Optimization of hydrogen dynamic heat treatment and re-calcination for preparation of strontium hexaferrite nanocrystalline powder

Article abstract of DOI:10.1016/j.jallcom.2009.01.020

Strontium hexaferrite is a hard magnetic material which under hydrogen treatment and re-calcination, its phase composition and also particles size and morphology change completely. Strontium hexaferrite was prepared by conventional route with calcination

Full text of DOI:10.1016/j.jallcom.2009.01.020

Journal of Alloys and Compounds 479 (2009) 638–641
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Optimization of hydrogen dynamic heat treatment and re-calcination for
preparation of strontium hexaferrite nanocrystalline powder
a
a,∗
a
a
b
H.R. Koohdar , S.A. Seyyed Ebrahimi , A. Yourdkhani , R. Dehghan , F. Zajkaniha
a
Center of Excellence in Magnetic Materials, School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran
Iranian Gas National Corporation, Tehran, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 May 2008
Received in revised form 5 January 2009
Accepted 10 January 2009
Available online 21 January 2009
Strontium hexaferrite is a hard magnetic material which under hydrogen treatment and re-calcination, its
phase composition and also particles size and morphology change completely. Strontium hexaferrite was
prepared by conventional route with calcination of strontium carbonate and hematite at 1100 C for 1 h.
◦
Then strontium hexaferrite was heat treated in hydrogen dynamic atmosphere at various temperatures
◦
and gas flows for different times. Optimum conditions of hydrogen treatment were obtained at 850 C
3
with 60 cm /min flow for 1 h. Subsequent re-calcination was carried out at various temperatures for
Keywords:
◦
the optimum hydrogen treated powder and its optimum conditions were obtained at 1000 C for 1 h.
Nano-structured materials
Gas–solid reactions
Chemical synthesis
The effect of dynamic hydrogen treatment and re-calcination on the phase composition and particles
size and morphology characterized by X-ray diffraction (XRD) and scanning and transmission electron
microscopes (SEM and TEM). The results showed decomposition of strontium hexaferrite and reduction of
the resultant hematite mainly to iron during hydrogen treatment. Nanocrystalline powder of strontium
hexaferrite was also reformed after the re-calcination. The magnetic properties of the initial and final
strontium hexaferrite powder were measured by a vibration sample magnetometer (VSM). The results
showed about 30% increase in the coercivity by application of this process on the strontium hexaferrite
powder.
©
2009 Elsevier B.V. All rights reserved.
1
. Introduction
In this work the effects of dynamic hydrogen heat treatment and
re-calcination on the microstructure and magnetic properties of
conventionally synthesized strontium hexaferrite have been stud-
ied systematically.
M-type hexaferrites with MFe12O19 (M = Ba, Sr and Pb) chem-
ical formula have been widely used as permanent magnets due
to their low cost of production, high uniaxial magnetic anisotropy
and excellent corrosion resistivity [1]. Hexagonal ferrites are also
promising materials for microwave devices and perpendicular
recording Media [2]. The conventional method of production of this
2. Experimental procedure
Starting materials for M-type strontium hexaferrite conventionally synthesis
were hematite (␣-Fe2O3) and strontium carbonate (SrCO3) without using any addi-
material is the solid-state reaction between SrCO3 and Fe O3 at
temperatures higher than 1100 C [3]. In order to get single domain
◦
2
tives. The calcination was carried out at 1100 C for 1 h in air. Hydrogen heat
◦
treatment was also carried out in a dynamic atmosphere at different temperatures
with various flows. A tube furnace with quartz reactor was used for heat treating
of strontium hexaferrite in H2 atmosphere. Subsequent calcination processes also
consisted of heating up to various temperatures in a muffle furnace, dwelling for 1 h
particles of strontium hexaferrite, different synthesis techniques
have been developed such as hydrothermal synthesis [4], salt melt
method [5], co-precipitation [6], self-propagating high temperature
synthesis [7] and sol–gel auto combustion method [8,9].
Static gas heat treatment and re-calcination is a method which
produces hexaferrite nanoparticles from conventionally synthe-
sized powder and have been investigated in last decade [10–13].
Dynamic carbon monoxide heat treatment and re-calcination has
also been reported recently [14].
◦
and then cooling. The heating and cooling rates were 10 C/min. X-ray diffraction
analysis (Cu K␣ radiation) was used for phase identification. Scanning and transi-
tion electron microscopy was also used to determine the morphology and size of
the particles. Finally, the magnetic properties were measured by a vibration sample
magnetometer.
3. Results and discussion
X-ray diffraction pattern of the single phase strontium hexafer-
rite synthesized conventionally has been shown in Fig. 1. Fig. 2 also
shows SEM image of the microstructure this powder. The hexagonal
shape particles with size of below 500 nm can be observed clearly.
∗ Corresponding author. Tel.: +98 21 88010879; fax: +98 21 88006076.
E-mail address: saseyyed@ut.ac.ir (S.A. Seyyed Ebrahimi).
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.01.020

H.R. Koohdar et al. / Journal of Alloys and Compounds 479 (2009) 638–641
639
Fig. 1. X-ray diffraction pattern of Sr-hexaferrite synthesized conventionally at
◦
1
100 C for 1 h (SH = Sr-hexaferrite).
X-ray diffraction patterns of the strontium hexaferrite powder
subjected to hydrogen atmosphere at various temperatures with
3
3
0 cm /min flow for 1 h have been shown in Fig. 3.
Hydrogen heat treatment makes the initial powder to decom-
pose into hematite and Sr7Fe10O22. Then the resultant hematite was
reduced by hydrogen to metallic iron and other iron oxides. Fig. 3
indicates that the extent of reduction increases with increasing the
temperature. XRD pattern of the powder heat treated in hydrogen
◦
atmosphere at 550 C in Fig. 3a shows that the reduction products
are iron and magnetite. There is no trace of wustite (FeO) because
◦
it is unstable at temperatures lower than 570 C. With increasing
◦
the temperature up to 650 C, the magnetite was reduced totally to
wustite and no traces of it could be seen any more (Fig. 3b). With
further increasing the temperature, the reduction of wustite to iron
◦
became faster (Fig. 3c) and at temperature of 850 C only a few
traces of wustite would be still observed (Fig. 3d).
Fig. 4 shows X-ray diffraction pattern of the strontium hexafer-
◦
3
rite powder heat treated at 850 C with 60 cm /min for 1 h. This
pattern indicates that with increasing the gas flow, the hematite
was reduced completely to iron phase by hydrogen.
Since the wustite completely removed from the phase composi-
◦
3
tion, the hydrogen heat treatment at 850 C with 60 cm /min flow
for 1 h was selected as optimum process.
The SEM image of this powder has been shown in Fig. 5. Com-
pared to Fig. 2 the particle size has been decreased significantly
due to expansion of the hydrogen penetrated inside the particles
and breakage of the big grains into much finer sub-grains.
This optimum heat treated powder was then calcined at 900
◦
and 1000 C for 1 h in air. Their X-ray diffraction patterns have been
shown in Fig. 6.
According to Fig. 6a, the present phases at the temperature of
◦
9
00 C are SrFe12O19 , Fe O and Sr7Fe10O22. Therefore, this temper-
2
3
Fig. 3. X-ray diffraction patterns of the strontium hexaferrite powder heat treated
◦
◦
◦
◦
in hydrogen atmosphere at (a) 550 C, (b) 650 C, (c) 750 C and (d) 850 C with
3
3
0 cm /min flow for 1 h (F = Fe, W = FeO, M = Fe3O4, S = Sr7Fe10 O22).
Fig. 4. X-ray diffraction pattern of the strontium hexaferrite powder heat treated at
◦
3
Fig. 2. SEM image of the powder synthesized conventionally.
850 C with 60 cm /min for 1 h (F = Fe, S = Sr Fe10 O22).
7

6
40
H.R. Koohdar et al. / Journal of Alloys and Compounds 479 (2009) 638–641
◦
Fig. 7. SEM image of the strontium hexaferrite powder after re-calcination at 1000 C
for 1 h.
◦
3
Fig. 5. SEM image of the hydrogen treated sample at 850 C with 60 cm /min gas
flow for 1 h.
ature is not enough to form the single phase strontium hexaferrite.
◦
With further increase of the calcination temperature up to 1000 C,
the single phase strontium hexaferrite was formed as it is shown in
Fig. 6b. During the re-calcination process, the metallic iron was oxi-
dized to hematite and then by reaction with Sr7Fe10O22 the single
phase strontium hexaferrite was re-formed.
The SEM image of the strontium hexaferrite powder after re-
◦
calcination at 1000 C has been shown in Fig. 7.
This image shows that the strontium hexaferrite particles after
re-calcination are much finer than those of the initial powder (Fig. 2)
due to reformation of the hexaferrite phase on the surfaces of very
fine grains obtained after hydrogen heat treatment.
Fig. 8. The TEM image of the strontium hexaferrite nanocrystalline powder after
◦
re-calcination at 1000 C for 1 h.
Fig. 8 also shows the TEM image of this powder. It seem that the
size of the particles is below 50 nm which is in consistence with the
measurement of the crystallite size of this sample by the X-ray line
broadening technique employing the Scherrer formula as 38 nm.
The magnetic properties of the initial powder along with the
powder hydrogenated and re-calcined optimally have been indi-
cated in Table 1.
As it is shown in this table, the intrinsic coercivity of the initial
strontium hexaferrite powder increased markedly after hydrogen
heat treatment and re-calcination from 263 to 342 kA/m due to
much finer structure obtained after this process. However, there
are also small decreases in Mr and Ms after the process.
Table 1
Magnetic properties of the initial powder and the powder hydrogenated and re-
calcined optimally.
Powder
Ms (J/Tkg)
Mr (J/Tkg)
Hci (kA/m)
Initial
66.9
55.8
37.0
31.9
263
342
Hydrogenated and re-calcined
optimally
Fig. 6. X-ray diffraction pattern of the optimum powder which calcined at (a) 900
◦
and (b) 1000 C for 1 h (SH = SrFe12 O19 , S = Sr7Fe10 O22, H = Fe2O3).

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641
4
. Conclusions
References
[
1] J.P. Jakubovics, Magnetism and Magnetic Materials, The Institute of Metals,
London, 1987.
During the hydrogen heat treatment, the strontium hexafer-
rite decomposed into Fe O3 and Sr7Fe10O22 and then the resultant
2
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◦
mum conditions of hydrogen treatment obtained at 850 C with
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3
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◦
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(
[
The main effect of this very fine microstructure was on the
noticeable increase of the intrinsic coercivity from 263 to 342 kA/m.
6
3
[
[
[
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[
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Financial and technical supports of the University of Tehran and
Iranian Gas National Corporation are gratefully acknowledged.