1. Introduction
Energetic materials (explosives, propellants, and pyrotechnics) are necessary material bases of high‐performance weapons and ammunition, which are used extensively for both civil, military, and space applications. In order to enable the energetic materials to possess a more powerful performance, such as the high quantity of heat release, the high combustion temperature, the fast burning rate, and so on, adding combustion catalysts is a quite effective method.
In recent years, researchers pay much attention to the preparation and application of the combustion catalysis of nanoscale. Many studies reported that catalysts in nanoscale exhibit the absolute advantages both in accelerating the thermal decomposition process of the main energetic materials such as cyclotrimethylene trinitramine (RDX), nitrocellulose (NC), cyclotetramethylene tetranitramine (HMX), 2,4,6,8,10,12‐hexanitro‐2,4,6,8,10,12‐hexanitro hexaazaisowurtzitane (CL‐20), and 3‐nitro‐1,2,4‐triazol‐5‐one (NTO), and in enhancing the ignition and combustion performances of the solid. For instance, the nano‐sized Cr2O3 particles decrease the ignition delay time by a factor 3.5 (16 ± 2 vs 54 ± 4 ms) and accelerate the combustion rate (340 ± 10 mm s-1) of the Al/Cr2O3 thermite, which is fabricated by Cr2O3 micro‐ or NPs (Φ ≈20 nm) and Al NPs (Φ ≈50 nm) [1]. Pantoya [2] reported that nanocomposite thermites (Al/MoO3) can significantly reduce the ignition delay time compared with micron‐composite thermites. Nitrocellulose nanofiber‐based thermite textiles were studied and compared with the pure nitrocellulose and nano‐aluminum incorporated nanofiber; the result indicates that the burning rates were enhanced by adding the Al/CuO thermite [3].
The abovementioned nanothermite contains two parts: metal fuel (Al, used due to its low cost, high density, and the efficient catalytic property [4]) and metal oxides (Fe2O3, CuO, MnO2, MoO3, PbO [5], Bi2O3, etc.). The nanothermite system, as the metastable intermolecular composites (MICs) [6], can enhance the reactivity [7-9] through the oxidation‐reduction reactions, which lead to high burning rate [10], high heat production [11], and negligible gas generation. The traditional thermite, Al/Fe2O3, is prepared in various nanoparticle size, shape, and composition [12] in order to be better applied in free‐standing heat sources, airbag ignition materials, hardware destruction devices, welding torches [13], and energetic material field. Both Al and Fe2O3 particles have been used as catalysts not only in the thermal decomposition process of the main energetic components but also in composite solid propellants [14-17]. However, the effects of Al/Fe2O3 nanoparticles on the thermal behavior and non‐isothermal decomposition kinetics of NC are barely investigated. And, to the best of our knowledge, there has been no report about the dependence of catalytic properties of Al/Fe2O3 thermites on the morphology of Fe2O3 particles in combustion reactions to date.
Nitrocellulose (NC) is extensively applied as a main component in gun, blasting gelatin, dynamites, and rocket propellants [18-21] owing to its high flammability and explosiveness. In order to obtain more information about NC, the thermal decomposition mechanism of NC has been investigated. It is shown that the fission of oxygen‐nitrogen bond is the first and rate‐determining step during the decomposition process [22-25]. Quantities of NO2 gases, derived from the O‐NO2 bond cleavage, could stagnate in the polymer skeleton and lead to promote the secondary autocatalytic reactions (i.e., the heterogeneous reactions in condensed phase) [26]. Furthermore, Mahajan et al. [27] reported that copper oxide influences the combustion/thermal decomposition of NC in a way so as to retard the breaking of O‐NO2 bonds in solid phase. With the excellent characteristics of nanomaterials, we study the influence of Fe2O3 particles and Al/Fe2O3 thermites on thermal behavior and non‐isothermal decomposition kinetics of NC in order to provide basic data for establishing the combustion model and studying the combustion process.
In this contribution, granular, oval, and polyhedral Fe2O3 particles have been prepared by the hydrothermal method and used to fabricate Al/Fe2O3 thermites by integrating Al nanopowders with Fe2O3 at a stoichiometric ratio of Fe2O3:Al (71.1wt%:28.9wt%). All the Fe2O3 and Al/Fe2O3 thermite samples were characterized using a combination of experimental techniques including scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), X‐ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM) and high‐resolution TEM. The effects of Fe2O3 nanoparticles and Al/Fe2O3 on the thermal decomposition of NC have been investigated by the differential scanning calorimetry (DSC) method and the thermogravimetry with Fourier transform infrared analysis (TG‐IR). The influences of Fe2O3 and the corresponding thermite on the combustion properties of the ammonium perchlorate/hydroxyl‐terminated polybutadiene (AP/HTPB) composite propellant were investigated and compared. Moreover, the combustion wave structures and the flame temperatures of AP/HTPB composite propellants containing thermites Al/Fe2O3 are obtained at 4 MPa.
