Date of Award

Summer 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering - (Ph.D.)

Department

Chemical, Biological and Pharmaceutical Engineering

First Advisor

Edward L. Dreyzin

Second Advisor

Rajesh N. Dave

Third Advisor

Norman W. Loney

Fourth Advisor

Robert Benedict Barat

Fifth Advisor

Joel Carney

Abstract

Many multifunctional nanocomposite materials have been developed for use in propellants, explosives, pyrotechnics, and reactive structures. These materials exhibit high reaction rates due to their developed reaction interfacial area. Two applications addressed in this work include nanocomposite powders prepared by arrested reactive milling (ARM) for burn rate modifiers and reactive structures. In burn rate modifiers, addition of reactive nanocomposite powders to aluminized propellants increases the burn rate of aluminum and thus the overall reaction rate of an energetic formulation. Replacing only a small fraction of aluminum by 8Al•MoO3 and 2B•Ti nanocomposite powders enhances the reaction rate with little change to the thermodynamic performance of the formulation; both the rate of pressure rise and maximum pressure measured in the constant volume explosion test increase.

For reactive structures, nanocomposite powders with bulk compositions of 8Al•MoO3, 12Al•MoO3, and 8Al•3CuO were prepared by ARM and consolidated using a uniaxial die. Consolidated samples had densities greater than 90% of theoretical maximum density while maintaining their high reactivity. Pellets prepared using 8Al•MoO3 powders were ignited by a CO2 laser. Ignition delays increased at lower laser powers and greater pellet densities. A simplified numerical model describing heating and thermal initiation of the reactive pellets predicted adequately the observed effects of both laser power and pellet density on the measured ignition delays.

To investigate the reaction mechanisms in nanocomposite thermites, two types of nanocomposite reactive materials with the same bulk compositions 8Al•MoO3 were prepared by different methods. One of the materials was manufactured by ARM and the other, so called metastable interstitial composite (MIC), by mixing of nano-scaled individual powders. Clear differences in the low-temperature redox reactions, well- detectable by differential scanning calorimetry (DSC), were established between MIC and ARM-prepared materials. However, the materials behaved similarly to each other in the ignition experiments. It is proposed that the ignition of both MIC and ARM-prepared materials at the same temperature can be explained by a thermodynamically driven transformation of a protective amorphous alumina into a crystalline polymorph.

Low temperature redox reactions in ARM-prepared Al-CuO nanocomposites were characterized using DSC and isothermal microcalorimetry. The results were interpreted using a Cabrera-Mott reaction model. Simultaneous processing of both experimental data sets identified the parameters for the respective Cabrera-Mott kinetics. The low temperature kinetic model was coupled with a multi-step oxidation model describing diffusion-controlled growth of amorphous and γ-Al2O3 polymorphs. The kinetic parameters for the multistep oxidation model from previous research were adjusted based on DSC measurements. The combined heterogeneous reactions model was used to interpret results of ignition experiments. It is proposed that the heterogeneous reactions considered serve as ignition triggers and ensuing gas release processes contributes to additional heat release and temperature runaway.

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