Coarsening kinetics of silica in copper
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AND
K. C. RUSSELL
C o a r s e n i n g ex-periments have b e e n conducted in the c o p p e r - s i l i c a s y s t e m to study the a g ing b e h a v i o r of compound p r e c i p i t a t e s in m e t a l l i c s y s t e m s u n d e r v a r i o u s conditions of t e m p e r a t u r e and a t m o s p h e r e . M o s t of the e x p e r i m e n t s w e r e conducted in a t w o - z o n e h e a t ing f u r n a c e , w h e r e i n a c o p p e r - c u p r o u s o x i d e m i x t u r e w a s h e a t e d in one zone and the c o p p e r - s i l i c a s a m p l e was h e a t e d in the o t h e r zone, a l l e n c l o s e d in a q u a r t z c a p s u l e . The t e m p e r a t u r e of the f o r m e r zone d e t e r m i n e d the oxygen p a r t i a l p r e s s u r e in the s y s t e m and the t e m p e r a t u r e of the l a t t e r zone d e t e r m i n e d the c o a r s e n i n g t e m p e r a t u r e . The s i l i c a p a r t i c l e s f r o m the c o a r s e n e d s a m p l e s w e r e e x t r a c t i o n - r e p l i c a t e d and p h o t o g r a p h e d in an e l e c t r o n m i c r o s c o p e . The a v e r a g e r a d i i obtained f r o m t h e s e p h o t o g r a p h s w e r e u s e d to d e t e r m i n e c o a r s e n i n g r a t e c o n s t a n t s and a c t i v a t i o n e n e r g i e s . I s o t h e r m a l c o a r s e n i n g k i n e t i c s followed the ~3 vs t law, i n d i c a t i n g v o l u m e diffusion control. A c t i v a t i o n e n e r g i e s w e r e o b t a i n e d f o r f o u r d i f f e r e n t s e t s of e x p e r i m e n t a l conditions. In e a c h c a s e the a c t i v a tion p l o t was l i n e a r . F o r c o a r s e n i n g u n d e r v a r i a b l e oxygen p r e s s u r e the e x p e r i m e n t a l a c t i v a t i o n e n e r g y of 530 kJ (127 Kcal) a g r e e s f a i r l y well with the p r e d i c t e d value of 514 k J (123 K c a l ) which t a k e s into c o n s i d e r a t i o n the h e a t s of s o l u t i o n and d i s s o l u t i o n , a s well a s the a c t i v a t i o n e n e r g y for diffusion. F o r c o a r s e n i n g u n d e r c o n s t a n t oxygen p r e s s u r e s the e x p e r i m e n t a l a c t i v a t i o n e n e r g i e s do not c o m p a r e w e l l with the p r e d i c t e d v a l u e s , c a l c u l a t e d on the b a s i s of c o m p l e t e e q u i l i b r a t i o n of oxygen b e t w e e n the g a s e o u s p h a s e and the s a m p l e . H o w e v e r , the e x p e r i m e n t a l a c t i v a t i o n e n e r g i e s of 524 kJ (125 Kcal) and 447 kJ (107 Kcal) l i e in the p r e d i c t e d r a n g e c a l c u l a t e d on the b a s i s of fixed a m o u n t s of oxygen in s o l u t i o n in c o p p e r . In no e a s e i s the a c t i v a t i o n e n e r g y for c o a r s e n i n g equal to that f o r d i f f u s i o n of e i t h e r s p e c i e s in the m a t r i x a s i s s o m e t i m e s a s s u m e d . COARSENING or "Ostwald ripening" is defined as the process where the average size of a dispersion of second phase particles increases with time at virtually c o n s
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