Combustion, Explosion, and Shock Waves, Vol. 37, No. 4, pp. 413 417, 2001
Oxidation of Powdered Alloys
of Aluminum and Cerium during Heating in Air
V. G. Shevchenko,1 V. I. Kononenko,1 I. A. Chupova,1 UDC 546.621 655-19
I. N. Latosh,1 and N. V. Lukin1
Translated from Fizika Goreniya i Vzryva, Vol. 37, No. 4, pp. 53 57, July August, 2001.
Original article submitted May 25, 2000; revision submitted October 19, 2000.
The effect of cerium on the oxidation of powdered alloys of aluminum and cerium
during heating in air at temperatures up to 1773 K was studied using derivatography,
x-ray phase analysis, and thermal desorption of argon. It is established that the rate
and completeness of the oxidation of aluminum increase if it is alloyed with cerium.
The effect of cerium is due to its polyvalence, high reactivity, and surface activity
toward aluminum. Structural and phase nonuniformities that arise during the growth
of oxide phases on the surface of oxidized particles of the alloys enhance the interaction
of these particles with air oxygen. It is shown that the assumptions that the surfaces
of the alloys have a micrononuniform structure and are enriched with CeAl2 and CeAl4
groups are valid.
Alloys based on aluminum and rare-earth elements Golubev, Kononenko, et al. [6] found that introduc-
have found wide industrial use. However, many of their tion of 1.5 at.% Ce in aluminum increases considerably
properties, in particular, the behavior of powdered al- the oxidation rate of powdered alloys, especially during
loys of aluminum and cerium in active gaseous media, transition to the liquid state. From analysis of experi-
have not yet been studied. mental results on the thermodynamics of alloying and
It is known [1] that the constitution diagrams data on the surface properties and viscosity of alloys,
of aluminum with rare-earth elements belong to eu- it was suggested that the oxidation is activated due to
tectic systems with the presence of chemical com- the microgroups that are present on the surfaces of liq-
pounds. In the Al Ce system, the richest in aluminum uid alloys and correspond to the composition of com-
is CeAl4 alloy [2] or, according to other data, Ce3Al4 [3]; pounds, in particular, to the composition of the most
CeAl2 alloy is second to them. Buschow and van high-melting compound CeAl2.
Vutch [3] found intermediate CeAl3 but it was iden- The present paper reports experimental results on
tified only after considerable standing (for more than the oxidation of CeAl2 and CeAl4 powders and Al Ce
10 days) at a temperature T = 850ć%C. Since the solu- alloys under conditions of programmed heating in air.
bility of cerium in aluminum is very low  0.05% (by
weight) at T = 910 K [4]  the properties of alloys rich
in aluminum will are determined by the amount of the
EXPERIMENTAL PROCEDURE
nearest compound in its structure.
The structure of an alloy in the solid state is de-
We used AV-000 aluminum and TsEM-1 cerium
termined only by its composition, whereas in the liquid
as the starting materials for the present study. The
state, the structure of the surface layer differs signif-
alloys were produced in alundum crucibles in a vac-
icantly from the structure of the bulk of the sample.
uum SShVL furnace, whose working-zone temperature
This is true for alloys of aluminum and cerium because,
reaches 1873 K with vacuum of at least 10-5 mm Hg.
as is shown in [5], cerium is a surface-active element
Before alloying with aluminum, cerium was subjected
with respect to aluminum.
to melting in vacuum to make it free of gaseous and
1
volatile impurities. The powders were prepared by me-
Institute of Solid-State Chemistry, Ural Division,
Russian Academy of Sciences, Ekaterinburg 620219. chanical grinding in an agathic mortar. The specific
0010-5082/01/3704-0413 $25.00 © 2001 Plenum Publishing Corporation 413
414 Shevchenko, Kononenko, Chupova, et al.
TABLE 1
Results of X-Ray Phase Analysis of Initial Samples
and Products of Their Stepwise Oxidation at a Heating Rate of 15 K/min
T , K
Phase composition of initial samples and products of their stepwise oxidation
293 Al, CeAl4 (Al + 10 at. % Ce) CeAl4 CeAl2
873 Al, CeAl4 CeAl4 CeAl2, CeAlO3
1233 Al, CeAl4, Ä…-Al2O3, CeAlO3 CeAlO3, Ä…-Al2O3, Al (very little) CeAl2, CeAlO3
1473 Al, Ä…-Al2O3, CeO2 Ä…-Al2O3, CeO2, Al (very little) CeO2, Ä…-Al2O3
1773 Ä…-Al2O3, CeO2 Ä…-Al2O3, CeO2 Ä…-Al2O3, CeO2
surface area determined by the method of thermal des-
orption of argon was equal to 0.2 0.5 m2/g. The oxida-
tion process was studied on a Q-1500D derivatograph at
heating rates of 7.5 and 15 K/min. The sample weight
of the alloys studied was 0.015 0.03 g. X-ray phase
analysis of starting alloys and their oxidation products
was performed on a DRON-2.0 diffractometer.
DISCUSSION OF RESULTS
Figure 1 shows curves of the degree of conversion of
aluminum and its alloys versus temperature and com-
position. Based on the results of oxidation of powdered
pure metals obtained in [7], the degree of conversion Ä…
Fig. 1. Variation in the degree of conversion of alu-
was calculated with allowance for the oxidation of alu-
minum and its alloys versus the Ce content in Al.
minum to Al2O3 and the oxidation of cerium to higher
oxide CeO2.
It follows from Fig. 1 that an increase in the Ce CeAlO3 in addition to the initial intermetallide.
content in the alloys leads to a considerable increase in The same phases exist during heating up to T =
the completeness and rate of interaction and a decrease 1233 K; only the ratio of line intensities changes. Stoi-
in the temperature of the onset of intense oxidation of chiometry suggests the presence of aluminum or its ox-
powders. Such behavior of the alloys can be explained ide in addition to the phases given in Table 1. However,
by the high reactivity of cerium, whose powder is ignited even annealing of the oxidation products in an evacu-
at T = 375 403 K [7]. Since the Gibbs energy of forma- ated ampoule at T = 873 K for 120 h did not yield
tion of cerium sesquioxide is higher than the Gibbs en- changes in x-ray photographs. This indicates both the
ergy of formation of aluminum oxide (-"G0 = 1705.9 insufficient sensitivity of x-ray phase analysis and the
298
and 1582 kJ/mole, respectively), in alloys, an alloying very small particle sizes of the Al and Al2O3 phases
element oxidizes mainly. Moreover, because of the vari- formed during heating and oxidation. Further increase
able valence of cerium, phases of varying composition in temperature leads to formation of dioxide CeO2 and
can be formed (besides CeO1.5 and CeO2), and this Ä…-Al2O3. Complete oxidation of the weighted samples
should deteriorate the protective properties of the bar- is terminated at T H" 1573 K with formation of indi-
rier layer of interaction products. vidual oxides CeO2 and Ä…-Al2O3. Since CeAl2 melts
Table 1 lists results of x-ray phase analysis of the congruently at T = 1753 K, the entire process of inter-
initial samples and the products of their stepwise oxi- action ends in the solid phase. The maximum oxidation
dation. rate of the alloy is observed at T H" 740 K.
The analysis shows that at T < 873 K the oxida- The curve of the degree of conversion for intermet-
tion products of CeAl2 contain monoaluminate oxide allide CeAl4 is more complex than that for CeAl2. This
Oxidation of Powdered Alloys of Aluminum and Cerium during Heating in Air 415
is obviously due to high aluminum content. In this case,
monoaluminate CeAlO3 is detected simultaneously with
Ä…-Al2O3 in the interaction products at higher tempera-
tures (see Table 1). At temperatures below T = 873 K,
although more than 40% of the weighted sample was ox-
idized (see Fig. 1), x-ray phase analysis showed only the
presence of CeAl4 (see Table 1). As noted above, this
can be due to the very small sizes of the oxide phases
formed, which makes them undetectable for x-ray anal-
ysis. As in the case with CeAl2, cerium aluminate is
not detected at T > 1373 K. However, in addition to
CeO2 and Ä…-Al2O3, weak lines are observed which are
assigned to unoxidized aluminum in the particles.
The oxidation of the alloy with 10 at.% Ce is even
less intense. Analysis of the oxidation products showed
that cerium aluminate CeAlO3 appears at T H" 1200 K.
In this case, the oxidation products contain aluminum
Fig. 2. Variation in specific surface during oxidation
of intermetallide CeAl2.
oxide Ä…-Al2O3. In addition, x-ray phase analysis indi-
cates a larger amount of pure aluminum in comparison
with CeAl4. At T > 1370 K, cerium aluminate was not
detected but, in addition to individual oxides CeO2 and
where aluminum is in the metal state, and the composi-
Ä…-Al2O3, the lines corresponding to aluminum are ob-
tion of the subsurface layer suggests separate oxidation
served. The low solubility of cerium in aluminum did
of aluminum and cerium at this temperature. They also
not affect the position and relative intensity of the max-
note that accelerated penetration of oxygen to the oxide
ima due to aluminum in the x-ray patterns. Complete
surface occurs not only by its diffusion but also directly
oxidation of the alloy with 10 at.% cerium in aluminum
via microcracks [12].
is terminated with the formation of CeO2 Ä…-Al2O3 at
In particular, measurements of the specific sur-
T H" 1770 1780 K.
face S even of a monophase sample of CeAl2 dur-
Analysis of literature data on the interaction of alu-
ing stepwise oxidation support the above assumption
minum and cerium oxides in systems shows that during
(Fig. 2). An increase in the specific surface at the stage
heating in air CeO2 does not form chemical compounds
of accelerated oxidation (up to T = 873 K) is evidence of
of definite composition or solid solutions with AlO1.5 the formation of microcracks during growth of dissim-
up to T = 1970 K. Only above this temperature are
ilar oxide phases on the surface of the intermetallide,
two compounds CeAlO3 and CeO1.5 · 11AlO1.5 formed
which agrees well with the results obtained earlier for
[8, 9]. In our case, cerium aluminate CeAlO3 is formed
pure polyvalent metals [13]. We note that it is not only
during oxidation of CeAl2 at T < 873 K, and it is also
phase nonequilibrium that plays an important role in
formed at higher temperatures for alloys richer in alu-
the growth of new phases in disperse systems but also
minum. The formation of aluminate of this composition
morphological, structural, and substructural nonequi-
in air is possible only during interaction of CeO1.5 and
libria, which activate diffusion processes because they
AlO1.5 [10].
change qualitatively the mechanism of diffusion in the
As noted in publications, the kinetics of synthesis
systems.
depends significantly on the dispersion and reactivity of
This oxidation pattern leads to activation of the
starting reagents [11].
interaction of residual aluminum. This is clearly seen in
The above data show that during oxidation of Al
the case of CeAl2, which is completely oxidized at T H"
Ce alloys, cerium is initially oxidized to CeO1.5, which
1537 K, unlike pure aluminum powder, which interacts
interacts with the aluminum oxide formed. Apparently,
at the same temperature by less than 50%.
a positive fact is that the oxides formed in surface layers,
In this connection, the results obtained by Braaten
which are first in the molecular and then in the nanosize
et al. [15] are of considerable interest. They studied
state, are in close contact with each other.
the effect of a thin cerium coating on aluminum oxi-
Using Auger spectroscopy, Kozhanov, et al. [12]
dation by x-ray photoelectron and ultraviolet electron
showed that aluminum oxidation in air is abruptly ac-
spectroscopy using synchrotron radiation. The inter-
tivated if the Ce content in aluminum is 10 at.% and
metallide Ce Al produced by deposition of Ce on a pure
T = 873 K. In this case, oxygen penetrates into a depth
aluminum surface is further oxidized to form mixed ox-
416 Shevchenko, Kononenko, Chupova, et al.
ide Ce Al O and aluminum oxide below it. In this case, The complex nature of phase formation in the bar-
the oxidation rate of aluminum under a layer of effective rier layers of interaction products on the surface of oxi-
thickness H"9 Å increases considerably, as in the inter- dizing particles during heating (AlCe + O2 CeO1.5 +
action with the pure surface. This is also explained by AlO1.5 CeAlO3 + O2 CeO2 + AlO1.5) facilitates
imperfections of various nature in the layer of interac- the access of the oxidizer to the oxidation reaction zone,
tion products. thus accelerating the reaction.
Returning to the oxidation of alloys with low This work was supported by the Russian Foun-
cerium content, for hypoeutectic alloys (up to H"3 at.% dation for Fundamental Research (Grant No. 99-03-
Ce), the oxidation pattern changes most drastically in 32710a).
transition to the liquid state. These alloys are char-
acterized by a considerable increase in the rate and
completeness of interaction in the temperature range of
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