Fuel additives and heat treatment effects on nanocrystalline zinc ferrite phase composition

Article abstract of DOI:10.1016/j.jmmm.2010.10.013

Nanocrystalline ZnFe₂O₄ powder was prepared by the auto-combustion method using citric acid, acetic acid, carbamide and acrylic acid as fuel additives. Pure spinel zinc ferrite with the crystallite size of about 15 nm can be obtained

Full text of DOI:10.1016/j.jmmm.2010.10.013

Journal of Magnetism and Magnetic Materials 323 (2011) 569–573
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Fuel additives and heat treatment effects on nanocrystalline zinc ferrite
phase composition
a
a
b
a
a
a,n
c
Ping Hu , De-an Pan , Xin-feng Wang , Jian-jun Tian , Jian Wang , Shen-gen Zhang , Alex A. Volinsky
a
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P.R. China
b
Beijing Electronic Science Vocational College, Beijing 100026, P.R. China
c
Department of Mechanical Engineering, University of South Florida, Tampa, FL 33620, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
2 4
Nanocrystalline ZnFe O powder was prepared by the auto-combustion method using citric acid, acetic
Received 15 January 2010
Received in revised form
acid, carbamide and acrylic acid as fuel additives. Pure spinel zinc ferrite with the crystallite size of about
5 nm can be obtained by using acrylic acid as fuel additive. Samples prepared using other fuel additives
1
1
1 October 2010
contain ZnO impurities. In order to eliminate ZnO impurities, the sample prepared with citric acid as fuel
additive was annealed at different temperatures up to 1000 1C in air and in argon. Annealed powders have
pure ZnFe O phase when annealing temperature is higher than 650 1C in air. Sample annealed at 650 1C
Available online 28 October 2010
Keywords:
2
4
Zinc ferrite
3 4
in air is paramagnetic. However, annealed powders become a mixture of Fe O and FeO after annealing at
Fuel additive
Heat treatment
Phase composition
1
000 1C in argon atmosphere due to Zn volatility and the reduction reaction.
&
2010 Elsevier B.V. All rights reserved.
1
. Introduction
The auto-combustion method has been demonstrated for zinc
ferrite production [18–22]. However, few studies considered the
influence of fuel additives and heat treatment conditions on the
phase composition of nanocrystalline zinc ferrite. In the present
work, nanocrystalline zinc ferrite was prepared by the auto-
combustion method using citric acid, acetic acid, carbamide, and
acrylic acid as fuel additives. The effects of different fuel additives on
crystallite phase composition and microstructure of zinc ferrite
were characterized. Then as-prepared powders were annealed at
different temperatures ranging up to 1000 1C for 1 h in air and in
argon. Influence of heat treatmentconditions on microstructure and
magnetic properties of annealed zinc ferrites were investigated.
Zinc ferrite (ZnFe
2
O
4
) has normal spinel structure; it is a
commercially important material and has been widely used in
magnetic applications [1,2], gas sensors [3], catalysts [4], photo-
catalysts [5,6] and absorbent materials [7,8] because of its excellent
electrical and magnetic properties.
Recently, nanocrystalline ferrites have been extensively studied
because of their superior physical and chemical properties com-
pared with bulk counterparts [9,10]. To prepare high electromag-
netic performance zinc ferrites, various synthesizing methods have
been reported, including high-energy ball milling [1–2], hydro-
thermal technique [4], co-precipitation [11,12], ferrocenyl pre-
cursor method [13], ultrasonic cavitation [14], thermal plasma [15],
etc. However, some of these methods encountered problems such
as the requirement of complicated equipment or long processing
time caused by multiple steps, thus being economically unfeasible
for large-scale production. Lately more attention has been paid to
the auto-combustion method, which is an exothermic redox
reaction between metal nitrates (oxidizing agents) and appropriate
fuel additives (reducing agents) and had been successfully used for
synthesizing nanocrystalline ferrites [16]. The advantages of this
method are chemically homogeneous composition, processing
simplicity, low energy loss, high-production efficiency and high-
purity products [17].
2. Experiment
Nanocrystalline zinc ferrite (ZnFe
2
O
4
) was prepared by the
ꢀ 9H O, zinc
as raw materials and citric acid
O, acetic acid CH COOH, carbamide CO(NH , and
acrylic acid C COOH as fuel additives for samples labeled S1,
auto-combustion method using ferric nitrate Fe(NO
nitrate Zn(NO
ꢀ 6H
ꢀ H
3
)
3
2
3
)
2
2
O
C
6
H
8
O
7
2
3
2 2
)
2 3
H
S2, S3 and S4, respectively. Appropriate proportion of metal
nitrates and fuel additives were dissolved in distilled water, and
the solution pH value was adjusted to 7 with ammonia. The
solution was heated to 60 1C and continuously stirred using
magnetic agitation. After 4 h the solution became a homogeneous
viscous gel. Then the gel was oven dried at 120 1C for 24 h to obtain
a dried gel. A loose and very fine zinc ferrite powder was produced
after the dried gel had spontaneously combusted in air.
n
Corresponding author. Tel./fax: +86 10 62333375.
E-mail address: zhangshengen@mater.ustb.edu.cn (S.-g. Zhang).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2010.10.013

5
70
P. Hu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 569–573
In order to investigate the influence of annealing temperature
on structural and magnetic properties, as-prepared auto-com-
busted S1 powders were annealed between 300 and 1000 1C for
incomplete reaction (1). Moreover, Fe
with ZnFe for the same spinel structure, but ZnO cannot be
dissolved in ZnFe due to its hexagonal crystal structure.
3 4
O can form a solid solution
2 4
O
2 4
O
1
h in air and in argon.
X-ray diffraction (XRD) was used to determine the phases and
Therefore, ZnO remains in samples S1, S2 and S3 as a separate
hexagonal phase.
crystallite size. XRD patterns were recorded on a Philips APD-10
diffractometer using Cu radiation. The morphology of particles was
observed by scanning electron microscopy (SEM) (LEO 1450).
LDJ 9600 (LDJ Electronics, Troy, MI, USA) vibrating sample mag-
netometer (VSM) was used to investigate the magnetic properties
at room temperature.
Table 1 shows the crystallite sizes and phases of all samples in
our experiments. The crystallite sizes were determined using
2 4
Debye–Scherrer’s formula [23]. The crystallite size of pure ZnFe O
sample S4 is about 15 nm. Owing to the intense and rapid
combustion using acrylic acid as fuel additive, the crystallite does
not have enough time to grow, so the crystallite size of S4 is quite
small. However, acrylic acid also causes imperfect crystallization
and lower crystallinity for sample S4 (Fig. 1). Therefore, it is very
important to select the appropriate fuel additives for phase
formation by the auto-combustion method.
3
. Results and discussion
3.1. Phase analysis of products with different fuel additives
3.2. Heat treatment in air
The dried gel for sample S4 combusted rapidly and more intense
than S1 during its ignition in air. However, the dried gel for sample
S2 and S3 began to smoke after 2–3 min preheating and combusted
slowly and mildly. Fig. 1 shows the XRD patterns for samples S1, S2,
S3 and S4 with different fuel additives. These measurements show
that only S4 has a pure spinel zinc ferrite structure (JCPDS # 22-
2 4
In order to eliminate ZnO impurities and obtain pure ZnFe O
from samples S1, S2, S3, we chose S1 as a representative sample and
subsequently annealed it at different temperatures ranging from
00 to 1000 1C for 1 h in air. The XRD patterns of annealed products
are shown in Fig. 2. Below 650 1C annealing temperature, powder
has ZnO phase. However, powders have pure spinel ZnFe O
2 4
4
1
012) by gel precursor auto-combustion, and samples S1, S2 and S3
contain XRD reflections coming from impurities due to the ZnO
phase (JCPDS # 36-1451).
structure when the annealing temperature is higher than 650 1C.
Moreover, with higher annealing temperature, powders have
stable structure and fine crystallization. The crystallite sizes of
annealed powder increase from 27.7 to 52.5 nm with the annealing
temperature increase from 400 to 1000 1C (Table 1).
The synthesis of ZnFe
as follows. Dried gels first generate metal oxide ZnO and Fe
2
O
4
by auto-combustion can be explained
in
3 4
O
the beginning of the combustion process. Combustion releases
large amounts of heat, and the following combination reaction
occurs:
Nanocrystalline ferrites produced by the auto-combustion
method have large specific surface area and good reactivity [17].
6
ZnO+4Fe
3
O
4
+O-6ZnFe
2
O
4
(1)
2 4
Annealed powders are pure ZnFe O and ZnO disappeared when
the annealing temperature was higher than 650 1C due to the
combination reaction (1) at 650 1C. However, annealed powders
also have ZnO impurity phase below 650 1C (400 and 600 1C
annealing temperatures), possibly due to very slow diffusion rate
at lower temperatures.
The sample S4 has pure ZnFe
tion and large amount of heat, so reaction (1) for sample S4 fully
completes and produces pure ZnFe . However, samples S1, S2
2
O
4
phase due to intense combus-
2 4
O
and S3 retain some ZnO phase due to their mild combustion and
3.3. Heat treatment in argon
The structure of ferrite depends on the environment and the
heat treatment temperature [16]. Sample S1 was annealed at
different temperatures ranging from 300 to 1000 1C for 1 h in
argon atmosphere and the resulting XRD patterns are shown in
Fig. 3. The products contain a mixture of ZnFe
2 4
O and ZnO phases
when sample S1 was annealed below 1000 1C in argon. However,
annealed powders become a mixture of Fe
3
O
4
(JCPDS # 19-0629)
Table 1
Crystallite size and phases for all samples.
Samples
Crystallite size (nm)
Phase
S1
23.4
29.2
20.3
15.1
27.7
35.1
35.9
39.6
44.1
52.5
25.6
35.9
39.2
55.8
39.7
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
ZnFe
2
2
2
2
2
2
2
2
2
2
2
2
2
2
4
O
O
O
O
O
O
O
O
O
O
O
O
O
O
4
4
4
4
4
4
4
4
4
4
4
4
4
4
and ZnO
and ZnO
and ZnO
S2
S3
S4
S1 annealed at 400 1C in air
S1 annealed at 600 1C in air
S1 annealed at 650 1C in air
S1 annealed at 700 1C in air
S1 annealed at 800 1C in air
S1 annealed at 1000 1C in air
S1 annealed at 300 1C in Ar
S1 annealed at 500 1C in Ar
S1 annealed at 650 1C in Ar
S1 annealed at 800 1C in Ar
S1 annealed at 1000 1C in Ar
and ZnO
and ZnO
and ZnO
and ZnO
and ZnO
and ZnO
3
Fe O and FeO
Fig. 1. XRD patterns for samples S1, S2, S3 and S4.

P. Hu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 569–573
571
It causes Zn to dissociate from ZnO, and the boiling point of Zn is
907 1C. Thus, Zn would be volatile if it dissociated from ZnO during
preparation and heat treatment of zinc ferrites. It is hard to get
2
+
rid of Zn in ferrites due to Zn fixed in zinc ferrite lattice for
ZnFe compound which is combined with ZnO and Fe
However, ZnFe conversion takes place at high annealing
temperature [25]:
2
O
4
3 4
O .
2 4
O
ZnFe
2
O
4
-(1ꢁx)ZnFe
2
O
4
+2/3xFe
3
O
4
+xZnO+1/6xO
2
(3)
In other words, ZnO can partially dissociate from ZnFe
2 4
O
at
high annealing temperature due to reaction (3) and its hexagonal
structure. This free ZnO then decomposes by reaction (2), which
removes Zn from the samples. The higher the annealing tempera-
ture, the more Zn is removed. Moreover, both reactions (2) and (3)
are oxygen evolution reactions; therefore they will aggravate Zn
volatility in anoxic atmosphere.
In our experiments Zn is volatilized because samples annealed
in argon atmosphere accelerate reactions (2) and (3). As a result Zn
volatilization increases with annealing temperature in argon
atmosphere. At 1000 1C annealing temperature, powders have
3 4
Fe O and FeO phases due to complete Zn volatilization and further
reduction in argon atmosphere.
Fig. 4 shows the effect of the annealing temperature on the
crystallite size of annealed S1 sample in argon. It shows that
the crystallite sizes of annealed powders increased from 25.6
to 55.8 nm with the annealing temperature increase from 300 to
Fig. 2. XRD patterns for sample S1 annealed at different temperatures in air.
800 1C. However, it decreased to 39.7 nm at 1000 1C due to
decomposition and reduction in ferrites.
3.4. Products microstructure evolution with different fuel additives
Typical SEM micrographs of samples S1, S2, S3 and S4 micro-
structure are shown in Fig. 5. It is clear that sample S4 is uniform in
both morphology and particle size due to the intense and rapid
combustion using acrylic acid as fuel additive. However, samples
S1, S2 and S3 are non-uniform in morphology and are agglomerated
to some extent due to their slow and mild combustion and ZnO
impurities.
3.5. Magnetic properties
Magnetic properties were measured by the VSM at room tempera-
ture for the sample S1 annealed at 650 1C in air, as shown in Fig. 6.
Fig. 3. XRD patterns for sample S1 annealed at different temperatures in argon.
and FeO (JCPDS # 06-0615) after annealing at 1000 1C in argon
atmosphere due to Zn volatility in the samples.
Normally it is impossible to remove ZnO from ferrite because of
the 1975 1C melting point of bulk ZnO. The following decomposi-
tion reaction in ZnO would occur with increased annealing
temperature [24]
Fig. 4. S1 crystallite size dependence on the annealing temperature in argon
ZnO-Zn+1/2O
2
(2)
atmosphere.

5
72
P. Hu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 569–573
Fig. 5. SEM micrographs for samples S1, S2, S3 and S4.
nanocrystalline zinc ferrite samples prepared by other techniques
12,26–30]. Taking into account normal spinel structure of zinc ferrite
[
2
+
2 4
(ZnFe O ) which has a tetrahedral A-site occupied by Zn ions and an
3
+
octahedral B-site occupied by Fe ions; the coercivity value of 15 Oe
suggests that in the as-prepared zinc ferrite cation distribution has
partially changed from normal to mixed spinel type. This can be
attributed to some Fe ions occupying tetrahedral A-sites and switch
by the A–B super-exchange interaction [26,31].
3
+
4. Conclusions
2 4
Nanocrystalline ZnFe O was prepared by the auto-combustion
method using different fuel additives. Pure spinel zinc ferrite can be
obtained by using only acrylic acid as fuel additive, with the
crystallite size of about 15 nm and uniform in both morphology
and particle size due to intense and rapid combustion. However,
the samples obtained using other fuel additives contain ZnO
impurities.
Fig. 6. Magnetic hysteresis loops for sample S1 annealed at 650 1C in air.
In order to eliminate ZnO impurities, sample S1 was annealed at
different temperatures ranging from 400 to 1000 1C for 1 h in air
2 4
and in argon. Annealed powders have pure ZnFe O when the
Table 2
annealing temperature is higher than 650 1C in air. With continu-
ously increased annealing temperature, powders have stable
structure and fine crystallization in air. Sample S1 annealed at
650 1C in air has good paramagnetism. However, annealed powders
Magnetic properties, crystallite size and phases of S1 annealed at 650 1C in air.
ꢁ
1
Samples
M
s
M
r
(emu g
)
H
c
(Oe) Crystallite Phase
size (nm)
ꢁ
1
(emu g
)
3 4
become a mixture of Fe O and FeO after annealing at 1000 1C in
argon atmosphere due to the volatilization of Zn and the reduction
reaction in argon atmosphere.
5.551 ꢂ 10ꢁ 15
3
35.9
Zinc
A annealed
2.583
at 650 1C in air
ferrite
Magnetic properties, crystallite size and phases of sample S1 annealed
at 650 1C in air are listed in Table 2. It is clear that ZnFe has good
Acknowledgments
2 4
O
paramagnetism. The magnetization value does not attain saturation at
the highest employed magnetic field of 20 kOe. A magnetization value
The authors gratefully acknowledge support provided by the
National Natural Science Foundation of China (no. 50972013, no.
50874010 & no. 50802008), National Key Project of Scientific and
Technical Supporting Programs funded by Ministry of Science &
Technology of China (no. 2009BAE74B03) and Guangdong Province
ꢁ
1
of 2.583 emu g
was obtained at room temperature. No magnetic
2 4
anisotropy was observed. The paramagnetic properties for ZnFe O in
3
ꢁ1
our experiment (M
r
¼5.551 ꢂ 10ꢁ emu g ) are superior to the

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573
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