Indian .l ou rll;iI of Pure & App li ed Physics Vol. :IX. March 2000. pp. 21 :1-21 I)

Dielectric constant, permeability and microwave absorption studies of Ba-NiTi hexaferrite composites in X-band

*Praveen Singh & T C Goel

Department of Physics. Indian Institute of Technology. ew Delhi

Received 22 .luly 191)1): rev ised 25 January 2000: accepted 7 Fehru ary 2000

The elTect of Ni ~+Ti .J+ suhsti tution on complex permeability and permi lli vity and microwave ahsorption has heen stud ied for Ba( N iTi ), Fe t ~.~x ()1 ') hex;! !'crrit e-epoxy compos it es. wherei n.r varies from 0.0 to 1.0 in steps 01'0.2. i nthc frequency range X.O-12...!

G HI. . The hexa ferrite-co rnposites wi th.r < O,(l ex hihit signi fi can t dispers ion in the complex permill i vit y (£' - j£") wi th Illa x i mU1l1

value of r" is ohserved for .r equal to 0.6. The dispersion in the complex permeab i lity (P' -jp" ) is not signi ficant which is attrihuted to the reduced interacti on het ween crystall ises of ferrit es and low alternating electromagnet ic fi eld app lied to the sample. The variation of rel'icction loss has heen studied a.s a function or frequency (f). Ni2+Ti4+ Content (x) and thi ckness of the absorber (ti). A minimum rellcclion loss of -30.0 dB is obtained for composites with .r equal to 0.6 and absorher thickness of 2.75 m1l1 . The ex perimental values of the matching frequency <1ndlhe matching thickness agree well wi th the theoreti ca l va lues ohtained by so lving the equations numericall y.

Introduction In the development of ferrite based microwave ab­

sorbers , the M- type hexagonal ferrites are o f importance

to ex tend the ran ge of microwave absorbers to hi gher

frequency region o f X-band l.These fen'ites absorb mi­

crowa ves due to vari ous interact i ve loss processes of

magnetization and po lari zati on in the material. The

complex permeability (~l'~i ~") and permittivity (£' -j£")

of the constituent material o f the microwave absorbers

determine the reflecti on and attenuation characteri sti cs

of the in vestigated material. In case of ferrite epoxy

composites, a number of investi gat ions had been re­

poned " ) for study ing the effect of compos iti on on their

microwave properti es . Many studies were carri ed out to

in vesti gate the effec ts of fe rrite material s and its vo lume

percentage in the compos ite o n the absorpti on of micro­

wave{' and the influence o f the addition of conducting

fiber7

on the microwave absorbing properti es. The pre­

sent work is aimed at findin g the optimum amount o r

N i2+Ti .J+ ions to be substituted in BaFe1 2()1 1J hexagonal

ferrite in o rder to y ield a minimum reflect ion loss over

hroad frequenc ies and also to in vesti gate the quantita­

t i ve rela ti onship between magnet ic and cI ie lectric char­

acteristics o r ferrite-epoxy compos ite and its microwave

ahsorbing properti es in the X-band frequencies. The

':' Present Address: Biophysics. Ele': tron Microscope and Instru1l1entation Section . Indian Veterinary Research Institute. I/.atnagdr. UP 24:1 122

theoretical determinati on of matching frequenc y and

matching thick ness was ta ken up with the help o r equa­

ti o ns g i ven by Naito e! o f". In order to ac t as lossy

materi als, the following two conditi ons are requirecllo

be satisfied ; ( I ) The normall y incident electromagnet ic

wave mu st be able, to enter the m aterial surface i .e.,

re fl ectivity at the surface must be minimum; (2) The

inc ident electromagnetic wave must be attenuated rap­

idly and effectively in the material. These conditi ons are

expressed here by llsi ng the complex re lati ve permeabi 1-

ity (~',- .i ~",) and comp lex relative permitti vi ty U:'.­/ £",) of the materi al.

characteristics impedance of absorber

(2;) = .~ ( ~l ,. /1o/£,. £0 )

free space impedance (20) = ~ ( /10/£0 )

For condition I to be satisfi ~d ; the impedance should

be matched at the absorber surface i .e.

2;=20

2, /20 = I = '1/ ( ~' ,.- .i /1",) / ( E ',. - j £ " ,. )

The condition I requires that:

/1./ = £/ and /1 /' = £/' and conditi on 2 requires that PI'.

~t,", £/ and £," parameters shou ld have hi g h va lul's .

2 Experimental Details A seri es o f samples was prepared by the conventi onal

so lid st a t e r eac ti o n method wi th co mpositio n

Ba(N iTi ), Fell.2xOI') wherein x vari es fro m 0.0 to I.n in

2 14 INDIAN J PURE & APPL PHYS, VOL 38, MARCH 2000

steps 01'0.2. The sample .~ were sy nthes ized from pulve r­

izing each sample and heating in the air at I OOO.O°C for

12 hr . The pre-sintered mat erials were thoroughl y ground and cold pressed into pallets and were finall y

sintered at 1300.0°C for 12 hr . X-ray diffraction analy··

ses of few typi cal sampl es were made by using Cu-KOt radi ati on from Ri gaku·-Denki diffractometer, whi ch confirmed the magnetoplu bite type of crystal structure for these samples. The ferrite-epoxy composites were prepared by homogeneously mi xin g the ferrite powder with 90 % epox y res i n (EPG 280) and 10 % hardener (No 10). The hexa ferritc compos ites of thi ckness ap­

proximate ly 2.0 and 3.0 mm were prepared by perspex di es of rectangu lar shape and size so as to fit exactly into the cav ity of the X-band microwave bench. These sam­

ples were cured at 80 .()OC for 2 hr and coated on alumi­num substrate. The variati ons of refl ec ti on loss versus frequency were studied hy measurin g VSWR in the

X- band on microwa ve bench. In order to measure the

compl ex relati ve permeability (p ,'- jp,") and compl ex

re lat ive permitti vit y (E,'- j E," ) of these ferrite compos­ites, toroidal shaped samples of 3.5 mm outer di ameter and 1.5 mm inner diameter were nrenared. T he samnles

5·0

4·0

u IE t; 2·0 w ...J W 5

1-0

were prepared with 50.0 % (weight percentage) ferrite

loading in epoxy resin. The measurements Of E,' , E,", p,'

and p," versus frequency were made by reflec ti on trans­miss ion tec hnique using Hewlett Pac kard Network An­al yser Mode l HP 851 () B . The tes t samples of toroidal shape were ti ghtl y inserted into the standard coax ia l line of I 5.0 cm air length and used to measu re re fl ected and tran smitted scattering parameters (5 11 , 52 1), The va lues

of E/, E,", p ,.' and p," parameters were calculated for a ll I . I R ') t lese compos l te samp es . .

3 Results and Discussion

3.1 Dicledric and magnetic parameters

The frequency dependence of E,.' and E," for Bil-M hexaferrite-epoxy compos ites with NiTi content , x, equal to 0.0, 0.2 and 0 .6 is shown in Fig. I. The rea l part

of re lati ve permitti vity (Er') increases slightl y with in­creased content of substituti on, x , equa l to 0 .2 and 0.6 from 8.0 to 10.8 GH z. There is signi fic.ant decrease in

the values of E/ for all the compos ites from 8.0 to 12.4

GHz. The dielectric loss co mponent (E," ) shows hi ghest va lues for compos ites with x equal to 0.6. The values of

die lec tric loss (E," ) increases with frequency up to I () .() GHz thereafter it dec reases. The di e lectric nroperti es of

X valu e

~: O· O

-_x;...--- = o· 2 -_e-:: 0. 6

0-08'0 g·o 10·0 11 ·0 12·0

FREQUENCY (GHz)

Fi g. I - Varial io lls of £', and £", as a funcliun for Ba(NiTi lxFc I2-2xOI') hexa fcrrite cO lllposiles for x = D.O. 0.2 . (J.(,

SING H & GOEL: Ba-NiTi HEXAFERRITE COMPOSITES

polyc rystali ine ferrite-pol ymer compos ites arise mainl y due to the interfac ial polari za ti on along with so me con­tribution from ioni c polari zati on and intrinsic electri c dipole polari zati on. The i'lterfacia l polarization results from heterogeneous ~tructure of fen-i tes compri sin g low conducti vity grains separated by the higher resisti vity grain boundaries as proposed by the KOO plO The mar­

gi nall y hi gher va lues of £,' for composites with x equal to 0.2 and 0.6 may be due to some contributions of F 1+ ,+ . II N' ?+T '-!+ I e' and Fe- Ions as we as 1- I comp exes to interfacial polarization. The dielectric loss increases very sli ghtl y for hi gher va lues ofx where the number of Fe.l+ ions is red uced .

The permeability ( p ,'~i !.l") of the composi tes shows

sma ll va riati ons in the va lues of ~L/ and Pr" in X-band .

The real part of relati ve permeability (p/) varies be­tween 1.1 to 1. 3 for these compos ites whereas imaginary

part (Pr" ) shows a linear increase in the values ofpr" for compos ites with x equal to (J .O and 0.6 as shown in Fig.

2. The va lue of Ilr" for composites with x equ al to 0.2 shows increase up to 11 .5 GH z. thereafter, it decreases marginally. The magneti c behav iour of compos ites pri­marily depends on the hexa!!onal fen-ites used as ma!!-

netic filler. The variation in the magneti c spect ra I S

attributed to the three magneti zati on processes name! y,

relaxation of magnetizati on (Xrel), resonance of magnet ic

domain (Xli) and spin rotational resonance (X, ). The relaxation of magnetization is a dominant process and mainl y responsible fo r higher microwave l osses~. More­over, the crystalli ses of ferrites are enve loped by non­magnetic coating of epoxy res in which enhances the effective reluctance of the composite thus resulting in the weak di spersion phenomena in the spectral I of

p," .The relations expressing the resonancc- relaxat ion phenomena near the characteri stics frequen cy of spi n rotati on or domain wa ll displacement are gi ve n by Ish ino el ([/1 2 The effec ti ve magnetic spectra of com pos­ites have been desc ribed in a model by Slama e l (If II which attempts to predicts effecti ve permeability. sus­ceptibility of composites by taking into account the grain and matrix reluctance and their dimension parame­ters.

3.2 Reflection loss measurement

A minimum value of -30.0 dB retlect ion loss at 10.0 GHz was measured for 2.8 mm thickn ess for Ba(NiTi)I\(,Fell, "0", hexarerri te co moos ites. The loss

1·5~-------------------------------------------------, x value

- - 0--- = a-a -- =0 ,2

I ~=0- 5 4y ~

r~--: --~ ----"-0-;---; -~~~ _--",c.....-. --

>­f----l CD « w ~ Ck: W 0-5 Cl..

8-0 g·o 10 ·0 11 ·0 12·0

FREQUENCY (GHz)

Fig . 2 -- Vari at ions or 1-1,.' and I-I r" as a rUllction or rrequency 1'01' Ba(NiTih FeI 2-2xOI'j hexarerr ite composites ror .\' = (J.O. 0.::. (l.()

216 INDIAN J PURE & APPL PHYS, VOL 38, MARCH 2000

peak exhibits a small hal f bandwidth of 250 MHz. The other minimum refl ecti on loss peak of -25.0 dB was obse rved 10 .2 GHz 1'0 1' 2 . 8 nim thi c kn ess for Ba(NiTi)o2FeI1601 <) hexak ril ite composite as shown in Fig. 3.The two small loss peaks of -I n.o dB and -7 .5 dB were observed at 10.8 GHz and 9.0 GHz respectively for the compos ites ex = 0.0, 0.4) of 2.8 mm thickness as shown in Fig. 4. The minimum refl ection loss is signifi­ca ntl y in creased for thv hexaferrite composites of 2.0mm thickness . This is due to increased impedance mismatch arising out of small er thickness mi crowave absorbing layer.

3.3 Microwave ahsorhing properties

The nonnali zed inpul impedance of a microwave absorbing layer backed by refl ector at the absorber surface ' (Zi ll ) is given by :

- 112 · 112! II ( I Zill = Zi I Zo = (I.1'/Er) tanh 1.f2n Ie (/-lltr) .L . ... )

where Zi and Zo are the impedances of absorber and

that of free space respecti vely, /-lr and Er are relati ve permeability and relati ve permittivity of medium, C the veloci ty of li ght,f is the frequency of microwave in free space and d is the thi ckness of absorber. The re fl ec ti on loss(' is re lated to Z;" as

0·0

-10·0

m "0

1Il 1Il

:3 z-20'0 o I-U W ....J LL W rr

-30,0

8·0 g·O 10·0

Reflection loss (dB) = 20 log lo I (Zill - I) I (Zill + I ) ... (2)

Thus surface refl ectance of an absorber is a functi on

of six characteri st ics parameters, viz,.!; d, E/, fr" , /1 ,' ami

/1 r" . It is possible to evaluate the numeri ca l values of the parameters f and d correspondin g to the zero reflec ti on condition (Zin = 1.0) by substituting the measured values

OfE,', E/', /-l/ and /-l/' into the Eqs ( I ) and (2). The va lues of rdlection loss for composite with x equal to 0.6

calculated by using Eqs ( I ) and (2) for var ious measured

va lues of E,', Er" , /-l,' and /1 ,'1 are shown in Fig. 5. The minimum refl ecti on point shifts to the lower frequency with increase of thickness.

The compari son of refl ec ti on loss measured and ca l­culated for the compos ite (x =: 0.6 with thi ck ness 2.X Ill ill is show n in Fig. 6. The peak of the ex perimental curve fall s almost at the same frequency as that of the theoreti ­cal curve. The experimental curve does not fit exactly on the theoretical curve due to spurious refl ec tions caused by vari ous components of wave guide whi ch cause errors in VSWR measurements and also because of surface irregularity of absorbin g sample. The g"p between waveguide dimensions and sam ple size allli also instabi lit v in the freouencv source or ll1icrowa ve

11·0

x Value ~- =0 ' 0

~- ;: 0 · 2

--6-- :: 0·4

----:: 0·6

d. ~ 2· 8 mm

12·0 130

FREQ.UENCY (GHz)

Fi g. :l - Rc ll ection loss as runct ion or composit ion (x :: 0.0.0.2 . !lA. 0.6 )

j...-.

SINGH & GOEL: 8a-NiTi HEXAFERRlTE COM POSITES 217

bench cause error in measurement of voltage standing wave rat io.

Fig. 7 shows the vari at ion of matching thi ckness w ith matching frequency fo r a Ba(N iT i)o r.Fe'tlSO,,) hexafer-

rite-compos ite. The matchi ng thickness decreases al­

most linearl y w ith increase in the matching freq uency. T his fo llows the re lati onship given by Eq. ( I ) by aSSlIlll-

0'0

z -10-0 o r-u w --.J LL W 0:

-20-0 ·0 g-O 10·0

FREQUENCY (GHz)

X Value

-&-- =0 · 2 --6- =0 · 4

---.- =0' 6

d~ 2.0mm

12·

Fig. 4 - Reflect ion loss for hexaferrite cO lllposites of 2.0 Ill ill th ickness

OO~ 8 0 Ni Tj Fe 0

0·6 0· 6 1()· 8 19

- 10.0

~-20.0

-1

z o ~

~-3 0.0 -1 lL. W 0:

-40.0

o tj.

- 50. O~. _.1.., _.L....-LI --.-l_ -L---.l._.L1 -l.--L_L..t:c.>.JI_--L---L_..l-~1 _ -'L---'-----'----.J

8·0 g·O 10·0 11- 0 12 '0

FRE Q. UENCY (GHz)

Fig. ::; - Microwa ve ahsorpt ion hehaviour of cOlllpmitc (x = 0.6) at different lllatChing thi ckness

2111 INDIAN J PURE & APPL PHYS, VOL 38, MARCH 2000

o.or-------------------------------------------------------~

rn :s

-10.0

(/) -20.0 g z o ~ u ~-30.0 lL W a:

-40.0

9·0

o o

10·0

FREQUENCY (6Hz)

o d ~ 2'Smm

EXPERIMENTAL

•

THEORETICAL

11-0 12·0

Fig. () - C() mflari~()n of theoret ica ll y determined and experimentall y measured values of reflection los~ with frequency

E E

3 ·5~----------------------------~~-------,

~3'0 Vl Vl Cl> c

..x u

.r:. t-

2· 0 '---'---'------'~_-'-----L---l'__-'-----'-_'____'_____L_.L.___'____'_ _ _____'

8·0 g·O 10·0 11 ·0 12·0

Mate h ing Frequency (GHz )

Fig. 7 -- Correlat ion hetween the match ing layer thi ck ness dM and the miltehing frequencY.l~1 for Ba(N,Ti >x Fc I2-2xO I"

SINGH & GOEL: Ba-N iTi HEXAFERRITE COMPOSITES 2 19

ing Il, and E, as constant III the given regIon of fre­quency.

Acknowledgement The Auth ors would like to thank Prof S L Srivastava,

Department of Phys ics , Allahabad University, Alla­habad, for providing the experimental facility for the measurement of dielectric constant and permeability on Hewlett Packard Network Analyser.

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