Combustion characteristics of TiO2/Al/C system

Article abstract of DOI:10.1016/S0025-5408(01)00612-2

The formation of TiC-Al₂O₃ composite powder was studied by a combustion reaction on the system TiO₂/Al/C. The effects of the molar ratios of raw materials, compaction pressure and initial temperature of reactants on the products and combustion process were studied. The most important variable affecting the synthesis of TiC-Al₂O₃ was the molar ratio of C and Al. The highest yield of TiC-Al₂O₃ was obtained at the molar ratio of TiO₂:Al:C=3.0:4.0:2.7～3.0. The combustion temperature and the combustion velocity were increased with increasing preheating temperature. On the other hand, the cooling rate was decreased with increasing of preheating temperature. The cooling rate after completing the combustion was related to grain size of products. The grain size was increased with decreasing cooling rate.

Full text of DOI:10.1016/S0025-5408(01)00612-2

Pergamon
Materials Research Bulletin 36 (2001) 1157–1167
Combustion characteristics of TiO /Al/C system
2
J.H. Lee*, S.K. Ko, C.W. Won
Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, 305-764, Taejon,
South Korea
(Refereed)
Received 11 July 2000; accepted 6 February 2001
Abstract
The formation of TiC-Al O composite powder was studied by a combustion reaction on the
2
3
system TiO /Al/C. The effects of the molar ratios of raw materials, compaction pressure and initial
2
temperature of reactants on the products and combustion process were studied. The most important
variable affecting the synthesis of TiC-Al O was the molar ratio of C and Al. The highest yield of
2
3
TiC-Al O was obtained at the molar ratio of TiO :Al:Cϭ3.0:4.0:2.7ϳ3.0. The combustion temper-
2
3
2
ature and the combustion velocity were increased with increasing preheating temperature. On the other
hand, the cooling rate was decreased with increasing of preheating temperature. The cooling rate after
completing the combustion was related to grain size of products. The grain size was increased with
decreasing cooling rate. © 2001 Elsevier Science Ltd. All rights reserved.
Keywords: A. Ceramics; B. Chemical synthesis; C. X-ray diffraction; D. Thermodynamic properties
1
. Introduction
Self-propagating High Temperature Synthesis is potentially an energy-efficient pro-
cess to synthesize many inorganic materials, including intermetallics, ceramics, and
ceramic composites [1–3]. Characteristics of the process in the combustion-wave mode
o
are self-generated high temperature (800 to 3500 C), relatively rapid propagating com-
6
bustion fronts (0.1 to 10cm/sec), high rates of heating (up to 10 deg/sec), and thermal
gradients (up to 107 deg/cm) at the combustion front. The exact values of temperature,
*
Corresponding author. Fax: ϩ82-42-822-9401.
E-mail address: jong-lee@cnu.ac.kr (J.H. Lee).
0
025-5408/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0025-5408(01)00612-2

1
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J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
Table 1
Thermochemial data for TiC, Al O , TiO , Al and C [9,10]
2
3
2
Element Temp.
Range(K)
⌬H°
(KJ/mole)
⌬Hm
(KJ/mole)
Cp(J/Kmole)
49.5 ϩ (3.35 ϫ 10 T) Ϫ (14.98 ϫ 10 T
Ϫ3
Ϫ3
5
Ϫ2
TiC
298ϳ1800
00ϳ3290
Ϫ183.69
)
Ϫ2
5
8
34.2 ϩ (11.58 ϫ 10 T) ϩ (74.161 ϫ 10 T
)
Ϫ3
5
Ϫ2
Al O
298ϳ1800
800ϳ2327
327ϳ3000
Ϫ1677.44
111.085
106.61 ϩ (17.78 ϫ 10 T) Ϫ (28.54 ϫ 10 T
)
2
3
Ϫ3
5
Ϫ2
1
2
128.0 ϩ (5.28 ϫ 10 T) Ϫ (80.235 ϫ 10 T
)
192.464
TiO2
Al
C
Ϫ944.79
–
–
–
0
0
wave velocity, thermal gradients, and rate of heating are functions of the particular
chemical system and experimental parameters. Both solid-solid and gas-solid combus-
tion reactions are used to produce a variety of advanced technological materials.
Al O -TiC, Al O -ZrO , Si N and TiC which have high temperature strength, high
2
3
2
3
2
3 4
thermal shock resistance have used as advanced structural materials. Especially, the TiC-
Al O composite used for making abrasive tool and wear resistant coating to protect
2
3
components of oil refining equipment has been produced by hot pressing TiC and Al O
2
3
powder. The preparation of TiC-Al O composite powder by the SHS process was widely
2
3
studied in several former researches [4–7], however the effect of carbon sources on the
combustion characteristics are rare. Hence, in this study, the SHS process was applied to a
system of TiO /Al/C for the production of TiC-Al O composite. The reaction in this system
2
2 3
was discussed in terms of the mixture ratio and the preheating of the pellet for the formation
of fine TiC-Al O powder when activated charcoal, carbon black and graphite used as carbon
2
3
sources.
2
. Experimental
The raw materials used were TiO (98.56% rutile, Ͻ0.5␮m; Korea Titanium Co.,
2
Korea) and Al (99.5%, Ͻ44␮m; Chang Seong Co., Korea). Activated charcoal(Duksan
Pharm. Co., Japan), Carbon black(LG Chemical Ltd., Korea) and Graphite (Kanto
Chemical Co., Inc., Japan were used as carbon sources. The reactants(TiO :Al:Cϭ3.0:
2
3
.4ϳ4.6:1.8ϳ3.6), were mixed by milling for 5 hours using an alumina ball mill. The
mixed powders were pressed to form a pellet 40mm diameter and 50ϳ60mm in height
o
under a pressure of 80MPa and then dried in a drying oven of 70 C for 1 hour. The green
pellet was ignited in a reactor using a tungsten coil connected to a power supply under
atmosphere of argon. The green pellet was preheated by small furnace installed in
o
combustion chamber from room temperature to 600 C. The combustion temperature, the
combustion velocity and the cooling rates of the pellet were measured by a c-type
(
W-5%Re vs. W-26%Re) thermocouples embedded into the pellet using a data acqui-
sition system and a personal computer. The crystal structures of reaction products were

J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
1159
Fig. 1. X-ray diffraction patterns of reaction products varying with carbon molar ratio at TiO : Alϭ3.0:4.0(carbon
2
source:charcoal activated):(a)1.8 (b) 2.1 (c) 2.4 (d) 2.7 (e) 3.0 (f) 3.3 (g) 3.6.

1
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J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
Fig. 2. X-ray diffraction patterns of reaction products varying with aluminum molar ratio at TiO : Cϭ3.0:3.0
2
(carbon source: charcoal activated): (a) 3.4 (b) 3.6 (c) 3.8 (d) 4.0 (e) 4.2 (f) 4.6.

J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
1161
Fig. 3. The effect of the preheating temperature on the combustion temperature.
analyzed using X-ray powder diffraction (XRD) and the microstructure was investigated
using scanning electron microscopy (SEM).
3
. Results and discussion
The adiabatic temperature (T ) can be used as a general indication of the temperature at
ad
the combustion front. It can also be used in a semiquantitative way to ascertain whether the
synthesis of a given material can be accomplished by a self-propagating method. It has been
empirically suggested that combustion reactions will not become self-sustaining unless Tad
Ն 1800K [8]. The reaction of this experiment can be represented as follows:
3
TiO ϩ 4Al ϩ 3C 3 3TiC ϩ 2Al O
3
(1)
2
2
The theoretical adiabatic temperature of this reaction can be calculated using thermody-
namics data in Table 1 as follows;

1
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J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
Fig. 4. The effect of the preheating temperature on the combustion velocity.
T
ad
0
Q ϭ (Ϫ⌬H 98) ϭ
͸
C ͑products͒dT
(2)
2
͵
p
2
98
0
0
298,TiC
0
298,Al O
0
298,TiO2
0
298,Al
0
⌬
H
ϭ ͑3⌬H
ϩ 2⌬H
͒ Ϫ ͑3⌬H
3
ϩ 4⌬H
ϩ 3⌬H
͒
298,C
2
98
2
ϭ Ϫ 1071.58kJ/mol
T
ad
0
(
Ϫ⌬H 98) ϭ
͸
C ͑products͒dT
2
͵
p
2
98
1
800
Finally, 1071580 ϭ
͑3Cp,TiC͑s͒ ϩ 2Cp,Al2O3͑s͒͒dT
͵
2
98

J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
1163
Fig. 5. Arrhenius plot of Al O -TiC combustion reaction varying with carbon source.
2
3
2
327
ϩ
͑3Cp,TiC͑s͒ ϩ 2Cp,Al2O3͑s͒͒dT
͵
1
800
T
ad
ϩ ⌬H
͑3Cp,TiC͑s͒ ϩ 2C
l͒͒dT
p,Al O3͑
m,Al O
2
͵
3
2
2
327
ІTad Ϸ 2546K
Therefore, we can assume that the SHS reaction of Eq. [1] will be possible from the
preceding theoretical result. Actually, the combustion temperature, T , measured in this
c
system was about 1700ϳ2250K because of heat losses.
The effect of the carbon molar ratio on the XRD of product without preheating sample is
represented in Fig. 1. The molar ratio of carbon was changed from 1.8 to 3.6mole, because
the value of x in TiC compound is in the range of 0.6ϳ1.0, then the content of the total
x

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J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
Fig. 6. X-ray diffraction patterns of reaction products varying with carbon source(TiO : Al: Cϭ3.0:4.0:3.0); (a)
2
charcoal activated (b) carbon black (c) graphite.
carbon in the product should be between 1.8 and 3.0 mole from eq. [1]. As the carbon content
increased, the unreacted Ti diminished and was completely eliminated when the carbon
content was over 2.7 mole shown in Fig. 1. Further, the XRD patterns in Fig. 1 now show
the presence of carbon when the value of C in the ratio TiO :Al:C exceeds 2.7. Fig. 2 shows
2
the X-ray diffraction patterns of the products with various Al molar ratios. As the Al molar
ratio increases, the amount of unreduced TiO is seen to decrease until the Al:TiO reaches
2
2
a ratio of 4.0:3.0 when all the TiO is completely reduced. Peaks of Al appear above that ratio
2
because of an excess of Al over stoichiometry.
Figures 3 and 4 show the effect of the preheating temperature on the combustion
temperature and the combustion velocity of the reactions varying with carbon source. As
expected, the combustion temperature and the combustion velocity of the reaction increased

J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
1165
Fig. 7. SEM photomicrographs of synthesized products varying with carbon source: (a) charcoal activated (b)
carbon black (c) graphite.

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J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
Fig. 8. SEM photographs of synthesized products with various cooling rate.
as the preheating temperature increased. According to these figures, charcoal as carbon
source has the highest combustion temperature and velocity of all. In room temperature, the
combustion velocity was 0.5ϳ1.5mm/sec varying with carbon source. When the initial
o
temperature of reactants is 600 C, the combustion velocity of charcoal activated, carbon
black and graphite were 14.2mm/sec, 8.65mm/sec and 6.06mm/sec respectively. Fig. 5
shows the activation energy which is calculated using measured combustion temperature and
velocity on the combustion reaction. It shows that activated charcoal, graphite and carbon
black values are 308kJ/mol, 316kJ/mol and 345kJ/mol respectively.
Figures 6 and 7 show that the X-ray diffraction patterns and the microstructure of the
product vary with the carbon source at TiO :Al:Cϭ3.0:4.0:3.0 mole. These figures show
2
that crystal structure and microstructure of the product was not affected by carbon
sources.
In this work, we used a wedge type copper mold to investigate microstructure of reaction
product as to the different cooling rates. The cooling rate is changed according to the
diameter of the wedge by the large heat conductivity of copper. Fig. 8 shows the temperature

J.H. Lee et al. / Materials Research Bulletin 36 (2001) 1157–1167
1167
profile and microstructure at different mold positions (TiO :Al:Cϭ3.0:4.0:3.0, activated
2
charcoal without preheating). As the cooling rate become faster, the grain size become
smaller. We can see the result that the Al O particles grew bigger and TiC particles seem
2
3
to be sintered in Fig. 8(a), the slowest cooling rate. In the case of fastest cooling rate, Fig.
(d), the grain size of alumina becomes smaller (up to 3ϳ4␮m) and the grain size of TiC is
8
about 2␮m independently. So, the grain size of product depends on the cooling rate after the
combustion reaction.
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