Bulletin of the American Physical Society
21st Biennial Conference of the APS Topical Group on Shock Compression of Condensed Matter
Volume 64, Number 8
Sunday–Friday, June 16–21, 2019; Portland, Oregon
Session B3: AETD: Ultrafast Methods |
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Chair: Mike Armstrong, LLNL Room: Pavilion East |
Monday, June 17, 2019 9:15AM - 9:30AM |
B3.00001: Ultrafast Shock Induced Mid-Infrared Vibrational Changes in Thin Film Explosives Michael Powell, Pamela Bowlan, Steven Son, Cynthia Bolme, Kathryn Brown, David Moore, Marc Cawkwell, Alejandro Strachan, Shawn McGrane There are many chemical reactions and pathways predicted to occur during shock loading of explosive materials. Direct experimental evidence of intermediate formation from shock induced chemistry is very limited. Reactive models can provide insight into the chemistry and physics that occur during shock; however, experiments have typically been on orders of magnitude longer time and length scales resulting in limited direct experimental comparison. This work aims to bridge that gap using ultrafast laser spectroscopies to probe electronic and vibrational functional group changes at comparable scales. Broadband mid-infrared (MIR) and visible (VIS) absorption spectroscopy were performed on shocked thin films of explosives materials. Strong absorbance changes were measured in the MIR with peak disappearance as well as a broad absorptive feature in time. VIS absorbance also showed strong absorbance changes indicating electronic structure changes under shocked loading conditions. These results were compared to reactive molecular dynamics and accelerated chemistry models [Preview Abstract] |
Monday, June 17, 2019 9:30AM - 9:45AM |
B3.00002: Pairing Ultrafast Spectroscopy to Nanosecond Shock Generation Kathryn Brown, Michael Powell, Shawn McGrane Time-resolved spectroscopy on shocked explosives can help elucidate the evolution of the chemical reactions that result from shock compression. We have previously demonstrated the transient visible absorption of explosives and other materials in the first 300 ps of a sustained shock, combining ultrafast shock generation with ultrafast visible absorption spectroscopy. In order to expand the sustained shock duration and probe the evolving chemistry of materials at later times, we couple laser-driven flyer plates that impart a 10 ns shock into a target with ultrafast transient visible absorption spectroscopy. [Preview Abstract] |
Monday, June 17, 2019 9:45AM - 10:15AM |
B3.00003: Dynamic characterization of energetic materials with Ultrafast Transmission Electron Microscopy Invited Speaker: Volkan Ortalan Fundamental understanding and prediction capability of energy localization and evolution in reactive materials are limited by the lack of understanding of the details of the processes occurring at different time and length scales. Under impact, mechanical energy is localized due to the formation of stress-concentration sites such as localized contact points, friction, and void collapse inside the heterogeneous materials. Energy localization leads to localized temperature rise, and eventually the formation of hot spots that lead to ignition. While this general concept has been widely accepted, direct experimental evidence of such hot spots is exceptionally limited and the mechanisms for their generation are poorly understood. Such investigations in energetic solids have proven to be difficult due to the complexity of material microstructures, chemical reactivity of the solids and short lifetimes of transient hot spots and this requires advanced characterization tools with the temporal and spatial resolution of hotspots. Transmission electron microscopy is one of the techniques which has a great potential in investigating the dynamic processes in energetic materials. In-situ electron microscopy is moving forward at a rapid pace with the development of gas/liquid stages that permit reaction processes to be imaged and analyzed at atomic resolution. Moreover, the development of nanosecond and faster photoemission electron sources offers the chance to move the high spatial resolution world of electron microscopy into the ultrafast world of materials dynamics. In this presentation, examples of dynamic ultrafast transmission electron microscopy studies of energetic crystals, such as HMX, with embedded plasmonic nanoparticles will be presented. Additionally, the potential of novel in-situ stages in multimodal data acquisition scheme to push the envelope of electron microscopy for the investigation of materials under extreme conditions will be discussed. [Preview Abstract] |
Monday, June 17, 2019 10:15AM - 10:30AM |
B3.00004: Heat propagation in energetic materials in the first 50 pico-seconds Nhan Dang, Jennifer Gottfried, Frank DeLucia Under dynamic shock conditions, pressure, temperature and strain rate can be convoluted into the material response, therefore complicating studies of the importance and role of each factor. Our focus here is to determine the role of temperature (T) jump in the initial events using the indirect flash heating technique coupled with transient visible absorption spectroscopy. Previously we have shown that the thermal energy from T-jump (from a femto-second laser-heated gold (Au) layer) couples into explosive molecules by changing electronic configurations of the molecules in the excited state [Appl. Opt. 56, B85 (2017)], and the changes are T-jump dependent [Proceedings for the 16th International Detonation Symposium (2018)]. In this work we expand the previous studies to investigate how and how fast thermal energy from T-jump (generated by a 35 fs laser pulse with an energy density of 20 mJ cm$^{\mathrm{-2}}$ on 110 nm thick films of Au) transfers and propagates throughout 5$\mu $m thick films of RDX, PETN and HMX in the first 50 pico-seconds. The evidence of heat propagation through the samples will be reported in terms of transient absorption intensities and the propagation rates will be discussed. [Preview Abstract] |
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