Published: August 29, 2011 r2011 American Chemical Society 10982 dx.doi.org/10.1021/ie2003544 | Ind. Eng. Chem. Res. 2011, 50, 10982–10988 ARTICLE pubs.acs.org/IECR Mechanism of Molten-Salt-Controlled Thermite Reactions Khachatur V. Manukyan,* ,†,‡ Khachatur G. Kirakosyan, † Yeva G. Grigoryan, † Ofik M. Niazyan, † Armenuhi V. Yeghishyan, † Artavazd G. Kirakosyan, † and Suren L. Kharatyan †,‡ † Laboratory of Kinetics of SHS Processes, A. B. Nalbandyan Institute of Chemical Physics, National Academy of Sciences of the Republic of Armenia (NAS RA), 5/2, P. Sevak Street, Yerevan 0014, Armenia ‡ Department of Inorganic Chemistry, Yerevan State University, 1, A. Manoogian Street, Yerevan 0025, Armenia ABSTRACT: The present work was undertaken to study the chemistry and phase formation mechanism in the salt-controlled MoO 3 + Mg + NaCl thermite reaction. It was found that the structure and phase formation mechanism in the studied system primarily depend on the salt content in the initial mixtures. In salt-poor mixtures, nucleation of product particles takes place in the molten magnesium, whereas under salt-rich conditions, products are mainly formed in molten sodium chloride. Analyses of combustion temperature profiles and product microstructures and thermal analysis of reacting mixtures suggested that the molybdenum oxide reacts with the salt at early stages of the process. The formed intermediate molybdenum oxychloride and sodium molybdate then react with magnesium, yielding Mo, MgO, and NaCl phases. The low value of the activation energy (50 kJ/mol) of the combustion process also suggests that gaseous (liquid) intermediates play an important role in the phase formation mechanism. 1. INTRODUCTION The term “thermite reaction” is used to describe a class of reactions that involves a metal reacting with a metal or nonmetal oxide. This form of oxidationÀreduction reaction can be written in general form as M þ AO ¼ MO þ A þ ΔH where M (typically Mg, Al, Ti, Zr, Zn, etc.) is a metal, A (MoO 3 , WO 3 , Fe 2 O 3 , Cr 2 O 3 , TiO 2 , SiO 2 , CuO, etc.) is either a metal or a nonmetal, MO and AO are their corresponding oxides, and ΔH is the heat generated by the reaction. 1À4 Because of the large ex- othermic effect, thermite reactions can generally be initiated locally and become self-sustaining, a feature that makes their use ex- tremely energy-efficient. Many thermite reactions yield a molten product consisting of a heavier metallic phase and a lighter oxide phase that can be separated by gravity and surface tension forces. 3 The latter makes these reactions potentially useful in a variety of metallurgical applications. 4À7 More recently, thermite reactions have become important in the synthesis of refractory ceramics, 8,9 composite materials, 9À13 and metal powders. 14,15 However, in- tense gas evolution due to the decomposition/vaporization of initial oxides and/or reducing elements coupled with high reaction temperatures make it difficult to control the microstructure of the obtained materials. Therefore, some approaches have been adopted to soften violent reaction conditions and tune the mor- phology of the products. One of the most recognized methods is the application of so-called inert diluents. Addition of diluents to thermite mixtures effectively reduces the combustion tempera- ture and reaction rate because of the production of less heat and the longer transport distances between reactants. A modified pro- cess of conventional thermite reactions with halide salt additives is known as molten salt-controlled combustion synthesis. 13À15 The basic precursors for the process are known higher oxides of transition metals such as WO 3 , Ta 2 O 5 , MoO 3 , and TiO 2 . Metallic magnesium and zinc are frequently used as reduction agents. 14,15 For certain oxides (WO 3 , MoO 3 ), sodium azide (NaN 3 ), and sodium boron hydride (NaBH 4 ) can also be used as reducing agents. 15 Recently, it was shown that one can use this method to synthesize not only nanopowders of pure metals but also di fferent carbides (e.g., TiC, 16 WC 17 ), silicides (e.g, MoSi 2 13 ), and complex compositions such as WCÀCo. 16 Two main factors are important in controlling the microstruc- ture of the products in salt-controlled thermite reactions. The first factor is mild reaction conditions, such as low temperatures, which prevent intense grain growth. Second is the presence of a molten “inert phase” in the reaction zone. Because of the heat generated by self-sustaining reaction, the salt melts at about 800 °C, and further nucleation of product particles occurs in the molten salt environment, which protects them from agglomera- tion and grain growth. In all published works, however, the effect of sodium chloride on the chemistry of combustion process was not studied, and salt was always considered as only an inert diluent. This does not rule out the possibility that, in the initial stages of the reaction, metal oxides might react with salt yielding various intermediates. For instance, it is well-documented 18,19 that MoO 3 reacts intensely with NaCl at 400À800 °C, forming MoO 2 Cl 2 and Na 2 MoO 4 . Early research 20,21 on the interaction of transition metal oxides, namely, Ta 2 O 5 , WO 3 , MoO 3 , and TiO 2 , with sodium chloride showed intense weight loss at 650À950 °C, which is conditioned by evaporation of sodium chloride, volatile initial metal oxide, metal chlorides, and oxyclorides. The aqueous solutions obtained after water treatment of the metal oxideÀ NaCl reaction products contains oxyanions and chloride species. The concentrations of soluble metal species varied from several Received: February 21, 2011 Accepted: August 23, 2011 Revised: August 2, 2011