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TAILORABLE ENERGETIC MATERIALS: PROPELLANT MANUFACTURING AND MODIFICATION OF EXPLOSIVES’ WAVE SHAPES AND SENSITIVITIES

thesis
posted on 2024-04-20, 20:22 authored by Joseph Robert LawrenceJoseph Robert Lawrence

Tailorable energetics are energetic materials that can be modified to alter their performance and sensitivity. Examples of tailoring energetic materials include additive manufacturing, manufactured hot spots, switchable energetics, and cocrystallization. Developing novel energetic material is a difficult and cost intensive process, because of this, tailoring the performance and sensitivity of existing energetic materials is critical for continued improvement. Additive manufacturing has provided new methods for generating complex geometries of composite materials. Additive manufacturing of composite materials through direct-ink-write (DIW) experiences extrusion limitations due to the high viscosities of highly solids loaded mixtures; the limitations being more severe with smaller syringe tip diameter. A novel printing technique called vibration-assisted printing (VAP) was developed as a method to extend the extrudability limits and extrusion speeds observed with direct-ink-write systems. Printability envelopes were shown in previous work to extend extrudability of monomodal glass bead composites in VAP systems over conventional DIW systems. This study compares the mass flowrates and extrudability limits for bimodal mixtures of glass beads suspended in a hydroxyl-terminated polybutadiene (HTPB) binder for both VAP and DIW printing as a function of volume percent solids loading. The bimodal glass bead mixtures showed a linear response in extrusion rate versus solids loading for both VAP and DIW systems. The VAP system was able to print higher volume loadings than the direct-ink-write system. In addition to extending the extrudability limits, the mass flowrate for the VAP system was significantly higher at all volume loadings tested compared to the DIW. Interestingly, bimodal mixtures were shown to extrude quicker than the monomodal mixtures at all volume loadings and across both printing systems.


Inhomogeneities within explosives affect the sensitivity and detonation wave shape of energetic materials. The influence of voids on explosive initiation has been well documented; however, the effects that voids between 0.1 mm and 10 mm have on a propagating detonation wave remains largely unexplored. The effect of single cylindrical voids on detonation wave shape and re-initiation was examined here using manufactured voids in a rubberized 1,3,5-trinitro-1,3,5-triazinane (RDX) explosive. Two streak imaging techniques were fielded to investigate void influence. For the first, back-surface streak imaging, the location of the void on the samples was changed and the resulting change in detonation wave shape at the downstream breakout was captured using a streak camera in cut-back experiments. The results from this experiment showed the effects of an initial jet form for short cut-back distances and as shock propagation progressed, the jet formation was absorbed by the unaffected portions of the wave front. The second method, top-surface streak imaging, was used to investigate the re-initiation/downstream propagation of the detonation front and the detonation velocity of the rubberized explosive. These experiments were compared to similar experimental results from machined voids in PBX 9501, a 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX)-based explosive, to investigate the interaction of a detonation wave with a 0.5 mm void for different explosives. The experiments were also compared to simulations using a multi-dimensional and multi-material hydrodynamic code (CTH). These results showed the influence that small features can have on detonation wave shaping and how explosive properties play a key role in that interaction. In addition to air-filled voids, this study examined the effects of 0.5 mm diameter voids filled with different inert metals on the detonation wave shape for an RDX-based rubberized explosive. The metals selected for experiments were 1066 aluminum, brass, copper, and tungsten. Experimental results showed that the extent of detonation wave shaping was closely tied to the density differential between the bulk explosive and metal insert. Simulations were performed using CTH to further analyze material inclusions. Forty-four different filler materials were simulated to isolate the driving factors for wave shaping of the detonation front. The main factors of interest were bulk sound speed, shock impedance, and filler material density. Understanding the influence of material inclusions on detonation performance and wave shape allows for tailoring of detonations as well as characterizing how unintentional defects will alter the explosive.


Improving the safety of explosive materials through the synthesis of insensitive explosives has been studied extensively. However, little work has focused on creating switchable explosives. A switchable explosive is normally insensitive to detonation, and therefore safe to handle and transport, but can be sensitized when needed to create a functional explosive. Similarly, it may be desired to desensitize an explosive to prevent its function. This study examined the ability to create a switchable RDX-based rubberized explosive using thermally-expandable microspheres (TEMs). The addition of TEMs to the explosive formulation allowed for microstructural changes and potential hot spot locations such as voids to form as the microspheres expanded. Small voids (less than about 10 µm) are more likely to be critical hot spots when shocked, and likewise larger voids are less likely to ignite successfully (sub-critical) when shocked. Consequently, both sensitization and desensitization are possible. The rubberized explosive considered here with unexpanded microspheres was unable to sustain a detonation for the size used, but after specific heating followed by cooling to produce small voids, a detonation was achieved. That is, the TEMs addition to the RDX-based rubberized explosive resulted in an explosive that is detonation insensitive when unheated but becomes a functional explosive after it is sensitized through heating. This paves the way to create insensitive explosive formulations with on-demand switchable detonation function through the incorporation of thermally-expandable microspheres. Desensitization was also demonstrated with specific heating of TEMs in an initially detonable explosive charge. And finally, we also demonstrated that deflagration can be affected by heating TEMs.


Energetic cocrystallization is a technique that produces a cocrystal that is formed using two known explosives to potentially gain the benefits of one or both without the drawbacks for a particular application. A comparison of cocrystals to a physical mixture of the same coformers can be considered. Cocrystals have unique material properties and crystal structure, whereas a physical mixture is just a mixed combination of the known materials at the same molar ratio. This study used photon Doppler velocimetry (PDV) to compare the particle velocity for 2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and 1-methyl-3,5-dinitro-1,2,4-triazole (MDNT) at a 1:1 molar ratio for both a cocrystal and a physical mixture of the two energetic materials. This cocrystal was previously shown to detonate faster than a physical mixture. However, the PDV results here were not consistent with this result. In addition to measuring output particle velocity with PDV, the cocrystal was characterized to examine phase purity and possible signs of deterioration of the material over time. Evaluating the cocrystal with Fourier-transform infrared spectroscopy (FT-IR), bomb calorimetry, and powder X-ray diffraction (PXRD) allowed for more accurate comparison and greater confidence in the particle velocity measurements obtained in these experiments. The most significant difference in the material characterization results was the difference in enthalpy of formation, as the material tested in this study had a substantially lower enthalpy of formation than previously measured for a CL-20/MDNT cocrystal.

History

Degree Type

  • Doctor of Philosophy

Department

  • Mechanical Engineering

Campus location

  • West Lafayette

Advisor/Supervisor/Committee Chair

Steven F. Son

Additional Committee Member 2

Weinong Chen

Additional Committee Member 3

Marisol Koslowski

Additional Committee Member 4

Terrence R. Meyer