Effect of NaNO3 foaming agent on barium ferrite hollow microspheres prepared by self-reactive quenching technology

Article abstract of DOI:10.1016/j.jallcom.2015.02.015

Al + Fe₂O₃ + BaO₂ and NaNO₃ as the reactive system and the foaming agent, respectively, are used to prepare barium ferrite hollow microspheres (BFHMs) by self-reactive quenching technology based on flame sprayin

Full text of DOI:10.1016/j.jallcom.2015.02.015

Journal of Alloys and Compounds 636 (2015) 348–356
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Effect of NaNO3 foaming agent on barium ferrite hollow microspheres
prepared by self-reactive quenching technology
a
b
c
a
a
a
Xudong Cai a, , Jianjiang Wang , Guanhui Liang , Jinshu Guo , Yongshen Hou , Haitao Gao , Liang Yu
⇑
a Mechanical Engineering College, Advanced Material Institute, Shijiazhuang 050003, PR China
b Artillery Investigation Radar Office, Mechanical Engineering College, Heping West Road, 97, Shijiazhuang 050003, PR China
c Shijiazhuang Environment Monitoring Center, Beixin Street, 169, Shijiazhuang 050022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Al + Fe2O3 + BaO2 and NaNO3 as the reactive system and the foaming agent, respectively, are used to pre-
pare barium ferrite hollow microspheres (BFHMs) by self-reactive quenching technology based on flame
spraying technology, self-propagating high-temperature synthesis (SHS) technology and quick chilling
technology. Effects of NaNO3 on the particle size, morphology, phase structure and microwave absorption
properties of BFHMs are investigated through SEM, XRD, particle size analyzer, high-speed camera and
vector network analyzer. The results show that, after adding 10 wt.% NaNO3, the average particle size
Received 8 November 2014
Received in revised form 29 January 2015
Accepted 2 February 2015
Available online 3 March 2015
Keywords:
NaNO3
Self-reactive quenching technology
Hollow microspheres
Microwave absorption properties
of BFHMs decreases initially from 28
Micro-nano lamellar crystals appear on the surface of BFHMs, with the dimension ranging from
500 nm to 2 m. BaFe2O4 and Fe3O4, which have spinel structures, can be seen in XRD, and they are
lm to 6 lm, as well as the particle distribution gets narrower.
l
beneficial for microwave absorption properties. The real part (e0), imaginary part (e00) of permittivity,
and the imaginary part of permeability (l00) increases in 0.5–18 GHz. The real part (l0) of permeability
increases in 0.5–10.3 GHz, while decreases in 10.3–18 GHz. The microwave absorption properties are
improved greatly, and the minimum reflectivity decreases from ꢀ3.1 dB to ꢀ9.8 dB. The reasons for
improvement of microwave absorption properties after adding NaNO3 foaming agent may be the
decreasing of the particle size of BFHMs, the appearance of ferrites (BaFe2O4 and Fe3O4) with spinel struc-
ture and special micro-nano tabular crystals. Magnetoplumbite-type barium ferrites (BaFe12O19) hollow
microspheres are obtained after heat-treatment and the microwave absorption properties are further
improved.
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
rare-earth ions (LRE) such as La, Pr, and other metal cations in sub-
stitution for Ba and Fe, respectively, taking into account the ionic
Magnetoplumbite-type barium ferrites (BaFe12O19) are widely
used because they have multiple EM (electromagnetic) wave lossy
mechanisms such as magnetic hysteresis loss, natural resonance,
residual loss, and eddy current loss [1]. Additional attention is
drawn due to their high stability, high-frequency response and
narrow distribution of transmission field properties. BaFe12O19
materials have been prepared by various methods, including the
chemical coprecipitation method [2,3], the glass crystallization
[4], the sol–gel method [5,6] and the ceramic process [7]. In
addition, many studies have also been concerned with cationic
substitutions to improve the fundamental magnetic properties of
BaFe12O19 [8–9]. For example, some experiments have used light
radius of the elements, and the magnetic properties are investigat-
ed [10].
These methods mentioned above have their own advantages.
However, BaFe12O19 materials prepared by them are always with
great density and poor high-temperature properties, which makes
it difficult to meet the requirement of light absorbents [11]. If
BaFe12O19 materials are fabricated into hollow structures, the
weight can be significantly reduced without any reduction of
microwave absorption properties. Hollow microspheres have spe-
cial properties in physics and chemistry, namely, micro-particles,
hollow structure and light weight [12,13]. Self-reactive quenching
technology is a favorable method to prepare barium ferrite hollow
microspheres (BFHMs), which combines flame spraying
technology, self-propagating high-temperature synthesis (SHS)
technology and quick chilling technology [14–16]. With a low
input–output ratio, self-reactive quenching technology can also
⇑
Corresponding author at: Heping West Road 97, Shijiazhuang 050003, PR China.
Tel.: +86 15512109353; fax: +86 0311 87994008.
E-mail address: caixudong87@qq.com (X. Cai).
http://dx.doi.org/10.1016/j.jallcom.2015.02.015
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

X. Cai et al. / Journal of Alloys and Compounds 636 (2015) 348–356
349
Then the ceramic droplets are quenched quickly into the cooling medium (the dis-
tilled water). As the gas cannot escape from the ceramic droplets, BFHMs are
obtained immediately after the quenching products are dried and filtered. The reac-
tive equation is showed in Eq. (1). In the experiment, the flame field is used to ignite
the SHS reaction. And once the reactions occur, they can be self-sustaining based on
the heat released by themselves and the flame field is not required.
control the particle size, morphology, and phase structure of
BFHMs. Meanwhile, the hollow structure effectively reduces the
weight of BFHMs and forms special microwave absorption
mechanisms, which has the development potential to meet the
need of new-style absorbents ‘‘thin, light, wide and strong’’ [16].
Controlling the particle size of BFHMs by adjusting the tech-
nology parameters based on self-reactive quenching technology
has being considered as a potential research since this technology
was invented by our team. Considering previous investigation [17],
it is indicated that polyethylene glycol (PEG) foaming agent can
effectively improve the fluidity of agglomerate powders and
increase the particle size of BFHMs. In addition, the microwave
absorption properties are innovated. However, it has not been
referred that how to decrease the particle size and obtain BFHMs
with micron, micro-nano even nanometer particle size. Therefore,
this study is established. Due to its high foaming quantity and
excellent visbreaking properties, NaNO3 has the potential to
decreasing the particles of BFHMs.
In this study, micron-sized BFHMs absorbent is prepared using
Al + Fe2O3 + BaO2 + sucrose and NaNO3 as the reactive system and
the foaming agent respectively based on self-reactive quenching
technology. Effects of NaNO3 foaming agent on particle size, mor-
phology, phase structure and microwave absorption properties of
BFHMs are investigated, and the mechanisms are studied. This
thesis is carried out to master a controllable preparation method
of BFHMs absorbent and lay the foundation on the future
investigations.
In addition, the vibration of high-energy flame spraying gun parameters plays
an important role in the products. According to the previous study, it is indicated
that the atmosphere (such as O2, N2, and Ar) has great effect on the BFHMs, and
it is found that oxygen provides an oxygen-enriched atmosphere, which benefits
for the formation of hollow structures. In addition, the rate of O2 and acetylene
ꢀ
ꢁ
b ¼ VO =VC
affects the temperature and oxidizability of flame, and in this
experiment, the rate is set as 1.4. The quenching distance was set to 500 mm, which
guarantee the high balling rates of BFHMs.
2 H2
2
Moreover, different amounts of sucrose and epoxy resin will generate different
amount of gases during the reaction, which will affect both the morphology and the
properties of BFHMs. In this experiment, their amounts are 50 g and 50 ml,
respectively.
2.2. Characterization of the samples
The morphology of the quenching products was detected by scanning electron
microscope (SEM, QUANTA FEG-250). The phase composition of the quenching
products was studied by X-ray diffraction (XRD, BRUKER D2 PHASER). Particle size
distribution was measured by using Laser Particle Size Analyzer (Beckman Coulter
LS 13 320, testing from 0.04 lm to 2000 lm). The density of BFHMs was deter-
mined by Archimedes Method. High-speed camera (Fastcam-Ultima512) was
selected to observe the flight combustion behavior of agglomerate powders in the
flame field. The resolution factor of sensor was 512 ꢁ 512 picture dots, and the
mutograph speed was 500 fps. Xiandai F112 High-temperature Resistance
Furnace was used to heat-treat BFHMs.
2.3. Measurement of microwave-absorbing behavior for BFHMs-paraffin composite
The absorbing composite were prepared by molding and curing the mixture of
BFHMs and paraffin. Paraffin was used as a polymer matrix due to its good flexibil-
ity and wave-transparent property. And it has little effect on investigating the
microwave absorption properties of BFHMs–paraffin composite. The mix ratio of
BFHMs-to-paraffin was 3:2 by weight. The testing specimens have a toroidal shape
with the thickness at 3 mm, and the outer and inner diameters are respectively
2. Experiments
2.1. Materials and methods
Analytical reagent raw materials of Al powders, BaO2 powders, Fe2O3 powders,
sucrose (precursor of C, 50 g), epoxy resin (bonding agent, 50 ml) and NaNO3 foam-
ing agent were selected. Related information is showed in Table 1. Considering pre-
vious investigations, it is indicated that 10 wt.% (the mass percentage of reactive
system including all the raw materials) is the best quantity of NaNO3 foaming
agent. Therefore, 10 wt.% NaNO3 was selected. Eq. (1) shows the main reactive sys-
tem. According to the stoichiometry ratio, two experiments were conducted: one
was without NaNO3, and the other was with 10 wt.% NaNO3. Agglomerate powders
were prepared as the following processes.
Firstly, the raw materials were put into LJM-5L ball mill, and anhydrous ethyl
alcohol was taken as the medium sphere to be grinded for 6 h. After that, epoxy
resin–alcoholic solution was added into the mill to be stirred for another 2 h.
Epoxy resin was used to cooperate with sucrose to enlarge the contact area of the
components in the agglomerate powders and to increase the bonding strength of
reactive components. It can be transformed to CO, CO2 and H2O in SHS reaction
due to the high temperature. Secondly, the mixtures were dried and carbonized
at 200 °C in 101-2A horizontal drying cabinet until no smoke released, and then
they were comminuted by the FW177 disintegrator. Thirdly, two kinds of agglom-
erate powders before and after adding NaNO3 were selected for the experiment
after the sieving process. Finally, BFHMs after adding NaNO3 are heat-treated in
Xiandai high-temperature resistance furnace at 1100 °C for 4 h with the heating
rate of 3 °C/min.
7.0 mm and 3.0 mm. The e0 e00 l0 and l00 versus frequency were measured by
, ,
coaxial reflection/transmission method using Vector Network Analyzer (Agilent-
N5242A) in 0.5–18 GHz range.
The absorbing characteristics can be represented as the reflection loss (R.L.), as
shown in Eqs. (2) and (3).
Where Zin is the normalized input impedance related to the impedance in free
space, er
=
e0 ꢀ je00
,
lr
=
l0 ꢀ jl00 is the complex relative permeability and permit-
tivity of the material, d is the thickness of the absorber, and C and f are the velocity
of light and the frequency of microwave in free space, respectively.
To represent the perfect absorbing properties, the impedance matching condi-
tion is given by Zin = 1. The impedance matching condition is determined by the
, , , lr, the
combination of six parameters e0 e00 l0 l00, f and d. Also, knowing er and
R.L. value versus frequency can be evaluated by using metlab 8.0 at a specified
thickness.
3. Results and discussion
3.1. Effect of NaNO3 on the morphology, particle size and phase
structure of BFHMs
Fig. 1 shows the preparation diagram of BFHMs. As shown in the figure, the
agglomerate powders are sprayed into the flame field (about 3500 K) through a
CP-D type high-energy flame-spraying gun, and the temperature of the materials
increases gradually. When reaching the ignition temperature, the SHS reactions
occur promptly, and the temperature of reactive system exceeds the melt point
of products. So ceramic droplets are generated. Simultaneously, large volume of
gases are produced, which results in the hollow structure of ceramic droplets.
Fig. 2(A) illustrates the SEM images of the quenching products
under the Al + Fe2O3 + BaO2 + sucrose + epoxy resin reactive system
without NaNO3. The adiabatic combustion temperature (Tad) is
2526 K, and the heat of reaction (
shows the SEM images of Al + Fe2O3 + BaO2 + sucrose + epoxy
resin + NaNO3 reactive system (Tad = 2620 K,
D
H°f) is ꢀ820.6 kJ/mol. Fig. 3(B)
D
H°f = ꢀ836.5
kJ/mol). Because Tad = 1800 K, two SHS reactions could occur.
Images A1–A3 are magnified 200, 5000 and 10,000 times respec-
tively, while Images B1–B3 are enlarged 1000, 5000, and 20,000
times. According to the SEM images, the particle size is getting
smaller and the particle distribution is more uniform after adding
NaNO3. In addition, the morphology of BFHMs changes drastically.
As shown in Image A3 and B3, the surfaces are relatively smooth
without NaNO3, on which there are a small number of fine spherical
and irregular particles. After adding NaNO3, the spheres are
Table 1
Raw materials and their chemical composition used in the experiments.
Material
Particle size (
l
m)
Chemical Purity (wt.%)
Al
BaO2
Fe2O3
Sucrose
NaNO3
Epoxy resin (6101)
640
635
640
65
635
Null
Al P 99
BaO2 P 97%
Fe2O3 P 99.5
C12H22O11 P 99.9 H2O < 0.1
NaNO3 P 99.5%
Epoxy resin P 99

350
X. Cai et al. / Journal of Alloys and Compounds 636 (2015) 348–356
Fig. 1. Principle diagram for preparation of BFHMs absorbent by self-reactive quenching technology.
Fig. 2. SEM photographs of the BFHMs without (A1–A3) and with NaNO3 (B1–B3).
composed of micro-nano lamellar crystals, ranging from 500 nm
to 2 m.
sufficient aluminothermic reactions result in the decreasing quan-
tity of Al element. The exact reasons will be clarified in the follow-
ing passage.
l
The energy spectrums of hollow microspheres before and after
adding NaNO3 are analyzed in Fig. 3. Fig. 3(a) and (b) shows the
EDS patterns of Spot 1 (marked in Fig. 2A2) and Spot 2 (marked
in Fig. 2B3), respectively. The results show that the quenching
products both consist of Ba, Fe, Al, and O elements, whether
NaNO3 is added or not. However, C element disappears and the
content of Al element declines after adding NaNO3. This may be
ascribed to the follows reasons. Properties of the agglomerate pow-
ders are improved so drastically that each reaction unit is mixed
more uniformly and reacts more sufficiently. As a result, all sucrose
are burned and released in forms of gas. In addition, the further
The particle size distributions of BFHMs sample is shown in
Fig. 4. The particle size has an average value of 28
between 5 and 65 m without NaNO3. After adding NaNO3, the
average particle size decreases to 6 m with 68% between 1 and
10 m. It is indicated that the particle size is getting smaller and
lm with 92%
l
l
l
the distribution is getting narrower after adding NaNO3.
XRD patterns of BFHMs before and after adding NaNO3 foaming
agent are showed in Fig. 5. From XRD, it can be concluded that
changes of phase components have taken place. Without NaNO3,
BFHMs are constituted of BaFeO3ꢀx, Fe2O3 and Al2O3. While the

X. Cai et al. / Journal of Alloys and Compounds 636 (2015) 348–356
351
Fig. 3. EDS patterns of the microspheres marked in Fig. 2.
Fig. 4. Particle size distribution of BFHMs samples without (a) and with NaNO3 (b).
phase components are BaFe2O4, Fe3O4, Fe2O3 and Al2O3 after add-
ing NaNO3. The results show that, BaFe2O4 is obtained and some
of Fe2O3 transforms to Fe3O4. Integrating the results of Fig. 3(b),
it is concluded that micro-nano lamellar crystals may consist of
Fe3O4, Fe2O3, BaFe2O4 and Al2O3. Among these phases, BaFe2O4 is
an important precursor to form BaFe12O19. Moreover, BaFe2O4
and Fe3O4 have spinel structures, which greatly influence the
microwave absorption properties. As is known in Fig. 1, self-reac-
tive quenching technology has great subcooled temperature when
the droplets are quenched into the distilled water. Droplets are
solidified so fast that sufficient crystallization cannot occur. As a
result, BaFe12O19 have not been present, which needs a heat-treat-
ment experiment to obtain.
capillary osmosis. During SHS reaction progress, the temperature
of agglomerate powders increases drastically and reaches to the
SHS ignition temperature immediately, then Al + Fe2O3
(Tad = 3000 K) reaction happens promptly and releases much heat,
as is shown in Eq. (4). The releasing heat guarantees the continuing
reaction of Fe + BaO2 + Fe2O3 (Tad = 2170 K), as is shown in Eq. (5).
In Eq. (5), Fe is produced by the reaction of Al + Fe2O3. The heat
released by Eqs. (4) and (5), together with the high temperature
of oxygen–acetylene flame, guarantee the products keeping in a
melting status. Meanwhile, the combustion of epoxy resin and
sucrose releases much CO2 and the gas stores inside the droplets.
So hollow droplets are generated. During post-combustion pro-
gress, convection and radiation of the flame field make the surface
temperature of droplets reach to about 1000 K. With the flying dis-
tance increasing, the temperature decreases gradually. However,
there are still many complexed chemical reactions inside the dro-
plets. When sprayed into the cooling medium, droplets are solidi-
fied rapidly in a great subcooled temperature, and gases keep
inside the droplets instead of escaping. Therefore, microspheres
with hollow structure are obtained.
3.2. Effect mechanism analysis of NaNO3 foaming agent on the
preparation of BFHMs
When flying into oxyacetylene flame field provided by the
flame-spraying gun, agglomerate powders produce series of physi-
cal and chemical reactions and undergo three progresses including
energy storage progress, SHS reaction progress, and post-combus-
tion progress [12]. During energy storage progress, agglomerate
powders are preheated gradually from surface to inside. Particles
with low melting point including Al (933 K) and BaO2 (723 K) are
first melted and spread on the surface of Fe2O3 (1838 K) by
Effects of NaNO3 foaming agent on the particle size of BFHMs
could be closely ascribed to its special foaming mechanism.
Characteristics of high foaming quantity and stable foaming pro-
cesses, NaNO3 decomposes gradually and releases a large amount
of gas under high temperature. It decomposes into NaNO2 and O2

352
X. Cai et al. / Journal of Alloys and Compounds 636 (2015) 348–356
Eqs. 6–9 [16]. As NaNO3 and Na2O have no microwave absorption
properties themselves, NaNO3 is only used as the foaming agent.
Moreover, though Na2O exists in the sample, it is not marked in
XRD patterns.
Without adding NaNO3, gases mainly release from the combus-
tion of epoxy resin and sucrose. The quantity of gases is limited.
The melting phases of the droplets including Al2O3 have the capa-
city to restrain the gas from escaping the droplets [18]. By adding
NaNO3, the restraining capacity and the viscosity of the droplets
are significantly reduced. In addition, massive gases are generated
due to decomposition of NaNO3. And the droplets are filled with
thousands and hundreds of bubbles immediately, then grow
gradually when their internal air pressure increases continuously.
Fig. 5. XRD patterns of the BFHMs before and after adding NaNO3.
when it is 653 K, and releases N2 and O2 between 673 and 873 K.
NO sets free in 973 K and NO2 as well as N2O release between
1048 and 1138 K. The decomposition processes are showed from
Fig. 7. TEM photograph of BFHMs after adding NaNO3.
Fig. 6. High-speed photograph of the droplets explosion after adding NaNO3.

X. Cai et al. / Journal of Alloys and Compounds 636 (2015) 348–356
353
When the resultant force of all bubbles exceeds the surface tension
of droplets, the droplets break into several drips with small particle
size. The reaction progresses can be proved by high-speed photog-
raphy experiment [19]. Fig. 6 illustrates the flying progress of
Droplet A in the flame field (0.002 s/frame). According to the figure,
it can be concluded that significant decrepitation phenomenon of
the droplet appears when t = 0.006 s. And the broken droplets
transform to BFHMs with small particle size after being quenched
into the coolant.
Fig. 7 shows the TEM image of microspheres after adding
NaNO3, which demonstrates that hollow structures exist. By using
the Archimedes Method it is found that the densities of micro-
spheres are 1.79 g/cm3 and 2.05 g/cm3 before and after adding
NaNO3, respectively. While the destiny of hexagonal BaFe12O19
solid powder is 5.364 g/cm3, which further indicates that the
microspheres are of hollow structures prepared by self-reactive
quenching technology before and after adding NaNO3. According
to the data mentioned above, it can be concluded that although
large amount of gas are released in the decrepitation progress, part
of gas are still stored in the small droplets. As a result, micro-
spheres with hollow structure are obtained after adding NaNO3
foaming agent.
Al and BaO2 rapidly enter the agglomerate powders by capillary
osmosis and spread on the surface of Fe2O3. This mechanism suffi-
ciently accelerates SHS reactions. As the melting point of agglom-
erate powder is reduced, the preheated time decreases obviously.
As a result, the reaction units of agglomerate powder have suffi-
cient time to react in the flame field. The droplets are formed ear-
lier and transform to BFHMs with a uniform distribution.
Adding NaNO3 makes much influence on the morphology of
BFHMs, which may be attributed to its effect on the particle size
of droplets. After the droplets break, the particle size decreases
obviously, and the radiating becomes faster in coagulation. So
the crystallization is relatively fast. In contrast, droplets are of
big particle size and slow radiating without adding NaNO3, so its
coagulation and crystallization are slow. The growth of crystals is
restrained. Therefore, the surface of BFHMs is smooth without
NaNO3, while special micro-nano lamellar crystals appear on the
surface after adding NaNO3. In addition, some irregularity particles
are present after adding NaNO3. It is possibly because the broken
droplets easily fly to the edge of the flame field where the tem-
perature is lower, and the droplets solidify in the cooling medium
without sufficient reaction.
By adding NaNO3, some Fe2O3 transforms to Fe3O4 phases, so
some Fe2+ turns to Fe3+. In addition, BaFe2O4 with spinel structure
is generated after adding NaNO3. As demonstrated, BaFe2O4 is an
important precursor to form BaFe12O19, which might influence
the microwave absorption properties. The exact mechanism will
be shown in the following section.
The particle size distribution of BFHMs gets narrower after add-
ing NaNO3. It is mainly ascribed to the following two causes. On
one hand, droplets break into small drips with uniform particle
size. On the other hand, NaNO3 (580 K) reduces the melting point
of agglomerate powder [18]. The liquid phases including NaNO3,
Fig. 8. Permittivity and permeability of BFHMs before and after adding NaNO3.

354
X. Cai et al. / Journal of Alloys and Compounds 636 (2015) 348–356
Fig. 9. Reflection–frequency curves of the BFHMs before and after adding NaNO3.
permeability for BFHMs is shown in Fig. 8. It is indicated that, by
adding NaNO3, the real part of permittivity (e0) increases and main-
tains at about 6.5. It is possible that surfaces of micro-nano tabular
crystals on BFHMs have different polarity and conductivity. In the
action of electrical field, charges accumulate rapidly in grain
boundaries and space-charge polarization is generated. In addition,
BaFe2O4 and Fe3O4 which have spinel structure are formed after
adding NaNO3. Compared with intermediate phases (such as
BaFeO3ꢀx), BaFe2O4 and Fe3O4 are of cubic crystal with a high sym-
metry, and they have high real part of permittivity [21]. The ima-
ginary part of permittivity (e00) stays relatively stable in 0.5–
8 GHz. However, e00 has a noticeable enhancement in 8–18 GHz.
The e00 value refers to the dielectric loss ability of electromagnetic
wave. From that it can be concluded that dielectric loss increases
apparently in 8–18 GHz by adding NaNO3.
From Fig. 8(c) and (d), it can be indicated that the real part of
permeability (l0) increases apparently in 0.5–10 GHz and the ini-
tial permeability is high by adding NaNO3. The amplification of
permeability (l0) decreases gradually with the increasing of the
frequency. However, the l0 value is less than that without NaNO3
in 10–18 GHz. The imaginary part of permeability (l00) has an obvi-
ous enhancement in 0.5–18 GHz, and the span is between 0.08 and
0.35.
Fig. 10. XRD patterns of the BFHMs after heat-treatment.
Fig. 9 shows the calculated reflectivity curve of BFHMs before
and after adding NaNO3. From the picture, it is concluded that
the minimum reflectivity is ꢀ3 dB at 10 GHz without NaNO3.
After adding NaNO3, the minimum reflectivity is ꢀ9.8 dB at
9.5 GHz, and there is an obvious absorption of electromagnetic
waves in 6–14 GHz. Consequently, the microwave absorption
properties are greatly improved after adding NaNO3 which mainly
decided by the following aspects:
3.3. Effect of NaNO3 foaming agent on the microwave absorption
properties of BFHMs
Complex permittivity and permeability represent the dielectric
and dynamic magnetic properties of materials [20]. The real parts
(
e0 and l0) of complex permittivity and permeability symbolize the
Firstly, micro-nano tabular crystals are generated on the surface
of BFHMs after adding NaNO3. On one hand, the surface of BFHMs
is smooth before adding NaNO3, and the main contacting method
storage and transform capability of electromagnetic energy. The
imaginary parts (e00 and l00) represent the loss of electromagnetic
energy. The frequency dependence of complex permittivity and
Fig. 11. Reflection–frequency curves of the BFHMs after heat-treatment.

X. Cai et al. / Journal of Alloys and Compounds 636 (2015) 348–356
355
among BFHMs is point-contact. By adding NaNO3, multiple con-
tacting methods including point-contact, line-contact and sur-
face-contact appear. And conducting networks easily generate
among BFHMs. As a result, the action of electron transition in
microscopic structures of BFHMs is enhanced, which increases
the dielectric loss. On the other hand, micro-nano tabular crystals
have large flakiness ratio and high magnetocrystalline anisotropy.
When impressed alternating magnetic field exists, more magnetic
domain structures on the surface of crystalline grains will absorb
energy. Natural resonance effect will appear when the frequency
of impressed alternating magnetic field equals the natural frequen-
cy of magnetic moment precession inside the micro-nano tabular
crystals. In the action of natural resonance, BFHMs will absorb
the energy of alternating magnetic field drastically. In addition,
more grain boundaries form due to the appearance of micro-nano
tabular crystals, which restrains the relative motions of magnetic
domain walls. Consequently, the magnetic loss increases.
Secondly, with the decreasing particle size of BFHMs, the speci-
fic area increases. Accordingly, the proportion of atoms on the sur-
face increases and become more active. In addition, the dangling
bonds and defects increase, which make it easy to form surface
polarization. Moreover, stronger rayleigh scattering are generated
when incidence waves go to the surface of BFHMs. As a result,
dielectric loss of BFHMs increases obviously. Therefore, it is clear
that surface polarization and multiple scattering increase the
microwave absorption properties of BFHMs.
(2) NaNO3 foaming agent releases high quantity of gas and
makes the droplets break into small liquid drops in the flame
field. This is the main reason for decreasing of BFHMs’ parti-
cle size. After adding NaNO3, the melting point of agglomer-
ate powders decreases, and the reactive mechanisms are
improved. As a result, BFHMs with a uniform particle distri-
bution are obtained. The decreasing particle size of droplets
promotes the rate of heat dissipation, which is beneficial for
the coagulation and crystallization. Therefore, micro-nano
tabular crystals with a favorite grain growth appear on the
surface of BFHMs.
(3) The real part (e0) and imaginary part (e00) of complex permit-
tivity increase obviously after adding NaNO3. The real part of
complex permeability (l0) increases in 0.5–10.2 GHz, while
decreases in 10.2–18 GHz. The imaginary part of complex
permeability (l00) increases in 0.5–18 GHz. In addition, the
microwave absorption properties of BFHMs are improved
in the whole frequency band. The minimum reflectivity
decreases from ꢀ3 dB to ꢀ9.8 dB. Appearance of micro-nano
tabular crystals, decreasing particle size of BFHMs and the
formation of BaFe2O4 and Fe3O4 are main reasons for the
improvement of microwave absorption properties. After
BFHMs with NaNO3 being heat-treated at 1100 °C for 4 h,
magnetoplumbite-type ferrite BaFe12O19 is obtained and
the microwave absorption properties are further improved.
The minimum reflectivity is ꢀ14.7 dB at 9.5 GHz, and the
frequency range lower than ꢀ10 dB is 7.8–11.2 GHz, with a
bandwidth of 3.4 GHz.
Thirdly, BaFe2O4 and Fe3O4 are formed. They are of spinel struc-
ture and have a high electrical resistivity. The electron vacancies
inside them transform from an equilibrium position to another in
the electric field, which need to remove the energy barriers and
consequently drop behind the electromagnetic field. As a result,
electromagnetic waves are absorbed by strong polarization
relaxation.
Acknowledgements
Authors would like to express their sincere appreciation to
National Natural Science Fund of China (No. 51172282) and Pre-re-
search of the Weapons Equipment (9140A12040211JB34) for
financial support. Tremendous thanks are owed to Hongjun
Huang, who provides us with great help in the investigation.
Heat-treatment experiment of BFHMs with NaNO3 is conducted
to further improve microwave absorption properties, according to
Literature [22]. The heat-treatment temperature is 1100 °C and the
heating rate is 3 °C/min. After the temperature reaching to 1100 °C,
the samples are held at that temperature for 4 h. Fig. 10 shows XRD
patterns of BFHMs with NaNO3 after heat-treatment. And it is
found that magnetoplumbite-type ferrite BaFe12O19 is obtained,
as well as Fe3O4, Fe2O3, and Al2O3. Fig. 11 shows reflectivity of
BFHMs with NaNO3 after heat-treatment. The minimum reflec-
tivity is ꢀ14.7 dB at 9.5 GHz, and the frequency band lower than
ꢀ10 dB is 7.8–11.2 GHz, with a bandwidth of 3.4 GHz. Compared
with BaFe2O4, BaFe12O19 has a higher effective field of magne-
tocrystalline anisotropy. which results in a higher natural reso-
nance frequency. When alternating magnetic field frequency of
incident wave equals the frequency of effective field of magne-
tocrystalline anisotropy, natural resonance with great amplitude
happens and massive electromagnetic waves are absorbed. This
may be the reason for improvement of microwave absorption
properties after heat-treatment.
Appendix A
Equations
2Al þ BaO2 þ 6Fe2O3 þ C þ 2O2 ! Al2O3 þ BaFe12O19 þ CO2
ð1Þ
ð2Þ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ðZin ꢀ 1Þ
ðZin þ 1Þ
R:L:ðdBÞ ¼ 20log10
"
#
ꢃ
ꢄ
ꢃ
ꢄꢃ
ꢄ
1=2
1=2
lr
er
2
p
fd lr
Zin
¼
tanh
j
ð3Þ
C
er
Fe2O3 þ 2Al ! 2Fe þ Al2O3
ð4Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
2Fe þ 3BaO2 þ 17Fe2O3 ! 3BaFe12O19
2NaNO3 ! 2NaNO2 þ O2 " ð653 KÞ
4. Conclusions
BFHMs containing micro-nano tabular crystals have been pre-
pared by self-reactive quenching technology with NaNO3 foaming
agent. Effect of NaNO3 on the particle size, morphology, phase
structure and microwave absorption properties is investigated.
4NaNO3 ! 2Na2O þ 2N2 " þ5O2 " ð673—873 KÞ
4NaNO3 ! 2Na2O þ 4NO " þ3O2 " ð973 KÞ
(1) The average particle size of BFHMs is 28 lm, and the surface
8NaNO3 ! 4Na2O þ 4NO2 " þ2N2O þ 5O2 " ð1048—1138 KÞ ð9Þ
is relatively smooth without obvious crystals before adding
NaNO3. By adding NaNO3, the average of particle size is
6 lm and micro-nano tabular crystals appear on the surface.
References
BaFe2O4 and Fe3O4 with spinel structure are obtained in the
phase component.
[1] S.H. Liu, J.M. Liu, X.L. Dong, Electromagnetic Wave Shielding and Absorbent,
Chemical Industrial Press, Beijing, 2007.

356
X. Cai et al. / Journal of Alloys and Compounds 636 (2015) 348–356
[2] A. Ataie, S. Heshmati-Manesh, H. Kazempour, Synthesis of barium hexaferrite
[12] D.G. Wang, Y.H. Zhang, A.G. Dong, Conversion of fly ash cenosphere to hollow
microspheres with zeolite/mullite composite shells, Adv. Funct. Mater. 13
(2003) 563–566.
by the co-precipitation method using acetate precursor, J. Mater. Sci. 37 (2002)
2125–2128.
[3] T. Ogasawara, M.S. Oliveira, Microstructure and hysteresis curves of the
barium hexaferrite from co-precipitation by organic agent, J. Magn. Magn.
Mater. 217 (2000) 147–154.
[4] K. Oda, T. Yoshio, K. O-Oka, F. Kanamaru, Morphology and magnetic properties
of BaFe12O19 particles prepared by the glass-ceramic method, J. Mater. Sci. Lett.
4 (1984) 876–879.
[5] H.J. Zhang, Z.C. Liu, C.L. Ma, X. Yao, L.Y. Zhang, M.Z. Wu, Preparation and
microwave properties of Co-and Ti-doped barium ferrite by citrate sol–gel
progress, Mater. Chem. Phys. 80 (2003) 129–134.
[6] F. Khademi, A. Poorbafrani, P. Kameli, H. Salamati, Structural, magnetic and
microwave properties of Eu-doped barium hexaferrite powders, J. Supercond.
Novel Magn. 25 (2012) 525–531.
[7] D. Lisjak, M. Drofenik, Synthesis and characterization of A–Sn-substitute
(A = Zn, Ni, Co) BaM–hexaferrite powders and ceramics, J. Eur. Ceram. Soc. 24
(2004) 1841–1845.
[8] X.S. Liu, W. Zhong, S. Yang, Z. Yu, B. X Gu, Y.W. Du, Structure and magnetic
properties of La3+ substituted strontium hexaferrite particles prepared by sol–
gel method, Phys. Status Solidi A 193 (2002) 314–319.
[13] G. Scrotts, T.G. Park, Preparation of porous and nonporous biodegradable
polymeric hollow microspheres, Controll. Release 35 (1995) 91–94.
[14] X.D. Cai, J.J. Wang, B.C. Xu, H.F. Lou, X.J. Huo, Hollow multi-phase ceramic
microspheres of Al + Cr2O3 + SiO2 system prepared by self-reaction quenching
technology, Key Eng. Mater. 512–515 (2012) 621–625.
[15] X.D. Cai, J.J. Wang, B.C. Xu, H.F. Lou, Y.S. Hou, Effect of polyethylene glycol on
the hollow multiphase ceramic microspheres prepared by self-reaction
quenching technique, J. Synth. Cryst. 41 (2012) 1778–1784.
[16] X.D. Cai, Study on the controllable preparation and microwave
electromagnetism property of hollow multi-phase ceramic micro-spheres,
Master degree thesis of Mechanical Engineering College, 2012.
[17] X.D. Cai, J.J. Wang, R.C. Yang, X. Xiong, H.F. Lou, Influence of the foaming agent
on the particle size and microwave absorption properties of hollow ceramic
microspheres, Int. J. Appl. Ceram. Technol. 8 (2013) 1–9.
[18] H.F. Yang, C.G. Dang, F. Ma, L.L. Jing, Effect of sodium nitrate on improvement
in the properties of ceramsite using red mud, J. Univ. Sci. Technol. Beijing 33
(2011) 1260–1264.
[19] J.J. Wang, H.F. Lou, X.D. Cai, J.H. Wen, Research on fly combustion process of
self-reaction quenching agglomerate powders based on high-speed camera,
Therm. Spray Technol. 4 (2012) 1–5.
[20] K.M. Lim, K.A. Lee, M.C. Kim, C.G. Park, Complex permeability and
electromagnetic wave absorption properties of amorphous alloy–epoxy
composites, J. Non-Cryst. Solids 351 (2005) 75–83.
[21] D.G. Xia, B.S. Lu, H.K. Wang, Preparation Principle and Technology of Soft
Magnetic Ferrites, Shanxi Press, Xi’an, 2010.
[9] W. Zhong, W.P. Ding, N. Zhang, J.M. Hong, Q.J. Yan, Y.W. Du, Key step in
synthesis of ultrafine BaFe12O19 by sol–gel technique, J. Magn. Magn. Mater.
168 (1997) 196–202.
[10] X.S. Liu, W. Zhong, S. Yang, Z. Yu, B.X. Gu, Y.W. Du, Influences of La3+
substitution on the structure and magnetic properties of M-type strontium
ferrites, J. Magn. Magn. Mater. 238 (2002) 207–214.
[11] X.D. Cai, J.J. Wang, B.C. Xu, H.F. Lou, Y.S. Hou, Effect of heat-treatment
temperature on structure and microwave electromagnetic properties of
hollow multiphase ceramic microspheres, J. Chin. Ceram. Soc. 41 (2013)
1331–1338.
[22] Y.H. Lan, X.X. Gao, Y. Q, J. G, X.Q. Bao, Study of barium ferrite based on self-
propagating high-temperature synthesis, Mater. Rev. 21 (2007) 333–335.