Vapor pressures have been established by Langmuir vaporization of C-saturated ZrC and by Knudsen effusion studies of ZrC in equilibrium with graphite. These are plotted in Figure 10. Langmuir studies are internally consistent, but give higher pressures than for the Knudsen method. Pollock37 and Coffman eta/.38 assumed the congruent evaporation composition
to be stoichiometric, that is, equal evaporation rates for Zr and C. However, Langmuir evaporation of ZrC0.74_0.96 by Nikol’skaya et a/.39 found the con­gruently evaporating composition to lie in the range ZrC0 8-087, decreasing with increasing temperature between 2300 and 3100 K. Vidale40 computed Zr and C vapor pressures from tabulated H and S functions for Zr and C, AHf for ZrC of —185.5 kJmoP1, and an estimated ASf for ZrC of —11.3 kJmol—1 K—1, and the trend is consistent with Langmuir data. Storms2 com­puted Zr vapor pressure over ZrC + C from thermo­dynamic functions derived by the author for ZrC0 96, values in the 1963 JANAF thermochemical tables for Zr^g) and C(s), AHf for ZrC of —196.6 kJ mol—1, and AH^ for ZrC of 608 kJ mol—1, with the prediction consistent with Knudsen data. Evaporation rate as a function of temperature is plotted in Figure 11. Stan­dard enthalpy of vaporization of ZrC at 298 K has been reported as —1520 kJ mol—1 for Langmuir studies and —805 kJ mol—1 for Knudsen studies.37,38

Storms and Griffin13 coupled Knudsen effusion from TaC cells with mass spectrometry between 1800 and 2500 K to determine the Zr activity of ZrC0 55-“197” by comparing ion currents from pure Zr with those of the carbide. Carbon activity was obtained via a Gibbs-Duhem integration; activity of both as a function of C/Zr ratio at 2100 K is plotted

Temperature (K)
3000 т	2800 Т	2600 Т
10-4

10-5

10-8

10-9

3.2 X 10-4 3.4X10-4 3.6 X 10-4 3.8 X 10-4 4.0X10-4 4.2X10-4 4.4 X 10-4

1/T (K-1)

Figure 11 Langmuir rate of evaporation of ZrCx as a function of temperature.

in Figure 12. Activity of Zr exceeds that of C for carbon-deficient compositions up to the cross-over composition at 2100 K of ZrC089. The change in Zr activity with C/Zr ratio is most rapid at high-carbon compositions and becomes near-constant as the com­position drops below approximately ZrC0.8. Partial standard molar enthalpies of vaporization for Zr and C as a function of C/Zr ratio are plotted in Figure 13. Total enthalpies obtained by Pollock37 and Coffman et al3 are consistent with the values of Storms and Griffin.1 Partial enthalpy of Zr decreases monotonically as C is removed from the lattice. Partial enthalpy of C exceeds that of Zr for most of the homogeneity range, approaching that of Zr at a composition of ZrC0.99.