Combustion characteristics of A1 nanoparticles and nanocomposite A1+MoO3 thermites



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Scientific advances in material synthesis such as exploding wire technology, plasma nucleation and wet precipitation have enabled industrial manufacturers to produce metal and metal oxide powders with nanometer-sized particles. These processes have enabled better overall quality control (i.e. more definitive particle size, smaller particle size distributions, oxide coating control and decreased contaminate concentration) and faster production rates.

Much interest has been formed in the science and application of nano-sized aluminum (nm-Al) combustion. A thermite (or aluminothermic) reaction is an oxidation reaction between aluminum and a metal oxide with highly exothermic energy release. Thermite reactions of traditional Al powder (typically micron-sized particles) and Iron-oxide have been used for decades in welding and other intense heat applications. Nano-thermite reactions, have shown unique properties in ignition sensitivity and deflagration (flame propagation) speeds which have propelled thermites to new realms of applications. The decrease in required ignition stimuli of nano-thermites is an improvement for many payload critical applications, but the ignition sensitivity also creates various hazards during material handling and seems to be a factor in decreased reactivity of aged nano-thermites. Nano-thermites have displayed reaction rates near detonation speeds presenting applications as more efficient incendiary devices. The precise particle size control of nano-thermites is leading researchers to develop highly-tunable energy release mechanisms that can be applied as heat signature flare decoys.

Studies have shown that the thermite reaction of nm-Al+MoO3 has a large theoretical energy density [19], increased ignition sensitivity [23][8], and near detonation flame propagation speeds [5][6] in comparison to traditional micron-particle thermites. This work will present macroscopic combustion behaviors (such as flame speed) along with experimental results focusing on the molecular reactions and thermal properties of nanocomposite Al+MoO3 thermite materials

This work will outline the successes and precautions of several nm-Al+MoO3 powder mixing methods and several cold-pressing techniques used to form compressed solid samples. A general relationship of sample density as a function of pressing force and with a systematic methodology is presented to allow other researchers to produce similar samples for future comparison.

Second, results from laser experiments performed to determine flame speeds of nano and micron-sized Al+MoO3 composites through a range of sample densities. Flame propagation speeds were measured using high-speed digital video. Samples were also tested to determine thermal conductivity, specific heat and thermal diffusivity as a function of compressed sample density. Theories are presented for the unique trends of the nano and micron-composite results.

Third, experimental work is presented analyzing the effects of pre-heated compressed nm-Al+MoO3 samples. Sample pre-heating is achieved by volumetric heating using an isothermal oven and by varying the applied laser power to allow conductive heating. Both methods of preheating show unique behaviors and elevated flame propagation speeds compared to previous results. Results and discussion of this work also discuss the difficulties and critical time response of using bare-wire thermocouples to accurately measure nano-thermite reaction temperatures.

Fourth, a series of DSC/TGA experiments were performed on the reaction of Al and gaseous oxygen to analyze the purest and ¡¥simplest¡¦ form of the Al oxidation (void of any reaction mechanisms dependent on the metal-oxide decomposition). Results are presented showing unique reaction onset temperatures, oxidation rates and activation energies for nano and micron-Al reacting in a gaseous oxygen environment.

Fifth, a series of DSC/TGA experiments were performed on the reaction of Al and nano-MoO3. Results are presented for reaction onset temperatures, peak temperatures, heat of reaction values, and activation energies for Al+MoO3 composites with Al particles ranging from 50 nm to 20 ƒÝm.

A final set of experiments was designed using the DSC/TGA to determine reaction duration and reaction self-propagation criteria for Al particle sizes ranging from 50 nm to 20 ƒÝm. Heating programs were manipulated for micron and nano-Al+MoO3 samples to determine the relationship between sample heating rate and reaction mechanisms. DSC tests were done using isothermal time intervals displaying that the nm-Al+MoO3 reactions are temperature dependent and not self-sustaining. Isothermal time intervals applied to ƒÝm-Al+MoO3 reactions displayed a delayed peak temperature.

Finally, all of the results and experiments are combined as evidence in support of a single theory of the oxidation reaction of spherical Al particles. The presented results portray unique evidence in support of the nano and micron-sized Al reaction characteristics.