Thermal diffusion in kinetic and equilibrium measurements by levitation technique
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T H E l e v i t a t i o n of a d r o p l e t of m e t a l in an e l e c t r o m a g netic f i e l d o f f e r s , a s an e x p e r i m e n t a l t e c h n i q u e , c e r t a i n a d v a n t a g e s o v e r c o n v e n t i o n a l c r u c i b l e and f u r n a c e t e c h n i q u e s . F o r g a s - l i q u i d m e t a l s t u d i e s , the t e m p e r a t u r e r a n g e i s i n c r e a s e d at both l i m i t s and the p r o b l e m of m e t a l c o n t a m i n a t i o n i s r e d u c e d . V i g o r o u s m i x i n g in the m e t a l p r e c l u d e s a s i g n i f i c a n t r e s i s t a n c e to m a s s t r a n s f e r in that p h a s e . The p o s s i b i l i t y of m e a s u r i n g s u r f a c e tension during a kinetic experiment 1 allows a direct m e a s u r e of s u r f a c e r e s i s t a n c e . The final and p e r h a p s m o s t i m p o r t a n t a d v a n t a g e i s the s p e e d at which e x p e r i m e n t s m a y be p e r f o r m e d . The l e v i t a t i o n a p p a r a t u s i n v o l v e s a r e l a t i v e l y s m a l l m a s s and c o n s e q u e n t l y , t e m p e r a t u r e s t e a d y - s t a t e m a y be r e a c h e d in m i n u t e s r a t h e r than h o u r s o r even d a y s . H o w e v e r , an i n t r i n s i c d i s a d v a n t a g e of the technique i s that d u r i n g l e v i t a t i o n e l e c t r i c a l e n e r g y i s c o n t i n u o u s l y c o n v e r t e d to heat withir the d r o p . To m a i n t a i n a t i m e - i n v a r i a n t t e m p e r a t u r e , heat i s d i s s i p a t e d v i a a cold g a s s t r e a m flowing p a s t the l e v i t a t e d d r o p . The n e c e s s a r y s t e e p t e m p e r a t u r e g r a d i e n t at the g a s - m e t a l i n t e r f a c e r e s u l t s in p r o p e r t y v a r i a t i o n s in the g a s p h a s e and m a y give r i s e to a s i g nificant t h e r m a l diffusion flux. The l a t t e r i s p a r t i c u l a r l y t r u e when the g a s m i x t u r e c o n t a i n s s p e c i e s whose m o l e c u l a r s i z e s a r e quite d i f f e r e n t . The net flux of a s p e c i e s m a y t h e r e f o r e be a r e s u l t of m o l e c u l a r and t h e r m a l diffusion and c o n v e c t i o n , and at s t e a d y - s t a t e a c o n c e n t r a t i o n g r a d i e n t m a y e x i s t in the gas p h a s e n e a r the g a s - m e t a l i n t e r f a c e . To p r o p e r l y i n t e r p r e t k i n e t i c and e q u i l i b r i u m m e a s u r e m e n t s one m u s t account q u a n t i t a t i v e l y f o r s i m u l t a n e o u s m o l e c u l a r and t h e r m a l diffus i o n , and convection. So f a r this h a s not b e e n done and c o n s e q u e n t l y the l e v i t a t i o n technique h a s not b e e n fully e x p l o i t e d a s an e x p e r i m e n t a l tool. The l e v i t a t i o n technique was f i r s t s u g g e s t e d in 1923,2 with i t s f i r s t s i g n i f i c a n t a p p l i c a t i o n by O k r e s s e t a l . 3 Since then, the technique h a s b e e n u s e d in m a n y d i f f e r ent w a y s with v a
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