Microwave-absorbing characteristics for the composites of TP(9)

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

Table4

Re ectionlossofcompositesmadefromsynthesizeddopedandundopedferritepowdersannealedat500 Cfor1hSpecimen

R.L.(dB)d=2mm

Co-dopedNiZnferriteCu-dopedNiZnferriteMg-dopedNiZnferriteUndopedNiZnferrite

35

LocationofmaximumR.L.(GHz)

d=5mm 42.51 37.07 29.32 24.34

d=2mm

11.1810.5210.179.52

d=5mm5.145.586.045.82

Bandwidtha(GHZ)d=2mm0.340.210.120.10

d=5mm0.570.430.460.33

30.74 27.61 23.41 21.52

a

ForR.L.≤20dB.

tolargehysteresisattenuationandabsorbingbehavior.Ourexperimentalresultsagreewiththisperspective.

Ontheotherhand,whentheM2+ion(M=Co,Cu,orMg)substitutionsareintroducedintothesublatticesinthereversedNiZnspinelferrite,itmayoccupytheoctahedralsitesinreplacingtheZn2+orNi2+ionsduetotheirnearlythesameradii.Thestrengthofthesuperexchangeinterac-tioncanbereducedwhereasthemagneticmomentscanbeimproved,resultinginanincreaseinthespontaneousmagne-tization[27].Someresearcheshavebeenmadealsotoverifythatnotonlycanthesaturationmagnetization(Ms)andcoer-civeforce(Hc)ofNiZn2+ferritesbeenhancedbysubstitutingaportionofZn2+orNiionsbydivalentmetalionssuchascobalt,butalsotheinitialpermeability[6–9,28].Inthiswork,fortheCo-doped,Cu-doped,andMg-dopedNiZnferrite,thepermeabilityandthehysteresisloopcanbemodi edduetothepresenceofthesedopants,whichareinagreementwiththestatementmentionedabove(seeTable2andFig.6).Thefurther-improvedmagneticpropertiesofthesedopedferriteswerealsoreportedinRefs.[6–9,26],andtherebyitwouldbeinstructivetosuggestthatthedoped-ferritecompositionspossessbetterabsorbingcharacteristicswhencomparedwiththeundopedone.Thisopinionmaybeveri edusingthedatalistedinTable4.

Insummary,inthiswork,theexperimentalresultsindi-catethatthegrainsizeoftheNiZnferritesandthepresenceofthedopantaffectthemicrowave-absorbingbehavioroftheferrite-TPUcomposite.Therefore,theimprovedre ectionlosseswerethoughttoberelatedtoenhancedpermeability,largercoerciveforce,andlargehysteresisloop.Theseresultscanbeattributedtobefromthedomainwallmotionandspon-taneousmagnetization.Furtherinvestigationfortheeffectsofbothferriteanddopantcontentontherelationshipbetweentheelectromagneticcharacteristicsofnano-sizedferritecom-positesandtheirmicrowave-absorbingpropertiesareunder-wayandwewillsoondisclosewherewestand.

5.Conclusion

Inthisstudy,wehavecarriedoutexperimentstodemon-stratethedependenceofparticlesizeofNiZnferriteonthere ectionlossofferrite-TPUcompositebyusingcombus-tionsynthesizedpowders(20–80nm)andcommercialmicro-sizedpowders.ItisfoundthatnanocrystalliteofNiZnfer-

ritearound35–45nmexhibithigherre ectionlossthanboththoseofmicro-sizedpowdersandthosewithsizelessthan25nm.Inaddition,theCo-doped,Mg-doped,andCu-dopedNiZnferrite-TPUcompositeshavemoreeffectiveabsorptioneffectthantheundopedone.Thesecompositescouldbede-signedtobemorenegativethan 20dBinafrequencyrangeof2–12GHztobeabletobecomepromisingmaterialsformicrowave-absorbingapplication.

Acknowledgements

SupportforthisresearchbytheNationalScienceCounciloftheRepublicofChinaunderGrantno.NSC93-2623-7-014-005isgratefullyacknowledged.

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