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Chemistry away from local equilibrium: shocking high-energy density and energy absorbing materials
The response of materials to shock loading include plastic deformation and viscoelasticity, phase transformation, and even chemical reactions. This talk will focus on how the energy in a shockwave is effectively transferred to the molecular modes that can lead to chemical reactions and how, in turn, these reactions can affect the propagation of the leading shock. I will describe reactive molecular dynamics simulations of the shock to deflagration transition in the molecular high-energy density material RDX. We find that energy localization during pore collapse leads to ultra-fast, multi-step chemical reactions that occur under non-equilibrium conditions. The formation of exothermic product molecules during these first few picoseconds of the process prevents the nanoscale hot spot from quenching and within 30 ps, a deflagration wave develops. Quite surprisingly an artificial hot-spot matching the dynamical one in size and thermodynamic conditions quenches, providing strong evidence of the important of non-statistical processes in nanoscale hotspot criticality. To achieve time and lengths beyond what is possible in MD we developed a coarse grain model that enables the description of stress-induced chemical reactions. We used this model to explore shock loading on materials that exhibit volume-reducing chemical reactions. The simulations show that such chemical reactions can attenuate the propagating shockwave indicating that materials with such characteristics could be used for protection. We characterized how the characteristics of the chemical reactions affect shock attenuation. The simulations show that the magnitude of the volume collapse and velocity at which the chemistry propagates are critical to weaken the shock, whereas the energetics in the reactions play only a minor role. Such knowledge has proven valuable in experimental efforts to design and optimize such materials.
Bio: Alejandro Strachan is a Professor of Materials Engineering at Purdue University and the Deputy Director of the Purdue’s Center for Predictive Materials and Devices (c-PRIMED) and of NSF’s Network for Computational Nanotechnology. Before joining Purdue, he was a Staff Member in the Theoretical Division of Los Alamos National Laboratory and worked as a Postdoctoral Scholar and Scientist at Caltech. He received a Ph.D. in Physics from the University of Buenos Aires, Argentina, in 1999. Among other recognitions, Prof. Strachan was named a Purdue University Faculty Scholar (2012-2017), received the Early Career Faculty Fellow Award from TMS in 2009 and the Schuhmann Best Undergraduate Teacher Award from the School of Materials Engineering, Purdue University in 2007.
Prof. Strachan’s research focuses on the development of predictive atomistic and molecular simulation methodologies to describe materials from first principles, their application to problems of technological importance and quantification of associated uncertainties. Application areas of interest include: coupled electronic, chemical and thermo-mechanical processes in devices of interest for nanoelectronics and energy as well as polymers and their composites, molecular solids and active materials, including shape memory and high-energy density materials. He has published over 110 articles in the peer-reviewed scientific literature.