Session K04: Explosives Sensitivity

Chemical reactivity changes in energetic crystals upon phase transformation at elevated temperature and pressure

Energetic crystals (such as CL20) exhibits numerous phases at different pressure and temperature. Apart from changes to the molecular crystal structure, the primary difference in the different phases of CL20 are associated with the orientations of the nitro group. Even though it is quite well known that changes in molecular structure of a crystal results in changes to the physical response of a material, but changes in chemical surface reactivity of the material are not quite well understood. In this study, changes in surface chemical reactivity of different phases of CL20 have been investigated. Studies involving surface chemical reactivity includes detailed estimation of quantum chemical parameters such as chemical hardness, electrophilicity, fukui indices. It is anticipated that changes in surface chemical reactivity affects the activation energy as well as the bonding of the energetic crystal with the polymeric binder.  

    

Determining effects of molecular structure variation in nitrate esters on explosive sensitivity through structurally similar energetics.

Current design of novel explosives attempts to accurately predict the sensitivity of potential molecular candidates prior to costly and time intensive synthetic exploration. Subtle chemical, molecular, and physical properties can contribute significantly to overall sensitivity of an explosive, though these can be interrelated and extremely difficult to isolate. For example, modifying one molecular characteristic (e.g. chemical structure) will often adversely result in altering other molecular or physical properties (e.g. melting point). Sabatini and coworkers studied the effects of stereo- and regiochemistry that resulted in altering the physical properties and sensitivity of explosives, from extremely low-melting energetic liquids to melt-castable explosive eutectics. However, structural alterations result in changes to intra- and intermolecular forces which have been suggested to aid in the stability of explosive molecules. A systematic study is needed to determine the interplay of molecular forces and explosive sensitivity. Here we describe pentaerythritol tetranitrate (PETN) derivatives with varying degrees of molecular forces and the affects on explosive sensitivity and molecular physical properties.   

Modeling Impact Induced Damage and Ignition in Unconfined High Explosives

We have recently conducted a series of tests in which unconfined explosive disks mounted

onto a polycarbonate plate are impacted at low velocities (15-40 m/s) by a rounded projectile.

We used high speed videography to record the impact event from the back side of the plate.

We observe the damage evolution prior to ignition, and monitor the velocities where ignition

occurs in different explosive formulations. We use these observations to inform the

development and calibration of the High Explosive Response to Mechanical Stimuli (HERMES)

explosive safety model [1]. In this work we assess the affects of various material parameters in

the HERMES model on the observed mechanical behavior, which includes dynamic fracture and

comminution. We calibrate our ignition model for comparison with low-velocity impact

ignitions in other, more confined geometries such as the Steven test.

[1] J. E. Reaugh, et al. “A Computer Model to Study the Response of Energetic Materials to a

Range of Dynamic Loads,” Propellants, Explosives, Pyrotechnics, 43(7), p. 703-720, 2018.   

Microscale Electrostatic Discharge (ESD) Sensitivity Characterization of Energetic Materials

Prior to formulating and large-scale testing, all novel energetics must undergo a variety of safety testing, including characterizing the electrostatic discharge (ESD) sensitivity, where the material is exposed to a spark discharge-generated shockwave at varying energy levels to determine the ignition threshold. The current standards for these tests include a variety of technique implementations and subjective and operator-dependent go/no-go criteria. As a result, values reported for the same material vary widely across different laboratories. Furthermore, these tests typically require grams of material to complete, which is typically unavailable during the early development process when only milligrams are synthesized at a time. Waiting to test until after developing scale-up procedures is nonideal since it is often too late to modify the synthesis procedure without significant additional cost and labor; if the material does not pass, it is often abandoned. Therefore, an objective microscale method to pre-screen the ESD sensitivity of novel energetic materials was developed in our laboratory. The required amount of material is reduced by an order of magnitude (less than ten milligrams per test) and objective go/no-go criteria are incorporated, including time-resolved integrated emission measurements, emission spectroscopy, and high-speed imaging. This presentation details our unique microscale ESD sensitivity procedure and provides example characterizations of aluminum powders.   

Evaluating the Role of Radiation in High-Explosive Shock Initiation by the use of Aluminum & Lithium Flouride-Loaded Explosives

Aluminum (Al) and lithium fluoride (LiF) powders are High Explosive (HE)-like in many of their physical properties: density, molecular weight, melting point and shock Hugoniot. This means that when Al and LiF powders are added to HEs to form a mixture, they (to first order) alter reactive rather than physical properties. Al-loaded explosives are well known to alter HE reactivity and are often used to prolong reaction and add thermal energy. While LiF apparently does not undergo reaction when mixed with HE, there is reason to believe that the tremendous light output of shocked LiF can contribute to HE initiation via radiation ahead of the CJ state. Such early-time light intensity is not created by similarly-formulated aluminized explosives, in which is decreased shock sensitivity in those relative to LiF is observed. A variety of sensitivity tests, such as Gapstick, ECOT, front curvature will be employed to further examine this effect.