Fabrication and characterization of micro- or nano-scale organic energetic materials
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Organic energetic material (OEM) research has attracted a lot of attention due to its wide application in mining, demolition, military weaponry, and aeronautical engineering. OEMs often exhibit a tradeoff between properties, associated with the energy content of the materials, and the potential for inadvertent ignition to detonation. Microstructures have been shown to have a significant impact on properties of OEMs. Thus, properties of OEMs can be tailored by manipulating their microstructures. In this dissertation, several techniques are developed for fabricating micro- or nano-scale OEMs. The thermal stability, burn rate, and laser ignition properties of some prepared micro- or nano-scale OEMs are also investigated. In addition, the prepared highly nanoporous graphene oxide (GO)-nitrocellulose (NC) films are used to synthesize three dimension (3D) graphene networks and nitrogen-doped (N-doped) 3D graphene networks. In chapter I, the author provides a comprehensive description of the progress and development of microfabrication techniques in OEMs and the motivation of the dissertation. The developed techniques are divided into two categories: (1) fabricating micro- or nano-scale void structures on the surface or inside of OEM films or bulk materials; and (2) controlling the crystalline structure and particle size of OEMs. In addition, this chapter will discuss properties of micro- or nano-scale OEMs. In chapter II, purification methods of various organic materials, preparation methods of various substrate and organic thin films, and procedures of different patterning methods are addressed. The author also demonstrates the procedures to fabricate pentaerythritol tetranitrate (PETN) micro-crystals, graphene oxide (GO)-PETN micro-composites, and various nanostructured nitrocellulose films. In chapter III, two novel techniques, adhesive reagent assisted lift-off lithography and tip induced crystallization lithography (TICL), are developed to pattern OEM thin films. For the adhesive reagent assisted lift-off lithography, an adhesive agent is coated on a PDMS stamp to increase the adhesion force between the stamp and the organic thin film. In comparison to traditional lift-off lithography techniques, the adhesive reagent assisted lift-off lithography does not require high contact pressures or external heating, which potentially allowing for patterning of a wide range of thermally sensitive compounds. The TICL technique depends on coating an amorphous organic thin film on a substrate and then inducing crystallization of the thin film using an AFM tip. After removing the non-crystalline materials from the substrate, the organic crystal arrays can be obtained on the substrate. In chapter IV, two novel techniques are developed to manipulate the crystalline structure of PETN. Firstly, the author investigates the fabrication of PETN thin films through spin coating. The crystalline structure of PETN is found to have a strong dependence on the size of non-crystalline PETN particles in amorphous thin films. The non-crystalline PETN particles can be controlled by adjusting the spin coater rotational speed and solution concentration. The author also sets up a model to describe the relationship between the non-crystalline particle size, spin coater rotational speed and solution concentration. Secondly, the author indicates that GO can be used to manipulate the crystalline structure of PETN. The irregular PETN micro-crystals can be fabricated by using a solvent/non-solvent method; however, after introducing 0.5% or more GO, the PETN formed regular microrods. In comparison to pure PETN micro-crystals, GO-PETN micro-composites have higher thermal stability and slower sublimation rate and vapor pressure at temperatures ranging from 105 oC to 135 oC. In chapter V, nanostructured pure NC and GO-NC films are fabricated via using an evaporating method and a mixing method, respectively. The morphology of pure NC films can be controlled by the solvent and growth temperature. Using dimethylformamide (DMF) at a growth temperature was 5 oC, reproducibly yielded spherical NC particles. And the final diameter of the prepared NC particles can be further tuned by solution concentration. The highly nanoporous NC films can be prepared via introducing 0.5% to 3% GO. In comparison to the bulk NC film, the pure NC films with sub-micro spherical particles and nanoporous GO-NC films have faster burn rate. The thermal stability and NIR laser ignition properties of NC films are also obviously improved when doped with 0.5% or more GO. Furthermore, the high-quality and uniform 3D graphene networks and N-doped 3D graphene networks can be prepared through thermal decomposition and combustion of GO-NC composites, respectively. In chapter VI, the author summarizes the dissertation and presents the future works.