Atomistic investigation of deformation characteristics of energetic material TKX-50 under shock by developing a quantum chemistry-based forcefield

Abstract

TKX-50 is an emerging energetic material with a high potential. Computational investigations facilitate the understanding of materials' behavior, characteristics, and safety concerns. A proper understanding of the hot spot formation in explosive materials require thorough atomic scale investigations for which an accurate classical forcefield is must. This study examines TKX-50's atomistic structure, thermodynamic characteristics, and mechanical behavior to create an accurate classical forcefield based on quantum chemistry. The ultimate goal is to comprehend TKX-50's performance under extreme situations which led to hot spot formation and detonation and facilitate energetic material development. This dissertation studies the recently synthesized energetic/explosive material TKX-50 (Dihydroxylammonium-5,5’-bistetrazole-1,1’-diolate) at the atomistic level to develop a classical molecular dynamics forcefield using conventional First Principle Calculation. A post-self-consistent field (SCF) method was used for developing the forcefield for TKX-50 which adds electron correlation effects and improves accuracy over the currently available forcefields. The Electrostatic Potential (ESP) method estimates nonbonded electrostatic potential to maintain charge neutrality and crystal structure integrity. The developed forcefield reproduces the material’s crystal structure, cohesive energy, and density very well. Bulk modulus and shear modulus of TKX-50 is further calculated which matches quite well with the experimental values. Thermodynamic investigations are done to evaluate constant volume heat capacity and fares well with experimental results. The elastic constantsare estimated, a first which is vital for any continuum scale study. The developed new forcefield is used to investigate the material’s behavior under shock to understand deformation characteristics. Constant stress uniaxial Hugoniot shock compression simulations show TKX-50's behavior under extreme conditions by tracking temperature, volume fluctuation, and shear stress. The Hugoniot Elastic Limit (HEL) shock pressure is about 6 GPa with a potential overdriven point about 8 GPa. The shock simulation reveals that TKX-50 undergoes plastic deformation by forming shear bands at high shear. Width of the shear band depends on the shock pressure. The classical forcefield developed accurately reproduces TKX-50’s geometric and thermo-mechanical properties. This research opens the door to machine learning-based forcefields, non-equilibrium molecular dynamics simulations, and multiscale modeling, to understand its sensitivity, ensure its safety and smooth its advancement.