Theoretical study on the effect of different surfaces on structure, excess energy, electronic structure and impact sensitivity in 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane
Graphical abstract
Introduction
High energetic materials have been of increasing interest over the last few decades since there are a wide range of demands in the civilian and military domains [[1], [2]–3]. Unfortunately, the inherent contradictions between sensitivity and high detonation performance of the energetic materials are taxing the minds of scientists [4–5]. Explosives with excellent detonation performance mostly exhibit poor insensitivity and vice versa. The sensitivity of energetic compounds is used as a measure of their stability to external stimuli such as shock, impact, friction, heat and electric spark. Each stimulus corresponds to specific sensitivity [6,7]. Current studies show impact sensitivity is the most widely used in comparison with other sensitivities. Drop hammer test is one of the test methods to measure the impact sensitivity in experiment. Explosion or non-explosion are recorded by subjecting a sample to the impact of the standard mass falling from different heights [[8], [9]–10], usually with a 2.5kg weight. A height of 50% probably in causing an explosion is denoted by h50. Although drop hammer test is quite simple to operate, it is hard to obtain reliable experimental results since the impact sensitivity tests have a strong dependence on the external conditions and are often unrepeatable. In addition, the potential applications and safe handling are also determined by the sensitivity of energetic materials. Therefore, the theoretical sensitivity prediction can provide a significant insight into its aging, storage and transportation.
A great quantity of research has been devoted to investigate the factors affecting the sensitivity of materials, finding that sensitivity depends on many factors: oxygen balance [11], electronegativity [12], Mulliken charge of the nitro group [13], surface electrostatic potentials [14,15], free space per molecule in the unit cell [16], band gap and so on [17]. Kamlet and Adolph found that there is a significant linear correlation between the logarithmic 50% impact heights and the oxygen balance OB100 for classes of energetic compounds with similar decomposition mechanism. As the oxygen balance OB100 increases, the logarithmic of h50 decreases [11]. Zhang et al. reported that nitro group charge can be deemed as a structural parameter to assess the impact sensitivity for nitro compounds. The compound with less -QNO2 will be more sensitive [13]. Politzer et al. described the effects of several molecular properties on impact sensitivity, including surface electrostatic potentials, lattice free space and maximum heat of detonation per unit volume [14], [15], [16]. Although there is no correlation, impact sensitivity is inclined to become larger as molecular properties increase. Zhu et al. investigated the relationship between impact sensitivity of energetic materials and band gap using first-principles, finding that the smaller band gap is, the higher impact sensitivity is for energetic compounds with similar structure or similar decomposition mechanism [17]. It is well-known that the sensitivity of an energetic compound is anisotropic in different crystal directions. What about the situations on the sensitivity of the different surfaces for an energetic crystal? Here we study the effect of different surfaces on structure, excess energy, electronic structure and impact sensitivity in a typical energetic molecular crystal β-Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine with the first-principles.
Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) is known as octogen, where HMX is an abbreviation for High Melting Point Explosive [18]. It has widespread applications ranging from propellants and high explosives to the space shuttle and rocket engine fuel due to its high detonation velocity and excellent thermal stability [[19], [20], [21]–22]. There are four well-known solid phases for HMX at room temperature, labeled as α, β, γ and δ. The β-HMX has the highest density with the most stable form at ambient conditions, so the β-HMX is selected as the object of study in this article. The following work is composed of three parts: first, a brief description of the computational method is introduced; and then, the results and discussion are displayed; followed by a summary of the conclusion at last.
Section snippets
Computational details
First-principles calculations have been carried out employing the plane-wave pseudopotential method as implemented in the CASTEP code [23]. The effects of exchange−correlation energy of valence electrons were described in the light of the generalized gradient approximation (GGA) parametrized by Perdew-Burke-Ernzerhof (PBE) functional [24]. TS correction of DFT-D method dealed with the van der Waals interactions, which has been predicted with success for the structural parameters, adsorption
Results and discussion
The calculated lattice parameters of β-HMX with the P21/c space group are a=6.63 Å, b=11.22 Å and c=8.82 Å [3], which are in good agreement with experimental values a=6.54Å, b=11.05Å and c=8.70Å [31]. The crystal and surface structures are shown in Fig. 1. The unit cell of β-HMX possesses a ring-chain conformation in which the NO2 groups employ a chair-like arrangement. This makes the entire molecule center-symmetric.
The bond population and bond length in molecules are efficient for the
Conclusions
In this study, we investigate the structure, excess energy, electronic properties and impact sensitivity of different surfaces in β-HMX based on the DFT calculations. The results show that the (010) surface has the shortest average bond length and the greatest average bond population compared to other surfaces, indicating the (010) surface is less sensitive. It can be observed that there is a strong elastic anisotropy in different directions based on the curved surface of Young's and shear
CRediT authorship contribution statement
Wei-Hong Liu: Writing - original draft, Formal analysis, Investigation, Methodology, Software. Wei Zeng: Investigation, Methodology, Writing - review & editing. Han Qin: Data curation, Methodology. Yun-Dan Gan: Conceptualization, Visualization. Fu-Sheng Liu: Data curation, Methodology, Writing - review & editing. Bin Tang: Methodology, Software, Writing - review & editing. Qi-Jun Liu: Conceptualization, Project administration, Resources, Supervision, Writing - review & editing.
Declaration of Competing Interest
None.
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