Shock Initiation Microscopy with High Time and Space Resolution

Abstract

We describe studies of shock initiation and shock‐to‐detonation transitions in energetic materials using a tabletop shock compression microscope with nanosecond time resolution and micrometer spatial resolution. Planar input shocks with durations of 4–20 ns are produced using 0–4.5 km/s laser‐launched flyer plates. Emphasis is on measurements of temperature, velocities, pressure, and microstructure using photon Doppler velocimetry (PDV), optical pyrometry and high‐speed videography. Techniques are discussed for fabricating disposable shock target arrays of tiny plastic‐bonded explosives (PBX), liquid and powder explosives, and single‐crystal explosives for high‐throughput studies. Optical temperature measurements of shocked triaminotrinitrobenzene (TATB) are discussed. Since TATB is yellow, we developed methods to correct for the blue absorption to obtain more accurate temperatures. Hot spots in shocked polymer‐encased HMX (octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine) crystals are observed in real‐time, showing a hot spot produced in a collapsing void that ignites a deflagration. Despite the small dimensions of our explosive charges (typically 1 mm diameter and 250 μm length), we produced reproducible detonation states in solid and liquid explosives using short‐duration shocks near the von Neumann spike (VNS) pressure. We show the VNS pressure is associated with a transition to high‐efficiency gas production from the explosives. In studies of NM, prior to detonation, we see reaction originating at hot spots which coalesce to form a superdetonation.

A Hotspot’s Better Half: Non-Equilibrium Intra-Molecular Strain in Shock Physics

“…Hotspots are routinely characterized and analyzed by their temperature fields, across atomistic and continuum modelling [ 14,24,29 ] and in experiments [ 30 ]. That is, hotspots are considered as sites of localized thermal energy.…”

“…Hotspots are routinely characterized and analyzed by their temperature fields, across atomistic and continuum modeling ,,, and in experiments . It is well understood that applying external forces to a molecule can accelerate and change chemical reactions through mechanochemistry .…”

Observing Hot Spot Formation in Individual Explosive Crystals Under Shock Compression

“…The input shocks were about 4 ns in duration, which produced a calculated pressure of 19 GPa in the PU polymer and 25 GPa in the explosive crystal (Supporting Information (SI), Section I). The shock compression microscope with a high-speed camera and optical pyrometer is depicted in Figure b and has been described in detail elsewhere. ,− A photon Doppler velocimeter (PDV) was used to measure the flyer plate velocity, and the entire sample manifold is held under rough vacuum (100 mTorr). , A spatial filter at an auxiliary focus was used to select the region of interest for viewing by the camera or pyrometer. The spatial filter was also used to exclude light artifacts, including triboluminescence, resulting from flyer launching. , …”

“…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.…”

Estimating Void Nucleation Statistics in Laser-Driven Spall

“…The spall response is characterized through analysis of the free-surface velocity history obtained with photon Doppler velocimetry (PDV) employing a probing spot size of ~ 50 μm, ensuring that the material response of multiple grains is captured during the experiment. Both the laser-driven micro-flyer apparatus and the velocimetry are described in detail by Mallick et al [21,25,26] and were developed with assistance from the Dlott research group [27].…”