Study On The Methods To Evaluate The Reactivity And Excess Stored Energy Of Aluminum Nanopowders

Because of their excellent abilities of heat release and oxidation under lowtemperature, Al nanopowders have an important application in energetic materials such aspropellants, explosives and thermits, etc. Due to the size and surface effect, however, thereactivity of Al nanopowders is so high that atoms on the surface of the particles areoxidized once they are produced, which results in the decrease of metallic Al content. Thehigh reactivity brings difficulties to the determination of metallic Al content at the sametime. The metallic Al content of nanopowders decreases but the reactivity of them is high,meanwhile, the reactivity of Al nanopowders are closely related to the preparationmethods, so it is urgent to set up the methods to evaluate the reactivity of Al nanopowders.As a kind of energetic materials, the ability of heat release of Al nanopowders receivesmuch concern. The excess stored energy brought by the none-equilibrium when preparedis one aspect of the reactivity. In this thesis, methods to determine the metallic Al contentsof the Al nanopowders prepared by different methods were studied systematically, then themethods to evaluate the reactivity of Al nanopowders below 100nm of the same meandiameters were set up, the excess stored energy of Al nanopowders brought by differentpreparation methods were also investigated.Alumina passivated Al nanopowders were prepared by laser and induction heatingevaporation methods in this thesis. Many means were employed to characterize the twonanopoweders as well as the commercial nanopowders produced by electro explosionwires (EEW) and plasma arc (PA), then a method of storing the powders to prevent themfrom the decrease of reactivity. As were characterized, all the four kinds of Alnanopowders had a mean diameter of about 50nm and the particle diameters followednormal distribution. These powders had a core-shell structure with a thickness of 2～5nmalumina shell which varied with different production methods.Volumetric, thermogravimetry, reductant-oxidant titration and Rietveld refinementwere applied to determine Al content, and then the errors and their reasons were studiedthoroughly. Studies revealed that there were unreacted small sized nanoparticles due to theexistence of oxide shells in volumetric method, which resulted in the lower result. The uncompleted oxidation of Al nanopowders before 1000℃resulted in the lower result inthermogravimetry, too. In conventional reductant-oxidant titration, the results were lowbecause hydrogen evolution reaction was easy to react beteewn Al nanopowders andaqueous solution. The accuracy of Al content determined by the following tworeductant-oxidant titration was improved: a permanganatometric method of ethanol as itssolvent and Fe(NO₃)₃ as its oxidizer and a titration method of Ce(SO₄)₂ as its oxidizer.The phase contents determined by Rietveld quantitative phase refinement were accurateon the premise that the phase composition and the crystal structure were accuratelydetermined; however, the existence of amorphous would make the quantitative phaserefinement difficult. Hydrogen Evolution ReactionThe thermal behavior of Al nanopowders was heavy influenced by the heating rates,and the heating rates were classified into two kinds: high heating rates and low heatingrates by the TG-DTA curves under different heating rates. In the low heating rates, theoxidation behavior could be divided into three stages: the first oxidation stage whichbegan at about 520℃was dominated by chemical reaction and reacted acutely; the secondoxidation stage which began at about 600℃was dominated by diffusion of oxygen andchemical reaction and reacted slow; the intensive oxidation of the last stage began at700℃because of the fracture of the oxidize shell under the pull force dress. In the highheating rates, Al nanopowders were heated above 560℃rapidly, and then the oxidize shellcracked and combusted due to the fusion of the core part Al. This oxidation process wasacute, and had only one phenomenon of exothermal and mass gain.The temperature of intensive oxidation onset T_on which could be determined by themethod of extrapolate onset temperature decreased as the decrease of the mean diameterof Al nanopowders. T_on changed little for Al nanopowders in same mean diameters. Thedegree of conversion of metallic aluminumαbelow 1000℃was a factor relate to Alcontent. Maximum rate of oxidation v_ox which reflected the intensity of oxidation reactionwas determined by the maximum value of DTG curve at about 580℃. The specific heatrelease H/Δm^* was an intrinsic characteristic of Al nanopowders, which could representthe ability of energy release. The reactivity for Al nanopowders in same mean diametersshould be comprehensively evaluated by the thermal parametersα,v_ox and H/Δm^* obtained from the TG-DTA curves under a same low heating rate. Al nanopowdersproduced by LCHE had the highest reactivity, and then the reactivities of powders byEEW and PA were second and third respectively. The reactivity of powders by IHE hadthe lowest value.The DSC results of Al nanopowders under inert gas atmosphere revealed that thelevel of excess stored energy was different for Al nanopowders produced by differentmethods, which was brought by the defects when Al nanopowders prepared undernonequilibrium conditions. The types of defects characterized in Al nanopowders werelattice distortion, dislocation, twin boundaries and absence of body in the particles. Thenpositron annihilation lifetime experiment was performed to investigate vacancy-typedefects in bulk, micro-powders and aluminum nanopowders and the evolvement rules ofvacancy as annealing temperature changed. Finally the mechanism of excess stored energybrought by different preparation conditions was analyzed, which revealed that the level ofexcess energy could be improved by tuning the parameters of preparing parameters, andthen the goal of improving the reactivity of Al nanopowders was reached.