The kinetics of diffusion of hydrogen in niobium
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p e r m e a b i l i t y
of h y d r o g e n t h r o u g h n i o b i u m h a s b e e n d e t e r m i n e d by Rudd1 a n d t h e a c t i v a t i o n e n e r g y f o r p e r m e a t i o n w a s f o u n d t o b e 5208 c a l p e r m o l e . I t has also been shown t h a t h y d r o g e n does n o t p e r m e a t e a l o n g g r a i n b o u n d a r i e s but t h r o u g h t h e c r y s t a l l a t t i c e . G u l b r a n s e n a n d A n d r e wz i n v e s t i g a t e d t h e r a t e of a b s o r p t i o n o f h y d r o g e n in niobium o v e r t h e t e m p e r a t u r e r a n g e o f 250 ° t o 9 0 0 ° C f o r h y d r o g e n p r e s s u r e s up t o 57 t o r r . T h e y f o u n d n o c o n s i s t e n t r a t e l a w g o v e r n i n g a b s o r p t i o n . A l b r e c h t e t a l .3 p e r f o r m e d s i m i l a r a b s o r p t i o n r a t e d e t e r m i n a t i o n s i n t h e t e m p e r a t u r e r a n g e 300 ° t o 700°C. T h e y o b s e r v e d that although the initial r a t e o f a b s o r p t i o n w a s l i n e a r up t o a b o u t 40 t o 50 p c t s a t u r a t i o n , f r o m t h e n o n it d e c r e a s e d a n d finally s t o p p e d w h e n e q u i l i b r i u m w a s r e a c h e d . F o r 600 ° t o 7 0 0 ° C Albrecht et al. found that t h e diffusion activation ene r g y a n d t h e p r e e x p o n e n t i a l c o n s t a n t w e r e 9370 & 600 c a l p e r m o l e , a n d 0 . 0 2 1 5 sq c m p e r s e c . , r e s p e c t i v e l y . O a k w o o d a n d D a n i e l s4 a l s o w o r k i n g i n t h e s a m e t e m p e r a t u r e r a n g e obtained diffusion coefficients that a g r e e v e r y w e l l w i t h t h o s e of A l b r e c h t e t a l . T h e i r a c t i v a t i o n e n e r g y f o r d i f f u s i o n i s a b o u t 9000 c a l p e r mole. H o w e v e r , A l b r e c h t et al. also c o n d u c t e d s o m e desorption experiments and t h e diffusion coefficients obtained from t h e s e experiments w e r e s m a l l e r than t h o s e o b t a i n e d f r o m a b s o r p t i o n m e a s u r e m e n t s . It w a s concluded that a s u r f a c e reaction must have an i n f l u e n c e o n t h e d e s o r p t i o n of h y d r o g e n . A l l i n v e s t i g a t o r s studying the Nb-H s y s t e m have noted the p r o f o u n d e f f e c t of h y d r o g e n g a s p u r i t y a n d t h e s u r f a c e c o n d i t i o n o n a l l r a t e p r o c e s s e s . M o s t of t h e d i s c r e p a n c i e s i n t h e d i f f u s i o n r e s u l t s of d i f f e r e n t i n v e s t i g a t o r s a r e p r o b a b l y due t o s u r f a c e r e a c t i o n s . In t h e p r e s e n t w o r k a s e r i e s o f a b s o r p t i o n e x p e r i m e n t s w e r e p e r f o r m e d by v a r y i n g t h e t i m e a n d t e m p e r a t u r e of exposure. T h e room temperature h a r d n e s s profile t a k e n f r o m each specimen a f t e r t h e diffusion annealing t r e a t m e n t s h o w e d e r r o r function-type beh a v i o r w i t h r e s p e c t t o p e n e t r a t i o n d e p t h . It w a s c o n c l u d e d that t
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