Synthesis of TiN from FeTiO3 by microwave-assisted carbothermic reduction-nitridation

Article abstract of DOI:10.1016/j.jallcom.2013.08.128

Titanium nitride (TiN) was synthesized from ilmenite (FeTiO₃) through the microwave-assisted carbothermic reduction-nitridation followed by acid leaching treatment. The predominance area diagrams of Ti-Fe-N-C-O system at different temperatures

Full text of DOI:10.1016/j.jallcom.2013.08.128

Journal of Alloys and Compounds 583 (2014) 121–127
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Synthesis of TiN from FeTiO3 by microwave-assisted carbothermic
reduction–nitridation
⇑
Juanjian Ru, Yixin Hua , Cunying Xu, Qibo Zhang, Ding Wang, Kai Gong
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2013
Received in revised form 18 August 2013
Accepted 19 August 2013
Available online 3 September 2013
Titanium nitride (TiN) was synthesized from ilmenite (FeTiO3) through the microwave-assisted carbo-
thermic reduction–nitridation followed by acid leaching treatment. The predominance area diagrams
of Ti–Fe–N–C–O system at different temperatures were established by Outokumpu HSC Chemistry to
analyze the stability of the species concerned. It is demonstrated that titanium and iron oxides can be
transformed into TiN–Fe composite by controlling suitable CO and N2 partial pressures at high temper-
atures. The TiN–Fe composite can be prepared from FeTiO3 with microwave heating at 1000 °C for
40 min. TiN was obtained with the removal of iron from TiN–Fe composite by acid leaching at room tem-
perature. The reaction mechanism for the carbothermic reduction–nitridation process was proposed
according to experimental results.
Keywords:
Titanium nitride (TiN)
Ilmenite (FeTiO3)
Microwave
Carbothermic reduction–nitridation
Predominance area diagram
Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction
atmosphere [14,16–18]. FeTiO3 is the most important titanium
ore which is a naturally occurring and economically mineral with
TiN has been recently attracted increasing interest as a coating
material on cutting tools, aircrafts, jigs and fixtures due to its excel-
lent characters such as high hardness, high melting temperature,
and good thermal and electrical conductivity [1–4]. The existence
of hard TiN particles among the materials increases the wear resis-
tance, shear modulus, and creep property.
a known world total resource of ꢀ2 ꢁ 1012 kg [19]. From the view-
point of thermodynamics, FeTiO3 can be easily reduced to TiN in N2
atmosphere by carbon powder compared with TiO2 raw material.
Therefore, it is likely to be more economical for the production of
TiN powder with superior quality. However, with traditional heat-
ing, a long time up to 21 h and high reaction temperature caused
by slow diffusion rates in solids is required to obtain TiN [20–
22]. It is necessary to find other more promising and effective heat-
ing ways of promoting the reduction process. In recent years there
has been a growing interest of microwave heating in the field of
material synthesis due to rapid heating, selective material cou-
pling, and enhanced reaction kinetics [23–26]. It has been found
that FeTiO3 and carbon powder have strong ability to absorb
microwave energy and can be preferentially and rapidly heated
to elevated temperatures [27]. Therefore, the applications of
microwave irradiation to the carbothermic reduction–nitridation
of FeTiO3 would be advantageous in shorting reaction time, reduc-
ing operation temperature and accelerating reactions.
These applications are calling for the synthesis of TiN powders
with homogeneous chemical composition and small grain size. A
number of physical and chemical methods have been reported in
literatures for synthesizing nitride. TiN can be produced by direct
nitridation of metallic Ti in N2 atmosphere [5–7], carbothermal
reduction of TiO2 in N2 atmosphere [8,9] or mechanic alloying
and subsequent heat treatment [10,11]. In the last few years, nano-
meter TiN powder could be synthesized by carbothermal reduction
nitridation [12], self-propagating high temperature synthesis [3],
low temperature mechanochemical formation [13], pyrolysis of
poly(titanylcar-bodiimide) [14], and microwave plasma chemical
vapor deposition [15]. Although these methods are considered suc-
cessful from a commercial point of view, they require expensive
raw materials, high temperatures, long reaction time and/or
expensive equipment. Another method that has attracted consider-
able attention is the carbothermic reduction of FeTiO3 in N2
In this paper, the microwave heating has been used to the car-
bothermic reduction–nitridation of FeTiO3 with the aim of obtain-
ing TiN–Fe composite with lower operating temperature in N2
atmosphere. The products prepared were purified by acid leaching
process to remove metallic Fe. Its phase transition procedure and
influence factors including reaction temperature and reaction time
were investigated.
⇑
Corresponding author. Tel.: +86 871 65162008; fax: +86 871 65161278.
E-mail address: huayixin@gmail.com (Y. Hua).
0925-8388/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.08.128

122
J. Ru et al. / Journal of Alloys and Compounds 583 (2014) 121–127
3. Experimental
2. Thermodynamics analysis
3.1. Materials
The synthesis of TiN from FeTiO3 includes the preparation of
TiN–Fe composite from FeTiO3 by carbothermic reduction–nitrida-
tion and the removal of Fe form the prepared TiN–Fe composite by
acid leaching. The carbothermic reduction–nitridation process for
synthesizing TiN–Fe composite from FeTiO3 is a very complex pro-
cess which involves a series of intermediate reactions. The possible
reactions in the process and the corresponding changes in Gibbs
free energy are shown in Table 1. The proceeding sequence of the
reduction reactions depends on the CO partial pressure, while the
formation of TiN depends on both N2 and CO partial pressures. In
order to describe the effect of N2 and CO partial pressures on the
formation of TiN–Fe composite, the predominance area diagrams
for the system Ti–Fe–N–C–O at 800 °C, 900 °C and 1000 °C are
depicted by Outokumpu HSC Chemistry as shown in Fig. 1, where
PCO and PN2 are the equilibrium partial pressures of CO and N2 in
gas phase, and P° is standard atmospheric pressure.
Fig. 1 demonstrates that the TiN–Fe composite can be prepared
from the oxides by controlling CO and N2 partial pressures at
elevated temperatures, as shown in the shaded areas in the figures.
By comparing the shaded areas in Fig. 1, it can be seen that curve
abdefg shifts upward gradually as the temperature rises. This im-
plies that the rising of temperature is favorable for the formation
of TiN–Fe composite. In addition, the TiN formed is in equilibrium
with carbon and oxygen via reaction C + 1/2O2 = CO [28], hence the
residual oxygen and carbon as interstitial atoms may exist in the
TiN lattice, which will result in the formation of N, C and O contain-
ing phases, such as TiC, TiNxC1ꢂx (0 6 x 6 1), TiCyO1ꢂy (0 6 y 6 1)
and TiNxCyO1ꢂxꢂy (x + y = 1; x, y P 0). Due to lack of the thermody-
namic data for TiNxC1ꢂx, TiCyO1ꢂy and TiNxCyO1ꢂxꢂy, their possible
predominance areas are speculated in accordance with literatures
[28,29] and the principles of phase equilibrium, as shown in
Fig. 1. Lines ck, ij, bi and hi are the phase boundary between
The reduction experiments were conducted with mixtures of FeTiO3 and carbon
powders (99 wt% C). FeTiO3 powder was synthesized via co-precipitation combined
with microwave calcination method and the XRD revealed no evidence of other
phases being present. The particle size of FeTiO3 and carbon powders are both
ꢀ75
lm. High purity nitrogen (99.99 wt% N2) which was dried using silica gel
and anhydrous potassium hydroxide was used to maintain a reducing atmosphere
with a flow rate of 100 mL/min in all experiments.
3.2. Procedures
Before mixing, all the reagents were dried in an oven at 100 °C for 24 h to re-
move the moisture existed in the raw materials. The samples with a 1:3 M ratio
of FeTiO3 to carbon powder were thoroughly mixed and ground with 6 wt% polyvi-
nyl alcohol (PVA). Then the mixtures were pressed with a pressure of 10 MPa in a
closed die to obtain a cylindrical pellet (2 mm in height and 13 mm in diameter).
The reduction experiments were carried out in a tubular high-temp microwave
reactor (2.45 GHz, 3 kW) under nitrogen atmosphere. The pellets were placed in a
crucible which was enclosed in a quartz tube (100 cm in length and 8 cm in diam-
eter) and then subjected to microwave irradiation. The sample temperatures with
microwave heating were determined by inserting a metallic-sheathed Ni(Cr) ther-
mocouple into the center of sample. The pellets reduced were ground to ꢀ75
lm
and then leached by HCl solution in a mechanical-agitated vessel for 24 h at room
temperature. The agitation speed was held at 500 rpm and the ratio of solid to
liquid was maintained constant at 1:4 for all the leaching experiments. The initial
concentrations of HCl in leaching solution were 30 wt%. After filtering, the remain-
ing powder was washed several times with distilled water until the pH is about 7,
and then dried in a vacuum oven at 100 °C for 12 h.
3.3. Characterization
The products were analyzed by XRD (D/Max-2200 model) with Cu Ka radiation
at a scan rate of 10o/min in the range of 2h = 10–90°. The product X-ray diffraction
results were used for determining the lattice parameters of the TiN phase for a
given set of processing conditions. SEM (XL 30 ESEM TMP model) was used to
estimate the particle size of samples and characterize the morphology of products.
Ti3O5 and TiCyO1ꢂy, TiNxC1ꢂx and TiN, TiNxCyO1ꢂxꢂy and TiNxC1ꢂx
,
TiNxCyO1ꢂxꢂy and TiN, respectively. The values of x, y, and z are
determined by composition of N, C, and O. The locations of these
lines in the diagram depend on the reaction equilibrium constant
K and the values of x, y, and z. Fig. 1 also shows that the reduction
of iron oxides is dependent on CO partial pressure and independent
off N2 partial pressure, while the formation of TiN from TiO2 is
determined by CO and N2 partial pressures at the same time. For
example, TiO2 is firstly reduced to sub-oxides and then reacted
with carbon and nitrogen to form TiNxCyO1ꢂxꢂy, which is further
nitridized to TiN at 1000 °C when lg(PN2/P°) is lower than 2.88.
According to Fig. 1, the phase transition sequence of TiO2 as a
function of N2 partial pressure can be obtained, as shown in Table 2.
It is observed that the intermediate steps from TiO2 to TiN are
increased with decreasing in N2 partial pressure.
4. Results and discussion
As mentioned above, the carbothermic reduction–nitridation of
FeTiO3 is a complex process. In order to study the effect of micro-
wave heating on the phase changes occurring during the reduction
process, the XRD patterns of reaction products for FeTiO3:C = 1:3
after microwave-assisted carbothermic reduction–nitridation at
800 °C, 900 °C and 1000 °C for different reaction time (10, 20, 40,
and 60 min) are shown in Fig. 2. Depending on atmosphere, tem-
perature and reaction time, FeTiO3 can be reduced and nitridized
to different products such as Fe, TiO2, Ti2O3, Ti4O7, TiN. From
Fig. 2, it is found that FeTiO3 can be transformed into TiN–Fe com-
posite, which is in a good agreement with the results predicted by
thermodynamics.
Table 1
Equations for the changes in Gibbs free energy and lg(PCO/P°)–lg(PN2/P°) relationshipsa.
GoT (J)
Chemical reaction
lg(PCO/P°)–lg(PN2/P°) relationships
D
800 °C
900 °C
1000 °C
FeO + C = Fe + CO(g)
111505–154.87T
195676–188.94T
284019–251.51T
245699–208.90T
1101300–836.84T
622144–526.34T
lg(PCO/P°) = 0.61
lg(PN2/P°) = ꢂ2.04
lg(PN2/P°) = ꢂ3.35
lg(PN2/P°) = ꢂ3.77
lg(PN2/P°) = ꢂ4.18
lg(PCO/P°) = 1.24
lg(PN2/P°) = ꢂ1.06
lg(PN2/P°) = ꢂ2.13
lg(PN2/P°) = ꢂ2.54
lg(PN2/P°) = ꢂ3.09
lg(PCO/P°) = 1.78
lg(PN2/P°) = ꢂ0.27
lg(PN2/P°) = ꢂ1.06
lg(PN2/P°) = ꢂ1.51
lg(PN2/P°) = ꢂ2.18
9TiO2 + C = Ti9O17 + CO(g)
Ti9O17 + 5/4C = 9/4Ti4O7 + 5/4CO(g)
3Ti4O7 + C = 4Ti3O5 + CO(g)
Ti3O5 + 8C = 3TiC + 5CO(g)
2TiO2 + 4C + N2(g) = 2TiN + 4CO(g)
lg(PN2/P°) = 0.25ꢃlg(PN2/P°)–2.43 lg(PN2/P°) = 0.25ꢃlg(PN2/P°)–1.65 lg(PN2/P°) = 0.25ꢃlg(PN2/P°)–0.99
Ti9O17 + 17C + 9/2N2(g) = 9TiN + 17CO(g) 2603970–2179.58T lg(PN2/P°) = 0.27ꢃlg(PN2/P°)–2.46 lg(PN2/P°) = 0.27ꢃlg(PN2/P°)–1.68 lg(PN2/P°) = 0.27ꢃlg(PN2/P°)–1.03
Ti4O7 + 7C + 2N2(g) = 4TiN + 7CO(g)
Ti3O5 + 5C + 3/2N2(g) = 3TiN + 5CO(g)
2TiN + 2C = 2TiC + N2(g)
1031090–856.92T
711893–590.46T
259602–164.25T
lg(PN2/P°) = 0.29ꢃlg(PN2/P°)–2.38 lg(PN2/P°) = 0.29ꢃlg(PN2/P°)–1.65 lg(PN2/P°) = 0.29ꢃlg(PN2/P°)–1.03
lg(PN2/P°) = 0.3ꢃlg(PN2/P°)–2.31
lg(PN2/P°) = ꢂ6.21
lg(PN2/P°) = 0.3ꢃlg(PN2/P°)–1.60
lg(PN2/P°) = ꢂ4.97
lg(PN2/P°) = 0.3ꢃlg(PN2/P°)–1.01
lg(PN2/P°) = ꢂ3.92
a
For pure solid phases, the all activities of them are assumed to be 1. All thermodynamic data used above comes from Outokumpu HSC Chemistry.

J. Ru et al. / Journal of Alloys and Compounds 583 (2014) 121–127
123
Fig. 1. The predominance area diagram in Ti–Fe–N–C–O system at (a) 800 °C, (b) 900 °C, and (c) 1000 °C. (solid line and dash line: Ti–N–C–O; dash dot line: Fe–N–C–O).
Table 2
The relationship between phase transition sequence and lg(PN2/P°) at different temperaturesa.
Phase transition sequence
lg (PN2/P°) range at different temperatures
800 °C
900 °C
1000 °C
TiO2 ? Ti9O17 ? Ti4O7 ? Ti3O5 ? TiCyO1ꢂy ? TiC
TiO2 ? Ti9O17 ? Ti4O7 ? Ti3O5 ? TiCyO1ꢂy ? TiNxCyO1ꢂxꢂy ? TiNxC1-x
TiO2 ? Ti9O17 ? Ti4O7 ? Ti3O5 ? TiCyO1ꢂy ? TiNxCyO1ꢂxꢂy ? TiN
TiO2 ? Ti9O17 ? Ti4O7 ? Ti3O5 ? TiNxCyO1ꢂxꢂy ? TiN
TiO2 ? Ti9O17 ? Ti4O7 ? TiNxCyO1ꢂxꢂy ? TiN
TiO2 ? Ti9O17 ? TiNxCyO1ꢂxꢂy ? TiN
ꢂ8 ꢀ ꢂ6.21
ꢂ6.21 ꢀ a1
a1 ꢀ b1
ꢂ8 ꢀ ꢂ4.97
ꢂ4.97 ꢀ a2
a2 ꢀ b2
ꢂ8 ꢀ ꢂ3.92
ꢂ3.92 ꢀ a3
a3 ꢀ b3
b1 ꢀ ꢂ4.85
ꢂ4.85 ꢀ ꢂ3.38
ꢂ3.38 ꢀ 1.56
1.56 ꢀ 4
b2 ꢀ ꢂ3.12
ꢂ3.12 ꢀ ꢂ1.69
ꢂ1.69 ꢀ 2.36
2.36 ꢀ 4
b3 ꢀ ꢂ1.67
ꢂ1.67 ꢀ ꢂ0.12
ꢂ0.12 ꢀ 2.88
2.88 ꢀ 4
TiO2 ? TiNxCyO1ꢂxꢂy ? TiN
a
a1,
a
2 and
a3 are defined as the lg(PN2/P°) of point i at 800 °C, 900 °C and 1000 °C respectively in Fig. 1. b1, b2 and b3 are defined as the lg(PN2/P°) of point c at 800 °C, 900 °C
and 1000 °C respectively in Fig. 1.

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J. Ru et al. / Journal of Alloys and Compounds 583 (2014) 121–127
Fig. 3. The peak intensity of X-ray diffraction pattern of different product phases
after microwave-assisted carbothermic reduction–nitridation for 20 min.
intensities of different product phases for reaction time of 20 min
as a function of temperature is shown in Fig. 3, where the intensity
of the strongest XRD peak of Fe is used as a measure of the relative
amounts present. It is obvious that the peak intensity of TiN in-
creases gradually with the rising of reaction temperature whereas
those of titanium oxides, such as Ti9O17, Ti7O13, Ti2O3, TiO2 and
Ti4O7 decrease. Therefore, increasing in temperature is favorable
for the formation of TiN.
The lattice parameters of synthesized TiN were calculated from
the X-ray diffraction peaks by MDI Jade. The lattice parameter
values calculated as a function of temperature is shown in Fig. 4.
It is evident that when temperature is elevated from 900 °C to
1000 °C, the lattice parameter of the TiN phase formed in 20 min
increases sharply. This can be contributed to the fact that the C
atoms dissolution in the interstitial sites in TiN structure result
in an increasing of the lattice parameter [28]. By 1100 °C, the lat-
tice parameters decrease for all samples. On the one hand, the con-
stant removal of CO from the system while keeping a constant
pressure of N2 will result in the replacement of C by N in the TiNx-
C1ꢂx lattice to form TiN, which is indicated by the decrease in the
lattice parameter [30]. On the other hand, the rising of temperature
is benefit to the formation of TiN from the predominance diagrams.
The above characteristics in variation of titanium-containing
phases can be analyzed by predominance area diagrams of Ti–N–
C–O system. It can be seen from Fig. 1 that as the CO partial pres-
sure decreases, TiO2 will be reduced to TinO2nꢂ1, TiCyO1ꢂy and TiC
successively. These titanium-containing intermediates formed
can react with N2 gas at the same time to form TiNxCyO1ꢂxꢂy, TiNx-
C1ꢂx and TiN in the presence of carbon. The experimental results
from Table 3 show that the main titanium-containing phases of
products are TiN, Ti9O17, Ti4O7, Ti2O3 and Ti7O13, but none of
Ti3O5, TiNxC1ꢂx, TiCyO1ꢂy, TiNxCyO1ꢂxꢂy and TiC was observed. This
is because the stable region of Ti3O5 is very narrow as shown in
Fig. 1 and thence Ti3O5 is not stable and can be easily nitridized
to TiN as the N2 partial pressure increases. Moreover, the formation
of TiNxC1ꢂx, TiCyO1ꢂy, TiNxCyO1ꢂxꢂy and TiC need to be carried out
Fig. 2. X-ray diffraction pattern for FeTiO3:C = 1:3 (carbon powder) over a range of
temperatures and time periods in N2 atmosphere indicated in the figure. (N – Fe, ꢀ –
TiC, H – TiO2, j – Ti2O3, d – Ti4O7, } – Ti7O13
,
– Ti9O17, 4 – C, h – FeTiO3).
4.1. Effect of reaction temperature on products
According to Fig. 2, the effect of reaction temperature on prod-
ucts can be obtained, as shown in Table 3. It is observed from
Table 3 that the main phases in the products are Fe, Ti9O17
,
Ti7O13, Ti4O7, Ti2O3, and TiO2 at 800 °C. This indicates that iron
can be easily reduced from FeTiO3 to metallic iron and the reduc-
tion rate is very fast, while TiO2 can be partially reduced to
titanium sub-oxides only. However, no TiN is observed in the prod-
ucts at 800 °C, implying that the nitridation of titanium oxide and
sub-oxides need to be carried out at higher temperatures. When
temperature is elevated to 900 °C, in addition to Fe and titanium
oxides, a small amount of TiN begins to form. At 1000 °C, the
amount of TiN increases rapidly whereas those of titanium oxides
decrease gradually, indicating that the rising of temperature is in
favor of the transformation of titanium oxides into TiN. By
1100 °C, only Fe and TiN are present which means that FeTiO3
has already transformed into TiN completely.
In order to observe the variation of characteristic peak intensity
of XRD with reaction temperature clearly, the strongest XRD peak
Table 3
Phases observed after microwave-assisted carbothermic reduction–nitridationa.
Time (min)
Temperature (°C)
800
900
1000
1100
10
20
40
60
–
–
FeTiO3, Fe, TiO2, Ti9O17
Fe, TiN, Ti9O17, Ti4O7, TiO2, Ti2O3, Ti7O13
Fe, TiN
Fe, TiN
–
Fe, Ti9O17, Ti7O13, Ti2O3, TiO2, Ti4O7
–
–
Fe, Ti9O17, Ti2O3, TiO2, Ti4O7,Ti7O13, TiN
–
–
,
Fe, TiN
Fe, TiN
Fe, TiN
a
Phases are arranged in a decreasing order of relative diffraction peak intensities.

J. Ru et al. / Journal of Alloys and Compounds 583 (2014) 121–127
125
Fig. 4. Effect of temperature on the lattice parameters of synthesized TiN.
Fig. 5. The peak intensity of X-ray diffraction pattern of different product phases
after microwave-assisted carbothermic reduction–nitridation at 1000 °C.
at much lower CO partial pressure. Besides, they will be also trans-
formed rapidly into TiN at higher N2 partial pressure even though it
can be formed.
Based on above experimental results and analysis, the reaction
steps for microwave-assisted carbothermic synthesis of TiN–Fe
composite from FeTiO3 and carbon powder are as follows:
Step 1: FeTiO3 þ C ¼ Fe þ TiO2 þ COðgÞ
Step 2: nTiO2 þ C ¼ TinO2nꢂ1 þ COðgÞ ð2 6 n 6 9Þ
Step 3: TinO2nꢂ1 þðnxþ2nyþnꢂ1ÞCþnx N2ðgÞ ¼ nTiNxCyO1ꢂxꢂy
þ
2
ðnxþnyþnꢂ1ÞCOðgÞðxþy ¼ 1;x;y P 0;2 6 n 6 9Þ
Step 4: TiNxCyO1ꢂxꢂy
COðgÞðx þ y ¼ 1; x; y P 0Þ
þ
1ꢂx N2ðgÞþð1ꢂxꢂ2yÞC ¼ TiNþð1ꢂxꢂyÞ
2
The overall carbothermal reduction reaction is given as follows:
Fig. 6. SEM micrographs of samples after microwave-assisted carbothermic
reduction–nitridation. (a and b) 1000 °C/40 min, (c and d) 1100 °C/40 min.
1
2
FeTiO3 þ 3Cþ N2ðgÞ ¼ Fe þ TiN þ 3COðgÞ
time is 10 min, the main phases of products are FeTiO3, Fe, TiO2
and Ti9O17. As reaction time is extended to 20 min, in addition to
titanium oxides such as Ti2O3, Ti7O13 and Ti9O17, there is a little
amount of TiN in products, while FeTiO3 is disappeared entirely.
This implies that FeTiO3 can be easily transformed into metallic
Fe and titanium oxides with a fast reaction rate but the nitridation
of titanium oxides into TiN is difficult relatively. However, the
4.2. Effect of reaction time on products
In order to observe the characteristics in the variation of peak
intensity of XRD as reaction time clearly, the peak intensity of
XRD of different product phases as a function of reaction time at
1000 °C is shown in Fig. 5, which shows that when the reaction

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J. Ru et al. / Journal of Alloys and Compounds 583 (2014) 121–127
titanium oxides can be gradually nitridized to TiN as reaction time
prolongs. There is no other substance existing except for Fe and TiN
when reaction time is 40 min, as shown in Fig. 5. Therefore, FeTiO3
can be transformed into TiN–Fe composite completely if the tem-
perature is higher than1000 °C and the reaction time is greater
than 40 min when microwave heating is used to assist carbother-
mic reduction–nitridation.
However, FeTiO3 cannot be completely transformed into TiN–Fe
composite when using conventional heating until reaction time
exceeds 60 min at 1400 °C [12]. Thus, the microwave heating can
significantly lower reaction temperatures and shorten reaction
time as compared with conventional heating. This may be mainly
attributed to the following microwave-heating characteristics
[31,32]. First of all, FeTiO3 and carbon are good microwave absorp-
tion materials. In the microwave heating process, they can be pref-
erentially heated to high temperatures in a short time, and the high
temperatures localized within carbon and FeTiO3 can enhance
their reactivity. Secondly, microwave has a large transmission
depth to materials which can solve the ‘‘cold center’’ problems
associated with conventional heating [33,34]. Therefore, micro-
wave can considerably promote the carbothermic reduction–
nitridation process of FeTiO3 due to the localized high
temperatures and the large transmission depth.
Fig. 8. SEM micrograph of products leached with 10% HCl solution at room
temperature.
5. Conclusions
(1) TiN was successfully synthesized from FeTiO3 through the
microwave-assisted carbothermic reduction–nitridation
followed by acid leaching treatment.
(2) Thermodynamic analysis of Ti–Fe–N–C–O system indicates
that titanium and iron oxides can be transformed into
TiN–Fe composite by controlling suitable CO and N2 partial
pressure at high temperatures.
(3) TiN–Fe composite can be prepared from FeTiO3 by micro-
wave-assisted carbothermic reduction–nitridation at
1000 °C for 40 min. The reaction process for the preparation
of TiN–Fe composite from FeTiO3 and carbon under N2 atmo-
sphere includes four steps:
4.3. Micrographs of products
The micrographs of carbothermic reduction–nitridation prod-
ucts at different temperatures for 40 min are shown in Fig. 6. It is
noticed that particles of the products are sub-micrometer sized
and uniformly shaped. When the temperature applied is 1000 °C,
the microstructure of products is approximately flake (Fig. 6a
and b). The size of flakes is increased when temperature is raised
to 1100 °C (Fig. 6c and d).
Step 1: FeTiO3 þ C ¼ Fe þ TiO2 þ COðgÞ
Step 2: nTiO2 þ C ¼ TinO2nꢂ1 þ COðgÞ ð2 6 n 6 9Þ
Step 3: TinO2nꢂ1 þðnxþ2nyþnꢂ1ÞCþnx N2ðgÞ ¼ nTiNxCyO1ꢂxꢂy
þ
4.4. Acid leaching treatment
ðnx þ ny þ n ꢂ 1ÞCOðgÞx þ y ¼ 1; x; y P 20; 2 6 n 6 9Þ
Step 4: TiNxCyO1ꢂxꢂy
þ
1ꢂx N2ðgÞ þ ð1 ꢂ x ꢂ 2yÞC ¼ TiN þ ð1 ꢂ x ꢂ yÞ
In order to obtain pure TiN, acid leaching treatment is used to
remove metallic Fe from the TiN–Fe composite prepared at
1000 °C for 60 min in N2 atmosphere. Figs. 7 and 8 show the XRD
patterns and SEM micrograph of samples after leached with 10%
HCl aqueous solution at room temperature respectively. It can be
seen that purified TiN is obtained with the removal of metallic Fe
(Fig. 7). The SEM micrograph reveals that the morphology of TiN
particles becomes irregular as compared with those of the samples
before leached (Fig. 8).
COðgÞx þ y ¼ 1; x; y P 02Þ
Acknowledgements
The authors acknowledge the financial support of the National
Natural Science Foundation of China (Project No. 21263007,
51274108), Applied Basic Research Foundation of Yunnan province
(Project No. 2011FA009) and Talents Cultivation Foundation of
Kunming University of Science and Technology (Project No.
14118441) for this work.
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