Thermomigration of hydrogen in Palladium
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f r a m e of r e f e r e n c e . T h e s t e a d y s t a t e s o l u t e d i s t r i b u t i o n f o r s u c h s y s t e m s i s g i v e n , by i r r e v e r s i b l e t h e r m o d y namics, 1 as ~Y in
Ci : (Qi*/R) w ( I / T )
w h e r e Ci i s t h e s o l u t e c o n c e n t r a t i o n . T h e p a r a m e t e r Qi* i s c a l l e d t h e ' h e a t of t r a n s p o r t ' a n d i s t h e q u a n t i t y c h a r a c t e r i s t i c of t h e p h e n o m e n o n ; i t s s i g n a n d m a g n i t u d e d e t e r m i n e t h e d i r e c t i o n a n d m a g n i t u d e of s o l u t e m o v e m e n t , i.e., a p o s i t i v e Q* indicates movement to l the colder regions. For those interstitial solutes which exist as gases in t h e p u r e s t a t e a t o r d i n a r y t e m p e r a t u r e s the techn i q u e of t h e r m o o s m o s i s m a y b e u s e d t o e v a l u a t e Qi*. T h e o n l y p r e v i o u s s t u d y of t h e r m o o s m o s i s in m e t a l s a p p e a r s to b e t h a t of G o n z a l e z a n d O r i a n i . 2 A t e m p e r ature gradient is maintained across the metal memb r a n e , e a c h f a c e of w h i c h i s c o n n e c t e d t o a r e s e r v o i r of s o l u t e g a s . T h e c o n c e n t r a t i o n of s o l u t e d i s s o l v e d in t h e m e m b r a n e a t e a c h s u r f a c e , a n d t h u s t h e c o n c e n tration gradient through the membrane, may be obtained by m e a s u r i n g t h e s o l u t e g a s p r e s s u r e s in t h e c o n n e c t ing reservoirs. At the steady state, the equation relati n g Qi* to t h e m e a s u r e d g a s p r e s s u r e s i s 3
ln Ph/Pc = - 2 ( H i + Q / )
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w h e r e H/ i s t h e h e a t of s o l u t i o n of t h e s o l u t e in t h e m e m b r a n e on a g r a m - a t o m b a s i s , a n d Ph a n d Pc a r e t h e p r e s s u r e s of t h e g a s i n e q u i l i b r i u m w i t h t h e m e m brane faces at temperatures T h a n d Tc, r e s p e c t i v e l y . The experimental arrangement used for the H-Pd
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Thermomigration of Hydrogen in Palladium W. A. O A T E S AND J. G. S H A W T H E h e a t of t r a n s p o r t f o r h y d r o g e n t h e r m o m i g r a t i o n in p a l l a d i u m h a s b e e n m e a s u r e d o v e r t h e t e m p e r a t u r e r a n g e 4 0 0 ~ t o 560~ u s i n g a t h e r m o o s m o s i s technique. T h e v a l u e s o b t a i n e d w e r e e s s e n t i a l l y i n d e p e n d e n t of temperature, the average being +1.5 • 0.5 kcal per g a t o m . T h i s i n d i c a t e s a s l i g h t s e g r e g a t i o n of h y d r o g e n to t h e c o l d e r r e g i o n s . P e r h a p s t h e s i m p l e s t m e t a l l i c s y s t e m to c h o o s e f o r e x p e r i m e n t a l i n v e s t i g a t i o n of t h e r m o m i g r a t i o n is the interstitial solid solution. Solute mobility is high at quite moderate temperatures and effectively only the solute moves, the solvent lattice serving as a fixed W. A. OATES is Senior Lecturer, Department of Metallurgy, University of Newcastle,
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