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
It is known that superfine aluminum powders (d 0.05 m) are contact zones which slightly
(SFAP) can successfully replace the micron- change the shape of the particles. The presence
sized aluminum powders (10 100 m) in pro- of agglomerates of particles leads to heteroge-
pellants. Such substitution leads to an increase neity of the mixtures and to coalescence of
in the combustion efficiency of aluminum and agglomerates in the heat penetration zone dur-
also leads to decreased agglomeration of the ing combustion. In this case large drops are
combustion products and reduction of two- formed, so the advantages of using SFAP are
phase losses [1, 2]. The decrease in the size of lost.
aluminum particles and increase in the reac- It has been experimentally established that
tion s surface area, considerably increase the additives to the Ar gas used in EEW, such as
combustion rate of the propellant composition chemically active gases (O2 and N2), lead to the
[1]. SFAP obtained by the electrical explosion of products from EEW being more dispersed [4].
wires (EEW) has been studied in detail [3 9], Reduction of particle size in this case is because
and interest in such SFAP continues to rise [10]. of a decrease in agglomeration and sintering
The formation of particles under conditions of during EEW. The presence of high-melting
electrical explosion (power density 1013 point non-metallic compounds (AlN, Al2O3) on
W/cm3, time of process 1 10 s) can lead to the Al particles also reduces agglomeration during
stabilization of metastable energy-saturated the heating of SFAP during combustion, analo-
structures, which relax at relatively low temper- gous to when aluminum particles are encapsu-
atures and increase the reactivity of SFAP [11]. lated by high-melting point metals (Cu, Ni, Fe)
An increase in the dispersiveness of SFAP leads [12]. If the stabilization of SFAP is because of
to an increase in their reactivity. But the basic the formation of an oxide film, it leads to the
problem of using SFAP is the relatively low loss of 3 to 5 mass % of aluminum and to a
content of metallic aluminum (93 97 mass %) decrease in the combustion enthalpy of the
[5, 6] simultaneously with the high reactivity of powder. In other words, to stabilize SFAP it is
SFAP. Another problem of using superfine necessary to cover them by a film obstructing
powders as additives to propellants is the orig- future oxidation. If such a film is Al2O3 the
inal agglomeration of SFAP when produced by content of metallic aluminum is just 93 to 97
EEW. The original agglomeration of SFAP is mass %. Additionally, the Al2O3 is the sub-
connected with the reactivity of the particles stance containing aluminum in its highest de-
surfaces, and the necessity for particles to con- gree of oxidation (Al3 ), which is inert during
centrate and collect in the gas-phase. The ag- combustion. By adding nitrogen to argon during
glomerates of SFAP in the porous structures EEW, the dispersity of the powders obtained
increases, and a coating of AlN can be produced
*Corresponding author. E-mail: [e-mail] in the electrical explosion [4]. During passiva-
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
350 Y.-S. KWON ET AL.
Fig. 2. SEM photograph of SFAP with AlB2-coating.
Fig. 1. EPMA pattern of SFAP with AlB2-coating.
EXPERIMENT AND DISCUSSION
Boride-encapsulated particles of SFAP were
tion in air (this stage is necessary to stabilize
obtained by EEW of aluminum wires with a
powders), however, AlN is oxidized and hydro-
boron-containing coating. From the EPMA
lyzed; therefore the protecting film in this case
(Electron Probe Micro Analysis) data plotted in
is Al2O3 with some Al(OH)3. Thus, qualitative
Fig. 1, it was shown that the composition of the
improvements in the reactivity and characteris-
coating is close to AlB2. X-ray diffraction
tics of SFAP are needed.
(XRD) analysis of SFAP only indicates the
As an alternative to a protective film of oxide,
presence of an aluminum phase; thus AlB2 is
aluminum diboride (AlB2) as a coating is of-
not detected, clearly, because of the amorphous
fered in this present work. Such a coating of
characteristics of the coating. A SEM-photo-
particles forms during EEW [13] in contrast to
graph of these SFAP is given in Fig. 2, which
the oxide coating, which forms after EEW by
reveals that the particle sizes are not uniform. In
passivation. In this case, unlike the inert alumi-
fact, most particles have a diameter less than
num oxide coating, the combustion of alumi-
100 nm. Producing EEW-powders with a nar-
num diboride coating is exothermic by 41337
rower particle size distribution is possible by
kJ/kg of heat: the passivation layer is thus a
increasing the energy liberated in the wire and
material with a high enthalpy.
by using chemically active gases [4] or reagents.
TABLE 1
Characteristics of SFAP Obtained by EEW Method
SFAP e/eb, arb. Ss (BET), [Al0], [Al2O3]d,
s
No (cover film) Gas-media unit m2g mass % mass %
1 Al(AlB2) Ar 1.38 17.0 78.0 18.0% [AlB2] 1.0
2 Al(Al2O3) Ar 1.45 9.3 88.5 5.5
3 Al(Al2O3) Ar N2 1.64 16.0 89.0 5.0
4a Al(Al2O3) Ar 2.15c 12.1 94.8 4.0
a
Alex (Argonide Corp.) [15].
b
e/es-specific electrical energy liberated in the wire [4] (ratio of electrical energy liberated in the aluminum wire to the
energy of sublimation of Al eAl 12208 kJ/kg).
s
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
ton, 1 ( 660°C) 2 ( 1000 °C), vox, mg/min, S/ m, Calculated
No °C (%) (%) (t, °C) arb. unit H298, kJ/kg
c
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
4a 550 39.4 45.0 3.0 (541 555)  29406
a
Alex (Argonide Corp.) [15].
The characteristics of SFAP with aluminum (from the area of the peak under a DTA-
diboride coatings are given in Table 1, together curve) to the corresponding value of the mass
with those of other types of SFAP for compar- increase (from a TGA-curve) of the analyzed
ison. sample (S/ m, arb. unit).
During EEW the initial products of electrical
The standard mass of SFAP samples under
explosion (T 104 K) are cooled below the
investigation was 5 10 5 kg; the rate of
upper temperature boundary of the chemical
heating was 10°C/min in the DTA/TGA-
reactions (T 5 103 K). At such tempera-
analyzer. Parameters for the reactivity of SFAP
tures, because of the presence of reagents, the
and of SFAP  Alex (Argonide Corp.) are
formation of refractory compounds occurs. This
compared in Table 2. According to Table 2,
decreases the original agglomeration of parti-
coating the particles with aluminum diboride
cles and their sintering. The presence of boron
increases the thermal stability of SFAP. Thus,
(sample No. 1) or the addition of nitrogen to
the temperature for the onset of intensive oxi-
argon (sample No. 3) during the electrical ex-
dation rises by 30 to 40°C in comparison with
plosion actually leads to an increase by almost a
SFAP with a coating of oxide-hydroxide. The
factor of 2 in the specific surface area (Ss) of
degree of oxidation of sample No. 1 (with the
SFAP, in comparison with the SFAP obtained
AlB2-coating) during heating to 660°C ( 1) is
in argon (sample No. 2 in Table 1). In this case
lower by 6 to 16% than with sample Nos. 2 to
the specific electrical energy liberated in the
4, which begin to be oxidized at a lower temper-
wire (e/es) increases insignificantly for samples
ature. During the heating to 1000°C, the major-
No. 1 to 3, that is, the power expenditure for
ity (70 78.5%) of the SFAP is oxidized: there
metal dispersion (formation of 1 m2 of surface)
remained only either drops, which are formed
because of the presence of additives (samples
during the coalescence and sintering of SFAP
No. 1 and No. 3) is reduced about a factor of 2
particles, or particles of micron sizes. Less than
(see Table 1). The analysis of the reactivity of
half (45.0%) of sample No. 4 is oxidized during
SFAP was carried out through the previously
heating to 1000°C. Oxidation of sample No. 4 at
proposed parameters [14], derived from differ-
a temperature above 1000°C occurs analogously
ential thermal analysis of SFAP samples at
with the oxidation of micron-sized powders [5,
standard conditions. The reactivity of SFAP,
14]. The values of the maximum rate of oxida-
which characterizes their behavior in oxidized
tion (Vox) for sample Nos. 1, 3, and 4 are not
media was determined from four parameters:
much different, but the maximum value of Vox is
The temperature for the onset of intensive that for sample No. 2 with the largest particles
o
oxidation (ton, C), (see Table 1). The higher rate of oxidation of
The maximum rate of oxidation (vox, mg/ sample No. 2 can be attributed to relaxation
min), processes (defects annihilation, amorphous
The degree of conversion (degree of oxida- phases crystallization, and so forth). An inverse
tion) of Al in a certain range of temperatures dependence between the maximum rate of oxi-
( , %), dation (Vox) and the experimentally determined
The ratio of the oxidation thermal effect thermal effect (S/ m) is also observed. It is
352 Y.-S. KWON ET AL.
Metals in Active Media. Nauka Moscow (1972). See
possible to explain this dependence as because
also FTD-MT-24 551-73 translated from Russian by
of an increase in the heat losses at higher rates
Foreign Technology Division. Wright Patterson Air
of oxidation (see Table 2). Thus, the SFAP
Force Base. Ohio. Oct (1973).
obtained with the AlB2 coating has a higher
2. Tepper, F., Ivanov, G., Lerner, M., and Davidovich,
reactivity than the SFAP obtained by EEW
V., International Pyrotechnology Seminars, Proceedings.
under the offered parameters [14]. The calcu- Chicago, IL, ITT Research Institute No.24. p. 519 530
1998.
lated combustion enthalpies of all the samples
3. Jones, D. E. G., Brousseau, P., Fouchard, R. C.,
being studied show that replacing the oxide-
Turcotte, A. M., and Kwok, Q. S. M., J. Thermal Anal.
hydroxide film on particles with a film of AlB2
Calorimetry 61:805 (2000).
during combustion gives an additional (2.2
4. Kotov, Y. A., and Samatov, O. M., Nanostructured
4.1) 103 kJ/kg of heat. Moreover, the pres-
Mater. 12:119 (1999).
ence of boron as AlB2 in a composition can
5. Ilyin, A. P., Gromov, A. A., Reshetov, A. A., Tihonov,
D. V, and Yablunovskii, G. V., Proceedings of the 4th
promote the gasification of the metal and in-
Korea-Russia International Symposium on Science and
crease the combustion temperature [16].
Technology (KORUS 2000). Ulsan, South Korea, 2000,
Part 3. p. 299.
CONCLUSIONS
6. Ilyin, A. P., Gromov, A. A., Tihonov, D. V, and
Yablunovskii, G. V., The Fifth Korea-Russia Interna-
tional Symposium on Science and Technology
Aluminum diboride coating, as applied to SFAP
(KORUS 2001). Tomsk, Russia, 2001, Vol.2. p.278.
particles in the process of electrical explosion of
7. Rhee, C. K., Lee, G. H., Park, J. H, and Kim, W. W.,
aluminum wires, leads to an increase in the
Proceedings of the Second International Symposium on
dispersiveness by 2 times because of lower
Pulsed Power and Plasma Applications (ISPP-2001).
agglomeration. The properties of SFAP parti-
Chang-Won, Korea, 2001, p.314.
cles with the diboride coating are changed: their
8. Ushakov, V. Ya., Ilyin, A. P., Nazarenko, O. B.,
Krasnjatov Yu. A, An, V. V., Tikhonov, D. V., and
thermal stability (on 30 40°C) is higher than
Yablunovsky, G. V., Proceedings of the Firstt Korea-
SFAP obtained in an Ar medium or in a me-
Russia International Symposium on Science and Tech-
dium of Ar with N2. The significant part (more
nology (KORUS 97). Ulsan, South Korea, 1997, Vol.2.
than 40 mass %) of SFAP, passivated with AlB2,
p.167.
is oxidized in the range of temperatures from
9. Kwon, Y. S., Jung, Y. H., Yavorovsky, N. A., Ilyin,
660°C to 1000°C. SFAP with a covering of AlB2
A. P., and Kim, J. S., Scripta Mater. 44:2247 (2001).
10. Proceedings of the Vth All-Russia conference.  Phys-
gives an additional (2.2 4.1) 103 kJ/kg of
ics and Chemistry of Ultrafine Systems. 2000, Octo-
combustion enthalpy than other SFAP obtained
ber. Moscow, MEPhI. 2000.
by EEW. In the future it would be interesting to
check the reactivity of SFAP coated with AlB2 11. Ilyin, A. P., Phizika I khimiya obrabotki materialov
(Physics and Chemistry of Material Processing) 3:94
in AP/HTPB/Al propellants.
(1994).
12. Breiter, A. L., Maltsev, A. L., and Popov, E. I., Fizi.
Gore. Vzriva 4:97 (1990).
This work has been supported by the Korean
13. Russian Federation Patent 2139776.
Science and Engineering Foundation (KOSEF)
14. Il in, A. P., Gromov, A. A., and Yablunovskii, G. V.,
through the Research Center for Machine Parts
Comb. Expl. Shock Waves 37:418 (2001).
and Materials Processing (ReMM) at the Univer-
15. Mench, M. M., Kuo, K. K., Yeh, C. L., and Lu, Y. C.,
sity of Ulsan. The authors also are grateful to Drs
Comb. Sci. Tech. 135:269 (1998).
D.V. Tikhonov and G.V. Yablunowsky for useful
16. Spalding, M. J., Krier, H., and Burton, R.L., Comb.
Flame 120:200 (2000).
discussions.
REFERENCES
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