTION Synthesis Fe/C nanocapsules were prepared by an arc-discharge technique. The detailed process is described as the following: Fe (99.9%) ingot and graphite needle served as the anode and cathode, respectively. High-purity argon was introduced into an evacuated chamber (10 Pa) to serve as the plasma source, and 20 mL ethanol as the C resource was introduced into the evacuated chamber. At the end of the arc-discharge process, residual gases were pumped out. After being passivated in air for 24 h, the prepared powders were collected at the top of the chamber. Subsequently, the collected powders were embedded inside PPy polymer by in-situ oxidative polymerization method. A typical preparation process is described in the following manner: the collected Fe/C powders were added to ethanol solution under



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ultrasonication for 30 minutes to obtain a uniform suspension. An aqueous solution of HCl was added to pyrrole monomer to obtain HCl-protonated pyrrole. The well-dispersed Fe/C powders were mixed with the HCl-protonated pyrrole monomers. APS (ammonium persulphate, (NH4)2S2O8) was then added to the aforementioned solution as an oxidant for the polymerization reaction under vigorous mechanical stirring for 12 h. In this experiment, the molar ratio of pyrrole monomers to APS was retained at 1:1.Three different Fe/C/PPy composites were prepared by controlling the weight ratios of Fe/C powders to pyrrole monomers at 1:1, 1:2, 1:3, respectively. Beside these, pure PPy polymer was also synthesized. Finally, the precipitated solutions were centrifuged and washed five times with distilled water repeatedly. The obtained sticky matters were dried in a drying cabinet at 40 °C for 48 h. The preparation method of two-step approach is necessary in this work. The one-pot approach is proper to prepare the composites of single-layered structure. However, for double or more layers, the one-pot approach may lead to the components uncontrollable. In this work, if only one-pot approach is employed, the C layer and the polypyrrole layer will be disordered, and the layer component is uncontrollable. Therefore, for double (or more) layered structure, the preparation method of two-step approach is proper and necessary. Preparation of samples for the measurements of electromagnetic properties The Fe/C/PPy composites were uniformly mixed with paraffin and then pressed into a cylindrical-shaped compact with an outer diameter of 7.00 mm, an inner diameter of 3.04 mm, and an approximate height of 2.00 mm for the electromagnetic parameter measurements, wherein paraffin, due to its transparent properties, not only can agglomerate samples, but also can provide a good passageway of microwave absorption. The weight concentrations of pure Fe/C, PPy and Fe/C/PPy composites with different ratios of Fe/C to PPy (1:1, 1:2 and 1:3) in paraffin matrices were all set at 50 wt%. The weight concentrations of Fe/C/PPy composites with 1:2 weight ratio of Fe/C to PPy in paraffin matrices were set at 15wt%, 20wt%, 30 wt%, 40 wt%, 50wt%, 60wt%, respectively.


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Material characterization The size distribution, morphology and structures were investigated using transmission electron microscopy (TEM, JEOL 2010EX). The surface information of prepared samples was determined by X-ray photoelectron spectroscopy (XPS) with an Al Ka line X-ray source using ESCALAB-250 multi-functional surface analyzer. The data of XPS core level spectra was analyzed with the software XPSPEAK4.1. Fourier transform infrared spectra (FT-IR) were recorded with resolution of 2 cm−1 in the method of attenuated total reflection by using a Thermo Nicolet Nexus. A variety of weight and thermal values of samples were investigated by thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) at ambient pressures from 100 °C to 600 °C with a heating rate of 4 °C per minute. The electromagnetic parameters of the samples were determined in the frequency range from 2 to 18 GHz using an Agilent N5230C network vector analyzer with a transverse electromagnetic mode.

3. RESULTS AND DISCUSSION The morphologies and microstructures of samples were investigated by TEM. Figure1 (a) shows TEM image of Fe/C nanocapsules synthesized via the arc-discharge process, which reveals clearly that their shapes are spherical with diameters from 5 to 20 nm. The C shells covered well over the Fe nanoparticles, as shown in high-resolution TEM (HRTEM) images of Figure1(b and c). It can be seen that the C shells consist of an ordered stack of fragmentary graphite layers. Those graphite layers are in a parallel array with interplanar spacing of 0.34 nm, which corresponds with the standard (002) plane of graphite phase. It is worth to note that the C shells contain some structural defects, such as C-layer breakage and serious blending, as shown in Figure1 (d and e). These defects are easy to form the collapse, unevenness and terraces.19However, the existence of defective sites in C shells is beneficial to the microwave absorption. The defective sites can generate additional energy states near the Fermi level, reducing the electron transition energy.22 The microwave can be



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absorbed easily by electronic transitions from quasi-continuous states induced by the defects near the Fermi level.23Figure 1(f)illustrates the clear image of Fe lattices. It can be observed that the Fe lattices are in a regular and ordered array with interplanar spacing of 0.21 nm (calculated by the inset in Figure 1(f)), which corresponds with the standard (110) plane of the Fe lattices.

Figure1.(a) TEM and (b) (c), (d), (e) HRTEM images of prepared Fe/C nanocapsules. (f) HRTEM image of Fe lattice area of prepared Fe/C nanocapsules.

When in–situ oxidative polymerization reaction was carried out, the Fe/C nanocapsules were embedded in PPy polymer. Figure2 (a and b) show the TEM image of as-prepared Fe/C/PPy composites. It can be observed that the surfaces of the Fe/C nanocapsules are surrounded by a large quantity of PPy polymer. The PPy polymer is firmly attached to the Fe/C nanocapsules even though ultrasonication is applied during the preparation of TEM specimens. It indicates an excellent adhesion between Fe/C nanocapsules and PPy polymer. The obtained samples are not very uniform due to the special uncontrollable in-situ oxidative polymerization process. The sizes of obtained Fe/C/PPy composites vary from several nanometers to tens of nanometers.


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The Fe/C/PPy composites can be prepared in large-scale mode. The output of Fe/C composites can reach scale of kilograms by the improved arc-discharge system, which controls the output of Fe/C/PPy composites in the process. The marginal thicknesses of Fe/C/PPy composites have an obvious increase due to cover of PPy, as shown in Figure. 2(c and d).The outlines of both Fe/C and C/PPy interfaces can be observed clearly, as shown in Figure 2(e). Those heterogeneous interfaces can induce many beneficial

physical effects on

microwave

absorption,

such

as

interfacial

polarization,9,10reﬂectingand scattering of microwave.11-14The image in Figure 2(f) shows that the covered PPy polymer has no stationary interplanar spacing, indicating a relatively disordered array in microstructures of PPy polymer.

Figure2.(a), (b) TEM and (c), (d), (e) HRTEM images of prepared Fe/C/PPy composites. (f) HRTEM images of PPy area of prepared Fe/C/PPy composites.

XPS is a very effective and convenient technique to investigate the surface states of materials, which not only can present information about the atomic compositions, but also can identify the type of bonds between atoms.24 Figure3 (a) displays the XPS survey spectra of Fe/C nanocapsules and Fe/C/PPy composites. Three strong peaks



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centered at 707 eV, 532 eV and 284 eV can be observed from the XPS survey spectra of Fe/C nanocapsules, corresponding to the Fe, O and C, respectively. The quantitative results from XPS analysis reveal that the atomic percentages of Fe, O and C in the surface are 3.37%, 14.75% and 81.89%, respectively. As a result of the high surface sensitivity (less than 10 nm in depth) of XPS, the major presence (81.89%)of C in the surface shows that the Fe nanoparticles have been well encapsulated by C layers, which is in accordance with the TEM results. The C (1s) core level spectra were divided into four peaks corresponding to C=C (283.5 eV), C-C (284.8 eV), C-O (286.5V), and C=O (288.5 eV),25 as shown in Figure 3(b).The intensity ratio of C-C to C=C can be used to estimate the perfection degree of the graphite layers. It can be known from Figure 3(b) that the intensity ratio of C-C to C=C is close to 1, indicating relatively imperfect array of graphite layers. The amount of O in the surface is somewhat large (14.75%). The detected O is resulted from chemisorbed O. In the arc-discharge process, O atoms as well as C atoms in ethanol molecules got rid of the bound of chemical bonds, and escape to vacuum space. Some O atoms may enter into the C layers due to the non-equilibrium thermodynamic process of the arc-discharge, increasing the amount of O in C layers.26 The quantitative results from XPS analysis reveal that the atomic percentages of C, O, Fe and Nin the surface of Fe/C/PPy composites are 66.52%, 22.29%, 2.96% and 8.24%, respectively. The O content has an obvious increase in contrast with that of Fe/C nanocapsules, which may be ascribed to the remnants of reactants with O-element in PPy during the in-situ polymerization reaction. The O (1s) core level spectra of Fe/C/PPy composites were divided into three peaks. The peaks at 530.6, 532 and533.4 eV are ascribed to O-C=O, C=O and C-O-C/C-OH, respectively,27,28 as shown in Figure 3(e). The increase in the carboxylic groups (O-C=O) demonstrates the high level of oxidation due to the presence of increasing amount of PPy polymer. The N element exists in the surface of Fe/C/PPy composites, which confirms the formation of PPy in the surface of the samples. Figure3 (f)represents the N (1s) core level spectra which can be divided into four peaks. The binding energy centered at


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396.6, 398.3, 399.5 and 400.8 eV can be assigned to quinoid [=N-], benzenoid amine [-NH-],

cationic

nitrogen

atoms

(=NH+)

and

protonated

amine

units

(-NH+),respectively.29The peak at binding energy of 400.8 eV is assigned to protonated amine units (-NH+), which is ascribed to the stronger electron localization associated with poor conjugation at sp3 bonded sites.30Thepositively charged nitrogen atoms with protonation (=NH+) are correlated with the cationic nitrogen atoms. The high peak (=NH+) indicates a relatively good protonation level (N+/N) of PPy, implying good electrical properties of the composites.31

Figure3. (a) XPS survey spectra of Fe/C and Fe/C/PPy composites. (b) C (1s) and (c) O (1s) core level spectra of Fe/C nanocapsules. (d) C (1s), (e) O (1s) and N (1s) core level spectra of Fe/C/PPy composites.

FT-IR can provide information about the bonding of organic functional groups.27The FT-IR absorption spectra of the Fe/C, PPy and Fe/C/PPy composites with different ratios of Fe/C to PPy (1:1, 1:2 and 1:3) were displayed in Figure 4.It can be observed that the Fe/C spectra have no obvious vibration peak, which may be due to the low C content. The Fe/C/PPy composites display many vibration peaks due to the abundant organic functional groups. The features of vibration peaks are summarized as below: the strong band at about 3423 cm−1 was associated with the N-H stretching vibration. The peak at 1545 cm−1 was attributed to C-N asymmetric



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and ring stretching. The broadband at 1326, 1165 and 903 cm−1 were ascribed to the C-H in-plane deformation vibrations, C-N stretching vibrations, and C-H out-of-plane vibration, respectively.32,33The band around 1050 cm-1 was attributed to C-O stretching vibrations. The band around 1720 cm-1 was related to stretching vibrations from C=O. The band at 842 cm-1 was ascribed to C-H rocking.27Compared to FT-IR spectra of Fe/C/PPy and PPy, their characteristic peaks appearing in the spectra are similar, such as 3423, 14211165,and 842cm−1, indicating that PPy chains integrated with Fe/C to form Fe/C/PPy composites. Nevertheless, some small differences can be observed in the 1500-1800 cm-1.The differences of characteristic peaks between Fe/C/PPy and PPy may be a result of the interaction of intermolecular hydrogen bands between Fe/C and PPy. It has been reported that the structure of PPy doped with common inorganic counterions is essentially changed.34

Figure 4.FT-IR spectra of Fe/C, PPy, and Fe/C/PPy composites with different ratios of Fe/C to PPy (1:1, 1:2 and 1:3).

Raman spectroscopy is an effective and non-destructive technique to identify the bonding and microstructure of C-species materials. Usually, the Raman spectra exhibit two broad peaks around 1332 cm-1,known as the D peak for ′disordered′ C, and around 1580 cm-1, known as the G peak for ′graphite C′. The Raman spectra of Fe/C and Fe/C/PPy composites are displayed in Figure 5. The D peak centered around 1348 cm-1 is associated with the sp2–bonded C phase, which shifts upwards lightly in comparison with that of bulk diamond (1332 cm). Such a peak shift may be attributed


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to phonon-confinement effects resulting from the serious aberrance and uneven distribution in the graphite layers. The G peak centered around 1567 cm-1 has a shift, compared with bulk graphite (1580 cm-1), which may be due to the tensile strain resulting from massive aberrance in the graphite layers. The peak at 1065 cm-1 is attributed to the C-H in-plane deformation. The bands located at about 962 and 930 cm-1 correspond to ring deformation of PPy and the out-of-plane C-H deformation.35 After undergoing in-situ oxidative polymerization, the formed Fe/C/PPy exhibits the characteristic peaks of PPy at 930, 962, 1065, 1348, and 1567 cm-1, indicating the successful polymerization of pyrrole along Fe/C nanocapsules.

Figure 5. Raman spectra of Fe/C and Fe/C/PPy composites

The DTA-TGA plots of the Fe/C and Fe/C/PPy composites with different ratios of Fe/C to PPy (1:1, 1:2 and 1:3) are shown in Figure6 (a and b), respectively. For the DTA plot of Fe/C nanocapsules, three stages can be observed: in the first stage (50–200 oC), the Fe/C nanocapsules nearly have no thermal variety with increasing temperature. In general, the metal nanoparticles are easy to transform spontaneously into metal oxides in nanometer sizes, and have an obvious exothermic behavior. Here, the Fe/C noanocapsules have no thermal variety even at 200 oC, which can be ascribed to the fact that the outer C shells prevent the Fe cores from oxidation. The second stage (200–260 oC) may be associated with oxidative behaviors of Fe cores and C shells. The strong exothermal peak occurs at about 220oC, which can be



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attributed to the strong burning of C shells. In the third stage (260–600 oC), the thermal values only have slight change as a result of much weaker oxidative behaviors. The Fe/C/PPy composites have distinct exothermic behaviors. It can be observed that the strong exothermal peak of Fe/C/PPy composites are located at about 350oC. This temperature is much higher than that (220oC) of Fe/C nanocapsules, which can be ascribed to the higher burning temperature of PPy polymer. Moreover, the temperature range of the exothermic peak is very wide (200–450 oC) due to the slow burning feature of PPy polymer. In Figure6 (b), it can be observed that the weight of Fe/C nanocapsules has an obvious increase in 210–250 oC due to the Fe oxidation. Subsequently, the weight nearly has no change, which indicates that the Fe core has been nearly oxidized completely after 250 oC. In contrast, the weight variation of Fe/C/PPy composites is distinct greatly. As the temperature increases, the composite weight has a slow decline. It is an integrated result of the weight increase induced by Fe oxidation and the weight decrease induced by burning C and PPy. With the continuous increasing of temperature, the composite weight only have a slight change due to the gradually weakened oxidative behaviors.

Figure6. (a) DTA and (b) TGA plots of Fe/C and Fe/C/PPy composites with different ratios of Fe/C to PPy.

The real part (ε′) and the imaginary part(ε′′) of the complex permittivity verse frequency curves for Fe/C-paraffin, PPy-paraffin and Fe/C/PPy-paraffin composites


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with different ratios of Fe/C to PPy (1:1, 1:2 and 1:3) are shown in Figure7(a and b), which constitute a very good illustration of the influence of the embedded structure on the dielectric properties. It can be seen that the ε′ of Fe/C/PPy-paraffin composites with different ratios of Fe/C to PPy is situated between those of Fe/C-paraffin and PPy-paraffin composites, and the ε′ shows an increase with increasing the ratios of Fe/C to PPy from 1:1 to 1:3.In comparison, the ε′′ has somewhat difference with ε′. The ε′′ shows an increase with the ratios of Fe/C to PPy increasing, and the Fe/C/PPy composites with 1:3 ratio have higher ε′′ in contrast with Fe/C-paraffin and PPy-paraffin composites, implying enhanced dielectric properties by this embedded structure. It is well known that the ε′′ values represent the dielectric loss ability.36The higher ε′′values also suggest that this structure is favorable to generate stronger dielectric loss. When the incident microwave penetrates into the heterogeneous interfaces, the interfacial polarization phenomenon will occur and a mass of polarized charges will accumulate at the heterogeneous interfaces, enhancing dielectric loss.19The real part (µ′) and the imaginary part (µ′′) of the complex permeability verse frequency curves for Fe/C-paraffin, PPy-paraffin and Fe/C/PPy-paraffin composites are represented in Figure7(c and d), respectively. It can be found that the Fe/C/PPy-paraffin and PPy-paraffin composites have similar variation of µ′ and µ′′ as frequency increases, which vary slightly around 1 and 0.Whilefor Fe/C-paraffin, the µ′ have an obvious decrease from 1.14 to 0.89 and the µ′′ values are increased from 0.06 to 0.21.The curve of µ′′ verse frequency exhibits three broad peaks at about 4.5 GHz, 9.5 GHz and 13.9 GHz. These multi-resonance peaks can be due to the ‘exchange mode’ resonance, which may be associated with the small size effect, surface effect and spin-wave excitations.37



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Figure7.Frequency dependences of (a) ε′, (b) ε′′, (c) µ′′ and (d) µ′′ for 50% weight concentrations of pure Fe/C, pure PPy, and Fe/C/PPy composites in paraffin matrices, respectively.

The microwave-absorption performances depend on their magnetic and dielectric losses. Figure8 (a and b) illustrate the frequency dependences of the dielectric loss factor tan (δε)(viz. ε′′/ε′) and magnetic loss factor tan (δµ)(viz. µ′′/µ′). The tan (δε) shows an increase with increasing the ratios of Fe/C to PPy, and both the Fe/C/PPy-paraffin composites with 1:2 and 1:3ratios have higher tan (δε) in contrast with Fe/C-paraffin and PPy-paraffin. The Fe/C/PPy-paraffin and PPy-paraffin composites have similar variation of tan (δµ) as frequency increases, which only have slight fluctuation near zero in 2-18 GHz frequency. However, Fe/C-paraffin composites have an obvious increase of tan (δµ) due to the ferromagnetic property of Fe. The tan (δµ) values vary from 0.05 to 0.25, suggesting strong magnetic loss ability. In general, the contributors to magnetic loss include magnetic hysteresis, domain-wall displacement, eddy current and natural resonance. Here, the natural resonance loss


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may be the main contributor to the magnetic loss due to the weak applied electromagnetic field.19,38

Figure8.The frequency dependences of (a) tan (δε) and (b) tan (δµ)for 50% weight concentrations of pure Fe/C, pure PPy, and Fe/C/PPycomposites in paraffin matrices, respectively.

The electromagnetic properties are associated not only with the intrinsic nature of filler and host polymer matrix, but also with the weight concentrations of filler in host polymer matrix.26,39Theε′, ε′′, µ′ and µ′′ versus frequency curves for Fe/C/PPy-paraffin composites with different Fe/C/PPy weight concentrations in paraffin matrices are plotted in Figure9(a-d).For a fixed Fe/C/PPy weight concentration, the ε′ and ε′′ values show a slow decline as frequency increases from 2 to 18 GHz. For a fixed frequency, the ε′ and ε′′ values increase as Fe/C/PPy weight concentrations increase from 15% to 60%.It can be seen that the dielectric constant has moderate values that are essentially close to that of the host polymer matrix at low concentrations, and only decrease slightly with increasing frequencies. When the weight concentration increases to 60%, the ε′ has an abrupt decline from 44 to 21, and ε′′ decreased from 53 to 13 in the 2-18 GHz range. As Fe/C/PPy composite concentrations increase, the µ′ and µ′′ values only have slight variation around 1 and 0, respectively, as shown in Figure9(c and d).



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Figure9.Frequency dependences of (a) ε′, (b) ε′′, (c) µ′′and (d) µ′′ for different weight concentrations of Fe/C/PPycomposites in paraffin matrices.

The attenuation constant of Fe/C/PPy-paraffin composites is calculated using the following expressions: A=1-T-R, T=S211r+ S211i, R= S221r+ S221i, where A is the attenuation constant, T transmission constant, R reflection constant, and S11r, S11i, S21r,S21ithe measured parameters. Figure 10 shows attenuation constant of Fe/C/PPy-paraffin composites with different weight concentrations. It clearly indicates that the attenuation constant increases as the concentration increasing from 15% to 30%. With the further increase of concentration, the attenuation constant varies slightly. In general, a high filler concentration in the dispersion-matrices has the tendency to obtain higher attenuation constant. Nevertheless, the further increasing of concentration may induce the


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conductivity of system, and the percolation effect may occur, which may weaken the attenuation ability. For the 30% weight concentration of Fe/C/PPy composites in paraffin matrices, the attenuation constant has a decrease from 0.42 to 0.35 in 2.0-7.2 GHz range, and then it increases from 0.35 to 0.55 in 7.2-18.0 GHz range. For the 60% weight concentration of Fe/C/PPy composites in paraffin matrices, the attenuation constant increases from 0.28 to 0.54 with increasing the frequency.

Figure 10.Frequency dependences of attenuation constant for different weight concentrations of Fe/C/PPy composites in paraffin matrices.

To further reveal the microwave-absorption properties, the reflection loss (RL) of Fe/C/PPy-paraffin composites is calculated using the relative complex permeability and permittivity at a given frequency and thickness layer according to the transmission-line theory by means of the following expressions:40,41 Zin=Z0 (µr/εr)1/2tanh[ j(2πfd/c) (µrεr)1/2], RL=20log| (Zin-Z0)/(Zin+Z0) |, where f is the microwave frequency, d the thickness of absorber, c the velocity of light, Z0 the impedance of air and Z the input impedance of the absorber. Figure11(a-f) show color maps of RL of Fe/C/PPy-paraffin composites with different weight concentrations, which clearly indicate that the Fe/C/PPy-paraffin composites have optimal microwave-absorption properties at the 30% weight concentration due to the much larger valuable microwave-absorption areas (

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