Application Of Nanometer Materials And Nanocomposites In Thermites

The term of "reactive materials" denotes a class of energetic materials that generally combine two or more non-explosive solids.This kind of mixtures/composites always have high density,high reactivity and of low sensitivity and can be ignited to release a mass of chemical energy when they undergo a strong impact action.At present,in order to enhance the power of warhead,there is also interest in and initial work underway to examine the application of reactive materials in fabricating reactive fragments,charging in high-speed projectile and replacing standard metal liners in shaped charges or explosively formed penetrators.Some studies indicated that many kinds of metastable materials may be used as reactive materials,in which thermites are most promising because of their large exothermic heat,high density,reliable safety,and desired thermal properties that can be readily tailored by varying ratios of oxidizer to fuel.However,traditional thermites are usually composed of coarse Al particles and micron metal oxide.This kind of thermites usully exhibit the poor reactivity,such as lower burnning rate,deficient heat release,and hard ignition(T_ign.＞900℃), which can not be used as reactive materials and find no application further.This study intends to improve the reactive property of thermites via promoting the reactivity of Al powder or enhancing the combination between the fuel and oxidizer.Concretely,the paper aims at exploring the thermites made up of nanocomposites and investigating the thermites with high reactivity and potential application in reactive materials.Firstly,to promote the thermal reactivity of micron Al powder,a kind of core-shell metal/Al nanocomposite has been prepared by a facile displacement method in the near neutral aqueous solution via controlling the dosage of surfactant,temperature,initial concentration of metallic salts and NH₄F,in which the micron Al cores were tightly coated in nanometer Ni,Co,Fe and Cu particles.After preparation,these metal/Al nanocomposites were blended with metal oxides that included WO₃,SnO₂,PbO,CuO and Fe₂O₃ powders. Using the onset and peak temperature,heat release,maximal heat flow and active energy as the parameters to evaluate the performance of thermite reaction,the reactivity of the above mixtures has been investigate via thermal analysis.The results indicated that metal/Al particles have great higher thermal reactivity than raw Al powders,among which Ni/Al is the best one.In addition,the possible oxidation mechanisms of raw Al and metal/Al were discussed.It is found that the progress of oxidation process for raw Al was severely limited by the Al₂O₃ layers wrapped on the surface of active Al core,but the metallic nanoparticles within metal/Al nanocomposites improved the oxidation mechanism of Al by reducing the deleterious influences of the Al₂O₃ layer and bridging the gap between fuel and oxidizer.So, the oxidizer can react with active Al easliye,which is confirmed by great improvement in the thermal reactivity of micron Al powder.Next,in order to enhance the combination and contacting area between fuel and oxidizer, a kind of Fe₂O₃/Al nanocomposite has been prepared by sol-gel method under a mild and low-toxicity condition via introducing of 1,2-epoxypropane as the agent for speeding up the hydrolyzation of Fe(â…¢) ions,in which nanometer,submicron,micron and coarse four kinds of Al particles were combined tightly with nanometer ferric oxide,respectively.The optimal technical parameters were located at initial temperature of 19℃,Fe(NO₃)₃·9H₂O initial concentration of 0.37mol·L^-1 and 1,2-epoxypropane dosage of 2.005 mL·g_{硝酸铁}^-1.Results of thermal analysis revealed that the composing between Al and Fe₂O₃ and the particle size of Al obviously affected the reactivity of thermites.If nanometer Fe₂O₃ particles compactly coated on the surface of Al,the peak temperature of the thermite reaction is lower than that of mixed sample by 50℃～70℃.Moreover,as the particle size of Al decreases,the values of peak temperature decrease and the heat release and of max heat flow increases linearly.The study of reactive mechanisms reveals that if the fraction of active Al in thermite system is low,the residue will present many intermediate products such as FeAl₂O₄,Fe₃O₄ or FeO etc.,which is attributed to an incomplete reaction process and results in a low reactivity.Thirdly,we tried to promote the reactivity of micron Al particles via changing their crystal structure.Accordingly,three kinds of Mg-Al alloys have been prepared by bidirectional-rotation milling,in which the content of Mg is 10wt.%,20wt.%and 30wt.%, respectively.Sample characterization showed that the crystallite size of micron Al had been decreased from＞100nm to about 25nm after alloying with Mg.Thermal analysis illustrated that the reactivity of Al has been enhanced obviously due to the introducing of Mg and decreasing of Al crystallite.The thermite reaction of Al with Fe₂O₃ was initiated even before the melting point of Al(Mg),which is a solid-solid reaction.Moreover,as the fraction of Mg increased in Mg-Al alloy,the peak temperature in DSC traces decreased and the maximum heat flow increased.Finally,to confirm that whether the thermite with high reactivity also can be impact initiated,the traditional thermite and the thermites made up of nanocomposites were pressed into pillars with the size ofΦ10mm×10mm after adding some accessorial ingredients.The pillars were charged into steel projectiles,and then were shot to impact against the sealed steel-pot that filled up with diesel oil.This test aims at igniting the diesel oil by the energy released from the thermite reaction.The experimental results showed that although the pillars pressed by traditional thermite can be ignited by impact action in the course of penetration, but the reaction heat and flame are not mighty enough to light up the diesel oil with low volatility.As a result,the combustion flame was stamped out by the diesel oil.Successfully, the pillars fabricated by nanocomposites thermites can not only be ignited after impacting the target,but also exhibit a blazing deflagration with igniting the part of the diesel oil. Undoubtedly,the thermites made up of nanocomposites can become a kind of impact initiated energetic materials,and will find promising applation in as reactive materials in the warhead.