Combustion characteristics of aluminum nanoparticles and nanocomposite aluminum+moly-trioxide thermites

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Title: Combustion characteristics of aluminum nanoparticles and nanocomposite aluminum+moly-trioxide thermites
Author: Granier, John Joseph
Abstract: 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 .
URI: http : / /hdl .handle .net /2346 /20218
Date: 2005-05


Combustion characteristics of aluminum nanoparticles and nanocomposite aluminum+moly-trioxide thermites. Doctoral dissertation, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /20218 .

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