Thermal expansion behaviors and phase transitions of HMX polymorphs via ReaxFF molecular dynamics simulations
TT Zhou and FL Huang, ACTA PHYSICA SINICA, 61, 246501 (2012).
The response to temperature of HMX polymorph is important for understanding the sensitivity, stability, and phase transitions of energetic material. Using ReaxFF-lg with isothermal-isobaric molecular dynamics (NPT-MD) methods, the crystal and molecular structures of beta-, delta-, and alpha-HMX crystals in a temperature range of 303-503 K and at atmospheric pressure are investigated. The calculated crystal structures and thermal expansion coefficients are in general agreements with experimental results, indicating that ReaxFF-lg potential can correctly describe the thermal expansion of HMX polymorph. The linear thermal expansion coefficients indicate that the thermal expansion of beta-HMX is anisotropic, the thermal expansion along c axis is slightly different from those for a and b axes for delta-HMX, and the thermal expansion along a, b, and c axes are almost the same for alpha-HMX. The volume expansion coefficients for the three phases decrease in the following sequence: delta-HMX> alpha-HMX> beta-HMX, showing that delta- HMX is the most sensitive to temperature in the three crystals, which may be the reason for higher sensitivity of this phase. Sharp changes in lattice parameter and molecular conformation transformation from "chair" to "boat-chair" occur for beta-HMX when temperature reaches 443 K. Helmholtz free energy derived from the two-phase thermodynamics (2PT) model suggests a phase transition for beta-HMX at T = 423-443 K. The beta-, delta-, and alpha-HMX crystals are stable in the temperature ranges of 303-423 K, 443-503 K, and 363-423 K, respectively.
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