Thermodynamics of the cadmium-neptunium system: Solute-solvent interaction in liquid alloys
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+ 2780.3T in T) XNp , in which XCd and XNp a r e the atom f r a c t i o n s of c a d m i u m and n e p t u n i u m . The Cd-U, Cd-Np, and C d - P u s y s t e m s a r e d i s c u s s e d in t e r m s of a model which involves the f o r m a t i o n of l o c a l ized bonds between the actinide and c a d m i u m atoms in the solid and the liquid state. T
L I Q U I D m e t a l s o l u t i o n s a r e often c o n s i d e r e d as r a n dom m i x t u r e s of a t o m s . However, in r e c e n t y e a r s a p h e n o m e n o n has b e e n r e p o r t e d with i n c r e a s i n g f r e quency which is c a u t i o u s l y d e s c r i b e d a s " i n t e r a c t i o n u n i t , " " c l u s t e r i n g , " " l o c a l o r d e r i n g , " o r in other s i m i l a r t e r m s , T h e s e i n t e r a c t i o n s m a y be r e l a t e d to the fact that the a t o m i c o r b i t a l s a r e diffuse at high a t o m i c n u m b e r s and can o v e r l a p quite e a s i l y with o r b i t a l s of a d j a c e n t a t o m s . Neptunium as an actinide e l e m e n t has a high at. no. and the 5f o r b i t a l s a r e p a r t i c u l a r l y diffuse. An i n t e r a c t i o n with c a d m i u m would, t h e r e f o r e , be l i k e l y in Np-Cd s o l u t i o n s . We have obt a i n e d evidence for the e x i s t e n c e of such s o l u t e - s o l v e n t i n t e r a c t i o n f r o m s t u d i e s of the t h e r m o d y n a m i c s of liquid and solid Np-Cd alloys u s i n g a high t e m p e r a t u r e galvanic cell method.
The n e p t u n i u m e l e c t r o d e was a o n e - g r a m piece of m e t a l l i c n e p t u n i u m held b e t w e e n two t a n t a l u m nuts on the t h r e a d e d end of a 1/16 in. diam t a n t a l u m w i r e lead. The Cd-Np a l l o y s were held in t a n t a l u m cups (6 m m d i a m by 10 m m high) welded to 1/16 in. diam t a n t a l u m wire leads. The t a n t a l u m cups, leads and nuts used to p r e p a r e the e l e c t r o d e s were c l e a n e d by sand b l a s t i n g and a n n e a l i n g at 800~ and 10 -6 m m Hg. Without a n nealing, the cell p o t e n t i a l s were found to be higher and u n s t a b l e . The n e p t u n i u m m e t a l was p r e p a r e d f r o m NpO2 by a p y r o c h e m t c a l r e d u c t i o n p r o c e d u r e d e s c r i b e d e a r l i e r . 2 In c o n t r a s t to the e a r l i e r work the 1 pct plut o n i u m i m p u r i t y was r e m o v e d f r o m the oxide s t a r t i n g m a t e r i a l by i o n - e x c h a n g e . 3'4 The m a j o r m e t a l l i c i m p u r i t y r e m a i n i n g in the n e p t u n i u m was 200 ppm iron.
ALUMINA THERMOCOUPLE~---_~__~ WELL
APPARATUS AND MATERIALS The thermodynamic properties of the Cd-Np system were determined from electromotive force measurements of cells of the type:
O-RINGTANTALUMFITTINGLEADS ~1 ~IT--~'j'HEL UMRuBBER I ORsERuMVACUUMcAP
Npl NpCIs, KCI-LiCI (eutectic) INp-Cd The cells were similar to those used in the study of the Cd- Pu system;1 a schematic cross-sectionalview is shown in Fig. I. The stainless steel fu
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