Structure and Density Predictions for Energetic Materials

The discovery of new energetic materials could be facilitated, accelerated and made more cost effective with the use of computer modeling and simulations for the identification of compounds that have significant advantages over materials currently in use. The quantitative estimation of properties, such as the heat of formation, density, detonation velocity, detonation pressure and sensitivity, to screen potential energetic candidates would permit the selection of only the most promising substances [1] for laboratory synthesis, measurement of properties, scale-up, testing, etc. The most significant properties or characteristics of a high performance energetic material are the molecular structure, elemental composition, heat of formation, solid-state density and microstructure. Performance characteristics such as the detonation velocity and pressure are proportional to the density. The detonation velocity, for example, increases linearly with density while the Chapman-Jouguet pressure is proportional to the square of the initial density [2]. An increase in the solid-state density also is desirable in terms of the amount of material that can be packed into a volume-limited warhead or propulsion configuration. Density has been termed “the primary physical parameter in detonation performance” [3]. Microstructure is a catchall term that refers to the three-dimensional structure with various dislocation motifs and imperfections. The sensitivity of a material in response to impact or shock stimuli is associated with a number of factors, among which are the molecular and micro structures. Initial efforts were directed to density prediction. Without question, the simplest method is by so-called “group or volume additivity.” This is truly a back-of-the-envelope or spreadsheet calculation and involves the summation of appropriate atom and functional group volumes to give an effective solid-state volume for a molecule, then a density. Overall, volume additivity parameterization cannot readily account for molecular conformation, crystal packing efficiency or positional isomerism. The cyclic nitramine explosives RDX and the three HMX polymorphs with CH2NNO2 as the basic structural unit are a case in point. The experimental crystal densities are 1.806 (RDX), 1.839 (α-HMX), 1.902 (β-HMX) and 1.759 (δ-HMX) g cm . The additivity densities of 1.838 for RDX and 1.847 g cm