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Enhanced Microwave Absorption Properties of Electrospun PEK-C Nanofibers Loaded With [Fe.sub.3][O.sub.4]/CNTs Hybrid Nanoparticles.

HIGHLIGHTS

1. The thermal stability of nanofibers is improved with increasing CNTs loading.

2. The permittivity and dielectric loss are enhanced obviously by adding CNTs.

3. A better impedance matching characteristic is obtained by mixing CNTs with [Fe.sub.3][O.sub.4].

4. The microwave absorption properties are enhanced by adding CNTs.

INTRODUCTION

Electromagnetic (EM) wave absorbing materials have received extensive attention due to their particular important role in civil and military applications [1-5]. Nanoresins consisting of magnetic nanoparticles (NPs) embedded into a polymer matrix are generally used as absorbing coating or resin matrix applied in radar stealth composites. However, NPs with high surface energy tend to form aggregation in polymer resins, which will affect their activity and selectivity for absorbing EM wave. Therefore, much effort has been devoted recently, seeking efficient nanoresins with light weight, strong absorption, thin thickness, and excellent thermal stability [6-11]. Electrospinning is a novel and highly efficient technique for large-scale preparation of nanofibrous materials [12, 13]. This innovative method provides a versatile and effective approach to produce traditional nanoresins in the form of polymeric nanofibers with diameters ranging from nanometers to microns. The top-down laye-by-layer processing technology can avoid aggregation by confining NPs loaded in the polymeric nanofibers [14, 15]. Furthermore, electrospinning technology produces absorbing NPs with low-dimension, microporosity, and large specific surface area, which can enhance the absorbing ability per unit mass [16, 17]. Therefore, the unique structural advantages of polymeric composite nanofibers make it more potential in serving as absorbing coating or resin matrix materials for application in stealth composites.

The polymeric nanofibers containing magnetic absorbers such as [Fe.sub.3][O.sub.4] NPs have shown good microwave absorbing performance by combining the typical three-dimensional network structure of nanofibers with the high magnetic loss of [Fe.sub.3][O.sub.4] NPs [18]. However, pure [Fe.sub.3][O.sub.4] NPs have a poor impedance matching characteristic due to their high permeability and low permittivity, which will restrict their application in further improvement of microwave absorption properties [19-21]. Carbon nanotubes (CNTs) have a unique ID tubular structure and high electronic conductivity, which are considered as typical dielectric loss material with high complex permittivity [22-24]. Therefore, it is efficient to tailor EM parameters by employing [Fe.sub.3][O.sub.4]/CNTs hybrid absorbers with a better impedance matching characteristic, so as to conveniently achieve the polymeric composite nanofibers with excellent absorbing properties and simplicity workmanship [25].

In this work, [Fe.sub.3][O.sub.4]/CNTs hybrid NPs loaded in PEK-C nanofibers were synthesized by electrospinning technique. The microstructure and thermal stability of composite nanofibers were characterized by scanning electron microscope (SEM) and thermogravimetric analysis (TGA), respectively. The complex permittivity and permeability of composite nanofibers in the microwave frequency range of X band (8.2-12.4 GHz) were measured by vector network analyzer using wave-guide method. The enhanced microwave absorption properties are mainly attributed to tunable EM parameters and thus a better impedance matching characteristic by mixing CNTs with [Fe.sub.3][O.sub.4] NPs within proper ranges loaded in PEK-C nanofibers.

EXPERIMENTAL

Materials

[Fe.sub.3][O.sub.4] NPs (20 nm in diameter, 99.5%) and CNTs (multiwalled carbon nanotubes, 8-15 nm in outer diameter, 50 [micro]m in length, 98%) were obtained from Beijing Dao King, Ltd., China. The phenolphthalein polyether ketone (PEK-C) resin used for electrospunning nanofibers was received from Xuzhou engineering plastic factory. PEK-C is an amorphous engineering thermoplastic polyetherketone with a phenolphthalein side group. It shows an intrinsic viscosity of 0.3 dl/g and a glass transition temperature [T.sub.g] of 230[degrees]C. The chemical structure of the PEK-C is shown in Fig. 1. The dimethyl acetamide (DMAC) solvent was received from Tianjin Fuyu fine chemical company. The surfactant sodium dodecyl benzene sulfonate (SDBS) and triton X-100 were purchased from Sinopharm chemical regents limited company.

Preparation of Fej04-CNTs/PEK-C Composite Nanofibers

The [Fe.sub.3][O.sub.4]/CNTs hybrid NPs (1.25 g) along with SDBS and triton X-100 were added into DMAC (22.78 g) under ultrasonic dispersion for 45 min, followed by addition of PEK-C resin (5 g) under mechanical stirring at 65[degrees]C for 4 h combined with mechanical shear mixing for 10 min to form a homogeneous solution. The obtained solution (10 ml) was then loaded into a plastic syringe with a stainless-steel needle (0.7 mm in inner diameter). The needle used as the positive electrode was connected to a high-voltage supply (Dingtong High Voltage Power Co., China, DPS 200). During the electrospinning, the applied voltage was kept at 23 KV, when the distance between the spinneret and a metal collector was optimized and around 14 cm. The composite nanofibers in the form of nonwoven mats were collected, and were dried off at 120[degrees]C for 1 h and then 180[degrees]C for 3 h. Several composite nanofibers absorbers were obtained by mixing [Fe.sub.3][O.sub.4] with CNTs at various weight ratios ([W.sub.CNTs] = 0, 2, 5, and 10%) of the 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid NPs loaded in PEK-C nanofibers.

Characterizations

The surface morphologies and composition of composite nanofibers were observed by an SEM (Hitachi company, Japan, SU3500) combined with energy disperse spectrum. Nanofibers average diameter was determined by analysis of SEM images manually. The average nanofiber diameter has been calculated based on 50 number of nanofibers. The thermal stability of composite nanofibers was characterized by TGA (Perkin Elmer company, USA, Pyris 1). Samples of approximately 5 mg were heated from room temperature to 800[degrees]C at a heating rate of 10[degrees]C [min.sup.-1] under a nitrogen gas flow rate of 20 ml [min.sup.-1]. The EM characteristics including complex permittivity and permeability of composite nanofibers in the form of nonwoven mats (22.86 X 10.16 mm) were performed by means of wave guide measurements in X-band (8.2-12.4 GHz) using a vector network analyzer (Agilent Technologies Inc., USA, 8720ET). In this type of measurement, the sample is placed in a wave guide and illuminated by EM waves that are partially reflected and partially transmitted through the material, depending on its EM intrinsic properties. The reflection loss (RL) of composite nanofibers was calculated by Matlab based on the relatively complex permeability and permittivity at a given frequency and matching thickness according to the following equations:

[Z.sub.in] = [Z.sub.0] [square root of [[mu].sub.r]/[[epsilon].sub.r]]tanh[j 2[pi]fd/c)[square root of [[mu].sub.r][[epsilon].sub.r]] (1)

RL(dB) = 20[log.sub.10] [absolute value of [Z.sub.in] - [Z.sub.0]/ [Z.sub.in] + [Z.sub.0]] (2)

where [[epsilon].sub.r] ([[epsilon].sub.r] = [epsilon]' - j[epsilon]") is the relative complex permittivity, pT ([[mu].sub.r] = [mu]' - j [mu]") is the relative complex permeability, j represents the imaginary part, dB represents the unit of attenuation values of incident radar wave, f is the frequency of the EM wave, d is the coating thickness, c is the velocity of light, [Z.sub.0] is the impedance of free space, and [Z.sub.in] is the input impedance of the absorber.

RESULTS AND DISCUSSION

Morphology Analysis

Figure 2 shows the SEM images of PEK-C nanofibers containing 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid NPs with different [W.sub.CNTs]. Correspondingly, the distribution histograms of fiber diameters are shown in Fig. 3. It can be observed that all these electrospun nanofibrous materials have shown interpenetrating networks structure consisted of fine fibers in microscopic view. As shown in Fig. 2a, the pure PEK-C nanofibers show relative smooth surfaces and homogeneous characteristics in the fiber diameters. As the CNTs loading in hybrid absorbers increases from 2% to 10% shown in Fig. 2b-d, there are more protrusions formed on the fibers surface and the nanofibers diameter become fine accompanied with a nonuniform distribution. It is also found that obvious cohesive entanglement and bending of nanofibers took place in local region especially for [W.sub.CNTs] = 10%. As shown in Fig. 3, increasing CNTs loading in hybrid absorbers, we can observe that the nanofibers diameter range between 400 and 650 nm for sample (a), 200-600 nm for sample (b), 100-450 nm for sample (c), and 150-450 for sample (d). Increasing CNTs loading in PEK-C matrix will decrease the distance between adjacent CNTs, which leads to their tendency to tangle together especially for their big curvature characteristic. Consequently, the PEK-C nanofibers loaded with hybrid absorbers appear bending and entanglement phenomenon. Furthermore, increasing CNTs loading will decrease the intermolecular force of PEK-C matrix and enhance kinetic energy of molecular chains, thus increases the instability and stretching force of liquid jets of polymer solutions in the process of electrospinning. Therefore, increasing CNTs loading leads to uneven nanofibers surface and narrow nanofibers diameter.

Thermal Gravimetric Analysis

The thermal stability of PEK-C nanofibers containing 20 wt% Fe3(VCNTs hybrid NPs with different [W.sub.CNTs] was studied by TGA. Figure 4 shows the TG curves as a function of temperature for the composite nanofibers upon heating in Ni. Thermal stability can be expressed in terms of parameters such as 5% weight loss temperature ([T.sub.5%]) and char residue at 800[degrees]C, which are summarized in Table 1. It can be observed from Table 1 that as the [W.sub.CNTs] increases to 10%, [T.sub.5%] and char residue of the composite nanofibers increase from 517[degrees]C and 63.6% to 536[degrees]C and 72.7%, respectively. It indicates that the thermal stability of the composite nanofibers is improved to a certain extent by adding CNTs. The CNTs which serve as good thermal conductors in resin matrix is in favor of heat conduction and reducing system temperature, thereby improving the thermal stability of the PEK-C nanofibers. In addition, increasing CNTs loading in composite nanofibers is equivalent to increasing cross-linking points due to good dispersion and interfacial bonding between the CNTs and the resin matrix, which is also good for improving the thermal stability of the PEK-C nanofibers.

Permittivity and Permeability

Figure 5 shows the complex permittivity of PEK-C nanofibers containing 20 wt% [Fe.sub.3][O.sub.4]CNTs hybrid NPs with different [W.sub.CNTs]. The real parts ([epsilon]') and imaginary parts ([epsilon]") of permittivity represent storage capability and loss of electric energy, respectively. It is apparent that both the real part s' and imaginary part s" are nearly constant over the whole frequency range (8.2-12.4 GHz). As for PEK-C nanofibers including 20 wt% [Fe.sub.3][O.sub.4] only ([W.sub.CNTs] = 0%), the [epsilon]' and [epsilon]" keep almost constant at 3.5 and 0.05 with increasing frequency range, respectively. When the weight fraction (20 wt%) of [Fe.sub.3][O.sub.4]/CNTs hybrid absorbers loaded in PEK-C matrix keeps unchanged, the increasing proportion of CNTs causes a significant increase in both the real part [epsilon]' and imaginary part [epsilon]". With the [W.sub.CNTs] increasing to 5%, the [epsilon]' and are, respectively, increased to 18.5 and 3.7 remarkably; meanwhile, some slight fluctuations can be observed in the [epsilon]" curve. As the [W.sub.CNTs] further increases to 10%, the [epsilon]' and [epsilon]" are significantly increased to around 23.5 and 8. The enhancement of permittivity including [epsilon]' and [epsilon]" benefits from the addition of CNTs with high electronic conductivity and low percolation threshold, which tend to form conductive network in the resin matrix and thus is in favor of enhancing the dielectric loss. Therefore, it suggests that the storage capability and the loss of the electric energy are enhanced remarkably by introduction of CNTs into [Fe.sub.3][O.sub.4]/PEK-C composite nanofibers.

Figure 6 shows the complex permeability of PEK-C nanofibers containing 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid NPs with different [W.sub.CNTs]. The real parts ([mu]') of permeability represent the storage capability, which accounts for the ability of the system to store magnetic energy, and the imaginary parts ([mu]") of permeability symbolize the loss of magnetic energy. Both [mu]' and [mu]" are found to decline in the whole X band with the increase of [W.sub.CNTs]. From Fig. 6a, with [W.sub.CNTs] = 0%, there is a sharp decrease of [mu]' varying from 3.0 to 1.3 at 8.2-9.2 GHz, and then occurring two resonance peaks respectively at 9.3-9.6 GHz and 10.3-10.6 GHz, and finally tending to be stable at higher frequency region (>11.2 GHz). However, as the [W.sub.CNTs] increases to 5%, the spectrum characteristic becomes more distinguishable with the resonance peak almost disappeared at 10.3-10.6 GHz. Especially when the [W.sub.CNTs] increases to 10%, the [mu]' shows a further reduction varying from 2.1 to 0.8 and only one resonance peak at 9.6-9.8 GHz. As observed from Fig. 6b, as for the sample of [W.sub.CNTs] = 0%, the [mu]" decreases from 1.6 to around 0.1 with increasing frequency. Meanwhile, three resonance peaks could be observed at 9.0-9.8, 10.3-11.0, and 11.6-12.2 GHz. As the [W.sub.CNTs] continues to increase to 5% and 10%, the [mu]" is found to decline with increasing frequency from 1.1 and 1.3 to around 0. As CNTs are nonmagnetic, the multiresonance behavior observed in the [mu]' and [mu]" curves are ascribed to [Fe.sub.3][O.sub.4] particles which have both high magnetic loss and low dielectric loss. Increasing [W.sub.CNTs] means to reduce the [Fe.sub.3][O.sub.4] content in composite nanofibers, thereby leading to the decrease in the storage capability and the loss of the magnetic energy by introduction of CNTs into [Fe.sub.3][O.sub.4]/PEK-C composite nanofibers.

EM loss capacity can be described by dielectric loss tangent (tan [[delta].sub.[epsilon]] = [epsilon]"/[epsilon]') and magnetic loss tangent (tan [[deltya]s.ub.[mu] = [mu]"/[mu]'). Figure 7 shows the dielectric loss tangent (tan [[delta].sub.[epsilon]]) and magnetic loss tangent (tan [[delta].sub.[mu]]) of PEK-C nanofibers containing 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid NPs with different [W.sub.CNTs]. From Fig. 7a, as the [W.sub.CNTs] increases to 10%, the tan [[delta].sub.[epsilon]] curves become relatively flat in the whole frequency range with the values increasing from around 0.05 to 0.34. The dielectric loss of composite nanofibers is attributed to multiple polarization mechanisms of [Fe.sub.3][O.sub.4]/CNTs hybrid absorbers and unique microstructure of nanofibers with interpenetrating networks. First, the CNTs play a key role in elevating dielectric loss due to the enhanced electrical conductivity and resonance loss with increasing CNTs loading. Second, [Fe.sub.3][O.sub.4] NPs can introduce extra dissipation due to polarization processes which may involve space charge polarization, dipole polarization, and interfacial polarization at microwave band. For the metal-based composites, as there is a different electric potential between conductive metal NPs and insulating resin matrix, the charges will gather in the interfaces between [Fe.sub.3][O.sub.4] NPs and PEK-C nanofibers, thereby forming space charge polarization. Meanwhile, interfacial polarization is greatly enhanced due to large specific surface area of NPs and corresponding big interfacial area between NPs and nanofibers. In addition, the NPs with many unsaturated bonds on surface would serve as dipoles, which will form dipole polarization under the electric field. Third, the nanofibrous materials have interpenetrating networks structure consisted of fine fibers, simultaneously, the loaded [Fe.sub.3][O.sub.4] NPs would create defects inside the fibers. All these microstructure characteristics would increase travelling path of the EM waves inside the material and thus enhances energy dissipation. As observed from Fig. 7b, the variation trend of tan [[delta].sub.[mu]] with [W.sub.CNTs] is opposite from that of tan <S" showing a decrease in the tan [[delta].sub.[mu]] and less resonance behavior in the whole frequency range with increasing [W.sub.CNTs]. The magnetic loss of composite nanofibers is mainly ascribed to natural resonance of the [Fe.sub.3][O.sub.4] NPs and exchange resonance benefiting from the addition of the CNTs, but mostly depends on [Fe.sub.3][O.sub.4] NPs. As a result, increasing [W.sub.CNTs] leads to a gradual decrease in tan [[delta].sub.[mu]], owing to the decreasing weight fraction of [Fe.sub.3][O.sub.4] NPs. With the [W.sub.CNTs] increasing to 5%, the tan [[delta].sub.[epsilon]] at equilibrium position (0.2) is closest to tan [[delta].sub.[mu]] (0.25), indicating a better balance of dielectric loss and magnetic loss. Thus, the complementarity between the relative permittivity and permeability can be efficiently tailored by mixing CNTs with [Fe.sub.3][O.sub.4] NPs within proper ranges.

Microwave Absorption Properties

To investigate the effects of [W.sub.CNTs] on microwave absorption properties of composite nanofibers, the relationship between the reflection loss ([R.sub.L]) and the microwave frequency in the 8.2-12.4 GHz range of PEK-C nanofibers with the same thickness (1.7 mm) containing 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid NPs with different [W.sub.CNTs] is shown in Fig. 8. It can be observed that the microwave absorption properties of the composite nanofibers are improved significantly by adding CNTs. The PEK-C nanofibers containing only [Fe.sub.3][O.sub.4] NPs ([W.sub.CNTs] = 0%) have high magnetic loss and low dielectric loss, which thus induces a poor microwave absorption property due to the mismatch of magnetic loss and dielectric loss. By mixing a small amount of CNTs ([W.sub.CNTs] = 2%) with [Fe.sub.3][O.sub.4] NPs, the minimum [R.sub.L] value is -21.6 dB at 9.3 GHz with the bandwidth of 2.7 GHz under -10 dB, indicating an improved microwave absorption properties due to enhanced dielectric loss by adding CNTs. With the [W.sub.CNTs] increasing to 5%, the corresponding minimum [R.sub.L] value reaches -41.4 dB with a bandwidth of 3.2 GHz under -10 dB, meaning that there is a better balance by modulating dielectric loss and magnetic loss, and thus obtains an excellent microwave absorption performance. However, as the [W.sub.CNTs] further increases to 10%, there is an obvious decrease in the amplitude of [R.sub.L], with the minimum RL value reduced to -19.4 dB and a bandwidth of 0.6 GHz under -10 dB. It indicates that although excessive CNTs can increase relative permittivity, but mismatch between high dielectric loss and low magnetic loss would also induce poor microwave absorption properties. Therefore, excellent microwave absorption performance results from good EM impedance match of materials; only in this way EM waves could enter into the material interior and be dissipated rather than reflected on the material surface.

Figure 9 shows the relationship between the reflection loss ([R.sub.L]) and the microwave frequency in the 8.2-12.4 GHz range of PEK-C nanofibers containing 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid NPs ([W.sub.CNTs] = 5%) with different thicknesses. It can be observed that the minimum [R.sub.L] gradually moves toward lower frequency with increasing thickness. The [R.sub.L] values exceeding -10 dB, which means more than corresponds 90% of the incident EM wave is absorbed, almost cover the whole X band with matching thicknesses of 1.4--1.8 mm. When the thickness is 1.5 mm, the minimum [R.sub.L] value is -28.2 dB at 9.7 GHz, with an absorption bandwidth of 3 GHz under -10 dB. As the thickness increases to 1.6 mm, the minimum [R.sub.L] value increases to -32.6 dB at 9.5 GHz with a bandwidth of 3.2 GHz under -10 dB. With the thickness increasing to 1.7 mm, the minimum [R.sub.L] value reaches up to -41.4 dB at 9.3 GHz with a bandwidth of 3.2 GHz under -10 dB. However, as the thickness further increases to 1.8 mm, the minimum RL value decreases to -23.2 dB at 9.2 GHz with a bandwidth of 1.1 GHz under -10 dB. It can be concluded that increasing thickness of PEK-C nanofibers loaded with [Fe.sub.3][O.sub.4]/CNTS hybrid NPs is better for lower frequency absorbing properties, but further increase in the thickness could induce mismatch of dielectric loss and magnetic loss, thereby leading to reduced microwave absorption properties.

CONCLUSIONS

The PEK-C nanofibers loaded with [Fe.sub.3][O.sub.4]/CNTs hybrid NPs were prepared by electrospinning technique. SEM images showed that electrospun nanofibers presented interpenetrating networks structure consisted of fine fibers. Increasing CNTs loading led to uneven nanofibers surface and narrower nanofibers diameter, as well as improved thermal stability of the composite nanofibers. The EM characteristics showed that the permittivity and the dielectric loss of composite nanofibers increased with the increasing [W.sub.CNTs]. Therefore, the PEK-C nanofibers containing 20 wt% [Fe.sub.3][O.sub.4]/CNTS hybrid absorbers with [W.sub.CNTs] = 5% showed excellent microwave absorption properties with a minimum [R.sub.L] value of -41.4 dB at 9.3 GHz corresponding to a thickness of 1.7 mm and exceeding -10 dB with thickness of 1.4-1.8 mm in the whole X band. The enhanced microwave absorption properties are mainly attributed to tunable EM parameters and thus a better impedance matching characteristic by mixing CNTs with [Fe.sub.3][O.sub.4] NPs within proper ranges loaded in PEK-C nanofibers.

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Qi Yu, (1) Mingbo Ma, (1) Ping Chen, (2) Qi Wang, (1) Chun Lu, (1) Yu Gao, (1) Rongchao Wang, (1) Hanlin Chen (1)

(1) School of Aerospace Engineering & Liaoning Key Laboratory of Advanced Polymer Matrix Composites, Shenyang Aerospace University, Shenyang 110136, China

(2) Schooi of Chemical Engineering & State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China

Correspondence to: P. Chen; e-mail: chenping_898@126.com Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 51303106, 51073094; contract grant sponsor: Liaoning Excellent Talents in University; contract grant number: LJQ2015085; contract grant sponsor: National Defense 12th 5-year program Foundational Research Program; contract grant number: A352011XXXX; contract grant sponsor: Liaoning Key Laboratory Fundamental Research Project; contract grant number: LZ2015057; contract grant sponsor: Key Laboratory of Materials Modification by Laser. Ion and Electron Beams of Ministry of Education; contract grant number: LABKF1502. DOI 10.1002/pen.24496

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. Chemical structure of PEK-C.

Caption: FIG. 2. SEM images of PEK-C nanofibers containing [Fe.sub.3][O.sub.4]/CNTs hybrid nanoparticles with different [W.sub.CNTs]: (a) pure PEK-C; (b) 2%; (c) 5%; (d) 10%. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. The diameter distribution histograms of PEK-C nanofibers containing [Fe.sub.3][O.sub.4]/CNTs hybrid nanoparticles with different [W.sub.CNTs]: (a) pure PEK-C; (b) 2%; (c) 5%; (d) 10%.

Caption: FIG. 4. TG curves of PEK-C nanofibers containing [Fe.sub.3][O.sub.4]/ CNTs hybrid nanoparticles with different [W.sub.CNTs]. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Inlluence of [W.sub.CNTs] on complex permittivity of PEK-C nanofibers containing 20 wt.% [Fe.sub.3][O.sub.4]/CNTs hybrid nanoparticles. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Influence of WCNXs on complex permeability of PEK-C nanofibers containing 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid nanoparticles. [Color figure can be viewed al wileyonlinelibrary.com]

Caption: FIG. 7. Influence of [W.sub.CNTs] on dielectric loss and magnetic loss of PEK-C nanofibers containing 20 wt% [Fe.sub.3] [O.sub.4]/ CNTs hybrid nanoparticles. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. Reflection loss ([R.sub.L]) curves versus frequency of PEK-C nanofibers with the same thickness (1.7 mm) containing 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid nanoparticles with different [W.sub.CNTs]. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. Reflection loss ([R.sub.L]) curves versus frequency of PEK-C nanofibers containing 20 wt% [Fe.sub.3][O.sub.4]/CNTs hybrid nanoparticles ([W.sub.CNTs] = 5%) with different thicknesses. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Characteristic temperature of thermal
decomposition of PEK-C nanofibers containing
[Fe.sub.3][0.sub.4/CNTs complex nanoparticles
with different [W.sub.CNTs].

[W.sub.CNTs] (%)   [T.sub.5%] (a)   RW (b)
                    ([degrees]C)     (%)

0                       517          63.6
2                       524          67.2
5                       530          70.3
10                      536          72.7

(a) Temperature at 5% weight loss.

(b) Residual weight percentage at 800[degrees]C.
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Author:Yu, Qi; Ma, Mingbo; Chen, Ping; Wang, Qi; Lu, Chun; Gao, Yu; Wang, Rongchao; Chen, Hanlin
Publication:Polymer Engineering and Science
Article Type:Report
Date:Nov 1, 2017
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