Jiang 2015

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RSC Advances

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This article can be cited before page numbers have been issued, to do this please use: L. Jiang, Z. Wang,

D. Geng, Y. Wang, J. An, J. He, D. Li, W. Liu and Z. Zhang, RSC Adv., 2015, DOI: 10.1039/C5RA06212H.

http://dx.doi.org/10.1039/c5ra06212h

http://pubs.rsc.org/en/journals/journal/RA

http://crossmark.crossref.org/dialog/?doi=10.1039/C5RA06212H&domain=pdf&date_stamp=2015-04-15

aShenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang

110016, P. R. China. E-mail: [email protected]; Tel: +86-24-83978846

bDivision of Functional Material Research, Central Iron & Steel Research Institute, Beijing 100081, P. R. China

Excellent microwave-absorption performances by matched

magnetic-dielectric properties in double-shelled Co/C/polyaniline

nanocapsules

Linwen Jiang,a Zhenhua Wang,

*a Da Li,

a Dianyu Geng,

a Yu Wang,

b Jing An,

b Jun He,

b Wei

Liua and Zhidong Zhang

a

Abstract

The double-shelled Co/C/polyaniline (Co/C/PA) nanocomposites were prepared by

combing the arc-discharge process and in-situ chemical oxidative polymerization reaction.

The effects of PA shells on the magnetic properties of Co/C/PA nanocomposites were studied,

and the electromagnetic properties of Co/C/PA-paraffin composites were investigated in the

2-18 GHz frequency range. The reflection loss (RL) exceeding -10 dB is obtained in 9.9–16.4

GHz for the absorber thickness of 2.5 mm, which cover almost half of X-band (8-12 GHz)

and most of Ku-band (12-18 GHz). Moreover, some strong absorption peaks exceeding -40

dB can be observed in low frequency range (3.5-5.5 GHz). The excellent absorbing

performances show that the Co/C/PA nanocomposites have great prospects for application in

microwave-absorption field for their strong absorption and broad bandwidth.

1. Introduction

Microwave-absorption materials have many promising applications in military,

electronic devices, and communication instruments. Exploiting a type of

microwave-absorption material with strong absorption in a wide frequency range has been

attracting an increased attention.1-5

Currently, the research of nanocomposites, composed of a

magnetic core (Ni, FeNi, Co, etc.) and a dielectric shell (C, ZnO, polyaniline, etc.) in the

nanometer size, is very active in the microwave-absorption field due to their excellent

microwave-absorption performances. Some core-shell structured nanocomposites, including

Page 1 of 21 RSC Advances

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View Article OnlineDOI: 10.1039/C5RA06212H


Ni/C,6 FeNi/C,

7 Co/C,

8 CoNi/C,

9 Zn(Fe)/ZnO,

10 Ni/polyaniline,

11 are believed to be attractive

candidates for the new types of microwave-absorption materials. The excellent

microwave-absorption performances of nanocomposites were reported to be associated with

the special core-shell structure. This core-shell structure can induce beneficial physical

effects on microwave absorption at an applied electromagnetic field, including reflecting and

scattering inside materials,12-15

cooperative effects of dielectric properties and magnetic

properties,6,16

and multi-polarizations at core-shell interfaces.8,17

Recent interest has been expanded to the multi-shelled nanocomposites. It is expected

that the microwave-absorption properties of nanocomposites may obtain a great breakthrough

by adding more shells of different types due to the stronger physical effects of multi-shelled

structure. Some multi-shelled nanocomposites have been reported, such as double-shelled

FeCo/C/BaTiO3 nanocomposites,16

graphene/Fe3O4/SiO2/SnO2 nanocomposites.18

and Fe3O4

cores/double-shelled SnO2 nanocomposites,19

The research on multi-shelled nanocomposites

is becoming more and more active in the microwave-absorption field.

The choice of material types of cores and shells is very important for the enhancement

of microwave-absorption properties. The combining of magnetic-loss-type and

dielectric-loss-type materials may generate a remarkable microwave-absorption performance

due to the balance of electromagnetic properties. Polyaline (PA) with good dielectric

properties has been studied on the microwave-absorption properties,20

and the single-shelled

Co/C nanocomposites with large saturation magnetization also have been investigated.8 In

this work, the unique double-shelled Co/C/PA nanocomposites were prepared by combining

the arc-discharge method and in-situ chemical oxidative polymerization method. The effects

of PA shells on the magnetic properties of Co/C/PA nanocomposites were investigated. The

electromagnetic properties of Co/C/PA-paraffin composites were investigated in the 2-18

GHz frequency range, and the excellent microwave-absorption performances were explained

by the proper magnetic-dielectric match set up by the special double-shelled structure.

2. Experimental section

2.1 Material Synthesis

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The single-shelled Co/C nanocomposites were prepared by an arc-discharge technique.

The detailed process is described as below: Co (99.98%) ingot was served as anode, and the

graphite needle was served as cathode. High purity argon was introduced into an evacuated

chamber (10 Pa) to serve as the source of plasma, and 20 mL C2H5OH was also introduced

into the evacuated chamber as the C resource. At the end of the arc-discharge process,

residual gases were pumped out. After being passivated in air for 24 h, the Co/C

nanocomposites were collected on the top of the chamber.

Subsequently, the double-shelled Co/C/PA nanocomposites were synthesized by the

in-situ chemical oxidative polymerization method. A typical preparation process is described

as below: the HCl aqueous solution was added to aniline monomer to obtain HCl-protonated

aniline. The Co/C nanocomposites of calculated amount were added to C6H8O7 solution

under ultrasonication for 30 min to obtain a uniform suspension. The well-dispersed Co/C

nanocomposites were mixed with the prepared HCl-protonated aniline monomer under

ultrasonication for 30 min. Then the APS (Ammonium persulphate, (NH4)2S2O8) was added

to the above solution as oxidant for polymerization reaction under vigorous mechanical

stirring. In this experiment, the molar ratio of aniline monomer, HCl and APS was retained at

1:1:1. The precipitated powders were centrifugated and washed with distilled water

repeatedly for five times, and finally dried in a drying cabinet at 40 °C for 48 h.

2.2 Preparation of samples for the measurements of electromagnetic properties

The double-shelled Co/C/PA nanocomposites were mixed with paraffin uniformly and

then be pressed into a cylindrical-shaped compact of 7.00 mm outer diameter, 3.04 mm inner

diameter, and an approximate height of 2.00 mm for the electromagnetic parameter

measurements, wherein paraffin not only can agglomerate samples, but also can provide a

good passageway of microwave absorption due to its transparent properties. The mass

fraction of samples in paraffin matrix was set at 50 wt. %.

2.3 Material characterization

Phase analysis of the products was performed by powder X-ray diffraction (XRD) with

Cu Kα radiation at 50 kV and 300 mA. The scattering range (2θ) was set in 20o–85

o with a


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scan rate of 4o/min

-1. The size distribution, morphology and structures were investigated by

using a transmission electron microscope (TEM, JEOL 2010EX). The surface information of

prepared samples was determined by X-ray photoelectron spectroscopy (XPS), with an Al Kα

line X-ray source. Magnetic properties of samples were measured using a vibrating sample

magnetometer (VSM). The variety of weight and thermal values of samples were investigated

by thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) at ambient

pressure from 50oC to 800

oC with a heating rate of 4

oC/min. The electromagnetic parameters

of samples were determined in the frequency range of 2 to 18 GHz using an Agilent 8722ES

network vector analyzer with a transverse electromagnetic mode.

3. Results and discussion

In the arc-discharge process, Co atoms in the Co ingot and C atoms in C2H5OH

molecules obtained energy and escaped to the vacuum space by the sputtering of high-energy

Ar ions. The highly active Co and C atoms reassembled rapidly and nucleated through the

rapid energy exchange. After a series of thermodynamic process, the surfaces of Co were

covered by the light mass C. The formed Co/C nanocomposites with high speed were

stochastically diffused around the region of the plasma to all direction. When they entered

into a cool area, the speed of Co/C nanocomposites slowed down and finally deposited on the

wall of chamber. A schematic diagram of preparing Co/C nanocomposites is shown in Fig. 1a.

Covering of PA over the Co/C nanocomposites was carried out using in-situ chemical

oxidative polymerization reaction. The HCl as dopant was added into the aniline monomers

in order to obtain HCl-protonated aniline. During the polymerization reaction, the Co/C

nanocomposites were served as efficient nucleation centers of polymerization. The aniline

monomers were polymerized around the Co/C nanocomposites by the addition of APS

aqueous solution under vigorous mechanical stirring.21,22

Here, the APS as oxidant has

extremely strong oxidization ability which can participate in oxidation-reduction reaction

even at a very low concentration of aqueous solution. After the polymerization reaction, the

formed chained PA polymer covered over Co/C nanocomposites. A schematic diagram of

preparing Co/C/PA nanocomposites is shown in Fig. 1b.


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Fig. 1 Schematic diagrams of preparing (a) Co/C nanocomposites and (b) Co/C/PA

nanocomposites.

Fig. 2a shows TEM images of Co/C nanocomposites, which reveal clearly that they are

spherical in shape with diameters from 10 to 20 nm. The C shells are covered well over the

Co cores, and the shell thickness is about 3 nm, as shown in Fig. 2b. After the in-situ

oxidative polymerization reaction, the surfaces of Co/C nanocomposites were covered by PA.

Fig. 2c shows TEM images of as-prepared Co/C/PA nanocomposites. The shell thicknesses of

the Co/C/PA nanocomposites have an obvious rise as a result of cover of PA shells, as shown

in Fig. 2d. The inset in Fig. 2d shows that the Co/C/PA nanocomposites present

core-shell-shell structure. Fig. 2e depicts XRD patterns of as-prepared Co/C/PA, Co/C, C, PA.

Here, the C and PA were also prepared via arc-discharge method and in-situ oxidative

polymerization method, respectively. It can be observed that the strong sharp diffraction

peaks at 2θ = 44.2°, 51.5°, and 75.9° in the XRD patterns of Co/C/PA are in good accord with

the (111), (200) and (220) planes of Co PDF [04-0836], respectively. The XRD patterns of

as-prepared C and PA reveal that both of them have crystalline structure. The broad

bread-shaped diffraction peak at 2θ≈25° in the XRD pattern of C corresponds to the (0 0 2)

lattice plane of a graphite phase. The broad peak at 2θ≈26° in the XRD pattern of PA


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corresponds to the repetition units of regular aniline monomers.23

However, the C and PA

components have not been detected in the XRD patterns of Co/C/PA, which may be due to

their low contents. Moreover, since the C and PA are located in the shells of the

nanocomposites, it is also difficult to detect their XRD patterns because of breaking down of

the periodic boundary condition (translation symmetry) along radial direction.7 The average

size of Co cores is calculated from the broadening of the (111) diffraction peak by the

well-known Scherrer equation as follows.24

D = θβ

λ

cos

89.0

where D is crystallite size in nm, λ is the radiation wave length (0.15405 nm for Cu Kα), β is

the corrected full width at half maximum, and θ is the diffraction angle. The average size of

the Co cores is calculated to be 18 nm, which is close with that of Co/C/PA nanocomposites.

Fig. 2 (a) TEM and (b) high-resolution TEM images of prepared Co/C nanocomposites, (c)

TEM and (d) high-resolution TEM of Co/C/PA nanocomposites, (e) XRD patterns of

Co/C/PA, Co/C, C, and PA.

XPS is a very effective and convenient technique to investigate the surface states of


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materials, which not only can present information about the atomic compositions, but also

can identify the type of bonds between atoms.25

Here, XPS was employed to identify the

surface information of the prepared samples. Fig. 3a displays the XPS survey spectrum of

Co/C/PA, Co/C and PA. Two strong peaks centered at 778.7 eV and 284.6 eV, corresponding

to the Co and C, can be observed from the XPS survey spectra of Co/C. The quantitative

results from XPS analysis reveal that the atomic percentages of Co and C in the surface are

11% and 89%, respectively. The major presence (89%) of C atoms in the surface shows that

the C atoms covered over the Co nanoparticles, which is accordance with the results of TEM

image. Three strong peaks centered at 283.9 eV, 398.5 eV and 530.6 eV, corresponding to the

C, N and O, are observed from the XPS survey spectra of Co/C/PA. The appearance of N

peaks confirms the formation of PA in the surface of the samples. The high-resolution C (1s)

core level spectra is displayed in Fig. 3b. The spectra are made of two components that can be

identified as follows: (1) a main peak at 284.6 eV (generated by photoelectrons emitted from

sp2-bond carbon atoms in graphite configuration); (2) a second peak centered at 285.4 eV

(from the photoelectron contribution of the sp3-bond carbon atoms).

26 Fig. 3c represents the

N (1s) core level spectra which can be de-convoluted into four peaks. The binding energy

centered at 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

+) in PA,

respectively.27

Furthermore, the positively charged nitrogen (=NH+) is associated with the

cationic nitrogen atoms. The peak at binding energy of 400.8 eV assigned to protonated

amine units (-NH+) is ascribed to the stronger electron localization associated with poor

conjugation at sp3 bonded sites.

28 A relatively weak peak centered at 776.8 eV, corresponding

to Co (2p), is observed in the XPS survey spectra of Co/C/PA. The quantitative results from

XPS analysis reveal that the atomic percentage of Co is 0.9%, far lower than 11% of Co in

Co/C. As a result of the high surface sensitivity (less than 10 nm in depth) of XPS, the

decrease of Co content in the surface shows that the shell thickness of Co nanoparticles has

been increased. This indicates again that the PA has covered over the nanoparticles. Fig. 3d

shows the C (1s) core level spectra in Co/C/PA. The C (1s) peak could be de-convoluted into

four peaks with binding energies centered at 283.9 eV, 284.7 eV, 285.9 and 287.4 eV

corresponding to C-H, C-C, C-N and C=C species, respectively.27

One can observe that O


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atoms exist in the surface of Co/C/PA nanocomposites. The presence of the O atoms may be

ascribed to the remnants of reactants with O-element in PA during the in-situ chemical

polymerization. To prove it, the XPS spectra of single-component PA were investigated, as

shown in Fig. 3a. It can be observed that the similar O-peak also exists in the survey spectra,

and the peak intensity is close with that of Co/C/PA. Therefore, it is concluded that the

presence of O atoms originates from the remnants of reactants with O-element.

Fig. 3 (a) XPS survey spectra of synthesized Co/C/PA, Co/C and PA, (b) deconvolution

spectra of C (1s) of Co/C, deconvolution spectra of (c) N (1s) and (d) C (1s) of Co/C/PA.

The TGA-DTA plots of Co/C and Co/C/PA nanocomposites are shown in Fig. 4. The

TGA-DTA plots of Co/C nanocomposites display three stages: in the first stage (50–240 oC),

the weight of Co/C nanocomposites nearly has no variety. In general, the metal nanoparticles

are easy to transform spontaneously into metal oxides in room temperature as the size

decreases from micron to nanometer, thus have an obvious weight increase. Here, the

unchanged weight shows that the Co/C noanocapsules have not been oxidized even at 240 oC


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since protective C shells prevent the Co cores from oxidation. The result indicates that the Co

cores are well coated by C shells, which is in good agreement with the TEM results. The

second stage (240–270 oC) may be associated with oxidative behaviors of the Co cores and C

shells, which is similar with that of Ni/C nanocomposites (250–280 oC).

29 The strong

exothermal peak at about 250 o

C can be attributed to the strong burning of C shells. The Co

cores are oxidized quickly after the burning of protective C shells, thus an obvious weight

increase is observed in the TGA curve. In the third stage (270–800 oC), the weight and

thermal values only have slight change as a result of much weaker oxidative behaviors. It can

be observed that a strong exothermal peak of Co/C/PA nanocomposites locates at about 310

oC. This temperature is higher than that (250

oC) of Co/C nanocomposites, which can be

ascribed to the higher burning temperature of PA shells. The inner C shells have not been

burned before the outer PA shells start burning. The weight increase of Co/C/PA

nanocomposites is slow as the temperature increases, which is an integrated result of the

weight decrease induced by the burning of C/PA and the weight increase induced by the

oxidation of Co cores.

Fig. 4 TGA-DTA plots of Co/C nanocomposites and Co/C/PA nanocomposites.

The magnetic properties of both Co/C and Co/C/PA nanocomposites are measured in order

to investigate the effects of PA shells on the magnetic properties. The M-H curves at 300 K of

Co/C/PA are shown in Fig. 5a, wherein the saturate magnetization (Ms), remanent


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magnetization (Mr), and coercive field (Hc) are 97 emu/g, 15 emu/g, and 280 Oe, respectively.

The Ms is relatively lower than that of Co/C (110 emu/g), and is only about 58% that of bulk

Co. The relatively lower Ms in Co/C/PA may be due to the following reasons: (1) the

magnetic properties are strongly dependent on the particle size as well as the crystalline

structure and the chemical bonding at the surface. The magnetic properties may be affected

by a combination of magnetization mechanisms related closely to several anisotropies. The

coating of PA on the Co/C nanocomposites will likely affect the contributions of surface,

shape, and interface anisotropies to the net anisotropy.30

(2) The decrease of content of

magnetic matters (Co) weakens the magnetic properties of total materials as a result of the

introduction of PA. It is noteworthy that the additive PA has diamagnetic properties, as shown

in the inset 2 of Fig. 5. However, the diamagnetic properties of PA are quite smaller, only

reaching -0.11 emu/g at an applied field of 20 kOe, thus negligible.

Fig. 5 M-H curves of Co/C/PA and Co/C nanocomposites at 300 K. Inset 1: low field part of

M-H curves. Inset 2: M-H curve of PA at 300 K. Inset 3: the parameters of magnetic

properties at 300 K.

Fig. 6a shows the frequency dependence of the real parts (ε′) and imaginary parts (ε′′) of

relative permittivity of the Co/C/PA-paraffin composites. It can be found that the values of ε′

and ε′′ declined from 8 to 5 and from 3.5 to 1, respectively, with a slight fluctuation in 2–18

GHz. The large ε′′ values are attributed to the lags of polarization between the core/shell/shell

interfaces, as shown in Ni/C,6 FeCo/C/BaTiO3,

16 FeCo/C.

31 The ε′ is mainly associated with

the amount of polarization occurring in the material, and ε′′ is related to the dissipation of


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energy. The performance of relative permittivity depends on ionic, electronic, orientational,

and space charge polarization.32

Of these polarization mechanisms, space charge polarization

(interfacial polarization) is more frequency-dependent. The observed decrease in ε′ and ε′′

with increasing frequency may be attributed to the decrease in interfacial polarization.33

In a

heterogeneous system, the accumulation of space charges at the interface of two media will

lead to interfacial polarization and is known as Maxwell–Wagner polarization.34

Here, plenty

of heterogeneous interfaces in double-shelled Co/C/PA nanocomposites contribute to the

interfacial polarization.

Conventionally, the permittivity can be represented by the Debye dipolar relaxation

expression,11

�� = �� +�� − ��

1 + 2�� = �� + ��,

where f is the microwave frequency, τ is the relaxation time, εs and ε∞ is the stationary and

optical dielectric constant, respectively. From the equation, the results below can be deduced:

�� = �� +�� − ��

1 + �2�� ,

�� =2�� − ��

1 + �2�� .

It can be found that the ε′ is a function of ε′′/f, viz. �� = �� +��

��. Thus, if the

dielectric loss is only a consequence of Debye dipolar relaxation, the plot of ε′(f) versus

ε′′(f)/f would be linear. Fig. 6b shows the plots of ε′ versus ε′′/f of Co/C/PA-paraffin composite

samples. Here, it presents approximate four beelines, suggesting that four Debye dipolar

relaxation processes may occur in the Co/C/PA nanocomposites. Each beeline corresponds to

a Debye dipolar relaxation process.8 The four Debye dipolar relaxation processes may mainly

result from the electron displacement polarization, the interfacial polarizations at Co/C, C/PA

and PA/air interfaces, respectively. According to the different slopes of four beelines, we can

obtain the following results: τ1=7.8×10-11

s, τ2=2.6×10-10

s, τ3=6.2×10-10

s, τ4=1.5×10-9

s. In

general, the building time of electric relaxation process is about in the range of 10-2

-10-9

s.

Here, the relatively short time of relaxation processes may be attributed to the weak applied

field.

The real part (µ′) and the imaginary part (µ′′) of relative permeability of the


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Co/C/PA-paraffin composite as a function of frequency in 2–18 GHz are shown in Fig. 6c.

The µ′ values exhibit a nearly linear decrease from 1.4 to 0.8 in the 2–18 GHz frequency

range. Meanwhile, as the frequency is varied, µ′′ values exhibit three broad peaks at about 4.9

GHz, 9.1 GHz and 13.6 GHz, respectively. These multi-resonance peaks of µ′′ can be due to

the ‘exchange mode’ resonance, which may be associated with the small size effect, the

surface effect and spin wave excitations, etc.29

Zhang et al. reported the relation of µ′′ versus

frequency of single-shelled Co/C nanocomposites, wherein the resonance peaks present at

about 6 GHz and 12 GHz.8 The different resonance peaks of µ′′ of the Co/C nanocomposites

and Co/C/PA nanocomposites may be due to the additional PA shell. Moreover, the resonance

frequency is dependent on the particle’s radius, lattice defects and interior stress resulting

from the special double-shelled structure, which can bring a great increase in the effective

anisotropy field and further lead to the resonance frequency appearing at a different

frequency.30,35

The microwave-absorption performances of the Co/C/PA-paraffin composites

depend on their magnetic and dielectric loss. Fig. 6d shows the frequency dependence of the

dielectric loss factor tan (δε) (viz. ε′′/ε′) and magnetic loss factor tan (δµ) (viz. µ′′/µ′). The

dielectric loss factor tan(δε) exhibits the fluctuation behaviors from 0.24 to 0.45 in the 2–18

GHz range, which can be ascribed to the relaxation process.36,37

The magnetic loss factor

tan(δµ) varies from 0.06 to 0.24, and exhibits three broad peaks at about 5.0 GHz, 9.1 GHz

and 13.8 GHz, respectively. Magnetic loss factor tan (δµ) is a significant parameter for

magnetic loss. In general, the contributors to magnetic loss include magnetic hysteresis,

domain-wall displacement, and eddy current and natural resonance. Here, the natural

resonance loss may be the main contributor to the magnetic loss due to the weak applied

electromagnetic field.6 It is expected that the natural resonance will result in strong magnetic

loss abilities, implying enhanced microwave absorption during gigahertz frequency range.


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Fig. 6 The frequency dependence of (a) ε′ and ε′′ of the Co/C/PA-paraffin composites. (b) the

plots of ε′ versus ε′′/ f of Co/C/PA-paraffin composites. The frequency dependence of (c) µ′,

µ′′, and (d) tan (δε), tan (δµ) of the Co/C/PA-paraffin composites.

To further reveal the microwave-absorption properties, the reflection loss (RL) of

Co/C/PA-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:38,39

Zin=Z0 (µr/εr)1/2

tanh[ j(2πfd/c) (µrεr)1/2

],

RL=20log| (Zin-Z0)/(Zin+Z0) |,

where f is the frequency of microwave, d is the thickness of absorber, c is the velocity of light,

Z0 is the impedance of air, and Z is the input impedance of absorber.

Fig. 7a shows the relationship between the calculated RL values and the frequency for

the Co/C/PA-paraffin composites in 2–18 GHz. In general, the thickness of absorber,

absorption bandwidth, and absorption intensity are used to evaluate the absorbers in


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engineering applications.40

It can be seen that the Co/C/PA nanocomposites present excellent

absorption performances due to the high absorption intensity and wide absorption bandwidth

at different thicknesses. The RL exceeding -10 dB is obtained in the 9.9–16.4 GHz for a

given thickness of 2.5 mm, which cover almost half of X-band (8-12 GHz) and most of

Ku-band (12-18 GHz). The RL exceeding -10 dB corresponds to 90% attenuation of the

microwave absorption, which is believed to be the valuable bandwidth in engineering

applications.24

Some strong absorption peaks exceeding -40 dB are located in 3.5-5.5 GHz for

the given thicknesses range from 5 to 7 mm. The excellent absorbing performances show that

the Co/C/PA nanocomposites have great prospects for application in microwave absorption

for its strong absorption and broad bandwidth in low frequency range. The excellent

microwave-absorption performances can be explained by the following facts. Firstly, the

double-interfaces are advantageous for microwave absorption due to the multi-interfacial

polarizations.41

Secondly, the core-shell interfaces can provide more active sites for reflection

and scattering of microwave. The microwave will be reflected and absorbed repeatedly inside

the nanocomposites, and is hard to escape from the nanocomposites before being absorbed

and transferred to heat.15

Ling et al. reported that the microcellular foaming method can

enhance the microwave absorption since the interface of microscale air bubbles in the foams

could attenuate the microwave by reflecting and scattering between the cell wall and

nanofillers.14

To explain clearly the effects of core-shell interfaces on the microwave

absorption, a physical model of reflection and scattering of microwave in double-shelled

Co/C/PA nanocomposites is shown in Fig. 7b. Thirdly, the void space at the interfaces can

effectively interrupt the spread of electromagnetic wave and generate dissipation due to the

existing impendence difference and enhanced the microwave absorption properties.18


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Fig. 7 (a) Relationship between the calculated RL values and the frequency for the

Co/C/PA-paraffin composites in the 2–18 GHz frequency range. (b) Physical model of

reflection and scattering of microwave in double-shelled Co/C/PA nanocomposites.

The physical properties of materials are associated greatly with the constituent structures.

A good constituent structure may generate some beneficial physical effects on the

microwave-absorption performances of materials,42,43

e.g., multi-polarizations at interfaces,44

multi-reflections inside materials,15

cooperative effects of dielectric-magnetic properties.45

These beneficial effects may lead to the enhancement of microwave-absorption properties. In

the case of Co/C/PA nanocomposites, the excellent microwave-absorption performances may

be associated with the particular double-shelled structures. To prove the conclusion, the

microwave-absorption properties of the mixture of single-shelled Co/C and PA are

investigated. The electromagnetic parameters of the mixture of single-shelled Co/C and PA as

a dependence of frequency are shown in Fig. 8a. It can be found that the values of ε′ and ε′′

declined from 9.3 to 5.6 and from 7.0 to 2.4, respectively, in the 2-18 GHz frequency range.


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Meanwhile, the values of µ′ declined from 1.40 to 0.87, and the values of µ′′ exhibit an

approximate constant (0.23) in 2-9 GHz and then decline from 0.23-0.11 in 9-18 GHz. One

can observe that the permeability of the mixture of single-shelled Co/C and PA is very close

with that of double-shelled Co/C/PA, while the permittivity of the mixture of single-shelled

Co/C and PA is higher than that of double-shelled Co/C/PA. Herein, the higher permittivity

probably results from the following reason: the Co/C and PA are separated from each other in

the mixture of single-shelled Co/C and PA, while Co/C and PA are connected tightly by

core-shell interfaces in double-shelled Co/C/PA nanocomposites. In other words, the

conductive matters (include Co/C and PA) in the mixture has a more dispersed state in

comparison with those in double-shelled Co/C/PA nanocomposites, resulting in the formation

of a qualitatively more conductive network.

Fig. 8 (a) Electromagnetic parameters and (b) RL as a dependence of frequency in 2–18 GHz

for the mixture of single-shelled Co/C and PA.

The RL as a dependence of frequency of the mixture of single-shelled Co/C and PA is

shown in Fig. 8b. It can be observed that the maximum RL reaches -23.3 dB at 17.2 GHz

with 2 mm thickness layer. With increasing the thickness layer, the RL peak value exhibits a

slow decline, and the absorption peak shifts to lower frequency range. When the thickness

layer reaches a maximum 7 mm, the RL peak value is reduced to -12.8 dB. Obviously, the

absorption performances of the mixture of single-shelled Co/C and PA are much weaker than

those of double-shelled Co/C/PA nanocomposites, especially in low frequency range. The

results indicate that the double-shelled structure is more advantageous in obtaining good

microwave-absorption performances in contrast with the single-shelled structure. In general,


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higher permittivity usually leads to higher dielectric loss and obtains better

microwave-absorption properties. However, in this work, the mixture of single-shelled Co/C

and PA with higher permittivity exhibits weaker microwave-absorption properties, in

comparison with the double-shelled Co/C/PA nanocomposites. It may be attributed to the

mismatched dielectric-magnetic properties. The excellent microwave-absorptions result from

the efficient complementarities between the permittivity and permeability in materials. Either

only the magnetic loss or only the dielectric loss may induce a weak microwave-absorption

property due to the imbalance of the dielectric-magnetic match.6 Zhang et al. reported that the

excellent microwave-absorption performances of Fe/C nanocomposites result from a good

dielectric-magnetic match between the dielectric C shells and magnetic Fe cores by the

particular core-shelled microstructures.45

In the case of double-shelled Co/C/PA

nanocomposites, a similar dielectric-magnetic match also may be set up by the particular

double-shelled microstructures, resulting in the excellent microwave-absorption

performances. The core-shell-structural nanocomposites with dielectric shells and magnetic

cores may become attractive candidates in the microwave-absorption field for their special

magnetic-dielectric match.

4. Conclusions

The double-shelled Co/C/PA nanocomposites were prepared by combing the

arc-discharge process and the in-situ chemical oxidative polymerization reaction. The results

of XRD, TEM, XPS and TGA-DTA show that both the C and PA were covered over the Co

cores. The M-H curves at 300 K of Co/C/PA nanocomposites show that the saturate

magnetization (Ms), remanent magnetization (Mr), and coercive field (Hc) are 97 emu/g, 15

emu/g, and 280 Oe, respectively. The RL exceeding -10 dB is obtained in a wide frequency

range in the 2-7 mm thickness range. Moreover, some strong absorption peaks exceeding -40

dB can be observed in low frequency range (3.5-5.5 GHz). The results show that the Co/C/PA

nanocomposites have great prospects for application in microwave-absorption field for its

strong absorption and broad bandwidth. By the contrastive experiments, it indicates that the


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double-shelled structure is more advantageous in obtaining good microwave-absorption

performances in contrast with the single-shelled structure.

Acknowledgements

This research project has been supported by the National Basic Research Program

No.2012CB933103 of China, Ministry of Science and Technology China, and the National

Nature Science Foundation of China No.51102244, 51171185 and 51371055,

and International S&T Cooperation Program of China No. 2012DFA51300.

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