On the calculations of the diffusion coefficients of oxygen and nitrogen in niobium
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AND
R. E. REED-HILL
The Snoek peaks for oxygen and n i t r o g e n in n i o b i u m have been r e m e a s u r e d u s i n g a t o r s i o n p e n d u l u m equipped with LVDT t r a n s d u c e r s connected to an o s c i l l o g r a p h for a u t o m a t i c data r e c o r d i n g . The peak t e m p e r a t u r e s were d e t e r m i n e d u s i n g a new n u m e r i cal a n a l y s i s of the peak shape. T h e s e new low t e m p e r a t u r e data have b e e n c o m b i n e d with high t e m p e r a t u r e d i r e c t data and i n t e r m e d i a t e t e m p e r a t u r e high f r e q u e n c y i n t e r n a l f r i c t i o n data r e p o r t e d in the l i t e r a t u r e . When it is a s s u m e d that the oxygen and n i t r o gen occupied o c t a h e d r a l p o s i t i o n s in the n i o b i u m m a t r i x , the diffusion coefficients for both oxygen and n i t r o g e n a t o m s follow a s i m p l e A r r h e n i u s b e h a v i o r . In the t e m p e r a t u r e r a n g e of this a n a l y s i s (140 to 1000~ for oxygen and 270 to 1800~ for n i t r o g e n ) the and D = (2.6E - 6) exp equations D = (5.3E - 7) exp - (1.095E5) RT
(1.523E5) apply for the RT
diffusion of oxygen and n i t r o g e n , r e s p e c t i v e l y . Entropy of a c t i v a t i o n c a l c u l a t i o n s were made u s i n g the above a c t i v a t i o n e n e r g i e s and f r e q u e n c y f a c t o r s , and the r e s u l t s a g r e e well with r e p o r t e d v a l u e s for other s y s t e m s as well as with the t h e o r y of K e y e s . T h i s good a g r e e m e n t s u g g e s t s that the a s s u m p t i o n that oxygen and n i t r o g e n p r e f e r o c t a h e d r a l s i t e s is p r o b a b l y c o r r e c t .
THE
l i t e r a t u r e c o n c e r n i n g the diffusion of oxygen and n i t r o g e n in n i o b i u m exhibits s e v e r a l i n c o n s i s t e n c i e s . F i r s t , c a l c u l a t i o n s by B e s h e r s I of the e n t r o p y of a c t i vation for the diffusion of oxygen in n i o b i u m , b a s e d on P o w e r s and D o y l e ' s 2 i n t e r n a l f r i c t i o n data, have indicated that the o x y g e n - n i o b i u m data a r e in conflict with those obtained with o t h e r bcc m e t a l - i n t e r s t i t i a l atom s y s t e m s . This has b e e n i n t e r p r e t e d as p o s s i b l y i m p l y i n g that oxygen a t o m s favor t e t r a h e d r a l s i t e s . l ' 3 However, in m o s t bcc m e t a l s it is b e l i e v e d that o c t a h e d r a l s i t e s a r e p r e f e r r e d . Second, O g u r t a n i and U y g u r 4 have proposed, as a r e s u l t of t h e i r e x p e r i m e n t s , that the a c t i v a t i o n e n e r g y for the diffusion of n i t r o g e n i n n i o b i u m d e c r e a s e s with t e m p e r a t u r e . This is in conflict with the u s u a l concept of A r r h e n i u s behavior. In this p a p e r new low t e m p e r a t u r e i n t e r n a l f r i c t i o n data a r e c o m b i n e d with high t e m p e r a t u r e diffusion m e a s u r e m e n t s and i n t
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