Synthesis of FeTi from mixed oxide precursors

Article abstract of DOI:10.1016/j.jallcom.2008.07.018

A study was carried out on the synthesis of FeTi intermetallics from mixed oxide precursors using the method of electro-deoxidation. Fe₂O₃ and TiO₂ mixed in molar proportions of 1:2 were sintered at temperatures ranging fr

Full text of DOI:10.1016/j.jallcom.2008.07.018

Journal of Alloys and Compounds 475 (2009) 368–372
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of FeTi from mixed oxide precursors
∗
˙
Serdar Tan, Taylan Örs, M. Kadri Aydınol, Tayfur Öztürk , Ishak Karakaya
Department of Metallurgical and Materials Engineering, Middle East Technical University, 06531 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 April 2008
Received in revised form 3 July 2008
Accepted 7 July 2008
Available online 15 August 2008
A study was carried out on the synthesis of FeTi intermetallics from mixed oxide precursors using the
method of electro-deoxidation. Fe2O3 and TiO2 mixed in molar proportions of 1:2 were sintered at tem-
◦
◦
peratures ranging from 900 C to 1300 C. The sintered pellet of mixed oxides, connected as cathode, was
◦
then electrolyzed in a molten CaCl2 at 900 C using a graphite anode at a potential of 3.2 V. The electrolysis
yielded the target composition FeTi in substantial amounts only when the sintering temperature was close
◦
to or above 1100 C. The process of deoxidation was followed with the use of interrupted experiments.
Keywords:
This has shown that the two-phase structure of Fe2TiO5 and TiO2 in the oxide pellet reacts with the molten
salt even before the electrolysis forming CaTiO3, transforming the rest into a mixture of ilmenite (FeTiO3)
and a spinel phase (Fe2TiO4). During electrolysis these complex oxides were converted into simpler ones.
Fe is the first element to be produced followed by the intermetallic Fe2Ti. FeTi evolves quite late in the
electrolysis which seemed to have followed the reduction of CaTiO3. It is shown that interrupted exper-
iments and examination of partially reduced pellets yield considerable information with regard to the
sequence of changes that occur during the deoxidation process.
Electrodeoxidation
FeTi
Molten calcium chloride
Fe2TiO5
©
2008 Elsevier B.V. All rights reserved.
1
. Introduction
added consideration is the stoichiometry of the product itself. As
has been shown by Qiu et al. [7], oxides of the same mixture could
lead to different products depending on the sintering conditions. In
There has been considerable interest in recent years in the
solid-state deoxidation of oxides which would yield alloys and
compounds at targeted compositions, e.g. [1,2]. It is an electroly-
sis process in molten salt, first developed in the 1990s by Fray et
this study, the target compound, TbFe , can be obtained when the
2
◦
pellet was sintered at an elevated temperature, i.e. 1200 C.
The current work deals with the deoxidation of mixed oxides;
Fe O and TiO2 and concentrates on the choice of sintering con-
al. [3], for extraction of Ti from TiO . The process makes use of sin-
2
2
3
tered oxide pellet as a starting material, connected to a current
collector as the cathode and electrolyzed in a molten salt using a
graphite anode. During electrolysis, oxygen in the cathode is ion-
ized, dissolved in the electrolyte and discharged at the anode in the
ditions which would yield FeTi intermetallics. Here, the target
composition FeTi, is a well known compound that reversibly stores
hydrogen at room temperature [8].
2
.
Materials and methods
form of CO or CO . Thus unlike conventional electrolysis where the
2
metal is deposited onto the cathode, here the cathode is gradually
de-oxidized leaving the metallic elements behind.
Starting materials, Fe2O3 and TiO2 (rutile), were of technical grades. Fe2O3 pow-
ders were needle-like and TiO2 were rounded as given in Fig. 1. Both were roughly
1 ␮m in size but in an agglomerated state. A mixture of Fe2O3 and TiO2 in molar
proportions of 1:2 was prepared, i.e. Fe:Ti ratio of 1:1, using a Spex mill at ball-to-
powder ratio (B/P) of 1. After 30 min of mixing, the mixture was further hand mixed
adding some PVA solution and allowed to air dry for 24 h. The powder mixture was
then cold compacted into a cylindrical pellet at a pressure of 110 MPa. The pellets
were 18-mm in diameter and 2–3 mm in thickness weighing approximately 2 g.
Pellets were sintered at elevated temperatures. For this purpose they were
heated up in a tube furnace in air atmosphere to the predetermined temperatures
Due to this special nature of the cathode, the preparation of
oxide pellet as a cathode material has attracted considerable atten-
tion. In the case where the target is pure metal, e.g. Nb [4], Ta [5],
efforts are mainly concentrated on controlling the porosity of the
pellets. Normally pellets of high porosity are preferred since this
is believed to improve the kinetics of deoxidation. In this context,
especially in Ti, the formation of a perovskite phase, i.e. CaTiO3,
◦
with a heating rate of 5 C/min. The pellets were held at that temperature for 2 h.
which occurs as a result of reaction of TiO with the molten salt has
2
Electro-deoxidation experiments were conducted in an electrolytic cell, shown
schematically in Fig. 2. It comprises a stainless steel crucible for holding molten elec-
trolyte and electrodes mounted on a top cover (not shown in the figure) immersible
into the electrolyte. There were two pairs of electrodes; one pair was the auxil-
iary and the other was the working electrodes. The reactor was gas tight, allowing
electrolysis in an argon atmosphere (99.995% purity) maintained at a flow rate of
received considerable attention [6]. In the case of compounds, an
∗ Corresponding author. Tel.: +90 312 210 5935; fax: +90 312 210 2518.
E-mail address: ozturk@metu.edu.tr (T. Öztürk).
◦
150–250 ml/min. 1 kg of CaCl2 was used as electrolyte. This was heated to 900 C, i.e.
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.07.018

S. Tan et al. / Journal of Alloys and Compounds 475 (2009) 368–372
369
perature, the reactor lid was opened and the sample with its stainless steel connector
was removed. The sample which was in the form of sponge-like agglomerate was
washed in hot water and all undissolved material was collected for XRD and SEM
analysis.
3. Results and discussion
◦
◦
SEM images of pellets sintered at 900 C and 1300 C are given
in Fig. 3. It is seen that the structure which was highly porous at
◦ ◦ ◦
00 C (and also at 1100 C), was quite dense at 1300 C. The porosity
values were ≈47% and 45.1% with sintering at 900 C and 1100 C
9
◦
◦
Fig. 1. SEM images of oxide powders: (a) Fe2O3 and (b) TiO2 used in the experiments.
Note that Fe2O3 powders are needle-like whereas TiO2 are in the form of rounded
particles.
the electrolysis temperature, slowly so as to reduce its moisture content. The elec-
trolyte was further purified by pre-electrolysis using a graphite anode and stainless
steel cathode, at a potential of 3.0 V for 6 h. The current, during the pre-electrolysis
following an initial transient rise, decreases gradually over a period of typically 4 h,
finally settling down to a constant value. After pre-electrolysis, these auxiliary elec-
trodes were removed and the electrolysis was initiated by immersing the working
electrodes, i.e. oxide pellet (cathode) and a fresh graphite rod (anode) into the molten
salt. The electrolysis was carried out at a constant potential of 3.2 V for 24 h. Cur-
rent vs. time data were collected by a computer. The temperature of the system was
◦
controlled ± 10 C of the selected temperature.
Following electrolysis, the pellet and the graphite rod were removed from the
electrolyte by positioning them above the melt. After cooling it down to room tem-
◦
◦
Fig. 3. Micrographs of the oxide pellets sintered for 2 h at (a) 900 C and (b) 1300 C.
The graphs refer to broken pellets as viewed in the through thickness section, (c) is
the same as (b) but the section is polished and viewed in back-scattered mode to
reveal the two-phase structure.
Fig. 2. Schematic drawing of an electrolytic cell used in the deoxidation experi-
ments.

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S. Tan et al. / Journal of Alloys and Compounds 475 (2009) 368–372
Fig. 4. X-ray diffractograms of the oxide pellets Fe2O3 and TiO2 mixed in molar
◦
◦
proportions of 1:2 and sintered at (a) 900 C, (b) 1100 C. Note the presence of Fe2TiO5
Fig. 6. SEM image of metallic agglomerate, FeTi, obtained from deoxidation of mixed
oxide pellet sintered at 1300 C.
◦
◦
◦
along with Fe2O3 and TiO2 at 900 C. Also note that at 1100 C the sintered pellet has
a two-phase structure: Fe2TiO5 and TiO2.
◦
FeTi. Sintering at 900 C, however, was not successful in yielding
◦
respectively, whereas that arising from sintering at 1300 C had a
value of 17.3%.
the target composition in substantial amounts.
◦
◦
The sample sintered at 1100 C and 1300 C yielded mainly FeTi,
which is the target composition plus Fe2Ti as a minor phase. Fe
X-ray diffractograms of the sintered pellets are given in Fig. 4.
The diffractogram of the sample sintered at 900 C comprises the
◦
◦
which was quite dominant at 900 C was insignificant in these sam-
initial phases, i.e. Fe O3 and TiO , as well as a new phase; pseu-
ples. The same was true for Ti, implying that this element, upon
reduction, reacted with the existing phases yielding the target com-
position FeTi. The SEM image of one such sample is given in Fig. 6.
The conditions of sintering reported above for the successful
synthesis of FeTi are consistent with that reported in a similar study
conducted by Ma et al. [10]. The sintering in this study was carried
2
2
dobrookite (Fe TiO5). The formation of this phase via the reaction
2
of TiO2 with Fe O is expected since the associated Gibbs energy
2
3
change is negative at the current sintering temperatures [9]. In pel-
◦
lets sintered at 1100 C, Fe O is consumed totally by the reaction
2
3
with the result that the structure is made up of two phases; Fe TiO5
2
◦
and TiO , see Fig. 4(b). The same was also the case for the pellet sin-
out at 1050 C, which after electrolysis, yielded a product with X-ray
2
◦
tered at 1300 C. The two-phase structure is clearly visible in this
diffractogram very similar to the one reported in Fig. 5(b).
To follow the details of electro-deoxidation, samples sintered
latter sample, brought out by the back scattered image given in
Fig. 3(c).
◦
at 1300 C were selected for further study. A current–time plot for
X-ray diffractograms of the pellets after deoxidation are given in
Fig. 5. It was observed that deoxidation was successful in varying
degrees. The greater portion of the pellet sintered at 900 C after
deoxidation was Fe and a Ti bearing compound. The compound was
TiC, most probably formed, at the end of electrolysis, following the
reaction:
this sample covering the first 6 h of electro-deoxidation is given in
Fig. 7. It is seen that the current, following a rapid rise to a peak
value, drops down to a smaller value which then decreases grad-
ually with time. Three positions were selected on this curve for
structural examinations and accordingly three samples were pre-
pared. The samples which were maintained above the salt bath
during pre-electrolysis were then processed sequentially. One sam-
ple was immersed into the salt bath, maintained there for 30 min
and then lifted out. The others were electrolyzed as soon as they
were immersed; one for 30 min and the other for 6 h.
◦
Ti + 2CO = TiC + CO2
(1)
Here CO is the anode product (see below) which can accumulate
in the cell vessel and could react with Ti reduced from TiO2 at
the cathode. The minor constituents of the sample were Fe Ti and
2
◦
Fig. 5. X-ray diffractograms of the pellets deoxidized at 900 C for 24 h at 3.2 V. The
Fig. 7. Current–time data collected during deoxidation of the oxide pellet sintered
at 1300 C.
◦
◦
◦
diffractograms refer to pellets sintered at (a) 900 C, and (b) 1100 C.

S. Tan et al. / Journal of Alloys and Compounds 475 (2009) 368–372
371
reduction of Ti is quite sluggish. After 6 h of electrolysis, the greater
portion of Ti is still tied up in CaTiO3.
Using the thermodynamic data, the decomposition voltages of
various oxides have been calculated and are listed below with
descending order. The values here were determined for the reac-
◦
tions taking place at 900 C, the anode product being CO(g).
Fe TiO5 + 3C = 2Fe + TiO + 3CO, ꢀE = 0.212
(2)
(3)
(4)
(5)
(6)
2
2
Fe TiO + 2C = 2Fe + TiO + 2CO, ꢁE = 0.051
2
4
2
FeTiO + C = Fe + TiO + CO, ꢁE = 0.036
3
2
TiO + 2C = Ti + 2CO, ꢁE = −0.784
2
CaTiO + 2C = Ti + CaO + 2CO, ꢁE = −1.015
3
It should be noted that with the reactions (2)–(4) which give
rise to Fe have reduction potentials more positive than those which
give rise to Ti, i.e. the reactions (5) and (6). The early formation of
Fe and the persistence of Ti bearing oxides until the very end of
electrolysis are consistent with the above values. It is also worth
noting that CaTiO has a higher decomposition potential than pure
3
TiO2 which implies that the reduction process is made difficult by
CaTiO3 formation.
Electrolysis, with the sequence of changes described above,
may be contrasted with the sintering process. While during sin-
tering Fe O3 and TiO2 are combined into more complex oxides, i.e.
2
Fe TiO5, with electrolysis, the structure first attacked probably by
2
CaO which is normally present in CaCl2 as it forms during the dry-
ing process, is then gradually reduced into simpler oxides yielding
first Fe and then Ti. Even though the structure formed by sintering
is later disintegrated, it appears that the state, which had evolved
during sintering, exercises a considerable effect on the nature of the
reduction process. This effect of sintering is most probably related
to the length scale of the chemical species in the oxide preforms. At
◦
Fig. 8. X-ray diffractograms of the oxide pellets sintered at 1300 C and deoxidized
◦
◦
900 C, the sintered pellet is grossly heterogeneous in that it con-
in CaCl2 at 900 C (3.2 V). The diffractograms refer to pellets; (a) in the sintered
condition, (b) sintered and immersed into the salt bath for 30 min, (c), (d) and (e)
refer to sintered pellets electrolyzed for 30 min, 6 h and 24 h respectively.
tains unreacted Fe2O3 and TiO2 phases as well as Fe2TiO5. With the
porosity of 47%, elements that are obtained as a result of deoxida-
tion, i.e. Fe and Ti, especially from the unreacted phases, are not
close enough to react with one another. Accordingly deoxidation
yields a range of phases, namely Fe, Ti (i.e. TiC see above) as pure
The sequence of changes that occurred in the samples dur-
ing electrolysis can be seen in Fig. 8. The X-ray diffractogram
shows a systematic change from the sintered condition to that after
elements, and Fe Ti and FeTi as intermetallics.
2
the 24 h electrolysis. In the immersed sample, Fe TiO5 and TiO ,
At higher sintering temperatures, the reaction goes to comple-
2
2
i.e. the phases in the sintered sample, continue to be the major
constituents, but there are also other phases. These are ilmenite
tion. Thus all Fe O3 is consumed and the preform is made up of
2
two phases Fe TiO5 and TiO . The structure, therefore, is more
2
2
(
FeTiO ), a spinel phase (Fe TiO ), and a perovskite phase (CaTiO ).
homogenous than that given above. Even though this structure is
modified upon immersion into the salt bath, this modification prob-
ably occurs with a length scale that is comparable to the original
structure. Thus upon reduction, elements are close to each other
and with diffusion over small distances they react with each other
yielding the intermetallics. The result, at the end of electrolysis, is
that FeTi makes up the considerable portion of the product.
As a final remark, it should be pointed out that, with the current
3
2
4
3
After 30 min of electrolysis, the phases did not change much, though
CaTiO3 is much stronger and a metallic phase Fe is already present
in the sample. After 6 h, the Fe TiO5 and TiO2 are all consumed.
Fe is now the dominant phase accompanied by the intermetallic
Fe Ti. The target composition FeTi can be identified in this sample,
2
2
but it is far from being a major phase. It is interesting to note that
CaTiO3 continues to be the major phase in this sample. It appears
that beyond this, the electrolysis mainly consumes CaTiO and with
processing conditions, the phases Fe Ti and a small amount of Ti
3
2
the production of Ti as a result, the greater portion of the sample is
converted into FeTi, see Fig. 8(e).
The observations reported above imply that, from the oxide state
to the final composition FeTi, electroreduction follows quite a sys-
tematic route. It appears that the oxides are already modified upon
immersion into the salt bath even before the electrolysis. Thus a
(i.e. TiC) do form in the final product. Since the target composition
is FeTi, the formation of these other phases need to be minimized
or should be prevented altogether. Following the approach given
above, it may be suggested that one method of achieving this,
would be to make use of mixed oxides with refined structures, i.e.
to employ a processing route that would yield the mixed oxide pel-
let with as fine a structure as possible. The other route would be to
two-phase structure, i.e. Fe TiO5 and TiO , is in part converted into
2
2
a mixture of phases comprising CaTiO , FeTiO3 and Fe TiO
produce a single-phase pellet, namely ilmenite (FeTiO ). It appears
3
2
4.
3
With the progress of electrolysis, as the oxygen is discharged,
these complex oxides are consumed and instead the remaining oxy-
gen is tied up in simpler oxides such as FeO and CaO. It appears that
Fe is reduced quickly from the iron bearing oxides. In contrast, the
that the ilmenite approach, with Fe:Ti in 1:1 proportion, would be
particularly worthwhile since this would make the electrolysis pos-
sible in a variety of sintering conditions. For instance, pellets of
increased porosity could be electrolyzed which might accelerate

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S. Tan et al. / Journal of Alloys and Compounds 475 (2009) 368–372
the reduction process, an approach which would create difficulties
in the mixed oxide approach.
Acknowledgements
We would like to thank Prof. Derek J. Fray for useful discus-
sions. Support for this work was provided by DPT with project
number BAP-03-08-DPT.200305K120920-20, which we gratefully
acknowledge. The authors also thank Cara Keyman for reading the
manuscript.
4
. Conclusion
In the current work, a study was carried out on the synthe-
sis of FeTi intermetallics from mixed oxide precursors. Fe O3 and
2
TiO2 mixed in molar proportions of 1:2 were sintered at elevated
◦
◦
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◦
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