Peter Dickson

The β-δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine: Thermodynamics

In this paper we present second harmonic generation (SHG) experiments designed to confirm the mechanism and quantify the transformation kinetics of the β–δ solid state phase transition in the organic nitramine molecule octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). The β phase adopts a centrosymmetric crystallographic configuration (P21/c) while the δ phase adopts a noncentrosymmetric one (P61(P65)). As expected, this results in a very poor generation of SHG intensity from the β phase, while the δ phase is very efficient, rivaling KH2PO4 in absolute efficiency. SHG thus provides a very high sensitivity zero background probe of the δ phase. We discuss the use of this signal as a quantitative measure of the δ phase mole fraction in ensembles of free HMX crystals and crystals embedded in a visco–elastic polymer matrix. We report imaging experiments where the spatial characteristics of the transformation are shown to be consistent with nucleation from a low density of initial sites, followed by rapid...

On the nucleation mechanism of the β-δ phase transition in the energetic nitramine octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

In our previous work on the mechanism for the β-δ solid-solid phase transition in octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), we used an empirical mechanism for the nucleation step and hypothesized a defect mechanism that was greatly affected by the presence of a nitroplasticizer/estane binder in the HMX formulation. Since then, we have acquired further evidence for this and have separated out the components of the binder to confirm that it is the nitroplasticizer that controls the nucleation energy in HMX formulations containing a nitroplasticizer/estane binder. While the exact distribution of nucleation energies as a function of synthesis route/defect type has not been worked out, it is likely that the solubility of the HMX in the nitroplasticizer is responsible for lowering the nucleation energy at the crystal surface, and therefore determines the nucleation rate for the formulation.

On the Difference in Impact Sensitivity of Beta and Delta HMX

The beta-to-delta phase transition in HMX has been implicated as the primary reason behind the increased sensitivity of the explosive as it is heated. Both physical and chemical changes accompany the transition, but no study has conclusively shown which specific change, or set of changes, is responsible. We present evidence that shows that the mechanical differences, in and of themselves, do not result in increased sensitivity to shock compression of HMX.

Ignition Chemistry in HMX from Thermal Explosion to Detonation

We present a global chemical decomposition model for HMX based materials. The model contains three component processes, the initial beta to delta phase transition, solid to gas decomposition and gas phase ignition, for which all kinetic and thermodynamic parameters are fixed by independent measurement. We present an isothermal ignition calculation over the range of temperatures from thermal explosion to detonation. The calculation is performed for a sphere of material and the critical diameter and time for ignition are determined. The sample diameter, and thus the balance of heat generation and dissipation, is the only degree of freedom in the calculation. The results of the calculation are in good agreement with data with respect to both the ignition times and length scales over the full temperature range of energetic response in HMX.

Thermal cook-off response of confined PBX 9501

The thermal cook–off response of energetic materials (ignition resulting from direct, bulk thermal heating) is important from a safety point of view, but also challenges our understanding of these materials. Explosives are not designed to be cooked off, and, especially in the case of slow cook–off, by the time the material ignites it is substantially different, both chemically and physically, from its original state. In attempting to model such a process numerically, it has generally been assumed that combustion proceeds, from an ignition point, in a more or less planar manner, as has been observed many times in pristine energetic materials at room temperature. To investigate directly the spread of reaction following cook–off in one such energetic material (PBX 9501), small discs of PBX 9501 were heated with a confining glass or sapphire window through which the early stages of the combustion process could be observed directly by high–speed photography. The resulting combustion was found to vary with temperature of ignition, but in all cases was quite different to the laminar burn model. The results of these tests are presented, together with some possible explanations of the behaviour and discussion of the implications to modelling this response.

Frictional heating and ignition of energetic materials

For many years, powder friction tests have been an integral part of explosives sensitivity and safety testing. More recently, oblique impact tests have been used in the hazard assessment of monolithic charges. However, these tests are simply threshold tests for reaction, and relatively little work has been done to try to examine the processes that lead to frictional heating and ignition of energetic materials. We report the results from a series of experiments in which energetic materials are subjected to frictional heating under closely‐controlled conditions (normal load, sliding speed, grit quantity and composition, substrate). The response of the energetic material and grit, if present, is observed by optical and infrared high‐speed photography to determine the nature of the interactions between the test material, grit and substrate, and the mechanisms by which the energetic material may be heated to ignition.

Measurement of phase change and thermal decomposition kinetics during cookoff of PBX 9501

Models describing the kinetics of HMX slow decomposition in terms of reduced 3-step Arrhenius kinetics have been very successful in predicting time-to-explosion for HMX-based explosives over a wide range of heating rates. However, the detailed reaction and temperature profiles predicted by these codes have not been tested, and in particular, the accuracy of the predicted ignition location, which is thought to be an important factor in determining the subsequent reaction violence, is unknown. Experiments have been conducted to make spatially and temporally resolved temperature measurements in a cylindrical charge of PBX 9501 during external heating to cookoff. These data provide a direct comparison with the predictions of thermochemical models, and indicate that the existing models do not capture the detailed response of the process. The data have been used to develop a modified reduced kinetic scheme which models the process more accurately.

Interplay of explosive thermal reaction dynamics and structural confinement

Explosives play a significant role in human affairs; however, their behavior in circumstances other than intentional detonation is poorly understood. Accidents may have catastrophic consequences, especially if additional hazardous materials are involved. Abnormal ignition stimuli, such as impact, spark, friction, and heat may lead to a very violent outcome, potentially including detonation. An important factor influencing the behavior subsequent to abnormal ignition is the strength and inertia of the vessel confining the explosive, i.e., the near-field structural/mechanical environment, also known as confinement (inertial or mechanical). However, a comprehensive and quantified understanding of how confinement affects reaction violence does not yet exist. In the research discussed here, we have investigated a wide range of confinement conditions and related the explosive response to the fundamentals of the combustion process in the explosive. In our experiments, a charge of an octahydrotetranitrotetrazine-...

Direct measurement of strain field evolution during dynamic deformation of an energetic material

We previously reported results showing displacement fields (at a single instant in time) on the unconfined surface of an explosive during deformation using white light speckle photography. We have now successfully obtained similar data in confined samples showing the evolution in time of the strain field using laser-induced fluorescence speckle photography. A modified data analysis technique using methods borrowed from particle image velocimetry was used in conjunction with an eight frame electronic CCD camera. For these tests, projectiles of varying shape were fired into an explosive sample. Localization of strain was observed in all cases and was found to be a strong function of the projectile shape, with ignition occurring in those cases where shear appears to play a dominant role. Results from this and continuing studies provide experimental evidence for strain localization, and for the first time allow the direct comparison to computer model predictions. The data are also being used in the design of ...

Impact-induced friction ignition of an explosive: Infrared observations and modeling

A contaminant (grit) trapped between an explosive and an impacted surface can significantly reduce the impact energy required to initiate a secondary high explosive. Several severe accidents have occurred when an explosive charge was dropped from a height insufficient to cause ignition by heating due only to plastic deformation; the frictional heating from embedded grit has been implicated. Here, we describe an idealization of this situation where a small sample of a polymer-bonded cyclotetramethylenetetranitramine explosive (HMX-PBX 9501), with a 400 μm diameter sphere of silica embedded in the surface, was impacted between instrumented transparent anvils and infrared images were recorded. The instrumentation provided temperature and the work done by the friction between the grit and the anvil surface for the impact process, up to ignition. All experiments were conducted under impact conditions insufficient to cause ignition without grit. Ignition occurred at approximately 500 μs, a grit temperature of 1...