碳纤维,碳管

Fabrication and microwave absorption properties of magnetite nanoparticle–carbon nanotube–hollow carbon fiber
composites
Jun Qiu
a ,
b ,*
,Tingting Qiu
b
a Key Laboratory of Advanced Civil Engineering Materials (Tongji University),Education of Ministry,Shanghai 201804,PR China b
School of Materials Science and Engineering,Tongji University,Shanghai 201804,PR China
A R T I C L E I N F O
Article history:
Received 27March 2014Accepted 4September 2014Available online 19September 2014
A B S T R A C T
Absorbents with ‘‘tree-like’’structures,which were composed of hollow porous carbon fibers (HPCFs)acting as ‘‘trunk’’structures,carbon nanotubes (CNTs)as ‘‘branch’’structures and magnetite (Fe 3O 4)nanoparticles playing the role of ‘‘fruit’’structures were prepared by chemical vapor deposition technique and chemical reaction.Microwave reflection loss,per-mittivity and permeability of Fe 3O 4–CNTs–HPCFs composites were investigated in the fre-quency range of 2–18GHz.It was proven that prepared absorbents possessed the excellent electromagnetic wave absorbing performances.The bandwidth with a reflection loss less than À15dB covers a wide frequency range from 10.2to 18GHz with the thickness of 1.5–3.0mm,and the minimum reflection loss is À50.9dB at 14.03GHz with a 2.5mm thick sample layer.Microwave absorbing mechanism of the Fe 3O 4–CNTs–HPCFs composites is concluded as dielectric polarization and the synergetic interactions exist between Fe 3O 4and CNTs–HPCFs.
Ó2014Elsevier Ltd.All rights reserved.
1.Introduction
With the explosive growth of information technology and the widespread use of high-frequency electromagnetic (EM)waves,the phenomena of EM radiation and interference are becoming ever more seriously.EM radiation and interference not only have a direct impact on the routine use of medical equipment,industrial equipment,but also cause potential damages to biological systems,and direct threats to human health.Therefore,it is urgent to enhance the research of solv-ing the EM pollution problem.Conventional microwave absorbing materials,especially magnetic metals,have been extensively investigated and are commonly utilized for EM wave interference shielding.However,some disadvantages,
such as high density,poor environmental adaptability,and uneconomic processing,restrict their applications.Therefore,microwave absorbing materials,with a low density,strong ability of absorbing EM waves,excellent environmental adaptability and wide EM wave absorption,are becoming highly desirable and necessary.
Recently,carbon materials,including carbon fibers [1–2],CNTs [3–4],carbon black [5–6]and silicon carbide fibers [7–9]have been found to be good candidates for EM wave absorp-tion due to their low specific mass.Among these carbon materials,porous carbon fibers,CNTs as well as carbon nanofibers become increasingly important due to their excel-lent dielectric polarization properties and low density,which resulting in outstanding EM wave absorption with a reduction
/10.1016/j.carbon.2014.09.011
0008-6223/Ó2014Elsevier Ltd.All rights reserved.
*Corresponding author at:School of Materials Science and Engineering,Tongji University,Shanghai 201804,PR China.Fax:+8602169582101.
E-mail address:qiujun@ (J.Qiu).
in thickness and weight[10–12].For example,hollow porous carbonfibers/epoxy composites possessed a minimum reflec-tion loss ofÀ15.5dB and a bandwidth of10.9GHz under À10dB in the range of4–18GHz[13].Composites involving 30wt%CNTs with high helicity could reach a reflection loss ofÀ20dB at10.5GHz,with a bandwidth of reflection loss belowÀ5dB from9.5to12.5GHz[14].A varnish containing 8wt%CNTs had considerable absorbing peak at15.3GHz and achieved minimum absorbing value ofÀ24.27dB[15]. Composites including15and22wt%twin carbon nanocoils exhibited the minimum reflection loss ofÀ15dB at11GHz andÀ27dB at13GHz,respectively[16].However,carbona-ceous absorbing materials have the shortcoming of poor magnetic loss performance,which restricts their further improvement of absorbing properties.Therefore,hybrids con-taining carbon materials and metal particles with dielectric polarization,magnetic loss performance and interface inter-actions may exhibit great potential as high-performance microwave absorbents[17–18].Porous carbon/Co composites possessed a minimum reflection loss ofÀ40dB at4.2GHz with a thickness of5mm,which showed larger dielectric loss as compared with carbonfibersfilled composites[19].FeCoNi-filled CNTs/epoxy possessed a minimum reflection loss value ofÀ28.2dB at15.2GHz[20].Ag nanowirefilled MWNT/epoxy composites had the minimum reflection loss ofÀ19.19dB at 7.8GHz,with a bandwidth of reflection loss belowÀ10dB from7.2to9.0GHz in thickness of1.0±0.1mm[21].Despite a great deal of research on carbon materials for absorbents, until now there have been few reports on the EM wave absorption performance of hollow porous carbonfibers and their pared to the other carbon materials,hollow porous carbonfibers possessed the superior properties of the large surface area,low density and multiform framework[22]. Therefore,it could be expected that hollow porous carbon fibers decorated with metal nanoparticles could show the excellent microwave absorption properties.
In this article,Fe3O4–CNTs–HPCFs composites were pre-pared through growing CNTs and anchoring Fe3O4nanoparti-cles on the surface of HPCFs via a chemical vapor deposition technique and chemical reaction,respectively.The complex permittivity and permeability of the composites in paraffin wax were analyzed,and the microwave absorbing perfor-mance was evaluated.It was found that the absorbents exhi-bit excellent EM wave absorbing performances in the GHz range.The fabrication of such composites would pave a way for preparation of novel high-performance microwave absorbents.
2.Experimental
2.1.Preparation of HPCFs
A polyacrylonitrile(PAN)hollowfiber membrane(850–950l m I.D.,100–150l m wall thickness,0.05–0.1l m pore size)from Shenzhen Shuifuyuan Environmental Science and Technol-ogy Co.,Ltd.(Shenzhen,China)was used as the precursor [23–24].Firstly,a PAN hollowfiber membrane cut into3cm was immersed into alcohol for24h for purification.After dry-ing,the purified membranes were oxidized at230°C for 60min in air,and then heated at the rate of2.0°C/min,car-bonized at700°C for60min under nitrogen(N2)atmosphere, HPCFs samples can be prepared.
2.2.Preparation of CNTs–HPCFs
Initially,above prepared HPCFs were placed into nickel nitrate solution(0.1mol/L)for5h at room temperature to ensure nickel ions were adsorbed onto surface sites.After being dried,a thermal reduction process was carried out at400°C for60min under the atmosphere of reductive hydrogen(H2) and shielding gas N2in the volume ratio of1:3.Then CNTs were grown on the nickel coated HPCFs with ethanol (C2H5OH)vapor as carbon source and N2as the carrier gas by a chemical vapor deposition technique.The vapor deposi-tion process was carried out in the quartz tube of the tube fur-nace at700°C for a growth period of50min.And CNTs–HPCFs carbonaceous composites were prepared.
2.3.Preparation of Fe3O4–CNTs–HPCFs
On the basis of preparation of CNTs–HPCFs as‘‘trunk-branch’’structure,Fe3O4nanoparticles were anchored on the CNTs–HPCFs.Firstly;CNTs–HPCFs(100mg)were dispersed in a 20mL ethyl acetate(TREG)solution in an ultrasonic bath to obtain a dispersed CNTs–HPCFs suspension.The iron precur-sor(450mg),iron(III)acetylacetonate(Fe(acac)3),was then added into the suspension,and this mixture was further son-icated for30min to form a brown colored stable solution.The resulting mixture was subsequently heated to180°C at a rate of3°C/minÀ1under vigorous stirring and argon protection and kept at reflux for15min.Then,under the same atmo-sphere,the mixture was rapidly heated to287°C by chemical reaction of30min.After cooling to room temperature,the products were magnetically separated by a magnet and washed with ethanol several times and dried in a vacuum oven.The strategy for fabrication of Fe3O4–CNTs–HPCFs com-posites is illustrated in Fig.1.
2.4.Characterization
X-ray diffraction(XRD)patterns of the products were obtained on a Bruker D8Advance diffractometer equipped with Cu Ka source.Scanning electron microscope(SEM)images were taken on a Quanta200Ffield emission scanning electron microscopy.The transmission electron microscope(TEM) images were taken on a JEOL JEM-2100F electron microscope under an accelerating voltage of200kV.TEM samples were prepared by dropping a diluted dispersion of the as-prepared products onto amorphous carbon-coated copper grids and drying in air.The X-ray photoelectron spectroscopy(XPS) analyses were carried on an ESCALab220i-XL electron spec-trometer from VG Scientific using300W Al K a radiation.Mag-netic measurements were carried out using a Lake Shore7410 vibrating sample magnetometer with a magneticfield up to 2.4T.
Electromagnetic parameters were measured on a vector network analyzer(Agilent,N5224A)with transmission–reflection mode in the range of2–18GHz band at room tem-perature.The composite samples for electromagnetic
parameters measurements were prepared by evenly mixing the product with a paraffin wax in mass ratio of1:4,and then being pressed into a mold with an outer diameter of7.0mm, inner diameter of3.0mm,and thickness of about2.5mm.
3.Results and discussion
3.1.Morphology and structure analysis
The morphologies,surface structures,and dimension of HPCFs,CNTs–HPCFs and Fe3O4–CNTs–HPCFs composites were characterized by SEM and TEM at different magnifications and view angles.Fig.2a1shows the typical SEM micrograph of the morphology of the obtained HPCFs.It can be observed that thermal decomposition of PAN hollowfibers can prepare hollow structure of carbonfibers.The external and internal diameters of the HPCFs are about900and660l m,respec-tively,and the wall thickness of thefiber is about120l m. Fig.2a2shows the mesoporous and microporous structures on the surface of thefiber,which are the source of high specific surface area and strong absorbing properties[25]. Di-finger like porous structure which plays a role in support-ingfiber can be seen in Fig.2a3.And from the enlarged view of the cross-section of HPCFs(Fig.2a4),it can be seen that a large number of small cracks,which can contribute to the loss of the incident electromagnetic wave through multiple reflec-tions and refractions,are on its surface.
Fig.2b1presents the SEM images of the CNTs–HPCFs through CVD process,which apparently showed that plenty of CNTs are grown on the external surface of the substrate HPCFs,and a novel carbon composite absorbent is success-fully fabricated.The magnified SEM image of CNTs grown on the surface of HPCFs is displayed in Fig.2b2,showing a large number of dense CNTs grows on the substrate of HPCFs which results in the formation of‘‘trunk-branch’’structures,that is,the prepared HPCFs acts as the‘‘trunk’’structures, and the grown CNTs as the‘‘branch’’structures.
After anchoring monodisperse Fe3O4nanoparticles as ‘‘fruit’’structures,CNTs grown on the HPCFs have been high-density coated,as shown in Fig.2c1.Fig.2c2shows the high-resolution TEM image of the Fe3O4–CNTs–HPCFs.It can be seen that the(311)lattice plane of the spinel-structured Fe3O4with the two adjacent planes at a distance of0.25nm is well matched with the XRD analysis of Fe3O4and the core size of Fe3O4nanocrystals is around7nm.The considerably sharp ring-like features of the corresponding selected elec-tron diffraction pattern(Fig.2c3)indicate the polycrystalline nature of the obtained sample with clear(220),(311),(400), (511)and(440)diffractions.
To further understand the obtained Fe3O4–CNTs–HPCFs absorbents,XRD measurements were carried out,and the patterns are presented in Fig.3a.For the HPCFs sample,there are two obvious broad peaks observed at25.0°and44.0°,corre-sponding to the(002)and(110)planes of graphite structure of HPCFs.After growth of CNTs on the substrate HPCFs,it can be found that(110)reflection become stronger which demonstrates that the structure order degree of the CNTs–HPCFs composites have increased significantly than that of HPCFs bined with the SEM images,this result can be indirectly proved that CNTs have successfully grown on HPCFs.Simultaneously,a peak centered at52.00°is found in addition to the peaks at25.00°and44.00°,which can be cor-responded to the(200)plane of the face-centered cubic phase of nickel.The main reason for this result should be the pres-ence of nickel elements as a catalyst in the CNTs–HPCFs com-posites.After the load of the Fe3O4nanoparticles on the CNTs–HPCFs as‘‘fruit’’structures,eight major diffraction peaks observed for the Fe3O4–CNTs–HPCFs hybrids.The peak at25°is assigned to(002)planes of the HPCFs and CNTs. Diffraction peaks at30.00°,35.45°,43.10°,53.44°,57.04°,
illustration of Fe3O4–CNTs–HPCFs composites formation.(A color version of thisfigure
62.58°and74.25°represent the Bragg reflection from(220), (311),(400),(422),(511),(440)and(533)planes of the cubic spinel crystal structure of magnetite(JCPDS card No.19-0629).The XRD results predominantly indicate the Fe3O4 nanoparticles as‘‘fruit’’structures were successfully attached on the CNTs–HPCFs.Fig.3b shows the XPS spectra of Fe3O4–CNTs–HPCFs composites,and Fig.3c shows the high-resolu-tion XPS spectra of Fe2p.It is clear that in the as-prepared absorbents,some Fe3O4exists as Fe2p and O1s are observed from Fig.3b.T wo distinct peaks at710.6and724.1eV appear in Fig.3c.The former is attributed to Fe2p3/2and the latter to Fe2p1/2.The above results confirm the information of Fe3O4in the composites.Furthermore,the content fraction of C,O and Fe elements in Fe3O4–CNTs–HPCFs absorbents were obtained from XPS measurement(C1s:O1s:Fe 2p=58.38%:34.94%:6.68%),which means the weight fraction of Fe3O4in Fe3O4–CNTs–HPCFs composites is31.62%.The nanosized Fe3O4particles endow the Fe3O4–CNTs–HPCFs composites with a superparamagnetic property and a satu-rated magnetization up to17.123emu gÀ1(Fig.3d)at room temperature which helps the as-prepared absorbent with ‘‘tree-like’’structures have the performance of magnetic energy loss.
3.2.Permittivity and permeability characterizations of the three absorbents
In terms of features of EM wave absorbers,the Fe3O4–CNTs–HPCFs composites should exhibit outstanding microwave absorbing properties,especially the properties of dielectric loss and magnetic loss.The coaxial ring specimens for elec-tromagnetic parameters measurements are shown in Fig.4a.From which it can be seen that the sample with an outer diameter of7.0mm,inner diameter of3.0mm,and homogeneous composition.Fig.4b shows the sectional view of the coaxial ring sample,which apparently showed that plenty of Fe3O4–CNTs–HPCFs absorbents were evenly wrapped in paraffin wax.
The frequency dependence of relative complex permittiv-ity and relative complex permeability,used for characteriza-tion of dielectric loss and magnetic loss properties of absorbents,for the product/paraffin composites is shown in
image of HPCFs sample and(a2)is magnified image of the surface of HPCFs.(a3)and(a4)SEM image sample.(b1)and(b2)SEM images of CNTs–HPCFs.(c1)TEM image of the Fe3O4–CNTs sample.(c2)HRTEM –CNTs–HPCFs sample showing interlayer spacing(red lines)and enlarged view of lattices(encircled by ).(c3)is the electron diffraction pattern of Fe3O4.(A color version of thisfigure can be viewed
Fig.5.From Fig.5a,the real part(e0)of complex permittivity for HPCFs declines from13.70to9.45with a relatively gentle incline in the range2.0–9.67GHz and some minorfluctuations in the range9.67–18GHz.For CNTs–HPCFs,e0decreases from 11.7to6.35with the increase of frequency and exhibits a broad peak at around15GHz.e0for Fe3O4–CNTs–HPCFs shows only a gentle curve in the range2–18GHz with the value at about6.0.In addition,comparing the e0curves,it can be observed that the growth of CNTs on the substrate HPCFs can increase the value of e0.Instead,the load of Fe3O4nano-particles can reduce the e0of the composites.The main rea-sons for these phenomena may be due to CNTs having excellent dielectric properties which Fe3O4almost no.The imaginary part(e00)curves of the complex permittivity are shown in Fig.5b.e00for HPCFs shows only some minorfluctu-ations in the range2–11GHz.For CNTs–HPCFs,e00rises slightly from3.78to4.74with the increase of frequency,followed by variousfluctuations up to18GHz.The e00curve of Fe3O4–CNTs–HPCFs exhibits a similar behavior to the e0curve,except for the value of e00at around 2.60.And comparing the e00 curves,the same phenomena with e0curves have been found, except for the e00for HPCFs slightly lower than the e00for CNTs–HPCFs in the range of6.0–11.2GHz.
As commonly known,in the relative complex permeabil-ity,the real part(l0)and imaginary part(l00)represent the stor-age and loss capability for EM wave energy,respectively.The frequency dependence of l0and l00for the product/paraffin composites is shown in Fig.5c and d,respectively.It is
1000020000
2H
3
O4–CNTs–HPCFs.
composites,insert showing
thisfigure can be
coaxial ring samples,(b)SEM image of the coaxial ring samples showing circles)were wrapped in paraffin wax.(A color version of thisfigure can be viewed
observed that the real part of the permeability (l 0)of the three prepared samples have almost the same values (Fig.5c),which reflect the ability of magnetic energy storage of the three absorbents is about the same.Nevertheless Fig.5d dis-plays that for Fe 3O 4–CNTs–HPCFs composites,the values of l 00are slightly larger than other samples in the range of 2–11.3GHz.From this result,it can be concluded that Fe 3O 4nanoparticles anchored on CNTs–HPCFs carbonaceous composites contribute to the improving of the magnetic loss performance of the prepared absorbent.
3.3.Microwave absorbing properties of the three
absorbents
According to transmission line theory,the EM wave absorbing performances of the composites were calculated [26–27].The equations are as follows,
Z in ¼Z 0ðl r =e r Þ1=2tan h ½j ð2p fd =c Þðl r e r Þ1=2 ð1ÞR L ðdB Þ¼20log Z in ÀZ 0in 0
ð2Þ
where Z in is the input impedance of the absorbent,Z 0is the
impedance of air,l r and e r are,respectively,the relative com-plex permeability and permittivity,f is the frequency of the electromagnetic waves,d is the thickness of the absorbent and c is the velocity of electromagnetic waves in free space,R L (dB)is the reflection loss of the absorbent.
Fig.6shows the relationship between reflection loss and frequency in different thickness for the obtained sample/paraffin composites.The minimum reflection loss reaches
À26.7dB with a coating thickness of 2.0mm for HPCFs at 14.6GHz,À38.4dB with a thickness of 3.0mm for CNTs–HPCFs at 7.3GHz,and À50.9dB with a thickness of 2.5mm for Fe 3O 4–CNTs–HPCFs at 14.03GHz.It can be drawn from the above results that the growth of CNTs as ‘‘branch’’struc-tures and the load of Fe 3O 4nanoparticles as ‘‘fruit’’structures contribute the minimum reflection loss of the obtained absor-bent with ‘‘tree-like’’structure superior to other samples,demonstrating that the prepared Fe 3O 4–CNTs–HPCFs compos-ites possess the better EM wave absorbing performance.Reflection loss values of less than À15dB (97%absorption)were achieved in the range 10.2–18GHz with thicknesses of 1.5–3.0mm for Fe 3O 4–CNTs–HPCFs.However,the correspond-ing absorbing bandwidth is 6.5GHz for CNTs–HPCFs and 6.05GHz for HPCFs,which also means the self-prepared absorbent with ‘‘tree-like’’structure,has better absorption bandwidth 7.8GHz with a reflection loss below À15dB.It is worth nothing that the absorbing bandwidth of Fe 3O 4–CNTs–HPCFs composites is significantly broader than those reported in the literature,i.e.,MWCNT/Fe 3O 4(about 5.8GHz)[28],PANI/Fe 3O 4/MWCNT (around 5.5GHz)[29]porous carbon fibers (about 2.2GHz)[30].A comparison of the reflection loss curves for the three samples at the thickness of 2.5mm (Fig.6d)indicates that both the absorbing intensity and the bandwidth of lower than À15dB for the Fe 3O 4–CNTs–HPCFs composites improve significantly with the growth of CNTs and load of Fe 3O 4nanoparticles.Therefore,the Fe 3O 4–CNTs–HPCFs ‘‘tree-like’’structure composites prepared in this work exhibit an excellent electromagnetic wave attenuation perfor-mance,including high absorption efficiency,strong absorp-tion,and a wide operation frequency bandwidth.
3.4.Microwave absorbing mechanism of the Fe3O4–CNTs–HPCFs composites
In order to further understand the roles of Fe3O4and CNTs on the performance of Fe3O4–CNTs–HPCFs absorbent,the com-parison of microwave reflection losses of each sample at the thickness of2.5mm is shown in Fig.7.It can be demonstrated from Fig.7a that the minimum reflection loss reaches À28.1dB at8.9GHz with a thickness of2.5mm for CNTs–HPCFs,À26.1dB at11.2GHz for Mixture of CNTs and HPCFs,À21.5dB at12.7GHz for HPCFs andÀ17.6dB at10.4GHz for CNTs.The result suggests that the growth of CNTs on the substrate HPCFs can help to improve the microwave absorp-tion abilities of the absorbents and synergetic interactions exist between HPCFs and CNTs which lead to some extent improved microwave absorption.The Fe3O4exhibited little microwave absorption(Fig.7b),similar to a previous report [28].When mixing the Fe3O4nanoparticles and CNTs–HPCFs composites together,there was an obvious reduction in the ability of the microwave absorption compared to the CNTs–HPCFs;the best reflection loss value achieved being À14.35dB at10.0GHz.A substantial increase to the micro-wave absorption abilities was noted after the CNTs–HPCFs was decorated by high-density Fe3O4nanoparticles.Minimum reflection losses reached as low asÀ50.9dB at14.03GHz, which was comparable to the almost100%of EM wave absorption.And the absorption bandwidth with a reflection loss belowÀ15dB is up to2.98GHz(from12.59to15.57GHz)
26C A R B O N81(2015)20–28
for the Fe3O4–CNTs–HPCFs absorbent with the thickness in 2.5mm.However,the corresponding absorbing bandwidth is 1.68GHz(8.42–10.10GHz)for the CNTs–HPCFs carbonaceous composites and0GHz for the Fe3O4and the mixture of CNTs–HPCFs and Fe3O4.The main reasons for these phenom-ena may be due to synergetic interactions exist between CNTs–HPCFs and Fe3O4,that is,the synergetic interactions exist between the dielectric loss and magnetic loss lead to the greatly improvement of the absorbing properties of the composites.
As mentioned above,the microwave absorbing properties of the Fe3O4–CNTs–HPCFs absorbent with‘‘tree-like’’structure could be attributed to several reasons.Firstly,the presence of a high density of defects from pores and cracks on HPCFs are unstable and could be easily excited under an EM wavefield [31–33],which could be seen as polarized centers that are prone to space charge polarization by trapping space charges, thus contributing to enhancing microwave absorption.Mean-while,the presence of many pores in HPCFs could also pro-vide another additional pathway for the absorption of EM waves.The SEM image of the porous structure of HPCFs is illustrated in Fig.8a,which demonstrates a large number of pores presented in the HPCFs.The presence of many pores in HPCFs is equivalent to the occurrence of variety transmis-sion ways for microwaves at the same time,which increases the propagation paths of the microwaves in the absorbent,as shown in Fig.8b.Therefore,multiple reflections and refrac-tions of the incident EM waves among the porous structure will also consume some EM wave energy[34–35],also contrib-uting to the enhancement of EM wave absorption.Secondly, with the growth of CNTs on the HPCFs,the dangling bonds and lattice distortions in the CNTs,and the interfaces between CNTs and HPCFs,are also prone to space charge polarization by trapping space charges,thus helping to the wastage of EM waves.In addition,as discussed above,syner-getic interactions exist between HPCFs and CNTs which lead to some extent improved microwave absorption.Thirdly,the synergetic interactions exist between Fe3O4and CNTs–HPCFs which also contributes to the enhancement of microwave absorption.In conclusion,the excellent microwave absorbing properties of Fe3O4–CNTs–HPCFs absorbent not only due to dielectric polarizations caused by the defects,porous struc-tures,dangling bonds of CNTs,but also attributed to the syn-ergetic interactions exist between Fe3O4and CNTs–HPCFs, that is,the synergetic interactions exist between the dielectric loss and magnetic loss lead to the greatly improve-ment of the absorbing properties of the composites.
4.Conclusion
The Fe3O4–CNTs–HPCFs absorbents with‘‘tree-like’’structure, which were composed of HPCFs act as‘‘trunk’’structures, CNTs play the role of‘‘branch’’structures and Fe3O4as‘‘fruit’’structures,exhibit excellent electromagnetic absorbing abili-ties.The bandwidth with a reflection loss less thanÀ15dB covers a wide frequency range from10.2to18GHz with the thickness of1.5–3.0mm,and the minimum reflection loss is À50.9dB at14.03GHz with a2.5mm thick sample layer.The dielectric polarization arising from the defects,porous struc-tures,dangling bonds of CNTs and the synergetic interactions exist between Fe3O4and CNTs–HPCFs contribute to the out-standing EM wave absorbing performances.And related to the excellent properties of physical,chemical and mechanical of HPCFs,CNTs and Fe3O4nanoparticles,the Fe3O4–CNTs–HPCFs composites with lightweight are promising for utiliza-tion in aeroplanes and spacecraft.
Acknowledgements
This work was supported by the National Natural Science Foundation of China(Grant no.51173133),the Key Basic Research Project of Shanghai Municipal Science and Technol-ogy Commission(13JC1405300).We also thank Dr.Yida Deng in School of Materials Science&Engineering,Shanghai Jiaotong University for the help of microwave absorption measurements.
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