34
Propellants, Explosives, Pyrotechnics 27, 34 ± 38 (2002)
Experimental Research of the Effects of Superfine Aluminum
Powders on the Combustion Characteristics of NEPE Propellants
Fang Chong and Li Shufen*
Department of Chemical Physics, University of Science and Technology of China, Hefei 230026 (P. R. China)
Summary
The combustion behavior of grading aluminum powders
containing superfine aluminum powder (SAl) in NEPE propellant
has been studied by several kinds of experimental techniques. The
results indicate that the usage of grading aluminum powders
containing SAl can effectively improve the combustion characteristics of NEPE propellant and the combustion efficiency of
aluminum. The reason is that SAl has the different combustion
and thermochemical properties from those of generally powdered
aluminum (Al). SAl is inclined to burn in a single step, hence
greatly increasing the heat released during the thermal decomposition of NEPE propellant.
Wang(2) et al. selected four different aluminum particle
sizes (30 mm, 5 ± 7 mm, 1 mm, and < 1 mm) in HTPB/HMX/
AP/Al propellants, in which the solid content was 87 wt% to
investigate their effects. Corresponding experimental results showed that SAl could raise the burning rate, reduce
the pressure exponent and improve the propellant×s combustion characteristics. In NEPE propellants, the similar
research work should be carried out more rigorously,
especially for the effects of SAl, due to the complex
interactions among all the solid propellant ingredients and
their different thermal behaviors.
1. Introduction
2. Experiments and Results
Nitrate ester plasticized polyether (NEPE) propellants
are a series of new-style propellants that possess good
energetic and mechanical properties. They have many
advantages existing in composite propellants and modified
double base propellants, but their combustion characteristics are somewhat non-ideal. For example, their pressure
exponent is rather high and the range over which the burn
rate can be adjusted is relatively small. Powdered aluminum
is one of the main components in NEPE propellants, with
the general content of about 15 to 20 wt%. Many factors,
such as the content and particle size of ammonium
perchlorate (AP) and aluminum powders and the types of
catalyst, can influence the combustion characteristics of a
NEPE propellant. So in the same basic formulation, besides
the first examination of the effects of AP particle size, the
aluminum particle size was also considered. Both the
experimental results and theoretical calculations(1) have
shown that in formulations with the same oxidizer particle
size the reduction of the aluminum particle size results in the
more serious aluminum agglomeration on the combustion
surface of NEPE propellant and the formation of bigger
aluminum agglomerates. Recent research(2,3) has indicated
that, compared with powdered aluminum whose particle
size is between 1 and 10 mm, SAl (< 1 mm) has quite different
combustion performance. SAl can resist the agglomeration
more efficiently, so in the series of HTPB propellants, it can
improve their ignition and combustion characteristics.
* Corresponding author; e-mail: lfs@ustc.edu.cn
¹ WILEY-VCH Verlag GmbH, 69469 Weinheim, Germany, 2002
2.1 Experimental Formula
In the basic formula, the solid content AP/HMX/Al is 17/
40/18 wt%, respectively. The rest is the polyethylene glycolNE binder. Three different particle sizes of aluminum were
used to formulate propellants, and the burning rates and
pressure exponents were then measured. Details are given
in Table 1.
These data reveal that the decrease of aluminum particle
size can increase a propellant×s burning rate, and that effect
can be more significant with a component of fine grain SAl.
2.2 Observation of Flame Images
For the sake of investigating the combustion behavior and
taking flame photographs, we cut the propellant into a slab
with the size of 5 2 15 mm3. The test slabs, without
inhibitor, were then stuck on to a stainless-steel plate with a
dowel. A nickel-chromium wire with diameter of 0.3 mm
was used for ignition, and it had a tight contact with the
upper surface of the slab. Two windows with a lighttransmitting area of 70 26 mm2 were designed to the
diametrically opposite direction of the combustion chamber, and they were utilized for macrophotographic observations and back lighting before ignition.
At three pressures (0.98, 2.94 and 4.90 MPa) in N2
atmosphere, the flame characteristics of the burning samples were observed in the combustion chamber. We find that
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Propellants, Explosives, Pyrotechnics 27, 34 ± 38 (2002)
Experimental Research of the Effects of Superfine Aluminum Powders
Table 1. Burning rates and pressure exponents of the samples.
Propellant
Samples×
No.
Aluminum
particle size
(mm)*
Burning
rate
r (mm/s)
Pressure
exponent
n
NE-A
NE-B
NE-C
30
30; and < 1**
7
9.41
9.94
9.75
0.60
0.66
0.69
* Average particle size of the industrial aluminum powders in use.
** The amount of 30 mm aluminium of < 1 mm aluminium 5 : 1.
at the low pressures, the combustions all formed many
aluminum agglomerates, while sample NE-B has the smallest agglomerates in a fairly even flame. At the high
pressures, all the samples× combustion becomes more
violent, which produce smaller aluminum agglomerates.
This tendency is more manifest in sample NE-B, which
forms a succession of brighter flames.
2.3 Propellants× Aluminum Agglomerates and Optical
Density Analysis
Large quantities of negative films on the whole flame zone
of burning samples were taken by the specially designed
close-range photographic system, at 0.98, 2.94 and 4.90 MPa
of N2 atmosphere pressures. Utilizing the 20-fold spectral
projector to count up the aluminum agglomerates on the
negatives (at 2.94 and 4.90 MPa), we obtained the size
distribution of aluminum agglomerates (Table 2). The
optical density of the flames on the negatives was also
determined through the transmission densitometer.
The above data indicate that,
(1) in the gas phase of the same sample, aluminum agglomerates× size will decrease when the pressure increases;
(2) at the same pressure, aluminum agglomerates× size is the
smallest in sample NE-B that contains SAl, which
means the addition of SAl can reduce the aluminum
agglomeration effectively.
From the optical density analysis of the combustion flame
negatives, we can also conclude that whether the pressure is
high or low, sample NE-B×s flame brightness is the highest.
That is because its combustion is the most turbulent, while
the light radiation of its burning aluminum powders in the
gas phase is the strongest.
Table 2. Size distribution of aluminum agglomerates in the gas
phase.
Sample
NE-A
NE-B
NE-C
Pressure
( MPa)
< 100 mm
(%)
100 200 mm
(%)
> 200 mm
(%)
2.94
4.90
2.94
4.90
2.94
4.90
76.2
79.4
99.7
99.5
91.1
99.0
10.4
10.7
0.3
0.5
6.3
1.0
13.4
9.9
±
±
2.6
±
35
2.4 Gravimetric Analysis of the Combustion Residue
Since there is aluminum in propellants, the combustion
products contain solid Al2O3 and remaining Al that has not
burned completely. These solid products compose the
residue. Observing carefully, we found out that the residue
consisted of two patterns of components: one is the white
powder, relatively thin, which is Al2O3 formed after the
burning of aluminum; the other is the gray powder, normally
thick, the majority of which is the remaining aluminum.
The residue on the bottom of the chamber was collected
and weighed. Because Al2O3 initially forms in the gas phase,
mainly solidifies and deposits on the wall of the chamber, the
residue staying at the bottom of the chamber is almost
entirely consisted of gray powders. Due to the very small
amount, we cannot determine the content of activated
aluminum in the residue by ordinary chemical methods.
The results also reveal that at the three pressures, the
residue amount of sample NE-B is always the smallest and
its color is white. That means SAl can reduce the quantity of
remaining aluminum and therefore raise the combustion
efficiency of powdered aluminum.
2.5 Deflagration Heat Experiment
The propellant×s deflagration heat was measured by the
thermostatic oxygen bomb calorimeter, at 1.96 MPa in N2
atmosphere. Table 3 lists the average values of the three
experimental results.
Together with the analysis data of the residue, it can be
seen that the more completely aluminum powders in
propellants burn, the higher is the deflagration heat of
those propellants.
2.6 DSC Analysis
In order to investigate the effect of aluminum particle size
on decomposition behavior of the propellant, and to
quantitatively inspect its influence on the condensed-phase
reaction heat, DSC technique was adopted to measure the
three propellant samples several times. Table 3 has the
experimental results.
The grading aluminum powders with SAl, as we can infer,
will make the first exothermal peak of the propellant
emerge more than 20 8C sooner, and greatly increase the
total heat released during the propellant×s thermal decomposition.
2.7 Ignition Temperature
At the room temperature and 1atm air pressure, the
ignition temperatures of three samples were measured at
the heating rate of 10 8C/min. See Table 3.
Compared with the total amount of released heat
obtained through DSC analysis, the conclusion can be
36
Chong, Shufen
Propellants, Explosives, Pyrotechnics 27, 34 ± 38 (2002)
Table 3. Deflagration heat, DSC and ignition temperature measuring results of the propellants.
Propellant
sample
Average deflagration heat
( J/g)
NE-A
NE-B
5703
9500
NE-C
8907
DSC results
Ignition
temperature
(8C)
Decomposition
Peak Temperature (8C )
Total amount
of released heat ( J/g)
205.03
183.75
279.49 346.38
195.39
547.69
812.44
147
132
509.21
157
made that the propellant×s ignition temperature will decrease with the increase of released heat during the
propellant×s thermal decomposition.
3. Analysis and Discussions
The above experimental results indicate that in NEPE
propellants, the large aluminum particle size is helpful to
reduce the pressure exponent, while the fine-grained
aluminum can increase the burning rate, especially when
some SAl (< 1 mm) is added. At the same time, we cannot
deny that the fine-grained aluminum will raise the pressure
exponent of the propellant to some extent, but the situations
will be improved for grading aluminum powders containing
SAl.
Two mechanisms are therefore put forward, through
which SAl improves the propellant×s combustion characteristics.
3.1 SAl×s Combustion Behavior in Propellants is Different
from that of Al
First, the experiment(2) show that SAl×s antiagglomeration
properties are much better than Al×s at high temperatures.
Hence, it won×t be easy for SAl to agglomerate seriously on
the burning surface of the propellant. For Al, they will easily
break through the zone of oxidation to connect with each
other after melting on the burning surface, and finally form
aluminum agglomerates.
Second, aluminum powder×s ignition energy will increase
at the order of magnitude with the increase of its particle
size(3). Compared with Al, SAl possesses a lower ignition
energy.
After making a comprehensive survey on the effect of
surface reactions, heat radiation and heat conduction of
aluminum particles, T. A. Roberts, et al.(4) constructed a
particle-heating model, in which they also presumed that the
aluminum particle will ignite when its temperature reaches
the melting point of Al2O3. Through this model, they
discovered that when the gas temperature was less than
3000 K, the contribution of heterogeneous surface reactions
for aluminum powders× ignition energy could not be
neglected. In fact, when the propellant burns, aluminum
powders will ignite on its combustion surface, which
temperature is normally less than 1000 K. Under such
circumstances, the heterogeneous surface reaction becomes
the main source of the aluminum powders× ignition energy.
Since the surface oxidation-reduction reaction heat is
approximately proportional to the surface area, when the
aluminum particle size is smaller than a certain critical
value, heat released during its surface oxidation reaction will
be enough to ignite the aluminum particle, and it tends to
ignite in a single step.
What×s more, SAl experiences different physical and
chemical processes from Al on the propellants× combustion
surface. The latter will stay long there, due to its long ignition
delay time; as a result of the surface viscous force, aluminum
powders will then gather, melt, connect and agglomerate
with each other on the surface, then finally ignite to enter the
gas phase. On the other hand, by virtue of SAl×s rather good
antiagglomeration properties and its fairly low ignition
energy, when the burning surface withdraws to expose it,
SAl can rely on its own surface oxidation reaction heat to
reach its ignition temperature quickly, and to shorten its
ignition delay time. The majority of those aluminum
particles can directly enter the gas phase to burn in single,
and their agglomeration can be effectively prevented. The
decrease of aluminum amount on the combustion surface
can therefore reduce the chance for aluminum particles to
melt, connect and agglomerate with each other; the size of
forming agglomerates consequently decreases.
3.2 SAl×s Thermochemical Behavior in Propellants is
Different from that of Al
3.2.1 Thermal Analysis of Powdered Aluminum
The DTA experiments on four kinds of powdered
aluminum with the different particle size (30, 5 7, 1,
< 1 mm) were carried out, at the temperature scanning-rate
of 10 8C/min, and the results are shown in Figure 1. The [uv]
on the ordinate is a kind of measurement for voltage, which
value can reflect the heat effect of samples. The voltage
range is related to the sensitivity of DTA analysis. For easy
comparison, we made corresponding translations to put four
experimental curves into one figure, so the ordinate values
do not possess exact meanings any more. The symbol (a. u.)
was therefore used to represent the ™approximate unit∫.
It can be seen that SAl vigorously releases heat near its
melting point (660 8C for pure aluminum), which indicates
Propellants, Explosives, Pyrotechnics 27, 34 ± 38 (2002)
Experimental Research of the Effects of Superfine Aluminum Powders
37
burning rate of the propellant will then be raised at high
pressures not because of the grain size of aluminum, but
because of aluminum itself. That×s why the pressure
exponent of grading aluminum powders containing SAl is
smaller than that of fine-grained aluminum alone, this
tendency is also in agreement with the data represented by
Table 1.
Here another reason for us to use grading aluminum
powders lies in that the complete usage of Al results in the
combustion instability of the propellant and the instantaneous dramatic increase of its burning rate(8), while the
initial step to make the propellant containing SAl only is
rather difficult. The activated aluminum content inside SAl
is also less than Al(2).
Figure 1. DTA curves of the four kinds of powdered aluminum.
its two obvious exothermic peaks in Figure 1; Al does not
absorb or release heat significantly between 0 and 500 8C,
and only has an obvious endothermic peak near its melting
point. In fact, all kinds of aluminum powders will absorb
heat near 660 8C. This heat of transformation, which is
illustrated as the negative heat effect in Figure 1, increases
when the aluminum particle size becomes larger. These
differences also foreshadow the distinct characteristics
between SAl and Al.
We know from the discussion in section 3.1 that SAl×s
acting place locates on the combustion surface and its
approaching area, where the surface-melting layer forms
when the propellant burns. Since the temperature of this
layer is between 600 and 900 K(5), and SAl will significantly
release heat near 600 8C (illustrated in Figure 1), the effect
of SAl to raise the propellant×s burning rate is therefore
attributed to SAl×s combustion and its released heat. During
the combustion process, the thickness of the surface-melting
layer will decrease with the increase of pressure, so will the
condensed-phase reaction fraction. We look on them as two
parameters.
At intermediate pressures, the numerical values of these
parameters are relatively large, so the probability for the
oxygen-rich products given out during AP×s decomposition
to diffuse through the binder surface-melting layer to SAl×s
surface is relatively big. Then SAl will mainly ignite on the
combustion surface and the increased heat feedback will
efficiently raise the propellant×s burning rate(6). Due to its
rather high ignition energy and its rather small heat released
in surface ignition, fine-grained aluminum obviously would
not exhibit this effect.
When the pressure increases, those parameters× being
smaller will make less SAl to burn in single particle on the
surface-melting layer and more aluminum particles are
easily pushed to the gas-phase flame position by an impetus
from combustion gases. A. A. Zenin(7) elucidated that heat
interaction between the reaction layer of the condensed
phase and hot portions of the gas phase is very slight because
the distances between the burning surface and the hot flame
position are much larger than conductive size. The heat
feedback will therefore decrease especially for SAl, and the
3.2.2 Thermochemical Behavior of Aluminum Powders in
Propellants
In the several propellant samples studied in this paper, the
aluminum particle size is the only varied factor, so the
differences appearing in thermoanalysis results origin
mainly from this factor. The obvious effect of sample NEB, which has grading aluminum powders containing SAl, to
significantly raise the heat released during the thermal
decomposition of the propellant can be explained by the
interaction between aluminum particles and the oxidizer.
When the oxidizer decomposes, heat will be released and
transferred to aluminum powders. If the powdered aluminum particles cannot get in contact with each other freely,
the thermal conductivity is low and it becomes difficult to
lose heat through conduction. Hence the aluminum particle
can be easily heated to a higher temperature, and the ™local
heat point∫ can be formed. In consequence, the ™local heat
point∫ strengthens the interaction and releases extra heat,
which can further increase the propellant×s thermal decomposition reaction heat. Conversely, the relatively easy
contact between aluminum particles will make it difficult
to form the ™local heat point∫.
Based upon the above-mentioned viewpoint, Al will be
classified as the latter situation, which predicts less decomposition reaction heat. Meanwhile grading aluminum powders containing SAl belong to the former condition, which
points to the easier formation of ™local heat point∫ with the
aluminum particle as the center, the increase of total heat
release and the improvement of the propellant×s ignition
characteristics. The reason lies in the fact that SAl is much
easier to ignite than Al and in the grading aluminum
powders, SAl cannot contact with each other because of the
separation by larger aluminum particles. That implies the
existence of the ™local heat point∫.
4. Concluding Remarks
In NEPE propellants, grading aluminum powders containing SAl can raise the propellant×s burning rate, improve
38
Chong, Shufen
its combustion characteristics and increase the specific
impulse efficiency of the solid motor, while the propellant×s
pressure exponent is lower than one with fine-grained
aluminum alone. Besides the adjustment of the basic
formula, the further reduction of the pressure exponent
can be accomplished by many new and exciting methods,
such as the surface treatment of powdered aluminum and
the addition of appropriate catalysts, based on more precise
physical and numerical models established in the future.
5. References
(1) Jin Leji, Li Shufen, ™A Sponge Model for Aluminum
Agglomeration in Solid Composite Propellants∫, Yuhang
Xuebao 3, 25 ± 32 (1989), [Chinese].
(2) Wang Guilan, Li Shufen, et al., ™Research on Combustion
Performance of Superfine Aluminum Powder∫, Bing Gong
Xue Bao 18(2), 23 ± 26 (1996), [Chinese].
(3) Jin Leji, Deng Kangqing, et al., ™Primary Research on
Combustion Performances of Superfine Aluminum Powder∫,
Journal of Propulsion Technology 6, 68 ± 72 (1993), [Chinese].
Propellants, Explosives, Pyrotechnics 27, 34 ± 38 (2002)
(4) Ted A. Roberts, et al., ™Ignition and Combustion of Aluminum/Magnesium Alloy Particles in O2 at High Pressures∫,
Combustion and Flame 92, 125 ± 143 (1993).
(5) J. P. Renie, ™Temperature and Pressure Sensitivity of Aluminized Propellants∫, AIAA Paper 80 ± 1166.
(6) Deng Kangqing, et al., ™The Characteristics and Model for
Superfine Aluminum Powder Combustion∫, Journal of Solid
Rocket Technology 19 (1), 28 ± 35 (1996), [Chinese].
(7) A. A. Zenin, ™Thermophysics of Stable Combustion Waves of
Solid Propellants∫, in: L. DeLuca, E. W. Price, M. Summerfield (eds), ™Nonsteady Burning and Combustion Stability of
Solid Propellants∫, Vol. 143, Progress in Astronautics and
Aeronautics, AIAA, Washington D. C. 1992, pp. 197 ± 231.
(8) V. N. Simonenko, V. E. Zarko, ™Comparative Studying the
Combustion Behavior of Composite Propellants Containing
Ultrafine Aluminum∫, 30th Int. Annual Conference of ICT,
Karlsruhe, Germany, June 29 ± July 2, 1999, pp. 21/1 ± 14.
Acknowledgement
The authors are grateful to the National Natural Science
Foundation of China for financial support.
(Received February 1, 2001 ; Ms 2001/012)