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Materials Science and Engineering B 139 (2007) 119–123

Preparation and electromagnetic wave absorption properties of Fe-dopedzinc oxide coated barium ferrite composites

Xin Tang ∗, Ke-ao HuState Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 1954, Huashan Road,

Shanghai 200030, PR China

Received 20 July 2006; received in revised form 28 December 2006; accepted 20 January 2007

bstract

Fe-doped zinc oxide (ZnO) coated barium ferrite composite particles were prepared by a heterogeneous precipitation and thermal treatmentrocess. The prepared composite particles were characterized with X-ray diffraction (XRD) and transmission electron microscopy (TEM). Theomplex relative permittivity and permeability of ZnO/barium ferrite composites were measured in the frequency range of 2–12 GHz. The resultshow that the coverage of ZnO has a great influence on microwave response of barium ferrite particles. The formation of a ZnO thin layer on the

urface of a barium ferrite particle changes the character of the frequency dispersion of the complex relative permittivity and permeability. Byhanging the thickness of ZnO coverage, the frequency dependence of the microwave electromagnetic and absorbing properties could be adjusted,hich provides us an opportunity for the synthesis of tailored particles. 2007 Elsevier B.V. All rights reserved.

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eywords: Doped zinc oxide; Barium ferrite; Composites; Complex relative pe

. Introduction

As a specific kind environmental pollution, the electromag-etic interference (EMI) is drawing more attention recently,ue to the increasing use of gigahertz electromagnetic wavesn industrial, commercial and military application. Usually,onducting materials are used to shield electromagnetic wave.ut, the reflecting wave from conducting materials mayause unimaginable badness aftereffects. Electromagnetic wavebsorption materials have attracted considerable attention due tohe facts that they can absorb radiation energy from microwaveenerated from an electric source, and are promising candi-ates for replacement of conducting metal materials. Extensivetudy has been carried out to develop new electromagnetic wavebsorbing materials with a high magnetic and electric loss [1–3].lectromagnetic wave absorption materials from magnetic to

ielectric related materials used in a high frequency were par-icularly noticed. But, pure dielectric or magnetic materialsre insufficiently absorbing radiation energy. The efficiency of

∗ Corresponding author. Tel.: +86 21 62933751; fax: +86 21 62822012.E-mail addresses: tangxin98@sjtu.edu.cn, rivertx2003@yahoo.com.cn

X. Tang).

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vity and permeability; Microwave absorption

agnetic/semiconductor absorbers is high because the complexermittivity (εr = ε′ − jε′′) and permeability (μr = μ′ − jμ′′) dif-er from unity. As a result, the materials thickness decreeases byε′μ′)1/2 times [4]. Modifying the properties of one materialy coating it with another type of material has been a pop-lar approach widely documented in the literature [5–9]. Inhis work, the concept of coating one material with anothers used to develop a novel electromagnetic wave absorptionaterial. This study focused on the investigation on a type ofagnetic–semiconductor composites made of Fe-doped ZnO

nd barium ferrite fine particles. The barium ferrite is chosens a magnetic core because it has high saturation magnetiza-ion, high anisotropy field and excellent magnetic propertieshich make it potential good magnetic loss materials in aigh frequency such as X band [10–12]. However, negligiblepoor) dielectric loss, that is to say, a very low complex per-ittivity, confined its applications. On the other hand, ZnO

s an important inorganic semiconductor, it was selected dueo its advantages such as temperature and environmentallytable dielectric properties, and also transition-metal-doped

nO shows some magnetism properties [13]. ZnO whisker

ZnOw) appears to have a good microwave absorption prop-rty in the X-band (8–12 GHz) and Ku-band (12–18 GHz) ashe reported earlier [14,15], it has the potential to be used as

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20 X. Tang, K.-a. Hu / Materials Science

light weight absorber. ZnO/barium ferrite composites com-ine the merits of ZnO and magnetic barium ferrite particles.o our best knowledge, there are few articles concerning thelectromagnetic wave absorption properties of ZnO/barium fer-ite composites. It is those factors that motivate us to workn the synthesis of ZnO-coated barium ferrite composite par-icles suitable for microwave absorption application. In thisork, sugar-nitrates technique was used to prepare barium fer-

ite particles. Fe-doped ZnO coating on barium ferrite particlesas prepared by a heterogeneous precipitation of zinc chlo-

ide (ZnCl2) onto the surface of barium ferrite particle and thenhermal treatment the composite particles. The crystal structure,

agnetic properties of the composites were measured and com-ared with that of the uncoated barium ferrite particles. Theffects of the content of the ZnO on the complex permittiv-ty, complex permeability and microwave absorption propertiesf composites in the frequency range of 2–12 GHz have beennvestigated.

. Experimental

.1. Materials

Ferrite nitrate (Fe(NO3)3·9H2O), barium nitrate (Ba(NO3)2),inc chloride (ZnCl2) and sodium hydroxide (NaOH) werebtained from Shanghai Lingfeng Chemical CorporationShanghai, China). All reagents were of analytical grade andere used without further purification. White granulated sugaras obtained from Shanghai Sugar & Tobacco Industry Limitedompany (Shanghai, China, GB317, sucrose >98%). Deionizedater was used in the all experiments.

.2. Preparation of barium ferrite particles

Barium ferrite powders were prepared by a method simi-ar to previously reported method [16]. A certain amount ofe(NO3)3·9H2O, Ba(NO3)2 and white granulated sugar wereissolved in deionized water, respectively. The mole ratio ofa to Fe was fixed at 1:11.5 which gives a composition ofaFe12O19 (barium ferrite). Ferric nitrate and sugar solutionsere mixed, and the barium nitrate solution was added with

ontinuous stirring for 2 h. Then the mixed solution was evap-rated slowly at 90 ◦C, during which the Fe3+ and nitrate ionrovided an in situ oxidizing environment for sugar beingydrolyzed and converted into carboxylic acids, and the nitrateshemselves were decomposed to give out brown fumes ofitrogen dioxide. When the reaction completed, the obtainedolution was cooled. In order to make carboxyl acid be ion-zed and carboxylic groups chelate Fe2+ and Ba2+, dilutedmmonia solution (10 wt.%) was slowly added to adjust theH value of the solution to 6.5. The precursors were pre-ipitated from the reaction medium by adding ethanol. Thehallow green product was then filtered, and dried at 80 ◦C

or 24 h, and turned into a dried precursor. Finally, powders ofhe dried precursor were annealed at 1100 ◦C for 1.5 h with aeating rate of 8 ◦C per minute to obtain barium ferrite fineowders.

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Engineering B 139 (2007) 119–123

.3. Preparation of Fe-doped ZnO coated barium ferriteomposite particles

The barium ferrite particles were coated with uniform layersf Fe-doped ZnO by a heterogeneous precipitation and thermalreatment process. A certain amount of the barium ferrite pow-ers was dispersed in the aqueous solution of ZnCl2-ferric nitrateith a Zn/Fe molar ratio of 9:1. The suspension was sonicated

or 20 min in an ultrasonic cleaning bath and subsequently vigor-usly stirred for 2 h. And then, 1 M NaOH aqueous solution waslowly dropped into the above suspension. A slow and gradualuper-saturation is essential to achieve the heterogenous nucle-tion of zinc hydroxide onto the barium ferrite particles. Thetock suspension was stirred for at least 8 h to ensure the com-lete coating. The precipitates were separated from the motheriquor by filtration, washed with deionized water and absolutethanol in order to minimize particle agglomeration from hydro-en bonding. This washing process was repeated three times ateast. Such obtained composite particles were dried at 100 ◦C for4 h and calcined at 450 ◦C for 1 h, and the final product of ZnOoated barium ferrite composite was obtained. The compositesith the ZnO/barium ferrite molar ratio were 0, 5, 10, 15%, and

he corresponding samples are marked with ZB0, ZB1, ZB2 andB3, respectively.

.4. Characterization

The obtained samples were analyzed for their composition,icrostructure, the complex relative permittivity and perme-

bility, and microwave absorption properties by using multipleethods and apparatus. The phase composition of the samplesas determined by X-ray powder diffraction, and the XRD dataere collected using a Bruker AXS D8 Advance diffractometerith the wavelength of 1.5418 A of Cu K� radiation in the rangeθ = 20–80◦. Transmission electron microscopy was performedn a JEOL JEM-2010 instrument. Samples were dispersed onoley carbon films on copper grids. A network analyzer (Agilentechnologies, E8363) was employed to determine the values of

he complex relative permittivity (εr = ε′ − jε′′) and permeabilityμr = μ′ − jμ′′) in the 2–12 GHz range by using coaxial reflec-ion/transmission technique [17]. For this, the samples wererepared with 70.0 wt.% barium ferrite and ZnO/barium ferriteomposite powders loading in epoxy resin. The powder–resinomposites were die-pressed to form cylindrical toroidal shapedpecimens with 3.0 mm inner diameter, 7.0 mm outer diameternd 3–4 mm thickness. The test samples of toroidal shape wereightly inserted into the standard coaxial line, the measured val-es of reflected and transmitted scattering parameters were usedo determine ε′, ε′′, μ′, μ′′.

. Results and discussion

.1. Crystal structures and morphology

To identify the crystalline structure of the ZnO layer, XRDnalysis is performed on the ZnO and ZnO-coated barium ferriteomposite particles calcined at 450 ◦C for 1 h. The results of the

X. Tang, K.-a. Hu / Materials Science and Engineering B 139 (2007) 119–123 121

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ig. 1. XRD patterns of: (a) ZnO particles calcined at 450 ◦C for 1 h. (b)nO/barium ferrite composites calcined at 450 ◦C for 1 h.

RD are presented in Fig. 1. Fig. 1a shows that the peaks of ZnOt 2θ = 31.78◦, 34.38◦, 36.28◦, 47.70◦, 56.66◦, 62.86◦, 68.00◦ppeared, which is identified as wurtzite structure (space group63mc (JCPDS No. 79-2205)) from the XRD pattern. The broadeak shows that the crystallite size is small. The crystalline sizef ZnO calculated from the most strong peak corresponding tohe (0 0 1) plane of the ZnO phase at 2θ = 36.28◦ using Scherrerquation is 10 nm. Fig. 1b shows the presence of ZnO and bar-um ferrite (JCPDS No. 79-2205, 74-1121, respectively) [18].he most intense peaks at 2θ = 36.28, 31.78, 34.38 among thenO peaks are detectable in the XRD patterns of ZnO coatedarium ferrite composite particles, however, the rest peaks ofnO are not observable since the ZnO loading molar percent isnly 15%, and the crystal size of the ZnO is much smaller thanhat of barium ferrite, and the intensity of the peaks from bariumerrite, which is much stronger than that of ZnO. There were noxtra peaks, that is, there were no impurities formation at thenterface between ZnO and barium ferrite, indicating no solidtate reaction between the barium ferrite and ZnO.

The representative TEM images for the barium ferrite andhe ZnO/barium ferrite composite particles are shown in Fig. 2.he particles are hexagonal platelet crystals. They stack eachther due to their magnetic attraction between the particles, ashown in Fig. 2. The comparison of two pictures (refer to Fig. 2and b) shows that a ZnO coating layer is made on the surfacef the platelet barium ferrite particles, and the thickness of thenO layer is estimated as about 15–30 nm. And the ZnO layer

s uniformly formed covering all the surface of barium ferritearticles, is clearly discernible in the TEM image. Barium ferrites magnetic material that has more absorbed electron than thatf ZnO, which is semiconductor material. Therefore, the barium

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ig. 2. TEM micrographs of barium ferrite powders (a) and representative ZnO-oated barium ferrite composite particles (b).

errite particle is dark, while the ZnO layer is brighter in the TEMmage of ZnO/barium ferrite composite particles.

.2. Complex relative permittivity and permeability spectrat 2–12 GHz

Fig. 3 shows the real and imaginary parts of the complex rel-tive permittivity (ε′, ε′′) spectra for pure barium ferrite–resinomposites and ZnO/barium ferrite (molar ratio of ZnO = 5, 10,5%)–resin composites. As shown in Fig. 3, the complex rel-tive permittivity of the ZB0 is low and the values are almostonstant behavior through out the 2–12 GHz frequency range.he real part of the complex relative permittivity of the speci-ens remained almost constant in the whole frequency range,

nd the ε′ increased slightly with the increase of the molar ratiof ZnO. Compared with the pure barium ferrite, the imaginary

arts of the complex relative permittivity for ZB1, ZB2 and ZB3how a small resonance peak at 10.3 GHz for ZB1, 8.5 GHzor ZB2 and 7.6 GHz for ZB3. The characteristic feature ofhe ZnO is dielectric materials, the dominant dipolar polariza-

122 X. Tang, K.-a. Hu / Materials Science and Engineering B 139 (2007) 119–123

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ig. 3. The complex relative permittivity curves poltted against frequency forhe ZnO/barium ferrite–resin composites.

ion and the associated relaxation phenomena constitute the lossechanisms. Composite materials, in which magnetic particles

re coated with dielectric nano-layers, introduce an additionalnterfaces and more polarization charges on the surface of thearticles, this makes the dielectric relaxation behaviour moreomplex. It has been shown by the refs. [19,20] that the proper-ies of interfaces could have a dominant role in determiningielectric performance. Additionally, Joule-heating loss willlso occur due to finite conductivity of the composites. Sincehe ZnO-coated barium ferrite composites is a heterogeneousystem, and interfacial polarization is an important polariza-ion process and associated relaxation also will give rise to loss

echanism. In the present study, the higher values of the realnd imaginary parts of the complex relative permittivity for thenO-coated barium ferrite composite may be due to signifi-ant contributions to polarization and interfacial polarization.ggregate dielectric losses in the samples can be described asue to the contributions from both the dc/ac conductivity andipole relaxation [21]. In sample ZB0, contribution from inter-acial polarization is expected to be less, and no resonance peakas observed. When the composites consisted of ZnO and bar-

um ferrite, polarization and interfacial polarization occurred.onsequently, the permittivity spectra curve exhibited a reso-ance peak. Through the dielectric loss peaks may due to higherodes from microwave measurements in a coaxial line, we can

xclude this situation by measurements with different thicknessf the samples, which was suggested by Lefrancois and Pasquet22]. So, the resonance peaks come from intrinsic properties ofrepared materials.

The real and imaginary parts of the complex relative per-eability (μ′, μ′′) spectra of the uncoated barium ferrite–resin

omposites and the ZnO/barium ferrite (molar ratio of ZnO = 5,

0, 15%)–resin composites are shown in Fig. 4. The real partsf the complex relative permeability for the samples show aecreasing trend with increasing frequency. Compared the ZnO-oated barium ferrite composites with the uncoated barium

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ig. 4. The complex relative permeability curves poltted against frequency forhe ZnO/barium ferrite–resin composites.

errite, the real parts of the complex relative permeability ofhe ZnO-coated barium ferrite composites is lower than thatf the uncoated barium ferrite. The lower values of the com-lex relative permeability for the ZnO-coated barium ferritebserved in these samples may be attributed to the presencef nonmagnetic polymer between the neighboring crystalliteshich weakens the inter-granular magnetic interaction. We cannd that the μ′′ of the ZnO/barium ferrite–resin composites areigher than that of pure barium ferrite–resin composites in theost frequency range. This result can be explained by the mag-

etic dissipation. According to Vander Zaag’s suggestion [23],he magnetic dissipation of ferrite includes: hysteresis loss, eddyurrent loss, residual loss, ferromagnetic resonance loss andomain wall loss. In this study, however, the enhancement ofonductance of Fe-doped ZnO might be contributed to higher′′ of the ZnO/barium ferrite–resin composites because of Fe-oped ZnO being a diluted magnetic semiconductor. Especially,hen the molar ratio of ZnO is 10%, the μ′′ of the ZnO/barium

errite–resin composites shows the maximum value.

.3. Microwave absorbing properties

According to transmission line theory, the reflection loss oflectromagnetic radiation, RL (dB), under normal wave inci-ence at the surface of a single-layer material backed by a perfectonductor can be obtained [24,25]. The microwave absorbingroperties of the powder–resin composites have been calcu-ated for various measured values of ε′, ε′′, μ′, μ′′ previouslybtained as shown in Figs. 3 and 4, assuming the layer thickness= 2.5 mm. The calculated reflection losses of composites as a

unction of frequency for all samples are shown in Fig. 5. Ashown in Fig. 5, the sample ZB0 shows the maximum reflectionoss of −5.0 dB at 10.4 GHz, the sample ZB1 shows the max-mum reflection loss of −8.7 dB at 9.6 GHz, the sample ZB2

X. Tang, K.-a. Hu / Materials Science an

Fig. 5. Frequency dependence of the reflection loss for pure barium ferrite–resincomposites and ZnO/barium ferrite–resin composites at same sample thickness(thickness = 2.5 mm).

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ig. 6. Frequency dependence of the reflection loss of the ZB2 composites atarious sample thickness (d = 2.0, 2.5, 3.0 and 3.5 mm).

hows the maximum reflection loss of −16.8 dB at 8.6 GHz andhe sample ZB3 shows the maximum reflection loss of −11.0 dBt 8.4 GHz. The difference on reflection loss maximum as a func-ion of the samples is associated with the magnetocrystallinenisotropy of synthesized materials. The maximum reflectionoss increases from −5.0 dB to about −16.8 dB for the molaratio of ZnO ≤ 10%. When the molar ratio of ZnO is 10%,he composites have good compatible dielectric and magneticroperties, and hence the microwave absorbing properties showhe maximum value. However, when the molar ratio of ZnOs 15%, the maximum reflection loss decreases to −11.0 dB,hich may be due to deteriorating the permeability when theolar ratio of ZnO exceeds a critical value. The reflection lossaximum is equivalent to the occurrence of minimal reflec-

ion of the microwave power for the particular thickness. Fig. 6

hows the frequency dependence of the reflection loss of theB2 composites at sample thickness of 2.0, 2.5, 3.0 and 3.5 mm.learly demonstrated is that the intensity and frequency of

eflection loss maximum for the sample depends on the mate-

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d Engineering B 139 (2007) 119–123 123

ial’s thickness. The dip of the maximum reflection shifted tolower frequency with increasing the thickness. The matching

requency is 8.6 GHz, with corresponding matching thicknessalue of 2.5 mm.

. Conclusions

In this work, the novel Fe-doped ZnO coated barium ferritearticles have been obtained. The morphology, microstructurend microwave electromagnetic properties have been charac-erized. Compared the ZnO-coated barium ferrite compositesith the uncoated barium ferrite, the complex permittivity of

he ZnO-coated barium ferrite composites is higher than that ofhe uncoated barium ferrite. The real part of the complex rel-tive permeability of composites was found to decrease withncreasing the frequency as well as with the molar ratio of ZnO.owever, the imaginary part of the complex relative perme-

bility of composites was found to enhance. When the molaratio of ZnO is 10%, the imaginary part of the complex per-eability of ZnO/barium ferrite–resin composites shows theaximum value, and the maximum reflection loss was obtained.he results show that the ZnO coverage on barium ferrite has areat influence on its microwave absorption properties.

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