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Preparation, thermal, magnetic and microwave absorption properties
of thermoplastic natural rubber matrix impregnated with NiZn ferrite
nanoparticles
Moayad Husein Flaifel
a,
⇑
, Sahrim Hj Ahmad
a
, Mustaffa Hj Abdullah
a
, Rozaidi Rasid
a
,
Abdul Halim Shaari
b
, Ayman A. El-Saleh
c
, Sivanesan Appadu
d
a
School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia
b
Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia
c
Department of Telecommunication Engineering, Faculty of Engineering (FOE), Multimedia University, Jalan Multimedia, 63100 Cyberjaya, Selangor, Malaysia
d
Radiation Processing Technology Division, Malaysian Nuclear Agency, Bangi, 43000 Kajang, Selangor, Malaysia
article info
Article history:
Received 18 November 2013
Received in revised form 24 February 2014
Accepted 19 March 2014
Available online 27 March 2014
Keywords:
A. Nanocomposites
A. Nanoparticles
A. Polymer–matrix composites (PMCs)
B. Magnetic properties
B. Thermal properties
abstract
In this research, NiZn ferrite nanoparticles were incorporated in thermoplastic natural rubber matrix
(TPNR) as reinforcing magnetic nanofiller by melt blending technique. TPNR was prepared from high-
density polyethylene (HDPE), natural rubber (NR) and liquid natural rubber (LNR) in a ratio of
70:20:10 and later impregnated with 4, 8, 12 (wt%) of NiZn ferrite nanoparticles. Thermal stability, tem-
perature dependence of coercivity and microwave absorption characteristics of the nanocomposite
samples were investigated using thermogravimetric analyser (TGA), vibrating sample magnetometer
(VSM) and vector network analyser (VNA), respectively. The results showed that the thermal stability
of the nanocomposite samples has increased with increasing filler content in the TPNR matrix. Temper-
ature dependence of coercivity for pure NiZn ferrite and nanocomposite samples exhibited immeasurable
values below blocking temperature (T
B
), above which an abrupt increase was observed. In addition, the
microwave absorption results showed multi-absorbing peaks characteristic with a great enhancement
attained for 12 wt% nanocomposite at a sample thickness of 7 mm with a potential application in the
X-band frequency.
Ó2014 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, microwave absorbing materials (MAMs) received
great attention due to their ballast in widespread applications they
are involved in. Hazardous environmental pollution associated
with the vast utilization of electrical and electronic devices as well
as the rapid development of radar technology in military applica-
tions being as a countermeasure to stealth technology [1], are con-
sidered to be the major challenges that must be taken into account
when selecting or developing MAMs.
Magnetic polymer nanocomposites (MPNCs) are class of nano-
composites which combine the properties of both magnetic and
polymeric materials and can be used in several applications such
as information storage, magnetic recording media, magnetic refrig-
eration [2] microelectromechanical systems (MEMSs), microsen-
sors [3], anticorrosion [4], cell separation, bioprocessing, medical
diagnosis, controlling drug delivery [5,6]. Above all, these materials
were found to be competent candidate that can be employed as
MAMs in electromagnetic interference shielding [5] and in micro-
wave absorption [7–9] due to their merit of combining the charac-
teristics of both dielectric and magnetic materials.
It has been reported that the incorporation of magnetic nano-
particles into polymeric matrices confers stabilization or enhance-
ment in the physical properties of the produced nanocomposites
[10] as was evident from the overall MPNCs characteristics, partic-
ularly, the enhancement in their microwave absorption properties
[11]. This can be realised by considering the unique phenomena
associated with the magnetic particles’ behaviour when they are
at nano scale size, such as quantum size effect, high surface to vol-
ume ratio, quantum tunnelling of magnetization and spin reversal
in single domain particles (superparamagnetism) [12].
Different researchers have reported on the preparation and
properties of several polymeric matrices impregnated with various
magnetic nanofillers [13]. For instance, Zhu et al. have prepared
Fe@SiO
2
/PU nanocomposites via surface initiated polymerization
http://dx.doi.org/10.1016/j.compscitech.2014.03.016
0266-3538/Ó2014 Elsevier Ltd. All rights reserved.
⇑
Corresponding author. Tel.: +60 387343141; fax: +60 389213777.
E-mail address: physci2007@hotmail.com (M.H. Flaifel).
Composites Science and Technology 96 (2014) 103–108
Contents lists available at ScienceDirect
Composites Science and Technology
journal homepage: www.elsevier.com/locate/compscitech
(SIP) method. The nanocomposite produced has shown better ther-
mal stability than the PU matrix with a maximum single micro-
wave absorption dip of approximately 20 dB at 11.3 GHz
frequency and 1.8 mm absorber thickness when NPs loading was
71 wt% [14]. In another study, a combination of polyaniline con-
ducting polymer matrix with La
0.6
Sr
0.4
MnO
3
NPs prepared using
in situ polymerization technique has resulted in maximum single
reflection loss dip of 61.3 dB at 11.5 GHz and 2.5 mm absorber
thickness but with high NPs loading of 51 wt% [15]. In recent study,
a hybrid nanocomposite containing strontium ferrite, carbon black
and nitrile rubber has been fabricated and the microwave absorp-
tion characteristics were investigated in both S and X-frequency
bands [16]. The hybridization of rubber matrix with carbon and
magnetic fillers has improved the mechanical properties of the
nanocomposite, and the maximum reflection losses at S
(2.55 GHz) and X (11.2 GHz) frequency bands have been found to
be 28 and 16 dB at 7 and 5 mm absorber thicknesses, respec-
tively [16].
In actual sense, very few researches have discussed the impor-
tance of MPNCs in microwave absorption using TPNR as a matrix
[10]. The selection of NR and HDPE to form TPNR matrix was made
due to their cost effectiveness, light weight, high flexibility and
mouldability, the demand of low energy for processing and their
capability of being produced in high amounts [17]. On the other
hand, opting for NiZn ferrite nanoparticles as a magnetic filler
was due to their unique properties such as low production cost,
high electrical resistivity, good chemical stability, mechanical
hardness [18] and their capability of absorbing EM wave caused
by their large electric or magnetic loss due to their natural reso-
nance phenomena.
2. Materials and methods
2.1. Preparation
NiZn ferrite nanoparticle powder prepared using conventional
precipitation technique [19] with a chemical formula of (Ni
0.5
Zn
0.5-
Fe
2
O
4
) and size ranging from 10 to 30 nm, were incorporated into
thermoplastic natural rubber (TPNR) matrix by melt blending tech-
nique. This method has proven to be economical, simple as it uses
conventional processing equipment and environmentally friendly
as it does not require any solvent [20].
TPNR was comprised of high-density polyethylene (HDPE), nat-
ural rubber (NR), and liquid natural rubber (LNR) in a ratio of
70:20:10. LNR served as compatibilizer between the two polymeric
phases. NR matrix phase was first introduced to thermo-Haake
internal mixer (Model Rheomix 600P). Two minutes later, a pre-
mixed blend of LNR and NiZn ferrite nano-powder was allowed
to mix with NR for 5 min, after which HDPE matrix was added.
The nanocomposite was blended at a temperature of 140 °C for
about 13 min at a rotating speed of 100 rpm. The samples then
were removed from the mixer and were of 0, 4, 8 and 12 wt% filler
content. Afterwards, by using hydraulic compression moulding
machine at 145 °C, the samples were pressed into a rectangular
shaped specimens with ½ and 1 mm thicknesses for magnetic
and thermal characterization, respectively. However, for micro-
wave absorption characterization, the samples were moulded into
toroidal shape with inner diameter of 2.9 mm and outer diameter
of 6.9 mm using injection moulding instrument (Model Ray-Ran).
2.2. Characterization
Microstructural characterization was used to determine
the homogeneity of magnetic nanofiller in the TPNR matrix. The
thermal stability of all nanocomposite samples was determined
by thermogravimetric analysis (TGA model Mettler Toledo SDTA
851
e
) in the temperature range of 25–600 °C at a heating rate of
10 °C/min in a nitrogen gas atmosphere. Differential scanning cal-
orimetry (DSC model Mettler Toledo 822
e
) was used to identify the
samples’ transition glassy temperature (T
g
) in the temperature
range of 100–100 °C using nitrogen gas pressure at a heating rate
of 10 °C/min. Vibrating sample magnetometer (VSM model Lake-
shore 7404) aided with cryostat assembly was used to investigate
the temperature dependence of coercivity from 10 K to 300 K. The
microwave absorption characteristics were measured in frequency
range of 0.5–12 GHz by transmission/reflection method using
microwave vector network analyser (MVNA model Anritsu
37247D) with GPC-7 mm coaxial line.
3. Results and discussion
3.1. SEM
Scanning electron micrographs (SEM) illustrated in Fig. 1(a–c),
were meant to examine the homogeneity of the nanoparticles dis-
persion in the TPNR matrix. It is clearly seen that NiZn ferrite nano-
particles appear in all filled nanocomposite samples with bright
phase and spherical shape. Generally, all nanocomposite samples
exhibit good particle’s dispersion within TPNR matrix. This good
dispersion is believed to be due to nanoparticles–polymer interac-
tions, which have improved at the expense of interparticle interac-
tions due to agglomeration, and hence formed a uniform
interconnected network within the host.
3.2. Thermal stability measurements
Fig. 2 shows the thermogram of percentage weight change as
a function of temperature of all nanocomposites samples. It is
evident that all nanocomposite samples exhibit two weight loss
stages. In the beginning of measurement, insignificant weight
loss appears due to the loss of adsorbed moisture and/or evapo-
ration of trapped solvent (toluene). The first stage of the weight
loss for TPNR matrix and all nanocomposite samples, took place
at about 300 °C and represents the total elimination of volatile
products as well as the initial degradation of high-density poly-
ethylene (HDPE) component, since the dehydration reaction on
the polymer chains occurred at this particular temperature [21].
The second weight loss which took place in the temperature
range of 417–510 °C, is attributed to almost complete degrada-
tion of both matrix phases of the TPNR nanocomposite, i.e. NR
and HDPE, which afterwards, turned into carbon and hydrocar-
bons residues [21]. Finally, as the temperature reached to
600 °C, the weight loss change as a function of temperature be-
came negligible due to the complete degradation of TPNR matrix
into carbonaceous products, and the observed weight percentage
residues were referred to the remaining non-decomposed mag-
netic nanoparticles.
From Table 1, it is apparent that the initial decomposition tem-
perature and degradation temperature (T
d
) of the TPNR nanocom-
posite increase with increasing filler content, which indicates an
improvement in the thermal stability of TPNR matrix. Similar re-
sults were also observed for ferrites/PET nanocomposites [20],
and for silicon carbide/polyurethane nanocomposites [22]. In addi-
tion, from Table 1 it is clearly seen that the temperature of the
maximum weight loss rate (T
p
), which corresponds to the second
degradation loss, has increased considerably with increasing con-
tent of NiZn ferrite nanoparticles in the TPNR matrix. This is an-
other significant sign that shows the enhancement in the thermal
stability of TPNR matrix when impregnated with NiZn ferrite nano-
particles. This improvement could be due to the interaction be-
tween NiZn ferrite nanoparticles and the TPNR matrix in which
104 M.H. Flaifel et al. / Composites Science and Technology 96 (2014) 103–108
the nanoparticles served as a heating barrier, impeding thereby the
dehydration process and hence, shifting T
p
to higher values. These
findings are in good agreement with the results obtained for differ-
ent magnetic polymer nanocomposites [20,23]. The increment of
T
p
value with the increase of the magnetic nanofiller content in
the TPNR matrix can be ascribed originally to the high surface to
volume ratio exhibited by the magnetic nanoparticles, which is
believed to be the main reason behind the interaction of the
nanoparticles–TPNR matrix. Due to this phenomena, large
number of nanoparticles available at the interface of the polymeric
phases of the TPNR matrix have hampered the polymeric chain
mobility, and in turn reduced the diffusivity within the polymer
matrix [24].
Moreover, Table 1 shows that the weight losses of the 1st and
2nd stage have decreased with increasing amount of magnetic
nanofiller loading in the TPNR matrix. On the other hand, with
referring to Fig. 2, the final stage, which corresponds to the remain-
ing non-decomposed magnetic filler nanoparticles, shows negligi-
ble weight loss due to the complete degradation of the TPNR
matrix into carbonaceous products as previously stated. Thus, in
this region it is apparent that the non-decomposed residue in-
creased with increasing content of NiZn ferrite nanoparticles in
TPNR matrix.
Table 1 also shows the glass transition temperature (T
g
) for all
nanocomposite samples obtained using differential scanning calo-
rimetry (DSC). The role of NiZn ferrite nanoparticles is vividly seen
in altering the glass transition temperature (T
g
) of TPNR matrix.
The T
g
value of the TPNR matrix increased slightly with the in-
crease of the magnetic filler loading in the TPNR matrix. The expla-
nation of this effect could be related to the results that have been
found earlier in the TGA curves. These results indicate that the
improvement attained in the thermal stability of NiZn–TPNR nano-
composite, was a consequence of the addition of NiZn ferrite nano-
particles, which served as a heating barrier and impeded the
polymeric chain mobility in the TPNR matrix. On the contrary, flex-
ible and easy moving polymeric chains are yielded with lowering
the value of T
g
. Taken altogether, this justifies why the increase
in the filler loading has increased the T
g
value of the TPNR matrix.
Conclusively, the enhancement in the thermal stability of TPNR
matrix with the addition of NiZn ferrite nanoparticles essentially,
indicates a good particle dispersion and suggests a strong chemical
bonding between the magnetic filler nanoparticles and TPNR ma-
trix interface [22].
3.3. Temperature dependence of coercivity
Magnetic measurements through the hysteresis loops were
carried out to investigate the temperature dependence of coerciv-
ity for pure NiZn ferrite and NiZn–TPNR nanocomposite of differ-
ent filler content in the temperature range of 10–300 K. It is
apparent that at very low temperatures the coercivity value be-
comes much higher than when it is measured at room tempera-
ture. The translation of the hysteresis loops measurement for
the coercivity values are plotted in Fig. 3. The blocking tempera-
NiZn ferrite MNPs
NiZn ferrite MNPs
(a)
(c)
(b)
NiZn ferrite MNPs
Fig. 1. SEM micrographs of NiZn–TPNR nanocomposite with (a) 4%, (b) 8% and (c)
12% filler loadings taken at 500 times magnification.
Fig. 2. TGA thermograms of NiZn–TPNR nanocomposite of different filler loadings.
M.H. Flaifel et al. / Composites Science and Technology 96 (2014) 103–108 105
tures of pure NiZn ferrite nanoparticles, 4, 8 and 12 wt% nano-
composite samples are 92, 90, 89, 88 K, respectively. The coerciv-
ity values from room temperature to blocking temperature were
fluctuating within the experimental error of 10–20 Oe. These
measurements show that the coercivity of all samples was almost
immeasurable with a monotonous trend above their blocking
temperature T
B
. This can be ascribed to the occurrence of super-
paramagnetic behaviour in the nanoparticles, which is in good
agreement with previous reported results [6]. The immeasurable
value of coercivity due to superparamagnetic phenomenon is ex-
plained by considering the fact that the spins’ magnetic moment
above T
B
has different orientations due to higher thermal activa-
tion energy. This thermal energy, which is responsible for the
spins fluctuation, overcame the magnetic anisotropy barrier of
the nanoparticles, and hence lowered the coercivity value. Similar
results showing immeasurable values of coercivity above T
B
have
also been reported [25].
On the other hand, when the temperature decreased to a value
below the blocking temperature, i.e. T<T
B
, a dramatic increase in
the coercivity value of the pure and nanocomposite samples has ta-
ken place with further decrease in temperature. This happens be-
cause the thermal activation energy that is dominant above T
B
,
was no longer capable of flipping spins’ magnetic moment orienta-
tions below T
B
value, and hence begun to pin along their easy axis
below T
B
gradually. Consequently, this situation resulted in an in-
crease in the anisotropy energy barrier, and hence the observed in-
crease in value of coercivity.
3.4. Microwave absorption properties
Electromagnetic waves incident on MAMs can be completely
absorbed and then dissipated to heat by the concept of Joule effect
through magnetic and dielectric losses. Complete absorption is a
goal of which most researchers are fond of to obtain. This absorp-
tion is expressed in terms of attenuation constant (
C
) and reflec-
tion loss (R
L
) which both are indications of the effectiveness of
MAMs [26]. The reflection loss minimum or the dip in R
L
is equiv-
alent to the occurrence of minimum reflection of the microwave
power for a particular thickness. The lower R
L
implies better
absorption properties.
In the case of a metal-backed single-layered absorber, the value
of the R
L
is dependent on the measured values of complex permit-
tivity and permeability of a material through the following
relations:
R
L
¼20log
10
j
C
jð1Þ
C
¼ðZ
in
Z
0
Þ
ðZ
in
þZ
0
Þð2Þ
Z
in
¼Z
0
ffiffiffiffiffiffi
l
r
e
r
stan h½
c
tð3Þ
where R
L
is the reflection loss,
C
is the reflection/attenuation coef-
ficient, Z
in
is the input impedance at the air–material interface,
Z
0
¼ffiffiffiffi
l
0
e
0
q= 377
X
is the intrinsic impedance of free space,
c¼
j
x
ffiffiffiffiffiffiffiffiffiffi
ð
l
r
e
r
Þ
p
½
c
is the propagation factor in the EM wave absorber
material, e
r
and
l
r
are relative complex permittivity and permeabil-
ity, respectively, tan his the hyperbolic tangent function and tis the
thickness of the absorber.
To assess the microwave absorption effectiveness of NiZn–TPNR
nanocomposite with different ferrite content, the variation of the
minimum reflection loss (R
L
) with frequency for the sample thick-
ness of 7 mm is illustrated in Fig. 4. This plot is quite revealing in
several ways. First, multi absorbing peaks or dips appeared in the
plot are not expected according to the impedance matching condi-
tions. Two matching conditions appear usually for ferrites materi-
als, where one occurs when
e
r
¼
l
r
for zero reflection and second
one takes place due the geometrical cancellation of the incident
and reflected waves at the surface of the absorber provided that
the sample’s thickness equals to odd number multiple of quarter
of the propagating wavelength in the material [27]. However, from
the literature, very few had reported on multi absorbing peaks
behaviour in our studied frequency range [28]. Such kind of behav-
iour could be ascribed to different mechanisms. In general, the
simultaneous and synergistic effect of both dielectric and magnetic
losses could be mainly the cause behind such behaviour. In another
word, we believe that the significant interaction between NiZn fer-
rite NPs and the TPNR matrix had resulted in unexpected absorp-
tion behaviour. Furthermore, the damping parameter (
a
) could
be one of the reasons behind the creation of many peaks in our
studied frequency range. It was reported by Bregar [29] that larger
value of damping coefficient could significantly broaden the ferro-
magnetic resonance, and hence create strong subsequent absorp-
tion peaks at higher frequencies. Another reason could be due to
TPNR matrix permittivity value which prompted the creation of
multi absorption peaks in the stipulated frequency range.
Secondly, a remarkable notice appears in Fig. 4 has highlighted
on the reflection loss dips in three different frequency bands S, C
and X to which many electronic and military applications are re-
lated. It is apparently seen that there are three absorption peaks
corresponding to the former mentioned bands in which the band-
width range for each has attained absorption of 610 dB. At rela-
tive lower frequency (S-band) and moderate frequency (C-band),
the R
L
dip has increased and its position was observed to shift to
lower frequencies with increasing ferrite content in the nanocom-
posite. The minimum R
L
values achieved in S- and C-band frequen-
cies were for 12 wt% sample at 7 mm thickness with values 18.00
and 17.6 dB at 2.8 and 6.1 GHz, respectively. However, at higher
frequency (X-band), the R
L
dip has also increased and reached to
38.3 at 9.6 GHz for the same sample, but its position was noticed
to shift to higher frequency, which is attributed to the enhance-
ment of anisotropy field with increasing ferrite content. Further-
more, the bandwidth range in which minimum absorption was
610 dB, has increased in the S and C bands but remained con-
stant in the X-band.
Table 1
The TGA and DSC thermal data for NiZn–TPNR nanocomposites of different loadings.
Sample Initial decomposition
temperature (°C)
Degradation
temperature (T
d
)(°C)
Max. weight loss
temperature (T
p
)(°C)
Weight loss of
1st stage (wt%)
Weight loss of
2nd stage (wt%)
Filler residue
(wt%)
T
g
(°C)
TPNR 164.1 366.4 467.7 23.6 76.3 0.26 58.2
NiZn–TPNR 4% 173.9 371.0 477.2 22.2 73.3 4.90 56.9
NiZn–TPNR 8% 188.9 376.2 482.6 20.9 71.2 8.10 55.1
NiZn–TPNR 12% 198.9 381.5 488.1 20.5 67.3 12.38 52.0
106 M.H. Flaifel et al. / Composites Science and Technology 96 (2014) 103–108
The estimated enhancement of the R
L
dip for 12 wt% NiZn–TPNR
nanocomposite in S and C bands was significant when comparing it
to the R
L
values obtained for TPNR sample in the same bands and
was determined to be 17% and 29%, respectively. This enhance-
ment of the R
L
dip might be ascribed to the synergistic interaction
of dielectric loss through interface polarization and multi scatter-
ing with magnetic loss through magnetocrystalline anisotropy
and shape anisotropy [30]. On the other hand, the R
L
value ob-
tained in the X-band was considered to be of tremendous achieve-
ment since no R
L
dip was detected for the TPNR matrix in this
frequency band. This can be attributed to the non-magnetic behav-
iour of the TPNR matrix previously mentioned as the occurrence of
the R
L
minimum in this frequency band was owed to the relative
complex permeability and its effect on the ferromagnetic
resonance.
The variation of the minimum reflection loss R
L
with sample
thickness for TPNR matrix and NiZn–TPNR nanocomposite of
12 wt% NiZn ferrite loading measured in the frequency range of
0.5–12 GHz is illustrated in Figs. 5 and 6, respectively. In case of
Fig. 5, two peaks were observed in the S and C bands where it
might be due to the relative complex permittivity contribution of
the TPNR matrix. It is clearly seen that the microwave absorbing
peaks in both frequency bands moved to higher frequency when
sample thickness was below 7 mm. However, the R
L
minimum
dropped off with increasing of sample thickness.
As depicted in Fig. 6, it is proven that unlike the former plot
specified for the TPNR matrix, the results from this figure exhibit
a shift in the R
L
dip into lower frequency above sample thickness
of 4 mm in the S-band and the absorbing peaks diminished with
increasing sample thickness. However, in C-band the absorbing
peaks shifted to higher frequency and its strength has decreased
with increasing sample thickness. Interestingly, for the situation
in the X-band frequency range, the position of the absorbing peaks
has shifted to lower frequency with increasing thickness as it is a
conclusive pattern of the quarter wavelength principle [31], but
in this time the strength of the R
L
minimum has increased with
increasing sample thickness.
4. Conclusion
Melt blending technique was used to prepare TPNR matrix
impregnated with different filler loadings of NiZn ferrite nanopar-
ticles and the nanocomposite was characterized using various
instruments. The results obtained by TGA curves and DSC data re-
vealed an enhancement in the thermal stability of the TPNR matrix
when increasing the magnetic filler content in the matrix. It is be-
lieved that these magnetic nanoparticles acted as a heating barrier,
which hindered the polymeric chain mobility in the TPNR matrix.
Moreover, the enhancement of thermal properties suggests good
Fig. 3. Temperature dependence of coercivity of pure NiZn and NiZn–TPNR
nanocomposite of 4%, 8%, and 12% filler loadings.
Fig. 4. Effect of filler content on the reflection loss R
L
values of all attenuation dips
for NiZn–TPNR nanocomposite with an optimum sample thickness of 7 mm.
Fig. 6. Reflection loss dependence on the frequency for 12% NiZn–TPNR nanocom-
posite of 4, 5, 6, 7 mm sample thicknesses.
Fig. 5. Reflection loss dependence on the frequency for 0% NiZn–TPNR nanocom-
posite of 4, 5, 6, 7 mm sample thicknesses.
M.H. Flaifel et al. / Composites Science and Technology 96 (2014) 103–108 107
particles dispersion and strong interaction between NiZn ferrite
nanoparticles and TPNR matrix. Temperature dependence of coer-
civity of NiZn–TPNR nanocomposites of different filler loadings
were measured through hysteresis loop curves. The results showed
a typical behaviour of superparamagnetism, where the measured
coercivity value was almost immeasurable until blocking temper-
ature T
B
below which an abrupt increased was observed. Micro-
wave absorption properties were determined through the
evaluation of refection loss as a function of frequency. The results
showed multi absorbing peaks behaviour in frequency range of
0.5–12 GHz. Larger value of damping coefficient (
a
) as well as
simultaneous and synergistic effect of both dielectric and magnetic
losses could be mainly the cause behind such behaviour. The influ-
ence of NiZn ferrite NPs was reported to enhance the absorption
properties of TPNR matrix in all frequency bands especially in
the X-band with a R
L
minimum of 38.3 dB at 9.6 GHz for
12 wt% sample at 7 mm thickness. Further, the operating band-
width frequency was also affected by the addition of NiZn ferrite
NPs into TPNR matrix in S and C bands, but had left no mentioned
sign in the X-band frequency. The shift in the R
L
peak positions,
which was observed as a result of different NiZn ferrite loadings
and various sample thicknesses, was believed to be due to different
effects such as the increase in the values of
l
0
and
l
00
or enhance-
ment of anisotropy field.
Acknowledgments
The authors would like to thank the Centre of Research and
Instrumentation Management (CRIM) at the National University
of Malaysia for sponsoring and supporting this research work.
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