Pressure-induced phase transitions in energetic materials

Abstract

Explosives and propellants, known generically as energetic materials, are widely used in applications that include mining, munitions, and automotive safety. Key properties of these materials include: reliable performance under a range of environmental conditions; long-term stability; environmental impact; processability; sensitivity to accidental initiation through stimuli such as impact, shock, friction, and electrostatic discharge. Many of these properties are affected by the crystal structure of the energetic material. Explosives experience elevated pressures and temperatures under detonation conditions -such conditions often induce phase transitions in the energetic material. Hence detailed studies of pressure-induced structural changes in these materials are essential in order to understand and model fully their behaviour. This presentation will describe some recent high-pressure studies (using a combination of X-ray and neutron diffraction techniques) on 2,4-dinitroanisole (DNAN), an insensitive explosive that is replacing TNT in some applications [1,2]. DNAN shows rich pressure-induced polymorphism, with at least four high-pressure forms having been identified to date. One of the structures provides insight into as to why DNAN is particularly insensitive to initiation by shock. The presentation will also describe the interplay between experiment and theory, which will be illustrated by experimental and computational high-pressure studies of 1,1-diamino-2,2-dinitroethene (DADNE or FOX-7). A very subtle phase transition has been identified at a pressure of ~2.0 GPa and the implications of this will be discussed in relation to the observed structural changes and properties of this material.

High-pressure crystallography of periodic and aperiodic crystals

“…In the search for new energetic materials with enhanced safety properties, such as stability against changes in temperature and humidity, chemical stability, stability against electrical charges and shock resistivity, while simultaneously performing well against the detonation stimulus, simple organic molecules have recently become the focus of attention, e.g. 2,4-dinitroanisole (DNAN), 1,1-diamino-2,2-dinitroethene (DADNE or FOX-7), 1,3,5-trinitrohexahydro-1,3,5-triazine (RDX), cyclotetramethylene (HMX), hexanitrohexaazaisowurtzitane (CL-20) and 3-nitro-1,2,4-triazol-5-one (NTO) (Pulham et al, 2014;Coster et al, 2014;Lloyd et al, 2014;Hunter et al, 2013;Millar et al, 2012;Pulham et al, 2010;Millar, Maynard-Casely et al, 2010;Oswald et al, 2010;Davidson et al, 2008). All of these molecules have the common feature of oxidizing nitro groups in close proximity to an oxidizable hydrocarbon skeleton.…”

“…At the microsymposium 'High-Pressure Crystallography of Periodic and Aperiodic Crystals' of the 2014 IUCr Congress and General Assembly in Montreal, Canada, contributions were presented on the high-pressure behaviour of molecular crystal structures (Dziubek et al, 2014;Ivanenko et al, 2014;Kapustin et al, 2014a;McGregor et al, 2014;Saouane et al, 2014), energetic materials (Pulham et al, 2014), phases relevant to the Earth's interior (Boffa Ballaran et al, 2014;Miletich et al, 2014;, materials with a pressure-induced expansion in one or two directions Adamson et al, 2014), dealing with high-pressure data from crystals with twinning and pseudosymmetry (Friese & Grzechnik, 2014a, pressure-induced phase transitions including an incommensurate phase , and technical developments (Munaka et al, 2014). Although the presented contributions do not cover the whole spectrum of research, they do demonstrate the broad diversity covered by the title and have inspired the chairs of the microsymposium to summarize selected recent results of high-pressure singlecrystal studies of molecular and aperiodic structures.…”

Spectroscopic pressure investigation of a novel high energy system

“…They have many applications in both the civilian and military realms including fireworks, explosives, and propellants. Of course, one major area of concern re-volving around these energetic systems is safety [1][2][3]. Energetic systems must be as safe as possible while still meeting performance, cost, and reliability demands.…”

“…Accidental detonations are often induced by shock, such as from high-speed fragments or sympathetic reaction from adjacent energetic systems. However, other common triggers for detonation, and therefore a release of high energy, may include electrostatic discharge, impact, and friction [1,2]. These conditions may result in phase transitions and may even lead to the initiation of chemical reactions [1][2][3][4][5][6][7].…”

“…However, other common triggers for detonation, and therefore a release of high energy, may include electrostatic discharge, impact, and friction [1,2]. These conditions may result in phase transitions and may even lead to the initiation of chemical reactions [1][2][3][4][5][6][7]. Understanding these phase transitions at high pressures is therefore critical to ensure a very reliable and safe energetic system.…”