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T H E effects of r e a c t a n t gas dissociation on the k i n e t ics and m e c h a n i s m of g a s / s o l i d r e a c t i o n s a r e b e i n g a c t i v e l y i n v e s t i g a t e d for both p r a c t i c a l and m e c h a n i s tic r e a s o n s . In view of the r e p o r t e d 1-~ enhanced r e a c t i v i t y of d i s s o c i a t e d s p e c i e s it is t e m p t i n g to conclude that a r a d i c a l l y d i f f e r e n t m e c h a n i s m is r e s p o n s i b l e for the f o r m a t i o n of v o l a t i l e r e a c t i o n p r o d u c t s , although the g e n e r a l i t y of this i n f e r e n c e m u s t be i n v e s t i g a t e d f u r t h e r . F o r e x a m p l e , in d i s c u s s i n g the s t r i k i n g l y enhanced, t e m p e r a t u r e - i n s e n s i t i v e , and o x y g e n - p r e s s u r e - i n d e p e n d e n t r e a c t i o n p r o b a b i l i t i e s o b s e r v e d for a t o m i c oxygen i n t e r a c t i n g with Mo and W,* R o s n e r and A l l e n d o r f ~ ' 2 s u g *In the filament temperature range: 1400 to 2000 K and O-atom pressure range: 3 X 10 "t to 6.7 X 10~ Pa.

gested that m e t a l oxide f o r m a t i o n in d i s s o c i a t e d oxygen v e r y l i k e l y o c c u r s via a Rideal (direct gas atom impact) m e c h a n i s m r a t h e r than a L a n g m u i r - H i n s h e l w o o d (ada t o m - a d a t o m ) m e c h a n i s m . Our attention is now d i r e c t e d at a t o m / m e t a l r e a c t i o n s a c c o m p a n i e d b y gas d i s s o l u t i o n and, in view of the a b o v e m e n t i o n e d information, we a d d r e s s e d the specific q u e s t i o n of whether the R i d e a l - t y p e m e c h a n i s m is r e s p o n s i b l e for the f o r m a t i o n of the v o l a tile p r o d u c t s TaO and TaO2 in the O / T a ( s ) r e a c t i o n at t e m p e r a t u r e s above ~ 2000 K. As d e m o n s t r a t e d below, we have found that for this d i s s o c i a t e d g a s / m e t a l r e a c tion the R i d e a l - t y p e m e c h a n i s m can be r u l e d out as the p r e d o m i n a n t m e c h a n i s m for the f o r m a t i o n and d e s o r p tion of oxides. While the oxidation k i n e t i c s of Ta(s) have b e e n s t u d ied e x t e n s i v e l y in m o l e c u l a r oxygen, 4-~s the e x p e r i m e n DANIEL E. ROSNER is Professor, Department of Engineeringand Applied Science, Chemical Engineering Section, Yale University, New Haven, Conn. 06520. }lEE CHUNG, formerly Post-Doctoral Research Associate, Department of Engineeringand Applied Science, Yale University, New Haven, Conn., is now with the Materials Science Division, Argonne National Laboratory, Argonne, Ill. HENG FENG is Graduate Student, Department of Engineeringand Applied Science, Yale University, New Haven, Conn. This work was supported by the United States Air Force Office of Scientific Research, Energetics Division under Grant 73-2487. Manuscript submitted September 4, 1973. METALLURG[CALTRANSACT[ONS

tal data presented here constitutes the first available information on the kinetics of engas