Microwave-absorbing characteristics for the composites of TP(6)

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Fig.5.Room-temperaturehysteresisloopsofNi0.5Zn0.5Fe2O4powders.( )As-synthesizedferritewithoutheattreatment,( )synthesizedferriteannealedat500 Cfor1h,( )synthesizedferriteannealedat900 Cfor1h,and( )commercialferrite.(AplotmagnifyingthevicinityoftheoriginisinsetintoFig.4.ThevaluesofHc(i.e.,theintersectionpointonpositivex-axis)andMr(i.e.,theinterceptonpositivey-axis)foreachspecimencanbeobserved).

500 Cfor1h.ThedatapresentedinFig.6andTable2il-lustratethatthehysteresisparametersofthesedopedNiZnferritesareconsiderablylargerthanthoseoftheundopedone.Evidently,introducingthesedopantscanmodifythemagneticpropertiesofNiZnferrite.Itisthusexpectedthatthecompos-itesfabricatedbyusingdoped-NiZnferritemightpossessdif-ferentmicrowave-absorbingcharacteristicswhencomparedwiththecompositeofundopedNiZn

ferrite-TPU.

Fig.6.Room-temperaturehysteresisloopsofsynthesizeddoped-andundoped-ferritepowdersannealedat500 Cfor1h.( )Undoped,( )Co-doped,( )Cu-doped,and( )Mg-doped.(AzoomplotshowingthevicinityoftheoriginisinsetintoFig.5.ThevaluesofHc(i.e.,theintersectionpointonpositivex-axis)andMr(i.e.,theinterceptonpositivey-axis)foreachspecimencanbeobserved.).

3.4.Measurementofelectromagneticproperty

Fig.7(a–d)manifeststherealpartandtheimaginarypartofthe relativepermittivity(ε andε )andpermeability(µ andµ)spectraforNi0.5Zn0.5Fe2O4-TPUcomposites.Therealpartandtheimaginarypartofpermittivityarealmostconstant(ε =6–7andε =0.45–0.65,respectively).Thisismostlikelytobecausedbytheintrinsicallysmalldielectriclosstangent(tanδand )oftheferritepowderusedinthisstudy.Thevaluesofµ µ dependonthefrequencyandcalci-nationtemperature.Thespectrumofµ exhibitsafallatthebeginningandthenremainsconstantin5–12GHz,whereastheµ decreaseswithincreasingfrequency.

Fig.8givestherelativepermittivityandtherelativeper-meabilityspectrafordopedandundopedferrite-TPUcom-posites.Therealpartandimaginarypartofpermittivityre-mainpracticallyconstantandtheydependonthedopanttobevariedfrom6to7.5andfrom0.45 to1.0,respec-tively.Withincreasingfrequency,theµexhibitsadecreas-ingtendency(exceptforCo-dopedsample)intheorderofundoped>Mg-doped>Cu-doped.Themaximumvalueofµ canbeshiftedtohigherfrequencyintheorderofCo-doped>Cu-doped>Mg-doped>undoped.Inaddition,thevalueofµ isalsoaugmentedinsuchanorder,especiallyinthehigh-frequencyregion(>8GHz).3.5.Re ectionloss

Fig.9showsthere ectionlossesandthematchingfre-quenciesoftheNiZnferrite(annealedat500 Cfor1h)-TPUcomposite,whichcanbeobtainedbyusingEqs.(3)and(4).Thematchingfrequency(fm)isequalto3.16GHzinthelower-frequencyregionanditisequalto9.43GHzinthehigher-frequencyregion.Thecorrespondingvaluesofmatchingthickness(dm)are5.92mmand2.13mm,re-spectively.Table3liststhedataofabsorptioncharacteristicsfortheabsorbingcompositesfabricatedfromthecommer-cialNiZn ferrite,as-synthesizedandannealedat500 Cand900Cfor1h.ItisclearthattheseNiZnferrite-TPUcompos-itescanbeconsideredasamicrowave-absorbingmaterialin～2–4GHzlower-frequencyrangeandin～9–10GHzhigher-frequencyrange. Inparticular,theNiZnferriteafteranneal-ingat500Cfor1hexhibitsthelargestre ectionlossandthewidestbandwidththanthoseobtainedfromotherspeci-mens.Thisphenomenon,coupledwiththeresultspresentedinTable1,suggeststhatthecrystallitesizeofNiZnferritehassigni canteffectsonmagneticpropertiesandmicrowave-absorbingcharacteristics.

Table4showstheabsorptionef ciencyfortheundoped,Co-doped,Mg-doped,andCu-dopedNiZnferrite-TPUcom-positestobeobtainedbytakingthespeci edthicknessof2mmand5mm,respectively.Itshowsthatthere ectionlossofthesecompositescouldbedesignedtobemoreneg-ativethan 20dBinthefrequencyrangeof2–12GHz.Inaddition,itisevidentthatthedopedferrite-containingcom-positeshavemuchmoreeffectiveelectromagneticabsorp-