The effects of high magnetic field on the morphology and microwave electromagnetic properties of MnO2 powder

Article abstract of DOI:10.1016/j.jssc.2010.04.027

MnO₂ with a sea urchin-like ball chain shape was first synthesized in a high magnetic field via a simple chemical process, and a mechanism for the formation of this grain shape was discussed. The as-synthesized samples were characterized by XRD, SEM, TEM, and vector network analysis. The dielectric constant and the loss tangent clearly decreased under a magnetic field. The magnetic loss tangent and the imaginary part of the magnetic permeability increased substantially. Furthermore, the theoretically calculated values of reflection loss showed that the absorption peaks shifted to a higher frequency with increases in the magnetic field strength.

Full text of DOI:10.1016/j.jssc.2010.04.027

ARTICLE IN PRESS
Journal of Solid State Chemistry 183 (2010) 1490–1495
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
The effects of high magnetic field on the morphology and microwave
electromagnetic properties of MnO2 powder
Ã
Jia Zhang a, Duan Yuping a, , Li Shuqing b,1, Li Xiaogang c,2, Liu Shunhua a
a Department of Materials Processing Engineering, School of Materials Science & Engineering, Dalian University of Technology, Linggong Road 2, Ganjingzi District, Dalian 116085,
Liaoning Province, PR China
b Beijing Aeronautical Manufacturing Technology Research Institute, 1 Jun Zhuang east Road, Chaoyang District, Beijing 100024, PR China
c School of Materials Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
MnO2 with a sea urchin-like ball chain shape was first synthesized in a high magnetic field via a
simple chemical process, and a mechanism for the formation of this grain shape was discussed.
The as-synthesized samples were characterized by XRD, SEM, TEM, and vector network analysis. The
dielectric constant and the loss tangent clearly decreased under a magnetic field. The magnetic loss
tangent and the imaginary part of the magnetic permeability increased substantially. Furthermore, the
theoretically calculated values of reflection loss showed that the absorption peaks shifted to a higher
frequency with increases in the magnetic field strength.
Received 11 December 2009
Received in revised form
14 April 2010
Accepted 17 April 2010
Available online 28 April 2010
Keywords:
Manganese dioxides
High magnetic field
Chain shape
& 2010 Elsevier Inc. All rights reserved.
Electromagnetic properties
1. Introduction
Several research groups have investigated the MnO2 with
different crystalline structures. They have successfully prepared
nanorods [1,11–13], nanofibers [14,15], cuneiforms [2], micro-
spheres [16], and nanowires [17] of -MnO2, -MnO2, -MnO2,
and -MnO2. Manganese oxide resembling a ‘‘sand rose’’ with
large blades was synthesized by Beaudrouet [18]. Nest-like
hierarchical architecture of manganese oxide was studied by Liu
et al. [19]. Cheng et al. [20] synthesized a 2-D hexagonal star-like
Shape- and size-controlled dispersible inorganic nanocrystals
such as metals, semiconductors, and metal oxides, have been the
subject of intensive research because of their potential applica-
tions in electronics, photonics, and catalysis [1,2]. They are also
ideal subjects for research on optical, electrical, magnetic,
microwave electromagnetic, mechanical properties, and the
interdependence of these characteristics on morphology and size
confinement [3–5]. Consequently, particles with novel shapes and
sizes have become especially interesting.
In this context, manganese dioxides and their derivative
compounds have attracted special attention. Their outstanding
structural flexibility, in combination with their novel chemical
and physical properties has paved the way for their wide use as
catalysts [6], molecular sieves [7], and Li/MnO2 batteries materials
[8]. The different crystallographic structural forms of MnO2 that
a
b
g
d
b-MnO2. Zhang et al. [21] obtained hollow octahedral of
manganese oxides. However, most of these studies used the
method of hydrothermal synthesis to obtain manganese oxides
without any external field, and there are only a few research
papers on the hydrothermal synthesis under the high magnetic
field. One of these is by Zhong and the co-author [22], which
focused only on the electrochemistry property of the prepared
MnO2 powder. The microwave electromagnetic characteristics
of the prepared MnO2 under the high magnetic field were not
dealt with.
exist in nature, such as the a-, b-, g-, d-, and other types [9], are
Many researchers have synthesized other materials in high
magnetic field and obtained good morphology. Gong et al. [23] were
able to fabricate Ni fibers of different lengths by applying external
magnetic fields. Polyaniline with an open-ramified rod-like pattern
was obtained under the high magnetic field by Duan [5]. External
magnetic fields can therefore be viewed as important components of
synthetic processes for generation of unusually shaped materials.
In the present paper, manganese oxide with the sea urchin-like
ball chain shape was first synthesized in a high magnetic field,
and the microwave electromagnetic characteristics of the
generally believed to be responsible for their unique properties.
For example, Bach et al. have pointed out that the electrochemical
properties of MnO2 are strongly dependent on parameters such as
powder morphology, crystalline structure, or bulk density [10].
Ã
Corresponding author. Fax: +86 411 8470 9284.
E-mail addresses: duanyp@dlut.edu.cn (D. Yuping),
lsq6668@126.com (L. Shuqing), lixiaogang99@263.net (L. Xiaogang).
1
Fax: +86 10 85701102.
2
Fax: +86 10 62327878.
0022-4596/$ - see front matter & 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.04.027

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1491
prepared MnO2 were also studied firstly. Moreover, a mechanism
for formation of the observed grain shape by a high magnetic field
was discussed.
began to form sea urchin-like ball chain structures (Fig. 1 and 2 (b)),
but this was not very pronounced. With magnetic field strengths
of 3 to 8 T, the sea urchin-like ball chain pattern became more
evident (Figs. 1 and 2 (c–e)). From Fig. 1, the effect of high
magnetic field on the microspheres size was only slightly, but it
is obvious from Fig. 2 that the particle size of the sample 3 T is
smaller and more uniform than others. The inserted diffraction
pattern images in Fig. 2 show that the crystallization degree of the
MnO2 in a high magnetic field was improved effectively compared
with that without high magnetic field. And the HRTEM images
provide more detailed structural information about the stick on
the surface of the micro-sphere, showing the apparent lattice
fringes of the crystal. But the influence of the high magnetic field
on lattice fringes of the crystal is not very evident.
2. Experimental
All chemical reagents were commercially available and were
used as-received without further purification. Manganese sulfate
(MnSO4 ꢀ H2O) and ammonium persulfate ((NH4)2S2O8) were
purchased from the Dalian Shenlian Chemical Reagent, Co.
(China). The procedures of a typical synthesis are described
below.
2.1. Synthesis
One possible mechanism for the formation of the sea urchin-
like ball chain shape is the following. The original sea urchin-like
microspheres were arrayed randomly in the absence of a high
magnetic field. Nevertheless, the microspheres arrayed them-
selves along a given direction under the external magnetic field
(Fig. 1 (c–e)). This is because, for any kind of crystals, differences
in the arrangement and the density of the atomic structure will
result in differences in the magnetic susceptibilities of the same
crystal at different crystallographic orientations. Small size
microspheres arrayed along the direction of their maximum
magnetic susceptibility (which is also along the magnetic field)
were equivalent to small magnetic bodies. Consequently, they
assembled to form the sea urchin-like ball chain shaped.
A solution of MnSO4 was prepared by dissolving 5.1 g in
100 mL distilled water. The obtained solution was stirred until the
solution was transparent, then 17.1 g (NH4)2S2O8 was added and
stirring was continued for another 30 min at room temperature.
The transparent solution was then transferred to a glass test tube
and heated at 70 1C for 4 h in a high magnetic field, at selected
magnetic field strengths of 1, 3, 5 or 8 T. Subsequently, the glass
test tube was allowed to cool to room temperature. The black
products were filtered off, washed several times with distilled
water and absolute ethanol, and then dried under vacuum at 60 1C
for 24 h. The product,
characterization.
a black powder, was collected for
3.2. Phase structure and compositional analysis
2.2. Characterization
According to Fig. 3, the
a-MnO2 and g-MnO2 are the major
Phase identifications were performed by the X-ray powder
diffraction (SHIMADZU, XRD-6000, 40 kV/30 mA) with CuK
radiation ( range of 101 to 1001 in steps
¼0.15406 nm) in the 2
MnO2 products, and all of the diffraction peaks can be indexed to
tetragonal and orthorhombic structures (PDF# 721982, PDF#
822169, PDF# 721983 and PDF# 731539). The broadened
diffraction peaks indicate that the crystalline size of the samples
is small and that the crystallinity is not yet ideal. Further more,
it is obvious that the crystalline nature and percentage of
-phase MnO2 in each sample are different from the X-ray results,
which show that the high magnetic field has a significant effect on
the crystalline nature and the phase structure.
a
l
y
of 0.041, using the detector technique for measuring intensities.
Microstructural features of the MnO2 were observed by electron
microscopy (SEM, JSM-5600LV). Transmission electron micro-
scope (TEM) images, high resolution TEM (HRTEM) images, and
selected area electron diffraction (SAED) patterns were generated
using a TECNAI G220 S-Twin TEM.
a- and
g
The measurements of complex relative permittivity and
permeability versus frequency were carried out by reflection/
transmission technology using an Agilent 8722ES network
analyzer. The as-obtained MnO2 was dispersed in molten paraffin
wax, and the uniform mixtures were molded into toroid-shaped
samples of 7.00 mm outer diameter and 3.00 inner diameter. The
specimens, consisting of 30 wt% MnO2 powders, were measured at
2–18 GHz. The microwave absorption properties of the MnO2/
paraffin wax samples were calculated according to the transmit
line theory.
3.3. Electromagnetic properties
The complex permittivity, permeability, and their loss tangent
represent the dielectric and magnetic properties of an absorbing
material. The real parts (e0 and m0) of complex permittivity and
permeability symbolize the storage capability of electric and
magnetic energy. The imaginary parts (e00 and m00) represent the
loss of electric and magnetic energy [24]. The loss tangent (tgd)
represents the loss properties of incidence electromagnetic wave
in the microwave absorber. In terms of microwave absorption, the
imaginary parts and the loss tangent are expected to be larger.
3. Results and discussion
Fig.
4 shows the relatively complex permittivity and
3.1. Morphology analysis
permeability of the five samples in the frequency range
2–18 GHz. As shown in Fig. 4 (a), the e0 values for the five
samples declined from 9.29, 9.39, 7.77, 4.64 and 5.06 to 5.26, 5.29,
5.03, 3.38 and 3.72, respectively, with increasing frequency in the
2–18 GHz range. All of the e00 values for the five samples also
exhibited a decrease from 3.43, 3.72, 2.54, 0.93 and 1.04 to 2.20,
2.27, 1.64, 0.75 and 0.83, respectively, as frequency was increased
from 2 to 18 GHz. Increases in magnetic field strength from 0 to
8 T resulted in decreases in e0 values of the samples, except for 8 T
and e00 values, decreased. Fig. 4 (b) shows that the m0 and m00 values
of samples, except for 3 and 5 T, were almost constant, with less
variation throughout the whole frequency range. All m0 E 1.04 and
A series of experiments were carried out by varying the
magnetic field strength, and the morphologies of the resulting
MnO2 products obtained were examined by SEM and TEM. Figs. 1
and 2 show the SEM and TEM images with inserted diffraction
patterns and the HRTEM images prepared in the present study.
The five samples corresponding to Fig. 1 and Fig. 2 (a–e) were
obtained at 70 1C for 4 h with the magnetic field strengths of 0, 1,
3, 5, and 8 T, respectively. As can be seen in Fig. 1 and 2 (a), sea
urchin-like microspheres can be observed in the precipitated
sample. At the magnetic field strength of 1 T, the microspheres

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Fig. 1. SEM images of the MnO2 samples synthesized at 70 1C for 4 h. (a) Without a high magnetic field, (b) with a magnetic field strength of 1 T, (c) with a magnetic field
strength of 3 T, (d) with a magnetic field strength of 5 T, and (e) with a magnetic field strength of 8 T.
m00 E0.017. When the external magnetic field strength was 3 T, the
values of m0 and m00 appeared as a large peak at 5.76 GHz, which is
expected when magnetic loss has been improved.
tangent value reached an expected improvement in the whole
frequency and a maximum peak at 5.76 GHz, which corresponded
to the data depicted in Fig. 4 (b). However, with further sequential
increases in magnetic field strength, the magnetic loss tangent
values gradually decreased. The magnetic loss tangent values of
the five samples were 0.001–0.025, 0.002–0.035, 0.029–0.420,
0.020–0.126, and 0.001–0.047. Therefore, the electromagnetic loss
property of the 3 T sample was more favorable among the five
samples. The possible reasons are as follows:
Fig. 5 shows the frequency dependence of the electric loss
tangent and magnetic loss tangent of the five samples. In Fig. 5 (a),
the electric loss tangent values of the five samples are fluctuant
with increasing frequency in the 2–18 GHz range. Magnetic field
strength increased, the electric loss tangent values for the five
samples decreased, except for 1 T. The electric loss tangent
values of the five samples were 0.37–0.42, 0.39–0.43, 0.29–0.33,
0.20–0.24, and 0.20–0.23. As shown in Fig. 5 (b), the magnetic loss
tangent values of 0 and 1 T were almost constant, with less
variation throughout the whole frequency range. However, the
magnetic loss tangent values of 3, 5, and 8 T had clearly increased
compared with those of 0 and 1 T. As 3 T, the magnetic loss
The attenuation of electromagnetic wave in the x department
I(x) is given by [25]
I
ðxÞ
¼ I expðꢁn
sexxÞ
ð1Þ
0
where I(0) is the incident energy of electromagnetic waves, n the
number of absorbent particles per unit volume, and sex the

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Fig. 2. TEM images with inserted diffraction patterns and the HRTEM images of the MnO2 samples synthesized at 70 1C for 4 h. (a) Without a high magnetic field, (b) with a
magnetic field strength of 1 T, (c) with a magnetic field strength of 3 T, (d) with a magnetic field strength of 5 T, and (e) with a magnetic field strength of 8 T. The HRTEM
images were obtained from the stick on the surface of the MnO2 micro-sphere, which were marked with the rectangular regions.
extinction cross section of the particles. According to Eq. (1), for a
larger n, a larger extinction cross section sex is obtained, so there
is a greater electromagnetic wave attenuation of the particles.
Due to the smaller particle size of the 3 T sample, compared to the
substantially increased, while the real part of the magnetic
permeability, except for 3 T, was almost constant, with less
variation, compared to MnO2 prepared in the absence of an
external magnetic field.
other samples, the extinction cross section sex is larger, which will
aid in the improvement of the magnetic loss. Furthermore,
3.4. Electromagnetic wave absorption properties
according to the phase structure and composition in Fig. 3, the
crystalline nature and percentage of a- and g-phase MnO2 in each
To further confirm the dependence of the microwave absorp-
tion properties on the permittivity and the permeability, the
reflection losses [R(dB)] were calculated according to the trans-
mission line theory [27]. The normalized input impedance Zin of a
microwave absorber is given by
sample will play an important role for their electromagnetic
properties [26].
Based on this analysis, an external high magnetic field can be
seen to have substantial effects on the electromagnetic properties
of a prepared MnO2 powder. The dielectric constant and the loss
tangent clearly decreased under the magnetic field. The imaginary
part of the magnetic permeability and magnetic loss tangent
rffiffiffiffiffi
ꢀ
ꢁ
m
er
2
p
pffiffiffiffiffi
Zin
¼
rtanh j
mrerfd
ð2Þ
c

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J. Zhang et al. / Journal of Solid State Chemistry 183 (2010) 1490–1495
where mr and er are the relative permeability and permittivity of
the composite medium, c the velocity of electromagnetic waves in
free space, f the frequency of the microwave, and d the thickness
of the absorber. The reflection loss is related to Zin as
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
Z ꢁZ
0
ZiinnþZ0
Reflection loss ðdBÞ ¼ 20log
ð3Þ
ꢂ
ꢂ
ꢂ
ꢂ
pffiffiffiffiffiffiffiffiffiffiffiffi
where Z0 ¼ m0
=
e0 is the characteristic impedance of free space.
Fig. 6 shows the theoretically calculated values of reflection
loss versus frequency according to Eqs. (2) and (3). Clearly,
when the magnetic field strength increases, except for 1 T, the
absorption peaks become smoother and shift to
a higher
frequency. Over the whole range from to 18 GHz, the
2
reflection loss values of 0, 1 and 8 T only had one absorption
peak. However, the reflection loss values for 3 and 5 T have two
absorption peaks, which attributes mainly to the resonance peak
frequency of the corresponding complex relative permeability
(see Fig. 4) and wave interference on the fore and after surface
of the sample. In the frequency range 2–18 GHz, the 0 and 1 T
Fig. 3. XRD patterns of the as-synthesized samples with 0, 1, 3, 5, and 8 T.
Fig. 4. The relative complex permittivity and permeability of the five samples.
Fig. 5. The electric loss tangent and magnetic loss tangent of the five samples.

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1495
the Weapon Equipment of China (No: 2009317), The National
R&D Infrastructure and Facility Development Program of China
(2005DKA10400) and the Growth Foundation for the Youth
Teachers of the Dalian University of Technology.
Appendix A. Supplementary data
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.jssc.2010.04.027
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a
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The authors acknowledge the support from The Research
Fund for the Doctoral Program of Higher Education of China
(No.20090041120038), The Aero Science Foundation of China
(20095463008), The Research Fund for the Advanced Research of