Preparation of strontium hexaferrite nano-crystalline powder by carbon monoxide heat treatment and re-calcination from conventionally synthesized powder

Article abstract of DOI:10.1016/j.jallcom.2008.03.021

In this work strontium hexaferrite nano-crystalline powders were prepared by carbon monoxide heat treatment and re-calcination from conventionally synthesized powder. First strontium hexaferrite was obtained by the conventional route with calcination of s

Full text of DOI:10.1016/j.jallcom.2008.03.021

Journal of Alloys and Compounds 470 (2009) 561–564
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Preparation of strontium hexaferrite nano-crystalline powder by carbon
monoxide heat treatment and re-calcination from conventionally
synthesized powder
A. Yourdkhani, S.A. Seyyed Ebrahimi∗, H.R. Koohdar
Center of Excellence in Magnetic Materials, School of Metallurgy and Materials Engineering,
University of Tehran, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work strontium hexaferrite nano-crystalline powders were prepared by carbon monoxide heat
treatment and re-calcination from conventionally synthesized powder. First strontium hexaferrite was
obtained by the conventional route with calcination of strontium carbonate and hematite at 1100 ◦C for
1 h. Then strontium hexaferrite was isothermally subjected to carbon monoxide dynamic atmosphere
at various temperatures and flows for different times. Optimum carbon monoxide heat treatment was
achieved at 850 ◦C with 20 cm3/min flow for 0.5 h. The resultant powder was then calcined at 900 and
1000 ◦C for 1 h. The single-phase strontium hexaferrite nano-crystalline powder was finally obtained after
calcination at 1000 ◦C. The phase identification of the powders was recorded by an X-ray diffractometer
(XRD) with Cu K␣ radiation. Morphology and size of the particles were studied by scanning electron
microscopy (SEM) and transmission electron microscopy (TEM) techniques. The magnetic properties were
also measured by a vibration sample magnetometer (VSM). The results show a good enhancement in the
coercivity by applying this method on the hexaferrite powder.
Received 9 December 2007
Received in revised form 4 March 2008
Accepted 6 March 2008
Available online 9 June 2008
Keywords:
Nano-structured materials
Gas–solid reactions
Chemical synthesis
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental procedure
M-type strontium hexaferrite was produced conventionally by calcination of
M-type hexaferrites have been widely used as permanent mag-
nets due to their low cost of production, high uniaxial magnetic
anisotropy and excellent corrosion resistivity [1].
hematite (␣-Fe2O3) and strontium carbonate (SrCO3). The weight ratio of iron oxide
to strontium carbonate was 5.5:1 without using any additives. The calcination was
carried out at 1100 ◦C for 1 h in air. Carbon monoxide 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 CO
atmosphere. Subsequent calcination processes also consisted of heating up to vari-
ous temperatures in a muffle furnace, dwelling for 1 h 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 electron microscopy (SEM) and transmission
electron microscopy (TEM) were also used to determine the morphology and size of
the particles. Finally, the magnetic properties were measured by a vibration sample
magnetometer.
The conventional method of production of this material is
solid-state reaction between SrCO3 and Fe2O3 at temperatures
higher than 1100 ◦C [2]. For preparation of strontium hexaferrite
nano-particles, different production methods such as mechani-
cal alloying [3], hydrothermal [4], co-precipitation [5] and sol–gel
auto-combustion [6,7] have been employed in the last few years.
Gas heat treatment and re-calcination is a novel method which
produce the hexaferrite nano-particles from conventionally syn-
thesized powder. Hydrogen and nitrogen heat treatments and
re-calcination have been investigated in the last decade [8–11].
In this work, the effects of carbon monoxide heat treatment
and re-calcination on the microstructure and magnetic proper-
ties of conventionally synthesized strontium hexaferrite have been
reported.
3. Results and discussion
Fig. 1 shows the X-ray diffraction pattern of the powder calcined
at 1100 ◦C for 1 h which indicates the formation of single-phase
strontium hexaferrite by the conventional route.
Fig. 2 also shows the SEM micrograph of this sample. The parti-
cles size is below 500 nm with hexagonal shape, smooth surfaces
and sharp edges.
Fig. 3 shows the XRD patterns of strontium hexaferrite pow-
ders heat treated in carbon monoxide dynamic atmosphere at
550–850 ◦C with flow of 30 cm3/min for 1 h.
∗
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 © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.03.021

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Fig. 1. The X-ray diffraction pattern of strontium hexaferrite synthesized conven-
tionally at 1100 ◦C for 1 h (SH, strontium hexaferrite).
During carbon monoxide heat treatment at different temper-
atures, strontium hexaferrite decomposed into Fe2O3 and SrO
according to the following reaction:
SrO·6Fe2O3 → SrO + 6Fe2O3
(1)
The resultant Fe2O3 was then reduced to Fe3O4, FeO and finally
Fe according to reactions (2), (3) and (4), respectively. Fig. 3 shows
that the extent of reduction depends on the temperature of carbon
monoxide heat treatment. The traces of Fe3O4 and FeO in Fig. 3a
shows that the reduction process of hematite was not completed
yet. The disappearance of Fe3O4 and FeO in Fig. 3b also shows the
progress of the reduction process to the formation of the Fe phase.
Then according to reaction (6), the Fe phase and the carbon which
resulted from reaction (5), reacted again to form Fe3C phase. SrCO3
also formed according to reaction (7).
3Fe2O3 + CO → 2Fe3O4 + CO2
Fe3O4 + CO → 3FeO + CO2
FeO + CO → Fe + CO2
2CO → C + CO2
(2)
(3)
(4)
(5)
(6)
(7)
3Fe + C → Fe3C
SrO + CO2→ SrCO3
With increasing the temperatures up to 750 and 850 ◦C (Fig. 3c
and d) it could be seen that the cementite phase decomposed again
to carbon and iron phases. This is more visible at 850 ◦C rather than
at 750 ◦C.
Fig. 3. The X-ray diffraction patterns of the powders heat treated in carbon monox-
ide atmosphere with 30 cm3/min flow for 1 h at: (a) 550 ◦C, (b) 650 ◦C, (c) 750 ◦C and
(d) 850 ◦C (C, Fe3C; W, FeO; M, Fe3O4; SC, SrCO3; F, Fe).
Fig. 4. X-ray diffraction pattern of strontium hexaferrite heat-treated at 850 ◦C for
Fig. 2. The micrograph of the powder synthesized conventionally, showing
hexagonal-like particles.
1 h with flow of 20 cm3/min (F, Fe; SC, SrCO3; C, Fe3C).

A. Yourdkhani et al. / Journal of Alloys and Compounds 470 (2009) 561–564
563
Fig. 5. The X-ray diffraction pattern of the strontium hexaferrite after carbon
monoxide heat treatment at 850 ◦C for 30 min with 20 cm3/min flow (SC, SrCO3;
F, Fe).
Fig. 4 shows the X-ray diffraction pattern of strontium hexafer-
rite heat treated at 850 ◦C for 1 h with flow of 20 cm3/min. As it is
shown in this figure the extent of iron carbide has been decreased
with 20 cm3/min flow of carbon monoxide at 850 ◦C for 1 h com-
pared to the sample prepared with flow of 30 cm3/min.
For further decrease of the non-magnetic iron carbide phase,
carbon monoxide heat treatment was carried out at 850 ◦C with
20 cm3/min flow for 30 min rather than 1 h. The X-ray diffraction
pattern of this powder which has been shown in Fig. 5 confirms that
the cementite is eliminated totally. Therefore, the optimum condi-
tion of carbon monoxide heat treatment was selected at 850 ◦C with
20 cm3/min flow for 30 min.
Fig. 7. X-ray diffraction patterns of the optimized powder which calcined at (a)
900 ◦C and (b) 1000 ◦C for 1 h (SH, SrFe12 O19 ; S, Sr7Fe10 O22; H, Fe2O3).
Table 1
Magnetic properties of the initial and carbon monoxide heat treated and re-calcined
powders
Fig. 6 shows the morphology of nano-powder heat treated in
CO atmosphere at 850 ◦C for 30 min with flow of 20 cm3/min. It
can be seen that the size of the particles was below 200 nm. Com-
parison of Fig. 2 with Fig. 6 indicates that the particle size of the
initial powder decreased dramatically after the gas heat treatment.
The average crystallite size of the heat-treated powder was also
calculated by the X-ray line broadening technique employing the
Scherrer formula as 33 nm.
The resultant optimized powder was then calcined at 900 and
1000 ◦C for 1 h. Fig. 7 shows the X-ray diffraction patterns of the
powders calcined at 900 and 1000 ◦C for 1 h. Fig. 7a shows that after
re-calcination at 900 ◦C the formation of single-phase strontium
hexaferrite was not completed and Fe2O3 and Sr7Fe10O22 phases
are still present. The single-phase Sr hexaferrite was only obtained
at temperatures higher than 1000 ◦C as it is shown in Fig. 7b.
The morphology of SrFe12O19 powders after re-calcination at
1000 ◦C for 1 h is shown in Fig. 8. Compared to Fig. 2, it is clear
Powder
MS (J/T kg)
Mr (J/T kg)
Hci (kA/m)
Initial
66.9
60.9
37.0
33.5
262.6
318.3
Carbon monoxide
heat treated and
re-calcined
that the particles size of the powder after gas treatment and re-
calcination became much finer. This is attributed to nucleation of
hexaferrite crystallites during re-calcination, on the surfaces of very
fine grains obtained after gas treatment as it is shown in Fig. 6.
The TEM image of the SrFe12O19 after re-calcination is also
shown in Fig. 9 showing the size of crystallites below 50 nm. This
is consistent with the measurement of the crystallite size of this
sample by the Scherrer formula as 38 nm.
The magnetic properties of the initial strontium hexaferrite
powder before and after optimum gas heat treatment and re-
calcination processes have been compared in Table 1.
As it is indicated in Table 1, the carbon monoxide heat treatment
and re-calcination processes resulted in a noticeable increase of the
intrinsic coercivity from 262.2 to 318.3 kA/m due to the much finer
Fig. 6. The morphology of the powder after CO heat treatment at 850 ◦C for 30 min
with 20 cm3/min flow.
Fig. 8. SEM image of SrFe12 O19 powders after re-calcination at 1000 ◦C for 1 h.

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A. Yourdkhani et al. / Journal of Alloys and Compounds 470 (2009) 561–564
than that of the initial powder which is responsible for the very fine
structure of strontium hexaferrite powder re-formed again after a
re-calcination process.
Single-phase strontium hexaferrite powder with as aver-
age 38 nm crystallite size was successfully achieved by carbon
monoxide heat treatment of conventionally synthesized stron-
tium hexaferrite at 850 ◦C for 0.5 h with 20 cm3/min flow and
re-calcination at 1000 ◦C for 1 h.
The coercivity of strontium hexaferrite powder after carbon
monoxide heat treatment and re-calcination was increased signif-
icantly due to the decrease of the crystallite size of the powder
during these processes.
Acknowledgements
Financial and technical support from the University of Tehran,
the University of Birmingham and the Iranian Gas National Corpo-
ration are gratefully acknowledged.
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Fig. 9. TEM image of the SrFe12 O19 nano-crystalline powder after re-calcination at
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Strontium hexaferrite decomposed into hematite and strontium
oxide during the carbon monoxide heat treatment and the resul-
tant iron oxide was then reduced by carbon monoxide mainly to
metallic iron. This process made the microstructure much finer