Molecular dynamics studies of the pressure- and temperature-dependent thermodynamics and kinetics of melting in β-HMX
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[EMBARGOED UNTIL 08/01/2026] High-energetic materials (HEMs) are crucial for various applications, including propellants and explosives. Among these, β-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (β-HMX) is a widely used energetic molecular crystal due to its high density and powerful explosive performance. A comprehensive understanding of its phase transitions, particularly melting behavior under extreme conditions, is paramount for predicting and controlling its initiation and detonation mechanisms. The melting process, a precursor to chemical reactions in the shock front, influences energy release rates and overall performance. However, experimental and theoretical data on the melting curve and kinetics of β-HMX at pressures relevant to detonation (1-40 GPa) remain limited. This dissertation presents a systematic investigation into the melting curve and melting kinetics of β-HMX across a pressure range of 1 to 40 GPa. Using all-atom molecular dynamics (MD) simulations employing the two-phase coexistence method, we precisely determined melt temperatures at various pressures within this range. Furthermore, we characterized the thermodynamics of melting, including volume change (∆Vm) and enthalpy change (∆Hm), and elucidated the kinetic aspects of melting, quantifying the rates of melting, activation energy for melting, and velocity of melting. Our findings reveal a exponential increase in the melt temperature with pressure, exhibiting a steeper slope at lower pressures that gradually flattens at higher pressures, consistent with the expected behavior of molecular solids under compression. The results indicate that kinetic barriers to melting increase significantly with pressure, suggesting a more sluggish transition at higher compressions. The insights gained from this research provide fundamental data for developing more accurate equations of state and reactive flow models for β-HMX. These findings are essential for enhancing the predictive capabilities of detonation simulations, optimizing energetic material design, and improving safety protocols by offering a deeper understanding of β-HMX’s response under extreme dynamic loading conditions.
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Ph. D