1694
FOLOMEIKIN et al.
(4) vaporizes congruently and Al O C, incongruently with
Al O C + 6C = Al C + 4CO.
4
4
4
3
4
4
accumulation of aluminum oxide in the condensed
phase. The components of the Al O SiC system
Aluminum oxide first completely converts into
oxycarbide and then to carbide. Secondary processes
2
3
varopize at a lower temperature that those of the
Al O C system.
(5) can occur on cold parts of the apparatus.
2
3
Sitnikov and co-workers [17, 18] studied phase
formation in the course of carbothermal reduction of
aluminum oxide. The X-ray phase analysis of solid
reaction products of aluminum oxide with carbon
revealed in their structure aluminum oxycarbides
Al O C and Al OC, aluminum carbide Al C , a phase
Al O C + Al C = 8Al + 4CO.
(5)
4
4
4 3
As the carbon content in the condensed phase in-
creases, the following conversions take place: Al O3
2
Al O C Al OC Al C .
4
4
2
4 3
4
4
2
4 3
Elyutin et al. [13] detected no other products than
Al O C in the reduction of Al O with carbon at
with the spinel structure, as well as aluminum oxide
and unreacted carbon. It was noted that thermal treat-
ment of reaction mixtures of Al O and C is accom-
4
4
2
3
1
973 K. Aluminum carbide Al C appeared in the
4 3
2
3
condensed phase at 2083 K, while at 2273 K, Al OC
panied by gas evolution; therewith, the weight loss
reaches 95%, which cannot be explained in terms of
reactions (1) (3). The fact that the weight loss is
larger than expected by reactions (1) (3) is explained
by vaporization of aluminum compounds.
2
was found in the reaction products instead of Al O C.
4
4
Condensates collected from different parts of the
apparatus contained, in order of decreasing tempera-
ture, Al C , Al O , or Al OC. The mechanism of the
4
3
2
3
2
reduction of aluminum oxide with carbon is described
by Eq. (6) at temperatures below 2073 K and Eqs. (6)
and (7) at 2073 2273 K.
We studied vapor composition and partial pressures
of molecular forms by vaporizing Al O from a one-
temperature effusion chamber, as well as from a gra-
phite-armoured chamber. The mass spectrum at an
ionizing voltage of 25 V of the vapor over aluminum
2
3
Al O C + 3C = Al OC + 2CO,
(6)
(7)
4
4
2
2
Al OC + 3C = Al C + 2CO.
oxide vaporized from a tungsten chamber in the range
2
4 3
+
2
220 2350 K contained Al, Al O, AlO , Al O , WO ,
2
2
2
2
Vodop’yanov et al. [14, 15] reduced Al O with
and WO ion peaks. The intensity ratio of the Al,
2
3
3
carbon in such conditions that the reducer and con-
densed reaction products did not contact with the
starting aluminum oxide. It was found that (1) the rate
Al O, AlO, and Al O ions was, on average, 100:9.3:
2
2
2
6.7:1.4. The origin of the ions was determined by
measuring their appearance energies by the method of
vanishing ion current, using gold as standard [19].
The following appearance energies were obtained, eV
of Al O3 sublimation is strongly temperature-de-
2
pendent, (2) the rate of Al O sublimation much (by
2
3
+
+
two orders of magnitude) increases when the former is
heated near carbon, and (3) Al O sublimation in the
( 0.3 eV): 6.2 (Al), 7.8 (Al O ), 9.5 (AlO ), 10.0
2
(WO ), and 11.6 (WO ). The appearance energy of
2
3
2
3
+
presence of carbon occurs at appreciable rate at a
temperature close the initial reduction temperature
1873 K) of aluminum oxide with carbon directly
Al O was not measured because of the low intensity
2 2
of its current. The appearance energies of ions in the
mass spectrum of the vapor over Al O were coin-
(
2
3
contacting with it.
cident, within error, with the ionization energies of
the corresponding molecules [7, 8]. Consequently, the
ions are formed by direct ionization of the latter. The
principal components of the vapor at 2220 2350 K
were oxygen and atomic aluminum, and the sum of
the partial pressures of aluminum oxides was no more
than 15% the aluminum pressure. From the tempera-
Analysis of the resulting data, as well as thermo-
dynamic calculations led us to suggest that aluminum
oxide heated in the presence of carbon first evolves
oxygen. The latter reacts with carbon to form CO that,
in its turn, reacts with aluminum oxide by the absor-
ption mechanism. Therewith, a number of processes
can occur; for instance, Al O and aluminum formed
by partial reduction of Al O with CO can react with
carbon to give Al C .
ture dependence of the ion currents of Al, Al O, and
2
2
AlO+ in the range 2228 2352 K and the partial pres-
2
3
sures Al, Al O, and AlO we deduced Eqs. (8) (10) for
2
4
3
the partial pressures of the components of the vapor
over Al O .
Yokokawa et al. [16] performed a thermogravi-
metic analysis of component volatilities in the Al O
C system in a vacuum. Vaporization of aluminum
oxycarbides, a mixture of aluminum oxide with
carbon, and a mixture of aluminum oxide and carbide
with silicon dioxide and carbon. Aluminum oxide
2
3
logp(Al, Pa)= (25036 1993)/T + (10.86 0.87), (8)
logp(AlO, Pa)= (27676 2660)/T + (10.86 1.16), (9)
logp(Al O, Pa)= (27336 2522)/T + (10.69 1.10). (10)
2
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 76 No. 11 2006