CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
THE EFFECT OF ADDITIVES ON THE DETONATION
CHARACTERISTICS OF A LIQUID EXPLOSIVE
P J Haskins, M D Cook, R I Briggs
Defence Evaluation & Research Agency, Fort Halstead, Sevenoaks, Kent TN14 7BP, England
In this paper we report new experimental results on the detonation characteristics of nitromethane
containing high volume percentages of essentially inert additives. In particular, we have studied the
detonation of packed beds of small spherical glass and aluminium particles saturated with pure
nitromethane. These mixes are found to have reduced detonation velocities and critical diameters
compared to the liquid explosive alone. We conclude with a general discussion of the propagation
mechanism in such materials.
INTRODUCTION
particles saturated with pure NM. We interpret the
results of these new experiments, and draw some
general conclusions about the effect of inert
additives in general.
The effect of inert additives, both solid and liquid,
on the detonation of liquid explosives has been the
subject of a number of previous studies (e.g. 1-5). In
general, the effect of any inert additive will be to
reduce the detonation velocity and pressure since
some of the energy released will be used in heating
and accelerating the inert material. For a miscible
liquid additive, mixing will be at the molecular level
and full thermal and mechanical equilibrium can be
assumed. However, for solid additives the degree of
equilibrium achieved will depend on the size of the
additive particles. Solid additives also have the
effect of introducing hot spots, thus changing the
behaviour of the explosive from homogeneous to
heterogeneous. This latter effect means, that despite
the decrease in available energy, it is possible for
solid additives to give rise to an increase in
sensitivity, and a reduction in critical diameter.
The most commonly used explosive in such work
has been nitromethane (NM), often sensitised by an
organic amine. A systematic study of packed beds of
inert spherical beads saturated with sensitised NM
has been reported by Lee et al. (3, 4). In contrast,
here we report some experiments carried out with
packed beds of spherical glass or aluminium
EXPERIMENTAL
The experiments were all carried out in 300mm
long glass tubes, of various diameters, that were
completely filled with the composition under test.
The tubes were sealed at the bottom with a steel
witness plate, and were initiated at the top by a
booster charge. The booster had a length and
diameter equal to the diameter of the glass tube.
Experiments were carried out on pure NM (to
establish a baseline detonation velocity and critical
diameter in the glass tubes) and on packed beds of
glass and aluminium saturated with NM. The mixes
were prepared by part filling the tube with NM and
then slowly adding the solid until the mix just
became dry at the surface, and then repeating this
procedure in an incremental fashion until the tube
was filled. A very small amount of excess NM was
left on the top surface of the charge to assist takeover from the booster, and as an insurance against
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evaporation. All increments were weighed and the
mass ratio of solid to liquid was calculated.
The tests were filmed using a high-speed framing
camera operating at an inter-frame time of 2.1
microseconds, and the charges were front
illuminated with an argon flash bomb.
Smaller diameters were not tested due to the
difficulty in obtaining a uniform filling in smaller
diameter tubes. The detonation velocity extrapolated
to infinite diameter (see Fig. 1) was estimated to be
4.64mm/jLis.
The second series of tests employed aluminium
particles in two grades. For both grades the
compositions contained approximately NM 35% / Al
65% by mass. The first material was a spherical
grade with a mean diameter of 10.5|im. As with the
glass beads it was found that these mixes also
detonated down to diameters of 15mm. The
detonation velocity extrapolated to infinite diameter
(see Fig. 1) was estimated at 5.57mm/jLis.
RESULTS
Tests carried out on pure NM in a range of
diameters showed the critical diameter in the glass
tubes to lie between 20mm (no detonation) and
25mm (detonation). At 25mm diameter the
detonation velocity was measured at 6.32mm/jLis.
The pure NM results are plotted in Fig. 1, from
which the infinite diameter detonation velocity is
estimated to be 6.42mm/us. Figure 2 shows a frame
from the high-speed record of a NM test at 25mm
diameter.
0.01
0.02
0.03
0.04
0.05
0.06
FIGURE 2. Frame from a high-speed record showing detonation
of NM in a 25mm diameter glass tube.
0.07
1
1/d(rmf )
FIGURE 1. Detonation velocity versus inverse charge diameter
for (a) pure NM, (b) NM/Al(10.5jim), (c) NM/glass beads.
In addition to the tests using the 10.5urn material,
one experiment was carried out using a nanometric
grade of aluminium. The Argonide Corporation
supplied the nanometric grade, known as Alex. This
material is manufactured by an exploding wire
process and has a mean particle diameter of ca.
lOOnm, although there are some considerably larger
particles present. The composition based on this
material was tested in a 15mm diameter tube, and
again a stable detonation was observed. The
detonation velocity, at 5.66mm/us, was a little
higher than that observed with the larger aluminium
In the bulk of the experimental work we were
concerned with studying packed beds of small
particles saturated with NM. The first series of tests
were carried out using glass beads. The beads were
spherical and 0-44 um in diameter. The compositions
using these beads were approximately NM 23% /
Glass 77%, by mass. It was found that the critical
diameter for these mixes was reduced from that of
the pure liquid. Tests were carried out down to
15mm diameter, and all exhibited stable detonations.
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particles. It should be noted that whilst, in principle,
aluminium is a reactive additive it is not believed
that there is time for any significant reaction of even
the Alex material within the reaction zone of NM
(6). However, the after burning of the aluminium is
clearly visible in Fig. 3, which shows a frame from
the high-speed record of the 15mm diameter test
using Alex.
and a reduced velocity of detonation, compared with
NM alone. The detonation velocities of the NM/A1
compositions were higher than that for the
NM/glass, but it should be noted that the
NM/particle mass (and volume) ratio obtained was
higher for the aluminium mixes.
TABLE 1. Calculated detonation velocities for NM and
NM35%/A165%.
Composition
NM
NM / Reactive Al
NM / Inert Al
P (g/cc)
1.13
1.698
1.698
D (mrn/^is)
6.06
4.46
4.41
It is interesting to compare these results with those
obtained by Lee et al. (4) with sensitised NM. By
studying packed beds of spherical glass beads of
different sizes they found that the critical diameter
was a maximum for bead diameters of the order of
the critical diameter of the liquid explosive (ca. 12mm for amine sensitised NM). They reasoned that
there were two regimes, in which different
propagation mechanisms operated, depending on the
bead size. For large beads the detonation is thought
to merely propagate around the obstacles.
Consequently, as the bead size increases the
diffraction has less effect and the critical diameter
decreases towards that of the pure liquid. However,
for small beads detonation cannot propagate around
the beads, but shock transmission through them
continues to propagate reaction of the liquid
explosive in the interstitial pores. In this "smallbead" regime the critical diameter decreases as the
bead size decreases, but remains above that of the
liquid explosive alone. However, the smallest beads
studied by Lee et al. were in the 44-88um range, and
were therefore larger than those considered here.
The other, very significant, difference between the
studies lies in our use of pure, as opposed to
chemically sensitised, NM. Clearly, the large critical
diameter of pure NM means that the "small-bead"
regime would be expected to apply unless very large
(ca. 20mm) beads were used.
Clearly, the addition of high percentages of inert
additives represents a large potential dilution of the
energy available to support a detonation, and this
might be expected to lead to an increase in the
FIGURE 3. Frame from a high-speed record showing detonation
of NM/Alex in a 15mm diameter glass tube.
CHEETAH (version 2.0) (7) equilibrium code
calculations have been carried out for these mixes
assuming the aluminium to be inert or fully reactive.
Table 1 gives the detonation velocities calculated
using the BKWC equation of state at the
experimental densities. It can be seen that the
calculated velocity for pure NM is a little below
(0.36mm/|is) the infinite diameter value estimated
from the experiments. However, the calculated
values for the NM/A1 composition are considerably
(>lmm/|Lis) below those observed experimentally,
and this is discussed in the next section.
DISCUSSION
The results we have presented here show that
packed beds of both glass and aluminium particles
saturated with NM have smaller critical diameters,
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critical diameter. This is certainly true when
miscible liquids are added to NM. The addition of
acetone, in particular, has been extensively studied
(5) and shown to lead to a very rapid increase in
critical diameter with increasing dilution (critical
diameter > 200mm at 25% by volume acetone).
However, the addition of solid particles differs in
two respects from the addition of a miscible liquid.
Firstly, unless the particles are extremely small they
are unlikely to be in full thermal and mechanical
equilibrium within the detonation reaction zone, and
hence will not be fully effective as a diluent.
Secondly, small particles are capable of acting as hot
spots that can significantly sensitise the explosive.
Since, experimentally, we observe a reduction in the
critical diameter on addition of particles it seems
reasonable to assume that, for the particles
considered here, this effect far outweighs any
dilution effects.
The CHEETAH calculations provide further
evidence for lack of equilibrium between the
particles and the detonation products. The
CHEETAH calculations (Table 1), which assume
equilibrium, are seen to predict a significantly lower
detonation velocity for the NM/A1 composition than
that found experimentally, regardless of whether the
Al is assumed reactive or inert. It would therefore
appear that even for particles as small as lOOnm (the
Alex material) the very short reaction zone length of
NM means there is insufficient time for full
equilibrium.
The smaller particles used in this study are more
likely to be effective as a source of hot spots
(through shock interactions) than the larger ones
used by Lee et al. (4). This is probably at least part
of the reason that we observe a decrease in critical
diameter upon addition of particles, whereas the
earlier studies with larger particles see an increase
over that of the liquid alone. However, it is probable
that the large difference in sensitivity between pure
and chemically sensitised NM also plays a part. This
follows since the lower activation energy required
for decomposition of the sensitised NM means there
is a smaller gain in sensitivity available through the
introduction of hot spots.
It is not possible to draw any firm conclusions
about the differences in detonation velocity
observed between the NM/glass and NM/A1 mixes
because we did not achieve the same NM/solid
ratios. However, in view of the proposed
propagation mechanism it is likely that the shock
velocity in the solid particles will play a role in
determining the detonation velocity. As a
consequence we might expect a correlation of
observed velocity with the sound speed of the
additive. This would certainly be consistent with our
observations with glass and aluminium, but further
work is required to test this hypothesis.
CONCLUSIONS
We have shown that a packed bed of small glass or
aluminium particles saturated in NM can be
detonated at diameters less than that of the pure
liquid. We have also observed that the propagation
velocities of such mixes are less than that of NM,
but are higher than would be expected if the
particles and detonation products were in
equilibrium within the reaction zone.
The role of particles is important with regard to
understanding the important mechanisms controlling
the detonation process in non-ideal explosives.
Consequently, we hope to extend this work to
quantify the critical diameter changes, and study the
effects of different additives.
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