Indi an Journal of Engineering & Materials Sciences
Vol. 7. October-December 2000. pp. 422-425
Microwave absorption studies of BaCoTi ferrite composites
Archana Razdan". V K Babbar" & Praveen Singh b*
"Department of Applied Physics. Guru Nanak Dev University. Amritsar 143005. Indi a
bDepartment of Physics. Indian Institute of Technology. New Delhi 110016. Indi a
Received 25 FebruGlY 2000; accepted 5 Septelllber 2000
A ferrit e series of the compos iti on BaFeI2_2,CoxTixOI9. wherein x varies from 0.0 to 0.9 in steps of 0.2 . has been
prcpared by the standard ceramic technique. The microwave absorption studies of ferrite-polymer composites with different
till-factors have been made in the freq uency range from 8.6- 12.4 GHz. A maximum return loss of 14.7 dB has been
obtained for compos ites with x equ al 0.3 and absorber thickness of 4.0 mm at 9.2 GH z. The dependence of matching
frequency on compositio n is appa rent for this thickness. The maximum values of dc res istivity and th e Curie tem perature
have been found to be 234.8 kQ and 469°C respectively. These values correspond to the same composition for which the
return loss is the maximum. A dependence of return loss on complex dielectric co nstant and permeability has been
investigated in the X-band.
The magnetic dielectrics, such as M-type hexaferrites,
are known to be effective microwave absorbers due to
their lossy interaction with the electric and magnetic
fields of microwaves, which transforms the
microwave energy to thermal energyl.5. The complex
permeability and permittivity affect the reflection and
atten uation characteristics of these materials. The
composites of such ferrites with suitable polymers are
quite useful as they can be easily shaped and
converted into a paste or a paint that can be applied on
the objects of interest. The absorption characteristics
of ferrite-po lymer composites depend on the
composition of ferrite, nature of polymer, fill factor
and thickness of composite layer and can be tailormade by altering these parameters. Barium hexaferrite
has been extensively used for producing permanent
magnets and microwave devices 6 . The substitution of
C02+Ti 4+ ions for Fe 3+ ions in Ba-ferrite is expected to
change the anisotropy constant and modify the
properties so as to make them effective microwave
absorbers. In the present work, we report the
microwave absorpt ion characteristics of substituted
BaCoTi hexaferrites obtained fro m the observed
variations of permeability and permittivity as a
function of C02+Ti 4 +-content at different frequencies.
It helps to determine the optimum amount of C0 2+Ti 4 +
ions in Ba-ferrite, which yields a minimum reflection
loss over a wide freq uency range.
*Present address : Indian Veterinary Research Institute. Izatnagar.
Bareilly 243 122. India
Experimental Procedure
A ferrite series of composition BaFe I2-2,CoxTi xOI 9,
wherein x varies from 0-0.9 in steps of 0.2 was
prepared by the conventional ceramic method. The
samples were synthesised fro m stoichiometric
mixtures of BaC0 3 , CaC0 3 , Ti0 2 and Fe203 of 99.9%
purity. The mixture was wet-milled in di stilled water
and calcined in air at 1000a C for 12 h. The material
was again wet-milled and granulated using a small
quantity of polyvinyl alcohol as binder. It was then
cold pressed into pellets followed by sintering at
l300 a C for 24 h. The ferrite-epoxy composites with
50% of ferrite by weight were prepared by
homogeneously mixing the ferrite powder with 90%
epoxy resin (EPG 280) and 10% hardener (Number
10). The hexaferrite composites of differe nt thickness
were prepared by using rectangular perspex dies.
These samples were cured at 80 a C for 2 h and coated
on aluminium substrate. The variations of return loss
versus frequency were studied by measuring VSWR
using X-band microwave bench. In order to measure
the complex relative permeability (J.1' - jJ.1') and
permittivity (E' - jE'), the toroidal-shaped samples of
3.5 mm outer diameter and 1.5 mm inner diameter
were prepared. The measurements of E', E'~ J.1' and J.1"
as a function of frequ ency were performed by a
reflection/transmission technique using a Network
Analyser (HP 851OB). For this, the toroidal-shaped
samples were tightly inserted into the standard coaxial
line of 15 cm air length and the reflection and
RAZDAN
el
al.: MICROWAVE ABSORPTION OF BaCoTi FERRITE COMPOSITES
transmission scattering parameters (SII, S21) were
determined. These parameters were employed to
7
calculate the values of E', E'~ f-l' and f-l'~ The dc
resistivity of these samples was measured by a two
terminal method using spring-loaded copper
electrodes. The gravity method 8 was used to
determine the Curie temperature.
Results and Discussion
X-ray
diffraction
analysis
revealed
the
magnetoplumbite structure for all the compositions of
BaCoTi ferrites. The variations of return loss and
percentage absorption with frequency have been
studied for Ba-ferrite composites as a function of
C02+Ti 4+-content, sample thickness and fillfactor. The
return loss is found to be maximum when the fillfactor is 50% as shown in Fig. I for a typical
composition, BaCoo.3Tio.3FeI1.401 9. The remaining
data is, therefore, reported for 50% fill-factor only.
The lines in figures are not theoretical curves but are
drawn only to guide the eye. Fig. 2 shows the return
loss and the percentage absorption versus frequency
for different compositions and the thickness of 4.0
mm. A maximum absorption of 14.7 dB was obtained
at frequency of 9.2 GHz for C02+Ti 4+-content, x, equal
to 0.3. A lower absorption of 13.9 dB was observed at
a frequency of 9.l GHz for the composition with x
equal to 0.5. The return loss variation for composites
of 3.0 mm thickness is shown in Fig. 3. It shows
16
---x=O .O
-"-·x=0.1
.. . .. x=0.3
-o-·x=0.5
--6--x=0.7
___ x=0.9
14 12 -
.
m 10
~
II)
<J)
0
...J
8 -
C
....
:::J
Qi
6 ·
0::
4 -
Or----.----,----,----~==~~~~
8.5
9.5
9.0
10.0
11 .5
9-r------------------------------.
8
I
,
I.
I
,
I
I
---x=O.O
-"-·x=0.1
.. . .. x=O .3
II
I
I
14
\
I
•
I
m
12
~
<J)
II)
6
~.
5
,~
,"
I
Qi
8
I
0::
C
:;
.
I
I
E
4
:::J
II)
II)
I
\
I'
...J
~
I
,; :
o
CD 10
0:::
11.0
Fig. 2 - Variation of return loss with frequency for different
compositions of BaCoxTi xFel2_2xOl9 hexaferrite composites of
thickness 4.0 mm
7
iii
10.5
Frequency (GHz)
16~-----------------------------.
0
-l
423
I
..
I
3
I
I
I
I
l,
\
,,
,
I
,,
*'
".
I
6
\
...
I
2
I
I
4
I
1
2
..
.. ;;'.""
I
'"
"'....
.,ro'ji."
" A
O-~--~--~--~--~~~~~
0
8,5
8.5
9.0
9.5
10.0
10.5
Frequency (GHz)
Fig. I - Variation of return loss as a function of frequency and
fill -factor for BaCoo.3Tio.3FeI1.4019 hexaferrite composites
9.0
9.5
10.0
10.5
11 .0
11.5
Frequency (GHz)
Fig. 3 - Variation of return loss with freq uency for different
compositions of BaCoxTixFe l2_2xOl9 hexaferrite composites of
thickness 3.0 mm
INDIAN J. ENG. MATER. SCI., OCTOBER-DECEMBER 2000
424
absorption of less than 8.0 dB at 9.2 and 9.6 GHz for
compositions with x equal to 0.0 and 0.1 respectively.
Such a small absorption indicates a high impedance
mismatch at air-sample interface. Other composites
with x equal to 0.5, 0.7 a
nd 0.9 show a very small absorption of about 2.0 dB
and are not shown. It is apparent that the return loss is
greater for the sample thickness of 4.0 mm than that
of 3.0 mm. For other values of thickness, the return
loss is further low and is not shown. Fig. 2 shows two
absorption maxima for compositions with x equal to
0.7 and 0.9 and sample thickness of 4.0 mm. Fig. 3
shows similar maxima for x equal to 0.0 and 0.3 with
sample thickness of 3.0 mm. These may be attributed
to the resonance phenomena due to domain wall
displacement at lower frequencies and spin rotation at
higher frequencies. The dependence of matching
frequency on composition is apparent particularly for
sample thickness of 4.0 mm.
The dc resistivity is found to increase from 172.9
kQ to 234.8 kQ as x increases from 0 to 0.3 followed
by a continuous decrease to 16.5 kQ as x approaches
0.9 in barium ferrite. The conduction in ferrites is
attributed to the simultaneous presence of Fe2+ and
Fe 3+ ions on equivalent lattice sites, which are mainly
4fvl and 2b sites present in the R-block of the M-type
barium ferrite 9 . The electrons can hop between Fe2+
and Fe3+ ions on the neighbouring equivalent sites and
cause conduction. As C02+ and Ti4+ ions have site
preference for 4fvl and 2b sites9, the substitution of
these ions in barium ferrite forces some of the Fe3+
ions to migrate to 12k sites, thus weakening the
hopping mechanism and increasing the resistivity.
The decrease in resistivity for x>O.3 may be due to the
contribution of C0 2+ and Ti 4+ ions to hopping and is
being investigated. The Curie temperature, Te,
increases marginally from 425°C to 469°C when x
increases from 0 to 0.3, and decreases slightly to
457°C for x equal to 0.9. All the measurements have
an inaccuracy of ±5°C. It is, thus, apparent that the dc
reststtvtty, Curie temperature and microwave
absorption all show their peak values for x equal to
0.3.
The variation of complex permittivity, (E' - jE''),
and complex permeability, (J..l'-jJ..l''), with frequency
for BaFel2019 ferrite is shown in Fig. 4. The real part
of complex permittivity, E', shows insignificant
decrease from 3.2 to 2.8 in the X-band as shown in
Fig. 5. However, with C0 2+Ti 4 + substitution, as shown
in Fig. 5, E' exhibits a significant variation. It
increases to 4.1 at 8.6 GHz and then decreases to 3.2
at 12.4 GHz. The dielectric properties of
~------------------------~
4.5
4.0 -
4.0
3.5 .
3.5
3.0 .
-+--;:-
2.5
2.5
-l
E
~
-k-E'
-:1.
~ E"
-=: 1.5 .
-:l.
--~"
...... E'
w.. 2.0
---~l"
·w 2.0 ·
'w
1
w
-+- fl'
....... e"
1.5
1.0 -
1.0
0.5 -
0.5 .
0.0 .1 - - - - - . - - - - . - - - - . - - , - - - - - 1
8.0
9.0
10.0
11 .0
12.0
13.0
0.0
+--,----,----r---.,---r----j
8.0
Variation of E', E'~ 11' and 11" with frequency for
BaFel2019 hexaferrite composite
10.0
11.0
12.0
13.0
Frequency (GHz)
Frequency (GHz)
Fig. 4 -
9 .0
Fig. 5 -
Variation of E~ E'~ 11' and l1"with frequency for
BaCoo.3Tio.3Fe11.4019 hexaferrite composite
RAZDAN et al.: MICROWA VE ABSORPTION OF BaCoTi FERRITE COMPOSITES
polycrystalline ferrite-polymer composites arise
mainly due to the interfacial pol arisation and intrinsic
electric dipole
polarisation.
The
interfacial
polarisation results from the heterogeneous structure
of ferrites comprising low conductivity grains
separated by high resistivity grain boundaries as
proposed by KOOpSID. The higher values of E' for
composites with x equal to 0.3 may be due to
significant contributions of C0 2+Ti 4+ ions in addition
to Fe 2+ and Fe3+ ions to interfacial polarisation .
Besides these, the epoxy resin has also got some
degree of electronic conductivity due to a large
number of polar groups present in the polymer chains.
The conductivity of epoxy-resin contributes to
dielectric loss in these composites. The permittivity
originated from
electronic polarisation, ion
polarisation and intrinsic electric polarisation is given
by
E = E.,.,
+ (Eo - E.,.,)/(l + jon)
where Eo and £= respectively represent permittivity for
angular frequency U)--10 and U)--1oo, and r is the
relaxation time for polarisation of intrinsic electric
dipole.
The permeability (p'-jp") of these composites
shows only a small variation in the X-band. The real
part of permeability varies between 1.1 and 1.3. The
imaginary part of permeability shows a significant
425
increase with frequency from 8.6 to 12.4 GHz. The
effective magnetic spectra of composites have been
described in a model!!, which attempts to predict
resonance-relaxation phenomenon by taking into
account the relative values of internal wavelength.
The lower values of permeability and weak dispersion
effects observed in these samples may be attributed to
the presence of nonmagnetic epoxy-resin between the
neighbouring crystallites which weakens the
intergranular magnetic interaction.
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