Oxidation of iridium
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WIMBER
AND
HAL
G. KRAUS
I r i d i u m w i r e s s e l f - r e 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 the r a n g e of 1675 to 2260~ (1948 to 2533 K) were oxidized in n a t u r a l l y convected oxygen at p r e s s u r e s in the r a n g e of 0.00132 to 1.32 a t m o s p h e r e s (134 to 1.34 • 105 Pc). The 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 lated by a t h e o r e t i c a l r a t e equation b a s e d upon c o n t r o l of the oxidation r a t e s b y diffusion of Ir (g), IrO 2 (g) and IrO 3 (g) through the gaseous b o u n d a r y l a y e r . Values w e r e obtained for the s t a n d a r d - s t a t e f r e e - e n e r g i e s of f o r m a t i o n of IrO, (g) and IrO, (g), and t h e i r t e m p e r a t u r e d e p e n d e n c i e s were d e s c r i b e d by e m p i r i c a l equations.
T H E i n h e r e n t l y slow r a t e s of oxidation of i r i d i u m at t e m p e r a t u r e s g r e a t e r than a p p r o x i m a t e l y l l 0 0 ~ (1373 K) a r e a s s o c i a t e d with the f o r m a t i o n of oxides which a r e v o l a t i l e and thus c a u s e the s u r f a c e of the m e t a l to r e m a i n b a r e . Of the oxides f o r m e d , IrO2 (g) and IrOs(g) have b e e n m o s t r e a d i l y identified 1-5 while the e x i s t e n c e of IrO (g) and Ir203 (g) has b e e n suggested but not e x p e r i m e n t a l l y s u b s t a n t i a t e d . 4-7 A few s t u d i e s of the k i n e t i c s of oxidation of i r i d i u m have b e e n made u s i n g d i f f e r e n t e x p e r i m e n t a l a p p r o a c h e s and have yielded r e s u l t s that w e r e n o t i c e a b l y dependent upon the e x p e r i m e n t a l a p p r o a c h employed in each case. 8-12 The s t u d i e s did not yield an u n d e r s t a n d i n g of the m e c h a n i s m ( s ) by which the r a t e of oxidation of i r i d i u m is c o n t r o l l e d or a t h e o r e t i c a l r a t e e x p r e s s i o n which can be used in p r e d i c t i n g oxidation r a t e s u n d e r specified c o n d i t i o n s of oxygen p a r t i a l p r e s s u r e , total p r e s s u r e , t e m p e r a t u r e , s p e c i m e n s i z e (and g e o m e t r y ) , gas c o m p o s i t i o n and flow conditions (for f o r c e d or n a t u r a l c o n v e c t i o n flow). The p r e s e n t p a p e r c o n t a i n s a d e s c r i p t i o n of the r e s u l t s of oxidizing s e l f - r e s i s t a n c e heated i r i d i u m w i r e s in n a t u r a l l y convected oxygen and a c o r r e l a t i o n of the e x p e r i m e n t a l r e s u l t s with a t h e o r e t i c a l r a t e equation which i n c o r p o r a t e s a d e p e n dency on the j u s t - m e n t i o n e d p a r a m e t e r s .
EXPERIMENTAL AND
MATERIALS, APPARATUS PROCEDURE
E m i s s i o n - s p e c t r o g r a p h i c and m a s s - s p e c t r o m e t r i c a n a l y s e s of the two lots of the c o m m e r c i a l l y p u r e i r i d i u m w i r e used in the p r e s e n t study a r e contained in Table I along with a n i n d i c a t i o n of the s o u
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