Quantitative Characterization of Internal Defects in RDX Crystals, CHEMIA I PIROTECHNIKA, Chemia i Pirotechnika

[ Pobierz całość w formacie PDF ]
Propellants, Explosives, Pyrotechnics 24, 255±259 (1999)
255
Quantitative Characterization of Internal Defects in RDX Crystals
Lionel Borne and Jean-Claude Patedoye
French-German Research Institute of Saint-Louis (ISL), F-68301 Saint-Louis Cedex (France)
Christian Spyckerelle
SociÂt Nationale des Poudres et Explosifs (SNPE), F-84706 Sorgues Cedex (France)
Quantitative Charakterisierung der Fehlstellen im Innern von
RDX-Kristallen
GrÈ ûe, Gestalt, Fehlstellen sind sehr wichtige Eigenschaften von
Explosivstoffkristallen. Diese Parameter spielen eine Rolle sowohl bei
der Herstellung als auch beim detonativen Verhalten von Spreng-
stoffformulierungen. Die Verwendung von Kristallen frei von
L
È
sungsmitteleinschlÈssen fÈhrt zur Abnahme der Schlag-
emp®ndlichkeit bei gegossenen Sprengstoffformulierungen. Viele
Anstrengungen zur Herstellung solch hochwertiger Explosiv-
stoffkristalle sind durchgefÈ hrt worden und werden noch weitergefÈhrt
Qualitative Beobachtungen von Kristallen mit Fehlstellen kÈnnen
durchgefÈhrt werden mittels optischer Mikroskopie und Vergleich mit
dem Brechungsindex. Ziel dieser Arbeit ist, zwei genaue quantitative
Methoden fÈr Messungen der Kristallfehlstellen zu entwicheln. Die
erste Methode basiert auf genauen Messungen der scheinbaren Kri-
stalldichte. Die zweite Methode bestimmt die Masse der EinschlÈsse
im Kristallinnern. Die Versuche wurden durchgefÈhrt mit zwei RDX-
Chargen. Die strenge Korrelation, die zwischen den Ergebnissen der
beiden Methoden auftritt, beweist die Richtigkeit der Messungen. Die
Meûmethoden sind komplement
È
r. Die Dichtemessungen liefern eine
genaue allgemeingÈltige Charakteristik der FehlstellenanhÈufung einer
Kristallcharge und erlauben eine Eingruppierung der Kristalle nach
ihrer Scheindichte. Mit der zweiten Methode werden die einge-
schlossenen Fremdsubstanzen bestimmt.
Caract
Â
risation quantitative des d
Â
fauts internes dans des cristaux
d'hexogÁne
La taille, la forme, les d
Â
fauts internes sont des propri
Â
t
Â
s impor-
tantes des cristaux d'explosif. Ces paramÁtres in¯uencent d'une part la
fabrication des compositions explosives, et d'autre part le comporte-
ment dÂtonique de ces mÃmes compositions. L'emploi de cristaux
d'explosif contenant peu de dÂfauts internes permet de rÂduire la
sensibilit au choc de formulations explosives coulÂes. Des tentatives
pour fabriquer des cristaux d'explosif sans dÂfauts ont Ât rÂalisÂes et
des travaux sont en cours. La microscopie optique avec adaptation
d'indice permet une observation qualitative de certains dÂfauts inter-
nes dans les cristaux tels que des cavitÂs occluses. L'objectif de notre
travail est de dÂcrire deux mÂthodes quantitatives de caractÂrisation de
ces cavit
Â
s internes dans les cristaux d'explosif. La premi
Á
re m
Â
thode
repose sur une mesure trÁs prÂcise de la densit apparente des cristaux.
La seconde m
Â
thode permet de mesurer la masse des esp
Á
ces con-
tenues dans les cavitÂs internes aux cristaux. Des mesures rÂalisÂes sur
deux lots de cristaux d'hexog
Á
ne sont pr
Â
sent
Â
es. Les deux techniques
de mesure des dÂfauts internes fournissent des rÂsultats com-
pl
Â
mentaires. La forte corr
Â
lation obtenue entre les r
Â
sultats issus des
deux mÂthodes de mesure valide les techniques et protocoles expÂri-
mentaux. La mesure de la densit apparente des cristaux fournit une
information quantitative globale sur les populations de dÂfauts internes
aux cristaux. Cette mÂthode de mesure est Âgalement un outil de tri des
cristaux en fonction de leur densit apparente. La seconde mÂthode
permet l'identi®cation et la quanti®cation des espÁces chimiques
contenues dans les cristaux.
Summary
crystal quality is a possible solution. Some important crystal
properties are studied and pointed out in several published
studies. The crystal size
(1)
, the crystal surface and shape
(2,3)
and the internal crystal defects population
(4±6)
are important
crystal properties which play a role on the shock to detona-
tion transition of cast explosive formulations. These crystal
parameters are also important for formulation processing.
They can modify the bonding between the crystals and the
surrounding (the polymer for PBX's). Many efforts have
been done and are in progress to ®nd methods of processing
explosive crystals with the suited properties
(7±10)
.
All these works need some metrology tools to quantify the
explosive crystal properties. Measurements of the crystal size
distributions are performed using several tools: sieving,
Coulter counter, laser diffraction or image analysis. Gas
adsorption methods and mercury intrusion porosimetry are
quantitative experiments to characterize crystal shape and
crystal surface. The aim of this paper is to present quantita-
tive tools to characterize the internal crystal defects popula-
tions. An accurate record of the crystal apparent density is
proposed to measure the global amount of the internal
defects
(4,6)
. Fine measurements of the mass of the entrapped
Size, shape, internal defects are very important properties of
explosives crystals. These parameters play a role on both the explosive
formulation processing and the detonic behavior of the explosive
formulations. The use of explosive crystals free of solvent inclusions
leads to decrease the shock sensitivity of cast explosive formulations.
Many efforts for processing such high quality explosive crystals have
been done and are still in progress. Qualitative observations of internal
crystal defects can be performed by optical microscopy with matching
refractive index. The purpose of this paper is to provide two accurate
quantitative tools for internal crystal defects measurements. The ®rst
method is based on accurate measurements of the crystal apparent
density. The second method records the mass of the species entrapped
in the crystal internal cavities. Experiments are performed on two
RDX batches. The strong correlation recorded between the results of
the two complementary methods validates the measurements. Appar-
ent density measurements provide an accurate global characterization
of the internal defects population of a crystal batch sorting the crystals
in function of their apparent density. The second method is a tool to
identify the species entrapped in the crystals.
1. Introduction
To reduce the vulnerability of explosive formulations,
several parameters can be tuned. Improving the explosive
# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0721-3115/99/0306±0255 $17.50:50=0
256 L. Borne, J. C. Patedoye, and C. Spyckerelle
Propellants, Explosives, Pyrotechnics 24, 255±259 (1999)
species in the internal cavities are performed. Based on two
dedicated RDX batches, the two kinds of measurements give
strongly correlated results, underlining the accuracy of the
experiments. RDX crystal internal cavities are mainly ®lled
with air, water and recrystallization solvent.
particle. This gives the accuracy required to be able to
characterize the global amount of internal defects in a
crystal using the record of the crystal apparent density. An
accuracy of 0.0001 g=cm
3
for the apparent density measure-
ments will allow the detection of an internal defect of 10 mm
®lled with air in a particle with a diameter of 250 mm.
The crystal apparent density measurement is performed
using a ¯otation method. Crystals are immersed into a
mixture of toluene and methylene iodide (CH
2
I
2
) to sort the
crystals of higher apparent density in the lower part of the
container. The mixture density can be tuned between
0.9 g=cm
3
and 3.3 g=cm
3
. The main points of the experiment
are:
2. RDX Batches
Two dedicated RDX batches have been used. The manu-
facturing processes of the two batches are similar but two
different solvents are used: acetone and cyclohexanone.
Particle sizes of both batches are located between 200 mm
and 300 mm. In both cases the water used during the crystal
processing has been marked by adding potassium bromide
(KBr). This allows to measure the amount of water entrapped
in the crystal internal cavities.
Figure 1a and Figure 1b are obtained using optical
microscopy with matching refractive index. These qualita-
tive observations show the internal defects populations of
different crystals. These crystal internal cavities are the
macroscopic part (optically observable) of larger crystal
defects
(11)
. Baillou and co-workers have shown that the
internal cavities contain recrystallization solvent and
mineral salt
(5)
.
A good accuracy for the mixture density measurements.
This is achieved using a PAAR densimeter using the
vibrating tube principle. The accuracy is 0.0001 g=cm
3
.
A good homogeneity and temperature stability of the
¯otation mixture. A double wall container and a ¯ow of
water between the two walls limit the residual tempera-
ture variations below 0.1
C.
The crystal solubility in the liquid mixture must be lim-
ited. The ¯otation mixture must exhibit a good wetting
for the crystals.
The experiment starts using a liquid density higher than the
apparent density of any particle.
Then the following steps are performed until all the
crystals have been extracted from the container:
Particles are dispersed in the mixture by stirring;
separation of crystals is performed by decanting. The
decanting time is a function of the crystal size. Two hours
is a convenient time for 200 mm crystals;
the lower part of the container is extracted, washed with
toluene, dried and weighted;
3. Experimental Tools
3.1 ISL Flotation Method and Apparent Density
Measurement
Table 1 gives the expected apparent density variation
generated by a spherical cavity in an academic spherical
Figure 1a. RDX 200=300 mm ± recrystallization with acetone.
Figure 1b. RDX 200=300 mm ± recrystallization with cyclohexanone.
Propellants, Explosives, Pyrotechnics 24, 255±259 (1999)
Quantitative Characterization of Internal Defects in RDX Crystals 257
Table 1. Apparent Density Variations for a Spherical Particle of Diameter D with a Spherical Cavity of Diameter d
(Material density: 1.8 (RDX), air density: 0.001205, acetone density: 0.792)
D
d
Dr
Dr
Dr
(mm)
(mm)
(cavity ®lled with air)
(cavity ®lled with water)
(cavity ®lled with acetone)
(g=cm
3
)
(g=cm
3
)
(g=cm
3
)
250
5 ÿ1.4E-05
ÿ0.6E-05
ÿ0.8E-05
250
10 ÿ11.4E-05
ÿ5.1E-05
ÿ6.5E-05
200
40 ÿ143E-04
ÿ64E-04
ÿ80E-04
250
40 ÿ73E-04
ÿ32E-04
ÿ41E-04
300
40 ÿ42E-04
ÿ19E-04
ÿ23E-04
250
80 ÿ585E-04
ÿ262E-04
ÿ330E-04
A small volume of toluene is added to the decanted
¯otation mixture in order to decrease its density of
0.001 g=cm
3
. Then the previous three steps are executed
again.
First it is important to identify the species entrapped in the
crystals. It is reasonable to assume that in internal crystal
cavities we may ®nd species present at the time of the
formation of the crystal: recrystallization solvent and water.
We proved also that air may be entrapped. One crystal-
lization experiment was realized under pure argon atmo-
sphere; the gas released after dissolution of the crystals was
identi®ed as argon using gas chromatography.
The quantity of entrapped crystallization solvent is deter-
mined by gas chromatography (internal standard method)
after dissolution of the crystals in a solvent (N-methyl
pyrrolidone).
An indirect method was set up for the determination of the
water content of the two dedicated batches. The water used
during the crystal processing was marked by adding a de®ned
amount of potassium bromide (KBr). For water content
measurement, crystals are dissolved in a solvent (cyclohex-
anone), and the solution is extracted by water. Water
entrapped in the crystals is deduced from the bromide ion
concentration measured by ion chromatography in the
extract. Results have been compared to those obtained by
the standard Karl Fischer determination on some samples,
and a good correlation has been found.
A special glass apparatus was designed to measure micro-
liters of gas evolved. Presence of air in the crystals has been
evidenced by volumetric measurements on some samples,
but because of a lack of accuracy of the method for small
volumes, air content was determined using volume computa-
tions.
The experimental results are given by plotting on the Y-
axis the cumulative weight in percent of crystals with an
apparent density higher than the given value on the X-axis.
Figure 2 gives the experimental results for the two RDX
batches. The four measurements performed on each batch
give the reproducibility of the experiment. Its accuracy will
be discussed later. The use of acetone as the recrystallization
solvent in this case leads to a smaller crystal apparent density
than the use of cyclohexanone.
During each experiment, crystals are sorted according to
their apparent density. For the two RDX batches, 11 classes
of crystals with a narrow apparent density distribution have
been obtained (Table 2). The amount of entrapped species
would be measured for crystals of most of the classes.
3.2 SNPE Entrapped Species Measurements
Table 3 gives the porous volume for several academic
spherical particles with spherical cavities of various sizes.
This leads to the amount of particles needed to get a total
porous volume of 1 ml. This illustrates the accuracy required
to set up the measurements of the entrapped species. Appro-
priate methods have been developed to search and to measure
the traces of the entrapped species.
Figure 2. Comparative crystal apparent density distribution for the
two RDX batches.
Table 2. Various Classes of RDX
Crystals Processed for Each RDX Lot
Classes
1:80005r g=cm
3
1:79905r51:8000 g=cm
3
1:79805r51:7990 g=cm
3
1:79705r51:7980 g=cm
3
1:79605r51:7970 g=cm
3
1:79505r51:7960 g=cm
3
1:79405r51:7950 g=cm
3
1:79205r51:7940 g=cm
3
1:79005r51:7920 g=cm
3
1:78805r51:7900 g=cm
3
r51:7880 g=cm
3
258 L. Borne, J. C. Patedoye, and C. Spyckerelle
Propellants, Explosives, Pyrotechnics 24, 255±259 (1999)
Table 3. Porous Volume (vp) for a Spherical Particle of Diameter D with only One Spherical
Cavity of Diameter d and Amount of Such Particles Needed to get 1 ml of Total Porous Volume
D
d
vp
Amount (g) of particles with only one cavity
(mm)
(mm)
(ml)
to get 1 ml of acetone if pores are saturated
with acetone
250
5
6.5E-08
225 g
(15 278 875 particles)
250
10
5.2E-07
28.12 g
(1 909 859 particles)
200
40
3.3E-05
0.22 g
(29 842 particles)
250
40
3.3E-05
0.44 g
(29 842 particles)
300
40
3.3E-05
0.76 g
(29 842 particles)
250
80
2.7E-04
0.05 g
(3 730 particles)
The amount of entrapped species (crystallization solvent
and water) were measured for crystals of most of the 11
crystal classes sorted from each of the two RDX raw batches.
The results are plotted on Figure 3a for RDX crystals
processed with acetone and on Figure 3b for RDX crystals
processed with cyclohexanone. The average crystal apparent
density of the class is reported on the X axis. The amount of
solvent and the amount of water entrapped in the crystal are
reported on the Y axis. A strong experimental correlation is
recorded between the crystal apparent density measurement
and the concentration of solvent and water entrapped in the
crystals. These results demonstrate the ef®ciency and the
accuracy of the various measurement tools employed.
performances of the sorting experiment in a disadvantageous
range of apparent density. Between 1.7950 g=cm
3
and
1.7980 g=cm
3
the apparent density distribution curves of
the two raw batches have the highest slopes.
For apparent density values below 1.7880 g=cm
3
, the slope
of the apparent density distribution curve is reduced and the
ef®cacy of the sort experiment is increased. A second sort on
the class of crystals grown in cyclohexanone whose apparent
density should be lower than 1.7880 g=cm
3
shows that 93%
in weight of crystals have an apparent density lower than
1.7880 g=cm
3
and 99% have an apparent density lower than
1.7900 g=cm
3
.
The ef®ciency of the sort experiment is a function of the
ratio between the volume forces and the surface forces and
consequently is a function of the crystal size and the viscosity
of the sorting liquid mixture. The ef®ciency of the sort
experiment decreases when the crystal size is reduced.
Practically below 100 mm the sort experiment needs to be
improved to reduce the role of the surface forces.
Nevertheless, the very good correlation between crystal
apparent density and the solvent and water content is a
validation of the measurements.
Evidence of air entrapped in the crystals has been proved
as described before. Solubility of air in solvents may not be
neglected. Part of the air released after dissolution of the
crystals may be solubilized by the solvent, that is why
volumetric measurements were found to be not accurate
enough and may only be used for determination of orders
of magnitude. Work is under progress to de®ne another
technique for accurate measurement of air entrapped in
crystals.
4. Results and Discussion
To check the accuracy of the sort experiment a second sort
was performed with some of the previous crystal classes. The
experimental results are compared with theoretical values
and plotted on Figures 4a and 4b. After the second sort some
crystals remain out of the apparent density limits resulting
from the ®rst sort.
For the selected class of crystals grown in acetone, the
second sort shows that 44% in weight of crystals remain
between the apparent density bounds (1.7970 g=cm
3
5r5
1:7980 g=cm
3
) and that 78% have an apparent density higher
than 1.7960 g=cm
3
. The ®rst sort gave 47% in weight of raw
crystals whose apparent density is higher than 1.7960 g=cm
3
.
The ef®cacy of the sort is not 100%. These data illustrate the
Figure 3a. Trapped species in the various classes sorting from the
RDX lot using acetone as the recrystallization solvent.
Figure 3b. Trapped species in the various classes sorting from the
RDX lot using cyclohexanone as the recrystallization solvent.
Propellants, Explosives, Pyrotechnics 24, 255±259 (1999)
Quantitative Characterization of Internal Defects in RDX Crystals 259
Figure 4a. Accuracy of the sorting experiment (acetone).
Figure 4b. Accuracy of the sorting experiment (cyclohexanone).
Accurate measurements of the air content would allow
calculations of the crystal apparent density and provide a
more complete checking of the apparent density measure-
ments.
Symposium (International) on Detonation, Portland, August 28±
September 1, 1989, Oregon, pp. 18±24.
(2) A. C. Van Der Steen, H. J. Verbeek, and J. J. Meulenbrugge,
``In¯uence of RDX Crystal Shape on the Shock Sensitivity of
PBX's'', Proc. 9th Symposium (International) on Detonation,
Portland, August 28±September 1, 1989, Oregon, pp. 83±88.
(3) L. Borne, D. Fendeleur, and A. Beaucamp, ``Explosive Crystal
Properties and PBX's Sensitivity'', DEA 7304 Physics of
Explosives, Berchtesgaden, Germany, September 1997 and ISL
Report ISL=PU 358=97.
(4) L. Borne, ``In¯uence of Intragranular Cavities of RDX Particle
Batches on the Sensitivity of Cast Wax Bonded Explosives'' ,
Proc. 10th Symposium (International) on Detonation, Boston,
July 12±16, 1993, Massachusetts, pp. 286±293.
(5) F. Baillou, J. M. Dartyge, C. Spyckerelle, and J. Mala, ``In¯uence
of Crystal Defects on Sensitivity of Explosives'', Proc. 10th
Symposium (International) on Detonation, Boston, July 12±16,
1993, Massachusetts, pp. 816±823.
(6) L. Borne, ``Microstructure Effect on the Shock Sensitivity of Cast
Plastic Bonded Explosives'', Europyro 95 6i
Á
me Congr
Á
s Inter-
national de Pyrotechnie, Tours, 5±9 juin, 1995, France, [Proc.]
pp. 125.
(7) P. M. Gallagher, M. P. Coffey, V. J. Krukonis, and W. W.
Hillstrom, ``Gas Anti-Solvent Recrystallization of RDX: For-
mation of Ultra-®ne Particles of a Dif®cult-to-Comminute
Explosive'', The Journal of Supercritical Fluids 5, 130±142
(1992).
(8) M. Y. Lanzerotti, J. Autera, L. Borne, and J. Sharma, ``Crystal
Growth of Energetic Materials during High Acceleration'' ,
Symposium Proceedings of the Materials Research Society,
Boston, November 27±30, 1995, Massachusetts, Vol. 418.
(9) J. H. ter Horst, R. M. Geertman, A. E. van der Heijden, and G. M.
van Rosmalen, ``Bench Scale, Cooling Crystallization of RDX'' ,
Proceedings 27th International Annual Conference of ICT,
Karlsruhe, 1996, pp. 126=1±126=13.
(10) U. Teipel, U. FÈrter-Barth, P. Gerber, and H. H. Krause, ``For-
mation of Particles of Explosives with Supercritical Fluids'',
Propellants, Explosives, Pyrotechnics 22, 165±169 (1997).
(11) D. Spitzer and M. Samirant, ``Shock Solicitation of PETN Single
Crystals Presenting Defects and Visualization of Hot Spots
Initiation'', Proc. 10th Symposium (International) on Detonation,
Boston, July 12±16, Massachusetts, 1993, pp. 831±840.
5. Conclusion
Experimental results and theoretical models underline the
important role of internal crystal defects on the shock
sensitivity of cast explosive formulations. This paper
described accurate quantitative measurement tools to char-
acterize the internal defects in a lot of crystals. The measure-
ment of the apparent density of the crystals provides
quantitative information on the global amount of internal
defects.
This work shows that quantitative tools exist for the
measurements of the chemical species entrapped in the
defects. A good correlation is found between the crystal
apparent density measurements and the amount of entrapped
species in the internal cavities.
The detection of an internal cavity of 10 mm in a crystal
whose average size is 200 mm is the low limit of the
measurements.
Theoretical models and computations suggest that the
sizes of the internal crystal cavities could play a role on the
formulation sensitivity. Tests for convenient metrological
tools are in progress.
Strong experimental correlation has been recorded between
the amount of internal defects and the shock formulation
sensitivity
(4,6)
. Accurate correlation between the formulation
sensitivity and the nature of the entrapped chemical species
could now be performed taking care to control the other
parameters (amount of defects, crystal size, . . .).
6. References
Acknowledgements
This work was performed under SNPE contract with a govern-
mental (DRET) ®nancial support.
(1) H. Moulard, ``Particular Aspect of the Explosive Particle Size
Effect on Shock Sensitivity of Cast PBX Formulations'', Proc. 9th
(Received December 4, 1998; Ms 32=98)
[ Pobierz całość w formacie PDF ]

  • zanotowane.pl
  • doc.pisz.pl
  • pdf.pisz.pl
  • tejsza.htw.pl

    •