Combustion Synthesis of Mosi 2 -al 2 O 3 Composites from Thermite-based Reagents

Formation of MoSi 2 –Al 2 O 3 composites with a broad range of the MoSi 2 /Al 2 O 3 ratio was conducted by thermite-based combustion synthesis in the SHS mode. The addition of two thermite mixtures composed of MoO 3 + 2Al and 0.6MoO 3 + 0.6SiO 2 + 2Al into the Mo–Si reaction systems facilitated self-sustaining combustion and contributed to in situ formation of MoSi 2 and Al 2 O 3. The samples adopting the former thermite reagent were more exothermic and produced composites with MoSi 2 /Al 2 O 3 from 2.0 to 4.5, beyond which combustion failed to proceed. Because of lower exothermicity of the reactions, the final products with MoSi 2 /Al 2 O 3 from 1.2 to 2.5 were fabricated from the SHS process involving the latter thermite mixture. Combustion temperatures of both reaction systems decreased from about 1640 to 1150 • C with increasing MoSi 2 /Al 2 O 3 proportion, which led to a phase transition of MoSi 2. It was found that the dominant silicide was β-MoSi 2 when the combustion temperature of the synthesis reaction exceeded 1550 • C and shifted to α-MoSi 2 as the combustion temperature fell below 1320 • C. The results of this study showed an energy-efficient fabrication route to tailor the phase and content of MoSi 2 in the MoSi 2 –Al 2 O 3 composite.


Introduction
Silicides of many transition metals (typically in the IVb, Vb, and VIb groups) are promising candidate materials for high-temperature structural applications, due to their high melting point, low density, high thermal conductivity, and excellent oxidation and corrosion resistance at elevated temperatures [1,2].A variety of processing techniques, often with two or more in combination, including arc-melting and casting, mechanical alloying, hot pressing, reaction sintering, spark plasma sintering, combustion synthesis, and solid-state displacement reactions have been employed to fabricate transition metal silicides and composites on their basis [3][4][5].Among these methods, combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) takes advantage of the highly exothermic reaction, and hence, has the merits of low energy requirement, short processing time, simplicity, high productivity, and a structural and functional diversity of final products [6][7][8].
Molybdenum disilicide (MoSi 2 ) is one of the intermetallic silicides with broad applications for constructing combustion chamber parts, missile nozzles, diesel engine glow plugs, and industrial gas burners [9].In microelectronic devices, MoSi 2 thin layers are used as interconnections and contacts [9].Moreover, the addition of a metallic phase or ceramic compound contributed to an improvement in the creep resistance, fracture toughness, and high-temperature strength of MoSi 2 .Studies have been performed on many reinforcements, such as Al, W, Cu, SiC, Si 3 N 4 , ZrB 2 , and Al 2 O 3 [10][11][12][13][14][15][16][17][18].The incorporation of Al 2 O 3 or Si 3 N 4 to MoSi 2 has received particular attention due to excellent Metals 2016, 6, 235 2 of 8 thermal stability and a close match of their thermal expansion coefficients [15][16][17][18].Preparation of the MoSi 2 -Si 3 N 4 composites was investigated by Manukyan et al. [17,18] using molten salt-assisted combustion synthesis, in which a series of experiments adopting the Mo-5Si-0.5NaCl-mSi3 N 4 (0.7 ≤ m ≤ 1.65) samples was conducted in nitrogen from 1 to 5 MPa.NaCl acted as an inert diluent to control the combustion temperature and aided in transportation of the reactant species.The products obtained at nitrogen pressures higher than 3 MPa were composed of MoSi 2 , β-Si 3 N 4 and a small amount of α-Si 3 N 4 .
Preparation of transition metal silicides of the Ti-Si, Zr-Si, Nb-Si, Ta-Si, and Mo-Si binary systems was widely studied by the classical SHS method using elemental powder compacts of their corresponding stoichoimetries [19][20][21][22][23][24].It should be noted that a preheating temperature of 300 • C was required to establish a planar self-propagating combustion wave for combustion synthesis of NbSi 2 and TaSi 2 [21,22], and 200 • C for the elemental powder compact of Mo:Si = 1:2 [23].When combined with thermite reactions using Al as the reducing agent, combustion synthesis represents an in situ approach to preparing intermetallic and ceramic composites reinforced by Al 2 O 3 [25].Moreover, aluminothermic reduction of metal oxides is thermally beneficial for the SHS process.This study takes advantage of aluminothermic reduction of MoO 3 to obtain Mo, Al 2 O 3 , and a large amount of heat for fabricating the MoSi 2 -Al 2 O 3 composite through self-sustaining combustion.It is useful to point out another energy-saving route to obtain elemental Mo was direct reduction of ammonium molybdate tetrahydrate by a mixture of Zn and Mg [26].
The objective of this study was to fabricate the MoSi 2 -Al 2 O 3 composite with a broad range of the phase composition by thermite-based combustion synthesis.Fabrication of the MoSi 2 -Al 2 O 3 composites with different composition ratios is to achieve potential for tailoring material properties to meet the demands of different applications.For example, the increase of MoSi 2 enlarges the thermal conductivity and heat capacity of the composite.Fracture toughness and flexural strength are improved as the content of Al 2 O 3 increases.Two thermite mixtures, Al-MoO 3 and Al-MoO 3 -SiO 2 , were adopted and integrated into the Mo-Si reaction system.The synthesis reaction was conducted with no prior heating on the specimen and the effect of sample stoichiometries was studied on the flame-front velocity, combustion temperature, and phase constituents of the final products.
where the stoichiometric coefficients, x and y, signify the number of mole of MoSi 2 formed in the composite containing 1 mole of Al 2 O 3 from Reactions (1) and (2), respectively.The values of x and y also represent the molar ratio of MoSi 2 /Al 2 O 3 of the composite.The thermite reaction of MoO 3 + 2Al is extremely exothermic with ∆H = −930.7 kJ and T ad = 4280 K [27].Based upon the heat of formation of MoSi 2 (∆H f = −131.4kJ/mol), T ad = 1770 K is obtained for the Mo + 2Si reaction [27].Therefore, the addition of Mo and Si into the thermite mixture had a dilution effect on combustion.In this study, Reaction (1) was conducted with x varying from 2.0 to 4.5, within which stable and self-sustaining combustion was attained.For Reaction (1) with x < 2.0, it was difficult to recover the end product because violent combustion accompanying massive melting of the sample occurred.In contrast, combustion ceased to proceed in Reaction (1) with x > 4.5 due to insufficient reaction exothermicity.For Reaction (2), co-reduction of MoO 3 and SiO 2 by Al was carried out.Despite weaker reaction exothermicity compared with the WO 3 + 2Al reagent, the 0.6WO 3 + 0.6SiO 2 + 2Al thermite mixture with ∆H = −683.7 kJ and T ad = 3150 K [27] is thermally adequate to initiate Reaction (2).The steady self-sustaining combustion for Reaction (2) was found in the range of 1.2 ≤ y ≤ 2.5.A combination of Reaction (1) with 2.0 ≤ x ≤ 4.5 and Reaction (2) with 1.2 ≤ y ≤ 2.5 broadens the MoSi 2 /Al 2 O 3 ratio from 1.2 to 4.5 for the composites.Formation of the MoSi 2 -Al 2 O 3 composite through combustion synthesis involving aluminothermic reduction proceeds in consecutive stages.Reduction of MoO 3 by Al to produce Mo and Al 2 O 3 is believed to be the initiation step, followed by aluminothermic reduction of SiO 2 in the case of Reaction (2).Thereupon, MoSi 2 is formed from the interaction of Mo and Si.
The reactant powders were well mixed and compressed into cylindrical test specimens with 7 mm in diameter, 12 mm in length, and a relative density of 60%.The SHS reaction was conducted under high-purity (99.99%) argon of 0.15 MPa.The combustion wave velocity (V f ) was precisely determined from the time series of recorded combustion images.The combustion temperature (T c ) of the sample was measured by a fine-wire (125 µm) Pt/Pt-13%Rh thermocouple attached on the sample surface.The synthesized products were analyzed by X-Ray Diffraction (XRD) to identify the phase composition.Details of the experimental methods were reported elsewhere [28].

Combustion Front Velocity and Combustion Temperature
Two typical combustion sequences are illustrated in Figure 1a,b, which are respectively associated with Reaction (1) of x = 3.0 and Reaction (2) of y = 2.0.It is evident that for both SHS processes a distinct combustion front forms upon ignition and propagates along the sample compact in a self-sustaining manner.Figure 1a shows a faster combustion wave and a deformed product caused by melting of the sample, implying its higher combustion exothermicity.Serious melting and collapse of the samples were observed in Reaction ( 1 because violent combustion accompanying massive melting of the sample occurred.In contrast, combustion ceased to proceed in Reaction (1) with x > 4.5 due to insufficient reaction exothermicity.For Reaction (2), co-reduction of MoO3 and SiO2 by Al was carried out.Despite weaker reaction exothermicity compared with the WO3 + 2Al reagent, the 0.6WO3 + 0.6SiO2 + 2Al thermite mixture with ΔH = −683.7 kJ and Tad = 3150 K [27] is thermally adequate to initiate Reaction (2).The steady self-sustaining combustion for Reaction (2) was found in the range of 1.2 ≤ y ≤ 2.5.A combination of Reaction (1) with 2.0 ≤ x ≤ 4.5 and Reaction (2) with 1.2 ≤ y ≤ 2.5 broadens the MoSi2/Al2O3 ratio from 1.2 to 4.5 for the composites.Formation of the MoSi2-Al2O3 composite through combustion synthesis involving aluminothermic reduction proceeds in consecutive stages.Reduction of MoO3 by Al to produce Mo and Al2O3 is believed to be the initiation step, followed by aluminothermic reduction of SiO2 in the case of Reaction (2).Thereupon, MoSi2 is formed from the interaction of Mo and Si.
The reactant powders were well mixed and compressed into cylindrical test specimens with 7 mm in diameter, 12 mm in length, and a relative density of 60%.The SHS reaction was conducted under high-purity (99.99%) argon of 0.15 MPa.The combustion wave velocity (Vf) was precisely determined from the time series of recorded combustion images.The combustion temperature (Tc) of the sample was measured by a fine-wire (125 μm) Pt/Pt-13%Rh thermocouple attached on the sample surface.The synthesized products were analyzed by X-Ray Diffraction (XRD) to identify the phase composition.Details of the experimental methods were reported elsewhere [28].

Combustion Front Velocity and Combustion Temperature
Two typical combustion sequences are illustrated in Figure 1a,b, which are respectively associated with Reaction (1) of x = 3.0 and Reaction (2) of y = 2.0.It is evident that for both SHS processes a distinct combustion front forms upon ignition and propagates along the sample compact in a self-sustaining manner.Figure 1a shows a faster combustion wave and a deformed product caused by melting of the sample, implying its higher combustion exothermicity.Serious melting and collapse of the samples were observed in Reaction ( 1   The melting of the sample was alleviated with increasing content of MoSi 2 formed in the composite.The powder compacts of Reaction (2) also experienced lesser melting due to lower reaction enthalpies.As indicated in Figure 1b, only slight sample deformation was observed and the combustion wave was slower than that of Figure 1a.
Figure 2 presents the variations of flame-front propagation velocity (V f ) of Reactions ( 1) and ( 2) with respect to their corresponding stoichiometric coefficients, x and y.For Reaction (1), a significant decrease in the combustion wave speed from about 6.5 to 1.7 mm/s was observed with increasing x from 2.0 to 4.5.An increase in the stoichiometric coefficient of the reaction means a larger amount of Mo and Si powders added to the sample.The decrease of combustion front velocity was attributed to the dilution effect of Mo and Si additions, because the formation of MoSi 2 was less energetic than aluminothermic reduction of oxide reagents.Similarly, the decrease of the combustion velocity of Reaction (2) from 6.0 mm/s at y = 1.2 to 1.8 mm/s at y = 2.5 is revealed in Figure 2. A comparison between Reactions (1) and ( 2) at x = y = 2.0 and 2.5 points out that the flame-front velocities of Reaction (1) are much faster than those of Reaction (2).This is most likely caused by the fact that the thermite mixture of MoO 3 + 2Al is more exothermic than that of 0.6MoO 3 + 0.6SiO 2 + 2Al.The melting of the sample was alleviated with increasing content of MoSi2 formed in the composite.The powder compacts of Reaction (2) also experienced lesser melting due to lower reaction enthalpies.As indicated in Figure 1b, only slight sample deformation was observed and the combustion wave was slower than that of Figure 1a.
Figure 2 presents the variations of flame-front propagation velocity (Vf) of Reactions ( 1) and ( 2) with respect to their corresponding stoichiometric coefficients, x and y.For Reaction (1), a significant decrease in the combustion wave speed from about 6.5 to 1.7 mm/s was observed with increasing x from 2.0 to 4.5.An increase in the stoichiometric coefficient of the reaction means a larger amount of Mo and Si powders added to the sample.The decrease of combustion front velocity was attributed to the dilution effect of Mo and Si additions, because the formation of MoSi2 was less energetic than aluminothermic reduction of oxide reagents.Similarly, the decrease of the combustion velocity of Reaction (2) from 6.0 mm/s at y = 1.2 to 1.8 mm/s at y = 2.5 is revealed in Figure 2. A comparison between Reactions (1) and ( 2) at x = y = 2.0 and 2.5 points out that the flame-front velocities of Reaction (1) are much faster than those of Reaction (2).This is most likely caused by the fact that the thermite mixture of MoO3 + 2Al is more exothermic than that of 0.6MoO3 + 0.6SiO2 + 2Al.2) with different stoichiometries.The abrupt rise in temperature signifies rapid arrival of the combustion wave and the peak value stands for the reaction front temperature.Figure 3 shows that the combustion front temperature of Reaction (1) decreases from 1645 to 1198 °C as the molar ratio of MoSi2/Al2O3 increases from x = 2.0 to 4.5, confirming a decline in the overall reaction exothermicity with Mo and Si additions.As presented in Figure 4, for the samples of Reaction (2) a decrease in the peak combustion temperature from 1587 to 1146 °C was detected with increasing proportion of MoSi2/Al2O3 from y = 1.2 to 2.5.A comparison between Figures 3 and 4 verifies that Reaction (1) is more exothermic than Reaction (2).In addition, the temperature profiles of Reaction (1) appear to be broader than those of Reaction (2).This is because the amount of heat generated by Reaction (1) is larger than that generated by Reaction (2).Consequently, the rate of temperature decline behind the combustion front is slower for the sample of Reaction (1).Furthermore, it is useful to note that for Reactions (1) and ( 2), the composition dependence of the combustion temperature is in agreement with that of combustion wave velocity.2) with different stoichiometries.The abrupt rise in temperature signifies rapid arrival of the combustion wave and the peak value stands for the reaction front temperature.Figure 3 shows that the combustion front temperature of Reaction (1) decreases from 1645 to 1198 • C as the molar ratio of MoSi 2 /Al 2 O 3 increases from x = 2.0 to 4.5, confirming a decline in the overall reaction exothermicity with Mo and Si additions.As presented in Figure 4, for the samples of Reaction (2) a decrease in the peak combustion temperature from 1587 to 1146 • C was detected with increasing proportion of MoSi 2 /Al 2 O 3 from y = 1.2 to 2.5.A comparison between Figures 3 and 4 verifies that Reaction (1) is more exothermic than Reaction (2).In addition, the temperature profiles of Reaction (1) appear to be broader than those of Reaction (2).This is because the amount of heat generated by Reaction ( 1) is larger than that generated by Reaction (2).Consequently, the rate of temperature decline behind the combustion front is slower for the sample of Reaction (1).Furthermore, it is useful to note that for Reactions (1) and ( 2), the composition dependence of the combustion temperature is in agreement with that of combustion wave velocity.5a-c.The phase change is due probably to the decrease of combustion temperature.As reveals in Figure 5c, the silicide phase is governed by α-MoSi2 in the product with x = 4.0, for which Tc,max = 1280 °C.β-MoSi2 is a metastable phase with excellent thermoelectric properties, while α-MoSi2 is a stable phase with a low electric resistance and high oxidation resistance [29].Formation of β-MoSi2 in the final product could be attributed to the high thermal gradient and rapid cooling rate inherent in the SHS process.Experimental evidence of this study indicates that the as-synthesized composite features β-MoSi2 as the major silicide when the combustion temperature exceeds 1550 °C, whereas α-MoSi2 dominates in the final product with combustion temperature lower than 1320 °C.

Phase Constituents of Synthesized Composites
In addition, the XRD signature of aluminum silicate rather than Al2O3 is indexed in the spectra of Figure 5. Aluminum silicate or called mullite is a solid solution of Al2O3 and SiO2.This verified that, in agreement with the initial hypothesis, Al2O3 was produced.Because there was Si in the reaction system, dissolution of a small amount of Si into Al2O3 led to the formation of aluminum

Phase Constituents of Synthesized Composites
Figure 5a-c displays the XRD patterns of SHS-produced composites from Reaction (1) with different MoSi2/Al2O3 ratios.Formation of both α and β phases of MoSi2 was observed.It is noticed that β-MoSi2 (the high-temperature phase) is the dominant silicide in Figure 5a of x = 2.5, which corresponds to Tc,max = 1580 °C.With the increase of the coefficient x, a phase transformation from β-MoSi2 to α-MoSi2 (the low-temperature phase) is shown in Figure 5a-c.The phase change is due probably to the decrease of combustion temperature.As reveals in Figure 5c, the silicide phase is governed by α-MoSi2 in the product with x = 4.0, for which Tc,max = 1280 °C.β-MoSi2 is a metastable phase with excellent thermoelectric properties, while α-MoSi2 is a stable phase with a low electric resistance and high oxidation resistance [29].Formation of β-MoSi2 in the final product could be attributed to the high thermal gradient and rapid cooling rate inherent in the SHS process.Experimental evidence of this study indicates that the as-synthesized composite features β-MoSi2 as the major silicide when the combustion temperature exceeds 1550 °C, whereas α-MoSi2 dominates in the final product with combustion temperature lower than 1320 °C.
In addition, the XRD signature of aluminum silicate rather than Al2O3 is indexed in the spectra of Figure 5. Aluminum silicate or called mullite is a solid solution of Al2O3 and SiO2.This verified that, in agreement with the initial hypothesis, Al2O3 was produced.Because there was Si in the reaction system, dissolution of a small amount of Si into Al2O3 led to the formation of aluminum   5a-c.The phase change is due probably to the decrease of combustion temperature.As reveals in Figure 5c, the silicide phase is governed by α-MoSi 2 in the product with x = 4.0, for which T c,max = 1280 • C. β-MoSi 2 is a metastable phase with excellent thermoelectric properties, while α-MoSi 2 is a stable phase with a low electric resistance and high oxidation resistance [29].Formation of β-MoSi 2 in the final product could be attributed to the high thermal gradient and rapid cooling rate inherent in the SHS process.Experimental evidence of this study indicates that the as-synthesized composite features β-MoSi 2 as the major silicide when the combustion temperature exceeds 1550 • C, whereas α-MoSi 2 dominates in the final product with combustion temperature lower than 1320 • C.

Phase Constituents of Synthesized Composites
In addition, the XRD signature of aluminum silicate rather than Al 2 O 3 is indexed in the spectra of Figure 5. Aluminum silicate or called mullite is a solid solution of Al 2 O 3 and SiO 2 .This verified that, in agreement with the initial hypothesis, Al 2 O 3 was produced.Because there was Si in the reaction system, dissolution of a small amount of Si into Al 2 O 3 led to the formation of aluminum silicate in the final product.The EDS analysis of the final product indicated a slightly Si-lean atomic ratio of Mo:Si = 37.73:62.27for the MoSi 2 (Mo:Si = 33.33:66.67)phase.Therefore, the amount of Si dissolved into Al 2 O 3 was estimated to be about 6.6 atm %. ratio of Mo:Si = 37.73:62.27for the MoSi2 (Mo:Si = 33.33:66.67)phase.Therefore, the amount of Si dissolved into Al2O3 was estimated to be about 6.6 atm %.
Phase constituents of the composites synthesized from Reaction (2) are identical to those from Reaction (1).The dependence of phase evolution of MoSi2 produced by Reaction (2) on combustion temperature is in a manner consistent with that observed in Reaction (1).For the SHS-derived composites of Reaction (2), the silicide of the product of y = 1.2 was dominated by β-MoSi2 because of a high combustion temperature close to 1590 °C, but α-MoSi2 prevailed in the products of y = 2.0 and 2.5 on account of their low combustion temperatures of 1320 and 1146 °C, respectively.The XRD pattern of the composite with y = 2.5 is depicted in Figure 6, which has the same MoSi2/Al2O3 ratio as that shown in Figure 5a.This demonstrates that the phase of MoSi2 is tailorable through the reaction systems with different thermite reagents.Phase constituents of the composites synthesized from Reaction (2) are identical to those from Reaction (1).The dependence of phase evolution of MoSi 2 produced by Reaction (2) on combustion temperature is in a manner consistent with that observed in Reaction (1).For the SHS-derived composites of Reaction (2), the silicide of the product of y = 1.2 was dominated by β-MoSi 2 because of a high combustion temperature close to 1590 • C, but α-MoSi 2 prevailed in the products of y = 2.0 and 2.5 on account of their low combustion temperatures of 1320 and 1146 • C, respectively.The XRD pattern of the composite with y = 2.5 is depicted in Figure 6, which has the same MoSi 2 /Al 2 O 3 ratio as that shown in Figure 5a.This demonstrates that the phase of MoSi 2 is tailorable through the reaction systems with different thermite reagents.silicate in the final product.The EDS analysis of the final product indicated a slightly Si-lean atomic ratio of Mo:Si = 37.73:62.27for the MoSi2 (Mo:Si = 33.33:66.67)phase.Therefore, the amount of Si dissolved into Al2O3 was estimated to be about 6.6 atm %.Phase constituents of the composites synthesized from Reaction (2) are identical to those from Reaction (1).The dependence of phase evolution of MoSi2 produced by Reaction (2) on combustion temperature is in a manner consistent with that observed in Reaction (1).For the SHS-derived composites of Reaction (2), the silicide of the product of y = 1.2 was dominated by β-MoSi2 because of a high combustion temperature close to 1590 °C, but α-MoSi2 prevailed in the products of y = 2.0 and 2.5 on account of their low combustion temperatures of 1320 and 1146 °C, respectively.The XRD pattern of the composite with y = 2.5 is depicted in Figure 6, which has the same MoSi2/Al2O3 ratio as that shown in Figure 5a.This demonstrates that the phase of MoSi2 is tailorable through the reaction systems with different thermite reagents.

Conclusions
The SHS process involving thermite reduction of MoO 3 and SiO 2 was conducted to prepare the in situ MoSi 2 -Al 2 O 3 composites with a broad range of the MoSi 2 /Al 2 O 3 proportion.Two thermite mixtures made up of MoO 3 + 2Al and 0.6MoO 3 + 0.6SiO 2 + 2Al were incorporated into the Mo-Si reaction system.Aluminothermic reduction of metal oxides released a large amount of the reaction enthalpy and generated Al 2 O 3 .The increase of Mo and Si for the production of a higher content of MoSi 2 reduced the overall reaction exothermicity and decelerated the combustion wave.The SHS process involving the MoO 3 + 2Al thermite was more exothermic than that containing the 0.6MoO 3 + 0.6SiO 2 + 2Al thermite.As a consequence, the former was adopted to produce composites with the MoSi 2 /Al 2 O 3 ratio from 2.0 to 4.5, and the latter aimed for MoSi 2 /Al 2 O 3 from 1.2 to 2.5.
Both reaction systems showed a substantial decrease in combustion temperature from around 1640 to • C and a reduction in combustion wave velocity from approximately 6.5 to 1.7 mm/s with the increase of the molar ratio of MoSi 2 /Al 2 O 3 .The phase of MoSi 2 formed in the composite was dependent on the combustion temperature.It was found that β-MoSi 2 dominated in the products with combustion temperatures higher than 1550 • C and α-MoSi 2 prevailed in those with combustion temperatures below 1320 • C. In addition, dissolution of Si into Al 2 O 3 led to the presence of aluminum silicate in the final product.
) with x = 2.0-3.0, which generated high combustion temperatures because these samples contained single oxide MoO 3 and relatively less amounts of Mo and Si.Moreover, MoO 3 and Al have very low melting points of 795 • C and 660 • C, respectively.Metals 2016, 6, 235 3 of 8 ) with x = 2.0-3.0, which generated high combustion temperatures because these samples contained single oxide MoO3 and relatively less amounts of Mo and Si.Moreover, MoO3 and Al have very low melting points of 795 °C and 660 °C, respectively.

Figure 2 .
Figure 2. Effects of stoichiometric coefficients of reactions with different thermite reagents on flame-front velocity for formation of MoSi2-Al2O3 composites.

Figures 3
Figures 3 and 4 plot combustion temperature profiles measured from the powder compacts of Reactions (1) and (2) with different stoichiometries.The abrupt rise in temperature signifies rapid arrival of the combustion wave and the peak value stands for the reaction front temperature.Figure3shows that the combustion front temperature of Reaction (1) decreases from 1645 to 1198 °C as the molar ratio of MoSi2/Al2O3 increases from x = 2.0 to 4.5, confirming a decline in the overall reaction exothermicity with Mo and Si additions.As presented in Figure4, for the samples of Reaction (2) a decrease in the peak combustion temperature from 1587 to 1146 °C was detected with increasing proportion of MoSi2/Al2O3 from y = 1.2 to 2.5.A comparison between Figures3 and 4verifies that Reaction (1) is more exothermic than Reaction (2).In addition, the temperature profiles of Reaction (1) appear to be broader than those of Reaction (2).This is because the amount of heat generated by Reaction (1) is larger than that generated by Reaction(2).Consequently, the rate of temperature decline behind the combustion front is slower for the sample of Reaction (1).Furthermore, it is useful to note that for Reactions (1) and (2), the composition dependence of the combustion temperature is in agreement with that of combustion wave velocity.

Figure 2 .
Figure 2. Effects of stoichiometric coefficients of reactions with different thermite reagents on flame-front velocity for formation of MoSi 2 -Al 2 O 3 composites.

Figures 3
Figures 3 and 4 plot combustion temperature profiles measured from the powder compacts of Reactions (1) and (2) with different stoichiometries.The abrupt rise in temperature signifies rapid arrival of the combustion wave and the peak value stands for the reaction front temperature.Figure3shows that the combustion front temperature of Reaction (1) decreases from 1645 to 1198 • C as the molar ratio of MoSi 2 /Al 2 O 3 increases from x = 2.0 to 4.5, confirming a decline in the overall reaction exothermicity with Mo and Si additions.As presented in Figure4, for the samples of Reaction (2) a decrease in the peak combustion temperature from 1587 to 1146 • C was detected with increasing proportion of MoSi 2 /Al 2 O 3 from y = 1.2 to 2.5.A comparison between Figures3 and 4verifies that Reaction (1) is more exothermic than Reaction (2).In addition, the temperature profiles of Reaction (1) appear to be broader than those of Reaction (2).This is because the amount of heat generated by Reaction (1) is larger than that generated by Reaction(2).Consequently, the rate of temperature decline behind the combustion front is slower for the sample of Reaction (1).Furthermore, it is useful to note that for Reactions (1) and (2), the composition dependence of the combustion temperature is in agreement with that of combustion wave velocity.

Figure 3 .
Figure 3. Variation of combustion temperature with MoSi2/Al2O3 ratio of composites synthesized by self-propagating high-temperature synthesis (SHS) involving reduction of MoO3 by Al.

Figure 4 .
Figure 4. Variation of combustion temperature with MoSi2/Al2O3 ratio of composites synthesized by SHS involving co-reduction of MoO3 and SiO2 by Al.

Figure
Figure 5a-c displays the XRD patterns of SHS-produced composites from Reaction (1) with different MoSi2/Al2O3 ratios.Formation of both α and β phases of MoSi2 was observed.It is noticed that β-MoSi2 (the high-temperature phase) is the dominant silicide in Figure 5a of x = 2.5, which corresponds to Tc,max = 1580 °C.With the increase of the coefficient x, a phase transformation from β-MoSi2 to α-MoSi2 (the low-temperature phase) is shown in Figure5a-c.The phase change is due probably to the decrease of combustion temperature.As reveals in Figure5c, the silicide phase is governed by α-MoSi2 in the product with x = 4.0, for which Tc,max = 1280 °C.β-MoSi2 is a metastable phase with excellent thermoelectric properties, while α-MoSi2 is a stable phase with a low electric resistance and high oxidation resistance[29].Formation of β-MoSi2 in the final product could be attributed to the high thermal gradient and rapid cooling rate inherent in the SHS process.Experimental evidence of this study indicates that the as-synthesized composite features β-MoSi2 as the major silicide when the combustion temperature exceeds 1550 °C, whereas α-MoSi2 dominates in the final product with combustion temperature lower than 1320 °C.In addition, the XRD signature of aluminum silicate rather than Al2O3 is indexed in the spectra of Figure5.Aluminum silicate or called mullite is a solid solution of Al2O3 and SiO2.This verified that, in agreement with the initial hypothesis, Al2O3 was produced.Because there was Si in the reaction system, dissolution of a small amount of Si into Al2O3 led to the formation of aluminum

Figure 3 . 8 Figure 3 .
Figure 3. Variation of combustion temperature with MoSi 2 /Al 2 O 3 ratio of composites synthesized by self-propagating high-temperature synthesis (SHS) involving reduction of MoO 3 by Al.

Figure 4 .
Figure 4. Variation of combustion temperature with MoSi2/Al2O3 ratio of composites synthesized by SHS involving co-reduction of MoO3 and SiO2 by Al.

Figure 4 .
Figure 4. Variation of combustion temperature with MoSi 2 /Al 2 O 3 ratio of composites synthesized by SHS involving co-reduction of MoO 3 and SiO 2 by Al.

Figure
Figure 5a-c displays the XRD patterns of SHS-produced composites from Reaction (1) with different MoSi 2 /Al 2 O 3 ratios.Formation of both α and β phases of MoSi 2 was observed.It is noticed that β-MoSi 2 (the high-temperature phase) is the dominant silicide in Figure 5a of x = 2.5, which corresponds to T c,max = 1580 • C. With the increase of the coefficient x, a phase transformation from β-MoSi 2 to α-MoSi 2 (the low-temperature phase) is shown in Figure5a-c.The phase change is due probably to the decrease of combustion temperature.As reveals in Figure5c, the silicide phase is governed by α-MoSi 2 in the product with x = 4.0, for which T c,max = 1280 • C. β-MoSi 2 is a metastable phase with excellent thermoelectric properties, while α-MoSi 2 is a stable phase with a low electric resistance and high oxidation resistance[29].Formation of β-MoSi 2 in the final product could be attributed to the high thermal gradient and rapid cooling rate inherent in the SHS process.Experimental evidence of this study indicates that the as-synthesized composite features β-MoSi 2 as the major silicide when the combustion temperature exceeds 1550 • C, whereas α-MoSi 2 dominates in the final product with combustion temperature lower than 1320 • C.In addition, the XRD signature of aluminum silicate rather than Al 2 O 3 is indexed in the spectra of Figure5.Aluminum silicate or called mullite is a solid solution of Al 2 O 3 and SiO 2 .This verified that, in agreement with the initial hypothesis, Al 2 O 3 was produced.Because there was Si in the reaction