Microwave-absorbing characteristics for the composites of TP

MaterialsScienceandEngineeringB117(2005)

27–36

Microwave-absorbingcharacteristicsforthecompositesofthermal-plasticpolyurethane(TPU)-bondedNiZn-ferritespreparedbycombustion

synthesismethod

Cheng-HsiungPenga,Chyi-ChingHwangb, ,JunWanb,Jih-ShengTsaib,San-YuanChena

ab

DepartmentofMaterialsScienceandEngineering,NationalChaoTungUniversity,Hsinchu300,TaiwanDepartmentofAppliedChemistry,ChungChengInstituteofTechnology,TashiJen,Taoyuan335,Taiwan

Received6June2004;receivedinrevisedform6October2004;accepted14October2004

Abstract

NiZnferritewiththechemicalformulaNi0.5 xZn0.5 xM2xFe2O4(M=Co,Cu,orMg,andx=0or0.05)wassynthesizedbyacombustionsynthesismethodusingmetalnitratesandurea((NH2)2CO)asreactants.Thenanocrystalliteofthesematerialsweremixedwithathermal-plasticpolyurethane(TPU)elastomertobeconvertedintoamicrowave-absorbingcomposite.Thecomplexrelativepermittivity(εr=ε jε )andpermeability(µr=µ jµ )oftheabsorberweremeasuredinafrequencyrangeof2–12GHz.There ectionloss(R.L.),matchingfrequency(fm)andmatchingthickness(dm)werecalculatedusingthetheoryoftheabsorbingwall.Effectsofboththeparticlesizeofferriteandthedopantpresentedintheferriteontheelectromagneticpropertiesandmicrowave-absorbingcharacteristicswereinvestigated.Itwasfoundthatnanoparticlesaround40nmexhibithigherre ectionlossthanboththoseobtainedfrommicro-sizedpowdersandthosewithsizelessthan25nm.Also,itwasshownthattheCo-doped,Mg-doped,andCu-dopedNiZnferrite-TPUcompositescouldbedesignedtobemorenegativethan 20dB,inafrequencyrangeof2–12GHz,tobecomepromisingmaterialsformicrowave-absorbingapplication.Thesedopedferrite-containingcompositeshavemoreeffectivemicrowave-absorbingcharacteristicscomparedtoundopedone.©2004ElsevierB.V.Allrightsreserved.

Keywords:NiZnferrites;Microwaveabsorbers;Re ectionloss;Combustionsynthesis

1.Introduction

Muchattentionhasbeenattractedbymicrowave-absorbingmaterialsduetothefactsthattheycanabsorbenergyfrommicrowaveandthattheycanbewidelyusedinthestealthtechnologyofaircraft,televisionimageinter-ferenceofhigh-risebuildings,andmicrowavedark-roomandprotectionetc.[1,2].Extensivestudyhasbeencarriedouttodevelopmicrowave-absorbingmaterialswithhighef ciencyandnewabsorptionmaterials[3–5].Onecriterioninselect-ingtheabsorbingmaterialistodiscoverthelocationofitsnaturalresonanceregion.Thus,astudyofthefrequencyde-pendenciesofthecomplexrelativepermittivity(εr=ε jε )

Correspondingauthor.Tel.:+88633891716x312;fax:+88633892494.E-mailaddress:cchwang1@ccit.edu.tw(C.-C.Hwang).

andpermeability(µr=µ jµ )ontheabsorbingmaterialhasbeena eldofinterest.

Inthepast,thespinelferriteshavebeenutilizedasab-sorbingmaterialsinvariousformsformanyyears.Also,theferrite-polymercompositesareusefulasabsorbersduetotheirlightweight,lowcost,andgooddesign exibility[1,5,6].Amongthespinelferrites,NiZnferritesarecommerciallyusedaselectromagneticdevicesoperatedinhighfrequencies(>10MHz),arisingfromthefactthatmetaldopantscanbeusedtomodifytheelectromagneticpropertiesofNiZnfer-ritesbyincreasingtheresistivityandpermeability[7].Forthispurpose,someattemptsweremadetoverifythecorrela-tionbetweenthematerialconstants,εrandµr,andmicrowaveabsorptioninthesinteredferritesthatcontaindivalentmetalions[6–9].

Recently,theresearchpursuingtonanocrystallinema-terialsandtheirpropertieshasbeengivenmuchattention

0921-5107/$–seefrontmatter©2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.mseb.2004.10.022