Facile synthesis of crumpled ZnS net-wrapped Ni walnut spheres with enhanced microwave absorption properties

Article abstract of DOI:10.1039/c4ra15411h

Controllable magnetic-dielectric hybrids with cores of walnut-like Ni and shells of ultra-thin and crumpled ZnS nets have been successfully synthesized by a facile two-step approach. The morphology, microstructure and microwave absorption properties of the as-synthesized core-shell Ni/ZnS composites were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, transmission electron microscopy and network analysis. The shapes and microwave absorption properties of Ni/ZnS can be tuned by the hydrothermal temperatures. The core-shell Ni/ZnS composites present significantly enhanced microwave absorption compared with pristine Ni walnuts. When the reaction temperature was 60°C, the reflection loss (RL) could be as low as -42.4 dB at 12.3 GHz. Moreover, the effective bandwidth (RL < -10 dB) can be recorded in the 11.3-15.6 GHz range with the absorber thickness of only 2.2 mm. The excellent microwave absorption properties were attributed to impedance match, the synergetic effect between the dielectric loss and magnetic loss, interfacial relaxation and conduction loss of unique cross-linked ZnS shells. These results suggest that the as-synthesized crumpled ZnS net-wrapped Ni composites may be an attractive candidate for microwave absorption application.

Full text of DOI:10.1039/c4ra15411h

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PAPER
Facile synthesis of crumpled ZnS net-wrapped Ni
walnut spheres with enhanced microwave
absorption properties
Cite this: RSC Adv., 2015, 5, 9806
a
a
a
a
a
Biao Zhao, Gang Shao,* Bingbing Fan, Wanyu Zhao, Yongqiang Chen
ab
and Rui Zhang*
Controllable magnetic–dielectric hybrids with cores of walnut-like Ni and shells of ultra-thin and crumpled
ZnS nets have been successfully synthesized by facile two-step approach. The morphology,
a
microstructure and microwave absorption properties of the as-synthesized core–shell Ni/ZnS
composites were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy,
X-ray diffraction, transmission electron microscopy and network analysis. The shapes and microwave
absorption properties of Ni/ZnS can be tuned by the hydrothermal temperatures. The core–shell Ni/ZnS
composites present significantly enhanced microwave absorption compared with pristine Ni walnuts.
ꢀ
When the reaction temperature was 60 C, the reflection loss (RL) could be as low as ꢁ42.4 dB at
12.3 GHz. Moreover, the effective bandwidth (RL < ꢁ10 dB) can be recorded in the 11.3–15.6 GHz range
with the absorber thickness of only 2.2 mm. The excellent microwave absorption properties were
attributed to impedance match, the synergetic effect between the dielectric loss and magnetic loss,
interfacial relaxation and conduction loss of unique cross-linked ZnS shells. These results suggest that
the as-synthesized crumpled ZnS net-wrapped Ni composites may be an attractive candidate for
microwave absorption application.
Received 28th November 2014
Accepted 6th January 2015
DOI: 10.1039/c4ra15411h
www.rsc.org/advances
metallic magnetic Ni-based materials with high permeability as
1
. Introduction
11–14
well as low cost have been extensively investigated,
which
With the rapid proliferation of electronic devices and high should be expected to avoid the eddy current effect. It stimu-
frequency circuit devices in the gigahertz (GH) range, electro- lates scientists to prepare core–shell structured microwave
magnetic interference (EMI) pollution has become a vital absorption absorbers, such as Fe
3
O
4
/graphene and carbon
1,2
concern. The EMI pollution can not only affect various nanocoils coated with Fe O or Ni by atomic layer deposi-
3 4
1
5,16
17
commercial and industrial equipment but also threatens the tion,
Fe O @ZnO nanohybrid, CuO/Cu O-coated Ni nano-
3 4 2
18
19
20
health of living things by breaking down DNA strands, accel- capsules, Ni/polyaniline, carbon-coated nickel, Ni@Ni
O
2 3
3–5
21
22
erating heart rates, causing cancer, and so on. An effective particles, carbon-coated iron, Fe O microspheres/polyani-
3 4
24
23
3 4
way to deal with these problems is to pursue a new type of line, and Fe O /carbon nanorods, which exhibit better
material with strong electromagnetic wave absorbing proper- microwave absorption performance than the pure core or shell
ties. The main goal of this absorber is to achieve the attenuation materials. These kind of absorber materials, which combine
of electromagnetic energy through its dielectric loss or with the magnetic loss of cores and dielectric loss of shells as
magnetic loss, and could be used to minimize the electromag- well as special core–shell structures, give rise to the enhanced
6,7
netic reection. Microwave absorbing materials are currently microwave absorption capabilities. Therefore, preparation of
needed to have the features of strong absorption, wide wave- magnetic–dielectric composite with core–shell structure is
8,9
band, light-weight, thin-thickness, and simple operation.
benecial for excellent microwave absorption abilities.
The microwave absorption performance can be determined
ZnS is a wide band-gap semiconductor with a band-gap
g
by the complex permittivity/permeability, EM impedance match energy (E ) of 3.6 eV. It has been used widely in displays,
10
25,26
and the microstructures. Among the microwave absorber, sensors, and lasers for many years.
To the best of our
knowledge, there are scarce literatures about the microwave
absorption properties of ZnS-based were presented. In our
earlier report, wall-like ZnS-coated Ni composites were
successfully prepared. Compared with pure Ni microspheres
with the diameter of 0.9 mm, Ni/ZnS composites exhibit the
enhanced microwave absorption properties. However, the size
a
School of Materials Science and Engineering, Zhengzhou University, Zhengzhou
27
4
50001, China. E-mail: gang_shao@zzu.edu.cn; zhangray@zzu.edu.cn; Fax: +86-
71-60632600; Tel: +86-371-60632007
3
b
Laboratory of Aeronautical Composites, Zhengzhou Institute of Aeronautical Industry
Management, Zhengzhou 450046, China
9806 | RSC Adv., 2015, 5, 9806–9814
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of Ni spheres were too large and the effect of morphologies on 2.4. Characterization
the microwave absorption properties of Ni/ZnS composites were
not investigated.
In this present work, the walnut-like Ni particles with the
average size of 500 nm and crumpled ZnS nets-wrapped Ni
walnuts have been successfully synthesized. The morphologies
and microwave absorption properties of core–shell Ni/ZnS were
The crystal structure of the as-obtained samples were analyzed
by X-ray diffraction (XRD, XD-3, Beijing Purkinje General
Instrument Co. Ltd.). The morphologies of samples were
observed by eld emission scanning electron microscopy
(FESEM, JEOL JSM-7001F) and transmission electron micro-
scope (TEM, JEOL JEM-2010). The element composition was
carried out by an energy dispersive X-ray spectroscopy (EDS,
Oxford Instruments) equipped with FESEM. To probe the
microwave absorption properties of Ni walnut spheres and
core–shell Ni/ZnS composites, the electromagnetic parameters
of the Ni walnut spheres and Ni/ZnS composites were measured
using a vector networker (Agilent, N5244A) in the frequency of
determined by the hydrothermal temperature. The ultra-thin
ꢀ
and crumpled ZnS-wrapped Ni walnuts prepared at 60
C
show the best microwave absorption capability. The optimal
reection loss is ꢁ42.4 dB at 12.3 GHz. These results suggest
that we have paved a new way to fabricate novel microwave
absorbing materials and extend the application of semi-
conductor ZnS materials.
1–18 GHz. Paraffin composite samples containing 40 wt% as-
obtained products were pressed into toroidal-shape (Fout
:
2
. Experimental section
7.00 mm, Finner: 3.04 mm) for microwave measurement. The
paraffin wax was employed as the binder, which is transparent
for electromagnetic wave.
2.1. Materials
All the chemical reagents were of analytical grade and used
without any further purication. Nickel chloride hexahydrate
3
2 2 3 2
$6H O), glycerol, trisodium citrate dihydrate, NH $H O
. Results and discussion
(
NiCl
and sodium acetate were supplied by Xilong Chemical Reagent
Co. Ltd. (Guangdong, China). Zn(CH COO) $2H O, and
NaH PO $H O were provided by Kemiou Chemical Reagent Co.
Ltd. (Tianjin, China). Na S$9H O was purchased from Feng-
Fig. 1 presents the X-ray diffraction (XRD) patterns of as-
prepared Ni walnut spheres and core–shell Ni/ZnS compos-
ites. As shown in Fig. 1a, all the diffraction peaks are perfectly
indexed to the face-centered cubic phase of Ni (JCPDS no.
3
2
2
2
2
2
2
2
chuan Chemical Reagent Technologies Co. Ltd. (Tianjin,
China). The thin and crumpled ZnS nanonet-wrapped Ni walnut
spheres were prepared through a two-stage method.
04-0850). Aer wrapped by ZnS nanonet, the diffraction peaks
corresponding to both Ni and ZnS (JCPDS Card no. 05-0566) can
be clearly seen. No other phases are observed except for the Ni
and ZnS phases. According to the Fig. 1, it can be inferred that
the as-synthesized core/shell structured composites are
2.2. Preparation of Ni walnut spheres
In a typical procedure, NiCl $6H O (1.2 g), trisodium citrate comprised of crystalline Ni and ZnS.
2
2
dihydrate (0.2 g) and sodium acetate (3.0 g) were dissolved in
The morphologies of Ni particles and Ni/ZnS composites
mixture solution containing 30 mL glycerol and 30 mL distilled were investigated using the FESEM, as shown in Fig. 2.
water. Then NaOH (1.6 g) was added into the solution. Aer- According to the Fig. 2a, the as-synthesized Ni shows the narrow
wards, sodium hypophosphite (NaH PO $H
2 2 2
O, 3.2 g) was added size distribution of walnut-like shapes with the average diam-
to the above mixture. Finally, the solution was transferred into a eter of 500 nm. In addition, the synthesized Ni products appear
Teon-lined stainless steel autoclave, which was then sealed to possess superior dispersibility. Fig. 2b–d exhibit the different
ꢀ
and kept in an oven at 140 C for 15 h. The black precipitate was magnication images of Ni/ZnS composites. From the pano-
collected and rinsed with distilled water and ethanol, and ramic images of Ni/ZnS (Fig. 2b), it can be found that the

nally dried in a vacuum oven.
surfaces of composites were comprised of ZnS nano-nets
2
.3. Preparation of ZnS nanonet-wrapped Ni walnut spheres
In a typical experiment, the as-prepared Ni walnut spheres
0.05 g) and Zn(CH COO) $2H O (0.45 g) were dispersed in a
(
3
2
2
mixture of ethanol (30 mL) and distilled water (30 mL). Then,
Na S$9H O (0.50 g) and ammonia solution (4 mL) were intro-
2
2
duced into the above mixture solution with intensely stirring
over 20 min. Then the mixture was transferred into a Teon-
ꢀ
lined stainless steel autoclave, and maintained at 60 C for
15 h. When cooled to room temperature, the nal precipitates
were washed with distilled water and absolute ethanol. In order
to investigate the effect of temperature on the morphologies
and microwave absorption properties of the Ni/ZnS composites.
ꢀ
ꢀ
ꢀ
Three temperatures (40 C, 60 C, 80 C) were conducted in this
experiment. The corresponding products were denoted as S-1, S-
Fig. 1 XRD patterns of (a) walnut-like Ni particle and (b) core–shell
Ni/ZnS composites. Inset is the XRD profile of Ni/ZnS composite.
2
and S-3 samples, respectively.
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possibly comes from the carbon conductive tape to support the
samples during the test and the oxygen residual or oxide in the
composite, respectively. To illustrate the spatial distribution of
Ni, Zn and S elements in the composite spheres, elemental
mapping was performed on a representative Ni@ZnS core–shell
composite (Fig. 3c–e) under FESEM observation. The elemental
mapping results demonstrate the generally uniform distribu-
tion of Ni/ZnS composite microspheres (Fig. 3c–e).
Furthermore, to further validate the core–shell structure of
Ni/ZnS composites, the TEM images of the attained composites
are applied to characterize the morphology in detail (Fig. 4). The
thin ZnS shells have a crumpled and rippled structure. The
interfaces between Ni and ZnS could be clearly observed.
Moreover, the Ni/ZnS composites show a typical cross-linked
framework structure. The HRTEM image (inset of Fig. 4b) of
the ultra-thin shell region of core–shell structure shows that the
fringe spacing is about 0.31 nm, corresponding to the (111)
crystal planes of the zinc blende ZnS, which is in accordance
with XRD result. From the SEM and TEM images, it can be
concluded that the walnut-like Ni powders are wrapped by thin
and crumpled ZnS nets to form core–shell structure.
Fig. 2 (a) FESEM image of Ni walnuts and (b–d) different magnification
FESEM images of ZnS nanonet-wrapped Ni composites.
compared with pristine walnut-like Ni particles. Fig. 2c and d
show the amplied image of Ni/ZnS composite microspheres,
which indicate the existence of ultra-thin and crumpled struc-
tured ZnS nets on the surfaces of Ni particles. The peanut-like
Ni particles were densely wrapped by ultra-thin and crumpled
ZnS nets to form core–shell structures. The unique crumpled
and rippled structured ZnS nano-nets might give rise to multi-
reection when the microwaves are incident on the Ni/ZnS
composites, which is benecial for the microwave attenuation.
In order to get more information of core–shell structured
Ni/ZnS composites, the morphology of an individual Ni/ZnS
composite microsphere and energy-dispersive X-ray spectros-
copy (EDS) were performed (Fig. 3a and b). The EDS result
shown in Fig. 3b indicates that the obtained composites are
composed of Ni, Zn and S elements. The carbon and oxygen
elements are also appeared in the EDS spectrum, which
Fig. 5 shows the morphologies of the Ni/ZnS composites
prepared at various hydrothermal temperatures. It is worth
noting that the surfaces of all samples turns coarser compared
Fig. 3 (a) FESEM image of an individual Ni/ZnS composites, (b) EDS Fig. 4 TEM images of crumpled ZnS net-wrapped Ni walnut sphere
pattern of Ni/ZnS composite and (c–e) corresponding elemental composite with different magnifications. Inset of (b) shows the HRTEM
mappings of Ni, Zn and S.
of ZnS shell.
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structures between Ni particles and thin ZnS nets endow the
composite with the features of high surface area and allow more
microwave enter, which is favorable to the microwave energy
attenuation. When the reaction temperature is further
ꢀ
increased to 80 C (S-3), the densely ZnS particles are deposited
on the surfaces of Ni powders. Meanwhile, the thickness of ZnS
shells was increased in comparison with S-1 and S-2 samples.
The dense and thick shells might damage the electromagnetic
impedance, which hamper the microwave enter in the
absorbing materials. From the above results, it can be deduced
that the structures of ZnS shells can be effectively determined by
an appropriate hydrothermal temperature.
Microwave absorption properties of a absorber are highly
associated with its complex permittivity and complex perme-
0
00
ability. Fig. 6 shows the complex relative permittivity (3
3
r
¼ 3 ꢁ j3 ,
0
00
and 3 are the real and imaginary part of complex permittivity)
0
00
0
00
and permeability (m
r
¼ m ꢁ jm , m and m are the real and
Fig. 5 FESEM images of hierarchical Ni/ZnS core–shell composites
imaginary part of complex permeability) of walnut-like Ni parti-
ꢀ
ꢀ
prepared at different temperature: (a) S-1 (40 C); (b and c) S-2 (60 C);
ꢀ
cles and core–shell Ni/ZnS composites. It is well known that the
(
d) S-3 (80 C).
0
real part of complex permittivity (3 ) is relative to the polarization
0
0
and imaginary part (3 ) represents the dielectric loss capability of
2
8,29
absorbing materials.
As shown in Fig. 6a, it can be found that
with the pure Ni walnuts, which could be attributed to the
successful coating of the ZnS on the primary Ni surfaces.
Furthermore, the shapes of ZnS shells could be controlled by
0
the 3 values of Ni/ZnS composites were higher than those of Ni
walnuts, which implies more polarization for the Ni/ZnS
composites. It can be attributed to the more interfaces of
ꢀ
the reaction temperatures. At low temperature (40 C, S-1), the
0
unique core–shell structures. Notably, the 3 of S-1 sample is
walnut-like Ni submicrospheres were coated by relatively loose
ZnS nanoparticles (Fig. 5a). When the hydrothermal tempera-
ture was increased to 60 C (S-2), ultra-thin and crumpled
structured ZnS nets were produced on the surfaces of Ni walnut
spheres (Fig. 5b and c). The unique cross-linked framework
higher than that of S-2 and S-3 samples. However, according to
30–32
the previous reports,
high real part of complex permittivity
ꢀ
might lead to impedance mismatch, which is harmful to the
microwave absorption properties. It can be deduced that an
0
appropriate 3 values is necessary for the better microwave
Fig. 6 Frequency dependence of the (a) real and (b) imaginary parts of complex permittivity, and the (c) real and (d) imaginary parts of complex
permeability for the Ni and three Ni/ZnS samples.
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0
0
attenuation. Fig. 6b shows the imaginary parts (3 ) of walnut-
like Ni particles and core–shell Ni/ZnS composites. Interest-
0
0
ingly, one can see that S-2 sample exhibits the highest 3 values
among the four samples, which means the highest dielectric
loss for the S-2 sample. It is worthy to notice that the four
0
0
samples present multi-peaks on the 3 values, suggesting a
resonance behavior, which is expected when the composite is
3
3
highly conductive and skin effect becomes signicant.
Moreover, especially for the Ni/ZnS composites, these
3
4
phenomena are also called the nonlinear resonant behaviors,
which originates mainly from the cooperative consequence of
the Ni cores, the core/shell interfaces and the dielectric ZnS
3
5,36
37,38
00
shells.
According to the free electron theory,
3
z
1
/p3 rf, where r is the resistivity. The electric resistivity of the
0
Ni samples is higher than that of the ZnS@Ni samples. The
high electric resistivity of the Ni samples was assigned to the
low ller ration (40 wt%) and high dispersion in the paraffin
composites. However, for the Ni/ZnS composites, especially for
the S-2 samples, the ultra-thin and crumpled cross-linked ZnS
nets can connect with each other and a continuous micro-
current gradually forms, which will give rise to low electric
3
9
resistivity and cause electric and conduction loss.
0
00
The real part (m )and imaginary part (m ) of the relative
complex permeability are plotted as a function of frequency in
0
Fig. 6c–d. The m of Ni particles and core–shell Ni/ZnS composites
Fig. 7 (a) The dielectric loss factor and (b) magnetic loss factor of Ni
particles and Ni/ZnS–paraffin wax samples versus frequency.
gradually decreased with increasing frequency (Fig. 6c). However,
the m values of S-2 sample show large uctuation, which was due
0
to the unique crumpled ZnS nets-wrapped Ni walnut composites.
00
Imaginary parts (m ) stand for the magnetic loss of the microwave
dissipated capability. From the Fig. 6d, it can be seen that S-2
0
0
3
s
ꢁ 3
N
0
00
sample exhibits the highest m values, which indicates the
highest magnetic loss among the four paraffin-composites.
3
r
¼ 3
N
þ 1
¼ 3 ꢁ j3
(1)
þ j2pf s
0
0
Furthermore, for the Ni/ZnS composites, the m values show
the multi-peaks, which suggest the resonance behaviors. Gener-
ally, the resonance can be assigned to natural resonance in the
Where 3
s N
, 3 , f, s are the static permittivity, relative dielectric
permittivity at the high-frequency limit, frequency and polari-
zation relaxation time, respectively. Thus, 3 and 3 can be
0
00
40
low frequency, while other resonances could be attributed to
described by
4
1,42
exchange resonances in the high frequency.
3
s
ꢁ 3
2
2
N
30 ¼ 3N
0
0
0
þ
(2)
(3)
The dielectric loss tangent (tan d
3
¼ 3 /3 ) and magnetic loss
1
þ ð2pf Þ s
00
0
tangent (tan d
tromagnetic loss capacity. The higher values of tan d
tan d indicate the greater dielectric loss and magnetic loss. The
m
¼ m /m ) are universally used to evaluate elec-
43
3
and
2
pf sð3
s
ꢁ 3
N
Þ
00
3
¼
1
m
2
2
þ ð2pf Þ s
0
0
0
dielectric loss factor (tan d ¼ 3 /3 ) and magnetic loss factor
3
00
0
0
00
(
tan d ¼ m /m ) of the Ni particles and Ni/ZnS composites are based on eqn (2) and (3), the relationship between 3 and 3 can
m
presented in Fig. 7. S-2 sample exhibits the highest dielectric be deduced
loss and magnetic loss among the four samples, which might
indicates the outstanding capability of microwave absorption. It
is noteworthy that both the dielectric loss and magnetic loss
exhibit multi-peaks from the Fig. 7. Moreover, for the S-2
ꢀ 0
ꢁ
2
ꢂ
ꢃ
2
3
ꢁ 3
N
2
3
s
ꢁ 3
N
00
þ ð3 Þ ¼
(4)
2
2
0
00
sample, the dielectric loss shows the maximum values while
magnetic loss exhibits minimum values. Such complementar- generally denoted as the Cole–Cole semicircle. Each semi-
Thus, the plot of 3 versus 3 would be a single semicircle,
4
9
ities between the dielectric loss and the magnetic loss endows circle corresponds to one Debye relaxation process. Fig. 8
0
0
0
the S-2 with the best microwave absorption ability.
shows the curve of 3 as function of 3 for the paraffin matrix
To further understand the mechanism of dielectric loss in composite containing Ni particle and core/shell structured
the absorbing materials, the Debye dipolar relaxation was Ni/ZnS composites. Five semicircles were found in the curves
accounted for the dielectric loss. According to the Debye dipolar of S-2 sample, indicating the contribution of the Debye
relaxation, the relative complex permittivity can be expressed by relaxation process to the enhanced dielectric properties.
44–48
the following equation:
However, the Cole–Cole semicircles are distorted, indicating
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0
0
0
ꢁ2 ꢁ1
Fig. 9 The value C
0
(m (m )
f
) of Ni particles and Ni/ZnS composite
as a function of frequency.
0
0
0
Fig. 8 Typical Cole–Cole semicircles (3 versus 3 ) for Ni particles and
Ni/ZnS composites in the frequency range of 1–18 GHz.
pffiffiffi
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
pf
2
2
00
00
0
0
00 00
0
0
0
00
00 0
a ¼
ꢂ
ðm 3 ꢁ m 3 Þ þ ðm 3 ꢁ m 3 Þ þ ðm 3 þ m 3 Þ
c
(5)
that except for the Debye relaxation, other mechanisms, such
5
0
as Maxwell–Wagner relaxation and electron polarization where f is the frequency of the EM-wave and c is the velocity of
also exist in ultra-thin and crumpled ZnS-wrapped Ni light. As shown in Fig. 10, the dependence of a on frequency
paraffin-composite. The former behavior appears in hetero- shows that S-2 sample holds the maximum a value among the
geneous media owing to charges generated at the surfaces four samples in most of the tested frequency range. Therefore, it
between Ni and ZnS. Due to the connective conductor path of can be speculated that the S-2 sample (crumpled ZnS net-
cross-linked Ni/ZnS composites, the microcurrent could be wrapped Ni) may exhibit better EM-absorption properties than
produced, which induce electron polarization.
those of Ni particles and other Ni/ZnS samples.
Generally speaking, the magnetic loss of absorbing
To disclose the microwave absorption capability, commonly
materials originates from hysteresis loss, domain wall denoted by the reection loss (RL), could be simulated on the
displacement loss, eddy current loss, natural and exchange basis of the relative permeability and permittivity for a given
resonance. In current case, the hysteresis loss is commonly frequency and absorber thickness, by means of the following
5
1
3,51,60,61
negligible in weak applied eld.
displacement loss oen occurs in the MHz range rather than
The domain wall equations:
5
2
RL ¼ 20 log|(Z ꢁ Z )/(Z + Z0)|
(6)
(7)
GHz, so the domain wall resonance can also be excluded.
The eddy current loss is correlated to the diameter (d) and the
electric conductivity (s), which can be described as:
pm (m ) sd f/3. In which f is the applied frequency, m is the
vacuum permeability. From the above equation, if magnetic
loss only originates from eddy current loss, the values of
in
0
in
rffiffiffiffi
ꢀ
pffiffiffiffiffiffiffiffiꢁ
m
2pfd m 3
r
r
r
5
3,54 00
Z
in ¼ Z
0
tan h j
m z
3
r
c
0
2
2
2
0
0
0
where Z is the impedance of free space, Zin is the input
characteristic impedance, f is the frequency, c is the velocity
of light, and d is the thickness of the composites. Fig. 11
0
0
0
ꢁ2 ꢁ1
2
C
0
(C
frequency is varied. In Ni and Ni/ZnS composite, the value of
decrease gradually with increasing frequency in the whole
0
¼ m (m )
f
¼ 2pm
0
sd /3) should be constant as the
C
0
range of 1–18 GHz (Fig. 9). Thus, the eddy-current loss can be
precluded. Therefore, microwave magnetic loss is attributed
to the natural resonance and the exchange resonance.
Moreover, it is commonly suggested that the natural reso-
nance should occur at lower frequency than the exchange
5
5
resonance.
To obtain absorbing materials with excellent microwave
absorption capabilities, there are two key problems to be
considered. One is the impedance match between the material
56
and free space, which needs the permittivity is close to
permeability. The other one is the EM attenuation in the inte-
rior absorber. The attenuation constant a determines the
attenuation properties of materials. Based on transmission line
theory and EM-wave propagation constant, the a can be
Fig. 10 Attenuation constant of Ni particles and Ni/ZnS–paraffin
composites versus frequency.
57–59
expressed:
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Fig. 11 The reflection losses of (a) Ni particles, (b) S-1, (c) S-2 and (d) S-3 samples with various thicknesses.
Table 1 Some Ni-based composites for microwave absorption in recent literatures
Minimum RL value
(dB)
Optimum thickness
(mm)
Optimum frequency
(GHz)
Frequency range
(RL < ꢁ10 dB)
Sample
Ref.
Octahedral Ni
Ni ber
Urchin-like Ni
Ni chains
ꢁ40.44
ꢁ39.5
ꢁ43
2.5
3.0
2.0
0.8
7.0
2.0
2.0
5.0
8.8
4.8
10
17.2
14.7
13.0
9.2
7.1ꢁ11.2
6.6ꢁ8.8
11
12
14
13
30
20
32
19
8.7ꢁ11.5
16ꢁ18
ꢁ19.9
ꢁ18.6
ꢁ32
Ni/SnO
Carbon-coated nickel
Al -coated Ni
Ni/polyaniline
2
13.8ꢁ15.3
11.2ꢁ15.5
87.5–13.3
4.9ꢁ6.1
2
O
3
ꢁ30.03
ꢁ35
17.2
1
6.2ꢁ18
Ni/Sn
6
O
4
(OH)
4
ꢁ32.4
5.0
13.2
7.2ꢁ8.0
35
1
2.7ꢁ13.8
Ni/polypyrrole
Ni/TiO
Ni/SiO
Crumpled ZnS nets-wrapped Ni
ꢁ15.2
ꢁ35.4
ꢁ40.0
ꢁ42.4
2.0
4.0
1.5
2.2
13.0
17.8
12.6
12.3
11ꢁ15.4
63
64
64
2
17.0ꢁ18.0
10.9ꢁ14.4
11.3ꢁ15.6
2
This work
shows the calculated theoretical walnut-like Ni and core–shell increasing absorber thickness, the position of RL peaks almost
Ni/ZnS paraffin composites with 40 wt% loadings at various remains unchanged without shiing to lower frequency,
2
7
thicknesses in the frequency range of 1–18 GHz. It is worth which has also been reported by our previous work and other'
6
2
noting that the core–shell Ni/ZnS composites exhibit the groups. The positions of RL peaks were consistent with the
enhanced microwave absorption properties compared natural and exchange resonances, which means the resonance
with pristine walnut-like Ni particles, which might be attrib- affects the maximum microwave absorption. Table 1 shows
uted to the additional interfacial polarization in the core–shell some Ni-based composites and their corresponding micro-
structures. Among the three Ni/ZnS samples, S-2 sample wave
possesses the best electromagnetic wave absorption ability tures.
absorption
performances
According to the comparison, the ultra-
in
recent
litera-
1
1–14,19,20,30,32,35,63,64
(
1
Fig. 11c). The optimal RL of ꢁ42.4 dB was obtained at thin and crumpled ZnS nets-wrapped Ni walnut spheres core–
2.3 GHz and RL values less than ꢁ10 dB (90% microwave shell composites are very promising to be used as thin-
attenuation) were observed in the 11.3–15.6 GHz rang with thickness, and high microwave absorptive materials in a wide
only absorber thickness of 2.2 mm. Interestingly, with frequency range.
9812 | RSC Adv., 2015, 5, 9806–9814
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1
7 Z. Wang, L. Wu, J. Zhou, B. Shen and Z. Jiang, RSC Adv., 2013,
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4
. Conclusion
, 3309–3315.
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ꢀ
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