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An experimental investigation on combustion synthesis of transition metal silicides V5Si3, Nb5Si3, and Ta5Si3
Journal of Alloys and Compounds 439 (2007) 59–66
An experimental investigation on combustion synthesis of transition metal silicides V5Si3, Nb5Si3, and Ta5Si3 C.L. Yeh ∗ , W.H. Chen Department of Mechanical and Automation Engineering, Da-Yeh University, 112 Shan-Jiau Rd., Da-Tsuen, Changhua 51505, Taiwan Received 4 August 2006; received in revised form 18 August 2006; accepted 18 August 2006 Available online 18 September 2006
Abstract Preparation of transition metal silicides V5 Si3 , Nb5 Si3 , and Ta5 Si3 was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts at their stoichiometries. The propagation mode of self-sustaining reactions, flame-front velocity, combustion temperature, and phase composition of end products were extensively studied. For the reactant compacts from the mixtures of V:Si = 5:3 and Ta:Si = 5:3, a planar flame front propagating in a steady manner was observed, with or without sample preheating. On the other hand, a preheating temperature of 200 ◦ C or above was required by the Nb–Si powder compact to initiate the self-sustaining combustion. Moreover, it was found that the planar reaction font was not stable in the samples of Nb:Si = 5:3 and was transformed into a localized reaction zone propagating in a spinning mode. For all three types of the samples, as the sample density and preheating temperature increased, the combustion temperature was found to increase and the flame-front velocity was correspondingly enhanced. According to the XRD analysis, formation of single-phase silicides V5 Si3 and Nb5 Si3 from the reactant compacts of 5V + 3Si and 5Nb + 3Si, respectively, was confirmed. The samples of 5Ta + 3Si yielded predominantly the silicide Ta5 Si3 , along with an intermediate phase Ta2 Si in a small amount. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of V5 Si3 , Nb5 Si3 , and Ta5 Si3 were determined to be 111.5, 259.2, and 105.2 kJ/mol, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Transition metal silicide; V5 Si3 ; Nb5 Si3 ; Ta5 Si3 ; SHS; Activation energy
1. Introduction Transition metal silicides have been of considerable interest as high-temperature structural materials, because of their stability and resistance to oxidation, high melting points, good creep tolerance, low densities, and excellent mechanical strength at elevated temperatures [1–3]. Among the silicides of transition metals, the Mo–Si and Ti–Si compounds have been widely investigated [3–9]. Other silicides, such as those of the transition metals in the VB group (including V, Nb, and Ta) have received relatively little attention. In the V–Si phase diagram shown in Fig. 1(a), four silicides are present: V3 Si, V5 Si3 , V6 Si5 , and VSi2 [10]. With the exception of V6 Si5 , all the compounds show a congruent melting point. The V5 Si3 phase has the highest melting point at 2010 ◦ C. Silicide compounds in the systems
∗
Corresponding author. Tel.: +886 4 8511888x2118; fax: +886 4 8511215. E-mail address: [email protected] (C.L. Yeh).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.075
of Nb–Si and Ta–Si are among the most refractory metal silicides. In the Nb–Si system, as indicated in Fig. 1(b), there are three silicides of niobium: Nb3 Si, Nb5 Si3 , and NbSi2 [11]. The Nb5 Si3 has the highest melting point (2515 ◦ C) among those of the 5:3 transition metal silicides with densities below that of nickel-based superalloys [12]. The phase diagram of the Ta–Si system in Fig. 1(c) shows the presence of four intermetallic compounds (Ta3 Si, Ta2 Si, Ta5 Si3 , and TaSi2 ) with melting points in the range of 2200–2500 ◦ C [13]. Transition metal silicides and their composites have been produced by a wide variety of powder processing techniques, often with two or more in combination, including conventional arcmelting and casting, mechanical alloying, hot pressing, reaction sintering, plasma spraying, self-propagating high-temperature synthesis (SHS), and solid-state displacement reactions [5,14]. With the advantages of time saving, high energy efficiency, and good purification capability, the SHS technique has been considered as an alternative route to the conventional methods of producing advanced materials, including carbides, borides, nitrides,
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Fig. 1. Phase diagrams for: (a) V–Si system [10], (b) Nb–Si system [11], and (c) Ta–Si system [13].
silicides, hydrides, intermetallics, chalcogenides, etc. [15–18]. By utilizing a modification of the classical SHS approach, called field-activated combustion synthesis (FACS), some of the metal silicides in the systems of V–Si and Nb–Si were prepared [19,20]. In the absence of an electrical field, only the compacts with a composition V:Si = 5:3 reached self-propagating reactions [19]. Under the field values of 14.8–18.5 V/cm, combustion front along a sample of Nb:Si = 5:3 propagated in a steady mode and the final product was identified as a mixture of ␣-Nb5 Si3 and -Nb5 Si3 [20]. For the Ta–Si compounds, combustion synthesis of TaSi2 and Ta5 Si3 was conducted by Maglia et al. [21] using the reactant compacts from mechanically activated powders. The product composition was found to vary with starting stoichiometry and milling time [21]. The objective of the present investigation was to perform a comparative study on preparation of transition metal silicides V5 Si3 , Nb5 Si3 , and Ta5 Si3 by the classical SHS method using compacted samples from elemental powder mixtures. The dependence of combustion characteristics, including the propagation mode of the self-sustaining combustion, flame-front velocity, and reaction temperature, on the sample green density,
initial sample temperature, and starting composition of the reactant compact was explored. The phase composition of as-synthesized products was characterized by the XRD analysis. In addition, the activation energy associated with the synthesis reaction was determined by correlating the experimental data acquired in this study. 2. Experimental methods of approach Particles of the transition metals (Me) in the VB group, including vanadium (Aldrich Chemical, 99.5% purity), niobium (Strem Chemicals, 99.8% purity), and tantalum (Aldrich Chemical, 99.9% purity), were used to mix with silicon (ProChem Inc., 99.5% purity) powders as the starting materials in this study. All of the constituent powders had a size specification of −325 mesh. Powder blends were prepared at a stoichiometric ratio Me:Si = 5:3 by dry mixing in a ball mill and then were cold-pressed into cylindrical test samples with a diameter of 7 mm and a height of 12 mm. The V–Si and Nb–Si powder compacts were formed with a compaction density between 50 and 65% of the theoretical maximum density (TMD) of the reactant mixture. Due to the relatively high density of Ta (16.6 g/cm3 ) in comparison to those of V (6.1 g/cm3 ) and Nb (8.57 g/cm3 ), the green density of the Ta–Si compact was in the range from 35 to 45% TMD. The SHS experiments were conducted in a stainless-steel windowed combustion chamber under an atmosphere of high purity argon (99.99%). The sample
C.L. Yeh, W.H. Chen / Journal of Alloys and Compounds 439 (2007) 59–66 holder was equipped with a 600 W cartridge heater used to raise the initial temperature of the test sample prior to ignition. Reaction was normally initiated by a heated tungsten coil. Experiments were performed at different sample preheating temperatures (Tp ) from room temperature to 300 ◦ C. In the absence of sample preheating, only the samples of V:Si = 5:3 and Ta:Si = 5:3 reached selfsustaining reactions. The powder compacts of 5Nb + 3Si required not only a preheating temperature of 200 ◦ C or above, but also an ignition enhancer made up of titanium (Ti) and carbon black (C) powders (with a ratio of Ti:C = 1:1) to ensure a self-propagating combustion wave throughout the entire compact [22]. Detailed description of the experimental set-up and measurement approach was given previously [23,24].
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3. Results and discussion 3.1. Observation of combustion characteristics Typical SHS processes recorded from the V–Si, Nb–Si, and Ta–Si powder compacts are respectively illustrated in Fig. 2(a–c). Upon ignition, as shown in Fig. 2(a), a distinct combustion front develops on a sample of 5V + 3Si and propagates in a steady and self-sustaining manner. After the flame
Fig. 2. Recorded combustion images of SHS processes associated with: (a) a 50% TMD V–Si sample at Tp = 100 ◦ C, (b) a 55% TMD Nb–Si sample at Tp = 300 ◦ C, and (c) a 35% TMD Ta–Si sample at Tp = 200 ◦ C.
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front reaches to the bottom of the sample at about t = 1.30 s, Fig. 2(a) reveals that the luminosity on the burned sample is gradually reduced. Instead of a constant losing of the brightness, however, a noticeable afterglow emerges on the sample from approximately t = 2.50 s and lasts for about 1 s, which suggests further phase conversion taking place volumetrically. According to Merzhanov [15], formation of metastable intermediate compounds, which subsequently convert into end products, could be responsible for the afterburning glow observed in the solid-phase SHS process. For the reactant compact of Nb:Si = 5:3, Fig. 2(b) indicates that shortly after the ignition the planar combustion front is transformed into a localized reaction zone moving along a spiral trajectory on the sample surface. It is believed that the heat generated from the Ti–C igniter pellet along with the thermal energy liberated from the Nb–Si reaction within the combustion zone support the steady propagation of a planar front before the appearance of a spinning combustion wave. With the fading of the heat input from the ignition enhancer, combustion becomes self-sustained. As proposed by Ivleva and Merzhanov [25], once the heat flux generated from the self-sustaining combustion is no longer sufficient to
maintain the steady propagation of a planar front, the flame front forms one or several localized reaction zones. Due to progression of the spinning combustion wave, spiral marks left on the surface of the burned sample can be clearly seen in Fig. 2(b). Furthermore, similar to that observed in Fig. 2(a) for the V–Si sample, Fig. 2(b) shows evidence of afterburning reactions starting roughly from t = 2.10 s in the Nb–Si powder compact. The SHS process of the test specimen with an initial stoichiometry Ta:Si = 5:3 is presented in Fig. 2(c), which depicts a steady combustion wave travelling in a nearly parallel fashion. As also revealed in Fig. 2(c), the burned region behind the reaction front continues to glow incandescently, implying that the reaction is not only restricted to the combustion front, but also continues vigorously in bulk. When compared with test samples of the other two compositions (i.e., V–Si and Nb–Si powder compacts), the propagation rate of the reaction front associated with the Ta–Si compact appears to be higher. As indicated in Fig. 2(c), it took only about 0.73 s for the combustion wave to arrive at the bottom of the sample. In addition, it is useful to note that no afterburning reactions were observed after the passage of the flame front. According to Merzhanov [15], the lack of suffi-
Fig. 3. Effects of initial sample density and preheating temperature on flame-front velocity of sample compacts with starting stoichiometries of: (a) V:Si = 5:3, (b) Nb:Si = 5:3, and (c) Ta:Si = 5:3.
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cient reaction time might cause phase conversion uncompleted, thus resulting in the presence of intermediate phases in the final composition. 3.2. Measurement of flame-front propagation velocity The flame-front propagation velocity (Vf ) in the longitudinal direction was deduced from the recorded sequences of combustion images and reported in Fig. 3(a–c) as a function of initial sample density and preheating temperature. First of all, it was found that the flame-front velocity increases with sample green density for all three types of the samples. The influence of sample compaction density on the propagation rate of the reaction front can be attributed to two competing phenomena [17]. As the sample density increases, the intimate contact between the reactant particles is augmented, thus enhancing the reaction and consequently increasing the propagation rate of the combustion front. On the other hand, the thermal conductivity of the reactant compact also increases with sample density, which might cause more thermal loss by conduction from the reaction zone and therefore could lead to a decrease in the reaction rate [17]. It is evident to see in Fig. 3(a–c) that the former reason dominates within the density range adopted in this study.
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Secondly, on account of the increase of reaction temperature with initial sample temperature, Fig. 3(a–c) indicates that the propagation rate of the reaction front is increased by increasing the preheating temperature. As mentioned earlier, a preheating temperature of 200 ◦ C or above was required by the compacts of Nb:Si = 5:3 to achieve self-sustaining reactions. Therefore, only the data under the conditions of Tp = 200 and 300 ◦ C were reported for the Nb–Si test specimens and plotted in Fig. 3(b). However, as shown in Fig. 3(a and c), self-propagating reactions can be completed in the compacts of 5V + 3Si and 5Ta + 3Si with no prior heating. In addition, Fig. 3(a and c) indicates that for the samples of these two types, the increase of initial sample temperature by preheating to 200 or 300 ◦ C almost doubles the reaction front velocity. Finally, a comparison between sample compacts of different compositions shows that under the same initial sample temperature the Ta–Si sample has the highest flame speed regardless of its low relative density, whereas because of the reaction zone propagating in a spinning mode the Nb–Si powder compact exhibits the slowest propagation rate in the longitudinal direction. The velocity of the reaction front associated with the sample of V:Si = 5:3 was found slightly lower than that of the Ta–Si powder compact.
Fig. 4. Effects of initial sample density and preheating temperature on combustion temperature of sample compacts with starting stoichiometries of: (a) V:Si = 5:3, (b) Nb:Si = 5:3, and (c) Ta:Si = 5:3.
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Fig. 5. Relation between flame-front velocity and combustion temperature for determination of activation energy associated with combustion synthesis of: (a) V5 Si3 , (b) Nb5 Si3 , and (c) Ta5 Si3 .
3.3. Measurement of combustion temperature Typical temperature profiles detected in the self-sustaining combustion regime from the burning samples with starting compositions of V:Si = 5:3, Nb:Si = 5:3, and Ta:Si = 5:3 are plotted in Fig. 4(a–c), respectively. As illustrated in Fig. 4(a–c), the abrupt rise in the temperature profile represents the rapid arrival of the reaction front and the peak value signifies the flame-front temperature, which generally increases with either sample green density or preheating temperature. For example, Fig. 4(a) indicates an obvious increase in the peak reaction temperature from about 1190 ◦ C for a 50% TMD V–Si compact at Tp = 100 ◦ C to 1425 ◦ C in the case of 60% TMD and Tp = 200 ◦ C. For the powder compacts of Nb:Si = 5:3, it was found that the influence of sample density and preheating temperature on the reaction temperature was not as pronounced as that observed in the test specimens of V:Si = 5:3. Peak reaction temperatures measured from the Nb–Si samples were found to vary between 1600 and 1700 ◦ C, as shown in Fig. 4(b). On the contrary, as can be seen in Fig. 4(c), the samples of Ta:Si = 5:3 exhibit great dependence of combustion temperature on sample density and
preheating temperature. The flame-front temperature in the range from 1250 to 1680 ◦ C was observed for the 5Ta + 3Si compacts under different test conditions. The apparent activation energy of a self-sustained reaction can be determined by means of constructing the dependence of reaction front velocity on combustion temperature. A simplified relation used is expressed in the following [22–24]:
Vf Tc
2
= f (n)
R Ea
Ea K exp − RTc
(1)
where Vf is the velocity of combustion front, Tc the combustion temperature, Ea the activation energy of the reaction, R the universal gas constant, f(n) a function of the kinetic order of the reaction, and K is a constant which includes the heat capacity of the product, thermal conductivity, and the heat of reaction. According to Eq. (1), the slope of a straight line correlating ln(Vf /Tc )2 with 1/Tc can provide the apparent activation energy of the reaction process. Based upon the slopes of best-fitted lines for the data plotted in Fig. 5(a–c), the activation energy with values of 111.5, 259.2, and 105.2 kJ/mol was obtained for combustion synthesis of V5 Si3 , Nb5 Si3 , and Ta5 Si3 , respectively. In
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Fig. 6. Typical XRD patterns of combustion products obtained from sample compacts with starting stoichiometries of: (a) V:Si = 5:3, (b) Nb:Si = 5:3, and (c) Ta:Si = 5:3.
comparison to those of V5 Si3 and Ta5 Si3 , the activation energy required to form Nb5 Si3 is substantially higher, which could explain the need of prior heating and an ignition enhancer to initiate the reactions in the Nb–Si powder compact.
3.4. Composition analysis of combustion products The phase composition of as-synthesized products was identified by XRD analysis. No variation of the product composition is shown with initial sample density and temperature. The XRD pattern plotted in Fig. 6(a) represents the expected phase composition of end products synthesized from the samples of 5V + 3Si. Formation of a single-phase silicide V5 Si3 in both tetragonal and hexagonal forms was identified. For the Nb–Si powder compacts, Fig. 6(b) indicates that combustion yields a monolithic silicide Nb5 Si3 existing in two tetragonal modifications (␣ and  forms). However, as shown in Fig. 6(c), the reaction product obtained from the compact of Ta:Si = 5:3 consists of Ta5 Si3 as the dominant phase and a lesser amount of Ta2 Si as the intermediate phase. The presence of Ta2 Si in the final product from a 5Ta + 3Si compact could be caused by the lack of sufficient reaction time in the Ta–Si compact, as discussed earlier.
4. Conclusions This study presents an experimental investigation on preparation of the silicides of transition metals (the VB group), including V5 Si3 , Nb5 Si3 , and Ta5 Si3 , by self-propagating hightemperature synthesis from elemental powder compacts at their corresponding stoichiometries. Experimental evidence shows once initiation, planar and self-sustaining reaction fronts propagating steadily along the samples composed of V:Si = 5:3 and Ta:Si = 5:3. For the powder compacts from the mixture of Nb:Si = 5:3, however, preheating the samples at 200 ◦ C or above was required to achieve self-sustaining reactions which were confined to a small region travelling in a spinning mode. After the passage of the flame front, the emergence of afterburning glows resulting from further phase transformation was noticed in the V–Si and Nb–Si powder compacts. The flame-front propagation velocity was found to increase with the sample density, because of the improvement in the contact between particles. Moreover, as a result of the increase of reaction temperature, preheating the reactant compact enhanced the reaction front velocity substantially, in particular for the samples of 5V + 3Si and 5Ta + 3Si. Under the same initial sample temperature, the Ta–Si compact has the fastest flame front. Due
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to the spinning propagation mode, the velocity of the reaction front along the Nb–Si sample is the slowest. On the basis of the dependence of reaction front velocity on combustion temperature, the activation energy with values of 111.5, 259.2, and 105.2 kJ/mol was deduced for combustion synthesis of V5 Si3 , Nb5 Si3 , and Ta5 Si3 , respectively. The XRD analysis on the synthesized products verifies complete conversion from the powder compacts of V:Si = 5:3 and Nb:Si = 5:3 to single-phase compounds. Silicide V5 Si3 in both tetragonal and hexagonal forms was yielded from the 5V + 3Si sample. Monolithic silicide Nb5 Si3 existing in two tetragonal modifications (␣ and  forms) was obtained from the 5Nb + 3Si sample. However, in addition to formation of Ta5 Si3 as the dominant phase, the presence of an intermediate phase Ta2 Si was detected in the final product from the 5Ta + 3Si compact. Acknowledgement This research was sponsored by the National Science Council of Taiwan, ROC, under the grants of NSC 95-2221-E-212-053MY2. References [1] P.J. Meschter, D.S. Schwartz, JOM 41 (1989) 52–55. [2] D.M. Shah, D. Berczik, D.L. Anton, R. Hecht, Mater. Sci. Eng. A155 (1992) 45–57.
[3] J.J. Petrovic, A.K. Vasudevan, Mater. Sci. Eng. A261 (1999) 1–5. [4] A.K. Vasud´evan, J.J. Petrovic, Mater. Sci. Eng. A155 (1992) 1–17. [5] Y.L. Jeng, E.J. Lavernia, J. Mater. Sci. 29 (1994) 2557– 2571. [6] S.C. Deevi, N.N. Thadhani, Mater. Sci. Eng. A192/193 (1995) 604– 611. [7] L.L. Wang, Z.A. Munir, Metall. Mater. Trans. B 26 (1995) 595–601. [8] I.J. Shon, H.C. Kim, D.H. Rho, Z.A. Munir, Mater. Sci. Eng. A269 (1999) 129–135. [9] J. Trambukis, Z.A. Munir, J. Am. Ceram. Soc. 73 (5) (1990) 1240– 1245. [10] J.F. Smith, Bull. Alloy Phase Diag. 6 (1985) 266. [11] M.E. Schlesinger, H. Okamoto, A.B. Gokhale, R. Abbaschian, J. Phase Equilib. 14 (1993) 502–509. [12] J.D. Rigney, P.M. Singh, J.I. Lewandowski, JOM 44 (8) (1992) 36– 41. [13] M.E. Schlesinger, Chem. Rev. 90 (4) (1990) 607–628. [14] N.S. Stoloff, Mater. Sci. Eng. A261 (1999) 169–180. [15] A.G. Merzhanov, Combust. Sci. Technol. 98 (1994) 307–336. [16] Z.A. Munir, U. Anselmi-Tamburini, Mater. Sci. Rep. 3 (1989) 277–365. [17] A. Varma, J.P. Lebrat, Chem. Eng. Sci. 47 (1992) 2179–2194. [18] P. Mossino, Ceram. Int. 30 (2004) 311–332. [19] F. Maglia, U. Anselmi-Tamburini, C. Milanese, N. Bertolino, Z.A. Munir, J. Alloys Compd. 319 (2001) 108–118. [20] S. Gedevanishvili, Z.A. Munir, Mater. Sci. Eng. A211 (1996) 1–9. [21] F. Maglia, C. Milanese, U. Anselmi-Tamburini, S. Doppiu, G. Cocco, Z.A. Munir, J. Alloys Compd. 385 (2004) 269–275. [22] C.L. Yeh, W.H. Chen, J. Alloys Compd. 402 (2005) 118–123. [23] C.L. Yeh, C.C. Yeh, J. Alloys Compd. 388 (2005) 241–249. [24] C.L. Yeh, W.Y. Sung, J. Alloys Compd. 384 (2004) 181–191. [25] T.P. Ivleva, A.G. Merzhanov, Doklady Phys. 45 (2000) 136–141.
An experimental investigation on combustion synthesis of transition metal silicides V5Si3, Nb5Si3, and Ta5Si3 C.L. Yeh ∗ , W.H. Chen Department of Mechanical and Automation Engineering, Da-Yeh University, 112 Shan-Jiau Rd., Da-Tsuen, Changhua 51505, Taiwan Received 4 August 2006; received in revised form 18 August 2006; accepted 18 August 2006 Available online 18 September 2006
Abstract Preparation of transition metal silicides V5 Si3 , Nb5 Si3 , and Ta5 Si3 was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts at their stoichiometries. The propagation mode of self-sustaining reactions, flame-front velocity, combustion temperature, and phase composition of end products were extensively studied. For the reactant compacts from the mixtures of V:Si = 5:3 and Ta:Si = 5:3, a planar flame front propagating in a steady manner was observed, with or without sample preheating. On the other hand, a preheating temperature of 200 ◦ C or above was required by the Nb–Si powder compact to initiate the self-sustaining combustion. Moreover, it was found that the planar reaction font was not stable in the samples of Nb:Si = 5:3 and was transformed into a localized reaction zone propagating in a spinning mode. For all three types of the samples, as the sample density and preheating temperature increased, the combustion temperature was found to increase and the flame-front velocity was correspondingly enhanced. According to the XRD analysis, formation of single-phase silicides V5 Si3 and Nb5 Si3 from the reactant compacts of 5V + 3Si and 5Nb + 3Si, respectively, was confirmed. The samples of 5Ta + 3Si yielded predominantly the silicide Ta5 Si3 , along with an intermediate phase Ta2 Si in a small amount. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of V5 Si3 , Nb5 Si3 , and Ta5 Si3 were determined to be 111.5, 259.2, and 105.2 kJ/mol, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Transition metal silicide; V5 Si3 ; Nb5 Si3 ; Ta5 Si3 ; SHS; Activation energy
1. Introduction Transition metal silicides have been of considerable interest as high-temperature structural materials, because of their stability and resistance to oxidation, high melting points, good creep tolerance, low densities, and excellent mechanical strength at elevated temperatures [1–3]. Among the silicides of transition metals, the Mo–Si and Ti–Si compounds have been widely investigated [3–9]. Other silicides, such as those of the transition metals in the VB group (including V, Nb, and Ta) have received relatively little attention. In the V–Si phase diagram shown in Fig. 1(a), four silicides are present: V3 Si, V5 Si3 , V6 Si5 , and VSi2 [10]. With the exception of V6 Si5 , all the compounds show a congruent melting point. The V5 Si3 phase has the highest melting point at 2010 ◦ C. Silicide compounds in the systems
∗
Corresponding author. Tel.: +886 4 8511888x2118; fax: +886 4 8511215. E-mail address: [email protected] (C.L. Yeh).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.075
of Nb–Si and Ta–Si are among the most refractory metal silicides. In the Nb–Si system, as indicated in Fig. 1(b), there are three silicides of niobium: Nb3 Si, Nb5 Si3 , and NbSi2 [11]. The Nb5 Si3 has the highest melting point (2515 ◦ C) among those of the 5:3 transition metal silicides with densities below that of nickel-based superalloys [12]. The phase diagram of the Ta–Si system in Fig. 1(c) shows the presence of four intermetallic compounds (Ta3 Si, Ta2 Si, Ta5 Si3 , and TaSi2 ) with melting points in the range of 2200–2500 ◦ C [13]. Transition metal silicides and their composites have been produced by a wide variety of powder processing techniques, often with two or more in combination, including conventional arcmelting and casting, mechanical alloying, hot pressing, reaction sintering, plasma spraying, self-propagating high-temperature synthesis (SHS), and solid-state displacement reactions [5,14]. With the advantages of time saving, high energy efficiency, and good purification capability, the SHS technique has been considered as an alternative route to the conventional methods of producing advanced materials, including carbides, borides, nitrides,
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Fig. 1. Phase diagrams for: (a) V–Si system [10], (b) Nb–Si system [11], and (c) Ta–Si system [13].
silicides, hydrides, intermetallics, chalcogenides, etc. [15–18]. By utilizing a modification of the classical SHS approach, called field-activated combustion synthesis (FACS), some of the metal silicides in the systems of V–Si and Nb–Si were prepared [19,20]. In the absence of an electrical field, only the compacts with a composition V:Si = 5:3 reached self-propagating reactions [19]. Under the field values of 14.8–18.5 V/cm, combustion front along a sample of Nb:Si = 5:3 propagated in a steady mode and the final product was identified as a mixture of ␣-Nb5 Si3 and -Nb5 Si3 [20]. For the Ta–Si compounds, combustion synthesis of TaSi2 and Ta5 Si3 was conducted by Maglia et al. [21] using the reactant compacts from mechanically activated powders. The product composition was found to vary with starting stoichiometry and milling time [21]. The objective of the present investigation was to perform a comparative study on preparation of transition metal silicides V5 Si3 , Nb5 Si3 , and Ta5 Si3 by the classical SHS method using compacted samples from elemental powder mixtures. The dependence of combustion characteristics, including the propagation mode of the self-sustaining combustion, flame-front velocity, and reaction temperature, on the sample green density,
initial sample temperature, and starting composition of the reactant compact was explored. The phase composition of as-synthesized products was characterized by the XRD analysis. In addition, the activation energy associated with the synthesis reaction was determined by correlating the experimental data acquired in this study. 2. Experimental methods of approach Particles of the transition metals (Me) in the VB group, including vanadium (Aldrich Chemical, 99.5% purity), niobium (Strem Chemicals, 99.8% purity), and tantalum (Aldrich Chemical, 99.9% purity), were used to mix with silicon (ProChem Inc., 99.5% purity) powders as the starting materials in this study. All of the constituent powders had a size specification of −325 mesh. Powder blends were prepared at a stoichiometric ratio Me:Si = 5:3 by dry mixing in a ball mill and then were cold-pressed into cylindrical test samples with a diameter of 7 mm and a height of 12 mm. The V–Si and Nb–Si powder compacts were formed with a compaction density between 50 and 65% of the theoretical maximum density (TMD) of the reactant mixture. Due to the relatively high density of Ta (16.6 g/cm3 ) in comparison to those of V (6.1 g/cm3 ) and Nb (8.57 g/cm3 ), the green density of the Ta–Si compact was in the range from 35 to 45% TMD. The SHS experiments were conducted in a stainless-steel windowed combustion chamber under an atmosphere of high purity argon (99.99%). The sample
C.L. Yeh, W.H. Chen / Journal of Alloys and Compounds 439 (2007) 59–66 holder was equipped with a 600 W cartridge heater used to raise the initial temperature of the test sample prior to ignition. Reaction was normally initiated by a heated tungsten coil. Experiments were performed at different sample preheating temperatures (Tp ) from room temperature to 300 ◦ C. In the absence of sample preheating, only the samples of V:Si = 5:3 and Ta:Si = 5:3 reached selfsustaining reactions. The powder compacts of 5Nb + 3Si required not only a preheating temperature of 200 ◦ C or above, but also an ignition enhancer made up of titanium (Ti) and carbon black (C) powders (with a ratio of Ti:C = 1:1) to ensure a self-propagating combustion wave throughout the entire compact [22]. Detailed description of the experimental set-up and measurement approach was given previously [23,24].
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3. Results and discussion 3.1. Observation of combustion characteristics Typical SHS processes recorded from the V–Si, Nb–Si, and Ta–Si powder compacts are respectively illustrated in Fig. 2(a–c). Upon ignition, as shown in Fig. 2(a), a distinct combustion front develops on a sample of 5V + 3Si and propagates in a steady and self-sustaining manner. After the flame
Fig. 2. Recorded combustion images of SHS processes associated with: (a) a 50% TMD V–Si sample at Tp = 100 ◦ C, (b) a 55% TMD Nb–Si sample at Tp = 300 ◦ C, and (c) a 35% TMD Ta–Si sample at Tp = 200 ◦ C.
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front reaches to the bottom of the sample at about t = 1.30 s, Fig. 2(a) reveals that the luminosity on the burned sample is gradually reduced. Instead of a constant losing of the brightness, however, a noticeable afterglow emerges on the sample from approximately t = 2.50 s and lasts for about 1 s, which suggests further phase conversion taking place volumetrically. According to Merzhanov [15], formation of metastable intermediate compounds, which subsequently convert into end products, could be responsible for the afterburning glow observed in the solid-phase SHS process. For the reactant compact of Nb:Si = 5:3, Fig. 2(b) indicates that shortly after the ignition the planar combustion front is transformed into a localized reaction zone moving along a spiral trajectory on the sample surface. It is believed that the heat generated from the Ti–C igniter pellet along with the thermal energy liberated from the Nb–Si reaction within the combustion zone support the steady propagation of a planar front before the appearance of a spinning combustion wave. With the fading of the heat input from the ignition enhancer, combustion becomes self-sustained. As proposed by Ivleva and Merzhanov [25], once the heat flux generated from the self-sustaining combustion is no longer sufficient to
maintain the steady propagation of a planar front, the flame front forms one or several localized reaction zones. Due to progression of the spinning combustion wave, spiral marks left on the surface of the burned sample can be clearly seen in Fig. 2(b). Furthermore, similar to that observed in Fig. 2(a) for the V–Si sample, Fig. 2(b) shows evidence of afterburning reactions starting roughly from t = 2.10 s in the Nb–Si powder compact. The SHS process of the test specimen with an initial stoichiometry Ta:Si = 5:3 is presented in Fig. 2(c), which depicts a steady combustion wave travelling in a nearly parallel fashion. As also revealed in Fig. 2(c), the burned region behind the reaction front continues to glow incandescently, implying that the reaction is not only restricted to the combustion front, but also continues vigorously in bulk. When compared with test samples of the other two compositions (i.e., V–Si and Nb–Si powder compacts), the propagation rate of the reaction front associated with the Ta–Si compact appears to be higher. As indicated in Fig. 2(c), it took only about 0.73 s for the combustion wave to arrive at the bottom of the sample. In addition, it is useful to note that no afterburning reactions were observed after the passage of the flame front. According to Merzhanov [15], the lack of suffi-
Fig. 3. Effects of initial sample density and preheating temperature on flame-front velocity of sample compacts with starting stoichiometries of: (a) V:Si = 5:3, (b) Nb:Si = 5:3, and (c) Ta:Si = 5:3.
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cient reaction time might cause phase conversion uncompleted, thus resulting in the presence of intermediate phases in the final composition. 3.2. Measurement of flame-front propagation velocity The flame-front propagation velocity (Vf ) in the longitudinal direction was deduced from the recorded sequences of combustion images and reported in Fig. 3(a–c) as a function of initial sample density and preheating temperature. First of all, it was found that the flame-front velocity increases with sample green density for all three types of the samples. The influence of sample compaction density on the propagation rate of the reaction front can be attributed to two competing phenomena [17]. As the sample density increases, the intimate contact between the reactant particles is augmented, thus enhancing the reaction and consequently increasing the propagation rate of the combustion front. On the other hand, the thermal conductivity of the reactant compact also increases with sample density, which might cause more thermal loss by conduction from the reaction zone and therefore could lead to a decrease in the reaction rate [17]. It is evident to see in Fig. 3(a–c) that the former reason dominates within the density range adopted in this study.
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Secondly, on account of the increase of reaction temperature with initial sample temperature, Fig. 3(a–c) indicates that the propagation rate of the reaction front is increased by increasing the preheating temperature. As mentioned earlier, a preheating temperature of 200 ◦ C or above was required by the compacts of Nb:Si = 5:3 to achieve self-sustaining reactions. Therefore, only the data under the conditions of Tp = 200 and 300 ◦ C were reported for the Nb–Si test specimens and plotted in Fig. 3(b). However, as shown in Fig. 3(a and c), self-propagating reactions can be completed in the compacts of 5V + 3Si and 5Ta + 3Si with no prior heating. In addition, Fig. 3(a and c) indicates that for the samples of these two types, the increase of initial sample temperature by preheating to 200 or 300 ◦ C almost doubles the reaction front velocity. Finally, a comparison between sample compacts of different compositions shows that under the same initial sample temperature the Ta–Si sample has the highest flame speed regardless of its low relative density, whereas because of the reaction zone propagating in a spinning mode the Nb–Si powder compact exhibits the slowest propagation rate in the longitudinal direction. The velocity of the reaction front associated with the sample of V:Si = 5:3 was found slightly lower than that of the Ta–Si powder compact.
Fig. 4. Effects of initial sample density and preheating temperature on combustion temperature of sample compacts with starting stoichiometries of: (a) V:Si = 5:3, (b) Nb:Si = 5:3, and (c) Ta:Si = 5:3.
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Fig. 5. Relation between flame-front velocity and combustion temperature for determination of activation energy associated with combustion synthesis of: (a) V5 Si3 , (b) Nb5 Si3 , and (c) Ta5 Si3 .
3.3. Measurement of combustion temperature Typical temperature profiles detected in the self-sustaining combustion regime from the burning samples with starting compositions of V:Si = 5:3, Nb:Si = 5:3, and Ta:Si = 5:3 are plotted in Fig. 4(a–c), respectively. As illustrated in Fig. 4(a–c), the abrupt rise in the temperature profile represents the rapid arrival of the reaction front and the peak value signifies the flame-front temperature, which generally increases with either sample green density or preheating temperature. For example, Fig. 4(a) indicates an obvious increase in the peak reaction temperature from about 1190 ◦ C for a 50% TMD V–Si compact at Tp = 100 ◦ C to 1425 ◦ C in the case of 60% TMD and Tp = 200 ◦ C. For the powder compacts of Nb:Si = 5:3, it was found that the influence of sample density and preheating temperature on the reaction temperature was not as pronounced as that observed in the test specimens of V:Si = 5:3. Peak reaction temperatures measured from the Nb–Si samples were found to vary between 1600 and 1700 ◦ C, as shown in Fig. 4(b). On the contrary, as can be seen in Fig. 4(c), the samples of Ta:Si = 5:3 exhibit great dependence of combustion temperature on sample density and
preheating temperature. The flame-front temperature in the range from 1250 to 1680 ◦ C was observed for the 5Ta + 3Si compacts under different test conditions. The apparent activation energy of a self-sustained reaction can be determined by means of constructing the dependence of reaction front velocity on combustion temperature. A simplified relation used is expressed in the following [22–24]:
Vf Tc
2
= f (n)
R Ea
Ea K exp − RTc
(1)
where Vf is the velocity of combustion front, Tc the combustion temperature, Ea the activation energy of the reaction, R the universal gas constant, f(n) a function of the kinetic order of the reaction, and K is a constant which includes the heat capacity of the product, thermal conductivity, and the heat of reaction. According to Eq. (1), the slope of a straight line correlating ln(Vf /Tc )2 with 1/Tc can provide the apparent activation energy of the reaction process. Based upon the slopes of best-fitted lines for the data plotted in Fig. 5(a–c), the activation energy with values of 111.5, 259.2, and 105.2 kJ/mol was obtained for combustion synthesis of V5 Si3 , Nb5 Si3 , and Ta5 Si3 , respectively. In
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Fig. 6. Typical XRD patterns of combustion products obtained from sample compacts with starting stoichiometries of: (a) V:Si = 5:3, (b) Nb:Si = 5:3, and (c) Ta:Si = 5:3.
comparison to those of V5 Si3 and Ta5 Si3 , the activation energy required to form Nb5 Si3 is substantially higher, which could explain the need of prior heating and an ignition enhancer to initiate the reactions in the Nb–Si powder compact.
3.4. Composition analysis of combustion products The phase composition of as-synthesized products was identified by XRD analysis. No variation of the product composition is shown with initial sample density and temperature. The XRD pattern plotted in Fig. 6(a) represents the expected phase composition of end products synthesized from the samples of 5V + 3Si. Formation of a single-phase silicide V5 Si3 in both tetragonal and hexagonal forms was identified. For the Nb–Si powder compacts, Fig. 6(b) indicates that combustion yields a monolithic silicide Nb5 Si3 existing in two tetragonal modifications (␣ and  forms). However, as shown in Fig. 6(c), the reaction product obtained from the compact of Ta:Si = 5:3 consists of Ta5 Si3 as the dominant phase and a lesser amount of Ta2 Si as the intermediate phase. The presence of Ta2 Si in the final product from a 5Ta + 3Si compact could be caused by the lack of sufficient reaction time in the Ta–Si compact, as discussed earlier.
4. Conclusions This study presents an experimental investigation on preparation of the silicides of transition metals (the VB group), including V5 Si3 , Nb5 Si3 , and Ta5 Si3 , by self-propagating hightemperature synthesis from elemental powder compacts at their corresponding stoichiometries. Experimental evidence shows once initiation, planar and self-sustaining reaction fronts propagating steadily along the samples composed of V:Si = 5:3 and Ta:Si = 5:3. For the powder compacts from the mixture of Nb:Si = 5:3, however, preheating the samples at 200 ◦ C or above was required to achieve self-sustaining reactions which were confined to a small region travelling in a spinning mode. After the passage of the flame front, the emergence of afterburning glows resulting from further phase transformation was noticed in the V–Si and Nb–Si powder compacts. The flame-front propagation velocity was found to increase with the sample density, because of the improvement in the contact between particles. Moreover, as a result of the increase of reaction temperature, preheating the reactant compact enhanced the reaction front velocity substantially, in particular for the samples of 5V + 3Si and 5Ta + 3Si. Under the same initial sample temperature, the Ta–Si compact has the fastest flame front. Due
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to the spinning propagation mode, the velocity of the reaction front along the Nb–Si sample is the slowest. On the basis of the dependence of reaction front velocity on combustion temperature, the activation energy with values of 111.5, 259.2, and 105.2 kJ/mol was deduced for combustion synthesis of V5 Si3 , Nb5 Si3 , and Ta5 Si3 , respectively. The XRD analysis on the synthesized products verifies complete conversion from the powder compacts of V:Si = 5:3 and Nb:Si = 5:3 to single-phase compounds. Silicide V5 Si3 in both tetragonal and hexagonal forms was yielded from the 5V + 3Si sample. Monolithic silicide Nb5 Si3 existing in two tetragonal modifications (␣ and  forms) was obtained from the 5Nb + 3Si sample. However, in addition to formation of Ta5 Si3 as the dominant phase, the presence of an intermediate phase Ta2 Si was detected in the final product from the 5Ta + 3Si compact. Acknowledgement This research was sponsored by the National Science Council of Taiwan, ROC, under the grants of NSC 95-2221-E-212-053MY2. References [1] P.J. Meschter, D.S. Schwartz, JOM 41 (1989) 52–55. [2] D.M. Shah, D. Berczik, D.L. Anton, R. Hecht, Mater. Sci. Eng. A155 (1992) 45–57.
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