A novel technique to extract Bi from mechanochemically prepared Bi-Fe 3O4 nanocomposite

Article abstract of DOI:10.1016/j.physb.2005.10.095

The solid-state reduction of Bi₂O₃ to bismuth (Bi) nanoparticles by high-energy ball milling of raw materials (Bi₂O ₃ and Fe) in air and argon atmospheres has been described. XRD results show that in addition to bismuth, a second phase of nanocrystalline magnetite is also formed. This is due to the formation of Fe₂O ₃ and the subsequent change to Fe₃O₄ in the course of ball milling. Mean particle sizes of the obtained Bi and Fe ₃O₄ particles were 22 and 18 nm, respectively, using Scherrer's formula. A saturation magnetization of 80 emu/g is achieved for magnetic phase (Fe₃O₄). As both Bi and magnetite were nanosized particles, it was not possible to separate these two phases by the magnetic separation technique. A novel technique based on different thermal expansions of the Bi and Fe₃O₄ was then used to extract metallic Bi from the as-milled powders.

Full text of DOI:10.1016/j.physb.2005.10.095

ARTICLE IN PRESS
Physica B 371 (2006) 309–312
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A novel technique to extract Bi from mechanochemically prepared
Bi–Fe3O4 nanocomposite
Ã
M. Mozaffaria,b, , J. Amighiana,b, A. Hasanpoura,b,c
aPhysics Department, Faculty of Sciences, Isfahan University, Isfahan 81746-73441, Iran
bNanophysics Research Group, Research Center for Nanosciences and Nanotechnology, The University of Isfahan, Isfahan, Iran
cPhysics Department, Faculty of Sciences, Payame Noor University, Tehran, Iran
Received 12 July 2005; received in revised form 19 October 2005; accepted 19 October 2005
Abstract
The solid-state reduction of Bi2O3 to bismuth (Bi) nanoparticles by high-energy ball milling of raw materials (Bi2O3 and Fe) in air and
argon atmospheres has been described. XRD results show that in addition to bismuth, a second phase of nanocrystalline magnetite is
also formed. This is due to the formation of Fe2O3 and the subsequent change to Fe3O4 in the course of ball milling. Mean particle sizes
of the obtained Bi and Fe3O4 particles were 22 and 18 nm, respectively, using Scherrer’s formula. A saturation magnetization of 80 emu/g
is achieved for magnetic phase (Fe3O4). As both Bi and magnetite were nanosized particles, it was not possible to separate these two
phases by the magnetic separation technique. A novel technique based on different thermal expansions of the Bi and Fe3O4 was then used
to extract metallic Bi from the as-milled powders.
r 2005 Elsevier B.V. All rights reserved.
PACS: 81.20.Ym; 75.50.Dd; 81.20.Ev; 65.40.De; 82.33.Pt
Keywords: Purification; Ferrimagnetics; Powder processing; Thermal expansion; Solid-state chemistry
1. Introduction
with Fe and then by a novel technique, metallic Bi
component was extracted. This is due to the fact that Fe
strongly reduces Bi2O3 with regards to a negative enthalpy
(DH ¼ ꢀ58:5 kJ=mol) associated with the following oxido-
reduction reaction:
Mechanical alloying is basically a dry high-energy
milling process, which has been primarily devoted to the
production of composition metallic powders with a fine
controlled microstructure [1–3]. This process has been used
since 1960 and its field of application and scientific scope
has been increased very rapidly [4]. In addition to alloys,
various metallic materials, such as intermetallic com-
pounds [5–7], magnets [8,9] and nanocomposite powders
[10–12], etc., have been prepared by this method. The
mechanochemistry of inorganic substances has also been
widely studied in order to understand the change in the
reactivity of solids by and after action of mechanical energy
[13,14]. In this work, mechanical alloying was used to
prepare Bi–Fe3O4 nanocomposite by reduction of Bi2O3
Bi2O3 þ 2Fe ! 2Bi þ Fe2O3;
and can explain the self-sustained combustion of the
reactants during mechanical alloying [3].
2. Experimental
The raw materials were powders of Bi2O3 and Fe both
from E. Merck Co., with minimum purity of 99%. The
powders were weighed in 1:2-mole ratio, respectively, with
10% Fe deficiency to compensate iron wear in the course of
milling. A total of 30 g of the powders together with
different sizes of hardened-steel balls were loaded into an
air-filled hardened-steel vial (500 ml). The process was also
repeated in an argon-filled vial, using a glove bag. The
Ã
Corresponding author. Physics Department, Faculty of Sciences,
Isfahan University, Isfahan 81746-73441, Iran. Tel.: +98 311 793 2441;
fax: +98 311 793 2409.
E-mail address: mozafari@sci.ui.ac.ir (M. Mozaffari).
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2005.10.095

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M. Mozaffari et al. / Physica B 371 (2006) 309–312
310
milling times in both processes were up to 240 min in a
high-energy planetary mill (FRITSCH ‘‘Pulverisett6’’). The
number and size of the balls were chosen so that a ball to
powder ratio of 10 was achieved. In order to maintain an
equal powder to ball mass ratio in all experiments, the vial
was cleaned and reloaded with 30 g of new raw materials.
Phase formation of the as-milled composition was studied
by a diffractometer (BRUKER, D8 model) using Cu Ka
and Fe3O4 in the composite were calculated from the XRD
peak broadening, using the Scherrer’s formula. Magnetic
measurements were carried out on cold pressed as-milled
samples, using a vibrating sample magnetometer (VSM).
Magnetic separation was attempted to separate the
magnetic phase (Fe3O4) from the nonmagnetic phase (Bi),
but this was unsuccessful. A novel technique based on
different thermal expansions of the Bi and Fe3O4 was then
used to extract metallic Bi from the as-milled powder. In
˚
radiation (l ¼ 1:5405 A). The mean particle sizes of the Bi
120
Bi=Bismuth
44-1246
19-0629
Bi
100
M= Magnetite
80
60
40
Bi
Bi
20
0
Bi
Bi
Bi
Bi
M
Bi
Bi
M
Bi
M
Bi
20
30
40
50
2theta
60
70
80
Fig. 1. XRD pattern of the as-milled product.
Fig. 2. Magnetization curve of the as-milled composite.

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311
6000
5000
4000
3000
2000
1000
0
20
30
40
50
60
70
80
90
100
Fig. 3. XRD pattern of the extracted Bi. All peaks belong to a pure metallic Bi.
40
35
30
25
20
15
10
5
3. Results and discussion
The XRD pattern of the as-milled powder, Fig. 1, shows
that a nanocomposite of only two phases, bismuth (Bi) and
magnetite (Fe3O4), has been formed. The mean particle
sizes of the Bi and Fe3O4 were 22 and 18 nm, respectively,
using Scherrer’s formula. Fig. 2 shows the magnetization
curve of the as-milled composite. A saturation magnetiza-
tion of 142 mT was obtained, which is related to the
magnetic portion of the nanocomposite. Using Fig. 2 and
calculations based on the weight percent of each phase, a
saturation magnetization of 80 emu/g is achieved for
magnetic phase (Fe3O4). This of course is smaller than
the value of 90 emu/g, related to bulk Fe3O4. This is due to
the fact that as the particle size is inclined, the value of
magnetization decreases [15].
Magnetic separation was attempted to separate the
magnetic phase (Fe3O4) from the nonmagnetic Bi phase.
This was not successful, because both phases were
agglomerated together and precipitated simultaneously by
the magnetic driving force of a strong magnet located
beneath the beaker containing the mixture of distilled
water and the nanocomposite. The cause of the simulta-
neous precipitation is that during attraction of the
magnetite particles, they will collide with Bi nanoparticles
and push them towards the bottom of the beaker. In order
to obtain Bi metallic phase, the nanocomposite was pressed
into pellets, and heated in a vacuum-sealed quartz tube. It
was observed that a pure metallic Bi phase had been
squeezed out of the body of the pellets, which was carefully
separated from the samples. The reason for the squeezing
was that thermal expansion coefficients of Bi and Fe3O4 are
different and is higher for metallic Bi. The XRD pattern of
the squeezed Bi has been shown in Fig. 3. As it can be seen
from this pattern, all the peaks belong to a pure Bi phase.
Fig. 4 shows the variation of extraction percentage of the
metallic Bi as a function of temperature for a constant
0
0
200
400
600
800
1000
T(°C)
Fig. 4. Variation of extraction percentage of the metallic Bi with
temperature for constant soaking time (1 h) and constant shaping pressure
(7 kbar).
50
40
30
20
10
0
0
2
4
6
8
10
12
p(kbar)
Fig. 5. Extraction percentage of metallic Bi as a function of pressure
heated at 700 1C.
this technique, the as-milled powder was shaped into pellets
of about 10 mm in diameter and about 6 mm in height
under different pressures (1–10 kbar). The pressed samples
were then heated at different temperatures up to 800 1C
with different soaking times of 15, 60, 120 and 180 min in a
vacuum-sealed quartz tube in order to avoid any oxidiza-
tion of the metallic Bi. In the course of the heating process,
the molten Bi was squeezed out of the samples and then
removed and characterized carefully.

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20
15
10
5
nanocomposite. In order to maximize the amount of Bi
extracted from the nanocomposite, the parameters such as
temperature, pressure and soaking time were optimized.
The results show that the optimum temperature is 700 1C,
but the values for pressure and soaking time should be
chosen as low as possible.
0
0
1
2
3
4
t(hour)
Acknowledgement
Fig. 6. Evaporation percentage of Bi as a function of soaking time (with a
constant annealing temperature of 700 1C).
This study was completed at The University of Isfahan
and supported by The Office of Graduate studies. The
authors are grateful to this office for their support.
soaking time (1 h) and a constant shaping pressure (7 kbar).
This shows that the driving force to push the Bi out of the
nanocomposite is stronger at higher temperatures up to
700 1C. At higher temperatures, as can be seen in Fig. 4, the
extraction percentage reaches a nearly constant value and it
is not necessary to heat the samples above 700 1C. Fig. 5
shows the variation of extraction percentage of Bi at
different shaping pressures. As can be seen, the extraction
of the Bi is more or less independent of the applied
pressure. It was also found that the shorter soaking times
are much better than longer ones, because the latter will
increase the evaporation percentage of the Bi phase, Fig. 6.
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4. Conclusion
Bi–Fe3O4 nanocomposite has been formed during
mechanochemical processing of Fe and Bi2O3 powders as
the starting materials. The phase formation was checked by
XRD. VSM measurements also show that there is a
magnetic phase in the nanocomposite. A novel technique
has been established to extract metallic Bi from the
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