D. D. JACKSOX, G. W. BARTON, JR.,0. H. KRIKORIAS, AND R. S. XEWBURY
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Vaporization of Thorium Dicarbide’
by Donald D. Jackson, George W. Barton, Jr., Oscar H. Krikorian, and Ray S. Newbury Lawrence Radiation Laboratory, University of California, Liuermore, California
(Received January 24>1964)
X mass spectrometric investigation was carried out on the vaporization processes and thermodynamics of vaporization of solid ThCa. The carbide was heated jn a graphite Knudsen cell and the vapors were analyzed. The partial pressures of Th(g) and ThCa(g) in equilibrium n-ith the carbon-saturated solid were found to be of comparable magnitude in the temperature range investigated, 2371-2642OK. The enthalpy of sublimation of ThCz(s) to ThClz(g) is 198.1 i 3.5 kcal./g.f.w., and to Th(g) is 168.4 f 3.5 kcal./g.f.w. The calculated AHf0z98of formation of ThCz is -30.7 f 3.7 kcal./g.f.w.
Introduction
h mass spectrometric investigation has been made of the gaseous species present in the carbon-rich region of the Th-C system, as well as the theriiiodynamics of the vaporization processes and the enthalpy of formation of ThCz. The gaseous species have not been studied previously, whcreas several prior deterininations have been made of the enthalpy of formation of ThCz. Considerable uncertainties are present in the AHf” values, resulting partly froiii a lack of heat capacity data and partly from experiinental procedures. Free energy data mere given by Prescott and Hincke2 for the equilibrium Th02(s) 4C(graphite) = ThC2(s) 2CO(g) a t temperatures between 2057 and 2494’K. Krikorian3 made a third-law treatment of the Prescott and Hincke data by using estimated free energy functions and obtained a AHfo298of formation of -33.0 f 8 kcal./g.f.m. for ThC2. Kubaschewski and Evans4estiof ThCz as -44.8 kcal./g.f.w. and mated the AHf0298 So298 as 19.3 cal./deg.-g.f.w. Egan and Bracker5reported high-temperature galvanic cell measurements of i l F ~ O l o 7 3 = -29.6 kcal. 1g.f.w. and AHioloT3 = -37.35 kcal./ g.f.w. for forination of ThCs.Sa Lonsdale and Gravess calculated a AH*’ of ThC2of -46 i. 6 kcal./g.f.w. between 2300 and 29OOOK. on the basis of target-effusion expcriments by assuming that Th(g) was the only gaseous species. Lofgren and I!ICz,where M = La, Ce, Pr, Gd, and Lu. It should be noted that the oxides of these rare earths produce >4O(g) most abundantly in v a p o r i ~ a t i o n , just ~ ~ , ~as~ ThOz(s) produces ThO(g) as a major species.3e Moving across the actinide series, we observe somewhat similar relationships. As previously mentioned, UC,(g) is a vapor component over UCz(s),although not so important in the case of ThC2. UO(g) has been
1523
found to be the most important species in equilibrium with a mixture of U(l) and U O , ( S ) , ~just ~ as Th(s) containing dissolved 0 produces large amounts of ThO(g).38 No detailed theoretical explanation of such behavior has yet been made. However, it is likely that other gaseous carbide and oxide complexes will soon be identified and that species ratios as a function of stoichiometry will be better explored. For example, from the above observations, and from similarities between certain of the lanthanides and actinides, we may, by analogy, expect to detect vapor molecules such as CmCdg).
Acknowledgment. A preliminary version of this study appeared in the Proceedings of the Symposium on the Thermodynamics of Nuclear Materials, held by the International Atomic Energy Agency, Vienna, 1962.
Appendix We have recalculated the heat of formation of ThCz from the data of Prescott and Hincke2using the following information. 1. The temperature scale is converted to ITS-1948 by using Cz = 1.4380 cm.-deg. 2. Free-energy functions are derived (a) for ThOz, from high-temperature enthalpy and entropy data of K e l l e ~the , ~ ~high-temperature enthalpies of Hoch and Johnston,40and the Sozgs given by Kelley41; (b) for graphite, as given by Stull and Sinke30; (c) for ThCz(s), as presented in Table 111; (d) for CO, from data by Kelle~.~~ From a third-law treatment, we find ThOz(s)
+ 4C(graphite) = ThCz(s) + 2CO(g)
AH'298
=
208.79
f
2.0 kcal./g.f.w.
Using the values AHf0298 = -293.2 f 0.4 kcal./ and AHfozg8 = -26.42 f 0.01 kcal./ g.f.w. of g.f.w. of C 0 , 4 2we calculate AHro2g8 = -31.6 f 2.4 kcal./g.f.w. of ThCz. (34) M. B. Panish, J . Chem. Phys., 34, 1079 (1961). (35) M. B. Panish, ibid., 34, 2197 (1961). (36) R. J. Ackermann, E. G. Rauh, R. J. Thorn, and M. C. Cannon, J . Phys. Chem., 67, 762 (1963). (37) G. De Maria, R. P. Burns, J. Drowart, and M. G. Inghram, J . Chem. Phvs., 32, 1373 (19GO). (38) A. J. Darnell, W. A . McCollum, and T. A. Milne, J . Phys. Chem., 64, 341 (1960). (39) K. K. Kelley, U.S.Bureau of Mines Bulletin 584, U. 9. Govt. Printing Office, Washington, D. C., 1960. (40) M. Hoch and J. L. Johnston, J . Phys. Chem., 6 5 , 1184 (19Gl). (41) K . K. Kelley and E. G. King, U. S. Bureau of Mines Bulletin 592, U. 8 . Govt. Printing Office, Washington, D. C., 1961. (42) J. P. Coughlin, U. S.Bureau of Mines Bulletin 542, U. S. Govt. Printing Office. Washington, D. C., 1954.
Volume 68, Number 6
June, 1964
