Evidence of clustering in liquid Zn-Sb alloys from viscosity measurements
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MANYa t t e m p t s
have b e e n made to obtain i n f o r m a tion about the atomic s t r u c t u r e of liquids f r o m v i s c o s i t y data. McLaughlin and Ubbelohde 1 have a n a l y z e d such data by c o n s i d e r i n g that any deviation from a s t r i c t exp o n e n t i a l dependence on t e m p e r a t u r e at t e m p e r a t u r e s c l o s e to the m e l t i n g point i s indicative of c i u s t e r i n g or p r e f r e e z i n g phenomena in the melt. They e s t i m a t e d that c l u s t e r s of about 100 a t o m s in size e x i s t e d in s o m e m e t a l l i c m e l t s . More r e c e n t i n v e s t i g a t i o n s have tended to discount such d e p a r t u r e s of data f r o m the t h e o r e t i c a l c u r v e s , and a t t r i b u t e such d e v i a t i o n s to i m p u r i t i e s or oxides. 2 V i r t u a l l y all s t u d i e s of p r e f r e e z i n g p h e n o m e n a have b e e n p e r f o r m e d on pure e l e m e n t s , and little data i s a v a i l a b l e on a l l o y s . F i s h e r and P h i l l i p s , 3 however, s t u d i e d the v i s c o s i t y of a l l o y s i n the P b - S n and Cd-Sb s y s t e m s , and found that while the v i s c o s i t y behaved n o r m a l l y just above the n o r m a l liquidus t e m p e r a t u r e , c e r t a i n alloys in the Cd-Sb s y s tem did show a m a r k e d i n c r e a s e in v i s c o s i t y when s u p e r c o o l e d below t h e i r s t a b l e liquidus t e m p e r a t u r e . In m e t a l s y s t e m s v i s c o s i t y m i n i m a f r e q u e n t l y were o b s e r v e d at e u t e c t i c compositions~ and v i s c o s i t y m a x i m a at the c o m p o s i t i o n s at which compounds f o r m e d in the solid. 4 Some e v i d e n c e has b e e n r e p o r t e d i n d i cating that the v i s c o s i t y also i n c r e a s e s at the compos i t i o n c o r r e s p o n d i n g to the solid solubility l i m i t s , s'6 b u t m o r e r e c e n t data on s e v e r a l s y s t e m s do not a g r e e with this c o n c l u s i o n , s In o r d e r to obtain m o r e i n f o r m a t i o n r e g a r d i n g the c o m p o s i t i o n dependence of v i s c o s i t y , it was decided to i n v e s t i g a t e the Zn-Sb s y s t e m , since the t h e r m o d y n a m i c p r o p e r t i e s of this s y s t e m exhibit l a r g e negative d e v i a t i o n s from ideality and three i n t e r m e t a l l i c c o m pounds, ZnSb, Zn4Sbs, and Zn3Sb2 f o r m all c l o s e l y spaced at the c e n t e r of the phase d i a g r a m . Yao ~ has studied this s y s t e m p r e v i o u s l y , but he c o n c e n t r a t e d his m e a s u r e m e n t s on this s y s t e m in the v i c i n i t y of the two pure c o m p o n e n t s , and d e t e r m i n e d only a few e x p e r i m e n t a l points in the composition r e g i o n in which the compounds f o r m . Our r e s u l t s were found to i n d i cate a much m o r e c o m p l i c a t e d and p r o n o u n c e d dependence of v i s c o s i t y on c o m p o s i t i o n than h a s b e e n found p r e v i o u s l y b
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