Thin Film Crystallization of Energetic Materials for Performance and Stability Enhancement

The study of energetic materials encompasses a wide variety of applications, spanning from construction and pyrotechnics to military devices and space exploration, amounting to a multibillion-dollar industry. The main goal in research and development of energetics is to balance material performance and stability to ensure effective and safe use for a designated application. Crystallization and formulation of energetic materials and energetic composites provide challenges as important structure-property relationships are not fully understood. Influences of crystal properties (e.g., polymorphism, morphology and habit, particle size, defects) on material behavior both independently and when interfaced with other materials in composite formulations must be well characterized for optimal use; however, conventional characterization techniques are often unable to probe bulk crystal geometries when dispersed in composites. Thin film crystallization offers a platform to control, study, and eventually develop a thorough understanding of structure-property relationships for energetic crystals both independently and when interfaced with other materials.

Meniscus-guiding coating (MGC) is an evaporative, thin film crystallization technique that has a proven ability to highly control the crystallization pathway for other organic molecules. Varying the processing conditions produces a variety of different crystal structures, morphologies, particle sizes, and preferential orientations. In this work, MGC is utilized to crystallize thin films of 2,4,6,8,10,12-hexanitrohexaazaisowurtzitane (CL-20) and ammonium perchlorate (AP).

In chapter 2, thin film crystallization of a high-performing EM (CL-20) is explored using MGC. CL-20 crystallizes in several different polymorphic structures, each with variable performance and stability parameters; therefore, understanding material stability and potential phase transformations is crucial for commercial implementation of CL-20. A large parameter space was studied, and a wide range of morphology, film thickness, film coverage, and crystal structures were observed at different processing conditions. Most interestingly, the metastable β-CL-20 polymorph was observed for films crystallized at temperatures below 140 °C, and the high temperature γ-CL-20 polymorph was observed for some films crystallized above this temperature. After thermal annealing films at 180 °C, films with thickness > 500 nm underwent a β → γ phase transformation while films with thickness < 500 nm remained in the β-CL-20 metastable structure. This finding suggests β-CL-20 is stabilized in thin film geometry and could have serious implications for composite formulations.

In chapter 3, highly tunable crystalline templates, provided by 2D metal halide perovskites, are utilized to study the influence of interfacial interactions on the crystallization of CL-20 thin films. Four perovskite surfaces were chosen with different surface termination and ligand density, and MGC was used to crystallize CL-20 on top of each unique surface. Morphology, film thickness, crystal structure, and preferential crystallographic orientation were characterized, and different perovskite surfaces were observed to promote unique morphology and preferred orientation of β-CL-20. Our results suggest that interfacial energy minimization between the energetic crystal face and the interfacial material influences CL-20 nucleation and growth, which could inform composite material selection for enhanced stability.

In chapter 4, AP, a common oxidizer in solid composite propellants, is crystallized in thin film geometry using MGC, and a large parameter space was explored. Variable morphology, particle sizes, and preferred crystallographic orientations were observed by varying the processing conditions. The coherence length was calculated from the diffraction peak spreading, and the relative defect density was determined for a smaller subset of samples. Thermogravimetric analysis and differential scanning calorimetry results provide information regarding the AP decomposition behavior and suggest that higher defect density is associated with a higher energy release at lower temperatures. This finding can be utilized to improve design and tailor energy release rates for propulsion formulations.

Chapter 5 briefly summarizes the work presented in this dissertation, as well as provides future directions for study based on findings discussed here within. Ultimately, developing an understanding of how energetic materials behave, both independently and when interacting with other materials, will allow for performance and stability enhancement in future formulations.