Microwave Absorption and Shielding Property of Composites with FeSiAl and Carbonous Materials as Fil
J.Mater.Sci.Technol.,2012,28(10),
913–919.
Microwave Absorption and Shielding Property of Composites with FeSiAl and Carbonous Materials as Filler
Wenqiang Zhang,Yonggang Xu,Liming Yuan,Jun Cai and Deyuan Zhang ?
Bionic and Micro/Nano/Bio Manufacturing Technology Research Center,School of Mechanical Engineering and Au-tomation,Beihang University,Beijing 100191,China
[Manuscript received January 10,2012,in revised form April 24,2012]
Silicone rubber composites ?lled with FeSiAl alloys and multi-walled carbon nanotubes (MWCNT)/graphite have been prepared for the ?rst time by a coating process.The complex permittivity and permeability of the composites were measured with a vector network analyzer in a 1–4GHz frequency range,and the DC electric conductivity was measured by a standard four-point contact method.These parameters were then used to calculate the re?ection loss (RL)and shielding e?ectiveness (SE)of the composites.The results showed that the added MWCNT increased the permittivity and permeability of composites in the L-band,while the added graphite increased only the permittivity.The variation lies in the interactions between two carbonous absorbents.Addition of 1wt%MWCNT enhanced the RL in the L-band (minimum ?5.7dB at 1mm,?7.3dB at 1.5mm),while the addition of graphite did not.Addition of MWCNT as well as graphite reinforced the shielding property of the composites (maximum SE 13.3dB at 1mm,18.3dB at 1.5mm)owing to the increase of conductivity.The addition of these carbonous materials could hold the promise of enforcing the absorption and shielding property of the absorbers.
KEY WORDS:Absorbing materials;Electromagnetic;Re?ection loss;Shielding e?ectiveness
1.Introduction
With the rapid advancement in applications of wireless communications and the widespread use of microwave devices over the 1–4GHz range,the emerging hazards of microwaves on human health and electrical equipment have garnered the interest of many researchers.Electromagnetic interference (EMI)shielding o?ers an e?ective means to solve the problem.The conventional shielding materials (mainly conductive composites)?xed on electronic de-vices may interfere with the electrical circuits and re-sult in work program turbulence or malfunction [1,2].So,fabrication of absorbing/shielding materials will e?ciently overcome the aforesaid problems.The in-cident microwaves undergo attenuation due to these materials and the re?ected microwaves become much
?Corresponding author.Prof.,Ph.D.;Tel./Fax:+861082316603;E-mail address:zhangdy@https://www.360docs.net/doc/5f13508262.html, (D.Y.Zhang).
weaker as compared with the incident waves re?ected from the traditional shielding materials.At present,the absorbent is key factor dominating their absorbing and shielding properties.It is widely agreed that to achieve the ideal absorption property,the absorbing composites should satisfy two important conditions [3]:(1)the intrinsic impedance of the absorbing materials is equal to the impedance of the free space (match-ing characteristic),(2)the incident electromagnetic wave must enter and get attenuated rapidly through the material layer (attenuation characteristic).More-over,the promising absorbers should be thinner,with a wider absorption band,lighter weight and higher absorption ratio,as compared with the traditional shielding materials.
FeSiAl alloys,being soft metallic magnetic materi-als,have been extensively used in absorbing materials due to the large values of their saturation magnetiza-tion and Snoek s limit at gigahertz (GHz)frequency ranges.With a ?aky shape,they have two types of
914
W.Q.Zhang et al.:J.Mater.Sci.Technol.,2012,28(10),
913–919.
Fig.1SEM/TEM images of morphology of:(a)MWCNT,(b)?aky FeSiAl,(c)graphite
magnetic anisotropy:easy-plane anisotropy and easy magnetization plane anisotropy,so they can easily ex-ceed the Snoek s limit of the spherical particles [4,5].The ?aky FeSiAl alloy with a thickness lower than its skin depth exhibits a higher complex permeabil-ity,and e?ectively suppresses the eddy current e?ect.Zhou s results [6]have shown that the re?ection loss (RL)of composites ?lled with ?aky FeSiAl is ?7.5dB at 1GHz,which is 4.6dB less than that of composites added with spherical particles of the same thickness i.e.,2.5mm.It has been demonstrated that FeSiAl is a suitable absorbent for use in the L-band (1-2GHz)and the S-band (2–4GHz)[7].
The carbonous materials,mainly including car-bon nanotube (CNT),carbon black (CB),carbon ?ber (CF)and graphite,have been studied as light-weight,and thermally and electrically conductive multifunctional materials and https://www.360docs.net/doc/5f13508262.html,T,in particular,was used to prepare complex ab-sorbers (CoFe 2O 4/CNT [8],Fe/CNT [9],etc .),which had excellent absorption due to their properties of large magnetic loss and dielectric loss.Simi-lar characteristics have also been found in hybrid graphite/metal absorbents [10].However,the shielding property is rarely reported in those hybrid materials,while complicated technological conditions,prepara-tion processes and rare production restrict the exten-sive use of these materials.Therefore,directly mixing the carbonous materials and FeSiAl as the absorbents could be an e?ective and low-cost approach to fabri-cate the required absorbing/shielding materials due to their dielectric/magnetic loss property.
This paper presents the use of the carbo-nous materials (multi-walled carbon nanotube (MW-CNT)/graphite)and FeSiAl composites as microwave absorbing/shielding materials for the ?rst time.The absorbents were added to a silicone rubber matrix and the electromagnetic (EM)parameters were measured in the frequency range of 1–4GHz.The main focus was the e?ects of the carbonous materials on the EM property of the composites.2.Experimental 2.1Materials preparation
Methyl vinyl silicone rubber was used as the ma-trix and 2,5-dimethylhexane was used as the vulcan-
ized assistant;both were supplied by Laizhou Jintai Silicon Industry Co.Ltd,China.The MWCNT of 10–30nm in diameter and 1–2μm in length were pur-chased from Anhui Gold Sun Nano Techonology Co.Ltd,China.The graphite was provided by Qingdao Shenshu Graphite Manufacturing Co.Ltd,China,with an average diameter of 5μm and thickness of 0.5μm.Flaky FeSiAl particles were prepared by a mechanical milling method using raw commercial spherical particles with average diameter of 75μm,supplied by Changsha Hualiu Power Co.Ltd,China.Pre-ground FeSiAl particles,steel balls and alcohol were introduced into a steel jar.The ratio of the masses of the steel balls,the FeSiAl particles and al-cohol was 16:1:4.The mechanical milling process was carried out in SP2planetary ball milling machine at 300r/min for 80h.The morphology of the three par-ticles is shown in Fig.1.
The silicone rubber and absorbents were mixed in a two-roll mixer for 15–30min.FeSiAl was added into the silicone rubber with a 50%volume content,and then the carbonous absorbent was added to the compound obtained.The content of MWCNT or graphite was 1wt%or 3wt%relative to the afore-said compound,respectively.The mixer would pro-vide a shearing force,which could overcome the in-termolecular van der Waals force between the parti-cles and disperse the absorbents uniformly in the rub-ber matrix [11,12].The absorbing/shielding composites were fabricated using a coating process that involved stacking several sheets with each of 0.1–0.2mm in thickness;?aky FeSiAl could be oriented in the thin ?lms caused by the aforesaid shearing force.The composites were then vulcanized into pieces of 2mm in thickness at 180?C for 5min under pressure of 10MPa.The testing samples for measurement of EM parameters were toroid-shaped with outer diameter of 7.0mm,inner diameter of 3.04mm,and thickness of 2mm.
2.2Sample testing
The morphology of the absorbents and fractured cross-section of the composites were observed by scan-ning electron microscopy (SEM,CamScan CS3400).The complex dielectric permittivity and magnetic per-
W.Q.Zhang et al.:J.Mater.Sci.Technol.,2012,28(10),913–919.
915
1.0 1.5
2.0 2.5
3.0 3.5
4.0
Frequency / GHz R e a l p a r t o f p e r m i t t i v i t y , '
1.0 1.5
2.0 2.5
3.0 3.5
4.0
I m a g i n a r y p a r t o f p e r m i t t i v i t y ,
''
Frequency / GHz
1.0
1.5
2.0 2.5
3.0 3.5
4.0
R e a l p a r t o f p e r m e a b i l i t y ,
'
Frequency / GHz 1.0 1.5 2.0 2.5 3.0 3.5 4.0
I m a g i n a r y p a r t o f p e r m e a b i l i t y ,
''
Frequency / GHz
Fig.2EM parameters of the composites as a function of frequency:(a)real part of permittivity,(b)imaginary
part of permittivity,(c)real part of permeability,(d)imaginary part of permeability
meability of the absorbers were measured using the transmission method with an AV3627vector network analyzer in the 1–4GHz frequency range.The DC electrical conductivity was measured on pressed rec-tangular of the composites by means of the standard four-point contact method.Then the RL could be calculated to evaluate the absorption property using the above EM parameters.For a single-layer absorb-ing material,the RL of a normal incident EM wave at the absorber surface can be de?ned as the ratio of re?ected power to incident power;the EMI shield-ing e?ectiveness (SE)can be de?ned as the ratio of transmitted power to incident power.They can be represented by the following equations [13]
Γ0=Z in ?Z 0Z in +Z 0
(1)Z in =
μr μ0εr ε0tanh(j 2πd λ
√
μr ·εr )(2)RL (dB)=20log |Γ0|
(3)SE (dB)=?20log (1?Γ20)T 1?T 2Γ20
(4)T =e
?j 2πd λ
√
μr ·εr
(5)
where Z in is the normalized input impedance of the
absorbing material,Z 0= μ0/ε0=120π?,is the in-trinsic impedance of free space.Γ0is the re?ection
coe?cient,while T is the transmission coe?cient.εr ,μr and ε0,μ0are complex permittivity and complex permeability of the absorbing material and free space,respectively.λis the wavelength of the microwave,and d is the thickness of the absorbing material.3.Results and Discussion
3.1EM parameters of the composites ?lled with Fe-SiAl and carbonous absorbents In absorbing composites,ε was mainly depen-dent on the conductivity of the composites,and ε was dependent on the dielectric loss tangent and ε value.From Table 1it can be observed that compos-ites ?lled with 3wt%MWCNT had the highest con-ductivity,while the conductivity of composites added with only FeSiAl was the lowest.Fig.2shows the
Table 1Conductivity (σ)characteristics of
each composite FeSiAl
0.77FeSiAl and 1wt%graphite 1.01FeSiAl and 3wt%graphite 1.25FeSiAl and 1wt%MWCNT 1.82FeSiAl and 3wt%MWCNT
2.50
916
W.Q.Zhang et al.:J.Mater.Sci.Technol.,2012,28(10),913–919.
1.0 1.5
2.0 2.5
3.0 3.5
4.0
Frequency / GHz D i e l e t r i c l o s s t a n g e n t , t a 1.0 1.5 2.0 2.5 3.0 3.5 4.0
M a g n e t i c l o s s t a n g e n t , t a n
Frequency / GHz
Fig.3Dielectric/magnetic loss tangents of the composites as a function of frequency
complex permittivity and permeability of the compos-ites ?lled with FeSiAl and carbonous materials (MW-CNT or graphite)as absorbents in the 1–4GHz fre-quency range.In Fig.2(a),the composites ?lled with FeSiAl/MWCNT or FeSiAl/graphite had a larger real part of permittivity (ε )than those ?lled with sin-gle FeSiAl particles (maximum 126.7at 1GHz,98.1at 4GHz),and ε was increased as more MWCNT or graphite was added.The composites with added MWCNT or graphite have a larger ε than composites added with only FeSiAl due to the better conductiv-ity of carbonous materials.Also the imaginary part of permittivity (ε )was enlarged as MWCNT was added (Fig.2(b)).The maximum value was 21.1at 1GHz,which is 19.4times larger than the composites added with only FeSiAl,while ε of composites added with graphite declined as more graphite was added.It could be attributed to the best dielectric loss of the MWCNT.The dielectric loss of graphite is lower be-cause the particles were easy arranged,which caused most of the microwave to be re?ected back.
In Fig.2(c)and (d),the real part of permeabil-ity (μ )was reduced slightly as MWCNT or graphite was added but the maximum decreased value was less than 1.It would be attributed to the carbonous ma-terials which do not have magnetic properties.The imaginary part of permeability (μ )was increased as MWCNT was added (maximum 6.73at 1GHz),while as graphite was added,μ didn’t change obviously.μ of composites ?lled with 1wt%MWCNT was nearly 0.8times larger than that ?lled with 3wt%MWCNT,and increasing the weight content of graphite did not in?uence μ of the composites signi?cantly.This may be caused by the coupling e?ect in the composite with 1wt%MWCNT and FeSiAl added.
Fig.3shows the e?ects of the added carbonous materials on the dielectric/magnetic loss tangent of the composite.Results show that the dielectric loss tangent (tan δe =ε /ε )increased as MWCNT was added but reduced as the graphite was added.The magnetic loss tangent (tan δμ=μ /μ )also increased as MWCNT was added.The maximum value was
3.2at 4GHz as 1wt%MWCNT was added,while tan δμremained nearly unchanged as 1wt%or 3wt%graphite was added (the changing magnitude was less than 0.5).The results of Fig.3were consistent with those of Fig.2.According to the reported electro-magnetic theory [15]:
tan δe =f tan δet +(1?f )tan δes (6)tan δμ=f tan δmt +(1?f )tan δms
(7)
where f is the volume content of the absorbents in the matrix,tan δe (ε /ε )is the dielectric loss tangent of the composites and tan δμ(μ /μ )is the magnetic loss tangent of the composites tan δes and tan δms ,tan δet and tan δmt denote the magnetic loss tangents of ab-sorbents and matrix,respectively.Considering the result of Micheli et al.[16],δe of MWCNT was larger than that of the composites added with FeSiAl and ε of the graphite was not.As a result,tan δe of the com-posites ?lled with MWCNT could be increased,and with increasing weight content of MWCNT,ε of the composites increased,in contrast to the composites added with graphite.Meanwhile,tan δμof MWCNT and graphite was less than that of FeSiAl particles,because the magnetic loss tangent of carbonous mate-rials was nearly 0[16].As more MWCNT or graphite was added,tan δμwould decrease according to the previous two equations.The results showed that the changing tendency of tan δe was consistent with the two equations,but the trend of tan δμwas not.The more the carbonous materials were added,the less the volume content of FeSiAl would be in the composites.So the added carbonous materials would lessen the permeability according to the Maxwell–Garnett mix-ing rule.Unfortunately,the added MWCNT enlarged the permeability.The interaction between FeSiAl and the MWCNT should be considered,which also in?u-enced the permeability of the composites.Similar re-sults could also be found in composites ?lled with car-bonyl iron and MWCNT [14].
Fig.4shows the microstructure of composites ?lled with FeSiAl,which was oriented by the coating
W.Q.Zhang et al.:J.Mater.Sci.Technol.,2012,28(10),913–919.
917
Fig.4Microstructure of composites ?lled with ?aky FeSiAl:(a)the longitudinal plane,(b)the horizontal plane
1.0 1.5
2.0 2.5
3.0 3.5
4.0
R e f l e c t i o n l o s s / d B
Frequency / GHz 1.0
1.5
2.0
2.5
3.0
3.5
4.0
Frequency / GHz
R e f l e c t i o n l o s s / d B
Fig.5RL of the absorbing composites as a function of frequency:(a)at thickness 1mm,(b)at thickness 1.5mm
process.The carbonous materials were dispersed in the rubber matrix and located among the FeSiAl par-ticles.So the conductive network of FeSiAl could be built up easily,and the domain wall motion and gy-romagnetic spin rotation could be enhanced,which made the magnetic loss tangent increase.A similar re-sult has also been reported in literature [14,17,18].The interfaces between the FeSiAl particles and the car-bonous materials played an important role in increas-ing the imaginary part of permeability;however,the di?erences of the enhancement mechanism between MWCNT and graphite on the magnetic loss of sili-cone rubber/FeSiAl composites still need further in-vestigations.
3.2Absorption property of the composites ?lled with
FeSiAl /carbonous absorbents Fig.5shows the RL of each composite in the 1–4GHz frequency range with a variable thickness.For each composite,as the thickness increased,the match-ing frequency decreased.It was in accordance with the relationship between the matching frequency and the matching thickness,which was shown in the equa-tion as [19]
f m =c (1+tan 2δμ/8)/(4d m
|μr εr |)
(8)where c is the speed of light in free space,d m is the
matching thickness,and f m is the matching frequency.As d m was a constant,the composite ?lled with Fe-SiAl and 1wt%MWCNT had the largest |μ ε |,so the f m was the lowest (2.7GHz at 1mm,1.7GHz at 1.5mm).
Moreover,when thickness was 1mm,the added MWCNT/graphite made the RL of the FeS-iAl/silicone rubber composites decrease at L-band.The RL value was reduced from ?3.7to ?5.7dB at 2GHz.The absorption band under ?5dB was broad-ened from 1.8to 4GHz as 1wt%MWCNT was added.The minimum RL was reduced to ?7.0dB,while the RL was lowered slightly as graphite was added at L-band (less than 0.5dB).When the thickness was 1.5mm,the added MWCNT or graphite enhanced the absorption ratio at L-band,and RL of composites added with 1wt%MWCNT was the lowest.The rea-son may be that the composites added with 1wt%MWCNT,has biggest the dielectric/magnetic loss tangent at the L-band.Since the minimum absorp-tion ratio was partially dependent on the thickness of the absorbers,and the RL curves were changed con-tinuously,the peak moved to low frequency,and the absorbing properties were improved at L-band when the carbonous materials were added into the compos-ite.
918
W.Q.Zhang et al.:J.Mater.Sci.Technol.,2012,28(10),913–919.
1.0 1.5
2.0 2.5
3.0 3.5
4.0
Frequency / GHz S h i e l d i n g e f f e c t i v e n e s s / d B
1.0 1.5
2.0 2.5
3.0 3.5
4.0
Frequency / GHz
S h i e l d i n g e f f e c t i v e n e s s / d B
Fig.6SE of the absorbing composites as a function of frequency:(a)with thickness 1mm,(b)with thickness
1.5mm
3.3Shielding property of the composites ?lled with
FeSiAl /carbonous absorbent Fig.6shows the e?ects of added MWCNT/graphite on the SE of the composites at thickness 1mm and 1.5mm.It could be apparently observed that the SE value of each composite increased in the 1–4GHz frequency range.As the thickness increased from 1to 1.5mm,the SE value of each composite was enlarged,and the SE of composites added with FeSiAl was increased from 2.6dB to 4.4dB at 1GHz,and from 10.1dB to 13.5dB at 4GHz.The SE of the com-posites ?lled with FeSiAl was enhanced as MWCNT or graphite was added.The SE was mainly deter-mined by the conductivity of the composites,so com-posites added with carbonous materials had higher SE.And the formation of more networks of conduc-tive ?ller and larger source of free electrons in the material could interact with incident electromagnetic waves [20].Meanwhile,it was observed that the SE of the composites added with MWCNT was larger than that of composites with added graphite.Since the MWCNT has a larger special surface area as com-pared with graphite,it can be dispersed in the gaps between the FeSiAl particles more easily and build the conductive network more expediently.
The total SE can be represented by the sum of con-tributions from absorption loss (re?ection loss)and re?ection.So the absolute value of the RL was always lower than the SE value,which has also been demon-strated by the experimental results.According to the RL and SE results,the absolute RL value of the com-posites at L-band was close to the SE value,and the SE value of composites with 3wt%MWCNT ranged from 8.1dB to 11.6dB,while the absolute RL value ranged from 5.6dB to 7.3dB.It was revealed that the absorption of the microwaves occupied a domi-nant part of the shielding property at L-band.The absolute RL of composites added with 3wt%MW-CNT was 51.5%–55.6%greater than the SE value at 2GHz with thickness 1mm,and 50.7%–65.2%greater
than that with thickness 1.5mm.On the contrary,the re?ection of the microwave gradually dominated the shielding property at S-band,and the absolute RL was only 38.1%and 22.5%larger than the SE value at 4GHz with thickness 1mm and 1.5mm,respec-tively.The reason is that the RL peak of the com-posite moved to low frequency with the carbonous materials adding and the absorbing properties were declined at S-band.Because the particles in the com-posite were well arranged,which were easy to form the conductive network,the Z in may rise fast with increasing the frequency added,and the re?ection oc-cupied a dominant part of the shielding property at S-band.
However,how to simultaneously improve the ab-sorbing/shielding property of FeSiAl/rubber compos-ites with an optimum content of carbonous material will be established in the further investigation.4.Conclusion
Carbonous materials could enhance the RL and SE of the rubber/FeSiAl composites with 1mm or 1.5mm in thickness.The added MWCNT could im-prove the absorption and shielding property of the composites at L-band.With 1wt%MWCNT adding the RL was less than other composites.And increas-ing the weight content of MWCNT did not lower the RL monotonously,which was opposite to the SE value.It was the interface between FeSiAl and MW-CNT that made the imaginary part of permeability and DC conductivity increase.The added graphite could enhance the SE of the composites and improve the RL at L-band insigni?cantly,due to the enlarged conductivity of the composites added with graphite and the almost unchanged permeability.
Acknowledgements
This work was supported by the National Natural Sci-ence Foundation of China (No.50805005),the National
W.Q.Zhang et al.:J.Mater.Sci.Technol.,2012,28(10),913–919.919
High Technology Research and Development Program of China(“863Program”,No.2009AA043804),the Foun-dation for the Author of National Excellent Doctoral Dis-sertation of PR China(No.2007B32)and the Innovation Foundation of BUAA for Ph.D.Graduates.
REFERENCES
[1]A.A.Al-Ghamdi and F.El-Tantawy:Compos.Pt.A-
Appl.Sci.Manuf.,2010,41,1693.
[2]W.S.Jou,H.Z.Cheng and C.F.Hsu:J.Alloy.
Compd.,2007,434–435,641.
[3]Y.C.Qing,W.C.Zhou, F.Luo and D.M.Zhu:J.
Magn.Magn.Mater.,2009,321,25.
[4]T.L.Gilbert:IEEE Trans.Magn.,2004,40,3443.
[5]D.S.Xue,F.S.Li,X.L.Fan and F.S.Wen:Chin.Phys.
Lett.,2008,25,4120.
[6]T.D.Zhou,P.H.Zhou,D.F.Liang and L.J.Deng:J.
https://www.360docs.net/doc/5f13508262.html,pd.,2009,484,545.
[7]T.D.Zhou,D.F.Liang,L.J.Deng and D.C.Luan:J.
Mater.Sci.Technol.,2011,27,170.
[8]R.C.Che,C.Y.Zhi,C.Y.Liang and X.G.Zhou:Appl.
Phys.Lett.,2006,88,033105.
[9]R.C.Che,L.M.Peng,X.F.Duan,Q.Chen and X.L.
Liang:Adv.Mater.,2004,16,401.
[10]X.G.Liu,Z.Q.Ou,D.Y.Geng,Z.Han,J.J.Jiang,W.
Liu and Z.D.Zhang:Carbon,2010,48,891.
[11]J.Sandler,M.S.P.Sha?er,T.Prasse,W.Bauhofer,K.
Schulte and A.H.Windle:Polymer,1999,40,5967.
[12]D.R.Iosif and V.H.Suong:Carbon,2009,47,1958.
[13]J.Joo and A.J.Epstein:Appl.Phys.Lett.,1994,65,
2278.
[14]Y.Li,C.X.Chen,X.Y.Pan,Y.W.Ni,S.Zhang,J.
Huang,D.Chen and Y.F.Zhang:.Physica B,2009, 404,1343.
[15]K.C.Pitman,M.W.Lindley, D.Simkin and J.F.
Cooper:Radar Absorbers:Better by Design,in Proc.
of RSP-5Conference,British,June,1991,223. [16]C.A.Micheli,R.Pastore and M.Marchetti:Compos.
Sci.Technol.,2010,70,400.
[17]Y.C.Qing,W.C.Zhou,F.Luo and D.M.Zhu:Carbon,
2010,48,4074.
[18]G.X.Tong,W.H.Wu,Q.Hua,Y.Q.Miao,J.G.Guan
and H.S.Qian:https://www.360docs.net/doc/5f13508262.html,pd.,2011,509,451. [19]S.S.Kim,S.B.Jo,K.I.Gueon,K.K.Choi,J.M.Kim
and K.S.Churn:IEEE Trans.Magn.,1991,27,5462.
[20]M.Arjmand,M.Mehdi,A.G.Genaro,S.Park and U.
Sundararaj:Carbon,2011,49,3430.