A thermodynamic assessment for synthesizing transition metal silicides by the combustion synthesis process

Scripta METALLURGICA et MATERIALIA

Vol. 27, pp. 1277-1281, 1992 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

A THERMODYNAMIC ASSESSMENT FOR SYNTHESIZING TRANSITION METAL SILICIDES BY THE COMBUSTION SYNTHESIS PROCESS

S.B. BHADURI Department of Metallurgical & Mining Engineering University of Idaho, Moscow, ID 83843 (Received July 23, 1992) (Revised September 8, 1992)

Introduction Transition metal silicides have important applications in various disciplines. These include uses as interconnects in chips, as coatings, as heating elements, etc. [I,2]. As their uses increased, various processing techniques were adopted to produce them. These vary from chemical/physical vapor deposition, rapid thermal processing, and sputtering for thin film processing; traditional vacuum casting and powder metallurgical routes are used to produce bulk samples. In the present paper, we are interested in those transition metal silicides which have the potential for use in the bulk form, possibly in the aerospace or other demanding applications. The primary requirements are high refractoriness, low density, high strength and good oxidation resistance. It is the oxidation resistance that makes the siliddes better candidates as compared to the other intermetallics. Meschter [3] argued that most engineering materials retain substantial strength up to 80% of their melting points. Therefore, for an operating temperature of 1500C, the melting point of a typical candidate may be 1950C, while the limit in density can be defined by the density of Ni ( 8.75 g m / c c ). Based on the above argument, Table 1 shows some of the potential transition metal silicides which may be used for demanding applications. The materials listed in Table 1 have been produced by different ways. For example, MoSi2 powders were produced by reacting elements at a low temperature of 1200C for many hours [4]. MoSi2 compacts have also been fabricated by mechanical alloying followed by v a c u u m hot pressing [5]. Likewise, Nb5Si3 has been produced by ingot metallurgy [6] as well as by the P / M route

As an alternative, combustion synthesis (CS) processing may prove to be a viable route for producing these transition metal silicides. In this process, an exothermic reaction is initiated in a compact containing stoichiometric mixture of elemental powders. The benefits of this process have been elaborated in recent reviews [8 - 11]. These include generation of substantial heat in situ, use of a simple reactor, ease of scaling up, tailoring of the composition and reduction in impurity. Indeed, there are several studies in the literature on the combustion synthesis of MoSi2 [12 - 17]. Reports of synthesis of Ti5Si3 have also appeared in the literature [18 - 20]. NbSi2 is reported to have been produced [21] as well.

1277 0956-716X/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.

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This paper will examine the feasibility of synthesizing some of the important transition metal silicides by the combustion synthesis process purely from a thermodynamics point of view. There are two basic modes of a CS reaction-- the propagating reaction and the explosion reaction. A propagating reaction is ignited locally and spreads by propagation of the combustion wave through the reactants. On the other hand, in the explosion mode, the pressed reactants can be uniformly heated in a furnace whereupon the entire sample is ignited simultaneously. The propagating reactions occur in highly energetic systems. However, these reactions quench themselves in sluggish systems unless extra energy is supplied in order to help the reaction. Thermodynamics has a big role to play in determining the kinetic behavior of the reactions.

Thermodynamic Considerations Once ignited, quite high temperatures can be achieved in very short times due to the high exothermicity of the reaction. The enthalpy of the synthesis reaction for the propagating case is-

AHreaction = AHf,298

(1)

where AHf,298 is the standard enthalpy of formation of the product. For an exothermal reaction, is negative and is used to heat products. For an adiabatic reaction, the reaction temperature is denoted by Tad: the heating of the product from 298 to Tad then takes place in response to the input of AHreaction. Depending upon whether Tad is smaller than, equal to, or larger than the product melting temperature Tm, the following three cases arise.

AHreaction

(1) If Tad < Tm,

-AHf,298

Tad = ~29S C p ( p r o d u c t ) d T

(2)

(2) If Tad = Tm,

-- A H f , 2 9 8

Tad = ~ 9 8 C p ( p r o d u c t ) d T + x)AHm

(3)

where u is the fraction of the liquid phase in the mixture of liquid and solid phases since both of them are expected to exist at the melting point. (3) If Tad > Tin, T - A H f , 2 9 8 = ~98adCp(product)d T + AH m + ~Fta~pC p(liquid) dT (4) Based on the observations of a number of reactions, primarily on carbides, h i , i d e s and borides [8 - 10], a linear correlation between AHf,298/ Cp,298 and Tad has been found. It has been

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suggested in the literature that the combustion reaction will not become self sustaining if Tad values are less than 1800C ( or AHL298/ Cp,298 < 2000K). This work will examine this criterion in order to assess the viability for producing some transition metal silicides of engineering interest. The relevant thermodynamic data for the various silicides were obtained from Kubaschewski [22] and are tabulated in TabLe 2. Tad temperatures for Mo5Si3, MoSi2, Ti5Si3, Zr5Si3 and ZrSi were obtained from the paper of Azatyan et al. [18]. The data for Nb5Si3 and NbSi2 are obtained from the paper of Sarkisyan et al. [21]. Figure I shows a plot of AHf/Cp vs Tad. This figure differentiates between the reactions that are self propagating to those that extinguish themselves. The seven silicides can be broadly grouped into three classes-- very sluggish, on the borderline of being self-propagating, and energetic selfpropagating reactions. From this figure, Mo5Si3 seems to be very sluggish and must be produced in the explosion mode, and this, in fact, has been observed experimentally. MoSi2, Nb5Si3 and NbSi2 are on the boderline of being self-propagating, which has been corroborated experimentally. In fact, MoSi2 has been produced in both of the modes. However, the propagation reaction in MoSi2 cannot be repeatedly achieved unless there is an extraneous input of energy [17]. Such is also the case for the Nb based silicides. Of the seven silicides, Ti5Si3, ZrSi and Zr5Si 3 can be grouped into the highly energetic self-propagating reactions. Ti5Si3 has been produced as a single phase material [18]. Sarkisyan et al. claimed that ZrSi and Zr5Si3 decompose [14]; however, Cellis and Ishizaki [23] have shown that stable Zr5Si 3 can be produced by melting. Since the thermodynamic criterion holds good for all the materials for which experimental data exist, in the opinion of this author, Zr5Si3 may be a prime candidate for production by combustion synthesis. Conclusions

In conclusion, we have assessed the possibility of producing some important transition metal silicides by the combustion synthesis technique. Of the seven silicides considered, Mo5Si3 reaction is not self propagating. The formation of MoSi2, NbSi2 and Nb5Si 3 are at the borderline of being self-propagating . The results of this study predict that the formation of Ti5Si3, ZrSi and Zr5Si3 may take place in a propagation mode. These predictions match whatever experimental data are available in the literature.

Acknowledgements v

The author acknowledges the support of an AMAX Foundation grant. References

1. S.P.Muraraka, Silicides for VLSI Applications, Academic Press, N.Y. 1983. 2. J. Schlichting, High Temperature & High pressures, 10, 241 (1978). 3. P.J. Meschter and D.S. Schwartz, J.O.M., 41, 52 (1989).

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4. R. Weherman in High Temperature Materials and Technology, edited by I.E. Campbell and E.M. Sherwood, Wiley, N.Y. 1967, pp 399. 5. Y.S. Kim., M.R. Johnson, R. Abbaschian and M.J. Kaufman, pp 839 in M.R.S. Symp. Proc., v. 213. edited by L.A. Johnson, D.P. Pope and J.O. Steiger, M.R.S., Pittsburgh, PA (1991). 6. J.D. Rigney, J.J. Lewandowski, L. Matson and M.G. Mendiratta and D.M. Dimiduk, pp. 1001 in reference 5. 7. J. Kajuch and K. Vedula, Advances in Powder metallurgy-1990, edited by E.R. Andreotti and P.J. McGeehan, APMI-MPIF, Princeton, NJ (1990). 8. Z.A. Munir, Bull. Am. Ceram. Soc., 67, 342 (1988). 9. H.C. Yi and J. J. Moore, J. Mater. Sci., 25, 1169 (1990). 10. Z.A. Munir and U. Anselmi-Tamburini, Mater. Sci. Reports, 3, 277 (1989). 11. A.G. Merzhanov, pp. 1 in Combustion and Plasma Synthesis of High Temperature Materials, edited by Z.A. Munir and J.B. Holt, VCH Publishers (1990). 12. D.H. Killeffer and A. Linz, Molybdenum compounds: Their Chemistry & Technology, Interscience NY, (1952). 13. J.B. Huffadine, pp. 220 in Special Ceramics, edited by P. Popper, Academic press (1960). 14. A.R. Sarkisyan, S.K. Dolukhanyan, I.P. Borvinskya and A.G. Merzhanov, Comb. Expl. Shock Waves, 14, 49 (1978). 15. S.C. Deevi, J. Mater. Sci., 26, 3343 (1991). 16. S.C. Deevi, Mater. Sci. Eng., A149, 241(1992). 17. S. Zhang and Z.A. Munir, J. Mater. Sci., 26, 3685 (1991). 18. T.S. Azatyan, V.M. Maltsev, A.G. Merzhanov and V.A. Seznev, Comb. Expl. Shock Waves, 15, (1979) 35. 19. J. Trambukis and Z.A. Munir, J. Am. Ceram. Soc., 73, (1990) 2140. 20. B.R. Kruger, A.H. Mutz and T. Vreeland, Met. Trans., 23A, (1992) 55. 21. A.R. Sarkisyan, S.K. Dolukhanyan and I.P. Borvinskya, Comb. Expl. Shock Waves, 15, (1979) 95. 22. I. Barin, O. Knacke and O. Kubaschewski, Thermal Properties of Inorganic Substances, Springer Verlag, Berlin (1977) 23. P.B. Cellis and K. Ishizaki, J. Mater. Sci., 26, (1991) 3497. Table 1: Physical Properties of Silicides under Consideration

Material Mo5Si3 MoSi2 Ti5Si3 Zr5Si3 ZrSi Nb5Si3 NbSi2

Density

M.p.

(8/cc)

(C)

8.96 6.24 4.32 6.04 5.66 7.13 5.66

2180 2020 2130 2210 2095 2480 1930

Structure

D88 C11b D88 D88 B27 D88 Cllb

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Table 2: Thermodynamic Properties of Silicides

Mate.riM

AHf(298) (KCal Mo1-1)

Cp (Cal Mo1-1 K -1)

Tad

(IO

Mo5Si 3

-13.4

43.8 - 8.4 X 10-3T - 2.86 X 105T-2

MoSi2

-31.4

16.2 + 2.86 X 10"3T - 1.57 X 10ST"2

TisSi3

-138.4

46.95 + 10.7 X 10-3T - 4.8 X 105T"2

2500

ZrsSi3

-137.6

45.2 + 7.35 X 10-3"1"- 3.6 X 105T"2

2800

ZrSi

-37.0

10.8 + 2.1 X 10-3"i"- .71 X 105T"2

2700

NbsSi3

-108.0

45.21 + 7.36 X 10-3T - 3.6 X 105T-2

1850

NbSi2

-33.0

15.10 + 3.67 x 10-3T - .668 X 105T'2

1950

4.0

ZrSi o / TisSi3~//

3.0

[] ZrsSis

NbsSi~ 0 / NbSiz o / MoSiz U /

6 " " 2.0

¢2

/

t.0

0 Mo~$i3

<3 0.0

1200

1600

2000

T.(K)

2400

2800

.3200

Figure 1. The plot of AHf/Cp vs.Tad for the transition metal silicides.

1600 1900