Microwave-absorbing characteristics for the composites of TP(2)

28C.-H.Pengetal./MaterialsScienceandEngineeringB117(2005)27–36

anditiswellknownthatnano-scaleparticlespossessdis-tinctivephysicalandchemicalpropertiesbecauseoftheirnano-sizedcrystallite,largesurfaceareaanddifferentsur-faceproperties(suchassurfacedefect)etc.Yet,avarietyofwet-chemicalmethods(suchassol–gel,co-precipatationandhydrothermal,etc.)havebeenreportedtobeeffectiveingeneratingnano-sizedandhomogeneouspowdersofce-ramicoxides[10–12].However,relativelycomplexsched-ules,expensiveprecursors,andlowproductionratearethecommonproblemsofthewet-chemicalmethod[13].Inthepastdecade,asolutioncombustionmethodhasbeennarratedandutilizedtosynthesizesimpleand/ormixedmetaloxides[14–17].Withthismethod,theheatingandevaporationofdesiredmetalnitratesolutionemployinganorganiccom-poundcanresultinself- ringtogenerateheatbyexother-micreaction.Thisliberatedheatisusedtosynthesizetheceramicoxidepowders.Together,thismethodhasadvan-tagesofapplyinginexpensiverawmaterialsandmaintainingarelativelystraightforwardpreparationprocess,andachiev-inga nepowderwithhighhomogeneity.Inaddition,theprocessofcombustionreactionisrelativelyquickascom-paredtoothersynthesistechniques.Thisprocesswithitsfastpaceinascendinganddescendingthetemperaturecanpro-ducemeta-stablephase,un-equilibriumphaseandproductsoftinygrains;therefore,superiorpropertiesareattainable[18].

Inourpreviousresearch,afurthermodi edcombus-tionsynthesismethodhasbeendevelopedforprepara-tionofnanocrystallineceramicoxides[19].Utilizationofanorganiccompound(e.g.,glycine,urea,citricacid,ala-nineorcarbohydrazide)andmetalnitratesinconjunctionwithmixingthereactantsdirectlywithoutaddingwaterarethekeytechniquesofthismethod.Itwasfoundthatthesinterabilityandsoft-magneticpropertiesoftheresultantNiZnferritepowderaresuperiortothoseobtainedbyothermethods.

Sincetherewasnotmuchliteraturereportingtheappli-cationofusingcombustionmethodtosynthesizeNiZnfer-ritesinregardtomakingelectromagneticwaveabsorbers,wecontinueourresearchtousecombustionsynthesismethodtosynthesizeNiZnferriteandMg-,Co-,andCu-dopedNiZnferrites(chemicalformula:Ni0.5 xZn0.5 xM2xFe2O4,M=Mg,CoorCu,andx=0or0.05),andthenthere-sultantferritepowderswereblendedwithathermal-plasticpolyurethane(TPU)elastomertobemadeintomicrowave-absorbingcompositesandusedinourrelevantresearch.Inthiswork,theeffectsofboththeparticlesizeofferriteandthedopantpresentedintheferriteontheelectromag-neticpropertiesandmicrowave-absorbingcharacteristicsareevaluated.

2.Experimentalprocedure

The owchartoftheexperimentalprocedureisshowninFig.1.

2.1.Preparationofferrite

Ferritepowderswerepreparedbyanamountof25gperbatchfromanexothermicreactionusingmixturesofmetalnitrates(purity:>99%,Aldrich)andurea((NH2)at2CO,purity:99.5%,Alfa).Reactantsweredirectlymixedan‘equiva-lentstoichiometricratio’withoutaddingwater.Notethatthe‘equivalentstoichiometricratio’isdesignatedastheoxygencontentofmetallicnitratesthatcanbecompletelyreactedtooxidize/consumeureaexactly.Thereactantmixturecaneasilyabsorbmoisturefromtheairandbecomeaslurrysub-stance,sincemetallicnitratespossesshygroscopicity.Thisslurrymixturewashomogenizedandheatedonahotplateat～230 Ctodehydrateuntilself-ignitionwastakenplace.Then,thedriedmixturewasignitedatroomtemperaturetostartcombustionreaction,givingrisetotheevolutionofalargevolumeofgasesandproducingadry,loose,andvo-luminousash.Whencompletecombustionisassumed,thereactionequationcanbeexpressed(greatly-simpli ed)asfollows:

(0.5 x)Ni(NO3)2+(0.5 x)Zn(NO3)2+2xM(NO3)2+2Fe(NO3)

3+6.67CO(NH2)2 →

Ni0.5 xZn0.5 xM2xFe2O4+6.67CO2+10.67N2+13.34H2O

(1)

Tounderstandthein uencesofheattreatmentongrainsizeandmagneticproperties,aportionoftheas-synthesizedfer-ritepowderswasannealedat500 Cand900 Cfor1h.Forcomparison,acommercialNi0.5Zn0.5Fe2O4(purity:99%,av-eragediameter:1.5 m,source:CoreconnectorCo.,Taiwan)manufacturedbytheconventionalmixedoxidemethodwasalsousedinthepresentstudy.2.2.Characterizationofferrite

Phaseformationofproductwasidenti edbyusingX-raydiffraction(XRD;SIEMENSD5000)withCuK radiation(λ=0.15418nm).Thecrystallitesize(D)ofNiZnferritewascalculatedfromthediffractionpeakofthe(311)planefromtheXRDpro le,inaccordancewiththeDebye–Scherrerfor-mula[20]:D=

0.9λ(βcosθ)

(2)

whereλistheX-raywavelength,βisthehalf-maximumbreadth,andθistheBraggangleofthe(311)plane.Themorphologicalfeaturesandtheelectrondiffractionpatternsoftheferritepowderswereimagedbytransmissionelec-tronicmicroscope(TEM;HitachiH-7100,Japan).Vibrat-ingsamplemagnetometer(VSM;Toei,VSM-5)wasusedtodeterminethehysteresisloopsofferritepowdersatroomtemperature.