The inert gas effect on the rate of evaporation of zinc and cadmium
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T. J. O'KEEFE,
AND
F. KISSLINGER
An e x p e r i m e n t a l study has b e e n made to i n v e s t i g a t e the effect of a r g o n and h e l i u m on the r a t e of e v a p o r a t i o n of zinc and c a d m i u m u n d e r one a t m o s p h e r e p r e s s u r e at t e m p e r a t u r e s r a n g i n g f r o m 500 to 850~ The e x p e r i m e n t a l r e s u l t s w e r e c o m p a r e d with the m a x i m u m r a t e s c a l c u l a t e d using the effusion f o r m u l a a s well as with v a l u e s o b t a i n e d u s i n g two diff e r e n t types of equations b a s e d on k i n e t i c t h e o r y , diffusion t h e o r y , and e m p i r i c a l data. Equations have been d e r i v e d for e x p r e s s i n g the r a t e of e v a p o r a t i o n of zinc and c a d m i u m in both a r g o n and h e l i u m as functions of t e m p e r a t u r e of the liquid zinc and c a d m i u m . It was found that the r a t e s of e v a p o r a t i o n of zinc and c a d m i u m were higher in h e l i u m than in argon, with the d i f f e r e n c e i n c r e a s i n g with i n c r e a s i n g t e m p e r a t u r e . It was a l s o found that the e x p e r i m e n t a l r e s u l t s obtained in a r g o n a g r e e with the c a l c u l a t e d v a l u e s b e t t e r than those obtained in h e l i u m . T H E m a x i m u m r a t e at which m o l e c u l e s of a v a p o r can leave the s u r f a c e of a liquid d u r i n g e v a p o r a t i o n is g i v e n by the effusion f o r m u I a , d e r i v e d f r o m the k i n e t i c t h e o r y of g a s e s :1 W o = P o ( M / 2 I I R T ) 1/~
[l]
where : W o is the r a t e of e v a p o r a t i o n in g m / c m 2 - s , P o is the v a p o r p r e s s u r e of the liquid at t e m p e r a t u r e T in d y n e s / c m ~, M is the m o l e c u l a r weight of the liquid, in g, R is the gas c o n s t a n t , in c a l / m o l - d e g r e e Kelvin, and T is the a b s o l u t e t e m p e r a t u r e , in d e g r e e s K e l v i n .
T h e m a x i m u m e v a p o r a t i o n r a t e would be obtained if a p e r f e c t v a c u u m could be m a i n t a i n e d over the liquid. However, in o r d e r to a c c o u n t for the p r o b a b i l i t y that s o m e of the m o l e c u l e s a r e " r e f l e c t e d " d u r i n g e v a p o r a t i o n , Knudsen 2 has i n s e r t e d alpha, the " e v a p o r a t i o n c o e f f i c i e n t " into Eq. [1]. Since zinc and c a d m i u m e v a p o r a t e as m o n a t o m i c g a s e s , it is r e a s o n a b l e to a s s u m e that the e v a p o r a t i o n c o e f f i c i e n t s a r e unity in the p r e s ent i n v e s t i g a t i o n . E v a p o r a t i o n in the p r e s e n c e of a f o r e i g n gas has b e e n d i s c u s s e d in a n u m b e r of p a p e r s . 3'4 U n d e r t h e s e c i r c u m s t a n c e s the p h a s e t r a n s i t i o n is followed by diffusion into the f o r e i g n g a s . In the case of s t e a d y - s t a t e c o n d i t i o n s both p r o c e s s e s , the phase t r a n s i t i o n and the d i f fusion, occur at the s a m
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