Estimating the Relative Energy Content of Reactive Materials Using Nanosecond-Pulsed Laser Ablation
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Estimating the Relative Energy Content of Reactive Materials Using Nanosecond-Pulsed Laser Ablation Jennifer L. Gottfried, Steven W. Dean, Eric S. Collins, Chi-Chin Wu U.S. Army Research Laboratory, RDRL-WML-B, Aberdeen Proving Ground, MD 21009, U.S.A.
ABSTRACT
Recently, a laboratory-scale method for measuring the rapid energy release from milligram quantities of energetic material has been developed based on the high-temperature plasma chemistry induced by a focused, nanosecond laser pulse. The ensuing exothermic chemical reactions result in an increase in the laser-induced shock wave velocity compared to inert materials. Laser-induced air shock from energetic materials (LASEM) provides a method for estimating the detonation performance of novel organic-based energetic materials prior to scale-up and full detonation testing. Here, the extension of LASEM to non-organic energetic materials is discussed. The laser-induced shock velocities from reactive materials such as Al/PTFE, Al/CuO, Al/Zr alloys, Al/aluminum iodate hexahydrate, and porous silicon composites have been measured; in many cases, the high sensitivity of the samples resulted in propagation of the reaction to the surrounding material, producing significantly higher shock velocities than conventional energetic materials. Methods for compensating for this effect will be discussed. Despite this limitation, the relative comparison of the shock velocities, emission spectra, and combustion behavior of each type of material provides some insight into the mechanisms for increasing the energy release of the material on a fast (s) and/or slow (ms) timescale.
INTRODUCTION The development of laboratory-scale methods for measuring the energy release of novel materials is essential for energetic material research. Typically, only milligram quantities of energetic materials are synthesized initially, and scale-up to the tens of grams (or more) required for sensitivity and detonation testing may require significant resources. In our laboratory, we use pulsed lasers to initiate self-sustaining exothermic reactions that result in the formation of gaseous products. When the focused laser pulse interacts with a material, heating, melting and vaporization lead to particle ejection off the sample surface and expansion of the evaporated material into the background gas. Because the evaporated material is at very high temperatures (tens of thousands of Kelvin), the vapor is ionized – forming a laser-induced microplasma. In the plasma, atoms, molecules, and ions undergo collisions. The tail end of the near-infrared nanosecond laser pulse is strongly absorbed by the plasma through inverse Bremsstrahlung (absorption of a photon by a free electron), further heating the plasma and shielding the sample surface from the laser pulse. Following cessation of the laser pulse, the plasma cools and becomes less ionized, resulting in recombination and Downloaded from https://www.cambridge.org/core. Cornell University Library, on 18 Jan 2018 at 16:11:51, subject to the Cambridge Core term
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