【作者】 陈亮；

【导师】 宋武林；

【作者基本信息】 华中科技大学 ， 材料学， 2009， 博士

【摘要】 纳米铝粉由于其优异的热释放和低温氧化能力，在推进剂、火炸药和铝热剂等含能材料领域中发挥着重要的作用。但是由于纳米铝粉所具有的小尺寸效应和表面效应使其反应活性很高，一经制备表面原子便被氧化致使单质铝含量降低，同时其高活性也给单质铝含量的测定带来极大困难。纳米铝粉的单质铝含量降低但其反应活性却很高，而且纳米铝粉活性与其制备方法密切相关，因此迫切需要建立纳米铝粉活性的评判方法。作为含能材料其热释放能力备受关注，纳米铝粉由于非平衡的制备条件所引起的额外储能也是活性的重要反映。本文对不同方法制备的纳米铝粉的单质铝含量的测定方法进行了系统研究，建立了100nm以下平均粒径相同的纳米铝粉的活性评判方法，并对制备方法不同引起的纳米铝粉的额外储能进行了研究。本文分别用激光加热蒸发法和感应加热蒸发法制备了慢氧化钝化的纳米铝粉，用多种手段对制备的两种纳米铝粉以及电爆炸丝法和等离子体电弧法制备的商用纳米铝粉进行了表征，然后用真空包装的方法对纳米铝粉进行储存以防止其活性降低。经微观表征，这四种纳米铝粉平均粒径约50nm，粒径呈正态分布，它们均具有核壳结构，但是不同方法制备的纳米铝粉的壳层厚度存在差异，在2～5nm之间变化。分别用气体容量法、热分析法、氧化还原滴定法和Rietveld精修法对纳米铝粉的单质铝含量进行了测定，系统研究了这些方法对实验结果产生的偏差及其来源。研究表明，气体容量法中氧化物壳层的存在会使小尺寸纳米颗粒反应不完全，而使测定结果偏低，纳米铝粉在1000℃以前氧化不完全致使热分析法测定结果偏低。传统以三价铁盐为氧化剂的高锰酸钾滴定法中纳米铝粉易于与水溶液发生析氢反应，也造成测定结果偏低。采用以乙醇为溶剂、硝酸铁为氧化剂的锰滴法和硫酸高铈为氧化剂的氧化还原滴定法使测定结果更加准确。采用Rietveld精修无标定量物相分析法在所有物相及其晶体结构准确确定的前提下，可以得到准确的物相含量结果，但是非晶物质的存在将给定量分析带来较大困难。加热速率对纳米铝粉的热学行为产生了重要影响，根据不同加热速率下热重—差热曲线（TG-DTA）的不同可将加热速率分为低加热速率和高加热速率两类。低加热速率下纳米铝粉的氧化分为三个阶段：第一阶段在520℃左右开始，受化学反应控制，反应剧烈；到600℃左右进入氧化缓和期，此步骤受氧的扩散和化学反应控制，反应较慢：700℃以后纳米铝粉氧化物壳层受拉应力破裂发生了剧烈的氧化反应。在高加热速率下，纳米铝粉温度迅速被加热到560℃以上，芯部的铝熔化造成氧化铝壳层的破裂燃烧，氧化反应剧烈，仅有一次放热和质量增重现象。氧化起始温度T_on的确定可采用外推起始温度法，随着铝粉粒径的减小，T_on减小，但对于平均粒径相同的纳米铝粉T_on相差不大。1000℃以前铝粉转化率α是与单质铝含量相关的参数。最大氧化速率v_ox可由DTG曲线在580℃左右的极大值来确定，它反映了铝粉氧化反应的激烈程度。铝粉放热热效应H／Δm^*反映了纳米铝粉的热释放能力，是一个衡量纳米铝粉热效应的重要参数。平均粒径大致相同的纳米铝粉的活性评判方法是在相同的低加热速率下，在TG-DTA曲线中提取α、v_ox和H／Δm^*等热学参数进行综合评判。基于此评判方法，对四种纳米铝粉的活性评判结果表明，激光法制备的活性最高、电爆法制备的活性次之、等离子体法制备的活性第三、感应法制备的活性最低。纳米铝粉在惰性气氛下的DSC分析结果表明不同方法制备的纳米铝粉额外储能水平存在较大差异，这是由于制备过程中的非平衡制备条件引起纳米铝粉缺陷造成的，经微观表征纳米铝粉中存在的缺陷类型包括晶格畸变、位错、孪晶界和颗粒体缺失等。随后用正电子湮没实验研究了块体铝、微米铝和各种纳米铝粉的空位型缺陷，并研究了纳米铝粉空位缺陷随退火温度变化的规律。最后对制备条件不同引起的纳米铝粉额外储能机理进行了分析，研究表明可以通过调整纳米铝粉制备工艺参数提高纳米铝粉额外储能水平，并达到提高活性的目的。更多还原

【Abstract】 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.更多还原