Crystal packing driven comparable thermal expansion of a biimidazole-based energetic material

Abstract

The thermal expansion is one of the important reasons for the stress increase in polymer bonded explosive (PBX) and the damage of the long-term storage structure of explosives. Herein, the thermal expansion characteristics and crystal damage behavior under thermal stimulation of three crystal forms (β, α and γ) of 4,4′,5,5′-tetranitro-1H,1′H- [2,2′-biimidazole]-1,1′-diamine (DATNBI) were analyzed. The results show that DATNBI has obvious anisotropic thermal expansion, in which β- and α-DATNBI are first-order thermal expansion, and γ-DATNBI is second-order thermal expansion. The thermal expansion of DATNBI is reversible, but the crystal damage caused by thermal expansion is irreversible. Combined with Rietveld Refinement of powder X-ray diffraction (XRD) patterns, the volumetric expansion coefficients are 17.98 × 10⁻⁵ °C⁻¹ (β-DATNBI), 23.28 × 10⁻⁵ °C⁻¹ (α-DATNBI) and 14.39 × 10⁻⁵ °C⁻¹ (γ-DATNBI), respectively. The relationship between crystal packing and thermal expansion characteristics of DATNBI at different temperatures was studied. It is demonstrated that the thermal expansion can probe certain intermolecular interactions resulting in anisotropic expansion behavior. Usually, the thermal expansion of most crystals (β-DATNBI) is positive, but slip or dislocation may cause a smaller negative expansion of a certain axis of the crystal cell (α-DATNBI). However, the unique cross-cross grid crystal packing (γ-DATNBI) will make a certain axis of the cell appear obvious negative expansion, which may lead to the overall decrease in the volumetric expansion coefficient of the crystal.