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(Eq. [16] in their article) may be questioned. In this connection, it would have been useful if the authors had examined the electrodes after the galvanic cell experiments by suitable methods for the phases present. In a similar way, even the compound V2C has a wide range of compositions, as can be seen in Figure 2. Hence, a combination of the A G 0 for reaction
0
.20o0o
................
......
...............
.........
2V + C - - - V 2 C
-10000(
[2]
(Eq. [11] in their article) with AG O for the reaction -14000(
r
5._
2. 1739V2C + Mo2C = 4.3478VC0.73 -I- 2Mo
9
(Eq. [19] in their article) to arrive at the AG O for the formation of VC0.73 as Coltters and Nava had done (their Eq. [23]) does not take consideration of the energetics of carbon dissolution in V2C. For the above-mentioned reasons, a comparison of the results of one of the present authors 18] with the results of Coltters and Nava is not included in this discussion.
-16000(
-20000(
-220000
I
|
600
|
I
m
800
e
i
I
I
i
i
|
1000 TEMP. (K)
Fig. l - - G i b b s energy of formation C = I/2BaC2; 171 (2) l / 2 B a + (3) 1/2Ba + C : 1/2BaC2(aBac2 l/2BaC2(aBac2 = 0.001); (5) 1.94V V2C; "l and (7) 2Ta + C = Ta2C. ul
I
I
I
1200
i
[3]
I
1400
of some carbides: (1) l / 2 B a + C = 1/2BaC2(aB,c2 = 0.1); = 0.01); (4) 1/2Ba + C = + C = Vt ~C; 181 (6) 2V + C =
VC0.73, V2CffBaF2-BaC2ffCr, Cr23C 6 [E] VC0.73, V2CffBaF2-BaC2ffMo, Mo2C [F] ([D], [E], and [F] are symbols used by Coltters and Nava.t l]) From their measurements, the authors arrive at the standard Gibbs energies of formation of V2C as well as VC0.73. The phase diagram for the system V-C 9 is reproduced in Figure 2. The compound VC in Figure 2 has a range of compositions varying from VC0.923 at the carbon-rich side to VC0.739 at the vanadium-rich side around 1073 K. The composition VC0.73 c a n n o t be in equilibrium with VRC but V4C3-x, which has a composition corresponding to VC0.67. Consequently, the cell reactions of the type
REFERENCES 1. R.G. Coltters and Z. Nava: Metall. Trans. B, 1991, vol. 22B, pp. 661-68. 2. R.G. Cohters and G R . Belton: Metall. Trans. B, 1980, vol. 11B, pp. 525-29. 3. R.G. Coltters and G.R. Belton: Metall. Trans. 11, 1983, vol. 14A, pp. 1915-19. 4. R.G. Coltters and G.R. Belton: Metatl. Trans. B, 1984, vol. 15B, pp. 517-21. 5. Du Sichen: Scand. d. Metall., 1989, vol. 18, pp. 226-34. 6. T.A. Ramanarayanan: in Solid Electrolytes and Their Applications, E.C. Subbarao, ed., Plenum Press, New York, NY, 1980, pp. 81-98. 7. I. Barin, O. Knacke, and O. Kubaschewski: Thermochemical Properties of Inorganic Substances, Supplement, Springer-Verlag, Berlin, 1977. 8. Du Sichen: Metall. Trans. B, 1990, vol. 21B, pp. 313-20. 9. Binary Alloy Phase Diagrams, 1st ed., T.B. Massalski, ed., ASM, Metals Park, OH, 1986, p. 598.
2.1739V2C + 1/6Cr23C 6 : 4.3478VC0.73 + 23/6Cr
Weight Percent Carbon
Authors' Reply R. C O L I T E R S and Z. NAVA
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