Microwave-absorbing characteristics for the composites of TP(4)

30C.-H.Pengetal./MaterialsScienceandEngineeringB117(2005)27–36

Theabsorbingcharacteristicscanberepresentedasthere ectionloss(R.L.),tobedescribedas[22]:

R.L.(dB)=20log (Z10in 1)

(Z (3) 1/2 in Zin=µr2πfd µr

1/2

rtanhjr(4)

whereZinisthenormalizedinputimpedancerelatingtothe

impedanceinfreespace,εr=ε jε ,µr=µ jµ isthecom-plexrelativepermeabilityandpermittivityofthematerial,disthethicknessoftheabsorber,andCandfaretheveloc-ityoflightandthefrequencyofmicrowaveinfreespace,respectively.

TheimpedancematchingconditionisgivenbyZin=1torepresenttheperfectabsorbingproperties.Theimpedancematchingconditionis determinedbythecombinationsofsixparametersε ,ε ,µ,µ ,fandd.Also,knowingtheεrandµr,theR.L.valueversusfrequencycanbeevaluatedataspeci edthickness.

3.Results

3.1.PhaseIdenti cation

Fig.2showsthecrystallinephasesofas-synthesizedun-dopedanddopedferritepowders.Itisobservedthatonlysin-glespinelphaseexists.Inthedopedferritecases,the

dopants

Fig.2.XRDpatternsofas-synthesizedNiZnferritepowders.(a)Undoped,(b)Co-doped,(c)Cu-doped,and(d)Mg-doped.

ofCo2+,Cu2+orMg2+seemtodissolve/arrangeinthespinelstructuretoful lltheformationofsinglespinelphase.Itisgenerallyrecognizedthatthevacancysitesofpartialdepri-vationofNi2+andZn2+canbe lledbythese2+dopantionsarisingfromthefactsthattheionicradiiofCo,Cu2+andMg2+areactually0.060nm,0.058nmand0.057nm,respec-tively,whichareclosetotheionicradiiofNi2+(0.055nm)andZn2+(0.060nm).(Theseionicradiimentionedabovearesimplyforthecaseofcoordination2+number(C.N.)=4.WhenC.N.=6,theionicradiiofNi,Zn2+,Co2+,Cu2+andMg2+are0.069nm,0.074nm,0.075nm,0.077nmand0.072nm,respectively[23],whicharenearlyequaltoeachother.)Fig.3showsthe morenarrowpeaksofNiZnferritesafteranneal-ingat500Cand900 Cfor1h.Itisexpectedthatcrystallitesizeafterannealingwillbecomelargerduetograingrowth.Table1depictstheresultofthecrystallitesizedeterminedbyXRD.Onecanobservefromthistablethatthegrainsizeincreaseswithrespecttotheincreaseofannealingtempera-ture.

3.2.Morphologyobservation

Themorphologyofas-synthesizedNiZnferritepowdersandthepowderproductsannealedat500 Cfor1hwiththeircorrespondingelectrondiffractionpatternsareshowninFig.4.Thecrystallitesizeswereestimatedtobe～20nmfortheas-synthesizedpowdersand～35–45nmforthe

annealed

Fig.3.XRDpatternsofas-synthesizedNi0.5Zn0.5Fe2O4powders.(a)With-outheattreatment,(b)annealedat500 Cfor1h,and(c)annealedat900 Cfor1h.