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39XV International Symposium on Self-Propagating High-Temperature Synthesis
THE INITIATION TEMPERATURES IN NANOTHERMITE REACTIONS V. G. Myagkov
Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk, 660036 Russia e-mail: miagkov@iph.krasn.ru
DOI: 10.24411/9999-0014A-2019-10106
Over the past decade, investigations focused on the nanoenergetic materials, such as reactive multilayer thin films and nanostructured reactive mixtures. Thermite mixtures belong to a wide class of energetic materials that comprise a metal fuel (e.g. Al, Mg, or B) and an oxidizer (e.g. Fe2O3, MoO3, CuO, Bi2O3, or WO3). These mixtures react with a lot of heat release; therefore, the thermite reactions often occur in a self-sustaining mode. In recent years, there has been an increasing interest in nanothermites (superthermites) where the particle size is reduced to a few nanometers. Studies of nanothermite combustion are mainly aimed at measuring the combustion wave velocity, burning temperature, delay time, and their dependences on density, morphology, and composition of the reaction mixture. Despite the intense investigations of thermite reactions, their general regularities and mechanisms remain unclear. Currently, the classical nanothermite Goldschmidt reaction Fe2O3 + 2Al = AhO3 + 2Fe and other Al-based reactions are well studied.
In this work, we extend the existing concepts of the first phase and its initiation temperature Tin which describe the initial stage of solid-state reactions in nanofilms onto thermite reactions and demonstrate that in all Al-based nanothermite mixtures the synthesis of the AhO3 phase starts at the same initiation temperature Tin ~ 510°C. These results open up a way for understanding the exclusive role of the initiation temperature Tin in the solid-state reactions at the nanoscale.
The enthalpy of formation of the first phase is a good measure of the free energy variation during the solid-state interaction; therefore, the heats of formation were used in several initial models to predict the first phase and phase sequence formation. Pretorius et. al. [1] proposed an effective model for the enthalpy of formation, which was successfully used for predicting the first phase formation in many binary systems.
As mentioned above, an increase in the temperature of the bilayer above Tin leads to the beginning of intermixing of the reagents and first phase synthesis on the interface and consequently physical characteristics of the film samples, such as electrical resistance, magnetization, transparence, and heat release, begin to radically change. Obviously, the start temperature of these changes is the reaction initiation temperature Tin. In most cases the energetic properties of thermite nanocomposites were investigated by differential thermal analysis (DTA), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). In this case the initiation temperature Tin is the temperature at which heat release starts. An important characteristic of the DSC curves is also the exothermic peak temperature, which, unlike the initiation temperature Tin, depends on the heat removal conditions from the reaction zone. It is important to note that the contaminants that form on the reagent interface during various methods of sample preparation (especially for chemically produced samples) can form thin barrier layers that slightly change the initiation temperature Tin but do not suppress the reaction. An error in finding the exact value of the initiation temperature Tin can also follow from the certain inaccuracy in determining Tin from DTA, TGA, and DSC plots. To find the exact Tin value, low heating rates are required. Therefore, we referred only to the studies in which the heat release curves were obtained at minimum heating rates (5, 10 or 20°C/min).
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V. G. Myagkov
_SI IS 2019_Moscow, Russia
The main results of the work based on an analysis of more than 60 papers presented in the literature and our papers are summarized in the schematic diagram in Fig. 1. showing the initiation temperature Tin ~ 510°C of the AI2O3 phase in Al/Fe2O3, Al/Co3O4, Al/NiO, Al/MnO2, Al/Bi2O3, Al/CuO, Al/MoO3 nanothermite reactions and oxidation of Al nanomaterials.
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O
Fig. 1. The schematic illustration of the initiation temperature Tin ~ 510°C for the Al-based nanothermite reactions with Fe2O3, Co3O4, NiO, MnO2, Bi2O3, CuO, MoO3 oxidizers and the oxidation of Al nanomaterials. The initiation temperature Tin ~ 510°C is characteristic of the leading AhO3 phase (first phase), which has a high negative enthalpy of formation (AHf = -1676 kJ/mol) and is the driving force of all the Al-based nanothermite reactions.
These results suggest the following reaction mechanism: below the initiation temperature Tin < ~ 510°C, Al and O atoms remain chemically neutral. At Tin > ~ 510°C, strong chemical interactions occur between Al and O atoms that break old chemical bonds causing the directed atomic migration to the reaction zone and the synthesis of Al2O3 regardless of the system they exist in. Therefore, the initiation temperature Tin ~ 510°C is a universal parameter of all Al-based nanothermite reactions.
The main concepts of this study are the first phase and its initiation temperature Tin, which describe thin-film solid-state reactions and were extended onto nanothermite reactions. The paper results prove that all Al-based nanothermite reactions have the same initiation temperature and they only start higher Tin > ~ 510°C. Analysis presented in the literature and our papers of Zr-, Mg-, and In-based nanothermite reactions show the same initiation temperatures ~ 250, ~ 450, and ~ 180°C, respectively. Finally, these findings predict that nanothermite reactions based on other fuels (e.g. Ti and B) must have their own initiation temperatures. This approach can be widely applicable in the study of the multicomponent thin-film solid-state reactions.
1. R. Pretorius, C.C. Theron, A. Vantomme, J.W. Mayer, Compound phase formation in thin film structures, Crit. Rev. Solid State Mater. Sci., 1999, vol. 24, pp. 1-62.
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