Thermal stability of aluminum oxocarbides

Article abstract of DOI:10.1023/B:RUGC.0000045852.62299.10

The composition of vapor over Al₂OC and 2Al₂O ₃·Al₂OC aluminum oxycarbides was determined. The thermal stability of the oxycarbides increases in the series Al ₂OC-2Al₂O₃·Al₂

Full text of DOI:10.1023/B:RUGC.0000045852.62299.10

Russian Journal of General Chemistry, Vol. 74, No. 7, 2004, pp. 989 992. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 7, 2004,
pp. 1072 1075.
Original Russian Text Copyright
2004 by Sitnikov, Lopatin, Ryabkov, Grass.
Thermal Stability of Aluminum Oxocarbides
P. A. Sitnikov, S. I. Lopatin, Yu. I. Ryabkov, and V. E. Grass
Institute of Chemistry, Komi Scientific Center, Ural Division,
Russian Academy of Sciences, Syktyvkar, Komi Republic, Russia
St. Petersburg State University, St. Petersburg, Russia
Received April 3, 2003
Abstract The composition of vapor over Al2OC and 2Al2O3 Al2OC aluminum oxycarbides was deter-
mined. The thermal stability of the oxycarbides increases in the series Al2OC 2Al2O3 Al2OC Al2O3.
Aluminum carbides and oxycarbides are formed in
the reaction of carbon with aluminum oxide phases.
The reactions of aluminum oxide with carbon were
studied extensively [1 7].
which these compounds begin to vaporize and the
study of the vaporization dynamics and the composi-
tion of the gaseous products will allow prediction of
the stability of properties of the above-listed com-
posite materials.
By now we confirmed the fact that stable alumi-
num oxycarbides, Al4O4C and Al2OC, can form in a
high-temperature reaction of aluminum oxide, Al2O3,
with carbon and Al4C3-type carbides [5, 6]. Alu-
minum monoxycarbide Al2OC is unstable on heating
and seems to dissociate with vaporization of gaseous
compounds. Elyutin et al. [4] believe that aluminum
monoxycarbide has a high volatility at the tempera-
tures of the synthesis. Lihrmann et al. [8] suggested
that vaporization of aluminum monoxycarbide is
accompanied by its decomposition to simpler com-
pounds (Al, Al2O, and CO). No unambiguous con-
clusions about the thermal stability of monoxycarbide
can be found in the literature.
In this work we studied the vaporization of Al2OC
obtained in the reaction of aluminum oxide with
carbon in the molar ratio of 1:3 (1750 C, 8 h) and a
single-phase sample of the spinel phase of the com-
position Al2OC 2Al2O3 obtained in the reaction of
aluminum oxide with carbon in the molar ratio of 1:1
(1750 C, 1 h).
Vaporization of Al2OC. In the mass spectrum of
vapor over aluminum oxycarbide in the temperature
range 1620 1755 K, we found peaks of Al+ and
Al2O+ ions; their intensity ratio depended on tempe-
rature. To determine the nature of these ions, we
measured their appearance potentials by the method
of vanishing ion current. We used gold as a reference
substance with well-known ionization potential of
9.22 eV [11]. The appearance potentials (eV, 0.3)
were 6.2 (Al+) and 7.8 (Al2O+). These values almost
coincide with the ionization potentials of atomic
aluminum and Al2O, respectively, which suggests
their molecular origin.
We have found [5, 6] that a phase with the spinel
structure is formed in a high-temperature reaction in
the Al2O3 C system. This phase resembles aluminum
oxynitrides ( -AlON or xAlN Al2O3) [9] in its struc-
tural characteristics. Taking into account the known
structural similarity of Al2OC and AlN [10], and also
the absence of nitrogen-containing compounds in our
experiment, we have good grounds to believe that the
resulting spinel phase is a solid solution of the com-
position xAl2OC Al2O3. We also synthesized a single-
phase sample of this phase of the composition Al6O7C
or Al2OC 2Al2O3.
The vapor mass spectra, the dependences of the ion
currents on the vaporization temperature (T) and time
( ), and the appearance potentials of the ions in the
mass spectrum suggest that, in the temperature range
1620 1755 K, aluminum oxocarbide vaporizes almost
congruently with complete dissociation; the vapor
consists of a mixture of Al2O and Al, and, probably,
of oxygen and carbon oxides. Quantitative measure-
ments of the partial pressures of O2, CO, and CO2 are
impossible because of the presence of N2, CO, O2,
and CO2 in the vacuum system of the mass spectrom-
eter and, as a consequence, of strong background for
The aim of this work was to study the thermal
stability of aluminum oxycarbide -Al2OC and of the
phase with the spinel structure Al2OC 2Al2O3, since
their formation in the condensed phase is possible on
high-temperature heating of such important composite
refractory materials as Al2O3 SiC, Al2O3 TiC, and
Al2O3 C. The determination of the temperatures at
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990
SITNIKOV et al.
0
pi, Pa
log pi
0.5
1650 K
1760 K
1950 K
1.4
1.2
1.0
1
2
1.0
1.5
0.8
0.6
0.4
0.2
1
2
2.0
2.5
0
50
100
150
200
250
, min
0.58
0.60
0.62
1
103/T, K
Fig. 2. Plot of the logarithms of partial pressures of the
Fig. 1. Plot of partial pressures of the vapor components
over Al OC vs. vaporization time. (1) Al O and (2) Al.
vapor components over Al OC vs. reciprocal tempera-
2
ture. (1) Al O and (2) Al.
2
2
2
m/z 28, 32, and 44. Nevertheless, the deterioration of
the vacuum inside the mass spectrometer on heating
of the sample and the ion peaks corresponding to O2+
and CO+, which are partially eliminated with a shutter,
suggest the presence of these gases in the vapor over
the sample. The intensities of the ion currents of CO+2,
which are proportional to the partial pressures of
carbon dioxide over the samples, are low and remain
almost unchanged during the experiment. We deter-
mined the partial pressures of atomic aluminum and
Al2O by the procedure of comparing the ion currents
by Eq. (1):
dependences of the intensities of Al+ and Al2O+ ion
currents allowed us to derive temperature dependences
(2), (3) of the partial pressures of the vapor com-
ponents over the starting composition in the range
1626 1755 K.
Al2O:logp [Pa] = (38254 1882)/T + (21.53 1.12), (2)
Al: logp [Pa] = (31564 1609)/T + (17.09 0.95). (3)
The dependences of the logarithms of Al and Al2O
partial pressures on reciprocal temperature are shown
in Fig. 2. It is seen that, as the temperature increases,
the relative content of Al2O in the vapor increases,
and the content of atomic aluminum decreases, which
can be accounted for by an increase in the concentra-
tion of oxygen in the saturated vapor and by the shift
of gas-phase equilibrium (4) to the left.
p2I1T1 2 2a2
I2T2 1 1a1
p1 =
.
(1)
Here pi is the partial pressure of ith vapor component;
Ii, intensity of the ion current; i, full ionization cross
section of the molecules; , conversion coefficient of
the secondary electron multiplier of the mass spec-
trometer; and ai, natural abundance of an isotope. The
indices 1 and 2 refer to the substance under study and
a reference substance, respectively. The ionization
cross sections were determined by the additivity rule
using atomic cross sections from [12]. The depen-
dence of Al2O and Al partial pressures on the vapori-
zation time and temperature is shown in Fig. 1.
Al2O(gas)
2Al(gas) + 1/2O2(gas).
(4)
Vaporization of the 2Al2O3 Al2OC spinel phase.
We have detected peaks of Al+ and Al2O+ ions in the
vapor mass spectrum starting from 1650 K. Examina-
tion of the vapor mass spectrum, its dependence on
the vaporization time and temperature, and also the
appearance potentials of ions show that the sample
vaporizes incongruently. In the vapor there are
aluminum oxide Al2O, atomic aluminum (the ratio of
their partial pressures changed during the experiment),
and, presumably, O2 and CO. The dependence of the
partial pressures of Al and Al2O on the vaporization
time and temperature is shown in Fig. 3. It is seen that
the pressures of Al2O and atomic aluminum decrease
during the whole experiment. This is characteristic for
the vaporization of solid solutions or melts with com-
At 1650 1800 K, the partial pressures of Al and
Al2O and their ratio remain almost unchanged during
the whole time of vaporization. The vapor pressure of
Al2O is higher than the vapor pressure of aluminum
in this temperature range, and their ratio depends on
temperature. Only at the end of the experiment, on
passing to the unsaturated vapor, the pressure of Al2O
decreases and becomes equal to the pressure of atomic
aluminum. The measurements of the temperature
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 7 2004

THERMAL STABILITY OF ALUMINUM OXOCARBIDES
991
pi, Pa
0.4
ponents differing in their volatility. The vapor pres-
sure of aluminum was higher than the vapor pressure
of Al2O in the vaporization temperature range. Only
on increasing the temperature by 100 150 K for sev-
eral minutes the pattern became opposite. After a
short isothermal heating, p(Al) again became greater
than p(Al2O). As the sample vaporizes, less volatile
Al2O3 seems to accumulate in the condensed phase,
vaporizing only above 2100 K. To check this assump-
tion, we studied the vaporization of pure aluminum
oxide from a molybdenum cell.
1760 K
1950 K 2110 K
1
0.3
0.2
2
0.1
0.0
Vaporization of Al2O3. We have detected peaks of
0
20 40 60 80 100 120 140 160 180 200
, min
+
Al+, Al2O+, AlO+, Al2O2+, MoO2+, and MoO3 ions
in the mass spectrum of the vapor over aluminum
oxide at the ionizing voltage of 25 V. The Al+ :Al2O+ :
AlO+ :Al2O+2 ratio in the temperature range 2220
2350 K is, on the average, 100:8.7:6.5:1.2. The
appearance potentials (eV, 0.3) are 6.2 (Al+), 7.8
(Al2O+), 9.5 (AlO+), 9.5 (MoO2+), and 12.0 (MoO+3).
We have not measured the appearance potential of
Al2O+2 because of the low intensity of the ion current.
The appearance potentials of ions in the mass spec-
trum of the vapor over Al2O3 coincide with the ap-
pearance potentials of the corresponding molecules
[11] within the limits of the measurement error,
suggesting that these ions are formed by the direct
ionization of gaseous Al, Al2O, AlO, Al2O2, MoO2,
and MoO3. The main components of the vapor at
2220 2350 K are oxygen and atomic aluminum; the
sum of the partial pressures of aluminum suboxides
does not exceed 15% of the aluminum pressure
[scheme (5)].
Fig. 3. Plot of partial pressure of vapor components over
the Al O Al OC system vs. vaporization time (starting
2
3
2
composition of the condensed phase 2Al O Al OC).
2
3
2
(1) Al O and (2) Al.
2
atomic aluminum, and oxygen. Aluminum oxide is
the most thermally stable. It begins to dissociate, with
atomic aluminum and oxygen passing into vapor,
above 2200 K. The data obtained support our assump-
tion that the spinel phase is a solid solution of
aluminum monoxycarbide in aluminum oxide of the
composition xAl2OC Al2O3. The thermal stability of
the spinel phase seems to depend on the concentration
of aluminum oxide and oxycarbide in it. The activity
of aluminum monoxycarbide decreases with the
formation of the spinel phase, which results in a
decrease in the partial pressures of Al and Al2O vapor
and leads to an increase in the thermal stability of the
phase.
Al2O3(cr.) + Mo(cr.)
Al(gas) + O2(gas)
EXPERIMENTAL
+ Al2O(gas) + AlO(gas) + MoO2(gas) + MoO3(gas). (5)
The work was carried out on an MS-1301 mass
spectrometer at an ionizing voltage of 25 V. We
vaporized the samples from a molybdenum twin cell
heated by electron bombardment. The temperature was
measured with an EOP-66 optical pyrometer. The cell
was first calibrated against the vapor pressure of gold
and CaF2 [13, 14]. We determined the partial pres-
sures of the vapor components by comparing the ion
currents. We used gold as internal and external
reference of pressure, as recommended by IUPAC
[13].
From the measured intensities of the ion currents
of Al+, Al2O+, and AlO+ and the partial pressures of
Al, Al2O, and AlO as functions of temperature in the
range 2228 2352 K, we derived Eqs. (6) (8) for the
temperature dependences of the partial pressures of
the vapor components over Al2O3.
logp [Al, Pa] = (25050 2000)/T + (10.9 0.9), (6)
logp [AlO, Pa] = (27700 2700)/T + (10.9 1.2), (7)
logp [Al2O, Pa] = (27350 2500)/T + (10.7 1.1). (8)
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