Investigation of the convective-diffusion limited oxidation of molybdenum by the rotating disk method
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o x i d a t i o n of m o l y b d e n u m h a s been i n v e s t i g a t e d by a v a r i e t y of t e c h n i q u e s o v e r the p a s t d e c a d e , x-~ The m a j o r i t y of t h e s e s t u d i e s s-~ w e r e p e r f o r m e d at high t o t a l g a s p r e s s u r e s (> ~ 50 t o r r ) w h e r e the o x i d a t i o n r a t e is i n f l u e n c e d by m a s s t r a n s f e r in the g a s a d j a c e n t to the s u r f a c e . The r e s u l t s of t h e s e i n v e s t i g a t i o n s have been r e v i e w e d by R o s n e r . v The o b j e c t of the p r e s e n t work w a s to m e a s u r e the o x i d a t i o n k i n e t i c s of m o l y b denum u s i n g low c o n c e n t r a t i o n s of oxygen in an i n e r t diluent at a t o t a l p r e s s u r e of 1 a t m in a flow s y s t e m in which the c o n t r i b u t i o n of c o n v e c t i v e diffusion could be p r e d i c t e d t h e o r e t i c a l l y . The i m p o r t a n c e of a c c u r a t e a s s e s s m e n t of g a s p h a s e d i f f u s i o n a l r e s i s t a n c e h a s been d i s c u s s e d by R o s n e r . ~ The r o t a t i n g d i s k m e t h o d u t i l i z e d in the p r e s e n t w o r k h a s b e e n a p p l i e d p r e v i o u s l y to the G e - I 2 r e a c t i o n . 9 THERMOCHEMISTRY O x i d a t i o n of m o l y b d e n u m at t e m p e r a t u r e s g r e a t e r than l l 0 0 ~ p r o c e e d s by the o v e r a l l r e a c t i o n : Mo(s) + -}Os(g) = ml----(MoO3)m(g)
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The -~ : 1 o x y g e n - t o - m o l y b d e n u m s t o i c h t o m e t r y h a s been d e m o n s t r a t e d b y G u l b r a n s e n et al." and was v e r i fied in the p r e s e n t study. The a v e r a g e d e g r e e of p o l y m e r i z a t i o n of the p r o d u c t t r i o x i d e , m , r a n g e s b e t w e e n one and t h r e e , depending upon the t e m p e r a t u r e and oxygen p a r t i a l p r e s s u r e in the r e a c t a n t g a s . Between 1100 ~ and 2000~ a tough, a d h e r e n t c o a t of MoO2 is p r e s e n t on the o x i d i z i n g s p e c i m e n . The oxide c o a t was o b s e r v e d in the p r e s e n t w o r k and is evident in the p h o t o g r a p h s of o x i d i z e d s p e c i m e n s in Ref. 4. Solid s t a t e diffusion through t h i s c o a t was not the p r i m a r y r e s i s t a n c e to oxidation; the o x i d a t i o n r a t e was t i m e - i n d e p e n d e n t under a l l c o n d i t i o n s and at high t e m p e r a t u r e s , the r a t e was g o v e r n e d c o m p l e t e l y by oxygen D. R. OLANDER is with the Inorganic Materials Research Laboratory, Lawrence Radiation Laboratory, and the Department of Nuclear Engineering, College of Engineering, University of California, Berkeley, Calif. J. L. SCHOFILL, Jr., is with Cogswell Polytechnic Institute, San Francisco, Calif, Manuscript submitted March 23, 1970. METALLURGICAL TRANSACTIONS
m a s s t r a n s f e r t h r o u g h the g a s b o u n d a r y l a y e r . N e v e r t h e l e s s , the g a s p h a s e at the s p e c i m e n s u r f a c e was in c o n t a c t
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