Combustion Synthesis of MoSi₂-Al₂O₃ Composites from Thermite-Based Reagents

Abstract

Formation of MoSi₂–Al₂O₃ composites with a broad range of the MoSi₂/Al₂O₃ ratio was conducted by thermite-based combustion synthesis in the SHS mode. The addition of two thermite mixtures composed of MoO₃ + 2Al and 0.6MoO₃ + 0.6SiO₂ + 2Al into the Mo–Si reaction systems facilitated self-sustaining combustion and contributed to in situ formation of MoSi₂ and Al₂O₃. The samples adopting the former thermite reagent were more exothermic and produced composites with MoSi₂/Al₂O₃ from 2.0 to 4.5, beyond which combustion failed to proceed. Because of lower exothermicity of the reactions, the final products with MoSi₂/Al₂O₃ 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 MoSi₂/Al₂O₃ proportion, which led to a phase transition of MoSi₂. It was found that the dominant silicide was β-MoSi₂ when the combustion temperature of the synthesis reaction exceeded 1550 °C and shifted to α-MoSi₂ 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 MoSi₂ in the MoSi₂–Al₂O₃ composite.

1. 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 (MoSi₂) 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, MoSi₂ 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 MoSi₂. Studies have been performed on many reinforcements, such as Al, W, Cu, SiC, Si₃N₄, ZrB₂, and Al₂O₃ [10,11,12,13,14,15,16,17,18]. The incorporation of Al₂O₃ or Si₃N₄ to MoSi₂ has received particular attention due to excellent thermal stability and a close match of their thermal expansion coefficients [15,16,17,18]. Preparation of the MoSi₂–Si₃N₄ 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–mSi₃N₄ (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 MoSi₂, β-Si₃N₄ and a small amount of α-Si₃N₄.

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 NbSi₂ and TaSi₂ [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 Al₂O₃ [25]. Moreover, aluminothermic reduction of metal oxides is thermally beneficial for the SHS process. This study takes advantage of aluminothermic reduction of MoO₃ to obtain Mo, Al₂O₃, and a large amount of heat for fabricating the MoSi₂–Al₂O₃ 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 MoSi₂–Al₂O₃ composite with a broad range of the phase composition by thermite-based combustion synthesis. Fabrication of the MoSi₂–Al₂O₃ 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 MoSi₂ enlarges the thermal conductivity and heat capacity of the composite. Fracture toughness and flexural strength are improved as the content of Al₂O₃ increases. Two thermite mixtures, Al–MoO₃ and Al–MoO₃–SiO₂, 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.

2. Materials and Methods

The starting materials of this study included Mo (Strem Chemicals, Newburyport, MA, USA, <45 μm, 99.9%), Si (Strem Chemicals, <45 μm, 99.5%), Al (Showa Chemical Co., Tokyo, Japan, <45 μm, 99.9%), MoO₃ (Acros Organics, Pittsburgh, PA, USA, 99.5%), and SiO₂ (Strem Chemicals, 99%). Two thermite reagents were prepared to mix with Mo and Si powders. One combines MoO₃ with the reductant Al at a molar ratio of MoO₃:Al = 1:2. The other employs two metal oxides, MoO₃ and SiO₂, in a proportion of MoO₃:SiO₂:Al = 0.6:0.6:2. Reactions (1) and (2) represent two combustion systems with different thermite mixtures for the production of the MoSi₂–Al₂O₃ composites.

$$(x-1)\mathrm{Mo}+(2x)\mathrm{Si}+{\mathrm{MoO}}_3+2\mathrm{Al}\to x{\mathrm{MoSi}}_2+{\mathrm{Al}}_2{\mathrm{O}}_3$$

(1)

$$(y-0.6)\mathrm{Mo}+(2y-0.6)\mathrm{Si}+0.6{\mathrm{MoO}}_3+0.6{\mathrm{SiO}}_2+2\mathrm{Al}\to y{\mathrm{MoSi}}_2+{\mathrm{Al}}_2{\mathrm{O}}_3$$

(2)

where the stoichiometric coefficients, x and y, signify the number of mole of MoSi₂ formed in the composite containing 1 mole of Al₂O₃ from Reactions (1) and (2), respectively. The values of x and y also represent the molar ratio of MoSi₂/Al₂O₃ of the composite. The thermite reaction of MoO₃ + 2Al is extremely exothermic with ΔH = −930.7 kJ and T_ad = 4280 K [27]. Based upon the heat of formation of MoSi₂ (ΔH_f = −131.4 kJ/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 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 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 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 (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].

3. Results and Discussion

3.1. 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) 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.

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 (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 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.

Figure 3 and Figure 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. 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 Figure 3 and Figure 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.

3.2. 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 T_c,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 T_c,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 silicate in the final product. The EDS analysis of the final product indicated a slightly Si-lean atomic ratio of Mo:Si = 37.73:62.27 for 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.

4. Conclusions

The SHS process involving thermite reduction of MoO₃ and SiO₂ was conducted to prepare the in situ MoSi₂–Al₂O₃ composites with a broad range of the MoSi₂/Al₂O₃ proportion. Two thermite mixtures made up of MoO₃ + 2Al and 0.6MoO₃ + 0.6SiO₂ + 2Al were incorporated into the Mo–Si reaction system. Aluminothermic reduction of metal oxides released a large amount of the reaction enthalpy and generated Al₂O₃. The increase of Mo and Si for the production of a higher content of MoSi₂ reduced the overall reaction exothermicity and decelerated the combustion wave. The SHS process involving the MoO₃ + 2Al thermite was more exothermic than that containing the 0.6MoO₃ + 0.6SiO₂ + 2Al thermite. As a consequence, the former was adopted to produce composites with the MoSi₂/Al₂O₃ ratio from 2.0 to 4.5, and the latter aimed for MoSi₂/Al₂O₃ from 1.2 to 2.5.

Both reaction systems showed a substantial decrease in combustion temperature from around 1640 to 1150 °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 MoSi₂/Al₂O₃. The phase of MoSi₂ formed in the composite was dependent on the combustion temperature. It was found that β-MoSi₂ dominated in the products with combustion temperatures higher than 1550 °C and α-MoSi₂ prevailed in those with combustion temperatures below 1320 °C. In addition, dissolution of Si into Al₂O₃ led to the presence of aluminum silicate in the final product.