Directed Crystal Growth and Solid-State Analysis of the Secondary Explosives RDX and HMX
Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are two of the most commonly used secondary explosives in military and industrial applications. For use as explosives, they are often formulated by mixing the crystalline material with a polymer binder to form a composite known as a plastic bonded explosive. The performance of these materials can be affected by several factors, many of which depend on solid-state properties, e.g. sensitivity to detonation, detonation velocity and pressure, crystal density and crystal morphology. Though widely studied, a detailed understanding of how the crystal growth conditions affect the properties of these compounds remains incomplete.In this work, the crystal growth of RDX and HMX was explored using different solvent systems and a variety of self-assembled monolayer templates. Grown crystals were analyzed via diffraction, microscopy and spectroscopic techniques to determine phase, morphology and defect structure. A number of important findings were made.α-RDX growth morphology and defect structure could be modified though changes in the solvent. Oriented growth of these crystals was accomplished using templates with different end functionalities. The crystallization of the metastable form, β-RDX, was found to occur readily using drop cast crystallization methods. Crystallization conditions that stabilized this form for longer periods than previously realized enabled the examination of its crystalline properties for the first time. β-RDX crystals were found to be smaller and have a rougher surface topography than α-RDX crystals. Thermal analysis showed them to melt 15° lower than α-RDX. β-RDX transformed to the thermodynamically stable α-RDX upon mechanical stimulation. α-RDX and β-RDX decomposed differently in the presence of electromagnetic radiation.HMX forms were found to crystallize concomitantly depending on the growth solvent used. Nitrogen rich templates were able to nucleate HMX crystals. Crystals grown in the presence and absence of templates had qualitatively different types of defects. The former had grain boundary defects, while the latter showed more lattice distortions.These studies demonstrate that RDX and HMX exhibit different solid-state properties depending on the growth condition. Understanding the intimate relationship between the growth conditions and the resultant properties is an important prerequisite to improving the performance of these compounds.
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