Physical and chemical characterization of energetic nano-materials via theoretical modeling and spectrochemical analysis.

摘要

The present work aims to characterize structural, morphological, and chemical properties of energetic nanoparticles with the aid of theoretical and experimental studies.; The first part develops a kinetic Monte Carlo algorithm to study the effect of particle-particle collision and exothermic coalescence events on primary particle sizes in a growing nano-aerosol. The results indicate that under certain process conditions, two coalescing nanoparticles can release sufficient energy and substantially exceed the background gas temperature. This increases atomic diffusivities and hastens the coalescence process. Typical results for silicon and titania nanoparticles show that increasing volume loading and decreasing pressure enhances non-isothermal sintering rates and produces larger primary particles. These results suggest the use of process parameters heretofore not considered in tailoring the microstructure of nanoparticles.; The second part develops a hybrid nodal model to study the effect of size-dependent surface tension on nanoparticle evolution during gas-particle conversions. It uses chemical reaction-based formalism to compute nuclei formation rates as opposed to the commonly used kinetic homogeneous nucleation expression based on steady-state assumption and capillarity approximation (size-independent constant surface tension). The results indicate that although quasi-steady state assumption is reasonable, the capillarity approximation significantly over-predicts particle sizes.; On the experimental side, Laser Induced Breakdown Spectroscopy (LIBS) is used for quantitative estimation of elemental composition of nano-aerosols using an internal standard. The methodology uses time-resolved atomic emission spectra and plasma temperatures to calculate species densities calibrated with background gas densities under the same experimental conditions. The method is applied to determine the extent of oxidative coating on aluminum nanoparticles. The aim is to understand the reactivity of nano-materials and the stability of passivation coatings, such as metal oxides. The use of background gas as the internal standard mitigates the need for material standards and can be extended to the characterization of other multi-component aerosol systems.; This thesis also presents laser ablation technique to produce aluminum nanoparticles. Using various laser fluences, Al nanoparticles were generated with a production rate of 4--4.2 mg/hr having a size range of 17--75 nm and number concentrations of 3 x 105--3 x 106 #/cc.