Effects of Fe/Si Stoichiometry on Formation of Fe 3 Si/FeSi-Al 2 O 3 Composites by Aluminothermic Combustion Synthesis

: Aluminothermic combustion synthesis was conducted with Fe 2 O 3 –Al–Fe–Si reaction systems under Fe/Si stoichiometry from Fe-20 to Fe-50 at. % Si to investigate the formation Fe 3 Si/FeSi– Al 2 O 3 composites. The solid-state combustion was sufﬁciently exothermic to sustain the overall reaction in the mode of self-propagating high-temperature synthesis (SHS). Dependence of iron silicide phases formed from SHS on Fe/Si stoichiometry was examined. Experimental evidence indicated that combustion exothermicity and ﬂame-front velocity were affected by the Si percentage. According to the X-ray diffraction (XRD) analysis, Fe 3 Si–Al 2 O 3 composites were synthesized from the reaction systems with Fe-20 and Fe-25 at.% Si. The increase of Si content led to the formation of both Fe 3 Si and FeSi in the ﬁnal products of Fe-33.3 and Fe-40 at.% Si reaction systems, and the content of FeSi increased with Si percentage. Further increase of Si to Fe-50 at.% Si produced the FeSi–Al 2 O 3 composite. Scanning electron microscopy (SEM) images revealed that the fracture surface morphology of the products featured micron-sized and nearly spherical Fe 3 Si and FeSi particles distributing over the dense and connecting substrate formed by Al 2 O 3 .

Among various fabrication routes to prepare iron silicides in monolithic and composite forms, the reaction synthesis methods associated with mechanical alloying and combustion process have been of great interest. For example, FeSi and β-FeSi 2 were produced by mechanically-activated combustion reaction of the Fe + 2Si powder mixture pretreated by shock-assisted ball milling for a long period of time [11,12]. In addition to mechanical activation, Gras et al. [13] adopted KNO 3 of 20 wt.% to chemically promote self-sustaining combustion reaction of the Fe + 2Si mixture for the formation of FeSi and α-FeSi 2 composites. Zakeri et al. [14] conducted mechanical alloying of SiO 2 and Al powders with stainless steel balls for 45 h to fabricate FeSi-Al 2 O 3 nanocomposite powders from induced reactions. According to Guan et al. [15], Fe 3 Si-Al 2 O 3 nanocomposites were produced from Fe 3 O 4 , Al, and Si reactant powders through 4-h mechanical alloying followed by an annealing process at 900 • C for 1 h. Besides, Li et al. [16] produced iron silicide nanoparticles of various phases by thermal annealing of Fe-FeSi 2 samples with a core-shell structure and identified the phase transformation from Fe-FeSi 2 to FeSi and Fe 3 Si under an annealing time of 2 h and temperatures of 600 and 700 • C. Recently, an in situ fabrication approach combining the chemical interaction between Fe and Si with aluminothermic reduction of Fe 2 O 3 and SiO 2 has been attempted to produce FeSi-Al 2 O 3 and α-FeSi 2 -Al 2 O 3 composites in the mode of self-propagating high-temperature synthesis (SHS) [17,18]. With the advantages of energy efficiency, rapid reaction, simplicity, and high-purity products [19], the SHS scheme has been recognized as one of the most effective methods for preparing transition metal silicides in monolithic and composite forms [20][21][22][23].
As an extension of the previous studies [17,18], this work aims to investigate the production of Fe 3 Si/FeSi-Al 2 O 3 composites by aluminothermite-based combustion synthesis in the SHS mode, with an emphasis on exploring the effect of Fe/Si stoichiometry on the formation of Fe 3 Si and FeSi. So far, no studies have been reported in the literature on combustion synthesis of Fe 3 Si which exists in a wide stoichiometric range from 10 to 25 at.% Si. In this work, the reactant mixtures with different Fe/Si stoichiometries were prepared for combustion experiments, reaction exothermicity and combustion wave kinetics of the SHS process were studied, and compositional and microstructural analyses of the final products were performed.
The stoichiometric coefficient x was varied between 0.75 and 3.0 to examine the influence of Fe/Si stoichiometry on the formation of silicide phases. Specifically, Reaction (1) was carried out with x = 0.75, 1.0, 1.5, 2.0, and 3.0, i.e., the test specimens with Fe/Si stoichiometries of Fe-20, 25, 33.3, 40, and 50 at.% Si were formulated. According to the value of x, the coefficients m and n were calculated and summarized in Table 1. Note that with respect to Fe 3 Si, Reaction (1) with x = 0.75 represents an off-stoichiometric condition with a Si-lean mixture of Fe-20 at.% Si, under which the silicide phase to be produced is Fe 3 Si owing to its wide formation stoichiometry. The Fe/Si proportions of Reaction (1) with x = 1.0 and 3.0 match the exact stoichiometries of Fe 3 Si and FeSi, respectively; thus, they are expected to be the only silicide formed in the corresponding conditions. Based on the Fe-Si phase diagram, the products containing two silicide phases, Fe 3 Si and FeSi, are considered for Reaction (1) with x = 1.5 and 2.0 in Table 1.
where ∆H r o is the reaction enthalpy at 298 K, n j is the stoichiometric constant, C p and L are the heat capacity and latent heat, and P j refers to the product.
Reactant powders were well mixed and compressed into cylindrical samples with 7 mm in diameter, 10 mm in length, and a relative density of 55%. The relative density of the test specimen is related to its initial components. The theoretical density (ρ TD ) of the test specimen is calculated from the mass fraction (Y) and density (ρ) of each component through the following equation: The SHS experiment was conducted in a stainless steel combustion chamber equipped with two quartz viewing windows and filled with high-purity argon (99.99%, Hochun Gas Co., Taichung, Taiwan) at 0.25 MPa. Based upon the time sequence series of combustion images, the flame-front trajectory as a function of time was constructed. The time derivative of the trajectory was determined as the combustion wave velocity (V f ). To facilitate the accurate measurement of instantaneous locations of the combustion front, a beam splitter (Rolyn Optics Co., Covina, CA, USA), with a mirror characteristic of 75% transmission and 25% reflection, was used to optically superimpose a scale onto the image of the test sample. The flame-front propagation velocity was slightly higher in the early stage right after the ignition, and then the linearity of time derivative of the trajectory implied that propagation of the flame front can be treated as a constant-velocity event. The relatively high propagation velocity in the beginning was attributed to the thermal energy supplied by the igniter, and the constant velocity in the later stage represents self-sustained propagation of the flame front.
The combustion temperature was measured by a fine-wire thermocouple with a bead diameter of 125 µm. R-type thermocouples (Omega Engineering Inc., Norwalk, CT, USA) with an alloy combination of Pt/Pt-13%Rh were used. The thermocouple bead was firmly attached on the sample surface at a position~5 mm below the ignition plane. At this location, self-sustaining combustion was well developed so that measurement of the combustion front temperature (T c ) was justified. Phase constituents of the end products were identified by an X-ray diffractometer (Bruker D2 Phaser, Billerica, MA, USA) with CuK α radiation. Scanning electron microscopy (SEM, Hitachi, S3000H, Tokyo, Japan) examination and energy dispersive spectroscopy (EDS) analysis were performed to study the fracture surface microstructure and elemental composition of the final products. Details of the experimental methods were reported elsewhere [26].

Combustion Exothermicity and Combustion Wave Kinetics
Calculated ∆H r o and T ad of Reactions (1) with different values of x are presented in Figure 1. The calculation was based on the products with phase compositions specified by the values of m and n listed in Table 1. Results showed that the increase of x from 1.0 to 3.0 led to an increase of ∆H r o from −881.9 to −1118.6 kJ for Reaction (1), but a decrease of T ad from 3059 to 2872 K. The increase of ∆H r o is caused by the fact that both the aluminothermic reduction of Fe 2 O 3 and formation of iron silicides are heat-releasing reactions. Moreover, the number of moles of iron silicides (i.e., the sum of m and n) formed in the final product increases with increasing x value. Specifically, the reaction of Fe 2 O 3 + 2Al → 2Fe + Al 2 O 3 is extremely exothermic with ∆H r o = −852.3 kJ and the formation enthalpies of Fe 3 Si and FeSi are −79.4 and −73.1 kJ/mol, respectively [24]. On the other hand, the decrease of T ad was because the reaction exothermicity (i.e., ∆H r o /C p ) of aluminothermic reduction of Fe 2 O 3 is much higher than that of the formation of Fe 3 Si and FeSi. Consequently, the combustion temperature of the overall reaction decreased as the molar fraction of iron silicides in the product increased. Figure 2 illustrates a representative sequence of combustion images recorded from Reaction (1) with x = 1.5. It is evident that upon ignition a well-defined combustion front forms and propagates along the powder compact in a self-sustaining style. The progression of combustion wave arrived at a nearly constant rate after the ignition energy was faded out and the high exothermicity of combustion caused partial melting of the burned sample.
Stoichiometric Coefficient, x  The measured combustion wave velocity reported in Figure 3a decreases from about 2.7 to 1.6 mm/s when the value of x increases from 0.75 to 3.0. The decline of combustion front speed was mainly ascribed to the dilution effect of Fe and Si additions and to the increase of iron silicides formed in the product. The combustion wave propagation rate is mostly governed by the layer-by-layer heat transfer from the reacting region to unburned zone and is likely affected by the combustion front temperature. Typical temperature profiles of Reaction (1) under different stoichiometries are depicted in Figure 3b. The temperature profile suggested that the sample experienced a steep thermal gradient and a rapid cooling rate, both of which are SHS characteristics. The peak value of the profile was defined as the combustion front temperature (Tc). As shown in Figure 3b, the value of Tc  (1) with different stoichiometric coefficients, x.
As calculated adiabatic temperatures are higher than the criterion proposed by Merzhanov [19], combustion synthesis based on Reaction (1) is thermally satisfactory to be self-sustaining. Besides thermodynamic considerations, the SHS process must overcome the kinetic limitation of the reaction. Kinetic restraints are caused by inadequate reactivity owing to the presence of diffusion barriers. It is believed that the reduction of Fe 2 O 3 by Al to produce Fe and Al 2 O 3 acts as the initiation step, followed by the interaction between Fe and Si to generate FeSi and/or Fe 3 Si [27]. Figure 2 illustrates a representative sequence of combustion images recorded from Reaction (1) with x = 1.5. It is evident that upon ignition a well-defined combustion front forms and propagates along the powder compact in a self-sustaining style. The progression of combustion wave arrived at a nearly constant rate after the ignition energy was faded out and the high exothermicity of combustion caused partial melting of the burned sample. the combustion temperature of the overall reaction decreased as the molar fraction of iron silicides in the product increased. As calculated adiabatic temperatures are higher than the criterion proposed by Merzhanov [19], combustion synthesis based on Reaction (1) is thermally satisfactory to be self-sustaining. Besides thermodynamic considerations, the SHS process must overcome the kinetic limitation of the reaction. Kinetic restraints are caused by inadequate reactivity owing to the presence of diffusion barriers. It is believed that the reduction of Fe2O3 by Al to produce Fe and Al2O3 acts as the initiation step, followed by the interaction between Fe and Si to generate FeSi and/or Fe3Si [27]. Figure 2 illustrates a representative sequence of combustion images recorded from Reaction (1) with x = 1.5. It is evident that upon ignition a well-defined combustion front forms and propagates along the powder compact in a self-sustaining style. The progression of combustion wave arrived at a nearly constant rate after the ignition energy was faded out and the high exothermicity of combustion caused partial melting of the burned sample.
Stoichiometric Coefficient, x  The measured combustion wave velocity reported in Figure 3a decreases from about 2.7 to 1.6 mm/s when the value of x increases from 0.75 to 3.0. The decline of combustion front speed was mainly ascribed to the dilution effect of Fe and Si additions and to the increase of iron silicides formed in the product. The combustion wave propagation rate is mostly governed by the layer-by-layer heat transfer from the reacting region to unburned zone and is likely affected by the combustion front temperature. Typical temperature profiles of Reaction (1) under different stoichiometries are depicted in Figure 3b. The temperature profile suggested that the sample experienced a steep thermal gradient and a rapid cooling rate, both of which are SHS characteristics. The peak value of the profile was defined as the combustion front temperature (Tc). As shown in Figure 3b, the value of Tc The measured combustion wave velocity reported in Figure 3a decreases from about 2.7 to 1.6 mm/s when the value of x increases from 0.75 to 3.0. The decline of combustion front speed was mainly ascribed to the dilution effect of Fe and Si additions and to the increase of iron silicides formed in the product. The combustion wave propagation rate is mostly governed by the layer-by-layer heat transfer from the reacting region to unburned zone and is likely affected by the combustion front temperature. Typical temperature profiles of Reaction (1) under different stoichiometries are depicted in Figure 3b. The temperature profile suggested that the sample experienced a steep thermal gradient and a rapid cooling rate, both of which are SHS characteristics. The peak value of the profile was defined as the combustion front temperature (T c ). As shown in Figure 3b, the value of T c decreased from 1730 • C at x = 0.75 to 1506 • C at x = 3.0, confirming the dilution effect on combustion with Fe and Si additions. It is important to note that the dependence of combustion front temperature on Fe/Si stoichiometry was in a manner consistent with that of combustion wave velocity. The descending trend of T c with x value is in agreement with that of T ad . However, because burning samples suffered considerable heat losses to surrounding argon by conduction and convection and to the inner wall of the chamber by radiation, the measured T c was lower than the calculated T ad . decreased from 1730 °C at x = 0.75 to 1506 °C at x = 3.0, confirming the dilution effect on combustion with Fe and Si additions. It is important to note that the dependence of combustion front temperature on Fe/Si stoichiometry was in a manner consistent with that of combustion wave velocity. The descending trend of Tc with x value is in agreement with that of Tad. However, because burning samples suffered considerable heat losses to surrounding argon by conduction and convection and to the inner wall of the chamber by radiation, the measured Tc was lower than the calculated Tad.  Figure 4a,b plots XRD patterns of two products respectively produced from Reaction (1) with x = 0.75 and 1.0. As indicated in Figure 4a, the formation of Fe3Si and Al2O3 with almost no other phases was obtained from an off-stoichiometric and Si-lean sample of Fe-20 at.% Si (i.e., x = 0.75). For Reaction (1) with Fe/Si stoichiometry of Fe-25 at.% Si (i.e., x = 1.0), Figure 4b reveals that in addition to two target compounds, Fe3Si and Al2O3, a minor compound, aluminum silicate or mullite, was found. Mullite is a stable solid solution in the Al2O3-SiO2 system and refers to Al4+2zSi2−2zO10−z with z varying between around 0.2 and 0.9 [28]. Formation of mullite was essentially due to dissolution of a small amount of Si into Al2O3 during the SHS process [29,30]. The yield of Fe3Si as the only silicide phase under these two test conditions is justified by the fact that Fe3Si exists in a composition range from 10 to 25 at.% Si. In view of no impurities in the final product, the Si-lean mixture of Fe-20 at.% Si was more favorable for the formation of Fe3Si-Al2O3 composite.

Composition and Microstructure of Synthesized Products
On account of its large negative Gibbs free energy change (G o = -840.8 kJ) [24], the reaction pathways of Reaction (1) were proposed to be initiated by the aluminothermic reaction of Fe2O3 + 2Al → 2Fe + Al2O3. In addition to the kinetic aspect, the reduction of Fe2O3 by Al was also a thermodynamically favored reaction which supplied a large amount of heat (Hr o = −852.3 kJ) to maintain the synthesis reaction in the SHS mode. After the initiation step, the chemical interaction between Fe and Si proceeded to produce Fe3Si.   Figure 4b reveals that in addition to two target compounds, Fe 3 Si and Al 2 O 3 , a minor compound, aluminum silicate or mullite, was found. Mullite is a stable solid solution in the Al 2 O 3 -SiO 2 system and refers to Al 4+2z Si 2−2z O 10−z with z varying between around 0.2 and 0.9 [28]. Formation of mullite was essentially due to dissolution of a small amount of Si into Al 2 O 3 during the SHS process [29,30]. The yield of Fe 3 Si as the only silicide phase under these two test conditions is justified by the fact that Fe 3 Si exists in a composition range from 10 to 25 at.% Si. In view of no impurities in the final product, the Si-lean mixture of Fe-20 at.% Si was more favorable for the formation of Fe 3 Si-Al 2 O 3 composite.  For Reaction (1) with x = 1.5 and 2.0 (i.e., Fe-33.3% Si and Fe-40% Si), Figure 5a,b shows the presence of two silicide phases-Fe3Si and FeSi-along with Al2O3 and aluminum silicate. The content of aluminum silicate appeared to be larger in the case of Fe-40% On account of its large negative Gibbs free energy change (∆G o = −840.8 kJ) [24], the reaction pathways of Reaction (1) were proposed to be initiated by the aluminothermic reaction of Fe 2 O 3 + 2Al → 2Fe + Al 2 O 3 . In addition to the kinetic aspect, the reduction of Fe 2 O 3 by Al was also a thermodynamically favored reaction which supplied a large amount of heat (∆H r o = −852.3 kJ) to maintain the synthesis reaction in the SHS mode. After the initiation step, the chemical interaction between Fe and Si proceeded to produce Fe 3 Si.

Composition and Microstructure of Synthesized Products
For Reaction (1) with x = 1.5 and 2.0 (i.e., Fe-33.3% Si and Fe-40% Si), Figure 5a,b shows the presence of two silicide phases-Fe 3 Si and FeSi-along with Al 2 O 3 and aluminum silicate. The content of aluminum silicate appeared to be larger in the case of Fe-40% Si. Formation of Fe 3 Si and FeSi was justified because of coexistence of both phases in the composition range from Fe-25 to Fe-50 at.% Si. Figure 5a,b indicates that the content of FeSi relative to Fe 3 Si increases with increasing Si percentage. The strongest XRD peaks associated with FeSi and Fe 3 Si are, respectively, located at 2θ = 45.062 • (Inorganic Crystal Structure Database (ICSD) card number: 88-1298) and 45.337 • (ICSD card number: 65-0994). It is believed that the addition of Si transformed a part of the Si-lean phase Fe 3 Si to monosilicide FeSi, thus resulting in a decrease of Fe 3 Si but an increase of FeSi. As proposed by Guan et al. [15], the reaction of Fe 3 Si with additional Si generated an intermediate phase    Figure 6 presents the XRD spectrum of the product synthesized from Reaction (1) with x = 3.0 (i.e., Fe-50% Si). Apparently, monosilicide FeSi was the only silicide formed in the end product. Besides, Al 2 O 3 and a small amount of aluminum silicate were detected. It should be noted that in spite of a high Si percentage, aluminum silicide is at a much less quantity in Figure 6 when compared with that in Figure 5. This could be explained by the fact that FeSi possesses a very narrow composition range. As a result, few Si particles dissolved in Al 2 O 3 .  The SEM image of Figure 9 was associated with the FeSi-Al2O3 composite synthesized from Reaction (1)

Conclusions
Fabrication of Fe 3 Si/FeSi-Al 2 O 3 composites was conducted by the aluminothermic SHS reactions with Fe 2 O 3 -Al-Fe-Si systems under Fe/Si stoichiometry from Fe-20 to Fe-50 at.% Si. Experimental results showed that self-propagating combustion was achieved upon ignition. The increase of Si percentage lowered combustion exothermicity and thus decreased not only the combustion front temperature from 1730 to 1506 • C, but flame-front velocity from 2.7 to 1.6 mm/s. Both Fe-20 and Fe-25 at.% Si reaction systems produced Fe 3 Si-Al 2 O 3 composites and the off-stoichiometric case with a Si-lean composition of Fe-20 at.% Si yielded almost no impurity, aluminum silicate. Two silicide phases-FeSi and Fe 3 Si-were present in the final products of reaction systems with Fe-33.3 and Fe-40 at.% Si and the content of FeSi increased with increasing Si percentage. For the reaction with Fe-50 at.% Si, the phase conversion was almost completed, and the product was FeSi-Al 2 O 3 composite. SEM micrographs of the products revealed that Al 2 O 3 formed a dense and connecting substrate, over which granular Fe 3 Si and FeSi particles were uniformly distributed. The size of iron silicide particles varied in the range from submicron to 5 µm.