Nitromethane (NM), a flammable liquid, has been a model system for the shock-to-detonation transition in homogeneous condensed-phase explosives for over 50 years, but we do not understand the fast processes at the molecular scale in the detonation front at the molecular scale. That is largely because prior studies triggered detonations in bomb-sized charges with input shock durations and times-to detonation that were typically microseconds, which made it impossible to observe the faster processes in real time. We studied NM shocked with 4 ns duration input pulses using a tabletop apparatus with laser-launched flyer plates and arrays of tiny disposable optical cuvettes, where the pressure and temperature were probed in real time (1 ns) with photon Doppler velocimetry, optical pyrometry, and high-speed video. Using a 4 ns shock with an input pressure close to the von Neumann spike pressure of 19 GPa, we achieved the minimum time-to-detonation, about 12 ns, where the time-to-detonation is controlled by fundamental molecular processes. We demonstrated the reproducibility of our detonations and showed that they had the same properties as in bomb-sized charges: our detonation velocity, von Neumann spike and Chapman-Jouguet pressures, temperatures, and reaction zone lengths were the same as in bomb-sized charges. Being able to trigger realistic reproducible detonations from a short pulse makes it possible to investigate molecular and fluid dynamics in the detonation by measuring transient responses in real time. We found that it took 6 ns for the temperature to reach 3430 K. The high pressure was observed at about 8 ns, when there was a volume explosion to nearly twice the von Neumann spike pressure before settling down to a steady detonation.
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.
Experimental studies of high-velocity (1-4 km/s) impacts of laser-launched flyer plates with polycrystalline, glassy or polymer windows were performed, with the intent of understanding and assessing the use of optical windows as high-speed solid-state shock wave detector materials. The problem is that the detector material undergoes radical physical transformations during the measurement. Here we present a simplified model where the detector material transformations are described as velocity and time dependent changes in shock impedance. The window materials were used to study the detonation shock produced by nitromethane, which is about 20 GPa and 10 ns. Reference experiments producing shocks in this pressure and duration range characterized how the shock impedances changed with impact velocity and shock duration. Sapphire, CaF2 and LiF windows had the fastest response to the detonation shocks, while the polymer windows had a sluggish response. Pyrex and BK7 were the best of the glassy windows.
The merger of the absorption coefficient dispersion, retrieved from transmission by the modified Urbach rule introduced by Ullrich and Bouchenaki [Jpn. J. Appl. Phys. 30, L1285, 1991], with the extended Roosbroeck-Shockley relation reveals that the optical absorption in ZnO distinctively determines the photoluminescence lineshape. Additionally, the ab initio principles employed enable the accurate determination of the carrier lifetime without further specific probing techniques.
The intrinsic photoluminescence Stokes shift, i.e., the energy difference between optical band gap and emission peak, of 350 μm thick semi-insulating GaAs wafers is found to be 4 meV at room temperature. The result is based on the determination of the optical bulk band gap from the transmission trend via modified Urbach rule whose result is confirmed with the transmission derivative method. The findings reveal the detailed balance of the optically evoked transitions and disclose the intrinsic link between Stokes shift and the Urbach tail slope parameter.
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