ELECTRICAL CONDUCTIVITY PROFILES IN DETONATING LOW-DENSITY EXPLOSIVES OF VARIOUS GRAIN SIZES

1. ELECTRICAL CONDUCTIVITY PROFILES IN DETONATING LOW-DENSITY EXPLOSIVESOF VARIOUS GRAIN SIZES A. P. Ershov, N. P. Satonkina Lavrentyev Institute of Hydrodynamics, 630090 Novosibirsk, Russia Presented by Eduard R. Pruuel

2. Low-density explosives:an interesting but mostly ignoredmatterHow shock wave looks like in loose packed powder? Can ZND model work?In solid explosives the reaction is believed to proceed from hot spots. Fine-grained materials should have more hot spots which agrees with the shock sensitivity data. To the contrary, in the stationary detonation no effect of the grain size on the reaction kinetics was found:A.N. Dremin et al., ~ 1970, ~ 100 ns resolutionModern studies mostly avoid grain size effect at low , exceptions are: A.V. Utkin et al., A.V. Fedorov et al. This work: electrical conductivity diagnosticapplied to low-density explosives.

3. Coaxial differential cell Coil gauge thin slit feedingcircuit аxialignition outer copper electrode Conductivity gauge signal U(t) Resolution: a quarter of slit width 50 m, or 10 ns • Low gas-dynamic disturbances • Natural averaging around the axis Proc. 13th Detonation Symp., Norfolk, 2006.

4. Calibration and conductivity evaluation HMX fine 1.35 g/cc Gauge inductancecorrection: Russ. Journ. Phys. Chem. B, 1, no.6 (2007)

5. Previous  dense explosives: Conductivity peaks were of about the same width as thechemical peaksmeasured by the conventional methods (40-70 ns).No grain sizeeffect was found. Possible reason: the compaction may produce the charge structure which is more or less independent of theinitial grain size. Present  Low-density explosives: In low-density explosives the initial grains are intact. To provide sufficient homogeneity, the charges were slightly compacted (by several %). Comparative tests with different grain sizes were done.

6. Scalebars: Scalebars: MATERIALS 100 m 10 m 300 m 30 m 30 m 200 m Fine explosives: ~ an order of magnitude smaller grains PETN: ~3 times smaller

7. RESULTS Conductivity profiles 3 explosives, 18 shots: 3 fine and 3 coarse each RDX HMX • Peaks in fine-grained explosives are about 2 times shorter. • Peak conductivity depends on explosive, but not on grain size. • “Coarse” records are more noisy. PETN

8. RESULTS

9. DISCUSSION In contrast with high-density case, at low initial density the conductivity peaks are shorter for smaller grains. Moreover, they may be narrower than in the same explosive pressed to almost TMD. One may suppose that the reaction rate is higher in fine-grained explosives. The reaction rate concept of Dremin et al.seems capable to explain the results. Hot spot mechanism and homogeneous decomposition proceed in parallel; contribution of hot spots prevails at low pressures, while at high pressure/density the reaction is mainly homogeneous.

10. Acknowledgements This work was supported by the Russian Foundation for Basic Research (grant 09-03-00127a) and by research program 11 of the Russian Academy of Sciences Presidium (project no. 18). Authors are grateful to: Ya.L. Lukyanov, E.R. Pruuel for helpin experiments; E.R. Pruuel for valuable discussions and this speech. More detailed account see in: Combustion & Flame 2010, in press Thank you for attention