Observation of oxide particles below the apparent oxygen solubility limit in tantalum
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Observation of Oxide Particles Below the Apparent Oxygen Solubility Limit in Tantalum STEPHAN STECURA T h e a p p a r e n t s o l u b i l i t y of oxygen in p o l y c r y s t a l l i n e t a n t a l u m a s d e t e r m i n e d by the X - r a y d i f f r a c t i o n l a t t i c e p a r a m e t e r t e c h n i q u e i s about 1.63 at. p c t at 820~C. H o w e v e r , oxide p a r t i c l e s w e r e i d e n t i f i e d in s a m p l e s containing a s low a s 0.5 at. pct oxygen. T h e s e p a r t i c l e s w e r e p r e s e n t at the g r a i n b o u n d a r i e s and within the g r a i n s . T h e n u m b e r of oxide p a r t i c l e s i n c r e a s e d with i n c r e a s i n g oxygen c o n c e n t r a t i o n in t a n t a l u m . T h e p r e s e n c e of oxide p a r t i c l e s s u g g e s t s that the t r u e s o l u b i l i t y in the p o l y c r y s t a l l i n e t a n t a l u m m e t a l i s p r o b a b l y l o w e r than that r e p o r t e d in the l i t e r a t u r e . T H E p r e s e n c e of oxygen in t a n t a l u m a f f e c t s many of i t s p r o p e r t i e s . T h u s the u s e s of t a n t a l u m m a y be l i m i t e d o r e n h a n c e d b y the p r e s e n c e of d i s s o l v e d oxygen. In the c a s e of t a n t a l u m h e a t p i p e s , high t e m p e r a t u r e f a i l u r e s w e r e a t t r i b u t e d to the p r e s e n c e of oxygen in t a n t a l u m . 1'2 A r e v i e w of the l i t e r a t u r e showed that the r e p o r t e d s o l u b i l i t y v a l u e s f o r oxygen in t a n t a l u m v a r y . F o r e x a m p l e , the s o l u b i l i t y of oxygen at 820~C has b e e n r e p o r t e d a s 2.8 at. p c t , ~ 1.9 at. p e t , 4 and 1.0 at. p c t ) I t i s p o s s i b l e t h a t t h e s e d i f f e r e n c e s might be due to the p r e s e n c e of t a n t a l u m s u b o x i d e s a n d / o r o x i d e s at t h e s e low oxygen c o n c e n t r a t i o n s . If such o x i d e s a r e p r e s e n t in t a n t a l u m , t h e y would affect not only the c o m p a t i b i l i t y of t a n t a l u m with o t h e r m e t a l s but a l s o s u c h p r o p e r t i e s a s d u c t i l i t y and e l e c t r i c a l and t h e r m a l conductivities. A s p a r t of a l a r g e r study involving l i t h i u m c o r r o s i o n of o x y g e n - d o p e d t a n t a l u m , the d i s t r i b u t i o n of oxygen in t a n t a l u m was e x a m i n e d . T h i s s t u d y d i s c u s s e s the d i s t r i b u t i o n of oxygen in t a n t a l u m at 820~C. M A T E R I A L S AND PROCEDURE T a n t a l u m cups (2.29 c m o u t s i d e d i a m . x 1.19 c m long • 0,095 c m w a l l t h i c k n e s s ) and t a n t a l u m coupons (3.18 x 1.91 • 0.095 cm) w e r e c l e a n e d in the m a n n e r d e s c r i b e d in Ref. 6. A f t e r c l e a n i n g , cups and coupons w e r e a n n e a l e d at 1200~ f o r 2 h at 10 -s T o r r p r e s s u r e . R a n d o m l y s e l e c t e d cups and coupons w e r e analyzed for impurities. The major impurities are i n d i c a t e d in T a b l e I. T h e m a x i m u m t o t a l
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