【作者】 彭亚晶；

【导师】 杨延强；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2008， 博士

【摘要】 纳米金属复合含能材料作为一种新兴的能量体系具有高度可调的能量释放率等优点。目前的应用和研究表明:需要对纳米含能材料的反应动力学过程尽可能详细地了解以提高其综合性能。激光诱导纳米含能材料反应过程（如点火或烧蚀）的机理存在着两种基本的解释:一个是热点火;另一个是冲击压力点火,即激光感生冲击压力引发化学反应从而实现点火。这两种机制哪一个占主要地位,分别在什么情况下适用尚不清楚。针对这一背景问题,本文选取纳米金属铝复合硝化纤维含能材料（Al/NC）为研究对象,采用实验测量和数值模拟的方法研究Al-Al₂O₃纳米粒子的光吸收性质以及脉冲激光引发纳米含能材料的点火初期热反应动力学机制。对镶嵌在硝化纤维中的Al-Al₂O₃核壳纳米粒子的近红外和可见吸收光谱的实验测量和数值模拟表明:金属的带间跃迁起始频率是和尺寸相关的,这种尺寸相关性是由纳米粒子的小尺寸效应和表面效应导致电子结构的改变引起的,并且是吸收峰红移的主要原因。为了进一步探求带间跃迁起始频率与纳米粒子尺寸之间的关系,对其它几种尺寸的Al-Al₂O₃核壳纳米粒子的吸收光谱进行了测量和模拟。研究发现带间跃迁起始频率随纳米金属直径（25-100nm）的减小近似线性的减小。这种变化关系与最近文献报道的纳米粒子内压力随粒子直径的变化关系相接近。带间跃迁起始频率与尺寸关系的确立使金属介电函数的尺寸效应不再仅仅表现在自由电子阻尼系数上,电子的带间跃迁尺寸效应也可以被引入到金属的介电函数中,从而更有效的描述纳米粒子的光学性质。引入带间跃迁的尺寸关系研究了Al-Al₂O₃纳米粒子的核壳尺寸（包括氧化）、粒子的形状以及点火光波长对其光吸收性能的影响。一个反直觉的现象被发现:纳米金属粒子的氧化可以增强光吸收,但是这种增强性要依赖于点火光波长和初始时刻纳米粒子的核壳尺寸。在同体积条件下,椭球形纳米粒子因其表体比较大而具有比球形纳米粒子更强的光吸收性能,这表明用椭球形纳米粒子作为含能材料的添加剂可以减小点火时间;这些结果在纳米含能材料的激光点火和材料制备等应用方面具有重要的指导意义。给出镶嵌在介质中的纳米金属粒子吸收脉冲激光的瞬时功率密度的解析表达式,进而分别利用双温模型和Fourier定律计算短脉冲（1ps、10ps、100ps）和长脉冲（10ns、25ns）激光加热Al/NC薄膜的温度场分布。结果表明:当脉冲宽度大于或等于100ps时,Fourier热传输模型可以用来计算温度场分布;此时,需要考虑纳米金属Al粒子与周围NC介质之间的热量交换。当脉冲宽度小于100ps时,需要用双温模型求解温度场分布。以热分解机制为基础发展了热点模型数值模拟了100ps、10ns、25ns脉冲激光激发Al/NC纳米含能材料的热反应动力学过程。热点模型涉及纳米金属Al吸收脉冲激光能量,热量传播引发放热的化学反应以及其释放的能量反馈给热点。计算中,考虑了脉冲激光作用期间纳米Al粒子和周围NC介质之间的热交换和化学反应,金属Al核的消耗以及反应后原物质的改变。体系中发生化学反应的位置是时间、空间和温度的函数。Matlab有限差分数值方法用来求解体系的温度场分布。反应过程中体系内部的热量和化学反应的进展由不同时刻的三维空间温度形貌图展示出来。计算的化学反应直径与实验结果相比较,表明了热分解机制主导着纳秒脉冲激光引发的反应过程,验证了实验上发展的Al/NC烧蚀阈值标准的正确性。然而对于100ps脉冲激发的过程,热分解不再重要,定性分析表明了冲击压力引发了此过程的化学反应。更多还原

【Abstract】 Nanometallic particles based composite energetic materials as a novel energetic system have the merits of high and tunable energetic release rate Recently, practical applications and researches show: It is necessary to kown more detailed of the reaction mechanism of energetic materials in order to enhance its comprehensive properties. There exist two basical explanations for the reacted process （such as ignition and ablation） of laser-induced nanoenergetic materials: one is the heat ignition; the other is the shock pressure ignition, that is, shock pressure induced by laser causes chemical reaction to realize ignition. It is not evident for the two mechanisms that which one dominates the reaction process and their accordingly applicable conditions. In view of this problem, we choose nanometallic aluminum composite nitrocellulose energetic materials to be the study object. Experimentally study and numerically simulate are used to research the optical property of Al-Al₂O₃ nanoparticles and the thermal reaction dynamic mechanism of Al-Al₂O₃/NC composite thin film excited by pulse laser.The experimental measurement and numerical simulation of near-infrared and visible absorption spectra of Al-Al₂O₃ core-shell nanoparticles embedded in the nitrocellulose show that the interband transition onset frequency of nanometallic Al is size-dependent, and this size-dependent property is caused by variation of electronic structure due to the small size effect and surface effect of nanoparticles and is the main reason of redshift of absorption peak by simulations.To explore the dependence of interband transition onset frequency and particle size further, measuring and simulating the absorption spectra of composites containing another several sized nanoprticles. Results show that interband transition onset frequency linearly increases with the increase of metallic particle diameter approximately. This change relationship is generally agreed with that of inner pressure of nanoparticle and metallic particle diameter reported by recent document. This descover of relation of interband transition onset frequency and metallic particle size provides convenience to incorporate the interband transition size effect into the metallic dielectric function, which makes the size effect of dielectric function is not only embodied in the free electron damping coefficient any longer as before.Incorporating the linear relation between interband transition onset frequency and nanoparticle size, we calculate the effects of core and shell sizes of nanoparticle （including oxidization）, particle shape and the ignition optical wavelength on the absorption spectra. A counterinstinctive phenomenon is found: oxidization of nanometallic particle may enhance the optical absorption, but the enhancement relies on the excited optical wavelength and the initial core shell sizes of nanoparticles. For the same volume, ellipsoidal nanoparticles have the higher optical absorption ability due to its larger surface volume ratio than the spherical nanoparticles. This indicates that adding ellipsoidal nanoparticles into the energetic materials may reduce the igniton time. These results have important indicative significance in laser ignition and preparation of nanoenergetic materials.An analytic expression of instantaneous power density of pulse laser absorbed by nanoparticles embedded in matrix is derivated, Based on which, the temperature distribution of Al/NC system heated by short pulse （1ps,10ps,100ps） and long pulse （10ns,25ns） laser are calculated by using two-temperature model and Fourier law, respectively. Calculation results show: Fourier Law can be used to calculate the temperature distribution when the pulse duration is larger than the 100ps, while the two-temperature model is needed to calculate the temperature distribution when the pulse duration is smaller than 100ps. But during long pulse heating, the heat exchange between Al and surrounding NC has to be considered.A hot spot model based on thermal decomposition is developed to simulate numerically the chemical reaction process of Al/NC nanoenergetic materials excited by 100ps, 10ns, and 25ns pulse laser. The hot spot model involves in laser energy decoposition with the nanoparticle, exothermic chemical reaction triggered by heat propagation, and the energy feedback of hot spot. In this calculation, heat exchange and chemical reaction between Al nanoparticle and surrounding NC, the consumption of Al core, and conversion of matter due to chemical reaction are considered. The material position occurring chemical reaction are the functions of time, spatial and temperature. The evolutions of heat and chemical reaction in this process are exhibited by drawing three dimensional spatial images of temperature at different time. Calcuated chemical reaction diameters are compared with experimental results, which show thermal decomposition mechanism dominates the reaction process heated by nanosecond pulse laser and experimental ablation threshold ceritea of Al/NC is proven theoretically. However, for 100ps regime, thermal decomposition is not important any longer, but it is shown qualitatively that the shock pressure induces the chemical reaction.更多还原