Structure-Function Relationships of Oxidizers for Fuel Oxidation
Zachariah, Michael R
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Nanothermite is a class of composite that combines nanoscale fuel and metal oxide to allow for the rapid release of large amounts of energy through redox reaction. The use of nanomaterials has been illustrated to increase reactivity by multiple orders of magnitude as a result of the larger specific surface area and reduced diffusion length scales. However for this seemingly simple oxygen exchange process, there can be large differences in initiation temperatures among different fuel-metal oxide formulations. The large numbers of variables among metal oxides have limited our understanding of what properties of metal oxide control the initiation and reaction kinetics. For my dissertation, I have employed mainly two systematic doped metal oxides as oxidizers that minimize numbers of variables, allowing to probe the transport of solid-state oxygen for fuel oxidation with the goal of understanding the fundamental reaction mechanisms. Doped perovskite oxides synthesized by aerosol spray pyrolysis with the same crystal structure, morphology and size distribution were used as oxidizers mixed with Al as fuel. Ignition temperatures were measured by the temperature-jump/time-of-flight mass spectrometer (T-Jump/TOFMS). Remarkably we found a linear relationship between average bond energy and electronegativity with ignition temperature. Doped δ-Bi2O3 with high oxygen ion conductivity is another systematic oxidizer with even higher oxygen reactivity. The oxygen ion transports of doped δ-Bi2O3 were measured by impedance spectroscopy and it was found that oxygen ion transport from oxidizer controls the reaction initiation. Similar trend between average bond energy, oxygen vacancy concentration and ignition temperature was found for Al/doped δ-Bi2O3. With boron as fuel, the ignition temperature and combustion reactivity (pressurization rate and burn time) can also be correlated with the average bond energy and oxygen vacancy concentration of doped δ-Bi2O3. By employing fuels with different melting points for core/passivation layers (Al, B, Ta) and carbon that with no shell and two systematic doped metal oxides (doped perovskite oxides and doped δ-Bi2O3), in general, within each systematic metal oxide, I found linear relationships between average bond energy, oxygen vacancy concentration, electronegativity of the metal oxides with initiation temperature for all four fuels, despite their very different physical/chemical properties. These results indicate that it is generic that intrinsic microscopic properties of metal oxide control the fuel-metal oxide reaction initiation. In addition, the reaction kinetics between carbon and doped δ-Bi2O3 in the application for chemical looping combustion (CLC) was measured by in-operando synchrotron X-ray diffraction. Again, I found that lower metal-oxygen bond energy and higher oxygen vacancy concentration of doped δ-Bi2O3 led to lower onset temperature, faster reaction rate and smaller activation energy for carbon oxidation. Finally, size-tunable uniform metal iodates were synthesized by electrospray co-precipitation method. Common nano metal iodates including Mn(IO3)2, Zn(IO3)2, Cr(IO3)3, Bi(IO3)2, Fe(IO3)3, and Ni(IO3)2 were synthesized successfully. In addition, by tuning the experimental parameters in electrospray co-precipitation setup, Bi(IO3)3 nanoparticles of size range 10 nm to 200 nm could be successfully prepared.