Process Of Optical And Thermal Reaction Of Nanometallic Al Composite Nanoenergetic Materials

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.