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Mat. Res. Soc. Symp. Proc. Vol. 423 ©1996 Materials Research Society

FIGURE 1. Trends in the TM-Si-C systems. The shaded area indicates systems with binary carbides. The bold outline encloses systems with one or more ternary phases. Silicides exist in all of these systems. chosen as a representative of the early transition metals. The Ni-Si-C and Co-Si-C systems are chosen to represent the late transition metals. The effect of temperature on the phase equilibria was also examined. Two approaches were taken to determine the applicability of the high temperature isotherms (typically reported between 10000 and 1727 0 C) for temperatures of interest either for operation or processing of high temperature devices (300-i1100 0C). The first approach involved examination of the effect of temperature on the phase diagrams when isotherms for more than one temperature were available in the literature. The second approach involved the use of thermochemical data to estimate the lower temperature phase equilibria. A combination of both approaches was preferred but not always possible due to lack of diagrams or data. The results suggest that phases in thermodynamic equilibrium with SiC will often remain unchanged over a wide temperature range. This observation is not very surprising since the entropies of formation for solid phases from solid elements are generally quite small; so the Gibbs energies of formation, which govern the phase equilibria, are often not a strong function of temperature. The early transition metal systems are characterized by the presence of silicides, carbides and ternary phases. The Ta-Si-C ternary diagram has been investigated experimentally at 1000 °C [12] and 1727 °C [9]. The diagram for 1000 °C is shown in Fig. 2. Despite the large difference in temperature, the phases in thermodynamic equilibrium with SiC are the same at both 1000 °C and 1727 °C: TaSi 2, TaC (at the C-rich end of its range of homogeneity), Si, and C. There is a slight discrepancy between the two diagrams in the Ta-rich portion; however, this portion of the diagram from the study at 1727 °C must be questioned since it does not agree with the most recent Si-Ta binary phase diagram [13]. An estimate of the phase equilibria at 25 °C using thermodynamic data [12-17] once again predicts the same tie-lines to SiC, despite the fact that the ternary phase Ta5 Si 3C1~x was omitted from the calculation due to a lack of thermodynamic data. Consistent with the Ta-Si-C phase diagrams and thermodynamic calculations, Edmond et al. [21 found that TaSi 2 contacts exhibited good thermal stability as ohmic contacts to n-SiC with electrical characteristics that remained constant for at least 8 hours at 400 °C. The tie-lines between both C-rich TaC and TaSi2 are characteristic of the early transition metal systems. In fact, for the group IVB and VB metals, all of the metal disilicides and monocarbides are in thermodynamic equilibrium with SiC, at least over a portion of their ranges of homogeneity. In some systems, other silicides and/or ternary phases al