Electrospray Formation Of Nano Energetic Materials And Characterization Of The Combustion Properties

In this thesis, the assembly technique of aluminum nanoparticles based micro/nano energetic materials into microparticles by electrospraying was firstly established. The corresponding technical parameters were systemically optimized and certificated. The successfully produced microparticles include aluminum nanoparticles/nitrocellulose microspheres, aluminum nanoparticles/copper oxide nanoparticles/nitrocellulose microparticles, aluminum nano-/micro-particles/ammonium perchlorate microparticles, aluminum nanoparticles/ammonium perchlorate/copper oxide nanoparticles microparticles, aluminum nanoparticles/copper oxide nanoparticles/iodine microparticles, aluminum nanoparticles/metal iodate nanoparticles/nitrocellulose microspheres.And the size distribution and morphology, the thermal decomposition properties, the burning pressure and velocity, the ignition temperature and the reaction kinetics of these microparticles/microspheres were systemically investigated. The results were systemically discussed and analyzed, and the possible methods for improving the combustion performance of nanoenergetic materials were proposed. This thesis also will contribute to expanding the applicability and improving the efficacy of nanoenergetic materials in propellants, explosives and pyrotechnics, and further providing a feasible technical proposal to develop more novel energetic materials.Firstly, under the help of a little bit of energetic binder-nitrocellulose, assemble the aluminum nanoparticles into microspheres with condensed surface but porous inner structure by electrospraying. The microspheres have a quite narrow size distribution and the spherical structure was strong enough and not easy to be broken. Moreover, the microspheres have similar high specific surface area as aluminum nanoparticles do. The size of microspheres can be easily tuned by adjusting the loading of aluminum nanoparticles, the concentration of nitrocellulose, or the feeding rate in electrospraying. The microspheres with different nitrocellulose content have different performance on combustion velocity, ignition delay time and the burning time. In consideration of the increase, a combustion mechanism of these microspheres was come up with and the technology trajectory of producing nanoenergetic materials by electrospraying was established.Secondly, also employ the nitrocellulose as the binder, aluminum nanoparticles and common metal oxide nanoparticles (such as copper oxide, bismuth oxide, iron oxide) were assembled into microparticles with high reactivity by the above technology trajectory. The results show the fuel (aluminum nanoparticles) and the oxidizer (metal oxide) were mixed much better than the physical mixing case, the contact between the two was largely increased and the mass transportation distance was largely reduced, thus obtaining high reactivity. Moreover, the size of the microparticles can be increased by increasing the concentration of nitrocellulose while the size remains unchanged when the loading of nanoparticles in the precursor increases. A certain amount of microparticles were ignited in a confined cell, the burning pressure, pressurization rate and the burning time can be simultaneously obtained. The results show that the microparticles with 5% nitrocellulose has the largest pressure, which is 2 times larger than the physical mixing case. Moreover, if we reduce the loading of nanoparticles in the precursor to 1/3 of the primary value, the peak pressure can be further doubled. The burning products of the thermite reaction were collected and characterized by scanning electron microscope and X-ray energy spectrometer, the results indicate that the electrosprayed microparticles have much smaller burning residues coming from post-burning agglomeration than the physically mixed case, which is owing to the nitrocellulose in the microparticles can release gas once ignited to prevent the possible sintering of nanoparticles, thus permitting the aluminum nanoparticles remain their nano features during combustion. Moreover, the excellent mixing condition between the fuel and oxidizer and the unique microstructure also contribute to the high reactivity.Thirdly, strong and soluble oxidizer-ammonium perchlorate was dissolved in precursor beforehand, aluminum nanoparticles/ammonium perchlorate/nitrocellulose and aluminum nanoparticles/copper oxide nanoparticles/ammonium perchlorate microparticles were successfully produced by the above technology trajectory. The results show the ammonium perchlorate was successfully incorporated with nanoparticles and if the content is proper, the ammonium perchlorate can coat on the surfaces of nanoparticles. Combustion experiments show that the electrosprayed ammonium perchlorate coated micro- and nano- aluminum particles burn violent and homogenous, the size of burning residues is quite small. And the aluminum nanoparticles/copper oxide nanoparticles/ammonium perchlorate microparticles with 13% ammonium perchlorate has the highest burning pressure. Ammonium perchlorate was also supposed act as gas generator besides oxidizer, which produce gas to prevent possible sintering among nanoparticles, thus achieving high reactivity. Three possible means of increasing the reactivity of nanoenergetic materials were provided and summarized.Moreover, based on the previous work, functional nanoenergetic materials such as iodine-containing thermite microparticles were successfully produced by electrospraying with dissolving iodine in the precursor beforehand. The iodine content can be tuned from 5% to 50%. With the increase of iodine mass fraction, the size of iodine crystals in the microparticles is increasing, and the reactivity of the microparticles was highly reduced, with increasing burning time. The reaction mechanism of iodine-containing thermite was raised and details in the reaction process and reaction produces were revealed.At last, strong oxidizers with high iodine content as copper iodate nanoparticles were synthetized by milling technique, bismuth iodate and iron iodate nanoparticles were synthetized by chemical precipitation method. The thermal properties of the three metal iodates were investigated by Thermo Gravimetric Analyzer/Differential Scanning Calorimetry. The Three metal iodates show similar thermal decomposition curves and so as their crystal transformation mode. The two crystal forms of bismuth iodates were found for the first time. Under the help of nitrocellulose, the above three metal iodates nanoparticles were assembled with aluminum nanoparticles into uniform and well mixed microspheres. All the three novel nanothermites show high reactivity with peak burning pressure can be as high as 5MPa, which is several times even tens of times of the corresponding metal oxides based nanothermites. It is confirmed that the metal iodates-as a novel strong oxidizer-can decompose very quickly and release plenty oxygen and free iodine, contributing to the early ignition and high reactivity of nanothermites. The nanothermites can produce high pressure and temperature, promoting the vaporized iodine gas entering into the spores, thus obtaining a high inactivation efficiency of spores.