Knudsen Cell Studies of the Vaporization of Gadolinium and Gadolinium Dicarbide

Vapor pressure of gadolinium metal and carbon-rich gadolinium dicarbide was measured by the Knudsen effusion technique using an automatic recording balance. Knudsen pressures calculated on the basis that Gd(g) is the major gaseous species did not vary significantly as a function of orifice diameter or sample surface area. Corrections were made for a minor gaseous species, GdC2(g). The entropy and heat content of GdC2 were estimated. For the reaction Gd(l) = Gd(g) the third-law ΔH°298 of 95.2 ± 0.3 kcal/g-atom was in agreement with the second-law value of 97.3 ± 0.8 kcal/g-atom. Third- and second-law ΔH°298 were combined to give a value of 125 ± 9 kcal/mole for the reaction GdC2(s) = Gd(g) + 2C(s). From these values the enthalpy of formation of carbon-rich GdC2 was calculated to be — 30 ± 9 kcal/mole. Studies of rare earth dicarbide vaporization behavior are briefly reviewed and discussed, and their possible application to a self-purifying, high-temperature nuclear reactor is considered.