Synthesis and characterization of nanosized urchin-like α-Fe 2O3 and Fe3O4: Microwave electromagnetic and absorbing properties

Article abstract of DOI:10.1016/j.jallcom.2011.01.058

Urchin-like α-Fe₂O₃ and Fe₃O ₄ nanostructures were prepared from the precursor urchin-like α-FeOOH under reducing atmosphere. The dependence of reduction temperature on their morphology, microstructure, and microwave electromagnetic and absorbing characteristics were systematically studied. It is found that the reduction temperature plays an important role in the microstructure and electromagnetic characteristics of the resulting products. In present study, the urchin-like α-Fe₂O₃ with dual absorption peaks can be formed at the relatively low temperature (e.g. 300 °C). Urchin-like Fe₃O₄ can be obtained just at 350-400 °C, which presents excellent microwave absorption property, with the minimum reflection loss of -29.96 dB and below -20 dB in 3.76-8.15 GHz corresponding to 3-4 mm thickness. The excellent microwave-absorption properties are a consequence of a proper electromagnetic matching and enhanced absorbing abilities resulting from the urchin-like shape and inverse spinel-type crystal structure.

Full text of DOI:10.1016/j.jallcom.2011.01.058

Journal of Alloys and Compounds 509 (2011) 4320–4326
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of nanosized urchin-like ␣-Fe O and Fe O :
2
3
3
4
Microwave electromagnetic and absorbing properties
a,∗
a
b
a
a
b
Guoxiu Tong , Wenhua Wu , Jianguo Guan , Haisheng Qian , Jinhao Yuan , Wei Li
a
College of Chemistry and Life Sciences, Zhejiang Key Laboratory for Reactive Chemistry on Solid Surface, Zhejiang Normal University, Jinhua 321004, PR China
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
2 3 3 4
Urchin-like ␣-Fe O and Fe O nanostructures were prepared from the precursor urchin-like ␣-FeOOH
Received 11 September 2010
Received in revised form 5 January 2011
Accepted 7 January 2011
under reducing atmosphere. The dependence of reduction temperature on their morphology, microstruc-
ture, and microwave electromagnetic and absorbing characteristics were systematically studied. It is
found that the reduction temperature plays an important role in the microstructure and electromagnetic
characteristics of the resulting products. In present study, the urchin-like ␣-Fe2O3 with dual absorption
Available online 15 January 2011
◦
peaks can be formed at the relatively low temperature (e.g. 300 C). Urchin-like Fe3O4 can be obtained just
Keywords:
Urchin-like nanostructure
Iron oxide
Complex permittivity
Complex permeability
Microwave absorption
◦
at 350–400 C, which presents excellent microwave absorption property, with the minimum reflection
loss of −29.96 dB and below −20 dB in 3.76–8.15 GHz corresponding to 3–4 mm thickness. The excellent
microwave-absorption properties are a consequence of a proper electromagnetic matching and enhanced
absorbing abilities resulting from the urchin-like shape and inverse spinel-type crystal structure.
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
With the rapid development of wireless telecommunication
from satisfactory for practical application. Thus, it is still a great
challenge to find new types of complex nanostructures with rel-
atively light weight, flexible and sound structure, and especially
excellent microwave absorbing properties.
systems, electromagnetic interference (EMI) and EM-wave pol-
lution are increasingly deteriorated nowadays. For this purpose,
extensive studies have been carried out to exploit new and high
efficient absorbents to prevent such phenomena. Among all the
contenders, new type nanomaterials with complex nanostruc-
tures have attracted considerable attention owing to their unique
chemical and physical properties [1–6], especially as microwave
absorbing and shielding materials [7–12]. For example, Cao et al.
Iron oxides (e.g. ␣-Fe O3 and Fe O ), are generally recog-
2
3
4
nized as most of promising materials because of their low cost,
non-toxicity, environmental benignity, and interesting magnetic
behaviors, which is of great importance to have potential applica-
tions as catalysts, gas sensors, magnetic filters, electrode materials,
magnetic storage media, as well as materials for microwave absorb-
ing and shielding research [13–17]. Interestingly, these intrinsic
properties of magnetic nanostructures may be tailored by control-
ling their shape, composition, size, and so forth.
[
8] prepared cagelike ZnO/SiO2 nanocomposites with the maximal
attenuation of 10.68 dB at 12.79 GHz. As reported by Yan et al. [9,10]
the maximal attenuation values for ZnO nanotrees and 3D sponge-
like Mn O porous network were 42 dB at 3.6 GHz with a thickness
of 5.0 mm and 27.1 dB at 3.1 GHz with a thickness of 6.0 mm, respec-
tively. Zou et al. [11] found that the plane antenna absorbent
presented an effective bandwidth. Recently, Kang and co-workers
Despite its fundamental and technological importance, the chal-
lenge of systematically controlling the morphology, structure, and
properties of iron oxides with complex nanostructures have been
met with limited success [14,18–21]. Meanwhile, the reported
literatures mainly focused on the investigation of the photocat-
alytic, magnetic, and electrochemical properties of iron oxide
complex nanostructures [14,18–21]. To the best of our knowl-
edge, a systematic investigation of microwave electromagnetic and
absorbing properties of urchin-like ␣-Fe O , especially urchin-like
2
3
[
12] observed that FeCo alloy particles/graphite nanoflake com-
posites showed a strong low-frequency absorption of −30.6 dB at
.4 GHz with a coating thickness of 2.0 mm. Notwithstanding, the
7
aforementioned new-type complex nanostructures display some
enhanced electromagnetic characteristics and novel absorption
mechanism, the corresponding absorption performances are far
2
3
Fe O is still unavailable so far. In present study, we developed a
3
4
facile glucose-guided hydrolyzing-reducing process for the prepa-
ration of urchin-like ␣-Fe O and Fe O , which exhibited excellent
2
3
3
4
microwave absorption properties. Furthermore, structural evolu-
tion, electromagnetic characteristics, and microwave absorption
mechanism were discussed in detail.
∗ Corresponding author. Tel.: +86 579 82282269; fax: +86 579 82282269.
E-mail address: tonggx@zjnu.cn (G. Tong).
0
925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.01.058

G. Tong et al. / Journal of Alloys and Compounds 509 (2011) 4320–4326
4321
◦
Fig. 1. (a and b) SEM, (c) XRD pattern and (d) IR spectrum of urchin-like ␣-FeOOH nanostructures synthesized at 60 C for 12 h.
2
.
Experimental
(FTIR) absorption spectra were captured with a Nicolet FTIR-170SX spectrometer
−
1
in the range of 4000–400 cm at room temperature with the sample in a KBr disk.
The transmission/reflection method was applied to determine the relative com-
plex permeability and permittivity of the sample-wax composites through an
Agilent N5230 vector network analyzer system. The cylindrical toroidal samples
with 3.0 mm in inner diameter, 7.0 mm in outer diameter and 3.5 mm in thickness
were fabricated by uniformly mixing wax with 50 wt% the absorbents and then
pressed into cylindrical compacts. Based on the above relative permeability and
permittivity at the given frequency, the reflection loss (RL) curves were calculated
according to transmission line theory [9].
2.1. Materials
All reagents including d-(+)-glucose (C6H12O6·H2O, AR) and iron (II) sulfate hep-
tahydrate (FeSO4·7H2O, AR) were obtained from Shanghai Sinopharm and used
without any further purification.
2
2
.2. Preparation
.2.1. Synthesis of the precursor urchin-like ˛-FeOOH
The synthesis method-the glucose-guided hydrolyzing approach has been
described elsewhere previously in detail [21,22]. In a typical synthesis, certain
3. Results and discussion
amount of d-(+)-glucose, FeSO4·7H2O, and deionized water were added into a
◦
4
00 mL three-naked conical flask, and refluxed at 60 C for 12 h under magnetic
3.1. The morphology and structure of the precursors
stirring and a constant air flow of 1.0 L/min. The urchin-like ␣-FeOOH precursors
were collected by filtrating and washed with deionized water and absolute ethanol
for 3 times, respectively, finally dried in a vacuum oven at 90 C for 12 h.
◦
Fig. 1a and b shows the low- and high-magnification SEM images
of the precursors obtained by a facile glucose-guided hydrolyz-
ing approach. It can be seen that urchin-like aggregates with
2
.2.2. Synthesis of urchin-like ˛-Fe2O3 and Fe3O4
According to the conventional reaction (1) [21,22], the urchin-like ␣-Fe2O3 was
0
.5–1.0 ␮m in diameter consist of sea shuttle fibrous bunches with
◦
obtained after calcinations of the urchin-like ␣-FeOOH precursor at 300 C for 1 h
with a heating rate of 5 C/min. The urchin-like ␣-Fe2O3 conversion to urchin-like
Fe3O4 was achieved at 350–400 C for 2 h in H2/Ar (1:4 volume ratio) mixing gas at
a flow of 1.0 L/min. Finally, the system was allowed to cool down naturally to room
temperature under the umbrella of N2.
◦
40–100 nm in diameter and ca. 200 nm in length, radially growing
from a center of the aggregate. Fig. 1c shows that the XRD pat-
terns of the precursors, in which all the characteristic peaks are
in good agreement with ␣-FeOOH (JCPDS card no. 29-0713). This
is further confirmed by the FTIR analysis. Observed from Fig. 1d,
◦
2
␣-FeOOH−→ ␣-Fe2O3 + H2O
(1)
ꢀ
−1
the sharp peaks at 3178, 892, and 794 cm are attributed to the
2
.3. Characterization
characteristic vibrations in the FeO-(OH) product [23], while the
strong broad peak at 3384 cm and the sharp peak at 1638 cm
correspond to the stretching vibration and the bending vibration of
hydroxyl groups in the surface-adsorbed molecules [24,25], respec-
tively. Thus the as-obtained precursors are urchin-like ␣-FeOOH.
−
1
−1
The morphologies of the as-prepared samples were observed by a field-emission
scanning electron microscope (FE-SEM, Hitachi S-4800), operated at 10.0 kV. The
phase analysis of the samples was performed by X-ray diffraction (XRD) on a RIGAKU
D/MAX-IIIA diffractometer with CuK␣ radiation. Fourier-transforming infrared

4
322
G. Tong et al. / Journal of Alloys and Compounds 509 (2011) 4320–4326
ity and the growth rate of the crystals, resulting in the decrease
of the internal strain and the increase of the average crystal
size.
3.3. The morphology dependence on reduction temperature
Fig. 3 displays the SEM images of the products obtained at var-
◦
ious Tr. The ␣-Fe O3 samples gained at Tr = 300 C still keep the
2
initial shapes, namely, sea shuttle fibrous bunches with 20–50 nm
in diameter radially spread and assemble into the spherical aggre-
gates of 0.5–1.0 ␮m (shown in Fig. 3a). Also, the Fe O samples
3
4
◦
obtained at Tr = 350–400 C exhibit the urchin-like morphology.
Interestingly, their building-blocks are not the fibrous bunches
but the rod-like nanoparticles with the diameter of 20–50 nm and
length of 50–200 nm (Fig. 3b and c). Furthermore, the aspect ratio of
the nanorods decreases as the Tr increases (Fig. 3c). This is because
at the relatively high reduction temperature, the nanoparticles
reduced sharply grow via fusing the neighboring nanoparticles
after phase transformation and nucleation, causing the formation of
the nanorods. However, the size and the percentage of the urchin-
Fig. 2. XRD patterns of the products obtained at various Tr.
◦
like Fe O particles obtained at Tr = 450 C decrease and the rod-like
3
4
3
.2. The microstructure dependence on reduction temperature
particles begin to appear (Fig. 3d), which can be deduced as ␣-Fe
◦
from the above XRD data. Further increasing the Tr to 500 C, the
Fig. 2 presents the XRD patterns of the products obtained at var-
ious Tr. Compared with the data in JCPDS card no. 24-0072, all peaks
urchin-like Fe O particles completely disappeared and rod-like ␣-
3
4
Fe particles with 20–50 nm in diameter and 50–200 nm in length
formed. The aforesaid results demonstrate that the urchin-like ␣-
◦
denoted by the ꢀ sign, from the products obtained at Tr = 300 C,
can be identified as ␣-Fe O with orthorhombic system. Whereas,
2
3
Fe O and Fe O4 are apt to form at the moderate temperature,
2
3
3
all the peaks indicated by the ♥symbol disclose that the products
whereas the only rod-like ␣-Fe may be obtained at the relatively
high temperature.
◦
obtained at Tr = 350–400 C are face centered cubic Fe O (JCPDS
3
4
card no. 65-3107). Owing to the enhanced crystallization and the
enlarged crystal size, the intensity of peaks significantly sharpens as
the Tr goes up. Known by JCPDS card no. 65-3107 and 06-0696, the
The above morphological evolution can be interpreted as fol-
◦
lows: at the relatively low temperature (e.g. Tr = 300 C), the
remained morphology is associated with the mild phase trans-
◦
products obtained at Tr = 450 C may be a mixture of Fe O and ␣-Fe.
3
4
formation from ␣-FeOOH to ␣-Fe O , as well as the same and/or
2
3
◦
However, at Tr = 500 C the only three peaks denoted by the ꢁ sym-
bol can be accurately indexed as (1 1 0), (2 0 0) and (2 1 1) directions
of body-centered cubic (bcc) ␣-Fe, respectively, consistent with the
reported data (JCPDS card no. 06-0696). This demonstrates that
the chemical activity of H2 is gradually boosted with the elevated
◦
similar crystallographic system. While, at Tr = 350–400 C, the
maintenance of the urchin-like structures is mainly ascribed to
the FeO6 coordination octahedrons between the interphases of ␣-
Fe O and Fe O , which interact by sharing their vertices and/or
2
3
3
4
◦
edges. Besides, at Tr = 450 C or higher, The morphological evolution
from urchin-like to rod-like is relative to the different crystal-
lographic system and the low melting point of Fe nanocrystals.
As-reduced Fe nanocrystals automatically depart from the sur-
temperature. Hereinto, pure Fe O products can be gained just at
3
4
◦
3+
Tr = 350–400 C, in which H begins to reduce partially Fe into
Fe as shown in the reaction (2). However, at Tr = 500 C or higher,
the chemical activity of H is improved enough to completely revert
Fe to Fe (see reaction (3)), resulting in the formation of the pure
metallic Fe products.
2
2
+
◦
2
face of the Fe O4 precursors, causing the decreased size of the
3
3+
0
urchin-like Fe O , and subsequently are self-assembled into one-
3
4
dimensional structure under the induced process of the carrier gas
flow and the spontaneous magnetization of magnetic nanoparti-
cles [28]. Thus, the phase and morphology can be well tailored by
modulation of the Tr.
3
␣-Fe O + H −→ 2Fe O + H O
(2)
(3)
2
3
2
3
4
2
ꢀ
␣
-Fe O + 3H −→ 2␣-Fe + 3H O
2
3
2
2
ꢀ
The average crystal size (ı) and internal strain (ε) of the prod-
3.4. The permittivity dependence on reduction temperature
ucts are calculated by the Hall–Williamson equation [26,27] and
the results are listed in Table 1. It can be clear that varying
the Tr from 300 C to 500 C, the ı fluctuates at the range of
ꢀ
Fig. 4 shows the frequency dependence of real (ε ) and imag-
◦
◦
ꢀꢀ
inary (ε ) parts of the relative complex permittivity for the
1
2.4–104.1 nm and the ε changes at 0.052–0.0083% range. In addi-
wax-composites including 50 wt% samples obtained at various Tr.
◦
tion, the ı and ε display an abnormal fluctuation at Tr = 450 C
due to the phase transformation. It indicates that the increasing
Tr is conducive to the enhancement of the H2 chemical activ-
In general, ␣-Fe O3 with corundum structure is an n-type semi-
2
ꢀ
ꢀꢀ
ꢀ
conductor with relatively low ε and ε [20]. Seen from Fig. 4, the ε
value of the urchin-like ␣-Fe O3 obtained at Tr = 300 C is about
◦
2
Table 1
Average crystal size (ı), internal strain (ε), and microwave absorption properties of the as-obtained products at various reduction temperatures (Tr).
◦
Tr ( C)
Crystallite
size ı (nm)
Strain D (%)
Minimum RL
value (dB)
fm (GHz)
(minimum RL)
dm (mm)
(RL < −20 dB)
Frequency range (GHz)
(Ri < −20 dB)
300
350
400
450
500
12.4
30.5
67.2
51.7
104.1
0.052
0.015
0.014
0.017
0.0083
−9.20
−24.89
−29.96
−21.22
−19.07
16.4
4.4
5.2
8.4
13.2
/
/
3–4
3–4
3
4.20–6.40
3.76–8.15
7.89–8.23
/
/

G. Tong et al. / Journal of Alloys and Compounds 509 (2011) 4320–4326
4323
◦
◦
◦
◦
◦
Fig. 3. SEM images of the samples obtained at different Tr of: (a) 300 C, (b) 350 C, (c) 400 C, (d) 450 C, and (e) 500 C. The bars in (a)–(e) are 200 nm and the bar in (f) is
2
␮m.
ꢀ
ꢀ
5
.0, and the ε value is about 0.5. It indicates that the urchin-
resonance loss (tg ıres), and other [30]. They are strongly depen-
dent on the composition, dimension, morphology, conductivity,
and polarization model [31]. In this study, the improvement of the
like ␣-Fe O possesses enhanced dielectric loss. The permittivity
2
3
originates from orientation polarization, atomic polarization, and
electronic polarization [7,29], and the dielectric loss may be clas-
sified in conductance loss (tg ıe), dielectric relaxation loss (tg ırel),
dielectric characteristics for ␣-Fe O3 is mainly attributed to the
2
enhanced orientation and interface polarization in the urchin-like
ꢀ
ꢀꢀ
Fig. 4. (a) Real part ε and (b) imaginary part ε of the permittivity for the wax-composites including 50 wt% samples obtained at various Tr.

4
324
G. Tong et al. / Journal of Alloys and Compounds 509 (2011) 4320–4326
ꢀ
ꢀꢀ
Fig. 5. (a) Real part ꢁ and (b) imaginary part ꢁ of the complex permeability for the wax-composites including 50 wt% samples obtained at various Tr.
structures [9], which is different from the rod-like and spherical
particles.
result reveals that the permittivity resonance behavior reported
here is the intrinsic characteristic of our urchin-like material.
◦
By contrast, urchin-like Fe O obtained at Tr = 350 C exhibits
3
ꢀꢀ
4
ꢀ
significantly increasing ε and ε , suggesting the higher storage and
loss capability of the electric energy owing to the enhanced electri-
cal conductivity of Fe O arising from its inverse spinel-type crystal
3.5. The permeability dependence on reduction temperature
3
4
Fig. 5 presents the complex permeability of the wax composites
3
+
2+
3+
2−
structure. Formally, it is written as Fe [Fe , Fe ] O to show
that the first (A type) Fe is tetrahedrally coordinated, whereas the
bracketed Fe and Fe ions occupy (B type) octahedrally coordi-
r
with 50 wt% samples obtained at various T . As for the urchin-like
A
B
4
3
+
◦
ꢀ
ꢀꢀ
␣-Fe2O3 gained at Tr = 300 C, the ꢁ and ꢁ values are around
1.0 and 0.25, respectively. This suggests the presence of the weak
ferromagnetic behavior and magnetic loss. Usually, ␣-Fe2O3 is an
antiferromagnetic material below Morin temperature (TM = 260 K)
and exhibits weak ferromagnetic behavior (called parasitic fer-
romagnetism) when the temperature is between 260 and 955 K
(Néel temperature). In this study, the oxygen vacancies inside the
urchin-like ␣-Fe2O3 may destroy the balance of antiferromagnetic
behavior, resulting in the extremely weak ferromagnetism and
magnetic loss [41]. However, for the urchin-like Fe3O4 obtained
2
+
3+
nated sites. At room temperatures, hopping of the minority-spin
3+
“
extra” electron between the B site Fe2+ and Fe ions enhances the
electronic conductivity ꢂ [32,33]. On the one hand, known from
1
0
tg ıc = 1.8 × 10 (ꢂ/fεr) [34], the enhanced electrical conductivity
ꢂ is good for the elevation of the conductance loss (tg ıc). On the
other hand, the space charge polarizations stemming from both
the inverse spinel-type crystal and urchin-like structure cause the
significant increase of the dielectric relaxation loss (tg ırel).
ꢀ
ꢀꢀ
◦
ꢀꢀ
For the same phase structure, the ε and ε of urchin-like
at Tr = 350–400 C, the ꢁ significantly increases, suggesting the
enhanced magnetic loss. This arises from its inverse spinel-type
crystal structure, in which the incomplete cancellation of the two
B site magnetic moments by the antiparallel A site cation moment
results in ferrimagnetism and enhanced magnetic loss [32]. Mean-
◦
Fe O obtained at Tr = 400 C decrease. The change can be inter-
3
4
preted as following two aspects: One is the decreased dipoles. It
is well known that the dipoles will contribute to the dielectric
loss [35,36]. However, an increase in the Tr will lead to a decrease
in the dipoles. This is because the number of surface atoms with
unsaturated bonds decreases with the increasing crystalline size
and the decreasing internal strain [37]. The other is the varia-
tion of the urchin-like structure. The urchin-like nanostructure can
enhance the dielectric characteristics due to the special orientation
and interface polarization. However, the above characteristics are
closely relative with its structure parameters, such as crystal size,
diameter, surface area, and aspect ratio of the nanorod. Therefore,
the decreased aspect ratio with the increasing Tr may induce the
weakened orientation polarization. Further, elevating the Tr from
ꢀ
ꢀꢀ
while, the ꢁ and ꢁ fluctuate at the range of 0.85–1.25 and
0.05–0.35, respectively, and slightly increase as the Tr increases as
a result of the morphology and microstructure variation.
It is worthy of notice that the two sharp resonance peaks of the
ꢀ
ꢀꢀ
ꢁ and ꢁ curves appear at 2–7 GHz and 14–18 GHz, respectively.
Usually, for ferromagnetic materials the occurrences of two types
of magnetic resonance are ferromagnetic and spin resonances.
The former normally emerges at low frequency region (<2 GHz)
owing to domain wall motion; while, the latter appears at high
frequency region thanks to spin rotational component [42]. How-
ever, in our studies, the first resonance peak (around 7 GHz) may be
related with the “exchange mode” resonance as a result of the small
size effect, surface effect, and spin wave excitations. Whereas the
second resonance peak (around 16 GHz) may be associated with
natural resonance. This indicates that the multi-resonance behav-
ior of the ferromagnetic matter is very sensitive to its composition,
size, and morphology.
◦ ◦ ꢀ ꢀꢀ
4
50 C to 500 C, the ε and ε of the as-prepared products fur-
ther decrease owing to the disappearance of urchin-like Fe O4 and
3
appearance of the rod-like ␣-Fe. This further confirms the above
results.
In addition, two strong peaks of 6–10 GHz and 11–16 GHz can
ꢀ
ꢀ
be observed in the ε plots, implying a resonance behavior. Also,
the similar multiresonance behaviors have been reported in the
others literatures, which are ascribed to the existence of polariz-
able Fe2+ ions [38], the heterogeneous structure of the composite
3.6. The absorption property dependence on reduction
temperature
[
39], the vacancy and space-charge polarization [40], respectively.
Therefore, the general dispersive behaviors of dielectric response
rely heavily on the composition, morphology, and microstructure
of the samples. Seen from Fig. 4b, the resonance peaks are obvi-
Fig. 6 shows a typical relationship between the calculated RL
and frequency for the wax composites containing 50 wt% samples
ously enhanced after the appearance of the urchin-like Fe O4 and
obtained at various T . The entire curves present a similar variation
3
r
weakened after the disappearance of the one. So we may deduce
that electronic spin and charge polarization originating from the
urchin-like Fe O may be responsible for the above resonance. The
trend and the R are found to be dependent sensitively on the coat-
L
ing thickness. On the one hand, R peaks gradually shift to the lower
L
3
4
frequency with increasing coating thickness. On the other hand,

G. Tong et al. / Journal of Alloys and Compounds 509 (2011) 4320–4326
4325
Fig. 6. Reflection loss curves of the as-prepared products obtained at various Tr.
the minimum RL value changes with the coating thickness. This
isotropic quasi-antennas, which favors the electromagnetic wave
to penetrate the composites containing the urchin-like iron oxide
nanostructures. The above EM energy further is induced into a dis-
sipative current and then consumed in the discontinuous networks
ꢀ
−
1
ꢀ
ꢀ
2
is in accord with fm = (c/4dm)(1/ ε ꢁ )(1 + (1/8) tan ıM) [43],
where dm is matching thickness, fm is the matching frequency, and c
is the velocity of light. Therefore, the best matching and absorbing
performance can be achieved via adjusting the coating thickness
and electromagnetic parameters.
[
10]. However, to clarify the mechanism, further experimental and
theoretical work will be carried out in the future.
The EM-wave absorption properties of the wax composites con-
taining 50 wt% samples obtained at various Tr are summarized in
Table 1. As shown in Fig. 6a and Table 1, as for the urchin-like ␣-
4
. Conclusions
◦
Urchin-like ␣-Fe O3 and Fe O4 were synthesized after calcina-
Fe O obtained at Tr = 300 C, the absorbing peaks of 4–13 GHz and
2
3
2
3
tion of the urchin-like ␣-FeOOH precursors at diverse temperatures
under reducing atmosphere, respectively; and the urchin-like
␣
ing approach. The phase, morphology, and microstructure of the
urchin-like nanomaterials can be well controlled and carried
out via modulation of the temperature, which resulted in well
controlled the electromagnetic characteristics correspondingly.
Excellent microwave absorbing performances have been observed
in urchin-like Fe O4 obtained at 400 C, with a minimum RL value
1
4–18 GHz can be observed, respectively, with a minimum RL value
of −9.2 dB and coating thickness between 1.0 and 5.0 mm. How-
-FeOOH precursors were obtained by a glucose-guided hydrolyz-
ever, the absorption properties of urchin-like Fe O obtained at
3
4
◦
Tr = 350–400 C are significantly enhanced. Among them, urchin-
like Fe O obtained at Tr = 400 C exhibits the optimal absorption
◦
3
4
properties, with a minimum RL value of −29.96 dB and below
−
20 dB in 3.76–8.15 GHz corresponding to 3–4 mm thickness. It is
worth noting that the RL values (below −20 dB in the 3.76–8.15 GHz
◦
frequency range) are broader frequency ranges than those reported
3
of −29.96 dB and below −20 dB in 3.76–8.15 GHz corresponding to
in the literatures, i.e. carbon fibers coated by Fe O4 nanoparticles
3
3
–4 mm thickness. Our results give a novel insight of the absorp-
[
15], Fe O nanoparticles [39], Fe O –carbon sphere composite
3 4 3 4
tion mechanism of urchin-like ␣-Fe O and Fe O for the first time.
[
44]. Further increasing the Tr leads to a decrease in the number
2
3
3
4
The urchin-like magnetic nanostructures would find applications
in a variety of fields, such as catalysis, energy conversion, electro-
magnetic devices, electrode materials, sensors, biomedicine, and
microwave absorbing and shielding materials, etc.
of urchin-like Fe O4 and an increase in the number of rod-like ␣-
3
Fe. Consequently, the absorbing intensity reduces and the effective
bandwidth (below −20 dB) pinched down from 3.39 GHz to 0 GHz
(
see Table 1 and Fig. 6d and e).
Apparently, the mechanism for the microwave attenuation of
Acknowledgements
urchin-like nonmaterial is different from that of conventional
microwave absorbing materials due to the especially geometrical
morphology. As can be seen from the aforesaid figures, urchin-like
samples demonstrate diverse electromagnetic characteristics from
the sphere-like and rod-like samples: (1) enhanced value of all
parameters, especially for permittivity; (2) extra multi-resonance
behaviors. Additionally, the following facts are well worth consid-
ering for the microwave attenuation of urchin-like nonmaterial:
When the urchin-like samples are filled in the wax matrix, some
urchin-like nanostructures interconnect and form the continu-
ous micronetwork. This may generate the microcurrent network
on a large scale, resulting in the enhancement of conductive
loss [8]. Whereas, others with discontinuous networks can form
Financial support was provided by the National High-
Technology Research and Development Program of China (No.
2
006AA03A209), Natural Scientific Foundation of Zhejiang
Province (No. Y4100022), Special Grand Science-Technology
Project in Zhejiang Province (No. 2010C11053), Science and Tech-
nology Projects from Jinhua City (No. 2010A12066), and Doctoral
Start-up Foundation (No. ZC304009094).
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