Tracking hot spot dynamics in solid state explosives with shock compression microscopy

Pacheco, Belinda

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https://hdl.handle.net/2142/112999 Copy

Description

Title

Tracking hot spot dynamics in solid state explosives with shock compression microscopy

Author(s)

Pacheco, Belinda

Issue Date

2021-07-13

Director of Research (if dissertation) or Advisor (if thesis)

Dlott, Dana D

Doctoral Committee Chair(s)

Dlott, Dana D

Committee Member(s)

• Glumac, Nick G
• Murphy, Catherine J
• Moore, Jeffrey S

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• explosives
• shock waves
• pyrometry
• high-speed imaging

Abstract

Experimentally investigating the shock initiation of explosives is difficult because of the violence and speed at which explosive processes occur. This dissertation systematically investigates the initiation behavior of solid-state explosives when subjected to shock compression using experimental capabilities pioneered by the Dlott Group. Laser-driven impactors that are 500 µm in diameter are used to impart initiating shocks into tiny arrays of polymer-bound explosives (PBXs). The ignition and initiation behavior of these materials are studied using a suite of optically-coupled diagnostics which afford high temporal and spatial resolution. A 32-channel pyrometer was used to determine explosive temperature histories with 2 ns resolution, while multi-frame, high-speed photography simultaneously captured short movies of explosive behavior with 2 µm resolution. The entire shock-pump, visible emission-probe platform, referred to as a shock compression microscope, is tabletop-based and has uniquely permitted high-throughput experiments of PBXs at the time and length scales where data is severely lacking. Two explosive sample systems were studied- bulk PBXs and a model PBX. PBXs are highly heterogenous materials whose microstructures can create ignition sites, or “hot spots”, and cause deviations from predicted behavior. Bulk PBXs contain a statistical number of explosive crystals, and therefore, provide an ensemble description on how PBXs behave and perform. However, bulk PBXs do not inform our understanding of individual hot spot behavior, which is a crucial aspect of developing high-fidelity, reactive models that predict explosive performance. To address this need, a model PBX was developed to pinpoint fundamental ignition phenomena. A model PBX was made by deconstructing bulk PBXs into their primary components- a single explosive grain embedded in a polymer. Various bulk PBXs were studied in multiple experimental configurations. Using a 32-channel pyrometer, we found that large internal voids created high temperature hot spots via in situ gas generation and compression. We observed overdriven, detonative-like dynamics in a PBX by shocking hundreds of bulk PBX samples and interrogating them with pyrometry, high speed imaging, and photon doppler velocimetry. By varying the chemical composition of the bulk PBX, we gained insight into the hot spot and reaction zone behavior of 4 different explosives. The model PBX allowed us to “see” individual hot spots forming on an explosive crystal. We found that the location of hot spot formation varied depending on crystal morphology. In cyclotetramethylene-tetranitramine (HMX) single crystals (HMX-SC), hot spots consistently formed on the crystal facets and corners. Whereas, on highly defective HMX grains, hot spots were seen to emanate from internal voids first. By shocking hundreds of HMX-SC at increasing shock strengths, we observed thresholds in behavior. Furthermore, we determined that increased shock pressure led to miniscule rises in temperature allowing us to relate hot spot volume growth to the extent of reaction. While precise hot spot formation mechanisms could not be directly elucidated from experiment, combining theory with experiment has yielded preliminary conclusions on hot spot formation mechanisms in our model PBXs. The methodologies outlined in this dissertation provide a new avenue for the direct observation and study of hot spots. Explicit interrogation of fundamental hot spot behavior, as with these model PBX experiments, is crucial for providing quantitative information to calibrate and test predictive explosive models.

Graduation Semester

2021-08

Type of Resource

Thesis

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http://hdl.handle.net/2142/112999

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Copyright 2021 Belinda Pacheco

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