Investigations of Laser Ablation and Laser-Induced-Plasma Ignition of Cyclotrimethylenetrinitramine (RDX)

Abstract

Performance and sensitivity are two factors almost equally weighed when an explosive is considered in application. With a superb combination of high detonation power and low sensitivity to external stimuli, secondary explosives, instead of primary explosives, have been mainly used as a charge in military and mining field. One of the main objectives of this dissertation is to study the mechanism of one of the most common secondary explosives, cyclotrimethylenetrinitramine (RDX), under 266nm UV-laser ablation both in air and argon environments. Preliminary investigations on laser-induced plasma initiation on RDX have been carried out. Chapter 1 presents a comprehensive literature review on explosives, laser ablation on explosive and plasma initiation. Detailed information was provided of background knowledge and recent advances of explosive initiation. In chapter 2, the products of the ultraviolet (UV) laser ablation of RDX in air havebeen investigated using infrared (IR) spectroscopy. The total carbon and nitrogen content of the observed gas phase species account for less than 10% of the material removed by the ablation process. The low fraction is explained by an ablation process specific to molecular solids, where a large fraction of the RDX is removed in the ablation process, > 83% in this case, is simply sublimed from the surface only to deposit on the walls of the container without reaction. The chemical products observed by IR include HCN, N2O, CO, CO2, NO2. Chapter 3 presents a comparative study of the products obtained under an argon environment vs. in air as in the previous chapter. The IR detection of gas products showed all previously detected compounds (HCN, NO2, N2O, CO, CO2) with addition of a new peak, NO. The understanding of the dependence of plasma formation on the laser fluence provides insight into the chemical mechanism of each direct gas product. Comparison of the anticipated line shape of gas distribution over laser fluence with the experimental data implies the following possible mechanisms under Argon environment: a) HCN, N2O and NO are formed via multi-photon process; b) NO2 may be a single photon product; c) CO and CO2 are formed via thermal process of plasma chemistry. Further comparison of gas distribution over laser fluence under air and Ar indicates that the above-mentioned direct products may be formed in air following the same mechanism, except the reactive species O2 in air reacts secondarily with some direct products, such as NO, alternated the line shapes to some degree. Chapter 4 presents a preliminary investigation on laser-induced-plasma initiation of RDX. A thin polycarbonate (PC) film has been used as the plasma generating source. A sample of PC film spin-coated on top of RDX has been exposed to laser pulse irradiation. The laser –induced plasma initiation has been probed and proved through a variety of approaches, namely, gas products pressure detection, observation of reflected light, witness board experiment and dynamic pressure measurement. All approaches provided evidence for an initiation of RDX films by a laser-induced-plasma.