Document Type

Dissertation

Date of Award

5-31-2021

Degree Name

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

Department

Chemical and Materials Engineering

First Advisor

Edward L. Dreyzin

Second Advisor

Mirko Schoenitz

Third Advisor

Rajesh N. Dave

Fourth Advisor

Gennady Gor

Fifth Advisor

Melissa Liberatore-Moretti

Abstract

This work explores inorganic fluorides as oxidizers for fuel-rich reactive materials. A preliminary assessment of metal fluorides accounting for their enthalpy of formation points to bismuth (III) fluoride, BiF3 and cobalt (II) fluoride, CoF2 as oxidizers of interest. Initially, composite powders of aluminum with chosen fluorides at 50-50 wt. % are prepared by arrested reactive milling. Despite an increase in reactivity and lowtemperature ignition, the prepared composite powders are insensitive to initiation by electro-static discharge (ESD), making them attractive alternative to analogous thermites having very high ESD sensitivity. In air, the composite powder particles burn faster than reference aluminum particles of the same size. Very high combustion temperatures are observed suggesting gasification of a significant fraction of the fluorinated combustion products. However, in hydrogenated environments, fluorination of the fuel is hindered due to cannibalistic side-reaction between fluoride and water vapor; as a result the burn rates for composite particles are the same or even lower than for pure Al.

Further, nickel (II) fluoride is considered as an oxidizer in more aluminum-rich compositions. Milling protocol is refined to achieve low ignition temperatures for the selected composition. Similar fast burn times and low ignition temperatures in air is achieved with only 30 wt. % of NiF2 suggesting it is possible to prepare even more fuelrich composites with attractive reactivities by further refining the mixing scale between fuel and oxidizer. The aerosol of aluminum-nickel fluoride composite burns with higher efficiency than spherical aluminum powder with comparable size distribution.

Boron-based compositions with 50 wt. % of both fluoride oxidizers are similarly prepared and characterized. The nascent hydrated boron oxide layer is found to react with the fluoride and initiates low-temperature ignition. The composites burned in air faster than boron yielding gaseous reactive fluorinated products of interest to chemical and biological agent-defeat applications. The fluoride content is reduced to characterize the effect of composition to develop boron-replacement fuel. Additionally, solvent-based nanometric BiF3 coating is deposited on boron particle to homogenously disperse smaller quantity of fluoride. It is observed that only 10 wt. % of fluoride is sufficient for both milled, and coated boron powders to ignite readily and burn much faster than boron in air. During combustion, the reduced metal, Bi, functions as a catalytic oxygen-shuttle accelerating the particle burn rate. Finally, silicon-based compositions with the same fluoride oxidizers are prepared and characterized. For all the three fuels, ignition is found to be driven by lowtemperature oxidation initiated by fluorination. The fluorination mechanism is based on multiple factors such as fluoride stability in air, fuel reactivity and alloying tendency between metal fuel and metal reduced from the fluoride. Fluoride decomposition-driven ignition is observed in boron and silicon-based composites for different fluorides. For composites of Al with CoF2 and NiF2, fluorination occurs through redox-reaction; for Al·BiF3, the reaction was driven by decomposition of BiF3.

Directions of possible future work are outlined based on the results and properties of different inorganic fluorides.

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