ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 11, pp. 1309–1313. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © M.I. Ivanovskaya, A.I. Tolstik, V.V. Pan’kov, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 11, pp. 1398–1403.
Synthesis of Zn0.5Mn0.5Fe2O4
by Low-Temperature Spray Pyrolysis
M. I. Ivanovskayaa, A. I. Tolstikb, and V. V. Pan’kovb
a
Research Institute of Physicochemical Problems, Belarussian State University,
Leningradskaya ul. 14, Minsk, 220030 Belarus
b Belarussian State University, Leningradskaya ul. 14, Minsk, 220030 Belarus
e-mail: ivanovskaya@bsu.by
Received September 15, 2008; in final form, January 12, 2009
Abstract—The effect of synthesis temperature on the structural perfection of the Zn0.5Mn0.5Fe2O4 ferrite synthesized via spray pyrolysis of a solution of Zn(II), Mn(II), and Fe(III) nitrates has been studied using X-ray
diffraction, scanning electron microscopy, and IR spectroscopy. The material obtained at 650°C is shown to
have a nanocrystalline structure. IR spectroscopy results indicate that the synthesized Zn0.5Mn0.5Fe2O4 spinel
ferrite is highly homogeneous in composition and structure.
DOI: 10.1134/S0020168509110223
INTRODUCTION
An important point in the synthesis of magnetic
materials based on Zn–Mn ferrite is the ability to obtain
a phase-pure spinel-structure product uniform in composition and structure. The formation of phase-pure
Zn–Mn ferrite and its magnetic properties are adversely
influenced by the oxidation of Mn2+ to Mn3+, which
occurs most rapidly at temperatures above 900°ë.
One way of preventing the Mn2+
Mn3+ process
is by reducing the ferrite synthesis temperature. The
temperature of ferrite formation can be reduced by utilizing nanotechnology instead of conventional ceramic
processing techniques (sintering of oxides). The use of
nanophase precursors accelerates diffusion of reactants
and ensures the formation of highly homogeneous solid
solutions. In particular, as shown by Sviridov et al. [1]
highly homogeneous, fine-particle zinc ferrite can be
prepared through hydroxide coprecipitation. Zn(II) and
Fe(III) hydroxide coprecipitation products can be converted to zinc ferrite by aging the precipitate at 100°ë.
The formation of manganese ferrite is known to be
less energetically favorable than that of zinc ferrite: the
activation energy for ferrite formation is 410.5 kJ/mol
in the ZnO–Fe2O3 system and 754.1 kJ/mol in the
MnO–Fe2O3 system. As reported by Novosadova et al.
[2], when hydroxide coprecipitation products with a
Zn : Mn ratio of 1 : 1 are converted to the
Zn0.5Mn0.5Fe2O4 phase at 100°ë, the yield is about 65%.
Therefore, the synthesis of Zn–Mn ferrite requires a
higher temperature in comparison with ZnFe2O4, independent of the procedure. However, as mentioned
above, the oxidation of Mn2+ to Mn3+ is more likely at
higher temperatures. This process may be accompanied
by the reduction of Fe3+ to Fe2+ and cation redistribution
over the tetrahedral and octahedral sites of the spinel
structure. The cation distribution in the spinel structure
of Zn–Mn ferrite plays a key role in determining its
magnetic properties.
There are reports that, during solid-state synthesis
from oxides, the spinel structure of Zn–Mn ferrite
begins to form at 600°ë, and the process involves the
formation of intermediate oxide phases. The first to
form is ZnFe2O4 with an excess of uncombined Fe2O3
because the Zn2+ ion is rather mobile and always occupies the tetrahedral site in the spinel structure, with no
magnetic hindrances during diffusion. Mn2+ diffuses
into the structure of ZnFe2O4 · Fe2O3 at higher temperatures [3]. The optimal heat treatment temperature,
which ensures the required magnetic performance of
ferrites, is 1000–1200°ë.
The synthesis temperature of Zn–Mn ferrite can be
reduced by using spray pyrolysis. This method has been
shown to be effective in the synthesis of magnetic materials based on barium hexaferrite, BaFe12O19 [4]. Owing
to the small reaction volume, the hexaferrite structure
can be obtained by spray pyrolysis of nitrate solutions
without intermediate oxide phases, which allows the
synthesis temperature to be reduced and enables the
preparation of nanocrystalline material. Previous
results [4] suggest that the synthesis temperature of
spinel ferrites can also be reduced.
In this paper, we describe the preparation of the
Zn0.5Mn0.5Fe2O4 ferrite by low-temperature spray
1309
1310
IVANOVSKAYA, TOLSTIK
OH
Powder XRD patterns were collected on a DRON2.0 diffractometer with Coäα radiation (λ = 0.178896
nm, Ni monochromator) in the angular range 2θ = 20°–
80°. The results were analyzed by standard procedures
using JCPDS PDF data.
Mass magnetization was measured as a function of
temperature by the Faraday method in a magnetic field
H = 0.86 T during heating and cooling in the range 77–
700 K.
NOx
6
5
Transmission
4
3
2
1045
1277
1389
1471
1
1800 1700 1600 1500 1400 1300 1200 1100 1000
Wavenumber, cm–1
Fig. 1. Portions of the IR spectra of samples obtained by
thermal decomposition of an aqueous solution of Zn(II),
Mn(II), and Fe(III) nitrates in air at (1) 100, (2) 200, (3) 300,
(4) 400, (5) 500, and (6) 650°C.
pyrolysis of a mixture of zinc, manganese, and iron
nitrate solutions.
EXPERIMENTAL
To synthesize Zn–Mn ferrite by spray pyrolysis, we
used a solution of Zn(II), Mn(II), and Fe(III) nitrates
with a concentration corresponding to 0.25 M
Zn0.5Mn0.5Fe2O4. The solution was prepared by mixing
solutions of the individual metals in the required proportions. Ferrite samples were prepared in a tubular furnace through which droplets of the solution dispersed
by an ultrasonic atomizer were propelled by air.
The structure of the samples was examined by X-ray
diffraction (XRD), scanning electron microscopy
(SEM) on a LEO 1420 instrument, and IR spectroscopy.
IR spectra were taken on a Nicolet Avatar 330 spectrometer in diffuse reflection mode (without KBr) in the
range 400–4000 cm–1. A small amount of the powder
was applied to a steel substrate.
RESULTS AND DISCUSSION
Synthesis temperature. To select the synthesis
temperature sufficient for the synthesis of the ferrite, an
aqueous solution of Zn(II), Mn(II), and Fe(III) nitrates
with the same composition as was used in spray pyrolysis was heated in air, and the decomposition products
were characterized by IR spectroscopy. Analysis of the
IR spectra allowed us to determine the temperature
range where the nitrate/nitrite residues were removed.
The nitrates of the metals in question are known to
melt at t ≤ 50°ë. When heated to 100°ë, Zn(II), Mn(II),
and Fe(III) nitrates have the form of viscous paste, and
their IR spectra show strong absorption bands at 1471,
1389, 1277, and 1045 cm–1, due to bond vibrations in
–
the NO 3 . ion.
According to earlier results, the stretching frequen–
–
cies of the NO 3 and NO 2 radical anions in inorganic
compounds lie in the range 970–1500 cm–1.
Figure 1 illustrates the temperature effect on the IR
spectra of the decomposition products in the stretching
region of the nitrate and nitrite ions. It follows from
these data that both the number of absorption bands and
their intensity decrease with increasing temperature.
The concentration of nitrogen-containing radical
anions is low after heating at 400°ë, and there are no
such anions after heating at 500–650°ë. The weak feature at 1347 cm–1 in the spectrum obtained after heating
at 500°ë may be due not to residual NO2 groups but to
CO2 adsorption on the surface of the forming oxide
structure. At the same time, there is evidence that ther–
mal dehydration of hydroxides in the presence of NO 3
•
may lead to stabilization of NO 2 , radical anions, which
cause partial amorphization of the resulting oxides [5].
•
NO 2 radical anions were detected by ESR in highly
dispersed Al2O3, MÓO3, In2O3, and SnO2 prepared by
sol–gel processing from colloidal suspensions stabi–
lized with NO 3 anions [5, 6].
It follows from our IR spectroscopy data that the
decomposition of the nitrate/nitrite residues during
heating of a mixture of zinc, manganese, and iron
nitrates reaches completion in the range 500–650°ë. In
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SYNTHESIS OF Zn0.5Mn0.5Fe2O4 BY LOW-TEMPERATURE SPRAY PYROLYSIS
1311
2
1
75
70
65
60
55
50
45
40
35
2θ, deg
Zn0.5Mn0.5Fe2O4
Mn2O3
ZnO
α-Fe2O3
Fig. 2. XRD patterns of the Zn–Mn ferrite samples synthesized at (1) 450 and (2) 650°C.
(a)
(b)
2
828
949
1038
1205
1162
1122
1082
1040
1
560
430
1402 1633
1331
1038
Transmission
1
3408
Transmission
2
4000 3000 2000 1000
Wavenumber, cm–1
0
1400 1200 1000 800 600
Wavenumber, cm–1
Fig. 3. IR spectra of the Zn–Mn ferrite samples synthesized at (1) 450 and (2) 650°C: (a) 400–4000 cm–1, (b) 500–1500 cm–1.
view of this, two temperatures were chosen for the synthesis of Zn–Mn ferrite by spray pyrolysis: 450 and
650°ë.
The lower synthesis temperature (450°ë) was
intended to ascertain whether dehydration, nitrate
decomposition, and ferrite crystallization in the system
under investigation occur more rapidly when they are
localized in a microvolume (in a droplet). We took into
account the Mössbauer results reported by Sviridov et
al. [7], which demonstrate that, in highly dispersed
Fe(III) and M(II) (M = Zn, Cu, Ni) hydroxide coprecipitation products, radical structural changes resulting in
ferrite formation begin and 400°ë and reach completion
at 500°ë.
Phase composition, particle size, and crystal
structure. The XRD pattern of the sample prepared at
450°ë shows one, broad line, which can be identified as
the 311 reflection from the spinel ferrite phase (JCPDS,
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2009
no. 10-0467) (Fig. 2). This indicates that the temperature 450°ë is too low for the formation of the
Zn0.5Mn0.5Fe2O4 spinel. The crystallite size is 4–5 nm. In
addition, the XRD pattern shows a weak halo in the 2θ
range of the strongest reflections from ZnO (39.30°),
α-Fe2O3 (38.69°), and Mn2O3 (38.40°), which may be
interpreted as evidence that the sample contains small
amounts of these oxides in a highly dispersed state. The
same is evidenced by IR spectroscopy data: the broad
absorption band in the frequency range characteristic of
M–O (M = Fe, Zn, Mn) bond vibrations (500 cm–1)
suggests that the ferrite phase is poorly ordered and that
the sample may contain binary oxides (Fig. 3).
The features at 828, 949, and 1038 cm–1, characteristic of the Zn–O–H, Mn–O–H, and Fe–O–H bending
modes in the corresponding oxides, lend support to the
assumption that the sample contains ZnO, MnO, and
Fe2O3.
1312
(‡)
IVANOVSKAYA, TOLSTIK
2 µm
2 µm
(b)
Fig. 4. SEM micrographs of Zn–Mn ferrite particles synthesized at (a) 450 and (b) 650°C.
The strong, broad absorption band at 3408 cm–1
attests to a large amount of adsorbed water, which is
rather typical of highly dispersed α-Fe2O3. The feature
at 1633 cm–1, due to the O–H bending mode, indicates
that not all of the OH groups were removed from the
material, which is also typical of the various forms of
FeOOH present as reaction intermediates when dehydroxylation of the oxide system is incomplete.
The strong absorption bands at 1331 and 1402 cm–1
are due to the residual nitrate and nitrite ions in the sample synthesized at 450°ë.
The above experimental data indicate that this temperature is too low for the synthesis of phase-pure
spinel-structure Zn0.5Mn0.5Fe2O4 by spray pyrolysis.
The forming fine-particle material seems to be a mixture of poorly crystallized (ZnMn)xFe2O4, Fe2O3, ZnO,
–
and MnOx, with large amounts of H2O, NO x - , and OH
groups.
According to SEM results (Fig. 4), the sample consists of regularly shaped, spherical particles. The surface morphology of the particles is typical of amorphous or glassy materials. The particle diameter d
ranges from 150 nm to 1.6 µm. Most of the particles
have d = 650–800 nm. It seems likely that these are
aggregates of finer particles, given that the crystallite
size determined by XRD is almost two orders of magnitude smaller than the average diameter of the spherical particles.
The XRD pattern of the sample prepared at 650°ë
shows broad lines attributable to a spinel ferrite phase
(Fig. 2). No other phases were detected by XRD. The
crystallite size is 7–8 nm. The unit-cell parameter of
this sample (aav = 8.470 Å, ‡511 = 8.443 Å) is smaller
than those reported for high-temperature Zn–Mn ferrite
samples of different compositions (8.459–8.472 Å). It is
known that, for a given composition, the unit-cell
parameter of Zn–Mn ferrites increases with calcination
temperature.
IR spectroscopy results confirm that this sample has
the spinel structure with a rather uniform cation distribution. In the region characteristic of Fe–O bond vibrations
in the spinel structure, its IR spectrum shows two symmetric absorption bands, centered at 430 and 560 cm–1, which
are assignable to Fe–O–Zn and Fe–O–Mn combined
bond vibrations (Fig. 3).
The shift of the IR absorption bands characteristic of
the Fe–O stretching mode in Fe3O4 to lower frequencies
430 cm–1) indicates the
(580
560 cm–1, 440
presence of Zn2+ and Mn2+ in the spinel structure [8, 9].
The symmetric shape of these bands suggests that the
Zn0.5Mn0.5Fe2O4 ferrite has a homogeneous spinel structure and that the Zn2+ and Mn2+ cations are evenly distributed over its sites.
The chemical homogeneity of the Zn–Mn spinel ferrite
is also evidenced by the absence of the absorption band
characteristic of Mn–O–H bends in Mn2O3 (949 cm–1), the
reduced intensity and shift (836
828 cm–1) of the
Zn−O–H bending band, and the symmetric splitting of the
absorption band characteristic of Fe–O–H bends in the
oxide into five equally spaced (∆ 40 cm–1) components
(1040, 1082, 1122, 1162, and 1205 cm–1). The splitting of
the Fe–O–H bending band may be due to the reduction
of the symmetry of the octahedral oxygen coordination
of Fe(III) in the structure of the ferrite as a result of the
accommodation of the large-sized cations Zn2+ and
Mn2+ in the tetrahedral site.
From the temperature dependence of mass magnetization for the Zn–Mn ferrite sample synthesized at
650°ë (Fig. 5), its Curie temperature TC was determined
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No. 11 2009
SYNTHESIS OF Zn0.5Mn0.5Fe2O4 BY LOW-TEMPERATURE SPRAY PYROLYSIS
σ, A m2/kg
22
1313
Heating
Cooling
Second
heating
20
18
16
14
12
10
8
TC = 375–380 K
6
4
2
0
0
100
200
300
400
500
600
700
800
Temperature, K
Fig. 5. Temperature dependence of mass magnetization for the Zn–Mn ferrite sample synthesized at 650°C.
to be 375–380 K. The shape of the σ (T) curve and the
value of TC confirm the formation of a phase-pure ferrite and indicate that the material contains no uncombined iron oxide [10].
3. Pankov, V.V., Interaction of (Mn,Zn)O Solid Solution with
Fe2O3 As Intermediate Stage of Formation of Mn–Zn Ferrites, Ceram. Int., 1988, vol. 20, pp. 87–91.
CONCLUSIONS
Nanocrystalline Zn0.5Mn0.5Fe2O4 with the spinel
structure was synthesized via spray pyrolysis of a solution of Zn(II), Mn(II), and Fe(III) nitrates at 650°C.
Single-phase material with the spinel structure and a
unit-cell parameter ‡ = 8.443–8.470 Å was shown to
form at 650°ë. The ferrite particles thus produced have
a regular spherical shape and range in diameter from
650 to 800 nm, which is typical of spray pyrolysis
because structure formation processes take place in
microscopic droplets of the starting solution. IR spectroscopy results indicate that the zinc and manganese
cations are evenly distributed in the structure of the synthesized ferrite. The crystallite size is 7–8 nm as determined by XRD.
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Vol. 45
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