Reactivity of superfine aluminum powders stabilized by aluminum diboride, CHEMIA I PIROTECHNIKA, Chemia i ...

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Brief Communication
Reactivity of Superfine Aluminum Powders Stabilized by
Aluminum Diboride
YOUNG-SOON KWON
Research Center for Machine Parts and Materials Processing, School of Materials and Metallurgical Engineering,
University of Ulsan, San-29, Mugeo-2Dong, Nam-Ku, Ulsan 680-749, South Korea
ALEXANDER A. GROMOV*, and ALEXANDER P. ILYIN
High Voltage Research Institute, Tomsk Polytechnic University, 2a, Lenin Ave., Tomsk, 634050, Russia
INTRODUCTION
consist of superfine particles in the state of the
primary stage of sintering. Between the particles
(d
It is known that superfine aluminum powders
(SFAP) can successfully replace the micron-
sized aluminum powders (10 –100
m) are contact zones which slightly
change the shape of the particles. The presence
of agglomerates of particles leads to heteroge-
neity of the mixtures and to coalescence of
agglomerates in the heat penetration zone dur-
ing combustion. In this case large drops are
formed, so the advantages of using SFAP are
lost.
It has been experimentally established that
additives to the Ar gas used in EEW, such as
chemically active gases (O
2
and N
2
), lead to the
products from EEW being more dispersed [4].
Reduction of particle size in this case is because
of a decrease in agglomeration and sintering
during EEW. The presence of high-melting
point non-metallic compounds (AlN, Al
2
O
3
)on
Al particles also reduces agglomeration during
the heating of SFAP during combustion, analo-
gous to when aluminum particles are encapsu-
lated by high-melting point metals (Cu, Ni, Fe)
[12]. If the stabilization of SFAP is because of
the formation of an oxide film, it leads to the
loss of 3 to 5 mass % of aluminum and to a
decrease in the combustion enthalpy of the
powder. In other words, to stabilize SFAP it is
necessary to cover them by a film obstructing
future oxidation. If such a film is Al
2
O
3
the
content of metallic aluminum is just 93 to 97
mass %. Additionally, the Al
2
O
3
is the sub-
stance containing aluminum in its highest de-
gree of oxidation (Al
3
), which is inert during
combustion. By adding nitrogen to argon during
EEW, the dispersity of the powders obtained
increases, and a coating of AlN can be produced
in the electrical explosion [4]. During passiva-
m) in pro-
pellants. Such substitution leads to an increase
in the combustion efficiency of aluminum and
also leads to decreased agglomeration of the
combustion products and reduction of two-
phase losses [1, 2]. The decrease in the size of
aluminum particles and increase in the reac-
tion’s surface area, considerably increase the
combustion rate of the propellant composition
[1]. SFAP obtained by the electrical explosion of
wires (EEW) has been studied in detail [3–9],
and interest in such SFAP continues to rise [10].
The formation of particles under conditions of
electrical explosion (power density
10
13
s) can lead to the
stabilization of metastable energy-saturated
structures, which relax at relatively low temper-
atures and increase the reactivity of SFAP [11].
An increase in the dispersiveness of SFAP leads
to an increase in their reactivity. But the basic
problem of using SFAP is the relatively low
content of metallic aluminum (93–97 mass %)
[5, 6] simultaneously with the high reactivity of
SFAP. Another problem of using superfine
powders as additives to propellants is the orig-
inal agglomeration of SFAP when produced by
EEW. The original agglomeration of SFAP is
connected with the reactivity of the particles’
surfaces, and the necessity for particles to con-
centrate and collect in the gas-phase. The ag-
glomerates of SFAP in the porous structures
*Corresponding author. E-mail: rrc@uou.ulsan.ac.kr
COMBUSTION AND FLAME
131:349 –352 (2002)
© 2002 by The Combustion Institute
0010-2180/02/$–see front matter
Published by Elsevier Science Inc.
PII S0010-2180(02)00414-5
0.05
W/cm
3
, time of process 1–10
350
Y.-S. KWON ET AL.
Fig. 2. SEM photograph of SFAP with AlB
2
-coating.
Fig. 1. EPMA pattern of SFAP with AlB
2
-coating.
EXPERIMENT AND DISCUSSION
tion in air (this stage is necessary to stabilize
powders), however, AlN is oxidized and hydro-
lyzed; therefore the protecting film in this case
is Al
2
O
3
with some Al(OH)
3
. Thus, qualitative
improvements in the reactivity and characteris-
tics of SFAP are needed.
As an alternative to a protective film of oxide,
aluminum diboride (AlB
2
) as a coating is of-
fered in this present work. Such a coating of
particles forms during EEW [13] in contrast to
the oxide coating, which forms after EEW by
passivation. In this case, unlike the inert alumi-
num oxide coating, the combustion of alumi-
num diboride coating is exothermic by 41337
kJ/kg of heat: the passivation layer is thus a
material with a high enthalpy.
Boride-encapsulated particles of SFAP were
obtained by EEW of aluminum wires with a
boron-containing coating. From the EPMA
(Electron Probe Micro Analysis) data plotted in
Fig. 1, it was shown that the composition of the
coating is close to AlB
2
. X-ray diffraction
(XRD) analysis of SFAP only indicates the
presence of an aluminum phase; thus AlB
2
is
not detected, clearly, because of the amorphous
characteristics of the coating. A SEM-photo-
graph of these SFAP is given in Fig. 2, which
reveals that the particle sizes are not uniform. In
fact, most particles have a diameter less than
100 nm. Producing EEW-powders with a nar-
rower particle size distribution is possible by
increasing the energy liberated in the wire and
by using chemically active gases [4] or reagents.
TABLE 1
Characteristics of SFAP Obtained by EEW Method
No
SFAP
(cover film)
Gas-media
e/e
b
, arb.
unit
S
s
(BET),
m
2
g
[Al
0
],
mass %
[Al
2
O
3
]
d
,
mass %
1
Al(AlB
2
)
Ar
1.38
17.0
78.0
18.0% [AlB
2
]
1.0
2
Al(Al
2
O
3
)
Ar
1.45
9.3
88.5
5.5
3
Al(Al
2
O
3
)
Ar
N
2
1.64
16.0
89.0
5.0
4
a
Al(Al
2
O
3
)
Ar
2.15
c
12.1
94.8
4.0
a
Alex (Argonide Corp.) [15].
b
e/e
s
-specific electrical energy liberated in the wire [4] (ratio of electrical energy liberated in the aluminum wire to the
energy of sublimation of Al e
s
Al
12208 kJ/kg).
c
Calculated theoretically by correlation equation [8].
d
SFAP also includes some absorbed gases (not calculated in this Table).
REACTIVITY OF AL POWDERS
351
TABLE 2
Reactivity Parameters from DTA/TGA Analyses of SFAP
t
on
,
°C
1
(
660°C)
(%)
2
(
1000 °C),
(%)
v
ox
, mg/min,
(t, °C)
m,
arb. unit
Calculated
No
H
298
, kJ/kg
1
580
34.0
77.8
3.2 (580–600)
6.3
31635
2
540
40.0
70.0
5.6 (545–570)
5.6
27451
3
540
49.7
78.5
3.0 (550–605)
8.7
27607
4
a
550
39.4
45.0
3.0 (541–555)
—
29406
a
Alex (Argonide Corp.) [15].
The characteristics of SFAP with aluminum
diboride coatings are given in Table 1, together
with those of other types of SFAP for compar-
ison.
During EEW the initial products of electrical
explosion (T
(from the area of the peak under a DTA-
curve) to the corresponding value of the mass
increase (from a TGA-curve) of the analyzed
sample (S/
m, arb. unit).
10
4
K) are cooled below the
upper temperature boundary of the chemical
reactions (T
The standard mass of SFAP samples under
investigation was
5
10
5
kg; the rate of
10
3
K). At such tempera-
tures, because of the presence of reagents, the
formation of refractory compounds occurs. This
decreases the original agglomeration of parti-
cles and their sintering. The presence of boron
(sample No. 1) or the addition of nitrogen to
argon (sample No. 3) during the electrical ex-
plosion actually leads to an increase by almost a
factor of 2 in the specific surface area (S
s
)of
SFAP, in comparison with the SFAP obtained
in argon (sample No. 2 in Table 1). In this case
the specific electrical energy liberated in the
wire (e/e
s
) increases insignificantly for samples
No. 1 to 3, that is, the power expenditure for
metal dispersion (formation of 1 m
2
of surface)
because of the presence of additives (samples
No. 1 and No. 3) is reduced about a factor of 2
(see Table 1). The analysis of the reactivity of
SFAP was carried out through the previously
proposed parameters [14], derived from differ-
ential thermal analysis of SFAP samples at
standard conditions. The reactivity of SFAP,
which characterizes their behavior in oxidized
media was determined from four parameters:
5
10°C/min in the DTA/TGA-
analyzer. Parameters for the reactivity of SFAP
and of SFAP “Alex” (Argonide Corp.) are
compared in Table 2. According to Table 2,
coating the particles with aluminum diboride
increases the thermal stability of SFAP. Thus,
the temperature for the onset of intensive oxi-
dation rises by 30 to 40°C in comparison with
SFAP with a coating of oxide-hydroxide. The
degree of oxidation of sample No. 1 (with the
AlB
2
-coating) during heating to 660°C(
6 to 16% than with sample Nos. 2 to
4, which begin to be oxidized at a lower temper-
ature. During the heating to 1000°C, the major-
ity (70 –78.5%) of the SFAP is oxidized: there
remained only either drops, which are formed
during the coalescence and sintering of SFAP
particles, or particles of micron sizes. Less than
half (45.0%) of sample No. 4 is oxidized during
heating to 1000°C. Oxidation of sample No. 4 at
a temperature above 1000°C occurs analogously
with the oxidation of micron-sized powders [5,
14]. The values of the maximum rate of oxida-
tion (V
ox
) for sample Nos. 1, 3, and 4 are not
much different, but the maximum value of V
ox
is
that for sample No. 2 with the largest particles
(see Table 1). The higher rate of oxidation of
sample No. 2 can be attributed to relaxation
processes (defects annihilation, amorphous
phases crystallization, and so forth). An inverse
dependence between the maximum rate of oxi-
dation (V
ox
) and the experimentally determined
thermal effect (S/
1
)is
●
The temperature for the onset of intensive
oxidation (t
on
,
o
C),
The maximum rate of oxidation (v
ox
, mg/
min),
●
●
The degree of conversion (degree of oxida-
tion) of Al in a certain range of temperatures
(
,%),
●
The ratio of the oxidation thermal effect
m) is also observed. It is
S/
heating was
lower by
352
Y.-S. KWON ET AL.
possible to explain this dependence as because
of an increase in the heat losses at higher rates
of oxidation (see Table 2). Thus, the SFAP
obtained with the AlB
2
coating has a higher
reactivity than the SFAP obtained by EEW
under the offered parameters [14]. The calcu-
lated combustion enthalpies of all the samples
being studied show that replacing the oxide-
hydroxide film on particles with a film of AlB
2
during combustion gives an additional (2.2–
4.1)
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CONCLUSIONS
2 times because of lower
agglomeration. The properties of SFAP parti-
cles with the diboride coating are changed: their
thermal stability (on 30 – 40°C) is higher than
SFAP obtained in an Ar medium or in a me-
dium of Ar with N
2
. The significant part (more
than 40 mass %) of SFAP, passivated with AlB
2
,
is oxidized in the range of temperatures from
660°C to 1000°C. SFAP with a covering of AlB
2
gives an additional (2.2– 4.1)
10
3
kJ/kg of
combustion enthalpy than other SFAP obtained
by EEW. In the future it would be interesting to
check the reactivity of SFAP coated with AlB
2
in AP/HTPB/Al propellants.
This work has been supported by the Korean
Science and Engineering Foundation (KOSEF)
through the Research Center for Machine Parts
and Materials Processing (ReMM) at the Univer-
sity of Ulsan. The authors also are grateful to Drs
D.V. Tikhonov and G.V. Yablunowsky for useful
discussions.
REFERENCES
1. Pokhil, P. F., Belyaev, A. F., Frolov, Yu. V., Logachev,
V. S., and Korotkov, A. I.,
Combustion of Powdered
Received 3 December 2001; revised 23 May 2002; accepted 18
June 2002
10
3
kJ/kg of heat. Moreover, the pres-
ence of boron as AlB
2
in a composition can
promote the gasification of the metal and in-
crease the combustion temperature [16].
Aluminum diboride coating, as applied to SFAP
particles in the process of electrical explosion of
aluminum wires, leads to an increase in the
dispersiveness by
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