Kinetics of evaporation/oxidation of iridium
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a n e a r l i e r s t u d y~ i r i d i u m w i r e s w e r e s e l f - r e s i s t a n c e - h e a t e d to t e m p e r a t u r e s in t h e r a n g e of 1675 to 2260°C ( 1 9 4 8 to 2533 K) in n a t u r a l l y c o n v e c t e d o x y g e n at p r e s s u r e s in the r a n g e of 1.32 × 10-3 to 1.32 a r m (134 to 1.34 × 105 P a ) . T h e e x p e r i m e n t a l r e s u l t s w e r e c l o s e l y c o r r e l a t e d by a t h e o r e t i c a l r a t e e q u a t i o n b a s e d upon a m o d e l in w h i c h the r a t e s of o x i d a t i o n w e r e a s s u m e d to b e c o n t r o l l e d by the r a t e of d i f f u s i o n of I r (g), IrO2 (g) and IrO3 (g) t h r o u g h t h e g a s e o u s b o u n d a r y l a y e r a s s o c i a t e d with the n a t u r a l - c o n v e c t i o n flow p r o c e s s . V i r t u a l e q u i l i b r i u m conditions w e r e b e l i e v e d to e x i s t at the g a s - m e t a l interface. The p r e s e n t study involved heating iridium w i r e s in n a t u r a l l y c o n v e c t e d o x y g e n a n d a i r to the s a m e t e m p e r a t u r e s e m p l o y e d in the e a r l i e r s t u d y but with the r a n g e of p r e s s u r e s e x t e n d e d down to 9.9 × 10-8 a r m (10-2 P a ) . O x i d a t i o n in f o r c e - c o n v e c t e d s t r e a m s of o x y g e n a n d a i r w a s a l s o i n v e s t i g a t e d in t h e p r e s e n t study.
T h e a p p a r a t u s d e s c r i b e d in d e t a i l e a r l i e r1 w a s m o d i f i e d s l i g h t l y a t the o u t s e t of t h e p r e s e n t s t u d y . A n oil d i f f u s i o n p u m p w a s hung f r o m t h e b o t t o m of the c y l i n d e r upon w h i c h the o x i d a t i o n cell r e s t e d ; a n o p t i cally dense, water-cooled baffle prevented line-ofs i g h t - t r a v e l of s i l i c o n e p u m p - o i l m o l e c u l e s f r o m e n t e r i n g d i r e c t l y into the o x i d a t i o n c e l l . P r e s s u r e s < 1 . 3 2 × 10 -4 a r m ( < 1 3 . 4 Pa) w e r e m e a s u r e d and a u t o m a t i c a l l y c o n t r o l l e d b y m e a n s of a high p r e s s u r e ion gage s y s t e m w h i c h w a s c a l i b r a t e d a g a i n s t a M c L e o d gage. T h e e x p e r i m e n t a l p r o c e d u r e u s e d e a r l i e r in p h o t o g r a p h i c a l l y m e a s u r i n g the w i r e d i a m e t e r a s a f u n c tion of t i m e w a s i m p r o v e d s l i g h t l y in the p r e s e n t s t u d y ; a m i n o r c o r r e c t i o n w a s m a d e f o r the v a r i a t i o n of the w i d t h of t h e f r i n g e z o n e at t h e e d g e o f the film i m a g e with v a r i a t i o n of t h e o x i d a t i o n t e m p e r a t u r e .
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T h e i r i d i u m w i r e u s e d in the p r e s e n t a n d p r e v i o u s1 s t u d i e s w a s c o m m e r c i a l l y p u r e . T h e w i r e u s e d in the p r e v i o u s s t u d y w a s f r o m two lots l a b e l e d " L o t 1 and Lot 2 . " Of t h e i m p u r i t i e s w h o s e c o n c
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