The isotope effect and the influence of interstitial impurities on the hydrogen solubility limit in niobium and vanadium
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ppm for sheet stock used in an e a r l i e r study. 8 I n t e r s t i t i a l i m p u r i t i e s w e r e r e m o v e d f r o m the n i o b i u m w i r e b y the UHV t r e a t m e n t below the d e t e c t a b l e level. 5 I m p u r i t i e s in the v a n a d i u m a r e d i s c u s s e d l a t e r . The s o l u b i l i t y l i m i t of h y d r o g e n in each s p e c i m e n was d e t e r m i n e d r e s i s t o m e t r i c a l l y by methods d e s c r i b e d in Ref. 3. A plot of r e s i s t a n c e v s t e m p e r a t u r e T is n e a r l y l i n e a r so long as T is high enough that all h y d r o gen r e m a i n s in solid solution. The c r i t i c a l t e m p e r a t u r e for the n u c l e a t i o n of h y d r i d e p a r t i c l e s in a given m e t a l hydrogen alloy is that t e m p e r a t u r e at which t h e r e is a d i s c o n t i n u i t y in the slope of that p l o t J RESULTS AND DISCUSSION In an e a r l i e r study, 6 the i n i t i a t i o n of h y d r i d e p r e c i p i t a t i o n in Nb c a u s e d a d i s c o n t i n u i t y in the slope of a plot of r e s i s t a n c e vs t e m p e r a t u r e . F o r c o n c e n t r a t i o n s lower than about 0.5 at. pct H, it is difficult to a s c e r t a i n the t e m p e r a t u r e of the d i s c o n t i n u i t y . But, if one plots 649, the d i f f e r e n c e b e t w e e n the r e s i s t i v i t y of n i o b i u m m e t a l and the Nb-H alloy, v s t e m p e r a t u r e , then the d i s c o n t i n u i t y is quite a p p a r e n t (Fig. 1). One can see in Fig. 1 that Ap = 0.099/lf~.cm for 0.14 at. pct D in solid solution. About 0.01/~f~.cm was a t t r i b u t a b l e to a b s o r p t i o n of other i n t e r s t i t i a l i m p u r i t i e s d u r i n g the a n a l y s i s for H which p r e c e d e d the c h a r g i n g with D. Thus, the c o n t r i b u t i o n to the r e s i s t i v i t y was a p p a r e n t l y about 0 . 6 3 / ~ 2 . c m p e r at. pct D. F o r c o n c e n t r a t i o n s of 0.67, 0.75, and 2.67 at. pct D, the v a l u e s were 0.64, 0.65, and 0 . 6 5 / ~ 2 . c m per at. pct D. The p r e s e n t work on p r o t i u m c o n f i r m s the v a l u e 0 . 6 5 5 t.t~2 9cm p e r at. pct H 6, so the isotope effect on r e s i s t i v i t y is v e r y s m a l l , if it e x i s t s at all. The following e x p e r i m e n t was n e c e s s i t a t e d by the fact that a t t a i n m e n t of e q u i l i b r i u m at a given t e m p e r a t u r e r e q u i r e d a waiting t i m e . 6 A s p e c i m e n of Nb-0.14 at. pct D was cycled 10 t i m e s b e t w e e n r o o m t e m p e r a t u r e and 77~ to a s s u r e the e x i s t e n c e of n u c l e a t i o n s i t e s for d e u t e r i d e p r e c i p i t a t e s . 6 Then an e x p e r i m e n t was conducted in which the r e s i s t i v i t y was d e t e r m i n e d at d e c r e a s i n g t e m p e r a t u r e s (Fig. 1). Each value was obtained a f t e r the t e m p e r a t u r e was c o n s t a n t for about 5 min. The a p p a r e n t c r i t i c a l t e m p e r a t u r e T c for d e u t e r i d e p
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