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Processing of single phase Mo5Si3 by microwave activated combustion synthesis
Materials Science and Engineering A323 (2002) 478– 483 www.elsevier.com/locate/msea
Processing of single phase Mo5Si3 by microwave activated combustion synthesis J.R. Jokisaari, S. Bhaduri, S.B. Bhaduri * Department of Materials and Metallurgical Engineering, Uni6ersity of Idaho, Moscow, ID 83844 -3024, USA Received 17 April 2001; received in revised form 31 July 2001
Abstract A ‘microwave activated combustion synthesis (MACS)’ process was utilized in synthesizing Mo5Si3, an important silicide material. This paper reports the following results. First, it independently confirms that metallic powders can be heated by microwave irradiation. Second, it shows that propagating combustion reactions can be sustained in sluggish reactions by means of microwave activation. Third, this paper demonstrates synthesis of pure Mo5Si3 using microwave activation. The formation of secondary phases during processing was minimized by using off-stoichiometric starting mixtures. These were prepared and processed to produce close to single phase product. A mixture of 16 wt.% Si and 84 wt.% Mo produced the best result. © 2002 Published by Elsevier Science B.V. Keywords: Microwave activated combustion synthesis (MACS); Mo5Si3; Silicides
1. Introduction Over the last decade, silicides received considerable attention for their optimum combination of strength, high temperature oxidation, and creep resistance. Among these, silicides of Mo – Si binary system (e.g. MoSi2, Mo5Si3 etc.) are of interest due to their excellent properties [1,2]. The Mo – Si binary system has three compounds, Mo3Si; MoSi2; and Mo5Si3. Of these MoSi2 has been the most popular and has been extensively studied by many researchers [3,4]. While MoSi2 has excellent oxidation resistance up to 1700 °C, its creep resistance is poor. The creep resistance of MoSi2 can be improved by alloying with other silicides, or by adding reinforcements [5,6]. A simple strategy to improve the creep resistance of MoSi2 is to make a composite with Mo5Si3, thus limiting oneself to the Mo –Si binary system [7,8]. Improved creep resistance is expected because Mo5Si3 has a higher melting point than that of MoSi2 and furthermore, Mo5Si3 belongs to a class of more complex body-centered tetragonal struc* Corresponding author. Tel.: + 1-208-885-7228; fax: +1-208-8852855. E-mail address: [email protected] (S.B. Bhaduri).
tures. However, the oxidation resistance of Mo5Si3 is poorer than that of MoSi2. Of late, a significant breakthrough has been achieved in improving the oxidation resistance of Mo5Si3 through the addition of boron [9,10]. Therefore, Mo5Si3 is becoming an important material on its own right. Mo5Si3, being a brittle material, is difficult to process either by the casting or by the mechanical working route. ‘Combustion synthesis (CS)’ is an alternate, energy efficient and well-proven technique to produce silicides [11,12]. From a thermodynamic viewpoint, a self-sustaining exothermic combustion requires reaction products with high negative enthalpy of formation (DHf,298) and moderate to high adiabatic temperature (Tad), assuming that adiabatic conditions are maintained during an experiment [11]. Although, in practice adiabatic conditions are difficult to maintain, thermodynamic calculations of Tad serve as a guideline for the potential of using a CS reaction for synthesis of advanced materials. In systems where reactions are not sufficiently exothermic to be self-sustaining, the propagation of exothermic fronts is erratic and non-uniform [4]. These reactions lead to non-uniform microstructures and properties. To avoid such problems, these sluggish reac-
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Table 1 Thermodynamic data and calculated Tad values Compound
A (J mol−1K−1)
b10−3 (J mol−1 K−2)
C105 (J mol−1 K)
DHf (kJ mol−1)
DHm (kJ mol−1)
Tm (K)
Tad (K)
MoSi2 Mo5Si3
67.44 182.3
11.90 34.84
−6.53 −11.9
130.96 307.84
74.83 56.65
2303 2463
1872 1672
tions must be further activated. Strategies for activation include application of thermal [11], mechanical [13], chemical [14] or electromagnetic energy [15,16]. This paper utilizes a ‘microwave activated combustion synthesis (MACS)’ process involving electromagnetic activation. Microwave energy has been used in triggering combustion reactions in the past. Dalton et al. [17] and Ahmad et al. [18] recognized that microwave energy can be helpful in triggering combustion reactions in the Ti and C system, as well as in a mixture of TiO2, Al and C. These reactions are highly exothermic and triggering them from local hot spots (by microwave absorption) is easy. The presence of non-metallic carbon further aids in triggering the reaction process since carbon is known to absorb microwaves well. Vaidhyanathan and Rao reported the synthesis of several disilicides in a home microwave oven using amorphous carbon as susceptor [19]. They also showed that metallic powders could be heated by microwave energy. However, combustion issues (e.g. thermodynamics) were not addressed. The concept also works in some aluminides [20]. The present paper uses the MACS process to activate a CS reaction in metallic reactants to synthesize an intermetallic compound (e.g. Mo5Si3), the formation of which is known to require activation. This paper further expands on the concept of microwave activation. There are several objectives of the paper. The first and foremost is to explore the MACS process in order to synthesize single phase Mo5Si3 from an elemental mix of Mo and Si. An activated CS process is necessary in synthesizing Mo5Si3 as reported by Zhang and Munir [21]. According to them, a stable propagating reaction took place in Mo5Si3 only when the samples were pre-heated to 500 °C (thermal activation). Subrahmanyam reported using an induction coil to initiate combustion reactions in Mo5Si3 [22]. It was not reported whether there was a stable propagation. It may be possible that there was activation by the induction coil. The second objective is to compare the MACS process with other variants of the CS process e.g. ‘thermally activated combustion synthesis (TACS)’ and conventional (unactivated) propagation. Finally, it is also the intention that compositions be carefully optimized in order to get pure Mo5Si3.
2. Thermodynamic calculations Eq. (1) represents the general thermodynamic equation for CS where the products of reaction do not melt. The formation of Mo5Si3 from elemental reactants follows this model. Calculated values for Mo5Si3 are shown in Table 1. Values for MoSi2 are also included in the calculations for the purpose of comparison. Since in the two specific cases under consideration, Tad is smaller than the product melting temperature Tm, the equation can be written as follows: − DHf,298 =
&
Tad
Cp(product) dT
(1)
298
For many compounds, the thermodynamic data required to calculate Tad are available in the literature. Table 1 shows the results of Tad calculations for the two materials in question. DHf,298 values were obtained from [23]. Cp (in J mol − 1 K − 1) is written as a+bT+ c/T 2, where the coefficients a, b and c are taken from [24]. These were confirmed using a recent version of the HSC™ thermodynamic software package (V 4.1, Outokumpu Corp., Finland). In an activated process, some slight modifications must be made to Eq. (1). In order to activate the combustion, energy must be added. This energy, in this case supplied by microwaves, is used to heat the sample prior to combustion. This changes the lower limit of the integral in Eq. (1), as well as the value for the enthalpy of reaction. Because of this, the adiabatic temperature of reaction changes with the pre-heat temperature of the sample; Table 2 shows the effect of the ignition Table 2 Effect of pre-heat temperature (T0) on Tad To (K)
Tad (K)
H (kJ mol−1 K−1)
200 300 400 500 600 700 800 900 1000 1100 1200 1300
1438 1521 1602 1680 1758 1836 1914 1992 2071 2150 2230 2311
−302.503 −297.352 −295.383 −293.307 −291.123 −288.852 −286.526 −284.186 −281.874 −279.639 −277.529 −275.592
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Fig. 1. Equilibrium phase diagram of Mo – Si binary system.
temperature on Tad, where To is the ignition (pre-heat) temperature. The literature suggests a temperature of 2000 K is required for a combustion reaction to be self-propagating. A pre-heat temperature of 1000 K brings Tad above this limit. Thus the reaction should be activated if energy sufficient to increase the temperature above 1000 K is added. 3. Experimental procedure Elemental molybdenum (99.9%, − 250 mesh) and silicon (99.5%, − 325 mesh) were procured from (Johnson Matthey Electronics, Ward Hill, MA). Initially, the two constituents were mixed in the exact stoichiometric ratio containing 15 wt.% of Si. Three more compositions containing 15.5, 16 and 16.5 wt.% of Si were also mixed in order to optimize the composition range within the stable region of the phase diagram (Fig. 1). These four compositions were designated as MS0, MS1, MS2 and MS3. The powders were ball milled for 2 h to ensure homogeneous mixing. Pellets were cold isostatically pressed (CIP’ed) at 100 MPa to a cylindrical form of 13 mm in diameter. The pellets were degassed in an Argon atmosphere at 550 °C for 3 h to remove any absorbed moisture and impurities. The samples were then ignited in a microwave furnace (MMT, Knoxville, TN). The MMT system is provided with a variable power output magnetron source capable of operating from 0 to 3 kW at 2.45 GHz. The cavity is large and ‘overmoded’ ensuring mixing of the microwave modes and producing a homogeneous temperature distribution.
The samples were contained in a container of insulating Al2O3 board and covered with alumina fiber insulation. The Al2O3 fiber mats are low density, insulating but not significantly absorptive at the operating frequency. The samples were placed on SiC susceptors, which couple microwaves effectively and thus provide pre-heating. The system is provided with a thermocouple feedback for temperature measurement. The main controller of the system recorded the temperature signals from a MoSi2 sheathed C-type thermocouple embedded into the sample through a swagelock fitting. Independently, temperature measurements were made using an optical pyrometer (Mikron, N.J.). The temperature was recorded continuously at a sample rate of 2 Hz throughout the experiment. The temperature of the sample prior to ignition and at the time of the ignition (maximum temperature) were also recorded. Multiple measurements were made through several sample runs and the average of these measurements is reported. Pellets were also ignited in a conventional reaction chamber, which was evacuated and subsequently backfilled with argon before triggering the reaction using a tungsten coil. For a thermally activated reaction, pellets were reaction sintered in a furnace at 1200 °C for 2 h in an inert atmosphere. A video camera was used to observe the propagating front in MACS as well as in the conventional process. The video recording was further digitized and analyzed using commercially available multimedia software such as Corel Photopaint™. X-ray diffraction (XRD) patterns of the products were recorded by a Siemens diffractometer (D5000)
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using Cu –Ka radiation operating at 40 kV and 30 mA. The microstructures of the as-polished and etched samples were examined by a scanning electron microscope (SEM model AMRAY 1830) which was equipped with an energy dispersive X-ray analyzer. At selected sites on the samples, the local composition was examined using energy dispersive analyses (EDAX).
4. Results and discussion The thermodynamic calculations indicate that the formation of MoSi2 is more exothermic than that of Mo5Si3. Zhang and Munir [21] reported that the Tad value for Mo5Si3 is 1725 K while Subrahmanyam [22] obtained a value of 1739 K. We obtained a somewhat lower value of 1672 K. It is known that a reaction is not going to be self-propagating if the Tad value is less
Fig. 2. (a) Wave propagation during microwave activated synthesis (Mo– 15wt.%Si). (b) Wave propagation during conventional CS reaction, (Mo– 15wt.%Si).
Fig. 3. Time – temperature profile of Mo5Si3 phase formation.
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than 2000 K [11,12]. From this viewpoint, MoSi2 is at the borderline of being self-propagating while Mo5Si3 is below this limit. In spite of the differences in the calculations, it is clear that a self-propagating reaction should not take place in Mo5Si3 under normal circumstances. Previous experimental results validate this finding [21]. Thus, the efficacy of the MACS process is tested in this case. Fig. 2a and b show propagating reactions in the cases of MACS and the conventional reaction scenario using the MS0 composition. In both of these figures, the brighter zone indicates the higher temperature area, where the combustion has taken place or is taking place. The dark area ahead of it indicates the virgin material. In Fig. 2a, one front seems to be propagating at an angle 45° from the bottom while another one moves radially from the center to the outer edge. Thus, it is believed that ignition took place at two different sites. This can be explained by recalling the fact that the pellet was placed on a SiC susceptor which coupled microwaves initially and triggered the reaction. Also, because of the peculiarity of microwave absorption, the interior was heated faster and to a higher temperature than the exterior surfaces because of radiation cooling of the exterior. Therefore, the second initiation point was within the interior of the sample. At some point of time, the two fronts met and engulfed the entire sample. The propagation of these fronts was uniform, because the microwaves continuously supply energy to the fronts. Combustion was complete approximately 4 s after ignition. Temperature measurements indicate that the sample reached a temperature of 2073 K (1800 °C) (Fig. 3). This is consistent with the thermodynamic calculations and predictions (Table 2). Therefore, the microwave energy provided is sufficient to pre-heat the compacts to approximately 1000 K, making the reaction selfpropagating. The presence of two fronts (as seen in Fig. 2a) is the reason why temperature measurements show an initial peak followed by a second shallower peak. In conventional CS, as represented in Fig. 2b, the ignition took place at the top and the propagation proceeded from the top to bottom. However, the reaction front was non-uniform and chaotic as shown in the figure. The products of different processing routes, MACS, TACS and conventional CS, differ considerably. Fig. 4 compares XRD data from three samples of MS0 composition processed by MACS, TACS, and conventional CS techniques. Analysis of conventional CS samples shows the presence of several secondary phases. Both MoSi2 and unreacted Mo are present. The presence of Mo was a result of incomplete reaction, where MoSi2 formed before Mo5Si3 and the reaction ceased with free Mo remaining. Analyses of thermally activated samples show the presence of both unreacted Si and Mo, with
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According to Fig. 1, Mo5Si3 is stable between 15 and 16.5 wt.% of silicon content, 15 wt.% being the stoichiometric composition. In combustion synthesized samples, the relative amounts of secondary phases change with increasing silicon content. Samples MS0– MS3 were prepared and processed by the MACS process in order to determine the optimum composition to produce a single-phase compound. Fig. 6 compares XRD data for each of these compositions. The only secondary phase present in MS2 is a trace amount of Mo. The amount of other phases increases both above and below 16 wt.% Si in pure Mo5Si3. Therefore, a composition of 16 wt.% Si and 84 wt.% Mo was found to produce the purest product.
5. Conclusions It was shown that the MACS process successfully activated and allowed for stable propagation of combustion fronts in elemental mixtures of Mo and Si. These metallic constituents couple microwaves and with some pre-heating, a propagating combustion reaction ensues. The present results compare well with previous
Fig. 4. Comparison of Mo5Si3 produced by different methods.
the Si being present as spherical droplets and the Mo appearing as bright regions within the Mo5Si3 grains. MACS samples show a nearly pure-phase product with only a small amount of Mo as an impurity, but with no MoSi2 present. Fig. 5a and b compare SEM micrographs for samples produced by the MACS and TACS methods, respectively. The MACS sample shows a more uniform microstructure that its TACS counterpart. Fig. 5b depicts pure Si particles (black sphere) surrounded by bright Mo– Si alloy of 4 wt.% Si as determined by EDAX (not shown). These unreacted particles are found throughout the microstructure. The presence of both elemental Si and Mo indicates that the reaction is incomplete, after 2 h at 1200 °C.
Fig. 5. (a) MACS Mo5Si3 shows a uniform, porous microstructure. (b) TACS sample shows inhomogeniety in the microstructure (EDAX pattern is shown).
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microwaves add sufficient energy to allow the reaction to be self-propagating. Conventional CS and TACS were also attempted, and the results compared with the MACS process. Under CS, stable combustion could not be maintained. The combustion front and correspondingly the product was non-uniform and contained large quantities of MoSi2, Mo, and Si. The TACS process allowed combustion to occur, but the product was not of the same quality as that achieved through the MACS process. Specifically significant amounts of unreacted constituents (e.g. Mo, Si) as well as secondary phases such as MoSi2 were found interspersed throughout the microstructure along with the primary Mo5Si3 phase.
Acknowledgements This work was partially supported by NSF-DMII 9800009.
References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] Fig. 6. Comparison of several off-stoichiometric mixtures of Mo5Si3 within its stable range. A mixture containing 16 wt.% Si produced the purest Mo5Si3 phase.
results. The maximum temperature recorded during the chemical reaction was 1800 °C (2073 K). This is above the calculated adiabatic temperature, and indicates that
[19] [20] [21] [22] [23] [24]
A.K. Vasudevan, J.J. Petrovic, Mater. Sci. Eng. A155 (1992) 1. J.J. Petrovic, MRS Bull. 18 (1993) 35. R.M. Aikin, Scr. Metall. Mater. 26 (1992) 1025. R. Radhakrishnan, S.B. Bhaduri, C.H. Henager Jr, J.O.M. 49 (1997) 41. W.S. Gibbs, J.J. Petrovic, R.E. Honnell, Ceram. Eng. Sci. Proc. 8 (1987) 645. J.J. Petrovic, R.E. Honnell, Ceram. Eng. Sci. Proc. 11 (1990) 734. R. Gibala, A.K. Ghosh, D.C. van Aken, D.J. Srolovitz, A. Basu, H. Chang, D.P. Mason, W. Yang, Mater. Sci. Eng. A155 (1992) 1201. D.P. Mason, D.C. van Aken, Acta Mater. 43 (1995) 1201. M.K. Meyer, M. Akinc, J. Am. Ceram. Soc. 79 (1996) 938. M.K. Meyer, M.J. Kramer, M. Akinc, Adv. Mater. 8 (1996) 85. Z.A. Munir, U. Tamburini-Anselmi, Mater. Sci. Rep. 3 (1989) 277. J.J. Moore, A. Feng, Prog. Mater. Sci. 39 (1995) 243. N.N. Thadhani, J. Appl. Phys. 76 (1994) 2129. W.-C. Lee, C.-L. Tu, C.-Y. Weng, S.-L. Chung, J. Mater. Res. 10 (1995) 774. Z.A. Munir, K.H. Ewalds, US Patent 5,380,409, 1995. Z.A. Munir, Met. Mater. Trans. 27A (1996) 2080. R.C. Dalton, I. Ahmad, D.E. Clark, Ceram. Eng. Sci. Proc. 11 (1990) 1729. I. Ahmad, R.C. Dalton, D.E. Clark, J. Microwaves Power Electromag. Energy 26 (1991) 128. B. Vaidhyanathan, K.J. Rao, J. Mater. Res. 12 (1997) 3225. S. Gedevanishvili, D. Agrawal, R. Roy, J. Mater. Sci. Lett. 15 (1999) 665. S. Zhang, Z.A. Munir, J. Mater. Sci. 26 (1991) 3685. J. Subrahmanyam, J. Mater. Res. 9 (1994) 2620. I. Barin, O. Knacke, O. Kubaschewski, Thermal Properties of Inorganic Substances, Springer, Berlin, 1977. T.G. Chart, High Temperature –High Pressures 5 (1973) 1241.
Processing of single phase Mo5Si3 by microwave activated combustion synthesis J.R. Jokisaari, S. Bhaduri, S.B. Bhaduri * Department of Materials and Metallurgical Engineering, Uni6ersity of Idaho, Moscow, ID 83844 -3024, USA Received 17 April 2001; received in revised form 31 July 2001
Abstract A ‘microwave activated combustion synthesis (MACS)’ process was utilized in synthesizing Mo5Si3, an important silicide material. This paper reports the following results. First, it independently confirms that metallic powders can be heated by microwave irradiation. Second, it shows that propagating combustion reactions can be sustained in sluggish reactions by means of microwave activation. Third, this paper demonstrates synthesis of pure Mo5Si3 using microwave activation. The formation of secondary phases during processing was minimized by using off-stoichiometric starting mixtures. These were prepared and processed to produce close to single phase product. A mixture of 16 wt.% Si and 84 wt.% Mo produced the best result. © 2002 Published by Elsevier Science B.V. Keywords: Microwave activated combustion synthesis (MACS); Mo5Si3; Silicides
1. Introduction Over the last decade, silicides received considerable attention for their optimum combination of strength, high temperature oxidation, and creep resistance. Among these, silicides of Mo – Si binary system (e.g. MoSi2, Mo5Si3 etc.) are of interest due to their excellent properties [1,2]. The Mo – Si binary system has three compounds, Mo3Si; MoSi2; and Mo5Si3. Of these MoSi2 has been the most popular and has been extensively studied by many researchers [3,4]. While MoSi2 has excellent oxidation resistance up to 1700 °C, its creep resistance is poor. The creep resistance of MoSi2 can be improved by alloying with other silicides, or by adding reinforcements [5,6]. A simple strategy to improve the creep resistance of MoSi2 is to make a composite with Mo5Si3, thus limiting oneself to the Mo –Si binary system [7,8]. Improved creep resistance is expected because Mo5Si3 has a higher melting point than that of MoSi2 and furthermore, Mo5Si3 belongs to a class of more complex body-centered tetragonal struc* Corresponding author. Tel.: + 1-208-885-7228; fax: +1-208-8852855. E-mail address: [email protected] (S.B. Bhaduri).
tures. However, the oxidation resistance of Mo5Si3 is poorer than that of MoSi2. Of late, a significant breakthrough has been achieved in improving the oxidation resistance of Mo5Si3 through the addition of boron [9,10]. Therefore, Mo5Si3 is becoming an important material on its own right. Mo5Si3, being a brittle material, is difficult to process either by the casting or by the mechanical working route. ‘Combustion synthesis (CS)’ is an alternate, energy efficient and well-proven technique to produce silicides [11,12]. From a thermodynamic viewpoint, a self-sustaining exothermic combustion requires reaction products with high negative enthalpy of formation (DHf,298) and moderate to high adiabatic temperature (Tad), assuming that adiabatic conditions are maintained during an experiment [11]. Although, in practice adiabatic conditions are difficult to maintain, thermodynamic calculations of Tad serve as a guideline for the potential of using a CS reaction for synthesis of advanced materials. In systems where reactions are not sufficiently exothermic to be self-sustaining, the propagation of exothermic fronts is erratic and non-uniform [4]. These reactions lead to non-uniform microstructures and properties. To avoid such problems, these sluggish reac-
0921-5093/02/$ - see front matter © 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 7 7 4 - 9
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Table 1 Thermodynamic data and calculated Tad values Compound
A (J mol−1K−1)
b10−3 (J mol−1 K−2)
C105 (J mol−1 K)
DHf (kJ mol−1)
DHm (kJ mol−1)
Tm (K)
Tad (K)
MoSi2 Mo5Si3
67.44 182.3
11.90 34.84
−6.53 −11.9
130.96 307.84
74.83 56.65
2303 2463
1872 1672
tions must be further activated. Strategies for activation include application of thermal [11], mechanical [13], chemical [14] or electromagnetic energy [15,16]. This paper utilizes a ‘microwave activated combustion synthesis (MACS)’ process involving electromagnetic activation. Microwave energy has been used in triggering combustion reactions in the past. Dalton et al. [17] and Ahmad et al. [18] recognized that microwave energy can be helpful in triggering combustion reactions in the Ti and C system, as well as in a mixture of TiO2, Al and C. These reactions are highly exothermic and triggering them from local hot spots (by microwave absorption) is easy. The presence of non-metallic carbon further aids in triggering the reaction process since carbon is known to absorb microwaves well. Vaidhyanathan and Rao reported the synthesis of several disilicides in a home microwave oven using amorphous carbon as susceptor [19]. They also showed that metallic powders could be heated by microwave energy. However, combustion issues (e.g. thermodynamics) were not addressed. The concept also works in some aluminides [20]. The present paper uses the MACS process to activate a CS reaction in metallic reactants to synthesize an intermetallic compound (e.g. Mo5Si3), the formation of which is known to require activation. This paper further expands on the concept of microwave activation. There are several objectives of the paper. The first and foremost is to explore the MACS process in order to synthesize single phase Mo5Si3 from an elemental mix of Mo and Si. An activated CS process is necessary in synthesizing Mo5Si3 as reported by Zhang and Munir [21]. According to them, a stable propagating reaction took place in Mo5Si3 only when the samples were pre-heated to 500 °C (thermal activation). Subrahmanyam reported using an induction coil to initiate combustion reactions in Mo5Si3 [22]. It was not reported whether there was a stable propagation. It may be possible that there was activation by the induction coil. The second objective is to compare the MACS process with other variants of the CS process e.g. ‘thermally activated combustion synthesis (TACS)’ and conventional (unactivated) propagation. Finally, it is also the intention that compositions be carefully optimized in order to get pure Mo5Si3.
2. Thermodynamic calculations Eq. (1) represents the general thermodynamic equation for CS where the products of reaction do not melt. The formation of Mo5Si3 from elemental reactants follows this model. Calculated values for Mo5Si3 are shown in Table 1. Values for MoSi2 are also included in the calculations for the purpose of comparison. Since in the two specific cases under consideration, Tad is smaller than the product melting temperature Tm, the equation can be written as follows: − DHf,298 =
&
Tad
Cp(product) dT
(1)
298
For many compounds, the thermodynamic data required to calculate Tad are available in the literature. Table 1 shows the results of Tad calculations for the two materials in question. DHf,298 values were obtained from [23]. Cp (in J mol − 1 K − 1) is written as a+bT+ c/T 2, where the coefficients a, b and c are taken from [24]. These were confirmed using a recent version of the HSC™ thermodynamic software package (V 4.1, Outokumpu Corp., Finland). In an activated process, some slight modifications must be made to Eq. (1). In order to activate the combustion, energy must be added. This energy, in this case supplied by microwaves, is used to heat the sample prior to combustion. This changes the lower limit of the integral in Eq. (1), as well as the value for the enthalpy of reaction. Because of this, the adiabatic temperature of reaction changes with the pre-heat temperature of the sample; Table 2 shows the effect of the ignition Table 2 Effect of pre-heat temperature (T0) on Tad To (K)
Tad (K)
H (kJ mol−1 K−1)
200 300 400 500 600 700 800 900 1000 1100 1200 1300
1438 1521 1602 1680 1758 1836 1914 1992 2071 2150 2230 2311
−302.503 −297.352 −295.383 −293.307 −291.123 −288.852 −286.526 −284.186 −281.874 −279.639 −277.529 −275.592
480
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Fig. 1. Equilibrium phase diagram of Mo – Si binary system.
temperature on Tad, where To is the ignition (pre-heat) temperature. The literature suggests a temperature of 2000 K is required for a combustion reaction to be self-propagating. A pre-heat temperature of 1000 K brings Tad above this limit. Thus the reaction should be activated if energy sufficient to increase the temperature above 1000 K is added. 3. Experimental procedure Elemental molybdenum (99.9%, − 250 mesh) and silicon (99.5%, − 325 mesh) were procured from (Johnson Matthey Electronics, Ward Hill, MA). Initially, the two constituents were mixed in the exact stoichiometric ratio containing 15 wt.% of Si. Three more compositions containing 15.5, 16 and 16.5 wt.% of Si were also mixed in order to optimize the composition range within the stable region of the phase diagram (Fig. 1). These four compositions were designated as MS0, MS1, MS2 and MS3. The powders were ball milled for 2 h to ensure homogeneous mixing. Pellets were cold isostatically pressed (CIP’ed) at 100 MPa to a cylindrical form of 13 mm in diameter. The pellets were degassed in an Argon atmosphere at 550 °C for 3 h to remove any absorbed moisture and impurities. The samples were then ignited in a microwave furnace (MMT, Knoxville, TN). The MMT system is provided with a variable power output magnetron source capable of operating from 0 to 3 kW at 2.45 GHz. The cavity is large and ‘overmoded’ ensuring mixing of the microwave modes and producing a homogeneous temperature distribution.
The samples were contained in a container of insulating Al2O3 board and covered with alumina fiber insulation. The Al2O3 fiber mats are low density, insulating but not significantly absorptive at the operating frequency. The samples were placed on SiC susceptors, which couple microwaves effectively and thus provide pre-heating. The system is provided with a thermocouple feedback for temperature measurement. The main controller of the system recorded the temperature signals from a MoSi2 sheathed C-type thermocouple embedded into the sample through a swagelock fitting. Independently, temperature measurements were made using an optical pyrometer (Mikron, N.J.). The temperature was recorded continuously at a sample rate of 2 Hz throughout the experiment. The temperature of the sample prior to ignition and at the time of the ignition (maximum temperature) were also recorded. Multiple measurements were made through several sample runs and the average of these measurements is reported. Pellets were also ignited in a conventional reaction chamber, which was evacuated and subsequently backfilled with argon before triggering the reaction using a tungsten coil. For a thermally activated reaction, pellets were reaction sintered in a furnace at 1200 °C for 2 h in an inert atmosphere. A video camera was used to observe the propagating front in MACS as well as in the conventional process. The video recording was further digitized and analyzed using commercially available multimedia software such as Corel Photopaint™. X-ray diffraction (XRD) patterns of the products were recorded by a Siemens diffractometer (D5000)
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using Cu –Ka radiation operating at 40 kV and 30 mA. The microstructures of the as-polished and etched samples were examined by a scanning electron microscope (SEM model AMRAY 1830) which was equipped with an energy dispersive X-ray analyzer. At selected sites on the samples, the local composition was examined using energy dispersive analyses (EDAX).
4. Results and discussion The thermodynamic calculations indicate that the formation of MoSi2 is more exothermic than that of Mo5Si3. Zhang and Munir [21] reported that the Tad value for Mo5Si3 is 1725 K while Subrahmanyam [22] obtained a value of 1739 K. We obtained a somewhat lower value of 1672 K. It is known that a reaction is not going to be self-propagating if the Tad value is less
Fig. 2. (a) Wave propagation during microwave activated synthesis (Mo– 15wt.%Si). (b) Wave propagation during conventional CS reaction, (Mo– 15wt.%Si).
Fig. 3. Time – temperature profile of Mo5Si3 phase formation.
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than 2000 K [11,12]. From this viewpoint, MoSi2 is at the borderline of being self-propagating while Mo5Si3 is below this limit. In spite of the differences in the calculations, it is clear that a self-propagating reaction should not take place in Mo5Si3 under normal circumstances. Previous experimental results validate this finding [21]. Thus, the efficacy of the MACS process is tested in this case. Fig. 2a and b show propagating reactions in the cases of MACS and the conventional reaction scenario using the MS0 composition. In both of these figures, the brighter zone indicates the higher temperature area, where the combustion has taken place or is taking place. The dark area ahead of it indicates the virgin material. In Fig. 2a, one front seems to be propagating at an angle 45° from the bottom while another one moves radially from the center to the outer edge. Thus, it is believed that ignition took place at two different sites. This can be explained by recalling the fact that the pellet was placed on a SiC susceptor which coupled microwaves initially and triggered the reaction. Also, because of the peculiarity of microwave absorption, the interior was heated faster and to a higher temperature than the exterior surfaces because of radiation cooling of the exterior. Therefore, the second initiation point was within the interior of the sample. At some point of time, the two fronts met and engulfed the entire sample. The propagation of these fronts was uniform, because the microwaves continuously supply energy to the fronts. Combustion was complete approximately 4 s after ignition. Temperature measurements indicate that the sample reached a temperature of 2073 K (1800 °C) (Fig. 3). This is consistent with the thermodynamic calculations and predictions (Table 2). Therefore, the microwave energy provided is sufficient to pre-heat the compacts to approximately 1000 K, making the reaction selfpropagating. The presence of two fronts (as seen in Fig. 2a) is the reason why temperature measurements show an initial peak followed by a second shallower peak. In conventional CS, as represented in Fig. 2b, the ignition took place at the top and the propagation proceeded from the top to bottom. However, the reaction front was non-uniform and chaotic as shown in the figure. The products of different processing routes, MACS, TACS and conventional CS, differ considerably. Fig. 4 compares XRD data from three samples of MS0 composition processed by MACS, TACS, and conventional CS techniques. Analysis of conventional CS samples shows the presence of several secondary phases. Both MoSi2 and unreacted Mo are present. The presence of Mo was a result of incomplete reaction, where MoSi2 formed before Mo5Si3 and the reaction ceased with free Mo remaining. Analyses of thermally activated samples show the presence of both unreacted Si and Mo, with
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According to Fig. 1, Mo5Si3 is stable between 15 and 16.5 wt.% of silicon content, 15 wt.% being the stoichiometric composition. In combustion synthesized samples, the relative amounts of secondary phases change with increasing silicon content. Samples MS0– MS3 were prepared and processed by the MACS process in order to determine the optimum composition to produce a single-phase compound. Fig. 6 compares XRD data for each of these compositions. The only secondary phase present in MS2 is a trace amount of Mo. The amount of other phases increases both above and below 16 wt.% Si in pure Mo5Si3. Therefore, a composition of 16 wt.% Si and 84 wt.% Mo was found to produce the purest product.
5. Conclusions It was shown that the MACS process successfully activated and allowed for stable propagation of combustion fronts in elemental mixtures of Mo and Si. These metallic constituents couple microwaves and with some pre-heating, a propagating combustion reaction ensues. The present results compare well with previous
Fig. 4. Comparison of Mo5Si3 produced by different methods.
the Si being present as spherical droplets and the Mo appearing as bright regions within the Mo5Si3 grains. MACS samples show a nearly pure-phase product with only a small amount of Mo as an impurity, but with no MoSi2 present. Fig. 5a and b compare SEM micrographs for samples produced by the MACS and TACS methods, respectively. The MACS sample shows a more uniform microstructure that its TACS counterpart. Fig. 5b depicts pure Si particles (black sphere) surrounded by bright Mo– Si alloy of 4 wt.% Si as determined by EDAX (not shown). These unreacted particles are found throughout the microstructure. The presence of both elemental Si and Mo indicates that the reaction is incomplete, after 2 h at 1200 °C.
Fig. 5. (a) MACS Mo5Si3 shows a uniform, porous microstructure. (b) TACS sample shows inhomogeniety in the microstructure (EDAX pattern is shown).
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microwaves add sufficient energy to allow the reaction to be self-propagating. Conventional CS and TACS were also attempted, and the results compared with the MACS process. Under CS, stable combustion could not be maintained. The combustion front and correspondingly the product was non-uniform and contained large quantities of MoSi2, Mo, and Si. The TACS process allowed combustion to occur, but the product was not of the same quality as that achieved through the MACS process. Specifically significant amounts of unreacted constituents (e.g. Mo, Si) as well as secondary phases such as MoSi2 were found interspersed throughout the microstructure along with the primary Mo5Si3 phase.
Acknowledgements This work was partially supported by NSF-DMII 9800009.
References [1] [2] [3] [4] [5] [6] [7]
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] Fig. 6. Comparison of several off-stoichiometric mixtures of Mo5Si3 within its stable range. A mixture containing 16 wt.% Si produced the purest Mo5Si3 phase.
results. The maximum temperature recorded during the chemical reaction was 1800 °C (2073 K). This is above the calculated adiabatic temperature, and indicates that
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