Reactivity of Aluminum Powders, CHEMIA I PIROTECHNIKA, Chemia i Pirotechnika

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Combustion, Explosion, and Shock Waves, Vol. 37, No. 4, pp. 418{422, 2001
Reactivity of Aluminum Powders
A. P. Il'in,
1
A. A. Gromov,
1
and G. V. Yablunovskii
1
UDC 541.16:182
Translated from Fizika Goreniya i Vzryva, Vol. 37, No. 4, pp. 58{62, July{August, 2001.
Original article submitted April 11, 2000; revision submitted September 5, 2000.
The reactivity of aluminum powders is determined using the following parameters: the
temperature of the onset of oxidation, maximum oxidation rate, degree of conversion
(degree of oxidation) of aluminum, reduced (conditional) ratio of the thermal eect
to the weight increment. These parameters for estimating the activity of aluminum
powders were chosen from the results of nonisothermal oxidation of powders of various
particle sizes under conditions of programmed heating (oxidizer{air). In accordance
with the testing method proposed, the most reactive powder studied was STPA-4
ultrane aluminum powder produced by electrical explosion of conductors.
INTRODUCTION
known, such powders (with equal aluminum contents)
have dierent oxidation rates, degrees of oxidation (con-
version), etc., depending on their forms, sizes, and parti-
cle distribution functions. With increase in particle size,
the metal content of such powders usually decreases but
the oxidation rate can increase. Agglomeration, incom-
plete burning, and two-phase losses are serious problems
in using coarsely dispersed powders, especially, in highly
metallized compositions [1]. It is possible to raise the
activity of coarsely dispersed powders by melting them
with rare-earth elements [3], doping particle surfaces
with high-melting metals [4], and introducing oxidation
catalysts into the powders [5]. A common disadvan-
tage of all these methods is that being an energetic bal-
last, the additives reduce the heat of combustion fuel
combustion. Another method of increasing the reac-
tivity of aluminum powders is to increase their surface
area by using dusts with scaly particles. However, the
at particles of dusts deteriorate the physicomechanical
properties of fuel compositions and contribute to re
and explosion hazards during processing. This raises
the question of using new types of spherical aluminum
powders, which combine high reactivity with relatively
high metal content.
The goal of the work described here was to show
experimentally that the reactivity of aluminum pow-
ders, especially that of ultrane powders, depends not
only on the metal content but on dierent parameters
as well. For a rapid analysis of the reactivity of pow-
ders, we propose to use the following parameters: the
temperature of the onset of oxidation (T
on
[
C]), the
Aluminum powder is an eective fuel: its mass
caloricity is more than twice that of magnesium, and
although ranking below boron and beryllium in this
parameter, aluminum greatly surpasses them in den-
sity [1]. In addition, beryllium and its combustion
products are toxic, and boron is a heat-resistant ma-
terial, whose viscous low-melting oxide prevents the ox-
idizer from entering the combustion zone. Therefore,
aluminum is the most appropriate fuel for metallized
mixed compositions [1].
A problem that arises in the use of aluminum as a
fuel is to determine the activity of aluminum powders.
On the one hand, low reactivity of aluminum powder is
preferred during its production, storage, transportation,
and processing, and, on the other hand, a high rate of
the process is required during oxidation.
OBJECT OF STUDY AND EXPERIMENTAL
METHODS
Traditionally, the reactivity (reactive aluminum)
implies the content of metallic aluminum in powders [2].
This does not involve problems for coarsely dispersed
powders. However, with this approach to the denition
of the reactivity it is dicult to explain the dierent
behavior of ultrane powders during oxidation. As is
1
High-Voltage Institute, Tomsk Polytechnical University,
Tomsk 634050.
418
0010-5082/01/3704-0418 $25.00 c 2001
Plenum Publishing Corporation
Reactivity of Aluminum Powders
419
maximum oxidation rate (v
ox
, mg/sec), the degree of
conversion (the degree of oxidation) of aluminum in a
specied temperature range (, %), the reduced (con-
ditional) ratio of the thermal eect to the weight incre-
ment (S=m). These parameters can be obtained by
processing the results of nonisothermal oxidation under
conditions of programmed heating (oxidizer is air).
In recent years, along with the design of new grades
of powders using the technology of producing spherical
disperse aluminum, there is a tendency for conversion
to ultrane aluminum powders in fuel systems (particle
characteristic size less than 1 m) [6]. It should be noted
that we do not use the term \ultradisperse powders" for
the powders studied because this term corresponds to
the physical state of a substance [7]. Among the numer-
ous methods of producing ultrane aluminum powders,
the electric explosion method is distinguished by the
low energy inputs, high output, and high quality of the
powders produced [8]. Determination of the parameters
of nonisothermal oxidation under standard conditions of
programmed heating in air [9] was developed and tested
in sucient detail. This makes it possible to compare
results obtained and determine the reactivity of a pow-
der using several parameters [10].
Other methodological approaches to studying the
kinetics of oxidation of ultrane aluminum powders in
air can also be found in the literature. Thus, Ivanov
and Gavrilyuk [11] used derivatography as a research
method. However, their results are presented in the
form of kinetic equations, which demonstrate only for-
mal parameters of the processes. This complicates anal-
ysis of the results and comparison of the reactivity pa-
rameters of powders.
Fig. 1. Derivatograms of sample Nos. 2 (a) and 7 (b)
(sample numbers correspond to those of Table 1): m =
5 10
5
kg, the heating rate in air is 10
C/min, and
-Al
2
O
3
is the reference.
University (sample Nos. 6 and 7). The specic sur-
face area (S
sp
) of the samples studied was determined
by the BET (Brunauer{Emmett{Teller) method (low-
temperature adsorbtion of nitrogen). The metal content
was determined by the volumetric technique, i.e., from
the volume of hydrogen evolved during interaction of
powders with a 5% solution of NaOH. The temperature
of the onset of oxidation was determined by the Piloyan
method [9] from the curve of mass loss of dierential
thermal analysis (DTA).
The data given in Table 1 suggest that with rise
in S
sp
(decrease in surface-average diameter of par-
ticles a
s
), the mass concentration of the unoxidized
metal (C
Al
0
) in the powders increases, and, simultane-
ously, the bulk density
0
decreases. For sample Nos. 1
and 2, the temperature of the onset of intense oxida-
tion far exceeded the melting temperature of aluminum
(660
C). The values of T
on
for the remaining samples
lie below the melting point of aluminum (especially for
sample No. 7, whose temperature T
on
is 120
C below
the melting point of aluminum). For sample Nos. 1
and 2, the degree of oxidation of the metal before melt-
EXPERIMENTAL STUDY OF POWDERS
The comparability of the results of derivatography
of the powders is provided for by identical experimental
conditions. The standard weight of ultrane aluminum
samples was 510
5
kg, the heating rate was equal to
10
C/min, and the remaining parameters were found
during numerous experiments [12].
Table 1 lists characteristics of the powders studied:
commercial aluminum powders (sample Nos. 1 and 2),
PY87 dust (\Pechiney") (sample No. 3), STPA-IK (ul-
trane powder produced by vaporization{condensation
in argon) (sample No. 4), ultrane aluminum pow-
ders produced by electrical explosion of conductors,
the \Alex" powder (\Argonide Corp.") [13] (sample
No. 5), and STPA-1 and STPA-4 ultrane aluminum
powders produced at the semicommercial division of
the High-Voltage Institute of the Tomsk Polytechnical
420
Il'in, Gromov, and Yablunovskii
TABLE 1
Sample No. S
sp
(BET), m
2
/g a
s
, m C
Al
0
, %
0
, g/cm
3
T
on
,
C
, %
up to 660
C up to 1000
C
1
0.15
80.0
99.5
1.60
920
0.65
52.2
2
0.38
9.0
98.5
0.87
820
2.5
41.8
3
5.91
Powder
96.0
0.315
580
8.0
40.5
4
11.00
0.20
86.0
0.21
555
39.9
69.3
5
12.10
0.18
94.8
|
548
39.4
45.0
6
7.80
0.28
91.0
0.13
560
23.9
74.3
7
16.00
0.13
89.0
0.11
540
50.1
78.6
Sample No.
v
ox
, mg/sec
(in the temperature range,
C)
S=m, rel. units
Notes
1
0.04 (920{950)
2.1
Sample weight 86.2 mg
2
0.05 (970{980)
|
|
3
0.025 (580{650)
|
Scale diameter from 2{3 to 5{6 m;
width 0.15 m
4
0.125 (560{570)
7.7
Sample weight 26.8 mg
5
0.05 (541{554)
|
[13]
6
0.04 (565{590)
7.0
|
7
0.05 (550{605)
8.7
|
ing did not exceed 3% and for sample No. 3 it did not
exceed 10%. For ultrane aluminum powders (sample
Nos. 4{7), the degree of conversion of aluminum before
the melting point was more than 20% (the maximum
value of 50.1% was obtained for sample No. 7). The
zone of the most intense oxidation was determined from
the TG curve (segments AB and A
0
B
0
in Fig. 1). The
highest oxidation rate was observed for sample No. 4.
Sample Nos. 1, 2, 6, and 7 had comparable oxidation
rates, whereas intense oxidation of sample Nos. 1 and 2
began at 920 and 820
C, respectively, and oxidation
of sample Nos. 6 and 7 began at about 400
C below
than that for samples 1 and 2. The specic heat re-
lease S=m was determined by dividing the area of the
peak of heat release (DTA curve) by the corresponding
increase in sample weight (mg) (TG curve). The pa-
rameter S=m is maximal for sample No. 7 and more
than four times larger than that for sample No. 1.
Fig. 1. The oxidation of sample No. 7 proceeds in two
macroscopic stages: the rst stage begins at a temper-
ature of 550
C and the second, less intense stage be-
gins at 750
C and continues up to complete oxidation
of aluminum (more than 1000
C). For sample No. 2,
it is possible to distinguish four stages of oxidation:
1) 560{640
C; 2) 810{970
C; 3) 970{980
C; 4) 980
C
and then until complete oxidation. The degrees of ox-
idation of sample Nos. 7 and 2 before melting were
50.1 and 2.5%, respectively. The rst macrostage of
oxidation of sample No. 7 also includes several stages:
one can see four segments of increase and decrease in
temperature on the DTA and DTG (dierential ther-
mogravimetry) curves. Several powder fractions of close
sizes are unlikely to burn out separately at T > 2000
C:
the particle distribution of ultrane aluminum powders
produced by electrical explosion is not tetramodal but
bimodal (maxima are in regions 1{3 and 0.1 m). We
can explain this unique phenomenon for ultrane alu-
minum powders if we assume that combustion proceeds
under quasiadiabatic conditions. Apparently, an abrupt
increase in temperature leads to the \actuation" of en-
dothermic processes, primarily, aluminum boiling, ni-
tration with further formation of AlN or AlON, and va-
porization and dissociation of aluminum oxide. Thus,
the heat expended in vaporization of 1 mole of Al
2
O
3
DISCUSSION OF RESULTS
An analysis of the data given in Table 1 shows that
in accordance with the testing parameters proposed, the
most active powder is sample No. 7 (STPA-4 powder).
Derivatograms of sample Nos. 2 and 7 are shown in
Reactivity of Aluminum Powders
421
a
CONCLUSIONS
To test ultrane aluminum powders, one should use
several characteristics that are standard for ordinary
powders: particle shape and particle size distribution,
specic surface area, etc. [1]. At the same time, the
reactivity of ultrane powders is characterized by the
following parameters:
temperature of the onset of oxidation;
maximum oxidation rate;
x 5500
degree of conversion (degree of oxidation) of alu-
minum;
b
ratio of the thermal eect to the weight increase
measured under standard conditions (see Fig. 1).
x 5500
These parameters can be obtained under conditions of
nonisothermal oxidation in air under linear heating.
This set of parameters reects not only the reactivity
of the powders but also their special features, i.e., can
be used as a test for a particular powder (see Table 1).
If other oxidizers are used, the activity of such pow-
ders can also be determined from the above parameters
taking into account the special features of the system
\aluminum powder{oxidizer."
This work was supported by Ministry of Education
of the Russian Federation (Grant No. 98-8-5.2-74).
Fig. 2. Electron photomicrographs of products of ox-
idation in air for sample Nos. 2 (a) and 7 (b) (sample
numbers correspond to those of Table 1).
REFERENCES
at the boiling point is equivalent to the amount of heat
released in the combustion of 1.2 moles of the metal in
oxygen under adiabatic conditions [1]. Consuming the
heat of the reactive system, endothermic processes de-
crease the temperature, and this can occur several times
(for example, four times for sample No. 7). The self-
organization processes involved in the high-rate com-
bustion of the powders occur via the feedback described.
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chemical analysis and electron microscopy. In the pho-
tographs shown in Fig. 2, the combustion products of
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microstructure of the combustion products of STPA-4
ultrane powder changed radically in comparison with
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bustion products of the commercial powder remained
unchanged. Obviously, the appearance of combustion
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