Study On The Preparation And Reactivity Of Core-shell Structure Aluminum Nanopowders

Aluminum (Al) nanopowders are a new metal fuel for energetic materials and have attracted considerable attention in the last few years because of their unique characteristics that cannot be obtained in the conventional Al powders. Compared with conventional Al powders, Al nanopowders in composite solid propellants have been shown to increase the burning rate, the specific impulse, and decrease the signal characteristic etc. Al nanopowders which lost the reactivity have a negative effect on the performance of energetic materials, so it is necessary and urgent to study the reactivity and the maintaining mechanism of Al nanopowders thoroughly.In this work, three kinds of core-shell Al nanopowders with different surface coatings (Al2O3 passivation coating, carbon coating, and propellant organic component coating) were prepared by laser-induction complex heating technology. The characterization of these Al nanopowders was revealed by XRD, SEM, TEM, HRTEM, XPS and FTIR, and the thermal properties and the active Al contents were measured by TG-DSC/DTA and the volumetric method. The influence of nano-Al characteristic factor and environmental factor on the reactivity of Al nanopowders was studied.Al2O3-passivated Al nanopowders were obtained by introducing small doses of oxygen before exposure to air, and this process can be explained through the mechanism of"passivation-up-to-selfsaturation". Carbon-coated Al nanopowders were synthesized in methane atmosphere, and carbon atoms decomposed by methane deposited on the surface of Al particles because the solubility of carbon in Al metal is relatively low. Organic-coated Al nanopowders were obtained by two ways, organic molecules binded to the surface of the Al particle through the functionality (such as hydroxyl terminated polybutadiene, HTPB) and organic molecules directly deposited on the surface of the Al particle through the adsorption effect (such as di-n-octyl sebacate, DOS). These Al nanopowders all show a core-shell structure and spherical morphology with the size ranging from 10 to 90 nm. Microstructure characteristics revealed that the three kinds of core-shell Al nanopowders consist of inner crystalline Al core and outer different material shell (3.5nm thick), which is amorphous Al₂O₃ shell, onion-like graphite shell, and amorphous organic.Three kinds of core-shell Al nanopowders with different surface coatings all show an early oxidation below the melting point of Al (⁶60℃). However, the early oxidation of carbon-coated Al nanopowders and organic-coated Al nanopowders has a lower onset temperature (about 30℃), a lower peak temperature (about 20℃), a higher enthalpy change and a higher mass gain than that of Al₂O₃-passivated Al nanopowders. One possible explanation for this phenomenon is that carbon coating and organic coating are firstly burnt and pyrolyzed in an oxidative environment during particle heat up, which could trigger ignition and accelerate oxidation much eatlier than inert Al₂O₃ coating. The DTA results of Al₂O₃-passivated Al nanopowders show that the enthalpy change for oxidizing reactivity of Al nnaopowders is reduced from 3.72kJ/g to 0.93kJ/g as Al particle size decreases from 50 nm to 20 nm. The small exotherm observed for Al nanopowders of 20 nm may result from its relative lower content of Al. The enthalpy change of Al nanopowders produced by laser-induction complex heating is 3 times higher than that of Al nanopowders produced by high-frequency induction heating, which may be relative to the evaporating condition and the energy input behavior of defferent preparation method. So the reactivity of Al nanopowders in air depends on many factors.To further assess the effectiveness of different surface coatings in protecting the reactivity of Al nanopowders, the effects of environmental factors (humidity, temperature and time) on the reactivity of Al nanopowders with different surface coatings were studied. The results show Al₂O₃-passivated Al nanopowders are more sensitive to humidity and Al(OH)₃ (bayerite) is the major product of hydrolysis. Humidity affects the reactivity of carbon-coated Al nanopowders to some extent, which may result from the presence of some incompletely coated powders. Interestingly, the organic coating significantly decreases the aging of Al nanopowders in humid atmospheres, which is due to the hydrophobic nature of organic HTPB and DOS. Al₂O₃-passivated Al nanopowders was thermally stable up to at least 300℃. However, because of the combustion of carbon coating and the pyrolysis of organic coating before 400℃, the reactivity of carbon-coated Al nanopowders and organic-coated Al nanopowders were deeply influenced by the environment temperature. The reactivity of Al₂O₃-passivated Al nanopowders decreases as the stored time is prolonged, and the reactivity of carbon-coated Al nanopowders depends on coating effectiveness. HTPB-coated Al nanopowders stored for two years in ambient environment has still higher reactivity, which indicates that the protective organic coating with hydrophobic groups is essential to protect Al nanopowders from oxidation.Finally, the mechanism of Al nanopowders oxidation was discussed. The oxidation of Al nanopowders can proceed in two regimes. At temperatures below the melting point, a slow oxidation regime occurs through the diffusion of oxygen through the oxide shell. Above the melting point of aluminium, a fast oxidation regime with diffusion of both aluminium and oxygen occurs.