Partial Heats of Mixing in the Bi-Sn system
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AND
M. J. POOL
P a r t i a l heats of m i x i n g have been m e a s u r e d at 725 K for liquid Bi-Sn a l l o y s u s i n g a liquid m e t a l solution c a l o r i m e t e r . I n t e g r a l heats of m i x i n g w e r e c a l c u l a t e d and w e r e found to be e n d o t h e r m i c . The e x p e r i m e n t a l data a r e a n a l y z e d u s i n g the s u b r e g u l a r model and the modified q u a s i c h e m i c a l model.
IN a p r e v i o u s p a p e r 1 a model was
p r o p o s e d for d e s c r i b i n g heat of m i x i n g c u r v e s for b i n a r y solid and liquid a l l o y s . The p r o p o s e d m o d e l was applied to 84 b i n a r y s y s t e m s , with the B i - S n s y s t e m b e i n g one of the s y s t e m s not fitting the model. The B i - S n data u s e d in the c a l c u l a t i o n s w e r e taken f r o m H u l t g r e n e t al. 2 They a r e b a s e d on e l e c t r o m o t i v e force m e a s u r e m e n t s of Seltz and D u n k e r l e y 3 (608 K) and supported by the d i r e c t c a l o r i m e t r i c m e a s u r e m e n t s of Huber 2 (748 K). T h e s e heats of m i x i n g a r e e n d o t h e r m i c , being in the r a n g e 0 to 25 cal p e r g - a t o m at 608 K. However, o l d e r i n v e s t i g a t i o n s b a s e d on c a l o r i m e t r i c m e a s u r e m e n t s by Magnus and M a n n h e i m e r 4 (600 K) and K a w a k a m i 5 (623 K) r e s u l t in e x o t h e r m i c heats of m i x i n g in the r a n g e 0 to -160 cal p e r g - a t o m . B e c a u s e of the d i s c r e p a n c i e s between these i n v e s t i g a t i o n s and the a v a i l a b i l i t y of e q u i p m e n t capable of d i r e c t m e a s u r e m e n t of p a r t i a l heats of m i x i n g , a r e i n v e s t i g a t i o n of this s y s t e m by solution c a l o r i m e t r y was u n d e r t a k e n . E X P E R I M E N T A L PROCEDURE The liquid m e t a l s o l u t i o n c a l o r i m e t e r u s e d in this work is of the t w i n - w e l l type o p e r a t e d in a d i f f e r e n t i a l mode. It is s i m i l a r in d e s i g n to other d i f f e r e n t i a l t w i n well c a l o r i m e t e r s d e s c r i b e d in the l i t e r a t u r e 6'7 and is c o m p l e t e l y d e s c r i b e d by S b a r k e y . 8 Two i d e n t i c a l c a l o r i m e t e r wells a r e located s y m m e t r i c a l l y within a 12-in. d i a m a l u m i n u m block. Each well c o n t a i n s a m u l t i j u n c t i o n t h e r m o p i l e c o m p o s e d of 96 c h r o m e l a l u m e l t h e r m o c o u p l e s . F o r t y - e i g h t of these couples lie a d j a c e n t to a s t a i n l e s s s t e e l tube which c o n t a i n s the s o l v e n t bath. The r e m a i n i n g 48 couples a r e adj a c e n t to the a l u m i n u m block. T h i s c r u c i b l e - t h e r m o pile a r r a n g e m e n t is v e r y s i m i l a r to that d e s c r i b e d by Kleppa 6 The two t h e r m o p i l e s a r e connected in s e r i e s so that the e l e c t r o m o t i v e force output f r o m one opposes the e l e c t r o m o t i v e f
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