Solid State Sciences 113 (2021) 106553
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Solid State Sciences
journal homepage: http://www.elsevier.com/locate/ssscie
Electromagnetic and radar absorbing properties of γ Fe2O3/
Ba4Co2Fe36O60-epoxy polymeric composites for stealth applications
Vivek Pratap a, *, Amit K. Soni b, Himangshu B. Baskey b, S.M. Abbas b, A.M. Siddiqui a,
N. Eswara Prasad b
a
b
Department of Physics, Jamia Millia Islamia, New Delhi, 110025, India
Defence Materials and Stores Research and Development Establishment, Kanpur, 208013, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Strategic materials
Barium hexaferrite
Gamma iron oxide
Magnetic properties
Electromagnetic properties
Reflection loss
Microwave absorbers
Various constituent ratios with 50:30, 40:40 and 30:50 wt % of U-type barium hexaferrite (Ba4Co2Fe36O60) and
Gamma iron oxide (γ Fe2O3) have been used to designing the radar absorbing composites within epoxy resin
system. In the current work, studies are done on electromagnetic and absorbing properties of γ Fe2O3 powder
dispersed Ba4Co2Fe36O60-epoxy composites. X-Ray Diffraction (XRD), Field Emission Scanning Electron Micro­
scope (FESEM) and Vibrating Sample Magnetometer (VSM) tools have been used for structural, surface
morphology and magnetic properties respectively of the synthesized powder and fabricated composites. The
electromagnetic properties (ε′ , ε", μ’ & μ") of fabricated composites were retrieved from measured reflection (S11
& S22) and transmission (S12 & S21) parameters in 2–18 GHz frequency range. Experimental finding show that the
fabricated composite having 50 wt % of γ Fe2O3 of designed Ba4Co2Fe36O60/γ Fe2O3-epoxy composites possesses
maximum absorption of 98.8% (RLmin of − 19.89 dB) at 13.2 GHz for sample thickness of 3.2 mm. The fabricated
microwave absorbers confirm the effective absorption performance which has potential for strategic applications
and stealth technology.
1. Introduction
Electromagnetic spectrums having a part of frequency range between
0.3 GHz and 300 GHz known microwaves are important for smooth
functioning electronic devices and system, and also for design and
development of microwave absorbers in this frequency region. Due to
the tremendous advancement in wireless communication, electronics
and aviation sectors, microwave absorbers are working in various sys­
tems and subsystems level under different frequencies region. Conse­
quently, electromagnetic compatibility is a major issue as the radio
frequency (RF) radiation causes concern for human health. In order to
mitigate such issues, it is of paramount importance for the synthesis of
microwave/radar absorbing filler materials which can be used for
electromagnetic interference/shielding applications. On the other hand
for military applications, microwave/radar absorbing materials can be
used in various forms such as paints and composites for reducing the
radar cross section (RCS) of various strategic targets such as aircrafts,
missiles etc [1,2]. From the past decades, microwave/radar absorbing
materials (MAMs/RAMs) have been designed and intensively explored
to remove such problems with scientific interest [3,4]. Single domain
iron oxide nanoparticles with super paramagnetic behavior; maghemite
(γ Fe2O3) is individually most demanding materials among the oxides
and are used in various applications such as; magnetic tape, recording
chip, motors, transformer cores, refrigerators and medical applications
[5,6]. While, U-type barium hexaferrite (Ba4Co2Fe36O60; BaU) have
been extensively used as radar/EM wave absorbers. Due to wide range of
application of these magnetic materials, scientific community has been
working tremendously in the field of microwave absorbing coatings and
composites. It has been observed from literature findings that particle
size and morphology of synthesized filler particulates influence the EM
properties to a large extent [7–9]. The particle size and structural
properties of synthesized materials can be controlled significantly by
optimizing various parameters of solid state synthesis technique as a
result, which ultimately helps in improving the overall microwave
absorber performances of synthesized composites [10]. Literature
studies also reveals that ferrites serve as more potential electromagnetic
* Corresponding author.
E-mail addresses: vivek.can.pratap@gmail.com (V. Pratap), amitsoni.kgp@gmail.com (A.K. Soni), himanshudmsrde@gmail.com (H.B. Baskey), abbas93@
rediffmail.com (S.M. Abbas), amsiddiqui@jmi.ac.in (A.M. Siddiqui), neswarap@rediffmail.com (N.E. Prasad).
https://doi.org/10.1016/j.solidstatesciences.2021.106553
Received 9 November 2020; Received in revised form 22 January 2021; Accepted 24 January 2021
Available online 30 January 2021
1293-2558/© 2021 Elsevier Masson SAS. All rights reserved.
V. Pratap et al.
Solid State Sciences 113 (2021) 106553
Fig. 1. XRD patterns (a) for U type BHF powder sintered at 1300 ◦ C temperature for 4 h, (b) for γ Fe2O3 powder and (c) for various wt. % ratios of Ba4Co2Fe36O60 & γ
Fe2O3 within epoxy matrix.
interference (EMI) shielding/radar absorbing materials than their
dielectric counterparts. Ferrite based absorbers consist various losses
such as; magnetic loss factor by natural, exchange resonance and
dielectric loss factor from interfacial polarization and relaxation results
it can be suitably used for scientific and industrial applications [11–13].
In this proposed scientific work, BaU and γ Fe2O3 have been used as
potential filler materials for microwave absorber, further in order to
improve its EM properties, a solid state reaction technique is employed
for synthesis of barium hexagonal U-type ferrite (Ba2Co2Fe36O60).
Further various microwave absorber composites have been fabricated
using synthesized U type ferrite along with maghemite powder in resin
system. Magnetic absorbing materials and fabricated composites have
also been characterized for structural, surface and magnetic properties
using X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM),
and Vibrating Sample Magnetometer (VSM) respectively. Further under
this frame work of current research, EM properties such as complex
permittivity and complex permeability have also been retrieved in the
frequency range of 2–18 GHz using measured reflection and trans­
mission coefficients of test specimen. Moreover, studies on the micro­
wave absorption performance of various fabricated composites have
been caused entire the frequency region of 2–18 GHz.
Sigma Aldrich, USA consisting of analytical grade were used for syn­
thesis of U type hexagonal ferrite material. All starting materials have
been weighted in stoichiometric proportion for the synthesis of U-type
hexaferrite. The other prominent filler material Gamma iron oxide; γ
Fe2O3 nano particles purchased from M/s Alfa Aesar, USA along with
epoxy resin; (trade name: Araldite® LY 5052 and Aradur® HY 5052)
was purchased from M/s Huntsman Ltd., USA, used to design the
absorbing composites.
2.2. Synthesis of Ba4Co2Fe36O60 powder
U-type barium hexaferrite (Ba4Co2Fe36O60) powder was synthesized
using solid state reaction route having high purity starting materials;
BaCO3, Co3O4, Fe2O3 as previously described. Starting materials were
used as received and further mixture was prepared in stoichiometric
proportion of Ba4Co2Fe36O60 in acetone medium using ball milling for
18 h. Further wet mixture was dried and further sintered at 1300 ◦ C in
air medium using high temperature box furnace. Finally, synthesized
powder were sieved using a standard mesh in order to achieve particle
size having dimension in range lesser than 40 μm.
2.3. Fabrication of Ba4Co2Fe36O60/γ Fe2O3 -epoxy composite
2. Experimental details
A series of microwave absorbing composites was fabricated using
wet mixing technique having various compositions of 50:30, 40:40 and
30:50 wt % (Ba4Co2Fe36O60: γ Fe2O3) in Ba4Co2Fe36O60/γ Fe2O3-epoxy
composites. The thermosetting resin system were used to fabricate the
2.1. Materials
The starting materials such as BaCO3, Co3O4 and Fe2O3 from M/s
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Solid State Sciences 113 (2021) 106553
Fig. 2. Scanned micrographs of (a) synthesized Ba4Co2Fe36O60 powder sintered at 1300 ◦ C temperature for 4 h (b) γ Fe2O3 powder, (c–e) morphological images of
various wt. % ratio (50:30, 40:40 and 30:50 wt %) of Ba4Co2Fe36O60 and γ Fe2O3 within epoxy matrix.
polymeric composites based on two pack of Araldite® LY 5052 and
Aradur® 5052 purchased from Huntsman Ltd., USA. The wet mixture
with proportionate ratios of filler materials and resin system was
embedded in desired toroidal shaped steel mould for preparation of
samples having inner and outer diameter of 3.0 mm and 7.0 mm
respectively. Subsequently, the loaded mould was kept in hot press at
60 ◦ C temperature under pressure of 10 MPa (MPa) for 2 h for finished
test specimens [14].
dispersive spectrometer (EDS) detector was employed to analyze the
chemical composition and crystallographic nature of sample with the
elemental attachment available together with scanning electron micro­
scope. Magnetic properties of synthesized powder and prepared com­
posite specimens were characterized using Vibrating sample
magnetometer (VSM, MicroSense, LLC technologies) at room tempera­
ture under an applied magnetic field ranging from − 18 to +18 kilo
Oersted (kOe). Further, the electromagnetic properties in terms of
complex permittivity (εr) and complex permeability (μr) for designed
composites were computed from measured scattering (reflection and
transmission) parameters using Agilent vector network analyzer (VNA)
employing a co-axial transmission line technique for the frequency
range of 2–18 GHz.
2.4. Characterization and measurement of properties
The synthesized BaU (Ba4Co2Fe36O60) powder was characterized
using X-ray diffractometer (XRD, PANalytical make, Xpert-Pro model) in
the diffraction angle (2θ) ranging from 20◦ to 60◦ with scanning rate of
0.0167◦ /s. Field emission scanning electron microscope (FE-SEM, FEI
make, Quanta 200 model) was used to examine the grain size and
morphology of synthesized powder and prepared composites. Energy
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Solid State Sciences 113 (2021) 106553
Fig. 3. Corresponding EDS analysis of (a) synthesized Ba4Co2Fe36O60 powder sintered at 1300 ◦ C temperature for 4 h (b) γ Fe2O3 powder, (c–e) various wt. % ratio
(50:30, 40:40 and 30:50 wt %) of Ba4Co2Fe36O60 and γ Fe2O3 within epoxy matrix.
3. Results and discussion
of magnetic filler (50:30, 40:40 & 30:50 wt % of Ba4Co2Fe36O60 and γ
Fe2O3) in Ba4Co2Fe36O60/γ Fe2O3-epoxy composites as shown in Fig. 1
(c).
3.1. Phase confirmation and crystal structure
The structural characteristics of synthesized Ba4Co2Fe36O60, γ Fe2O3
powder and prepared composites were identified using XRD pattern and
shown in Fig. 1(a–c). It can be observed from the corresponding figure
that the synthesized powder gives evidence for U-type hexagonal phase
formation with space group of ‘R3m’. As per the literature from previous
findings, no other phase was identified and all diffraction peaks
consistently reveal the hexagonal structure.
The PANalytical’s X’pert-Pro scanning tool has been used to identify
the peaks under the angle 2θ in degree in the region from 20◦ to 60◦ .
Further, XRD peaks were indexed and the lattice parameters (a & c) were
obtained through formula (1) as given below;
) ( 2)
(
1 4 h2 + hk + k2
l
=
+ 2
(1)
d2 3
a2
c
3.2. Surface morphology
The size and morphology of synthesized Ba4Co2Fe36O60 and γ Fe2O3
powder were characterized by field emission electron microscope
(FESEM) in Fig. 2 (a & b) while, the surface and fractured morphology of
lower to higher dispersion of γ Fe2O3 filler in Ba4Co2Fe36O60/γ Fe2O3epoxy composites shown in Fig. 2(c–e). These images depicts that there
is uniform dispersion between fillers and epoxy matrix. The optimum
value of Ba4Co2Fe36O60 particles size is less than 50 μm and grain size
varies from 100 nm to 120 μm. The micrographs show that the absorbing
filler materials have hexagonal platelet like morphology which is most
suitable for radar absorbing purpose. Based on these initial findings, the
proposed composites can be suitably used for the designing of electro­
magnetic interference shielding materials and radar absorbers [16–20].
According to EDS analysis of Fig. 3(a–e), the percentage of the
chemical elements was closer to the estimated theoretical stoichiometry
in U-type barium hexaferrites and gamma iron oxide also for the
designed composites. For a comparison, in a recent study of filler ma­
terials and its composites studied through EDS detector, which unable to
identified other elements. The characteristics EDS spectra of filler ma­
terials and its composites are shown in Fig. 3(a and b) and Fig. 3(c–e)
respectively. Consequently, observed elemental composition ratio of
metallic ions are consisting expected stoichiometry is well maintained
Using the above formula (1), calculated values of lattice parameter
are: a = 5.86 Å and c = 113.7 Å; these values are shown in the same
range as previous reported in literatures for the U-type barium hex­
aferrite powder [15].
Although γ Fe2O3 powder shows cubic structure with almost equal
lattice constant (0.834 nm) as shown in Fig. 1(b), which are due to the
ordered occupancy of octahedral sites by Fe+3. The estimated cuboids
shape enhances the shape anisotropy which results in the increased
magnetic loss. Its XRD patterns are reproducible with the various ratios
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Solid State Sciences 113 (2021) 106553
Fig. 3. (continued).
during synthesis process of material and designing composites.
its composites have shown a narrow hysteresis loop. Definite combina­
tion of saturation magnetization (Ms) and low coercivity (Hc) are suit­
able for high frequency in gigahertz applications. It is observed that the
saturation magnetization (Ms) value was increased from 49.32 to 55.49
emu/g of increasing the γ Fe2O3 contents in Ba4Co2Fe36O60/γ Fe2O3epoxy composites. The low coercivity of ≤153 Oe and low squareness
ratio (Mr/Ms) ratio of ≤0.051, categorize the prepared Ba4Co2Fe36O60/γ
Fe2O3-epoxy composites as soft ferrites based absorber. It can be noted
that the densification of magnetic powders results in grain growth which
evolves the multi domain particle regime. These multi domain natures of
3.3. Study of magnetic properties
The magnetic properties of both, magnetic fillers and fabricated
composites have been measured using vibrating sample magnetometer
(VSM) under the applied magnetic field from − 18 kOe to +18 kOe. The γ
Fe2O3 powder and its composites were characterized for its histogram
and properties at room temperature as shown in Fig. 4 with values
tabulated in Table 1. Synthesized Ba4Co2Fe36O60, γ Fe2O3 powder and
5
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Solid State Sciences 113 (2021) 106553
optimized level for prepared series of Ba4Co2Fe36O60/γ Fe2O3-epoxy
composites. Here, we have been found a hysteresis loop and an
arrangement of saturation magnetization with low coercive values
suitable for gigahertz frequency applications.
4. Electromagnetic parameters and absorption phenomena
Electromagnetic (EM) properties of prepared composites are gener­
ally characterized in terms of is functional parameters such as complex
permittivity (ε′ -jε") and complex permeability (μ’-jμ"). These parameters
can be tuned for desired microwave absorption when the incident EM
wave interacts with the surface of absorbing materials. Absorbers are
capable to absorb the incident radar/EM energy significantly and
convert this energy into other forms (heat), which thermal imagers are
unable to detect. When an incident EM wave fall on the absorber’s
surface; attenuation, reflection and transmission of EM waves are three
major phenomena take place, as shown in Fig. 5. The complex permit­
tivity (ε′ -jε") and complex permeability (μ’-jμ") parameters of composite
specimen have been computed from the measured scattering parameters
using Agilent VNA E8364B in the 2–18 GHz frequency range. The real
parts of complex permittivity and complex permeability (ε′ and μ’)
represents the storage of electric and magnetic energy respectively.
Whereas, the imaginary parts of complex permittivity and imaginary
parts of complex permeability (ε" and μ") are responsible for the lossy
manners corresponding to dielectric and magnetic energy respectively.
Fig. 4. Magnetic hysteresis loop of (a) γ Fe2O3 as maghemite powder, (b) 30 wt
% and (c) 50 wt % dispersion of γ Fe2O3 in Ba4Co2Fe36O60/γ Fe2O3
-epoxy composites.
Table 1
Magnetic properties of synthesized U-type hexaferrite [14], γ Fe2O3 powder and
prepared composites for lower (30 wt %) and higher (50 wt %) dispersion of γ
Fe2O3 in Ba4Co2Fe36O60/γ-Fe2O3-epoxy composites.
System
Magnetization;
Ms (emu/g)
Coercivity;
Hc (Oe)
Remanence;
Mr (emu/g)
Squareness
ratio; Mr/Ms
Ba4Co2Fe36O60
powder
γ Fe2O3 powder
50:30 wt % of
ferrite and γ
Fe2O3 in
composite
30:50 wt % of
ferrite and γ
Fe2O3 in
composite
55.81
107.31
2.17
0.038 [15]
74.70
49.32
243.09
96.34
4.09
1.591
0.054
0.032
55.49
153.84
2.872
0.051
4.1. Complex permittivity
Fig. 6(a, b and c) shows the real and imaginary values of permittivity
for fabricated Ba4Co2Fe36O60/γ Fe2O3 epoxy composites with respect to
the variation in filler content. It can be observed that the values of real
permittivity increases as the filler content of γ Fe2O3 in the composites
has been increased from 30 wt % to 50 wt % with constant wt. % of
epoxy matrix. It is obvious from the corresponding figure that all the
composites show almost constant values of real permittivity through the
whole frequency region for particular filler content. The increase of real
permittivity with the high loading of γ Fe2O3 can be interpreted by the
hopping mechanism which is established for electric polarization and
dielectric loss, increases with maximum wt. % of filler materials. The
maximum value achieved of real permittivity (ε′ ) achieved 4.23 for
higher dispersion (30:50 ratio of U-ferrite and γ Fe2O3) of γ Fe2O3 in
prepared Ba4Co2Fe36O60/γ Fe2O3-epoxy composites. The imaginary part
of complex permittivity or dielectric loss factors (ε′′ ) is also constant
throughout the 2–18 GHz frequency region for an individual loading.
The increase of dielectric constant at lower frequencies is remarkable.
Such types of frequency dependent dielectric constants have improved
loss mechanism of electrical energy inside the Ba4Co2Fe36O60/γ Fe2O3epoxy composites. This is because γ Fe2O3 nano-particle having the
greater surface to volume ratio than Ba4Co2Fe36O60 particles and sub­
sequently, high dielectric permittivity. These phenomena could be
attributed to the inhomogeneity of used magnetic absorbing (micro and
nano) materials. In case of Ba4Co2Fe36O60/γ Fe2O3-polymer composites,
the contribution to real permittivity (ε′ ) and imaginary permittivity (ε′′ )
rise to heterogeneity due to interfacial polarization and interfacial
relaxation as the electric charge particles are separated by insulating
matrix grains which enhanced heterogeneity. The resonance peak at 6.2
GHz frequency is preliminary attributed to the dipole-dipole interaction
as well as interfacial polarization by adding the γ Fe2O3 content [14,21,
22]. The dielectric constant and relaxation parameters of filler disper­
sion are changed with the gradual transformation between Ba4Co2
Fe36O60 and γ Fe2O3. However, as γ Fe2O3 increases in Ba4Co2Fe36O60/γ
Fe2O3-epoxy composites from 30 wt % to 50 wt %, high and smooth
complex permittivity curves {Fig. 6(a, b and c)} are obtained and no
further peak is observed above 13.2 GHz frequency.
Fig. 5. Schematic diagram of designed toroidal shape composites for absorp­
tion mechanism.
the magnetic fillers are responsible for a lowered coercivity value of
fabricated Ba4Co2Fe36O60/γ Fe2O3-epoxy composites.
It can be observed that, increasing the γ Fe2O3 content depicts the
increased values of saturation magnetization (Ms) and coercivity (Hc) at
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Solid State Sciences 113 (2021) 106553
Fig. 6. Frequency response of the complex permittivity for different loading; (a) 30 wt %, (b) 40 wt % and (c) 50 wt % of γ Fe2O3 in Ba4Co2Fe36O60/γ Fe2O3epoxy composites.
4.2. Complex permeability
frequencies. It is to be noted, that the electromagnetic properties of the
fabricated composites depends upon the fillers contents of hexagonal
ferrite obeying with the ferromagnetic materials which creates vacuum
phenomena such as eddy current effect, natural and exchange resonance
rather than magnetic permeability and space charge polarization [19,
23].
Fig. 7(a, b and c) represents the frequency versus complex perme­
ability (μ’- jμ") characteristics of fabricated Ba4Co2Fe36O60/γ Fe2O3
composites in the frequency range of 2–18 GHz. The aim of incorpo­
rating Ba4Co2Fe36O60/γ Fe2O3 into the epoxy matrix is to increases real
permeability value above the unity and to increase the magnetic loss in
the working frequency region. It can be observed from Fig. 7(a) that real
permeability for 30 wt % of γ Fe2O3 loaded composite decreases sharply
from 2 to 6.4 GHz, further remains at constant value in the frequency
range of 6.4–18 GHz. The highest value of real part of complex perme­
ability is observed in the range of 1.73–0.72, whereas its imaginary part
varies in the range of 0.72–0.24 across the entire 2–18 GHz frequency.
Fig. 7(b) shows that for the real permeability for 40 wt % of γ Fe2O3
loaded composite is in the range of 1.63–0.82, whereas its imaginary
part is in the range of 0.71–0.09. High (50 wt %) filler concentration of γ
Fe2O3 in designed composite as depicted in Fig. 7(c), the real part of
permeability increased substantially in the range of 2.06–0.85 between
the frequency ranges from 2 to 5.02 GHz, whereas magnetic loss factor
(μ") is in the range of 1.5–0.085 in the whole frequency range of 2–18
GHz. The dispersion of γ Fe2O3 nano-particles minimize the eddy current
of designed composites due to small size than skin depth, which in turns
is optimize the electromagnetic parameters. The optimized complex
permeability is present in Ba4Co2Fe36O60/γ Fe2O3-epoxy polymeric
composites; which gives rise to the circumferential demagnetizing field
as a result, diminishes the real permeability (μ′ ) at resonance
4.3. Loss tangent
Fig. 8(a and b) shows the nonlinear deviation of the dielectric
tangent loss (tanδe = ε"/ε′ ) and magnetic tangent loss (tanδμ = μ"/μ’)
with frequency. In Fig. 8(a), at starting frequency of 2.0 GHz, the nu­
merical value of dielectric tangent loss (0.023) remains almost same up
to 9.8 GHz for all wt. % of γ Fe2O3 dispersion in Ba4Co2Fe36O60/γ Fe2O3
polymer composite. However, this value is obtained greater than 0.03
for 30, 40 and 50 wt % loaded in Ba4Co2Fe36O60/γ Fe2O3-epoxy com­
posites. Maximum value of tanδe achieved was 0.34 for 50 wt %
dispersed γ Fe2O3 in Ba4Co2Fe36O60/γ Fe2O3-epoxy composites at the
corresponding frequency of 10.2–11.9 GHz. In Fig. 8(b), at starting
frequency of 2–8 GHz, the numerical value of magnetic tangent loss
increases to 1.2 for 50 wt % dispersed γ Fe2O3 in Ba4Co2Fe36O60/γ
Fe2O3-epoxy composites. Further the value of magnetic tangent loss
(tanδμ) was found to be decreasing sharply for all prepared composites
in the frequency range of 8.0–18 GHz. It can be observed from the
corresponding figure that highest value of tanδμ; 1.2 for 50 wt % was
greater than 0.68 & 5.6 for 30 wt % & 40 wt % loaded γ Fe2O3 in
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Solid State Sciences 113 (2021) 106553
Fig. 7. Frequency response of the complex permeability for different loading; (a) 30 wt %, (b) 40 wt % and (c) 50 wt % of γ Fe2O3 in Ba4Co2Fe36O60/γ Fe2O3epoxy composites.
Fig. 8. Frequency response of dielectric loss tangent (a) and magnetic loss tangent (b) of Ba4Co2Fe36O60/γ Fe2O3-epoxy composites.
Ba4Co2Fe36O60/γ Fe2O3-epoxy composites. The prepared composites are
the heterogeneous combination of conducting/magnetic particles in the
insulated epoxy grains. Further the complex permittivity and complex
permeability are mainly attributed to the amount of dielectric polari­
zation and magnetic polarization for the prepared composites. The well
dispersion of hexaferrite (Ba4Co2Fe36O60) and maghemite (γ Fe2O3) in
epoxy matrix induces the mobility of charge carriers and spin of elec­
trons that produce polarization and magnetization respectively. As a
result of this, the governing polarization and magnetization with the loss
mechanism comprise the association of relaxation and resonance
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Solid State Sciences 113 (2021) 106553
layer absorber by using the following equation (2) proposed by Naito
and Suetake [24]:
[
]
z− 1
(2)
RL(dB) = − 20log 10
z+1
The ratio of material’s impedance (Zin) to impedance of free space
(Zo) is accepted as normalized impedance (Z) calculated through
following equation (3).
√̅̅̅̅̅̅̅̅̅̅̅
{
( )̅
}√̅̅̅̅̅̅̅̅̅̅̅̅
zin
μr
− j2πf .d
where, z = =
tanh
(3)
(μr εr )
c
zo
εr
In the above expression RL is the reflection loss represented in
√̅̅̅̅̅̅̅
decibel (dB), j is an imaginary unit for the value of − 1, μr and εr are the
complex permeability and complex permittivity respectively, f is the
frequency of the incident EM wave, c is speed of light in free space and
d being thickness of the composite specimens.
Fig. 9(a, b and c) shows the calculated reflection loss for fabricated
Ba4Co2Fe36O60/γ Fe2O3-epoxy composites as a function of thickness in
the frequency range of 2–18 GHz. These results are analyzed by metal
backing model as proposed by Naito and Suetake. RL has been calcu­
lated for various thicknesses (2.6, 2.9 and 3.2 mm) of fabricated com­
posites through a MATLAB codes. Consequently, dip of RL gets shifted
towards the lower frequency region by increasing the absorbers thick­
ness from 2.6 to 3.2 mm. The impedance matching condition can be
justified to the microwave absorption mechanism. RL performance for
30 wt % of γ Fe2O3 in Ba4Co2Fe36O60/γ Fe2O3-epoxy composite shows
that the minimum RL (Maximum Absorption) of − 8.86 dB, − 10.36 dB
and − 11.86 dB have been observed corresponding to matching fre­
quency of 6.48 GHz, 5.84 GHz and 4.88 GHz respectively. Fig. 9 (b)
depict RL performance for 40 wt % of γ Fe2O3 in Ba4Co2Fe36O60/γ
Fe2O3-epoxy composite at 4.7 GHz has been observed from the corre­
sponding graphs that the minimum reflection minima of − 10.02 dB,
− 13.82 dB and − 14.8 dB have been observed corresponding to match­
ing frequency of 13.3 GHz, 13.0 GHz and 12.3 GHz respectively. It can
be also observed from the corresponding graphs that the 10 dB ab­
sorption bandwidth (11.7–13.3 GHz) has improved substantially by
adding equal wt. % of γ Fe2O3 and Ba4Co2Fe36O60 in Ba4Co2Fe36O60/γ
Fe2O3-epoxy composites. Similarly Fig. 9 (c) shows the variation of RL
performance for 50 wt % of γ Fe2O3 in Ba4Co2Fe36O60/γ Fe2O3-epoxy
composite, it can be analyzed from corresponding figure that the mini­
mum reflection loss of − 13.89 dB, − 15.39 dB and − 19.89 dB have been
observed corresponding matching thickness 14.5 mm, 13.8 mm and
13.2 mm respectively. The optimum performance in RLmin as compared
to maximum bandwidth 4.8 GHz (RL ≤ − 10 dB) was observed for the
absorber having composite thickness of 3.2 mm (RLmin ≤ − 19.89 dB
implies 98.9% absorption). Consequently, Ba4Co2Fe36O60/γ Fe2O3
loaded absorbers with maximum (50 wt % of γ Fe2O3) content can be
used as effective microwave absorbers for the X-band and Ku-band fre­
quency region.
Fig. 9. (a, b and c) calculated reflection loss of different percentage loading; (a)
30 wt %, (b) 40 wt % and (c) 50 wt % of γ Fe2O3 in Ba4Co2Fe36O60/γ Fe2O3epoxy composites.
5. Conclusions
In summary, U-type barium hexaferrite (Ba4Co2Fe36O60) powder was
synthesized based on solid state reaction route, the polycrystalline phase
of synthesized powder was validated using XRD technique. The surface
morphology and elemental composition of used absorbing powder and
composites were scanned together with the elemental attachment
available with the FESEM. Further, radar absorbers were fabricated
through wet mixing method based on various wt. % of magnetic filler
contents. The magnetic properties were also studied for the composites
with increasing wt. % of γ Fe2O3 in Ba4Co2Fe36O60/γ Fe2O3-epoxy
composites. The intrinsic electromagnetic parameters of prepared
polymeric composites were significantly improved with increasing the
wt. % of γ Fe2O3. It is found that the matching frequency of prepared
absorbing sample shifted towards the lower frequency band with
phenomena which are subsequently responsible for increasing the
dielectric tangent loss and magnetic tangent loss governing the ab­
sorption phenomena.
4.4. Reflection loss
The microwave absorption which is quantified in terms of reflection
loss (RL) can be estimated from measured value of complex permittivity
and complex permeability of fabricated Ba4Co2Fe36O60/γ Fe2O3-epoxy
composites using metal backed coaxial line measurement technique
using the transmission line approach. It can be measured as the single
9
V. Pratap et al.
Solid State Sciences 113 (2021) 106553
increasing the thickness of absorbers. The minimum RL was observed as
− 19.89 dB for designed absorber having the thickness of 3.2 mm (RLmin
≤ − 19.89 dB implies 98.8% absorption) corresponding its matching
frequency 13.2 GHz. The optimized absorber with highest loading in wt.
% of γ Fe2O3 nano powder in designed composites seems strong candi­
date as radar absorber and in high frequency devices for stealth
applications.
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Credit author’s contribution statement
Vivek Pratap: Conceptualization, Design of study, Analytic and
interpretation of data, Writing review and editing investigation, Writing
– original draft preparation. Amit K. Soni: Validation, Investigation,
Formal analysis. Himangshu B. Baskey: Validation, Formal analysis. S.
M. Abbas: Methodology, Formal analysis, Supervision. A. M. Siddiqui:
Formal analysis, Supervision. N. Eswara Prasad: Resources, Formal
analysis.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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