【作者】 郭连贵；

【导师】 宋武林；

【作者基本信息】 华中科技大学 ， 材料学， 2008， 博士

【摘要】 纳米铝粉是含能材料的一种新型金属燃烧剂,由于其具有普通铝粉不具备的特性近年来倍受关注。纳米铝粉可显著提高推进剂的燃烧速率、比冲,降低其特征信号等。然而活性丧失的纳米铝粉在含能材料中反而起到相反的作用,因此系统地研究纳米铝粉的活性及其活性保持机制显得尤为重要和迫切。本论文基于激光-感应复合加热技术,制备了慢氧化钝化、碳包覆及推进剂有机组分包覆三类不同核/壳结构的纳米铝粉,采用XRD、SEM、TEM、HRTEM、XPS及FTIR等手段对制备的粉末进行表征,采用TG-DSC/DTA及气体容量法测量粉末的热学性能及活性铝含量,研究了纳米铝粉特征因素及环境因素对纳米铝粉活性的影响。氧化铝钝化纳米铝粉通过引入小剂量氧气获得,是一个“钝化-自饱和”的过程。碳包覆纳米铝粉是在甲烷气氛中制备的,由于碳在铝中的溶解度较低,甲烷分解的碳原子沉积于铝粒子表面。有机物包覆纳米铝粉的形成则通过有机分子的官能团结合（如粘结剂HTPB）或吸附沉积于铝粒子表面（如增塑剂DOS）。表征结果表明这些纳米铝粉呈球形的核/壳结构,粒径范围10-90nm,平均粒径50nm;粒子的内核均为结晶铝,而外壳分别为非晶氧化铝、类洋葱状石墨及非晶有机物,且外壳厚度约3.5nm左右。三种不同包覆核/壳结构纳米铝粉在铝熔点（⁶60℃）之前均出现早期氧化,但碳包覆及有机物包覆纳米铝粉的早期氧化展现了比氧化铝钝化纳米铝粉较低的氧化开始温度（Ton,约30℃）及氧化峰温（Tpeak,约20℃）,较高的氧化热焓（△H）及氧化增重（△m）,这是由于石墨碳层及有机物层在加热过程中首先燃烧和热解,从而较早的触发纳米铝粉点火并加速氧化,这表明包覆层类型对纳米铝粉的活性有影响。氧化铝钝化纳米铝粉的DTA结果表明,随着铝粒子粒径从50nm减少到20nm,早期氧化的放热焓变从3.72 kJ/g减少到0.93 kJ/g,小尺寸粒子活性反而降低与其相对低的活性铝含量有关。激光-感应复合加热方法制备的纳米铝粉的氧化放热焓变是高频感应方法制备的3倍,这与不同制备方法的蒸发情况及能量注入方式有关。因此,纳米铝粉特征因素对其活性的影响是多方面的。为了评价不同包覆层对纳米铝粉活性的保持能力,研究了环境因素（湿度、温度及时间）对不同包覆纳米铝粉活性的影响。结果表明,氧化铝钝化纳米铝粉对湿度极为敏感,其水解产物是Al（OH）₃;碳包覆纳米铝粉由于包覆不均匀在一定程度上仍受湿度的影响,而有机物包覆则由于有机物的疏水特性使其活性较少受湿度影响。氧化铝钝化纳米铝粉在300℃之前基本可以稳定存在,说明了致密的非晶氧化铝钝化层在低温阶段具有一定的抗氧化能力;在400℃之前,由于石墨碳壳存在燃烧而有机物存在热解,外壳对纳米铝粉活性的保持能力随着温度的升高而下降,表明碳包覆纳米铝粉与有机物包覆纳米铝粉受温度的影响很大。随储存时间的延长,氧化铝钝化纳米铝粉活性下降,开始阶段的氧化尤为明显;时间对碳包覆纳米铝粉活性也有一定影响;而在大气环境下储存2年的HTPB包覆纳米铝粉仍有较高的活性表明随着储存时间的延长有机物能有效的保持纳米铝粉的活性。最后讨论了纳米铝粉氧化的机理。纳米铝粉的氧化分为两个阶段:在铝的熔点之前是慢氧化阶段,氧气通过氧化铝壳层扩散;高于铝的熔点是快速氧化阶段,铝和氧同时通过氧化铝壳层扩散。更多还原

【Abstract】 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.更多还原