Date of Award

Summer 2019

Document Type

Dissertation

Degree Name

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

Department

Chemical and Materials Engineering

First Advisor

Dreyzin, Edward L.

Second Advisor

Barat, Robert Benedict

Third Advisor

Schoenitz, Mirko

Fourth Advisor

Khalizov, Alexei

Fifth Advisor

Khalizov, Alexei

Abstract

Boron has received much attention as a potential additive to explosives and propellants due to its high theoretical gravimetric and volumetric heating values. The challenge, however, is that boron particles tend to agglomerate, have lengthy ignition delays and very low combustion rates. Prior research indicates that boron’s long ignition delays are due to its inhibiting naturally occurring oxide layer, impeding the diffusion of reactants for oxidation. For combustion, current studies report that boron particles have two consecutive stages, but the actual reaction mechanism is poorly understood. Despite many years of relevant research, quantitative combustion data on micron-sized boron particles are limited and most of the proposed modifications of boron powder for its improved ignition and combustion substantially diminish the energy density of the produced composites. Such modifications affect low-temperature oxidation kinetics, and thus, aim to reduce the ignition delay rather than accelerate high-temperature reactions affecting combustion rates and efficiencies.

The objectives of this research are to achieve higher burn rates for boron powders without jeopardizing their thermochemical performance, safety and stability, and to develop an experimentally validated model adequately describing boron oxidation kinetics that can be used in practical simulations for a broad range of temperatures. The study is also aimed to close the gap in data for combustion of fine boron particles in varying oxidizing environments.

In this work, burn times as a function of particle size, ignition delays and temperatures of commercial and modified boron powders are collected from optical emissions and images of single particles burning in air, steam, and gases formed by combusting hydrocarbons. In each case, the oxidizing gas environment is described accounting for thermodynamic equilibrium and using computational fluid dynamics. Unlike previous work, the complex morphology of boron aggregates is explicitly accounted for by correcting for their fractal dimension. The fractal dimension is determined by scanning electron microscopy (SEM) image analysis by box counting and diffusion limiting cluster morphology theories. Strategies to modify boron’s heterogeneous reactions by functionalizing its surface by organic solvents and using transition metals as “shuttle catalysts” are explored. It is found that washing boron with acetonitrile removes hydrated surface oxide and reduces ignition delays while preventing rapid aging and re-oxidation at ambient conditions. Doping boron with less than 5wt% transition metals (Fe or Hf) by high energy ball milling or wet synthesis, accelerates surface reaction rates leading to shorter particle burn times compared to the starting commercial powder.

A kinetic model is derived from low-temperature thermo-analytical measurements to describe the oxidation of complex aggregated boron particles accounting for their surface morphology. Comparison with particle combustion experiments shows that the same model can describe reactions at high temperatures typical of the full-fledged boron combustion, suggesting that the same heterogeneous reactions govern both ignition and combustion of boron. It is found that the morphology of as received boron powders comprising micron-sized agglomerates of finer primary particles does not always change to spherical droplets even at temperatures exceeding the boron melting point. This leads to variation in burn rates and temperatures for various particles.

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