Calculated equilibrium compositions of products for the MoO 3 + NaCl mixture. 

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... concentrations of the products for the MoO 3 + NaCl mixture as a function of temperature were calculated as well. Figure 1 shows that, at T < 770 °C, no reaction occurred between the salt and molybdenum trioxide. In the 770 < T < 1200 °C range, liquid Na 2 MoO 4 and gaseous MoO 2 Cl 2 were the main products of the reaction between sodium chloride and molybdenum oxide. ...

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... particle size <5 μ m), magnesium (MPF-3, AVISMA, Verkhnaya Salda, Russia, particle size 150 À 300 μ m), and sodium chloride (high grade, Michailovskii Factory of Chemical Reagents, Michai- losvk, Russia, purity 99.9%, particle size <20 μ m) powders. The precursors were thoroughly hand-mixed in the desired ratio in a ceramic mortar for 1 h to ensure homogeneity of the reaction medium, which was then uniaxially cold-pressed into cylindrical pellets (diameter 20 mm, height 40 mm) at a pressure of 20 MPa to relative densities in the range of 0.45 À 0.5. Two thermocouple holes (2 mm in diameter, 10 mm deep) were drilled into each specimen perpendicular to the cylinder axis at a spacing of ∼ 10 mm. Combustion experiments were conducted in a laboratory constant- pressure reactor (CPR-3 L, Sapphire Co., Abobyan, Armenia). Before reaction initiation, the reaction chamber was sealed, evacuated, and purged with argon (purity 99.8%, oxygen content no more than 0.1%) for three cycles and finally filled with argon to the desired pressure (1 MPa). A tungsten coil, positioned at the upper surface of the sample, was electrically heated until the reaction was initiated locally, after which the power was immedi- ately turned off, as the reaction wave propagated along the sample. The temperature À time distributions (temperature pro fi le) at given points of the reacting samples were recorded by two 0.1-mm-diameter C-type thermocouples. To ensure the stability of the measurements, the thermocouples fi rst were sputtered by a thin layer of boron nitride. The output signals of the thermocouples were transformed by a multichannel data acquisition board at a rate of 2 kHz and were recorded on a computer. The maximum combustion temperatures ( T c ) were determined from the maxima of the temperature pro fi les. The average values of combustion velocity ( U c ) were calculated from the physical distance between the thermocouples and the temporal distance between the signals of the thermocouples. All experimental data points for the combustion parameters were determined as averages of at least three measurements. The standard measurement errors for T c and U c were ( 20 ° C and 5%, respectively. Average heating rates of initial reagents in the combustion wave were determined from the temperature À time pro fi les. 2.2. Characterization of Materials. After the combustion process, reacted samples were kept in the reactor to complete cooling. Typical purification operations for crushed solid products include treatment with warm deionized water and dilute hydro- chloric acid (5 wt %). After the water treatment the content of metals (Mo, Mg, Na, Ca, etc.) in the obtained solution were analyzed by ELAN-9000 ICP mass spectrometer. The drying of purified material was performed in a vacuum furnace at a temperature of 80 ° C for 4 h. The combustion products were studied by XRD analysis with Cu K α radiation (di ff ractometer DRON-3.0, Burevestnik, Russia). XRD analyses of samples were performed at 25 kV and 10 μ A. Nova 230 and Hitachi S4800 fi eld-emission scanning electron microscopes were used to study the microstructure and compo- sition of the produced materials by the energy-dispersive spec- troscopy (EDS) method. The spatial resolution of the EDS analysis was 1.5 À 2 μ m. The speci fi c surface areas of the obtained molybdenum powders were determined by the Brunauer À Emmett À Teller (BET) method using nitrogen adsorption (Gasometer, GKh-1). The oxygen content in the molybdenum powder was determined with a LECO TC400 analyzer. Di ff erential thermal analysis (DTA) of initial reagents and reacting mixtures was performed using a Derivatograph Q1500 instrument (MOM, Budapest, Hungary). DTA investigations were conducted in argon fl ow (7 mL/s) at 20 ° C/min heating rate. Oxidation of the prepared molybdenum powder in air was investigated by the DTA technique at a heating rate of 20 ° C/min. 3.1. Thermodynamic Considerations. Before experiments, ISMAN-THERMO software 23 was used to consider the thermodynamics in the MoO 3 + 3Mg + n NaCl (where n is the number of moles of salt) system. This software is specially designed to calculate adiabatic temperature ( T ad ) and product equilibrium compositions in heterogeneous chemical processes. The inert gas pressure in these calculations was kept at 1 MPa. The results (Table 1) suggest that the calculated T ad value for the diluent-free mixture ( n = 0) is about 3500 ° C. Increasing n to 12 decreases the reaction temperature to 800 ° C. The equilibrium products at T ad < 2000 ° C consist of Mo, MgO, and NaCl, as well as small amounts of different molybdenum oxychlorides. At elevated temperatures, the amounts of molybdenum oxychlorides (MoOCl 2 and MoO 2 Cl 2 ) in the products are relatively higher (Table 1). Some amounts of MoO and MoO are also present in the products. Equilibrium concentrations of the products for the MoO 3 + NaCl mixture as a function of temperature were calculated as well. Figure 1 shows that, at T < 770 ° C, no reaction occurred between the salt and molybdenum trioxide. In the 770 < T < 1200 ° C range, liquid Na 2 MoO 4 and gaseous MoO 2 Cl 2 were the main products of the reaction between sodium chloride and molybdenum oxide. Thus, these calculations predicted that, for molten-salt-assisted combustion reactions, sodium chloride cannot be considered as an inert medium. 3.2. Combustion of MoO 3 + Mg + NaCl Mixtures. Fol- lowing the thermodynamic analysis, combustion processes in the MoO 3 + 3Mg + n NaCl mixtures were studied under 1 MPa inert gas pressure. Combustion temperature profiles for these mixtures are shown in Figure 2. Inspection of all of the data suggests (Table 1 and Figure 2) that the maximum combustion temperature ( T ) decreased dramatically as the salt content grew. The salt amount was found to have a critical point ( n = 5) above which the combustion wave did not propagate throughout the sample. The measured maximum temperature at this point was about 770 ° C. Note that the experimentally measured values for the combustion temperature were significantly lower than the ther- modynamically calculated adiabatic temperatures (Table 1). This disagreement is a result of heat losses that usually occur during the combustion process. In all cases, the combustion wave propagated within the sample in the steady-state regime. Another combustion feature that is signi fi cantly a ff ected by the salt amount is the heating rate ( V , ° C/s) of the reagent in the reacting zone, determined from the temperature pro fi les (Table 1). This parameter for n = 2 is about 1500 ° C/s. For n = 3, V = 350 ° C/s. Unusually low heating rates, from 20 to 50 ° C/s, for combustion synthesis processes were observed at high n . The temperature pro fi les (Figure 2) recorded for most n values contained constant-temperature plateaus at 450 À 510, 650, and 790 À 820 ° C. As can be seen, these plateaus were well-de fi ned, especially at low heating rates (high n values). The salt concentration strongly in fl uences the fl ame propagat- ing velocity ( U c ) as well. U c fell by more than a factor of 10, from 0.33 to 0.03 cm/s, as the salt content increased to 5 mol (Table 1). U c exhibited a dramatic dependence on T c , which provides a basis for the determination of the apparent activation energy of the combustion process. The relationship correlating combustion temperature, fl ame velocity, and activation energy is expressed by the simpli fi ed equation ...