Effect of Carbon Black on Dielectricand Microwave Absorption Properties

of Carbon Black/Cordierite Plasma-SprayedCoatings

Jinbu Su, Wancheng Zhou, Yi Liu, Yuchang Qing, Fa Luo, and Dongmei Zhu

(Submitted September 13, 2014; in revised form March 11, 2015)

Carbon black (CB)/cordierite composite coatings with different CB contents were fabricated by a multi-function micro-plasma spraying system developed by the Second Artillery Engineering College. Scanningelectron microscopy was employed to investigate the microstructure of the spray-dried powders andas-sprayed coatings. The complex permittivities of the coatings and powders with different CB contentswere investigated at the frequency of 8.2-12.4 GHz. The results show that both real and imaginary part ofthe permittivity increase with increasing CB content, which can be ascribed to the increase of the numberof micro-capacitors and the polarization centers. Reflection loss of the as-sprayed coatings with differentCB contents and thicknesses was calculated according to the transmission line theory. The coating with4.54% CB content and 3.0 mm thickness shows optical microwave absorption with a minimum reflectionloss of223.90 dB at 10.13 GHz and reflection loss less than29 dB over the whole investigated frequency.

Keywords carbon black/cordierite coating, dielectric prop-erty, plasma spraying, reflection loss

1. Introduction

Recently, because of the increasing electromagneticinterference problems and of the essential role of stealthdefense in military platforms, microwave absorbingmaterials have attracted much attention. Excellentmicrowave absorbing materials are required to providespecified performance of wide absorption frequencyrange, strong absorption properties, low density, goodthermal stability, and oxidation resistance (Ref 1-3).

Nowadays, various metallic and ceramic materials, suchas carbonyl iron (Ref 4, 5), ferrites (Ref 6), and bariumtitanate (Ref 7), have been used and researched widelybecause of their properties of high magnetic loss anddielectric loss. However, the property of heavyweight andlow resistance to elevated temperature limits their appli-cations. Therefore, carbon-based materials, such as single-and multi-walled carbon nanotubes (CNTs) (Ref 8-10),carbon black (CB) (Ref 11), carbon foams (Ref 12, 13),carbon fibers (Ref 14), and graphene (Ref 15), haveprompted intensive study because of their unique struc-tures and excellent properties.

The first use of CB particles as microwave absorbingmaterial can date back to 1936 in the Netherlands (Ref 16).Kwon et al. (Ref 17) investigated the absorbing propertiesof carbon black/silicon rubber with different carbon blackcontents and thicknesses in the frequency of 2-18 GHz andconcluded that the samplewith 10 wt.%of carbonblackandthickness of 1.9 mm showed the best reflection loss with lessthan �10 dB in the frequency of 9.6-13.5 GHz. Liu et al.(Ref 18) prepared single-layer absorbers with a thickness of2 mmusing SiC andCB as absorbents.When the compositecontained 5 wt.% CB and 50 wt.% SiC with a thickness of2 mm, the maximum reflection loss was �41 dB at 9 GHz,and the bandwidth below �10 dB reached 6 GHz. Chenet al. (Ref 19) fabricated double-layer microwave absorp-tion coatings using carbonyl iron and carbon black as ab-sorbents in the matching layer and absorption layer,respectively, and the optimal RL reached�17.3 dB and theeffectual absorption band (better than �4 dB) was5.7 GHz. In our previous investigation (Ref 20),MWCNTs/cordierite nanocomposite coatings were pre-pared via atmosphere plasma spraying (APS). When theMWCNT content increased to 7%, the nanocompositecoating reveals the highest dielectric constant and optimalmicrowave absorption property. The influence of CB ad-dition is different from that of MWCNTs in microwaveabsorption field, which is mainly ascribed to the geometrydifference. Micheli et al. (Ref 21) investigated the X-bandmicrowave characterization of five carbon-basednanocomposite materials, and clearly showed how the ge-ometry of the nano-inclusions could significantlymodify theelectromagnetic behavior at microwave frequencies.

Thermal spraying technique, suitable to make metal-lic or ceramic coatings, has potential application forfabricating radar absorbing coatings, due to the advantage

Jinbu Su, Wancheng Zhou, Yi Liu, Yuchang Qing, Fa Luo, andDongmei Zhu, State Key Laboratory of Solidification Processing,Northwestern Polytechnical University, Xi�an 710072, China.Contact e-mail: nwpu_sujinbu@163.com.

JTTEE5 24:826–835

DOI: 10.1007/s11666-015-0238-y

1059-9630/$19.00 � ASM International

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of high achievable deposition efficiency, cost effectiveness,and very specific coating properties (Ref 22, 23). Yuanet al. (Ref 22) reported a multiscale effective fractal modelto characterize the microstructure-property relationshipfor high-velocity oxygen fuel (HVOF) sprayed compositecoatings and sprayed nanometer (Li2O-B2O3-SiO2)-SiCb

composite coatings as an example. Lou et al. (Ref 23)prepared three kinds of hollow multiphase ceramicmicrospheres via flame thermal spraying. Results showedthat feeding gas had an important effect on composition,morphology, and surface organization, and initialagglomerate powder size could affect dimension and surfaceorganization significantly.

Cordierite has a combination properties includinglow dielectric constant (e= 5-6), high resistivity(q > 1012 X m), elevated thermal and chemical stabilities,and very low thermal expansion coefficient (a=1-2 9 10�6 �C�1), which makes it a potentially suitablematerial for the electronic industry (Ref 23).

In the present investigation, the carbon black/cordieritecomposite microwave absorbing coatings were fabricatedby APS technique. The complex permittivities of thecoatings and powders with different CB contents wereinvestigated at the frequency of 8.2-12.4 GHz. Further-more, the reflection loss of the coatings was calculated,and the possible microwave absorbing mechanisms werediscussed.

2. Experimental

2.1 Materials

A commercial grade of carbon black (CB) in this studywas purchased from Tianjin Ebory Chemical Co. Ltd.(China), and the physical properties of the CB are shownin Table 1. Industrial grade raw cordierite (Mg2Al4Si5O18,

MAS) powders with an average diameter of 5 lm,according to the supplier, were purchased from HebeiXingtai New Refractory Materials Co. Ltd. (China).

2.2 Powder Preparation

The nano-sized particles should be granulated tomicron-sized granules since they cannot be used assprayable feedstock directly. The preparation process ofspray-dried powders includes two steps: (i) the MAS andCB powders were blended uniformly to produce viscousslurry with the addition of binder (PVA) and deionizedwater by planetary wet ball milling (QM-3SP4, NanjingNanDa Instrument Plant, China). The CB and MASpowders were taken in the weight ratio of 4/96, 6/94,8/92, 10/90, and 12/88, which were abbreviated as 4CB,6CB, 8CB, 10CB, and 12CB. In order to enhance thewettability between CB particles and the slurry, ethanolwas added into the slurry. Al2O3 balls (6 mm in di-ameter) were used as the milling media, and the wetball milling process continued for 3 h with a rotaryspeed of 300 r/min in order to blend the nano-sizedparticles homogeneously. The weight ratio of the ballsand solid is 2:1. (ii) The slurry of powder mixture wasspray dried to prepare nanostructured composite pow-ders by centrifugal spray dryer (LGZ-5, Wuxi Dong-sheng Spray-drying Machinery Plant, China). Theagglomeration parameters are reported in Table 2.

2.3 Sample Preparation

Because of low dielectric constant, paraffin is alwaysadopted as composite matrix to measure dielectricproperty of powders. CB/MAS-paraffin composites withhybrid of 50 wt.% CB/MAS powders and 50 wt.%paraffin were compacted into a rectangle flange with thesize of 22.86 mm 9 10.16 mm for the dielectric propertymeasurement of CB/MAS powders with different CBcontents.

APS is a process in which molten, semi-molten, or solidparticles are deposited on a substrate, in which plasma isused as the heat source. In this study, a plasma sprayingsystem with an internally fed powder torch developed bythe Second Artillery Engineering College was used toprepare the CB/MAS coatings (Ref 24). The plasmaspraying system adopted the cylindrical nozzle with achannel diameter of 5 mm and a nozzle length of 15 mm.Argon and nitrogen were adopted as primary gas andsecondary gas at a flow rate of 20 and 3 standard liters per

Table 1 Some physical parameters of carbon black

Parameters Value

DBP absorption number (mL/g) 1.5-5Iodine absorption (g/kg) 90-105Particle diameter (nm) 9-17Resistivity (X m) ~2.5PH 6-8Ash content £ 0.2

DBP: dibutyl phthalate

Table 2 Powder agglomeration parameters

Slurry parameters Spray drying parameters

CB+MAS 50 wt.% Entry air temperature (�C) 350Chamber air temperature (�C) 180

Deionized water 37 wt.% Exit air temperature (�C) 120-130Ethanol 10 wt.% Rotational speed of nozzle (r/min) 32,000Poly vinyl alcohol (PVA) 3 wt.% Slurry feed rate (g/min) 100

Atomizing air flow rate (m3/h) 20

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minute (slpm), respectively. The power of spraying isabout 9.8 kW. More details about the parameters arelisted in Table 3. The coatings were deposited on agraphite substrate with a thickness of 2.5 mm andmechanically removed from the substrate.

2.4 Phase and Microstructure Characterization

The morphology of the coatings was observed usingscanning electron microscope (SEM, Model JSM-6360,Tokyo, Japan). The crystalline phases of the coatings wereidentified by x-ray diffraction (XRD, Philips, Netherlands)with Cu Ka radiation. The size distribution of spray-driedpowders was checked by laser light diffusion (Winner2000E, Jinan, China). Conductivity measurements at roomtemperature were carried out on the composites using thefour-point probe technique. The test samples were cutfrom sprayed coatings into squares of 10 mm and coatedwith the conductive silver on both opposite surface.

2.5 Dielectric Property Measurement andMicrowave Reflection Loss Calculation

The complex permittivity can be simply expressed bye ¼ e0 � je00. The real part e0 is dielectric constant andrepresents the storage ability. The imaginary part e00 isthe lossy part, which is closely related to the conductivity.The complex permittivity of spray-dried powders andcoatings was measured by the wave-guide method at thefrequency of 8.2-12.4 GHz using an E8326B PNA net-work analyzer. The tested samples were cut into rectan-gular block with dimension of 22.86 mm 9 10.16 mm 92 mm.

According to the transmission line theory, the theore-tical reflection loss value for a single-layer absorber can becalculated by the following equations (Ref 25):

RL ðdB) = 20 logZin � Z0

Zin þ Z0

����

����

ðEq 1Þ

Z0 ¼ffiffiffiffiffil0e0

r

ðEq 2Þ

where RL (dB) represents the reflection loss of coatings indB unit. Z0 is the characteristic impedance of free space,which equals 120p in this situation. Zin is the input char-acteristic impedance of the absorber/free space interface,which can be calculated as follows:

Zin ¼ Z0

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

lrertanh j

2pfdc

ffiffiffiffiffiffiffiffiffiffilr�er

p� �

s

ðEq 3Þ

where f is the frequency of the electromagnetic wave, d isthe thickness of the microwave absorbing coating, c is thevelocity of light in free space, lr and er are the relativepermeability and permittivity of the coating, respectively.In our study, owing to the poor magnetic property of CB,the term lr can be referred as 1.

3. Results and Discussion

3.1 Morphology of the Powders and SprayedCB/MAS Coatings

The SEM image and size distribution curve of thespray-dried powders are illustrated in Fig. 1 and 2,respectively. As observed by SEM, the spray-dried pow-ders show a full spherical shape. The size of powdersranges from 20 to 100 lm and centers around 55 lm.

Figure 3(a)-(c) shows the typical polished cross-sec-tional SEM morphologies of the as-sprayed CB/MAS

Table 3 Plasma spraying parameters

Parameters Value

Arc current (A) 280Arc voltage (V) 35Primary gas (Ar) flow rate (slpm) 20Secondary gas (N2) flow rate (slpm) 3Spray distance (mm) 100Powder carrier gas (N2) flow rate (slpm) 3Powder feed rate (g/min) 2.5

Fig. 1 SEM micrograph of the spray drying CB/MAS powders

Fig. 2 Size distribution and cumulative curve of the spray-driedpowders

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coating (6CB). A lamellar structure, typical for otherthermally sprayed coatings, was not observed. The coat-ings were formed by a mixture of particles that were fullymolten and semi-molten in spray process, which weredescribed as a ‘‘bimodal microstructure’’ (Ref 26). It wasalso observed that CB agglomeration was distributedthroughout the coatings, which influenced on the dielectricproperty of the coatings.

XRD patterns of the raw MAS powders, CB powders,and the as-sprayed CB/MAS coatings with 6% CB contentare shown in Fig. 4, which indicates that the main phaseconstitution of the sprayed coatings is MAS. Moreover, abroad peak in the range from 20� to 30� 2h, which ischaracteristic of amorphous materials, is observed in thecoating diffraction spectrum. This suggests that the oxidephases have transformed from crystalline to amorphousstate due to the spraying process, which is also observedin SiC coatings deposited using high-frequency pulsedetonation (HFPD) thermal spraying technique (Ref 27)However, peaks of CB cannot be seen in the XRD spec-trum of sprayed CB/MAS coatings obviously. This ismainly ascribed to the oxidation of CB during the spray

process. Therefore, the actual CB content in the sprayedcoatings is relatively low. Meanwhile, the highest line forthe XRD pattern CB is 26.11� 2h, which is close to arelatively high line of 26.338� 2h for MAS XRD pattern.

Because of the CB oxidation during the spray process,the actual CB content in the sprayed CB/MAS coatings isdifferent from the added CB content. The difference willhave an important influence on the permittivity versuscarbon black content. Therefore, it would definitely beimportant to know the actual content of carbon black inthe sprayed coatings. In order to identify the actual CBcontent in the sprayed coatings, the sprayed coatings weregrinded into fine particles and annealed at 1000 �C in theair for 2 h. It could be supposed that the CB oxidizedcompletely and the weight loss was the mass of CB in thesprayed coatings. In order to ensure the accuracy of theexperiment, a comparison of only MAS sprayed coatingswas carried out in the same procedure. The result showsthat the actual CB contents in sprayed coatings are 2.11,4.54, 6.32, 7.21, and 9.98%, corresponding to the 4CB,6CB, 8CB, 10CB, and 12CB, respectively. For the purposeof discussing conveniently, the CB/MAS coatings werestill abbreviated as the former ones.

3.2 Conductivity of the Sprayed CB/MAS Coatings

The conductivity of CB/MAS coatings as a function ofCB content in the range of 2.11-9.98% is shown in Fig. 5. Itcan be observed from Fig. 5 that the conductivity of theCB/MAS composites increases from 0.02 ± 0.001 to0.0625 ± 0.003 S/cmwith an increase in themass fraction ofCB from 2.11 to 9.98%. At a CB content of 2.11%, theconductivity of the composite shows the lowest value of0.02 ± 0.001 S/cm. As the CB content increases, theconductivity of the composites increases slightly. However,when the CB content is more than 4.54%, the conductivityof the composites increases obviously. At a low CB content,the tunnel effect exists in the conductive particles, where thespaces among the neighboring particles are large. As theCBcontent increases, the distance between two CB particles issmaller and conductive network forms. The rapidly chang-ing area of the high conductivity state to the low conduc-tivity state is known the percolation zone (Ref 17).

Fig. 3 Cross-sectional SEM morphologies of the as-sprayed CB/MAS coating (6CB): (a) low magnification, (b) middle magnification,(c) high magnification

Fig. 4 X-ray diffraction patterns of MAS powders, CB powders,and the as-sprayed CB/MAS coating (6CB)

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3.3 Complex Permittivity of CB/MAS Powders

Figure 6(a) and (b) shows the real part and imaginarypart of complex permittivity of the CB/MAS powders withdifferent CB contents at the frequency range of 8.2-12.4 GHz. Both the real part and imaginary part of com-plex permittivity increase with the CB content increasingfrom 4 to 12%. It is obvious that the permittivity increasesslowly when the CB content is lower than 8%. However, itincreases sharply when the CB content is higher than 8%.It can be ascribed by the concept of percolation threshold(Ref 28). However, as illustrated in Fig. 5, the percolationzone is about 6% CB content, which is different from thatof CB/MAS powders. According to the preceding text, thespecimen for CB/MAS powders dielectric measurementconsists of 50 wt.% CB/MAS powders and 50 wt.%paraffin. Actually, the CB content in the specimen is halfvalues. Meanwhile, the actual CB content in sprayedcoating abbreviated as 6CB is 4.54%, which coincides withthe former discussion. When the CB content is low, it is

difficult for CB powders to contact each other and formconductive networks. There are no significant influenceson dielectric property of the composites, especially for theimaginary part of the complex permittivity. When the CBcontent reaches 8% and the actual CB content is 4%,which is called the percolation threshold, the CB powderscan contact each other and form a conductive network inthe paraffin, resulting in a sharp increase in complexpermittivity, especially for imaginary part, which can beobserved obviously in Fig. 6.

3.4 Complex Permittivity of Sprayed CB/MASCoatings

Figure 7(a) and (b) illustrates the real part and imagin-ary part of complex permittivity of the sprayed CB/MASpowders with different CB contents at frequency of 8.2-12.4 GHz. Both real part e0 and imaginary part e00 of thecomplex permittivity decreasewith increasing frequency, asshown inFig. 7(a) and (b), respectively. For the coatingwith9.98%CBcontent, the e0 and e00 values decline from10.7 and12.9 to 8.9 and 9.7, respectively. This phenomenon indicatesthat the complex permittivity has a weakening response toincreasing frequency. At low-frequency, the molecules andelectrons have enough time to polarize. However, at high-frequency, the polarization of molecules and electronscould not have enough time to catch up with the change inelectromagnetic field frequency.

Furthermore, the concentration-dependent dielectricproperty of as-sprayed CB/MAS coatings has also beeninvestigated. Both real and imaginary parts of the complexpermittivity are sensitive to the CB content and increasewith the CB content. The magnitude of real part of per-mittivity and imaginary part of permittivity for the coatingwith 9.98% CB content is about 2 and 20 times largerwhen compared with 2.11% CB content.

The CB is a conductive material and does not showdielectric property by itself. However, when conductivematerials are encapsulated with insulation materials, they

Fig. 6 (a) The real (e0), and (b) imaginary (e00) part of permittivity of the CB/MAS powders vs. frequency

Fig. 5 Electrical conductivity of the CB/MAS coatings vs. CBcontent

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show dielectric property. The microwave absorption in thesystem of the conductive materials mixed with non-con-ductive materials is related to the interfacial polarization(Ref 17). In CB/MAS coatings, conductive CB powdernetworks act as dissipating mobile charge carriers.According to the effective medium theory, the increase ofthe number of micro-capacitors and polarization centerswith increase of CB content leads to the increasing e0.

According to the electromagnetic theory, the imaginarypart of the permittivity has a relationship with conduc-tivity, which can be expressed (Ref 13) as

e00 ¼ rxe0

¼ r2pf e0

;

r ¼ 2pf e0e00;

ðEq 4Þ

where r is the electric conductivity (S/m), e0 is the freespace permittivity (8.8549 9 10�12 F/m), and f is the fre-quency (Hz). As described in the former section, theconductivity of CB/MAS coatings increases with theenhancing CB content. Consequently, according to Eq 4,the imaginary of the permittivity increases with theenhancing CB content.

3.5 Reflection Loss of As-Sprayed CB/MASCoatings

To further reveal the influence of CB content andcoating thickness on the microwave absorption properties,reflection loss of as-sprayed coatings was calculatedaccording to Eq 1-3 based on the measured complexpermittivity. Details of the minimum values of reflectionloss, peak frequency, and bandwidth (RL £ �5 and�10 dB) of the coatings are listed in Table 4.

3.5.1 Effect of the CB Content on the MicrowaveAbsorption Property of the Sprayed Coatings. The CBcontent is the most important factor that influences thereflection loss of the coatings, which determines thecomplex permittivity of the coatings. An ideal microwaveabsorbing material must satisfy two qualifications, (i) theimpedance matching between free space and the materialsurface to reduce wave reflection, which requires thecomplex permittivity close to complex permeability; (ii)materials can absorb as much incident wave intensity aspossible inside the absorbers, which requires strong mag-netic and/or dielectric loss. The two qualifications arecontradictory, and the microwave absorbing materials

Table 4 Details of the minimum values of reflection loss and bandwidth (RL £ 2 5 and 210 dB) of the samples

CB content Thickness (mm) Minimum RL (dB) Peak frequency (GHz) Bandwidth (RL £ 25 dB) Bandwidth (RL £ 210 dB)

4CB 2.5 �2.26 12.34 0 06CB 1.0 �0.41 12.4 0 0

1.5 �1.93 12.4 0 02.0 �7.03 12.4 0.47 02.4 �20.85 12.4 3.21 1.532.5 �32.08 12.4 3.63 2.022.6 �29.04 11.90 4.05 2.483.0 �23.45 10.05 4.2 4.013.5 �19.71 8.39 4.2 2.23

8CB 2.5 �7.91 11.01 4.2 010CB 2.5 �7.42 10.70 4.2 012CB 2.5 �6.45 9.06 4.2 0

Fig. 7 (a) The real (e0), and (b) imaginary (e00) part of permittivity of the CB/MAS coatings vs. frequency

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Fig. 8 Two-dimensional color maps for the calculated reflection loss of CB/MAS coatings: (a) 4CB, (b) 6CB, (c) 8CB, (d) 10CB,(e) 12CB

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should have proper permittivity and permeability. Konget al. (Ref 29) calculated the relation of permittivity andreflection loss, assuming the permeability equals 1. Theyconcluded that the optimum real and imaginary parts ofthe permittivity were equal to 7.3 and 3.3 to get the lowestRL at a frequency of 10 GHz and a thickness of 2.86 mm.

The detailed two-dimensional color maps of reflectionloss for sprayed coatings with different CB contents andthicknesses are illustrated in Fig. 8. From Fig. 8, we cansee that the coating with 2.11% CB content shows aproper reflection loss at a thickness of more than 8 mm,the coating with 4.54% CB content shows a properreflection loss at a thickness of about 3.5 mm, and thecoatings with 6.32, 7.21, and 9.98% CB content show aproper reflection loss at a thickness of about 2 mm.

Figure 9 shows the calculated reflection loss ofas-sprayed coatings with a thickness of 2.5 mm for dif-ferent CB contents in the frequency of 8.2-12.4 GHz. It isobserved that the coating with 2.11% CB content showsthe highest reflection values of more than �5 dB in thewhole frequency, which can be ascribed to the low com-plex permittivity. The coating with 4.54% CB contentshows the optimal reflection loss with the minimumreflection loss of �32 dB at 12.4 GHz. The effectiveabsorption bandwidth ( £ �10 dB) can reach to 2.02 GHzfrom 10.38 to 12.4 GHz, while the absorption range under�5 dB over 8.77-12.4 GHz. The coating with more than4.54% CB content shows a relative higher reflection lossvalues, which can be ascribed to the too high permittivity,especially the imaginary part of the permittivity, resultingin strong reflection and weak absorption. From the insetfigure, the reflection loss curves were found to graduallyshift to lower frequency with the increasing CB content. Itis worthy noticing that all the coatings with CB contenthigher than 4.54% exhibit a proper microwave absorptionproperty with an effective absorption bandwidth( £ �5 dB) across the whole measured frequency.

3.5.2 Effect of the Thickness on the MicrowaveAbsorption Property of the Sprayed Coatings. Thethickness of the coating is one of the crucial parameters thataffect the intensity and the position of the frequency of theRL minimum. As illustrated in Fig. 8(b), when the thick-nesses are about 3.0 mm, the corresponding reflection lossvalues are much smaller than that of their adjacent, indi-cating much better microwave absorbing abilities.

Figure 10 shows the calculated reflection loss ofas-sprayed coatings with 4.54% CB content for differentthicknesses (from 1.0 to 3.5 mm) at the frequency of 8.2-12.4 GHz. When the thickness is less than 2.0 mm, thesprayed coating reveals high reflection loss values of largerthan �2 dB at the whole measured frequency. As thethickness increases from 2.0 to 2.5 mm, an excellentabsorption property at higher frequency can be obtained.Meanwhile, the peak intensity decreases and peak fre-quency increases when the thickness increases, and theminimum RL reaches �32.08 dB at the frequency of12.4 GHz when the thickness is 2.5 mm. When the thick-ness is 3.0 mm, the sprayed coating shows the optimalabsorbing property with the minimum reflection loss of�23.90 dB in 10.13 GHz and reflection loss less than

�9 dB at the whole frequency, whereas compared with the3.0 mm coating, the coating with thickness of 3.5 mmshows a high reflection loss, which is attributed to resonantpeaks shifting to lower frequency regions.

Moreover, it can be seen that the absorbing peakmoves to a lower frequency band with an increase of thethickness, which is attributed to the resonant absorptioncaused by the quarter-wavelength attenuation when thereflected waves and incident waves meet in a material.This result could be explained by the following expression(Ref 30):

fm ¼ c

4dffiffiffiffiffiffiffiffiffiffier�lr

p ; ðEq 5Þ

where fm is the matching frequency of the absorbingpeaks. With the increase of thickness d, the matchingfrequency fm will decrease, which exhibits the peaksshifting toward lower frequency regions.

Fig. 9 Calculated reflection loss of CB/MAS coatings with dif-ferent CB contents vs. frequency (a thickness of 2.5 mm)

Fig. 10 Calculated reflection loss of CB/MAS coatings (6CB)with different sample thicknesses vs. frequency

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4. Conclusions

Carbon black (CB)/cordierite composite coatings withdifferent CB contents were fabricated by a multi-functionmicro-plasma spraying system developed by the SecondArtillery Engineering College. The complex permittivitiesof the coatings and powders with different CB contentswere investigated at the frequency of 8.2-12.4 GHz. Bothreal part e0 and imaginary part e00 of the complex permit-tivity decrease with increasing frequency. Furthermore,the complex permittivity is sensitive to the CB content andincreases with the CB content. The calculated reflectionloss results show that when the coating thickness is2.5 mm, the coating with 4.54% CB content shows theoptimal reflection loss with the minimum reflection loss of�32 dB at 12.4 GHz, and the coatings with CB contenthigher than 4.54% exhibit a proper microwave absorptionproperty with an effective absorption bandwidth( £ �5 dB) across the whole measured frequency. Thethickness has a noticeable influence on the reflection lossof the coatings, and when the thickness is 3.0 mm, thesprayed coating with 4.54% CB content shows optimalabsorbing property with the minimum reflection loss at�23.90 dB in 10.13 GHz and reflection loss less than�9 dB over the entire investigated frequency range.

Acknowledgments

This work was supported by the Chinese NationalNatural Science Foundation (Grant No. 51072165). Thiswork was financially supported by the fund of the StatesKey Laboratory of the Solidification Processing in NWPU,No. KP201307.

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15. Y.-J. Chen, Z.-Y. Lei, H.-Y. Wu, C.-L. Zhu, P. Gao, Q.Y.Ouyang, L.-H. Qi, and W. Qin, Electromagnetic AbsorptionProperties of Graphene/Fe Nanocomposites, Mater. Res. Bull.,2013, 48, p 3362-3366

16. F. Qin and C. Brosseau, A Review and Analysis of MicrowaveAbsorption in Polymer Composites Filled with CarbonaceousParticles, J. Appl. Phys., 2012, 111, p 061301

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18. X.-X. Liu, Z.-Y. Zhang, and Y.-P. Wu, Absorption Properties ofCarbon Black/Silicon Carbide Microwave Absorbers, Compos. B,2011, 42, p 326-329

19. L.-Y. Chen, Y.-P. Duan, L.-D. Liu, J.-B. Guo, and S.-H. Liu,Influence of SiO2 Fillers on Microwave Absorption Properties ofCarbonyl Iron/Carbon Black Double-Layer Coatings, Mater.Des., 2011, 32, p 570-574

20. J.-B. Su, W.-C. Zhou, Y. Liu, F. Luo, and D.-M. Zhu, Atmo-sphere Plasma-Sprayed Carbon Nanotubes/CordieriteNanocomposite Coatings for Microwave Absorption Applica-tions, J. Therm. Spray Technol., 2014, 23(7), p 1065-1072

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834—Volume 24(5) June 2015 Journal of Thermal Spray Technology

PeerReviewed

29. K.H. Wu, T.H. Ting, G.P. Wang, W.D. Ho, and C.C. Shih, Effectof Carbon Black Content on Electrical and MicrowaveAbsorbing Properties of Polyaniline/Carbon Black Nanocom-posites, Polym. Degrad. Stab., 2008, 93, p 483-488

30. L. Kong, X.-W. Yin, F. Ye, Q. Li, L.-T. Zhang, and L.-F. Cheng,Electromagnetic Wave Absorption Properties of ZnO-Based

Materials Modified with ZnAl2O4 Nanograins, J. Phys. Chem. C,2013, 117, p 2135-2146

31. B.-M. Wang, Z.-Q. Guo, Y. Han, and T.-T. Zhang, Electromag-netic Wave Absorbing Properties of Multi-walled Carbon Nan-otube/Cement Composites, Constr. Build. Mater., 2013, 46, p 98-103

Journal of Thermal Spray Technology Volume 24(5) June 2015—835

PeerReviewed

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