Surpassing the Ambient Packing Limit of Energetic Crystals: Squeezing Effect of Molecular Level "Net-fishing"

TL;DR

This study demonstrates a novel molecular stacking method to surpass traditional ambient packing limits of energetic crystals, resulting in higher density and improved energetic properties suitable for advanced propellants.

Contribution

The paper introduces a molecular level compression technique using 2-D TAGP stacking to enhance the packing density of HMX crystals under ambient conditions.

Findings

Achieved higher density HMX crystals with 2.13 g/cm³.

Enhanced detonation velocity of 10.40 km/s.

Improved specific impulse of about 292 s.

Abstract

High energy density is always a key goal for developments of energy storage or energetic materials (EMs). Except exploring novel EMs with high chemical energy, it is also desirable if the traditional EMs could be assembled at a higher density. However, it is very difficult to surpass their theoretical maximum molecular packing density under ambient conditions, even though a higher density could be achieved under ultra-high pressure (Gigapascals). Such solid-state phase changes are reversible, and hence this high density packing is not able to maintain under ambient conditions. Alternatively, in this research, we demonstrated a molecular level compression effect by stacking of 2-D TAGP, resulting in a higher density packing of the HMX molecules with changed conformation. The HMX crystal formed under compression in the solvent has a unit cell parameter very close to the reported one observed under pressure of 0.2 GPa. It shows that the compressed HMX molecules are trapped in the TAGP layers, resulting in a higher density (e.g. 2.13 g cm-3) and also higher heat of formation. The resulted HMX crystals are free of defects, and unlike the pristine HMX, no polymorphic transition and melting point were observed upon heating. Experiments and relevant calculations show that the best resulted hybrid HMX crystal has a detonation velocity of 10.40 km s-1 and pressure of 53.9 GPa, respectively. Its ground specific impulse reaches about 292 s, even much better than CL-20, making it a promising propellant component for future space explorations.