Hot Spot Chemistry in Several Polymer‐Bound Explosives under Nanosecond Shock Conditions

Abstract

Initial hot spot temperatures and temperature evolutions for 4 polymer‐bound explosives under shock compression by laser‐driven flyer plates at speeds from 1.5–4.5 km s−1 are presented. A new averaging routine allows for improved signal to noise in shock compressed impactor experiments and yields temperature dynamics which are more accurate than has been previously available. The PBX formulations studied here consist of either pentaerythritol tetranitrate (PETN), 1,3,5‐trinitro‐1,3,5‐triazinane (RDX), 2,4,6‐trinitrotoluene (TNT), or 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) in a 80/20 wt.% mixture with a silicone elastomer binder. The temperature dynamics demonstrate a unique shock strength dependence for each base explosive. The initial hot spot temperature and its evolution in time are shown to be indicative of chemistry occurring within the reaction zone of the four explosives. The number density of hot spots is qualitatively inferred from the spatially‐averaged emissivity and appears to increase exponentially with shock strength. An increased emissivity for formulations consisting of TNT and TATB is consistent with carbon‐rich explosives and in increased hot spot volume. Qualitative conclusions about sensitivity were drawn from the initial hot spot temperature and rate at which the number of hot spots appear to grow.

Observing Hot Spot Formation in Individual Explosive Crystals Under Shock Compression

“…A HMX PBX was synthesized using previously detailed procedures and consisted of 80% HMX and 20% Sylgard 182 binder. ,,,, These samples had a 96.1% theoretical maximum density and an average particle size of 30 ± 27 μm (SI, Section II). PBX charges were 1 mm in diameter and 40 μm thick.…”

“…The formulas for temperature calculations and additional information over the averaging algorithm are found in SI, Section IV. The original pyrometer probed a region of less than 100 μm at the center of the flyer plate. ,,, For the present study, we modified the optical path (Figure S4) so the pyrometer observed the entire region surrounding the shocked HMX, about 400 μm in diameter.…”

“…The resulting images show bright spots arising from thermal emission (Figure b). Generally, we are measuring temperatures primarily at the hottest parts of the sample since the thermal intensity varies as a high power of temperature (about six in the optical pyrometer’s spectral region, Figure S5) according to the Stefan–Boltzmann law. ,, The images are processed using MATLAB. To facilitate comparisons among images, the intensity scale was adjusted so each image had the same dark and maximum intensity level.…”

“…The shock compression microscope described in this work uses laser-launched flyer plates to deliver planar, short-duration shocks in a convenient tabletop format. Previously, this microscope has been used to study tabletop detonations in nitromethane and hot spot temperature transients in several PBXs. − Unlike prior studies, the roughly 100 shocked HMX crystals in this study were probed by a combination of the nanosecond video and optical pyrometry. The video produces a movie with four frames showing the hot spots with about 4 μm spatial resolution and 30 ns temporal resolution.…”

“…The original pyrometer probed a region of less than 100 μm at the center of the flyer plate. 6,7,9,35 For the present study, we modified the optical path (Figure S4) so the pyrometer observed the entire region surrounding the shocked HMX, about 400 μm in diameter.…”