Insight into material behavior via surface free energy calculations for common energetic materials

Abstract

The Gibbs Free energy is a driving force for equilibrium crystal shapes and the formation of crystal facets in molecular crystals. Orientation dependence of interfacial properties is linked to surface free energy (SFE). Prediction of orientation‐dependent properties such as thermal stability, mechanical response, and compatibility with binders require a systematic approach to the quantification of SFE. In molecular crystals, entropy has a much larger contribution among all ordered crystalline materials. In this paper, we extend our previously developed method to quantify SFE and entropy of β‐HMX to other common energetic materials–TATB, α‐RDX, and PETN. Two complimentary approaches, Nonequilibrium Thermodynamic Integration (NETI) and Steered Molecular Dynamics (SMD) methods are used to obtain insight into interfacial phenomena along with surface free energy estimates. We discuss the relevance of surface free energy and the importance of surface entropy for facetted molecular crystals in understanding crystal properties, activation of slip planes, and potential pathways for fracture. These values allow us to predict theoretical crystal shape using Wulff Construction, better understand the effect of hydrogen bonding on SFE, and the diversity of bonding environment in energetic crystals. In particular, in crystals with low stacking fault energy, the SMD values can be inconclusive due to the triggering of slip plane motions. In cases where SMD simulations lead to large deformations and high uncertainty, the NETI approach can still provide SFE estimates.