Date of Award

Spring 2016

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

Robert Benedict Barat

Third Advisor

Xianqin Wang

Fourth Advisor

Mirko Schoenitz

Fifth Advisor

Stephen Da-Yen Tse

Abstract

Aluminum and magnesium are widely used in pyrotechnic formulations and other energetic materials; they are also common components of reactive alloys, e.g., Al-Mg and B-Mg, and others, which are potential fuels for explosives and propellants. Reaction mechanisms and oxidation kinetics of aluminum, magnesium and Al-Mg alloy powders in different oxidizing environments are investigated using thermo-analytical measurements. New methods of data processing are developed, relying on measured particle size distributions of the reactive spherical powders. It became possible to identify the reaction interface location for many heterogeneous metal oxidation processes; for several reactions, detailed kinetic descriptions are obtained.

For aluminum powders, location of the reaction interface is established for oxidation in steam and liquid water. Stage-wise oxidation behavior is observed and interpreted. The oxidation of aluminum covered by a thin natural oxide layer in oxygen occurring at relatively low temperatures is quantitatively characterized using different types of thermo-gravimetric (TG) measurements with increased amount of powder for greater sensitivity. Activation energy and the pre-exponent are determined as a function of reaction progress using isoconversion processing and assuming a diffusion-limited reaction mechanism. The reaction kinetics is also established for aluminum nanopowders. It is shown that the oxidation mechanism established for micron sized aluminum remains valid for particles as small as 10 nm. Aluminum oxidation model is combined with a heat transfer model to describe ignition of aluminum particles exposed to a heated oxidizing environment.

For magnesium powders, their oxidation by both oxygen and steam was studied by thermo-analytical measurements for micron-sized powders. The location of reaction interface is identified using experiments with spherical powders with different but overlapping particle size distributions. The reaction is found to occur at the interface of metal and the growing oxide layer for all oxidizing conditions. Thus, the reaction is rate limited by diffusion of oxidizer to the metal surface. Reaction rates for low and elevated temperatures are quantified using heat flow calorimetry and TG measurements, respectively. Simplified diffusion-limited reaction models are developed for oxidation of magnesium in both oxygen and steam. The models enable one to predict both pre-ignition reactions and the time of Mg powder aging when exposed to moisture or oxygen at different temperatures.

Finally, the mechanisms of low-temperature, heterogeneous oxidation of differently prepared Al-Mg alloy powders in oxygen are studied using thermo-gravimetric measurements. Fully and partially oxidized samples are recovered and characterized using scanning electron microscopy and x-ray diffraction. Voids grow within oxidized alloy powders for both atomized and mechanically alloyed powders. Two oxidation stages are identified for both alloy powders. Both magnesium and aluminum are oxidized at first oxidation stage, producing MgO and amorphous alumina. Spinel MgAl2O4 is produced during the second stage. The reaction is found to occur at the internal surface of the oxide shell as determined by matching the oxidation dynamics for particles with the same size but belonging to powders with different particles size distributions. Apparent activation energies for both oxidation stages are obtained as a function of the thickness of the growing oxide layer. The switchover between oxidation stages occurs when the spinel structure starts forming.

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