DETONATION STUDY OF ENERGETIC MICRO-SAMPLES

I. Plaksin, J. Campos, P. Simões*, A. Portugal*, J. Ribeiro,

R. Mendes and J. Góis

Laboratory of Energetics and Detonics Mech. and *Chem. Eng. Departments

Fac. of Sciences and Technology, Polo II-Univ. of Coimbra Pinhal de Marrocos, 3030 Coimbra

PORTUGAL

Detonation study of PBX on the meso-scale level of resolution is presented. Direct registration of 3D phenomena using an optical non-intrusive method allows to understand the micro-detonics mechanisms of HMX large crystals. Effects of particle sizes and binder nature are discussed. The existence of clusters and oscillating process allows to clarify the global phenomena of the detonation of PBX.

INTRODUCTION

Stability of PBX detonation is one of the fundamental problems of detonation studies. Results of previous works1-9,11, on the meso-scale level, show strong evidences that PBX detonation means an irregular reaction front followed by clusters of reacted zones, in a quasi-periodic structure, depending on the front curvature, particle size distribution, inter-particle distance and nature of binder.

Local instabilities of detonation wave (DW) are expected due to the heterogeneities of the system and due to the cells or clusters formation in detonation front (DF) surface, that starts in a heterogeneous media based in a homogeneous mix of particles of HMX (fine and coarse particles) and binder.

The results1 allow to conclude that self-sustained state the PBX detonation represents a global stable oscillating system, with a local non homogeneous distribution of reaction zones. This complex phenomena can be explained by the synergetic effect, between particles inside

binder, that took place in a non-linear field, far away from the equilibrium, when the system is loosing its stability. Then the existed small fluctuations2,5 lead to a new regime of more organised structures, that represents now the co-operative motion of large "associations" of reactive and reacted clusters8,9.

EXPERIMENTAL

Characterization of the samples

Two PBX compositions (PBX0 and PBXb) based on HMX and GAP binder (density ρ0 = 1.290 g/cm³) have been studied.

PBX0 was formed by 82% in mass of HMX, presenting a bimodal size distribution (80% of d50 = 204 µm and 20 % d50 = 18.4 µm) and 18% of non-cured energetic binder GAP.

PBXb was formed by 82% of HMX fine particles (d50 = 17 µm) and 18% of GAP. The compositions were used in a low-pressure pressing technique and non-polymerized way. HTPB was also used as

inert binder with a density of ρ0 = 0.901 g/cm³.

FIGURE 1. MICROPHOTO OF A COARSE (200 µm) HMX CRYSTAL

The density of fine and coarse particles of HMX, and of GAP/HMX based PBX, was measured by Helium gas pycnometry (Micromeritics - Accupyc 1330). The results presented in Table I. shown the density of HMX fine particles practically equal to TMD. The different density values found for HMX crystals are probably due to crystal micro-defects in coarse particles. A micro-photo of a typical coarse HMX crystal can be seen in Figure 1.

TAB. I - MEASURED DENSITIES.

Sample ρ / (g cm-3) HMX Coarse 1.894 ± 0.001

HMX Fine 1.917 ± 0.003* PBX0 1.745 ± 0.001 PBXb 1.759 ± 0.001

*Standard deviation.

Recording optical system

A high resolution optical method, based on multiple 64 fibber strips (with the diameter of each fibber - as well as the inter-axis distance between two adjacent fibbers- equal to 250±1µm), has been used for the DW propagation registration in PBX mini and macro samples, with a maximum temporal resolution of 0.6 ns. The fibber strip is connected directly to a fast electronic streak camera (THOMSON-TSN 506 N) without any intermediate optics.The light associated to the DW or shock wave [SW] is then transmitted through the optical fibber line to the streak camera. A Multi-Fibber Optical Probes [MFOP] were developed for the direct view

registration of light emitted by shock or detonation front placing several bands in predefined positions. The MFOP allow the characterisation of 2D shock and detonation waves in PBX, within a few mm of the propagation, before touching the optical probe. This non-shocked layer of PBX (2~2.5 mm of thickness) is partially transparent of DF light emission.

Placing a Kapton (Polyimide) film, between the PBX sample and MFOP operating surface, it is possible to filter the transmitted light, when the SW crosses the sample/Kapton interface, due to the shock destruction of the polyimide layer. A stack of Kapton films, with µ-gaps between adjacent surfaces, filled by air or heavy inert gas (Argon), allows a set of sub-nanosecond successive flashes, in the process of SW run through the multi-layer barrier. The registration of (z-t) diagram using the MFOP (z is the geometrical axis of the explosive charge) of SW propagation into the Kapton stack barrier, provides the way of 2D measurements not only of the SW velocity and pressure fields (that are representing the post detonation effects), but also the light irradiation effect of proper detonation. For calculations of shock pressure it was considered the shock Hugoniot properties of Kapton10.

Standard SW generator

SW Generator (SWG), based on double explosive charge (∅ 25 mm, total length 110 mm, in brass and steel confinement) is connected to the Kapton (polyimide) stack barrier (3-4 layers of Kapton film, each with 125 µm thickness) and separated by argon or air gaps with predefined values (5, 10 or 12.5 µm).

Explosive charge is formed by an RDX plastic explosive (PE-4A) mounted in brass confinement. The second stage is 10 mm thick charge of PBXb. , that was selected for the terminal part of SWG, due to its detonation characteristics, like high DW stability, minimum irregularities in DF and

relatively high values of velocity and pressure.

SW induced by PBXb detonation in Kapton stack barrier have been measured in standard Long Charge Test5. Experimental set up and typical photochronogram are shown in Figure 2.

Time 240 ns

FIGURE 2 EXPERIMENTAL SET-UP AND PHOTOCHRONOGRAM.

Mean value of detonation velocity (D = 8.03 ± 0.09 mm/µs ) was obtained from twelve experimental points of DF (z-t) diagram, recorded in the terminal zone (3.25 mm) of its run in PBXb charge (z is the direction of DW propagation). The diagrams of shock velocity US(z) and pressure PKap(z), are presented in Figure 3, showing the attenuation of SW induced in Kapton stack barrier.

Values of PKap were obtained from the measures values of US , using the Hugoniot properties of Kapton. As it can be seen from US(z) diagram, the measured SW becomes stable after the SW run of the first three Kapton layers (380 µm) with final lower values. In these conditions, SF is highly smooth, without local fluctuations in its entire spherical surface of 150 mm radius. Consequently SWG allows the simultaneous parallel testing up to four EM micro-samples in the same experiment.

0

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6

8

0 200 400 600 800 1000

z (µm)

Us

(mm

/µs)

0

10

20

30

40

P (

GP

a)

PBXb = HMXf + 18% GAPUs,Kapton f(z)

PKapton f(z)

D = 8,03+0,09mm/µs

FIGURE 3. ATENUATION OF SW INDUCED BY DETONATION OF PBXb, IN KAPTON STACK BARRIER.

Shock characterization of GAP and HTPB binders.

Two microsamples of pure outgased liquid binders GAP and HTPB, encapsulated in cells and organised in a Kapton Stack barrier, were tested in parallel using the standard SWG. A schematic representation of the used experimental set-up and obtained photochronogram are presented in Figure 4.

200

ns

FIGURE 4. EXPERIMENTAL SET-UP AND PHOTOCHRONOGRAM

It shows the simultaneous registration of the input SW (Us=5.55 µm/ns; PK=15.3 GPa), the following propagation in both HTPB and GAP micro-samples and the final run of the output SW in the afterward Kapton barrier. Shock reaction of GAP is

attended by intense light irradiation that appears instantaneously at the interface and quickly increases to a maximum. There is no light irradiation during the SW propagation in the HTPB sample, beyond the moment of the SW crossing from Kapton to the HTPB.

The z-t diagram of this experiment, presented in Figure 5, shows a big difference between the shock behavior of the energetic binder GAP and inert binder HTPB. Mean velocity of SW in GAP is about 40 to 50% less than in HTPB. The velocity and pressure amplitude of the transmitted SW (to the afterward Kapton layer) are respectively 1.3 and 1.9 times higher. The results also show an excellent shock response of HPTB and Kapton.

0

100

200

300

400

0 500 1000 1500 2000

Distance (µm)

tim

e (n

s)

GAPHTPBkapton

Us= 5,55 mm/µs

Us= 6,44 mm/µs Us= 5,02

mm/µs

Us= 5,90 mm/µs

Us= 4,10 mm/µs

Us= 5,00 mm/µs

FIGURE 5. SW PROPAGATION IN GAP AND HTPB µ-SAMPLES AND IN THE KAPTON CONFINEMENT.

The diffrences of SW behavior observed between GAP and HTPB can not be explained only by the difference (30%) in their initial density values,

ρ(HTPB)=0.901 g/cm³ and ρ(GAP)=1.290 g/cm³ and also it suggest a more precise study of their kinetics.

DF IRREGULARITIES AND INSTABILITY OF PBX DETONATION

Records of DF in the long PBX0 charge (5×5×80 mm), in copper confinement, are presented in Figure 6. In this charge, in which the cross dimension is 2.9 times bigger that the detonation critical diameter, the DF has a constant mean curvature, cf = +(Rf)-1 = 0.052mm-1. In this configuration, the DW propagates in a regime with local oscillation and with the formation of cells in DF. Existing cells is clearly identified not only by the optical method but also by the copper witness plates, which were used in the same experiment. Mean dimension of individual cell is on order of 1.0-1.25 mm corresponding to 5 to 6 times the mean size (d50) of coarse HMX particles of PBX.

D

Quasi regular pattern 40 ns FIGURE 6. EXPERIMENTAL SET-UP FOR THE GLOBAL DF CURVATURE AND RESULTS.

The case of the divergent detonation flow (cf>0 and dcf/dt<0) was evaluated with a

5 mm thick PBX sample, confined by copper plates (see Figure 7). For this situation a DF is characterised by the existence of a quasi-regular cellular structure, with the local accelerations-slowing down and with the temporal oscillations in the light emission. Cells are observed not only in the photochronograms but also printed in the witness plates. During this spreading process (up to a radius equal to 3.5 times the characteristic dimension of the initial charge) the cross dimension of cells increase from 5 to 8 times d50.

MICRODETONICS PHENOMENA IN COARSE HMX CRYSTALS

In order to clarify the mechanisms of the cells formation and the origins of oscillating flow in PBX detonation, experiments with individual coarse HMX crystals have been performed. The importance of the study of coarse HMX particles, under the condition of strong SW, allow to clarify its contribution in common PBX compositions.

200 ns

400 ns

FIGURE 7. EXPERIMENTAL SET-UP FOR THE DIVERGENT DETONATION AND RESULTS.

Two experiments with coarse crystals (d = 953 to 1050 µm) were performed with

a mesoscale level of temporal and spatial resolution using the set-up presented in Figure 8.

120 nsH

MX

+GA

PH

MX

+HT

PBTime

FIGURE 8. MESOSCALE LEVEL EXPERIMENTS WITH HMX CRYSTALS AND RESULTS.

In the first experiment (see Figure 8 a)) HMX crystals (d = 1004 µm in HTPB and 953 µm in GAP) were encapsulated in cells organized in a Kapton barrier, and then filled and outgased by liquid binder (GAP and HTPB). The input SW was simultaneously induced in both cells, with an amplitude of 15.4GPa using the standard SWG. The light irradiation is assumed to be directly associated with the shock reaction intensity (inside the crystals and in the surrounding media) and the following propagation of the SW in the forward Kapton stack barrier. It was recorded by matrix of MFOP with 98 channels of registration. The used experimental technique allows a 3D visualization, in real time, of the propagation process, inside and around the crystal, and to build up the field

of Us (Us=F(x,y,z,t)) of the transmitted (to Kapton stack) 3D SW. The temporal histograms of the light intensity are presented on Figure 9.

0

50

100

150

200

250

3002223

2425

2627

28LIG

HT

IN

TE

NS

ITY

(Arb

itrar

y U

nits

)

t / ns

FIBER

Col 1 vs Col 2 vs Col 3

40 ns FIGURE 9 TEMPORAL HISTOGRAM OF THE RECORDED LIGHT INTENSITY

The analysis of the photochronogram and histograms show the unstable propagation of the shock reaction in both systems crystal, with GAP and crystal with HTPB. In both situations, the shock reaction [SR] is accompanied by a non-monotonous light irradiation, during the process of its propagation along the crystal-binder system. Initial phase of propagation (25 ns of duration) is characterized by a low intensity light irradiation (that also is significantly higher for the GAP configuration). After this initial phase, for the crystal-GAP system it is possible to observe a high intensity reaction zone, (high intensity emission of light) for almost 50 ns, followed by a not so intense reaction that keeps on during the rest of the SR propagation distance (with no significant variation in the intensity of the emitted light). For the crystal-HTPB system the initial phase is followed by a not so intense reaction (compared with the GAP situation). The intensity of the reaction is increased, in a non monotonous way, until the terminal zone of the SW propagation, where it reaches its maximum.

The explanation for this (crystal-HTPB) non-monotonous light irradiation can be found in the 3D geometry of crystal structure and in the SW wrapping phenomena that propagates in binder, generating at the same time a complex flow of shock waves inside the crystal. Situation for crystal-GAP is different, because GAP is a strongly reactive media with a shock interaction inside the crystal is enhanced by its reaction at the interface with crystal. These differences in the kinetic instabilities (non-monotonous SR propagation process) lead to different values in the mean propagation velocity and in the intensity of the transmitted SW. The obtained values for the mean SR propagation velocity were 6.9 mm/µs for crystal-GAP system and 5.6 mm/µs for the crystal –HTPB system. Selected parts of the Us = f(x,y,t) fields, of the 3D SW transmitted to a 4×125 µm Kapton stack barrier, are presented in Figure 10. The analyzed situation (crystal-GAP and crystal HTPB) for reference (Kapton), crystal and the relative position of the fibers configuration are shown in Figure 11. The analysis of the results show a latter arrival of the SR that propagates through the crystal, in comparison with the kapton (reference), with some anisotropical effect in the zone of the crystal-binder interface. The zones bellow the crystal, presents a delay in the arrival of the SR and show an amplitude of the SW transmitted to the Kapton stack barrier significantly higher than the amplitude that characterizes the SW in the reference zone. The obtained results for the crystal-HTPB system confirms those obtained in the past9,12.

Large HMX crystal embedded in PBX-b

Experiment of large HMX crystal (955 µm) embedded in a HMX fine particles (d50 = 17 µm) plastic explosive (HMX 82% + GAP 18 %) was performed. The objective of this experiment was to observe the

different phases of the shock reaction of a HMX crystal in a situation as close as possible to a usual PBX. A schematic representation of the experimental set-up is shown in Figure 12, where it is also possible to observe the microscope photo of the relative position and dimensions of the optical fibers and the analyzed crystal.

x / µm

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

t / n

s

250

260

270

280

US

/ µm

(ns)

-1

0

2

4

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10

100 ns

F43 F44 F45 F46 F47 F48F49

x / µm

200 400 600 800 1000 1200 1400 1600 1800 2000 2200

t / n

s

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250

US

/ µm

(ns)

-1

0

2

4

6

8

10100 ns

F22 F23 F24 F25 F26 F27 F28

x / µm

400 600 800 1000 1200 1400 1600 1800 2000 2200

t / n

s

345

355

365 US

/ µm

(ns)

-1

0

2

4

6

8

10

100 ns

F15F16 F17 F18 F19 F20 F21

FIGURE 10. SHOCK REACTION ARRIVING TIME AND US PROFILES IN A 4×× 125 µm KAPTON STACK BARRIER, CORRESPONDING TO THREE DIFFERENT ROWS OF FIBERS

F49F44

F23

F28

KAPTON

HMX GAP

F16F18

FIGURE 11. RELATIVE POSITIONS OF THE OPTICAL FIBERS AND CRYSTAL FOR THE RESULTS SHOWN IN FIGURE 10.

B2C2

D2A2

126789101112

24

36

48

60

72

84

96

13

25

37

49

61

73

85

C

D

FIGURE 12. REPRESENTATION AND MICROPHOTO OF THE CRYSTAL AND MFOP MATRIX.

40 n

s

First toutchof the DWin the upperpart of thecoarse HMXcrystal

FIGURE 13 PHOTOCHRONOGRAM FROM THE EXPERIMENTAL SET-UP SHOWN IN FIGURE 12

# GAP

# HTPB

KAPTON

Temporal histograms of the light intensity, obtained from the recorded photochronogram, are shown in Figure 14.

0

65

130

195

260373839

4041

42434445

464748LI

GH

T

INT

EN

SIT

Y

(Arb

itrar

y U

nits

)

t / ns

FIBER

40 ns

FIGURE 14. TEMPORAL HISTOGRAMS OF PHOTOCHRONOGRAM SHOWN IN FIGURE 13.

The identification of the moment of the DW arrival to the upward point of the coarse crystal (well identified in the photochronogram by the characteristic local light flash) is in the very good accordance with the DW propagation time in the standard PBXb for D = 8.03 mm/µs. The long (75 ns of 140 ns of total propagation time) initial phase of the reaction of the crystal presents a non monotonous, low intense, light irradiation. During this phase the DW is sweeping around the crystal generating a multiple interaction between the waves refracted inside the crystal. This first phase is continuing almost up to the embracing of 2/3 in crystal height. The following phase of high intensity light irradiation, which is observed only in the crystal zone, extends to the end of propagation in the crystal, can be explained by the result of the converging shock that was forming in the initial phase. The 3D surface of the “time arrival surface” [t=f(x,y,z=const.)] of the detonation in the surrounding PBXb (outside crystal) and in the crystal, to the sample-kapton interface, was obtained directly from the recorded photochronogram presented in Figure 15,

showing a significant delay (up to 20 ns) for the SR that is propagating through the crystal.

-60

-40

-20

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z

x

y

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t / n

s

x

y

A

B

C

D

C2

D2

C1

D1

A1

B1

B2

A2

F2

F3

F12F11

F13

F25F37

F49

F61F73

F85

F92

F3F5 F6

F7F8

F9F10

FIGURE 15. 3D “TIME ARRIVAL SURFACE”.

Two fragments of the Us field, built-up from the analysis of the SW propagation in a 4×50 µm, kapton stack barrier, for a region bellow the medium cross section of the crystal, and for a region outside its vertical projection, are presented in Figure 16, showing a commulative effect, due to the overdriving internal shock formed in a narrow local zone bellow the crystal. The results of these experiments are showing that the interface factor (means nature of binder and its amount) plays a more important role, in the initial phase of crystal reaction, and in the rate of following liberation of the stored energy, than the energetic nature of the crystal itself. Our earlier experiments with the substitution of HMX coarse crystals by the crystalline sugar 9 confirms this conclusion.

Such delay phenomena of the energy liberation in the HMX coarse crystal, following by the origination of the overdriving micro shocks, can play a fundamental role in PBX composite, namely in the process of the cells formation and their quasi regular oscillations.

x / µm

0 400 800 1200 1600 2000 2400 2800 3200

t / n

s

0

10

20

30

US

/ µ

m (n

s)-1

0

4

8

12

1630 ns

F61F62 F63 F64 F65 F66 F67

F68 F69 F70 F71 F72

x / µm

0 400 800 1200 1600 2000 2400 2800 3200

t / n

s

0

10

20

30

US

/ µ

m (n

s)-1

0

4

8

12

16

30 ns

F25F26 F27 F28

F29 F30 F31 F32 F33 F34 F35 F36

FIGURE 16 PROFILES OF THE SW TRANSMITTED TO THE 4×× 50 µm KAPTON STACK BARRIER.

CONCLUSION

Direct registration of 3D phenomena using an optical non-intrusive method allows to understand the micro-detonics mechanisms of HMX large crystals. Effects of particle sizes and binder nature are discussed. The existence of clusters and oscillating process allows to clarify the global phenomena of the detonation of PBX

REFERENCES

1. Howe, P. Frey, R. and Melani, G., Combustion Science and Technology, Vol. 14, N. 1, 2 3, 1976, p.p. 63-74.

2. Plaksin, I., Campos, J., Mendes, R., and Góis, J., Interaction of Double Corner Turning Effect in PBX, Shock Compression in Condensed Matter – 1997, edited by S. C. Schmidt, D. P. Dandekar, and J. W. Forbes, AIP CP 429, New York, 1998, p.p. 755-758

3. Plaksin, I., Campos, J., Mendes, R., Ribeiro, J., and Gois, J., Pulsing Behaviour and Corner Turning Effect of PBX, in

Eleven International Symposium on Detonation, pp. 679-685.

4. Plaksin, I., J. Campos, R. Mendes, J. Ribeiro and J. Góis, Mechanism of Detonation Wave Propagation in PBX with Energetic Binder, Shock Compression in Condensed Matter – 1999, edited by M. D. Furnish, L. C. Chhabildas, and R. S. Hixon, AIP CP 505, New York, 2000, p.p. 817-820

5. Plaksin, I., Campos, J., Ribeiro, J., and Mendes, R., “Irregularities of Detonation Wave Structure and Propagation in PBX”, 32nd International Annual Conference of ICT – Energetic Materials, Ignition Combustion and Detonation, Karlsruhe, July 3-6, 2001, pp. 31-1 to 31-14.

6. R. Mendes, I. Plaksin and J. Campos, Single and Two Initiation Points of PBX Proceedings of the Conf. of the American Phys. Soc. Topical Group on Shock Compression of Condensed Matter, Amherst, Massachusetts, July 27-August 1, 1997, p.p. 715-718

7. Plaksin, A. Portugal, L. Pedroso, P. Simões and J. Campos, Detonation Properties of HMX-DNAM-GAP Compositions, International Conference in Shock Waves in Condensed Matter, St Petersburg, September 8-13, 2000. (to be published in Chem. Phys. Reports)

8. Plaksin, J. Campos, J. Ribeiro, R. Mendes and A. Portugal, Detonation Study of the PBX Micro-samples International Conference in Shock Waves in Condensed Matter, St Petersburg, September 8-13, 2000. (to be published in Chem. Phys. Reports)

9. Plaksin, J. Campos, J. Ribeiro, R. Mendes, J. Gois, A. Portugal P. Simões and L. Pedroso, Detonation meso-scale tests for energetic materials, Shock Compression in Condensed Matter – 2001.

10. LASL, Shock Hugoniot Data, ed. S. P. Marsh, Univ. California Press, Berkley, 1980. 11. R. Mendes, I. Plaksin, J. Campos and J. Ribeiro, Double Slapper Initiation of PBX Proceedings of the Conf. of the American Phys. Soc. Topical Group on Shock Compression of Condensed Matter, Snowbird, Utah, June 27-July 2, 1999. p.p. 915-918

12. Plaksin, J. Campos, J. Ribeiro, R. Mendes and A. Portugal, Detonation Study of the PBX Micro-samples, Shock Compression in Condensed Matter – 2001