We collaborate with LLNL experts in computational chemistry to predict the performance of energetic materials formulations. Our speeds of sound results facilitate develop of materials equations-of-state (EOS), which are required to semi-empirically predict e.g., using the CHEETAH thermochemical code, thermodynamic states at detonation, deflagration, and combustion conditions. We also determine shock Hugoniot and, when combined with diamond-anvil cell technology, off-Hugoniot state properties of a wide range of materials using our ultrafast shock interrogation technique (USI). Moreover, USI provides a window into physico-chemical transitions that occur on the picosecond timescale. Shock-synthesis of new materials is also explored. By integrating our experimental and computational capabilities and expertise, we systematically advance science-based stockpile stewardship agendas, provide guidance to large-scale energetic materials formulation tests, and elucidate thermochemical processes of direct relevance to global security.
Nuclear Forensics and Energy Sciences
We provide experimental evidence to guide nuclear materials forensics attribution efforts. Currently we are characterizing the high-temperature chemistry of UO2 reactant when in the presence of common gas and fluid specie environments. Here we utilize our custom-made vibrational spectroscopy systems and key ex situ characterization tools, to characterize uranium chemistry that is likely to occur in engineer-controlled environments or under explosive conditions. Ultimately this work provides fundamental data required to optimize the safety and design performance of nuclear power plant systems. In addition, our results provide insight to the nuclear forensics/attribution community: It is important to develop a priori knowledge of the chemical partitioning of UO2, occurring at temperatures representative of critical or subcritical detonations.
Chemical Energy Systems
The ability to synthesize and characterize novel compounds that store significant amounts of chemical energy -with on-demand energy release- is an increasingly important scientific and engineering issue. The motivation for our research stems in part by ongoing reductions of our planetary fossil fuel energy reserves concomitant with production of greenhouse gas production. As part of a search for "green" chemical energy systems, we synthesize polynitrogen compounds at high temperatures and modestly low pressures. In collaboration with LLNL computational chemistry experts, our experimental results are used to guide the characterization of material structures, phyiscio-chemical stability fields, and potential energy and release performance.
Characterization of Improvised Explosives
The threat of improvised explosives (IEs) continues to challenge our global and national security. It is important to recognize to what extent detonability can be achieved with IEs. Here we study IEs to quantify physical and chemical parameters and elucidate thermodynamic states that make the difference between deflagration (low damage yield) and detonation (high damage yield). Our semi-empirical assessments of detonability, grounded in speeds of sound and molecular phase stability measurements, provide rapid and accurate results that may otherwise be difficult to obtain by large-scale trial-an-error tests. Moreover, our approach is potentially applicable to myriad classes of IE formulations e.g., liquids, solids, and mixtures.
Hydrogen and Deuterium
We have been awarded a LLNL Laboratory directed research and development (LDRD) grant to shock precompressed hydrogen or deuterium. Samples are first brought up to many 10's of GPa using conventional diamond-anvil cell technology, and then they are mechanically shocked using our USI method. The aim is to characterize these materials at high-density shock states, which are not accessible by any known experimental methods. Our results will guide the development of fundamental and theoretical knowledge of simple molecular interactions –how they proceed at conditions relevant to myriad of energetically dynamic processes. Phase stability curves will be delineated. There is the possibility of transitioning these materials to the metallic state.
Tholin Chemistry and Titan's Methane Mystery
In collaboration with scientists at NASA's Ames Research Center and LLNL computational chemistry experts, we are attempting to resolve a long-standing issue regarding the presence of methane (5 % by volume) in Titan's atmosphere. The lifetime of CH4 on Titan is estimated to be 10 – 100 million years. Numerous scientific studies have effectively ruled out many possible production mechanisms; hence, one is lead to believe that methane is produced below the surface. Using our custom-designed micro-FTIR instrumentation and, mass spectrometry capabilities and expertise at NASA, we will characterize pressure and/or temperature induced products formation from Titan-Tholin starting material.
Terahertz Radiation Sources
We are funded by DARPA to investigate methods to amplify and modulate THz acoustic waves in materials, ultimately in the interest of developing more versatile THz radiation sources. Previously, we developed a method to coherently generate THz radiation from a THz acoustic wave (Armstrong et al., Nature Physics 2009); however, this method requires ultrafast laser system to generate THz acoustic waves. Here we are exploring different approaches to generate and launch THz acoustic waves where large ultrafast systems are not required. A portable THz generator would also facilitate spatial imaging of materials with nanometer length scale resolution.
Ph.D., Physical Chemistry, University of Washington, Seattle,
B.S., Chemistry, Minor in Physics, Illinois Institute of Technology, 1988
Staff Chemist, Founder of High-Pressure Material Science Group, Principal Investigator, Equation of State and Reaction Chemistry of Supercritical Fluids and Energetic Materials,
Chemical Sciences Division