Thermodynamics of formation of Th-Co alloys
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SKELTON, N.J.
MAGNANI, AND J . F. SMITH
E l e c t r o m o t i v e f o r c e c e l l s have b e e n u s e d to d e t e r m i n e the Gibbs f r e e e n e r g i e s , e n t h a l p i e s , and e n t r o p i e s of f o r m a t i o n for Th2Colv, ThCos, Th2CoT, T h C o , and ThTCo3. Solid CaFz w a s e m p l o y e d a s the e l e c t r o l y t e , and m e a s u r e m e n t s w e r e m a d e o v e r the t e m p e r a t u r e r a n g e 917 ~ to 1233~ C o m p a r i s o n of the T h - C o enthalpy v a l u e s with T h - N i v a l u e s shows that t h e r e l a t i v e bond s t r e n g t h s a s s o c i a t e d with t h o r i u m - t r a n s i t i o n m e t a l i n t e r a c t i o n s in the two s y s t e m s i n v e r t b e t w e e n the t h o r i u m - p o o r and t h o r i u m - r i c h r e g i o n s .
THERMODYNAMIC data for thorium alloy systems are sparse. The compilation of Hultgren e t a l . ~ together with supplemental data sheets2 current to June 1970 list a total of only eight binary systems for which some thermodynamic data have been compiled. Sincethorium is a fertile material with a significant probability of future importance in nuclear reactor technology, thermodynamic data on thorium alloy systems have relevance to the future development of fuel reprocessing schemes. The present report on the Th-Co system is the third in a series. The initial report3 dealt with the thermodynamics of formation of seven thorium-poor phases in the Th-Fe, Th-Co, Th-Ni, and Th-Cu systems. At the time of publication of the initial report, published information indicated that the most thorium-poor phase in the Th-Cu system was ThCu4 and the electromotive force measurements were interpreted in terms of that stoichiometry. A more recent structural study4 based on X-ray powder diffraction has reported the stoichiometry of this thorium-poor phase to be ThCu~isomorphous with CeCu6. This stoichiometry and structure have now been corroborated by X-ray diffraction from single crystals at this laboratory. Sincethe original electromotive force measurements were made on 2phase alloys containing 5 and 7 at. pct Th, the electromotive force data are valid for ThCu8 and a revised analysis of the data indicates for ThCuoa Gibbs free energy of formation of -3.13 kcal per g-atom, an enthalpy of formation of -2.09 kcal per g-atom, and an entropy of formation of +1.06 eu per g-atom. The second reports completed the evaluation of the thermodynamics of phase formation for all five intermediate phases in the Th-Ni system, and the present report does the same for the Th-Co system. Thomson~ has investigated the phase relationships in the Th-Co system, and the temperature-composition diagram in Fig. 1 is drawn primarily from her data. Florio e t a l . v were the first to examine the crystallography of the phases in the Th-Co system. They found ThTCo3to be isomorphous with both ThTFe3 and ThTNi3, ThCo to be isomorphous with CrB, and ThCo~ to be isomorphous w i t h CaCu~. In a d d i t i o n F l o r i o e t a l . r e p o r t e d ThzCo~7 to c r y s t a l l i z
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